Ca0.9Mn0.5Ti0.5O3−δ: A Suitable Oxygen Carrier ... - ACS Publications

Jun 6, 2014 - oxygen carrier material for use in fixed-bed CLC reactors is investigated. The main ..... oxidation and reduction (inner box) reactions...
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Ca0.9Mn0.5Ti0.5O3−δ: A Suitable Oxygen Carrier Material for Fixed-Bed Chemical Looping Combustion under Syngas Conditions Mehdi Pishahang, Yngve Larring,* Michael McCann, and Rune Bredesen SINTEF Materials and Chemistry, Sustainable Energy Technology Sector, P.O. Box 124, Blindern, NO-0314 Oslo, Norway ABSTRACT: Power generation using chemical looping combustion (CLC) technology has emerged as a promising CO2capture-based alternative to conventional technology. In this study, the performance of a Ca0.9Mn0.5Ti0.5O3−δ perovskite-type oxygen carrier material for use in fixed-bed CLC reactors is investigated. The main focus of the study is on the material’s oxygencarrying capacity and reactivity with and conversion of syngas in the fixed-bed reactor. Pressurized thermogravimetric analysis in model gases indicates neither Boudouard coking nor reactivity of the oxygen-carrier material toward CO2. A fuel gas conversion of 95% was achieved in the fixed-bed reactor using this oxygen-carrier material.

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INTRODUCTION Atmospheric concentrations of CO2 have risen drastically during the past century, and it is widely acknowledged that increased human-related emissions of CO2 contribute to climate change. Fossil-fuel power plants are globally responsible for the biggest fraction of CO2 emission,1 and future demands for sustainable energy production can only be met using carbon capture and storage (CCS) as part of the solution. Presently, CO2 capture represents the main cost of the CCS chain, and further research is required to lower the cost significantly. Technologies for large-scale power generation and inherent CO2 capture are usually categorized in three classes, namely, postcombustion and precombustion CO2 capture processes and oxyf uel combustion.2 In postcombustion CO2 capture, air is used as the oxidizing agent in the combustion of fossil fuel and CO2 is captured from the flue gas, which contains a relatively high concentration of N2. This CO2/N2 separation is coupled with a significant economic penalty. For precombustion techniques, where the fossil fuel is converted to a mixture of CO2 and H2, significant cost is related to CO2 separation upstream of the gas turbine. An advantage of this process is the possibility of combined H2 gas and power production. The sorptionenhanced water−gas shift3,4 and membrane-assisted water− gas shift5,6 processes are among the most promising processes for combined H2 production and power generation with CO2 capture. In oxyfuel processes, oxygen is used as the oxidizing agent and resulting flue gas, with which a high concentration of CO2 can be separated from H2O simply by condensation. O2 production by conventional cryogenic air separation is still costly in addition to the fact that the infrastructure for cryogenic O2 separation is highly priced.7 Chemical looping combustion (CLC) is an oxyfuel process with the potential for cost-effective power generation in combination with CO2 capture. The chemical looping concept is based on the idea of a combustion reaction in two separate subprocesses;8,9 the first is O2 capture from an air stream using an oxygen chemisorbent (often named “oxygen carrier material”, OCM), and in the second subprocess, the OCM releases oxygen in the fuel stream in order to achieve combustion. The (often endothermic or slightly exothermic) reduction of the OCM with fuel during combustion yields CO2 © 2014 American Chemical Society

and steam as the products, which can simply be separated by condensation. In the second step, the reduced OCM is transported to the air reactor, where it is reoxidized by air. Oxidation of the OCM is exothermic, resulting in a hot air stream lean on O2, which is used for power generation. CLC has the additional advantage of eliminating problems associated with the thermal formation of NOx as nitrogen is not exposed to extremely high temperature flames. Coal-employed CLC has been focused on lately and reported as very competitive, with only a 2.5% ideal CO2 capture penalty10 and a calculated net efficiency of 43−45% for a combined cycle with CLC with a turbine inlet temperature of 1200−1350 °C.11 Reaching the lowest CO2 capture penalty requires an OCM, which can fully deliver the required amount of oxygen for complete combustion, thus avoiding the need for a secondary oxygen supplier (e.g., air separation unit). Higher efficiencies can also be achieved by pressurizing the process and therefore adding the possibility of including a gas turbine in the process, as notably implemented in natural gas combined cycle12 and integrated gasif ication combined cycle13 processes. Although pressurization of the process can technically be achieved in nearly all CLC technologies, it is less convenient for fluidized circulating beds. One approach to ease the challenges associated with pressurization is the selection of a fixed-bed reactor in which the reactor volume is significantly reduced and the packing density of the OCM is high. In the alternating f ixedbed reactors,14 transport of the OCM between the reactors is avoided. Instead each reactor repeatedly becomes a “fuel reactor” and an “air reactor” when fuel and air are introduced to the reactor periodically.15 The CLC alternating fixed-bed reactor system is schematically presented in Figure 1. Ca1−yMn1−xTixO3−δ (CMT) as the OCM. Perovskite-type oxides (with the general formula ABO3−δ) are known to be a very promising family of materials to be used as the OCM for CLC.16,17 The oxygen nonstoichiometry, δ, varies depending Received: Revised: Accepted: Published: 10549

March 4, 2014 May 16, 2014 June 6, 2014 June 6, 2014 dx.doi.org/10.1021/ie500928m | Ind. Eng. Chem. Res. 2014, 53, 10549−10556

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Figure 1. Schematic flow sheets for the two steps in the alternating fixed-bed CLC reactors.

on the partial pressure of oxygen in the surroundings and temperature.18−22 In isothermal conditions, as the surrounding oxygen partial pressure decreases, the defect concentration increases by releasing oxygen to the gas. CaMnO3−δ shows very promising results because of its fast reduction at relatively high partial pressures of oxygen.23 It is, however, previously shown by Bakken et al.24 that at 1000 °C the CaMnO3 material continuously loses oxygen when the oxygen partial pressure is reduced from 1 to 10−5 atm, and the material reduction (oxygen release) is compensated for by the reduction of Mn4+ to Mn3+. They also show that this material tends to decompose to Ca2MnO4 and CaMn2O4 at higher temperatures.24 Partial decomposition might restrict the full recovery of material by quick oxidation. In fact, CaMnO3−δ releases its oxygen capacity too easily for fixed-bed applications (at relatively high oxygen partial pressures). This leads to the release of oxygen to the inert gas when the OCM bed is flushed during the postoxidation purge step. It is therefore crucial to adjust the oxidation enthalpy, reflecting the oxygen partial pressure range where the material releases oxygen. One way to tailor this thermodynamically fundamental property of the material is by adjusting its chemical composition.16,25−27 The addition of titanium to the B-site sublattice in CaMnO3−δ stabilizes the structure because titanium is more redox-stable (higher enthalpy of oxidation) and helps to keep the perovskite structure at lower partial pressures of oxygen.28 Partial substitution of manganese with titanium at the B-site sublattice stabilizes the structure chemically but at the cost of a reduced oxygen capacity. This disadvantage, however, is offset by the fact that the reduced oxygen capacity helps to prevent the detrimental decomposition during the reduction reaction. Selection of the OCM is the most important aspect in the development of CLC processes. The OCM should possess adequate reduction−oxidation (redox) reactivity, structural stability, and a reasonable capacity of oxygen. In previous publications, we have developed and investigated the redox thermodynamic properties of several perovskite-related OCMs suitable for CLC applications.20,21,28−30 The CaMn0.8Ti0.2O3−δ

composition shows CO2 uptake in a gas stream containing a partial pressure of CO2 equal to 4 atm due to the marginal formation of secondary phases. The stability of CMT toward CO2 may be improved by lowering the calcium content and increasing the titanium content. Cation nonstoichiometry is a known phenomenon for CaTiO3, where the ratio of Ti/Ca exceeds unity, i.e., Ca1−xTiO3−δ.31 An intentional decrease in the calcium content inhibits the formation of secondary phases because of this phenomenon. CaTiO3 and CaMnO3 are very similar in both terms of the formation enthalpy and the Goldschmidt tolerance factor.32 Still, they show different redox behaviors due to the difference in the redox characteristics of titanium and manganese. Unlike CaMnO3, CaTiO3 does not show oxygen nonstoichiometry and, therefore, does not participate in the redox reaction, thus keeping the structure together during the deep reductions that the solid solution CMT undergoes. By acting as the skeleton in the solid solution, replacement of manganese by titanium in the solid solution increases the structural stability and restricts the reactivity of secondary phases with CO2. In this paper, we report the results of the performance of stabilized Ca0.9Mn0.5Ti0.5O3−δ as OCM for fixed-bed CLC. The experiments are conducted in an in-house fixed-bed reactor suitable for alternating the inlet gases in cycles of air−inert− fuel−inert−air. The experiments are performed under syngas conditions. Because the syngas consists of more than one active reducing agent, several chemical reactions may occur during the reduction and oxidation steps. The main reactions (1)−(3) are listed below: During the reduction: Ca 0.9Mn 0.5Ti 0.5O2.9(s) + CO(g) ⇌ Ca 0.9Mn 0.5Ti 0.5O2.9 − δ (s) + CO2 (g)

(1)

Ca 0.9Mn 0.5Ti 0.5O2.9(s) + H 2(g) ⇌ Ca 0.9Mn 0.5Ti 0.5O2.9 − δ (s) + H 2O(g) 10550

(2)

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Figure 2. Schematic view of the fixed-bed reactor apparatus and the auxiliary components.

controller was kept constant during the experiment, therefore setting an invariable temperature profile across the upper section of the furnace where the OCM was placed. The bottom of the reactor was filled with a penetrable bed consisting of clay ash (packed in the bottom 300 mm of the reactor). The OCM bed, 200 mm in height, was placed on top of the inert bed in the hot zone of the furnace with an average bed porosity of ∼60%. A rather dense OCM used was in the form of cylindrical granulates with only 5% porosity (fabricated by CTI, Salindres, France) and had an average diameter and length of 2.7 and 15 mm, respectively. The length distribution was such that 70% of the granulates had a diameter between 10 and 20 mm). A high-temperature filter was located above the OCM in order to avoid damaging sensitive downstream equipment by the potential fines abraded from the granulates and carried in the gas stream. A gas mixing system consisting of six mass flow controllers (MFCs) and two back-pressure controllers (BPCs) was used to supply the reactive gas mixtures in the reactor and control the reactor’s pressure. This gas mixing system allowed mixtures of argon (Ar) and O2 with a total flow of 4500 mL/min and Ar, CO, and H2 with a total flow of 900 mL/min for oxidizing and reducing gas mixtures, respectively. An automatic alternating four-way valve (VICI, Norway) was used to switch between oxidizing and reducing conditions. For safety reasons, a manual pressure relief valve was installed on the gas feed line with an opening pressure of 10 atm, in order to vent the entire feed gas to the exhaust in case the reactor or any of the outlet gas lines became clogged. In order to avoid mixing of the oxidizing and reducing gases, an inert purge with 1000 mL/min of Ar was

During the oxidation: Ca 0.9Mn 0.5Ti 0.5O2.9 − δ (s) + O2 (g) ⇌ Ca 0.9Mn 0.5Ti 0.5O2.9(s)

(3)

Equation 3 is an oxidation reaction, involving a decrease in the entropy but the creation of some new bonds, which makes this reaction exothermic. Reactions (1) and (2) are the sums of two subreactions, the endothermic reduction of the OCM and the exothermic oxidation of CO to CO2 or H2 to H2O, and therefore the net reaction can be either exothermic or endothermic depending on the oxidation enthalpy of the OCM.

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EXPERIMENTAL PROCEDURE The CLC experiments were conducted in a vertical fixed-bed reactor, schematically presented in Figure 2. The fixed-bed reactor consists of a boiler-grade stainless steel (MA253) tube (o.d. = 34 mm, i.d. = 18 mm, and l = 600 mm), which is resistant to temperatures as high as 1100 °C. A set of axial thermocouples, consisting of 6 K-type thermocouples (Skotselv, Norway), was inserted into the reactor, which allowed measurement of the temperature at six positions along the reactor (40 mm between each). The reactor was positioned in a vertical tubular furnace (i.d. = 40 mm and l = 500 mm), which was controlled separately by an external controller. A separate K-type thermocouple was used to measure the furnace temperature, and the thermocouple was placed well below the hot zone of the furnace. Thus, the thermocouple’s temperature was not affected by the reactions occurring in the reactor. As a result, the power output of the furnace 10551

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The total flow was kept constant at 1000 mL/min. Each reduction was followed by an oxidation in 20% O2 for 30 min. Ca0.9Mn0.5Ti0.5O3−δ (fabricated by CTI, Salindres, France) was used as the OCM. A paste corresponding to the final composition was made from the raw minerals and directly extruded to the form of cylindrical granulates. The granulates were sintered at 1300 °C for 5 h. Ca0.9Mn0.5Ti0.5O3−δ has the perovskite structure, and the XRD shows asingle phase with only traces of the minor secondary phase in the material.

used between the oxidizing and reducing periods. In this study, the inert part of the syngas, N2, is replaced by Ar in order to differentiate between the inert part and CO in the mass spectrometer signal (N2 and CO have very similar molecular weights of 28 g/mol). A separate liquid MFC was used to control the required flow of H2O into the reactor. The feed gas to the reactor was preheated to 200 °C by heating tapes in order to make sure that the H2O supplied by the liquid MFC is evaporated completely before entering the reactor. Preheating also decreases the temperature change in the reactor because of forced convection caused by forcing the cool gas into the hot bed. The fixed-bed experimental procedure consists of periodic reduction and oxidation cycles; the experimental conditions used are summarized in Table 1.

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RESULTS AND DISCUSSION Figure 3 shows the thermogravimetric results giving the reduction and oxidation capacities of the OCM under isothermal conditions at 900, 1100, and 700 °C. The only cation taking part in the redox reaction is Mn ions, which under oxidizing conditions mostly have a charge of 4+, giving an oxygen content of 2.9 (Ca0.9Mn0.5Ti0.5O2.9). During the reduction, the calcium and titanium ions remain unreacted, and the Mn4+ cations are reduced to Mn3+. If all of the Mn4+ ions are reduced to Mn3+, the chemical composition becomes Ca0.9Mn0.5Ti0.5O2.65 and the corresponding weight loss becomes 3 wt %. From Figure 3, it can be estimated that the material’s capacity is slightly more than 3 wt %, indicating the further reduction of some Mn3+ to Mn2+. As is also seen from Figure 3, the material recovers its oxygen capacity fully during a fast oxidation. Another finding of this experiment is the resilience of this OCM toward reactive coking. Because a significant portion of the reactive gas consists of CO, the formation of soot in the reactor according to the Boudouard, as shown by reaction (4), is also a possibility; however, no indication of soot formation is observed even though the reducing gas is charged completely dry (Figure 3). The soot formation could have been revealed as a weight increase in TG and as CO2 release under reoxidation of the fixed-bed experiment.

Table 1. Experimental conditions in the fixed bed experiments parameter

range

pressure P (atm) oxidation temperature (°C) reduction temperature (°C) inlet oxygen fraction (oxidation) (%) inlet CO and H2 fraction (reduction) (%) inlet H2O fraction (reduction)

1−7 400−800 400−800 20 60 and 20 equal to molar CO + H2

An in-house LabView program was used to control the experiment and record the data from the MFCs and BPCs, as well as log the temperatures, pressures, and mass spectrometer data. A Spectra Mini-Lab mass spectrometer (MKS Instruments) was used to analyze the outlet gas of the reactor. The mass spectrometer was calibrated prior to experiments with corresponding gas mixtures. A pressurized thermogravimetric analyzer with magnetic suspension (Rubotherm, Bochum, Germany) was used to study the reduction and oxidation capacities of the OCM under isothermal conditions at 700, 900, and 1100 °C. The total pressure was constant at 10 atm during the whole experiment.

2CO(g) ⇌ CO2 (g) + C(s)

(4)

Figure 3. Weight change imposed by cyclic reduction and oxidation in air of Ca0.9Mn0.5Ti05O3−δ at different temperatures and gas compositions under high pressure (10 atm). No stability issues toward CO2 were observed. 10552

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in a more elaborate way. The 3D plot in Figure 5 shows how the temperature increase varies with time and reactor length. The horizontal lines in this figure represent the thermocouple positions, and the temperature between the thermocouple points is extrapolated linearly. The gas flow used is in the regime of plug flow, which is mainly due to the fairly long tubular reactor with a significant ratio of length/diameter. A plug-flow regime is crucial to ensure that the gas is pushed out of the reactor as it is periodically switched between the oxidizing and reducing gases. It ensures that these gases will not mix within the reactor, both for safety reasons and to prevent the fuel from being diluted with nitrogen from air (which is the main idea behind CLC). Another interesting observation from Figure 5 is the time frame in which the energy is released during the oxidation and reduction steps. It is evident that a significant portion of the energy during the oxidation is released in only 200 s. The gas velocity is 0.137 m/s (corresponding to a flow of 4500 mL/ min), and thus the oxidation kinetics of the reaction are fast enough such as not to be the controlling factor in the reaction rate; rather, the physical availability of oxygen is the controlling parameter in the oxidation kinetics. This is more pronounced in Figure 6, which represents the maximum temperature achieved at three different gas speeds. As illustrated in the inset of Figure 6, the maximum achieved temperature increase during the oxidation is almost linearly proportional to the oxidizing gas speed. Unlike many other OCM systems (such as ilmenite), the reduction reaction is also exothermic (as shown in Figures 4 and 5). Although the maximum temperature increase observed during the reduction is only 30 °C, this is still very beneficial because it can at least help maintain the reactor temperature throughout the reduction step. The slightly exothermic nature of the reduction step avoids several challenges related to strong endothermic reactions such as reduced conversion related to cooling of the bed and unreacted fuel in the effluent gas stream. The reaction rate of the reduction is not as fast as that of the oxidation, which may be attributed to two factors. First, a lower gas speed of 0.0274 m/s was used during this experiment for the reduction (corresponding to a flow of 900 mL/min). Second, the syngas (i.e., H2 and CO) reactivity is reduced because of dilution of the fuel with H2O vapor. This yields a higher oxygen partial pressure of the fuel and therefore lower chemical reactivity. In practice, for stationary applications, more than two reactors could be coupled in parallel with cyclic switching between the oxidation and reduction modes to adjust for the difference in reactivity between fuel and air conditions. Another finding of this investigation is the effect of the starting bed temperature on the maximum temperature increase and the maximum temperature reached during the oxidation. The results are summarized in Figure 7. Here the results for Ca0.9Mn0.5Ti0.5O3−δ are compared with our previous results for another material of the same family, CaMn0.8Ti0.2O3−δ in similar and comparable conditions. It should be noted that the results presented in this figure are obtained from breakthrough reduction experiments at the same bed temperature followed by an oxidation. As this figure shows, the maximum temperature increase during the oxidation increases with increased starting bed temperature. The temperature increase is correlated to the chemical energy release as heat, due to the oxidation enthalpy of the material. The portion of manganese ions (the only cations participating in the redox reaction) is significantly higher in CaMn 0.8 Ti 0.2 O 3−δ compared to

Figure 4. Temperature increase evolution during the oxidation and reduction (inner box) reactions. The initial bed temperature is 700− 750 °C.

Figure 4 shows the temperature increase evolution during the oxidation and reduction (inner box) reactions. During the experiment, the initial bed temperature (T1−T6) was between 700 and 750 °C. As observed in Figure 4, both the oxidation and reduction reactions are exothermic. The oxidation is, however, far more exothermic than the reduction, as expected. During the oxidation step, the temperature is increased by as much as 230 °C, resulting in outlet gas temperatures as high as 950 °C. Figure 5 represents the temperature increase evolution

Figure 5. Bed temperature versus time and the reactor length: (a) during the reduction; (b) during the oxidation. 10553

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Figure 6. Highest bed temperature increase during the oxidation at different gas speeds; 4500, 3000, and 1500 mL/min. Inset: Maximum temperature increase achieved during the oxidation versus gas speed.

Figure 7. Effect of the starting bed temperature on the maximum temperature increase and the maximum temperature reached during the oxidation for Ca 0 . 9 Mn 0 . 5 Ti 0 . 5 O 3 − δ (CMT0955) and CaMn0.8Ti0.2O3−δ (CMT1082).

Figure 8. Syngas conversion versus starting bed temperature during the reduction, using a 200 mm bed height of the granulates.

hypothesis. As the temperature increases, the reaction rate increases, and therefore more of the chemical energy of the fuel is stored in the material, in the form of Mn2+ and Mn3+. Because the oxidation reaction is exothermic, increasing the temperature leads to less thermodynamic motivation for the reaction to move forward, leading to stabilization of the maximum temperature increase at a maximum value. In order to investigate the chemical reactions in the reactor during the oxidation and reduction, the gas outlet of the reactor is analyzed by in-line mass spectrometry at different starting bed temperatures. The results are presented in Figure 9. It should be noted that H2O in the outlet gas is removed via a condenser before mass spectrometry in order to avoid damaging the mass spectrometer. As a result, H2O is not visible in the gas analysis curves in this figure. It is evident that

Ca0.9Mn0.5Ti0.5O3−δ, indicating that the energy capacity of CaMn0.8Ti0.2O3−δ is much higher than that of Ca0.9Mn0.5Ti0.5O3−δ. Therefore, much more energy is expected to be released during the oxidation of CaMn0.8Ti0.2O3−δ, which leads to higher values for the maximum temperature increase. A higher oxidation enthalpy of Ca0.9Mn0.5Ti0.5O3−δ reduces the expected impact of having close to twice the oxygen capacity in CaMn0.8Ti0.2O3−δ (the maximum temperature increase is less than a factor of 2, with the inlet temperature above 600 °C). At lower temperatures, the reaction rate of the reduction is not high enough, and some of the fuel can leave the reactor unreacted. Post gas analysis by the mass spectrometer during the reduction process is plotted in Figure 8 and confirms this 10554

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the reactivity of the feed fuel increases as the starting bed temperature increases, and ultimately at temperatures above 700−750 °C, the conversion reaches its highest value. In this temperature range, the hydrogen in the fuel is completely consumed, and only a small minimal amount of CO remains in the flue gas (Figure 9c). The amount of CO in the flue gas was 5%, and no hydrogen was detected in the flue gas. Given the very short length of the OCM bed (200 mm) and the relatively high speeds at which the gases are fed into the reactor, this is a significant achievement. As expected, the reactivity of H2 is much faster than that of CO. The concentration of CO in the feed is 3 times higher than the concentration of H2, but the addition of H2O eases the consumption of CO via the water− gas-shift reaction, as illustrated by equation (5). The higher reactivity of H2 compared to CO is due to the higher gas diffusion rate for H2, as well as the higher thermodynamic driving force of H2 reacting with O2 compared to CO. CO(g) + H 2O(g) ⇌ CO2 (g) + H 2(g)

(5)

Because a significant portion of the syngas consists of CO, the formation of soot in the reactor, according to the Boudouard reaction in eq 4, is also a possibility. One problem with the solid carbon produced during the coking is the damage it will cause to the downstream processing equipment (i.e., filters, valves, turbine) as well as the steel reactor itself. The solid carbon formed during the reduction step will be oxidized in the oxidation step and lead to unwanted CO2 emission. Mixing the syngas with steam shifts the CO content of the syngas to CO2 and H2 according to eq 5 and therefore reduces the coking driving force in the reactor by decreasing the CO partial pressure. In our case, all of the experiments in the fixedbed reactor were performed with a syngas/steam ratio of 1:1, and it is evident that the Boudouard coking is inhibited in the reactor.

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CONCLUSION The thermogravimetric measurements confirm that Ca0.9Mn0.5Ti0.5O3−δ shows neither any coking under 5 atm of CO nor any CO2 uptake under 4 atm partial pressure of CO2. Both lowering the calcium content and increasing the titanium content contributed to stabilizing the material. The temperature increase during the oxidation for Ca0.9Mn0.5Ti0.5O3−δ is lower than that of CaMn0.8Ti0.2O3−δ. This is explained by the lower amount of manganese, which is the cation controlling the amount of oxygen uptake. Because of the high oxidation enthalpy, Ca0.9Mn0.5Ti0.5O3−δ does not give full combustion of the syngas; still 95% conversion is achieved.

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AUTHOR INFORMATION

Corresponding Author

*Tel: +47 9828 3956. Fax: +47 2206 7350. E-mail: Yngve. [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The work presented in this article is conducted under the European Union Seventh Framework Programme (FP7/20072013) under Grant 268112 (Project acronym DEMOCLOCK). The authors thank CTI for supplying the materials and, in particular, Eric Louradour and Didier Tournigant for providing the material used in this investigation.

Figure 9. Flue gas analysis by mass spectrometry at different starting bed temperatures. 10555

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dx.doi.org/10.1021/ie500928m | Ind. Eng. Chem. Res. 2014, 53, 10549−10556