In my previous post, I talked about three of the most promising carbon capture technology systems that could be used to generate electricity from coal while separating out a purified carbon-dioxide stream, reducing the carbon-dioxide emissions of a coal power plant significantly (in the ballpark of a 90% reduction). However, these technologies only solve half of the technical challenge of carbon capture and sequestration (or CCS for short). What to do with the massive amounts of captured carbon-dioxide is still unclear. In this post, I will take a look at three of the most promising technological options for long-term storage of carbon-dioxide that isolate carbon-dioxide from the atmosphere for very long time periods (on the order of hundreds to thousands of years).

Carbon-Dioxide Sequestration Technologies

1. Geologic sequestration

Geologic sequestration is the injection and long-term storage of carbon-dioxide in deep geologic formations in the earth’s upper crust, primarily porous and permeable rock bodies at depths around 1 kilometer below the surface. At such depths, the pressure and temperature of the environment puts carbon-dioxide into its supercritical phase where it has similar viscosity and density to oil.

1.1. Geologic sequestration in saline aquifers

One of the most promising types of geologic formation for carbon-dioxide sequestration (in terms of natural ability to trap carbon-dioxide and availability in needed regions) are saline aquifers. Saline aquifers are underground reservoirs that contain brine in their pore volumes and have an impermeable layer of rock above the reservoir chamber. Over short time scales, injected carbon-dioxide spreads through the reservoir as an underground plume, and over longer times scales, the injected carbon-dioxide dissolves into other fluids in the reservoir (particularly the salt brine) and eventually may precipitate as a mineral. According to the Department of Energy’s National Energy Technology Laboratory, in North America alone, saline aquifers may be able to store between 1.3-3.0 gigatonnes of carbon-dioxide. However, there remain significant unanswered questions about geologic sequestration in saline aquifers; for example it is still unknown how permanent sequestered carbon-dioxide will remain trapped over very long time scales.

1.2. Geologic sequestration with enhanced oil recovery

In addition to injection in saline aquifers, carbon-dioxide can also be sequestered in mature oil fields via a process called “enhanced oil recovery,” or “EOR.” EOR using carbon-dioxide injection, first demonstrated in the early 1970s, is a relatively mature technique that can make significant additional quantities of oil in a reservoir recoverable. Injecting a gas like carbon-dioxide into an oil reservoir causes the gas to expand underground and push additional oil towards an extraction well. In some cases, EOR has added as much as 25 years to the life of an oil field. EOR is, on the one hand, a promising avenue for carbon sequestration because it provides positive benefit to the firm storing carbon dioxide (in the form of additional recovered oil that can be sold); but on the other hand, EOR has only limited potential in terms of scalable quantity of carbon-dioxide storage capacity. Therefore, while carbon-dioxide sequestration through EOR provides a positive incentive for private actors to sequester carbon-dioxide, EOR will not be a long-term feasible option for carbon sequestration because there simply isn’t enough EOR capacity in the geology of the planet. Looking to the future, EOR may play a crucial role in getting the first CCS plants off the ground and operating profitably. In my next post I will take a look at the economics of CCS, but I will mention briefly here that the potential for EOR would allow CCS plant operators to profit from the carbon-dioxide they capture (by allowing for the extraction of additional oil), and this additional revenue may make the crucial difference between a CCS plant’s overall profitability and its unprofitability.

2. Ocean sequestration

Today, the world’s oceans contain about 50 times as much carbon-dioxide as the atmosphere and naturally remove about 7 billion tons of carbon-dioxide from the atmosphere every year. However, carbon-dioxide could also be deliberately injected into the ocean for long-term sequestration. Ocean sequestration of carbon-dioxide consists of injecting carbon-dioxide into the ocean at great depths, and because the overturning of the deep ocean is a very slow process, the injected carbon-dioxide would be sequestered from the atmosphere for several centuries. The deep oceans provide a nearly limitless sink for carbon-dioxide, but the potential risks of ocean sequestration are large. For example, it is likely that injected carbon-dioxide will harm marine life and may damage ecosystems.

3. Carbon-dioxide mineralization

Carbon-dioxide mineralization is the process of reacting carbon-dioxide to form a solid byproduct, such as a silicate. Such a byproduct would be stable and could be stored for very long time scales. Carbon-dioxide mineralization is seen in nature in weathering processes (e.g. the White Cliffs of Dover); however this process is kinetically very slow. Therefore, in order to speed up the mineralization reaction to be relevant for CCS systems, large quantities of external energy must be used, making the process very expensive. Currently, the cost of carbon-dioxide mineralization is the primary barrier to the technology’s deployment.