The gas produced by anaerobic digestion of wastes consists mainly of methane (50-75%) and smaller amounts of carbon dioxide and hydrogen, and has an energy content of 20-25 MJ/kg, less than that of 100% methane which has energy content of 50.2 MJ/kg (Table 5.2). However, this is sufficient energy to be used in boilers and engines but if it is to be used as a transport fuel it will need to be upgraded to ~95% methane. The upgrading consists of the removal of contaminants such as carbon dioxide, hydrogen sulfide, ammonia, particles and water so that the gas contains 95-98% methane. The most common methods available to remove carbon dioxide are water scrubbing and pressure-swing absorption. Hydrogen sulfide can be removed during anaerobic digestion by adding iron chloride and air-oxygen dosing of the resulting gas.

Light-duty vehicles running on biogas will normally be fitted to petrol engines where the vehicle retains the ability to run on petrol. In contrast, heavy-duty vehicles are normally run on biogas only. The gas is stored compressed or liquefied where compressed gas is the most common at 200 bar. The energy density of compressed gas is much less than in liquids so that the vehicle range is reduced or the tank needs to be much larger (Table 5.2). In the heavy-duty vehicles where long range is impor­tant, liquefied gas is generally used. As a transport fuel methane, like LPG, CNG and LNG, has seen only limited use because of the costs of modification and installation, despite being a cheaper fuel due to tax concessions. The number of alternative fuelled vehicles in the USA from 1993 is shown in Fig. 5.10. The number of CNG vehicles has remained static as has LNG vehicles, whereas the number of LPG vehicles has declined. There has been a slow increase in electric vehicles where the number excludes electric hybrids, and a very small number of hydrogen power vehicles. It is those vehicles capable of running on ethanol E85 (85% ethanol) that have shown a rapid increase and the EIA estimates that there are some 6 million vehicles capable of using E85.

The Energy Saving Trust (2007) states that in 2007 there were 1490 LPG stations and 18 dispensing natural gas (NG) in the UK but by comparison there were only 622 in Germany and 521 in Italy.

The properties of the transport fuel gases methane, propane and butane are compared with diesel and petrol in Table 5.2. Methane has a higher energy content than petrol but a much lower octane number. The octane number is a measure of the resistance of the fuel to pre-ignite when compressed. A low octane fuel will pre-ignite causing a condition known as ‘pinking’ with a loss of power.

Table 5.2. Properties of methane, propane, butane, petrol and diesel. LPG 60% propane Properties Methane (CH4) Hydrogen (H2) Propane (C3H8) Butane (C4H10) 40% butane LNG Petrol (C7HJ Molecular weight 16.07 2 44.11 58.13 — — 100.2 Carbon (%) 37.5 0 82 96 — — 85-88 Liquid density (kg/l) 0.72 0.071 0.5 0.58 0.5 0.5 0.74 Gas density (kg/l) 0.0007 0.000084 0.0018 0.002 0.002 0.5 — Liquid energy (MJ/kg) 50.2 141.9 46.4 45.7 48.8 50.0 44.0 Gas energy (MJ/l) 0.039 0.012 0.079 — 0.024 0.023 — Boiling point (°C) -161 -253 -42 -0.5 35-200 Cetane number 5 — 5 20 — — 0.5 Octane number 10 — 112 — — — 90-100 Sulfur (%) 0 0 0 0 0 0 0.05

250,000
200,000 ф
150,000
ф J2 Е
100,000
50,000
1997	1999 2001	2003 2005
□ CNG □ Elec DLPG □ LNG DE85

Whether biogas is to be used for electricity generation or as a transport fuel, the total biogas available in the UK at present has been estimated as just over 6 Mtoe (Table 5.3). In 2006, the UK used over 60 Mtoe as transport fuels, 37.2 Mtoe in elec­tricity and 232.2 Mtoe in total. So biogas can make a contribution to the renewable portion of energy used in the UK but the contribution will only be small at 2.7%.

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 pro­vided 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.

• A cetane number of around 50 is a measure of the ignition quality of the fuel.

• The viscosity of the fuel is important as it affects the flow of the fuel through 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 waste.

• It must be available in large quantities. For example the current use of diesel in 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 agricul­tural 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 cracked to form diesel, is a first-generation biofuel.

• Long-chain hydrocarbons (C 30) accumulated by some microalgae, which can 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 a second-generation biofuel.

• Gasification of biomass followed by Fischer-Tropsch synthesis of diesel (FT diesel) is a second-generation biofuel.

f 136

• Transesterification of plant, animal and waste oils and fats to methyl esters (biodiesel) is a first-generation biofuel.

• Oil accumulated by some microalgae, extracted and transesterified into biodiesel, is a third-generation biofuel.

The scale of the problem of providing a significant replacement for fossil fuels using first-generation biofuels is illustrated in Table 8.4. The agricultural land required to supply 15% of the US transport fuels for a number of crops has been calculated. For

the temperate crops such as soybean and rapeseed, this would require 67 and 42% of the agricultural land, respectively, and even with the highest yield crop, oil palm, 7.2% of the land would be needed. Unfortunately crops such as oil palm are unsuit­able for growth in temperate climates. The table does make a good case for biodiesel from alternative sources such as microalgae.

One method of reducing or stabilizing atmospheric carbon dioxide levels is to trap and lock away carbon dioxide produced by the large carbon dioxide emitters such as power generation and cement works (Fig. 3.10). It is clear that electricity generation, particularly using coal, is the largest stationary source of carbon dioxide, followed by cement production and refineries.

Carbon sequestration can be defined as stable storage of carbon, but it has been suggested that storage in soil and plants cannot be regarded as stable as microbial degradation can lead to carbon dioxide release. However, the degree of permanence can vary greatly with carbon sequestered in soils with some components lasting up to 1000

-У

Fig. 3.11. Various methods that could be used for carbon dioxide sequestration from a large stationary source of carbon dioxide.

The possible methods of sequestering carbon dioxide from stationary sources are as follows (Fig. 3.11):

• Carbon capture and storage in the deep oceans, oil and gas reservoirs, aquifers and coal beds (geosphere sink).

• Planting more trees or reforestation (biosphere sink).

• Chemical sequestration (material sinks).

• Trapping the carbon dioxide in material such as plastics (material sinks).

• Agricultural practices (biosphere sink).

Introduction

Oil makes up 35% of the world’s primary energy supply and the majority of this oil is used to produce the transport fuels petrol, diesel and kerosene. If biofuels are to be used to replace the liquid fuel produced from oil, the scale of the replacement needs to be appreciated. The world used in 2005 about 1900 Mtoe oil for transport, the USA 601 Mtoe, the EU 25 342 Mtoe and the UK 58 Mtoe. The fuels used for trans­port in the UK, the EU 25 and world are given in Table 6.1. At present almost all transport fuels are liquid and used in internal combustion, compression ignition and jet engines. The liquid fuels are predominantly petrol, diesel and kerosene. The pat­tern of fuel use will vary depending on the country as many use more diesel than petrol. In the EU, diesel use exceeds petrol use and the more recent figures for the UK indicate that diesel use has exceeded petrol for the first time.

The possible biofuel replacements for petrol and diesel were sketched out in Chapter 4, section ‘The Nature of Biofuels: First-, Second — and Third-generation Biofuels’, where the three generations of biofuels were explained. This chapter deals with biofuels which can supplement or replace petrol and include methanol, bioetha­nol, biobutanol and FT-petrol.

Table 6.1 gives the amount of liquid fuels used for transport in the UK in 2006 and in the EU 25 and world in 2005. In the UK, diesel use has increased rapidly and now exceeds petrol, following the trend set in the EU, whereas petrol use is greater on a global basis.

Untreated plant-derived oils have been used to replace diesel in emergency situations and Diesel’s patent included plant-derived oils as one of the fuels. However, long-term use of untreated oil does cause problems with diesel engines. Recently interest has been revived in the use of oils as a renewable and carbon-neutral replacement for diesel.

Oils are produced from plants throughout the world in considerable quantities (Shay, 1993). Plant oils are normally extracted from oil-containing seeds, where the plant uses oil rather than starch as an energy store for the seed. Seed oil can be extracted from a wide range of annual crops such as soybean, sunflower, rapeseed (canola) and the perennial oil palm. A list of high oil-producing plants is given in Table

7.9 where it is clear that perennial crops have a higher yield of oil per hectare. Despite the higher yields from the perennial plants, annual crops like rapeseed and soybean have commanded most interest, probably because there is already a market for their oil and annuals are a more flexible crop which are often grown in rotation.

The advantages of using plant-derived oils are:

• They are liquid.

• Their calorific content is 80% of diesel.

• They are readily available in large quantities disregarding competition with food crops.

• They are renewable/sustainable as derived from crops.

• They are non-toxic and much more biodegradable than diesel.

• They are CO2-neutral, combustion releases CO2 previously fixed by the plant.

• They are contain no sulfur.

However, the following are problems associated with the use of untreated plant oils:

• They have high viscosity.

• They have low volatility and high flash point.

• They contain reactive unsaturated hydrocarbon chains.

• They have carbon deposits.

To function correctly in a diesel engine, the fuel must form a fine mist, which should burn rapidly and evenly. Untreated plant-derived oil contains residual components such as waxes, gums and high molecular weight fatty components, which clog the fuel lines and filters. High oil viscosity cause poor atomization, affecting ignition and combustion, which gives carbon deposits on injectors, combustion chamber walls and pistons. The polymerization of unsaturated fatty acids in the combustion chamber also causes deposits on the wall, and some components mix with the lubricating oils increasing their viscosity (Peterson et al., 1996; Ma and Hanna, 1999). The presence of water in the oils can allow microbial growth that can block the fuel filters. To illustrate the problem of viscosity, a comparison of the properties of diesel and plant oil is shown in Table 7.10.

Different methods have been used to reduce the viscosity of the oil, which includes blending with diesel, microemulsification, pyrolysis and transesterification. Of these methods, only transesterification has been successful, and the mixture of esters formed is called biodiesel.

Forest trees are not just sources of building material and paper pulp but are also sys­tems for carbon dioxide sequestration and a source of biofuel. It is proposed that genetic manipulation could increase carbon partition to woody tissues, and increasing cellulose availability for digestion. Lignin content can be changed by modification of gene expression (Groover, 2007). Plant material having less lignin is more digestible when lignocellulose needs to be broken down into sugars for ethanol production. Reduction in lignin content has been achieved in trees in order to reduce the bleaching required when making paper pulp. Changes in lignin content have been carried out in maize, sorghum, poplar and pine (Gressel, 2008). Some concerns have been aired that a reduction in lignin content will weaken plants, causing flattening of crops (lodging), but the evidence for this is limited as many of the crops used now are short stemmed.

The demand for electricity varies daily and seasonally and therefore some centralized power stations may only be required for short periods or to operate at limited capacity. In general, a fully interconnected electricity network will use low-cost, very large power stations for the base load and more expensive units for peak loads. The fossil fuels used for either base load or peak units are easily stored and trans­ported. Most renewable energy sources with the exception of biomass cannot be stored and transported easily, unless they are converted into electricity or other energy carriers. In addition, some renewable power systems only supply electricity intermittently, for example wind, wave and photovoltaics. In order to incorporate these intermittent electricity sources and to deal with peaks and troughs in electricity

demand some form of storing either electricity or energy which can be converted back into electricity is required. The advantages of storage would be the following:

• Bulk storage of energy would allow the decoupling of production from supply.

• Allows the incorporation of smaller power stations into the network.

• Improves power quality and reliability.

• Reduces transmission losses as transmission distances reduced.

• Cost reduction, as smaller, more efficient power stations can be constructed.

• Allows the use of intermittent renewable power sources.

• Decreased environmental impact associated with renewable sources.

• Strategic advantages of generating energy from indigenous energy sources, avoid­ing imports.

At present two large-scale energy storage systems are in operation: pumped hydro storage and compressed air energy storage (Dell and Rand, 2001).

In the case of pumped hydro storage, excess electricity at times of low demand is used to pump water into a lake or reservoir some distance above a hydroelectric power station. When a peak in electricity demand occurs conventional power stations are too slow to respond but the stored water can be released and the hydroelectric plant comes online rapidly. In the UK, there is such a system in Wales. The second large-scale energy storage system is to compress air in large reservoirs when electricity is in excess and release this to drive electricity-producing turbines. Such systems have been operat­ing for some time in Germany and the USA (van der Linden, 2006).

On the small scale a number of systems are under development including the following:

• Flywheels.

• Hydrogen production.

• Batteries.

• Thermal storage.

• Superconducting magnetic coils.

Flywheels have also been used to store energy and using new technology small high- density systems have been constructed and megawatt modules can be installed.

On the island of Utsira, Norway, electricity is provided by wind turbines as there is no link to a mainland power station. Wind power is intermittent so that any excess electricity generated when the wind blows is used to electrolyse water, producing hydrogen. The hydrogen is stored and burnt to produce electricity when the wind is insufficient to run the turbines. The feasibility of a wind-photovoltaic system using compressed hydrogen has also been tested in Australia, where the costs of the hydro­gen storage was the most critical factor (Shakya et al., 2005).

Five types of batteries can be used to store electricity. The lead-acid battery was developed a long time ago and is used widely in the automotive industry. These batter­ies have also been used for small wind and solar installations but they require periodic maintenance and are poor at low and high temperatures. Alkaline batteries, nickel-iron and nickel-cadmium, were also developed a long time ago, around 1900. The best is the nickel-cadmium which performs better than the lead-acid at high and low temperatures. It is however more expensive but the nickel-metal-hydride has been developed. This battery, although more expensive, holds more charge and has seen widespread use in mobile phones and laptop computers. It has also been used in

electric and hybrid vehicles. The third type of battery is the flow batteries, sometimes known as ‘regenerative fuel cells’ (rated to 12 MW). The cells are charged, converting electricity into chemical energy. The two compartments of the cell are separated by an ion-exchange membrane and the electrolyte in the compartments is circulated in a closed-loop system. The last two types of battery are the high temperature battery and the rechargeable lithium battery. The high temperature battery uses molten sodium at 300-400°C, and both these types have problems for large-scale use, although lithium ion batteries are widely used in portable electronic devices.

The thermal storage of energy from electricity using hot water or solid material is used to heat buildings in the form of night storage radiators where off-peak electri­city is used. The heat cannot efficiently be converted back to electricity so this is not suitable for energy storage. However, phase-change materials have been used to store solar energy (Kenisarin and Mahkamov, 2007).

In systems where there is fluctuating power, superconductive magnetic energy storage can be used, and though the system is expensive, it can respond in millisec­onds. Energy is stored in a magnetic field formed by a DC current in super-cooled superconductive coils.

In the UK, the total energy consumption in 2006 was 232 Mtoe with the consumption of gas at 89.2 Mtoe and coal 43.4 Mtoe (Fig. 1.4). Both gas and coal are used to gener­ate electricity and it is these two fuels that biomass may replace. An estimate of the possible biomass available in the UK is given in Table 4.8. A total of 7 million t of biomass represents an energy content of 0.149 EJ as 1 t of biomass represents19 GJ. This constitutes 1.53% of the total energy consumed in the UK. If the biomass is con­verted into electricity at an efficiency of 30%, it would generate 9.8 TWh. The UK electricity use is 346.4 TWh and therefore 9.8 TWh represents 2.83% of total demand.

In another study the potential of a mixture of energy crops and forestry and straw wastes combined with a conversion of grassland to produce electricity was determined (Powlson et al., 2005) (Table 4.9). The mixture of energy sources was capable of producing 12.2% of the electricity required from 7.43 Mha, some 40% of agricultural land. In contrast it has been suggested that 2.7 Mha would be required to produce 100% of the UK’s electricity (Rowe et al., 2009).

Lignocellulose consists of three polymer types, cellulose, hemicellulose and lignin, which are the main constituents of plant cell walls (Fig. 6.4). The primary cell wall consists of cellulose fibres embedded in a polysaccharide matrix of hemicellulose and pectin. Cellulose is the most abundant plant compound and the second most abundant is lignin which provides mechanical support and protection in plants. The composition of some lignocellulose sources is given in Table 6.7 in terms of lignin, cellulose and hemicellulose. Cellulose is a polymer of glucose linked together by 1,4-glycosidic bonds. Hemicellulose is heterogeneous polymers with a backbone of 1,4-linked xylose residues but contains short side chains containing other sugars such as galactose, ara — binose and mannose. Xylose is the predominant sugar in hardwoods and arabinose in agricultural residues.

Lignin is a highly branched polymer of phenyl-propanoid groups such as con — iferyl alcohol (Fig. 6.4). Cellulose represents 40-50% of dry wood, hemicellulose 25-35% and lignin 20-40% depending on the plant type. Although lignocellulose is abundant, it cannot be metabolized by yeast and therefore needs to be broken down to its constituent sugars before it can be used. In addition not all yeast can metabolize the pentose sugars derived from the hemicellulose. The composition of agricultural lignocellulose sugars is shown in Table 6.8. Saccharomyces cerevisiae can ferment

glucose, mannose and galactose, but not the other sugars. Depending on the source of lignocellulose the sugar produced will change. Glucose and xylose are the main sugars in the agricultural lignocellulose except in switchgrass. Investigations are under way to find organisms which can ferment these other sugars and produce etha­nol or to genetically manipulate yeasts to be able to metabolize these sugars.

Lignocellulose is difficult to break down into sugars, but a number of technolo­gies are under investigation including enzymes. Because of the presence of hemicel — lulose and lignin and the crystalline nature of cellulose in lignocellulose some form of pretreatment is required before enzymatic or chemical hydrolysis. These pretreat­ments are shown in Fig. 6.10, and include carbon dioxide, steam and ammonia explo­sion, mechanical grinding, acid, white rot fungi treatment and ozonolysis.