Summary

This project, started in 1993, involves a power plant fuelled by gas from a landfill site outside the city of Bergen, on the west coast of Norway. The electrical efficiency of the power plant is 37%, with a net electrical output of 1.3 MW. The use of a specially tuned, medium speed, lean-burn gas engine is expected to prolong the life of the power plant and decrease the running costs per kWh produced, when compared with similar plants. The lean burn engine combustion principle offers a combination of high power, high efficiency and reduced exhaust emissions.

Project Background

Landfill gas is extracted from approximately ten landfill sites in Norway, and this number is increasing. Since the direct use of gas by industry and the domestic sector is limited in Norway, the production of electricity is often the only way to use landfill gas for energy purposes. Using landfill gas for electricity production also has the advantage that it avoids extensive flaring. Nevertheless, until recently electricity was produced at only two of the landfill sites in Norway. Therefore, and in order to contribute to environmental improvements, Bergen Lysverker (the local utility company in Bergen) decided to offer a firm price, equivalent to US$ 0.03/kWh (where US$ is the US dollar), for electricity from a power plant fuelled by gas from a landfill site outside the city of Bergen. In order to increase life-time profitability in power production based on biogas from the landfill site at Raadalen, it was decided to invest in a demonstration plant, based on a medium speed, lean-burn gas engine from Ulstein Bergen AS.

The Project

The Raadalen power plant runs on gas from a landfill site outside the city of Bergen, the second largest city in Norway. The project, which started in 1993, has been carried out in collaboration with the local authorities, who built the gas collection system. Ulstein Bergen AS built and now operates the power station. Gas is drawn from the landfill and supplied to the power station at 50 mbar g by means of centrifugal blowers; it is then compressed to 3.8 bar g. After compression, the gas is chilled to approximately 2°C in two stages, in order to dry the gas. Following this, the gas is heated to further reduce the relative humidity of the gas. Gas pressure to the engine, after filtration and drying, is approximately 3.5 bar g. The engine is an Ulstein Bergen AS spark-ignition, lean-burn gas engine, specially tuned for running on biogas. The engine is based on a diesel/heavy fuel oil engine, with which it shares most of its mechanical components. It is designed for a high degree of reliability and it can operate for long intervals between overhauls, which makes it well suited for baseload power generation under varying conditions. The lean-burn engine combustion principle is unique in offering a combination of high power, high efficiency and reduced exhaust emission. These normally contradictory properties are achieved through controlled combustion of a homogeneous and lean mixture of gas and air in a high turbulence compact combustion chamber. The gas feed pressure can be varied to keep the energy supply to the engine constant. Other essential engine features include:

• A low pressure gas supply system promoting the mixing of air and gas. The system can be accurately tuned for each cylinder;
• A pre-chamber combustion system.

The net electrical output from the plant is 1.3 MW. Heat can be utilized from the engine’s exhaust, lubricating oil system, cooling water system and air cooler. Part of the heat generated by the engine (60%) is bought by the local authorities for use in a nearby agricultural college, while the remaining 40% is available for other purposes. Economics The local authorities are required by law to invest in the gas collection Technical Brochure No.11 and flaring system. The total cost for additional investment in buildings, power generation and ancillary plant, excluding VAT, was approximately US$ 1.46 million (1993 prices). Operating costs, based on the exported power and including consumables, have amounted to around US$ 0.01/kWh. The power plant is owned and operated by the manufacturer of the gas engine, Ulstein Bergen AS. Exported power is sold to Bergen Lysverker, the local utility company, at a firm price equivalent to approximately US$ 0.03/kWh, based on a ten-year contract.

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