Natural Ores as Oxygen Carriers in Chemical Looping Combustion

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Natural Ores as Oxygen Carriers in Chemical Looping Combustion Hanjing Tian,†,‡ Ranjani Siriwardane,*,† Thomas Simonyi,†,‡ and James Poston† †

U.S. Department of Energy, National Energy Technology Laboratory, 3610 Collins Ferry Road, P.O. Box 880, Morgantown, West Virginia 26507-0880, United States ‡ URS, 3610 Collins Ferry Road, P.O. Box 880, Morgantown, West Virginia 26507-0880, United States ABSTRACT: Chemical looping combustion (CLC) is a combustion technology that utilizes oxygen from oxygen carriers (OC), such as metal oxides, instead of air to combust fuels. The use of natural minerals as oxygen carriers has advantages, such as lower cost and availability. Eight materials, based on copper or iron oxides, were selected for screening tests of CLC processes using coal and methane as fuels. Thermogravimetric experiments and bench-scale fixed-bed reactor tests were conducted to investigate the oxygen transfer capacity, reaction kinetics, and stability during cyclic reduction/oxidation reaction. Most natural minerals showed lower combustion capacity than pure CuO/Fe2O3 due to low-concentrations of active oxide species in minerals. In coal CLC, chryscolla (Cu-based), magnetite, and limonite (Fe-based) demonstrated better reaction performances than other materials. The addition of steam improved the coal CLC performance when using natural ores because of the steam gasification of coal and the subsequent reaction of gaseous fuels with active oxide species in the natural ores. In methane CLC, chryscolla, hematite, and limonite demonstrated excellent reactivity and stability in 50-cycle thermogravimetric analysis tests. Fe2O3-based ores possess greater oxygen utilization but require an activation period before achieving full performance in methane CLC. Particle agglomeration issues associated with the application of natural ores in CLC processes were also studied by scanning electron microscopy (SEM).

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INTRODUCTION

Fuel (CO, H 2) + metal oxide

The accumulation of CO2 released into the atmosphere can result in natural greenhouse effects and global climate changes, which have gained increasing international attention. It is necessary to develop efficient technologies that safely use fossil fuels to generate electricity while reducing greenhouse gas emissions.1,2 Commercial CO2-capture technologies can be expensive and energy intensive. The main disadvantage of these technologies is that they require a large amount of energy to separate CO2, reducing the overall efficiency of the power plant. Novel CO2 capture methods are being developed to address this problem, but they are not yet commercially available. Chemical looping combustion (CLC) is considered as an efficient method for producing high-purity CO2 from fossil fuel combustion. In CLC, an oxygen carrier (OC) transfers oxygen from air to fuel while avoiding contact between the materials during the transfer. Similar to oxy-fuel combustion, CLC combustion produces only CO2 and water vapor. However, unlike oxy-fuel combustion, no direct supply of oxygen is needed, avoiding the additional cost and energy of producing and supplying oxygen to the power generation system. The main advantage of the CLC process versus normal combustion is the concentrated CO2 stream obtained without expending any major energy for separation.3 The economic evaluations reported in the literature indicate that the energy penalty for CO2 capture in the CLC process is significantly lower than that for both integrated gasification combined cycle and oxy-fuel combustion processes.4 CLC systems have two reactors, an air reactor and a fuel reactor. In the fuel reactor, fuel reacts with the metal oxide:5,6 This article not subject to U.S. Copyright. Published 2013 by the American Chemical Society

→ CO2 + H 2O + metal/reduced metal oxide

(1)

The reduced-form metal oxide is then oxidized in the air reactor to form metal oxide: metal or reduced metal oxide + O2 → metal oxide

(2)

The oxygen carrier then initiates the second cycle. The exit gas from the fuel reactor contains CO2 and H2O, and after condensation of the water, a pure stream of CO2 is produced, which can be used for carbon capture and storage. The applicable oxygen carrier is essential to successfully operate a CLC system. Various fuels (coal, natural gas/ methane, or synthesis gas) have been extensively studied in the past decade.7−18 The oxygen carrier requires a sufficient combustion rate suitable for various reactor systems, sufficient oxygen release to facilitate the coal−oxygen carrier interactions, stable reactivity during multiple cycles, high attrition resistance, and low reactivity with ash and other contaminants. The majority of research conducted on oxygen carriers has focused on synthetic metal oxide materials, including single or mixed CuO, NiO, Fe2O3, MnOx, CoO, CrOx, WO3, BaO, SrO, as well as those oxide materials supported on Al2O3, SiO2, ZrO2, TiO2, bentonite, sepiolite, and Yttrium-stabilized zirconia, etc. Among those metal oxides, it is generally accepted that NiO, Fe2O3, Special Issue: Accelerating Fossil Energy Technology Development through Integrated Computation and Experiment Received: September 11, 2012 Revised: December 28, 2012 Published: January 2, 2013 4108

dx.doi.org/10.1021/ef301486n | Energy Fuels 2013, 27, 4108−4118

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Table 1. Elemental Analysis of Natural Ore Materials Oxide component (wt %) Chryscolla Cuprite Malachite Hematite Ilmenite Limonite Magnetite Taconite

Place of origin

Active component in calcined sample

Al2O3

CuO

Fe2O3

SiO2

TiO2

AZ, U.S.A. AZ, U.S.A. Anhui, China MI, U.S.A. OR, U.S.A. NY, U.S.A. OR, U.S.A. OR, U.S.A.

CuO Cu2O CuO/Fe2O3 Fe2O3 Fe2O3/TiO2 Fe2O3 Fe3O4 Fe2O3

7.73 18.18 12.82 2.55 6.26 8.78 2.9 8.55

64.41 15.7 15.08 0.76 0.76 2.184 0.88 2.51

1.34 2.66 12.12 94.23 46.01 66.97 88.23 79.46

24.59 61.04 52.64 1.39 10.84 18.11 6.27 4.5

0.93 2.09 7.08 1.01 36 3.79 1.64 4.83

and CuO are promising OC materials for their high combustion performance, lower production cost, and long-term stability. Recently, the applications of natural ores and inexpensive industrial byproducts have been investigated as oxygen carriers for low cost and wide availability.19−24 Leion et al. analyzed the behavior of several iron-containing ores, such as ilmenite, Carajas, Malmbergret, iron-based byproducts, and manganesecontaining ores, such as Colormax, Tinforss, Elkem, Eramet, in multiple redox cycles and concluded that ilmenite, a natural mineral mainly composed of FeTiO3, has the potential for CLC. Moreover, ilmenite was also tested in a 10-kW CLC unit for long periods (37 h, 37 cycles) at about 975 °C without encountering issues with defluidization. Among manganese ores, Colormax EF also met all of the CLC reaction performance criteria and is considered a suitable oxygen carrier for CLC when using solid fuel. An attempt was made to promote the reaction performance of ilmenite by adding NiO. A series of manganese ores from Norway has been studied by thermogravimetric measurement using methane as fuel. Calcium oxide was added into manganese ores to modify the bulk structure and to improve the reactive performance. Results showed that excess amounts of calcium can promote the reactivity of manganese ores because the formation of calcium manganite perovskite has potential advantages in terms of kinetics, chemical, and mechanical stability for methane CLC at low temperatures. The major problem with using natural ores in CLC systems is the decreased stability during multiple reduction/oxidation cycles. Volume changes occur during the reversible reduction/ oxidation state between Mn3O4−MnO and Fe2O3−Fe3O4 that induce mechanical stress, which will likely decrease the mechanical efficiency. Aside from mechanical inefficiencies, particle agglomeration and sintering during the CLC process, which has been reported for most Fe2O3 and Mn3O4-based ores, poses another risk. In the present work, three CuO ores and five iron ores from different sources were studied for their possible application in direct coal and natural gas CLC systems. Multiple-cycle thermogravimetric analysis (TGA) was used to determine the rates of reaction and percentage of combustion/oxidation. Bench-scale flow-reactor tests were also conducted to verify TGA data. SEM images were used to understand morphological changes of these natural materials during the CLC process.

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Figure 1. Reduction profile of coal−chryscolla in N2 up to 1000 °C. concentrations of Al, Si, Fe, Cu, and Ti were measured and normalized as Al2O3, SiO2, Fe2O3, or Fe3O4 (for magnetite), CuO, and TiO2 with a total weight of 100%. Trace elements include carbon, sodium, magnesium, phosphorus, chlorine, potassium, and calcium indicated by SEM-EDS. The results are also summarized in Table 1. The received natural samples were first calcined at 500 °C in an air flow (100 standard cubic centimeters per minute or sccm) for 4 h to remove moisture and volatiles. Metal hydroxides and carbonates decompose, forming stable metal oxide species after calcination. The calcined samples were ground and sieved to obtain the particle size range 74−88 μm. The Brunauer−Emmett−Teller (BET) area of the calcined samples was in the range 4−7 m2/g. These calcined ore samples (74−88 μm) were directly used as oxygen carriers in methane CLC tests. For direct coal CLC tests, coal (Illinois #6, 100 μm) was physically mixed with the calcined ore samples (weight ratios of 0.2:4.5 coal/ores) in a Crescent WIG-BUG shaker. The composition of Illinois #6 coal has been listed in our previous paper.25 In this study, the reaction performance of natural ores was also compared with standard CuO (99%, Aldrich) and Fe2O3 (99.98%, Fisher) within the same particle-size range. Thermogravimetric Analysis. TGA experiments were conducted in a TA Model 2050. The samples were placed in a 5 mm deep and 10 mm diameter crucible. Samples of approximately 40 mg were heated in a quart bowl from ambient to 900/1000 °C at a heating rate of 15 °C/ min in ultrahigh pure nitrogen. For direct coal CLC tests, the coal/ natural ore mixture was then kept isothermal at the final temperature for 60 min. Following, air was introduced for 60 min. For methane CLC tests, about 40 mg natural ores were heated to the reaction temperature, and then 20% methane (balanced by ultra-high-purity nitrogen) was used for the reduction segment, while zero air was used for the oxidation segment. All purge/reaction gas flow rates were maintained at 45 sccm. Reduction and oxidation reaction times for the methane CLC reaction were fixed at 10 and 30 min, respectively, to avoid carbon deposition. The system was flushed with ultra-high-purity

EXPERIMENTAL SECTION

Sample Preparation and Characterization. Three CuO ores (chryscolla, cuprite, malachite) and five Fe2O3 ores (hematite, ilmenite, limonite, magnetite, taconite) were purchased from Ward’s natural science. ICP (inductively coupled plasma) analysis was conducted to measure elemental components of the samples. The 4109


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Table 2. Coal CLC Reactivity of Various Ores Obtained from TGA Tests CuO Chryscolla Cuprite Malachite Fe2O3 Magnetite Limonite Hematite Taconite

Reaction temp. (°C)

Reduction rate (min−1)

% Combustion

Oxidation rate (min−1)

% Oxidation

780 876 1100 1100 977 1000 973 900 900

0.079 0.02 negligible negligible 0.05 negligible negligible negligible negligible

100 35.0 12.9 11.6 91.6 10.4 15.6 12.1 26.4

0.174 0.01 0.11 0.13 0.78 0.57 0.35 0.08 0.08

99.2 67.2 46.1 23.2 90.6 70.3 65.2 34.7 35.6

Moxd = weight of completely oxidized sample after introducing air The fractional conversion data was used as a function of time and fitted to obtain the polynomial regression equation. The global rates of reactions (dX/dt) at different fractional conversions (X) were calculated by differentiating the fifth order polynomial equation. The percentage of combustion and percentage of oxygen uptake were obtained using the weight-change data from TGA using the following equations:

% combustion = (actual weight loss from TGA/theoretical weight loss based on carbon content in coal sample) × 100 % oxygen consumption = (experimental oxygen consumption/theoretical capacity of oxygen present in the metal oxide) × 100 Oxygen transfer capacity was used to characterize the amount of oxygen from natural ores consumed for the methane CLC process. The oxygen transfer capacity is defined as

oxygen transfer capacity = 100(Mo − M red)/Moxd Bench-Scale, Fixed-Bed Flow-Reactor Tests. Bench-scale, fixedbed reactor tests were conducted to evaluate the reaction performance and stability of the ore samples during CLC reactions. An approximately 5g sample (∼3 mL) was packed between quartz wool in a 10.6-in. long stainless steel reactor tube with 1-in. inner diameter. The reactant gases and argon gas were introduced to the reactor from the gas cylinder through mass flow controllers. Using an online mass spectrometer (Quadrupole Prisma, Pfeiffer), the composition of the product gas was continuously analyzed and the temperature of the sample continuously monitored. For direct coal CLC, the sample containing coal/ore mixtures (prepared similarly to the samples used for TGA tests) were heated from 25 to 250 °C in ultra-high-purity argon with a flow rate of 100 sccm and kept isothermal at 250 °C for 60 min after the MS signal of argon stabilized. The reactor was heated to 800 °C and kept isothermal for 60 min to complete the reduction; then, the sample was exposed to 4% O2/N2 (100 sccm) until oxygen breakthrough (the observation of 1% O2 from reactor outlet). For methane CLC, a 7-cycle reduction/oxidation CLC test was conducted at 800 °C and at 10 psi using 20% CH4/N2 for reduction and 4% O2/ N2 for oxidation. The reduction reaction time was 15 min, while the oxidation reaction continued until oxygen breakthrough. LABTECH NOTEBOOK Pro was used to monitor the reactor temperature and pressure during the experiments.

Figure 2. Reduction profile of coal, magnetite, and limonite in N2 up to 1000 °C. nitrogen for 5 min before and after each reaction segment. All TGA tests were repeated three times to avoid system error introduced by the heterogeneous ore samples. Fractional conversions (fractional reduction and fractional oxidation) were calculated using the TGA data. The fractional conversion (X) is defined as

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fractional reduction (X ) = (Mo − M)/(Mo − M f )

RESULTS AND DISCUSSION Direct Coal Chemical Looping Combustion with Thermogravimetric Analysis. Various reaction mechanisms of coal CLC have been reported in the literature. One possible reaction mechanism involves the oxygen carrier with gaseous products from coal pyrolysis at the lower temperature range

fractional oxidation (X ) = (M − M f )/(Moxd − M f ) M = instantaneous weight of metal oxide-coal mixture Mo= initial weight of metal oxide-coal mixture Mf = weight of metal oxide-coal mixture after reaction in N2 (i.e., reduced metal + ash + unreacted coal) 4110


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Figure 3. TGA profile of coal−hematite (a) and coal−taconite (b) during reduction and oxidation reaction in N2 up to 900 °C (reduction time, 5 h).

400−500 °C.10 When enough solid−solid contacts are present in a fixed bed, oxygen carriers may directly combust coal char at higher temperature (700−1000 °C).26 In our previous TGA tests,25 coal/CuO (63−173 μm) systems reacted at the maximum rate of 780 °C with full reduction/oxidation conversion. For coal/Fe2O3 (44 μm) systems, the maximum combustion rate takes place at a higher temperature (973−977 °C) with 95% combustion. Steam is generally used during the coal CLC reactions and steam gasification of coal produces synthesis gas that reacts with oxygen carriers. Steam is required specifically in fluidized-bed reactors in which solid−solid reactions are minimal. The weight changes in coal−chryscolla system and global reaction rates were obtained by differentiating the fractional conversion data for combustion of coal in the TGA in nitrogen (without steam), as shown in Figure 1. A broad peak from 250−400 °C with a maximum at 278 °C was observed. This continuous reaction is due to the combustion reaction of copper oxides in chryscolla and coal devolatilization products. Moreover, there is a broad combustion peak between 600 and 1000 °C with the peak 876 °C, which can be attributed to the reaction of coal char and CuO species of chryscolla. Notably, the reaction rate of char/chryscolla is comparable with the reaction rate between coal volatiles/chryscolla, and significantly slower than the reduction of coal with pure CuO as shown in

Figure 4. Bench-scale, fixed-bed reactor test in 20% steam/Ar of (a) coal−chryscolla; (b) coal−limonite, and (c) coal−magnetite.

Table 2. The slower reaction rate of coal/ores system is expected since the active CuO species in mineral ores is only 33% as compared with 100% pure CuO and also diluted by inactive species (SiO2, etc.), which also contributed to less coal−oxygen carrier contacts. The combustion percentage of chryscolla is 35%, which is also lower than the 100% conversion observed with coal/CuO. The oxidation rate and oxygen uptake of coal/chryscolla are listed in Table 2. Only 67% reduced CuO species in chryscolla can be reoxidized by air with a significantly slower reaction rate than coal/pure CuO at 900 °C. Similar TGA screening tests were also conducted with the mixtures of coal and Fe2O3-based ores. The TGA reaction profiles are shown in Figures 2 and 3. The reactivates of ironbased ores were very low, and it was not possible to obtain accurate rates by differentiating the TGA data. The overall 4111


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Figure 5. MS profile of bench scale fixed bed reactor test of coal gasification with 20% steam/He.

Figure 7. TGA profile of 10-cycle CH4 CLC tests over CuO based ores (900 °C; 20% CH4/N2; reduction, 10 min; oxidation, 20 min). Figure 6. TGA profile of 10-cycle CH4 CLC tests over bentonite supported CuO and Fe2O3 (900 °C; 50% CH4/N2; reduction, 60 min; oxidation, 30 min).

reaction performances of coal/natural ores systems summarized in Table 2. Data with chryscolla indicates lowest reaction temperature, highest reduction rates, highest combustion percentages. Its oxidation percentage

also the second highest among these natural ores. All ironbased ores showed poor performance without steam. To obtain better reactivity with natural ores, it will be necessary to add steam in the fuel gas steam. Coal Chemical Looping Combustion in the Presence of Steam. The coal CLC reactions with natural ores were conducted in a bench-scale, fixed-bed reactor. To understand

are the and was 4112


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Figure 8. TGA profile of 10-cycle CH4 CLC tests over Fe2O3 based ores (900 °C; 20% CH4/N2; reduction, 10 min; oxidation, 20 min).

better coal combustion efficiency in the presence of steam and indicated that the addition of steam effectively improves the reaction performance of natural ores in the direct coal CLC process. To understand steam’s positive effect on coal CLC, the reaction of coal with steam was conducted in the bench-scale, fixed-bed reactor tests with 20% steam/Ar as temperature increased to 800 °C. The outlet concentrations of steam gasification products are shown in Figure 5. The data indicates that the coal gasification reaction initiated at temperatures as low as 670 °C. Observations show that the increase in gaseous products, CO, H2, and CO2, corresponded with the decrease in steam concentration. While hydrogen and CO were the most abundant species in steam gasification, these gases were not observed in coal/20% steam CLC reactions while oxygen carriers were present. Therefore, it is reasonable to conclude that the oxygen carriers reacted continuously with gaseous H2/ CO produced from steam gasification reaction initiating at a relatively low temperature (∼670 °C), accelerating the combustion reaction of oxygen carrier/coal with the presence of steam. The oxygen carrier may have converted CO to CO2 and H2 to water, which was condensed prior to mass spectrometer. Thus, the fixed-bed reactor test data suggests that coal gasification is the major reaction when steam is introduced in the CLC process and that gasification products readily react with the ore-based oxygen carriers. Steam gasification of coal may be accelerated due to fast removal of

Table 3. Reaction Rates and Oxygen Transfer Capacities of Natural Ores in Methane CLC Reaction in 5th Cycle TGA Tests

Chryscolla Cuprite Malachite Ilmenite Taconite Limonite Magnetite Hematite

Reduction rate (min−1)

Oxidation rate (min−1)

Oxygen transfer capacity

0.811 0.28 0.67 0.18 0.31 0.21 0.31 0.33

0.74 0.8 1.3 0.9 0.55 0.64 0.58 0.52

6.4 0.8 3.2 4.6 7.8 8.8 8.5 10

the effect of steam in CLC conversion, 20% steam was added during reduction, and the results are shown in Figure 4. The maximum operational temperature for the fixed bed reactor is 800 °C, and the reduction reaction information was only obtained up to 800 °C. As shown in Figure 4, with the addition of 20% steam, the reaction peak temperature decreased from >800 to 703 °C for the coal−chryscolla mixture. The reaction rate was also slightly higher as indicated by the higher CO2 concentration in the presence of steam. The reaction peak temperatures did not change for the coal−limonite and coal− magnetite systems, but the outlet CO2 concentration increased significantly at 800 °C after steam addition. This demonstrated 4113


Rred (min−1)

1.033 1.75 1.46

sample

hematite chryscolla limonite

Rox (min−1)

0.28 0.55 0.42

Roc (%)

10 4.59 11.4

cycle 5

1.02 1.71 1.44

Rred (min−1) 10 4.7 11.4

Roc (%)

cycle 10

0.273 0.54 0.42

Rox (min−1) 1.02 1.69 1.43

Rred (min−1)

Rox (min−1) 0.27 0.53 0.41

Roc (%) 10.2 4.77 11.4

cycle 15

1.01 1.67 1.42

Rred (min−1) 10.27 4.81 11.3

Roc (%)

cycle 20

0.27 0.53 0.41

Rox (min−1) 1 1.66 1.42

Rred (min−1)

Rox (min−1) 0.26 0.53 0.42

Roc (%) 10.33 4.85 11.4

cycle 25

1 1.67 1.41

Rred (min−1) 10.35 4.83 11.2

Roc (%)

cycle 30

0.26 0.53 0.42

Rox (min−1)

Table 4. Reaction Rates and Oxygen Transfer Capacities of Natural Ores in Methane CLC Reactions in Long-Term TGA Tests (Rred, reduction rate; Rox, oxidation rate; Roc, oxygen transfer capacity)

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Figure 9. TGA profile of 50-cycle CLC tests of hematite at 900 °C (reduction time, 10 min; oxidation time, 30 min; 20% CH4/N2).

Figure 10. TGA profile of 50-cycle CLC tests of chryscolla at 900 °C (reduction time, 10 min; oxidation time, 30 min; 20% CH4/N2).

Figure 11. TGA profile of 50-cycle CLC tests of limonite at 900 °C (reduction time, 10 min; oxidation time, 30 min; 20% CH4/N2).

gasification products, H2, and CO, from the reaction with the oxygen carrier. Therefore, steam is essential to obtain a

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shorter reduction time (10 min.) and a diluted methane (20%) concentration was used to evaluate the reaction performance of natural ores in TGA tests for methane CLC. The 10-cycle TGA data of three CuO-based ores with methane CLC reaction are shown in Figure 7. For all CuObased ores, the initial weight and final weight of reduction/ oxidation were constant during multiple cycle tests. The data indicated that CuO ores possess excellent stability for methane CLC. The 10-cycle TGA tests of Fe2O3-based ores in methane CLC are shown in Figure 8. For the fixed reduction time (10 min) and oxidation time (60 min.), weight change corresponding to carbon formation and removal reaction was not observed. After initial activation, all Fe 2O3-based ores demonstrated good stability in methane CLC during the 10cycle tests, but the activation period must be considered when designing a CLC process with natural ores in methane CLC. The reaction rates and oxygen use of natural ores during methane CLC reaction are summarized in Table 3. Chryscolla demonstrated a stable reduction rate (∼0.8 min−1), oxidation rate (∼0.7−0.8 min−1), and oxygen transfer capacity (∼6.4% in the fifth cycle) for the 10-cycle tests. The reduction rate was the highest for chryscolla containing CuO. Hematite and limonite showed the highest oxygen transfer capacity (∼9%) and remain stable over 10-cycle tests. The taconite sample possesses higher oxygen transfer capacity than chryscolla, but its oxygen transfer capacity is lower than other Fe2O3-based ores. Ilmenite has the lowest oxygen transfer capacity in the study. Long-Term Thermogravimetric Analysis Tests. Chryscolla, hematite, and limonite demonstrated high oxygen transfer capacities and reaction rates in 10-cycle TGA tests under screening conditions and were selected for long-term cycle tests. The reaction performances and oxygen capacity data of hematite, chryscolla, and limonite for 30-cycle (2000 min) long-term TGA tests are summarized in Table 4. For hematite and limonite, after the initial activation period, the reduction rate and oxygen transfer capacity were constant during the 30cycle test, but the oxidation rate slightly decreased. Chryscolla demonstrated great stability in reaction rates and oxygen transfer capacity. Hematite, chryscolla, and limonite also demonstrated stability during a 50-cycle TGA test of methane CLC, as shown in Figures 9−11. Seven-Cycle Bench-Scale Fixed-Bed Reactor Tests. The reaction performances of hematite, limonite, and chryscolla with methane were evaluated in a bench-scale, fixed-bed flow reactor. A 7-cycle methane CLC reaction was conducted. Figure 12 shows a typical outlet gas concentration during the test with chryscolla. The reduction time was limited to 15 min to avoid carbon formation. During reduction, the methane was introduced to the reactor and CO2 production was observed immediately. The doublet CO2 peaks may be associated with the 2-step reduction of CuO to Cu2O and then full conversion to metallic Cu, respectively. A trace amount of CO was also observed during the tests. The amount of oxygen transferred for methane combustion was computed by integrating CO2 peaks. The oxygen consumption of various oxygen carriers in 7cycle bench-scale, fixed-bed reactor tests are shown in Figure 13. All three samples demonstrated a stable reaction during cyclic tests. Chryscolla showed the highest oxygen consumption during the 15 min reduction time since the reduction rate of chryscolla is greater than that of hematite and limonite. Analyses of the flow reactor tests show that methane was mainly converted to CO2 with a trace amount of CO. However,

Figure 12. Outlet gas composition from MS during 7-cycle methane CLC reaction in bench-scale, fixed-bed reactor with chryscolla as the oxygen carrier.

Figure 13. Total oxygen consumption in the combustion reaction during 7-cycle methane CLC reaction in bench-scale, fixed-bed reactor: 5 g ores reduced by 20% CH4/N2 (100 sccm) for 15 min.

reasonable reactivity with metal oxide ores for coal CLC reactions. Methane Chemical Looping Combustion. The reaction performances of natural ores in the methane CLC reaction were evaluated by TGA. For comparison, 10-cycle methane CLC reaction tests were also performed with synthetic materials (60% CuO/bentonite and 60% Fe2O3/benonite) at 900 °C. The results are shown in Figure 6. For 60% CuO/ benonite sample, the weight decrease during reduction and weight increase during oxidation reaction were constant, demonstrating excellent stability during multicycle methane CLC tests. For the 60% Fe2O3/benonite sample, the TGA profile consisted of four reactions: (1) the reduction reaction, (2) carbon deposition shown as weight gain after initial reduction, (3) carbon removal shown as weight loss in air, and (4) oxidation reaction indicated by weight gain. The TGA data indicated that CuO is more resistant to carbon formation than Fe2O3 for methane CLC reaction. For the Fe2O3 oxygen carrier, weight loss during reduction increased during each successive cycle until the fifth cycle, indicating a slow activation process for Fe2O3 during CLC reaction. To avoid carbon deposition, 4115


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Figure 14. SEM images of various ores (oxygen carriers) before and after 7-cycle bench-scale, fixed-bed reactor tests (left column, fresh sample; right column, used sample).

with suitable space velocities, full combustion of methane should be achievable. Particle Agglomeration Evaluation. Particle agglomeration was evaluated during heating and cyclic tests. Natural ores were heated in an oven at 800 °C for 4 h to evaluate particle agglomeration due to heating. Severe particle agglomeration was observed with a calcined ilmenite sample, but hematite,

chryscolla, and limonite did not show any agglomeration due to heating. SEM images of fresh oxygen carrier samples and the reacted sample after 7-cycle bench-scale, fixed-bed reactor methane CLC tests are also shown in Figure 14. Particle breakage (smaller particle size of the reacted sample as compared to the fresh sample) was observed in the reacted chryscolla indicating a loss in mechanical strength of the 4116


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Table 5. Analysis of Extent of Reduction of Various Ore Oxygen Carriers During Coal CLC and Methane CLCa Coal/carbon CLC

Oxygen carriers/reduction reaction b

Methane CLC

Reduction percentage (%)

Species after reduction reaction

CuO (CuO→Cu )

99.4

Cu

Chryscolla (CuO→Cu)

35.0

Cuprite (CuO→Cub)

12.9

Malachite (CuO→Cub) (Fe2O3→FeOb)

11.6

Fe2O3 (Fe2O3→FeOb)

54

CuO (major), Cu2O (minor) CuO (major), Cu2O (minor) CuO, Fe2O3 (major), Cu2O/Fe3O4 (minor) Fe3O4

Magnetite (Fe3O4→FeOb)

10.4

Limonite (Fe2O3→FeOb) Hematite (Fe2O3→FeOb) Taconite (Fe2O3→FeOb)

15.6 12.1 26.4

Fe2O3 (major) Fe3O4 (minor) Fe3O4 Fe3O4 Fe3O4

Oxygen carriers/Reduction reaction

Normalized oxygen transfer capacity (%)

Species after reduction reaction

CuO→Cu2O CuO→Cu chryscolla (CuO→Cu)

6c 12c 10

Cu2O Cu Cu/Cu2O

cuprite (CuO→Cu)

5.1

Cu2O

malachite (CuO→Cu) (Fe2O3→Fe3O4)

6.6

Cu2O/Fe3O4

Fe2O3→Fe3O4 Fe2O3→FeO Fe2O3→Fe Magnetite (Fe3O4→Fe)

3.3c 10c 30c 9.5

Fe3O4 FeO Fe FeO/Fe

Limonite (Fe2O3→Fe) Hematite (Fe2O3→Fe) Taconite (Fe2O3→Fe)

13.1 10.6 9.8

FeO/Fe FeO/Fe FeO/Fe3O4

a Oxygen transfer capacities were normalized by the concentration of active oxide CuO or FeOx species. bThe reduction percentages were calculated assuming the reactions indicated in the parentheses. cTheoretical capacities corresponding to the reactions are reported.

particles during reduction/oxidation. No significant changes in particle size were observed with the reacted limonite and hematite samples, but smooth/soft points were found in some regions of used hematite, as shown in Figure 14. Oxidation States of Reduced Sample in Chemical Looping Combustion Reaction. As previously reported,26 the combustion temperature/percentage depends on the type of fuel. In the present work, the extent of reduction of various ores with coal and methane was calculated from TGA data. Based on theoretical values for possible reactions, the final reduced oxide species were identified and are summarized in Table 5. As shown in Table 5, for the coal/carbon CLC reaction, the combustion percentage (calculated assuming the reactions indicated in the parentheses of column 1, Table 5) of most CuO ores are less than 50% because of the low CuO concentration, thus, the major component of the reduced sample after coal CLC was CuO with some Cu2O. The Febased ores (a mixture of Fe2O3/Fe3O4 oxide) were observed after reduction, but all Fe-based ores had slower reaction rates than Cu-based ores. The major component of the reduced Febased ores was Fe2O3 with some Fe3O4. Therefore, the applications of natural ores as oxygen carriers for coal CLC are limited because of low active components and slow reactivity. However, our results from bench-scale, fixed-bed reactor tests suggest that the addition of steam improved the reaction performance of coal CLC with various ores due to the reaction with synthesis gas from the steam gasification process. Therefore, steam is required for the coal CLC process, if natural ores are to be used as oxygen carriers. In methane CLC reactions, the TGA weight changes during reduction and elemental composition data of metal oxides from the ICP analysis of the ores were used to calculate corresponding normalized oxygen transfer capacities for 100% metal oxide, as reported in Table 5. For the methane CLC reaction, comparing normalized weight and theoretical capacities based on possible reactions indicates that CuO species in Cu-based ores were reduced to Cu2O/Cu, and Fe2O3

species in Fe-based ores were reduced to either FeO/Fe3O4 or FeO/Fe during the 10-min reduction time. The reduced chryscolla, hematite, and limonite mainly contain metallic Cu or Fe species. The deeper combustion conversion of methane CLC compared with coal CLC is clearly due to the faster gas− solid transfer/reaction of methane CLC. In coal CLC, both solid−solid interactions and steam gasification of coal are slow reactions, which contributed to the slower reaction rates. Chryscolla, hematite, and limonite demonstrated stable reactivity with methane in both TGA cyclic tests and benchscale, fixed-bed reactor tests. Therefore, natural ores, especially chryscolla, hematite, and limonite, could be used directly as oxygen carriers in methane CLC processes.

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CONCLUSIONS The feasibility of using CuO and Fe2O3-based natural ores as oxygen carriers for CLC was investigated by using TGA and bench-scale, fixed-bed flow reactor tests. It was found that most ores could supply oxygen for direct coal combustion at 700− 1000 °C, but their combustion and reoxidation performances are worse than pure CuO or Fe2O3 due to the low active species concentration in ores. Chryscolla (Cu-based), magnetite, and limonite (Fe-based) demonstrate better reaction performances than other materials and are potential oxygen carriers for both coal and methane CLC as oxygen carriers. Steam greatly improved the coal CLC performance with natural ores due to steam gasification of coal and subsequent reaction with natural ores. Thus, steam is necessary to achieve reasonable reaction rates for the coal CLC process when natural ores are used as oxygen carriers. In the methane CLC process, CuO ores showed faster reaction rates than Fe2O3-based ores, but Fe2O3 ores had greater oxygen transfer capacity. The carbon formation associated with Fe2O3-based ores in methane CLC can be effectively avoided by using shorter reduction times and using diluted methane. Fe2O3-based natural ores required several activation cycles before achieving full performance in methane CLC. Chryscolla, hematite, and limonite demonstrated 4117


Energy & Fuels

Article

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excellent reaction performance in both long-term TGA tests and cyclic bench-scale, fixed-bed reactor tests. However, high attrition issues with chryscolla must be addressed.

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AUTHOR INFORMATION

Corresponding Author

*Phone: + 01-304-285-4513. Fax: + 01-304-285-0903. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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REFERENCES

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Natural Ores as Oxygen Carriers in Chemical Looping Combustion

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