Mass and energy balances over the lime kiln in a kraft pulp mill

Abstract: This report describes the construction of a model that calculates mass- and energy balances over the lime kiln in a kraft pulp mill. The model was implemented in FORTRAN to serve as the lime kiln block in the WinGEMS simulation software. The mass balance is based on a number of user specified parameters as well as chemical relations found in the literature. A general method to approach the different concentrations and mass flows when constructing a WinGEMS block is also included in the report. In the energy balance the fuel consumption of the lime kiln is calculated. The model is equipped to handle up to three different fuels at a time which compositions and heating values may either be specified by the user or calculated by the model. Methods to approximate heating values and adiabatic flame temperatures are presented. The fuel consumption is calculated analytically by establishing an energy balance over the kiln where the energy for calcination, drying as well as the heat energy of the in- and outgoing streams is accounted for. To calculate the heat energy of the different streams, polynomials were used to describe the temperature dependent heat capacities of the different components. The polynomials were fitted to thermodynamic data from the literature. Examples on how the heat rate of the kiln is affected when changing different parameters are shown. Two kinds of simulations were also performed with the lime kiln model. In the first simulation the possible effects of increased intake of non-process elements with the use of bio fuels are studied. In the second simulation the amounts of energy that may be retrieved from the flue gases when combining a more efficient lime mud filter with an external dryer is studied.