Innovative technological paths for carbon dioxide capture
A review of advances in carbon capture technologies. Continuous improvement of existing processes for efficient removal of CO2 from flue gases is essential.
Formerly Scientist ‘G’ CSIR-Indian Institute of Petroleum
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There is a clear scientific consensus that emissions of carbon dioxide (CO2) and other greenhouse gases are the major cause of global warming. In a country such as India, transportation and electricity generation contribute to 45% of the country’s total greenhouse gas emissions (Ashkanani, et al., 2020). While the primary goal must be to reduce the world’s reliance on fossil fuels, by switching to renewable energy sources (RES), a secondary goal is to capture emissions from processes fuelled by hydrocarbons. For this reason, the technological paths of carbon capture and sequestration/storage (CCS) from power plants and other industrial plants are the subject of intensive investigations. While extensive literature reviews on CO2 capture processes exist (Boot-Handford, et al., 2014), (Bui, et al., 2018), (Global CCS Institute, 2018), Table 1 provides an overview of the three main categories for CO2 capturing processes: pre-, post- and oxy-fuel combustion.
In terms of the technology used for separation, CO2 scrubbing via liquid solvents (chemical or physical) is a mature process that builds upon several commercial solutions employed in hundreds of plants (Cuellar-Franca and Azapagic, 2014).
A fourth category is direct air capture (DAC), which has been demonstrated at a commercial scale by companies such as Climeworks and Global Thermostat. Challenges include integrating current and future technologies with renewable sources of electricity, heat, and the need for significant cost reductions (Norskov, Latimer and Dickens, 2019).
The commercial benchmark technology for carbon capture from point sources is liquid scrubbing, which uses amine mixture to absorb CO2. It is commercially available from several companies and has been applied at scale with no major challenges (Norskov, Latimer and Dickens, 2019). Recent research efforts have been directed towards improving existing technologies and developing innovative technological paths to improve CO2 capturing/trapping efficiency, resulting in lower process costs and improved process safety to facilitate environmental verification. This article summarises the current state of the art for conventional solvents and the most recent innovative investigations (second- and third-generation technologies) in this field as applicable to carbon capture from point sources (Norskov, Latimer and Dickens, 2019).
CO2 capture: innovative research paths
Since 2000 the chemical industry has invested in the development of new processes that are less energy-intensive, have higher efficiency, and are more environmentally acceptable. New methods and techniques are being developed for pre-combustion, post-combustion, and oxy-fuel combustion. Dedicated research programmes, mainly in the US and Europe, have set ambitious targets to achieve a carbon capture cost approaching $20/t (Lockwood, 2017). Novel solvents with phase change systems, ionic liquids, and other non-aqueous solvents aim to achieve lower regeneration energy requirements than conventional amines. Ionic liquids (ILs) are most promising as they have shown incredible potential for CO2 absorption, have negligible vapour pressure, have adjustable structures, and are eco-friendly (Yan, et al., 2018), (Hasib-ur-Rahman, Siaj and Larachi, 2010). Techniques used in other commercial gas separations, such as solid sorbents, membranes, and cryogenic separation, may be applied to carbon capture. Electrochemical carbon capture and the co-production of hydrogen with carbon capture are two alternative paths under development.
Application of ‘green/new solvents’
A solvent can be the key to a good chemical process, as it determines the solubility and the stability of excited states, thus affecting the potential-energy curves of activation. Over the past two decades, the applications of green chemistry have led to improvements in the capabilities of conventional solvents, with a new class of so-called master solvents, also termed ‘green’ or ‘designer’ solvents (Nematollahi and Carvalho, 2019). By definition, an ideal fully sustainable green solvent would not have an ecological impact at any stage and would ease process conditions, making them milder and more sustainable. Such solvents are likely to provide productivity and economic and environmental benefits (Hessel, et al., 2021). Water is considered to be nature’s ‘green solvent’ for its bio-catalytic processes.
Green solvents considered include ILs and supercritical CO2, as well as deep eutectic, theomorphic and fluorous solvents. Some green solvents, such as ILs, are widely used commercially, while others, such as flouros and supercritical CO2, have more specific uses. Figure 1 summarises green solvents for CO2 sorption and CCUS technologies.
Ionic liquids (ILs) as green solvents ILs comprise a large category of salts that, due to differences in their cation and anion sizes, are normally liquids at a temperature less than 100⁰C (Ohno, 2005), (Mohammad and Inamuddin, 2012). Over the past decade, ILs have been extensively studied as scrubbers of greenhouse gases (Zheng, et al., 2017). Many studies used imidazolium-based ILs for CO2 capture (Wang, et al., 2019), (Nguyen and Zondervan, 2018).
Due to the low stability, high volatility, and corrosiveness of materials used in conventional CO2 capture processes, such as amine-based absorption, water and organic liquid scrubbing, and hollow fibre membranes, ILs potentially infract green chemistry principles. (Zheng, et al., 2017), (Daryayesalameh, Nabavi and Vaferi, 2021), (Lu, et al., 2018). Advantages of ILs include good dissolution properties, low volatility, high decomposition temperature, energy, and cost-efficient separation of CO2 from post-combustion flue gases. These solvents can be readily removed and recycled. Their properties enable their use as room temperature ionic liquids (RTIL) to give a wider acceptance as ‘green’ solvents, replacing classical volatile organic solvents in a variety of processes, including industrial chemical processes (Mukherjee, 2021).
One class of ILs, the cation ILs, consists of imidazolium, pyridinium, and amines. The anion class consists of organic/inorganic ILs, (carboxylate, azolate, phenoxide) (Shukla, et al., 2019). When compared with cation ILs, the basicity of the anion ILs was noted to increase CO2 uptake. Process simulations showed that substituting MDEA (methyl diethanolamine) with ILs can reduce the total electrical and thermal energy by 42.8% and 66.04%, respectively (Liu, et al., 2016). The regeneration energy demand also decreased by 15% when IL (1-butylpyridinium tetrafluoroborate) was used in place of MEA (Mumford, et al., 2015).
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