Green marks for black stuff

A method of generating energy from coal without actually burning it could form the basis of power stations whose carbon emissions can be easily captured and stored.

Combining two types of chemical process — one well-established, the other new — the technique, currently being developed at CambridgeUniversity, could have huge potential in the energy-hungry, rapidly-industrialising Chinese and Indian markets.

Once seen as yesterday’s fuel, coal is making a comeback. Vast untapped reserves — and a high growth in demand for power — in India and China mean that coal-fired stations are likely to be the fastest-growing power generation sector in the coming decades.

But coal can still be a threat to the environment. Its energy content is far lower than natural gas, meaning that for every unit of heat produced, it gives off a larger amount of carbon dioxide.

But because of the potential size of the market, ‘clean coal’ technologies, are attracting much attention. At Cambridge, chemical engineer John Dennis is working on a method which releases the energy content of the coal, but should produce only steam and virtually pure carbon dioxide as by-products.

‘That means that you can just condense the steam to collect the carbon dioxide, which is simple and cheap,’ said Dennis. This makes capturing the carbon dioxide, for sequestration in exhausted oil wells, for example, much cheaper.

‘For conventional coal-fired power stations, the flue gases are about 10 per cent carbon dioxide, with large amount of nitrogen oxides,’ he said. ‘The cost of separating out the CO2 is considerable.’ Energy researchers at ChalmersUniversity in Gothenburg, Sweden, have estimated the cost of sequestration of pure CO2 to be $2–8 (£1–5)/tonne of carbon, compared with $100–200/tonne for removing CO2 from flue gases.

Dennis’s technology ‘combusts’ the coal chemically, first converting its carbon content into a gaseous mixture of carbon monoxide and hydrogen, known as synthesis gas or syngas, then bringing this into contact with a solid metal oxide which can easily lose its oxygen.

The system uses a technique known as chemical looping, where a single reactor serves several different functions in succession. The reactor is a type known as a fluidised bed, where a thick layer of granules is made to act like a liquid by a gas pumped upwards through it at high pressure; in Dennis’s system, the granules initially consist of a porous solid such as aluminium or titanium oxide, supporting a layer of a copper oxide.

In the first phase, the bed is fluidised by steam at 800–1,000°C, and powdered coal is fed into the reactor to mix with the copper-containing granules. The hot steam reacts with the carbon to produce syngas. Known as coal gasification, this has been used for many years in oil-poor, coal-rich countries — syngas will itself burn readily, although this produces the same type of flue gas as burning any other hydrocarbon.

The next phase of the process, occurring almost simultaneously, sees both syngas components reacting with the copper oxide, transforming hydrogen into steam and CO into CO2. Many metal oxides could perform these reactions, said Dennis, but only copper will do it while releasing heat from both the hydrogen and the CO. This energy can be captured in a heat exchanger and used to generate the high-energy steam needed to spin turbines.

After a time, the researchers will stop the coal feed into the reactor and allow the remaining coal in the reactor to gasify and react, using up the remaining oxygen content of the copper oxide.

Once this process is completed, the flow of hot steam into the reactor stops and is replaced by hot air, whose oxygen recombines with the copper to regenerate the oxide. The process then ‘loops around’, with the steam and coal feeds beginning again.

Chemical looping is being pioneered at Chalmers, said Dennis, but the team there is working with pure methane and natural gas feeds. ‘Nobody has ever tried this with solid hydrocarbon feeds before,’ he said. ‘We think it might also work with biomass feeds, which would be even better for the environment, because they’re classified as carbon neutral.’

Dennis is currently overseeing the construction of his first pilot reactor, which will allow him to test different types of feed, and assess the potential for increasing the scale. He will also start refining the oxygen carrier oxide.

‘Copper seems to be the most promising, but we need to find a form which is robust enough to cope with the cycling of temperatures,’ he said. At this stage, Cambridge is not seeking a commercial partner for the research — ‘it’s a pre-competitive technology at the moment, and some of it should be patentable,’ Dennis said. If the team can perfect the system, export opportunities are likely to follow.