59202 - Co Emission Modeling in a Heavy Duty Annular Combustor Operating With Natural Gas 

This paper presents the development of a Large-Eddy Simulation (LES) based approach for the prediction of CO emission in an industrial gas turbine combustor. The constrain to keep the NOx emission low in a wide range of the engine operability pushes the combustion in a leaner and leaner regime, really close to the lean blow out limit where also carbon monoxide emission becomes critical. So, having a process able to provide a qualitative trend as well as a quantitative assessment of CO emission plays a crucial role, especially during the preliminary design phase of both the premixer and the combustor air split.

For an accurate prediction of CO emission, the non-equilibrium effects cannot be neglected in the combustion model. For this reason, in the present work, the consumption speed has been considered instead of the laminar flame speed, including both the heat loss and strain rate effects on to the flame brush for the closure of the reaction progress source term. An external and widely validated User-Defined Function (UDF) has been embedded in a commercial solver at this purpose, storing the consumption speed through an additional 4D look-up table as a function of the equivalence ratio, the aerodynamic strain rate and the heat loss.

Furthermore, in order to de-couple the CO time of formation from the flamelet assumption, a characteristic Damköhler number has been here considered. This parameter was used to separately assess the “in-flame” CO production from the “post-flame” contribution where the oxidation takes place. The time needed to oxidize the in-flame CO to the equilibrium value was chosen as the characteristic chemical time concurring to the definition of the Damköhler number. Both the in-flame and the post-flame inputs form the source term of the dedicated transport equation associated to the CO.

The process has finally been validated considering a fully premixed burner operating at relevant pressure and temperature for an industrial annular combustor burning pure methane. Different flame temperatures and off-design conditions were considered to assess the capability of the model to predict CO emission.  The comparison against the experimental data shows that the process is not only able to capture the trend in CO emission but also to predict CO in a quantitative manner. In particular, the interaction between the flame and the air fluxes at some critical sections of the combustor leading the CO emission from the equilibrium value to the super-equilibrium have been correctly reproduced.