Summary 

Older 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 turbine

To 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.

Combined cycle gas turbine

The 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.

Coal fired power station

In 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. 

Coal subsidies in NSW

Critical po​wer stations

Newer 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. 

Source: IEA 2008, p.257, Table 7.2

Efficiency

The 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%.

Heat values of methane and coal

Methane 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.

Effect of leakage of methane

This 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 CO₂

High hydrogen content     88% of CO₂
Twice heat value               50%
Can burn efficiently           50%

Greenhouse gas               22% of coal
 

Effect of methane leakage

Leakage = 3-9% of CH4 used
Say 6%
CH4 is 72 x worse than CO2 over 20 yrs
            25 x worse over 100 yrs
Say      50 x  over our lifetime
50 x 6% = 300%
Ch4 is used 22-44% of coal. Say 33%
33% x 300%  = 100%  +33%  = 133%
So methane is worse than coal.

Greenhouse emissions from gas vs coal

A new report commissioned by the peak body of the oil and gas industry suggests that claims coal seam gas exports are up to 70 per cent cleaner than coal exports over their life cycle may not actually be valid. The report by Worley Parsons finds that this is true when compared to coal technologies that are no longer deployed,

In practice, then, the best comparisons lie between CSG and the ultra-supercritical coal plants, which has a base-case emissions of 0.78t/Co2e for every MWh produced. Combined-cycle baseload plants with CSG have a base case of 0.55t/Co2e for every MWh, while open cycle peaking plants have a base case of 0.75t/Co2e for every MWh. The graphs below show the ranges – the first is based on different scenarios on all emissions sources, while the second is just on power plant efficiencies, which can vary widely.

Quoted from...

OVGT - Open Cycle Gas Turbine = a normal gas turbine used for standby and peak power. Starts very quickly.

CCGT - Combined cycle Gas Turbine = Gas turbine with steam turbine using exhaust gast from turbine.

 

The future for coal

According to the International Energy Agency,

“around one-third of the 340 GW of coal-fired capacity is more than 30 years old and relatively inefficient, and coal-fired generation faces more stringent pollution standards, for example on mercury, which may well lead to accelerated retirements of older plants. There has been hardly any net increase in coal-fired capacity since 2000.”

Investment in power stations

“King coal still reigns” was the headline emblazoned across a full page article in The Weekend Australian on the 28-29 April 2012, by Environment Editor Graham Lloyd. The article’s subtitle was, “The world is in the grip of a fossil fuel boom that shows no sign of fading”.

Nothing could be further from the truth. The latest data on global investment in new power production shows the dramatic decline in fossil fuel investment, and an astonishing increase in renewables investment.

See table:

The role of Murdoch’s media empire in talking up Old King Coal seems to be one of the “fiddlers three”, trying desperately to help keep the king merry as his kingdom collapses.

Old king coal is losing his grip - The Conversation

This article was co-authored by Ray Wills, CEO of the Sustainable Energy Association.