### Carbon Capture

March 2, 2020

Tikalon's home in northern New Jersey is located in one of the colder parts of the contiguous United States. Our house is heated by natural gas; and, although it's a smaller house by suburban New Jersey standards and well insulated, it still takes a lot of energy to heat. One reason for this is that it's a traditional ranch-style house with all rooms on a single floor, so there's no passive heating of a lower floor to an upper floor.

Our monthly natural gas bill conveniently includes the energy usage in therms. A therm is equal to 100,000 British thermal units (BTU), a non-SI unit that's commonly used to describe heating and cooling in the United States. One therm is obtained through the combustion of nearly 100 cubic feet of natural gas. In terms more common to scientists and engineers, a therm is equal to 25,200 kilocalories (chemistry), 105.5 megajoules (physics), or 29.3 kilowatt-hours (electrical engineering).[1]

Natural gas therms used for heating our home in 2019. The total usage in 2019 was about 1,350 therms. Natural gas use in warmer months is for heating water and to fuel a backup electrical generator. (Graphed using Gnumeric. Click for larger image.)

I've been familiar with the concept of carbon capture and sequestration for more than a decade. My 2012 science fiction novel, Mother Wode, has a scene in which an evacuation caused by an alarm at a carbon sequestration plant is used as a cover for the physical hacking of some desktop computers. At that time I investigated how homeowners might take charge of their own carbon emissions by capturing the carbon right at their home. My assumption was that a capture device shouldn't be any larger than the furnace creating the emission. The captured carbon might somehow be buried in the backyard of my third acre lot.

One quick calculation quickly killed that idea. Combustion of a therm of natural gas produces about 5.3 kilograms (11.7 pounds) of carbon dioxide. Ignoring the fact that we've captured carbon dioxide, and not carbon itself, five kilograms of graphite (density about 2.1 grams/cc) would occupy 10,500 cc. Our 1,350 therm annual natural gas usage would produce about 14 million cc of graphite, or 14 cubic meters. This would be a cube 2.4 meters (7.9 feet) on a side.

Since this residential point-of-emission solution is clearly untenable, what are the options? A retrofit to all electric heating would allow the sequestration to happen at the power plant if combustion fuel sources are used. Ideally, the electricity would be generated using renewable energy sources, such as hydroelectric, solar and wind. Such a major change would require a change in the electrical distribution infrastructure, since the power lines feeding homes in my area are not designed for everyone to take that much power.

That would be the case if a house required 1,350 therms annually for heating; but, proper thermal design can reduce this heating considerably. I wrote in an earlier article (Thermal Filtering, January 21, 2019), that there's an interesting anecdote about radio astronomy pioneer, Grote Reber. Reber built a radio telescope at his home in Wheaton, Illinois, in 1937, but he moved to Tasmania in his later years to conduct radio astronomy at very low frequencies. It's recalled that his Tasmanian house, built with eight inch thick walls and double-glazed window panes, was so well insulated that use of the kitchen oven would bring the room to 120 °F (50 °C).

Atmospheric carbon dioxide concentrations at Mauna Loa, Hawaii, 1958-2018. This curve is known as the Keeling curve.

The data, shown in red, demonstrate the seasonal variations in carbon dioxide caused by land plant uptake. The blue curve is a smoothed trend.

(Wikimedia Commons image, Data from R.F. Keeling, S.J. Walker, S.C. Piper, and A.F. Bollenbacher, reformatted for clarity. Click for larger image.)

In a recent issue of APS News, Amory Lovins, chair of the Rocky Mountain Institute, describes the design of his own house near Aspen, Colorado, where temperatures can get as low as 44 degrees below zero Celsius.[2] His house, constructed in 1983, uses no combustion heating. Just as in Reber's house, it is heavily insulated and has windows with very low heat loss.[2] Its heating is accomplished almost entirely by passive solar heating, with an additional 1% done by active solar heating.[2] Savings in the capital and operating costs of conventional heating allowed a payback for the energy efficiency add-ons used in the construction in less than a year.[2]

There's an old saying, "Put your money where your mouth is." At a recent luncheon with my former corporate research colleagues, we were talking about how much of a financial hit we might take if we wanted to mitigate our personal carbon footprint. The 2016 per capita carbon dioxide equivalent for the United States was 17.75 metric tons. I didn't know this figure at that time, so I estimated just the cost of mitigating the 7 metric tons of carbon dioxide from my natural gas usage. My estimate was that it would take about two thousand dollars and I would be willing to absorb this cost for the benefit of my children and grandchildren.

The technology for offset of all my present carbon emissions is direct air capture in which carbon dioxide is captured directly from the ambient air. One advantage of this method is that direct air capture plants can be sited anywhere. Another advantage is that it's a path to negative emissions in which we might reduce atmospheric carbon dioxide rather than just stabilizing it. Carbon dioxide can be extracted my various means, including chemical containment in a liquid solution of sodium hydroxide or amines, semi-permeable membranes, and metal-organic frameworks. The major problem is that vast amounts of air need to be passed through a direct air capture system, since the atmospheric concentration of CO2 is just 400 ppm. I'm reminded of the Barsoom atmosphere plant in the Edgar Rice Burroughs' novel, A Princess of Mars.

Don't worry, Princess! I'll save you from global warming!

According to the summary at Fandom.com, the Barsoom (Mars) atmosphere plant converts solar radiation to the air necessary to sustain life on the planet.

It's a huge and highly fortified installation, four square miles in area and 200 feet in height.

(A 1917 cover illustration by Frank Schoonover for A Princess of Mars, via Wikimedia Commons.)

In 2011, the American Physical Society published a report on direct air capture that estimated the cost at \$641-\$819 per ton of CO2.[3] By that estimate, the capture cost for my 7,155 kilograms of carbon dioxide from natural gas usage would be nearly \$6,000, quite a bit larger than my \$2,000 lunchtime estimate. However, costs have been reduced over the years to about \$100 per metric ton, which is well within my budget.[4] An early entry into this industry is Global Thermostat, which has a pilot plant in Huntsville, Alabama.[5] Global Thermostat uses amine-based absorbents for CO2 removal, and it claims a cost of just \$120 per metric ton at its Huntsville facility.

Global Thermostat's CO2 ends up as soft drink carbonation.[5] There exists an established billion dollar CO2 market for things such as carbonated beverages, a stimulant for greenhouse plant growth, and for enhanced oil recovery.[5] Carbon dioxide is miscible in petroleum and it lowers its viscosity, which is an aid to extraction.[6] Carbon dioxide is worth \$100 per metric ton in this market,[5] but you can't count on that money after direct air capture produces more CO2 than needed.

Estimate by the National Academies of Sciences, Engineering, and Medicine of carbon dioxide removal needed to attain net zero carbon emissions by 2090 while keeping global warming below 2°C. Some carbon capture will be needed even after 2090 to mitigate problematic or expensive sources of carbon dioxide, such as those from agriculture, land-use change, and aviation fuels. (Created using Inkscape from data in fig. 8.1 of ref. 4.[4] Click for larger image.)

There's been a growing interest in carbon capture, as indicated by the rate of publication of articles on this topic, including a lengthy article in the January, 2020, issue of Physics Today.[6] The article reports on a major investment in Global Thermostat by ExxonMobil, and a partnership with another DAC startup, Carbon Engineering, by Occidental Petroleum in the design of a plant with the capability of extracting a million tons of CO2 from the atmosphere annually.[6] As a cost check, the Swiss company, Climeworks, sells small-scale commercial DAC systems suitable for CO2 supply for applications such as greenhouses with a cost of \$600/ton and an expected cost reduction to \$200/ton in three years.[6]

One problem, of course, is that DAC plants need energy to operate. If this energy does not come from carbon-free sources, the actual cost per ton is higher, since its own carbon use needs to be mitigated. Energy is needed for the giant fans that drive air through the system, extracting the CO2 from the absorbents, compressing the CO2 and transporting it to storage sites,[6] One study referenced in the Physics Today article estimates that DAC might require a quarter of the world's total energy demand by 2100.[6] A technology under study at Arizona State University involves a less energy-intensive concept of artificial trees with leaves that contain an ion exchange resin for carbon capture.[6] Such artificial trees would be hundreds of times more efficient at capturing CO2 than Amazon rainforest trees.[6]

As I wrote earlier in this article, carbon capture is just the first part of a process that requires storage of the captured carbon. One option is to pump the captured CO2 into the pores of sedimentary rock. The United States Department of Energy estimates that the total sedimentary rock storage capacity for CO2 in the United States alone could be as high as 22 trillion tons, with an equivalent capacity in China.[6] That's enough for storage of 600 years of current CO2 emissions.[6] The additional transport and storage costs are between \$12-\$50 per ton.[6]

### References:

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