Evaluation of a cyclone gasifier design to be used for biomass fuelled gas turbines
Abstract: This thesis is a result of experimental studies carried out as a part of the efforts made in Sweden and within the European Union to develop a technology that will make it technically and economically possible to use biomass as a gas turbine fuel. The process that has been studied is based on cyclone gasification of biomass powder and no further cleaning of the product gas from the gasifier. The study covers four issues that are important for assessment of the possibilities to develop such a process to commercialisation: The feeding of biomass to the gasifier The efficiency of the gasification process The product gas quality The integration of the gas turbine and the gasifier. The studies of the feeding system are presented in papers 1, 2 and 3. Screw feeders were used to control the fuel flow and ejectors driven by pressurised air or pressurised steam were used for injection of the powder into the cyclone. Two approaches for achieving feeding with small temporal variations were studied. One is based on use of a vibrator device in the feeding train, the other on use of a brush-like device positioned after the metering screws. Both systems were found to work reliably during the feeding tests and the subsequent gasification experiments. The gasification tests aimed at determination of the quality of the gas produced from the cyclone gasifier. Stable operating ranges, generation of char residue, heating value of the product gas and amounts of contaminants in the product gas were studied. The results were compared with gas quality criteria provided by ABB Stal. Experiments were made at atmospheric pressure and elevated pressure. The atmospheric gasification tests covered five different biomass powders. The results of these tests are shown in paper 4. Two of the fuels appeared as less suitable in the type of cyclone gasifier that was used. The pressurised gasification tests included commercial Swedish wood powder fuel only. The results of the tests are shown in paper 5. For this fuel, stable gasification could be attained both at the atmospheric and elevated pressure. The gas quality fulfilled the criteria with exception for the amount of large particles (above 8µm). The implications of this will depend on the particle properties. The experiences from attempts to operate the cyclone gasifier integrated with a gas turbine are presented in paper 6. Many difficulties where encountered during these tests. Stable operation of the gas turbine could be however achieved on a few occasions. The longest uninterrupted operation period was 34 minutes. Pressure transients appearing upon ignition in the combustion chamber of the gas turbine and design of the char discharge system at the cyclone bottom are the main unresolved problems.
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