Overview

Since it is clear that the replacement of all fossil fuels by renewable sources of energy is going to take a very long time, interim measures must be devised to utilize the remaining reserves in as efficient a manner as possible and to minimize the pollution that will arise from their combustion. In this Chapter, we discuss the actions that are already under way and the future prospects for developing cleaner forms of fossil fuels.

By 'clean fuels', we mean clean-burning fuels that emit less pollution to the atmosphere. Among common pollutants are sulfur dioxide formed from sulfur in the fuel, nitrogen oxides (NO _x), produced during combustion of hydrocarbons in air, and the 'greenhouse gas' carbon dioxide that is an inevitable consequence of burning any hydrocarbon fuel. Other possible pollutants are carbon monoxide from incomplete combustion and unburnt hydrocarbon vapours. With correct control of the ignition conditions, the amounts of nitrogen oxides, carbon monoxide and unburnt hydrocarbons should be kept to a minimum. Finally, it should be noted that the combustion of waste materials and plastics can give rise to dioxins, which are highly toxic, and to chlorocarbons, which are damaging to the stratospheric ozone layer that protects the Earth from the sun's harmful ultraviolet rays.

Sulfur may be removed from liquid fuels by catalytic processes at the refinery. For solid fuels, the only practical process may be scrubbing of sulfur dioxide from the combustion gases. In the case of internal-combustion engines, the residual nitrogen oxides, along...