Cracking Characteristics of Simulated Dust-Containing Coal Pyrolysis

Dec 17, 2014 - of tar in coal pyrolysis volatiles to investigate the interrelationship between dust ... Furthermore, dust deposition from the ash in t...
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Cracking Characteristics of Simulated Dust-Containing Coal Pyrolysis Volatiles over Regenerated Nickel-Based Catalysts Peng Liang,* Yaqing Zhang, Aifang Wei, Xiaohang Wang, and Tao Liu College of Chemical and Environmental Engineering, Shandong University of Science and Technology, QingDao, Shandong 266590, P.R. China ABSTRACT: Two catalysts, 2%NiO/1%CeO2−Al2O3 and 2%NiO/1%CeO2/2%ZrO2−Al2O3, were employed in the conversion of tar in coal pyrolysis volatiles to investigate the interrelationship between dust collection and tar cracking as well as the effect of Ce/Zr doping on catalyst performance. In a fixed-bed catalytic reactor, a tar model mixture (57 wt % toluene, 14 wt % methylnaphthalene, 14 wt % cyclohexane, and 15 wt % dodecane) and simulated dusts (various metal oxides) were introduced to mimic real coal pyrolysis volatiles. The catalytic cracking activity of the latter catalyst increased with increasing temperature, space velocity, and water/model tar ratio, whereas the former catalyst exhibited the opposite trends, except with respect to temperature. Because of the formation of a Ce−ZrO2 solid solution, the latter catalyst showed higher stability. During cycling tests, the deactivation of 2%NiO/1%CeO2−Al2O3 was observed in the seventh reaction-regeneration cycle. Nickel aggregation, observed on the surface of the catalyst by XRD analysis, may account for the deactivation. Dust deposition on the catalyst surface had a significant influence on tar modification, and the interaction mechanism between dust deposition and coke formation was clarified. The electron probe microanalysis results showed that SiO2 tends to promote carbon deposition, whereas MgO and Fe2O3 components have a negative influence on carbon deposition. This study can serve as a reliable reference for the practical development and modification of purification processes for coal pyrolysis volatiles.

1. INTRODUCTION Worldwide, a number of clean coal technologies (CCT) have been developed to comply with stringent environmental regulations and dwindling fossil fuel energy resources.1 Pyrolysis, as the first step of coal conversion, can generate a considerable amount of tar, especially for low rank coals. Furthermore, dust deposition from the ash in the coal tar will form intractable tar residues, which can not only occlude the cooling system and corrode metal equipment but also increase the difficulty of the subsequent tar processing and reduce the utilization rate of hydrocarbon resources. Therefore, for heavily tar-laden pyrolysis gases, the dust removal process has become a bottleneck for industrialization.2,3 From energy-economy and environmental considerations, the catalytic cracking method for the conversion of tar holds great promise. For this process, much interest has been paid to the development of low-cost nickel-based catalysts for their better activities and mechanical properties. It has been reported that the addition of Ce compounds to the catalysts can improve both catalytic activity and surface acidity.4 However, CeO2 may sinter at high temperature because of its poor thermal stability, which will decrease its ability to store oxygen. A CeO2−ZrO2 solid solution that was produced by simultaneously introducing Ce and Zr additives exhibited better thermal stability and oxygen storage capacity than CeO2 alone.5−7 However, despite a tremendous amount of research, the rapid deactivation of nickel-based catalysts is still observed due to sintering or carbon deposition at high temperature.8,9 Therefore, methods to regenerate the catalysts as well as evaluate their performance must be developed.10 Typical regenerative approaches have involved the use of organic solvents,11 abrasion or ultrasonication,12 or supercritical fluids or plasmas13 to remove deposited carbon from the surfaces of the catalysts. Never© 2014 American Chemical Society

theless, the high cost or complexity of these methods has limited their large-scale development. Calcination, because of its cost-efficiency and convenience, is the most common, industrially used technology to eliminate carbon. The calcination process has been used to show that deactivation is reversible, and partial or complete regeneration of the catalysts is possible.14,15 Unfortunately, little attention has been paid to the influence of dust-laden tar on catalytic activity or the mechanism for the deposition of dust on the catalyst particles. Coal pyrolysis volatiles contain numerous organic compounds in the tar and complex mixtures in the dust, together with complex free radicals. For the coal pyrolysis volatiles which are exhausted from a pyrolysis reactor, direct catalytic cracking over a fixed catalyst bed is a promising technology. On the one hand, it advantageously makes full use of the sensible heat in the high-temperature coal pyrolysis volatiles and modifies the heavy tar into low-molecular-weight gases and light hydrocarbons. On the other hand, the packed catalyst particles may act as a filtration medium for dust collection, which can purify the dust-laden volatiles. Therefore, it is expected that dust removal from the pyrolysis volatiles and heavy tar modification can be achieved in one step. Because of the chemical complexity of the high-temperature dust-laden coal pyrolysis volatiles, both lab experiments and theoretical analyses are difficult to conduct, and related research in this area has not been reported. To obtain a general understanding of the characteristics of the catalytic reaction, model systems become necessary. To study the catalytic Received: September 19, 2014 Revised: November 28, 2014 Published: December 17, 2014 70

DOI: 10.1021/ef502125j Energy Fuels 2015, 29, 70−77

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were combined with water, and this feed was injected into the reactor simultaneously with the ingredients. The particle size of these materials did not exceed 74 μm. The simulated dust content was limited to less than 20 wt % of the catalyst mass. 2.2. Catalyst Characterization. The catalyst morphologies were observed by scanning electron microscopy (SEM, Zeiss EVO MA 10/LS 10) under an accelerating voltage of 10 kV. The surface phase compositions were determined by X-ray diffractometry (XRD, Rigaku D/max-2550) using Cu Kα radiation at 200 mA and 40 kV over 2θ = 10−80°. The specific surface area and pore size analyses were determined using the Brunauer−Emmett−Teller (BET) method in an ASAP 2020 adsorption instrument (Micromeritics) using N2 as adsorbent. The amount of formed coke was determined in a Mettler STARe thermogravimetric analyzer (TGA, TGA/sDTA851e) under air flow. Samples were heated from 20 to 1000 °C at a heating rate at 20 °C/min. Elemental analysis of the catalyst surface after the experiment was carried out in an electron probe microanalyzer (EPMA, JEOL JXA-8230). 2.3. Catalyst Performance Evaluation Criteria. The catalytic activity was evaluated by the yields (Y) of H2, CO, CH4, C2+, and CTotal. Note that because water is involved in the reaction, YH2 may exceed 100% in some of the experimental data.

cracking of tar in the pyrolysis products, a representative mixture of tar model compounds and simulated dust was used to approximate real dust-containing coal pyrolysis volatiles. The tar model compounds comprised a mixture of aromatics and alkanes. The dust, which typically contains ash, metal oxides, and various minerals as well as fly particles from the crushing process during coal pyrolysis, was modeled with different metal oxides. The experiments were carried out in a fixed-bed catalytic cracking apparatus. In this paper, 2%NiO/1%CeO2− Al2O3 and 2%NiO/1%CeO2/2%ZrO2−Al2O3 catalysts were selected to modify the simulated dust-containing coal pyrolysis volatiles. The effects of the additives Ce and/or Zr on the catalyst stability and regeneration performance as well as the interaction mechanism between dust deposition and the catalytic cracking of tar in the coal pyrolysis volatiles were investigated.

2. EXPERIMENTAL SECTION 2.1. Experimental Apparatus and Procedures. The catalysts were prepared by the impregnation method. γ-Al2O3 particles (1−2 mm) were impregnated with an aqueous solution of Ce(NO3)3·6H2O or a mixture of Ce(NO3)3·6H2O and Zr(NO3)4·5H2O for 12 h. After impregnation, the particles were dried overnight at 110 °C and then calcined at 500 °C for 3 h. Then the particles were impregnated with an aqueous solution of Ni(NO3)2·6H2O for 12 h, dried overnight at 110 °C, and calcined at 800 °C for 4 h. As shown in Figure 1, either

YH2 (%) =

YCO (%) =

FHout2 − FHin2 4FCin7H8 + 5FCin11H10 + 6FCin6H12 + 13FCin12H26

× 100

out FCO

7FCin7H8

+

11FCin11H10

+ 6FCin6H12 + 12FCin12H26

× 100 out FCH 4

YCH4 (%) =

7FCin7H8 + 11FCin11H10 + 6FCin6H12 + 12FCin12H26 × 100

YC2+ (%) = Figure 1. Schematic diagram of fixed-bed reaction equipment. (1. Ingredients; 2, 3. Peristaltic pump; 4. Mixture of dust and water; 5. Hydrogen; 6. Nitrogen; 7. Preheater; 8. Preheater furnace; 9. Reactor; 10. Catalyst; 11. Reactor furnace; 12. Cooler; 13. Gas−liquid separator; 14. Condensate; 15. Gas chromatograph; 16. Regenerate gases).

+ 3FCout + 4FCout 2FCout 2 3 4 7FCin7H8 + 11FCin11H10 + 6FCin6H12 + 12FCin12H26 × 100

YCTotal (%) =

out out + FCH + 2FCout + 3FCout + 4FCout FCO 4 2 3 4

7FCin7H8 + 11FCin11H10 + 6FCin6H12 + 12FCin12H26 × 100

where Fi and Fiin are the flow rates of the outlet and inlet gas i, respectively. out

catalyst (2 g) was loaded in the fixed bed reactor. A mixture comprising 57 wt % toluene, 14 wt % methylnaphthalene, 14 wt % cyclohexane, and 15 wt % dodecane was prepared as a model tar (“ingredients” in Figure 1). For the initial reduction of the catalyst, a mixture of H2 in N2 (15:85 v/v) was introduced at a flow rate of 120 mL/min and 800 °C for 3 h. When the reducing process was finished, a carrier gas with various space velocities comprising 50:50 (v/v) N2:H2 was sent to the preheating furnace. After the cracking reaction of the tar was conducted under the predetermined conditions for 5 h, the catalysts were regenerated under a mixture of 10:90 (v/v) O2:N2 at 800 °C for 20 min. One cycle was defined as one catalytic reaction pass followed by catalyst regeneration. To simulate actual dust-containing coal pyrolysis volatiles, typical metal oxides such as SiO2, Fe2O3, and MgO (or their mixture)

3. RESULTS AND DISCUSSION 3.1. Effect of Process Conditions on the Tar-Modified Reaction. 3.1.1. Effect of Temperature on the Yield of Gas Products. Figure 2 compares the average gas yields after 5 h reaction over 2%NiO/1%CeO2−Al2O3 and 2%NiO/1%CeO2/ 2% ZrO2−Al2O3 as a function of reaction temperature. It can be seen that temperature is the major factor affecting the two catalysts. The yields of H2, CO, and CH4 increase with the increasing reaction temperature. This is due to the rapid and irreversible tar steam reforming reaction16 that preferentially takes place for both reaction systems at high temperature. Concomitantly, the methane steam reforming reaction and 71

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Figure 4. Effect of space velocity on the activity of the two catalysts: (1) 2%NiO/1%CeO2−Al2O3 and (2) 2%NiO/1%CeO2/2%ZrO2− Al2O3. (Reaction conditions: (mH2O:mIngr.)1 = (mH2O:mIngr.)2 = 4:1, T1 = T2 = 800 °C, t1 = t2 = 5 h).

Figure 2. Effect of reaction temperature on the gas yields: (1) 2% NiO/1%CeO2−Al2O3 and (2) 2%NiO/1%CeO2/2%ZrO2−Al2O3. (Reaction conditions: (mH2O:mIngr.)1 = (mH2O:mIngr.)2 = 4:1, SV1 = SV2 = 12000 h−1, t1 = t2 = 5 h).

gas production. However, for the 2%NiO/1%CeO2/2%ZrO2− Al2O3 catalyst, the yields of H2 and CO improve gradually while those of C2+ and CTotal decline smoothly with the increase of SV. This may be because, with the increase of the SV, the adsorption of the reactant molecules on the 2%NiO/1%CeO2− Al2O3 becomes gradually saturated. As the residence time of the reactant tar model compounds in the reactor is prolonged at lower SV, deep cracking reactions probably occur. As the present experiments were conducted at atmospheric pressure, the residence time played the major role on the product distribution. For the Zr-doped catalyst, the adsorption capacity gradually increases with the increasing space velocity because of its improved micro- and mesoporosities. 3.1.3. Effect of H2O/Ingredients Ratio on the Yield of Gas Products. Figure 5 shows the effects of the H2O/Ingredients

steam carbon reaction17 contributes to the increases in the H2 and CO yields at high temperature. The C2+ conversion yields of the two catalysts first increase and then decrease with the rise in temperature; this can be considered a result of the high temperature, which is conducive to producing smaller gas molecules such as H2, CO, CH4, C2, C3, C4, etc. It is known that the thermal cracking reaction of tar is relatively vigorous at 900 °C. Therefore, to investigate the catalytic performance of the two catalysts at 900 °C, catalytic and thermal (blank) cracking experiments were performed (Figure 3). In the presence of the two catalysts, the yields of H2

Figure 3. Gas yield comparisons of the catalysts and the blank test at 900 °C.

and CO are distinctly increased, with the Zr-containing catalyst having a slightly larger effect. The comparison reveals that 2% NiO/1%CeO2/2%ZrO2−Al2O3 is probably strongly resistant to sintering. 3.1.2. Effect of Space Velocity on the Yield of Gas Products. The effects of space velocity (SV) on the activities of 2%NiO/1%CeO2−Al2O3 and 2%NiO/1%CeO2/2%ZrO2− Al2O3 are shown in Figure 4. In the reaction over 2%NiO/ 1%CeO2−Al2O3, as the SV increases, the yields of H2, CO, CH4, C2+, and CTotal rise initially and then decline (Figure 4-1). This means that the 2%NiO/1%CeO2−Al2O3 catalyst is more sensitive to the space velocity in the studied range. Furthermore, a SV value of 11000 h−1 results in the highest

Figure 5. Effect of H2O/Ingredients ratio on the activity of the catalysts: (1) 2%NiO/1%CeO2−Al2O3 and (2) 2%NiO/1%CeO2/2% ZrO2−Al2O3. (Reaction conditions: T1 = T2 = 800 °C, SV1 = 11000 h−1, SV2 = 16000 h−1, t1 = t2 = 5 h).

ratio on the activities of the 2%NiO/1%CeO2−Al2O3 and 2% NiO/1%CeO2/2%ZrO2−Al2O3 catalysts. After adding Ce, when the ratio of H2O/Ingredients increases initially, increasing yields of H2, CO, CH4, C2+, and CTotal are observed. However, a sudden decrease occurs when the H2O/Ingredients ratio reaches 3:1. The presence of increased amounts of steam can lead to hydrothermal deactivation,18 followed by changes in the 72

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tion. This may be related to the ability of Zr to achieve a better dispersion of active components.7 Thermogravimetric analysis (in air) shows that the weight loss rates for 2%NiO/1%CeO2− Al2O3 and 2%NiO/1%CeO2/2%ZrO2−Al2O3 were 4.2 and 3.8 wt %, respectively, which indicates that the presence of Zr results in better anticarbon performance.1 The BET analysis results are summarized in Table 1. Compared with the fresh 2%NiO/1%CeO2−Al2O3 catalyst, the BET surface area of the fresh 2%NiO/1%CeO2/2%ZrO2− Al2O3 increased from 106 m2·g−1 to 135 m2·g−1. Additionally, the pore diameter decreased from 14.1 to 11.4 nm, although the pore volume was unchanged. These results may be explained by the greatly improved micro- and mesoporosities imparted by the Ce/Zr additives. After reaction for 5 h, surface areas of both catalysts were significantly reduced, which can be explained by the coke blocking of the micro- and mesopores. 3.2. Evaluation of the Stability and Regeneration of the Catalysts. 3.2.1. Stability Tests. The data in Figure 8 show the stability testing results of the 2%NiO/1%CeO2− Al2O3 and 2%NiO/1%CeO2/2%ZrO2−Al2O3 catalysts under their optimized conditions. Compared with the 2%NiO/1% CeO2−Al2O3 catalyst, the Zr-doped catalyst exhibits higher gas yields ∼70% H2, 50% CTotal, 25% C2+, 18% CO, and 8% CH4. Moreover, the activity of 2%NiO/1%CeO2/2%ZrO2−Al2O3 remained stable throughout the examined period of time. Thus, Zr simultaneously improves both the catalytic activity and stability. During the first 14 h of the test, the activity of the 2%NiO/1%CeO2−Al2O3 catalyst was relatively little changed: ∼55% H2, 40% CTotal, 23% C2+, 15% CO, and 6% CH4. Then the yields began to decline, affording only ∼38.5% H2, 30.4% CTotal, 15.8% C2+, 7.8% CO, and 6.8% CH4 after 20 h reaction. This decline may be related to the formation of carbon-blocked pores or the sintering of the nickel. Therefore, BET measurements (Figure 9) were conducted to evaluate the role played by pore size. Previous studies19,20 have reported that coke is preferentially deposited at the strongest acid sites. In Figure 9, the cumulative pore volumes of the micro- and mesopores decrease substantially after reaction for both catalysts. The values change from 0.33 to 0.09 cm3g−1 for 2%NiO/1%CeO2−Al2O3 and from 0.35 to 0.16 cm3g−1 for 2%NiO/1%CeO2/2%ZrO2− Al2O3. The results reveal that the strongest acid sites of the two catalysts are located in the micro- and mesopores. 3.2.2. Regeneration Tests. In order to obtain better catalyst regeneration and performance in a relatively short period of time, the optimized conditions but a smaller ratio of water/ ingredients (mH2O:mIngr. = 2:1) in the feed were employed. Figure 10 shows the results for multiple regenerations of the two catalysts. In the case of the 2%NiO/1%CeO2/2%ZrO2− Al2O3 catalyst, high activity can still be observed after eight reaction-regeneration cycles. Unfortunately, deactivation of 2% NiO/1%CeO2−Al2O3 begins after six reaction-regeneration cycles, where the average yields of H2 and Ctotal at the first cycle drop from 54.4 and 36.5% to 36.9 and 25.3% at the seventh cycle, respectively. This may be related to the cumulative sintering of nickel with the increasing number of regeneration cycles. Therefore, 2%NiO/1%CeO2/2%ZrO2−Al2O3 catalyst exhibits greater durability with respect to regeneration testing. Figure 11 shows the XRD patterns of the two catalysts after the regeneration tests. Peaks assigned to NiAl2O4 are absent in pattern (b) (Zr-doped catalyst) but are obviously present in pattern (a), while Ni is observed in both patterns. These results

microscopic structure of the catalyst and nickel sintering. Therefore, the excess steam produces a negative effect on the catalytic performance of 2%NiO/1%CeO2−Al2O3. After adding Ce/Zr, however, the yield of each product shows a clear upward trend with the increasing ratio of H2O/Ingredients. This may be explained by the formation of a Ce−ZrO2 solid solution, which effectively improves the high-temperature stability and activity of the catalyst. Ultimately, the optimized process conditions for 2%NiO/1%CeO2−Al2O3 were determined as follows: T = 800 °C, mH2O:mIngr. = 3:1, and SV = 11000 h−1. Similarly, the optimized process conditions for 2% NiO/1%CeO2/2%ZrO2−Al2O3 are T = 800 °C, mH2O:mIngr. = 5:1, and SV = 16000 h−1. 3.1.4. Comparison of Catalysts before and after Reaction. In this section, the properties of fresh and reacted catalysts are compared. The reaction conditions are as follows: (mH2O:mIngr.)1 = (mH2O:mIngr.)2 = 4:1, T1 = T2 = 800 °C, SV1 = SV2 = 12000 h−1, t1 = t2 = 5 h. XRD patterns of both catalysts are shown in Figure 6. The appearance of NiAl2O4 peaks at 2θ

Figure 6. XRD patterns of catalysts before and after reaction. Before reaction: (a) 2%NiO/1%CeO2−Al2O3 and (b) 2%NiO/1%CeO2/2% ZrO2−Al2O3. After reaction: (c) 2%NiO/1%CeO2−Al2O3 and (d) 2% NiO/1%CeO2/2%ZrO2−Al2O3.

= 37.6°, 45.7°, and 67.8° suggests that the additions of Ce and Ce/Zr are unable to prevent the formation of the spinel compound. In the diffraction patterns of fresh 2%NiO/1% CeO2−Al2O3 and 2%NiO/1%CeO2/2%ZrO2−Al2O3, the peak due to CeO2 at 2θ = 28.7° indicates that Al2O3 and CeO2 do not form a crystalline structure but exist independently. After 5 h reaction, for both catalysts the CeO2 peak at 2θ = 28.7° disappears, and Ni peaks at 2θ = 44.53°, 51.81°, and 76.23° appear. These changes are due to the thermal instability of Ce and the reduction of NiO in the reaction process. In the diffraction pattern of fresh 2%NiO/1%CeO2/2%ZrO2−Al2O3, a peak attributable to (Zr0.85Ce0.15)O2 appears at 2θ = 29.7°, which is due to the formation of a Ce−ZrO2 solid solution during calcination. Figure 7 shows SEM surface comparisons of the two catalysts before and after 5 h reaction. The surface morphologies exhibit small changes before and after reaction, but fibrous coke deposits can still be observed. This result indicates that, after the reaction, it will be necessary to take measures to prevent deactivation caused by excessive carbon deposition. Nevertheless, compared with the obvious agglomerated coke deposition on the surface of the 2%NiO/1%CeO2−Al2O3 catalyst, a more dispersed, grainy coke deposition is observed on the 2%NiO/1%CeO2/2%ZrO2−Al2O3 catalyst after reac73

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Figure 7. SEM images of catalysts before and after reaction (3000×).

Table 1. BET Surface Area (SBET), Pore Volume (Vp), and Pore Diameter (dp) of the Catalysts before and after Reaction content of catalyst

SBET/m2·g−1

Vp/cm3·g−1

dp/ nm

2%NiO/1%CeO2 before reaction 2%NiO/1%CeO2 after reaction 2%NiO/1%CeO2/2%ZrO2 before reaction 2%NiO/1%CeO2/2%ZrO2 after reaction

106 73 135

0.36 0.26 0.37

14.1 9.0 11.4

79

0.31

7.8

suggest that despite the presence of the difficult-to-reduce spinel in the fresh catalyst, the Ce/Zr composition is capable of breaking the strong chemical bonds in NiAl2O4 during the regeneration process, to form more easily reducible components. The particle size of Ni was calculated on the basis of Xray line broadening using the Scherrer equation. Compared to 20.2 nm (44°) for 2%NiO/1%CeO2/2%ZrO2−Al2O3, the particle size of Ni increases to 40.2 nm (44°) for 2%NiO/1% CeO2−Al2O3. It appears that the increased size of the active metallic Ni must account for the deactivation of 2%NiO/1% CeO2−Al2O3. 3.3. Effect of Dust Deposition on Tar-Modified Reaction. In this section, we selected 2%NiO/1%CeO2/2% ZrO2−Al2O3 as the catalyst based on its superior performance and durability. Under the optimized conditions, both the effects of the dust species and dust contents on catalytic activity were investigated.

Figure 8. Stability test of the catalysts under the optimal conditions: (1) 2%NiO/1%CeO2−Al2O3 and (2) 2%NiO/1%CeO2/2%ZrO2− Al2O3.

3.3.1. Effect of Dust Species on Catalytic Activity. Figure 12 shows the effect of dust species on catalytic activity, using SiO2, MgO, and Fe2O3 as typical dusts with a dust-to-catalyst ratio of 10%. After introducing MgO and Fe2O3, the gas yields were much higher than in the blank experiment. This is consistent with the better distribution of acid sites discussed in relation to Figure 9. The effect of dust on gas yields increases in the order SiO2 < Blank < MgO < Fe2O3. The H2 yield with Fe2O3 (86.8%) was slightly higher than in the presence of MgO 74

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Figure 12. Effect of dust species on catalytic activity. (Reaction conditions: T = 800 °C, SV = 12000 h−1, mH2O:mIngr. = 5:1, mDust/mCat. = 10%).

Figure 9. Cumulative pore volumes and pore size distributions of catalysts before and after stability test: (1) 2%NiO/1%CeO2−Al2O3 and (2) 2%NiO/1%CeO2/2%ZrO2−Al2O3.

out on the used catalysts recovered after catalytic reactions performed with different dust species. From the EPMA results (Table 2), it can be observed that the C/Al ratios were in the order Fe2O3 < MgO