Coal power rises to carbon capture challenge

Mott MacDonald is applying its knowledge of carbon capture technologies to the challenge of making coal fired generation environmentally acceptable.

(Article taken from our customer magazine Momentum)

Mott MacDonald is co-ordinating design of what is hoped will be the UK’s first full scale, and the world’s largest, carbon capture and storage demonstration project at Scotland’s Longannet Power Station. The UK government has shortlisted three consortia to develop designs for plants capable of capturing CO2 from a coal fired power station of at least 300MW.

Acting as owner’s engineer for ScottishPower, Mott MacDonald is providing conceptual design and integrating input from other consortium members – oil company Marathon Oil, specialist technology provider Aker Clean Carbon and contractor Aker Solutions.

“We’re aiming to show that carbon capture and storage is technically and commercially viable,” says Mott MacDonald sector leader for pollution control technologies Guillaume Wolf, “and that it can be retrofitted to existing as well as new power stations. Retrofitting can potentially play an important role in reducing emissions in countries like China, India, Poland and South Africa where coal is the main fuel for generation and continues to feature strongly in the energy mix.”

A number of capture options exist, each with attractions and challenges. None has been demonstrated at a large scale.

Post-combustion capture

Post-combustion capture separates CO2 from the flue gas stream using solvents, sorbents, separation membranes or cryogenics. However, only absorption is considered technically proven and use of the organic solvent, amine, is the only commercially available absorption technology. Amine is extremely expensive. Care is needed to prevent contamination with SO2 and NOX which form corrosive salts, causing operational problems and solvent losses. SO2 and NOX have to be removed from the flue gas stream before carbon capture occurs. CO2 is ‘stripped’ from the amine by heating it. Lean amine can then be reused. The technology needs to be scaled up and optimised to be applied to large power plants.

Precombustion capture

Under highly controlled conditions coal is partially oxidised to release syngas, a mixture of carbon monoxide and hydrogen. The carbon monoxide is reacted with steam to produce CO2 and additional hydrogen. CO2 can be separated from the hydrogen, which in turn is used to fuel gas turbines. Precombustion carbon capture is cheaper than post-combustion capture and the volume of CO2 arising is smaller. But the technology cannot be retrofitted to existing coal fired power stations because of the fundamentally different combustion process. Significant turbine design differences also rule out substituting hydrogen for natural gas in existing gas fired power stations.

Oxyfiring

Using 95% oxygen rather than air in the combustion process significantly reduces the presence of other gases in the exhaust stream, meaning that the dominant flue gas is CO2. Though efficient, the cost of separating nitrogen and oxygen makes oxyfiring uneconomic.

Carbon capture pros and cons

Post-combustion and oxyfiring technologies are energy hungry, impacting the efficiency of a coal fired power station by 10-12%. A power station operating at 40% efficiency would be down to 28-30%. At present there is no financial driver to fit capture plant to power stations. But tightening European Union limits on carbon emissions and stiffer penalties for breaching limits will create market conditions that make capture commercially practical. New power stations need to be capture ready. Plants of over 300MW capacity will be allowed to emit no more than 350g of CO2/kW from 2020.

Storage

Deep coal seams, empty oil and gas reservoirs and saline aquifers are considered options for CO2 storage. Natural gas has been stored for millennia in each and it is thought that all could be relied on to prevent re-release of CO2 into the atmosphere. Storage offers the attraction that injecting carbon dioxide would help with extraction of oil and gas from depleting reservoirs. But before rushing ahead with storage, tools for assuring CO2 is not leaking have to be developed.

Transportation

For large volumes and over distances of up to 1000km, the preferred means of transporting compressed carbon dioxide from power station to storage site is by pipeline.

CO2 undergoes phase change at a pressure of approximately 74 bar – though still gaseous it behaves like a liquid. It should be possible to reverse flows on disused existing gas pipelines, which are designed to withstand pressures of 80-85 bar. But this pressure margin is too small to enable CO2 to be transported in very high volumes. In the UK it is being suggested that new purpose-built high pressure pipelines should be built. These would be shared by clusters of coal fired power stations in the Thames Estuary, Humber and Firth of Forth regions.

To prevent corrosion of steel pipelines, moisture would have to be condensed out of the CO2 stream. Depending on the length of the pipeline and the terrain crossed, interstage pumping may be necessary to maintain operating pressure.

Coal: back in the energy mix

As global demand for electricity climbs, countries worldwide face the challenge of providing secure supplies while reducing carbon emissions. Economic powerhouses China and India, among others, have forged ahead with construction of new coal fired power stations but Europe has held off.

Growing fears about the capacity of renewables, reliability of gas and timescales for construction of new nuclear generating capacity led the UK government to end a 30 year moratorium on coal in April 2009.

The turnaround hinges on the introduction of technologies to enable the capture of pollutants arising from combustion. Methods for removing SO2, NOX and particulate matter from the exhaust stream are well established in the thermal generation sector. The big challenge lies in capturing and disposing of the major greenhouse gas CO2, which accounts for 12%-14% of a coal fired power station’s emissions.

Combined with a new generation of more efficient ‘supercritical’ coal fired boilers, carbon capture and storage is seen as part of the solution.