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bioweb.sungrant.org » Technical » Biofuels » Technologies » Biofuels from Syngas » Ethanol from Syngas

Ethanol from Syngas—Microbial Conversion	Printer Friendly

The production of biofuels from biomass-generated syngas is an emerging technology that can utilize a wide variety of biomass resources. The process, as outlined in Figure 1, involves first converting biomass (e.g. prairie grasses, wood chips, paper wastes, agricultural wastes, etc.) to syngas (a mixture of carbon monoxide, carbon dioxide, and hydrogen) via a process called gasification.  Gasification is the partial oxidation of biomass at high temperatures. The syngas can then be converted to biofuels such as methanol, ethanol and hydrogen using either a metal catalyst or a microbial catalyst (e.g. bacteria). In addition to biomass, a number of other feedstocks can be used to produce syngas including natural gas (methane), coal, and a combination of coal and biomass denoted as co-firing (Dayton, 2003). Though many of these processes are well-established, interest in producing syngas from biomass is increasing due to its abundant availability and renewable nature.          The use of metal catalysts is a reliable technology and currently is used in industry to produce a variety of biofuels and chemicals. However, this method is limited by the need for extreme reaction conditions (high pressures and temperatures) which require high energy inputs. The selectivity of conversion is low, and some catalysts are very sensitive to contaminants in the syngas and are easily poisoned. Additionally, the reactions generate substantial quantities of heat which must be removed for the reactions to proceed. These challenges contribute to the high cost of producing biofuels and chemicals from syngas using metal catalysts. An alternative method is to use microbial catalysts to convert syngas to ethanol.     Current commercial production of fuel ethanol primarily involves fermentation of sugars, rather than syngas, that are typically derived from corn grain or sugarcane. Interest has greatly expanded to obtaining the sugars from the breakdown of cellulosic materials although not all cellulosic components are composed of sugars. The gasification process can potentially utilize the entire biomass feedstock, both the sugar-containing components of cellulose and hemicellulose and the non-sugar containing components of lignin, which provides a greater conversion efficiency of biomass to energy or chemicals (McKendry, 2002a).   A typical process for the conversion of syngas to ethanol involves (1) production of the syngas, (2) removal of contaminants from the syngas before entering the microbial conversion process, (3) if needed, adjustment of the syngas H2/CO ratio (which differ by feedstock and gasifier conditions) to fit the microbial process, (4) passage of the syngas through the microbial reactor at the appropriate temperature, and (5) collection and purification of the products as they exit the reactor.

Gasification
In the gasification process, steam or oxygen (in the form of air or pure oxygen in lower than stoichiometric amounts) are fed to a gasifier at high temperatures (greater than 700°C) to convert carbonaceous biomass into CO, CO2 and H2 which form the building blocks for ethanol production via fermentation.    Gasification of biomass to produce syngas involves three main steps--initial heating to dry out any moisture embedded in the feedstock; pyrolysis (heating the feedstock up to 300-500oC in the absence of oxidizing agents) to produce gas, tars, oils and solid char residue; and gasification of the solid char, tars and gas to yield the primary components of syngas (Bridgwater, 2003).    The gasification of carbonaceous biomass occurs via three main reactions—partial oxidation (equation 1), complete oxidation (equation 2), and the water gas reaction (equation 3) (McKendry, 2002b).             C + ½O2 → CO               ΔH298K = -268 KJ/mol               (1)           C + O2 → CO2                    ΔH298K = -406 KJ/mol               (2)           C + H2O → CO + H2        ΔH298K = 118 KJ/mol                 (3)   In addition, the water gas shift reaction plays an important role in the composition of the CO, CO2, and H2 (equation 4).              CO + H2O → CO2 + H2                                               (4)   Syngas can also contain a number of other compounds such as methane (CH4), acetylene, ethylene, ethane, nitric oxide (NO), sulfur dioxide (SO2), tars, and ash which can have complex effects on the metal catalyst or microorganisms used. Equations 5 through 10 show some of these reactions (Kuo, 2005).                          The impurities differ by feedstock and gasification process. Coal gasification produces more NOx and SOx impurities, whereas biomass gasification produces more hydrocarbon impurities (Kuo, 2005; McIlveen-Wright, 2006). Impurities are typically dealt with by including a gas clean-up process downstream of the gasifier or by modifying the design of the gasifier to produce lower levels of impurities (e.g., lower tar production). The addition of catalysts (such as CaO) can also be used to reduce tar (Corella, 2006). Research is needed to better understand how impurities affect microorganisms and their associated enzymatic functions.   The mix of gases contained in the syngas varies as a result of the feedstock type and oxidizing agent used (air, pure O2 or steam).   Air (because of the high inert N2 content) increases the volume of nitrogen in the syngas which increases the downstream cost of cleaning and storing syngas. However, the added cost of the air process must be balanced with the extra cost of using pure O2 or steam as the oxidizing agent. Steam produces a higher H2 content because of the water gas-shift reaction (equation 4), but requires more energy to create (Bridgewater, 2006). The differences in the mixture of gases using air-steam and air-catalysts (i.e., 20-30 wt% silica sand and dolomite) for a variety of different feedstocks are shown in tables 1 and 2 respectively (McIlveen-Wright, 2006). The air-catalyst process produces less methane and has a lower hydrocarbon volume percentage than occurs with the air-steam process.                          Gasification Reactors. Several different types of gasifiers can be used to produce syngas from biomass.   Counter-current fixed bed (updraft) gasifiers consist of a fixed bed of biomass with a counter current flow of steam, oxygen and/or air flowing upward through the fuel bed. In order to form a fixed bed that is permeable to the flow of the oxygen source, the biomass fuel must have high mechanical strength and not cake into an impermeable mass. Gas exit temperatures are low, which improves thermal efficiency, but increases tar and methane impurities in the gas (McKendry, 2002b; Bridgwater, 2006).  Co-current fixed bed (downdraft) gasifiers are similar to updraft gasifiers except that the steam, oxygen or air flows co-currently downward with the fuel. Because the gas passes through the hot char at the bottom of the bed before exiting, some impurities such as tars are trapped in the char and the final product has a higher purity. The exit temperature of the gas is higher, resulting in a lower overall efficiency (McKendry, 2002b; Bridgwater, 2006). The fuel in a fluidized bed gasifier is gasified in an oxygen/air and steam mixture. The ash is often removed using a cyclone. Fuel throughput is higher than for fixed bed gasifiers, and has the advantage of uniform temperature distribution achieved in the gasification zone resulting in cleaner reactions. However, conversion rates are lower, requiring the recycling of part of the exit gas back to the gasifier. Fluidized beds work particularly well for biomass, as biomass resources contain higher levels of corrosive ashes that can harm fixed bed reactors (McKendry, 2002b; Bridgwater, 2006).  In entrained flow gasifiers, the fuel is fed either as a dry pulverized solid or a fuel slurry in tandem with oxygen (or sometimes air). Gasification takes place in a dense cloud of fine particles and is particularly useful for coals which can be easily pulverized into fine particles. This gasifier has the highest operating temperature and pressure, which decreases the amount of tars and methane formed during gasification (McKendry, 2002b; Bridgwater, 2006).    Gasifier manufacturers indicate that of the commercial gasifiers in use, 75% are co-current, 20% are fluidized beds, 2.5% are counter-current, and 2.5% are other designs (Van Loo, 2003).

Syngas Microbial Catalysts
The commercial production of chemicals from syngas typically uses metal catalysts, but an alternative route to produce ethanol from syngas uses microbial catalysts (e.g. bacteria). Several genera of microorganisms are capable of consuming syngas as part of their metabolism and producing chemicals such as ethanol and other products (e.g. acetic acid). The overall stoichiometry for the formation of ethanol using syngas substrates are shown in equations 11-13 (Vega, 1989).           6 CO + 3 H2O → CH3CH2OH + 4 CO2                        (11)                2 CO2 + 6 H2 → CH3CH2OH + 3 H2O                         (12) 6 CO + 6 H2 → 2 CH3CH2OH+2 CO2                              (13)             Clostridium ljungdahlii and Clostridium autoethanogenum were among the first organisms identified that convert CO, CO2 and H2 (syngas) to ethanol and acetic acid (Abrini, 1994; Vega, 1990). C. ljungdahlii, first isolated in 1987, is a gram-positive, rod-shaped anaerobe capable of fermenting sugars such as xylose and fructose in addition to syngas (Klasson, 1992). This organism favors the production of acetate during its active growth phase while ethanol is produced primarily as a non-growth related product (Klasson, 1992). The production of acetate is favored at higher pH (5-7), whereas the production of ethanol is favored at lower values (pH 4 to 4.5).   Clostridium autoethanogenum is a strictly anaerobic, gram-positive, spore-forming, rod-like, motile bacterium which metabolizes CO to form ethanol, acetate and CO2 as end products. It is also capable of using CO2 and H2, pyruvate, xylose, arabinose, fructose, rhamnose and L-glutamate as substrates (Abrini, 1994).   Eubacterium limosum has been isolated from various habitats including the human intestine, rumen, sewage and soil. It has a high growth rate under high CO concentrations and can ferment syngas to produce acetate, ethanol, butyrate and isobutyrate (Chang 1998, 1999, 2001).    Peptostreptococcus productus is a gram-positive anaerobic coccus, found in the human bowel, and capable of metabolizing CO2, H2 or CO to produce acetate (Lorowitz, 1984). Studies have shown that although acetate is one of the primary end-products of its metabolism, P. productus can also form additional products in response to CO2 limitations (Misoph, 1996).   Clostridium carboxidivorans P7T is a novel solvent-producing anaerobic microbe which was isolated from the sediment of an agricultural settling lagoon. It is motile, gram-positive, and spore-forming and forms acetate, ethanol, butyrate, and butanol as end-products. The optimum pH range for this strain is 5.0-7.0 and the optimum temperature range is 37-40 ºC (Liou, 2005).   All of these microorganisms are acetogens. Acetogens are a versatile group of microorganisms that can use gases like CO2, H2 and CO, as well as sugars and other substrates (Drake, 1994; Wood, 1986b,c). They also are anaerobic microorganisms that utilize the acetyl-CoA pathway as their predominant mechanism to produce acetyl-CoA from CO2. Acetyl-CoA is subsequently a precursor to the production of other compounds such as lipids, amino acids, nucleotides and carbohydrates (Ljungdahl, 1986).   The acetyl-CoA pathway (Wood-Ljungdahl pathway) consists of two components—the methyl branch and the carbonyl branch (figure 2). As acetogens are anaerobic, CO2 rather than oxygen serves as the electron acceptor and H2 usually serves as the electron donor. The synthesis of acetyl-CoA from CO2 and H2 requires the reduction of CO2 and involves (1) the formation of the carbonyl precursor of acetyl-CoA by the enzyme acetyl-CoA synthase, also known as carbon monoxide dehydrogenase (CODH), (2) the formation of the methyl precursor of acetyl-CoA, and (3) condensation of the above two precursors to form acetyl-CoA (Wood, 1986a). Acetyl-CoA can then be used to make products such as ethanol, acetic acid, butanol, butyric acid, and/or cell mass (figures 2 and 3).                   Acetyl-CoA is a versatile intermediate chemical in the metabolic pathway of acetogenic microorganisms and is a precursor to the production of lipids, amino acids, nucleotides and carbohydrates (Ljungdahl, 1986). It is the source for cellular carbon as well as cellular energy (figure 3).                 In the methyl branch, CO2 (or CO2 formed from CO via CODH) is reduced to formate (HCOO-) which is subsequently converted to a methyl-corrinoid protein (Ljungdahl, 1986; Ragsdale, 1991). In the carbonyl branch, CO2 is reduced to form a bound CO complex with CODH. Acetyl-CoA is produced from the combination of the methyl-corrinoid protein and the CO complex (Diekert, 1994).   Electrons are required during several steps of the metabolic process.   In conjunction with other enzymes, hydrogenase enzymes utilize hydrogen to produce electrons required for the formation of ethanol, acetic acid, and/or cell mass. In the absence of hydrogen, electrons can be generated through the oxidation of CO to CO2 via the CODH enzyme.  Often microorganisms contain several different hydrogenases, and in many cases the functions of these enzymes are difficult to determine. Inhibition of hydrogenase is of particular interest as it reduces the ability of the microorganism to consume hydrogen, which causes the microorganism to use CO as a source of electrons at the expense of using the CO for ethanol. Gases such as O2, acetylene, CO, and nitric oxide (NO) are known inhibitors of hydrogenase (Seefeldt, 1989; Acosta, 2003; Byung Hong Kim, 1984; Krasna, 1954; Tibelius, 1984;  Girbal, 1995a; Meyer, 1985, 1986; Hyman, 1988, 1991).   Acetyl-CoA goes through the catabolic pathway (figure 3) in order to make ATP (adenosine triphosphate) which provides the microorganism with energy. It is also by this route that the acetyl-CoA is converted to acetate, and is the pathway favored by the microorganism during its rapid growth phase. This phase of metabolism is known as the acidogenic phase due to the increased acidity of the growth medium from the production of acids (Rao, 1989). The solventogenic phase is where ethanol is produced. This phase is characterized by slower growth of the microorganism with little or no production of ATP. Butanol and butyric acid can also be produced by combining two molecules of acetyl-CoA to form butyryl-CoA.   Several factors affect the switch from acidogenesis to solventogenesis including pH, ATP demand, availability of nutrients, availability of reducing equivalents, and enzyme activities (Meyer, 1989; Girbal, 1995a,b; Grube, 2002; Kashket, 1995; Kutzenok, 1952). Due to the uncertainty in the switch from acidogenesis to solventogenesis, several research teams have studied methods of inducing solventogenesis including the addition of acetate and butyrate (Gottschal, 1981) or yeast extract and reducing agents (Klasson, 1992) to the culture medium, as well as altering the availability of nutrients (nitrogen, sugar, iron) (Girbal, 1995a,b; Meyer, 1985; Byung Hong Kim, 1984). Addition of artificial electron carriers (e.g., methyl viologen, benzyl viologen, and neutral red) has been shown to promote alcohol production (Girbal, 1995a,b; Klasson, 1992).

Fermentation Reactors
Several reactor designs can be used for the fermentation process. Trickle-bed reactors (TBR) consist of a vertical tubular reactor, packed with solid material that the microorganisms can attach to. The direction of fluid-flow is normally counter current, with the liquid trickling downwards as the gases flow upwards (Amos, 2004; Wolfrum, 2002).   Continuous stirred-tank reactors (CSTR) are commonly used in syngas fermentation. A CSTR has a continuous flow of gas bubbling through the liquid which typically consists of a dilute solution of essential nutrients for the microorganism to grow and survive on. The liquid is continuously added and removed from the reactor.  High agitation is needed to enhance the transfer rate of the CO, CO2, and H2 from the syngas to the organisms (Klasson, 1992). If the transfer is not fast enough, the production of cellular products will be limited to how fast the gas is transferred to the organism. Microbial cell recycle systems can be used in conjunction with the CSTR to increase cell density within the reactor. In such a system, the fermentation broth is pumped through a recycle filter and the retentate containing the microbial cells is separated from the permeate (cell-free media) and recycled back to the bioreactor. This process prevents loss of cell mass from the bioreactor during continuous operation and also allows the CSTR to be operated at dilution rates greater than the maximum growth rate of the microbial catalyst. Recycling has been shown to provide a 2.6 fold increase in cell concentration (Klasson, 1993a,b).   Packed-bed reactors (immobilized-cell reactors) are columns packed with biocatalyst particles to which the microorganisms are immobilized (Bailey, 1986). These reactors are usually operated concurrently where the liquid and gas flow in the same direction (Klasson, 1992). Advantages of this reactor include high density of the microorganisms and easy separation of the microbial cells from the fermentation broth. However, the rate at which syngas components are transferred to the organism is usually slow.   Commercial Production of Ethanol Via Microbial Conversion of Syngas. Ethanol is not currently produced on a commercial basis using microbial fermentation of syngas. This is a new technology still in the research phase, with most research conducted at a laboratory scale using synthetic syngas (mixed from commercial gases). As summarized in table 3, a number of research groups have reported product yields for the conversion of syngas to ethanol using microorganisms.          Compared with metal catalytic processes, microbial ethanol from syngas processes can be operated at relatively low pressures and temperatures. Most biological enzymes operate at close to ambient temperatures which reduce costs (Wolfrum, 2002) and occur in the dark, enabling the use of closed reactors which use simpler reactor designs and have lower costs. Microbial enzymes have a higher tolerance for syngas contaminants than metal-catalysts (which are very susceptible to poisoning), and are even capable of adapting to contaminants like tar within certain limits (Ahmed, 2006; Ragauskas, 2006). The production of ethanol from biomass-derived syngas circumvents problems such as solids handling and disposal of the unconverted lignin that occur in conventional lignocellulosic fermentation processes.   Disadvantages of microbial processes include the long reaction time of the water-gas shift reaction due to the slow cell growth of the microorganisms. Since the reaction is anaerobic, it does not provide as much energy for cellular metabolism as do photosynthetic and aerobic reactions. Syngas fermentation is often limited by low productivity and the rate at which syngas can transfer to the liquid (Worden, 1997), which places a premium on good bioreactor design and high cell-densities of the microorganism to make the process economically feasible. At high partial pressures, nitric oxide (NO) and carbon monoxide (CO) contaminants in the syngas can inhibit the hydrogenase enzyme that is involved in the conversion of syngas to ethanol (Krasna, 1954; Tibelius, 1984).

References
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