Catalytic Tar Reforming during Brown Coal Pyrolysis: Effects of

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Catalytic Tar Reforming during Brown Coal Pyrolysis: Effects of Heating Rate and Activation Time on Char Catalysts Yonggang Wang, Zongding Chen, Xiuqiang Xu, Shu Zhang, Deping Xu, and Yuming Zhang Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b02530 • Publication Date (Web): 28 Nov 2017 Downloaded from http://pubs.acs.org on December 4, 2017

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Catalytic Tar Reforming during Brown Coal Pyrolysis: Effects of Heating Rate and Activation Time on Char Catalysts

Yonggang Wanga, Zongding Chena, Xiuqiang Xua, Shu Zhanga,b,*, Deping Xua, Yuming Zhangc

a)

School of Chemical and Environmental Engineering, China University of Mining and Technology (Beijing); Beijing, 100083, China;

b)

College of Materials Science and Engineering, Nanjing Forestry University, Nanjing 210037, Jiangsu, PR China;

c)

State Key Laboratory of Heavy Oil Processing, China University of Petroleum-Beijing, Beijing 102249, China

1

ACS Paragon Plus Environment

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ABSRACT: Chars from low rank fuels (i.e. brown coal and biomass) have attracted increasing attention as a promising candidate for catalytically reforming tarry material into light hydrocarbons and gases. The effects of heating rate and activation time on the preparation of char catalysts were examined. The catalytic activity of the char catalysts was tested for tar reforming during the pyrolysis of brown coal at different temperatures. The experiments of char preparation and tar reforming were conducted in bench-scale quartz reactor. Results indicate that the char derived from fast-heating rate showed a much better activity on tar reforming than the one from slow-heating rate. The char with 3-5 min activation time gave the best catalytic performance, achieving a very low tar yield of 1.18 wt%. The increase in the surface area of char catalysts due to the char-steam reaction did not always increase the catalytic activity of char catalysts. Raman spectra of chars revealed that the char properties (i.e. O-containing functional groups, and sizes of aromatic fused rings) played very important roles in determining the activity of char catalysts on tar reforming. KEYWORDS: Brown coal; Char; Catalyst; Tar reforming; Pyrolysis

2


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1. INTRODUCTION Low-rank coal gasification is one of the most promising techniques to produce syngas in a large scale

1,2

. However, the mixture of condensable aromatic compounds-tar, as the

unwanted byproduct of coal gasification, often leads to a series of problems. It blocks or corrodes the pipes, poisons catalyst and even poses threat to environment 3,4. Therefore, it’s a necessity to eliminate the tar from the gasification product before any potential application. Some physical methods (such as wet scrubbing and electrostatic precipitation) were popularly used for the removal of tar

5,6

. The water scrubber would inevitably generate

contaminated water, while the electrostatic precipitation was highly expensive. The thermal cracking was an effective method to crack tar molecules into lighter ones

7,8

, but requiring

elevated temperatures (above 1200 ℃). By contrast, catalytic tar reforming could not only reduce tar contents, but also convert tar into light gases at moderate temperatures, which is believed to be a very promising approach for tar elimination. A variety of tar reforming catalysts have been reported in literature, such as natural mineral (e.g. olivine or dolomite), alkali and alkaline earth metallic species (AAEM), nickel supported catalysts and so on

9-11

. The natural minerals as tar reforming catalysts are of low

lost, high availability and easy disposal, while they also have obvious disadvantages, such as showing lower activity than man-made ones and featuring low mechanic strength

12

.

Generally, the catalytic activity of olivine was slightly higher than that of dolomite, likely due to the higher contents of Fe and Ni in olivine. The attrition resistance of dolomite was however higher than that of olivine13. AAEM species have also been widely examined to be used for reforming tar derived from gasification and pyrolysis of biomass and coal, of which, alkali metals (Na and K) often showed higher tar reforming activity than alkaline metals (Mg and Ca) 12. Although it has been demonstrated that the alkali metals could effectively reform the tarry materials into light gaseous products, the low evaporation temperature and difficult recovery have restricted its potential applications. For example, Feng et al10. have recently compared the catalytic performance of K-char and Ca-char on tar reforming, concluding that the K-char catalysts exhibited much higher activity than Ca-char catalysts did, while the higher evaporation rate of K was also clearly observed due to its monovalent bond with char matrix. Ni-based catalysts with high tar reforming activity have attracted great attentions in academic society. Cao et al14. Reported that the nickel loaded on resin char could reduce the tar content in the product gas to as low as 84.5 mmol/g at 650 oC. In some case15, the activity 3


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of Ni-based catalysts could be very comparable to noble metals such as Pt. The most unbearable disadvantage of Nickel catalysts is its propensity to be deactivated by coke formation. Comparatively, char from pyrolysis or gasification of brown coal/biomass is more affordable and has nearly zero waste disposals as it can be simply gasified or burned after being used

16-18

.

It is reported that significant reductions of primary tar vapor could be

achieved by using different char catalysts as long as the residence time was no less than 3 s 19. Feng et al 20. also investigated the different tar reforming activity using different char species, showing that the sawdust char catalysts yielded a higher tar conversion than the cornstalk/rice husk chars. Our recent study has demonstrated that the char-supported catalysts in an integrated fixed-bed reactor could reduce the tar content to lower than 100 mg/m3 in a pilot scale gasifier

17, 21

. In addition, due to their neutral or weak base properties, char-supported

catalysts show high resistances to deactivation by depositions of coke and/or heavy metals, which were superior to other solid acid catalysts 22. As a catalyst, char consisted of carbon matrix as catalyst support and inherent metal species (e.g. Na, K, Mg, Ca, Fe and so on) as reactive species. The importance of char structure was reflected by its direct influence on reforming activity as well as its dominant role in affecting the interactions between metal species (reactive sites) and carbons. Yao et al.23 found out that the nascent char with abundant O-containing functional groups are preferred for enhancing tar destruction during the volatile-char interactions. The concurrence of reactive sites, such as unpaired electrons and highly-active polorized functional groups, in the nascent char would be the main reason for its high activity in tar reforming reactions. Therefore, it is of great significance to have a deep understanding on the effects of char structure on its activity for reforming tarry matters. Char structure was largely influenced by the preparation conditions, such as heating rate and char-steam reaction.

Soltani et al.

24

pointed out that heating rate could determine the surface area as well as pore structure, thus affecting the stability and strength of char. Tay et al.

25

claimed that the time of interaction

between char and steam could change the carbon skeleton structure and thus influence char reactivity. The change in carbon structure could be induced by the selective gasification of highly active matter and/or the transformation from small aromatic rings to large aromatic rings.

In the previous studies

26-28

, attentions were mainly paid to the effects of char

structure on its reactivity with gasifying agents, however little was known about its influence 4


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on char activity when reacting with volatile (tarry materials). In this paper, in-situ catalytic reforming of tar from pyrolysis of brown coal was carried out using chars as catalysts, and the char catalysts were prepared from the pyrolysis at both fast and slow heating rates. The components of tar before and after being reformed were analysed by gas chromatography-mass spectrometry (GC-MS). The char structure was characterized by BET (Brunauer-Emmertt-Teller) method, SEM (scanning electronic microscopy) and Raman spectroscopy in an attempt to explore the relevance between char activity and its structure. 2. EXPERIMENTAL SECTION 2.1. Sample preparation A Chinese Shengli brown coal sample

29

was chosen in this study. It was pulverized,

sieved to a range of 96-150 µm, and then dried at 70 ℃ in a vacuum oven for 24 h before being used. The proximate and ultimate analyses were shown in Table 1. A one-stage quartz reactor which was similar to the reactor in Figure 1 but without top two frits was used for the preparation of char catalysts. The chars from slow pyrolysis were prepared at a heating rate of 15℃/min to 900 ℃ with 30 min holding time. The coal powders were entrained into the 900 ℃ hot zone directly via the top tube within 1 min to prepare char catalysts at fast heating rate, also holding for 30 min. The mass of raw coal samples used for each experiment was 5±0.1 g and the coal bed was about 6 ±0.1 mm in height. For preparing the activated char, steam (15 vol.% steam in argon with the total flow rate of 1.8 L/min) was supplied into the reactor to activate the char using a peristaltic pump for 10 min at 900 ℃ right after the 30 min holding in argon. The four chars were named as the SI-char, SA-char, FI-char and FA-char, representing for the slow-heating inactivated char, slow-heating activated char, fast-heating inactivated char and fast-heating activated char respectively. Chars with different activation time (char-steam reaction time) were prepared at the fast-heating rate, and the activation time was 0 min, 1 min, 3 min, 5 min and 10 min respectively. Hereafter, the term of “catalyst” in this study mainly refers to “chars” prepared here. 2.2. Catalytic reforming of tar The experimental set-up for catalytic reforming of the tar is shown in Fig.2. About 1 g 5


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catalyst (char prepared in Section 2.1) and 1 g coal sample were pre-loaded on the middle and bottom frits inside the quartz reactor as shown in Figure 1, respectively. The quartz reactor features a reaction zone of 40 mm (diameter) x 130 mm (height). The volatile produced from brown coal on the bottom frit were forced to travel through the “char catalyst layer” on the middle frit, thus being reformed by the char catalyst. Argon with a flow rate range from 1.30 L/min to 1.57 L/min was purged into the reactor before heating up, to ensure the same gas velocity inside the reactor when the reaction temperature varied. The reactor was heated to 700-900 ℃ at 15 ℃/min, holding for 1 h at the final temperature. 2.3. Collection and analysis of tar The method for tar collection and analysis was the same for all the experiments in this work no matter if the char catalysts were used or not. Four washing bottles connected in series were used for capturing tar vapor (Fig.2, Part 2): the first two were filled with dry ice while the latter two were holding the organic solvent mixture: analytically pure chloroform (≥ 99.0 wt.%, content of ethanol 0.3-1.0 wt.%) and methanol (purity (GC):≥ 99.5 %) with volume ratio of 4:129. The amount of the blended solvent in two bottles was 50 mL. The four bottles were buried in a dry ice bath to be cooled for achieving effective tar condensation. The effectiveness of this method on tar condensation has been verified by connecting a fifth bottle with solvent which showed no any color after experiments. This indicates that the amount of tar escaping out of the fourth bottle was negligible. To collect the tar solution, the solvent in the fourth bottle was firstly used to wash other three bottles and the outlet of reactor before some extra fresh solvent was utilized to wash all bottles. A part of tar solution (accurately weighed) was dried at 35 ℃ in an aluminum tray for 8 h to ensure a complete evaporation of the moisture and solvent. The yield of tar could be calculated by Eq.(1). Ytar =

M1 M S × 100% M2 MC

(1)

Ytar—tar yield, wt. %; M1, M2—the weight of tar solution used for the analysis after/before drying, g; MS—the total weight of tar solution collected from the experiment (before drying), g; MC—the weight of coal sample, g. The composition of tar was determined using gas chromatography-mass spectrometer (GC-MS-4000, VARIAN), and the components were quantified with the area normalization 6


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method. A column (VF-5ms, 60 m×0.25 mm) was adopted and its initial temperature was 50 ℃. After holding for 6 min, it was heated up to 290 ℃ at a rate of 5℃/min with further holding of 30 min. Ion source in the MS was electron ionization (25 µA of the filament current, 50-1000 u of the scanning range with 1.64 s). 5 µL of sample (tar dissolved in the chromatographically pure chloroform) was injected into the column every time. 2.4. Characterization of char structure The carbon skeleton structural features were determined by a 193 LAS-NY532/50 confocal microprobe Raman spectroscopy. The details of the apparatus/method can be found elsewhere

29

. Briefly, the original Raman spectra in the range of 800 cm-1-1800 cm-1 was

curve-fitted using the GRAMS/32 AI software with 10 Gaussian bands, which has been summarized in previous studies 28,30, representing the typical structures in char from low rank coal. D (1300 cm-1) band in the Raman spectrum usually comes from large aromatic ring systems with no less than 6 fused rings. The three bands of Gr (1540 cm-1), Vr (1380 cm-1) and Vl (1465 cm-1) together mainly represent smaller aromatic ring systems as well as the semi-circle breathing of aromatic rings. The difference in structure characteristics of char samples can be obtained by comparing the areas of different bands. IGr, ID and I(Gr+Vl+Vr) denote the band areas of Gr, D and (Gr+Vl+Vr) respectively.

The ratios of IGr/ID and

I(Gr+Vl+Vr)/ID could broadly reflect the ratio between the large aromatic ring systems (≥6 fused rings) and the small aromatic ring systems (3-5 fused rings) 31. The porous structures of the chars were determined by an automatic volumetric sorption analyzer (Quantachrom, Autosorb-1). The measurement was conducted by N2 adsorption at 77 K. The total specific surface area of the samples was estimated using the multilayer adsorption model developed by Brunauer, Emmett, and Teller (i.e., the BET method). 3. RESULTS AND DISCUSSION 3.1. Effects of heating rate and steam activation on char catalysts for tar reforming at different temperatures Fig.3 shows the tar yields after catalytic reforming reactions by four different chars (catalysts) at different temperatures. As expected, the tar yields generally decreased with increasing temperature irrespective of the presence of catalysts. The increased reaction temperature has enhanced thermal cracking and/or intensified reforming by pyrolytic CO2 and 7


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H2O as shown in (a)-(c) 32. CnHm→C+CxHy+CO+H2

(a)

CnHm+nCO2→(m/2)H2+(2n)CO

(b)

CnHm+nH2O→(m/2+n)H2+nCO

(c)

CnHm represented for tar precursor, while CxHy denoted light gaseous hydrocarbons. The addition of char (especially activated char from fast pyrolysis) as catalysts has caused the drastic reduction in tar yields. When the catalysts were not employed, the removal of tar mainly depended on thermal cracking, which was very limited considering the moderate temperatures used in this study. In the presence of chars as catalysts, the tarry materials could accumulate on char surface and promote condensation reactions as well as the reforming reactions. Under the same condition, the char catalyst prepared from fast-heating rate pyrolysis performed better for tar reforming than that from slow-heating rate pyrolysis. In the meanwhile, the activated char always showed higher catalytic activity than the inactivated one. The difference in catalytic performance of chars could be largely attributed to the pore structure and/or carbon skeletal structure as the char structure could not only determine the ease for reactants (tarry materials) to be absorbed, but also affect the state and distribution of inherent metallic species in char. Therefore, the char structure including pore structure and carbon skeletal structure has been examined and will be discussed later. In order to gain more detailed information on the effects of the chars on tar reforming, the chemical components of tars were analyzed by GC-MS. Aromatic compounds were always the main compositions for all the tars analyzed. As 1-4 fused rings accounted for the majority of tar, the content of each size of aromatic compound was calculated by the following Eq.(2): C = C1 × Y

(2)

C—the percentage of each tar component on basis of coal as shown in Fig.4,; C1—the content of each component in tar measured by GC-MS; Y—tar yield. Fig.4 shows the change in the relative abundance of various sizes of aromatic compounds as a function of char catalysts at 800 oC. The 2-4 aromatic ring systems seem to be the major portion in each coal tar sample investigated. The comparison between Fig.4(a) 8


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and (b) again indicated that the chars from the fast-heating rate pyrolysis were better candidates for tar reforming than those from the slow-heating rate pyrolysis. Particularly, the activated char showed very promising activity on the reforming of 2-4 fused rings. Compared to the inactivated char, the high porosity and abundant functional groups in the activated char were favorable structural conditions for the removal of aromatic ring systems. It has been pointed out that after the activation by steam, some O-containing functional groups may form on the surface of char, which acts as the “active sites” that could easily capture the tarry matters for its negatively-charged π electron cloud system 29. The absorption of tarry materials on the “active sites” could disturb the π electron cloud of those hydrocarbon molecules. The decrease in stability of chemical bonds would thus promote the tar reforming/cracking to a great extent. 3.2. Effects of activation time on char catalysts for tar reforming The activated char above was prepared by further reaction with steam for 10 min after the pyrolysis at 900 ℃. The char-steam reactions not only consumed carbon and produced gaseous products of H2 and CO, but also rearranged carbons structure via steam-generated H-radicals

25, 30

. The change in char structure would greatly influence char activity as

reforming catalysts. To reveal the effects of char-steam reaction on reforming performance of chars, the chars from different activation (char-steam reaction) time were prepared and tested as catalysts to reform tar at 800 ℃. As shown in Fig.5 (see the stars for the change in tar yields), the activity of chars for tar reforming initially increased with increasing char-steam reaction time, eventually achieving the highest performance at around 3 min. After that, the char’s catalytic reactivity gradually decreased. The content of aromatic ring systems at a range of 1-4 rings in tar after reforming by chars from various activation time was also measured and shown in Fig.5 (see the columns for the change in tar contents). The contents of large aromatic rings broadly decreased with increasing char activity while the content of small aromatic rings could not see a clear trend, implying that the chars derived from various char-steam reaction time possessed different amounts/types of reactive sites and could selectively eliminate aromatic rings in tar. Xu et al. 32

found that the mesopores (2-50 nm) of the activated carbon played an important role for the

effective conversion of heavy hydrocarbon compounds into lighter fractions. When the 9


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activation time was extended, the physio-chemical structure of char catalysts (i.e. surface area, pore size distribution and carbon structure) may be altered, which would change the catalytic performance of char catalysts for tar reforming. The large aromatic ring systems owned higher π electron cloud density than the small ones, which had a strong propensity to associate with reactive sites on the surface of char. By contrast, the structure of 1 or 2 aromatic rings (i.e. benzene or naphthalene) in tar was more stable and relatively inert for reforming reactions 33. 3.3. Structural features of chars 3.3.1. Pore structure Fig.6 exhibits the SEM images of four chars (SI, SA, FI and FA). It can be seen that char prepared from fast-heating pyrolysis featured more coarse surface morphology than that from slow-heating pyrolysis. In the meantime, it appears that the activated char showed more ordered structure and debris on its surface than the inactivated one. The specific surface area was also obtained by BET method as is shown in Fig.7. The data in Fig.7a indicate that the order

of

the

specific

surface

area

of

the

four

chars

was

as

follows:

FA-char>SA-char>FI-char>SI-char, which was in consistent with the SEM images as well as char activity on tar reforming. This again suggested that the large surface area could provide more “sites” for tar precursor to accumulate and react. However, it was found that the surface area of SA-char was far bigger than that of the FI-char (the difference was about 403 m2/g), while the difference in tar yield after being reformed by these two chars was relatively small, suggesting that some other factors (e.g. carbon structure and so on) also played important roles for the catalytic performance of activated chars. The data in Fig.7b exhibits that the char yield continued to decrease as a function of activation time while the specific surface area sharply increased at the early stage and then very slowly after 3 min. When the difference in surface area was big (such as activated for 0 min, 1 min, 3 min), the char’s capacity on tar removal was well correlated to their surface area. The big surface area could potentially provide more reactive sites on the char surface and promote the transmitting and reforming of tarry matters. However, when the difference in surface area was small (such as from 3 min to 10 min activation), the relationship between surface area and char catalytic activity became very poor (see Fig.5), demonstrating that the 10


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specific surface area was no longer the main factor influencing the char activity. To further explore the pore structural characteristics, the pore size distribution of the char catalysts with various activation time was shown in Figure 8. The pore sizes in the chars were in the range of 2-50 nm (mesopores) although most of them were below 10 nm. Except the char from 1 min activation time, the char catalysts from 3 to 10 min activation time shared the similar pore size distribution. This basically means that the continual char-steam reactions after 3 min was less selective, thus maintaining the analogous pore structure. 3.3.2. Carbon structure The char activation (char-steam reaction) could have an influence on char activity from the following three aspects

29

: firstly, it could result in the changes of pore size and the

specific surface area of char; secondly, the carbon skeleton structure may vary because of the char-steam reactions as well as the radicals-induced change; thirdly, after the activation by steam, the formation of some O-containing functional groups on the surface of char could act as the reactive sites to promote the tar reforming. Raman spectroscopy has been widely used to characterize the structural features of char derived from coal (especially low rank coal) by correlating the deconvoluted Raman bands to structural parameters

30,31,34

. The total Raman peak area could also reflect the electron-rich

structures such as O-containing functional groups as well as the extent of fused aromatic ring systems

31, 35

. Therefore, Raman spectroscopy was used for the determination of carbon

structure of chars in this study. Fig.9a and Fig.9b show the total Raman intensity and area ratios between small and large aromatic ring systems respectively in the region 800-1800 cm-1 for chars with different activation time. The total Raman area increased and then decreased with extending the activation time. The Raman intensity was affected by the Raman scattering ability and light absorptivity of the char. O-containing functional groups that are rich in electron would enhance the Raman intensity, while the condensation of aromatic ring systems would lead to a low Raman intensity 36-38. The former one normally has much more important effects on the Raman intensity than the latter one. Therefore the Raman intensity could reflect the abundance of O-containing functional groups in char. When steam reacted with char, the H2O molecule firstly dissociated into •OH and •H 11


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which then bonded with its adjacent active sites on char surface. The •OH reacting with char could form O-containing functional groups while •H may further penetrate into char matrix and initiated the alteration of char structure 27, 29. The trend indicated in Fig.9 may be ascribed to the actual operation of experiments. The fast pyrolysis was carried out by feeding about 5 g coal into hot reaction zone in a very short time. Following 30 min holding time at 900 ℃, the steam was supplied into the reactor to react with about 2.5 g char (roughly 2.5 mm in height). It may take some time to allow all the char particles to have sufficient contacts with steam, especially considering that the “0” point of holding time (i.e. 0 min of activation time) was counted right after turning on the water pump. It has been reported that the sudden supply of steam in short time could create abundant small aromatic ring systems and amorphous carbon structure in char 38. However, the active structures would be easily consumed for extending gasification time (e.g. more than 3 min). These may plausibly elucidate the trend in Fig.8(a) and (b) where both total Raman intensity and ratios of (I(Gr+Vl+Vr) /ID and IGr/ID) for the char with 3 min activation time showed the highest values. Correspondingly, the high catalytic performance of char from around 3 min activation time strongly suggested that the carbon structure and O-containing complexes were very critical components in affecting the activity. Fig.10 depicts the total Raman peak area as well as the band area ratios (I(Gr+Vl+Vr) /ID and IGr/ID) of chars from slow and fast-heating rate pyrolysis. Data from Fig.10a indicated that the total Raman area of char from fast-heating rate pyrolysis was always lower than that from slow-heating rate pyrolysis. As mentioned before, the total Raman intensity was mainly determined by electron-rich elements (e.g. oxygen) on char surface as well as extents of aromatization in char. Since the change in the degree of aromatization (see I(Gr+Vl+Vr)/ID and IGr/ID in Fig.10b) has no clear relationship with the trend in the total Raman intensity, the difference in the total Raman area seems to be mainly due to the change in the O-containing functional groups. Steam-char reactions during the activation time could form oxygen complexes on char surface which could exert resonant effects on aromatic ring systems, thus enhancing Raman intensity. Regarding to the slow pyrolysis, the coal particles would slowly decompose and volatile gradually evolved out of char matrix (inner particles). Coal sample used in this study contains about 30 wt.% oxygen (Table 1). Therefore, the O-containing species/radicals may repeatedly dissociate and associate with char matrix, and eventually 12


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could have chances to form relatively stable O-containing structure remaining in char. In contrast, the volatile was released from coal particles in a very short period when coal was fed into a hot reaction zone suddenly (fast-heating pyrolysis). Therefore, the total Raman area for the chars from slow-heating pyrolysis was apparently higher than that from fast-heating pyrolysis. However, the stable O-containing structure formed during the slow pyrolysis could be likely inside char bodies, thus showing less catalytic reactivity. Fig.9b and Fig.10b together indicate that the char-steam interaction was more effective to change carbon skeletal structure of chars than the heating rate.

4. CONCLUSIONS The catalytic performance of chars that have experienced different heating rates and activation time was examined for tar reforming using Shengli brown coal in a bench scale quartz reactor. Compared to the slow heating rate, the fast heating pyrolysis was more favorable conditions for preparing chars (catalysts) with large surface area and high catalytic reforming activity. The tar reductions by activated char from the fast heating rate were nearly double those by activated char from the slow heating rate. The activation time for char was very vital for its reforming activity. The char with 3-5 min activation (char-steam reaction) showed the highest activity in this study. Longer activation time by steam would apparently lead to the more condensation of aromatic ring system and less oxygen-containing functional groups in char catalysts. The Raman analysis appears to suggest that the specialty of char with medium activation time (3-5 min) was due to its relatively high contents of electron-rich elements (i.e. oxygen) and abundant small aromatic ring systems in char. AUTHOR INFORMATION Corresponding Authors * (S. Zhang). [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGEMENTS

The authors gratefully acknowledge the financial support from 12th Five-Year Plan of National Science and Technology Support (2012BAA04B02) and National Nature Science Foundation of China (21406264). 13


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REFERENCES [1] Zhang, L.X.; Matsuhara,T.; Kudo,S;. Hayashi,J.; Norinaga,K. Rapid pyrolysis of brown coal in a drop-tube reactor with co-feeding of char as a promoter of in situ tar reforming. Fuel 2013,112, 681-686. [2] Hayashi,J.I.; Hosokai,S.; Sonoyama,N. Gasification of low-rank solid fuels with thermochemical energy recuperation for hydrogen production and power generation. Process Saf. Environ. Prot. 2006,84,409-419. [3] Devi,L.; Ptasinski,K.J.; Janssen,F.J.J.G.; Paasen,S.V.B.; Bergman,P.C.A.; AKiel,J.H. Catalytic decomposition of biomass tars: use of dolomite and untreated olivine. Renew. Energy 2005,30,565-587. [4] Virginie,M.; Adánez, J.; Coursona,C.; Diego,L.F.; García-Labiano,F. Niznanskyc,D.; Kiennemann, Gayán,A. P.; Abad, A. Effect of Fe-olivine on the tar content during biomass gasification in a dual fluidized bed. Appl. Catal., B Environ. 2012,121,214-222. [5] Paasen, S.V.B.; Rabou, L.P.L.M.; Bar, R. Tar removal with a wet electrostatic precipitator (ESP): a parametric study. The Second World Conference and Technology Exhibition on Biomass for Energy-Industry and Climate Protection, Rome, Italy, 10-14 May, 2004. [6] Han,J.; Kim,H. The reduction and control technology of tar during biomass gasification/pyrolysis: an overview. Renew. Sustainable Energy Rev. 2008, 12, 397-416. [7] Anis,S.; Zainal,Z.A. Tar reduction in biomass producer gas via mechanical, catalytic and thermal methods: a review. Renew. Sustainable Energy Rev. 2011, 15,2355-2377. [8] Doolan,K.R. Mackie,J.C.; Tyler,R.J. Coal flash pyrolysis secondary cracking of tar vapours in the range 870-2000K. Fuel 1987, 66, 572-578. [9] Hassan, S.S.A.; Wang, Y.; Hu, S.; Su S.; Xiang, J. Thermochemical processing of sewage sludge to energy and fuel: Fundamentals, challenges and considerations. Renew. Sustainable Energy Rev. 2017, 80, 888-913. [10] Feng, D.D.; Zhao, Y.J.; Zhang, Y.; Zhang, Z.B.; Sun, S.Z. Roles and fates of K and Ca species on biochar structure during in-situ tar H2O reforming over nascent biochar. Int. J Hydrogen Energ. 2017, 42, 21686-21696. [11] He, L.; Hu, S.; Jiang, L.; Liao, G.; Zhang, L.; Han, H.; Chen, X.; Wang, Y.; Xu, K.; Su, S.; Xiang,

J.

Co-production

of

hydrogen

and

carbon

14


nanotubes

from

the

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Energy & Fuels

decomposition/reforming of biomass-derived organics over Ni/a-Al2O3 catalysts: Performance of different compounds. Fuel 2017, 210, 307-314. [12] Guan, G.q.; Kaewpanha, M.; Hao, X.G.; Abudula, A. Catalytic steam reforming of biomass tar: Prospects and challenges. Renew. Sustainable Energy Rev. 2016, 58, 450-461. [13] Rapagna, S.; Gallucci, K.; Foscolo, P.U. Olivine, dolomite and ceramic filters in one vessel to produce clean gas from biomass. Waste Management 2017, In press, https://doi.org/10.1016/j.wasman.2017.07.038. [14] Cao, J.P.; Liu, T.L.; Ren, J.; Zhao, X.Y.; Wu, Y.; Wang, J.X.; Ren, X.Y. Preparation and characterization of nickel loaded on resin char as tar reforming catalyst for biomass gasification. J. Anal. Appl. Pyrolysis 2017, 127, 82-90. [15] Hua, X.; Lu, G.X. Comparative study of alumina-supported transition metal catalysts for hydrogen generation by steam reforming of acetic acid. Appl. Catal. B Environ. 2010, 99, 289-297. [16] Wang, Y.; Jiang, L.; Hu, S.; Su, S.; Zhou, Y.B.; Xiang, J.; Zhang, S.; Li, C.Z. Evolution of structure of char-supported iron catalysts prepared for steam reforming of bio-oil. Fuel Process. Technol. 2017, 158, 180-190. [17] Zhang, S.; Asadullah, M.; Dong, L.; Tay, H.L.; Li, C.Z. An advanced biomass gasification technology with integrated catalytic hot gas cleaning. Part II: Tar reforming using char as a catalyst or as a catalyst support. Fuel 2013, 112, 646-653. [18] Qian, K.Z.; Kumar, A. Catalytic reforming of toluene and naphthalene (model tar) by char supported nickel catalyst. Fuel 2017, 187, 128-136. [19] Park, J.J.; Lee, Y.W.; Ryu, C.K. Reduction of primary tar vapor from biomass by hot char particles in fixed bed gasification. Biomass and Bioenergy 2016, 90, 114-121. [20] Feng, D.D.; Zhao, Y.J.; Zhang, Y.; Zhang, Z.B.; Che, H.W.; Sun, S.Z. Experimental comparison of biochar species on in-situ biomass tar H2O reforming over biochar. Int. J Hydrogen Energ. 2017, 42, 24035-24046. [21] Zhang, S.; Song, Y.; Song, Y.C.; Yi, Q.; Li, D.; Li, T.T.; Zhang, L.; Feng, J.; Li, W.Y.; Li, C.Z. An advanced biomass gasification technology with integrated catalytic hot gas cleaning. Part III: Effects of inorganic species in char on the reforming of tars from wood 15


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and agriculture wastes. Fuel 2016, 183, 177-184. [22] Xu, C.C.; Donald, J.; Byambajav, E.; Ohtsuka, Y. Recent advances in catalysts for hot-gas removal of tar and NH3 from biomass gasification. Fuel 2010, 89, 1784-1795. [23] Song, Y.; Wang, Y.; Hu, X.; Hu, S.; Xiang, J.; Zhang, L.; Zhang, S.; Min, Z.H.; Li, C.Z. Effects of volatile-char interactions on in situ destruction of nascent tar during the pyrolysis and gasification of biomass. Part I. Roles of nascent char. Fuel 2014, 122, 60-66. [24] Soltani, S.M.; Yazdi, S.K. The effect of pyrolysis time and heating rate on the surface area and pore size properties of porous carbon obtained from the pyrolysis of a cellulosic waste. 2nd International Conference on Environment and Industrial Innovation, Singapore, 2012. [25] Tay, H.L.; Kajitani, S.; Zhang, S.; Li, C.Z. Effects of gasifying agent on the evolution of char structure during the gasification of Victorian brown coal. Fuel 2013, 103, 22-28. [26] Zhang, S.; Min, Z.H.; Tay, H.L.; Wang, Y.; Dong, L.; Li, C.Z. Changes in char structure during the gasification of Mallee wood: effects of particle size and steam supply. Energy Fuels 2012, 26, 193-198. [27] Barea, A.G.; Ollero, P.; Leckner, B. Optimization of char and tar conversion in fluidized bed biomass gasifiers. Fuel 2013, 103, 42-52. [28] Asadullah, M.; Zhang, S.; Min, Z.H.; Yimsiri, P.; Li, C.Z. Effects of biomass char structure on its gasification reactivity. Bioresour. Technol. 2010, 101, 7935-7943. [29] Wang, F.J.; Zhang, S.; Chen, Z.D.; Liu, C.; Wang, Y.G. Tar reforming using char as catalyst during pyrolysis and gasification of Shengli brown coal. J. Anal. Appl. Pyrolysis 2014, 105, 269-275. [30] Zhang, S.; Min, Z.H.; Tay, H.L.; Asadullah, M.; Li, C.Z. Effects of volatile-char interactions on the evolution of char structure during the gasification of Victorian brown coal in steam. Fuel 2011, 90, 1529-1535. [31] Li, X.; Hayashi, J.; Li, C.Z. Volatilisation and catalytic effects of alkali and alkaline earth metallic species during the pyrolysis and gasification of Victorian brown coal. Part VII. Raman spectroscopic study on the changes in char structure during the catalytic gasification in air. Fuel 2006, 85, 1509-1517. 16


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Energy & Fuels

[32] Xu, C.; Hamilton, S.; Ghosh, M. Hydro-conversion of Athabasca vacuum tower bottoms in supercritical toluene with highly porous biomass-derived activated carbon and metal-carbon composite. Fuel 2009, 88, 2097-2105. [33] Shen, Y.; Yoshikawa, K. Recent progresses in catalytic tar elimination during biomass gasification or pyrolysis-A review. Renew. Sustainable Energy Rev. 2013, 21, 371-392. [34] Lin, X.C.; Wang, C.H.; Ideta, K.K.; Miyawaki, J.; Nishiyama, Y.; Wang, Y.G.; Yoon, S.; Mochida, I. Insights into the functional group transformation of a Chinese brown coal during slow pyrolysis by combining various experiments. Fuel 2014, 118, 257-264. [35] Min, Z.H.; Zhang, S.; Yimsiri, P.; Wang, Y.; Asadullah, M.; Li, C.Z. Catalytic reforming of tar during gasification. Part IV. Changes in the structure of char in the char-supported iron catalyst during reforming. Fuel 2013, 106, 858-863. [36] Li, X.; Li, C.Z. Volatilisation and catalytic effects of alkali and alkaline earth metallic species during the pyrolysis and gasification of Victorian brown coal. Part VIII. Catalysis and changes in char structure during gasification in steam. Fuel 2006, 85, 1518-1525. [37] Li, X.; Hayashi, J.I.; Li, C.Z. FT-Raman spectroscopic study of the evolution of char structure during the pyrolysis of a Victory brown coal. Fuel 2006, 85, 1700-1707. [38] Keown, D.M.; Hayashi, J.I.; Li, C.Z. Drastic changes in biomass char structure and reactivity upon contact with steam. Fuel 2008, 87, 1127-1132.

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Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Table 1. Proximate and ultimate analyses of Shengli brown coal Proximate analysis（wt.%）

Ultimate analysis（wt.%）

Moisture

Asha

Volatileb

Fixed carbonb

Cb

Hb

Nb

Sb

Ob,c

4.11

7.99

46.26

53.74

64.39

4.50

1.21

0.42

29.48

a

Dry basis

b

Dry ash free basis

c

By difference

18


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Energy & Fuels

List of Figures Fig.1. A schematic diagram of the two-stage quartz reactor used for tar reforming experiments Fig.2. A schematic diagram of the experimental rig for tar reforming and collection Fig.3. Tar yields as a function of char catalysts and temperatures during brown coal pyrolysis Fig.4. Comparison of aromatics compounds (1-4 fused rings) in tar reformed by activated/inactivated char at 800 oC Fig.5. Comparison of aromatics compounds (1-4 fused rings) in tar reformed by chars with different activation time at 800 oC Fig.6. Surface morphology of char catalysts revealed by SEM analysis Fig.7. The specific surface area and yields of chars prepared from brown coal pyrolysis Fig.8. The pore size distribution of char catalysts with different activation time at 900 oC Fig.9. Raman analysis of chars with different activation time Fig.10. Raman analysis of chars from the pyrolysiss at low and fast heating rate

19


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Fig.1. A schematic diagram of the two-stage quartz reactor used for tar reforming experiments

20


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Energy & Fuels

Fig.2. A schematic diagram of the experimental rig for tar reforming and collection 1.Electrical furnace; 2.Thermocouple; 3. Solid inlet; 4.Gas outlet; 5. Gas inlet; 6.Tar traps (with dry ice); 7.Dry ice bath; 8.Tar traps (with solvent); 9. Pump; 10. Distilled water; 11. Fluidizing gas.

21


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Fig.3. Tar yields as a function of char catalysts and temperatures during brown coal pyrolysis. (a) SI-char: char (catalyst) prepared from slow-heating pyrolysis without steam activation; SA-char: char (catalyst) prepared from slow-heating pyrolysis with 10 min steam activation (b) FI-char: char prepared from fast-heating pyrolysis without steam activation; FA-char: char prepared from fast-heating pyrolysis with 10 min steam activation

22


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Page 23 of 29

4.0

(b)

(a)

3.5

Content of components in tar/%

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

3.0

Four rings Three rings Two rings One ring

2.5 2.0 1.5 1.0 0.5 0.0

None-char

SI-char

SA-char None-char FI-char

FA-char

A C B Fig.4. Comparison of aromatics compounds (1-4 fused rings) in tar reformed by

activated/inactivated char at 800 oC (a) Chars from slow-heating rate pyrolysis at 900 oC; (b) Chars from fast-heating rate pyrolysis at 900 oC

23


Energy & Fuels

3

Content of components in tar/%

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Four rings Three rings Two rings One ring

Tar yield

2

1

0

0

1

3

5

10

Activation time/min

Fig.5. Comparison of aromatics compounds (1-4 fused rings) in tar reformed by char catalysts at 800 oC. (The char catalysts were prepared at 900 oC with different activation time).

24


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Energy & Fuels

SI-char

SA-char

FI-char

FA-char

Fig.6. Surface morphology of char catalysts revealed by SEM analysis (a) SI-char: char (catalyst) prepared from slow-heating pyrolysis without steam activation; SA-char: char (catalyst) prepared from slow-heating pyrolysis with 10 min steam activation (b) FI-char: char prepared from fast-heating pyrolysis without steam activation; FA-char: char prepared from fast-heating pyrolysis with 10 min steam activation

25


Energy & Fuels

600

(a)

60

-1

500 400 50

300

45

200

Char yield/wt %

55

2

Specific surface area/m .g

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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40

100 0 SI-char

SA-char

FI-char

FA-char

35

Fig.7. The specific surface area and yields of chars prepared from brown coal pyrolysis. (a) slow and fast heating pyrolysis; (b) different activation time (char-steam reaction time)

26


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0.030

1 min 3 min 5 min 10 min

0.025

3

dV/dD,cm /g.nm

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

0.020 0.015 0.010 0.005 0.000 0

5

10

15

20

25

30

35

Diameter, nm

Fig.8. The pore size distribution of char catalysts with different activation time at 900 oC

27


Energy & Fuels

55000

(a)

Total area(a.u.)

50000 45000 40000 35000 30000 25000

0

1

5

3

10

Activation time/min

0.5

(b) 1.6

0.3

Gr D

1.5

I /I

/I

(Gr+Vr+Vl) D

0.4

I

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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1.4 0.2 1.3 0.1 0

2

4

6

8

10

Activated time/min Fig.9. Raman analysis of chars with different activation time

(a) Total Raman peak area of chars; (b) Band area ratios I(Gr+Vl+Vr) /ID and IGr /ID of chars

28


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55000

(a)

Total area(a.u)

50000 45000 40000 35000 30000 25000

SI-char

SA-char

FI-char

FA-char

0.5 (b)

1.6

0.3

Gr D

1.5

I /I

/I

(Gr+Vr+Vl) D

0.4

1.4

I

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

0.2 1.3 SI-char

0.1 SA-char

FI-char

FA-char

Fig.10. Raman analysis of chars from the pyrolysiss at low and fast heating rate

(a) Total Raman peak area of chars; (b) Band area ratios I(Gr+Vl+Vr) /ID and IGr /ID of chars

29


Catalytic Tar Reforming during Brown Coal Pyrolysis: Effects of

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