The technical route proposed by Thunman et al. [1] to use OC to aid fluidized bed combustion is very creative, although it is likely that the OCAC technology was inspired by chemical looping combustion (CLC) technology developed at the same department of Chalmers University of Technology [2]. OCAC shares many characteristics with CLC as well as with three-way catalyst (TWC) technology in terms of adopting the oxygen-carrying material, gas–solid redox reaction, improving fuel conversion and reducing pollutants, et al. Therefore, the TWC and CLC will be briefly reviewed here to provide some preliminary knowledge before introducing working principle of OCAC.

2.1 Three-Way Catalyst (TWC) Technology

The OCs in OCAC technology are similar to the oxygen storage material of three-way catalysts (TWC) in gasoline engines, which was developed to simultaneously reduce CO, hydrocarbons, and NOx in a combustion situation close to the stoichiometric ratio [3]. In the TWC technology, the catalyst not only acts as an oxygen buffer to ensure the complete combustion of fuel, but it also catalyzes the NOx reduction [4, 5]. During the catalysis, the continuous fluctuation of the air–fuel ratio in the engine alternates between atmospheres of mild reduction and mild oxidation. Therefore, the catalyst can oxidize unburnt species in a reducing atmosphere with the donation of oxygen, but it absorbs oxygen back to its original state in an oxidizing atmosphere [6]. A schematic of TWC technology is shown in Fig. 2.1.

Fig. 2.1
A schematic of T W C technology. A steel canister has catalyst substrate and catalyst such as P t, P b, R h, and so on. Oxygen buffer capacity is inside the canister. The catalytic reaction is given separately in a box.

Schematic illustration of TWC technology

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At present, almost all catalytic converters for automobiles have an integral honeycomb structure, and the catalyst support is divided into a ceramic carrier and a metal carrier. Commonly used catalysts are precious metals such as Pt, Pd, Rh, and rare-earth metals [6]. The three-way catalyst contains about 0.1–0.15% of precious metals, and some metal oxides, such as La2O3, BaO, CeO2, and Zr are used as auxiliary catalysts to improve the catalytic efficiency and catalyst lifespan. Generally, the main factors affecting the life time and conversion efficiency performance of the catalytic converter are the working temperature of the catalytic converter and the content of Pb, P and S in the fuel [3]. Excessive Pb, P, S and other elements in the fuel may results in catalyst poisoning, because high temperature will lead to a chemical reaction between the noble metal and the support, and to sintering of the catalyst etc., which can further lower the lifetime and conversion efficiency. In order to reduce the cost of catalysts while ensuring adequate reactivity and longevity, researchers have focused on the development of catalysts by using cheaper high-performance transition metals with lowered metal loading [3].

2.2 Chemical Looping Combustion (CLC) Technology

The concept of CLC technology for CO2 capture was realised by Ishida et al. [7] from Tokyo Institute of Technology in the middle of 1990s, but it was only when Lyngfelt and coworkers [2] developed the subject and in continuation promoted it in the form of several research projects, supported by the European Union, that it was spread internationally. Then, research activities started to develop. In the European Union projects there is a requirement to co-operate between the European countries and to involve research organizations and companies, and that was done. Much knowledge on CLC was accumulated and spread in publications among many researchers, particularly Lyngfelt et al. and Adánez et al. from Instituto de Carboquímica of Spain [8]. Subsequently, CLC technology was spread back to Asia (for instance, Laihong Shen [9] got the idea during a stay at Chalmers University) and to America.

The CLC system consists of two interconnected fluidized bed reactors (air reactor and fuel reactor) and cyclic transport of OCs between them, which can avoid direct contact between the fuel and air [2]. As shown in Fig. 2.2, the OCs are oxidized by air in the air reactor, then the oxidized OCs enter the fuel reactor and react with the fuel, finally, the reduced OCs are returned to the air reactor to complete the cycle [8, 10]. In this system, the exhaust gas from the fuel reactor is mainly CO2 and H2O. High concentration of CO2 is obtained after the condensation of water vapour, which is conducive to CO2 capture, storage, and utilization (CCSU). In the CLC process, a solid oxygen carrier supplies the oxygen needed for completing combustion to CO2 and water, and this leads to a nitrogen-free CO2 mixture. As a result, the requirement of CO2 separation from flue gases, a major cost for CO2 capture, is circumvented. Furthermore, since there is no N2 in the fuel reactor, the formation of NOx is also reduced. Economic assessments have suggested that the CLC technology holds great promise for combustion processes with the potential for achieving efficient and low-cost CO2 capture compared to other CCS processes [11].

Fig. 2.2
A schematic of C L C technology. The oxidation and reduction of the O C takes place in air and fuel reactor, respectively. The O C forms a cycle between air and fuel reactor.

Schematic illustration of CLC technology

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The availability of high-performing and scalable oxygen carriers has become one of the crucial concerns in scaling up and demonstrating the CLC applications. An oxygen carrier is typically formed by a reactive metal oxide and inert support, which provide, respectively, oxygen storage and mechanical strength, but also simple natural ores are used. Over the last 15 years, many research groups have been studying the formulation and preparation of active and stable oxygen carriers.

2.3 Basic Working Principles of OCAC

2.3.1 OCAC Technology Evolution

The main innovation of OCAC technology is to use oxygen carriers (OCs) as bed material instead of using inert bed material. When the OCs undergo oxidation and reduction reactions at different spots of a CFB reactor, the distribution of oxygen becomes more even, and its utilization efficiency is improved [1]. A schematic diagram is displayed in Fig. 2.3. In the CFB operation, the OCs, typically natural ores, are injected into the CFB and heated up. The lattice oxygen reacts with combustible gaseous reactants in an oxygen-lean region, resulting in the reduction of OCs and the formation of CO2 and H2O. Then, the reduced oxygen carriers are oxidized by oxygen from the air in oxidizing regions, resulting in fully oxidized OCs with reactive lattice oxygen. As the OCs move throughout the reactor, redox reactions take place cyclically. Ideally, the primary gas can provide enough oxygen for the OC oxidation, but not enough for fuel combustion in the dense bed. When the oxidized metal (Me) OCs (denoted as MexOy) are transported upward and downward through the entire combustor, the MexOy will be reduced by combustible gases to reduced OC particles (denoted as MexOy−1). These small size reduced OC particles are separated from the flue gas by the cyclone separator and returned to the bottom of the dense bed. Then they are re-oxidized in the oxidizing regions of the combustor [12]. The entire process of oxidation and reduction of OCs during reaction with hydrocarbons CiHj can be expressed as:

$$ \left( {2i + 0.5j} \right){\text{Me}}_{\text{x}} {\text{O}}_{{\text{y}} - 1} + (i + 0.25j){\text{O}}_2 = (2i + 0.5j){\text{Me}}_{\text{x}} {\text{O}}_{\text{y}} $$
(2.1)
$$ C_i H_j + (2i + 0.5j){\text{Me}}_{\text{x}} {\text{O}}_{\text{y}} = (2i + 0.5j){\text{Me}}_x {\text{O}}_{y - 1} + i{\text{CO}}_2 + 0.5j\,{\text{H}}_2 {\text{O}} $$
(2.2)
Fig. 2.3
A schematic of O C A C technology. Primary air is passed through wind box forming a cycle with oxidizing and reducing regions. On passing fuel, it gives Oxygen and volatiles which further gives M e subscript x and O subscript y and M e subscript x O subscript y minus 1 which again passes through the wind box. The fuel gas is emitted.

Schematic illustration of OCAC technology

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The OCs are capable of taking up and donating oxygen, which can improve the uniformity of oxygen distribution and the homogeneity of the oxygen-fuel mixture in the combustion chamber. Many advantages may be achieved. Less excess air can be one of those.

Theoretically, the role of OC is to transport oxygen from an O2-rich region to an O2-lean region. In the real CFB reactor operation, the dense bed presents a reducing atmosphere with local oxidizing spots. Thus, the redox reactions between MexOy and combustible gases always take place in the reducing regions of the dense bed. Similarly, the volatiles can also react with OCs (MexOy) in the riser. Due to the typical arrangement of secondary air in the boiler, the oxygen in the upper part of the combustor oxidizes the OCS (MexOy−1).

Compared to CLC, the OCAC has a simpler process system, which only contains one FB reactor, and there is no capture of CO2. Therefore, during the OCAC process, the role of the OCs is to donate and acquire oxygen while moving between the reducing and oxidizing regions in the same reactor. The OCAC technology shows great advantages in application potential such as (1) it can be directly adapted to the existing CFB boilers without system modification; (2) the knowledge of OC materials selection and applicability from CLC investigations [13, 14] provides valuable experience for the application of OCAC technology [12].

2.3.2 Suggested OCAC Principles

OCAC is a relatively new concept derived from the use of solid OCs as bed material in assisting fuel conversion. Although the fundamental study and practical application of this technology are rapidly progressing, the principles of the OCAC process are still vague to some extent. The concept of OCAC still lacks a fixed and precise definition. OCAC is similar to the technologies of oxidation catalysis (e.g., the TWC) and CLC in some aspects, but there are also considerable differences. The differences among the three processes are mainly in the following aspects: the reaction time interval, the spatial characteristics, and the reaction temperatures in oxidation and reduction, as well as in the way of generating the desired products (see Table 2.1).

Table 2.1 Comparisons between various aspects of TWC, CLC and OCAC technologies
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It has been accepted that the reactions involved in the oxidation catalysis either follow the Mars-Van Krevelen (MvK) or Langmuir–Hinshelwood (LH) mechanisms, in which the former is more widely accepted [15]. When it follows the MvK mechanism, the catalyst is oxidized and reduced promptly and almost simultaneously with the progress of the reaction. At this stage, the catalyst behaves as an oxygen vector transferring oxygen radicals, atoms, ions or molecules among different reactants. However, the time intervals between oxidation and reduction reactions are too short to be separated in terms of time during observations. From the functional point of view, oxidation catalysis requires the synergy between the oxidation and reduction reactions to obtain the desired products. As a result, the redox reactions, i.e., the oxidation and reduction reactions, cannot be spatially segregated during the oxidation catalysis. On the contrary, the oxidation and reduction reactions are always segregated spatially during the CLC process. They take place either simultaneously or in sequence depending on the reactor and process design of the CLC processes. Unlike oxidation catalysis, the desired products of the CLC process can be generated from either oxidative or reductive reaction or even both [8]. Nevertheless, the OC involved in the CLC process functions similarly to the catalyst for oxidation catalysis, but transports oxygen among different physicochemical environments in terms of multiple reactors. The reaction temperature is another obvious difference between oxidation catalysis and CLC. The redox reactions always take place isothermally in oxidation catalysis, while the reaction temperatures for the oxidation and reduction reactions can be varied in the CLC processes.

Despite its own characteristics, the OCAC shares several similarities with the oxidation catalysis and the CLC technologies. The redox reactions of oxidation and reduction for the OCAC process always take place isothermally within the same reactor but can be distinguished both timely and spatially, which is significantly different from the oxidation catalysis and CLC technologies [1]. In general, the OCAC technology can be deemed a versatile process, in which OC materials are employed in delivering the oxygen species from one physicochemical environment to another. Besides, such oxygen transportation is accomplished via space and time-resolved redox reactions in terms of reductive and oxidative reactions within a single isothermal reactor. Benefiting from the ability of the OCAC technology in transporting the oxygen species from oxygen-enriched regions to the oxygen-deficient areas, the reactants, which are usually solid and gaseous fuels, can be oxidized more evenly and efficiently within the reactor. Although the current oxygen transferring materials used in OCAC technology are mostly derived from the CLC processes, it raises higher requirements for OCs in response to the subtle environment change. In the CLC system, the oxygen partial pressure (\(P_{{\text{o}}_2 }\)) and temperature can be significantly different among the different reactors. Hence, the reductive and oxidative reactions which take place on the OCs in the CLC process can be altered easily due to the wide changes of \(P_{{\text{o}}_2 }\) and temperature. However, the \(P_{{\text{o}}_2 }\) and temperature vary only slightly in different regions inside the OCAC reactor since only a single isothermal reactor is concerned. Therefore, the OCs designed for OCAC should respond to a change of \(P_{{\text{o}}_2 }\) and temperature much more sensitively in order to alter the reactions promptly between oxidation and reduction. Moreover, the time in which the OC materials are retained at different physicochemical environments within the OCAC reactor is much shorter than the residence time of OCs in CLC reactors. Therefore, the reaction rate of OCs during either reduction or oxidation reaction should be much faster in OCAC than that in a CLC system. The major differences between the TWC, CLC, and OCAC technologies are summarized in Table 2.1.