(261f) Emission Behaviour during Oxy-Coal Combustion

This paper highlights major results during oxy-coal combustion in a 20 kW electrically heated once through PF furnace when using medium volatile bituminous coal from South Africa and Lignite coal from Eastern Germany.

Emissions during oxy-coal combustion with different oxygen concentrations have been established. Results have demonstrated that emission behaviour during oxy-coal combustion with 27% O₂/73% CO₂ is almost similar to air-blown combustion. Fuel NO_x reduction potential during oxy-coal combustion with the application of oxidant-staging by utilising over-fire oxidant have also been established and includes in-flame measurement of NO_x precursors like HCN and NH₃ as well. Results strongly indicate that fuel-NO_x formation mechanism in fuel rich condition during oxy-coal combustion is similar to air-blown combustion and the state-of the art Low NO_x technology is applicable to reduce NO_x emissions under oxy-coal conditions as well. Fate of NO recycled back into the furnace during staged as well as un-staged combustion have been investigated by injecting known concentration of NO into the furnace and there are firm evidences that reduction of recycled NO_x is completely dependent on local stoichiometry, the time available to interact with hydrocarbon radicals and the location of injection of NO. Investigation carried out so far on the fate SO₂ when using Lignite coal has revealed that recycled SO₂ is not reduced at all in the high temperature radiative section of the furnace. However, depending on the combustion condition, between 5-20% SO₂ is reduced in the flue-gas path where the temperature is around 450°C. Results also indicate that due to SO₂ accumulation in the furnace, the H₂S concentration during oxy-coal combustion is around 2.5 times more than corresponding air-fired combustion.