Some Insight Into the Energy Costs Associated with Direct Air Capture of Carbon Dioxide. [View all]
The paper to which I will briefly refer in this post is this one: Exploring Geometric Properties and Cycle Design in Packed Bed and Monolith Contactors Using Temperature-Vacuum Swing Adsorption Modeling for Direct Air Capture, Valentina Stampi-Bombelli and Marco Mazzotti, Industrial & Engineering Chemistry Research 2024 63 (45), 19728-19743.
It is - often rightly - remarked that the concept of direct air capture, (DAC) is a means, like hyping "hydrogen" to rebrand fossil fuels as "green," is a potentially a pathway to greenwashing fossil fuels. This argument has some merit. This said, because we have left future generations with a destroyed planetary atmosphere, the research is, to my mind, worthy, since, depending on the resources available to them - which are diminishing in our orgy of consumerism coupled with wishful thinking - they may wish to attempt restoration of the world that once was, should the concept of "history" survive, which it may not do in Orwellian times.
The problem is that to capture CO2 from the air, or for that matter, flue gas, requires energy, significant quantities of it. The more dilute the CO2 is, the more energy is required to overcome the entropy of mixing. In addition, if one wishes to put the CO2 to use, rather than to propose vast unsustainable CO2 dumps, which are in fact, like hydrogen, an approach to greenwashing dangerous fossil fuels.
I have my own ideas about DAC, which involve process intensification using air based Brayton cycles, with both electricity and CO2 being side products, rather than the main product, by increasing exergy capture, essentially increasing the recovery of thermal energy, i.e., raising the energy efficiency in the use of nuclear generated heat.
The text refers to the 2015 Paris Agreement on what was then called "climate change" and to which I now refer as "extreme global heating," as this is what we now observe in our Godotian approach to energy, which is to wait for a so called "renewable energy" nirvana that did not come, is not here, and won't come. In Beckett's "Waiting for Godot," Vladimir and Estragon consider suicide, whereas in waiting for the so called "renewable energy" nirvana, we are actively committing suicide, slowly, by degrees, literally and figuratively, a less than minor difference. What is relevant to the issue is not the details of the paper, which is wonderful as a tool in considering the issues of gas/liquid equilibria expressed in mathematical terms, but rather some insight to the energy cost of direct air capture. This is the energy associated with overcoming the entropy of mixing by taking concentrated carbon - what the dangerous fossil fuels were before combustion and dealing with them in a dilute form, the low, but dangerously rising levels found in air.
From the text:
In alignment with the 2015 Paris Agreement, global efforts to limit warming to a 1.5 °C increase above preindustrial levels call for substantial reductions in CO2 emissions. (1) Although decarbonizing mobility, households, industry, and power generation is crucial, these measures are slow to implement and incomplete, leaving residual emissions in so-called hard-to-abate sectors that need to be addressed. Thus, negative emission technologies (NETs) that allow for carbon dioxide removal (CDR) from the atmosphere, such as afforestation, bioenergy with carbon capture and storage, and direct air capture and carbon storage, play a vital role. Direct air capture (DAC), a technology that extracts CO2 directly from the atmosphere, emerges as a promising CDR technology. Employing both solid sorbents (adsorption) and liquid solvents (absorption), DAC research and deployment are brought forward by both academia and industry, with companies like Climeworks, Global Thermostat, and Carbon Engineering leading the way. The lower energy demand for sorbent regeneration in adsorption offers an advantage over absorption (with heats of desorption of ca. 40–90 kJ/mol vs 200 kJ/mol in adsorption and absorption, respectively (2,3)), yet DAC’s widespread adoption hinges on resolving significant technical and economic challenges.
The low concentration of CO2 and the unavoidable humidity in the air are two of the primary challenges associated with DAC. As a result, a large portion of DAC research has centered around developing materials with high CO2 capacity and selectivity over N2 and O2, (4,5) evaluating sorbent stability (4,6−8) and accurately characterizing CO2–H2O coadsorption. (9−13) Amine-functionalized materials have emerged as promising sorbents, offering high CO2 capacities even in dilute CO2 conditions, often enhanced in humid environments. (12,14,15) However, despite their favorable thermodynamic properties, amine-functionalized sorbents have been shown to exhibit kinetic limitations. (11,16−20) Various studies have shown that external and internal diffusion resistances in the gas phase are the limiting mechanisms defining the adsorption kinetics, (20−22) with their limitations increasing as the feed concentration decreases. (21,22) This is particularly relevant in DAC, where a low CO2 concentration significantly affects gas-phase mass transport, contributing to slow kinetics, early breakthrough, and inefficient sorbent utilization. Experimental characterization of mass transfer kinetics under conditions relevant to DAC remains sparse despite its critical importance. (12,22,23) However, precise modeling of these dynamics is essential, as evidenced by the significant impact that the mass transfer coefficient, k, has on DAC performance through cyclic temperature-vacuum swing adsorption calculations. (24−26)
Another major challenge associated with the dilute CO2 concentration in the feed is that large volumetric amounts of air need to be processed to capture a significant amount of CO2. Coupled with the desire to operate at high air velocities to maintain short cycle times, this requirement might result in a large pressure drop across the air–solid contactor, thus increasing the energy demand of the blowers. Given that pressure drop increases with longer beds, lower bed porosities, and higher velocities, (27,28) it is intuitive to reduce bed lengths or to increase bed porosities to accommodate high air velocities. Multiple new contactor designs have been proposed to achieve the goal of reducing pressure drop while trying to maintain a compact structure. These include thin-layered packed beds arranged in compact geometries, such as radial bed contactors, (12,24−26,29−33) and structured sorbents like laminates (34) and monoliths, (2,21,23,30,31,35−40) which are compact and feature high porosities. Monoliths have gained particular attention due to their wide use in the catalyst industry and readiness at a technical level. With their parallel channel configuration, they have been shown not only to reduce pressure drop but also to enhance mass transfer kinetics in point-source CO2 capture applications, (41−46) thereby also potentially addressing the kinetic limitations inherent in DAC. Both packed bed and monolith structures present distinct advantages and drawbacks for DAC applications, each influenced by current technological and commercial realities. (28) Monoliths, although beneficial for their lower pressure drops and higher mass transfer rates, are typically constrained by lower sorbent loadings, while at the same time, monoliths tailored for DAC are not readily commercially available. On the other hand, packed beds exhibit a higher pressure drop but benefit from high sorbent loadings and the availability of commercially viable pelletized sorbents like Lewatit VP OC 1065 for easier scalability...
What is interesting is the sentence I have bolded, which gives the energy cost of two approaches, absorption and adsorption, to regeneration.
A mole of carbon dioxide (its molecular weight) is 44.0095 grams. This means to regenerate the adsorptive agent, it will take a minimum of 40 kJ of energy to regenerate an adsorptive agent to capture 44.01 grams of CO2.
Ignoring land use changes, it appears that emissions from the combustion of dangerous fossil fuels, as of 2023 is on the order of 37.8 billion tons per year.
Breakdown of Carbon Emissions by Source; 2003-2023
This translates to 3.78 X 1016 grams of CO2 which further translates to 8.59 X 1014moles of CO2.
This means that for the low end of adsorptive cost 40 kJ per mole, the energy requirement of carbon free energy would be roughly 34.4 Exajoules (EJ) of energy. This is slightly more than twice the 16 Exajoules that our multitrillion dollar combined solar and wind energy produced in 2023 for all purposes, this on a planet that was consuming 642 EJ in that year.
IEA World Energy Outlook 2024
Table A.1a: World energy supply Page 296.
At the upper limit for adsorptive processes, 90 kJ/mole, the energy requirement would be 77.3 EJ.
For the absorptive process, at 200 kJ/mole, the energy cost would be 171 EJ.
In the "percent talk" used to justify the useless solar and wind industries, at 40 kJ/mole, 90 kJ/mole, and 200 kJ, these energy costs are respectively 5.3%, 12.0%, and 26.8% of the current world energy supply, not to do any useful work, merely to remove a year's worth of carbon dioxide.
Note that this is simply to capture carbon dioxide from the air - that is to over overcome the entropy of mixing, not to reduce it to a liquid or solid form, which will require reproducing all of the energy, and then some (for the entropy of combustion/reduction) that put it there in the first place, nor to compress it for the putative fantasy of building huge geological carbon dioxide dumps that will inevitably leak but, like the hydrogen fantasy, is pure fossil fuel greenwashing bullshit.
It is feasible, to my mind, to recover more energy from nuclear fuels than is currently captured in the major usage thus far, which is simply to generate thermodynamically degraded electricity. But the technology for doing this is "pie-in-the-sky" as of now. We're in deep shit. I don't really weep for us on this score. We clearly never gave a shit. I do weep, instead, for future generations.
History will not forgive us, nor should it.
Happy New Year.
= new reply since forum marked as read