Solving differential–algebraic equation systems by means of index reduction methodology

With the overall goal of optimizing the design and operation of steam boilers, a model for optimizing the dynamic performance has been developed. The model has been developed as three sub-models that are integrated into an overall model for the complete boiler. Each of the sub-models consist of a number of differential equations and algebraic equations—a so called DAE system. Two of the DAE systems are of index 1 and they can be solved by means of standard DAE-solvers. For the actual application, the equation systems are integrated by means of MATLAB’s solver: ode23t, that solves moderately stiff ODEs and index 1 DAEs by means of the trapezoidal rule. The last sub-model that models the boilers steam drum consist of two differential and three algebraic equations. The index of this model is greater than 1, which means that ode23t cannot integrate this equation system. In this paper, it is shown how the equation system, by means of an index reduction methodology, can be reduced to a system of ordinary differential equations—ODEs.