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

FIGURE 5.12 A schematic of VEGA process for biomass gasification.

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

Biomass Condensate — Syngas FTS Product BtL Synfuel Feed Char Slurry FIGURE 5.15 FZK process concept for BtL synfuel.

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].

TABLE 5.12 Physicochemical and Transport Properties of Water at Supercritical and Ambient Conditions Ambient Water Properties Compared Supercritical Water Negligible to Low Organic Solubility Very High Very High to High Inorganic Solubility Negligible to Very Low Higher Density Medium to High Higher Viscosity Lower Lower Diffusivity Higher ~80 Dielectric Constant 5.7 at Critical Point High Polarity Low Not Corrosivity Somewhat Lower Energetics Highly Energized Fire Extinguishing Oxidation Ideal Combustion Medium 9.2 mg/L Oxygen Solubility In any Proportion Low H2 Solubility Very Low

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.