The major obstacle for the large-scale implementation of biomass gasification is the ‘tar problem’. The circulating fluidised bed gasifier typically produces ~10 g/ M03. Tars in the synthesis gas give rise to fouling, as they deposit on the walls of the system when the tars, due to cooling, condensate. There are numerous purification processes, but few of them are suitable for our application. The OLGA3 concept seems to be most promising for this process. This concept reaches highest efficiency, has been tested thoroughly and is commercially available. Besides that, this system is designed especially for this purpose. For thermal tar cracking, high temperatures will have to be reached, which leads to extra loss of energy and therefore lower efficiency. The catalytic tar cracking also seems to be promising, but isn’t commercially obtainable yet. Besides that, one of the boundary conditions to the plant is that as few as possible additives are to be used. The catalyst in this concept needs extra care especially as deactivation can be a problem and the higher investment costs.

17.3.3 Fermentation

After gasification and gas purification the next process step to produce ethanol is fermentation. Fermentation occurs when bacteria metabolise a material into another one. In this case the cleaned synthesis gas can be converted anaerobically into ethanol. Biological production of chemicals from synthesis gas offers several advantages over catalytic techniques:

• Biological conversion occurs under mild temperatures and pressures, whereas catalytic reactors are operated at high temperatures and pressures.

• The reaction specificity of enzymes is typically higher than that of inorganic catalysts.

• Most biological catalysts are tolerant to sulphur gases, reducing the cost of gas cleanup prior to the conversion step.

• In the fermenter the waste gas shift takes place biologically so preventing the use of a separate shift reactor for adjusting the CO/H2 ratio.

Type of bacteria

Several acetogenic microbes are capable of metabolising synthesis gas into ethanol. Two of the more promising strains are described below. Both are gram­positive bacteria, this means they are characterised by having as part of their cell wall structure peptidoglycan as well as polysaccharides and/or teichoic acids. The peptidoglycans are heteropolymers of glycan strands, which are cross-linked through short peptides.4

• Butyribacterium methylotrophicum (Bredwell et al., 1999): It is a gram­positive, motile, rod-shaped anaerobic bacterium, which grows on a wide variety of substrates, including glucose, formate and methanol, H2 and CO2, and CO. The latter two are of interest to us. The products achieved are acetic acid, butyric acid, ethanol and butanol. Production of ethanol and butanol is usually low. When production of ethanol and butanol is increased, butanol is dominant.

• Clostridium species: In the case of clostridial fermentation it has been proposed that acetyl-CoA is the central intermediate (Roger, 1986). The first of the clostridium species was isolated from chicken waste and grew well on the components of synthesis gas to produce acetate and ethanol. The first optimisations of this species resulted in an approximate 1:1 ratio of ethanol and acetate under optimal conditions (Vega, 1989). After that the developments of the clostridium species have been many.

— Clostridium ljungdahlii: It is a gram-positive, motile, rod-shaped anaerobic bacterium, which converts CO, H2 and CO2 into a mixture of acetate and ethanol. The ratio of these products can be adjusted by pH. When the pH is lowered to 4 the ratio ethanol: acetate becomes 3: 1 (Gaddy and Fayetteville, 1995). Further medium adjustment has reportedly nearly eliminated acetate production and led to an ethanol concentration of 48 g/L (approximately 1 mol/l) on day 25 when using an optimised medium (Phillips et al, 1993). This means the separation of ethanol from water is feasible.

— Clostridium carboxidovorans (P7) (Rajagopalan et al., 2002): P7 is a gram­positive, motile, rod-shaped anaerobic bacterium, which converts CO, H and CO2 into a mixture of acetate, butanol and ethanol. The ratio ethanol: butanol:acetate is 6:3:1 in absence of hydrogen. Further developments are expected to include hydrogen and inhibition of the butanol step.

The clostridium species has the most potential and is best suited for the development of ethanol. From the clostridium species P7 has a lot of potential, but because of the early stages of the development of this bacterium and therefore lack of data, Clostridium ljungdahlii is most promising for our goal to produce ethanol from synthesis gas at this moment.

Bacterial fermentation of CO, CO2 and H2 to ethanol using Clostridium ljungdahlii gives the following equations (Klasson et al., 1993):

(1) 6 CO + 3 H2O ^ C2H5OH + 4 CO2 AG = 216 kJ/mol

(2) 6 H2 + 2 CO2 ^ C2H5OH + 3 H2O AG = -97 kJ/mol

The distinctive feature of the followed pathway of these microorganisms seems to involve the reduction of carbon dioxide to a methyl group and then its combination with a molecule of carbon monoxide and CoA to form acetyl-CoA (Ljungdahl, 1986). This combination of reactions has been designated as the acetyl-CoA pathway (Wood et al., 1986).

After cleaning the synthesis gas, the next part of equipment needed is for fermentation. In this equipment the bacteria must be able to convert synthesis gas into ethanol and after this the product (ethanol) has to be removed from the rest of the stream. For energetic, environmental and economic reasons the rest stream (consisting of nutrients, water and a small amount of ethanol) should be recycled into the reactor. This is schematically shown in Fig. 17.3. Here a gas-sampling vessel disengages the gas from the liquid. At the recycle-bottle port, fresh nutrients are added, the liquid effluent is withdrawn, and the pH is adjusted. Experimental studies have shown that the rate-limiting step in synthesis gas fermentation is typically gas-to-liquid mass transfer. This means when the gas gets easier in the liquid the reaction-rate will increase. A common approach to enhance gas-to — liquid mass transfer in stirred tanks is to increase the agitator’s power-to-volume ratio. Increasing the power input increases bubble break-up, thereby increasing the interfacial area available for mass transfer.

However, this approach is not economically feasible for the very large reactors being considered for commercial synthesis gas fermentation, due to excessive power costs. Consequently, alternative bioreactor configurations that may provide more energy-efficient mass transfer are needed. The most interesting option is a monolith reactor.

Monolith reactor

A monolith reactor is columnar and does not require mechanical agitation and thus offers the potential for lower power costs than stirred tanks. A monolith reactor (Fig. 17.4) is a packed bed of channels where the liquid and gaseous phase flow in co/current downwards. High gas and low liquid flow rates are typically used, giving relative low-pressure drops. The cells are immobilised on the wall. This development is promising, but requires a lot of knowledge and equipment and is still in the experimental stage for fermenters (Salim et al., 2008).