Process and technoeconomic analysis of leading pretreatment technologies for lignocellulosic ethanol production using switchgrass

Six biomass pretreatment processes to convert switchgrass to fermentable sugars and ultimately to cellulosic ethanol are compared on a consistent basis in this technoeconomic analysis. The six pretreatment processes are ammonia fiber expansion (AFEX), dilute acid (DA), lime, liquid hot water (LHW), soaking in aqueous ammonia (SAA), and sulfur dioxide-impregnated steam explosion (SO(2)). Each pretreatment process is modeled in the framework of an existing biochemical design model so that systematic variations of process-related changes are consistently captured. The pretreatment area process design and simulation are based on the research data generated within the Biomass Refining Consortium for Applied Fundamentals and Innovation (CAFI) 3 project. Overall ethanol production, total capital investment, and minimum ethanol selling price (MESP) are reported along with selected sensitivity analysis. The results show limited differentiation between the projected economic performances of the pretreatment options, except for processes that exhibit significantly lower monomer sugar and resulting ethanol yields.     

Comparative data on effects of leading pretreatments and enzyme loadings and formulations on sugar yields from different switchgrass sources

Dilute sulfuric acid (DA), sulfur dioxide (SO(2)), liquid hot water (LHW), soaking in aqueous ammonia (SAA), ammonia fiber expansion (AFEX), and lime pretreatments were applied to Alamo, Dacotah, and Shawnee switchgrass. Application of the same analytical methods and material balance approaches facilitated meaningful comparisons of glucose and xylose yields from combined pretreatment and enzymatic hydrolysis. Use of a common supply of cellulase, beta-glucosidase, and xylanase also eased comparisons. All pretreatments enhanced sugar recovery from pretreatment and subsequent enzymatic hydrolysis substantially compared to untreated switchgrass. Adding beta-glucosidase was effective early in enzymatic hydrolysis while cellobiose levels were high but had limited effect on longer term yields at the enzyme loadings applied. Adding xylanase improved yields most for higher pH pretreatments where more xylan was left in the solids. Harvest time had more impact on performance than switchgrass variety, and microscopy showed changes in different features could impact performance by different pretreatments.     

Ethanol production from rice straw using optimized aqueous-ammonia soaking pretreatment and simultaneous saccharification and fermentation processes

Rice straw was pretreated using aqueous-ammonia solution at moderate temperatures to enable production of the maximum amount of fermentable sugars from enzymatic hydrolysis. The effects of various operating variables including pretreatment temperature, pretreatment time, the concentration of ammonia and the solid-to-liquid ratio on the degree of lignin removal and the enzymatic digestibility were optimized using response surface methodology. The optimal reaction conditions, which resulted in an enzymatic digestibility of 71.1%, were found to be 69 °C, 10 h and an ammonia concentration of 21% (w/w). The effects of different commercial cellulases and the additional effect of a non-cellulolytic enzyme, xylanase, were also evaluated. Additionally, simultaneous saccharification and fermentation was conducted with rice straw to assess the ethanol production yield and productivity.     

Sugar yields from dilute sulfuric acid and sulfur dioxide pretreatments and subsequent enzymatic hydrolysis of switchgrass

Dacotah switchgrass was pretreated with sulfuric acid concentrations of 0.5, 1.0, and 2.0 wt.% at 140, 160, and 180 °C and with 1 and 3 wt.% sulfur dioxide at 180 °C over a range of times. Sulfur dioxide loadings of 0%, 1%, 3%, 5%, and 10%wt.% of dry biomass were also tested at 180 °C for 10 min. Sugar yields were tracked for pretreatment and subsequent enzymatic hydrolysis to identify conditions for the highest total sugar yields. Pretreatment with 1 wt.% dilute sulfuric acid at 140 °C for 40 min followed by enzymatic hydrolysis with 48.6 mg enzyme/g initial glucan in raw biomass resulted in ～86% of theoretical yield for glucose and xylose combined. For sulfur dioxide pretreatment, the highest total sugar yield of about 87% occurred at 5% SO? for 10 min and 180 °C. However, xylose yields were higher at shorter times and glucose yields at longer times.     

Surface and ultrastructural characterization of raw and pretreated switchgrass

The US Department of Energy-funded Biomass Refining CAFI (Consortium for Applied Fundamentals and Innovation) project has developed leading pretreatment technologies for application to switchgrass and has evaluated their effectiveness in recovering sugars from the coupled operations of pretreatment and enzymatic hydrolysis. Key chemical and physical characteristics have been determined for pretreated switchgrass samples. Several analytical microscopy approaches utilizing instruments in the Biomass Surface Characterization Laboratory (BSCL) at the National Renewable Energy Laboratory (NREL) have been applied to untreated and CAFI-pretreated switchgrass samples. The results of this work have shown that each of the CAFI pretreatment approaches on switchgrass result in different structural impacts at the plant tissue, cellular, and cell wall levels. Some of these structural changes can be related to changes in chemical composition upon pretreatment. There are also apparently different structural mechanisms that are responsible for achieving the highest enzymatic hydrolysis sugar yields.     

Investigation of enzyme formulation on pretreated switchgrass

This work studied the benefits of adding different enzyme cocktails (cellulase, xylanase, β-glucosidase) to pretreated switchgrass. Pretreatment methods included ammonia fiber expansion (AFEX), dilute-acid (DA), liquid hot water (LHW), lime, lime + ball-milling, soaking in aqueous ammonia (SAA), and sulfur dioxide (SO₂). The compositions of the pretreated materials were analyzed and showed a strong correlation between initial xylan composition and the benefits of xylanase addition. Adding xylanase dramatically improved xylan yields for SAA (+8.4%) and AFEX (+6.3%), and showed negligible improvement (0-2%) for the pretreatments with low xylan content (dilute-acid, SO₂). Xylanase addition also improved overall yields with lime + ball-milling and SO₂ achieving the highest overall yields from pretreated biomass (98.3% and 93.2%, respectively). Lime + ball-milling obtained an enzymatic yield of 92.3 kg of sugar digested/kg of protein loaded.     

Efficient pretreatment of lignocellulose in ionic liquids/co-solvent for enzymatic hydrolysis enhancement into fermentable sugars

Ionic liquids (ILs) have been widely used as alternative solvents for biomass pretreatment, however, efficient methods that enable economically use of ILs at large scale have not been established. In this study, a new method in which ILs and polar organic solvents (ILs/co-solvent systems) was proposed for efficient pretreatment of lignocellulosic materials. The combination use of appropriate ILs and organic co-solvents can significantly enhance the solubility of lignocellulose due to the lower viscosity of ILs/co-solvent mixture as compared to those of pure ILs while the hydrogen bond basicity was maintained. In addition, the solubility of lignocellulosic materials in ILs/co-solvent system was found to be correlated with the Kamlet-Taft solvent parameters. Moreover, the use of microwave heating also enhances the efficiency of lignocellulose pretreatment. For example, the microwave-assisted [Emim][OAc]-DMSO (1:1 volume ratio) treated-rice straw could be hydrolyzed at least 22 times faster than that of untreated-rice straw by cellulase from Trichoderma reesei. This enhancement was attributed by several factors including more efficient lignin extraction, less crystalline cellulose and lower residual ILs in treated-rice straw. The produced sugars can be effectively fermented by Pichia stipitis for ethanol production. Moreover, [Emim][OAc]-DMSO mixture could be reused at least 5 times without significantly decrease in effectiveness demonstrated that the use of ILs/co-solvent was potential alternative method for large-scale biomass pretreatment.     

Low temperature alkali pretreatment for improving enzymatic digestibility of sweet sorghum bagasse for ethanol production

A low temperature alkali pretreatment method was proposed for improving the enzymatic hydrolysis efficiency of lignocellulosic biomass for ethanol production. The effects of the pretreatment on the composition, structure and enzymatic digestibility of sweet sorghum bagasse were investigated. The mechanisms involved in the digestibility improvement were discussed with regard to the major factors contributing to the biomass recalcitrance. The pretreatment caused slight glucan loss but significantly reduced the lignin and xylan contents of the bagasse. Changes in cellulose crystal structure occurred under certain treatment conditions. The pretreated bagasse exhibited greatly improved enzymatic digestibility, with 24-h glucan saccharification yield reaching as high as 98% using commercially available cellulase and β-glucosidase. The digestibility improvement was largely attributed to the disruption of the lignin-carbohydrate matrix. The bagasse from a brown midrib (BMR) mutant was more susceptible to the pretreatment than a non-BMR variety tested, and consequently gave higher efficiency of enzymatic hydrolysis.     

Effect of xylanase supplementation of cellulase on digestion of corn stover solids prepared by leading pretreatment technologies

Solids resulting from pretreatment of corn stover by ammonia fiber expansion (AFEX), ammonia recycled percolation (ARP), controlled pH, dilute acid, lime, and sulfur dioxide (SO₂) technologies were hydrolyzed by enzyme cocktails based on cellulase supplemented with β-glucosidase at an activity ratio of 1:2, respectively, and augmented with up to 11.0 g xylanase protein/g cellulase protein for combined cellulase and β-glucosidase mass loadings of 14.5 and 29.0 mg protein (about 7.5 and 15 FPU, respectively)/g of original potential glucose. It was found that glucose release increased nearly linearly with residual xylose removal by enzymes for all pretreatments despite substantial differences in their relative yields. The ratio of the fraction of glucan removed by enzymes to that for xylose was defined as leverage and correlated statistically at two combined cellulase and β-glucosidase mass loadings with pretreatment type. However, no direct relationship was found between leverage and solid features following different pretreatments such as residual xylan or acetyl content. However, acetyl content not only affected how xylanase impacted cellulase action but also enhanced accessibility of cellulose and/or cellulase effectiveness, as determined by hydrolysis with purified CBHI (Cel7A). Statistical modeling showed that cellulose crystallinity, among the main substrate features, played a vital role in cellulase–xylanase interactions, and a mechanism is suggested to explain the incremental increase in glucose release with xylanase supplementation.     

Effect of additives on the digestibility of corn stover solids following pretreatment by leading technologies

Bovine serum albumin (BSA), Tween‐20, and polyethylene glycol (PEG6000) were added to washed corn stover solids produced by ammonia fiber expansion (AFEX), ammonia recycled percolation (ARP), dilute sulfuric acid (DA), lime, controlled pH, and sulfur dioxide (SO₂) pretreatments and to untreated corn stover (UT) and pure Avicel glucan prior to adding cellulase supplemented with β‐glucosidase at an activity ratio of 1:2/g and a moderate enzyme loading of 16.1 mg/g glucan in the raw corn stover. The additives were applied individually at 150, 300, and 600 mg/g glucan in the pretreated solids and in combinations of equal amounts of each that totaled 600 mg/g. The greatest increase in total sugar release was by Tween‐20 with SO₂ pretreated solids followed by PEG6000 with ARP solids and Tween‐20 with lime solids. The effectiveness of the additives was observed to depend on the type of sugars left in the solids, suggesting that it may be more beneficial to use the mixture of these additives to realize a high total sugar yield. In addition, little enhancement in sugar release was possible beyond a loading of 150 mg additives/g glucan for most pretreatments, and combinations did not improve sugar release much over use of additives alone for all except SO₂. Additives were also found to significantly increase concentrations of cellobiose and cellooligomers after 72 h of Avicel hydrolysis. Biotechnol. Bioeng. 2009;102: 1544–1557. © 2008 Wiley Periodicals, Inc.     

Comparative study on chemical pretreatment methods for improving enzymatic digestibility of crofton weed stem

In order to utilize and control the invasive weed, crofton weed (Eupatorium adenophorum Spreng), a potential pathway was proposed by using it as a feedstock for production of fermentable sugars. Three chemical pretreatment methods were used for improving enzymatic saccharification of the weed stem. Mild H2SO4 pretreatment could obtain a relatively high yield of sugars in the pretreatment (32.89%, based on initial holocellulose), however, it led to only a slight enhancement of enzymatic digestibility. NaOH pretreatment could obtain a higher enzymatic conversion ratio of cellulose compared with H2SO4 pretreatment. Peracetic acid (PAA) pretreatment seemed to be the most effective for improving enzymatic saccharification of the weed stem in the three chemical pretreatment methods under the same conditions. The conversion ratio of cellulose in the sample pretreated by PAA under the "optimal" condition was increased to 50% by cellulase loading of 80 FPU/g cellulose for 72 h incubation. A number of empirical quadratic models were successfully developed according to the experimental data to predict the yield of sugar and degree of delignification.     

Comparative studies on hydrothermal pretreatment and enzymatic saccharification of leaves and internodes of alamo switchgrass

Hydrothermal pretreatment was performed on the leaves and internodes portions of Alamo switchgrass, Panicum virgatum L., to enhance the digestibility of cellulose towards cellulase. It was observed that extractives free leaves portion provided 18.1% lower pretreatment gravimetrical yield and 33.8% greater cellulose-to-glucose yield than internodes portion. The degree of polymerization (DP) and ultrastructure of cellulose were determined by gel-permeation chromatography and solid-state cross polarization/magic angle spinning ¹³C NMR experiments. The results suggested that hydrothermal pretreatment hydrolyzed amorphous cellulose and yielded a product enriched in paracrystalline cellulose. Furthermore, the DP of cellulose was reduced to one third of the origin value after hydrothermal pretreatment. The resulting biomass after pretreatment for leaves and internodes has similar cellulose ultrastructure and chemical profiles. The results of the enzymatic hydrolysis studies of cellulose suggest that the reduced DP of cellulose of pretreated switchgrass was an important factor influencing the enhanced digestibility of pretreated switchgrass.     

Comparative sugar recovery and fermentation data following pretreatment of poplar wood by leading technologies

Through a Biomass Refining Consortium for Applied Fundamentals and Innovation among Auburn University, Dartmouth College, Michigan State University, the National Renewable Energy Laboratory, Purdue University, Texas A&M University, the University of British Columbia, and the University of California at Riverside, leading pretreatment technologies based on ammonia fiber expansion, aqueous ammonia recycle, dilute sulfuric acid, lime, neutral pH, and sulfur dioxide were applied to a single source of poplar wood, and the remaining solids from each technology were hydrolyzed to sugars using the same enzymes. Identical analytical methods and a consistent material balance methodology were employed to develop comparative performance data for each combination of pretreatment and enzymes. Overall, compared to data with corn stover employed previously, the results showed that poplar was more recalcitrant to conversion to sugars and that sugar yields from the combined operations of pretreatment and enzymatic hydrolysis varied more among pretreatments. However, application of more severe pretreatment conditions gave good yields from sulfur dioxide and lime, and a recombinant yeast strain fermented the mixed stream of glucose and xylose sugars released by enzymatic hydrolysis of water washed solids from all pretreatments to ethanol with similarly high yields. An Agricultural and Industrial Advisory Board followed progress and helped steer the research to meet scientific and commercial needs. © 2009 American Institute of Chemical Engineers Biotechnol. Prog., 2009     

Comparative efficiency of different pretreatment methods on enzymatic digestibility of Parthenium sp.

The potential of Parthenium sp. as a feedstock for enzymatic saccharification was investigated by using chemical and biological pretreatment methods. Mainly chemical pretreatments (acid and alkali) were compared with biological pretreatment with lignolytic fungi Marasmiellus palmivorus PK-27. Structural and chemical changes as well as crystallinity of cellulose were examined through scanning electron microscopy, fourier transform infra red and X-ray diffraction analysis, respectively after pretreatment. Total reducing sugar released during enzymatic saccharification of pretreated substrates was also evaluated. Among the pretreatment methods, alkali (1 % NaOH) treated substrate showed high recovery of acid perceptible polymerised lignin (7.53 ± 0.5 mg/g) and significantly higher amount of reducing sugar (513.1 ± 41.0 mg/gds) compared to uninoculated Parthenium (163.4 ± 21.2) after 48 h of hydrolysis. This is the first report of lignolytic enzyme production from M. palmivorus, prevalent in oil palm plantations in Malaysia and its application in biological delignification of Parthenium sp. Alkali (1 % NaOH) treatment proves to be the suitable method of pretreatment for lignin recovery and enhanced yield of reducing sugar which may be used for bioethanol production from Parthenium sp.     

Comparative material balances around pretreatment technologies for the conversion of switchgrass to soluble sugars

For this project, six chemical pretreatments were compared for the Consortium for Applied Fundamentals and Innovation (CAFI): ammonia fiber expansion (AFEX), dilute sulfuric acid (DA), lime, liquid hot water (LHW), soaking in aqueous ammonia (SAA), and sulfur dioxide (SO₂). For each pretreatment, a material balance was analyzed around the pretreatment, optional post-washing step, and enzymatic hydrolysis of Dacotah switchgrass.All pretreatments + enzymatic hydrolysis solubilized over two-thirds of the available glucan and xylan. Lime, post-washed LHW, and SO₂ achieved >83% total glucose yields. Lime, post-washed AFEX, and DA achieved >83% total xylose yields. Alkaline pretreatments, except AFEX, solubilized the most lignin and a portion of the xylan as xylo-oligomers. As pretreatment pH decreased, total solubilized xylan and released monomeric xylose increased. Low temperature-long time or high temperature-short time pretreatments are necessary for high glucose release from late-harvest Dacotah switchgrass but high temperatures may cause xylose degradation.     

The study of factors affecting the enzymatic hydrolysis of cellulose after ionic liquid pretreatment

Cellulose resource has got much attention as a promising replacement of fossil fuel. The hydrolysis of cellulose is the key step to chemical product and liquid transportation fuel. In this paper a serials of chloride, acetate, and formate based ionic liquids were used as solvents to dissolve cellulose. The cellulose regenerated from ILs was characterized by FTIR and X-ray powder diffraction. From the characterization and analysis, it was found that the original close and compact structure has changed a lot. After enzymatic hydrolysis, different kinds of ionic liquids (ILs) have different yields of the reducing sugar (TRS). They are 100%, 90.72%, and 88.92% from 1-butyl-3-methylimidazolium chloride ([BMIM]Cl), 1-ethyl-3-methylimidazolium acetate ([EMIM][OAc]), 1-butyl-3-methylimidazolium formate ([BMIM][HCOO]) respectively after enzymatic hydrolysis at 50 °C for 5 h. The results indicated that the yields and the hydrolysis rates were improved apparently after ILs pretreatment comparing with the untreated substrates.     

Enhanced methane production from wool textile residues by thermal and enzymatic pretreatment

Methane production from two types of wool textile wastes (TW1 and TW2) was investigated. To improve the digestibility of these textiles, different pretreatments were applied, and comprised thermal treatment (at 120 °C for 10 min), enzymatic hydrolysis (using an alkaline endopeptidase at different levels of enzymatic loading, at 55 °C for 0, 2, and 8 h), and a combination of these two treatments. Soluble protein concentration and sCOD (soluble chemical oxygen demand) were measured to evaluate the effectivity of the different pretreatment conditions to degrade wool keratin. The sCOD as well as the soluble protein content had increased in both textile samples in comparison to untreated samples, as a response to the different pretreatments indicating breakdown of the wool keratin structure.The combined treatments and the thermal treatments were further evaluated by anaerobic batch digestion assays at 55 °C. Combined thermal and enzymatic treatment of TW1 and TW2 resulted in methane productions of 0.43 N m³/kg VS and 0.27 N m³/kg VS, i.e., 20 and 10 times higher yields, respectively, than that gained from untreated samples. The application of thermal treatment by itself was less effective and resulted in increasing the methane production by 10-fold for TW1 and showing no significant improvement for TW2.     

Application of cellulase and hemicellulase to pure xylan, pure cellulose, and switchgrass solids from leading pretreatments

Accellerase 1000 cellulase, Spezyme CP cellulase, β-glucosidase, Multifect xylanase, and beta-xylosidase were evaluated for hydrolysis of pure cellulose, pure xylan, and switchgrass solids from leading pretreatments of dilute sulfuric acid, sulfur dioxide, liquid hot water, lime, soaking in aqueous ammonia, and ammonia fiber expansion. Distinctive sugar release patterns were observed from Avicel, phosphoric acid swollen cellulose (PASC), xylan, and pretreated switchgrass solids, with accumulation of significant amounts of xylooligomers during xylan hydrolysis. The strong inhibition of cellulose hydrolysis by xylooligomers could be partially attributed to the negative impact of xylooligomers on cellulase adsorption. The digestibility of pretreated switchgrass varied with pretreatment but could not be consistently correlated to xylan, lignin, or acetyl removal. Initial hydrolysis rates did correlate well with cellulase adsorption capacities for all pretreatments except lime, but more investigation is needed to relate this behavior to physical and compositional properties of pretreated switchgrass.     

Effect of ozonolysis pretreatment on enzymatic digestibility of wheat and rye straw

Wheat and rye straws were pretreated with ozone to increase the enzymatic hydrolysis extent of potentially fermentable sugars. Through a 2(5-1) factorial design, this work studies the influence of five operating parameters (moisture content, particle size, ozone concentration, type of biomass and air/ozone flow rate) on ozonization pretreatment of straw in a fixed bed reactor under room conditions. The acid insoluble lignin content of the biomass was reduced in all experiments involving hemicellulose degradation. Near negligible losses of cellulose were observed. Enzymatic hydrolysis yields of up to 88.6% and 57% were obtained compared to 29% and 16% in non-ozonated wheat and rye straw respectively. Moisture content and type of biomass showed the most significant effects on ozonolysis. Additionally, ozonolysis experiments in basic medium with sodium hydroxide evidenced a reduction in solubilization and/or degradation of lignin and reliable cellulose and hemicellulose degradation.     

Influence of pyrolysis condition on switchgrass bio-oil yield and physicochemical properties

The poor and inconsistent physicochemical properties of bio-oil are inhibiting its industrialized production. We investigated the variability in properties of switchgrass bio-oil produced at three pyrolysis temperatures (T = 450, 500, and 550 °C) and three feedstock moisture contents (MC = 5%, 10%, and 15%) in a 3 × 3 factorial experiment in order to exploit opportunities to improve bio-oil properties through optimization of pyrolysis parameters. Results showed that even with the single type of feedstock and pyrolysis system, the two main factors and their interaction caused large variations in bio-oil yield and most of the measured physicochemical properties. Following improvements of bio-oil properties could be individually achieved by selecting an optimal pyrolysis condition (shown in parenthesis) comparing with the worst case: increase of bio-oil yield by more than twofold (MC = 10%, T = 450 °C), increase of pH by 20.4% from 2.74 to 3.3 (MC = 10%, T = 550 °C), increase of higher heating value by 18.1% from 16.6 to 19.6 MJ/kg (MC = 10%, T = 450 °C), decrease of density by 5.9% from 1.18 to 1.11 g/cm³ (MC = 5%, T = 550 °C), decrease of water content by 36% from 31.4 to 20.1 wt.% (MC = 5%, T = 450 °C), decrease of viscosity by 40% from 28.2 to 17 centistokes (MC = 5%, T = 550 °C), decrease of solid content by 57% from 2.86 to 1.23 wt.% (MC = 15%, T = 550 °C), and decrease of ash content by 41.9% from 0.62 to 0.36 wt.% (MC = 15%, T = 550 °C). There is no single, clear-cut optimal condition that can satisfy the criteria for a bio-oil product with all the desired properties. Trade-offs should be balanced according to the usage of the end-products.