Effect Of Impregnation And Catalytic Tar Conversion On Biomass Pyrolysis For Gas Production

Biomass energy had gradually become a good alternative to traditional fossil fuels. Biomass resources can be effectively utilized by biomass pyrolysis or gasification. However, there were some problems, such as low thermal efficiency, high tar content and low combustible gas content in gas prodution, limited the development and application of biomass gasification.The process of biomass pyrolysis can be improved by catalytic method. As a result, the tar content in the gas production was effectively reduced and the product gas composition was also improved. The catalytic process can be accomplished by pretreatment or downstream catalytic process. Impregnation pretreatment can improve catalytic pyrolysis of biomass to produce more gas. The introduction of downstream catalytic bed had obvious catalytic reforming effect to the crude gas with tar in it. In this paper, the effect of pretreatment and catalytic tar reforming on the biomass pyrolysis for gas production was studied.(1) The mechanism of metal-impregnation pretreatment and effect of metal-impregnation on the kinetic characteristics of biomass catalytic pyrolysis was studied. The results showed that:biomass pyrolysis moved to the low temperature zone; the maximum pyrolysis rate increased; and the coke production decreased. The activation energy of pyrolysis reaction can be reduced by metal impregnation. The catalytic activitys of Ni, Fe and K were slightly higher than those of Ca and Mg. Impregnation pretreatment was a complex process, in which charge adsorption theory and stable compounds theory may simultaneously exist.(2) The effect of Ni-pretreatment on oak wood pyrolysis gas and the reuse feasibility of biomass char containing nickel were studied. The results showed that the pyrolysis temperature had obvious influence on the distribution of three phase products. Ni-impregnation can effectively promote the catalytic pyrolysis and chang the tar fraction. The gas product yield and heat value of Ni-impregnated biomass were higher than those of direct mixing. Co-pyrolysis of biomass and char containing nickle showed no obvious effect on the promotion of gas production.(3) With biomass char in the second bed, the catalytic reforming of primary gas containing tar was studied. The results showed that more pore structure in oak char caused better catalytic effect than that of rice husk char and quartz sand. Under the condition of this study, with optimumized 800 "C catalytic temperature and 21cm bed length, the tar conversion reached 83.77% and the gas product yield reached 635.87 mL·g-1 feedstock. After catalysis, the oxygen content of tar decreased, and the low molecular weight hydrocarbons increased.(4) The changes of quality and nature of biomass char in the catalytic process, and the mechanism of catalytic process were studied. The results showed that the increase of gas prodution was due to tar reforming, steam and CO2 reforming of carbon. The catalytic reaction and carbon consumption happened from the inside of the char bed. As long as the char bed was stable, a certain range of carbon consumption would not significantly reduce the tar conversion rate. The mechanism and process model of catalytic tar reforming on biomass char was proposed, and the key to ensure the catalytic activity was the adsorption and reactivity of biomass char.Based on research results above, a new biomass pyrolysis process combined impregnation and catalytic tar conversion was developed for more efficient gas production. Based on biomass char which containing Ni in it, this new process can effectively convert biomass to low-tar content and high quality gas. This process can effectively solve the problems of low efficiency, high tar content and poor quality gas in our biomass gasfication plants. Therefore, this process had important significance to promote the sustainable utilization of biomass resources.