Predicting Non-Process Element Behavior in Modern Pulp Mills

These days, many pulp mills are moving away from using fossil fuels, and changing the lime kiln fuel is often the first step. However, many renewable fuel alternatives, such as gasified bark, may cause operational problems by introducing non-process elements (NPEs) into the pulping cycle. Understanding and predicting the behavior of these elements is necessary to ensure the operational efficiency of the mill and to minimize the dead load in the mill. This work presents a model for the simulation of the NPEs in the chemical and aqueous streams of the pulp mill. The applied methodology combines calculating mass balances of the mill and evaluating the thermodynamic equilibrium of multi-phase systems in selected process units. Empirical data is used to validate the modeling results. In the case of adapting to non-fossil fuels, the developed model can be used to plan purge strategies for the NPEs. Furthermore, the NPE model can help to utilize the waste and side streams of the pulp mill, decrease the use of fresh water in the mills (e.g., by recycling bleach plant filtrates, which is the second example in the paper), and develop new innovative technologies.