Numerical modelling and simulation of atmospheric entrained flow gasification of surrogate fuels

Entrained flow gasification is the prevailing gasification technology for the production of tar-free synthesis gas. However, in view of the rapid climate changes, the technology needs to be adapted to biogenic and anthropogenic feedstocks. Therefore, new validated mathematical models are required for design, optimisation and scale-up.

This thesis focusses on the numerical modelling and simulation of atmospheric entrained flow gasification of surrogate fuels to accelerate the model development for high-pressure entrained flow gasification of biomass. Firstly, RANS based models were developed to describe the gasification of ethylene glycol and of mixtures of ethylene glycol and wood char. Specifically, improved inlet conditions and injection properties and new wood char devolatilisation kinetics were derived using data from laboratory-scale experiments. Subsequently, numerical simulations of gasification experiments with ethylene glycol and with mixtures of ethylene glycol and wood char were performed using improved implementations and meshes to close the elemental and energy balances. The predictions were compared with experimental flame shape observations, experimental axial droplet velocities and experimental radial profiles of gas temperature and dry gas species concentrations. ... mehrFurthermore, sensitivity analyses were carried out to investigate the impact of the homogeneous reaction kinetics, the vaporisation model, the turbulence model, the thermal gas radiation property model, the inlet conditions, the injection properties and the wood char kinetics.

The comparisons show that (i) appropriate inlet conditions and injection properties are essential for accurate predictions of flame shape and recirculation strength when using the RANS approach and (ii) predictions of gas temperature and dry gas species concentrations outside the flame region can even be accurate when applying strongly simplified injection properties or kinetics. Specifically, for both the gasification of ethylene glycol and the gasification of mixtures of ethylene glycol and wood char, the predictions of gas temperature and dry gas species concentrations outside the flame region were mainly in good to excellent agreement with the experimental data. Larger deviations were found for the conversion of ethylene glycol and of wood char in the near-axis region as the adopted common models for turbulent mixing and turbulent dispersion led to erroneous predictions. Thus, the mathematical description of the physical and thermo-chemical process steps in the flame region of entrained flow gasification processes should be further improved in future studies. Furthermore, the sensitivity analyses have shown three major findings. Firstly, the HVI1 mechanism is highly recommended for preliminary simulations due to its low stiffness while the DLR2017/RM mechanism provides the most reasonable flame temperature predictions at adequate computing times. Secondly, the specific models for turbulence and the thermal gas radiation properties are not decisive for accurate predictions of gas temperature and dry gas species concentrations. Thirdly, for the gasification of mixtures with wood char contents of up to 30 %, accurate predictions of gas temperature and dry gas species concentrations outside the flame region are possible within the uncertainty limits of the wood char kinetics.