| Summary: | 碩士 === 國立臺灣大學 === 機械工程學研究所 === 92 === Biomass is an important renewable energy resource produced from trees, agriculture crops, and aquatic plants. Using biomass as fuel for power generation is carbon neutral and helps to lower the emission of global warming gases. However, the application of biomass should be shifted from its traditional low-efficiency applications for heating to high-efficiency applications for combined heat and power generation. The gasification process for biomass converts renewable substances to fuel gas that can be used in a modern conversion device, such as gas turbine or engine, for the production of electricity and heat. Downdraft stratified gasifier is suitable for small scale heat and power production due to the fact that its producer gas contains low tar and low particulates. Therefore, a laboratory scale downdraft gasifier with 750mm height and 150 mm diameter is used in this study to investigate the relations between quality of producer gas and operation conditions.
Steady state condition is reached within ninety minutes after start-up, then the producer gas is generated continuously. Different reaction zones including heating and drying, pyrolysis, gasification, combustion and char reduction, can be identified inside the gasifier reactor. Distribution of reaction zones affects the heating value of producer gas and the rate of gas generation, as well as tar content and particulates. Factors influence the distribution of reaction zones including fuel type, fuel load, moisture content, combustion temperature and air staging.
Optimum operation condition of the gasifier varies in the range of 2.79 to 3.79 Kg-air/Kg-fuel at the feeding rate between 2.55 and 2.81 Kg/hr. HHV of producer gas is 3.81-4.40 MJ/Nm3 at a volumetric flow rate of 8.8-12.4 Nm3/hr. The producer gas composition, heating value, and generation rates vary in a wide range more less in synchronization of the fuel feeding cycles. Improvement is suggested by shortening the feeding intervals with lower charge to even out the fluctuations in output.
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