Oxygen-Carrier Development of Calcium Manganite–Based Materials with Perovskite Structure for Chemical-Looping Combustion of Methane

2.1.1 300 W Unit

The laboratory-scale continuous 300 W unit required approximately 250–400 g of oxygen-carrier particles, depending on their density. The reactor was 300 mm high and had two reactor chambers: an AR fluidized with air and an FR, here fluidized with methane. The FR had a cross-section of 25 mm × 25 mm, whereas the base of the AR was 25 mm × 42 mm and contracted to 25 mm × 25 mm in the riser section. Figure 3 shows the working principle of the reactor unit: high gas velocities caused the particles to lift out of the AR and to enter the gas–solid separator placed on the top flange of the reactor unit (not shown in the figure). The particles fell and entered the inlet of the J-type loop seal (downcomer). This caused particles at the outlet of the loop seal to fall onto the bubbling bed of the FR (through the return orifice). From the bottom of the FR, the particles flowed back into the AR via the lower loop seal. The AR was fluidized with air, the loop seals were fluidized with argon, and the FR was fluidized with either methane (fuel operation) or argon (determination of CLOU behavior).

The off-gases from the reactors were separately cooled to 4 °C and filtered. The FR flue gas passed a water seal, which generated a slight overpressure in the FR and reduced leakage of air from the AR into the FR. The dried gases were analyzed for CO, CO₂, CH₄ (all IR sensors), and O₂ (paramagnetic sensors). The FR flue gas was also analyzed in a gas chromatograph to verify these gas species and to further quantify N₂ and H₂.

2.1.2 10 kW Unit

The 10 kW laboratory-scale chemical-looping reactor system was constructed in 2002, and was used to perform the first successful demonstration of continuous CLC in 2003.34 A schematic of the unit is shown in Figure 4a.

The AR (ID 150 mm) tapered down to the riser (ID 80 mm), which drove the global circulation of solids. After the riser, gas and particles were separated in the cyclone. From the cyclone, particles fell into a loop seal that prevented gas leakage between AR and FR. From the exit of the loop seal, particles flowed into the bubbling bed of the FR. The FR was equipped with a vertical separation wall to prevent particles from bypassing the bed (see Figure 4b). Particles left the FR through an overflow exit, which led the particles, via the second loop seal, back to the AR. Approximately 14–20 kg of oxygen-carrier particles were required to operate this unit.

The flue gases from AR and FR were first passively cooled through finned pipes before sample streams were extracted and led to a gas conditioning system. After the gas conditioning system, where the gas was cooled to 4 °C and filtered, CO, CO₂, CH₄, and O₂ were measured continuously by infrared and paramagnetic sensors, respectively. An additional analysis of the FR flue gas by a gas chromatograph was conducted to quantify N₂ and H₂. Not all particles were removed in the cyclone and, therefore, the cyclone off-gas, which was the AR flue gas, was led to a bag filter, which captured elutriated particles and fines, before the filtered gas went to the stack. The FR flue gas passed through a water seal, which had the dual purpose of collecting condensed steam and controlling the pressure in the FR.

2.4.1 Fuel Conversion (300 W and 10 kW Units)

2.4.2 Solids Circulation and Oxygen-Carrier Conversion (10 kW Unit)

The true circulation of solids was approximately and indirectly determined with a method that consists of two parts.

Here, the method was adapted and tested for the 10 kW unit. A similar method of determining solids circulation through injection of char batches had previously been applied in the 300 W chemical-looping reactor.38, 39

2.4.3 Oxygen-Carrier Characterization

The different oxygen-carrier samples were characterized by measuring particle size distribution, bulk density, and crystalline phase composition. Attrition was measured in situ and in a customized jet-cup attrition test rig.

Particle size distribution was measured through stacked test sieves placed in a Retsch sieve shaker. A cumulative particle size distribution curve yielded the particle sizes that correspond to 10, 50, and 90 wt% of the sample, i.e., D₁₀, D₅₀, and D₉₀.

The bulk density of a sample of particles was determined on the basis of standard ISO 3923-1:2008. A powder sample was poured through a funnel, which had a defined outlet, and which was placed at a defined height above a test container. Excess material on top of the container was leveled without compressing the sample, and the resulting content, i.e., weight and volume, yielded the poured bulk density.

Semiquantitative X-ray powder diffraction (XRD) analysis was performed with two different X-ray powder diffractometers, at VITO using a Philips X'Pert diffractometer with PANalytical X'Pert Pro software with Cu Kα radiation (λ = 1.5406 Å) at 40 kV and at 2Θ-interval 5°…120°, and at Chalmers University of Technology using a Siemens D5000 diffractometer with a thin-film detector and Cu Kα_1/2 radiation at 45 kV and 40 mA at 2Θ-intervals 10°…105° together with the software used is DIFFRAC.SUITE EVA, version 4.2.2.

Attrition in the 10 kW unit was determined by quantifying fines captured in the bag filter. The off-gas from the cyclone, Stream 3.1 in Figure 4a, was cooled and led through a bag filter, where particles and fines larger than 2 μm were captured. Here, fines are the fraction of the bag filter content smaller than 63 μm. In parallel, the off-gas from the FR, i.e., Stream 5.2 in Figure 4a, was cooled and led through a water seal, where particles were retained. Due to the low gas velocities, only fines were normally elutriated from the FR. The bag filter downstream the AR was emptied, and its content sieved at least once per day, and the water seal was emptied and filtered about once per week.

The attrition index is a standardized measurement performed with 5 g of oxygen-carrier particles in the size range 125–180 μm to determine mechanical attrition. The test was conducted at room temperature and the particles were exposed to an air jet at about 100 m s⁻¹ while rotating in a conical cup. The stresses induced in that way were assumed to resemble the grid region and the cyclone inlet in industrial-scale units,40 albeit the conditions were more severe to give significant results in only 1 h.

3.1.1 CLOU Properties

Figure 5 shows the release of gaseous oxygen in the FR under inert conditions at different temperatures. All materials show a considerable CLOU effect, i.e., oxygen released into inert atmosphere, that increases with fuel-reactor temperature. The CLOU properties vary clearly for different raw materials and sintering temperatures used (see Figure 5a). For upscaled materials, i.e., in Figure 5b, a clear correlation to the sintering temperature is not visible: the initial, unoptimized materials (C28-E3-12801 and C28-E3-1320) show the lowest CLOU effect, followed by the material C28-E5-1100 and the material with highest sintering temperature (C28-E5-1310). The strongest CLOU effect is achieved by C28-E3-12802 which also had the lowest concentration of oxygen in the AR. It should be stressed that the combination of relatively high circulation rates and relatively low flow of inert gas means that the concentration of oxygen at the outlet of the FR is likely limited by thermodynamics. Thus, the rates of release cannot be estimated from this data. However, the results clearly show a strong oxygen uncoupling effect of all calcium manganite–based materials produced.

The fitted lines in Figure 5 are simple linear regressions models based on data generated in the CLOU characterization test. Such a test consisted of a variation of reactor temperature, while oxygen-carrier particles were circulated between AR and FR. The FR was fluidized with 1 L_n min⁻¹ of argon, and the AR was fluidized with air. Here, the resulting concentration of oxygen at the outlet of the AR was between 17.0 and 19.6 vol%. Table 3 shows details of the CLOU characterization tests, i.e., temperature profile and number of data points used for the fitting of the model, as well as modeling results, i.e., coefficient of determination, adjusted R². The coefficient of determination was usually well above 0.9, which indicates a good agreement between the models and the experimental data.

3.1.2 Fuel Conversion

Figure 6 shows methane conversion against specific fuel-reactor bed mass for the oxygen carriers produced with substituted raw materials, whereas Figure 7 shows conversion of the upscaled materials. The values shown are averages over periods of steady-state operation at 900 and 950 °C, respectively.

All oxygen-carrier materials achieved very high levels of methane conversion, which was well above 90% for most materials. An exception from this is the two initial upscaled materials that were not optimized, i.e., C28-E3-12801 and C28-E3-1320. At 900 °C, the performance of the material C28-E1S2-1335 came close to that of the reference oxygen-carrier from the Innocuous project, C28-TA-1300, whereas at 950 °C its performance was somewhat lower. During upscaling, all materials, except the high-density samples, which had reacted with the saggars during sintering (see Figure 7), were able to match the performance of the material C28-E1S2-1335, i.e., the reference material using low-cost raw materials.

3.1.3 Oxygen-Carrier Properties

Figure 8 shows the correlation of reactivity of the different oxygen-carrier materials produced during the upscaling process and the bulk density of the materials. The reactivity was chosen to be expressed through fuel conversion, i.e., the CO₂ yield ${\gamma }_{{\text{CO}}_2}$, at 900 °C in the FR and at a specific fuel-reactor bed mass of 300 kg MW_th⁻¹, based on the data shown in Figure 7. The density of the particles clearly increases with increasing sintering temperature. However, initial sinterings with stacked-saggar configuration (samples C28-E3-1320 and C28-E3-12801), which is required for large-scale production, showed a higher density and a significantly lower fuel conversion in the 300 W tests (see Figure 8). The main difference between single- and stacked-saggar firings is believed to be the extent of gas exchange, which is limited when using a stacked configuration, and which could cause conditions with low oxygen concentration. Consequently, the sintering temperature had to be reduced significantly to achieve a density and reactivity similar to that of materials sintered in single saggars: the materials C28-E5-1100 and C28-901 were sintered in stacked saggars and show a similar density and reactivity as compared with materials sintered in single saggars at a temperature more than 200 °C higher.

3.2.1 Solids Circulation and Oxygen-Carrier Conversion

The circulation of the solid oxygen carrier was measured by means of addition of batches of solid fuel, here lignite char, for different superficial gas velocities in the riser. Solids circulation is expressed as a mass flux, i.e., normalized with respect to the cross-sectional area of the riser, to allow comparison to other CFB units. Based on the solids circulation determined via the char injection method, a linear model for the solids circulation as a function of the circulation index (see Equation 2) was established. Both measured data and model are shown in Figure 9 together with upper and lower bounds for a prediction interval with 95% confidence. Close to the measured data with respect to the circulation index, the interval between the upper and lower bounds corresponds to about 40% of the target value.

Oxygen-carrier conversion and oxygen deficiency are estimated based on the modeled solids circulation and the oxygen consumption in the AR (Equation 5). Figure 10 shows the solids conversion, expressed as the mass-based difference between AR and FR, ω_AR − ω_FR, as well as the difference in oxygen deficiency in the oxygen-carrier material between FR and AR, δ_FR − δ_AR. Oxygen-carrier conversion is shown against specific fuel-reactor bed mass (Figure 10a) and against AFR (Figure 10b). Data are shown for two different intervals of solids circulation, i.e., 15–22 and 22–29 kg m⁻² s⁻¹, and for three different intervals between 920 and 980 °C, respectively. Two general trends can be seen in the figure. If it is assumed that oxidation in the AR is constant and complete, the results indicate that bed material reduction in the FR 1) increases with less bed material in or more fuel input to the FR; 2) decreases if the circulation of bed material is increased; and 3) increases at higher temperatures in the FR.

Observations (1) and (2) can be explained by the commonly observed decreasing oxygen-carrier reactivity as oxygen-carrier reduction increases. Consequently, changes in oxygen-carrier reduction per unit of fuel added are highest in the beginning and decrease as more fuel is added. At higher rates of solids circulation, more oxidized oxygen carrier is added to the FR, which decreases the average degree of reduction in the FR. This, in turn, increases the average oxygen-carrier reactivity, and the average oxygen-carrier reduction per unit of fuel added increases. Observation (3) can be explained by a change in thermodynamic equilibrium, which causes more oxygen to be released (cf. Figure 5) or, in case of a direct reaction of the fuel with the oxygen carrier (in addition to CLOU), a higher reactivity.

3.2.2 Fuel Conversion

Fuel conversion expressed as the CO₂ yield, ${\gamma }_{{\text{CO}}_2}$, is shown against the AFR for different intervals of the fuel-reactor temperature (see Figure 11). Fuel conversion increases if the AFR is increased or if the fuel-reactor temperature is increased. A higher temperature increases oxygen release by the oxygen carrier (see Figure 5) as well as methane reactivity. A higher AFR, i.e., higher oxygen-carrier circulation and/or lower fuel input, could mean more reactive oxygen carrier in case of a direct reaction (in addition to CLOU) between fuel and oxygen carrier (cf. Section 3.2.1).

Figure 12 shows fuel conversion of different materials, i.e., Success materials C28-901 and C28-E1S2 as well as reference materials (Innocuous) C14-T and C28-TA, at varied fuel input (the bed height in the FR of the 10 kW unit can be considered as constant), i.e., specific fuel-reactor bed mass, and varied fuel-reactor temperature. Fuel conversion increases for all materials with decreasing fuel input, which means that the fuel-reactor bed on average is less reduced and has thus a higher reactivity.

As can be seen from Figure 12, the material C28-E1S2 was able to achieve comparable fuel conversion as the reference materials, i.e., C14-T and C28-TA, yet at a slightly higher temperature. For the upscaled material C28-901, the fuel conversion achieved was significantly lower, usually between 70% and 85%. It should be mentioned that the upscaled material was operated for over 60 h at high temperature with a significant gas leak upstream of the FR, Stream 5.1 in Figure 4a, which could have affected the oxygen-carrier material, although this is not known. Note that all experimental data shown for material C28-901 in the 10 kW unit were recorded after the gas leak was mended.

The reference materials C14-T and C28-TA have also been tested in a continuous CLC reactor system at the TU Wien. The results were comparable to the ones achieved here, though higher levels of fuel conversion were achieved in the reactor system of the TU Wien. With the material C14, the CO₂ yields achieved at the TU Wien were between 95% and 99% at about 400 kg MW⁻¹ in the FR at 900–970 °C, whereas with the material C28 only 300 kg MW⁻¹ were needed to achieve a similar fuel conversion at similar temperatures.22, 23 The reason for the higher conversion in the unit at the Wien is believed to be due to the circulating bed in the FR in contrast to the bubbling bed of the 10 kW unit used here. A circulating bed is likely to have better mixing, i.e., better contact between oxygen carrier and fuel, and might have a longer contact time as the oxygen carrier is distributed along the height of the reactor.

The oxygen-carrier material C28-902, which is similar to the material C28-901 used here, was tested at TU Darmstadt in a continuous CLC unit with a nominal fuel input of 1 MW.29 The raw materials used for C28-902 and the production method were the same as for C28-901, but due to the larger batch size, 2.2 t in total, the calcination process might have led to different material properties. The group at TU Darmstadt compared their results with the results from TU Wien with a C28 material that was similar to C28-TA used here, and which was made from different raw materials than C28-901 and C28-902.22 Conversion at TU Darmstadt was lower but in line with the results from TU Wien as the specific fuel-reactor bed mass was significantly lower. This differs somewhat from the results achieved here, where the material C28-901 achieved significantly lower fuel conversion than the reference material C28-TA. The reason for the seemingly better performance at TU Darmstadt might have been due to the fact that the FR of their unit is a CFB and/or that the material properties were different from those of the material C28-901 used here.

3.2.3 Oxygen-Carrier Analysis and Lifetime

Particle analysis reveals that particle size distribution and bulk density do deviate significantly between the fresh sample and the sample taken from the 10 kW unit at the end of the campaign, i.e., after 110 h of fuel operation (see Table 4). However, the sample of the material that was used in the 300 W unit indicates that bulk density may change at different operational conditions. It seems unlikely that the different bulk densities are a consequence of differences in size distribution; the used 10 kW sample from the AR contains less fines and more coarse particles than the fuel-reactor sample as would be expected, and the used 300 W sample has a similar, slightly narrower size distribution than either of the 10 kW samples.

The interpretation of the XRD diffractograms shows that most of the materials consist of the desired perovskite structure throughout the whole test campaign in the 10 kW unit. The perovskite-structured phases that were detected contain Ca, Mn, Ti, and O and Ca, Ti, and O, respectively (see Table 5). About a third of the samples consist of calcium manganese oxides CaMn₂O₄, Ca₂MnO₄, and Ca₂Mn₂O₅. Magnesium was only detected in the oxide form MgO, and not as part of any other phase. The analysis performed by VITO (see Table 5) seems to indicate that the fraction of the phase CaMn_0.9Ti_0.1O_2.961 is slightly reduced over time, which might be a result of a material decomposition. However, this is uncertain as the analysis is semiquantitative and no reliable species balance can be conducted.

The rate of fines production during operation in the 10 kW unit was used to estimate particle lifetime (see Figure 13). Fines production changed significantly throughout the campaign. The fresh materials contained up to 4 wt% of particles below 90 μm, and, additionally, a fraction of irregular particles, e.g., microagglomerates, hollow spheres, and donut-shaped particles, which is common for spray-dried materials. These initial fines and irregular particles are likely the reason for the initially high rates of fines production, i.e., during phase I. After these initially high rates, fines production was rather low until about 80 h of fuel operation, i.e., throughout phase II. During this phase there was a significant leakage of fuel upstream of the FR, which impaired fluidization in the FR and circulation in the whole 10 kW unit. After the fuel line was repaired and fluidization and circulation were reestablished, the rate of fines production increased significantly from phase II to phase III. Samples of the filter content in the fraction of 125–180 μm were used to determine the attrition index throughout the campaign. The development of the attrition index over fuel operation time fits the observed rates of fines production well (see Figure 13). Relatively high attrition indices were recorded when fines production in the 10 kW was high and vice versa. This indicates that particles inside the reactor were not damaged during phase II, but became less resistant to attrition during phase III.

Values for lifetimes estimated based on operation in the 10 kW unit as well as attrition indices determined for fresh and used particles are shown in Table 6. In comparison to other, similar materials operated in the 10 kW unit (see Table 6), the material C28-901 achieved a rather low lifetime and exhibited relatively high attrition indices at the end of the campaign. The tests conducted at TU Darmstadt with the material C28-902 yielded a similar lifetime to the one estimated here with the material C28-901, i.e., 500 h as compared with 700 h.29 The material C28-902 was produced in a similar manner as the material C28-901; the only differences between the two materials are the calcination furnaces used and the calcination parameters, which differed slightly.

Upon opening and emptying the 10 kW unit, agglomerates were detected in the FR and in the upper loop seal of the unit. The agglomerates were soft and could be easily crushed between fingers. About 20 wt% of the content of each vessel consisted of agglomerates between 0.5 mm up to about 6 cm. The agglomerates in the loop seal may have been formed as a result of high temperature as there was much heating cable wrapped around the loop seal, which also caused stains on the outside of the loop seal. The agglomerates in the FR may have been formed as a result of local zones of bad mixing and/or high reduction. An XRD analysis performed with a sample of the agglomerate indicated that the agglomerate has a phase composition that corresponds to that of the samples from AR and FR. If the XRD analysis is representative, agglomeration may have been caused by high temperatures rather than high reduction.

Figure 14 shows light microscope images of fresh particles, particles at the end of the campaign from AR and FR, and an agglomerate from the FR. The fresh batch visibly contains a fraction of small and very small particles, i.e., about 5 wt% are below 100 μm. Most of the particles appear to be spherical and intact. Visible defects include satellites, nonspherical particles, and microagglomerates, i.e., two particles molten together. The samples taken at the end of the campaign appear mostly spherical and undamaged, though a significant fraction consists of broken, hollow, donut-shaped or cracked spheres, and microagglomerates. The sample from the FR contains fines, whereas the sample from the AR does not. This is believed to be due to the higher gas velocities in the AR. The agglomerate from the FR looks like a tight packing of particles, and no molten bridges are visible between the particles. This confirms the observation that the agglomerates are soft and easily crushed between the fingers. It is speculated that in an industrial-scale process such agglomerates could only be formed in zones of improper fluidization, but would dissolve into individual particles once they enter a zone of proper fluidization. Therefore, the agglomerations observed are believed to be reversible and no threat to an industrial-scale process.