Saccharomyces cerevisiae has been engineered to express the a-amylase and glucoamylase enzymes from the filamentous fungus Aspergillus shirousamii (Shiuya et al., 1992) and has shown a high level of enzyme activity with starch as a substrate. Other constructs have been made with enzymes from Bacillus subtilis, Aspergillus awamori and mouse pancreatic a-amylase (Birol et al., 1998). Ethanol production levels in starch and glucose-containing media were found to be comparable.

Biomass can be converted into gas by heating at 1300°C in an oxygen-limited atmos­phere. The gas produced contains mainly hydrogen, carbon monoxide, methane, carbon dioxide and nitrogen and can be used in a boiler or turbine for the generation

of heat, steam or electricity. Small-scale gasification systems (up to 100 MWth; mega­watts thermal) have been developed for heat and power systems. Small fixed-bed gasifiers linked to diesel or gas engines (100-200 kW) have an electrical efficiency of 15-20% but as yet have not been installed. The variations in the biomass used for gasi­fication makes the small systems difficult to operate. Larger gasifiers over 100 MWth use a variety of systems, including circulating fluidized beds, atmospheric gasifiers (ACFB), integrated gasification/combined cycle (IGCC) (Faaij, 2006). An example of simple-cycle gas turbine and biomass integrated gasification combined cycle (BIGCC) is shown in Fig. 4.7. The efficiency of the simple-cycle gas turbine can be improved from 40 to 60% with a combined cycle turbine system (Rukes and Taud, 2004).

Alkali treatment reduces the lignin and hemicellulose content and can be carried out at lower temperatures and pressures than acid treatment. The treatment also increases the surface area of the biomass.

Ozonolysis

Ozone can be used to degrade lignin and hemicellulose (Sun and Cheng, 2002). The advantages are it removes lignin, produces no toxic residues and it is carried out at room temperature and pressure.

Enzymes

Fungal enzymes from white and brown rot fungi, such as Sporotrichum pulverulentum and Pleurotus osteatus, can be used to pretreat lignocellulose. The brown rot fungal enzymes degrade cellulose whereas white rot fungal enzymes degrade lignocellulose.

Cellulose breakdown

After pretreatment the cellulose is suitable for hydrolysis to glucose and a number of methods can be used (Fig. 6.10).

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Concentrated acid

Concentrated acid hydrolysis of cellulose gives a rapid and complete conversion to glucose using 70% sulfuric acid for 2-4 h. The problems are associated with the dif­ficulties of handling concentrated sulfuric acid and the cost of the acid, which requires recovery and reuse to be economical.

Dilute acid hydrolysis

The cellulose is broken down by dilute acid in complex reaction at a higher tempera­ture than the hemicellulose reaction and has a sugar recovery of around 50%.

Cellulase enzymes

Once the structure of the cellulose has been opened up by the pretreatment, enzym­atic hydrolysis can proceed. The crude cellulase enzyme is a consortium of enzymes, which operate under mild conditions, pH 4.8 and 45-50°C. Although cellulase is commercially available it is usually obtained from fungi such as Trichoderma reesei, and the yields are better than acid hydrolysis. Cellulases can be produced by both fungi and bacteria which can be grown both aerobically, and anaerobically. The bac­teria include Clostridium, Cellulomonas, Bacillus, Thermomonospora, Bacteroides, Erwinia, Acetovibrio, Microbispora and Streptomyces. Three enzymes are involved in the hydrolysis, endo-1,4-P-glucanases (endoglucanases), cellobiohydrolyases (exoglucanases), and P-glucosidases. The endoglucanases cleave the cellulose chain randomly and the exoglucanases hydrolyse the cellulose chain, releasing glucose and cellobiose. The P-glucosidases catalyse the conversion of cellobiose to glucose.

The fungi produce all three types of cellulase but the exoglucanases are the major enzymes with T. reesei. The Trichoderma sp. are considered the best of cellulase enzyme producers. Cellulose on hydrolysis liberates cellobiose which is cleaved into two molecules of glucose by the enzyme P-glucosidase. The disadvantage of the enzyme process is that both products glucose and cellobiose act as inhibitors of cel — lulase and P-glucosidase enzymes.

Ethanol can be used in petrol engines up to a concentration of about 24%, any more and either engine modifications are required or a flexible fuel engine is required. However, with modifications, a petrol engine can run on 100% ethanol, although it contains less energy than petrol. Biobutanol is a possible replacement for ethanol, as it has a higher energy density but there is little information about its use in engines. FT petrol is essentially the same as petrol and should cause no problems in supply and use. Methanol has been used in petrol engines, but it is not used as a fuel at the moment but could be a very useful fuel in the supply of hydrogen to fuel cells.

Diesel replacements

Biodiesel can be used up to 100% in a conventional diesel engine and jet engine with­out modification, but bio-oil requires considerable processing before it can be used because of its high viscosity and acidic nature. FT diesel is the same as diesel and can be used without modification. The microalgal biodiesel appears to have similar prop­erties to biodiesel, but so far there is no information on its use in diesel engines.

Gas turbine engines are used in aircraft, marine propulsion and electricity gener­ation and can function on a variety of fuels, paraffin, natural gas and propane, but the formulation for aircraft engines is regulated by international specifications DEF

169 I

STAN 91-91 and ASTM D1655. Jet A and Jet A-1 are known as aviation kerosene, and Jet B is a wide cut fuel. Biodiesel has been used at concentrations of 2, 20 and 30% in jet fuels to power aviation turbine engines with no apparent ill effects (Wardle, 2003). However, the biodiesel results from diesel engines cannot be transferred directly to aircraft engines as the combustion system responds differently, influenced by density more than chemistry (Ebbinghaus and Weisen, 2001). The problems with the supply of first-generation biofuels may preclude the use of biodiesel in aircraft but it is another option.

Thus, biofuels are capable of supplementing or replacing fossil fuels for transport and electricity generation and could be integrated into the present fuel infrastructure. The only exception is hydrogen which may require a completely new infrastructure.

Chemical sequestration involves the conversion of carbon dioxide to inorganic car­bonates, and the use of carbon dioxide in the production of urea and plastics.

Agricultural practices

Agriculture uses a very large area of land and because of this size it is responsible for the emission of large quantities of greenhouse gases. Agriculture has been regarded as responsible for 25% of carbon dioxide, 50% of methane and 70% of the nitrous oxide released (Hutchinson et al., 2007). The World Resources Institute (2006) gives the values of 27% carbon dioxide, 53% methane and 75% nitrous oxide. The green­house gas emissions from agricultural sources are given in Table 3.9. These emissions arise from the following:

• Fossil fuel use in cultivation, harvesting, etc.

• Nitrogen fertilizer use.

• Rice production.

• Deforestation.

• Livestock (enteric fermentation).

• Manure.

Mitigation of greenhouse gases, sometimes known as stabilization, may require meth­ods that are expensive, therefore low-cost options have been investigated. Agriculture offers a number of low-cost technologies that include:

• Altering land use.

• Changing cultivation methods.

• Better management of livestock.

• Altering crop mix and fertilization methods.

• Expanding the production of biofuels.

An example of the changes that can be made to carbon dioxide sequestration is shown in Table 3.10. A number of agricultural methods have been shown to increase the retention of carbon in soils for Canadian Prairies (Hutchinson et al., 2007). Another example of changes in agricultural methods that reduce greenhouse gas emis­sions is the anaerobic treatment of slurry and liquid manure. Rather than placing liquid manure on the land or retaining it in lagoons where methane and nitrous oxide are produced, it can be anaerobically digested. Slurry or liquid manure anaerobically digested produces a mixture of methane (CH4) and carbon dioxide (CO2) which can be use as a gaseous fuel (Clemens et al., 2006).

Anaerobic breakdown of organic material can yield hydrogen in a number of cases. Anaerobic digestion of sewage sludge by a consortium of microorganisms can produce small amounts of hydrogen in addition to the major product biogas (section ‘Anaerobic digestion’, Chapter 5, this volume). Anaerobic digestion or fermentation of organic compounds by Clostridium sp. and some microalgae was also found to produce hydrogen under specific conditions. One of the best-known examples is the production of acetone and butanol by Clostridium acetobutylicum growing anaerobically on glucose (molasses). This process was used from 1915 until the 1950s to produce acetone and butanol for the munitions and chemical industries. The biological process has now been replaced by the production of acetone and butanol from petrochemicals. However, other products are formed along with acetone and butanol and include ethanol, butyrate, acetic acid, carbon dioxide and hydrogen. Depending on the culture conditions, and the strain used, the amount of the various products formed can vary including the amount of hydrogen produced. The pathway involved in the production of acetone and butanol is shown in Fig. 5.11. It has been estimated that 2 mol of hydrogen are formed per mole of glucose consumed (Ni et al., 2006).

Green algae such as C. reinhardtii respond to anaerobic conditions or nutrient reduction (sulfur) by producing hydrogen. The induction of anaerobic conditions switches the organism’s metabolism to fermentative which produces a number of harmful end products such as ethanol and organic acids. Under these conditions hydrogenase activity is inhibited and hydrogen acts as an electron sink, avoiding some of the problems of aerobic conditions. The low sulfur condition causes the downregulation of the photosystem II where the lack of sulfur-containing amino acids blocks the repair cycle for photosystem II.

This is carried out with a hydrogen-providing solvent in the presence of catalysts (Co-Mo, Ni-Mo) at high temperatures and pressures.

Catalytic cracking

Oxygen can be removed from bio-oils by catalytic decomposition in the presence of cata­lysts. Although this is cheaper than hydrodeoxgenation, it suffers from high coking.

Steam reformation for hydrogen production

The production of hydrogen from the reforming of bio-oils has been investigated and shows some promise.

Emulsification

Bio-oils have been combined directly with diesel to form a fuel, but a surfactant is required as the bio-oil is immiscible with diesel. Chiaramonti et al. (2003) showed that the optimum level of bio-oil addition was between 0.5 and 2%, but above these values the viscosity was too high. Light fractions of bio-oil have been obtained by centrifugation and used at 10-30% in emulsions with diesel (Ikura et al., 2003). The viscosity of the mixture was lower than the bio-oil and the cetane number was reduced by 0.4 for each 10% addition. In both cases, the long-term effect on the engine needs to be determined. The cost of bio-oil based on 2000 prices has been determined (Brammer et al., 2006) at a value of €32/MWth which was not competi­tive with conventional energy sources.

Application Product

Heat

Electricity

Combined heat and power (CHP) Electricity

Combined heat and power (CHP) Electricity

Combined heat and power (CHP) Combined heat and power Emulsion use for transport

Some of the applications of bio-oil as a heating fuel, diesel fuel and gas turbine fuel are listed in Table 7.4 (Brammer et al., 2006). In six European countries, one application of bio-oil was competitive due to low biomass costs.

From the figures above, it is clear that no single biofuel will be able to fully replace either diesel or petrol but a combination of fuels may be suitable (Table 8.6). The UK has 18,166,000 ha of agricultural land available for biofuel crops. If the first-generation fuels are considered using a 5% addition to petrol and diesel, the total land required would be 9.58% of the agricultural land (Table 8.6). Any more land would begin to conflict with food crops. Therefore, second — and third-generation biofuels have to be considered for biofuel contributions above 10% of the total. The two best candidates are FT biodiesel and FT petrol and microalgal biodiesel, as the FT system uses the

whole plant and microalgae non-agricultural land. To provide 20% of both petrol and diesel requirements using FT synthesis would need 36.1% of the agricultural land. This is probably not acceptable even if 9.6% of the land is not used for first-generation biofuels. This suggests that if a large proportion of fossil fuels are to be replaced, fuel generated from non-agricultural land is required. The minimum yields have been used for the FT diesel and petrol but these may be increased through research, and at a yield of 5.7 t/ha, 9.2% of the agricultural could provide 40% of the total fuels. If microalgal biodiesel could provide 40% of the total fuel as biodiesel which would require 2.32% of the land, the total land used would be 11.52% if the microalgae were using agri­cultural land. This would leave a shortfall of 20% diesel and 60% petrol, but given the assumption that 20% of the fuel use can be saved by increases in efficiency and the continued use of fossil fuels, this shortfall may also be filled. Biodiesel is also being tested as a jet fuel which would reduce the amount of fuel needed from FT synthesis.

There are five categories of solid biofuels, depending on their source. The yield and energy content of some of the biomass types are given in Table 4.2.

1. Wood from forests and forest residues.

2. Crop residues.

3. Crop specifically grown for energy generation.

4. Animal waste.

5. Municipal waste.

Wood biomass

Materials, such as wood, have always been used by humans as a source of energy, but depending on the development of a country the amount of biomass used can differ greatly (Table 4.1).

The wood from forests consists of felled wood, thinnings, logging residues, wood-processing residues and waste from clearing. The wood available will be that surplus to the need for construction and industrial wood products from forests, plant­ations and trees outside forests.

Crop residues

Crop residues include haulms of legumes, stalks of sorghum, maize and millet and straw from rice, wheat and barley. Although there is a considerable quantity of bio­mass as residues, it is widely distributed and seasonal in its availability. The yields can be up to 20 t/ha with an energy content of 14 GJ/t giving a yield of 280 GJ/ha.

The concentration of ethanol used in petrol differs greatly from country to country. Hydrous ethanol which contains 4.5% water (alcool) has been used in all-ethanol vehicles in Brazil, but sales of these vehicles ceased in the 1990s to be replaced with a

blend containing 24% ethanol (Table 6.4). This change was probably introduced in order to avoid the modification of car engines to use 95.5% (E95) ethanol allowing the unmodified engines to use both petrol and the 24% blend. The modifications to run on E95 were a heated inlet manifold due to the cooling effect of ethanol, changes to the carburettor, the fuel tank and fuel line replaced by one in tin and cadmium brass. The fuel filter was changed to accommodate a higher fuel flow and the compres­sion increased to 12.1 because of the higher octane rating. Changes were also needed to the valve housings and catalytic converter.

In the USA, the initial blend contained 10% ethanol (Gasohol) but more recently a blend containing 85% ethanol (E85) has been introduced and flexible fuel engines have been developed which can use either E85 or petrol. Ethanol is also used in the USA to increase the oxygen levels in petrol with an addition of 7.6% and as a replace­ment for MTBE in reformulated petrol. Ethanol contains 35% oxygen which increases combustion and therefore reduces particulate and NOx emissions.