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Intrinsic Conversion Mechanism on Nitrous Oxide and Nitrogen Oxide During Circulating Fluidized Bed Combustion of Oil Shale

ABSTRACT At a 5-m-high large-scale thermal state circulating fluidized bed (CFB) test rig adopting an automatic discrete control system, an experimental investigation on continuous combustion of oil shale was carried out, analyzing in detail the variation trade of bed temperature distribution and flue gas concentration, including oxygen (O2), moisture (H2O), carbon dioxide (CO2), nitrogen oxide (NO), and nitrous oxide (N2O) at different operational factors consisting of bed temperature and O2 concentration at cyclone outlet. The characteristics of nitrogen (N) conversion during continuous CFB combustion were calculated and analyzed emphatically. The results indicate that N2O and NO emissions decrease gradually when raising bed temperature, and increase considerably when lifting O2 concentration during continuous combustion of oil shale. Adopting a detailed mechanism calculation method, the reaction mechanism during oil shale CFB combustion was studied. The conversion paths about key formation and decomposition mechanism between nitrogen components are that, first NO converting to NO2 and N2O is reversible, then NO2 converts to N2O easily, and finally N2O converts to N2. Promoting N2O formed by NO converted to N2 but not NO is an effective method to decrease N2O and NO emissions. In addition, the high content of CaO in oil shale promotes catalysis on N2O decomposition. It plays an important role on combustion reaction rate of bed temperature and oxidative free radical, which shows that N gross conversion would remain at a lower level at the operational condition with bed temperature reaching to or above 850°C, and low N2O and NO emissions require excess air to be limited. The above finding can provide guidance for the operation and pollution control of oil shale-fired CFB boilers.