| With the progress of investigation and application of biomass gasification technology, the influence of biomass gasification tar arouses much attention. On the basis of the investigation of medium calorie biomass gasification and centralized gas supply technology, extensive study was performed on the catalytic cracking of biomass tar, both experimental and kinetic.The production of biomass tar is influenced by gasifier type biomass feedstock, gasification agent and operation parameters, and it may cause great harm to the gasification system and gas consummation equipment. However, traditional tar removal technology, such as filtration, wet cleaning and thermal decomposition, can't meet with the need for relatively clean gas. Catalytic cracking of tar is viewed as one prominent tar reduction technology that has become the hot spot of investigation. A extensive review was made on the development of the technology of catalytic cracking of tar, and several type of catalysts commonly used were compared including dolomite, nickel based and alkali metals.Extensive experiment on tar cracking was performed on bench-scale tar catalytic cracking test system in ITPE. Cold trapping was used for tar sampling and gravimetric analysis of tar samples was performed to achieve tar content in flue gas. Gas from rice straw, rice husk and wood chips gasification was used as the source of tar. Tar content in the gas was influenced by the gasification conditions, such as the gasification temperature, residence time of feedstock in the gasifier and the type of feedstock, which can help us to investigate the mechanism of tar production in gasification process. Catalytic cracking of tar was performed in a downstream secondary fixed-bed cracker with dolomite, limestone and alumina brick as catalysts. By comparison, thermal cracking of tar was also performed with silica carbide. Operation parameters like cracking temperature(650-950℃), gas residence time(0.5-ls) and catalyst type was investigated for their influence on tar cracking and gas quality. The performance of different catalysts was compared in order to seek for catalyst suitable for commercial utilization in gasification systems. Tar cracking efficiency higher than 90 percent was achieved in catalytic cracking conditions, compared with lower than 60 percent in thermal cracking.One set of composition analysis method was erected for biomass gasification tar, in which tar samples are separated into Aliphatics, Aromatics, Ester, Polar and Aspheltenes fractions of increased polarity by one kind of chromatography, and GC/MS is used for the characterization of each fraction except Aspheltenes. Much information was achieved by analysis of different kind of tar samples, including the production and evolution of tar in gasification process, the variation of tar composition under catalytic or thermal cracking conditions. And the mechanism of tar cracking was discussed in-depth with the variation of tar composition under different cracking conditions.The process mechanism of tar cracking was investigated in kinetics view, in which a five-lump kinetic model was erected. The whole tar was divided into five lumps of different kinetic behavior in cracking process, and the kinetic parameters used to construct the kinetic model of each lump was achieved byoptimally fitting experiment data. The overall cracking model was constructed on the basis of the kinetic models of each lump. This five-lump model was used to simulate the cracking process of tars from different sources and under different cracking conditions, and good results were achieved by comparison with experiment data. Also, one overall tar-cracking model was erected for comparison with the five-lump model. The results reveal that the five-lump model can work with only the compositional information of the raw tar as the input parameters, and it can be used to predict the results of different cracking conditions and the final composition of residual tar. The simulation of the cracking process was satisfying.Some i...
|
PDF Full Text Request