Traditionally, torrefaction is a thermal process for roasting biomass operated at 200° to 300° and for a relatively long residence time (30 to 60 minutes) under an inert atmospheric condition. The name torrefaction is adapted from the process used to roast coffee beans, which is performed at lower temperatures

Summary of Advantages and Disadvantages of Various Biomass Pretreatments

Source: Modified from Shah and Gardner, in press. Biomass Torrefaction: Applications in Renewable Energy and Fuels. In Encylopedia of Chemical Processes, Boca Raton, FL: CRC Press.

and in the presence of air. Nevertheless, the important mechanical effect of torrefaction on biomass is similar to the effect on coffee beans. In the open literature, torrefaction is also referred to as roasting, slow-and-mild pyroly­sis, wood cooking, and high-temperature drying [18-25]. The drying and grinding of biomass is not as easy as torrefaction and grinding due to the physical nature of biomass.

The process of torrefaction dehydrates and depolymerizes the long poly­saccharide chains of biomass. This results in a product that is hydrophobic and has a higher energy density and improved grinding and combusting capabilities [26-32]. This process is best illustrated through the Van Krevelen plot shown in Figure 7.1 [18-24]. The figure illustrates that torrefaction results in the reduction of oxygen content and increased heating value of the bio­mass. Generally, during torrefaction an increase in both mass and energy density occurs because about 30% (by weight) of the biomass is transformed

TABLE 7.7

Aspects of Torrefied Biomass for Gasification and Other Applications

Torrefied Product

Has lower moisture content and higher heating value Is easy to store and transport

Is hydrophobic, does not gain humidity in storage and transportation Is less susceptible to fungal attack Is easy to burn, forms less smoke and ignites faster Significantly conserves the chemical energy in biomass

Has heating value (11,000 BTU/lb) that compares well with coal (12,000 BTU/lb)

Generates electricity with a similar efficiency to that of coal (35% fuel to electricity) and considerably higher than that of untreated biomass (23% fuel to electricity)

Has grindability similar to that of coal

Requires grinding energy 7.5 to 15 times less than that for untreated biomass for the same particle size

Has mill capacity 2 to 6.5 times higher compare to untreated biomass Possesses better fluidization properties in the gasifiers

Is suitable for various applications in heating, fuel, steel and new materials manufacturing industries

Source: Bergman and Kiel, 2005. Torrefaction for biomass upgrading. Proceedings of the Fourteenth European Biomass Conference and Exhibition," Paris, October, pp. 17-21. Bergman al. 2004. Torrefaction for entrained flow gasification of biomass. In: W. P.M. Van Swaaij, T. Fjallstrom, P. T. Helm, and P. Grassi (Eds.), Proceedings of the Second World Biomass Conference on Biomass for Energy, Industry, and Climate Protection, Rome, Italy, May 10-14, pp. 679-682, Energy Research Centre of the Netherlands (ECN), Petten, The Netherlands, Report No. ECN-RX—04-046; Bergman. 2005. Combined Torrefaction and Pelletization: The TOP Process. Energy Research Centre of the Netherlands (ECN), Petten, The Netherlands, Report No. ECN-C-05-073; and Bergman et al., 2004. Torrefaction for Entrained Flow Gasification of Biomass. Energy Research Centre of the Netherlands (ECN), Petten, the Netherlands, Report No. ECN-C-05-067.

into volatile gases. These gases carry only 10% of original biomass energy content [29, 30, 33-35]. This implies that during torrefaction, a substantial amount of chemical energy is transformed from the raw material to the product resulting in the enhanced fuel properties of the torrefied biomass. Mild pyrolysis of biomass results in gases such as H2,CO, CO2, CH4,CxHy; the liquids such as toluene, benzene, H2O, sugars, polysugars, acids, alcohol, furans, ketones, terpenes, phenols, fatty acids, waxes, and tannins and solids that contain char and ash.

As shown in Table 7.7, torrefied biomass possesses very valuable properties. It has lower moisture content and therefore higher heating value compared to untreated biomass. The storage and transportation capabilities of torrefied biomass are superior to those of untreated or only dried biomass. Torrefied biomass is hydrophobic and does not gain humidity in storage and trans­portation. It shows little water uptake on immersion (7-20% of mass) and is more stable and more resistant to fungal attack compared to charcoal and

an untreated biomass. Pelletization, by itself, produces biomass with higher mass density; however, the pellets are not hydrophobic and are susceptible to fungal attack. Torrefied biomass significantly conserves the chemical energy present in the biomass. The heating value of torrefied wood is approximately 11,000 BTU/lb and is nearly equal to that of a high volatile bituminous coal which is 12,000 BTU/lb. It generates electricity with an efficiency comparable to that of coal of approximately 35%, on a fuel to electricity basis [36, 37], and much higher than that of untreated biomass which has an efficiency of 23%, on a fuel to electricity basis [36, 37]. Bergmann et al. showed that torrefied biomass has better fluidization properties than that of untreated biomass, but similar to that of coal [38-41].

Untreated biomass requires many times the grinding energy (by a factor of 7.5 to 15) to achieve a similar particle size compared to torrefied biomass. This energy difference is significantly larger than the energy loss of biomass and energy supplied during torrefaction. The grindability of torrefied bio­mass versus that of untreated biomass is compared in Figure 7.2. The mill capacity of the torrefied biomass can also be as high as 6.5 times that of the untreated one. Finally, torrefied biomass is suitable for various applications such as working fuel, residential heating, new materials for the manufacture of fuel pellets, reducer in the steel smelting industry, the manufacture of charcoal and active carbon and gasification, and co-firing with other fuels in gasifiers, boilers, and so on [38-41]. Such wide usefulness makes torrefied biomass a valuable and marketable product.

Ratafia-Brown et al. [6] identified various feed preparation techniques needed for an entrained bed gasifier based on the nature of biomass and the nature of the feeding mechanism. For wet feeding, they proposed a pyrolysis process that produces a bioslurry which can be fed into the entrained bed gasifier by a feeder or an injector. This technique is analogous to the coal slurry feed used in the GE gasifier, and it is most appropriate for strawlike crops. For dry feeding they proposed three alternatives. For woody biomass, the feed can be milled (and cut) and broken down to size of 1 mm particles and fed to the entrained bed gasifier by either a screw feeder or a piston compressor. The process of milling can also be preceded by torrefaction. The torrefied biomass in this case can be fed to the gasifier by a pneumatic feeder, a screw feeder, or a piston compressor. For all other types of biomass they presented two options. One option is to follow the path of torrefaction, mill­ing, and feeding just like the one for woody biomass. The other is to gasify the biomass in a pressurized fluidized bed to generate gas product and char and feed these materials to the entrained bed gasifier for the production of biosyngas.

Algal oil can be extracted from microalgae using an effective chemical sol­vent [35]. Hexane, cyclohexane, benzene, ether, acetone, and chloroform have proven to be effective in oil extraction of microalgae paste [36]. Among these, hexane has long been used as an oil extraction solvent in the food industry and is relatively inexpensive. A chemical method is usually faster in terms of extraction speed and requires a lower energy input for the extraction process itself. One of the drawbacks of using chemical solvents for algae oil extrac­tion is the safety issues involved in working with the chemicals. Care must be taken to avoid exposure to chemical vapors and direct contact with the chemical, either of which can cause serious personal injury. Benzene is clas­sified as a carcinogen, and most ethers are highly flammable. Another disad­vantage of using a chemical is the additional cost of recovery of the chemical for reuse in the process.

In a broader classification of algae oil extraction technology, chemical extraction methods include: (1) hexane solvent method, (2) Soxhlet extrac­tion, and (3) supercritical fluid extraction.

For the past two centuries in the United States, corn refiners have been devel­oping, improving, and perfecting the process of separating corn into its com­ponent parts to create a variety of value-added corn products and by-products. The corn wet milling process separates corn into its four basic components, viz., starch, germ, fiber, and protein. There are eight basic steps involved to accom­plish this corn refining and alcohol fermentation process [20].

1. First, the incoming corn is visually inspected and cleaned. Corn refin­ers use #2 yellow dent corn, which is removed from the cob during harvesting. One bushel of yellow dent corn weighs about 56 pounds on average. Refinery people inspect arriving corn shipments and clean them two or three times to remove cob, dust, chaff, and any other foreign unwanted materials before the next processing stage of steeping. An effective screening process can save a great deal of trouble in the subsequent stages. The inspected and screened corn is then conveyed to storage silos holding up to 350,000 bushels.

2.

Second, it is steeped to initiate polymeric bond cleavage of starch and protein into simpler molecules. Steeping is typically carried out in a series of stainless steel tanks. Each steep (or steeping) tank may hold about 2,000-13,000 bushels of corn soaked in water at 50-52°C for 28-48 hours. During steeping, the kernels (as shown in Figure 3.5) absorb water, thereby increasing their moisture levels from 15% to 45% by weight and also more than doubling in size by swelling [20]. The addition of 0.1% sulfur dioxide (SO2) to the water suppresses excessive bacterial growth in the warm water environment. As the corn swells and softens, the mild acidity of the steeping water begins to loosen the gluten bonds within the corn and eventually release the starch [20]. A bushel is a unit of volume measure used as a dry measure of grains and produce. A bushel of corn or milo weighs about 56 pounds, a bushel of wheat or soybeans weighs about 60

FIGURE 3.5

Corn kernel.

pounds, and a bushel of sunflowers weighs about 25 pounds. Or, a U. S. bushel is equivalent to 35.23907 liters as a volume unit.

3. The third step is the germ separation. It starts with coarse grind­ing of the corn in the slurry to separate/break the germ from the rest of the kernel. The germ is the embryo of a kernel of grain, as shown in Figure 3.5. This germ separation is accomplished in cyclone separators, which spin the low-density corn germ out of the slurry. Therefore, this cyclone separator is called a germ separa­tor. It is also called a degerminating mill. The germs, which contain about 85% of corn’s oil, are pumped onto screens and washed repeat­edly to remove any starch left in the mixture [20]. A combination of mechanical and solvent processes extracts the oil from the germ. The oil is then refined and filtered into finished corn oil. The germ resi­due is saved as another useful component of animal feed. Both corn oil and germ residues are important by-products of this process.

4. As the fourth step, the remaining slurry, consisting of fiber, starch, and protein, is finely ground and screened to separate the fiber from the starch and protein. After the germ separation step described in Step 3, corn and water slurry goes through a more thorough grind­ing in an impact or attrition-impact mill to release the starch and gluten from the fiber in the kernel. The suspension of starch, glu­ten, and fiber flows over fixed concave screens, which catch fiber but allow starch and gluten to pass through. The fiber is collected, slur­ried, and screened again to reclaim any residual starch or protein, then piped or sent to the feed house as a major ingredient of animal feed. The starch-gluten suspension, called mill starch, is piped or sent to the starch separators [20].

5. Fifth, starch is separated from the remaining slurry in hydrocyclones. By centrifuging mill starch, the gluten is readily spun out due to the density difference between starch and gluten. Starch is denser than gluten. Separated gluten, a type of protein composite, can be used for animal feed. Corn gluten meal (CGM) is a by-product of corn processing and is used as animal feed. CGM can also be used as an organic herbicide. The starch, now with just 1-2% protein remain­ing, is diluted, washed 8 to 14 times, rediluted and rewashed in hydrocyclones to remove the last trace of protein and produce high — quality starch, typically more than 99.5% pure. Some of the starch is dried and marketed as unmodified cornstarch, another portion is modified into specialty starches, but most is converted into corn syrups and dextrose [20]. Cornstarch has a variety of industrial and domestic uses. All these are important by-products of the process that contribute to the corn distillers’ profitability.

6. Sixth, the cornstarch then is converted to syrup (corn syrup) and this stage is called the starch conversion or starch-to-sugar conversion step. The starch-water suspension is liquefied in the presence of acid or enzymes. Enzymes help convert the starch to dextrose that is soluble in water as an aqueous solution. Treatment with another enzyme is usually carried out, depending upon the desired process outcome. The process of acid and enzyme reactions can be stopped or terminated at key points throughout the process to produce a proper mixture of sugars such as dextrose (a monosaccharide, C6H12O6) and maltose (a disaccharide, C12H22O11) for syrups to meet desired specifications [20]. For example, in some cases, the conver­sion of starch to sugars can be halted at an early stage to produce low-to-medium sweetness syrups. In other situations, however, the starch conversion process is allowed to proceed until the syrup becomes nearly all dextrose. After this conversion process, the syrup is then refined in filters, centrifuges, or ion-exchange columns, and excess water is evaporated to result in concentrated syrup. Syrup can be sold directly as is, crystallized into pure dextrose, or processed further to produce high-fructose corn syrup (HFCS). Across the corn wet milling industry, about 80% of starch slurry goes to corn syrup, sugar, and alcohol fermentation.

7. Seventh, the concentrated syrups can be made into several other products through a fermentation process. Dextrose is one of the most fermentable forms of all of the sugars. Dextrose is also called corn sugar and grape sugar, and dextrose is a naturally occurring form of glucose, that is, D-glucose. Dextrose is better known today as glucose. Following the conversion of starch to dextrose, dextrose is piped and sent to fermentation reactors/units/facilities where dextrose is con­verted to ethanol by traditional yeast fermentation. Using a continuous process, the fermenting mash is allowed to flow, or cascade, through several fermenters in series until the mash is fully fermented and then leaves the final tank. In a batch fermentation process, the mash stays in one fermenter for about 48 hours before the distillation process for alcohol purification is initiated. Generally speaking, a continuous mode is more effective with a higher fermenter throughput, whereas higher-quality product may be obtained from a batch mode.

8. As the eighth step, ethanol separation or purification follows the fermentation step. The resulting broth is distilled to recover etha­nol or concentrated through membrane separation to produce other by-products. Carbon dioxide generated from fermentation is recap­tured for sale as dry ice and nutrients still remaining in the broth after fermentation are used as components of animal feed ingredi­ents. These by-products also contribute significantly to the overall economics of the corn refineries.

Even though the term "by-product" was used throughout the process description, "coproduct" may be a better term in corn refining technol­ogy, inasmuch as these products are not only valuable but also targeted in the master plan of corn distillers. The corn-to-alcohol process detailed above can be summarized in a schematic process diagram, as shown in Figure 3.6.

The operating cost of the SSF process is generally lower than that of SHF as long as the process integration is synergistically done. As the name of the process implies, both the hydrolysis and fermentation are carried out in the same vessel. In this process, yeast ferments the glucose to ethanol as soon as the glucose is produced, thus preventing the sugars from accumulating and causing end-product inhibition. Using the yeast, Candida brassicae, and the Genencor enzyme (by Genencor International), the yield is increased to 79% and the ethanol concentration produced is 3.7% [43, 77].

Even in SSF, cellobiose (the soluble disaccharide sugar) inhibition occurs to an appreciable extent. The enzyme loading for SSF is only 7 IU/g of cellulose, compared to 33 IU/g in SHF. The cost of energy and feedstock is somewhat reduced because of the improved yield, and the increased ethanol concen­tration significantly reduces the cost of distillation and utilities. The cost of the SSF process is slightly less than the combined cost of hydrolysis and fer­mentation in the SHF process. The decreasing factor of the reactor volume due to the higher concentration of ethanol offsets the increasing factor in the reactor size caused by the longer reaction times (seven days for SSF vs. two days for hydrolysis and two days for fermentation). Earlier studies showed that fermentation is the rate-controlling step and the enzymatic hydrolysis process is not. With recent advances and developments in recombinant yeast strains that are capable of effectively fermenting both glucose and xylose, these process-configurational considerations for commercial exploitation, as well as the determination of a rate-limiting step for the overall process tech­nology, may have to change accordingly.

The hydrolysis is carried out at 37°C and increasing the temperature increases the reaction rate; however, the ceiling temperature is usually limited by the yeast cell viability. The concentration of ethanol is also a limiting fac­tor. (This was tested by connecting a flash unit to the SSF reactor and remov­ing the ethanol periodically. This technique showed productivities up to 44% higher.) Recycling the residual solids may also increase the process yield. However, the most important limitation in enzyme recycling comes from the presence of lignin, which is inert to the enzyme. High recycling rates increase the fraction of lignin present in the reactor and cause handling difficulties.

Two major types of enzyme recycling schemes have been proposed: one in which enzymes are recovered in the liquid phase and the other in which enzymes are recovered by recycling unreacted solids [43]. Systems of the first type have been proposed for SHF processes that operate at 50°C. These systems are favored at such a high temperature because increasing tempera­ture increases the proportion of enzyme that remains in the liquid phase. Conversely, as the temperature is decreased, the amount of enzyme adsorbed on the solid increases. Therefore, at lower temperatures encountered in SSF processes, solids recycling becomes a more effective option.

4.5.3.4.1 Comparison between the SSF and SHF Processes

SSF systems offer large advantages over SHF processes thanks to their reduc­tion in end-product inhibition of the cellulase enzyme complex. The SSF pro­cess shows a higher yield than SHF (88% vs. 73% in an earlier example) and greatly increases product concentrations (equivalent glucose concentration of 10% vs. 4.4%). The most significant advantage of the SSF process is the enzyme loading, which can be reduced from 33 to 7 IU/g cellulose; this cuts down the cost of ethanol appreciably. With constant development of low-cost enzymes, the comparative analysis of the two processes will inevitably be changing. A comparative study of the approximate costs of the two processes was reported in Wright’s 1988 article [43]. The results show that, based on the estimated ethanol selling price from a production capacity of 25,000,000 gallons/year, SSF is found to be more cost-effective than SHF by a factor of 1:1.49; that is, $SHP/$SSP = 1.49. It should be clearly noted that the number quoted here is the ratio of the two prices, not the direct dollar value of the ethanol selling price. Furthermore, this study was also based on the enzymes and bioconversion technologies available in the mid-1980s, which are significantly different from the most advanced current technologies of the twenty-first century. However, this ethanol production cost comparison between two different process con­figurations provides an idea about the complexity of interrelated cost factors among the reaction rates, temperature, processing time, enzyme adsorption, enzyme loading and recoverability, product inhibition, and more.

For the very same process economic reasons, it is anticipated that a hybrid hydrolysis and fermentation (HHF) process configuration is going to be widely accepted as a process of choice for production of lignocellulosic fuel ethanol, which begins with a separate prehydrolysis step and ends with a simultaneous saccharification (hydrolysis) and fermentation step. In the first stage of hydrolysis, higher-temperature enzymatic cellular saccharification is taking place, whereas in the second stage of SSF, mesophilic (moderate- temperature) enzymatic hydrolysis and biomass sugar fermentation are tak­ing place simultaneously. The optimized process configurational scheme would have to change if a specific enzyme, proven to be highly efficient and cost-effective, is found to be intolerant against certain inhibitors that are associated with any of these processing steps.

About 90% of incinerators treating MSW use grate incinerators because of their simplicity and ability to handle a wide range of waste particle sizes. Grate incinerators are also used for commercial and industrial nonhazard­ous waste. This type of incinerator contains (a) a waste feeder unit, (b) incin­erator grate on which waste materials are placed, (c) incinerator chamber, (d) incinerator air duct system, (e) auxiliary burner, and (f) bottom ash dis­charger [13].

6.5.1.1 Rotary Kilns

The rotary kiln consists of a cylindrical vessel slightly inclined on its hori­zontal axis. The vessel is usually located on rollers, allowing the kiln to rotate or oscillate around its axis. Solid, liquid, gaseous, or sludge waste is conveyed through the kiln as it rotates. For most fluids, direct injection is preferred. Rotary kilns are used more for hazardous and clinical wastes and less for MSW. Although the rotary kiln is normally operated at 850°C, the temperature can vary from 500°C (as a gasifier) to 1,450°C for an ash-melting kiln [7]. For hazardous waste, the kiln is operated between 900-1,200°C. The residence time in the kiln normally varies between 30 to 90 minutes. For complete destruction of toxic compounds, a postcombustion chamber may be required [7, 11].

6.5.1.2 Fluidized Beds

For RDF, sewage sludge, and finely divided waste, fluidized bed incin­erators are widely used. These types of incinerators are normally used for large-scale operations. Various types of fluidized bed—stationary, bubbling (atmospheric or pressurized), rotating, or circulating—have been used in industrial practice. These fluidized beds differ in gas velocities, the design of the nozzle plate, and their internal design. The waste is injected from the top in an inert sand or ash fluidized bed (by air) which is preheated at the desired level [15]. Sometimes the waste is fluidized by simple injection of air (no inert solids) through holes in the bed plate. The inert solids provide a better heat transfer to the waste. For most waste, the space above the bed is kept at 850-950°C and the temperature of the bed is kept around 650°C [7]. A fluidized bed combustor generally gives uniform temperature and oxygen concentration distributions as well as more stable operation due to a high level of mixing within the bed.

A fluidized bed generally operates with waste particle size of 50 mm. For heterogeneous waste, this may require a pretreatment and sizing step that may add to the overall cost. For a rotating fluidized bed, a particle size of 200-300 mm [16] is possible due to additional mixing caused by the bed rota­tion. The unburned waste and ash in a fluidized bed incinerator are removed at the bottom of the reactor [11, 13]. The combustion heat can be either cap­tured by an internal device, at the exit, or both together.

A significant cost factor comes from product purification of a biofuel prod­uct. Even if a highly selective chemical transformation process is adopted, the nonuniform and heterogeneous nature of biomass feedstock invariably produces products of a broad spectrum of chemical species, targeted ver­sus nontargeted, or desired versus not desired. The unwanted species in the product composition include source-specific and treatment-specific ingredi­ents that need to be separated out from the principal products. Examples include ethanol purification from crude ethanol beer, separation of methanol and salt from crude glycerin that was produced by the transesterification process, removal of trace minerals from biosyngas and bio-oil for upgrad­ing, char removal from bio-oil, denitrification of bio-oil, removal of carbon dioxide from biosyngas, and so on.

Biodiesel has a color varying from golden or light brown to dark brown. The color depends upon the originating biodiesel feedstock. Biodiesel is immis­cible with water, and has a high boiling point and a low vapor pressure. The boiling point data of biodiesel are very scarcely reported in the literature

[48] because a smoke point is usually reached before a boiling point during its measurement, thus making the measurement itself quite difficult. At a smoke point, ingredients of biodiesel are degrading, that is, going through pyrolytic decomposition. Based on the published literature values, pure methyl esters of C18:0, C18:1, and C18:2 are 625, 622, and 639 K, respectively

[49] . Yuan, Hansen, and Zhang [48] developed models, based on the Antoine equation and a group contribution method, for predicting vapor pressure and the normal boiling point of pure methyl esters and biodiesel fuels. The flash point of biodiesel (>130°C) is significantly higher than that of petroleum diesel (64°C) or gasoline (-45°C), which makes biodiesel substantially easier to handle and less flammable. Biodiesel has a density of approximately 0.880 g/cm3, which is higher than that of petrodiesel (0.85 g/cm3) or gasoline (0.71­0.77 g/cm3).

Fermentation, one of the oldest chemical processes known to humans and most widely practiced by them, is used to produce a variety of useful prod­ucts and chemicals. In recent years, however, many of the products that can be made by fermentation are also synthesized from petroleum feedstock, often at lower cost or more selectively. It is also true that modern efforts of exploiting renewable biological resources rather than nonrenewable petro­leum resources as well as focusing on green technologies, thereby allevi­ating the process involvement of harmful chemicals, are strong drivers for biological treatment processes such as fermentation. The future of the fer­mentation industry, therefore, depends on its ability to utilize the high effi­ciency and specificity of enzymatic catalysis to synthesize complex products and also on its ability to overcome variations in the quality and availability of the raw materials.

Ethanol can be quite easily derived by fermentation processes from any material that contains sugar(s) or sugar precursors. The raw materials used in the manufacture of ethanol via fermentation are classified as sugars, starches, and cellulosic materials [15, 16]. Sugars can be directly converted to ethanol by simple chemistry, as fully discussed in Chapter 3. Starches must first be hydrolyzed to fermentable sugars by the action of enzymes. Likewise, cellulose must first be converted to sugars, generally by the action of mineral acids (i. e., inorganic acids such as the common acids sulfuric acid, hydro­chloric acid, and nitric acid). Once the simple sugars are formed, enzymes from yeasts can readily ferment them to ethanol.

5.1.2 Similarities and Differences between Biomass and Coal as Feedstock

Even though biomass gasification has long been practiced on a variety of scales with and without modern scientific understanding, the subject itself has greatly benefited from the coal science and technology which has been studied far more in depth [15]. Most of the scientific tools developed for and applicable to coal technology are more or less relevant to biomass utilization technology and they include analytical methods, solids handling technol­ogy, chemical reaction pathways, reactor designs and configurations, pro­cess integration, waste heat recovery and energy integration, gas cleanup, product separation, safety practice and measures, and much more. However, the compositional differences between coal and biomass feedstock as well as their impact on processing technologies must be clearly understood for full and beneficial exploitation of the advances and innovations made in coal processing technology. The differences between biomass and coal feedstock are summarized below.

1. The hydrogen content in biomass is significantly higher than that of coal. Coal is a very mature product of a lengthy and slow coalifi — cation process whose principal chemical reaction is carbonization, whereas biomass is not. The coal rank basically indicates the degree
of carbonization progressed and a higher rank coal is a petrologically older coal. Therefore, the H/C ratio of coal is much lower than that of biomass. The H/C ratio of a higher rank coal is also lower than that of a lower rank coal.

2. A higher H/C ratio of biomass feedstock makes it generally more reactive than coal for conventional transformational treatments.

3. Biomass typically has much higher moisture content than coal. This statement is applicable to both forms of moisture: equilibrium and chemically bound. Among various ranks of coal, lignite, the lowest rank coal which is also the youngest, has the highest mois­ture content. Therefore, of all ranks of coal, lignite may be consid­ered the closest to biomass in terms of both proximate analysis and ultimate analysis.

4. Biomass contains a significantly higher oxygen content than coal due to its oxygenated molecular structures of carbohydrates (or polysac­charides), cellulose, glycerides, fatty acids, and the like. Weathered coals (i. e., coal exposed to atmosphere after mining) show an increased level of oxygen content over that of freshly mined coal. However, the oxygen content of weathered coal is still far lower than that of typical biomass. Due to the high oxygen content of biomass fuel, its heating value is substantially lower than that of coal.

5. Coal contains 0.5-8 wt% sulfur (S), whereas biomass has little or no sulfur content. Coal with lower than 1 wt% sulfur may be classified as low-sulfur coal, whereas coal with higher than 3 wt% sulfur may be considered high-sulfur coal. No such designation or classification is needed for typical biomass. In coal syngas sulfurous compounds, if not removed, affect the downstream processing severely by poi­soning the catalysts and also by starting corrosion on metallic parts and equipment. In this regard, biomass is considered a sulfur-free raw material. Sulfurous compounds in coal syngas typically include H2S, carbonyl sulfide (COS), and mercaptans (R-SH), whose prevail­ing abundance depends largely on the gasifying environment as well as the feed coal composition. Furthermore, coal sulfur is subdi­vided largely into three different forms: pyritic sulfur, organic sul­fur, and sulfatic sulfur [14]. However, such a subcategorization for forms of sulfur is unnecessary for biomass.

6. Alkali metals such as sodium (Na) and potassium (K) as well as low — boiling heavy metals such as lead (Pb) and cadmium (Cd) are typi­cally present in raw biomass syngas [16]. This is not as severe for coal syngas and the trace element problems with coal syngas are more source-specific. Due to the trace mineral components in the biomass syngas, downstream processing of biomass syngas, in particular catalytic processing, requires rather comprehensive purification

pretreatment of feed syngas or use of robust and poison — and foul — ing-resistant catalysts.

7. Fuel analysis of both coal and biomass is represented by proximate analysis and ultimate analysis. Proximate and ultimate analyses of a variety of biomass samples found in the literature are presented in Tables 5.1 and 5.2, respectively.

8. Due to the high abundance of moisture, high oxygen content, and noncombustible impurities in biomass, the heating value of biomass is typically much lower than that of coal. The energy density of bio­mass feedstock on a volume basis is therefore substantially inferior to that of coal.

9. Biomass has substantially higher volatile matter (VM) content than coal, although it has much lower fixed carbon (FC) content than coal. Therefore, a large amount of hydrocarbon species can be extracted/ obtained from biomass simply via devolatilization or pyroly­sis, whereas devolatilization or pyrolysis of coal generates a high amount of char.

10. Biomass is generally composed of softer organic materials and its grindability or pulverizability is poor using common size reduc­tion equipment. Considering the irregular shapes and nonuniform compositions of untreated biomass components, cost-effective size reduction for manageable transportation as well as continu­ous reactor feeding often becomes a technological challenge. Pretreatment of biomass feedstock is usually required for industri­alized utilization.

11. Both biomass gasification and coal gasification encounter varying degrees of tar formation during thermal/chemical transformation, however, the severity of tar formation is typically more significant with biomass gasification. Although tar is collectively a carcinogenic species, it condenses at reduced temperatures, thereby blocking and clogging pipelines and valves as well as fouling process equipment and parts.

5.1.3 Analysis of Biomass

As mentioned in the previous section, the proximate and ultimate analy­ses of specific biomass material provide very valuable information about the biomass feedstock. This compositional information provides the science and engineering information needed to identify or determine the fuel heating value, ash amount projected, maximum achievable gasification and liquefac­tion efficiency, moisture content of feedstock, predicted behavior of the feed­stock in a processing environment, and much more. The proximate analysis is a procedure for determination, by prescribed methods, of moisture (MO), volatile matter (VM), fixed carbon (FC), and ash. The amount of fixed carbon is determined by difference. The term proximate analysis involves neither determination of quantitative amounts of chemical elements nor determina­tion other than those categorically named or prescribed. The group of analy­ses involved in proximate analysis is defined in ASTM D 3172. On the other hand, the ultimate analysis is a procedure of the determination of the ele­mental composition of the organic portion of carbonaceous materials, as well as the total ash and moisture. The ultimate analysis is also called elemental analysis. And this analysis is also determined by prescribed methods.

An extensive tabulation of both proximate and ultimate analysis data on over 200 biomass species was presented in Channiwala’s PhD dissertation (1992) [17]. Some representative values of proximate and ultimate analyses of a variety of biomass species, as found from the literature sources, are pre­sented in Tables 5.1 and 5.2, respectively. For comparison, the classification of coal and typical analysis is also shown in Table 5.3.

Comparing between the analyses of coal and biomass, the following gen­eralized statements can be made.

1. Biomass has a very high oxygen (O) content, which is the second most abundant atomic species present in biomass and is nearly as much as the carbon (C) content. However, the oxygen content in coal is much lower than the carbon content and this trend is even more noticeable with higher rank coals. The higher the rank of a coal, the lower its oxygen content is. It may be said that deoxygenation (i. e., oxygen rejection) was an important part of a petrochemical process of coalification or carbonization.

2. Due to the higher oxygen content in biomass, the heating value of biomass is much lower than that of coal. Bio-oil derived from bio­mass also has a high oxygen content, which makes the oil more corrosive to metallic parts and piping. Therefore, efficient use of bio­mass as fuel or fuel precursor will involve a certain level of oxygen rejection (i. e., deoxygenation) in its process scheme.

3. The H/C ratio of biomass is substantially higher than that of coal. The reactivity of biomass is generally higher than that of coal and its processability is also better than coal’s.

4. Among various ranks of coal, lignite is the closest to biomass in a number of properties including its high moisture content, high oxy­gen content, low carbon content, and low heating value. As such, lignite has often been considered as a co-fed companion fuel with biomass.

The standardized analysis of biomass fuel is conducted following the ASTM standards and Table 5.4 shows the list of these codes for specific analyses.

In addition to water, carbon dioxide and alcohols have been used for super­critical extraction. The use of supercritical carbon dioxide to extract oil from coal, tar, and various crops or crop waste has been extensively studied [101­105]. These studies have shown that supercritical carbon dioxide can be very effective in extracting certain types of chemicals from various carbohydrates, lignin, and organic materials. More work is, however, needed.

Xiu et al [106] reported an interesting study of supercritical extraction of swine manure by ethanol in the temperature range of 240°C to 360°C and at a pressure of 6.37 MPa under noncatalytic conditions. The maximum yield of oil at 26.7% was obtained at 340°C. At the same temperature, the highest lique­faction yield of 62.77% was obtained. The study concluded that the supercriti­cal ethanol liquefaction was an effective way to remove oxygen and utilize carbon and hydrogen in swine manure to produce energy-condensed biofuel.