Mechanistic and kinetic aspects of nitrogen oxides formation in coal char and model char oxidation processes

Emission of nitrogen oxides NO_x is one of the major environmental problems associated with pulverized coal combustion. A significant fraction of NO_x is proposed to originate from the oxidation of char nitrogen. In view of the well-known variability and heterogeneity of coals and the lack of understanding of the precise structure and origin of some nitrogen forms in coal samples, let alone in coal-derived chars, availability of well-defined nitrogen-rich model chars served as an important prerequisite for the systematic studies of the mechanisms and kinetics of NO_x formation during oxidation of chars.; These studies were performed by oxidizing various chars (Carbosieve, model chars and coal chars) in a well controlled environment by examining various time- and temperature-resolved evolution profiles of carbon (CO, CO ₂), hydrogen (H₂O) and nitrogen (HCN, N₂O, NO) products. The overall effect of oxygen level on the mechanisms of NO, N ₂O and HCN formation was also investigated. Furthermore, the catalytic effect of molecular carbon sieve in the heterogeneous and homogeneous secondary reaction mechanisms of NO, N₂O and HCN, and their involvement in the formation and reduction of NO_x, were examined.; Two different reactor configurations were utilized to help detect and evaluate the possible effects of mass transport limitations. The original reactor (Perkin Elmer TGA7 furnace tube) data indicated that the early stages of char oxidation processes are under chemical control. During the middle portion of the oxidation reaction, falling oxygen levels within the reactor bed strongly affect measured reaction rates. The observed effects are in good agreement with the first order relationship between oxygen levels and char oxidation rates most commonly proposed in the literature. Towards the end of the process, oxygen levels within the char bed as well as the corresponding reaction rates tend to recover to the initial levels.; The modified version of the TG system (tubular reactor) configuration was mainly used for mechanistic studies of NO, HCN and N₂O studies. The formation of a major part of NO was found to result from the homogeneous oxidation of HCN. An investigation of HCN and H₂O profiles suggests that H₂O is a hydrogen provider for the CN functionality in char to form HCN. The bimodal behavior of NO was discovered to be a result of desorption of the chemisorbed exited NO onto the active carbon sites. The same reactive NO molecules also appear to be responsible for the formation of N₂O molecules. In the presence of carbon, NO and N₂O can undergo reactions at temperatures in the excess of 900°C to form HCN.