1. Introduction - Firenze University Press

NOx emissions from cement kilns depend on both the kiln type and the fuel type. In general, preheater
and precalciner kilns have lower NOx emission rates than long dry and wet kilns, due to the
higher fuel efficiency and lower firing rates in the kiln firing zone [4].
NOx is formed during fuel combustion by oxidation of molecular nitrogen of combustion air as well
as nitrogen compounds of the fuel. Significant oxidation of molecular nitrogen of combustion air
takes place in oxidizing flames with a temperature above 1200°C. The NOx formed in this way is
named thermal NOx, otherside the NOx formed by oxidation of nitrogen compounds in the fuel
which is named fuel NOx.
1.3 – Co-processing
Cement companies are introducing the industrial waste co-processing from a perspective that
combines environmental policies with the interests of companies. Industrial wastes with residual
energy and low content of chlorides and heavy metals can be appropriate to provide part of the
energy required to make cement.
The process of clinker burning in rotary kiln creates favourable conditions for the use of industrial
waste like alternative fuel. These conditions are high temperature, alkaline environment, oxidizing
atmosphere, large heat-exchange surface, good mixture of gases and products, and sufficient time
(over 2 seconds) for the disposal of hazardous wastes [5]. The Table 1 presents examples of
alternative fuels used in cement industry.
Table 1 – Examples of Alternative Fuels
Alternative Fuel Examples
gaseous Landfill gas, pyrolysis gas
Liquid Pasty wastes, solvents, waste oils, greases
Solid Paper, used tires, rubber wastes, plastics
The used tires are material residual special, based on higher heating value (28–32 MJ/kg of tyres),
they are excellent sources of energy, mainly when used as secondary fuels. The high temperature,
the high time of residence, the high effect of absorption of the raw material in the pre-heating and
the incorporation of the ashes generated to the clinker, are favourable conditions for burning of
tyres in rotary kilns of clinker production, so that, is an adequate form to final disposition for these
wastes. Besides, due to the high calorific value of the tires, the co-processing contributes to a
decrease in consumption of others fossil fuels utilized (as coal, petroleum coke and fuel oil), saving
the natural resources [6].
1.4 – Mineralizers
The use of mineralizers in the cement industry is widely known. The incorporation of compounds
other than those usual in low proportions improves the clinkering conditions as well as decreases
the maximum clinkering temperature or improves the phase formation in the clinker without
altering the final properties of the product [7]. The mineralizing properties of the compounds CaF2
and CaSO4 have already been described in the literature. Their properties are different when they
are added separately or jointly in the raw materials.
It has been verified that the combined addition of CaF2 and CaSO4 to raw clinker materials results
in a decrease of the maximum clinkering temperature to approximately 1350°C, in a clinker with a
good proportion of alit and a cement with satisfactory mechanical properties [7]. The reduction of
temperature by 100°C is expected to result in a saving of fuel by 80-100 kcal/kg of clinker [8].
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