Techno-economic comparison of centralized versus decentralized biorefineries for two alkaline pretreatment processes

In this work, corn stover subjected to ammonia fiber expansion (AFEX™)¹ pretreatment or alkaline pre-extraction followed by hydrogen peroxide post-treatment (AHP pretreatment) were compared for their enzymatic hydrolysis yields over a range of solids loadings, enzymes loadings, and enzyme combinations. Process techno-economic models were compared for cellulosic ethanol production for a biorefinery that handles 2000tons per day of corn stover employing a centralized biorefinery approach with AHP or a de-centralized AFEX pretreatment followed by biomass densification feeding a centralized biorefinery. A techno-economic analysis (TEA) of these scenarios shows that the AFEX process resulted in the highest capital investment but also has the lowest minimum ethanol selling price (MESP) at $2.09/gal, primarily due to good energy integration and an efficient ammonia recovery system. The economics of AHP could be made more competitive if oxidant loadings were reduced and the alkali and sugar losses were also decreased.

Evolved strains of Scheffersomyces stipitis achieving high ethanol productivity on acid- and base-pretreated biomass hydrolyzate at high solids loading

BACKGROUND

Lignocellulosic biomass is an abundant, renewable feedstock useful for the production of fuel-grade ethanol via the processing steps of pretreatment, enzyme hydrolysis, and microbial fermentation. Traditional industrial yeasts do not ferment xylose and are not able to grow, survive, or ferment in concentrated hydrolyzates that contain enough sugar to support economical ethanol recovery since they are laden with toxic byproducts generated during pretreatment.

RESULTS

Repetitive culturing in two types of concentrated hydrolyzates was applied along with ethanol-challenged xylose-fed continuous culture to force targeted evolution of the native pentose fermenting yeast Scheffersomyces (Pichia) stipitis strain NRRL Y-7124 maintained in the ARS Culture Collection, Peoria, IL. Isolates collected from various enriched populations were screened and ranked based on relative xylose uptake rate and ethanol yield. Ranking on hydrolyzates with and without nutritional supplementation was used to identify those isolates with best performance across diverse conditions.

CONCLUSIONS

Robust S. stipitis strains adapted to perform very well in enzyme hydrolyzates of high solids loading ammonia fiber expansion-pretreated corn stover (18% weight per volume solids) and dilute sulfuric acid-pretreated switchgrass (20% w/v solids) were obtained. Improved features include reduced initial lag phase preceding growth, significantly enhanced fermentation rates, improved ethanol tolerance and yield, reduced diauxic lag during glucose-xylose transition, and ability to accumulate >40 g/L ethanol in <167 h when fermenting hydrolyzate at low initial cell density of 0.5 absorbance units and pH 5 to 6.

Sugar loss and enzyme inhibition due to oligosaccharide accumulation during high solids-loading enzymatic hydrolysis

BACKGROUND

Accumulation of recalcitrant oligosaccharides during high-solids loading enzymatic hydrolysis of cellulosic biomass reduces biofuel yields and increases processing costs for a cellulosic biorefinery. Recalcitrant oligosaccharides in AFEX-pretreated corn stover hydrolysate accumulate to the extent of about 18-25 % of the total soluble sugars in the hydrolysate and 12-18 % of the total polysaccharides in the inlet biomass (untreated), equivalent to a yield loss of about 7-9 kg of monomeric sugars per 100 kg of inlet dry biomass (untreated). These oligosaccharides represent a yield loss and also inhibit commercial hydrolytic enzymes, with both being serious bottlenecks for economical biofuel production from cellulosic biomass. Very little is understood about the nature of these oligomers and why they are recalcitrant to commercial enzymes. This work presents a robust method for separating recalcitrant oligosaccharides from high solid loading hydrolysate in gramme quantities. Composition analysis, recalcitrance study and enzyme inhibition study were performed to understand their chemical nature.

RESULTS

Oligosaccharide accumulation occurs during high solid loading enzymatic hydrolysis of corn stover (CS) irrespective of using different pretreated corn stover (dilute acid: DA, ionic liquids: IL, and ammonia fibre expansion: AFEX). The methodology for large-scale separation of recalcitrant oligosaccharides from 25 % solids-loading AFEX-corn stover hydrolysate using charcoal fractionation and size exclusion chromatography is reported for the first time. Oligosaccharides with higher degree of polymerization (DP) were recalcitrant towards commercial enzyme mixtures [Ctec2, Htec2 and Multifect pectinase (MP)] compared to lower DP oligosaccharides. Enzyme inhibition studies using processed substrates (Avicel and xylan) showed that low DP oligosaccharides also inhibit commercial enzymes. Addition of monomeric sugars to oligosaccharides increases the inhibitory effects of oligosaccharides on commercial enzymes.

CONCLUSION

The carbohydrate composition of the recalcitrant oligosaccharides, ratios of different DP oligomers and their distribution profiles were determined. Recalcitrance and enzyme inhibition studies help determine whether the commercial enzyme mixtures lack the enzyme activities required to completely de-polymerize the plant cell wall. Such studies clarify the reasons for oligosaccharide accumulation and contribute to strategies by which oligosaccharides can be converted into fermentable sugars and provide higher biofuel yields with less enzyme.

Novel thermostable endo-xylanase cloned and expressed from bacterium Geobacillus sp. WSUCF1

A gene encoding a GH10 endo-xylanase from Geobacillus sp. WSUCF1 was cloned and expressed in Escherichia coli. Recombinant endo-xylanase (37kDa) exhibited high specific activity of 461.0U/mg of protein. Endo-xylanase was optimally active on birchwood xylan at 70°C and pH 6.5. The endo-xylanase was found to be highly thermostable at 50 and 60°C, retaining 82% and 50% of its original activity, respectively, after 60h. High xylan conversions (92%) were obtained with oat-spelt xylan hydrolysis. Higher glucan and xylan conversions were obtained on AFEX-treated corn stover with an enzyme cocktail containing WSUCF1 endo-xylanase (71% and 47%) as compared to enzyme cocktail containing commercial fungal endo-xylanase (64% and 41%). High specific activity, active at high pH's, wide substrate specificity, and higher hydrolytic activity on recalcitrant lignocellulose, make this endo-xylanase a suitable candidate for biofuel and bioprocess industries.

A comparative study of ethanol production using dilute acid, ionic liquid and AFEX™ pretreated corn stover

BACKGROUND

In a biorefinery producing cellulosic biofuels, biomass pretreatment will significantly influence the efficacy of enzymatic hydrolysis and microbial fermentation. Comparison of different biomass pretreatment techniques by studying the impact of pretreatment on downstream operations at industrially relevant conditions and performing comprehensive mass balances will help focus attention on necessary process improvements, and thereby help reduce the cost of biofuel production.

RESULTS

An on-going collaboration between the three US Department of Energy (DOE) funded bioenergy research centers (Great Lakes Bioenergy Research Center (GLBRC), Joint BioEnergy Institute (JBEI) and BioEnergy Science Center (BESC)) has given us a unique opportunity to compare the performance of three pretreatment processes, notably dilute acid (DA), ionic liquid (IL) and ammonia fiber expansion (AFEX(TM)), using the same source of corn stover. Separate hydrolysis and fermentation (SHF) was carried out using various combinations of commercially available enzymes and engineered yeast (Saccharomyces cerevisiae 424A) strain. The optimal commercial enzyme combination (Ctec2: Htec2: Multifect Pectinase, percentage total protein loading basis) was evaluated for each pretreatment with a microplate-based assay using milled pretreated solids at 0.2% glucan loading and 15 mg total protein loading/g of glucan. The best enzyme combinations were 67:33:0 for DA, 39:33:28 for IL and 67:17:17 for AFEX. The amounts of sugar (kg) (glucose: xylose: total gluco- and xylo-oligomers) per 100 kg of untreated corn stover produced after 72 hours of 6% glucan loading enzymatic hydrolysis were: DA (25:2:2), IL (31:15:2) and AFEX (26:13:7). Additionally, the amounts of ethanol (kg) produced per 100 kg of untreated corn stover and the respective ethanol metabolic yield (%) achieved with exogenous nutrient supplemented fermentations were: DA (14.0, 92.0%), IL (21.2, 93.0%) and AFEX (20.5, 95.0%), respectively. The reason for lower ethanol yield for DA is because most of the xylose produced during the pretreatment was removed and not converted to ethanol during fermentation.

CONCLUSIONS

Compositional analysis of the pretreated biomass solids showed no significant change in composition for AFEX treated corn stover, while about 85% of hemicellulose was solubilized after DA pretreatment, and about 90% of lignin was removed after IL pretreatment. As expected, the optimal commercial enzyme combination was different for the solids prepared by different pretreatment technologies. Due to loss of nutrients during the pretreatment and washing steps, DA and IL pretreated hydrolysates required exogenous nutrient supplementation to ferment glucose and xylose efficiently, while AFEX pretreated hydrolysate did not require nutrient supplementation.

Probing the nature of AFEX-pretreated corn stover derived decomposition products that inhibit cellulase activity

Sequential fractionation of AFEX-pretreated corn stover extracts was carried out using ultra-centrifugation, ultra-filtration, and solid phase extraction to isolate various classes of pretreatment products to evaluate their inhibitory effect on cellulases. Ultra-centrifugation removed dark brown precipitates that caused no appreciable enzyme inhibition. Ultra-filtration of ultra-centrifuged AFEX-pretreated corn stover extractives using a 10 kDa molecular weight cutoff (MWCO) membrane removed additional high molecular weight components that accounted for 24-28% of the total observed enzyme inhibition while a 3 kDa MWCO membrane removed 60-65%, suggesting significant inhibition is caused by oligomeric materials. Solid phase extraction (SPE) of AFEX-pretreated corn stover extractives after ultra-centrifugation removed 34-43% of the inhibition; ultra-filtration with a 5 kDa membrane removed 44-56% of the inhibition and when this ultra-filtrate was subjected to SPE a total of 69-70% of the inhibition were removed. Mass spectrometry found several phenolic compounds among the hydrophobic inhibition removed by SPE adsorption.

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.

Proteomics-based compositional analysis of complex cellulase-hemicellulase mixtures

Efficient deconstruction of cellulosic biomass to fermentable sugars for fuel and chemical production is accomplished by a complex mixture of cellulases, hemicellulases, and accessory enzymes (e.g., >50 extracellular proteins). Cellulolytic enzyme mixtures, produced industrially mostly using fungi like Trichoderma reesei, are poorly characterized in terms of their protein composition and its correlation to hydrolytic activity on cellulosic biomass. The secretomes of commercial glycosyl hydrolase-producing microbes was explored using a proteomics approach with high-throughput quantification using liquid chromatography-tandem mass spectrometry (LC-MS/MS). Here, we show that proteomics-based spectral counting approach is a reasonably accurate and rapid analytical technique that can be used to determine protein composition of complex glycosyl hydrolase mixtures that also correlates with the specific activity of individual enzymes present within the mixture. For example, a strong linear correlation was seen between Avicelase activity and total cellobiohydrolase content. Reliable, quantitative and cheaper analytical methods that provide insight into the cellulosic biomass degrading fungal and bacterial secretomes would lead to further improvements toward commercialization of plant biomass-derived fuels and chemicals.

Hemicellulases and auxiliary enzymes for improved conversion of lignocellulosic biomass to monosaccharides

BACKGROUND

High enzyme loading is a major economic bottleneck for the commercial processing of pretreated lignocellulosic biomass to produce fermentable sugars. Optimizing the enzyme cocktail for specific types of pretreated biomass allows for a significant reduction in enzyme loading without sacrificing hydrolysis yield. This is especially important for alkaline pretreatments such as Ammonia fiber expansion (AFEX) pretreated corn stover. Hence, a diverse set of hemicellulases supplemented along with cellulases is necessary for high recovery of monosaccharides.

RESULTS

The core fungal cellulases in the optimal cocktail include cellobiohydrolase I [CBH I; glycoside hydrolase (GH) family 7A], cellobiohydrolase II (CBH II; GH family 6A), endoglucanase I (EG I; GH family 7B) and β-glucosidase (βG; GH family 3). Hemicellulases tested along with the core cellulases include xylanases (LX1, GH family 10; LX2, GH family 10; LX3, GH family 10; LX4, GH family 11; LX5, GH family 10; LX6, GH family 10), β-xylosidase (LβX; GH family 52), α-arabinofuranosidase (LArb, GH family 51) and α-glucuronidase (LαGl, GH family 67) that were cloned, expressed and/or purified from different bacterial sources. Different combinations of these enzymes were tested using a high-throughput microplate based 24 h hydrolysis assay. Both family 10 (LX3) and family 11 (LX4) xylanases were found to most efficiently hydrolyze AFEX pretreated corn stover in a synergistic manner. The optimal mass ratio of xylanases (LX3 and LX4) to cellulases (CBH I, CBH II and EG I) is 25:75. LβX (0.6 mg/g glucan) is crucial to obtaining monomeric xylose (54% xylose yield), while LArb (0.6 mg/g glucan) and LαGl (0.8 mg/g glucan) can both further increase xylose yield by an additional 20%. Compared with Accellerase 1000, a purified cocktail of cellulases supplemented with accessory hemicellulases will not only increase both glucose and xylose yields but will also decrease the total enzyme loading needed for equivalent yields.

CONCLUSIONS

A diverse set of accessory hemicellulases was found necessary to enhance the synergistic action of cellulases hydrolysing AFEX pretreated corn stover. High glucose (around 80%) and xylose (around 70%) yields were achieved with a moderate enzyme loading (~20 mg protein/g glucan) using an in-house developed cocktail compared to commercial enzymes.

Switchgrass water extracts: extraction, separation and biological activity of rutin and quercitrin

Switchgrass (Panicum virgatum L.) has recently received significant attention as a possible feedstock for the production of liquid fuels such as ethanol. In addition, switchgrass may also be a source of valuable co-products, such as antioxidants, and our laboratory recently reported that switchgrass contains policosanols and alpha-tocopherol. Motivation for this work began when a switchgrass sample was extracted with water at 50 degrees C and was then tested for low-density lipoprotein (LDL) oxidation inhibition activity using the Thiobarbituric Acid Reactive Substances (TBARS) assay. The TBARS results showed that the switchgrass water extracts inhibited LDL oxidation by as much as 70% in comparison to the control. Liquid chromatography coupled with mass spectrometry (LC-MS) and high performance liquid chromatography (HPLC) were used to identify the compounds that were responsible for LDL oxidation inhibition activity as flavonoids: quercitrin (quercetin-3-O-rhamnoside) and rutin (quercetin-3-O-rutinoside). To maximize flavonoid concentrations, switchgrass was then extracted with water and 60% methanol at different temperatures. The 60% methanol treatment resulted in higher rutin and quercitrin yields when compared to water-only extraction; however, the use of this solvent would not be practical with current biorefinery technology. Centrifugal partition chromatography (CPC) was then used to purify rutin and quercitrin from the switchgrass water extract, which were then tested via the TBARS assay and shown to exhibit lipid peroxidation inhibition activity similar to that obtained with pure flavonoid standards. This is the first report on the presence of rutin and quercitrin in switchgrass. The results support the extraction of viable coproducts from switchgrass prior to conversion to liquid fuel.

Policosanol, alpha-tocopherol, and moisture content as a function of timing of harvest of Switchgrass (Panicum virgatum L.)

Using switchgrass ( Panicum virgatum L.) as a cellulosic feedstock for the production of ethanol could lead to the extraction of co-products prior to the pretreatment step, thereby adding value to the ethanol conversion process. Policosanols, registered as 142583-61-7, are present in Poaceae and are a mixture of long-chained primary alcohols. Policosanols are composed mainly of docosanol (C(22)), tetracosanol (C(24)), hexacosanol (C(26)), octacosanol (C(28)), triacontanol (C(30)), and dotriacontanol (C(32)). This study determined changes in moisture, policosanol, and alpha-tocopherol concentrations of Cave-in-Rock and Blackwell switchgrass cultivars during maturation from July to December in Arkansas and Oklahoma. Moisture content on a dry weight basis declined from 150 to 50% with progressive harvests. The total policosanol concentration ranged between 89 mg/kg for July harvested Cave-in-Rock switchgrass from Arkansas and 182 mg/kg for August harvested Cave-in-Rock switchgrass for Oklahoma, and these values remained relatively constant throughout the season. This is the first report on the presence of policosanols in switchgrass. Total switchgrass policosanol concentrations were lower than those typically reported for sorghum grains; however, switchgrass-extracted policosanols contained different policosanol ratios, wherein C(30) and C(32) alcohol ranges were 36-41 and 43-50%, respectively. alpha-Tocopherol extracted from both switchgrass cultivars varied between 320 and 400 mg/kg but decreased in the October harvest after frost.