Figures released for large UK coal fired power
plants - for example Drax which is the 2nd largest coal fired power station
in Europe.

http://en.wikipedia.org/wiki/Drax_power_station

It burns between 7 and 11 million tonnes of coal per year and produces on
average 24 TWh of electricity to the grid. I'll let you do the ground work
and calculate the approximate efficiency - but I think that you will fid
that it is well below 40%.

For the figures to be meaningful you have to specify the calorific value of
the coal being burned. Lignite (Germany, Poland, N. Dakota) is only good
for 8500 BTU/lb. Bituminous and semi-bituminous coals are generally around
12,000 BTU/lb.

The efficiency of the plant can be increased by designing it for higher
steam temperatures and pressures -

Explained here

http://www.mhi.co.jp/en/technology/review/pdf/e423/e423094.pdf

However this adds significant additional cost - and most power companies
have been reluctant to follow this route - they're running a business for
profit after all, not wishing to get involved in high-risk super-effiecient
thermal designs - and so compromise between cost/complexity versus
efficiency is a deciding factor.

This 2005 Japanese plant manages 43% efficiency

http://www.mhi.co.jp/technology/review/pdf/e451/e451011.pdf

Here's a wiki entry for typical efficiencies of turbine based thermal
powerstations.

http://wapedia.mobi/en/Thermal_power_station

This document explains increases in efficiency since 1920 about 20%, rising
to about 30% in the 1950s. Modern stations are often taken as an avearge
efficiency of 38% stating that the best we are ever likely to achieve is
about 55%, but for practical reasons summarised on the last page, suggest
that we are likely only ever to see efficiencies of between 40% and 45% when
the station is connected to a real world fluctuating load.

For practical purposes, you can assume that 60% of the thermal energy in the
coal is lost to the atmosphere

http://www.sealnet.org/s/8.pdf

Coal was traditionally gasified to make lighting gas and for running gas engines. With improved pipelines, natural gas began to become popular in the 1940s in the US and so manufacturing gas from coal fell almost into obscurity.

In the 1980s a coal gasification plant was built in North Dakota, with the purpose of converting the local brown coal into high purity methane that could be used for domestic and industrial heating in the local community.

At the time it was built, oil was cheap and so the economics didn't quite work out.

However, one of the by-products of the gasification was large volumes of pure CO2, and a pipeline was built to run 205 miles north up into Sascatchewan, so that the CO2 could be used to pressurise an oil field and get another 20 years of production from the declining field.

The process is detailed here:

http://www.dakotagas.com/Companyinfo/Gasification_Process.html

Coal gasification will likely become a viable energy conversion technology and it has the following advantages:

1. It converts low calorific value brown coal into high energy natural gas that is easier to transport and use.
2. The by-products are easily separated from the process and have commercial value for fertilisers and phenol feedstocks.
3. The CO2 is captured and can be sequestered in oil wells allowing extended production
4. Waste products - toxic metals etc are separated from the mix and can be appropriately handled
5. There is no requirement for an exhaust stack so atmospheric pollution is low.

It is fairly obvious that the world will have to continue to use its coal reserves for producing primary energy - more so with declining petroleum reserves. Coal gasification could be one technique that greatly improves the utilisation of what is generally considered to be a dirty polluting fuel.

As coal fields are exploited, the most valuable grades are consumed first. Coal gasification allows the exploitation of the remaining poorer grades.

The natural gas can be added into the existing pipeline distribution network or can be used at the plant in a combined cycle gas turbine power plant. So the energy in the coal can be exported either as electricity or as methane.

If the coal gasification process runs continuously, the methane can fuel CCGT power plant at times of peak power demand, or compressed and stored at times of low power demand. So by storing the energy in the form of methane it allows you to meet the changing demands put on the power plant. Extra CCGT generators could be spooled up quickly using the stored methane when needed.

By locating these gasification plants close to the coalfields - coal transport costs could be reduced. Energy exported in the form of electricity - possibly using a new HVDC supergrid, would be in great demand during summer months for air conditioning, whilst in winter months, natural gas for home heating may be more appropriate. Either way there is flexability to tailor the exports to meet the changing demands.

Coal gasification is a scaleable technology. Plants could be designed to suit the output of the local coal fields and the demands of the population.

The ND plant uses nearly 6 million tons of lignite per year and from this produces 54 billion cubic feet of natural gas per year - enough to power a CCGT power plant of between 800MW and 1GW.