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A novel combustion system was applied to a 600 MWe Foster Wheeler (FW) down-fired pulverized-coal utility boiler to solve high NOx emissions, without causing an obvious increase in the carbon content of fly ash. The unit included moving fuel-lean nozzles from the arches to the front/rear walls and rearranging staged air as well as introducing separated overfire air (SOFA). Numerical simulations were carried out under the original and novel combustion systems to evaluate the performance of combustion and NOx emissions in the furnace. The simulated results were found to be in good agreement with the in situ measurements. The novel combustion system enlarged the recirculation zones below the arches, thereby strengthening the combustion stability considerably. The coal/air downward penetration depth was markedly extended, and the pulverized-coal travel path in the lower furnace significantly increased, which contributed to the burnout degree. The introduction of SOFA resulted in a low-oxygen and strong-reducing atmosphere in the lower furnace region to reduce NOx emissions evidently. The industrial measurements showed that NOx emissions at full load decreased significantly by 50%, from 1501 mg/m3 (O2 at 6%) to 751 mg/m3 (O2 at 6%). The carbon content in the fly ash increased only slightly, from 4.13 to 4.30%.

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The present study is a further effort to extend our knowledge of the included and excluded mineral characteristics responsible for the formation of particulate matter (PM). A Chinese bituminous coal was first separated into three density fractions using the float-sink method: heavy (>2.0 g/cm 3 ), medium (1.4-2.0 g/cm 3 ) and light (<1.4 g/cm 3 ). Then, combustion and pyrolysis of coal with different density fractions were carried out in a laboratory-scale drop tube furnace to understand the formation mechanism of inhalable particulate matter, less than 10 µm (PM 10 ) and less than 1 µm (PM 1 ). PM 10 was collected with a 13-stage low pressure impactor (LPI) having aerodynamic cutoff diameters ranging from 10.0 to 0.03 µm for a size-segregated collection. The experimental results indicated that the light fraction of the coal produced 44 wt % of total PM 10 and 45 wt % of total PM 1 . The medium fraction of the coal contributed 52 wt % of total PM 10 and 49 wt % of total PM 1 . The heavy fraction contributed 4 wt % of total PM 10 and 6 wt % of total PM 1 . The light fraction and the medium fraction of the coal contained mostly included mineral and the heavy fraction contained largely excluded minerals. The PM 10 and PM 1 contents formed by the excluded minerals were very low compared to those formed primarily from included minerals. The proportion of the minerals in the light density fraction converted into PM 1 and PM 10 was the highest, with their weight percentages being 9.59% and 43.49%, respectively. There were three reasons for this. One of reasons was the mineral particle size. The median mineral size in the light density fraction coal was smallest. However, the median size of each coal fraction was almost the same. Another reason was mineral transformation during combustion. The light fraction and the medium fraction of the coal contained mostly included minerals, and the heavy fraction contained largely excluded minerals. The transformations of included and excluded minerals were largely different and played a different role during coal combustion. The last reason was char fragmentation. Char formed by the light coal fraction was easier to fragment and subsequently formed more fine ash particles. This was because the swelling ratio, BET surface area, and total pore volume of char decreased with increasing parent coal density.

To reduce NO x emissions without introducing an obvious increase in the carbon content of fly ash, a novel combustion system was applied to a 600 MWe Foster Wheeler (FW) down-fired boiler. This approach mainly consisted of moving fuel-lean nozzles from the arches to the front/rear walls, rearranging staged air, and introducing separated overfire air (SOFA). The aim of this work was to evaluate the overall performance of the novel combustion system relative to different SOFA ratios (i.e., 15, 20, 25, and 30%) and different SOFA locations in the upper furnace (1.0, 2.0, and 3.0 m above the arches) using numerical simulations and experimental measurements. Both numerical and experimental results showed that, with increasing the SOFA ratio from 15 to 20%, NO x emissions were greatly reduced but the carbon content in the fly ash increased slightly. With a further increase from 20 to 30%, NO x emissions slightly decreased but the carbon content in the fly ash increased substantially. Considering both the environmental and economic effects, 20% was chosen as the optimal SOFA ratio. With increasing the SOFA location height in the upper furnace from 1.0 to 3.0 m above the arches, the average gas temperature after the superheater and the carbon content in the fly ash at the furnace outlet somewhat increased but NO x emissions decreased. Considering various factors, the location (2.0 m above the arches) was chosen as the optimal SOFA location. The performance of this boiler in actual operation was good after performing modifications with the optimal SOFA ratio and location.