Carbon Capture and Storage: Why and How?

Available CO₂ Capture Technologies

Regardless of the source, CO₂ capture can be achieved through several technologies like absorption, adsorption, membrane, cryogenic and biological processes.

In the absorption process, CO₂ capture can be achieved by either using physical solvents or reactive organic chemical solvents (amine solvents). Physical solvents absorb CO₂ through physical affinity with CO₂. Commercial physical solvents include dimethylethers of polyethylene glycol (Selexol), propylene carbonate (Fluor solvent), cold methanol (Rectisol) and N-methyl-2-pyrolidone (Purisol).

However, CO₂ capture using reactive chemicals, known as amine solvents, is the most mature and applied technology when compared to other technologies (adsorption, membrane, cryogenic and biological processes). This is because it is capable of more than 90% CO₂ capture from a low- and high-pressure feed gas and produces high purity CO₂ (≥ 98%).

Amine solvents absorb CO₂ by chemically interacting with CO₂ to form ionic species. This chemical reaction also takes place when other acid gases are present in the feed gas like carbonyl sulphide (COS), carbon disulphide (CS2), hydrogen sulphide (H₂S), sulphur dioxide (SO₂) and sulphur trioxide (SO₃), nitric oxide (NO) and nitrogen dioxide (NO₂).

From Figure 1, which shows the process flow diagram of a typical CO₂ capture plant, note that the feed gas containing CO₂ (25 ^oC to 60 ^oC) from an upstream process unit is fed to the absorber (contactor) bottom at the desired flow rate. The feed gas can be a high-pressure gas (natural gas, liquefied petroleum gas, etc.) or flue gas (combustion and/or non-combustion sources) that comes at atmospheric pressure. Though the flow rate of the feed gas is mostly constant, operational challenges, such as foaming, can prompt the CO₂ capture plant to run at reduced gas production rate. In such a scenario, a valve installed on the feed gas line will serve the purpose of controlling the gas flow rate.

As the feed gas flows upward through the absorption column it counter-currently meets the CO₂-lean amine solution flowing downwards from the top. The absorption column is fitted with either trays (sieve tray, valve tray, etc.) or packings (random packings, structured packings) that provide the effective contact area for the amine solution and the feed gas. Mass transfer takes place and CO₂ in the gas phase is absorbed by the amine solution by chemical reaction forming ionic species like carbamate (AmineCOO–) and/or bicarbonate (HCO₃–), etc.

The CO₂-lean gas leaving the top of the absorber is condensed to recover any amine or water (H₂O) carryover. The recovered liquid is refluxed back to the absorber top (slightly above the CO₂-lean amine entrance tray or packing) and a control valve controls the flow rate and liquid level in the absorber overhead separator.

For high-pressure-gas stream (natural gas, liquefied petroleum gas etc.), the treated gas is sent to downstream gas dehydration unit. On the contrary, for feed gas at slightly above atmospheric pressure (flue gas, etc.), the treated gas is flared.

The amine solution saturated with CO₂ (CO₂-rich amine solution) exits from the bottom of the absorber. A control valve located at the bottom exit of the absorber (Figure 1) controls the flow rate of the CO₂-rich amine solution entering the flash drum or cross-exchanger. This control valve also maintains the liquid level in the absorber bottom (liquid sump).

For high-pressure feed gas, the CO₂-rich amine solution is routed to the flash drum to reduce the pressure of the amine solution prior to entering the cross-exchanger. At the bottom exit of the flash drum, a control valve is installed to control the flow rate of the amine solution entering the cross-exchanger and to control the liquid level in the flash drum. The flash drum also helps to remove any absorbed hydrocarbons, thereby preventing them from entering the high-temperature stripper.

The flash gas produced from the flash drum is mostly used as supplementary fuel for the process heater. For low-pressure feed gas, the CO₂-rich amine solution is sent directly to the cross-exchanger with a pump providing the required pressure to pass through the exchanger and to the stripper. The cross-exchanger is the heart of heat integration in CO₂ capture plant because the CO₂-rich amine solution exchanges heat with the hot CO₂-lean amine solution from the bottom of the reboiler.

For a high-pressure system, the valve installed at the exit of the cross-exchanger not only controls flow rate but also reduces the pressure of the CO₂-rich amine solution to the desorber pressure.

The stripper operates at high temperature (110 ^oC to 130 ^oC) to reverse the products of the chemical reaction in the absorber, hence recover the amine solution for reuse. The vapor phase (mostly CO₂ and H₂O and some amine solvents) leaving the top of the stripper is condensed to recover all the condensable components. The control valve located at the bottom of the stripper overhead separator controls both the level in the separator (to avoid overflow) and the flow rate of the reflux entering the stripper. The CO₂ from the top exit of the separator is compressed and dried for either storage in depleted reservoirs or for enhanced oil and gas recovery.

The reboiler at the bottom of the stripper is where the amine solution is heated to strip the absorbed CO₂, while the vaporized phase (mostly CO₂ and H₂O with some amine solvents) is sent back to the stripper. The heating is usually provided by steam supply (low- or medium-pressure steam) or proprietary heating fluids, and their choice will depend on availability and cost. The flow rate and liquid level of the stripper and reboiler are controlled by control valves located at their bottom exit location.

The hot CO₂-lean amine solution is recycled to the absorber for CO₂ absorption, however, amine is added (amine make-up) to maintain the design amine concentration for CO₂ absorption. The flow rate of the CO₂-lean amine solution is controlled by a control valve prior to being cooled to the required absorption temperature.

In order to maintain the performance of the control valves and to extend operating (service) life in CO₂ capture plants, proper valve design, selection, installation, operation, inspection and maintenance is essential.

Chikezie Nwaoha is a researcher at the Clean Energy Technologies Research Institute (CETRI), University of Regina in Regina, Saskatchewan.