Chemical-looping combustion: Status and research needs
Introduction
Chemical-Looping Combustion (CLC) is an unmixed combustion process based on the transfer of oxygen from the air to a fuel by means of an active metal oxide (MxOy), preventing direct contact between fuel and air. CLC is a very promising technology for power plants due to its inherent CO2 capture, which circumvents the energetic penalty associated with CO2 or air separation present in other technologies, resulting in low capture costs. The process is typically performed in two reactors or in two steps, as shown in Fig. 1.
The fuel is oxidized to CO2 and H2O by an oxygen-carrier, most commonly a metal oxide (MxOy), which is reduced to a metal (M) or a reduced form MxOy-1 inside the fuel-reactor according to reaction (R1). After water condensation and purification, a highly concentrated stream of CO2 ready for transport and storage is achieved. This concept is the main advantage of the process in relation to other CO2 capture technologies. CO2 capture is inherent to this process, as the air is not mixed with the fuel, and the energy penalty for the separation is very low. The oxygen-carrier is further oxidized with air inside the air-reactor (AR), and the regenerated material is ready to start a new cycle (R2). The flue gas contains N2 and unreacted O2. The global enthalpy change of the process is the same as that of conventional combustion where the fuel is burned in direct contact with the oxygen in air (R3).
The principle was proposed in a patent by Lewis and Gilliland [1] to produce pure CO2 from any oxidizable carbonaceous material. The term Chemical-Looping Combustion was coined by Ishida, Zheng and Akebata [2]. The CLC process using interconnected fluidized beds was a paper concept in 2000 and has now reached a development level of TRL 6. An overview of the CLC process can be found in a number of review publications [3], [4], [5], [6]. A specific review on CLC with solid fuels was produced by Adánez et al. [7].
CLC was initially developed for gaseous fuels (natural gas, CH4, syngas) under atmospheric pressure. Different configurations have been proposed to apply the CLC concept to the combustion of gaseous fuels, including the use of two interconnected, moving [8] or fluidized-bed reactors [9], fixed-bed reactors [10], or a rotating reactor [11]. The most used configuration for the fuel-reactor is a fluidized bed either in the bubbling or high-velocity regime, with the air-reactor acting as a riser to drive oxygen-carrier circulation. The use of a moving-bed has been also considered in the CLC configuration [6]; see Table 1. The concept of circulation based on two interconnected fluidized beds has several advantages over alternative designs because it provides good mixing of solids and heat transfer management, as well as a high flow of solid material between fuel- and air-reactors.
CLC offers great flexibility in regard to fuel characteristics, making the use of gaseous, liquid and solid fuels feasible. The use of coal in CLC is highly attractive in scenarios with restrictions on CO2 emissions, given that coal will continue to be a major energy source in the medium-term. In cases where biomass wastes (BECCS) are used as fuel, the captured CO2 can be considered negative emissions because this CO2 has been removed from the atmosphere through photosynthesis in the plants.
With respect to the power cycle when burning gaseous fuels, in order to achieve competitive energy efficiencies, it is necessary to operate at high temperatures and high pressures (1-3 MPa) [17]. There are some concerns regarding operation of pressurized interconnected fluidized beds owing to the difficulties in preventing solids entrainment to the gas turbine found with pressurized fluidized-bed combustion boilers. Moreover, deep loop seals combined with active backpressure control must be considered for stable operation. There is little experience in the use of interconnected fluidized-beds for pressurized-CLC [18], [19]. Instead, dynamically operated packed-bed reactors have been proposed for the operation of CLC under pressure [10]. The process consists of alternate oxidation and reduction cycles in separate reactors, with a large number of parallel reactors required to assure a continuous gas stream to the downstream gas turbine.
The Chemical-Looping concept can also be applied to hydrogen production through reforming processes of CH4 in different ways [6], although the focus of this work is on the combustion process.
Section snippets
Fundamentals
The cornerstone of Chemical-Looping processes is the oxygen-carrier. The CLC concept is based in the transfer of oxygen from the air to the fuel by means of a solid oxygen-carrier. For a system of two interconnected fluidized beds, there must be a high enough solids circulation between the reactors to transfer the oxygen necessary for combustion of the fuel and the heat required to maintain the heat balance in the system, if necessary.
Oxygen-carriers suitable for gaseous fuels
Characteristics of the oxygen-carrier are crucial for the design of the CLC system and its performance. Significant efforts have been made in the field of oxygen-carrier development in recent years. A few redox pairs have the capability to be reduced by fuel gases and regenerated by air under boiler conditions, and at the same time achieve a high selectivity towards CO2 and H2O. Thus, most of the oxygen-carriers proposed in the literature as suitable for gas combustion are synthetic materials
CLC with solid fuels
Several design configurations are being considered for solid fuels, with the fuel-reactor in the form of a moving or fluidized-bed, or even with discontinuous operation [7]. Among them, the use of interconnected CFBs is a preferred configuration because it is more flexible, easy to control and scalable. The following presents the main aspects of its use.
Process modelling and reactor design
Promising oxygen-carriers have been specifically developed for the combustion of every kind of fuel, and some of them have been used at the 0.1 MWth scale for gaseous fuels, and 1 MWth scale for solid fuels. The good results obtained during the experimental demonstration of CLC encourage its future scale-up, e.g. to 10 MWth [64], with modelling tasks being a valuable tool in order to create a safe design for the CLC unit. Mechanistic models can predict the CLC performance by taking into account
Final considerations and research needs
The suitability of the CLC process with gaseous fuels has been demonstrated in units with very different design concepts and mainly at small scale (10–120 kWth). Therefore, the next step in the development of CLC technology is the scaling-up of the process to demonstration scale. Scaling up to 10 MW for demonstration during long periods needs to be addressed in order to reach the next level of maturity for the scale-up step before pre-commercial units.
The core of the performance of CLC with
Acknowledgement
Authors wish to thank to EU Framework Programmes 5, 6 and 7; RFCS-EU; Spanish Research and Development National Plan; Alstom Power Boilers; CCP Project (1 and 2); and Shell for funding the research mentioned in this work.
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