Thermally Modulated Fiber Sorbents for Rapidly Cycled Vacuum-Pressure Swing Adsorption of Post-Combustion Flue Gas

Abstract

Thermally Modulated Fiber Sorbents for Rapidly Cycled Vacuum Pressure Swing Adsorption of Post Combustion Flue Gas

Stephen J.A. DeWitt

339 Pages

Directed by Dr. Ryan P. Lively

The continued rise in the concentration of CO2 in the Earth’s atmosphere driven by society’s rising standard of living and continued reliance on carbon-containing fossil fuels has led to several significant environmental challenges, which will continue to face humanity in the coming decades and centuries. Developing technological solutions to accelerate the reduction of carbon dioxide emissions and curtail the effects of climate change will continue to serve one of the critical directions society will pursue to combat these effects. The capture of CO2 from point sources like coal-fired power plants will likely be a direction for the short and long-term reduction in emissions needed to stabilize the environment. With this in mind, developing technologies to serve this purpose with the least impact on people’s way of life serves as a critical challenge over the coming decades. The removal of CO2 from point sources has been an area of interest for a number of years now, with absorption technology appears best suited to make an immediate impact with pilot and full-scale plants currently being built. In the future, methods of separating and capturing CO2 will look to technologies with the potential to be considerably lower energy requirements, like adsorption and membranes, and these are still in the early stages of development. While these approaches lag behind absorption in technology readiness for CO2 capture today, the potential impact of their adoption in terms of cost reduction has led to considerable research investment in developing new materials and processes to enable their application. In this dissertation, a novel strategy is proposed, and materials are developed to enable the proposed process for post-combustion CO2 capture from coal. As part of this dissertation, four objectives were pursued to understand and enable this process. xxvii i) A process model was created and studied to develop a deeper understanding of the approach’s potential and the necessary materials developments to enable it. ii) Promising materials were synthesized and manufactured into ready-made devices for bench-scale testing. iii) Fundamental challenges of adsorptive separations related to heat management were considered in detail, and a novel manufacturing approach was developed to enable improvement in materials performance. iv) The potential of fiber sorbents for sub-ambient CO2 capture was examined through the operation of single bed PSA cycles using the manufactured materials from previous objectives. The first objective showed that a sub-ambient pressure-driven separation process, coupled with downstream liquefaction, could be used without the need for external refrigeration. Excess low-quality cooling could also be used for the dehydration of the flue gas, a key challenge facing adsorptive CO2 capture. This process would be significantly limited by its capital costs, consistent with expectation. Regardless of the separation considered, the capital and energy costs of pre- and post-treatment of CO2 lead to costs of CO2 capture exceeding $47/tonneCO2, and the best case study of sub-ambient pressure swing adsorption led to costs of capture around $60/tonneCO2. Preliminary analysis shows a more complicated process, where the post-treatment liquefaction is removed or replaced with a low-cost membrane or adsorbent system that could allow for additional cost reductions. The second objective focused on the production and spinning of MOF fiber sorbents, which showed the potential, in simulation, when combined with the work from objective 3 to reach 8-10x the performance of traditional pellet packed bed systems. Work here focused on the development of scale-up and manufacture of multiple MOF fiber xxviii sorbents, overcoming challenges in dope composition formulation and particle size resulting in sorbent leaching not previously reported in fiber sorbent spinning. The third objective focused on the development of a passive internal heat management strategy for pressure swing adsorption in the fiber sorbent morphology. Microencapsulated phase change materials were, for the first time, incorporated into fiber sorbents in the spinning step, allowing for a reduction in the manufacturing complexity of heat management in PSA systems. A 20-25% improvement in the breakthrough capacity of the sorbent and 30-40% reduction in amplitude of the thermal front prove the manufacturing process works and will enable more efficient sorbent performance. The final objective looked at the operation of a pressure swing adsorption unit for the removal of CO2 from simulated flue gas within the sub-ambient process framework. MOF fiber sorbents with and without phase change materials were compared in terms of the tradeoffs between purity, recovery, and CO2 productivity. This preliminary analysis showed there was much potential for sub-ambient CO2 capture, with productivities as high as 0.01 mol kg-1 sec-1 achieved using MOF fibers. Due to the single bed cycles used in these experiments, the recovery of the system suffered (never exceeding 45%), but future work focused on optimizing more complex cycles should allow for improvements in this area. Thermally modulated fiber adsorbents were also considered in the sub-ambient PSA system, showing higher purities and productivities than fibers without heat management at comparable recovery levels.