Buy As Gift. Overview A monograph collection on various topics with implications on new or possible advances with ligno-cellulose research in regards to animal feeding and also lending itself to bioenergy feedstock. It is an informative discussion for the research scientist, and in particular, the specialist in ruminant nutrition covering such topics as follows.
Enzyme technology, applied to crop post-harvest technology, with novel microbial anaerobic lignases, aerobic lignases and other extracellular fibrolytic enzymes EFEs , boosting water-soluble carbohydrate WSC content in new tropical forage-type feeds, action of proteases in plant feed material and digestion, lowering lignin content and use of lacasse for bio-bleaching lignocellulose. Feed resources discussed, in particular in Asia, including sugarcane and use of bagasse and tops, grasses and legumes, with resources for food and feed farming systems and legume browse tree and shrubs for feed.
If favorable conditions prevail after overcoming these hurdles, then a high capital of about — million dollars is required to establish a commercial grade biorefinery that could produce several million gallon of ethanol per year. It is important that a biorefinery should be established in an appropriate location that has good water resources, access to feedstocks, and energy that is needed to process the feedstock.
Few big corporations e. Once this is done, then the group of technologies will be sublicensed to different biofuel manufactures. To facilitate this biofuel production process, several millions of dollars are currently provided by the government to stimulate large scale biofuel production. In the past five years, only a few companies have been successful in demonstrating their technology in their pilot scales and are now gradually progressing to establish their commercial plants. Based on the public information that is available, it looks like lignocellulosic ethanol will first hit the US market, later followed by several advanced biofuels e.
Due to the challenges discussed in this paper, it is anticipated that there may be a considerable delay in the commercial availability of lignocellulosic biofuels from the previously projected timeline of by the USDA and DOE. The author would like to thank Dr. Mingjie Jin, Michigan State University, who was instrumental in shaping up this review. The author also thanks Ms.
Sumathi Venkatesh, Dr. Sunil Nityanand, and Mr. Cory Sarks from Michigan State University for helping to revise the paper and giving their valuable suggestions. The author declares that there is no conflict of interests regarding the publication of this paper. National Center for Biotechnology Information , U. ISRN Biotechnol. Published online May 4. Author information Article notes Copyright and License information Disclaimer. Received Dec 15; Accepted Feb This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
This article has been cited by other articles in PMC. Abstract Biofuels that are produced from biobased materials are a good alternative to petroleum based fuels. Introduction to Biorefineries Biorefineries are manufacturing facilities that convert biobased materials such as agricultural residues to various products such as food, feed, fuels, chemicals, and energy [ 1 , 2 ]. Open in a separate window. Figure 1. Figure 2. First Generation Biorefineries The first generation biorefineries used corn, wheat, cassava, barley, rye, soybean, sugarcane, sugar beet, or sweet sorghum as feedstocks Figure 3.
Figure 3. Second Generation Biorefineries Producing fuels from food grade materials has become a controversial topic as there are several million people in the world without sufficient food. Bioenergy Feedstock Bioenergy plants are broadly classified into two categories, i gymnosperms soft woods like pine, spruce, fir, and cedar and ii angiosperms [ monocots : all perennial grasses e. Feedstocks for Biorefineries The cost of feedstocks will significantly influence the cost of biofuel production. Figure 4. Biomass Harvesting Biomass harvesting is an energy intensive process that requires large machinery and demands large amounts of fuel for transportation [ 45 ].
Biomass Yield About one-third of biofuel production cost is associated with biomass cost. Table 1 Potential lignocellulosic biomass that is available in US and their average yield dry ton per acre. Biomass Supply Chain and Logistics The biomass supply chain will include several key processing steps, which are i collection, ii storage, iii preprocessing densification by compaction, pelleting, and briquetting , iv transportation from field to biorefinery , and v postprocessing at the biorefinery [ 47 , 57 ]. Biomass Densification Biomass densification is an energy intensive process done after size reduction by traditional milling processes followed by a compaction process to increase the density by severalfold [ 64 ].
Figure 5. Biomass Transportation Woody biomass can be transported in four different forms: whole tree residue, wood chip, bundle, and pellet. Biomass Storage Harvested biomass can be protected from rain by storing in a building or by covering using a polythene wrap before being shipped to the biorefinery. Table 2 Different pretreatment technologies used in biorefinery and their advantages and disadvantages. High i Carbohydrate losses are generally low and ii Degradation products are significant only at severe conditions.
Biofuel production from straw hydrolysates: current achievements and perspectives
Medium i Volatile ammonia can be recovered and reused, ii Lesser degradation product formation, iii Dry to dry process, iv Lignin relocation to surface help to densify the biomass i Safety precautions for handling ammonia, ii Ammonia recovery step is added cost, iii Not efficient for hardwood biomass. High i Milder pretreatment condition. Physical Pretreatment Physical pretreatments include mechanical processing and extrusion where the objective is to reduce the particle size but increase the surface area.
Chemical Pretreatment Chemical pretreatments are carried out at acidic, neutral, or basic conditions. Biological Pretreatment Microorganisms like brown, white, and soft-rot fungi are used to pretreat biomass in the biological pretreatment process. Catalyst Recovery Most of the catalysts either acid or base used in the pretreatment processes are miscible in water and end up in the waste water stream. Influence of Pretreatment and Degradation Products on Downstream Processing To allow enzymes to have easy access to the sugar polymers, pretreatment helps to open up the complex plant cell wall.
Enzyme Hydrolysis Microorganisms secrete enzymes to degrade biomass for producing monomeric sugars for their own survival. Cost of Enzymes The enzyme quantity required to hydrolyze lignocellulosic biomass is onefold higher than for starch. Enzymatic Hydrolysis Time The time taken to completely hydrolyse biomass to monomeric sugars depends on several factors: lignin content in biomass [ ], pretreatment effectiveness [ , ], cellulose crystallinity [ ], substrate concentration, and enzyme activity. Enzyme Recycling Since biomass degrading enzymes are expensive, they need to be recycled to reduce the processing cost.
Effect of Solid Loadings on Sugar Conversion Enzyme conversion rate is dependent on the solid loading used during hydrolysis. Microbial Fermentation Microbial fermentations convert sugars produced from lignocellulosic biomass into biofuels e. Table 3 Examples of microbial strains that are used for biofuel production. Incorporated XI gene from piromyces; overexpression of endogenous xylulokinase, ribose 5-phosphate isomerase, ribulose 5-phosphate epimerase, transketolase and transaldolase genes; knockout of GRE3 gene, which encodes an aldose reductase. Ethanol 47 0. Incorporated pyruvate decarboxylase and alcohol dehydrogenase genes PET operon from Zymomonas mobilis.
Incorporated many different heterologous genes and endogenous genes to build butanol synthesis pathways in either mitochondria or cytosol. Microbes for Producing Biofuels Cofermentation is believed to be superior than separate fermentation in terms of cost savings. Figure 6. Two possible xylose metabolic pathways that are commonly used in yeast and bacteria. Separation of Biofuels from Fermentation Broth Traditionally, distillation is used to separate alcohol and water. In Situ Biofuel Separation to Improve Fermentation Performance Since biofuels are known to inhibit microbes during fermentation, a large number of studies have been conducted to remove biofuels during the fermentation process.
Figure 7. Coproduct Generation and Its Influence of Biofuel Production Cost Coproduct generation is very essential for producing cost competitive biofuels. Lignin Lignin is a randomly linked aromatic polymer and constitutes about one-third of lignocellulosic biomass. Protein Protein is another potential coproduct that could be separated and sold in the second generation biorefinery. Microbial Biomass One of the potential coproducts in a biorefinery is microbial biomass [ ].
Biofuels Economy of Scale and Cost Using key process parameters developed by national renewable energy laboratory NREL on biomass conversions, several cost models have been developed in the past to understand the required capital investments [ — ]. Water Requirement in a Biorefinery and the Necessity for Recycling Producing biofuels using the sugar platform is a water intensive process.
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Environmental Issues It is well reported that biofuels offers several environmental benefits over fossil fuels. Energy Associated with Biomass Processing The goal of establishing a biorefinery is to produce energy in the form of liquid transportation fuels from biomass [ ]. Conclusion This review highlighted several bottlenecks that are being faced by big corporations to commercialize the biofuel production technology pretreatment, hydrolysis, microbial fermentation, and biofuel separation. Conflict of Interests The author declares that there is no conflict of interests regarding the publication of this paper.
References 1. Naik S.
- Sustainable Bioenergy: Genomics and Biofuels Development | Learn Science at Scitable.
- The wifes sister; or The forbidden marriage.
- A Compilation of Ligno-cellulose Feedstock And Related Research for Feed, Food and Energy.
- A Compilation of Ligno-cellulose Feedstock And Related Research for Feed, Food and Energy.
- Automatic Translation!
- Current Challenges in Commercially Producing Biofuels from Lignocellulosic Biomass.
- Careers in Hydroelectric Power & Marine Energy (PixyJack Renewable Energy Series).
- Supplementary files.
- Biofuels: Producing Ethanol from Cellulosic Material | Protocol.
- Current Challenges in Commercially Producing Biofuels from Lignocellulosic Biomass.
Production of first and second generation biofuels: a comprehensive review. Renewable and Sustainable Energy Reviews. Demirbas A. Energy Conversion and Management. Jones D. Handbook of Petroleum Processing. Mohr A. Lessons from first generation biofuels and implications for the sustainability appraisal of second generation biofuels. Energy Policy. Paris, France: International Energy Agency; Lee R. From first- to third-generation biofuels: challenges of producing a commodity from a biomass of increasing complexity.
Animal Frontiers. Greenwell H. Biofuels, science and society. Interface Focus. Perlack R.
chapter and author info
Department of Agriculture; Mata T. Advanced Biofuels and Bioproducts. Berlin, Germany: Springer; Valorization of waste frying oils and animal fats for biodiesel production; pp. Buijs N. Advanced biofuel production by the yeast Saccharomyces cerevisiae. Current Opinion in Chemical Biology. Rabinovitch-Deere C. Synthetic biology and metabolic engineering approaches to produce biofuels. Chemical Reviews. Gowen C. Applications of systems biology towards microbial fuel production. Trends in Microbiology.
Martin M. First generation biofuels compete. New Biotechnology. Yang S. Integrated biorefinery for sustainable production of fuels, chemicals, and polymers. In: Yang S. Timilsina G. How much hope should we have for biofuels? Bothast R. Biotechnological processes for conversion of corn into ethanol. Applied Microbiology and Biotechnology. Rosentrater K. A review of corn masa processing residues: generation, properties, and potential utilization. Waste Management. Kamm B. Biorefinery systems—an overview. In: Kamm B. Cherubini F.
Crop residues as raw materials for biorefinery systems—a LCA case study. Applied Energy. Sims R. An overview of second generation biofuel technologies. Bioresource Technology. Balan V. Biochemical and thermochemical conversion of switchgrass to biofuels. In: Monti A. London, UK: Springer; Review of US and EU initiatives towards development, demonstration and commercialization of lignocellulosic biofuels. Biofuels, Bioproducts and Biorefining. Carriquiry M. Second generation biofuels: economics and policies. Hoekman S. Biofuels in the U. Renewable Energy. Menon V. Progress in Energy and Combustion Science.
Luo L. Biorefining of lignocellulosic feedstock—technical, economic and environmental considerations. A short review on ammonia based lignocelluloses' biomass pretreatment. In: Simmons B. Royal Society of Chemistry; Tao G. Biomass properties in association with plant species and assortments I: a synthesis based on literature data of energy properties.
Vassilev S. An overview of the chemical composition of biomass. An overview of the organic and inorganic phase composition of biomass. Boerjan W. Lignin biosynthesis. Annual Review of Plant Biology. Garlock R. Bioenergy Research. Sokhansanj S. Large-scale production, harvest and logistics of switchgrass Panicum virgatum L. David K. Switchgrass as an energy crop for biofuel production: a review of its ligno-cellulosic chemical properties. Energy and Environmental Science. Sannigrahi P.
Poplar as a feedstock for biofuels: a review of compositional characteristics. Bermuda grass as feedstock for biofuel production: a review. Zegada-Lizarazu W. Are we ready to cultivate sweet sorghum as a bioenergy feedstock? A review on field management practices. Biomass and Bioenergy. Zub H. Agronomic and physiological performances of different species of Miscanthus, a major energy crop. A review. Agronomy for Sustainable Development. Dohleman F. More productive than maize in the Midwest: how does Miscanthus do it?
Plant Physiology. Somerville C. Feedstocks for lignocellulosic biofuels. McKendry P.
What is Kobo Super Points?
Energy production from biomass part 1 : overview of biomass. Thorsell S. Economics of a coordinated biorefinery feedstock harvest system: lignocellulosic biomass harvest cost. Biomass supply logistics and infrastructure. In: Mielenz J. Biofuels: Methods and Protocols. Methods in Molecular Biology. Hoskinson R. Engineering, nutrient removal, and feedstock conversion evaluations of four corn stover harvest scenarios. Hess J. Corn stover availability for biomass conversion: situation analysis. Huggins D. Introduction: evaluating long-term impacts of harvesting crop residues on soil quality.
Duffy M. Switchgrass Production in Iowa: Economic Analysis. Iowa State University; Rojas C. Genetically modified crops for biomass increase. Genes and strategies. GM Crops. Dubouzet J. Potential transgenic routes to increase tree biomass. Plant Science. Vanholme R. Metabolic engineering of novel lignin in biomass crops.
New Phytologist. Sticklen M. Plant genetic engineering to improve biomass characteristics for biofuels. Current Opinion in Biotechnology. Bhatnagar-Mathur P. Transgenic approaches for abiotic stress tolerance in plants: retrospect and prospects. Plant Cell Reports. Sultana A. Development of agri-pellet production cost and optimum size. Caputo A. Economics of biomass energy utilization in combustion and gasification plants: effects of logistic variables.
Kumar A. Development of a multicriteria assessment model for ranking biomass feedstock collection and transportation systems. Applied Biochemistry and Biotechnology. Badger P. Biomass transport systems. Encyclopedia of Agricultural, Food and Biological Engineering. Biomass power cost and optimum plant size in western Canada.
Pipeline transport and simultaneous saccharification of corn stover. Switchgrass Panicum vigratum , L. Mani S. Specific energy requirement for compacting corn stover. Tumuluru J. A review of biomass densification systems to develop uniform feedstock commodities for bioenergy application. Kaliyan N. Natural binders and solid bridge type binding mechanisms in briquettes and pellets made from corn stover and switchgrass.
Miao Z. Energyrequirement for lignocellulosic feedstock densifications in relation to particle physical properties, preheating, and binding agents. Energy Fuels. Mort P. Scale-up and control of binder agglomeration processes—flow and stress fields. Powder Technology. Samuelsson R. Effect of biomaterial characteristics on pelletizing properties and biofuel pellet quality. Fuel Processing Technology. Audsley E. Modelling the value of a rural biorefinery—part I: the model description. Agricultural Systems. Annetts J.
Modelling the value of a rural biorefinery—part II: analysis and implications. You F. Life cycle optimization of biomass-to-liquid supply chains with distributed-centralized processing networks. Industrial and Engineering Chemistry Research. Kurian J. Feedstocks, logistics and pre-treatment processes for sustainable lignocellulosic biorefineries: a comprehensive review.
Carolan J. Technical and financial feasibility analysis of distributed bioprocessing using regional biomass pre-processing centers. Journal of Agricultural and Food Industrial Organization. Sharma B. Biomass supply chain design and analysis: basis, overview, modeling, challenges, and future.
Eranki P. Comparative life cycle assessment of centralized and distributed biomass processing systems combined with mixed feedstock landscapes. GCB Bioenergy. Advanced regional biomass processing depots: a key to the logistical challenges of the cellulosic biofuel industry. Egbendewe-Mondzozo A.
Can dispersed biomass processing protect the environment and cover the bottom line for biofuel? Environmental Science and Technology. A mathematical model to design a lignocellulosic biofuel supply chain system with a case study based on a region in Central Texas. Zhu X. Challenges and models in supporting logistics system design for dedicated-biomass-based bioenergy industry. Optimal configuration and combination of multiple lignocellulosic biomass feedstocks delivery to a biorefinery. Rentizelas A. Logistics issues of biomass: the storage problem and the multi-biomass supply chain.
Johansson J. Transport and handling of forest energy bundles—advantages and problems. Gray B. Spontaneous ignition hazards in stockpiles of cellulosic materials: criteria for safe storage. Mood S. Lignocellulosic biomass to bioethanol, a comprehensive review with a focus on pretreatment. Pauly M. Plant cell wall polymers as precursors for biofuels. Current Opinion in Plant Biology. Hideno A. Wet disk milling pretreatment without sulfuric acid for enzymatic hydrolysis of rice straw.
Yoo J. Thermo-mechanical extrusion pretreatment for conversion of soybean hulls to fermentable sugars. Teymouri F. Optimization of the ammonia fiber explosion AFEX treatment parameters for enzymatic hydrolysis of corn stover. Tae H. Pretreatment and fractionation of corn stover by ammonia recycle percolation process. Bioethanol production from corn stover using aqueous ammonia pretreatment and two-phase simultaneous saccharification and fermentation TPSSF Bioresource Technology. Taherzadeh M. Pretreatment of lignocellulosic wastes to improve ethanol and biogas production: a review. International Journal of Molecular Sciences.
Banerjee G. Scale-up and integration of alkaline hydrogen peroxide pretreatment, enzymatic hydrolysis, and ethanolic fermentation. Biotechnology and Bioengineering. Lime pretreatment of switchgrass at mild temperatures for ethanol production. Banerjee S. Evaluation of wet air oxidation as a pretreatment strategy for bioethanol production from rice husk and process optimization.
Dadi A. Mitigation of cellulose recalcitrance to enzymatic hydrolysis by ionic liquid pretreatment. Kim Y. Liquid hot water pretreatment of cellulosic biomass; pp. Effect of ozonolysis pretreatment on enzymatic digestibility of wheat and rye straw. Lloyd T. Combined sugar yields for dilute sulfuric acid pretreatment of corn stover followed by enzymatic hydrolysis of the remaining solids.
Qin L. Mass balance and transformation of corn stover by pretreatment with different dilute organic acids. Hemicelluloses for fuel ethanol: a review. Galbe M. A review of the production of ethanol from softwood. Sun F. Organosolv pretreatment by crude glycerol from oleochemicals industry for enzymatic hydrolysis of wheat straw. Liu H. Eliminating inhibition of enzymatic hydrolysis by lignosulfonate in unwashed sulfite-pretreated aspen using metal salts. Tengborg C. Comparison of SO 2 and H 2 SO 4 impregnation of softwood prior to steam pretreatment on ethanol production.
Kim K. Supercritical CO 2 pretreatment of lignocellulose enhances enzymatic cellulose hydrolysis. Schacht C. From plant materials to ethanol by means of supercritical fluid technology. Journal of Supercritical Fluids. Patel S. Comparative study of ethanol production from microbial pretreated agricultural residues. Journal of Applied Sciences and Environmental Management. Wan C. Fungal pretreatment of lignocellulosic biomass.
Biotechnology Advances. Hendriks A. Pretreatments to enhance the digestibility of lignocellulosic biomass. Zhu J. Pretreatment of woody biomass for biofuel production: energy efficiency, technologies, and recalcitrance. Agbor V. Delignification has been attempted using white-rot fungi such as Trametes versicolor or Phanerochaete chrysosporium or isolated lignin-degrading enzymes such as laccases. Moreover, experiments with biopretreatment require moistening of the material and in most cases start with sterilised material e.
Cianchetta et al. Moist straw is quite susceptible to mould infection Passoth et al. Straw is usually dried in the field, which implies that rainfall can affect drying and thus later utilisation of the material Nilsson Establishing a low energy preservation system would enable even the utilisation of moist straw and thus increase the amount of raw materials accessible for straw-based production of biofuels in areas with high precipitation. Potential useful preservation systems have been developed for moist cereals, using airtight storage together with biocontrol organisms.
This kind of biopreservation efficiently inhibited the growth of undesirable microbes and improved the grain characteristics for use as animal feed, such as decreased amounts of phytate Olstorpe et al.
Biofuel production from straw hydrolysates: current achievements and perspectives | SpringerLink
Moreover, the starch was better accessible for enzymatic degradation, resulting in improved bioethanol production from moist stored cereals Passoth et al. The concept of airtight preservation was extended to wheat straw. Biopreservation of wheat straw with the help of appropriate biocontrol yeasts prevented mould infections, and there was even enhanced biofuel production from the biopreserved moist straw compared to dry material, indicating that biopreservation during storage can also be a part of the pretreatment Passoth et al.
After physico-chemical pretreatment, monosaccharides are released from the polysaccharides by using enzymes. For obtaining a maximum release of sugar monomers, enzymes should be used that degrade all three major polymers: cellulose, hemicellulose and lignin Gupta et al. However, due to a lack of good lignin-degrading enzymes, commercial enzyme mixtures usually degrade cellulose and hemicellulose Jaramillo et al.
Cellulose-degrading enzymes are formed by a variety of organisms, including anaerobic and aerobic thermophilic and mesophilic bacteria, and fungi Obeng et al. Commercial cellulolytic enzymes are usually derived from various fungal species. The most extensively studied cellulolytic enzyme systems are from the ascomycete Trichoderma reesei teleomorph name Hypocrea jecorina Jaramillo et al. These enzymes are classified as belonging to the glycoside hydrolase GH families in the carbohydrate-active enzyme CAZy classification system Lombard et al.
LPMOs generate chain breaks in the polysaccharide molecule, yielding additional sites for GH-enzyme activity. Due to the synergistic action of LPMOs and GHs, a lower enzyme load can be used for degrading lignocellulosic biomass, which is an important step towards economically feasible lignocellulose conversion Obeng et al. Swollenins, also called expansins, represent an additional group of proteins involved in lignocellulose degradation. The mechanism of their activity has not yet been discovered.
Hemicelluloses are usually solubilised during thermal pretreatment, and some acid pretreatments obviously release sufficient amounts of sugar monomers to perform subsequent microbial cultivations on the substrate e. Brandenburg et al. However, it has been pointed out that thermochemical pretreatment in many cases releases oligosaccharides, which cannot be assimilated by most of the relevant fermentation organisms. Therefore, treatment with hemicellulases has a great potential to improve the efficiency of lignocellulose-based processes Girio et al.
Alkaline pretreatment of straw yields deesterified arabinoglucuronoxylan, while non-alkaline pretreatments result in partially acetylated saccharides. Accordingly, after acid or steam pretreatment, acetylxylan esterases are required to achieve complete saccharification of straw hemicellulose Biely et al. Oligosaccharides derived from cereal plant hemicelluloses can also be used as prebiotics Broekaert et al.
Those oligosaccharides are produced by GH11 xylanases. The shortest product of their activity consists of four xylose residues substituted with one or two arabinose residues at the penultimate xylose from the non-reducing end Biely et al. Interactions of different hemicellulase systems with components of the cell wall have to be further elucidated to develop an optimal system for hemicellulose degradation Biely et al. After several decades of developmental work, commercial cocktails of both cellulases and hemicellulases are now available, e.
As mentioned above Fig. Complete removal of the straw from the field is not desirable, since this over the long term decreases the amount of soil carbon in the fields Karlsson et al. However, even after considering all these alternative utilisations, there is a surplus of straw that can be used in a biorefinery, although the annual amounts of available straw can vary, depending on for instance weather conditions and chosen cultivars Scarlat et al. There are a variety of methods to further convert the monosaccharides derived from straw pretreatment to biofuels, including ethanol, methane biogas , butanol or biodiesel Chandel et al.
Genes overexpressed in industrial strains to obtain xylose-fermenting Saccharomyces cerevisiae suitable for commercial ethanol production from lignocellulose hydrolysate. The genes include the S. Modified from Passoth a. Methane biogas production is another option when producing biofuels from straw.
Anaerobic digestion includes four process steps: hydrolysis of biopolymers, acidogenesis, acetogenesis and methanogenesis. During hydrolysis, carbohydrates including celluloses and hemicelluloses from lignocellulose material , lipids and proteins are degraded into their monomers, for instance monosaccharides, amino acids and short-chain fatty acids.
In acidogenesis, these compounds are further converted to organic acids volatile fatty acids, VFAs. As side products, ammonia, carbon dioxide and other compounds are produced. In methanogenesis, acetate and hydrogen are converted to methane and CO 2. Since hydrolytic bacteria form part of the microbial consortium during anaerobic digestion, a pretreatment of lignocellulosic biomass is not essential for biogas production. However, due to the complexity and recalcitrance of lignocellulose, the hydrolysis step is frequently the limiting factor for methane production from lignocellulose.
Therefore, a variety of physical, thermochemical or biotreatments of lignocellulose has been investigated. In principle, the same pretreatments can be used as for bioethanol production. However, losses of hemicellulose should be avoided and fermentation inhibitors may also negatively affect biogas processes. The use of sulphuric acid during pretreatment should be kept to a minimum, since sulphate-reducing bacteria may outcompete methanogens. Biopretreatments by white-rot fungi have frequently been tested, to decrease lignin content and increase availability of polysaccharides for the hydrolysis steps.
Biotreatment generates less inhibitors and requires less energy than thermochemical methods; however, it is time consuming and the fungi may degrade some organic material resulting in lowered methane yields. In general, pretreatment can have positive or negative impacts on the final biogas production, and optimisation for the specific material and biogas process is required Carrere et al.
There have also been attempts to combine storage of corn stover and pretreatment Cui et al. When ethanol was produced from oat straw and biogas from the residue of ethanol production, biogas production rate was considerably enhanced and the total energy output was higher than in either bioethanol or biogas production alone.
This indicated that ethanol production from the initial material can serve as a pretreatment for enhanced conversion of the remainder to biogas Dererie et al. In general, ethanol production should be connected with valorisation of residues, for instance biogas production, to obtain an energy output similar to biogas production Lantz et al.
Anaerobic co-digestion of some straw materials modified from Sawatdeenarunat et al. Biogas generation is in general more sustainable and produces less emissions of greenhouse gases and health-threatening compounds than consumption of fossil fuels or open field burning of straw. However, there is still net greenhouse gas emission and global warming potential by biogas production. Under Chinese production conditions, biogas purification, biogas residue disposal and total electricity consumption are main factors to optimise for reducing negative impacts of biogas production Wang et al.
Butanol, both n -butanol and iso-butanol, has excellent fuel characteristics because it has higher energy density, is less corrosive and more compatible with existing engines than ethanol. Acetone-butanol-ethanol production from starchy material has been established on an industrial scale using solventogenic Clostridia Xin et al. However, the process suffers from high costs for raw materials and too low final titres of butanol. Conversion of lignocellulosic materials to butanol has been tested, using metabolically engineered Clostridia, adding cellulolytic enzymes to fermentations or by using mixed cultures of the solventogenic Clostridia with cellulolytic bacteria.
For instance, 5. Although certain progress was made, the final butanol titres and production rates did not reach the levels necessary for commercially viable butanol production Jiang et al. The fermentation strain was Clostridium beijerinckii P Optimising process parameters could further increase butanol production. Fed-batch cultivation with immobilised cells high cell densities and continuous removal of the butanol was most promising.
After ethanol, biodiesel is currently the second most abundant biofuel in the world. Biodiesel is produced from vegetable oil, for instance rape seed, palm or soya oil. Microbial lipids produced from lignocellulose such as straw can provide a sustainable alternative to vegetable oils. At carbon surplus, the citrate cycle is inhibited in oleaginous yeasts, and citrate is transported out of the mitochondria.
The latter is transported back to the mitochondria, while acetyl-CoA is the basis of fatty acid synthesis, which is achieved by acetyl-CoA carboxylase and the fatty acid synthase FAS enzyme complex, under consumption of NADPH. Many oleaginous yeasts can convert the different hexoses and pentoses and organic acids released from lignocellulose pretreatment to lipids Passoth b ; Sitepu et al. Lipid production from different lignocellulosic materials has been tested, including rice straw final lipid concentration Inhibitors in the hydrolysate act also against oleaginous yeasts, setting a limit for feedstock dry matter in the fermentation process, which in turn limits the amount of potential end product.
Lipid-accumulating strains may be adapted to inhibitors by sophisticated feeding techniques. When the medium feed was connected to pH regulation, a self-regulating fed batch could be established, yielding the highest lipid concentration to date from the hemicellulose fraction of lignocellulose. In oleaginous Rhodosporidium spp.
On the other hand, microbial biodiesel production from lignocellulose has the potential to reach a similar energy balance as bioethanol Karlsson et al. A number of steps in the production process of microbial biodiesel could be improved, which would significantly increase the efficiency of the whole process. This includes lipid extraction from the cells, identification of rapid lipid producers since lipid production is an aerobic process, requiring much more energy per fermentation time compared to the in principle anaerobic ethanol production process , utilising all residues to generate co-products such as biogas , and conversion of the crude glycerol, which is a residue of the transesterification of the microbial triglycerides to fatty acid methyl esters, to lipids Biddy et al.
Production of high-value biodiesel and co-products will also be a means to reach competitive production prices. Carotenes are widely used as colourants and antioxidants in the food, feed, pharmaceutical and cosmetic industries. Co-production with lipids can improve the economic viability of biodiesel production Schneider et al. Furfural is another high-value compound that can be produced from wheat straw.
This dehydration product of pentoses is one major platform chemical to produce biofuels, fuel additives and other compounds. It is a side product of thermochemical pretreatment and acts as an inhibitor in the fermentation broth. Currently, there is no technology for synthetic furfural production; it has to be produced from lignocellulose. Current technologies for furfural production are not very efficient and they damage the cellulose, so that its glucose monomers cannot be converted to biofuels Machado et al.
At Latvian State Institute of Wood Chemistry, a novel technique for furfural extraction has been developed, allowing an efficient extraction of furfural from lignocellulosic material without extensively damaging the cellulose fraction. Pentoses were in principle completely converted to furfural, and the hydrolysate contained the easily fermentable glucose.
Moreover, the hydrolysate had a low content of fermentation inhibitors Brandenburg et al. It is also possible to utilise the lipids of oleaginous yeasts for other purposes, for instance as ingredient in fish feed, to replace vegetable oil such as palm oil. As it is not necessary to extract the oil or to run transesterification, this approach can also be a valuable step towards a sustainable economy, taking into account the environmental impact of palm oil production see above Blomqvist et al.
Polyhydroxybutyrate PHB is a polyhydroxyalkanoate PHA , which is produced by certain bacteria as intracellular storage compound. PHAs can be used as bioplastics, replacing fossil-based plastics produced from mineral oil components. Production of PHAs by microorganisms has been investigated during the last years; however, production costs were usually too high to achieve a substantial replacement of plastic from fossil resources. Identification of cheap carbon sources for the microbial production of PHA may be one approach to decrease production costs.