Why Retrofitting OTSGs with CCS Costs More Than Building New GT-OTSG CCS Systems

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.

A Better Approach to Cogeneration Choices 

Building carbon capture into the initial design of a Once-Through Steam Generator (OTSG) can yield stronger project returns than a later retrofit. A Gas Turbine paired with an OTSG (GT-OTSG) can improve process integration and further reduce the cost of capture. Designing carbon capture into a new facility can optimize usable space and reduce integration complexity. Standalone OTSGs produce flue gas with low CO₂ concentration, while an integrated GT-OTSG configuration creates a much higher CO₂ concentration in the exhaust. Capturing CO₂ from a concentrated stream requires smaller equipment and less energy. This directly reduces capital and operating costs.

Whether you are developing a natural gas power plant or a heavy oil Steam Assisted Gravity Drainage (SAGD) facility, you need both electricity and steam. Developers traditionally look at steam and power separately. They build standalone gas turbines for power plants and standalone OTSGs for steam. They may also use cogeneration, utilizing a Heat Recovery Steam Generator (HRSG) to capture waste heat and generate steam from the gas turbine. 

HRSG systems are large and complex. They contain multiple pressure sections and circulation loops. During start-up, these units require bypass stacks and diverter dampers to vent emissions and heat before they can safely generate steam. This extra equipment significantly increases the capital cost and physical footprint of a project. 

A GT-OTSG cogeneration design fixes this. Unlike HRSGs, an OTSG uses a continuous tube matrix. Subcooled feedwater enters cold and exits as high-purity superheated steam in a single pass. This steam can be used directly for SAGD oil production. It can also be used to spin a secondary steam turbine for more grid electricity, or it can provide the thermal heat needed to regenerate solvents in a carbon capture facility. 

The Cost Penalty of Carbon Capture Retrofits 

Retrofitting an existing standalone OTSG or gas turbine with carbon capture faces economic barriers. It requires high capital and it forces a large physical footprint into a space not designed for it. These are the same economic challenges facing CCS retrofits for other heavy industrial emitters.   
 
Designing a natural gas cogeneration facility with carbon capture built-in from day one is the lower-cost option, as the layout can be optimized for the space available. Integrating the steam production and capture systems in the design phases reduces the complexity of tying two facilities together in the field. 

The Impact of CO₂ Concentration 

Different industries emit drastically different concentrations of CO₂ in their exhaust gases. Heavy industrial processes like cement and steel production naturally generate highly concentrated CO₂ streams, often between 20% and 30%. Coal-fired power plants sit in the middle, typically exhausting around 12% to 15% CO₂. 

Natural gas exhaust sits at the low end of this spectrum. Standard gas turbine exhaust contains a CO₂ concentration of roughly 3% to 5% by volume, with a high concentration of residual oxygen. This low concentration of carbon dioxide limits the capture performance, necessitating large absorber columns and high solvent circulation rates to achieve target capture rates. 

By introducing supplemental fuel into this oxygen-rich exhaust stream via a duct burner, the system consumes residual oxygen and generates additional CO₂, increasing the exhaust concentration to 8% or higher. This drives down capture costs by reducing the amount of solvent required to capture each tonne of CO₂ and reducing the amount of heat needed for solvent regeneration.  

Physical Footprint and Modular Construction 

The GT-OTSG arrangement reduces the facilities footprint. Engineering out the steam drums, downcomers, and circulation networks required by a HRSG creates a more compact footprint.  

The mechanical simplicity of the OTSG eliminates thick-walled pressure vessels, large circulation pumps, and bypass dampers, substantially reducing raw material costs. The continuous tube matrix design also supports modular construction. That means these OTSGs can be fabricated, assembled, and hydrostatically tested before being moved to site. Traditional HRSG designs require on-site construction and expensive skilled labor to complete complex connections in the field. 

Summary 

The lowest total cost path for carbon capture is building it into the facility design from day one. Pairing a gas turbine directly with an OTSG allows the developer to create higher concentration CO₂ in the exhaust gas, which lowers the overall cost per tonne of CO₂ captured.