Progress in O2 separation for oxy-fuel combustion–A promising way for cost-effective CO2 capture: A review
Graphical abstract
Introduction
Oxygen is widely used in industry in numerous ways, with some primary applications related to combustion and gasification [1]. Although air is frequently used as an oxidizer, the nitrogen in air can have negative impacts on combustion processes. Essentially, the heat absorbed by nitrogen during combustion is wasted through thermodynamic cycles, despite efforts at heat recovery, and often will form harmful NOx compounds. Combustion processes can thus be improved by lowering the amount of nitrogen used in the combustion air and increasing the amount of oxygen [2]. Although increasing oxygen concentration would lead to higher adiabatic flame temperature, in actual applications, oxygen is diluted by recirculated CO2 so as to control the temperature to the required level set by boiler metallurgical constraints. The benefits of combustion in oxygen include higher combustion efficiencies due to lower heat loss, easier CO2 capture due to very high CO2 concentration in the flue gas, lower NOx emissions, and improved temperature stability [3].
Although alternative energy technologies such as solar and wind energy are emerging and under rapid development in recent years, advanced combustion technologies will remain highly relevant in the next few decades. In the U.S., approximately 83% of greenhouse gas emissions are produced from the combustion and nonfuel uses of fossil fuels, which currently supply over 85% of the energy needs in both the United States and other nations [4]. CO2 emission mitigation options include the development of higher efficiency power generation cycles and equipment, the use of less carbon-intensive fuels, and carbon capture and sequestration (CCS) [5].
Post-combustion, pre-combustion, and oxy-combustion are the three major carbon capture technologies. Post-combustion capture, which uses chemical absorption/desorption processes with amines to scrub CO2 directly from the exhaust, is the primary conventional method [6]. Since post-combustion capture units are installed as “add-ons” at the exit of the flue gas stack, the upstream power plant does not require significant modification [6]. However, due to the low CO2 concentrations typical of hydrocarbon oxidation in air [5] and thermal energy requirements for chemical scrubbing processes, among other factors [7], post-combustion capture is both energy-intensive and expensive. Pre-combustion capture is an alternative CCS technology that essentially removes carbon from the fuel prior to combustion via processes such as steam reforming, partial oxidation, or auto-thermal reforming, and transfers part of the chemical bond energy stored in the original hydrocarbon into pure hydrogen [8], [9]. This method requires complex processing equipment and incurs exergetic losses for each conversion into another chemical form, thus making it expensive, and barriers to commercial application of gasification are common to pre-combustion capture [6].
In contrast to pre- and post-combustion techniques, oxy-combustion is a promising CCS technology with the potential to significantly reduce the penalty associated with the CO2 separation process. Along with significant CCS cost reduction compared to post-combustion capture, the International Energy Agency (IEA) presented a technology roadmap for oxy-fuel combustion in coal-fired power plants using CO2 capture [10], with clear indication that this technology can be commercialized by 2020. In oxy-combustion, fuel is oxidized in a nearly nitrogen-free, CO2 diluted mixture such that the products consist mainly of CO2 and water vapor, thus enabling a relatively simple and inexpensive condensation separation process [6]. However, oxy-combustion requires the separation of oxygen from air prior to combustion.
Since high-purity oxygen (O2) is required for both technologies, air separation units are required for both pre-combustion and oxy-combustion systems. Although all three major carbon capture approaches are highly important in CCS technology, this article focuses mainly on air separation methods related to oxy-combustion. Fig. 1 presents a typical oxy-combustion system. In oxy-combustion, the concentration of CO2 in the flue gas can reach more than 80% (flue gas consists mainly of CO2 and water, with additional components including particulates, SOx, and NOx) and after a simple purification process can exceed 95%, to meet the needs of large-scale transfer and storage via pipeline. Not only can this technology facilitate the recovery of CO2 in flue gas, it can also greatly reduce SO2 and NOx emissions in order to achieve the synergistic removal of pollutants for near-zero emissions in clean coal utilization technology.
In addition, CO2 sequestration in geologic formations shows promise because of the large number of potential geologic sinks [4]. Also, with higher petroleum prices, there is increased interest in using CO2 flooding for enhanced oil recovery; and with higher natural gas prices, there will be growing interest in using CO2 for increased coal bed methane production [4]. However, none of these activities would be possible in the first place unless CO2 were captured. Hence, in order to capture CO2 effectively, oxygen is essential.
Oxygen also plays a very important role in coal gasification [1], [11] by enhancing the efficiency of the gasifier and downstream processes [12]. Because of the high operating pressures of these processes, economics favor the use of oxygen purities of 90% or higher [13]. In contrast, when coal burns or is reacted in air, 79% of which is nitrogen (N2), the resulting CO2 is diluted and more costly to separate. Thus, improved air separation methods are vital if all these benefits in combustion and gasification are to be realized.
Section snippets
Oxygen separation methods
To meet the needs of large-scale oxy-fuel combustion technologies, cryogenic air separation (CAS, also known as cryogenic distillation) is currently the most feasible technique for producing oxygen, nitrogen, and argon as either gaseous or liquid products [14]. This technique relies on fractional distillation of air at low temperature and pressure for separation. In other words, air is separated according to the different boiling temperatures of its components. An air separation unit (ASU)
Membranes
The primary challenge with membranes for oxygen separation is in the materials area, with focus on the fundamental need to produce membranes with both sufficient thinness and mechanical integrity to produce high fluxes and reasonable durability. Polymer membranes that maintain integrity at higher operating temperatures are needed. Mixed matrix membranes that combine, for example, polymers and zeolites, require better contact and adhesion between the components to avoid pinholes and other
Conclusion
The preceding discussion provides a review of three major technologies as alternatives to cryogenic air separation for the production of oxygen. Based on this review, membrane methods are simple and economical compared to cryogenic methods. However reliable membranes are still hard to find and ITMs must be operated at relatively high temperatures. Adsorption air separation methods are also very promising, yet currently suited only for small and medium-sized markets. Decreased purity due to
Acknowledgments
This work was supported by the State of Wyoming. We greatly appreciate the help of the other members in Dr. Maohong Fan's research group.
Fan Wu is currently studying for a Ph.D. degree from University of Wyoming. He received B.S. (2012) from Huazhong University of Science and Technology, and M.S. (2014) from West Virginia University. His research interest is sorption separation for Carbon Capture and Sequestration technologies.
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Fan Wu is currently studying for a Ph.D. degree from University of Wyoming. He received B.S. (2012) from Huazhong University of Science and Technology, and M.S. (2014) from West Virginia University. His research interest is sorption separation for Carbon Capture and Sequestration technologies.
Morris D. Argyle became interested in catalysis as process engineer for an Exxon Baytown Texas Refinery fluid catalytic cracking unit. After earning his Ph.D. at the University of California, Berkeley, he moved to the University of Wyoming, becoming Associate Professor and Department Head, before joining the Chemical Engineering faculty at Brigham Young University in 2009.
Paul A. Dellenback serves as an Associate Dean in the College of Engineering and Applied Science at the University of Wyoming. He received his Ph.D. from Arizaona State University in 1986. His research interests focus on convective heat transfer, turbulent fluid mechanics, and optical diagnostics.
Maohong Fan is a SER Professor of Chemical and Petroleum Engineering in University of Wyoming. He has led and worked on many projects in the areas of chemical production, clean energy production and environmental protection. These projects have been supported by various domestic and international funding agencies such as NSF, DOE, EPA, USGS and USDA in the United States. Also, he has led various projects funded by various international agencies such as RITE/NEDO. He has published over 300 refereed books, book chapters, and papers in different chemical and environmental engineering, energy, and chemistry journals.
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Co-first author