Progress in O₂ separation for oxy-fuel combustion–A promising way for cost-effective CO₂ capture: A review

Abstract

Oxy-fuel combustion has received significant attention because it is widely considered to be a promising method for cost-effective CO₂ capture. The key to the success of oxy-fuel combustion processes is the availability of inexpensive O₂. Therefore, development of low-cost and robust technologies for air separation or separating O₂ from air is critical. Great progress has been made in this research and development area. However, a recent review of the progress made in the area is lacking. This review is designed to fill the gap. The review focuses on three major technologies for separating oxygen from air, including membranes, adsorption, and chemical-looping with oxygen uncoupling. The separation principles, characteristics, challenges and the associated solutions of these major technologies are systematically discussed. Future opportunities are outlined for each.

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 NO_x 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 NO_x 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, SO_x, and NO_x) 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 NO_x 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.