Due to the health and environment impacts of fossil fuels utilization, biofuels have been investigated as a potential alternative renewable source of energy. Bioethanol is currently the most produced biofuel, mainly of first generation, resulting in food-fuel competition. Second generation bioethanol is produced from lignocellulosic biomass, but a costly and difficult pretreatment is required. The pulp and paper industry has the biggest income of biomass for non-food-chain production, and, simultaneously generates a high amount of residues. According to the circular economy model, these residues, rich in monosaccharides, or even in polysaccharides besides lignin, can be utilized as a proper feedstock for second generation bioethanol production. Biorefineries can be integrated in the existing pulp and paper industrial plants by exploiting the high level of technology and also the infrastructures and logistics that are required to fractionate and handle woody biomass. This would contribute to the diversification of products and the increase of profitability of pulp and paper industry with additional environmental benefits. This work reviews the literature supporting the feasibility of producing ethanol from Kraft pulp, spent sulfite liquor, and pulp and paper sludge, presenting and discussing the practical attempt of biorefineries implementation in pulp and paper mills for bioethanol production.

Second-generation bioethanol production's main bottleneck is the need for a costly and technically difficult pretreatment due to the recalcitrance of lignocellulosic biomass (LCB). Chemical pulping can be considered as a LCB pretreatment since it removes lignin and targets hemicelluloses to some extent. Chemical pulps could be used to produce ethanol. The present study aimed to investigate the batch ethanol production from unbleached Kraft pulp of Eucalyptus globulus by separate hydrolysis and fermentation (SHF). Enzymatic hydrolysis of the pulp resulted in a glucose yield of 96.1 ± 3.6% and a xylose yield of 94.0 ± 7.1%. In an Erlenmeyer flask, fermentation of the hydrolysate using Saccharomyces cerevisiae showed better results than Scheffersomyces stipitis. At both the Erlenmeyer flask and bioreactor scale, co-cultures of S. cerevisiae and S. stipitis did not show significant improvements in the fermentation performance. The best result was provided by S. cerevisiae alone in a bioreactor, which fermented the Kraft pulp hydrolysate with an ethanol yield of 0.433 g·g −1 and a volumetric ethanol productivity of 0.733 g·L −1 ·h −1 , and a maximum ethanol concentration of 19.24 g·L −1 was attained. Bioethanol production using the SHF of unbleached Kraft pulp of E. globulus provides a high yield and productivity.Energies 2020, 13, 744 2 of 15 sustainability, has a low and stable price, and practically does not demand extra land [6,7]. There are some facilities producing 2G bioethanol on a commercial scale. However, large-scale production still faces some technological barriers that must be overcome in order to achieve a cost-competitive production [8]. Due to the recalcitrance of LCB, a costly pretreatment step is required, which is the main technological bottleneck of 2G bioethanol production. The release of enzymatic and fermentation inhibitors during pretreatment is another limitation [9].Pulp and paper mills have the infrastructures and logistics to handle LCB, and chemical mills employ technology required for LCB fractionation and conversion [10]. Bioethanol has been produced from different feedstocks such as Kraft pulp, spent sulfite liquor, and pulp and paper sludge [11]. Chemical pulping processes can be considered as a LCB pretreatment since they promote delignification and target hemicelluloses to some degree [12]. Chemical pulping represents about 77% of the virgin pulps produced globally, and more than 95% of these chemical pulps are Kraft pulps [13]. These pulps are produced by Kraft pulping involving the reaction of white liquor, i.e., an alkaline aqueous solution of sodium hydroxide and sodium sulfide with a pH of 14, with lignin at high temperature (150-170 • C). This reaction promotes lignin breakdown and degradation with the release of phenolic fragments, removing almost 90% of the lignin from the wood. Kraft pulping also leads to hemicelluloses and some cellulose loss and decreases the degree of polymerization of cellulose [14]. The utilization of Kraft pulping as a pretreatment method for LCB has ...

No abstract