The Energy Cost of Direct Air CO2 Capture Using Zeolite Driven Vacuum/Temperature Swing Absorption.

[NNadir]

Last edited Fri May 8, 2020, 07:36 AM - Edit history (2)

The paper I'll discuss in this post is this one: Direct Dry Air Capture of CO2 Using VTSA with Zeolites (Sean M. W. Wilson* and F. Handan Tezel, Ind. Eng. Chem. Res. 2020, 59, 18, 8783-8794.)

Recently I've been thinking quite a bit about the ever broadening concept of process intensification, and in fact, was writing a rather long and typically jargon laced barely intelligible musing on the integration of two or more thermochemical carbon dioxide splitting schemes which would produce electricity as a side product while providing very high thermodynamic efficiency. I reserve the right to suffer through finishing it in some future date, and publishing it here, irrespective of whether it has any value at all outside of making me learn stuff I otherwise wouldn't learn.

If one is inspired to suffer through my posts in this section, well, thanks for listening and thanks for your tolerance.

Process intensification is the practice of maximizing exergy, exergy being that portion of primary energy that can be put to practical use, in one form or another. The second law of thermodynamics requires that one can never utilize 100% of any form of energy, but certainly we can raise it to a higher percentage beyond current common practice.

As I often point out, we have not only dumped responsibility for dealing with our chemical wastes - and perhaps the most problematic is carbon dioxide - but we have also dumped a great deal of entropy on all future generations, inasmuch as we have distributed many elements in the periodic table haphazardly, consumed the best ores and left them diluted largely in problematic and often toxic repositories, that is, landfills. There has been a lot of talk about urban mining, which can be considered cowboy recycling, but anyone attempting to do urban mining will need access to sustainable energy, since unsustainable energy, the most widely used form of energy, dominates current energy use on this planet. It's not getting better either; it's getting worse.

Carbon itself, of course, which increasingly dominates our energy supply has been subjected to an enormous amount of entropy. When I was born, there were lots of ores with high carbon content, dangerous coal, dangerous oil, and dangerous natural gas, but in my generation, using ever more dastardly and destructive schemes, we have wrung more and more of this stuff our of the land and sea, combusted it and dumped the waste directly into the planetary atmosphere with nothing more than lip service to doing something else.

History will not forgive us, nor should it.

Future generations, to the extent they will require carbon, will thus need to produce it from one of our two major waste dumps, our atmosphere and our bodies of water. Collecting it will require significant energy, since in general, to reverse entropy, one must expend energy to do so. Future generations will thus need to pay for the energy we used.

The paper under discussion here is about one much debated approach, direct air capture. There are oodles upon oodles of scientific papers on the topic of direct air capture of carbon dioxide. This one caught my eye because it seems to lend itself to process intensification since it is amenable to utilizing waste heat, as well incorporation in the line of a Brayton type heat engine utilizing air as a working fluid. (A jet engine is a Brayton heat engine utilizing air as the working fluid.) It also includes an explicit description of its energy cost, the amount of energy that future generations will need to find to clean up our mess. (They'll think we hated them, and they won't be far from being correct, since we have definitely treated them with contempt.)

So let me cut to the chase:

The abstract is available at the link.

From the introduction to the paper:

Increasing the levels of CO2 in the atmosphere has been shown to correlate to the increasing average global surface temperature, which has led many researchers to invest time and effort into finding ways to effectively reduce the amount of CO2 entering the environment or to capture CO2 directly from the air. The latter, the subject of this study, has been dubbed direct air capture (DAC) and has significantly gained interest as a true carbon-negative strategy to reduce the amount of CO2 in the atmosphere.(1?3) DAC, although thermodynamically less favorable than capturing CO2 from large stationary postcombustion point sources,(4,5) has important benefits, including the following: all greenhouse gas emissions (including transportation emissions) can be captured from one spot, previous CO2 emissions can be captured, and the location, as well as the size, is not limited unlike postcombustion capture of a power plant. Additionally, studies have shown that using methods such as the reverse water gas shift reaction in tandem with the Fischer–Tropsch process could allow DAC of CO2 to be used to generate hydrocarbon fuels instead of traditional sources, which would allow for the continuous use of the transportation infrastructure well into the future.(6?8)

With DAC gaining interest, the scientific challenge of implementation is to find a low-cost sorbent that has high uptakes, selectivity for CO2, and fast kinetics, with strong physical and chemical stabilities, whose regeneration is not overly energy-intensive.

The authors screened a number of zeolites. Zeolites are minerals having a defined structure that is porous and cage like. Since the pores can have different sizes, they can be utilized to separate gases (and other fluids) by showing a preference for the molecular size of the gases. The authors screened a number of different types of zeolites, and found that the best one for separating carbon dioxide from air was a zeolite that appears in nature as a relatively rare mineral Faujasite, that can in fact be synthesized and is in fact commercially available.

The structure of the Faufasite zeolite can be seen with the abstract, but for convenience it is shown here:



A four step process is utilized to recover carbon dioxide. A container (in these experiments a column) packed with the zeolite is evacuated with a vacuum pump, pressurized at room temperature, and maintained at pressure until the concentration of the carbon dioxide coming out was equivalent to the carbon dioxide concentration going in, the point called the "breakthrough" point, which takes place when the zeolites are saturated with carbon dioxide. The concentrations were between 410 ppm and 460 ppm; this week we've measured in the atmosphere at the Mauna Loa CO2 observatory as high as 418 ppm. The chamber is slowly depressurized by pumping the air out, to give a weak vacuum, about a tenth of an atmosphere (10 kPa), whereupon it was heated to various test temperatures. The highly enriched carbon dioxide released by the zeolites was then pumped out of the chamber.

It may be useful to look at some graphics from the paper.

Here is a graph of the isotherms, which give a feel for the selectivity of the gas separations for the major components of air:



The caption:

Figure 1. CO2, N2, O2, and Ar pure adsorption isotherms at 22 °C for G5CO2M, APG-III Na-X HP, Ca-X, Na-LSX, Z10-01, and Z10-02ND. Each isotherm is fitted to Langmuir, Freundlich, Sips, and Toth and is represented by the model with the best fit.

The selectivity as a function of the maximum pressure in the pressurization step.



The caption:

Figure 2. Adsorption capacity ratio of CO2 at 400 ppm over the summation of adsorption capacities of N2, O2, and Ar at 780 840, 209 460, and 9300 ppm, respectively, at different total pressures calculated from their isotherms at 22 °C.

Breakthrough volumes as a function of regeneration temperature:



The caption:

Figure 3. Breakthrough curves for a column packed with APG-III with a positive CO2 concentration step of 431 ± 10 ppm of CO2 (indicated in blue) at 23.5 °C and a GHSV of air at 13 400 h–1. These breakthroughs happened after regeneration temperatures of 62, 116, 194, and 261 °C while applying a vacuum over the course of 3 h.

GHSV is the "Gas Hourly Space Velocity" which is a function of the flow rate, the free void volume and the column volume, basically an expression of the amount of gas that flows over the zeolites. It is clear from this graphic that the Faujusite zeolite is not usable as a simple vacuum swing absorption (or pressure swing absorption) material, heat is required.

This graphic shows the effect of flow rate on time to breakthrough:

Figure 5. Breakthrough curves for a column packed with APG-III with a positive CO2 concentration step of 421 ± 11.5 ppm of CO2 (indicated in blue) at 23.5 °C after a regeneration temperature of 194 °C and vacuum over the course of 3 h for GHSV of air of 34 400, 27 000, 13 400, and 6720 h–1. Corresponding breakthrough adsorption capacities are next to the displayed GHSVs.

I'll skip over some other graphics in the paper reflecting the amount of heat required to maximize the absorption capacity of the zeolite.

Let's cut to the money shot, which is how much energy this system is reported to require on a ton scale.

Figure 9. Energy required to remove water from air for the DAC of CO2 using APG-III using either silica gel (blue) or zeolite 3A (green) as a function of temperature and humidity levels 20, 40, 60, 80, and 100% between the temperature of ?20–10 °C (above) and 5–35 °C (below).

Ev here is the energy required for generating a vacuum, Ec, energy required to compress the air, Ef the energy required to drive the flow over the sorbent, Es, the energy required to heat the sorbent, and ED the energy required to remove the gas from the sorbent.

Here the authors refer to primary heat.

Before noting the consequences of the requirement that air needs to be dry for this system to work, a requirement that requires additional energy for drying the air, let's take a look at the energy required to remove 35 billion tons of carbon dioxide from the air. This is the amount we dump each year as dangerous fossil fuel waste, and does not include the additional tonnage connected with land use changes, for agriculture or for putting roads through pristine wilderness to turn it into industrial parks for "green" wind farms.

Let us assume that the temperature at 194°C strikes a balance between the cost of the zeolite and the completeness with which it releases the carbon dioxide. The sum of the energy figures for the different components of the process in the bar graph is 16928 MJ per ton. To recover 35 billion tons of carbon dioxide from the air using this process is thus on the order of 590 exajoules of energy. This is uncomfortably close to the amount of energy that humanity consumed, according the the IEA World Energy Outlook (2019 Ed.) by all of humanity for all purposes, which is 600 exajoules (as of 2018).

However this doesn't tell the whole story, because the air must be dry. The consequence of drying the air on the energy consumption is actually quite enormous, as the following graphic from the paper shows:



The authors thus suggest that were this system to be industrialized, it should located in dry areas. They write:

Investigating different locations on the planet concludes that polar deserts are excellent locations for this process. This is because polar deserts are arid frigid locations, which would reduce the energy required to dehydrate the air as well as provide higher CO2 adsorption capacities at lower temperatures. One advantageous place, in particular, is the super arid McMurdo Dry Valley in the Antarctic, which has a yearly average temperature of ?20 °C and an average humidity of 54%.(62) This particularly dry arctic weather is due to the katabatic winds,(63) which could be harnessed to provide energy to run the DAC process. Another ideal location is the Atacama Desert located in Chile. This 105 000 km2 elevated desert located on the Tropic of Capricorn has temperatures and humidity levels between ?5 and 20 °C, and 5–20%, respectively, near the top of Cerro Paranal.(64) Due to its location and aridness, the Atacama Desert is one of the best locations for solar power, which houses both photovoltaic and concentrated solar power plants.(65)

From where I sit, these proposals to use intermittent so called "renewable energy" for these purposes is obscene. You cannot run a system like this on a meaningful scale using intermittent energy, since the heat cycles cannot stand interruption. In this space, I showed, using the wind turbine database of the Danish Energy Agency, that the average lifetime of a wind turbine in Denmark is about 17 years. All of the wind turbines in Denmark combined produce as much energy as two small power plants. The blades, which use large amounts of carbon fibers are made using dangerous fossil fuels as a feedstock need replacement every 3 or 4 years, and they go to landfills since recycling them is problematic.

Now imagine hauling this crap into the interior of Antarctica, huge steel posts, for devices that might, in conditions far more extreme than Denmark, for a lifetime considerably shorter than what the Danes see in their offshore oil and gas drilling hellhole of a country.

The same constraints of intermittency apply to putative solar facilities in the Atacama desert.

After 50 years of wild cheering, the entire planet, at a cost of a couple of trillion dollars, has managed to produce (as of 2018) 12.27 exajoules of energy from wind and solar combined, with a little geothermal and tidal energy thrown in for good measure. The steel, aluminum, lanthanide metal, carbon fiber, glass, silicon processing of these schemes is already unacceptably high, and now we argue that to engage in shoving buckets of water against a storm surge, that we can address climate change by using as much energy as we use for all purposes to capture carbon dioxide that we scale up this disastrous failing scheme by 4,800% just to capture the carbon dioxide we routinely dump, is just, well, silly.

(What's even worse, is that the authors suggest sequestering this carbon, as if we really might have a place in Antarctica to dump a few hundred billion tons of carbon dioxide.)

Despite this silliness, the paper is interesting in terms of the physical chemistry of the zeolites and these are reported experimental results. If the claims of the paper hold up, there are ways to actually make this practical to recover carbon to be utilized in various capacities, such as long term use polymers, refractory metal carbides and MAX phases, carbon fibers for use in useful systems. These systems for use are well explored, and with heat, carbon dioxide can be even used to oxidize waste biomass and municipal wastes containing plastics, paper, food waste and construction waste.

Earlier I spoke of process intensification. There are heat engine cycles, Brayton cycles in particular, that operate at very high temperatures, including some using carbon dioxide as the working fluid. I wrote about a variant of the carbon dioxide driven cycle called the Allam cycle, which (as it can be biomass driven) is an indirect way to capture carbon dioxide: Considering an Alternative Hybrid Allam Heat Engine Cycle for the Removal of CO2 from the Air. The Allam cycle in the papers referenced there ran at 860°C, whereas I was suggesting a higher temperature, my default target temperature, 1400°C. It is very clear that a heat engine operating at these temperatures offer many opportunities for process intensification, including, for example, the heat component required to regenerate the zeolites in the scheme in the paper under consideration. With a continuous and reliable heat source to drive an Allam cycle - the only environmentally acceptable source is nuclear energy - we could recover carbon dioxide from the air using energy efficiency rather than generating new energy.

With the annoying flaw of suggesting the unsustainable use of so called "renewable energy" put aside, this paper, if it holds up and is scalable, is quite interesting.

Stay safe, stay well, stay home, and remember, for all this tragedy, how beautiful life is, and how very much it is worth living.