Nitrous Oxide Emission from Circulating Fluidized Bed Combustion

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

Nitrous Oxide Emission from Circulating Fluidized Bed Combustion

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

FTIR Analysis of Nitrogen Species in a Fluidized Bed Combustion Chamber

The present paper shows how the FTIR measurements in the combustion chamber of a commercial fluidized bed boiler was performed and evaluated. Problems with interfering compounds are discussed. It is concluded that the present instrument is not suitable for measurement of hydrogen cyanide. The influence of instrument resolution and the removal of the main interfering component, carbon dioxide are demonstrated. However, the problem with interfering species is much less severe for ammonia, and the present instrument can be used for the determination of this species

FTIR Analysis of Nitrogen Species in a Fluidized Bed Combustion Chamber

The present paper shows how the FTIR measurements in the combustion chamber of a commercial fluidized bed boiler was performed and evaluated. Problems with interfering compounds are discussed. It is concluded that the present instrument is not suitable for measurement of hydrogen cyanide. The influence of instrument resolution and the removal of the main interfering component, carbon dioxide are demonstrated. However, the problem with interfering species is much less severe for ammonia, and the present instrument can be used for the determination of this species

EMISSIONS OF NITROUS OXIDE (N2O) FROM FLUIDIZED BED BOILERS

ABSTRACT Measurements of emissions of nitrous oxide (N20) have been carried out on two kind of fluidized bed boilers. One of them, a circulating fluidized bed boiler has been run with three kind of fuels at different operating conditions. The results of the measurements show that the emission of N20 depends on bed temperature and fuel. The temperature dependence is explained by the strong influence of temperature on the rate of destruction of nitrous oxide by radicals in the gas phase

Primary Measures to Reduce the Emissions of Nitrogen Oxide in Circulating Fluidized Bed Boilers

During the combustion of coal in a circulating fluidized bed boiler the conversion of the fuel\uadbond nitrogen to either nitrogen oxides or nitro\uadgen can be influenced by design and operational measures. The paper gives a summary of results obtained in a series of tests on a 40 MW circulating fluid\uadized bed boiler in Nykoping, designed by Gota\uadverken Energy. In this commercial boiler it was not possible to study the influence of primary to secondary air addition independent of thebed temperature. For this reason, a smaller boiler designed by the same manufacturer was adapted to permit independent variations of the primary air ratio and the bed temperature. The results from the 40 MW and the 8 MW boilers are compared and the influence of the distribution of the air supply and of the bed temperature on the reduction of nitrogen oxide emissions are dis\uadcussed

The Role of Fuel Volatiles for the Emission of Nitrogen Oxides from Fluidized Bed Boilers - A Comparison Between Designs

The nitrogen oxides formed during fluidized bed combustion of solid fuels originate from fuel nitrogen in both char and volatiles. In the course of combustion these fuel constituents not only contribute to the production but also take part in the reduction of the emission. In order to study the role of the nitrogen-containing volatiles for the production and re\uadduction of nitrogen oxides in fluidized bed boilers a series of tests has been carried out with fuels having different contents of volatiles in a range from anthracite to wood in two types of boiler, a circulating and a stationary fluidized bed boiler. It was found that the different designs of boilers result in different behaviours of the ni\uadtrogen-containing volatiles. In stationary fluidized bed boilers the volatiles tend to reduce the emissions of NO comparable to what takes place in a pulverized coal flame, whereas in circulating fluidized bed boilers the volatiles do not have the same beneficial effect. In cir\uadculating fluidized bed boilers the emission of NO tends to increase with an increasing content of volatiles in the fuel, whereas the emission decreases from the stationarry fluidized bed type of boiler burning fuels with a larger amount of volatiles

Ammonia Addition for NOx Reduction in Fluidized Bed Boilers

Ammonia is added to the gases in the combustion chamber of fluidized bed briilers in order to investigate its effect ori the emission of nitrogen oxides. A stationary and a circulating fluidized bed boiler are used. In the sta\uadtionaryfluidized bed boiler an addition corresponding to a (NH3/NOexit-molar ratio of 3 gives a 50% reduction of the NO emission. Larger doses lead to escape of ammonia and other disadvantages. In the circulating fluidized bed boiler no effect from injection of ammonia into the combustion chamber is noted. The ammonia is decomposed. If NH3 is to be used in circulating fluidized bed boilers it should be introduced in the particle-free gas after the particle separator

Primary Measures to Reduce the Emissions of Nitrogen Oxide in Circulating Fluidized Bed Boilers

During the combustion of coal in a circulating fluidized bed boiler the conversion of the fuel\uadbond nitrogen to either nitrogen oxides or nitro\uadgen can be influenced by design and operational measures. The paper gives a summary of results obtained in a series of tests on a 40 MW circulating fluid\uadized bed boiler in Nykoping, designed by Gota\uadverken Energy. In this commercial boiler it was not possible to study the influence of primary to secondary air addition independent of thebed temperature. For this reason, a smaller boiler designed by the same manufacturer was adapted to permit independent variations of the primary air ratio and the bed temperature. The results from the 40 MW and the 8 MW boilers are compared and the influence of the distribution of the air supply and of the bed temperature on the reduction of nitrogen oxide emissions are dis\uadcussed