Modelling and simulation of pressurised black liquor gasification at high temperature

Abstract: Black liquor (BL), a by-product of the chemical pulping process, is an important liquid fuel in the pulp and paper industry. It contains almost all of the inorganic cooking chemicals along with the lignin and other organic matter separated from the wood during pulping in the digester. To recover the organic combustion heat and cooking chemicals (sodium and sulphur) from the produced black liquor in kraft pulping, Tomlinson kraft recovery boilers are traditionally used. However, the kraft recovery boiler is not thermodynamically efficient and suffers from problems with explosion phenomena, fouling, and emissions. A potential technology for improving the recovery cycle is pressurised gasification of black liquor. However, uncertainties about the reliability and robustness of the technology are preventing a large-scale market introduction. One important step towards a greater trust in the process reliability is the development of simulation tools that can provide a better understanding of the process and improve performance through optimisation. This licentiate thesis is concerned with the modelling and simulation of an entrained flow gasifier for pressurised gasification of black liquor at high temperature. The thesis comprises three papers presenting the simulation results obtained from different models using CFD (Computational Fluid Dynamics). In paper A a simplified gasification model, considering just the gas phase, is used to make a qualitative study of the effect of burner spray angle on the gas flow in the gasifier. In paper B the simulation results from a more advanced model with gas/droplet interaction, are compared to the results presented in paper A. Finally, in paper C a complex model for pressurised gasification of black liquor droplets is presented and used to study the influence of uncertainties in model parameters.