New insight to migration and influence of potassium element on combustion of coal/biomass char-slag interface

Co-combustion of biomass and coal can both support heat requirement of a boiler and reduce carbon emission while the high contents of alkali metals (e.g. Na, K) in ash are key factors in the stable operation. In this study, the migration behavior and influence of K on the combustion of coal/biomass char particles on the slag surface were investigated with visualization experiments and surface analytical techniques. The results showed that the addition of K in the slag can accelerate the combustion process of coal char particles on the slag surface, which proved that K+ was not deactivated as ionic state in the slag. The promotion effect of potassium was mainly reflected in accelerating the combustion rate at the beginning stage, where the carbon conversion was below 0.2, compared to the resting stage. K was proved to migrate from the slag side to the interface of char particles and slag and aggregated with Al during the combustion process, while other elements were enriched on the slag surface outside the combustion zone. Results also showed that the combustion of biomass char particles (rice straw char) on the slag surface with less K was inhibited. However, K was found to aggregate on the interface of biomass char and slag throughout the combustion process with no diffusion. A migration mechanism was proposed that the combustion of char particles on the slag surface caused an increasing temperature and further depolymerization of aluminosilicates, with alumina replacing silicon to form aluminum tetrahedra while adsorbing K+ to the interface and promote the combustion.