Enhanced Reforming of Tar Based on Double-Effect Ni/CaO–Ca12Al14O33 Catalysts: Modified by Ce, Mg, and Fe

The double-effect Ni-based catalysts, modified with Ce, Mg, and Fe and synthesized by the coprecipitation method, were applied into the enhanced steam reforming process of real tar. The effects of the catalysts with different doping mass proportions (3, 6, 9, and 12%) of Ce, Mg, and Fe on the H2 yield, and H2 and CO2 concentrations were studied. The results revealed that the tar reforming efficiency was improved with appropriate proportions of the additives added. The Ce- or Mg-doped catalyst could change the distribution or morphology of the active component Ni. The modified catalyst with 6% Ce or 3% Mg doping showed the best catalytic activity in the reforming experiment, with the H2 yield reaching 86.84% or 85.22%, respectively. The Fe-doped catalyst could form an Ni–Fe alloy and improve the stability of the catalyst, and the better catalytic activity can be obtained at 9 and 12% Fe doping, with the H2 yield reaching 85.54 and 85.80%, respectively.


INTRODUCTION
Coal gasification plays an important role in the coal chemical industry, while tar is the main by-product of this process. In China, the total output of coke was 473.1 million tons in 2019, with about 19 million tons of tar as the by-product. The composition of tar is complex, and it is estimated to include more than ten thousand compounds. Distinguish by boiling point, tar includes light oil, phenol oil, naphthalene oil, washing oil, onion oil, and coal tar pitch (Li and Suzuki, 2010;Li et al., 2013;Duan et al., 2017). Tar is gaseous at high temperature and liquid at low temperature, which will easily corrode the pipeline equipment. In addition, it can cause harm to the ecology and the human body if discharged into the environment (Xie et al., 2016a;Zeng et al., 2018;Li et al., 2021;Zuo et al., 2021). The methods of tar removal include physical and chemical methods. The physical method is to remove the tar from the syngas through the principle of absorption and adsorption, such as water washing, oil washing to absorb tar, or using porous media to adsorb tar onto its surface. The physical method has the advantages of simple operation and a low equipment cost, but there are some disadvantages such as secondary pollution and low removal efficiency (Xie et al., 2016c). The chemical method mainly includes the pyrolysis method and reforming method (partial oxidation reforming and steam reforming). The pyrolysis method refers to the tar cracking at high temperature to produce non-condensable small molecules of gas, which increases the cost due to the need of adding additional heat sources (Torres et al., 2007). The partial oxidation reforming method can effectively convert tar into small molecular gases such as H 2 and CO, but the process is mainly conducted at a higher temperature (about 1,200°C), by consuming a large amount of pure oxygen and additional fuel to supply heat (Onozaki et al., 2006). The steam reforming method prefers to obtain hydrogen-rich gas under the action of steam and catalyst. Compared with the partial oxidation reforming method, the temperature of this method is lower (about 800°C), with no additional O 2 and less heat. Thus, steam reforming of tar and its model compounds for hydrogen production has received extensive attention in recent years (Furusawa et al., 2013;Duan et al., 2015;Li et al., 2015;Dou et al., 2016;Duan et al., 2018;Xie et al., 2018).
Catalysts are key to steam reforming of tar, and many catalysts have been studied so far. The commonly used catalysts mainly include dolomite (Gusta et al., 2009;Sarıoğlan, 2012), olivine (Virginie et al., 2012), alkali metal (Jiang et al., 2015), Ni-based catalysts (Richardson et al., 2010;Chan and Tanksale, 2014;Xie et al., 2015), and noble metal catalysts (Furusawa et al., 2013;Mei et al., 2013). Considering the catalyst efficiency and cost, the Ni-based catalyst is one of the most promising catalysts for tar reforming. However, inhibiting its carbon formation and improving its catalytic stability are the urgent problems to be solved (Yue et al., 2010;Gao et al., 2015). As a carrier, Ca 12 Al 14 O 33 can improve the reactivity and the anti-carbon property of the Ni-based catalyst. It itself can also be used as a catalyst for cracking tar, and the synergistic with Ni addition can further inhibit the carbon formation (Li et al., 2009). However, common steam reforming reactions yield only 70% (volume fraction) H 2 and more than 20% CO 2 , limiting the use of such a hydrogencontaining gas (Xie et al., 2016d). So, researchers try to add adsorption components to the original catalyst and combine the catalytic reforming process with the CO 2 adsorption process. The adsorbent components can break the balance of the original reforming reaction, improving the quality and concentration of H 2 , and the carbon deposition on the catalyst can be effectively reduced (Kinoshita and Turn, 2003;Dou et al., 2016;Xie, et al., 2016b;Zuo et al., 2020). In the adsorption-enhanced process, CaO is widely used as the CO 2 adsorbent due to its low cost and easy accessibility. However, CaO is easy to be sintered at high temperatures, resulting in a decrease in the adsorption capacity (Zamboni et al., 2011). As mentioned above, as a carrier, Ca 12 Al 14 O 33 has a special free oxygen storage structure, which can also effectively inhibit the sintering of CaO when carried within Ca 12 Al 14 O 33 (Li et al., 2009). Thus, the double-effect Ni/ CaO-Ca 12 Al 14 O 33 catalyst was prepared and applied to the steam reforming process of 1-methylnaphthalene (C 11 H 10 ) as a tar model component in our previous study, showing a good catalytic activity and CO 2 adsorption capacity (Zhang et al., 2021a;Zhang et al., 2021b). However, the doubleeffect catalyst has not been applied in the reforming of real tar, and because of its component's complexity, the catalytic performance on the reforming process of real reforming may be decreased compared to that on the model component. According to the literature, the catalytic activity and anti-carbon property of the Ni-based catalyst can be improved by adding additives such as precious metals (Pt, Rh, Ru, etc.) (Mei et al., 2013;Cai et al., 2014), rare earth metals (La, Ce, etc.) (Kimura et al., 2006;Tomishige et al., 2007;Mazumder and Lasa., 2015), alkali earth metals (Ca, Mg, etc.) (Wang et al., 2006;Ashok et al., 2015;Nakhaei and Mousavi, 2015), and transition metals (Fe, Co, etc.) (Wang et al., 2011;Koike et al., 2012;Wang et al., 2013). Considering the cost, Ce (rare earth metal), Mg (alkali earth metal), and Fe (transition metal) were studied in this article.
In this article, modified double-effect Ni-based catalysts were prepared and applied to the enhanced reforming process of real tar. The double-effect Ni-based catalysts with different additives (Ce, Mg, and Fe) of different doping mass proportions (3, 6, 9, and 12%) were synthesized by the coprecipitation method and were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). Then, the effects of different modified double-effect catalysts on the hydrogen production from the enhanced reforming of tar were studied, and the result was compared with that over the unmodified doubleeffect Ni-based catalyst.

EXPERIMENTAL Tar Sample
The raw tar sample came from a coking plant in Liaoning province, China. The sample was preheated by distillation to remove moisture and solid-phase impurities prior to the chemical composition analysis. Elementar (vario MACRO cube, Germany) was used for the elemental analysis of tar, and the results show that the molar content of C is 54.50%, H is 39.27%, O is 5.32%, N is 0.70%, and S is 0.21%. The chemical formula of the sample can be simplified to C 10 H 7.21 O 0.98 , ignoring nitrogen and sulfur that were too low compared with other elements. Therefore, the steam reforming (SR) reaction of tar can be expressed by the following equation: and following the water-gas shift (WGS) reaction, we obtain Thus, the total steam reforming reaction can be written as follows: GC-MS (Agilent 5975-7890A, Shanghai Tianmei) was used for the composition analysis of tar. Specific operating conditions are as follows: 1) Chromatographic conditions: Hp-5 chromatographic column; oven temperature program was as follows: maintain 1 min at 60°C as the starting temperature, heat to 280°C with the heating rate of 15 C/min, and maintain for 5 min; and He as the carrier gas with the shunt ratio of 1:100. 2) Mass spectrometry conditions: solvent delay for 4 min; ionization source: EI; electron bombardment energy: 70 eV; October 2021 | Volume 9 | Article 729919 electronic multiplier voltage: 1200 V; and quality range: 30-600 amu, with the scanning interval of 0.5 s. Through the GC-MS analysis, a total of 22 major compounds were detected, among which the naphthalene content was the highest (given in Table 1).

Catalyst Preparation
The modified double-effect Ni-based catalysts were prepared by the coprecipitation method, with the mass proportion of Ni (

Experimental Apparatus
The experimental apparatus for the reforming experiment is shown in Figure 1. The modified double-effect catalyst was kept in a three-stage temperature control tube furnace. The tar and deionized water were pumped into the stainless-steel downstream reactor drop by dropping through an injection pump and a peristatic pump with 600 ml/min N 2 carrier gas.
To ensure the good fluidity of tar, the syringe was wrapped with a heating belt, keeping the wall temperature above 50°C. The product gas in the reactor was cooled and dried successively, and finally detected by a gas meter and a gas analyzer.

Experimental Design and Assessment
In this article, the catalytic performances of the modified doubleeffect catalysts were assessed under the optimal experimental condition based on the previously enhanced reforming experiment result with the unmodified catalyst (temperature 740°C, S/C (the mole ratio of steam to carbon in tar) 15, and WHSV (weight hourly space velocity) 0.0447 h −1 ). The experimental process of the tar reforming can be divided into two stages: enhanced reforming stage and common reforming stage (shown in Figure 2). For the enhanced reforming stage, CO 2 generated via the WGS reaction (Eq. 2) was adsorbed by CaO in the doubleeffect catalyst (adsorption reaction, Eq. 4). Then, the equilibriums of the steam reforming reactions of tar were transferred to the direction of hydrogen production, with the CH 4 and CO concentrations being very low and the H 2 concentration being quite high. As the time went by, the concentration of CO 2 was increased and that of H 2 was decreased because the CO 2 adsorption capacity of the double-effect catalyst tended to be saturated. Finally, the H 2 and CO 2 concentrations tended to be stable, about 70 and 25%, respectively, which were similar with those of the dry reformed gas without adding any CO 2 adsorbent, and thus, such a stage was considered as a common reforming stage (Xie et al., 2018;Zhang et al., 2021b), The main indicators in this article were as follows.  H 2 yield (Y H2 ) was defined as the ratio of the output of H 2 (Q H2, out ) in the reformed gas to the theoretical output of H 2 (Q H2 ) when all the input tar is converted completely via Eq. 3.
The dry reformed gas (Q total, out ) was considered to be composed of H 2 , CO, CO 2 , and CH 4 . The concentration of the reformed gas (C i, i was H 2 , CO, CO 2 , or CH 4 ) was defined as the ratio of the output of i (Q i, out ) to the total output of the reformed gas (Q total, out ). In this article, only the concentrations of H 2 and CO 2 were studied in consideration of the extremely low concentrations of CO and CH 4 ,

Catalyst Characterization Results
Prior to characterization, the prepared catalysts were activated at 800°C with 10% mol H 2 /N 2 stream for 3 h. The structure of the catalysts was characterized by XRD (Shimazu, Japan, XRD-7000) using Cu Kα radiation operated at 30 kV and 40 mA, with the scanning angle of 10-80 rad and the scanning speed of 2 rad/min. The morphology of the catalysts was observed by SEM with the aid of a Hitachi su-8010 microscope. Figure 3 shows the XRD patterns of Ce-modified catalysts with different doping mass proportions. As shown in the figure, the Ce-modified catalysts include the active components, Ni and CeO 2 ; adsorption component, Ca(OH) 2 (formed by CaO absorbing water in the air); and carrier component, Ca 12 Al 14 O 33 . As the doping proportion increases, the peak value of CeO 2 significantly increased. Figure 4 shows the SEM spectra of Ce-modified catalysts as well as the undoped sample (shown in Figure 4A). In the spectra, the bright part is mainly the active component Ni (shown by arrows 1, 3, 4, and 6), and the gray part is the adsorption component CaO and the carrier component Ca 12 Al 14 O 33 (shown by arrows 2, 5, and 7); the element contents of the different parts are shown in Table 2. The different particles were based on a CaO-Ca 12 Al 14 O 33 matrix, with Ni and CeO 2 dotting on the surface of the matrix by small spherical particles. Proper addition of Ce can improve the distribution of Ni, which showed that part of Ni no longer relied on or just relied on a small amount of the CaO-Ca 12 Al 14 O 33 matrix after Ce doping, with the small particles of Ni-CeO 2 also slightly increased.

Reformation Result
CeO 2 generally has a strong oxygen storage capacity due to its face-centered cubic structure. CeO 2 can provide oxygen atoms to the adjacent nickel metal and promote the oxidation reaction of carbon intermediate products on nickel metal, thus improving the catalyst reactivity and reducing the formation of carbon deposition (Kimura et al., 2006;Tomishige et al., 2007). Figure 5A shows the H 2 yields of tar reforming over the catalysts with different Ce doping mass proportions. Overall, the catalysts, H 2 yields, and concentrations at the enhanced reforming stage were higher than those at the common reforming stage. It could also be seen that Ce doped in the catalyst could improve the H 2 yield at both the stages. As the doping proportion rose, the H 2 yields in the two stages first increased, then decreased, and finally reached the highest at the doping proportion of 6% (86.84% for the enhanced reforming). It was corroborated that a small amount of Ce doping made the active component no longer completely rely on the CaO-Ca 12 Al 14 O 33 matrix, and the active component distribution was more uniform,  thus promoting the SR reaction (Eq. 1), with the increase in the H 2 yield. However, the excessive Ce (>6%) doping produced more Ni and CeO 2 aggregated particles, which reduced the effective catalytic activity area, with the decrease in the H 2 yield. Figures 5B,C show the H 2 and CO 2 concentrations obtained over the catalysts with different Ce doping proportions. Due to the in-situ CO 2 adsorption, the H 2 concentrations of the enhanced reforming all over the catalysts with Ce doping were >90%, and among them, the sample with 6% Ce had the highest H 2 concentration (95.44%) and a very little CO 2 concentration, on some extent indicating that proper Ce addition can also improve the dispersion of CaO. After the adsorption saturation, i.e., at the common reforming stage, the H 2 concentrations were all about 72% and CO 2 concentrations were about 23-24%.

Effect of the Mg-Modified Double-Effect Ni-Based Catalyst
Catalyst Characterization Results Figure 6 shows the XRD patterns of Mg-modified catalysts with different doping mass proportions. With the rise of the doping proportion, the peak value of Ni decreased, while those of MgO and MgNiO 2 spinels increased significantly. From Figure 7 and Table 3, we can see that the CaO-Ca 12 Al 14 O 33 matrix was still the main composition in the catalyst (shown by arrows 2, 4, and 6), and the active components (shown by arrows 1, 3, 5, 7, and 8) on the matrix were changed from the original globular particles to spiny pellets, caused by the formation of MgNiO 2 . With the rise of the doping proportions, the Mg-modified catalysts also showed the phenomenon that the active components detached from the CaO-Ca 12 Al 14 O 33 matrix and then aggregated alone, resulting in the decrease in the catalytic area (shown by arrows 7 and 8).

Reformation Result
Mg is a common additive in the process of steam reforming. MgO has a certain catalytic property and can also promote the oxidation of CO. For the Ni-based catalyst, the doping of Mg enables the catalyst to form one Ni-Mg-O solid solution, which has good stability and resistance to carbon deposition (Wang et al., 2006). Figure 8 shows the H 2 yield and concentrations over the catalysts at different Mg doping proportions. For the two reforming stages, the Compared with the undoped catalyst, the increase in the H 2 yield and concentration corroborated that the increase in the catalytic area, causing the active components to change from the original globular particles to spiny pellets, promoted the SR reaction. Then, the decrease in the H 2 yield and concentration was because too much of the Mgdoped catalyst caused the active components to detach and aggregate, reducing the catalytic area. In the enhanced reforming stage, the CO 2 concentration gradually increased with the rise of the Mg doping proportion, to some extent corroborating the result found in the literature that doping Mg in the catalyst could promote the oxidation of CO (Wang et al., 2006). For the common reforming, Mg addition showed a slight increase in the H 2 concentration and a slight decrease in the CO 2 concentration with the increase in the doping proportion, which almost had no effect.

Effect of the Fe-Modified Double-Effect Ni-Based Catalyst
Catalyst Characterization Results Figure 9 shows the XRD patterns of Fe-modified catalysts with different doping mass proportions. The XRD spectra of Femodified catalysts were similar to those of unmodified catalysts, and the peak value of Fe 2 O 3 was weak and nearly did not change with the rise of the doping proportion. The SEM spectra and EDS results of Fe-modified catalysts are shown in Figure 10 and Table 4. It could be seen that Ni (shown by arrows 1, 3, 5, 7, and 8) adhered to the surface of the CaO-Ca 12 Al 14 O 33 matrix (shown by arrows 2, 4, 6, and 8) in a

Reformation Result
The oxide of Fe can be used as a catalyst for the reforming reaction, and its catalytic activity is lower than that of the Nibased catalyst. For example, the active component of olivine is mainly Fe 2 O 3 . Ni and Fe can closely combine to form a Ni-Fe alloy and improve the stability of the catalyst. In addition, Fe doping in the catalyst can provide oxygen to neighboring nickel species and promote the oxidation of carbon, reducing the amount of carbon formation (Wang et al., 2011).
The effects of the modified catalysts on the H 2 yield and concentration are shown in Figures 11A,B. From the figure, it is clear that the H 2 yield and concentration gradually increased with the rise of the Fe doping proportion and then flattened out when the Fe doping proportion was over 9%. The H 2 yield of the enhanced reforming can reach 85.54 and 85.80% at the doping proportions of 9 and 12%, respectively. The result corroborated that Fe doping in the catalyst (forming a Ni-Fe alloy) could improve the catalyst stability, which promoted the SR reaction for the hydrogen production. For the common reforming, the concentrations of the products were relatively stable over the catalysts with different Fe loads, around 73% for H 2 and around 24% for CO 2 . However, for the enhanced reforming, the CO 2 concentration showed an increasing trend with the increase in the Fe load, like the H 2 yield, to some extent corroborating that doping Fe in the catalyst could promote the oxidation of CO.

DATA AVAILABILITY STATEMENT
The original contributions presented in the study are included in the article/Supplementary Material; further inquiries can be directed to the corresponding authors.