The use of a mixed feedstock in combustion, pyrolysis, gasification, plasma technology, liquefaction, and supercritical technology is becoming more and more prevalent and popular. Going forward, there are, however, sev­eral issues that need to be addressed to make the use of mixed feedstock in each of these technologies more economical, environmentally accept­able, and technologically feasible. Some of these issues are briefly outlined below.

1. Although there are several successful commercial co-firing instal­lations, the effects of increased biomass/coal ratio and the use of low-quality biomass on the effectiveness of co-firing need to be further examined. Some of the issues include transportation, han­dling, storage, milling and feeding problems, slagging and fouling due to deposit formation, agglomeration, corrosion or erosion, and ash utilization. There are several methods to address these issues. One method is the cleaning of deposits by soot blowing or exchange of agglomerated materials. It is also possible to avoid the effects of agglomeration and deposit formation by adding chemicals to reduce corrosion and increase the ash melting point. It is also possible to create an independent infrastructure for feeding, milling, storage, and conveying biomass just like one for coal. More expensive paral­lel and indirect firing modes where both feed treatments and gas­ification infrastructure can be separate at different levels are also options that need to be further examined for specific types of mixed feedstock. For each combustion process and mixed feedstock, an optimum logistic detail for biomass transportation, storage, and pre­treatment needs to be examined.

2. Unlike combustion, co-gasification is more complex because the process of gasification can be used to generate fuels, chemicals, and materials along with heat and electricity. Syngas generated via gas­ification has many downstream applications, but it needs to be pure and of the right composition. It is generally accepted that a future gasification reactor, even for a mixed feedstock, is most likely to be a pressurized oxygen-blowing entrained bed reactor [6]. For this type of reactor, an optimum burner design for solid biomass feeding and optimum gasification conditions with respect to biomass particle size (1 mm or less), maximum efficiency, maximum heat recovery, minimum flux use, minimum inert gas consumption, complete con­version, production of biosyngas with low CH4, and no tars will need to be obtained for each type of mixed feedstock [6]. In a slagging gas­ifier the ash and flux are present as a molten slag that protects the inner wall against high temperature. The slag must have the right viscosity and flow behavior at the temperature in the gasifier and its behavior as a function of gasification temperature, biomass ash properties, and selected flux should be known. The reactor should be operated at the lowest temperature possible to get high cold gas efficiency and oxygen consumption [6]. In a gas cooling and puri­fication system, maximum energy efficiency is desirable. Multiple cooling steps should be further optimized. The reactor should also be capable of processing feedstock of a wide variety of properties and produce no waste. The bottom ash should be recoverable as non — leachable slag with a value as construction material whereas fly ash needs to be used for mineral recycling [6]. This goal will require a continuous and steady flow of mixed feedstock for safe and steady operation. An appropriate pretreatment of feedstock, which may include leaching, torrefaction, and pelletizing, and an optimum process configuration are critically important to achieve the desired process objectives.

As shown in Section 7.5 [6], there are numerous process options possible. Milling of woody biomass and pressurization in a piston compressor with negligible inert consumption and feeding to the gasifier with a screw feed system (or a stamet type of system) is one option. The use of flash pyrolysis for grassy and straw biomass mate­rials to produce bioslurry that can be fed to the gasifier is another option. The torrefaction can allow the generation of fine and homo­geneous particles (with low energy consumption). This can then be homogeneously mixed with coal and pressurized by a piston com­pressor and fed to the reactor by a screw or a pneumatic feeding system. Finally, all feed materials can be gasified in a fluidized bed gasifier and the product gas then be processed in an entrained bed gasifier. These are some of the options when using coal and biomass of different properties. For other mixtures, which include materi­als such as oil shale, plastics, rubber tires, tar sand, and the like, additional modifications of the above-described options may be required.

3. Just as with combustion and gasification, the use of a plasma reac­tor for a variety of feedstock needs to be optimized. Plasma reactors often generate valuable chemicals along with fuel gas. The reactor design and operating conditions will need to be such that maximum production of valuable chemicals and fuel gas is possible. More experience with a larger scale plasma process is needed.

4. For high severity pyrolysis, the above-described statements for gas­ification apply. For low severity pyrolysis, a focus needs to be on the liquid production rate and its quality. As shown earlier, there is no synergy between pyrolysis of coal and biomass although there is a significant synergy between pyrolysis of biomass and plastics. Thus, an appropriate optimization of the operating conditions for the pyrolysis process will need to be examined based on the nature of the feedstock mixture. More experience with a larger scale pyroly­sis operation using mixed feedstock is needed.

5. An optimization of liquefaction of mixed feedstock is challenging and interesting. There seems to be very little synergy between liq­uefaction of coal and biomass. There is, however, synergy between liquefaction of biomass and polymeric products. This is true for both organic (hydrogen donor) solvents as well as water. More data on synergy for other types of feed mixture are needed. At the present time, just as in direct coal liquefaction, the liquefaction process for the mixture is, in general, economically unattractive for the produc­tion of high-quality fuels. More research is needed to test the lique­faction process which uses less pressure, less gaseous hydrogen, and recycled solvent. More markets for lower quality liquefaction prod­ucts need to be found.

6. Supercritical technology, both for gasification and liquefaction, is becoming more popular. The application of this technology using water, carbon dioxide, and some alcohols for mixed feedstock needs to be fully explored. The technology may be very attractive for selected feedstock mixtures and products.

Until now at larger scales, co-utilization of waste and coal has received considerably more attention than co-utilization of coal and biomass. This has been true for all thermochemical processes. The pricing factor for the raw material and government subsidies often make the use of waste for energy and products more attractive. New concepts of sustainable resource management and enhanced landfill management will make thermochemical conversion of mixed waste more popular. Investigated waste has been (a) municipal solid waste that has had minimal presorting, (b) refuse derived fuel that has had significant pretreatment such as mechanical shredding and screening, and (c) shredded rubber tires, paper and pulp waste, and plastic waste.

Process configuration is more important and more complex for gasification and pyrolysis than combustion because the products are used for down­stream upgrading as well as power generation. Process configuration has many options [6]: (a) gasify both coal and biomass together in the same gas­ifier and have a unified downstream operation, (b) gasify both coal and bio­mass in separate gasifiers and then combine the product gas for a unified downstream operation, (c) have separate syngas cleanup steps but combine syngas coming from coal and biomass at some downstream step, (d) use different forms of feed (dry or liquid) for biomass, or (e) share a common or separate air separation unit to produce oxygen for the biomass and coal gasifiers. All these options lead to numerous process configurations. Ratafia — Brown et al. [6] considered six possible configurations and their advantages and disadvantages. The analysis of Ratafia-Brown et al. [6] is briefly sum­marized below. This analysis will allow others to consider other possible configurations.

Configuration 1: Co-feeding coal and biomass to the gasifier as a mix­ture, either in dry or slurry form.

Configuration 2: Co-feeding biomass and coal to the gasifier using sep­arate gasifier feed systems, either in dry or slurry form.

Configuration 3: Pyrolyzing as-received biomass followed by co-feed­ing pyrolysis char and coal to the gasifier and separately feeding pyrolysis gas to the syngas cleanup system.

Configuration 4: Biomass and coal are co-processed in separate gasifi­ers followed by combined syngas cleanup.

Configuration 5: Biomass and coal are co-processed in separate gasifi­ers followed by separate syngas cleanup trains, and the syngas feeds are combined prior to sulfur and CO2 removal unit operations.

Configuration 6: Same as 4 and 5 but share common air separation unit (ASU) for oxygen feed to the separate gasifiers.

Ratafia-Brown et al. [6] have given an extensive assessment of these six process options. The following paragraphs briefly summarize their assess­ments of each option.

According to Ratafia-Brown et al. [6], the advantages of the first two con­figurations are: (a) they take advantage of economy of scale; gasification can proceed with or without biomass; (b) in Configuration 1, a separate biomass feed system is not required and it reduces gasifier complexity via the use of a single feedstock injection; (c) in Configuration 2, a separate biomass system can be designed without affecting the coal system; and (d) both Configurations 1 and 2 depend on a single syngas processing system. The disadvantages of the first two configurations are: (a) gasifier design should avoid tar production or the syngas system should treat tars, (b) the common gasifier will need to be larger than a single coal processing gasifier because of the low energy content of biomass, (c) control of the syngas composition (H2/CO ratio) for subsequent FT processes may become an issue with a sin­gle gasifier processing both coal and biomass, and (d) the value of biomass ash which contains phosphate and potash for fertilizer purposes will be reduced due to the addition of coal ash.

As shown by Ratafia-Brown et al. [6],the advantages of Configuration 3 are: (a) it is thermodynamically more efficient than off-site biomass process­ing, (b) the pyrolysis gas can be used as a feed to syngas, (c) and the pyroly­sis process can handle biomass with different properties and char can be directly injected to the gasifier with coal, although this will depend on the gasifier type and design and properties of char. The disadvantage of this configuration is that it may not be cost-effective depending on the scale of the gasification facility and level of biomass consumed.

According to Ratafia-Brown et al. [6], the advantages of Configurations 4, 5, and 6 are: (a) they allow maximum flexibility in coal and biomass properties and an independent control of biomass gasification for syngas production, (b) they take advantage of the economy of scale of syngas processing, par­ticularly in Configuration 4, (c) depending on the type of gasifier employed, they allow separate recovery of biomass ash, and (d) for Configurations 5 and 6, independent control of two gasification processes but taking advan­tage of economy of scale for the air separation unit.

The disadvantages of Configurations 4, 5, and 6 are: (a) they do not take care of coal gasification economy of scale; (b) parallel biomass gasification increases investment cost, design, and operational complexity; (c) a tar and particulate removal system may be required depending on the type of bio­mass gasifier used; and (d) more land area and parallel ash handling and storage systems for coal and biomass are required.

Although the above discussion is mainly focused on a mixed coal and bio­mass feedstock, the same thought process can be applied to the different types of mixed feedstock (including different types of biomass) with other variable properties. Earlier in Section 7.3, various process configurations and feed preparation options for biomass with different types of properties as identified by Ratafia-Brown et al. [6] were briefly outlined. These options are also valid for the mixed (coal and biomass, coal and waste, etc.) feedstock.

Currently, the entire energy industry is striving for more alternate, eco­nomical, and environmentally friendly options. The industry is divided into three parts. Old fossil energy relies on the energy sources coming from the ground. The use of this energy source releases carbon to the atmosphere which is becoming more and more unacceptable due to its impact on the environment. Although over the next decade, oil and coal may be replaced more and more by a cleaner natural gas, this form of energy will continue to receive some environmental objections unless the problems of carbon diox­ide and volatile hydrocarbons released to the environment are appropriately handled. Over the next several decades, fossil energy will still be the domi­nant force in the energy industry.

Renewable bioenergy comes from materials that grow on the ground and in its life cycle, it is carbon neutral to the environment. Thus it is more acceptable to the public on environmentally friendly grounds. Bioenergy coming from bio­mass is, however, of limited supply and carries some serious logistic problems of transportation, storage, and pretreatment. Waste (which is predominantly a form of bio or cellulosic energy) is an interesting source for energy and prod­ucts, and it will play an increasingly important role in the energy industry.

The third and final source of energy is green and clean energy such as wind, solar, hydroelectric, hydrothermal, hydrogen, nuclear, and so on which do not release any carbon into the atmosphere. Wind, solar, hydro­electric and hydrothermal energy sources are also renewable, but they are location-dependent and more work is needed to make them economical. At the present time they are largely used for stationary sources of energy. Hydrogen is the purest and most abundant form of energy. Public accep­tance of nuclear energy at this time is uncertain.

The resources available for these three types of energy industries vary greatly among countries and in the future each country will optimize its own local situation. It is clear that the use of mixed feedstock for produc­ing energy has to be an important part of the strategies for future energy industry development. It will allow more flexibility and a greater acceptance by the public on environmental grounds. As summarized in the previous section, more work is needed to make this strategy more practical and attrac­tive. The future for mixed feedstock is, however, bright, and novel ideas will further accelerate its use in the energy industry.

There are those who believe that in the long term, electricity and hydrogen are the solutions to our energy needs. Even if this is true, a mixed feedstock should play an important role in achieving this goal.

In principle, plasma technology process options are the same as those described above for gasification. Because plasma technology is used either for heat and electricity generation as well as for fuels and chemical produc­tion, it is more analogous to the gasification or pyrolysis process than a com­bustion process. The use of a mixed feedstock is very prevalent in plasma technology. A challenging feedstock is automobile rubber tires. They can be pyrolyzed (with or without the use of plasma technology) as is, however, they are often shredded and then converted to useful chemicals by high — temperature pyrolysis. However, just as with switchgrass, the feed prepara­tion for rubber tires can be expensive.

7.4.3 Industrial Processes

There are numerous gasification processes with mixed feedstock currently in operation [1, 6, 114]. We briefly describe three such processes. The descrip­tions of two gasification processes that use an entrained bed gasifier follow the ones outlined in the report by Ratafia-Brown [6].

2.1.2 Introduction

Despite the ever-escalating price for petroleum products and rapidly grow­ing concerns regarding carbon dioxide emissions, the world still remains heavily dependent upon fossil fuels. The 2011 International Energy Outlook [16] predicts by its Reference Case Scenario that the total world consumption of marketed energy will increase by roughly 42% by 2035 from 2010 with an increase in liquid fuels consumption of 30% by 2035 from 2010 in the transportation sector. This increase in demand, however, cannot be met by petroleum alone.

Many commodities, such as biodiesel and bioplastics, are made from the triglycerides found in vegetable oils, and other petrochemicals can also be derived or synthesized using processing by-products including glycerin. Algae, specifically microalgae, are a promising source of oil because, com­pared to other crops, they have fast growth rates, potential for higher yield rates, and the ability to grow in a wide range of conditions [17]. The yield of oil per unit area of algae is at least seven times greater than that of palm oil, the second highest yielding crop [18]. Another benefit of using algae is that they overcome the "food versus fuel" issue of other vegetables and grains because algae is not a universal food crop, and it does not take arable land from other crops. Algae grown on 9.5 million acres, compared to the 450 mil­lion acres used for other crops, could provide enough biodiesel to replace all petroleum transportation fuels in the United States [18]. Based on the cur­rently available technologies, harvesting of algae and the extraction of oil is technologically challenging and energy intensive [17]. There are several dif­ferent methods proposed and developed for extracting oil, an important step in making biofuels as well as bioplastics. Unlike straight vegetable oils (SVOs) and conventional biodiesels based on crop oils, algae fuels are classified as second-generation biofuels.

The lipid containing oil from algae must be separated from the proteins, carbohydrates, and nucleic acids. The steps for extracting the oil involve breaking the cell wall, separating the oil from the remaining biomass, and purifying the oil [19]. Oil can be extracted from algae by either mechanical or chemical methods. The three well-known methods for extraction of oil from algae are expeller pressing (or oil pressing), subcritical solvent extraction, and supercritical extraction [17]. Other methods include enzymatic extrac­tion, osmotic shock, and ultrasonic-assisted extraction [18]. Expeller pressing and ultrasonic-assisted extraction are mechanical processes, and subcritical and supercritical extraction methods are chemical processes. Each of the methods has its advantages and drawbacks. Extraction of oil by expeller pressing is simple and straightforward, but requires the algae to be fully dried, which is energy intensive. Furthermore, the extraction efficiency is not very high, and a substantial amount of unextracted oil is left behind. The benefit of solvent extraction is that the algae do not need to be fully dried, but common subcritical solvents, such as hexane, pose environmental, health, and safety concerns [19]. In addition, although most solvent is recovered and reused, its associated cost is also burdensome. Supercritical fluid extraction may be the most efficient method as it can extract almost all the oil and pro­vide the highest purity inasmuch as supercritical fluids are selective [17]. Furthermore, extraction with supercritical CO2 eliminates the use of harmful solvents. However, its high-pressure operation and required high-pressure equipment increase the overall process cost.

Algae oil has been proposed as both a sustainable and economically fea­sible solution to alternative liquid transportation fuels.

Thermal energy and electricity are the main types of energy used in both types of milling plants. Dry milling corn ethanol plants have traditionally used natural gas as their process fuel for production. The choice of natural gas as a process fuel may turn out to be advantageous costwise due to the sharp decrease in natural gas price in recent years as well as the shale gas boom in the United States. Natural gas is used to generate steam for mash cooking, distillation, and evaporation as well as also being used directly in DGS dryers and thermal oxidizers that destroy the volatile organic com­pounds (VOCs) present in the dryer exhaust [15]. DGS stands for "distillers grain with solubles." Due to increased production efficiencies and expanded fuel capabilities, combined heat and power (CHP) has become increasingly popular as an efficient energy option for many new ethanol plants. CHP is an efficient, clean, and reliable energy services alternative, based on cogen­eration of electricity and thermal energy on site. Therefore, CHP achieves avoiding line losses, increases reliability, and captures much of the thermal energy otherwise normally wasted in power generation to supply steam and other thermal energy needs at the plant site.

A CHP system typically achieves a total system efficiency of 60-80% com­pared to only about 50% for conventional separate generation of electricity and thermal energy [15]. By efficiently providing electricity and thermal energy from the same fuel source at the point of use, CHP significantly reduces the total fuel usage for a commercial ethanol plant, along with reductions in corresponding emissions of carbon dioxide (CO2) and other pollutants. Generally speaking, electrical energy is used mostly for grinding and drying corn, whereas thermal energy is used for fermentation, ethanol recovery, and dehydration. On the other hand, flue gas is used for drying and stillage processing as part of waste heat recovery and energy integra­tion efforts. The carbon dioxide generated from the fermentation process is also recovered and utilized to make carbonated beverages as well as to aid in the manufacture of dry ice as a by-product of the ethanol process. As mentioned earlier, based on the 2008 survey of 150 dry milling corn etha­nol plants in the United States [14], 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 was 28.2 and 32.1% lower than the 2001 values of 36,000 BTU and 1.09 kWh, respectively. Ethanol productivity per bushel of corn also increased by 5.3% from 2.64 gallons in 2001 to 2.78 gallons per bushel in 2008 [12, 14]. It was also found that on average 5.3 pounds of dried distillers grains and 2.15 pounds of wet distillers grains (WDGs) as well as 0.06 gallons of corn oil per every gallon of ethanol are also produced as process coproducts. One U. S. bushel as a volume unit is equivalent to 35.23907 liters. Even though the U. S. corn ethanol industry has been considered a mature industry, the recent enhancements made on their process and energy efficiencies as well as the overall profitability are quite remarkable.

A typical autohydrolysis process [48] uses compressed liquid hot water at a temperature of about 200°C under a pressure that is higher than the saturation pressure, thus keeping the hot water in liquid phase, to hydro­lyze hemicellulose in minutes. Hemicellulose recovery is usually high, and unlike the acid-catalyzed process, no catalyst is needed. The process is represented as shown in Figure 4.8. Very high temperature processes may lead to significant pyrolysis, which produces inhibitory compounds. The ratio of the rate of hemicellulose hydrolysis to that of sugar degra­dation (more pyrolytic in nature) is greater at higher temperatures. Low-temperature processes have lower xylose yields and produce more degradation products than well-controlled, high-temperature processes that use small particles.

According to a study by Dekker and Wallis [49], pretreatment of bagasse by autohydrolysis at 200°C for 4 min and explosive defibration resulted in a 90% solubilization of the hemicellulose (a heteroxylan) and in the pro­duction of a pulp that was highly susceptible to hydrolysis by cellulases from Trichoderma reesei. Saccharification yields were 50% after 24 hours at 50°C (pH 5.0) in enzymatic digests containing 10% (w/v) bagasse pulps and 20 filter paper cellulase units (FPU), and their saccharification yield could be increased to 80% at 24 hours by the addition of exogenous p-glucosidase from Aspergillus niger.

In general, xylose yields in autohydrolysis are low (30-50%). An autohy­drolysis system is used as the pretreatment in separate hydrolysis and fer­mentation (SHF). The reaction conditions are 200°C for 10 minutes, with a xylose yield of 35%.

Steam consumption in autohydrolysis is strongly dependent upon the moisture content of the starting material. Wet feedstock requires consid­erably more energy because of the high heat capacity of retained water. An important advantage of autohydrolysis is that it breaks the lignin into relatively small fragments that can be easily solubilized in either base or organic solvents.

The steam explosion process [50] was first developed in 1925 for hardboard production and more recently was applied on aspen wood in the early 1980s. In a typical steam explosion process, cellulosic material is heated using high — pressure steam (20-50 atm, 210-290°C) for a short period (seconds to min­utes). At the increased pretreatment pressure, water molecules diffuse into the inner microporous structure of the lignocellulose [47]. In this process, some steam condenses under high pressure, thereby wetting the material. The wetted material is then driven out of a reactor (i. e., ejected from a reac­tor) through a small nozzle by a pressure difference. Due to a rapid decrease in the pressure, the material is ejected through the discharge valve. The term "explosion" is used due to the process characteristics of ejection driven by a sudden large pressure drop of steam.

Tar is neither a chemical name for certain molecular species, nor a clearly defined terminology in materials. Tar has been operationally defined in gas­ification work as the material in the product stream that is condensable in the gasifier or in downstream processing steps or subsequent conversion devices and parts [45]. This physical definition is inevitably dependent upon the types of processes, the nature of the feedstock, and specifically appli­cable treatment conditions. Producer gases from both biomass gasification and coal gasification contain tars. The generalized composition of tars is mostly aromatic and the average molecular weight is fairly high. Even with the very same biomass feed, the amount and nature of tars formed are dif­ferent depending upon the gasifiers used and process conditions employed. Similarly, the same gasifier would generate different amounts and types of tars depending upon the feedstock properties and compositions. Therefore, successful implementation of efficient gasification technology depends on the effective control of tar formation reactions as well as the efficient removal or conversion of tar from the produced gas.

A number of investigators studied various aspects of tar in terms of its for­mation, maturation scheme, properties, molecular species, and relationship

between the tar yield and the reaction temperature. Elliott extensively reviewed the composition of biomass pyrolysis and gasifier tars from vari­ous gasification processes and proposed a tar maturation scheme [46], as shown in Figure 5.2.

Nickel-based catalysts are known to be effective in biomass gasification for tar reduction to produce synthesis gases, because of their relatively lower cost and good catalytic effects. Several different types of nickel-based cata­lysts for biomass gasification were reviewed with respect to tar reduction efficiency by Wu and Williams [47].

Several methods for the sampling and analysis of tar have been devel­oped. Most of these methods are based on the condensation of tar in a liquid phase or adsorption of tar on a solid material. The collected sam­ples are subsequently analyzed gravimetrically or by using a gas chro­matograph (GC). The SPA (solid-phase absorption) method was originally developed by KTH, Sweden, and according to the SPA method a gas sam­ple is passed through an amino-sorbent which collects all tar compounds [48]. The ensuing step is to use different solvents to collect aromatic and phenolic compounds separately. These compounds are then analyzed on a gas chromatograph and positive identification of the condensed mate­rial is achieved by a gas chromatograph-mass spectrometer (GC-MS). The tar amount determined by the GC analysis is called GC-detectable tar. Tar can also be analyzed gravimetrically and the gravimetrically determined tar is called gravimetric tar. Gravimetric tar is evaporation/distillation residue from particle-free solution(s) determined by gravimetric analy­sis. Both GC-detectable and gravimetric tar are reported in mg/m3. Both chromatographic and gravimetric determination of tar is based on the European Technical Specification, TC BT/TF 143 WI CSC 03002.4: 2004 (E), developed by Technical Committee CEN/BT/TF 143 "Measurement of Organic Contaminants (Tar) in Biomass Producer Gas" [49]. This technical specification is applicable to sampling and analysis of tars and particles in the concentration range between 1 mg/m3 to 300 g/m3 at all relevant sampling port conditions (0-900°C and 0.6-60 bars). The application of the

technical specification allows determination of four different analytical values [49]:

• The concentration of gravimetric tars in mg/m3

• The sum of the concentrations GC-detectable tars in mg/m3

• The concentration of individual organic compounds in mg/m3

• The concentration of particles in mg/m3

Over the last several decades most investigations on mixed feedstock were carried out for gasification technologies. This is because gasification technol­ogy in its different formats (i. e., combustion, pyrolysis, gasification, plasma technology, supercritical gasification, etc.) is very versatile and capable of generating heat and electricity as well as fuels, chemicals, and materials.

7.1.1 Literature Studies

Numerous outstanding reviews [43-63] have addressed the laboratory studies carried out on combustion, gasification, high-severity pyrolysis, plasma technology, and supercritical gasification for mixed feedstock. Sami, Annamalai, and Wooldridge [7] and Maciejewska et al. [1] have outlined a detailed review of combustion of mixed feedstock carried out at laboratory and demonstration scales. Similarly, Ricketts et al. [5] and Davidson [63] have presented outstanding reviews of the gasification, plasma technology, and pyrolysis of mixed feedstock. Some of recent gasification studies for mixed feedstock are summarized in Table 7.8. A summary of the technology devel­opers in western Europe and the United States for gasification, pyrolysis, and plasma is given by Ricketts et al. [5].

The literature studies for mixed feedstock indicate that there seems to be very little or no synergistic effects on gasification kinetics between biomass and coal. The situation is different when polymeric waste is added into the mix. The compositions of the gases produced from this mixture are not sim­ple additions of individual behaviors. More systematic work is needed to understand the nature of the synergy that may exist in polymeric and other

TABLE 7.8

Typical Studies of Gasification of Mixed Feedstock

(Continued)

kinds of feedstock. The basic issues, as described in this section, for mixed feedstock gasification are the effects of mixture on the nature of the gases and solids produced as well as on the reactor and the downstream opera­tions. These issues are extensively addressed in the literature, and they are detailed in the remaining part of this section.

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Sunggyu Lee

Preface

Humans have a long history of using a wide variety of biomass resources as sources of energy and fuel. The discovery and use of fossil energy, repre­sented largely by coal, natural gas, and petroleum, have drastically reduced the utilization of biomass fuels. Technologies for generating electricity using biomass, producing bioliquid fuels, and powering motor vehicles using bio­alcohols and blended gasolines have been developed and practiced since the early twentieth century. Up until recently, however, development interest in biofuels had lessened due to the availability of relatively inexpensive fossil energy resources as well as the handling and transportation convenience of these conventional fuel sources.

Due to the strong growth of global transportation fuel demand, sharply escalating worldwide fossil energy prices, fear over the dwindling supply of petroleum and natural gas for the near future, and credible evidence link­ing global warming and climate change issues with the emission of green­house gases, global interest and R&D efforts in renewable alternative fuels have become intense and fiercely competitive, targeting both short — and long-term solutions to alternative energy needs. Although there are a num­ber of options and routes for energy sustainability and independence via renewable alternative energy, bioenergy and biofuels certainly possess out­standing potential to provide solutions and relief to many of the immediate, intermediate, and long-term societal needs of clean energy and their associ­ated challenges. Bioenergy and biofuels are quite broadly defined, includ­ing fuels derived from biological resources such as agricultural and forestry products, biomass and biomass-derived energy, fuels and fuel precursors secondarily derived from the primary biofuels and their by-products, fuels and energy derived from biological activities and processes, biodiesel from microalgae, and much more.

Accordingly, the resources for biofuels are ultimately renewable and totally independent of fossil energy availability and its distribution pattern. Biofuels are still, in some sense, similar to fossil fuels in that: (a) both are carbon energy sources, (b) direct combustion generates carbon dioxide emis­sion, (c) both can be used as gas, liquid, and solid fuels, (d) both can be used for heat and power generation, and (e) both can be used as transportation fuels for conventional and futuristic motors. However, biofuels are drasti­cally different from fossil energy in that biofuel resources are renewable, biomass is distributed worldwide, biomass typically contains much less sulfur, biofuels can also originate from nonmanufacturing and nonmining industrial sectors such as agriculture and forestry, and carbon dioxide gen­erated by combustion of biofuels essentially originated from carbon dioxide removed from the atmosphere by plants through photosynthesis.

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Because plant material in nature utilizes sunlight and carbon dioxide to produce energy and hydrocarbons, the combustion of biofuel derived from biomass by itself does not contribute to a net increase of carbon dioxide in the atmosphere. When biofuel is processed in conjunction with other car­bon-neutral energies such as solar, wind, and hydrothermal energy, biofu­els could be made available as a nearly carbon-neutral fuel. Furthermore, if biomass feedstock is chosen from abundantly available but nonfood crops whose growth does not require arable land, both sustainability and renew — ability would be warranted. Biofuel process R&D has directly benefited from the technological and scientific advances in Q-chemistry, coal science and technology, petroleum science and technology, natural gas science and engi­neering, hydrocarbon processing, environmental science and engineering, separation science and technology, heat and mass transfer, catalysis and reactor engineering, materials science and engineering, biochemistry and biotechnology, agriculture, food science, and more. For example, the pro­cesses for fast pyrolysis, gasification, and liquefaction of biomass are quite similar to those developed and adopted for coal and oil shale processing. Gas cleaning and upgrading of bioliquid fuels also share common routes in conventional fuel processing with, in particular, those of natural gas and petroleum, with modifications and adaptations. Ingenious energy integra­tion schemes are also receiving technological enhancements and know-how support from the well-practiced petroleum, petrochemical, and power gen­eration industries. Biodiesel and bioethanol technologies have also benefited from the lessons-learned approaches of internal combustion engine develop­ers and fuel engineers. Needless to say, the bioethanol industry is a direct and successful example of advances made in biotechnology, separation science and technology, agriculture, and animal food science. As such, the subject of biofuels cannot be treated as a new and independent stand-alone discipline, but rather as a multidisciplinary subject that has its foundation in C1-chemistry, petroleum and coal science, natural gas engineering, envi­ronmental science and technology, process engineering and design, separa­tion science and technology, biotechnology and biological engineering, heat and mass transfer, reactor engineering, catalysis and enzymology, energy management, public policies, and more. Therefore, this book is written as a comprehensive source book on the subject area covering all of the aforemen­tioned subjects of relevance from the standpoints of fuel process engineers and fuel scientists as well as from the viewpoints of energy technologists.

This book is intended to provide the most comprehensive background in the science and technology of biofuels and bioenergy, including the most up-to-date and in-depth coverage of definitions and classifications of biofu­els and related matters, characterization and analysis of biofuels, primary processing technologies of biofuel resources, secondary processing technol­ogies of biofuels and biofuel precursors, upgrading of crude biofuels, issues involving the ethanol economy and the hydrogen economy, chemistry of process conversion, process engineering and design of biofuel production

and associated environmental technologies, combined cycle processes, coprocessing of biomass with other fuels, energy balances and energy effi­ciencies, reactor designs and process configurations, energy materials and process equipment, commercial biofuel processes and significant practices, energy integration strategies and schemes, flowsheet analysis, relation to and integration with other conventional fossil fuel processes, by-product uti­lization, process economics of biofuel technology, environmental and eco­logical impacts and benefits, sustainability issues, governmental regulations and policies, global bioenergy trend and outlook, and much more.

Chapter 1 focuses on the introductory subjects of the book including the definition of biofuels, global energy outlook in reference to bioenergy and biofuels, issues of sustainability and carbon neutrality, generalized view of biomass feedstock and its availability, brief overview of techno­logical trends of biofuel and bioenergy processes, and a general discus­sion of environment and ecology in relation to the utilization of bioenergy and biofuels.

Chapter 2 provides scientific and technological details of crop oils, algae fuels, and biodiesel. Vegetable oils, or plant oils, and their utilization as straight biofuels and biofuel feedstock are discussed in depth, including characteriza­tion, extraction, purification, and chemical conversion to biodiesel via trans­esterification. Also presented are algae biofuel and its technological details involving harvesting, extraction, and chemical conversion with detailed descriptions of the proposed or practiced technologies and their flowsheets.

Chapter 3 deals with the science and technology of producing bioetha­nol from starch crops, specifically corn. The fuel properties of ethanol as an oxygenated fuel for the internal combustion engine and as a blend fuel for conventional gasoline are presented. Particulars of corn ethanol process technologies, both dry milling and wet milling, are explained in necessary detail. Socioeconomic issues of "food versus oil" for corn ethanol produc­tion as well as technoeconomic issues of net energy balance of corn ethanol production are also discussed based on the literature data and recently pub­lished information. Also mentioned is the beneficial and profitable utiliza­tion of process by-products and coproducts in a variety of industrial sectors.

Chapter 4 is devoted to the production of ethanol from lignocellulose. An historical perspective of alcohol fermentation is presented along with its tech­nological evolution and trend. Essential scientific and technological details of cellulose, hemicellulose, and lignin are provided and a variety of enzymes that are found and developed for processing these components are also dis­cussed. Processing steps and options for lignocellulosic alcohol production involving prehydrolysis and pretreatment, hydrolysis and enzymatic treat­ment, and fermentation are elucidated. Particular attention is also paid to the fermentation of xylose, C5-sugar, and to the co-fermentation of xylose with glucose, C6-sugar, using genetically engineered micro-organisms. Also included in the chapter is the beneficial utilization of lignin in areas other than cellulosic ethanol production.

Chapter 5 is mainly focused on the thermochemical conversion of bio­mass into an assortment of gaseous products (biomass syngas), bioliquid (bio-oil), and solid fuel (biochar). Details of process technologies devel­oped or proposed for fast pyrolysis of biomass as well as for gasification of biomass are presented with explanations of the merits and poten­tial limitations associated with the specific technological steps. Process parameters, operating conditions, product spectra and properties, and pertinent reactor design issues are also discussed. Process options for fur­ther upgrading of crude products of thermochemical intermediates are also examined.

Chapter 6 provides an in-depth overview of the conversion of wastes to biofuels, bioproducts, and bioenergy. More specifically, the chapter deals with strategies for waste management, methods for waste preparation and pretreatment, an extensive list of waste-to-energy conversion technologies, socioeconomic and environmental issues of waste conversion, and the future of the waste management and conversion industry. Conversion technologies covered in the chapter include incineration, gasification, pyrolysis, plasma technology, supercritical processing, transesterification, anaerobic digestion, fermentation, and product upgrading.

Chapter 7 discusses various thermochemical technologies for processing a mixed feedstock such as a mixture of coal and biomass. The discussions are divided into two principal parts: (a) the technologies that are mainly gener­ating gases such as combustion, gasification, and high-severity pyrolysis or plasma technology, and (b) those that are for generating liquids such as low — severity pyrolysis, liquefaction, and supercritical process. The chapter also reviews essential topics of reactor configurations and associated technolo­gies, handling of product streams, process configurations, and examples of commercial processes, while discussing various technoeconomic benefits of the technology including early expansion of bioenergy utilization, mitigated environmental concerns, and enhanced thermal and economical efficiencies.

Even though this book covers in-depth knowledge on the subject of inter­disciplinary biofuels and bioenergy, it is written for readers with college — level backgrounds in chemistry, biology, physics, and engineering. This book can be used ideally as a textbook for a three-credit-hour semester course in bioenergy or biofuels. If used as a textbook, two weeks each may be allocated for each chapter of this book, with one final week opted for open-ended dis­cussions on regionally important topics or contemporary or arising issues related to the subject area. This book can also be adopted as a textbook or reference book for upper-level undergraduate or graduate-level courses in the fields of energy and fuels, renewable energy, alternative fuels, and fuel science and engineering.

This book will also serve as an excellent desk reference book for profes­sionals who are engaged in renewable bioenergy — and biofuel-related indus­tries as well as environmental engineering fields. Furthermore, this book will serve as a single source encompassing the most comprehensive and

up-to-date scientific and technological information for researchers in the fields of biofuels as well as alternative and renewable energies.

Finally, the authors wish to acknowledge and thank their families, former and current graduate students, friends, and colleagues for their support and assistance in completing this book project.

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