(117m) Effects of Flue Gas Recycle on Residence Time Distribution and NO Reduction

NOx concentration in flue gas in oxy-combustion is much lower than that in air-combustion. Two main mechanisms are attributed to the low NOx during oxy-combustion: reburning reduction and reverse zeldovich mechanism. The former mechanism -reburning mechanism, governed at low temperature, is fast to reach equilibrium. The reverse zeldovich mechanism takes into effect when the temperature is above 1400 ºC, which is highly dependent on the residence time and is far from reaching steady state during the operation. In oxy-combustion, N2 is replaced by the recycled product gas, mainly CO2. Flue gas recirculation effectively enhances the mean residence time of NO and pushes the reverse zeldovich reactions toward equilibrium. The relationship between the mean residence time and different recycle ratio has been deducted in this work. The experiments designed here is just to recycle dry flue gas (including CO2, O2, NO) without burning of coal, aimed at the direct influence of residence time on NOx reduction. The calculated result by use of the software HSC shows that NO concentration in the dry flue gas is much higher than that under equilibrium. Experiments' results demonstrate that NO concentration in recycled flue gas is indeed lower than that without recycle.