The Migration Characteristics And Catalysis Contribution Of Alkali Metals During Coal And Biomass Co-Gasification

In the 21st century, to solve the fossil fuel resources shortage and fossil fuels producing CO2 problem, countries continuously began to explore biomass rational utilization of energy way. Research of biomass/coal gasification becomes a concern. At the same time, synergistic effect produced by co-gasification process attract wide interest, most people think that the synergistic effect during gasification process is mainly caused by the catalysis of biomass to coal gasification, and studies have shown that adding biomass in the process of coal gasification can improve efficiency of coal gasification; CO2 atmosphere reaction activity of char produced by pyrolysis of coal mixed with biomass was significantly higher than char from coal pyrolysis, and researchers consider this phenomenon is due to the catalytic activity of alkali/alkaline metal which is rich in biomass in coal gasification. However, no related research systematically point out that how the alkali/alkaline metal of biomass migrates to the coal in the coal and biomass co-gasification process and how to play catalytic role in the process of coal gasification.In this paper, a series of experiments were carried out in the assembled and two stages fluidized-bed reactor including coal/biomass co-gasification, co-pyrolysis, single gasification and single pyrolysis experiment. Adopting Pressure digestion method can digest solid material in the reaction, and the content of alkali metal K and Na was accurately determined by atomic absorption spectroscopy (AAS). The temperature、O/C ratio、H2O/C ratio and the Biomass ratio of Materials was investigated to how to effect on alkali metal migration. The Specific surface area、pore size distribution and reaction activity of char with different alkali metal contents was determined, and the contribution value of alkali metal to improve the efficiency of coal gasification was investigated during co-gasification process. The conclusions are as follows:Firstly, in the coal gasification and the biomass gasification process, the content of alkali metal K and Na in the gas phase is far more than one of staying in solid phase. But the migration of alkali metal changed in the biomass/coal co-gasification process, most of the alkali metal K and Na remained in the gasification ashes after coal and biomass co-gasification, only a few volatilized into the gas phase. Co-gasification can make most of the alkali metal fixed in a solid phase in order to provide a good condition for the catalysis of alkali metal to coal gasification.Secondly,70% of K and Na have evaporated into the gas phase after biomass pyrolysis, but 15% of K and Na volatilized into the gas phase after coal/biomass co-pyrolysis. It is shown that most of the alkali metal K and Na remained in the solid phase in the process of coal/biomass co-pyrolysis, preventing it volatilized into the gas phase. Alkali metal K and Na of the biomass moved into the char during the pyrolysis stage.Thirdly, in the co-pyrolysis stage, most alkali metals of biomass firstly volatilized into the gas phase. In gas phase, alkali metal compounds are adsorbed by the char, completing the transmission of alkali metal from biomass to char. Therefore, the transmission of alkali metal in biomass to char is through the air-solid contact way.Fourthly, raising temperature and increasing the biomass content of Materials are beneficial to the alkali metal K and Na migrating to the solid phase during co-gasification process, and biomass rate plays a bigger impact. Improve O/C ratio and H2O/C ratio will decline the migration of alkali metal to the solid phase with a certain degree.Fifthly, the coal/biomass co-gasification will improve the reactivity of coal gasification, reduce the gasification temperature and shorten the gasification time. When the content of biomass is 20%, the gasification temperature of co-pyrolysis char is lower by 100℃, and the gasification rate reaches the highest at 1100℃ which is about three times of char from pyrolysis of coal.Finally, a number of alkali metal remained in solid phase after coal/biomass co-gasification, at the same time, it can affect specific surface area and pore size distribution of char in pyrolysis stage, which make more holes about 4 nm existed in the char, thus increase the specific surface area. But the contribution of high specific surface area to the increasing reaction activity of char is far less than the contribution of alkali metal as coal gasification catalyst.