The flue-gases from total energy installations can be used for CO2 fertilization in greenhouses. However, the gases contain too much toxic chemicals to be led into the greenhouses untreated. At Delft University of Technology a process has been developed for the removal of these toxins. This so called ’Delft NOx/urea process’ has been tested successfully downstream of a natural gas fired engine. Our group has described that the process could be well used for so called lean-bum engines. However, for non-lean-bum engines, which have a very high NOx output (1500 ppm), the required NOx conversion was not reached. Which was caused by high NOx concentration. Hence, the process should be optimized further for this purpose. Especially, the decomposition of urea should be given more attention. This paper focuses on this aspect. The decomposition has been investigated both theoretically and experimentally.