Biodiesel has been around for quite some time, but it was not considered a viable fuel until recently. The transesterification of triglycerides was discovered by E. Duffy and J. Patrick as early as 1853; in fact, this hap­pened many years before the first functional diesel engine was invented. It was not until 1893 that Rudolf Diesel invented the first diesel engine and designed it to run on peanut oil. Later in the 1920s, the diesel engine was redesigned to run on petrodiesel, a fossil fuel derived from petro­leum crude [43]. Petrodiesel had been much cheaper to produce compared to any biofuel, thus there had not been many active developments in the biodiesel infrastructure.

It was not until 1977 that the first industrial biodiesel process using etha­nol was patented. Later, in 1979, South Africa started research on the trans­esterification of sunflower oil. After four years, South African Agricultural Engineers published a process for fuel-quality, engine-tested biodiesel. An Australian company called Gaskoks used this process to build the first biodiesel pilot plant in 1987 and later built an industrial-scale plant in 1989. The industrial-scale plant was capable of processing 30,000 tons of rapeseed per year [43]. As a matter of fact, rapeseed oil has also become the primary feedstock for biodiesel in Europe (estimates for 2006: more than 4 million tons of rapeseed oil went into biodiesel) [44]. During the 1990s, many European countries such as Germany and Sweden started building their own biodie­sel plants. By 1998 approximately 21 countries had some sort of commercial biodiesel production.

In September of 2005, the state of Minnesota became the first U. S. state to mandate that all diesel fuel sold in the state contain a certain part biodiesel, requiring a content of at least 2% biodiesel (B2 and up). This established that biodiesel blend fuel is no longer a choice, but a standard and mandate. On April 23, 2009, the European Union (E. U.) adopted the Renewable Energy Directive (RED) which included a 10% target for the use of renewable energy in road transport fuels by 2020. It also established the environmental sus­tainability criteria that biofuels consumed in the European Union have to comply with, covering a minimum rate of direct GHG emission savings as well as restrictions on the types of land that may be converted to production of biofuel feedstock crops [45, 46].

4.1 Lignocellulose and Its Utilization

4.1.1 Lignocellulose

The structural materials that are produced by plants to form cell walls, leaves, stems, and stalks are composed primarily of three different types of biobased macromolecular chemicals, which are typically classified as cel­lulose, hemicellulose, and lignin. These biobased chemicals are collectively called lignocellulose, lignocellulosic biomass, or lignocellulosic materials. As shown in Figure 4.1 here and also in Chapter 3, a generalized plant cell wall structure is like a composite material in which rigid cellulose fibers are embedded in a cross-linked matrix of lignin and hemicellulose that binds the cellulose fibers.

Generally speaking, the dry weight of a typical cell wall consists of approx­imately 35-50% cellulose, 20-35% hemicellulose, and 10-25% lignin [1]. Others claim that cellulose typically accounts for 40-50% of woody biomass, whereas lignin and hemicellulose each account for about 20-30%. Although lignin comprises only 20-30% of typical lignocellulosic biomass, it provides 40-50% of the overall heating value or total available energy of the biomass, due to its higher calorific value (CV) than cellulose and hemicellulose. This explains why chemical conversion or beneficial use of lignin is very impor­tant in fuel/energy utilization of lignocellulosic resources.

Lignocellulosic biomass structures also contain a variety of plant-specific chemicals in the matrix; these include extractives (such as resins, pheno — lics, and other chemicals) and minerals (calcium, magnesium, potassium, and others) that will leave behind ash when the biomass is combusted. The trace minerals and major elements in lignocellulosic materials display a high degree of variability for most of the elements between different spe­cies, between different organs within a given plant, and also depending on the growing conditions including the soil characteristics [2]. In addition to their potential health and environmental effects, trace minerals can exhibit nontrivial effects on the next stage chemical treatment, including catalytic conversion of thermochemical intermediates of lignocellulose.

Cellulose is a large polymeric molecule composed of many hundreds or thousands of monomeric sugar (glucose) molecules, and in this regard it may be considered a polysaccharide. The molecular linkages in cellulose form linear chains that are rigid, highly stable, and resistant to chemical attack. Due to its linear polymeric structure, cellulose exhibits crystalline proper­ties [3]; for example, cellulose may be somewhat soluble in a suitable solvent. However, cellulose molecules in their crystalline form are packed so tightly that even small molecules such as water cannot easily permeate the struc­ture. Logically, it would be even more difficult for larger enzymes to perme­ate or diffuse into the cellulose structure. Cellulose exists within a matrix of other polymers, mainly hemicellulose and lignin, as illustrated in Figure 4.1.

On the other hand, hemicellulose consists of short and highly branched chains of sugar molecules. It contains both five-carbon sugars (usually D-xylose and L-arabinose) and six-carbon sugars (such as D-galactose, D-glucose, and D-mannose) as well as uronic acid. For example, galactan, found in hemicellulose, is a polymer of the sugar galactose, whose solubility in water is 68.3 g per 100 grams of water at room temperature. Uronic acid is a sugar acid with both a carbonyl and a carboxylic function. Hemicellulose is amorphous due to its highly branched macromolecular structure [3] and is relatively easy to hydrolyze to its constituent simple sugars, both five-car­bon and six-carbon sugars. When hydrolyzed, the hemicellulose from hard­woods releases sugary products high in xylose (a five-carbon sugar), whereas the hemicellulose contained in softwoods typically yields more six-carbon sugars. Even though both five-carbon and six-carbon sugars, illustrated in Figure 4.2, are simple fermentable sugars, there is a discerning difference

OH

H

OH

in their fermentation chemistry and process characteristics with regard to specific yeasts and enzymes involved.

Humans have had far more extensive and successful experience in the fer­mentation of six-carbon sugars (hexoses) than five-carbon sugars (pentoses or xyloses), as well evidenced by a long history of manufacturing alcoholic beverages throughout the world. This statement is still valid for fuel ethanol fermentation as well. Many years ago, it was believed that xylose could not

CH2OH

OH O OH O c

OH

HOft,,..

HOH2C OH

P-D-Galactofuranose

FIGURE 4.2 (CONTINUED)

Molecular structures of five-carbon and six-carbon sugars.

be fermented by yeasts, but in recent years, a number of yeasts have been found to be capable of fermenting xylose into ethanol [4, 5]. Genetic engi­neering of xylose fermentation in yeasts has also been carried out with suc­cessful outcomes [6].

Lignin is a complex and highly crosslinked aromatic polymer that is cova­lently linked to hemicellulose, as shown in Figure 4.1. Lignin contributes to the stabilization of mature cell walls. Lignin yields more energy than cel­lulose when burned due to its higher calorific value. Lignin is a macromol­ecule whose typical molecular weight exceeds 10,000. Due to its crosslinked structure, lignin is generally more difficult to process, extract, hydrolyze, or react than cellulose or hemicellulose. Therefore, degradation or biodegrada­tion of the crosslinked structure becomes the first step for biofuel production from the cellulosic feedstocks. Needless to say, efficient conversion of lignin would result in a substantial increase in fuel yield as well as an enhanced economic outlook with utilization of lignocelluloses.

5.1 Biomass and Its Utilization

5.1.1 Definition of the Term Biomass

The term biomass has been an important part of legislation enacted by Congress for many decades and has evolved over time, resulting in a vari­ety of differing and sometimes conflicting definitions [1]. These definitions are critical to all parties engaged in the research, development, finance, and application of biomass to produce energy. The term biomass is more gener­ally defined as "different materials of biological origin that can be used as a primary source of energy" [2-5]. Alternately, biomass is defined as "plant materials and animal wastes used especially as a source of fuel" [6]. These biomass definitions contain the generalized statements for the origins of the materials or their intended uses and applications, however, the definitions are not meant to provide sufficient and necessary conditions for certain spe­cific material to be classified, or qualified, as biomass.

Riedy and Stone (2010) explained the evolving nature of biomass defini­tions and analyzed its trend in biomass-related legislation [1]. Based on the common definition that biomass is biologically originated matter that can be converted into energy, more readily conceivable and common examples of biomass include food crops, nonfood crops for energy generation, crop residues, woody materials and by-products, animal waste, and residues of biological fuel-processing operations. Over the past decades, however, the term biomass has grown to encompass algae and algae-processing residues, municipal solid waste (MSW), yard waste, and food waste. The term still remains highly flexible and open to divergent interpretations, including spe­cific inclusions and special exceptions, often based on a number of factors involving technoeconomic considerations, technological advances, and new scientific findings, renewability and sustainability issues, environmental and climate change concerns, ecological issues, strategic directions of local and federal governments, regional economic strengths and weaknesses, and more. Simply put, biomass is a very broad term and encompasses a wide variety of matters. This is why the term biomass itself has been a part of modern legislation promulgated by the U. S. Congress. Legislation can have many purposes: to regulate, to authorize, to provide funds and incentives, to sanction, to grant, to proscribe, to declare, or to restrict. Using a globally generic definition of biomass in specific legislation would be not only grossly insufficient and inappropriate, but also potentially conflicting and contro­versial [1].

This book deals with conversion of biomass into biofuels and bioenergy, generally speaking. For the same reason described above, the individual chapters of this book are subdivided, based on the specific types of bio­mass and their associated transformation technologies, into the technolog­ically categorized topics of corn ethanol, cellulosic ethanol, biodiesel, algae biodiesel, waste-to-energy, biomass pyrolysis and gasification, and so on.

The discussion of biomass definitions lately has centered around the issues involving: (i) the types of forestry products considered eligible biomass sources, (ii) the lands where biomass removal can occur, specifically Federal and Indian lands, and (iii) the kinds and types of waste that qualify as biomass, specifically municipal solid waste and construction and debris (C&D) [1].

The process of liquefaction has been widely used for fossil energy such as direct liquefaction of coal, shale, bitumen, heavy oil, and the like. The same concept can be easily applied to biomass which produces a water-insoluble bio-oil at high pressure (50-200 atm) and low temperature (250-450°C). These process conditions are less severe than the ones normally used for coal. The liquefaction involves all kinds of processes such as solvolysis, depolymer­ization, decarboxylation, hydrogenolysis, and hydrogenation (which can be accompanied by mild hydrocracking). The overall objective of biomass (and waste) liquefaction is to control the reaction rate and reaction mechanisms using pressure, gases, and catalysts to produce a high-quality liquid oil. The reactor feed generally consists of solid biomass feed (or a suitable waste), sol­vent, reducing gas such as H2 or CO, or a catalyst. The bio-oil produced via liquefaction has a lower oxygen content, lower viscosity, and higher energy density than pyrolysis-derived oil. Fundamentally, the nature of the lique­faction process can be broken down into several categories depending on the nature of the solvent and gas. For example, hydrothermal liquefaction uses water as the solvent, hydropyrolysis uses H2 or a reducing gas but no solvent, and solvolysis uses a reacting or hydrogen donor solvent. A review of biomass liquefaction research done between 1920 and 1980 is presented by Moffatt and Overend [77].

The nature of solvent, gas, and catalyst dictate the operating conditions and the quality of the liquefaction product. A number of solvents such as water, creosote oil, ethylene glycol, methanol, and recycled oil have been tested. Water is the most attractive because it is cheaper, and it does not require drying of waste or biomass. Hydrothermal liquefaction is sepa­rately described in this section. The recycled oil increases the selectiv­ity, and it also makes the process self-sufficient with no need to add new solvent in the process. Creosote oil, ethylene glycol, tetralin, phenathrene, alcohols, and phenols among other hydrogen donor solvents are used in the solvolysis process. They react with the solid materials during the liq­uefaction process. Hydrogen, carbon monoxide, and even methane act as reducing agents during hydropyrolysis in the presence of a catalyst and give a higher quality bio-oil. A number of catalysts have been used for liquefaction including alkali (from the alkaline ash components in the wood, alkaline oxides, carbonates, and bicarbonates) and metals such as zinc, copper, nickel, formate, iodine, cobalt sulfide, zinc chloride, and fer­ric hydroxide, as well as Ni, Mo, Ru, Co (which aid in hydrogenation/ hydrocracking).

Akhtar and Amin [80] studied the effects of various reaction variables on the hydroliquefaction process. They concluded that the major parameters influencing the yield and composition of bio-oil are temperature, proper­ties of solvent, solvent density, and type of biomass or waste. Temperature is the most important parameter, and they recommended a temperature range of 300-374°C depending upon the biomass type and specifications for the composition of bio-oil. At temperatures higher than 350°C, too much gas is formed and for temperature less than 280°C, conversion to oil is low. They recommended the use of water, methanol, ethanol, acetone, tetralin, and benzene, among others as suitable solvents. Residence time, heating rates, pressure, biomass particle size, presence of reducing gas, or hydrogen donor species are found to be of secondary importance in the hydrolique­faction process. The nature of the biomass waste also influences the yield of liquid production.

The diesel engine is named after the original developer of the engine, Rudolf Diesel, who initially attempted to run the engine with coal dust and later redesigned the engine to run with vegetable oil. Several similar efforts of the early 1900s were reported. Since then, the R&D efforts of using straight vegetable oils as a diesel substitute or alternative diesel fuel have continued or received public interest in response to escalating and fluctuating petro­leum prices.

If properly modified, most diesel engines on automobiles can be run on vegetable oils, that is, SVO or PPO. Principal modifications involve reduc­tion of viscosity and surface tension of SVO before injection of the fuel. High viscosity and surface tension of vegetable oils, if not altered, can cause poor atomization of the fuel, which results in incomplete combustion. Incomplete combustion causes coking or carbonization, which produces polycyclic aro­matic hydrocarbons (PAHs), soot, or coke. The reduction of viscosity and surface tension can be achieved via preheating of the fuel before its injection, which can be accomplished using the waste heat of the engine or the automo­bile’s electric power. In such a case, an additional tank of normal diesel (i. e., petrodiesel or biodiesel) would still be needed in addition to the main SVO tank for smooth engine operation. The cold engine is started with normal diesel and once the engine gets warmed up, the fuel is switched from nor­mal diesel fuel to vegetable oil which is preheated using a heat exchanger. In a very cold climate, this limitation in cold start is potentially a significant hurdle to overcome.

Alternately, an innovative blend chemistry may be devised for property modification of vegetable oils for their direct use in diesel engines. Vegetable oil can be mixed with other fuels or chemicals such as kerosene, diesel, and gasoline, thereby reducing the viscosity (in particular, kinematic viscosity) and surface tension of the blended fuel. The blending chemical/fuel typically

Fatty Acid Compositions of Common Edible Oils3

TABLE 2.3 Oil Unsat./Sat Ratio Saturated Monounsaturated Polyunsaturated Capric Acid 00:0 Laurie Acid 02:0 Myristic Acid 04:0 Palmitic Acid 06:0 Stearic Acid 08:0 Oleic Acid 08:1 Linoleic Acid (u>6) 08:2 Alpha Linolenic Acid (u>3) 08:3 Almond Oil 9.7 — — — 7 2 69 17 — Canola Oil 15.7 — — — 4 2 62 22 10 Cocoa Butter 0.6 — — — 25 38 32 3 — Cod Liver Oil 2.9 — — 8 17 — 22 5 — Coconut Oil 0.1 6 47 18 9 3 6 2 — Corn Oil (Maize Oil) 6.7 — — — 11 2 28 58 1 Cottonseed Oil 2.8 — — 1 22 3 19 54 1 Flaxseed Oil 9.0 — — — 3 7 21 16 53 Grape seed Oil 7.3 — — — 8 4 15 73 — Olive Oil 4.6 — — — 13 3 71 10 1 Palm Oil 1.0 — — 1 45 4 40 10 — Palm Olein 1.3 — — 1 37 4 46 11 — Palm Kernel Oil 0.2 4 48 16 8 3 15 2 — Peanut Oil 4.0 — — — 11 2 48 32 — Safflower Oilb 10.1 — — — 7 2 13 78 — Sesame Oil 6.6 — — — 9 4 41 45 —

Source: http://www. scientificpsychic. com/fitness/fattyacidsl. html; http://www. coimectworld. net/whc/images/chart. pdf; http://curezone. com/ foods / fatspercent. asp

Note: Percentages may not add to 100% due to rounding and other constituents not listed in the table. Where percentages vary, average values are used. a By weight of total fatty acids. b Not high-oleic variety.

is chosen from lower molecular weight hydrocarbons. This type of blending is often referred to as "cutting," "diluting," or "cosolvent mixing." However, there are some concerns regarding blending, which are largely based on higher rates of wear and tear in conventional fuel pumps and piston rings when using such blends. Advances in automobile component design includ­ing fuel injectors, cooling system, and glow plugs as well as in materials of construction are expected to be made.

As an example of recent advances, automobiles powered by indirect injec­tion engines equipped with in-line injection pumps are capable of running on pure SVO in most moderate climates, except during cold winter tem­peratures. Some of these vehicles are equipped with a coolant-heated fuel filter, which functions as a fuel preheater that helps reduce the viscosity of the SVO.

In 2007, the U. S. Congress enacted the Energy Independence and Security Act as a result of President Bush’s Advanced Energy Initiative (AEI) chal­lenging the United States to change the way its citizens fuel their vehicles to improve the nation’s energy security. The important message delivered by the AEI was "Keeping America competitive requires affordable energy." The EISA of 2007 is an energy policy act designed to improve energy efficiency and to increase the supply of clean renewable fuels, thereby reducing U. S. energy consumption by 7% and greenhouse gas (GHG) emissions by 9% by 2030. The initiative requires a mandatory Renewable Fuel Standard requiring transportation fuels sold in the United States to contain a minimum of 36 bil­lion gallons of renewable biofuels by 2022, including advanced and cellulosic biofuels and biomass-based diesel. The EISA further specifies that 21 billion gallons of the 2022 total biofuel blends in gasoline must be derived from noncornstarch products. Additionally, the EISA requires that the Corporate Average Fuel Economy (CAFE) standard increase to 35 miles per gallon by the year 2020. Although the EISA somewhat appears to discourage further growth of conventional corn-based ethanol as a blend fuel for the future and instead promotes rapid growth and market expansion of cellulosic ethanol, it still provides ample room for advancement of the U. S. ethanol industry.

Acid hydrolysis of cellulosic materials has long been practiced and is rela­tively well understood. Among the important specific factors in chemical hydrolysis are surface-to-volume ratio, acid concentration, temperature, and time. The surface-to-volume ratio is especially important in that it also deter­mines the magnitude of the yield of glucose. Therefore, smaller particle size results in better hydrolysis, in terms of the extent and rate of reaction [41]. With respect to the liquid-to-solids ratio, a higher ratio leads to a faster reac­tion. A trade-off must be made between the optimum ratio and economic feasibility because the increase in the cost of equipment parallels the increase in the ratio of liquid to solids. For chemical hydrolysis, a liquid/solids ratio of 10:1 seems to be most suitable [41].

In a typical system for chemically hydrolyzing cellulosic waste, the waste is milled to fine particle sizes. The milled material is immersed in a weak acid (0.2 to 10%), the temperature of the suspension is elevated to 180 to 230°C, and a moderate pressure is applied. Eventually, the hydrolyzable cel­lulose is transformed into sugar. However, this reaction has no effect on the lignin which is also present. The yield of glucose varies, depending upon the nature of the raw waste. For example, 84-86 wt% of kraft paper or 38-53 wt% of the ground refuse may be recovered as sugar. The sugar yield increases with the acid concentration as well as the elevation of temperature. A suit­able concentration of acid (H2SO4) is about 0.5% of the charge.

A two-stage, low-temperature, and ambient-pressure acid hydrolysis process that utilizes separate unit operations to convert the hemicellulose and cellulose to fermentable sugars was developed [42] and tested by the Tennessee Valley Authority (TVA) and the U. S. Department of Energy (DOE). Laboratory and bench-scale evaluations showed more than 90% recovery and conversion efficiencies of sugar from corn stover. Sugar product concen­trations of more than 10% glucose and 10% xylose were achieved. The inhibi­tor levels in the sugar solutions never exceeded 0.02 g/100 ml, which is far below the level shown to inhibit fermentation. An experimental pilot plant was designed and built in 1984. The acid hydrolysis pilot plant provided fer­mentable sugars to a 38 L/h fermentation and distillation facility built in 1980. The results of their studies are summarized as follows.

• Corn stover ground to 2.5 cm was adequate for the hydrolysis of hemicellulose.

• The time required for optimum hydrolysis in 10% acid at 100°C was 2 hours.

• Overall xylose yields of 86 and 93% were obtained in a bench-scale study at 1- and 3-hr reaction times, respectively.

• Recycled leachate, dilute acid, and prehydrolysis acid solutions were stable during storage for several days.

• Vacuum drying was adequate in the acid concentration step.

• Cellulose hydrolysis was successfully accomplished by cooking stover containing 66 to 78% acid for six hours at 100°C. Yields of 75 to 99% cel­lulose conversion to glucose were obtained in the laboratory studies.

• Fiberglass-reinforced plastics of vinyl ester resin were used for con­struction of process vessels and piping.

Among the endothermic gasification reactions of hydrocarbons, the speed of carbon dioxide gasification reaction is the slowest at practical operat­ing temperatures. Most advanced gasification technologies produce carbon dioxide as a component in their syngas products. The gasification using CO2 has not been popularly attempted, due to its poorer thermal efficiency and inferior energetics compared to steam gasification. However, due to the growing concerns of greenhouse gas emissions as well as the roles of carbon dioxide as a major greenhouse gas, various technologies includ­ing the capture of CO2, its reduction, utilization in carbon gasification, and conversion into other petrochemicals are actively pursued and developed. Gasification of biomass or coal coupled with CO2 management is also an environmentally prudent option.

Complete combustion of biomass or fossil fuels generates carbon dioxide. Because carbon dioxide is chemically very stable, its reactivity is limited. Therefore, the conversion of carbon dioxide into far more reactive carbon monoxide is one of the technological options, whereas the direct conversion of carbon dioxide into hydrocarbons is another. The two types of reactions are categorized under the reduction of carbon dioxide, and finding ener­getically prudent pathways for CO2 reduction is a challenge in modern fuel chemistry. The first group of chemical reactions includes the Boudouard reaction and the reverse water gas shift reaction:

C(s) + CO2(g) = 2CO(g)

CO2(s) + H2(g) = CO(g) + H2O(g)

As can be seen from Table 5.9, the temperature for Kp > 1 for the forward reactions as written to proceed for the Boudouard reaction and the reverse water gas shift (RWGS) reaction are 697°C and 814°C, respectively. Also, the RWGS reaction requires hydrogen as a reactant, which generally makes the process conversion costly.

Lee et al. [34] studied the kinetics of carbon dioxide gasification of various coal char samples for a temperature range between 800°C and 1,050°C using a unified intrinsic kinetic model and compared with the literature values obtained for various carbon, coal, and char samples. The Arrhenius acti­vation energy values obtained for the carbon dioxide gasification for these samples are shown in Table 5.10 [34].

Obtained from independent investigations by various investigators on diverse carbonaceous materials, the activation energy values for the kinetic rate equations for carbon dioxide gasification are around 60 kcal/mol or 250 kJ/mol. This high activation energy is also indicative of the nature of chemical reaction which requires a high temperature reaction to attain a

practically significant reaction rate. If we check the (E/RT) value for carbon dioxide gasification at 1,000°C, then the value becomes

E = 60,000

RT 1.987 * 1273

This (E/RT) value is within the range of the values for most industrially prac­ticed petrochemical reactions, often used as a rule of thumb. The study [34] also established that the kinetic rate of the noncatalytic carbon dioxide gas­ification of coal char at practical operating conditions, such as 900°C and 250 psi, is substantial. The rate was found to be about two to four times slower than that of steam gasification at the same T and P conditions.

The advantages and disadvantages of the co-gasification of coal and biomass mixture are briefly illustrated in Table 7.3. [12-14]. As shown in this table, co-gasification provides many advantages to the production of the syngas as well as generation of power. The pure biomass gasification process is limited to the small scale, has high capital (fixed) cost, has lower thermal efficiency, and carries shutdown risk. All of these are alleviated by the use of coal. A mixture of coal and biomass provides a stable and reliable feed supply for large-scale operations. Coal can be considered as the "fly wheel" that allows

TABLE 7.3

Advantages and Disadvantages of Use of Coal and Biomass Mixture Feed for Gasification

Disadvantages

Source: Prins, Ptasinski, and Janssen, 2006. Torrefaction of wood. Part 1. Weight loss kinetics, J. Anal. Appl. Pyrolysis, 77: 28-34; Shafizadeh, 1985. Pyrolytic reactions and products of biomass. In R. P. Overend, T. A. Mime, and L. K. Mudge (Eds.), Fundamentals of Biomass Thermochemical Conversion, London: Elsevier, pp. 183-217; and Shafizadeh, 1983. Thermal conversion of cellulosic materials to fuels and chemicals. In Wood and Agricultural Residues, New York: Academic Press, pp. 183-217.

a continuous plant operation when biomass fuel is not easily available. This concept in principle can be applied to any mixed feedstock. Co-gasification reduces the cost associated with fossil fuel consumption although some types of biomass can add significant cost to fuel production.

Co-gasification also reduces CO2 discharge in the atmosphere. When emis­sions related to harvesting, transportation, and other elements of the biomass supply chain are not included, biomass is considered to be a CO2-neutral fuel. A life-cycle assessment study shows that in comparison with coal-based systems, the use of biofuels for gasification results in environmental benefits. Unlike coal, the use of biomass fuel sources result in the generation of sig­nificantly lower quantities of anthropogenic CO2 emissions during power or fuel productions. A 70/30 mixture of coal and biomass generally produces a carbon-neutral process [15]. These advantages provide more security and less risk for the project financiers than the use of pure biomass, and are likely to engender more positive public attitudes toward the use of a fossil fuel sup­ply as a part of the mixed feedstock.

The mixed feedstock of biomass and coal also carries some disadvantages. As shown in Table 7.3, feed preparation and complex feed systems for mixed feedstock can be expensive. Two separate feed injectors, versus a single feed injector, may affect the gasifier performance. The gas cleaning system has the additional complications due to impurities in biomass. Although SOx emissions during gasification with mixed feedstock generally decreases,

NOx emission can increase, decrease, or remain the same depending on fuel type, firing, and operating conditions. Also, depending on the gasifica­tion technology, the presence of more tar or oil in the product gas may be problematic [1, 6]. Co-firing mixed feedstock with high chlorine content can increase corrosion in the system.

The ash coming from coal gasification processes is different from that coming from biomass gasification processes. When coal and biomass are gasified separately and ashes are kept separate, coal ash is generally used for the construction industry and biomass ash is recycled to the biomass origin or used as fertilizer, a building material, or as a fuel for power and heat gen­eration. The last option is possible only for biomass ash with a high energy content such as that from a fluidized bed gasification process. Biomass ash is often sent to a landfill for disposal. When coal and biomass are gasified together, the combined ash may not be useful for concrete and other con­struction industry applications. The slagging behavior of the combined ash of coal and biomass in the gasifier may also have a negative impact [1, 6]. The hydrophilic character of biomass and alkaline metals in biomass ash can also create fouling of heat transfer surfaces within gasifiers.

The effects of various physical properties and chemical constituents of biomass on the various aspects of gasification process are summarized in Table 7.4. These and other constraints of mixed feedstock processing as well as possible solutions are further discussed in subsequent sections.

TABLE 7.4

Effects of Various Physical and Chemical Properties of Biomass on Gasification Process

Relevant Physical or Chemical Properties

Source: Modified from Maciejewska et al. 2006. Co-Firing of Biomass with Coal: Constraints and Role of Biomass Pre-Treatment, DG JRC Institute for Energy Report, EUR 22461 EN.

Expeller pressing methods have a long history of extracting vegetable oil from oil seeds; the methods involve some means of mechanically squeezing oil from crushed oil-containing seeds under an applied pressure. Most cooking oils are produced via expeller pressing of a variety of feedstock including maize, sunflower, soya, sesame, coconut, mustard seed, and groundnuts. Even though this mechanical extraction method of oil from oil seeds is tech­nologically very simple, the process has been significantly enhanced with the development of energy-efficient and mechanically superior machinery. Furthermore, large processing plants have advantages over small plants in terms of reduced processing cost and extraction efficiency; most countries have such centralized large plants for cooking oil manufacture. These plants are also known as "oil refineries," that is, vegetable oil refineries. However, logistical burdens and transportation costs of raw materials and finished products also make small-scale highly efficient plants relevant and viable options. This is especially true for algae oil and biodiesel processing.

Oil presses are typically used for vegetable oils and biodiesel process­ing on both large and small scales. There are two different kinds of large — scale processing methods involving oil presses, one being hot processing
and the other being cold processing. In hot processing, the system includes a steam cooker and oil press. The steam cooker is mainly for pretreatment of oil seeds. In cold processing, the machine operates at a low temperature (e. g., 80°C) when it presses the seeds. An advantage of cold processing is that the extraction environment is not destructive to the nutrients in the oil. Dry algae can also be processed, quite similarly to oil seeds, using an oil press for extraction of algae oil, by mechanically rupturing the cell walls and collect­ing the extracted oil.