Improving CO₂ Capture Efficiency Through Novel CLOU-Based Fuel Reactor Configuration in Chemical Looping Combustion

Abstract

Climate change and global decarbonization targets drive the search for more efficient and cost-effective combustion technologies. Chemical looping combustion (CLC) using solid oxygen carriers with chemical looping with oxygen uncoupling (CLOU) functionality has attracted growing interest due to its inherent potential for CO₂ capture without requiring additional separation processes. This study introduces a conceptual proof-of-concept design of a novel fuel reactor design for a dual-fluidized bed CLC system operating with solid fuels. The new configuration incorporates a perforated conveyor for circulating CLOU-type oxygen carriers, thereby avoiding direct contact between the carriers and the fuel–ash mixture. This approach prevents the slip of unburned fuel and ash into the air reactor, minimizes the loss of oxygen carriers, and improves combustion efficiency. The proposed reactor concept enables the generation of flue gas with a high CO₂ concentration, which facilitates its subsequent capture and reduces the energy penalty associated with traditional CCS techniques. The improved phase separation and better control of oxygen carrier residence time contribute to enhanced system performance and reduced operating costs. Preliminary process simulations conducted in the CeSFaMB environment, using boundary and initial conditions from a CLC test rig, were included to illustrate the potential of the design. Experimental validation is outside the scope of this study and will be presented in future work once the dedicated test facility is operational. This contribution should therefore be regarded as a conceptual proof-of-concept study, and experimental validation together with techno-economic benchmarking will be reported in follow-up publications once the dedicated test facility is operational.

1. Introduction

Climate change and the growing urgency to reduce greenhouse gas emissions have led to intensified research into low-emission energy technologies. Among the strategies aimed at decarbonizing the energy sector, carbon capture and storage (CCS) technologies play a crucial role. However, conventional CCS approaches such as post-combustion separation, pre-combustion reforming, and oxy-fuel combustion often suffer from high energy penalties and operational costs [1,2,3].

Chemical looping combustion (CLC) has emerged as a promising alternative due to its inherent ability to separate CO₂ without the need for additional gas separation units [4]. In CLC systems, solid oxygen carriers—typically metal oxides—transport oxygen between two interconnected reactors: an air reactor (AR) and a fuel reactor (FR). This indirect oxygen transfer eliminates direct contact between air and fuel, which not only enables CO₂ capture but also suppresses NOx formation [5]. The overall concept of the CLC process is illustrated in Figure 1, which shows the cyclic transfer of oxygen through the oxidation and reduction of the carrier material.

Oxygen carriers with chemical looping with oxygen uncoupling (CLOU) functionality represent an advanced variant of CLC. These materials can release gaseous oxygen directly into the fuel reactor environment without requiring direct interaction with the fuel [6,7]. This mechanism makes CLOU-based systems particularly attractive for the combustion of solid fuels with low volatile content and facilitates faster and more complete fuel conversion [8].

Numerous pilot-scale CLC facilities have been constructed worldwide, demonstrating the feasibility of the technology using various oxygen carriers such as nickel, copper, iron, and manganese oxides [9,10,11,12]. However, the effective use of CLOU carriers in solid fuel combustion remains a technical challenge, especially concerning optimizing fuel conversion, preventing particle loss, and minimizing ash contamination in the air reactor [13,14].

This study introduces a novel design of a fuel reactor for dual-fluidized bed CLC systems operating with CLOU-type oxygen carriers. The new configuration incorporates a perforated conveyor that enables the circulation of oxygen carriers while avoiding direct contact with the fuel–ash mixture. This approach improves phase separation, reduces oxygen carrier losses, and enhances CO₂ capture efficiency. The concept builds upon earlier pilot-scale experiences but introduces a structural innovation aimed at improving reactor performance and economic feasibility.

To overcome the limitations of existing designs, the proposed reactor facilitates controlled circulation of CLOU carriers while maintaining their separation from fuel and combustion residues. This prevents the transport of ash and unburned fuel particles from the fuel reactor to the air reactor—a common drawback in interconnected fluidized bed systems. As a result, the configuration is expected to improve combustion efficiency, preserve the reactivity and mechanical integrity of the oxygen carriers, and enhance the overall performance and reliability of the CLC system. The scope of the present research includes the development and evaluation of this novel concept as a step toward more effective and economically viable CLOU-based combustion technologies.

2. Materials and Technological Background for CLC

2.1. Oxygen Carrier Materials

The selection of an appropriate oxygen carrier is a critical aspect of the CLC process, as its physical and chemical properties significantly influence combustion efficiency, process stability, and the quality of CO₂ separation. Oxygen carriers are required to meet specific criteria, including environmental safety, cost-effectiveness, resistance to attrition and agglomeration, and high oxygen transport capacity. Additionally, they should exhibit sufficient reactivity during both oxidation and reduction, a high melting point, good fluidization properties, and mechanical robustness [5,15].

To enhance these properties, active materials such as metal oxides are often supported by inert substrates, including silicon dioxide (SiO₂), zirconium dioxide (ZrO₂), titanium dioxide (TiO₂), aluminum oxide (Al₂O₃), and yttria-stabilized zirconia (YSZ). These supports improve fluidization behavior and resistance to erosion, which is particularly important in the harsh environment of fluidized bed reactors [16].

A fundamental classification of oxygen carriers is based on the type of active metal or metal oxide used. The most widely studied carriers are based on nickel, copper, iron, manganese, and cobalt [8,9,10]. One of the key parameters used to assess oxygen transport capability is the oxygen transport capacity, R_OC, calculated from the difference in mass between the oxidized and reduced forms of the carrier:

$$R_{OC}={\displaystyle \frac{m_{OX}-m_{red}}{m_{OX}}}$$

(1)

where $m_{OX}$ and $m_{red}$ denote the masses of the fully oxidized and reduced forms, respectively [17].

2.1.1. Nickel-Based Oxygen Carriers

Nickel-based oxygen carriers are widely recognized for their high oxygen transport capacity, fast redox kinetics, and strong catalytic properties, particularly for gaseous fuels. Their active redox couple involves nickel oxide (NiO) and metallic nickel (Ni), which exhibit excellent reactivity in the typical temperature range of 900–1100 °C. These properties make nickel one of the most effective materials in chemical looping combustion (CLC), especially in laboratory and early pilot-scale studies [18,19].

One of the significant advantages of nickel-based carriers is their ability to facilitate complete and rapid oxidation of fuel gases, such as CH₄, CO, and H₂, without requiring an external source of oxygen. The oxygen transport capacity of Ni-based materials is relatively high, allowing for improved combustion efficiency and greater fuel conversion within the fuel reactor. Furthermore, due to their catalytic activity, they can promote reforming reactions and gas-phase cracking, enhancing the utilization of complex gaseous or volatile fuels [20].

However, nickel-based oxygen carriers also present several significant disadvantages that limit their practical deployment. First and foremost, nickel is toxic and carcinogenic, which raises environmental and occupational health concerns. As a result, strict handling protocols and containment strategies must be adopted when using nickel in pilot or industrial-scale systems. Second, nickel-based carriers are relatively expensive, both in terms of raw material cost and regeneration/recycling complexity, which impacts the overall economic viability of large-scale CLC systems [21].

Another major issue is their susceptibility to sintering and agglomeration, especially under cyclic redox conditions. Over time, repeated oxidation and reduction cycles can lead to grain growth and particle fusion, significantly degrading the carrier’s reactivity and fluidization behavior. To address these challenges, various support materials have been investigated to improve thermal stability and mechanical strength. Among the most effective supports are aluminum oxide (Al₂O₃), nickel aluminate (NiAl₂O₄ spinel), yttria-stabilized zirconia (YSZ), titanium dioxide (TiO₂), and silicon dioxide (SiO₂) [22].

Nickel aluminate spinels, in particular, offer superior resistance to sintering while maintaining high reactivity even at elevated temperatures of up to 1300 °C. The spinel structure immobilizes the nickel phase within the ceramic matrix, thereby preventing migration and agglomeration during redox cycling. Yttria-stabilized zirconia is also notable for its ability to enhance reactivity through improved oxygen ion mobility, although it is more costly. On the other hand, supports like TiO₂ and SiO₂ are more commonly used to improve mechanical durability, albeit with some compromise on redox performance [23,24].

Despite their limitations, nickel-based oxygen carriers continue to be used in experimental CLC studies, particularly for validating novel reactor designs or for benchmarking performance. Their well-understood redox behavior, high reaction rates, and substantial fuel flexibility make them valuable reference materials in the development of more advanced, cost-effective, and environmentally friendly oxygen carriers for future CLC systems [18].

2.1.2. Copper-Based Oxygen Carriers

Copper-based oxygen carriers are considered one of the most promising materials for chemical looping combustion (CLC), particularly due to their high reactivity, favorable oxygen uncoupling properties, and relatively low cost compared to nickel-based carriers. The active redox couple in these materials is copper oxide (CuO) and metallic copper (Cu), which participate in oxidation-reduction reactions with rapid kinetics over a broad range of operating conditions [25,26].

One of the defining advantages of Cu-based oxygen carriers is their CLOU (chemical looping with oxygen uncoupling) capability. This means that CuO can spontaneously release molecular oxygen (O₂) in the fuel reactor at typical CLC operating temperatures, without requiring direct contact with the fuel. As a result, copper-based carriers enable non-catalytic combustion of solid and gaseous fuels, which is particularly beneficial for fuels with low volatile content or in cases where fuel conversion efficiency is critical [27].

Additionally, both the oxidation and reduction reactions involving copper oxides are exothermic, which enhances the overall thermal efficiency of the CLC process by eliminating the need for external energy input in the fuel reactor. This distinguishes copper from materials like iron, which require endothermic reduction. The relatively high oxygen transport capacity of copper oxides further contributes to improved combustion performance and CO₂ capture efficiency [28].

Despite these benefits, copper-based oxygen carriers also face notable challenges, particularly related to their thermal stability. The melting point of Cu is approximately 1085 °C, which is significantly lower than that of other oxygen carrier materials. Under high-temperature redox cycling, copper tends to migrate, sinter, and form agglomerates, leading to a loss of reactivity and poor fluidization behavior. To mitigate these problems, it is essential to incorporate appropriate support materials that stabilize the copper phase and inhibit sintering [27,29].

Commonly used supports for Cu-based oxygen carriers include aluminum oxide (Al₂O₃), copper aluminate (CuAl₂O₄ spinel), magnesium oxide (MgO), magnesium aluminate (MgAl₂O₄), zirconium dioxide (ZrO₂), titanium dioxide (TiO₂), and silicon dioxide (SiO₂). Among these, copper aluminate spinels are especially effective in immobilizing the active phase and increasing thermal resistance. Supports like MgAl₂O₄ and Al₂O₃ also enhance mechanical strength and cyclic stability [30].

The synthesis of copper-based carriers can be achieved through several methods, including mechanical mixing, wet impregnation, and spray drying. Each method influences the dispersion of the active CuO phase, the porosity, and the overall stability of the material. The impregnation method is especially popular due to its simplicity and ability to control the copper loading on the support [26,27,28].

Overall, copper-based oxygen carriers represent a compelling compromise between performance and economic feasibility. While they are more reactive and more easily regenerable than iron-based carriers, they are also less expensive and less toxic than nickel-based alternatives. However, their successful application in large-scale systems depends on advancements in material stabilization to overcome sintering and ensure long-term operability [4].

2.1.3. Iron-Based Oxygen Carriers

Iron-based oxygen carriers are among the most widely studied and utilized materials in chemical looping combustion (CLC) due to their combination of low cost, non-toxicity, and excellent mechanical strength. Despite their relatively low intrinsic reactivity, particularly in the reduction step with solid fuels, their advantages make them attractive for large-scale applications. One of the key drawbacks of iron-based carriers is their tendency to agglomerate during redox cycling, especially under high-temperature conditions typical of fuel reactors. This agglomeration can lead to poor fluidization behavior, channeling, and ultimately a loss of process stability and efficiency [17].

To mitigate these issues, iron-based carriers are often formulated with structural supports that enhance their physical and chemical resilience. Commonly used supports include aluminum oxide (Al₂O₃), magnesium aluminate (MgAl₂O₄), silicon dioxide (SiO₂), and titanium dioxide (TiO₂). These supports help maintain the structural integrity of the oxygen carrier particles during repeated oxidation-reduction cycles and improve fluidization performance [31].

Among iron-based carriers, ilmenite stands out as one of the most practical and widely tested natural minerals. Ilmenite primarily consists of iron titanium oxide (FeTiO₃) and titanium dioxide (TiO₂) and often includes traces of Fe₂O₃ and neutral fractions. Before use in CLC systems, ilmenite is typically pre-calcined at temperatures ranging from 900 to 950 °C, a process that enhances its oxygen release and uptake behavior by activating redox-active phases and stabilizing its crystalline structure [13,17,32].

Ilmenite is favored not only for its low cost and natural abundance but also for its favorable thermal and mechanical stability, which makes it suitable for long-term operation under harsh fluidized-bed conditions. Unlike some synthetic oxygen carriers, ilmenite exhibits a relatively low oxygen-carrying capacity, which can limit its overall fuel conversion efficiency, particularly for high-demand systems. Nevertheless, this drawback is offset by its superior resistance to attrition and sintering, allowing for extended operational lifespans and reduced material loss [13].

Although its reactivity is modest in comparison to copper- or nickel-based carriers, ilmenite performs consistently across multiple redox cycles and does not require extensive regeneration procedures. As a result, it is frequently used in pilot-scale and demonstration-scale CLC facilities, particularly in systems designed for solid fuels such as coal or biomass, where mechanical stability and cost-effectiveness are prioritized over high oxygen release rates [33].

2.1.4. Oxygen Carriers Based on Manganese and Cobalt

Manganese-based oxygen carriers have received considerable attention in recent years due to their favorable cost, low toxicity, and variable oxidation states, which enable flexible redox behavior in chemical looping combustion (CLC). Manganese oxides can exist in several valence states, most commonly as MnO₂ (manganese dioxide), Mn₂O₃ (manganese(III) oxide), and Mn₃O₄ (trimanganese tetraoxide). Among these, Mn₃O₄ is considered the most stable and effective for CLC applications. MnO₂ begins to decompose at relatively low temperatures (around 500 °C), and Mn₂O₃ becomes unstable above 800 °C, making Mn₃O₄ the optimal choice for thermal and operational stability in practical systems [8].

A significant advantage of manganese-based carriers is their relatively low environmental impact and low cost of synthesis, especially when compared to nickel- or cobalt-based alternatives. They also offer moderate oxygen transport capacity and can support partial oxygen uncoupling, although typically not as efficiently as copper-based carriers. Depending on the support material and operating conditions, manganese oxides may exhibit behavior aligned with the iG-CLC (in situ gasification) mechanism or limited CLOU functionality [34].

To enhance the reactivity and cyclic stability of manganese-based oxygen carriers, supports such as Al₂O₃, TiO₂, and MgAl₂O₄ are commonly used. These supports improve structural resistance against sintering and promote better dispersion of the active manganese phase, especially under cyclic thermal and chemical stresses typical for fluidized bed systems [30].

In contrast, cobalt-based oxygen carriers, though less commonly employed due to their toxicity and high cost, exhibit excellent redox performance. Cobalt oxides participate in redox cycling primarily through the transformation between Co₃O₄ (cobalt(II, III) oxide), CoO (cobalt(II) oxide), and metallic Co, depending on the operating temperature and oxygen partial pressure. The Co₃O₄ phase provides high oxygen uncoupling ability, classifying cobalt-based carriers as CLOU-functional materials. However, Co₃O₄ is thermally unstable above ~900 °C, where it decomposes into CoO, limiting its application range unless stabilized with appropriate support matrices [35].

Despite their high oxygen transport capacity, cobalt-based materials face critical challenges that restrict their large-scale implementation. These include not only environmental and health concerns but also issues with phase transformation reversibility and material degradation over extended operational cycles. Nevertheless, in research environments or high-value niche applications, Co-based carriers continue to serve as valuable reference materials for benchmarking performance [36].

In summary, manganese- and cobalt-based oxygen carriers present complementary profiles in CLC applications. Manganese offers a sustainable and cost-effective alternative with moderate performance, while cobalt provides exceptional reactivity and CLOU capability at the expense of environmental and economic feasibility. Their practical use depends strongly on support material selection, reactor configuration, and the target fuel type [30].

2.2. CLC Pilot Facilities

Numerous classifications of CLC systems are proposed in the literature, yet the most common approach focuses on the design configuration of interconnected fluidized-bed reactors. A typical CLC system consists of two separate reactors—a fuel reactor (FR) and an air reactor (AR)—that are hydraulically interconnected to ensure continuous circulation of solid oxygen carriers, while simultaneously preventing gas backflow between the reactors. A well-established design element enabling this function is the loop seal, which serves both as a gas barrier and a mechanical passage for solids [37].

One of the most representative and extensively studied examples of chemical looping combustion systems is the Dual Circulating Fluidized Bed Reactor (DCFB) developed at the Vienna University of Technology. The pilot unit, with a nominal thermal capacity of 120 kW_t, is specifically designed for the combustion of gaseous fuels, including natural gas and synthetic gas mixtures composed of hydrogen (H₂), carbon monoxide (CO), and methane (CH₄). The reactor setup consists of two interconnected circulating fluidized beds—an air reactor and a fuel reactor—each operating under distinct hydrodynamic regimes to optimize performance [9].

The fuel reactor operates in the fast fluidization regime, which promotes intensive mixing of the gaseous fuel with solid oxygen carriers, thereby enhancing mass and heat transfer rates and enabling high fuel conversion efficiencies [38]. The air reactor, on the other hand, serves to regenerate the oxygen carriers via oxidation, with its design facilitating stable and continuous circulation of the solids. The system employs loop seals and specially engineered standpipes to prevent gas leakage between reactors and to ensure accurate solids recirculation [39].

A notable advantage of the DCFB design is its ability to achieve independent control over gas and solids residence times in each reactor, allowing for greater operational flexibility and a more detailed evaluation of reaction kinetics and material behavior. The unit has been used extensively to investigate the performance of various oxygen carriers, most notably nickel-based carriers and ilmenite. These materials were subjected to repeated redox cycles to assess their reactivity, thermal stability, attrition resistance, and tendency toward agglomeration under real process conditions. The tests provided critical insights into the deactivation mechanisms of oxygen carriers and the role of support materials in improving long-term durability [40].

Moreover, the DCFB reactor served as a benchmark installation for validating numerical models of CLC processes, including multiphase flow, reaction kinetics, and particle transport. Data obtained from this facility have been widely used for scaling strategies and the development of next-generation reactors, including those intended for solid fuels and CLOU-based applications [41]. The simplified design of the DCFB system is presented in Figure 2.

A larger-scale system based on a similar dual-reactor design is the 1 MW_t chemical looping combustion (CLC) pilot plant constructed at the Darmstadt University of Technology in Germany. This facility represents a key milestone in the development of CLC technology, serving as a bridge between laboratory-scale systems and potential commercial-scale applications. The architecture of the installation includes a fuel reactor (FR) with a height of approximately 11.5 m and an air reactor (AR) measuring around 8.5 m in height. The internal diameters are 0.4 m for the fuel reactor and 0.6 m for the air reactor, resulting in distinct hydrodynamic behaviors tailored to the specific functional demands of each section [10].

A simplified schematic of the installation is presented in Figure 3 [14].

The reactor system is configured to allow continuous circulation of oxygen carrier particles between the AR and FR while maintaining gas-phase separation, ensuring a high-purity CO₂ stream at the fuel reactor outlet. The design also incorporates advanced control systems and instrumentation, enabling precise regulation of gas flows, temperatures, and solid circulation rates—essential for process optimization and scale-up studies [12].

This installation was designed to operate primarily with solid fuels, including bituminous coal, and has been used extensively to investigate the combustion performance of low-volatile fuels. A wide range of oxygen carriers has been tested, including copper-, iron-, and manganese-based oxides, as well as natural ilmenite, to evaluate their behavior under realistic process conditions. These materials were assessed for their oxygen transport capacity, redox reactivity, mechanical durability, and resistance to attrition and agglomeration over multiple operating cycles [11].

Beyond material testing, the pilot plant facilitated comprehensive parametric investigations, including the influence of fuel type, oxygen carrier properties, reactor temperature, and gas composition on overall fuel conversion efficiency and CO₂ capture performance. The system also enabled the study of phenomena such as carbon slip, ash interaction, and the formation of pollutants (e.g., SOx, NOx), providing valuable insights into the environmental implications of CLC technology [10].

Due to its scale and flexibility, the Darmstadt unit has become a reference installation in Europe for demonstrating the technical feasibility of chemical looping with solid fuels. It has also supported the validation of computational fluid dynamics (CFD) models and reactor-scale simulations, which are instrumental in the design of next-generation CLC reactors, including those based on CLOU oxygen carriers. Moreover, CFD-based models turned out to be feasible and are pivotal for simulating the fluid dynamics, thermal characteristics, and chemical reactions within fluidized bed reactors [42,43].

The CLC units developed at Chalmers University of Technology in Sweden are among the most extensively studied and well-documented in the literature. In 2008, a 10 kW_t prototype reactor was commissioned, marking one of the earliest interventions in solid-fuel CLC using ilmenite as the oxygen carrier and coal as fuel [44]. Building on this foundation, a larger 100 kW_t unit was introduced in 2012, in which experiments were conducted with coal, petroleum coke, and biochar, while ilmenite and iron ore served as oxygen carriers—a setup allowing valuable comparative evaluations of carrier performance and operational robustness [45].

In 2016, Chalmers advanced to pilot scale with the construction of a 4 MW_t CLC unit, integrated into a circulating fluidized bed boiler. This installation utilized biomass as the primary fuel and tested ilmenite and manganese ore as oxygen carriers. The facility demonstrated excellent performance, achieving CO₂ capture efficiencies of 98–99%, confirming the high efficacy of the technology in bioenergy applications [10].

In Poland, a dual fluidized bed system for chemical looping combustion of solid fuels (DFB-SF-CLC) has been developed at the Institute of Advanced Energy Technologies, Czestochowa University of Technology. This pilot-scale installation was designed to enable flexible experimental research on solid fuels—such as coal and biomass—under both iG-CLC and CLOU operating modes. Two vertical heater assemblies electrically heat the reactor, each equipped with internal electric heating elements, providing a total power of 72 kW. Each heater is divided into three independent temperature control zones, allowing precise axial temperature regulation and thermal stabilization during operation [25].

A schematic representation of the reactor system is shown in Figure 4.

The system consists of a compact fuel reactor (FR) with internal dimensions of 0.15 m (width), 0.085 m (depth), and 0.5 m (height), designed to operate in the bubbling or turbulent fluidization regimes, depending on the process parameters. In contrast, the air reactor (AR) features a more complex and vertically extended geometry: a bottom cylindrical section (diameter: 0.098 m; height: 0.285 m), a conical transition zone (diameter: 0.12 m), and a top cylindrical riser (diameter: 0.04 m; height: 2.14 m), yielding a total reactor height of approximately 2.55 m [6].

The installation enables continuous circulation of solids between reactors and supports operation with various oxygen carriers, including natural ilmenite and synthetic CuO-based materials. These carriers were tested under both oxygen uncoupling (CLOU) and in situ gasification (iG-CLC) modes, offering insights into their reactivity, oxygen transport efficiency, and redox stability under varied fuel types and temperature profiles [5].

In Germany, the Institute of Combustion and Power Plant Technology (IFK) at the University of Stuttgart operates a 10 kW_t dual fluidized bed pilot plant that can be configured for either circulating or bubbling fluidized bed operation. The installation consists of a carbonator riser and a bubbling fluidized bed regenerator, with integrated loop seals to ensure continuous solids circulation between the reactors. A notable design feature is the cone valve system, which enables precise control of the oxygen carrier or sorbent circulation rate, allowing stable operation across a wide range of process conditions. The unit is equipped with advanced instrumentation for measuring solids inventory, pressure profiles, and gas–solid flow characteristics, making it a versatile platform for investigating hydrodynamics, residence time control, and scale-up strategies relevant to both CaL and CLC processes. Its modular configuration permits the testing of different oxygen carriers and operational modes, supporting fundamental studies as well as process optimization and model validation efforts [46].

In Spain, the Instituto Nacional del Carbón (CSIC-INCAR) operates a 30 kW pilot-scale facility designed for long-term evaluation of oxygen carrier performance under realistic process conditions. The unit enables systematic testing of material durability, reactivity, and resistance to contaminants such as sulfur compounds. Recent experimental campaigns have also explored process modifications aimed at enhancing CO₂ capture efficiency, including the creation of a controlled low-temperature zone in the upper section of the carbonator. This approach, combined with optimized sorbent distribution, has demonstrated a significant improvement in capture performance and provided valuable operational insights for potential large-scale applications [47].

Norway’s SINTEF Energy Research, in collaboration with the Norwegian University of Science and Technology (NTNU), has developed and operates a 150 kW circulating fluidized bed CLC pilot plant intended for gaseous fuel applications. The installation consists of two interconnected reactors that allow continuous circulation of an oxygen carrier between the fuel and air reactor while maintaining gas-phase separation. The system is equipped to handle porous copper oxide-based oxygen carriers with a bulk density of approximately 800 kg/m³. It includes advanced instrumentation for monitoring gas composition, temperature profiles, and solids circulation rates. Its design enables flexible adjustment of fuel input, solids inventory, and air-to-fuel ratios, supporting both steady-state operation and parametric testing. The facility has been used as a reference platform for Nordic scale-up activities, demonstrating stable hydrodynamics and reliable solids transfer between the reactors [48].

At the French Alternative Energies and Atomic Energy Commission (CEA) in Grenoble, a pilot-scale solar thermochemical reactor has been developed to explore the integration of chemical looping principles with renewable heat sources. The facility utilizes ceria-based oxygen carriers within a high-temperature cavity receiver, enabling redox cycles driven by concentrated solar power. The reactor design allows direct irradiation of the reactive material, achieving temperatures above 1500 °C necessary for splitting H₂O and CO₂ into syngas. This configuration has been tested under realistic solar flux conditions, demonstrating stable redox performance and validating the feasibility of coupling CLC concepts with solar thermochemical fuel production [49].

Another unconventional solution in chemical looping combustion (CLC) is the rotary reactor, which remains the least widely adopted configuration in this technology. This reactor is primarily intended for gaseous fuels, as the handling of solid fuels in a rotating environment poses significant engineering challenges. The reactor is composed of three coaxially arranged concentric rings, where the inner and outer rings are stationary, while the middle ring slowly rotates and contains the bed of oxygen carriers. The design is subdivided into four distinct functional zones: a fuel sector, an air sector, and two inert gas sealing zones, which isolate the reactive zones and prevent direct mixing between the oxidizing and reducing gases (Figure 5) [15,50].

The key advantage of this design lies in the enhanced fuel conversion efficiency, which is achieved through extended and continuous contact between gaseous fuel and the oxygen carriers. Since the oxygen carrier bed rotates cyclically through the oxidizing and reducing zones, this configuration enables continuous regeneration of the material without the need for external recirculation. However, despite its conceptual attractiveness, the rotary reactor faces significant practical limitations. One of the primary issues is gas leakage between sectors, which arises from the mechanical complexity and difficulties in achieving perfect sealing between rotating and stationary components [50].

As a result of these drawbacks, rotary reactors are not widely implemented at pilot or industrial scale, and research has focused more on circulating fluidized bed combustors (CFBCs) as alternative solutions. CFBC systems exhibit high fuel flexibility (including solid fuels), low NOx and SO₂ emissions, and excellent gas–solid contact due to the intense mixing of particles in the fast bed regime. These features promote efficient combustion and uniform temperature distribution, minimizing the risk of hot spots [51].

Chemical looping combustion (CLC) is a promising CO₂ capture technology that enables inherent separation of combustion products through the use of oxygen carriers cycled between two interconnected reactors. The choice of oxygen carrier—whether based on nickel, copper, iron, manganese, cobalt, or natural minerals like ilmenite—has a critical impact on the efficiency, reactivity, and durability of the process. Various reactor designs, including dual fluidized beds, moving beds, and rotary configurations, have been developed to facilitate the redox cycle, each with specific operational advantages and limitations. Despite significant progress demonstrated by numerous pilot-scale installations worldwide, there remains a clear need for innovative reactor configurations and design improvements to further enhance process efficiency, operational stability, and scalability—especially for solid fuels. These considerations form the basis for exploring alternative fuel reactor concepts that address the existing design and operational challenges.

3. Results and Discussion

3.1. Description of the Novel Fuel Reactor Concept

A novel concept of a fuel reactor for chemical looping combustion (CLC) with CLOU-type oxygen carriers is proposed in this study. The invention, protected by Polish patent No. PL 232612 (Czakiert et al., 2017) [52] is designed to improve the performance of the CLC process when solid fuels are used. As shown in Figure 6a,b, the reactor features a rectangular horizontal cross-section and is equipped with a fluidization grid and an ash drain at its base. These elements ensure the stable operation of a bubbling fluidized bed, which is composed mainly of fuel and ash particles.

Fuel is fed through the right side wall, and combustion gases are extracted from the top. The core innovation lies in a centrally located perforated conveyor that transports CLOU-type oxygen carriers through the reactor. This conveyor acts as a distributor of gaseous oxygen, releasing it into the surrounding bed through its perforated walls. Notably, the oxygen carriers remain physically separated from the fuel and ash mixture, avoiding direct contact and improving process selectivity and control.

3.2. Functional Advantages and Expected Performance

The separation of the oxygen carriers from the fuel and ash offers several operational benefits. First, it prevents the undesirable transport of unburned fuel and ash particles to the air reactor, a known issue in conventional dual fluidized bed systems [13,53]. This minimizes carbon and sulfur slip, ensuring that these elements are fully oxidized in the fuel reactor and not transferred into the air reactor, where their oxidation would reduce overall CO₂ capture efficiency.

Second, the new design reduces the risk of oxygen carrier losses through entrainment or drainage with bottom ash. This improves material utilization and lowers operating costs. Additionally, the absence of ash in the air reactor reduces flow resistance, improving fluidization quality and thermal efficiency in that part of the system. Moreover, the described configuration prevents oxygen carriers from being entrained with the flue gas stream or unintentionally removed together with bottom ash, thus improving the economic feasibility of the system.

3.3. System Integration and CO₂ Capture Process

The novel fuel reactor is integrated into a complete dual-reactor CLC system, as shown in Figure 7. The system includes a conventional air reactor, where oxygen carriers are oxidized, and a cyclone that separates solids from the oxygen-depleted air. The oxidized oxygen carriers are then sent to the fuel reactor, where they release oxygen via CLOU mechanisms and are subsequently reduced. The reduced oxygen carriers return to the air reactor, thus closing the circulation loop.

Combustion gases from the fuel reactor are directed to a gas–solid separator, where fly ash is removed, and the cleaned flue gas is fed to a CO₂ processing unit (CPU). The system allows for partial recirculation of CO₂ to the fuel reactor as a fluidizing gas. The high CO₂ concentration at the outlet of the CPU facilitates cost-effective capture and utilization.

The schematic presented in Figure 7 illustrates all major components and process streams of the dual-reactor system, including the novel fuel reactor, air reactor, gas–solid separators, and CO₂ processing unit. Key material flows such as oxygen carriers, solid fuel, recycled and cleaned flue gas, and captured CO₂ are marked. This closed-loop configuration enables effective oxygen carrier regeneration, high fuel conversion, and the production of a highly concentrated CO₂ stream ready for capture or utilization.

3.4. Novelty and Technological Advantages of the Proposed Reactor

3.4.1. Key Innovative Aspects

The proposed fuel reactor configuration introduces a set of distinctive design features specifically developed to overcome persistent operational limitations of conventional dual fluidized bed (DFB) and rotary CLC/CLOU systems [54]. The novelty lies not only in the physical separation of the oxygen carrier from the fuel–ash mixture but also in the ability to control the residence time of the oxygen carrier independently from the hydrodynamic regime of the fuel bed. This combination enables optimization of redox kinetics, minimizes material losses, and enhances CO₂ capture efficiency.

The principal innovative aspects are as follows:

(1)

Physical separation of solid phases—The perforated conveyor ensures complete isolation of CLOU-type oxygen carriers from fuel and ash particles, preventing fouling and chemical deactivation of the active material. This preserves reactivity and mechanical integrity over extended operating periods.

(2)

Independent control of oxygen carrier residence time—By adjusting the conveyor speed independently from the fuel bed dynamics, the redox cycle duration can be precisely optimized, ensuring high oxygen release efficiency and better utilization of the active phase.

(3)

Minimization of oxygen carrier losses—Eliminating direct contact with bottom ash significantly reduces the risk of active material loss through ash removal streams, improving process economy and sustainability.

(4)

Enhanced CO₂ stream purity—Physical separation limits the transfer of unburned carbon and sulfur compounds to the air reactor, preventing their secondary oxidation and increasing the purity of the captured CO₂ stream.

(5)

Reduced mechanical complexity compared to rotary systems—Unlike rotary reactors, the static perforated conveyor eliminates the need for high-precision dynamic seals, lowering leakage risks, mechanical wear, and maintenance demands, and thereby improving long-term reliability.

3.4.2. Comparative Analysis with State-of-the-Art Designs

A meaningful evaluation of the proposed fuel reactor configuration requires positioning it within the broader landscape of circulating fluidized bed technologies used for chemical looping processes. Currently, two major design categories dominate CLC/CLOU development efforts: conventional dual fluidized bed (DFB) systems, such as dual circulating fluidized bed (DCFB) designs, and rotary reactor systems, which employ mechanical rotation to alternate between oxidizing and reducing environments.

DFB systems are well established, having been demonstrated at pilot and pre-commercial scales for both gaseous and solid fuels. Their main strengths include mature solids handling, reliable hydrodynamics, and proven operational stability. However, they inherently allow direct contact between oxygen carriers and fuel–ash mixtures, which leads to several drawbacks:

−

Fouling and deactivation of the oxygen carrier surface due to ash deposition,

−

Attrition and mechanical degradation from abrasive interaction with ash particles,

−

Material losses when oxygen carriers are entrained with bottom ash or carried over to the air reactor,

−

Reduction in CO₂ capture efficiency caused by oxidation of unburned carbon in the air reactor (carbon slip).

Rotary reactor systems, on the other hand, address some of these limitations by mechanically isolating the oxidizing and reducing zones using rotating chambers and sealing sections. They can achieve higher control over the residence time of solids in each zone and potentially limit cross-contamination between streams. Nevertheless, their benefits come at the expense of

−

Increased mechanical complexity,

−

Gas leakage through imperfect seals between rotating and stationary components,

−

Higher maintenance requirements due to moving parts and wear-prone sealing surfaces.

The proposed conveyor-based CLOU reactor incorporates the strengths of both concepts while mitigating their principal weaknesses. Physical separation of the oxygen carrier from fuel and ash is achieved without resorting to moving the entire reactor chamber, thereby avoiding the mechanical complications of rotary systems. At the same time, solids circulation remains straightforward and does not require the complex hydrodynamic tuning often needed in DFB systems to minimize carryover losses. Furthermore, preliminary numerical simulations performed in CeSFaMB indicate that, with optimized operating parameters, the new design can reach CO₂ concentrations in the outlet gas exceeding 98%, which surpasses the typical performance range of both reference configurations.

The parameters selected for comparison in

Table 1. Qualitative comparison of operational features for conventional DFB and rotary CLC systems and the proposed CLOU-based conveyor reactor.

Feature	Conventional DFB (e.g., DCFB)	Rotary Reactor	Proposed Conveyor-Based Reactor
Physical separation of the oxygen carrier from fuel/ash	No	Partial (gas sealing zones)	Yes
Control of carrier residence time	Limited	Possible	Yes
Risk of carrier loss via bottom ash	High	Low	Very low
Typical CO2 capture efficiency [%]	85–95	90–96	≥98 *
Mechanical complexity	Moderate	High	Low
Risk of inter-zone gas leakage	Low–moderate	High	Low
Maintenance requirements	Medium	High	Low

* Based on preliminary CeSFaMB simulations for a pure CuO oxygen carrier under optimized conditions.

reflect critical performance and operational criteria identified in the literature as determinants of reactor viability:

−

Solids phase separation (physical or functional),

−

Ability to control oxygen carrier residence time,

−

Risk of material losses via bottom ash or entrainment,

−

Expected CO₂ capture efficiency,

−

Mechanical complexity,

−

Risk of inter-zone gas leakage,

−

Maintenance demands over long-term operation.

A qualitative assessment of these criteria highlights the potential operational advantages of the proposed CLOU-based conveyor reactor over conventional designs, particularly concerning CO₂ capture efficiency, oxygen carrier retention, and mechanical simplicity.

As shown in Table 1, the proposed CLOU-based conveyor reactor combines the high CO₂ capture potential of rotary systems with the operational simplicity and robustness of DFB designs while offering superior oxygen carrier retention and reduced risk of gas leakage. These projected advantages highlight its potential for achieving both higher process efficiency and lower maintenance demands compared to conventional configurations.

It should be emphasized that this benchmarking is qualitative and based on representative literature data and preliminary simulations. A rigorous, quantitative comparison under strictly matched operating conditions will only be feasible after experimental validation of the proposed design.

3.4.3. Integration of the Novel Design into Numerical Modeling

The performance of the proposed CLOU-based conveyor reactor was evaluated through numerical simulations conducted in the CeSFaMB simulator. These simulations are presented as preliminary, non-calibrated scoping analyses intended to illustrate the conceptual potential of the reactor rather than to provide final, validated predictions. To ensure that the modeling reflected realistic operating conditions, key thermodynamic parameters—such as bed temperature, inlet gas composition, and system pressure—were directly taken from measurements performed in the CLC experimental unit [6]. While the physical test rig did not incorporate the patented perforated conveyor, this feature was implemented in the simulation model to investigate its potential impact on hydrodynamics and gas–solid conversion efficiency. The selected operating conditions were also consistent with those reported for pilot-scale CLOU reactors in the literature, providing an additional basis for ensuring that the modeling framework reflects realistic and widely accepted process parameters.

To reflect the anticipated technical implementation of the concept, additional engineering assumptions were incorporated into the simulation framework. The fuel reactor was assumed to operate in the typical CLOU temperature range of 800–950 °C at near-atmospheric pressure, with the inlet gas flow rates matching those measured in the experimental unit. The perforated conveyor section was considered to be fabricated from high-temperature oxidation- and wear-resistant alloys (e.g., heat-resistant stainless steels or nickel-based alloys), selected to withstand prolonged exposure to both mechanical attrition and oxidizing/reducing environments.

Mechanical reliability aspects were also addressed at the conceptual stage. Potential clogging of the perforations was mitigated in the design by specifying an orifice size-to-particle diameter ratio that prevents blockage under stable fluidization conditions. At the same time, wear resistance was enhanced by considering replaceable perforated plates or protective surface coatings. Sealing arrangements between the conveyor section and adjacent reactor zones were based on labyrinth-type configurations, which are widely applied in high-temperature gas–solid systems to minimize leakage without introducing complex moving components.

Preliminary scale-up considerations indicated that maintaining uniform solids distribution and preventing localized pressure drops across the perforated section will be critical in large-scale operation. Strategies such as optimized perforation patterns, staged solids feeding, and careful matching of gas distributor characteristics to the conveyor geometry were included in the conceptual assessment. These considerations will guide future pilot-scale testing to validate the reactor’s hydrodynamic stability, phase separation efficiency, and long-term mechanical reliability under industrially relevant conditions. At this stage, no experimental validation, calibration, or uncertainty quantification has been performed, as the experimental facility is still under construction. These aspects will be addressed in future work. While not experimentally validated at this stage, these measures were introduced into the simulation model to enable a more realistic representation of the proposed reactor’s potential hydrodynamic performance.

The reactor geometry, dense bed height, and solids circulation layout were kept identical to those of the reference experimental configuration, with the sole structural modification being the inclusion of the perforated conveyor for physical separation of the oxygen carrier from the fuel–ash mixture. The oxygen carrier was modeled as pure CuO, in line with the laboratory-grade material used in previous experimental campaigns.

This modeling approach enabled a direct comparison between conventional fluidized bed operation and the proposed configuration under otherwise identical process conditions. It also allowed the influence of the conveyor on CO₂ capture efficiency, CO reduction, and temperature stability to be identified. The resulting axial concentration and temperature profiles along the fuel reactor are presented and discussed in Section 3.4.4.

3.4.4. Numerical Simulation Results

Numerical simulations were conducted using the CeSFaMB simulator (Comprehensive Simulator of Fluidized and Moving Bed equipment, Campinas, Brazil), with input operating conditions derived from the CLC experimental unit. While the physical test facility did not include the patented perforated conveyor, this element was incorporated into the simulation model to evaluate its potential impact on process performance. The modeled reactor geometry, bed height, and operating parameters (temperature, gas composition, solids circulation) were based on experimental data, ensuring that the simulated conditions remain representative of realistic operation.

Figure 8 shows the axial concentration profiles of CO₂, CO, and H₂O along the height of the fuel reactor. The distribution of components is highly favorable, with CO₂ maintained at a consistently high level from the reactor inlet to the outlet, indicating efficient conversion of carbon to its fully oxidized form. The final CO₂ concentration at the outlet reaches 98.19%, while CO remains at only 1.036%. Water vapor is present at a low level of 0.71%. The uniformity of these profiles along the reactor height confirms stable combustion conditions and effective utilization of the oxygen carrier throughout the process.

The observed trends demonstrate that the proposed configuration, featuring physical separation of the oxygen carrier from the fuel–ash mixture, facilitates near-complete combustion under the tested conditions. The flue gas composition, with approximately 98% CO₂ and only about 1% CO, confirms the high conversion efficiency and minimal carbon slip. This performance is attributed to the capability to control solids’ residence time independently of fuel bed hydrodynamics as well as to maintain the active material in optimal contact with the fluidizing gas.

Figure 9 presents the temperature distribution along the reactor height. The temperature remains stable at approximately 1120 K across most of the reactor length, with only a minor decrease near the outlet. This stability supports optimal redox activity of the oxygen carrier and uniform reaction conditions in the fuel bed.

Overall, the CeSFaMB simulation outcomes confirm that integrating the perforated conveyor into the fuel reactor design enables high CO₂ capture efficiency and stable operation, while keeping CO emissions at low levels. The numerical results presented in this section are consistent with the comparative assessment in Section 3.4.2, confirming that the proposed design exceeds the typical CO₂ capture performance range reported for both conventional DFB and rotary reactor systems. These results provide a strong foundation for subsequent numerical analyses and targeted experimental validation.

Potential operational challenges of the proposed design include clogging of perforations and material wear caused by abrasive particles. Selecting corrosion-resistant and wear-resistant materials, as well as integration of conveyor cleaning systems, will be essential to ensure system durability and reliability. Furthermore, sealing and hydrodynamic control at industrial scales present challenges that will require further investigation and development.

3.5. Discussion in the Context of Existing Technologies

Building on the simulation outcomes presented in Section 3.4, it is essential to assess the proposed reactor concept relative to existing chemical looping configurations. Compared to conventional dual fluidized bed or rotary reactor systems, the design introduces a distinctive feature—physical isolation of the oxygen carrier from direct fuel contact. This structural modification addresses key limitations observed in pilot-scale studies, including oxygen carrier deactivation due to ash fouling, carbon slip, and operational instability related to material losses.

While rotary reactors attempt to improve oxygen carrier circulation through mechanical motion, they often suffer from gas leakage and increased mechanical complexity [15,50]. In contrast, the proposed system achieves controlled circulation and oxygen release through a static, perforated conveyor mechanism, thereby enhancing reliability and reducing maintenance demands.

This concept aligns with broader objectives in CLC development, namely achieving high fuel conversion rates, lowering CO₂ capture costs, and improving operational robustness for solid fuel applications. The capability to decouple oxygen carrier residence time from that of the fuel–ash mixture offers a valuable degree of process control over redox kinetics, supporting more complete combustion and improved oxygen carrier utilization. Consequently, the configuration represents a promising step toward scalable and efficient CLC systems, complementing and extending current technological approaches.

4. Conclusions

The present study introduces and evaluates a novel fuel reactor configuration for chemical looping combustion (CLC) with CLOU-type oxygen carriers, featuring a perforated conveyor that physically separates the oxygen carrier from the fuel–ash mixture. This contribution should be regarded as a conceptual proof-of-concept study, providing the foundation for future experimental and numerical validation. Numerical simulations performed in the CeSFaMB environment indicate that this structural innovation enables CO₂ concentrations exceeding 98% at the fuel reactor outlet, significantly reducing carbon slip and oxygen carrier losses compared to conventional dual fluidized bed and rotary systems.

Beyond the performance benefits, the design offers mechanical simplicity, reduced risk of gas leakage, and the capability to independently control oxygen carrier residence time, which is essential for optimizing redox kinetics and extending material lifetime. These results suggest that the proposed reactor could enhance both the technical feasibility and economic viability of CLOU-based solid fuel combustion.

Nevertheless, the present work is limited to numerical modeling under idealized assumptions. The simulations consider perfect solids separation, stable hydrodynamics, and constant material properties, while in real operation, wear, fouling, and gas maldistribution may occur. Additionally, the mechanical performance of the perforated conveyor, its sealing effectiveness, and long-term stability have not yet been experimentally verified. The simulations were carried out using boundary and initial conditions obtained from the CLC test rig at Czestochowa University of Technology, which does not include the perforated conveyor section but provides a realistic baseline for evaluating the proposed design. We emphasize that the absence of experimental validation, calibration, and quantified uncertainty is deliberate at this stage, since the dedicated test facility is still under construction. Consequently, the benchmarking presented in this study should be interpreted as an initial, qualitative assessment rather than a rigorous matched-condition comparison. Detailed benchmarking across all relevant performance indicators will be feasible only after experimental data and calibrated numerical models become available.

In addition to these limitations, the proposed design may face practical challenges during operation. Potential risks include perforation clogging, conveyor surface wear, sealing degradation, and localized pressure drops across the perforated section, especially under long-term high-temperature conditions. Such factors could affect reactor hydrodynamics, solids distribution, and overall system stability. While the present modeling assumes idealized conditions, these technical aspects should be addressed through pilot-scale testing and durability evaluation to confirm the feasibility and reliability of the reactor in industrial applications.

Future research should therefore focus on pilot-scale testing of the reactor under realistic thermal and mechanical conditions with representative fuels and oxygen carriers, long-term durability and fouling resistance evaluation, and hydrodynamic optimization using coupled CFD–DEM approaches. A techno-economic assessment will also be essential to quantify potential savings compared to existing CLC configurations and to support scale-up strategies.

In conclusion, the proposed CLOU-based conveyor reactor represents a promising advancement in CLC reactor engineering, combining high CO₂ capture efficiency with operational simplicity. Addressing the identified limitations through targeted experimental tests will be crucial to confirm its scalability and accelerate the transition from concept to industrial application.

Planned future work includes computational fluid dynamics (CFD) modeling, artificial intelligence (AI)-based analysis, and pilot-scale experiments to quantitatively evaluate the performance of the proposed reactor configuration. The key efforts will focus on advanced numerical simulations, AI-driven optimization, pilot trials, and scale-up studies aimed at addressing technical challenges and fully demonstrating the reactor’s industrial applicability. According to European Commission and DOE definitions, the Technology Readiness Level (TRL) of the reactor concept is currently TRL 2 (technology concept formulated). Its advancement to TRL 3–4 will only be possible after experimental validation on the planned pilot facility. Consequently, the benchmarking presented in this study should be interpreted as an initial, qualitative assessment. Comprehensive matched-condition comparisons and techno-economic analysis will follow once experimental data and calibrated models are available.

5. Patents

The fuel reactor design presented in this study is protected under Polish patent No. PL 232612: Combustion chamber of a fluidized-bed duo-reactor with CLOU-type oxygen carriers for solid fuel combustion.