Process Design and Economics for Biochemical Conversion of Lignocellulosic Biomass to Ethanol: Dilute-Acid Pretreatment and Enzymatic Hydrolysis of Corn Stover
Abstract
This report describes one potential biochemical ethanol conversion process, conceptually based upon core conversion and process integration research at NREL. The overarching process design converts corn stover to ethanol by dilute-acid pretreatment, enzymatic saccharification, and co-fermentation. Building on design reports published in 2002 and 1999, NREL, together with the subcontractor Harris Group Inc., performed a complete review of the process design and economic model for the biomass-to-ethanol process. This update reflects NREL's current vision of the biochemical ethanol process and includes the latest research in the conversion areas (pretreatment, conditioning, saccharification, and fermentation), optimizations in product recovery, and our latest understanding of the ethanol plant's back end (wastewater and utilities). The conceptual design presented here reports ethanol production economics as determined by 2012 conversion targets and 'nth-plant' project costs and financing. For the biorefinery described here, processing 2,205 dry ton/day at 76% theoretical ethanol yield (79 gal/dry ton), the ethanol selling price is $2.15/gal in 2007$.
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- In contrast, in the case of on-site enzyme production the overall cost of enzyme was reported to be $ 90 per cubic meter of ethanol that is lower than Novozymes. Humbird et al. [12] had estimated the cost of enzyme prepared from pure glucose as the carbon and energy source and reported that the carbon source is the most significant expense in enzyme production. In the present work, P. pinnata seed residue, after extracting oil from the seeds, was utilised as the carbon source during the enzyme preparation .
[Show abstract] [Hide abstract] ABSTRACT: Lignocelluosic biomass represents a promising feed stock for biofuels production. However currently the costs associated with lignocellulases represent a key limiting factor in the development of biomass conversion process. The aim of this work was to exploit Pongamia pinnata seed residue, a cheap inedible bioresource, for both lignocellulases and ethanol production. Spingomonas echinoides and Iprex lacteus were selected as novel sources of lignocellulases during solid state fermentation. Both organisms produced an array of lignocellulases (exoglucanase, endoglucanase, xylanase and laccase). Subsequent hydrolysis of the seed residue for sugars production using this crude enzyme from S. echinoides and I. lacteus were compared with those produced using commercial cellulase from Aspergillus niger (10 U g⁻¹), resulted in sugars yields of 233, 302 and 330 mg g⁻¹, respectively. Ethanol yields of 81.5, 104.5 and 157.6 mg g⁻¹ and ethanol concentrations of 4.0, 5.3 and 7.9 mg mL⁻¹ were achieved from the fermentation of the three hydrolyzates using Saccharomyces cerevisiae. The study demonstrated the feasibility of using the seed residue for enzyme preparation and its subsequent application in hydrolysis of the same seed residue and the potential of using the hydrolysis product for ethanol production.- Ethanol selling price also varies considerably due to market fluctuations. An ethanol price of $0.634 L -1 ($2.20 gallon À1 ) was selected for the baseline scenario [15]. Given price uncertainty, we also looked at how sensitive the optimum enzyme loadings are by varying ethanol price by 20%.
[Show abstract] [Hide abstract] ABSTRACT: Response surface methodology was used to investigate the interaction of pH, temperature, and enzyme loadings on corn stover hydrolysis rates following soaking in aqueous ammonia pretreatment. Economic tradeoffs were estimated for cellulase and hemicellulase loadings under different hydrolysis conditions. Enzyme loadings had a more significant effect on rates than did pH or temperature. The effect of hy-drolysis pH was independent of temperature and enzyme loadings, and the optimal pH for glucose and xylose yields were 4.5 and 4.3, respectively. Conducting hydrolysis at 50 C rather than 37 C enables either a 10% glucose yield increase, or a comparable yield with 40% and 65% reduction in cellulase and hemicellulase loadings, respectively. Although yield models showed that hydrolysis rates increase with higher enzyme loadings, economic models showed that optimal cellulase and hemicellulase loadings were as much as 47% and 23% lower, respectively, than the maximum loadings tested. Optimal enzyme loadings change with fluctuations in enzyme costs and ethanol price, but cellulase loadings were more sensitive to these changes than hemicellulase loadings. Enzyme loadings were also more sensitive to enzyme price at lower processing temperatures. Enzyme loadings can be adjusted to increase return based on enzyme costs, ethanol price, and process temperature.- However, the greatest cost savings associated with CBP will likely be obtained by reducing the costs of saccharification. A detailed cost analysis has been performed for cellulosic ethanol production from corn stover using dilute acid pretreatment, enzymatic saccharifica‐ tion, and cofermentation [37]. In this analysis, on‐site production of fungal enzymes was estimated to contribute $0.34 per gallon of ethanol (assuming enzyme loadings of 20 mg enzyme per gram of biomass), which could in principle be eliminated using a CBP microbe.
- Similar trend was reported in previous studies [2,8,12], confirming a significant positive effect of high-solid, fed-batch SSF on the process economy. Although the MESP in this study is higher than the previously reported MESP of 19.58 Baht/L (approximated 2.1 USD/gal) [12,30] , the MESP in our process may not be comparable with their processes due to the difference in process flowsheet , production scale (demo-scale vs. industrial scale), feedstock composition and raw materials used (sugarcane bagasse vs. corn stover). Nevertheless, the techno-economic framework reveals that the estimated minimal ethanol selling price of high-solid, fed-batch process was still below the current selling price of ethanol from cassava-based process (27.19 baht/L, www.thaiethanol.
[Show abstract] [Hide abstract] ABSTRACT: An efficient simultaneous saccharification and fermentation (SSF) at high-solid loading are keys to the successful commercialization of lignocellulose-based process. In the present work, technological and economical potentials of high-solid SSF for sugarcane bagasse-to-ethanol conversion process [6] was analyzed based on process flowsheet simulation for an estimation of the minimal ethanol selling price (MESP). Based on techno-economic assessment a high-solid SSF process platform for a low-cost lignocellulosic ethanol production was designed composing of (1) yeast consortium for C5 and C6 sugars co-fermentation and (2) cellulase/on-site hemicellulase enzyme mixtures acting synergistically for efficient saccharification. Implementing the integrated SSF process with on-site enzymes and yeast consortium, the MESP could be reduced to as low as 15.7 Baht/L equivalent to 1.66 USD/gal which is a 6% lower than the current market selling price of 1.76 USD/gal. Thus, the on-site enzymes together with cellulase-hemicellulase synergism to lower enzyme demand as well as the yeast consortium technology to increase ethanol titer from C5/C6 co-fermentation would provide economic feasibility for the future cellulosic ethanol production in the industrial scale. Such process platform is also an important strategy for the development of low-cost biorefinery industry that can outperform the current sugar-based process for the production of biofuels.- In 2009, NREL studied the whole slurry enzymatic saccharification of the acid-pretreated corn stover for fermentable sugar production. It reported that when 20 mg cellulase (per 1 g glucan) was applied in the enzymatic hydrolysis process, the cellulose-to-glucose conversion was approached 90 % at 20 wt % total acid-pretreated corn stover, also with the xylan-to-xylose conversion yield reached to 75 % (Humbird et al. 2011). In the lignocellulosic material cell wall, hemicellulose coats the cellulose microfibrils and forms a barrier for efficient cellulose degradation.
[Show abstract] [Hide abstract] ABSTRACT: Every year, an abundance of agricultural wastes are produced after harvest, which are generally burned or discarded in the farmland. Agricultural wastes are an attractive lignocellulosic material for fermentable sugar production (glucose and xylose) since they are polysaccharide-rich resources. Unfortunately, the intimate associations between the main components of the cell wall create barriers for the enzymatic saccharification of cellulose and hemicellulose. Pretreatment plays a critical role in increasing enzymatic saccharification to obtain the glucose and xylose from pretreated agricultural wastes by different enzymes. The present review is a comprehensive evaluation to describe the advancements in pretreatment and enzymatic saccharification processes to produce the fermentable sugars from agricultural wastes. Using these agricultural wastes for the sugar production is ideal for developing bio-based chemicals while converting unwanted agricultural waste streams into valuable resources.- Primary refined samples showed 62.5 % total sugar yields after 144 h enzymatic hydrolysis, which were higher than the secondary refined samples' total sugar yields (50.1 %) after 48 h enzymatic hydrolysis [69]. Tao et al. [70] performed techno-economic analysis of deacetylated, dilute acid-pretreated, and mechanically refined samples based on the experimental data generated by Chen et al. [58], which was compared to a techno-economic analysis that was published by National Renewable Energy Laboratory (NREL) in 2011 [3]. The biggest difference between the 2011 and 2012 experimental designs is the dilute acid conditions.
[Show abstract] [Hide abstract] ABSTRACT: Production of advanced biofuels from woody and herbaceous feedstocks is moving into commercialization. Biomass needs to be pretreated to overcome the physicochemical properties of biomass that hinder enzyme accessibility, impeding the conversion of the plant cell walls to fermentable sugars. Pretreatment also remains one of the most costly unit operations in the process and among the most critical because it is the source of chemicals that inhibit enzymes and microorganisms and largely determines enzyme loading and sugar yields. Pretreatments are categorized into hydrothermal (aqueous)/chemical, physical, and biological pretreatments, and the mechanistic details of which are briefly outlined in this review. To leverage the synergistic effects of different pretreatment methods, conducting two or more pretreatments consecutively has gained attention. Especially, combining hydrothermal/chemical pretreatment and mechanical refining, a type of physical pretreatment, has the potential to be applied to an industrial plant. Here, the effects of the combined pretreatment (combined hydrothermal/chemical pretreatment and mechanical refining) on energy consumption, physical structure, sugar yields, and enzyme dosage are summarized.
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