High-resolution numerical simulations of turbulent non-catalytic reverse water gas shift

TL;DR

This paper investigates catalyst-free RWGS as a pathway to syngas for e-SAF, focusing on kinetics and turbulence–chemistry coupling in endothermic conditions. It employs DNS (Pencil Code) and LES (OpenFOAM) to compare fully resolved and modeled turbulence–chemistry interactions, validating several syngas mechanisms and selecting Li2015 for detailed study. A key finding is that trace O$_2$ in the CO$_2$–H$_2$ feed can dramatically boost CO formation via OH pathways, especially at atmospheric pressure, while the effect weakens at high pressure. The authors demonstrate that a PaSR-based LES framework can reproduce DNS results for endothermic RWGS and provide a practical algebraic estimate for CO conversion time, enabling scalable predictions for reactor design, with a clear path toward experimental validation.

Abstract

A green transition in aviation requires a drastic upscaling of Sustainable Aviation Fuel (SAF). The power-to-liquid process for the production of CO2-neutral jet fuel via electricity, called e-SAF, directly replaces fossil jet fuel without having to change infrastructure, aeroplanes, or jet-engines. The process combines green hydrogen with industrial exhaust gas, or captured carbon dioxide, in a circular economy concept. A key element of the e-SAF production plant is the reactor where syngas is produced. Traditional reactors use catalytic technology, which faces severe challenges due to the reduced performance over time because of catalyst degradation, clogging, and breakup due to embrittlement. A high-potential alternative is the catalyst-free reverse water-gas-shift (RWGS) reactor concept. The primary aim of this paper is to investigate the fundamental aspects of the catalyst-free RWGS process, such as reaction kinetics and the interactions between turbulence and chemistry. The secondary aim is to identify how a typical combustion subgrid scale models for Large Eddy Simulations (LES) perform when the chemical reactions are endothermic, in contrast to the strong endothermicity associated with classical combustion. It is found that even small traces of O2 in the CO2 stream can significantly increase the production rate of CO. This is attributed to the increased pool of OH. The effect is strongest at atmospheric pressure and less pronounced at higher pressure. By using the temporal jet framework to study turbulence-chemistry interactions, an algebraic equation for the prediction of the CO conversion time in a turbulent flow as a function of Damkohler number and chemical timescale is employed. Finally, it is concluded that the PaSR LES subgrid model designed for combustion reactions perform well also for the endothermic reverse water-gas-shift reaction.

Figures (21)

• Figure 1: Comparison of species concentrations (kmol/m$^3$) for global and selected detailed mechanisms in conditions corresponding to experiment of Bustamante et al. Bustamante2004. The two detailed mechanisms are marked with green and blue symbols.
• Figure 2: Comparison of syngas mechanisms predictions of species concentrations (kmol/m$^3$) in conditions corresponding to experiment of Bustamante et al. Bustamante2004
• Figure 3: Comparison of selected mechanisms predictions of species concentrations (kmol/m$^3$) in conditions relevant for the studied reactor
• Figure 4: Predictions of syngas mechanisms for the conditions corresponding to the experiment of Graven & Long Graven1954
• Figure 5: Effect of 0.5% volumetric O$_2$ content at atmospheric pressure.