Lecture 2 - Supply side Flashcards
How much primary energy demand is met by fossil fuels currently?
80%
How can we decarbonise the electricity sector?
- More efficient coal-fired power generation
- Switch from coal to natural gas
- Switch to (or co-fire with) biomass
- CCS
- Nuclear
Coal - key facts
- 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
What is pulverised coal combustion
- 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.
What impacts the efficiency of a coal fired power plant?
- 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.
Name the types of coal plants in order from least efficient to most efficient
- 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%
Describe the key features of a sub-critical power plant
- Conventional pulverised coal combustion
- Steam produced below the critical pressure of water
- Efficiency = 38%
- Overnight cost = 600-1,980 USD/kWh
Describe the key features of a supercritical power plant
- Steam generated above the critical point of waters.
- Efficiency = 42-43%
- Overnight cost = 700-2,310 USD/kWh
Describe the key features of a ultra-supercritical power plant
- Similar to SC but works at higher T & P
- Efficiency = up to 45%
- Overnight cost = 800-2,530 USD/kWh
Describe the key features of an advanced ultra-supercritical power plant
- Under development
- Even higher T & P than USC
- Efficiency = up to 50%
Describe the key features of an integrated gasification combined cycle power plant
- Under development
- Gasification of coal to produce syngas, which is then combusted and the hot gas is used to run the turbine.
- Efficiency = >50%
What category of power plant are most new coal plants?
Supercritical and ultra-supercritical
What is the CO2 reduction potential of the different types of coal power plants
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)
What does the 2DS require from coal?
- 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.
What is happening to coal use in China?
- Levelled off - due to pollution concerns.
- Perhaps better able to impose changes than other countries and have a long term view -> political situation.
What are the key natural gas technologies?
- Open cycle gas turbine: 35% efficiency
- Combined cycle gas turbine: 60% efficiency
What are the advantages of gas over coal?
- Plants are cheaper and quicker to build
- Higher efficiency
- Less polluting
- Lower CO2 intensity
- Can be operated move flexibly
Describe the key features of the combined cycle gas turbine
- 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.
How much more low-carbon is natural gas than coal?
- 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%
What impacts did the shale gas boom have?
- 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)
What is the role of natural gas fired generation in the future?
- 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.
What is CCS?
- 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.
Why is CCS challenging?
Because we burn fuels in air which is ~78% nitrogen and separating CO2 from nitrogen is hard and expensive.
CCS: describe post combustion capture
- 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%
CCS: describe pre combustion capture
- Fuel is gasified to produce syngas
- Shift reaction increases CO2 content
- CO2 is removed
- 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%
CCS: describe oxyfuel capture
- Oxygen is separated from nitrogen in an air separation unit (ASU)
- Fuel is burnt in oxygen
- Flue gas consists of CO2 and H2O
- H2O is condensed
- 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%
Advanced CCS: describe chemical looping combustion capture
- Metal oxide reacts with fuel in presence of steam in fuel reactor.
- Produces CO2 and steam
- 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.
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.
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
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.
What are the three main biomass co-firing strategies?
- 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.
- 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
- Co-combustion of fuel at any ration depending on market price, availability and local supply conditions: difficult to do with most boiler designs.
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
What are the key controversies associated with nuclear power?
- 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.
- 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.
- Safety - danger of a major disaster should not be ignored. We can’t completely rule out human error.
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.
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.
