New Black Liquor Drop Burning Model, 1998 International Chemical Recovery Conference Proceedings

A new elemental-based black liquor drop burning model for incorporation into computational fluid dynamics (CFD) based computer models of recovery furnaces has been developed. The new model allows rigorous material and energy balances including rigorous treatment of drop-gas energy exchanges using a thermodynamic approach. A set of thermodynamic properties is computed for the black liquor solids based on the heating value of the liquor, the elemental analysis, and a heat-capacity algorithm. The processes of drying, devolatilzation (pyrolysis) and char burning are treated as occurring in parallel, eliminating the need for arbitrary criteria for transitions between process steps. Drying rates are computed using the local water vapor pressure (dependent on solids content and temperature) and a mass transfer resistance. The rates of volatilization of carbon and sulfur during pyrolysis are determined from Kobayashi-type models. Stochiometric algorithms tied to the volatilizing gas species are used to compute the rates of release of the elements hydrogen and oxygen. Evolution of Na, K, Cl, and residual C occur as a result of char burning reactions. These include reactions between char components and with furnace gases. Kate equations for the latter type of reactions combine mass transfer and chemical kinetic resistances. A NO, formation model tied to fuel N content is also included. The new model allows computation of parameters that relate directly to boiler fouling and plugging (the amount and composition of carryover, the amount and composition of other aerosols, and furnace exit gas velocity and temperature profiles). It also allows the prediction of the concentrations of air emissions, including TRS, CO, VOC, SO2 , and NOx .