N2O and NO Emissions from CFBC Cofiring Dried Sewage Sludge, Wet Sewage Sludge with Coal and PE

Experiments on cofiring dried sewage sludge, wet sewage sludge with coal and polyethylene (PE) were carried out on a pilot scale 0.15MWt circulating fluidized bed combustion (CFBC) plant, and the influence of furnace temperatures, cofiring rates on N2O and NO emissions was investigated. Temperature is an effective parameter influencing N2O emission, and higher temperature leads to significant N2O reduction and decrease of conversion ratio of fuel-N to N2O. Increasing in cofiring rates leads to higher nitrogen content in the mixed fuel, which could result in higher NO and N2O emissions from combustion. With more sewage sludge addition, higher NO but lower N2O emissions are observed. N2O emission from cofiring wet sewage sludge with coal is higher than that from cofiring dried sewage sludge with coal and PE, and fuel-N conversion ratio to N2O and NO is much higher in cofiring wet sewage sludge with coal than that in cofiring dried sewage sludge with coal and PE. 1. Introduction Vast quantities of sewage sludge are produced as by-product of wastewater treatment in recent years, and the production is expected to rise significantly [1]. Meanwhile, huge quantities of plastic waste are produced due to the ever-increased consumption of plastic all over the world. So much sewage sludge and plastic waste disposal have posed a very serious environmental challenge, and much attention has been paid on this problem. Volatile contents of sewage sludge as dry ash-free basis are generally higher than 80% [2–4], with heating value similar to that of brown coal, which indicates that sewage sludge could be incinerated with the advantage of thermal recycling and volume reduction [1, 4, 5]. But the low heating value of the sewage sludge is usually very low, because of the high water content in it. Therefore, supplementary fuel is required to ensure stable combustion and burnout. Coal and plastic waste have much higher heating value compared with sewage sludge and is suitable for cofiring with sewage sludge to provide supplementary energy [4, 6]. Circulating fluidized bed combustion (CFBC), as an established technique, is feasible to deal with these waste materials, with the advantages of burning a wide variety of solid fuels and low emissions [7]. However, CFBC emits much N2O because of its low combustion temperature [8]. In addition, nitrogen content in the sewage sludge is generally in the range of 4–8%, much higher than that in the coal [9, 10], and fuel-N is the main source of NO and N2O emissions at low combustion temperatures [11]. As a result, there is a high