Methanol from CO2: a technology and outlook overview
Optimal capture of CO2 towards methanol production compels development of sustainable renewable solutions like green methanol.
Pattabhi Raman Narayanan
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Methanol 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, adsorbents/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 report1 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 chemical 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 (liquid) 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 [CO2] and hydrogen produced from renewable electricity). Also, the term ‘renewable methanol’ has emerged, with projects coming online that utilise renewable feedstocks and captured CO2. In essence, all these methanol categories significantly reduce GHG intensity and contribute to energy transition markets. The following sections will explore available technologies, summarise industry projects, and address the market outlook.
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. Typical methanol plant carbon efficiencies can range from 89-95%. Among several opportunities like carbon efficiency 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 reactor 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, improving the heat distribution with the reactor helps to prevent catalyst sintering, extending catalyst life and minimising interruptions in the process.
Technology development is under way to tune processes towards the different requirements of CO2 conversion/utilisation driven by global climate change. One such utilisation option is the nearly carbon-neutral methanol that can be produced using green hydrogen and captured CO2.
There are two pathways for converting CO2 to methanol. One pathway is to reduce CO2 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 CO2 with hydrogen over a heterogeneous catalyst through a one-step process that converts CO2 directly to liquid fuels. The technology can use CO2 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 CO2 source, there may be a need to ‘polish’ the CO2 before it can be used for methanol synthesis. Of the two bespoke approaches, reacting hydrogen produced by electrolysis of water with CO2 is the closest to market.
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.
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 CO2 to CO, such as electrochemistry2 or photochemistry,3 are interesting.
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 has recently been created at TU Wien (Vienna). Liquid methanol is formed from CO2 with the aid of a unique catalyst material consisting of sulphur and molybdenum.
Increasingly abundant low-cost renewable electricity enabled electrochemical3 processes to compete with traditional thermocatalysis methods. 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, unlike low-temperature electrolysis, SOECs are not able to reduce CO2 directly to other hydrocarbons and oxygenates.
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 byproducts necessary in the recirculating process.
Several alternatives for CO2 reduction to methanol have emerged in recent years involving homogeneous, enzymatic catalysis, photocatalysis, and electrocatalysis. The advantages of the emerging processes include temperatures 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.
Green/renewable methanol synthesis
The simplest and most mature method is to make hydrogen through the electrolysis of water using renewable electricity, followed by catalytic reaction with CO2 to form methanol (see Figure 1).
In the presence of catalysts like Cu/ZnO/Al2O3, CO2 reacts with hydrogen to form methanol at a pressure of 5-10 MPa and temperature of 210-270°C. Produced methanol is separated from water and residual gases and purified through distillation. To produce 1,000 kg of methanol, about 1,400 kg of CO2, ~200 kg of hydrogen, and ~1,700 kg of water are needed. Around 10-11 MWh of renewable electricity is required to produce 1,000 kg of methanol, a predominant part of which is used for the electrolysis of water.
Methanol synthesis starting from pure CO2 and hydrogen greatly simplifies the reaction products. In terms of chemistry, it is reduced to the following three reactions: CO2 hydrogenation (1), reverse water gas shift (2), and CO hydrogenation (3):
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