Cement Energy and Environment

( Considering the operating parameters and fuel being used for the kiln under study, possibility of anhydrite formation in the upper regions of preheater sections cou ld not be ruled out altogether. Further, Anhydrite is known to have difficult to remove characteristics and usually it is difficult to remove such fan material deposits. TG/% 100~- , --- - i3S • 100 Hence it was felt necessary to reduce the preheater exit gas temperatures and/or control inlet gas temperature to fa n in addition to avoiding local reducing conditions in pre-calciner and kiln inlet area for tackling the CaS04 formation problem. Peaf 673 5 ·: ~ I \\ (1) J _ _.,...-J 00 020 -0 40 -060 '00 8CO 000 100C Figure 6: Thermal characteristics of fan deposition sample showing the loss of water molecules from Kaolinite at 494.7°C and phase transformation of Kaolinite to Mullite at 873.5°C in addition to presence of Chlorite at around 708°C It should be noted here that a water spray through four atomized lances is provided in the gas exit duct (at about 75 meters height above the fan inlet) for lowering gas temperatures . However, it was observed that particles were getting collected on the duct walls just after the spraying systems. These collected particles fell off periodically in the form of wet cake (after gai ning critical mass) in preheater fan s1de casings with high velocity causing large sounds similar to small explosions. It was also observed that the portion of exit gas duct introducing water sprays was not insulated properly from the atmosphere . This was therefore suspected to result in condensation on the walls. This condensation of water droplets on the duct inside walls and consequent cooling of the walls may thus aid in collection of particles due to the phenomena of thermophoresis. Thermophoresis basically refers to the separation of particles suspended in the hot gases on the relatively colder wall surface due to high thermal gradients (14]. Further, the possibility of generation of large water droplet size was also examined by assessing atomizing nozzle characteristics. From the process data it can be observed that although the spraying system is designed for 75° temperature drop, actual temperature drop ranges between 50-60°C. The effect of droplet diameter on the evaporation time is known to follow exponential relation [7]. For example 20011m diameter water droplet typically takes around 0.95 seconds to evaporate as compared to only 0.25 seconds for 10011m diameter in the temperature range of interest in the present context. Thus by reducing the droplet size just by half a reduction in evaporation time of almost 4 times is expected . Optimum droplet size for gas cooling applications reported in the literature lies in the range of 50-60~tm requiring about 0.065 seconds evaporation time [8]. It should also be noted that although the average droplet size is important, maximum size and their percentage in the total spray is also important for determining evaporation effectiveness. Subsequently, in the light of above effects appropriate attention was given to water spraying system and insulation surrounding lances to minimize related problems.