Biogas, hydrogen and DME have been proposed as gaseous replacements for fossil fuels used in transport and electricity generation.

Gaseous fuels have problems of storage and supply not encountered with either solid or liquid fuels. Storage of gas at atmospheric pressure is not practical, and so the gas has to be either compressed at high pressure or liquefied at low temperatures to reduce its volume. Compression to pressures of 200 bar and liquefaction, which for hydrogen requires a temperature of -253°C, expends a considerable amount of energy, and subsequent storage has to be in strong pressure vessels or in well-insulated tanks. The lower energy density of the gaseous fuels compared with liquid fuels means that larger fuel tanks are required in vehicles. There is a small number of modified internal combustion engines using gases derived from fossil fuels such as liquid natural gas (LNG), liquid petroleum gas (LPG) and compressed natural gas (CNG). After treat­ment biogas is the same as natural gas, and therefore could be used as a replacement for liquid and CNG. Biogas has been used as a fuel for boilers, dual fuel engines and the generation of electricity. DME has a boiling point of -24.9°C and so can be lique­fied and stored easily. DME has lower energy content than diesel, 28.6 MJ/kg com­pared to 38-45 MJ/kg, but it has a higher cetane value of 55-60 compared with diesel at 40-55. DME could be used as a replacement for diesel but its properties are similar to propane and butane so that it could also replace these fuels for distributed power generation, heating and cooking. Hydrogen is a high energy clean fuel producing no carbon dioxide on combustion and has been used as a fuel for internal combustion engines and fuel cells. There are problems with the sustainable production of hydro­gen, its storage and distribution which may require a completely new infrastructure. The number of alternative-fuelled vehicles in the USA from 1993 is shown in Fig. 5.10. The number of CNG and liquefied natural gas vehicles has remained static, whereas the numbers of liquefied petroleum gas vehicles have declined. Thus, there must be some doubt about the introduction of gaseous biofuels as transport fuels.