With the CO2-to-methanol route gaining more interest, what emerging technology do you see accelerating this interest?Feb-2023
Pattabhi Raman Narayanan, Becht, email@example.com
Methanol synthesis is a mature technology. The feedstock is typically a mixture of CO2, CO, and hydrogen, and catalysts are mainly based on copper or copper/zinc oxide. Technology development is underway to tune them towards the different requirements of CO2 conversion driven by global climate change. There are two pathways for converting CO2 into methanol. One is to reduce CO2 to carbon monoxide (CO) and then reduce CO with hydrogen to make methanol. The second is the direct hydrogenation of CO2 with hydrogen over a heterogeneous catalyst.
In the first pathway, the reverse water gas shift (RWGS) reaction is receiving increased attention as a method for converting CO2 into syngas using renewable hydrogen. RWGS is attractive as it allows existing, high technology readiness level (TRL) processes to be run in two steps from CO2. The key issues are selectivity to methane, carbon lay-down, and the high temperatures needed to drive the reaction forward, and they are being addressed at present. Also, a range of catalysts is being evaluated. Many of these are based on copper, but iron, nickel, platinum, and molybdenum carbide catalysts are also under investigation. In light of the challenges to develop a commercialised RWGS process, other methods for activation of CO2 to CO such as electrochemistry or photo-chemistry are interesting.
The George Olah methanol plant in Iceland, commissioned in 2011, follows the second method. The production unit captures CO2 from flue gas released by an adjacent geothermal power plant, which is purified to make it suitable for downstream methanol synthesis. Following adequate compression, synthesis gas containing green hydrogen generated by electrolysis of water and CO2 is catalytically reacted to form methanol.
Direct CO2 hydrogenation to produce methanol is licensed by several leading companies. Recently, China made great progress in employing copper-based and oxide catalyst systems. Also, a novel technique for converting CO2 to methanol was recently created at TU Wien (Vienna). Liquid methanol is formed from CO2 with the aid of a unique catalyst material consisting of sulphur and molybdenum. The new technology has already been patented, and now it must be ramped up to industrial size in collaboration with business partners.
Increasingly abundant low-cost renewable electricity enabled electrochemical processes to compete with traditional thermocatalysis methods. The George Olah plant mentioned above couples the hydrogen through electrolysis with thermocatalysis to produce methanol. The availability of electrolysis at the scale needed to supply hydrogen to methanol plants is a key challenge, and significant efforts are being made to scale up electrolysers. Also, the high-temperature electrolysis to produce CO and syngas using solid oxide electrolyser cell (SOEC) systems could be advantageous if coupled with thermochemical processes to reduce heating cycles. However, SOECs cannot reduce CO2 directly to other hydrocarbons and oxygenates, unlike low-temperature electrolysis.
Methanol synthesis from CO2 over heterogeneous catalysts suffers from several shortcomings, such as harsh operating conditions like high pressure and temperature. Also, CO2 thermocatalytic hydrogenation is limited by thermodynamics, and continuous separation of methanol from CO2 and by-products is necessary for the recirculating process.
Several alternatives for CO2 reduction to methanol have emerged in recent years involving homogeneous, enzymatic catalysis, photocatalysis, and electrocatalysis. Advantages of these emerging processes include temperature lower than in heterogeneous catalysis, alternative sources of energy (light or electricity) use, and potentially higher methanol selectivity. In some alternatives, water is used for CO2 reduction instead of costly green hydrogen.
Troels Juel Friis-Christensen, Topsoe, firstname.lastname@example.org
Green methanol produced by biogenic CO2 and hydrogen from electrolysis powered by renewable energy is one of the possible solutions for decarbonising the maritime sector. Demand for green methanol is therefore predicted to increase significantly in the future. Conversion of CO2 into methanol changes the chemical processing conditions for the methanol catalyst. The concentration of water and CO2 is much higher than that of a traditional operation.
Based on Topsoe’s knowledge of copper-based methanol catalysts and years of experience within CO2 utilisation for the production of methanol, Topsoe has developed MK-317 Sustain, which can achieve a high and stable conversion rate over a long period of time. The dependency on fluctuating renewable energy for the generation of hydrogen requires a robust plant design with the ability to change load fast and frequently, and with extended turndown requirements. Topsoe’s eMethanol process provides the required flexibility in a simple and efficient solution by combining a methanol catalyst with a reliable and proven process design.
Joris Mertens, KBC (A Yokogawa Company), joris.mertens@kbcglobal
For CO2-to-methanol, the primary accelerator will not be technical but rather a market pricing strategy based on the carbon intensity of the methanol product. In brief, ‘green’ methanol needs a higher price. The International Maritime Organization (IMO) has set a 50% emission reduction target for shipping that will require the use of low-carbon fuels. In addition to being more technically mature, the e-methanol and bio-methanol paths are easier to apply to existing shipping infrastructure. In their energy transition outlook, DNV predicts e-methanol demand for bunkering will reach 360 and 1800 PJ in 2030 and 2050, respectively, which corresponds with 18 and 90 million tonnes per year.
Regulations are crucial, with technical developments important as well. On the one hand, renewable electricity and the development of electrolyser technology can reduce hydrogen costs. On the other hand, the well-established methanol synthesis technology needs to be further developed, particularly the methanol synthesis catalyst. Conventional methanol synthesis uses a syngas mixture rich in CO, not CO2. E-methanol technology will evolve faster if stable and selective catalysts tailored to CO2/H2 feeds are developed. They will reduce the yield of lower value by-products, as well as the capital cost (for example, reactor size) and operating cost (for example, reduced recycling of unconverted product). Reducing operating costs will reduce the carbon intensity of the process, which may further increase the product value.
The final parameters in the equation are the price and availability of CO2, which will be determined both by regulations and cost reductions through further technological developments. CO2 captured from large point sources is likely to be used over the short and medium terms as it will become more readily available at lower costs. Ultimately, however, CO2 from direct air capture should be the preferred CO2 source.