Influence of Biomass Reburning on NOx Reductions during Pulverized Coal Combustion

The NOx reductions by reburning using sawdust, corn straw, and cotton straw were studied in a lab-scale and self-heating drop-tube furnace with varied reaction temperatures, reburning fuel fractions, stoichiometric ratios, and residence times in the reburning zone. Results show that NOx reduction efficiency increased with reburning biomass ratio; however, the NOx reduction efficiency increased first and then decreased with the increased reburning temperature, stoichiometric ratio, and residence time with an optimal point around 1100 °C, 0.6–0.7, and 1.88 s, respectively. Above 1100 °C, thermal-NOx that originated from reburning biomass resulted in a decrease in NOx reduction efficiency. High stoichiometric ratio oxidized reducing radicals and thus decreased NOx reduction efficiency, but the NOx reductions were also inhibited when the excess air ratio was too low, which hindered the further conversion of the intermediates during NOx reductions. In a fixed-size furnace, long residence time in the reburning zone improved volatile release and NOx reductions, but shortened residence time of pulverized coal in the primary zone resulted in incomplete burnout and excess oxygen which oxidized the reducing radicals and thus decreased NOx reduction efficiency. In addition, high volatiles and heating value (influencing furnace temperature distribution) of the reburning biomass caused high NOx reduction efficiency, and consequently, the woody biomass (sawdust) showed higher NOx reduction efficiency in comparisons with agricultural residues such as corn straw and cotton straw. Meanwhile, the Van Krevelen diagram where the fuel far from the zero point shows high NOx reduction efficiency may be used as a guideline for the selection of reburning biomass.