Improving Sugar Yields and Reducing Enzyme Loadings in the

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Improving Sugar Yields and Reducing Enzyme Loadings in the Deacetylation and Mechanical Refining (DMR) Process through Multistage Disk and Szego Refining and Corresponding TechnoEconomic Analysis Xiaowen Chen,*,† Wei Wang,‡ Peter Ciesielski,‡ Olev Trass,§ Sunkyu Park,∥ Ling Tao,† and Melvin P. Tucker*,† †

National Bioenergy Center, National Renewable Energy Laboratory, 15013 Denver West Parkway, Golden, Colorado 80401, United States ‡ Bioscience Center, National Renewable Energy Laboratory, 15013 Denver West Parkway, Golden, Colorado 80401, United States § Department of Chemical Engineering and Applied Chemistry, University of Toronto, 200 College Street, Toronto, Ontario M5S 3E5, Canada ∥ Department of Forest Biomaterials, North Carolina State University, 2820 Faucette Drive, Campus box 8005, Raleigh, North Carolina 27695, United States S Supporting Information *

ABSTRACT: Deacetylation and mechanical refining (DMR) has the potential to be a highly efficient biochemical conversion process for converting biomass to low toxicity, high concentration sugar streams. To increase the costeffectiveness of the DMR process, improvements in enzymatic sugar yields are needed, in addition to reducing the refining energy consumed, and decreasing the enzyme usage. In this study, a second refining step utilizing a Szego mill was introduced, resulting in significant improvements in sugar yields in enzymatic hydrolysis at equivalent or lower refining energy inputs. The multistage DMR process increased the monomeric glucose and xylose yields to approximately 90% and 84%, respectively, with an energy consumption of 200 kWh/ODMT. SEM imaging revealed that Szego milling caused significant surface disruption and severe maceration and delamination of the biomass structure. Our results show that the DMR process is a very promising process for the biorefinery industry in terms of economic feasibility. KEYWORDS: Deacetylation, Alkaline pretreatment, Mechanical refining, Disk refining, Szego milling, Ethanol, Sugar, Biorefinery, Techno-economic analysis

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dilute acid,3−5 SPORL6 or green liquor pretreatment7−9 can severely deactivate the catalysts and eventually terminate the desired reactions. Therefore, reducing the levels of chemical poisons while achieving high sugar yields at high concentrations are the ultimate goals for conversion of biomass to sugars. However, conventional chemical and thermal pretreatments normally require harsh conditions to overcome biomass recalcitrance to improve biomass enzymatic digestibility, which generates high concentrations of poisons. One method to improve sugar purity is to separate the impurities from the sugar stream through solid/liquid separation followed by ion exchange.10 The large capital investments and expensive

INTRODUCTION Production of fuels and chemicals via renewable lignocellulosic derived biomass sugars is a promising pathway to replace partially current nonsustainable fossil-oil based fuels and chemicals. The research as well as the commercialization of the biochemical biomass to sugar conversion pathways all focus on two major aspects: improving sugar purity and reducing sugar costs. Improving sugar purity is beneficial for downstream sugar utilization and upgrading, either via fermentation, or chemically, using catalysts for conversion of the sugars and upgradable intermediates to fuels and bioproducts. Chemicals, such as furfural and HMF from acid degradation of biomass sugars, or acetic acid from partial saponification of the acetyl substituents on the xylan backbone, significantly inhibit fermentations and reduce product yields.1,2 On the other hand, sulfur introduced by pretreatment processes such as © 2015 American Chemical Society

Received: October 6, 2015 Revised: November 4, 2015 Published: November 17, 2015 324

DOI: 10.1021/acssuschemeng.5b01242 ACS Sustainable Chem. Eng. 2016, 4, 324−333

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ACS Sustainable Chemistry & Engineering Table 1. Compositional Analysis (wt % dry basis) of Native and Deacetylated Corn Stover native corn stover deacetylated corn stover a

ash

lignin

glucan

xylan

galactan

arabinan

acetyl

2.3(0.1)a 0.6(0.2)

14.9(0.0) 12.6(0.4)

36.4(0.0) 43.6(0.1)

30.8(0.4) 33.1(0.1)

1.8(0.0) 1.4(0.0)

3.4(0.0) 2.5(0.1)

2.7(0.2) 0.3(0.2)

±one standard deviation.

pretreated corn stover with a bench scale PFI (Paper and Fiber Research Institute, Stockholm, Sweden) mill as compared to a bench scale disk refiner.14 SEM pictures and other related evidence suggested that the internal surface area and pore volume caused by internal fibrillation resulting from PFI milling increased glucose yields to 80% during enzymatic hydrolysis, whereas disk refining provided less internal fibrillation but more external fibrillation,15 resulting in sugar yields of ∼70% in that earlier publication. These yields were not improved much even though refining energy was further increased by reducing the clearance between refining disks. This result suggested that a mechanical refining technique that provides effective internal fibrillation might be an important factor to achieving higher glucose yields in enzymatic hydrolysis while keeping the refining energy and enzyme loading at lower levels. However, the PFI mill is a batch laboratory refiner that has not been scaled up. To achieve a similar internal fibrillation refining effect in commercial scale refiners, there are multiple alternative pathways: (1) modify the high consistency disk refiner plates with design patterns to enhance the crushing effect that causes internal fibrillation; (2) introduce a different mechanical refiner with similar refining effects as found in PFI milling. We are currently exploring the first option, whereas a search of different refining and crushing mills resulted in the rental and integration of a Szego mill in addition to disk refiner to achieve a multistage mechanical refining option for this process. Other crushing mills are used in the pulp and paper industry and may prove suitable (Fengel and Wegener, 2003).16 The Szego mill, as described in a previous paper, is a continuous wet mill available in commercial scale equipment sizes and throughput.14 The Szego mill is basically a planetary ring-roller mill, generating crushing and shearing forces by the radial acceleration of the rollers and by the high velocity and pressure gradients between the roller ridges and the stator, respectively. The biomass is fed by gravity or pumped into the top of the mill and is discharged out of the bottom. Inside the mill, biomass is mechanically refined by the repeated crushing and shearing forces on biomass trapped between the rollers and internal surface of the cylindrical wall, resulting in fiber cutting, crushing, and internal and external fibrillation. A brief comparison of Szego mill with disk refiner and PFI mill is summarized in Table S1 (see the Supporting Information). In this study, the integration of Szego milling into the DMR process was focused on biomass feeding issues, refining strategy, energy consumptions, the effects on enzymatic hydrolysis, and the effects on final sugar and ethanol costs. Both experiment data and TEA results are provided to evaluate the feasibility of using a Szego mill in applications in the biorefinery industry.

operational costs associated with the purification option make such processes uneconomical at the commercial scale.10 Therefore, in conventional chemical pretreatment, it is difficult to improve sugar purity and reduce sugar costs at the same time. A highly efficient biomass to sugar conversion process known as the deacetylation and mechanical refining (DMR) process has been reported.11 In addition to the minimal usage of chemicals, low energy consumptions, and a simple process design, enzymatic hydrolysis of the DMR processed corn stover solids features high sugar yields at high solids loadings, and result in high sugar concentrations with low levels of toxic compounds and chemicals to poison biological and chemical upgrading catalysts. This is in contrast to enzymatic hydrolysis of dilute acid pretreated slurries where enzymatic hydrolysis yields drop off significantly above 17 wt % solids in enzymatic hydrolysis, and require increasing enzyme loadings and costs to achieve high yields (Kristensen et al., 2009).12 Dilute alkali deacetylation at very mild conditions saponifies acetyl groups from the xylan backbone of biomass, which not only increases the fermentability of the sugar stream that is produced (if acetic acid is carried away in the deacetylation black liquor) but also reduces the recalcitrance of biomass. We have found that the enzymatic hydrolysis yield of native corn stover used in this work is ∼29%. Performing dilute alkali deacetylation only on this corn stover substrate under the conditions used in this work improved enzymatic digestibility to ∼69% without any pretreatment. We have found that disk refining after dilute alkali deacetylation increased the enzymatic hydrolysis yield of the deacetylated corn stover (DCS) solids from 69% to approximately 79% when ∼128 kWh/ODMT of refining energy was applied by the disk refiner. However, sugar yields gradually plateaued with further increases in refining energy, where we found that increasing refining energy from ∼128 to ∼468 kWh/ODMT using a commercial scale disk refiner resulted in minimal enzymatic glucose yield increases from 79% to 83%. In addition, reducing enzyme loadings from 20 to 16 mg protein/g of cellulose resulted in decreasing the enzymatic glucose yields from 82% to 78%, when the substrate was disk refined at ∼212 kWh/ODMT.11 Sugar yields of approximately 80% are cost-effective enough in biofuels production according to the techno-economic analysis (TEA).13 A recent paper showed that the minimum sugar selling price (MSSP) of the DMR process increases linearly with increasing refining energy and enzyme usage, indicating the 2−4% sugar yield increase (79% to 83% for increasing disk refiner input energy from 128 to 468 kWh/ ODMT) will be offset against the increased cost of electricity (2−4 times) and enzymes (1.5−2 times).13 Therefore, to reduce further the MSSP, additional improvements in sugar yields are needed, while keeping the same or reducing the refining energy applied and reducing the enzyme loadings. To accomplish this goal, a better refining and enzymatic yield outcome at lower refining energy consumptions is key to the success of this process. A previous paper showed that increased digestibility was found for mechanically refining dilute acid-

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METHODS AND MATERIALS

Materials. Corn stover harvested in Hurley County, SD, USA in 2009 was supplied by Idaho National Laboratory. Upon receipt at NREL, the corn stover was knife milled (Jordan Reduction Solutions, Birmingham, AL) to pass a 19 mm (0.75 in) round screen and stored 325

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ACS Sustainable Chemistry & Engineering indoors in 200 kg lots in supersacks. The compositional analysis of the native corn stover is shown in Table 1. Deacetylation. Corn stover deacetylation was performed in a 1900 L paddle mixer (American Process Systems, Gurnee, IL). Dry knifemilled corn stover (120 dry kg) was added to the paddle mixer along with a dilute 0.1 M sodium hydroxide solution. The 8 wt % slurry was heated to 80 °C and held for 2 h, and then the deacetylation black liquor was allowed to drain overnight through screens (2 mm wire spacing). Water was added to the mixer to rinse the solids and the rinsewater was then drained through the screens. The solids were pumped to a continuous screw press (Vincent Corp. Model CP10, Tampa, FL) for dewatering to 45 wt % total solids (TS). Nine batches of deacetylated feedstock (∼1000 kg combined) were prepared in this manner, composite samples taken, and then the deacetylated corn stover substrate was sealed in plastic bags, loaded into 55 gal drums, strapped to pallets, and shipped to Andritz’s R&D Facility in Springfield, Ohio for mechanical refining in their 36 in. commercial scale disk refiner. Note that a portion of the mass of the input corn stover solids was solubilized by deacetylation and that the deacetylated solids were only acid impregnated to improve the stability (prevent microbial growth) of the material during the extended time frame required to perform extensive mechanical refining testing. The pH of the entrained liquor in the deacetylated corn stover substrate was measured at pH 3. The compositional analysis of deacetylated corn stover is shown in Table 1. Mechanical Refining. PFI Milling. A PFI mill is a laboratory scale refiner used to beat pulp fiber. It can cause internal fibrillation, which generates smaller diameter fibers and fines and increases void spaces. Approximately 30 g of deacetylated corn stover was placed in the PFI mill and rotation of the rotor causes the deacetylated corn stover slurry to be thrown against the wall of the mill housing (bedplate) by centrifugal forces. As the slurry forms a smooth film against the bedplate the fibers are subjected to impact by the rotating bars of the rotor. The shearing and compression forces produced by the impacts of the rotor bars cause intrafiber bond breaking and internal fibrillation, external fibrillation, and fiber cutting. Disk refined deacetylated corn stover (DRDCS) was subjected to PFI refining at 20% total solids at 4000 revolution counts. The refining experiments were conducted in Dr. Sunkyu Park’s laboratories at North Carolina State University. The PFI milled DRDCS substrate was named as PMDRDCS throughout this study. Small Commercial Scale Disk Refining of Deacetylated Corn Stover. A small, commercial scale 36 in (91 cm) Sprout Model 401 atmospheric double disk refiner was used in this study to investigate the effects of disk refining power consumption on the enzymatic digestibility of deacetylated corn stover. The Durametal 36104 plate pattern used in the Sprout 401 disk refiner consisted of a fine-bar design formulated for fiber strength development in pulping was used to configure the plates in the refiner. A 36 in. disk refiner provides a reliable, accurate, and repeatable size for a refiner for both refining effects and specific electrical refining energy that is being applied to the substrate. And the 36 in. disk refiner is large enough to measure “real” energy usage levels (as compared to small pilot scale machines that was used for our previous work14). Five energy levels were investigated in this study. The operating parameters during refining included energy consumptions with corresponding plate gaps and throughputs were described elsewhere.11 Refining energy is controlled mainly by adjusting refining plate gap and throughput. Decreasing the throughput from 32.0 to 17.3 ODMT/d as well as reducing the plate gap from 1.78 to 0.00 mm, the refining energy increased from ∼128 to ∼468 kWh/ODMT for DCS.11 The disk refined DCS substrates were named as DRDCS throughout this study. Szego Milling. The Szego SM-160 mill features a 160 mm inside diameter heavy walled cylindrical chamber, with three 160 mm long 60 mm diameter steel rollers with helical grooves consisting of 5 mm ridges and 5 mm grooves with 10 mm pitch. The refining of DCS with the Szego mill was performed at a rotational speed of 1160 rpm. It was found necessary to dilute the DCS with deionized water to 10% total solids to facilitate feeding this small mill. The biomass was fed at a rate of approximately 20 oven-dried kg/h. Sequential multiple passes

through the Szego mill was also tested. The schematic diagram of the Szego mill can be found elsewhere.14 The Szego milled substrates were easily dewatered using a close mesh laundry bag by simply squeezing the bag. The Szego milled DCS substrates were named as SMDCS, whereas the Szego milled DRDCS substrates were named as SMDRDCS throughout this study. Measurement of Refining Energy. Energy usage by the Szego mill during the refining experiments was acquired using a Fluke 1735 Power Logger instrument. For each experiment, the mill was run dry for 1 min prior to adding the deacetylated biomass to obtain the baseline. After the baseline was acquired, 6.0 kg of DRDCS substrate at 10 wt % total solids was fed to the Szego mill within 2 to 3 min, and the energy consumed was recorded by the Fluke Power Logger instrument. The refining energy applied to the substrate was calculated by subtraction of the baseline energy when the mill was running idle. Determination of Particle Size. The particle size distribution of the biomass samples was measured using laser diffraction on a Mastersizer 2000 with a Hydro 2000G module (Malvern Instruments). The instrument measures particle sizes over a range from 0.02 to 2000 μm in a recirculating liquid suspension. For the analysis, 0.05−0.2 g of each DMR sample was dispersed in water in a 15 mL centrifuge tube. Thereafter, individual dispersed samples were vortex mixed and transferred to the Hydro 2000G module that contained 0.8 to 1.0 L of deionized water, with a stirrer setting of 600 rpm and a pump setting of 1250 rpm. After 30 s delay, three, 15 s readings (30 s apart) of the circulating samples were acquired and averaged. The volume weighted mean value was used to represent the mean particle diameter (MPD). Each sample was run in triplicate, and MPD is shown as the average of the triplicates. Scanning Electron Microscopy. Imaging by scanning electron microscopy (SEM) was performed using a FEI Quanta 400 FEG instrument (FEI, Hillsboro, OR). Samples were freeze-dried prior to imaging and mounted on aluminum stubs by sprinkling particles onto conductive carbon tape. Unbound particles were removed by applying a nitrogen stream to shear the surface of the tape. Samples were then sputter-coated with 10 nm of gold. Images were obtained using beam accelerating voltages from 10 to 20 keV. Enzymatic Hydrolysis. Low Solids Enzymatic Hydrolysis. The commercial enzyme preparations Novozymes Cellic CTec3 and HTec3 (Novozymes, Franklin, NC, USA) were used to digest the corn stover samples. Digestions were performed in 125 mL Erlenmeyer flasks containing 50 mM citrate buffer, pH 4.8, at biomass loadings of 1.0% glucan (w/v). Digestion conditions were 130 rpm at 50 °C. The digestions were carried out at an enzyme loading of 32 mg protein/g cellulose (21.5 FPU or 0.118 mL/g of cellulose) CTec3 and 5 mg/g cellulose HTec3. Error bars represent ± one standard deviation. Samples were taken after 108 h (4.5 days) and analyzed for sugar release using HPLC. Cellulose conversion was defined as the percentage of glucose released divided by the theoretical maximum. High Solids Enzymatic Hydrolysis. The enzymatic digestibilities of the SM-DRDCS were measured under high solids conditions. Hydrolysis was conducted with whole slurries in 250 mL capped Schott media bottles with a final loading of 60 g at a total solids concentration of 15% or 20% (w/w) insoluble solids. The bottles were autoclaved empty, and then the substrates were manually introduced into the bottles using a small funnel to reach the target total solids concentration of 15% or 20% (w/w) insoluble solids. 2 mL of citrate buffer (pH 5.1, 1.0 M) was added to each flask to help maintain pH at approximately 5.0 throughout the experiments. Enzymatic hydrolysis was started by adding aliquots of enzyme preparations to achieve the target enzyme dosages, then placing the fully loaded and capped bottles in a shaking incubator operating at 150 rpm and 48 °C. A NIST-certified thermometer (Thermo Scientific, Waltham, Massachusetts, United States) immersed in water in an Erlenmeyer flask was used to verify shaker incubator temperature. Duplicate flasks were performed at all enzyme loadings. The experiments were run for 4 days, with time course samples taken once daily throughout the fourday run time. Time zero concentration values were calculated based on composition of the pretreated slurry and then adjusted based on the weight additions of water, citrate buffer, and enzymes added. Final 326

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low flowability of biomass at high solids, biomass bridging, and clogging. The deacetylated corn stover (DCS) was first fed at 40, 20, and finally 10% total solids, where the 40% and 20% solids did not feed in a continuous manner through the small opening in the top of the milling chamber, whereas the 10% solids slurry fed through the mill nicely. Thus, at the higher solids loadings, the continuous Szego mill was functioning as a batch refiner (residence time was measured). However, at 10% solids, the DCS could be discharged freely due to the higher flowability of the biomass slurry after adding more water. It was also found that large corn stover particles (1/4 in.), including kernel residues clogged the grooves of the rollers and prevented the refined biomass from discharging. In addition, an internal baffle (as circled in Figure 1a) prevented continuous flow at the

samples were taken at day four and analyzed for density, total and insoluble solids, as well as monomeric, oligomeric, and calculated total sugar concentrations. Glucose yields during enzymatic saccharification was calculated from the net amount of monomeric glucose produced, which also used measurements of total weight, fraction insoluble solids, and liquor density to calculate liquid volumes in the high solids slurries. The sugar yields discussed in the experiment part were all based on pretreated corn stover. The overall process sugar yields used in the techno-economic analysis were corrected with 10% xylan loss in the deacetylation stage. Techno-Economic Analysis. The TEA model developed for this paper included a conceptual process design using a process flow diagram, detailed process modeling for rigorous calculation of the material, and energy balances using Aspen Plus.17 The resulting capital investment, project, and operating cost estimates were translated into discounted cash flow calculations. From this information, a MSSP and a MESP were established based on a stipulated 10% internal rate of return (IRR). The OPEX calculation for the designed facility was based on material and energy balance calculations using Aspen Plus process simulations.18 Raw material unit costs were based on literature or existing models, summarized in the 2011 NREL cellulosic ethanol design report.17 Major raw materials included sodium hydroxide, sulfuric acid, diammonium phosphate, ammonia, corn steep liquor, purchased sugar for enzyme production, water, and cooling tower chemicals. All costs were inflated to 2011 U.S. dollars using the Plant Cost Index from Chemical Engineering Magazine,19 the Industrial Inorganic Chemical Index from SRI Consulting,20 and the labor indices provided by the U.S. Department of Labor Bureau of Labor Statistics.21 Salaries for personnel were inflated from 2009 dollars to 2011 dollars. Ninety percent of the total salaries are added for labor burden, and 3.0% of the inside boundary limit (ISBL) capital expenses was designated for maintenance. Property insurance and taxes accounted for 0.7% of the fixed capital investment. Material, energy balance, and flow rate information was used to size equipment based on the Aspen Plus simulation of the material and energy balances. CAPEX was calculated from equipment cost obtained from vendor quotations, prior published NREL design reports22,17 or from internal equipment costing databases. For most equipment, scaling factors were applied for variations in the throughput or other key design parameters relative to the original design basis using standard methodologies as described in the NREL 2002 and 2011 design cases.17,23 The discounted cash flow assumed 40% equity financing with a loan interest at 8% for 10 years. Working capital was assumed to be 5% of the fixed capital investment. The plant depreciation period was set for seven years. The plant was assumed to take three years to construct with a quarter of a year spent on start-up. The MSSP and the MESP were the prices at which sugar or ethanol must be sold to reach an IRR of 10%. The purpose of cost analysis for sugar was merely to separate the cost of producing sugars from the downstream costs of producing ethanol or other products. The sugar and ethanol TEA work is based on the design and models discussed in our previous work with the following changes:17 (1) All process design through enzymatic hydrolysis was kept the same for the sugar model. (2) A lignin press with counter-current washing was added after hydrolysis to separate lignin and unreacted insoluble solids from the dilute mixed sugar stream.17 (3) A triple-effect evaporator system was added to the model, with heat input specified to achieve 50% water in the sugar syrup. The MSSP was calculated using dry weight sugar basis, although the sugar syrup product contained 50% water and other nonsugar compounds that may require further cleanup, at additional cost.

Figure 1. Comparison of clogging of rollers by large corn stover particles after Szego milling with DCS and DRDCS at 10% solids. Red circle in panel A shows internal baffle in the rotor.

higher solids. The clogged biomass eventually prevented continuous operation, resulting in lower digestibility of the refined corn stover. To resolve this issue, DRDCS at an energy input of ∼128 kWh/ODMT was used as starting material for Szego milling. At 20% and 40% solids, again low flowability prevented the discharge of the refined DRDCS biomass from the Szego mill. At 10% solids, all of the DRDCS were recovered from the bottom discharge. The clogging by large particles was significantly reduced due to the smaller particle size as shown in Figure 1b. The results suggest that feeding the small Szego mill needs to be conducted at 10% solids or lower to realize continuous refining of biomass without discharging issues. For corn stover with firm corn kernel residues, predisk refining is suggested to reduce the particle size of corn kernels to prevent clogging. In some recent trials, a set of newly designed rollers by removing the baffles (which was originally designed to increase residence time for coal grinding) and with wider and deeper grooves could significantly reduce the possibility of clogging by larger particles and thus improve the milling effect. The 10% Szego milled deacetylated corn stover (SMDCS) was collected in a close mesh Tide washing bag and dewatered to about 23% solids simply by squeezing out the free water manually. Very small quantities of fines were observed to be lost during the dewatering step. The ease of water removal shows that the dilution water added prior to Szego milling could be simply removed and recycled using inexpensive solid− liquid separation equipment such as a wire press or twin roller press that is widely applied in pulp and paper industries. As a result, we speculate that the addition of a Szego milling operation should not increase the amount of water passed onto downstream processing.

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RESULTS AND DISCUSSION Challenges in Continuous Operation of the Szego Mill. One of the more difficult operations of a continuous reactor/refining system is feeding and discharging the reactor under pressure and temperature. For the second stage Szego milling operation, the challenge came from well-known issues: 327

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Figure 2. Enzymatic hydrolysis yields using various substrates including: native corn stover (NCS), deacetylated corn stover (DCS), Szego milled deacetylated corn stover (SMDCS), disk refined deacetylated corn stover (DRDCS), PFI milled predisk refined deacetylated corn stover (PMDRDCS), and Szego milled predisk refined deacetylated corn stover (SM-DRDCS). The digestions were carried out at 1% (w/w) cellulose loadings with an enzyme loading of 32 mg protein/g cellulose CTec3 and 5 mg/g cellulose HTec3. Error bars represent ± one standard deviation.

Effects of Szego Milling on Sugar Yields Using Both DCS and DRDCS. Figure 2 shows the monomeric glucose and xylose yields for enzymatic hydrolysis at 1% cellulose loadings for native corn stover (NCS), DCS, SMDCS, SM-DRDCS substrates. The enzymatic hydrolysis of NCS resulted in 29% glucose and 28% xylose yields, respectively, showing the inherent recalcitrance of the biomass. Deacetylation greatly increased the glucose yield from 29% to approximately 70% and the xylose yield from 28% to 54% without mechanical refining or pretreatment. Szego milling of DCS increased the glucose yields further to 73% to 76% at various solids content at feeding and residence times (as a batch refiner due to discharging issue). Varying the solids content at feeding and residence time of refining showed negligible effects on digestibility. The sugar yields were not increased as expected after Szego milling, most likely due to the high clogging of biomass in the grooves as shown in Figure 1a. DRDCS was used as a feeding material in this study because the smaller particle sizes of the DRDCS substrates could solve the clogging and discharging issues as discussed previously. In a previous paper, single stage DRDCS at refining energies of ∼128 kWh/ODMT without a second stage Szego milling step showed glucose and xylose yields of ∼78% and ∼69%, respectively.11 In this work, the addition of a second stage Szego milling step increased the monomeric glucose yields to approximately 90% and xylose yields to 84%, respectively. The over 10% higher sugar yields of the SM-DRDCS substrate compared to the DRDCS substrate shows that Szego milling could notably increase biomass digestibility even when the substrate was previously disk refined. This improvement could be attributed to a different refining mechanism involved with Szego milling, which the disk refiner did not provide even at refining energies as high as ∼468 kWh/ODMT. We speculate that the crushing and compression effects provided by Szego

milling was probably the major driver for the improved sugar yields. Similar digestibility improvements (89% glucose yield and 79% xylose yield) was also found for predisk refined PFI milled deacetylated corn stover (PM-DRDCS) substrates, where PFI milling was thought to impart a typical compression effect caused by the rotor bars beating the biomass against the inner wall of the bedplate causing internal fibrillation and increasing internal surface area of the fiber structure.14,15 In addition, the shear forces involved with the rotor bars moving past the inner wall of the bedplate with differential speeds causes surface defribillation of the fibers and increases exposed surface area to enzyme accessibility. All the trials resulted in similar sugar yields. The over 90% sugar yields suggests that multiple-stage refining with a first stage disk refiner followed by a second stage Szego milling is a promising technology that a TEA analysis might show is able to justify the costs of the extra refining energy and capital added by incorporating a second stage Szego milling are economical. Effects of Refining Energy of Szego Milling on Sugar Yields. Refining energy plays a key role in the economic feasibility of the mechanical refining application in a biorefinery industry. In our previous work, the TEA analysis showed refining energies over 200 kWh/ODMT will require a bioethanol plant to purchase electricity from the grid due to the high energy demand.13 External energy input not only increases the sugar and final product costs, but also increases the greenhouse gas emissions. In addition, refining energy could also affect the biomass digestibility. Typically, lowering refining energy may lead to lower sugar yields. Therefore, the energy consumption of Szego milling was measured, and the results are shown in Table 2. Table 2 shows the effects of substrate, solids content, and number of passes through the Szego mill on the refining energy consumption. DCS was first fed at 40% into the Szego mill, 328

DOI: 10.1021/acssuschemeng.5b01242 ACS Sustainable Chem. Eng. 2016, 4, 324−333

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ACS Sustainable Chemistry & Engineering

extra refiners and refining energy required, leading to a higher sugar/ethanol production cost. However, multistage disk and Szego milling is attractive because we achieved 90% monomeric glucose and 80% xylose yields, respectively, using the SMDRDCS substrates, even at a lower total refining energy of approximately 205 kWh/ODMT. Nearly 10% sugar yield improvement was achieved by applying an additional 80 kWh/ ODMT to the DRDCS substrate through Szego milling. Apparently, the mechanism and effects of Szego milling on the substrate are different from disk refining. Effects of Mechanical Refining on Biomass Particle Size Distribution and Fiber Structure. The changes in physical properties of the refined corn stover substrates were investigated through laser diffraction measurement for particle size distribution and scanning electron microscopy (SEM) for surface morphology and surface texture. One of the major outcomes of mechanical refining is size reduction of the biomass. Table 3 shows the mean particle size distribution of

Table 2. Energy Consumptions of Szego Milling Using Various Substrates

SMDCS SMDRDCS

pass no.

total solids (feeding %)

energy consumption (kWh/ODMT)

1 2 1 2

40 20 10 10

219 75 68 85

which consumed 220 kWh/ODMT. The SMDCS was then diluted to 20% and fed into the Szego mill again. However, the lower solids still could not make the product flowable enough to be discharged from the bottom of the mill. The energy consumed for the second milling of DCS was measured at ∼75 kWh/ODMT. Our work showed that the refining energy required for Szego milling of the DRDCS substrate was much lower, possibly due to the reduced particle size and lower solids of feeding material. Disk refining at ∼128 kWh/ODMT using the commercial scale disk refiner effectively reduced the particle size of the substrate and thus facilitated the Szego milling. At 10% solids, water lubricated the grinding surface of the Szego mill, which also reduced the refining energy as shown in Table 2. The first and second pass of the DRDCS substrate through the Szego mill consumed 68 and 85 kWh/ODMT, respectively. Figure 3 shows the effect of the combined (total) Szego and disk refining energies on enzymatic hydrolysis yields compared to single stage disk refining. The substrates were all hydrolyzed at an enzyme loading of 16 mg protein/g of cellulose (∼11 FPU or 0.059 mL/g of cellulose) CTec3 and 4 mg protein/g of cellulose HTec3. As a reference, DRDCS at ∼128 and ∼468 kWh/ODMT is presented here along with the SM-DRDCS at 205 (1 pass), 285 (2 passes), and 365 (3 passes) kWh/ODMT. Deacetylated corn stover refined in a single stage commercial scale disk refiner at ∼128 kWh/ODMT yielded approximately 79% monomeric glucose and 71% monomeric xylose yields, respectively. Increasing the refining energy to 468 kWh/ ODMT increased sugar yields by approximately 4 to 6% to ∼82%. According to the previously reported TEA work,13 the marginal sugar yield improvement did not justify the cost of the

Table 3. Volumetric Mean Particle Size of Mechanically Refined Corn Stover

sample ID DRDCS-1 DRDCS-2 PM-DRDCS SM-DRDCS-1 SM-DRDCS-2 SM-DRDCS-3

total specific mechanical energy (kWh/ODMT) 128 468 205 285 365

mean particle size (μm) 302 198 190 218 134 122

glucose monomer yield per unit of energy consumption (1/GJ) 1.7 0.5 1.2 0.9 0.7

mechanically refined deacetylated corn stover substrates using various refining techniques at different energy levels. The NCS used in the current study was knife milled to pass through a 3/4 in. screen. Particle size distribution of both NCS and DCS were not measured because they could not be fed into the Mastersizer 2000 instrument. Disk refining of deacetylated corn stover at ∼128 kWh/ODMT reduced the mean particle size of DCS to approximately 300 μm. Increasing refining energy in the single stage disk refiner to ∼468 kWh/ODMT

Figure 3. Effects of refiner type and refining energy on the enzymatic digestibility of deacetylated corn stover. DRDCS: deacetylation and disk refined corn stover. SM-DRDCS: Szego milled disk refined deacetylated corn stover. Total refining energy includes the energy applied in both 1st stage disk refining and 2nd stage Szego milling. 329

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Figure 4. SEM micrographs of corn stover subjected to various thermochemical and mechanical treatments. (a,b) Deacetylated corn stover particles appear as largely intact cluster of cells with a relatively smooth surface texture. (c,d) Disk refining in addition to deacetylation produces much smaller particles, many of which have been reduced to individual disjoined cells, although some cellular clusters remain intact. The surface texture of the material is substantially rougher than the DCS material. (e,f) Szego milled, predisk refined, deacetylated corn stover also produces very small particles, many of which are individual cells. The surface area is increased dramatically relative to the DCS material, and shows evidence of sever disruption and delamination. (g,h) PFI milled, predisk refined, deacetylated corn stover particles are also much smaller than particles produced by DCS material, and again many individual cells are present. Although the surfaces do not display disruption to the extent of that observed on the SMDRCS particles, the filamentous texture of the cellulose fibrils is more evident in these samples relative to the others, which suggests a large amount of easily accessible cellulose on the particle surface.

caused further size reduction to a mean particle size of approximately 200 μm. PFI milling on predisk refined corn stover (disk refined at ∼128 kWh/ODMT) decreased the mean particle size to approximately 190 μm. Calculating the refining energy imparted by a PFI mill has many measurement difficulties,15,24 and according to Kerekes, unbleached softwood pulp in a PFI mill can consume about 0.19−0.20 kWh/ton-rev. If we assume similar amounts of energy are needed, then the energy in PFI milling of our DRDCS substrate at 4000 rev is ∼760−800 kWh/ODMT, leading to a total refining energy consumption of approximately 890−930 kWh/ODMT including the energy from the disk refining step. Therefore, in addition to not being scalable, PFI mill has much higher power requirements compared to disk refiner and Szego mill. Szego milling also led to significant particle size reduction. As shown in Table 3, the first pass through the Szego mill using a DRDCS substrate lowered the mean particle size to approximately 220 μm, which was about 80 μm reduction compared to the starting substrate DRDCS refined at 128 kWh/ODMT (302 μm). However, it is still 20 μm higher than the DRDCS refined at 468 kWh/ODMT, indicating particle size reduction was not the only factor related to biomass digestibility. Higher sugar yields occurred at larger mean particle size when comparing refining of the SM-DRDCS substrate at ∼205 kWh/ODMT to DRDCS at 468 kWh/ ODMT. A second pass through the Szego mill further reduced the mean particle size to approximately 130 μm and third pass lowered the mean particle size to approximately 120 μm, whereas sugar yields plateau. The finding shows that lower mean particle sizes may not result in increased sugar yields in enzymatic hydrolysis. Apparently Szego milling was more energy efficient for size reduction compared to disk refining under the current experiment conditions. However, it should not be concluded that disk refiner is less efficient because the disk refining experiment was completed at 40% total solids, whereas Szego milling was performed at 10% solids. A large amount of refining energy is lost as heat and steam when refining is done at higher energy levels using substrates at

higher solids. In future work, we would like to explore medium to low consistency (solids) refining as a comparison. Table 3 also shows the mechanical refining energy efficiency using the previously reported “pretreatment energy efficiency” as an evaluation criteria.25 The mechanical energy efficiencies in the DMR process using corn stover are approximately 7−10 times higher compared to that of the SPORL process using softwood and 3−5 times higher using hardwood.25 Biomass variety is the major reason causing the large difference in energy efficiency. Therefore, the economic feasibility of the application of DMR process on woody biomass needs further investigation. The morphology and surface texture of the particles resulting from various chemical and mechanical treatments was investigated by SEM. Low magnification images of the particles are shown in the top row of Figure 4, and higher magnification of the surface texture of the particles are shown in the bottom row. For the high magnification images, care was taken to select fiber cells for imaging to ensure that differences in the surface texture could be attributed to differences in the treatment of the material rather than natural variation between biomass tissue types. Corn stover particles subjected to deacetylation (DCS) are shown in Figure 4a,b, appear similar to those produced by conventional dilute acid pretreatment in terms of general particle morphology,26 and consist of intact clusters of parenchyma cells and vascular bundles. However, disk refining the material in addition to deacetylation produces much smaller particles as shown in Figure 4c. Many of these particles consist of individual cells that have become completely separated from other cells. Such complete disjoining of cells from one another greatly enhances the surface area of the particle ensemble and has been observed previously for corn stover subjected to high degrees of lignin extraction by clean fractionation27 and alkaline pretreatment,28 as well as of Arabidopsis mutants that contain predominantly S-type lignin treated with maleic acid.29 This highly effective means of particle size reduction was observed in corn stover treated by Szego milling, predisk refining, and deacetylation (SM-DRDCS, Figure 4e) as well as PFI milling, predisk refining, and deacetylation (PM-DRDCS, Figure 4g). 330

DOI: 10.1021/acssuschemeng.5b01242 ACS Sustainable Chem. Eng. 2016, 4, 324−333

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ACS Sustainable Chemistry & Engineering

integrating a Szego milling step to reduce enzyme loadings regarding to the economic feasibility will be investigated in the future. Techno-Economic Analysis. A corresponding TEA was performed to investigate the economic viability of the multistage disk refining/Szego milling integration with the current DMR process. Compared to the TEA based on NREL 2012 dilute acid SOT (Tao et al., 2014), the current TEA analysis eliminated the expensive pretreatment reactor and associated OPEX costs of sulfuric acid and ammonia used for neutralizing the acid, but added the costs for the refiners, electrical energy, and water usage. Figure 6 shows the TEA

Substantial differences between the surface textures of the corn stover fiber cells were observed to result from the various mechanical treatments. The surface of the DRDCS substrate (Figure 4b) displayed an increase in surface roughness relative to the DCS sample with some evidence of wall delamination and fibrillation. The SM-DRDCS sample (Figure 4f) showed more evidence of surface disruption of all the samples investigated here, and displayed severe maceration and delamination, which likely greatly increased the nanoscale surface roughness of this material and contributed to its enhanced digestibility. Although the PM-DRDCS sample (Figure 4f) had the most obvious characteristic texture of exposed cellulose fibrils, although it did not display the extent of surface disruption of the SM-DRDCS substrate. This nanofibrillated surface texture has also been observed previously in materials subjected to lignin-extracting pretreatments, and is thought to contribute to enhanced enzymatic conversion.30 In a summary, surface disruption and severe maceration and delamination caused by Szego milling are probably the major driving forces for improved biomass digestibility. Reducing Enzyme Loading by Szego Milling. Enzyme loading also plays a key role in economic feasibility of biochemical conversion of biomass to fuels and chemicals. Previous TEA work revealed that enzyme loading at 20 mg total protein per gram of cellulose (14 FPU or 0.074 mL CTec3 product/g of cellulose) may not be economic because of the large tank volume needed and the associated high cost for air.13 Therefore, we investigated the multistage SM-DRDCS process for potential reductions in enzyme loading. Figure 5 shows the

Figure 6. TEA analysis of the multistage disk refining/Szego milling of deacetylated corn stover (DMR process with corn stover residues.

results using the multistage disk refining/Szego milling process at various enzyme loadings. The calculated MESP is approximately $2.10 for enzyme loadings varying from 10 to 20 mg total protein/g of cellulose. The lower MESP compared to the $2.15 achieved in 2012 NREL dilute acid pretreatment SOT31 is achieved due to increased ethanol yields from the increased sugar yields at higher enzyme loadings. When enzyme loading increased from 10 to 20 mg total protein/g cellulose in the residue, the cost reductions due to higher yields are offset by increased costs of the higher enzyme loadings. In contrast, when the enzyme loading is decreased to 5 mg total protein/g of cellulose, a significant decrease in sugar yields occurs, leading to a 25 cents increase in the calculated MESP.

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CONCLUSIONS We carried out the multistage integration of a Szego milling step into a deacetylation and disk refining (DDR) process in this study. A schematic diagram of the modified process is shown in Figure S1 (see the Supporting Information). We found that incorporating a Szego milling step into the deacetylation and mechanical refining process could effectively improve the enzymatic digestibility while reducing refining energy consumption and enzyme loadings. The investigation using SEM revealed that Szego milling caused significant surface disruption and severe maceration and delamination and internal fibrillation of the biomass ultrastructure, leading to 10% or more sugar yield enhancements on top of the enhancements achieved by disk refining the deacetylated corn stover substrate. The extra refining energy needed for Szego milling was found to be relatively low. The integration of Szego milling with the DDR process also allowed the reduction of enzyme loadings to approximately 10 mg per gram of cellulose, whereas the glucose yields were kept above 80%. This glucose yield is similar to that

Figure 5. Effects of enzyme loading on enzymatic digestibility using the SM-DRDCS-1 substrate.

enzymatic sugar yields using the SM-DRDCS-1 substrate at cellulase/hemicellulase enzyme loadings of 5, 10, 15, and 20 mg of total protein per gram of cellulose in the refined residues. Although the sugar yields decreased with lower enzyme loadings, we obtained ∼80% monomeric glucose and 70% monomeric xylose sugars yields at 10 mg protein loadings (7 FPU or 0.037 mL CTec3 product/g of cellulose). In addition, we found that the total glucose and xylose yields were approximately 90% and 80%, respectively, indicating more fermentable monomeric sugars could potentially be produced if further hydrolysis of the oligomers occurs during fermentation or by using additional accessory enzymes. The impact of 331

DOI: 10.1021/acssuschemeng.5b01242 ACS Sustainable Chem. Eng. 2016, 4, 324−333

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(2) Franden, M. A.; Pienkos, P. T.; Zhang, M. Development of a high-throughput method to evaluate the impact of inhibitory compounds from lignocellulosic hydrolysates on the growth of Zymomonas mobilis. J. Biotechnol. 2009, 144, 259−267. (3) Tucker, M. P.; Kim, K. H.; Newman, M. M.; Nguyen, Q. A. Effects of Temperature and Moisture on Dilute-Acid Steam Explosion Pretreatment of Corn Stover and Cellulase Enzyme Digestibility. Appl. Biochem. Biotechnol. 2003, 105−108, 165−178. (4) Schell, D. J. F. J.; Newman, M.; McMillan, J. D. Dilute-Sulfuric Acid Pretreatment of Corn Stover in Pilot-Scale Reactor: Investigation of Yields, Kinetics, and Enzymatic Digestibilities of Solids. Appl. Biochem. Biotechnol. 2003, 105−108, 69−86. (5) McMillan, J. Bioethanol production: Status and prospects. Renewable Energy 1997, 10 (2−3), 295−302. (6) Zhu, J. Sulfite pretreatment (SPORL) for robust enzymatic saccharification of spruce and red pine. Bioresour. Technol. 2009, 100 (8), 2411−2418. (7) Wu, S.-f.; Chang, H.-m.; Jameel, H.; Philips, R. Novel Green Liquor Pretreatment of Loblolly Pine Chips to Facilitate Enzymatic Hydrolysis into Fermentable Sugars for Ethanol Production. J. Wood Chem. Technol. 2010, 30 (3), 205−218. (8) Yu, Z. Y.; Jameel, H.; Chang, H. M.; Park, S. The effect of delignification of forest biomass on enzymatic hydrolysis. Bioresour. Technol. 2011, 102 (19), 9083−9089. (9) Yoon, S. H.; van Heiningen, A. Green liquor extraction of hemicelluloses from southern pine in an Integrated Forest Biorefinery. J. Ind. Eng. Chem. 2010, 16 (1), 74−80. (10) Aden, A.; Ruth, M.; Ibsen, K.; Jechura, J.; Neeves, K.; Sheehan, J.; Wallace, B.; Montague, L.; Slayton, A.; Lukas, J. Lignocellulosic Biomass to Ethanol Process Design and Economics Utilizing Co-Current Dilute Acid Prehydrolysis and Enzymatic Hydrolysis for Corn Stover; NREL/TP-510-32438; NREL: Golden, CO, 2002. (11) Chen, X.; Shekiro, J.; Pschorn, T.; Sabourin, M.; Tao, L.; Elander, R.; Park, S.; Jennings, E.; Nelson, R.; Trass, O.; Flanegan, K.; Wang, W.; Himmel, M.; Johnson, D.; Tucker, M. A highly efficient dilute alkali deacetylation and mechanical (disc) refining process for the conversion of renewable biomass to lower cost sugars. Biotechnol. Biofuels 2014, 7 (1), 98. (12) Kristensen, J. B.; Felby, C.; Jørgensen, H. Yield-determining factors in high-solids enzymatic hydrolysis of lignocellulose. Biotechnol. Biofuels 2009, 2 (1), 1−10. (13) Chen, X.; Shekiro, J.; Pschorn, T.; Sabourin, M.; Tucker, M.; Tao, L. Techno-economic Analysis (TEA) of the Deacetylation and Disk Refining Process (DDR): Characterizing the Effect of Refining Energy and Enzyme Usage on Minimum Sugar Selling Price (MSSP) and Minimum Ethanol Selling Price (MESP). Biotechnol. Biofuels 2015, DOI: 10.1186/s13068-015-0358-0. (14) Chen, X.; Kuhn, E.; Wang, W.; Park, S.; Flanegan, K.; Trass, O.; Tenlep, L.; Tao, L.; Tucker, M. Comparison of different mechanical refining technologies on the enzymatic digestibility of low severity acid pretreated corn stover. Bioresour. Technol. 2013, 147 (0), 401−408. (15) Kerekes, R. J. Characterizing Refining Action in PFI Mills. TAPPI Paper Summit, Atlanta, GA, March 3−7, 2002. (16) Fengel, D.; Wegener, G. Wood-Chemistry, Ultrastructure, Reactions; W.de.Grupter: New York, 1984. (17) Humbird, D.; Davis, R.; Tao, L.; Kinchin, C.; Hsu, D.; Aden, A.; Schoen, P.; Lukas, J.; Olthof, B.; Worley, M.; Sexton, D.; Dudgeon, D. Process Design and Economics for Biochemical Conversion of Lignocellulosic Biomass to Ethanol: Dilute-Acid Pretreatment and Enzymatic Hydrolysis of Corn Stover; NREL/TP-510-47764; NREL: Golden, CO, 2011. (18) AspenPlus, Release 7.2; Aspen Technology Inc.: Cambridge, MA. 2007. (19) Plant Cost Index. Chemical Engineering Magazine. (20) U.S. Producer Price Indexes − Chemicals and Allied Products/ Industrial Inorganic Chemicals Index. In Chemical Economics Handbook; IHS: Douglas County, CO, 2008.

found with the single-stage disk refining of deacetylated corn stover at double the enzyme loadings. A TEA using an existing ethanol process model17 where the expensive dilute acid pretreatment reactor and OPEX costs were replaced by less expensive mechanical refiners and refining energy consumed, showed that the MESPs for multistage disk refining/Szego milling process was approximately $2.10 at enzyme loadings in the range of 10−20 mg per gram of cellulose. This is about 5 cents lower than that reported in the 2012 DOE bioethanol demonstration via deacetylation and dilute acid pretreatment.31

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.5b01242. Comparison of various mechanical refining equipment including disk refiner, PFI refiner, and Szego mill (Table S1); process flow diagram of the multistage deacetylation and mechanical refining process (Figure S1) (PDF).

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AUTHOR INFORMATION

Corresponding Authors

*Xiaowen Chen. E-mail: [email protected]. Tel: 303547-8705. *Melvin P. Tucker. E-mail: [email protected]. Author Contributions

X.C. designed and conducted deacetylation and mechanical refining work. X.C. was also involved in TEA. W.W. designed and conducted particle size measurement. P.C. conducted SEM experiment. O.T. designed and conducted partial of the Szego milling work. S.P. designed and conducted PFI milling experiment. L.T. conducted partial of the TEA work. M.P.T. led the whole project and helped conduct deacetylation and mechanical refining work. Funding

We greatly appreciate the funding support from USA DOE’s Bioenergy Technology Office. Notes

The authors declare no competing financial interest.

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ABBREVIATIONS DCS = deacetylated corn stover DI = dynamic impregnator DRDCS = disk refined deacetylated corn stover HMF = hydroxymethylfurfural INL = Idaho National Laboratory LAPs = laboratory analytical procedures MPD = mean particle diameter NREL = National Renewable Energy Laboratory PCS = pretreated corn stover PMDRDCS = PFI milled disk refined deacetylated corn stover SEM = scanning electron microscope SMDRDCS = Szego milled disk refined deacetylated corn stover

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REFERENCES

(1) Klinke, H. B.; Thomsen, A.; Ahring, B. K. Inhibition of ethanolproducing yeast and bacteria by degradation products produced during pre-treatment of biomass. Appl. Microbiol. Biotechnol. 2004, 66 (1), 10−26. 332

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Research Article

ACS Sustainable Chemistry & Engineering (21) Bureau of Labor Statistics Data website National Employment, Hours, and Earnings Catalog, Industry: Chemicals and Allied Products, 1980−2009. (22) Tao, L.; Schell, D.; Tan, E. C.; Elander, R.; Bratis, A. Achievement of Ethanol Cost Targets: Biochemical Ethanol Fermentation via Dilute-Acid Pretreatment and Enzymatic Hydrolysis of Corn Stover; NREL/TP-5100-61563; NREL: Golden, CO, 2014. (23) Aden, A.; Ruth, M.; Ibsen, K.; Jechura, J.; Neeves, K.; Sheehan, J.; Wallace, B.; Montague, L.; Slayton, A.; Lukas, J. Lignocellulosic Biomass to Ethanol Process Design and Economics Utilizing Co-Current Dilute Acid Prehydrolysis and Enzymatic Hydrolysis for Corn Stover; NREL/TP-510-32438; NREL: Golden, CO, 2002. (24) Chakraborty, A.; Sain, M. M.; Kortshot, M. T.; Ghosh, S. B. Modeling Energy Consumption for the Generation of Microfibres from Bleached Kraft Pulp Fibres in PFI Mill. Bioresources 2007, 2 (2), 210−222. (25) Zhu, J. Y.; Pan, X.; Zalesny, R., Jr. Pretreatment of woody biomass for biofuel production: energy efficiency, technologies, and recalcitrance. Appl. Microbiol. Biotechnol. 2010, 87 (3), 847−857. (26) Wang, W.; Chen, X.; Donohoe, B.; Ciesielski, P.; Katahira, R.; Kuhn, E.; Kafle, K.; Lee, C.; Park, S.; Kim, S.; Tucker, M.; Himmel, M.; Johnson, D. Effect of mechanical disruption on the effectiveness of three reactors used for dilute acid pretreatment of corn stover Part 1: chemical and physical substrate analysis. Biotechnol. Biofuels 2014, 7 (1), 57. (27) Katahira, R.; Mittal, A.; McKinney, K.; Ciesielski, P. N.; Donohoe, B. S.; Black, S. K.; Johnson, D. K.; Biddy, M. J.; Beckham, G. T. Evaluation of Clean Fractionation Pretreatment for the Production of Renewable Fuels and Chemicals from Corn Stover. ACS Sustainable Chem. Eng. 2014, 2 (6), 1364−1376. (28) Karp, E. M.; Donohoe, B. S.; O’Brien, M. H.; Ciesielski, P. N.; Mittal, A.; Biddy, M. J.; Beckham, G. T. Alkaline Pretreatment of Corn Stover: Bench-Scale Fractionation and Stream Characterization. ACS Sustainable Chem. Eng. 2014, 2 (6), 1481−1491. (29) Ciesielski, P. N.; Resch, M. G.; Hewetson, B.; Killgore, J. P.; Curtin, A.; Anderson, N.; Chiaramonti, A. N.; Hurley, D. C.; Sanders, A.; Himmel, M. E.; Chapple, C.; Mosier, N.; Donohoe, B. S. Engineering plant cell walls: tuning lignin monomer composition for deconstructable biofuel feedstocks or resilient biomaterials. Green Chem. 2014, 16, 2627. (30) Resch, M. G.; Donohoe, B. S.; Ciesielski, P. N.; Nill, J. E.; Magnusson, L.; Himmel, M. E.; Mittal, A.; Katahira, R.; Biddy, M. J.; Beckham, G. T. Clean Fractionation Pretreatment Reduces Enzyme Loadings for Biomass Saccharification and Reveals the Mechanism of Free and Cellulosomal Enzyme Synergy. ACS Sustainable Chem. Eng. 2014, 2 (6), 1377−1387. (31) Tao, L.; Schell, D.; Davis, R.; Tan, E. C.; Elander, R.; Bratis, A. Achievement of Ethanol Cost Targets: Biochemical Ethanol Fermentation via Dilute-Acid Pretreatment and Enzymatic Hydrolysis of Corn Stover; NREL/TP-5100-61563; NREL: Golden, CO, 2012.

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