Cement Energy and Environment

I to approximately 2 per cent, volatility of sulphur is progressively reduced while increasing the oxygen beyond 2 per cent has a limited effect. CaS04 starts to decompose slowly at temperatures above 1220oC. CaS04 + heat = CaO + S02 + li02 In a reducing atmosphere (presence of C and CO), both alkali and calcium sulphates decompose releasing S02. Generated S02 travels back to the preheater. With higher sulphur recirculation the plugging problems in the preheater increase significantly. The location of spreader box on kiln riser plays critical importance. If it is possible to place the spreader on the smoke chamber shoulder, the introduced hot meal will absorb the S02 before it sticks to the riser wall. Thus a higher S02 content in the smoke gas is allowable, which means that kiln can run with higher excess sulphur, sometimes up to more than 1000 gm/1 00 kg clinker; however, with consequences of increasing tendency to form dusty clinker. The introduced sulphur ends up in the clinker if not removed elsewhere. Typically, the limit for sulphur in clinker is 1.6 per cent, as S03, to assure good quality. Removal of Sulphur Dioxide There are methods to remove and prevent the formation of S02 by modifying or controlling conditions in the cement pyro-processing systems. Sufficient oxygen level can be maintained in exhaust gases to stabilise alkali and calcium sulphate compounds formed in the process. The burning zone flame shape can be modified to reduce the possibility of forming localised reducing conditions. Raw materials can be altered to affect the alkali/sulphur molar ratio and also to affect absence of su lphide sulphur, organic sulphide or carbon, may reduce S02 emissions. Increasing alkali input may not be possible because of product quality limits on total alkali concentration in the cement. Installation of SOx reduction cyclone directs naturally occurring CaO present in the pyro system, especially in the calciner, to the upper stages. The dust laden gas from the calciner (near the outlet) is withdrawn and passed through an LP cyclone located towards the top of the preheater. The separated dust, rich in CaO, is fed to cyclone 1 or 2. The gas from this cyclone goes to stage two or three cyclone inlet. Scrubber technologies that capture S02 after the kiln system can be divided into four classes, dry reagent injection, hot meal injection, lime/limestone spray dryer absorber, and wet scrubbers. NOx Emission Control Technology NOx (NO and NO) is formed in cement pyro system by following mechanisms. Thermal NOx Formation Thermal NOx is formed at a temperature greater than about 1200°C by direct oxidation of atmospheric nitrogen. Since the flame temperature in cement rotary kiln is about 2000°C, considerable amount of thermal NO is generated. The thermal reaction between oxygen and nitrogen to form NO takes place as per Zeldovic reaction : 0 . + N2 ? NO + N. N. + 02 ? NO + 0. NO formation increases exponentially with temperature and in the presence of excess oxygen . Factors affecting the concentration of NO in the kiln gases are: • Maximum theoretical (adiabatic) flame temperature • Flame shape (burner type) • Excess air rate • Maximum necessary material temperature • Material retention time in burning zone • Gas retention time in burning zone Kiln loading (TPD/ m 3 ) Lower secondary air temperatures and presence of dust increases NOx formation. Dust reduces radiation from the flame which in turn reduces heat transfer to material. Fuel NOx Formation Addition of lime in kiln feed helps in absorb ing NOx also results from the oxidation of nitrogen released S02 to form CaS04 and gets back to the compounds present in fuel, other than gaseous. system. 39