Studies On Biomass Gasification Characteristics And Migration And Release Behavior Of Alkali Metal K

Among the thermochemical conversion technologies,gasification is one of the most important technologies for efficient and clean utilization of biomass,and is considered to be the most promising means for the utilization of biomass.However,biomass,especially crop straw,is rich in alkali metal K,resulting in more problems,such as agglomeration,ash accumulation,slagging and corrosion.Therefore,the research on biomass gasification characteristics and alkali metal K release and migration behavior is of great significance for the efficient and clean utilization of biomass and the solution of problems caused by alkali metals.In this paper,a combination of chemical thermodynamic equilibrium calculation and experimental analysis was used to fully study the gasification characteristics of rice straw and the release and migration behavior of alkali metal K.The migration and release of alkali metals K and Cl during the process of char CO₂gasification were studied on a fixed bed and the effects of reaction temperature and reaction time on the migration and release of K and Cl were investigated.The results show that the gas-phase release ratio of K and Cl increases rapidly with the increase of the reaction temperature and K and Cl show the relationship of simultaneous release.At a temperature of 650℃,the release of K is mainly caused by the decomposition of carboxyl combined with potassium.In the range of 750-900℃,the release of K and Cl is mainly due to the evaporation of KCl,and the decomposition of char-K and K₂CO₃ also causes K release.In this paper,the effects of reaction temperature and equivalence ratio on the gasification characteristics of rice straw were studied on the pilot-scale fluidized bed gasification system.The results show that as the temperature of the gasification reaction increased,the volumetric concentration of CO gradually increased,the volumetric concentration of CO₂ gradually decreased,the volumetric concentration of H₂ and CH₄ showed a slight upward trend,the gas yield Yg and the carbon conversion rateηincreased.Under different reaction temperatures（650℃and 800℃）,the gasified combustible gas components show different trends with the increase of the equivalent ratio,but the gas production rate and coke conversion rate gradually increase with the increase of the equivalent ratio.The thermodynamic equilibrium calculation module in the HSC Chemistry software was used to simulate the change of K and Cl components during the gasification process of straw with an equivalent ratio ER=0.25 at 400-1200℃.The results show that during low temperature（<700℃）gasification,K is mainly formed as KCl（s）in the fly ash.As the temperature of the gasification reaction further increases（>700℃）,KCl（s）begins to evaporate to the gas phase in the form of KCl（g）.And when the gasification reaction temperature is higher than 800℃,there is obvious amount of KOH（g）releasing to gas phase.During the gasification reaction,a small amount of K₂O·Si O₂（s）,K₂O·4Si O₂（s）and KAl Si O₄ are formed in the fly ash.The effects of the reaction temperature and equivalence ratio on the release and migration behavior of K were investigated on the pilot-scale fluidized bed gasification system.The results show that as the temperature of the gasification reaction increases,the proportion of alkali metal K gas phase release gradually increases.When the gasification reaction temperature is less than700℃,the proportion of alkali metal K gas phase release is small.When the gasification reaction temperature is greater than 700℃,the alkali metal K gas phase release ratio increases significantly.As the temperature of the gasification reaction increases,the content of water-soluble K gradually decreases,the content of ion-exchanged K gradually decreases,and the content of HCl-soluble K and insoluble K increases slightly.Relative to temperature,the equivalence ratio has little effect on the release and migration of alkali metal K.With the increase of the equivalence ratio,the gas phase release ratio of alkali K increased slightly,the content of water-soluble K and ion-exchanged K decreased slightly,and the content of HCl-soluble K and insoluble K increased slightly.On the 1.5MWth two-stage coupled fluidized bed gasification system,the gasification characteristics of rice husk and the migration and release behavior of alkali metal K were studied.The results show that increasing the equivalence ratio can promote the release of K,but the further increase of the equivalence ratio leads to a reduction in the residence time of the coke particles in the furnace,resulting in insufficient gasification of the coke particles and a reduction in the gas phase release ratio of alkali K.