Study Of Calcium Oxide Dechlorination In Biomass Fluidized Bed Gasifier

As a renewable energy sources, biomass energy has the advantages of renewable, low pollution and huge reserves. The techonology of biomass fluidized bed gasification is one of the effective ways of biomass utilization. However, the high chlorine content in biomass, especially in agricultural crops, is an important factor which leads to the high temperature corrosion of equipment during gaisification. Chlorine can be removed through the addition of alkaline sorbent such as CaO in the gasifier. Therefore, CaO dechlorination in biomass fluidized bed gasifier is studied by the methods of both experimental tests and numerical simulation in this paper.Thermogravimetric tests of CaO particles with HCl gas were performed in a thermogravimetric analyzer (TGA). The effect of temperature on CaO conversion ratio was investigated. The kinetic parameters of CaO-HCl reaction required for the numerical simulation were calculated by shrinking core models using thermogravimetric experimental results.Experimental study on the dechlorination characteristics of CaO during biomass (straw briquettes fuel) gasification was carried out in a bubbling fluidized bed. Firstly, the effects of temperature and the fuel-air equivalence ratio (ER) on chlorine emission during biomass gasification were researched. Further experiments were performed to evaluate the effects of Ca/Cl molar ratio, temperature, ER and CaO particle size on dechlorination efficiency. The variation laws of the dechlorination efficiency under different experimental conditions could be concluded as follows:The dechlorination efficiency of CaO increases with the increasing of Ca/Cl molar ratio, and it enters in the stable area at Ca/Cl molar ratio of 4.73. CaO dechlorination efficiency increases at first and then decreases with the increasing of temperature, and it reaches the peak at 923 K. CaO dechlorination efficiency gradually increases with the increasing of ER, but the change is not obvious when ER more than 0.16. A slight rising of CaO dechlorination efficiency emerges when the CaO particle size decreasing.Numerical simulation of CaO dechlorination characteristics in the biomass fluidized bed gasifier was carried out. A three-dimensional numerical model consistenting with the experimental conditions was developed to simulate the biomass fluidized bed gasification. The CaO dechlorination model obtained by TGA was then coupled. An Eulerian-Eulerian method was adopted to simulate the gas-solid flow and a self-developed chemical reaction module was used to simulate chemical reactions. Flow patterns, gasification results and dechlorination efficiency were obtained. Meanwhile, further simulations were performed to evaluate the effects of Ca/Cl molar ratio and temperature on CaO dechlorination efficiency. Through the comparison of the simulation results and the experimental datas, the suitability of the model to obtain the characteristics and optimum parameters of CaO dechlorination in gasifier was verified.