As mentioned earlier, the nonfermentable solids in distilled mash (stillage) contain variable amounts of fiber and protein, depending on the feedstock. The liquid also contains soluble protein and other nutrients and as such remains valuable. The recovery of the protein and other nutrients in stillage for use as livestock feed can be essential to the overall profitability of etha­nol fuel production. Protein content in stillage varies with feedstock. Some grains such as corn and barley yield solid by-products that are called dried distillers grains (DDGs). Protein content in DDG typically ranges from 25 to 30% by mass and makes an excellent feed for livestock.

The enzyme of interest is cellulase, needed for the hydrolysis of the cellulose, that is, cellulolysis. Cellulase is a multicomponent enzyme system consisting of endo-p-1,4-glycanases, exo-^-1,4-glucan glucohydrolases, and exo-p-1,4- glucan cellobiohydrolase. Cellobiose is the dominant product of this system but is highly inhibitory to the enzymes and is not usable by most organisms. Cellobiase hydrolyzes cellobiose to glucose, which is much less inhibitory and highly fermentable. Many fungi produce this cellobiase and most of the work that is presently being conducted is on Trichoderma reesei (viride). The cellulase produced by T. reesei is much less inhibited than other cellulases that have a major advantage for industrial purposes [58].

The type of inhibition exhibited by cellulases is the subject of much debate in research. Although most of the researchers favor competitive inhibition [59-64], some cellulases are noncompetitively [46, 62, 65, 66] or uncompeti­tively inhibited [60]. Uncompetitive inhibition takes place when an enzyme inhibitor binds only to the complex formed between the enzyme and the substrate, whereas noncompetitive inhibition takes place when an enzyme inhibitor and the substrate may both be bound to the enzyme at any given time. On substrates such as Solka Floc® (purified cellulose), wheat straw, and bagasse (biomass remaining after sugarcane stalks are crushed to extract their juice), Trichoderma reesei produced enzyme is competitively inhibited by glucose and cellobiose. On the other hand, some enzymes are noncom­petitively inhibited by cellobiose using other substrates such as rice straw and Avicel®. Avicel is a registered trade name for microcrystalline cellulose that has been partially hydrolyzed with acid and reduced to a fine powder; it is used as a fat replacer. Trichoderma viride is uncompetitively inhibited by glucose in a cotton waste substrate [60].

Many mutants have been produced following Trichoderma reesei. The most prominent among these is the Rut C-30 [67], the first mutant with p-glucosidase production [43]. Other advantages of the strain include its hyperproduc — ing properties and the fact that it is carbolite-repression resistant. The term hyperproduction means excessive production.

Cellulases from thermophilic bacteria have also been extensively exam­ined. Among these, Clostridium thermocellum is perhaps the most extensively characterized organism. C. thermocellum is an anaerobic, thermophilic, cel­lulolytic, and ethanogenic bacterium capable of directly converting cellulosic substrate into ethanol. The enzymes isolated from thermophilic bacteria may have superior thermostability and hence will have longer half-lives at high temperatures. Although this is not always the case, cellulases isolated from Clostridium thermocellum have high specific activities [68], especially against crystalline forms of cellulose that have proven to be resistant to other cel — lulase preparations.

Enzyme production with Trichoderma reesei is difficult because cellulase production discontinues in the presence of easily metabolizable substrates. Thus, most production work has been carried out on insoluble carbon sources such as steam-exploded biomass or Solka-Floc [69]. Solka-Floc is composed of beta-1, 4-glucan units, is white, odorless, and flavorless, and has varying particle sizes [70]. In such systems, the rate of growth and cellulase produc­tion is limited because the fungi must secrete the cellulase and carry out slow enzymatic hydrolysis of the solid to obtain the necessary carbon. Average productivities have been approximately l00 IU/L/hr. [Hydrolytic activity of cellulose is generally in terms of international filter unit (IU). This is a unit defined in terms of the amount of sugar produced per unit time from a strip of Whatman filter paper.] The filter paper unit is a measure of the combined activities of all three enzymes on the substrate. High productivities have been reported with Trichoderma reesei mutant in a fed-batch system using lactose as a carbon source and steam-exploded aspen as an inducer. Although lactose is not available in sufficient quantities to supply a large ethanol industry, this does suggest that it may be possible to develop strains that can produce cel- lulases with soluble carbon sources such as xylose and glucose.

Productivity increases dramatically reduce the size and cost of the fer­menters used to produce the enzyme. More rapid fermentation technolo­gies would also decrease the risk of contamination and might allow for less expensive construction. Alternatively, using a soluble substrate may allow simplification of fermenter design or allow the design of a continuous enzyme production system. Low-cost but efficient enzymes for lignocellu — losic ethanol technology must be developed in order to reduce the opera­tional cost and improve the productivity of the process.

The waste originates from numerous sources: residential community (i. e., municipal solid waste or MSW), commercial and light-industrial communi­ties, manufacturing activities such as heavy-industrial and chemical indus­tries (generally classified as hazardous waste), agricultural and forestry waste, human and animal waste, paper and pulp industry waste, automobile and other transportation waste, hospital waste (generally considered as infec­tious waste), nuclear waste, and so on. In addition to man-made waste, there are numerous naturally occurring waste materials generally classified under the category of "lignocellulosic materials (LCM)" and certain forms of crop oils (e. g., algae, waterweed, water hyacinth). These are biomasses that do not have a useful purpose for food or a direct use for human or animal purposes.

Table 6.1 lists the heating values of various waste-derived fuels. This table shows the valuable energy content of various types of waste materials. This chapter does not consider special types of waste such as nuclear and infectious waste as well as several types of hazardous and nonhazardous industrial waste such as glass, metals, and other noncombustible waste. The chapter does, however, examine the appropriate conversion processes for all cellulosic-based waste as well as some polymeric waste such as plastic and rubber tires. A typical material distribution of MSW collected in the United States is illustrated in Figures 6.1a and b. In 1988, approximately 80% of 180 million tons of waste generated in the United States was cellulose-based [10]. This percentage has not changed in the 1990s and 2000s [4]. In Europe, MSW is expected to increase up to 300 million tons by 2015 [5].

In processing waste, noncombustible materials such as glass and dirt are removed. The glass is either recycled or sent to glass-melting furnaces. The heavy metals such as ceramics, heavy metals, and aluminum are routed to the landfill for disposal. A significantly important component of MSW is polymeric materials. Although polymer waste only accounts for 8.5% by mass of the total MSW disposed of in the United States, plastic represents

over 28% by volume [6]. Plastic waste is not biodegradable. Thus it is mostly recycled either for reuse or to recover basic monomers. Polymeric waste ranges from packaging materials used in the food industry to various parts in automobiles to high-density polyethylene (HDPE) containers such as pop bottles, laundry detergent bottles, milk jugs, and so on. It is estimated that over 65% of the food packaging in the United States is from plastics. As of 1978, the Ford Motor Company estimated that the average junked car con­tained 80 kg of plastic and nontire rubber. This number is increasing every year because of the increased use of polymers in various automobile parts.

Some types of plastic waste are recycled directly. For example, milk bot­tles, juice containers, laundry detergent bottles, motor oil cans, spring water bottles, and other similar containers are subjected to a thorough cleaning process and are reused. Some rubber tires are retread and put back for reuse. Polymeric materials including rubber are generally not discarded in a land­fill and are subjected to a conversion process either to recover basic chemi­cals and materials or to generate energy.

Plasma gasification uses an external electric source for heat and this results in the conversion of feedstock to fuel gas and the elimination of all tars, chars, and dioxins due to high temperatures. Unlike the reactors described above, plasma reactors are applied more to MSW and as shown in an earlier section they can be easily adapted to process a mixed feedstock including other types of waste such as rubber tires, polymer and plastic waste, and coal waste. The amount of electricity and temperature in the plasma reactor depends on the reactor configuration, energy content of the feedstock, and amount of air (or oxygen) allowed in the reactor. A plasma reactor can pro­duce a variety of products by careful control of reaction conditions.

Westinghouse Plasma manufactures and supplies plasma torches to the industry. The Westinghouse Plasma gasifier is essentially a classic downflow moving bed system operating at 1 atm and about 2,300°F. Various types of plasma reactors described in the previous chapters can also be used for pro­cessing the mixed feedstock.

In addition to Westinghouse, the Solena group uses the Solena gasifier which locates the torch at the bottom of the gasifier to vitrify inorganics in the feed, forming glass aggregate. This reactor uses less energy, and it also uses a carbon-based catalyst to enhance gasification. This group is planning to develop a 10 MWe plant in Malaysia using Padi husks and a number of 130 MWe integrated plasma gasification combined cycle plants in the east­ern United States to use waste coal and coal fines in partnership with Stone and Webster [107, 108]. Startech Environmental Corporation (SEC) is also developing a plasma gasification unit that feeds shredded materials using a pump, screw, or ram, depending on the consistency of the feed. In this reac­tor, the plasma torch is located at the top of the reactor and is then directed to dissociate organics and to melt inorganics in the feed, forming clean gas and glass aggregate. Acid gases, volatile metals, and particulate matter are removed from the cooled gasifier effluent. Starcell™ technology can also pro­duce hydrogen. SEC is also planning a 200 T/D MSW plant in Panama jointly with Hydro-Chem (a division of Linde) and a 10T/D plant in China [6].

Biomass is the oldest fuel known to and used by humans in every conceiv­able aspect of their lives. Biomass is of a widely diverse kind and is also the most versatile. Considering this long history of biomass use in energy gen­eration, its development as a modern energy source has been slow-paced, mainly due to the rapidly gained public acceptance and popularity of other competing fuel sources such as fossil fuels and nuclear energy. However, biomass has clear advantages over fossil fuels in terms of renewability, sustainability, carbon neutrality, political independence, and regional eco­nomic benefits.

Biomass occurs in many different forms with widely varying chemical compositions. Biomass can be readily converted into solid, liquid, and gas­eous fuel products. For example, wood is typically chopped into chunks, chipped for easier handling, or pelletized for pumping. Biomass can be pyrolyzed into bio-oil liquid fuel as a major product along with the by­products including solid biochar and gaseous fuel. Or, biomass can be gas­ified to generate synthesis gas which is rich in hydrogen. Biomass can also be burned to generate heat (such as hot water or steam) or to produce elec­tricity (or power). Thus, biomass combustion can be used in a combined heat and power (CHP) mode. Alternately, some biomass can be coprocessed with other conventional fossil fuels such as lignite coal to generate power. However, some biomass is too wet to be efficiently burned and instead is fermented to produce liquid fuel such as bioethanol. In addition, some bio­mass such as oil crops and microalgae can be processed into biodiesel via both physical and chemical treatments. Furthermore, bio-oil and biosyngas can be upgraded for subsequent chemical and fuel processing as well as for other diverse end uses.

Different forms and shapes, diverse origins, varying feedstock composi­tion, and substance-specific properties of biomasses inevitably require dif­ferent types, levels, and sequences of process treatments before they can be used effectively and cleanly in modern technological society. Typical process treatments in biomass conversion and utilization technologies are discussed in the next sections.

The name biodiesel comes from the fact that the fuel is derived from biologi­cal sources and the fuel is, at least originally, meant to be used in diesel engines. Biodiesel is considered a renewable fuel as it is produced from plant oils and animal fats [40]. However, it should be noted that "biodiesel" as a name potentially implies and encompasses more than what is technologi­cally defined as biodiesel. In other words, biodiesel is far more specific than simply "a biologically derived fuel that can be used on diesel engines."

Biodiesel is a renewable alternative fuel for diesel engines comprised of a long-chain mono-alkyl ester and is the product of the transesterification reaction of triglycerides with low molecular weight alcohols such as metha­nol and ethanol. According to the National Biodiesel Board (NBB), which is the national trade association representing the biodiesel industry in the United States, biodiesel is defined as "a domestic, renewable fuel for die­sel engines derived from natural oils like soybean oil, and which meets the specifications of ASTM D 6751" [41]. Although ASTM D 6751 provides the original specifications for 100% pure biodiesel (B100), there are other objec­tive fuel standards and specifications for biodiesel fuel blends such as:

• Biodiesel blends up to 5% (B5) to be used for on — and off-road diesel applications (ASTM D975-08a)

• Biodiesel fuel blends from 6 to 20% (B6-B20; ASTM D7467-09)

• Residential heating and boiler applications (ASTM D396-08b)

There is a major global push enacted by the Kyoto Protocol (adopted on December 11, 1997; entered into force on February 15, 2005) to reduce emis­sions of greenhouse gases (GHGs), more particularly, carbon dioxide [42]. Currently, the world is seriously concerned with energy sustainability and affordability, as many industrialized and developing nations are economi­cally hurting from escalating costs of energy and fuels, in particular, petro­leum-based transportation fuels. Biodiesel is one of the alternative fuels that can help the world address these issues. Biodiesel is considered a mostly carbon-neutral fuel and is completely biodegradable.

In the presence of an acid catalyst (typically, sulfuric acid) ethanol reacts with carboxylic acids to produce ethyl esters. The two largest-volume ethyl esters are ethyl acrylate (from ethanol and acrylic acid) and ethyl acetate (from ethanol and acetic acid).

Ethyl acetate is a common solvent used in paints, coatings, and in the phar­maceutical industry. The most familiar application of ethyl acetate in the household is as a solvent for nail polish. A typical reaction that synthesizes ethyl acetate is based on esterification:

C2H5OH + CH3COOH = C2H5OOCCH3 + H2O

This chemical reaction follows very closely a second-order reaction kinet­ics, an often-used example problem for second-order elementary reactions in chemical reaction engineering textbooks.

Recently, Kvaerner Process Technology developed a process that produces ethyl acetate directly from ethanol without acetic acid or other cofeeds. Considering that both acetic acid and formaldehyde can also be produced from ethanol, this ethanol-to-ethyl acetate process idea is quite innovative and significant. The Kvaerner process allows the use of fermentation etha­nol, produced from biorenewable feedstock, as a sustainable single-source feed, which is remarkable. Furthermore, the process elegantly combines both dehydrogenation and selective hydrogenation in its process scheme, thus producing hydrogen as a process by-product which makes the process eco­nomics even better.

Ethyl acrylate, which is synthesized by reacting ethanol and acrylic acid, is a monomer used to prepare acrylate polymers for use in coatings and adhe­sives. Ethanol is a reactant for ethyl-t-butyl ether, as is the case for methanol to methyl-t-butyl ether. ETBE is produced by reaction between isobutylene and ethanol as

C2H5OH + CH3C(CH3)=CH2 ^ C(CH3)3 OQH5

Vinegar is a dilute aqueous solution of acetic acid prepared by the action of Acetobacter bacteria on ethanol solutions. Ethanol is used to manufacture ethylamines by reacting ethanol and ammonia over a silica — or alumina-sup­ported nickel catalyst at 150-220°C. First, ethylamine with a single amino group in the molecule is formed and further reactions create diethylamine and triethylamine. The ethylamines are used in the synthesis of pharmaceu­ticals, agricultural chemicals, and surfactants.

Ethanol can also be used, instead of methanol, for transesterification of tri­glycerides in biodiesel production using vegetable oils or algae oils, as dis­cussed in Chapter 2. In the United States, methanol is currently more popularly used for this purpose, mainly due to its more favorable process economics.

In addition, ethanol can be used as feedstock to synthesize petrochemicals that are also derived from petroleum sources. Such chemicals include ethyl­ene and butadiene, but are not limited to these. This option may become via­ble for regions and countries where the petrochemical infrastructure is weak but agricultural produce is vastly abundant. This is particularly true for the times when petroleum prices are very high. Ethanol can also be converted into hydrogen via reforming reaction, that is, chemical reaction with water at an elevated temperature typically with the aid of a catalyst. Even though this method of hydrogen generation may be economically less favorable than either steam reforming of methane or electrolysis, the process can be used for special applications, where specialty demands exist or other infrastruc­ture is lacking.

More recently, supercritical water reformation of crude ethanol beer was developed for hydrogen production [41]. The process utilizes super­critical water (T > 374 C and P > 218 atm) functioning both as a highly energetic reforming agent and as a supercritical solvent medium, thus effectively eliminating the service of any noble metal catalyst or the need of pure ethanol. Furthermore, its direct noncatalytic reformation of unpu­rified crude ethanol beer alleviates the need for any energy-intensive pre­distillation or distillation of a water-ethanol solution, thereby achieving overall energy savings.

Poly(lactic acid) or polylactide (PLA) is a thermoplastic aliphatic polyester derived from cornstarch. Poly(lactic acid) is one of the leading biodegradable polymers, which is derived from renewable biosources, more specifically corn in the United States. A variety of applications utilizing poly(lactic acid) are being developed, wherever biodegradability of plastic materials is desired. PLA can be used by itself, blended with other polymeric materials, or as composites. As biodegradable polymer technology further develops, the PLA market is also expected to grow and so is the cornstarch market.

McAloon et al. [97] studied the cost of ethanol production from lignocellu — losic materials in comparison to that from corn starch. As properly pointed out in their study, the cost comparison was made between the mature corn — ethanol industry and the emerging lignocellulosic-ethanol industry. Based on the fixed price of the year 2000, the cost of fuel ethanol production from lignocellulose processes was determined to be $1.50/gal, whereas that from corn processes was $0.88/gal [97]. Needless to say, the cost values deter­mined in 2000 cannot be considered valid for the current year, due to signifi­cant changes during the period in infrastructural and raw material costs as well as variable operating costs. In order to make lignocellulosic biorefinery technology a success, the following must be resolved.

1. The lignocellulose feedstock collection and delivery system has to be established on an economically sound basis. Feedstock prepara­tion also becomes an issue.

2. Each step of the process technology needs to be separately inves­tigated for various options and the interactions and connectivity between the steps must be completely evaluated. Interactive effects between stages become very important, because one stage’s product and by-products may function as the next stage’s inhibitors.

3. A thorough database for a variety of different feedstock must be established. A different feedstock can be chosen as a model feed­stock for different countries and regions, depending upon the local availability, logistical constraints, and infrastructural benefits. Furthermore, conversion technologies should be readily adaptable to other lignocellulosic feedstock and agricultural residues [93].

4. Large-scale demonstration is crucially important for commercial operational experience as well as to minimize the risk involved in scale-up efforts. In addition, such an operation on a large scale helps demonstrate environmental life-cycle analysis whose results are more meaningful.

5. Low-cost but highly efficient enzymes for the technology must be developed in order to reduce operational cost and improve produc­tivity. Current efforts by the National Renewable Energy Laboratory, Genencor International, and Novozymes Biotech are very significant and noteworthy in this regard. Further advances will make the full- scale commercialization of cellulosic ethanol closer and more eco­nomically feasible.

The single-stage process was developed and commercialized by Westing — house Corporation (Madison, Wisconsin, United States) and Europlasma Corporation (Mocenx and Cenon, France and Shimonoseki, Japan). The scale of the process varied from about 10 tons/day to about 42 tons/day in different locations [68, 69]. The largest commercial plant was established at Utashinai in Japan (180 tons/day) and a smaller (22 tons/day) at Mihama-Mikata in Japan both using Westinghouse plasma gasification technology [7]. Both of these plants are called Alter NRG/Westinghouse plasma gasification process [68-70]. In 2007, NRG acquired the Westinghouse Plasma Corporation and combined the Westinghouse updraft gasification reactor concept that uses plasma torches to provide part of the energy input, with synthesis gas clean­ing in order to convert the synthesis gas to heat and electricity and other value-added products. The process can use a variety of feedstock such as MSW, MSW plus tires, RDF, ASR, coal plus wood, petcoke, and other haz­ardous wastes. The ability to process heterogeneous, unsorted, or differ­ently sized feedstock reduces the cost required for feed handling prior to gasification.

The plasma gasification reactor (PGR) used by Alter NGR is graphically illustrated in Figure 6.7. The reactor has a refractory lining to withstand high temperatures and corrosive conditions. The reactor is first packed

with metallurgic coke which absorbs and retains the thermal energies from plasma torches and creates an environment to melt inorganic materials. The coke is consumed as the reaction proceeds. The temperature of the plasma plume varies from 5,000 to 7,000°C and the temperature at the bottom is about 2,000°C [7].

The reactor uses a mixture of oxygen and steam to improve hydrogen yield in the synthesis gas. As shown in Figure 6.7, whereas the synthesis gas leaves the reactor at the top at about 890-1,100°C, the molten slag at the bottom containing nocombustible inorganics and recoverable metals leaves the reactor at about 1,650°C. The molten slag then goes through a slag handling system for further processing. High residence time in the reactor assures the conversion of tar and it minimizes the particulate car­ryover. The electrical energy supplied by plasma torches counterbalances the heating value of the waste feed and thereby controls the temperature and the quality of the synthesis gas. The vitrified slag can be used for the construction industries, although the quality of this slag can be deterio­rated by the presence of ASR in the waste feed. The major problem with the single-stage process is the different types of harmful contaminants present in the synthesis gas which requires a series of downstream processes to remove them. These downstream processes add significantly to the overall cost of the gasification process [7].

6.5.4.2.1 Two-Stage Process

A two-stage Gasplasma™ process [70] was developed by Tetronics Corp. as well as Plasco Energy Group (Ottawa, Canada). Integrated Environmental Technologies of the United States and Pyrogenesis Corporation of Canada also use this process. The pilot-scale process is used by Advanced Plasma Power of Swindon, UK [72-74]. The process contains two stages, gasification followed by plasma conversion, which are followed by a number of processes to clean and cool the synthesis gas prior to delivering it to gas engines for conversion to mechanical/electrical energy.

The Gasplasma process converts waste feedstock into a clean, hydrogen — rich synthesis gas and a vitrified recyclate called Plasmarok™ that can be used for building material or replacement aggregate. The process is capable of producing synthesis gas which, after passing through further treatment, is suitable for use as fuel in a gas engine [75].

In the first stage, the pretreated waste stream (RDF, pretreated commer­cial and municipal waste and refined biomass) is gasified in a fluidized bed gasifier in the presence of oxygen and steam at a temperature around 800- 900°C. The reactor uses a portion of heat contained in the waste material. The synthesis gas generated in the reactor contains tar and soot and solid char and ash contained in the feed material are removed at the bottom of the reac­tor and processed in the plasma converter [7].

In the second stage, the plasma converter cracks tars and soot to synthesis gas to form a gas comprised primarily of hydrogen, carbon monoxide, car­bon dioxide, and nitrogen. The ash and inorganic fraction from the gasifier are vitrified to form Plasmarok™. The plasma converter is designed in such a way as to allow the maximum amount of residence time for the synthe­sis gas under most energy-intensive conditions. The synthesis gas leaves the reactor around 1,200°C, and it is then cooled to about 200°C [7]. The heat release during cooling is recovered for further use in the reactor via steam. The synthesis gas is further cooled and the acidic components in the gas are absorbed by the alkaline solutions. The final synthesis gas is introduced in the engines at constant pressure to generate electricity. The two-stage pro­cess shows great promise for the conversion of waste into heat, electricity, and other valuable products. It gives higher throughput rate, higher conver­sion efficiency to a clean high calorific syngas, and better control over VOCs/ tars compared to single-stage operation. Process control and engineering are critical in both single — and two-stage processes [7, 70].

Vegetable oils may or may not be edible, as explained in Section 2.1.1. Even though most efforts in producing biodiesel have been using edible oils, biofuel and, in particular, biodiesel can also be produced using inedible oils [6].

The most frequently used method of characterizing the chemical structure of a vegetable oil is the fatty acid composition, which shows the distribu­tion of different kinds of fatty acids, the carbon numbers of ingredient fatty acids, location of double bonds in the fatty acid molecular structure, ratio of saturated versus unsaturated fatty acids, and more. Table 2.3 shows fatty

acid compositions of common edible oils in terms of percent by weight of total fatty acids.

The specific gravity of most edible vegetable oil ranges between 0.91 and 0.93, depending upon the kind of vegetable oil, specific composition of the oil, purity, and other factors. The specific gravity of castor oil, an inedible oil, is approximately 0.957-0.961.