CFD Simulation of Biomass Gasification using Detailed Chemistry

The use of biomass as a CO2-neutral renewable fuel and the only carbon containing renewable energy source is becoming more important due to the decreasing resources of fossil fuels and their effect on global warming. The projections made for the Renewable Energy Road Map [1] suggested that in the EU, the use of biomass can be expected to double, to contribute around half of the total effort for reaching the 20 % renewable energy target in 2020 [2]. To achieve this goal, efficient processes to convert biomass are required. At the Karlsruhe Institute of Technology (KIT), Germany, a two-stage process called bioliq [3], for the conversion of biomass into synthetic fuel, is being developed. In this process, straw or other abundant lignocellulosic agricultural by-products are converted to syngas through fast pyrolysis and subsequent entrained flow gasification. After gas cleaning and conditioning, the syngas is converted into different chemicals via known processes such as direct methanol synthesis or Fischer-Tropsch synthesis. The prime goal of this thesis was the modeling and simulation of the gasification of biomass-based pyrolysis oil-char slurries in an entrained flow gasifier, which is an important step of the bioliq process. Computational Fluid Dynamics (CFD), as a powerful tool for modeling and simulation of fluid flow processes, was utilized in this thesis. A lab scale entrained flow gasifier, located at KIT, was simulated using the CFD code ANSYS FLUENT 12.0. Due to the turbulent nature of the flow, the realizable k-epsilon model was used to model the turbulence. The discrete phase model (DPM) was employed to describe the fluid phase, consisting of char particles suspended in ethylene glycol. Ethylene glycol served as non-toxic model fuel for pyrolysis oil, mainly because of its similar C/H/O-ratio and its similar physical properties to biomass derived liquid pyrolysis products. A detailed reaction mechanism for high temperature oxidation of ethylene glycol was implemented in the CFD code. The mechanism comprised of 43 chemical species and 629 elementary reactions. The use of detailed chemistry enables one to have a deeper insight into the gasification process. Turbulence-chemistry interactions were modeled with the eddy dissipation concept (EDC). The in-situ adaptive tabulation (ISAT) procedure was employed to dynamically tabulate the chemistry mappings and reduce computer time for the simulation. The effect of the thermal radiation was taken into account by using the discrete ordinates model (DOM). The radiative properties of the gas were described with the weighted sum of gray gases model (WSGGM). The simulation results were compared with the experimental measurements wherever possible, with good agreement. The simulations depicted the importance of the recirculation zone in entrained flow gasification. Furthermore, the main reaction path of ethylene glycol gasification could be observed and analyzed. In order to study the effect of boundary conditions on the gasification process, a series of simulations were done to perform sensitivity analysis. Four parameters were varied, namely: oxidizer and fuel inlet temperatures, the oxidizer composition, the air-fuel ratio and the operating pressure of the gasifier. Effects of the parameter variations on the gasification efficiency and the composition of the product gas were studied. Three different chemistry models (i.e. equilibrium chemistry, flamelet model and EDC) were studied in this thesis. Their relative advantages and disadvantages for the simulation of gasification processes were examined. The EDC model proved to be the better choice for entrained flow gasifiers with recirculation zones. The slurry gasification simulations were performed to study the effects of the mass fractions of the char particles on the process. With the aid of the detailed chemistry model, sub-processes could be analyzed and suggestions for the improvement could be made. The simulations performed in this work help to better understand the gasification process inside entrained flow gasifiers and considerably reduce the number of experiments needed to characterize the system. The simulations produced spatial and temporal profiles of different system variables that are sometimes impossible to measure or are accessible only by expensive experiments. However, more experimental measurements help to validate and optimize the CFD model. The sensitivity analyses performed in this study are considered as a basis to find optimized operating conditions and assist the successful scale-up of entrained flow gasifiers