Managing Excess Heat in a Capture Facility

Optimizing Capture Costs: A Blog Series

We have created a series of blogs detailing how carbon capture isn’t just about capturing carbon dioxide (CO₂), it’s about designing an entire system to do it efficiently. In this optimization series, we explore the engineering decisions that influence the cost, performance, and reliability of carbon capture and storage (CCS) facilities, and the trade-offs engineers make to optimize project outcomes.

Managing Excess Heat in a Capture Facility

One aspect of carbon capture that many people don’t think about is heat management. Capturing CO₂ from industrial emissions generates heat during the absorption and solvent regeneration processes. This heat must be removed to keep the system operating efficiently, much like the radiator in a car removes excess heat to keep the engine from overheating. Because the ability to cool the system depends on the ambient air temperature, the design of the cooling system plays a critical role in the performance and efficiency of a capture facility.

The Canadian Prairies present a unique challenge, as temperatures fluctuate between highs near 40°C in the summer and lows as cold as minus 40°C in the winter. Cooling systems have to dissipate excess heat during the hottest days and continue to operate reliably through winter conditions. Designing equipment that performs efficiently across these temperatures is an important consideration for capture projects in the region.

So, how does an engineer design a cooling system for the right ambient temperature?

At first glance it might be tempting to design for the hottest temperature recorded at the project site. While this would help ensure the facility can operate during extreme heat, it would also require the cooling system to be larger than typically needed, with larger heat transfer surfaces, additional structural steel, more piping and higher fan power. These additional costs would be difficult to justify for an event that might only occur for a few hours over the life of the facility.

Instead, engineers look for the sweet spot – a balance between capital costs and operational performance. Rather than designing for the extremes, they use weather data to determine the design temperature that provides reliable performance and keeps costs under control. The design temperature is based on statistical exceedance values, not the temperatures reported in the weather forecast.

Two temperatures

Not all cooling systems work the same. Some rely on the surrounding air to remove heat, while others use the evaporation of water to improve cooling. As a result, engineers use two different measures of ambient conditions to design a system: dry-bulb temperature and wet-bulb temperature.

Dry-bulb temperature is the one most people are familiar with. This is the actual air temperature reported in the weather forecast. Wet-bulb temperature is how much cooling can happen by evaporating water. Effective cooling system design needs engineers to understand both temperatures. On a dry day evaporation is more effective, so the wet-bulb temperature is lower than the dry-bulb temperature. On a humid day, evaporation is less effective and the two temperatures are closer to one another. It’s easiest to understand if you think about how our bodies cool us down. Imagine stepping outside on a 35° day in the Prairies. Your body sweats, and because the air is dry, your sweat evaporates quickly, carrying the heat away from your skin and cooling you down. Now imagine stepping outside on a humid 35-degree day in Toronto. Your body still sweats, but because the air already contains a lot of moisture, your sweat doesn’t evaporate as quickly. With less evaporation, your body can’t cool down as effectively, and the day feels much hotter.

Cooling System Designs

The type of cooling system determines how much heat can be removed under different ambient conditions. Whether a facility uses air cooling, evaporative cooling, or a combination of the two changes how much heat can be removed, and which ambient temperature – dry-bulb or wet-bulb – is used for design.

Air-cooled heat exchangers transfer heat directly to the surrounding air. Their performance depends on the dry-bulb temperature. Evaporative cooling systems, like cooling towers, improve the cooling capacity by allowing a small amount of water to evaporate. Because evaporation removes additional heat, these systems can cool fluids to temperatures below the ambient dry-bulb temperature. Their ultimate cooling limit is the ambient wet-bulb temperature. This explains why evaporative cooling systems generally provide more cooling capacity than air-cooled systems, especially in hot weather.

Balancing Performance and Cost

When selecting a design temperature, engineers are looking at decades of historical weather data. They are balancing the costs of building a larger cooling system against the operational impacts of occasionally operating during hotter-than-designed-for conditions.

One of the tools engineers use to make these decisions is an exceedance value. For example, a 10% dry-bulb exceedance temperature means ambient temperatures are expected to be hotter than the design temperature for approximately 876 hours per year. A 15% exceedance would be hotter for roughly 1,314 hours per year. Different projects select different exceedance values depending on the desired balance between capital costs and operational flexibility.

A lower exceedance value means a larger, more expensive cooling system that maintains performance during hotter weather. A higher exceedance value means capital costs might be lower, but cooling performance will decline during hot weather.

Operating Considerations

When a facility is designed for a 15% temperature exceedance, ambient conditions may be above the design limit for approximately 1,314 hours per year. During those hotter periods, the cooling system might not be able to remove enough heat to support full-capacity operations. Rather than shutting down the capture facility, operators can reduce the plant throughput to bring cooling demand back within the system’s capacity.

For an amine-based capture facility, this typically means reducing the flue gas flow to the absorber and lowering the amine circulation rate. While these adjustments allow the plant to operate safely, they also reduce the overall CO₂ captured. Depending on the facility, this can increase carbon compliance costs, reduce CO₂ deliveries to transport and storage systems, and potentially lower revenues. These potential operational impacts have to be weighed against the capital costs of a larger cooling system.

While summer temperatures typically determine the size of cooling systems, winter conditions introduce a different design challenge. As temperatures fall well below freezing, cooling equipment needs to keep operating without the process fluid or cooling water  freezing.

To maintain reliable operations in Canadian winters, cooling systems incorporate freeze protection and turndown strategies. These may include installing Variable Frequency Drives (VFDs) that slow fan speeds on motors, adjustable louvers to limit airflow, and bypass systems to keep warm fluid circulating in the equipment. Together these measures can keep the facility operating safely across the highs and lows of Canadian seasons.

Design Margins

Engineers typically include a thermal design margin of 10% to 15% when sizing cooling equipment. This additional capacity accounts for things like heat exchanger fouling, equipment aging and normal uncertainties in future operating conditions. The margins help to ensure the system continues to perform reliably throughout its operating life.

Planning for long-term climate change is a different design consideration. There is a projected 1.5°C or 2°C global temperature rise twenty years in the future. While project owners could install larger cooling systems to account for these increases, doing so may not be an effective use of capital.

Instead, many facilities can build flexibility into their design. For example, piping headers can include additional tie-in points, structural layouts can have additional space for future equipment, and electrical systems can be designed to accommodate expansion. If future climate conditions or operating requirements increase cooling demand, additional cooler bays or towers can be added with minimal disruption to the facility.

Engineering for the Real-World

Designing a cooling system for a carbon capture facility is an exercise in balancing performance, reliability and costs. While it might be tempting to design for the hottest and coldest temperatures recorded, doing so would require larger equipment that might not be needed for more than a handful of hours over the life of the facility.

Instead, engineers optimize cooling systems by selecting appropriate design temperatures based on historical data, understanding the performance of different cooling technologies, and planning for operational adjustments. Combined with thoughtful margins and provisions for expansion, this approach delivers a cooling system to support reliable operations and efficiently use capital over the lifecycle of the facility.