Fig. 6.15. Reductive acetyl-CoA pathway. THF, tetrahydrofolate. (From Henstra et al., 2007.) CO2
CO2
H2 CO
HCOOH 2 [H]
H2O
HCO-THF
CH-THF
CH2-THF
CH3-THF HSCoA [CO]
CH3-CO-S-CoA
Acetate
Ethanol
Acetoacetyl-CoA
not able to grow in the presence of air. The industrial production of acetone and butanol by fermentation has a long history that started in 1914. Acetone and butanol were some of the first biotechnological products and the process that developed was one of the largest. Before 1914 acetone was produced by heating (dry distillation) calcium acetate.
Calcium acetate was produced by the dry distillation or pyrolysis of wood. The wood distillate contained about 10% acetic acid that was either distilled off into calcium hydroxide to form calcium acetate or directly neutralized with lime. Between 80 and 100 t of wood was required to produce 1 t of acetone. In 1910 Chaim Weizmann had been working in Manchester as part of a group working for Strange and Graham Ltd trying to produce butanol by fermentation. Butanol was needed as it could be used to form butadiene, a precursor of synthetic rubber. At the time natural rubber was in short supply, as Brazil was the only source and they did not allow the export of rubber trees from their country. By 1914 Weizmann and co-workers had isolated an anaero- bic organism which was later named as Clostridium acetobutylicum that produced both acetone and butanol when grown on starch. In 1914, at the start of the First Fig. 6.16. An outline of the production of ethanol from biomass via gasification. (Redrawn from BRI, 2007.)
Biomass waste
ASH Gasifier
Gas clean-up and heat recovery
Steam power
Cell filtration Distillation
~99% ethanol 95% ethanol
Water recycle
Molecular sieve
Fermenter Nutrients
Exhaust gas
World War, the demand for acetone increased rapidly as acetone was used as a solvent for nitrocellulose in the manufacture of cordite, a smokeless explosive. By 1915 the demand had exceeded supply and the Nobel Company approached Weizmann and the process of biological production of acetone was adopted rapidly. Brewing capacity was commandeered and by 1916 the bioreactor capacity had reached 700 m3.
At the end of the First World War the demand for acetone reduced but butanol was still in demand as a solvent for the nitrocellulose paints used in the rapidly developing motor industry. Acetone was also being used as a solvent in the production of aircraft dopes and for the production of textiles and isoprene. Only certain Clostridia are cap- able of producing reasonable levels of acetone and butanol and C. acetobutylicum has been the one most studied and used in industrial processes. C. acetobutylicum is a gram-positive anaerobic spore-forming rod 0.6–0.9 μm wide and 2.4–4.7 μm long. It is motile and will ferment arabinose, galactinol, fructose, galactose, glucose, glycogen, lactose, maltose, mannose, salicin, starch, sucrose, trehalose and xylose. The optimum growth temperature is 37°C. As the bacterium will form spores readily when the nutri- ents are exhausted it can be easily maintained as spores mixed with sterile soil. Loss of solvent-forming potential is a common problem with C. acetobutylicum cultures but heat treatment restores solvent-forming ability. The concentration of substrate normally used was 6.0–6.5% and the maximum yield of solvent formed was 37% of the substrate used. However, in practice the yields are around 30% with a ratio of buta- nol/acetone/ethanol of 6:3:1 with small amounts of hydrogen and carbon dioxide being formed as well. Thus 100 t of substrate will yield about 22 t of butanol. The yields depend upon a number of factors including the strain of microorganism, temperature, pH and substrate. In the 1930s, a bacterium C. saccharobutylicum was isolated which when grown on sucrose formed acetone and butanol only.
During the exponential phase little solvent is produced but butyric and acetic acids were formed causing the pH of the medium to drop from 6.0 to below 5.5 (Fig. 6.17). In the stationary phase the accumulation of acetone, butanol and ethanol
Fig. 6.17. The anaerobic production of acetone and butanol.
Relativeunits
Acids
Gas production
Solvent
9 18 27 36
Time (hours)
proceeds rapidly at the expense of the acids and therefore the pH rises. In culture C. acetobutylicum can be in three states: acidogenic where acetic and butyric acids are formed at neutral pH; solventogenic where acetone, butanol and ethanol are formed at low pH; and alcohologenic where butanol and ethanol are formed but no acetone at neutral pH, so that it is important to monitor or maintain pH.
Although molasses-based fermentations were more economical than the original starch substrate the expansion of the petrochemical industry from 1945 onwards meant that by the 1960s the process had ceased to be used. The reasons for the decline of the acetone/butanol process were:
Low yield of solvents (30–35% of substrate).
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Low solvent concentration in medium due to the toxicity of butanol and ethanol
●
at 20–25 g/l.
Phage sensitivity.
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Autolysin-induced autolysis in stationary phase.
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Cost of distillation.
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Production of considerable amounts of waste.
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High cost of molasses.
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Petrochemical production was cheaper.
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However, since the late 1990s the process has been reevaluated in the light of modern developments in genetic manipulation and waste treatment and the sudden increase in oil prices in 1973 (Durre, 1998). The reasons for the possible re-introduction are:
The process uses renewable substrates.
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Butanol can replace ethanol as a liquid fuel.
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The newer strains can grow on waste starch and whey and metabolic engineering
●
is being attempted so that it can be grown on cellulose.
The waste can now be treated anaerobically forming biogas.
●
The process may be able to operate at 60°C so that the solvents can be removed
●
as they are formed.
Solvent may be recovered during fermentation using reverse osmosis, perstrac-
●
tion, pervaporation, membrane evaporation, liquid–liquid extraction, adsorption and gas stripping (Durre, 1998). Any process that avoids distillation will be con- siderably cheaper and able to compete with fossil fuels.
It will be interesting to see how ethanol and butanol develop as liquid fuels in the EU and UK as in the short term much of the ethanol will have to be imported from Brazil.
Conclusions
At present the infrastructure is in place to use liquid fuels and therefore the replace- ment or addition to petrol will be ethanol at least in the short term. The main prob- lem with ethanol is that when it is produced from starch considerable processing is required, which means a substantial input of energy. The most economical method is to make ethanol using sugar from sugarcane as in Brazil. However, the quantity of sugar needed to supply the volumes needed to replace ethanol may start to com- promise the rainforest in Brazil as more and more land is use to grow sugarcane. The
drivers for ethanol use are the directives in the EU and the USA to include ethanol in all petrol sold. If the amount of ethanol added to petrol is increased the increase in sugar and starch crops used for ethanol production may compromise food crops.
It is therefore important that ethanol production from the more abundant ligno- cellulose becomes industrial. Lignocellulose requires treatment and enzymatic degrad- ation before it can be converted into ethanol. This processing requires energy and increases costs and these need to be reduced before lignocellulose ethanol can com- pete. It may be that lignocellulose will not be able to compete with Fischer-Tropsch fuels from biomass and wastes. Butanol may also supersede ethanol as the liquid fuel of choice.
7 Liquid Biofuels to Replace Diesel
Introduction
Both transport and industry rely heavily on the diesel engine that is widely used to power lorries, trains, tractors, ships, pumps and generators. The USA uses 50 billion gallons (1 gallon = 3.8 l) annually (Louwrier, 1998) and the consumption in the UK was 23.9 million t (106) in 2006 (IEA, 2008). The engine designed by Diesel ran for the first time on 10 August 1893, and the patent when filed proposed that the fuel could be powdered coal, groundnut oil, castor oil or a petroleum-based fuel (Shay, 1993; Machacon et al., 2001). At this time, the growing petrochemical industry pro- vided the best fuel, a crude oil fraction, now called diesel, which has been the fuel of choice for diesel engines ever since this time. Conventional diesel is produced by the distillation of crude oil and collecting middle distillate fractions in the range of 175–370°C. The fuel contains hydrocarbons such as paraffins, naphthenes, olefins and aromatics containing from 15 to 20 carbon molecules. To replace diesel without modifying the engine, any substitute will have to be similar to diesel in the following properties:
A calorific value of 38–40 MJ/kg is a measure of the energy available in the fuel.
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A cetane number of around 50 is a measure of the ignition quality of the fuel.
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The viscosity of the fuel is important as it affects the flow of the fuel through
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pipelines and injector nozzles where a high viscosity can cause poor atomization in the engine cylinder.
The flash point is a measure of the volatile content of the fuel and gives a measure
●
of the safety of the fuel. The flash point for diesel is 64–80°C.
It must be obtained from renewable resources such as biomass, oil crops and
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waste.
It must be available in large quantities. For example the current use of diesel in
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the UK is 23,989,000 t where a 5% addition (on an energy basis) requires an addition of 5.75% by volume, which is equal to 1,199,450 t (1499 million l).
There are a number of possible sources of diesel replacements produced from agricul- tural products or microbial cultures, which are first-, second- and third-generation biofuels (Fig. 7.1). Some of the sources are as follows:
Long-chain hydrocarbons (C 30) extracted from herbaceous plants, which can be
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cracked to form diesel, is a first-generation biofuel.
Long-chain hydrocarbons (C 30) accumulated by some microalgae, which can
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also be cracked to form diesel, is a first-generation biofuel.
Pyrolysis of biomass or waste to form bio-oil, which can be converted to diesel, is
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a second-generation biofuel.
Gasification of biomass followed by Fischer-Tropsch synthesis of diesel (FT diesel)
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is a second-generation biofuel.
Transesterification of plant, animal and waste oils and fats to methyl esters
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(biodiesel) is a first-generation biofuel.
Oil accumulated by some microalgae, extracted and transesterified into biodiesel,
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is a third-generation biofuel.