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bioweb.sungrant.org » Technical » Biofuels » Technologies » Biofuels from Syngas » Hydrogen from Syngas
| Hydrogen from Syngas—Microbial Conversion |
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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 (e.g. Fischer-Tropsch Synthesis) is a reliable technology and currently used industrially to produce a variety of biofuels and chemicals, such as liquid hydrocarbons, hydrogen, and methanol. Today, hydrogen can be produced commercially by converting methane to syngas and then using metal catalysts to further convert carbon monoxide to carbon dioxide and hydrogen. However, metal catalytic methods are 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.
Research is on-going to develop hydrogen from biomass feedstocks using microorganisms, rather than metal catalysts, to convert the syngas to hydrogen. A typical process for the microbial conversion of syngas to hydrogen involves (1) production of the syngas, (2) if needed, 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 hydrogen as it exits the reactor.
Gasification Process
In the gasification process, steam or oxygen (in the form of air or pure oxygen in lower than stoichiometric amounts of oxygen) are fed to a gasifier at high temperatures (greater than 700°C) to convert carbonaceous biomass into CO, CO2 and H2. CO can then be added to a microbial system to produce additional H2 via the water gas shift reaction.
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 can convert some of the CO to 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 microorganisms being used. Equations 5-10 show some of these reactions (Kuo, 2005).
CO + 3H2 → CH4 + H2O (5)
CxHy + 3/2O2 → CO2 + H2O +Cx-1Hy-2 (6)
N + OH → NO + H (7)
NO + O2 → NO2 + O (8)
S + OH → SO + H (9)
SO + O2 → SO2 + O (10)
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 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 as 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). |
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| Syngas Microbial Catalysts |
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Several genera of microbial catalysts are capable of consuming syngas as part of their metabolism and producing H2. These bacteria convert the CO component of syngas to H2 using a biological water-gas shift reaction similar to equation 4 (Amos, 2004; Klasson, 1993a,b; Wolfrum, 2002). This reaction is carried out by the enzymes carbon monoxide dehydrogenase (CODH) and hydrogenase contained within the bacteria. CODH catalyzes the oxidation of CO to CO2 (equation 11).
CO + H2O → CO2 + 2e- + 2H+ (11)
Hydrogenase enzymes then catalyze the formation of H2 (equation 12).
2H+ + 2e- → H2 (12)
Equation 4 is the sum of equations 11 and 12 that occur in a biological system. Inhibition of hydrogenase is of particular interest as it changes the metabolic pathway of the microorganism. Gases like O2, acetylene, CO, and nitric oxide (NO) are known inhibitors of hydrogenase (Acosta, 2003; Byung Hong Kim, 1984; Krasna, 1954; Seefeldt, 1989; Tibelius, 1984).
Photosynthetic bacteria such as Rhodospirillum rubrum, Rhodopseudomonas palustris, Rhodopseudomonas gelatinosa and Rubrivivax gelatinosus CBS, are capable of converting CO to H2 (Amos, 2004; Klasson, 1993a,b; Wolfrum, 2002).
Rhodospirillum rubrum is a purple-red non-sulfur bacterium that is known for its high CO uptake rates and hydrogen yields (Najafpour, 2004). This bacterium is spiral shaped, and is usually found in mud, sewage and pond water. Rsp. rubrum can utilize organic acids as well as CO for its metabolism. The presence of acetic acid in the medium affects CO uptake by this organism and 1-2 gram/liter is the optimum range of acetic acid for the production of high H2 yields (Najafpour, 2004).
Rhodopseudomonas palustris is a purple, non-sulfur phototrophic bacterium that grows in the presence of light, but metabolizes CO to produce H2 in the absence of light. Rps. palustris is a nutritionally versatile microorganism that can degrade a wide variety of aromatic compounds under aerobic and anaerobic conditions (Harwood, 1988).
Rubrivivax gelatinosus CBS is a purple non-sulfur bacterium which has a curved rod shape. Once the hydrogen producing pathway is induced in this organism, it can proceed equally well in both light and darkness. In the absence of light, the organism can utilize CO as the sole carbon source (Maness, 2002). |
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| Fermentation Reactors |
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The microbial production of hydrogen from biomass first involves gasification of the biomass to produce syngas, followed by conversion of the syngas to hydrogen in a fermentation reactor.
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). Among the potential biofuel reactors, trickle-bed reactors (TBR) have been used to produce hydrogen from CO (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.
Bubble-column reactors can be used for the production of hydrogen as well as ethanol from syngas. These reactors have a large height-to-diameter ratio (aspect ratio) in which high mass transfer can be obtained even without the use of additional agitation. Smaller bubble size and improved gas dispersion can be obtained by using porous fritted discs to disperse the syngas (Vega, 1990). However, bubble columns are associated with high pressure drops at large capacities. |
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| Commercial Production of Hydrogen Via Microbial Conversion of Syngas |
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Today, most commercial production of hydrogen from syngas uses metal catalysts and natural gas as the feedstock. Efforts to develop microbial processes using biomass are on-going. Product yields for the conversion of syngas to hydrogen using microorganisms are shown in table 3.
Compared with metal catalytic processes, microbial hydrogen 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 reduce 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).
Disadvantages of microbial processes include the long reaction time of the water-gas shift reaction due to the slow cell growth of the microorganisms. 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 hydrogen (Krasna, 1954; Tibelius, 1984). |
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| References |
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