Coal as a reburn fuel for NOx reduction
Experiments were conducted on a 200 kW downward-fired, pilot-scale furnace where residence times and temperatures are comparable to practical units. Nine bituminous coals were used as reburning fuels to investigate various aspects of the reburning process, including key process parameters (operating stoichiometries, reburn fuel fraction, primary zone NO concentration, reburn zone residence time, temperature and mixing effects) and to assess the effectiveness of pulverised coal, including microfine, as reburn fuel. The results obtained showed that the extent of NO reduction was dependent on optimising the different process variables, and the maximum reduction achieved by doing so was 75%. The most influential variables were those coupled to the reburn zone, with the reburn zone stoichiometry being the dominant impact variable. For the range of reburn zone stoichiometries studied (0.85 - 1.03) no optimum value was obtained, however, higher reductions were generally achieved under fuel rich operations. The direct effect of varying primary zone stoichiometry on reburn performance was of minor significance, however, secondary effects such as variation in the reburn zone stoichiometry can be significant. The NO reduction process was mostly completed in the reburn zone where the optimum reburn zone residence time was around 450 ms, and only marginal gains were achieved beyond this point. The NO reduction efficiency increased with increasing primary NO concentration up to around 600 - 700 ppmv, after which the trend levels off, however, at low primary NO (<200 ppmv) it was difficult to obtain a positive NO reduction efficiency. The amount of reburn fuel or Rff required to generate the hydrocarbon radicals necessary for effective NO control was not conclusively quantified, however, from the results obtained the optimum amount of reburn fuel was in the region of 20-25% of the primary fuel input. Lower inlet gas temperature in the reburn zone generally enhanced NO reduction, however, this effect diminished under sufficiently fuel rich conditions. Furthermore, the effect of temperature in the reburn zone was dependent on residence time, with high temperature (1773 K) and long residence time (>500 ms) achieving higher reduction. Improved mixing conditions in the reburn zone enhanced reburning effectiveness, however, in fuel lean operations poorer mixing was found to improve NO reduction through local fuel rich pockets. Finer particle size distribution of the reburning coal gave rise to better NO reduction and higher burnout efficiency. The carbon burnout efficiency was around 85% - 95%, and higher gas temperature improved carbon burnout efficiency, however, under fuel rich conditions (SR2=0.85) burnout efficiency was hampered by the low oxygen concentration. Finally, the results of the multi-variate analysis undertaken to determine the importance of some of the above operational parameters on NO reduction as well as the influence of reburn coal properties such as fuel nitrogen content and volatile matter, confirmed the importance of SR2 as the dominant variable in coal reburning. The proximate volatile matter content was the most influential characteristic of the reburn fuel affecting reburn performance, while fuel nitrogen content was not as influential a parameter for the range of operating conditions and coals studied.