Mechanism of NOx Formation from Nitrogen in the Combustion of the Coals Used in Sintering Process

The released NO_x during fuel combustion is one of the major air pollutants, directly related to acid rain and photochemical smog. At present, there is not an economical and effective method of inhibiting NO_x emission from sintering flue gas. Therefore, controlling the conditions of fuel combustion is extremely required for the reduction in NO_x. The current research investigated the effects of different combustion parameters on the formation of NO_x for three type of coals. The formation of NO_x decreased with increasing temperature and heating rate while it increased when oxygen content in an atmosphere increased. Anthracite coal replacement with coke up to 100 pct promoted a decrease in the formed NO_x amount and X_NO by 21 and 53 pct, respectively. The addition of CaO and FeO enhanced the conversion of nitrogen to NO_x, especially at temperatures below 1273 K (1000 °C). Taking both the oxidation reaction to form NO_x and reduction reaction of NO_x to form N₂ into account, the activation energy (E_A) values of anthracite and coke were evaluated to be 2.2 and 3.6 kJ/mol, respectively. The combustion of semi-anthracite coal emitted the largest amount of NO_x, but the anthracite coal containing less fuel-N than other coals showed the highest value of X_NO, which indicates that nitrogen content in coal is not the major factor affecting the formation of NO_x. The change in nitrogen functionality after combustion process might be the reason for the unpredicted results in the X_NO. The pyrrolic-N in anthracite coal might be considered to be the major functional form that directly affects the conversion of nitrogen to NO.