Large scale experiments and modeling of black liquor gasification

Abstract: Biomass gasification could provide a basis for increased electricity and engine fuel production from a renewable source in the pulp and paper industry. This work focuses on the largest byproduct available at the pulp mills, black liquor. Black liquor is a mixture of spent cooking chemicals, dissolved lignin, dissolved carbohydrates and a small portion of inorganic compounds found in the wood. The conventional technology to recover the cooking chemicals and the chemical energy as heat is combustion in large boilers. Here, gasification could be an alternative, replacing or complementing the boilers. The gasification technology produces a combustible gas that can be cleaned to produce electricity in a gas turbine/engine or, be synthesized into valuable chemicals or liquid engine fuels. The technology has been demonstrated in development scale since 2005 and appears to be promising. Still, commercial plants have not yet been built. This thesis focuses on the understanding of the oxygen blown, pressurized, entrained flow, black liquor gasification technology. The main goals have been to increase the understanding about the dominating mechanisms in black liquor gasification and to develop an engineering tool that can be used to design and optimize, pressurized, entrained flow, black liquor gasifiers. To accomplish these goals gas samples were extracted from the gasification reactor using a gas sampling probe that was developed within this work. Gas samples were also collected downstream the quench located underneath the reactor and the results were compared. Finally, an existing numerical model was developed so it can predict the behavior of the black liquor gasifier within reasonable accuracy.Even though the actual mechanisms in the reactor and quench are very complex it appears that they can be described with relatively simple global mechanisms. The main gas components are dictated by the water gas shift reaction. At the outlet of the reactor the gas composition is not in global thermodynamic equilibrium. However, the main gas components are close to partial equilibrium whilst CH4 and H2S are not. Very little of the available CH4 is reformed outside the flame region and the primary consumption occurs in the flame through oxidation and reformation. When the system pressure is increased, H2S concentration in the gas will increase, the same will happen if the oxygen-fuel ratio is decreased. In the quench, the primary spray flow rate/load (mass flow of black liquor and oxygen) ratio has a critical value of about 0.6 below which the gas concentration of CO2, CO, and H2, is significantly changed. The H2/CO ratio can be changed from about 1.15 to 1.4 by changing the primary spray flow rate/load ratio. The mechanism is associated with the water gas shift reaction and the quenching rate of the gas stream. The computational fluid dynamics reactor model predicts most of the trends when operating conditions are changed and is in good agreement with the experimental results with respect to gas composition and char carbon conversion.