Critical decision

The UK energy sector is likely to be urged to spend hundreds of millions of pounds on supercritical boiler technology in a bid to cut pollution from the nation’s ageing coal-fired power plants.

Energy and environmental group Mitsui Babcock is poised to launch a government-backed study into the feasibility of retrofitting the plants with supercritical equipment and other advanced CO2 abatement systems.

The energy sector sees reducing emissions from coal-fired plants as vital to its long-term future in the UK’s power economy, and to meeting the nation’s international commitments on the environment.

The supercritical refit is the key first step in a three-stage process needed to cut emissions, according to Les King, project leader at Mitsui Babcock, which will work with partners including Air Products, Alstom and Imperial College on the DTI-funded inititiative.

‘The supercritical retrofit is the first building block towards CO2 reduction,’ said King. ‘By fitting supercritical boilers we can increase fuel efficiency by up to 20 per cent.’

Existing coal-fired plants are based on a sub-critical design, which operates at 160-bar pressure and with a steam temperature of around 540°C. When a supercritical boiler is fitted, the steam temperature is raised to 600°C and the pressure to 300 bar. As the steam pressure increases, the cycle efficiency also rises and converts more heat into electrical energy, reducing CO2 emissions accordingly.

The UK’s power plants are all past their planned design life, according to King, with even the youngest, DRAX, already 20 years old.

‘A supercritical retrofit can be done for £116m, as opposed to spending £365m on a brand-new plant,’ he said. ‘New boilers are smaller and lighter and can easily fit inside existing infrastructure at the plant. Then we just need to modify the high-pressure and intermediate-pressure cylinders on the turbine.’

King said the second phase of reducing carbon emissions is the use of biomass fuels, which are CO2 neutral. The retrofit would allow for a boiler to be designed which could accommodate a higher percentage of biomass fuels.

‘Combined with the fuel efficiency of the supercritical boiler, these two technologies together could reduce emissions by up to 40 per cent,’ claimed King.

The third stage involves the use of new CO2-capture technology. The captured gas could then be used in carbon sequestration, which needs a carbon concentration level of 90 per cent to be effective.

Researchers are developing technologies to achieve this, and the two possible systems being investigated are amine scrubbing and oxyfuel firing.

Amine scrubbing passes the flue gas through an amine solvent that removes nitrogen from the gas. Oxyfuel firing is a pre-combustion technology that uses only oxygen rather than a nitrogen/oxygen mix to fire the fuel, leaving pure CO2.

The DTI will spend almost £1m on the Mitsui-led study, which forms part of a larger initiative announced by energy minister Malcolm Wicks last week.

Wicks unveiled the Carbon Abatement Technology Strategy, which will dedicate £25m towards improving the efficiency of power plants and the development of carbon capture technology.

Wicks said: ‘We’ve consulted the industry closely and it’s clear that the long-term benefits of capture and storage, which could reduce emissions from power plants by up 85 per cent, merit significant investment now.’