Effects of different catalysts on steam gasification ofbiomass char at low temperature

Abstract: The low-temperature catalytic gasification of biomass is a promising technology for hydrogen production from the energy point of view due to its relatively low heat input. And it has attracted the worldwide interests. However, the lower char conversion efficiency and higher tar yield at low-temperature are unsolved problems to the technical application of low-temperature catalytic gasification. In order to overcome these issues, numerous researches are being focused on the catalyst development. As the studies concerning catalytic gasification of biomass char under relatively low temperature (T≤750℃) are fairly limited, catalytic gasification of two biomass char samples, rice husk and wheat straw, were investigated at relatively low temperature in this paper. The impregnated alkali metal catalysts were prepared in this study. In which, Al2O3 was used as carrier of the catalyst, and potassium carbonate and sodium carbonate were used as active ingredient, respectively. In addition, the catalyst components were detected by X-ray diffraction analysis (XRD). Using the prepared catalysts, catalytic steam gasification of biomass char was carried out in a lab-scale fluidized bed reactor at the temperature between 600℃ and 700℃ to investigate the performance of catalysts. And the effects of catalyst parameters including active ingredient, ingredient contents and calcination temperature, and gasification temperature on the behavior of char gasification were also studied. The results revealed that both potassium based catalyst and sodium based one have significant catalytic action on biomass char conversion. Moreover, potassium based catalyst exhibited better catalytic performance than sodium based one with the same content of active component in rice husk char gasification process. Compared with non-catalytic rice husk char gasification, carbon conversion efficiency was increased by 18.2% and 13.5% using 30KAl(600) and 30NaAl(600), respectively. Increasing K2CO3 content was beneficial to improving char conversion efficiency, and also increasing CO and H2 yield. However the active component contents were inadvisable beyond 30%. The catalyst prepared at different calcination temperature generated the different existing forms of active components. 30KAl(800) catalyst calcinated at 800℃ showed the optimal catalytic action on the char gasification. X-ray diffraction analysis (XRD) showed that the 30KAl(800) catalyst contained K2Al2O2(CO3)2·3H2O, and thus it may be a more effective component. The results showed that both gas yield and carbon conversion efficiency of wheat straw char were higher than those of rice husk char under the same gasification conditions. Thus it also reveals that alkali metal contained in biomass ash has catalytic action on char gasification. The gas yield and carbon conversion efficiency of wheat straw char reached 130.0 mol/kg-char and 95.9% respectively using 30KAl(800) catalyst at the gasification temperature of 700℃, which were 57.0% and 34.1% higher than those of non-catalytic gasification. It was also found that the gas yield and carbon conversion efficiency significantly reduced with decrease of gasification temperature. When gasification temperature was decreased to 600℃, the carbon conversion efficiency was only 19.1% using 30KAl(800) catalyst. Thus it was not appropriate for gasification at temperature below 700℃ based on the study.