The loss of biodiversity, due to pollution and global warming, is more important and relevant now than it has ever been. Man made climate warming has its roots in the huge amount of CH4 and CO2 emissions, which have increased by 47% and 156% respectively since 1750. To minimise the effects of the already heavily damaged climate, the share of renewables, as well as their efficiency, need to be increased. Besides solar and wind, a promising alternative for countries with a large forest industry is energy through biomass based fuels. Gasification of biomass, already a mature technology, produces a mixture of CO, CO2 and H2 called synthesis gas (syngas). Syngas can be refined into high added value biofuels or used directly in a gas turbine. Syngas combustion in a gas turbine will not replace the aforementioned renewable energy sources but can be used in conjunction with them to account for the electrical grid peak load demand and intermittency issues. However, syngas combustion in a gas turbine gives rise to quite a few challenges as these machines have been optimised for hydrocarbons, and more specifically, methane. Another challenge is that the composition of syngas varies depending on the biomass and gasification method used for its production. It is then necessary to use both advanced experimental methods and Computational Fluid Dynamics (CFD) simulations to assess the combustion behaviour of syngas in a gas turbine environment.

In this work, Reynolds Averaged Navier Stokes (RANS) and Large Eddy Simulation (LES) approaches are used to study the behaviour of syngas combustion from different biomass and gasification principles. Unteady RANS (URANS) was also used to assess good practice guidelines when dealing with complex geometry and flow fields in a burner setup. The numerical modelling is verified and validated against advanced experiments for both burner geometries used in this thesis. The first burner that was investigated (Paper I) was the TECFLAM swirl burner. This work was aimed at comparing the flame shape and other characteristics of methane, 40% hydrogen enriched methane and four typical syngas compositions from practical application (e.g., black liquor gasification). It was found that the syngas fuels experience lower swirl intensity, due to the higher flame speed of the syngas fuels that weaken the inner recirculation zone. A strong correlation was found between the laminar flame speed and the flame shape.

Due to the computational demand of the CFD simulations, a detailed parametric analysis on fuels can become very expensive. Thus, in Paper II, in order to speed up the parametric analysis a Two-Step, One Way (TSOW) coupled method was assessed. The domain under question was the CeCOST swirl burner (similarly for Papers III-V). Using the TSOW method, the solution is divided into an upstream and downstream part, where the inlet conditions from the downstream part (TC) are derived from the upstream solution. The full domain solution (FS) was then compared to the TC solution and they were found to be in good agreement. In addition, for the TC domain a RANS approach could be used for further simplification (an average based method could not be used on the FS due to the flow created by the swirler). However, this introduced further discrepancies between the numerical solutions. The TC numerical solutions using a RANS approach instead of URANS showed a speedup of almost one order of magnitude.

Combustion in the CeCOST burner was investigated experimentally in Paper III for three CO2 dilutions of a black liquor based syngas with a composition of 53% H2, 44% CO and 3% CH4 on a volume basis and a Reynolds number (Re) of 10 000. It was found that the safe operability range is shifted to richer equivalence ratios with increased dilution. In addition, for the higher dilutions, combustion occurred in isolated pockets rather than producing a continuous flame. In Paper IV a LES model, with 53 species and 325 chemical reactions using artificial thickening of the flame, is verified using a quality measure from the LES_IQ method and validated against the experiments on the CeCOST burner. The overall velocity comparison against experiments was significantly good, which enabled the use of the LES model for flame predictions at higher Re. Higher Re cases (20 000 and 25 000) predicted an arrower Inner Recirculation Zone and higher velocity magnitude at the Outer Recirculation Zone, which induced the edges of the flame to move upstream. Unfortunately, the model was not able to capture the flame pocket structure of the experiments. This was attributed to the preferential diffusion mechanism in low Lewis number fuels (hydrogen rich) and lack thereof in the version of the Flamelet Generated Manifold sub model that was used for reactive flow modelling. The flammability limits of these dilutions were also investigated and compared to experimental data (Paper V). The lean blowout limit could be resolved for all cases, but the attempts at flashback simulations, being more complex than blowout, led to severe numerical instabilities. Instead, an assumed flashback limit is introduced prior to where the solution becomes unstable. From these, a numerical stable flame region map is shown, which covers approximately 72% of the experimental range. Similarly to experiments, the numerical results predict the same trend of the stable flame region shifting to richer conditions with increased CO2 dilution. In addition, it was found that going from lean blowout to flashback the flame shape changed from a compact V-shape to a M-shape.