Fluid solutions

A process using solvents to strip away minerals from coal to remove impurities could lead to more efficient power stations, according to researchers at Nottingham University.

The most efficient way to use any fossil fuel in a power station is the combined cycle firing system, where the hot gases from burning the fuel are used to drive a turbine, then to raise steam which drives a second turbine. Most new gas-fired stations use this mechanism. However, worries about the availability and price of gas are leading energy researchers to look at ways of adapting the technique for fuels such as coal.

Large reserves are still available, but coal-fired stations are far less efficient than gas-fired ones and, as Nottingham researcher Karen Steel explained, coal can't currently be used for combined-cycle generation.

Even if it is gasified, she said, it contains mineral impurities, such as silica and sulphur, which survive the gasification process. When the gas is burned, these minerals can remain in the combustion gas stream as abrasive particles, or are converted into acidic gases. Both of these cause damage to the turbine blades as the gases pass through.

Steel's research, using a £120,000 EPSRC grant, aims to remove these unwanted minerals without causing any further environmental damage. In a process known as leaching, the coal is finely ground and suspended in hydrofluoric acid — a compound so aggressive it can dissolve silica. It's then filtered and suspended in another solution containing an oxidising agent, which removes the sulphur-containing mineral pyrite, and any remaining traces of fluoride.

Another filtration recovers the now-cleaned coal, with a mineral content reduced from around 10 to 0.05 per cent. Steel then mixes the two solutions containing the impurities, which ensures the fluoride is all combined in one place. 'When you combine them, the silica crashes out of solution with very high purity, about 98-99 per cent,' she said.

Steel explained that leaching processes to clean coal were investigated in the 1970s and 80s. 'The conclusion then was that it was just too expensive,' she said. However, what was neglected in the previous studies is that the solvent solution can be regenerated, recovering both the original solution and the minerals.

This has several advantages, said Steel. First, the aggressive chemicals used for leaching are never released into the environment, so there is no need for extensive and expensive water treatment. And, as they are recycled, they only have to be bought once, reducing running costs. The process also strips away substances such as mercury and arsenic which occur naturally in coal and become toxic pollutants if burned in the fuel.

Steel said that the high-purity silica is itself a valuable product. 'Pure silica has many uses in the semiconductor industry and solar cells, and we're working on adapting the process to control its form when it comes out of solution, to give predictable particle sizes and increase the surface area. That will give it even more applications, and increase the potential value,' she said.

The cleaned coal also has other applications, notably in the aluminium industry, said Steel. Aluminium smelters currently use petroleum coke as the anode to purify the metal, but this tends to contain high levels of nickel, vanadium and sulphur, which cause pollution. 'Clean coal has fewer impurities, and a lower level of sulphur, which could be attractive.'

The other impurities could also be recovered as raw materials, she said, but this depends on the particular coal used. 'If you analyse it, you'll find virtually every element in there, but profiles vary. One thing we're looking at is, rather than just using the kinds of coal burned in power stations, being much more coal-specific.'

More research is needed before the process can be commercialised, said Steel. 'One problem is that we can only remove sulphur that's in the form of pyrites, which is about two-thirds of its content; we can't remove organic sulphur.' This means that post-treatment of flue gases to remove sulphur dioxide would still be needed.