Black liquor conbustion in Karft Recovery Boiler-Numerical Modelling

by Fakhrai, Reza

Black liquor is a by-product, which results from digestion of wood chip in alkaline pulping processes. After the evaporation process the solid in black liquor increases up to 80% and it is combustible. Black liquor is conventionally burned in a large unit called Kraft recovery boiler, for the dual purposes of energy production and recovery of the pulping chemicals. Kraft recovery boiler model in the present context refers to the numerical simulation for solving partial differential equations governing the characteristics phenomenon in a Kraft recovery furnace. The model provides an analytical tool and it is best appreciated when the numerical simulations and the measurement techniques are linked to the real industrial problem and the industry that used it. The purpose of this study was to enhance the understanding of the processes involved in a Kraft recovery furnace through mathematical modelling and to keep the code in state-of-art. It is essential to consider the path of every drop in the cavity of a Kraft recovery furnace. In this process the liquor may accumulate on the walls and depending on the gravity or the flow pattern at the wall it periodically sloughs off and falls to the bed. As new components in the general framework of the Kraft recovery boiler model, the interaction of burning drops and walls in a recovery boiler considering the above mentioned was modelled. The importance of a bed model and its effect on the predicted temperature in the furnace cavity was examined. The heterogeneous conditions in a Kraft recovery furnace with significant local variation of concentration of constituent gas components and temperature level/gradient could affect NO production rate. The NOx model developed in this work considers the NO formation from fuel NO and prompt NO. It is assumed that the fuel nitrogen in black liquor is released either via devolatilization or char combustion. Further work focussed on estimation of the temperature level near/on the bed based on the mass distribution on the char bed. The model was also used to examine the effects of changes in black liquor properties (used in Kraft recovery furnace model) namely effect of the swelling and solid content versus the Kraft recovery furnace performance. The results illuminate also the potential of numerical modelling method as a promising tool to deal with the complicated combustion processes even for practical application in the industry.