Application of steam explosion to the fractionation and rapid vapor-phase alkaline pulping of wheat straw

Abstract

The utilization of steam explosion technology for the production of cellulose pulps was evaluated at a bench scale using wheat straw as raw lignocellulosic material. Steam explosion was used either as a pretreatment method to achieve the fractionation of the straw into its constitutive polymers, or as a rapid pulping method for the production of unbleached chemical pulps from alkali-impregnated straw. In the fractionation process straw was pretreated by steam explosion at temperatures comprised between 205 and 230°C, and a reaction time of 2 min. The exploded fiber was washed three successive times to yield a hemicellulosic sugars-rich solution. The recovered fiber was delignified by alkali at 160°C for 60 min. The alkali lignin was recovered by filtration after acidification of the black liquor. The resulting fiber was screened to separate the fines from the pulp. The latter was bleached and viscose-grade cellulose obtained. By-products of the process were lignin, and molasses rich in hemicellulose-derived oligomers. The optimization of the process led to the following results at a steam explosion severity of log 10( R 0)=3.80: viscose-grade cellulose pulp yield=70% of the potential; lignin recovery=70% of the Klason lignin present in the original straw; hemicellulose sugars=55% of the potential pentosan, recovered as molasses. The production of chemical pulp from wheat straw was studied using a conventional soda process and a two-stage cooking sequence, consisting of straw impregnation with the caustic liquor followed by rapid (≤4 min) steam explosion treatment (160–215°C) of the impregnated material following withdrawal of the excess impregnation liquor. The impregnation/steam treatment sequence for wheat straw pulping shortens the total processing time (impregnation+cooking) to less than 15 min. Unbleached IRSP pulps, with yields of 33–34% (screened and fines removed) show strength properties comparable both to those of conventional unbleached wheat straw soda pulps and hardwood Kraft pulps, refined to similar freeness values (around 300 ml CSF).