Techno-Economic Analysis of the Optimum Softwood Lignin Content for the Production of Bioethanol in a Repurposed Kraft Mill

Abstract

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. Keywords 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...

Techno‐economic analysis of various biochemical conversion platforms for biosugar production: Trade‐offs of co‐producing biopower versus pellets for either a greenfield, repurpose, or co‐location siting context

“…Green liquor pretreatment achieves only minor delignification relative to traditional kraft pulping with oxygen delignification . Kraft pulping of biomass encompasses white liquor (NaOH + Na 2 S) cook and O 2 treatment for delignification prior to enzymatic hydrolysis for monomeric sugar production.…”

Second Generation Bioethanol Production: On the Use of Pulp and Paper Industry Wastes as Feedstock

“…One example of this conversion is the reorientation of closed Kraft pulp mills to bioethanol production [23,24]. Wu et al (2014) evaluated the techno-economic potential of repurposing a Kraft mill for bioethanol production from loblolly pine [173]. Fornell et al (2012) discussed the energy efficiency and performed an economic analysis of a conceptual Kraft pulp mill that was converted into a lignocellulosic ethanol plant [174,175].…”

Second-generation bioethanol from industrial wood waste of South American species

“…As examples, the combination of enzymes recycling and decreases in hydrolysis time led to decreased ethanol production cost by 27% for hardwoods and 38% for softwood feedstocks (Gregg et al, 1998). The co-location of the bioethanol plant into a softwood kraft-pulping mill, using the kraft process plus oxygen delignification as pretreatment were also shown to result in economic production of bioethanol (Wu et al, 2014). More specifically, an economic analysis showed that through such implementations, an ethanol yield of 285 L/ton of dry wood with a total production cost of USD 0.55/L could be obtained (Wu et al, 2014).…”

“…The co-location of the bioethanol plant into a softwood kraft-pulping mill, using the kraft process plus oxygen delignification as pretreatment were also shown to result in economic production of bioethanol (Wu et al, 2014). More specifically, an economic analysis showed that through such implementations, an ethanol yield of 285 L/ton of dry wood with a total production cost of USD 0.55/L could be obtained (Wu et al, 2014). In this sense, it would be interesting to evaluate how the incorporation of a different pretreatment to a different raw material (like pin-chips or sawdust) would work, taking advantage of the existing infrastructure of the mill.…”