The effect of solid CaO on the production of NOx and N2O in fluidized bed combustors: Studies using pyridine as a prototypical nitrogenous fuel

Pyridine is typical of the nitrogenous species found in coal which decompose during devolatilization to yield HCN. Hence, pyridine was burned in an electrically heated bed of sand fluidized by O-2 + N-2. One product, NO, is found to have a concentration proportional to that of the pyridine in the fluidizing gases. A second, N2O, has a concentration proportional to the square of the pyridine concentration. The mechanistic implications of this are discussed; it turns out that the production of NO and N2O in this system is different from in a homogeneous gas-phase system. In particular, it appears that CN radicals adsorbed on sand are oxidised relatively rapidly to NO in: - CN --> - CNO --> CO + NO. The presence of 2 wt.% CaO in the bed increases the yield of NO by up to a factor of 20 and decreases that of N2O slightly, without changing the dependences of [NO] and [N2O] on [pyridine] noted above. The effects of oxygen concentration and temperature are investigated. It appears that CaO promotes a new heterogeneous reaction, whereby species in the pool of HCN and CN radicals are converted relatively rapidly to NO, resulting in a lower yield of N2O. Certainly, NH3 is found to react with CaO to produce NO. Addition of NO to the fluidizing air when burning pyridine establishes that N2O is produced by reaction between NO and an intermediate which is most probably the NCO radical. Another effect of the presence of CaO is to reduce emissions of CO. It appears that CO burns catalytically on a CaO surface. Finally, CaO probably catalyses the reduction of NO and N2O in: CO + NO --> CO2 + 1/2 N-2 and CO + N2O --> CO2 + N-2.