18.2: Capturing CO2 from the Atmosphere

Trees

Trees are the first form of natural carbon storage that most of us think of when we imagine removing carbon dioxide from the atmosphere. There are at least three trillion trees on Earth, and they hold about 500 petagrams of carbon, or in more common terms, 500 billion tons. When we talk about living organisms, we talk about carbon as the chemical species, C (molecular weight 12), as opposed to when we talk about the atmosphere, where carbon takes the chemical form CO₂ (molecular weight 44 with the addition of the two oxygen molecules). The 500 billion tons of carbon in forests (trees, roots, and dead material) came originally from CO₂ in the air: 500 × 44 / 12 = 1,833 billion tons of carbon dioxide, or roughly about twice as much CO₂ as exists in the atmosphere today

Figure 18.2.1 Aspen grove, Rocky Mountain National Park. National Park Service photograph by J. Westfall.

Adding more forested area to the Earth is an obvious and fairly rapid way to remove carbon dioxide from the atmosphere, and reforesting land that has been denuded by logging or clearing for agriculture is the first option for negative emissions. This is happening naturally in places like Maine in the United States, where former farmland has been allowed to return to forest, and because of conservation activity in places like Bhutan, where in 2016 at the request of the king, 108,000 trees were planted to honor the birth of the new prince, Jigme Namgyel Wangchuck (the number 108 is auspicious in Bhutan’s Bhuddist tradition).

There are two important limitations to how much CO₂ we can remove by reforestation. The first is the total land area to be covered in trees. We can’t cover farmland that is in active use, or much of our urban landscape. This limits the total CO₂ that can be removed to about 100 billion tons, although some estimates are as high as 260 billion tons. The second limitation is the rate of growth, and this restricts the rate of removal to between 1 and 3 billion tons of CO₂ per year. The good news is that this is a relatively inexpensive option, with costs as low as $10/ton of CO₂ removed.

Figure 18.2.2 Gain and loss of carbon from Canada’s forests in recent years. Since 2002, Canadian forests have been emitting carbon dioxide, not absorbing it, because of fires and insect kills. (MT CO₂e = million tons of carbon dioxide equivalent.) Data from Natural Resources Canada.

Climate change is hard on forests, however. Figure 18.2.2 shows the carbon uptake and loss from Canada’s extensive forests. Despite being mostly undisturbed by direct human activity, Canada’s forests are currently emitting CO₂, not absorbing it. In California, trees are also dying at a rapid rate because of drought and insects. In 2010 there were fewer than 5 million dead trees in California’s forests; today there are more than 145 million. Maintaining healthy forests will be a major challenge in the Anthropocene, and using forests as a negative emissions sink requires us to solve the forest health challenge. We can’t just assume that planting trees, and ignoring existing forests, will achieve a climate benefit.

Soils

Soils store a large amount of carbon in the form of decaying plant matter, organic chemicals derived from plants and soil organisms, and also a large pool of living organic matter such as microbes and fungi. In healthy soils these compounds are in constant flux. Plants absorb CO₂ and grow roots; roots exude chemicals that are absorbed by the microbial community, which is preyed upon by viruses; new microbes consume the decaying plant and animal matter, emitting CO₂; and some soil organic material becomes associated with soil minerals. This results in a constant flux of carbon in and out of healthy soils, such that the carbon content of soils is never static but is a stock and flow problem. Soils that are rich in organic matter have healthy ecosystems and vice versa, as you learned in Chapter 16.

The rich soils of the US Midwest were formed by these processes operating around the roots of perennial plants, such as grasses, that live for multiple years (as opposed to the annual plants that die and regrow from seeds every year, like most of our crop plants). These perennial plants typically put down deep roots—often 4 meters deep—in search of reliable water. Switchgrass is one of the most common of the perennial grasses of the Midwest.

Agricultural practices like plowing can decrease soil carbon by exposing it to the atmosphere, which causes it to oxidize to CO₂ and be lost. An even larger loss of carbon can occur from rich surface soils washing or even blowing away, as they did in the US Dust Bowl of the 1930s. Similarly, harvesting a crop like corn and then leaving the ground uncovered for the winter allows organic matter to be lost by all these mechanisms and discourages the healthy microbial activity that forms good soils. Today farmers seek to diminish these effects by using soil conservation plowing that decreases erosion, no-till agriculture where seeds are planted without plowing, and cover crops in winter to avoid bare soil and reduce loss of organic material.

When American farmers began plowing the soils of the prairies, they rapidly released carbon from the soils. Figure 18.2.3 shows how the loss continued until recent years, when the new soil management practices were put into place.

Figure 18.2.3 Soil organic carbon (SOC) in the central US. Conventional tillage is plowing every year, without using cover crops in winter. Reduced tillage uses things like no-till planting, cover crops, and contour plowing to reduce erosion and runoff. Adapted from Donigan, A. S., Jr., et al. 1994.

These kinds of soil carbon losses have occurred around the world and are responsible for approximately 133 billion tons of carbon loss from farmed land, which represents about 450 billion tons of CO₂ in the atmosphere. If we could return that carbon to soils, it would increase their productivity while decreasing the carbon dioxide in the atmosphere. Since soils formerly had these carbon contents, we assume they could be returned, but the challenge is: how fast can that occur? This is a major target for negative emissions studies, because returning that carbon to agricultural soils would erase much of the excess CO₂ in the atmosphere today. As you can see in Figure 18.2.3, it is possible for the soil to slowly regain carbon, but can we regain all of it? And, can we greatly speed up the process? Research in this area includes better farm practices, which you learned about in Chapter 16, using perennial crops instead of annual crops, and finding ways to encourage deep root growth. Changing the plants we grow today to do more for soil carbon enrichment—through deeper roots or genetic modification to increase root growth and longevity—is an active research area. There is not yet an obvious silver bullet—much work needs to be done.

Biofuels and carbon dioxide capture

An important link between forestry, agriculture, and negative emissions is the production of biofuels. Today in the United States 10% of automobile fuel is the biofuel ethanol, produced from corn and increasingly from other lignocellulosic feedstocks like corn stover, which is the cornstalks left over after the ears of corn are removed. That ethanol is made by fermentation in which yeast breaks down the sugar in the corn, turning it into ethanol and CO₂ in about equal parts. Today that carbon dioxide is simply allowed to bubble out of the vat and return to the atmosphere where it started at the beginning of the growing season. If we could catch that CO₂ and permanently keep it out of the atmosphere, it would be an easy form of negative emissions. Since the CO₂ bubbling out of the fermentation vat is nearly pure (it has some water vapor in it), it is relatively easy to capture. Today much of the CO₂ in fizzy drinks comes from that source. Of course, that does not constitute negative emissions, since it immediately returns to the atmosphere when we drink the beverage.

The production of other biofuels also emits carbon dioxide. Anaerobic digesters that process manure, sewage, and food waste create methane for use in vehicles or in our natural gas pipelines, and carbon dioxide is a by-product. That CO₂ has to be separated from the methane before it can be used, and today the CO₂ is simply dumped into the atmosphere. This represents another readily obtained negative emission. Typically, one carbon dioxide molecule is created for every two methane molecules in an anaerobic digester. Decomposition of trash in landfills creates a similar mix of gases. Today we try to control the methane emissions from these sources because it is such a potent greenhouse gas. Capturing and storing the CO₂ can turn this control into a double benefit.

Another way to obtain energy from biomass is to burn it for electricity. This is an old way to make electricity, but now we can consider also capturing the CO₂ and putting it underground. This leads to a negative emissions concept that is prominent in recent Intergovernmental Panel on Climate Change (IPCC) reports—bioenergy carbon capture and storage, or BECCS. An attractive aspect of this approach is that in principal all of the carbon in the biomass could be captured, yielding the maximum amount of negative emissions. A challenge is that the electricity generated by burning biomass is relatively expensive compared with solar and wind power—typically about 12¢ per kilowatt hour (kWh) compared with 5¢. The added cost of capturing the CO₂ from a biomass power plant would today increase the cost of the power by at least 50%, to 18¢ per kWh, making it dramatically more expensive as a source of electricity. The question to be asked is, How much are we willing to pay to remove carbon dioxide from the atmosphere, and is this a practical way to do it? We could certainly subsidize BECCS power to achieve this goal, but since biomass power plants are struggling to be competitive today, it seems that this will be an expensive and potentially less attractive way to remove carbon dioxide from the atmosphere.

Much of the biomass that could be burned for electrical power can also be processed into liquid fuel by using a variety of methods that heat the organic material to extract organic molecules. One method is pyrolysis, or the rapid heating of organic material, as mentioned in Figure 18.2.4. This volatile organic material comprises most of the smoke from typical fires. Pyrolysis attempts to keep the volatile organic material from burning by performing the heating rapidly in an oxygen-free or low-oxygen environment. The released organic chemicals, called bio-oil, are then condensed and processed into fuel in a refinery, much like fossil petroleum is made into gasoline.

Figure 18.2.4 The smoke coming off a pile of burning leaves is volatile organic compounds being cooked out of the biomass. It could be condensed to form a biofuel in a process called pyrolysis. Photograph by Jorge Royan via Wikimedia.

Once the volatiles have been removed, pyrolysis leaves behind a carbon-rich residue that is fundamentally charcoal and has been labeled biochar. Typically making up about 20% of the original weight of the biomass, this material can be added to poor soil to enrich its water-holding and nutrient properties, as well as encouraging microbial activity by providing a substrate for growth. This carbon is the major opportunity for negative emissions when pyrolysis is used to create fuels from biomass. Although the carbon in the fuel (from the volatiles) is rapidly returned to the atmosphere for net neutral emissions, the biochar can be stable for as long as hundreds of years (this is highly dependent on the soil environment), providing negative emissions.

The practice of adding charcoal to poor soil through deliberate burning campaigns has been used by agricultural societies for centuries. In Brazil, soils labeled by European settlers as terra preta (“black earth”) are now known to have been deliberately created by the Indigenous people as an enrichment process for depleted rain forest soils. However, pyrolysis is in its infancy as a combined negative emissions and energy technology, with many issues yet to be worked out. One perceived advantage is the ability for pyrolysis facilities to be relatively small and be economically located near the source of the biomass. A major cost in any biomass-to-energy system is transporting the biomass. Trucks can typically carry biomass about 50 miles before the costs begin to overwhelm the profitability of the operation. This is particularly important for forest biomass such as the slash that is left over from logging operations that are often far from potential places to use biomass. Currently these small trees, limbs, and unmarketable wood are piled up and burned in the forest (have you heard the term slash and burn?). This material could be converted into bio-oil and transported to refineries, where it could be made into transportation fuel. If the biochar is left in the forest soils (or otherwise permanently stored out of the atmosphere), the resulting transportation fuels can be carbon negative; that is, even after burning the fuel, the net impact on the atmosphere is that carbon dioxide has been removed (discussed in Section 18.4).

The final method for converting biomass into energy with ultimate negative emissions is gasification. In this approach the biomass is heated to high temperatures, above 1,200°C, in the absence of oxygen, and it breaks down into a mixture of hydrogen, carbon monoxide, and carbon dioxide called synthesis gas. This process has been used industrially for more than 180 years. The synthesis gas can be converted, using catalysts, into organic chemicals and fuels or even burned directly to generate heat or electricity.

This is an interesting way to create hydrogen for a modern carbon-free economy. Running the conversion process in such a way as to generate maximum amounts of hydrogen and carbon dioxide, and minimal carbon monoxide, is a good way to make hydrogen. The carbon dioxide can then be separated from the hydrogen with either solvents or membranes, leaving pure hydrogen for energy use, as well as pure carbon dioxide for permanent storage out of the atmosphere. Gasification plants tend to be large in order to achieve efficient operations, and industrial development in the past depended on coal as the feedstock. It is hard to get enough biomass in a small radius around a gasification plant to keep it operating efficiently (without excessive distances for trucks to travel, which is an additional cost and which generates additional pollution), but modern developers are trying to overcome these hurdles. Like burning biomass for direct energy production, gasification is in principal capable of capturing all of the carbon, for maximum negative emissions.

Biomass negative emissions benefits and limitations

I discussed biomass and soil negative emissions methods first in this chapter because they have many positive attributes, but they also have some significant limitations.

On the positive side, we can improve soils while also harvesting biomass, yielding double benefits. These approaches tend to be the least expensive of negative emissions technologies because the plants have done the hard work of accumulating CO₂ and solar energy for us. Afforestation, biochar, and soil carbon enhancement all can be done for less than $100/ton of CO₂ removed, and they have, respectively, maximum capacities of 4 billion, 2 billion, and 5 billion tons per year worldwide, capable of making a significant dent in our 20-billion-ton need.

Each of these methods requires land, however, which is also needed for other purposes. Producing the food we need is the primary competition with energy uses—of course it is the good-quality land that is best for both needs. If farmers are paid higher prices to grow energy-related crops, then they will grow less food. The US Department of Energy has estimated the amount of biomass that could be provided in the United States for energy purposes in 2050, without reducing food production. The 2016 Billion-Ton Report found that it was realistic to expect that in 2040, the United States would have about 1.5 billion tons of biomass available for energy or negative-emissions-related use, without significant impact on food production or other land uses like housing and transportation. This amount of biomass could be used to create all of the airplane fuel used in the United States.

That 1.5 billion tons of biomass includes trash, sewage, manure, crop residues like almond shells and straw, and also growing new crops like poplar trees or switchgrass on land not suitable for high-value agricultural crops. About half of that total would be from those new energy crops, so the impact on food and water needs to continue to be evaluated. Not surprisingly, the trash resources are located in cities, and the agriculture-based biomass is in the center of the country, where agriculture is vibrant and widespread (Figure 18.2.5).

Figure 18.2.5 Summary map of biomass resources in 2040 in the United States available at a cost less than $60/ton of dry biomass. Map colors show the available tons of dry biomass (dt) per square mile of area. From US Department of Energy 2016, Figure ES4.

Since biomass tends to be around 50% carbon (waste like sewage can be much less, however), 1.5 billion tons would represent about 750 million tons of carbon. In the form of the carbon dioxide that was pulled from the atmosphere to make the biomass, this would be about 1.5/2 * 44 / 12 = 2.75 billion tons of carbon dioxide (remember that carbon and carbon dioxide have molecular weights of 12 and 44, respectively). While it is infeasible to assume that all of that biomass could be collected, and that energy crops could achieve their maximum contribution with no impact on other important aspects of society, it is clear that biomass could be a very real contributor to US negative emissions.

In countries with significant biomass-based industry like the timber harvesting in Sweden and Finland, there may be significantly more op-tions for negative emissions using existing resources. These countries get a significant amount of their energy supply from wood waste today, and capturing the CO₂ from that combustion could be a very substantial component of their greenhouse gas activities, by some estimates making both countries carbon negative overall. Europe is investigating using its trash for negative emissions in a project called Northern Lights, where the CO₂ from the trash-burning facility in Oslo, Norway, will be captured for true negative emissions, expected to come on line in 2022.

Some plans for negative emissions involve growing additional energy crops, such as switchgrass or poplar trees, entirely for the purpose of bioenergy with carbon capture and storage. These plans face serious criticism today because of the competition for land and water, and the impact on food supplies. Most current assessments conclude that the most obvious sources of biomass are those that are thrown away today. However, the future need for negative emissions may require us to evaluate whether additional biomass resources can be brought to bear on the problem without undesired consequences.

Direct capture of carbon dioxide from the atmosphere

Carbon dioxide can be removed from the air with strong chemicals like sodium hydroxide (known as caustic soda, or lye) and liquid amine or ammonia solutions. Both of these work because they are chemical bases, while CO₂ is an acid. The stronger the base, the more reactive it is with the acid carbon dioxide. Carbon dioxide is not an acid until it dissolves in water, at which point the reaction occurs to create carbonic acid (\(\ce{H2CO3}\)), a weak acid:

\[\ce{CO2 + H2O ↔ H2CO3}\]

Carbonic acid, \(\ce{H2CO3}\), can give up a proton (\(\ce{H+}\)) to react with a base like sodium hydroxide, \(\ce{NaOH}\), yielding water and two new ions in solution, and releasing heat (chemical energy):

\[\ce{H2CO3 + NaOH ↔ H+ + HCO3 − + Na+ + OH− ↔ H2O + Na+ + HCO3 −}\]

A solution of sodium hydroxide will spontaneously absorb carbon dioxide by reactions (1) and (2), releasing heat and heating the solution or surrounding air. The solution will only contain the air’s carbon dioxide, and not all the other gases—oxygen, nitrogen, argon, etc.—that were mixed with the carbon dioxide. If you heat that solution—add back into the system the heat that was released in dissolving the CO₂ (plus a little extra heat, of course; no chemistry is free!)—pure CO₂, plus some water vapor, will bubble out of the solution. This way of making pure CO₂ has been known for more than 100 years and was used to make dry ice and carbonated drinks before other sources of CO₂ became available from industry. If you use amines or ammonia in place of sodium hydroxide, a similar reaction occurs.

These reactions are being used to harvest CO₂ from the atmosphere in experimental systems we call direct air capture, or DAC. Today they are relatively expensive to operate because of the heat that has to be added to the system to release the CO₂ in pure form, and because the systems need to be large to harvest significant amounts of CO₂ from air. In this book, we are very worried about CO₂ at a concentration of 415 parts per million (ppm) because of its blanket effect, but to a chemical engineer, 415 ppm is a very low concentration and requires large, expensive machines to both contact the air with the solution to catch the CO₂, and process that solution once it is enriched in CO₂.

A variety of schemes are being tested today to examine whether this direct air capture can realistically be used to remove CO₂ from the air. Most estimates place the current cost at around $600/ton of CO₂ removed, with the possibility in the future of decreasing to $300 to $100/ton. Barring dramatic breakthroughs, direct air capture will always be more expensive than the biomass-based systems we previously discussed. But an advantage of direct air capture systems is that they are only limited by the amount of space we are willing to allocate to their operation and the amount of carbon-free energy we can supply to run them (obviously you cannot use something like coal-fired electricity to power such an endeavor, or you will emit more CO₂ than you catch).

Jennifer Wilcox has evaluated the amount of energy needed for direct air capture systems, and it is significant. Today’s methods require about 250 megawatts (MW) of power supply to remove CO₂ at a rate of 1 million tons per year. A 250 MW solar farm is among the largest built today. The area required for a direct air capture facility would predominantly be for the energy production, and not for the actual capture devices (like those in Figure 18.2.6). For scale, the Topaz Solar Farm in California takes up 7.3 square miles of land and generates 550 MW of peak power. Considering intermittency (the solar farm produces no power at night), this would be about what is needed for 1 million tons of direct air capture per year. While it is certainly possible to allocate large areas of land for renewable power for direct air capture, this will be a major land use challenge. Today direct air capture methods are being studied and seriously evaluated, even though their large-scale operation may be many decades in the future. If we are lucky and work hard at other options, we may not need direct air capture to meet our climate goals, but if we need it in 2040 or 2050, it will be too late then to start developing it.

Figure 18.2.6 The Climeworks direct air capture facility near Zurich, Switzerland, uses 18 modular capture units to remove CO₂ from the air. The purified CO₂ is then used to enhance the growth of vegetables in the greenhouses in the background—not really negative emissions yet, but a demonstration of the costs and issues associated with separating CO₂ from air. This facility catches about 1,000 tons of CO₂ per year. Photograph by Roger Aines.

Carbon mineralization

Next, a brief mention of the mechanisms that the Earth uses to control CO₂ in the atmosphere, and how we might speed them up. Limestone, or calcium carbonate, \(\ce{CaCO3}\), is the most stable solid form of CO₂ in the earth. Its stability is attested by its use to construct buildings, particularly beautiful facades. But even as the most stable solid form of a CO₂-containing substance, it will still dissolve slowly in rainwater and turn into bicarbonate, \(\ce{HCO3 −}\), in solution, which is even more chemically stable than limestone and is one of the most important ions in seawater. Marine organisms use bicarbonate to form their shells and solid structural elements (like coral).

Bicarbonate and calcium carbonate come from the weathering of rocks containing calcium. These rocks tend to come from deep in the earth and are brought to the surface by volcanism (in basalt like that found in Hawaii) or faulting and plate tectonics, which can bring up large slabs of rock from deep in the earth called ultramafic rocks. Some examples are shown in Figure 18.2.7. These rocks are dissolved readily by seawater for the same reason that CO₂ is absorbed by a sodium hydroxide solution—the calcium dissolves to form calcium hydroxide, which reacts by the same mechanisms as sodium hydroxide (see equations [1] and[2]) forming calcium ions and bicarbonate in rivers that empty into the ocean. There the bicarbonate builds up until marine organisms like corals precipitate it into their homes and bodies, which eventually turn into limestone rock, permanently storing the CO₂. This natural cycle of CO₂ in the air reacting with rocks, forming calcium and bicarbonate ions that travel to the ocean in rivers, where they eventually precipitate into solid calcium carbonate shells and skeletons that accumulate on the ocean bottom and turn into limestone rock, has been the primary control on the average amount of carbon dioxide in the atmosphere throughout time. (This process does not acidify the ocean, because the acidity of the CO₂ was neutralized by the base in the rock—ocean acidification occurs when CO₂ in the air dissolves directly into the ocean and, as in equation [1], turns into carbonic acid.)

Figure 18.2.7 Lawrence Livermore National Laboratory scientists examine ultramafic rocks in Washington state. These rocks are rapidly reacting with CO₂ and breaking down to calcium and bicarbonate ions, leaving clays and unreacted blocks of rock behind. This site is a landslide—as the rocks break down, they lose structural integrity and slide downhill. Photograph by Roger Aines.

Researchers are examining whether this process can be speeded up, either by circulating water through rocks and dissolving the calcium or by grinding up calcium-rich rocks and reacting them with air and rainwater. This is an attractive approach because it mimics the processes already active in the earth and, most importantly, uses very little added energy because the reaction of CO₂ with dissolved calcium hydroxide actually releases energy (heat). There is no need to heat the solutions up again, as the direct air capture facilities must do to recover pure CO₂, because in this carbon mineralization or enhanced weathering approach the CO₂ forms either solid calcium carbonate or dissolved bicarbonate like that already in the ocean.

Much needs to be worked out before the benefits of this approach can be estimated, but since ultramafic rocks are found in a wide variety of locations, including California, it is worth pursuing. Current estimates are that a process like this is less expensive than direct air capture and could be quite inexpensive. Since it is still quite uncertain, we estimate that the costs would be from $50 to $200 per ton removed, and the capacity would be several billion tons per year.

Ocean carbon uptake

Finally, a brief mention of one of the earliest ideas for removing carbon dioxide from the air: enhancing the primary productivity of the oceans. In this approach, fertilizer would be applied to encourage the growth of plankton, which upon their death would sink to the abyssal depths of the ocean where carbon is out of contact with the atmosphere. This approach has proven to be difficult to experimentally test, because of limitations imposed by international treaties and also public opinion objecting to addition of the fertilizer components, like iron, to the ocean. However, the extraordinary size of the oceans, and therefore the amount of CO₂ that could be absorbed by this method, makes this an interesting option to continue evaluating.