Transformation of alkali and alkaline earth metallic species during pyrolysis and CO₂ gasification of rice straw char

The release of alkali and alkaline earth metallic species (AAEMs) can cause serious technical problems during biomass thermal utilization. The release characteristics of AAEMs from rice straw char during pyrolysis in N₂ and CO₂ gasification at 900℃ were investigated using a fixed-bed reactor. The release and transformation results of AAEMs under both atmospheres were obtained based on analysis of solid residues sampled after different residence time, and effect of CO₂ atmosphere on AAEMs migration was discussed. Under both pyrolysis and gasification conditions, K release ratio initially increased rapidly and the growth rate reduced with time. At early stage of gasification, K release ratio is higher than that after pyrolysis for the same time, while the K release ratio is almost same at late stage of gasification and pyrolysis. Little amounts of Ca and Mg were released during both pyrolysis and gasification. During pyrolysis, fractions of acid-soluble K and Ca first decreased and then remained nearly constant, while the fraction of acid-soluble Mg always remained constant. During gasification, the fraction of acid-soluble K first decreased slowly and then decreased rapidly, while that of Ca and Mg increased first and then reduced with time. At early stage of gasification, the fractions of acid-soluble AAEMs were higher than that after pyrolysis. At last stage of gasification, significant higher fractions of insoluble AAEMs were formed. CO₂ atmosphere facilitated release of K by enhancing decomposition and release of char-K. Insoluble char-associated AAEMs are transformed to be acid-soluble due to consumption of char matrix. At the end of gasification, significant insoluble AAEM-silicates compounds were formed.