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.