Effect of leaching pretreatment on the inhibition of slagging/sintering of aquatic biomass: Ash transformation behavior based on experimental and equilibrium evaluation

Abstract

In this study, aquatic biomasses of Ulva lactuca and Hydrilla verticillata were pretreated by water and acid washing. A comprehensive assessment strategy, including systematic experiments and equilibrium calculations, was used to study the migration of elements and transformation characteristics of minerals in aquatic biomass ash before and after leaching pre-treatment. The effect of leaching pre-treatment on slagging inhibition was analyzed. Owing to the different mineral compositions and characteristics of the two types of aquatic biomass, the serious slagging caused by Ulva lactuca was related to the presence of alkali (Na and K) sulfate. In contrast, the presence of silicate/aluminosilicates in Hydrilla verticillata had a greater effect on slagging. The removal of alkali metals (especially Na and K) and Cl from aquatic biomass by leaching pretreatment was conducive to alleviating aquatic biomass slagging. Following leaching pretreatment, the alkali index of the aquatic biomass was reduced to <0.17, and the Cl content was reduced to <0.3%. Furthermore, the abundant alkaline earth metals (Ca and Mg) in aquatic biomass can inhibit slagging by generating high-melting-point alkaline earth metal silicates or phosphates. The removal rate of alkaline earth metal oxides by acid washing was >96% and >78% in Ulva lactuca and Hydrilla verticillata, respectively, while water washing was less effective. Although water and acid washing can alleviate slagging, a comparative investigation showed that water washing of aquatic biomass had better outcomes than acid washing. Therefore, water washing is a better way to comprehensively pre-treat aquatic biomass. The results obtained can help promote the thermochemical utilization of aquatic biomass on an industrial scale.