The Lowdown on CCS

Part 2: Use and Sequestration

In Part 1 last year, I described five categories of approaches to carbon capture: 

1. flue gas scrubbing; 
2. pipeline-ready CO₂ capture; 
3. direct air capture; 
4. photosynthetic capture; and 
5. hydrological capture. 

Capture, however, is only half the story. Once the CO₂ has been captured, what’s to be done with it? As with capture, there are many options for use / sequestration.

Ocean Storage

Historically, ocean storage was one of the first options considered for large scale carbon sequestration. With an estimated 37,000 gigatons of stored carbon, the deep oceans constitute by far the largest carbon reservoir in the global carbon cycle. 37,000 gigatons is 46 times larger than the atmosphere’s current 800 gigatons, and 67 times the estimated 550 gigatons of standing biomass. 

Ocean storage is also easy to access. No need to drill injection wells thousands of feet into porous sedimentary rock formations. As anyone who has popped the tab on a can of soda knows, the solubility of CO₂ in water increases with pressure. Water containing a high content of dissolved CO₂ is heavier than water with a lower content. So all that’s needed to store CO₂ in deep ocean waters is a pipe carrying pressurized CO₂ to a depth where it can fully dissolve in a descending column of water. 

The scientific community, by consensus, now rejects this type of ocean storage for captured CO₂. The problem is that simple dissolution of CO₂ in deep ocean waters, in the absence of other actions, would lower the pH --  raise the acidity -- of those waters. Once the water loaded with dissolved CO₂ had become well mixed with deep ocean waters, the increase in acidity would be very small. However, within the plume of descending seawater around the injection pipes, it would be substantial. When that plume reached the seafloor and spread out, it could potentially be very disruptive to benthic sea life. We really don’t know, but the potential for unintended consequences is high.

That’s not to say that there’s no safe way to sequester CO₂ in ocean waters. The risk from unknown impact on sea life applies to direct injection of gaseous CO₂ into deep waters. The potential impact is from the reduction of pH -- increased acidity -- in waters with high dissolved CO₂ gas content. But weathering of rocks has been slowly adding alkalinity to ocean waters for billions of years. The added alkalinity draws in a balancing amount of CO₂ from the atmosphere. In fact, one of the more promising options for CDR, as mentioned in Part 1 last year, is to accelerate the addition of alkalinity to ocean surface waters. The entire ocean surface then serves as a vast “air contactor” for CO₂ removal. 

The activity coefficients for the reactions involved in ocean alkalinity are such that roughly 0.9 moles of CO₂ will (eventually) be removed from the atmosphere for every mole of alkalinity added. The removed CO₂ is safely stored in the form of bicarbonate ions (HCO₃⁺). It becomes part of the ocean’s vast load of “dissolved inorganic carbon” (DIC). The remaining 0.1 moles of added alkalinity go toward raising the average pH of upper ocean waters. That’s a helpful and non-controversial thing for ocean shellfish, corals, and calcareous phytoplankton -- lifeforms that are increasingly endangered by ocean acidification.

Biomass

Biomass, produced by photosynthetic capture of CO₂, brings a broad spectrum of options for utilization or storage. On one end of the option spectrum are regenerative agricultural practices. Regenerative agriculture aims to reverse the loss of topsoil that tends to accompany large scale industrial agriculture. Thick topsoil, among other things, is an important carbon reservoir. It stores carbon in the form of nutrient-rich humus, partially decomposed organic matter, and root mass. Durable carbon in the form of biochar can also be added to amend the soil. The addition sequesters carbon while boosting the soil’s capacity for retention of water and nutrients. 

A related option for sequestration of carbon in biomass is to replace shallow-rooted annual crops with deep-rooted perennials. The deep root systems increase soil carbon, while the woody parts above ground contribute to the global inventory of standing biomass. The ultimate for this approach is tree planting. When trees are planted on sparsely vegetated scrub or grassland, CO₂ is drawn from the atmosphere into tree growth. It remains sequestered for at least the life of the trees.

The duration of sequestration can be extended when mature trees are harvested and used in construction. Though small in relation to the amount of carbon that needs to be captured and sequestered, the amount of carbon that can be sequestered in the human built environment is potentially significant. It isn’t just a matter of the wall studs, floor joists, and plywood we use in residential construction. Surprisingly, when used in commercial building construction, modern glu-lam beams and cross-laminated panels can outperform steel reinforced concrete (q.v.). First employed widely in Europe, laminated timber is now seeing growing acceptance and popularity in the U.S. and Canada. This year, a 25-story timber skyscraper is set to open in Milwaukee, Wisconsin. There is a double benefit from this type of construction: it sequesters carbon in the structure, while avoiding the carbon cost of the concrete and steel production that it replaces.

When biomass is harvested for use as fuel (or for making fuel), the carbon story is not so straightforward. In theory, fuels made from biomass are “carbon neutral”, since the carbon released when they are burned was previously taken from the atmosphere by photosynthesis. Biomass burned as fuel can substitute for fossil carbon fuels, and in that sense can reduce long term carbon emissions. But it’s still moving carbon from a reservoir where it had been temporarily sequestered, and releasing it back into the atmosphere. The issue is further complicated by the fact that both the rate of net production of biomass and the amount of carbon stored peak much later in the development of forest ecosystems than the point at which harvesting is usually performed [q.v.]. Harvesting before carbon storage has peaked has a negative effect on sequestration of atmospheric carbon.

Advocates for biofuels will be quick to point out that trees are not the only option for biofuel. And probably not the best option. But even the most productive crops for biofuel production capture only about a tenth as much solar energy, per acre of land occupied, as solar panels. They do have some advantages, however. Capital cost can be very low, they can sometimes be grown on land that is too arid or nutrient-starved to be used for growing food, and storage of the energy they capture is built in. They can be used as a source for firm power generation to back intermittent renewables.

Synthetic fuel production

Given enough energy, captured CO₂ can be combined with hydrogen to synthesize hydrocarbon fuels. This merits a special category in the lineup of use / sequestration for waste CO₂: aside from enhanced oil recovery, it’s the only utilization option for captured CO₂ that could scale sufficiently to make a difference. 

Hydrocarbons can be synthesized from mixtures of H₂, CO, and CO₂ via the Fischer-Tropsch (FT) process. It’s well-established technology, famously employed by Nazi Germany in World War II when the Reich was cut off from oil supplies. The synthesis gas that they employed was produced from coal by steam reforming. Electrical energy was only needed to operate fans and compressors. But if cheap electrical energy is available, synthesis gas for the FT process can be made from CO₂ and electrolytic hydrogen. Or, with less electrical energy, it can be made from CO₂ and “blue” hydrogen. The source of hydrogen is irrelevant to the synthesis process.

Synthetic fuel production could, in principle, consume 100% of the world’s captured carbon emissions. There are no limits on its scalability, other than the amount of hydrogen produced to drive it. If scaled to the ultimate level, annual production of synthetic fuels could equal the total amount of hydrocarbons burned annually. Transportation would be able to keep running on gasoline and diesel indefinitely.

Scaling synthetic fuel production to that ultimate level is neither necessary nor desirable. It would be quite inefficient. Direct air capture would be needed for the bulk of the CO₂ that synthesis operations would consume. The cost of new clean energy and chemical plant infrastructure that would have to be built would dwarf whatever could be saved by allowing transportation to avoid electrification. 

Nonetheless, for the CO₂ that is easily captured, fuel synthesis is a viable utilization. It’s not long term sequestration, since the fuel produced will subsequently be burned. But it does replace its equivalent in fossil fuel. For long distance air travel and other applications that require the high energy density of hydrocarbon fuels, it offers a carbon neutral alternative to newly extracted and refined fossil fuels. Whether it would be economically competitive with other alternatives is a more difficult question to answer.

Enhanced Oil Recovery (EOR)

Injection of compressed CO₂ into an aging oil field is a well-established way to stimulate oil production from the field. It increases the fraction of the reservoir’s original oil-in-place (OOIP) that is ultimately recoverable. At high pressure, supercritical CO₂ mixes with oil; the two form a low viscosity fluid that flows easily through the pore spaces of the reservoir rock to a collection well. CO₂ injection in effect scrubs oil from the pore spaces of the formation and leaves large volumes of supercritical CO₂ behind. 

There’s a common misconception about the use of CO₂ for EOR. Because EOR increases the amount of oil that can be recovered from oil fields using it, a natural assumption is that it leads to a net increase in oil production. That’s incorrect. World oil consumption is demand-limited, not supply limited. The ability to get more production from existing fields simply undercuts the need for new production. It allows oil producers to curtail exploration and development (E&D). The CO₂ used for EOR remains sequestered in the field, while the environment is spared the access road cutting and new well drilling that E&D entails. There’s also a bonus in reduced oil demand, as E&D activity itself accounts for a noticeable percentage of world oil consumption.

Other Geological Sequestration

Global emissions of CO₂ are currently greater than the amount that EOR can profitably utilize. CO₂ that is captured but not needed for anything more valuable must simply be sequestered. That generally means pumping it into a suitable long term geological reservoir. Two types of formation are widely available: depleted oil and gas reservoirs and deep saline aquifers.

Depleted oil and gas fields

Even after old oil and gas fields have become too depleted to be candidates for EOR, they remain good candidates for CO₂ storage. The fact that they held oil and gas over a span of millions of years demonstrates that they’re suitable: porous sedimentary rock formations below an impermeable cap layer. CO₂ pumped into the formation can be expected to remain there for geological time. 

That’s of course provided that gas is not piped in with enough pressure to fracture the cap layer, and that injection wells that pierce the cap layer are properly sealed. Best practices must be followed, and formations used for carbon storage must be monitored. However, CO₂ is not radioactive waste, and porous rock is not something that any fluid will flow through quickly. If a formation used for carbon storage were to leak, it would generally not be a disaster.

Deep saline aquifers

Deep saline aquifers are the other option for long term sequestration of CO₂. “Deep”, in this context, means 800 meters or more below the surface. At that depth, the hydrostatic pressure of water in the pore space is above the critical pressure of CO₂. The solubility of CO₂ within the brine is high. Provided the injection depth is a few hundred meters below the top of the aquifer, CO₂ can be safely sequestered even if there is no impermeable cap layer above the saline aquifer to seal it off. The reason is that injected CO₂ will become fully dissolved in brine within the aquifer before it can rise to the top and escape. Brine saturated with pressurized CO₂ is denser than non-carbonated brine. It will slowly seep to the bottom of the aquifer, carrying its load of CO₂ along with it.

There’s an interesting alternative to direct injection of CO₂ into the aquifer. In certain cases, permanently sequestering CO₂ could also produce useful amounts of geothermal energy. The geothermal potential stems from the high temperatures encountered in geothermal brines. At a depth of 2000 m below the surface, ordinary rock temperature is typically about 75 ℃ (167 ℉).  At 3000 m, it’s around 100 ℃.That’s for crustal rock far removed from geothermal hotspots. Near hotspots, deep pressurized brine will be much hotter than 100 ℃.

An extraction well could draw brine from that depth and use its heat enthalpy in various ways. One would be to drive organic Rankine cycle turbines for power generation. Another would be for production of fresh water through thermal desalination of seawater. Yet another would be for regeneration of CO2 sorbent solutions in large-scale flue gas scrubbing or direct air capture. 

Whatever the use, once its heat energy had been spent, the cooled brine would be recycled back to the deep aquifer. But first it would be mixed with CO₂. As the foamy mix of gaseous CO₂ and brine descended into the injection well, hydrostatic pressure would rise. By the time the solution reached the injection depth to reenter the aquifer, the CO₂ would be fully dissolved into the brine. The solution would be denser than the surrounding aquifer brine; it would naturally diffuse downward, eliminating any possibility of leakage.

Choices ahead

The IPCC has warned that global carbon emissions are not on track to achieve the 1.5 ℃ cap on global warming that the Paris accords were intended to achieve. Far from it. Not only are global CO₂ emissions not dropping at the rate necessary to cap atmospheric CO₂ at safe levels, they’re moving in the wrong direction. They’re still rising. 

At this late date, major overshoot is guaranteed. It won’t be sufficient for humanity to reduce global carbon emissions to zero. To avoid the worst effects of extreme warming, we will have to get to zero as quickly as possible and then go on to negative emissions. I.e., we will have to begin actively removing CO₂ from the atmosphere. The sooner we can get to zero, the less excess CO₂ we will subsequently have to find a way to remove from the atmosphere.

One way or another, we will have to embrace large scale CCS. But it need not be done all at once, nor involve all of the available approaches. We can choose the approaches that will deliver the “most bang for the buck” and start implementing those first. The most bang for the buck, in terms of carbon capture, would be those that produce pipeline-ready waste CO₂ streams. By the time those have been mostly exploited, it’s likely that one or both of the new flue gas scrubbing technologies that I referenced in Part 1 will have been commercialized. That should make it practical to capture CO₂ emissions from existing fossil-fueled power plants.

For sequestration, the options present a more complex “bang for the buck” picture. There is no single option that stands out as the most cost-effective starting point. Economics for particular options will vary depending on characteristics of locations. Nonetheless, the opportunity to combine CO₂ sequestration in deep saline aquifers with geothermal power generation should place that option high on the list of choices.