Fuel staging tests at the Kymijaervi power plant

The aim of this study was to measure nitrogen oxide (NOx) emissions and find the methods to reduce them in plants using coal and natural gas as fuel. The tests involved were made at the Kymijaervi Power Plant, Lahti, Finland. Coal and natural gas was used alone or mixed. With natural gas when using flue gas recirculation, the NOx emission level dropped from 330 mg/m3 down to 60 mg/m3. A negative side effect was that the flue gas temperature increased. At coal combustion and staged combustion, the flue gas recirculation had no significant effect on the NOx emission level. At coal combustion, the staging of combustion air halved the NOx emission but the combustibles increased strongly. With fuel staging, using coal as main fuel and gas as staging fuel, the NOx emission level was decreased from 340 mg/m3 to 170 mg/m3. At the same time the combustibles increased 2 %- units. Also the flue gas temperature increased a little. At the tests, the proportion of natural gas was rather high, one third of the fuel energy input, but it could not be decreased, because the gas flow ratio was already too low to ensure good mixing