Biotechnological processes for conversion of corn into ethanol

Ethanol has been utilized as a fuel source in the United States since the turn of the century. However, it has repeatedly faced significant commercial viability obstacles relative to petroleum. Renewed interest exists in ethanol as a fuel source today owing to its positive impact on rural America, the environment and United States energy security. Today, most fuel ethanol is produced by either the dry grind or the wet mill process. Current technologies allow for 2.5 gallons (wet mill process) to 2.8 gallons (dry grind process) of ethanol (1 gallon = 3.785 l) per bushel of corn. Valuable co-products, distillers dried grains with solubles (dry grind) and corn gluten meal and feed (wet mill), are also generated in the production of ethanol. While current supplies are generated from both processes, the majority of the growth in the industry is from dry grind plant construction in rural communities across the corn belt. While fuel ethanol production is an energy-efficient process today, additional research is occurring to improve its long-term economic viability. Three of the most significant areas of research are in the production of hybrids with a higher starch content or a higher extractable starch content, in the conversion of the corn kernel fiber fraction to ethanol, and in the identification and development of new and higher-value co-products.     

2008 National dry mill corn ethanol survey

Emerging regulations require an examination of corn ethanol’s greenhouse gas emissions on a life cycle basis, including emissions from energy consumed at the plant level. However, comprehensive survey data of the industry’s average performance dates back to 2001, prior to the industry’s expansion phase. Responding to the need for updated data, we conducted a survey to collect energy and processing data for average dry mill ethanol produced during 2008. The study finds that the average liter of anhydrous corn ethanol produced during 2008 requires 28% less thermal energy than 2001 ethanol: 7.18 MJ/l compared to 10 MJ/l. Also, 2008 ethanol requires 32% less electricity: 0.195 kWh/l compared to 0.287 kWh/l, but anhydrous ethanol yields from corn are 5.3% higher and total 0.416 l/kg compared to 0.395 l/kg. Findings also suggest that older plants installed energy efficiency retrofits.     

Availability of corn stover as a sustainable feedstock for bioethanol production

The amount of corn stover that can be sustainably collected is estimated to be 80-100 million dry tonnes/yr (t/yr), a majority of which would be available to ethanol plants in the near term as only a small portion is currently used for other applications. Potential long-term demand for corn stover by non-fermentative applications in the United States is estimated to be about 20 million dry t/yr, assuming that corn stover-based products replace 50% of both hardwood pulp and wood-based particleboard, and that 50% of all furfural production is from corncobs. Hence, 60-80 million dry t/yr of corn stover should be available to fermentative routes. To achieve an ethanol production potential of 11 billion L (3 billion gal) per year (a target level for a non-niche feedstock), about 40% of the harvestable corn stover is needed. This amount should be available as long as the diversion of corn stover to non-ethanol fermentative products remains limited.     

Life cycle assessment of fuel ethanol derived from corn grain via dry milling

Life cycle analysis enables to investigate environmental performance of fuel ethanol used in an E10 fueled compact passenger vehicle. Ethanol is derived from corn grain via dry milling. This type of analysis is an important component for identifying practices that will help to ensure that a renewable fuel, such as ethanol, may be produced in a sustainable manner. Based on data from eight counties in seven Corn Belt states as corn farming sites, we show ethanol derived from corn grain as E10 fuel would reduce nonrenewable energy and greenhouse gas emissions, but would increase acidification, eutrophication and photochemical smog, compared to using gasoline as liquid fuel. The ethanol fuel systems considered in this study offer economic benefits, namely more money returned to society than the investment for producing ethanol. The environmental performance of ethanol fuel system varies significantly with corn farming sites because of different crop management practices, soil properties, and climatic conditions. The dominant factor determining most environmental impacts considered here (i.e., greenhouse gas emissions, acidification, eutrophication, and photochemical smog formation) is soil related nitrogen losses (e.g., N2O, NOx, and NO3-). The sources of soil nitrogen include nitrogen fertilizer, crop residues, and air deposition. Nitrogen fertilizer is probably the primary source. Simulations using an agro-ecosystem model predict that planting winter cover crops would reduce soil nitrogen losses and increase soil organic carbon levels, thereby greatly improving the environmental performance of the ethanol fuel system.     

Life Cycle Performance of Cellulosic Ethanol and Corn Ethanol from a Retrofitted Dry Mill Corn Ethanol Plant

This study conducts a life cycle assessment of a simulated dry mill corn ethanol facility in California’s Central Valley retrofitted to also produce ethanol from corn stover, a cellulosic feedstock. The assessment examines three facility designs, all producing corn ethanol and wet distiller’s grains and solubles as a co-product: a baseline facility with no cellulosic retrofit, a facility retrofitted with a small capacity for stover feedstock, and a facility retrofitted for a large capacity of stover feedstock. Corn grain is supplied by rail from the Midwest, while stover is sourced from in-state farms and delivered by truck. Two stover feedstock supply scenarios are considered, testing harvest rates at 25 or 40 % of stover mass. Allocation is required to separate impacts attributable to co-products. Additional scenarios are explored to assess the effect of co-product allocation methods on life cycle assessment results for the two fuel products, corn ethanol and stover ethanol. The assessment tracks greenhouse gas (GHG) emissions, energy consumption, criteria air pollutants, and direct water consumption. The GHG intensity of corn ethanol produced from the three facility designs range between 61.3 and 68.9 g CO₂e/MJ, which includes 19.8 g CO₂e/MJ from indirect land use change for Midwestern corn grain. The GHG intensity of cellulosic ethanol varies from 44.1 to 109.2 g CO₂e/MJ, and 14.6 to 32.1 g CO₂e/MJ in the low and high stover capacity cases, respectively. Total energy input ranges between 0.60 and 0.71 MJ/MJ for corn ethanol and 0.13 to 2.29 MJ/MJ for stover ethanol. This variability is the result of the stover supply scenarios (a function of harvest rate) and co-product allocation decisions.     

An engineering and economic evaluation of wet and dry pre-fractionation processes for dry-grind ethanol facilities

An engineering-economic model was developed to compare the profitability of the wet fractionation process, a generic dry fractionation process, and the conventional dry grind process. Under market conditions as of January 2011, only fractionation processes generated a positive cash flow. Reduced unit manufacturing costs and increased ethanol production capacity were two major contributions. Corn and ethanol price sensitivity analysis showed that the wet fractionation process always outperformed a generic dry fractionation process at any scenario considered in this research. A generic dry fractionation process would provide better economic performance than the conventional dry grind process if corn price was low and ethanol price was high. All three processes would perform more resiliently if the DDGS price was determined by its composition.     

Allocation procedure in ethanol production system from corn grain i. system expansion

We investigated the system expansion approach to net energy analysis for ethanol production from domestic corn grain. Production systems included in this study are ethanol production from corn dry milling and corn wet milling, corn grain production (the agricultural system), soybean products from soybean milling (i.e. soybean oil and soybean meal) and urea production to determine the net energy associated with ethanol derived from corn grain. These five product systems are mutually interdependent. That is, all these systems generate products which compete with or displace all other comparable products in the market place. The displacement ratios between products compare the equivalence of their marketplace functions. The net energy, including transportation to consumers, is 0.56 MJ_net/MJ of ethanol from corn grain regardless of the ethanol production technology employed. Using ethanol as a liquid transportation fuel could reduce domestic use of fossil fuels, particularly petroleum. Sensitivity analyses show that the choice of allocation procedures has the greatest impact on fuel ethanol net energy. Process energy associated with wet milling, dry milling and the corn agricultural process also significantly influences the net energy due to the wide ranges of available process energy values. The system expansion approach can completely eliminate allocation procedures in the foreground system of ethanol production from corn grain.     

Economic analysis of a modified dry grind ethanol process with recycle of pretreated and enzymatically hydrolyzed distillers' grains

A modification of the conventional dry grind process for producing ethanol from yellow dent corn is considered with respect to its economic value. Process modifications include recycling distillers' grains, after being pretreated and hydrolyzed, with the ground corn and water to go through fermentation again and increase ethanol yields from the corn starch. A dry grind financial model, which has been validated against other financial models in the industry, is utilized to determine the financial impact of the process changes. The hypothesis was that the enhanced process would yield higher revenues through additional ethanol sales, and higher valued dried distillers' grains (DDGS), due to its higher protein content, to mitigate the drop in DDGS yields. A 32% increase in net present value (NPV) for the overall operation is expected when applying the process modifications to a 100million gallon ethanol plant, and an enzyme cost of $0.20 for each additional gallon of ethanol produced. However, there may be no value added to the enhanced dried distillers' grains (eDDGS), even in light of its higher protein levels, as current pricing is expected to be more sensitive to the amino acid profile than the total protein level, and the eDDGS has lower lysine levels, a key amino acid. Thus, there is a decrease in revenue from eDDGS due to the combination of no price change and loss of DDGS yield to ethanol. The financial improvements are a result of the increased revenue from higher ethanol yields outpacing the sum of all added costs, which include higher capital costs, larger loan payments, increased operating costs, and decreased revenues from dried distillers' grains.     

Dry biorefining maximizes the potentials of simultaneous saccharification and co‐fermentation for cellulosic ethanol production

Despite the well‐recognized merits of simultaneous saccharification and co‐fermentation (SSCF) on relieving sugar product inhibition on cellulase activity, a practical concomitance difficulty of xylose with inhibitors in the pretreated lignocellulose feedstock prohibits the essential application of SSCF for cellulosic ethanol fermentation. To maximize the SSCF potentials for cellulosic ethanol production, a dry biorefining approach was proposed starting from dry acid pretreatment, disk milling, and biodetoxification of lignocellulose feedstock. The successful SSCF of the inhibitor free and xylose conserved lignocellulose feedstock after dry biorefining reached a record high ethanol titer at moderate cellulase usage and minimum wastewater generation. For wheat straw, 101.4 g/L of ethanol (equivalent to 12.8% in volumetric percentage) was produced with the overall yield of 74.8% from cellulose and xylose, in which the xylose conversion was 73.9%, at the moderate cellulase usage of 15 mg protein per gram cellulose. For corn stover, 85.1 g/L of ethanol (equivalent to 10.8% in volumetric percentage) is produced with the overall conversion of 84.7% from cellulose and xylose, in which the xylose conversion was 87.7%, at the minimum cellulase usage of 10 mg protein per gram cellulose. Most significantly, the SSCF operation achieved the high conversion efficiency by generating the minimum amount of wastewater. Both the fermentation efficiency and the wastewater generation in the current dry biorefining for cellulosic ethanol production are very close to that of corn ethanol production, indicating that the technical gap between cellulosic ethanol and corn ethanol has been gradually filled by the advancing biorefining technology.     

Bacterial contaminants of fuel ethanol production

Bacterial contamination is an ongoing problem for commercial fuel ethanol production facilities. Both chronic and acute infections are of concern, due to the fact that bacteria compete with the ethanol-producing yeast for sugar substrates and micronutrients. Lactic acid levels often rise during bouts of contamination, suggesting that the most common contaminants are lactic acid bacteria. However, quantitative surveys of commercial corn-based fuel ethanol facilities are lacking. For this study, samples were collected from one wet mill and two dry grind fuel ethanol facilities over a 9 month period at strategic time points and locations along the production lines, and bacterial contaminants were isolated and identified. Contamination in the wet mill facility consistently reached 10⁶ bacteria/ml. Titers from dry grind facilities were more variable but often reached 10⁸/ml. Antibiotics were not used in the wet mill operation. One dry grind facility added antibiotic to the yeast propagation tank only, while the second facility dosed the fermentation with antibiotic every 4 h. Neither dosing procedure appeared to reliably reduce overall contamination, although the second facility showed less diversity among contaminants. Lactobacillus species were the most abundant isolates from all three plants, averaging 51, 38, and 77% of total isolates from the wet mill and the first and second dry grind facilities, respectively. Although populations varied over time, individual facilities tended to exhibit characteristic bacterial profiles, suggesting the occurrence of persistent endemic infections.Names are necessary to report factually on available data; however, the USDA neither guarantees nor warrants the standard of the product, and the use of the name by USDA implies no approval of the product to the exclusion of others that may also be suitable.     

Production of ethanol from carbohydrates from loblolly pine: a technical and economic assessment

Ethanol from lignocellulosic biomass has the potential to contribute substantially to bioethanol for transportation. We have evaluated the technical and economic feasibility of producing ethanol from the carbohydrates in loblolly pine. In the process evaluated, prehydrolysis with dilute sulfuric acid was employed to hydrolyze hemicellulose and make the cellulose more accessible to hydrolysis by enzymes. Residual biomass from hydrolysis and extraction of carbohydrates was burned in a CHP plant to generate power and process steam. Our analysis indicates that ethanol can be produced at a cost of dollars 1.53/gal, based on a delivered wood cost of $63.80/dry metric ton and 75% conversion of the carbohydrates in wood to sugars for ethanol production. Improving the conversion of wood carbohydrates to sugars to 95% would reduce the production cost to dollars 1.29/gal. These values are for a plant producing 74 million gal/yr and 93 million gal/yr, respectively. At current feedstock prices, ethanol produced from loblolly pine would be competitive with ethanol produced from corn or other lignocellulosic biomass. Based on our analysis, discounted cash flow rates of return would be 18% and 25%, respectively for plants of this capacity.     

Production of ethanol and xylitol from corn cobs by yeasts

Saccharomyces cerevisiae and Candida tropicalis were used separately and as co-culture for simultaneous saccharification and fermentation (SSF) of 5-20% (w/v) dry corn cobs. A maximal ethanol concentration of 27, 23, 21 g/l (w/v) from 200 g/l (w/v) dry corn cobs was obtained by S. cerevisiae, C. tropicalis and the co-culture, respectively, after 96 h of fermentation. However, theoretical yields of 82%, 71% and 63% were observed from 50 g/l dry corn cobs for the above cultures, respectively. Maximal xylitol concentration of 21, 20 and 15 g/l from 200 g/l (w/v) dry corn cobs was obtained by C. tropicalis, co-culture, and S. cerevisiae, respectively. Maximum theoretical yields of 79.0%, 77.0% and 58% were observed from 50 g/l of corn cobs, respectively. The volumetric productivities for ethanol and xylitol increased with the increase in substrate concentration, whereas, yield decreased. Glycerol and acetic acid were formed as minor by-products. S. cerevisiae and C. tropicalis resulted in better product yields (0.42 and 0.36 g/g) for ethanol and (0.52 and 0.71 g/g) for xylitol, respectively, whereas, the co-culture showed moderate level of ethanol (0.32 g/g) and almost maximal levels of xylitol (0.69 g/g).     

Composition of corn and distillers dried grains with solubles from dry grind ethanol processing

Increase in the demand for ethanol has resulted in growth in the dry grind (DG) ethanol industry. In DG processing, the whole corn kernel is fermented, resulting in two main coproducts, ethanol and distillers dried grains with solubles (DDGS). Marketing of DDGS is critical to the economic stability of DG plants. The composition of DDGS can vary considerably; this reduces market value. Factors that cause variation in composition need to be evaluated. The objective was to determine the relationship between composition of corn and composition of DDGS. Samples of corn and DDGS were obtained from a DG ethanol plant and analyzed for protein, fat, starch and other nutrients. Concentrations of protein, fiber and starch were similar to published data for corn but were higher for DDGS. Coefficients of variation for protein fat and fiber concentrations were similar for corn and DDGS. There were no significant correlations between concentrations of components in corn and those in DDGS. Variation in the composition of DDGS was not related to variation in corn composition and probably was due to variation in processing streams or processing techniques. This implies that reducing the variation in composition of DDG will require modification of processing strategies.     

Effect of soy skim from soybean aqueous processing on the performance of corn ethanol fermentation

The feasibility of using soy skim, a co-product of the aqueous processing of soybeans, in ethanol production from corn was evaluated. Specific growth rates were compared when Saccharomyces cerevisiae was grown in soy skim and peptone-yeast extract media supplemented with glucose. Such soy skim was proved to be a good nitrogen source for yeast growth. Next, fermentation of dry-ground corn to ethanol using soy skim as the media was simulated on 1.5-L scale. Replacing water with soy skim increased the initial ethanol production rates by 4-32% while final ethanol yield was about 39 g/100 g dry corn, similar to the result when water was used. Solid and protein contents in the finished beer increased with the addition of soy skim. Thus, replacing water in corn-ethanol fermentation with soy skim is feasible, and may improve the economics of both aqueous soybean processing and corn ethanol fermentation.     

Grain pearling and very high gravity (VHG) fermentation technologies for fuel alcohol production from rye and triticale

A SATAKE laboratory abrasive mill was used for rye and triticale grain processing. About 12% of dry grain mass was removed after three and five successive abrasions for triticale and rye, respectively. Starch contents in the pearled grain were increased by 8·0% for triticale, and by 7·1% for rye. The pearled rye and triticale were ground and fermented by active dry yeast for fuel alcohol production by very high gravity (VHG) fermentation at 20°C. VHG technology was applied to increase final ethanol concentrations in the fermentors from 9·5–10·0% (v/v) (normal gravity) to 12·9–15·1% (v/v). The grain pearling process coupled with VHG technology further raised the ethanol concentration to 15·7–16·1% (v/v). Partial removal of outer grain solids in an alcohol plant would improve plant efficiency and decrease energy requirements for mash heating, mash cooling, and ethanol distillation.     

Ethanolic fermentation of hydrolysates from ammonia fiber expansion (AFEX) treated corn stover and distillers grain without detoxification and external nutrient supplementation

External nutrient supplementation and detoxification of hydrolysate significantly increase the production cost of cellulosic ethanol. In this study, we investigated the feasibility of fermenting cellulosic hydrolysates without washing, detoxification or external nutrient supplementation using ethanologens Escherichia coli KO11 and the adapted strain ML01 at low initial cell density (16 mg dry weight/L). The cellulosic hydrolysates were derived from enzymatically digested ammonia fiber expansion (AFEX)-treated corn stover and dry distiller's grain and solubles (DDGS) at high solids loading (18% by weight). The adaptation was achieved through selective evolution of KO11 on hydrolysate from AFEX-treated corn stover. All cellulosic hydrolysates tested (36-52 g/L glucose) were fermentable. Regardless of strains, metabolic ethanol yields were near the theoretical limit (0.51 g ethanol/g consumed sugar). Volumetric ethanol productivity of 1.2 g/h/L was achieved in fermentation on DDGS hydrolysate and DDGS improved the fermentability of hydrolysate from corn stover. However, enzymatic hydrolysis and xylose utilization during fermentation were the bottlenecks for ethanol production from corn stover at these experimental conditions. In conclusion, fermentation under the baseline conditions was feasible. Utilization of nutrient-rich feedstocks such as DDGS in fermentation can replace expensive media supplementation.     

Amylolysis of raw corn by Aspergillus niger for simultaneous ethanol fermentation

The novelty of this approach was hydrolysis of the raw starch in ground corn to fermentable sugars that are simultaneously fermented to ethanol by yeast in a non-sterile environment. Thus, the conventional cooking step can be eliminated for energy conservation. A koji of Aspergillus niger grown on whole corn for 3 days was the crude enzyme source. A ratio of 0.2 g dry koji/g total solids was found sufficient. Optimum pH was 4.2. Ethanol concentration was 7.7% (w/w) in the aqueous phase with 92% raw starch conversion. Agitation increased rate. Sacharification was the rate-limiting step. The initial ethanol concentration preventing fermentation was estimated to be 8.3% by weight.     

Conversion of corn milling fibrous co-products into ethanol by recombinant Escherichia coli strains K011 and SL40

Corn hulls and corn germ meal were both evaluated as feedstocks for production of ethanol for biofuel. Currently, these fibrous co-products are combined with corn steep liquor and the fermentation bottoms (if available) and marketed as cattle feed. Samples were obtained from wet and dry corn mills. The corn hulls and germ meal were evaluated for starch and hemicellulose compositions. Starch contents were 12 to 32% w/w and hemicellulose (arabinoxylans) contents were 23 to 64% w/w. Corn fibrous samples were hydrolysed, using dilute sulphuric acid, into mixed sugar streams containing arabinose, glucose and xylose. Total sugar concentrations in the hydrolysate varied from 8.4 to 10.8% w/v. The hydrolysates were fermented to ethanol using recombinant E. coli strains K011 and SL40. Ethanol yields were 0.38 to 0.41g ethanol produced/g total sugars consumed and fermentations were completed in 60h or less. However, residual xylose was detected for each hydrolysate fermentation and was especially significant for fermentations using strain SL40. Strain K011 was a superior ethanologenic strain compared with strain SL40 in terms of both ethanol yield and maximum productivity.     

Improvement in fermentation characteristics of degermed ground corn by lipid supplementation

With rapid growth of fuel ethanol industry, and concomitant increase in distillers dried grains with solubles (DDGS), new corn fractionation technologies that reduce DDGS volume and produce higher value coproducts in dry grind ethanol process have been developed. One of the technologies, a dry degerm, defiber (3D) process (similar to conventional corn dry milling) was used to separate germ and pericarp fiber prior to the endosperm fraction fermentation. Recovery of germ and pericarp fiber in the 3D process results in removal of lipids from the fermentation medium. Biosynthesis of lipids, which is important for cell growth and viability, cannot proceed in strictly anaerobic fermentations. The effects of ten different lipid supplements on improving fermentation rates and ethanol yields were studied and compared to the conventional dry grind process. Endosperm fraction (from the 3D process) was mixed with water and liquefied by enzymatic hydrolysis and was fermented using simultaneous saccharification and fermentation. The highest ethanol concentration (13.7% v/v) was achieved with conventional dry grind process. Control treatment (endosperm fraction from 3D process without lipid supplementation) produced the lowest ethanol concentration (11.2% v/v). Three lipid treatments (fatty acid ester, alkylphenol, and ethoxylated sorbitan ester 1836) were most effective in improving final ethanol concentrations. Fatty acid ester treatment produced the highest final ethanol concentration (12.3% v/v) among all lipid supplementation treatments. Mean final ethanol concentrations of alkylphenol and ethoxylated sorbitan ester 1836 supplemented samples were 12.3 and 12.0% v/v, respectively.Mention of brand or firm names does not constitute an endorsement by University of Illinois or USDA above others of similar nature not mentioned     

Implications of corn prices on water footprints of bioethanol

Previously reported water footprints (WFPs) of corn ethanol have been estimated based on the assumption that corn ethanol feedstock could be supplied by the same states where the corn is grown. However, ethanol conversion facilities may choose out-of-state feedstock suppliers depending on the total price of feedstock they have to pay including both the corn price and transportation costs. The purpose of this study is to evaluate the WFPs and total water use (TWU) of corn ethanol considering an optimal allocation of corn with heterogeneous corn feedstock prices across states. The results show that the WFPs of corn ethanol are less than 100 l of water per liter of ethanol (Lw/Le) for all ethanol-producing states based on both the 2008 corn price and transportation costs for rail and truck. Results also reveal that WFPs are very sensitive to the market price of corn and that additional greenhouse gas emissions due to corn trade between states are not significant.