English

• With the global warming and the depletion of fossil fuels, hydrogen becomes more and more important which can reduce consumption of oil^{[1, 2]}. Hydrogen can be produced by ion exchange membrane^[3], methanol steam reforming^[4], biomass pyrolysis gasification^[5], and other chemical processes like fossil fuel reforming^[6]. Biomass pyrolysis gasification is considered to be one of the most effective thermal chemical conversion method to produce hydrogen^{[7, 8]}. However, the hydrogen content was low in the usual pyrolysis gasification, and it was required catalytic reforming to improve the hydrogen ratio^[9]. Various types of catalysts such as calcination rock, zeolite, iron ore, alkali metal, Ni and precious metal catalysts were studied in gas reforming reactions^{[10, 11]}. Among these catalysts, Ni catalysts received extensive attention, which were usually supported by metal oxide (Al₂O₃ and MgO) or natural materials (dolomite, olivine, activated carbon)^{[12, 13]}. The price of these carriers are relatively expensive, limiting the Ni catalysts are widely used. Using char as the catalyst carrier because of its abundant pore structure and surface functional groups can not only reduce the cost of catalyst, but also recycle the waste catalyst easily by simple combustion and gasification to avoid the tedious catalyst regeneration process^[14-16]. Wang et al^[17] and Shen et al^[18] prepared Ni catalysts with coal and rice char as supports in biomass pyrolysis gas reforming experiment, and the effect is remarkable to get hydrogen-rich gases. Using coal char as the carrier to support Ni and Fe catalysts, Min et al^[19] found that the coal char carrier had the effect not only to disperse active component but also to improve the living of catalyst. Alrahbi et al^[20] used tire char as catalysts and compared the effects of hydrogenation on biomass gasification under primitive and acid washing conditions.

  This experiment used tire-pyrolytic char (which is rich in metal elements) as the carrier for the preparation of nickel-based catalysts, and researched its catalytic performance in the straw pyrolysis gas reforming, explore its feasibility as a nickel-based catalyst carrier, and determine impact factors (the catalytic reaction temperature, the holding time, Ni loading and catalysts use time) of gas reforming.

  1.   Methods and materials

  1.1   Straw pretreatment and related analysis

  Wheat straw was first broken into about 3 mm size and then dried for use. The C, H, N and S elements of the sample were determined by FLASH2000 elemental analysis instrument (air drying base), and the content of O elements was determined by the difference method. Industrial analysis adopts GB/T 28731—2012 industrial analysis method for solid biomass fuel. The results are shown in Table 1.

  Table 1

  

  Table 1.  Ultimate and proximate analyses of wheat straw sample

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  Ultimate analysis w/%						Proximate analysis w/%
  C	H	O	N	S		M	A	V	FC
  33.65	4.91	51.88	0.89	0.15		11.15	8.43	78.1	2.69

  1.2   Catalyst preparation

  Firstly, the tire pyrolysis char (TPC) was broken, sealed and soaked in 5mol/L hydrochloric acid solution, and filtered, washed and dried. Secondly, certain proportion of Ni (NO₃)₂·6H₂O and treated TPC were added in a 250mL flask, and then 40g urea and 150mL deionized water were added. The sample was heated 2h at 115℃. Finally, after filtering, washing, drying in vacuum, and heat preservation for 3h in N₂ atmosphere under 850℃, Ni/TPC catalyst was synthesized.

  1.3   Characterization of catalyst

  The element composition and material structure of the catalyst were tested by EDX-7000 X-ray fluorescence spectrometer (EDX) and X-ray diffractometer (XRD). The surface morphology of catalyst particles was observed by S-3000N scanning electron microscope (SEM). The surface area was analyzed by Autosorb-l-c automatic chemical adsorption instrument. The thermal stability of the catalyst was evaluated using the Q600 heat retest instrument (TG/DTG).

  1.4   Evaluation of catalytic activity

  The activity of Ni/TPC catalyst was tested in a fixed pyrolysis furnace shown in Figure 1. The porcelain boat with wheat straw and catalyst bed were fed into the pyrolysis furnace through the flange mouth before the trial. The high-purity nitrogen was used to exclude the furnace tube air. The reactor temperature was programmed to the set value and preserved a certain period. The evolution gases were analyzed by Gasboard-3100P infrared gas analyzer.

  Figure 1

  

  Figure 1.  Flow scheme of bench scale reactor for catalytic pyrolysis of straws biomass

  1: nitrogen; 2: pyrolysis gasifier and thermocouples; 3: insulation brick; 4: porcelain boat; 5: catalytic bed and catalyst; 6: temperature controller; 7: waterproof valve; 8: water channel; 9: particle filter; 10: gas dryer (silica gel); 11: pump; 12: gas buffer package; 13: flowmeter; 14: gas analyzer; 15: fire prevention

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  2.   Results and discussion

  2.1   Characterization of Ni catalysts

  2.1.1   EDX analysis

  The components of TPC, Ni/TPC and wasted Ni/TPC were analyzed and shown in Table 2. It can be seen that the carrier TPC is mainly composed of C, S, Zn, Si, Ca, K and Fe. The Ni content in the fresh Ni/TPC catalyst was 28.44% which was close to the theoretical load of 30%. The Ni was 27.91% in the waste Ni/TPC catalyst after 850min reaction. Compared with the fresh Ni/TPC catalyst, Ni and other metal contents of the waste Ni/TPC catalyst were decreased. The increase of C content in the waste catalyst was caused by carbon deposition on the surface of the catalyst^{[21, 22]}.

  Table 2

  

  Table 2.  EDX element analysis of three samples

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  Sample	Main composition and content w/%
  	Ni	C	S	Zn	Si	Ca	Fe	K
  TPC	-	61.49	15.47	13.94	5.85	1.16	0.93	0.73
  30% Ni/TPC	28.44	42.81	11.25	10.46	4.66	0.86	0.71	0.54
  Waste Ni/TPC	27.91	43.75	11.44	9.96	4.61	0.82	0.65	0.55

  2.1.2   XRD analysis

  Figure 2 shows the XRD spectra of TPC, Ni/TPC and waste Ni/TPC. Besides the characteristic peaks of ZnS (PDF2004/36-1450), there are also some others for the TPC. For the Ni/TPC catalyst the peaks in 42.76°, 49.78°, 73.14° and 88.62° are mainly the characteristic peak of Ni₃ZnC_0.7 (PDF2004/28-0713). Previous research showed that Ni active component was present in the form of Ni₃ZnC_0.7 in the Ni/TPC catalyst^[23]. For the waste Ni/TPC catalyst there isn't Ni₃ZnC_0.7 structure, and peaks in 44.16°, 51.46° and 75.82° are mainly the characteristic peak of FeNi₃ (PDF2004/38-0419). It shows that the active component structure of Ni/TPC catalyst was converted after repeatedly used^[24].

  Figure 2

  

  Figure 2.  XRD patterns of TPC and Ni/TPC and waste Ni/TPC

  (a): TPC; (b): Ni/TPC; (c): waste Ni/TPC
  ■: ZnS; ●: Ni₃ZnC_0.7; ▲: FeNi₃

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  2.1.3   SEM analysis

  Figure 3 shows the SEM images of TPC, Ni/TPC and waste Ni/TPC. For the carrier TPC particles are scattered and the particles size is not uniform and regular distribution. For the Ni/TPC catalyst the particles are fluffy like a piece of sponge. Some particles of waste Ni/TPC catalyst become unevenly. There is the deposition carbon in the surface of Ni/TPC catalyst because of the repeated use of the catalyst^[25].

  Figure 3

  

  Figure 3.  SEM images of TPC, Ni/TPC and waste Ni/TPC

  (a): TPC; (b): Ni/TPC; (c): waste Ni/TPC

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  2.1.4   TG analysis

  Figure 4 shows the TG/DTG curves of Ni/TPC catalyst and wheat straw. The pyrolysis process of wheat straw is mainly divided into three stages. The wheat straw begins to dehydration at 30℃ and its physical water is completely lost at blow 160℃ in region 1, which is physical dehydration and drying stage^[26]. Region 2 from 160 to 550℃ is mainly the devolatilization stage, and the wheat straw begins to devolatilize and decompose into gas, carbon and tar shown in equation (1)^[27]. The maximum weight loss peak is at 320℃ with weight loss rate of 7.1%/℃. Region 3 is the secondary pyrolytic cracking stage. There were homogeneous reactions (between the gas and steam) and heterogeneous reactions (between char and gas and steam) in this stage with the main reactions of equation (2)-(6)^{[28, 29]}.

  Figure 4

  

  Figure 4.  TG/DTG curves of Ni/TPC catalyst and wheat straw biomass

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  Meanwhile, with the temperature increase, the quality of the catalyst has few changes from the TG curve of Ni/TPC catalyst, which indicates that the thermal stability of Ni/TPC catalyst is excellent^[30].

  $ {{\rm{C}}_x}{{\rm{H}}_y}{{\rm{O}}_z} \to {{\rm{H}}_{\rm{2}}}{\rm{ + CO + C}}{{\rm{O}}_{\rm{2}}}{\rm{ + }}{{\rm{H}}_{\rm{2}}}{\rm{O + C}}{{\rm{H}}_{\rm{4}}}{\rm{ + C + }}{{\rm{C}}_m}{{\rm{H}}_{n}} $

  (1)

  $ {\rm{C + }}{{\rm{H}}_{\rm{2}}}{\rm{O }} \to {\rm{ CO + }}{{\rm{H}}_{{\rm{2}}}} $

  (2)

  $ {\rm{CO + }}{{\rm{H}}_{\rm{2}}}{\rm{O }} \to {\rm{ C}}{{\rm{O}}_{{\rm{2}}}}{\rm{ + }}{{\rm{H}}_{\rm{2}}} $

  (3)

  $ {\rm{C + 2}}{{\rm{H}}_{{\rm{2}}}} \to {\rm{C}}{{\rm{H}}_{{\rm{4}}}} $

  (4)

  $ {\rm{C + C}}{{\rm{O}}_{{\rm{2}}}} \to {\rm{ 2CO}} $

  (5)

  $ {\rm{C}}{{\rm{H}}_{\rm{4}}}{\rm{ + }}{{\rm{H}}_{\rm{2}}}{\rm{O }} \to {\rm{CO + 3}}{{\rm{H}}_{\rm{2}}} $

  (6)

  2.1.5   BET analysis

  Table 3 shows the BET surface area of TPC and Ni/TPC and waste Ni/TPC. The BET surface area of the carrier TPC is 84m²/g, the Ni/TPC catalyst is 62m²/g, and the waste Ni/TPC catalyst is 49m²/g. The BET surface area decrease of Ni/TPC catalyst is mainly caused by the load of active component^[31]. The BET surface area decrease of waste Ni/TPC catalyst is mainly caused by the deposition of carbon in the reaction^[32].

  Table 3

  

  Table 3.  BET surface area of TPC and Ni/TPC waste Ni/TPC

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  Sample	TPC	Ni/TPC	Waste Ni/TPC
  BET surface area A/(m2·g-1)	84	62	49

  2.2   Catalytic performance of Ni/TPC catalyst

  The wheat straw sample with or without TPC or Ni/TPC catalyst were put in biomass pyrolysis reactor, and the experiments were carried out under the same condition. The catalytic activity of Ni/TPC in wheat straw pyrolysis was evaluated by comparing H₂ and CO content in gas. Meanwhile, the performance of Ni/TPC catalyst including reforming temperature, holding time, Ni loading and use time on the straw pyrolysis was investigated.

  2.2.1   Effect of reforming temperature

  Figure 5 shows the influence of pyrolysis temperature on wheat straw pyrolysis gas production. As the pyrolysis temperature increase, the contents of H₂ and CO also increase. The H₂ and CO gas productions are almost the same for TPC catalyst and without catalyst, which indicates that the TPC does not have catalytic activity in wheat straw pyrolysis. When use Ni/TPC catalyst the contents of H₂ and CO are significantly high, which shows that Ni/TPC catalyst has a remarkable catalytic effect in pyrolysis of wheat straw.

  Figure 5

  

  Figure 5.  Effect of temperature on pyrolysis gas reforming

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  Compared the gas products at 750℃ with or without Ni catalyst, the H₂ gas content increases from 19% to 33.6%, and the CO gas content increases from 18.5% to 28.6%. The relative improvements of H₂ and CO contents are the highest compared with other pyrolysis temperature. So the catalytic efficiency of Ni/TPC catalyst is highest at the pyrolysis temperature of 750℃, and 750℃ was chosen as the following reaction temperature.

  2.2.2   Effect of holding time

  Figure 6 shows the influence of different holding time on wheat straw pyrolysis gas production. With the holding time increase, the H₂ and CO contents also increase. When the holding time is more than 10min the H₂ and CO increase slowly. When the holding time is 10min for Ni/TPC catalyst, the content of H₂ gas increases from 16.2% to 28.2%, and the CO gas content increases from 16% to 26%. So the holding time of 10min was chosen as Ni/TPC catalyst had highest products increment. When the holding time is increased, the contact time between the tar macromolecule and the catalyst is prolonged, and the catalytic reaction time is also increased, so the catalytic reaction will be increased. However, when the holding time exceeds 20min, most of the tar molecules have already been decomposed, the continue increase of holding time will not improve the gas product significantly.

  Figure 6

  

  Figure 6.  Effect of holding time on pyrolysis gas reforming

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  2.2.3   Effect of Ni loading

  Figure 7 shows the influence of different Ni load of Ni/TPC catalyst on wheat straw pyrolysis gas production.

  Figure 7

  

  Figure 7.  Effect of Ni loading on pyrolysis gas reforming

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  As the nickel loading increases the contents of H₂ and CO are first increase and then decrease. When the Ni loading is 30%, the H₂ content increases from 18.5% to 32.5%, and the CO content increases from 18% to 27%. When the nickel loading is 30%, the Ni/TPC catalyst has highest catalytic efficiency in wheat straw pyrolysis. When the Ni loading is too high, a large amount of Ni enters to the carrier pores, clogs the pores of the carrier, and the active components inside the catalyst particles cannot contact the tar macromolecules resulting in a decrease in the catalytic reforming activity^[33].

  2.2.4   Effect of usage time

  Figure 8 shows the influence of different reaction time of the Ni/TPC on wheat straw pyrolysis gas production.

  Figure 8

  

  Figure 8.  Effect of catalytic usage time on pyrolysis gas reforming

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  With the increase of using time the H₂ and CO gas contents slowly decrease. The contents of H₂ and CO are gradually stable after 600min. According to the XRD pattern of Figure 2, the active component Ni₃ZnC_0.7 changes to FeNi₃ after the catalyst is used. Therefore, it is considered that the decrease of the catalytic activity after long-term use is due to the change of the active component, and the catalytic performance of Ni₃ZnC_0.7 is higher than that of FeNi₃.

  2.2.5   Thermal catalytic process analysis

  The catalytic pyrolysis of wheat straw process is shown in Figure 9.

  Figure 9

  

  Figure 9.  Simple thermal catalytic process analysis of Ni/TPC catalyst

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  The tar macromolecules and water vapor contact at the surface of the Ni/TPC catalyst, and some tar macromolecules are catalytic split into small molecular gases such as hydrogen and carbon monoxide. At the same time, the other tar macromolecules which do not contact with the catalyst are also thermally cracked into lighter, smaller molecular tar and H₂ ^[34]. During the tar macromolecules reforming reactions and cracking reactions, carbon deposition on the catalyst surface can occur and cause the decrease of the catalytic activity. After a long period use, the active component Ni₃ZnC_0.7 can change and convert to the FeNi₃ structure. Through the long time experimental data, it is found that FeNi₃ also has relative strong reforming catalytic activity and can be used for a long reforming reaction period.

  3.   Conclusions

  The results show that Ni loaded on TPC with a specific surface area of 62m²/g has strong catalytic activity for wheat straw pyrolysis gas reforming. Meanwhile, nickel combines with Zn and C species in the carrier to form Ni₃ZnC_0.7 structure. The active components of Ni₃ZnC_0.7 is changed to FeNi₃ after long-term used.

  The Ni/TPC catalyst has the highest catalytic efficiency at 750℃ with a 10min holding time and 30% Ni loading. The content of H₂ is still higher after continuous use of 850min.

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• 

  Figure 1  Flow scheme of bench scale reactor for catalytic pyrolysis of straws biomass

  1: nitrogen; 2: pyrolysis gasifier and thermocouples; 3: insulation brick; 4: porcelain boat; 5: catalytic bed and catalyst; 6: temperature controller; 7: waterproof valve; 8: water channel; 9: particle filter; 10: gas dryer (silica gel); 11: pump; 12: gas buffer package; 13: flowmeter; 14: gas analyzer; 15: fire prevention

  下载: 全尺寸图片 幻灯片
  

  Figure 2  XRD patterns of TPC and Ni/TPC and waste Ni/TPC

  (a): TPC; (b): Ni/TPC; (c): waste Ni/TPC
  ■: ZnS; ●: Ni₃ZnC_0.7; ▲: FeNi₃

  下载: 全尺寸图片 幻灯片
  

  Figure 3  SEM images of TPC, Ni/TPC and waste Ni/TPC

  (a): TPC; (b): Ni/TPC; (c): waste Ni/TPC

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  Figure 4  TG/DTG curves of Ni/TPC catalyst and wheat straw biomass

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  Figure 5  Effect of temperature on pyrolysis gas reforming

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  Figure 6  Effect of holding time on pyrolysis gas reforming

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  Figure 7  Effect of Ni loading on pyrolysis gas reforming

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  Figure 8  Effect of catalytic usage time on pyrolysis gas reforming

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  Figure 9  Simple thermal catalytic process analysis of Ni/TPC catalyst

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  Table 1.  Ultimate and proximate analyses of wheat straw sample

  Ultimate analysis w/%						Proximate analysis w/%
  C	H	O	N	S		M	A	V	FC
  33.65	4.91	51.88	0.89	0.15		11.15	8.43	78.1	2.69

  下载: 导出CSV

  Table 2.  EDX element analysis of three samples

  Sample	Main composition and content w/%
  	Ni	C	S	Zn	Si	Ca	Fe	K
  TPC	-	61.49	15.47	13.94	5.85	1.16	0.93	0.73
  30% Ni/TPC	28.44	42.81	11.25	10.46	4.66	0.86	0.71	0.54
  Waste Ni/TPC	27.91	43.75	11.44	9.96	4.61	0.82	0.65	0.55

  下载: 导出CSV

  Table 3.  BET surface area of TPC and Ni/TPC waste Ni/TPC

  Sample	TPC	Ni/TPC	Waste Ni/TPC
  BET surface area A/(m2·g-1)	84	62	49

  下载: 导出CSV
•