Prozess- und numerische Strömungssimulation bei der Neuentwicklung einer Verbrennungsanlage für ballenförmige Biomasse

Despite numerous alternative technologies for thermal biomass utilisation, combustion by modern standards remains to be the most prominent route offering a considerable potential for the satisfaction of the growing global primary energy demand in the face of climate change and dwindling fossil resources. Development and optimisation of novel combustion technologies typically are highly demanding regarding time and cost due to present-day's high standards. Modern simulation tools like process simulation and computational fluid dynamics (CFD) own the capability to powerfully act as support and acceleration factors during this phase. To unfold their full potential physically well-founded models with high degree of detail have to be applied. Current work addresses the development of these simulation technologies during the evolution of an innovative combustion system for baled herbaceous biomass. Initially, an introduction of the analysed combustion concept and some experi-mental results from a 2 MWth pilot plant are given. Subsequently, process simulation and combustion calculation for the considered process are discussed. A three-zoned combustion model has been implemented in this work having the ability to reproduce zonal under-stoichiometric combustion and combustible gases formation under global excess air conditions. For this purpose the stoichiometric gas reaction model has been extended with a reaction kinetic approach. Finally, the inlet boundary conditions for CFD calculations have been derived from the results of the process simulation works. Concerning the CFD simulation a complex model algorithm capable of describing the heterogeneous biomass combustion steps (drying, volatilisation, and char burnout) has been developed and implemented in an external solver environment. The solid fuel phase as well as the interaction with the gaseous phase have been modelled rigorously with three-dimensional spatial discretisation. Flow, turbulence, kinetic gas reactions and radiative heat transfer have been accounted for applying existing models within a commercial CFD-solver. Results of present work clearly indicate the high potential of modern simulation technologies for the development and optimisation of innovative biomass combustion concepts. Analysis of the current case shows that a complete volatilisation of the fuel can not be achieved within the primary combustion zone due to the very slow lignin thermolysis process. Subsequently, the fuel load of the afterburning grate is significantly higher than assumed in the initial plant layout. Furthermore, the non-ideal positioning of air nozzles in the primary combustion zone leads to a poor coverage of the burning bale surface, unbalanced bale burnout, thermal hot spots and local ash sintering. Additionally, several weak points in the flow inside the secondary combustion zone have been identified leading to reduced carbon burnout and increased pollutant emissions. Finally and ultimately, simulation allows for a better understanding of the physical be-haviour of the analysed process and provides an insight to experimentally hardly accessible parameters.