Centralized versus decentralized biorefinery configurations for cellulosic ethanol: Can we reconcile environmental sustainability and profitability?
This paper compares environmental and profitability outcomes for a centralized biorefinery for cellulosic ethanol that does all processing versus a biorefinery linked to a decentralized array of local depots that pretreat biomass into concentrated briquettes. The analysis uses a spatial bioeconomic model that maximizes profit from crop and energy products, subject to the requirement that the biorefinery must be operated at full capacity. The model draws upon biophysical crop input-output coefficients simulated with the Environmental Policy Integrated Climate (EPIC) model, as well as market input and output prices, spatial transportation costs, ethanol yields from biomass, and biorefinery capital and operational costs. The model was applied to 82 cropping systems simulated across 37 sub-watersheds in a 9-county region of southern Michigan in response to ethanol prices simulated to rise from $1.78 to $3.36 per gallon. Results show that the decentralized local biomass processing depots lead to lower profitability but better environmental performance, due to more reliance on perennial grasses than the centralized biorefinery. Simulated technological improvement that reduces the processing cost and increases the ethanol yield of switchgrass by 17% could cause a shift to more processing of switchgrass, with increased profitability and environmental benefits.
Michigan State University’s invention, Gaseous Ammonia Pretreatment (GAP), uses hot ammonia gas rather than liquid ammonia to pretreat biomass in a reactor. The hot ammonia gas condenses on the biomass and reacts with the water that is used to wet the biomass prior to adding the ammonia gas. During the GAP process, the raw biomass is more uniformly treated by ammonia and requires much shorter pretreatment time. Therefore, this method has the potential to be a less expensive and more effective raw biomass pretreatment process. At present, pretreatment cost is considered one of the primary bottlenecks for the development of biorefinery technology.
Effect of storage conditions on the stability and fermentability of enzymatic lignocellulosic hydrolysate
To minimize the change of lignocellulosic hydrolysate composition during storage, the effects of storage conditions (temperature, pH and time) on the composition and fermentability of hydrolysate prepared from AFEX™ (Ammonia Fiber Expansion – a trademark of MBI, Lansing, MI) pretreated corn stover were investigated. Precipitates formed during hydrolysate storage increased with increasing storage pH and time. The precipitate amount was the least when hydrolysate was stored at 4 °C and pH 4.8, accounting for only 0.02% of the total hydrolysate weight after 3-month storage. No significant changes of NMR (Nuclear Magnetic Resonance) spectra and concentrations of sugars, minerals and heavy metals were observed after storage under this condition. When pH was adjusted higher before fermentation, precipitates also formed, consisting of mostly struvite (MgNH4PO4·6H2O) and brushite (CaHPO4·2H2O). Escherichia coli and Saccharomyces cerevisiae fermentation studies and yeast cell growth assays showed no significant difference in fermentability between fresh hydrolysate and stored hydrolysate.
Proﬁling of diferulates (plant cell wall cross-linkers) using ultrahigh-performance liquid chromatographytandem mass spectrometry
Grasses serve society in myriad ways, providing food in the form of grain (e.g., rice, wheat, maize, and oats), and nutrition for livestock as grain and/or silage. Society has turned increasing attention to developing sustainable resources to provide biomass for conversion to renewable liquid fuels...
Historical Perspective of Ammonia-based Pretreatments:
Plant biomass is a plentiful resource which can be used to co-produce fuels, chemicals and nutritional feed, driving forward a nation’s bio-based economy . However, the costs of chemical/biological processes that facilitate the conversion of plant polymers into monomers and ﬁnally into desired end-products have stymied the development of cellulosic bioreﬁneries . The choice of thermochemical pretreatment has a substantial impact on the economic and environmental viability of bioreﬁneries within the gates of the reﬁnery and beyond . Ammonia ﬁber expansion, or AFEX (a trademark of MBI International, Lansing), is an ammonia-based pretreatment which has shown tremendous promise at cost-effectively reducing the recalcitrance of lignocellulosics towards biologically catalyzed deconstruction into fermentable sugars. Unlike other aqueous pretreatments, AFEX is a dry-to-dry process (i.e., AFEX-treated biomass composition is unchanged) that reduces plant cell-wall recalcitrance through a unique physicochemical mechanism. The aim of this chapter is to explore the recent advances in AFEX and provide a general overview of this technology from a microscopic (physicochemical mechanisms) and macroscopic (distributed regional processing and life-cycle analysis) perspective.
Performance of AFEX™ pretreated rice straw as source of fermentable sugars: the influence of particle size
It is widely believed that reducing the lignocellulosic biomass particle size would improve the biomass digestibility by increasing the total surface area and eliminating mass and heat transfer limitation during hydrolysis reactions. However, past studies demonstrate that particle size influences biomass digestibility to a limited extent. Thus, this paper studies the effect of particle size (milled: 2 mm, 5 mm, cut: 2 cm and 5 cm) on rice straw conversion. Two different Ammonia Fiber Expansion (AFEX) pretreament conditions, AFEX C1 (low severity) and AFEX C2 (high severity) are used to pretreat the rice straw (named as AC1RS and AC2RS substrates respectively) at different particle size
Evaluation of storage methods for the conversion of corn stover biomass to sugars based on steam explosion pretreatment
Effects of dry and wet storage methods without or with shredding on the conversion of corn stover biomass were investigated using steam explosion pretreatment and enzymatic hydrolysis. Sugar conversions and yields for wet stored biomass were obviously higher than those for dry stored biomass. Shredding reduced sugar conversions compared with non-shredding, but increased sugar yields. Glucan conversion and glucose yield for non-shredded wet stored biomass reached 91.5% and 87.6% after 3-month storage, respectively. Data of micro-structure and crystallinity of biomass indicated that corn stover biomass maintained the flexible and porous structure after wet storage, and hence led to the high permeability of corn stover biomass and the high efficiency of pretreatment and hydrolysis. Therefore, the wet storage methods would be desirable for the conversion of corn stover biomass to fermentable sugars based on steam explosion pretreatment and enzymatic hydrolysis.
Comparative metabolic profiling revealed limitations in xylose-fermenting yeast during co-fermentation of glucose and xylose in the presence of inhibitors
During lignocellulosic ethanol fermentation, yeasts are exposed to various lignocellulose-derived inhibitors, which disrupt the efficiency of hexose and pentose co-fermentation. To understand the metabolic response of fermentation microbes to these inhibitors, a comparative metabolomic investigation was performed on a xylose-fermenting Saccharomyces cerevisiae 424A (LNH-ST) and its parental strain 4124 with and without three typical inhibitors (furfural, acetic acid, and phenol). Three traits were uncovered according to fermentation results. First, the growth of strain 424A (LNH-ST) was more sensitive to inhibitors than strain 4124. Through metabolomic analysis, the variance of trehalose, cadaverine, glutamate and γ-aminobutyric acid (GABA) suggested that strain 424A (LNH-ST) had a lower capability to buffer redox changes caused by inhibitors. Second, lower ethanol yield in glucose and xylose co-fermentation than glucose fermentation was observed in strain 424A (LNH-ST), which was considered to be correlated with the generation of xylitol, as well as the reduced levels of lysine, glutamate, glycine and isoleucine in strain 424A (LNH-ST). Accumulation of glycerol, galactinol and mannitol was also observed in strain 424A (LNH-ST) during xylose fermentation. Third, xylose utilization of strain 424A (LNH-ST) was more significantly disturbed by inhibitors than glucose utilization. Through the analysis of fermentation and metabolomic results, it was suggested that xylose catabolism and energy supply, rather than xylose uptake, were the limiting steps in xylose utilization in the presence of inhibitors. Biotechnol. Bioeng. 2013;xxxx: xx–xx. © 2013 Wiley Periodicals, Inc.
The watershed-scale optimized and rearranged landscape design (WORLD) model and local biomass processing depots for sustainable biofuel production: Integrated life cycle assessments
An array of feedstock is being evaluated as potential raw material for cellulosic biofuel production. Thorough assessments are required in regional landscape settings before these feedstocks can be cultivated and sustainable management practices can be implemented. On the processing side, a potential solution to the logistical challenges of large biorefineries is provided by a network of distributed processing facilities called local biomass processing depots. A large-scale cellulosic ethanol industry is likely to emerge soon in the United States. We have the opportunity to influence the sustainability of this emerging industry. The watershed-scale optimized and rearranged landscape design (WORLD) model estimates land allocations for different cellulosic feedstocks at biorefinery scale without displacing current animal nutrition requirements. This model also incorporates a network of the aforementioned depots. An integrated life cycle assessment is then conducted over the unified system of optimized feedstock production, processing, and associated transport operations to evaluate net energy yields (NEYs) and environmental impacts.
A sustainability assessment was conducted in a nine-county region of Michigan for the categories of cellulosic ethanol production, soil characteristics, water quality, and greenhouse gas (GHG) emissions. Making significant changes such as introducing perennial grasses, riparian buffers and double crops in current landscapes provides the largest absolute NEYs of about 53 GJ/ha while also attaining 120% gains in soil organic carbon, 103% lower nitrogen leaching, and 68% reductions in net GHG emissions (compared to a baseline of current conventional landscapes). Interestingly, minimizing certain environmental impacts also provides greater NEYs. © 2013 Society of Chemical Industry and John Wiley & Sons, Ltd
Substrate binding is typically one of the rate-limiting steps preceding enzyme catalytic action during homogeneous reactions. However, interfacial-based enzyme catalysis on insoluble crystalline substrates, like cellulose, has additional bottlenecks of individual biopolymer chain decrystallization from the substrate interface followed by its processive depolymerization to soluble sugars. This additional decrystallization step has ramifications on the role of enzyme–substrate binding and its relationship to overall catalytic efficiency. We found that altering the crystalline structure of cellulose from its native allomorph Iβ to IIII results in 40–50% lower binding partition coefficient for fungal cellulases, but surprisingly, it enhanced hydrolytic activity on the latter allomorph. We developed a comprehensive kinetic model for processive cellulases acting on insoluble substrates to explain this anomalous finding. Our model predicts that a reduction in the effective binding affinity to the substrate coupled with an increase in the decrystallization procession rate of individual cellulose chains from the substrate surface into the enzyme active site can reproduce our anomalous experimental findings.