NO and N2O emissions and mitigation in circulating fluidized-bed coal combustion: complementary experimental studies

Intermediate-temperature circulating fluidized-bed (CFB) combustion suppresses thermal NO formation but can promote N 2 O accumulation, making simultaneous mitigation of both pollutants challenging. Three complementary experimental systems were used to examine different aspects of NO/N 2 O behavior: a cold-flow CFB for hydrodynamic characterization, a micro-fluidized-bed reactor for controlled coal-combustion and simulated-gas conversion experiments, and a 10 kW CFB combustor for hot-state emission measurements. Because these systems differ in geometry, gas properties, temperature, solids inventory, and reaction timescale, the cold-flow and microreactor results are used as qualitative context rather than direct quantitative predictors of the hot-CFB response. The weighted geometric mean particle diameter best described the minimum-fluidization behavior of the bimodal bed materials, with a mean absolute percentage error ( MAPE ) of 14.34%, while the empirical correlation between onset-of-circulation velocity and particle terminal velocity gave a MAPE of 11.45% over the tested range. Micro-fluidized-bed tests showed that higher temperature and O 2 concentration promoted net NO emissions, whereas N 2 O peaked near 800 °C and decreased at higher temperatures. Fine coal favored NO emissions, whereas coarse particles showed higher N 2 O emissions. In the 10 kW CFB, increasing the bed temperature from 700 to 900 °C reduced NO and N 2 O from 513 and 237 to 318 and 88 mg·m −3 , respectively (dry gas, 273.15 K, 101.325 kPa). At 800 °C with 7.5 wt% MnO 2 , NO and N 2 O decreased to 101 and 18 mg·m −3 . Secondary-air ratios of 30–40% formed a lower-emission window, with 40% giving the lowest combined NO and N 2 O emissions under the tested conditions. Overall, the three systems provide complementary evidence regarding the effects of hydrodynamic state, coal particle size, metal-oxide addition, and air distribution on net NO/N 2 O emissions, while quantitative coupling among the systems remains to be established.