Experiments on an electrically heated entrained flow combustion reactor were carried out in order to test the air-staging behavior of four bituminous coals of industrial interest. Through measurements of gaseous nitrogen-containing species profiles (NO, HCN) and sampling of char particles at different conversion levels, a study was elaborated about the impact of process parameters and coal type on NO formation and reduction, as well as on the nitrogen fate during the course of combustion. While the air-staging abatement efficiency was observed to be correlated with the volatile-nitrogen release from the coal, the presented analysis reveals that the contribution of char-nitrogen release cannot be neglected. This study shows that nitrogen release rates change significantly during the various phases of combustion, also revealing the effect of the operating conditions on the release rates. A simple computational modeling has been carried out in order to estimate the relative influence of the process parameters on char-nitrogen conversion into NO in the burnout zone. The results exhibit the influence of the NO concentration level in the gas phase as one possible explanation of the differences exhibited by the coals. The comparison of experimental data and the computational modeling also displays the necessity of a more detailed kinetic approach to describe char-nitrogen evolution by computer codes for the optimization of staged combustion processes.

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Reformulation of the standard equations governing internal reaction of carbon, used to predict the Zone I−Zone II transition temperature, T c, shows that T c is, primarily, an inverse function of particle radius (a). The expression obtained has the form:  log(a) ∝ (1/T c). This result is supported by the (limited) experimental data available. The numerical values also show that, for particles of pulverized coal size, the values of T c are in the flame temperature range, thus possibly allowing a discontinuous drop in reaction rate toward the end of a flame; this has relevance to increased LOI due to staging for NOX control. The results also permit evaluation of the internal diffusion coefficient indicating that internal reaction most probably involves only the macropores, and total internal surface area measurements are probably irrelevant in determining reaction rates.

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