Detailed measurements of in-furnace gas temperature and species concentration distribution regarding the primary-air distribution mode in a spreader and reversal chain-grate furnace

Abstract

In order to uncover the coal combustion and NOx formation characteristics of chain-grate furnaces and evaluate the effect of the primary-air distribution mode, industrial-size tests with measurements taken of the overall gas temperature distribution in the furnace height direction and gas components (O2, CO, and NO) in the grate zone in a 35 t/h reversal chain-grate furnace were performed at four primary-air distribution modes of the uniform air mode, front-lean air mode, middle-concentrated air mode, and rear-enhanced air mode. Gas temperature decreased continuously in the furnace height direction while showed an increase-to-decrease trend as the grate proceeded. The high-temperature zones with levels below 1350 °C and a small suspension combustion share in the furnace height direction not only confirmed that the primary NOx formation relied on fuel-NO but also ruled against the availability of air staging along the furnace height for NOx reduction. The uniform air mode achieved the highest levels in both burnout and NOx emissions. The other three setups with a sharp air flux reduction in the former air box, decreased apparently NOx emissions while affected burnout to some extent. The rear-enhanced air mode delayed coal combustion with the largest extent and achieved the lowest NOx emissions but highest carbon in slag. In contrast, the middle-concentrated air mode relying on the highest air flux at the central air boxes for strengthening the primary combustion stage, resulted in both NOx emissions and carbon in slag showing acceptable levels of 493 mg/m3 (at 6% O2) and 9.27%, respectively. NOx emissions were reduced by about 18% and meanwhile carbon in slag raised only a limited extent in comparison to the conventional uniform air mode. Therefore, the middle-concentrated air mode was recommended as the optimal primary-air distribution pattern among the four setups.