Characterization of acid-mechanical milling pretreated rice straw for subcritical water hydrolysis

Abstract

Abstract Ball milling of rice straw impregnated with sulfuric acid (RS-S), hydrochloric acid (RS-H), acetic acid (RS-A), or nitric acid (RS-N) were carried out in this study. Physicochemical analysis and subcritical water hydrolysis were performed to evaluate the effect of acid species on ball milling treatment of rice straw. Acetic acid and solo ball milling treatment showed little effect on solubility, thermal stability and crystalline structure of rice straw, while hydrochloric acid, acetic acid and nitric acid significantly improved the solubility and decreased the crystallinity index and thermal stability of rice straw. Sulfuric acid was found to be the most efficient acid to destroy the rice straw structure during ball milling followed by nitric acid and hydrochloric acid, attributed to its long retention on rice straw surface after drying. The effective cleavage of holocellulose-lignin chemical linkages in RS-S during pretreatment made the hydrolysis products of RS-S easier to be hydrolyzed to biochar at high reaction temperatures, resulting in the increased solid residue yield. The breakage of crystallites and holocellulose-lignin chemical linkages greatly improved the reactivity of RS-S, resulting in the lower temperature and activation energy required to initiate the hydrolysis reaction compared with those of rice straw and RS-A.