Electricity from coal and gas
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SummaryOlder coal fired stations are on average 31% efficient. The newer ones are up to 43%. Gas can give 66%, however on standby ready for an instant start, they are only 33% efficient. Gas (methane) also has twice the heating value for the same CO2 emissions so is being pushed as a green alternative to coal. However over 50 years, methane in the atmosphere is 50 times worse than CO2. With average leakages of 4%, it works out worse than coal. If all the world's gas reserves were burnt, it would increase the CO2 level to 440 ppm. 450 is nominated as the limit of safe climate change. This is not allowing for oil and coal. Brown coal power plants generate between 1.25 and 1.4 tonnes of CO2 per megawatt/hour, versus 0.90 for black coal and 0.4 to 0.5 for new-build gas-fired power plants. A hybrid plant that gasified brown coal would emit about 40 per cent less CO2 than existing brown coal plants. |
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Gas turbineTo generate electricity, the methane is burnt in a gas turbine. The turbine is built of two parts. The first set of blades draws in air and compresses it. The methane is then pumped into the compressed air and set alight. The gas now expands about 4 times and rushes past the next set of blades, turning them and spinning a generator to produce electricity. But a gas turbine is only about 33% efficient, as the exhaust gases carry away a lot of heat. This has usually been wasted as gas has been very cheap. |
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Combined cycle gas turbineThe heat can be used by passing the hot exhaust gasses over water pipes to produce steam. The steam then turns another turbine to produce more electricity. This gives another 30 % efficiency, so the total adds up to over 60%. So it produces half the CO2 This is called a combined cycle gas turbine, or CCGT. |
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Coal fired power stationIn a coal fired power station only the steam turbine stage is used. You can’t burn coal in a gas turbine because the ash would melt onto the blades and damage them. Although coal burns at 2.500 deg C, the steam can only be heated to around 650 deg C because the pipes and valves etc would fail unless made of very expensive alloys, and this would push the price of electricity up. So a lot of temperature is wasted. This is why coal fired power stations are only about 30% efficient. But then who cares, coal has been cheap? There are trials on gasifying the coal and using the gas in a CCGT. |
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Critical power stationsNewer coal fired power stations operate at higher temperatures and pressures. Coal sub critical = burn pulverised coal and produce steam below the critical pressure of water which is 220 bar. (3206 psia) They are about 37% efficient. Coal supercritical = As above above the critical pressure and 40% efficient. Coal ultra super critical = Operate at a higher pressure and temperature and 42-45% efficient. The subcritical label does not make sense.The critical temperature of water is 374 deg C. As the power plant superheats heat the steam above this point Above the critical temperature steam cannot be liquefied by pressure alone and is regarded as a gas rather than a vapour.
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Source: IEA 2008, p.257, Table 7.2 |
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EfficiencyThe maximum theoretical efficiency is determined by the Carnot theorum. The maximum possible amount of energy that can be recovered from the heat is dependent on the temperature difference. The larger the temperature difference, the more efficient. Maximum efficiency = 1- T cold/T hot. Temperature of hot and cold sides are expressed in degrees Kelvin. In practice the efficiencies in industry is usually about half the theoretical. For a power station producing steam at 650 deg C, the maximum theoretical efficiency is 66%. The average around the world is 31%. With more expensive materials the steam can be heated more and the efficiency can reach 43%. |
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Heat values of methane and coalMethane has another advantage, it has twice as much energy as coal, so if we burn it in a combined cycle gas turbine power station that should give us one quarter of the CO2 of coal.
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Effect of leakage of methaneThis beautiful efficiency unravels if there is any leakage of methane. As methane is 25-70 times worse than CO2 as a greenhouse gas, the effect of leaks become magnified. Even a small leak makes methane worse than coal. 4% leakage as the USA CSG industry admits, makes gas just as bad as coal burnt in an old coal fired plant. It is worse if compared to a new coal fired plant. If the leakage rate is 6%, then methane is worse. |
Methane produces less CO2High hydrogen content 88% of CO2 Greenhouse gas 22% of coal Effect of methane leakageLeakage = 3-9% of CH4 used
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Greenhouse emissions from gas vs coal
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The future for coalAccording to the International Energy Agency,
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Investment in power stations
Old king coal is losing his grip - The Conversation This article was co-authored by Ray Wills, CEO of the Sustainable Energy Association. |
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