Potential synergies and challenges in refining cellulosic biomass to fuels,chemicals, and power

Lignocellulosic biomass such as agricultural and forestry residues and dedicated crops provides a low-cost and uniquely sustainable resource for production of many organic fuels and chemicals that can reduce greenhouse gas emissions, enhance energy security, improve the economy, dispose of problematic solid wastes, and improve air quality. A technoeconomic analysis of biologically processing lignocellulosics to ethanol is adapted to project the cost of making sugar intermediates for producing a range of such products, and sugar costs are predicted to drop with plant size as a result of economies of scale that outweigh increased biomass transport costs for facilities processing less than about 10,000 dry tons per day. Criteria are then reviewed for identifying promising chemicals in addition to fuel ethanol to make from these low cost cellulosic sugars. It is found that the large market for ethanol makes it possible to achieve economies of scale that reduce sugar costs, and coproducing chemicals promises greater profit margins or lower production costs for a given return on investment. Additionally, power can be sold at low prices without a significant impact on the selling price of sugars. However, manufacture of multiple products introduces additional technical, marketing, risk, scale-up, and other challenges that must be considered in refining of lignocellulosics.     

Techno-economic analysis of biomass to fuel conversion via the MixAlco process

MixAlco is a robust process that converts biomass to fuels and chemicals. A key feature of the MixAlco process is the fermentation, which employs a mixed culture of acid-forming microorganisms to convert biomass components (carbohydrates, proteins, and fats) to carboxylate salts. Subsequently, these intermediate salts are chemically converted to hydrocarbon fuels (gasoline, jet fuel, and diesel). This work focuses on process synthesis, simulation, integration, and cost estimation of the MixAlco process. For the base-case capacity of 40 dry tonne feedstock per hour, the total capital investment is US $5.54/annual gallon of hydrocarbon fuels (US $5.54/annual gallon of hydrocarbon fuels (US 3.79/annual gallon of ethanol equivalent), and the minimum selling price [with 10% return on investment (ROI), internal hydrogen production, and US $60/tonne biomass] is US $60/tonne biomass] is US 2.56/gal hydrocarbon, which is equivalent to US $1.75/gal ethanol. If plant capacity is increased to 400 tph, the minimum selling price of biomass-derived hydrocarbon fuels is US $1.75/gal ethanol. If plant capacity is increased to 400 tph, the minimum selling price of biomass-derived hydrocarbon fuels is US 1.76/gal hydrocarbon (US $1.20/gal ethanol equivalent), which can compete without subsidies with petroleum-derived hydrocarbons when crude oil sells for about US $1.20/gal ethanol equivalent), which can compete without subsidies with petroleum-derived hydrocarbons when crude oil sells for about US 65/bbl. At 40 tph, using the average tipping fee for municipal solid waste (US $45/dry tonne) and current price of external hydrogen (US $45/dry tonne) and current price of external hydrogen (US 1/kg), the minimum selling price is only US $1.24/gal hydrocarbon (US $1.24/gal hydrocarbon (US 0.85/gal ethanol equivalent).     

Chemical and morphological characterization of sugarcane bagasse submitted to a delignification process for enhanced enzymatic digestibility

Background

While advantages of biofuel have been widely reported, studies also highlight the challenges in large scale production of biofuel. Cost of ethanol and process energy use in cellulosic ethanol plants are dependent on technologies used for conversion of feedstock. Process modeling can aid in identifying techno-economic bottlenecks in a production process. A comprehensive techno-economic analysis was performed for conversion of cellulosic feedstock to ethanol using some of the common pretreatment technologies: dilute acid, dilute alkali, hot water and steam explosion. Detailed process models incorporating feedstock handling, pretreatment, simultaneous saccharification and co-fermentation, ethanol recovery and downstream processing were developed using SuperPro Designer. Tall Fescue (Festuca arundinacea Schreb) was used as a model feedstock.

Results

Projected ethanol yields were 252.62, 255.80, 255.27 and 230.23 L/dry metric ton biomass for conversion process using dilute acid, dilute alkali, hot water and steam explosion pretreatment technologies respectively. Price of feedstock and cellulose enzymes were assumed as $50/metric ton and 0.517/kg broth (10% protein in broth, 600 FPU/g protein) respectively. Capital cost of ethanol plants processing 250,000 metric tons of feedstock/year was $1.92, $1.73, $1.72 and $1.70/L ethanol for process using dilute acid, dilute alkali, hot water and steam explosion pretreatment respectively. Ethanol production cost of $0.83, $0.88, $0.81 and $0.85/L ethanol was estimated for production process using dilute acid, dilute alkali, hot water and steam explosion pretreatment respectively. Water use in the production process using dilute acid, dilute alkali, hot water and steam explosion pretreatment was estimated 5.96, 6.07, 5.84 and 4.36 kg/L ethanol respectively.

Conclusions

Ethanol price and energy use were highly dependent on process conditions used in the ethanol production plant. Potential for significant ethanol cost reductions exist in increasing pentose fermentation efficiency and reducing biomass and enzyme costs. The results demonstrated the importance of addressing the tradeoffs in capital costs, pretreatment and downstream processing technologies.     

An economic analysis of using alternative fuels in a mass burn boiler

In this study the economic feasibility of using alternative fuels in a mass burn boiler for a chemical plant in northeastern Missouri is analyzed. The key consideration is whether biomass (switchgrass and crop residues) is economically preferred to other available fuels. Research reveals an abundance of alternative fuels for which the plant would receive a tipping fee, including municipal solid waste and used tires. Since the plant would have to pay for biomass, it does not appear in the optimal solution. An economic optimization model shows the marginal cost to the plant of using biomass would increase as more biomass is used, displacing quantities of more valuable (in terms of tipping fees per BTU) waste materials.     

An Olive Tree Pruning Biorefinery for Co-Producing High Value-Added Bioproducts and Biofuels: Economic and Energy Efficiency Analysis

This work presents a conceptual design of an integrated biorefinery using olive tree pruning as feedstock. The biorefinery combines a state-of-the-art thermochemical technology for producing high value-added antioxidants with an energy self-sufficient biochemical platform for lignocellulosic ethanol production. These plants are integrated by exchanging energy and feedstock. The process and design parameters employed in the plant designs are based on the authors’ own lab and pilot-scale data. The paper discusses the economic dilemma of using this feedstock for producing high value-added products in small amounts versus producing large amounts of low-profit biofuels. The feasibility of this production strategy at medium scale is demonstrated via a techno-economic analysis based on total production cost for each co-product. Each plant is energy integrated, and the energy performance of the bioethanol plant is assessed by calculating the end-use-energy ratio. Both analyses are parameterized with respect to plant capacity (100–1500 t dry weight (dw)/day) and raw material price (20–100 €/ton dry weight).     

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.     

Softwood forest thinnings as a biomass source for ethanol production: a feasibility study for California

A plan has been put forth to strategically thin northern California forests to reduce fire danger and improve forest health. The resulting biomass residue, instead of being open burned, can be converted into ethanol that can be used as a fuel oxygenate or an octane enhancer. Economic potential for a biomass-to-ethanol facility using this softwood biomass was evaluated for two cases: stand-alone and co-located. The co-located case refers to a specific site with an existing biomass power facility near Martell, California. A two-stage dilute acid hydrolysis process is used for the production of ethanol from softwoods, and the residual lignin is used to generate steam and electricity. For a plant processing 800 dry tonnes per day of feedstock, the co-located case is an economically attractive concept. Total estimated capital investment is approximately $70 million for the co-located plant, and the resulting internal rate of return (IRR) is about 24% using 25% equity financing. A sensitivity analysis showed that ethanol selling price and fixed capital investment have a substantial effect on the IRR. It can be concluded that such a biomass-to-ethanol plant seems to be an appealing proposition for California, if ethanol replaces methyl tert-butyl ether, which is slated for a phaseout.     

Conversion of paper sludge to ethanol in a semicontinuous solids-fed reactor

Conversion of paper sludge to ethanol was investigated with the objective of operating under conditions approaching those expected of an industrial process. Major components of the bleached Kraft sludge studied were glucan (62 wt.%, dry basis), xylan (11.5%), and minerals (17%). Complete recovery of glucose during compositional analysis required two acid hydrolysis treatments rather than one. To avoid the difficulty of mixing unreacted paper sludge, a semicontinuous solids-fed laboratory bioreactor system was developed. The system featured feeding at 12-h intervals, a residence time of 4 days, and cellulase loading of 15 to 20 FPU/g cellulose. Sludge was converted to ethanol using simultaneous saccharification and fermentation (SSF) featuring a -glucosidase-supplemented commercial cellulase preparation and glucose fermentation by Saccharomyces cerevisiea. SSF was carried out for a period of 4 months in a first-generation system, resulting in an average ethanol concentration of 35 g/L. However, steady state was not achieved and operational difficulties were encountered. These difficulties were avoided in a retrofitted design that was operated for two 1-month runs, achieving steady state with good material balance closure. Run 1 with the retrofitted reactor produced 50 g/L ethanol at a cellulose conversion of 74%. Run 2 produced 42 g/L ethanol at a conversion of 92%. For run 2, the ethanol yield was 0.466 g ethanol/g glucose equivalent fermented and >94% of the xylan fed to the reactor was solubilized to a mixture of xylan oligomers and xylose.     

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.     

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.     

Conversion of cellobiose and xylose to ethanol by immobilized growing cells of Clostridium saccharolyticum on charcoal support

Summary An immobilization technique has been developed for the conversion of both cellobiose and xylose to ethanol, which may be considered as one stage of a process for the conversion of cellulosic biomass to ethanol. Relatively inexpensive charcoal was used as a support material, with 23 mg dry weight of Clostridium saccharolyticum cells per g dry weight of support. Tests were run for 170 h at 0.15 1/h dilution rate. From a 3% (w/v) sugar mixture, 0.7% (w/v) ethanol was obtained with over 97% cellobiose and 62% xylose utilization.     

Advanced monitoring of high-rate anaerobic reactors through quantitative image analysis of granular sludge and multivariate statistical analysis

Four organic loading disturbances were performed in lab-scale EGSB reactors fed with ethanol. In load disturbance 1 (LD1) and 2 (LD2), the organic loading rate (OLR) was increased between 5 and 18.5 kg COD m(-3) day(-1), through the influent ethanol concentration increase, and the hydraulic retention time decrease from 7.8 to 2.5 h, respectively. Load disturbances 3 (LD3) and 4 (LD4) were applied by increasing the OLR to 50 kg COD m(-3) day(-1) during 3 days and 16 days, respectively. The granular sludge morphology was quantified by image analysis and was related to the reactor performance, including effluent volatile suspended solids, indicator of washout events. In general, it was observed the selective washout of filamentous forms associated to granules erosion/fragmentation and to a decrease in the specific acetoclastic activity. These phenomena induced the transitory deterioration of reactor performance in LD2, LD3, and LD4, but not in LD1. Extending the exposure time in LD4 promoted acetogenesis inhibition after 144 h. The application of Principal Components Analysis determined a latent variable that encompasses a weighted sum of performance, physiological and morphological information. This new variable was highly sensitive to reactor efficiency deterioration, enclosing variations between 27% and 268% in the first hours of disturbances. The high loadings raised by image analysis parameters, especially filaments length per aggregates area (LfA), revealed that morphological changes of granular sludge, should be considered to monitor and control load disturbances in high rate anaerobic (granular) sludge bed digesters.     

Plant genetic engineering to improve biomass characteristics for biofuels

Currently, most ethanol produced in the United States is derived from maize kernel, at levels in excess of four billion gallons per year. Plant lignocellulosic biomass is renewable, cheap and globally available at 10-50 billion tons per year. At present, plant biomass is converted to fermentable sugars for the production of biofuels using pretreatment processes that disrupt the lignocellulose and remove the lignin, thus allowing the access of microbial enzymes for cellulose deconstruction. Both the pretreatments and the production of enzymes in microbial tanks are expensive. Recent advances in plant genetic engineering could reduce biomass conversion costs by developing crop varieties with less lignin, crops that self-produce cellulase enzymes for cellulose degradation and ligninase enzymes for lignin degradation, or plants that have increased cellulose or an overall biomass yield.     

A sustainable pathway of cellulosic ethanol production integrating anaerobic digestion with biorefining

Anaerobic digestion (AD) of animal manure is traditionally classified as a treatment to reduce the environmental impacts of odor, pathogens, and excess nutrients associated with animal manure. This report shows that AD also changes the composition of manure fiber and makes it suitable as a cellulosic feedstock for ethanol production. Anaerobically digested manure fiber (AD fiber) contains less hemicellulose (11%) and more cellulose (32%) than raw manure, and has better enzymatic digestibility than switchgrass. Using the optimal dilute alkaline pretreatment (2% sodium hydroxide, 130°C, and 2 h), enzymatic hydrolysis of 10% (dry basis) pretreated AD fiber produces 51 g/L glucose at a conversion rate of 90%. The ethanol fermentation on the hydrolysate has a 72% ethanol yield. The results indicate that 120 million dry tons of cattle manure available annually in the U.S. can generate 63 million dry tons of AD fiber that can produce more than 1.67 billion gallons of ethanol. Integrating AD with biorefining will make significant contribution to the cellulosic ethanol production. Biotechnol. Bioeng. 2010;105: 1031–1039. © 2009 Wiley Periodicals, Inc.     

Reactor scale up for biological conversion of cellulosic biomass to ethanol

The absence of a systematic scale-up approach for biological conversion of cellulosic biomass to commodity products is a significant bottleneck to realizing the potential benefits offered by such conversion. Motivated by this, we undertook to develop a scale-up approach for conversion of waste paper sludge to ethanol. Physical properties of the system were measured and correlations were developed for their dependence upon cellulose conversion. Just-suspension of solid particles was identified as the scale up criterion based on experiments at lab scale. The impeller speed for just solids suspension at large scale was predicted using computational fluid dynamics simulations. The scale-up strategy was validated by analyzing mixing requirements such as solid–liquid mass transfer under the predicted level of agitation at large scale. The scale-up approach enhances the prediction of reactor performance and helps provide guidelines for the analysis and design of large scale bioreactors based on bench scale experimentation.     

Life cycle cost analysis of fuel ethanol produced from cassava in Thailand

Background, aim, and scope

As a net oil importer, Thailand has a special interest in the development of biofuels, especially ethanol. At present, ethanol in the country is mainly a fermentation/distillery product of cane molasses, but cassava holds superior potential for the fuel. This study aims to assess the economics of cassava-based ethanol as an alternative transportation fuel in Thailand. The scope of the study includes the cassava cultivation/processing, the conversion to ethanol, the distribution of the fuel, and all transportation activities taking place within the system boundary.

Materials and methods

The life cycle cost assessment carried out follows three interrelated phases: data inventory, data analysis, and interpretation. The functional unit for the comparison between ethanol and gasoline is the specific distance that a car can travel on 1 L ethanol in the form of E10, a 10% ethanol blend in gasoline.

Results

The results of the analysis show, despite low raw material cost compared to molasses and cane-based ethanol, that cassava ethanol is still more costly than gasoline. This high cost has put an economic barrier to commercial application, leading to different opinions about government support for ethanol in the forms of tax incentives and subsidies.

Discussion

Overall, feedstock cost tends to govern ethanol’s production cost, thus, making itself and its 10% blend in gasoline less competitive than gasoline for the specific conditions considered. However, this situation can also be improved by appropriate measures, as discussed later.

Conclusions

To make ethanol cost-competitive with gasoline, the first possible measure is a combination of increasing crop yield and decreasing farming costs (chemical purchase and application, planting, and land preparation) so as to make a 47% reduction in the cost per tonne of cassava. This is modeled by a sensitivity analysis for the cost in the farming phase. In the industrial phase of the fuel production cycle, utilization of co-products and substitution of rice husk for bunker oil as process energy tend to reduce 62% of the price gap between ethanol and gasoline. The remaining 38% price gap can be eliminated with a 16% cut of raw material (cassava) cost, which is more practical than a 47% where no savings options in ethanol conversion phase are taken into account.

Recommendations and perspectives

The life cycle cost analysis helps identify the key areas in the ethanol production cycle where changes are required to improve cost performance. Including social aspects in an LCC analysis may make the results more favorable for ethanol.     

Starch self-processing in transgenic sweet potato roots expressing a hyperthermophilic α-amylase

Sweet potato is a major crop in the southeastern United States, which requires few inputs and grows well on marginal land. It accumulates large quantities of starch in the storage roots and has been shown to give comparable or superior ethanol yields to corn per cultivated acre in the southeast. Starch conversion to fermentable sugars (i.e., for ethanol production) is carried out at high temperatures and requires the action of thermostable and thermoactive amylolytic enzymes. These enzymes are added to the starch mixture impacting overall process economics. To address this shortcoming, the gene encoding a hyperthermophilic α-amylase from Thermotoga maritima was cloned and expressed in transgenic sweet potato, generated by Agrobacterium tumefaciens-mediated transformation, to create a plant with the ability to self-process starch. No significant enzyme activity could be detected below 40°C, but starch in the transgenic sweet potato storage roots was readily hydrolyzed at 80°C. The transgene did not affect normal storage root formation. The results presented here demonstrate that engineering plants with hyperthermophilic glycoside hydrolases can facilitate cost effective starch conversion to fermentable sugars. Furthermore, the use of sweet potato as an alternative near-term energy crop should be considered.     

Techno-economical study of ethanol and biogas from spruce wood by NMMO-pretreatment and rapid fermentation and digestion

Given that N-methylmorpholine-N-oxide (NMMO) is a promising alternative for the pretreatment of lignocelluloses, a novel process for ethanol and biogas production from wood was developed. The solvent, NMMO, is concentrated by multistage evaporation, and the wood is pretreated with the concentrated NMMO. Thereafter, ethanol is produced by the non-isothermal simultaneous saccharification and fermentation (NSSF) method, which is a rapid and efficient process. The wastewater is treated by upflow anaerobic sludge blanket (UASB) digester for rapid production of biogas. The process was simulated by Aspen plus®. Using mechanical vapor recompression for evaporators in the pretreatment and multi-pressure distillation columns, the energy requirements for the process were minimized. The economical feasibility of the developed biorefinery for five different plant capacities was studied by Aspen Icarus Process Evaluator. The base case was designed to utilize 200,000 tons of spruce wood per year and required M€ 58.3 as the total capital investment, while the production cost of ethanol is calculated to be €/l 0.44.     

Acetaldehyde and ethanol biosynthesis in leaves of plants

Leaves of terrestrial plants are aerobic organs, and are not usually considered to possess the enzymes necessary for biosynthesis of ethanol, a product of anaerobic fermentation. We examined the ability of leaves of a number of plant species to produce acetaldehyde and ethanol anaerobically, by incubating detached leaves in N₂ and measuring headspace acetaldehyde and ethanol vapors. Greenhouse-grown maize and soybean leaves produced little or no acetaldehyde or ethanol, while leaves of several species of greenhouse-grown woody plants produced up to 241 nanograms per milliliter headspace ethanol in 24 hours, corresponding to a liquid-phase concentration of up to 3 milligrams per gram dry weight. When leaves of 50 plant species were collected in the field and incubated in N₂, all higher plants produced acetaldehyde and ethanol, with woody plants generally producing greater amounts (up to 1 microgram per milliliter headspace ethanol concentration). Maize and soybean leaves from the field produced both acetaldehyde and ethanol. Production of fermentation products was not due to phylloplane microbial activity: surface sterilized leaves produced as much acetaldehyde and ethanol as did unsterilized controls. There was no relationship between site flooding and foliar ethanol biosynthesis: silver maple and cottonwood from upland sites produced as much acetaldehyde and ethanol anaerobically as did plants from flooded bottomland sites. There was no relationship between flood tolerance of a species and ethanol biosynthesis rates: for example, the flood intolerant species Quercus rubra and the flood tolerant species Quercus palustris produced similar amounts of ethanol. Cottonwood leaves produced more ethanol than did roots, in both headspace and enzymatic assays. These results suggest a paradox: that the plant organ least likely to be exposed to anoxia or hypoxia is rich in the enzymes necessary for fermentation.     

The economics of current and future biofuels

This work presents detailed comparative analysis on the production economics of both current and future biofuels, including ethanol, biodiesel, and butanol. Our objectives include demonstrating the impact of key parameters on the overall process economics (e.g., plant capacity, raw material pricing, and yield) and comparing how next-generation technologies and fuels will differ from today’s technologies. The commercialized processes and corresponding economics presented here include corn-based ethanol, sugarcane-based ethanol, and soy-based biodiesel. While actual full-scale economic data are available for these processes, they have also been modeled using detailed process simulation. For future biofuel technologies, detailed techno-economic data exist for cellulosic ethanol from both biochemical and thermochemical conversion. In addition, similar techno-economic models have been created for n-butanol production based on publicly available literature data. Key technical and economic challenges facing all of these biofuels are discussed.