Bed Agglomeration Mechanisms At Biomass Dual Fluidized Beds Gasification Atmospheres

Biomass gasification in dual fluidized beds is capable to produce fuel gas with a high heating value and with a high carbon conversion.However,during biomass gasification or combustion in fluidized bed,bed particles have a tendency to form agglomerates.Consecutive operation is severely hindered by agglomeration problems.Formation of agglomerates are closely related to melting properties of biomass ash.Gas atmosphere in gasifier is completely different from that in combustor,but agglomeration mechanisms in different gas atmospheres are still unclear.Dual fluidized bed gasification technology in pilot plant scale needs further studies.This study focuses on investigation of biomass ash melting characteristics,agglomeration mechanism and dual fluidized bed gasification in a pilot plant.First of all,a new method based on Thermal Mechanical Analyzer was proposed to measure the melting temperature of biomass ash.Ts,Tm and T90 were defined as sintering,severely melting and completely melting temperatures,respectively.Compared with cone method,this method had higher reproducibility and can measure slight shape changes caused by ash melting or sintering.At the severely melting stage,the shrinkage of ash samples were caused by the melting of potassium silicates which could cause bed particles sticking to each other.Therefore,Tm could be used to predict the tendency of defluidization caused by the melting of different biomass ash in fluidized bed.Furthermore,the mechanisms of bed agglomeration induced by biomass ash in different gas atmospheres were studied.Compared to air or CO₂ atmosphere,the defluidization temperatures,in H₂ or steam atmospheres,were much lower.In air or H₂ atmosphere,bed agglomeration is caused by ash melting,while the agglomeration in steam atmosphere was related to the formation of coating on bed particle surface.In combustion and gasification atmosphere,bed agglomeration was caused by ash melting,but the defluidization temperature in gasification atmosphere was lower than that in combustion atmosphere.FactSage simulation results indicated that compared to combustion atmosphere,in gasification atmosphere,a larger amount of K was transformed into the molten phase,which increased the fraction of molten phase in biomass ash and fluidized bed.The differences of agglomeration characteristics in different atmospheres were caused by the transformation of K species.The agglomeration mechanisms of K salts and quartz sand were studied.The effects of atmospheres on agglomeration mechanisms and transformation of K salts were also studied.KCl could react with SiO₂ to form potassium silicates in steam atmosphere while K₂SO₄ could react with SiO₂ to form potassium silicates in H₂ atmosphere.Bed agglomeration was caused by the melting of potassium silicates.The defluidization temperature of K₂CO₃ and quartz sand in steam or H₂ atmosphere was much lower than that in air,which may be caused by transformation of K₂CO₃ to KOH in steam or H₂ atmosphere.During pine sawdust gasification or combustion,no defluidization occurred in fluidized bed.Among three types of biomass,pine sawdust had the lowest of K content and K/Ca.During biomass dual fluidized gasification,agglomeration could be avoided through controlling the ratio of K to bed particles and choosing proper combustion or gasification temperatures.During steam gasification,the heating value of fuel gas can amount to 13MJ/Nm3.Circulating ratio of bed particles in air gasification and steam gasification were 11.4 and 14.0,respectively.