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.

KEYWORDS:

• Circulating fluidized bed
• Continuous combustion
• N conversion
• Oil shale
• Reaction mechanism

Funding

This work was supported by the National Natural Science Foundation of China (Grant No. 50906051) and Project 2015021124 supported by Natural Science Foundation for Young Scientists of Shanxi Province, China.

Additional information

Funding

This work was supported by the National Natural Science Foundation of China (Grant No. 50906051) and Project 2015021124 supported by Natural Science Foundation for Young Scientists of Shanxi Province, China.