Delignification of softwood by glycerol from biodiesel by-product I: model reaction using glycerol and fatty acid sodium soap mixture for pretreatment on bioethanol production

Abstract

This study investigated the use of glycerol–fatty acid sodium soap mixtures to delignify woody biomass as a model for utilization of crude glycerol by-product from biodiesel manufacture. Lignin-bearing glycerol was also produced. Delignification was carried out using glycerol mixed with sodium salts of laurate, stearate, oleate, or linoleate at 100–250 °C for 0.5–3 h. Oak, beech, bamboo, and rice straw were easily delignified by 20% sodium oleate dissolved in glycerol at around 150 °C for 1 h. For softwood (Japanese cedar and spruce), delignification did not occur below 200 °C. However, the lignin content decreased from 37.5% in untreated Japanese cedar to 10.6% and from 29.4% in untreated spruce to 11.2% by treatment at 250 °C. Japanese cedar was not delignified in glycerol mixtures with oleic acid or sodium acetate. It is suggested that the surfactant activity of soap assists delignification, but the alkaline action of sodium acetate was not effective. Enzymatic saccharification of delignified Japanese cedar (9.3% lignin) and oak (3.6% lignin) samples gave glucose yields of 0.55–0.67 g/g after 72 h and these yields were comparable with that from pure cellulose (0.77 g/g). Lignin dissolution also increased the calorific value of the collected glycerol fraction from 20 to 25 MJ/kg. The results suggest that a waste-free delignification method can be achieved based on the combined processes of biodiesel and bioethanol production.