CHP schemes are energy efficient because they capture waste heat from fuel combustion for use in applications such as district heating. The Government sees the technology as one option to help curb the UK's emissions of carbon dioxide, and last year increased its target for CHP capacity by 2000 from 4,000MW to 5,000MW.
However, the CHP Association has warned that obstacles and uncertainties in the power market may make the target hard to achieve. Few urban CHP schemes have got off the ground in the UK, although the technology is well established elsewhere in Europe. In Denmark, CHP and district heating supplies some 40% of heating needs.
One of the most advanced new projects in the UK is Citigen's 90MW station next to Smithfield meat market in the City of London. Citigen is a joint venture between British Gas and Utilicom, the UK arm of two French electricity companies which runs Southampton's geothermal district heating scheme. The first 32MW phase of the CHP project is due for start-up in May. Two diesel engines will burn natural gas, with 5% low-sulphur fuel oil needed to maintain combustion. Oil will be used as the base fuel during interruptions to the gas supply. The plant will be brought to full capacity over the next four years with the addition of further engines or gas turbines.
Citigen has designed the plant carefully to fit within the shell of a 100-year old listed building - originally a coal-fired power station. The return of power generation to the city centre will allow hot water to be piped to the Corporation of London, the Barbican Centre, the Guildhall and commercial customers. Cooled water for use in office air conditioning systems and Smithfield market will be prepared by absorption chillers. Citigen says that this will replace the use of ozone-depleting chemicals, and also remove the risk of Legionella contamination from traditional cooling systems.
The overall efficiency of the plant is claimed to be 85-90%. Citigen is keen to set up long-term supply contracts for the bulk of its energy, heat and chilled water output to allow it to run at base load, and also to avoid the vagaries of the electricity pool system.
However, the plant's location has several drawbacks. Notably, the poor air quality in the region has forced Citigen to meet unusually tight emission standards. The firm agreed with HM Inspectorate of Pollution to curb NOx emissions using SCR equipment based on ammonia injection. SCR is used on over 130 European power plants (ENDS Report 217, pp 5-6 ), but the Citigen project is believed to be its first application in the UK. During periods of oil firing, NOx emissions will be cut by 90-95% to a typical level of 90ppm.
A lime slurry injection system will reduce emissions of SO2 by roughly 90% to achieve a typical level of 27ppm on oil firing. The scrubber will produce gypsum wastes which will be sent to landfill.
Citigen will not disclose the impact of these abatement measures on its generation costs. However, it says that the high efficiency of CHP, the avoidance of transmission losses, and income from sale of hot and cold water mean that it is able to offer electricity at a competitive price.
The company is also working on a smaller CHP plant in Edinburgh, and is hoping to reach agreement with a local authority in North London for a third scheme.