Influence of Biomass Reburning on NO x Reductions during Pulverized Coal Combustion

The NO x 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 NO x reduction efficiency increased with reburning biomass ratio; however, the NO x 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-NO x that originated from reburning biomass resulted in a decrease in NO x reduction efficiency. High stoichiometric ratio oxidized reducing radicals and thus decreased NO x reduction efficiency, but the NO x reductions were also inhibited when the excess air ratio was too low, which hindered the further conversion of the intermediates during NO x reductions. In a fixed-size furnace, long residence time in the reburning zone improved volatile release and NO x 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 NO x reduction efficiency. In addition, high volatiles and heating value (influencing furnace temperature distribution) of the reburning biomass caused high NO x reduction efficiency, and consequently, the woody biomass (sawdust) showed higher NO x 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 NO x reduction efficiency may be used as a guideline for the selection of reburning biomass.