Pyrolysis char derived from waste peat for catalytic reforming of tar model compound

Applied Energy 263 (2020) 114565

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Pyrolysis char derived from waste peat for catalytic reforming of tar model compound

T

⁎

Shuxiao Wanga,b,c,d, Rui Shana,b,c,d, Tao Lua,b,c,d, Yuyuan Zhange, Haoran Yuana,b,c,d, , Yong Chena,b,c,d a

Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences, Guangzhou 510640, China Southern Marine Science and Engineering Guangdong Laboratory, Guangzhou 511458, China c CAS Key Laboratory of Renewable Energy, Guangzhou 510640, China d Guangdong Provincial Key Laboratory of New and Renewable Energy Research and Development, Guangzhou 510640, China e College of Materials Science and Energy Engineering, Foshan University, Foshan 528000, China b

H I GH L IG H T S

pyrolysis peat char as a new carrier to prepare composite catalyst. • The porosity of char was significantly enhanced by KOH and CO activation. • The syngas yield of 88.1% was obtained over the Fe/CPC catalyst. • AThehighpresence of FeC and FeSiO can ensure the activity and stability of catalyst. • The mol% of syngas decreased from 88.1% to 64.2% after 8 h. • 2

3

A R T I C LE I N FO

A B S T R A C T

Keywords: Tar removal Pyrolysis char Waste peat Catalytic cracking

The pyrolysis char derived from solid waste peat was used in the removal of biomass tar. A laboratory dual-stage reactor was designed to obtain a cost-effective and eco-friendly tar removal approach using peat pyrolysis charbased catalyst. Rich pore structure of pyrolysis char can enhance the adsorption and removal performance of tar, the KOH and CO2 activation method were used to increase the pore structure of pyrolysis char. Toluene was chosen as the model compound of biomass tar for basic research. The effects of pyrolysis char and transition metal Fe on toluene removal were studied. The investigated reforming parameters were reaction temperature (700–900 °C), residence time (0.3–0.8 s) and steam-to-carbon ratio (1.5:1–4:1). The results indicated that the peat pyrolysis char-based Fe catalysts showed excellent catalytic performance (toluene conversion > 89%) and gas selectivity, especially the catalyst that activated by CO2 had the best selectivity for syngas (88.1 mol%), and the waste peat catalyst was compared with other waste pyrolysis char-based catalysts. Textural characterization showed that the excellent catalytic activity and stability of the catalysts are due to the presence of FeC and FeSiO3 structures. Such the peat pyrolysis char can as a carrier be used to remove tar and produce high content syngas in pyrolysis process.

1. Introduction Pyro-gasification technology is considered as one of the effective ways for production of sustainable fuels. Tar, gases, and a solid char are

the three main products during pyro-gasification [1]. Tar, which includes 1- to 5-ring aromatic compounds along with other oxygen-containing hydrocarbons and complex polycyclic aromatic hydrocarbons, is unacceptable because of its difficulty of use, ease of coking, pollution

Abbreviations: PC, peat char; KPC, KOH activated peat char; CPC, CO2 activated peat char; Fe/PC, peat char-based Fe catalyst; Fe/KPC, KOH activated peat charbased Fe catalyst; Fe/CPC, CO2 activated peat char-based Fe catalyst; XRF, X-ray fluorescence; SEM, scanning electron microscopy; EDS, energy dispersive spectroscopy; FTIR, Fourier transform infrared spectroscopy; XRD, X-ray diffraction; BET, Brunauer-Emmett-Teller; DFT, Density Function Theory; τ, residence time; S/C, steam-to-carbon ratio ⁎ Corresponding author at: Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences, No. 2 Nengyuan Road, Wushan, Tianhe District, Guangzhou, Guangdong 510640, China. E-mail address: [email protected] (H. Yuan). https://doi.org/10.1016/j.apenergy.2020.114565 Received 9 September 2019; Received in revised form 14 January 2020; Accepted 25 January 2020 0306-2619/ © 2020 Elsevier Ltd. All rights reserved.

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of the environment, destruction of equipment and so on [2,3]. Effective control and conversion of tar is a key problem in pyro-gasification technology. Among tar elimination, catalytic reforming is a universal method which has a high reaction rate and reliability, and it is considered a safe, environmentally friendly and cost-effective technology [4]. Various types of catalysts such as alkali [5], natural zeolite [6], carbon-supported [7], metal-based catalysts [8] were used for the reforming of tar. For example, Masnadi [9,10] and Ellis [11] showed that alkali and alkaline earth metals are effective for tar removal in their studies. Moreover, many researchers such as Mueangta [12], Santamaria [13], Guan [14] have proven that steam reforming method has better tar conversion ability, because adding water can effectively convert tar into useful gases, such as H2, CO, CO2, and, CH4 based on the following reactions formula:

Here, we choose waste peat pyrolysis char as carrier to prepare composite catalyst by supporting metal Fe. Through tailored design of the reactor and precise control of experimental conditions, we utilize the bonding mechanism between silica and Fe to improve the catalytic activity and stability of the catalyst of peat char-based Fe catalysts. In order to further improve the catalytic performance, KOH and CO2 activation methods were used to increase the pore structure of the char. In catalytic cracking experiments, a simplified model tar, toluene was selected to facilitate fundamental research. In addition, the influence of the reaction temperature, residence time (τ) and steam-to-carbon ratio (S/C) on the catalytic reaction and the molar ratios of H2, CO, CO2 and CH4 in the generated gas were investigated to study the influencing factors of catalytic performance.

Cx Hy Oz + (x − z) H2 O → xCO + [(x + y /2 − z ) H2]

2. Experimental

CO + H2 O ↔ CO2 + H2

2.1. Materials

In recent studies, using a solid waste pyrolysis char material as a carrier and combine with the metal to prepare catalysts has become more popular, this method can realize the reuse of solid waste, and has the advantages of low cost, renewable and environmental friendly [15,16]. Char which produced by pyrolysis or gasification, has abundant mesoporous and microporous structures and high specific surface area [17]. Thus, the porosity of carbon is good for metal loading and can make tar more easily exposed to active sites. In addition, deactivation is usually a limiting factor for other catalysts, char is a continuously produced and available as a by-product of gasification which can avoid the problem of deactivation [18,19]. In this scenario, waste from pine sawdust [20], food [21], wood pallets [22], rice husk [23], ash [24] have been studied for the preparation of catalysts. Hervy et al. [25] have designed a catalytic reaction system of ethylbenzene and benzene, they used wood and food waste char as materials to produce syngas. Authors have studied the physicochemical properties of the chars, the cracking temperature and the relationships among the deactivation mechanisms. Shen et al. [26] have reported a novel rice husk char and char-supported Ni-Fe catalysts which from biomass pyrolysis were used to remove the biomass tar, and their study proved that char-supported Ni-Fe catalysts exhibited more advantages in terms of convenience and energy savings. Ravenni et al. showed that gasification char and active carbon were effective as adsorbents for tar species and active as catalysts for tar conversion [27]. In addition, to the best of our knowledge, the catalyst deactivation is a key issue in catalytic cracking reactions and an important evaluation index for catalysts, it is a critical technical point to choose a suitable catalyst support. In our previous paper [28], we have reported that when using waste peat pyrolysis char (C-SiO2) as a catalyst support, the deactivation of the peat char-based catalysts can be slowed because of the bonding mechanism between the silica structure and metal ions such as Si-O-K and Si-O-Al [29]. Peat char contains a large amount of carbon and silica which can be combined with active substances, and the existence of stable bonds can prevent the loss of active ions and increase the service life and recycling times of the catalyst. Suppose that peat char was served as a support to synthesize metal catalysts for tar reforming, on the one hand, the presence of the active metal will ensure the activity of the catalyst, at the same time, the bonding between the carrier and metal can effectively prevent the deactivation of catalysts, this combination not only can ensure the catalytic activity, but also can enhance its stability. On the other hand, the porosity of peat char is good for metal loading and can be highly exposed to tars, and as a byproduct of gasification, the peat char has the ability to absorb tar due to its abundant pore structure and surface functional groups. In addition, the high catalytic activity of metallic Fe has been proven in many cases [30,31], and Fe-based catalysts are widely used due to their inexpensive and environmental friendly nature.

Peat was collected from peatland in the Baekdu Mountain range (China). Potassium hydroxide (KOH, AR, 98%) and ferric trichloride hexahydrate (FeCl3·6H2O, AR, 98%) were purchased from Shanghai Macklin Biochemical Co. Ltd, and toluene (AR, 99.8%) and isopropyl alcohol (AR, 99.8%) were supplied by Guangzhou Chemical Reagent Co. Ltd, nitrogen gas (N2, purity > 99.999%) and carbon dioxide (CO2, purity > 99.99%) were purchased from the Guangzhou Shengying Gas Co. Ltd, China. 2.2. Pyrolysis char and catalyst preparation The raw material peat was dried at 105 °C for 24 h. Then, the dried peat was calcined at 800 °C for 2 h under N2 atmosphere to acquire the peat char (PC). The prepared PC was analyzed by elemental analysis, XRF (X-ray fluorescence) analysis and ash analysis, and the data are shown in Table 1. The main components of PC were C, O, Si and a small amount of metallic elements (Al, K, Fe and Ca), and the peat char has a high ash content (63.43%). The PC was subsequently sieved to 0.15–0.18 mm in diameter. Then, the KOH or CO2 activation methods were used to increase the surface area and pore volume of PC. The details of the KOH activation method was as follows: 2 M KOH solution 500 ml with 2 g of peat char were mixed and stirred for 1 h, then mixed solution was dried overnight in an oven at 105 °C to get the catalyst precursor. The precursor was calcined at 800 °C under N2 atmosphere, and the calcined product was filtered washed by deionized water for several times until the pH of the filtrate maintained stable to obtain KPC (KOH activated peat char). The CO2 activation method proceeded follow: the dry chars were loaded in a vertical fixed-bed quartz reactor and heated at 10 °C/min under pure N2 flow to 800 °C. Then, the nitrogen flow was replaced by carbon dioxide (99.99%) flow. After 30 min at 800 °C, the carbon dioxide flow was replaced by nitrogen flow, and the reactor was cooled to room temperature to obtain CPC (CO2 activation peat char). In this paper, we used the wet impregnation method to prepare the catalysts. Typically, 4.8 g of FeCl3·6H2O powder dissolved in 100 ml of water, then add 10 g of dried PC to dissolve. The resulting solution was stirred for 3 h at 800 rpm. Subsequently, the suspension was dried at 105 °C for 24 h, followed by activation (1 h at 800 °C) under N2 atmosphere. The catalyst was designated as Fe/PC. The catalysts Fe/KPC Table 1 Element and ash content of peat char. Sample

PC

2

Elemental content (wt.%)

Ash (wt.%)

C

O

Si

Al

Ca

Fe

K

others

39.06

24.45

16.50

7.76

3.88

3.04

2.45

2.86

63.43

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min, and that of H2O was 0.36 ml/min; moreover, the factor of steamto-carbon ratio (S/C) and residence time (τ) was regulated by the rates of toluene and H2O and the flow velocity of nitrogen, respectively. The raw material entered the reaction chamber in gas-phase through the vaporizer and heater band, and then the liquid-phase and gas-phase products were separated through the receiving flask. Gas-phase products were collected by the air pocket method, and the unreacted toluene was absorbed by isopropyl alcohol in the receiving flask.

and Fe/CPC were obtained similarly. 2.3. Analysis and characterization 2.3.1. Microscopic morphological characterization Scanning electron microscopy (SEM) images and Energy Dispersive Spectroscopy (EDS) were obtained by using SEM system equipped (Japan's Hitachi, S-4800, cold field-emission), and the sample was sprayed with gold before this. The N2 adsorption/desorption (77 K) and CO2 adsorption/desorption (273 K) of the catalysts were evaluated using an automated gas sorption analyzer (Quadrasorb, Quantachrome, USA). The Specific surface area and pore distribution of N2 adsorption/ desorption was calculated by the BET (Brunauer-Emmett-Teller) method, the Specific surface area and pore distribution of CO2 adsorption/desorption was calculated by the DFT (Density Function Theory) method.

2.4.2. Experimental data evaluation The contents of generated gas (H2, CO, CH4 and CO2) were detected by gas chromatograph (Agilent 7890A), and the unreacted toluene was determined by liquid chromatograph (Agilent 1290-6540). The toluene conversion (C) was estimated using Eq. (1)

C= 2.3.2. Textural characterization Fourier transform infrared spectroscopy (FTIR) spectra were recorded on a FT-IR spectrometer (TENSOR27, Bruegel, Germany) in the range of 400–3700 cm−1. Powder samples The sample was thoroughly ground and mixed with potassium bromide and then pressed into a uniform and transparent round cake. X-ray diffraction (XRD; X’Pert Pro MPD, PANalytical, D8 Advance) patterns of the catalyst were analyzed in the range from 10° to 80°. Powder coupled with Cu Kσ radiation and a step size of 0.02° was used for all samples. The data were processed with Jade 6.5 software.

mtol,in − mtol,out 100% mtol,in

(1)

where mtol,in was the amount of toluene injected, mtol,out was the amount of unreacted toluene. The performance of catalysts was investigated for catalytic toluene removal, and the influence factors of reaction temperature, residence time (τ) and steam-to-carbon ratio (S/C). The value of τ was estimated according to Eq. (2)

τ=

S∗h f N2

(2)

where S is the area of the catalysts, h is the height of the catalyst bed, f N2 is the flow velocity of nitrogen. S/C was estimated according to Eq. (3)

2.4. Apparatus and experimental parameters 2.4.1. Experimental reactor The experimental reaction system consisted of a bench scale plant (Fig. 1). Approximately 0.5 g of catalyst was preloaded into the central location of the quartz reactor followed by protection by nitrogen purging at 100 ml/min for 20 min, then heating at a rate of 10 °C/min to the desired temperature 800 °C under N2. When the temperature stabilized at 800 °C, H2O and toluene were fed by a peristaltic pump and injection pump, respectively. The feeding rate of toluene was 0.1 ml/

S/C =

m H2O/MH2O 7 ∗ mtol /Mtol

(3)

where m H2O is the weight of the injected water, MH2O is the molar mass of water, mtol is the mass of the injected toluene, and Mtol is the molar mass of the injected toluene.

Fig. 1. Experimental schematic of toluene reforming. 3

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Fig. 2. SEM images of (a) PC, (b) KPC, (c) CPC, (d) Fe/PC, (e) Fe/KPC, (f) Fe/CPC.

3. Results and discussion

Table 3 Textural properties of the samples.

3.1. Catalyst microscopic and textural characterization

Samples

3.1.1. Microscopic morphological characterization Fig. 2 shows the SEM images of (a) PC, (b) KPC, (c) CPC, (d) Fe/PC, (e) Fe/KPC and (f) Fe/CPC. PC particles have a blocky massive structures with few holes. After activation by KOH, KPC displays an irregular ridged outer layer, and the number of small clumps increases. After activation by CO2, it can be seen that the outer surface of CPC becomes thinner and ridged with petal-like morphology. The results illustrate that KOH activation and CO2 activation significantly increase the surface area of the char, and makes it more favorable to be used as a catalyst support. In addition, (d), (e) and (f) show the morphology of the Fe catalysts. It can be seen that the outer surfaces of Fe/PC, Fe/KPC and Fe/CPC are rough and corrugated with a globular outer layer, and many cavities with varying sizes have been formed. Table 2 lists the elemental compositions of the chars and catalysts. As observation, the main components of PC were C, O, Si and a small amount of metallic elements (Al, K and Ca), the DES test results are consistent with the results of elemental analysis mentioned above. The KOH and CO2 activation treatments had no significant effect on elemental composition. When the chars were loaded with iron, the Fe content ranged from 7.98% to 8.90%, which indicated that metallic matter was successfully inserted into the char framework. Table 3 shows the textural properties of the chars and catalysts. The mesopore and macropore were detected by N2 adsorption/desorption method, and micropore was detected by CO2 adsorption/desorption method. As a result, the BET and DFT surface area of pure PC were 79.36 m2/g and 93.94 m2/g, the pore radius of macropore and micropore were 83.32 nm and 1.84 nm, respectively. When using the KOH or CO2 activation method, there was a significant increase in char textural

PC KPC CPC Fe/PC Fe/KPC Fe/CPC

Fig. Fig. Fig. Fig. Fig. Fig.

1a 1b 1c 1d 1e 1f

O

Si

Fe

Al

K

Ca

Others

45.23 46.05 46.19 37.23 38.34 38.11

25.07 26.65 27.03 32.62 31.50 31.89

18.87 18.51 17.26 13.09 14.43 13.63

/ / / 7.98 8.90 8.67

5.06 4.10 4.12 3.42 3.05 2.97

2.61 1.53 1.89 2.33 1.64 2.08

1.28 0.87 1.01 1.31 0.96 1.63

1.88 2.29 2.50 2.02 1.18 1.02

Pore radius (nm)

BET

DFT

BET

DFT

BET

DFT

79.36 209.86 239.78 54.73 136.71 147.63

93.94 170.58 199.40 75.32 147.62 131.60

83.73 253.57 259.67 76.73 128.67 132.01

45 62 53 37 46 41

83.32 163.78 156.74 73.01 68.27 87.34

1.84 1.05 0.92 0.61 0.50 0.48

3.1.2. Textural characterization The FT-IR analysis results of the chars and catalysts are shown in Fig. 3, which shows that the chars and catalysts consisted of silane, ketones and aromatic oxygenated functional groups. Detail assignments are as follows. The peaks at 465 and 1037 cm−1 were attributed to Si-O stretching and Si-O-Si vibrations. The presence of functional group Si-O and Si-O-Si indicates the presence of a silica material in the peat char. The peaks at approximately 1600 cm−1 corresponded to C]C alkene stretching. Furthermore, for the Fe/KPC and Fe/CPC catalysts, the new peak at approximately 3430 cm−1 could though to be the stretching vibrations of eOH groups [35–37]. Compared with peat char, the catalyst surface has more abundant functional groups, and the existence of various functional groups provides more potential capabilities for catalytic performance. Fig. 4 shows the XRD patterns of the chars and catalysts. The several

Elemental content (wt.%) C

Pore volume (mm3/g)

properties. The BET surface area increased to 209.86 m2/g and 239.78 m2/g, while the DFT surface area increased to 170.58 m2/g and 199.40 m2/g, and the pore volume increased significantly. However, BET pore radius increases obviously, while the DFT pore radius decreases (from 18.4 nm to 1.05 nm and 0.92 nm), This proves that activation can enlarge macropores and generate new micropores, and activation can increase the surface area to form active sites on chars. When the chars were loaded with iron, the BET and DFT surface area showed a significant decrease to 54.73 m2/g, 136.71 m2/g, 147.63 m2/ g and 75.32 m2/g, 147.62 m2/g, 131.60 m2/g for Fe/PC, Fe/KPC and Fe/CPC, respectively. In addition, pore radius and pore volume of the catalyst decreased by different degrees. The results also indicated that the functional groups of the outer surface of the catalysts interacted with iron and led to an evident decrease in specific surface area, and the coverage of the active sites and the production of the new material caused the decrease of the pore size and pore volume [32–34].

Table 2 EDS analysis of the surface of the samples. Image

Surface area (m2/g)

4

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Fig. 5. Effect of catalyst type on conversion and gas content. Fig. 3. FT-IR analysis of chars and catalysts.

percentage (mol%), while CO had the second highest. The maximum mol% of syngas (H2 + CO) was 88.1% for the Fe/CPC catalyst, 80.5% for the Fe/KPC catalyst and 76.5% for the Fe/PC catalyst. It is well known that a slightly higher syngas yield was attained by the Fe/CPC catalyst. In this scenario, the mol% values of CH4 and CO2 decreased by less than 10% and 20%, respectively. It is especially pointed out that the Fe based peat char catalyst had better selectivity for syngas. To further explore the gas selectivity of the Fe/PC, Fe/KPC and Fe/ CPC catalysts, the major components (H2, CO, CO2 and CH4) were investigated with reaction time, and the results were showed in Fig. 6. Specifically, Fig. 6(a) shows the effect of catalyst Fe/PC on gas content, as the reaction time increased from 0 to 150 min, the maximum mol% (76.5%) of syngas (H2 + CO) was obtained at 15 min after the reaction started, and with the reaction time increases, the mol% of H2 and CO decreased slightly (from 58.0% to 51.6% and from 18.5% to 11.0%, respectively), while the mol% of CO2 and CH4 increased gradually (from 14.1% to 22.6% and from 8.5% to 16.6%, respectively). This phenomenon indicates that the catalyst's selectivity to syngas will be weakened by the increase in the use time of the catalyst. Further analysis showed that the deactivation of catalyst has a great influence on the breaking of the CeH bond than CeC bond, and leading to an increase in CH4. As a whole, after a 150-minute experiment, the mol% of syngas dropped from 76.5% to 62.6%, the mol% was reduced by 13.9%. The catalytic performance of Fe/KPC catalyst was shown in Fig. 6(b). It can be observed that compared to the catalyst Fe/PC, the Fe/KPC is more selective to H2 with the mol% of H2 > 60%. As the reaction time increased, the mol% of syngas reached a maximum of 80.5% at 15 min. The mol% of H2 decreased from 64.5% to 58.4%, while the mol% of CO decreased from 16.0% to 10.6% with the reaction

intense and sharp characteristic peaks at 2θ = 26.6° and the small peaks at 2θ = 22.0°, 50.2°, 59.9° and 68.0° could be identified as amorphous silica. Meanwhile, the peaks that appeared at 2θ = 20.0° corresponded to the diffraction peak of C. When the chars were loaded with iron, two different types of phases (FeC and FeSiO3) were observed in the XRD patterns of Fe/PC, Fe/KPC and Fe/CPC. The peak at 2θ = 35.9° could be identified as FeC which was formed by the reaction between C and Fe. In addition, reflections of FeSiO3 due to the interaction between SiO2 and Fe appeared at 2θ = 27.6° and 51.2°. Compared with Fe/PC, the intensity of the FeC and FeSiO3 peaks for Fe/KPC and Fe/CPC increased, which may be because the KOH or CO2 activation methods increased the surface area and exposed more active sites on the surface of the chars, thereby loading more active Fe. The newly formed FeC and FeSiO3 structures on the catalyst surface are important active sites for catalytic tar cracking. 3.2. Analytical toluene cracking experiment 3.2.1. Influence of catalyst type Fig. 5 shows the effect of catalysts Fe/PC, Fe/KPC and Fe/CPC on the conversion and gas content. For toluene conversion, it could be intuitively observed that the Fe/CPC catalyst had better catalytic performance. The toluene conversion of catalysts Fe/PC, Fe/KPC and Fe/ CPC were 89.1%, 91.8% and 92.7%, respectively. To further explain the gas selectivity of the catalyst, the gas mole percentages of the major components (CO, CO2, CH4 and H2) were investigated at 15 min after the reaction begins. It can be seen that the H2 had the largest mole

Fig. 4. XRD patterns of the chars and catalysts. 5

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100

Fe/PC Fe/KPC Fe/CPC

conversion (%)

95

90

85

80 10 0

700

800

o

900

temperature ( C) Fig. 7. Effect of reaction temperature on conversion.

and prevent the CeC and CeH bonds from breaking [38,39].

3.2.2. Influence of reaction conditions The reaction conditions (reaction temperature, residence time and steam-to-carbon ratio) in the toluene cracking experiment were optimized using catalysts Fe/PC, Fe/KPC and Fe/CPC. Fig. 7 summarizes the variation in toluene conversion with reaction temperature, it can be found that when the react temperature was 700 °C, the toluene conversion of Fe/PC, Fe/KPC and Fe/CPC was 87.3%, 91.1% and 92.1%, respectively. As the temperature increased from 700 °C to 900 °C, the conversion increased to 89.1%, 91.8% and 92.7%, respectively, and reached a maximum. It's worth noting that with the temperature increased from 800 °C to 900 °C, a little increased of conversion could be found. The results show that higher temperature will increase the toluene conversion efficiency, but it is not that higher temperature is more favorable. Too high temperature will cause the sintering of the catalyst, which to some extent is not conducive to CeC and CeH bond breaking at active site to some extent [40–42]. The effect of steam-to-carbon ratio (S/C) were shown in Fig. 8. As S/ C varied from 1.5:1 to 4:1, the toluene conversion reached a maximum of 89.1% for Fe/PC at 3:1, 91.8% for Fe/KPC at 2.5:1, and 92.7% for Fe/CPC at 3:1. A further increase in S/C to 4:1 led to a decline in toluene conversion. Too much water will reduce the contact probability between toluene and the active site of the catalyst, and too much scouring action will also cause the loss of the active substance, excessive S/C ratio can cause the decline of catalytic performance. The Fe-

Fig. 6. (a) Effect of the catalyst Fe/PC on gas content, (b) effect of the catalyst Fe/KPC on gas content, (c) effect of the catalyst Fe/CPC on gas content.

time increased from 0 to 150 min. Compared to the catalyst Fe/PC, the deactivation of catalyst Fe/KPC was reduced relatively. Fig. 6(c) show the catalytic performance of Fe/CPC, it is worth mentioning that comparing with the Fe/PC and Fe/KPC, the Fe/CPC catalyst had the best selectivity for syngas with a mol% of syngas of 88.1%. The mol% of syngas decreased from 88.1% to 72.9%, while the mol% of CO2 and CH4 increased from 7.2% to 16.6% and from 4.7% to 11.6% with the reaction time increased, respectively. The catalysts Fe/PC, Fe/KPC and Fe/CPC showed different degrees of decrease in syngas selectivity over time. This phenomenon may be caused by the carbon deposition in the reaction process, the resulting carbon can cover some of the active sites

Fig. 8. Effect of steam-to-carbon ratio on conversion. 6

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Fig. 9. Effect of residence time on conversion. Fig. 11. XRD patterns of fresh and used Fe/CPC catalyst.

based peat char catalyst film clearly favored a maximum toluene conversion rate at a higher S/C ratio (S/C > 2.5), similar to the results reported for other catalysts [43]. Addition of H2O can reduce catalyst deactivation due to carbon formation via Eqs. (4) and (5).

H2 O + C ↔ CO + H2

(4)

2H2 O + C ↔ CO2 + 2H2

(5)

of syngas (H2 + CO) produced by the three catalysts over time. As the time increased from 0 min to 480 min, the mol% of syngas showed a significant decline with from 76.5% to 51.5% for Fe/PC, from 80.5% to 56.7% for Fe/KPC and from 88.1% to 64.2% for Fe/CPC, respectively. As a result, after an 8-hour experiment, the relative decreases for syngas of the three catalysts were 25%, 23.8%, 23.9%, respectively. It could be intuitively observed that the peat char-based Fe catalysts have a certain deactivation effect with the increase of time. The deactivation of the catalyst is mainly caused by carbon deposition. The amount of carbon deposition increases with the time of the catalyst, and a large amount of carbon will cover the active site, which will reduce the probability of CeC and CeH bond break. In addition, analysis of the reduction leads conclusions that the stability of Fe/KPC and Fe/CPC is slightly better than that of Fe/PC, illustrating that the activation can promote the stability of catalyst to some extent. In addition, to further understand the deactivation of the catalysts, characterizations of the fresh and used Fe/CPC catalyst was conducted via XRD analysis. Fig. 11 shows the XRD patterns of fresh and used Fe/ CPC catalyst, the intensity of peaks for the used Fe/CPC was slightly different from that of the fresh Fe/CPC catalyst, illustrating that the main structures of the catalysts were not changed. Nevertheless, slightly different peaks were found at 2θ = 18.2° and 31.5°, which are attributed to the C diffraction peak. The result illustrating that carbon deposition occurred in the catalytic process. Moreover, it can be seen that the peaks of FeC and FeSiO3 were still existed on the used Fe/CPC, which suggested that the FeC and FeSiO3 structures improved the stability of the catalyst to some extent because of the existence of SieOeFe bond.

Fig. 9, shows the effect of residence time (τ) on toluene conversion. Residence time is important factor for catalytic performance. As shown, when the residence time was investigated from 0.3 s to 0.8 s. Toluene conversion gradually increased with increased τ for all three catalysts reached a maximum at 0.7 s for Fe/PC, at 0.6 s for Fe/KPC, at 0.6 s for Fe/CPC. There was no obvious increase in conversion when the residence time was prolonged to 0.8 s, and similar tendency was observed for all three catalysts. In our experiment, the residence time was determined by the flow rate of N2, too short residence time will lead to incomplete reaction of toluene, appropriate increase of residence time can improve the conversion efficiency, but when the time is enough to make toluene saturated on the catalyst surface, further increase of time will not resume the catalytic effect. 3.3. Pyrolysis char catalyst deactivation 3.3.1. Pyrolysis char catalyst life analysis In order to verify the stability of peat char-based Fe catalysts, the service life of catalysts was tested for 480 min. Fig. 10 shown the mol%

3.3.2. Comparison with reported waste pyrolysis char catalysts In order to comprehensively analyze the properties of peat charbased Fe catalyst prepared in this study, the peat pyrolysis char-based catalysts were compared with other reported similar waste pyrolysis char catalysts catalyst. As shown in Table 4, the reported catalysts with similar structure was selected for comparative analysis. Compared with reported catalysts, the peat pyrolysis char catalyst in this paper has a slightly higher toluene conversion rate, but it's worth mentioning that the catalyst has good gas selectivity, the peat char-based Fe catalyst has a higher syngas ratio than other reported catalysts. The mol% of syngas was 88.1% at 900 °C and 85.7% at 800 °C. The catalytic conversion of toluene and the selectivity of syngas are evaluated, the peat char-based Fe catalyst prepared in this paper has certain advantages over other catalysts, and the catalysts are thus promising for biomass tar conversion and syngas conditioning.

Fig. 10. Effect of reaction time on the mol% of syngas. 7

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Table 4 Comparison of the peat char-based Fe catalyst with other reported catalysts. Catalyst

Fe/rice husk char Fe/rice husk char Fe/bamboo char Ni-Fe/rice husk Fe/peat char Fe/peat char

Reaction temperature (°C)

800 800 600 800 900 800

Yield

Reference

Conversion (%)

H2 + CO

CO2

CH4

92.6% / 73.5% 92.3% 92.7% 92.4%

342.7 mg/L 67% 81.14% 77.9% 88.1% 85.7%

/ 22% 10% 15.8% 8.2% 9.2%

/ 11% 8.86% 6.3% 3.7% 5.1%

4. Conclusions [6]

In this paper, the waste peat pyrolysis char catalysts were successfully prepared and used to catalyze the cracking of tar model compounds. The results indicated that peat pyrolysis char is an ideal catalyst carrier and can combine well with active metal Fe. The peat pyrolysis char-based catalyst has excellent catalytic activity and gas selectivity for toluene catalytic reforming. The optimal toluene conversion was 92.7% under reaction conditions of 900 °C, steam-tocarbon ratio of 3:1 and residence time of 0.6 s. In addition, the catalyst showed good selectivity to syngas, and the maximum mol% of syngas can reach 88.2% in the generated gas. We found that the excellent catalytic activity and stability of the peat char-based Fe catalyst was due to the presence of FeC and FeSiO3 structures. The results indicated that the peat pyrolysis char catalysts can be used for catalytic reforming of biomass tar and production of syngas.

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[9] [10]

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CRediT authorship contribution statement

[13]

Shuxiao Wang: Conceptualization, Data curation, Writing - review & editing. Rui Shan: Methodology, Investigation, Writing - original draft. Tao Lu: Investigation, Data curation. Yuyuan Zhang: Formal analysis, Validation. Haoran Yuan: Resources, Funding acquisition, Writing - review & editing. Yong Chen: Supervision.

[14] [15]

[16] [17]

Declaration of Competing Interest

[18]

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

[19] [20]

Acknowledgements

[21]

This paper is financially supported by National Key R&D Program of China (2018YFC1901200), National Natural Science Foundation of China (51608507 and 51906248), Key Special Project for Introduced Talents Team of Southern Marine Science and Engineering Guangdong Laboratory (GML2019ZD0101), Science and Technology Planning Project of Guangdong Province (2017B040404009), and Guangdong Province Key Laboratory of Efficient and Clean Energy Utilization (2019ECEU02).

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