3.2.2 Sugar Content of Biological Materials

Figure 3.3 shows a much generalized view of plant cell wall composition. The base molecules that give plants their structure can be processed to produce sugars, which can be subsequently fermented to ethanol. As such, feedstock that can generate sugars more readily and cost-effectively automatically become prime candidates for ethanol fermentation.

The principal components of most plant materials are commonly described as lignocellulosic biomass. This type of biomass is mainly composed of the compounds called cellulose, hemicellulose, and lignin. Cellulose is a pri­mary component of most plant cell walls and is made up of long chains of

the 6-carbon sugar, glucose, that are arranged in bundles (often described as crystalline bundles). The cellulose molecules in the plant cell wall are interconnected by another molecule called hemicellulose. The hemicellulose is primarily composed of the 5-carbon sugar, xylose. In addition to cellu­lose and hemicellulose, another macromolecule called lignin is also present in significant amounts and provides the structural strength for the plant. As also explained in Chapter 4, lignin is not easily converted into sugars or starches by current technology and therefore, has not been a target for alcohol fermentation. However, economically beneficial utilization of lignin is quite important in enhancing the overall process economics of ethanol production as well as minimizing process wastes. Technological develop­ments have recently introduced a variety of processes of extracting and dis­solving the cellulose and hemicellulose to produce sugars in such a form that can be readily fermented to ethanol. Generally speaking, efficient pretreat­ment can liberate the cellulose and hemicellulose from the plant material. Further treatment using chemicals, enzymes, or micro-organisms can also be applied to liberate simple sugars from the cellulose and hemicellulose, thus making them available to micro-organisms for fermentation to ethanol.

The overall hydrolysis is based on the synergistic action of three distinct cel — lulase enzymes and is dependent on the concentration ratio and the adsorp­tion ratio of the component enzymes: endo-p-glucanases, exo-p-glucanases, and p-glucosidases. Endo-p-glucanases attack the interior of the cellulose polymer in a random fashion [43], exposing new chain ends. Because this enzyme catalyzes a solid phase reaction, it adsorbs strongly but reversibly to the microcrystalline cellulose (also known as Avicel). The strength of the adsorption is greater at lower temperatures. This enzyme is necessary for the hydrolysis of crystalline substrates. The hydrolysis of cellulose results in a considerable accumulation of reducing sugars, mainly cellobiose, because the extracellular cellulase complex does not possess cellobiose activity. Sugars that contain aldehyde groups that are oxidized to carboxylic acids are classified as reducing sugars.

Exo-p-glucanases remove cellobiose units (which are disaccharides with the formula ([HOCH2CHO(CHOH)3]2O) from the nonreducing ends of cel­lulose chains. This is also a solid-phase reaction, and the exo-p-glucanases adsorb strongly on both crystalline and amorphous substrates. The mecha­nism of the reaction is complicated because there are two distinct forms of both endo — and exo-enzymes, each with a different type of synergism with the other members of the complex. As these enzymes continue to split off cellobiose units, the concentration of cellobiose in solution may increase. The action of exo-p-glucanases may be severely inhibited or even stopped by the accumulation of cellobiose in the solution.

The cellobiose is hydrolyzed to glucose by the action of p-glucosidase. Glucosidase is any enzyme that catalyzes hydrolysis of glucoside. P-Glucosidase catalyzes the hydrolysis of terminal, nonreducing beta-D — glucose residues with release of beta-D-glucose. The effect of p-glucosidase on the ability of the cellulase complex to degrade Avicel has been investi­gated by Kadam and Demain [73]. They determined the substrate specific­ity of the p-glucosidase and demonstrated that its addition to the cellulase complex enhances the hydrolysis of Avicel, specifically by removing the accumulated cellobiose. A thermostable p-glucosidase form, clostridium thermocellum, which is expressed in Escherichia coli, was used to deter­mine the substrate specificity of the enzyme. The hydrolysis of cellobi — ose to glucose is a liquid-phase reaction and p-glucosidase adsorbs either quickly or not at all on cellulosic substrates. p-Glucosidase’s action can be slowed or halted by the inhibitive action of glucose accumulated in the solution. The accumulation may also induce the entire hydrolysis to a halt as inhibition of the p-glucosidase results in a buildup of cellobiose, which in turn inhibits the action of exo-glucanases. The hydrolysis of the cel — lulosic materials depends on the presence of all three enzymes in proper amounts. If any one of these enzymes is present in less than the required amount, the other enzymes will be inhibited or lack the necessary sub­strates upon which to act.

The hydrolysis rate generally increases with increasing temperature. However, because the catalytic activity of an enzyme is also related to its shape, the deformation of the enzyme at high temperature can inactivate or destroy the enzyme. To strike a balance between increased activity and increased deactivation, it is preferable to run fungal enzymatic hydrolysis at approximately 40-50°C.

Although enzymatic hydrolysis is preferably carried out at a low tempera­ture of 40-50°C, dilute acid hydrolysis is carried out at a substantially higher temperature. Researchers at the National Renewable Energy Laboratory (NREL) reported results for a dilute acid hydrolysis of softwoods in which the conditions of the reactors were as follows [74].

1. Stage 1: 0.7% sulfuric acid, 190°C, and a 3-min residence time

2. Stage 2: 0.4% sulfuric acid, 215°C, and a 3-min residence time

Their bench-scale tests also confirmed the potential to achieve yields of 89% for mannose, 82% for galactose, and 50% for glucose, respectively. Fermentation with Saccharomyces cerevisiae achieved ethanol conversion of 90% of the theoretical yield [75].

If the MSW is used in an as-received condition as input to WtE or WtP pro­cesses, it can lead to variable (and even unstable) operating conditions due to variable properties of the feedstock. This, at least, will result in fluctuating product quality. Depending upon the technology used, convenient and stable feeding of waste to the conversion process is important. Refuse-derived fuel (RDF), which is a processed form of MSW, is often used to prepare waste for various WtE and WtP processes. This preparation or pretreatment usually consists of size reduction, screening, sorting, and in some cases, drying or pelletization to improve the handling characteristics and homogeneity of the waste materials. Therefore, for a given technology, a trade-off between the increased cost of producing RDF from MSW and potential cost reductions from system design and operations needs to be found. The main advantages of RDF are higher heating value; homogeneous physical and chemical struc­ture of the feed; easy storage, handling, and transportation; lower pollutant emissions; and reduced excess air requirement during combustion. In addi­tion to pelletization, biomass, in general, can undergo a variety of pretreat­ments. The advantages and disadvantages of these pretreatment methods are described in Table 6.2.

Because waste can contain numerous impurities, which during the con­version process mostly end up either in the gas phase or the solid phase, these impurities can also be removed as a part of waste preparation meth­ods. Once again it is a choice of economics between removing the impuri­ties up front or at the end of the conversion process. Waste can, in general, contain both inorganic and organic impurities. Depending upon the tech­nology used, some CO, HCl, HF, HBr, HI, NOx, SO2, VOCs, PCDD/F, PCBs, and heavy metal compounds (among others) can be formed or remain [7]. Solids from various conversion processes can also contain numerous impurities including ash.

Efficient control of heat transfer to and from biomass in every step of the process treatments becomes crucially important. In particular, depending upon the desired heat transfer characteristics of biomass conversion technol­ogy, a wide variety of process designs and reactor configurations have been proposed and demonstrated. Some of the examples in the reactor design include bubbling fluidized bed, circulating fluidized bed, entrained flow, ablative tube, rotating cone, vacuum, twin-screw, and others. Examples of a heat transfer medium utilized in such processes include hot sand which is usually recirculated in the process system as a direct-contact heat transfer medium as well as a means for thermal energy preservation.

Biodiesel can be produced in a few different ways. The process can be operated either as a batch process or as a continuous process. It is usu­ally performed catalytically, using a strong base or acid as the catalyst. Alternatively, it can be operated noncatalytically, using supercritical meth­anol. The most popular process in the industry currently uses methanol as the alcohol, sodium hydroxide (NaOH) as the base catalyst, and is a con­tinuous process.

Biodiesel is most popularly produced by the transesterification reaction of triglycerides. Triglycerides are found in plant oils and animal fats and the molecular structure of a triglyceride is shown in Figure 2.5. Transesterification occurs when the triglycerides are mixed with an alcohol, typically either methanol or ethanol. As the hybridized terminology of “trans — + esterifica­tion" implies, transesterification is a chemical reaction in which the aliphatic organic group (R-) of an ester is exchanged with another aliphatic organic group (R-) of an alcohol, thereby producing a different ester and a differ­ent alcohol. In other words, the starting ester (triglyceride) and monohydric alcohol (methanol) are converted by the transesterification reaction into a simpler form of esters (biodiesel) and a more complex form of alcohol, that is, trihydric alcohol (glycerin). In this reaction of transesterification of triglyc­erides, three alcohol molecules liberate the long-chain fatty acids from the glycerin backbone by bonding (i. e., esterification) with the carboxyl group carbons in the triglyceride molecule, as shown in Figure 2.6. The products of the transesterification reaction are a glycerin molecule and three long — chain mono-alkyl ester molecules, otherwise commonly known as biodiesel. More specifically, if a biodiesel is produced by transesterification reaction between soy triglycerides and methanol, the resultant biodiesel ester is often referred to as methyl soyate. The transesterification reaction is usually cata­lyzed using a strong base such as NaOH or KOH. The base helps to catalyze this reaction by removing the

Before oils and fats can react to form biodiesel they must go through a pretreatment process. The first stage of the pretreatment process involves fil­tering to remove dirt and other particulate matters from the oil. Next, water must be removed from the oil because it will hydrolyze the triglycerides to form fatty acid and glycerin instead of biodiesel and glycerin. Free fatty acids can directly react with base catalyst to form soap, which is certainly not desir­able for biodiesel manufacture. If soap formation is active, the process would require an additional amount of base catalyst to compensate for the reactive depletion. Finally, the oil must be tested for free fatty acid (FFA) content. Typically, less than 1% FFA in oil is acceptable for processing without further provisional treatments. Free fatty acids are long-chain carboxylic acids that have broken free from the triglycerides, typically from thermal degradation of triglycerides as a result of prolonged exposure to heat. These acids can increase soap formation in the reactor, as mentioned earlier. Too much soap in the reactor causes substantial difficulties: (a) soap formation becomes a reason for an additional amount of base catalyst usage to overcome its reac­tive depletion in the soap formation; (b) additional problems arise in product separation; and (c) as an extreme case, the formed soaps mix with water from the fuel wash stage to create an emulsion that can seriously slow down or even prevent settling of the wash water layer from the product biodiesel layer. There are two ways to deal with the free fatty acids in the oil. An acid can be added to the oil to convert the free fatty acids into biodiesel; this is the case with an acid-catalyzed esterification reaction. Alternatively, they can be neu­tralized, turned into soap, and removed from the oil. After being pretreated the oil is then sent to the reactor. The methanol that reacts with the oil also has to go through some pretreatment. Before the methanol is sent to the reac­tor it goes through a mixer where it is combined with the sodium hydroxide catalyst. The oil and methanol/catalyst mixture are then fed into the reac­tor to undergo the transesterification reaction. Methanol is fed in excess of around 1.6 times the stoichiometric amount and the reactor is kept at around 60°C. With the aid of the base catalyst the reaction is able to proceed at up to 98% conversion. The exit stream from the reactor is fed into a separator. The glycerin by-product has a much greater density (glycerin specific gravity at 25°C = 1.263) than the biodiesel (specific gravity at 60°C = 0.880) and is there­fore easily removed via gravity separation. After the biodiesel is separated from the glycerin by-product it goes through a purification process. The first step is to neutralize the remaining catalyst by adding an acid to the biodie­sel. Then the biodiesel is sent through a stripper to remove any methanol left from the reactor. This methanol is then recycled back to the methanol/ catalyst mixer.

After the methanol removal, the biodiesel goes through a water wash to remove all the soaps and salts (e. g., neutralized salt of NaOH catalyst) gen­erated during transesterification and neutralization. The biodiesel is then dried and stored as the final product. The glycerin by-product, or crude glycerin, also goes through some purification. The crude glycerin contains a considerable amount of methanol, which comes out unreacted due to its excess amount of feed to the reactor. The glycerin goes through a distilla­tion separation that recovers a great deal of the methanol for recycling. The recycled methanol collects most of the water that entered the process and therefore it must go through a separate distillation column to be purified. The glycerin that comes out of the distillation process is pure glycerin that can be marketed for other industries including the pharmaceutical and cosmetics industries. A schematic of the biodiesel manufacturing process via transesterification is shown in Figure 2.7.

As mentioned above, crude glycerin is a mixture of glycerin, methanol, and salts. Crude glycerin can be sold as is or further purified into phar­maceutical-grade glycerin. A marketable grade of crude glycerin is gener­ally at least 80% glycerin with less than 1% methanol. Crude glycerin that has lower levels of glycerin or higher levels of methanol often has little or no value; this is especially true in the current era of an oversupply of glycerin on the market. Although glycerin is overly abundant in the world marketplace due to rapidly increased biodiesel production, the purification of crude glycerin into pure glycerin is quite energy-intensive and costly. Efficient chemical conversion of glycerin or crude glycerin into other value — added chemicals and petrochemicals, in addition to the conventional end — uses established in the food, pharmaceutical, and cosmetics sectors, would help stabilize the market price of glycerin and provide additional income to the biodiesel industry, thus improving the industry’s gross margin and profitability.

Developing countries with characteristically weak economies and insuf­ficient industrial infrastructures have been more seriously hurt by the energy crises. To fully exploit the potential of available fossil-based fuels and other alternative renewable resources, significantly large capital funds, which developing countries may be lacking in, are required. The trend has thus been shifted toward the small-scale or localized utilization of energy resources. One potentially promising area where developing countries can achieve relatively quick success is the supplementing of their fossil fuel sup­plies with alternative renewable fuels derived from food and agricultural
crops such as sugarcane, cassava, maize, and sorghum. This option is also viable for developed economies.

Focused primarily upon petroleum as a primary source of transportation fuels, ethanol has garnered a great deal of attention as a liquid fuel source alternative to gasoline or as a gasoline blend to reduce the consumption of conventional gasoline. The ethanol alternative fuel program has been most seriously pursued by Brazil and the United States. In Brazil, all cars are run on either gasohol (a 22 vol% mixture of ethanol with gasoline, or E22, man­dated in 1993; a 25% blend, or E25, mandated since 2007) or pure ethanol (E100). In Brazil, the National Program of Alcohol, PROALCOOL, started in November 1975, was created in response to the first oil crisis of 1973. This program effectively changed the consumption profile of transportation fuels in the country. In 1998, these ethanol-powered cars consumed about 2 billion gallons of ethanol per year and about 1.4 billion gallons of ethanol were addi­tionally used for producing gasohol (E22, i. e., 22 vol% ethanol and 78 vol% gasoline) for other cars [7]. In March 2010, a milestone of 10 million flex-fuel ethanol-powered vehicles produced in Brazil was achieved. The Brazilian program has successfully demonstrated large-scale production of ethanol from sugarcane and the use of ethanol as a sustainable motor fuel. In 2010, Brazil produced about 6.92 billion gallons [8] of ethanol, whereas the United States produced just over 13.2 billion gallons; 2010 U. S. production was more than two times greater than that of 2007 (6.5 billion gallons) and about eight times greater than that of 2000 (1.62 billion gallons). As shown, ethanol has been the fastest growing chemical in the United States for the past decade. These two countries alone were responsible for about 90% of the world’s industrial ethanol production in 2010.

Regarding the atmospheric concentrations of greenhouse gases (GHGs), the National Research Council (NRC), responding to a request from Congress and with funding from the U. S. Department of Energy, emphasizes the need for substantially more research and development on renewable energy sources, improved methods of utilizing fossil fuels, energy conservation, and energy-efficient technologies. The Energy Policy Act (EPAct) of 1992 was passed by the U. S. Congress to reduce the nation’s dependence on imported petroleum by requiring certain fleets to acquire alternative fuel vehicles, which are capable of operating on nonpetroleum fuels. Alternative fuels for vehicular purposes, as defined by the Energy Policy Act, include ethanol, natural gas, propane, hydrogen, biodiesel, electricity, methanol, and p-series fuels. P-Series fuels are a family of renewable fuels that can substitute for gasoline. The Energy Policy Act of 2005 changed U. S. energy policy by pro­viding tax incentives and loan guarantees for energy production of various types, which included tax reductions for alternative motor vehicles and fuels including bioethanol [9].

The United States does not suffer from a lack of energy resources (it has plenty of coal and oil shale reserves), but it is in need of conventional liquid transportation fuels. The market for transportation fuels has been dominated by petroleum-based fuels and that trend is expected to continue for a while. A great many researchers of the world have worked on the biological pro­duction of liquid fuels from biomass and coal [10]. They have found micro­organisms that can produce ethanol from biomass, convert natural gas into ethanol, and convert syngas derived from coal gasification into liquid fuels. These micro-organisms are found to be energy efficient and promising for industrial production. The microbial process works at ordinary temperature and pressure and offers significant advantages over chemical processes, such as direct coal liquefaction and Fischer-Tropsch synthesis, which oper­ate under severe conditions to produce liquid fuels from coal.

Researchers have focused on using lignin as a renewable source to derive traditional liquid fuel. Lignins are produced in large quantities in the United States as by-products of the paper and pulp industry. As a consequence, the prices of some lignin products, such as lignosulfonates or sulfonated lignins, are relatively low. Lignosulfonates are used mainly as plasticizers in making concrete and also used in the production of plasterboard. Global production of lignin for various industrial applications is estimated to be quite high, even though reliable statistical data are unavailable. In China, the national lignin production has grown from 32 million metric tons in 2006 to 45 mil­lion metric tons in 2010, at a relatively fast growth rate.

An aspect that is quite attractive in biomass utilization is its renewability that ultimately guarantees nondepletion of the resource. Considering all plants and plant-derived materials, all energy is originally captured, trans­formed, and stored via a natural process of photosynthesis. Strictly following the aforementioned definitions of biomass, it can be safely said that energy from biomass has been exploited by humans for a very long time in all geo­graphical regions of the world. The combustion, or incineration, of biological substances such as woody materials and plant oils has long been exploited to provide warmth, illumination, and energy for cooking. It has been estimated that, in the late 1700s, approximately two-thirds of the volume of wood removed from the American forest was for energy generation [3]. Because wood was one of the only renewable energy sources readily exploitable at the time, its use continued to grow until the mid-to-late 1800s, when petro­leum was discovered and town gas infrastructure based on coal gasification was introduced. It was reported that during the 1800s, single households consumed an average of 70 to 145 m3 of wood annually for heating and cooking [7, 8]. A small percentage of rural communities in the United States still use biomass for these purposes. Countries including Finland use the direct combustion of wood for a nontrivial percentage of their total energy consumption [9]. Furthermore, Finland has spent significant R&D efforts in biomass utilization programs and has successfully developed a number of advanced biomass conversion technologies. Finland and the United States are not the only countries that use biomass consumption for supplementing their total energy usage. In fact, the percentage of biomass energy of the total energy consumption for a country is far greater in African nations and many other developing countries.

An assessment by the World Energy Council (WEC) [10] reported that the 1990 biomass usage in all forms accounted for 1,070 MTOE, which is approxi­mately 12% of global energy consumption of 8,811 MTOE assessed for the same year. MTOE stands for metric tonnes of oil equivalent. In 2010, about 16% of global energy consumption came from renewables, of which about 10% was contributed from traditional biomass, which was mainly used for heating, 3.4% from hydroelectricity, and 2.8% from so-called "new renew­ables" which included small hydro, modern biomass, wind, solar, geother­mal, and biofuels [11]. The last category of new renewables has been growing very rapidly based on the development of advanced technologies as well as the global fear of depletion of conventional petroleum fuel.

On a larger scale, biomass is currently the primary fuel in the residential sector in many developing countries. Their biomass resources may be in the form of wood, charcoal, crop waste, or animal waste. For these countries, the most critical function of biomass fuel is for cooking, with the other principal uses being lighting and heating. The dependence on biomass for the critical energy supply for these countries is generally decreasing, whereas that for industrialized countries is more strategically targeted for new generation bio­mass energy. According to REN21 [11], the top five nations in terms of exist­ing biomass power capacity in 2011 are the United States, Brazil, Germany, China, and Sweden in order of one to five. The top two nations of this list are also the top two nations in bioethanol production, not coincidentally. In other words, the biomass power category has so far been propelled and dominated mostly by the bioethanol transportation fuel sector.

Waste feedstock containing lignocellulose, fatty acids, and protein derivatives can be hydrothermally transformed to produce a range of products such as biocrude, methane, hydrogen, biodiesel, and biogasoline. Many waste prod­ucts such as agricultural residues, food processing wastes, and municipal and agricultural sludge contain large amounts of water. The removal of this water in gasification, pyrolysis, and other thermochemical processes con­sumes a significant amount of energy. The hydrothermal process avoids the need for this water removal. Also, the considerable variations in the physical properties of water that occur with changes in temperature and pressure can facilitate efficient separations of product and by-product streams with low energy requirements. For feedstock that contains inorganics such as sulphates, nitrates, and phosphates, the hydrothermal method can facilitate recovery and recycling of these chemicals in their ionic form for eventual use as fertilizer. The product streams from hydrothermal processing are also completely sterilized against any possible pathogens including biotoxins, bacteria, and viruses.

In general, hydrothermal processing can be divided into three regions: liquefaction, catalytic gasification, and high-temperature gasification [76]. These three regions are graphically illustrated in Figure 6.8. Both catalytic and high-temperature gasifications occur under supercritical conditions producing gases with high hydrogen content. Also, depending upon the operating conditions, a significant amount of reforming reactions can take place under gasification conditions, particularly when a suitable reforming catalyst is used. The gasification process under supercritical water is cov­ered in the later section on supercritical technology. Hydrothermal liquefac­tion generally occurs at temperatures between 200 and 370°C and pressure between 4 and 20 MPa, and it can be applied to a stream of mixed waste. A significant literature on hydrothermal processing of biomass and waste has been reported over the last two decades [78-83].

Two other variations within hydrothermal liquefaction (HTL) have also been examined in the literature [83, 84]. At low temperatures (between about 180-220°C) and at saturated pressure, hydrothermal carbonization of bio­mass occurs. Although carbonized biomass is inferior to liquid or gaseous fuels, process requirements for hydrothermal carbonization are comparably low while producing a fuel that is easy to handle and store because it is stable and nontoxic. Thus, HTC may provide some advantages when considering small-scale, decentralized applications.

HTC is produced for residence time between 1 and 72 hours and with water pH below 7. Alkaline conditions produce substantially different products

[81-83]. The process goes through numerous reactions such as hydrolysis, dehydration, decarboxylation, polymerization, and aromatization. For a variety of feedstock such as cellulose, lignin, wood, peat bog, and the like, as reaction severity increases, more carbonization occurs reducing H/C and O/C ratios of the product. In a typical HTC operation, 48-50% of HTC coal (which contains lignitelike material in a dispersed powder form), about 35-37% of water and total organic carbon (which contains sugars and deriva­tives, organic acids, furanoid, and phenolic compounds), and about 15-16% of gas (which contains mainly CO2, with some CH4 and CO and traces of H2 and CnHm) are generated.

Unlike HTC, HTL (hydrothermal liquefaction) is carried out in a tem­perature range of about 200 to 400°C, and it produces products often called bio-oil or biocrude. Although HTL is a promising technology to treat waste streams from various sources, its commercial growth is inhibited due to the high transportation costs of cellulosic waste, poor conversion efficiency, and lack of understanding of complex reaction mechanisms. Kranich [85] was one of the first to use an HTL process to convert MSW. He used three differ­ent types of materials from MSW; primary sewage sludge, settled digester sludge, and digester effluent. In a laboratory autoclave, he performed experi­ments at temperatures ranging from 570-720 K, pressure of 14 MPa, and resi­dence time ranging from 20 to 90 minutes. The results showed the organic conversion rates from 45 to 99% and oil production rates from 35 to 63.3%.

Subsequent works were reported by Suzuki et al. [86], Itoh et al. [87], and Inoue et al. [88]. Most recently, Changing World Technology Co. in Carthage, Missouri, United States [89] reported the effectiveness of hydrothermal liq­uefaction for the treatment of turkey waste.

The process of hydrothermal upgrading (HTU) combines the liquefac­tion and upgrading steps. A schematic of this HTU process is described by Demirbas [84]. In this process the feed is pretreated, preheated, and pumped into the reactor. The products are separated in gas, liquid, and solid streams. In normal operating conditions the product composition is: gaseous prod­uct (>90% CO2) about 25%, process water about 20%, water-soluble organ­ics about 10%, and biocrude about 45%. The gaseous products, which have some heating value, are catalytically combusted with air to generate flue gas. The process water stream and soluble organics go through an anaero­bic digestion to produce biogas (largely containing methane) which can go through a combustion process to generate heat and electricity. The solid biocrude is separated into light and heavy biocrude. The heavy biocrude is co-combusted with coal to generate electricity. The lighter biocrude is hydro — deoxygenated to produce upgraded products such as premium diesel fuel, kerosene, and other feedstock for biorefineries. The process has an overall thermal effciciency of about 70-90%.

The hydrothermal upgrading process is generally carried out at 575 K; however, the process can be operated in the temperature range of 575-625 K, pressure range of 12-18 MPa, and residence time range of 5-20 minutes. The process carries out a series of depolymerization, decarboxylation, dehy­dration, hydrodeoxygenation, and hydrogenation reactions. The oxygen is removed as water and carbon dioxide. Typically the feed slurry contains 25% solids in water which may include wood and forest wastes, agricultural and domestic residues, municipal solid waste, or organic industrial residues. Kumar and Gupta [90] examined the effect of temperature on molecular structure and enzymatic activity of cellulose in subcritical hydrothermal technology. They indicated that the percentage of crystallinity of microcrys­talline cellulose increased with treatment with water.

A thorough and excellent review of biofuel production in hydrothermal media was given recently by Peterson et al. [76].

Higher heating values (HHVs) of vegetable oils range between 39,000-48,000 kJ/kg, or 16,770-20,650 BTU/lb, depending upon the kind of vegetable oil. The HHV of vegetable oil is higher than that of anthracite, spent tire rubber, or wood. Table 2.4 shows a comparison of higher calorific values of common fuels and energy sources [7].

The higher heating value is also known as the gross calorific value, higher calorific value, gross energy, or gross heat. The HHV of a fuel is defined as the amount of heat released per unit mass (initially at 25°C) once it is com­busted and the products have returned to a temperature of 25°C. This means that the HHV value is the total heat recoverable, including the energy con­tained in water vapor released due to combustion reaction. In other words, the higher (or gross) heating value is the gross calorific value (gross CV) when all products of combustion are cooled back to the precombustion tem­perature, water vapor formed during combustion is also condensed, and necessary corrections have been made.

On the other hand, the lower heating value (LHV) is also known as net calorific value, or net CV. The LHV of a fuel is defined as the amount of heat released due to combustion of a unit mass of fuel (initially at 25°C or another reference state) and returning the temperature of the combustion products to 150°C. As such, the energy contained in water vapor released during com­bustion is not wholly included in the LHV. A major portion of the energy amount excluded from the HHV, in obtaining the LHV, is the latent heat of vaporization of water.

As shown in Table 2.4, the heating value of vegetable oil is quite high, higher than other naturally derived fuels such as coal and wood. Vegetable oils have long been used for cooking, lighting, and heating throughout the world. Residential furnaces and boilers that are designed to burn heating oil No. 2 can be modified to burn vegetable oils, including filtered waste vegetable oil. The required modification is based on a similar ground as

TABLE 2.4

Higher Heating Values (HHVs) of Common Fuels

Higher Heating Value
(Gross Calorific
Value—GCV)

that for diesel engines for SVO, in which the viscosity of vegetable oil is reduced by appropriate preheating. Although this method results in sub­stantial cost savings, it has not been popularly practiced in the United States.

In analyzing and discussing the energy balance of alternative fuel produc­tion, a term called net energy value (NEV) is often used. The net energy value is the difference between the energy content of product ethanol and the total energy used/consumed in producing and distributing ethanol. Higher corn yields of modern agricultural industry, lower energy consumption per unit of output in the fertilizer industry, and recent advances in fuel conver­sion technologies have significantly enhanced the economic and technical feasibility of producing ethanol from corn, when compared with that of just a decade ago. Therefore, studies based on the older data may tend to overestimate energy use (input), because the efficiency of growing corn as well as converting it to fuel ethanol has improved significantly over the past decade [5].

A large number of studies have been conducted to estimate the NEV of ethanol production. However, variations in data and model assumptions resulted in a widely differing range of estimated values (conclusions), rang­ing from a very positive to a negative value. A negative net energy value would mean that it takes more energy to produce the energy content of prod­uct ethanol. According to the study by Shapouri, Duffield, and Graboski [5] of the USDA, the net energy value of corn ethanol was calculated as +16,193 BTU/gal, assuming that fertilizers were produced by modern (1995 or so) processing plants, corn was converted in modern (also about 1995) ethanol facilities, farmers achieved normal corn yields, and energy credits were allocated to coproducts. Updated values for the NEV of corn ethanol by Shapouri et al. show +21,205 BTU/gal (July 2002) [6] and +30,528 BTU/gal (October 2004), respectively [7]. The first value of 21,205 BTU/gal was based on the higher heating value of ethanol, whereas the second value of 30,528 BTU/gal was based on the lower heating value of ethanol. However, another study conducted by Pimentel and Patzek (2005) showed that the NEV of eth­anol was -1,467 kcal/liter (equivalent to -16,152 BTU/gal), which was based on the LHV [8]. A recent study thoroughly conducted by Argonne National Laboratory (2005) shows that ethanol generates 35% more energy than it takes to generate [9].

As shown, sharp differences in the calculated NEV of ethanol produc­tion among the studies still existed and they stemmed from several factors, which were comprehensively identified and directly compared in a report by MathPro Inc. [10]. According to MathPro’s analysis, the differences in the NEV reflected sharp differences in four energy usage categories and they are [10]

1. Energy used in corn production: The USDA estimates (20.2 K BTU/ gal in 2002 and 18.7 BTU/gal in 2004) are about half of Pimentel — Patzek’s (37.9 K BTU/gal).

2. Energy used in corn transport: The USDA estimates (2.1 K BTU/gal in 2002 and 2004) are less than half of Pimentel-Patzek’s (4.8 K BTU/gal).

3. Energy used in ethanol production: The USDA estimates (46.7 K BTU/gal in 2002 and 49.7K BTU/gal in 2004) are substantially lower than that of Pimentel-Patzek (56.4 K BTU/gal).

4. Coproduct energy credit: The 2002 USDA estimate (-13.5 K BTU/gal) is twice that of Pimentel-Patzek (-6.7 K BTU/gal). The 2004 USDA estimate (-26.3 K BTU/gal) is twice the 2002 USDA estimate and four times Pimentel-Patzek’s estimate [10].

The "true" and "actual" value of ethanol’s NEV would depend on vari­ous factors that involve the geographical region, agricultural productivity, efficiency of the ethanol production process, energy efficiency of fertil­izer manufacture, and much more. It has been observed that the ethanol proponents have claimed positive NEVs, whereas the ethanol critics have referred to negative NEVs. As such, this subject has been controversial, from analytical and technoeconomical standpoints. However, it is cer­tain that modern corn ethanol plants use substantially less energy and produce more ethanol per bushel of corn than older plants, and it also appears certain that the claims of negative NEVs have been based on obso­lete material and energy balance data of the corn ethanol industry [11]. In 2008, Mueller conducted a very extensive milestone survey of the nation’s ethanol plants in terms of new energy use and coproduct data as well as land use and his conclusions published in 2010 clearly showed significant improvements over the 2001 data [12].

The 2001 survey by BBI International [42] found that dry mill plants use, on average, 36,000 BTU of thermal energy and 1.09 kWh of electrical energy per gallon of ethanol produced, while producing an average of 2.64 gallons of ethanol per bushel of corn. However, ethanol plants in 2008 used an aver­age of 25,859 BTU of thermal energy and 0.74 kWh of electricity per gallon of ethanol produced, which is 28.2 and 32.1% lower than the values of 2001, respectively. Ethanol produced per bushel of corn, meanwhile, increased by 5.3% to 2.78 gallons per bushel in 2008 [12]. This survey clearly supports that the NEV of ethanol production based on modern technology data is on the positive side.