Experimental and thermodynamic study on interaction of copper

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Experimental and thermodynamic study on interaction of copper oxygen carriers and oxide compounds commonly present in ashes Esraa Darwish, Duygu YILMAZ, and Henrik Leion Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b04060 • Publication Date (Web): 12 Feb 2019 Downloaded from http://pubs.acs.org on February 14, 2019

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EXPERIMENTAL AND THERMODYNAMIC STUDY ON INTERACTION OF COPPER OXYGEN CARRIERS AND OXIDE COMPOUNDS COMMONLY PRESENT IN ASHES Esraa Darwish, Duygu Yılmaz*, Henrik Leion Department of Chemical and Biological Engineering, Chalmers University of Technology, S-412 96 Göteborg, Sweden

ABSTRACT

The chemical-looping combustion (CLC) and chemical-looping oxygen uncoupling (CLOU) processes are unique and efficient methods for direct separation of carbon dioxide in combustion. In these processes, metal oxides are used under reducing atmosphere as an oxygen carrier to transfer oxygen between an air and a fuel reactor. The fuel is converted by oxygen provided by the oxygen carrier. In the case of using coal, or any ash containing fuel interaction between coal derived ash and the oxygen carrier is likely to occur and can lead to deactivation and agglomeration of the oxygen carriers. Since the amount of the possible compounds and compositions of ash can

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vary widely, thermodynamic equilibrium calculations can be used to represent the formed compounds during the CLC process to reveal the interaction between oxygen carrier and commonly present oxide compounds in ash.

In this study, the interaction between oxide

compounds commonly present in ash and CuO oxygen carriers were studied both experimentally and thermodynamically. CuO is a widely used oxygen carrier with CLOU properties, the ability to release gaseous oxygen under inert atmosphere. Experiments were carried out at 900°C under both oxidizing and inert atmosphere using where CuO or Cu2O (CuO/Cu2O) as oxygen carrier and SiO2, Al2O3, Fe2O3, CaO and K2O to represent oxide compounds present in ashes. To observe the interaction of oxygen carriers with each oxide compound used, equal mole of copper oxide and oxide compound were mixed. Further, oxide compound fractions with elemental composition relevant to coal ash were mixed with oxygen carriers to investigate the interaction under conditions approaching realistic operation. In all cases, a significant amount of copper oxides survived without any interaction. However, it was observed that silicate based formations, especially potassium silicates lead to strong agglomeration which most likely would decrease the lifetime and oxygen releasing ability of the oxygen carriers. As the results showed that thermodynamic equilibrium based calculations were well in line with the experiments, these calculations can be a good first approach in these type of investigations.

INTRODUCTION In recent years, CO2 emission from the combustion has increased due to the high energy demand across the world. Since CO2 emission is one of the most important reasons of global warming, methods have been developed to capture and store carbon (CCS) 1. Chemical looping combustion (CLC) is one of the most promising capturing methods due to its low energy demand

1,2

. In this

technique, interconnected fluidized beds are used as air and fuel reactors. The oxygen carrier cycles


2

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between the fuel and air reactor and pure CO2 is obtained from the fuel reactor. In the air reactor (Reaction 1), oxygen carriers are oxidized in air and in the fuel reactor they are reduced by a fuel (Reaction 2) 2,3. MxOy and MxOy−1 are the fully oxidized and reduced forms of the oxygen carrier. The scheme of the process is shown in Figure 1. 2𝑀𝑥 𝑂𝑦−1 + 𝑂2 → 2𝑀𝑥 𝑂𝑦

(1)

(2𝑛 + 𝑚)𝑀𝑥 𝑂𝑦 + 𝐶𝑛 𝐻2𝑚 → (2𝑛 + 𝑚)𝑀𝑥 𝑂𝑦−1 + 𝑛𝐶𝑂2 + 𝑚𝐻2 𝑂

(2)

CO2, H2O

N2, O2

Mx O y Air Reactor

Fuel Reactor

MxOy-2

Fuel ( +H2O/ CO2)

Air

Figure 1. Scheme of the CLC process. In a CLC system, solid, liquid and gaseous fuels can be used 4. For the use of solid fuels, the solids are first gasified to H2 or CO before further converted by the oxygen carrier to form H2O or CO2. The initial gasification step is comparably slow 5. In order to increase the reactivity of the solid fuel, an additional step should be created for the release of the gaseous oxygen from the oxygen carrier via chemical looping oxygen uncoupling (CLOU) 6. With CLOU, gaseous oxygen released by the oxygen carrier, according to reaction 3, reacts with the gasified fuel products or the fuel char leading to accelerated overall fuel conversion. The forward reaction can take place


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also in an oxygen-lean atmosphere such as pure CO2 flow, then gaseous oxygen can be released which can be used to oxidize a fuel 7. For these reason, CuO/Cu2O is a good candidate as oxygen carrier in CLOU. 4CuO(s) ⇌ 2Cu2O(s) + O2(g)

(3)

The CLOU oxygen carriers can be either oxides of transition metals (Mn, Fe, Co, Ni and Cu), their mixtures or natural ores of transition metals 8. However, it is known that using solid fuels can create problems due to possible interactions between oxygen carrier and ash forming matters, which can result in the agglomeration of oxygen carriers and decrease oxygen releasing ability 9,10. A considerable amount of research has been conducted on CuO/Cu2O based oxygen carriers both for CLC and CLOU 11–18. Cu-based oxygen carriers have shown high solid conversion rates and high oxygen transfer capacity. Further, Cu based oxygen carriers have no thermodynamic restrictions for complete conversion of fuel to CO2 and H2O

12,19

. In addition, Cu is not so

expensive and its use causes less environmental problems than nickel and cobalt based oxygen carriers 20. Cu-based oxygen carriers have received a great deal of interest in the past few years, due to their characteristics such as good oxygen transport and releasing ability, high reactivity to both solid and gaseous fuels and good thermodynamic properties to complete the oxidation of fuel 13

. CuO/Cu2O systems have been widely used mainly with supporting materials, but also as pure

oxides 7,12,13,21,22. However, studies about the effect of coal ash on the copper oxide oxygen carriers still do not reflect more than a fraction of the possible interaction between copper oxide and ash forming matters. Since the composition of ashes can be different depending on fuel, it is important to see the effect of each compound on the specific oxygen carrier.


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Since ash forming matters can show a significant effect on the lifetime and the oxygen releasing ability of oxygen carriers, understanding of the interaction between ash forming matters and oxygen carriers are of great importance. The literature on the effect of solid fuel derived ash on the copper based oxygen carriers in CLOU is still very limited, although some investigations have been recently carried out on solid fuels since solid fuels are the most abundant fuel type 10,23,24. For this reason, investigation of interaction between oxygen carriers and ash forming matters is needed to match fuel and oxygen carrier. Interaction between the ash forming matters and oxygen carriers can lead to agglomeration and deactivation of oxygen carriers 24. Although separating the ash and oxygen carrier may be possible via utilizing the difference in particle density and size, a significant solid-solid contact in the fuel reactor where the fuel and oxygen carriers are mixed will occur. Previous investigations have shown that the effect of different coal ashes on oxygen carriers vary depending on the ash content, composition of the ash as well as system parameters such as temperature and number of cycles

25–28

. The depositions of coal ash on the surface of oxygen

carriers can also be a problem encountered in CLC and CLOU 29. This study was focused on ash forming matters with form of common oxides such as SiO2, Al2O3, Fe2O3, CaO and K2O. This work also investigates the effect of one by one interaction between pure oxide compounds present in ashes and copper oxides and only pure oxides were used to provide basic understanding of the interaction independent of fuel. In the literature, there are a few studies on CuO based systems, both experimental work and thermodynamic calculations 10,24, but no work has been performed on the pure CuO system and one-by-one interaction of oxide compounds with CuO or Cu2O (CuO/Cu2O). In this work, the aim was to make a systematic study to compare experimental results and thermodynamic equilibrium based calculations on the interaction between common ash forming matters and pure copper oxides. This study, was carried


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out by means of experiments and thermodynamic equilibrium calculation simulations at conditions relevant to CLOU. EXPERIMENTAL SECTION Preparation of the Oxygen Carriers Copper oxide granules were prepared by mechanical mixing. Pure copper oxides as Cu2O (Fisher ScientificTM, >97.6%) or CuO (MerckTM, >99%) were used to obtain granules via wet granulation technique. In order to obtain a well dispersed slurry, 500 ml of deionized water and one mole of CuO or Cu2O were mixed thoroughly. The obtained slurries were mixed for 24 hours at room temperature and then dried at 110 °C for 24 hours. Coarse particles were ground and sieved to prepare oxygen carriers with a size range of 80-125 µm. Figure 2 shows the oxygen carriers used as starting materials. To observe the effect of heat treatment on oxygen carriers without using any oxide compound, heat treatment was carried out. Cu2O oxygen carriers formed CuO oxygen carriers (Figure 2.a) after heat treatment at 900°C for 10 hours under oxidizing atmosphere (OA) and CuO oxygen carriers formed Cu2O (Figure 2.b) oxygen carriers after heat treatment at 900°C for 10 hours under inert atmosphere (IA).


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Figure 2. CuO (a) and Cu2O (b) oxygen carriers after heat treatment at 900°C for 10 hours. Experimental procedure and characterization To investigate the interaction between copper oxides and oxide compounds commonly present in ashes, CuO or Cu2O particles were mixed with a number of oxide compounds commonly present in ash in different combinations. Throughout this paper, the oxide based ash forming matters used in this study will be referred to as oxide compounds and the CuO or Cu2O particles will be referred to as oxygen carrier. The first group of the experiments was carried out with a one to one molar mix between copper oxide oxygen carriers (CuO or Cu2O) and the oxides used. The same molar amounts of each of the oxide compound and oxygen carriers were mixed in an agate mortar for 4 hours. For the second group of the experiments, a definite amount of oxides representing a common fuel ash composition was used according to Table 1. The composition was obtained from Åbo


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Akademi Chemical Fractionation Database and a literature study about coal ash30. Detailed information about elemental analysis, such as max. and min. values and standard deviation can be found both in the literature study and Åbo Akademi Chemical Fractionation Database, which 21 different coal samples were investigated to reveal chemical fractionations and elemental analysis as oxides 30,31. Mixtures were prepared as 1 gram synthetic ash for 1 gram of oxygen carrier. To obtain synthetic ash, chemical composition of the ash in Table 1 was taken as a reference. Then, oxide compounds were mixed to prepare mixtures as CuO/Cu2O-aSiO2-bAl2O3-cFe2O3-dCaO-eK2O (a-e correspond to mole amount), depending on the selected combination of compounds based on amounts in Table 1. To investigate as many interactions as possible between oxide compounds and CuO/Cu2O systems, experiments were carried out for all systems by using CuO/Cu2O in a combination with 2, 3 4 and last 5 different oxide compounds. The mixtures of Cu2O and oxide compounds were placed in a tube furnace under oxidizing atmosphere to simulate the air reactor so Cu2O could be oxidized to form CuO. The mixtures of CuO and oxide compounds were placed in a tube furnace under inert atmosphere (N2) to simulate the fuel reactor so CuO could be reduced to form Cu2O at 900°C. In both cases the samples were left in the furnace for 10 hours. Throughout this paper, oxidizing and inert conditions will be mentioned as oxidizing atmosphere (OA) and inert atmosphere (IA), respectively. The gas flow rate was set to 100 ml N2 or air per minute for all experiments. The obtained powders were characterized by X-ray diffraction (Siemens TM D5000, Cu-Kα, 40 kV, 40 mA) in the range 2ϴ=1580° with a step size of 0.01 and Scanning Electron Microscope (ZeissTM LEO Ultra 55 FEG) for phase analysis and morphological characteristics, respectively. Quantitative phase analysis was carried out via using RIR (reference intensity ratio) method to make an approach to reveal the


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qualification and quantification of the phases and these results were combined with Rietveld Refinement technique 32–34. For this reason, it should be noted that the obtained results should be considered as an approach since the accuracy of the methods is limited. SEM-EDX was also used to investigate the chemical analysis of the agglomerates. Table 1. The chemical composition of the reference ash and the amount of the compounds used in the second group of the experiments in the study (mole). Reference Ash

Prepared Mixtures

Mean Amount Amount of

Amount of

of Oxide Based Oxide

component used

component per

per mole of

mole of CuO

Cu2O (mole)

(mole)

Ash Forming Compound

Matter in Ash30,31* (%)

SiO2

55

a

1.31

0.73

Al2O3

26

b

0.36

0.20

Fe2O3

7

c

0.06

0.03

CaO

5

d

0.13

0.07

K2O

2

e

0.03

0.02

Others

5

* Ash forming matters are presented as oxides.

Thermodynamic equilibrium calculations FactSage® 7.2 software was used for thermodynamic equilibrium calculations of the oxygen carriers and oxide compounds interactions. Calculations were performed for 900°C using the


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FactSage-Equilibrium Module, under the presumption of isothermal and standard state with the use of Pure Substance (FactPS) - Oxide (FToxid) databases. Equilibrium calculations were based on the Gibbs energy minimization method. In this method, elements or compounds react or partially react to reach a state of chemical equilibrium. In order to calculate the multi-component equilibrium compositions, amounts and properties of raw materials were given and the chemical reaction characteristics included phases and species were specified in the system. Calculated gaseous species within the amount of less than 0.001 moles were neglected to simplify the results. Throughout this paper, thermodynamic equilibrium calculations will be called as TEC. RESULTS Part I. Interaction Between CuO/Cu2O and Single Oxide Compound To investigate the interaction between copper oxides and oxide compound, each experiment was carried out at 900°C under both OA and IA, respectively. Generally, there was an unexpected and small amount of copper oxide which could not be converted totally by the used atmosphere. Some color differences between top and bottom of the sample showed that the complete conversion of copper oxide could not be obtained, most certainly due to the lack of the gas contact with the inner part of the powder mixture. Table 2 presents the obtained phases analysed using XRD and observed phases using TEC on the interaction between CuO/Cu2O to one of the oxide compounds.

Table 2. Comparison of the experimentally obtained phases analysed using XRD and observed phases using TEC on the interaction between CuO/Cu2O to one of the oxide compounds.

Conditions CuO-SiO2

OA

Compounds XRD Analysis 72.9% CuO 3.6% Cu2O 23.5% SiO2

TEC 72.69% CuO 27.31% SiO2


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IA OA CuO-Al2O3 IA

OA CuO-Fe2O3 IA

OA CuO-CaO IA CuO-K2O

OA IA

49.7% Cu2O 4.3% CuO 45.9% SiO2 55.4% CuO 44.6% Al2O3 47.2% Cu2O 40.7% Al2O3 12.1% Cu2Al2O4 80.9% CuO 11.7% Fe2O3 4.4% CuFe2O4 33.1% CuO 12.7% Cu2O 19.6% Fe2O3 24.8% CuFe2O4 9.8% Cu2Fe2O4 66.1% CuO 1.6% CaO 19.4% CaCu2O3 12.9% Ca2CuO3 6.3% CuO 31.5% Cu2O 62.25% CaO 100% CuO 100% Cu2O

54.35% Cu2O 45.65% SiO2 60.92% CuO 39.08% Al2O3 32.49% Al2O3 67.51% Cu2Al2O4 24.95% CuO 75.05% CuO.Fe2O3 34.53% Fe2O3 65.47% Cu2O.Fe2O3

73.94% CuO 26.06% CaO

56.06% Cu2O 43.94% CaO 100% CuO 100% Cu2O


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Figure 3. XRD patterns of the CuO/Cu2O - SiO2 mixtures after experiments. The XRD patterns in Figure 3 show that there was no interaction between copper oxides and silica for any conditions. This was confirmed by TEC where no new phase could be obtained. XRD analysis revealed that CuO formation over Cu2O and Cu2O formation over CuO can take place under OA and IA (Eqn. 3), respectively. However, during oxidizing conditions, when SiO2 was used as oxide compound, the gas-solid contact was insufficient and some Cu2O could not be converted to CuO. Among various supporting materials for CuO based oxygen carriers, Al2O3 has received considerable attention. On the other hand, it is also known that in the case of using Al2O3 as a supporting material in copper based oxygen carriers, a difficulty arises due to the facile interaction between CuO and Al2O3. This can occur either during synthesis or during operation which results in partial loss of CuO by formation of copper (Ⅱ) aluminate (CuAl2O4) and copper


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(Ⅰ) aluminate (CuAlO2) 7. However, since the copper-aluminate phases are highly reducible, the interaction between the supporting material and the active phase does not always constitute a problem with respect to CLC application 11. Also the use of CuAl2O4 as an oxygen carrier has been suggested as oxygen carrier due to its resistance to agglomeration and attrition. Moreover, the CuAl2O4 spinel has higher oxygen transport capacity than CuO included-Al2O3 supported oxygen carriers. For CLOU, it is necessary to avoid these kind of interactions and preserve CuO as an active phase 40. Under these circumstances, as Al2O3 can be considered as a support for CuO in CLOU systems, a lot of studies have focused on using CuO as an oxygen carrier with cheap and readily-available alumina (Al2O3) as a support material 7. For the CuO-Al2O3 system, there were no new phases detected during oxidizing condition as can be seen in the XRD patterns in Figure 4 and the TEC also concluded that no new phases were formed. With the TEC, CuAlO2 and CuAl2O4 phases could not be calculated as the database does not include these phases. It is known that, CuO can interact to form CuAl2O4 (Reaction 4) if γalumina is present in the system below 950°C 7. Since it has been known that CuAl2O4 formation over CuO and Al2O3 is thermodynamically favorable in air below 1000°C35–37, it has a great importance to investigate the CLOU properties of CuAl2O4 phase (Reaction 5)11. It has been known that CuAl2O4 is a good oxygen carrier, however the studies showed that it is incapable of releasing oxygen11. In addition to this, studies showed that reaction 5 is only possible under reducing atmosphere such as methane or syngas38. 𝐶𝑢𝑂(𝑠) + 𝐴𝑙2 𝑂3 (𝑠) → 𝐶𝑢𝐴𝑙2 𝑂4 (𝑠)

(4)

4𝐶𝑢𝐴𝑙2 𝑂4 (𝑠) → 4 𝐶𝑢𝐴𝑙𝑂2 (𝑠) + 2𝐴𝑙2 𝑂3 (𝑠) + 𝑂2 (𝑔)

(5)

Since all experiments were carried out at 900°C, the results also revealed that there was no formation of CuAl2O4, as expected. On the other hand, under IA the thermodynamic equilibrium


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calculations showed that Cu2Al2O4 compound can be formed after the formation of Cu2O from CuO. This was confirmed by the XRD analysis, but following the Cu2O formation, only a small amount of Cu2Al2O4 was formed over Cu2O and Al2O3. A significant amount of copper oxide presented in the system without forming any new compound. In addition to this, thermodynamic calculations showed that Cu2Al2O4 decompose into CuO and Al2O3 (Reaction 6) in presence of oxygen. In addition to this, reaction 7 and 8 were also reported in the literature studies which take place during oxidation below 900°C11,38,39. 𝐶𝑢2 𝐴𝑙2 𝑂4 (𝑠) + 0.5 𝑂2 (𝑔) → 2 𝐶𝑢𝑂(𝑠) + 𝐴𝑙2 𝑂3 (𝑠)

(6)

4𝐶𝑢𝐴𝑙𝑂2 (𝑠) + 𝑂2 (𝑔) → 2𝐶𝑢𝑂(𝑠) + 2𝐶𝑢𝐴𝑙2 𝑂4 (𝑠)

(7)

8𝐶𝑢𝐴𝑙𝑂2 (𝑠) + 𝑂2 (𝑔) → 2𝐶𝑢2 𝑂(𝑠) + 4𝐶𝑢𝐴𝑙2 𝑂4 (𝑠)

(8)

When the inert atmosphere is present in the system, copper oxide will release gaseous oxygen to form Cu2Al2O4, since the reaction is favorable at any temperature less than 1000°C.


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Figure 4. XRD patterns of the CuO/Cu2O - Al2O3 mixtures after experiments.

Similar to the CuO-Al2O3 system, strong interaction has been found between CuO/Cu2O and Fe2O3 which resulted in the formation of CuFe2O4 and Cu2Fe2O4 compounds as well as the CuO/Cu2O-Fe2O3 based solid solutions, as can be seen in Figure 5. The same phases were identified by the TEC. Spinel metal oxides such as CuFe2O4 has high catalytic performance during combustion and it can be also used as an oxygen carrier 28,41. It is known that CuFe2O4 can form over CuO and Fe2O3 above 900°C. In order to form Fe-included CuO and Cu2O based solid solutions, higher temperatures are needed 22,42.


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Figure 5. XRD patterns of the CuO/Cu2O - Fe2O3 mixtures after experiments. In the CuO-CaO system (Figure 6), formation of new compounds based on CuO/Cu2O and CaO was observed as CaCu2O3 and Ca2CuO3 phases in the experiment carried out under OA. However, it is difficult to compare this result with the TEC since there is no information about these two compounds in the thermodynamic database.


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Figure 6. XRD patterns of the CuO/Cu2O - CaO mixtures after experiments. The oxygen releasing behavior of the compounds formed over CuO and CaO has been reported in the literature, where the Ca2CuO3 phase probably could form between 880–940℃ 43. Although the results also showed that the oxygen releasing ability of Ca2CuO3 was lower than other CuOCaO based compounds, it has been reported that there was no serious agglomeration or attrition issue related to Ca2CuO3 43. In the experiment, to investigate the interaction between copper oxides and potassium oxide, pure and totally converted copper oxides were obtained (Figure 7); but there was no phase related to K2O, since K2O creates volatile species above 740℃. There was no transitional compound formed as there was no reaction between CuO/Cu2O and K2O.


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Figure 7. XRD patterns of the CuO/Cu2O – K2O mixtures after experiments.

Part II. Interaction Between CuO/Cu2O and Combined Oxide Compounds Interaction Between CuO/Cu2O and Two of the Oxide Compounds Table 3 shows the phases during interaction between CuO/Cu2O and two of ash forming matter based oxide compounds that were either experimentally obtained and analysed by XRD or were predicted by TEC.


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Table 3. Comparison of the experimentally obtained phases analysed using XRD and observed phases using TEC on the interaction between CuO/Cu2O to two of the oxide compounds

Compounds

Conditions

OA CuO-SiO2-Al2O3 IA

OA CuO-SiO2-Fe2O3 IA

OA CuO-SiO2-CaO IA

OA CuO-SiO2-K2O IA

XRD Analysis 92.6% CuO 4.5% SiO2 2.9% Al8SiO14 58.7% Cu2O 19.7% SiO2 18.6% Cu2Al2O4 3% Al2O3 34% CuO.Fe2O3 33.8% CuO 21.9% SiO2 10.3% Fe2O3 56.1% SiO2 29.6% Cu2O 8% Fe2SiO4 4.8% Cu2O.Fe2O3 1.5% CuFeO2 62.44% CuO 10.1% SiO2 1.8% Ca2SiO4 ~1% CaCuO2 76% Cu2O 15% SiO2 8.1% CuO ~1% CaO 44.9% SiO2 43.3% CuO 11.9% K2Si4O9 55.6% Cu2O 34.8% SiO2 7.2% CuO 1.4% K2Si4O9

TEC 62.71% CuO 21.47% SiO2 15.82% Al6Si2O13 39.74% Cu2O 30.77% SiO2 29.49% Cu2Al2O4 51.54 CuO.Fe2O3 35.51% CuO 12.95% SiO2 72.53% Cu2O.Fe2O3 14.39% SiO2 13.07% Cu2O

64.97% CuO 29.01% SiO2 6.02% CaSiO3 60.06% Cu2O 33.09% SiO2 6.85% CaSiO3 63.47% CuO 32.48% SiO2 4.05% K2Si4O9 58.43% Cu2O 36.96% SiO2 4.61% K2Si4O9


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When a mixture of CuO-SiO2-Al2O3 was investigated, no significant interaction between CuO and SiO2 or Al2O3 was obtained under OA. This is since Al2O3 and SiO2 are more prone to form aluminium silicate compounds than to form CuO based compounds. However, in case of IA, Cu2Al2O4 formation was observed. For the CuO-SiO2-Fe2O3 system, both TEC and XRD analysis showed that a significant amount of CuO.Fe2O3 or Cu2O.Fe2O3 solid solutions was formed. In addition to this, the formation of Fe2SiO4 was observed. For CuO-SiO2-CaO system, there was no strong interaction between CuO/Cu2O and the oxide compounds neither under OA nor IA. This result was also verified by TEC. Besides, TEC showed that SiO2 and CaO should react to form CaSiO3, so the interaction between CuO/Cu2O and CaO could possibly be minimized or completely prevented, however this compound could not be detected via XRD analysis. When both SiO2 and K2O are present in the system, it is well known that potassium silicates form. These are problematic compounds that can melt, cause agglomerations and decrease the lifetime of the oxygen carriers. In the CuO-SiO2-K2O system, there was no interaction between CuO/Cu2O and the oxide compounds for any atmosphere. However, in both experiments and TEC, a high amount of K2Si4O9 compound was obtained which would exist as liquid around 800°C and cause a lot of problems such as agglomeration of oxygen carriers 44,45. In accordance with these, a significant amount of agglomeration occurred when K2Si4O9 was obtained in the experiments.


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Energy & Fuels

Table 3. (cont.) Comparison of the experimentally obtained phases analysed using XRD and observed phases using TEC on the interaction between CuO/Cu2O to two of the oxide compounds.

Compounds

Conditions

OA CuO-Al2O3-Fe2O3 IA

OA CuO-Al2O3-CaO IA

OA CuO-Al2O3-K2O IA

OA CuO-Fe2O3-CaO IA

CuO-Fe2O3-K2O

OA

XRD Analysis 65.4% CuO 19.6% CuO.Fe2O3 13% Al2O3 1.7% CuAlO2

Experimental and thermodynamic study on interaction of copper

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