Pilot-Scale Batch Alkaline Pretreatment of Corn Stover - ACS

Dec 18, 2015 - Recently, bench-scale alkaline pretreatment of extractives-free corn stover was performed over a wide range of conditions more severe t...
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Pilot-Scale Batch Alkaline Pretreatment of Corn Stover Erik M. Kuhn,*,† Marykate H. O’Brien,† Peter N. Ciesielski,‡ and Daniel J. Schell† †

National Bioenergy Center, National Renewable Energy Laboratory, 16253 Denver West Parkway, Golden, Colorado 80401, United States ‡ Biosciences Center, National Renewable Energy Laboratory, 16253 Denver West Parkway, Golden, Colorado 80401, United States S Supporting Information *

ABSTRACT: The goal of biomass pretreatment is to increase the enzymatic digestibility of the plant cell wall polysaccharides to produce sugars for upgrading to biofuels. Alkaline pretreatment has the ability to solubilize much of the lignin in biomass while the carbohydrates remain insoluble. With an increased research focus to produce high-value products from lignin, a low molecular weight, lignin-rich stream in a biorefinery is desirable. This work reports on batch alkaline pretreatment of corn stover conducted using a three-factor, two-level central composite experimental design in a pilot-scale reactor to determine the relationship between sodium hydroxide (NaOH) loading, temperature, and anthraquinone (AQ) charge on solids solubilization, component yields, and enzymatic digestibility of the residual solids. Operating conditions were 100 to 140 °C, 40 to 70 mg NaOH/g dry corn stover, and 0.05% to 0.2% (w/w) AQ loading. An enzymatic hydrolysis screening study was performed at 2% cellulose loading. Empirical modeling results showed that NaOH loading and temperature are both significant factors, solubilizing 15% to 35% of the solids and up to 54% of the lignin. Enzymatic hydrolysis of the residual solids produced good monomeric glucose (>90%) and xylose (>70%) yields at the more severe pretreatment conditions. We also found that the AQ charge was not a significant factor at the conditions studied, so efforts to reduce xylan and increase lignin solubilization using this compound were not successful. While good lignin solubilization was achieved, effectively recovering this stream remains a challenge, and demonstrating performance in continuous reactors is still needed. KEYWORDS: Lignocellulose, Pretreatment, Alkaline, Sodium hydroxide, Lignin, Scale-up, Enzymatic hydrolysis

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INTRODUCTION Applied biorefinery research and development (R&D) using lignocellulosic feedstocks continues to gain support due to energy security and environmental concerns and the need to responsibly develop rural regions. The Intergovernmental Panel on Climate Change (IPCC) Contribution of Working Group II to the IPCC’s Fifth Assessment Report states that “managing the risks of climate change involves adaptation and mitigation decisions with implications for future generations, economies, and environments.” According to the report, the anthropogenic impact on the climate is undeniable, and a portfolio of mitigation strategies is vital to address future impacts of climate change, including the development and deployment of sustainable biofuels and bioproducts.1 The U.S. Department of Energy’s (DOE) Office of Energy Efficiency and Renewable Energy (EERE) has the goal of reducing greenhouse gas (GHG) emissions in the range of 17% and oil imports by 50% by 2020, and the EERE budget has doubled since 2006.2 In the same report, EERE outlines a three-phase investment pathway to identify, develop, and deploy novel renewable energy technologies with the goal of industry adoption in competitive markets. One of the investments that EERE has made since © XXXX American Chemical Society

2006 is the Integrated Biorefinery Research Facility (IBRF) expansion project detailed in the National Renewable Energy Laboratory (NREL) Ten-Year Plan: FY2007-FY2018.3 The applied R&D capabilities at the IBRF play an integral role in phase two of the EERE investment strategy, bioprocess development, and demonstration to drive down fuel production price and become cost competitive in the open market. This was previously performed by demonstrating cost-competitive cellulosic ethanol at the IBRF,4 and there are other, recently commissioned commercial cellulosic ethanol plants in operation.5 Many of the current lignocellulosic biofuels production strategies involve pretreatment and saccharification to produce a sugar-rich intermediate stream that is upgraded through fermentation.6−11 The lignin is generally a waste carbon stream and is burned to generate electricity and heat to run the process.4,12 If the goal of the U.S. Energy Independence and Security Act (EISA) of 2007 is met, 79 billion liters of biofuels Received: September 8, 2015 Revised: December 8, 2015

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DOI: 10.1021/acssuschemeng.5b01041 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering

ionic liquids in a manner that renders the solvents reusable. Research has been performed to address this issue, but more work is needed to solve the problem.33−35 In producing cellulosic ethanol, a specific mild alkaline treatment known as deacetylation has been employed, where corn stover was soaked in 0.4% (w/w) sodium hydroxide (NaOH) at 7% total solids (TS; hereafter these values are always reported as weight percent) and 80 °C for 2 h prior to dewatering then performing dilute acid pretreatment, enzymatic hydrolysis, and fermentation.36 The process removed 80% to 90% of the acetate, less than 5% of the xylan, and 11% to 22% of the lignin. Several benefits to cellulosic ethanol production were observed by implementing deacetylation including higher xylan to xylose yields from dilute-acid pretreatment at lower severities than without deacetylation, enhanced glucose yields from enzymatic hydrolysis, and reduced toxicity due to lower concentrations of acetic acid, furfural, and 5-hydroxymethlyfurfural (HMF) in the hydrolysate.36−39 TEA has shown that the minimum ethanol selling price (MESP) improves by properly incorporating deacetylation into the bioethanol process.40,41 A mechanical refining step to further improve enzymatic digestibility was used within the process for some of the TEA studies. We also performed further work to test different refining techniques on mild dilute-acid pretreated corn stover in an effort to optimize enzymatic hydrolysis yields with respect to the amount of power input into mechanical refining.42 Recently, bench-scale alkaline pretreatment of extractivesfree corn stover was performed over a wide range of conditions more severe than the deacetylation work previously discussed.43 In this work, 18 alkaline pretreatment conditions were examined at 10% TS, 2 mg anthraquinone (AQ)/g dry solids, 30 min, temperatures ranging from 100° to 160 °C, and alkali loadings between 35 to 660 mg NaOH/g dry solids. No mixing was conducted during the pretreatment. The lower bound of the alkali loading is consistent with the deacetylation condition, and the upper bound is nearly twice that used in pulping.43 AQ addition was based on results from the pulp and paper industry showing reduced carbohydrate degradation and faster delignification through an AQ/AHQ−2 redox cycle, where carbohydrates reduce AQ to AHQ, forming its dianion AHQ−2 in alkali. The AHQ−2 is oxidized to AQ during lignin hydrolysis, thereby completing the cycle.44 The simple redox shuttle hypothesis was scrutinized when research showed that both lignin and carbohydrates were oxidized and reduced and a small amount of 3,4-dideoxyhexonic acid was observed in AQ-pulping liquors, suggesting that an α,β-dicarbonyl compound was reduced by AHQ, resulting in the observed carbohydrate acid.45 Further shortcomings for the simplified redox cycle include the very low solubility of AQ in aqueous alkali at 170 °C because the reaction rates between two insoluble species, AQ and carbohydrates, is very low.46 Dimmel et al. proposed two mechanisms for lignin fragmentation, an adduct mechanism for AHQ−2-induced β-aryl ether dimer hydrolysis and a singleelectron transfer (SET) mechanism where AHQ ion radicals and dianions undergo redox reactions that lead to lignin fragmentation and carbohydrate stabilization. The evidence supporting the SET is substantial.47,48 The fundamental research performed by Dimmel et al. could be the foundation to finding cheaper and more effective redox shuttle catalysts potentially tailored for herbaceous feedstocks. The effect of AQ addition was unclear in the recent bench-scale study because all of the experiments incorporated the same AQ loading,49 and TEA results would likely show that unless a substantial benefit

will be produced annually in 2022, which could result in approximately 62 million tons of lignin produced every year.13 This is an order of magnitude greater than the amount of lignin currently in demand in the industrial chemicals market, and unless new technologies create an increased demand for lignin, it may be most beneficial to use it for electricity generation in a biofuels refinery, depending on outcomes of techno-economic (TEA) and life cycle analyses (LCA).4,14,15 Producing high-value products from lignin has been extensively researched, but new technologies must be discovered and existing upgrading technologies require further development before commercialization may be considered. Lignosulfate produced from Kraft pulping can be processed to separate the low molecular weight fraction through solvent extraction or size exclusion membranes and then meltprocessed into fibers.16,17 The fibers may then be oxidized and pyrolyzed to make carbon fiber, but the carbon fiber produced from Kraft lignin has unacceptably poor material properties compared to carbon fiber produced from poly acrylonitrile (PAN) due to greater fiber porosity and a lack of oriented graphitic structure.18,19 Thermoplastic elastomers and polyurethane foams have been produced from lignin with some chemical properties similar to petroleum products.20 One study showed that polyurethane synthesis by oxypropylation of lignin yields desirable physical strength properties.21 Another study showed that subjecting lignin to Lewis acid treatment increased the concentration of hydroxyl groups available to react with diisocyanate monomers resulting in better integration of lignin into the urethane network.22 Most of these studies have been performed on Kraft lignin, or lignosulfate, because of its availability, but this is likely to be a feedstock for lower value commodity products due to its high sulfur conten, heterogeneous mixture of phenolics with different reactivities causing varied cross-linking and large molecular weight distribution.13 The ability to genetically manipulate feedstocks so the lignin is more amenable to upgrading for a given process, such as carbon fiber or thermoplastic elastomers, is essential. Genetically engineering plants to reduce lignin content or manipulate their structure has been performed in an effort to reduce recalcitrance for bioenergy feedstocks and improve forage digestibility.23−25 Ragauskas et al. recently reviewed biosynthetic pathways to monolignols, recently discovered nonclassical lignins, and modified lignins that could be used for a variety of lignin-based, high-value materials.13 To reach longterm goals of replacing petroleum-derived products with their biobased counterparts, further research building on this foundation will be vital. With a lack of available genetically modified plants with tailored lignin properties for pilot-scale and near-term commercialization opportunities, wild-type lignin extraction using varying fractionation methods has been studied. Regardless of the fractionation method employed, if the carbohydrates and lignin are to produce different products within a biorefinery, it is essential to tailor the fractionation method to ensure maximum utilization of each component. Clean fractionation, a type of organosolv treatment using methyl isobutyl ketone, ethanol, water, and sulfuric acid, is effective for solubilizing lignin in herbaceous and woody feedstocks while retaining the carbohydrates in insoluble form.26−28 Studies performing ionic liquid treatments of biomass have shown it is effective in fractionating carbohydrates from lignin.29−32 Organosolv and ionic liquid treatments are not cost-effective without recycling the organic solvents and B

DOI: 10.1021/acssuschemeng.5b01041 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering with the addition of AQ in the form of carbohydrate retention and lignin extraction and quality is observed. Its use in a biorefinery would be unwarranted. In this work, we scale-up batch alkaline (NaOH) pretreatment to a 130-L, horizontally agitated, jacketed vessel and investigate corn stover fractionation with and without the addition of AQ. A three-factor, two-level central composite experimental design with triplicate runs at the center point is presented to investigate the relationship between NaOH loading, temperature, and AQ charge on solids solubilization and carbohydrate retention in the insoluble solids. The lowest NaOH loading, 30 mg/g dry corn stover, nearly matches the deacetylation condition that was developed in the previous study.36 The experimental design factor levels were chosen based on the lower severity ( 0.05). The R2 values for the glucose 3 and 5 day models are between 93% and 96%, and the R2 values for the xylose 3 and 5 day models are between 71% and 82%. The difference between adjusted and predicted R2 is well within the 0.20 limit. Adequate precision values greater than 4 are desirable, and the pretreatment component models range from 10.9 to 27.6.

Figure 5. Glucose (A and C) and xylose (B and D) enzymatic hydrolysis 3 day (A and B) and 5 day (C and D) yield contour plots at an [AQ] of 0.125% (w/w). The data shown includes the 17 unwashed pressed alkaline pretreatment conditions as defined in the Experimental Section. The number next to each bold line is the yield value for that contour, and there is a 10% yield difference between each contour for all of the plots. Enzymatic hydrolysis was performed at 2% (w/w) cellulose loading and 20 mg protein/g glucan enzyme dose using an 80/20 mixture of Novozymes’ Cellic CTec3 and HTec3.

design. Equations 6−9 were used to calculate all the plots in Figure 5 and are all first order and at a constant [AQ] of 0.125% (w/w). Glucose yields at 3 and 5 days are shown in plots A and C of Figure 5, respectively. The 95% CI of the glucose yield was calculated from triplicate enzymatic hydrolysis runs from each of the 22 alkaline pretreatment experiments. The variation in the 95% CI is random, with no observed trend with respect to pretreatment severity. The

YG,3day = −0.9876 + 1.2961e−2[NaOH] + 7.0100e−3T + 0.3096[AQ]

(6)

YG,5day = −0.8258 + 1.2156e−2[NaOH] + 7.0100e−3T + 0.1948[AQ]

(9)

(7)

Table 3. Enzymatic Hydrolysis Monomeric Glucose and Xylose Yield Modeling Results Including Model Process Order, Resulting ANOVA P Values for Main Factors, and Residuals Analysis P values response

model process order

glucose yield 3-day xylose yield 3-day glucose yield 5-day xylose yield 5-day

1-linear 1-linear 1-linear 1-linear

[NaOH] < < <
85%) may be achieved at temperatures greater than 140 °C and at alkali loadings lower than 40 mg NaOH/g dry corn stover. We intend to investigate these operating conditions using pilot-scale (25 dry kg biomass/d) continuous reactors in the near future.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.5b01041. Component yield data used to develop the empirical models presented in the manuscript. (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Phone: 303-384-6829. E-mail: [email protected]. Author Contributions

The manuscript was written through the contribution of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest. The authors would like to thank Justin Sluiter and Eric Karp for their useful discussions.

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ACKNOWLEDGMENTS The U.S. Government retains and the publisher, by accepting the article for publication, acknowledges that the U.S. Government retains a nonexclusive, paid-up, irrevocable, worldwide license to publish or reproduce the published form of this work, or allow others to do so, for U.S. Government purposes.

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DOI: 10.1021/acssuschemeng.5b01041 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX