Old King Coal

Clean coal technology is heralding a greener future for the once dirty energy source. Andrew West investigates the clean coal options

Think of coal and you probably think of dirty mines, sooty coal fires and smog over London. Currently, few people would argue that coal is a clean source of energy, but with the soaring cost of oil and gas, new nuclear power stations largely more than a decade away and renewable energy sources underdeveloped, it is undergoing a renaissance as a widely available fuel source with a steady price. 

While the global demand for coal is increasing in order to sustain growing energy demands, most people recognise that a widespread return to using coal for conventional energy generation would be environmentally disastrous. CO₂ emissions in the UK alone continue to rise, despite efforts by government and industry to reduce them. Around one third of UK electricity is already generated using coal-fired power stations, so increasing this percentage would only make CO₂ emissions worse, not to mention sulfur and nitrogen-based pollutants and mercury. 

In short, if governments intend to meet their emission targets while maintaining energy supplies, coal needs to clean up its act. Research is now underway to try to do this under the banner of clean coal technology (CCT). 

The term CCT is widely used to describe a complete process whereby coal is converted to a gas, or gasified, the gas burnt to power a turbine and the emissions captured for storage or burial.  

With the increasing cost of other fuels, CCT is becoming both economically viable and politically favourable. China is actively researching CCT, along with Japan, Australia and the EU. Last year the UK government announced a 
?25 million funding package to develop demonstration plants while recently, US president George Bush earmarked an additional $300 million (?160 million) in funds for CCT research on top of the $1 billion already allocated to FutureGen, an ambitious project developed by the US Department of Energy (DoE). The goal is to demonstrate by 2013 the viability of a zero-emission power plant. Coal gasification will produce syngas, composed primarily of CO and H₂, which will be used as a fuel to produce electricity. The project will also investigate the possibility of hydrogen capture for other uses, such as advanced automotive applications.  

Finally, the CO₂ produced will be captured and its geological storage evaluated. Four possible sites for the demonstration 275MW prototype plant were shortlisted in July this year, with construction at the winning site expected to begin in the second half of 2007. The plant should finally be online by 2012. 

Power for the future 

Outside the core definition of CCT, there is a large research base on a whole host of other technologies which promise to improve the efficiency and cleanliness of coal. 

New power station designs have been developed to increase efficiency. Integrated gasification combined cycle systems produce syngas from coal and burn it in a conventional gas turbine. The hot exhaust gases are then passed into a boiler to make steam, which powers a second turbine to generate more electricity. These plants are predicted to be around 45 per cent efficient in converting fuel to electricity, while producing fewer emissions.  

The FutureGen project intends to use such systems, but there are considerable technical design challenges. While some demonstration units have already been built, the high-pressure requirement for syngas production limits the final plant size.  

Underground coal gasification (UCG) may remove this size limitation. Using this technology, coal mining is no longer required. Instead, two wells are drilled into a stable coal seam and an oxidising stream of air or oxygen is pumped down one drill hole into the seam. Partial oxidation of the coal occurs, producing mainly syngas, which rises to the surface through the second drill hole.  

UCG has been investigated throughout the 20th century, but has only recently become practical with improvements in drilling technology in the oil and gas industry. The economics for UCG look good too. At the end of 2005, the average electricity price using natural gas was ?7/GJ, whereas the estimated price for electricity produced using UCG syngas was just ?1.5/GJ. A feasibility study of UCG under the Firth of Forth in Scotland will soon be completed and could pave the way for many UCG operations around the world. 

However coal or syngas is burnt, CO₂ and contaminants are still produced and released into the atmosphere via flue gases. Technology is already in place to reduce sulfur and nitrogen emissions but current methods of flue gas cleaning work only at relatively low temperatures. Plant operators often want to run the plant at higher temperatures to increase power output, giving a trade-off between efficiency and emissions. With stricter controls on emissions and a demand to reduce CO₂ releases, much of the current technology is unlikely to be sustainable. 

Carbon capture and storage (CCS) is a current hot topic in the media. In this process, almost all of the CO₂ produced by a fossil fuel-burning power station is captured for storage or burial. There are already a number of trials of this technology around the world; Shell and Norwegian firm Statoil are involved in a $1.4 billion project to bury under the North Sea CO₂ produced from a gas-fired power station in Norway; BP is planning an advanced CCS power station in Peterhead, Scotland, where natural gas will be converted to hydrogen and CO₂. The hydrogen will then be burnt to produce energy while the CO₂ will be buried in BP’s Miller oilfield. This negates the problem of CO₂ capture from the power station chimney.  

Liquid solutions 

However, once more, CCS has its drawbacks. One major issue is that fitting current technology to coal-fired power stations reduces their efficiency by up to a quarter. This means more stations would have to be built and more fuel would need to be burnt, dramatically increasing running costs. To overcome this, research into new methods of CO₂ capture is already underway.  

The Castor CCS project is one example. It involves 30 European partners, including Imperial College, London and the UK electricity company Powergen, and is investigating capturing CO₂ from flue gas from a coal-fired power station near Esbjerg, Denmark, using monoethanolamine (MEA) as a capture solvent. Initial results are promising: around 90 per cent of CO₂ in the flue gas can be captured, but losses of MEA are relatively high at 2.4kg per tonne of CO₂. The project is now looking at using other capture solvents. 

Research at the Oak Ridge National Laboratory (ORNL), US, uses task-specific ionic liquids as capture solvents to absorb CO₂ and mercury from flue gas streams. Ionic liquids are salts that are liquid at room temperature and are receiving a lot of academic interest thanks to their unique properties. ’Ionic liquids are ideal for this application because they do not evaporate easily, they are stable in a wide temperature range and they have adjustable properties,’ says David DePaoli of ORNL’s nuclear science and technology division. ’We hope to find the best compounds that can be used together to extract both CO₂ and mercury from stack gases.’ 

’I think that this is a very ambitious project, although there is a high risk of failure.’ says John Holbrey, a research chemist at Queen’s University Ionic Liquid Laboratories in Belfast, UK. ’However, I doubt that the high cost often associated with ionic liquids would be a major impediment to successful implementation here due to the low volumes required.’ 

The FutureGen programme has yet to announce which method it will use to capture CO₂, but the DoE is actively researching selective membranes that will separate CO₂ from other flue gases. The most successful capture method is the one most likely to make it into the final plant design. 

While CCT can lead to large improvements in efficiency and emissions over conventional power stations, there is still room for improvement. One possible example of the future of coal for energy production is being tested at the Wabash River power station in Indiana, US. Here, syngas produced by coal gasification is cleaned before being fed into a solid oxide fuel cell (SOFC). In many fuel cells, CO acts as a poison, making syngas an unsuitable fuel. However, due to high operating temperatures, SOFCs can use CO to produce electricity. ’It’s not as good as hydrogen as it doesn’t have the energy content,’ explains David Bayless, an expert in SOFCs from Ohio University, US, ’but planar SOFCs can use it.’ 

Reliability and cost will be the major issues. ’If this is going to be viable in the long term, the cost of the fuel cell has to be competitive,’ continues Bayless. However, he is a firm believer in the system. ’The efficiencies of coal plants right now are about 37 per cent. With fuel cells, you’re talking about a theoretical efficiency of 70 per cent.’  

Bayless sums up the potential of the research enthusiastically. ’This is good for coal long term. If you are using it more efficiently, it makes it a more valuable fuel. And less input for the same useable output just has to be good for the environment.’ 

The statement could equally be applied to CCT in general. If coal is once more going to be used on a grand scale for electricity generation, efficiency and emission control gains must be made to avoid an environmental catastrophe. With the help of researchers around the world, new technology will be developed that should help to remove the perception of coal as the dirtiest of fossil fuels and support ever-growing global demands for energy. 

Andrew West is an assistant project formulator with Chemistry Innovation KTN.