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Clean coal

Some see the term “clean coal” as an oxymoron – you can’t burn coal without producing carbon dioxide as well as other environmentally problematic solids, liquids and gases. But it is possible to move towards “cleaner coal” by filtering out or capturing some of the emissions. We already do this for acid emissions, but since the sulphur content of most fuels is relatively small, SO2 is a relatively small issue: the big issue is of course carbon dioxide – the main product of combustion.

“Carbon Capture” is the buzz word – separating out the CO2 produced in power stations. It requires complex and relatively expensive chemical processes, and then you have to find somewhere to store the resultant gas. The options for that include old coal seams, exhausted oil and gas wells, and as yet undisturbed geological strata of various types.

Along with many others, the IEA Clean Coal Centre has been promoting Carbon Capture and Storage as vital for the future. They point out that around 40% of the electricity generated globally comes from coal and that this is bound to grow – as countries like China expand. If they don’t adopt CCS, then climate impacts could be significant.

There are some CCS-type projects in China, including the Green Gen project due to come online next year, eventually planned to expand to 650 MW, but most of the running is being made in the US, and to a lesser extent the EU.

The IEA group suggests that globally there is the potential for replacing 300 GW of existing coal plant with new CCS plants and for 200 GW of upgrading with CCS. But it currently looks like we will only see around 29 GW of coal-fired CCS in place globally by 2020. So what’s stopping us?

Firstly, and mainly, the cost. CCS adds perhaps 50% to the capital cost of a of plant ($735/950 m for a new/retrofitted 400 MW plant). It also reduces overall energy conversion efficiency – some energy is needed for CCS. The IEA team says that the “energy penalty” is somewhere between 10–15% at present, although higher figures have been quoted, especially for CCS added on to existing plants. However, there are hopes of reducing the energy penalty to below 6% of output by 2030. Improved technology may also reduce the costs.

The cheapest option at present, which can be added on to existing plants, is simple post-combustion capture, but it’s inefficient – it is hard to extract the CO2 from the large volumes of low-pressure exhaust gases. It’s much cleverer to extract CO2 at an earlier stage – as in pre-combustion capture systems. In these Integrated Gasification Combined Cycle (IGCC) plants the coal is gasified, to produce a mix of methane, carbon monoxide, carbon dioxide and hydrogen. The carbon dioxide is then separated out while the other gases are used as fuel for power production. An extension of this approach is provided by firing with oxygen rather than just air – that increases overall efficiency, but adds complexity. Vattenfall has built a 30 MW (thermal) oxy-fuel demonstration plant. Meanwhile, there are many plans and programmes underway around the world (e.g. the EU has already put €1 bn into CCS, and the US $3.4 bn. Within the EU, RWE is planning a 450 MW IGCC unit, while the UK is planning four CCS plants, two pre-combustion, two post-combustion, to be backed by a new levy of 2–3% on electricity charges.

As this indicates, CCS is still a relatively expensive option for carbon reduction, at $35–70/tonne of CO2. Although there are hopes of it falling to $25–35t CO2, that’s still much more than the current value of carbon under the EU Emission Trading System. So far CCS has not be eligible for support under the Clean Development Mechanism – India and Brazil amongst others have objected to its proposed inclusion. The fear is that supporting CCS will deflect resources away from renewables and other low-carbon options more relevant to them.

The second issue is eco impacts. It is conceivable that stored CO2 might suddenly be released in large amounts – and the resultant ground-hugging cloud of dense cold gas could asphyxiate any people it engulfed. The CCS lobby sees this as unlikely – we already pump gas into part-empty wells for Enhanced Oil Recovery, and oil and gas strata have stored methane and oil for millennia, so replacing them with CO2 should not lead to any risk of sudden catastrophic release. That may not be as certain with as yet undisturbed aquifers though (e.g. undersea earthquakes do occur). And on land storage in, for example, old coal seams, seems even more potentially risky, given nearby populations.

The IEA Group says that globally there is room for perhaps 40 Gt CO2 in coal seams, and that is being followed up in the USA – where there has been some local opposition on safety grounds. There is more room, possibly for 1000 GT globally in old oil and gas wells, but the big option is saline aquifers – maybe 10,000 GT. For comparison, according to Vaclav Smil’s Energy at the Crossroads: Global Perspectives and Uncertainties, in 2005 world CO2 production was around 28 GT p.a.

The main driver for CCS is clearly the fossil-fuel industries desire to stay in business despite pressures to reduce emissions. In theory CCS can cut CO2 emissions from coal burning by 85% or more, and of course it’s not just coal – some of the CCS projects involve natural gas. For example, French oil company Total has retrofitted a gas-fired plant at Lacq in the South of France with CCS in a £54 m pilot project, with the CO2 being sent down the existing pipeline, back to a major local gas well at Rousse, which used to supply to the plant, for storage at a depth of 4,500 metres. Moreover, power production may not be the only option – as noted above, coal gasification can produce a range of synfuels, some of which can be used for heating or for fuelling vehicles. Indeed it could be that this may offer a way to improve the economic prospects for fossil-fuel CCS – by moving into new/additional markets.

However, a rival option is biomass. That could be burnt just for power production or gasified to also provide synfuels, with, in either case, the CO2 being separated and stored. And if the biomass feed stock is replaced with new planting, then in effect, CCS would mean not just zero- or low-net emissions, but an overall reduction in carbon in the atmosphere – negative emissions. Production of biochar from biomass, with CCS, is seen by some as an even better option. But then there are land-use limits to the widespread development of biomass, whereas there is claimed to be lot of coal available around the world, with large reserves in China, N America and Russia/Eastern Europe, enough for more than 150 years at current use rates (although estimates vary).

While many environmentalist would like coal to be left in the ground, some see coal CCS as not only inevitable, but also as positively attractive, at least in the interim, and possibly as being a low-carbon alternative to nuclear. As long as it does not detract from the development of renewables.

For more information about Clean Coal, visit www.iea-coal.org.uk.

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2 comments

  1. Eddie Breeveld

    I’m not sure I understand this bit:
    “In these Integrated Gasification Combined Cycle (IGCC) plants the coal is gasified, to produce a mix of methane, carbon monoxide, carbon dioxide and hydrogen. The Carbon dioxide is then separated out while the other gasses are used as fuel for power production.”
    Surely when the methane and carbon monoxide are burnt then carbon dioxide is created? Is this not captured?

  2. John

    IGCC uses a combined cycle format with a gas turbine driven by the combusted syngas, while the exhaust gases are heat exchanged with water/steam to generate superheated steam to drive a steam turbine. Using IGCC, more of the power comes from the gas turbine. Typically 60-70% of the power comes from the gas turbine with IGCC, compared with about 20% using PFBC.
    Coal gasification takes place in the presence of a controlled ‘shortage’ of air/oxygen, thus maintaining reducing conditions. The process is carried out in an enclosed pressurized reactor, and the product is a mixture of CO + H2 (called synthesis gas, syngas or fuel gas). The product gas is cleaned and then burned with either oxygen or air, generating combustion products at high temperature and pressure. The sulphur present mainly forms H2S but there is also a little COS. The H2S can be more readily removed than SO2. Although no NOx is formed during gasification, some is formed when the fuel gas or syngas is subsequently burned.
    Three gasifier formats are possible, with fixed beds (not normally used for power generation), fluidized beds and entrained flow. Fixed bed units use only lump coal, fluidized bed units a feed of 3-6 mm size, and entrained flow gasifiers use a pulverised feed, similar to that used in PCC.
    IGCC plants can be configured to facilitate CO2 capture. The new gas is quenched and cleaned. The syngas is ‘shifted’ using steam to convert CO to CO2, which is then separated for possible long-term sequestration.

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