Catalytic upgrading of coal pyrolysis tar over char-based catalysts

Fuel Processing Technology 122 (2014) 98–106

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Catalytic upgrading of coal pyrolysis tar over char-based catalysts Jiangze Han a,b, Xingdong Wang a, Junrong Yue a, Shiqiu Gao a,⁎, Guangwen Xu a,⁎ a b

State Key Laboratory of Multi-phase Complex Systems, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, China University of Chinese Academy of Sciences, Beijing 100049, China

a r t i c l e

i n f o

Article history: Received 24 June 2013 Received in revised form 24 January 2014 Accepted 29 January 2014 Available online 14 February 2014 Keywords: Coal pyrolysis Catalytic upgrading Tar quality Char-based catalyst

a b s t r a c t Catalytic upgrading of coal pyrolysis tar was investigated in a dual-stage reactor over char and metalimpregnated char (Co-char, Ni-char, Cu-char, Zn-char). The catalytic upgrading caused the lower total tar yield and the higher non-condensable gas yield but the fraction of light tar (boiling point b 360 °C) obviously increased to allow slightly higher total yield of light tar. When the catalytic upgrading was at 600 °C over a layer of char having a mass of 20% of the tested coal, the resulting light tar fraction in the tar increased by 25% in comparison with the coal pyrolysis only at 600 °C. Over the metal-impregnated char, which was 5% of the tested coal in mass, good upgrading effect was obtained at 500 °C. The catalytic tar-upgrading activity decreased in an order of Cochar N Ni-char N Cu-char N Zn-char, and over Ni-char the realized light tar yield and its content in the tar increased by 17.2% and 32.7%, respectively. The upgrading effect also lowered the contents of element N and S in the resulting tar by 45.6% and 43.5%, respectively. NH3-TPD clarified that the order in acidity of the char-based catalysts was the same as for the upgrading activity shown above. © 2014 Elsevier B.V. All rights reserved.

1. Introduction Pyrolysis provides mild conversion of coal volatiles into low-carbon fuels and chemicals to allow cascade utilization of coal resource. Coking is a kind of commercialized coal pyrolysis technology which produces a large amount of tar [1]. This coal tar, also including many from the other coal pyrolysis technologies, usually has high content of heavy components with boiling points above 360 °C (pitch in fact), for example, it may account for about 50–70 wt.% of the total tar mass [2–4]. The heavy components in tar are difficult to be treated and it may lead to operational troubles by blocking and fouling the downstream equipments such as engines and turbines [5–8]. On the other hand, the heavy components in tar can be cracked into desired valuable oil components to upgrade the tar so as to get more high-value light tar having boiling points below 360 °C (i.e. BP b 360 °C). However, tar is very refractory and its thermal cracking usually occurs at high temperature by requiring sufficient energy supply and reaction time, leading to the decrease of overall tar yield and energy efficiency [9,10]. Adopting catalysts is one of the promising techniques for upgrading tar at moderate conditions [11]. Literature studies also reported catalytic cracking of coal pyrolysis volatiles over catalysts. Chareonpanich et al. studied the pyrolysis of Millmerran coal and secondary catalytic cracking of its resulting pyrolysis products in a two-stage reactor [12–14]. The total yield of benzene, toluene and xylenes (BTX), reached 10 wt.% (daf) over a

⁎ Corresponding authors. Tel.: +86 10 82544885; fax: +86 10 82629912. E-mail addresses: [email protected] (S. Gao), [email protected] (G. Xu). 0378-3820/$ – see front matter © 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.fuproc.2014.01.033

Ni–Mo–Al2 O 3 catalyst and 14 wt.% (daf) over a USY-zeolite catalyst under 5.0 MPa hydrogen pressure. Sonoyama et al. investigated the catalytic cracking the coal tar obtained from pyrolyzing the Loy Yang coal in a steam atmosphere and a fixed bed at 773 K over an iron-oxide catalyst containing metal promoters [15]. The results showed that about 97 wt.% of the initial heavy components (BP N 350 °C) in tar were decomposed. Nursen et al. tested pyrolysis of Zonguldak bituminous coal mixed with metal chlorides (CoCl2, NiCl2, ZnCl2 and CuCl2) and clarified the better catalytic effects for ZnCl2, CoCl2 and NiCl2 [16]. Zou et al. impregnated several different metal chlorides on Huolinhe lignite and via pyrolysis they demonstrated that KCl, NiCl2 and CoCl2 promoted the conversion of organic matters into light components during pyrolysis [17]. Min et al. investigated the in-situ catalytic reforming effects of char and metal-impregnated char on tar derived from pyrolyzing mallee wood in a steam atmosphere [18]. The char-supported iron/nickel exhibited much higher activity for reforming tar than the char itself did. The preceding studies on upgrading tar are usually in hydrogencontaining atmosphere and pressurized conditions [19,20], while their results mainly focused on the variations of specific components such as BTX. The studies rarely considered the overall quality of the tar in terms of, for example, the distillation temperatures. Coal char is very cheap and easy to get, and it also has some good catalytic effects on tar cracking. There are many cavities on the char surface and some inherent metallic minerals in the char [21–23]. The cavities on the char surface can prolong the residence time of tar kept in the char to promote the interaction of tar with both the active sites on the char and the metals in the char structure. The inherent metal species are mainly alkaline and alkaline earth metals which have obvious catalysis on tar upgrading [24]. When the volatile product of pyrolysis passes through

J. Han et al. / Fuel Processing Technology 122 (2014) 98–106

a char layer, some of the heavy components in the tar can be cracked into light components. The transition metal chlorides, including NiCl2, CoCl2 and ZnCl2, have certain acidity, which is beneficial for cracking heavy components and hydrogenation of the cracked tar fragments [17]. Using char as support can make full use of its surface, while there is also a kind of synergetic effect between its containing metal minerals and metallic chlorides on the heavy tar cracking [16–19,25]. The objective of this study is to explore the upgrading effect by catalytic cracking for coal pyrolysis tar over char-based catalysts. Char and a few metal-impregnated chars (Co-char, Ni-char, Cu-char and Zn-char) were used as the catalysts to upgrade the primary volatile product of coal pyrolysis in a dual-stage fixed bed reactor. The distribution and composition of the final pyrolysis product were tested at laboratory with respect to different char-base catalysts for secondary upgrading. How and what the heavy components in the tar are converted into light components during upgrading are analyzed on the basis of the experimental results. 2. Experimental section 2.1. Coal and catalyst preparation The Fugu bituminous coal from Shanxi province, China, was used and it was crushed and sieved to the sizes of 0.4 to 1.0 mm for the experimental tests. Table 1 shows the results of proximate and ultimate analyses for the tested Fugu bituminous coal and char. The coal sample was dried at 110 °C for 2 h before experiments. The used char was prepared by pyrolyzing the above-mentioned raw coal in N2 at 800 °C for 3 h in a fixed bed reactor. Fig. 1 shows the TG/DTG curves of the resulting char measured by a temperature program that heated the sample in high-purity N2 (100 ml/min) up to 105 °C, kept it there for 10 min, further heated to 700 °C at a rate of 5 °C/min, and finally holding the sample at 700 °C for 30 min. In Fig. 1 the detected total weight loss was 2.2 wt.%, but the loss before 105 °C, which should be mainly the moisture removal, was about 1.2 wt.%. Thus, the char weight loss should contribute little to the volatile product of coal pyrolysis over the char (b700 °C). The metal-impregnated char was prepared by dispersing the char sample into a deionized aqueous solution containing 10 wt.% CoCl2, NiCl2, CuCl2 or ZnCl2. The resulting slurry was in turn vibrated at 30 °C by an ultrasonator for 12 h to enhance the dispersion of the metal composition. The resulting suspension liquid was finally dried at 110 °C for 4 h and calcined in N2 at 600 °C for 4 h to produce the metal-impregnated char catalyst. The metal chlorides in the catalyst was 10 wt.%. 2.2. Apparatus and test procedure As shown in Fig. 2, the experimental apparatus consisted of a gas supply secton, a dual-stage fixed bed reactor, an electric furnace, and the tar and pyrolysis gas collection parts. The reactor was made of quartz glass and it contained three parts, upper tube, lower tube and a cover. The upper tube used for coal pyrolysis was 30 mm in inner diameter and 400 mm in length, with a porous sintering quartz plate as the distributor at 330 mm from the reactor top. The lower tube was used for catalytic cracking of the primary pyrolysis products, which was 38 mm in inner

Table 1 Proximate and ultimate analyses of the tested Fugu bituminous coal and char. Proximate (wt.%, air dried)

Coal Char

Ultimate (wt.%, dry and ash-free)

Moisture

Ash

V.M.

F.C.

C

H

O

N

S

4.57 2.25

4.44 6.35

33.75 13.83

57.24 77.57

82.92 87.43

4.66 2.126

10.94 8.778

1.26 1.343

0.22 0.323

99

Fig. 1. Experimental TG and DTG diagrams for the tested char.

diameter and 850 mm in length and its porous sintering quartz distributor was at 570 mm from the reactor top. Two K-type thermocouples measured the pyrolysis and cracking temperatures inside the reactor. High-purity N2 (99.999%) controlled with a mass flow meter provided the basic reaction atmosphere of pyrolysis in the dual-stage reactor. Being the pyrolysis zone, 20 g of coal was loaded into the upper tube, and as the secondary cracking or upgrading zone, char-based catalyst was loaded into the lower tube. The coal pyrolysis product from the upper tube was catalytically reformed when it passed through the catalyst bed in the lower tube. The final pyrolysis product was cooled down in the pipe condensers and washed in three acetone bottles to collect tar. The non-condensable gas was dewatered in a silica gel column and collected in a bottle filling with saturated sodium bicarbonate solution. The gas volume was measured via the volume of replaced solution. All pipelines until the acetone bottle was washed using acetone, and the resulting liquid was mixed with the tar-dissolved acetone in the gaswashing bottles. The liquid mixture was first dehydrated with MgSO4, and then filtrated and vaporized at about 25 °C and 0.08 MPa in a rotary vacuum evaporator to extract tar [26]. A series of preliminary tests showed that the pyrolysis of 20 g of coal can fully finish in 40 min in the adopted dual-stage reactor. The methods testing the pyrolysis without and with secondary catalytic upgrading were as follows. When without the secondary upgrading, the temperatures in the upper and lower tubes of the dual-stage reactor were both controlled at 600 °C and a N2 flow of 100 ml·min−1 introduced into upper tube. The 20 g coal sample was loaded into the upper tube by opening instantaneously the top cover of the reactor at the desired temperature. When the secondary upgrading was included, a given amount of catalyst was put into the lower tube in advance of heating the reactor. The reactor heating was in the same N2 flow of 100 ml·min−1 but the upper pyrolysis and lower cracking stages might have been set for different temperatures. The 20 g coal sample was similarly loaded from the instantaneously opened top cover when the temperatures for pyrolysis and cracking both steadily reached their setting values.

2.3. Analysis and characterization The composition of non-condensable pyrolysis gas including H2, CH4, N2, CO, CO2, C2 and C3 hydrocarbons was analyzed by a gas chromatograph (Agilent 3000A). Tar (a mixture of aliphatics, aromatics and other organic chemicals) was analyzed via a simulated distillation GC (Agilent 7890A) and vario MACRO CHNS Elemental Analyzer. The fractions of coal tar were typified in terms of a few boiling point ranges, as listed in Table 2. Herein, the light tar was defined as the total fraction having boiling point below 360 °C, which is the sum of the fractions for light oil, phenol oil, naphthalene oil, wash oil and anthracene oil shown in Table 2. The heavy tar refers to the pitch with boiling points above 360 °C.

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5

3

1

2 7 7 8

4

N2

Micro GC

5

6

9

10

Fig. 2. A schematic diagram of the adopted experimental apparatus. (1) Mass flow controller; (2) electric furnace; (3) upper tube; (4) lower tube; (5) thermocouple; (6) condenser; (7) acetone trap; (8) dry silica gel bottle; (9) gas collecting bottle; and (10) measuring cylinder.

The catalysts were characterized using an X-ray diffraction (XRD, X'Pert MPD Pro, PANalytical) that has a copper radiation source worked at 40 kV and 30 mA and a scanning rate of 0.42°/s in 10°–90° for 2θ. The catalyst acidity was detected using the temperature-programmed desorption of ammonia (NH3-TPD) equipped with an automated chemisorption analyzer (chem-BET pulsar TPR/TPD, Quantachrome). The measurement was carried out in a quartz U-tube reactor loaded with a 50 mg catalyst sample. The catalyst was heated to 110 °C at a rate of 5 °C/min and held there for 60 min in a He stream to remove the catalyst-containing moisture. After cooling the pretreated catalyst sample to 90 °C, the He stream was switched to NH3 for 1 h. Then, the gas stream was switched to He again at the same temperature for 2 h for removing the physically absorbed NH3. At last the sample was heated to 700 °C at a rate of 5 °C/min in the He stream to desorb NH3. The emitted NH3 was measured on-line with a mass spectrometer (MS, Proline, AMETEK). The tar yield Ytar (wt.%), light tar fraction flight (wt.%), light tar yield Ylight (wt.%) and gas yield Yi (ml·g−1) were calculated with Eqs. (1) to (4) to evaluate the distribution of pyrolysis products. Y tar ¼

ð1Þ

W tar W

ð2Þ

Y light ¼ Y tar  f light

Yi ¼

3. Results and discussion Pyrolysis without secondary upgrading was first conducted by keeping the entire reactor at 550 °C to 700 °C to determine the temperature leading to the maximal tar yield. About twenty grams coal was put into the upper tube and Fig. 3 shows the results. The tar yield reached its highest value of about 9.1 wt.% (daf) at 600 °C (Fig. 3(a)) (the detailed tar compositions in terms of the boiling point range being presented later in Fig. 8). Rather higher temperatures facilitated the volatile release but led to more extensive secondary cracking reaction to monotonically decrease the tar yield and increase the yields of H2, CH4 and CO (Fig. 3(b)). The light tar yield and its fraction in the tar reached the highest value at 650 °C. The tests herein selected the temperature allowing the highest tar yield, which was 600 °C. 3.1. Tests with coal char

W light W tar

f light ¼

where W, Wtar and Wlight are respectively the mass of coal on dry–ashfree (daf) basis, tar and light tar in grams, and i in Yi refers to the gas components of H2, CH4, CO, CO2, and C2–C3 hydrocarbons, and Vi is the volume of non-condensable pyrolysis gas i (ml).

ð3Þ

Vi W

ð4Þ

Table 2 Typical boiling point range for characterizing the coal tar composition. Products

Boiling point range (°C)

Light oil Phenol oil Naphthalene oil Wash oil Anthracene oil Pitch

b170 170–210 210–230 230–300 300–360 N360

Pyrolysis with secondary cracking was conducted by loading coal and char into the upper and lower tubes of the dual-stage reactor, respectively. The pyrolysis temperature (Tpy) and cracking temperature (Tcr) were both kept at 600 °C, and corresponding to a 20 g coal sample and 0–10 g char (about 0–50 wt.% of tested coal) were used, respectively. The average residence time of the primary pyrolysis products from the upper tube in the char bed of the lower tube is 0–10 s. One can see from Fig. 4(a) that the tar yield continuously decreased with the increasing char catalyst amount, whereas the light tar had its highest fraction in the produced tar when the char-to-coal mass ratio was 30%. The light tar yield estimated as the product of the tar yield and light tar fraction thus varied little until the char-to-coal ratio of 30%. With rather more char catalyst, the pyrolysis products had longer interaction time in the char bed, which led to the deeper cracking of tar and caused the lower yields in both tar and light tar but the higher pyrolysis gas yield (Fig. 4(b)). In order to ensure the possibly highest light tar yield, the char amount in the coming tests was kept at 30 wt.% of the tested coal. Fig. 5 shows the effect of secondary cracking temperature on the product distribution for the char catalyst. The pyrolysis conditions in

J. Han et al. / Fuel Processing Technology 122 (2014) 98–106

Fig. 3. (a) Pyrolysis tar yield and composition and (b) pyrolysis gas composition varying with temperature for pyrolysis without secondary catalytic upgrading.

the upper tube were fixed, and 4 g of char (20 wt.% of the pyrolyzed coal) were adopted for the catalytic upgrading in the lower tube. The tar yield remained almost constant by increasing the cracking temperature from 400 °C to 600 °C. At the cracking temperature of 600 °C over char, the light tar fraction in the tar was relatively 25% higher than that from the pyrolysis without the secondary upgrading shown in Fig. 3(a). While, the light tar yield at 600 °C was almost the same in Figs. 3(a) and 5(a). Fig. 5(b) presents the yield variation of various pyrolysis gas components in terms of ml/g with changing the cracking temperature. Evident increase in the production of H2, CH4 and CO when the cracking temperature was over 500 °C, whereas the production yields for CO2 and C2–C3 hydrocarbons changed little at the tested cracking temperatures. Generally, more H2 is generated from polymerization, cyclization and aromatization of tar components at higher temperatures [27]. In this process the element O in the tar is transmitted into pyrolysis gas in the form of CO and CO2 [28]. As a result, the increasing percentage in the total gas yield reached 31.2% in comparison with the pyrolysis without the secondary upgrading in the lower stage of the reactor (Fig. 3(b)). 3.2. Tests with metal-impregnated char Although char manifested certain catalytic effect on upgrading the pyrolysis tar, its realized increases in the light tar yield and fraction were slight. A few modified char catalysts by loading metal chlorides on the char were thus prepared and tested. The test was first performed to understand how the catalyst activity and composition varied in upgrading reactions, for getting the catalyst with stable composition and free of Cl species. Taking Co-char as an example, Fig. 6 shows the pyrolysis product yields and pyrolysis tar composition varying in two times of upgrading tests at a temperature of 600 °C over a layer of catalyst having a mass of 20% of the tested coal. While the data show that the

101

Fig. 4. (a) Pyrolysis tar yield and composition and (b) pyrolysis gas composition varying with the amount of char catalyst (Tpy = 600 °C, Tcr = 600 °C).

catalyst activity was stable in the tested two times for Co-char, our tests for the other catalysts including Ni-char, Cu-char and Zn-char catalysts generated the similar results. Characterizing the spent catalysts using XRD (see data in Fig. 11) further revealed that the fresh Ni-char and Co-char had peaks for chlorides, no chlorine specie was identified for the spent Ni-char and Co-char after only one upgrading test. Consequently, all the cracking tests reported herein adopted the spent metal-impregnated char catalysts after their first upgrading tests. Fig. 7 shows the pyrolysis product distribution and pyrolysis gas composition varying with the cracking temperature over the Co-char catalyst (20 wt.% of the tested coal) in the lower tube of the reactor. The tar yield decreased with increasing the cracking temperature, but the light tar fraction showed a variation trend opposite to that for the total tar yield. The light tar fraction reached near 65 wt.% at 600 °C, about 7 percentage points higher than that at 400 °C in the same figure and more than 10 percentage points higher than that in Fig. 3. The highest light tar yield was obtained at a cracking temperature of 500 °C in Fig. 7(a), while the pyrolysis gas yield increased slightly in 450–600 °C especially for H2 (Fig. 7(b)). The similar change tendency was obtained for the other catalysts (Ni-char, Cu-char, Zn-char). Comparing Fig. 5 and Fig. 7 indicates that the dependence of catalytic activity on temperature is more obvious for Co-char in Fig. 7 than for char in Fig. 5. Thus, the test below for investigating the effect of catalyst amount was conducted at the cracking temperature of 500 °C. Fig. 8 shows the effects of different amounts of Co-char, Ni-char, Zn-char and Cu-char catalysts for the secondary upgrading on the pyrolysis tar yield and composition. The tar yield decreased with the increasing catalyst amount and the lowest tar yield was shown for the Co-char catalyst (Fig. 8(a)). This means that the Co-char catalyst had the strongest catalytic activity for cracking tar. The light tar fraction reached its maximal value over the tested metal-char

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Fig. 5. (a) Pyrolysis tar yield and composition and (b) pyrolysis gas composition at different cracking temperature over char catalyst (Tpy = 600 °C, 20% catalyst).

Fig. 7. (a) Pyrolysis tar yield and composition and (b) pyrolysis gas composition at different cracking temperatures over Co-char catalyst (Tpy = 600 °C, 20% catalyst).

catalysts of Co-char, Ni-char and Cu-char (Fig. 8(b)) at the catalyst amount of 5 wt.% of the pyrolyzed coal (in the upper stage). With regard to Zn-char in Fig. 8(b), the light tar fraction in the tar had an obvious increase in raising the catalyst amount from 0 to 5 wt.% of the pyrolyzed coal, and then the variation with catalyst amount was slight. Possibly because the Zn-char had the lowest catalytic activity for tar cracking, the light tar fraction in Fig. 8(b) still slightly increased with the increasing catalyst amount. In fact, the catalytic activity of such metal-char catalysts for tar cracking is subject to an order of Co-char N Ni-char N Cu-char N Zn-char (Fig. 8(a)), if based on their realized tar cracking capability. Considering the light tar yield, the Ni-char catalyst manifested the best performance for catalytic upgrading the primary pyrolysis tar into a rather highquality one. In Fig. 8(c), the highest light tar yield is truly shown for the Ni-char catalyst at 5 wt.% of the pyrolyzed coal (pyrolysis and

cracking temperatures being 600 °C and 500 °C, respectively). In comparison with the pyrolysis at 600 °C without catalyzed secondary upgrading, the catalytic upgrading over the Ni-char catalyst increased (relatively) the light tar yield and light tar fraction by 17.2% and 32.7%, respectively. Consequently, impregnating metal chlorides on char facilitated the catalytic activity of char for tar upgrading, which not only decreased the cracking temperature but also lowered the catalyst amount to some extent.

Fig. 6. Pyrolysis product yield and light tar fraction varying in two times of upgrading tests over the Co-char catalyst (Tpy = 600 °C, Tcr = 600 °C, 20% catalyst).

3.3. Tar characterization Fig. 9 summarizes the composition of tar (Fig. 9(a)) and the fraction and yield of light tar (Fig. 9(b)) realized under the conditions allowing the highest tar yield for pyrolysis without secondary upgrading (Blank in the figure) and enabling the highest light tar yield for the cause with secondary catalytic upgrading over various char-based catalysts. For all the compared data, the coal pyrolysis in the upper stage was at 600 °C. The used char catalyst was 20 wt.% of the pyrolyzed coal and the cracking temperature was 600 °C, while for all metal-impregnated chars, the adopted catalyst was 5 wt.% of the pyrolyzed coal and the cracking temperature was 500 °C. In comparison with the pyrolysis without secondary upgrading, adopting the secondary upgrading all evidently decreased the content of pitch and increased the fractions of all light components including light oil, phenol oil and naphthalene oil. The changes in the factions of washing oil and anthracene oil were limited. Consequently, the fraction and yield of light tar plotted in Fig. 9(b), representing the sum of all oil fragments in Fig. 9(a) except for pitch, were higher than the pyrolysis without secondary upgrading for all the tested char-based catalysts. As one can see, the light tar fraction increased from about 52 wt.% to about 69 wt.% for the secondary upgrading over the Ni-char catalyst.

J. Han et al. / Fuel Processing Technology 122 (2014) 98–106

103

Fig. 9. (a) Tar composition and (b) light tar fraction and yield under various conditions allowing the highest tar production for pyrolysis without secondary upgrading and the highest light tar yield in the case with secondary catalytic upgrading (pyrolysis at 600 °C, char catalyst: 20 wt.% of the tested coal at 600 °C, metal-impregnated char catalyst: 5 wt.% of the tested coal at 500 °C).

slightly the H/C ratio and decreased the S and N contents of the tar. For the most efficient Ni-char catalyst (against the case without catalyst), the H content and the H/C molar ratio in its produced tar increased (relatively) by 33.4% and 36.3%, while the contents of N and S elements decreased (relatively) by 45.6% and 43.5%. The Zn-char catalyst showed the strongest ability to remove the heteroatom O, N and S from tar. Overall, the metal-impregnated char is thus more efficient in upgrading the tar product by both decreasing its pitch content and removing its pollutant elements. 3.4. Catalyst characterization Fig. 8. (a) Tar yield, (b) light tar fraction and (c) light tar yield varying with the amount of metal-impregnated char catalysts (Tpy = 600 °C, Tcr = 500 °C).

Among all the catalysts, Zn-char led to the lowest light fraction, but the fraction was also about 61 wt.%. These results showed that the secondary upgrading greatly improved the tar quality so that the downstream treatment could become easier and more efficient. Considering the light tar yield shown in Fig. 9(b), only the char catalysts with impregnated metal species resulted in certain increase in comparison with that of the pyrolysis without secondary upgrading. Although the secondary upgrading obviously increased the light fraction, as depicted above, it decreased the total tar yield so that the light tar yield thus varied little. For example, the char catalyst increased the light tar fraction from 52 wt.% to 65 wt.% but the tar yield decreased 20.7% (relative percentage). From both the light tar fraction and light tar yield shown in Fig. 9(b), one can see that the Ni-char catalyst results in the best upgrading effects, and next to this is the Co-char catalyst. Table 3 lists the data of ultimate analysis of all the tars corresponding to Fig. 9. For all the tested catalysts the secondary upgrading increased

The NH3-TPD spectra were measured for all the catalysts to clarify the relationship between their acidities and activities for tar upgrading. Fig. 10 compares the acquired spectra for all spent char-based catalysts after the first catalytic cracking test. No obvious acidic site was detected on char, while there were obvious acidities for all the char-supported metallic catalysts. Two different peaks appeared for the Co-char and Ni-char catalysts to indicate the presence of both weak Lewis acidic sites and strong Bronsted acidic sites, respectively. Ma et al. [29] reported that two NH3 desorption peaks at about 207 °C and 297 °C were Table 3 Ultimate analysis of tars corresponding to the plot in Fig. 9 (ad, wt.%). Catalyst

C

H

O⁎

N

S

H/C(mol/mol)

Without Char Co-char Ni-char Cu-char Zn-char

81.9 79.65 82.33 80.18 81.56 82.83

4.383 4.884 5.204 5.849 5.062 4.788

11.562 13.687 11.245 12.785 12.257 11.327

1.528 1.207 0.827 0.832 0.77 0.721

0.627 0.572 0.394 0.354 0.351 0.334

0.642 0.736 0.759 0.875 0.745 0.694

⁎ By difference.

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J. Han et al. / Fuel Processing Technology 122 (2014) 98–106

Fig. 10. NH3-TPD spectra for all spent char-based catalysts after the first catalytic cracking test.

identified for three zirconia polymorphs, which corresponded to weak acid sites and moderate acid sites, respectively. Thus, the intensity of acidity was lower for the Ni-char catalyst because its NH3 release was earlier or had a lower temperature. The Lewis acidic sites were present at about 200 °C on the catalysts of Co-char, Zn-char and Cu-char, while they appeared at about 150 °C on the Ni-char catalyst. These indicated that the acidity of the Ni-char catalyst was weaker than that of the other three catalysts. However, the number of the Lewis acidic sites, estimated from the integrated area of each spectrum [30], was highest for the Co-char catalyst, moderate for the Ni-char catalyst and obviously lower for the Znchar and Cu-char catalysts. The Lewis acidic sites are beneficial to the cracking of organic macromolecule [31], making the secondary upgrading over the Co-char and Ni-char catalysts have the higher light tar yield and higher light tar fraction in their produced tar (Fig. 9). Nonetheless, if the acidity of the catalyst is too strong, excessive tar cracking would occur to increase the gas production and also form some coke. These would surely decrease the desired light tar fraction and yield. According to Fig. 10, the catalytic acidity for upgrading decreases in the order of Co-char N Ni-char N Cu-char N Zn-char N char, which is in accordance with the tar yield variation shown in Fig. 8(a). The Co-char had a slightly lower light tar yield than the Ni-char catalyst, indicating that the Co-char may be too active (too strong acidity) for tar upgrading.

Correspondingly, the upgrading activity appeared too weak for Cu-char and Zn-char catalysts. Fig. 11 presents the XRD patterns of all the tested catalysts including both fresh catalysts and spent ones after the first catalytic cracking test. For the tested char, graphite (G) was obviously identified and the detected quartz should be attributed to coal ash. For some metalimpregnated catalysts, their quartz peak at 2θ of about 20° was not obvious (e.g., Cu-char), which should be due to the strong peak (intensity) of the metal-related species. Of all the metal-impregnated catalysts, metal chlorides were presented in the fresh Co-char and Ni-chat but not obvious in Cu-char and Zn-char. Element metal was presented in the fresh catalyst for all the tested ones except for Zn-char. Comparing the XRD diagrams of the spent catalysts, the graphite peak was reduced in the char catalyst after experiencing tar upgrading reactions. The catalytic upgrading reaction made Co and Ni chlorides disappear from the catalysts so that only element metal was identified for the catalysts except for Zn-char. On both fresh and spent Zn-char, only ZnO was identified. The above different features for catalysts impregnated with different metal chlorides reflect essentially their different interactions with char and water in the process of metal impregnation and with various gas components in upgrading. For example, the chlorides of Co, Ni and Cu should be reduced by char to cause these element metals to present on the catalysts, while in the upgrading process most gas components including tar further reduce the remaining CoCl2 and NiCl2 on the char to only Co and Ni. And the released Cl should become HCl in the preceding reduction reaction [32–35]. The copper chloride (CuCl2) is possibly easier to be reduced by char so that the fresh Cu-char does not have any chloride on the char support. For ZnCl2, it is easy to react with water during aqueous impregnation to form ZnO that is difficult to be reduced by char as well as pyrolysis gas products [36], making only ZnO present on Zn-char. Therefore, the Cl in the catalysts was completely removed after the reduction by pyrolysis products. The different chemical status of metal species on the tested charbased catalysts is responsible for their different catalytic performances for upgrading the pyrolysis tar. For example, the metallic particles generated from releasing Cl could become the catalytic site to promote the tar upgrading reactions [37]. As for the Zn-char catalyst only ZnO presented on its surface, which was difficult to be reduced to form additional catalytic sites. Its catalytic activity for upgrading coal pyrolysis tar was thus lower in comparison with the other catalysts. More studies are

Fig. 11. XRD spectra of different char-based fresh and spent catalysts tested in this article. (a) Fresh catalysts, and (b) spent catalysts after the first catalytic cracking test.

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surely needed for justifying why such metal composition has the different chemical status shown in Fig. 11 and thus the different performances of the tested catalysts in upgrading the coal pyrolysis tar. 4. Conclusions Catalytic upgrading of coal pyrolysis tar was investigated in a laboratory dual-stage reactor over char and metal-impregnated char catalysts. The tar quality was greatly improved by catalytic cracking of the primary volatile product of coal pyrolysis. (1) The coal pyrolysis with a secondary catalytic upgrading over the char-based catalysts resulted in lower total tar yield and higher non-condensable gas yield, but it obviously increased the fraction of light tar, the sum of all tar components except for pitch, in the produced tar. The tar produced from the pyrolysis integrated with a secondary upgrading over the Ni-char catalyst realized a light tar fraction of 69 wt.% (relative increase: 32.7%), which means a high-quality tar product that can be efficiently refined downstream. Carefully examining the tar composition found that the secondary upgrading mainly increased the fraction of light oil, phenol oil and naphthalene oil, while the fractions of wash oil and anthracene oil varied little. It was also shown that the secondary upgrading over the char-based catalysts obviously decreased the contents of elements N and S and increased the H/ C ratio of the produced tar. The light tar yield manifested a slight increase over some upgrading catalysts, such as Ni-Char and Cochar, but the extent of the increase was not large. Consequently, the secondary upgrading contributes mainly to the improvement on the tar quality, while its induced increase in the light tar yield is limited. (2) The necessarily required char for secondary upgrading was about 20 wt.% of the tested coal, while this ratio can be reduced to 5 wt.% for the metal-impregnated char catalysts. In terms of cracking activity all the tested catalysts are subject to an order Co-char N Ni-char N Cu-char N Zn-char N Char. From the realized upgrading effects, the Ni-char catalyst showed the best performance to indicate that the Co-char may have too strong cracking activity for tar upgrading. The preceding order of cracking activity was justified by the catalyst acidity determined by NH3-TPD analysis, while no acidity was identified for pure char, the metal-impregnated chars all had certain acidity characterized by the existence of both weak Lewis acidic sites and strong Bronsted acidic sites. Characterization via XRD revealed that CuCl2 and ZnCl2 were not present in the fresh char-supported catalyst. For the metal-impregnated catalysts except for Znchar, element metals Ni, Co and Cu were presented respectively on the spent Ni-char, Co-char and Cu-char, while Zinc existed as ZnO on the Zn-char catalyst, even after reduction by pyrolysis product. Acknowledgments The authors are grateful to the financial support from the National Basic Research Program of China (2011CB201304), the International Science & Technology Cooperation Program of China (2013DFG60060), the National Natural Science Foundation of China (21376250) and the Strategic Priority Research Program of Chinese Academy of Sciences (XDA07010100). References [1] C.S. Li, K. Suzuki, Resources, properties and utilization of tar, Resources, Conservation and Recycling 54 (2010) 905–915. [2] J.G. Wang, X.S. Lu, J.Z. Yao, W.G. Lin, L.J. Cui, Experimental study of coal topping process in a downer reactor, Industrial & Engineering Chemistry Research 44 (2005) 463–470.

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[3] P. Liang, Z.F. Wang, J.C. Bi, Process characteristics investigation of simulated circulating fluidized bed combustion combined with coal pyrolysis, Fuel Processing Technology 88 (2007) 23–28. [4] X. Qu, P. Liang, Z.F. Wang, R. Zhang, D.K. Sun, X.K. Gong, Z.X. Gan, J.C. Bi, Pilot development of a polygeneration process of circulating fluidized bed combustion combined with coal pyrolysis, Chemical Engineering & Technology 34 (2011) 61–68. [5] D. Velegol, M. Gautam, A. Shamsi, Catalytic cracking of a coal tar in a fluid bed reactor, Powder Technology 93 (1997) 93–100. [6] L. Devi, K.J. Ptasinski, F.J.J.G. Janssen, A review of the primary measures for tar elimination in biomass gasification processes, Biomass and Bioenergy 24 (2003) 125–140. [7] S.J. Juutilainen, P.A. Simell, A.O.I. Krause, Zirconia: selective oxidation catalyst for removal of tar and ammonia from biomass gasification gas, Applied Catalysis B: Environmental 62 (2006) 86–92. [8] T. Kimura, T. Miyazawa, J. Nishikawa, S. Kado, K. Okumura, T. Miyao, S. Naito, K. Kunimori, K. Tomishige, Development of Ni catalysts for tar removal by steam gasification of biomass, Applied Catalysis B: Environmental 68 (2006) 160–170. [9] J. Han, H. Kim, The reduction and control technology of tar during biomass gasification/pyrolysis: an overview, Renewable and Sustainable Energy Reviews 12 (2008) 397–416. [10] A.V. Bridgwater, The technical and economic feasibility of biomass gasification for power generation, Fuel 74 (1995) 631–653. [11] L. Li, K. Morishita, H. Mogi, K. Yamasaki, T. Takarada, Low-temperature gasification of a woody biomass under a nickel-loaded brown coal char, Fuel Processing Technology 91 (2010) 889–894. [12] M. Chareonpanich, T. Takeda, H. Yamashita, A. Tomita, Catalytic hydrocracking reaction of nascent coal volatile matter under high pressure, Fuel 73 (1994) 666–670. [13] M. Chareonpanich, Z.G. Zhang, A. Nishijima, A. Tomita, Effect of catalysts on yields of monocyclic aromatic hydrocarbons in hydrocracking of coal volatile matter, Fuel 74 (1995) 1636–1640. [14] M. Chareonpanich, T. Boonfueng, J. Limtrakul, Production of aromatic hydrocarbons from Mae-Moh lignite, Fuel Processing Technology 79 (2002) 171–179. [15] N. Sonoyama, K. Nobuta, T. Kimura, S. Hosokai, J. Hayashi, T. Tago, T. Masuda, Production of chemicals by cracking pyrolytic tar from Loy Yang coal over iron oxide catalysts in a steam atmosphere, Fuel Processing Technology 92 (2011) 771–775. [16] N.A. Öztaş, Y. Yürüm, Effect of catalysts on the pyrolysis of Turkish Zonguldak bituminous coal, Energy & Fuels 14 (2000) 820–827. [17] X.W. Zou, J.Z. Yao, X.M. Yang, W.L. Song, W.G. Lin, Catalytic effects of metal chlorides on the pyrolysis of lignite, Energy & Fuels 21 (2007) 619–624. [18] Z.H. Min, P. Yimsiri, M. Asadullah, S. Zhang, C.Z. Li, Catalytic reforming of tar during gasification. Part II. Char as a catalyst or as a catalyst support for tar reforming, Fuel 90 (2011) 2545–2552. [19] R. Jolly, H. Charcosset, J.P. Boudou, J.M. Guet, Catalytic effect of ZnCl2 during coal pyrolysis, Fuel Processing Technology 20 (1988) 51–60. [20] W. Li, N. Wang, B.Q. Li, Process analysis of catalytic multi-stage hydropyrolysis of lignite, Fuel 81 (2002) 1491–1497. [21] P. Brandt, E. Larsen, U. Henriksen, High tar reduction in a two-stage gasifier, Energy & Fuels 14 (2000) 816–819. [22] L.X. Zhang, T. Matsuhara, S. Kudo, J. Hayashi, K. Norinaga, Rapid pyrolysis of brown coal in a drop-tube reactor with co-feeding of char as a promoter of in situ tar reforming, Fuel (2012), http://dx.doi.org/10.1016/j.fuel.2011.12.030. [23] S. Hosokai, N.K. Koyo, T. Kimura, M. Nakano, C.Z. Li, J.J. Hayashi, Reforming of volatiles from the biomass pyrolysis over charcoal in a sequence of coke deposition and steam gasification of coke, Energy & Fuels 25 (2011) 5387–5393. [24] X. Zeng, Y. Wang, J. Yu, S.S. Wu, J.Z. Han, S.P. Xu, G.W. Xu, Gas upgrading in a downdraft fixed-bed reactor downstream of a fluidized-bed coal pyrolyzer, Energy & Fuels 25 (2011) 5242–5249. [25] M. Blazsó, B. Zelei, Thermal decomposition of low-density polyethylene in the presence of iron and copper chlorides, Journal of Analytical and Applied Pyrolysis 36 (1996) 149–158. [26] R. Xiong, L. Dong, J. Yu, X.F. Zhang, L. Jin, G.W. Xu, Fundamentals of coal topping gasification: characterization of pyrolysis topping in a fluidized bed reactor, Fuel Processing Technology 91 (2010) 810–817. [27] L.P. Wiktorsson, W. Wanzl, Kinetic parameters for coal pyrolysis at low and high heating rates—a comparison of data from different laboratory equipment, Fuel 79 (2000) 701–716. [28] X. Zeng, Y. Wang, J. Yu, S.S. Wu, S.P. Xu, G.W. Xu, Coal pyrolysis in a fluidized bed for adapting to a two-stage gasification process, Energy & Fuels 25 (2011) 1092–1098. [29] Z.Y. Ma, C. Yang, W. Wei, W.H. Li, Y.H. Sun, Surface properties and CO adsorption on zirconia polymorphs, Journal of Molecular Catalysis A: Chemical 227 (2005) 119–124. [30] T. Viinikainen, H. Rönkkönen, H. Bradshaw, H. Stephenson, S. Airaksinen, M. Reinikainen, P. Simell, O. Krausea, Acidic and basic surface sites of zirconia-based biomass gasification gas clean-up catalysts, Applied Catalysis A: General 362 (2009) 169–177. [31] N.D. Taylor, A.T. Bell, Effects of Lewis acid catalysts on the cleavage of aliphatic and aryl–aryl linkages in coal-related structures, Fuel 59 (1980) 499–506. [32] D.M. Quyn, H.W. Wu, J. Hayashi, C.Z. Li, Volatilisation and catalytic effects of alkali and alkaline earth metallic species during the pyrolysis and gasification of Victorian brown coal. Part IV. Catalytic effects of NaCl and ion-exchangeable Na in coal on char reactivity, Fuel 82 (2003) 587–593. [33] L. Wang, D.L. Li, M. Koike, H. Watanabe, Y. Xu, Y. Nakagawa, K. Tomishige, Catalytic performance and characterization of Ni–Co catalysts for the steam reforming of biomass tar to synthesis gas, Fuel (2012), http://dx.doi.org/10.1016/j.fuel.2012.01.073. [34] T. Miyazawa, T. Kimura, J. Nishikawa, S. Kado, K. Kunimori, K. Tomishige, Catalytic performance of supported Ni catalysts in partial oxidation and steam reforming of

106

J. Han et al. / Fuel Processing Technology 122 (2014) 98–106

tar derived from the pyrolysis of wood biomass, Catalysis Today 115 (2006) 254–262. [35] D.M. Quyn, H. Wu, C.Z. Li, Volatilisation and catalytic effects of alkali and alkaline earth metallic species during the pyrolysis and gasification of Victorian brown coal. Part I. Volatilisation of Na and Cl from a set of NaCl-loaded samples, Fuel 81 (2002) 143–149.

[36] H.C. Hsu, C.I. Lin, H.K. Chen, Zinc recovery from spent ZnO catalyst by carbon in the presence of calcium carbonate, Metallurgical and Materials Transactions B 35 (2004) 55–63. [37] G. Guan, G. Chen, Y. Kasai, E.W.C. Lim, X. Hao, M. Kaewpanha, A. Abuliti, C. Fushimi, A. Tsutsumi, Catalytic steam reforming of biomass tar over iron- or nickel-based catalyst supported on calcined scallop shell, Applied Catalysis B: Environmental 115–116 (2012) 159–168.