Agglomeration and transformation of different types of inorganic potassium in biomass during co-gasification with coal

Abstract

Co-gasification of biomass and coal has many advantages, which also mitigates some of the ash-related problems induced by potassium (K) in biomass. However, this effect varies with the occurrence mode of inorganic K in biomass, which varies with the species of biomass. Therefore, the present work investigated the effect of a bituminous coal on the transformation of K and the ash chemistry of bed agglomeration (without considering the effect of fluidization) during co-gasification (steam and CO2 atmospheres) for different types of inorganic K (K2CO3, K2SO4 and KCl) in biomass. The results show that for both K2CO3 and K2SO4 in biomass, K is mainly retained as KAlSiO4 and K2SO4 in the gasification ash with KAlSiO4 being the dominant phase. In addition, for K2CO3 in biomass, more KAlSiO4 and less K2SO4 are formed under the steam atmosphere than under the CO2 atmosphere. However, the K retention of the ash or the agglomerate for both K2CO3 and K2SO4 in biomass is similar and is much higher than that for KCl in biomass, which tends to release and to induce other ash-related problems. In addition, both the organic compounds of coal and the gasification atmosphere significantly alter the reaction between coal with KCl in biomass: The majority of KCl reacts with coal to form KAlSiO4, the formation of which is quite limited for the coal ash; More K is retained in the ash and more KAlSiO4 is formed under the steam atmosphere than under the CO2 atmosphere. Lastly, although co-gasification with coal (silica sand as the bed material) cannot stop the formation of agglomerates for K2CO3, K2SO4 and KCl in biomass, these agglomerates are more easily to be broken than those produced from the gasification of biomass alone, thus mitigating bed agglomeration in fluidized-bed reactors.