CSTRs are the most commonly employed bioreactors in syngas fermentation. The CSTR has a continuous gas supply into the liquid phase, while agitator controls the gas-liquid mass transfer (Vega et al., 1990). Higher agitation speeds lead to a higher mass transfer rate between the substrate gases and the microbes. However, in industrial-scale fermentors, higher agitation speeds increase the agitator’s power consumption, thus increasing the operational cost of the plant. Table 2 summarizes some of the performance parameters for various bioreactors.

combined with a single-point gas entry

n/a, not applicable.

aAgitation speed. bSulfate-reducing bacteria.

Investigation and large-scale application of co-gasification and co-pyrolysis of biomass and coal are becoming more common recently. In addition to the reduction of CO2 emission, cogasification of biomass provides several advantages over biomass or coal gasification (Kumabe et al., 2007). One of the advantages is the reduction of sulfur and ash that cause equipment corrosion and environmental problems in coal gasification (Chmielniak and Sciazko, 2003; McLendon et al., 2004). It can also reduce the high cost of the feedstock and high tar generation in biomass gasification.

Likewise, co-pyrolysis has advantages over sole biomass or coal pyrolysis. Although pyrolysis of coal is a good method for producing liquid fuels, the yields of these products are limited because of the low H/C ratio in coal. The high H/C ratio in biomass renders bio­mass to act as a hydrogen donor in co-pyrolysis of biomass/coal blends. Moreover, the high thermochemical reactivity and high content of volatiles of biomass facilitate the conversion and the upgrading of the fuel. Therefore, it’s considered promising to co-fire the two fuels as a step toward valid, sustainable utilization of coal and biomass and to minimize the impact on the environment.

Pyrolysis gas has a high heating value, 17 MJ/kg (http://www. nh. gov/oep/programs/ energy/documents/biooil-nrel. pdf), and both pyrolysis oil and char can be gasified to pro­duce syngas; it is a promising technique to further process the pyrolysis products through gasification to produce syngas more efficiently. Here, the process consists of the pyrolysis and subsequent gasification sections. In the first reactor, biomass is pyrolyzed with coal at 500-700 °C. The pyrolysis gas is quenched to produce liquid oil, and the char is flowed to the gasifier where steam and limited air are supplied to produce syngas.

Biomass-derived syngas often contains additional constituents such as CH4, some higher hydrocarbons (C2H2, C2H4 and C2H6), tar, ash, and char particles. Since most of the researchers use bottled synthetic gas mixtures for syngas fermentation studies, there are limited studies that examine the effects of impurities on syngas fermentation (Ahmed and Lewis, 2007). The authors reported the effects of NO on hydrogenase activity, cell growth, and product distribution using C. carboxydivorans. The authors further concluded that NO concentration below 40 ppm had no significant effect on syngas fermentation process. Ahmed et al. (2006) reported that tars could promote cell dormancy and product redistribution (etha­nol and acetic acid) during syngas fermentation. Kundiyana et al. (2010) examined the ability of Clostridium ragsdalei ((ATCC BAA-622), previously known as strain Pll) to grow and metabolize CO under microaerophilic condition (5% O2) in a pilot-scale fermentor. Further, the growth of acetogens such as M. thermoacetica and Clostridium magnum in a medium supplemented with 21% O2 was reported by Karnholz et al. (2002). These findings provide the prospects of scaling-up syngas fermentation for commercialization. Furthermore, the microbial catalysts used in syngas fermentation had higher tolerance to toxic gases and trace contaminants such as hydrogen sulfide (H2S) and carbonyl sulfide (COS) than that of bio­chemical pathway (Worden et al., 1991; Younesi et al., 2005).

A vortex tube has certain advantages as a chemical reactor, especially if the reactions are endothermic, the reaction pathways are temperature dependent, and the products are temperature sensitive. With low-temperature differences, the vortex reactor can transmit enormous heat fluxes to a process stream containing entrained solids. This reactor has nearly plug flow and is ideally suited for the production of pyrolysis oils from biomass at low pressures and residence times to produce about 10 wt% char, 13% water, 7% gas, and 70% oxygenated primary oil vapors based on mass balances. This product distribution was verified by carbon, hydrogen, and oxygen elemental balances. The oil production appears to form by fragmenting all of the major constituents of the biomass. Cyclonic fast pyrolysis, also called vortex fast pyrolysis, separates the solids from the noncondensable gases and returns them to the mixer.

The bubble column consists of a glass column mounted on a steel base (Datar et al., 2004). The substrate gases are introduced into the reactor through a fritted glass disk with a pore size of 4-6 microns. These reactors are mainly designed for industrial applications with large reac­tor volumes. High mass transfer rate and relatively low operational and maintenance cost are among the merits of this type of reactor, while back mixing and coalescence are common drawbacks of the system (Datar et al., 2004).

4.3.1 Trickle-bed Reactors

Trickle-bed reactor is a slender column with a packing media. The microbial culture media continuously flows down and the gaseous substrate flows either upward (countercurrent) or downward (cocurrent) direction depending on the application (Munasinghe and Khanal, 2010). The trickle-bed reactors are operated under the atmospheric pressure and no agitation is necessary. Therefore, trickle-bed reactors consume less energy than the conventional CSTR (Bredwell et al., 1999).

Hydrolysis pathways are appropriate for lignocellulose processing if higher selectivity is desired in biomass utilization, for example, in the production of chemical intermediates or targeted hydrocarbons for transportation fuel. Selective transformations require isolation of sugar monomers, a step which is complex and expensive for lignocellulosic feedstocks. Once sugar monomers are isolated, however, they can be processed efficiently at relatively mild conditions by a variety of catalytic technologies (Alonso et al., 2010).

The ability to recover and use the major components of lignocellulosic biomass (cellu­lose, hemicellulose, lignin) is critical in developing economically viable bioproducts and biorefineries. This project focuses on the biomass pretreatment step of hemicellulose acid hydrolysis to recover the hemicellulose sugars and prepare the biomass for subsequent enzymatic or acid cellulose conversion. The ultimate goal is to identify promising routes to reduce the sugar production cost by 30% compared with established methods. Researchers are investigating three hydrolysis systems: water-rich hydrolysis, water-restricted, and near neutral pH. Using different reactor configurations (e. g., batch tube, Parr, flow through) with varying solids and pH levels, researchers have developed comprehensive data on the destructuring, disaggregation, and depolymerization of hemicellulose to sugars. Flow rate has been found to enhance hemicellulose removal, which is inconsistent with models typi­cally applied to describe hemicelluloses hydrolysis. New models have been defined that reveal mass transfer could be important in explaining this anomaly. The flow through reactor experiments showed that lignin is modified as hemicellulose reacts, and the resulting disruption of lignin may play a significant role in enhancing cellulose digestion. In addition, researchers have shown that nonproductive adsorption on lignin can be reduced by prior treatment with low-cost proteins, thereby substantially cutting enzyme costs (Iranmahboob et al., 2002; Mosier et al., 2005; Patrick Lee et al., 1997; Wang et al., 2007; Yat et al., 2008).

The ideal process for cellulosic biomass conversion would be the production of liquid fuels from biomass in a single step at a short residence time. The liquid product produced in pyrolysis is called bio-oil, which is an acidic combustible liquid containing more than 300 compounds (Wang et al., 2008). Bio-oils are not compatible with existing liquid

transportation fuels including gasoline and diesel. To use bio-oil as a conventional liquid transportation fuel, it must be catalytically upgraded (Carlson et al., 2008). Zeolite catalysts added into the pyrolysis process can convert oxygenated compounds generated by pyrolysis of the biomass into gasoline-range aromatics.

In general, the gasification of biomass is often followed by a gas clean-up and conditioning. The gas mixture is passed through a series of cyclones and filters to remove most of the unde­sirable pollutants (e. g., tar, particulate matter, and char). Datar et al. (2004) employed a con­densation tower followed by acetone scrubbers to remove tar and moisture from the producer gas. A series of 0.025-gm filters were successfully used to clean producer gas mixture to pre­vent cell dormancy and product redistribution (Ahmed et al., 2006). Further, trace amounts of NO can be removed by using chemicals such as sodium hypochlorite, potassium permanga­nate, or sodium hydroxide. Syngas fermentation shows a high tolerance toward sulfur gases such as hydrogen sulfide (H2S), and COS. Vega et al. (1990) found that CO-utilizing methanogenic microbes can grow in the presence of H2S up to 2%.

The rotating cone reactor is a novel reactor type for fast pyrolysis of biomass with negli­gible char formation, in which rapid heating and short residence time of the solids can be realized. Particles fed into the reactor first enter an impeller which is mounted in the base of the heated cone. After leaving the impeller, the particles flow outward over the conical surface and experience a high heat transfer rate due to their small distance from the heated surface. Biomass materials like wood, rice husks, or even olive stones can be pulverized and fed to the rotating cone reactor. Flash heating of the biomass will suppress coke-forming cracking reactions. Since no carrier gas is needed (cost reducing), the pyrolysis products will be formed at high concentrations. If additional thermal quenching of the gas outlet flow is applied, the amount of secondary tar decomposition reactions can be suppressed. In the rotating cone reactor, wood particles fed to the bottom of the rotating cone, together with an excess of inert heat carrier particles, are converted while being transported spirally upward along the cone wall. The cone geometry is specified by a top angle of p/2 radians and a max­imum diameter of 650 mm. Products obtained from the flash pyrolysis of wood dust in a rotating cone reactor are noncondensable gases, bio-oil, and char. The biomass decomposes into 70% condensable gases with 15% noncondensable gases and 15% char.

Microbubble sparged reactor is a combination of a CSTR reactor and a microbubble dis­perser. In this reactor configuration, the large specific interfacial area of the gas bubbles and the longer retention time stimulate the high gas-liquid mass transfer.

4.3.2 Membrane-based Reactors

CHFM have been proposed as a technologically and economically feasible method for syn­gas fermentation. Even though the technology is yet to be adopted for syngas fermentation, it has been widely studied in water and wastewater treatment. In CHFM reactors, the gas is introduced through the membrane fibers. The microbes grown as a biofilm on the surface of the membrane fibers utilize the CO and H2 and produce biofuels. This novel CHFM system offers several advantages such as higher microbial cell retention, higher yield, and higher tol­erance to toxic compounds (tar, acetylene, NOx, etc.).

4.4 pH

pH is a critical parameter to obtain optimal microbial activity in the culture media. In gen­eral, acidogenic reactions are more favorable at higher pH values (6.0-7.0), whereas solventogenesis (alcohol production) requires low pH values (4.0-4.5) (Klasson et al., 1993). Researchers employed pH shift approach in order to achieve high alcohol yields. Low pH, however, inhibits the cell growth (Klasson et al., 1992). The optimum pH for most of the syngas-fermenting microbes varies between 4.5 and 7.3 depending on the species. For example, C. Ijungdahlii has an optimum pH of 5.8-6.0.

In addition to being a resource for energy generation, lignocellulosic biomass has potential to serve for multiple purposes. There is not necessarily a concurrence of various options. Highest value can be achieved by diverting individual components to optimum routes, thus aiming to achieve complete valorization of the material. Among possible target utilization options are to be mentioned in particular: (i) electricity and fuel generation; (ii) production of chemicals; (iii) precursors for industrial products such as biodegradable plastics; (iv) utilization as soil amendment.

The idea of so-called biorefineries is to process bioresources such as agricultural or forest biomass to produce energy and a wide variety of precursor chemicals and bio-based materials (Sigrid and Morar, 2009). Petroleum refineries are already built, and use of this existing infra­structure for the production of biofuels requires little capital investment (Marinangeli et al.,

2006) . Furthermore, the infrastructure for blending fuels as well as their testing and dis­tribution is already in place at oil refineries. Three options are available for using petroleum refineries to convert biomass-derived feedstocks into fuels and chemicals: (i) fluid catalytic cracking (FCC), (ii) hydrotreating-hydrocracking, and (iii) utilization of biomass-derived synthesis gas (syngas) or hydrogen.

Cofeeding biomass-derived molecules into a petroleum refinery could rapidly decrease our dependence on petroleum feedstocks. Petroleum-derived feedstocks are chemically dif­ferent than biomass-derived feedstocks; therefore a new paradigm in how to operate and manage a petroleum refinery is required. Another improvement toward the production of biofuels in a petroleum refinery would be if governments were to offer tax exemptions and subsidies to all types of biofuels, and not only for selected biofuels such as ethanol and biodiesel. As the price of petroleum continues to increase, we project that refining tech­nology will be developed to allow the coproduction of bio — and petroleum-based fuels in the same (petroleum) refinery and even using the same reactors. A realistic practical sce­nario will be one in which both industries cooperate, with one producing the biofuel precursors and the other processing and converting them into valuable fuels (Huber and Corma, 2007).