Cleaner gas in the nursery

20 July 2001

A new CHP installation with six Cummins gas-powered generating sets is reducing emissions and fuel costs for tomato grower Hazlewood VIHB of Whitstable, UK, as well as providing 12 MWe of energy to heat 15.2 ha of greenhouse, free CO2 to feed the plants and 9 MWe of electricity for export. The installation was commissioned by the recently formed Gas Projects division of Cummins Power Generation In winter as many as two trucks a day would deliver heavy oil to fire the boilers for the greenhouse heating system and liquid CO2 for feeding the tomato plants. Today the site's heat and CO2 requirements are met by four dual-fuel boilers and a chp plant built, owned and operated by Nedalo (UK), a Texas Utilities company, who are part of Eastern Electricity and one of Europe's largest suppliers of energy related equipment. Nedalo has a number of similar installations in the UK, including two at other VBB nurseries, but Hernhill is first to operate 24 hours a day At the heart of the system are six Cummins QSV91 generating sets, each comprising a natural gas-fired Cummins engine coupled to a Stamford Newage alternator, rated 1900 kVA, 400 V, 50 Hz. The six sets are arranged for automatic synchronisation together and with the mains supply. All six units operate around the clock, monitored by a Nedalo control system. A small amount of the output is used to meet the plant's low baseload demand. Most of the electricity, however, is exported to the mains via two feeders from an 11 kVA transformer. Each set provides 2000 kW of heat energy. This is recovered using a combination of exhaust gas heat exchangers, condensing heat exchangers, pre-coolers and plate heat exchangers. From an initial temperature of 550°C, exhaust gas leaves the stack at 54°C. The recovered energy is metered and used to provide hot water for the greenhouse heating system. The system also incorporates exhaust gas purifiers. These remove impurities, leaving clean CO2, which is fed to the greenhouse free of charge. According to site manager Vince John-Charles, the plant is about 84 per cent efficient. The six sets are each housed in enclosed silenced canopies reducing sound levels to 75 dB(A) at 1 m, all installed within a purpose built plant room.

As part of the turnkey project, Nedalo replaced the existing oil-fired boilers with four high-efficiency dual-fuel units that run on natural gas or kerosene. The units operate in pairs, with two always in standby mode.

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