A Fundamentals-based Approach for Scale-up of DAC Technology
Geoeng Info
A Fundamentals-based Approach for Scale-up of DAC Technology
Unique scale-up approaches are needed to accelerate the development of cost-effective DAC technologies.
Energy touches, directly or indirectly, every aspect of daily life. As the global population increases, so does the demand for energy. The challenge is to provide affordable, reliable, and clean energy to support sustainable development (ExxonMobil 2019).
Background
To minimize the environmental and associated social and financial impacts of climate change, the Intergovernmental Panel on Climate Change set a goal to limit the rise in global average temperature to less than 1.5°C above preindustrial levels (IPCC 2014). Meeting this goal will likely require a combination of reduced greenhouse gas emissions from point source streams and removal of CO2 from the atmosphere. Direct air capture (DAC) technology may help with the latter, similar to natural sinks, but at much higher productivity (Sinha and Realff 2019).
Technical challenges for DAC include efficient capture of CO2 at low concentrations and minimization of its energy consumption. In addition, large-scale deployment is required to capture up to 10 billion tons of CO2 per year (NASEM 2019).
The large scale needed and the sense of urgency to address current climate issues necessitates unique scale-up approaches to accelerate the development of cost-effective technologies. The development strategy presented in this paper relies on an understanding of process fundamentals coupled with detailed modeling, parallel work across different scale-up activities, and collaborations.
Scale-up Considerations
Technology scale-up is a multidisciplinary activity involving science and engineering across different scales (see figure 1). At one end of the scale, active materials define process characteristics that have an impact at the other end of the scale, such as plant size, mechanical and civil considerations, and potential for process integration.
FIGURE 1 Multiscale nature of technology scale-up.
To accelerate technology development, activities across different areas need to be efficiently progressed in parallel, through identification of relevant tie points across scales. For instance, the contactor geometry will have implications for larger-scale aspects such as equipment selection (to accommodate pressure drop or to provide the energy required), structural needs, and their associated implications for cost and lifecycle assessment.
The wide range of scale-up activities requires expertise across several science and engineering disciplines, such as material synthesis and characterization, and chemical, mechanical, and civil engineering. And collaborations are needed among universities, national labs, startups, and industry to accelerate scale-up and deployment.
ExxonMobil has entered a joint development agreement with Global Thermostat to advance technology that will capture and concentrate CO2 from air. Global Thermostat brings more than 10 years’ experience in the DAC space with a focus on materials and demonstration units, while ExxonMobil brings expertise in process and materials scale-up, phenomenological modeling, process development, and project execution.
Use of a Model-centric Scale-up Method
The nature of DAC involves a number of challenges across different scales. They are being addressed using a model-based method that focuses on understanding the fundamentals that drive these processes in order to identify optimal scalable solutions and potential R&D opportunities.
Process fundamentals are targeted by decoupling integrated phenomena into simpler elements that can be characterized and validated through small-scale experiments (see figure 2). This strategy improves understanding of fundamentals and increases confidence in model results when validated phenomena are integrated in larger-scale experiments and systems.
FIGURE 2 Model-based scale-up approach decouples integrated phenomena, targeting model validation through simpler experiments.
Active Materials
Another fundamental area of focus is the performance of active materials. DAC process performance using solid sorbent materials depends on the properties of active materials for capture and regeneration. During the capture step, air flows over/through the material loading it with CO2. Once the material reaches a certain capacity, the CO2 is desorbed, through heat and/or vacuum, to be further processed, leaving the material ready to restart the capture-regenerate cycle.
The main R&D objective is to maximize the working capacity of the active material, minimize the energy expended in regeneration through appropriate materials selection, and minimize energy requirements due to pressure drop to move large amounts of air by using an appropriate form factor for the process module and heat integration either between modules or with process facilities.
Two challenges for active materials in DAC technology are (1) the low concentration of CO2 in air, which requires the use of materials that can selectively and efficiently capture CO2 from dilute conditions, and (2) material regeneration at low temperatures. Amine-based sorbents are appropriate materials for this separation because of their CO2 affinity at low concentrations and low regeneration temperature. Understanding of their fundamentals, such as isotherms and heat of adsorption, and the associated modeling are both critical to establish the maximum potential capacity for capture and the required regeneration conditions.
Other Performance Factors
While the fundamentals of active materials define the potential process limits, the actual performance is strongly dependent on the process conditions and the form factor in which the materials contact the CO2 in air. These determine transport properties.
Monoliths provide a large number of parallel channels per cross-sectional area where air flows along the channels to contact the active material coated on the walls. These configurations provide a lower pressure drop than packed beds. Design parameters such as channel density, monolith length, and coating characteristics, together with process conditions such as air flow rate, determine performance metrics such as CO2 capture rate, efficiency, and pressure drop. The regeneration energy required depends on the thermal properties of the different components of the contactor and the intrinsic heat of adsorption of the active material.
Model Validation
The use of a model-based method relies on the development and validation of models that cover detailed phenomena such as flow and pressure drop in channels, diffusion, and adsorption in porous media integrated with heat and mass transfer, including spatial variations across scales in the contactor. Furthermore, the cyclic nature of these processes necessitates the application of dynamic models to determine the performance of the process for a given design and a set of conditions by finding the cyclic steady state for the capture and regeneration cycle. Identification of these elements and validation through targeted experimentation are crucial for confidence in the results generated by the models.
Validated models allow exploration of different design contactors by varying geometry, channel density, and coating characteristics defined by a large number of variables. In addition, they make it possible to study the impact of varying process operation such as feed conditions, cycle timing, and regeneration schemes.
The large space available for design and operating decisions, coupled with the model complexity, require advanced computing techniques. The use of high-performance computing clusters can accelerate analysis through parallel processing of numerous options. The results are then further processed with technoeconomic analyses and lifecycle assessments for comparison of important scale-up considerations for DAC technology.
Conclusion
The use of model-based methods is enabling acceleration of the development and scale-up of direct air capture technologies. This approach targets validation of key phenomena to increase confidence in results for further exploration and development. It provides sensitivities of process indicators as a function of different design and operating decisions, which are useful when looking beyond the current modeling scope to assess impacts on mechanical and civil considerations, process integration, cost, and carbon footprint. And model-based methods are extremely useful for identifying improvement areas to push the boundaries of innovation in science and engineering in order to provide scalable and cost-effective solutions to address global climate change.