Within decades, measures have been developed and established aiming for the reduction of nitric oxides (NO_x) emissions from combustion processes. Primary measures like low NO_x burners or air staging affects the combustion process directly and are widely used. However, secondary, i.e. post-combustion measures are necessary to comply with environmental restrictions. In the early 1990s, flameless oxidation (FLOX) became a promising solution for nitric oxides abatement from flame applications. This technology proved its NO_x reduction potential first for gaseous fuels, but soon the focus was directed to other fuels like pulverized coal. It was investigated by different researchers under the name of High Temperature Air Combustion (HiTAC) and Moderately and Intensive Low-oxygen Dilution (MILD) combustion. The recent EU founded project FLOX-COAL-II seeks for an optimized PC-FLOX burner design, a suitable methodology for up-scale from pilot scale to full scale and validated Computational Fluid Dynamic (CFD) tools. In the course of this project, a first prototype PC-FLOX burner was tested in 2012 at IFK's 500 kW_t KSVA facility. During these experiments, air and CO_2 were used as coal carrier to investigate the effect of a low external flue gas recirculation on NO_x emissions. The 2012 experiments highlighted the effectiveness of CO_2 as coal carrier to lower the NO_x formation (391 instead of 529 mg/m~3_(stp) @ 6 vol-% O_2) but with a penalty on the combustion efficiency (97.3% and 99.4% burnout respectively). Detailed measurements like in-flame gas concentrations and temperatures as well as velocity field characterization by Laser-Doppler-Velocimetry (LDV) were conducted allowing the validation of IFK's in-house CFD program code AIOLOS to this combustion process.
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