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Energy Issues in the Cement Industry 21 Figure 3.5. Flow diagram of suspension preheating/precalcination system 3.4 Concluding Remarks Since the cement industry is an energy intensive high-temperature-process, attention should be given to the recovery of waste heat from the various exhaust streams. The housekeeping measures can lead to lower energy consumption as well as proper functioning of processes. The modification and replacement of sub-processes by adapting advanced technologies can also save significant amount of energy. The low grade fuel substitution in cement industry is found to be beneficial both at the macro and micro levels. The fuel substitution needs strategic planning at the plant management level as well as institutional support from the national authorities. Cogeneration and computerization are also the improvements which should be incorporated along with process conversion. In most industrialized countries, the wet process has been completely eliminated. The conversion of wet to dry process is the necessary improvement for developing countries. Therefore, medium and long term goals should be set to take gradual action in this direction.
22 Technology, Energy Efficiency and Environmental Externalities of the Cement Industry 4. ENVIRONMENTAL POLLUTION AND MANAGEMENT 4.1 Sources and Characteristics of Pollutants 4.1.1 Water pollution The largest part of the water used in cement manufacturing is essentially non polluting. Process water is evaporated and most cooling water is not contaminated. The water pollution problems originating from cement plants are generally directly related to dust collection and/or dust disposal. The main sources are: - raw material washing and beneficiation - produces high pH and alkalinity, total dissolved and suspended solids - process water - only in the event of spillage - dust control - uses wet scrubbers to collect kiln dust from effluent gases. - dust leaching - dry dust is mixed in a slurry and placed in a clarifier for settling, the under flow of which is returned to the kiln. The overflow containing high pH, alkalinity, suspended solids, dissolved solids, potassium and sulfate is discharged. This constitutes the most severe water pollution problem in the industry. - dust disposal - collected dust is mixed into slurry and fed into a pond for solid settling. Settled solids are not recovered and the overflow (leachate) is discharged. Only in exceptional cases, seepage water from dumps or stockpiles will have to be considered. The process water used in cement manufacturing is required for conditioning the exit gases or for the treatment of the raw meal in wet-process and grate preheater kilns. As this water evaporates into the atmosphere, it is therefore not discharged as waste water. 4.1.2 Air pollution Among the pollution problems associated with cement industry, air pollution is undoubtedly the most significant one. Making 1 ton of cement requires the grinding of about 2.5 tons of raw materials, intermediate products and solid fuels to a dust-like fineness. Furthermore, even with heat-saving methods, about 100-110 kg of coal equivalent (with a flame temperature of over 1500 o C) is needed per ton of cement. Depending on the process employed and the degree of sophistication of a cement plant, the manufacture of 1 kg of cement gives rise to between 6 and 14 m 3 of exhaust air and gas. These quantities of air and gas have to be cleaned before being discharged into the atmosphere. Besides the particulate emissions, these gaseous pollutants play an important part in the air pollution. The gases from the kilns are generally identified as CO, CO2 and nitrogen oxides. Oxides of sulfur are either absent or present only as a trace quantity depending on the sulfur content of the coal used and also because sulfur oxides are absorbed in the kiln during the clinkering process. Hydrocarbons and other organic identities in the exit gases are absent if coal is used as the fuel.
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The Asian Institute of Technology (
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