Page 172 - CW E-Magazine (18-6-2024)

Special Report

gas emissions. Specifications for CO There will also be cross-sectoral and intolerance to feed impurities. One
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purity are helpful for understanding competition for that energy, which of the main advantages is that electro-
the potential to integrate with chemical could act as a limiting factor on the catalytic CO conversion can be inte-
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manufacturing. The typical ‘pipeline’ viability of DAC. grated with renewable energy sources:
specifications are set to >95% CO with without doing so, the technology would
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parts per million level limitations on CO to chemicals processes cause a net increase in emissions and
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the quantity of water, oxygen, carbon CO is already used to make chemi- be counterproductive to any efforts to
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monoxide, sulphur and nitrogen oxide cals including urea (for fertilisers), transition the chemical sector to help
contaminants. There is a slightly higher methanol, carbonates and polymers. meet net zero targets.
(a few percent) level tolerance of nitrogen The thermodynamic stability of CO
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(N ), argon and methane as impurities. means that many transformations Carbon monoxide can be made by
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These specifications are likely to be require significant energy input. This the reaction of CO with hydrogen that
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quite well aligned with some chemical energy must be low carbon to reduce can be produced from water. This is the
processes but may present challenges in associated Scope 1 and 2 emissions. high temperature shift reaction, which
others, most obviously catalyst poisoning Life Cycle Assessments of the thermo- is a well-known process. A poten-
in electrochemical processes. dynamics and economics will be needed tial alternative to this shift reaction is
to understand the net contribution of thermochemical cycling, which pro-
Direct air capture (DAC) of CO 2 specific uses of CO . vides an opportunity to split water and
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DAC differs from point source CCU reduce CO into syngas using reduction-
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in that the technology removes CO The potential chemistries to convert oxidation (redox) cycles. This techno-
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directly from the atmosphere, rather CO into chemicals are almost all cata- logy has high theoretical efficiencies.
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than from a specific source. There is lytic processes and often require both However, it currently suffers from low
clearly a very significant difference in vast energy input and other chemicals to efficiencies due to the large tempera-
CO concentration between the two, work. There are different ways the energy ture swing between redox steps. Iso-
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with DAC needing to capture and can be input to these processes, including thermal or near-isothermal operation
concentrate starting from 0.04% by heating (thermochemical), electrically with implementation has been demon-
CO . The highly dilute CO in the air or, in the longer-term, by sunlight. strated by pressure-swing, enhancing
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means that any uses in chemical manu- heat efficiency and fuel yield. Further
facturing will require a very signifi- The most technologically advanced research should be directed towards
cant energy input both to capture and catalytic routes for the conversion of high temperature energy storage, solid-
concentrate it. CO include methanation to produce solid heat recuperation, and oxygen
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methane, direct methanol synthesis and separation for achieving high solar-to-
There are two main types of DAC other alcohols, and syngas production fuel conversion efficiencies.
technology being explored: solid DAC, using the water-gas shift reaction fol-
using adsorbents, where capture occurs lowed by methanol synthesis or Fisher- At the very early research stage,
at relatively low pressures and medium Tropsch to produce hydrocarbons. All photoelectrochemical routes can po-
temperatures; and liquid DAC, which these processes require a large amount tentially convert CO into syngas by
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uses a solution and high temperatures of hydrogen and heat. However, these combining light and electrochemical
to extract CO . technologies require further develop- driven steps, whilst other studies have
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ment to be commercially viable. explored novel photoelectrochemical
DAC is currently a nascent techno- routes that combine CO conversion
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logy, which appears to be viable in Electrocatalysis converts CO into with plastic-to-chemical conversion.
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countries with specific geologies and a variety of products in an electro- However, all these routes are at very
readily available renewable energy. chemical cell through the application early stages of development and are a
Because it is so energy intensive to of an electrical voltage. The main chal- long way off being realisable. This field
concentrate atmospheric levels of CO , lenges of the current technologies are: of research remains at an academic
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DAC would require a vast supply of maintaining selectivity to the desired discovery stage and there are still
energy. To limit emissions associated carbon-based product; the very low fundamental challenges to be overcome.
with this energy and DAC as a process, solubility of CO in water; the need
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this energy supply would have to be for extra voltage to drive the reactions, Another option to use CO is to
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sourced from renewable energy. which leads to energy inefficiencies; produce carbonates and polymers by

172 Chemical Weekly June 18, 2024

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