In amine technology, CO2 is captured by an amine solvent, a liquid comprising of water and amines, which is being used to absorb the CO2 from the flue gas.
Amine technology has been used for decades in other applications and is therefore considered to have a moderate technical risk. However, TCM will evaluate opportunities for improvements in process design, construction methods and operations with the purpose to qualify the technology for use in large scale post-combustion plants. Aker Clean Carbon has included several technology improvements in the plant such as; improved amines, energy saver and emission control. The flexible absorber is about 60 m high and is constructed in concrete with a liner.
How does the technology work?
Exhaust gas containing the CO2 is routed into a large absorption tower. The exhaust gas enters the bottom of the absorber flowing upwards where it comes into contact with the liquid amine flowing downwards allowing the CO2 to be absorbed from the flue gas.
The Water Wash:
After absorbing CO2, the remaining exhaust gas is treated in a water wash placed in the upper part of the absorber tower to remove amines before the cleaned exhaust gas is released back to the atmosphere.
The CO2 Desorbers:
The CO2 rich amine solvent is pumped via heat exchangers to a regenerator where the chemical reaction between the amine and CO2 is reversed by steam flowing upwards in the regenerator column. The separated CO2, would then be ready for compression, transport and storage and the CO2 lean liquid amine can be pumped back into the absorber for reuse and the cycle repeated. The amine plant at TCM will have two dedicated regenerators – one designed for the refinery cracker flue gas, and the second for the combined heat and power flue gas.
What is amine?
An amine is basically an ammonia compound with one or more of the hydrogen atoms replaced by a substitute. (Formula of monoethanolamine: H2NCH2CH2OH)
- Amines are chemical solvents that undergo a reversible reaction with carbon dioxide (CO2) and other acid gases such as hydrogen sulfide The chemistry is quite complex but in simple terms, when a flue gas (or any gas) containing CO2 is contacted in an “Absorber” with an amine in solution with water (lean solution), the CO2 reacts with the amine molecule and chemically binds (attaches) to it and is thus removed from the gas stream. This absorption reaction is exothermic, meaning heat is generated.
The solution (now called rich solution) is then pumped to a separate column called “Regenerator” or “Stripper”. In the Regenerator, the amine solution is heated to provide the energy of dissociation so that the CO2 is released (detached) from the rich amine solution.
The resulting lean amine solution is then pumped back to the Absorber to repeat the absorption/regeneration cycle. The liberated (almost pure but wet) CO2 stream is then sent to the next step (dehydration and compression) so it can be disposed of (used for enhanced oil recovery or injected into a depleted reservoir for storage etc.). The treated flue gas is finally scrubbed with water before being released to atmosphere to recover the bulk of any traces of entrained amine in order to minimize any environmental impact.
Amine Process: Absorption
The details of the mechanisms of CO2 absorption into an amine solution in an absorption column are quite complex. There are many references about the chemistry involved in the process, and many references and models comprising mass transfer mechanisms and chemical reaction kinetics.
First, CO2 has to be transported from the gas to the liquid surface, and then it is absorbed in the liquid solution. Following are some of the reactions normally assumed to take place when CO2 reacts in a primary amine like MEA in an aqueous solution.
CO2 + NRH2 ↔ RH2+NCOO-
RH2+NCOO- + NRH2 ↔ RH2NCOO-NRH2+
2H2O ↔ H3O+ + OH-
2H2O + CO2 ↔ H3O+ + HCO3-
H2O + HCO3- ↔ H3O+ + CO32-
H2O + NRH3+ ↔ H3O+ + NRH2
H2O + NRH2 + CO2 ↔ H3O+ + NRH2COO-
Note: In the case of MEA (NH2C2H2OH), R is C2H2OH.
During regeneration of the solvent, the absorption reactions described above are reversed. The reverse reactions are endothermic, meaning heat has to be applied to the solution to liberate the CO2.
Amine Plant: Absorption Tower
In the absorber, the flue gas is contacted with an amine solution to remove the CO2 and the secondary function is to scrub the treated flue gas with water to recover any entrained amine solution to minimize losses and also protect the environment.
The absorber column at TCM is rectangular and is built of lined concrete since it operates essentially at atmospheric pressure. It can also be cylindrical but at the sizes required for full scale capture plants, rectangular columns are considered to be more cost effective.
This section is the tallest section, typically 8 m – 12 m in height. At TCM, there are multiple sections of various heights installed due to the research nature of the facility. The sections are filled with metallic structured packing which enhances the efficiency of the capture process by providing a large contact area for the gas and liquid. The amine solution is fed at the top of the packing and is evenly distributed by specially designed liquid distributors. The flue gas flows upwards, counter-current to the liquid and exits at the top after being washed by circulating water.
The concentration of CO2 in the flue gas decreases steadily as it makes its way up through the column. The CO2 loading of the solution increases steadily as it makes its way down the column.
The Regenerator is also a column with a bed of structured packing. However, the column is much smaller in diameter as the volume of gas to be handled is much less than in the Absorber.
In the Regenerator, the rich amine solution, after heating, is fed at the top of the stripping section. In the bottom of the Regenerator, heat is applied by means of steam which heats the solution that has made its way down to the bottom.
A heat-exchanger (Reboiler) is used to transfer the heat indirectly from steam into the amine solution. As a result of this heat input, steam is produced (by vaporizing the water in the solution) which then flows up through the column.
As the steam makes its way up the column, it comes in contact with the CO2 loaded amine solution flowing down and provides the heat of dissociation, resulting in liberation of CO2 and condensation of the steam. The liberated CO2 and the remaining steam keep flowing up until they exit the stripping section.
The rich solution as it makes its way down steadily gives up the absorbed CO2 and becomes leaner until it leaves from the bottom of the column to be recycled.
At the top of the column, there is a reflux section where water is supplied to wash the CO2 and steam leaving the stripping section. The washed vapours then leave the column to be cooled in a Condenser. The residual steam is condensed and separated from the CO2 gas in the Reflux Drum and then the condensate is recycled back as reflux.
The CO2 product stream is then sent to another processing unit for dehydration (removal of any moisture) and compression so that it meets the end use specifications. At TCM, this stream is vented at present but in the future may be recovered.