Decarbonisation Technology - February 2023

based solvent in the first tower. The CO2 -lean flue gas flows to the atmosphere. The CO2-rich amine is pumped to a second stripper tower, where steam is used in vast quantities to boil the CO2 away from the amine solvent. The regenerated, CO2-lean amine solvent is pumped back to the absorber tower to collect more CO2, and the process operates continuously, with the amine being recirculated from the absorber to the stripper. Variations of the amine-based carbon capture process use chilled ammonia, methanol or potassium bicarbonate as the solvent to absorb the CO2. The process configuration is similar. All these processes require a huge heat energy input to boil the CO2 out of the solvent. Unfortunately, this additional energy is not always available at the site where the CO2 must be captured. The ramp-up in natural gas or fuel oil supplies may stress the local infrastructure beyond its capability (see Figure 4 ). Vacuum swing adsorption (VSA) and temperature swing adsorption have also been used for refinery SMR CO2 capture. VSA locks the CO2 into a solid molecular sieve adsorbent. In the VSA system, a rapid pressure reduction releases the CO2 from the adsorbent. Electrical power is required in large quantities, and the local power supply infrastructure may not be sized for the additional demand. VSA has been demonstrated effective in removing CO2 from SMR process gases. Air Products operates two SMRs at Port Arthur, Texas, to supply hydrogen to the neighbouring Valero refinery. Both SMRs use VSA technology to capture the CO2 by-product. The resultant

CO2 is compressed, dried, and transferred to a nearby location through a pipeline. The CO2 is utilised and permanently stored underground in an enhanced oil recovery scheme. This is an example of carbon capture, utilisation, and storage (CCUS). SMRs generally use natural gas, refinery gas or naphtha as their feedstock. The hydrocarbon feedstock is combined with steam and converted to syngas over various reforming and water gas shift catalysts. The resultant syngas has a typical composition of 76% hydrogen, 17% CO2, and 7% unreacted methane and other gases. The syngas pressure is around 25 bar at this stage in the process (see Figure 5 ). An SMR typically emits 9.5 kg of CO2 per kg of hydrogen, making hydrogen produced from fossil fuels without carbon capture unsustainable. To capture much of the CO2 from the process gas on the Air Products SMRs, the VSA units were installed between the water gas shift reactors and the hydrogen purification PSA unit. For established CO2 capture technologies such as amine wash or VSA, this is a highly cost-effective location to capture the CO2 due to the high pressure and high CO2 concentration, which combine to result in a high partial pressure of CO2. CO2 can be produced from the VSA units with a purity of 95% and a recovery rate (from this process stream) of over 90%. In addition to the process CO2, there are additional post-combustion CO2 emissions on the SMR. It is more challenging to capture CO2 from the post-combustion flue gases using conventional amine wash or VSA

CO lean ue gas Absorber tower

Stripper tower

CO lean ue gas

Regeneration gas

Pure CO gas

CO lean amine

Unsaturated absorbent

Saturated absorbent

Raw CO rich ue gas

CO rich ue gas

CO descorbed under vacuum

Change-over valves alternate the regeneration gas & the ue gas ow from one bed to the other.

CO rich amine solution

Steam reboiler

Figure 4 Established carbon capture technology – VSA and amine solvent