(425i) Acid Gas to Syngas(TM) Technology As New Path for Low-Cost Ccu

CO₂ is dramatically impacting our life: climate change and consequent social-economic problems are identified as Grand Challenges of current century. Le Quéré et al.¹ identified that 87% of anthropogenic CO₂ emissions are due to combustion of fossil fuels and, according to data from other sources^{2, 3}, it is estimated that overall anthropogenic carbon emissions were 35 Gt in the last years. In the last two decades, a large portion of scientific and engineering community has faced the problem from upstream, therefore focusing on mitigation of CO₂ impacts by reducing its production (i.e. renewable energies); on the other hand, the problem can be faced from downstream also by implementing neutralization processes of waste CO₂. Use of atmospheric CO₂ for direct methanol production (by CO₂ hydrogenation), as proposed by Olah⁴, represents an example of downstream approach to the problem. In line with such an approach, this contribution describes an innovative CO₂ neutralization pathway⁵, including relevant technology, for reducing CO₂ emissions. It is based on the thermo-chemical conversion of CO₂; nevertheless, instead of reacting CO₂ either with H₂ or with other valuable, reducing species (e.g., ammonia and methane), here it is proposed to combine CO₂ with another pollutant species, the hydrogen sulphide (H₂S).

H2S is a compound present in raw sources (e.g., natural gas) and produced in desulphurization processes applied to fossil fuels, often in co-presence of CO₂. Commonly, in industrial plants, H₂S is sequestered by means of a chemical washing unit, as in case of CO₂, and then burnt in so-called Claus process according to the overall reaction 2 H₂S + O₂ => S₂ + 2 H₂O to generate elemental sulphur and water, which can be safely disposed. The proven and spread Claus process, however, has a latent disadvantage: H₂S is, amazingly, a potential source of hydrogen, yet its hydrogen atoms are fixed into H₂O during the Claus thermal and catalytic reactions, and consequently they are lost forever (assuming, for instance, water hydrolysis is too energy intensive for bulk production sustainability). On the contrary, reaction between CO₂ and H₂S realizes to be undoubtedly superior from environmental and sustainable standpoint since H₂S is not only neutralized, but its hydrogen content is made also available. As per above, it has been presented a completely new synthesis route leading to the following overall reaction⁶:

2 H₂S + CO₂ => H₂ + CO + S₂ + H₂O [R1]

H₂S and CO₂ are reacted and a mixture rich in H₂ and CO (known as syngas) is obtained. Therefore, the two costly and troubling contaminants are regenerated into a syngas mixture by means of a complex oxy-reduction reaction mechanism. Water and elemental sulphur are harmless by-products. Reaction [R1] takes place in a special gas-phase reactor, the Regenerative Thermal Reactor or “RTR”, which is a key subject of the present work. Theoretical conversion is high (3 moles of reactants into 2 moles of syngas); in practice, the conversion is supposed slightly less due to a small amount of oxygen injected for RTR energy self-sustainability. According to preliminary studies, estimated cut of CO₂ emissions amounts to 1.2 Gt/y (3% of anthropogenic emissions and 5 times the current reuse).

The acid gas to syngas technology has been successfully implemented in an Italian oil refinery and it is currently a hot technology for coal-to-methanol and gas field-to-methanol in Africa and Europe. A sus-techno-economic analysis for both the fields of application will be provided.

References

[1] Le Quéré, C.; Jain, A. K.; Raupach, M. R.; Schwinger, J.; Sitch, S.; Stocker, B. D.; Viovy, N.; Zaehle, S.; Huntingford, C.; Friedlingstein, P.; Andres, R. J.; Boden, T.; Jourdain, C.; Conway, T.; Houghton, R. A.; House, J. I.; Marland, G.; Peters, G. P.; Van Der Werf, G.; Ahlström, A.; Andrew, R. M.; Bopp, L.; Canadell, J. G.; Kato, E.; Ciais, P.; Doney, S. C.; Enright, C.; Zeng, N.; Keeling, R. F.; Klein Goldewijk, K.; Levis, S.; Levy, P.; Lomas, M.; Poulter, B., The global carbon budget 1959-2011. Earth System Science Data Discussions 2021, 5, 1107-1157.

[2] International Energy Agency, CO2 Emissions from Fuel Combustion 2012. Paris: Organisation for Economic Co-operation and Development 2012.

[3] Olivier, J. G. J.; Janssens-Maenhout, G.; Muntean, M.; Peters, J. A. H. W., Trends in global CO2 emissions: 2014 Report. PBL Netherlands Environmental Assessment Agency, The Hague 2014.

[4] Olah, G. A., Beyond Oil and Gas: The Methanol Economy. Angewandte Chemie International Edition 2005, 44, 2636-2639.

[5] Manenti, F.; Pierucci, S.; Molinari, L., Process for reducing CO2 and producing syngas. WO 2015/015457 A1 2015.

[6] Manenti, F., CO2 as feedstock: A new pathway to syngas. In Computer Aided Chemical Engineering, 2015; Vol. 37, pp 1049-1054.