Transport equations for all chemical species are formulated
together with boundary conditions originated from the chemical
reactions. The formulation leads to the nonlinear equations
system. The linearized system is solved iteratively by the same
linear solver what is used for the temperature equation, see
**Computation** ->
** Numerical Parameters** ->
** Forward** dialog tab.

The numerical parameters applied for the solution procedure are

**Underrelaxation factor****Maximal outer iterations****Desired residuum**

No underrelaxation is usually required for the execution of the chemical model. It means the value 1 of the underrelaxation may be appropriate. If the stability problems occur during the iterative solution run, the underrelaxation value should be reduced.

The parameter **Desired residuum** means
the relative decrease of the overall residual of the equations
system. Here the value of 1.e-9 is usually sufficient if
equations are solved the first time.

The required number of iterations is problem dependent, it depends also on the applied underrelaxation value. For the problem size about 30000 nodes and 5 transported species with 4 heterogeneous chemical reactions in two transport media, about 100 iterations was enough. That corresponds to a several minutes execution on a single PC.