Article Cite This: Energy Fuels 2019, 33, 5452−5463
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Dynamic Simulation and Mass Transfer Study of Carbon Dioxide Capture Using Biochar and MgO-Impregnated Activated Carbon in a Swing Adsorption Process Saeed Ghanbari* and Girish Kamath
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Department of Chemical and Biological Engineering, College of Engineering, University of Saskatchewan, Saskatoon, Saskatchewan S7N 5A9, Canada ABSTRACT: Carbon dioxide capture and utilization has been considered as one of the solutions to the current climate change challenge. Swing adsorption processes can be used to capture carbon dioxide from industrial gases. Developing a selective highperformance adsorbent is key in this process. Cost-effective bio-based adsorbents, such as biochar, have shown a promising performance in recent decades and have been extensively studied. In this work, dynamic modeling and mass transfer study of carbon dioxide capture using biochar and MgO-impregnated activated carbon adsorbents were performed to provide insights on the adsorption mechanisms and improve the process. The results suggested that CO2 adsorption was physisorption and diffusion in the micropore was the controlling step. However, MgO impregnation enhanced the crystalline structure of the sample and increased the diffusion flux in macropores, which resulted in a higher adsorption capacity. The effects of various biochar activation methods and MgO impregnation techniques on the macro- and micropore mass transfer coefficients were reported as well. ASPEN Adsim was used to simulate temperature swing adsorption (TSA) and vacuum swing adsorption (VSA) processes to investigate their feasibility based on the experimental data obtained. The TSA process was not feasible as a result of high heating and cooling duties and the long time required for these cycle steps; however, the VSA process worked well, and product purity and recovery of 99.9 mol % and 90% were achieved, respectively. The VSA process operates at room temperature and atmospheric pressure and has potential for industrial application, especially in biofuel plants, where biochar is already available as a waste material.
1. INTRODUCTION In recent decades, as a result of climate change, carbon dioxide capture has become an attractive research topic and a great deal of research has been performed to address the challenges of climate change.1−5 Numerous review papers were published reporting the progress in carbon capture technologies, such as microporous organic and functionalized polymers,6,7 membranes,8 microporous adsorbents,9,10 ionic liquids,11 and metal−organic frameworks (MOFs).12 It is known that carbon dioxide is adsorbed in micropores by micropore diffusional activation forces based on the adsorption critical diameter of CO2 (2.8 Å).13,14 The micropore diffusional activation force is inversely proportional to the difference between the effective pore diameter and adsorption critical diameter. In general, the review papers suggest two main research approaches toward carbon capture adsorption systems: (1) engineering the effective pore size of adsorbents and (2) surface functionalization toward chemisorption. The first approach showed reasonable improvements in CO2 adsorption capacity and selectivity, i.e., over nitrogen in flue gas treatment; however, because the critical adsorption diameter of other gases, such as nitrogen, methane, and water vapor, are very close to that of carbon dioxide,13,14 it is extremely difficult to synthesize an adsorbent material with a precise and uniform pore size, especially at large scale for industrial application. As for the second approach, engineering the surface functional groups for selective chemisorption of CO2 from a mixture of gases seems to be a better approach toward carbon capture. An ideal adsorbent material should have no micro-/mesopore volume © 2019 American Chemical Society
and abundant surface functional groups that can bond with CO2. Without any micro-/mesopore volume, other small molecules in the gas, i.e., methane in biogas/natural gas or nitrogen and oxygen in flue gas, cannot be adsorbed according to the micropore diffusional activation force principle,14 while surface functional groups can selectively adsorb CO2 from a gas mixture. Ghanbari et al. reported a similar scenario for gas dehydration using biosorbents that were 100% selective to water vapor and did not adsorb nonpolar gases, such as nitrogen and methane.15−17 Their experimental and modeling results showed that the biosorbents had negligible micro-/ mesopore volume and abundant hydroxyl and carboxyl functional groups on the surface that selectively adsorbed moisture from gas. Amine functional groups showed potential for CO2 adsorption, which are further discussed in the next paragraph. High-performance MOFs have been developed and tested for CO2 adsorption.2,18,19 Vismara et al.20 used 3-amino-4,4′bipyrazole to functionalize a novel MOF, M(BPZNH2) (M = Zn, Ni, and Cu), and achieved a CO2 adsorption capacity of 3.07 mmol g−1 (13.5 wt % CO2) and CO2/N2 selectivity of 17. They reported that the amine functional groups increased the adsorption capacity of the MOF; however, selectivity was not very high, which is due to the high micropore volume of the adsorbent (69%). This result illustrates the challenge in Received: March 25, 2019 Revised: May 18, 2019 Published: May 24, 2019 5452
DOI: 10.1021/acs.energyfuels.9b00923 Energy Fuels 2019, 33, 5452−5463
Article
Energy & Fuels
cost-effective bio-based adsorbents, such as activated biochar.31,32,34−37 Jung et al. wrote a review on the strategic use of biochar for CO2 capture and sequestration for interested readers.38 Shahkarami et al. developed several adsorbents based on biochar. The authors used various activation methods, such as KOH, steam, and CO2, and enhanced the CO2 adsorption capacity by increasing the pore volume of biochar.36 In another work, they further enhanced the CO2 adsorption by impregnating biochar with MgO to enhance the crystallinity and micropore structure of biochar.32,37 Sevilla et al. optimize the pore structure of bio-based carbons to enhance the adsorption capacity and selectivity toward CO2.31 They developed a method to fine-tune a hierarchical micro-/ mesoporous carbon material and reported that narrow micropores had a prominent role at pressures of
