Cement producers worldwide are striving to lower their production costs. One effective method of achieving this is the use of alternative fuels. Use of low-grade alternative fuels such as waste coal, pellets, tyres, sewage sludge, and biomass fuels (such as wood products, agricultural wastes, etc.) in pre-calciners is a viable option because combustion in a pre-calciner vessel takes place at a lower temperature.
In pre-calciners where kiln exhaust gases pass through, the NOx emissions are much reduced due to re-burn reactions. There is an increased net global reduction in CO2 emissions when waste is combusted in the cement kiln systems as opposed to dedicated incinerators, resulting in reduction in the CO2 penalties. Since alternative fuels are often deemed cheaper than conventional fossil fuels, the possibility of a competitive edge is generated.
The conservation of non-renewable resources, and the reduction of waste disposal requirements. The use of alternative fuels in European cement kilns saves fossil fuels equivalent to 2.5 million tonnes of coal per year (Cembureau, 1999). The proportion of alternative fuels used in cement kiln systems between 1990 and 1998 in some European countries are as follows in order of importance:
France 52.4 percent; Switzerland 25 percent; Great Britain 20 percent; Belgium 18 percent; Germany 15 percent; Czech Republic 9.7 percent, Italy 4.1 percent; Sweden 2 percent; Poland 1.4 percent; Portugal 1.3 percent and Spain 1 percent (Mokrzycki et al., 2003).
The process of clinker production in kiln systems creates favourable conditions for use of alternative fuels.
These include: high temperatures, long residence times, an oxidising atmosphere, alkaline environment, ash retention in clinker, and high thermal inertia.
These conditions ensure that the fuel’s organic part is destroyed and the inorganic part, including heavy metals is trapped and combined in the product.
The wastes used as alternative fuels in cement kilns would alternatively either have to be landfilled or destroyed in dedicated incinerators with additional emissions as a consequence. Their use in cement kilns replaces fossil fuels and maximises the recovery of energy. Employing alternative fuels in cement plants is an important element of a sound waste management policy. This practice promotes a vigorous and thriving materials recovery and recycling industry (Cembureau, 1999).