The Urea-Water-Solution (UWS), known as AdBlue, serves as an ammonia precursor in Selective Catalytic Reduction (SCR) systems to reduce nitrogen oxide (NOx) emissions from combustion sources. In exhaust gas treatment systems, incomplete evaporation and thermolysis of injected UWS can lead to solid deposit formation in liquid films formed on the walls and mixing grids of the SCR systems. Studies on UWS decomposition exist and considerable progress has been made in modeling the decomposition. These models, however, are demanding with respect to implementation in CFD computations for the process optimization. In this study, a reduced model for UWS film decomposition is developed to be implemented in a Lagrangian CFD framework without requiring detailed chemistry during runtime. The approach is based on a progress-variable tabulation concept [3] based on one-dimensional film simulations incorporating detailed liquid-phase chemistry. The reduced model can be applied over a wide range of ambient temperatures and film thicknesses, with accuracy depending on tabulation resolution. The reduced model accurately reproduces the processes in ammonia production and solid residue formation observed in the detailed simulations while achieving significant computational efficiency, which makes it well suited for predictive SCR simulations. The results of a stand-alone application of the reduced model demonstrate that film geometry leads to substantially slower decomposition in comparison to droplets, promoting prolonged persistence of liquid intermediates and enhanced formation of solid residues, while ambient temperature strongly governs decomposition rates and ammonia release.