Effects of Reaction Mechanisms and Differential Diffusion in Oxy-Fuel Combustion Including Liquid Water Dilution

Abstract

The influence of chemistry and differential diffusion transport modeling on methane oxy-fuel combustion is analyzed considering different diluent characteristics. Analyses are conducted in terms of numerical simulations using a detailed description of the chemistry. Herein, different reaction mechanisms are employed to represent the combustion of methane. Simulations were performed with the computational fluid dynamics (CFD) code CHEM1D following different numerical setups, freely propagating flame, counter flow flame, and propagating flame in droplet mist reactors. The employed method is validated against experimental data and simulation results available in the literature. While the counter-flow flame reactor is exclusively used in the validation stage, different scenarios have been established for propagating flame simulations, as in single- or two-phase flow configuration. These comprehend variations in diluent compositions, reaction mechanisms, and different models to account for diffusion transport. Conducted investigations show that the choice for a specific reaction mechanism can interfere with computed flame speed values, which may agree or deviate from experimental observations. The achieved outcomes from these investigations indicate that the so-called GRI 3.0 mechanism is the best option for general application purposes, as a good balance is found between accuracy and computational efforts. However, in cases where more detailed information and accuracy are required, the CRECK C1-C3 mechanism demonstrated to be the best choice from the evaluated mechanisms. Additionally, the results clearly indicate that commonly applied simplifications to general flame modeling as the unitary Lewis number and mixture averaged approach strongly interfere with the computation of flame propagation speed values for single- and two-phase flows. While the application of unitary Lewis number approach is limited to certain conditions, the mixture averaged approach demonstrated a good agreement with the complex model for flame speed computations in the various tested scenarios. Such an outcome is not limited to oxy-fuel applications, but are straightly extensible to oxy-steam and air-blown combustion.

Influence of the simplifications applied to the surrounding gas composition of evaporating droplets in air-blown and oxy-fuel combustion atmospheres

“…Analysis were conducted with the CFD code CHEM1D [19], which has been extended with a Lagrangian module in [15] and gradually enhanced in [14] and [10] to investigate flames propagating in fuel and non-fuel droplet mists. Herein, two-phase flow simulations are performed within a Eulerian-Lagrangian framework, in which a full inter-phase two-way coupling is considered.…”

“…Analysis evolves gradually in a systematic fashion, considering the full description of the surrounding mixture composition up to the most simplified approaches. This work can be seen as a continuation of both [14] and [10], as it complement the modeling strategies proposed in [14] and improve the computational efficiency of water diluted oxy-fuel combustion from the methodology applied in [10].…”

“…The description of the carrier phase follows a variable-density low Mach number formulation. Following the strategies presented in [10,15,19], the set of equations employed here is…”

“…An exception occurs in [6], where Moderate or Intense Low-oxygen Dilution (MILD) combustion is considered. During the development of [10], where methane oxy-fuel flames diluted with water droplets were studied, it was noticed that the performance of simplifications applied to represent the surrounding gas composition differ from air-blown to oxy-fuel processes. Additionally, the consideration of a combustion flue gas species as the liquid vapor requires a special care when using previously proposed models, for instance the one presented in [14].…”

A review on combustion instabilities in energy generating devices utilizing oxyfuel combustion

“…Some literature are available looking into how the changes in oxidizer composition from air to oxyfuel mixture (premixed and non-premixed) affect the chemical kinetics as well as transport properties thereby influencing the stability of the combustion. 111,112 The diffusive-thermal effects as underscored by the Lewis number are important for oxy-combustion stability because of the significant departure of the oxyfuel mixtures from the unity Lewis number assumption. Furthermore, differences in thermal and mass diffusivities in oxy-combustion mixtures will influence the flame's response to strain, heat release characteristics, and ultimately the stability of the flame.…”