The complexities of equipment sealing in Carbon Capture, Utilisation, and Storage (CCUS) applications

Introduction

As the global community faces the urgent challenge of reducing greenhouse gas emissions to mitigate climate change, technological innovations such as Carbon Capture, Utilisation, and Storage (CCUS) are gaining momentum. CCUS represents a multi-faceted approach to mitigating carbon dioxide (CO₂) emissions by capturing CO2 from industrial sources or directly from the air, utilizing it in industrial applications, or storing it in geological formations deep underground.

While the potential of CCUS to reduce CO₂ emissions is promising, implementing it at a large scale introduces significant challenges in the development of equipment that can reliably handle CO₂ in its various phases.

James Walker is one of a number of companies taking a leading role in the development of sealing materials compatible with CO₂ applications. Drawing from its experience in the oil and gas industry, James Walker is validating solutions that address the specific challenges posed by CO₂ handling in CCUS systems, particularly in relation to valves.

Sealing challenges in CO₂ handling and storage

CO₂ exists in multiple phases—gaseous, liquid, and supercritical—depending on the temperature and pressure in application. These phase transitions present unique challenges for the materials used in CCUS systems. CO₂ enters a supercritical state at relatively modest conditions (around 31°C and 74 bar, or 1070 PSI), and in this state, it displays characteristics of both a liquid and a gas. This supercritical phase is particularly advantageous for transport and storage of CO₂ due to its high density, but it also poses significant challenges for the materials used in pipelines, compressors, and valves.

Key challenges:

Rapid Gas Decompression (RGD): RGD occurs when a gas-saturated elastomer is exposed to a sudden decrease in pressure, causing gas to escape from the elastomer’s matrix, leading to material rupture or even catastrophic failure. In CCUS applications, where CO₂ is frequently compressed to its supercritical state, rapid decompression can occur when CO₂ reverts to a gaseous phase, increasing its volume and placing stress on the elastomer. RGD is a well-known problem in the oil and gas industry and whilst many materials have been developed that are resistant to this phenomenon, less testing has yet been done with high concentrations of CO₂.

Swell and permeation: Supercritical CO₂ (scCO₂) can permeate and cause significant swelling in elastomeric materials. This swelling can lead to the deterioration of the seal’s mechanical properties, making it unable to maintain an effective seal. The degree of swelling varies depending on the composition of the elastomer, making material selection critical to minimising this risk. Housing design may also need to be optimised for highest performance.

Leaching: scCO₂ has solvent-like properties that can cause the leaching of additives, fillers, and plasticisers from elastomer compounds. This process reduces the mass and volume of the elastomer, leading to a decrease in sealing performance and longevity. The challenge in this instance is to develop elastomer formulations that resist the leaching effects of scCO₂ whilst retaining other desirable properties, such as flexibility and strength.

Glass transition temperature (Tg) suppression: One intriguing phenomenon associated with scCO₂ is its ability to suppress the glass transition temperature of some elastomers. This suppression can improve the elastomer’s performance at lower temperatures, enhancing its ability to maintain flexibility and sealing integrity in cold environments. However, this effect is not uniform across all elastomer types and must be carefully considered during material selection.

Expertise in CO₂ Sealing

Sealing companies leading the validation and development of solutions compatible with CO₂ are initially seeking to leverage the performance of existing materials - subjecting them to new testing regimes including high-pressure cycling, thermal variation testing, and long-term exposure to assess material degradation mechanisms in CO₂ environments. Many of these materials were originally developed for seals for CO₂ reinjection wells used in enhanced oil recovery (EOR) applications, experience that has become the basis for current efforts in CCUS, where the demands on materials and seals are even more stringent.

In James Walker’s case, testing of existing high performance materials has revealed two elastomers that are particularly suited to use with CO₂ and scCO₂. Vermilion Four and Vermilion Two, have been extensively tested in high-CO₂ environments, including supercritical conditions. These materials have demonstrated excellent resistance to CO₂-induced RGD and leaching, in addition to possessing resistance to the high levels of swell often seen in other similar compounds. This makes them ideal candidates for CCUS applications.

Ongoing development

Elastomers are just one of many material types used in sealing products that are currently being tested for new applications such as high concentrations of CO₂ and supercritical CO₂.

At the moment, although there is significant activity and interest in CCUS CO2 applications the aims of the end users are not always clear. With no clear industry acceptable criteria in place or test criteria laid down for CCUS applications, seal manufacturers and equipment OEMs must continue to use their initiative and work together to help end-users solve operational issues one step at a time. 

Find out more: https://www.jameswalker.biz/your-challenges/media-environments/carbon-dioxide

Formulation, compounding, moulding and testing of materials and final products is only the start of the long journey to a successful, fully operational and fully validated piece of equipment working in a CCUS application.

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