A team of researchers from Khalifa University asks: Are we missing something when evaluating adsorbents for CO2 capture at the system level?
We may be on the brink of global-scale change in the way we consume hydrocarbon fuels, but until the policies and agreements made at COP26 in Glasgow this month can be actualized, our relentless fossil fuel consumption continues to pump carbon dioxide into the atmosphere. These continuous emissions are the leading cause of climate change and it’s clearer than ever that we need to do something about the levels of carbon in our atmosphere.
In 2015, the international community adopted the Paris Climate Agreement, agreeing to limit the global average rise in temperature this century to less than 2° C, compared to pre-industrial levels, but with ambitions to limit the rise to less than 1.5° C. Along with a paradigm shift from fossil fuels to renewable energy sources, deployment of carbon capture, utilization and storage technologies was proposed as a core strategy to actively and significantly reduce greenhouse gas emissions. This is in addition to the clear economic benefit that could be derived from using CO2 as a feedstock material for chemical products in a resilient circular economy.
What are the most promising carbon capture technologies? There are as many ways of capturing carbon dioxide as there are of pumping it into the atmosphere. This means that carbon capture and storage technologies can be implemented across a range of industries from heating to electricity generation. To remove existing carbon dioxide from the atmosphere, we can use chemical solvents of different types, including membranes that adsorb carbon dioxide into porous molecules such as potassium hydroxide. However, this technology is currently expensive and energy intensive, as the amount of CO2 in the atmosphere is much diluted. Alternatively, CO2 capture from concentrated sources such as power plants is expected to play an important role in avoiding CO2 emissions, contributing to climate change mitigation. The more mature technology used in industry today for this purpose is absorption with chemical solvents.
Absorption works well but there’s a trade-off: many of our existing solvents come with an energy cost associated with heating the water for the removal of the CO2 to recover them. Ideally, we need processes that require less energy to capture and separate the CO2. Adsorption with porous solid materials is gaining attention as a promising carbon capture technology due to the potential improvement in energy efficiency and cost reductions, and a research team from Khalifa University is on the hunt.
Dr. Ahmed AlHajaj, Assistant Professor, Hammed Balogun, Research Engineer, Dr. Daniel Bahamon, Research Scientist, Saeed Almenhali, Master student, and Prof. Lourdes Vega, all from the Khalifa University Research and Innovation Center on CO2 and Hydrogen (RICH), developed a systematic tool uses various key performance indicators such as energy consumption and cost to screen novel adsorbents operating at a commercial scale, while maintaining the US Department of Energy requirements of 95 percent CO2 purity and 90 percent CO2 capture rate. They published their results in the prestigious journal Energy and Environmental Science.
“There have been many previous attempts to assess the technical performance of adsorbents using experimental and modelling approaches,” Dr. AlHajaj explained. “Ours goes further by considering non-monetized factors including the purity of the captured CO2 as well as the quantity captured, and the energy required for the whole process at commercial scale.”
Rather than individually testing each material at lab scale, the research team conducted simulations to guide their selection of potential adsorbent materials for carbon capture applications. The team used molecular simulations to generate missing experimental data on the efficacy of the adsorptive material – how much it could adsorb – and a dynamic process model to simultaneously determine its economic potential.
Then, they selected the five most promising candidates for the detailed assessment at industrial carbon capture conditions. These five materials included a zeolite, three metal organic frameworks (MOFs), and activated carbon, all of which were evaluated for capturing CO2 from the flue gas of an industrial coal-fired power plant. The materials were examined for their performance in terms of CO2 purity, CO2 capture rate, productivity, energy consumption, and unit cost of CO2 captured at a commercial scale.
Flue gas is the by-product gas that leaves a fossil fuel power plant via a chimney known as a flue. While its composition depends on the fuel being burned, it mostly comprises nitrogen, carbon dioxide, water vapor and a number of pollutants such as particulate matter, carbon monoxide, nitrogen oxides and sulfur oxides. The ‘smoke’ seen pouring from these flues is not smoke at all, but the water vapor in the gas forming a cloud as it meets cooler air. Carbon dioxide is the second largest component of flue gas at around four to 25 percent, depending on the fuel source. It is sent to the atmosphere unless a carbon capture unit is used to separate it from the flue gas.
“Since the performance of a process can be altered when we scale it up, it was essential to evaluate these materials at commercial and industrial scales,” Prof. Vega said. “The zeolite was included as a comparison as it is already widely used in industry for air separation, where CO2 needs to be removed as an impurity. While one particular MOF performed as well as the traditional zeolite, the zeolite was still the best performing low-cost material, as it’s cheaper to synthesize than the MOF. A very relevant result is that other MOFs appear to be very good for CO2 capture when examined at lab scale using technical performance indicators, but fail when considered at industrial carbon capture conditions.”
“This is very relevant in the search for the right materials for CO2 capture”, added Dr. AlHajaj. “Using the tool we have proposed to assess materials for carbon capture, including the right key performance indicators, will save time and economic efforts towards this goal.”
Zeolites are microporous materials commonly used as adsorbents and catalysts and are often considered “molecular sieves” as they can selectively sort molecules based primarily on a size exclusion process. However, they have limited capacity for CO2 capture and they are deactivated with water and other impurities. The best performing MOF would become a much more viable alternative if its production cost could be reduced. Hence the need for continued laboratory research on MOFs for use in carbon capture operations.
Jade Sterling
Science Writer
13 December 2021
