Edited by: Abdul-Sattar Nizami, King Abdulaziz University, Saudi Arabia
Reviewed by: Mohammad Rehan, King Abdulaziz University, Saudi Arabia; Yong Xu, Nanjing Forestry University, China
Specialty section: This article was submitted to Bioenergy and Biofuels, a section of the journal Frontiers in Bioengineering and Biotechnology
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Wet anaerobic storage of corn stover can provide a year-round supply of feedstock to biorefineries meanwhile serving an active management approach to reduce the risks associated with fire loss and microbial degradation. Wet logistics systems employ particle size reduction early in the supply chain through field-chopping which removes the dependency on drying corn stover prior to baling, expands the harvest window, and diminishes the biorefinery size reduction requirements. Over two harvest years, in-field forage chopping was capable of reducing over 60% of the corn stover to a particle size of 6 mm or less. Aerobic and anaerobic storage methods were evaluated for wet corn stover in 100 L laboratory reactors. Of the methods evaluated, traditional ensiling resulted in <6% total solid dry matter loss (DML), about five times less than the aerobic storage process and slightly less than half that of the anaerobic modified-Ritter pile method. To further demonstrate the effectiveness of the anaerobic storage, a field demonstration was completed with 272 dry tonnes of corn stover; DML averaged <5% after 6 months. Assessment of sugar release as a result of dilute acid or dilute alkaline pretreatment and subsequent enzymatic hydrolysis suggested that when anaerobic conditions were maintained in storage, sugar release was either similar to or greater than as-harvested material depending on the pretreatment chemistry used. This study demonstrates that wet logistics systems offer practical benefits for commercial corn stover supply, including particle size reduction during harvest, stability in storage, and compatibility with biochemical conversion of carbohydrates for biofuel production. Evaluation of the operational efficiencies and costs is suggested to quantify the potential benefits of a fully-wet biomass supply system to a commercial biorefinery.
Biomass is recognized as a renewable and sustainable energy resource, and presently 130 million tonnes of agricultural residues are available in the U.S. with the potential to supply up to 180 million dry tonnes of biomass for conversion to bioenergy by 2040 (Langholtz et al.,
Corn stover is one of the primary agricultural residues available for producing bioenergy, but there are challenges associated with dry storage as a result of available harvest techniques. For example, since the final moisture content of the stover is fixed by the timing of the grain harvest, it is often higher than optimal for dry storage, especially in northern latitudes (Shinners and Binversie,
Wet anaerobic storage (i.e., ensiling) is an attractive alternative to dry bale storage and can protect the biomass from biologically-mediated DML while reducing the risk of catastrophic fire loss. Wet storage through ensiling commonly utilizes silage bags, bunkers, or drive-over piles for oxygen limitation availability, followed by biological anaerobic organic acid production through fermentation, which lowers the feedstock pH and effectively preserves biomass during storage (McDonald et al.,
The preservation of corn stover through ensiling as a bioenergy feedstock has been evaluated in the laboratory (Richard et al.,
The “Ritter” pile was designed for the preservation of wet fiber and digestion of the non-fiber component into pulp and/or paper (Ritter,
Two anaerobic wet storage methods, ensiling and the modified-Ritter storage system, were evaluated in this study because they have tremendous potential to secure corn stover from losses due to fire or microbial degradation, which are both significant vulnerabilities of dry storage systems at industrially relevant scales. These storage studies were necessary to evaluate the applicability of ensiled storage for biochemical conversion pathways, where cellulose and hemicellulose in biomass are depolymerized to monomeric carbohydrates through a combination of pretreatment and enzymatic saccharification followed by fermentation to fuels and/or chemicals. Unfortunately, the majority of research on ensiling assesses carbohydrate composition in storage performance using forage industry-specific terms, which are not directly correlated to fuel yield at a biorefinery (Wolfrum et al.,
Laboratory-based anaerobic storage experiments were conducted to simulate ensiling in a drive-over pile and a modified-Ritter pile. For these experiments, Pioneer P1151 corn stover was harvested in September of 2014 in Stevens County, KS following the grain harvest using a forage harvester, and then
An outdoor storage pile experiment was performed concurrently with aerobic laboratory experiments using corn stover, Pioneer P1151AMX, which was harvested in Stevens County, KS in August 2015. The corn stover was harvested at an initial moisture content of 40.5 ± 1.6% (wb;
Field storage pile and locations of monitoring points for temperature, gas formation, and dry matter loss. Yellow dots indicate monitoring locations at various depths and pile locations. Monitoring locations are abbreviated as follows: N = north, S = south, E = east, W = west, B = bottom, M = middle, C = center.
The 2015 corn stover was shipped overnight to INL and stored aerobically for 111 days in laboratory reactors to simulate the outer regions of an uncovered pile. For the aerobic laboratory storage experiment, two air flow rates were evaluated in duplicate 100 L reactors as described previously (Wendt et al.,
To test for microbially generated greenhouse gases or air pollutants, the following were analyzed: CO2, CH4, H2, N2O, CO, NOx. Gas exiting the storage reactors was analyzed using an automated gas chromatograph (MicroGC 3000, Agilent, Santa Clara, CA, USA) as described previously (Wendt et al.,
Chemical compositional analysis was determined for duplicate corn stover composite samples (prior to and after storage) according to standard biomass procedures developed by the National Renewable Energy Laboratory (NREL) (National Renewable Energy Laboratory,
To determine the yields of sugars available in biomass, sugar release was measured following sample pretreatment with dilute acid/dilute alkaline and subsequent enzymatic hydrolysis. Samples analyzed include as-harvested stover from 2014 and 2015, the ensiled and 1 L/min samples, and the field composite. Soluble and structural sugars were considered in the analysis of sugar yields. Dilute acid pretreatment was performed with a Dionex ASE 350 Accelerated Solvent Extractor (Dionex Corporation, Sunnyvale, CA, USA) at 10% (w/w) solids loading by adding 30 mL of 1% sulfuric acid (w/w) in 66-mL Dionium cells, using a method similar to that described by Wolfrum et al. (
Dilute alkaline pretreatment was carried out in 25 mL Incoloy® tube reactors (Alloy Metal and Tubes, Houston, TX, USA) with a fluidized sand bath (Omega FSB-4, Stamford, CT, USA) to supply heat. Dry biomass (2 g) was soaked in a prepared sodium hydroxide solution overnight before being loaded into the tube reactors. Total weight per reaction was 20 g with a 10% (w/w) biomass loading and 0.05 g sodium hydroxide per g biomass. After 140°C pretreatment for 1,620 s (severity factor = 2.61), reactions were quenched by immersing tubes into an ice bath. Pretreatment liquor was collected after centrifugation at 3,000 × rpm (1,811 ×
Enzymatic hydrolysis of dilute acid-pretreated biomass was performed using 1 g (db) of washed solids at 10% (w/w) solids loading and 50 mM citrate buffer, pH 4.8 in triplicate, similar to the methods described by Wolfrum et al. (
Particle size distribution was determined for four representative samples of dry corn stover using a Ro-Tap RX-29 (W.S. Tyler, Mentor, OH, USA) with sieve sizes of 6.35, 3.34, 0.84, 0.42, 0.25, 0.177, and 0.149 mm. The weight percent of corn stover on each sieve was calculated after a 600 s vibration operating time (ASAE Standards,
Averages and one SD are presented with
To determine the impact of wet storage on biomass, corn stover was treated with two laboratory anaerobic wet storage methods (traditional ensiling and modified-Ritter storage) as well as two laboratory aerobic wet storage scenarios (0.5 and 1.0 L/min supplied airflow) that simulate wet aerobic field storage. Parameters including gas production, fermentation products, fate or loss of the total dry matter, chemical composition of the stored solids, and availability of sugars for fermentation were analyzed for each condition. In addition, a 272 t (db) corn stover drive-over pile was constructed to evaluate ensiling in the field.
The PSD of the forage-chopped corn stover used in the storage experiments was measured for the two harvest years (Figure
Particle size distribution of forage-chopped corn stover used in laboratory and field storage experiments.
CO2 released from biomass during storage can be linked to microbial degradation of sugars due to aerobic microbial respiration or to a lesser extent as a result of anaerobic fermentation (McGechan,
Gas production in laboratory reactors after 110 days (anaerobic) or 111 days (aerobic) in storage.
Gas production (mg/kg biomass) |
|||||
---|---|---|---|---|---|
Sample description | CO2 | CO | H2 | NOX | NO2 |
Ensiled | 6.44 × 103 (0.55) | 3.92 (0.41) | 2.78 (0.14) | 96.40 × 10−3 (0) | 0 (0) |
Modified-Ritter | 19.27 × 103 (1.05) | 14.89 (0.03) | 14.19 (0.05) | 482.00 × 10−3 (0) | 65.00 × 10−3 (0) |
Aerobic, 0.5 L/min | 161.09 × 103 (19.83 × 103) | 0 | 0 | BD | BD |
Aerobic, 1 L/min | 366.53 × 103 (88.43 × 103) | 0 | 0 | BD | BD |
Composition analysis including percent dry matter loss (DML) and organic acid production for corn stover after anaerobic storage by ensiling and the modified-Ritter method as well as field and aerobic storage.
Organic acids (% of biomass) |
||||||||
---|---|---|---|---|---|---|---|---|
Sample description | DML (%) | pH | Lactic | Acetic | Butyric | Propionic | Succinic | Formic |
Ensiled | 5.75 (1.04) | 4.62 (0.04) | 3.1 (0.07) | 0.86 (0.07) | BD | 0.78 (0.10) | 0.54 (0.02) | 0.003 (0.00) |
Modified-Ritter | 9.89 (0.61) | 5.18 (0.03) | BD | 5.41 (0.02) | 1.45 (0.02) | 0.09 (0.02) | 0.02 (0.002) | BD |
Aerobic, 0.5 L/min | 32.10 (1.54) | 5.91 (0.04) | 0.58 (0.04) | 0.86 (0.05) | 0.87 (0.06) | 2.10 (0.14) | 1.73 (0.11) | 0.07 (0.005) |
Aerobic, 1 L/min | 29.52 (5.33) | 6.46 (0.43) | 0.41 (0.13) | 0.80 (0.05) | 0.41 (0.41) | 1.10 (0.89) | 0.96 (0.61) | 0.05 (0.02) |
Field stored | 4.36 (3.25) | 4.92 (0.17) | 0.51 (0.26) | 0.46 (0.05) | 0.34 (0.14) | 1.14 (0.24) | 0.79 (0.24) | 0.28 (0.06) |
CO2 production ranged from 161 to 366 g/kg in the aerobic reactors and DML varied from 32.10 ± 1.54% to 29.52 ± 5.33% for the 0.5 and 1 L/min conditions, respectively. The other permanent gases listed in Table
Temperature profile of chopped stover in aerobic reactors.
For the field experiment, the pile exhibited self-heating during the initial 3 weeks (Figure
Temperature profile of chopped stover field pile. Each zone is an average of the north and south zones.
Organic acid concentrations and pH were measured in samples collected at the end of the storage period to characterize fermentation products (Table
Chemical analyses were performed on samples from each storage experiment alongside the respective starting material (as-harvested) to determine the effect of storage on composition (Tables
Percent chemical composition analysis of corn stover stored under conditions that were anaerobic, aerobic, and in the field.
Total ash |
Protein | Total extractives | Total carbohydrates |
Lignin | Acetate | |||||
---|---|---|---|---|---|---|---|---|---|---|
Sample description | Structural | Extractable | Glucan | Xylan | Galactan | Arabinan | ||||
2014, As-harvested | 3.2 (0.4) | 5.8 (0.6) | 4.5 (0.1) | 22.3 (0.2) | 33.7 (0.9) | 15.9 (0.0) | 1.5 (0.4) | 2.4 (0.5) | 14.6 (0.3) | 2.6 (0.3) |
Ensiled, Lab | 4.0 (0.4) | 6.3 (0.8) | 3.9 (0.3) | 21.0 (0.4) | 33.2 (0.8) | 16.2 (0.4) | 1.4 (0.2) | 2.4 (0.3) | 14.6 (0.2) | 2.6 (0.3) |
2014, Washed | 4.6 (0.3) | 3.5 (0.4) | 3.7 (0.3) | 15.6 (0.2) | 34.9 (0.8) | 16.6 (0.5) | 1.6 (0.2) | 2.8 (0.6) | 15.3 (0.4) | 2.6 (0.1) |
Modified-Ritter, Lab | 4.9 (0.6) | 3.5 (1.0) | 2.7 (0.1) | 12.9 (0.1) | 35.4 (0.3) | 17.1 (0.2) | 1.4 (0.1) | 2.4 (0.4) | 15.9 (0.2) | 2.9 (0.3) |
2015, As-harvested | 8.5 (0.8) | 10.9 (1.6) | 3.8 (0.4) | 28.7 (1.7) | 31.5 (0.9) | 14.9 (0.4) | 1.5 (0.0) | 2.9 (0.0) | 11.3 (0.4) | 2.3 (0.1) |
Aerobic, 0.5 L/min | 9.2 (1.3) | 5.8 (0.5) | 4.3 (0.0) | 19.2 (0.5) | 31.1 (1.2) | 16.8 (0.4) | 1.7 (0.1) | 3.2 (0.1) | 15.1 (0.3) | 1.3 (0.0) |
Aerobic, 1 L/min | 7.8 (0.7) | 6.4 (0.2) | 4.2 (0.2) | 33.5 (1.9) | 28.9 (0.5) | 15.0 (0.3) | 1.5 (0.2) | 2.6 (0.4) | 13.9 (0.6) | 1.1 (0.3) |
Field stored | 8.2 (1.6) | 6.1 (0.9) | 4.5 (0.4) | 32.3 (2.0) | 28.3 (1.1) | 14.4 (0.5) | 1.3 (0.2) | 2.8 (0.2) | 11.8 (0.5) | 1.7 (0.1) |
Analysis of percentage of corn stover sugars stored under conditions that were anaerobic, aerobic and in the field.
Soluble sugars |
Structural sugars |
|||||||
---|---|---|---|---|---|---|---|---|
Sample description | Glucan | Xylan | Galactan | Arabinan | Glucan | Xylan | Galactan | Arabinan |
2014, As-harvested | 1.6 (0.1) | 0.3 (0.0) | 0.3 (0.0) | 0.3 (0.0) | 32.1 (1.0) | 15.6 (0.0) | 1.2 (0.4) | 2.2 (0.5) |
Ensiled, Lab | 0.8 (0.0) | 0.2 (0.0) | 0.2 (0.0) | 0.2 (0.0) | 32.4 (0.9) | 16.0 (0.4) | 1.2 (0.2) | 2.2 (0.4) |
2014, Washed | 1.0 (0.0) | 0.2 (0.0) | 0.2 (0.0) | 0.2 (0.1) | 33.9 (0.8) | 16.4 (0.5) | 1.4 (0.2) | 2.6 (0.7) |
Modified-Ritter, Lab | 0.4 (0.0) | 0.1 (0.0) | 0.2 (0.0) | 0.1 (0.0) | 35.0 (0.3) | 17.0 (0.2) | 1.2 (0.1) | 2.4 (0.5) |
2015, As-harvested | 6.4 (1.0) | 0.3 (0.2) | 0.3 (0.1) | 0.2 (0.1) | 25.1 (1.0) | 14.6 (0.2) | 1.2 (0.0) | 2.7 (0.0) |
Aerobic, 0.5 L/min | 1.1 (0.0) | 0.8 (0.0) | 0.4 (0.0) | 0.5 (0.0) | 30.0 (1.2) | 15.9 (0.4) | 1.3 (0.1) | 2.8 (0.1) |
Aerobic, 1 L/min | 3.3 (0.6) | 3.2 (1.1) | 0.7 (0.0) | 1.1 (0.0) | 25.6 (0.4) | 11.8 (1.4) | 0.8 (0.2) | 1.5 (0.4) |
Field stored | 4.5 (0.7) | 0.6 (0.3) | 0.5 (0.1) | 0.6 (0.1) | 23.8 (1.3) | 13.9 (0.6) | 0.8 (0.2) | 2.2 (0.2) |
Aerobic storage in the two laboratory conditions resulted in many statistically significant changes compared to the as-harvested material; extractable inorganics decreased, acetate decreased, and lignin was enriched. Total extractives decreased from 28.7 to 19.2% in the 0.5 L/min scenario but increased to 33.5% in the 1 L/min case. Additional changes were observed in the carbohydrates in terms of soluble, structural, and total levels. For the 0.5 L/min scenario, structural glucan was enriched from 25.1 to 30.0% with a corresponding decrease in soluble glucan from 6.4 to 1.1%, resulting in no overall difference in total glucan. Total xylan, galactan, and arabinan were all significantly enriched as a result of this lower airflow condition, although only the increase in structural xylan and soluble arabinan was significant. For the 1 L/min scenario, total glucan was reduced from 31.5 to 28.9% due to the reduction of soluble glucan from 6.4 to 3.3%. Soluble xylan, galactan, and arabinan all increased significantly spurred by a decrease in the corresponding structural counterparts; however, the total levels were unchanged despite the fact that solubilization occurred.
Field-stored samples were taken from multiple zones in the pile and data were combined for simplicity (
The laboratory-ensiled corn stover, field-stored stover, and the stover from 1 L/min airflow laboratory condition, along with their as-harvested counterpart, were subject to biomass pretreatments using dilute acid or dilute alkali, both followed by enzymatic hydrolysis for determining total sugar release. Figures
Sugars released from corn stover with dilute acid pretreatment and enzymatic hydrolysis for the as-harvested or stored samples (A1, glucose; A2, xylose; A3, reactivity). Error bars represent the standards of deviation (
Sugars released from corn stover with dilute alkaline pretreatment and enzymatic hydrolysis for the as-harvested or stored samples (B1, glucose; B2, xylose; B3, reactivity). Error bars represent the standards of deviation (
Laboratory-ensiled corn stover from the 2014 harvest had minor yet statistically significant variations compared to as-harvested stover as a result of dilute acid pretreatment. For example, glucose was slightly higher for the as-harvested sample, but xylose yield following acid pretreatment was increased as a result of laboratory-ensiled storage. No changes were seen in subsequent enzymatic hydrolysis of these two samples, resulting in the net effect of no statistically measurable difference in feedstock reactivity as a result of laboratory ensiling with the dilute acid pretreatment approach. Within the 2015 samples, significant differences were observed as a result of dilute acid pretreatment and enzymatic hydrolysis. Stored samples released approximately half of the glucose after pretreatment compared to the as-harvested stover, yet increased glucose was released in enzymatic hydrolysis in the aerobic storage condition. This resulted in similar total glucose yields for the as-harvested and aerobically stored samples, significantly higher than the field-stored material, and this trend was conserved in final feedstock reactivity measurements.
An evaluation of the samples collected from the 2015 harvest showed markedly different response to dilute acid pretreatment. First, there was higher total soluble glucose released and ultimately higher reactivity as compared to the 2014 season samples; however, this was not surprising considering that the soluble glucan levels in the 2015 as-harvested field samples were also several fold greater than the 2014 as-harvested samples (5.9 vs 1.0–1.6%, respectively, Table
Dilute alkaline pretreatment and enzymatic hydrolysis results are presented in Figure
Forage chopping is a common practice employed when making silage for livestock feed, and the size reduction serves to improve packing density and, thus, limit oxygen infiltration during storage. Likewise, forage chopping of high-moisture feedstock can be used for biofuels-related crops and can reduce the preprocessing required to meet <6 mm target size specification for a biorefinery (Humbird et al.,
The preservation of biomass during storage is a key indicator in the success of a particular storage approach. Likewise, gas production is an important factor in evaluating a storage system for greenhouse gas and air pollutant potential and thus was measured in the wet storage conditions tested. The increased production of CO2 and other permanent greenhouse gasses in the modified-Ritter storage reactors along with higher DML suggested greater microbial activity than for the traditional ensiling material. The presence of CO has been documented in composting of green and municipal solid wastes and has been related to a physio-chemical process that occurs in the presence of oxygen during the initial stages of composting (Hellebrand and Kalk,
Successful ensiling results from the fermentative production of lactic acid from soluble sugars and is carried out by homolactic and heterolactic bacteria that help acidify material and prevent other microorganisms from degrading the dry matter; other compounds, such as acetic acid, may also be produced depending on the predominant fermentation pathways (McGechan,
In the present study, the lack of lactic acid and presence of butyric acid in the modified-Ritter material suggested that the washing process, used to simulate the slurrying of biomass in the Ritter method, may have had a negative effect on the lactic acid fermentation process. While a lactic acid bacteria inoculant could be added to the modified-Ritter storage method after the simulated slurrying, there is little practicality of doing this during field-scale storage as it would be challenging and costly to execute. The washing step of the as-harvested material was successful in removing about 40% of the soil contamination, which is measured as extractable inorganics. Similarly, total extractives and soluble glucan and xylan were significantly reduced by approximately one third as a result of the washing step. While there are benefits to removing these soluble components, primarily ash, prior to conversion, the additional DML observed in the modified-Ritter storage suggests that washing removed the necessary soluble sugars for successful fermentation. Overall, the organic acid profiles along with the DML and gas production observation suggest that the lactic acid fermentation associated with the ensiling storage method resulted in superior performance compared to the modified-Ritter and aerobic storage methods in the laboratory.
Anaerobic storage conditions preserved not only dry matter compared to aerobic storage but also the primary compositional components including the structural sugars. Similar observations for glucan and xylan preservation have been reported when DML in storage remains at 5% (Liu et al.,
Aerobic storage in laboratory reactors resulted in approximately 30% total loss of matter over the 111-day storage period. Significant self-heating was observed in the aerobic conditions likely due to microbial respiration of available carbohydrates, with the higher airflow condition exhibiting slightly higher temperatures and producing over twice the CO2 relative to the low-flow condition. Similar temperature profiles have been reported in piled corn stover (Shinners et al.,
In contrast to the relative stability of biomass components (i.e., carbohydrates, lignin, and inorganic nutrients) observed in the anaerobic storage laboratory experiments, aerobic and field storage resulted in measurable compositional changes. Extractable inorganics decreased in all samples, likely due to the consumption of macronutrients to support microbial respiration. A significant reduction in acetate was observed in all samples, suggesting that a mild form of pretreatment likely occurred. Acetate is one of the first structural components to be released during degradation, as acetyl bonds in the hemicellulose (xylan, galactan, and arabinan) are cleaved by microorganisms attempting to break down hemicellulose and later cellulose (primarily glucan). An assessment of the shift of structural xylan, galactan, and arabinan to soluble forms, along with significant acetate reduction, suggested that hemicellulose depolymerization occurred during aerobic storage with the 1 L/min airflow. A different story is evident as a result of storage at the lower airflow rate of 0.5 L/min. While total glucan was preserved under this condition, >75% of the soluble glucan was lost and a concomitant enrichment of structural glucan occurred. Hemicellulose depolymerization was not evident under low-airflow storage condition, as measured by an enrichment of structural xylan. Overall, the structural sugar profiles suggested that the lower airflow condition preserved the sugars better than in the high airflow condition, which were characterized by higher microbial consumption of the structural sugars as well as mild pretreatment effects. Interestingly, hemicellulose depolymerization was seen in the field-stored samples, with solubilization of galactan and arabinan at all pile depths and significant glucan and xylan solubilization in the shallower samples only (0.6–0.7 m depths). These results suggested that the microbial-mediated self-heating that occurred during the initial 3 weeks of storage was sufficient to produce the mild pretreatment effects seen in the 1 L/min airflow condition in the laboratory.
Lignin content was enriched as a result of aerobic storage, where approximately 30% of dry matter was lost. Lignin content is often proportionally enriched as a result of storage loss due to the biodegradation of cellulose and hemicellulose and the inaccessibility of lignin to microorganisms, and similar results have been reported for aerobically stored corn stover (Athmanathan et al.,
The pretreatment conditions chosen for this study are industrially relevant conditions and were selected to deliver maximum hydrolysis of structural sugars in cellulose and hemicellulose to monomeric sugars for subsequent fermentation to fuels and/or chemicals. In this study, enzymatic hydrolysis followed either acid or alkaline pretreatments in order to determine total glucose and xylose yield as well as feedstock reactivity. Dilute acid pretreatment resulted in glucose and xylose release several fold higher compared to alkaline pretreatment, however subsequent enzymatic hydrolysis improved depolymerization such that total glucose and xylose releases were only marginally less for the alkaline method. Similar trends in dilute acid and alkaline pretreatment have been reported for corn stover (Duguid et al.,
Laboratory-ensiling resulted in no change in feedstock reactivity or a slight increase in reactivity as a result of pretreatment with dilute acid and dilute alkali, respectively, followed by enzymatic hydrolysis. A similar result has been observed in ensiled sorghum subject to dilute alkali pretreatment (Sambusiti et al.,
Laboratory-based aerobic storage experiments (1 L/min airflow) using the same corn stover that was collected for the field study indicated that these samples were slightly more reactive than the field-stored stover, also evidenced by hemicellulose depolymerization (Table
The sugar release results provide strong support for the incorporation of wet stored biomass into commercial biochemical conversion processes. The nearly 100% release of sugars along with inhibitor formation suggests that the severity of the dilute acid pretreatment method was too high to accurately measure storage-related changes in this feedstock. This finding highlights two opportunities for cost reduction in conversion, either through reduced pretreatment severity through lower temperatures or acid levels, or for capturing the water extractives—including soluble carbohydrates—prior to pretreatment and defining a value added product stream for a conversion facility. Wolfrum et al. suggest that lowering severity levels is an effective approach to assessing biomass-related responses in conversion (Wolfrum et al.,
A primary challenge associated with the dry bale logistics system for providing herbaceous materials for bioenergy is the loss of feedstock in storage due to microbial degradation and potential fires. This study demonstrated that wet anaerobic storage is an active management approach for corn stover to preserve biomass in the supply chain. Storage performance was measured in terms of total DML, compositional analysis (i.e., carbohydrates, organic acids, ash, lignin, etc.), gas production, and the potential for sugar release for conversion to biofuels. Both laboratory and field studies showed that long-term stability can be achieved with little effect on feedstock performance in terms of sugar release. Furthermore, this study confirmed that field-chopping and particle size reduction early in the supply chain removed the bulk logistics system’s dependency on drying corn stover prior to baling and could be used to diminish the biorefinery size reduction requirements; in-field forage chopping was capable of reducing over 60% of the corn stover to a particle size of less than 6 mm. Additional opportunities beyond preservation are also possible with wet storage, for example with directed microbial preprocessing for improved convertibility. In summary, incorporating feedstock supply logistics systems centered around high-moisture biomass and wet anaerobic storage offer the potential for biorefineries to reduce the risks associated dry baled feedstock meanwhile providing a feedstock that is compatible with existing conversion technologies.
LW, JM, and WS performed a number of experiments and drafted the manuscript. TR and QN executed the field experimentation. TR, LL, and QH also performed experiments. DR, NS, AR, AH, and QN performed data analysis and revised the manuscript.
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest. The reviewer MR and handling Editor declared their shared affiliation.
The authors thank Karen Delezene-Briggs, Eric Fillerup, Sergio Hernandez, Sabrina Morgan, Kastli Schaller, and Brad Thomas at Idaho National Laboratory for their efforts in sample analysis and Vicki Thompson for review of the manuscript. Idaho National Laboratory and Lawrence Berkley National Laboratory would like to acknowledge core funding from the U.S. Department of Energy Office of Energy Efficiency and Renewable Energy’s Bioenergy Technologies Office, as well funding from the American Recovery and Reinvestment Act for the Advanced Biofuels Process Development Unit. This work is supported by the U.S. Department of Energy, under DOE Idaho Operations Office Contract DE-AC07-05ID14517. Accordingly, the U.S. Government retains a non-exclusive, royalty-free license to publish or reproduce the published form of this contribution, or allow others to do so, for U.S. Government purposes. The views and opinions of the authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof. Neither the United States Government nor any agency thereof, nor any of their employees, makes any warranty, expressed or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights.