Experimental And Modeling Study On NO_x Generation Mechanism In Circulating Fluidized Bed Boiler

With the implement of the national emission regulation（NER）of China,the circulating fluidized bed（CFB）boilers with the innate advantage of low NO_x emission are also under the pressure of NO_x removal.In order to achieve the low-cost control of NO_x emission,it is of great significance to further investigate the NO_x generation mechanism and emission rules for CFB boilers.Therefore the experimental and modeling study was conducted related to the key issues in the NO_x formation and reduction process:The char-NO_xformation mechanism in the dense bed of CFB boilers was experimentally studied focused on the influence of mass and heat transfer characteristics.A small-scale bubbling bed test rig was employed to simulate the dense bed,and single char particles were burnt in the typical temperature and O2 concentration of CFB dense bed.For three types of char derived from different coals,the conversion ratio of char-nitrogen to NO_x increases with the increase of bed material size due to the subsequent increase of mass transfer coefficient and the decrease of heat transfer coefficient in the emulsion phase of bubbling bed.Meanwhile,the burning process of chars was accelerated.Besides,the conversion ratio decreases with the increase of initial char particle diameter,and increases,but not significantly,with the increase of fluidization velocity.The central temperature of char was also measured,which demonstrated the previous experimental results.Furthermore,the model of NO_x generation from single char particle was established based on the detailed mass and heat transfer characteristics in the dense bed as well as the simplified chemistry mechanism,and the model was validated by the experimental results.The kinetic parameters of NO_x reduction reactions in CFB boilers,especially NO-CO and NO-char,were obtained.The influences of ash existence and composition on the reactions were highlighted.The kinetic parameters were fitted by the experimental results from the small-scale fixed bed test rig.Five typical coals were selected and the corresponding ash and chars were prepared on the specific conditions.Ordered by the catalytic activity on NO-CO reaction,the rank of main metallic components in the ash is Fe₂O₃ >CaO/MgO > Al₂O₃> SiO₂,and the effect of Fe₂O₃ is significantly higher.On the basis of unit carbon mass,the normalized reaction rate constant of NO reduction by char is positive to the catalytic activity of the corresponding ash on NO-CO reaction,but shows weak relationship with the ash content in the chars.An index was introduced in as γash,based on the weighted sum of mass fractions of ash components,to evaluate the impact of ash composition and revise the NO-CO and NO-char kinetic formulae in the Arrhenius forms.The detailed field tests of NO_x emission on practical CFB boilers were conducted and raw data was obtained for further processing.The back propagation artificial neural network（BP ANN）was used to preliminarily analyze the relation of NO_x emission and possible factors,and the NO_x emission correlations were established subsequently.Based upon the 1-D gas-solid flow and combustion model of CFB boiler,the NO_x generation and emission was modeled and validated by the field test results from the CFB boilers in Baode with the capacity of 480 t·h-1.The model is able to effectively predict the influences of conventional factors,such as bed temperature,coal type（especially sorted by volatile content）,air supply and in-situ desulphurization etc.Besides,it is also demonstrated by model calculation that the NO_x emission decreases with the decrease of the mean size of bed material.The reducing atmosphere in the dense bed and upper dilute region of CFB boiler would be enhanced by decreasing the bed material size as well as increasing the mass circulating rate.Meanwhile,the temperature uniformity would be improved to avoid the local over-temperature.A feasible technical route is proposed by means of decreasing the mean size of bed material and increasing the solid circulating rate of CFB boiler.