4.2 Developing current energy systems
4.2.1 Cleaner fossil fuels
A variety of techniques have been developed for manufacturing liquid or gaseous hydrocarbon fuels from coal or crude oil (in some cases to redress a deficit of a particular fuel in a single country). An historic example is 'town gas', mainly hydrogen and carbon monoxide with minor amounts of methane, which is produced by destructive distillation of coal in the absence of air (pyrolysis) and with steam injection. The process was associated with the production of coke, coal tar and other industrially useful materials. In the UK, town gas was a major domestic and industrial fuel from 1804 to the 1970s, when it was rapidly replaced by natural gas from the North Sea. It was also produced from oil in the UK between 1960 and 1975.
Plentiful supplies of oil and gas have curbed research into developing these technologies; after all, why bother with complex conversions when the stuff simply comes out of the ground virtually ready to use? However, the large reserves of coal may offer a source of oil by liquefaction, as well as more sophisticated gasification. Production of oil from coal was first developed on a commercial scale in Germany during the 1930s and 40s, and has operated in South Africa since the mid-1950s (Figure 17), prompted initially by the country's lack of oil, abundant coal deposits, and increasingly isolated political stance. Although three plants remain operational, the real costs of production are not competitive with crude oil, unless oil prices continue to rise. Although the South African plants employ a two-stage process (gasification followed by catalytic synthesis), research currently favours the development of a different, relatively low-temperature, catalytic process where the coal is first dissolved in a suitable solvent before treatment with hydrogen.
Other non-conventional sources of liquid petroleum are estimated to represent a vast, untapped resource (they include tar sands, oil shales and heavy oil. However, these are not clean fuels, and in the main they require extensive (and expensive) processing to produce petroleum comparable to crude oil. The difficulty in their exploitation, and their potential environmental effects, have retarded their development thus far, and may continue to do so unless the world remains addicted to petroleum as an energy source beyond the lifetime of conventional reserves. Methane hydrates, currently residing in seafloor sediments and some terrestrial settings, represent huge potential resources. They also present a major potential hazard to climate, should warming of deep water destabilise them. Yet their formation involves another gas hydrate that incorporates carbon dioxide, and methane-hydrate exploitation could be combined with sea-floor CO2 sequestration.