Intramolecular distribution of stable nitrogen and oxygen isotopes of nitrous oxide emitted during coal combustion.
The intramolecular distribution of stable isotopes in nitrous oxide that is emitted during coal combustion was analyzed using an isotopic ratio mass spectrometer equipped with a modified ion collector system (IRMS). The coal was combusted in a test furnace fitted with a single burner and the flue gases were collected at the furnace exit following removal of SO(x), NO(x), and H2O in order to avoid the formation of artifact nitrous oxide. The nitrous oxide in the flue gases proved to be enriched in 15N relative to the fuel coal. In air-staged combustion experiments, the staged air ratio was controlled over a range of 0 (unstaged combustion), 20%, and 30%. As the staged air ratio increased, the delta15N and delta18O of the nitrous oxide in the flue gases became depleted. The central nitrogen of the nitrous oxide molecule, N(alpha), was enriched in 15N relative to that occupying the end position of the molecule, N(beta), but this preference, expressed as delta15N(alpha)-delta15N(beta), decreased with the increase in the staged air ratio. Thermal decomposition and hydrogen reduction experiments carried out using a tube reactor allowed qualitative estimates of the kinetic isotope effects that occurred during the decomposition of the nitrous oxide and quantitative estimates of the extent to which the nitrous oxide had decomposed. The site preference of nitrous oxide increased with the extent of the decomposition reactions. Assuming that no site preference exists in nitrous oxide before decomposition, the behavior of nitrous oxide in the test combustion furnace was analyzed using the Rayleigh equation based on a single distillation model. As a result, the extent of decomposition of nitrous oxide was estimated as 0.24-0.26 during the decomposition reaction governed by the thermal decomposition and as 0.35-0.38 during the decomposition reaction governed by the hydrogen reduction in staged combustion. The intramolecular distribution of nitrous oxide can be a valuable parameter to estimate the extent of decomposition reaction and to understand the reaction pathway of nitrous oxide at the high temperature.
Department of Environmental Chemistry and Engineering, Interdisciplinary Graduate School of Science and Engineering, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8502, Japan. ogawa@chemenv.titech.ac.jp
Mitsuteru
M
Ogawa
Naohiro
N
Yoshida
D000393:Air Pollutants / Q000032:analysis; D002244:Carbon / Q000032:analysis; D003031:Coal / Q000032:analysis; D060729:Coal Ash; D006358:Hot Temperature; D009587:Nitrogen Isotopes; D009609:Nitrous Oxide / Q000032:analysis / Q000737:chemistry*; D010103:Oxygen Isotopes; D052638:Particulate Matter; D011210:Power Plants
D000393:Air Pollutants; D003031:Coal; D060729:Coal Ash; D009587:Nitrogen Isotopes; D010103:Oxygen Isotopes; D052638:Particulate Matter; D002244:Carbon; D009609:Nitrous Oxide