Tuning dry reforming of methane for F-T syntheses: A thermodynamic approach

In this research, a thermodynamic equilibrium model was established using FactSage to study the way to tune H₂/CO ratio of syngas produced via dry (CO₂) reforming of methane (DRM) for various F-T syntheses aiming at eliminating the use of a water–gas-shift unit. The effects of operating conditions, such as temperature, pressure and CH₄/CO₂ mole ratio, on CH₄ and CO₂ conversion, H₂ and CO yield, and solid carbon yield in DRM were investigated. These operating conditions were studied in a wide range, i.e., 550–1200 °C for temperature, 0.05–5 MPa for pressure and 0.5–2 for CH₄/CO₂ mole ratio. The results showed that lower CH₄/CO₂ ratios favoured high CH₄ conversion and CO selectivity, but hampered CO₂ conversion and H₂ selectivity. However, the increase in pressure hindered CH₄ conversion, CO₂ conversion, H₂ selectivity and CO selectivity except for carbon yield. Since the deactivation of catalyst associated with coke formation is the major obstacle for the industrialization of DRM process, a carbon-free regime of DRM was identified as CH₄/CO₂ mole ratio = 1 and pressure = 0.1 MPa and temperature >1000 °C. Although the H₂/CO ratio could be adjusted by adjusting CH₄/CO₂ mole ratio and/or pressure to satisfy the requirements of different F-T processes, the adjustment of CH₄/CO₂ mole ratio was found to be a more efficient way of tuning H₂/CO mole ratio than adjusting operating pressure. The dependence of H₂/CO ratio in syngas on operating conditions of the DRM process was also revealed in this research. With the assistance of this relationship, optimal operating conditions for DRM could be quickly determined based on the required H₂/CO mole ratio for various typical F-T processes. It is shown that when the operating temperature of DRM was raised to over 700 °C, the H₂/CO ratio obtained at CH₄/CO₂ ≤ 1 and P = 0.1 MPa was preferable for the synthesis of olefins, heavy hydrocarbons and oxygenated compounds. Otherwise the syngas was more suitable for the production of alkanes (C₁–C₅).