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CPE451 > Carbon Capture & Sequestration > Flashcards

Carbon Capture & Sequestration Flashcards

1
Q

What is involved in carbon dioxide capture and sequestration?

A
  1. Separating CO2 from other gases at emission sources
  2. Transporting it to a storage location
  3. Ensuring the long-term isolation of CO2 from the atmosphere.
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2
Q

What 3 static sources are good places for CCS implementation?

A

Fossil Fuel Power stations
Cement Manufacture
Steel Production

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3
Q

What are the 3 different approaches to CCS?

A
  1. Post-combustion capture (many existing conventional plants could be retrofitted)
  2. Oxy-fuel combustion and capture (a few pilot plants, similar to conventional technology)
  3. Pre-combustion decarbonisation (a few IGCC plants that could be retrofitted – new plant needed)
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4
Q

How does post-combustion CCS work?

A
  1. CO2 is separated from the other flue gases in the waste streams
  2. Gases are then compressed for subsequent storage or use.
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5
Q

Is post-combustion CCS best for coal or gas-fired plants? Why?

A

Coal.

Because coal plants have a higher partial pressure of CO2 in the flue gas which makes the separation process more efficient.

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6
Q

What is a disadvantage of post-combustion CCS?

A

Loss of energy conversion efficiency. Energy production reduced by 20-25% because some energy is used for CO2 capture process.

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7
Q

Compare the conditions for post-combustion & pre-combustion CCS.

A

Post-combustion requires higher T but lower P. Pre-combustion efficiency of separating CO2 is higher.

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8
Q

What technologies can be used for post-combustion CCS?

A
  1. Solvent Absorption
    e.g. chemical solvents : amine
    physical solvent : alcohol
  2. Adsorption (solids)
  3. Membranes
  4. Cryogenics
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9
Q

How does a chemical solvent work in post-combustion CCS?

A

Reaction of solvent (usually an alkanolamine) with gas to produce a CO2 containing compound in a reversible reaction.

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10
Q

How does a physical solvent work in pre-combustion CCS?

A

Solubility of gas / components of a gas determines effectiveness of absorbent. Henry’s Law dependent, operate best at high pressure and low temperature.

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11
Q

How does post-combustion CCS work with a chemical solvent such as alkanolamines (e.g. MEA)?

A
  • Absorber – spray tower approx 40 C (gas moves up, amine moves down)
  • Heat exchanger
  • Regeneration, by heating in stripper 140 C (steam taken from low power turbine)
  • Amine recycled
  • CO2 recovered from mixture of CO2 and water – compressed for transportation and storage
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12
Q

True or false: Only primary & secondary amines can be used in post-combustion CCS as chemical solvents.

A

True

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13
Q

What is the difference between a primary & secondary amine?

A

Primary amines have 1 carbon attached to the amino- functional group.
Secondary amines have 2 carbons attached to the amino- functional group.

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14
Q

Why don’t tertiary amines work a chemical solvents in post-combustion CCS?

A

Because they do not produce a carbamate species.

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15
Q

What are the advantages and disadvantages of chemical solvents in post-combustion CCS?

A

Advantages
1. Existing technology, proven to operate on fairly large scale industrial applications, proven for petrochemical processes.
2. Demonstrated to produce high purity CO2
3. Solvents easy to move around and can cope with water

Disadvantages
1. High amount of solvent is required - 2:1 ratio
2. Not proven for a large fossil-fuel burning power station
3. Corrosive solvents - problems with metal piping systems.
4. Regeneration cost - thermal regeneration, pumping and compression
5. Flue gases need to be desulphurised before they can be processed.
6. SOx and NOx react irreversibly with MEA to produce non-reclaimable corrosive salts.
7. Degradation of the amine means it needs to be replaced regularly
8. Overall MEA scrubbing systems can reduce the power output of a coal fired power plant by about 20% e.g. from 48 to 38% efficiency.

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16
Q

How does oxy-fuel combustion & capture work?

A

Where usually fossil fuels & air are fed into the combustion chamber. Instead, nitrogen is removed from the air so it is pure oxygen and fossil fuels. This creates 75% less flue gas during combustion.

After combustion, ash is removed from the flue gas and some of the gases (CO2 & water vapour) are recycled back into the process to create combustion conditions similar to those in air.

The remaining flue gas is cleaned to remove sulphur and H2O leaving only CO2.

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17
Q

What is the main disadvantage/challenge for oxy-fuel combustion?

A

Improving the oxygen production process - air separation.

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18
Q

What are the advantages and disadvantages of oxy-fuel combustion CCS?

A

Advantage:
1. Combustors could be fairly conventional
2. Removing the N2 means much less flue gas - 75% less flue gas
3. Easy CO2 separation because the flue gas mostly CO2 & H2O
4. Possible to use compact boilers with low quantities of flue gas
5. Oxyfuel CCS is more efficient than post combustion CCS

Disadvantage:
1. The process has only been tested on a small scale

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19
Q

How does pre-combustion CCS work?

A

Fossil fuels or biomass are not directly fed into the combustion chamber. They are first gasified or reformed to produce syngas. Then CO2 is separated using the Water Gas Shift Reaction : CO → CO2 shift.

CO2 is extracted from the fuel gas and subsequently stored or used.

The remaining fuel gas is burnt to generate electricity or used as a hydrogen source.

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20
Q

What reactions are involved in pre-combustion CCS when the fuel is gasified?

A

A combination of pyrolysis, combustion, gasification and water gas shift in a controlled amount of air / oxygen content so only a small amount of fuel burns.

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21
Q

What does IGCC stand for?

A

Integrated Gasification Combined Cycle

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22
Q

Is the water gas shift reaction endothermic or exothermic?

A

Exothermic

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23
Q

What is the main disadvantage/challenge for IGCC with pre-combustion CCS?

A

High capital cost compared to conventional power plants.

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24
Q

Why does the IGCC process work well with CCS?

A

The gas stream contains CO2 at high concentration ~35% and pressure ~50 bar.

The high pressure favours Henry’s law as more CO2 can be dissolved using the same quantity of physical solvent.

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25
Q

When are physical solvents preferred?

A

For gas based systems at elevated pressures

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26
Q

What determines how effectively the physical solvent can separate the CO2 in pre-combustion CCS?

A

The different physical solubility of gases in the solvent.

27
Q

Name 2 physical solvents used in pre-combustion CCS.

A
  1. Chilled methanol - Rectisol process
  2. Dimethyl ethers - Selexol process
28
Q

Why is methanol an effective physical solvent?

A

Well proven for H2S, Carbonyl sulfide (COS) and CO2 removal
Solubility increases significantly with decreasing temperature and increasing pressure

29
Q

What are the advantages and disadvantages of physical solvents with pre-combustion CCS?

A

Advantages:
1. They work well at high concentration of CO2 and high pressure
2. Compact equipment
3. Low energy requirement for CO2 separation
4. Potential to supply H2

Disadvantages:
1. New / unfamiliar technology for electricity generators
2. IGCC has higher costs without CO2 capture than pulverised fuel combustion
3. Air separation unit required
4. Gas separation not as selective as post combustion

30
Q

What happens during pyrolysis in an IGCC?

A

Carbonaceous feed fuel is heated and volatiles are released and char is produced, resulting in up to 70% weight loss for coal.

The process is dependent on the properties of the carbonaceous material (fuel type) and determines the structure and composition of the char, which will then undergo gasification reactions.

31
Q

What happens during combustion in an IGCC?

A

Volatile products and some of the char reacts with a limited supply of oxygen to form carbon dioxide and carbon monoxide, which provides heat for the subsequent gasification reactions.

This results in partial or complete combustion.

32
Q

What happens during gasification in an IGCC?

A

Char reacts with carbon dioxide and steam to produce carbon monoxide and hydrogen.

33
Q

Why are dimethyl ethers and polyetheylene-glycolan effective physical solvents?

A

They are thermally stable, have a low vapour pressure and CO2, H2S and COS are highly soluble in it.

34
Q

Why is the Selexol ® generally preferred to Rectisol ® pre-combustion CCS process?

A
  1. No refrigeration
  2. Less complex process
  3. High chemical & thermal stability
  4. Non-toxic & non-corrosive
  5. High H2S and COS solubility
35
Q

What other carbon capture technologies are available other than post, pre & oxy-fuel combustion with solvents?

A
  1. Solid Adsorbents
  2. Membranes
  3. Cryogenic Separation
36
Q

What are the benefits of using solid adsorption materials?

A
  • Ability to achieve greater uptakes of CO2 on a volumetric basis compared to solvents
  • Regeneration is potentially less energy intensive than amine scrubbers
  • Maintain relatively high CO2 pressure after water gas shift reaction (less compression later & better suited for physical absorbents)
  • Less sensitive to other acid gases (SOx and NOx)
  • Avoids corrosion problems which occur with liquid amine systems
  • Potential economic benefits over liquid amine systems
37
Q

What is an essential property of solid adsorbents?

A

Large surface area.

38
Q

What is the difference between an absorbent & adsorbent?

A

Absorption - fluid is dissolved by a liquid or solid absorbent.
Adsorption - atoms, ions or molecules adhere to a solid adsorbent.

39
Q

What are the disadvantages of solid adsorbents?

A
  • Not a proven technology, still in the development stage
  • Cost and lifetime of adsorbents is as yet unknown
  • Due to high temperature and low partial pressure of CO2, adsorption capacity and selectivity can be low.
40
Q

How do membrane technologies work?

A

They utilise the different physical and/or chemical properties of the substances in the gas mixture, and the fact that the different components will move through the membrane at different rates.

41
Q

What types of membrane separation are available?

A
  1. Gas separation membranes
  2. Gas absorption membranes
42
Q

How does gas separation membranes work?

A

Small gas molecules e.g. H2 passes through the membrane while other parts of the flue gas e.g. CO2 are excluded.

43
Q

How does gas absorption work?

A

The membrane functions as a contact point between the flue gas and an absorbing liquid e.g amine solution.

44
Q

What are the advantages & disadvantages of membrane separation technologies?

A

Advantages
1. Can be used in hybrid systems (e.g. membrane & solvent)
2. Provide high surface area for exchange – very compact system
3. Small size leads to capital cost reductions
4. Gas and liquid not in direct contact – avoids operational problems

Disadvantages
1. Currently only small scale
2. Problems with membrane stability
3. Thicker membranes are required for physical strength but this reduces membrane selectivity

45
Q

How does cryogenic separation work?

A

CO2 is physically separated from other gases by condensing it into a liquid form at an extremely low temperature.

46
Q

What are the advantages & disadvantages of cryogenic separation technologies?

A

Advantage:
1. The liquid CO2 produced would be immediately ready for transport to the disposal site.

Disadvantage:
1. Cryogenic separation of CO2 is handicapped by the cost of cooling such large quantities of gas, which makes the process un-economical on a large scale.

47
Q

In what state is CO2 transported in pipelines? Why?

A

Supercritical fluid - compressed to 150 times atmospheric.

In this state, they can be pumped like a liquid, but have very low resistance to flow, like a gas.

48
Q

What can be done with captured CO2?

A
  1. Stored
  2. Used in other industries as a feedstock.
49
Q

Where can CO2 be stored?

A
  1. Depleted oil/gas reservoirs
  2. Saline aquifers
  3. Shale & basalt formations
  4. Un-minable coal seams
  5. The ocean
50
Q

Where can CO2 be used in other industries?

A
  1. Carbonification of minerals which can then be returned to the environment
  2. Catalytic conversion to raw materials for organic synthesis - back into a useful fuel (this requires a lot of energy)
  3. Biological sequestration
51
Q

How do depleted oil reservoirs store carbon?

A

A layer of permeable rock with a layer of non-permeable cap-rock above traps and holds the oil and gas in place.

Every storage site is different, therefore each has to be individually assessed . Surveying techniques (seismic surveys) are used to map and visualize the subsurface and samples of rock and fluid are taken from the subsurface (core drilling).

52
Q

What is EOR?

A

Enhanced Oil Recovery (EOR) is used in mature oil reservoirs.

CO2 is injected into a mature oil reservoir which enables more oil to be recovered.

Some CO2 will dissolve in the oil, increasing its bulk volume and decreasing its viscosity (flows more easily).

EOR with a CO2 flood allows recovery of an additional 10-15%.

This only makes sense to do if the oil being extracted is used for products other than for fuel (to be burned).

53
Q

How do saline aquifers store carbon?

A

Saline aquifers are underground layers of very porous, permeable sediment filled with brackish, non-potable water.

These are much more common than coal, oil or gas bearing rock.

Less is known about saline aquifers because the fossil fuel industry has not been interested in them, but in the long term deep saline formations are the most promising storage formation.

However, to be suitable for CO2 storage:
1. Be at sufficient pressure to keep the CO2 in the dense phase – liquid or super critical liquid i.e. approx at depths >800m
2. Have a suitable seal of cap-rock (e.g. one or more layers of shale, anhydrite or evaporites) to prevent vertical flow of CO2 (geology similar to oil wells)
3. Have suitable hydro-geology such that the CO2 is isolated within the saline formation.
4. Extensive surveying is needed to ensure these conditions exist.

54
Q

What is meant by an un-minable coal seam? How do they work as a source of carbon storage?

A

Coal seams may be un-minable if they are too deep or thin.

All coals have methane adsorbed onto pore surfaces within the coal. Methane is extracted by drilling wells into the coal seam and de-watering or de-pressurising the formation.

This still leaves a considerable amount of methane adsorbed on the coal surface. Enhanced coal bed methane (ECBM) recovery uses CO2 to recover more of this methane by preferential adsorption of the CO2 onto the coal surface. (CO2 is twice as likely to adsorb onto the coal surface compared to methane).

This only makes sense to do if the methane being extracted is used for products other than for fuel (to be burned).

55
Q

What factors are the most important when considering a location to trap CO2?

A
  1. Geologic porosity - sedimentary rock better
  2. Depth - ideally >900m but pressure at depth reduces porosity therefore usually <1800m. 150bar ≈ pressure at 1600m
  3. Permeability of the selected site - ability of fluid to flow through the sediment – grain size, low porosity requires more CO2 wells. During injection the upwards buoyancy of the CO2 (e.g. relative to brine) acts against the downward injection pressure.
56
Q

What trapping mechanisms are used for long-term carbon storage (in order of short to long term)?

A
  1. Structural trapping - In oil and gas reservoirs and natural CO2 deposits a layer of impermeable cap-rock that overlies the porous rock formation prevents the upward flow of the fluid. This cap-rock can also be used to trap CO2 injected into such wells and also in suitable aquifers.
  2. Adsorption of CO2 - Coal and other organically-rich reservoirs will preferably adsorb CO2 onto their carbon-rich surfaces as a function of reservoir pressure.
  3. Residual capillary trapping - The surface of sandstone and other porous rock preferentially adheres to saline water over CO2. If there is enough saline water within a pore (75-90% of the pore volume), it will form a capillary plug that traps the residual CO2 within the pore space.
  4. Dissolution in saline water - CO2 is soluble in saline water so can be stored in brine.
  5. Mineralization - Over thousands of years dissolved CO2 reacts with minerals in the rock containing Mg or Ca to form solid carbonates.
57
Q

What are basalt formations? How do they trap CO2?

A

Basalt Formations consist of solidified lava. They contain high concentrations of calcium and magnesium ions that chemically react with CO2 to make chemically stable calcite, dolomite, and magnesite. Dissolving the CO2 above ground speeds up the mineralization process.

58
Q

What is shale rock? How does it trap CO2?

A

Shales (the most abundant rock type) consist of horizontal layers of rock with low permeability in the vertical direction. Many shales contain 1-2% hydrocarbons which could be used to adsorb CO2 in a similar way to coal bed methane recovery.

59
Q

What terrestrial options are available for carbon capture?

A

The terrestrial biological carbon cycle is about twenty times larger than the production of CO2 from fossil fuel combustion.

  1. Uptake of CO2 by soils and plants as organic material.
  2. It is a low cost option for CO2 reduction
  3. Wildlife habitat and water quality improvement.
  4. It includes activities such as tree planting, no-till farming, wetland restoration, grazing management, fire management and forest preservation.
  5. Advanced research efforts include fast growing trees and grasses and soil microbe studies.
60
Q

Why is it not good to directly put CO2 into the oceans for storage?

A

The environmental effects are generally negative, and are poorly understood.
1. Large concentrations of CO2 kills ocean organisms.
2. Dissolved CO2 would eventually equilibrate with the atmosphere, so the storage would not be permanent.
3. The oceans are know to be gradually increasing in acidity as they absorb atmospheric CO2

61
Q

What are the main safety concerns associated with CCS?

A
  1. It could leak and contaminate drinking water.
    Needs to be well surveyed and monitored
  2. It could poison the atmosphere.
    Unlikely that massive release of CO2 would occur – slow leaks possible so storage sites will to be monitored long term
  3. Pumping in the CO2 could cause earthquakes.
    Similar to fracking, pumping CO2 can result in seismic activity, so seismic monitoring and pumping limits should be in place
62
Q

What are the key barriers to CCS deployment?

A
  1. The high cost and energy penalties associated with of currently available capture technologies.
  2. Lack of experience operating CO2 capture equipment on commercial scale power plants.
  3. Lack of commitment from government to invest the required funds
  4. Lack of a CO2 pipeline infrastructure to transport CO2 from power plants in regions that lack suitable storage options.
  5. Lack of experience with storing extremely large volumes (3-5 million tons per year) of CO2 in geologic formations.
  6. An absence of rules for storage of CO2 in saline formations
63
Q

How could these barriers be overcome?

A
  1. Technology Maturation:
    Generally, the cost of all new technologies is highest at the time of market introduction
  2. Technology Advancement:
    The development of lower cost advanced CO2 capture technologies
    The development of system components that can bring down the overall cost of CCS equipment
  3. Financial Incentives:
    Governments can offer a variety of financial incentives for early or accelerated demonstration and deployment of CCS technology.

CPE451 (7 decks)

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