Introduction

In contrast to the solid biofuels, described in Chapter 4, gaseous biofuels can not only be used for both electricity generation and heating, but also most importantly as a transport fuel. A list of gaseous biofuels is given below:

Gaseous fuels:

• Methane (biogas).

• Hydrogen.

• Dimethyl ether (DME).

Methane or biogas can be used to replace natural gas (methane) which is a fossil fuel for electricity generation and for cooking and heating. For land transport, there are 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). Biogas, hydrogen and dimethyl ether have been proposed as replacements for these transport fuels. Hydrogen has also been proposed as a fuel for gas turbines.

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 so the gas has to be compressed to high pressure or liquefied at low temperatures to reduce its volume. Compression to pressures of 200 bar and liquefaction, which for hydrogen needs 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. One advantage is that transport of gaseous fuels can be carried out using pipelines which are used at present for natural gas although in the case of hydrogen its low density may encourage leaks. All gaseous fuels are inflammable, especially hydrogen, which introduces safety problems when these gases are stored in vehicles. The dangers of hydrogen fires are often illustrated by the crash of the airship Hindenberg, but as hydrogen diffuses so rapidly any spill in an open space may disperse before anything can happen.