Influence of SO₂ on the NO N₂O chemistry in fluidized bed combustion. 1. Full-scale experiments

The lime used for sulfur capture in circulating fluidized bed (CFB) boilers interferes with the NO N₂O chemistry. This has been explained as an effect of the surface of the lime. However, homogeneous reactions involving SO₂ could also be important. Large-scale experiments were carried out in a CFB boiler with the aim of separating the effect of lime from that of SO₂. This was achieved by comparing a test involving removal of SO₂ by lime addition, while burning a high sulfur coal, with a test where SO₂ was added to the combustion air for a low sulfur coal in a quantity corresponding to the emission of SO₂ from the high sulfur coal. The tests show that the SO₂ itself influences the NO N₂O chemistry independent of the catalytic effect of the lime surface. Carbon monoxide is believed to be an important intermediary species in the reactions, but laboratory investigations are needed to interpret the results further. The connection between NO reduction and N₂O formation is further studied by supply of NO to the primary air duct.