Numerical modelling of the CHEMREC black liquor gasification process : conceptual design study of the burner in a pilot gasification reactor

The work presented in this report is done in order to develop a simplified CFD model for Chemrec´s pressurised black liquor gasification process. This process is presently under development and will have a number of advantages compared to conventional processes for black liquor recovery. The main goal with this work has been to get qualitative information on influence of burner design for the gas flow in the gasification reactor. Gasification of black liquor is a very complex process. The liquor is composed of a number of different substances and the composition may vary considerably between liquors originating from different mills and even for black liquor from a single process. When a black liquor droplet is gasified it loses its organic material to produce combustible gases by three stages of conversion: Drying, pyrolysis and char gasification. In the end of the conversion only an inorganic smelt remains (ideally). The aim is to get this smelt to form a protective layer, against corrosion and heat, on the reactor walls. Due to the complexity of gasification of black liquor some simplifications had to be made in order to develop a CFD model for the preliminary design of the gasification reactor. Instead of modelling droplets in detail, generating gas by gasification, sources were placed in a prescribed volume where gasification (mainly drying and pyrolysis) of the black liquor droplets was assumed to occur. Source terms for the energy and momentum equations, consistent with the mass source distribution, were derived from the corresponding control volume equations by assuming a symmetric outflow of gas from the droplets and a uniform degree of conversion of reactive components in the droplets. A particle transport model was also used in order to study trajectories from droplets entering the reactor. The resulting model has been implemented in a commercial finite volume code (AEA- CFX) through customised Fortran subroutines. The advantages with this simple model are that no detailed information about the kinetic behaviour of the fuel is necessary and that it is numerically well behaved. The magnitude of the degree of conversion in the model was optimised during the solution by minimising the difference between the computed and a target value for the volume average temperature in the upper half of the reactor. The optimisation was done iteratively with the aid of a modified version of Brent’s algorithm that was implemented in the Fortran code. The primary use of the model was to study the influence of spray angle, hollow or full spray pattern, and degree of swirl for the burner. The results were post processed to yield, in addition to the velocity, pressure and temperature field, information about the magnitude of the degree of conversion, the residence time of particles of different sizes, heat flux to the walls and average temperatures. The optimum spray pattern according to the approximate model was found to be the wide angle, full cone pattern with moderate swirl.Validerat; 20101217 (root