The electric vehicle is ideal for use in cities as it emits no fumes and can be integrated into city-wide traffic systems where travelling distances are short. The electricity needed to charge the battery can be generated from renewable sources, which leads to a
considerable reduction in carbon dioxide emissions. However, for long journeys battery technology and electricity storage is the key point. New battery systems such as Li-ion, nickel hydride and high temperature are under development (Van Mierlo et al., 2006). Recently Mercedes has introduced a battery-powered Smart car. It is powered by a high temperature (260-330°C) sodium-nickel-chloride battery with an output of 15.5kWh. The battery can be recharged from the domestic supply and takes about 8 h and gives the car a range of 50 miles. Considering the developments in battery technology driven by the mobile phone and computer industries it is likely that the range will be extended.

Hybrid vehicles

For long-range travel, hybrid vehicles appear to offer the best option. The hybrid is a combination of a battery-powered electric engine plus an internal combustion engine or fuel cell. The internal combustion engine can be used to charge the battery when in use. There are a number of options for hybrid drive trains and a number of cars available from a range of manufacturers which combine electric motors with a small petrol engine, notably the Toyota Prius.

Landfill sites are used to dispose of a wide range of waste materials from domestic and industrial sources which contain organic materials. The composition of munici­pal solid waste (domestic) is given in Table 5.1. As the landfill is sealed, the conditions within the site soon become anaerobic, producing biogas. The stages of anaerobic degradation of the organic content are probably the same as for sewage sludge. In older landfill sites, the biogas generated was allowed to escape but at present the biogas is captured and used. In 2001 it was reported that there were 955 landfill sites globally where the gas was recovered.

The composition of municipal solid waste is very likely to change over time as recycling and reuse of waste continues to increase. In the UK, the composting of gar­den waste and the recycling of metals, glass and plastics along with degradable plastic will reduce the organic composition of landfill waste. The cost of landfill is also rising which will also reduce the amounts of waste which are placed in landfills.

In the construction of landfill sites an impermeable barrier to stop any leachate reaching the groundwater is required. The most suitable sites are abandoned quarries and opencast sites preferably with a non-porous substratum. The site is lined with clay, plastics and rubber and once sealed the site can be filled in cells or terraces (Fig. 5.6), each cell or terrace being covered with soil after compaction at the end of each day. The compaction reduces the amount of air trapped, helping anaerobic conditions form and avoiding spontaneous combustion by reducing oxygen content. In order to collect the biogas generated during construction, permeable horizontal trenches or perforated pipes are incorporated into the terraces and cells (Fig. 5.7). Once a landfill has reached its working level it is capped with clay, a drainage layer and soil. The drainage layer stops rainwater entering the landfill, reducing the leachate formed. Once capped the organic material in the landfill will begin to degrade and although aerobic at the start, oxygen will be soon exhausted and conditions will become anaerobic.

The biogas collected will, like that produced from sewage sludge, have about half of the calorific vale of natural gas because of the presence of carbon dioxide and the composition will vary depending on the waste composition in the landfill. The gas is normally extracted 1-2 years after capping of the site and at best yields 100 m3 gas

per tonne of waste. The total extracted is only 25% of the possible yield because of the slow rate of gas formation and migration within the site. Figure 5.8 shows the changes in the landfill gases as the site develops and the organic material is degraded. A typical timescale of gas production from a landfill site is given in Fig. 5.9.

Biogas from Anaerobic Digestion of Agricultural Wastes

Small anaerobic digesters have been installed on farms to treat excess animal slurries which cannot be placed on the land. The biogas formed is normally used for heating but can also be used in dual-fuel engines.

In a number of developing countries, such as China and India, both animal and human wastes are anaerobically digested on site. The biogas formed is used as a low pressure source of gas for domestic use.

It has been known for some time that some anaerobic bacteria can convert carbon monoxide, carbon dioxide and hydrogen into a mixture of ethanol, butanol and acetic acid. The pathway is shown in Fig. 6.15 where acetyl-CoA is produced via the acetogenic pathway (Henstra et al., 2007). Synthesis gas is a mixture of carbon mon­oxide and hydrogen produced by the gasification of coal, biomass and wastes. In the first step the oxidation of carbon monoxide and hydrogen yields protons which are used to reduce carbon dioxide to formate. The formate bonds to tetrahydrofolate (THF), and this complex adds more protons finishing with a methyl-THF complex.

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This complex reacts with carbon monoxide and CoA to produce acetyl-CoA cata­lysed by acetyl-CoA/carbon monoxide dehydrogenase. If insufficient carbon mon­oxide is available it can be produced from carbon dioxide. The production of acetyl — CoA has a negative energy balance which is recovered by the reduction of acetate formed from acetyl-CoA to ethanol. Butanol is formed from acetoacetyl-CoA. The overall balance in the formation of ethanol from syngas with an equimolar mixture of carbon monoxide and hydrogen is two-thirds of the carbon monoxide can be con­verted into ethanol (Rajagopalan et al., 2002).

The bacteria that have been demonstrated to be capable of growth on hydrogen and carbon monoxide include both mesophilic and thermophilic bacteria and Archaea. Examples of the mesophiles are Clostridium autoethanogenum and Eubacterium

129

methylotrophicum; thermophiles are Moorella thermoacetica, Archaea, and Methanosarcina acetivorans.

Recently the anaerobic fermentation of syngas has been developed from a labora­tory process to a commercial process by Bioengineering Resources Inc. (BRI, 2007). An outline of the process is given in Fig. 6.16. A single module will process 100,000 t of biomass producing 6-8 million gallons of ethanol (US) and 5-6 MW of energy per year. The energy is derived from heat recovered from the gasifier as the gas has to be cooled to around 37°C before it is introduced into the fermenter and the exhaust gases from the fermenter. Distillation gives 95% ethanol and a molecular sieve is used to separate the remaining 5% water. There are no details of whether this process is more efficient and economical than the conversion of lignocellulose biomass into sugar or the Fischer-Tropsch process.

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Fig. 6.15. Reductive acetyl-CoA pathway. THF, tetrahydrofolate. (From Henstra etal., 2007.)

Butanol

Butanol is another alcohol which has been considered as liquid fuel as it has similar properties to ethanol (Table 6.2) but has a higher energy content. Butanol will give a higher mileage and can be mixed at any proportion with petrol. Butanol has been used as an industrial solvent, paint thinner and a component of brake fluids. It is less corrosive than bioethanol and can be transported through existing pipeline whereas ethanol has to be carried in tankers, by rail or on barges. It is also safer as it has a higher flash and boiling point. With all these advantages over ethanol it is perhaps not surprising that a number of ethanol plants have switched to butanol. BP, DuPont and British Sugar’s plant in Wissington is being converted from ethanol to butanol and Virgin Fuels are interested in butanol produced from cellulose.

At present much of the butanol used is produced from petrochemicals but there is a renewable method of producing butanol using microorganisms. The biological produc­tion of butanol was first observed in 1861 when Pasteur isolated a butyric acid produc­ing bacterium. Studies also showed that acetone was also formed and the organism was

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 anaero­bic 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

132

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 cap­able 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 pm wide and 2.4-4.7 pm 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 nutri­ents 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

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).

• Low solvent concentration in medium due to the toxicity of butanol and ethanol at 20-25 g/l.

• Phage sensitivity.

• Autolysin-induced autolysis in stationary phase.

• Cost of distillation.

• Production of considerable amounts of waste.

• High cost of molasses.

• Petrochemical production was cheaper.

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.

• Butanol can replace ethanol as a liquid fuel.

• 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 con­siderably 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 replace­ment or addition to petrol will be ethanol at least in the short term. The main prob­lem 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 com­promise the rainforest in Brazil as more and more land is use to grow sugarcane. The

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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 degrad­ation before it can be converted into ethanol. This processing requires energy and increases costs and these need to be reduced before lignocellulose ethanol can com­pete. 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.

One of the main criticisms of the first-generation biofuels is that they appear incap­able of supplying the large quantities of transport fuels required without compromis­ing food crops. The first-generation biofuels were not intended to completely replace fossil fuels but rather to demonstrate that alternative fuels could be used in the inter­nal combustion engine and for electricity generation. Any calculation of the land required to replace 100% fossil fuel would indicate this. It was the development in the second and third generation that was intended to supply the bulk of the fuel. In addition, the introduction of some new fuels may require the installation of a new infrastructure which will take some time to install. The sustainable systems for elec­tricity generation such as wave and wind power can be integrated into the current electricity supply systems, but the introduction of new transport fuels will incur high costs if these fuels are not compatible with current supply infrastructure. To be com­patible with present infrastructure, the non-fossil fuels should be liquid as this fits the engine technology and supply systems. Gaseous fuels such as hydrogen and DME will require either the introduction of a new infrastructure or significant modification of the present systems. Once the first-generation biofuels were shown to be suitable for present engine technology their introduction was driven by the legislation produced under the Kyoto Protocol for carbon dioxide reduction.

The primary sources of transport fuels are all agricultural crops such as wheat and maize, apart from wood, woody and organic wastes, and many of these are food crops. Therefore, a conflict may occur between food and fuel crops.

The yield of fuel obtained per hectare varies depending on the crop, growth conditions, and climate. The yields of oil for biodiesel range from 5000 t/ha for oil palm to 1000 t/ha for rapeseed and 375 t/ha for soybean, which is mainly grown for its protein content. A number of studies have been carried out on the effect of biofuel crops on agriculture (Azar, 2005; Johansson and Azar, 2007). The detrac­tors of biofuels have perhaps been too simplistic in their approach to biofuels, and as a consequence biofuels have been blamed for food shortages and increases in food prices (Johansson and Azar, 2007). The adoption of large-scale production of

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first-generation biofuels has been recognized as having some unfortunate conse­quences in addition to their obvious advantages (OECD, 2007). The consequences are mainly the conflict between the growth of food crops and those for biofuels. This has been suggested as the reason for the shortage of certain foods and the rise in the price of others. However, food prices on a large scale are subject to a large variety of factors so that the reasons for price rises are complex and cannot be solely due to biofuels.

Producing biofuels on a large scale will require large land areas in countries where the land is required for food production. In two scenarios produced by the Inter­national Food Policy Research Institute (IFPRI), they predict that the price of maize and oilseed will increase by 26 and 18%, respectively, if the production of biofuels continues as currently planned. If biofuel production doubles, the price of maize would increase by 72% and oilseed by 44%. This increase in food prices would affect the poor populations who spend a higher proportion of their income on food. The use of non-food-producing land is the obvious solution to this problem but the yields on poor land will naturally be reduced. If the price of biofuel crops exceeds food crops, farmers will plant energy crops on normal agricultural ground. The outcome, at least for the US markets, will be that farm gate prices for all crops will increase substantially with a doubling of wheat prices. Increases in the farm gate prices are predicted not to affect food consumption as the price is low compared with commodity prices. How­ever, if biofuels are produced from lignocellulose the increase in the value of lignocel — lulose crops may stimulate the use of marginal land for non-food crops. Thus, the development of second and third generation biofuels needs to be pursued with some urgency to avoid conflict with food crops.

In order to reduce carbon dioxide emissions, carbon tax and trading schemes have been introduced, for example, €90/t carbon in the EU. However, once biofuel crops reach a certain price, driven by the carbon tax, commercial growers will use the most productive land for biofuel crops, replacing food crops. The conclusion was that at a carbon price of US$70/t carbon energy crops will dominate other agricultural options (Johansson and Azar, 2007).

To supply anywhere near the total requirement for liquid fuels either globally or in the UK will require the introduction of a mixture of fuels and propulsion systems coming from multiple sources rather than a single source. Perhaps one answer would be to stop using liquid fuels, abandon the internal combustion engine for alternative power sources such as fuel cells, and electric motors. However, liquid fuels are sup­ported by a vast infrastructure and industries which supply all the various compo­nents and employ a large number of people. Any change from our present position on fuels will need time, legislation and money and should go through a number of intermediate stages. A fossil fuel such as diesel may still be required for some time, as it is the motive force for the largest transport systems such as ships and trains, but we need to act now to mitigate the problem of global warming and fuel supply.

Although it is clear that energy generation from renewable resources will have to be incorporated into the overall consumption of energy, it has been slow to be adopted.

This has been in part due to the higher cost of renewable energy, reliance on the discov­ery of new fossil fuel sources rather than concentration on renewables, and the need for legislation to encourage these energy sources. In Chapter 1 the world’s current use of energy is given, where renewables contribute almost 12.7% of the total and nuclear 6.3% (Fig. 1.4). The use of hydroelectric, biomass and other renewables is expected to rise from 1.04 Gtoe in 2002 to 1.55 Gtoe in 2030 (Table 1.2) which is actually a slight drop on a percentage basis. A more detailed breakdown of the renewables contribution is given in Table 3.11. In 2005 the world’s total energy use was 11,434 Mtoe, where renewables, excluding nuclear power, contributed 3379 Mtoe.

Most of the renewable sources of energy are used to produce electricity and their contribution to world, EU (25) and UK electricity generation is given in Table 3.11. A modern power station, Didcot (in the UK), will have a peak output of 2000 MW and for nuclear station it is around 1320 MW. The peak electrical demand for England and Wales is 50,000 MW (50 GW). The potential contribution that the non-carbon-based renewable electricity generation could produce in the world is given in Table 3.12.

The possible electricity that will be required in 2100 has been included in the table to indicate the possible target. However, increased legislation and the Kyoto Protocol should encourage increases in renewable energy.

In the UK, the pattern has been much the same as the world’s with nuclear and renewables producing 22.5 Mtoe in 2006 (BERR, 2007). Much of this contribution was in electricity generation. In 2005, renewable electricity generation was 4.4% (Cockroft and Kelly, 2006) not including hydroelectricity, and Table 1.4 gives the renewable energy source in terms of 1000 tonnes of oil equivalent. The major proportion comes from landfill gas and biofuels. Table 3.13 gives the peak electrical power output from some of the renewable energy systems installed in the UK at present (Dti, 2006b) and indicates some of the potentials of these systems. Small power generation systems are often connected to the distribution network rather than the main grid (400,000 volts).

Dimethyl ether is a simple ether formula CH3OCH3 which has properties similar to propane, butane and LPG (Table 5.6). Dimethyl ether is volatile, non-toxic, non­mutagenic, non-carcinogenic, has a sweet ether odour and has been regarded as being

101

environmentally benign (Semelsberger et al., 2006). As DME is non-toxic and non­corrosive it is used mainly as a hairspray propellant, in cosmetics and in agricultural chemicals. DME has a high cetane value, no sulfur, little particulate matter (PM) emissions and can be competitive with LPG. Owing to these characteristics DME has been considered as a fuel for diesel engines, gas turbines and fuel cells. As its proper­ties are similar to those of propane and butane (Table 5.6), DME could be used to replace or supplement LPG for distributed power generation including gas turbines (Cocco et al., 2006). DME can also be reformed to produce hydrogen for fuel cells.

Dimethyl ether is produced in a two-step process where methanol is produced from syngas which is normally produced by the steam reformation of methane (natu­ral gas). The methanol is then dehydrated to form dimethyl ether. Syngas can be produced from waste and biomass so that DME could be produced from sustainable sources. To produce methanol the syngas needs a 1:1 ratio of carbon monoxide to hydrogen which can be adjusted by the water-shift reaction:

CO + 2H2 « CH3OH (5.12)

2CH3OH « CH3OCH3 + H2O (5.13)

DME has been shown to produce low noise, smoke-free combustion, and reduced NOx when used in an internal combustion engine (Huang et al., 2006). Because of its high cetane number and low boiling point, DME has been used at 100% or as an oxygenated addition to diesel. However, DME requires special fuel handling and storage as its properties are similar to LPG, and the lower energy means that a larger fuel tank will be required. The engine does not require modification but as the viscosity of DME is so low it can cause leakage in the pumps and injectors. Another consequence of the low fuel viscosity is a reduction in lubrication where lubricants need to be added to the fuel if used for long periods. The conclusions on DME were that it gave lower NOx and SOx, is soot-less and produces the highest well-to-wheel value compared with FT-biodiesel, biodiesel, methanol and ethanol (Semelsberger et al., 2006).

DME has also been shown to be suitable for gas turbines where performance and carbon dioxide emissions were improved when it was used in a chemically recuperated gas turbine (CRGT) (Cocco et al., 2006). One of the options to increase the efficiency of gas turbines is to recover the exhaust heat chemically. Most CRGT systems use methane

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steam reforming where carbon dioxide and hydrogen are formed and used as a fuel in the turbine. However, a high reforming temperature up to 600-800°C is required, which is higher than the exhaust temperature of commercial gas turbines. On the other hand methanol, DME and ethanol have lower reforming temperatures of 250-300°C, 300- 350°C and 400-500°C respectively which makes them more suitable. The overall reform­ing process is described below and a CRGT system is shown in Fig. 5.16:

The reformation is carried out at 290°C using Cu/SiO2 and HPA/Al2O3 catalysts. The CRGT system achieves an efficiency of 54%, 44% higher than a standard plant.

DME can also be used as a fuel for fuel cells as it can be reformed to produce hydrogen. The reformation is in two steps where the first step is an acid catalysis which converts dimethyl ether into methanol and the second step is the reformation of methanol over a Cu or Cu/Zn catalyst:

CH3OCH3 + H2O « 2CH3OH (5.17)

2CH3OH + 2H2O « 6H2 + 2CO2 (5.18)

Other similar compounds have been tested in diesel engines which include dimethyl carbonate and dimethoxy methane (Huang et al., 2006).

Fig. 5.16. A chemically recuperated gas turbine (CRGT) system using reforming dimethyl ether (DME) to utilize the exhaust heat. (From Cocco et al., 2006.)

Conclusions

Gaseous biofuels are a mixture of existing technologies and the promise of technology in the future. Methane is produced in landfill and anaerobic digesters and used for heating and electricity generation. Methane as a transport fuel has been proposed and the technology for the compression and liquefaction of the gas exists but may not be implemented. Methane is similar to LPG. LPG has been around for some years but the take-up of cars using this fuel has been very slow, probably due to the required expensive modifications to the car and the uneven distribution of LPG filling stations.

Much has been written about hydrogen and the ‘hydrogen economy’ but consider­able advances in technology will be required to make hydrogen a transport fuel that is sustainable. At present all hydrogen is made from natural gas and to be sustainable it needs to be produced from renewable resources such as biomass. The problem with hydrogen as a fuel is it needs to be stored so that sufficient fuel can be carried in a vehicle. At present, hydrogen can either be compressed gas or liquefied but both require considerable amounts of energy and special tanks. The production and distri­bution of hydrogen will also be required if it is to be used as a transport fuel. This will need the installation of a completely new infrastructure at a considerable cost. Despite the problems there is probably a future for hydrogen as a fuel for fuel-cell-powered vehicles; the question is whether hydrogen will be produced on-board or stored as hydrogen.

Microalgal biodiesel will need to comply with the standard EN 14214 in the EU and ASTM D 6751 in the USA before it can be universally accepted. Microalgal oils tend to contain more polyunsaturated fatty acids than plant oils, and those with four or five double bonds are more susceptible to oxidative degradation. However, the biodiesel produced from oil extracted from C. protothecoides had characteristics which were simi­lar to diesel (Miao and Wu, 2006) apart from a slightly higher viscosity (Table 7.8).

An increase in the oil content of the main oil-producing plants has obvious advan­tages, especially with soybean. Soybean is grown as a source of high protein animal feed but supplies large amounts of oil in the USA, although it only contains low levels of oil (18-22%). The limiting step in fatty acid synthesis would appear to be the acetyl-CoA carboxylase production of malonyl-CoA. However, genetic manipulation increasing the malonyl-CoA pool failed to increase oil yields significantly, which sug­gests that there are additional controls in fatty acid synthesis.

The quality of the oil produced can be altered by making single gene changes. The oleic acid content of soybean was increased to 86% by suppression of the enzyme oleoyl desaturase (Thelen and Ohlrogge, 2002). Rapeseed has been engineered to accumulate 58% short-chain lauric acid (12:0) in its oil by expression of a thioesterase from the California bay plant. However, not all gene transfers have been successful and breakdown of the target product can occur in some cases.

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One of the possibilities of increasing oil supplies is to introduce new plants, espe­cially those that can be grown on marginal land not suited to food crops. One group of plants is the oilseed shrubs which include castor bean (Ricinus communis), Ponga — mia pinnata, Calophyllum inophyllum and Jatropha curcas. Oil from these plants has been tested for its suitability as biodiesel (Forson et al., 2004; Chapter 7, Table 7.17). Castor oil is best suited as a lubricant, and for many other industrial uses. J. curcas has attracted interest as the oil is suitable as a biofuel and the plant can grow in the desert without addition of water and fertilizer (Openshaw, 2000). However, as a commercial crop J. curcas has a number of problems. It has to be harvested by hand and contains toxic alkaloids, phorbol esters and curcin which make the meal unsuitable as animal feed. It is perhaps why J. curcas is known as black vomit nut, purge nut, physic nut and the extracted oil, hell oil. The other plants also have toxic compounds in their seeds. Castor beans contain ricin, a neurotoxin, along with allergens and P. pinnata and C. inophyllum both have bitter and poisonous compounds in their seeds. To make these plants commercial crops they will need modification which could involve genetic manipulation. The characteristics which need to be introduced are as follows:

• Dwarf stalks for easy harvesting.

• Suppression of branching to allow for mechanical harvesting.

• Introduction of anti-shattering gene to stop fruit drying and scattering seeds.

• The elimination of the toxic compounds.

The genetic manipulation of microalgae has been demonstrated in a few algal species (Rosenberg et al., 2008). An increase in lipid content in the diatom Cyclotella cryp — tica was attempted by overexpression of ACCase but as found with higher plants it did not have a significant effect on lipid yields. More research is required to determine the control mechanisms of lipid synthesis.

Worldwide sources of biomass

One of the problems with biomass is the question of whether there is sufficient bio­mass to replace fossil fuels and can it be transported to where it is used. It has been estimated that biomass contributes 39.7-45 EJ/year to the global energy supply (9-15%) from a total 425 EJ (Parikka, 2004; Faaij, 2006). There have been a number of estimates of the biomass available globally and the range of estimates is given in Table 4.5. These estimates vary considerably from 1135 to 93 EJ. A potential of 100 EJ would supply around 25% of the present global energy requirement. Smeets and Faaij (2007) estimate that there may be a surplus of biomass, after directly used woodfuel and roundwood have been utilized, of 71 EJ but this may be an underesti­mate. Considering ecological and technical considerations the biomass may be only 2.4 Gm3 (28 EJ). The potential global biomass energy contained in various biomass types is given in Table 4.6. The great variation noted by Hoogwijk et al. (2003) appears to be in the estimate of the biomass grown on surplus or marginal land. Marginal lands are those with little economic value but before these can be used the environmental impacts need to be determined. The marginal land may be a specific wildlife habitat such as a wetland or forest.

The potential biomass energy consisting of wood, energy crops and straw in the various regions is given in Table 4.7, where the total is given as 103.8 EJ. All regions except Asia use only a small proportion of the available energy.

At present the two principal substrates that have been used commercially to produce ethanol are sugar (sucrose) and starch. There are problems with both these substrates in terms of can these crops supply sufficient ethanol, and in doing so will they com­promise the supply of food crops. There are other abundant, inexpensive, non-food substrates and the most obvious is lignocellulose.

Lignocellulose is the most abundant potential source for bioethanol production with a potential yield of 442 billion l (Balat et al., 2007). For countries where biofuel

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Glucose-6-phosphate

6-Phospho-gluconolactone

I
6-Phospho-gluconate

1

2-Oxo-3-deoxy-6-phosphogluconate

Carbon dioxide

Acetaldehyde

Fig. 6.9. The production of ethanol via the Entner-Doudoroff pathway.

crops are difficult to grow, lignocellulose is an attractive option. Lignocellulose can be obtained from trees species, wood residues, sawdust chips, construction residues, municipal wastes, paper, sewage sludge, maize stover, straws and grasses such as Miscanthus, switchgrass, sorghum, reed canary grass, bagasse, sugarbeet pulp, soft­wood, wheat straw, rice straw, pulp and paper mill residue, forest thinnings, munici­pal solid waste, winter cereals, and recycled paper.

Unfortunately, there are no ethanol-producing yeasts, other than genetically modified (GM) strains, that are capable of metabolizing starch and lignocellulose and only a few bacteria are capable of metabolizing lignocellulose. To make lignocellulose and starch suitable for fermentation both need to be converted into sugars in an inexpensive process.