Cement Energy Environment

63 As an example, municipal solid waste normally forms ash with a high content of alkalis and chlorine. Combined with sulfur-containing pet-coke this gives a high risk of coating on the refractory lining (Picture 2). inside the cement kiln system. With this the mix of sulfur, sodium, potassium and chlorine, the circulation of volatile salts increase and cause build-up and clogging, especially in the lower preheater Picture 4: Coating formed in smoke chamber and riser area Picture 2: Coating formed in Meal Pipes Picture 3: Refractory Crack due to Alkali Spalling Most of the refractories (except kiln basic bricks) used in cement plants are alumina-silica refractories. Alkalis (Na 2 O, K 2 O), sulphates (Na 2 SO 4 , K 2 SO 4 ), chlorides (NaCl, KCl) and carbonates (Na 2 CO 3 , K 2 CO 3 ) present in systems, infiltrate in refractory lining and react with alumina and free silica of refractory and form low melting compounds with refractory. This chemical corrosion starts at temperatures as low as 700°C and becomes more hazardous at elevated temperatures. The phases formed due to KCl and K 2 SO 4 were kalsilite, kaliophilte (both K 2 O·Al 2 O 3 ·2SiO 2 and leucite Chemistry Involved (K 2 O·Al2O3·4SiO 2 ) and exposure to Na 2 CO 3 led to the formation of nepheline (Na 2 O·Al 2 O 3 ·2SiO 2 ). The formation of these phases causes high volume expansions of between 20% to 25% in the refractory lining and leads to refractory crack and sometime bursting. cyclones, feed pipes, riser duct and kiln inlet (Picture 4).Besides build-up, alkali vapours and oxides released from firing organic waste may infiltrate the refractory lining and cause spalling as they react with the refractory minerals and form crystalline changes that may burst the lining (Picture 3). Alkali problems are most common when firing municipal solid waste or agricultural waste.