Article pubs.acs.org/EF
Effect of Production Parameters on the Spray-Dried Calcium Manganite Oxygen Carriers for Chemical-Looping Combustion Dazheng Jing,*,† Frans Snijkers,‡ Peter Hallberg,§ Henrik Leion,† Tobias Mattisson,§ and Anders Lyngfelt§ †
Department of Chemistry and Chemical Engineering, Division of Energy and Materials, Chalmers University of Technology, SE-41296 Gothenburg, Sweden ‡ VITO-Flemish Institute for Technological Research, B-2400 Mol, Belgium § Department of Energy and Environment, Division of Energy Technology, Chalmers University of Technology, SE-41296 Gothenburg, Sweden S Supporting Information *
ABSTRACT: The oxygen carrier CaMn0.9Mg0.1O3−δ was successfully tested in different chemical-looping units. High methane conversion and oxygen uncoupling properties have been found for this type of material. Most of the CaMn0.9Mg0.1O3−δ oxygen carrier particles tested so far have been produced using the spray-drying method. In this work, the focus has been on studying the effects of production parameters on the properties of this important oxygen carrier. The effects of three production parameters, i.e., milling time, calcination temperature, and calcination time, were examined for the spray-dried particles. The time of ballmilling for the slurry prepared for spray-drying was varied from 5 to 45 min, the calcination temperature from 1300 to 1350 °C, and the calcination time from 4 to 16 h. None of these parameters had any influence on the final crystalline phases of the oxygen carrier, yet some of the properties were clearly changed. The bulk density, crushing strength, and resistance against physical attrition can be enhanced by increasing the calcination temperature, calcination time, or milling time. Further, the BET specific surface area and porosity of the oxygen carrier particles decreased when the slurry was milled or particles were calcined for extended periods. The average methane conversion of the oxygen carrier varied in a wide range, from 99% to 55% at 950 °C, depending upon the production parameters used. However, no obvious influence of the examined production parameters was observed for the oxygen uncoupling property of the oxygen carrier, which may be due to the thermodynamic limitation during testing. 2MexOy → 2MexOy − 1 + O2
1. INTRODUCTION Chemical-looping combustion (CLC) is a promising combustion technology for carbon capture and storage (CCS).1−4 In the CLC process, the oxygen needed for combustion is transferred by metal oxides, i.e., oxygen carriers, which are oxidized in an air reactor and reduced in a fuel reactor according to reactions 1 and 2, respectively.5 Since there is no direct contact between the air and fuel, gas separation units with extensive energy input and cost are avoided in CLC. Also, 100% CO2 can be captured after steam condensation if full conversion of fuel is achieved in the fuel reactor.6,7 O2 + 2MexOy − 1 → 2MexOy
CnH 2m + (n +
(1)
(2)
When the oxygen partial pressure in the environment is low, i.e., in the fuel reactor, some oxygen carriers are able to release gaseous oxygen according to reaction 3. The released oxygen can directly react with the fuel in the fuel reactor according to reaction 4; thus, the reaction mechanism is altered.8 Oxygen carriers with such oxygen uncoupling properties are used in chemical-looping with oxygen uncoupling (CLOU). From a system perspective, CLOU is similar to CLC, with the difference being the route of fuel conversion. © 2016 American Chemical Society
1 m)O2 → nCO2 + mH 2O 2
(4)
A CLOU oxygen carrier has several advantages over a normal CLC oxygen carrier. Due to the release of gaseous oxygen, the reaction zone of fuel conversion can be extended to regions where there are no or few oxygen carrier particles. When CLOU oxygen carrier is applied for solid fuel combustion, the slow gasification step can be avoided as the solid fuel can react with the released oxygen.8,9 For a CLC process to be feasible, a key factor is the property of the oxygen carrier.4,5,10 An oxygen carrier should have suitable thermodynamic and kinetic properties to facilitate fast oxidation and reduction reactions in the air and the fuel reactor. Since the CO2 purity highly depends on the fuel conversion, the ability of the oxygen carrier to convert fuel to CO2 is a critical parameter. The performance of the oxygen carrier should be stable in long-term operation without fluidization problems. As the oxygen carrier will be used in a fluidized process, it is not likely that the process will be fully closed, because some oxygen carrier material is lost through attrition.
CnH 2m + (2n + m)MexOy → nCO2 + mH 2O + (2n + m)MexOy − 1
(3)
Received: December 8, 2015 Revised: March 7, 2016 Published: March 7, 2016 3257
DOI: 10.1021/acs.energyfuels.5b02872 Energy Fuels 2016, 30, 3257−3268
Article
Energy & Fuels Table 1. Production Parameters and Physical Properties of the Oxygen Carriers Investigated: CaMn0.9Mg0.1O3−δ
BET specific surface area (m2/g) notation
calcination temperature (°C)
calcination time (h)
milling time (min)
crushing strength (N)
bulk density (kg/m3)
fresh sample
used sample
1300C4h5m 1300C4h15m 1300C4h45m 1300C8h5m 1300C16h5m 1325C4h5m 1325C4h15m 1325C4h45m 1325C8h5m 1325C16h5m 1350C4h5m 1350C4h15m 1350C4h45m 1350C8h5m 1350C16h5m
1300 1300 1300 1300 1300 1325 1325 1325 1325 1325 1350 1350 1350 1350 1350
4 4 4 8 16 4 4 4 8 16 4 4 4 8 16
5 15 45 5 5 5 15 45 5 5 5 15 45 5 5
1.4 2.8 2.7 4.6 3.4 3.2 3.2 2.3 3.4 3.8 4.2 3.3 2.5 4.6 4.9
1420 1875 1976 2102 1960 2043 2119 2074 2109 2267 2211 2128 2112 2277 2337
0.3823 0.2234 0.1037 0.1121 0.0972 0.1634 0.1256 0.0543 0.1051 0.1231 0.0804 0.0623 0.0446 0.0735 0.0625
0.3375 0.3361 0.1737 0.2402 0.2285 0.1573 0.1736 0.1633 0.1850 0.2001 0.1389 0.1444 0.1747 0.1437 0.1386
and can release oxygen to the gas phase. It has a perovskite type of structure and is non-stoichiometric with respect to oxygen, thus enhancing the oxygen mobility in the lattice. The material has been investigated in 10 kW and 120 kW units.18,19 The reactivity of the material with gas has been very high, with full gas yield obtained in both units, and the estimated lifetime was long, albeit shorter than for Ni-based materials.15,17,18 The oxygen transfer capacity for this material has been investigated experimentally and found to be 0.075−0.101 for chemicallooping and 0.006−0.011 for CLOU, for fresh (higher value) and used particles (lower value).20 The combination of high reactivity, CLOU properties, and the fact that it can be made with relatively cheap and environmentally friendly raw materials, i.e., CaO and Mn3O4, makes it very interesting to investigate further in order to try and optimize oxygen carrier performance. Among the methods of oxygen carrier manufacture, spraydrying is an industrial granulation method which has shown to be capable for oxygen carrier production.16,21−23 In the spraydrying process, raw materials are homogenized by milling before injecting into a spray-dryer. After spray-drying, particles formed are calcined to facilitate chemical and physical changes. Almost all previous studied CaMnO3−δ oxygen carrier were produced by the spray-drying method.18,24−29 However, there are no studies on the influence of basic production parameters, such as calcination time, calcination temperature, and milling time, on the property of spray-dried CaMnO3−δ. In this work, the perovskite structure CaMnO3−δ with MgO additive manufactured by spray-drying was chosen as oxygen carrier, and the calcination temperature, calcination time, and milling time in the production process are varied. The aim of this study is to investigate if changing of these parameters can improve the mechanical stability while retaining reactivity of the oxygen carrier. This is the first systematic study on the effect of production parameters on the properties of spray-dried CaMnO3−δ oxygen carrier. By conducting reactivity test and particle characterization, the performance of the produced oxygen carrier is examined.
Thus, it is important that the oxygen carrier be nontoxic and has a high resistance against attrition. The attrition property of an oxygen carrier is closely related with the lifetime of the material which eventually has impact on the total cost of the capture process. One option is to find natural materials, such as metal oxide ores, which need only limited treatment and no synthesis step and have a much lower cost than manufactured materials. If the oxygen carrier is manufactured, a long lifetime is of importance as the cost of the oxygen carrier will be considerably higher than that of natural materials. Moreover, the reactivity of the synthesized materials are expected to be high, where full conversion of fuels can be achieved. Considering the balance between the cost and performance of the oxygen carrier materials, synthesized materials with high reactivity and long lifetime are promising candidates compared with natural materials. The aim of this study is to find such an oxygen carrier material by investigating of the influence of the production parameters. To optimize the properties of synthesized oxygen carriers, the influence of production parameters during manufacture has been investigated in previous works using a variety of oxygen carrier materials. Combined Cu−Fe oxide oxygen carriers were synthesized by physical mixing, wet impregnation, coprecipitation, and direct decomposition method by Siriwardane et al.11 The performance of the oxygen carriers produced was examined, and the influence of manufacture method was studied. Pans et al.12 and Tseng et al.13 manufactured combined Ni−Fe oxide oxygen carriers by physical mixing (physical mixing of NiO with Fe2O3-based oxygen carrier) and chemical mixing (impregnation of NiO on Fe2O3-based oxygen carrier and production of mixed oxide oxygen carrier by sol−gel method from metal nitrates) to study the impact of mixing on the behavior of oxygen carrier. Concerning calcination time, Azad et al. compared the behavior of CuO−Mn2O3 oxygen carrier manufactured by extrusion and calcined for 6 and 12 h, respectively.14 In recent years, several oxygen carrier materials have shown great promise and have been used in continuous operation in chemical-looping pilots, including Ni-based materials, ilmenite, and calcium manganite with MgO as additive.15−18 The latter material, with general formula CaMnO3−δ, is a CLOU material 3258
DOI: 10.1021/acs.energyfuels.5b02872 Energy Fuels 2016, 30, 3257−3268
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fluidized-bed system as presented in Figure 1. Oxygen carrier particles were placed on a porous plate located 370 mm from the bottom of an
2. EXPERIMENTAL SECTION 2.1. Oxygen Carrier Production. The production of oxygen carriers was conducted by VITO (Flemish Institute for Technological Research) using a spray-drying method, and the materials were designed to have a molar composition of CaMn0.9Mg0.1O3−δ. Raw materials, 50.5 wt% Ca(OH)2, 46.8 wt% Mn3O4, and 2.7 wt% MgO, were weighed and dispersed in deionized water with organic binders and dispersants. The water-based suspension was then milled in a planetary ball mill for homogenization. The planetary ball mill was equipped with 1 L Ertalon polymer jars and filled with about 0.5 L zirconia milling media of 3 mm diameter. Milling time was set to 5, 15, or 45 min individually. After milling, the spray-drying suspension is transferred to a container, and a propeller blade mixer is used to stir the homogenized slurry while it was pumped into an atomizer located in the drying chamber. The injected slurry first brakes into massive droplets due to the effect of surface tension. The droplets are dried during the time of flight by heated air and become dry particles that are separated from the hot air. The spray-dried particles were sieved to 150−300 μm in diameter for calcination. Calcination was conducted in natural convection furnaces (Entech, Sweden, and Bouvier, Belgium) under an air atmosphere. Here amounts of 100 g of the spray-dried particles were placed in an alumina crucible. Sieved particles were first heated from room temperature to 500 °C with a heating rate of 60 °C/h and then kept at 500 °C for 1 h. From 500 °C to the desired temperature, i.e., 1300, 1325, or 1350 °C (referred to as calcination temperature in the following), a heating rate of 100 °C/h was applied. At the calcination temperature, particles were kept for 4, 8, or 16 h before cooling down to room temperature with a rate of 180 °C/h. Calcined particles with diameter 125−180 μm were obtained by sieving for reactivity test, and 180−212 μm particles were used for bulk density and crushing strength measurement. Detailed information, i.e., milling time, calcination temperature, and calcination time, for individual samples can be found in Table 1. In the following text, fresh particles refer to oxygen carrier particles after calcination, and used particles are the ones collected after reactivity tests where particles have cooled down to room temperature in 5% oxygen. 2.2. Characterization. The bulk density of oxygen carriers produced was calculated by using the mass of fresh particles with diameter 125−180 μm divided by its volume, which was measured using a graduated cylinder. The mechanical strength of the sample, referred to as crushing strength (CS), was measured on fresh particles with a diameter of 180−212 μm by a digital apparatus (Shimpo FNG5). XRD (D5000 Advanced, utilizing Cu Kα1 radiation, Siemens) and BET (ASAP2000, Micromeritics) were employed to study the chemical composition, crystalline structures, and specific surface area of the material. The porous nature of selected oxygen carriers was measured on a few materials using a mercury intrusion (Poremaster, Quantachrome) method. The morphology of the oxygen carrier was studied by SEM microscope (TM3030, Hitachi). In Table 1, the manufacture composition, calcination parameters, and milling time of the oxygen carriers are presented. Note that material 1300C4h5m is also the material used in testing in 10 kW and 120 kW.18,19 A jet-cup attrition rig was used to examine the oxygen carrier resistance to mechanical attrition. The rig consists of a conical cup, a gravitational separator, and a filter. The conical cup, inner diameter of 13 mm at the bottom and 25 mm at the top, is located at the bottom of the gravitational separator. A mass of 5 g oxygen carrier particles was placed in the cup, and air with a velocity of approximately 100 m/s was applied through the nozzle. The particle filter (Parker P31FA12CGMN) placed at the top of the rig, where fines with a diameter less than ∼10 μm can pass through. Every 10 min, the filter was dismounted, and the amount of fines produced was weighed. Each test takes 1 h, and the results of the last 30 min were used to calculate an attrition index of the material. Detailed description and discussion about the jet-cup rig and the methodology can be found in Rydén et al.30 2.3. Batch Fluidized-Bed Reactor System and Reactivity Test. The reactivity of oxygen carrier was examined in a batch
Figure 1. Schematic representation of laboratory setup of a batch fluidized-bed system.
870 mm long and 22 mm in diameter quartz reactor. The quartz reactor was enclosed in an electron oven and heated to desired temperature while the inlet gas was sent via the porous plate to fluidize the oxygen carrier particles. The flue gas leaving from the top of the reactor was cooled, and the dry gas was analyzed by a gas analyzer (NGA-2000, Rosemount) where the volume fraction of O2, CO, CO2, and CH4 together with the total flow of the dry gas were measured. The pressure drop over the reactor, i.e., the pressure difference above and below the particle bed, was measured by a 20 Hz pressure transducer (Honeywell). The temperature in the reactor was measured by two thermocouples, one at 25 mm above and one at 5 mm below the porous plate. For the reactivity tests, a mass of 15 g of oxygen carrier particles with a diameter of 125−180 μm was placed in the quartz reactor and exposed to oxidizing (5% O2 in N2, 900 mLN/min) and reducing (100% CH4 or syngas, 450 mLN/min) gases cyclically at 950 °C. To investigate the oxygen carrier’s ability to release gaseous O2, i.e., the CLOU property, the particles were exposed to an N2 flow (100% N2, 600 mLN/min) for 360 s after being fully oxidized. The O 2 concentration in the outlet stream at the end of the 360 s N2 flushing cycle, also called the CLOU cycle, was selected to gauge the CLOU property of the oxygen carrier. Since more than one fuel/CLOU cycle was performed at one temperature, an average value of the fuel conversion/O2 concentration from cycles conducted at the same temperature was taken to gauge the material’s reactivity. It should be noticed that N2 (100% N2, 600 mLN/min) was used to flush the system for 60 s between each oxidizing and reducing period to prevent back mixing of fuel and inlet oxygen. In Table 2, an experimental scheme including the time of exposure and temperature for each period can be found. The period needed for oxidation is not given in the table since this depends on the temperature, previous cycle (CLOU cycle or fuel cycle), and material studied. But all oxygen carriers studied could be oxidized by 5% O2 without any problem. The
Table 2. Experimental Scheme for Reactivity Testsa no. of cycles
inert/ reducing gas
tCLOU (s)
3 3 3 3 2
Nitrogen CH4 Syngas Nitrogen Nitrogen
360
360 360
tIn (s)
tRed (s)
60 60
20 80
TOx (°C)
TRed/TCLOU (°C)
900 950 950 900 900
900 950 950 900 1000
a
The period of oxidation varies depending on previous inert or fuel cycles; thus, it is not presented in the table. Ti and ti are the temperature and period of i (i.e., oxidation (Ox), inert (In), and reduction (Red)).
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Figure 2. Crushing strength and bulk density as a function of (a) calcination time and (b) milling time. Oxygen carriers presented in (a) were milled for 5 min before spray-drying. The calcination time for the oxygen carriers in (b) was 4 h.
Figure 3. BET specific surface area of investigated oxygen carriers, both fresh and used, as a function of (a) calcination time and (b) milling time. Oxygen carriers presented in (a) were milled for 5 min before spray-drying. The calcination time for the oxygen carriers in (b) was 4 h. where mi is the mass of the oxygen carrier at time i, and mOx is the mass of the fully oxidized oxygen carrier. It should be noticed that mOx is simply the mass of the fresh particles used in the experiments, which is 15 g. Possible oxygen deficiency at higher temperatures31 was not taken into account for the calculation. In a fuel reduction period, ωi can also be calculated as a function of time. The oxygen carrier conversion during a methane and syngas reduction period is shown by eqs 8 and 9:
particles are regarded as fully oxidized when the outlet O 2 concentration reaches 5% in an oxidation cycle. 2.4. Data Evaluation. The reactivity of the oxygen carrier to fuels is quantified by fuel conversion which is defined as the fraction of the introduced fuel that has been fully converted into CO2. Methane conversion and the CO conversion to CO2 for the syngas experiments are given by eqs 5 and 6, γCH = 4
pCO
2
pCO + pCO + pCH 2
γsyn =
4
ωi =
(5)
ti 0
nout ̇ MO (4p + 3pCO,out − pH ,out ) dt 2 mOx ptot CO2,out
(8)
nout ̇ MO (2p + pCO,out − pH ,out ) dt 2 mOx ptot CO2,out
(9)
pCO
2
pCO + pCO 2
ωi =
(6)
where pi is the outlet partial pressure of component i. The definition of oxygen carrier conversion (or solid conversion) can be found in eq 7:
ωi =
∫t
mi mOx
∫t
ti 0
where ṅout is the molar flux of the dry flue gas, and MO is the molar mass of oxygen. For a specific reduction cycle, γCH4 is given as an average value from ω = 1 to 0.99. The concentration of H2 was not measured during experiments. It can be assumed to be low enough to be neglected since H2 generally has a higher reactivity than CO,32 and
(7) 3260
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Energy & Fuels the CO concentration detected out of the reactor was very low under all conditions.
noticed that oxygen carriers after reactivity test had higher BET surface area compared with the fresh ones. The porosimetry results below and the morphology of the particles presented in section 3.6 show a good match with the suggested mechanism. The porosity of some investigated oxygen carriers is listed in Table 3. Reading from the table, for oxygen carriers calcined at
3. RESULTS 3.1. Physical Properties of the Oxygen Carriers Investigated. The crushing strength, bulk density, and BET specific surface area of the oxygen carriers investigated can be found in Table 1. Further, the crushing strength and bulk density were shown as a function of calcination time and milling time in Figure 2. The oxygen carrier particles presented in Figure 2a were all ball-milled for 5 min, and the materials in Figure 2b were all calcined for 4 h. In general, the crushing strength and bulk density of the material increase with calcination temperature and calcination time, as seen in Figure 2a. The greatest increase was seen for the material sintered at 1300 °C, where more than a doubling of crushing strength was seen by increasing the calcination dwell time from 4 to 8 h. Also, there is a good correlation between the crushing strength and bulk density; i.e., the material with a higher bulk density has a higher crushing strength. Changing the ball-milling time of the water-based slurry before spray-drying had different effects on the physical properties of the oxygen carrier depending on the calcination temperature. For the material calcined at 1300 °C, milling the slurry for 15 min instead of 5 min can significantly improve the crushing strength and bulk density of the oxygen carrier. But further increase of the milling time to 45 min does not have any significant effect on material’s crushing strength and bulk density. In fact, increased milling time for the materials sintered at 1325 and 1350 °C actually showed a decrease in CS. Ryden et al.30 showed in an earlier study that it could be wise to use oxygen carrier particles with a crushing strength of above 2 N. From the discussion above, it seems that particles of CaMn0.9Mg0.1O3−δ should be calcined at 1325 °C or above. If 1300 °C is used, then it may be needed to increase the time of calcination. Of course, the exact behavior may depend on the actual raw material used as well as the physical properties of the raw material. The BET specific surface area of the oxygen carrier examined is shown in Figure 3 as a function of calcination time and milling time. Clearly, the BET surface area is very low for all materials, i.e.,
