OCR Text |
Show Cement Industry Assessment (15) of the cement industry determined that dry process kilns were the only practical sources of reject heat as opposed to the wet process kilns where reject energy is at too low a temperature to be of major use within the plant. Of the types of dry process kilns, the long dry kilns have the highest quality and quantity of recoverable energy followed by single-stage suspension preheater and the four-stage suspension preheater. Considering the sources of reject heat in each plant, kiln exit gas is the best option. It contains over 80% of the plant's reject heat and at 370 to 590°C is the highest temperature reject heat source in the plant. Clinker cooler exhaust gas (at about 175°C) is also a practical source of heat when used in conjunction with the kiln exhaust gas stream. The use of the energy contained in these streams to generate electricity for internal use in the cement plant was found to be the most cost effective of the uses considered (Fig. 4). Thermal storage in this application is used to provide reserve steam production capacity to meet peak loads, steam reheating, and feedwater heating during periods of high electrical demand. Rockbed storage using granite, cement clinker, or limestone as the storage medium was found to be the most economical type of storage. Of almost equal merit for large plants was a system using molten salt. Economic analysis showed an ROI of 44% for the long dry kiln and 31% for the single-stage preheater kiln. With a rockbed storage system included the ROI for the long kiln increases to 50% while that for preheater kiln remains constant. Iron and Steel Industry Because of the availability of relatively large amounts of high temperature (see Table 1) reject heat, a number of energy recovery options have been examined. These options can generally be categorized as either "add on" or process redesign options. A schematic (16) of a reject heat recovery "add on" system is shown in Fig. 5. In this application hot gases, nominally at 700°C, from a pair of electric arc steel remelting furnaces furnish energy for a power production system. Two energy storage beds are utilized. Once the system has begun operating the operational store rises in temperature to the mean of the fluctuating fume temperature. Thereafter, hotter inlet gases will be cooled when passing through the device while cooler gases will be warmed 8-9 |