Physical Address
304 North Cardinal St.
Dorchester Center, MA 02124
ISO/TR 27921:2020 — CO₂ Stream Composition — Cross-Cutting Issues for CCS
Risk-based framework for establishing CO₂ quality specifications balancing capture cost against transport and storage performance
ISO/TR 27921: CO₂ Stream Composition — Cross-Cutting Issues for CCS
ISO/TR 27921:2020 addresses one of the most critical cross-cutting issues in carbon capture and storage: the composition of the CO₂ stream and its implications across the entire CCS chain. The composition of captured CO₂ is not simply a technical specification — it affects pipeline transport hydraulics, compression energy requirements, material selection for equipment and pipelines, reservoir injectivity, geochemical interactions with storage formations, and long-term containment security. This Technical Report provides a comprehensive framework for establishing CO₂ quality specifications that balance capture cost against downstream impacts.
The CO₂ stream composition is the single most influential parameter affecting the cost and performance of the entire CCS chain. A 1% increase in impurity concentration can increase compression energy by 3-5% and significantly affect two-phase flow behavior in pipelines.
The standard identifies and characterizes the principal impurities found in captured CO₂ streams: nitrogen (N₂), oxygen (O₂), argon (Ar), hydrogen (H₂), carbon monoxide (CO), hydrogen sulfide (H₂S), sulfur oxides (SOx), nitrogen oxides (NOx), water (H₂O), methane (CH₄), and higher hydrocarbons. Each impurity originates from different capture technologies — for example, oxyfuel combustion produces CO₂ with elevated O₂ and Ar, pre-combustion capture yields residual H₂, and post-combustion amine scrubbing may introduce amine degradation products — and each has distinct impacts on CCS chain performance.
Impurity Impacts on CCS Chain Components
ISO/TR 27921 provides a detailed analysis of how each impurity affects CCS chain components. Water content is the most tightly controlled impurity due to its role in corrosion (forming carbonic acid with CO₂) and hydrate formation at pipeline operating conditions. The standard recommends water dew point specifications typically below -40°C for dense-phase CO₂ pipelines. H₂S and other acid gases require careful control to prevent stress corrosion cracking in carbon steel pipelines. Non-condensable gases (N₂, O₂, Ar, H₂, CH₄) increase the compression work, reduce pipeline capacity, and affect the two-phase envelope, potentially causing phase separation during transport.
| Impurity | Typical Concentration Range | Key Impact | Recommended Limit |
|---|---|---|---|
| Water (H₂O) | 10-500 ppmv | Corrosion, hydrate formation | < 50 ppmv (dense phase) |
| Hydrogen sulfide (H₂S) | 0-2000 ppmv | Toxicity, SCC of pipelines | < 200 ppmv |
| Oxygen (O₂) | 0-5% | Oxidative degradation, well integrity | < 100 ppmv (aquifer)< 4% (EOR) |
| Nitrogen (N₂) | 0-10% | Compression energy, phase behavior | < 4% by volume |
| Hydrogen (H₂) | 0-3% | Compression energy, material embrittlement | < 2% by volume |
| Nitrogen oxides (NOx) | 0-200 ppmv | Acid formation, solvent degradation | < 10 ppmv |
| Sulfur oxides (SOx) | 0-500 ppmv | Acid formation, health hazard | < 10 ppmv |
| Amines / degradation products | 0-50 ppmv | Formation damage, environmental | < 5 ppmv total |
The allowable impurity concentrations depend critically on the storage site characteristics. Injection into saline aquifers generally requires higher purity than injection for enhanced oil recovery (EOR), where certain impurities can actually improve oil recovery. ISO/TR 27921 stresses that CO₂ quality specifications must be site-specific and developed through risk-based analysis.
The standard addresses the challenge of variable CO₂ quality from different capture sources feeding into a common pipeline network — a scenario increasingly likely as CCS hubs develop. It recommends a combination of online quality monitoring (near-infrared spectroscopy, gas chromatography) and buffer storage to manage composition variability. Blending rules and quality banking systems are introduced as operational strategies to maintain pipeline stream composition within acceptable limits.
Engineering Design Insights for CO₂ Quality Management
A key engineering contribution of ISO/TR 27921 is its guidance on developing CO₂ quality specifications using a risk-based approach. Rather than prescribing universal limits, the standard provides a systematic methodology for deriving acceptable impurity concentrations from the performance requirements and risk tolerance of each CCS chain component. The methodology involves: (1) identifying damage mechanisms for each component-impurity pair, (2) establishing dose-response relationships from experimental data or literature, (3) defining acceptable risk levels, and (4) back-calculating allowable impurity concentrations at the point of injection.
The standard also provides practical guidance on CO₂ conditioning — the process of removing impurities to meet quality specifications. Conditioning technologies discussed include dehydration (glycol absorption, molecular sieve adsorption, solid desiccant), acid gas removal (amine polishing, activated carbon), mercury removal, and oxygen removal (catalytic reduction). The optimal conditioning train depends on the impurity profile of the raw CO₂ stream, which in turn depends on the capture technology.
CCS hub projects that have implemented the risk-based CO₂ quality specification framework from ISO/TR 27921 have reduced capture-side purification costs by 15-25% while maintaining acceptable transport and storage performance, demonstrating that optimized specifications benefit the entire CCS value chain.
Frequently Asked Questions
Q: Why can’t a single universal CO₂ quality specification be applied to all CCS projects?
A: The impacts of impurities vary significantly with transport distance, pipeline material, storage formation characteristics, and regulatory requirements. A specification suitable for a short pipeline to a robust saline aquifer would be inadequate for a long-distance pipeline to a sensitive storage formation.
A: The impacts of impurities vary significantly with transport distance, pipeline material, storage formation characteristics, and regulatory requirements. A specification suitable for a short pipeline to a robust saline aquifer would be inadequate for a long-distance pipeline to a sensitive storage formation.
Q: What is the most difficult impurity to remove from captured CO₂?
A: Nitrogen and argon are particularly challenging because they are chemically inert and have similar physical properties to CO₂, making separation energy-intensive. Cryogenic distillation or advanced membrane systems are required, adding significant cost.
A: Nitrogen and argon are particularly challenging because they are chemically inert and have similar physical properties to CO₂, making separation energy-intensive. Cryogenic distillation or advanced membrane systems are required, adding significant cost.
Q: How does CO₂ quality affect storage capacity?
A: Impurities reduce the effective storage capacity because they occupy pore space without being retained. Non-condensable gases also increase the reservoir pressure required for injection, potentially reducing total storage capacity by 10-30% depending on impurity levels.
A: Impurities reduce the effective storage capacity because they occupy pore space without being retained. Non-condensable gases also increase the reservoir pressure required for injection, potentially reducing total storage capacity by 10-30% depending on impurity levels.
Q: Does ISO/TR 27921 address CO₂ quality for utilization pathways?
A: Yes, the standard includes a discussion of quality requirements for CO₂ utilization applications including enhanced oil recovery, chemical synthesis, and food-grade applications, noting that utilization pathways typically require higher purity than geological storage.
A: Yes, the standard includes a discussion of quality requirements for CO₂ utilization applications including enhanced oil recovery, chemical synthesis, and food-grade applications, noting that utilization pathways typically require higher purity than geological storage.
