Study of the relationship between solid phase transformation and flow behavior transitions driven by carbothermal reaction in slag of BGL gasification

Co-gasification of solid waste or biomass with coal is usually performed in the British-Gas Lurgi (BGL) gasifier due to its wide adaptability to raw materials. However, carbothermal reactions in slag bath induce complex solid phase transformation and unfavorable flow behavior of slag, which are detrimental to the stable operation of BGL gasifiers. This study investigates the solid phase transformation (formation and floatation of metallic iron and refractory iron-carbon layer) driven by carbothermal reaction. Results indicate that as the relative carbon content (α) initially increases, the Fe3+ in slag is reduced to Fe2+, disrupting the silicate network structure and lowering the slag viscosity. When α exceeds 100, Fe2+ is further reduced to metallic iron (Fe0), which can combine with residual carbon to form a refractory iron–carbon layer on the slag surface through surface tension and buoyancy. The formation of this iron-carbon refractory layer increases the slag viscosity, resulting in a transition of the slag flow behavior. Furthermore, three sequential stages of the carbothermal reaction were identified with increasing temperature: solid reduction, liquid reduction, and silicon penetration. The primary crystalline phase of the sample slag is anorthite. As α exceeds 100, slag crystallinity increases, and crystal shape defects emerge due to solid-phase formation. This study elucidates the mechanisms of solid-phase formation and flotation in carbon-containing slag and reveals that the transition in its fluidity behavior stems from the solid-phase transformation induced by carbothermal reactions.