NNE5-1999-00097 Small-Scale CHP Plant Based on a Hermetic Four-Cylinder Stirling Engine for Biomass Fuels

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EESD (Energy, Environment and Sustainable Development) : Electricity : Thermochemical Conversion

Proposal No:	NNE5-1999-00097
Date Prepared:	February 2004
Source:	European Bio-Energy Projects (EUR 20808)

Introduction

For small-scale CHP systems using biomass as fuel, Stirling engines are a promising solution for installations with an electric power output below 150 kW corresponding to a thermal output of 600 - 800 kW. The advantage of the Stirling engine, compared to an IC-engine, is that the heat is not supplied to the cycle by combustion of the fuel inside the cylinder, but is transferred from the outside through a heat exchanger in the same way as in a steam boiler. Consequently, the combustion system for a Stirling engine can be based on well-known furnace technology, considerably reducing the problems of the utilisation of solid fuels.

Stirling engines are based on a closed cycle, where the working gas is alternately compressed in a cold cylinder volume and expanded in a hot cylinder volume. The heat input from the combustion of fuel is transferred from the outside to the working gas through a hot heat exchanger (the heater) at a high temperature, typically between 950 K and 1050 K.

Activities

The problems concerning the utilisation of biofuels in a Stirling engine are concentrated on transferring the heat from the combustion of the fuel into the working gas. The temperature must be high in order to obtain an acceptable specific power output and efficiency, and the hot heat exchanger must be designed so that problems with fouling are minimised.

The Stirling engine hot heat exchanger has been designed specifically for using biomass as fuel because of the high temperatures in the combustion chamber and the risk of fouling. Advanced design tools, including numerical simulation programmes (NSP), have been utilised for the calculation of main characteristic parameters of the Stirling engine. Numerical optimisation programmes have been used for the optimisation of more than 20 parameters describing the cylinders, heat exchangers, regenerators and other components.

The engine, which is designed for a nominal electric power output of 75 kW, has eight cylinders. Narrow passages in the heater section are avoided in order to for it to adapt to solid fuel combustion gases.

In order to avoid leakage of the helium working gas to the surroundings, the engine is designed as a hermetically sealed unit with the generator incorporated in the pressurised crankcase just like the electric motor in a hermetically sealed compressor for refrigeration.

The need for a high temperature in the Stirling engine hot heat exchanger also makes the design of the combustion system complicated. Therefore, the design of the combustion chamber is based on CFD calculations in combination with the above-mentioned MARS optimisation technique for improved performance and low emissions.

The minimisation of fouling of the heat exchanger surfaces by aerosol and fly ash particles contained in the flue gas has also been taken into consideration. A specially adapted automatic heater cleaning system has been developed to achieve extended cleaning intervals for the heat exchanger, preventing hard deposit formation and limiting corrosion.

Progress

Results from the optimisation of the Stirling engine design show that the new 8-cylinder engine should meet the design targets concerning power output, efficiency and service interval. The testing of the engine in the laboratory, with natural gas as fuel, has just been initiated and the results so far are promising.

The combination of CFD and the optimisation for the design of the furnace has opened new possibilities for adaptation of combustion to the application with a minimum of emissions. Results have shown that it is possible to transfer heat equally to all eight heat panels on the Stirling engine, and that it is possible to meet the targets concerning CO and NO _x emissions.

When the final erection of the biomass-fired CHP pilot plant is finished a comprehensive test programme is planned with wood chips as fuel. Results of the test programme will be evaluated for further development of the overall plant.

Impact and exploitation

The result of the project will offer a solution for simple, reliable, clean, efficient, safe and costeffective power production, utilising renewable energy. The new technology will have a positive socioeconomic impact on local communities and employment, especially in less favoured regions concerning employment and infrastructure.

Small-scale CHP plants, close to the site of biomass production, result in a reduction of transport costs and emissions compared to the utilisation of biomass in large centralised heat and/or power plants.

Based on the assumption of 10 000 installed Stirling CHP plants within the next ten years, the CO ₂ emissions can be reduced by approximately 540 000 tons per year (compared to oil furnaces for heating and electricity produced from coal). This corresponds to 1.8% of the 600 million tons of CO per year allowed for the EU according to the Kyoto objectives.

When a future production series of engines and plants has been established, it is expected that the cost target for biomass CHP technologies specified in the 5th Framework Programme can be met by the new technology.