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SiO2
Applied Energy 113 (2014) 1916–1923
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Chemical looping combustion of ultra low concentration of methane with Fe2O3/Al2O3 and CuO/SiO2 Yongxing Zhang a, Elham Doroodchi b, Behdad Moghtaderi a,⇑ a
Priority Research Centre for Energy, Chemical Engineering, School of Engineering, Faculty of Engineering & Built Environment, The University of Newcastle, Australia Priority Research Centre for Advanced Particle Processing & Transport, Chemical Engineering, School of Engineering, Faculty of Engineering & Built Environment, The University of Newcastle, Australia b
h i g h l i g h t s Chemical looping combustion can be extended to the combustion of ultra low concentration of methane. Compared with CuO/SiO2, Fe2O3/Al2O3 shows better performance in terms of reduction reactivity. The redox reactivity of Fe2O3/Al2O3 is stable over 60 cycles.
a r t i c l e
i n f o
Article history: Received 13 December 2012 Received in revised form 30 May 2013 Accepted 5 June 2013 Available online 3 July 2013 Keywords: Chemical looping combustion Ultra low concentration methane Fe2O3/Al2O3 CuO/SiO2
a b s t r a c t This study examines the performance of two metal oxide species in oxidizing ultra low concentration of methane (below 1% in volume). The focus on low methane concentrations are driven by its practical importance in applications such as abatement of ventilation air methane (VAM) in mining operations. Two mixed metal oxides, Fe2O3/Al2O3 and CuO/SiO2, were selected as oxygen carriers and prepared using dry impregnation method. The metal oxide loading contents are found to be 45 wt% and 48 wt%, respectively. The redox reactivity of the selected oxygen carriers were studied at various methane concentrations (i.e., 0.1%, 0.5% and 1% in volume) and temperatures between 873 K and 1073 K using a thermogravimetric analyzer. At low methane concentrations and low temperatures (below 1073 K) the conversion of Fe2O3 to Fe3O4 showed higher reduction reactivity than the reduction of CuO to Cu. The redox reactivity of Fe2O3/Al2O3 was also found to be quite stable even after 60 redox cycles at 1073 K. The respective weight percentages for oxidation and reduction were found to be 100% and 96.67%, corresponding to a full oxidized state Fe2O3 and a reduced state between Fe3O4 and FeO respectively. Moreover, the results for the global reactivity of reduction and oxidation (calculated at X = 0.5) showed that the reduction rates were temperature and concentration dependent, varying from 0.14%/s to 2.2%/s over the range of temperatures and methane concentrations of interest. The oxidation rates were much higher than their reduction counterpart. The values varied from 8.95%/s at 873 K to 10.65% at 1073 K. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction Chemical looping combustion is an emerging technology option with inherent ability for CO2 separation and hence the capacity to facilitate the post-combustion removal of carbon dioxide, a step common to all advanced low mission coal technologies. The CLC process (as shown in Fig. 1) is commonly carried out in a two-step reduction/oxidation (redox) reaction by circulating metal oxide particles between two connected reactors (see reactions ‘‘a’’ to ‘‘c’’). Fuel is burnt with oxygen carrier particles, e.g. metal oxide (MenOm), in a Fuel Reactor (FR) while oxygen carrier particles are reduced to MenOms. The oxygen carriers are then transferred to ⇑ Corresponding author. Tel.: +61 2 49854411; fax: +61 2 49216893. E-mail address: [email protected] (B. Moghtaderi). 0306-2619/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.apenergy.2013.06.005
an Air Reactor (AR) to react with air and oxidize to their original oxidation state (this is commonly referred to as regeneration). Carbon dioxide and steam are the main products from the fuel reactor although minute quantities of CO, CH4, H2 and other hydrocarbons can also form. The product gas from Air Reactor primarily consists of N2 and excess oxygen. As noted, CO2 and N2 do not mix in the CLC process and as such the operational costs and energy efficiency are both improved dramatically compared with conventional combustion.
Reduction : 4Men Om þ sCH4 ! 4Men Oms þ sCO2 þ 2sH2 O
ðaÞ
Oxidation : 2Men Oms þ sO2 ! 2Men Om
ðbÞ
Overall reaction : CH4 þ 2O2 ! CO2 þ 2H2 O
ðcÞ
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Depleted air
CO2 rich gas
Me Fuel Reactor
Air Reactor MeO
Fuel
Air
Fig. 1. Scheme of chemical looping combustion.
CLC combustion has been studied for many years since it was first proposed [1]. Much of the work, however, has focused on the investigation of oxygen carriers and their viability for use in different CLC systems [2–12]. Copper based oxygen carrier exhibits high reactivity with CH4 or syngas but the redox temperature is limited up to 1073 K due to low melting temperature. Adánez et al. [3] believed that the best Cu-based oxygen carriers were those supported by SiO2 or TiO2 sintered at 1223 K regarding the reactivity with CH4. The performance of CuO/TiO2 by wet-impregnation was investigated in a fixed bed of CLC for CH4 [13]. The performance of a copper oxide silica-supported was investigated in 20 cycles for CLC of CH4 [14]. High reactivity, almost complete fuel conversion to CO2 was achieved and no chemical or mechanical decay was observed. de Diego et al. [15] suggested that the chemical stability of copper based oxides was independent on binders or preparation methods while the mechanical properties were quite relied on the preparation methods through cyclic tests in TGA. Considering together chemical and mechanical stability, the impregnation method was better to be used for preparing Cu-based oxygen carriers. Fe-based metal oxides (Fe2O3/Fe3O4) are preferred as oxygen mediator due to good reactivity with gaseous fuels, high gas conversion and high melting temperature although the oxygen transfer capacity is not good (0.033). It is indicated that the transition state of Fe2O3 to Fe3O4 is able to fully converted CH4 or syngas to CO2 and H2O at 1073 K from thermodynamic aspects of view [16]. Mattisson et al. [17] believed that the reduction rate of Fe2O3 (dX/dt) with methane was a function of the solid conversion range (DX). To make an illustration, the conversion range is 11% and 33% for the reduction of Fe2O3 to Fe3O4 and Fe2O3 to FeO. Cho et al. [18] investigated the reactivity of 60 wt% Fe2O3/Al2O3 with 50 vol% CH4 balanced with H2O at 1223 K for the transition of Fe2O3–Fe3O4. Adánez et al. [3] thought that Fe2O3 supported by Al2O3 or ZrO2 shown high reactivity with CH4 at 1223 K for the transition phase of Fe2O3 to FeO. The solid conversion of 90% can be reached within less than 1 min. Mattisson et al. [19] developed 27 oxygen carriers composed of 40–80 wt% Fe2O3, together with Al2O3, ZrO2, TiO2 or MgAl2O4 by freeze granulation and sintered at temperatures of 1223–1673 K. The study showed that Fe2O3 supported with MgAl2O4, ZrO2 or Al2O3 sintered at temperature of 1223 K or 1373 K exhibited good reactivity with CH4 at 1223 K. Johansson et al. [20] investigated the reduction reactivity of Fe2O3/MgAl2O4 to Fe3O4 with 50 vol% CH4 at 1223 K. It was shown that 60 wt% Fe2O3/MgAl2O4 sintered at 1373 K was the best oxygen carrier considering together crushing strength and reactivity. Zafar et al. [21] studied the reactivity of Fe2O3 supported by SiO2 or MgAl2O4 with 10 vol% CH4 at 1073–1273 K. It seemed that Fe2O3/SiO2 maybe not feasible oxygen carrier due to the formation of silicate at high temperature. The Fe2O3/MgAl2O4 showed high reactivity during the phase of Fe2O3 to Fe3O4 but very slow for
Fe3O4 to FeO. Corbella and Palacios [22] tested the titania supported iron oxide as oxygen carrier in CLC of CH4 at 1173 K. The developed metal oxide showed acceptable reactivity and durability but low oxygen capacity due to the formation of irreversible FeTiO3 after the first cycle. Ortiz et al. [23] investigated the behavior of an iron waste from aluminum manufacturer as oxygen carrier for combustion the PSA tail gas from a steam methane reforming plant in a 500Wth CLC prototype. Although a large body of research [4,24] has been conducted on the reactivity of Fe2O3 and CuO with CH4, studies related to the reactivity of metal oxides under ultra low methane concentrations (i.e. 0.1–1 vol%) are generally scarce. However, the ultra low methane concentrations are of significant practical importance in applications such neutralization of residual methane in the retentate stream from a natural gas reforming process for hydrogen production as well as abatement of ventilation air methane (VAM) in mining operations which is responsible for approximately 64% of methane emissions from the coal mining sector (note methane global warming potential is 25 times greater than that of CO2). Mitigation and/or utilization of VAM are difficult primarily because: (i) the volume of the gas mixture is large (can be as high as 600 m3/ s); (ii) the methane concentration in the mixture is dilute (0.1–1%V/V), and (iii) the concentration of methane and the flow rate of the gas mixture show significant temporal variations. VAM mitigation systems based on principal use of methane (as opposed to ancillary use) are more attractive as they offer a more flexible and robust solution. Examples of such systems include: TFRR (thermal flow reversal reactors), CFRR (catalytic flow reversal reactors), CMR (catalytic monolith reactors), CLBGT (catalytic leanburn gas turbines), RLBGT (recuperative lean-burn gas turbines), and RAB (regenerative after burners). The above technologies suffer from a range of generic problems including: (a) performance degradation due to temporal variations in the VAM flow rate and concentration, (b) limited capacity to cope with VAM concentrations between 0.1% and 0.3%, (c) high energy demand and the need for secondary fuel at VAM concentrations less than 0.3%, (d) inherent safety and heat management issues, (e) limited degree of adaptability for integration with other technology options at mine site for combined heat and power generation. A chemical looping based process can resolve these shortcomings in an effective manner. The aim of the current study is to narrow down the above knowledge gap by investigating the reduction reactivity of Fe2O3/ Al2O3 and CuO/SiO2 with ultra low concentration CH4 over a range of temperatures between 873 K and 1073 K as they have proven to be promising oxygen carriers in chemical looping combustion of CH4.
Fig. 2. Scheme of TGA experiments.
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2. Experimental The reduction and oxidation reactivity were examined in a thermogravimetric analyser (TA Q50, refer to Fig. 2). The samples (10–15 mg) were placed in a platinum crucible (100 lL) and heated from the ambient temperature to the desired reaction temperatures (873–1073 K) at a constant heating rate of 20 K min1 under the inert gas flow of N2. For the reduction reaction, the reducing gas (i.e., CH4 balanced by N2) was introduced into the furnace and passed over the sample after the desired temperature was reached. In order to examine the true oxidation conversion, the sample was first reduced by 10 vol% H2 balanced by N2 for 30 min then purged with a nitrogen flow for over 10 min (this is a common practice given that the use of a hydrocarbon fuel may cause carbon deposition leading to errors in calculating the true conversion levels). Next, the oxidation step was started in the oxidant atmosphere at the reaction temperatures and lasted for 10 min. All experiments were conducted under the following conditions: (1) gas flow rate of 200 mL min1, (2) sample weight of around 10–15 mg and (3) sample particle size of 75–150 lm. These set conditions were selected to minimize the external and/or internal diffusion effects and hence promote chemically controlled reactions. The particle samples, Fe2O3/Al2O3 and CuO/SiO2, were prepared using dry impregnation method details of which can be found elsewhere [25]. All samples were calcined at 873 K for 3 h in a muffle furnace in air and sintered further at 1223 K for 6 h, and then sieved to a uniform size of 75–150 lm. Almost the same loading content of Fe2O3 and CuO were adopted (about 45 wt% and 48 wt%, respectively) to minimize the effect of loading content on the reaction reactivity. The reduction and oxidation reactivity figures were evaluated using the TGA data and represented by the fractional conversion of oxygen carriers, Xred and Xod, respectively. The conversion is defined as:
X red ¼
M ox M M ox M red
X od ¼ 1
M ox M M ox Mred
ð1Þ Fig. 3. A typical redox cycle of Fe2O3 (a) and CuO (b) by CH4.
ð2Þ
where Mox is the weight of metal oxide in its oxidation state; Mred the sample weight in reduction state and M the instantaneous weight of the sample. The transitional states of metal oxides are Fe2O3/Fe3O4 and CuO/Cu. When using Fe2O3 as oxygen carrier, the following redox reactions were involved:
12Fe2 O3 þ CH4 ! 8Fe3 O4 þ CO2 þ 2H2 O
ðdÞ
4Fe3 O4 þ CH4 ! 12FeO þ CO2 þ 2H2 O
ðeÞ
4Fe3 O4 þ O2 ! 6Fe2 O3
ðfÞ
While for CuO as oxygen carrier:
4CuO þ CH4 ! 4Cu þ CO2 þ 2H2 O
ðgÞ
2Cu þ O2 ! 2CuO
ðhÞ
4CuO ! 2Cu2 O þ O2
ðiÞ
The fractional conversion data as a function of time was fitted to obtain the polynomial regression equation. The global rates of reactions (dX/dt) at different fractional conversions (X) were calculated by differentiating a fifth-order polynomial equation.
3. Results 3.1. Redox reactivity of copper oxide and iron oxide To determine the reduction reactivity accurately, each test was conducted for five cycles and the data corresponding to the fifth cycle was used in relevant analyses. A typical redox cycle for Fe2O3 and CuO is illustrated in Fig. 3. As can be seen, the reduction of Fe2O3 to Fe3O4 is completed at a high reaction rate. Then Fe3O4 continues to convert to FeO with a lower rate. In contrast, either the oxidation of FeO to Fe3O4 or Fe3O4 to Fe2O3 is completed rapidly at a relatively steady rate. On the contrary, there is no evident turn point during the reduction of CuO to Cu and the possible reason for the decrease in the reaction rate is the increasing resistance on the surface layer as the reaction proceeds. It is also clear from Fig. 3 that the oxidation of Cu to CuO is remarkably faster than its reduction which is in good agreement with the date reported in the literature [26]. To simplify, the reduction reactivity was investigated with methane under three different concentrations (i.e., 0.1, 0.5 and 1 vol% in N2). The reaction temperatures varied from 873 K to 1073 K. The reduction conversion of Fe2O3 with CH4 is quite dependent on the reaction temperature, the methane concentration and the residence time. With the increases in the concentration of methane
Y. Zhang et al. / Applied Energy 113 (2014) 1916–1923
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Fig. 4. Fractional conversion as a function of time for Fe2O3/Al2O3 at temperatures of (j) 873 K, (h) 923 K, (N) 973 K, (4) 1023 K and (s) 1073 K: reduction in (a) 1 vol% CH4, (b) 0.5 vol% CH4 and (c) 0.1 vol% CH4; (d) oxidation in diluted air.
and the temperature, the conversion is ascending at a constant residence time (see Fig. 4a–c). For instance, with the reducing gas of 1 vol% CH4, at 873 K and t = 30 s, the conversion is about 20.3%. It increases to 37.7%, 50% and 73.7% as the temperature rises to 923, 973 and 1073 K, respectively. The conversion decreases to 14.3% and 5.8% with the decrease of methane concentration to 0.5 vol% and 0.1 vol%. Moreover, a longer residence time can produce a higher conversion. At the conditions of 873 K and 1 vol% CH4, the reduction conversion rises to 33.3% at t = 60 s from 20.3% at t = 30 s. Also it is demonstrated clearly in Fig. 4 that the reduction of Fe2O3 with CH4, especially at the lower temperatures, progresses at two stages. Apparently the initial stage has a high reaction rate which is then followed by a slow reaction. This is the result of different reaction resistances during these two stages, that is, chemical controlled reaction and diffusion controlled reaction. The conversion curves at 873 K are the examples of this phenomenon. However, the effect of diffusion is becoming negligible for the temperatures higher than 973 K. Interestingly, the reactivity difference between 1023 K and 1073 K is quite negligible for all investigated methane concentrations. The use of low reaction temperatures is of paramount importance because a low operation temperature has the capacity of saving energy and lowering the sintering risk. Similar results can be observed in the reduction of CuO/SiO2 with CH4 as shown in Fig. 5. In order to determine the reduction reactivity in an accurate manner, the test temperature is limited to 1023 K because the effect of the reaction (i) cannot be neglected for temperatures over 1023 K. As can be seen, the reduction rate is much slower than that for Fe2O3/Al2O3. The full conversion is
unable to complete even in 60 min over the most of test conditions. The shortest time to get X = 1 is almost 20 min at the condition of 1023 K and 1 vol% CH4. To be comparable, the required time for getting the conversion of X = 0.2, X = 0.5 and X = 0.8 over the range of conditions is listed in Table 1. Obviously, the reduction of Fe2O3 with CH4 is at least 10 times faster than that of CuO for the conditions listed here. In addition to the reduction reactivity, the oxidation reactivity of Fe2O3/Al2O3 was investigated in the air at different temperatures (873 K, 923 K, 973 K and 1073 K) and the results are shown in Fig. 4d. In order to obtain more details the oxidant is dilute and the oxygen concentration is 8.5 vol% O2 balanced with N2. For all range of the temperatures, the oxidation comprised of two steps (a fast chemical controlled step and a slow diffusion controlled step). For conversion (X) less than 0.8 the oxidation rate is very high and it takes 9–15 s to reach X = 0.8 (dependence on temperatures). Above 0.8 conversion the rate of conversion slows down and as a result the time to achieve 0.9 conversion becomes 12– 50 s for full conversion becomes 74–170 s. Also it is clear that the oxidation reactivity is increased with the increase in temperatures. However, the effect of temperature on the reactivity is quite subtle at the temperatures higher than 1023 K. Similar trends can be found during the oxidation of Cu/CuO shown in Fig. 5d. The Cu oxide is oxidized in the undiluted air at temperatures range between 873 K and 973 K. Though the oxygen concentration is higher, the oxidation rate of Cu/CuO is lower than Fe3O4/Fe2O3. Moreover, the turn point between first step and second step is around X = 0.65 which is lower than the transition of Fe3O4 to Fe2O3 at X = 0.8.
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Fig. 5. Fractional conversion as a function of time for CuO/SiO2 at temperatures of (j) 773 K, (h) 873 K, (s) 923 K , (N) 973 K and (4) 1023 K: reduction in (a) 1 vol%, (b) 0.5 vol%, (c) 0.1 vol% CH4 and (d) oxidation in air.
Table 1 The required time (s) to get the conversion of X = 0.2, X = 0.5 and X = 0.8 with Fe2O3/Al2O3 and CuO/SiO2. Conc.
Temp. (K)
Fe2O3/Al2O3
CuO/SiO2
t(X = 0.2)
t(X = 0.5)
t(X = 0.8)
t(X = 0.2)
t(X = 0.5)
t(X = 0.8)
1% CH4
T = 873 T = 923 T = 973 T = 1023 T = 1073
28.5 19 15 13 12.5
130 39.5 30 23.5 23
367 81 45.5 32.5 32
300 / 240 210 /
1074 / 612 450 /
2130 / 1140 750 /
0.5% CH4
T = 873 T = 923 T = 973 T = 1023 T = 1073
45.5 29.5 25.5 21 20.5
256 69 47.5 40.5 37.5
/ 186 72 59.5 53.5
1980 / 810 288 /
/ / 1980 612 /
/ / 3420 1020 /
0.1% CH4
T = 873 T = 923 T = 973 T = 1023 T = 1073
96 75 70.5 61 59
343 204 138.5 117 111
/ / 214 177.5 160
3120 / 1080 702 /
/ / 2640 1800 /
/ / / 3600 /
In summary, compared with 48 wt% CuO loading on SiO2, 45 wt% Fe2O3 supported by Al2O3 is a better oxygen carrier in terms of the redox reactivity with CH4 concentration as low as 1 vol% and air. The preferred reduction temperature is above 973 K because the reactivity decreases rapidly with the temperatures below 973 K. Additionally, the preferred oxidation conversion is between 80% and 90% given that the conversion curve plateaus for the conversions over 90%.
3.2. The global reaction rates (dX/dt) of reduction and oxidation of Fe2O3–Fe3O4 The reduction rates (dX/dt) at the conversions of X = 0.5 and various methane concentrations are shown in Fig. 6 as a function of temperature (873 K, 923 K, 973 K, 1023 K and 1073 K). The reaction rates at lower fractional conversion (<20%) were not analyzed because the concentrations of reaction gases cannot be accurately
Y. Zhang et al. / Applied Energy 113 (2014) 1916–1923
Fig. 6. The oxidation reactivity (}) and reduction reactivity in 1 vol% (s), 0.5 vol% (4) and 0.1 vol% CH4 (5) at X = 0.5.
determined due to the dilution of the reaction gas by the purge gas (N2) at the beginning of each reduction cycle. As can be seen, both the reaction temperature and methane concentration put a positive effect on the conversion rate, that is to say, the conversion rate is increasing with the increase in temperature and methane concentration. However, the curve as a function of temperatures is steeper with a higher methane concentration. The reduction rate at 1 vol% CH4 is increased by 1.81%/s to 2.2%/s at 1073 K from 0.39%/s at 873 K while it just increases by 1.13%/s to 1.32%/s and 0.31%/s to 0.45%/s at 0.5 vol% and 0.1 vol% CH4 respectively. It is also proven that the difference in the reduction rate between 1023 K and 1073 K is insignificant as mentioned earlier. The oxidation rate at X = 0.5 with air is also shown in the same figure (i.e. Fig. 6) at various temperatures. It is again shown that the oxidation rates are much higher than the reduction counterpart. The values are varied from 8.95%/s at 873 K to 10.65%/s at 1073 K.
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TGA at 1073 K. The reducing gas is 1 vol% CH4 balanced by N2. A redox cycle normally starts from a 10 min purge step, followed by the reduction which lasts for 10 min. After the reduction step, the purge gas is introduced again for 10 min. Then air goes through over the reduced sample and the oxidation starts and lasts for 5 min as well. After one cycle completes the next cycle starts. An extended 60-cycle set of redox test (the first cycle is not included as its reactivity is considerably low) was conducted in TGA at 1073 K. The results are shown in Fig. 7. The total operation time was around 15 h except for the purge time. Weight percentage equal to 100% means that the metal oxide is in the full oxidation state. However, when the sample is reduced the actual weight percentage would be between those of Fe3O4 (98.55%) and FeO (90%). Thus, the reduction weight percentage is typically around 96.7% indicating that sample in the reduced state is between Fe3O4 and FeO phases. It is demonstrated that the redox reactivity is quite stable during 60 cycles except for the first three cycles as the weight change for reduction and oxidation only has a slight difference which is contributed by the measurement error. The weight percentages for oxidation and reduction are almost stable at 100% and 96.7% respectively. The weight percentage for reduction state during the overall test period is slightly increasing to about 96.82% from 96.67%. This is equivalent of 4.5% decrease in the weight loss for the reduction step. It could be inferred that the global reaction rates (dX/dt) at X = 0.5 alter slightly. As the transition state of Fe2O3 to Fe3O4 considered, the global reduction and oxidation rates during 60 cycles were calculated and are illustrated in Fig. 8. For both reduction and oxidation rates, they show an initial increase followed by a decrease because the materials need to be activated. As expected, there is just a slight decrease in the reduction rates after 60 cycles. It is decreasing by 0.21%/s from 2.2%/s at fifth cycle to 1.99%/s at sixtieth cycle, equivalent to 9.5% loss in the total reaction rate. Similarly, the oxidation rate only decreases by 0.12%/s from 10.65%/s to 10.53%/s during 60 cycles. All in all, the deviation on either the weight percentage or global reaction rates is less than 10% which is promising in term of the long term operation.
4. Discussion 3.3. Cyclic tests of iron oxide To determine the stability of the selected oxygen carrier based on the reactivity principle, the cyclic test is carried out in the
The experimental results show that both Fe2O3/Fe3O4 and CuO/ Cu can be suitable oxygen carriers in chemical combustion of ultra low methane concentration even as low as 0.1 vol%. Although
Fig. 7. Weight change with reduction and oxidation during 60 cycles, (s) oxidation state, (4) reduction state and (—) theoretical state.
Fig. 8. The profile of global reaction rates during 60 cycles.
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Table 2 Theoretical oxygen transport capacity for different metal oxides with inert. MeO (wt%)
48 wt% CuO–Cu/SiO2
45 wt% Fe2O3–Fe3O4/Al2O3
OTC
0.096
0.01485
tion is 1 vol%. As shown in the plot, even with the same OTC, the reduction for copper oxide is substantially slower than iron oxide. It takes 172 s for CuO to achieve 100% conversion which is 4.5 times longer than that of iron oxide. The global reaction rates (dX/dt) for X = 0.2, 0.5, 0.8 have been calculated and plotted against the reduction conversion in Fig. 9b. It is clear that the reduction rates of Fe2O3/Al2O3 are in the range of 1.5%/s to 2.4%/s, which is about 4.3 times higher than that of CuO/SiO2 with the rates between 0.35%/s and 0.55%/s. 5. Conclusions In order to determine the performance of metal oxides in the chemical looping combustion of ultra low concentration methane, the redox reactivity of Fe2O3/Al2O3 and CuO/SiO2 were investigated at various methane concentrations (i.e., 0.1%, 0.5% and 1% in volume) over the temperatures between 873 K and 1073 K using a thermogravimetric analyzer. The key findings of this study are: The ultra low concentration of methane (below 1 vol%) can be oxidized by metal oxide over temperatures lower than 1073 K. The metal oxide transition of Fe2O3 to Fe3O4 shows higher reduction reactivity than CuO to Cu at low concentration methane (below 1 vol%) and low temperatures (below 1073 K). To achieve the full reduction conversion the temperatures over 973 K is preferred. Also the oxidation conversions less than 90% are more attractive as the plateau area in the conversion plot is avoid. The global reactivity (dX/dt) of reduction and oxidation is calculated at X = 0.5. The results showed that the reduction rates is temperature and concentration dependent, varying from 0.14%/s to 2.2%/s over the range of temperatures and methane concentrations. The oxidation rates are much higher than the reduction counterpart. The values are varied from 8.95%/s at 873 K to 10.65%/s at 1073 K. The cyclic stability of 48 wt% Fe2O3/Al2O3 is also studied over 60 cycles. It is realized that the redox reactivity of Fe2O3/Al2O3 is quite stable in 60 cycles at 1073 K. The weight percentages for oxidation and reduction are almost stable at 100% and 96.67% respectively. The deviation on weight loss and reaction rates is less than 10%.
Acknowledgments Fig. 9. Reduction conversion as a function of time (a) and reaction rates as a function of conversion (b) for Fe2O3/Al2O3 and CuO/SiO2 delivering the same OTC = 0.01485.
Fe2O3/Fe3O4 shows better redox reactivity at temperatures between 873 K and 1073 K, in terms of the oxygen transport capacity CuO/Cu results in much higher values. The oxygen transport capacity (OTC) for the respective active metal oxide can be defined by the oxygen content ratio in the reduced and oxidized forms through the following expression [3]:
OTC ¼
Mox M red Mox
ð3Þ
Table 2 shows the theoretical oxygen transport capacity for the selected metal oxides calculated from the above equation. It is shown that the transition of CuO to Cu deliver around 6.5 times higher OTC than the transition of Fe2O3 to Fe3O4. To be comparable, CuO/SiO2 with OTC equal to 0.01485 has been picked up and its reduction conversion as a function of time has been plotted in Fig. 9a. The reduction for both metal oxides has the same reaction conditions: (1) temperature is 1023 K and (2) methane concentra-
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Y. Zhang et al. / Applied Energy 113 (2014) 1916–1923 [9] Moghtaderi B. Review of the recent chemical looping process developments for novel energy and fuel applications. Energy Fuels 2012;26:15–40. [10] Siriwardane RV, Ksepko E, Tian H, Poston J, Simonyi T, Sciazko M. Interaction of iron–copper mixed metal oxide oxygen carriers with simulated synthesis gas derived from steam gasification of coal. Appl Energy 2013;107:111–23. [11] Zhang X, Jin H. Thermodynamic analysis of chemical-looping hydrogen generation. Appl Energy 2013. [12] Wang J, Anthony EJ. Clean combustion of solid fuels. Appl Energy 2008;85:73–9. [13] Corbella BM, De-Diego L, Garcia F, Adanez J, Palacios JM. The performance in a fixed bed reactor of copper-based oxides on titania as oxygen carriers for chemical looping combustion of methane. Energy Fuels 2005;19:433–41. [14] Corbella BM, de-Diego L, Garcia-Labiano F, Adanez J, Palacios JM. Characterization and performance in a multicycle test in a fixed-bed reactor of silica-supported copper oxide as oxygen carrier for chemical-looping combustion of methane. Energy Fuels 2005;20:148–54. [15] de Diego LF, Garcia-Labiano F, Adanez J, Gayan P, Abad A, Corbella BM, et al. Development of Cu-based oxygen carriers for chemical-looping combustion. Fuel 2004;83:1749–57. [16] Jerndal E, Mattisson T, Lyngfelt A. Thermal analysis of chemical-looping combustion. Chem. Eng. Res. Des. 2006;84:795–806. [17] Mattisson T, Lyngfelt A, Cho P. The use of iron oxide as an oxygen carrier in chemical-looping combustion of methane with inherent separation of CO2. Fuel 2001;80:1953–62. [18] Cho P, Mattisson T, Lyngfelt A. Comparison of iron-, nickel-, copper- and manganese-based oxygen carriers for chemical-looping combustion. Fuel 2004;83:1215–25.
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[19] Mattisson T, Johansson M, Lyngfelt A. Multicycle reduction and oxidation of different types of iron oxide particles application to chemical-looping combustion. Energy Fuels 2004;18:628–37. [20] Johansson M, Mattisson T, Lyngfelt A. Investigation of Fe2O3 with MgAl2O4 for chemical-looping combustion. Ind Eng Chem Res 2004;43:6978–87. [21] Zafar Q, Mattisson T, Gevert B. Redox investigation of some oxides of transition-state metals Ni, Cu, Fe, and Mn supported on SiO2 and MgAl2O4. Energy Fuels 2006;20:34–44. [22] Corbella BM, Palacios JM. Titania-supported iron oxide as oxygen carrier for chemical-looping combustion of methane. Fuel 2007;86:113–22. [23] Ortiz M, Gayan P, de-Diego LF, Garcia-Labiano F, Abad A, Pans MA, et al. Hydrogen production with CO2 capture by coupling steam reforming of methane and chemical-looping combustion: use of an iron-based waste product as oxygen carrier burning a PSA tail gas. J Power Sources 2010;196:4370–81. [24] Mattisson T, Jardnas A, Lyngfelt A. Reactivity of some metal oxides supported on alumina with alternating methane and oxygen application for chemicallooping combustion. Energy Fuels 2003;17:643–51. [25] Moghtaderi B, Song H. Reduction properties of physically mixed metallic oxide oxygen carriers in chemical looping combustion. Energy Fuels 2010;24:5359–68. [26] Garcia-Labiano F, Diego LFd, Adanez J, Abad A, Gayan P. Reduction and oxidation kinetics of a copper-based oxygen carrier prepared by impregnation for chemical-looping combustion. Ind Eng Chem Res 2004;43:8168–77.
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Applied Energy journal homepage: www.elsevier.com/locate/apenergy
Chemical looping combustion of ultra low concentration of methane with Fe2O3/Al2O3 and CuO/SiO2 Yongxing Zhang a, Elham Doroodchi b, Behdad Moghtaderi a,⇑ a
Priority Research Centre for Energy, Chemical Engineering, School of Engineering, Faculty of Engineering & Built Environment, The University of Newcastle, Australia Priority Research Centre for Advanced Particle Processing & Transport, Chemical Engineering, School of Engineering, Faculty of Engineering & Built Environment, The University of Newcastle, Australia b
h i g h l i g h t s Chemical looping combustion can be extended to the combustion of ultra low concentration of methane. Compared with CuO/SiO2, Fe2O3/Al2O3 shows better performance in terms of reduction reactivity. The redox reactivity of Fe2O3/Al2O3 is stable over 60 cycles.
a r t i c l e
i n f o
Article history: Received 13 December 2012 Received in revised form 30 May 2013 Accepted 5 June 2013 Available online 3 July 2013 Keywords: Chemical looping combustion Ultra low concentration methane Fe2O3/Al2O3 CuO/SiO2
a b s t r a c t This study examines the performance of two metal oxide species in oxidizing ultra low concentration of methane (below 1% in volume). The focus on low methane concentrations are driven by its practical importance in applications such as abatement of ventilation air methane (VAM) in mining operations. Two mixed metal oxides, Fe2O3/Al2O3 and CuO/SiO2, were selected as oxygen carriers and prepared using dry impregnation method. The metal oxide loading contents are found to be 45 wt% and 48 wt%, respectively. The redox reactivity of the selected oxygen carriers were studied at various methane concentrations (i.e., 0.1%, 0.5% and 1% in volume) and temperatures between 873 K and 1073 K using a thermogravimetric analyzer. At low methane concentrations and low temperatures (below 1073 K) the conversion of Fe2O3 to Fe3O4 showed higher reduction reactivity than the reduction of CuO to Cu. The redox reactivity of Fe2O3/Al2O3 was also found to be quite stable even after 60 redox cycles at 1073 K. The respective weight percentages for oxidation and reduction were found to be 100% and 96.67%, corresponding to a full oxidized state Fe2O3 and a reduced state between Fe3O4 and FeO respectively. Moreover, the results for the global reactivity of reduction and oxidation (calculated at X = 0.5) showed that the reduction rates were temperature and concentration dependent, varying from 0.14%/s to 2.2%/s over the range of temperatures and methane concentrations of interest. The oxidation rates were much higher than their reduction counterpart. The values varied from 8.95%/s at 873 K to 10.65% at 1073 K. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction Chemical looping combustion is an emerging technology option with inherent ability for CO2 separation and hence the capacity to facilitate the post-combustion removal of carbon dioxide, a step common to all advanced low mission coal technologies. The CLC process (as shown in Fig. 1) is commonly carried out in a two-step reduction/oxidation (redox) reaction by circulating metal oxide particles between two connected reactors (see reactions ‘‘a’’ to ‘‘c’’). Fuel is burnt with oxygen carrier particles, e.g. metal oxide (MenOm), in a Fuel Reactor (FR) while oxygen carrier particles are reduced to MenOms. The oxygen carriers are then transferred to ⇑ Corresponding author. Tel.: +61 2 49854411; fax: +61 2 49216893. E-mail address: [email protected] (B. Moghtaderi). 0306-2619/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.apenergy.2013.06.005
an Air Reactor (AR) to react with air and oxidize to their original oxidation state (this is commonly referred to as regeneration). Carbon dioxide and steam are the main products from the fuel reactor although minute quantities of CO, CH4, H2 and other hydrocarbons can also form. The product gas from Air Reactor primarily consists of N2 and excess oxygen. As noted, CO2 and N2 do not mix in the CLC process and as such the operational costs and energy efficiency are both improved dramatically compared with conventional combustion.
Reduction : 4Men Om þ sCH4 ! 4Men Oms þ sCO2 þ 2sH2 O
ðaÞ
Oxidation : 2Men Oms þ sO2 ! 2Men Om
ðbÞ
Overall reaction : CH4 þ 2O2 ! CO2 þ 2H2 O
ðcÞ
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Depleted air
CO2 rich gas
Me Fuel Reactor
Air Reactor MeO
Fuel
Air
Fig. 1. Scheme of chemical looping combustion.
CLC combustion has been studied for many years since it was first proposed [1]. Much of the work, however, has focused on the investigation of oxygen carriers and their viability for use in different CLC systems [2–12]. Copper based oxygen carrier exhibits high reactivity with CH4 or syngas but the redox temperature is limited up to 1073 K due to low melting temperature. Adánez et al. [3] believed that the best Cu-based oxygen carriers were those supported by SiO2 or TiO2 sintered at 1223 K regarding the reactivity with CH4. The performance of CuO/TiO2 by wet-impregnation was investigated in a fixed bed of CLC for CH4 [13]. The performance of a copper oxide silica-supported was investigated in 20 cycles for CLC of CH4 [14]. High reactivity, almost complete fuel conversion to CO2 was achieved and no chemical or mechanical decay was observed. de Diego et al. [15] suggested that the chemical stability of copper based oxides was independent on binders or preparation methods while the mechanical properties were quite relied on the preparation methods through cyclic tests in TGA. Considering together chemical and mechanical stability, the impregnation method was better to be used for preparing Cu-based oxygen carriers. Fe-based metal oxides (Fe2O3/Fe3O4) are preferred as oxygen mediator due to good reactivity with gaseous fuels, high gas conversion and high melting temperature although the oxygen transfer capacity is not good (0.033). It is indicated that the transition state of Fe2O3 to Fe3O4 is able to fully converted CH4 or syngas to CO2 and H2O at 1073 K from thermodynamic aspects of view [16]. Mattisson et al. [17] believed that the reduction rate of Fe2O3 (dX/dt) with methane was a function of the solid conversion range (DX). To make an illustration, the conversion range is 11% and 33% for the reduction of Fe2O3 to Fe3O4 and Fe2O3 to FeO. Cho et al. [18] investigated the reactivity of 60 wt% Fe2O3/Al2O3 with 50 vol% CH4 balanced with H2O at 1223 K for the transition of Fe2O3–Fe3O4. Adánez et al. [3] thought that Fe2O3 supported by Al2O3 or ZrO2 shown high reactivity with CH4 at 1223 K for the transition phase of Fe2O3 to FeO. The solid conversion of 90% can be reached within less than 1 min. Mattisson et al. [19] developed 27 oxygen carriers composed of 40–80 wt% Fe2O3, together with Al2O3, ZrO2, TiO2 or MgAl2O4 by freeze granulation and sintered at temperatures of 1223–1673 K. The study showed that Fe2O3 supported with MgAl2O4, ZrO2 or Al2O3 sintered at temperature of 1223 K or 1373 K exhibited good reactivity with CH4 at 1223 K. Johansson et al. [20] investigated the reduction reactivity of Fe2O3/MgAl2O4 to Fe3O4 with 50 vol% CH4 at 1223 K. It was shown that 60 wt% Fe2O3/MgAl2O4 sintered at 1373 K was the best oxygen carrier considering together crushing strength and reactivity. Zafar et al. [21] studied the reactivity of Fe2O3 supported by SiO2 or MgAl2O4 with 10 vol% CH4 at 1073–1273 K. It seemed that Fe2O3/SiO2 maybe not feasible oxygen carrier due to the formation of silicate at high temperature. The Fe2O3/MgAl2O4 showed high reactivity during the phase of Fe2O3 to Fe3O4 but very slow for
Fe3O4 to FeO. Corbella and Palacios [22] tested the titania supported iron oxide as oxygen carrier in CLC of CH4 at 1173 K. The developed metal oxide showed acceptable reactivity and durability but low oxygen capacity due to the formation of irreversible FeTiO3 after the first cycle. Ortiz et al. [23] investigated the behavior of an iron waste from aluminum manufacturer as oxygen carrier for combustion the PSA tail gas from a steam methane reforming plant in a 500Wth CLC prototype. Although a large body of research [4,24] has been conducted on the reactivity of Fe2O3 and CuO with CH4, studies related to the reactivity of metal oxides under ultra low methane concentrations (i.e. 0.1–1 vol%) are generally scarce. However, the ultra low methane concentrations are of significant practical importance in applications such neutralization of residual methane in the retentate stream from a natural gas reforming process for hydrogen production as well as abatement of ventilation air methane (VAM) in mining operations which is responsible for approximately 64% of methane emissions from the coal mining sector (note methane global warming potential is 25 times greater than that of CO2). Mitigation and/or utilization of VAM are difficult primarily because: (i) the volume of the gas mixture is large (can be as high as 600 m3/ s); (ii) the methane concentration in the mixture is dilute (0.1–1%V/V), and (iii) the concentration of methane and the flow rate of the gas mixture show significant temporal variations. VAM mitigation systems based on principal use of methane (as opposed to ancillary use) are more attractive as they offer a more flexible and robust solution. Examples of such systems include: TFRR (thermal flow reversal reactors), CFRR (catalytic flow reversal reactors), CMR (catalytic monolith reactors), CLBGT (catalytic leanburn gas turbines), RLBGT (recuperative lean-burn gas turbines), and RAB (regenerative after burners). The above technologies suffer from a range of generic problems including: (a) performance degradation due to temporal variations in the VAM flow rate and concentration, (b) limited capacity to cope with VAM concentrations between 0.1% and 0.3%, (c) high energy demand and the need for secondary fuel at VAM concentrations less than 0.3%, (d) inherent safety and heat management issues, (e) limited degree of adaptability for integration with other technology options at mine site for combined heat and power generation. A chemical looping based process can resolve these shortcomings in an effective manner. The aim of the current study is to narrow down the above knowledge gap by investigating the reduction reactivity of Fe2O3/ Al2O3 and CuO/SiO2 with ultra low concentration CH4 over a range of temperatures between 873 K and 1073 K as they have proven to be promising oxygen carriers in chemical looping combustion of CH4.
Fig. 2. Scheme of TGA experiments.
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2. Experimental The reduction and oxidation reactivity were examined in a thermogravimetric analyser (TA Q50, refer to Fig. 2). The samples (10–15 mg) were placed in a platinum crucible (100 lL) and heated from the ambient temperature to the desired reaction temperatures (873–1073 K) at a constant heating rate of 20 K min1 under the inert gas flow of N2. For the reduction reaction, the reducing gas (i.e., CH4 balanced by N2) was introduced into the furnace and passed over the sample after the desired temperature was reached. In order to examine the true oxidation conversion, the sample was first reduced by 10 vol% H2 balanced by N2 for 30 min then purged with a nitrogen flow for over 10 min (this is a common practice given that the use of a hydrocarbon fuel may cause carbon deposition leading to errors in calculating the true conversion levels). Next, the oxidation step was started in the oxidant atmosphere at the reaction temperatures and lasted for 10 min. All experiments were conducted under the following conditions: (1) gas flow rate of 200 mL min1, (2) sample weight of around 10–15 mg and (3) sample particle size of 75–150 lm. These set conditions were selected to minimize the external and/or internal diffusion effects and hence promote chemically controlled reactions. The particle samples, Fe2O3/Al2O3 and CuO/SiO2, were prepared using dry impregnation method details of which can be found elsewhere [25]. All samples were calcined at 873 K for 3 h in a muffle furnace in air and sintered further at 1223 K for 6 h, and then sieved to a uniform size of 75–150 lm. Almost the same loading content of Fe2O3 and CuO were adopted (about 45 wt% and 48 wt%, respectively) to minimize the effect of loading content on the reaction reactivity. The reduction and oxidation reactivity figures were evaluated using the TGA data and represented by the fractional conversion of oxygen carriers, Xred and Xod, respectively. The conversion is defined as:
X red ¼
M ox M M ox M red
X od ¼ 1
M ox M M ox Mred
ð1Þ Fig. 3. A typical redox cycle of Fe2O3 (a) and CuO (b) by CH4.
ð2Þ
where Mox is the weight of metal oxide in its oxidation state; Mred the sample weight in reduction state and M the instantaneous weight of the sample. The transitional states of metal oxides are Fe2O3/Fe3O4 and CuO/Cu. When using Fe2O3 as oxygen carrier, the following redox reactions were involved:
12Fe2 O3 þ CH4 ! 8Fe3 O4 þ CO2 þ 2H2 O
ðdÞ
4Fe3 O4 þ CH4 ! 12FeO þ CO2 þ 2H2 O
ðeÞ
4Fe3 O4 þ O2 ! 6Fe2 O3
ðfÞ
While for CuO as oxygen carrier:
4CuO þ CH4 ! 4Cu þ CO2 þ 2H2 O
ðgÞ
2Cu þ O2 ! 2CuO
ðhÞ
4CuO ! 2Cu2 O þ O2
ðiÞ
The fractional conversion data as a function of time was fitted to obtain the polynomial regression equation. The global rates of reactions (dX/dt) at different fractional conversions (X) were calculated by differentiating a fifth-order polynomial equation.
3. Results 3.1. Redox reactivity of copper oxide and iron oxide To determine the reduction reactivity accurately, each test was conducted for five cycles and the data corresponding to the fifth cycle was used in relevant analyses. A typical redox cycle for Fe2O3 and CuO is illustrated in Fig. 3. As can be seen, the reduction of Fe2O3 to Fe3O4 is completed at a high reaction rate. Then Fe3O4 continues to convert to FeO with a lower rate. In contrast, either the oxidation of FeO to Fe3O4 or Fe3O4 to Fe2O3 is completed rapidly at a relatively steady rate. On the contrary, there is no evident turn point during the reduction of CuO to Cu and the possible reason for the decrease in the reaction rate is the increasing resistance on the surface layer as the reaction proceeds. It is also clear from Fig. 3 that the oxidation of Cu to CuO is remarkably faster than its reduction which is in good agreement with the date reported in the literature [26]. To simplify, the reduction reactivity was investigated with methane under three different concentrations (i.e., 0.1, 0.5 and 1 vol% in N2). The reaction temperatures varied from 873 K to 1073 K. The reduction conversion of Fe2O3 with CH4 is quite dependent on the reaction temperature, the methane concentration and the residence time. With the increases in the concentration of methane
Y. Zhang et al. / Applied Energy 113 (2014) 1916–1923
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Fig. 4. Fractional conversion as a function of time for Fe2O3/Al2O3 at temperatures of (j) 873 K, (h) 923 K, (N) 973 K, (4) 1023 K and (s) 1073 K: reduction in (a) 1 vol% CH4, (b) 0.5 vol% CH4 and (c) 0.1 vol% CH4; (d) oxidation in diluted air.
and the temperature, the conversion is ascending at a constant residence time (see Fig. 4a–c). For instance, with the reducing gas of 1 vol% CH4, at 873 K and t = 30 s, the conversion is about 20.3%. It increases to 37.7%, 50% and 73.7% as the temperature rises to 923, 973 and 1073 K, respectively. The conversion decreases to 14.3% and 5.8% with the decrease of methane concentration to 0.5 vol% and 0.1 vol%. Moreover, a longer residence time can produce a higher conversion. At the conditions of 873 K and 1 vol% CH4, the reduction conversion rises to 33.3% at t = 60 s from 20.3% at t = 30 s. Also it is demonstrated clearly in Fig. 4 that the reduction of Fe2O3 with CH4, especially at the lower temperatures, progresses at two stages. Apparently the initial stage has a high reaction rate which is then followed by a slow reaction. This is the result of different reaction resistances during these two stages, that is, chemical controlled reaction and diffusion controlled reaction. The conversion curves at 873 K are the examples of this phenomenon. However, the effect of diffusion is becoming negligible for the temperatures higher than 973 K. Interestingly, the reactivity difference between 1023 K and 1073 K is quite negligible for all investigated methane concentrations. The use of low reaction temperatures is of paramount importance because a low operation temperature has the capacity of saving energy and lowering the sintering risk. Similar results can be observed in the reduction of CuO/SiO2 with CH4 as shown in Fig. 5. In order to determine the reduction reactivity in an accurate manner, the test temperature is limited to 1023 K because the effect of the reaction (i) cannot be neglected for temperatures over 1023 K. As can be seen, the reduction rate is much slower than that for Fe2O3/Al2O3. The full conversion is
unable to complete even in 60 min over the most of test conditions. The shortest time to get X = 1 is almost 20 min at the condition of 1023 K and 1 vol% CH4. To be comparable, the required time for getting the conversion of X = 0.2, X = 0.5 and X = 0.8 over the range of conditions is listed in Table 1. Obviously, the reduction of Fe2O3 with CH4 is at least 10 times faster than that of CuO for the conditions listed here. In addition to the reduction reactivity, the oxidation reactivity of Fe2O3/Al2O3 was investigated in the air at different temperatures (873 K, 923 K, 973 K and 1073 K) and the results are shown in Fig. 4d. In order to obtain more details the oxidant is dilute and the oxygen concentration is 8.5 vol% O2 balanced with N2. For all range of the temperatures, the oxidation comprised of two steps (a fast chemical controlled step and a slow diffusion controlled step). For conversion (X) less than 0.8 the oxidation rate is very high and it takes 9–15 s to reach X = 0.8 (dependence on temperatures). Above 0.8 conversion the rate of conversion slows down and as a result the time to achieve 0.9 conversion becomes 12– 50 s for full conversion becomes 74–170 s. Also it is clear that the oxidation reactivity is increased with the increase in temperatures. However, the effect of temperature on the reactivity is quite subtle at the temperatures higher than 1023 K. Similar trends can be found during the oxidation of Cu/CuO shown in Fig. 5d. The Cu oxide is oxidized in the undiluted air at temperatures range between 873 K and 973 K. Though the oxygen concentration is higher, the oxidation rate of Cu/CuO is lower than Fe3O4/Fe2O3. Moreover, the turn point between first step and second step is around X = 0.65 which is lower than the transition of Fe3O4 to Fe2O3 at X = 0.8.
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Fig. 5. Fractional conversion as a function of time for CuO/SiO2 at temperatures of (j) 773 K, (h) 873 K, (s) 923 K , (N) 973 K and (4) 1023 K: reduction in (a) 1 vol%, (b) 0.5 vol%, (c) 0.1 vol% CH4 and (d) oxidation in air.
Table 1 The required time (s) to get the conversion of X = 0.2, X = 0.5 and X = 0.8 with Fe2O3/Al2O3 and CuO/SiO2. Conc.
Temp. (K)
Fe2O3/Al2O3
CuO/SiO2
t(X = 0.2)
t(X = 0.5)
t(X = 0.8)
t(X = 0.2)
t(X = 0.5)
t(X = 0.8)
1% CH4
T = 873 T = 923 T = 973 T = 1023 T = 1073
28.5 19 15 13 12.5
130 39.5 30 23.5 23
367 81 45.5 32.5 32
300 / 240 210 /
1074 / 612 450 /
2130 / 1140 750 /
0.5% CH4
T = 873 T = 923 T = 973 T = 1023 T = 1073
45.5 29.5 25.5 21 20.5
256 69 47.5 40.5 37.5
/ 186 72 59.5 53.5
1980 / 810 288 /
/ / 1980 612 /
/ / 3420 1020 /
0.1% CH4
T = 873 T = 923 T = 973 T = 1023 T = 1073
96 75 70.5 61 59
343 204 138.5 117 111
/ / 214 177.5 160
3120 / 1080 702 /
/ / 2640 1800 /
/ / / 3600 /
In summary, compared with 48 wt% CuO loading on SiO2, 45 wt% Fe2O3 supported by Al2O3 is a better oxygen carrier in terms of the redox reactivity with CH4 concentration as low as 1 vol% and air. The preferred reduction temperature is above 973 K because the reactivity decreases rapidly with the temperatures below 973 K. Additionally, the preferred oxidation conversion is between 80% and 90% given that the conversion curve plateaus for the conversions over 90%.
3.2. The global reaction rates (dX/dt) of reduction and oxidation of Fe2O3–Fe3O4 The reduction rates (dX/dt) at the conversions of X = 0.5 and various methane concentrations are shown in Fig. 6 as a function of temperature (873 K, 923 K, 973 K, 1023 K and 1073 K). The reaction rates at lower fractional conversion (<20%) were not analyzed because the concentrations of reaction gases cannot be accurately
Y. Zhang et al. / Applied Energy 113 (2014) 1916–1923
Fig. 6. The oxidation reactivity (}) and reduction reactivity in 1 vol% (s), 0.5 vol% (4) and 0.1 vol% CH4 (5) at X = 0.5.
determined due to the dilution of the reaction gas by the purge gas (N2) at the beginning of each reduction cycle. As can be seen, both the reaction temperature and methane concentration put a positive effect on the conversion rate, that is to say, the conversion rate is increasing with the increase in temperature and methane concentration. However, the curve as a function of temperatures is steeper with a higher methane concentration. The reduction rate at 1 vol% CH4 is increased by 1.81%/s to 2.2%/s at 1073 K from 0.39%/s at 873 K while it just increases by 1.13%/s to 1.32%/s and 0.31%/s to 0.45%/s at 0.5 vol% and 0.1 vol% CH4 respectively. It is also proven that the difference in the reduction rate between 1023 K and 1073 K is insignificant as mentioned earlier. The oxidation rate at X = 0.5 with air is also shown in the same figure (i.e. Fig. 6) at various temperatures. It is again shown that the oxidation rates are much higher than the reduction counterpart. The values are varied from 8.95%/s at 873 K to 10.65%/s at 1073 K.
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TGA at 1073 K. The reducing gas is 1 vol% CH4 balanced by N2. A redox cycle normally starts from a 10 min purge step, followed by the reduction which lasts for 10 min. After the reduction step, the purge gas is introduced again for 10 min. Then air goes through over the reduced sample and the oxidation starts and lasts for 5 min as well. After one cycle completes the next cycle starts. An extended 60-cycle set of redox test (the first cycle is not included as its reactivity is considerably low) was conducted in TGA at 1073 K. The results are shown in Fig. 7. The total operation time was around 15 h except for the purge time. Weight percentage equal to 100% means that the metal oxide is in the full oxidation state. However, when the sample is reduced the actual weight percentage would be between those of Fe3O4 (98.55%) and FeO (90%). Thus, the reduction weight percentage is typically around 96.7% indicating that sample in the reduced state is between Fe3O4 and FeO phases. It is demonstrated that the redox reactivity is quite stable during 60 cycles except for the first three cycles as the weight change for reduction and oxidation only has a slight difference which is contributed by the measurement error. The weight percentages for oxidation and reduction are almost stable at 100% and 96.7% respectively. The weight percentage for reduction state during the overall test period is slightly increasing to about 96.82% from 96.67%. This is equivalent of 4.5% decrease in the weight loss for the reduction step. It could be inferred that the global reaction rates (dX/dt) at X = 0.5 alter slightly. As the transition state of Fe2O3 to Fe3O4 considered, the global reduction and oxidation rates during 60 cycles were calculated and are illustrated in Fig. 8. For both reduction and oxidation rates, they show an initial increase followed by a decrease because the materials need to be activated. As expected, there is just a slight decrease in the reduction rates after 60 cycles. It is decreasing by 0.21%/s from 2.2%/s at fifth cycle to 1.99%/s at sixtieth cycle, equivalent to 9.5% loss in the total reaction rate. Similarly, the oxidation rate only decreases by 0.12%/s from 10.65%/s to 10.53%/s during 60 cycles. All in all, the deviation on either the weight percentage or global reaction rates is less than 10% which is promising in term of the long term operation.
4. Discussion 3.3. Cyclic tests of iron oxide To determine the stability of the selected oxygen carrier based on the reactivity principle, the cyclic test is carried out in the
The experimental results show that both Fe2O3/Fe3O4 and CuO/ Cu can be suitable oxygen carriers in chemical combustion of ultra low methane concentration even as low as 0.1 vol%. Although
Fig. 7. Weight change with reduction and oxidation during 60 cycles, (s) oxidation state, (4) reduction state and (—) theoretical state.
Fig. 8. The profile of global reaction rates during 60 cycles.
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Table 2 Theoretical oxygen transport capacity for different metal oxides with inert. MeO (wt%)
48 wt% CuO–Cu/SiO2
45 wt% Fe2O3–Fe3O4/Al2O3
OTC
0.096
0.01485
tion is 1 vol%. As shown in the plot, even with the same OTC, the reduction for copper oxide is substantially slower than iron oxide. It takes 172 s for CuO to achieve 100% conversion which is 4.5 times longer than that of iron oxide. The global reaction rates (dX/dt) for X = 0.2, 0.5, 0.8 have been calculated and plotted against the reduction conversion in Fig. 9b. It is clear that the reduction rates of Fe2O3/Al2O3 are in the range of 1.5%/s to 2.4%/s, which is about 4.3 times higher than that of CuO/SiO2 with the rates between 0.35%/s and 0.55%/s. 5. Conclusions In order to determine the performance of metal oxides in the chemical looping combustion of ultra low concentration methane, the redox reactivity of Fe2O3/Al2O3 and CuO/SiO2 were investigated at various methane concentrations (i.e., 0.1%, 0.5% and 1% in volume) over the temperatures between 873 K and 1073 K using a thermogravimetric analyzer. The key findings of this study are: The ultra low concentration of methane (below 1 vol%) can be oxidized by metal oxide over temperatures lower than 1073 K. The metal oxide transition of Fe2O3 to Fe3O4 shows higher reduction reactivity than CuO to Cu at low concentration methane (below 1 vol%) and low temperatures (below 1073 K). To achieve the full reduction conversion the temperatures over 973 K is preferred. Also the oxidation conversions less than 90% are more attractive as the plateau area in the conversion plot is avoid. The global reactivity (dX/dt) of reduction and oxidation is calculated at X = 0.5. The results showed that the reduction rates is temperature and concentration dependent, varying from 0.14%/s to 2.2%/s over the range of temperatures and methane concentrations. The oxidation rates are much higher than the reduction counterpart. The values are varied from 8.95%/s at 873 K to 10.65%/s at 1073 K. The cyclic stability of 48 wt% Fe2O3/Al2O3 is also studied over 60 cycles. It is realized that the redox reactivity of Fe2O3/Al2O3 is quite stable in 60 cycles at 1073 K. The weight percentages for oxidation and reduction are almost stable at 100% and 96.67% respectively. The deviation on weight loss and reaction rates is less than 10%.
Acknowledgments Fig. 9. Reduction conversion as a function of time (a) and reaction rates as a function of conversion (b) for Fe2O3/Al2O3 and CuO/SiO2 delivering the same OTC = 0.01485.
Fe2O3/Fe3O4 shows better redox reactivity at temperatures between 873 K and 1073 K, in terms of the oxygen transport capacity CuO/Cu results in much higher values. The oxygen transport capacity (OTC) for the respective active metal oxide can be defined by the oxygen content ratio in the reduced and oxidized forms through the following expression [3]:
OTC ¼
Mox M red Mox
ð3Þ
Table 2 shows the theoretical oxygen transport capacity for the selected metal oxides calculated from the above equation. It is shown that the transition of CuO to Cu deliver around 6.5 times higher OTC than the transition of Fe2O3 to Fe3O4. To be comparable, CuO/SiO2 with OTC equal to 0.01485 has been picked up and its reduction conversion as a function of time has been plotted in Fig. 9a. The reduction for both metal oxides has the same reaction conditions: (1) temperature is 1023 K and (2) methane concentra-
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