Cellulosic materials have been touted as the feedstock for the “second generation” of biofuel production and the best alternative for replacing food and feed grains in ethanol production.

Proponents of cellulosic ethanol point out that its production generates a higher net energy gain and a lower level of greenhouse gas emissions relative to grain-based ethanol, due in part to the fact that a higher portion of the feedstock material is converted to fuel. As a result, the past decade has seen a tremendous increase in research related to ethanol production from feedstocks such as corn stover, switchgrass, rice hulls, wheat straw, landscape waste, paper processing waste, wood-processing waste, and sugarcane waste (U. S. Department of Energy, National Renewable Energy Laboratory 2007).

The U. S. Department of Energy committed more than $1 billion toward cellulosic ethanol projects in 2007, with a goal of making the fuel cost competitive at $1.33 per gallon by 2012 (U. S. DOE 2007) . Projects supported by the 2007 DOE commitment range from annual capacities of 11 million gallons of ethanol (Kansas) to 125 million gallons (Iowa) (U. S. DOE

2007) . The technologies utilized by these proposed plants also vary, as do their feedstocks. Feedstocks expected to be used by some of these proposed ventures include corn stover and cobs, rice and wheat straw, milo stubble, switchgrass, yard waste, wood and wood-processing residues, “green” wastes, and other wastes recovered from landfills. Some of the technologies to be utilized also generate coproducts such as electricity, hydrogen, ammonia, and methanol (U. S. DOE 2007).

A wide variety of cellulose-based biomass wastes and byproducts are available for conversion to biofuels. These include:

• Agricultural residues (corn stalks and cobs, straws, cotton gin trash, palm oil wastes, etc.)

• Paper (paper mill sludge, recycled newspaper, sorted municipal solid waste, etc.)

• Wood waste (sawdust, wood chips, prunings, etc.)

• Landscape waste (leaves, grass clippings, vegetable and fruit wastes, etc.)

Most of these materials are available at very low cost, and some even command tipping fees associated with their disposal as wastes.

Unlike grain-based ethanol, where processing technologies have become relatively stan­dardized and feedstock procurement is as simple as participating in the grain marketing system, cellulosic ethanol projects may have a wide range of technical efficiencies, conver­sion rates, and feedstock logistics. Decision makers, including agricultural producers, poten­tial investors, and rural community leaders, are interested in determining whether cellulosic ethanol production could be feasible in their area.

Cellulosic biomass is composed of cellulose, hemicellulose, and lignin. In order to produce ethanol from cellulosic biomass, complex cellulosic carbohydrates must be converted into simple sugars, which can then be fermented to ethanol by a variety of microorganisms. Cellulose conversion to sugars can be catalyzed by a variety of acids, including sulfuric, hydrochloric, hydrofluoric, and nitric acids. A decrystallized cellulosic mixture of acid and sugars reacts in the presence of water to produce individual sugar molecules (hydrolysis). The product from this hydrolysis is then neutralized, and yeast fermentation is used to produce ethanol. When inexpensive dilute acid is used to catalyze the hydrolysis reaction, biomass is impregnated with dilute sulfuric acid solution and treated with steam at tempera­tures ranging from 140 to 260°C (Katzen and Schell 2006). Concentrated acids can also be used to hydrolyze cellulose and hemicellulose to sugars.

Temperatures ranging from 100 to 120°C, which are lower than those with the dilute acid process, are typically used, and high yields of sugars are obtained with little production of degradation products. The economic viability of this process depends, however, on the suc­cessful recovery of the acid at low cost (Katzen and Schell 2006).

Enzymatic conversion of cellulose to sugars offers some promising advantages over acid hydrolysis. Sugar yields are limited during acid hydrolysis because sugars are also converted to degradation products. Cellulase is a multicomponent enzyme system that catalyzes cel­lulose hydrolysis and is 100% selective for conversion of cellulose to glucose; high yields are therefore possible. This enzyme is produced by a variety of microorganisms, most com­monly, the fungus Trichoderma reesei. Cellulose conversion rates are limited by the ability of the enzyme to access the cellulosic substrate. To increase accessibility, biomass is sub­jected to physical and chemical treatments that disrupt the biomass structure, usually by removing a fraction of the hemicellulose and/or lignin. Effective pretreatment is necessary to achieving good cellulose-to-glucose conversion yields (Katzen and Schell 2006).

Figure 8.2 provides a process flow that describes how cellulose processing might be incor­porated into an existing dry-mill ethanol production facility. With this configuration, the plant could continue to process corn feedstocks but could also have the capability of processing cellulosic feedstocks. Cellulosic feedstocks would be cleaned and ground to reduce particle size and expedite processing. The material would then be dried as needed to a moisture content acceptable for acid decrystallization (separation of the cellulose and hemicellulose from the lignin).

In Figure 8.2 example, a process using concentrated sulfuric acid is described. The con­centrated acid process offers some advantages over the dilute acid process. Even though the dilute acid is considerably cheaper to purchase, the concentrated acid process can compete successfully if the acid is recycled. While this would add capital expenditure up front, it would significantly reduce operating costs through lower expenditures for acid. Additionally, the concentrated acid process operates at lower temperatures (100-120°C) than the dilute processes, which operate at 140-260°C (Katzen and Schell 2006). Therefore, considerably less energy is required for the concentrated acid process.

In the near term, the concentrated acid process offers higher production rates than enzy­matic processes. However, enzymatic processes are safer and more environmentally friendly than those that use concentrated acid. Consequently, as the productivity of enzymatic hydro­lysis processes are improved, it is likely that the long-term prospects for this technology will be more favorable than either dilute or concentrated acid hydrolysis processes.

Conventional corn-to-ethanol conversion processes, baker’s yeast (Saccharomyces cerevisiae) , is commonly used in the fermentation step to produce ethanol from hexose (six — carbon sugar). The carbohydrates present in lignocellulosic biomass are considerably more complex than those derived from corn. Large quantities of xylose and arabinose, which are five-carbon sugars derived from the hemicellulose portion of the lignocellulose, are also present in the products derived from hydrolysis. For instance, when corn stover is hydrolyzed, approximately 30% of the total fermentable sugars produced are in the form of xylose. Consequently, the fermenting microorganisms used in cellulosic ethanol production must be capable of utilizing the entire range of sugars produced during hydrolysis. This will be vital to increasing the economic competitiveness of cellulosic ethanol and to incorporating cellulosic ethanol processes into existing corn-to-ethanol operations.

Metabolic engineering for microorganisms used in fuel ethanol production has made significant progress in recent years. In addition to S. cerevisiae, microorganisms such as Zymomonas mobilis and Escherichia coli have been targeted through metabolic engineering for cellulosic ethanol production (Jeffries and Jin 2004). Engineered yeasts have also been shown to effectively ferment xylose (Ohgren et al. 2006) and arabinose (Becker 2003) as well as both sugars together (Karhum et al. 2006). Utilizing yeast cells for cellulosic ethanol processes is particularly appealing because they have been used in biotechnology for hun­dreds of years. Yeast is tolerant to high ethanol and inhibitor concentrations because they can grow at low pH values, which prevent bacterial contaminations.

In order to successfully incorporate cellulosic ethanol production using concentrated acid hydrolysis into existing dry-mill processing, several key factors must be addressed. Concentrated acid is expensive; therefore, efficient methods for recovering and reconcentrat­ing acid must be incorporated. The sugars produced through the hydrolysis process must be of high concentration and high purity. Additionally, the process must have the ability to ferment both six-carbon and five — carbon sugars efficiently with conventional microbes. It is anticipated that early attempts to incorporate acid hydrolysis of cellulosic materials into exist­ing dry-mill operations will include dedicated fermentation vessels that are separate from the fermenters that process corn- derived sugars. As yeast strains that are equally capable of fermenting both six — carbon and five — carbon sugars are developed, the need for dedicated fermenters will no longer be necessary.

Several technologies that have been proven in small-scale facilities have been developed to produce cellulosic ethanol. These technologies are moving toward commercial production. However, challenges remain with respect to expanding the technologies to production scale, reducing production costs, and financing large-volume plants. To spur commercial develop­ment, the U. S. DOE announced grants of $385 million for six commercial-scale cellulosic ethanol biorefineries in February 2007. Chesterfield, Missouri-based Abengoa Bioenergy, and Range Fuels in Broomfield, Colorado, were each awarded $76 million; BlueFire Ethanol in Irvine, California, received $40 million; and Sioux Falls, South Dakota-based Poet was granted $80 million. These companies expect to complete commercial-scale facilities between 2009 and 2011. The two remaining firms, Alico, in La Belle, Florida, and Iogen in Ontario, Canada, which were awarded $33 million and $80 million respectively, have dropped out of the program (Greer 2008). The grants were awarded under Section 932 of the Energy Policy Act of 2005, which authorized the DOE to fund commercial demonstration of advanced biorefineries that use cellulosic feedstock to coproduce ethanol, bioproducts, heat, and power. Awards were capped at 40% of the total project cost, up to a maximum of $80 million (Greer 2008 ).

BlueFire Ethanol is developing a $150 million cellulosic ethanol facility in Riverside County, California. The so — called Mecca project will use concentrated acid hydrolysis to convert the green waste portion of a municipal solid waste stream and green agricultural waste, currently sent to landfills, into 17 million gallons per year of ethanol. The firm expects to begin construction in late 2009.

The first step in BlueFire’s process, which is described on their Web site, employs con­centrated acid hydrolysis to separate the cellulose and hemicellulose from the lignin and then hydrolyze the cellulose to produce simple sugars for fermentation (BlueFire Ethanol, n. d.) . After hydrolysis, a filtration and pressing process will remove the lignin and other insoluble materials from the sugar mixture.

BlueFire’s process utilizes a chromatographic system to separate the acid from the sugar. The process concentrates and recycles 98% of the sulfuric acid for reuse. The remaining 1%-2% of the acid left in the sugar solution is neutralized with lime, creating hydrated gypsum that can be easily separated from the sugar solution. Specially developed cultures of yeasts are then used to ferment the sugar stream into ethanol.

Byproducts from the process include lignin, which will be burned in solid fuel boilers to satisfy about 70% of the plant’s thermal needs, a yeast stream that can be sold into animal feed markets, and agricultural-grade gypsum that can be used as a soil amendment. The project proximity to Los Angeles is advantageous in reducing the cost of transporting feedstock to the plant and moving the ethanol to market.

Constructing this type of plant in California offers advantages over most other locations because curbside source separation and primary separation of municipal solid waste materi­als has already been implemented to a great extent. Feedstock used in the process includes green agricultural waste, commercial landscaping green waste, clean woody construction and demolition debris, and short paper fibers that cannot be recycled.

Poet and Abengoa Bioenergy plan to utilize enzymatic hydrolysis and fermentation tech­niques to produce cellulosic ethanol. The processes employ enzymes to liberate fermentable sugars locked in the complex carbohydrate structures that form the cell walls of plants. Microbes then ferment the sugars into ethanol (Greer 2008). Poet will convert an existing 50 million gallons per year corn ethanol facility in Emmetsburg, Iowa, into a 125 million gallons per year biorefinery, which will include a 25 million gallons per year cellulosic ethanol pro­cessing system. The new $200 million facility, called Project Liberty, will produce cellulosic ethanol from 770 tons of corncobs and corn fiber. Construction is expected to start in early 2010, with completion and commissioning in the second half of 2011 (Greer 2008).

Utilizing corncobs as a source of feedstock offers a less disruptive source of biomass than most other byproducts, wastes, and crops. In conventional harvesting systems, corn is har­vested by combines and the kernels are removed from the cob. The cobs are discarded and left on the surface, where they are plowed into the soil at a later date. By collecting the cobs separately from the kernels during harvesting, a relatively uniform source of biomass can be removed and stored for conversion to ethanol. The remainder of the crop residues can be left for incorporation into the soil and preservation of organic matter levels.

Poet estimates that farmers will receive between $30 and $60 per ton of corncobs and the average acre of corn yields between three-quarters to a ton of corncobs. Each ton of corncobs will produce approximately 85 more gallons of ethanol per acre. Consequently, about 27% more ethanol can be produced by adding the cellulosic production method to the current corn-to-ethanol technology (Greer 2008).

Colocating the corn and cellulosic ethanol facilities will allow Poet to leverage its relation­ships with the hundreds of farmers that already provide corn to the plant. Those same farmers will provide the cobs as well. The cellulosic ethanol facility will also take advantage of the existing biorefinery infrastructure, including roads, railroads, utilities, and land.

Waste from the cellulose-to-ethanol process will be used to produce steam in a solid fuel boiler and biogas in an anaerobic digester, generating process heat for the entire biorefinery. These alternative fuels will significantly reduce Poet’s usage of natural gas. The company completed a $9 million pilot-scale cellulosic ethanol plant, adjacent to its 9 million gallons per year corn ethanol refinery in Scotland, South Dakota, in late 2008. The facility will produce 20,000gallons of cellulosic ethanol from corncobs and fiber. Lessons learned from the development, testing, and validating of the technology at the Scotland facility will be applied to the design and engineering of the project (Greer 2008).

Abengoa Bioenergy is also building a hybrid biorefinery producing corn and cellulosic ethanol. The new facility in Hugoton, Kansas, will produce 85 million gallons per year of corn ethanol and 11.4 million gallons per year of cellulosic ethanol from 400 dry metric tons of biomass. Total project costs are estimated at $500 million, including $190 million for the cellulosic ethanol plant. Abengoa currently produces 198 million gallons per year of corn ethanol in the United States and 142 million gallons per year in Europe. The company also operates a cellulosic ethanol pilot facility in York, Nebraska.

Initially, biomass feedstock for the Hugoton plant will include corn stover, wheat straw, and milo stubble. In an effort to develop feedstock sources that are diverse and sustainable, the project will work with local farmers to establish energy crops, such as switchgrass, on nonfood-producing acres. A biomass gasification system producing syngas for thermal energy will reduce fossil fuel usage and greenhouse gas emissions.

Abengoa has received $15 million of its $76 million DOE grant to fund the preliminary design, permitting, and environmental review. Construction will begin shortly thereafter, with completion slated for 2011.

Most farm — based anaerobic digesters in Europe use co — digestion of animal manures with organic wastes, such as crop residues or energy crops. In Germany, for example, a high per­centage (over 90%) of agro-biogas plants use co-digestion. Organic materials of over 30 byproducts and wastes are used, but energy crops and crop residues are favored relative to off — farm wastes to avoid the cost of transportation to the farm. Most crops can be co-digested with manure, with maize and grasses as most important examples. The advantages of maize and

grasses are the relatively high biogas yield per hectare for maize and the low energy input required for grasses. Both maize and grasses can be readily stored to provide a year-long input for digesters (Chynoweth et al. 1993; Gunaseelan 1997; Lewandowski et al. 2003; Lehtomaki et al. 2008) . The disadvantage may be the very long digestion residence times required. Ensilage of whole crop maize, for example, is one potentially important process to conserve organic material and provide for pretreatment of feedstock (Driehuis et al. 1999).

The results of some of these studies show that up to 3000-5000m) of methane can be generated from a hectare of cultivated energy crops (Lehtomaki et al. 2008). Most crops have methane potentials in the range of 0.25 to 0.4 L CH4/g VSadded. In general, the methane poten­tial increased with crop maturity. Anaerobic digestion of dairy manure with crop materials (sugar beet tops, grass silage, and oat straw) in completely stirred tank reactors (CSTRs) was feasible with up to 40% VS for the feedstock (60% for manure; Chynoweth et al. 1993; Lewandowski et al. 2003; Bouallagur et al. 2005). This is in agreement with Lehtomaki et al. (2008), who obtained specific methane yields between 0.21 and 0.27 L CH4/g VSadded for straw, sugar beet tops, and grass as feedstocks with cow manure (30% and 70% based on VS, respectively) in a co-digestion approach.

Lignocellulose containing environments possess a natural degradative microbiome composed of a genomically diverse set of organisms. Often, these organisms, particularly those that are underrepresented, are missed in culture, but yet may supply significant metabolic contribu­tions to surrounding organisms in these complex environments (Ley et al. 2005, 2008a, b; Sogin et al. 2006; Turnbaugh et al. 2006). Microbiologists began using culture-independent methods, metagenomics, to circumvent low isolation numbers (less than 1%) often seen with culture-based techniques in environmental samples (Handelsman 2004; Schloss 2008). Metagenomics allows genomic access to the entire population of microorganisms and allows for independent analysis of these microbes in conjunction with their natural habitat.

Traditional metagenomic analyses generally begin with total extracted genomic DNA of that community. The DNA can be digested using restriction enzymes, ligated into a vector and propagated in a host, often Escherichia coli. For a sequence-driven analysis, clones can be chosen at random and subsequently sequenced. For function-driven analyses, clones can be screened for phylogenetic markers, enzymatic activity, or antibody binding. Heterologous gene expression then allows for physiological identification of small molecules or proteins. Metagenomic studies began with traditional Sanger sequencing. As microscopic enumeration and colony counts were compared to the resulting numbers of microbes cultured, it became apparent that there was a large majority of organisms that were overlooked in these traditional studies (Schloss and Handelsman 2003; Handelsman 2004) . Indeed, the sequencing and assembly of large gene insert libraries have also been hypothesized to lead to reconstruction of a nearly complete microbial genome (DeLong 2004). As demand for genomic tools arose that would allow for a more accurate picture of the functional distribution of the microbial diversity present, sequencing of large insert libraries, traditionally used in single organism genomics, was applied to total community DNA. This allows for the screening of clones for functional diversity resulting in novel gene discovery, providing a link for genetics and functional expression for each of the selected clones. Of particular significance to this review, metagenomic analysis of a bovine rumen expression identified 22 glycoside hydrolase clones of which four potentially represent previously undescribed families of glycoside hydrolases (Ferrer et al. 2005). A novel polyphenol oxidase (laccase) from this bovine rumen expression library has also been identified and characterized, and it was implied that this enzyme might play a role in ryegrass lignin digestion (Beloqui et al. 2006). Massive metagenome sequenc­ing was also recently applied to another lignocellulose- degrading community, the termite hindgut (Warnecke et al. 2007). This extensive data set showed a diversity of bacterial genes for cellulose and xylan hydrolysis, mainly from spirochete and fibrobacter species. Clearly, the termite hindgut, like the rumen, is a microbial community specialized toward plant cell wall degradation and is a potentially important source of novel enzymes for more woody substrates.

With the advent of next — generation sequencing technologies, sequenced — based metage­nomic approaches have strayed from cloning techniques, which introduce their own levels of bias, to a more random sequencing strategy, pyrosequencing (Ronaghi et al. 1996, 1998; Margulies et al. 2005). We have recently used this approach to examine randomly sampled pyrosequence data from three fiber-adherent rumen microbiomes and one pooled rumen liquid phase sample (Brulc et al. 2009). This genomic analysis revealed that, in the rumen micro — biome, the dominant enzymes are those that attack the easily available side chains of complex plant polysaccharides and not the more recalcitrant main chains, even when cellulose is present as a substrate. Furthermore, when compared to the termite hindgut microbiome, there are fundamental differences in the glycoside hydrolase content, with the termite hindgut microbiome containing more enzymes that are involved in degradation of cellulose (GH5, 9, 44, and 74) and xylan (GH10 and 11). Thus, it appears that in these lignocellulose-degrading microbiomes, CAZyme content appears to be diet driven (forages and legumes or wood). Therefore, when looking for novel microbial plant cell wall deconstructing enzymes, it is important to choose the environment that will serve as a genetic resource for plant cell-wall degrading microbial enzymes based on the substrates to be utilized.

Because of the lack of large commercial utilization of lignocellulosic biomass for liquid fuel production, very little commercial know-how and experience are available to learn from. Therefore, we will examine the inbound logistics of grain and corn cobs in a grain elevator and a dry-grind corn ethanol plant (biorefinery) in order to have an insight of the inbound logistical operations that take place at the plant gate during the delivery of feedstocks for further shipment or the production of bioproducts and biofuels. The brief description of these handling operations is taken from Mukunda (2007) and the material flowchart presented in Figure 7.8.

Grain Handling in a County Elevator

A brief overview of the grain handling in an elevator is presented. There are a number of layouts for inbound receipts of grain trucks, but the one described is from Berruto and Maier (2001). Grain is delivered to a county elevator with hopper bottom trucks with a capacity to carry about 25,500kg (1000bushel) grain. The first stop of the grain delivery truck at the elevator is the sampling station where samples of grain are pulled from the truck load using automatic telescopic probes. The pulled samples are graded to determine U. S. grades for grain quality by which the price to be paid is set. The variables tested to determine U. S. grade for corn include the moisture content, test weight (bulk density), broken kernel and foreign mate­rial (BCFM) and damaged kernels total (DKT), and heat-damaged kernels (Stored Product Management, E-912 1995). While the samples are being graded, the trucks drive up to an unloading station where it is weighed for gross weight, unloaded in a dump pit, and then weighed again for tare weight to determine the weight of the grain delivered. A ticket is made out for the delivery after this process. The moisture content of the grain determines if the grain is directly sent to one of the many storage silos or if it is first cleaned and dried to a safe moisture level before storage. A discount to the price is estimated for every percent point of moisture removed in drying.

The design layout of an elevator facility is very important and determines the efficiency at which inbound grain trucks flow through the facility, especially when delivering identity — preserved grain or different grain types during peak harvest season. Coordination of the opera­tions in delivering the grain from sampling to exit of the truck using a queue management system is vital to reducing the time it takes to service customers delivering grain to the facility. A goal is to minimize the average waiting time per truck so that the maximum designed capac­ity for the facility can be obtained for most of its service period. The cost of shipping grain is also reduced if trucks spend a minimal time waiting to receive their cargo. The inbound logistics of grain at an elevator does give us a glimpse of the challenges that would be faced processing large volumes of inbound trucks with plant biomass through a biorefinery.

Yuanhui Zhang

Abstract

All fossil fuels found in nature—petroleum, natural gas, and coal, based on biogenic hypothesis—are formed through processes of thermochemical conversion (TCC) from biomass buried beneath the ground and subjected to millions of years of high temperature and pressure. In particular, existing theories attribute that petroleum is from diatoms (algae) and deceased creatures and coal is from deposited plants.

TCC is a chemical reforming process of biomass in a heated and usually pressurized, oxygen deprived enclosure, where long-chain organic compounds (solid biomass) break into short — chain hydrocarbons such as syngas or oil. TCC is a broad term that includes gasification, including the Fisher-Tropsch process, direct liquefaction, hydrothermal liquefaction, and pyrolysis. Gasification of biomass produces a mixture of hydrogen and carbon monoxide, commonly called syngas. The syngas is then reformed into liquid oil with the presence of a catalyst. Pyrolysis is a heating process of dried biomass to directly produce syngas and oil. Both gasification and pyrolysis require dried biomass as feed­stock, and the processes occur in an environment higher than 600°C. The hydrothermal liquefaction (HTL) involves direct liquefaction of biomass, with the presence of water and perhaps some catalysts, to directly convert biomass into liquid oil, with a reacting temperature of lower than 400-C.

This chapter only covers the topic of HTL of biomass. Biomass feedstocks include biowaste (manure and food processing waste), lignocellulose (crop residue), and algae. The chapter is in two parts. The first part covers HTL fundamentals based on the current knowledge, and the second part is a summary of state-of-the-art knowledge of HTL for various feedstocks. The author has attempted to organize this chapter for a variety of readers who are interested in the topic of HTL, including students and professionals.

Biofuels from Agricultural Wastes and Byproducts Edited by Hans P. Blaschek, Thaddeus C. Ezeji and Ju rgen Scheffran 201 © 2010 Blackwell Publishing. ISBN: 978-0-813-80252-7

Process integration results in the reduction of capital and operational costs. There are pro­cesses known as separate hydrolysis and fermentation (SHF), simultaneous saccharification and fermentation (SSF), and simultaneous saccharification fermentation and recovery (SSFR). In SHF both hydrolysis and fermentation are carried out in separate reactors. In SSF the enzymatic hydrolysis continuously releases sugars that are used by the culture simultane­ously. The process integrated with product recovery is called SSFR. SSF can also be achieved by performing both hydrolysis and fermentation in a single bioreactor employing single or mixed microbial strains that produce all the necessary enzymes and use all the hexose and pentose sugars. This type of process is called consolidated/integrated bioprocessing (CBP/ IBP; Lynd et al. 2005). The CBP/IBP differs from SSF because CBP does not require addi­tion of exogenous hydrolytic enzymes (the ethanol-producing microbial strain produces them), unlike SSF, which requires addition of exogenous enzymes for substrate hydrolysis. These processes (CBP/IBP) can be integrated with simultaneous product recovery that can be called CBP/IBP with product recovery (CBPPR or IBPPR).

Anaerobic Digestion in China

Anaerobic digestion is a scalable process, which is often overlooked in the United States and is frequently stated to be economically viable only for large farms. Dairy farms, for instance, are thought to be economical for ~500 cows or more. The scalability feature is particularly evident in China, where by 2005 more than 18 million household digesters have been installed providing 7 billion m3 of biogas per year (van Nes 2006). A common operating system is a small inground digester fed with swine waste from a few pigs and human waste. The gener­ated biogas is used in the household for cooking and lighting. The Ministry of Agriculture (MOA) aims to increase the number to 27 million household digesters by 2010. In addition to the household digesters, in 2005 there were almost 3500 medium — and large-scale biogas digesters at livestock and poultry farms (van Nes 2006; Figure 4.7).

Dihydroxyacetone is a three-carbon sugar and a precursor for a number of fine chemicals and pharmaceuticals such as methotrexate. DHA is used in the cosmetics industry for the produc­tion of suntans. Since DHA causes pigmentation of the skin, it is also used in the treatment of vitiligo, an autoimmune disease in which the melanocytes are destroyed and irregular white patches are formed on the skin (Fesq et al. 2001). Gluconobacter oxydans is used industrially for the production of DHA. Glycerol is converted to DHA by the membrane-bound enzyme GldA. At high concentrations, DHA is toxic to microbial cells, and cell viability decreases exponentially with time. Overexpressing GldA gene (sldAB) in G. oxydans, however, increased the concentration of DHA from 18-25 g/L to 30g/L and the inhibitory effect of DHA on cell viability was also reduced (Gatgens et al. 2007).

Propionic Acid

Propionic acid is an important antifungal agent used in industrial production of cellulose — based plastics, solvents, herbicides, perfumes, flavors, thermoplastics, and so on. Propionic acid is formed from succinate via the dicarboxylic acid pathway. Propionic acid is natively produced by Propionibacterium acidipropionici, Propionibacterium acnes. and Clostridium propionicum. These bacteria are capable of fermenting crude glycerol to propionic acid. P. acidipropionici can produce up to 42 g/L of propionic acid, with a yield of 0.844 mol/mol glycerol and a productivity of 0.36 g/L/h (Barbirato et al. 1997).

Citric Acid

Citric acid is used as a flavoring and preservative agent in foods and beverages, as a water softener in detergents, and as a wax and color remover in shampoos. It is generally produced by Aspergillus niger and Yarrowia lipolytica. Y. lipolytica produced 11 g/L of citric acid in a glucose medium (Papanikolaou et al. 2002). When grown in a glycerol medium, this micro­organism produced up to 35 g/L citrate with a yield of 0.42-0.44 g/g glycerol (Papanikolaou et al. 2002 ).

The residues left in the forest as a byproduct of a timber harvest are often chipped and sold as a boiler fuel. This material is referred to as “fuel chips.” Whole trees are cut and brought to a preprocessing location in the woods known as a “landing.” Here the tree is de-limbed and sawed into logs of various lengths. The logs are loaded onto trucks for delivery to a sawmill (or pulp mill) and everything else, limbs and tops, is put through a chipper and blown into a chip van. If the logging contract calls for a clear-cut, then any non-merchantable trees are also brought to the landing and these trees are put through the chipper.

Some landings are mobile preprocessing plants. The chipper is mounted on a trailer so that it can be readily relocated to harvest a different tract of timber. Loggers try to optimize the in-forest transport of the raw biomass (skidding of whole trees along the ground) relative to the hauling of product (logs and chips) in highway trucks. The quicker they can get the mate­rial off the ground and onto a truck, the lower the total transport cost from stump to final use.

In the past, fuels chips have been sold at a price that basically just covers the cost of chip­ping and hauling, and perhaps a small part of the cost to bring the trees to the landing. The land owner gets his or her portion of the price for the logs but nothing for the fuel chips. The advantage to the landowner is that the site is cleared and ready for replanting when the logger leaves. The logger gets an advantage because the litter (limbs and tops) at the landing is cleared away and does not accumulate to slow operations. The fuel chip market, however, is changing rapidly as new bioenergy options compete for the chips, particularly in the Southeastern United States.

When there are no delays, a chip van can be filled in about 40 minutes. (High performance chippers are available that can fill a van in 15 minutes.) Average productivity is considerably less in a typical operation. Often, the hauler unhooks and leaves an empty van to be filled while delivering a full van. Waiting for the van to be loaded reduces the number of loads a truck tractor can pull in a given day.

The largest wood-fired electric-generating plant east of the Mississippi is an 80-MW plant in Hurt, VA. This plant unloads 150 chip vans on a typical day; their record is 311 vans in one 24-hour period. The trucks are weighed in, dumped by the truck driver, and weighed out. Total time required when there is no queue is about 10 minutes. For any bioenergy plant receiving fuel chips, the key to controlling hauling cost is the operation at the receiving facility. This was discussed previously in “Logistics of biomass feedstock handling at the plant gate " section. Haulers do not like to wait in the queue.

In the Southeastern United States, woody biomass is harvested year-round—it is stored in the forest until needed. Because just-in-time delivery is not practical, some at-plant storage is required. A host of operating variables (weather, traffic, equipment breakdowns) can inter­rupt deliveries. The 80iMW plant operates with about 20 days of aLplant storage in the summer and 45 days in the winter, when the potential for weather delays is greater. The
biomass is piled with two bulldozers that operate during the daytime delivery. These bulldoz­ers push material onto the storage pile and also push material into a twin-screw feedbox that meters material onto the belt conveyor into the plant. This conveyor must run continuously to maintain a flow of fuel into the boilers. The cost penalty for shutdown is very high. One bulldozer operates during the night shift to keep the conveyor feedbox continuously supplied.

Steam Stripping

Steam stripping, also known as steam distillation, is a process of removing temperature — sensitive compounds that cannot be separated by normal distillation due to decomposition at high sustained temperatures. It has been used to remove various organic contaminants from process plant waste water streams. Steam stripping can also be used to detoxify lignocellu — losic hydrolysates (Yu et al. 1987- . Leonard and Peterson — 1947) used steam stripping to remove inhibitory volatiles, such as furfural and acetic acids, from hydrolysates of maple and spruce. Maddox and Murray (1983) passed steam through hydrolysates of Pinus radiata to achieve a liquor temperature of 90°C for 15 minutes. Treatment of the hydrolysates by steam stripping followed by passage of the hydrolysates through activated carbon led to successful fermentation, but the treatment procedure caused about 30% sugar losses. The high stripping temperatures allow the removal of heavy and soluble organic compounds. The only waste generated in steam stripping is a small amount of concentrated organics. These organic wastes are easily processed by incineration, biological treatment, or recycling. However, steam strip­ping is not a good solution for hydrolysates that contain nonvolatile phenolics with high boiling points.

Evaporation

Evaporation when used as a detoxification method removes only volatile inhibitors. However, in previous studies, the volatile compounds did not affect either the enzymatic hydrolysis or

Detoxification Method Effectiveness

Physical

Steam stripping Remove volatile inhibitors or inhibiting end

products such as furfural and acetic acid

Evaporation

Solvent extraction Remove both volatile and nonvolatile

inhibitors pH dependently

Excellent biocompatibility; rapid mass transfer due to low-interfacial tension; remove both volatile and nonvolatile inhibitors; and potential for in situ extractive fermentation

Excellent biocompatibility; remove furans, phenolics, and aliphatic acids polarity dependently; high concentration factor of inhibitors

Alleviate the inhibitory effect of toxic compounds

Remove both volatile and nonvolatile inhibitors

The most commonly used method; precipitate a wide range of inhibitors

Extensive mediation and precipitation of inhibitors

Table 11.1. Continued. Detoxification Method Effectiveness Disadvantages References Reducing substances, NaHS03, Na2S205, KHS03, Na2S, etc. Overcoming unfavorable oxidation — reduction potentials in hydrolysates Less efficient than overliming Leonard and Flajny 1945; Larsson et al. 1999 Neutralization + zeolite Diatomaceous earth Activated carbon Wood charcoal Extensive removal of inhibitors Less efficient than overliming Sugar loss Need specially prepared wood charcoal and long treatment time Eken-Saragoglu and Arslan 2000 Ribeiro et al. 2001 Maddox and Murray 1983 Miyafuji et al. 2003 Polymeric adsorbents Mixed bed resin Ion-exchange resins Most efficient in inhibitors removal Costly and might cause sugar loss Weil et al. 2002 Tran and Chambers 1986 Florvath et al. 2005; Chandel et al. 2007 Biological Mutant S. cerevisiae Effective in utilizing acetic acid and keeping sugars intact Not effective in reducing other inhibitors Schneider 1996 Fugal isolate, Coniochaeta ligniaria NRRL 30616 Metabolizes furfural, HMF, as well as phenolics, aliphatic acids, and aldehydes Long treatment time Lopez et al. 2004; Nichols et al. 2008 Laccase and peroxidase from the white-rot fungus Highly efficient in reducing phenolics Not effective in reducing furans and aliphatic acids, etc.; long treatment time Jonsson et al. 1998; Larsson et al. 1999; Chandel et al. 2007 Adaptation Makes organisms more tolerant to the inhibitors Long treatment time and complicated manipulation Mussatto and Roberto 2004; Martfn et al. 2007; Agbogbo et al. 2008; Dinh et al. 2008

the fermentation significantly even at high concentrations. In contrast, the nonvolatile com­pounds severely affected both the hydrolysis and the fermentation (Palmqvist et al. 1996a, b). Palmqvist et al. (1996a) assessed the inhibitory effect of both the evaporation condensates and nonvolatile stillage by fermentation using S. cerevisiae. The most volatile fraction of a willow hemicellulose hydrolysate obtained by roto-evaporation (using a rotary evaporator) slightly decreased the ethanol productivity compared with a reference fermentation contain­ing no volatile fraction of hemicellulose hydrolysates. But in the nonvolatile fraction obtained by roto-evaporation, the ethanol yield decreased from 0.37g/g in the reference fermentation (glucose and nutrients) to 0.31 g/g in the treated lignocellulosic hydrolysates fermentation, and the average ethanol fermentation rate, r2h. decreased from 6.3 to 2.7 g/h. As a result, commercial application of evaporation for hydrolysates detoxification may be limited.