Burning characteristics of Victorian brown coal under MILD combustion conditions

In this work a vertical furnace is used to investigate the MILD (Moderate or Intense Low-oxygen Dilution) combustion characteristics of pulverized Victorian brown coal. This paper reports the effect of turbulence on the volatiles’ release and reactions under vitiated co-flow conditions as well as the impact on the pollutants’ formation and emission. Loy-Yang brown coal from the Latrobe Valley, Victoria, Australia, with particle sizes in the range of 53–125 µm, is injected into the furnace using CO2 as a carrier gas through an insulated and water-cooled central jet. The bulk jet Reynolds number was varied from Rejet = 5527 to Rejet = 20,000. The furnace walls as well as co-flow temperature and local oxygen concentrations are controlled by a secondary swirling burner using non-premixed natural gas combustion. The co-flow in the furnace was operated with an O2 concentration of 5.9% (db by volume). Detailed measurements of in-furnace temperatures and chemical species are presented and discussed, together with visual observations and CH chemiluminescence (CH*) imaging at the bottom, middle and top parts of the furnace. The CH* signal intensity is found to be significantly lower at the top part of the furnace which is an indication of slow rate of the heterogeneous combustion of char particles. The largest amount of CO concentrations are measured for the highest jet velocity (i.e., Rjet = 20,000) case which implies that with increasing turbulence there is a better mixing and a broad devolatilization zone is formed which produces more CO. The measured NO emission for any case was less than 125 ppmv (db at 3% O2) which provides evidence to the potential benefits of MILD combustion application to Victorian brown coal towards reducing NO emission. Complementary CFD model helped in shedding light on the flow field, turbulence intensity, volatiles’ release rate, combustion of volatile matters, and overall carbon consumption inside the furnace for the three cases. It was found that increasing the jet Reynolds number increases the volatiles release rates and decrease the rate of overall carbon consumption.