May-2024

Pre-combustion carbon capture

Pre-combustion carbon capture is the easiest route to rapid decarbonisation of the chemical and petrochemical sectors.

Stephen B Harrison
sbh4 Consulting

Article Summary

Through the years 2030 to 2050, it is inevitable that post-combustion carbon dioxide (CO2) capture will be used to decarbonise heavy industry and fossil-fired power generation. Capturing CO2 after air-fed combustion is expensive since the CO2 concentration is low, and a huge volume of nitrogen gas must be processed.

Pre-combustion CO2 capture has the benefit of operating at high pressure and often with a high CO2 concentration. The consequence is that the combined Opex and Capex costs per tonne of CO2 captured can be only 50% of post-combustion CO2 capture. Retrofitting CO2 capture to steam methane reformers (SMRs) for refinery hydrogen production represents a cost-effective way to achieve impactful decarbonisation.

Additionally, ammonia and ethylene oxide (EO) production must remove CO2 from the process gases to ensure the chemical reactions and catalyst performance are effective. In these cases, the Capex and Opex costs of CO2 capture are absorbed into the core process. These applications must represent some of the easiest routes to rapid decarbonisation of the chemical and petrochemical sectors.

Putting it into practice at Porthos
Air Products will retrofit a CO2 capture facility at its existing Botlek SMR in Rotterdam. The SMR was built in 2011 with a capacity of around 100,000 tonnes of hydrogen per year. The annual CO2 emissions at this production capacity would be more than 1,000,000 tonnes.

The retrofitted CO2 capture, drying, and compression facility is expected to be on-stream in 2026. Retrofitting CO2 capture equipment to an SMR of this size could cost in the order of €100 million. This represents an additional 50% of the original capital cost of the SMR. Steam is required to operate the CO2 capture facility, and power is needed to compress the dried pure CO2. These represent the main operating costs.

Once operational, this will be the largest low-carbon or ‘blue’ hydrogen plant in Europe. The hydrogen from the Botlek SMR will continue to serve ExxonMobil’s Rotterdam refinery and additional customers via Air Products’ hydrogen pipeline and hydrogen liquefier.

ExxonMobil aims to achieve net-zero Scope 1 and 2 emissions from its operated assets by 2050. Since Air Products owns and operates the hydrogen production facility, the CO2 emissions reduction is categorised as Scope 1 for Air Products and Scope 2 for ExxonMobil. This CO2 capture retrofit with CO2 sequestration in the Porthos scheme will allow Air Products to reduce its CO2 emissions in the port of Rotterdam by more than half.

The Porthos CCS scheme is the first large-scale CO2 transportation and storage infrastructure scheme in the Netherlands to achieve final investment decision (FID) and regulatory approval. CO2 will be sequestered 3km beneath the surface of the North Sea in depleted gas fields, which lie about 20km from the coast.

When capturing CO2 from an SMR, the location from which the CO2 is captured influences the cost (see Figure 1). The lowest unit cost is to capture CO2 from the pre-combustion syngas stream prior to the pressure swing adsorption (PSA) unit (Location A in Figure 1). At this point, the partial pressure of CO2 is at its highest.

On the other hand, the maximum CO2 recovery rate is capped at 70% because the post-combustion CO2 from the SMR burners is not captured. To achieve low-carbon hydrogen certification in some markets, such as the EU, capturing CO2 in the SMR flue gas may be required to achieve the necessary CO2 intensity of hydrogen production (Location C in Figure 1).

Low-carbon ammonia cracking and import
Blue ammonia may be produced at low cost in the US Gulf Coast, where natural gas prices are low and CO2 storage can be achieved in locations close by, such as the Permian Basin. Linde and OCI will collaborate to produce 1,100,000 million tonnes per year of blue ammonia. Partial oxidation (POx) will be used to convert natural gas to syngas. The process operates at high pressure which reduces the pre-combustion CO2 capture costs. CO2 liberated during hydrogen production will be captured and sequestered.

The added cost for carbon capture and storage (CCS) is around $120 per tonne of ammonia. This covers the additional equipment and energy costs to remove the CO2 from the gas stream, transport it to a sequestration location, and inject it for permanent storage.

Many projects have proposed to produce green hydrogen at scale. The optimal locations are where there is abundant renewable power generation potential from integrated wind and solar schemes, such as Western Australia. In the future, when electrolyser costs reduce and the efficiency of this technology improves, the cost of green hydrogen in these locations could potentially be comparable to blue hydrogen. Shipping and terminal infrastructure must be developed to connect the blue and green hydrogen producers with energy markets.

Air Products is also planning to make clean hydrogen available in Western Europe from cracked green or blue ammonia. The ammonia will be imported through the ports of Rotterdam and Hamburg. In Hamburg, Air Products will construct a new ammonia terminal for this purpose. At Rotterdam, Air Products has partnered with Gunvor to develop the import terminal. In Hamburg, Mabanaft will partner with Air Products.

In addition to the potential to crack ammonia to make hydrogen, low-carbon ammonia can be fired directly to generate steam in boilers or power on specially constructed gas turbines. On a smaller scale, it will also see application as a maritime fuel and may also be used for industrial transportation applications in rail and trucking operations.

CO2 capture is integral to ammonia production
In many large-scale ammonia plants, hydrogen is produced using a two-stage reforming process. The primary reformer is generally an SMR. The secondary reformer is an autothermal reformer (ATR), which requires oxygen for partial oxidation reactions. In this configuration, oxygen has traditionally been supplied by introducing air, as shown in Figure 2. The nitrogen that enters with the air remains in the process to be reacted with hydrogen in the ammonia synthesis loop.

Following the reformers, high-temperature shift and low-temperature shift reactors convert carbon monoxide (CO) and steam to hydrogen and CO2. Subsequently, the CO2 is removed using absorption and desorption in a solvent. The CO2 must be removed from the gas stream prior to ammonia synthesis since any oxygen-containing molecules such as CO2, CO, and water (H2O) would poison the ammonia synthesis catalyst.