CO₂ Product Specifications

Carbon dioxide (CO₂) captured by carbon capture, utilization, and storage (CCUS) projects must meet quality requirements before it can be transported and stored. These requirements are known as CO₂ product specifications, and they are set by transportation and storage operators to ensure system integrity and long-term reliability. 

Not all CO₂ product specifications are the same. They vary based on transportation method (i.e., pipeline, rail, truck, or ship), intended end use of the CO₂ (e.g., dedicated geologic storage or stored following enhanced oil recovery (EOR)), and the design of the transport and storage system. 

The variability doesn’t end there. The CO₂ product from each capture plant can vary due to flue gas sources, capture technologies, and purification or dehydration methods. These factors influence the characteristics of the CO₂ product, including its physical state (e.g., gas, liquid, or dense phase), impurity types, concentrations, and the pressure and temperature of the CO₂ exiting the capture process. 

To understand why CO₂ product specifications exist, it’s important to first recognize how impurities can impact transportation and storage systems. While rail, truck, and ship transport are all valid methods for transporting CO₂, particularly for smaller volumes or specific scenarios, pipelines are the primary transport method for delivering captured CO₂ to end users and storage sites. In Canada, pipelines are the dominant, safe and efficient method for large-scale CCUS operations. 

Capture, transport, utilization and/or storage are the value chain components of CCUS. Sometimes the entire process is for a single emitter, and other times transportation and storage networks are shared by multiple projects.

When there is only one source of CO₂ emissions (i.e., from one capture plant) entering the transport and storage network, the operating conditions are more stable, and the stream’s composition is typically consistent and predictable. When multiple capture plants are sharing transportation and storage infrastructure, variations are more likely. Different CO₂ sources may have different impurities, and when the CO₂ streams interact with one another, it can alter the behaviour of the product. These interactions may impact the safety and integrity of the system. Each CO₂ stream in the system must meet the product specifications to keep the system safe.

CO₂ product specifications are set by transportation and storage requirements. These specifications may vary depending on the impurities present for each respective system:

• Pipeline Transport: Pipelines have strict impurity limits to prevent corrosion while also minimizing operational hazards, such as two-phase flow. Additionally, higher levels of impurities reduce the density of a CO₂ stream, decreasing CO₂ flowrates under the same inlet pressure and temperature conditions. As impurity concentrations rise, the pipeline loses throughput capacity, resulting in decreased CO₂ flowrates unless additional compression is applied. 
• Dedicated Storage Reservoirs: Impurity limits are needed to prevent reactions with reservoir minerals, which can accelerate mineral dissolution and potentially affect reservoir integrity. These reactions can reduce injectivity and storage capacity by altering the permeability and porosity of a reservoir. Additionally, impurities can promote the precipitation of solids, increasing the risk of hydrate formation, or enhancing microbial activity which can block pore spaces in the reservoir.
• CO₂ EOR: Impurity limits are set to minimize interference that can negatively impact injectivity, mobility, and miscibility within the oil reservoir.

The type of impurities present in a CO₂ product stream depends on the source, which can be from naturally occurring sources, pre-combustion capture, or post-combustion capture.  Naturally occurring sources and pre-combustion capture produce a high-purity CO₂ product with few impurities, making them well suited for pipeline transport and storage with minimal conditioning. In contrast, post-combustion capture introduces impurities not typically found in other sources, such as nitrogen oxides (NOx), sulfur oxides (SOx), oxygen, and particulates. When transporting and storing CO₂, these impurities can create complexities. 

Some common impurities found in CO₂ product streams and their implications for transportation and storage are:

Oxygen (O₂)

Source: Oxygen, which is typically present in post-combustion flue gas, originates from processes where fuel is combusted with air.  Since fuel and air don’t mix perfectly in a combustor, the system is intentionally operated with excess air to ensure all fuel is burned. Any oxygen that is not part of the combustion reaction passes through the system and appears in the flue gas.

Impact on transportation: Oxygen is a non-condensable gas, meaning it cannot be condensed or separated during the capture process. Because of this, oxygen dilutes the stream, increasing the total volume that must be compressed and transported to deliver the required amount of CO₂. Since compressors are sized and operated based on the total volumetric flow, increased flow may also increase the size, cost, and power consumption of compression required for capture and transportation. The presence of oxygen can also prevent the natural formation of protective iron carbonate layers on carbon steel pipelines, increasing the system’s risk of corrosion. 

Impact on storage / CO₂ EOR: Oxygen can react with minerals in the storage reservoir, forming solid precipitates. These solids can plug pore spaces in the reservoir, reducing permeability and lowering injectivity and storage capacity. Oxygen can also enhance microbial activity, which may produce additional solids and further decrease a site’s injectivity and storage capacity. In EOR applications, oxygen can increase the minimum miscibility pressure, which can reduce the oil recovery efficiency.

Interactions with other impurities: When oxygen is present with water and acid-forming impurities such as SOx and/or NOx, it can form corrosive acids that can corrode carbon steel pipelines and CO₂ injection wellbores. These corrosive acids can also lead to mineral dissolution in the storage reservoir, impacting the integrity, injectivity, and storage capacity of a reservoir. 

Water (H₂O)

Source: In captured CO₂ streams, the sources of water differ between pre-combustion and post-combustion capture. In pre-combustion capture, the processes used to create and separate the CO₂ involve steam and water producing reactions. As a result, residual water can remain in the stream after separation. In post-combustion capture, water comes from any moisture present in the air or fuel during combustion and the capture process itself. Pre-treatment, such as direct contact cooling, or the amine-based capture process using an aqueous solvent, can introduce water into the CO₂ stream. Additionally, water can condense and drop out during pipeline transport if its solubility decreases due to changes in pressure, temperature, or composition.

Impact on transportation: The presence of free water is a significant concern for pipeline operations, as it presents the following challenges:

• Two-phase flow: This occurs when gas and liquids both exist within the pipeline. This causes operational concerns and can comprise the integrity of a pipeline due to unstable flows, increasing the risks of slugging, pressure drops, and corrosion.  
• Hydrate formation: In a CO₂ stream, hydrate formation is influenced by pressure, temperature, water concentration, and the presence of impurities. Hydrates are problematic as they can cause erosion or accumulate in a pipeline, potentially damaging or disrupting transport operations. Under typical operating conditions, hydrate formation in a pipeline system is generally not an issue, but may be of concern if the CO₂ stream is not properly dehydrated and reaches low temperatures.
• Dissolution of CO₂: This occurs when CO₂ readily dissolves in water, forming carbonic acid, which is corrosive and can impact carbon steel pipelines.  

Impact on storage / CO₂ EOR: Under certain temperature and pressure conditions, water that is introduced in the storage reservoir can form solid hydrates, which occupy pore space and reduce a site’s injectivity and storage capacity. 

Interactions with other impurities: When water is present with impurities such as NOx and SOx, it can react to form nitric acid (HNO₃) and sulfuric acid (H₂SO₄), both of which can potentially corrode carbon steel pipelines and CO₂ injection wellbores. These acids can also lead to mineral dissolution in a storage reservoir, impacting the reservoir’s integrity, injectivity, and storage capacity.

Sulfur Oxides (SO_X)

Source: Sulfur oxides are present in post-combustion flue gas. These are composed of primarily sulfur dioxide (SO₂) and are formed when sulfur-containing fuels such as coal, oil, and natural gas are combusted with air.  

Impact on transportation: Compared to other impurities, SOx significantly reduces the ability of water to dissolve in the CO₂ stream, increasing the chances of free water forming in the pipeline.

Impact on storage / CO₂ EOR: In storage reservoirs, SOx may cause enhanced dissolution and precipitation of certain materials, particularly carbonates, but its effects are typically limited to the near-wellbore area. In EOR applications, SOx is expected to negatively affect reservoir injectivity, mobility, and miscibility.

Interactions with other impurities: Even small amounts of water in the CO₂ stream can react with SOx to form H₂SO₄, a highly corrosive substance that can impact pipelines and injection wellbores. H₂SO₄ can also be formed when SOx and water are both present in the reservoir. This acid can cause mineral dissolution and precipitation within the reservoir, reducing the site’s injectivity and storage capacity and affecting its permeability and porosity. When SO₂ and H₂S are present together, their interaction can lead to the deposition of elemental sulfur, which can severely block pore spaces in the reservoir and reduce the site’s injectivity and overall storage capacity.   

Nitrogen Oxides (NOx)

Source: Nitrogen oxides are present in post-combustion flue gas. Nitrogen oxides, primarily nitric oxide (NO) and nitrogen dioxide (NO₂), form during high-temperature combustion of fossil fuels or biomass with air.

Impact on transportation: NOx can react with other impurities in the CO₂ stream, such as H₂S, which can promote water dropout within the pipeline. 

Impact on storage / CO₂ EOR: The presence of NOx in a storage reservoir may cause the dissolution of iron-bearing minerals, leading to secondary mineral precipitation that reduces permeability, occupies pore volume, and ultimately lowers storage capacity and reservoir injectivity. In EOR applications, NOx is expected to negatively impact reservoir injectivity, mobility, and miscibility. 

Interactions with other impurities: In the presence of water, NOx can form HNO₃, a corrosive acid that can damage pipelines and CO₂ injection wellbores. Nitric acid can cause mineral dissolution, impacting the storage reservoir’s porosity and permeability, affecting the site’s overall injectivity and storage capacity. However, NOx is not expected to have as significant of an impact on mineral dissolution as SOx. 

CO2 product specifications are not only influenced by the effects of individual impurities, but also by how these impurities interact with one another. For example, setting very low water limits can provide flexibility for relatively higher limits of impurities such as NOx, SOx, and O2, and vice versa. Achieving the right balance helps maintain pipeline integrity and storage safety while optimizing processing needs. For a more comprehensive look at the impacts that various impurities can have on pipeline transportation, refer to the materials published by a Joint Industry Project in Work Package 8 – Pipeline Transport here: Industry Guidelines for Setting the CO2 Specification in CCUS Chains.

As highlighted in Getting to FID Series Part 2, capture, transportation and storage are interdependent. Progress in one can’t happen without the others. Capture projects need to assess storage and transportation options early in the planning process, as CO2 product specifications can directly impact a capture system’s design and overall cost. To help emitters meet the CO2 product specifications required for their transport and storage selections, a range of pre-treatment and post-treatment options exist. For a detailed review of impurity removal technologies, refer to the Joint Industry Project materials published under Work Package 5 – Capture and Conditioning here:  Industry Guidelines for Setting the CO2 Specification in CCUS Chains.

To continue advancing our understanding of CO₂ transport and inform future standards, ongoing research is being conducted by industry groups such as the Canadian Standards Association (CSA) Z662 Technical Committee CO_₂ Task Force, International Standards Organization (ISO), American Petroleum Institute (API), and other international organizations. Lessons learned from operational CO₂ transport and storage projects, particularly those in Canada, provide current and future operators with valuable insights, existing industry standards, and recommended practices. To learn more about existing and planned CO₂ transport and storage projects, visit the CCUS in Canada map. 

To view examples of CO₂ product specifications and how allowable impurity limits vary across projects in North America, Europe and the United Kingdom, click here.