Recent excitement surrounding carbon capture largely focuses on certain technologies: absorber configurations, new solvents, tailored solid sorbents, and others. However, the fate of CO2 after it’s captured is often overlooked. Many techno-economic analyses on emerging carbon capture technologies exclude costs involved beyond the “gate,” or boundary limit of the project. Once captured, it is important to consider CO2 purity and how it is specified, custody transfer, and transport.
CO2 Purity Specifications
CO2 purity specifications are determined by whichever is stricter: transportation requirements or end use. For example, CO2 purity tolerances intended for food and beverage production are far more stringent than for Enhanced Oil Recovery (EOR). Similarly, the allowable moisture content in CO2 is greater for pipeline transport than liquified transport.
Table 1: Example CO2 pipeline quality specifications1
Pure, dry CO2 is inert towards most materials, but the presence of water creates a highly corrosive mix that can damage pipeline walls. When liquifying CO2, moisture can form hydrates and plug lines. The most common method for removing water from CO2 is a glycol contactor, which reduces moisture content to around 100 ppm. This method has the additional benefit of removing heavier hydrocarbon contaminants. To get even dryer CO2, an adsorption process is needed; common adsorbents include molecular sieves, silica gel, and aluminosilicate.
Oxygen also promotes corrosion, and is removed via catalytic reduction, distillation, or adsorption. H2S is destructive and highly toxic, and can be eliminated using distillation, electrochemistry, membranes, solvents, or sorbents. It’s important to keep nitrogen below 4% to maintain a dense phase in pipelines and can be removed using distillation. Hydrocarbon contaminants can create a separate phase in the pipe and are removed with catalytic oxidation or adsorption.
The cost of CO2 cleanup depends on the processes used; energy-intensive processes that require high-compression or cryogenic temperatures will be expensive in the long-run.
CO2 Custody Transfer
Once CO2 meets the right specification for transport or use, it undergoes a custody transfer to either a carrier or end user. Think of custody transfer as the of “point of sale” for the CO2 product; this means metering. The Inflation Reduction Act is providing substantial credits for sequestration of CO2, set on a dollars-per-ton basis. Consequently, accurate measurement of CO2 is necessary to ensure accurate flow of funds.
At the scale required for industrial or utility CO2 capture systems, CO2 is generally exported or sequestered as a supercritical liquid at pressures exceeding 1100 psia. While there are several modalities for the fiscal quantification of supercritical fluids, CO2 presents specific challenges, such as contaminants that cause density variations. Even small amounts of contaminants cause large changes in CO2 density. Even then, most meters are calibrated using water, and there is a lack of testing facilities capable of calibrating meters using supercritical CO2. To account for property variations–which result from changes in composition, temperature, or pressure–two approaches can be used: reference a database like NIST’s REFPROP or calculate the changed properties using an appropriate equation of state like Peng-Robinson or Span-Wagner. 2 Contracts and offtake agreements should specify what compensation method is used.
Coriolis meters are an accurate choice, with absolute average deviations of around 1%, but precautions are necessary to prevent the entrainment of compressible gas “bubbles” in the meter. 3 Ultrasonic flow meters can attain accuracies down to 0.5%, but the design must consider the high acoustic attenuation of supercritical CO2. 4 A more conventional metering option is a differential pressure orifice, and these can attain accuracies down to around 1.5%. All three measurement options have been used for CO2 metering; the decision ultimately comes down to accuracy versus cost.
Figure 2: Proposed (broken line) and existing (solid line) CO2 pipeline map.
Installing a spur line to a CO2 trunk line is a costly process and requires careful planning.
A one million TPD project, for example, requires a 6 to 8” pipeline (depending on the length of the spur line). Recent CO2 pipeline projects cost around $70-200k per diameter-inch-mile; a rough order estimate of $1-2 million per mile is reasonable. The rough order of magnitude for extended spur lines (250 miles and beyond) can cost the same as a carbon capture system.
Takeaway
While the capture technology usually takes the spotlight in a carbon capture project, it’s important to consider the fate of CO2 beyond the gate. How CO2 is transported or used ultimately determines purification requirements, which can be an additional cost. Furthermore, construction of CO2 infrastructure like pipelines can quickly become major expenditures. Questions such as “Where is the plant?” and “What will the CO2 be used for?” are as important as “How are we going to capture it?”
About the Authors
Rob Krumm is a senior chemical engineer with more than a decade of diverse industrial research and experience. He possesses a deep understanding of chemical engineering principles and has hands-on involvement with pyrolysis and gasification. Additionally, Rob has extensive knowledge surrounding methane valorization, gas to liquid, solids conversion, and the nuanced difficulties that come with thermal conversion processes. Along with project management, his skills encompass research and development, emerging technology, and intellectual property development.
John Lagomarsino is a senior project manager who has extensive experience in fossil power plant work, with a background in engineering economics and evaluation of technical factors. Throughout his career, John has worked with power generating cycles and air pollution control systems and has led feasibility studies.
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1 Race, J.M., Wetenhall, B., Seevam, P. M., & Downie, M. J. (2012). Towards a CO2 pipeline specification: defining tolerance limits for impurities. The Journal of Pipeline Engineering, 11(3), 173-190.
2 Zhao, Q., Mecheri, M., Neveux, T., Privat, R., & Jaubert, J-L. (2017). Selection of a Proper Equation of State for the Modeling of a Supercritical CO2 Brayton Cycle: Consequences on the Process Design. Industrial & Engineering Chemistry Research, 2017 56 (23).
3 Nazeri, M., Maroto-Valer, M. M., & Jukes, E. (2016). Performance of Coriolis flowmeters in CO2 pipelines with pre-combustion, post-combustion and oxyfuel gas mixtures in carbon capture and storage. International Journal of Greenhouse Gas Control, 54 (297-308).
4 Collie, G. J., Nazeri, M., Jahanbakhsh, A., Lin, C. W., & Maroto‐Valer, M. M. (2017). Review of flowmeters for carbon dioxide transport in CCS applications. Greenhouse Gases: Science and Technology, 7 (1).
Article originally published in on the POWER Engineers site.
