Nitrous Oxide Emission from Circulating Fluidized Bed Combustion

Doctoral thesis, 1994

In the present thesis N2O results are presented from several measurement projects on two commercial circulating fluidized bed (CFB) boilers. Traditional parameter studies have been carried out, as well as test programs where various nitrogen-containing species were injected into the furnace or gas-concentration profiles in the combustion chamber were measured. The parameter studies show the influence of fuel, temperature and air supply on the N2O emission. The N2O emission increases with rank of the fuel. For fuels of higher rank, such as bituminous coals, concentrations of 150-200 ppm can be measured in the flue gases, whereas negligible amounts of N2O are emitted from wood combustion. The emission also depends on combustion temperature and air supply. Increasing the temperature or strangling the air supply decreases the emission of N2O. These N2O abatement strategies cannot be used without affecting either the NO reduction performance or the sulphur capture process when lime is added. Special tests show that N2O is formed from nitrogen-containing volatiles (HCN and NH3) and when NO is reduced on char surfaces in the combustion chamber. This is in agreement with model calculations and laboratory tests carried out by others. The special tests also show that the N2O concentration increases with height in the combustion chamber and that a very high reduction capacity already exists under normal operating conditions, in the bottom part of the combustor. This leads to the conclusion that N2O abatement strategies should be applied to the top of the combustion chamber and/or cyclone. Care should be taken when such strategies are tested in the future in order not to disturb the excellent NO reduction performance and sulphur capture process by lime addition which are both important in order to fulfil stringent environmental requirements which is expected to be achieved by the CFB combustion process.