production of methanol, ammonia, or synthetic fuels, respectively. Reverse water gas shift The importance of reverse water gas shift (rWGS) technology stems from its utilisation of biogenic or captured CO 2 and green H 2 to produce syngas, and its scalability for cost- effective deployment across a wide range of project sizes. It is clearly the launchpad for the power-to-liquids (PtL) pathway for the production of e-methanol and e-SAF. Integrating rWGS into the syngas conversion value chain is a promising concept for CO 2 utilisation.3 While the rWGS pathway has attracted significant attention recently, prompting catalyst and process development, there is a need to quantify the net CO 2 and H 2 consumption in rWGS process schemes, while taking into account parameters such as thermodynamics alongside technoeconomic constraints for feasible process development. The mildly endothermic catalytic rWGS reaction ( Equation 1 ) converts CO 2 into CO and water while consuming H 2 . It generates CO as an intermediate product for the production of syngas (CO and H2 ). It is completed by a strongly exothermic CO 2 methanation (Sabatier) reaction (typically with Ni catalysts) at low temperatures ( Equation 2 ), resulting in a decreased CO yield. As a result, a mixture of CO and methane (CH 4) is formed, thereby burdening subsequent separation processes:
Thus, there is a growing need for rWGS catalysts with 100% CO selectivity. Improvement in the low-temperature activities and selectivity of the rWGS reaction is a key challenge for catalyst types. The catalytic conversion of CO 2 to CO via the rWGS reaction, followed by well-established synthesis gas conversion technologies, provides a potential approach to convert CO 2 into valuable chemicals and fuels. Operating rWGS at elevated pressures of ~30 bar allows direct integration with FT and methanol synthesis loops, as well as methanol-to-olefins (MTO), followed by olefins oligomerisation to produce SAF, avoiding cooling/ heating and compression steps. In both cases, rWGS represents a new challenge for process and energy integration, since it involves coupling high-temperature, endothermic reactions with low-temperature, exothermic reactions. Significant progress is currently being made towards the development of scalable rWGS processes. The key to successful adoption of this technology lies in the development and optimisation of rWGS catalysts by leading specialists such as Clariant, Johnson Matthey, Topsoe, Axens, and BASF. The current Technology Readiness Level (TRL) of the rWGS process is estimated to be 6-7. Clariant has successfully employed its ShiftMax 100 RE rWGS catalyst to convert ~8,000 MT of CO 2 annually to CO in one of the largest e-SAF plants recently commissioned in Germany by Ineratec, based on Sasol’s proprietary FT know- how. A similar capacity e-SAF plant utilising Johnson Matthey’s HyCOgen rWGS technology is under construction/commissioning in Spain by Repsol-Saudi Aramco. Co-electrolysis of CO 2 and CO 2 reduction reaction Solid oxide electrolysis cells (SOECs) are commonly used for the co-electrolysis of CO 2 and steam by using a solid oxide or ceramic electrolyte to produce H 2 , CO, and oxygen (O 2) through an electrochemical process. SOEC electrolysers are commonly used in co-electrolysis due to their inherent ability to operate at high temperatures, which significantly enhances reaction kinetics and efficiency.4 An SOEC is a thin (ca. 0.4mm) ceramic plate that contains different material layers with
ΔH = +41 kJ/mole (Eq. 1)
CO 2 + H 2 ⇋ CO + H 2O
CO 2 + 4H2 ⇋ CH 4 + 2H 2O ΔH = -165 kJ/mole (Eq. 2)
Under certain operating conditions and catalyst types (such as copper [Cu] catalysts), methanol is produced as a byproduct in a mildly exothermic reaction ( Equation 3 ):
CO 2 + 3H 2 ⇋ CH 3 OH + H 2O ΔH = -50 kJ/mole (Eq. 3)
The CO and byproduct(s) are influenced by temperature, pressure, space velocity, H 2/CO2 ratio, and the rWGS catalysts employed. It is therefore imperative that process conditions be tuned to sync with the selected catalyst to achieve high CO productivity.
Refining India
45
Powered by FlippingBook