An overview of the formation of SOx and NOx in various pyroprocessing systems

The regulations on SO₂ and NO_x emission from cement plants in the US are briefly described and compared with European regulations. The authors try to identify the SO₂ and NO_x emission levels of various kiln systems and explain the reasons for the variations ascertained. Various recent methods of reducing SO ₂ and NO_x emission from cement kiln systems are outlined. Due to the less intensive contact between gas and material in conventional wet and long dry kilns and their high specific heat consumption, on an average basis, specific SO₂ emission from these types of kilns is 4-8 times higher than that from efficient suspension preheater and precalciner kiln systems. Even preheater and precalciner kilns may have high SO₂ emission levels based on future standards if the raw materials contain considerable amounts of sulfides that will oxidize to SO₂ that is already in the upper preheater stages. Moreover, SO₂ may be let out into the atmosphere from large kiln gas bypass systems (in precalciner kiln systems). In such systems, it may be necessary to add burnt lime or calcium hydroxide to the upper preheater stages or the conditioning tower to reduce SO₂ emission. The difference in NO_x emission levels between various kiln systems is less pronounced. Use of kiln control systems, the new generation of LOWNOX kiln burners, and staged combustion in the secondary firing region may reduce the average NO_x emission from SP and precalciner kilns by up to 50%