Farm-Scale Cost of Producing Perennial Energy Cane as a Biofuel Feedstock

Energy cane varieties are high-fiber sugarcane clones which represent a promising feedstock in the production of alternative biofuels and biobased products. This study explored the crop establishment and whole farm production costs of growing energy cane as a biofuel feedstock in the southeastern USA. More specifically, total production costs on a feedstock dry matter biomass basis were estimated for five perennial energy cane varieties over alternative crop cycle lengths. Variable production costs for energy cane production were estimated to be in the $63 to $76 Mg^?1 range of biomass dry matter for crop cycles through harvest of fourth through sixth stubble crops. Total production costs, including charges for fixed equipment costs, general farm overhead, and land rent, were estimated to range between $105 and $127 Mg^?1 of feedstock biomass dry matter material.     

Integrated Economic and Environmental Assessment of Cellulosic Biofuel Production in an Agricultural Watershed

SWAT watershed model simulated biomass yield and pollutant loadings were integrated with associated economic costs of farm production and transport to study two dedicated energy crops, switchgrass and Miscanthus, and corn stover, as feedstocks for a cellulosic biorefinery. A multi-level spatial optimization (MLSOPT) framework was employed to get spatially explicit cropping plans for a watershed under the assumption that the watershed supplies biomass to a hypothetical biorefinery considering both the biochemical and the thermochemical conversion pathways. Consistent with previous studies, the perennial grasses had higher biomass yield than corn stover, with considerably lower sediment, nitrogen, and phosphorus loadings, but their costs were higher. New insights were related to the tradeoffs between cost, feedstock production, and the level and form of environmental quality society faces as it implements the Renewable Fuel Standard. Economically, this involved calculating the farthest distance a biorefinery would be willing to drive to source corn residue before procuring a single unit of perennial grasses from productive agricultural soils.     

The challenge of enzyme cost in the production of lignocellulosic biofuels

With the aim of understanding the contribution of enzymes to the cost of lignocellulosic biofuels, we constructed a techno-economic model for the production of fungal cellulases. We found that the cost of producing enzymes was much higher than that commonly assumed in the literature. For example, the cost contribution of enzymes to ethanol produced by the conversion of corn stover was found to be $0.68/gal if the sugars in the biomass could be converted at maximum theoretical yields, and $1.47/gal if the yields were based on saccharification and fermentation yields that have been previously reported in the scientific literature. We performed a sensitivity analysis to study the effect of feedstock prices and fermentation times on the cost contribution of enzymes to ethanol price. We conclude that a significant effort is still required to lower the contribution of enzymes to biofuel production costs.     

Specific energy requirement for compacting corn stover

Corn stover is a major crop residue for biomass conversion to produce chemicals and fuels. One of the problems associated with the supply of corn stover to conversion plants is the delivery of feedstock at a low cost. Corn stover has low bulk density and it is difficult to handle. In this study, chopped corn stover samples were compacted in a piston cylinder under three pressure levels (5, 10, 15 MPa) and at three moisture content levels (5%, 10%, 15% (wb)) to produce briquettes. The total energy requirement to compress and extrude briquette ranged from 12 to 30 MJ/t. The briquette density ranged from 650 to 950 kg/m3 increasing with pressure. Moisture content had also a significant effect on briquette density, durability and stability. Low moisture stover (5-10%) resulted in denser, more stable and more durable briquettes than high moisture stover (15%).     

Functional group and fertilization affect the composition and bioenergy yields of prairie plants

Prairies used for bioenergy production have potential to generate marketable products while enhancing environmental quality, but little is known about how prairie species composition and nutrient management affect the suitability of prairie biomass for bioenergy production. We determined how functional‐group identity and nitrogen fertilization affected feedstock characteristics and estimated bioenergy yields of prairie plants, and compared those prairie characteristics to that of corn stover. We tested our objectives with a field experiment that was set up as a 5 × 2 incomplete factorial design with C₃ grasses, C₄ grasses, legumes, and multi‐functional‐group mixtures grown with and without nitrogen fertilizer; a fertilized corn treatment was also included. We determined cell wall, hemicellulose, cellulose, and ash concentrations; ethanol conversion ratios; gross caloric ratios; aboveground biomass production; ethanol yields; and energy yields for all treatments. Prairie functional‐group identity affected the biomass feedstock characteristics, whereas nitrogen fertilization did not. Functional group and fertilization had a strong effect on aboveground biomass production, which was the major predictor of ethanol and energy yields. C₄ grasses, especially when fertilized, had among the most favorable bioenergy characteristics with high estimated ethanol conversion ratios and nongrain biomass production, relatively high gross caloric ratios, and low ash concentrations. The bioenergy characteristics of corn stover, from an annual C₄ grass, were similar to those of the biomass of perennial C₄ grasses. Both functional‐group composition and nitrogen fertility management were found to be important in optimizing bioenergy production from prairies.     

The Economics of Biomass Collection and Transportation and Its Supply to Indiana Cellulosic and Electric Utility Facilities

With cellulosic energy production from biomass becoming popular in renewable energy research, agricultural producers may be called upon to plant and collect corn stover or harvest switchgrass to supply feedstocks to nearby facilities. Determining the production and transportation cost to the producer of corn stover or switchgrass and the amount available within a given distance from the plant will result in a per metric ton cost the plant will need to pay producers in order to receive sufficient quantities of biomass. This research computes up-to-date biomass production costs using recent prices for all important cost components including seed, fertilizer, herbicide, mowing/shredding, raking, baling, storage, handling, and transportation. The cost estimates also include nutrient replacement for corn stover. The total per metric ton cost is a combination of these cost components depending on whether equipment is owned or custom hired, what baling options are used, the size of the farm, and the transport distance. Total costs per dry metric ton for biomass with a transportation distance of 60 km ranges between $63 and $75 for corn stover and $80 and $96 for switchgrass. Using the county quantity data and this cost information, we then estimate biomass supply curves for three Indiana coal-fired electric utilities. This supply framework can be applied to plants of any size, location, and type, such as future cellulosic ethanol plants. Finally, greenhouse gas emissions reductions are estimated from using biomass instead of coal for part of the utility energy and also the carbon tax required to make the biomass and coal costs equivalent. Depending on the assumed CO₂ price, the use of biomass instead of coal is found to decrease overall costs in most cases.     

Process Simulation and Cost Analysis for Removing Inorganics from Wood Chips Using Combined Mechanical and Chemical Preprocessing

Inorganic species (ash) in biomass feedstocks negatively impact thermochemical and biochemical energy conversion processes. In this work, a process simulation model is developed to model the reduction in ash content of loblolly logging residues using a combination of air classification and dilute-acid leaching. Various scenarios are considered, and it is found that costs associated with discarding high-ash material from air classification are substantial. The costs of material loss can be reduced by chemical leaching the high-ash fraction obtained from air classification. The optimal leaching condition is found to be approximately 0.1 wt% sulfuric acid at 24 °C. In example scenarios, total process costs in the range of $6–9?/dry tons of product are projected that result in a removal of 14, 62, 39, and 88 % of organics, total ash (inorganics), alkaline earth metals and phosphorus (AAEMS + P), and silicon, respectively. Sensitivity analyses indicate that costs associated with loss of organic material during processing (yield losses), brine disposal, and labor have the greatest potential to impact the total processing cost.     

Development of Sustainable Corn Stover Harvest Strategies for Cellulosic Ethanol Production

To prepare for a 2014 launch of commercial scale cellulosic ethanol production from corn/maize (Zea mays L.) stover, POET-DSM near Emmetsburg, IA has been working with farmers, researchers, and equipment dealers through “Project Liberty” on harvest, transportation, and storage logistics of corn stover for the past several years. Our objective was to evaluate seven stover harvest strategies within a 50-ha (125 acres) site on very deep, moderately well to poorly drained Mollisols, developed in calcareous glacial till. The treatments included the following: conventional grain harvest (no stover harvest), grain plus a second-pass rake and bale stover harvest, and single-pass grain plus cob-only biomass, grain plus vegetative material other than grain [(MOG) consisting of cobs, husks, and upper plant parts], grain plus all vegetative material from the ear shank upward (high cut), and all vegetative material above a 10 cm stubble height (low cut), with a John Deere 9750 STS combine, and grain plus direct baling of MOG with an AgCo harvesting system. Average grain yields were 11.4, 10.1, 9.7, and 9.5 Mg ha^?1 for 2008, 2009, 2010, and 2011, respectively. Average stover harvest ranged from 0 to 5.6 Mg ha^?1 and increased N, P, and K removal by an average of 11, 1.6, and 15 kg Mg^?1, respectively. Grain yield in 2009 showed a significant positive response to higher 2008 stover removal rates, but grain yield was not increased in 2010 or 2011 due to prior-year stover harvest. High field losses caused the direct-bale treatment to have significantly lower grain yield in 2011 because the AgCo system could not pick up the severely lodged crop. We conclude that decreases in grain yield across the 4 years were due more to seasonal weather patterns, spatial variability, and not rotating crops than to stover harvest.     

Crop Residue Considerations for Sustainable Bioenergy Feedstock Supplies

The anticipated 2014 launch of three full-scale corn stover bioenergy conversion facilities is a strong US market signal that cellulosic feedstock supplies must increase dramatically to supply the required biomass in a sustainable manner. This overview highlights research conducted by the USDA-Agricultural Research Service Renewable Energy Assessment Project (now known as the Resilient Economic Agricultural Practices) team as part of the National Institute for Food and Agriculture Sun Grant Regional Feedstock Partnership Corn Stover team. Stover and grain yield, soil organic carbon, soil aggregation, greenhouse gas, energy content of the stover, and several other factors affecting the fledgling bioenergy industry are addressed in this special issue of the journal.     

Effect of anatomical fractionation on the enzymatic hydrolysis of acid and alkaline pretreated corn stover

Due to concerns with biomass collection systems and soil sustainability there are opportunities to investigate the optimal plant fractions to collect for conversion. An ideal feedstock would require a low severity pretreatment to release a maximum amount of sugar during enzymatic hydrolysis. Corn stover fractions were separated manually and analyzed for glucan, xylan, acid soluble lignin, acid insoluble lignin, and ash composition. The stover fractions were also pretreated with either 0%, 0.4%, or 0.8% NaOH for 2 h at room temperature, washed, autoclaved and saccharified. In addition, dilute sulfuric acid pretreated samples underwent simultaneous saccharification and fermentation (SSF) to ethanol. In general, the two pretreatments produced similar trends with cobs, husks, and leaves responding best to the pretreatments, the tops of stalks responding slightly less, and the bottom of the stalks responding the least. For example, corn husks pretreated with 0.8% NaOH released over 90% (standard error of 3.8%) of the available glucan, while only 45% (standard error of 1.1%) of the glucan was produced from identically treated stalk bottoms. Estimates of the theoretical ethanol yield using acid pretreatment followed by SSF were 65% (standard error of 15.9%) for husks and 29% (standard error of 1.8%) for stalk bottoms. This suggests that integration of biomass collection systems to remove sustainable feedstocks could be integrated with the processes within a biorefinery to minimize overall ethanol production costs.     

Effect of stover fraction and storage method on glucose production during enzymatic hydrolysis

One avenue for overcoming the economic challenges associated with the production of ethanol from renewable resources is to reduce the cost of the biomass feedstock. The balance between storage costs and benefits depend on the storage method and composition changes of individual stover fractions. Corn stover from bales stored inside and outside of a barn was separated into an interior and exterior layer after approximately 10 months of storage. The cobs, stalks, and leaves and husks were separated, dried, and ground through a 2 mm screen. Stover, sodium acetate (buffer), cellulase, and deionized water were added to 125 ml flasks. The mixture was held at 50 degrees C in an incubator and samples taken for glucose determination. The average glucose concentration after 60 h of hydrolysis from cobs, leaves and husks, and stalks was 10.5, 9.6, and 3.1 g/l, respectively. Cobs, leaves, and husks produced over 300% more glucose than stalks. Storage outside of the barn decreased the glucose production from individual stover components between 4% and 8%. The effect of stover fraction type on glucose production was significant, while the storage treatment effect was not significant. Fractionation of corn stover may be a method to increase the value of corn stover as a feedstock for glucose production.     

Particle concentration and yield stress of biomass slurries during enzymatic hydrolysis at high‐solids loadings

Effective and efficient breakdown of lignocellulosic biomass remains a primary barrier for its use as a feedstock for renewable transportation fuels. A more detailed understanding of the material properties of biomass slurries during conversion is needed to design cost‐effective conversion processes. A series of enzymatic saccharification experiments were performed with dilute acid pretreated corn stover at initial insoluble solids loadings of 20% by mass, during which the concentration of particulate solids and the rheological property yield stress (τ_y) of the slurries were measured. The saccharified stover liquefies to the point of being pourable (τ_y ≤ 10 Pa) at a total biomass conversion of about 40%, after roughly 2 days of saccharification for a moderate loading of enzyme. Mass balance and semi‐empirical relationships are developed to connect the progress of enzymatic hydrolysis with particle concentration and yield stress. The experimental data show good agreement with the proposed relationships. The predictive models developed here are based on established physical principles and should be applicable to the saccharification of other biomass systems. The concepts presented, especially the ability to predict yield stress from extent of conversion, will be helpful in the design and optimization of enzymatic hydrolysis processes that operate at high‐solids loadings. Biotechnol. Bioeng. 2009; 104: 290–300 © 2009 Wiley Periodicals, Inc.     

Quantitative Sustainability Assessment of Seaweed Biomass as Bioethanol Feedstock

Since terrestrial biomass-based ethanol has environmental and economic vulnerability, seaweed-based bioethanol is emerging as a new biofuel. To investigate the sustainability of seaweeds as bioethanol feedstock, this study quantitatively assesses the energy, freshwater, and fertilizer requirements; land-related carbon balance; and bioethanol productivity of seaweed biomass through comparison with terrestrial biomass. Also, the metal resource potential of seaweeds is assessed because valuable metals can be recovered from seaweed fermentation residue. Compared to corn grain and stover, seaweeds exhibit competitive energy requirements and ethanol productivity. Seaweed cultivation does not incur carbon debt derived from land use change and requires less freshwater than corn grain but more than switchgrass in cultivation and fermentation. Seaweed cultivation also does not require fertilizer application despite the high content of nitrogen and phosphorus. Seaweeds exhibit high resource potential for gold and silver. Therefore, seaweed biomass has high potential as a sustainable bioethanol feedstock.     

Raw materials evaluation and process development studies for conversion of biomass to sugars and ethanol

A range of cellulosic raw materials in the form of agricultural crop residue was analyzed for chemical composition and assessed for potential yields of sugars through chemical pretreatment and enzymatic hydrolysis of these materials. Corn stover was used as a representative raw material for a preliminary process design and economic assessment of the production of sugars and ethanol. With the process as presently developed, 24 gal ethanol can be obtained per ton of corn stover at a processing cost of about $1.80/gal exclusive of by-product credits. The analysis shows the cost of ethanol to be highly dependent upon: (1) the cost of the biomass, (2) the extent of conversion to glucose, (3) enzyme recovery and production cost, and (4) potential utilization of xylose. Significant cost reduction appears possible through further research in these directions.     

Production, transportation and milling costs of sweet sorghum as a feedstock for centralized bioethanol production in the upper Midwest

Sweet sorghum has been identified as a possible ethanol feedstock because of its biomass yield and high concentration of readily fermentable sugars. It has found limited use, however, because of poor post-harvest storage characteristics and short harvest window in cooler climates. Previous research (Bennett, A.S., Anex, R.P., 2008. Farm-gate production costs of sweet sorghum as a bioethanol feedstock. Transactions of the ASABE 51(2), 603-613) indicates that fermentable carbohydrates (FC) can be produced at less expense from sweet sorghum than from corn grain. Previous research, however, did not include costs associated with off-farm transportation, storage, or capital costs associated with milling and energy recovery equipment that are required to provide FC suitable for biological conversion. This study includes these additional costs and reevaluates sweet sorghum as a biocommodity feedstock. A total of eight harvest-transport-processing options are modeled, including 4-row self-propelled and 2-row tractor-pulled forage harvesters, two different modes of in-field transport, fresh processing, on-farm ensilage and at-plant ensilage. Monte Carlo simulation and sensitivity analysis are used to account for system variability and compare scenarios. Transportation costs are found to be significant ranging from $33 to $71 Mg (-1) FC, with highest costs associated with at-plant ensilage scenarios. Economies of scale benefit larger milling equipment and boiler systems reducing FC costs by more than 50% when increasing annual plant capacity from 37.9 to 379 million liters. Ensiled storage of high moisture sweet sorghum in bunkers can lead to significant losses of FC (>20%) and result in systems with net FC costs well above those of corn-derived FC. Despite relatively high transport costs, seasonal, fresh processed sweet sorghum is found to produce FC at costs competitive with corn grain derived FC.     

Concentrated HCl Catalyzed 5-(Chloromethyl)furfural Production from Corn Stover of Varying Particle Sizes

5-(Chloromethyl) Furfural (CMF) is a potential chemical building block for replacing petroleum-derived chemicals derived from lignocellulosic feedstocks. In this study, hand harvested corn stover and mechanically forage chopped corn stover was processed in a 1 L hydrolysis reactor to produce CMF in a biphasic, two solvent system. Both 1,2 dichloroethane (DCE) and dichloromethane (DCM) were tested as organic solvents. The results showed that DCE performed better than DCM due to temperature and pressure limitations of the reactor system. Using DCE as the extracting solvent, the effects of solids loading, particle size, and moisture content of the corn stover on the hydrolysis efficiency were determined. One liter acid hydrolysis reactor provides consistent and reproducible yields of 63% CMF from hand harvested corn stover as feedstock at solid loading of 10% wt/v, 100C for 1 h. For the forage chopped corn stover, increasing particle size brings an increase in the feedstock sugar content. Foraged chopped corn stover (FCCS) particle sizes larger than 19 mm (0.75 in.) results in significant reduction in CMF yield from 43 to 35%.     

Biomass transportation model and optimum plant size for the production of ethanol

A detailed model based on a non-dimensional transportation factor is developed to assess the economics of biomass collection, transportation, and storage. The optimum plant size for bio-refineries is investigated; ethanol production from corn stover via dilute acid hydrolysis is presented as a case study. The conversion of straight-line, farm-to-plant distances to road distances via a winding factor leads to a shift in the distribution of transportation distances towards shorter hauls. The capital investment scaling exponent was calculated using the model developed at the National Renewable Energy Laboratory (Aden et al., NREL/TP-510-32438, 2002) and found to be 0.7. The cost of the delivered corn stover is proportional to the square root of the inverse of the farmer participation; as a consequence, bio-fuel producers intending to use agricultural residues as feedstock should work towards a farmer participation of fifty percent. Costs associated with storage represent a significant portion of the production cost.     

Quantification of morphochemical changes during in situ enzymatic hydrolysis of individual biomass particles based on autofluorescence imaging

Enzymatic hydrolysis of biomass is an established method for producing biofuels. Lignocellulosic biomass such as corn stover is very inhomogeneous material with big variation on conversion rates between individual particles therefore leading to variable recalcitrance results. In this study, we used noninvasive optical microscopy techniques, such as two-photon microscopy and fluorescence lifetime imaging microscopy, to visualize and analyze morphological and chemical changes of individual corn stover particles pretreated with sulfuric acid during hydrolysis. Morphochemical changes were interpreted based on the fluorescence properties of isolated building blocks of plant cell wall, such as cellulose, hemicellulose, and lignin. Enzymatic hydrolysis resulted in particle size reduction, side wall collapse, decrease of second harmonic signal from cellulose, redshifting of autofluorescence emission, and lifetime decrease attributed to the relative increase of lignin. Based on these observations, tracking compositional change after hydrolysis of individual particles was accomplished. The methodologies developed offer a paradigm for imaging and analyzing enzymatic hydrolysis in vitro and in situ, which could be used for screening enzymes cocktails targeting specific recalcitrant structures or investigating locally enzyme anti-inhibitory agents.     

Methane fermentation of selected lignocellulosic materials

Seven lignocellulosic materials: corn stover, napier grass, wood grass, newspaper, white fir and wheat straw from two different crops; two pure cellulosics: Solka Floc BW200 and Whatman No. 5 filter paper; and glucose, propionic and acetic acids were subjected to long-term batch methane fermentation. Ninety per cent of the original COD was recovered as methane gas from the two pure cellulosics and glucose. For the lignocellulosics, depending on the material, variations from over 80% conversion efficiency to methane for corn stover to less than 10% for white fir were observed. Generally, herbaceous materials were degraded faster and more extensively than woody biomass. A first-order rate model described well the methane fermentation process for the lignocellulosics tested, but was a poor model for the soluble substrates. It was not possible to predict either the biodegradability or the rate of methane fermentation with a reasonable degree of accuracy based solely on the lignin content of the lignocellulosic materials.     

Maximizing the liquid fuel yield in a biorefining process

Biorefining strives to recover the maximum value from each fraction, at minimum energy cost. In order to seek an unbiased and thorough assessment of the alleged opportunity offered by biomass fuels, the direct conversion of various lignocellulosic biomass was studied: aspen pulp wood (Populus tremuloides), aspen wood pretreated with dilute acid, aspen lignin, aspen logging residues, corn stalk, corn spathe, corn cob, corn stover, corn stover pellet, corn stover pretreated with dilute acid, and lignin extracted from corn stover. Besides the heating rate, the yield of liquid products was found to be dependent on the final liquefaction temperature and the length of liquefaction time. The major compounds of the liquid products from various origins were identified by GC-MS. The lignin was found to be a good candidate for the liquefaction process, and biomass fractionation was necessary to maximize the yield of the liquid bio-fuel. The results suggest a biorefinery process accompanying pretreatment, fermentation to ethanol, liquefaction to bio-crude oil, and other thermo-conversion technologies, such as gasification. Other biorefinery options, including supercritical water gasification and the effectual utilization of the bio-crude oil, are also addressed.