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Catalytic reforming of biomass pyrolysis tar using the low-cost steel slag as catalyst
Energy 189 (2019) 116161
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Catalytic reforming of biomass pyrolysis tar using the low-cost steel slag as catalyst Feiqiang Guo a, b, c, *, Shuang Liang a, b, Xingmin Zhao a, b, Xiaopeng Jia a, b, Kuangye Peng a, b, Xiaochen Jiang a, b, Lin Qian a, b a Jiangsu Province Engineering Laboratory of High Efficient Energy Storage Technology and Equipments, China University of Mining and Technology, Xuzhou, 221116, China b School of Electrical and Power Engineering, China University of Mining and Technology, 221116, Xuzhou, PR China c Center for Biorefining, Bioproducts and Biosystems Engineering Department, University of Minnesota, St. Paul, Minnesota, 55108, United States
a r t i c l e i n f o
a b s t r a c t
Article history: Received 7 July 2019 Received in revised form 14 September 2019 Accepted 17 September 2019 Available online 20 September 2019
In this work, the possibility of steel slag as an effective and low-cost catalyst for the decomposition of biomass pyrolysis tar has been explored based on the high content of iron oxides for sustainable syngas production from biomass. By simple calcination treatment at 800 C, the loose structure of the steel slag was formed with the main chemical composition of Fe2O3 and MgFe2O4. The steel slag exhibited good catalytic activity on the cracking of biomass pyrolysis tar, and even higher tar conversion efficiency can be obtained by reusing the steel slag, leading to the increase in syngas yield. The presence of additional steam can further promote the tar reforming reactions, leading to the significant increase in H2 and CO. At 800 C, the tar conversion efficiency reached 94.1% with a high gas yield of 493.5 mL/g. The interaction between steel slag and reductive gases resulted in the reduction of iron oxides into Fe3O4, and more pores were formed for the spent steel slag, which can enhance the contact between active sites and reactants. These characteristics indicate that steel slag has the potential to be used as an efficient catalyst with excellent stability in the long-term biomass tar removal applications. © 2019 Elsevier Ltd. All rights reserved.
Keywords: Biomass Steel slag Tar removal Catalytic reforming
1. Introduction Nowadays, biomass pyrolysis and/or pyrolysis is an increasingly attractive method for the production of fuel gas using abundant, low-cost and CO2 neutral biomass resource. However, one of the most challenging issues during biomass pyrolysis and/or gasification is to reduce the tar content in the product gas to a certain level [1,2]. The tar is mainly composed of condensable chemical compounds, such as aromatics (1e5 rings), poly-aromatic hydrocarbons and oxygen-containing compounds, which can cause coking, blockage and other operational problems in the downstream of the pyrolysis and gasification system [3,4]. Furthermore, these tars also contained large amounts of energy, and therefore lower the overall energy conversion of biomass. Thus, efficient tar removal method is of great importance to the biomass pyrolysis/gasification processes.
* Corresponding author. Jiangsu Province Engineering Laboratory of High Efficient Energy Storage Technology and Equipments, China University of Mining and Technology, Xuzhou, 221116, China. E-mail address: [email protected] (F. Guo). https://doi.org/10.1016/j.energy.2019.116161 0360-5442/© 2019 Elsevier Ltd. All rights reserved.
To date, the commonly used methods for tar removal in industrial applications are water scrubbing, thermal cracking and catalytic reforming [5]. Water scrubbing is a simple and effective method to reduce the tar content in the product gas, while wastewater containing aromatics is produced as well which may lead to new economic and environmental problems. The thermal cracking method generally requires a high operating temperature over 1100 C to achieve effective tar conversion efficiency [6]. In comparison, catalytic reforming of tar has been considered as the most promising approach for the chemical conversion of tar into permanent gases at relatively lower temperatures (600e800 C) without any production of wastewater [7]. The catalyst is crucial to the catalytic reforming method, which not only influences the tar decomposition, but also affects the overall economic feasibility of the method [8]. Hence, developing effective and low-cost catalysts is becoming increasingly attractive. Metal-based catalysts, such as Ni-based catalysts [9,10], Febased catalysts [11,12] and alkali metal catalysts [13,14], have been widely investigated due to the high catalytic activity and operational availability. However, considering the commercial
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application, the performance and the cost are two crucial parameters in the developing of catalysts for the catalytic pyrolysis/gasification processes. Thus, naturally available minerals and industrial waste, which are generally rich in metal oxides, have also been investigated as efficient and low-cost catalysts. For example, Ashok et al. [15] used municipal solid waste incineration bottom ash as a catalyst support material, and high catalytic activity was achieved for steam reforming of biomass tar at 700e800 C. Zou et al. [16] selected natural limonite as catalyst for catalytic cracking of toluene (a typical model compound of tar), finding that toluene can be decomposed into H2, CO, CH4 and CO, and the catalytic performance can be further improved by nickel loading. Yang et al. [17] reported that calcined eggshell possessed porous structure and alkaline property, which showed high catalytic activity on steam reforming of tar derived from cedar wood for the production of hydrogen-rich gas. In our previous study, high ash-containing paper sludge ash was investigated as a low-cost catalyst for biomass tar cracking, which showed that the tar can be successfully decomposed and the yields of H2, CH4 and CO2 were significantly improved [18]. Steel slag, the byproduct in the steelmaking process, is a cheap industrial waste that contains significant amounts of metal compounds such as iron, magnesium, aluminum and calcium, and thus has the potential to be used as catalyst for biomass tar reforming. Steel slag has been investigated as the catalyst for oil sludge pyrolysis, showing that steel slag can significantly intensify the secondary thermal cracking of volatile matters [19]. It has also been reported that steelmaking slag has good activity on coal pyrolysis, which can effectively convert coal-derived tar into fuel gas [20]. These studies confirmed that good activity can be expected using steel slag as a catalyst for biomass tar removal based on the selfcontained metal compounds. Also, the utilization of steel slag as a catalyst in the development of biomass energy can provide a promising approach for the reuse of steel slag.
Table 1 Ultimate and proximate analyses of PS. Sample
pine
Ultimate analysis (wt.%, daf)
Proximate analysis (wt.%, db)
C
H
Odiff
N
S
51.1
5.8
42.3
0.7
0.1
db- Dry basis; daf- Dry and ash free basis;
diff
Ash
Volatile
Fixed carbon
1.3
81.0
17.7
By difference.
In this study, steel slag after calcination was applied for catalytic cracking and steam reforming of biomass pyrolysis tar at 600e800 C. The experiment results are expected to provide substantial fundamental information for the application of steel slag as a catalyst in biomass pyrolysis/gasification processes. The catalytic performance of steel slag on biomass pyrolysis tar vapor was comprehensively investigated based on tar conversion efficiency and product gas yield. The structural properties of the fresh and spent steel slag samples were analyzed to study the evolution of the steel slag during tar reforming. 2. Experimental 2.1. Biomass and steal slag Pine sawdust (PS) was selected as the raw biomass material to produce the pyrolysis tar for catalytic cracking/reforming experiments. The pine sawdust was collected in the surrounding of Xuzhou, Jiangsu province, China, and washed using deionized water for several times to remove the impurities on its surface. The material was crushed using a mill and sieved to a particle size of 0.18e0.425 mm, and then dried in air at 105 C for 24 by a drying oven. Table 1 summarizes the ultimate and proximate analyses of pine sawdust, showing that PS has a high volatile content of 81 wt% on a dry basis. The steel slag (SS) used in this work was obtained from a local steel mill. After washing with deionized water, the steel slag was milled and sieved to a size of 0.18e0.425 mm as well, and then dried at 105 C for 24 h. The dried steel slag was calcined at 800 C for 4 h with a muffle furnace in air and then stored in sealable bags for further use as catalyst. 2.2. Experimental setup and procedure Fig. 1 illustrates the detailed schematic diagram of the experimental system. In this study, the experimental equipment employed to carry out the steel slag catalytic reforming of biomass pyrolysis tar experiments was a two-stage fixed bed reactor heated by a two-zone vertical electric heating furnace (220 V, 3 kW). The reactor was a 30 mm diameter quartz cylinder equipped with two porous quartz plates. The upper porous quartz plate was used for supporting the biomass material and the lower plate was employed for supporting the catalyst. There was a feeder above the reactor, where the biomass material was preloaded before each test. The
Fig. 1. Schematic diagram of experimental equipments for the process tests.
F. Guo et al. / Energy 189 (2019) 116161
3
Fig. 2. SEM images of (a, b) raw steel slag, (c, d) steel slag after calcination at 800 C.
Fe3O4 Fe2O3 MgFe2O4 Mg2SiO4
Intensity(a.u.)
(b)
(a) 10
20
30
40
50
60
70
80
90
2 theta (deg.) Fig. 3. XRD patterns of (a) raw steel slag and (b) steel slag after calcination at 800 C.
furnace has two separate heating zones with a stable heating height of 200 mm each, and the temperature was controlled employing a temperature controller. The temperature controller adopted the PID automatic temperature control system and could achieve a temperature control accuracy of plus or minus 1 C.
Nitrogen (N2, 99.99%) is used as carrier gas which is delivered by a mass flow controller, and the total gas flow is recorded. The steam was fed in as deionized water through a special pipe of the reactor by a high-performance peristaltic pump. The tar contained in the product gas was collected based on the condensation and adsorption mechanism. Thus, the tar collecting system consisted of four washing bottles filled with isopropanol which was placed in an ice bath. After the tar collecting devices, the product gas passed through a washing bottle with water and a drying bottle with silica gel to trap the escaping isopropanol. Then, the gas was pumped by a vacuum pump and collected using a sample bag. Before each test, 4 g of biomass was pre-loaded in the feeder and 20 g of steel slag was placed on the lower porous plate, and the N2 was fed continuously at a flow rate of 300 mL/min to remove the air in the system. After both the two stages of the reactor reached the desired temperatures (600 C for the upper stage, 600e800 C for the lower stage), the N2 flow rate was changed to 150 mL/min to provide an inert atmosphere for the reactions and carried the gas product out of the reactor. Then, the biomass sample was rapidly fed into the upper stage of the reactor (in around 3s), and the biomass pyrolysis started and the released volatile matters passed through the catalyst bed for tar reforming. For tar steam reforming experiments, the steam was fed directly to the catalyst bed at 0.12 g/min at the same time as the biomass feed. Each experimental run lasted 15 min to ensure that the reactions were completed, and at least 3 runs were carried out to ensure the repeatability and the mean values were calculated as the final results.
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2.3. Sampling and analysis The main composition of the dried gases (H2, O2, CO, and CH4) was analyzed using an SC-8000-010 gas chromatograph (Chongqing Chuanyi, China). The gas chromatograph is equipped with a thermal conductivity detector (TCD) and two columns (5A and GDX-104), and helium (He) was employed as the carrier gas. The pipelines between the reactor and tar collecting system were cleaned by isopropanol, and the solution was mixed with the solution in the tar collection system. Then, all isopropanol solutions were evaporated at 105 C with a rotary evaporator. The final remaining viscose liquid is defined as the tar in this study. The chemical composition was analyzed with the X-Ray Fluorescence (XRF, BRUKER S8 TIGER, Germany). The microstructure and morphology of the fresh and spent catalysts were analyzed with scanning electron microscopy (SEM, FEI Quanta 250, USA). Xray diffraction (XRD) patterns of the fresh and spent catalysts were measured using a Bruker D8 instrument (D8-ADVANCE, Germany) with a Cu Ka radiation. Based on the collection methods, the yields of gas and tar were calculated by different methods. For product gas, the volume flow of carrier gas N2 was recorded by a gas flowmeter during each test and the volume concentration N2 in the product gas was measured by the GC, and therefore the yield of the total syngas as well as each gas component can be calculated based on nitrogen balance method, as follow Eqn. 1：
Yx ¼ VN2 CN2 ðCx =mB Þ
(1)
where Yx represents the volume yield of total syngas or the gas component, mL/g; Cx is the volume concentration of the total gas (Cx ¼ 1) or the gas component, vol%; CN2 represents the volume concentration of N2 in the product gas, vol%; mB represents the mass of biomass material used for each test, g; VN2 represents the volume of N2 in the product gas recorded by the mass flowmeter, mL. The tar yield was calculated based on the weight of the tar sample in each test, as follows Eqn. 2:
YTar ¼ mTar =mB
(2)
where YTar represents the yield of the tar, mg/g; mTar is the weight of the tar sample, mg. The tar yield from pine sawdust pyrolysis at 600 C in the upper stage without any catalytic process (the lower stage of the reactor was set at 300 C to prevent tar from condensation) was defined as
100
90.8%
150
Cycle 1 Cycle 2 Cycle 3
Blank
199.4 mg/g
50
0
.
hTar ¼ YTar Y 0Tar
(3)
where hTar represents the tar conversion efficiency, %; Y 0Tar represents tar yield in the blank experiment, mg/g. 3. Experimental results and discussion 3.1. Characterization of steel slag XRF results showed that the steel slag was mainly composed of Fe2O3 and SiO2, which account for over 31.08 wt% and 33.22 wt% respectively. Additionally, the steel slag also contains many other metal oxides, such as Al2O3 (9.49 wt%), CaO (8.14 wt%), MgO (3.75 wt %), ZnO (1.87 wt %), Na2O (1.81 wt %) and K2O(0.74 wt %). SEM images of the steel slag before and after calcination at 800 C are shown in Fig. 2. It was observed in Fig. 2 (a) and (b) that the raw steel slag exhibited a smooth and compact surface without any porous structure. After calcination at 800 C, it was noted in Fig. 2 (c) and (d) that the smooth surface of steel slag was destroyed, and the surface became rough, showing a variety of irregular groove-like and large loose structures. The result can be attributed to oxidation reactions of the metal compounds, leading to the variation in the crystal structure of the slag [21,22]. Furthermore, the rough surface of steel slag with irregular groove-like and large loose structure is more conducive to the contact between the reactants and steel slag, which may in turn improve the catalytic activity of steel slag. The XRD patterns of the steel slag before and after calcination are depicted in Fig. 3. For the raw steel slag, a wide diffraction peak can be observed at 21.5 to 37.5 and centered at around 30 , which was attributed to the amorphous diffraction peak formed by silicon compounds. Also, some weak peaks corresponding to MgFe2O4, Fe2O3, and Mg2SiO4 were observed in the raw steel slag. Overall, the composition of steel slag is complex and most of the diffraction peaks are superimposed, leading to that the clear diffraction peaks cannot be effectively separated. After calcination at 800 C for 4 h, the XRD pattern of the steel slag can be seen in Fig. 3(b). Several clear diffraction peaks can be observed, corresponding to Fe2O3 and MgFe2O4, which indicates that the crystal structure of MgFe2O4 and Fe2O3 is gradually formed during the calcination of steel slag. Fe2O3 has been widely used in the preparation of catalysts for
(b) 400
Cycel 1 Cycel 2 Cycel 3
300
200
Blank
172.9
Tar yield (mg/g)
(a) 200
the tar yield in the blank experiment. Thus, the tar conversion efficiency was calculated to further evaluate the catalytic performance of the catalyst, as follows Eqn. 3:
Syngas yield (mL/g)
4
600
700
800 o
Temperature ( C)
100
600
700
800 o
Temperature ( C)
Fig. 4. (a) Tar yield and (b) syngas yield as function of temperature and steel slag recycling times.
F. Guo et al. / Energy 189 (2019) 116161
reforming process, and to investigate the influence of the related changes on the catalytic performance of the steel slag.
biomass pyrolysis/gasification and tar reforming. It has been reported by Bleeker et al. [23] that iron oxide had significant catalytic activity on the cracking of pyrolysis oil and led to the increased carbon-to-gas conversion during the gasification. Matsuoka et al. [24] also found that tar can be reformed by iron oxide with steam to form H2 during the steam reforming of woody biomass in a fluidized bed, while CO was consumed to reduce the iron oxide. Polychronopoulou et al. [25] reported a dolomite with relatively high iron content as a catalyst for the reforming of toluene, finding that the toluene can be effectively reformed during short times on stream. Consequently, the iron oxides play an important role in steam reforming biomass tar, and the calcined steel slag rich in iron oxides has the potential for catalytic reforming of biomass tar. 3.2. Catalytic cracking of biomass tar The vapor produced from biomass pyrolysis contains tar, permanent gas components and a small amount of water vapor, and therefore complex reforming reactions occur if the vapor passes through the high-temperature atmosphere. In the presence of catalyst, these reactions may be enhanced. Using steel slag as a catalyst, the main reactions of tar cracking and gas reforming can be explained by homogeneous and heterogeneous reactions of Eqs. (4)e(9) [26]. Besides, iron oxides in steel slag can be reduced by reducing gases (mainly CO and H2) via Eqs.(10)-(15) to form lowvalent iron oxides, even zero-valent iron, which have been reported having high activity on tar catalytic cracking [11]. Therefore, when steel slag is used as a catalyst, it is accompanied by the modification of steel slag itself in the process of tar reforming. Based on this, in this research, the steel slag was recycled three times to verify the variation of the steel slag in the process of tar
CO yield (mL/g)
H2 yield (mL/g)
(4)
Cn Hm þ nCO2 /2nCO þ ðm=2ÞH2
(5)
Cn Hm þ nH2 O/nCO þ ðn þ m=2ÞH2
(6)
CH4 þ CO2 42H2 þ 2CO
(7)
CO þ H2 O4CO2 þ H2
(8)
C þ H2 O4CO þ H2
(9)
3Fe2 O3 þ CO42Fe3 O4 þ CO2
(10)
3Fe2 O3 þ H2 42Fe3 O4 þ H2 O
(11)
Fe3 O4 þ CO43FeO þ CO2
(12)
Fe3 O4 þ H2 43FeO þ H2 O
(13)
FeO þ CO4Fe þ CO2
(14)
FeO þ H2 4Fe þ H2 O
(15)
Cycle 1 Cycle 2 Cycle 3
200
40
20
Tar / C þ Cn Hm þ gases
The tar and syngas yields under different conditions are shown in Fig. 4. The tar and syngas yields from pine sawdust pyrolysis in
Cycle 1 Cycle 2 Cycle 3
60
Blank
5
150
Blank 96.3
18.9
100
0
600
700
o
800
600
Temperature ( C) Cycle 1 Cycle 2 Cycle 3
80
700
o
800
Temperature ( C) Cycle 1 Cycle 2 Cycle 3
40
Blank
20 29.8
CO2 yield (mL/g)
40
Blank
20 27.9
CH4 yield (mL/g)
60
0
600
700
o
Temperature ( C)
800
0
600
700
o
Temperature ( C)
Fig. 5. The yield of the gas components as function of temperature and steel slag recycling times.
800
6
F. Guo et al. / Energy 189 (2019) 116161
Fe3O4 MgFe2O4
80
400 60 300 40 200 20
100
Mg2SiO4
(c) Intensity(a.u.)
Syngas yield (mL/g)
500
100
SS-Syngas SR-SS-Syngas SS-Tar SR-SS-Tar
Tar conversion efficiency (%)
600
(b)
(a) 0
600
700
0
800 o
20
Temperature ( C) Fig. 6. Tar and syngas yield under catalytic steam reforming.
the upper stage were shown as the blank experimental value (the blue dotted line). Since the tar cracking reactions are generally endothermic, it can be observed in Fig. 4 (a) that the tar yield decreased obviously with the increasing temperature. The steel slag showed good catalytic activity on tar reforming, leading to high tar conversion efficiency in the temperature range of 600e800 C. Particularly, at a certain temperature, the tar yield decreased with the increase of the recycling time, and the tar conversion efficiency reached 90.8% at 800 C for the third time reuse of steel slag. The result may be due to the reduction of iron oxides to low-valent iron during the process of tar cracking, which improves the overall catalytic performance of steel slag. The high tar conversion efficiency further confirms that steel slag has the potential as a lowcost catalyst for catalytic conversion of biomass tar. Fig. 4 (b) shows the variation of syngas with temperature and steel slag recycling times. Due to the enhanced tar reforming reactions at higher temperatures, the syngas yield increased significantly with the increase of temperature. Also, with the increase of the recycling time of steel slag, the yield of syngas was significantly improved, especially at relatively low temperatures of 600 C and 700 C, indicating that the recycled steel slag has higher catalytic
300
SS-600 SS-700 SS-800 SR-SS-600 SR-SS-700 SR-SS-800
Gas yield (mL/g)
250 200 150 100 50
40
60
80
2 theta (deg.) Fig. 8. XRD patterns of the spent steel slag (a) after tar cracking process for 1 recycling time, (b) after tar cracking process for 3 recycling times, (c) after steam reforming process.
activity for tar cracking and gas reforming. In comparison with the tar and syngas yields from the blank experiment, it can be seen that the Fe2O3, Mg2SiO4 and MgFe2O4 formed by calcined steel slag exhibit obvious catalytic effect on biomass tar decomposition for syngas production. After recycling, part of Fe2O3 may be reduced to low-valent iron, which can further improve the catalytic activity of steel slag and promote the cracking of tar to syngas. Fig. 5 shows the yield of the four main gas components (H2, CO, CO2, and CH4) under different reaction conditions. It can be seen that temperature has a great influence on the formation of the four gas components, and all their yields increased significantly with the increase of temperature due to the enhanced tar cracking and gas reforming reactions. At a certain temperature, the yield of H2 increased significantly with the recycling time, while no significant increase was observed in the yield of CO. The result can be explained based on the possible reactions during biomass tar cracking. The tar reforming reactions as well as reactions between gases lead to more yield of H2 and CO, while the reduction reactions between them and the steel slag result in a certain amount of consumption as well. After recycling for two times, a large amount of iron oxides on the surface of steel slag can be reduced to lowvalent iron oxides or zero-valent iron, which can further act as active sites for tar reforming reactions and promote the formation of more H2 and CO. The yield of CH4 remained stable with the recycling times, as shown in Fig. 4 (c), indicating that the steel slag exhibits similar catalytic activity on the CH4 related reactions. The tar cracking can generate some CH4, while the dry reforming of CH4 and CO2 leads to the conversion of some CH4 into H2 and CO. The formation of CO2 relates to many reactions, including the tar cracking (Eq. (4)), tar dry reforming (Eq. (5)), water-gas shift (Eq. (8)) and iron oxides reduction reactions (Eq. (12) and Eq. (15)), leading to the irregular changing trend of CO2 yield. 3.3. Catalytic steam reforming of biomass tar
0
H2
CO
CH4
CO2
Fig. 7. The yield of gas components under catalytic steam reforming.
The steam reforming of biomass pyrolysis tar was studied to further evaluate the catalytic performance of steel slag. Fig. 6 shows the tar and syngas yields after steam reforming, which are denoted as SReSSex (x stands for the reforming temperature), and the values without steam (SS-x) are also presented for comparison. It
F. Guo et al. / Energy 189 (2019) 116161
was observed that steam exhibited a significant effect on the tar conversion as well as syngas yield. The tar conversion efficiency after steam reforming was higher than that without steam, and the value reached 94.1% at 800 C, indicating the excellent catalytic performance of steel slag on biomass tar steam reforming. The tar steam reforming reactions can be significantly promoted by the additional steam supply, leading to the formation of more permanent gases. Furthermore, the steam also participated in many other gas reforming reactions, such as water-shift (Eq. (8)) and watercarbon reactions (Eq. (9)), which resulted in the generation of gases as well. As a result, the gas yield after tar reforming process was significantly improved in comparison with the condition without steam, and the maximum gas yield reached 493.5 mL/g at 800 C. The yields of the four gas components under catalytic steam reforming conditions are shown in Fig. 7, and the values without steam are presented for comparison as well. It can be observed that the presence of steam can significantly improve the yield of H2 and CO. For H2, the enhanced tar steam reforming reactions by the additional steam can promote the generation of H2, and the watergas shift and water-carbon reactions between H2 and CO as well as carbon can also produce H2. As a result, the yield of H2 increased from 23.3, 38.3 and 48.6 mL/g to 39.6, 67.4 and 91.3 mL/g at 600, 700 and 800 C respectively due to the presence of steam. For CO,
7
the increase of the yield was mainly due to the enhanced tar steam reforming and water-carbon reactions. Also, it can be speculated that the reduction reactions between the iron oxides and CO and H2 may be weakened by the high concentration of steam in the system, also leading to a higher yield of CO and H2. It can be seen that the CH4 yield is significantly affected by temperature, while the supply of steam shows almost no influence on its yield. For CO2, the yield shows a little increase by the presence of steam, which may be due to the enhanced water-gas shift reaction.
3.4. Characterization of the spent steel slag The XRD patterns of the spent steel slag are illustrated in Fig. 8, showing that the chemical composition of the spent steel slag samples after tar cracking and steam reforming is almost the same. It can be observed that the diffraction peaks of MgFe2O4 and MgSiO4 remained unchanged in the spent steel slag samples, while the peaks corresponding to Fe2O3 disappeared. Meanwhile, it was found that diffraction peaks corresponding to Fe3O4 were enhanced, representing the reduction of Fe2O3 during the tar cracking/reforming process, while no FeO or zero-valent iron was observed. It has been reported by Uddin et al. [27] that the iron oxide catalysts can be transformed to Fe3O4 after use which possessed good catalytic activity on the cracking of biomass tar.
Fig. 9. SEM images of the spent steel slag (a, b) after tar cracking process for 1 recycling time, (c, d) after tar cracking process for 3 recycling times, (e, f) after steam reforming process.
8
F. Guo et al. / Energy 189 (2019) 116161
Also, the iron oxides can be reduced into zero-valent iron which also has excellent catalytic activity on biomass tar decomposition, while it is unstable in air and easy to be oxidized again. As a whole, the main crystal structure of spent steel slag showed no significant change, leading to the high tar conversion efficiency after reuse several times. Fig. 9 shows the SEM images of the spent steel slag samples after the tar cracking/steam reforming experiments. After one-time tar cracking process, concave and convex grooves can be observed on the surface of the spent steel slag. Loose structure was formed and small particles with different colors appeared, indicating that complicated chemical reactions occur on the surface of steel slag during the tar cracking process. Reductive gases (H2, CO) from the biomass pyrolysis as well as tar cracking can react with the iron oxides on the surface of steel slag, leading to the variation in structure. This can be confirmed by the formation of Fe3O4 in the XRD results. The formation of pores on the surface of steel slag can in turn facilitate the contact between tar molecules and steel slag in the process of reforming reaction. Furthermore, the reduced lowvalent iron or even zero-valent iron on the surface of steel slag can further improve the catalytic activity of steel slag, which is the reason why the tar conversion efficiency increased with the recycling times of steel slag. After reuse for three times in tar catalytic cracking, the structure of the spent steel slag in Fig. 9 (c) and (d) does not show significant variation compared with the sample after used for one time, indicating that the surface structure of steel slag becomes relatively stable after tar catalytic cracking. The results are also coincident with the XRD results that the diffraction peaks of MgFe2O4 and MgSiO4 remain basically the same for the spent steel slag. The SEM images of steel slag after tar steam reforming in Fig. 9 (e) and (f) also showed stable loose structure, implying that the interaction between biomass tar and steel slag during the steam reforming was the same with catalytic cracking process. The stable porous structure of the spent steel slag contributes to the efficient tar removal in long-term biomass conversion applications.
4. Conclusion In this study, the steel slag was successfully used as a low-cost catalyst for tar cracking/steam reforming based on its high content of iron oxides. After simple calcination treatment, the steel slag can be converted into loose structure with many pores and clear crystal structure of Fe2O3 and MgFe2O4. The steel slag showed good catalytic activity on the decomposition of biomass-derived tar, and the reuse of the steel slag led to higher catalytic performance due to the reduction of iron oxides into low-valent iron. The steam reforming can further improve the tar conversion efficiency and syngas yield due to the enhanced tar steam reforming reactions. At 800 C, a tar conversion efficiency as high as 94.1% was achieved under steam reforming conditions, with a high syngas yield of 493.5 mL/g. After tar reforming, the spent steel slag still showed loose structure and the oxides were reduced into Fe3O4, which ensured the effective and long-term application of steel slag as a catalyst in biomass tar removal.
Acknowledgement This work was financially supported by the National Natural Science Foundation of China (No. 51876217).
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Catalytic steam reforming of biomass tar: prospects and challenges. Renew Sustain Energy Rev 2016;58: 450e61. [7] Guo F, Peng K, Liang S, Jia X, Jiang X, Qian L. One-step synthesis of biomass activated char supported copper nanoparticles for catalytic cracking of biomass primary tar. Energy 2019;180:584e93. [8] Gao N, Liu S, Han Y, Xing C, Li A. Steam reforming of biomass tar for hydrogen production over NiO/ceramic foam catalyst. Int J Hydrogen Energy 2015;40(25):7983e90. [9] Du ZY, Zhang ZH, Xu C, Wang XB, Li WY. Low-temperature steam reforming of toluene and biomass tar over biochar-supported Ni nanoparticles. ACS Sustainable Chem Eng 2019;7(3):3111e9. [10] Dai L, Wang Y, Liu Y, Ruan R, Duan D, Zhao Y, Yu Z, Jiang L. Catalytic fast pyrolysis of torrefied corn cob to aromatic hydrocarbons over Ni-modified hierarchical ZSM-5 catalyst. Bioresour Technol 2019;272:407e14. [11] Gai C, Zhang F, Lang QQ, Liu TT, Peng NN, Liu ZG. Facile one-pot synthesis of iron nanoparticles immobilized into the porous hydrochar for catalytic decomposition of phenol. Appl Catal B Environ 2017;204:566e76. [12] Guo F, Zhao X, Peng K, Liang S, Jia X, Qian L. Catalytic reforming of biomass primary tar from pyrolysis over waste steel slag based catalysts. Int J Hydrogen Energy 2019;44(31):16224e33. [13] Guo F, Liu Y, Liu Y, Guo C. Catalytic reforming of tar using corncob char and char-supported potassium catalysts. J Therm Anal Calorim 2017;130: 1297e306. [14] Feng D, Zhao Y, Zhang Y, Xu H, Zhang L, Sun S. Catalytic mechanism of ionexchanging alkali and alkaline earth metallic species on biochar reactivity during CO2/H2O gasification. Fuel 2018;212:523e32. [15] Ashok J, Das S, Yeo TY, Dewangan N, Kawi S. Incinerator bottom ash derived from municipal solid waste as a potential catalytic support for biomass tar reforming. Waste Manag 2018;82:249e57. [16] Zou X, Ma Z, Liu H, Chen D, Wang C, Zhang P, Chen T. Green synthesis of Ni supported hematite catalysts for syngas production from catalytic cracking of toluene as a model compound of biomass tar. Fuel 2018;217:343e51. [17] Yang J, Kaewpanha M, Karnjanakom S, Guan G, Hao X, Abudula A. Steam reforming of biomass tar over calcined egg shell supported catalysts for hydrogen production. Int J Hydrogen Energy 2016;41(16):6699e705. [18] Guo F, Peng K, Li T, Zhao X, Dong Y, Rao Z. Catalytic cracking of primary tar vapor from biomass over high ash-containing paper sludge ash. Energy Fuels 2018;32(12):12514e22. [19] Song Q, Zhao HY, Jia JW, Zhang F, Wang Z, Lv W, Yang L, Zhang W, Zhang Y, Shu X. Characterization of the products obtained by pyrolysis of oil sludge with steel slag in a continuous pyrolysis-magnetic separation reactor. Fuel 2019;255:115711. [20] Cahyono RB, Rozhan AN, Yasuda N, Nomura T, Hosokai S, Kashiwaya Y, Akiyama T. Integrated coal-pyrolysis tar reforming using steelmaking slag for carbon composite and hydrogen production. 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Catalytic reforming of biomass pyrolysis tar using the low-cost steel slag as catalyst Feiqiang Guo a, b, c, *, Shuang Liang a, b, Xingmin Zhao a, b, Xiaopeng Jia a, b, Kuangye Peng a, b, Xiaochen Jiang a, b, Lin Qian a, b a Jiangsu Province Engineering Laboratory of High Efficient Energy Storage Technology and Equipments, China University of Mining and Technology, Xuzhou, 221116, China b School of Electrical and Power Engineering, China University of Mining and Technology, 221116, Xuzhou, PR China c Center for Biorefining, Bioproducts and Biosystems Engineering Department, University of Minnesota, St. Paul, Minnesota, 55108, United States
a r t i c l e i n f o
a b s t r a c t
Article history: Received 7 July 2019 Received in revised form 14 September 2019 Accepted 17 September 2019 Available online 20 September 2019
In this work, the possibility of steel slag as an effective and low-cost catalyst for the decomposition of biomass pyrolysis tar has been explored based on the high content of iron oxides for sustainable syngas production from biomass. By simple calcination treatment at 800 C, the loose structure of the steel slag was formed with the main chemical composition of Fe2O3 and MgFe2O4. The steel slag exhibited good catalytic activity on the cracking of biomass pyrolysis tar, and even higher tar conversion efficiency can be obtained by reusing the steel slag, leading to the increase in syngas yield. The presence of additional steam can further promote the tar reforming reactions, leading to the significant increase in H2 and CO. At 800 C, the tar conversion efficiency reached 94.1% with a high gas yield of 493.5 mL/g. The interaction between steel slag and reductive gases resulted in the reduction of iron oxides into Fe3O4, and more pores were formed for the spent steel slag, which can enhance the contact between active sites and reactants. These characteristics indicate that steel slag has the potential to be used as an efficient catalyst with excellent stability in the long-term biomass tar removal applications. © 2019 Elsevier Ltd. All rights reserved.
Keywords: Biomass Steel slag Tar removal Catalytic reforming
1. Introduction Nowadays, biomass pyrolysis and/or pyrolysis is an increasingly attractive method for the production of fuel gas using abundant, low-cost and CO2 neutral biomass resource. However, one of the most challenging issues during biomass pyrolysis and/or gasification is to reduce the tar content in the product gas to a certain level [1,2]. The tar is mainly composed of condensable chemical compounds, such as aromatics (1e5 rings), poly-aromatic hydrocarbons and oxygen-containing compounds, which can cause coking, blockage and other operational problems in the downstream of the pyrolysis and gasification system [3,4]. Furthermore, these tars also contained large amounts of energy, and therefore lower the overall energy conversion of biomass. Thus, efficient tar removal method is of great importance to the biomass pyrolysis/gasification processes.
* Corresponding author. Jiangsu Province Engineering Laboratory of High Efficient Energy Storage Technology and Equipments, China University of Mining and Technology, Xuzhou, 221116, China. E-mail address: [email protected] (F. Guo). https://doi.org/10.1016/j.energy.2019.116161 0360-5442/© 2019 Elsevier Ltd. All rights reserved.
To date, the commonly used methods for tar removal in industrial applications are water scrubbing, thermal cracking and catalytic reforming [5]. Water scrubbing is a simple and effective method to reduce the tar content in the product gas, while wastewater containing aromatics is produced as well which may lead to new economic and environmental problems. The thermal cracking method generally requires a high operating temperature over 1100 C to achieve effective tar conversion efficiency [6]. In comparison, catalytic reforming of tar has been considered as the most promising approach for the chemical conversion of tar into permanent gases at relatively lower temperatures (600e800 C) without any production of wastewater [7]. The catalyst is crucial to the catalytic reforming method, which not only influences the tar decomposition, but also affects the overall economic feasibility of the method [8]. Hence, developing effective and low-cost catalysts is becoming increasingly attractive. Metal-based catalysts, such as Ni-based catalysts [9,10], Febased catalysts [11,12] and alkali metal catalysts [13,14], have been widely investigated due to the high catalytic activity and operational availability. However, considering the commercial
2
F. Guo et al. / Energy 189 (2019) 116161
application, the performance and the cost are two crucial parameters in the developing of catalysts for the catalytic pyrolysis/gasification processes. Thus, naturally available minerals and industrial waste, which are generally rich in metal oxides, have also been investigated as efficient and low-cost catalysts. For example, Ashok et al. [15] used municipal solid waste incineration bottom ash as a catalyst support material, and high catalytic activity was achieved for steam reforming of biomass tar at 700e800 C. Zou et al. [16] selected natural limonite as catalyst for catalytic cracking of toluene (a typical model compound of tar), finding that toluene can be decomposed into H2, CO, CH4 and CO, and the catalytic performance can be further improved by nickel loading. Yang et al. [17] reported that calcined eggshell possessed porous structure and alkaline property, which showed high catalytic activity on steam reforming of tar derived from cedar wood for the production of hydrogen-rich gas. In our previous study, high ash-containing paper sludge ash was investigated as a low-cost catalyst for biomass tar cracking, which showed that the tar can be successfully decomposed and the yields of H2, CH4 and CO2 were significantly improved [18]. Steel slag, the byproduct in the steelmaking process, is a cheap industrial waste that contains significant amounts of metal compounds such as iron, magnesium, aluminum and calcium, and thus has the potential to be used as catalyst for biomass tar reforming. Steel slag has been investigated as the catalyst for oil sludge pyrolysis, showing that steel slag can significantly intensify the secondary thermal cracking of volatile matters [19]. It has also been reported that steelmaking slag has good activity on coal pyrolysis, which can effectively convert coal-derived tar into fuel gas [20]. These studies confirmed that good activity can be expected using steel slag as a catalyst for biomass tar removal based on the selfcontained metal compounds. Also, the utilization of steel slag as a catalyst in the development of biomass energy can provide a promising approach for the reuse of steel slag.
Table 1 Ultimate and proximate analyses of PS. Sample
pine
Ultimate analysis (wt.%, daf)
Proximate analysis (wt.%, db)
C
H
Odiff
N
S
51.1
5.8
42.3
0.7
0.1
db- Dry basis; daf- Dry and ash free basis;
diff
Ash
Volatile
Fixed carbon
1.3
81.0
17.7
By difference.
In this study, steel slag after calcination was applied for catalytic cracking and steam reforming of biomass pyrolysis tar at 600e800 C. The experiment results are expected to provide substantial fundamental information for the application of steel slag as a catalyst in biomass pyrolysis/gasification processes. The catalytic performance of steel slag on biomass pyrolysis tar vapor was comprehensively investigated based on tar conversion efficiency and product gas yield. The structural properties of the fresh and spent steel slag samples were analyzed to study the evolution of the steel slag during tar reforming. 2. Experimental 2.1. Biomass and steal slag Pine sawdust (PS) was selected as the raw biomass material to produce the pyrolysis tar for catalytic cracking/reforming experiments. The pine sawdust was collected in the surrounding of Xuzhou, Jiangsu province, China, and washed using deionized water for several times to remove the impurities on its surface. The material was crushed using a mill and sieved to a particle size of 0.18e0.425 mm, and then dried in air at 105 C for 24 by a drying oven. Table 1 summarizes the ultimate and proximate analyses of pine sawdust, showing that PS has a high volatile content of 81 wt% on a dry basis. The steel slag (SS) used in this work was obtained from a local steel mill. After washing with deionized water, the steel slag was milled and sieved to a size of 0.18e0.425 mm as well, and then dried at 105 C for 24 h. The dried steel slag was calcined at 800 C for 4 h with a muffle furnace in air and then stored in sealable bags for further use as catalyst. 2.2. Experimental setup and procedure Fig. 1 illustrates the detailed schematic diagram of the experimental system. In this study, the experimental equipment employed to carry out the steel slag catalytic reforming of biomass pyrolysis tar experiments was a two-stage fixed bed reactor heated by a two-zone vertical electric heating furnace (220 V, 3 kW). The reactor was a 30 mm diameter quartz cylinder equipped with two porous quartz plates. The upper porous quartz plate was used for supporting the biomass material and the lower plate was employed for supporting the catalyst. There was a feeder above the reactor, where the biomass material was preloaded before each test. The
Fig. 1. Schematic diagram of experimental equipments for the process tests.
F. Guo et al. / Energy 189 (2019) 116161
3
Fig. 2. SEM images of (a, b) raw steel slag, (c, d) steel slag after calcination at 800 C.
Fe3O4 Fe2O3 MgFe2O4 Mg2SiO4
Intensity(a.u.)
(b)
(a) 10
20
30
40
50
60
70
80
90
2 theta (deg.) Fig. 3. XRD patterns of (a) raw steel slag and (b) steel slag after calcination at 800 C.
furnace has two separate heating zones with a stable heating height of 200 mm each, and the temperature was controlled employing a temperature controller. The temperature controller adopted the PID automatic temperature control system and could achieve a temperature control accuracy of plus or minus 1 C.
Nitrogen (N2, 99.99%) is used as carrier gas which is delivered by a mass flow controller, and the total gas flow is recorded. The steam was fed in as deionized water through a special pipe of the reactor by a high-performance peristaltic pump. The tar contained in the product gas was collected based on the condensation and adsorption mechanism. Thus, the tar collecting system consisted of four washing bottles filled with isopropanol which was placed in an ice bath. After the tar collecting devices, the product gas passed through a washing bottle with water and a drying bottle with silica gel to trap the escaping isopropanol. Then, the gas was pumped by a vacuum pump and collected using a sample bag. Before each test, 4 g of biomass was pre-loaded in the feeder and 20 g of steel slag was placed on the lower porous plate, and the N2 was fed continuously at a flow rate of 300 mL/min to remove the air in the system. After both the two stages of the reactor reached the desired temperatures (600 C for the upper stage, 600e800 C for the lower stage), the N2 flow rate was changed to 150 mL/min to provide an inert atmosphere for the reactions and carried the gas product out of the reactor. Then, the biomass sample was rapidly fed into the upper stage of the reactor (in around 3s), and the biomass pyrolysis started and the released volatile matters passed through the catalyst bed for tar reforming. For tar steam reforming experiments, the steam was fed directly to the catalyst bed at 0.12 g/min at the same time as the biomass feed. Each experimental run lasted 15 min to ensure that the reactions were completed, and at least 3 runs were carried out to ensure the repeatability and the mean values were calculated as the final results.
F. Guo et al. / Energy 189 (2019) 116161
2.3. Sampling and analysis The main composition of the dried gases (H2, O2, CO, and CH4) was analyzed using an SC-8000-010 gas chromatograph (Chongqing Chuanyi, China). The gas chromatograph is equipped with a thermal conductivity detector (TCD) and two columns (5A and GDX-104), and helium (He) was employed as the carrier gas. The pipelines between the reactor and tar collecting system were cleaned by isopropanol, and the solution was mixed with the solution in the tar collection system. Then, all isopropanol solutions were evaporated at 105 C with a rotary evaporator. The final remaining viscose liquid is defined as the tar in this study. The chemical composition was analyzed with the X-Ray Fluorescence (XRF, BRUKER S8 TIGER, Germany). The microstructure and morphology of the fresh and spent catalysts were analyzed with scanning electron microscopy (SEM, FEI Quanta 250, USA). Xray diffraction (XRD) patterns of the fresh and spent catalysts were measured using a Bruker D8 instrument (D8-ADVANCE, Germany) with a Cu Ka radiation. Based on the collection methods, the yields of gas and tar were calculated by different methods. For product gas, the volume flow of carrier gas N2 was recorded by a gas flowmeter during each test and the volume concentration N2 in the product gas was measured by the GC, and therefore the yield of the total syngas as well as each gas component can be calculated based on nitrogen balance method, as follow Eqn. 1：
Yx ¼ VN2 CN2 ðCx =mB Þ
(1)
where Yx represents the volume yield of total syngas or the gas component, mL/g; Cx is the volume concentration of the total gas (Cx ¼ 1) or the gas component, vol%; CN2 represents the volume concentration of N2 in the product gas, vol%; mB represents the mass of biomass material used for each test, g; VN2 represents the volume of N2 in the product gas recorded by the mass flowmeter, mL. The tar yield was calculated based on the weight of the tar sample in each test, as follows Eqn. 2:
YTar ¼ mTar =mB
(2)
where YTar represents the yield of the tar, mg/g; mTar is the weight of the tar sample, mg. The tar yield from pine sawdust pyrolysis at 600 C in the upper stage without any catalytic process (the lower stage of the reactor was set at 300 C to prevent tar from condensation) was defined as
100
90.8%
150
Cycle 1 Cycle 2 Cycle 3
Blank
199.4 mg/g
50
0
.
hTar ¼ YTar Y 0Tar
(3)
where hTar represents the tar conversion efficiency, %; Y 0Tar represents tar yield in the blank experiment, mg/g. 3. Experimental results and discussion 3.1. Characterization of steel slag XRF results showed that the steel slag was mainly composed of Fe2O3 and SiO2, which account for over 31.08 wt% and 33.22 wt% respectively. Additionally, the steel slag also contains many other metal oxides, such as Al2O3 (9.49 wt%), CaO (8.14 wt%), MgO (3.75 wt %), ZnO (1.87 wt %), Na2O (1.81 wt %) and K2O(0.74 wt %). SEM images of the steel slag before and after calcination at 800 C are shown in Fig. 2. It was observed in Fig. 2 (a) and (b) that the raw steel slag exhibited a smooth and compact surface without any porous structure. After calcination at 800 C, it was noted in Fig. 2 (c) and (d) that the smooth surface of steel slag was destroyed, and the surface became rough, showing a variety of irregular groove-like and large loose structures. The result can be attributed to oxidation reactions of the metal compounds, leading to the variation in the crystal structure of the slag [21,22]. Furthermore, the rough surface of steel slag with irregular groove-like and large loose structure is more conducive to the contact between the reactants and steel slag, which may in turn improve the catalytic activity of steel slag. The XRD patterns of the steel slag before and after calcination are depicted in Fig. 3. For the raw steel slag, a wide diffraction peak can be observed at 21.5 to 37.5 and centered at around 30 , which was attributed to the amorphous diffraction peak formed by silicon compounds. Also, some weak peaks corresponding to MgFe2O4, Fe2O3, and Mg2SiO4 were observed in the raw steel slag. Overall, the composition of steel slag is complex and most of the diffraction peaks are superimposed, leading to that the clear diffraction peaks cannot be effectively separated. After calcination at 800 C for 4 h, the XRD pattern of the steel slag can be seen in Fig. 3(b). Several clear diffraction peaks can be observed, corresponding to Fe2O3 and MgFe2O4, which indicates that the crystal structure of MgFe2O4 and Fe2O3 is gradually formed during the calcination of steel slag. Fe2O3 has been widely used in the preparation of catalysts for
(b) 400
Cycel 1 Cycel 2 Cycel 3
300
200
Blank
172.9
Tar yield (mg/g)
(a) 200
the tar yield in the blank experiment. Thus, the tar conversion efficiency was calculated to further evaluate the catalytic performance of the catalyst, as follows Eqn. 3:
Syngas yield (mL/g)
4
600
700
800 o
Temperature ( C)
100
600
700
800 o
Temperature ( C)
Fig. 4. (a) Tar yield and (b) syngas yield as function of temperature and steel slag recycling times.
F. Guo et al. / Energy 189 (2019) 116161
reforming process, and to investigate the influence of the related changes on the catalytic performance of the steel slag.
biomass pyrolysis/gasification and tar reforming. It has been reported by Bleeker et al. [23] that iron oxide had significant catalytic activity on the cracking of pyrolysis oil and led to the increased carbon-to-gas conversion during the gasification. Matsuoka et al. [24] also found that tar can be reformed by iron oxide with steam to form H2 during the steam reforming of woody biomass in a fluidized bed, while CO was consumed to reduce the iron oxide. Polychronopoulou et al. [25] reported a dolomite with relatively high iron content as a catalyst for the reforming of toluene, finding that the toluene can be effectively reformed during short times on stream. Consequently, the iron oxides play an important role in steam reforming biomass tar, and the calcined steel slag rich in iron oxides has the potential for catalytic reforming of biomass tar. 3.2. Catalytic cracking of biomass tar The vapor produced from biomass pyrolysis contains tar, permanent gas components and a small amount of water vapor, and therefore complex reforming reactions occur if the vapor passes through the high-temperature atmosphere. In the presence of catalyst, these reactions may be enhanced. Using steel slag as a catalyst, the main reactions of tar cracking and gas reforming can be explained by homogeneous and heterogeneous reactions of Eqs. (4)e(9) [26]. Besides, iron oxides in steel slag can be reduced by reducing gases (mainly CO and H2) via Eqs.(10)-(15) to form lowvalent iron oxides, even zero-valent iron, which have been reported having high activity on tar catalytic cracking [11]. Therefore, when steel slag is used as a catalyst, it is accompanied by the modification of steel slag itself in the process of tar reforming. Based on this, in this research, the steel slag was recycled three times to verify the variation of the steel slag in the process of tar
CO yield (mL/g)
H2 yield (mL/g)
(4)
Cn Hm þ nCO2 /2nCO þ ðm=2ÞH2
(5)
Cn Hm þ nH2 O/nCO þ ðn þ m=2ÞH2
(6)
CH4 þ CO2 42H2 þ 2CO
(7)
CO þ H2 O4CO2 þ H2
(8)
C þ H2 O4CO þ H2
(9)
3Fe2 O3 þ CO42Fe3 O4 þ CO2
(10)
3Fe2 O3 þ H2 42Fe3 O4 þ H2 O
(11)
Fe3 O4 þ CO43FeO þ CO2
(12)
Fe3 O4 þ H2 43FeO þ H2 O
(13)
FeO þ CO4Fe þ CO2
(14)
FeO þ H2 4Fe þ H2 O
(15)
Cycle 1 Cycle 2 Cycle 3
200
40
20
Tar / C þ Cn Hm þ gases
The tar and syngas yields under different conditions are shown in Fig. 4. The tar and syngas yields from pine sawdust pyrolysis in
Cycle 1 Cycle 2 Cycle 3
60
Blank
5
150
Blank 96.3
18.9
100
0
600
700
o
800
600
Temperature ( C) Cycle 1 Cycle 2 Cycle 3
80
700
o
800
Temperature ( C) Cycle 1 Cycle 2 Cycle 3
40
Blank
20 29.8
CO2 yield (mL/g)
40
Blank
20 27.9
CH4 yield (mL/g)
60
0
600
700
o
Temperature ( C)
800
0
600
700
o
Temperature ( C)
Fig. 5. The yield of the gas components as function of temperature and steel slag recycling times.
800
6
F. Guo et al. / Energy 189 (2019) 116161
Fe3O4 MgFe2O4
80
400 60 300 40 200 20
100
Mg2SiO4
(c) Intensity(a.u.)
Syngas yield (mL/g)
500
100
SS-Syngas SR-SS-Syngas SS-Tar SR-SS-Tar
Tar conversion efficiency (%)
600
(b)
(a) 0
600
700
0
800 o
20
Temperature ( C) Fig. 6. Tar and syngas yield under catalytic steam reforming.
the upper stage were shown as the blank experimental value (the blue dotted line). Since the tar cracking reactions are generally endothermic, it can be observed in Fig. 4 (a) that the tar yield decreased obviously with the increasing temperature. The steel slag showed good catalytic activity on tar reforming, leading to high tar conversion efficiency in the temperature range of 600e800 C. Particularly, at a certain temperature, the tar yield decreased with the increase of the recycling time, and the tar conversion efficiency reached 90.8% at 800 C for the third time reuse of steel slag. The result may be due to the reduction of iron oxides to low-valent iron during the process of tar cracking, which improves the overall catalytic performance of steel slag. The high tar conversion efficiency further confirms that steel slag has the potential as a lowcost catalyst for catalytic conversion of biomass tar. Fig. 4 (b) shows the variation of syngas with temperature and steel slag recycling times. Due to the enhanced tar reforming reactions at higher temperatures, the syngas yield increased significantly with the increase of temperature. Also, with the increase of the recycling time of steel slag, the yield of syngas was significantly improved, especially at relatively low temperatures of 600 C and 700 C, indicating that the recycled steel slag has higher catalytic
300
SS-600 SS-700 SS-800 SR-SS-600 SR-SS-700 SR-SS-800
Gas yield (mL/g)
250 200 150 100 50
40
60
80
2 theta (deg.) Fig. 8. XRD patterns of the spent steel slag (a) after tar cracking process for 1 recycling time, (b) after tar cracking process for 3 recycling times, (c) after steam reforming process.
activity for tar cracking and gas reforming. In comparison with the tar and syngas yields from the blank experiment, it can be seen that the Fe2O3, Mg2SiO4 and MgFe2O4 formed by calcined steel slag exhibit obvious catalytic effect on biomass tar decomposition for syngas production. After recycling, part of Fe2O3 may be reduced to low-valent iron, which can further improve the catalytic activity of steel slag and promote the cracking of tar to syngas. Fig. 5 shows the yield of the four main gas components (H2, CO, CO2, and CH4) under different reaction conditions. It can be seen that temperature has a great influence on the formation of the four gas components, and all their yields increased significantly with the increase of temperature due to the enhanced tar cracking and gas reforming reactions. At a certain temperature, the yield of H2 increased significantly with the recycling time, while no significant increase was observed in the yield of CO. The result can be explained based on the possible reactions during biomass tar cracking. The tar reforming reactions as well as reactions between gases lead to more yield of H2 and CO, while the reduction reactions between them and the steel slag result in a certain amount of consumption as well. After recycling for two times, a large amount of iron oxides on the surface of steel slag can be reduced to lowvalent iron oxides or zero-valent iron, which can further act as active sites for tar reforming reactions and promote the formation of more H2 and CO. The yield of CH4 remained stable with the recycling times, as shown in Fig. 4 (c), indicating that the steel slag exhibits similar catalytic activity on the CH4 related reactions. The tar cracking can generate some CH4, while the dry reforming of CH4 and CO2 leads to the conversion of some CH4 into H2 and CO. The formation of CO2 relates to many reactions, including the tar cracking (Eq. (4)), tar dry reforming (Eq. (5)), water-gas shift (Eq. (8)) and iron oxides reduction reactions (Eq. (12) and Eq. (15)), leading to the irregular changing trend of CO2 yield. 3.3. Catalytic steam reforming of biomass tar
0
H2
CO
CH4
CO2
Fig. 7. The yield of gas components under catalytic steam reforming.
The steam reforming of biomass pyrolysis tar was studied to further evaluate the catalytic performance of steel slag. Fig. 6 shows the tar and syngas yields after steam reforming, which are denoted as SReSSex (x stands for the reforming temperature), and the values without steam (SS-x) are also presented for comparison. It
F. Guo et al. / Energy 189 (2019) 116161
was observed that steam exhibited a significant effect on the tar conversion as well as syngas yield. The tar conversion efficiency after steam reforming was higher than that without steam, and the value reached 94.1% at 800 C, indicating the excellent catalytic performance of steel slag on biomass tar steam reforming. The tar steam reforming reactions can be significantly promoted by the additional steam supply, leading to the formation of more permanent gases. Furthermore, the steam also participated in many other gas reforming reactions, such as water-shift (Eq. (8)) and watercarbon reactions (Eq. (9)), which resulted in the generation of gases as well. As a result, the gas yield after tar reforming process was significantly improved in comparison with the condition without steam, and the maximum gas yield reached 493.5 mL/g at 800 C. The yields of the four gas components under catalytic steam reforming conditions are shown in Fig. 7, and the values without steam are presented for comparison as well. It can be observed that the presence of steam can significantly improve the yield of H2 and CO. For H2, the enhanced tar steam reforming reactions by the additional steam can promote the generation of H2, and the watergas shift and water-carbon reactions between H2 and CO as well as carbon can also produce H2. As a result, the yield of H2 increased from 23.3, 38.3 and 48.6 mL/g to 39.6, 67.4 and 91.3 mL/g at 600, 700 and 800 C respectively due to the presence of steam. For CO,
7
the increase of the yield was mainly due to the enhanced tar steam reforming and water-carbon reactions. Also, it can be speculated that the reduction reactions between the iron oxides and CO and H2 may be weakened by the high concentration of steam in the system, also leading to a higher yield of CO and H2. It can be seen that the CH4 yield is significantly affected by temperature, while the supply of steam shows almost no influence on its yield. For CO2, the yield shows a little increase by the presence of steam, which may be due to the enhanced water-gas shift reaction.
3.4. Characterization of the spent steel slag The XRD patterns of the spent steel slag are illustrated in Fig. 8, showing that the chemical composition of the spent steel slag samples after tar cracking and steam reforming is almost the same. It can be observed that the diffraction peaks of MgFe2O4 and MgSiO4 remained unchanged in the spent steel slag samples, while the peaks corresponding to Fe2O3 disappeared. Meanwhile, it was found that diffraction peaks corresponding to Fe3O4 were enhanced, representing the reduction of Fe2O3 during the tar cracking/reforming process, while no FeO or zero-valent iron was observed. It has been reported by Uddin et al. [27] that the iron oxide catalysts can be transformed to Fe3O4 after use which possessed good catalytic activity on the cracking of biomass tar.
Fig. 9. SEM images of the spent steel slag (a, b) after tar cracking process for 1 recycling time, (c, d) after tar cracking process for 3 recycling times, (e, f) after steam reforming process.
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F. Guo et al. / Energy 189 (2019) 116161
Also, the iron oxides can be reduced into zero-valent iron which also has excellent catalytic activity on biomass tar decomposition, while it is unstable in air and easy to be oxidized again. As a whole, the main crystal structure of spent steel slag showed no significant change, leading to the high tar conversion efficiency after reuse several times. Fig. 9 shows the SEM images of the spent steel slag samples after the tar cracking/steam reforming experiments. After one-time tar cracking process, concave and convex grooves can be observed on the surface of the spent steel slag. Loose structure was formed and small particles with different colors appeared, indicating that complicated chemical reactions occur on the surface of steel slag during the tar cracking process. Reductive gases (H2, CO) from the biomass pyrolysis as well as tar cracking can react with the iron oxides on the surface of steel slag, leading to the variation in structure. This can be confirmed by the formation of Fe3O4 in the XRD results. The formation of pores on the surface of steel slag can in turn facilitate the contact between tar molecules and steel slag in the process of reforming reaction. Furthermore, the reduced lowvalent iron or even zero-valent iron on the surface of steel slag can further improve the catalytic activity of steel slag, which is the reason why the tar conversion efficiency increased with the recycling times of steel slag. After reuse for three times in tar catalytic cracking, the structure of the spent steel slag in Fig. 9 (c) and (d) does not show significant variation compared with the sample after used for one time, indicating that the surface structure of steel slag becomes relatively stable after tar catalytic cracking. The results are also coincident with the XRD results that the diffraction peaks of MgFe2O4 and MgSiO4 remain basically the same for the spent steel slag. The SEM images of steel slag after tar steam reforming in Fig. 9 (e) and (f) also showed stable loose structure, implying that the interaction between biomass tar and steel slag during the steam reforming was the same with catalytic cracking process. The stable porous structure of the spent steel slag contributes to the efficient tar removal in long-term biomass conversion applications.
4. Conclusion In this study, the steel slag was successfully used as a low-cost catalyst for tar cracking/steam reforming based on its high content of iron oxides. After simple calcination treatment, the steel slag can be converted into loose structure with many pores and clear crystal structure of Fe2O3 and MgFe2O4. The steel slag showed good catalytic activity on the decomposition of biomass-derived tar, and the reuse of the steel slag led to higher catalytic performance due to the reduction of iron oxides into low-valent iron. The steam reforming can further improve the tar conversion efficiency and syngas yield due to the enhanced tar steam reforming reactions. At 800 C, a tar conversion efficiency as high as 94.1% was achieved under steam reforming conditions, with a high syngas yield of 493.5 mL/g. After tar reforming, the spent steel slag still showed loose structure and the oxides were reduced into Fe3O4, which ensured the effective and long-term application of steel slag as a catalyst in biomass tar removal.
Acknowledgement This work was financially supported by the National Natural Science Foundation of China (No. 51876217).
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