Numerical Simulation Of The SNCR Denitration Process In A Circulating Fluidized Bed

Circulating fluidized bed boiler (CFBB) is being widely used as a kind of clean coal combustion device. With the improvement of national requirements for environmental protection, adding denitration device has become an inevitable trend as the original concentration of NO_x emission in the CFBB cannot reach the environmental requirements. Selective non-catalytic reduction (SNCR) denitration technology is used extensively in CFBB for the cyclone separator in the CFBB is suitable for the SNCR process. In fact the non-uniformity of flue gas parameters such as velocity and NO_x concentration in the SNCR denitration entrance has great influence on the calculation accuracy of SNCR denitrationn reaction. This paper takes a 300MW CFBB as the research project and use numerical simulation method to research the influence of CFBB combustion process to the injection location, reducer injection quantity and so on in the SNCR denitration reaction. The calculated results are analysised and compared with actual running situation. Specific optimization method is then put forward.The numerical simulation of CFBB coal combustion and NO_x formation reaction are first be conducted and a three-dimensional euler-euler model is been built. The calculation results show that the non-uniformity coefficients of velocity in the entrance of the three high temperature cyclone separators are 0.26,0.42 and 0.47 respectively, NO_x concentration coefficients are 0.26,0.42 and 0.47 respectively. The product of the flue gas velocity and NO_x concentration is NO_x flux. NO_x flux coefficients are 0.30,0.59 and 0.65 respectively, which show obvious inhomogeneity.The numerical simulation of SNCR denitration is carried out on the basis of the combustion calculation. The calculated SNCR denitration efficiency is 61.1%when the ammonia nitrogen mole ratio is 1.4 and boundary conditions of the high temperature cyclone separator are the non-uniform velocity field and NO_x concentration field. The local measurement of the SNCR denitration efficiency is 62.7%when the ammonia nitrogen mole ratio is 1.4. The relative errors between the non-uniform boundary conditions and measured value is 2.6%. The calculated efficiency reaches 75%when using average value of velocity and NO_x concentration as uniform entrance, and the relative errors between the uniform boundary conditions and measured value is 19.6%. The calculated results show that using non-uniform boundary conditions can simulate more accuracy than the uniform boundary conditions.On the basis of SNCR denitration simulation and the non-uniform flue gas parameters in the inlet of high temperature cyclone separator, the existing reducer injection point and injection quantity are changed. The entrance of the cyclone separator is been divided into six equal sections and one reducer point is been set at the same height of the center of each section. The reducer quantity of each nozzle is been distributed according to the NO_x flux in each section. The calculated SNCR denitration efficiency reaches 70.5%when using non-uniform boundary conditions and ammonia nitrogen mole ratio is 1.4, which is 9%more than the original denitration efficiency.