Fixed-bed Reactor

In the fixed-bed reactor system, the gas feeds are alternated to a fixed-bed of oxygen carrier to establish cyclic reduction and oxidation stages (Figure 21), similar to the gas switching concept using fluidized-bed reactors discussed in section (2.3.1.2). The main benefits of this reactor concept are that solids circulation and solids attrition are intrinsically avoided, more compact reactor design with ease of pressurization in a single vessel ^[93]. The disadvantages of the fixed-bed reactors are the requirement of a high temperature switching valve system (in most targeted processes), and highly exothermic oxidation reaction creates large transient thermal gradients that can damage the oxygen carrier by sintering or other defect on the morphological properties of the OC ^[94]. Additionally, larger particles should be used to minimize the pressure drop, which may lead to intra-particle diffusion limitation lowering the utilization of the oxygen carriers ^[95]. A direct comparison of packed and fluidized gas switching configurations concluded that the plug flow nature of packed beds makes this configuration most suitable for achieving high efficiencies and high CO2 capture rates, but the material development is a large challenge due to the extreme thermochemical stresses imposed by the sharp heat and reaction fronts ^[96].

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Figure 21. A schematic diagram of a fixed-bed CLC reactor ^[97], "Adapted with permission, Copyright (2016) Elsevier BV".

Ishida et al. ^[98] used a fixed-bed reactor to study the effect of pressure on the reaction kinetics of chemical looping methane reforming to syngas with a Ni-based material as oxygen carrier.

It was found that the reduction rate at moderately pressurized conditions (3 bar) was lower than atmospheric pressure reduction rate, attributing this to the endothermic reaction of methane with NiO. Ishida et al. [101] suggested that increasing the H2O/CH4 ratio offer the capacity to improve the reactivity at high pressure.

Gallucci et al. ^[99,100] from Eindhoven University of Technology (TU/e) successfully demonstrated a cyclic steady state operation of chemical looping combustion of syngas in a 10 kWth pressurized fixed-bed reactor using NiO-based and ilmenite-based oxygen carriers. The reactor system has been demonstrated up to 7.5 bar. The mass flow rates were fixed during all experiments implying an increase in the residence time with the pressure. Using NiO-based oxygen carrier, the reactor performance at various pressure showed negligible effects of the pressure on the reduction and the oxidation cycle indicating that the increased gas residence time with the pressure had compensated the expected negative effects of gas dispersion and diffusion resistance to the particles. Carbon deposition enhanced at higher pressures, which could be the result of the higher level of oxygen carrier reduction achieved due to the higher fuel concentration as the pressure was increased. Addition of steam effectively suppressed carbon deposition, but also promoted CO conversion into CO2 and H2 through the WGS reaction. The maximum temperature rise achieved in the cyclic reduction/oxidation was 340°C with possibility of autothermal operation (no external heat supply) after about three full cycles.

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Using a fixed-bed reactor, pressurized hydrogen production with chemical looping water splitting system was investigated by a research group at Graz University of Technology ^[101–

105]. The authors proposed a new concept that combines conventional steam reforming and the steam-iron process in a single fixed-bed reactor containing both the oxygen carrier and the reforming catalyst (Figure 22). The process involved the following steps: 1) catalytic hydrocarbon reforming to syngas, 2) reduction of the iron-based OC using syngas, and 3) oxidation of the reduced OC using steam to produce pure hydrogen. Based on thermodynamic analysis they revealed that the oxidation could be achieved at pressurized conditions, however, the reforming and the reduction step should be carried out at atmospheric pressure to maximize the conversion efficiency ^[101]. The experiments of Zacharias et al. ^[105] were carried out at atmospheric pressure for the reforming/reduction step and at high pressure up to 95 bar for the steam oxidation step. The results revealed no negative effect of the elevated pressure on the reactor performance in the oxidation stage. High purity hydrogen was attained in the range of 99.95-99.999% with CO and CO2 only as impurities given that no air feed is needed in the process. The practicality of operating the oxidation and reduction stages at very different pressures and the feasibility of autothermal operation of the process are potential challenges of this configuration.

Figure 22. The reformer steam-iron process schematic in a fixed bed reactor ^[103], "Adapted with permission, Copyright (2016) RSC".

In-situ solid-fuel gasification under CLC mode has been investigated in a high-pressure fixed-bed system at Southeast University ^[106–108]. The study focused on the pressure effects on the cyclic performance rather than the reactor design, operation and scale-up. Chinese bituminous coal was used as fuel together with different types of iron ore-based oxygen carriers, while steam was used as a gasification agent. Initial investigation by Xiao et al. ^[106] for up to 5 reduction/oxidation cycles showed that the reduction rate increased with pressure up to 5 bar

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then slightly decreased at 6 bar. Subsequent study of 20 reduction/oxidation cycles showed an improvement of the reaction rate with increasing the pressure, which was attributed to the increase of the steam partial pressure and the gas residence time, thus simultaneously enhancing the coal char gasification and reduction of the iron ore ^[107,108].

The utilization of bulk monolithic OC for CLC in a fixed-bed reactor has been proposed by Gu et al. ^[109] to limit the temperature fluctuations, minimize the pressure drop and to decrease the intra-particle diffusion limitation associated with the use of large pellets in fixed-beds system.

The results of Gu et al. ^[109] showed high activity of Ce-Zr-F-O/Al2O3 oxygen carrier for methane combustion as a result of the strong active component to support interaction, that was similar to that of the powder oxygen carrier. Zhang et al. ^[110,111] extended this concept to a 10 kWth prototype using a honeycomb CLC reactor (Figure 23) with NiO-based and iron-based oxygen carriers. The results of Zhang et al. ^[110,111] showed superior performance in term of methane conversion, reduction kinetic, overall reactor stability and limited cyclic temperature fluctuation (50 K) benefiting from the homogeneous distribution of the reaction heat inside the surface of the honeycomb reactor. These preliminary studies proved the feasibility of the concept, but pressurized CLC operation using the monolithic structure yet to be completed to demonstrate the full potential of this configuration in solving the technical challenges facing pressurized chemical looping systems.

Figure 23. Illustration of the honeycomb CLC reactor ^[111], "Adapted with permission, Copyright (2018) Elsevier BV".