Recent data from NOAA, the National Oceanic and Atmospheric Administration, indicates that current atmospheric carbon dioxide (CO2) levels are at 409 ppm as of October 2018. This is a 36% increase from the highest historical CO2 level,1 and is increasingly being attributed to human activity, namely fossil fuel combustion in power generation, transportation, and industrial processes.
A myriad of issues stem from high CO2 levels, including global warming (1.6 ˚F in the past 35 years), rising sea levels (8 inches in the last century), glacial retreat, and ocean acidification (30% increase in acidity).2 No matter what one’s position is on the debate over the cause-and-effect of human-activity induced climate change, the issue at hand centers around responsible fossil fuel stewardship and controlling CO2 emissions. One strategy is to reduce fossil fuel use by implementing alternative energy sources such as solar, which will be the focus of a future article. This article will serve as an overview of how industrial, academic, and governmental efforts are aiming to tackle the issue of rising CO2 levels, as evidenced by their footprints in the recent patent literature. Particularly, these efforts can be broadly categorized as either CO2 capture methods or CO2 sequestration/conversion methods. The former is the focus of this article, the latter will be the focus of a future article.
While there are multiple methods for incorporating CO2 capture into the fossil-fuel based power generation processes currently utilized, the approach that is most amenable to existing infrastructure is a post-combustion approach where the flue gas exiting a coal or natural-gas fired power plant, consisting primarily of CO2 and water vapor, is subjected to a CO2 capture process. Currently, this is most frequently performed using aqueous solutions of chemical compounds with amine functionalities, such as monoethanolamine, in a CO2 capture unit, also known as a CO2 scrubber. During the operation of a CO2 scrubber, post-combustion flue gas is sent through an adsorption column that contains the lean amine solution, where lean indicates low CO2 content. The lean amine undergoes a reversible chemical reaction with CO2, resulting in the formation of rich amines, which are composed primarily of carbamate or bicarbonate.3 Rich amines are then transferred to a desorber system that converts rich amines back to lean amines via competitive water adsorption using steam. This process releases CO2, which can be collected for later conversion steps.
CO2 capture methods have been commercialized and implemented in power plants around the world.5 One prominent example is the Cansolv system from Cansolv Technologies Inc, a sister company of Shell Oil Company. Issued US7056482B2 relates to one of Cansolv’s processes with specific details regarding the mixture of amines utilized, specific properties of the amines, such as the pKa of the amine moeities (a chemical property that is related to proton affinity), and the conditions under which such a system is operable. There are further claims that mention specific oxidation inhibitors to protect the amines from degradation by molecular oxygen in the air, as well as methods to capture SO2 or NOx, other toxic contaminants in flue gas.
In addition to improving these already utilized technologies, other efforts have focused on the development and application of new classes of materials to act as CO2 adsorbers. One such class of materials being developed by an ExxonMobil affiliate (US20180250652A1 & US20180250653A1) are composed of an aluminum oxide support with silicon-modification and an alkali metal salt. The system is reported to improve the overall efficiency of CO2 capture by reducing the amount of steam needed during the desorption step. An additional application (US20180250654A1) was filed claiming that the calcination temperature, a heating step in the synthesis of the CO2 sorbent material, has a significant effect on CO2 adsorption ability.
In largely academic environments, a relatively new class of materials is being targeted for CO2 adsorption and separation applications, namely metal-organic frameworks (MOFs). These materials are solid phase and are microporous, meaning that they have typically nanometer-scale pores throughout the material, giving them massive internal surface area and the opportunity to adsorb gasses. Besides casual references to MOFs as a possible sorbent material in the patent literature, scientists are pursuing patents for the application of specific MOF materials to CO2 capture. While many of these examples are in the application stage, they paint a promising future for the emergence of new types of CO2 adsorption materials.
All of the previous examples focused on CO2 capture from flue gas, which contains up to 10% CO2, whereas the atmosphere contains only 0.04%.6 From a chemical perspective, it is exceedingly difficult to achieve efficient reaction yield when one of the reactants is at extremely low concentration. Because CO2 capture relies on a reversible chemical process, it follows that performing CO2 capture from atmospheric air with low CO2 concentration is difficult. Despite this challenge, scientists are developing and commercializing processes that are capable of CO2 capture from air. These include Global Thermostat LLC, Carbon Engineering, and Climeworks, among others. Using these technologies to capture carbon from both flue gas and air, it may be possible for power plants to achieve negative CO2 emissions, an important first step in remedying the high CO2 levels in our atmosphere.
The table below highlights patents and published applications related to CO2 capture technologies.
|Patent or Application Number||Title||Assignee||Inventor|
|Hydrophobic sorbents for CO2/H2O displacement desorption applications||TDS Research Inc., ExxonMobil Research and Engineering Co||Chuansheng Bai, Majosefina Cunningham, Patrick P. McCall, Hans Thomann, Jeannine Elizabeth Elliot, Vinh Nguyen|
|Application US20080289495A1||System and method for removing carbon dioxide from an atmosphere and global thermostat using the same||Peter Eisenberger, Graciela Chichilnisky||Peter Eisenberger, Graciela Chichilnisky|
-Brian Pattengale, PhD and Anthony Sabatelli, PhD, JD
Brian Pattengale is a Postdoctoral Associate in the Energy Sciences Institute at Yale University, where he is investigating the photodynamic properties of emerging materials and their catalytic/photocatalytic applications to reactions such as water splitting or carbon dioxide reduction. Prior to his position at Yale, Brian obtained his Ph.D. in Physical/Materials Chemistry at Marquette University, where he published numerous papers using ultrafast transient absorption and synchrotron X-ray absorption spectroscopies to study functional light absorbing and photocatalytic materials.
3 https://pubs.acs.org/doi/pdf/10.1021/am507465f – co2 capture review
5 Cebrucean, D.; Cebrucean V. and Ioana Ionel. “CO2 Capture and Storage from Fossil Fuel Power Plants” 2014, 63, 18-26
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