| Biomass gasification in a circulating fluidized bed(CFB) is quite a novel technology at home and abroad, this technology can use a broad spectrum of biomass as raw material to produce biomass gas, which promises to have benefits for the future. In recent years, the CFB gasification technology is developing rapidly, and achieved certain results, however, the solid particles residence time in CFB and numerical simulations of gas-solid flow still needs further study, the theoretical study and engineering applications has not been perfect, and the current biomass gasification equipment still exists a variety of problems in materials applicability, gasification project selection and amplification parameters, etc.The research of particle residence time distribution and the gas-solid flow can provide the basis for the optimization of CFB equipmentsThe subject investigated in this paper was the circulating fluidized bed gasifier, firstly, we use the GAMBIT software to establish a two-dimensional model of the CFB, then based on the CFD calculation method, particle residence time distribution (RTD) and gas-solid flow character were explored. The results showed that:the RTD curves of solids in circulating fluidized bed gasifier have the characteristic of bimodal distribution, the first peak is narrow and high, and the second peak is wide and low. With the increase of gas flow rate, the first peak value of the RTD curve increased significantly, while the second peak value increased in small amplitude and the particle mean residence time decrease with increasing of the inlet gas velocity. When the inlet gas velocity increases from3.5m/s to6m/s, the particle mean residence time decreases from5.86s to3.41s, so we chose4-5m/s as a best gas velocity in engineering applications. The increase of the gasifier height helps the mixing of particles more completely, and the mixing time became longer, resulting in the increase of the particle mean residence time. The particle size can also influence the mean residence time, the larger the particle size, the longer the mean residence time, and on the contrary, the smaller the particle size, the shorter the average residence time, the optimum range of the particle size is500～600μm when we chose4-5m/s as the inlet gas velocity. The simulated results show that the Euler two-fluid model can reflect the macroscopic flow characteristics of gas-solid in the circulating fluidized bed better, moreover, it is analyzed that the particle concentration distribution shows a "core-annular" flow characteristics, where the solid concentration is lower in the center than annular region. The comparison of simulated pressure drop with the theoretical result approves the validation of numerical simulation.Then based on the simulated results, we study the gasification on one kind of herb residues generated from Henan Wanxi Pharmaceutical Co., Ltd with the moisture content of about20%. An air preheater was used to raise the temperature of gasifying agent-Air from normal to about200℃, and a steam generator was used to generate stream. The effects of air equivalent ratio(ER) and steam to biomass ratio(S/B) on gasification at these two different temperatures were examined. The results showed that:with the increase of ER, the gasification temperature in the Circulating Fluidized Bed(CFB) gradually increased, and the calorific value and the tar content gradually reduced, but the gasification efficiency increased first and then decreased; The optimum range of ER for normal air as gasifying agent is0.26～0.30, the calorific value and the gasification efficiency reached4500kJ/m3and67%respectively; when the gasifying agent was200℃hot air, the optimum range of ER was0.24～0.29, the calorific value and gasification efficiency were5000kJ/m3and73%respectively. With the increase of the S/B, the gasification temperature gradually reduced, while the tar content increased, and the calorific value increased first and then decreased, and the calorific value reached6300kJ/m3at the optimum S/B value of0.4.Results indicated that the herb residues showed good gasification characteristics, and they are good raw materials for gasification. Besides, by using the air preheater and steam generator, the gasification results can be improved to a certain extent, providing a new method for harmless disposal and resource utilization of the herb residues. |
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