Study On Biomass Gasification Characteristics In Entrained Flow Gasifiers

With the fossil fuel resource decreasing and the serious pollution caused by fossil fuel combustion, the biomass energy attracts more and more attention. China has become a net importer of oil and oil products. The technology of indirect liquefaction from biomass for bio-fuel can be used for a supplement of liquid fuel.The first step of biomass indirect liquefaction technology is syngas production. Based on the analysis of extensive literatures, a bench-scale entrained flow gasifier for biomass gasification was built. Combining experimental and theoretical analysis, the characteristics of biomass entrained flow gasification and the transformation of alkali metals during the gasification were investigated.Rice husk, sawdust, cornstalk and algae dust were chosen as the feedstock for oxygen gasification. Experiments werw carried out to investigate the influence of gasification temperature, oxygen/biomass ratio, particle size of raw material on gasification characteristics. The results indicate that high temperature is in favor of gasification. And it is conducive to improving the rate of carbon conversion and cold gas efficiency. Comparison of adding different proportion of water vapor and CO2 to oxygen was carried out. Results show that the addition of CO2 can achieve the same effect of steam to some extent aimed at the heating value of syngas, while reducing the energy consumption and simplifying the gasification equipments. The characteristics of entrained flow gasification with or without the pre-processing of raw materials were compared. Results show that the pretreatment could improve the H2+CO yield and the low heating value of syngas. The H2+CO yield and LHV were increased by 45.9% and 67.1% respectively. Torrefaction can effectively solve the transmission problems of raw materials, and improve the quality of syngas.Alkali metal in the biomass may cause liquefaction equipment damage and catalyst poisoning. During the wheat straw gasification, the alkali in the syngas and bottom ash were collected. The distribution of alkali metal in the gas phase and solid phase in the gasification process was analyzed. The effects of acidic oxides SiO2, alkali oxides MgO, neutral oxide Al2O3 as additives on the alkali metal emission were investigated. The results indicate that the content of alkali metal in the gas increased with gasification temperature, whilst the content of alkali metal in the ash decreased. All the additives help to fixed more alkali metal in the ash, in which the effect of MgO is the best, the contents of potassium and sodium in the ash was increased by 20.23% and 21.24% respectively. The ash fusion point was determined by the natures of these additives. On the one hand, alkaline can react with additives to form eutectics with low ash fusion temperatures, which reduce the ash fusion temperature of straw ash. On the other hand, too much additive can lead to higher ash fusion temperature. Therefore, considering the alkali retain and the ash fusion point, the proportion of addition of additive for entrained flow gasification should be chosen appropriately.