Since converting solid fuels to combustible gases is gaining more and more importance it is essential to detect the quality of the produced gases. With the well known fact that the released volatiles contain a great amount of tars and the complexity of the composition of tars, simple standard analysers cannot directly be used to detect the calorific value. Hence it is very work and cost intensive to detect each gas fraction as well as the composition of the tars and calculate then the calorific value and the oxygen demand respectively. These parameters are of great interest concerning the optimization of gasification / combustion units. To overcome these difficulties a mobile analyser was designed, tested and applied to large scale plants. This mobile analyser determines the calorific value by a ‘Differential Scanning Calorimeter' (DSC). With this method it is not necessary to detect all components of the pyrolysis gases. The main principle of this measurement method and results of experiments with single particles (beech wood cubes and spruce wood pellets) were presented at the AIChE Annual Meetings 2004 and 2005. The content of this contribution is the presentation of the mobile measurement instrument, its capabilities and limitations. Furthermore results of tests with refuse derived fuels (RDF) at flash pyrolysis conditions will be shown and results of large scale experiments. These results are not only of major interest for gasification utilizations. They also mean a great improvement for existing computer simulations. On the one hand side one can determine with this analyser the calorific value and the oxygen demand of the volatiles dependant on time for single particles and on the other hand for fuel beds at each desired temperature, reaction atmosphere and so forth. This information is of great interest to optimize gasification / combustion units and hence to enhance the energy efficiency and to reduce the emissions (with the known geometry, transport velocity, …).