Hexane solvent extraction has long been used effectively for vegetable oil extrac­tion. A very good example is in the production of soybean oil, for which hex­ane solvent extraction is predominantly used in industrial production due to its lower energy consumption and higher extraction efficiency (oil yield) in comparison to hydraulic presses, that is, the expeller method. Furthermore, hexane extraction technology can be used as a stand-alone process for algae oil extraction or it can be used in conjunction with the physical extraction technology of the oil press/expeller method.

If a chemical extraction process based on cyclohexane as a chemical solvent is employed for algae oil extraction in conjunction with an expeller method, the envisioned process scheme is as follows.

The algae oil and lipids are first extracted using an expeller. The remaining pulp and biomass are then mixed with cyclohexane to extract the residual oil content remaining in the residue. The algae oil dissolves in cyclohexane, whereas the pulp and residues do not. The biomass is filtered out from the solution, and the biomass rejected here can be used for other energy gen­eration processes such as gasification. As the last stage, the algae oil and cyclohexane are separated by distillation. This two-stage extraction process of combined cold expeller press and hexane solvent extraction is capable of achieving an extraction efficiency of higher than 95% of the total oil present in the algae [35].

In comparison to the wet milling ethanol process where the corn kernel has to be separated into its components of germ, fiber, gluten, and starch prior to the fermentation step, the dry milling ethanol process first grinds the entire corn kernel into coarse flour form and then ferments the starch in the flour directly into ethanol. The dry milling corn ethanol process by ICM, Inc. is outlined below [21].

1. In the first step, corn receiving and storage, the corn grain is delivered by truck or rail to the corn ethanol fermentation plant. Grains are loaded in storage bins (silos) designed to hold sufficient amounts of grain to supply the plant operation continuously for 7-12 days.

2. The second step, milling, is where the grain is inspected and screened to remove debris (including corn cobs, stalks, finer materials, stones, and foreign objects) and ground into coarse flour. The screening is usually done using a blower and screen. Coarse grinding is typically
performed using a hammer mill. The feed rate from the milling step to the next stage of hot slurrying is typically controlled by the use of weighing tanks.

3. During the so-called cooking process, also called hot slurry, primary liquefaction, and secondary liquefaction, the starch in the flour is physically prepared and chemically modified for fermentation.

a. In the hot slurry method the coarsely ground grain is soaked in hot process water, the pH of the solution is adjusted to about 5.8, and an alpha-amylase enzyme is added. The slurry is heated to 180-190°F (82-88°C) for 30-60 minutes to reduce its viscosity. Agitation needs to be provided.

b. Primary liquefaction follows, where the slurry is then pumped through a pressurized jet cooker at 221°F (105°C) and held for 5 minutes. The jet cooker is also known as a steam injection heater. The mixture is then cooled by an atmospheric or vacuum flash condenser. The jet cooker is a critical component as steam helps to evenly hydrolyze and rapidly heat the slurry. The fluid dynamic relationship between the jet cooker’s steam injector and condens­ing tube produces a pressure drop to help maximize shear action to improve starch conversion [22].

c. The third process, secondary liquefaction, occurs after the flash condensation cooling. The mixture is held for 1-2 hours at 180- 190°F (82-88°C) to give the alpha-amylase enzyme sufficient time to break down the starch into short-chain, low-molecular- weight dextrins. This chemical conversion is called gelatinization. Generally speaking, during the gelatinization step, there is a sharp increase in the slurry viscosity that is rapidly decreased as the a-amylase hydrolyzes the starch into lower molecular weight dextrins. Dextrins are a group of low-molecular-weight carbo­hydrates produced by the hydrolysis of starch and are mixtures of polymers of D-glucose units linked by a-(1^4) or a-(1^6) gly — cosidic bonds. After pH and temperature adjustment, a second enzyme, glucoamylase, is added as the mixture is pumped into the fermentation tanks. Glucoamylase is an amylase enzyme that cleaves the last alpha-1,4-glycosidic linkages at the nonreducing end of amylase and amylopectin to yield glucose. In other words, glucoamylase is an enzyme that cleaves the chemical bonds near the ends of long-chain starches (carbohydrates) and releases maltose and free glucose. Maltose, or malt sugar, is a disaccha­ride that is formed from two units of glucose joined with an a(1^4)bond.

4. The fourth step is called simultaneous saccharification fermen­tation. Once inside the fermentation tanks, the mixture is now referred to as mash, because it is an end product of mashing (which involves mixing of the milled kernel and water followed by mixture heating). The glucoamylase enzyme breaks down the dextrins, oligosaccharides, to form simple sugars, that is, monosac­charides. Yeast is added at this stage to convert the sugar to ethanol and carbon dioxide via an alcohol fermentation reaction. The mash is then allowed to ferment for 50-60 hours, resulting in a mixture that contains about 15% ethanol as well as the solids from the grain and added yeast [21, 23].

5. In the distillation step the fermented mash is pumped into a multicol­umn distillation system. The distillation columns utilize the boiling point difference between ethanol and water to distill and separate the ethanol from the solution. By the time the product stream is ready to leave the distillation columns, it contains about 95% ethanol by volume (which is 190-proof). This point is just immediately below the azeotropic concentration of the ethanol-water binary system, as explained in Section 3.2.6.6. To overcome this azeotropic limitation of maximum achievable ethanol concentration via straight distilla­tion, several optional methods are being used, including jumping over the azeotropic point or bypassing the distillation. The residue from this process, called stillage, contains nonfermentable solids and water and is pumped out from the bottom of the distillation col­umns into the centrifuges.

6. The sixth step is that of dehydration. The 190-proof ethanol still con­tains about 5 vol.% water. This near-azeotropic binary mixture is passed through a molecular sieve to physically separate the remain­ing water from the ethanol based on the size difference between the two molecules [21]. This dehydration step produces 200-proof anhy­drous (waterless) ethanol, that is, near 100% ethanol.

7. Product ethanol storage is the seventh step. Before the purified eth­anol is sent to storage tanks, a small amount of denaturant chemi­cal is added, making it unsuitable for human consumption. There are so many different kinds of denaturants available on the market for diverse purposes other than fuel ethanol. However, only cer­tain gasoline-compatible blendstocks are suitable as denaturants for fuel ethanol. Some ethanol refineries also sell their denaturants for other ethanol industries. The ASTM D4806-11a specification covers nominally anhydrous denatured fuel ethanol intended for blending with unleaded or leaded gasolines for use as a spark-igni­tion automotive engine fuel. According to this specification, the only denaturants used for fuel ethanol shall be natural gasoline, (also known as natural gas liquid [NGL]), gasoline components,

or unleaded gasoline at the minimum concentration prescribed. Methanol, pyrroles, turpentine, ketones, and tars are explicitly listed as prohibited denaturants for fuel ethanol meant to be used as gasoline blendstock [24]. Most ethanol plants’ storage tanks are sized to allow storage of 7-12 days’ production capacity.

8. During the ethanol production process, two valuable coproducts are created: carbon dioxide and distillers grains. Their recover­able values are very important to the overall process economics and this is why they are called coproducts rather than simply by-products.

During yeast fermentation, a large amount of carbon dioxide gas is gen­erated. Because CO2 is a major greenhouse chemical, its release into the atmosphere is not desirable. The carbon dioxide generated by fermentation is of high concentration and its purification is relatively straightforward. Therefore, carbon dioxide from ethanol fermentation is commonly captured and purified with a scrubber so it can be marketed to the food processing industry for use in carbonated beverages and flash-freezing applications. Dry ice is a common coproduct of the ethanol refineries.

The stillage from the bottom of the distillation columns contains solids derived from the grain and added yeast as well as liquid from the water added during the process. The stillage is sent to centrifuges for separation into thin stillage (a liquid with 5-10% solids) and wet distillers grains [21].

Some of the thin stillage is recycled back to the cook/slurry tanks as makeup water, reducing the amount of fresh water required by the cook­ing (hot slurry) process. The rest is sent through a multiple-effect evapo­ration system where it is concentrated into a syrup containing 25-50% solids. This syrup, which is high in protein and fat content, is then mixed back in with the wet distillers grains [21]. This is a step intended to recover most of the nutritive components from the stillage. With the added syrup, the WDG still contains most of the nutritive value of the original feed­stock plus the added yeast and as such it makes excellent cattle feed. After the addition of the syrup, it is conveyed to a wet cake pad, where it is loaded for transport.

Many ethanol refinery facilities do not have enough nearby cattle farms or established markets to utilize all of their WDG products. However, WDG must be used soon after it is produced, because it gets spoiled rather easily. Therefore, WDG is often sent through an energy-efficient drying system to remove moisture and extend its shelf life. Dried distillers grains are com­monly used as a high-protein ingredient in cattle, swine, poultry, and fish diets. Modified forms of DDGs are also being researched for human con­sumption due to the outstanding nutritive values. In more practical senses, DDG is better known as a corn ethanol coproduct than WDG.

A schematic of ICM’s dry milling corn ethanol process [21] is shown in Figure 3.7.

Inasmuch as xylose accounts for 30-60% of the fermentable sugars in hard­wood and herbaceous biomass, the fermentation of xylose to ethanol becomes an important issue. The efficient fermentation of xylose and other hemicel- lulose constituents is essential for the development of an economically viable process to produce ethanol from lignocellulosic biomass. Needless to say, co-fermentation of both glucose and xylose with comparably high efficiency would be most ideally desirable. As discussed earlier, xylose fermentation
using pentose yeasts has proven to be difficult due to several factors includ­ing the requirement for O2 during ethanol production, the acetate toxicity, and the production of xylitol as by-product. Xylitol (or, xyletol) is a naturally occurring low-calorie sugar substitute with anticariogenic (preventing pro­duction of dental caries) properties.

Other approaches to xylose fermentation include the conversion of xylose to xylulose (a pentose sugar, part of carbohydrate metabolism, that is found in the urine of individuals with the condition pentosuria [78]) using xylose isomerase prior to fermentation by Saccharomyces cerevisiae, and the develop­ment of genetically engineered strains [79].

A method for integrating xylose fermentation into the overall process is illustrated in Figure 4.12. In this example, dilute acid hydrolysis was adopted as a pretreatment step. The liquid stream is neutralized to remove any mineral acids or organic acids liberated in the pretreatment process, and is then sent to xylose fermentation. Water is added before the fermentation, if necessary, so that organisms can make full use of the substrate without having the yield limited by end-product inhibition. The dilute ethanol stream from xylose fer­mentation is then used to provide the dilution water for the cellulose-lignin mixture entering SSF. Thus, the water that enters during the pretreatment process is used in both the xylose fermentation and the SSF process.

The conversion of xylose to ethanol by recombinant E. coli has been inves­tigated in pH-controlled batch fermentations [80]. Relatively high concentra­tions of ethanol (56 g/L) were produced from xylose with good efficiencies.

In addition to xylose, all other sugar constituents of biomass, including glucose, mannose, arabinose, and galactose, can be efficiently converted to ethanol by recombinant E. coli. Neither oxygen nor strict maintenance of anaerobic conditions is required for ethanol production by E. coli. However, the addition of base to prevent excessive acidification is essential. Although less base was needed to maintain low pH conditions, poor ethanol yields and slower fermentations were observed below the pH of 6. Also the addi­tion of metal ions, such as calcium, magnesium, and ferrous ions, stimulated ethanol production [80].

In general, xylose fermentation does not require precise temperature con­trol, provided the broth temperature is maintained between 25 and 40°C. Xylose concentrations as high as 140 g/L have been positively tested to evaluate the extent to which this sugar inhibits growth and fermentation. Higher concentrations slow down growth and fermentation considerably. Ingram and coworkers [80-83] demonstrated that recombinant Escherichia coli expressing plasmid-borne Zymomonas mobilis genes for pyruvate decar­boxylase (PDC) and alcohol dehydrogenase II (ADHII; adhB) can efficiently convert both hexose and pentose sugars to ethanol. Ethanologenic E. coli strains require simpler fermentation conditions, produce higher concentra­tions of ethanol, and are more efficient than pentose-fermenting yeasts for ethanol production from xylose and arabinose [84].

A study by Sedlak, Edenberg, and Ho [28] successfully developed geneti­cally engineered Saccharomyces yeasts that can ferment both glucose and xylose simultaneously to ethanol. According to their experimental results, following rapid consumption of glucose in less than 10 hours, xylose was metabolized more slowly and less completely. Although the xylose conver­sion was quite significant by this genetically engineered yeast strain, xylose was still not totally consumed even after 30 hours. Ideally, xylose should be consumed simultaneously [26] with glucose at a similar efficiency and speed; however, this newly added capability of co-fermentation of both glucose and xylose has given new promise in the lignocellulosic ethanol technology leading to technological breakthroughs. They also found that ethanol was the most abundant product from glucose and xylose metabolism, but small amounts of the metabolic byproducts glycerol and xylitol also were obtained [28]. Certainly, later studies will be focused on the development or refine­ment of more efficient engineered strains, ethanol production with higher selectivity and speed, and optimized process engineering and flowsheeting.

Unlike combustion, gasification is carried out with less than a stoichiomet­ric amount of required oxygen such that only partial oxidation of waste materials occurs. Gasification generates a producer gas of low, medium, or high BTU content depending on the operating conditions and amount of oxygen. The gasification temperature varies from 500°C to about 1,800°C depending upon the desired gas composition and the nature of the slag. The producer gas generally contains CO, CO2, H2, H2O, CH4, trace amounts of higher hydrocarbons such as ethane and ethylene, inert gases originat­ing from the gasification agent, and various contaminants such as small particles and others depending upon the impurities present in the waste [12, 17, 18]. The partial oxidation can be carried out using either air, oxy­gen, carbon dioxide, steam, or a mixture of these substances. Generally oxygen produces medium BTU producer gas whereas air produces low BTU producer gas. The producer gas can either be used for heating and the generation of electricity or it can be reformed to produce syngas that largely contains CO and H2 which can then be used as a raw material for second-generation biofuels via Fischer-Tropsch syntheses. Syngas can also be produced by operating a gasification reactor at a very high tempera­ture (greater than 1,400°C). Several types of gasification reactors have been developed to process MSW, hazardous waste, and dried sewage sludge. A stable and optimum operation of the gasification reactor with minimum generation of tar or slag formation requires feedstock of uniform size with some consistencies in its composition. This may require pretreatment of the feedstock which can be expensive.

The gasification process involves smaller gas volume (by a factor of 10 if pure oxygen is used) and smaller waste water from the producer gas cleaning process compared to incineration. High operating pressures applied in some gasification processes can also lead to smaller and more compact aggregates. Unlike incinerators, a gasification process mainly produces carbon monoxide instead of carbon dioxide. High-temperature

gasifiers capture inorganic impurities within the slag. Incinerators only produce heat (and thereby electricity), but syngas produced from a gasifier can be used to produce materials and transportation fuel along with heat and electricity.

Various types of gasification reactors (packed bed with upflow or down­flow mode of operation, bubbling or circulating fluidized bed, entrained bed, and cyclone) are used in commercial operations. The gasification tech­nology is very versatile and well developed, and it can process all types of waste. In recent years, new gasifiers tend to be entrained bed with both low — and high-pressure operational flexibility. The most preferred feed­stock for the gasifier is high-energy density solids and for efficient opera­tion of all gasifiers, waste materials must be finely ground before feeding into the gasifier. Hazardous waste may be gasified directly if it is liquid or finely granulated.

A typical simplified Texaco gasification process for conversion of MSW to a medium BTU gas is illustrated in Figure 6.5. Numerous other commer­cial processes for waste gasification are available, and they are described by Lee, Speight, and Loyalka [4]. SVZ Schwarze Pumpe GMbH operates both a packed bed gasifier for coal-waste mixtures (with waste up to 85%) and an entrained flow gasifier for hazardous waste. The entrained flow gasifier is operated at temperatures between 1,600°C and 1,800°C. The packed bed gasifier has a capacity of 8-14 tons per hour, and it operates between 800 and 1,300°C and 25 atm pressure and produces syngas using steam and oxygen as the gasification agents. A slag bed gasifier operates up to 1,600°C with a

FIGURE 6.6

Fluidized bed gasifier with high-temperature slagging furnace. (From EBARA. (2003). EUP — EBARAUBE. Process for gasification of waste plastics. Retrieved May, 2010 from http://www. ebarra. ch/) throughput rate of 30 tons per hour and slag is discharged as liquid [19, 20]. Recently, gasification technology has been used for numerous types of waste in addition to MSW. Among others, hazelnut shell, rice husk, salmon waste, and several other types of solids and liquid organic waste have been success­fully gasified to generate producer gas or syngas [21-25]. In all cases, gasifi­cation technology produced good quality producer gas. In some instances, producer gas was subsequently transformed into biomethanol.

A gasification process can also use two stages. An example of a two-stage waste gasification process using a fluidized bed and an entrained flow reac­tor (see Figure 6.6) is used in Japan for waste conversion to syngas. The fluidized bed gasifier operates at a lower temperature, and it converts hetero­geneous waste into syngas. The ash produced in this reactor is then passed onto a high-temperature cyclone gasifier where slag is collected. The syngas produced from this process is used for ammonia production and other appli­cations. Other modifications of this process for different types of wastes are described by Bridgewater [26].

A two-stage gasification system sometimes also uses a gasification reac­tor in combination with a combustion reactor. For example, a combina­tion of fluidized bed gasifier and a high-temperature combustor is used to process shredded MSW, plastics, and residues. In this process, the gasifier is generally operated at 580°C to produce gas and the combustor is oper­ated at 1,350-1,450°C for melting ash and other solid materials to further recover energy [7]. Generally particle size of 300 mm is preferred in such a process [20].

Sound management and efficient utilization of biomass can provide sub­stantial benefits in terms of increased biodiversity, local amenity, and even rehabilitation of land and water courses [13]. Examples include income-gen­erating management of native woodlands, growing energy crops such as short rotation coppice (SRC) that has biodiversity and soil conservation ben­efits, utilization of low-fertility and abandoned lands, returns and increase of certain declining animal populations, and energy independence of local or rural communities, among others. When biomass energy is exploited properly, the renewability of biomass via photosynthesis absorbing carbon dioxide as a pivotal species helps reduce the carbon footprint of the energy

generation and consumption cycle associated with human activities, thereby positively contributing to greenhouse gas management in both direct and indirect ways.

Furthermore, the utilization of biomass helps contribute to income genera­tion and increase for rural and agricultural communities, while reducing dependence on nonrenewable fossil fuels and helping establish a sustain­able future.

Pure biodiesel (B100) has poor cold-temperature properties as straight die­sel fuel. Biodiesel’s cloud point (CP) and cold filter plugging point (CFPP) are both high, thus making pure biodiesel (B100) unsuitable as a cold-climate fuel without blending or additives. When biodiesel is cooled below a certain temperature, some ingredient molecules of biodiesel start to aggregate and form crystals. This temperature varies depending on the biodiesel feedstock, but is consistently quite high. As the biodiesel is further cooled and the crys­tals become larger than one quarter of the wavelength of visible light, the fuel system starts to look cloudy. This point is known scientifically as the cloud point. The cloud point measurement follows the ASTM 2500. The low­est temperature at which biodiesel can pass through a 45-micron filter is called the cold filter plugging point. A high cold filter plugging point tends to clog up vehicle engines more easily. As biodiesel is further cooled below the CFPP, it will gel and eventually solidify. This point is called the gel point. Another important cold flow property is the pour point, which is defined as the lowest temperature where the fuel is observed to flow. As all these tem­peratures are generally higher for biodiesel (B100) than petrodiesel, biodiesel freezes faster than most petrodiesels. Commercial additives developed for diesel to improve its cold flow properties are mostly applicable to biodiesel and biodiesel blends [47].

A widely used form of sugar for ethanol fermentation is blackstrap molasses, which contains about 30-40 wt% sucrose, 15-20 wt% invert sugars such as glucose and fructose, and 28-35 wt% of nonsugar solids. The direct fermenta­tion of sugarcane juice, sugarbeet juice, beet molasses, fresh and dried fruits, sorghum, whey, and skimmed milk have been considered, but none of these could compete economically with molasses. From the viewpoint of industrial ethanol production, sucrose-based substances such as sugarcane and sug — arbeet juices present many advantages, including their relative abundance and renewable nature. Molasses, the noncrystallizable residue that remains after sucrose purification, has additional advantages: it is a relatively inex­pensive raw material, readily available, and already used for industrial etha­nol production. Molasses is used in dark brewed alcoholic beverages such as dark ales and also for rum. Bioethanol production in Brazil uses sugarcane as feedstock and employs first-generation technologies based on the use of the sucrose content of sugarcane. The enhancement potential for sugarcane ethanol production in Brazil was discussed by Goldemberg and Guardabassi [17] in the two principal areas of productivity increase and area expansion.

Park and Baratti [18] studied the batch fermentation kinetics of sugarbeet molasses by zymomonos mobilis, a rod-shaped gram-negative bacterium that can be found in sugar-rich plant saps. Z. mobilis degrades sugars to pyru­vate using the Entner-Doudoroff pathway [19]. The pyruvate is then fer­mented to produce ethanol and carbon dioxide as the only products. This bacterium has several interesting and advantageous properties that make it competitive with the yeasts and, in some aspects, superior to yeasts; impor­tant examples include higher ethanol yields, higher sugar uptake, higher ethanol tolerance and specific productivity, and lower biomass production.

When cultivated on molasses, however, Z. mobilis generally shows poor growth and low ethanol production as compared to cultivation in glu­cose media [18]. The low ethanol yield is explained by the formation of by-products such as levan and sorbitol. Other components of molasses such as organic salts, nitrates, or the phenolic compounds could also be inhibi­tory for growth [20]. As such, its acceptable and utilizable substrate range is restricted to simple sugars such as glucose, fructose, and sucrose. Park and Baratti [18] found that in spite of good growth and prevention of levan formation, the ethanol yield and concentration were not sufficient for the development of an industrial process.

In a study by Kalnenieks et al., potassium cyanide (KCN) at submillimolar concentrations (20-500 цМ) inhibited the high respiration rates of aerobic cultures of Z. mobilis but, remarkably, stimulated culture growth [21]. Effects of temperature and sugar concentration on ethanol production by Z. mobilis have been studied by scientists. Cazetta et al. [22] investigated the effects of temperature and molasses concentration on ethanol production. They used factorial design of experiments (DOE) in order to study varied conditions concurrently; the different conditions investigated included varying com­binations of temperature, molasses concentration, and culture times. They concluded that the optimal conditions found for ethanol production were 200 g/L of molasses at 30°C for 48 hours and this produced 55.8 g ethanol/L.

Yeasts of the "saccharomyces genus" are mainly used in industrial pro­cesses for ethanol fermentation. One well-known example is Saccharomyces cerevisiae, which is most widely used in brewing beer and wine. However, S. cerevisiae cannot ferment D-xylose, the second most abundant sugar form of the sugars obtained from cellulosic materials. One micro-organism that is naturally capable of fermenting D-xylose to ethanol is the yeast Pichia sti — pitis, however, this yeast is not as ethanol — and inhibitor-tolerant as tradi­tional ethanol-producing yeast, that is, Saccharomyces cerevisiae. Therefore, its industrial application is impractical, unless significant advances are made. There have been efforts that attempt to generate S. cerevisiae strains that are able to ferment D-xylose by means of genetic engineering [23]. Scientists have been working actively to ferment xylose with high productivity and yield by developing variants of Z. mobilis that are capable of using C5-sugars (pen­toses or xyloses) as a carbon source [24]. Advances with promising results are being reported in the literature.

As a significant advance in metabolistic changes brought about by genetic engineering, Tao [25] altered an Escherichia coli B strain, which is an organic acid producer, to E. coli strain KO11, which is an ethanol producer. The altered KO11 strain yielded 0.50 g ethanol/g xylose using 10% xylose solu­tion at 35°C and pH of 6.5. This result provides an example of how the output of a microbe can be altered.

Utilizing a combination of metabolic engineering and systems biology techniques, two broad methods for developing more capable and more toler­ant microbes and microbial communities are the recombinant industrial and native approaches [26]. The two methods differ as follows:

1. Recombinant industrial host approach: Insert key novel genes into known robust industrial hosts with established recombinant tools.

2. Native host approach: Manipulate new microbes with some complex desirable capabilities to develop traits needed for a robust industrial organism and to eliminate unneeded pathways.

The research on yeast fermentation of xylose to ethanol has been very actively studied; particular emphasis has been placed on genetically engi­neered Saccharomyces cerevisiae. S. cerevisiae is a safe micro-organism that plays a traditional and major role in modern industrial bioethanol produc­tion [27]. Saccharomyces cerevisiae has several advantages including its high ethanol productivity as well as its high ethanol and inhibitor tolerance. Unfortunately, this yeast does not have the capability of fermenting xylose. A number of different strategies based on genetic engineering and advanced microbiology have been applied to engineer yeasts to become capable of efficiently producing ethanol from xylose. These novel strategies included: (a) the introduction of initial xylose metabolism and xylose transport, (b) changing the intracellular redox balance, and (c) overexpression of xylulo — kinase and pentose phosphate pathways [27]. One of the pioneering studies involves the development of genetically engineered Saccharomyces yeasts that can co-ferment both glucose and xylose to ethanol by Sedlak et al. [28]. Even though their recombinant yeast Saccharomyces cerevisiae with xylose metabolism added was found to be the most effective yeast, they still utilized glucose more efficiently than xylose.

According to their experimental results, following rapid consumption of glucose in less than 10 hours, xylose was metabolized more slowly and less completely. In fact, xylose was not totally consumed even after 30 hours. Ideally, xylose should be consumed simultaneously [26] with glucose at a similar efficiency and speed; however, the newly added capability of co­fermentation of both glucose and xylose was a ground-breaking discovery. Furthermore, they found that although ethanol was the most abundant product from glucose and xylose metabolism, small amounts of the meta­bolic by-products of glycerol and xylitol also were obtained [28]. The above
two issues, viz. higher efficiency for xylose fermentation and optimization and by-product control, are the subjects of intense research investigation.

Thermochemical treatment of biomass can convert biomass into solid, liquid, and gaseous fuel products whose compositional distribution is governed by the imposed process treatment conditions. The solid product is usually char (or biochar), the liquid product is bio-oil, and the gaseous product is biosyn­gas. The process also involves formation of the unwanted product of tar. The thermochemical conversion process involves heating of the biomass feed­stock, which triggers a series of parallel and consecutive reactions including

devolatilization of volatile matter, pyrolytic decomposition of hydrocarbons and other carbonaceous matters, gas-solid type gasification reactions, coke and char formation, tar and its precursor formation, and more. Simply speak­ing, depending upon the processing temperature and reactor residence time, thermochemical treatment of biomass can be regrouped into three basic types of process treatment, namely carbonization, fast pyrolysis, and gasifi­cation. As shown in Table 5.5, the principal product, or intended product, of fast pyrolysis is a liquid fuel, whereas the desired product of gasification is a gaseous fuel [23].

Even though it is not listed in Table 5.5, indirect liquefaction via the biosyn­gas route is also a viable option for liquid fuel production, as well demon­strated in the fields of coal and natural gas syngas [5, 14, 24, 25]. As the name implies, indirect liquefaction goes through two stages of process treatment, viz., gasification followed by liquefaction by which liquid hydrocarbon fuels such as methanol, dimethylether (DME), higher alcohols, gasoline, diesel, and jet fuel are synthesized using the syngas produced by the gasification stage. In this case, biomass syngas is a thermochemical intermediate for the next stage synthesis of liquid fuel.

5.1.4 Analysis of Biomass Feedstock and Product Compositions

Fast pyrolysis of biomass generates a wide variety of organic and inorganic chemical compounds and the product compositions vary significantly depending upon the types of feedstock as well as the process treatment conditions to which the biomass is subjected. Therefore, studies of pro­cess modeling and technoeconomic analysis are often carried out using model compounds carefully chosen for the specific process and typical feedstock [26]. The analysis of corn stover samples used by Mullen et al. [27] for their fast pyrolysis study is presented in Table 5.6, as an example of the compositional analysis of biomass feedstock. They carried out the fast pyrolysis in a bubbling fluidized bed of quartz sand at a temperature of 500°C.

Inorganic elemental composition of corn stover used for the aforemen­tioned pyrolysis determined by x-ray fluorescence (XRF) is given in Table 5.7. Also compared in the same table are the XRF analysis data for corn cobs which were also tested for fast pyrolysis by Mullen et al. (2010) [27]. As can be seen, the elemental composition between corn cob and corn stover are quite different. The most abundant element in corn cob was K, whereas Si was the most abundant species in corn stover. High levels of K and P in both samples are expected, although the high levels of Ca, Mg, Al, Fe, and Mn in corn stover are noteworthy. Mineral matters in the biomass feedstock can reappear as contaminants or trace elements in bio-oils and biosyngas, which can potentially affect the catalytic activity of the downstream processing by fouling or poisoning.

The yield data of the USDA corn stover fast pyrolysis by Mullen et al. [27] is shown in Table 5.8. The pyrolysis product distribution in terms of the product phases was bio-oil 61.7%, biochar 17.0%, and noncondensable gas (NCG) 21.9%. As explained earlier and also summarized in Table 5.5, the principal product of fast pyrolysis of biomass, that is, corn stover in this example, is bio-oil.

From the product compositions, the following observations are deemed significant:

1. The gaseous effluent of fast pyrolysis has a heating value of only 6.0 MJ/kg. The gas composition is dominated by carbon oxides (CO and CO2), followed by methane and hydrogen.

2. High levels of oxygen in the effluent gas show that the gaseous efflu­ent served at least as an outlet for deoxygenation of biomass.

3. Bio-oil also showed a very high level of oxygen and its heating value was seriously affected. In order to enhance the fuel quality of bio-oil as well as to enhance the fast pyrolysis process, a systematic rejec­tion of oxygen from the products’ molecular structures (i. e., deoxy­genation) would become crucially important.

4. Biochar showed a heating value nearly as high as that of bio-oil, even though it contained a high level of ash.

5. Biochar showed a high C/H ratio, which is indicative of its lack of volatile hydrocarbons. Thus, biochar is a useful by-product of the fast pyrolysis process of biomass.

Transesterification is the reaction of triglycerides (or other esters) with alco­hols to produce alkyl esters (biodiesel) and glycerol, typically in the presence of acid and base catalysts (see Figure 6.10). Triglycerides are one of the three types of biomass obtained in 350 types of crops such as soybean, cotton seed, rapeseed, and algae, among others, as well as in fatty acids present in a vari­ety of fresh and waste cooking oils. The basic mechanism of transesterifica­tion of triglycerides is described in Figure 6.10. Methanol is most commonly used because of its low cost, although 2-propanol gives better biodiesel and ethanol is preferred in Brazil because of its easy and inexpensive availability. Alkyl esters or biodiesel are also called fatty acid methyl esters (FAME) and they can be directly used in diesel engines.

For the purpose of this chapter, it is the transesterification of waste cook­ing or frying oil that is of interest. Worldwide there is significant production of waste cooking and frying vegetable oils. The process of transestrification allows the conversion of these waste oils into useful biodiesel. The literature

O

FIGURE 6.10

Overall and intermediate reactions for transesterification of triglyceride and alcohol to pro­duce alkylester (biodiesel) and glycerol. (After Huber et al., 2006. Synthesis of transportation fuels from biomass: Chemistry, catalysis and engineering, Chem. Rev., ACS.

reported [99-117] for the conversion of waste oils into biodiesel indicates that the efficiency of this conversion process is lower than the one for fresh vegetable oil. This is due to the presence of water and free fatty acid in the waste oils that create a soap film and reduce the effectiveness of the trans­esterification process. Hossain and Mekhled [111] showed that the trans­esterification of waste canola (often called oilseed rape), which is the second largest oilseed crop in the world, results in the biodiesel yield of about 49.5% when transesterification is carried out for two hours with 1:1 molar ratio
of methanol and waste oil, and 0.5% sodium hydroxide catalyst. Normally, methanol/waste oil ratio used is about 6:1 or even 9:1 and this often results in nearly 100% conversion to biodiesel. The study showed that NaOH is a better catalyst than KOH. Numerous other studies [112-116] with frying oils and a mixture of fresh and used frying oils showed similar results. Al-Zuhair [114] used lipase immobilized on ceramic beads and entrapped in a sol-gel matrix for the production of biodiesel from waste cooking oil. The study showed that the immobilized lipase on ceramic beads was more capable of transesterifying waste cooking oil with high water content to biodiesel than lipase in free or entrapped in sol-gel matrix forms.

Studies [99, 117] have also been carried out to examine the effectiveness of supercritical alcohols for transesterification of vegetable oil in the absence of catalyst. These studies have produced some promising results. Demirbas, Ozturk, and Demirbas [99] and Demirbas and Kara [117] examined biodiesel production from vegetable oils via catalytic and noncatalytic transesterification using supercritical alcohols. The raw vegetable oils (as well as other 350 differ­ent types of crop oils) have high viscosity and low volatility; they do not burn completely and form deposits in the fuel injector of diesel engines. Vegetable oil viscosity (which is 11-17 times that of biodiesel) can be reduced by (a) dilu­tion, (b) microemulsion, (c) thermal decomposition, (d) catalytic cracking, and (e) transesterification with alcohols, preferably methanol and ethanol.

Demirbas et al. [99, 117] showed the transesterification of vegetable oils by supercritical alcohols to be a very effective process. Their results for hazelnut kernel oil under sub — and supercritical conditions for methanol showed a sharp increase in yield of methyl ester near the critical temperature of meth­anol. A further increase in temperature beyond critical temperature did not significantly affect the yield of methyl ester. The results of Demirbas et al. also indicate the molar ratio of waste to alcohol to be at least 1 to 9 to get significant yield of methyl ester. Within all the lower alcohols (e. g., methanol, ethanol, propanol, etc.) tested, methanol was found to be the best extracting agent. Afify et al. [108], among others, examined transesterification of algae (crop waste) to biodiesel and they found two-solvent systems to work more effectively. More work for the conversion of algae to diesel oil via improved transesterification process is needed.

Microalgae or microphytes are a division of algal organisms encompassing diatoms, the green algae, and the golden algae. These microscopic organisms are incredibly efficient solar energy converters that perform photosynthe­sis and are capable of rapid growth in either freshwater or saline environ­ments. Although this value varies from species to species and depends upon cultivation conditions, roughly 50% of the weight of algae is lipid oil. Algae are typically cultivated in either open or closed ponds, photobioreactors, or hybrid systems of both. Once the algae have matured they are harvested and processed to extract the algae’s oil.