Effects of algal hydrolyzate as reaction medium on enzymatic hydrolysis of lignocelluloses

Effects of an algal hydrolyzate on the enzymatic hydrolysis of lignocelluloses were examined using four bioenergy feedstocks (poplar, corn stover, switchgrass, and anaerobically digested manure fiber). Feedstocks were pretreated using dilute acid or alkali prior to hydrolysis. Hydrolysis reactions were conducted using the neutralized algal hydrolyzate, citrate buffer, or deioinized water as reaction media. Results demonstrated that algal hydrolyzate significantly improved the efficiency of enzymatic hydrolysis of lignin-rich or structurally recalcitrant biomass such as poplar and anaerobically digested manure fiber. This study showed that algal biomass can be used as not only a biofuel feedstock for direct diesel and ethanol production, but also a supplemental feedstock to enhance the performance of lignocellulosic biorefining.     

Effects of compressive force,particle size and moisture content on mechanical properties of biomass pellets from grasses

Mechanical properties of wheat straw, barley straw, corn stover and switchgrass were determined at different compressive forces, particle sizes and moisture contents. Ground biomass samples were compressed with five levels of compressive forces (1000, 2000, 3000, 4000 and 4400 N) and three levels of particle sizes (3.2, 1.6 and 0.8 mm) at two levels of moisture contents (12% and 15% (wet basis)) to establish compression and relaxation data. Compressed sample dimensions and mass were measured to calculate pellet density. Corn stover produced the highest pellet density at low pressure during compression. Compressive force, particle size and moisture content significantly affected the pellet density of barley straw, corn stover and switchgrass. However, different particle sizes of wheat straw did not produce any significant difference on pellet density. The relaxation data were analyzed to determine the asymptotic modulus of biomass pellets. Barley straw had the highest asymptotic modulus among all biomass indicating that pellets made from barley straw were more rigid than those of other pellets. Asymptotic modulus increased linearly with an increase in compressive pressure. A simple linear model was developed to relate asymptotic modulus and maximum compressive pressure.     

Hydrothermal pretreatment of switchgrass and corn stover for production of ethanol and carbon microspheres

Pretreatment of biomass is viewed as a critical step to make the cellulose accessible to enzymes and for an adequate yield of fermentable sugars in ethanol production. Recently, hydrothermal pretreatment methods have attracted a great deal of attention because it uses water which is a inherently present in green biomass, non-toxic, environmentally benign, and inexpensive medium. Hydrothermal pretreatment of switchgrass and corn stover was conducted in a flow through reactor to enhance and optimize the enzymatic digestibility. More than 80% of glucan digestibility was achieved by pretreatment at 190 °C. Addition of a small amount of K₂CO₃ (0.45-0.9 wt.%) can enhance the pretreatment and allow use of lower temperatures. Switchgrass pretreated at 190 °C only with water had higher internal surface area than that pretreated in the presence of K₂CO₃, but both the substrates showed similar glucan digestibility. In comparison to switchgrass, corn stover required milder pretreatment conditions. The liquid hydrolyzate generated during pretreatment was converted into carbon microspheres by hydrothermal carbonization, providing a value-added byproduct. The carbonization process was further examined by GC-MS analysis to understand the mechanism of microsphere formation.     

Switchgrass growth and morphological changes under established pine-grass agroforestry systems in the lower coastal plain of North Carolina,United States

Switchgrass (Panicum virgatum L.) intercropped with Loblolly pine (Pinus taeda L.) has been proposed as a potential biomass feedstock for biofuel production in the southeastern United States. This study investigated effects of treatments (intercropping vs. grass only) on biomass increment processes and morphological properties of switchgrass at two experimental plots (Lenoir1) located in the coastal plain of North Carolina. We also evaluated effects of trimming lower tree branches of pine trees on switchgrass growth at another watershed-scale site (Carteret7) in the same region. Results showed that biomass yield of intercropped switchgrass was reduced by adjacent trees and negatively affected by relative position of grass to trees at the 6th year after planting at Lenoir1. Relative grass-to-tree position was also found to be a significant (p < 0.001) factor affecting grass growth at Carteret7 site with tree age of 5 years old, which is irrespective to the trimming practice. Trimming lower tree branches did not significantly (p = 0.57) improve biomass yield of switchgrass at Carteret7. We also observed intercropped switchgrass typically had higher specific leaf area and grew taller compared to grass-only plots. Stem-to-leaf ratios of switchgrass were significantly (p = 0.02) affected by trees at Lenoir1, but not by trimming lower branches in Carteret7 and relative position of grass to trees at both study sites. Findings from this study are important for evaluating the viability of producing biofuel feedstocks using this proposed intercropping system in the southeastern United States.     

High dry matter whole-plant corn as a biomass feedstock

This research investigated the harvest, ambient pre-treatment, and storage of whole-plant corn as an alternative to conventional systems where corn grain and stover are fractionated at harvest. Harvesting the whole-plant, both grain and most of the above ground stover, after physiological maturity can reduce the intense logistics challenges typically associated with corn harvest and expand the harvest window. To determine the feasibility of the proposed system, corn was harvested at 350–840 g kg⁻¹ whole-plant dry matter (DM) using a forage harvester and then ensiled in pilot-scale silos. Ambient pretreatment during storage was investigated using both dilute acid and lime. Both pretreated and control whole-plant silages were very well conserved during anaerobic storage with DM losses generally less than 40 g kg⁻¹. Hydrodynamic separation of the grain and stover fractions after storage was found to be more effective at fractionating starch and fiber than conventional dry grain harvest, and both fractions had desirable composition. The effects of pretreatment on the silage were very pronounced at 30 and 100 g (kg DM)⁻¹ sulfuric acid loading with less than 100 g (kg DM)⁻¹ of the hemicellulose still bound in the cell wall at DM contents greater than 500 g kg⁻¹. The whole-plant harvest and storage system was shown to be a viable alternative to conventional corn grain and stover systems for producing feedstocks for biochemical conversion.     

Improvement of enzymatic saccharification of Populus and switchgrass by combined pretreatment with steam and wet disk milling

To reduce the recalcitrance of lignocellulosic biomass for subsequent biological processing, we pretreated energy crop feedstocks with mild steam treatment (ST; 130 and 150 °C for 60 min) and wet disk milling (WDM). We tested two phylogenetically different, but typical energy crop feedstocks: Populus trichocarpa and switchgrass (Panicum virgatum). WDM after ST facilitated the fibrillation of both types of biomass, resulting in an increase of specific surface area, improved enzymatic saccharification yield, and decrease in cellulose crystallinity. After steam treatment at 150 °C followed by 17 cycles of WDM, enzymatic hydrolysis resulted in almost complete glucan to glucose conversion in both feedstocks.     

Key techniques and parameters for briquetting corn stover sprayed with biogas slurry in a cold region in China

Densification of biomass is often necessary to combat the negative storage and handling characteristics of these low-bulk-density materials. Corn stover is an important feedstock being considered for production of renewable fuels and energy in China. Densification of corn stover would help reduce the problems and costs of bulk transportation, handling, and storage of biomass feedstock. In cold regions, some thermosetting binder will be added when briquette fuel is made with corn stover because of its high water and lignin content. It is important to understand the synthesis mechanism of thermosetting binder under certain temperature and pressure combined with strong alkaline slurry to explore develop ment of a new process of briquetting corn stover based on only alkaline slurry as an environment-friendly binder. In this study, flat-die briquetting machine has been used with a cone-shaped press roller, and a matching die hole on the press plate. At the same time, orthogonal experiments were designed from the results of single-factor experiments to obtain optimal briquetting conditions. The effects of spray quantity of biogas-slurry, size of die hole, and water content of corn stover on the durability and bulk density of corn stover briquettes were investigated. Moreover, the model for whole factors and the model for interaction between factors for indexes have been built, emulated the optimum processing parameters. The results show briquettes of corn stover with durability of 83.33% were produced when the spray liquid flow quantity of biogas-slurry, size of die hole, and water content of corn stover were 0.45 L/kg, 28 mm, and 20%, respectively. Bulk density of 616 kg/m³ was produced when the spray liquid flow quantity of biogas-slurry, size of die hole, and water content of corn stover were 0.3 L/kg, 24 mm, and 25%, respectively. These results have been checked by verification testing. The successful implementation of this project provides theoretical support and test instruction for the development of green biomass energy for sustainable development.     

Effect of biomass species and plant size on cellulosic ethanol: A comparative process and economic analysis

The effects of five different biomass species and their chemical composition on the overall process efficiency and economic performance considering feedstock availability and feedstock costs to manufacture ethanol from lignocellulose were studied. First is a comparison of ethanol production and excess electricity generated between different biomass species. Results show that, at the same feedstock rate of 2000 Mg day^?1, aspen wood has larger ethanol production than switchgrass, hybrid poplar and corn stover, while the excess electricity generated is as follows in increasing order: aspen < corn stover < hybrid poplar/switchgrass. Second, our results show that the ethanol production is largely linear with holocellulose (cellulose plus hemicellulose) composition of the various biomass species. However, the relationship between excess electricity generated and non-holocellulose combustible component is nonlinear. Last, on environmental performance, it is found that the water losses per unit ethanol production are in the following order: aspen wood < corn stover < hybrid poplar < switchgrass. While corn stover is a potential feedstock to produce cellulosic ethanol with the lowest ethanol production cost at the present time, hybrid poplar and switchgrass are the two promising future energy crops.The effects of plant size analysis showed that the estimated feedstock delivered costs, ethanol production, excess electricity generated and solid and gaseous waste emissions all increase with plant size for the various biomass species. The ethanol production costs decrease with the increase in plant size with optimal plant sizes for corn stover in the range from 2000 dry Mg day^?1 to 4000 dry Mg day^?1.     

Second-generation bioethanol of hydrothermally pretreated stover biomass from maize genotypes

Twelve maize genotypes, were agronomically evaluated and their stover hydrothermally pretreated in a temperature range of 210–225 °C to assess the effects of genotype and pretreatment severity on stover recalcitrance toward bioethanol conversion. Maize genotypes exhibited significant variation for biomass yield and all agronomic evaluated, while among all cell wall constituents measured in the unpretreated stover, only ash content showed differences among genotypes. The pretreatment severities assayed impacted most stover compositional traits, and the glucose recovered after enzymatic hydrolysis displayed a similar profile among genotypes with similar genetic background. Harsher pretreatment conditions maximized the potential cellulosic bioethanol production (208–239 L/t), while the mildest maximized the bioethanol from the hemicellulosic hydrolysates (137–175 L/t). Consequently, when both pentose and hexose sugars were considered, the total potential bioethanol produced at the lowest and highest pretreatment temperatures was similar in all genotypes (292–358 L/t), indicating that the lowest temperature (210 °C) was the optimal among all assayed. Importantly, the ranking of genotypes for bioethanol yield (L/ha) closely resembled the ranking for stover yield (t/ha), indicating that breeding for biomass yield would increase the bioethanol production per hectare regardless of the manufacturing process. Similarly, the genetic regulation of corn stover moisture is possible and relevant for efficient energy production as biomass moisture has a potential impact on stover transportation, storage and processing requirements. Overall, these results indicate that local landrace populations are important genetic resources to improve cultivated crops, and that simultaneous breeding for production of grain and stover bioethanol is possible in corn.     

Ash reduction strategies in corn stover facilitated by anatomical and size fractionation

There is growing interest internationally to produce fuels from renewable biomass resources. Inorganic components of biomass feedstocks, referred to collectively as ash, damage equipment and decrease yields in thermal conversion processes, and decrease feedstock value for biochemical conversion processes. Decreasing the ash content of feedstocks improves conversion efficiency and lowers process costs. Because physiological ash is unevenly distributed in the plant, mechanical processes can be used to separate fractions of the plant based on ash content. This study focuses on the ash separation that can be achieved by separating corn stover by particle size and anatomical fraction. Baled corn stover was hand-separated into anatomical fractions, ground to <19.1 mm, and size separated using six sieves ranging from 9.5 to 0.150 mm. Size fractions were analyzed for total ash content and ash composition. Particle size distributions observed for the anatomical fractions varied considerably. Cob particles were primarily 2.0 mm or greater, while most of the sheath and husk particles were 2.0 mm and smaller. Particles of leaves greater than 0.6 mm contained the greatest amount of total ash, ranging from approximately 8 to 13% dry weight of the total original material, while the fractions with particles smaller than 0.6 mm contained less than 2% of the total ash of the original material. Based on the overall ash content and the elemental ash, specific anatomical and size fractions can be separated to optimize the feedstocks being delivered to biofuels conversion processes and minimize the need for more expensive ash reduction treatments.     

Improved methods for the determination of drying conditions and fraction insoluble solids (FIS) in biomass pretreatment slurry

Accurate and precise chemical characterization of biomass feedstocks and process intermediates is a requirement for successful technical and economic evaluation of biofuel conversion technologies. The uncertainty in primary measurements of the fraction insoluble solid (FIS) content of dilute acid pretreated corn stover slurry is the major contributor to uncertainty in yield calculations for enzymatic hydrolysis of cellulose to glucose. This uncertainty is propagated through process models and impacts modeled fuel costs. The challenge in measuring FIS is obtaining an accurate measurement of insoluble matter in the pretreated materials, while appropriately accounting for all biomass derived components. Three methods were tested to improve this measurement. One used physical separation of liquid and solid phases, and two utilized direct determination of dry matter content in two fractions. We offer a comparison of drying methods. Our results show utilizing a microwave dryer to directly determine dry matter content is the optimal method for determining FIS, based on the low time requirements and the method optimization done using model slurries.     

Comparative net energy ratio analysis of pellet produced from steam pretreated biomass from agricultural residues and energy crops

A process model was developed to determine the net energy ratio (NER) for the production of pellets from steam pretreated agricultural residue (wheat straw) and energy crops (i.e., switchgrass in this case). The NER is a ratio of the net energy output to the total net energy input from non-renewable energy sources into a system. Scenarios were developed to measure the effects of temperature and level of steam pretreatment on the NER of steam pretreated wheat straw and switchgrass pellets. The NERs for the base case at 6 kg h⁻¹ are 1.76 and 1.37 for steam-pretreated wheat straw and switchgrass-based pellets, respectively. The reason behind the difference is that more energy is required to dry switchgrass pellets than wheat straw pellets. The sensitivity analysis for the model shows that the optimum temperature for steam pretreatment is 160 °C with 50% pretreatment (i.e. 50 % steam treated material is blended with the raw biomass and then pelletised). The uncertainty results for NER for steam pretreated wheat straw and switch grass pellets are 1.62 ± 0.10 and 1.42 ± 0.11, respectively.     

Production of butanol (a biofuel) from agricultural residues: Part II – Use of corn stover and switchgrass hydrolysates

Acetone butanol ethanol (ABE) was produced from hydrolysed corn stover and switchgrass using Clostridium beijerinckii P260. A control experiment using glucose resulted in the production of 21.06 g L^?1 total ABE. In this experiment an ABE yield and productivity of 0.41 and 0.31 g L^?1 h^?1 was achieved, respectively. Fermentation of untreated corn stover hydrolysate (CSH) exhibited no growth and no ABE production; however, upon dilution with water (two fold) and wheat straw hydrolysate (WSH, ratio 1:1), 16.00 and 18.04 g L^?1 ABE was produced, respectively. These experiments resulted in ABE productivity of 0.17–0.21 g L^?1 h^?1. Inhibitors present in CSH were removed by treating the hydrolysate with Ca(OH)₂ (overliming). The culture was able to produce 26.27 g L^?1 ABE after inhibitor removal. Untreated switchgrass hydrolysate (SGH) was poorly fermented and the culture did not produce more than 1.48 g L^?1 ABE which was improved to 14.61 g L^?1. It is suggested that biomass pretreatment methods that do not generate inhibitors be investigated. Alternately, cultures resistant to inhibitors and able to produce butanol at high concentrations may be another approach to improve the current process.     

Factors affecting strength and durability of densified biomass products

Effectiveness of a densification process to create strong and durable bonding in densified products such as pellets, briquettes, and cubes can be determined by testing the strength (i.e., compressive resistance, impact resistance, and water resistance), and durability (i.e., abrasion resistance) of the densified products. These tests can indicate the maximum force/stress that the densified products can withstand, and the amount of fines produced during handling, transportation, and storage. In this article, the procedures used for measuring the strength and durability of the densified products are discussed. The effects of constituents of the feed such as starch, protein, fiber, fat, lignin and extractives; feed moisture content; feed particle size and its distribution; feed conditioning temperature/preheating of feed; added binders; and densification equipment variables (forming pressure, and pellet mill and roll press variables) on the strength and durability of the densified products are reviewed. This article will help select process parameters to produce strong and durable densified products from new biomass feedstocks or animal feed formulations. Guidelines for developing standards on criteria for the acceptance levels of strength and durability of the densified products are presented.     

Utilization of excess corn kernels for hydrogen gas biofuel production

Corn kernels are good candidates for production of various value-added products such as gas biofuel, hydrogen due to the carbohydrate-rich composition. In this study, widely grown corn, field corn kernels were dissolved in subcritical water at different temperatures to determine optimal thermal hydrolysis condition. Organic-rich hydrolysate obtained from hydrolysis process was gasified by aqueous-phase reforming (APR) for hydrogen gas production.Since hydrolysis at 200 °C resulted in significantly more total organic carbon release than other temperatures and the lowest amount of insolubilized solid residue. Different concentrations of this hydrolysate (diluted with water at different ratios) were evaluated for high yielding hydrogen gas production. Gasification performance of corn kernels was also compared with lignocellulosic biomass using corn stover as a representative biomass material.The hydrolysate with 2486 mg/L TOC concentration showed the best performance for hydrogen gas production (130 mL H₂/g corn) and left less amount of ungasified solid residue. Corn kernels produced 2.3 times more hydrogen gas compared to corn stover biomass. Thus, corn kernels are promising feed materials for APR process, and excess production of corn can be utilized for hydrogen gas production in higher yield and richer composition.     

Improved hydrogen production via thermophilic fermentation of corn stover by microwave-assisted acid pretreatment

A microwave-assisted acid pretreatment (MAP) strategy has been developed to enhance hydrogen production via thermophilic fermentation of corn stover. Pretreatment of corn stover by combining microwave irradiation and acidification resulted in the increased release of soluble substances and made the corn stover more accessible to microorganisms when compared to thermal acid pretreatment (TAP). MAP showed obvious advantages in short duration and high efficiency of lignocellulosic hydrolysis. Analysis of the particle size and specific surface area of corn stover as well as observation of its cellular microstructure were used to elucidate the enhancement mechanism of the hydrolysis process by microwave assistance. The cumulative hydrogen volume reached 182.2 ml when corn stover was pretreated by MAP with 0.3 N H₂SO₄ for 45 min, and the corresponding hydrogen yield reached 1.53 mol H₂/mol-glucose equivalents converted to organic end products. The present work demonstrates that MAP has potential to enhance the bioconversion efficiency of lignocellulosic waste to renewable biofuel.     

Characterization of a Trichoderma atroviride strain isolated from switchgrass bales and its use to saccharify ammonia-pretreated switchgrass for biobutanol production

The feedstock-specific enzyme systems for saccharification of biofuel feedstocks like switchgrass may potentially provide better enzymatic systems for production of second-generation biofuels. One strategy to develop these enzyme systems could be to harness the microorganisms growing naturally on specific feedstocks. This study presents the isolation and screening of fungal cultures from switchgrass bales for saccharification of ammonia-pretreated switchgrass for subsequent biobutanol production. The best performing fungal isolate during screening was identified through Sanger sequencing of its ITS region to be a unique strain of Trichoderma atroviride and further characterized for production of an enzyme system for saccharification of ammonia pretreated switchgrass. The maximum FPase, CMCase and xylanase activity produced by T. atroviride CUA1 were 0.25 fpu/mL, 0.18 IU/mL and 5.8 IU/mL, respectively. T. atroviride CUA1 also produced considerable amount of β-glucosidase activity. This isolate was used to produce an enzyme system to convert switchgrass to soluble sugars that were then fermented to butanol, ethanol, acetate and butyrate. Glucose was the major product of hydrolysis of ammonia-pretreated switchgrass performed using the enzyme system produced by the isolate. This fungus may be useful for the hydrolysis for the bioenergy crop of switchgrass to overcome this rate-limiting step in the overall conversion of biomass to biofuels.     

Comparison of four different chemical pretreatments of corn stover for enhancing enzymatic digestibility

Four commonly used chemical pretreatment processes based on dilute acid, lime, aqueous ammonia steeping followed by dilute acid hydrolysis, and sodium hydroxide, were evaluated to provide comparative performance data. An obverse correlation between lignin removal and enzymatic digestibility of pretreated corn stover was observed. Compared with other three pretreatments, pretreatment of corn stover with 2% NaOH substantially increased the lignin removal and enhanced the accessibility and digestibility of cellulose. The hydrolysis yield of NaOH-pretreated corn stover reached 81.2% by 48 h at 8.0% substrate concentration and cellulase dosage of 20 FPU g⁻¹ substrate. Chemical analysis showed that the enzymatic hydrolysate from NaOH-pretreated corn stover contained higher content of fermentable sugars and less inhibitors, which is suitable for subsequent fermentation process to produce ethanol. The research results are meaningful in bioconversion and utilization of renewable lignocellulosic biomass.     

The effects of physical and chemical preprocessing on the flowability of corn stover

Continuous and reliable feeding of biomass is essential for successful biofuel production. However, the challenges associated with biomass solids handling are commonly overlooked. In this study, we examine the effects of preprocessing (particle size reduction, moisture content, chemical additives, etc.) on the flow properties of corn stover. Compressibility, flow properties (interparticle friction, cohesion, unconfined yield stress, etc.), and wall friction were examined for five corn stover samples: ground, milled (dry and wet), acid impregnated, and deacetylated. The ground corn stover was found to be the least compressible and most flowable material. The water and acid impregnated stovers had similar compressibilities. Yet, the wet corn stover was less flowable than the acid impregnated sample, which displayed a flow index equivalent to the dry, milled corn stover. The deacetylated stover, on the other hand, was the most compressible and least flowable examined material. However, all of the tested stover samples had internal friction angles >30°, which could present additional feeding and handling challenges. All of the “wetted” materials (water, acid, and deacetylated) displayed reduced flowabilities (excluding the acid impregnated sample), and enhanced compressibilities and wall friction angles, indicating the potential for added handling issues; which was corroborated via theoretical hopper design calculations. All of the “wetted” corn stovers require larger theoretical hopper outlet diameters and steeper hopper walls than the examined “dry” stovers.     

Roll press compaction of corn stover and perennial grasses to increase bulk density

The objective of this study was to investigate roll press compaction of coarsely ground corn stover and native perennial grasses to produce compacted products with a bulk density of at least 240 kg m⁻³ for effective truck transport. A pilot-scale roll press compaction machine (520.7 mm roll diameter and 127.0 mm roll width) was used to compress corn stover and perennial grasses (moisture contents of 10% to 18% wet basis) ground in a tub-grinder with three round-hole screen opening sizes (25.4, 76.2, and 127.0 mm). Bulk density of compacted materials (i.e., compacts) ranged from 263 kg m⁻³ to 365 kg m⁻³ for the roll compaction forces of 214–534 kN. Durability (tumbling test) of compacts ranged from 61% to 92%. Specific energy consumption (excluding no-load machine energy) for the roll press compaction ranged from 19 MJ Mg⁻¹ to 339 MJ Mg⁻¹. The results from this study would be useful for developing a biomass supply logistics system involving coarse-grinding followed by roll press compaction of biomass bales.