Modelling And Numerical Simulation Of Black Liquor Combustion

Kraft recovery boiler is special equipment for pulping enterprise; it was fueled by black liquor which is the by-product of pulping, and through the combustion of black liquor heat can produced as well as recovery the kraft. The design and operate of kraft recovery boiler is different from conventional coal-fired boilers, with the increasingly high demand in boiler design, the traditional method difficult to meet the requirement. The numerical simulation technology has been widely used in optimization of the boiler design abroad, but few researches have done in this field in our country. The combustion character of black liquor is different from coal, the model developed for coal combustion in Fluent is not suitable for simulate the combustion of black liquor, so it’s necessary to develop mathematic models specifically for black liquor.This paper presents a model which aimed at estimate the species in the black liquor. Based upon the characteristic of balck liquor during each combustion stage, the combustion process was divided into drying, devolatilization and surface combustion, the stage of combustion was decided by the component content in the particle, the black liquor droplet combustion model was presented under a serials hypothesis. Considering that the diameter of part of the particle is rather large, and the residence time in the boiler is short, these particles would fall into the bottom of furnace without burning out, so the char bed model was presented in this paper.The black liquor droplet combustion model and char bed model was integrated into the computational fluid dynamics package Fluent by UDF, and then the code was applied in a 2D mesh to test its availability and validity, the results showed that the program and run as expected and can simulate the combustion of black liquor droplet to some extent.For a kraft recovery boiler whose handling capacity is 300tds/d, after proper analysis of its combustion character, the simplification program of boiler geometry was proposed. According to the arrangement of the air injections and combustion character of combustion of the boiler, a proper meshing schem was presented. The combustion of black liquor in this boiler was simulated with the UDF presented above, the velocity, temperature and gas species concentration distribution as well as the trajectory of droplet with different initial size in the boiler was abtained through simulation, the operational status of the boiler were qualitatively predicted.