Impact of Pulverized Coal Particle Sizes on Combustibility and NOx Emission in Different Blending Methods

The main purpose of this study was to examine the effect of particle size on blending methods for pulverized coals, using a thermo-gravimetric analyzer (TGA) and an entrained drop tube reactor (EDTR). The experiment was performed with three particle sizes (small: 45–75 μm, medium: 75–90 μm, large: 150–180 μm) and two blending methods (bunker-blend and furnace-blend). The TGA results show that high rank coal (Trafigura), containing lower volatile matter, was more difficult to ignite than low rank coal (Berau). Also, faster ignition and higher reactivity were observed because of increase in the surface area with decreasing particle size. The EDTR results show that first, during bunker-blend, as the particle size decreased, the amount of carbon in ash (CIA) reduced. Moreover, the highest point of CIA shifted to a low blending ratio (LBR 50%) because a highly oxygen-deficient environment was rapidly created. In contrast, as the blending ratio of low rank coal increased, NOx emissions gradually increased at all conditions because of the fuel bonding N content in the coal, and the NOx reduction efficiency of a fine particle is larger than that of a coarse one. Second, during furnace-blend, CIA and NOx gradually decreased by approximately 33–52% (small sizes) and 9–27% (medium sizes) as the length of the tube increases. However, for the large sizes of coal, the amount of CIA and NOx increased because of the adverse effects of the greater particle size. The CO and O2 results among flue gases supported these phenomena. Thus, these results suggest that using fine particles in furnace-blend can effectively reduce CIA and NOx emissions.