Mobile, small-scale gasifiers are finding applications in industries that require economical on-site generation of electricity and waste heat from solid biomass waste. Most waste-to-energy gasifier systems pelletize the solid fuel waste prior to processing in the gasifier to minimize problems associated with materials handling. Pelletizers are expensive, require pretreatment (e.g. metals separation and removal), and have a large footprint. The objective of this study was to develop a shredded waste downdraft moving bed gasifier that converts high volatility municipal solid waste (MSW) and biomass fuels to a low tar producer gas, minimizes waste pretreatment problems, and at the same time results in faster reaction kinetics and higher conversion efficiencies. Shredded waste has a very high wall friction for typical gasifier refractory materials and geometries, resulting in non-uniform solid waste flow. The high surface area to volume ratio also results in low permeability flow, making it more difficult to inject secondary air into the gasifier to combust pyrolysis vapors for optimum conversion. A prototype downdraft packed bed gasifier was developed to process municipal solid waste and other biomass and tested in the laboratory. The walls of the gasifier were tapered with the cross-sectional area increasing from the inlet to the outlet of the reactor. A manually- operated grate, at the exit of the gasifier, was used to control the solid waste mass flow and to remove char and ash. The gasifier and downstream processing system were instrumented with thermocouples, pressure gauges and flow meters. The diverging tapered gasifier design resulted in bulk solids flow without any regions of stagnant flow due to bridging or arching. Shredded waste has a low permeability to gas flow compared to pellets and the extent of the secondary air gas penetration into the gasifier can be limiting. Flow simulation studies were carried out to determine the penetration of the secondary air into the solid waste through nozzles placed in the walls of the gasifier. The results showed at very high flow resistance, corresponding to shredded waste, the secondary air only penetrated about 33% into the center of the gasifier. Another objective of the study was to determine secondary air configurations that resulted in high producer gas energy. Secondary air injection ports were placed in three zones below the inlet to the gasifier and distributed around its periphery in each zone. In some experiments, a secondary air injection tube was also placed across the width of the gasifier. Gas samples were collected prior to the producer gas blower and their composition analyzed by gas chromatography. Vertical temperature profiles throughout the gasifier were obtained from the electronic data capture system at the time of gas sampling. High producer gas energies resulted from the injection of secondary air just below the pyrolysis zone in the gasifier and were a consequence of the combustion reactions generating CO2 and H2O, which in turn increased the production of CO and H2 through the Boudouard, water gas and water gas shift reactions, reducing char to ash. Further work is continuing and is focused on the development of other secondary air injection tube configurations, the use of feed stocks consisting of MSW (plastics, food, and paper/cardboard), and measuring the levels of tar and other contaminants in the producer gas.
展开▼
