Insight into the influence of water leaching on ash fusion behavior and alkali metal migration for corn and rice straw

Abstract

Alkali-induced slagging and silicate melt-induced slagging are the two mechanisms of the ash-related problem for herbaceous plants and agricultural residues. Water leaching changes ash-forming composition and mitigates the risk of alkali-induced slagging, but the fusion behavior and the related melt-induced slagging after water leaching are seldom studied. In this study, a comprehensive assessment strategy, including systematic experiments, optimized ash fusion test and equilibrium calculations, was used to study the influence of water leaching on alkali metal migration and fusion behavior of corn straw (CS) and rice straw (RS). The total potassium released in biomass after water leaching accounted for approximately 15% of the raw materials. Although water leaching increased the fusion temperatures of CS and RS, the residual potassium still induced the same lowest eutectic melting of K-Ca-Mg silicate, causing them to start sintering at around 700 °C. However, the changes in fusion behavior obtained from optimized ash fusion test showed that water leaching effectively mitigated the risks of deposition and slagging of RS. The characteristic of fusion behavior for RS transformed from the “melting-dissolution” mechanism to the “softening-melting” mechanism, which indicated a low risk of deposition after water leaching. The fusion behavior of CS after water leaching was less apparent due to the different mineral compositions and characteristics of the two types of biomass, as Ca and Mg in CS acted as modifiers for the silicate structure. Water-leaching is recommended for pretreating biomass with a predominance of K. For biomass with substantial alkaline earth metals, water leaching combined with other countermeasures is required. These findings can help promote the thermochemical utilization of agricultural residues on an industrial scale.