Laboratory study of interactions between biomass ash and alkali-feldspar bed material

Paper in proceeding, 2021

Bed material reactivity and lifetime are dependent on the materials’ interactions with biomass ash. Literature on reactions between feldspar and ash elements is scarce, as it is a relatively new bed material and therefore less established. To gain a better understanding on the mechanism behind the observed activity of feldspar, and thereby increase the viability of its industrial application, a mixture of Na- and K-rich feldspar was exposed in a lab-scale furnace under fixed bed conditions. Ash interactions were mimicked by adding major biomass ash elements, such as Ca and K. The interaction of the particles with ash components in terms of layer formation and agglomeration were studied using SEM-EDS and compared to thermodynamic modelling calculations with FactSage. Microscopic analysis of the synthetically formed surface layers revealed similarities to ash layers formed on alkali-feldspar during biomass gasification. Ca reacted with feldspar forming a Ca-rich layer of about 10 μm thickness after 24 hours exposure. It could be shown that Na-feldspar reacts with KOH to form K-feldspar by expelling Na from the particles structure which accumulated in the surface layer. From the laboratory exposures it was further found that fresh feldspar particles tend to agglomerate when exposed to KOH from the beginning. By pre-exposing the particles to a Ca-salt and thereby forming a Ca-rich layer on the particles’ surface prior to the exposure to KOH, the agglomeration tendency could be mitigated due to the higher melting point of the elemental composition present in the Ca-rich layer. Pre-exposure to CaCl2 was therefore found to be an effective method against agglomeration.