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%.