Carbon Capture: Part 4
By: Carly Anderson
Tags: Carbon Neutral, New Technologies, Carbon Capture, Chemical Engineering

Carbon Capture: Part 4

The next two posts review carbon capture technologies that are available now or are in development. This post focuses on technologies that capture CO2 from point-sources: emissions from power plants, chemical plants, cement production, etc. The last post will highlight technologies to capture CO2 directly from the air.

This list is not exhaustive, and some classes of technologies can be used to remove CO2 from either point-source emissions or air. Our goal is to highlight and explain some of the major approaches in this area.

Overview of Technologies

Before diving into specific technologies, here is a brief overview of carbon capture approaches that have seen significant traction. The most mature technologies are liquid absorption processes that are similar to the CO2 removal systems used in the oil and gas industry. The gas containing CO2 is bubbled through a liquid (called a “solvent”), which absorbs CO2 and lets the rest of the gas pass through. Many liquid-based technologies use a family of chemicals called “amines” that react strongly with CO2. Other liquid-based approaches use liquids that dissolve the CO2 but don’t chemically react with it (physical solvents).

Advances in membrane technologies over the last decade have led some teams to include membranes in their carbon capture designs, particularly for gases with high CO2 contents. Membranes are thin sheets of material which only allow certain gases to pass through, in this case CO2.

Some companies combine carbon capture and storage with mineralization technologies. In these technologies, CO2 is permanently converted to a mineral: limestone (a component of concrete), baking soda, or other useful inorganic products. These technologies avoid many of the challenges of gas pre-cleaning and CO2 purification, but may be limited by the demand for the final product.

The next post will discuss other technologies that utilize solid materials called “adsorbents” (note the “ad-” for solids vs “ab-” for liquids). These materials trap and later release CO2, like a rechargeable battery. Solid materials can be used in both point-source and direct air capture technologies. A final approach that has been used to capture CO2 from air uses water with chemicals that convert CO2 into a salt to make carbonates. This carbonate formation approach uses the same chemistry used in mineralization processes, but produces pure CO2 as the final product.

Liquid Absorption with Amines

The oil and gas industry has been using liquid “amine scrubbers” to remove the CO2 present in natural gas coming up from oil and gas fields since the 1930s. The process of capturing CO2 from emissions is not the same as removing CO2 from natural gas for several reasons. The pressure of the natural gas is 20–100 times higher and different contaminants are present. Still, much of the engineering and equipment design is the same, and this is by far the most mature carbon capture technology. A large number of companies including oil majors (Equinor/Statoil, Shell), established technology providers (e.g. Fluor), and startups (ION Engineering, Carbon Clean Solutions) are pursuing this approach to capture CO2 from point-sources like cement, coal and natural gas power plants.

Liquid amine scrubbing systems remove CO2 by bubbling the gas through water that contains 20–50% amine, a liquid chemical that strongly binds to CO2 but not other molecules. The gas leaving the top of this “absorber” is essentially CO2-free, and the liquid (with the amine and trapped CO2) is pumped to a separate large metal tower known as the CO2 “stripper”. The stripper heats the liquid water-amine-CO2 solution to force each amine to release its CO2 molecule. The liberated CO2 gas (plus some water) leaves the top of the stripper, and the liquid amine-water is recycled back to the absorber to trap more CO2.

Basic overview of a liquid absorption process

To bring down carbon capture costs, companies using liquid-based processes try to create better liquid solvents that 1) absorb more CO2 per liter of solvent, 2) react with CO2 faster, or 3) produce less heat when they react with CO2. By trapping more CO2 per liter of liquid solvent, the equipment can be smaller, reducing the upfront capital cost. Heating the water-amine-CO2 liquid to release CO2 also uses a large amount of energy, so reducing the amount of liquid needed reduces the energy consumed (a primary driver of operating costs). It is also important that the amine solvent doesn’t degrade over time.

An example of metal “packings” / Source: Joeravo (Creative Commons)

Often, the amount of CO2 absorbed by the liquid is limited by how fast the CO2 reacts with it, rather than how much CO2 the liquid can theoretically hold. To make the CO2 react faster, extra chemicals can be added, or the absorber can be filled with more elaborate shapes, bars, or other forms to mix the gas and liquid better. Some common absorber “packings” look like metal packing peanuts or steel wool. Improving absorber packing design and materials for liquid-based carbon capture is an important area of technical innovation.

To make things even more complicated, CO2 gives off heat when it reacts with amines: this heat is called an “exotherm”. At higher temperatures, less CO2 can be absorbed (remember that the liquid is heated to release CO2.) Finding ways to cool the liquid during CO2 absorption or otherwise manage the heat released is another important area for process optimization.

Companies active in this area include CanSolv (Shell), Mitsubishi Heavy Industries (MHI), ION Engineering, Carbon Clean Solutions Limited, Stanford Research International (SRI), and the Gas Technologies Institute (GTI)

Liquid Absorption with Physical Solvents

The processes for liquid absorption of CO2 with physical solvents vs amines are very similar. However, physical solvents dissolve the CO2 without a chemical reaction. As a result, less heat is released when CO2 is absorbed, and less heat is needed for physical solvents to release CO2 in the stripper.

The trade-off is that physical solvents are generally less selective for capturing CO2 — they also dissolve other gases (e.g. nitrogen and methane) at low levels. For gases with high CO2 contents (>15%), physical solvents may offer a less energy-intensive approach. Examples of physical solvents for CO2 removal include Rectinol and Selexol, which were developed to remove CO2 from natural gas but have seen limited adoption in the industry.

Membrane Systems

Membranes are also being explored to capture CO2 from gases with a high CO2 content. In a membrane separation process, gas is fed to one side of a long sheet of membrane material (typically wound into a cylindrical tube). As the gas travels along the membrane, some components of the gas (in this case, CO2) pass through the membrane and exit through a separate “permeate” outlet, while the remainder exits at the end of the tube (the “retentate” outlet).

An advantage of membrane systems is that they are easy to scale up or scale down for different applications. Unlike liquid absorption systems, which need to be redesigned for each new facility, the size of a membrane facility can be increased by adding more membrane cartridges. The disadvantages of membrane systems are that they are prone to fouling (dirt and contaminants build up), and the level of CO2 purity achieved by a single membrane is often low (so a series of membrane modules might be needed).

Companies active in this area include: Membrane Technology Research (MTR) and the Gas Technologies Institute (GTI).

Mineralization

CO2 can also be converted into minerals including sodium bicarbonate (baking soda, NaHCO3) and limestone (CaCO3), a common ingredient in concrete. In these processes, CO2 is first dissolved in water to make carbonates. The carbonate + water solutions then undergo a “mineralization” step, where the liquid is cooled slowly. As the solution cools, the carbonate in solution “un-dissolves” and forms solid rocks around a starter material (similar to the process of making rock candy). The mineralization process can be adjusted to make rocks of different sizes, from powders to small boulders. This material can be used to make concrete, which is a mixture of rocks, sand, and cement (cement acts as the glue to hold everything together).

Rock candy, another mineralization process / By Evan Amos

Concrete and cement are controversial products because the process of making cement is highly CO2 intensive (1 tonne of CO2 is emitted for every tonne of cement produced). Companies taking this approach cite increasing demand for concrete and cement, and the need for less carbon-intensive cement (or cement-like) materials. Blue Planet claims that each ton of CO2-sequestered limestone (not cement) traps 440 kilograms of carbon dioxide.

Companies active in this area include: Blue Planet, Carbicrete, C-Crete, Carbonfree Chemicals, Solidia, CarbonCure

Current State of the Art: Point-Source Carbon Capture

Today a handful of first-of-a-kind, commercial-scale carbon capture plants have been built for point-source CO2 capture, generally using liquid amine technologies. Examples include NRG’s Petra Nova coal retrofit project in South Texas, Shell’s Quest project in Saskatchewan, and Norway’s Snohvit project in the Barents Sea. The installed costs of the amine systems themselves are typically in the range of $80- $100/mt CO2. The Boundary dam plant, which uses Shell’s CanSolv amine technology, is able to capture 1mm mt/yr of CO2. For several reasons, the project went over budget, and the long-term economics were hurt by falling natural gas prices. [1] Currently, NRG’s Petra Nova project is the largest operational carbon capture facility in the US, capturing over 1mm mt CO2/yr for EOR. [2] Norway has ambitious plans to develop a CO2 transportation and sequestration hub for Europe under the “Northern Lights Project”, building on Equinor and other North Sea operators’ extensive experience reinjecting the CO2 produced from oil & gas production.

In the US, a large number of additional point-source pilot and demonstration projects are moving through the pipeline, supported by DOE funding from the past 10–15 years. [3] The companies developing these technologies estimate costs of $40–60/mt CO2 for point-source capture from a coal-fired power plant at commercial scale (4mm mt CO2/year) [4] and an expected time to commercial deployment of roughly 3–7 years if partners are found and project economics are favorable.

Many organizations receiving DOE funding for point-source carbon capture are research institutions or early stage companies that are not in growth mode. One exception is Svante (formerly Inventys), a Vancouver-based company that recently completed a 10,000 mt/yr pilot system at a Husky oil sands facility in Alberta, Canada. A possible larger project with Occidental Petroleum (Oxy) and Total to capture CO2 from a Colorado cement plant is in the feasibility study stage.

The previous post in this series covers major applications for carbon capture, and key metrics for evaluating carbon capture technologies. The next and final post in this series will continue to discuss carbon capture technologies, with a focus on direct air capture.

Carbon capture pilot system at the Imperial College London / Source: Mm907 (Creative Commons)

Notes