RECENT ADVANCES IN THE UNDERSTANDING OF PRESSURIZED BLACK LIQUOR GASIFICATION

Pressurized oxygen blown black liquor gasification, a new process for the recovery of energy and spent chemicals from chemical pulp mills, enables efficient power production or the production of motor fuels. In this paper, the experimental results of a 3 MWth process development plant are presented and compared with the results of a CFD model. The key parameters with the largest effect on gas composition are: operating pressure oxygen-to-fuel equivalence ratio, black liquor load and black liquor preheating temperature. The experimental gas composition during normal operation of the gasifier does not agree with equilibrium calculations. However, the agreement for the main components (CO, CO2 and H-2) becomes very good if the CH4 and H2S concentrations are prescribed in the equilibrium code. The impurities present in the syngas were also investigated, the observation being made that the particle content in the gas after cooling was very low, and the only significant trace elements were Cl and N. A theoretical model for the gasifier has been implemented in the commercial CFD code Ansys-CFX. The results of the code agree well with the experiments for the major components in the gas, while the CH4 concentration is under-predicted. A separate analysis with an equilibrium code (Factsage) shows that the CH4 content at equilibrium, under experimental conditions is very low, which agrees with the CFD code but disagrees with the experiments. The reason for the disagreement between the experiments and the theoretical models is assumed to be the limiting kinetics for CH4 conversion. A first approximation of the kinetics was implemented in the CFD code, which resulted in a very good agreement with the experiments.