NO emission and burnout characteristics in co-combustion of coal and sewage sludge following high-temperature preheating

Due to the high content of volatile nitrogen in sewage sludge, co-combustion of coal and sewage sludge easily leads to high emission of nitric oxides (NOx). To solve this problem, we applied the high-temperature preheating technology to promote the advance conversion of fuel-nitrogen into N2 so as to reduce NOx emission. In this study, a two-stage drop-tube furnace system was employed. The effects of sludge proportion, excess air ratio in the preheating zone (lambda 1), preheating temperature, and combustion temperature on NO emission and burnout were investigated. Our results showed that with the preheating treatment, increasing the sludge proportion reduced NOx emission and promoted char burnout. NO emission was the lowest when lambda 1 increased to 0.5. Under this condition, 62% of the fuel nitrogen could be converted to the gaseous species, in which the portion of N2 was 89.7%. By controlling lambda 1 to 0.5 and sludge proportion to 15%, the maximum NO reduction of 62% was achieved. The emitted NO and unburnt carbon in fly ash were reduced by 48.3% and 43.2%, respectively, when the preheating temperature increased to 1200 degrees C. The effect of combustion temperature strongly depended on the stoichiometry ratio. In oxygen-deficient regions, NO emission decreased with the increase of temperature, while an opposite trend appeared in oxygen-rich regions. The critical excess air ratio in our experiments was around 0.8.