Prehydrolysis soda pulping of Enset fiber for production of dissolving grade pulp and biogas

Abstract

Massive tonnes of fibrous residues are produced during the harvesting of the Enset plant for food preparation. The fibers are characterized by high cellulose and hemicellulose content and low lignin and extractive content. These make the fiber a good candidate for its concurrent valorization aimed at dissolving grade pulp and biogas. Prehydrolysis soda pulping was performed using steam pretreatment as a prehydrolysis step at a severity ranging from 2.95 to 4.13. The steamed fiber (PH fiber) was subjected to subsequent soda pulping under mild (160 °C and 16% alkali concentration) and severe (180 °C and 24% alkali concentration) pulping conditions. At higher steaming severity, a pulp with a xylose content of <4% and glucose content of 96% was obtained. A simple bleaching stage was envisaged to develop oxygen-peroxide (OQP₁), oxygen-double peroxide (OQP₂P₂, and OQP₂P₃) and oxygen-peroxide-chlorine dioxide (OQP₂D) sequences. Brightnesses up to ∼85% ISO could be reached for all sequences with CUEN viscosities of ∼350–500 ml g⁻¹. Higher viscosities with higher brightness were achieved mainly by OQP₂D sequence. However, even with OQP₁ and OQP₂P₃ sequences the pulps met the requirements for lyocell production. An intense steam treatment reduces the biochemical methane potential (BMP) of prehydrolysis liquid (PHL) from 462 ml g⁻¹ vs to 315 ml g⁻¹ vs. The reduction might be due to the inhibition effect of furan concentration increase in the corresponding PHL from 2 ppm to 24 ppm. However, due to the higher yield and carbohydrate concentration of the prehydrolysis liquid, the biogas production volumes per initial raw material were still higher at higher steaming severity.