Selecting Pipes for Carbon Capture (CCUS) Systems: Handling CO2, Amines, and Impurities

Selecting Pipes for Carbon Capture (CCUS) Systems: Handling CO2, Amines, and Impurities

The drive toward carbon capture, utilization, and storage (CCUS) is creating a new generation of industrial infrastructure. For engineers and project managers, designing these systems presents a unique materials challenge. The piping must handle not just pressurized CO₂, but also corrosive amine solvents, their degradation products, and unpredictable process impurities. Material failure here isn't just a maintenance issue; it risks system downtime, solvent loss, and compromised capture efficiency.

Selecting the right pipe material is a critical economic and technical decision. This guide breaks down the environmental factors and material options to ensure long-term integrity.

The Corrosive Landscape: More Than Just CO₂

A carbon capture piping system is a chemical plant in miniature, with distinct zones of aggression:

1. Carbonic Acid Attack: Wet CO₂ forms carbonic acid (H₂CO₃). While weak, it can cause uniform corrosion of carbon steel, especially in high-velocity areas like pump discharge lines and pipe elbows.

2. Amine Corrosion: The workhorse solvents like MEA, MDEA, or proprietary blends are alkaline but become corrosive:

   • Degradation Products: Over time, amines degrade, forming heat-stable salts (HSS) like oxalates, formates, and acetates. These are significantly more acidic and corrosive.

   • Oxidative Degradation: Ingress of oxygen (from flue gas or air) accelerates amine breakdown and can lead to severe localized pitting.

3. The "Deadly Trio": CO2, Amines, and Heat: The hottest sections of the system—the amine reboiler, rich/lean amine exchangers, and associated piping—see the highest corrosion rates. Temperature dramatically accelerates all chemical reactions.

4. Flue Gas Impurities: Despite pre-treatment, trace contaminants like SOx, NOx, HCl, and HF can slip through. These form strong acids when dissolved in the amine/water solution, creating highly localized, aggressive environments.

5. Stress Corrosion Cracking (SCC): The combination of tensile stress (from pressure, welding, or bending), temperature, and the amine environment can lead to catastrophic, sudden cracking in susceptible materials.

Material Selection Strategy: Matching the Zone

There is no single "best" material for an entire CCUS system. Selection is zone-specific, based on temperature, fluid composition, and pressure.

Zone 1: Raw Flue Gas Inlet & Pre-Treatment

• Conditions: Wet, acidic gas with impurities (SOx, particulates), lower temperatures.

• Common Choice: Carbon Steel (CS) with Corrosion Allowance.

  • Rationale: Cost-effective for large-diameter ducts and pipes. A substantial corrosion allowance (e.g., 3-6 mm) is added to the wall thickness. Internal linings (rubber, FRP) or coatings may be used in severe cases.

• Alternative: For high impurity loads or to minimize maintenance, 304/316L Stainless Steel may be specified for critical sections.

Zone 2: Amine Absorption & Low-Temperature Circulation

• Conditions: Lean and rich amine solutions at moderate temperatures (typically 40-70°C).

• Baseline Choice: Carbon Steel.

  • Consideration: Corrosion is manageable with proper chemical control (amine filtration, reclaiming to remove HSS), and the use of corrosion inhibitors. Continuous monitoring of wall thickness is a standard operating practice.

• Upgrade for Criticality: 304/316L Stainless Steel.

  • Rationale: Used for components where corrosion products cannot be tolerated (e.g., to prevent fouling of heat exchangers) or in high-velocity pump loops. It provides excellent resistance to amine and carbonic acid corrosion in this range.

Zone 3: The Hot Section (Stripper, Reboiler, Exchanger Shells)

• Conditions: Rich amine at temperatures exceeding 90°C, up to 120-130°C at the reboiler. This is the most severe general corrosion and SCC environment.

• Standard for Severity: Solid 316/316L Stainless Steel.

  • Reality: While better than CS, standard 316L can still experience localized corrosion and chloride-induced SCC if chlorides concentrate, or from amine degradation products.

• High-Performance Standard: 2205/2507 Duplex Stainless Steels.

  • Rationale: The mixed ferritic-austenitic structure provides roughly double the yield strength of 316L and superior resistance to chloride stress corrosion cracking and pitting. This allows for thinner walls (saving weight/cost) and enhanced safety margins. 2205 is often considered the optimal balance of cost and performance for hot amine service.

• For Maximum Resilience: Nickel Alloys (Alloy 825, Alloy 625).

  • Rationale: In systems with poor impurity control, high degradation, or where ultimate reliability is required (e.g., offshore platforms), these alloys are specified. Alloy 825 offers excellent resistance to chloride SCC and acidic by-products. Alloy 625 (Inconel) is the premium choice for the most aggressive hot spots, such as reboiler tubes and associated piping.

Beyond Material Grade: Critical Fabrication & Operational Factors

1. Welding & Post-Weld Treatment: For stainless and duplex steels, welding procedures must be qualified to preserve corrosion resistance. For carbon steel, post-weld stress relief may be specified for hot sections to reduce residual stresses and mitigate SCC risk.

2. Water Wash Sections: Areas where saturated water contacts CO₂ can be more corrosive than amine sections. 316L or duplex is often required here, even if upstream piping is carbon steel.

3. CO₂ Transportation & Injection Pipelines: For dried, compressed supercritical CO₂, carbon steel is standard. However, stringent control of water content (<500 ppm, often <50 ppm) is mandatory to prevent corrosive carbonic acid formation. For wet CO₂ scenarios or if impurity specs are lax, clad pipes (CS with 316L or 625 liner) or solid corrosion-resistant alloys become necessary.

4. Monitoring & Maintenance: Material selection is not a "set and forget" decision. A robust program of ultrasonic thickness testing, corrosion coupon racks, and fluid chemistry monitoring is essential for all materials, especially carbon steel.

The Selection Checklist for Your Project

• Map the Process: Divide your P&ID into distinct corrosion zones based on temperature, fluid phase, and chemistry.

• Define Impurity Limits: Establish and guarantee maximum concentrations for O₂, SOx, and chlorides in the flue gas feed.

• Lifecycle Cost Analysis: Compare upfront material costs against expected service life, maintenance (inspection, wall thinning), and risk of unplanned downtime. Duplex often wins over 316L in hot sections on this basis.

• Specify Fabrication Quality: Require proper welding procedures, passivation for stainless/alloys, and non-destructive testing (NDT) protocols.

• Plan for Monitoring: Design in inspection access points, coupon holders, and sample ports from the outset.

The Bottom Line

Piping for CCUS is a battle against a complex, evolving chemical environment. While carbon steel remains the economical backbone for non-severe sections, the industry standard is shifting toward corrosion-resistant alloys (CRA) for all hot, rich amine and critical service.

316L is often the minimum, 2205 Duplex is the robust default, and nickel alloys like 625 are the high-assurance solution for the most severe conditions. The correct choice hinges on a clear understanding of the full process chemistry, a realistic assessment of operational control, and a total cost-of-ownership perspective that prioritizes long-term integrity over lowest initial investment. In the race to decarbonize, the reliability of the capture plant itself will depend on these material decisions.