CE70035 - CCS and Energy Production

Question 1

What do FOAK, CCGT, and OCGT represent?

Answer 1

FOAK = First of a kind
CCGT = Combined / Closed Cycle Gas Turbine (gas turbine associated with steam cycle)
OCGT = Open Cycle Gas Turbine

Question 2

What is Cost of Lost Watts?

What are the issues?

Answer 2

The cost of not having electricity available when you need it.

Whilst it may be cheaper to generate electricity from solar in some circumstances, it is necessary to have energy storage available.

The cost of a “lost” kWh (i.e. not having electricity available when you need it) is a lot more than the cost of generating the power.

Question 3

How does energy supply vary for different energy resources?

Answer 3

Wind (zero fuel cost, switches on and off as the wind blows)

Nuclear = baseload (fuel costs very little compared to capex)

Fossil – relatively quick to run up and down, but not good from a cold start. (OCGT faster than CCGT faster than coal)

Hydro - very quick response, limited capacity

This is why you need a number of different power generation technologies – they each have different characteristics

Question 4

What are the largest industrial contributors to GHG emissions?

Answer 4

Iron and steel largest single sector by emissions

•Five main sectors considered responsible for 63% of industrial emissions are iron & steel, cement, aluminium, chemical, and paper.

•Different technologies are more appropriate in some industries compared to others

•Some sectors produce high partial pressure / purity CO2 and are therefore easier to capture from.

Question 5

What are the types of CCS?

Answer 5

1. Post-combustion capture
2. Pre-combustion
3. Oxy-combustion

Question 6

What happens in post-combustion CCS?

Answer 6

Exhaust gases with with CO2 are scrubbed by solvent. The CO2 rich solvent then passes through a solvent regeneration column, and the CO2 is compressed and stored.

Question 7

What happens in pre-combustion CCS?

Answer 7

At energy plants, following gasification reactors, gases containing COx pass through reforming and shift reactors to produce CO2 and H2.
This undergoes separation processes where hydrogen is sent to power plants and CO2 is compressed for storage.

Question 8

What is oxy-combustion CCS?

Answer 8

Oxy-fuel combustion is the process of burning a fuel using pure oxygen, or a mixture of oxygen and recirculated flue gas, instead of air. Since the nitrogen component of air is not heated, fuel consumption is reduced, and higher flame temperatures are possible.

Because oxyfuel combustion results in flue gas that already has a high concentration of CO2, it makes it easier to purify and store the CO2 rather than releasing it to the atmosphere.

Question 9

What factors effect energy decoupling?

Answer 9

Decoupling refers to the disassociation of a utility’s profits from its sales of the energy commodity.

The decoupling trend is explained by at least four factors:
- energy efficiency improvements
- saturation in the ownership levels and improved efficiency of the main domestic appliances
- the unresponsiveness of certain industrial uses, like space heating, to long run output growth
- a structural shift away from energy intensive activities (such as steel making) towards low energy industries (such as services).

Note: this often implies manufacturing off-shoring!

Question 10

What is energy decoupling?

Answer 10

In public utility regulation, decoupling refers to the disassociation of a utility’s profits from its sales of the energy commodity.

Instead, a rate of return is aligned with meeting revenue targets, and rates are adjusted up or down to meet the target at the end of the adjustment period.

This makes the utility indifferent to selling less product and improves the ability of energy efficiency and distributed generation to operate within the utility environment.

Question 11

What are IAMs?

Answer 11

Integrated Assessment Models (IAMs) are sophisticated tools that combine elements of natural science, economics, technology, demographics, and policy to understand complex interactions within human and natural systems.

A large time-dependent model, which marches forwards in time with sub-models of (in this case)
- time-dependent energy demand, pricing, etc.
- features of different energy supply technologies (capital cost, operational cost, ramp rates, CO2 emissions, fuel cost) etc
- any energy storage on the system
- almost all models assume some form of cost reduction with time through learning

They integrate multiple disciplines to produce an overall model.

By substituting the “allowed” technology types / rates of progress in their development, it is possible to explore (say) which technologies are critical to develop, and the effects of different constraints (say, how much biomass is available)

IAMs help policymakers and researchers explore different future scenarios, assessing trade-offs and synergies between different sectors and policies. By considering the interconnectedness of these systems, IAMs provide insights into the most effective pathways for achieving sustainability and addressing global challenges like climate change.

Question 12

What are disadvantages of IAMs? (Integrated assessment models)

Answer 12

• challenging to get risk profiles correct
• poor basis of information
• Will not necessarily choose the “best” path, can choose a pathway which is sub-optimal based on what is best at a particular time

Question 13

Why do IAMs choose CCS?

Answer 13

• CCS can be integrated into an existing energy system without making large changes to the overall system.
• Renewables tend to become more expensive to the system at higher penetration rates (their intermittency impacts more on the system as a whole).
• If the system includes industrial sources of CO2, CCS is one of the only ways of decarbonising these emissions.
• There are some other emissions that are exceptionally expensive to decarbonise (say air travel). Once the cost of decarbonising the emission at source is more than the cost of BECCS, the system will choose BECCS

Question 14

What do LCOE and VOTA stand for?

Answer 14

Levelised Cost of Electricity

Value of Technology Addition

Question 15

What are examples of unconventional oil?

Answer 15

Tight oil (or shale oil) – oil that is trapped in shale rocks. Forms much the same as conventional oil, but is not mobile within the rocks. Needs to be hydraulically fractured to get the oil out.

Oil Shale - rocks that contain kerogen but which have not been heated in the past and so have not matured to produce oil. Pyrolysis of the oil shale produces a synthetic crude oil. Frequently considered for potential fuel production when the cost of crude oil becomes very high – but bad environmental credentials

Oil Sands (tar sands) – a mixture of extremely heavy oil (bitumen) with sand. Huge deposits in Canada and Venezuela (Canada has more oil in oil sands than the rest of the world put together has oil). Questionable environmental credentials. Different methods exist to enhance production (though many areas are simply strip mined), many relying on advances in directional drilling.

Question 16

What are the conventional ways of mining coal?

Answer 16

Deep mines (pits)
Open-cast mining. (Digging down to the deposit from the surface, remove the overburden (the stuff above that isn’t coal) and dig out with a digger. Much cheaper than deep mining)

Question 17

What are unconventional methods of obtaining hydrocarbon gas?

Answer 17

Fracking

Gas hydrates – vast stores of methane, stored in ice-like molecular cages. One of the largest sources of hydrocarbons available.

Question 18

What are unconventional means of obtaining coal?

Answer 18

Underground coal gasification

– Dig a hole down to an unmineable coal seam
– Drill another hole a little way away in the seam
– Drill through the seam (directional drilling again) and link the holes (you can also do it by hand if you want the world’s worst job).
– Pump down air or oxygen to the first hole, and set the seam on fire.
– The hot combustion products move through and gasify the remainder of the coal, produce syngas at the other hole.

Question 19

How does Reserves compare to Resources?

Answer 19

Reserves = profitably recoverable using today’s resources and price.

Resources = how much is out there.

Question 20

List main uses of fossils fuels:

Answer 20

Coal:
- Chemicals (area of growth) – gasification covered in separate lecture(s)
- Power stations, Iron and Steel (coking coal), Cement production. Can be substituted for in some areas by biomass.
- Some heating (being phased out)

Oil:
- Transport fuels (vehicles, aviation)
- Petrochemicals
- Some heating and power (being phased out)

Natural Gas
- Power production
- Limited Chemicals
- Heating

Question 21

What is pulverised coal combustion?

Answer 21

The principal means of the generation of electricity from coal worldwide since the middle of the twentieth century
Technology for the large scale utilisation of coal (other solid fuels) for generation of power and heat

How does it work?
* A mixture of pulverised fuel (100 μm) and air is injected into the combustion chamber (boiler)
* Combustion takes place within the boiler while the fuel is in suspension
* Heat is transferred to the boiler walls and then to the heat exchange tube banks as the combustion gases pass through the boiler
* Steam is raised by passing water through tubes located in the boiler
Some ash falls to the bottom of the boiler (bottom ash), some is carried over with the flue gases (fly ash)
* The flue gas is later cleaned up (i.e., remove NOx, SOx, particulates)
* Around 15 % CO2 in the exhaust.

Question 22

What energy resource reserves are available and recoverable, from high to low?

Answer 22

Renewable:
1. solar
2. wind
3. geothermal
4. biomass
5. OTEC (ocean thermal energy conversion)
6. hydro

Non renewable:
1. coal
2. petroleum
3. natural gas
4. nuclear

Question 23

What is the most costly type of energy plant to construct?

Answer 23

Nuclear

(& Open cycle gas turbine plants (OCGT)?)

Question 24

How do open and closed cycle gas turbines differ?

Answer 24

In the open cycle gas turbine, the air enters from the atmosphere and passes through the compressor, combustor and turbine, so all working flow releases into the atmosphere.

In the closed cycle gas turbine, the working flow is continuously recirculated through the gas turbine.

Question 25

How do the costs of fossil plants with decarbonisation systems and renewable energy plants differ?

Answer 25

The cost (pre-development, variable and fixed O&M, capital, fuel etc) of some (gas and coal) decarbonised fossil plants is less than or equal to that of renewable energy.

Question 26

How will the best system of energy resources be organised in future years (I.e. by 2050)?

How can this be shown graphically (and compared to now* 2012)?

Answer 26

(Imagine time as x axis and power in GW as y axis)

By 2050, daily demand will roughly have doubled to around 80 GW.

• Demand will fluctuate based on day and time. Fluctuations will be greater by 2050.
• The steady baseline provided by nuclear will have increased (steady as in the supply will not fluctuate with date/time)
• There will be a higher proportion of energy from renewables (supply will fluctuate)
• The rest of the energy will come from convention means (non renewables)

Question 27

What milestones need to be met to reach Net Zero by 2050?

List by year

Answer 27

2021: No new unabated coal plants approved for development.
No new oil and gas fields approved for development and no new coal mines or mine extensions

2025: No new sales of fossil fuel boilers

2030: Universal energy access
All new buildings are zero-carbon-ready
60% of global car sales are electric
Most new clean techs in heavy industry demonstrated at scale
1,020GW annual solar and wind additions
Phase-out of unabated coal in advanced economies
150 Mt low-carbon hydrogen
850 Gw electrolsers

2035: Most appliances and cooling systems sold are best in class
50% of heavy truck sales are electric
No new ICE car sales
All industrial electric motor sales best in class
Overall net-zero emissions electricity in advanced economies
4 Gt CO2 captured

2040: 50% existing buildings retrofitted to zero-carbon-ready levels
50% fuels used in aviation are low emission
~90% existing capacity in heavy industries reach end of investment cycle
Net-zero emissions electric globally
Phase-out of all unabated coal and oil plants

2045: 50% heating demand met by heat pumps
435 Mt low carbon hydrogen
3,000Gw electrolysers

2050: > 85% buildings zero-carbon-ready
> 90% heavy industrial production is low-emission
~70% electricity generation globally from solar PV and wind

Question 28

How may the targets for the milestones for Net Zero by 2050 be displayed graphically?

Answer 28

Imagine year (2020-2050) along x axis and Gt CO2 (-5 to 40) along y axis.

Overall looks like half of bell curve - GtCO2 decreased for all sectors.

2020: Electric sector contributes ~13 Gt CO2
Industry ~ 10
Transport ~ 8
Buildings ~ 5
Other ~ 2

2035: Electric sector contributes ~5 Gt CO2
Industry ~ 9
Transport ~ 5
Buildings ~ 2
Other ~ 0

2050: Electric sector contributes ~ -1 Gt CO2
Industry ~ 1
Transport ~ 2
Buildings ~ 0
Other ~ -2

Question 29

How are our CO2 emissions expected to change over the next 100 years if we start acting on our emissions?

(Graphical)

Answer 29

(Time as x axis, GHG added or removed +/- as y axis)

Over time, gross emissions will increase slightly then decrease and plateau (in the positive). This is as some emissions from farming and heavy industry could persist, so removing their emissions directly from air may be necessary)

There will be a steady amount of GHG removed (in the negative) throughout the years, due to nature based carbon removal.

Around 2040 onwards, gross carbon removed increases (more negative) and then plateaus also.

Net emissions will follow the pattern of gross emissions - increase slightly then decrease and plateau, but will plateau in the negative.

Question 30

What’s embodied energy?

Answer 30

Embodied energy is a calculation of all the energy that is used to produce a material or product, including mining, manufacture and transport.

Question 31

What’s TPES?

Answer 31

Total primary energy supply

Question 32

How do we obtain the 2DS? (2C scenario?)

Answer 32

To achieve the 2DS, energy-related CO2 emissions must be halved by 2050.

On a global basis, total primary energy supply (TPES) will grow in all scenarios.
In the 2DS, TPES increases by some 35% in the period 2009 to 2050.

This is significantly lower than the 85% rise seen in the 6DS and the 65% increase in the 4DS.

Question 33

What is LCOE?

Answer 33

Measures lifetime costs divided by energy production.

The levelized cost of electricity (LCOE) is a measure of the average net present cost of electricity generation for a generator over its lifetime.

It is used for investment planning and to compare different methods of electricity generation on a consistent basis.

Question 34

What are the effects of adding new renewable technologies in our systems

[ in 5 GW increments from a 2030 central scenario (10 GW nuclear, 5 GW gas CCS, 28 GW wind, 20 GW of PV) at the origin].

Answer 34

As you build more of the zero or very low carbon technologies, the overall system cost generally increases, sometimes very sharply, but the CO2 intensity reduces (moving to the left).

[Graph showing total CO2 emissions along x axis and total system cost along y axis, and the total system cost for each type of energy resource steadily decreasing to a point at the 2030 scenario at 85 g/kWh]

Question 35

What is an oil trap?

Answer 35

Oil and gas traps, sometimes referred to as petroleum traps are below ground traps where a permeable reservoir rock is covered by some low permeability cap rock.

This combination of rock can take several forms, but they all prevent the upward migration of oil and natural gas up through the reservoir rock.

Question 36

How/where is coal formed?

Answer 36

Coal contains the energy stored by plants that lived hundreds of millions of years ago in swampy forests. Layers of dirt and rock covered the plants over millions of years. The resulting pressure and heat turned the plants into the substance we call coal.

Question 37

What’s a supply-cost curve?

Answer 37

The supply curve is a graphic representation of the correlation between the cost of a good or service and the quantity supplied for a given period. In a typical illustration, the price will appear on the left vertical axis, while the quantity supplied will appear on the horizontal axis.

Question 38

How does a circulating fluidised bed (CFB) work for fuel combustion?

Answer 38

A bed of fine inert material that has air/oxygen/steam (~ 5-10m/s) suspends material throughout the combustor

Fuel is fed in from the side, is suspended, and combusts providing heat

Steam is raised by passing water through tubes located in the combustor

The mixture of gas and particles are separated using a cyclone, with material returned into the base of the combustor.

Operates at temperatures below 900°C to avoid ash melting and sticking; can be pressurised.

Question 39

What’s tight gas?

Answer 39

Gas that is within a rock stratum of low permeability. Flowrate is low when normal drilling practices are used.

Question 40

What’s shale gas?

Answer 40

Gas from shale rock formations. Shales are semi-compacted mud rocks.

Shale gas is transforming the fossil fuel industry:
- cheap
- clean
- decreasing dependence on hydrocarbons from Middle East

Question 41

What’s gas in place?

Answer 41

All the gas that’s there

Question 42

How have gas reserves changed over the last decade?

Answer 42

Reserves have been increasing with time.

Causes of is include technology improvements and price increases

Question 43

What should/can policy makers do?

Answer 43

Policymakers can take action to accelerate structural changes that alleviate upward pressure on energy prices, including promoting energy efficiency and incentivizing new low- carbon sources of energy production.

These policies would also protect economies from future energy price volatility and accelerate the transition away from fossil fuels, helping to achieve climate change goals.

At present, however, many governments have focused on trade restrictions, price controls, and subsidies, which can be expensive and often exacerbate supply shortfalls and price pressures.

Question 44

What’re natural gas hydrates?

Answer 44

Gas hydrates consist of molecules of natural gas (the chief constituent of natural gas; methane) enclosed within a solid lattice of water molecules. When brought to the earth’s surface, one cubic meter of gas hydrate releases 164 cubic meters of natural gas.

Gas hydrate deposits are found wherever methane occurs in the presence of water under elevated pressures and at relatively low temperatures, such as beneath permafrost or in shallow sediments along deepwater continental margins.

Question 45

How can we minimise the carbon intensity of electricity?

Answer 45

Carbon intensity refers to how many grams of CO2 are released to produce a kWh of electricity.

• reduce (methane) leakage
• use high CO2 capture rates for CCS plants

Question 46

What are the 4 main ranks of coal?

Answer 46

Lignite (brown coal). Highly reactive, not much transformed from peat. Lowest carbon content. High moisture / low efficiency from power stations. Vast amounts burned by Germany.

Sub-bituminous. Black, dull surface. Higher heating value than lignite.

Bituminous. High heat and pressure have transformed this to be a hard black rock.

Anthracite. Highest carbon content. Slightly lower heating value than bituminous coal. Shiny.

Coal consists of C, H, O, N, S and trace and minor species.

Question 47

What’s the R/P ratio?

Answer 47

Reserves to production ratio

The R/P ratio measures the number of years a resource will last if production rates stay the same.

Question 48

What do Dry basis and Dry ash free basis refer to regarding coal?

Answer 48

Dry Basis : not including moisture content.
Dry, Ash Free basis: not including moisture and ash.

Question 49

What is an ultimate analysis, regarding coal?

Answer 49

Ultimate analysis: procedure to determine C, H, N, S (oxygen by difference) in the sample.

Generally, the dry, ash-free (DAF) value is quoted
Why DAF? - Moisture level can change, and no-one wants to pay for rocks.

Question 50

What is LHV?

Answer 50

Lower Heating Value (LHV): A measure of the thermal energy released from combustion of a fuel, NOT including the energy from condensing steam (from the combustion of the H content in the coal).

Question 51

What’s HHV?

Answer 51

Higher Heating Value (HHV): The energy released from combustion of coal or other fuel, INCLUDING the energy from condensing steam.

It is the amount of heat released by the unit mass of fuel (initially at 25C) once it has combusted and the products have returned to 25C. This includes the latent heat of vaporisation of water.

Question 52

How is coal produced?

Answer 52

Plants, trees etc. fall into swamps (anaerobic conditions), and get covered over, go through a number of stages in the production process.

Harder coals have been “cooked” in-situ by geothermal heat.
The older the coal, the higher the temperature and pressure it has been exposed to, the higher the rank (in general).

The process of producing coal from peat is “coalification”.

Question 53

What are the components of coal ash?

Answer 53

SiO2: 40-90
Al2O3: 20-60
Fe2O3: 5-25
CaO: 1-15
MgO: 0.4–4
Na2O: 0.5–3
K2OM 0.5–3
SO3: 0.5–10
P2O5: 0-1
TiO2: 0-2

List of trace elements:
antimony, arsenic, beryllium, boron, cadmium, chlorine, chromium, cobalt, copper, fluorine, lead, manganese, mercury, molybdenum, nickel, selenium, thallium, vanadium, and zinc.

Radioactive: thorium and uranium

Mercury is the main trace element of concern, since it is volatile and not captured easily by standard clean- up of the power station exhaust. Causes birth defects and mental issues (mad hatter).

Question 54

What is the main impurity in coal that is of concern?

Answer 54

Mercury is the main trace element of concern, since it is volatile and not captured easily by standard clean- up of the power station exhaust. Causes birth defects and mental issues (mad hatter).

Question 55

How does pulverised coal combustion work?

Answer 55

• A mixture of pulverised fuel (100 μm) and air is injected into the combustion chamber (boiler)

• Combustion takes place within the boiler while the fuel is in suspension

• Heat is transferred to the boiler walls and then to the heat exchange tube banks as the combustion gases pass through the boiler

• Steam is raised by passing water through tubes located in the boiler

• Some ash falls to the bottom of the boiler (bottom ash), some is carried over with the flue gases (fly ash)

• The flue gas is later cleaned up (i.e., remove NOx, SOx, particulates)

Question 56

Describe the steam cycle within power plants:

Answer 56

The power station heats and evaporates high pressure water to form steam (superheated steam), which is then expanded through a turbine (the High Pressure or HP turbine).

The exhaust from the turbine is then passed back to the boiler (a reheat) and expanded again through an Intermediate Pressure (or IP) turbine.

Finally, the steam is passed back to the furnace one final time, then expanded in the Low Pressure (LP) turbine. Because the steam has expanded quite a lot by this time, this might be two turbines in parallel.

The pressurised water may well be heated up (but not vapourised) via contact with the exhaust gas near the end of the
furnace. This is done in an economiser.

Question 57

Why is coal so ‘dirty’?
What does this lead to?

Answer 57

Due to large amounts of impurities, particularly S, N, and Hg (Mercury).

Unless gasified (which has its own challenges), coal is limited to the standard steam cycle, not the more efficient combined cycle (you would destroy gas turbines by putting solid coal through them).

This means that the efficiency of electricity production is 35 – 45%, as opposed to 55 – 60% (these values creep up slowly with time as metals get better – see discussions of metallurgical limits later in the course).

In addition, coal has a very high ratio of C to H compared to methane (CH4), or more specifically less heat given out per mole of C combusted.

The above two factors mean that coal produces approximately twice as much CO2 per kWh of electricity produced as does methane.

Question 58

How do coal furnaces handle processing different types of coal?

Answer 58

If an electricity generating or heating plant is designed to burn one type of coal then it must continue to be supplied with a similar coal or undergo an extensive and costly redesign in order to adapt to a different type of coal.

Similarly, furnaces designed to use coal that produces high amounts of heat will suffer severe losses in efficiency if they must accept coal that burns with substantially less heat

Question 59

What is natural gas used for?

Answer 59

Power generation
Space heating
Combined heat and power (CHP)
(Steam) reforming to Syngas

Question 60

Describe the distribution/proportion of proven gas reserves around the world:

Answer 60

Reserves have been increasing over the last few decades.

From highest to lowest proportion…

Middle East: 40.3%
CIS: 30.1%
Asia Pacific: 8.8%
North America: 8.1%
Africa: 6.9%
S & Central America: 4.2%
Europe: 1.7%

Question 61

What are the 3 channels through which gas markets respond to price shocks and policies?

Answer 61

Demand reduction
Substitution
Supply responses

Question 62

Key dates impacting energy and energy pricings:

Answer 62

1973/74 - Yom Kippur war

1979 - Iranian revolution

Early 2000s - emergency of China and India as global powers

2008 - financial crisis

2011 - Fukushima nuclear plant accident

2019 - Covid-19

2022 - Russia/Ukraine war

Question 63

Major LNG exporters:

Answer 63

Australia
Qatar
US
Russia
Malaysia
Nigeria
Algeria
Indonesia
Oman

By the 10-100s billion m3

Question 64

What is the issue with LNG production?

Answer 64

The liquefaction process is the largest contributor to GHG emissions of LNG overall. The liquefaction energy demand is normally assumed to be 8–12% of the natural gas throughput, or 4.09–7.66 g CO2eq/MJ.

Question 65

Describe the composition of coal:

Answer 65

Coal consists of C, H, O, N, S and trace and minor species. Mineral matter is often found associated with coal.

The sulfur content of coal may range from low (less than 1 weight percent), through medium (1 to 3 weight percent), to high (greater than 3 weight percent).

Ash yields may range from a low of about 3 percent to a high of 49 percent (if ash yields are 50 percent, or greater, the substance is no longer called coal).

Coal may produce high or low amounts of energy when burned, or contain high or low amounts of the substances that produce organic chemicals and synthetic fuels, or contain higher or lower amounts of the elements that are considered hazardous air pollutants (HAPs).”

Question 66

What may cause changes in reserves?

Answer 66

Technological advancements
New finds
Current fuel prices

Question 67

What determines oil prices?

Answer 67

The balance of supply and demand

OPEC (organisation of petroleum exporting countries) acts to try and keep the price within certain bounds (too high and renewables take over, too low and the cartel loses money), but only controls just under half of production.

Some OPEC members allegedly cheat, and produce too much.

OPEC basically changes the relationship between future and current production, but can’t do too much.
When new types of oil production come on stream that are not accounted for already, the oil price is depressed (see “tight” oil).

Question 68

What are the initial stages in the oil drilling/production process?

Answer 68

1. Geologists go out and survey. Sensitive magnetometers, seismology (generate shock waves to investigate underlying geology.
2. Environmental impact studies, survey the area.
3. Prepare the landscape – access roads, source of water – (drill water well?), pit for cuttings.
4. Construct drill rig
5. Various holes dug, first (large bore) drilling – first section of large diameter conductor pipe put in place.

Question 69

What are the two types of oil well?

Answer 69

1. Exploratory (Wildcat) well. Looking for a new reserve High risk, high reward.
2. Production well.
   Drilled where the company knows that hydrocarbons are present.

Question 70

What equipment is needed to drill a well?

Answer 70

Drill bit
Drill pipe
Bottom hole assembly
Drilling mud

Question 71

What are the requirements of drill bits for drilling an oil well?

Answer 71

Maximise rate of penetration.

Long service life (each time you replace it you have to pull possibly several miles of drill pipe up).

Convey drilling mud to and from the bottom of the well.

Question 72

What are the properties of a drill pipe?

Answer 72

Hollow, thin walled pipe (steel or Aluminium).

Transmits torque and drilling mud to drill bit / well head.

Tested and re-used after a drilling job.

Question 73

Describe the bottom hole assembly:

Answer 73

Includes drill bit (at the bottom)

Heavy pipe, used to apply
weight to the drill bit

Stabilisers

Measurement while drilling

Controls to build angle

The clever bit at the end of the drill pipe. Can be used to build angle for horizontal drilling.

Question 74

What are the functions of drilling mud?

Answer 74

1. Cool / lubricate the drill bit
2. Carry cuttings to surface
   – How? Viscosity! What rheology do we need?
3. Maintain well bore integrity
   – drilling through shale may lead to swelling
4. Hydrostatic pressure prevents fluid ingress into the well whilst it is being drilled.

Question 75

What is BOP?

Answer 75

Blow out preventer - important safety device connecting the rig and well bore.

Some close the annulus around the drill pipe
Some close off the entire wellbore
Some cut off the drill pipe as they shut off the well

Question 76

What are kicks?

Answer 76

A kick is where hydrocarbons overcome the hydrostatic pressure of the mud and enter the well.

Very dangerous – can lead to fire, explosion, etc at the drilling rig.

Blow-out protectors seal the well and prevent the hydrocarbons reaching the surface.

Once a kick is detected, operations are shut down and a heavier drilling mud (kill fluid) is circulated
(through the drill pipe to the BOP and out into the annulus) to increase hydrostatic pressure.

Question 77

What happens in cementing for oil well construction?

Answer 77

1. Cement is pumped down the casing
2. The cement forced up between casing and hole – annulus
3. This seals off wellbore from fresh water
4. The surface casing prevents contamination of aquifers

Cement pumped down to bottom of well Flows back upwards.
It sets the casing in place.
There can be many sections of casing. Each successive section will be smaller in diameter than the previous one.
Sometimes it is necessary to isolate particular strata and a different section of casing might be used for this.
The central casing is the production casing, and the last one drilled.
A “displacement fluid” is used to push the cement down into the annulus.

Question 78

What does a Christmas tree refer to?

Answer 78

Collection of valves used to produce the oil or gas.

Controls the flow out of the well.

Christmas trees are used for both above and below sea installations

Question 79

What are the 3 pathways of thermal conversion?

Answer 79

Pyrolysis

Gasification (biomass + oxygen -> fuel gas)

Combustion

Question 80

What’s syngas made up of?

Answer 80

Mainly CO and H2

Question 81

What are the 4 main gasifier types?

Answer 81

Updraft
Downdraft
Fluidised bed
Entrained flow

Question 82

What is coal gasification?

Answer 82

Coal gasification is the process of producing syngas—a mixture consisting primarily of carbon monoxide, hydrogen, carbon dioxide, methane, and water vapour —from coal and water, air and/or oxygen.

Question 83

What are some uses of syngas?

Answer 83

Ammonia/urea production
Methanol
SNG
Fischer tropsch
Oxy alcohols
Gas turbines

Question 84

Advantages of IGCC (integrated gasification combined cycle) to combustion:

Answer 84

Higher efficiencies
- Coal-firing plant (sub-critical) ~ 34-38%. UK average towards the low end of the range
- Similar efficiencies to operating supercritical power plants 41-43%
- Operating IGCC: 42-43%
- Combined Cycle: 56-59%

CO2 capture
Easier removal of sulphur and nitrogen

Question 85

Define IGCC

Answer 85

Integrated Gasification Combined Cycle

Question 86

Discuss tar regarding gasification:

Answer 86

Tar is one of the biggest issues in biomass gasification

•The amount of tar allowed downstream depends on the application.

•It can cause operational issues downstream,
and is never the desired output from a gasifier.

•Also, its production reduces the overall thermal efficiency of the production of the desired gases.

•Acceptable levels are ~ 0.05 g Nm3 , 0.005 g Nm3 and 0.001 g Nm3 for gas engines, gas turbines and fuel cells, respectively.

•Updraft gasifier – 10 – 20 wt % tar produced from biomass

Destruction of tar - ongoing area of research

Question 87

What is ‘market cap’?

Answer 87

Market capitalization, or market cap, is a measurement of a company’s size.

It’s the total value of a company’s outstanding shares of stock, which include publicly traded shares plus restricted shares held by company officers and insiders

Question 88

What is carbon intensity?

Which 5 countries have the highest carbon intensity of oil production?

Answer 88

Carbon intensity is a measure of how clean our electricity is. It refers to how many grams of carbon dioxide (CO2) are released to produce a kilowatt hour (kWh) of electricity. Electricity that’s generated using fossil fuels is more carbon intensive, as the process by which it’s generated creates CO2 emissions.

From highest to lowest C intensity;
1. Canada
2. Libya
3. Nigeria
4. Algeria
5. Iran

Question 89

How does gasification work?

Answer 89

Gasification is a process that converts organic or fossil-based carbonaceous materials at high temperatures (>700°C), without combustion, with a controlled amount of oxygen and/or steam into carbon monoxide, hydrogen, and carbon dioxide.

As opposed to combustion, which uses an abundance of oxygen to produce heat and light by burning, gasification uses only a tiny amount of oxygen, which is combined with steam and cooked under intense pressure. This initiates a series of reactions that produces a gaseous mixture composed primarily of carbon monoxide and hydrogen.

Question 90

How does an integrated gasification combined cycle (IGCC) work?

Answer 90

Gasify coal or biomass

Burn the gaseous products in a gas turbine, which produces electricity

Produce steam from the exhaust heat and any other sources in the plant to run a steam cycle, which also produces electricity

High efficiency – but very complex

Question 91

What are the main gasifier technology types?
(Industrial brands)

Answer 91

From largest to smallest operating plants…

Shell
Sasol Lurgi
GE
ECUST
E-GAS
Other

Shell is the gasification technology provider that has the largest installed syngas capacity at 28,822 MWth, followed by Sasol Lurgi at 17,753 MWth, and GE at 16,334 MWth. Six of the plants currently under construction will use Shell technology.
When planned capacity is added to current technology, GE switches places with Sasol Lurgi in terms of total capacity.

Question 92

How does an entrained flow gasifier work?

Answer 92

In an entrained-flow gasifier, solid fuel particles are typically fed into the gasifier from the top in a coaxial flow of the gasifying agent (e.g. oxygen and steam, in some cases, carbon dioxide or a mixture of them).

For an entrained flow gasifier, a high carbon conversion within shorter residence time demands high operating temperatures and the use of small, dry fuel particles.

Entrained flow gasifiers are often operated at pressures of 20-40 bar and at a temperature around 1400-1600ºC, above the ash-melting point which ensures the destruction of tar or oils, producing a tar-free-syngas but with the penalty of oxygen consumption.

The flow in an entrained flow gasifier can be represented with four different zones: the near-burner zone (NBZ), the jet expansion zone (JEZ), the external recirculation zone (ERZ) and the downstream zone (DSZ).

The near burner zone is a high temperature region where preheating and pyrolysis of the descending fuel particles take place along with gas phase oxidation reactions. Char gasification reactions are also initiated in the NBZ. However, the large majority of char particles are gasified in the JEZ.
The JEZ is characterized by high axial gas velocities with a considerable amount of flow expansion. Part of the flow in the JEZ is entrained into the ERZ which carries hot combustible gases into the NBZ to assist heat demand for preheating and pyrolysis of the descending fresh, solid fuel particles while the other part is directed to the downstream zone. The residual ash is drained from the bottom (in the DSZ), either as a molten slag or solid particles, depending on the temperature inside the gasifier.

Question 93

What are the features of entrained flow gasifiers?

Answer 93

First developed in the 1950s for gaseous and liquid products – refinery residues, etc

Dry feed, entrained flow, slagging gasifier

Pressurised lock-hoppers allow feeding

Pure oxygen and steam as oxidant / moderator

Flame goes up

Temperature of reactions 1500 – 1600oC

20 – 40 bar pressure

Key to operation is the membrane wall.

Cooling water circulates through, which cools the ash from the gasification process down; this then forms a protective layer on the wall, preventing erosion of the refractory lining.

At the outside of the slag layer, the slag is molten and flows down into a water bath, where it is removed through a lock hopper as a slurry.

Gas leaves the gasifier at 1300 – 1400oC

Recycled “quench” gas reduces the temperature of the gas and solidifies any ash fines remaining in the gas – reduces damage to the syngas cooler.

The gas passes through a HRSG (heat recovery steam generator) to produce steam for process use or power

Question 94

How do updraft gasifiers work?

Answer 94

In an updraft gasifier the downward-moving biomass is first dried by the upflowing hot product gas. After drying, the solid fuel is pyrolysed, giving char, which continues to move
down to be gasified, and pyrolysis vapours which are carried upward by the upflowing hot product gas.

The tars in the vapours either condense on the cool descending solid fuel or are carried out of the reactor with the product gas, contributing to its high tar content.

The product gas from an updraft gasifier thus contains significant amounts of tars and low-molecular hydrocarbons, which contribute to its high heating value. Usually this gas is directly used as a fuel in a closely coupled furnace or boiler.

The ash produced is not molten. Maximum T of ~ 1200 °C in the combustion zone, and 700 – 900oC in the gasification zone.

Residence time of fuel 30 – 60 minutes.
Three options for gasification media: (i) steam / O2, (ii) steam / air, (iii) steam / O2– enriched air.

Typical steam/fuel ratio is high: ~ 1.5

Question 95

What are the features of updraft gasifiers?

Answer 95

The updraft fixed bed (“counter-current”) gasifier consists of a fixed bed of carbonaceous fuel (e.g. coal or biomass) through which the “gasification agent” (steam, oxygen and/or air) flows in counter-current configuration. The ash is either removed dry or as a slag.

Gas in counter-current to feed

Bed continually moves down the gasifier

Char at bottom of the bed undergoes combustion – provides heat for the remaining sections of the bed.

Hot gases move up, gasify bed material.

Pyrolysis zone produces tar – large quantities and main issue with this type of gasifier.

Question 96

What is PF combustion?

How does it compare to entrained flow gasification?

Answer 96

Pulverized fuel combustion

It is similar to entrained flow gasification.

Pulverized fuel (PF) combustion involves grinding solid fuel into very fine particles (pulverized) and burning these particles in a combustion chamber. Entrained flow gasification, like PF combustion, involves breaking down solid feedstock into smaller particles, but instead of burning them, it gasifies the feedstock at high temperatures. Both processes use a fine particle size to facilitate efficient combustion or gasification.

Question 97

What are the features of downdraft gasifiers?

Answer 97

A throated gasifer has a restriction part-way down the gasifier where air or O2 is added. Throat-less gasifiers are also possible.

The temperature rises to 1200–1400 °C and the fuel feedstock is either burned or pyrolysed.

Combustion gases pass over the hot char at the bottom of the bed, where they are reduced to H2 and CO.

High throat temperature ensures that tars are significantly cracked, with further cracking on the hot char at the bottom of the bed. Much lower resulting tar than updraft.

Faster to start up than updraft.

Disadvantages: The constriction affects the types of biomass that can be successfully gasified.

A low moisture content is required (~ 25 wt%).

Ash and dust are still significantly present in the exhaust, so the gas still requires clean-up.

Inherently small – scale. Suitable for small-scale biomass exploitation.

Question 98

How do downdraft gasifiers work?

Answer 98

In a downdraft gasifier the fuel is loaded at the top and a fire is lit in the bottom. A suction blower draws in air either through an air jacket or down through the top. The incoming air allows partial combustion to take place in the lower hearth area.
The heat from that combustion produces pyrolysis above and reduction below. Once the gas leaves the hearth it’s piped out to the cooling and filtration system before being used for work.

Downdraft gasification generally produces a low particulate and low tar gas so it is suited for power generation in small scale applications.

The four basic processes of gasification are noted below.

Drying of the fuel
Pyrolysis
Oxidation (Combustion)
Reduction

Question 99

What is a BFB gasifier?
How does it work?

Answer 99

Bubbling fluidised bed (BFB) gasifier

A bed of fine inert material (e.g., sand) sits at the bottom of combustor, with air, oxygen or steam being blown upwards through the bed just fast enough (~1-3 m/s) to agitate or ‘fluidise’ the material

• Fuel is fed in from the side (usually), mixes with bed material, and combusts (or forms syngas) which leaves upwards
• Steam is raised by passing water through tubes located in the combustor
• Operates at temperatures below 900 °C to avoid ash melting and sticking (defluidisation); can be pressurised

Question 100

What is a CFBG gasifier?
How does it work?

Answer 100

Circulating fluidised bed gasifier

A bed of fine inert material has air/oxygen/steam (~5-10 m/s) suspends material throughout the combustor

Fuel is fed in from the side, is suspended, and combusts providing heat

Steam is raised by passing water through tubes located in the combustor

The mixture gas and particles are separated using a cyclone, with material returned into the base of the combustor

Operates at temperatures below 900°C to avoid ash melting and sticking; can be pressurised

Can incorporate a catalytic bed

Question 101

What are the features of the Kellogg, Brown and Root (KBR) Transport Gasifier (also known as TRIG)?

Answer 101

Advanced Circulating Fluidised bed Gasfier

Basis of the Kemper County IGCC (air-blown)

Based on Fluidised Catalytic Cracking technology (used in refining)

Solids are pneumatically conveyed upwards, gasify and then are returned to the bed via the cyclone

Internal temperature 800 °C (biomass) – 1100 °C (coal) – much lower than other gasifiers

Particularly suited for low-rank, high moisture, high ash fuels (basically, any fuel that requires a long residence time).

Coarse ash from the standpipe.

Question 102

What are the main products of gasification with air-blown producer gas?

Answer 102

CO 22%
H2 14%
CH4 5%
H2O 2%
CO2 11%
N2 46%

Question 103

What are the main outputs from gasification with oxygen-blown synthesis gas?

Answer 103

CO 38%
H2 20%
CH4 15%
H2O 4%
CO2 18%
C2H2 and higher 5%

Question 104

What are the main outputs from indirect-fired-steam gasification?

Answer 104

CO 19%
H2 20%
CH4 8%
H2O 38%
CO2 11%
C2H2 3%

Question 105

What is the mean tar content of the different gasifier types? (in g / Nm3)

Answer 105

Fixed updraft - 50
Fixed downdraft - 1

Fluidised bubbling - 12
Fluidised circulating - 8
Fluidised entrained - 10

Question 106

What are the issues with tar in gasification?

Answer 106

The amount of tar allowed downstream depends on the application.

Its production reduces the overall thermal efficiency of the production of the desired gases.

Acceptable levels are ~ 0.05 g Nm3 , 0.005 g Nm3 and 0.001 g Nm3 for gas engines, gas turbines and fuel cells, respectively.

Updraft gasifier – 10 – 20 wt % tar produced from biomass

Destruction of tar - ongoing area of research

Question 107

What is the Fischer-Tropsch process?

Answer 107

The Fischer–Tropsch process (FT) is a collection of chemical reactions that converts a mixture of carbon monoxide and hydrogen, known as syngas, into liquid hydrocarbons.

These reactions occur in the presence of metal catalysts, typically at temperatures of 150–300 °C and pressures of one to several tens of atmospheres.

Catalysts are needed to
- activate H2
- activate CO
- hydrogenate (CO)ads
- promote chain growth
- mildly promote hydrogenolysis
- promote dehydroxylation

Question 108

What are the 3 main mechanisms occurring in Fischer-Tropsch synthesis?

Answer 108

Carbene

Hydroxy-carbene

CO-insertion

Question 109

What is the Schulz – Flory product distribution?

Answer 109

Probability distribution that describes the relative ratios of polymers of different length that occur in an ideal step-growth polymerization process.

Question 110

What are the main components of the basic steam cycle?

Answer 110

To achieve the conversion of thermal energy to mechanical work energy there are four main components:

1. Boiler or hot-sink (e.g. coal fired boiler): hot combustion gases are used to raise the temperature of high-pressure water up to its saturation value. The water is evaporated to produce high-pressure saturated steam.
2. High-pressure steam is expanded in the turbine to low-pressure wet steam where a portion of the heat is converted to work.
3. Condenser where heat is rejected to a cold-sink. The low-pressure wet steam is completely condensed (The latent heat of condensation is rejected to the cooling water)
4. Pump (pumps condensate back to the boiler, i.e., low temperature saturated water is pumped up to boiler pressure becoming sub-saturated).

Question 111

How is steam cycle thermal efficiency calculated?

Answer 111

ᶯth = (Wout-Win)/Qin

= h2-h3/h2-h1

Question 112

What is dry and wet steam?

Answer 112

Dry or saturated steam does not contain any water droplets and you produce it by heating water in a closed chamber.

Wet or unsaturated steam does contain water droplets.

Saturated (dry) steam is an excellent source for heating.

Question 113

What techniques help to enhance steam cycle performance?

Answer 113

USE OF AN IP (Intermediate Pressure) turbine. Real power plants use 1 HP, 1 IP and likely 2 LP turbines (in parallel), with reheats between the HP / IP and IP / LP turbines. Steam for solvent scrubbing regeneration is extracted from the IP / LP turbine (see Niall’s slides).

Supercritical steam cycle, steam generated at pressure above the critical-point pressure of 221.1 bar gaining ~+ 2% efficiency

Overcome metallurgical limits with the use of new materials

‘Topping’ the steam cycle with a gas cycle, e.g. combined cycle plant: fuel combusted with high-pressure air or O2 and hot combustion gases expanded in a turbine, exiting at high temperature suitable to raise superheated steam ~+ 5% efficiency (gas turbine can operate at much higher temperature than steam turbine)

Combined heat and power (CHP), i.e., utilise the low-grade heat rejected from the cycle (rather than dumping it to the atmosphere via the condenser cooling water), e.g., for district heating.

Question 114

What is torrefaction?

Answer 114

The process of heating biomass in a non-oxidizing atmosphere to promote improvement in its proprieties: energy densification.

Question 115

What are the main dry biomass conversion processes?

Answer 115

Torrefaction: process of heating biomass in a non-oxidizing atmosphere to promote improvement in its proprieties: energy densification

Pyrolysis: Thermal decomposition of biomass wielding bio oil, gas and char at different shares depending on the process temperature

Gasification: Conversion of biomass into gaseous products in the presence of gasifying agent at 700-1300 C

Combustion: High temperature oxidation of biomass aimed at using the enthalpy of combustion in produced gas stream

Question 116

What is comminution?

Answer 116

The reduction in size of the biomass particles. This is a big issue regarding biomass use.

It’s required to make them sufficiently small that they will burn out within the boiler.

It uses a great deal of electricity.
Particles are generally left 1 mm or larger, compared to coal particles which are crushed to 100 microns or smaller.
Very heterogenous in shape – highly non spherical (implications for both modelling and experimentally).

Question 117

Advantages of renewable biomass:

Answer 117

Economic compared to any other renewable

Renewable energy – UK compliance by 2020

High degree of carbon saving – directly replacing coal

Existing plant – maintenance of capacity, speed of conversion

Existing transmission- no need to socialise additional line costs

Only renewable to operate independent of weather/season/time of day

High ramp rates allowing load following

Easier station compliance with Industrial Emissions Directive (IED) in 2016

Question 118

Disadvantages of renewable biomass:

Answer 118

Cost of fuel – much more expensive than coal for a given energy yield

Power stations are set up for coal

Logistics of transport – must be kept dry

Much less energy dense than coal (again, logistics)

Transportation as pellets, then re-grinding works

Concerns over sustainability

Energy cost for milling

Dusty (dust explosion possibility)

Question 119

What’s co-combustion?

Answer 119

Co-combustion is the conversion of two or more fuels in a boiler or boiler system.

Variations on this idea include conversion of two or more fuels in two separate boilers, but with sharing of either hot solids (heat) or gases, which may or may not have heating value.

Question 120

What are the pros and cons of different strategies for feeding fuels into burners?

Answer 120

Feed just after coal grinder, pneumatically convey to burner with coal (one burner):
Simple
Cheap
Possible ash synergies – reduces chances of sticky eutectics
Severely limits the total % of biomass to be burned

Feed coal and biomass separately to different burners:
More complex overall, but each line individually is less complex and can be designed to do the job it is supposed to do
Less coal/biomass ash synergy
Can have quite high biomass ratio

Separate Units:
Less complex for a particular fuel (can design the system better for biomass)
No synergies with coal ash
100 % biomass – less flexible?

Question 121

What are the stages of combustion?

Answer 121

Drying:
Water is released from the particle as steam
Can lead to explosion of particles if fast enough
Endothermic

Devolatalisation / pyrolysis:
Light, and then progressively heavier tars are released from the particle
The tars themselves will begin to burn once outside the particle (governed by mass transfer to the particles). This is exothermic.
The outflow of gas prevents oxygen getting in to the particle.
The process of making the tars volatile is endothermic.

Combustion:
Once the rate at which tar is leaving the particle has slowed sufficiently that oxygen can diffuse back into the particle, the fixed carbon (i.e. the non volatile stuff) will begin to burn.
Once burnout is complete, only ash is left.

Question 122

What’s pumped-storage hydro power?

Answer 122

Pumped-storage hydroelectricity (PSH), or pumped hydroelectric energy storage (PHES), is a type of hydroelectric energy storage used by electric power systems for load balancing. The method stores energy in the form of gravitational potential energy of water, pumped from a lower elevation reservoir to a higher elevation.

Question 123

Pros and cons of hydropower:

Answer 123

Advantages of Hydropower
Carbon Neutral… ish
One of the very few methods of storing energy at a large scale
Rapid response to changes in net demand
Allows control of flooding – Hoover dam originally built to prevent flooding, with electricity as a secondary objective

Disadvantages
People whinge if they are purposely flooded
Fish do not enjoy going through the turbines
Vast amounts of concrete are used
Flooding may lead to releases of GHG

Question 124

How is power output of hydro plants found?

Answer 124

P = η Q ρ g h
Where:
η - turbine efficiency
Q - flowrate
ρ - water density
g - 9.81 m/s2
h - height difference

Question 125

Main turbine types used for hydro power:

Answer 125

Francis turbine

Kaplan turbine

Bulb turbine

Pelton wheel

Question 126

How does a Francis hydro turbine work?

Answer 126

The process of water being diverted through the runner blades results in a force that propels the blades to the opposite side as the water is deflected. That reaction force (as we know from Newton’s third law) is what makes power to be carried from the water to the turbine’s shaft, continuing rotation

The water flowing from the reservoir or dam is made to pass through a pipe with high pressure to the turbine blades.

The blades of the turbines are circularly placed, which means the water striking the turbine’s blades should flow in the circular axis for efficient striking.
So the spiral casing is used, but due to the circular movement of the water, it loses its pressure.

The pressure at the exit of the runner of the reaction turbine is generally less than atmospheric pressure. The water at the exit, cannot be directly discharged to the tailrace. A tube or pipe of the gradually increasing area is used for discharging water from the exit of the turbine to the tailrace.

Question 127

What is a black start?

Answer 127

Black start is the ability of generation to restart parts of the power system to recover from a blackout. This entails isolated power stations being started individually and gradually reconnected to one another to form an interconnected system again.

Question 128

What is pyrolysis?

Answer 128

Pyrolysis is the rapid heating of fuels such as wood or coal in an inert atmosphere / in the absence of oxygen.

The main aim of pyrolysis is to produce pyrolytic oil (Bio oil) – a liquid mixture of hydrocarbons.

The bio-oil is further processed for applications such as chemicals, transport, and energy.

Question 129

Why produce biooil rather than simply process/combust biomass?

Answer 129

Biooil has improved properties/quality.

Biomass has low bulk density and poses challenges when trying to transport it. Pyrolysis can be done on small/local scale, producing biooil which can be transported more easily.

Biooils also have useful properties (such as smoke flavourings) which can be extracted and sold separately.

Question 130

What is biooil?

Answer 130

Bio-oil is the liquid product formed during pyrolysis. It is the main product of fast/flash pyrolysis of biomass with yields as high as 70%–80% .

It is basically a combination of about 80% polar organics and 20% water.

Utilisations pathways:

1) Used in boilers and combustion engines for heat and power generation. It is corrosive requiring the use of stainless steel and other corrosion-resistant materials when burnt directly in boilers and other combustion engines.

2) Is a potential precursor for the production of chemicals. However, separating these products in an economical way for the chemical market and for use as liquid fuels is still a challenge.

3) Fisher-Tropsch fuels and methanol which are useful transport fuels can be obtained from bio-oil using synthesis gas techniques. Bio-oil is usually regarded as a crude product which needs to be upgraded through further processing into high quality liquid fuels and chemicals. However, more research is still required to fully develop the process.

Question 131

What are the main factors affecting bio-oil yield?

Answer 131

Temperature
Oxygen content
Pressure - Bio-oil yield decreases with increasing pressure
Particle size - A higher bio-oil yield is expected from smaller particles
Heating rate - Bio-oil yield increases with increasing heating rate

At low temperatures (400C), yield is high and the composition consists of mall molecules with high oxygen content e.g. small chain alcohols, ketones and aldehydes

At high temperatures, yield is low and there are large carbonaceous molecules e.g. naphthalenes, phenanthrenes

Higher temperatures -> lower yield but also lower oxygen content

Question 132

What waste resources are key for energy production?

Answer 132

Not just for pyrolysis – also gasification and combustion!…

Agricultural Residues (straw, forestry, grasses) – 3 Billion Tonnes per yr1 (65 % forestry residues, 33 % residues of agricultural crops)

Rice husks and straw combined comprise 44 % of the crop residues.

Wood waste (treated, untreated)

Landfilled waste (mixture of plastics with food, wood, plus metal, glass, soil).

Medical Waste

Used Tyres

Food waste (wet)
Sewage sludge (wet)
Slurry from intensive pig / cattle farming (wet)

Question 133

Advantages and disadvantages of the pyrolysis of different materials:

Answer 133

Tyres, Sewage sludge, plastics – high HHV oil, low moisture, potentially high contamination with trace elements, Cl and S

Virgin Wood – low HHV, acidic, low trace elements, low Cl, S

Treated Wood – low HHV, acidic, high trace elements

All wastes potentially inhomogeneous – derived oils difficult to get to a global standard – a common problem with pyrolysis in general.

Two-stage pyrolysis can frequently be used to obtain a non-contaminated fraction (e.g. CCA / PVC)

Question 134

What’s biochar?

Answer 134

‘Biochar’ is a catch-all term describing any organic material that has been carbonised under high temperatures (300-1000°C), in the presence of little, or no oxygen. This process (called ‘pyrolysis’) releases bio-oils plus gases and leaves a solid residue of at least 80% elemental carbon which is termed biochar.

Question 135

What does OPC stand for?

Answer 135

Ordinary Portland cement (OPC) - it is mostly used for stabilization purposes and works best with sandy soils.

Question 136

What are the 3 main processes of the cement production process?

Answer 136

1) Precalciner –

CaCO3 -> CaO + CO2

2) Kiln

CaO + Silicates -> Cement Clinker

3) Post Kiln

Cement Clinker + Additives -> Cement

CO2 from the cement industry comes from the decomposition of CaCO3

Question 137

What is clinker?

Answer 137

Cement clinker is a solid material produced in the manufacture of portland cement as an intermediary product.

Clinker occurs as lumps or nodules, usually 3 millimetres (0.12 in) to 25 millimetres (0.98 in) in diameter.

It is produced by sintering (fusing together without melting to the point of liquefaction) limestone and aluminosilicate materials such as clay during the cement kiln stage.

Question 138

What does SCM stand for?

Answer 138

Supplementary cementing materials

The most commonly used SCMs in industry include:
* Fly Ash
* Ground Granulated Blast Furnace Slag
* Silica Fume
* Calcium Carbonate
* Natural Pozzolans - such as calcined clays, shale, and metakaolin

Question 139

What are the issues with cement production?

Answer 139

It’s responsible for around 6.5 % of Global CO2.

Cement Clinkeris the product of a cement kiln. It is the glue that holds concrete together

Majority (60 %) of emissions in producing cement clinker from intrinsic chemistry of reaction (CaCO3-> CaO + CO2)

Fuel responsible for the remaining emissions

Question 140

How can the cement industry be improved?

Answer 140

Use of alternative fuels: rotary kilns can flexibly operate with many different types of fuel. Increasingly, the industry has turned toward higher utilisation of alternative fuels, frequently municipal solid waste (MSW) and refuse-derived fuels (RDFs), produced from a diverse range of industrial wastes (e.g. tyres, plastics, textiles, sawdust, etc.)

CCS

Carbonation of waste for concrete production

Process improvements:
- Improve cement : clinker ratio to have as little clinker as possible. A portion of the clinker in cement can be replaced by materials with fewer associated CO2 emissions, lowering the net CO2 associated with the cement. Maximum clinker to cement ratio is 0.6. Current practices get as low as 0.74

• Switch to more efficient cement production processes: Globally there is still a considerable amount of installed capacity contributed to by inefficient processes (i.e. wet, semi-wet and semi-dry type processes) with older processing equipment.

Question 141

List current approaches to improve cement production sector:

Answer 141

Level 1. (saves money and CO2). Maximise Supplementary Cementitious Materials.
Many low carbon materials can be substituted for cement clinker, and up to a certain level of substitution yield cements which have almost identical properties to standard OPC. Many are by-products that would otherwise need disposal (Blast Furnace Slag, Coal Ash). However – WARNING! Many come from high carbon processes that themselves need decarbonising or shutting down. Both processes cannot claim a CO2 credit! Allowable substitution depends on the end use.

Level 2. (saves money and CO2). Maximise MSW (with high biogenic content).
Cemex, for example, has been running climafuel for years. Climafuel is carefully sorted MSW. Does need care to prevent nasty things getting in to the cement, which cuts down the potential for secondary applications of e.g. cement kiln dust.

Level 2a (from cost negative to cost positive, saves CO2). Carbonation of waste materials to produce aggregates to be used in cement. Can be a useful thing to do outside of CO2 savings, potentially prevents migration of nasty chemicals.

Level 3. (saves money and CO2). LC3 (Limestone Calcined Clay Cement). Very high potential for substitution, is within the decarbonisation strategies of many very large cement producers. Further research is ongoing.

Question 142

List future approaches to improve cement production sector:

Answer 142

Level 4. (Currently costs money and reduces CO2). Carbon Capture and Storage. A number of potential technologies, ranging from the poorly suited to cement (Amine scrubbing) to the highly integrated (oxyfuel kiln – potential to reduce cost), to the close to cost neutral (direct separation). Demonstrations ongoing – need CCS infrastructure. Calcium looping, oxyfuel and DSR make sense.

Level 5. Curing cement with CO2, and alternative cements. Good for pre-fabricated materials and non-structural cement (e.g. paving). Challenges over proven long term use, and if need a high CO2 atmosphere for the curing process very careful consideration of leakage, and a dedicated curing location. A long time to get into building codes for alternative cements and whether buyers will buy them is another serious issue.

Level 6. Electrify the calciner. Ok if you have serious oversupply of electricity (who has that?) but clinkering is a challenge (needs very high temperatures). Initial piloting going on. Rondo heat battery is possibly a very good idea.

Question 143

List techniques that are unlikely to be useful in improving the cement production process:

Answer 143

Level 7. Use hydrogen in the kiln. It is monumentally expensive, if you are making green hydrogen why not electrify and leave out a step? You still need CCS for the process emissions, so why not burn MSW and solve a different problem at the same time. Change in hazard class of fuel. Sealing the kiln against H2 permeation. If using blue hydrogen, you still need CCS and transport of H2 to the plant. There’s no chemical reason to use hydrogen (there is in iron production) and the hydrogen would be better used in industries where it makes more of an impact.

Level 8. Biomass / Synthetic natural gas. Likely to be crazily expensive – biomass possible if waste (also need to look at effects on cement).

Question 144

What is MSW?

Answer 144

Municipal solid waste

Question 145

How do primary and secondary steel production differ?

Answer 145

Primary steel production refers to that which uses iron ore as its main source of metallic input, whereas secondary production is that based on scrap

Iron is made in either blast furnaces (integrated steel mills) or in DRI furnace. The processes use Carbon monoxide and hydrogen as reducing agents to cleave the oxygen from these iron ore molecules.

Question 146

What is a blast furnace used for?

Answer 146

The blast furnace is a vertical counter-current heat exchanger as well as a chemical reactor in which burden material charged from the top descend downward and the gasses generated at the level ascend upward.

The throat is the top part of the furnace and includes the installation necessary for charging coke and burden materials and drawing off the top gasses . The top gas containing the flue dust is routed from the furnace top to the consumption zones.

Air for combustion in the blast furnace is blown from turbo blowers which are preheated in hot blast stoves to temperatures around 1300C , which is then blown through into the blast furnace.

The hearth is the lower cylindrical part of the furnace where the fluid slag and the hot metal accumulate.

Each blast furnace is equipped with two or more stoves which operate alternatively. Preheating of air helps in reducing fuel consumption in the furnace.

This is used for steel production.

Question 147

What’s a DRI furnance?

Answer 147

Directly reduced iron furnace

DRI is the removal of oxygen from iron ore or other iron bearing materials in the solid state, i.e. without melting, as in the blast furnace. The reducing agents are carbon monoxide and hydrogen, coming from reformed natural gas, syngas or coal. Approximately 50% of the reaction comes from hydrogen. Iron ore is used mostly in pellet and/or lumpy form.

Question 148

What’s EAF?
What’s it used for?

Answer 148

Electric arc furnace.

EAF route produces steel using mainly recycled steel and electricity (to melt recycled steel). Additives, such as alloys, are used to adjust to the desired chemical composition.

Electrical energy can be supplemented with oxygen injected into the EAF. Downstream process stages, such as casting, reheating and rolling, are similar to those found in the BF-BOF route.

About 28.9% of steel is produced via the EAF route.

Question 149

How do the costs of steel recycling facilities compare?

Answer 149

The initial CAPEX of EAF is lower than the integrated BF-BOF (blast furnace) mills, as there is no need to erect ironmaking facilities like a coke oven, sintering, blast furnace.

Variable OPEX are also lower (fuel cost). Since the process uses mainly only electricity.

KEY requirements (for low steel production): low carbon electricity

Question 151

Where is steel produced?

Answer 151

Around 50 % of world crude steel production takes place in China.

More than 70% of steel is produced in integrated steel mills. The integrated route relies on the use of coking coal and its mechanical properties to produce hot metal.

Countries with high share of steel production tend to have high coal reserves and use. Coking coal alone accounted for about 16% of global coal demand (5 530 Mtce) in 2019.

Largely owing to its reliance on coal, steel production is currently highly emissions intensive. Producing a tonne of crude steel results in, on average,1.4 t of direct CO2 emissions.

Question 152

Why is CO2 produced in steel production?

Answer 152

Carbon monoxide is one of the easiest reductants to have.

CO is used to strip oxygen from the metal (e.g. Fe2O3) ores.

Hydrogen could be used but is expensive.

Question 153

What is needed to produce steel?

Answer 153

Iron ore and coking coal.

The coal is put in a coke oven, produced coke. Coking coal is needed as it is mechanically strong and able to support the weight of materials above within a blast furnace.

The bigger the blast furnace, the more efficient.

Question 154

What’s a flux?

Answer 154

Any substance introduced in the smelting of ores to promote fluidity and to remove objectionable impurities in the form of slag.

Limestone is commonly used for this purpose in smelting iron ores.

Question 155

What does H-DR stand for?
What is it?

Answer 155

Hydrogen-based Direct Reduction

Hydrogen-based direct reduction is a process used in iron and steel production that utilizes hydrogen as a reducing agent to convert iron ore into metallic iron. Traditionally, the dominant method for producing iron involves using coke (a form of coal) as a reducing agent in a blast furnace, releasing carbon dioxide as a by-product. However, hydrogen-based direct reduction offers a more environmentally friendly alternative by using hydrogen gas to remove the oxygen from iron ore, producing water vapor instead of carbon dioxide.

In this process, iron ore, usually in the form of pellets or lump ore, is fed into a reactor along with hydrogen gas. The hydrogen reacts with the oxygen in the iron ore, reducing it to metallic iron and water vapor. The chemical reactions involved are:

Fe2O3 + 3H2 → 2Fe + 3H2O

This method significantly reduces greenhouse gas emissions because it eliminates the need for coke, which is a major source of carbon emissions in traditional steelmaking. Hydrogen used in this process is often produced through renewable energy sources like electrolysis of water, making it a more sustainable option.

The resulting sponge iron (direct reduced iron or DRI) can then be further processed in an electric arc furnace or used in other steelmaking processes to produce steel products. Hydrogen-based direct reduction is gaining attention as the industry seeks more environmentally friendly ways to produce steel, aiming to reduce carbon emissions and move towards a more sustainable future.

Question 156

What’s HBI?

Answer 156

Hot Briquetted Iron.
It’s a product obtained through the process of direct reduction of iron ore using natural gas or hydrogen as a reducing agent, similar to the process used to produce Direct Reduced Iron (DRI).

The key difference lies in the final form and handling of the product. HBI is compacted, high-density iron material in the form of briquettes. These briquettes are produced by compacting the hot DRI or sponge iron and then cooling it down. This process makes the iron easier to handle, transport, and use in steelmaking processes.

The production of HBI involves passing hot DRI through a press to form briquettes, which are then rapidly cooled, solidifying them into a dense, sturdy form. HBI typically has a higher density and strength compared to DRI, making it more suitable for use in electric arc furnaces (EAFs) or other steelmaking processes. It’s favored in scenarios where convenient handling, reduced dust, and improved efficiency in steel production are desired.

Question 157

What’s the easiest way to reduce steel sector emissions?

Answer 157

The cheapest and easy way to reduce steel sector emission is by increasing the amount of scrap recycling (using EAF route). The process requires the availability of low carbon electricity to achieve significant emission reduction.

Integrating CCS in BF-BOF plants is possible, but is complicated by the number of CO2 streams in the steel plants (many process steps involved to obtain the final product).

Producing steel via BF-CCS is more economical than via H2 reduction. If zero-carbon electricity is available H-DR might become viable by 2035, subject to capital cost reduction of H2 production processes.

Similar to cement, process emissions are the main source of CO2 in integrated steel mills (the dominant route for steel production).

Question 158

Explain demographic transition:

Answer 158

Demographic transition is a long-term trend of declining birth and death rates, resulting in substantive change in the age distribution of a population (particularly noted in 1963, when the birth control pill and other cheap effective contraceptive methods such as the IUD were adopted by the general population).

Question 159

What are the 2 main ways to model growth rate?

Answer 159

1. By assuming constant growth rate (not accurate/sustainable)
   g = 1/ P * dP/dt
   Where g is a growth constant, P is population, and t is time.
   Time for population to double is considered, td = ln2 / g (similar to half life equations)
2. Logistic model equation
   g = 1/ P * dP/dt = r (1 - P/K)
   Where K is max population (also known as carrying capacity, the y int is r.
   For small populations, when Population &laquo_space;Carrying capacity (K), g = r
   For max population, when P = K, g = 0

Question 160

How is oil production in year t + 1 calculated?

Answer 160

ΔN(t+1) = rNt((K-Nt)/K) = rNt(1-(Nt/K))

Question 161

What is peak oil?

Answer 161

The hypothetical point in time when the global production of oil reaches its maximum rate, after which production will irreversibly and gradually decline.

Question 162

What’s EOR?
Give examples:

Answer 162

Enhanced oil recovery

There are three primary techniques of EOR: gas injection, thermal injection, and chemical injection

Question 163

What is the logistic model for modelling population growth rate?

Answer 163

Logistic model equation:

g = 1/ P * dP/dt = r (1 - P/K)

Where K is max population (also known as carrying capacity, the y int is r.

For small populations, when Population &laquo_space;Carrying capacity (K), g = r
For max population, when P = K, g = 0

This model is more suitable for modelling varying population growth, and this can be used for modelling oil consumption in future.

Question 164

What are the 4 main assumptions for oil consumption behaviour regarding Hubbert Analysis?

Answer 164

1) Oil is a finite resource
2) Oil production started at some point in past
3) Initially, oil production underwent exponential growth
4) Oil production must peak at some time and then decline

Question 165

What does CCS involve?

Answer 165

Capturing CO2, particularly from a point source rather than air, and transferring it to infrastructure that will bury it underground.

CO2 is in supercritical state when being injected into aquifers. By buoyancy, CO2 rises to the top of the aquifer underground (drainage to the top).
Some CO2 also dissolves into brine/saline water present in the aquifer, making it more dense. Dissolving CO2 travelling downwards in the aquifer is preferred.
By dissolving, it also increases acidity of the fluids present.

Question 166

What mechanisms occur with CO2 during CCS injection into an aquifer?

Answer 166

1. By buoyancy, CO2 rises to the top of the aquifer underground (drainage to the top).
2. As it moves to the top, some CO2 also dissolves into brine/saline water present in the aquifer, making it more dense. As it becomes more dense it moves downwards in the aquifer, which is preferred.
3. By dissolving, it also increases acidity of the fluids present.
4. There is also precipitation and mineralisation which occurs over time.
5. 2 phase system produced by CO2 and brine. Inhibition of water into CO2 plume. Water enters via capillary forces and surrounds CO2 which separates the plume and breaks it into ganglia.

Trapping of CO2 into ganglia is the fastest of the mechanisms.

Question 167

What is the preferred situation for CCS, hydrocarbon reservoirs or saline aquifers?

Answer 167

Hydrocarbon reservoirs (emptied/drained) - proven fluid trap that will not lead to leaks.

Question 168

How does the CO2 stay underground?

Answer 168

• Dissolution
  CO2 dissolves in water – 1,000-year timescales
  Denser CO2-rich brine sinks
• Chemical reaction trapping
  acid formed carbonate precipitation – 103 – 109 years
• Hydrodynamic Trapping
  Trapping by impermeable cap rocks
• Capillary Trapping
  rapid (decades): CO2 as pore-scale bubbles surrounded by water

Only capillary trapping is fast, which can take days or decades. The rest are v. slow (thousands of years). This is the only one that can be modified by engineering.

Question 169

How could injected CO2 escape from aquifers (CCS)?

Answer 169

1. Backflow / returning up injection well
2. Upward travel via buoyancy forces
   (Natural flow (ground water) dissolves CO2 at the CO2/water interface and transports it out of closure
3. Leaking into neighbouring/other formations
   (CO2 escapes through gaps in rock into higher aquifers)
4. Diffusion through impermeable cap rock (CO2 pressure exceeds capillary entry pressure and passes through)
5. Travelling back through abandoned wells (CO2 escapes via poorly plugged old abandoned well)
6. Fault leakage

Question 170

What will happen to injected CO2 after thousands of years?

Answer 170

It dissolves in the water and sinks.

CO2 can combine with minerals in the water and form calcium carbonate (limestone) over 10^3-10^9 years.

CO2 + H2O <-> H2CO3 <-> HCO3- + H+
Ca 2+ + 2HCO3- <-> CO2 + H2O + CaCO3

Question 171

How could injected CO2 escape from hydrocarbon reserves (CCS)?

Answer 171

1. Backflow / returning up injection well
2. Upward travel via buoyancy forces
3. Travelling back through abandoned wells

Question 172

What does Re number show?

Answer 172

Ratio of inertial to viscous forces

Question 173

What does the Peclet number, Pe, show?

Answer 173

The ratio of advective to diffusion rates

Question 174

What does the Damköhler number, Da, show?

Answer 174

Ratio of reaction to advective rate

Question 175

What are the 3 main types of fluid/solid (reactive transport) dissolution regimes?

Answer 175

1. Compact
   Low Pe
   High PeDa
2. Wormhole
   High Pe
   High PeDa
3. Uniform
   High Pe
   Low PeDa

Question 176

What does the size of the value of the product of Pe and Da dimensionless values tell you?

Answer 176

For PeDa&raquo_space; 1
- reaction rate&raquo_space; transport rate
- transport limited
- transport is the slowest
- fast reaction

For PeDa &laquo_space;1
- reaction rate &laquo_space;transport rate
- reaction is the slowest
- fast transport

Question 177

What does the size of the Pe number tell you about flow and diffusion?

Answer 177

Pe &laquo_space;1
- fast diffusion
- advection limited
- advection the slowest

Pe&raquo_space; 1
- fast advection
- diffusion limited
- diffusion the slowest

Question 178

What is blue hydrogen?

Answer 178

Blue hydrogen is when natural gas is split into hydrogen and CO2 either by Steam Methane Reforming (SMR) or Auto Thermal Reforming (ATR), but the CO2 is captured and then stored.
As the greenhouse gasses are captured, this mitigates the environmental impacts on the planet.

Question 179

What is green hydrogen?

Answer 179

Hydrogen produced by splitting water via electrolysis. This produces only hydrogen and oxygen.

We use the hydrogen and vent oxygen to the atmosphere with no negative impact.

Question 180

Why store hydrogen?
Where is it stored?

Answer 180

We want to store it to save it and be able to reuse/withdraw it.

It can be stored underground, under cap rock.

It is stored around 100 m underground - much shallower than CO2 (kilometres underground)

Question 181

What processes and interactions arise during hydrogen storage?

Answer 181

1. Interactions with caprock
   - diffusion
   - capillary leakage
   - fracturing
   - buoyancy pressure
2. Fluid-rock interaction
   - forming hydrogen plumes
   - microbial activity
   - Dissolution and residual trapping
3. (Initial) Injection underground
   - P/T changes
   - Multiphase processes
   - Stress/stress changes
4. H2 Cushion gas
   - Unstable displacement and uncontrolled lateral spreading
   - Gas mixing
5. Cushion gas-brine
   - Fluid-rock interaction
   - Unstable displacement
   - Dissolution and residual trapping
6. Structural geology
   - Fault leakage
   - Far and near field stress changes
   - Reactivation
   - Overpressure

Question 182

What’s the first law efficiency?

Answer 182

The ratio of the amount of energy delivered to perform a task to the amount of energy that must be applied to achieve this task.

This first law approach is concerned only with the efficiency of one particular method of performing the task, and disregards alternative methods which may perform the same task with less energy consumption.

Question 183

What’s the second law efficiency?

Answer 183

A measure of thermodynamic effectiveness - Ratio of the thermodynamic minimum work required for a given process (e.g. gas separation) and the real/actual work required.

The second law efficiency or the exergy (maximum work possible) efficiency is used to compare the efficiency of a real process to a corresponding ideal process.
n = minimum work / real work

The second law efficiency or the exergy efficiency (or effectiveness) is used to compare the efficiency of a cycle to a corresponding ideal cycle.

Wreal is the sum of all the “work” (heating, cooling, pumping, compression…) required to operate a process.
All contributions must be expressed in equivalent units, i.e., heat ≠ electricity, conversion of ~ 0.33 is a typical number. (MWh thermal ≠ MWh elec)

Real separation processes typically have second law efficiencies ranging from 5–40%.

Question 184

How does 2nd law efficiency vary with concentration?

Answer 184

It drops with (decreasing) concentration (linearly on log-log plot).

The more dilute the exhaust gas, the more work required to separate the CO2.

Hence, it is “easier” to capture CO2from a coal plant than a gas plant

As system gets more dilute, the cost for separation increases.

Question 185

How does concentration affect minimum work for separation (1st law efficiency)?

Answer 185

Dilute /low conc systems should result in a higher minimum work necessary for separation.

Question 186

How can the 2nd law efficiency be increased?

Answer 186

By making changes which lead to a decrease in energy requirements

Heat integration

Thermal insulation

Question 187

What happens in chemical looping combustion?

Answer 187

In the fuel reactor, a solid oxygen carrier, usually a metal oxide, reacts with the fuel (such as natural gas or coal) in the absence of air. The oxygen carrier material releases oxygen atoms and is converted into its reduced form.

The reduced oxygen carrier moves to the air reactor, where it is exposed to air or oxygen-containing gas. In contact with the oxygen-rich environment, the oxygen carrier material gets oxidized back to its original state by capturing oxygen from the air, forming the metal oxide again.

The oxygen carrier material, now replenished with oxygen, is cycled back to the fuel reactor to repeat the process. This closed-loop system enables the continuous transfer of oxygen without direct contact between the fuel and air, thus preventing the mixing of nitrogen from the air with the fuel.

Question 188

What are the advantages of chemical looping combustion?

Answer 188

The closed-loop system enables the continuous transfer of oxygen without direct contact between the fuel and air, thus preventing the mixing of nitrogen from the air with the fuel.

The separated streams of carbon dioxide and steam resulting from the fuel reactor can be readily captured and sequestered, as they are not mixed with nitrogen from the air as in conventional combustion. This makes it easier and more energy-efficient to capture the CO2 for storage or utilization.

Question 189

What’s the difference between gross CO2 captured and net CO2 that doesn’t enter the atmosphere?

Answer 189

Gross CO2 - captured CO2

Net CO2 - avoided CO2

Avoided CO2 < Captured CO2 as some CO2 is always produced to operate the capture process.
The cost of Avoided CO2 > cost of Captured CO2.

Question 190

How does the cost of implementing new energy technologies vary with time?

Answer 190

Cost will tend to fall with increased deployment (although nuclear has increased rather than decrease).

Techniques that are used to estimate cost reductions include “learning curves”, engineering assessments and parametric modelling

The rate of cost reduction remains approximately constant for each doubling of installed capacity

Rates of cost reduction are significantly different for different technologies

Key factor in calculating cost reduction is extent of current deployment used

Question 191

What key factors contribute to the cost of CCS?

Answer 191

General:
- Location
- Brownfield or greenfield sites
- Labour rates
- Commercial factors (risks, contingencies, warranties, price of CO2)

Specific:
- Chemicals
- Fuel cost
- Tech choices
- Transport / routes and pipelines
- Storage and containment

Question 192

What’s TRL?

Answer 192

Technology readiness level

Ranges from 1 (basic principles observed) to 9 (deployed at commercial scale)

Question 193

Pros and cons of post-combustion capture with chemical absorption:

Answer 193

Pros:
- Chemical absorption is a mature gas-liquid separation technology
- Widely deployed at ‘large-scale’ with decades of experience in oil and gas industry
- Uses amine-based solvents, including MEA, AMP, DEA and MDEA as well as proprietary solvents
- Flexible technology option; applicable to a wide range of point sources and operating regimes

Cons
- High CAPEX associated with large scale equipment (large flue gas volumes with relatively low CO2 concentration, high solvent flow rates)
- Deployment of existing technology results in the reduction in the thermal efficiency from about 45 % to 35 % (equivalent to a ~20 % reduction in output)
- Major energy demand for regeneration (6% points), compression (3% points) and transportation of flue gas and solvents (1%)
- Less significant cost associated with solvent make-up (owing to thermal and oxidative degradation), disposal

Question 194

What is post-combustion capture with chemical absorption?

Answer 194

CO2-rich gas is exposed to solvent (~15 – 30 wt. %) in absorber, at 40 – 60 oC, at a pressure of ~ 1 bar, in counter current flow

Column packing increases fluid contacting.

CO2 bonds with solvent with a loading of CO2 at the exit of the absorber ~ 0.5 mol CO2 / mol solvent

The CO2 is then removed/desorbed by boiling the CO2-rich solvent (at a pressure of ~ 2 bar and a temperature of ~120oC) with heat from steam diverted from power station steam cycle

Question 195

What are issues with amine degradation and environmental emissions?

Answer 195

Amines are susceptible to degradation in the presence of O2, SO2, CO2, as well as thermal degradation.

Issues:
- corrosive
- cancerous
- solvent losses to the environment

However, reclamation processes also exist which recover the degraded solvents.

Question 196

Why do oxy-combustion for coal?

Answer 196

It increases flue gas CO2 concentration (> 80% vol.) by eliminating N2

Same combustion principles as for normal air-blown systems, meaning little effort in retrofit to existing technologies

Smaller flue gas volume saving capital costs due to scaling down of various equipment: smaller APC devices, smaller stacks, reduced fan duties

Some benefits in terms of emission control, e.g. reduced NOx emissions;

SOx emissions may also reduce somewhat, probably due to additional sulphur retention on ash deposits

Question 197

How do CCS, CDR, and CCU differ?

Answer 197

Carbon Capture & Storage: CO2 is captured from a point source and stored permanently

Carbon Dioxide Removal: CO2 is captured from the atmosphere and then stored geologically or in soil for a long period of time

Carbon Capture and Utilisation: CO2 is captured from a point source or the atmosphere and used in other products e.g. fuels or chemicals

Question 198

What is calcium looping?

What are the stages?

Answer 198

Calcium looping is a carbon capture technique used in industrial processes to reduce CO2 emissions.

It operates by using calcium-based materials to capture CO2 during the combustion process.

In the first stage, the fuel undergoes combustion in a reactor. However, instead of directly capturing CO2 from the flue gas, the combustion occurs in the presence of a calcium-based sorbent.

During combustion, the calcium-based sorbent captures CO2 by reacting with it to form limestone (CaCO3) and other products. The produced limestone is referred to as “spent” sorbent as it has reacted and absorbed the CO2.

The “spent” sorbent, containing the captured CO2, is then separated from the flue gas and transferred to a separate chamber called a calciner.
In the calciner, high temperatures are applied to the spent sorbent, causing it to release the captured CO2 and regenerate the sorbent back to its original form (calcium oxide, CaO).

The regenerated sorbent, now rich in calcium oxide, can be recycled back to the combustion chamber to capture more CO2, repeating the cycle.

By capturing CO2 in a solid form (calcium carbonate or oxide) and then subsequently separating it by heat, the process avoids the release of CO2 into the atmosphere.

It can also be used in cement plants, and occurs at sufficiently high temperatures that energy can be generated from it.

Question 199

What are the advantages of post-combustion capture with calcium looping?

Answer 199

• limestone cheap and non-toxic
• mature CFB technology
• low energy penalty
• re-power power station
• synergy with cement
• no SO2 removal required

Question 200

What are issues with calcium looping?

Answer 200

sintering
attrition and fragmentation
competing reaction with sulphur
ash fouling in calciner

Question 201

Discuss the reasons for utilizing CCS, making reference to integrated assessment models:

Answer 201

Integrated Assessment Models (IAMs) play a significant role in assessing its utility in mitigating climate change. Here are key reasons for utilizing CCS along with its relationship to IAMs:

1. CCS plays a crucial role in reducing greenhouse gas emissions from industries like power generation and heavy manufacturing.
   IAMs simulate the complex interactions between economic, social, and environmental systems to forecast the impact of different policies and technologies on climate change.
   They show that CCS, alongside renewable energy and efficiency measures, can be pivotal in achieving the ambitious climate targets set in agreements like the Paris Agreement.
2. Decarbonization of Hard-to-Abate Sectors: Certain industries, such as cement, steel, and chemicals, have processes that inherently produce significant CO2 emissions.
   IAMs help identify these “hard-to-abate” sectors and demonstrate that without CCS, achieving deep decarbonization in these industries to meet climate goals becomes considerably more challenging and costly.
3. Maintaining Energy Security and Transitioning Fossil Fuels: IAMs consider the energy transition from fossil fuels to cleaner alternatives.
   CCS allows for the continued use of fossil fuels while capturing emissions. In models, this is seen as a bridging technology, particularly in regions heavily reliant on coal, ensuring a more gradual shift to cleaner energy sources without destabilizing energy security.
4. Negative Emissions Technology (NET): IAMs explore pathways beyond emissions reductions, aiming for net-negative emissions to stabilize global temperatures. CCS in combination with bioenergy (BECCS - Bioenergy with Carbon Capture and Storage) is seen as a potential NET by capturing CO2 from bioenergy production, effectively removing CO2 from the atmosphere.

IAMs simulate these scenarios by considering technology costs, policy incentives, energy demand, economic development, and environmental impacts.
They demonstrate that, without CCS, it becomes significantly more expensive and challenging to achieve emission reduction targets and limit global warming to safe levels.

However, the successful deployment of CCS faces challenges such as high initial costs, regulatory frameworks, public acceptance, and infrastructure requirements. IAMs continue to evolve to incorporate these complexities and uncertainties, aiding in policy formulation and decision-making concerning CCS implementation for effective climate change mitigation strategies.

Question 202

What are the issues associated with energy decoupling internationally?

Answer 202

Energy decoupling refers to the process where economic growth becomes detached or decoupled from the increasing consumption of energy. In an ideal scenario, economic growth can continue or even accelerate while energy consumption remains stable or decreases.
Decoupling is often driven by advancements in technology and improvements in energy efficiency across industries. Another aspect of decoupling involves a shift from high-carbon intensity energy sources (like coal) to low-carbon or renewable sources.

ISSUES:
1. Balancing economic growth with efforts to reduce energy intensity can be challenging, especially in emerging economies when traditional industries demand substantial energy consumption.

2. Many regions lack the necessary funding, technology, or political will to facilitate the transition to low-carbon sources.
3. Subsidies on fossil fuels can deter the adoption of cleaner and more efficient energy sources.
4. As developing nations strive for economic growth and improved living standards, their energy demand tends to rise significantly. This can offset efforts made by more developed nations to reduce energy consumption and emissions.
5. Achieving global decoupling requires cooperation among nations. Disparities in development, resource availability, and geopolitical tensions can hinder international efforts.

Question 203

What’s BECCS?

Answer 203

Bioenergy with carbon capture and storage

Question 204

Why can the Levelised Cost of Electricity (LCOE) not be used solely to characterise the performance of a technology?

Answer 204

1. LCOE primarily focuses on the cost of generating electricity and doesn’t account for the full spectrum of benefits or external costs associated with a technology.
2. Some renewable energy sources, like solar and wind, have variable output due to weather conditions. LCOE doesn’t directly account for the costs of integrating these variable sources into the grid.
3. LCOE typically includes initial investment, operating, maintenance, and fuel costs but might overlook other lifecycle elements. For instance, decommissioning and waste management costs for nuclear or fossil fuel plants.
4. Reliability, dispatchability, or scalability of a technology aren’t reflected in the LCOE but are critical factors for grid stability and energy security.
5. Energy markets are dynamic, affected by policy changes, technological advancements, and market fluctuations. LCOE might not reflect future changes accurately, especially for rapidly evolving technologies like renewables.

Question 205

Describe key features of the Shell updraft gasifier:

Answer 205

Dry feed, entrained flow, slagging gasifier

Pressurised lock-hoppers allow feeding

Pure oxygen and steam as oxidant / moderator

Flame goes up

Temperature of reactions 1500 - 1600CC

20 – 40 bar pressure

Key to operation is the membrane wall. Cooling water circulates through, which cools the ash from the gasification process down; this then forms a protective layer on the wall, preventing erosion of the refractory lining.

At the outside of the slag layer, the slag is molten and flows down into a water bath, where it is removed through a lock hopper as a slurry.

Gas leaves the gasifier at 1300 – 1400C

Recycled “quench” gas reduces the temperature of the gas and solidifies any ash fines remaining in the gas – reduces damage to the syngas cooler.

The gas passes through a HRSG (heat recovery steam generator) to produce steam for process use or power.

Question 206

Give an example of a downflow gasifier:

Answer 206

GE (formally ChevronTexaco or Texaco gasifier)

Downwards flow, oxygen fed. Slurry-fed (easier to pump a slurry through a pressure differential). ~ 20 bar.

Question 207

Give an example of an upflow gasifier:

Answer 207

E-Gas gasifier (originally Conoco Philips)

Upflow, Oxygen fed, 2-stage, slurry fed ~ 30 bar. First stage 75 % of the slurry, 2nd stage injection produces tar and char (both recycled).

Question 208

Describe key features of the Sasol-Lurgi Fixed Bed Gasifier:

Answer 208

It has gasified more coal than any other type of gasifier.

Updraft gasifier

Largest models can gasify 1000 t/day

Good for low rank, high ash coals

Disadvantage – produces a lot of tar.
Where the scale is sufficiently high, this can be a source of high value chemicals / fuels, but the facilities have to be huge to allow this.

The ash produced is not molten.

Maximum T of ~ 1200 °C in the combustion zone, and 700 – 900C in the gasification zone.

Residence time of fuel 30 – 60 minutes.

Three options for gasification media:
(i) steam / O2
(ii) steam / air
(iii) steam / O2– enriched air.

Typical steam/fuel ratio is high: ~ 1.5.

Sasol: 55 million Nm3 / day of syngas (from coal, not biomass)

Question 209

Why do downdraft gasifiers produce less tar than updraft?

Answer 209

1. In a downdraft gasifier, the gasification process occurs with a downward flow of air or oxygen through the biomass, creating an oxygen-limited environment. This downward flow helps to minimize the interaction between the produced gas and the high-temperature char bed, reducing the formation of tar.
2. Downdraft gasifiers often have a longer residence time for the biomass in the high-temperature zone, allowing for more complete combustion and gasification of tars and other hydrocarbons.
3. Downdraft gasifiers usually have a secondary combustion zone where additional air or oxygen is introduced to further combust the produced gases, including tars. This secondary combustion helps reduce the tar content in the syngas.

Question 210

What does HRSG stand for?

Answer 210

Heat recovery steam generator

Question 211

Discuss the differences between coal and biomass in terms of combustion behaviour / what causes the differences in combustion behaviour?

Answer 211

Composition: Coal is primarily carbon and has a higher energy density than biomass. Biomass contains many materials e.g. cellulose, lignin etc - lower C content

Moisture content: Coal moisture content is generally low, whilst biomass typically has a relatively higher moisture content.

Volatility and combustion rate: Biomass has a higher volatility and can ignite more readily and combust at lower temperatures.

Ash: Coal produces ash and slag. Biomass produces ash with characteristics based on the type of biomass used.

Emissions: Coal releases more SO2, NOx, and particulates than biomass. But biomass produces VOCs and particulates still. Biomass often considered carbon neutral.

Stability: Coal burns with more stable combustion. Biomass combustion behaviour can vary, making the process hard to control sometimes.

Question 212

What’s the equivalence ratio, ER?

Answer 212

The number of moles of O2 supplied divided by that required for complete combustion

Question 213

What are the general steps to build a hydroelectric dam?

Answer 213

1. Find a site
2. Find a LOT of financing
3. Build a diversion tunnel around your chosen location
4. Build two temporary “cofferdams” (one upstream, one downstream) so you can build your actual dam in the dry
5. Build dam
6. Blow up downstream cofferdam
7. Shut off diversion – retain for emergencies – upstream cofferdam floods as you produce the new reservoir

Question 214

How does a Bulb hydro turbine work?

Answer 214

Turbine is set in front of a bulb containing the generator, etc.

The whole thing is placed in a tube and is used for low head, low flow applications (rivers or tidal estuaries).

The turbine is housed in a bulb-shaped structure that contains the generator and turbine blades.

Water flows into the bulb structure and passes through the turbine. The intake ports are designed to minimize disruptions to the environment.

As water flows through the turbine, it spins the blades. The Bulb turbine has fixed-pitch blades that are optimized for efficient operation at low heads and flows. These blades are often shaped to accommodate the specific characteristics of low-speed, high-volume water flow.

The rotating turbine shaft is connected to a generator housed within the bulb structure. As the turbine spins, it drives the generator to produce electricity.

Bulb turbines are designed to achieve higher efficiency at low heads and flow rates compared to other turbine types.
Their design minimizes cavitation (the formation of vapor bubbles in the water due to low pressure), ensuring smoother operation and higher reliability.

Question 215

How does a pelton hydro turbine work?

Answer 215

Most efficient for high head, low flow

Simple design: A large circular disk is mounted on some sort of rotating shaft known as arotor. Mounted on this circular disk are cup shaped blades known as buckets evenly spaced around the entire wheel.

Operation: high speed jets of water emerge from the nozzles that surround the turbine.
These nozzles are arranged so the water jet will hit the buckets at splitters - the centre of the bucket where the water jet is divided into two streams.

The two separate streams then flow along the inner curve of the bucket and leave in the opposite direction that it came in.
This change in momentum of the water creates an impulse on the blades of the turbine, generating torque and rotation in the turbine

Question 216

List features of hydro-pumped storage:

Answer 216

Around 75 % efficient (round trip)

99 % of global energy storage (by volume) is pumped hydro

Can use reversible Francis turbines

Exemplified by Dinorwig in Wales

In the UK – used for frequency management, not energy storage (at the moment).

Also functions for “black start” use.

Low “self discharge”

Question 217

List factors affecting bio-oil:

Answer 217

Heating rate:
- The higher the heating rate, the faster the release of bio-oil
- Minimises the occurrence of secondary reactions
- Bio-oil yield increases with increasing heating rate.

Particle size:
- The coarser the particle, the slower the heating rate
- Product yields are affected by the heating rate
- A higher bio-oil yield is expected from smaller particles

Pressure:
- Pressure suppresses the release of bio-oil
- Bio-oil yield decreases with increasing pressure

Temperature:
- Higher temp leads to lower bio-oil yield
- High temp leads to low oxygen content

Question 218

Briefly describe the cement production process:

Answer 218

In the preheater the feed/meal is heated by hot flue gas coming from the calciner and the rotary kiln.

The meal and the hot gases are mixed for heat transfer and then separated in cyclones arranged above one another.

Next, the raw meal enters the precalciner, where the major part of the calcination is performed (CALCNATION = THERMAL COMPOSITION OF RAW MEAL FROM LIMESTONE TO CALCIUM OXIDE AND CO2).

Around 2/3 of the plant’s total fuel input is consumed here to achieve the right temperature (~860 °C) and drive the endothermal reaction.

After the calciner, the raw meal enters the rotary kiln, where formation of the clinker takes place. Around 1/3 of the plant’s fuel is burnt in the main burner, which is placed in the other end of the kiln.

The precalciner is where the limestone gets heated and becomes CaO and CO2.
The kiln is where the reactions between CaO and Sio2 (sand) end up producing calcium silicates.

After the kiln, you grind in other materials and produce cement.

Question 219

Why stick with using Portland cement (OPC)?

Answer 219

It is difficult to get new cements in to building codes – which also tend to be composition (not performance) based.

Any new cement has to service 30 Billion tonnes / year of cement-based products. This means that in addition to the above points, it has to come from a small pool of super-abundant rocks that also form cementitious phases

Question 220

What’s LC3?

Answer 220

A new type of cement that is based on a blend of limestone and calcined clay. Can reduce CO2 emissions by up to 40%, is made using limestone and low-grade clays which are available in abundant quantities, is cost effective and does not require capital intensive modifications to existing cement plants.

The merit of LC3 is to allow the use of 50% clinker content in combination with cheaper and widely available constituents, like clay and limestone, without sacrificing cement performance.
Low grade limestone with impurities like quartz and dolomite can also be used, thereby increasing the potential geographic locations for LC3 manufacturing.

Question 221

How does gas CCS compare to coal CCS?

Answer 221

Natural gas is a less carbon intense fuel than coal.

CCGT (closed cycle gas turbine) plants are more efficient than coal-fired power plants. This means that they can generate more electricity per unit of fuel consumed.

Whilst GJ/tCO2 is greater for gas than for coal, the GJ/MWh is less for gas than for coal.

All else being equal, this implies that £/MWhgas < £/MWhcoal

Key Comparisons:
1. Gas CCS tends to be more efficient than coal CCS
2. The cost of implementing CCS technology on coal-fired power plants has been higher compared to gas-fired plants
3. Coal-fired power plants generally produce more CO2 emissions per unit of energy compared to natural gas plants
4. Both coal and gas CCS face similar challenges when it comes to safely storing the captured CO2 underground

Question 222

What are the issues associated with amine degradation?

Answer 222

Corrosion of equipment

Potential for solvent losses to the environment

Some amine solvents and degradation products present potential health risks (amides, aldehydes, nitrosamines, nitramines)

Reaction between aqueous solutions of amines and CO2 is well known
Uncertainty regarding degradation chemistry (amines susceptible to degradation in the presence of O2, SO2, CO2, also thermal degradation); 0.35 ̶ 2.0 kg of solvent per tonne CO2

Question 223

Give examples of CCS technologies:

Answer 223

Gasification, amine scrubbing and air-separation are very mature technologies.

There are more, such as Membranes, Cryogenic Separation, Calcium (Ca) looping and Chemical Looping Combustion (CLC). These are at a lower Technology Readiness Level (TRL) than the above three.

The three main categories are post-, pre-, and oxy-combustion.
- While not a storage method, EOR involves injecting captured CO2 into oil reservoirs to increase pressure and extract more oil.
- Mineralization is an emerging technology involves transforming CO2 into stable carbonate minerals through chemical reactions with certain types of rocks or industrial by-products.

Question 224

How is overall plant efficiency found?

Answer 224

As the product of all efficiencies.

E.g. thermal cycle efficiency * boiler efficiency * turbine efficiency * generation efficiency

Question 225

How is cycle thermal efficiency found?

Answer 225

ᶯth = (Wout-Win)/Qin = h2-h3/h2-h1

(enthalpy numbers dependent on streams)

Question 226

What is burnout?

Answer 226

The point in a combustion process where all the fuel is consumed, converting it into products like carbon dioxide (CO2) and water vapor.

Once burnout is achieved, all available fuel has been converted into CO2. Therefore, the total amount of CO2 produced isn’t affected by burnout since it’s a result of complete combustion.
Burnout affects the efficiency of fuel consumption. Incomplete burnout implies that not all fuel molecules have combusted fully. This inefficiency means that some fuel remains unburned or partially burned, leading to a lower fuel utilization efficiency.

Question 227

How is the weight of carbon in coal determined?

Answer 227

(1-MASH) x DAF C content

Where DAF C is dry ash-free carbon content and MASH is moisture and ash content.

Coal is made up of moisture, ash, and dry ash-free content. Of this DAF content, only a certain % is C. The rest is made up of H, O, N, and S.

Question 228

Suggested steps to calculate rate of CO2 production…
(3 main steps)

Answer 228

1. Calculate rate of heat transfer from the coal to the steam

Qin =Wnet/ ᶯp
Where Wnet is power output MWe and ᶯp is plant efficiency

2. Calculate rate of coal consumption
   Mfuel = Qin (MW) / LHV (MJ/kg) = kg/s
3. Calculate rate of CO2 considering the carbon content in the fuel

Rate of CO2 production = rate of coal consumption * (Mr CO2 / Mr C) * Burnout (B) * DAF * (1 - MASH)

Question 229

Describe the Sasol-Lurgi gasifier / how it works:

Answer 229

Updraft Gasifier

• Coal is fed into the top
• Steam and oxygen fed at the bottom
• The vessel is surrounded by a water jacket
• Coal contacts distributors at top of vessel which spread the coal around the vessel evenly
• There are 4 main stages within the vessel. From top to bottom, these are the:
  1. Drying zone
  2. Devolatisation/Pyrolysis zone
  3. Gasification/Reduction zone (700-900C)
  4. Combustion zone (~1200C)
• Solids flows downwards being dried (drying zone), transformed from coal to char (pyrolysis zone), gasified (gasification or reduction zone) and the remaining char is combusted (combustion zone), the remaining solids (ash) leaving the gasifier from the bottom.
• Steam and oxygen reach the zones through a grate at the bottom of the vessel
• Oxygen is consumed in the combustion zone.
• In the gasification zone a syngas is produced (CO, H2, with minor amounts of steam, CO2 and CH4).
• The gas leaving the devolatilization zone will have higher contents of CH4 and tars, and moisture from the coal will be transferred to the gas phase in the upper zone of the gasifier.
• High steam : fuel ratio of 1.5
• Pressure ~ 40 bar

Question 230

What is the current overall CO2 conc in atmosphere?
What is the annual change in atmospheric co2 conc?

Answer 230

Overall global concentration is around 414 ppm.

Annual change is around 2-3 ppm (coal is a major contributor)

Question 231

When wanting to estimate the moles of atmospheric carbon dioxide emissions, what are the steps needed to calculate moles CO2 released?

Answer 231

1. Determine mass of moles in atmosphere:
   mg = 4πr²*P
2. Calculate number of moles using n = m/Mr and assuming molar mass of air is approx that of N2 (28 g/mol, 0.028kg/mol)
3. To calculate ppm,
   ppm = (# moles of substance / total moles in solution)*10^6

Question 232

What are the trapping mechanisms for carbon dioxide injected into the subsurface for long-term storage? Explain the time-scales associated with each of them.

Answer 232

1. Hydrodynamic trapping in geological formations by impermeable cap-rock (1000 year timescales)
2. Capillary trapping, where CO2 is held in pore spaces by capillary forces and non-wetting phase ganglia (blobs) are surrounded by water.
   This is the only rapid mechanism that can be engineered at timescales of years to decades
3. Dissolution trapping where CO2 dissolves in water a 1000 year timescales, forms denser brine, and sinks down the reservoir
4. Chemical reaction trapping where acid formed by injection of supercritical CO2 in the in-situ brine dissolves the host rock then causes secondary carbonate precipitation. Time scales of 10^3 to 10^9 years.

Question 233

Define the following (regarding the global carbon cycle):

Metamorphism
Diagenesis
Subduction
Silicate weathering

Answer 233

Metamorphism - transformation of rocks due to high temperatures and pressures deep within the Earth’s crust or mantle

Diagenesis - chemical changes at low
temperatures during burial. the physical and chemical processes that affect sedimentary materials after deposition and before metamorphism and between deposition and weathering. (Mineral changes, compaction, lithification)

Subduction - the sideways and
downwards movement of plates of the Earth’s oceanic crust

Silicate weathering - where minerals containing silicate compounds undergo chemical reactions with CO2 and water, leading to the breakdown of these minerals and the release of elements such as Ca, Mg, and Si ions into the soil and water systems.

Question 234

Write the two fundamental equations for the global carbon cycle:

Answer 234

Organic Processes:
CO2 + H2O ↔ CH2O + O2

Inorganic:
CO2 + CaSiO3 ↔ CaCO3 + SiO2

Question 235

Describe the global carbon cycle:

Answer 235

Organic half:
- CO2 forms ‘Organic C in Sediments’ through photosynthesis and burial
- CO2 is then returned to the atmosphere from these organic sediments via metamorphism, diagenesis, and weathering
- Subduction also forms mantle C

Inorganic half:
- CO2 forms ‘Carbonate C in sediments’ via silicate weathering and CaCO3 deposition
- CO2 returns to the atmosphere via metamorphism and diagenesis
- Subduction also forms mantle C

Mantle C is returned to the atmosphere via volcanism

Question 236

At current rates of utilisation, what is the approximate global ratio of reserves to
utilisation for coal, oil and natural gas?

Answer 236

The ratios are around 120 for coal and 50 for oil and natural gas.

Question 237

Considering the 1st law efficiency, how is minimum work found?

Answer 237

W.min =∆G.sep (=∆Gout - ∆Gin)

Assuming an ideal mixture, the partial molar Gibbs free energy for each gas is given by:

𝜕𝐺/(𝜕𝑛.𝑖 ) = 𝐺.𝑖^𝑜 + 𝑅𝑇𝑙𝑛(𝑦.𝑖)

Therefore, the total Gibbs free energy of an ideal gas mixture is:
𝐺.𝑇𝑜𝑡𝑎𝑙=Σ𝑛.𝑖 * 𝜕𝐺/(𝜕𝑛.𝑖 )

Question 238

List processes that produce syngas:

Answer 238

Gasification
C + H2O → H2 + CO

Steam reforming
CH4 + H2O = CO + 3H2

Partial oxidation
CH4 + 0.5 O2 → CO + 2H2

Autothermal reforming
CH4 + 0.5 O2 + 0.5 H2O → CO + 3H2

Chemical looping
CH4 + 2MeO → CO2 + 2H2O + 2Me

Question 239

What flow regime occurs if Re &laquo_space;1?

Answer 239

Creeping flow aka Stokes flow.

Re &laquo_space;1 suggests a creeping flow regime with negligible inertial effects. Viscous forces dominate.

If Re ~ 1, interial forces are of the same order as viscous forces, and inertial effects are present.

Question 240

What are the key dimensionless numbers for flow, transport, and reaction?

Describe their ratios and equations:

Answer 240

1. Reynolds number
   Re = inertial : viscous forces
   Re = ρuL/μ
2. Peclet number
   Pe = advective : diffusive rates
   Pe = uL/Dm
3. Damköhler number
   Da = reaction : advective rate
   Da = kL/u
4. Peclet * Damköhler
   PeDa = reaction : diffusive rate
   PeDa = kL^2/Dm

Where:
L is characteristic length
u is average velocity
Dm is molecular diffusion coefficient
k is reaction rate

Question 241

How is flowrate found, considering porosity?

Answer 241

Q = AuΦ

Where:
Φ is porosity
u is average velocity
A is CSA

Question 242

What are typical efficiencies for a boiler, turbine, and generator, for use in calculations?

(These should be assumed and stated in exams - these won’t be given)

Answer 242

Boiler: 90%

Turbine: 95%

Generator: 97%

Question 243

How is work done by a fan or pump calculated?

Answer 243

W = (Q*ΔP) / η

Where:
Q is the volumetric flow of fluid (e.g. flue gas, CO2, solvent) in m3/s
ΔP is pressure drop in Pa
η is the efficiency of the chosen equipment

Question 244

How is minimum work calculated?

Answer 244

WMix = ∑(B,C) RT[ni CO2 ln(yi CO2) + ni i-CO2 ln(yi i-CO2)] - RT [nA CO2 ln(yA CO2) + nA A-CO2 ln(yA A-CO2)]

Question 245

How is real work calculated?

Answer 245

Wreal is the sum of all the “work” (heating, cooling, pumping, compression…) required to operate a process

W real = W fan + W pump + W regeneration

Question 246

How can work of solvent regeneration be calculated?

Answer 246

W regen = Qth / EF

Where:
Qth is the thermal energy cost of solvent regeneration
EF is the heat-to-work equivalence

Question 247

The main types of storage sites are saline aquifers and hydrocarbon reservoirs. State their capacity as estimated by IEA.

Answer 247

The main types of storage sites are saline aquifers (with IEA estimated capacity of 400-10,000 Gt) and hydrocarbon (gas and oil) reservoirs (with IEA estimated capacity of 920 Gt).

Question 248

Describe the 3 main Fisher-Tropsch mechanisms:

Answer 248

Carbene Mechanism:
- CO splits to C and O
- O binds with 2H to form H2O
- C binds with 2H to form CH2
- CH2 binds with another H to form CH3
- Further CH2 molecules join and bind with CH3 to form a longer chain
- The CH2 on the catalyst binds with another H to form another CH3 at the end, or forms a double bond and leaves the catalyst

Hydroxy-carbene Mechanism:
- CO binds with 2H to form CHOH
- 2CHOH + 2H form C(CH3)OH + H2O and the water leaves the catalyst
- C(OH)CH2R binds with 4H to form CH3CH2R + H2O which both leave the catalyst (or bind with 2H and a double bond form)

CO-insertion Mechanism:
- CH3 binds with CO to form COCH3
- COCH3 binds with 2H to form CH2CH3 and water which leaves the catalyst
- CH2CH2R + H becomes CH3CH2R which leaves the surface

Question 249

Write the logistic equation:

Answer 249

ΔP(t+1)=rPt(1 - Pt/K)

Where:
- ΔP_(t+1) is change in population at t+1
- r is growth constant for small populations
- Pt is population at time t
- K is carrying capacity

This is opposed to the equation with constant growth,

P=P0e^[g(t-t0)]

Question 250

List main ways of air separation (for ASU, for oxy-combustion)

Answer 250

1. Cryogenic:
   mature, high capacity, high purity (> 99%), high energy requirement
   multi-train cryogenic air separation (2 x 5000 t/d units needed for a 500 Mwe)
2. Pressure swing adsorption:
   mature, low to medium capacity, lower purity (~ 92%), lower energy requirement
3. Membrane:
   under development, high purity, possibly lower energy requirement, capacity unknown

Question 251

For each type of fossil fuel (oil, gas and coal), what is the chemical reaction equations for
burning?

Answer 251

Oil:
C8H18 + 25/2O2 → 8CO2 + 9H2O + energy

Gas:
CH4 + 2O2 → CO2 + 2H2O + energy

Coal:
C + O2 → CO2 + energy

Question 252

What physical property of CO2 is very different to that of water in deep saline aquifers?
How does this property affect the CO2 movement in the reservoir?
Compare its weight and flow characteristics to liquid and gas.

Answer 252

Viscosity.
Supercritical CO2 has low viscosity, around 10% that of water. The low CO2 viscosity enhances movement through the aquifer/reservoir which leads to higher speeds and easier escape. Supercritical CO2 weighs like a liquid and flows like a gas.

Question 253

List properties of CO2 for CCS:

Answer 253

Critical point of CO2 is 31C and 72 atm (7.2 MPa).

CO2 will be injected deep underground at supercritical conditions (depths greater than around 800 m).

CO2 is relatively compressible and its density,
although always less than water, is similar to oil.

Low viscosity – typically around10%
that of water.

Question 254

What’s the critical point of CO2?

Answer 254

31C and 7.2 MPa