Releasing behavior of alkali and alkaline earth metals during biomass gasification

The releasing characteristics of alkali and alkaline earth metals (AAEMs) during CO₂ gasification process of biomass and pyrolyzed char were explored in a fixed bed reactor. The amount of AAEMs releasing during the gasification of various feedstocks at different pyrolysis temperatures was determined by atomic absorption spectroscopy. Differences in weight loss of biomass during gasification are resulted from the diversity of volatile content in different biomass feedstocks. Along with the increasing of pyrolysis temperature, the reactivity of char gasification is decreased. It is found that the releasing amount of Na is larger than that of K, and that of alkaline earth metals is less, which is related to their existence form and metal valence. The increasing of pyrolysis temperature can improve the release of K at low temperatures, while it has little influence on the release of Na during the whole pyrolysis and gasification process. The type of biomass especially the Si content in ash has a significant impact on the migration of Ca and Mg, and higher char gasification conversion leads to a larger releasing amount of AAEMs. Results of thermodynamic calculations show that K and Na are released mostly in the form of chloride, while Ca and Mg mainly exist in the form of silicate which is hard to release.