[Show abstract][Hide abstract] ABSTRACT: Background:
A novel, highly efficient deacetylation and disk refining (DDR) process to liberate fermentable sugars from biomass was recently developed at the National Renewable Energy Laboratory (NREL). The DDR process consists of a mild, dilute alkaline deacetylation step followed by low-energy-consumption disk refining. The DDR corn stover substrates achieved high process sugar conversion yields, at low to modest enzyme loadings, and also produced high sugar concentration syrups at high initial insoluble solid loadings. The sugar syrups derived from corn stover are highly fermentable due to low concentrations of fermentation inhibitors. The objective of this work is to evaluate the economic feasibility of the DDR process through a techno-economic analysis (TEA).
A large array of experiments designed using a response surface methodology was carried out to investigate the two major cost-driven operational parameters of the novel DDR process: refining energy and enzyme loadings. The boundary conditions for refining energy (128-468 kWh/ODMT), cellulase (Novozyme's CTec3) loading (11.6-28.4 mg total protein/g of cellulose), and hemicellulase (Novozyme's HTec3) loading (0-5 mg total protein/g of cellulose) were chosen to cover the most commercially practical operating conditions. The sugar and ethanol yields were modeled with good adequacy, showing a positive linear correlation between those yields and refining energy and enzyme loadings. The ethanol yields ranged from 77 to 89 gallons/ODMT of corn stover. The minimum sugar selling price (MSSP) ranged from $0.191 to $0.212 per lb of 50 % concentrated monomeric sugars, while the minimum ethanol selling price (MESP) ranged from $2.24 to $2.54 per gallon of ethanol.
The DDR process concept is evaluated for economic feasibility through TEA. The MSSP and MESP of the DDR process falls within a range similar to that found with the deacetylation/dilute acid pretreatment process modeled in NREL's 2011 design report. The DDR process is a much simpler process that requires less capital and maintenance costs when compared to conventional chemical pretreatments with pressure vessels. As a result, we feel the DDR process should be considered as an option for future biorefineries with great potential to be more cost-effective.
Preview · Article · Oct 2015 · Biotechnology for Biofuels
[Show abstract][Hide abstract] ABSTRACT: Background: The deconstruction of renewable biomass feedstocks into soluble sugars at low cost is a critical component of the biochemical conversion of biomass to fuels and chemicals. Providing low cost high concentration sugar syrups with low levels of chemicals and toxic inhibitors, at high process yields is essential for biochemical platform processes using pretreatment and enzymatic hydrolysis. In this work, we utilize a process consisting of deacetylation, followed by mechanical refining in a disc refiner (DDR) for the conversion of renewable biomass to low cost sugars at high yields and at high concentrations without a conventional chemical pretreatment step. The new process features a low temperature dilute alkaline deacetylation step followed by disc refining under modest levels of energy consumption. Results: The proposed process was demonstrated using a commercial scale Andritz double disc refiner. Disc refined and deacetylated corn stover result in monomeric glucose yields of 78 to 84% and monomeric xylose yields of 71 to 77% after enzymatic hydrolysis at process-relevant solids and enzyme loadings. The glucose and xylose yields of the disc refined substrates in enzymatic hydrolysis are enhanced by 13% and 19%, respectively. Fermentation of the DDR substrates at 20% total solids with Z.mobilis utilized almost all sugars in 20hrs indicating the sugar hydrolyzate produced from the DDR process is highly fermentable due to low levels of chemical contaminants. The ethanol titer and ethanol process yield are approximately 70 g/L and 90% respectively. Conclusions: The proposed new process has been demonstrated using pilot scale deacetylation and disc refiners. The deacetylated and disc refined corn stover was rapidly deconstructed to monomeric sugars at 20% wt solids with enzymatic hydrolysis. High process sugar conversions were achieved, with high concentrations of monomeric sugars that exceeded 150 g/L. The sugar syrups produced were found to have low concentrations of known major fermentation inhibitors: acetic acid, furfural and HMF. The low levels of these fermentation inhibitors lead to high fermentation yields. The results suggest that this process is a very promising development for the nascent cellulosic biofuels industry.
Full-text · Article · Jun 2014 · Biotechnology for Biofuels
[Show abstract][Hide abstract] ABSTRACT: Dilute acid pretreatment is a promising process technology for the deconstruction of low-lignin lignocellulosic biomass, capable of producing high yields of hemicellulosic sugars and enhancing enzymatic yields of glucose as part of a biomass-to-biofuels process. However, while it has been extensively studied, most work has historically been conducted at relatively high acid concentrations of 1 - 4% (weight/weight). Reducing the effective acid loading in pretreatment has the potential to reduce chemical costs both for pretreatment and subsequent neutralization. Additionally, if acid loadings are sufficiently low, capital requirements associated with reactor construction may be significantly reduced due to the relaxation of requirements for exotic alloys. Despite these benefits, past efforts have had difficulty obtaining high process yields at low acid loadings without supplementation of additional unit operations, such as mechanical refining.
Recently, we optimized the dilute acid pretreatment of deacetylated corn stover at low acid loadings in a 1-ton per day horizontal pretreatment reactor. This effort included more than 25 pilot-scale pretreatment experiments executed at reactor temperatures ranging from 150 - 170 [degree sign]C, residence times of 10 - 20 minutes and hydrolyzer sulfuric acid concentrations between 0.15 - 0.30% (weight/weight). In addition to characterizing the process yields achieved across the reaction space, the optimization identified a pretreatment reaction condition that achieved total xylose yields from pretreatment of 73.5% +/- 1.5% with greater than 97% xylan component balance closure across a series of five runs at the same condition. Feedstock reactivity at this reaction condition after bench-scale high solids enzymatic hydrolysis was 77%, prior to the inclusion of any additional conversion that may occur during subsequent fermentation.
This study effectively characterized a range of pretreatment reaction conditions using deacetylated corn stover at low acid loadings and identified an optimum reaction condition was selected and used in a series of integrated pilot scale cellulosic ethanol production campaigns. Additionally, several issues exist to be considered in future pretreatment experiments in continuous reactor systems, including the formation of char within the reactor, as well as practical issues with feeding herbaceous feedstock into pressurized systems.
Full-text · Article · Feb 2014 · Biotechnology for Biofuels
[Show abstract][Hide abstract] ABSTRACT: Background
Historically, acid pretreatment technology for the production of bio-ethanol from corn stover has required severe conditions to overcome biomass recalcitrance. However, the high usage of acid and steam at severe pretreatment conditions hinders the economic feasibility of the ethanol production from biomass. In addition, the amount of acetate and furfural produced during harsh pretreatment is in the range that strongly inhibits cell growth and impedes ethanol fermentation. The current work addresses these issues through pretreatment with lower acid concentrations and temperatures incorporated with deacetylation and mechanical refining.
The results showed that deacetylation with 0.1 M NaOH before acid pretreatment improved the monomeric xylose yield in pretreatment by up to 20% while keeping the furfural yield under 2%. Deacetylation also improved the glucose yield by 10% and the xylose yield by 20% during low solids enzymatic hydrolysis. Mechanical refining using a PFI mill further improved sugar yields during both low- and high-solids enzymatic hydrolysis. Mechanical refining also allowed enzyme loadings to be reduced while maintaining high yields. Deacetylation and mechanical refining are shown to assist in achieving 90% cellulose yield in high-solids (20%) enzymatic hydrolysis. When fermentations were performed under pH control to evaluate the effect of deacetylation and mechanical refining on the ethanol yields, glucose and xylose utilizations over 90% and ethanol yields over 90% were achieved. Overall ethanol yields were calculated based on experimental results for the base case and modified cases. One modified case that integrated deacetylation, mechanical refining, and washing was estimated to produce 88 gallons of ethanol per ton of biomass.
The current work developed a novel bio-ethanol process that features pretreatment with lower acid concentrations and temperatures incorporated with deacetylation and mechanical refining. The new process shows improved overall ethanol yields compared to traditional dilute acid pretreatment. The experimental results from this work support the techno-economic analysis and calculation of Minimum Ethanol Selling Price (MESP) detailed in our companion paper.
Full-text · Article · Aug 2012 · Biotechnology for Biofuels
[Show abstract][Hide abstract] ABSTRACT: Enzymatic conversion of oligomeric xylose and insoluble xylan remaining after effective pretreatment offers significant potential to improve xylan-to-xylose yields while minimizing yields of degredation products and fermentation inhibitors. In this work, a commercial enzyme cocktail is demonstrated to convert up to 70 % of xylo-oligomers found in dilute acid-pretreated hydrolyzate liquor at varying levels of dilution when supplemented with accessory enzymes targeting common side chains. Commercial enzyme cocktails are also shown to convert roughly 80 % of insoluble xylan remaining after effective high-solids, dilute acid pretreatment.
[Show abstract][Hide abstract] ABSTRACT: Dilute acid pretreatment is a promising pretreatment technology for the biochemical production of ethanol from lignocellulosic biomass. During dilute acid pretreatment, xylan depolymerizes to form soluble xylose monomers and oligomers. Because the xylan found in nature is highly acetylated, the formation of xylose monomers requires two steps: 1) cleavage of the xylosidic bonds, and 2) cleavage of covalently bonded acetyl ester groups.
In this study, we show that the latter may be the rate limiting step for xylose monomer formation. Furthermore, acetyl groups are also found to be a cause of biomass recalcitrance and hydrolyzate toxicity. While the removal of acetyl groups from native corn stover by alkaline de-esterification prior to pretreatment improves overall process yields, the exact impact is highly dependent on the corn stover variety in use. Xylose monomer yields in pretreatment generally increases by greater than 10%. Compared to pretreated corn stover controls, the deacetylated corn stover feedstock is approximately 20% more digestible after pretreatment. Finally, by lowering hydrolyzate toxicity, xylose utilization and ethanol yields are further improved during fermentation by roughly 10% and 7%, respectively. In this study, several varieties of corn stover lots were investigated to test the robustness of the deacetylation-pretreatment-saccharification-fermentation process.
Deacetylation shows significant improvement on glucose and xylose yields during pretreatment and enzymatic hydrolysis, but it also reduces hydrolyzate toxicity during fermentation, thereby improving ethanol yields and titer. The magnitude of effect is dependent on the selected corn stover variety, with several varieties achieving improvements of greater than 10% xylose yield in pretreatment, 20% glucose yield in low solids enzymatic hydrolysis and 7% overall ethanol yield.
Full-text · Article · Feb 2012 · Biotechnology for Biofuels
[Show abstract][Hide abstract] ABSTRACT: To produce ethanol cost-effectively from herbaceous feedstocks such as corn stover, efficient xylan hydrolysis with monomeric xylose yields approaching 90% are necessary. Dilute acid pretreatment is well established as one of the pretreatment technologies for xylan hydrolysis; however, the accumulation of salts from neutralization, the production of toxic byproducts, and the release of acetic acid can inhibit enzymatic saccharification and fermentation, resulting in depressed ethanol yields. Successful removal of acetyl groups from native corn stover by alkali de-esterification could potentially increase monomeric xylose yields from pretreatment and enzymatic hydrolysis, improve cellulose digestibility, and reduce the cytotoxicity of the fermentation broth. Results presented in this article show that alkaline extraction removed significant amounts of acetyl groups from corn stover, improved xylan hydrolysis in high solids dilute acid pretreatment by more than 50%, and improved xylan and glucan hydrolysis in low solids enzymatic hydrolysis by 15% and 30% over control samples. In whole slurry enzymatic hydrolysis, a 30% improvement in cellulose digestibility was found over the control.
No preview · Article · Dec 2011 · Industrial & Engineering Chemistry Research
[Show abstract][Hide abstract] ABSTRACT: Dilute-acid pretreatment is a critical step in the biochemical conversion of biomass to fuels and chemicals, releasing hemicellulose as oligomeric and monomeric xylose and rendering the cellulose fraction readily digestible by enzymes. It has been difficult to achieve very high yields of monomeric xylose during pretreatment, since acid hydrolysis also converts xylan to various oligomers and non-fermentable sugar degradation products. An alternative strategy for the production of high yields of monomeric xylose is to perform secondary thermochemical hydrolysis on the dilute-acid pretreated material. This secondary hydrolysis, typically performed at lower temperatures than the primary dilute-acid pretreatment step, liberates additional monomeric xylose while minimizing the yield of sugar degradation products. This work presents the results of laboratory and pilot scale primary and secondary thermochemical hydrolysis experiments over a wide range of conditions. Our results suggest that reduced severity conditions for primary dilute-acid pretreatment provide the highest overall yields of monomeric xylose after secondary thermochemical hydrolysis for the conditions tested. The results of our LC-MS investigations suggest that higher-molecular weight xylo-oligomers are converted to xylose during secondary thermochemical hydrolysis, but that a small fraction of the xylan in corn stover is converted to stable but non-fermentable compounds, such as xylo-oligomers, during both primary and secondary hydrolysis.
[Show abstract][Hide abstract] ABSTRACT: NREL's Integrated Biorefinery Research Facility Pilot Scale Solids Handling Risk Mitigation
Jane C. Fisher, P.E., Daniel J. Schell, Richard T. Elander, David A. Sievers, Joseph Shekiro, and Timothy Johnston, National Renewable Energy Laboratory
To achieve commercial-scale production of cellulosic ethanol at a cost that is competitive with gasoline, it is crucial to understand the entire integrated biorefining process and how one stage of the process can impact the performance of the others. With the addition of the Integrated Biorefinery Research Facility (IBRF) at the National Renewable Energy Laboratory (NREL), the cellulosic biofuels industry has access to a significantly expanded pilot plant and biochemical conversion process research facility. At NREL, the goal is to improve the cost effectiveness of cellulosic biofuels production processes and thereby to accelerate commercial scale deployment of these technologies. A number of aggressive government policies are guiding NREL's approach to cellulosic ethanol and other cellulosic biofuels research and development (R&D), including the 2007 Energy and Independence and Security Act requiring 36 billion gallons of renewable fuels by 2022, and President Obama's New Energy for America Plan calling for 60 billion gallons of advanced biofuels by 2030. The IBRF's new integrated and flexible process R&D piloting capabilities will enable a wide variety of biofuels technology developers to reduce risks associated with scaling up to demonstration and full-commercial scales. Many of the risks to be mitigated are related to solids handling, which presents a variety of challenging technical issues that the cellulosic biofuels industry must resolve to successfully transition the conversion technologies from pilot to demonstration scale and ultimately to commercial scale. The IBRF was completed in two stages and includes provisions to evaluate a wide variety of sustainable biomass feedstocks (corn stover, switch grass, sorghum, etc). During completion of the first stage startup and commissioning a number of solids handing issues were encountered and to some extent resolved. Second stage design modifications were implemented to address the solids handling issues experienced during the first stage. The design changes were primarily associated with storage hoppers and discharge screws, pneumatic conveyance, cyclone separation, weigh belt feed, and dust collection. Design considerations for biomass feedstock handling as well as lessons learned during first and second stage facility commissioning and startup will be discussed.
[Show abstract][Hide abstract] ABSTRACT: The current “state-of-art” dilute acid pretreatment to produce bio-ethanol from corn stover requires severe conditions to overcome biomass recalcitrance, which, in turn, offers 70~80% xylose monomer yield and 5~7% furfural yield. However, high acid loadings and high reaction temperatures harm the economic feasibility of the process. In addition, the amount of acetate and furfural produced during pretreatment is in a range that strongly inhibits cell growth and impedes ethanol fermentation. To decrease the impact of these issues, deacetylation, pretreatment with lower acid concentrations, lower temperatures, and integration of disc-refining was carried out in this work. The results show that deacetylation with dilute NaOH (0.1M) combined with low severitydilute-acid pretreatment increased corn stover reactivity by increasing the xylose monomer yield by 20% in pretreatment while degradation of xylan to furfural remained under 2%. Glucose yield increased by 10% and xylose yield by 20% during dilute solids enzymatic hydrolysis as a result of deacetylation of the feedstock prior to pretreatment. Particle size reduction by mechanical refining after pretreatment is shown to assist achieving> 90% cellulose yield in high solids (20 wt% total solids) enzymatic hydrolysis. Bioscreen C growth assays and mini fermentation at high solids (20wt%) showed less toxicity of deacetylated pretreated corn stover and enhanced xylose utilization by 20%.
[Show abstract][Hide abstract] ABSTRACT: A range of industrially important fuels and chemicals such as ethanol can be produced from sugars. Xylose and glucose can be effectively extracted from renewable ligno-cellulosic biomass via depolymerization of hemicellulose and cellulose. A cost competitive bioconversion typically involves thermo-chemical pretreatment the ligno-cellulosic substrate followed by enzymatic deconstruction of the remaining cellulose into glucose. Dilute sulfuric acid pretreatment solubilizes much of the xylan into xylose and xylo-oligomers. However, fermentations typically require monomeric sugars to effectively convert sugars to higher value products. Xylo-oligomer conversion into monomers has been demonstrated by a secondary, mild thermochemical hydrolysis step and/or the addition of xylanases and side chain debranching enzymes. Thermochemical hydrolysis suffers from degradation due to lack of product specificity while enzymes are feedback inhibited, which makes it difficult to drive the reaction to completion. We seek a heterogeneous catalyst to selectively assist the hydrolysis of xylo-oligomers to xylose on a process relevant time scale.
We developed a reproducible method of generating a heterogeneous acid catalyst from a renewable source. Literature suggests that the conversion of ligno-cellulosic biomass through heterogeneous catalysis via carbonaceous materials is possible. In order to obtain a range of catalyst activities and support geometries, different polysaccharide candidates were chosen for catalyst generation, including Avicel, starch, cotton, and corn stover. Conditions such as carbonization temperature and method of sulfonation were varied during catalyst generation. The amount of thermal degradation and degree of sulfonation of the catalyst was determined through gravimetric analysis. Catalyst structural information was characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), and Raman spectroscopy.
Fundamental engineering principles state that significant mass transfer limitations exist when a solid substrate interacts with a solid catalyst. Therefore this study focuses on the conversion of solubilized xylo-oligomers to monomers in the dilute sulfuric acid pretreatment hydrolyzate in bench scale batch and continuous packed bed reactors. Solid catalysts could serve as a solid-liquid separation media, thereby increasing their utility to the process. These results are compared with whole slurry batch catalysis and dilute sulfuric acid catalysis. Catalyst selectivity for xylo-oligomers is investigated, and the yield of xylo-oligomer conversion into monomers determined.