Landfill power generation project produces 18MW of ‘green’ energy

In what is currently the largest electricity generation facility using spark ignition engines with landfill gas, the Arpley Landfill Power Generation Plant, owned by WRG and built, operated and maintained by CPL Energy, shows how environmental improvement and commercial success can be combined under the government's renewable energy policy. Atholl Donaldson, General Manager CPL Energy, describes the plant and the challenges met in its design.

With nearly one million tonnes a year of waste going into the site significant volumes of landfill gas, a mixture of methane, carbon dioxide, nitrogen and other gases, are produced as a result of biological action. This must be controlled to avoid air pollution and the dangers of gas migration from the site. It is, however, a valuable resource when managed effectively. To this end two contracts were won under the fourth round of the governments Non-Fossil Fuel Obligation and have been combined together on the Arpley site.

The landfill site, situated on the Southern edge of Warrington, extends over an area of some 130 hectares and the gas extraction network runs to over 13.5 kilometres of polyethylene pipe ranging in diameter from 1,000mm to 90mm. The power generation plant is situated at one end of the site on a narrow strip of land between the banks of the Mersey and the main access road to the landfill site. It is further bounded at one end by a major pipeline route and at the other by the landfill weighbridge and offices. The location, therefore, demanded an innovative approach to plant design and layout while paying particular regard to the environmental impact of noise and air quality on its surroundings.

Working with the chosen engine supplier, Jenbacher and its UK agent Clarke Energy, the standard ISO container package was reworked to allow adjacent sets to be positioned directly next to each other. All services: gas, cooling water, exhaust manifolds and electrical equipment (including 415V/11kV transformers, switchgear and RMU’s): are located on top of the containers. Each engine package includes a closed circuit cooling system circulating water through a plate heat exchanger again mounted on the container roof. Heat from these heat exchangers is extracted by cooling water circulating at a rate of 12,000 l/h to a bank of air blast coolers, located adjacent to the engines.

This design makes most efficient use of the available space and keeps the relatively noisy air blast coolers down at ground level. A 7 metre high noise attenuation bund screens the plant from the nearest houses and ensures that the noise levels meet the planning requirements. A 25 metre high stainless steel stack has been designed to exhaust the combustion gases at a height which, along with the Jenbacher Leanox system, ensures compliance with UK air quality standards.

Gas is extracted from the landfill via a network of wells and pipework by four Bryan Donkin 10×25 MKII Roots type blowers each powered by 132kW variable speed motors.

The rate of extraction is ~9000m3/h at 50% CH4 (although the total system is designed to operate down to 35%) discharged at a pressure of 250mb and at a temperature of up to 85°C. The filtered gas is conditioned prior to delivery to the engines, using HRS shell & tube heat exchangers and a refrigeration plant supplied by Tricool.

Power is exported into the grid at 33kV via a single 11/33kV transformer, the entire electrical system being provided by Power Systems. To fulfill the two NFFO contracts of 7.875MW each it is necessary to run 17 of the 18 JGC 320 GS L-L Jenbacher gas engines which are each rated at 1006kWE. The parasitic load imposed by transformer losses, the compression and cooling plant accounts for the net difference. The control of the system is provided by a state of the art SCADA package.

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