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Environmental Pollution and Management 33
4.3.4 Utilization of waste as raw material and fuel in cement industry
Because of the special features of the cement burning process - the strongly alkaline feed
material, kiln charge and kiln dust, oxidizing kiln atmosphere, temperature distribution in
the burning system, and the intimate contact between the solids and gases in the kiln - it is
possible, on the one hand, to employ waste materials as fuel and, on the other hand, to add
waste materials to the raw mix or the cement. The cement industry can thus make an
important contribution to the disposal of wastes arising from other sectors of industry.
The use of waste materials as “junk fuel” (waste-derived fuel) in the clinker burning process
is subject to limits due to requirements of environmental compatibility, product quality, and
economy in relation to primary fuels.
Such materials may be fired in a finely ground condition or in lump form. The feed-in
points for these fuels are the main firing system in the precalciner or the riser duct of the
cyclone preheater, the kiln inlet or the hot gas compartment of a grate preheater kiln, or as
an inter-ground admixture to the kiln feed meal.
The kiln inlet (feed end) can be used for this purpose only in plants equipped with
preheaters. The waste-derived fuels fed into the system at this point are mostly in the form
of coarse lumps. They may consist of scrap motor tires, rubber shreds, shredded
household refuse, compacted refuse, or textile and wood wastes. However, finely divided
waste materials such as acid sludge, low-grade coal and oil shale may also be fed in here.
As numerous measurements have shown, with proper process control the firing of these
waste-derived fuels does not cause any increase in emission of environmentally relevant
pollutants. Emission of dioxins and furans have received special attention because they
have been identified in the stack gases from number of solid waste incinerators. However,
survey of test results from trial burns at cement kilns indicates that emissions of dioxins
and furans from these facilities are not significant. When dioxins and furans have been
observed, they appear to be three orders of magnitude less than those reported for
municipal incinerators. Moreover, there is no change in dioxin or furan emissions due to
the use of waste-derived fuels. If waste materials with a high sulfur or chloride content are
used as fuels, attention must be paid not only to the quality of the clinker and cement
produced, but also more particularly to the process engineering requirements of kiln
control. It is advantageous to carry out an emission prognosis to ascertain what quantities
of waste-derived fuels can permissibly be used.
The waste-derived fuels currently used in USA and European cement plants are primarily
waste oils and spent organic solvents from the following industries: paint and coatings,
auto and truck assembly, solvent reclamation, ink and printing, cosmetics, toy, medical and
electronics. The following environmental and economical benefits can be achieved when
wastes are destroyed in a cement kiln: