Lignocellulose includes agricultural residues, forest industry wastes, and potentially perennial energy crops. Agricultural residues include corn stover e. Forest industry wastes include lumber scraps, pulping waste, as well as urban-generated cellulosic wastes. Perennial energy crops include warm season grasses e. Currently no crops are grown for energy, but warm season grasses are grown for forage, and trees, of course, are grown for pulping and to make lumber. Estimated availability of each is in the hundreds-of-million-ton range Figure 1 and all summed together could theoretically meet 20 percent of our total liquid transportation fuels by Perlack et al.
Recently the U. Department of Energy announced that it would help fund six commercialization efforts for converting biomass to biofuels, most of which are related to production of ethanol. While ethanol is the leading candidate for a renewably generated liquid fuel, there are other alternatives, some dating back nearly as far as ethanol.
These include butanol produced by acetone-butanol-ethanol ABE fermentation of biomass, synthetic gasoline gasification produced by gasifying biomass to syngas followed by Fisher-Tropsch reformation, ethanol. Source: Adapted from Perlack et al. Each is being actively pursued for commercialization; limited space prevents further discussion of these alternatives.
Carbohydrates are the only portion of the plant that can be fermented to ethanol. In fibrous biomass, carbohydrates are mostly present in the plant cell walls and are in the form of cellulose and hemicellulose. Cellulose can be converted to glucose and hemicellulose to a mixture of sugars, the composition of which varies with the source of biomass. Herbaceous hemicellulose contains mostly xylose and significant amounts of arabinose and glucoronic acids Dien et al. Other minor sugars include galactose and ribose.
This precludes the use of commercial yeast in converting lignocellulose to ethanol. Plants are approximately 60 percent wt. When fermented to ethanol, one CO 2 is produced for each ethanol, so the theoretical yield for neutral sugars is 0. For herbaceous biomass the theoretical yield of ethanol is gal. As a practical matter most experts use conversion factors of gal. Many processes have been conceptualized for converting fibrous biomass to ethanol. All have common aspects, so I will first discuss a conceptual design for a dilute acid pretreatment Figure 2 Aden et al.
Dry biomass that arrives at the facility is first cleaned and milled. The biomass is mixed with a dilute mineral acid solution to a solids consistency of percent wt. Treating in this manner physically reduces particle sizes and changes the consistency of the product from a damp fiber to sludge.
It also breaks down the plant cell wall, removing the hemicellulose to the syrup, displacing the lignin, and swelling the tightly highly crystalline. To recap, following the steam explosion the solids consist largely of lignin and cellulose, and the syrup contains most of the hemicellulose carbohydrates, water extractables, a little glucose, and minor amounts of released lignin products. Following pretreatment the solids are recovered and washed, possibly using a press. The syrup and wash water are mixed and the residual sulfuric acid neutralized by adding lime.
Pretreatment produces a wide variety of soluble side products, some of which are quite toxic to microbes. Therefore, the syrup often needs to be conditioned to reduce its toxicity prior to fermentation. Following this the syrup is remixed with the solids. At this point the biomass is too thick to ferment directly, so enzymes are added to thin the slurry and to begin saccharifying the cellulose to glucose.
Next, the biocatalyst is added, which begins to ferment the released sugars to ethanol. At the same time the fermentation is occurring the enzymes continue to release sugars for fermentation. As mentioned above, a special microbe needs to be used that is capable of fermenting the pentose sugars in addition to the glucose. A number of microbes are now available that ferment either xylose or both xylose and L-arabinose in addition to glucose. The fermentation could theoretically last up to 7 days, but is usually ended after 3 days.
The ethanol is stripped out of the beer, distilled, and finished by passing through a molecular sieve to remove the last of the water. The solids e. The recovered liquid is hopefully treated and recycled in the process. There are many process variations for converting biomass to ethanol. In a simultaneous saccharification and fermentation SSF , the microbe is added with the enzyme Takagi et al.
The key advantage of co-adding them is that the microbe ferments glucose immediately to ethanol, thereby avoiding any buildup of glucose in the culture. Maintaining a low glucose concentration has the advantage of avoiding end-product inhibition of the enzyme and helps minimize the risk for contamination.
Fermentations are run as open processes, and contamination is always a concern. Enzymes are a major cost of processing biomass to ethanol. If microbes were used that produce some of their own enzymes, it would be possible to eliminate a large expense item. Using microbes that produce their own carbohydrolytic enzymes is the central theme of consolidated bioprocessing CBP Den Haan et al.
An alternative to SSF would be to completely hydrolyze the carbohydrates and remove the solids prior to fermentation, which is referred to as SHF.
This is the process used by Iogen Corp. It has the advantages of making the fermentation faster because it is not enzyme limited, eliminates solids from the bioreactor, supplies a cleaner burning lignin, and may theoretically allow for recovering and recycling of enzymes. However, end-product inhibition of the cellulases is a major concern.
The major processing steps for converting biomass to ethanol are pretreatment, enzymatic saccharification, fermentation, and recovery. Lignocellulose contains primarily structural carbohydrates, which are highly resistant to enzymatic conversion to monosaccharides. Thermochemical pretreatment is needed to deconstruct the cell wall structure, allowing enzymes access to the carbohydrate polymers. The cell wall has been compared with reinforced concrete, where hemicellulose is the concrete, lignin the hydrophobic sealant, and cellulose microfibrils are the reinforcing bars Bidlack et al.
Specifically, pretreatment is needed to reduce particle size, dissolve the xylan, displace the lignin, and create broken ends in and swell the cellulose microfibers. There are numerous pretreatments available, some of which are summarized in Figure 3 reviews: Dien et al. There are three major categories of enzymes for converting pretreated biomass into fermentable sugars: cellulases, xylanases along with auxiliary enzymes for debranching xylan, and ligninases. A list of these enzymes is presented in Table 1. However, the acidic pretreatments also produce more inhibitors that are problematic for fermentation.
Lignin peroxidase LiP, EC Dilute acid pretreatment converts the hemicellulose carbohydrates directly to monosaccharides and therefore only requires cellulase blends; commercial blends containing xylanase activity can in some cases improve conversion efficiency. Other pretreatments will solubilize the xylan but require additional enzymes hemicellulases Saha, to saccharify. Ligninases have not been widely applied to biomass bioconversion as yet. The bioethanol industry is dependent upon S. Unfortunately S. The bacterium Zymomonas mobilis also selectively produces ethanol and has been offered as a substitute for S.
But like Saccharomyces , Z. Therefore, researchers have had to depend upon molecular methods for developing new biocatalysts for converting pentoses and especially xylose into ethanol. Two approaches have been taken to solve this problem: 1 engineering S.
Enzymatic conversion of biomass for fuels production, American Chemical Society Symposium Series No. edited by M. E. Himmel, J. O. Baker and R. P. . Bioconversion for Production of Renewable Transportation Fuels in the United States. A Strategic Perspective. John J. Sheehan. Chapter 1, DOI.
There has been considerable work on using the latter strategy to develop Gram-positive bacteria, but while progress is being made, full success has been elusive. Table 2 reviews the microorganisms available for fermenting xylose Dien et al. K b. RWB c. Ethanol production from wood dates back before World War II. Modern technology has allowed the potential of much higher yields with a smaller environmental footprint.
However, challenges remain, and further research will be needed to make lingocellulosic ethanol cost-competitive. Efforts will continue toward producing more robust pentose-fermenting microorganisms with higher productivity and more efficient, less-expensive enzymes. More work will also be directed at understanding the cell wall and the sources of biomass recalcitrance. Simultaneously there should be increased efforts to engineer plants for easier conversion to sugars e.
The U. In an attempt to jump start a lignocellulose ethanol industry the U. Abengoa Bioenergy Biomass, Poet Companies, and Iogen Biorefinery will focus on biochemical conversion of herbaceous biomasses, including corn cobs and fiber, switchgrass, and wheat straw. Alico Inc. Range Fuels will apply a strictly thermochemical approach. Blue Fire Ethanol Inc.
It is hoped by those working in the field that the combination of strong political and industrial interests will help to unlock lignocellulose as a commercially successful feedstock for ethanol. Aden, A. Ruth, K.
There are major differences in the concentrations of these elements between woody and herbaceous crops, and herbaceous crops generally have more N, Cl, and K, but less Ca than woody crops Vassilev et al. The pretreated bagasse resulted in doubling the yield of glucose by the hydrolysis compared to the untreated one. This suggests that the reduction in T 2 times of the different peaks in the pretreated lignocellulosic samples with increasing solids concentrations may be related to a shrinking of pores in the biomass matrix. FTIR-ATR-based prediction and modelling of lignin and energy contents reveals independent intra-specific variation of these traits in bioenergy poplars. Production of ethanol from lignocellulose has the advantage of abundant and diverse raw material compared to sources such as corn and cane sugars, but requires a greater amount of processing to make the sugar monomers available to the microorganisms typically used to produce ethanol by fermentation.
Ibsen, J. Jerchura, K. Neeves, J. Sheehan, B. Wallace, L. Montague, A. Slayton, and J. Bidlack, J. Malone, and R. Molecular structure and component integration of secondary cell walls in plants. Proceedings of the Oklahoma Academy of Sciences Den Haan, R. McBride, D.
Grange, L. Lynd, and W. Van Zyl. Functional expression of cellobiohydrolases in Saccharomyces cerevisiae towards one-step conversion of cellulose to ethanol. Enzyme and Microbial Technology Dien, B. Cotta, and T. Bacteria engineered for fuel ethanol production: Current status. Applied Microbiology and Biotechnology Iten, and C. Converting herbaceous energy crops to bioethanol: A review with emphasis on pretreatment processes. Nichols, P. Development of new ethanologenic escherichia coli strains for fermentation of lignocellulosic biomass.
Applied Biochemistry and Biotechnology 84 6 Emert, G.
Farrell, A. Plevin, B. Turner, A. Jones, M. Ethanol can contribute to energy and environmental goals. Science Jeffries, T. Metabolic engineering for improved fermentation of pentoses by yeasts. Karhumaa, K. Enzymes are known and available but the cost is often too high for them to be used on all feedstocks, particularly clean plant oils. One way of extending the life and thus lowering the cost of the enzymes is to immobilize them on a solid substrate to enable multiple cycles of use.
Another solution is to produce them in a more cost efficient system or to improve their activity. Cellulosic ethanol: For biomass conversion, cellulases are key for the digestion of cellulose into glucose for fermentation into biofuels. Over the past 10 years, intense investigations of issues surrounding the utilization of cellulosic feedstocks for biofuel production have been conducted.
Although in theory this process can utilize the tons of biomass available from farming and dedicated feedstocks, major problems of conversion are encountered. One is the cost of the enzymes needed for deconstruction of the cellulose and hemicellulose into usable sugar streams.
Several approaches have been pursued by researchers including changing the structure of the cell walls to lower the difficulty of digestion, finding better enzymes, using combined digestion and fermentation, and finding better pretreatment technologies to prepare the feedstock for the enzymes. Enzyme cost: Pretreated biomass is deconstructed with mixtures of enzymes. For the past 15 years, intense research on enzyme production platforms has yielded fungal enzyme mixtures that do not meet these cost requirements and in fact also require a huge infrastructure for production.
A relatively new technology utilizes genetically engineered plant seeds primarily maize to accumulate industrial enzymes. At scale, enzymes from this system can be less expensive to produce and formulate because of low requirements for capital infrastructure. Although the plant seed production system is more cost-competitive, it has not been tested at scale for efficacy.
Other research efforts are in multifunctional enzymes and combined bioprocessing organisms, the latter of which can decompose plant polymers as well as ferment them into biofuels. Chemical catalysis has been the method of choice for the efficient production of transportation fuels from fossil carbon sources so it is natural that it is a mainstay of biomass conversion technology.
Many of the routes to biomass transformation involve several steps including depolymerization followed by separation and upgrading processes. Pretreatment and Deconstruction: Often the first step is a destructive pretreatment of biomass with a strong acid and base hydrolysis. The ammonia treatment separates carbohydrates from lignin and opens up the structure. The method greatly improves the efficiency of enzymatic upgrading. It is likely that other downstream catalytic approaches will also be facilitated by pretreatment.
Multifunctional Catalysts: The products from depolymerization include highly oxygenated compounds and light gases that are unsuitable for use as fuels. These materials need to be upgraded by separate processes. However, the required separations and waste products from pretreatment steps increase the complexity and the cost of biofuel production. These requirements have led researchers to search for heterogeneous catalysts that can directly convert biomass to liquid fuels. Combining biomass depolymerization and upgrading to liquids by deoxygenation in a single step is particularly attractive.
Reforming light gases into liquid products is also desirable. An additional catalyst function is controlling the reactivity of intermediate products to prevent recombination into tars and high molecular weight molecules.
The direct conversion requires a method for contacting the biomass with the catalyst either by the use of solvent or volatilization by pyrolysis. Multifunctional catalyst systems that combine acid and metals are required to perform all of these tasks. It is difficult to balance the activity components of these systems to produce the optimum results.
Higher temperatures promote gasification to low value carbon oxides and acids, which require additional process steps to reform them to liquid fuels. Long contact times can allow recombination reactions. The most common approach to this problem is to perform reactions in different zones without interstage separation. Catalysts which promote conversion at low temperature are highly desirable.
Zeolite catalysts have revolutionized petroleum processing so it is not surprising they have received a lot of attention as potential biomass catalysts. ZSM-5 has proved to be the best of the commercial zeolites because of its deoxygenation activity, selectivity to lower molecular weight aromatics, and low coking properties. Recently attention has turned to the effects of adding lower-cost base metals to the zeolite to improve conversion and aromatic selectivity. Ni catalysts have received particular attention. A recent example is adding Ni to ZSM-5 to increase the yield of aromatic hydrocarbons while simultaneously increasing the conversion of oxygenates.