Aug-2023

Metal-organic frameworks for carbon capture

How the industrial-scale, cost-effective manufacture of MOFs and speciality nanomaterials could enable energy-efficient carbon capture and storage.

James Stephenson
Promethean Particles

Article Summary

Leading scientific opinion increasingly points to human activity having a large, detrimental impact on our planet. One effect of this activity has been an unprecedented rise in global warming driven predominantly by increased emissions of greenhouse gases such as carbon dioxide (CO2). The issues and consequences surrounding global warming have been well studied. Global surface temperatures have already reached 1.1°C above pre-industrial levels (IPCC, 2023). Exceeding a 1.5°C rise in average temperatures could risk the earth’s stability and life support systems (IPCC, 2018).

This startling recognition has led to an acceleration in demand for new solutions to help tackle climate change and, with it, a particular emphasis on decarbonisation. There are multiple decarbonisation approaches that can be employed to reduce the overall carbon footprint, which we have coined the ‘Decarbonisation Mix’. Many climate change proponents advocate strongly for the prioritisation of energy efficiency improvements and fuel switching to less carbon-intensive variants. However, many hard-to-abate industries (power generation, steel, cement, chemicals) cannot simply switch to different fuels or efficiently electrify their processes, at least in the near term. As such, a third element of the mix has to be carbon removals.

Carbon capture is therefore increasingly being recognised as a critical technology in the range of solutions needed to effect decarbonisation and help limit climate change. Despite the views of some, it is not an underhand way to ‘greenwash’ the continued use of fossil fuels. The UK government, the German government, and the Intergovernmental Panel on Climate Change (IPCC) have all recently opined that CO2 removal and carbon capture are now necessary approaches for the world to have any chance of limiting global warming to the 1.5°C goal established in the Paris Agreement. The UK government’s recent announcement of £20 billion in funding earlier this year to support the development of carbon capture and storage (CCS) projects highlights this emphatically.

Current technological options for CCS systems are limited
The most widely used commercial carbon capture technology today is amine scrubbing. The first amine scrubbers were designed and implemented in the 1930s, and the process is largely the same today, despite some significant improvements in the performance of the amine solvents used.

A typical configuration for a CO2 scrubbing system is shown in Figure 1. CO2 containing flue gas enters the bottom of the absorber and rises upwards. The CO2 is absorbed by the downward flowing amine, which then flows out of the bottom of the absorber and into the stripper. Here, the CO2 rich amine is intensively heated to desorb and separate the CO2. The reboiler at the bottom of the stripper provides the necessary heating for the desorption of the CO2. Reboiler bottoms are recycled back into the absorber, which provides regenerated lean amine ready to repeat the process.

Amine scrubbers have a high CO2 uptake capacity and relatively low initial capital investment. However, they do suffer from some limitations. Firstly, the operational cost of amine scrubbers is expensive, mainly driven by the energy required to regenerate the amine once it is saturated with CO2. Amines absorb CO2 through a strong chemical reaction, forming very stable bonds that require significant energy to break during regeneration. Our customers in the power generation space have pointed to energy penalties in the 30-40% range, representing a significant drop in productivity regarding both Capex and Opex.

Due to the high energy input required, the reboilers are often large, further resulting in operational footprint restrictions. High-duty amine scrubbers have large space requirements, limiting their utility for smaller, more modular carbon capture duties.

Lastly, certain amines present challenges from a material handling perspective. Most amines undergo oxidative degradation and thermally degrade at solvent regeneration temperatures above 120°C (Vega et al., 2014). It is estimated that this degradation leads to an average monthly replacement requirement of 5% of the amine. Amines also tend to corrode the process equipment and pipework, leading to higher overall Capex and maintenance-driven downtime.

Metal-organic frameworks (MOFs)
Initially discovered in 1965 as waste material from other chemical processes, MOFs are a class of materials with exciting chemical and structural properties. Well known for their ultra-high surface areas in excess of 7,000 m2 per gram, MOFs also have uniform pore structures, tuneable porosity, and significant flexibility in network topology and chemical functionality. The first permanently porous MOF was discovered in the late 1990s, and the term ‘metal-organic framework’ was coined.

MOFs are highly porous crystalline frameworks comprised of multivalent metals bonded to multitopic organic linkers (see Figure 2). The choice of metal ion and organic linker molecules is almost limitless, and it is estimated that more than 100,000 MOFs have so far been synthesised. Despite significant promise, the development of MOFs has been mainly the purview of academia, with novelty rather than utility being the main driver. This has resulted in MOFs acquiring a reputation for high-cost and low industrial-scale manufacturability.

Role of MOFs in energy-efficient CCS systems
Along with their high porosities and incredibly high surface areas, certain MOFs also have other advantageous properties for carbon capture applications. These include high thermal and chemical stabilities, tuneable selectivity, low energy of desorption, and recyclability (Britt et al., 2009). MOF-based CCS has the potential to deliver significant advantages over incumbent technologies, including increased energy efficiency, lower process complexity, and smaller operating footprints.

December 2022 represented a significant milestone for the technology. Promethean Particles and the University of Nottingham announced the completion of a MOF-based carbon capture pilot project at Drax‘s incubation facility in Selby, North Yorkshire. The aim of the project was to show how MOFs would perform outside of the laboratory and in relevant industrial conditions and, as such, demonstrate the achievement of technology readiness level (TRL) 5 (see Figure 3). The project was successful and not only showed that MOFs could capture the CO2 from the flue gas, but also helped inform future process design.

Rapid progress of the technology has since been demonstrated. Further pilots have been completed, including a more sophisticated, automated system that meets TRL6 criteria. This system can be transported to customer sites to provide in-situ demonstrations of the technology against the customer’s particular gas separation requirements. When not in use by customers, Promethean connects the system to its 1 megawatt (MW) gas-fired water boiler to help further inform process and application development activity.