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 electro-SAF (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. This results in large operational costs and low process efficiency. A high-potential alternative is the catalyst-free reverse water-gas-shift (RWGS) reactor concept, which has key advantages in energy efficiency, flexibility of operation, potential for upscaling, and applications in related fields.
The primary aim of this paper is to investigate the fundamental aspects of the catalyst-free (non-catalytic) 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. This is done by comparing results from LES with corresponding Direct Numerical Simulations (DNS), which does not rely on any turbulence modeling.
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, which benefits the CO production rate. 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 Damköhler number and chemical timescale is employed. This allows for an a priori estimation of the required residence time to reach full conversion of the RWGS reaction for various turbulent shear flows.
Finally, except for some smaller systematic deviations, it is concluded that the partially stirred reactor (PaSR) LES subgrid model designed for combustion reactions perform well also for the endothermic reverse water-gas-shift reaction.