Energy & Fuels, Vol.34, No.8, 9852-9861, 2020
Effect of CO2 on N Distribution in Pyrolysis and Oxidation of Volatile N and Char N in Oxy-Fuel Combustion at High Temperatures
In this paper, the coal combustion process was separated into a pyrolysis process, a volatile oxidation process, and a char oxidation process by utilizing a high-temperature fixed-bed reactor. Three types of experiments in different atmospheres were designed based on this reactor. The effects of CO2 on N distribution in pyrolysis (fractions of N in volatile and char) and oxidation of volatile N and char N in oxy-fuel combustion at different temperatures were investigated by comparing different experimental results. The results show that CO2 pyrolysis (coal pyrolysis in an O-2/CO2 atmosphere) inhibited the conversion of fuel N to volatile N at 1000 degrees C compared to Ar pyrolysis (coal pyrolysis in an O-2/Ar atmosphere). However, when coal was pyrolyzed between 1150 and 1600 degrees C, CO2 reduced the ratio distribution of char N and promoted the conversion of fuel N to volatile N. In the subsequent volatile oxidation process, CO produced by the char-CO2 gasification reaction during CO2 pyrolysis decreased the conversion rate of volatile N to volatile NO. During the CO2 oxidation (oxidation in an O-2/CO2 atmosphere) of volatile, CO2 increases the conversion rate of volatile N to volatile NO. This may result from that the conversion of N intermediates to N2O was promoted by CO2. In the CO2 oxidation of char, CO2 reduces the conversion rate of char N to char NO. Because of the char-CO2 gasification reaction, the formation of char NO was inhibited. In the whole process of coal combustion, the volatile-NO release amount increases, while the char-NO release amount decreases in a CO2 atmosphere compared to an Ar atmosphere at different temperatures. Besides, the ratio of volatile NO to fuel NO increases in a CO2 atmosphere.