The long-term future for coal in power generation, at least in the developed world, will depend on technology that is able to meet ever tighter environmental constraints at a price which is competitive with other forms of generation.
The most thermally efficient and least environmentally damaging of the cleaner technologies is the integrated gasifier and combined cycle (IGCC) system, which converts coal into a clean gas – a mixture of hydrogen, carbon monoxide and, in some cases, methane – and feeds it into a turbine.
Unfortunately IGCC is also the most expensive option, with an installed cost of up to 5p/kWhm – double that of gas-fired stations and significantly higher than that of conventional combustion coal plants. The construction of IGCC plants will depend, therefore, either on subsidy or a significant reduction in costs: in the near term probably both.
Air Products and Chemicals of the US may have found an answer to the cost issue with a new technology for producing methanol. It believes that fitting this system, which produces methanol at twice the rate of conventional methods, on the back end of the gasifier units on an IGCC plant can cut the capital cost by 25%.
The technology achieves this saving by reducing the number of gasifiers the IGCC plant needs – provided the full capacity of the power station is not required for baseload running. Dennis Brown, the research and development manager for Air Products Europe, says that on a 450MW IGCC plant the number of gasifiers could be reduced from four to three.
This enables the operator to make full use of the gasifers, which account for 50-60% of the cost of an IGCC and become prohibitively expensive under part-time operation. When power is not required, they can be switched to methanol production. This provides the additional fuel to meet full power output at times of peak demand. ‘You would get the extra electrical capacity by taking methanol out of your store,’ says Brown.
Air Products’ liquid phase methanol process passes the hydrogen and carbon monoxide produced by the gasifier through fine particles of a conventional methanol-producing catalyst suspended in an inert fluid rather than through a bed of dry catalyst pellets. The technology has completed 700 hours of stable operation at Eastern Chemical Company’s coal gasification facility in Kingsport, Tennessee, where it produced 260 tonnes of methanol a day.
Brown says, however, that potential clients have been more interested in boosting the economics of IGCC plants through exploiting the additional methanol production than in saving on the cost of a gasifier. While the methanol produced by the process would need further dewatering to achieve the grading for a chemical feedstock, it is a viable product for the power generation and fuel additive markets – its unit cost of about 9 cents a litre is 20% less than the current market price.
Brown accepts these additional benefits will not make an IGCC unit competitive with a combined-cycle gas-turbine (CCGT) plant where there is an adequate supply of natural gas. But he thinks a 500MW unit could compete with traditional coal-fired technology and could offer big opportunities in the countries that will build thousands of gigawatts of coal-fired plant over the next 10 years, such as China and India.
National Power, the UK generator which has joined RJB Mining’s study into the feasibility of building a 400MW IGCC plant at Kellingley colliery in West Yorkshire, is more cautious. ‘In our view it makes a marginal change, though it’s certainly not something to ignore,’ says Richard Hotchkiss, research engineer at National Power.
According to John Griffiths, technology development manager at energy consultant Jacobs Engineering, the difficulty is to secure a long-term purchase contract for the methanol that will allow the plant operator to keep the gasifiers in continuous operation. ‘If you can’t run them flat out all the time, you’re going to incur large capital charges.’
Jacobs was part of a consortium that designed a 500MW unit to produce 500-1,000 tonnes of methanol a day in the daylight hours and 1,500 tonnes overnight. But the scheme foundered on the problem of finding a long-term offtake contract for the methanol – the volatility of the crude product’s price makes this extremely difficult. ‘You can’t find a chemical market to match the electricity you’re making,’ says Griffiths.