Flue gas is a byproduct of combustion, released into the atmosphere through flue stacks of industrial facilities. For decades, flue gas has been treated as a waste stream, with monitoring and testing focused on meeting environmental regulatory requirements. As industries look toward reducing carbon dioxide (CO2) emissions through carbon capture, utilization, and storage (CCUS), understanding flue gas composition becomes increasingly important. Flue gas is the primary feedstock for a multi-million to multi-billion-dollar investment in CCUS.
For carbon capture projects, it is crucial to characterize the flue gas stream thoroughly to understand its composition and how it fluctuates across operating conditions. Impurities in the flue gas can negatively impact the performance of a carbon capture system, potentially leading to operational issues and unforeseen costs. To mitigate these risks, detailed flue gas characterization helps to identify contaminants early, informing key decisions for capture plant designs.
Standard Environmental Testing vs. Detailed Flue Gas Characterization
Standard environmental testing is designed to satisfy regulatory permit requirements. Industrial facilities rely on flue gas data collected from continuous emissions monitoring systems or from annual stack testing campaigns. The regulating authority determines the specific components that must be measured, and the acceptable limits for each component, and these parameters vary by jurisdiction.
This data is often not thorough enough when it comes to the optimal design for carbon capture facilities. Standard environmental testing focuses on the total amount of pollutants emitted into the atmosphere relevant to air quality. These tests typically measure broad categories such as total nitrogen oxides (NOx), total sulfur oxides (SOx), and particulate matter (PM), including two standard test size ranges of PM10 and PM2.5. The reporting limits are set strictly for environmental compliance, and any other contaminants that may be present are not tested or reported. When not properly accounted for, these unknown contaminants can significantly affect the performance, reliability, and operating costs of a carbon capture project.
Detailed flue gas characterization goes beyond environmental testing by identifying a wider range of chemical species and trace contaminants which may be present at very low concentrations. Instead of measuring broad categories, detailed characterization identifies specific compounds such as nitrogen dioxide (NO2), sulfur dioxide (SO2), oxygen (O2), aerosols, and acid halides. It also detects particulate matter smaller than PM2.5 and includes a comprehensive particulate analysis, measuring particle size distribution, particulate count, concentration, and composition. This level of flue gas characterization is essential as it can identify contaminants that may reduce capture performance or cause operational issues within the capture system.
Flue Gas Impurities and Performance Implications
The performance implications of common flue gas impurities on amine-based capture processes are outlined below:
Solvent Degradation
What It Is: Amines and flue gas impurities can react and form degradation products.
Flue Gas Impurities Involved: O2, SO2, NO2, acid halides, and metals
Performance and Cost Implications: Solvent degradation reduces the solvent’s capacity to capture CO2, lowering a project’s overall capture efficiency. It also increases operating expenses due to higher solvent make-up requirements, generation of additional waste for disposal, and the increased energy required for solvent regeneration. Certain amine degradation products (e.g., nitrosamines) can also be harmful to human health, and their emissions levels must be controlled according to project permits.
Formation of Aerosols
What It Is: Condensable particulate matter in the flue gas can form aerosols (mists). Amine molecules can attach to these fine droplets, leading to solvent carryover and loss through the absorber stack.
Flue Gas Impurities Involved: Sulfur trioxide (SO3), ammonium (NH4), volatile organic compounds (VOCs)
Performance and Cost Implications: When aerosols are vented out the absorber stack, if they carry amine molecules the solvent is lost to the atmosphere. Elevated solvent emissions can result in environmental exceedances which can potentially jeopardize a project’s operating permit compliance. Increased solvent loss also increases operating expenses due to higher solvent make-up requirements.
Solid Fouling
What It Is: Fouling occurs when solid particles in the flue gas deposit onto equipment surfaces such as absorber packing, heat exchanger internals, flue gas blowers, and pumps.
Flue Gas Impurities Involved: Particulate matter
Performance and Cost Implications: Solid fouling reduces the solvent’s capacity to capture CO2 as it reduces the contact between the flue gas and amine solution in the absorber. It also reduces the heat transfer efficiency in heat exchangers, further lowering the plant’s overall capture efficiency. In addition, fouling increases operating expenses by requiring more frequent cleaning and maintenance. Fouling can also cause unplanned downtime, impacting the plant’s operational reliability and availability.
Foaming
What It Is: Foaming occurs when the amine solution mixes with the flue gas, creating small bubbles. Impurities in the flue gas can alter the solvent’s surface tension and viscosity, increasing foam height and prolonging the foam’s collapse time.
Flue Gas Impurities Involved: Hydrocarbons, VOCs, particulate matter
Performance and Cost Implications: Excessive foaming reduces effective gas-liquid contact between amines and CO2 and increases pressure drops in both the absorber and regenerator units. Foaming lowers a project’s overall capture efficiency by reducing the solvent’s capacity to capture CO2. Foaming also increases operating expenses by requiring anti-foam systems and potentially increasing the frequency of operational interventions needed to maintain stable performance in the absorber and regenerator.
Impacts on Capture Plant Design
Early detailed characterization is important as it helps identify potential issues and informs decisions on capture plant design such as:
- Pre-treatment systems: Pre-treatment equipment can include various particulate removal technologies such as oxygen removal, NOx removal, SOx removal, or other chemical treatments used to condition the flue gas before it enters the capture unit. Detailed flue gas characterization allows for the proper sizing and selection of these units to ensure they can handle the impurities present.
- Capture technology: Different amines vary in their resistance to specific impurities present in the flue gas. Understanding the flue gas composition allows operators to select the most suitable amine for the capture process, optimizing both capital and operating costs. As new technologies develop, alternative capture methods may become more appropriate for certain flue gas profiles.
- Equipment margins and redundancy: Understanding the composition of the flue gas entering the capture unit can inform decisions about how much margin in flow rates and pressure drops should be incorporated into the design. It can also inform where equipment redundancy may be required, such as installing parallel heat exchangers and/or pumps. Having redundant equipment supports higher plant reliability and availability, as it allows for cleaning and maintenance to take place without interrupting operations.
- Amine reclamation and purification equipment: Amine technology providers may use thermal reclaimers or other processes to maintain solvent quality. Higher contaminant levels in the flue gas may require larger or additional pieces of equipment to handle the increased impurity loads. Understanding the flue gas composition also helps to determine the appropriate size and type of filtration equipment needed to protect downstream equipment.
When Should Detailed Flue Gas Characterization be Performed?
Detailed characterization should be done early in the project, ideally during the pre-FEED stage and long before the design freeze in the FEED phase. The flue gas should be tested multiple times, as its composition can change based on ambient weather conditions, plant operating conditions, and fuel characteristics. Testing multiple times throughout the year gives capture projects the best chance to fully understand the host facility’s seasonal variations. A continuous emissions monitoring system (CEMS) such as Fourier transform infrared spectroscopy (FTIR) can be a powerful tool to obtain real-time flue gas parameters and can be temporarily installed for several months or used permanently to provide an understanding of how operational fluctuations affect certain flue gas components.
Challenges
Flue gas characterization for CCUS is a niche field. In Alberta, most commercial laboratories are optimized for highly repeatable environmental compliance testing, and there are limited technical capabilities to perform a thorough analysis of flue gas components. Stack testing for CCUS requires specialized extraction systems and testing methods that are not commonly used in most laboratories. Accuracy is essential to detect impurities at the very low concentrations required for CCUS systems.
There is a lack of standardized protocols and guidelines for CCUS-specific flue gas characterization. Complex equipment and testing methods make this detailed analysis expensive, and the results are difficult to validate. Testing results may not be comparable between jurisdictions, different industries, or even testing campaigns at the same facility. Some stack testing guidance exists from the American Environmental Protection Agency (EPA) as well as European and Australian organizations, but data quality depends entirely on the expertise of the testing team. Working with experienced partners who understand flue gas and CCUS chemistry is critical to getting valid and useful data.
Conclusion
Understanding flue gas composition is critical for the effective design and operation of a carbon capture system. The variety of compounds and the low concentrations of impurities not identified through standard environmental compliance testing can cause significant performance and operational issues within the capture system. Conducting detailed flue gas characterization provides capture projects with an understanding of the impurities present in their flue gas, enabling more informed design decisions for the capture plant and helping to mitigate operational risks and unexpected costs.
To understand how early and thorough flue gas characterization can improve design and reduce uncertainty in the operation of a carbon capture system, visit our flue gas calculator.