Kraft pulping is one possible pretreatment for softwood to economically produce bioethanol. This work evaluates the techno-economic potential of using the kraft process for producing bioethanol from softwoods in a repurposed or co-located kraft mill. Pretreated loblolly pine was enzymatically hydrolyzed at low enzyme dosages of 5 and 10 FPU/g of substrate. Pretreated residue with 13% lignin content had the highest sugar recovery, 32.7% and 47.7% at 5 and 10 FPU/g, respectively. The pretreated residues were oxygen delignified and refined. In all cases, oxygen delignification improved sugar recovery, while refining was mostly effective for pulps with high lignin content. At 5 FPU/g, the sugar recovery for all kraft pulps was 51 to 53% with oxygen delignification and refining. Increasing the enzyme dosage to 10 FPU/g increased the sugar recovery for these pulps to greater than 60%. Economic analysis for the pulps with different initial lignin content showed that kraft pulps with an initial lignin content of 6.7% with oxygen delignification had an ethanol yield of 285 L/ODt wood and the lowest total production cost of $0.55/L. Pulps with initial lignin content of 18.6% had a total production cost of $0.64/L with an ethanol yield of 264 L/ODt wood.
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INTRODUCTIONLignocellulosic biomass is the most abundant renewable resource on earth. In the past decade, there has been a growing interest in using this biomass as feed stock for the production of bioethanol (Faaij 2006;Ragauskas et al. 2006;Jørgensen et al. 2007;Bozell 2008;Regalbuto 2009;Tilman et al. 2009). It is not practical to convert lignocellulosic biomass directly to ethanol; several unit operations must be employed. The modern biotechnical process of converting lignocellulosic biomass to ethanol includes pretreatment, enzymatic hydrolysis, and fermentation. In the past few decades, many pretreatment strategies have been developed to make the lignocellulosic substrate more susceptible to enzymatic hydrolysis. Following pretreatment, enzymatic hydrolysis is a key operation for the bioconversion of carbohydrates in lignocellulosic biomass into fermentable sugars by enzymatic hydrolysis. It is an important factor because the cost of the enzymes has a great impact on the economic feasibility of bioethanol production on a commercial scale. The price and dosages of cellulolytic enzymes have been progressively reduced due to intensive research by enzyme producers such as Novozymes and Genencor (Zhang et al. 2006). Although substantial progress has been achieved to improve the PEER-REVIEWED ARTICLE bioresources.com Wu et al. (2014). "Kraft pulping for bioethanol," BioResources 9(4), 6817-6830. 6818 enzymatic hydrolysis of lignocellulosic biomass, the rate of saccharification is slow and often incomplete, especially at low enzyme loadings. Lignocellulosic biomass, especially softwood, has natural resistance to biological degradation because of its morphological structure and chemical composition. Many softwood pretreatment methods have been studied, including ac...