Experimental and numerical investigation of flameless pulverised coal combustion

  • Experimentelle und numerische Untersuchung von flammenloser Kohlenverbrennung

Stadler, Hannes Alexander; Kneer, Reinhold (Thesis advisor)

Aachen : Publikationsserver der RWTH Aachen University (2010)
Dissertation / PhD Thesis

Aachen, Techn. Hochsch., Diss., 2010

Abstract

Aim of this work was to investigate the applicability of flameless combustion technology principles to pulverised coal combustion. Lab-scale experiments showed, that it is highly beneficial in terms of NOx reduction to use N2 as coal carrier instead of air. The finding has been supported by OH* chemiluminescence imaging which revealed a suppression of ignition in the coal jet when N2 is used. With the investigated settings, NOx emissions are always above the legislative limit of 200 mg/m3 (stp). However, emissions close to the limit can be achieved with a variety of settings of the thermal load, inlet velocity, wall temperature, burner air ratio, and primary combustion zone volume. The inlet velocity has no significant influence on NOx emissions whereas an increase of the wall temperature also increases NOx emissions. The parameters thermal load and primary combustion zone volume are interrelated by the residence time in the primary combustion zone. As long as the residence time is above a certain threshold, NOx emissions are low and constant for almost all settings. These high residence times have only been achieved with the burner air ratio below 0.8. Predictions of NOx emissions based on NO models from literature are not reliable for mainly one reason. The oxidation of intermediates (such as HCN and NH3) in the primary substoichiometric combustion zone is insufficient for burner air ratios below 1.0. With conventional models, intermediates are only oxidised by O2. However, O2 is consumed by other reactions as well so that its concentration approaches [O2]=0 mol/m3 in the substoichiometric zone and the intermediates are retained unto the injection of burnout air where they are quickly oxidised to NO. Thus, NO predictions with conventional models usually turn out too high. With a model which is based on intermediate oxidation by OH, predictions for stoichiometric conditions in the primary combustion zone are poor. In this work the concept of oxidation by OH has been adapted to yield a newly developed model, with which NO predictions for flameless pulverised coal combustion are in good agreement with experimental data over a wide range of boundary conditions. For flameless combustion experiments in the lab-scale facility, predictions for the share of thermal NO are in between 5 and 40% depending on the burner air ratio which correlates well with experimental data from literature. Simulations of a utility size boiler with the adapted model are in good agreement with experimental data, too. Based on the lab-scale experiments, burner modifications have been assessed with respect to boiler performance. According to the simulations, an increase in the combustion air velocity causes an increase of NOx emissions. Emissions can only be reduced by separating the combustion air inlet from the coal inlet and by increasing the diameter of the combustion ball. The latter option has to be thoroughly checked, as the CO concentration at the walls is increased compared to the reference which might cause corrosion. An increase of the primary combustion zone volume by shifting air from the over fire air ports to the secondary over fire air ports causes a slight increase in NOx emissions. Using flue gas as coal carrier instead of air in order to delay ignition causes a twofold increase in NOx emissions which is attributed to the decreased residence time due to the larger volume flow in the primary combustion zone. The experiments and numerical simulations showed that the NOx reduction potential by adapting the principles of the flameless combustion technique to pulverised coal combustion is only about 10-15% and thus significantly lower than in gas combustion. Where thermal NO can almost be prevented completely in flameless gas combustion, thermal NO still makes up a significant part of overall emitted NO in flameless pulverised coal combustion. In principle, modifications to facilitate flameless combustion are also applicable to utility size tangentially fired boilers, although NOx emission reduction is not as pronounced and post combustion flue gas cleaning systems have to be installed in order to comply with legislative emission limits. This work has helped in understanding the principles of NOx emission reduction in flameless pulverised coal combustion in lab-scale as well as utility size tangentially fired furnaces. The key to achieving low NOx combustion is the dilution of the reactants prior to ignition in order to reduce the local adiabatic flame temperature, and an intense mixing of the reactants with combustion products in order to achieve a homogeneous temperature distribution within the furnace. Furthermore, NOx emissions can be reduced by designing burners in such a way, that high NO concentrations overlap with high concentrations of intermediates like HCN or NH3.

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