The process involves two-stage sulfuric acid hydrolysis, relatively low tem­perature, and a cellulose prehydrolysis treatment with concentrated acid. Figure 4.6 is a schematic flow diagram of the TVA process. Corn stover is ground and mixed with the dilute sulfuric acid (about 10% by weight). The

hemicellulose fraction of the stover is converted to pentose (xylose) sugars by heating the solution to 100°C for two hours in the first hydrolysis reactor. Raw corn stover contains, on a dry basis, about 40% cellulose, 25% hemicel­lulose, and 25% lignin. Sulfuric acid for the hydrolysis reaction is provided by recycling the product stream from the second hydrolysis step, which con­tains the sulfuric acid and hexose sugars. The pentose and hexose sugars, which are primarily xylose and glucose, respectively, are leached from the reactor with warm water. The sugar-rich leachate is then neutralized with lime (calcined limestone, CaO or calcined dolomite, CaO • MgO), filtered to remove precipitated materials, and fermented to produce ethanol [43].

Residue stover from the first hydrolysis step (hemicellulose conversion) is dewatered and prepared for the second hydrolysis step (cellulose con­version) by soaking (prehydrolysis treatment step) in sulfuric acid (about 20-30% concentration) from one to two hours. The residue is then screened, mechanically dewatered, and vacuum dried to increase the acid concentra­tion to 75-80% in the liquid phase before entering the cellulose reactor. The second hydrolysis reactor operates at 100°C and requires a time of four hours. The reactor product is filtered to remove solids (primarily lignin and unre­acted cellulose). Because the second hydrolysis reactor product stream con­tains about 10% acid, it is used in the first hydrolysis step to supply the acid required for hemicellulose hydrolysis. Residue from the reactor is washed to recover the remaining sulfuric acid and the sugar not removed in the filtra­tion step.

Lignin is the unreacted fraction of the feedstock that can be burned as a boiler fuel. It has a heating value of about 5,270 kcal/kg (or, 9,486 BTU/ lb), which is comparable to that of subbituminous coal. Other products such as surfactants, concrete plasticizers, and adhesives can also be made from lignin. Stillage can be used to produce several products, including methane. Preliminary research showed that 30 liters of biogas containing 60% meth­ane gas was produced from a liter of corn stover stillage. For each liter of ethanol produced, 10 liters of stillage were produced [42].

All process piping, vessels, and reactors in contact with corrosive sul­furic acid were made of fiberglass-reinforced vinyl ester resin. The dryer was made of carbon steel and lined with Kynar®, which is a trademark of Arkema Inc. (formerly Atofina) for poly(vinylidene fluoride), or PVDF. Conveyor belts were also made of acid-resistant material. Mild steel agi­tator shafts were coated with Kynar or Teflon®, which is DuPont’s trade­mark for polytetrafluoroethylene, or PTFE. Heat exchangers were made with CPVC (chlorinated poly(vinyl chloride)) pipe shells and Carpenter 20 stainless steel coils. Carpenter 20, also known as Alloy 20, is a nickel-iron — chromium austenitic alloy that was developed for maximum corrosion resistance to acid attack, in particular sulfuric acid attack. Pumps were made with nonmetallic compound Teflon lining, or Carpenter 20 stain­less steel. The two filter press units had plates made of polypropylene (PP) [42].

The term hydrogasification is the reaction between carbonaceous mate­rial and hydrogen, strictly speaking. However, an augmented definition of "hydrogasification" involves the reaction of carbonaceous material in a hydrogen-rich environment to generate methane as a principal product. The gasification reaction in certain steam environments, such as in the copresence of steam and hydrogen, often qualifies for this hydrogen-rich environment for methane generation. The latter is called steam hydrogasifi­cation [38]. However, the steam gasification whose principal goal is to pro­duce syngas should still be referred to as "steam gasification," not simply as "hydrogasification." "Hydro-" as a prefix is used for "of water" or "of hydrogen," depending upon the situation. As far as the gasification of coal and biomass is concerned, the prefix "hydro-" means "of hydrogen," as in the case with "hydrocracking."

Carbonaceous materials undergo hydrocracking under high pressures of hydrogen at elevated temperatures. Hydrocracking generates lighter hydro­carbons as cleavage products from larger hydrocarbons. Although the
hydrocracking reaction is chemically distinct from hydrogasification of car­bon, the difference between the two becomes small when it is applied to coal or coal char whose molecular structure is deficient in hydrogen.

Unlike other gasification reactions involving steam and carbon dioxide, this gasification reaction is exothermic, that is, generating reaction heat, as

C(S) + 2 H2(g) = CH 4 (-ДН098) = 74.8 kj/mol

Coal char hydrogasification can be regarded as two simultaneous reac­tions differing considerably in their reaction rates [33], as also mentioned in the earlier section on steam gasification. This statement of apparent two — stage reactions of pyrolysis and gasification is valid for biomass gasification as well. Due to the high moisture content in raw untreated biomass, biomass hydrogasification always involves steam hydrogasification, where all three principal modes of gasification—including pyrolysis, steam gasification, and hydrogasification—take place simultaneously. Of these reactions, pyrolysis is by far the fastest chemical reaction at the operating conditions.

Hydrogasification of carbons and biomass can be catalyzed for faster and more efficient reactions [39]. Many metallic ingredients have been shown to have catalytic effects on hydrogasification of coal char and carbon and these catalysts include aluminum chloride [40], iron-based catalysts [41], nickel — based catalysts [42], and calcium salt-promoted iron group catalysts [43].

An interesting study was carried out by Porada [44], in which hydrogasifi­cation and pyrolysis of basket willow (Salix viminalis), bituminous coal, and a 1:1 mixture of the two were compared. Their study employed a nonisother­mal kinetics approach, in which the reaction temperature was increased at a constant rate of 3 K/min from ambient temperature to 1,200 K under the hydrogen pressure of 2.5 MPa. Of the test samples, the highest gas yields were obtained during hydrogasification of coal and the lowest yields were observed in the basket willow processing. It was also established that the conversion ratio to C1-C3 hydrocarbons from C under a relatively low H2 pressure was approximately five times higher than the pyrolysis conducted in an inert atmosphere. This clearly explains that the beneficial role of hydro­gen gas is very significant in gasification of biomass as well as coal.

In any mixed feedstock strategy, the transportation of raw materials to the plant is very important. Unlike coal, biomass is difficult and expensive to transport because of its low mass and energy densities (see Table 7.5). Generally, biomass is dispersed over a large area and will require to be trans­ported to a central location from numerous places. Unless the plant is built where a large amount of biomass is easily available, a question of whether to prepare the biomass on site or transport it to a common location from vari­ous sites and then prepare at this common site requires a careful assessment. If biomass is transported a long distance under a natural environment, its biological decay may also occur. The on-site storage of unprepared biomass can lead to its biological decay releasing some heat. Low density of biomass necessitates the requirement of a relatively large storage space for biomass compare to coal.

The transportation and storage of waste materials is also an issue. MSW can be collected and delivered to a common location, but its amount depends on the size of the municipality. For densely populated areas this can work well for a large-size plant; however, for rural locations a supply of sufficient MSW feedstock may become an issue. The same principle applies to paper and pulp, plastic and polymer waste, rubber tires, and so on. In general, bio­mass and waste transportation and storage can be more expensive than that for coal.

Ultrasonic extraction or ultrasonication can enhance and accelerate the algae oil extraction processes. In an ultrasonic reactor, intense sonication of liquids generates ultrasonic waves that propagate into the liquid medium. During the low-pressure cycle, high-intensity small vacuum bubbles are created in the liquid due to the pressure imbalance. When these bubbles attain a certain critical dimension (cavity size), they collapse violently during a high-pres­sure cycle. As these bubbles collapse vigorously near the algae cell walls (i. e., implode) they create shock waves, and locally high-pressure and high-speed liquid jets. The resultant shear forces cause algae cell walls to mechanically rupture and release or help release their contents (algae lipids) into the sol­vent medium. This process of bubble formation and subsequent collapse is mechanistically called cavitation [32]. The advantages of the process include:

(a) Dry cake is not required for oil extraction.

(b) No caustic chemical is involved.

(c) The ultrasonication process can be used in conjunction with enzy­matic extraction.

(d) Environmental impact is minimal.

However, the process is not yet proven on a large scale and the energy cost needs to be lower in order to be competitive. Ultrasonication can be employed in a solvent extraction process, however, the process is usually classified as mechanical extraction, largely based on its mechanically induced cell wall rupture mechanism.

Hydrolysis

FIGURE 3.4

Hydrolysis of cellulose.

sugars can be subsequently converted to ethanol using appropriately selected micro-organisms via a process called fermentation. The fermenta­tion of ethanol from 6-carbon sugars (such as D-glucose) follows the stoichio­metric equation.

Fermentation

2CH3CH2OH + 2CO2

According to the stoichiometric equation, one mole of D-glucose produces 2 moles of ethanol and 2 moles of carbon dioxide. Considering the molecu­lar weights of glucose, ethanol, and carbon dioxide being 180, 46, and 44, respectively, the maximum theoretical yield of ethanol by weight % from the process would be 92/180 = 51%. Nearly half the weight of the glucose 88/180 (49%) is converted to carbon dioxide at its theoretical maximum. As such, a significant amount of carbon dioxide is generated by the fermentation step, which needs to be captured or utilized for economically beneficial purposes.

Hemicellulose is made up of the 5-carbon sugar, xylose, arranged in chains with other minor 5-carbon sugars interspersed as side chains. Similarly to the cellulose case, the hemicellulose can also be extracted from the plant material and treated to liberate xylose that, in turn, can be fermented to produce ethanol. However, xylose fermentation is not as straightforward or efficient as glucose fermentation based on currently available technology. Depending on the micro-organism and conditions employed, a number of different fermentation paths are possible or conceivable. The array of prod­ucts can include ethanol, carbon dioxide, and water as

Fermentation

2CH3CH2OH + CO2 + H2O

Actually, three different reactions have been documented with yields of ethanol ranging from 30 to 50% of the weight of xylose as the starting mate­rial (i. e., weight ethanol produced/weight xylose). They are:

3 Xylose ^ 5 Ethanol + 5 Carbon Dioxide Xylose ^ 4 Ethanol + 7 Carbon Dioxide Xylose ^ 2 Ethanol + Carbon Dioxide + Water

The first reaction yields a maximum of 51% (= 5 * 46/(3 * 150)), the second 41% (= 4 * 46/(3 * 150)), and the third 61% (= 2 * 46/150), respectively. Although the maximum theoretical ethanol yields from these fermentation reactions range between 41 and 61%, the practical yields of ethanol from xylose as starting material are in the range of 30 to 50%.

In the discussion of potential yields of ethanol from various starting mate­rials, two different ranges of efficiencies of hemicellulose-to-xylose conver­sion and xylose-to-ethanol conversion have been combined to provide an overall conversion efficiency of hemicellulose to ethanol of about 50%. Just as with the glucose fermentation, the conversion of carbon dioxide to value — added products would vastly improve the overall process economics of etha­nol production, because the yield of carbon dioxide is not only significant in amounts but also inevitable. It must be noted that even though xylose fermentation to ethanol is also mentioned in this chapter, the main focus of this chapter is on glucose fermentation, more particularly corn sugars into ethanol. Ethanol-from-corn technology involves glucose fermentation, not xylose fermentation, as required in cellulosic ethanol technology. Xylose fer­mentation or hemicellulose fermentation is treated in depth in Chapter 4.

Cellulose hydrolysis and fermentation can be achieved by two different pro­cess schemes, depending upon where the stage of fermentation is actually carried out in the process sequence: (l) separate hydrolysis and fermentation, SHF, or (2) simultaneous saccharification and fermentation, SSF. The acro­nyms "SHF" and "SSF" are very commonly used in the field.

As mentioned before, the technologies for the conversion of waste to energy and products can be broken down into three categories: thermochemical, biochemical, and physicochemical technologies. Combustion/incineration is a special type of thermochemical technology which is mainly used for the generation of heat and electricity. As indicated earlier, various types of upgrading technologies are used to generate transportation fuels, chemicals, and materials [10]. The end results for the applications of these technologies are heat, electricity, transportation fuel, chemicals, or materials. In the fol­lowing sections, we briefly evaluate each of these technologies. Some of the detailed descriptions of the combustion, pyrolysis, gasification, and plasma technology outlined below closely follow the excellent review of Helsen and Bosmans [7].

TABLE 6.2

Summary of Advantages and Disadvantages of Various Biomass Pretreatments

Biomass

Pretreatment Advantages

(Continued)

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.

Note: This information is repeated in Table 7.6.

Crude products of biomass conversion technology include both principal products and by-products. Principal products of biomass processing include crude bio-oil from fast pyrolysis, biochar from slow pyrolysis, biomass syn­gas from gasification, biodiesel from transesterification, crude ethanol beer from corn ethanol, straight vegetable oils from oil crops, biogas from anaer­obic digestion, bioenergy and biofuels from waste and refuse, and more. These principal products can be used for direct end uses after purification, as raw feedstock for further upgrading and transformation into alternative fuels that supplant other conventional fuels and chemicals, or as captive energy sources for the fuel conversion and power generation technology and its ancillary processing. By-products include biochar from fast pyrolysis, bio­oil from gasification, dry distillers’ grains and corn syrups from the corn ethanol industry, crude glycerin from the transesterification process, and more. Profitable by-product portfolios for specific biomass technologies not only enhance the profitability of the biofuels industry, but also contribute to the sustainability of the technological society via promoted utilization of renewable materials.

2.3.3.1 Cetane Rating (CR)

The two most beneficial properties of biodiesel are its higher cetane rat­ings (CR) and better lubricating properties than the ultra-low sulfur diesel (ULSD). The CR of biodiesel ranges between 50 and 60, whereas that of ULSD ranges between 45 and 50. On the other hand, the CR of vegetable oil ranges

FIGURE 2.7

A schematic of biodiesel manufacturing process via transesterification.

between 35 and 45. Addition (or blending) of biodiesel in petrodiesel in a low concentration is reported to reduce fuel system wear, resulting in a beneficial effect to diesel engines.

4.1.2 Ethanol

4.1.2.1 Ethanol as Chemical and Fuel

Ethanol, C2H5OH, is one of the most significant oxygenated organic chemicals because its unique combination of physical and chemical properties make it suitable as a solvent, a fuel, a germicide, a beverage, and an antifreeze; its versatility as an intermediate to other chemicals and petrochemicals also contributes to its significance. Ethanol is one of the largest bulk-volume chemicals used in industrial and consumer products. The main uses for ethanol are as an intermediate in the production of other chemicals and as a solvent. As a solvent, ethanol is second only to water. Ethanol is a key raw material in the manufacture of plastics, lacquers, polishes, plasticizers, per­fume, and cosmetics. The physical and chemical properties of ethanol are primarily dependent upon the hydroxyl group which imparts the polarity to the molecule and also gives rise to intermolecular hydrogen bonding. In the liquid state, hydrogen bonds are formed by the attraction of the hydroxyl hydrogen of one molecule and the hydroxyl oxygen of another molecule. This makes liquid alcohol behave as though it were largely dimerized. Its association is confined to the liquid state, whereas it is monomeric in the vapor state.

Another important property of ethanol in its fuel application is that the ethanol-water binary system forms an azeotrope at a binary concentration of 95.63 wt% ethanol and 4.37 wt% water and this azeotropic mixture boils at 78.15°C, which is lower than ethanol’s normal boiling point (NBP) of 78.4°C. Therefore, straight distillation cannot boil off ethanol at a concentration higher than this azeotropic concentration. Most industrial grade ethanol has 95 wt% ethanol and 5 wt% water (190 proof). Therefore, fuel-grade ethanol is commonly produced by a combinatory process between distillation and zeolite-based absorption/adsorption in order to overcome the azeotropic concentration barrier encountered in the distillation separation process.