Evaluation of the Reactivity of Hematite Oxygen Carriers Modified Using Alkaline (Earth) Metals and Transition Metals for the Chemical Looping Conversion of Lignite

Abstract

Chemical looping (CL) technology is a novel technology for the clean and efficient use of energy. Oxygen carriers (OCs) are the cornerstone of CL technology. The development of low–cost, high–performance OCs is crucial for the application of CL conversion. Hematite, one of the natural Fe–based OCs, has several advantages (e.g., low cost and environmental friendliness), but its low reactivity limits its application in CL. The performance of hematite can be effectively improved by modifying some of its active components. This study explored the improvement of hematite reactivity by adding alkaline (earth) metals (K, Na, and Ca) and transition metals (Ni, Cu, and Mn). The crystal phases of the OCs were characterized using X-ray diffraction (XRD), and the results revealed that the addition of metals significantly changed the phase of the original hematite. The active solid solution of K–Fe–O and Na–Fe–O species exhibited strong catalytic activity to facilitate lignite char conversion. The addition of CaO promoted the devolatilization of lignite, while the formation of a solid CaFe₂O₄ solution with low reactivity inhibited the lattice oxygen release. The presence of CuO/CuFe₂O₄ in the Cu–modified sample could release a small amount of free O₂ to promote volatile conversion. The high activity phases of NiO and NiFe₂O₄ in the Ni–modified OCs could improve the reaction activity of hematite. However, the MnFeO₃ phase with low reaction activity was generated in the Mn–modified OC, decreasing the reaction rate of the Mn–modified OC with lignite char.

1. Introduction

Over the past decades, the combustion of fossil fuels has emitted excessive amounts of CO₂, contributing to climate change globally [1]. Carbon capture, utilization, and storage technologies have been widely investigated to achieve the target of carbon neutrality [2]. However, conventional carbon capture technologies (e.g., precombustion, post–combustion, and oxygen–enriched combustion) are inefficient and expensive [3,4,5,6]. Chemical looping (CL) combustion technology is a novel approach for implementing CO₂ capture with a low energy consumption since it directly generates high concentrations of pure CO₂ flue gas without the use of gas separation devices [7,8,9]. Oxygen carriers (OCs) can be used as oxidants to transfer the required oxygen for fuel conversion between different reactors (air reactor (AR) and fuel reactor (FR)) and to prevent direct contact between fuel and air (Figure 1) [10,11,12]. Generally, CL separates the reduction–oxidation (redox) process into two stages. In the reduction stage, a reducing agent (fuel) converts the OC into a reduced OC. In the oxidation stage, the reduced OC (Me) recovers the initial state of the lattice oxygen [13].

Hence, OCs are essential in CL systems, and their reactivity determines the reaction rate and conversion efficiency in CL processes [14]. Expensive raw materials are unsuitable for CL because of the high consumption of OCs [15]. Consequently, inexpensive natural iron ore–based OCs, such as hematite, have gained considerable interest because of their good cyclic stability, mechanical strength, and environmental friendliness [16,17,18,19].

However, iron ores have limited application potential in CL because of their low reactivity. Thus, it is necessary to modify iron ores to improve their reactivity [20]. Some studies [21,22] have discovered that multimetal composite oxides formed by two or more active components (e.g., Ni, Fe, Cu, and Mn oxides) have higher reactivities (e.g., excellent redox capacity, good thermal stability, and mechanical strength) than single metal oxides because multimetal elements can produce synergistic effects. Consequently, adding exogenous active components is an effective strategy for improving the reactivity of iron–based OCs [23,24,25]. Yu et al. [26] investigated the catalytic reduction mechanism of K₂CO₃–modified iron–based OCs and discovered that the addition of K₂CO₃ to generate a porous Fe/K spinel structure with good stability evidently enhanced the cyclic performance and reaction rate of the OC. Yan et al. [27] proved that the introduction of Na can significantly reduce the strength of the Fe–O bond and facilitate the release of lattice oxygen, thereby improving the reactivity of Fe–based OCs. Yang et al. [28,29] discovered that CuFe₂O₄ with a spinel structure was formed in Cu–modified Fe–based OCs and that it positively affected the rapid oxidation of coal. Wei et al. [30,31,32] investigated the performance of a CaO–modified Fe–based OC and discovered that the addition of CaO promoted CO and H₂ generation, while the main crystalline phase of CaO remained stable over 15 cyclic reactions. Meanwhile, it has been confirmed that CaO promotes lattice oxygen transfer in Fe–based OCs [33]. Sun et al. [34,35,36] discovered that the formation of NiFe₂O₄ species with a spinel structure in Ni–modified Fe–based OCs improved the tar cracking rate and the reactivity of the ilmenite–based OCs. Furthermore, Liu et al. [37,38] discovered that the addition of manganese formed abundant (Fe_1−xMn_x)₂O₃ crystal phases, which caused ilmenite–based OCs to exhibit significant O₂ uncoupling behavior and promoted fuel conversion.

It is crucial to investigate the effect of exotic metals on the properties of hematite. During CL, coal undergoes dehydration, devolatilization, and char conversion as the temperature increases. However, the detailed effect of external metal modification on the dominant reaction (devolatilization and char conversion) remains unclear. A better understanding of how external metal modification affects the dominant reaction would result in improved OC design, which further promotes coal conversion. Therefore, this work aims to determine the effect of alkaline (earth) metals (K, Na, and Ca) and transition metals (Ni, Cu, and Mn) on lignite conversion using both thermogravimetric (TG) experiments and XRD analysis. The results are expected to provide a reference for optimizing the design of hematite OCs and promoting lignite conversion.

2. Materials and Methods

2.1. Raw Material

Lignite has high reserves and a high content of combustible components; the utilization of it can alleviate the current situation of energy stress. CL technology is an efficient fuel utilization technology and a better means for studying fuel utilization; meanwhile, lignite is suitable for CL technology since it is a low–rank coal with a high volatile content [39,40]. In this study, lignite was collected from Yunnan Province, and its composition is presented in

Table 1. Proximate and ultimate analyses of Yunnan lignite.

Ultimate Analysis (wt.%)					Proximate Analysis (wt.%)
C	H	N	S	O a	Ash	Volatiles	Fixed carbon b	Moisture
51.72	4.13	1.14	1.22	25.22	16.57	43.53	32.98	6.92

^a O = 100 − C − H − N − S − ash. ^b Fixed carbon = 100 − ash − volatiles − moisture.

.

A natural iron ore (hematite) from Australia was used in this study, and its elemental composition is listed in

Table 2. Elemental compositions of hematite.

Element	Fe	O	Si	Al	K	Ca	P	Ti
Content/wt.%	61.82	32.33	3.35	1.69	0.27	0.14	0.09	0.09

. To prepare the modified OCs, hematite and selected metal salts or oxides (KNO₃, NaNO₃, CaO, NiO, MnO₂, or CuO) were mechanically mixed to the desired ratios in an agate mortar before being calcined in a muffle furnace at 950 °C for 3 h. Each sample was ground into powder after cooling. The mixed sample of original hematite and lignite is denoted as Hematite, and the mixed samples of hematite that were modified by adding 5%, 10%, and 15% mass ratios of foreign metal are denoted as M5, M10, and M15, respectively (where M = K, Na, Ca, Ni, Mn, or Cu). The particle diameter of the OCs and lignites used in this experiment are below 0.075 mm.

2.2. Experimental Apparatus and Procedures

The crystal structures of the OC samples were analyzed using an X-ray diffractometer (X’ Pert Pro MPD) with Cu Kα radiation ($\lambda$ = 0.1504 nm) operating at a voltage of 40 kV and a current of 40 mA. The scanning rate was 2°/min from 2θ = 5° to 80° at a step of 0.02°.

The reaction characteristics (redox reactivity) of the OCs and lignite were evaluated using a synchronous thermal analyzer (STA409, NETZSCH–Gerätebau GmbH). For all experimental runs, Ar was introduced as the protective gas and the purging gas at flow rates of 20 mL/min and 40 mL/min, respectively. The mixed samples were heated from 30 °C to 950 °C at a heating rate of 10 °C/min and then maintained at 950 °C for 1 h. The mass ratio of the OC to lignite was 1:1, and the total sample amount was 10 mg.

3. Results and Discussion

3.1. XRD Analysis

Figure 2 presents the XRD spectra of hematite and the various modified OCs. The predominant crystal phases of hematite were Fe₂O₃ and SiO₂. As shown in Figure 2a, the K5 sample had a small K content, and no new phase was detected. The K10 and K15 samples displayed the formation of a new crystal phase (a solid K–Fe–O solution) due to the reaction between the K element and Fe₂O₃. As shown in Figure 2b, different Na contents in the sample formed different new crystal phases. NaAlSiO₄ was formed in the Na5 sample, and a Na–Fe–Si–O structure was detected in the Na10 and Na15 samples. As shown in Figure 2c–f, the addition of Ca, Cu, Mn, and Ni contributed to the formation of an active solid solution of Ca₂Fe₂O₅, CaFe₂O₄, CuFe₂O₄, MnFeO₃, and NiFe₂O₄, respectively, in the modified OCs. Since some new crystal phases were formed, it can be predicted that the reactivity of the modified OCs changed noticeably.

3.2. Effect of Different Metal Modifications on the Reactivity of the Hematite OCs

The reactivity of the original hematite and lignite–modified hematite samples was thoroughly investigated to explore the effect of exogenous metals on the reactivity of the hematite OCs. Moreover, the reactivity of the doping metal salts or oxides (i.e., CaO, CuO, Mn₂O₃, and NiO) and the new crystal phases (i.e., CaFe₂O₄, CuFe₂O₄, MnFeO₃, and NiFe₂O₄) are discussed. The reactivity of the Fe–K–O and Na–Fe–Si–O species are not discussed because it was difficult to determine the new crystal phase in the K–modified and Na–modified OCs.

3.2.1. Effect of K and Na Modifications

Figure 3 presents the TG and DTG curves of the different mixtures. As shown in Figure 3a, compared to the inert SiO₂ sample, the active OC evidently promoted lignite conversion, and an apparent weightlessness process was observed. Furthermore, it was observed that the three mixtures (lignite–containing K–modified hematite) exhibited a slightly higher weight loss than hematite, indicating that the addition of K facilitates lignite conversion. Moreover, this trend became evident as the K content increased. These results preliminarily indicate that K modification improves the reactivity of hematite. As shown in Figure 3b, there are four weight loss peaks in the DTG curves, which correspond to the different reaction stages. The weight loss peak at 96 °C is attributed to the evaporation of the water in lignite and the OCs (stage I). The second weight loss peak at 420 °C is assigned to the devolatilization of lignite (stage II). The small weight loss peak at about 660 °C is due to the secondary devolatilization of lignite (stage III). The fourth weight loss peak, i.e., the largest weight loss peak, is attributed to the reaction between lattice oxygen and lignite char (stage IV) [41]. The former two weight loss peaks almost overlap in the DTG curves because the dehydration and devolatilization processes are not the rate–controlling steps of lignite conversion. The third weight loss peak for the inert SiO₂ sample is very small because of the extremely low degree of secondary lignite devolatilization. A fourth weight loss peak was not observed for this sample because SiO₂ could not provide lattice oxygen. An apparent weight loss peak (stage IV) was observed for the other four mixtures (the characteristics of this stage are shown in

Table 3. Characteristics of the char conversion using hematite and the K–modified OCs.

Sample	Initial Temperature a (°C)	Peak Temperature b (°C)	Peak Value c (wt.%/min)
Hematite	700	910	1.82
K5	698	900	1.39
K10	563	768	1.89
K15	540	735	1.87

^a The temperature corresponding to the initiation weight loss of the samples during the reaction phase between the OCs and the lignite char. ^b The temperature corresponding to the maximum rate of weight loss during the reaction phase between the OCs and the lignite char. ^c The maximum reaction rate between the OCs and the lignite char.

) because a large amount of active lattice oxygen reacted with the lignite char. Compared to those of the hematite sample (700 °C and 910 °C), the initial and peak temperatures of the K5 (698 °C and 900 °C), K10 (563 °C and 768 °C), and K15 (540 °C and 735 °C) samples exhibited a noticeable forward shift in stage IV. This is because the addition of K promoted the generation of an active solid solution of K–Fe–O species, which has a strong catalytic activity to facilitate lignite char conversion [42]. The lattice oxygen in K10 and K15 had a high reactivity and reacted with the lignite char at about 600 °C, causing the secondary devolatilization process to overlap with the char conversion process. Thus, the weight loss peak of the secondary volatilization of lignite was not observed. Moreover, the reactivity of the K–modified OCs exhibited an increasing trend as the K content increased because additional K–Fe–O species were generated. Thus, the K15 sample exhibited the highest reactivity among the four OCs in this work.

During the lignite conversion, the three mixtures (i.e., the lignite–containing Na5, Na10, and Na15 samples) exhibited weight losses of 43.6 wt.%, 46.7 wt.%, and 48.9 wt.%, respectively, which are slightly higher than that of hematite (43.5 wt.%), as shown in Figure 4a. These results preliminarily indicate that Na modification also enhances the reactivity of hematite, promoting lignite conversion.

Similar to the K–modified samples, the Na–modified samples contained lattice oxygen with a high reactivity (no weight loss peak of secondary volatilization of lignite was observed), and their DTG curves displayed three apparent weight loss peaks. The first and second weight loss peaks are ascribed to the dehydration and devolatilization processes, respectively. Since the Na element easily absorbs water in the air, the first weight loss peak sharpened as the Na content in the OCs increased. In terms of the third weight loss peak (corresponding to the secondary devolatilization and char conversion), the initial and peak temperatures of the Na5, Na10, and Na15 samples were 552 °C and 832 °C, 527 °C and 748 °C, and 516 °C and 705 °C, respectively, which are lower than those of the hematite sample (700 °C and 910 °C), as shown in

Table 4. Characteristics of the char conversion using hematite and the Na–modified OCs.

Sample	Initial Temperature (°C)	Peak Temperature (°C)	Peak Value (wt.%/min)
Hematite	700	910	1.82
Na5	552	832	1.91
Na10	527	748	1.69
Na15	516	705	1.68

. Furthermore, the initial and peak temperatures gradually decreased as the Na content in the OCs increased. These results suggest that Na modification increased the reactivity of hematite because of the formation of Fe–O–Na species in the modified OCs. Additionally, the OC reactivity increased as the Na content increased.

3.2.2. Effect of Ca Modification

The effect of Ca modification on the reactivity of hematite is discussed, as shown in Figure 5 and

Table 5. Characteristics of the char conversion using hematite and Ca–modified OCs.

Sample	Initial Temperature (°C)	Peak Temperature (°C)	Peak Value (wt.%/min)
Hematite	700	910	1.82
Ca5	709	851	0.86
Ca10	709	897	1.09
Ca15	721	897	0.92

. The four mixtures (lignite–containing Ca5, Ca10, Ca15, and hematite) exhibited similar weight losses, indicating that Ca modification did not promote lignite conversion. As shown in Figure 5b, two small weight loss peaks corresponding to the lignite dehydration (stage I) and devolatilization (stage II) processes, respectively, are almost overlapping, indicating that Ca modification did not promote the devolatilization of lignite. As the Ca content increased, the third weight loss peak (stage III) became more apparent (the weight loss peak at 673 °C for the Ca15 sample), suggesting that Ca modification can promote the secondary devolatilization of lignite, where the volatile products can react with lattice oxygen in this stage. The fourth weight loss peak is attributed to the lignite char conversion. The initial and peak temperatures of the Ca5 (709 °C and 851 °C), Ca10 (709 °C and 897 °C), and Ca15 (721 °C and 897 °C) samples were similar to those of hematite (700 °C and 910 °C). However, the reaction rates of the different lignite–containing OCs were substantially different. The Ca5, Ca10, and Ca15 samples had maximum reaction rates (i.e., the reaction rate of the OC and lignite char) of 0.86 wt.%/min, 1.09 wt.%/min, and 0.92 wt.%/min, respectively, which are lower than that of the hematite sample (1.82 wt.%/min). These results indicate that Ca modification decreases the reaction rate of OCs with lignite char.

Comparative experiments were conducted on the reactivity of the different lignite–containing OCs (i.e., the lignite–containing SiO₂, CaO, CaFe₂O₄, Ca15, and hematite) to determine why the reactivity of the Ca–modified hematite decreased, and the results are illustrated in Figure 6 and

Table 6. Characteristics of the char conversion using hematite and the Ca–modified OCs.

Sample	Initial Temperature (°C)	Peak Temperature (°C)	Peak Value (wt.%/min)
Hematite	700	910	1.82
CaO	543	661	0.73
CaFe2O4	640	850	0.89
Ca15	721	897	0.92

. CaO and CaFe₂O₄ were newly formed crystal phases in the Ca15 sample. Hematite was observed to exhibit the highest weight loss among those five mixtures, indicating that hematite of equal mass has the highest lattice oxygen content. As shown in Figure 6b, there is an overlap with the first weight loss peak ascribed to the dehydration process for all the samples. The CaO sample can be observed to exhibit two large weight loss peaks at 400 °C and 660 °C. The weight loss peak at 400 °C is noticeably stronger than that of the other four samples, which is attributed to the catalytic effect of CaO on the lignite devolatilization process [43]. The weight loss peak at about 660 °C is attributed to CaO, which can further promote the secondary devolatilization of lignite. Similar to the SiO₂ sample, the CaO sample did not exhibit a char conversion peak (stage IV) because it could not provide lattice oxygen. Although the DTG curves of CaFe₂O₄ and hematite basically coincided in the first three stages (the third peak is not noticeable), the reaction rate of CaFe₂O₄ (0.89 wt.%/min) was significantly lower than that of hematite (1.82 wt.%/min) in stage IV (the reaction between lattice oxygen and lignite char), indicating that CaFe₂O₄ has a lower reactivity than hematite. This is consistent with a previous finding [44]. Consequently, the Ca–modified hematite had a lower reactivity than the original hematite because of the formation of large amounts of CaFe₂O₄.

Generally, it can be concluded that CaO plays a major role in promoting lignite devolatilization, while the formation of a solid CaFe₂O₄ solution inhibits lattice oxygen release, thereby decreasing the reactivity of OCs.

3.2.3. Effect of Cu, Mn, and Ni Modifications

The effects of the modification of transition metals (i.e., Cu, Mn, and Ni) on the reactivity of hematite are also discussed in this paper. As shown in Figure 7a, the three mixtures (the lignite–containing Cu5, Cu10, and Cu15 samples) exhibited weight losses of 41.1 wt.%, 40.1 wt.%, and 43.3 wt.%, respectively, which are lower than that of hematite (43.5 wt.%). This is attributed to the CuO sample having a lower lattice oxygen content (20 wt.%) than the Fe₂O₃ sample (30 wt.%). As shown in Figure 7b, the first weight loss peaks of all the samples overlap because of the dehydration process, but the second weight loss peak exhibits a slight increase as the Cu content increases, which is due to the small amount of free O₂ released by CuO/CuFe₂O₄ [45,46]. The third weight loss peaks of all the samples are extremely small, and they also almost overlap. For the fourth weight loss peak, the initial temperature of the four mixtures was about 700 °C, but the peak temperatures of the Cu–modified OCs were higher than that of hematite (

Table 7. Characteristics of the char conversion using hematite and the Cu–modified OCs.

Sample	Initial Temperature (°C)	Peak Temperature (°C)	Peak Value (wt.%/min)
Hematite	700	910	1.82
Cu5	700	940	1.09
Cu10	700	930	0.89
Cu15	700	920	0.79

). Moreover, the Cu–modified OCs with lignite char had a lower reaction rate than hematite. These results reveal that Cu modification reduced the reactivity of hematite, inhibiting the reactions between lattice oxygen and lignite char; this trend became evident as the K content increased.

Furthermore, comparative experiments were conducted on the reactivity of the different OCs (i.e., the lignite–containing CuO, CuFe₂O₄, Cu15, and hematite), and the results are illustrated in Figure 8 and

Table 8. Characteristics of the char conversion using hematite and the Cu–modified OCs.

Sample	Initial Temperature (°C)	Peak Temperature (°C)	Peak Value (wt.%/min)
Hematite	700	910	1.82
CuO	560	650	0.49
CuFe2O4	720	901	0.64
Cu15	695	913	0.79

. The lignite–containing CuO and CuFe₂O₄ samples exhibited lower weight losses (35.5 wt.% and 39.5 wt.%) than hematite (43.5 wt.%) because the lattice oxygen content in CuO and CuFe₂O₄ was lower than that in Fe₂O₃. The CuO sample exhibited two noticeable weight loss peaks at 400 °C and 660 °C, which are attributed to the reactions between free O₂ released by CuO and the volatiles, as well as the conversion of residual lattice oxygen and lignite char, respectively. The second weight loss peaks of CuFe₂O₄ and Cu15 are also slightly higher than that of hematite, suggesting that Cu modification promoted lignite volatile conversion, and that the promotion effect of CuO was more pronounced than that of CuFe₂O₄ (the order is CuO > CuFe₂O₄ > Cu15 > hematite). In stage IV, the reaction rates of CuFe₂O₄ (0.64 wt.%/min) and Cu15 (0.79 wt.%/min) were significantly lower than that of hematite (1.82 wt.%/min), owing to the release of lattice oxygen (i.e., lattice oxygen was converted into gas phase oxygen) during the initial reaction stage of the Cu–modified OCs.

In summary, the crystalline phases of CuO and CuFe₂O₄ formed by Cu doping can promote the conversion of lignite volatiles but inhibit the conversion of lignite char.

Similar to the Ca–modified samples, the Mn–modified samples did not significantly promote lignite conversion because their weight loss values before and after Mn modification were almost consistent, as shown in Figure 9a. Meanwhile, the first three small weight loss peaks of the four mixtures (i.e., the lignite–containing Mn5, Mn10, Mn15 and hematite samples) almost overlap (Figure 9b), indicating that Mn modification had no noticeable promoting effect on the dehydration (stage I), devolatilization (stage II), and secondary devolatilization (stage III) of lignite. In stage IV, Mn15 (0.85 wt.%/min), Mn10 (0.73 wt.%/min), and Mn5 (0.78 wt.%/min) had substantially lower maximum reaction rates than hematite (1.82 wt.%/min), as shown in

Table 9. Characteristics of the char conversion using hematite and the Mn–modified OCs.

Sample	Initial Temperature (°C)	Peak Temperature (°C)	Peak Value (wt.%/min)
Hematite	700	910	1.82
Mn5	700	875	0.78
Mn10	700	885	0.73
Mn15	700	908	0.85

, indicating that the addition of Mn decreases the reaction rate of OCs and lignite char.

Experiments were conducted on the reactivity of the different OCs (i.e., the lignite–containing Mn₂O₃, MnFeO₃, Mn15, and hematite samples), and the results are shown in Figure 10 and

Table 10. Characteristics of the char conversion using hematite and the Mn–modified OCs.

Sample	Initial Temperature (°C)	Peak Temperature (°C)	Peak Value (wt.%/min)
Hematite	700	910	1.82
Mn2O3	560	650	0.54
MnFeO3	700	916	0.62
Mn15	700	908	0.85

. As shown in Figure 10a, the lignite–containing Mn₂O₃ sample exhibited a small weight loss (30.1 wt.%) because the lattice oxygen content in Mn₂O₃ (2.43 wt.%) was substantially lower than that in Fe₂O₃ (30 wt.%). Additionally, the lignite–containing MnFeO₃ sample exhibited a smaller weight loss (35.6 wt.%) than hematite (as shown in Table 10). A noticeable weight loss peak at 620 °C (in stage III) can be observed in the DTG curves of Mn₂O₃ (Figure 10b), which is attributed to the release of free O₂ from Mn₂O₃ to promote lignite conversion [47]. Similar to that of the CuO sample, the char conversion process of the Mn₂O₃ sample advanced and coincided with the secondary devolatilization process because of the release of free O₂. During the char conversion process, the weight loss rates of MnFeO₃ (0.62 wt.%/min) and Mn15 (0.85 wt.%/min) were significantly lower than that of hematite (1.82 wt.%/min), indicating that the formation of the MnFeO₃ phase decreased the reaction rate of the OCs and lignite char, thereby inhibiting the lignite conversion.

In conclusion, the Mn₂O₃ and MnFeO₃ phases in the Mn–modified hematite OCs both inhibit reactions between the OC and lignite char, but the Mn₂O₃ phase alone promotes reactions between the OC and lignite volatiles.

The effects of Ni modification on the reactivity of hematite were determined, and the results are shown in Figure 11 and

Table 11. Characteristics of the char conversion using hematite and the Ni–modified OCs.

Sample	Initial Temperature (°C)	Peak Temperature (°C)	Peak Value (wt.%/min)
Hematite	700	910	1.82
Ni5	760	910	1.68
Ni10	760	910	1.75
Ni15	760	910	1.39

. The TG curves (Figure 11a) show that the four mixtures (i.e., the lignite–containing Ni5, Ni10, Ni15, and hematite samples) exhibited weight losses of 43.88 wt.%, 46.09 wt.%, 41.31 wt.%, and 43.5 wt.%, respectively, with Ni15 exhibiting a noticeably smaller weight loss than hematite. This indicates that excessive Ni loading inhibits lignite conversion because high Ni contents cause surface carbon deposition [35,48]. Additionally, high Ni contents result in the sintering of OC particles. As shown in Figure 11b, the third weight loss peak increased slightly as the Ni content increased, demonstrating that the addition of Ni promotes the secondary devolatilization of lignite. Regarding the fourth weight loss peak, the initial and maximum reaction temperatures of the Ni5, Ni10, and Ni15 samples are consistent with those of the hematite sample (i.e., 700 °C and 910 °C, respectively), as shown in Table 11. Additionally, the reaction rate of the Ni15 sample (1.39 wt.%/min) was substantially lower than those of the Ni5 (1.68 wt.%/min), Ni10 (1.75 wt.%/min), and hematite (1.82 wt.%/min) samples, indicating that excessive Ni loading is ineffective for lignite char conversion.

Figure 12 presents the experimental results for SiO₂, hematite, Ni15, NiO, and NiFe₂O₄, which were all mixed with lignite. Similar to the CuO and Mn₂O₃ samples, the lignite–containing NiO samples exhibited a smaller weight loss (38.7 wt.%) than hematite (43.5 wt.%) because the lattice oxygen content in NiO (21.33 wt.%) was also lower than that in Fe₂O₃ (30 wt.%), as illustrated in Figure 12a. The DTG curves of the five mixtures almost overlap in the first two stages, indicating that the crystalline phases of NiO and NiFe₂O₄ did not affect the lignite dehydration and devolatilization processes, as illustrated in Figure 12b. The initial and peak temperatures for the maximum weight loss peak of NiO (526 °C and 720 °C) and NiFe₂O₄ (589 °C and 880 °C) were significantly lower than those of Ni15 (760 °C and 910 °C) and hematite (700 °C and 910 °C), as shown in

Table 12. Characteristics of the char conversion using hematite and the Ni–modified OCs.

Sample	Initial Temperature (°C)	Peak Temperature (°C)	Peak Value (wt.%/min)
Hematite	700	910	1.82
NiO	526	720	0.98
NiFe2O4	589	880	0.89
Ni15	760	910	0.92

, which is attributed to the high activity of NiO and NiFe₂O₄, which can react with lignite secondary devolatilization products and lignite char [49,50,51]. The third weight loss peak of the Ni15 sample is ascribed to the formation of the NiFe₂O₄ phase.

In conclusion, the high reactivity of the Ni–modified OCs was due to the formation of the NiO and NiFe₂O₄ crystal phases.

Comparative experiments were also conducted using the M10 OCs to compare the reactivity of the different metal–modified OCs containing lignite. The results are shown in Figure 13 and

Table 13. Characteristics of the char conversion using hematite and the exogenous metals–modified OCs.

Sample	Initial Temperature (°C)	Peak Temperature (°C)	Peak Value (wt.%/min)
Hematite	700	910	1.82
K10	563	768	1.89
Na10	527	748	1.69
Ca10	709	897	1.09
Cu10	700	930	0.89
Mn10	700	885	0.73
Ni10	760	910	1.75

.

As shown in Figure 13, the largest weight loss peak (i.e., the reaction between lattice oxygen with lignite char) of K10 and Na10 are significantly forward–shifted in comparison to that of the other samples, with a significantly lower initial and peak temperature than that of the other samples (Table 13), while the maximum weight loss rate of K10 and Na10 was less different from that of hematite. This indicates that the reactivity of the K–modified and Na–modified hematite OCs was higher than that of the other OCs. The peak value of K10 is larger than Na10, indicating that the modification of K is better than Na.

The peak temperature of Ca10, Cu10, Mn10, and Ni10 are all around 800 °C, so the performance is mainly compared by the peak value of the OC with lignite char. The addition of Ni promoted the conversion of lignite volatiles, and the reaction rates of lattice oxygen in the Ni10 sample and lignite char were almost the same as that of the hematite sample (the maximum reaction rates for hematite and Ni10 were 1.82 wt.%/min and 1.75 wt.%/min, respectively), demonstrating that using an appropriate amount of Ni for modification improves the performance of hematite. The Ca–modified, Cu–modified, and Mn–modified OCs exhibited relatively weak reactivity during the lignite char conversion. The order of reactivity is as follows: hematite (1.82 wt.%/min) > Ca10 (1.09 wt.%/min) > Cu10 (0.89 wt.%/min) > Mn10 (0.73 wt.%/min).

4. Conclusions

In this study, alkaline (earth) metals and transition metals were used to modify hematite to improve its reactivity in the CL of lignite. The addition of exogenous metals significantly changed the phase of the original hematite. Furthermore, the reactivity of the modified hematite OCs mainly depended on the newly formed crystal phase.

(1)

The reactivity of the K–modified and Na–modified OCs significantly increased as the K and Na contents increased, owing to the catalytic effect of the active solid solution of K–Fe–O and Na–Fe–O in the samples.

(2)

Ca modification promoted the conversion of lignite volatiles but inhibited the conversion of lignite char because CaO promotes lignite devolatilization, while the formation of a solid CaFe₂O₄ solution with a low reactivity reduced the conversion of lignite char.

(3)

CuO and CuFe₂O₄ can release a small amount of free O₂ at high temperatures to promote the conversion of volatiles; however, the conversion of lignite char is inhibited because of the partial depletion of lattice oxygen.

(4)

Regarding the Mn–modified OCs, the presence of MnFeO₃ with a low reactivity significantly decreased the reaction rate between the OC and lignite char, thereby inhibiting the conversion of lignite.

(5)

The NiO and NiFe₂O₄ phases in the Ni–modified samples with a high reactivity could promote reactions between lattice oxygen, secondary devolatilization products, and lignite char, enhancing the performance of hematite.

(6)

Generally, K, Na, and Ni modifications can enhance the reaction performance of hematite with lignite volatiles and lignite char, while Ca and Cu modifications can only enhance the reaction performance of hematite with lignite volatiles. However, Mn modification inhibits the conversion of lignite char. Among these OCs, the K–modified hematite OCs exhibit the best promoting effect.