UBC Theses and Dissertations
Nitrogen oxide emissions from circulating fluidized bed combustion Zhao, Jiansheng
A detailed study of NOT formation and destruction has been carried out on a pilot scale circulating fluidized bed combustor of cross-section 0.15 x 0.15 m and height 7.3 m. Coals of various ranks, including anthracite, bituminous, sub-bituminous and lignite, as well as one petroleum coke, were used in the study. Gas emissions in the flue gas and gas concentration profiles for 02, NOT, CO, CH4 and N20 inside the combustor were measured under various operating conditions. A multi-point gas sampling system was developed to accomplish the tasks. Parametric study of NOT emission shows that bed temperature and excess air ratio strongly influence NOT emissions. Increasing either parameter will increase NOT emission in the ranges studied. The effect of staged combustion on NOT emission in the pilot scale circulating fluidized bed combustion depends on the level at which the secondary air is introduced. A reduction of NOT emission was usually, but not always, observed when secondary air was introduced 3.4 m above the base. Limestone addition promotes NOT formation for high volatile bituminous coal due to the catalytic effect of limestone on oxidation of volatile-nitrogen. However, adding limestone during petroleum coke combustion reduced NOT emission. A relationship exists between fuel-nitrogen to NOT conversion and the volatile content of the fuel, with fuels of higher volatile content emitting more NO. Strong radial gas concentration gradients were found inside the combustor, with low concentration of oxygen and high concentrations of reducing gases near the wall where a relatively dense solid layer exists and the carbon concentration is usually high. This is consistent with the core-annulus hydrodynamic pattern often observed by earlier CFB investigators. NO concentration profiles also indicate that NO is mainly formed near the bottom of the reactor where volatile-nitrogen is released. At higher bed levels, competing reactions co-exist, including oxidation of char-N to NO and reduction of NO to nitrogen, mostly due to the char-NO reaction, and NO-CO reaction catalyzed by CaO. Based on the core-annulus flow pattern, a NO model was proposed in which 02 and CO concentration profiles were first calculated or fitted. Taking into account major hydrodynamic features of CFB and the most important reactions involving NO formation and reduction, the model correctly predicts the general qualitative trends in NO formation and reduction in a CFB combustor. Substantial N2O emissions were found in this study. Flue gas concentration ranged from 30 to 70 ppm for coals, and from 140 to 300 ppm for petroleum coke. Temperature and fuel type were found to be the most important operating parameters affecting N2O emissions, the N2O concentration decreasing with increasing temperature for the conditions studied. Excess air also significantly influenced the N20 level. Addition of limestone reduced N20 emissions. For the fuels tested in this study, there was no apparent correlation between N2O conversion and fuel properties. Both gas phase and gas-solid reactions are important in the formation and reduction of N20.
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