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International Nordic Bioenergy 2003 conference an Austrian development and engineering company. The design of the furnace and its adaptation to the specific requirements of the 35kWel Stirling engine was an important and difficult task. The plant should operate at a high temperature level to increase the electric efficiency of the Stirling engine but temperature peaks in the furnace should be impeded in order to reduce ash slagging and fouling. The new combustion system was developed and optimised using CFD simulations of gas phase combustion. Figure 8 shows a picture of the new CHP pilot plant. The furnace is equipped with underfeed stoker technology. The Stirling engine is mounted in a horizontal position downstream of the secondary combustion chamber for convenient maintenance (see Figure8). The air preheater and the economiser are placed on top of the furnace in order to achieve a compact design of the plant. The CHP plant should not require substantially more space than a normal biomass combustion plant with the same heat output. To remove fly ash particles from the hot gas heat exchanger, a pneumatic and fully automatic cleaning system was developed and installed. To keep the operating costs to a minimum, small- scale plants will have to run in unmanned operation for days or weeks. The system has therefore been fully automated. Any engine failure will be detected and the combustion system will immediately be shut down. The plant was put into operation at the end of summer 2002 and has been running fully automatically since autumn 2002. The engine has run for more than 4,300 hours until the end of June 2003 and for more than 2,900 hours from the beginning of February to the end of June 2003 at a high load level. In this period, the average availability of the pilot plant was more than 80% , in May and June 2003 even more than 92%. Table 3. Average results of test runs performed at the 35 kWel pilot plant compared to design targets at partial load than at full load operation. Furthermore, the air preheater does not work satisfactorily. The test runs performed show that the temperature of the preheated combustion air is about 190 °C lower than initially foreseen (see Table 3). This is due to high heat losses to the water cooled walls enclosing the air preheater. The overall electric efficiency of the CHP plant measured during the test runs amounts to approx. 9.2 %, which is about 25% less than expected. The overall efficiency of the CHP plant (electric + thermal) is higher than the design target, which shows that the economiser works very satisfactorily. During the test runs different wood chip qualities were used, and the plant was running well with water contents ranging from 10 to 55 wt.% (w.b.). The automatic cleaning system was improved several times during the test period. At the moment, manual cleaning of the hot heat exchanger is necessary after more than one month of operation. It is expected that further improvements of the automatic cleaning system will increase these intervals to 2 - 3 months. 3.3 State of development of the Stirling process Several research teams in Europe and the USA are working on the development of Stirling engines for CHP, and some of those are also working with biomass fuels. With more than 4,300 hours of successful operation the pilot plant described above can be considered as a breakthrough in the utilisation of Stirling engines for small-scale CHP plants utilising wood chip fuels. At the moment, the plant works fully automatically and the development of the control system is almost completed. But there are several problems to be addressed in future, primarily regarding enhanced electric efficiency. In this context, major emphasis should be placed on improving the efficiencies of the hot heat exchanger, of the air preheater and of the entire combustion system. Furthermore, the optimisation of the pneumatic cleaning system to reduce ash deposition in the hot heat exchanger and thus to achieve a higher availability of the whole system is of great relevance. In addition to these development goals, future activities should further prove the reliability and low service demand of the plant. Furthermore, the specific price of the plant has to be decreased by implementing serial production on a stage-by-stage basis in order to make the technology competitive. Small series production of Stirling engines is planned to be launched in 2004, and it is expected that the price of manufacture can be considerably reduced, once the first two or three small series have been built. In an ongoing EU research project (project “BIO- STIRLING”; project No. NNE5-1999-00097) a CHP plant with a 75 kWel eight cylinder Stirling engine has been developed. This plant will be put into operation in summer 2003. 4 CONCLUSIONS AND RECOMMENDATIONS FOR SMALL-SCALE BIOMASS CHP PLANTS Several technical side constraints are of great importance for decentralised biomass CHP plants. The technology must be robust and highly available and plants must be designed to run in unmanned operation. Therefore, a high level of process control and process Temperature of the preheated air Temperature of the cooling water at Stirling cooler inlet Electric power output Thermal power output - Stirling engine Thermal power output - CHP plant Fuel power input Fuel consumption (w.b.) (water content approx. 30 wt%) Electric efficiency - Stirling engine Overall electric efficiency - CHP plant Overall efficiency - CHP plant Design target °C 550 °C 55 kW 35 kW 105 kW 215 kW 291 kg/h 85 % 25.0 % 12.0 % 85.9 Obtained during test runs 360 61 31 124 272 337 96 20.0 9.2 90.0 In spring 2003 comprehensive test performed. Table 3 shows the results of the test runs compared to design targets. The average electric power output of 31 kWel is less than the expected nominal output of 35 kWel achieved in tests with natural gas. The decrease in power is partly due to a lower efficiency of the hot heat exchanger with wood chip fuel compared to natural gas. Furthermore, the temperature of the cooling water at the cold heat exchanger inlet of the Stirling engine was higher than initially foreseen, which also results in a reduced electric power output. The electric efficiency of the Stirling engine is also lower than expected (20 instead of 25%), which is mainly due to the fact that the Stirling engine is significantly less efficient runs werePDF Image | FUTURE DEVELOPMENTS REGARDING SMALL-SCALE BIOMASS CHP
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