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Heterogeneous cracking reaction of tar over biomass char, using naphthalene as model biomass tar Yun liang Zhang, Yong-hao Luo, Wen-guang Wu, Shanhui ZHAO, and Yu-feng Long Energy Fuels, Just Accepted Manuscript • Publication Date (Web): 25 Apr 2014 Downloaded from http://pubs.acs.org on April 26, 2014

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Heterogeneous cracking reaction of tar over biomass char, using naphthalene as model biomass tar Yun-liang Zhang†, Yong-hao Luo*,†, Wen-guang Wu‡, Shan-hui Zhao† and Yu-feng Long† †

Institute of Thermal Engineering, School of Mechanical Engineering, Shanghai Jiao Tong University, Shanghai 200240, P. R. China ‡

Institute of Renewable Energy Science and Engineering, School of Energy and Power

Engineering, University of Shanghai for Science and Technology, Shanghai 200093, P. R. China

Abstract

The tar problems are the major limit for development of biomass gasification. Biomass char has been proven to be an economical and effective catalyst of tar destruction for both utilizations inside gasifier and in downstream process after gasifier. In order to investigate the mechanism of catalytic cracking of tar over biomass char bed, experimental research was performed in a benchscale tube flow reactor, choosing rice straw char as the catalyst bed, naphthalene as the model tar compound and argon as the inert atmosphere. The Effects of temperature (700-1000 °C), tar concentration, time stream (0-330 min), presence of syngas and pretreatment of char (treated with deionized water or Ni(NO3)2 solution) on tar conversion were evaluated. The variation of

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inner pore structure of biomass char during process of tar removal was also investigated. Results showed that the original biomass char exhibited good catalytic activity in tar cracking and better stability compared with Ni(NO3)2 pretreated char. However, due to the naphthalene cracking reaction, soot was formed on the active sites of inner pore surface of char leading to the deactivation of char. A rapid decline in specific surface area of char was observed from 262 m2/g to 4.6 m2/g when the test had begun to run for 5 minutes with high tar concentration (25 g/Nm3) and temperature of 800 °C. The presence of syngas in atmosphere could slow the process of deactivation of char.

*

To whom correspondence should be addressed. Telephone: +86-21-34206047. E-mail:

[email protected]


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1 Introduction Biomass resources are promising substitute for fossil fuels as they are sustainable and environmental-friendly due to CO2 neutral nature. In China, about 1300 million tons of biomass waste are produced annually, including 789 million tons of agricultural by-products that are mostly from rice, corn and wheat 1. Thus China has great potential and huge demand for developing advanced technology of utilizing biomass energy. The thermochemical conversion of biomass has attracted more attention in recent few decades, especially for gasification due to its low cost, adaptability to loads of fuels and multiple use of syngas. However, the inevitable presence of tar in the producer gas can seriously influence the downstream processes, causing blocking and corrosion problems. Therefore, the tar problems are the major limit for development of biomass gasification. It was found that tar in pyrolysis gas could be greatly reduced by its sufficient contact with carbonaceous surfaces

2, 3

, which showed some extent of selectivity in reduction of tar

compounds 4. Biomass char, which is of complex and well developed pore structure and rich in alkali metal, is naturally produced from process of pyrolysis. It has been proven to be an effective catalyst for tar removal, by treatment either inside gasifier (primary methods)

5, 6

or in

downstream process after gasifier (secondary methods) 7. Some researchers investigated the effect of hot biomass char bed on destruction of tar derived from pyrolysis process of biomass in lab-scale reactors

8-13

. The majority of them found that biochar had better performance in tar

removal compared with homogeneous cracking conditions

8-12

. One of them had come to a

conclusion that biochar showed no significant increase in cracking of tar and homogeneous cracking was responsible for tar removal

13

. It was also mentioned that the oxygen-contained


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aromatics

10

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and polycyclic aromatic hydrocarbons (PAHs)

6, 12

were easily converted and

removed by biochar with respect to the composition of tar. However, the works referenced above mainly focus on trends and efficiencies of tar removal over biochar, with less attention to microscopic reaction mechanisms. In order to deeply investigate the mechanism of tar removal over biomass char, it is necessary to simplify the reaction processes, select typical tar compounds as research objects and choose more precise bench-scale platform. Some scientific works have been reported to study catalytic reforming of model tar compounds over biomass char

14-17

. Abu et al.14 compared the biomass

char with other known catalysts on catalytic reforming of phenol and naphthalene, with CO2 and steam in atmosphere and temperature range of 700-900 °C. Among the low cost catalysts, biomass char showed the highest conversion of naphthalene. The authors also indicated that the continuous production in gasifier and activation by steam and CO2 make biomass char the more stable catalyst for tar removal. Hosokai et al.15 investigated the catalytic decomposition of mixture of benzene and naphthalene over charcoal, with steam and H2 in atmosphere and temperature range of 700-900 °C. It was concluded that the main mechanism was coking (carbon deposition) rather than steam reforming of tar. The coking would lead to the deactivation of charcoal. However, with gasification of charcoal and coke, the activity of charcoal would be maintained. It was also mentioned that micropores of charcoal provided active sites for the coking. Fuentes-Cano et al.16 studied the catalytic reforming of mixture of toluene and naphthalene over three char (coconut char, coal char and char derived from dried sewage sludge), with steam and H2 in atmosphere and temperature range of 750-950 °C. It was revealed that the inner pore structure of char had minor influence on the activity of char for tar removal. The research mentioned above mainly focuses on catalytic reforming of tar. The presence of CO2 or


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steam in atmosphere makes dry/steam reforming of tar the predominant reaction pathway. The hydrogen in atmosphere can inhibit the polymerization of tar caused by thermal cracking

18

.

Nevertheless, the thermal cracking of tar is still the important reaction pathway for tar removal. The detailed study on catalytic cracking of model tar compound over biochar in inert atmosphere has not been found. The objective of this paper is to deeply study the mechanism of catalytic cracking of tar over biomass char. The Present work chose typical agricultural waste in China, rice straw, as the raw material for biochar and representative component of PAHs, naphthalene, as the model tar compound. Investigations on the heterogeneous cracking of naphthalene over rice straw char in inert atmosphere (only argon) were performed in a bench-scale micro reactor. The Effects of temperature (700-1000 °C), tar concentration, time stream (0-330 min), presence of syngas and pretreatment of char (treated with deionized water or Ni(NO3)2 solution) on tar conversion were evaluated. The variation of inner pore structure of biomass char during process of tar removal was particularly investigated. Conditions including homogeneous cracking, heterogeneous cracking over sand bed and heterogeneous cracking in the presence of syngas were performed for comparison.

2 Experimental section 2.1 Material Rice straw, the typical agricultural waste in China, was chosen as raw material for producing biomass char. The proximate analysis and ultimate analysis of rice straw raw material are shown in Table 1. The mineral content in rice straw are shown in Table 2. The amount of mineral was quantified by inductively coupled plasma-atomic emission spectrometry (ICP-AES), with model


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of IRIS Advantage 1000 from Thermo Jarrell Ash Co. The parameters were as follows: the working frequency was 27.1 MHz, the coupled power was 1.15 kW, the flow rate of carrier gas was 1.0 L/min, the pressure of nebulizer was 103 kPa and the integration time was 20 s for long wave and 10 s for short wave. The raw material was previously ground and sieved into smaller one with particle size range of 100-150 µm. The rice straw char was produced by heating rice straw powder at slow heating rate to 500 °C in inert atmosphere, then maintained for 1 hour. There have been three types of char performed in tests, including the original char, char treated with Ni(NO3)2 solution and char treated with deionized water. The procedure of pretreatment of char are listed. First, the char was immersed and sufficiently stirred in Ni(NO3)2 solution of 0.2 mol/L or deionized water for two hours. Then the char was dried at 105 °C for 48 hours to finally obtain the pretreated char samples.

2.2 Reactor and Procedures Figure 1 is the schematic diagram of experimental setup. The micro reactor was made of quartz tube, with inner diameter of 8 mm, outer diameter of 12 mm and length of 350 mm. It was vertically fixed in which a mesh-like platform was embedded in a position of 150 mm far from top. Some glass fiber was placed on the platform to support biomass char (about 0.25 g in every single test). The reactor tube was heated by an electrical furnace. Argon was used as a carrier gas for both micro reactor and naphthalene saturator. The gas flow rate was controlled by mass flowmeter. The naphthalene concentration was regulated by changing carrier gas flow rate and saturator temperature. All the gas and tar stream were passing through a unit of preheater and mixer before entering the reactor tube. Then the mixture stream was flowing downwardly through the reactor tube. The outlet stream was divided into two streams, one was led to a tar sampling unit and the other one to a gas analysis unit or vent. The length of isothermal section in


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reactor tube was 20-30 mm with maximum temperature deviation of 5 °C. Figure 2 is the temperature distribution in the reactor tube. The procedure of the experimental test is described as follows. Firstly, preparation work had been done before test begun. Specifically, the pre-weighed char and glass fiber were laid on the platform in tube. The reactor tube and lines were connected with each other. Then gas tightness of the system was inspected. Afterwards the whole system was swept by argon at 5 ml/min for 5 minutes to empty the oxygen. Next, the system stream was switched to bypass line without flowing through char bed. The reactor tube was heated to set temperature (700-900 °C). The temperature of mixer, inlet and outlet lines were maintained at 250 °C. The saturator was also heated to the set temperature (90-95 °C). Then the naphthalene steam had been flowing through bypass line for 30 minutes to stabilize inlet tar concentration. Secondly, when the test began, the system stream was switched to reactor tube, thus all the carrier gas and tar steam were flowing through the char bed in reaction zone. The gaseous products were led to the unit of tar sampling and unit of non-condensable gas analysis for further test. Thirdly, when the test had finished, all the heating devices were turned off and the carrier gas of naphthalene saturator was stopped. The compensatory gas of system would not be closed until the system temperature was lowered to ambience one. Then the char was taken out for further characterization. The naphthalene concentration in inlet gas was controlled by altering water bath temperature and carrier gas flow rate for saturator. Two different naphthalene concentrations were chosen in present tests. The specific parameters for high naphthalene concentration were: water bath temperature of 95 °C, carrier gas flow rate (for saturator) of 10 ml/min and system compensatory gas of 2 ml/min. The parameters for low naphthalene concentration were: water bath temperature of 90 °C, carrier gas of 2 ml/min and system compensatory gas of 10 ml/min.


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2.3 Tar sampling and analysis The cold solvent trap (CST) method was used for tar sampling in present study. Isopropanol was chosen as solvent for tar adsorption. A series of six impingers were used and each one of them was filled with isopropanol of 25 ml. Three impingers were laid in water bath of 20 °C and another three were laid in alcohol bath of -20 °C. The time for tar sampling is 3 min each time. After tar sampling, all the solvent were mixed together for further detection. This method was proven to be effective for tar sampling 19, 20. The model tar compound (naphthalene) was quantitatively analyzed by Agilent 6890N GC (gas chromatography) equipped with FID (flame ionization detector). The specific parameters of GC were as follows: The initial oven temperature was 70 °C, held for 2 min. Then it was heated to 150 °C at 10 K/min. The total flow rate of carrier gas was 30 ml/min and split ratio was 26.6:1. The HP-5MS capillary column was used, with length of 30 m, diameter of 250 µm and film thickness of 0.25 µm. The initial flow rate of the column was 1.0 ml/min, the initial inlet pressure was 13.02 psi and the average velocity was 26 cm/sec. The parameters for FID were as follows: temperature was 300 °C, hydrogen flow rate was 40 ml/min, flow rate of air (water and oil free) was 400 ml/min and flow rate of sweeping gas helium was 25 ml/min. The tar components form naphthalene cracking in product gas were qualitatively analyzed by Agilent 6890N GC equipped with QP2010NC MS (mass spectrometry) detector. The detail parameters setting could be seen in our previous research 12.

2.4 Char characterization and non-condensable gas The specific area and inner pore structure of char were characterized by TristarⅡ3020 auto adsorption analyzer from Micromeritics corp. The isothermal adsorption and desorption curves were obtained by using nitrogen as adsorption medium in the presence of liquid nitrogen (-196


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°C), with relative pressure P/P0 of 0.01~0.995. BET (Brunauer-Emmett-Teller) and BJH (Barrett-Joyner-Halenda) methods were used to calculate the specific area and pore volume distribution of biomass char. The non-condensable gas derived from naphthalene cracking was analyzed by GC 9560 equipped with TCD (thermal conductivity detector) from Huaai corp. The oven temperature was 60 °C and the carrier gas was argon. A six-way valve was used to feed gas sample. The TDX-01 packed column was performed with length of 2 m and inner diameter of 2 mm. The temperature for TCD was 150 °C.

3 Results and Discussions 3.1 Homogeneous cracking of naphthalene Results for homogeneous thermal cracking of naphthalene in inert atmosphere are shown in Figure 3. The inlet naphthalene concentration was chosen with high concentration of 25 g/Nm3. Naphthalene conversion efficiency increased from 26.2 % to 63.5 % with increasing temperature from 700 °C to 1000 °C. Process of tar cracking reaction is always accompanied by tar polymerization reaction that usually consists of dehydrogenation process. Therefore, hydrogen yield reflects the extent of tar polymerization reaction

21, 22

. Figure 3 shows that hydrogen concentration increased with rising

temperature. This trend indicates that polymerization of naphthalene is becoming severe with increasing temperature of thermal cracking. Moreover, increase of hydrogen concentration can promote hydrocracking process of tar fragments into smaller tar molecule and methane. The role of hydrogen (gaseous product) is more like a hydrogen (atom) transfer intermediate in process of thermal cracking of tar. The process of hydrogen transfer leads to cracking of larger tar


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fragments into smaller tar molecules, meanwhile, polymerizes smaller tar molecules into larger one 18. Only a small amount of naphthalene cracking products were detected by GC-MS analysis (the specific tar species can be seen in Figure 4). The inner surface of the reactor at isothermal section was found coated with a layer of dense soot when the temperature was 800 °C and higher. It indicates that naphthalene polymerization happens at higher temperatures and 800 °C is a transition temperature for soot formation. Results from Tesner et al.23 showed that naphthalene was of the highest soot formation tendency during thermal cracking process. The soot formation tendency of different hydrocarbons as relative to methane at 1623 K was calculated by formula N0/N0(CH4) (N0 refers to number of soot particles in 1 g of soot), considering with dispersion of soot. The values of different hydrocarbons were ranked from low to high as following: methane 1, ethylene 4, pxylene 4, toluene 5.5, benzene 7.4, acetylene 7.6, diacetylene 50, pyrene 74, anthracene 91 and naphthalene 112. The soot deposited on reactor wall was not detected by GC-MS, while small cracking tar products of naphthalene were sampled and identified by GC-MS. Results showed that the main products of tar were toluene, xylene (o-, m-, p-) and ethyl benzene. The first two species accounted for about 30% and 70% of products, respectively. As seen in Figure 4, the concentration of toluene slightly increased and that of xylene decreased with increasing temperature. The two main components (toluene and xylene) formed due to breaking of C-C bonds with aromatic ring opened and hydro-conversion. Soot precursor species (including anthracene, fluoranthene and pyrene etc.) derived from naphthalene cracking were not detected by GC-MS, due to their condensation on inner wall of sampling tube or adsorption by newly formed soot. Sánchez et al.24 investigated formation of


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PAHs and soot during acetylene pyrolysis. It was found that the amount of PAHs adsorbed on soot were much larger than that adsorbed on reactor wall and at outlet gas. It indicated that soot formation was in favor of conversion from PAHs to soot. Mathieu’s study

25

revealed that

adsorption of PAHs on soot was related to temperature and characteristics of soot formed. Conclusion was obtained that the smallest amount of adsorbed molecules of PAHs was found for the maximum soot yield. This might indicated that the PAHs adsorbed on soot could be converted to soot. An overall reaction for thermal cracking of naphthalene under inert atmosphere can be obtained in reaction 1. In reaction 1, CnHx represents tar species with smaller carbon atoms than naphthalene, CmHy represents tar species with larger carbon atoms than naphthalene, C represents soot which is solid phase. The results above demonstrates that both cracking and polymerization reactions exist in thermal cracking of naphthalene. Cracking reactions under oxidative atmosphere have been reported in some works 26, 27.

C10H8 → CnHx + CmHy + C + H2

(1)

3.2 Heterogeneous cracking of naphthalene Naphthalene conversion and hydrogen concentration as function of time and temperature, during the process of heterogeneous cracking of naphthalene over rice straw char, are shown in Figure 5. High inlet naphthalene concentration was chosen in conditions seen in Figure 5. Overall, the conversion efficiencies over rice straw char of all temperatures exhibit a gradual decline with time. In a specific period of time which is about 30 to 60 min, an increase in naphthalene conversion after a gradual decrease (“valley”) was observed in conditions of all temperatures. In works of Abu 7, the similar phenomenon can be seen. It was explained that the initial decrease in naphthalene conversion was due to deactivation of catalytic active site


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presented in biomass char. The subsequent increase was caused by gasification reaction of gasifying agent (CO2 and H2O) and biomass char or soot. However in present study, heterogeneous cracking of naphthalene took place in inert atmosphere without gasifying agent. As seen in Figure 5(b), the hydrogen concentration all decreased with time on char bed conditions. The release of hydrogen reflects the naphthalene polymerization reaction. The same “valley” phenomenon could not be found in period of 30-60 min of variation of hydrogen concentration. The phenomenon in present study is probably due to overall effect of carbon deposition and adsorption by newly formed soot. The newly formed soot can block the active sites of char, reduce its activity (carbon deposition) and further lower naphthalene conversion efficiency. Meanwhile, the soot itself can also adsorb naphthalene to increase conversion efficiency. The adsorption effect however could hardly counterbalance the previous effect. Thus the first half of the process exhibits a decreasing trend. When the active sites of char are almost all blocked, the soot formed can only promote naphthalene conversion by adsorption. Therefore the second half of the process shows an increasing trend. The naphthalene conversion efficiency over rice straw char increased with rising temperature. The conversion of 900 °C was close to that of 800 °C. When temperature was lower, the catalytic activity of rice straw char was lower and so was the corresponding conversion of naphthalene. When temperature was higher, the carbon deposition process was becoming severer to decrease catalytic activity of char. It was observed that a large amount of soot was formed on condition of homogeneous cracking at 900 °C, resulting in decrease in char activity and close results of 800 and 900 °C char bed conditions. Thus 800 °C is an economic and effective char bed temperature. As seen in Figure 3, the naphthalene conversion efficiency of homogeneous cracking at 800 and 900 °C, are 32.6 % and 44.8 % respectively. That is much lower than conversions in the presence


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of char at same temperatures, with average value of 77.0 % and 82.4 % respectively. The comparison also indicates the catalytic effects of char. It is shown in Figure 5(b) that, with increasing temperature, hydrogen concentration greatly increased, for the reason that higher temperature promoted dehydrogenation polymerization process. The gradual decline in naphthalene conversion and hydrogen concentration of all temperatures over the whole period of time was observed. These trends indicate that the catalytic activity of rice straw char decreases with extension of reaction time. Condition of 800 °C sand bed was chosen as comparison one. Its conversion was kept constant at much lower level (average value 36.1 %) over time compared with char bed conditions at 800 °C (average value 77.0 %). It also indicates that the inlet naphthalene concentration was also kept constant.

3.3 Naphthalene conversion over char in the presence of syngas Figure 6(a) shows variation of naphthalene conversion with time, in the presence of char and syngas at 800 °C. A mixture of four gases (H2, CO, CO2 and CH4) were used to stimulate the real atmosphere in biomass gasification process. Similarly, an increase in conversion after a gradual decrease are shown in Figure 6(a), followed by a gradual decrease till the end. However, the minimum value of the “valley” period appeared at around 90 min. The period is delayed compared with condition of 800 °C char bed with the absence of syngas. It may due to inhibition effect on the carbon deposition process by presence of syngas, especially CO2 and H2

28, 29

. The

average conversion of naphthalene (55.7 %) in the presence of syngas is lower than that (77.0 %) without syngas. It may due to shorter residence time in high temperature zone caused by increase of total flow rate. A comparison of gas concentrations before and after the char bed is necessary for analysis of the process. The gas concentrations before entering the char bed are: 7.45 %, 7.43 %, 8.58 % and


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6.24 %, for H2, CO, CO2 and CH4 respectively on the basis of volume fraction. Whereas gas concentrations after the char bed at 15 min are: 6.27 %, 8.53 %, 4.61 % and 4.31 %, for H2, CO, CO2 and CH4 respectively. The comparison demonstrates that, after heterogeneous reactions, CO concentration increased, CO2 and CH4 decreased considerably. These changes are possibly due to gasification reaction of biomass char (seen in reaction 2) and cracking reaction of methane (reaction 3). Hydrogen concentration decreased by a relatively small amount. This trend might because of dilution by increase in overall gas amount (reaction 2).

C + CO2 ↔ 2CO

∆Hθ273K = 172kJ / mol

(2)

CH4 ↔ C + 2H2

∆Hθ273K = 75kJ / mol

(3)

Figure 6(b) displays the gas concentration changes over time. As time passing, concentrations of CO and H2 decreased, while concentrations of CO2 and CH4 gradually increased. Because of cracking reactions of methane and naphthalene, carbon deposition on surfaces of biomass char becomes severer with time passing. Thus, the process of carbon deposition deactivates biomass char, further weaken gasification reaction of char (reaction 2) and cracking reaction of methane (reaction 3). The process of carbon deposition can well explain the phenomenon happened in Figure 6(b).

3.4 Effect of pre-treated char on naphthalene conversion Two types of char treated by Ni(NO3)2 solution and deionized water respectively were used to compare with original char on naphthalene reduction. Figure 7 shows naphthalene conversion at 800 °C, as a function of pretreatment of char and time. The variation of hydrogen over time is also demonstrated. As seen in Figure 7(a), the performances of three char at same temperature showed difference in tar conversion. At initial stage, sequence of naphthalene conversion over three char is: char


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treated by Ni(NO3)2 > original char > char treated by water. The corresponding conversion efficiencies at starting points are 96.6 %, 85.5 % and 69.4 % respectively. As time passing, three char all showed an increase after a gradual decrease process, which was previously explained as combination effect of carbon deposition and soot adsorption. After the “valley” process the conversions monotonously decreased. The char treated by Ni(NO3)2 is of the fastest decreasing rate among three char. Its conversion efficiency at 300 min is approaching to that of char treated by water, with conversions of 50.4% and 47% respectively. It can be deduced from the results that char treated by Ni(NO3)2 is of the highest catalytic activity at initial stage. However, its deactivation rate caused by carbon deposition is also the highest. After 50 min, naphthalene conversion over char treated by Ni(NO3)2 is lower than that over original char. This phenomenon indicates that biomass char is a stable catalyst for tar conversion. Hydrogen concentration of three types of char all monotonously decreased with time, as seen in Figure 7(b). Hydrogen concentration of char treated by Ni(NO3)2 is about double of that of the other two char. The large difference reveals that nickel has higher catalytic effect than alkali metal, which are rich in rice straw as seen in Table 2, on naphthalene cracking. Nickel is prone to be deactivated especially by carbon deposition 30, 31.

3.5 Inner pore structure characterization of char Figure 8 shows changes of specific surface area with time for rice straw char. On conditions of high inlet naphthalene concentration (25.2 g/Nm3), the specific surface area of rice straw char quickly decreased at initial stage. At 800 °C, the specific surface area fell sharply from 262 m2/g to 4.6 m2/g for 5 min after beginning. Then the specific surface area gradually increased with time, with the value of 63.8 m2/g for 120 min after beginning. The trend indicates that some new inner pore structures of char had gradually formed to enlarge the overall specific surface area.


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Conditions of other temperatures with same inlet naphthalene concentration show the similar trend. Furthermore, the increase after a sharp drop in specific surface area with higher temperature is larger than that with lower temperature. The rapid loss of specific surface area at initial stage is due to polymerization reaction of naphthalene. During the process, the active site on surface of char is quickly coated with soot, leading to catalyst deactivation and decrease in conversion. The process of the increase after sharp drop in specific surface area is probably explained by the fact that appropriate extending of residence time for char at high temperature is in favor of micropores formation. The fresh formed micropores greatly contribute to the increase in specific surface area of char. Therefore, during the rest of period (30-120 min), the fresh micropores and active sites were gradually formed for the initially deactivated char. On condition of low inlet naphthalene concentration (8.25 g/Nm3), variation of specific surface area is relatively smaller than that of high concentration conditions. As seen in Figure 8, at 800 °C, the specific surface area of biomass char gradually decreased from beginning of 245 m2/g to 215 m2/g for 10 min. Then it increased to 251 m2/g at 30 min. With time further proceeding, the specific surface area decreased till end of the test, with final value of 11.7 m2/g at 120 min. After the test was finished, the char no longer had abundant micropores and complex pore structures, due to the severe pore blocking and carbon deposition. Figure 9 shows the specific surface area increment distribution with pore diameter for char on conditions of 800 °C and low naphthalene feeding. Reduction in specific surface area mainly affects mesopores and micropores with pore diameters of less than 10 nm. However, the reduction in specific surface area has negligible influence on macropores (larger than 50 nm). Catalytic conversion of tar mainly happens in micropores

32

, while mesopores and macropores


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are chiefly for diffusion channels of reactants and products 33. In present tests, biomass char with low naphthalene concentration (8.25 g/Nm3) is effective for 60 min before deactivation, whereas char with high naphthalene concentration (25.2 g/Nm3) is only effective for 5 min (with respect to specific surface area). Decrease in reactivity and specific surface area of char are primarily caused by carbon deposition. The rate of carbon deposition is affected by tar concentration and temperature. Therefore it is feasible to keep biomass char effective (not deactivated) for a certain long time and maximum its ability for tar removal by appropriately control of the inlet tar concentrations and temperature for char bed in real process of biomass gasifier. Carbon deposition on active sites is the main pathway of catalytic decomposition of tar over biomass char, based on present experimental results and other researches 15, 16. Catalytic steam or dry reforming of tar is considered as another important pathway 7. The homogeneous pathways of polymerization, steam reforming and partial oxidation of naphthalene were reported elsewhere 28, 34

. The sequence of adsorption, reaction and desorption and active site are basic concepts for

heterogeneous catalysis 35. Liu et al. 36 reported the adsorption mechanism of phenol molecule by N, O-containing function group on surface of bio-char. The iron oxides and potassium were reported to be responsible for active sites on catalytic tar removal

37, 38

. Xu et al. concluded the

mechanism for catalytic cracking of toluene over Ni catalyst supported on Al2O3 or carbon. Detail steps includes: 1) Adsorption of toluene and gases on metal surfaces; 2) Dissociation of toluene and gases into radicals; 3) Desorption and reactions of radicals 33. Activated carbon was reported to be catalytically reactive for removing hydrogen atom from hydrocarbons to generate free radicals

39

. Based on above discussion, a probable mechanism for catalytic cracking of

naphthalene over biomass char has been considered. Naphthalene molecules are adsorbed on


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active sites of surface of biomass char, then they are catalytically dissociated into radicals of naphthyl and hydrogen. Then the radicals formed are desorbed and react with each other. When it is for inert atmosphere, radicals of naphthyl react with each other through polymerization reaction to produce soot precursor and finally soot. When there are plenty of oxidative species (CO2, H2O) in atmosphere, they are similarly adsorbed and dissociated into free radicals (O, OH, H). Then the oxidative radicals react with naphthyl to generate mono ring aromatics, aliphatic hydrocarbons and finally light hydrocarbons (C1, C2) and gases. Further detailed investigations are needed to figure out the mechanism of catalytic tar removal over bio-char.

4 Conclusion On conditions of homogeneous cracking with high tar inlet concentration (25.2 g/Nm3), naphthalene conversion increased from 26.2 % to 63.5 %, with rising temperature from 700 °C to 1000 °C. Soot and hydrogen are main products of cracking of naphthalene by polymerization reactions, especially when temperature is higher than 800 °C. Some smaller cracking products were also found, such as toluene, xylene and ethyl benzene. In tests of heterogeneous cracking with high tar inlet concentration, naphthalene conversion is much higher than that of homogeneous cracking at same temperature. The comparison indicates the catalytic effect of biomass char on tar removal. The average naphthalene conversion efficiency is higher with increasing temperature. Overall, conversions exhibit gradually decreasing trend over time. These trends are due to deactivation of biomass char. However, in a certain period (30-60 min), conversions show an increase after gradual decrease phenomenon which is probably due to competition between carbon deposition and model tar adsorption by soot. Besides, addition of syngas could effectively delay the deactivation of biomass char.


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Biomass char is a more effective and stable catalyst for tar removal, compared with char treated with water and Ni(NO3)2 solution respectively. Characteristics of inner pores of biomass char during model tar removal process were investigated. The rapid loss of specific surface area for char happens at initial stage with high tar inlet concentration. For instance it takes only 5 min for its decrease from 262 m2/g to 4.6 m2/g at 800 °C. The char with low tar inlet concentration is less prone to deactivation and remains high specific surface area for 60 min at 800 °C. Moreover, reduction in specific surface area mainly affects mesopores and micropores with pore diameters of less than 10 nm.

Acknowledgements The authors are grateful to the Science and Technology Commission of Shanghai Municipality for its financial support (under Grant 05dz12010).


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References (1) Chang, M.; Du, W.; Li, S.; etc. A study of potential and constraints of the biomass sector in China from the Europe-China clean energy centre (ec2). Presented at the 20th European Biomass Conference and Exhibition, Milan, Italy, June 18-22, 2012; Paper Number: 5BO.12.5. (2) Adams, W. N.; Gaines, A. F.; Gregory, D. H.; Pitt, G. J. Nature 1959, 183, 33. (3) Bond, A. L.; Godridge, A. M.; Murnaghan, A. R.; Napier, D.H.; Williams, D. J. Nature 1959, 184, 425-426. (4) Griffiths, D. M. L.; Mainhood, J. S. R. Fuel 1967, 46, 167-176. (5) Henriksen, U.; Ahrenfeldt, J.; Jensen, T. K.; Gøbel, B.; Bentzen, J. D.; Hindsgaul, C.; Sørensen, L. H. Energy 2006, 31, 1542–1553. (6) Brandt, P.; Larsen, E.; Henriksen, U. Energy Fuels 2000, 14, 816–819. (7) Abu El-Rub, Z. Biomass Char as In-situ Catalyst for Tar Removal in Gasification Systems. Ph.D. Dissertation, Twente University, Enschede, Netherlands, 2008. (8) Chembukulam, S. K.; Dandge, A. S.; Kovilur, N. L.; Seshagiri, R. K.; Vaidysewaran, R. Ind. Eng. Chem. Prod. Res. Dev. 1981, 20, 714-719. (9) Ekström, C.; Lindman, N.; Pettersson, R. Catalytic conversion of tars, carbon black and methane from pyrolysis/gasification for biomass. In Fundamentals of Thermochemical Biomass


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Conversion; Overend, R. P., Milne, T. A., Mudge, L., Eds.; Elsevier Science Ltd.: Oxford, U.K. 1985; pp 601-618. (10) Boroson, M. L.; Howard, J. B.; Longwell, J. P.; Peters, W.A. Energy Fuels 1989, 3, 735740. (11) Morf, P.O. Secondary Reactions of Tar during Thermochemical Biomass Conversion. Ph.D. Dissertation, Swiss Federal Institute of Technology Zurich, Swizerland, 2001. (12) Wu, W. G.; Luo, Y. H.; Su, Y.; Zhang, Y. L.; Zhao, S. H.; Wang, Y. Energy Fuels 2011, 25, 5394-5406. (13) Gilbert, P.; Ryu, C.; Sharifi, V.; Swithenbank, J. Bioresour. Technol. 2009, 100, 60456051. (14) Abu EI-Rub, Z.; Bramer, E. A.; Brem, G. Fuel 2008, 87, 2243-2252. (15) Hosokai, S.; Kumabe, Kazuhiro.; Ohshita, M.; Norinaga, K.; Li, C. Z.; Hayashi, J. I. Fuel 2008, 87, 2914-2922. (16) Fuentes-Cano, D.; Gómez-Barea, A.; Nilsson, S.; Ollero, P. Chem. Eng. J. 2013, 228, 1223-1233. (17) Juneja, A.; Mani, S.; Kastner, J. Catalytic cracking of tar using biochar as a catalyst. Presented at American Society of Agricultural and Biological Engineers Annual International Meeting, Pittsburgh, Pennsylvania, USA, June 20-23, 2010; Paper Number: 1009863. (18) Jess, A. Fuel 1996, 75, 1441-1448.


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(19) Simell, P.; Ståhlberg, P.; Kurkela, E.; Albrecht, J.; Deutsch, S.; Sjöström, K. Biomass Bioenergy 2000, 18, 19–38. (20) van Paasen, S. V. B.; Kiel, J. H. A.; Neef, J. P. A.; Knoef, H. A. M.; Buffinga, G. J.; Zielke, U.; Sjöström, K.; Brage, C.; Hasler, P.; Simell, P. A.; Suomalainen, M.; Dorrington, M. A.; Thomas, L. Guideline for sampling and analysis of tar and particles in biomass producer gases. Final Report Number: ECN-C-02-090; Energy research Centre of the Netherlands: Petten, the Netherlands, November, 2002. (21) Wu, W.G.; Luo, Y. H.; Chen, Y.; Su, Y.; Zhang, Y. L.; Zhao, S. H.; Wang, Y. Energy Fuels 2011, 25, 2721-2729. (22) Zhang, R. Q.; Brown, R. C.; Suby, A.; Cummer, K. Energy Convers. Manage. 2004, 45, 995-1014. (23) Tesner, P. A.; Shurupov, S. V. Combust. Sci. Technol. 1997, 126, 139-151. (24) Sánchez, N. E.; Callejas, A.; Millera, A.; Bilbao, R.; Alzueta, M. U. Energy 2012, 43, 3036. (25) Mathieu, O.; Frache, G.; Djebaïli-Chaumeix, N.; Paillard, C. E.; Krier, G.; Muller, J. F.; Douce, F.; Manuelli, P. Proc. Combust. Inst. 2007, 31, 511-519. (26) Devi, L.; Ptasinski, K. J.; Janssen, F. J. J. G.; Paasen, S. V. B. V.; Bergman, P. C. A.; Kiel, J. H. A. Renewable Energy 2005, 30, 565-587. (27) Abu EI-Rub, Z.; Bramer, E. A.; Brem, G. Ind. Eng. Chem. Res. 2004, 43, 6911-6919. (28) Garcia, X. A.; Hüttinger, K. J. Fuel 1989, 68, 1300-1310.


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List of captions Figure 1. Schematic diagram of micro reactor Figure 2. Temperature distribution in reactor tube Figure 3. Results for homogeneous cracking of naphthalene under different temperatures, conditions: naphthalene carrier gas 10ml/min, water bath 95 °C, compensatory gas 2 ml/min Figure 4. Tar components distribution of naphthalene cracking Figure 5. Results for conversion of naphthalene over rice straw char under different temperatures, conditions: naphthalene carrier gas 10ml/min, water bath 95 °C, compensatory gas 2 ml/min, (a) naphthalene conversion along with time (b) hydrogen concentration along with time Figure 6. Results for conversion of naphthalene over rice straw char at 800 °C in the presence of syngas, conditions: naphthalene carrier gas 10ml/min, water bath 95 °C, compensatory gas 2 ml/min, inlet syngas 7.2 ml/min,( H2: 25.09 %, CO: 25.02 %, CO2: 28.89 %, CH4: 21 %) (a) naphthalene conversion along with time (b) gas concentration along with time


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Figure 7. Results for conversion of naphthalene over rice straw char under different pre-treated char at 800 °C, conditions: naphthalene carrier gas 10 ml/min, water bath 95 °C, compensatory gas 2 ml/min, (a) naphthalene conversion along with time (b) hydrogen concentration along with time Figure 8. Rice straw char BET specific surface area variation along with time under different temperatures and inlet naphthalene concentrations (-low: naphthalene carrier gas 2 ml/min, water bath 90 °C, compensatory gas 10 ml/min; -high: naphthalene carrier gas 10 ml/min, water bath 95 °C, compensatory gas 2 ml/min) Figure 9. Specific surface area increment distribution along with pore diameter for rice straw char of different time at 800 °C, conditions: naphthalene carrier gas 2 ml/min, water bath 90 °C, compensatory gas 10 ml/min Table 1. Proximate and Ultimate Analysis of Rice Straw Table 2. Minerals content in rice straw material, wt. %


Energy & Fuels

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Ar

Bypass

MFC PI 3

CO

MFC

MFC 1 MFC

TIC

9

6

MFC

CO2

PI

4

TIC

H2

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2

1 Mass flow controller 2 Check valve 3 Model tar generator 4 Preheater and mixer 5 Furnace 6 Quartz tube 7 Char bed 8 Sampling pipe 9 GC-TCD/FID analyzer 10 GC/MS analyzer

GC/TCD GC/FID

5 8

Vent

Sample

7 TIC

Figure 1. Schematic diagram of micro reactor

Figure 2. Temperature distribution in reactor tube


10 293K

253K GC/MS

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Figure 3. Results for homogeneous cracking of naphthalene under different temperatures, conditions: naphthalene carrier gas 10 ml/min, water bath 95 °C, compensatory gas 2 ml/min

Figure 4. Tar components distribution of naphthalene cracking


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(a) (b) Figure 5. Results for conversion of naphthalene over rice straw char under different temperatures, conditions: naphthalene carrier gas 10 ml/min, water bath 95 °C, compensatory gas 2 ml/min, (a) naphthalene conversion along with time (b) hydrogen concentration along with time

(a) (b) Figure 6. Results for conversion of naphthalene over rice straw char at 800 °C in the presence of syngas, conditions: naphthalene carrier gas 10ml/min, water bath 95 °C, compensatory gas 2 ml/min, inlet syngas 7.2 ml/min,( H2: 25.09 %, CO: 25.02 %, CO2: 28.89 %, CH4: 21 %) (a) naphthalene conversion along with time (b) gas concentration along with time


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(a) (b) Figure 7. Results for conversion of naphthalene over rice straw char under different pre-treated char at 800 °C, conditions: naphthalene carrier gas 10 ml/min, water bath 95 °C, compensatory gas 2 ml/min, (a) naphthalene conversion along with time (b) hydrogen concentration along with time

Figure 8. Rice straw char BET specific surface area variation along with time under different temperatures and inlet naphthalene concentrations (-low: naphthalene carrier gas 2 ml/min, water bath 90 °C, compensatory gas 10 ml/min; -high: naphthalene carrier gas 10 ml/min, water bath 95 °C, compensatory gas 2 ml/min)


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

Figure 9. Specific surface area increment distribution along with pore diameter for rice straw char of different time at 800 °C, conditions: naphthalene carrier gas 2 ml/min, water bath 90 °C, compensatory gas 10 ml/min Table 1. Proximate and Ultimate Analysis of Rice Straw Proximate analysis a (wt.% as received) Moisture

13.45

Volatiles

62.79

Fixed carbon (by diff.)

15.92

Ash

7.84

Ultimate analysis (wt.% dry ash free) Cb

35.58

Hb

4.63

O (by diff.)

37.36

Nc

0.94

Sd

0.2


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Energy & Fuels

Q e (MJ/kg) 40

14.16

a

GB/T 212-2008

b

GB/T 476-2008

c

GB/T 19227-2008

d

GB/T 214-2007

e

High heating calorific value

Table 2. Minerals content in rice straw material, wt. %

Rice straw

Al

Ca

Fe

K

Mg

Na

Si

0.0545

0.3772

0.356

1.9998

0.18

0.1926

0.0705


Heterogeneous Cracking Reaction of Tar over ... - ACS Publications

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