As for the material compatibility, biodiesel is quite different from petro — diesel. Biodiesel is compatible with high-density polyethylene (HDPE), and it is incompatible with polyvinylchloride (PVC) and polystyrene (PS), as PS is readily soluble in biodiesel and PVC is slowly dissolved in biodiesel. Polypropylene (PP) is affected by biodiesel, showing a swell increase (by 8-15%) and reduced hardness (by about 10%). Polyurethane (PUR) is also affected showing some swell increase (by 6%). Biodiesel affects some natu­ral rubber and all nitrile rubber products, and biodiesel is compatible with commonly used viton-type synthetic rubbers in modern vehicles. Studies indicate that viton types B and F (FKM-GBL-S, and FKM-GF-S) are more resistant to acidic biodiesel [50]. Biodiesel affects, and is affected by, many metals including copper, zinc, tin, lead, cast iron, and brass, whereas biodie­sel does not affect stainless steel (304 and 316), carbon steel, and aluminum. Brass, bronze, copper, lead, tin, and zinc may accelerate, by catalytic activi­ties, the oxidation process of biodiesel creating fuel insolubles or gels and salts. As such these metals should be avoided as materials of construction for piping, regulators, and fittings in applications where biodiesel is expected to be in contact with them.

Neat biodiesel may degrade some hoses, gaskets, seals, elastomers, glues, and plastics with prolonged exposure. Acceptable storage tank materials for biodiesel include aluminum, steel, fluorinated polyethylene, fluorinated polypropylene, Teflon, and most fiber glasses [51].

4.1.2.2 Starches

The grains generally provide cheaper ethanol feedstock in most regions of the world and industrial conversion may be kept relatively inexpensive because they can be stored more easily than most sugar crops, which often must be reduced to a form of syrup prior to storage. Furthermore, the grain milling ethanol process produces a by-product that can be used for protein meal in animal feed [29]. Fermentation of starch from grains is inherently more complex, involving more steps than sugars because the starch must first be converted to sugar and then to ethanol. A simplified equation for the conversion of starch to ethanol can be written as

As shown in Figure 4.3, in making grain alcohol, the distiller produces a sugar solution from feedstock, ferments the sugar to ethanol, and then sepa­rates the ethanol from water through distillation.

Among the disadvantages in the use of grain are its fluctuations in price. Critics of corn ethanol have made remarks in relation to "fuel versus food" and stated that the recent food price increase has something to do with corn ethanol manufacture, whereas others strongly oppose this view with statis­tical data and logical reasons [31]. Ethanol in the gasoline boosts the fuel’s octane rating and also helps cleaner burning. In the United States, etha­nol is currently the most popular oxygenated fuel additive as discussed in Chapter 3.

Gasification is a conversion process that transforms macromolecular car­bonaceous matters contained in fossil fuels and biological substances into simpler gaseous molecular products. The gaseous products from a gasification reaction are called synthesis gas, syngas, or producer’s gas. Depending upon the original carbonaceous feedstock, the syngas may be further labeled as coal syngas, natural gas (NG) syngas, or biomass syngas. Gasification of biomass usually takes place at an elevated tem­perature with the aid of a gasifying medium, which may be regarded as a gaseous reactant. Because gasification involves both heat and chemical(s) that induce concurrent thermal decomposition and chemical reactions, the gasification process is classified as a thermochemical conver­sion process.

Anaerobic digestion (in the absence of oxygen) with anaerobic bacteria or methane fermentation is used worldwide for disposal of domestic, municipal, agricultural, and industrial biomass waste. This reaction generally produces methane and carbon dioxide and it also occurs in the ecosystem and in the digestive tract. As shown in the following reactions (6.1) and (6.2), hydrogen along with acetic and butyric acids can also be produced by dark fermentation processes using anaerobic and facultative anaerobic chemohetrotrophs [118].

C6O6H12 + 2H2O ^ 2CH3COOH + 4H2 (6.1)

C6O6H12 ^ CH3CH2CH2COOH + 2CO2 + 2H2 (6.2)

Different types of waste materials can also be used for hydrogen fermenta­tion. Hydrogen production is highly dependent on the pH, retention time, and gas partial pressure. Generally, hydrogen production increases with retention time. Biogas produced from landfills generally contains methane (about 55%) and carbon dioxide with traces of hydrogen, ethane, and other impurities. The following description of the sequence of biochemical reac­tions that occur to convert complex molecules to methane closely follow the excellent review of Weiland [118].

In general, methane fermentation can be divided into four phases: hydroly­sis, acidogenesis, acetogenesis/dehydrogenation, and methanation. As shown by Weiland [118], the degradation of complex polymers such as polysaccha­rides, proteins, and lipids results in the formation of monomers and oligomers such as sugars, amino acids, and long-chain fatty acids. The individual degra­dation steps are carried out by different consortia of micro-organisms, which place different requirements on the environment [119]. Hydrolyzing and fer­menting micro-organisms are responsible for the initial attack on polymers and monomers and produce mainly acetate, hydrogen, and varying amounts of fatty acids such as propionate and butyrate [118]. Hydrolytic micro-organ­isms excrete hydrolytic enzymes such as cellulose, amylase, lipase, and the like. Thus, a complex consortium of micro-organisms most of which are strict anaerobes such as Bacteriocides, Clostridia, and Bifidobacteria [118] participate in the hydrolysis and fermentation of organic material [118].

The higher volatile fatty acids are converted into acetate and hydrogen by obligate hydrogen-producing acetogenic bacteria. The maintenance of an extremely low partial pressure of hydrogen is very important for the acetogenic and hydrogen-producing bacteria. The present state of knowledge indicates that hydrogen may be a limiting substrate for methanogens [120]. This is because an addition of hydrogen-producing bacteria to the natural biogas-producing con­sortium increases daily biogas production. Studies [118] have shown that only two groups of methanogenic bacteria produce methane from acetate, hydrogen, and carbon dioxide. These bacteria are strictly anaerobes and require a lower radox potential for growth than most other anaerobic bacteria. Only few species are able to degrade acetate into CH4 and CO2, for example, Methanosarcina barkeri, Methanonococcus mazei, and Methanotrix soehngenii, whereas all methanogenic bacteria are able to use hydrogen to form methane [118].

The overall process of methane fermentation can be accomplished in two stages and a balanced anaerobic digestion process demands that in both stages the rates of degradation must be equal in size. If the first degradation step runs too fast, the acid concentration rises and pH drops below 7.0 which inhibits methanogenic bacteria. If the second phase runs too fast, methane production is limited by the hydrolytic stage. Thus the rate-limiting step

depends on the compounds of the substrate used for biogas production. Undissolved compounds such as cellulose, proteins, and fats take several days to crack whereas soluble carbohydrates crack in a few hours. Therefore the process design must be well adapted to the substrate properties for achieving complete degradation without process failure.

Numerous studies have been reported for anaerobic digestion to produce either methane or hydrogen from a variety of waste streams. Some of these studies are summarized in Table 6.7. Maximum gas yields and theoretical methane contents that can be generated from carbohydrates, raw protein, raw fat, and lignin are summarized in Table 6.8 [118]. The biogas generated from landfills generally contains about 50 to 55% methane and the remain­ing composition consists largely of CO2 and traces of water, hydrogen, and other impurities. It is clear from these studies that anaerobic digestion is a very widely used process to generate methane or hydrogen from a variety of organic wastes, both of which are important gaseous biofuels.

Collecting, concentrating, and processing algae consists of separating algae from the growth medium, drying, and processing it to obtain the desired product. Separating algae from its growth medium is generally referred to as algae harvesting. The term algae harvesting technologically refers to the activity of concentration of a fairly diluted (ca. 0.02-0.06% total suspended solids, TSS) algae suspension until a slurry or paste containing 5-25% TSS or greater is obtained. Specific harvesting methods depend primarily on the type of algae and growth media. The high water content of algae must be removed to enable further processing. The most common harvesting processes include: (1) microscreening, (2) flocculation, and (3) centrifuga­tion [20, 21]. The three methods represent different unit operations of filtra­tion, flotation, and centrifugation, respectively. Therefore, these harvesting steps must be energy-efficient and relatively inexpensive; as such, selecting easy-to-harvest algae strains becomes quite important. Macroalgae har­vesting requires substantial manpower, whereas microalgae can be har­vested more easily using microscreens, centrifugation, flocculation, or by froth flotation.

Starch may be regarded as a long-chain polymer of glucose (i. e., many glucose molecular units are bonded in a polymeric chain similar to a condensation polymerization product [16]). As such, macromolecular starches cannot be directly fermented to ethanol via conventional fermentation technology. They must first be broken down into simpler and smaller glucose units through a chemical process called hydrolysis. In the hydrolysis step, starch feedstock is ground and mixed with water to produce a mash typically containing 15 to 20% starch. The mash is then cooked at or above its boiling point and treated subsequently with two enzyme preparations. The first enzyme hydrolyzes starch molecules to short-chain molecules, and the second enzyme hydro­lyzes the short chains to glucose. The first enzyme is amylase. Amylase lib­erates "maltodextrin" by the liquefaction process. Such maltodextrins are not very sweet inasmuch as they contain dextrins (a group of low molecular weight carbohydrates) and oligosaccharides (a saccharide polymer contain­ing a small number of simple sugars, monosaccharides). The dextrins and oligosaccharides are further hydrolyzed by enzymes such as pullulanase and glucoamylase in a process known as saccharification. Complete sacchari­fication converts all the limit dextrans (complex branched polysaccharides of many glucose molecules) to glucose, maltose, and isomaltose. The mash is then cooled to 30°C, and at this point yeast is added for fermentation.

The organosolv process is a pulping technique that uses an organic solvent to solubilize lignin and hemicellulose. The process was first developed as an environmentally benign alternative to kraft pulping. Its main advantages include the production of high-quality lignin for added values and easy recovery and recycling of solvents used in the process, thereby alleviating environmental stress on the water stream.

In this type of pretreatment of lignocellulose, an organic solvent (etha­nol, butanol, or methanol) is added to the pretreatment reaction to dissolve and remove the lignin fraction. In the pretreatment reactor, the internal lignin and hemicellulose bonds (refer to Figure 4.1) are broken and both fractions are solubilized, whereas the cellulose remains as a solid. After leaving the reactor, the organic fraction is removed by evaporation (distil­lation) in the liquid phase. The lignin then precipitates and can be removed by filtration or centrifugation. Thus, this process cleanly separates the feed­stock into a solid cellulose residue, a solid lignin that has undergone a few condensation reactions, and a liquid stream containing xylon, as shown in Figure 4.9.

The organosolv process is usually carried out at an elevated temperature of 140-230°C under pressure. High temperature is somewhat dictated by the desired bond cleavage reactions involving the liberation of lignin, and the high pressure is needed to keep the solvent process operation in the liquid phase. Ethanol has been regarded as a preferred solvent for organosolv due to its low price, availability, and easy solvent recovery. Butanol, also, has shown promise because of its superior capability of high lignin yield and immisci — bility with water which make solvent recovery simple without energy-inten­sive distillation. Although butanol’s effectiveness is quite appealing, its cost is considered to be somewhat prohibitive. As explained, a principal concern in these processes is the complete recovery of the solvent, which affects the overall process economics; as such, process engineering and optimization become important factors in process economics.

Results have shown that there are some reactions occurring during the organosolv process that strongly affect the enzymatic rate [53]. These reac­tions could be due to the physical or chemical changes in lignin or cellulose.

In general, organosolv processes have higher xylose yields than the other processes because of the influence of organic solvent on hydrolysis kinetics. In a recent study, Pan et al. [54] applied the ethanol organosolv pretreatment to lodgepole pine killed by mountain beetle and achieved 97% conversion to glucose. They recovered 79% of the lignin using the conditions of 170°C, 1.1 wt% H2SO4, and 65 vol% ethanol for 60 minutes.

The first biomass gasification system investigated at the pilot scale was a fluidized bed that incorporated dry ash-free (daf) corn stover as the gasifier feed. Corn stover has been selected as the feed to the gasifier since 1977 [77], even when U. S. corn production was less than half of the 2010 production of 312 million metric tons [78]. The amount of corn stover that can be sustain­ably collected and made available in 2003 was estimated to be 80-100 million dry metric tons/yr [79]. Of this total, potential long-term demand for corn

FIGURE 5.11

KSU’s pilot-scale fluidized bed gasifier of corn stover.

stover by nonfermentative applications such as biomass gasification in the United States was estimated to be about 20 million dry tons/yr.

An early pilot-scale system designed and operated at Kansas State University, shown in Figure 5.11, has a 45.5 kg bed capacity [80]. Fluidizing gas and heat for biomass gasification were supplied by the combustion of propane in the presence of air. The particulates and char were removed using a high temperature cyclone. A Venturi scrubber was then used to separate the volatile matter into noncondensable gas, a tar-oil fraction, and an aque­ous waste fraction. Raman et al. [80] conducted a series of tests with tempera­tures ranging from 840 to 1,020 K. The optimal gas production was obtained using a feed rate of 27 kg/hr and a temperature of 930 K. At these conditions,

0. 25 x 106 BTU/hr of gas was produced. This was enough to operate a 25-hp internal combustion engine operating at 25% efficiency [80].

Another one of the extensively studied gasification systems for biomass conversion is Sweden’s VEGA gasification system. Skydkraft AB, a Swedish power company, decided in June 1991 to build a cogeneration power plant in Varnamo, Sweden to demonstrate the integrated gasification combined cycle (IGCC) technology. Bioflow, Ltd. was formed in 1992 as a joint ven­ture between Skydkraft and Alstrom to develop the pressurized air-blown circulating fluidized bed gasifier technology for biomass [81]. The biomass integrated gasification combined cycle (BIGCC) was commissioned in 1993 and fully completed in 1995. VEGA is a biomass-fuel based IGCC system that combines heat and power for a district heating system. It generated 6.0

MWe and 9.0 MWth for district heating of the city of Varnamo, Sweden. This was the first complete BIGCC for combined heat and power from biomass feedstock. As shown in Figure 5.12, the moisture of the entering biomass feedstock is removed via a "biofuel dryer" to decrease gaseous emissions [81]. The dried biomass is then converted into a "biofuel" in a combined cycle gasifier. The resulting gas is cooled before entering the heat recovery boiler and distribution to district heating. The gasifier is known as a bioflow gas­ifier or bioflow pressurized circulating fluidized-bed gasifier.

Biomass gasification and power generation technology has long been developed with significant technological advances in Finland, where about 20% of total energy consumption derives from biomass. This high percent­age of biomass energy utilization is mainly due to the recycling of biowaste produced as a by-product of the forest industry. VTT Energy and Condens Oy of Finland developed a new type of fixed bed biomass gasifier [58], whose configuration is based on forced feed flow that allows the use of low bulk — density fibrous biomass feedstock. This gasifier is a combination of updraft and cocurrent gasification technologies, where gasifying medium and solid feed move upward through the gasifying section of the reactor. In 1999-2001, a 500 kWth pilot plant was operated in a test facility and very positive test results were obtained. Some of the principal features of the technology include [58, 82]:

• Fuel feeding is not based on natural gravity alone.

• The process is suitable for various biomass residues and waste — derived fuels.

• The process achieves high carbon conversion and generates low tar content.

• The process can be scaled up to above 8 MW, unlike its predecessor technology of Bioneer gasifiers.

• There was no problem with leaking feeding system or blocking gas lines.

• The VTT successfully demonstrated a variety of feedstock including

• Forest wood residues chips (moisture level of 10-55wt%)

• Sawdust and wood shavings

• Crushed bark with maximum moisture of 58%

• Demolition wood

• Residue from plywood and furniture industry

• Recycled fuel manufactured from household waste

• Sewage sludge in conjunction with other fuels

Condens Oy is offering this technology for a wide range of fuel feedstock. The Kokemaki (Finland) CHP plant of 1.8 MWe/3.9 MWth based on this technology was started up in 2005 [82].

The most common method of gasifying biomass is using an air-blown cir­culating fluidized bed gasifier with a catalytic reformer, even though there are many different variations. Most fluidized bed gasification processes use closed-coupled combustion with very little or no intermediate gas cleaning [83]. This type of process is typically operated at around 900oC, and the prod­uct gas from the gasifier contains H2, CO, CO2, H2O, CH4, C2H4, benzene, and tars. Gasification uses oxygen (or air) and steam to help the process con­version, just as the advanced gasification of coal [5]. The effluent gas from the fluidized bed gasifier contains a decent amount of syngas compositions, however, the hydrocarbon contents are also quite substantial. Therefore, the gasifier effluent gas cannot be directly used as syngas for further processing for other liquid fuels or chemicals without major purification steps. This is why the gasifier is coupled with a catalytic reformer, where hydrocarbons are further reformed to synthesis gas. In this stage, the hydrocarbon content including methane is reduced by 95% or better. A very successful example is Chrisgas, an E. U.-funded project, which operates an 18 MWth circulating fluidized gasification reactor at Varnamo, Sweden. They use a pressurized circulating fluidized bed gasifier operating on oxygen/steam, a catalytic reformer, and a water gas shift conversion reactor that enriches the hydrogen content of the product gas. The process also uses a high-temperature filter. The project has been carried out by the VVBGC (Vaxjo Varnamo Biomass

Gasification Centre). The use of oxygen instead of air is to avoid a nitrogen dilution effect that, if not avoided, adds an additional burden of nitrogen removal as a downstream processing [83].

Another CFB gasification process by Termiska Processor AB (TPS) in Nykoping, Sweden developed for small — to medium-scale electric power generation is using biomass and refuse-derived fuel (RDF) as their feed­stock [84]. The process is based on an air-blown low-pressure CFB gasifier which operates at 850-900°C and at 1.8 bar. The raw product gas has a tar content of 0.5-2.0% of dry gas with a heating value of 107-188 BTU/scf. As such, the raw product gas is a low-BTU gas, if the coal syngas classifica­tion scheme is followed. The process has merits of good fuel flexibility, good process controllability, and low-load operation characteristics, uni­form gasifier temperature due to highly turbulent movement of biomass solids, high gasification yield, additional features of catalytic tar cracking, and fines recycling from a secondary solids separation. The tar in the syngas is catalytically cracked by dolomite [CaCO3 • MgCO3] in a sepa­rate reactor vessel at 900°C immediately following the gasifier. The full calcination of dolomite is active at this temperature, as the chemical equi­librium constant (Kp) for calcite (CaCO3) decomposition becomes unity at approximately 885°C [5]. A schematic of a pilot-scale TPS process with a tar cracker is shown in Figure 5.13. A waste-fueled gasification plant was

Air

FIGURE 5.13

A schematic of TPS biomass gasification system with a tar cracking unit.

constructed based on the TPS CFB process by Ansaldo Aerimpianti SpA in Greve-in-Chianti, Italy [85].

Indirect gasification is another gasification process technology that takes advantage of the unique properties associated with biomass feedstock. As such, indirect gasification of biomass is substantially different from most coal-based gasification process technologies. For example, biomass is low in sulfur, low in ash, highly reactive, and highly volatile. In an indirect gasifica­tion process, biomass is heated indirectly using an external means such as heated sands as in the Battelle process [86]. A typical gaseous product from an indirect gasifier is close to medium-BTU gas. Battelle began this process R&D in 1980 and has continued till now, accumulating a substantial amount of valuable data regarding biomass gasification and utilization through demonstration plant operation. Battelle’s process is known as the FERCO SilvaGas process, which has been commercialized by FERCO Enterprise. The principal gasifying medium for the process is steam. A commercial — scale demonstration plant of the SilvaGas process was constructed in 1997 at Burlington, Vermont, at a Burlington Electric Department (BED) McNeil Station. The design capacity of this plant is 200 tons/day of biomass feed (dry basis). McNeil Station uses conventional biomass combustion technology, a stoker gate, conventional steam power cycle, and an ESP (electrostatic pre­cipitator) based particulate matter (PM) removal system. The gas produced by the SilvaGas gasifier is used as a cofired fuel in the existing McNeil power boilers [86]. The product gas has a heating value of about 450-500 BTU/scf, which is in a medium-BTU gas range. A schematic of the FERCO SilvaGas process is shown in Figure 5.14.

CUTEC-Institut Gmb of Germany recently constructed and operated an oxygen-blown circulating fluidized bed (CFB) gasifier of 0.4 MWth capacity coupled with a catalytic reformer [87]. Part of their product gas is, after com­pression, directly sent to a Fischer-Tropsch synthesis (FTS) reactor for liquid hydrocarbon synthesis. The CFB gasifier of the CUTEC process was operated at 870°C, whereas the fixed bed Fischer-Tropsch synthesis reactor was oper­ated at 150-350°C and 0.5-4.0 MPa using a fused iron catalyst (Fe/Al/Ca/K/ Mg = 100/1.7/2.5/0.7). Their pilot-scale process system was successfully dem­onstrated with a variety of biomass feedstock with wide ranges of particle sizes and moisture levels, including sawdust (~3 mm), wood pellets (6-18 mm), wood chips (~10 mm), and chipboard residues (~30 mm). This process, once fully developed for large-scale operation, has a good potential for a sin­gle-train biomass-to-liquid (BtL) fuel conversion process [87]. The CUTEC’s idea of direct linking between the biomass gasification and Fischer-Tropsch synthesis is very similar to that adopted by the indirect coal liquefaction based on Fischer-Tropsch synthesis.

Another process option for biomass gasification for syngas production involves the use of an entrained flow reactor. This type of process is operated at a very high temperature, around 1,300°C, and without use of catalyst. The high temperature is necessary due to the fast reaction rate required for an

entrained flow reactor whose reactor residence time is inherently very short. If a specific biomass feed has a high ash content, which is not very typical for biomass, slag can be formed at such a high temperature. Learning from the research developments in coal gasification [14], a slagging entrained flow gasifier may be adopted for high-ash biomass conversion. Another important process requirement in addition to high temperature and short residence time is the particle size of the solid feed; it must be very fine for efficient entrainment as well as for better conversion without mass transfer limita­tions. However, pulverization or milling of biomass to very fine particle sizes is energy-intensive and costly, in general. To facilitate an efficient size reduc­tion of biomass feed, two options are most commonly adopted, viz., torrefac — tion and pyrolysis. Torrefaction is a mild thermal treatment at a temperature of 250-300°C, which converts solid biomass into a more brittle and easily pulverizable material that can be treated and handled just like coal [88]. This torrefied product is often called biocoal. Thus, pulverized torrefied biomass can be treated just as coal and most entrained flow gasifiers designed for coal can be smoothly converted for torrefied biocoal without much adapta­tion. Torrefaction as a process has long been utilized in many applications including the coffee industry. Torrefaction of biomass can alleviate some of the logistical problems involved with biomass feedstock collection and

transportation. However, more study is needed for the biomass industry to make it more tuned for biomass and optimized as an efficient pretreatment technique. Gases produced during the torrefaction process may be used as an energy source for torrefaction, thus accomplishing a self-energy supply cycle.

An example of entrained flow biomass gasification can be found from the Buggenum IGCC plant whose capacity is 250 MWe [89]. NUON has operated this process and their demonstration test program using, from 2001 through 2004, 6,000 M/T of sewage sludge, 1,200 M/T of chicken lit­ter, 1,200 M/T of wood, 3,200 M/T of paper pulp, 50 M/T of coffee, and 40 M/T of carbon black as cofeeds with coal. A typical particle size of biomass feed was smaller than 1 mm and pulverization of wood was more difficult than that of chicken litter and sewage [89]. In their test program, they also mentioned torrefaction as a pretreatment option. Their experience with a variety of biomass feedstock provides valuable operation data for future development in this area.

As explained in the fast pyrolysis section of this chapter, biomass pyroly­sis takes place actively at around 500oC and produces a liquid product via fast cooling (shorter than two seconds) of volatile pyrolytic products. As also mentioned earlier, the liquid product produced is called bio-oil. The pro­duced bio-oil can be mixed with char (biomass char or biochar) to produce a bioslurry. Bioslurry can be more easily fed, as a pumpable slurry, to the gasifier for efficient conversion. Bioslurry is somewhat analogous to coal-oil slurry (COM) [5]. A successful example of using bioslurry is found from the FZK process [90, 91]. FZK (Forschungszentrum Karlsruhe) developed a pro­cess that produces syngas from agricultural waste feeds such as straw. They developed a flash pyrolysis process that is based on twin screws for pyroly­sis, as explained earlier as an auger pyrolyzer. The process concept is based on the Lurgi-Ruhrgas coal gasification process [5, 15]. A 5-10 kg/hr PDU (process development unit) is available at the FZK company site. In this pro­cess, straw is flash-pyrolyzed into a liquid that is subsequently mixed with char to form a bio-oil/biochar slurry. The slurry is pumpable and alleviates technical difficulties involved in solid biomass feeding and handling. This slurry is transported and added to a pressurized oxygen-blown entrained gasifier. The operating conditions of the gasifier at Freiberg, Germany involve a slurry throughput of 0.35-0.6 tons/day, 26 bars, and 1,200-1,600oC. The current FZK process concepts involve gasification of flash-pyrolyzed wood products, slow-pyrolyzed straw char slurry (with water condensate), and slow-pyrolyzed straw char slurry (with pyrolysis bio-oil) [90]. Slurries from straw have been efficiently converted into syngas with high conversion and near-zero methane content [83]. Their ultimate objective is development of an efficient biomass-to-liquid plant. A simplified block diagram of the FZK process concept leading to BtL is shown in Figure 5.15.

Canadian developments in biomass gasification for the production of medium — and high-BTU gases have also received worldwide technological

attention. The BIOSYN gasification process was developed by Biosyn Inc., a subsidiary of Nouveler Inc., a division of Hydro-Quebec. The process is based on a bubbling fluidized bed gasifier containing a bed of silica (or alu­mina) and can be operated at a pressure as high as 1.6 MPa. They tested the process extensively during 1984 till 1988 on a 10 ton/hr demonstration plant that was comprised of a pressurized air — or oxygen-fed fluidized-bed gasifier [92]. The system has the ability to utilize a diversified array of feed­stock including: whole biomass, fractionated biomass, peat, and municipal solid waste. The primary end-use for the biogas is replacing the oil currently used in industrial boilers. It also has the added capability for producing syn­thesis gas for methanol or low-energy gas production. Later, they used a 50 kg/hr BIOSYN gasification process development unit and the test program also proved the feasibility of gasifying a variety of other feedstock, such as primary sludges, refuse-derived fuels, rubber residues containing 5-15% Kevlar, granulated polyethylene, and polypropylene [93].

Another emerging process option for biomass gasification involves super­critical water gasification of biomass. Supercritical water (SCW) is water exist­ing under a condition where both temperature and pressure are above critical temperature and pressure; that is, T > 374°C and P > 218 atm. At supercritical conditions, water exhibits extraordinary properties that are quite different and distinct from those of ambient water, as compared qualitatively in Table 5.12 [94].

Biomass feedstock can be gasified in a supercritical water medium at a temperature higher than about 650°C and pressure higher than 22.1 MPa [95, 96]. Although common gasification technology requires wet biomass to be sufficiently dried before gasification treatment, the technology based on gasification in supercritical water can handle wet biomass as is without energy — and cost-intensive drying of the feed material. Boukis et al. [96] stud­ied biomass gasification in near-critical and supercritical conditions using a pilot-scale process system called VERNA (a German acronym for "experi­mental facility for the energetic exploitation of agricultural matter") that had a throughput capacity of 100 kg/h and a maximum reaction temperature of 660°C at 28 MPa. The process system was capable of preparing large biomass particles into about <1 mm particle size using a cutting mill followed by a colloidal mill. The reactor was a downflow type and the reactor system could handle the separation of brines and solids from the bottom of the reactor [96].

The principal gaseous products of supercritical water gasification are hydrogen, carbon monoxide, carbon dioxide, methane, and ethane. A gen­eralized reaction scheme involves reformation (steam gasification) reaction of hydrocarbons and oxygenates as well as pyrolytic decomposition reac­tions involving the cleavages of both C-C and C-H bonds. Carbon dioxide concentration usually increases with the temperature of reaction, whereas carbon monoxide decreases. The increase of carbon dioxide in the product stream is due to the results of the forward water gas shift reaction that con­verts carbon monoxide and water into carbon dioxide and hydrogen [97]. The methane formation for all ranges of gasification temperature is believed to have originated from pyrolytic decomposition of hydrocarbons and their intermediates, not by methanation reaction of syngas; that is, CO + 3H2 = CH4 + H2O [98].

From the kinetics standpoint, the reformation reaction is more active than pyrolytic decomposition at higher temperatures, whereas the pyrolysis reac­tion is faster and more active than the reformation reaction at lower tem­peratures [99]. As such, the two pseudo first-order reaction rates meet and cross each other at some point in the temperature domain, when the rates are plotted against the temperature or a reciprocal of temperature. This is a crossover point in the rates between the two representative chemical reac­tions, where the two reactions have an identical pseudo first-order reaction rate. The higher the crossover temperature is, the more difficult the gasifica­tion (or reformation). The location of this kinetic cross-over point is different from chemical to chemical and from one biomass type to another.

The extent of the gasification or gasification efficiency, which is defined as the total carbon appearing in gas phase products divided by the total car­bon in biomass feedstock entering the reactor, depends very strongly upon imposed reaction conditions such as the reaction temperature, space time, feed biomass/water ratio, molecular structures of biomass pertaining to the H/C ratio, O and — OH content, isothermality/nonisothermality of the reac­tor, monolithic catalytic effect from the reactor wall materials, and so on. As the gasification temperature increases, the gasification efficiency generally increases to a certain point. However, gasification efficiency does not change much with the pressure, as long as the water is in its supercritical fluid region. Picou et al. [100] demonstrated that the hydrocarbon reformation in supercritical water can be operated in an autothermal mode with numerous advantages including (i) 100% gasification efficiency, (ii) energywise self-sus­tainable operation, and (iii) comparable or enhanced product gas composition and yield. They demonstrated their technology using jet fuel in supercritical water at temperatures up to 775°C on a high-nickel Haynes Alloy 282 reac­tor system. Autothermal reformation (ATR) has been successfully practiced in the field of steam reformation of methane (SMR) [101], wherein a substoi­chiometric amount of air (or oxygen) is fed to the reformer for sacrificial oxi­dation reaction of hydrocarbons, thus generating the exothermic heat and promoting reformation reaction. The conventional ATR has been operated catalytically on an industrial-scale subcritical reformation process at much higher temperatures, unlike the process Picou et al. demonstrated using non­catalytic supercritical reformation technology [100]. As shown in Table 5.12, oxygen fully dissolves in supercritical water, thereby easily facilitating an autothermal mode of operation in a supercritical water reactor.

Bouquet et al. [98] showed direct experimental evidence that hydroxyl functional groups present in the molecular structure of a feed chemical play an important role by making the reformation (or gasification) reaction proceed more efficiently via a mechanistically simpler pathway leading to syngas: hydrogen and carbon oxides. They experimentally compared the kinetic results of the supercritical water gasification reactions of three differ­ent kinds of C3 alcohols: iso-propyl alcohol, propylene glycol, and glycerin. The three alcohols used in the experiment represent monhydric, dihydric, and trihydric alcohols of C3 hydrocarbons, respectively. The results bear sig­nificance in biomass gasification in supercritical water, because biomass is rich in hydroxyl groups due to its abundant cellulosic ingredients. In par­ticular, biomass feedstock pretreatment should be carried out in such a way that hydroxyl groups in the molecular structure should not be prematurely destroyed or extracted if the biomass is to be gasified in supercritical water.

Currently, large-scale commercial coal gasification technology is dominated by high-temperature, high-pressure entrained flow gasifiers. In fact, about 85% of existing commercial coal gasification reactors are entrained bed reac­tors. As shown in the previous section, recent successful tests with crops and biomass [6] indicate that this type of gasifier is reasonably well suited to the gasification of mixed feedstock. They eliminate tar formation and are not greatly affected by ash content differences with coal/coke; however, the Buggenum plant [6] noted a tendency to foul the syngas cooler when using sewage sludge. The primary drawback of this type of gasifier for the process­ing of mixed feedstock is the handling of feed materials that are difficult to pulverize such as switchgrass and straw, among others, as well as some woody biomass and high-pressure feed into the gasifier using either a slurry or dry feed system. The dry feed system requires pulverized particles of about 100 mesh which may be difficult to achieve with certain types of bio­mass. The reactor operates at temperatures in excess of about 1250-1300°C. Such a high temperature results in the production of syngas (CO and H2) with no methane.

The entrained flow gasifier (a) is able to gasify all coal regardless of coal rank, caking characteristics, or amount of coal fines; (b) has uniform temper­ature and short residence time; and (c) slagging operation with some entrain­ment of molten slag in the raw gas. Although all new IGCC plant reactors will be entrained flow reactors, the design of various entrained flow reactors may vary depending on whether feed is dry or slurry, the internal design to handle the hot reaction mixture, and heat recovery configuration. One of the key technical issues is the method for cooling the product gases. Recent studies have shown [105 ] that this type of reactor is very suitable for mixed feedstock up to about 30% of biomass. Due to the high oxygen content of the biomass, the mixed feedstock will produce more water and carbon dioxide and less carbon monoxide, lower syngas heat content, and in general lower sulfur and ash content. In processing some mixed feedstock that contains biomass such as straw and other herbaceous biomass which contains chlo­rine can cause corrosion and biomass that contains calcium and sodium can alter ash melting point.

The global energy consumption outlook forecast by the U. S. Energy Information Administration (EIA) is shown in Figure 1.1, which shows two sets of data series for the OECD (Organization for Economic Cooperation and Development) and the non-OECD nations [3]. As shown in the chart, much of the increase in energy consumption is predicted for the non-OECD nations due to their strong long-term economic growth and continuous industrialization. Although about 45% of cumulative increase in the annual energy consumption is predicted from 1990 to 2035 for the OECD countries, more than a threefold increase is expected for the same period for the non — OECD countries. For the entire world, annual energy consumption for Year 2035 is predicted to be 770 quadrillion BTU, whereas that for year 1990 was recorded as 354 quadrillion BTU. A quadrillion BTU is 1 x 1015 BTU and is also called a quad. Assuming that the average heating value (HV) of gasoline is 125,000 BTU/gal, a quad of energy is equivalent to an aggregated heating value of approximately 8 billion U. S. gallons of gasoline.

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The marketed energy consumption of modern society is closely tied to its economic strength. It is also affected by the efforts of energy conservation and energy efficiency enhancement by various sectors. Petroleum prices on the global market have been largely dictated by the market’s supply and demand dynamics. Sharply increasing oil prices in recent years have been due to a lack of sufficient supply to meet the growing demand in the mar­ketplace, which was frequently intensified by regional supply disruptions caused by political unrest. High energy costs also add strong causes to infla­tion and adversely affect the global economy. Furthermore, it is difficult to predict or accurately assess technological advances in future energy tech­nologies with respect to the technoeconomic constraints of future societies. Therefore, an energy outlook forecast is by no means an easy task. The U. S. EIA presented five different cases/scenarios: viz. (1) reference case, (2) high oil price case, (3) low oil price case, (4) traditional high oil price, and (5) tra­ditional low oil price [4]. The reference case scenario is based on the baseline world economic growth at 3.5%/year for 2008-2015, 3.3%/year for 2015-2035, and the world light sweet crude oil prices growing to $125 per barrel by 2035 (2009 dollars). On the other hand, the high oil price case, a more pessimistic case scenario, is assuming that the world light sweet crude oil price grows to $200 per barrel by 2035 (2009 dollars) and the low oil price case, a more opti­mistic case scenario, is based on the assumption that the light sweet crude oil prices are going to be $50 per barrel (2009 dollars). The reference case scenario by the U. S. Energy Information Administration (EIA)’s forecast data for world energy consumption by fuel is presented in Table 1.1 [4].

As shown in Table 1.1, the lowest annual growth rate is estimated for liquid fuels or liquids among all fuel types. A very strong demand for transpor­tation vehicles is expected particularly for non-OECD nations, however, a more serious and rigorous global effort in vehicle fuel efficiency enhance­ment is also to be practiced, thus offsetting the effect from the increase in the number of vehicles. Even with the modest 1% increase, a large portion of the increase is expected to come from the increased consumption of biofuels as transportation fuels.

Table 1.2 shows the projections for world consumption of hydroelectricity and other renewable energy for the OECD and the non-OECD countries. As shown in Tables 1.1 and 1.2, the annual growth rate for this category is sub­stantially higher than the conventional fossil energy sources of petroleum, natural gas, and coal. However, it should be noted that common biofuels for transportation such as bioethanol and biodiesel are not included in this cat­egory of "Other," but in the category of "Liquids."

The category of "Liquids" in the EIA’s reporting, as also shown in Table 1.1, includes petroleum and other liquid fuels. Bioethanol and biodiesel, for example, are included in this category as other liquid fuels. Therefore, this category of "Liquids" includes both renewable and nonrenewable liq­uid fuels as well as conventional and unconventional supplies. Table 1.3 shows the world liquid fuel consumption by region for 1990-2035 [5]. The

Source: U. S. Energy Information Administration (EIA). 2011. International energy outlook 2011: World energy consumption by region and fuel. Tech. Rep. DOE/EIA-0484, September 19.

Note: (a) 1 quad = 1 x 1015 BTU. (b) The annual growth rate is taken for the period of 2008-2035. (c) "Other" in the table represents the hydroelectricity and other renewable energy.

worldwide consumption of petroleum and other liquid fuels in 1990 was 67 million barrels a day, 85.7 million in 2008, 83.9 million in 2009, and 86.0 mil­lion in 2010. A decrease in 2009 was due to the global recession. The total liquids consumption for the world is projected to increase to 112.2 mil­lion barrels a day (225 quadrillion BTU for the year) in 2035, which is an increase of 26.5 million barrels a day compared to 2008. Of the total increase from 2008 to 2035, 17.2 million barrels a day, or about 75% of the increase, is expected to come from non-OECD nations. This demand growth of liquids is driven by the projected world GDP growth of 3.6%/year for 2008-2020 and 3.2%/year for 2020-2035. This is also based on the aforementioned reference case scenario, where the world oil price rises to $125 per barrel (2009 dollars) by 2035. This also implies that in the long term, despite the high oil price assumed for the reference case, the liquids consumption will still increase steadily. In order to satisfy this increase in global liquids consumption in the reference case, liquids production has to increase by 26.6 million barrels per day from 2008 to 2035. Even with the expected increase in the world liq­uids consumption, the growth in demand for liquids in the OECD nations is expected to slow due to a number of factors including: (a) governmental policies, (b) efforts of increasing the fuel efficiencies of motor vehicles, and (c) various incentives. In Japan and OECD Europe, the consumption of liquids is predicted to decline by average annual rates of 0.4%/year and 0.2%/year, respectively [5].

The increased portion of the global liquids demand of 26.6 million barrels per day from 2008 to 2035 will have to come from both conventional supplies (such as crude oil and lease condensate, natural gas plant liquids, and refinery gain) and unconventional supplies (such as biofuels, oil sands, extra-heavy oil, coal-to-liquids (CtL), gas-to-liquids (GtL), and shale oil) [6]. Sustained high oil prices will provide incentives for the unconventional supplies by making them more competitive. Unconventional liquids production is predicted to be increasing at a rate of about 5%/year for 2008-2035, according to the refer­ence case. In all five oil price cases of projection models by the EIA, Canadian bitumen (oil sands) production is an important factor, making up more than 40% of the total non-OPEC unconventional liquids production, ranging from

Projected World Liquids Consumptions by Region: Reference Case

Source: Energy Information Administration (EIA). 2011. International Energy Outlook 2011: Liquid Fuels. Tech. Rep. DOE/EIA-0484, September 19.

Note: AGR = Annual growth rate in %/year.

3.1 million barrels/day (low oil and traditional low oil case scenarios) to 6.5 million barrels/day (high and traditional high oil price case scenarios).

Biofuels, which are the main topical area of this book, come under this unconventional liquids category in the EIA projections. The world biofuel production in 2010 exceeded 105 billion liters (28 billion U. S. gallons or 667 million barrels), which was a remarkable 17% increase from the 2009 pro­duction. The liquid biofuels, mostly made up of bioethanol and biodiesel, accounted for about 2.7% of the world’s transportation fuel in 2010 [7].

As a long-term projection, the global biofuels production in the reference case scenario of IEO2011 is projected to increase from 1.5 million barrels/day in 2008 to 4.7 million barrels/day in 2035, at an average annual growth rate of 4.3% per year. The largest increase in biofuels production and consump­tion is expected to be in the United States, whose annual biofuels production grows from 0.7 million barrels/day in 2008 to 2.2 million barrels/day in 2035 in the reference case scenario. Another very strong growth in biofuels pro­duction is expected to take place in Brazil, a traditionally strong biofuels nation, whose annual production will increase from 0.5 million in 2008 to 1.7 million barrels/day in 2035, based on the reference case scenario. To achieve these goals, many biofuel-using nations set mandates for the amount of bio­fuels used and provide tax credits or incentives for biofuel producers. For example, the United States mandates 36 billion gallons of biofuels by 2022 under the Energy Independence and Security Act of 2007 (EISA of 2007), which is explained in more detail in Chapters 3 through 5.

The combined total of the two nations accounts for about 84% of the world increase in biofuels production. It has to be noted here that the biofuel pro­jections by the EIA have received significant changes and adjustments in their predictions from the IEO2009 reference case to the IEO 2010 and 2011 reference cases. The revised projection of 2010 and 2011 shows a 40% lower projected biofuels production in 2030, compared to the IEO2009 reference case. This rather significant adjustment was made based on several com­pounded factors including:

• Some recent studies suggest that biofuels may not be as effective in reducing greenhouse gas (GHG) emissions as previously thought.

As a result, many countries such as Germany [8] have relaxed or postponed renewal of their mandates.

• The global economic recession of 2009 has dampened investment in biofuels development.

• Some of the timetables originally set for technological exploitation of new and enhanced biofuels technologies on commercial scales have been pushed back.

Biofuels will become more competitive with conventional petroleum prod­ucts over time, as the oil price remains high or continues to rise and new technologies are continuously introduced and enhanced. A strong relation­ship between future biofuels production and the future petroleum price trend is explicitly reflected in the IEO’s projections. The IEO2011’s low oil price case scenario predicts the total global biofuels production of 3.5 mil­lion barrels/day in 2035, whereas the IEO2011’s traditional high oil price case scenario predicts it at 6.2 million barrels/day in 2035[6].

It should also be noted that the future years’ projections of biofuels production will have to be continuously adjusted and modified based on evolving market conditions and changing technoeconomic constraints, which include: (a) oil price trend, (b) economic strengths of major markets, (c) global politics, (d) R&D progress of new technologies and commercializa­tion efforts, (e) changing views and concerns of environmental and sustain­ability issues with regard to biofuels utilization, (f) governmental mandates and incentives, and so on.