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Environmental Pollution and Management 23 4.1.2.1 Particulates While there are various sources of dust generation in a cement plant, the kiln generates the largest quantities of dust and gases. It is well known that the nature and quantity of dust and gases from kilns depend on the characteristics of raw materials, fuel, process, burning conditions, kiln dimensions, system used, etc., which in turn govern the choice of the dust collection system and its efficiency. The largest air pollutants in cement plants are the particulate emissions, which consist of carbonates, silicates, aluminates, fluorides and alkali halides, emitted through gasses at temperature of 120-350 o C. The chemical characteristics of the pollutants reflect the raw-mix composition and fuel quality. The use of lower grade raw materials leads to generation of kiln dust richer in SiO2 and alkali halides. The use of lower grade limestone also leads to relatively higher quantities of particulate matter and the particles in this case are relatively small. 4.1.2.2 Gaseous Substances Beside the airborne emissions (dusts), every combustion process gives rise to gaseous emission. The nature and quantity of the gases produced are specifically bound up with the process in question and depend on the fuels, the combustion atmosphere and the temperature. In firing systems involving direct contact between combustion gases and solid feed material, the initial materials employed are moreover to be rated among the principal influencing factors. The exit gases from cement kilns consist mainly of nitrogen oxides, carbon dioxide, oxygen and water vapor. In addition, they may contain small amounts of sulfur dioxide, nitrogen oxides, carbon monoxide and organic hydrocarbons. For product quality and process economy, the burning of cement clinker normally requires an oxidizing atmosphere and a temperature of over 1500 o C in the kiln, so that the exit gases contain only harmless amounts of carbon monoxide and hydrocarbons, if at all. Gaseous chlorine and fluorine compounds are not emitted, because they are combined with the alkaline kiln feed. Highly volatile compounds may eventually be released independently of burning process. (i) Sulfur dioxide Sulfur is introduced with the raw materials and fuels in the cement burning process. The sulfur compounds in the fuel first of all form SO2. If the raw materials contain pyrite or organic sulfides, some of these sulfides will oxidize to SO2 at temperatures as low as 450- 600oC, corresponding to the top stages in a preheater. Here, the absorption of SO2 is extremely low, and a substantial part passes out of the kiln system. In these cases, therefore, the kiln exit gases will always contain SO2. The sulfur dioxide formed by dissociation and combustion reacts chiefly with alkalis of the raw materials, giving rise to the formation of alkali sulfate which is incorporated in the clinker or the dust and thus discharged from the kiln system. In addition, sulfur dioxide reacts with calcium oxide from the calcimined raw meal to give calcium sulfate in an oxidizing kiln atmosphere. This reaction is not confined
24 Technology, Energy Efficiency and Environmental Externalities of the Cement Industry to the kiln itself, but continues in the conditioning tower and grinding/drying plant, in which the fresh reactive surface area formed in the grinding process strongly promotes this reaction in the presence of water vapor. If sulfuric or organically combined sulfur is present and the excess air is insufficient, SO2 may be released even at relatively low temperatures from the preheater of the kiln plant. The emission of SO2 can, however, be reduced by passing the gas through a grinding/drying mill to a conditioning tower. The cement burning process and grinding process are thus function as ideal desulfurising systems in which well in excess of 90 percent of SO2 is retained. That is why it is generally possible to use fuels with high sulfur content in the cement industry without harmful consequences to the environment. The introduction of a minimum amount of sulfur for combining the raw material alkalis as sulfate is indeed desirable to achieve better product quality. (ii) Nitrogen oxides NO formation takes place by means of two mechanisms. By the first mechanism, the thermal NO is formed in the kiln burning zone from the content of nitrogen in the atmosphere. The quantity is determined mainly by temperature and excess oxygen. By the second mechanism, the fuel NO is formed. In this instance, the content of volatiles and nitrogen in the fuel, as well as excess oxygen are the deciding factors. The fuel NO formation is of secondary importance in the burning zone as the temperature at this point is so high that considerable thermal NO is formed anyway. With secondary firing, as in precalciners, the fuel NO is of importance. The emissions of nitrogen oxides from the cement manufacturing process are much more difficult to reduce. For reasons of quality, the cement kiln has to be operated with high combustion temperatures and excess air; under these conditions the nitrogen oxide formed is more particularly the thermal NO. Gas measurements carried out in various parts of the world have revealed widely differing amounts of NOx emission from cement kilns, ranging from about 150 to over 1000 ppm (Kroboth et al. 1987). As opposed to what had been found with SO2 emission, there was no ascertainable elimination of NOx in conditioning tower or grinding/drying plants associated with the kilns. The reduced concentrations of NOx measured in those installations was entirely due to dilution with process air. A large number of short-term and long-term investigations have meanwhile revealed that the following factors are of qualitative importance to nitrogen oxide formation during the clinker burning process: the fuel used, the design and operation of precalcining system or the secondary firing methods employed, the characteristic properties of secondary fuels, the burnability of the feed material, the flame temperature, the flame shape, the burner or its setting, and the excess air factor. The variations in the NOx content of the cleaned gas discharged from a kiln plant, as determined in long-term measurements, are plotted in Figures 4.1.a and 4.1.b. During the course of the day as represented in Figure 4.1.a the kiln functioned trouble-free, producing clinker with between 0.7 and 1.2 percent of free lime. The NOx emission behavior of the same kiln over a long period is shown in Figure 4.1.b.
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The Asian Institute of Technology (
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