Chilled ammonia technology, although less widely used than amine technology, holds potential for lower energy consumption per tonne of CO2 captured. This process will be tested at large scale at TCM, and if this technology can be qualified, it will provide the world with a proven technology with reduced costs.
How does the technology work?
Flue Gas Cooling:
The exhaust gas, or flue gas is passed through a flue gas conditioning unit to condense the water and residual emissions which reduces the volume of gas to be treated – and thereby reducing the size of equipment necessary.
The conditioned gas then passes through a column where the CO2 is absorbed from the flue gas through contact with ammonia solution, forming ammonium bicarbonate solids. The treated flue gas then passes through additional columns to recover any ammonia vapour to release a cleaned flue gas to the atmosphere.
High Pressure Regeneration:
The ammonia solution, now rich in CO2, is pressurised and pumped into a regenerator column where heat is applied to separate CO2 from the solution, where the CO2 can then be further compressed to facilitate future transport and storage. The ammonia solution is then returned to the absorber for reuse.
What is ammonia?
Ammonia (NH3) is also a chemical solvent that acts much in a similar way to an amine. In order to improve the absorption efficiency, at TCM, the process is carried out at low temperatures – hence the name “Chilled Ammonia Process” which is a patented process by Alstom.
The process in more detail
The NH3 – CO2 chemistry is complex but it can be simplified thus. Flue gas containing CO2 is contacted with aqueous ammonia - ammonium carbonate solution in water in an “Absorber”. The CO2 in the flue gas reacts with the ammoniated solution, leading to formation of ammonium bicarbonate and ammonium bicarbonate.
This CO2 loaded rich solution is then pumped to a Regenerator, (similarly to an amine process), where heat is applied for regeneration of the solution and release of CO2. The resulting ‘lean’ ammonium carbonate solution (lean in CO2) leaves from the bottom of the regenerator column and is returned to the CO2 absorber.
The liberated CO2 is washed with water to recover ammonia and then further compressed and treated to produce clean CO2 at the desired specifications for end-use.
The treated flue gas also carries NH3 vapor with it because of NH3’s high volatility and hence a water wash system is incorporated in the process to recover this NH3.
Flue Gas Cooling: Before the flue gas is introduced into the ammonia process, it is cooled to condense water in order to improve the process efficiency.
The Chilled Ammonia Process is based on the chemistry of the NH3-CO2-H2O system and the ability of the ammoniated solution to absorb CO2 at low temperature and to release the absorbed CO2 at moderately elevated temperature.
The actual chemistry is quite complex; however, the primary chemical reactions for CO2 capture by ammonia are presented below. During absorption, CO2, NH3, and H2O combine to form ammonium carbonate [(NH4)2CO3] and ammonium bicarbonate [(NH4) HCO3].
These reactions are exothermic.
2NH3(aq) + H2O(l) + CO2(g) ↔ (NH4)2CO3(aq) (1)
NH3(aq) + H2O(l) + CO2(g) ↔ (NH4)HCO3(aq) (2)
H2O(l) + CO2(g) + (NH4)2CO3(aq) ↔ 2(NH4) HCO3(aq) (3)
(aq = aqueous g = gas l = liquid)
In addition to the primary reactions involving CO2 capture and release, a secondary reversible reaction between ammonium bicarbonate and ammonia forms ammonium carbamate.
(NH4)2CO3(aq) + NH3(aq) ↔ NH2COONH4(aq) (4)
The treated flue gas then passes through additional columns to recover ammonia vapour and to heat the gas prior to venting the clean flue gas to the stack.
After absorption, the aqueous ammonia solution now rich in CO2, is pumped to the regeneration system to release CO2 from the solution. During regeneration, the absorption reactions are reversed as heat is applied to release the CO2.
2NH4HCO3(aq) ↔ (NH4)2CO3(aq) + CO2(g) + H2O
(NH4)HCO3(aq) ↔ NH3(aq) + H2O(l) + CO2(g)
(NH4)2CO3(aq) ↔ 2NH3(aq) + H2O(l) + CO2(g)
The regenerated ammonium carbonate solution, now lean in carbon dioxide, is sent back to the process to be used again.
In the ammonia process, regeneration is carried out at much higher pressure than in the amine process which gives a CO2 product at a much higher pressure, thereby saving on compression cost. The liberated CO2 is washed with water to recover ammonia and then further compressed and treated to produce clean CO2 at the desired specifications for end-use.