Gas 2023 Issue

Methanol from CO 2 : a technology and outlook overview Optimal capture of CO 2 towards methanol production compels development of sustainable renewable solutions like green methanol

Pattabhi Raman Narayanan Becht

M ethanol is a highly versatile chemical mainly serving the chemical industry as a base material for a broad range of chemical products, such as polymer fibres for the textile industry, plastics for packaging, glues, adsor- bents/nappies, paints, adhesives, and solvents. Methanol also serves as a fuel or fuel additive. The production and use of methanol results in about 165 million t/y of carbon emissions, or about 0.3% of the world’s total, according to a May 2022 report 1 by the International Methanol Producers and Consumers Association (IMPCA). It is already acknowledged that sustainable methanol is a viable bridge to a low or net zero emission fuel and chemi- cal and its wide range of downstream applications. Drivers for the development of methanol are the existing market, already available infrastructure built for the fossil-based ‘grey methanol’, high energy density in comparison to (liq- uid) hydrogen, and applicability for long-distance transport and long-term storage of renewable energy. Emerging new categories are ‘green methanol’ (produced via a process that emits a minimal amount of greenhouse gases [GHG]), ‘bio-methanol’ (from sustainable biomass), and ‘e-methanol’ (from carbon dioxide [CO 2 ] and hydrogen produced from renewable electricity). Also, the term ‘renew- able methanol’ has emerged, with projects coming online that utilise renewable feedstocks and captured CO 2 . In essence, all these methanol categories significantly reduce GHG intensity and contribute to energy transition markets. The following sections will explore available technologies, sum- marise industry projects, and address the market outlook. Methanol synthesis Methanol synthesis is a mature technology. The feedstock is typically a mixture of CO 2 , CO, and hydrogen, and cata- lysts are mainly based on copper or copper/zinc oxide. Typical methanol plant carbon efficiencies can range from 89-95%. Among several opportunities like carbon effi - ciency improvement, adjustment of methanol loop process conditions, and improved catalysts, one of the largest gains in improving efficiency is the reactor design. The three commercially used designs of the methanol synthesis reac- tor are based on different heat transfer mechanisms: direct cool via feed gas injection (quench), counter-current gas exchange (tube-cooled converter [TCC]), and isothermal bed temperatures (or steam-raising converter [SRC]).

The TCC design enables the largest methanol production and carbon efficiency, whereas the quench-type reactor system typically contains the largest catalyst volume. The use of TCC is advantageous in terms of lower cost, higher efficiency, and relative simplicity of operation. Also, improv - ing the heat distribution with the reactor helps to prevent catalyst sintering, extending catalyst life and minimising interruptions in the process. Technology development Technology development is under way to tune processes towards the different requirements of CO 2 conversion/utili- sation driven by global climate change. One such utilisation option is the nearly carbon-neutral methanol that can be produced using green hydrogen and captured CO 2 . There are two pathways for converting CO 2 to methanol. One pathway is to reduce CO 2 to carbon monoxide (CO) and then reduce CO with hydrogen to make methanol, which is a two-step process. The second pathway is direct hydrogenation of CO 2 with hydrogen over a heterogeneous catalyst through a one-step process that converts CO 2 directly to liquid fuels. The technology can use CO 2 from multiple sources, such as direct air capture, point source capture, or other available biogenic sources like pulp mills and bio-waste to power plants. Depending on the CO 2 source, there may be a need to ‘polish’ the CO 2 before it can be used for methanol synthesis. Of the two bespoke approaches, reacting hydrogen produced by electrolysis of water with CO 2 is the closest to market. In the first pathway, the reverse water gas shift (RWGS) reaction is receiving increased attention as a method for con- verting CO 2 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 CO 2 . The key issues are selectivity to methane, carbon lay-down, and the high temperatures needed to drive the reaction forward. 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. Considering the challenges to develop a commercialised RWGS process, other methods for activating CO 2 to CO, such as electro- chemistry 2 or photochemistry, 3 are interesting. Direct CO 2 hydrogenation to produce methanol is licensed by several leading companies. Recently, China made great

Gas 2023