Lecture 2 - Supply side Flashcards
(35 cards)
1
Q
How much primary energy demand is met by fossil fuels currently?
A
80%
2
Q
How can we decarbonise the electricity sector?
A
- More efficient coal-fired power generation
- Switch from coal to natural gas
- Switch to (or co-fire with) biomass
- CCS
- Nuclear
3
Q
Coal - key facts
A
- The most abundant fossil fuel - proven reserves of 1 trillion tonnes
- Cheap
- In 2015, global demand for coal fell for the first time since 1990s
4
Q
What is pulverised coal combustion
A
- Principle means of electricity generation from coal worldwide.
- A mixture of pulverised fuel and air is injected into a combustion chamber (blast furnace/boiler)
- Heat is transferred to water in heat exchange tubes and steam rises.
- Steam passed into a turbine where it expands and turns the turbine to generate electricity.
- Steam is then condensed and cooled with the cooling water.
- Waste heat is removed in a cooling tower.
5
Q
What impacts the efficiency of a coal fired power plant?
A
- Quality of the coal
- Size of the plant
- Temperature and pressure - at higher T & P more ‘work’ can be extracted from the steam and therefore higher efficiency. Only recently able to build plants that can withstand this high T & P.
6
Q
Name the types of coal plants in order from least efficient to most efficient
A
- Sub-critical. Efficiency = 38%
- Supercritical. Efficiency = 42-43%
- Ultra-supercritical. Efficiency = up to 45%
- Advance ultra-supercritical. Efficiency = up to 50%
- Integrated gasification combined cycle. Efficiency = >50%
7
Q
Describe the key features of a sub-critical power plant
A
- Conventional pulverised coal combustion
- Steam produced below the critical pressure of water
- Efficiency = 38%
- Overnight cost = 600-1,980 USD/kWh
8
Q
Describe the key features of a supercritical power plant
A
- Steam generated above the critical point of waters.
- Efficiency = 42-43%
- Overnight cost = 700-2,310 USD/kWh
9
Q
Describe the key features of a ultra-supercritical power plant
A
- Similar to SC but works at higher T & P
- Efficiency = up to 45%
- Overnight cost = 800-2,530 USD/kWh
10
Q
Describe the key features of an advanced ultra-supercritical power plant
A
- Under development
- Even higher T & P than USC
- Efficiency = up to 50%
11
Q
Describe the key features of an integrated gasification combined cycle power plant
A
- Under development
- Gasification of coal to produce syngas, which is then combusted and the hot gas is used to run the turbine.
- Efficiency = >50%
12
Q
What category of power plant are most new coal plants?
A
Supercritical and ultra-supercritical
13
Q
What is the CO2 reduction potential of the different types of coal power plants
A
Sub-critical -> Supercritical = -21%
Sub-critical -> Ultra-supercritical = -33%
Sub-critical -> Advance ultra-supercritica = -40%
Sub-critical -> CCS = -90% (but efficiency loss of 7-12 percentage points)
14
Q
What does the 2DS require from coal?
A
- Early decommissioning of coal plants
- But many existing plants are ‘paid-for’ and therefore are a cheap ongoing source of power and revenue stream.
- By 2050, all plants need to be USC/SC, some with CCS.
15
Q
What is happening to coal use in China?
A
- Levelled off - due to pollution concerns.
- Perhaps better able to impose changes than other countries and have a long term view -> political situation.
16
Q
What are the key natural gas technologies?
A
- Open cycle gas turbine: 35% efficiency
- Combined cycle gas turbine: 60% efficiency
17
Q
What are the advantages of gas over coal?
A
- Plants are cheaper and quicker to build
- Higher efficiency
- Less polluting
- Lower CO2 intensity
- Can be operated move flexibly
18
Q
Describe the key features of the combined cycle gas turbine
A
- Combustion of gas heats water and generates steam which rises and then drives a turbine as it expands and cools.
- Hot gases from combustion are also used to drive a turbine generator.
19
Q
How much more low-carbon is natural gas than coal?
A
- CCGT: 350 gCO2/MWh; efficiency = >60%
- OCGT: 700 gCO2/MWh; efficiency = 35%
- SC: 880 gCO2/MWh; efficiency = 42-43%
- USC: 743 gCO2/MWh; efficiency = 45%
- AUSC: 669 gCO2/MWh; efficiency = 50%
20
Q
What impacts did the shale gas boom have?
A
- Drove down natural gas prices
- This made it hard for non-gas power generation technologies to compete
- Largely resulted in the displacement of coal (early retirement of plants)
21
Q
What is the role of natural gas fired generation in the future?
A
- Advancement of CCGT resulted in a boom of natural gas generation (in OECD countries, increased from 10% in 1990 to 24% in 2014, saving 1Gt (annual) of CO2 emissions).
- Two important roles to play: (i) displacing emissions from coal (ii) complementing the deployment of renewables by providing flexibility.
- Likely to play a role as a transition fuel but we need to be aware of the dangers of capital lock-in.
- Plants should be designed so that they can be easily retrofitted with CCS.
22
Q
What is CCS?
A
- The princess of preventing CO2 from entering the atmosphere by capturing it from large point emission sources and compressing and transporting it to a storage site and storing it permanently.
23
Q
Why is CCS challenging?
A
Because we burn fuels in air which is ~78% nitrogen and separating CO2 from nitrogen is hard and expensive.
24
Q
CCS: describe post combustion capture
A
- CO2 in flue gas is passed through an absorber counter current to the absorbent.
- CO2 is absorbed onto the absorbent.
- Spent absorbent is passed into a stripper where it is heated to release the CO2
- Regenerated absorbent is reused.
Advantages: easily retrofitted, closed to commercialisation
Disadvantages: might not be very flexible, relatively high efficiency penalty, absorbent degeneration on present of SOx and NOx.
Efficiency penalty = 10% - target of 8%
25
CCS: describe pre combustion capture
1. Fuel is gasified to produce syngas
2. Shift reaction increases CO2 content
3. CO2 is removed
4. H2 rich fuel is burned in air
Advantages: shift reaction and CO2 removal is a commercial practice, higher CO2 concentrations allow for easier separation of CO2, lower efficiency penalty than post combustion capture.
Disadvantages: not easy to retrofit, high capital cost, H2 fired boiler yet to be demonstrated commercially.
Efficiency penalty = 7-9% - target of 5-6%
26
CCS: describe oxyfuel capture
1. Oxygen is separated from nitrogen in an air separation unit (ASU)
2. Fuel is burnt in oxygen
3. Flue gas consists of CO2 and H2O
4. H2O is condensed
5. CO2 is compressed ready for transport and storage
Advantages: easily retrofitted
Disadvantages: ASU is expensive, might nt be very flexible due to operation of ASU.
Efficiency penalty = 10% - target of 8%
27
Advanced CCS: describe chemical looping combustion capture
1. Metal oxide reacts with fuel in presence of steam in fuel reactor.
2. Produces CO2 and steam
3. Reduced metal oxide is regenerated with air in air reactor
Advantages: doesn't require separate CCS unit, theoretically lower efficiency penalty because separation and combustion happen together
Disadvantages: still in very early research stage, requires a circulating fluid bed reactor, which is challenging to build at scale.
28
CO2 captured vs CO2 avoided
Compared to a reference plant with no CCS, a CCS plant will have:
- a small amount of CO2 generated that isn't captured (less than reference plant)
- the remaining CO2 will be captured
- but total CO2 generated will be higher than the reference plant due to the efficiency penalty of the CCS
- the total amount captured will be made up of the CO2 avoided compared to the reference plant, plus an additional amount associated with the efficiency penalty.
Therefore CO2 avoided < CO2 captured.
29
What is the public perception of CCS?
Mixed:
- Essential for abating emissions
- Dangerous distraction that may weaken.delay efforts to develop new technologies
- Western get-out clause avoiding difficult behavioural changes
- Doomed, but we've pumped millions into it
30
Describe the key features of biomass combustion
- Cheapest method of reducing CO2 from a large boilers assuming the additional fuels (biomass/waste) are considered CO2 neutral.
- Possibly the least complicated way of using biomass and waste for replacement of fossil fuels for stationary energy conversion.
- Drax power station in North Yorkshire - supplies 7%UK electricity, undertaking the largest co-firing project in the world.
31
What are the three main biomass co-firing strategies?
1. Fire a small amount (few %) of biofuel or waste into a coal-fired boiler to get rid of waste: easy, no obvious limitations in a large boiler, can claim GHG credits. But fails when legislation classifies such a boiler as a waste-firing until with stringent environmental regulations.
2. Fire a small amount of high heating value fuels with a fuel that has a low heating value (e.g. sludge) that needs thermal support to ensure combustion: applicable only to certain combustion technologies, similar approach to adding petcock to low rank coal
3. Co-combustion of fuel at any ration depending on market price, availability and local supply conditions: difficult to do with most boiler designs.
32
Describe the key features of nuclear power generation today
- ~13% power generation in 2013.
- In 2016: 30 countries operating 450 plants for electricity generation and 60 new plants under construction in 15 countries.
- Peak investment was in 1980s
- Dip around 2010 - Fukushima
33
What are the key controversies associated with nuclear power?
1. Dealing with nuclear waste - aim is for long term storage in geological site but no-one wants to be near them. None currently in operation.
2. High up-front cost - e.g. Hinkley point - £18bn. Government had to guarantee a price of £92.50/MWh which is double the current wholesale electricity price.
3. Safety - danger of a major disaster should not be ignored. We can't completely rule out human error.
34
Why do we need nuclear?
- Currently it is the only larger scale source of continuous low carbon electricity available in most parts of the world.
- Costs of mitigation are greatly increased if we have no nuclear (10-35%). Projection of 16% nuclear generation in 2050 in 2DS.
- Phasing out nuclear would have a significant impact on our ability to stay within 2C.
35
What are the key features of non-electricity supply?
- Refineries and other transformations, e.g. oil into other products -> big questions around how we decarbonise this sector.
- Role of hydrogen? Potential to play a role across all end use applications, e.g. fuel cell EVs, replace natural gas for heating and cooking, low carbon steel production, planes. But low carbon production on H isn't easy, will require a distribution and transportation network and demand side equipment would have to be redesigned, e.g. cookers.
