Hydroelectric power is a clean, non-polluting, long-lasting, renewable source, which does not produce carbon dioxide. Large-scale hydroelectric plants are responsible for about 17% of the electricity supply in developed countries and 31% in developing countries. However, hydroelectric systems have environmental impacts and can only be sited in certain areas, restricting their application.

One of the main problems with the use of hydrogen as a transport fuel and energy carrier is how to store it as it is a light gas (Coontz and Hanson, 2004; Zhou, 2005). Compression and subsequent storage at high pressure in cylinders is a common method of storing gases. However, the density of hydrogen is low (0.089 kg/m3) com­pared with methane (0.717 kg/m3) and therefore a pressure some four times higher than normal is required (345-690 bar; 10,000 psi) to contain sufficient hydrogen.

Even at these pressures it still requires the fitting of a tank eight times the size of the equivalent petrol tank, and tanks of this size are not available commercially. Therefore, compressed gas is not likely to be used at present.

Another common method of storing gas is as a liquid. Hydrogen has a boiling point of -253°C and the critical temperature for liquid hydrogen is -240°C. There­fore, the liquid hydrogen will need to be stored in well-insulated tanks in an open system to avoid pressure build-up and thus some gas will be lost on storage. Whether compression or liquefaction is used both require the input of energy, between 214 and 354 MJ/m3 for compression and 15.2 kWh/kg for liquefaction which is some 30% of the energy contained in the fuel (Midilli et al., 2005; de Wit and Faaij, 2007).

Despite these problems the USA has invested US$1.7 billion and the €2.8 billion in hydrogen power vehicles. Ford, Mazda and BMW have produced hydrogen-powered internal combustion engines and Honda, Ford, Toyota, General Motors, Daimler — Chrysler, Renault-Nissan and Volkswagen have produced fuel cell vehicles. Liquid hydrogen-filling stations have been installed in Munich and London. In 2006 there were 159 hydrogen vehicles in the USA not including electric hybrid vehicles (Fig. 5.10).

Because of the need to store hydrogen inexpensively, alternatives to compression and liquefaction have been investigated including complex hydrides, metallic hydrides, physisorption and nanostructures (Zhou, 2005).

Complex hydrides form between hydrogen and group I, II and III elements such as lithium, magnesium, boron and aluminium. The complex hydrides have a high hydrogen density (150 kg/m3) and can release hydrogen at moderate temperatures. This method is still under development as conditions for hydrogen release changes the particle and the effects of repeated adsorption/desorption need to tested.

Some metals absorb hydrogen, forming hydrides. These are usually a rare-earth metal such as lanthium combined with a transition metal such as nickel. Hydrogen density has reached 115 kg/m3 for the metal hydride LaNi5H6. In simple hydrides the metal can absorb and release the hydrogen at room temperature but their hydrogen density is low. The formation of hydrides is an exothermic reaction and the more stable the hydride the more heat is required to release the hydrogen.

Gases can be adsorbed on to a number of adsorbents in a variety of ways pro­vided the gas is below its critical point. The gas can be adsorbed as a single layer on the surface which depends on the surface area of the material and the temperature. Adsorption decreases with a rise in temperature.

Hydrogen can be stored in carbon nanotubes but the capacity appears much lower than was first estimated. As adsorption is dependent on surface area, it is dif­ficult for nanotubes to compete with super-activated carbon.

Not all microalgae accumulate high concentration of oil, but there are a number of freshwater and marine species that do. Some examples of the oil levels accumulated

are given in Chisti (2007) and Scragg (2005). However, not all microalgal oils are suitable for biodiesel production, as some contain high levels of unsaturated fatty acids which reduce the oxidative stability of the biodiesel. In many cases, high oil accumulation is only found under some form of stress such as nitrogen limitation (Illman et al., 2002), and so growth may have to be in two stages in order to obtain high levels of oil. In contrast, heterotrophic growth in glucose stimulated oil accu­mulation in Chlorella protothecoides (Miao and Wu, 2006; Xu et al., 2006). Strain selection will also be important depending on the type of cultivation system used.

The surface of the Earth is 510,072,000 km2 which on average receives 170 Wm-2 which is equal to 7500 times the world’s energy use of 450 EJ with a total of

3,375,0 EJ. In temperate zones, the amount of energy reaching the Earth’s surface is about 1.3 kW m-1 but only 5% of this energy is converted into carbohydrates by photosynthesis (Fig. 8.29). Any improvement in the efficiency of photosynthesis would have a considerable effect on crop and biofuel production. The rate-limiting steps in the fixation of carbon dioxide by photosynthesis are ribulose-1,5-bisphosphate car­boxylase (rubisco), regeneration of ribulose bisphosphate and the metabolism of triose phosphates. In addition, photosynthetic organisms stop growing and fixing carbon dioxide at light intensities lower than typical levels at midday in equatorial regions. The typical light intensity is about 2000 pmole m-2 s-1, whereas light saturation of photosynthesis occurs at 200 pmole m-2 s-1. Above the saturating light intensity inhibi­tion of photosynthesis, known as photoinhibition, occurs (Fig. 8.30). Improvements in the three stages of carbon fixation, the increase in the light saturation values and

f 196

Reflected and ^ transmitted 8%

Metabolism 19%

Fig. 8.29. The amount of energy converted into carbohydrates during photosynthesis.

reduction in photoinhibition would all be of value in increasing crop yields. During photosynthesis, leaves must also dissipate heat and improvements in heat tolerance would also be useful.

In Europe research on perennial grasses started with Miscanthus. The genus Miscanthus contains 17 species and originates from East Asia and a hybrid Miscanthus x giganteus was first introduced into Europe in 1930 from Japan as an ornamental. Miscanthus grows vigorously and can be harvested dry in one harvest. Miscanthus is a C4 metabolism grass which can reach 4 m in height and forms rhizomes. Different Miscanthus species have different rhizomes, which are persistent, with the oldest plantation some 18 years old. Miscanthus is wind pollinated with fan-shaped inflor­escences. Miscanthus can be grown on a wide range of soils but does not tolerate waterlogged soils. Most of the yields for Miscanthus reported in Europe have been determined using Miscanthus x giganteus. Yields are variable with values in the range of 5-44 t/ha/year. It does however suffer from some problems of poor resistance to cold and high costs of propagation as rhizomes have to be used.

Switchgrass

Switchgrass is a native of the North American grasslands, a perennial C4 grass with a high yields on poor soils. Like Miscanthus it was introduced into Europe as an ornamental grass, but based on the data obtained in the USA it has been considered as an energy crop. It is perhaps the best choice as it is drought-tolerant, gives high yields and can be harvested once a year.

The production of ethanol is considerably simpler in Brazil as there is no starch to process. The sugarcane is harvested and milled to extract sugar (sucrose) and the rest of the plant, known as ‘bagasse’, is retained as it can be burnt in boilers. The sugar can be processed to produce sugar and the residue and molasses used for fermentation or the sugar juice used directly (Fig. 6.6). The sugar and salts are run into 100,000­400,000 l open bioreactors and inoculated with yeast. After fermentation has ceased, the yeast is removed by flocculation or centrifugation and the liquid distilled. If more than 95.6% ethanol is required a second distillation is carried out with the ethanol blended with fusel oil. The residue from the first distillation can be used as a fertilizer. The economy of the process is improved greatly as the residue from the sugarcane (bagasse) is used to fire boilers which supply steam for the distillation process.

Biodiesel is subject to oxidative breakdown which is related to the double bond con­tent of the fatty acid methyl esters, and oxidation can lead to increased acidity, forma­tion of shorter fatty acids and the production of gums. This is an important feature, as stability is needed if biodiesel is to survive long-term storage, particularly for biodiesel from waste oils that have lost their natural antioxidants.

During oxidation the fatty acid methyl ester form a radical next to the double bond which binds oxygen forming a peroxide radical. The peroxide radical reacts with a fatty acid forming an acid releasing the radical and thus forming an autocata­lytic cycle.

The first study on the oxidative stability of biodiesel was by du Plessis et al. (1985) using sunflower-derived biodiesel stored at different temperature for 90 days. Other studies by Bondioli et al. (2004) and Mittelbach and Gangl (2001) on the stor­age of rapeseed-derived biodiesel were run over 1 year and 200 days, respectively. In the study by Mittelbach and Gangl (2001), rapeseed biodiesel was stored in poly­ethylene bottles at 20-22°C open or closed, in the light and in the dark. Samples were removed at intervals and the oxidative stability measured using the Rancimat system (Fig. 7.18). In the Rancimat system, the sample is heated to 100°C and air is passed

Fig. 7.18. The effect of storage conditions on the stability of biodiesel. (Redrawn from Mittelbach and Gangl, 2001.)

through the liquid. The air is run into a conductivity cell filled with water. After a few hours the conductivity in the cell increases rapidly as a result of volatile organic acid compounds produced by the oxidative breakdown of the sample collecting in the cell. It is the time required for the induction of the increase in conductivity that is taken as a measure of oxidative stability.

Bondioli et al. (2003) studied the long-term storage, 1 year, of 11 different biodie­sel preparations, some containing antioxidants. The list of samples and the results after 12 months’ storage are given in Table 7.16. After 1 year’s storage, a number of the parameters measured did not change but oxidative stability as revealed by the Rancimat data did change. Those samples with the lowest value changed less, such as the distilled sunflower biodiesel. The Rancimat values decreased with time and were dependent on both the state of the sample and the storage conditions. The addition of two antioxidants TBHQ and pyrogallol had little effect on stability in this case.

To combat the oxidation of biodiesel, natural and synthetic antioxidants have been added. Crude and distilled palm oil methyl esters were tested for their oxidative stability and these were 25.7 and 3.52 h in the Rancimat system (Liang et al., 2006). The crude palm oil methyl ester mixture contained 644 ppm vitamin E (a-tocopherol) and 711 ppm P-carotene, whereas the distilled version contained very little of these compounds. Various quantities of three antioxidants a-tocopherol, butylated hydro — xytoluene (BHT) and tert-butyl hydroquinone (TBHQ) were added to the distilled methyl ester mixture. The two synthetic antioxidants were better than the natural antioxidants and 50 ppm was sufficient to achieve the EN 14214 Rancimat standard. Dunn (2005) has also determined the effect of different antioxidants on soybean biodiesel and the NREL report indicates that antioxidants will be required for extended storage.

Another approach has been to blend Jatropha and palm oil biodiesel. Palm oil biodiesel contains high levels of palmitic (C16:0) and oleic (C18:1) acids which are

Peroxide value Viscosity Rancimat induction

(meqO2/kg) (mm2/s) time (h)

resistant to oxidation, whereas Jatropha methyl esters contain mainly oleic (C18:1) and linoleic (C18:2) acids. Thus, a mixture of the two esters will be more resistant to oxidation (Sarin et al., 2007).

The WTW values for gaseous fuels have been compared with CNG and petrol in Fig. 8.39. CNG requires more energy than petrol and diesel but produces about the same quantity of GHGs. When compressed biogas is used, the amount of energy used is greater than the fossil fuels, but biogas produced from liquid, solid manure and municipal solid waste saved considerable amounts of GHGs. Clearly improvements in energy use for biogas are required to make the process sustainable.

Fig. 8.38. The well-to-wheel (WTW) energy and emissions for the production of syn-diesel using the Fischer-Tropsch process with different starting materials. NG, natural gas. (From JRC, 2007.)

Fig. 8.40. The well-to-wheel (WTW) energy and emissions for gaseous fuels DME and hydrogen. DME produced from wood, coal and natural gas. Hydrogen compressed (C-H2) and liquid (L-H2) in a port injection internal combustion engine and compressed in a fuel cell (C-H2 FC). NG, natural gas; PISI, port injection spark ignition. (From JRC, 2007.)

DME requires more energy for its production than petrol and CNG due to gasifi­cation and gas cleaning steps, but if wood is used for its production the emissions are reduced to a very low level (Fig. 8.40). In contrast, DME produced from coal and natural gas has high energy and GHG values. Hydrogen is normally produced from natural gas when used in an internal combustion engine and requires more energy than petrol and produces more emissions, but these are considerably reduced if the hydro­gen is used in a fuel cell. Hydrogen stored as a liquid had higher energy and GHG values than hydrogen stored as a compressed gas. The hydrogen values are similar to CNG. The hydrogen figures are very dependent on the method used to generate

hydrogen and would be greatly improved if hydrogen was produced either biologically or from sustainable electricity. This can be seen in Fig. 8.41 where the methods of producing hydrogen have been compared. Electrolysis requires more energy than hydrogen production from wood, coal and natural gas, but electricity generated from wind, nuclear and wood reduces the GHG emissions considerably. The use of nuclear­generated electricity does, however, require the most energy. Hydrogen generated from wood also has a greatly reduced emission level. Natural gas reforming to produce hydrogen tends to be more efficient than gasification in terms of energy.

All the data on the WTW have been combined in a scatter figure plotting GHG against energy use (Fig. 8.42). The figure shows considerable variation in the WTW values for the biofuels. The low values for ethanol are from sugarcane production and the low GHG values for hydrogen, biogas and DME are due to using sustainable feedstocks such as wood. The higher GHG emissions are due to the use of fossil fuels such as coal. The ideal fuel would have low values for both GHGs and energy and this can perhaps be achieved by process and feedstock changes. The WTW study can be used to indicate which fuel and feedstock needs improving. Those fuels showing low GHG and energy are ethanol from sugarcane, lignocellulose and biodiesel from plant oils. All those with low GHG emissions have been produced from sustainable materials such as lignocellulose, wastes and plants.

Infrastructure

Biodiesel is a liquid fuel with a density close to that of diesel, non-toxic, biodegrad­able, with a high flash point which means that it can be used by the diesel supply infrastructure without any significant changes. In fact it is somewhat safer than diesel. Bioethanol has many of the characteristics of petrol except that it is hydro­scopic. The ability to accumulate water means that bioethanol cannot be transferred by pipeline because the water content causes corrosion (rust). This means that it has to be transferred by tanker and this is proving to be a problem in the USA where the transport provisions have not kept pace with ethanol production.

The gaseous fuels methane and hydrogen pose different problems from the liquid fuels. Methane is essentially the same as natural gas and could be transported through the natural gas infrastructure. Hydrogen once compressed can be run through pipe­lines with a loss of 0.77% per 100 km but with a low density a leak will disperse rapidly (Hammerschlag and Mazza, 2005).

If hydrogen is produced in situ by the electrolysis of water the efficiency is 74% and compression is 88% efficient, which means that only 65% of the energy will be delivered as hydrogen. Hammerschlag and Mazza (2005) quote: ‘In virtually any conceivable

211

arrangement for supplying renewable or carbon-neutral energy to electric customers, delivering the electricity directly is more efficient than manufacturing hydrogen.’ Table 8.8 gives the fuel stations capable of supplying a range of alternative transport fuels including biodiesel and bioethanol for the years 2005 and 2007. Worldwide there are 140 stations supplying hydrogen but none for private cars in the UK.

Conclusions

Biofuels should not be considered in isolation as alternatives to fossil fuels but as a part of a drive towards the production of sustainable products normally produced from crude oil. Biorefineries which can produce a range of products including biofu­els from renewable resources should be developed.

Liquid fuels of all types will continue to be used for some time because of the dif­ficulty of supplying alternative biofuels, and the existing extensive infrastructure will continue to be used. The use of gaseous fuels such as hydrogen, CNG and LNG require extensive and costly modification to vehicles and in the case of hydrogen a completely new infrastructure. Thus, these are long-term solutions. Fuel cells may replace the internal combustion engine but these are still under development and will require modification according to the supply infrastructure, depending on the fuel used in the fuel cells. Bioethanol and biodiesel are fuels that can be used now in present vehicles and infrastructure. The main restrictions on these fuels are insufficient supply and their cost, which could be improved by the tax structure. FT diesel, FT petrol and DME are in a developmental stage, as technical advances are required to make their production economic. In the longer term, hybrid or electric cars may be the best option for short-distance travel and city use provided the cars are charged using renewable­generated electricity. Diesel and biodiesel will probably be retained for heavy trans­port; trains and cars will also use more diesel as this has better fuel consumption. It is air transport that has not been addressed, probably because fuel is cheap. Biodiesel can replace kerosene and successful tests have been carried out with turboprop engines.

Electricity is mainly generated in large power stations (2000 MW and above) and 75% of home heating comes from gas supplied through a nationwide network. While centralized systems deliver economies of scale, safety and reliability, the transfer of

electricity to remote users loses 20.3 Mtoe, which is 8.7% of the total energy gener­ated (Table 1.4). However, new and existing technologies, especially advances in gas turbines, have achieved maximum efficiency in small power plants of up to 10 MW (Poullikkas, 2005). This makes it possible to generate energy close to where it is used, which is known as ‘distributed generation’. Distributed generation has been defined as ‘a small scale power generation technology that provides electric power at a site closer to customers than central station generation and is usually interconnected to the transmission or distribution system’ (Edinger and Kaul, 2000).

Distributed energy includes:

• All plants connected to a distribution network rather than transmission network.

• Small-scale plants that supply electricity to a building, industrial site or community.

• Microgeneration, small installations such as solar panels, wind turbines, biomass burners supplying one building or small community.

• Combined heat and power plants (CHPs), including large, community — or build­ing sized and micro-CHP, replacing domestic boilers in homes.

• Non-gas sources of heat such as biomass, wood, thermal, solar or heat pumps for households and small communities.

These smaller systems can be more flexible and reduce the distribution losses incurred with a centralized system. At present less than 10% of electricity comes from micro­generation and CHP plants but these are increasing. The advantages of the distrib­uted generation include:

• These plants can be more reliable.

• They are flexible in their energy source. These can handle renewable sources of power.

• They avoid transporting fuel long distances.

• Less power is lost in distribution.

• They enable the introduction of alternative power systems which can be intermit­tent such as wind power and photovoltaics.

These distributed systems could fundamentally change the way energy is supplied, and reduce transmission losses and fuel imports.

The anaerobic breakdown of organic material has only been studied in detail in the case of the degradation of sewage sludge and an outline of the process is given in Fig. 5.4. A consortium of microorganisms that develop under anaerobic conditions degrade the organic materials in series of stages.

In the first stage is the hydrolysis phase. Here complex organic materials consist­ing of carbohydrates, lipids, proteins, DNA and RNA are broken down by hydrolytic bacteria such as Clostridium sp., Eubacteria sp. and bacteroids. The result of the hydrolysis is simple sugars, acids, ketones and amino acids. In the next stage, acido — genesis, the simpler compounds are then broken down to acetate, lactate, propionate, ethanol, carbon dioxide and hydrogen. In the next stage, acetogenesis, the simple compounds are converted into acetate. Acetate is then combined with carbon dioxide

Fig. 5.4. Stages of the anaerobic breakdown of organic materials.

and hydrogen to form methane by a group of bacteria known as the methanogens in the last stage, methanogenesis. These bacteria are some of the most oxygen — sensitive bacteria found and include Methanobacterium sp., Methanobacillus sp., Methanococcoides sp. and Methanosarcina sp. The methanogens function in close contact with the acetogenic bacteria in time and space. This allows any hydrogen formed to be transferred without loss to the atmosphere. Strict anaerobes only grow and metabolize slowly so that anaerobic metabolism is much slower than aerobic metabolism and this is why the process can take up to 30 days. There are other pro­cesses that can lead to the production of methane, carbon dioxide and hydrogen.

Methanococcoides and Methanolobus form methane from acetate which can be the preferred substrate in cold wet anaerobic soils found in wetlands:

CH3COOH ^ CH4 + CO2 (5.1)

Another reaction which can occur converts carbon monoxide produced from acetate to carbon dioxide and hydrogen:

CH3COOH ^ CH3 + CO (5.2)

CO + H2O ^ CO2 + H2 (5.3)

Other methanogens produce methane by a complex series of reactions involving hydrogen and carbon dioxide:

4H2 + CO2 ^ CH4 + 2H2O (5.4)

The exact proportion of gases varies depending on the materials broken down and the process conditions. It is likely that similar stages occur in landfill and anaerobic digestion of animal slurries: