Cement Energy and Environment

median size (d5o) of around 12 ~~m. which are escaping the preheater system from cyclone gas outlet. Typical temperature of gases leaving top cyclones and that of gases entering the fan are in the range of 311-339°C and 250-26ooc respectively as indicated in Table 1. Table 1: Process conditions at the unit considered in the case study Parameter Range Type of fuels used in the kiln & precalciner Cyclone outlet temperature of gases Cyclone outlet temperature of material Fan inlet gas temperature Type of fan Impeller diameter Fan RPM Static pressure Fan drive power Gas flow rate Raw material feed rate Gas velocities in the downcomer duct Dust loss from top cyclone Designed duty of water spray in terms of temperature drop Actual temperature drop in the duct Water spray in gas duct Air in the atomized nozzle Probable causes of fan deposition: Considering the numerous parameters involved in the material deposition problem & difficulties in determining the most dominant mechanism responsible for the same, it is generally useful to categorize the probable mechanisms into various groups according to the prevailing nature. Gutzwiller et. al. [4] for example have categorized them in four classes namely Thermal , Chemical, Electrostatic and Mechanical. • Thermal parameters refer to the higher temperatures of preheater exit gases and material along with the high velocities. Due to higher temperatures some of the particles tend to get softened or sticky resulting in initiation of build up on the rotating parts. Additionally, fine particles having higher melting point, after near normal impact on the fan elements tend to get plastically deformed due to the severity of the conditions. This accelerates the deposition tendencies. Therefore in the continued presence of high centrifugal forces and temperature over the period of time , the particles tend to get consolidated resulting in hard to remove coatings. Petcoke (having sulphur content around 6%) 310-340°C 320-330°C 250-260°C Radial, backward curved 3120 mm after tipping 975 ::::::930 mmWG 1688 kW 185 m 3 /s 340-350TPH 20-22m/s :::: 6-7% of feed 75°C :::::: 53-55°C 9.30 Nm 3 /hr at 4.5-4.8bar pressure 573 Nm 3 /hr at 4.5-4.8bar pressure • Typical chemical parameters, which are suspected to affect the fan deposition problem, include increased sulfur levels in the gases due to increased petcoke utilization, chlorine levels in the material as well as that introduced from the water spraying systems and alkalies present in the systems. Presence of excessive sulfur and alkalies are suspected to aid the formation of gypsum in anhydrite form and lower melting alkali salts respectively on the fan surface due to favourable temperature conditions. Similarly some literature studies also indicate formation of low melting solids like FeCb, NaCI, etc. acting as initiator of the deposition mechanisms (6, 7]. • Since particles responsible for deposition primarily consist of fine particles, tribo– electrification of these particles due to high velocity impact, charge them positively. Since metals are generally electron receivers', particles impacting on metals assume positive charge by donating electrons. This results in electrostatic dust deposition on the surface due to static charge differential available in the rotating fan parts.