Synergistic reduction of PM and NOx in different preheating co-firing modes of coal and biomass

To achieve a deep-source reduction in particulate matter (PM) and nitrogen oxide (NOx) emissions, this study, for the first time, applied preheating technology to different co-firing modes of coal and biomass, namely fuel staging, biomass reburning, and air staging. Experiments were conducted on a two-stage drop-tube furnace system with a layout adjusted to fit different combustion modes. Preheating evidently reduced PM₁, PM_1–10, and NOx emissions during the single-firing of Huangling coal (HL), wheat straw (WS), or sawdust (SD). While PM_1–10 emissions showed an increasing linear relationship with the ash content of fuels, the reduction rates of PM₁ and NOx emissions were directly proportional to the volatile content. Furthermore, PM₁, PM_1–10, and NOx emissions in the preheating co-firing of SD with HL were lower than those in the preheating single-firing of HL, regardless of being coupled with fuel staging. Compared with the simultaneous feeding of HL and SD, when SD was fed first, the reduction rate of PM₁ emissions increased, whereas that of PM_1–10 and NOx emissions decreased. In contrast, when HL was fed first, the reduction rate of PM₁ emissions decreased, whereas that of NOx emissions increased. More interestingly, compared with the conventional biomass reburning mode, the mode of coal preheating biomass reburning reduced PM₁, PM_1–10, and NOx emissions by 13.23 %, 9.09 %, and 25.86 %, respectively, and its coupling with air staging reduced NOx emissions to 96 mg/m³.