SECTION II. CEMENT MANUFACTURING PROCESS Rules of Thumb 1.6 metric tons of raw mix gives 1 metric ton of clinker 0.95 metric tons of clinker + 0.5 metric tons of gypsum making 1 metric ton of clinker uses an average of 4.7 mmBtus. making 1 metric ton of cement uses an average of 158 kWh Process Description The cement manufacturing process consists of four main steps as shown in the figure overleaf. The method of grinding the raw meal defines the two broad classes of kilns. Wet. In which the raw materials are ground with water to give a slurry consisting of 30-40% water that is fed into the kiln The advantage of this process is that it 3. Burning changes raw mix chemically into cement clinker. Note four-stage prehester, flesh furnaces, and shorter kiln. this 4. Clinker with gypsum is ground into Pordand cement and shipped. New technology in dry-process cement manufacture. can handle wet or sticky materials and the raw materials can be effectively blended and homogenized in large tanks as a slurry. The disadvantage is the consumption of fuel needed to drive off the water. No plants using this technology have been built since 1975. Dry. In which the raw materials are ground dry and fed to the kiln as a fine On leaving the kiln, the clinkered material is cooled with air and transferred to storage. Step 4 Finish grinding. The clinker is mixed with 3 to 5% gypsum, ground to about 95% passing 325 mesh and stored for shipment. As indicated in the bar chart, the average fuel consumption of 5.89 mmBtus per metric ton of cement for wet process kilns is considerably greater than precalciners which average 3.74 mmBtus per metric ton. Carbon Dioxide Emissions and Cement Manufacturing Carbon dioxide emissions in cement manufacturing come directly from combustion of fossil fuels and from calcining the limestone in the raw mix. An indirect and significantly smaller source of CO, is from the consumption of electricity assuming that the electricity is from a fossil fuel-fired station. Taking a standard raw mix and the industry averages for fuel consumption per metric ton of cement, and assuming that 100% coal is used, then emissions range from 2,270 lbs CO, per metric ton of cement produced in wet kilns to 1,840 from precalciners. The CO, attributable to calcination is about 1,100 lbs/metric ton of cement in all cases. Carbon dioxide emissions attributable to electricity consumption, or from mobile equipment used on the plant site for cement or raw material transport are not included. Only the CO2 emissions from fuel combustion can be reduced by improving energy efficiency, those resulting from the raw material will not be affected. One approach to reducing the CO, emissions from limestone is to replace limestone in the raw mix with calcined materials such as lime wastes. However, the availability of these wastes is very limited. Despite the high concentration of CO, in kiln stack gases, there is currently no economically viable way of capturing it for beneficial use. Process stages and Energy Consumption The approximate energy consumption for each step of a modern precalciner plant indicates that 91.8 % of total energy is used in the pyroprocessing stage, Options for Improving Energy Efficiency By far the largest proportion of energy consumed in cement manufacture consists of fuel used to heat the kiln and the greatest gains in reducing energy input will come from improved fuel efficiency. Energy Consumption by Process Stage Kiln 91.8% Quarrying/Crushing 1.4% Raw Meal Grind 2.2% Source: Sample of 5 plants Optimization of the less fuel efficient dry plants has already shown considerable gains and will continue to progress through modifications such as conversion from direct to indirect firing and improved heat recovery from coolers. This type of optimization could achieve an average improvement of about 8 to 10%. From the plant operator's perspective there is a strong incentive to reduce the kilowatt hours of electricity used per ton of cement. Electricity averages only 10% of total energy but it |