Trends in NOx during Pulverized Fuel Oxy-fuel Combustion

This work presents the trends in NOx emissions during pulverized fuel (PF) oxy-fuel combustion using the coal combustion test facility (100 kW) at RWTH Aachen University. In this facility, three modes of pulverized coal combustion have been considered, namely, (1) firing in air (named as air mode), (2) firing in a mixture of oxygen and carbon dioxide (named as O-2/CO2 mode) by varying the O-2 concentration in the O-2/CO2 mixture, and (3) firing in a mixture of oxygen and recycled flue gas (RFG) (named as O-2/RFG mode). It was found that NOx emissions in the O-2/CO2 mode were about 20% lower than those in the air mode. This causes high temperatures in the burner vicinity to form large amounts of thermal NOx in the air mode. On the other hand, NOx emissions in the O-2/RFG mode are considerably reduced by approximately 50%. This is due to the fact that NOx contained in RFG is supplied back by a secondary stream to the flame, and thus, it is destructed by reduction gas in volatile matter. Moreover, in this investigation, the gas stream condition from the swirl burner is controlled. It was found that a decrease in the amount of primary stream leads to not only improved combustion but also an increase in NOx emissions for all combustion modes. A higher secondary stream leads to higher NOx emissions in both the air and O-2/CO2 modes. In the case of the O-2/RFG mode, however, it is clarified that NOx emissions decrease with an increasing secondary stream. The reason is that a higher secondary stream causes a higher circulating flow in the near burner region. It leads to an increase in NOx emissions because combustion is improved and a large amount of fuel NOx is formed. In the case of the O-2/RFG mode, as the amount of secondary stream increases, a large amount of NOx in the secondary stream, which includes that in RFG, is destructed by volatile matter in the flame. From these results, a method of predicting NOx emissions based on the mathematical model that is calculated by multiplex analysis has been applied to oxycoal conditions. Thus, the trends in NOx emissions obtained in a pilot-scale facility can be translated to industrial-scale boilers.