Of diverse biomass resources, woody biomass is of particular interest for biomass energy. Woody biomass is used to produce bioenergy and a variety of biobased products including lumber, composites, pulp and paper, wooden furniture, building components, round wood, ethanol, methanol, and chem­icals, and energy feedstock including firewood.

The U. S. national Energy Policy Act (EPAct) of 2005 recognized the importance of a diverse portfolio of domestic energy. This policy outlined 13 recommendations designed to increase America’s use of renewable and alternative energy. One of these recommendations directed the Secretaries of the Interior and Energy to re-evaluate access limitations to federal lands in order to increase renewable energy production, such as biomass, wind, geo­thermal, and solar. The Departments of Agriculture and Interior are jointly implementing the National Fire Plan (NFP), the President’s Healthy Forests Initiative, the Healthy Forest Restoration Act, and the Tribal Forest Protection Act of 2004 to address the risk of catastrophic wildland fires, reduce their impact on communities, assure firefighting capabilities for the future, and improve forest and rangeland health on federal lands by thinning biomass density. The NFP includes five key points: (1) firefighting preparedness, (2) rehabilitation and restoration of burned areas, (3) reduction of hazardous fuels, (4) community assistance, and (5) accountability [12].

On June 18, 2003, the U. S. Departments of Energy, Interior, and Agriculture jointly announced an initiative to encourage the use of woody biomass from forest and rangeland restoration and hazardous fuels treatment projects. The three departments signed a Memorandum of Understanding (MOU) on Policy Principles for Woody Biomass Utilization for Restoration and Fuel Treatment on Forests, Woodlands, and Rangelands, supporting woody bio­mass utilization as a recommended option to help reduce or offset the cost and increase the quality of the restoration or hazardous fuel reduction treat­ments [12, 13].

One of the gateway process technologies for bioenergy generation from woody biomass is gasification, whose resultant product is biomass synthesis gas, also known as biomass syngas or biomass gas. The biomass syngas is similar in nature and composition to coal-based or natural gas-based syngas, whereas differences are largely originated from the source-specific proper­ties. Similarly to the syngas generated via coal gasification or natural gas ref­ormation, biomass syngas is also rich in hydrogen, carbon monoxide, carbon dioxide, and methane and as such this syngas can be used as building block chemicals [14] for a variety of synthetic fuels and petrochemicals and also as a feedstock for electric power generation.

In recent years the application of supercritical technology for waste treat­ment has gained significant momentum. Two major areas have been the application of supercritical water for waste gasification and supercritical extraction by carbon dioxide, water, and other solvents. The application of supercritical technology to transesterification is briefly examined in the sub­sequent section on transesterification. Lastly, the interest in reforming (and in tri-reforming) under supercritical conditions has also gained significant momentum over last decade.

A number of manufacturers offer compressed ignition engine generators optimized to run on SVOs in which the waste engine heat is recovered. As with all types of generators, including diesel and jet fuel generators, the operational issues involve (1) fuel specificity; (2) extreme climate operation including hot, cold, wet, and dusty; (3) trouble-free operational longevity; (4) maintenance frequency and burden; and (5) overall energy efficiency, among others.

Although corn is an excellent source of starch and is heavily grown in the United States, its traditional use and value as a major food resource inevitably triggers a controversial debate of "food versus fuel." The supply-and-demand dynamics of corn in the U. S. marketplace for both fuel and food end-uses has significantly contributed to the recent escalation of corn prices, which in turn increased the production cost of ethanol as well as the price of corn-derived foods. This has been one of the principal reasons that drive the commercial­ization efforts of cellulosic ethanol production which is based on nonedible renewable feedstock.

As a fermentable carbohydrate, cellulose differs from other carbohydrates generally used as a substrate for fermentation. Cellulose is insoluble and is polymerized as 1-4, в-glucosidic linkage. Each cellulose molecule is an unbranched polymer of 15 to 10,000 D-glucose units. Hydrolysis of crys­talline cellulose is a rate-controlling step in the conversion of biomass to ethanol, because aqueous enzyme solutions have difficulty acting on insol­uble, impermeable, highly structured cellulose. Therefore, making soluble enzymes act on insoluble cellulose is one of the principal challenges in pro­cess development of cellulosic ethanol.

Cellulose needs to be efficiently solubilized such that an entry can be made into cellular metabolic pathways. Solubilization is brought about by enzy­matic hydrolysis catalyzed by a cellulase system of certain bacteria and fungi. Cellulase is a class of enzymes produced primarily by fungi, bacteria, and protozoans, that catalyze the hydrolysis of cellulose, that is, cellulolysis, as described below.

The water gas shift reaction plays an important role in manufacturing hydro­gen, ammonia, methanol, and other chemicals. Nearly all synthesis gas reac­tion involves a water gas shift reaction in some manner. The WGS reaction is of a reversible kind, whose proceeding direction can be relatively easily reversed by changing the gaseous composition as well as varying the tem­perature of the reaction. As shown in Table 5.9, the temperature dependence of chemical reaction equilibrium constant (Kp) is the mildest of the important syngas reactions considered in the table. Furthermore, the temperature when Kp becomes unity is around 814°C, where most carbon gasification reactions begin to be kinetically active.

The water gas shift reaction has dual significance. The forward water gas shift reaction converts carbon monoxide and water into additional hydrogen and carbon dioxide. This reaction is utilized to enhance the hydrogen yield from raw or intermediate syngas, such as raw product of steam reformation of hydrocarbons. If the water gas shift reaction is exploited in its reverse direction, that is, as a reverse water gas shift reaction, carbon dioxide can be reduced to carbon monoxide that is far more reactive than carbon dioxide. This enables further chemical conversion of carbon monoxide into other use­ful petrochemicals, instead of direct conversion of carbon dioxide, which is much more difficult as a task. The catalytic reverse water gas shift reaction could be very useful as a reduction method of carbon dioxide.

In many industrial reactions, the water gas shift reaction is a companion reaction to the principally desired reaction in the main stage, as evidenced in methanol synthesis and in steam reformation of methane. Whenever deemed appropriate, WGS is also carried out as a secondary stage reaction to result in additional conversion of water gas into hydrogen, as desired by a fuel reformer to generate hydrogen for proton exchange membrane (PEM) fuel cell application. Because the forward water gas shift is an exothermic reaction, low temperature thermodynamically favors higher CO conversion, and its intrinsic kinetic rate without an aid of an effective catalyst is inevi­tably low at low temperatures. Therefore, most water gas shift reaction is carried out catalytically at a low temperature such as 180-240°C. This type of catalyst is called a low-temperature shift (LTS) catalyst, which has long been used industrially. One of the LTS catalyst formulations is coprecipitated Cu/ ZnO/Al2O3 catalyst, whose formulation is better known for the low-pressure methanol synthesis catalyst [24].

In biomass gasification for generation of biosyngas, the water gas shift reaction plays an important role, inasmuch as it can produce additional hydrogen and it can also be used to control the ratio of H2:CO in the syn­thesis gas composition. The WGS reaction not only enhances the targeted gas composition with a higher selectivity, but also prepares a syngas more suitable for the next-stage conversion by adjusting it for an optimal syngas composition.

The goal of any gasification technology processing a mixed feedstock for specific applications such as IGCC, syngas generation, and the like is threefold. First, the gasifier operation and performance meet the desired

objectives. Second, the products must meet the requirements of all the down­stream operations such as gas turbine or FT catalysts, and so on, and third the gasifier must operate within its design limits. This goal is affected by the characteristics and properties as well as any synergistic effects of the mixed feedstock, composition of the total feed, the approach and equipment used to co-feed and the designs of the gasifier and the syngas clean-up equipment.

When possible, it is more economical to process a mixed feedstock in the process already being used for a single feedstock. For coal and biomass mixtures, it is more likely that coal processing plants will be retrofitted to accommodate the use of biomass in the feed. The pretreatment should allow this to happen as inexpensively as possible. Compared to coal, biomass has higher H/C and O/C contents (see Figure 7.1) which make biomass more active and easily degradable (more susceptible to fungal attack) when subject to the natural environment. Biomass also contains some inorganic elements (such as K, Na, Cl, Br, P, S, Ca, Mg, Si, Al, Fe, etc.) that if not removed can accumulate in the remaining solids of the various thermochemical processes causing slagging, reactor blockage, and other problems or end up with prod­uct gas requiring downstream cleaning operations. The ash from biomass gasification is different from that of coal gasification. Water removal from the biomass is also important for energy efficiency of the gasifier operation.

Pretreatment of biomass for its use in mixed feedstock is done for various reasons. A wide variety of biomass streams often does not match with nar­row fuel specifications of feeding systems and the desired conversion pro­cesses. Other reasons are to reduce the plant’s investment, maintenance, and personnel costs by using homogeneous fuel that is suitable for automatic fuel feeding and to reduce the need to invest in complex and novel gasification

systems. Pretreatment will also reduce the cost of transportation, storage, and handling of biomass. When coal and biomass are fed together as a mix­ture into a gasifier, the process of feeding must be uniform, consistent, and one that allows for easy particle fluidization in any type of gasifier. With popular entrained bed gasifiers, dry feeding is preferred because it allows maximum flexibility in allowable operating conditions and composition of the coal-biomass mixture feedstock. In addition to dry feeding a coal-bio­mass mixture successfully, the biomass needs to be prepared such that it forms a homogeneous mixture with coal.

The choice of pretreatment method used often depends on the downstream use of biomass as well as the local conditions and the need. As shown later, the choice of pretreatment method will also depend on the choice of the pro­cess configuration. The three most important parameters in feeding mixed feedstock are particle size and uniformity, level of inorganic impurities in the biomass, and moisture content of the biomass. The particle size affects the feeding and fluidization process within the reactor. A level of inorganics and other elements affects the quality of syngas and ash (and slag) formed within the reactor and the moisture content affects the energy efficiency of the gas­ifier. Various methods used to pretreat biomass for these purposes and their advantages and disadvantages are described in Table 7.6 [17]. The inorganic impurities can be partially removed by the leaching process mentioned in Table 7.6 [17]. Washing biomass with hot (around 60-80°C) water will largely remove ions of K, Na, Cl, and Br and somewhat remove the elements P, S, Ca, and Mg. Most other elements including Si, Fe, and Al will remain in the bio­mass. Process configuration and the downstream treatments are important for handling impurities in gas and solid product streams. The best method to handle volatile matter and particle size is torrefaction with or without pelletization. For an entrained bed reactor, a particle size of 1 mm. or less is required and this can be achieved through torrefaction. Sizing and milling of herbaceous biomass such as straw and switchgrass is very difficult and can be achieved via use of torrefaction. Pelletization of compressed biomass is possible, but it does not provide the additional benefits associated with the prevention of water absorption and fungal attack during storage and trans­portation of biomass that are provided by torrefaction. Furthermore, bio­mass pelletization alone does not easily allow the generation of fine particles on the order of several microns. In contrast, pyrolysis is a chemical process that significantly breaks down the chemical bonds within the biomass. It, however, requires biomass feeding in a slurry form.

Although many new technologies have been developed to extract straight vegetable oil from algae, this section specifically describes the single-step extraction process developed by OriginOil, Inc.

The single-step extraction process begins with the mature algae entering the system as an algae and water suspension. Before entering the extraction tank, the stream is subjected to pulsed electromagnetic fields and pH modifi­cation in a process known as quantum fracturing. As the terminology implies, quantum fracturing creates a fluid-fracturing effect, thereby mechanically distressing algae cells [33]. The electromagnetic fields are generated using a low-voltage power input and the pH is modified using carbon dioxide, which helps optimize electromagnetic delivery and assists in cell degrada­tion. The electromagnetic field created causes algae cells to release internal lipids. After quantum fracturing, the processed culture passes into a grav­ity clarifier and a return culture stream recycles into the inlet stream. The gravity clarifier separates the processed culture into layers of oil, water, and biomass. The lipid layer exit stream produces SVO and the water layer exits via a recycle stream to the bioreactor. The biomass can then be harvested for a number of purposes including livestock feed, ethanol processing, and bio­mass gasification. A schematic of the OriginOil single-step extraction pro­cess [34] is shown in Figure 2.4.

This innovative method can extract approximately 97% of the lipid oil contained in algae cells. Typical extraction values for expeller press extrac­tion and supercritical fluid extraction are approximately 75% and 100%, respectively, meaning this method would be competitive with industrial significance. This process does not require heavy machinery, chemicals, or dewatering of the feedstock and is thus claimed to use much less energy than traditional extraction processes.

The OriginOil single-step extraction process is an enhancement to the traditional algae oil extraction methods. The unique quantum fractur­ing method of extraction utilized by the process reduces the cost of pro­cessing by eliminating the need for chemical input or energy-intensive machinery. As shown in the flowsheet, the OriginOil process uses a radi­cally different approach in oil extraction which is based on a sequence of oil extraction^solids separation^dewatering basically in a single integrated

step, whereas the conventional approach follows a sequential order of sol­ids separation^dewatering^oil extraction [33]. As the result of this novel process alignment, no initial dewatering is required, substantial energy sav­ings can be expected, and the capital expenditure becomes reduced. With the biodiesel and bio-oil industries’ growth, single-step extraction technology will prove to be an invaluable tool for keeping algae-derived fuels competi­tive with petroleum.

Fermentation of sugars to ethanol, using commercially available fermenta­tion technology, provides a fairly simple, straightforward means of produc­ing ethanol with little technological risk. The system modeled assumes that the molasses is clarified, and then fermented via cascade fermentation with a yeast recycle. The stillage is concentrated by multiple-effect evaporation and a molecular sieve is used to dehydrate the ethanol. Corn ethanol is com­mercially produced in one of two ways, using either the wet mill or dry mill process. The wet milling process involves separating the grain kernel into its component parts (germ, fiber, protein, and starch) prior to yeast fermenta­tion. On the other hand, ICM-designed plants utilize the dry milling process, where the entire grain kernel is ground into flour form first. The starch in the flour is converted to ethanol during the fermentation process, also creating carbon dioxide and distillers grain as principal by-products.

In SHF, the hydrolysis is carried out in one vessel and the hydrolyzate is then fermented in a second reactor. The most expensive items in the overall process cost are the cost of feedstock, enzyme production, hydrolysis, and utilities. The feedstock and utility costs are high because only about 73% of the cellulose is converted to ethanol in 48 hr, and the remainder of the cel­lulose, hemicellulose, and lignin are burned or gasified. Enzyme production is a costly step due to the large amount of the enzyme used in an attempt to overcome the end-product inhibition as well as its slow reaction rate. The hydrolysis step is also expensive due to the large capital and operating costs associated with large size tanks and agitators. The most important param­eters are the hydrolysis section yield, the product quality, and the required enzyme loading, all of which are interrelated. Yields are typically higher in more dilute systems where inhibition of enzymes by glucose and cello — biose is minimized. Increasing the amount of enzyme loading can help to overcome inhibition and increase the yield and concentration, although it undoubtedly increases the overall cost. Increased reaction times also make higher yields and concentrations.

Cellulase enzymes from different organisms can result in markedly dif­ferent performances. Figure 4.11 shows the effect of yield at constant solid and enzyme loading and the performance of different enzyme loadings. Increase in enzyme loading beyond a particular point has turned out to be of no use. It would be economical to operate at a minimum enzyme loading level. Or, the enzyme could be recycled by appropriate methods. As the cel­lulose is hydrolyzed, the endo — and exoglucanase components are released back into the solution. Because of their affinity for cellulose, these enzymes can be recovered and reused by contacting the hydrolyzate with fresh feed. The amount of recovery is limited because of в-glucosidase, which does not adsorb on the feed. Some of the enzyme remains attached to the lignin and unreacted cellulose; in addition, enzymes are thermally denatured during hydrolysis. A major difficulty in this type of process is maintaining sterility; otherwise, the process system would be contaminated. The power consumed in agitation is also significant and affects the economics of this process [43]. Even though the effect of yield on the selling price of ethanol in the figure was based on more classical ethanol production processes, it does explain the importance of yield on the final product cost.

3.5

1.5 1

0.5 0

Um and Hanley [69] carried out an experimental study on high-solid enzy­matic hydrolysis and fermentation of Solka Floc into ethanol. To lower the ethanol distillation cost of fermentation broths, a high initial glucose concen­tration is desired. However, an increase in glucose concentration typically reduces the ethanol yield due to the decreased mass and heat transfer rate. To overcome the incompatible temperatures between the enzymatic hydro­lysis (50°C) and fermentation (30°C), saccharification, followed by fermenta­tion (SFF), was employed with relatively high solid concentrations (10% to 20%) using a portion loading method. Glucose and ethanol were produced from Solka Floc, which was first digested by enzymes at 50°C for 48 hours, followed by fermentation. In this process, commercial enzymes were used in combination with a recombinant strain of Zymomonas mobilis. The highest ethanol yields of 83.6%, 73.4%, and 21.8%, based on the theoretical amount of glucose, were obtained with substrate concentrations of 10%, 15%, and 20%, respectively. These values also correspond to 80.5%, 68.6%, and 19.1%, based on the theoretical amount of the cell biomass and soluble glucose present after 48 hours of SFF. In addition to the substrate concentration effects, they also investigated the effects of reactor configurations [69].

As a classic study of the mechanism of the enzymatic hydrolysis of cel­lulose, Fan, Lee, and Beardmore investigated the effects of major structural features of cellulose on enzymatic hydrolysis. They found that the hydrolysis rate is mainly dependent upon the fine structural order of cellulose which can be best represented by the crystallinity rather than the simple surface area [76].