At the International CCS Knowledge Centre (Knowledge Centre) our mandate is to accelerate the global deployment of carbon capture and storage (CCS).
As an engineer, my specialty is thermal performance and I have worked at SaskPower for over 20 years, with a heavy involvement in CCS since early 2006. A year ago, I moved over to join our technical team of research experts at the Knowledge Centre whose work is dedicated to taking the real-life experience and lessons learned at SaskPower’s Boundary Dam 3 (BD3), the world’s first commercial power plant to successfully use Carbon Capture and Storage (CCS) technology, and examine ways to improve the delivery and performance so that it can be effectively utilized around the world.
Our retooling and adapting work is producing exciting results. Without a doubt we are headed in the right direction. This blog is based on my abstract that can be found on our website here, and is one of a series that will share the significant improvements being achieved through our research and feasibility studies. The full paper on this topic will be delivered at the respected GHGT-14 Conference in Melbourne, Australia in October 2018.
The Thermal Energy Challenge
BD3’s carbon capture facility uses post-combustion capture technology. One of the challenges of post-combustion capture with solvents is the amount of thermal energy that is required to regenerate the solvent and release the CO2. In this instance, the source of this thermal energy is critical to how efficient and flexible the plant will operate.
Is the thermal energy most efficiently obtained at the crossover between the intermediate and low-pressure steam turbines of the coal plant or is adding a new gas-fired source to provide the thermal energy more efficient and economical? This is the topic of our study.
Why Our Study Was Different!
Most studies to date (Derate Mitigation Options) have focused on the full load performance of a plant with steam extraction, or thermal energy, coming from the coal plant steam turbine vs the combined performance of the coal plant and gas plant in a combined heat and power arrangement.
Naturally, these studies ultimately see a better overall efficiency if the gas turbine is added, as the efficiency of a modern natural gas plant is higher to begin with. But what these studies don’t consider are the realities or inefficiencies imposed on the gas turbine cycle by the combined heat and power arrangement. Combined heat and power typically looks good when compared to direct firing for production of process heat, but it may not look as good when you contrast it to taking the lowest quality heat from a larger coal power plant cycle.
In contrast, our study looked at:
- the combined performance for a coal plant and a modern natural gas combined cycle for a case where the two plants are totally separate; and,
- a case if the coal plant and modern natural gas plant are integrated - with the steam cycle of the new gas plant being the source of steam.
Our examination allowed for a true assessment of the impact that the process steam extraction had on the smaller steam cycle of a gas-fired plant.
So let’s break it down….
New Gas-Fired Steam Source
The first thermal energy option we studied was adding a new gas-fired source designed to provide the steam. Adding a gas-fired boiler to produce the steam was one idea but it made poor use of the potential energy of the fuel. Then we looked at a gas turbine with a heat recovery steam generator and tried to increase the efficiency by adding a steam turbine. However, applying a more efficient gas turbine cycle meant there was proportionally less, low-quality heat available. To compensate, an efficient gas plant would need to be even larger than the coal plant to support the regeneration heat demands.
Steam Extraction from a Coal Plant
In our examination of steam extraction from a coal plant, we replaced portions of the steam path to optimize the steam extraction pressure without imposing throttling losses or adding additional equipment. This provided the opportunity to apply upgraded blade technology and also recover accumulated degradation in turbine components. It also provided the best environment for the plant to operate with maximum flexibility to ebb and flow with the variables that impact power plants on a daily basis.
We were delighted to discover that these modifications impacted total project costs by only 5%.
A huge savings! Just as impressive is the minimal lost electrical output due to the process extraction. This was less than two thirds of what would have resulted from extracting the steam from a natural gas combined cycle. Any recovery of degradation in the existing steam turbine would be an additional benefit.
The Need for Flexibility
Flexibility is key here because many variables impact how a plant should be economically operated. The value of electricity, CO2 revenue, carbon taxes, other operating constraints can change throughout the day and all force the need for flexibility. If the coal-fired capture plant is integrated with the gas plant, then it becomes more difficult to dispatch the two generation sources independently.
We believe that extracting steam from the existing coal plant provides the most flexible and economical option. If the steam is coming from the coal plant, the quantity of steam available will follow the amount of CO2 to capture, as the load on the coal plant changes.
The cost of CCS will continue to rapidly decline by applying technological refinement at all stages of development. The International CCS Knowledge Centre continues to research and apply what we learned from the real-life experience at BD3. This knowledge is critical because it provides us with practical applications that can be used to support countries and investors in significantly reducing their costs and risks with their own large-scale CCS projects.
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For more blogs on advancements to 2nd generation CCS, see:
- How to Lower the Energy Penalty - Optimizing Steam Cycle
- New Heat Integration Strategy Improves the Efficiency of a CCS Facility
- Large-Scale CCS – It IS Cost Effective, Flexible and Efficient
- Tackling the Cooling Challenge of a Large-Scale CCS Retrofit