Catalytic Steam Reforming of Biomass-Derived Tar over the Coal

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Biofuels and Biomass

Catalytic steam reforming of biomass-derived tar over the coal/biomass blend char: effect of devolatilization temperature and biomass type Suwat Mueangta, Prapan Kuchonthara, and Supachita Krerkkaiwan Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.9b00320 • Publication Date (Web): 27 Mar 2019 Downloaded from http://pubs.acs.org on April 1, 2019

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Catalytic steam reforming of biomass-derived tar over the coal/biomass blend char: effect of devolatilization temperature and biomass type Suwat Mueangta†, Prapan Kuchonthara†, ‡,§, Supachita Krerkkaiwan‖,*

† Department

of Chemical Technology, Faculty of Science, Chulalongkorn University, Bangkok 10330, Thailand.

‡ Center

of Excellence on Petrochemical and Materials Technology, Chulalongkorn University, Bangkok 10330, Thailand.

§

Center of Excellence in Catalysis for Bioenergy and Renewable Chemicals (CBRC), Chulalongkorn University, Bangkok 10330, Thailand. ‖The

Joint Graduate School of Energy and Environment (JGSEE), King Mongkut's University of Technology Thonburi, Bangkok 10140, Thailand.

*Corresponding

author. Tel.: +66 2872 9014; Fax: +66 2872 6978;

E-mail address: [email protected], [email protected] Graphical Abstract

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ABSTRACT Catalytic tar steam reforming over char was conducted in a two-stage fixed bed reactor. The reactor was divided into two zones, “devolatilization zone”, where the biomass tar is released, and “tar reforming zone”, where the tar and char are contacted and reformed into the products. Temperature of tar reforming zone was maintained at 800 ºC for all experiments. The effects of devolatilization temperature (600, 700, and 800 ºC) and biomass type (rice straw, RS and Leucaena leucocephala wood, LN) on the catalytic performance of char during tar steam reforming were investigated. Biomass chars (RSC and LNC) and coal/biomass blended chars (C/RS and C/LN) were prepared as the catalyst for tar steam reforming. Results revealed that compared with biomass char, C/RS and C/LN performed a better catalytic activity in tar steam reforming, resulting in a higher carbon conversion into gas and a lower carbon conversion into tar. Main catalytic roles of the coal/biomass blend char can be explained by the improvement of the BET surface area during co-pyrolysis and the existence of alkali and alkaline earth metallic species, particularly K, as the stable silicate form. Considering the effect of devolatilization temperature, the tar released at 700 ºC can be mostly converted among all types of char. Tars containing the mixture of phenolic compounds (i.e. phenol, methyl phenol) and aromatic compounds (i.e. naphthalene, methyl naphthalene) were favorable to reform over the char surface to achieve the relatively high tar conversion. In addition, LN tar was easier to convert than the RS tar in both cases of with and without char because of the lower proportion of a stable aromatic compound. Outcome of this study can be beneficial for the design and operation of the tar removal process during biomass gasification using the low-cost carbon-based catalyst. Keywords: tar decomposition, coal/biomass blended char, catalytic tar reforming, biomass, steam reforming


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1. INTRODUCTION Tar is the main drawback of the biomass gasification process. It contaminates gas products and causes blockage or plugging problems, resulting in lower process efficiency. Generally, the tar removal process can be divided into two approaches; primary method, which the tar was removed inside the gasifier, and the secondary method which a tar cracking or tar steam reforming unit was installed after the gasifier1. The catalytic tar cracking or tar reforming is the most effective in tar reduction, especially in the large-scale application, because of its fast reaction rate, high reliability2. Nickel-based catalyst is the most prevailing catalyst for tar cracking unit because it has a high tar conversion and gas production. However, it has not been given much attention recently because of its rapid deactivation and relatively high cost. Carbon-based catalysts, such as coal char and biomass char, have been given attention for tar cracking in the previous decades because of their reasonable cost and also had the good catalytic performance on tar conversion3-5. In our previous studies 3, 6, coal char and biomass char showed a high catalytic performance in tar cracking and tar steam reforming, producing a higher carbon conversion into gas and relatively high H2 and CO yields compared to the absence of catalyst. The important aspects of the catalytic performance of the char on tar reduction were previously summarized into two properties: surface area of the char and content of alkali and alkaline earth metallic species (AAEMs) on the char surfaces3, 7, 8. The co-pyrolysis of coal and biomass was previously reported as the way to increase the catalytic performance of the char for tar reduction in terms of increasing the surface area in comparison with the pure coal and biomass chars9, 10. Moreover, the reactivity of the co-pyrolysis char under steam could increase the gasification rate and affect the catalytic activity of the char in tar cracking and tar steam reforming6, 11, 12.


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Aside from the properties of the catalyst, the chemical composition of tar is also influenced on the tar conversion and gas product composition during tar steam reforming13, 14. In biomass gasification, pyrolysis is always the first step to produce the primary products (tar, char, and gas), followed by tar cracking or tar reforming to generate the gaseous product in the second step. In the first step, such parameters as temperature, residence time and biomass type are the important factors that determine the tar composition. Many studies revealed the effect of pyrolysis temperature on tar composition and pyrolysis product yield15, 16. Nevertheless, none of the studies focused on the effect of tar composition on the catalytic performance of the char in tar decomposition or tar steam reforming. Consequently, in this work, the steam reforming of biomass-derived tar over the pyrolysis char was conducted in a two-stage fixed bed reactor. Rice straw (RS) and Leucaena leucocephala wood (LN) were selected as the biomass in this study because of their high potential in Thailand. The reactor was divided into two stages: the upper part called the devolatilization zone, where the biomass tar is released, and the lower part called the tar reforming zone, where the biomass tar is reformed into gaseous products over the char bed. The effect of temperature in the devolatilization zone on tar composition was examined. Moreover, four types of char were prepared and used as the catalysts for tar steam reforming including two types of biomass char (RSC and LNC) and two types of coal/biomass blend char (C/RS and C/LN). The effects of tar composition and char type on tar steam reforming in terms of tar conversion and gas product composition were investigated. The catalytic mechanism of each catalyst for tar steam reforming was also proposed. 2. MATERIALS AND METHODS 2.1 Coal, Biomass and Char Preparation Indonesian (sub-bituminous) coal, rice straw (RS) and Leucaena leucocephala wood (LN) were ground, sieved to 150–250 m size, oven dried


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at 110 oC for 6 h, and kept in a desiccator before use. The proximate and ultimate analyses of the sample are presented in Table 1. Table 1 Proximate and ultimate analyses of raw coal and biomass samples

Coal had higher carbon and lower oxygen contents than the biomass. LN had a lower ash content and a higher carbon content than RS. Four types of char, including pure biomass chars (RSC and LNC) and coal/biomass blend chars (C/RS and C/LN), were prepared from a slow heating pyrolysis at 600 ºC with a holding time 60 min in a typical fixed-bed reactor. The details of this process were explained elsewere9. C/RS and C/LN were prepared from the copyrolysis of the coal and biomass at coal to biomass blending ratio of 1:1 by weight. 2.2 Tar Steam Reforming in a Two-Stage Fixed Bed Reactor The steam reforming of biomass-derived tar was conducted in a two-stage fixed bed reactor, as presented in Figure 1.


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Figure 1 Schematic diagram of a two-stage fixed bed reactor consisting of the following: (1) N2 cylinder, (2) mass flow controller, (3) distillated water reservoir, (4) HPLC pump, (5) steam temperature controller, (6) electric furnaces, (7) quartz reactor, (8) sample feeder, (9) ice–tar trap, (10) bubble flow meter, and (11) moisture trap (filled with silica gel) The reactor consisted of an inner tube (ID: 9 mm, length: 60 cm) and an outer tube (ID: 19 mm, length: 89 cm), and it was divided into two parts, each with individual furnaces and temperature controllers (Carbolite model MTP 12 and Lenton model LTF 12 for the upper and lower part, respectively). The upper part is called the devolatilization zone, where the biomass pyrolysis takes place and the volatiles (or tars) are generated. The lower part is called the tar reforming zone, where the prepared char or inactive alumina ball (inert bed) is located, the biomass volatiles from the upper part are contacted with the char and the tar reforming occurs. About 0.5 g of the prepared char or 7.5 g of inert bed was packed at the lower part to give an approximate bed height of 2 cm. Then, N2 was fed into the reactor with flow rates of 80 and 30 mL min-1 for the inner and outer tubes, respectively. The residence time of the volatiles through the char bed was approximately 0.3 s. After the N2 purging for 1 h, the upper electric furnace for the devolatilization zone was heated up to the desired temperature (i.e., 600 oC,

700 oC, or 800 oC), while the temperature of the tar reforming zone was kept at 800 ºC for 6 ACS Paragon Plus Environment

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all experiments. Water with a flow rate of 0.14 µL min-1 was heated at 300 ºC. The hot steam was introduced into the lower part of the reactor at a steam-to-N2 volume ratio of 60:40 only in the outer tube. Then, 120 mg of fresh biomass was dropped into the inner tube at the upper part to enable the biomass pyrolysis to take place. The pyrolyzed residue remained over the quartz wool filter inside the inner tube. Only the biomass volatiles were allowed to pass through the filter, move to the lower part, and contact with the steam and the located char. Then, the catalytic tar reforming took place. A mass ratio of biomass feed and char was kept constant at 1:4.2 (g:g) for all experiments. For collecting the tar, a series of three impinger bottles filled with isopropanol were used. Some of 6 mm diameter-round glass beads were put in the first impinge bottle to enhance the capability for recovering the condensable compounds. The impinger bottles were put in an ice-mix-salt cooling bath at around -15ºC. Non-condensed (gaseous) products were then collected in a 2 L-gas bag for quantitative analysis by gas chromatography. The gas collection bag was changed every 10 min during 1 h of reaction time. The gas products generated from the char itself without the biomass feeding were also evaluated in the same manner and used as the reference data (blank experiment). Carbon conversion into gas was determined following Eq. (1):

 C in product gas  Carbon conversion  %     100 ,  C in biomass feed 

(1)

where C in product gas and C in biomass feed represent the moles of carbon in the gaseous product and in the biomass feed, respectively. The carbon conversion into char was obtained from the carbon content of the biomass char remaining in the inner tube in upper stage by a CHN analyzer. The carbon conversion into tar was calculated following Eq. (2):

Carbon conversion into tar  %   100  Cgas  Cchar ,

(2) 7

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where Cgas and Cchar represent the carbon conversion into gas and char, respectively. To evaluate the total amount of released tar from the devolatilization zone (upper part), the pyrolysis experiments of raw biomass (both RS and LN) were also performed in the same manner with the steam reforming testing but without the steam feeding and the presence of char. Carbon conversion into tar from the pyrolysis experiment was measured to further determine the tar conversion. Tar conversion by steam reforming was obtained by Eq. (3): C  Ctar ( steam ) Tar conversion  %    tar ( pyro )  Ctar ( pyro ) 

   100 , 

(3)

where Ctar ( pyro ) and Ctar ( steam ) represent the carbon conversion into tar from the pyrolysis experiment (without steam and char) and from the tar steam reforming experiment, respectively. 2.3 Characterization of the Sample and Products The proximate and ultimate analyses of raw coal, raw biomass, and char samples were performed following ASTM D3172-3175 and using a CHN analyzer (LECO CHN-2000), respectively. The specific surface area, average pore volume, and average pore size of the chars were measured by N2 adsorption at −196 °C using the BET method (model Quantachrome, Autosorb-1; instrument accuracy ± 0.11%). The samples were first degassed at 300 ºC for 12 h prior to the N2 adsorption. The mineral analysis of the chars was performed by the X-ray fluorescence (XRF; BRUKER model S8Tiger) technique. The gas product, which mainly comprised H2, CO, CH4, and CO2, was quantitatively analyzed by gas chromatography using a Shimadzu GC-2014 model with a thermal conductivity detector and a Unibeads C column (3.00 mm ID × 200 cm length). Some of the condensable tar in the ice–tar trap was analyzed to determine the chemical composition using the gas chromatography–mass spectrometry (GC-MS) on a Shimadzu Model QP2010 equipped with a DB-5 ms capillary column (0.25 mm OD × 0.25 mm film 8 ACS Paragon Plus Environment

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thickness × 30 m length; J & W Scientific) and with helium as the carrier gas. The molecular weight scanning range was set to 30–500 m/z with a 3.5 min solvent cut-off time. The column was maintained at 40 °C for 5 min, and the temperature was increased to 200 °C, with a heating rate of 10 °C min-1, and held for 25 min. 3. RESULTS AND DISCUSSION 3.1 Characterization of the prepared chars Table 2 shows the physical properties of the prepared chars. Compared with the biomass chars, the coal/biomass blended char had a higher carbon content but a lower oxygen content, resulting in a lower O/C molar ratio in both cases of C/RS and C/LN. It was due to the proportion of coal in the blended chars. Table 2 Physical properties of the prepared chars

Consider the mineral analysis that was reported on the basis of the char sample, the coal/biomass blended char had a lower K and Ca content than the biomass chars. This result can be attributed to the transformation of the allocated minerals, particularly the alkali and alkaline earth metallic species (AAEMs), during the co-pyrolysis of coal and biomass9. The 9 ACS Paragon Plus Environment

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K/Si molar ratio is an indicator of predicting the formation of K on the char surface. The low K/Si molar ratio may be due to the existing stable K in silicate form (K-Si)17, 18. The coal/biomass blended chars had a lower K/Si molar ratio rather than the biomass chars. It indicates that the K on the surface of C/RS or C/LN could exist in the silicate form rather than the reactive K bonded with the organic structure of the char. In other words, the K-O-C form is likely presented in the biomass chars (RSC and LNC). The surface properties of the prepared chars were also reported as the average BET surface area, total pore volume, and average pore size. The coal/biomass blended chars had a higher BET surface area and total pore volume than the biomass chars in both cases of C/RS and C/LN. This result indicates the synergetic effects during the co-pyrolysis of coal and biomass, resulting in the increased BET surface area of the co-pyrolyzed char9. Both the surface properties and the AAEMs transformation of the chars are the important factors influencing the catalytic performance in tar steam reforming. 3.2 Effect of devolatilization temperature on tar composition The composition of rice straw-derived tar (RST) at different devolatilization temperatures was characterized by the GC-MS technique. The result is summarized in Table 3. At the devolatilization temperature of 600 ºC, RST600 mainly consisted of the oxygenated hydrocarbons and benzene derivatives with a high value of relative peak intensity (4%–7%), such as furfural, phenol, methyl phenol, 1,2-benzenediol, and 2,3-dihydrobenzofuran. When the devolatilization temperature increased to 700 ºC, the main composition of tar changed into the mixture of phenolic compounds (phenol and methyl phenol) and aromatic hydrocarbons (naphthalene and 2,3dihydro, benzofuran). At the devolatilization temperature of 800 ºC, the composition of RST definitely changed. Naphthalene was the main composition of RST800, with an obviously high relative intensity of 17.2%. Some of the benzene derivatives, such as styrene, phenol, and indene, were also found to be the main compositions of RST800. This finding agrees


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well with the previous literature concluding that at a higher devolatilization temperature, the decomposition of lignin was promoted to generate a high concentration of aromatic compound13, 19. Table 3 Composition of rice straw-derived tar (RST) by GC/MS at different devolatilization temperatures

The composition of Leucaena leucocephala wood-derived tar (LNT) at different devolatilization temperatures was also characterized, as summarized in Table 4. LNT600 was


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mostly composed of the furfural and benzene derivatives such as 1,2-benzenediol and some phenolic compounds (i.e., phenol and methyl phenol). Table 4 Composition of Leucaena leucocephala derived-tar (LNT) by GC/MS at different devolatilization temperatures

When the devolatilization temperature increased to 700 ºC, the LNT700 gave a high peak intensity of phenol (11.73%), indene (8.5%), and naphthalene (7.4%). At 800 ºC, the relative peak intensities of naphthalene and indene significantly increased to 21.2% and 14.8%, respectively. Some of the polyaromatic hydrocarbons (PAHs), such as


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acenaphthylene, fluorene, and phenanthrene, were also observed. The different compositions of RST and LNT released from the different devolatilization temperatures will be affected in the tar steam reforming, as discussed in Section 3.4. 3.3 Effect of char as the catalyst for tar steam reforming The effect of char types on the carbon conversion from the steam reforming of biomass-derived tar at a devolatilization temperature of 800 ºC is shown in Fig. 2. Note that the presented gas product and the carbon conversion into gas were already subtracted from the gas generated from the char itself.

Figure 2 Effect of char type on the carbon conversion from the steam reforming of rice straw and LN wood at the devolatilization temperature of 800 ºC With the presence of chars, the carbon conversion into tar was obviously decreased while the carbon conversion into gas became higher in comparison with the presence of inert bed (without chars). It indicates that char can act as the catalyst for tar steam reforming for both the RS and LN feeding systems. Considering the effect of the char type, the coal/biomass blended char (C/RS and C/LN) possessed a better catalytic performance than the biomass chars (RSC and LNC), leading to a higher carbon conversion into gas and a lower carbon conversion into tar. The surface area and the formation of AAEMs on the char surfaces are the two important properties of char that are likely related to the catalytic activity 13 ACS Paragon Plus Environment

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for tar decomposition. As mentioned in section 3.1, the coal/biomass blended char had the higher BET surface area than the biomass chars of C/RS and C/LN. A key step in tar reduction, the enhancement of the surface area of the co-pyrolysis char could promote the tar deposition on the porous surfaces of the char. From previous studies20, the catalytic effect of char on tar decomposition was reported as a two-step mechanism. First, tar was deposited on the porous surface of the char to generate the coke or solid carbon. Second, the coke was gasified into gaseous products by the pyrolytic steam or external steam with the assistance of AAEMs. In terms of AAEMs, K is the main catalytic species to promote the tar and char steam reforming21-23. As shown in the AAEMs analysis of the char in section 3.1, K in cases of C/RS and C/LN could exist as the stable silicate form (K-Si) because of the low K/Si molar ratio (Table 2.). Conversely, K in the cases of RSC and LNC could present in the form of reactive K bonded with the carbon structure of char or the phenolate group (i.e., K-O-C). Therefore, K in the phenolate form is easier to volatilize during the steam reforming condition than K-Si24. In addition, the volatilization of AAEMs on the char surface was probably promoted by the existence of hydrogen radicals from the volatiles and steam, especially alkali metals (i.e., Na and K25), as described in Eq. (4):

CM  X  H  CM  H  X ,

(4)

where CM, H, and X represent the carbon/char matrix, hydrogen radical from the volatile, and AAEM species, respectively. To confirm the loss of K after the tar steam reforming, the K and Si contents of the spent C/LN and LNC were characterized by the XRF technique. The result revealed that the spent LNC had K 1.76 and Si 0.98 wt% and K/Si molar ratio of 1.28. The K/Si molar ratio of the spent LNC was significantly lower than that of the fresh LNC (K/Si = 2.96). This result confirmed the loss of K during the tar steam reforming in the case of LNC. Conversely, the spent C/LN had K 0.57 and Si 2.57 wt% and K/Si molar ratio of


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0.16. These values are close to those of fresh C/LN, as reported in Table 2. The K species in the case of C/LN was more stable than that in the LNC, thus preventing the loss of K during tar steam reforming. Therefore, the catalytic role of the coal/biomass blended chars is the combination effect between the enhancement of the BET surface area during the co-pyrolysis and the existence of K-Si forms on the char surfaces. This result is in good agreement with our previous study6, which also reported the high catalytic performance of the co-pyrolysis char between coal and wood in biomass-derived tar decomposition. The effect of char type on gas production from the tar steam reforming in both RS and LN feeding is illustrated in Fig. 3.

Figure 3 Effect of char type on gas production from the steam reforming of rice straw and LN wood at the devolatilization temperature of 800 ºC With the presence of all the chars, all of the gas species (H2, CO, CH4, and CO2) dramatically increased, especially H2. As mentioned above, the first step of the catalytic tar steam reforming over the char surface was the tar deposition or coke formation. At this step, the coke was formed via the condensation reaction of the hydrocarbon radicals in the released volatile over the char surfaces and H2 was consequently generated. Then, coke gasification 15 ACS Paragon Plus Environment

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was performed following Eq. (5) to produce the gas product species:

Ck  H 2O  H 2  CO  CO2  hydrocarbon ,

(5)

where Ck is the formed coke during the reaction. Considering the effect of char type, the presence of C/RS provided a higher total gas production, particularly H2 and CO production, than the presence of RSC. It could be due to the higher BET surface area of the C/RS that could induce the tar deposition to generate coke on the C/RS porous surface. The formed coke was then gasified by steam associated with the catalytic role of K in the K-Si form. In the case of RSC, the BET surface area was lower than that of the C/RS, thus leading to the lower amount of coke formation that causes a lower gas production. However, H2 and CO were the main gas product components in case of RSC. This indicates that the carbon steam reforming (in Eq. (5)) is the main mechanism reaction to generate gaseous products in the cases of C/RS and RSC. In the cases of C/LN and LNC, the presence of C/LN produced a lower total gas production, especially a lower H2 production, than the presence of LNC. Owing to the higher BET surface area of C/LN, the coke formation was promoted together with the catalytic role of K in the form of K-Si to produce high CO and CH4. Conversely, in the case of LNC, H2 production was extremely high, and CH4 production was quite low. It could be due to the promotion of the water–gas shift reaction and the methane steam reforming, which are expressed as Eqs. (6) and (7), respectively, as follows:

CO  H 2O  CO2  H 2 .

(6)

CH 4  H 2O  3H 2  CO .

(7)

The BET surface area of LNC was lower than that of the C/LN, but the AAEMs content, particular K, was relatively high and could exist in the form of a reactive phenolate group (K-O-C). Thus, the role of K could catalyze the side reactions during tar steam reforming, namely, the water–gas shift reaction26, 27 and the methane reforming in case of 16 ACS Paragon Plus Environment

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LNC. 3.4 Effect of devolatilization temperature on tar steam reforming The effect of devolatilization temperature on the tar conversion without char (with the presence of inert bed) is shown in Fig. 4. Based on the definition of tar conversion in Eq. (3), the result in Fig. 4 infers the effect of steam on tar reforming without the char (or catalyst). When the devolatilization temperature was increased, RST was converted more to achieve the higher tar conversion than the LNT. This result is directly related to the different compositions of tar derived from RST and LNT, as described in section 3.2.

Figure 4 Effect of devolatilization temperature on the tar conversion of the steam reforming of rice straw tar (RST) and LN wood tar (LNT) without the presence of char At the devolatilization temperature of 600 ºC, the ratio of peak intensity between an oxygenated compound with a high peak intensity (i.e., furfural) and a benzene derivative (i.e., 2,3-dihydrobenzofuran) was calculated to compare the composition of RST600 and LNT600. RST600 had a furfural to 2,3-dihydrobenzofuran ratio of 0.83, which was lower than that of LNT600 (3.32). It indicates that under a similar steam reforming condition, LNT600 could reform more easily than RT600 because of the relatively higher ratio of oxygenated compounds. At the devolatilization temperature of 700 ºC, the tar conversion of LNT700 was slightly higher than that of RST700. This result can be described by the relative peak ratio of 17 ACS Paragon Plus Environment

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phenol and naphthalene, which were the main components of both RST700 and LNT700. The relative peak ratio of phenol to naphthalene in the case of RST700 was 1.19, which was lower than that of LNT800 (1.56). RST700 contained a higher naphthalene fraction that was more stable than phenol. When the devolatilization temperature increased to 800 ºC, the tar conversion of RST800 became dramatically higher than that of LNT800. This finding can be explained by the different peak intensities of the main component (naphthalene and indene) between RST800 and LNT800, as shown in Tables 3 and 4. In the case of RST800, the peak intensity of naphthalene and indene was lower than that of LNT800. Therefore, the RST800 gave higher conversion than the LNT800. With the presence of char (catalyst), the effect of devolatilization temperature on the tar conversion is shown in Fig. 5. It was observed that the tar released at 700 ºC could mostly be converted into gaseous products during the steam reforming in all cases of char. It interprets that the tar which consists of the mixture of phenolic compounds (i.e., phenol and methyl phenol) and aromatic compounds (i.e., naphthalene) were favorable to crack with steam over the char surface. This result was confirmed by the BET surface area of the spent char after the experiment, as shown in Table 5.


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Figure 5 Effect of devolatilization temperature on the tar conversion of the steam reforming of rice straw and LN wood-derived tar with the presence of char To study the effect of tar on the char properties, the BET surface area of the char from the blank experiment (without biomass feeding) was characterized and compared with the BET surface area of the spent char. The BET surface area of char from the blank experiment was significantly higher than that of the fresh char presented in Table 2. It is due to the carbon–steam reaction or steam activation of the char, resulting in a high porous structure28, 29,

especially the LNC. Compared with the BET surface area of the blank experiment, the

BET surface area of all the spent chars was significantly lower. Therefore, the tar deposition to generate coke was the dominant step during tar steam reforming. Table 5 Average BET surface area of the blank experiment and the spent char after the experiments

Considering the effect of the devolatilization temperature on the BET surface area of 19 ACS Paragon Plus Environment

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the spent char, it revealed that the spent char at the devolatilization temperature of 700 ºC had the lowest percentage reduction of BET surface area for all types of char. It means that there are some interactions between the char and the tar released at 700 ºC. According to our previous study20, the positive interaction between tar and char to achieve a high tar conversion emerged when the rate of the tar deposition was slower than the rate of carbon/coke gasification. Owing to the relatively high concentration of the AAEMs on the char, especially K, the coke or carbon steam gasification could have been promoted, resulting in a relatively high tar conversion at the devolatilization temperature of 700 ºC. Hosokai et al.30 also reported that the coke steam gasification could be the main step to maintain the reactivity of the char. This is the reason why the BET surface area of the spent char at the devolatilization temperature of 700 ºC was higher than that of the other spent chars. Considering the effect of char type on tar conversion at different devolatilization temperatures (Figure 5.), the coal/biomass blended chars (C/RS and C/LN) had a higher tar conversion than the biomass chars (RSC and LNC) at all devolatilization temperatures. The coal/biomass blended char still performed a better catalytic activity in tar steam reforming independent of the tar composition. In terms of the effect of biomass type, RST was converted less than the LNT at all devolatilization temperatures, particularly at 600 ºC and 700 ºC. This can be explained by the different tar composition between RST and LNT, as discussed in section 3.2. 3.5 The proposed catalytic mechanism of the chars in tar steam reforming In this section, the catalytic mechanism of the coal/biomass blended char and biomass char at different devolatilization temperatures is proposed and summarized in Fig. 6. Tar released at 700 ºC could mostly be converted into a gaseous product for all types of char. The coal/biomass blended char had a catalytic mechanism in tar steam reforming through the 20 ACS Paragon Plus Environment

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combination effect of high porosity and the presence of stable K in silicate form (K-Si). The main catalytic reaction that occurred in the coal/biomass blended char was coke or carbon steam gasification, which produced high H2 and CO. Conversely, tar steam reforming was less catalyzed for the biomass char because of the lower surface area and the potential volatilization of the catalytic species in phenolate form (i.e., K-O-C). However, the presence of biomass char, especially LNC, provided a relatively high H2 production through the main catalytic reaction (i.e., coke steam gasification) together with the side reactions, such as the water–gas shift and methane steam reforming.

Figure 6 Summary of the catalytic mechanism of the coal/biomass blended char and the biomass char during tar steam reforming at different devolatilization temperatures 4. CONCLUSION In this study, the catalytic tar steam reforming of RS and LN wood over a char bed was conducted in a two-stage fixed bed reactor. The effect of devolatilization temperature and biomass type on the catalytic performance of chars during tar steam reforming was investigated. Four types of char were prepared, namely, biomass chars (RSC and LNC) and


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coal/biomass blended chars (C/RS and C/LN). The results showed that tar conversion and gas production increased drastically with the presence of all chars. Thus, all chars could act as a catalyst for tar steam reforming. At the same devolatilization temperature, the coal/biomass blended char (C/RS and C/LN) performed a better catalytic performance in tar steam reforming than the biomass char (RSC and LNC), resulting in a higher carbon conversion into gas and a lower carbon conversion into tar. This result can be explained by the enhancement of the BET surface area of the coal/biomass blended char during co-pyrolysis associated with the role of AAEMs (particularly K) in silicate form. The main catalytic reaction in the case of the coal/biomass blended char was the coke steam reforming, which produced high H2 and CO production. In the case of biomass char, especially LNC, some side reactions (i.e., water–gas shift and methane steam reforming) could occur to produce a relatively high H2 yield. In the view point of the different devolatization temperature, it was found that the tar released at 700 ºC mostly cracked over all the prepared chars in both RS and LN feeding. At the devolatilization temperature of 700 ºC, tar was composed of the mixture of phenolic compounds (i.e., phenol and methyl phenol) and aromatic compounds (i.e., naphthalene), which more easily cracked than the polyaromatic hydrocarbons (PAHs) presenting in the tar released at 800 ºC. LN tar was more easily converted than the RS tar in both cases of with and without char bed because of the lower proportion of a stable aromatic compound except at the devolatilization temperature of 800 ºC. The outcome of this result is beneficial for the design and operation of the tar removal process during biomass gasification by using low-cost, carbonbased catalysts such as the coal/biomass blended char. ACKNOWLEDGEMENT

The authors appreciate the financial support from Fuels Research Center, Department of Chemical Technology, Chulalongkorn University. Other support from the joint graduate


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school of energy and environment (JGSEE), King Mongkut's University of Technology Thonburi is also appreciated. REFERENCES 1.

Shen, Y., Chars as carbonaceous adsorbents/catalysts for tar elimination during biomass pyrolysis or gasification. Renewable and Sustainable Energy Reviews 2015, 43, 281-295.

2.

Huber, G. W.; Iborra, S.; Corma, A., Synthesis of Transportation Fuels from Biomass: Chemistry, Catalysts, and Engineering. Chemical Reviews 2006, 106, (9), 4044-4098.

3.

Krerkkaiwan, S.; Tsutsumi, A.; Kuchonthara, P., Biomass derived tar decomposition over coal char bed. ScienceAsia 2013, 39, 511-519.

4.

Gilbert, P.; Ryu, C.; Sharifi, V.; Swithenbank, J., Tar reduction in pyrolysis vapours from biomass over a hot char bed. Bioresource technology 2009, 100, (23), 60456051.

5.

El-Rub, Z. A.; Bramer, E.; Brem, G., Experimental comparison of biomass chars with other catalysts for tar reduction. Fuel 2008, 87, (10), 2243-2252.

6.

Krerkkaiwan, S.; Mueangta, S.; Thammarat, P.; Jaisat, L.; Kuchonthara, P., Catalytic Biomass-Derived Tar Decomposition Using Char from the Co-pyrolysis of Coal and Giant Leucaena Wood Biomass. Energy & Fuels 2015, 29, (5), 3119-3126.

7.

Park, J.; Lee, Y.; Ryu, C., Reduction of primary tar vapor from biomass by hot char particles in fixed bed gasification. Biomass and Bioenergy 2016, 90, 114-121.

8.

Zhao, Y.; Feng, D.; Zhang, Y.; Huang, Y.; Sun, S., Effect of pyrolysis temperature on char structure and chemical speciation of alkali and alkaline earth metallic species in biochar. Fuel Processing Technology 2016, 141, Part 1, 54-60.


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

9.

Page 24 of 26

Krerkkaiwan, S.; Fushimi, C.; Tsutsumi, A.; Kuchonthara, P., Synergetic effect during co-pyrolysis/gasification of biomass and sub-bituminous coal. Fuel Processing Technology 2013, 115, 11-18.

10.

Meng, H.; Wang, S.; Chen, L.; Wu, Z.; Zhao, J., Thermal behavior and the evolution of char structure during co-pyrolysis of platanus wood blends with different rank coals from northern China. Fuel 2015, 158, 602-611.

11.

Zhu, W.; Song, W.; Lin, W., Catalytic gasification of char from co-pyrolysis of coal and biomass. Fuel Processing Technology 2008, 89, (9), 890-896.

12.

Li, T.; Zhang, L.; Dong, L.; Zhang, S.; Qiu, P.; Wang, S.; Li, C.-Z., Effects of gasification temperature and atmosphere on char structural evolution and AAEM retention during the gasification of Loy Yang brown coal. Fuel Processing Technology 2017, 159, 48-54.

13.

Qin, Y.; Campen, A.; Wiltowski, T.; Feng, J.; Li, W., The influence of different chemical compositions in biomass on gasification tar formation. Biomass and Bioenergy 2015, 83, 77-84.

14.

Font Palma, C., Modelling of tar formation and evolution for biomass gasification: A review. Applied Energy 2013, 111, 129-141.

15.

Zhai, M.; Wang, X.; Zhang, Y.; Dong, P.; Qi, G., Characteristics of rice husk tar pyrolysis by external flue gas. International Journal of Hydrogen Energy 2015, 40, (34), 10780-10787.

16.

He, X.; Liu, Z.; Niu, W.; Yang, L.; Zhou, T.; Qin, D.; Niu, Z.; Yuan, Q., Effects of pyrolysis temperature on the physicochemical properties of gas and biochar obtained from pyrolysis of crop residues. Energy 2018, 143, 746-756.


Page 25 of 26 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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17.

Johansen, J. M.; Jakobsen, J. G.; Frandsen, F. J.; Glarborg, P., Release of K, Cl, and S during Pyrolysis and Combustion of High-Chlorine Biomass. Energy & Fuels 2011, 25, (11), 4961-4971.

18.

Okuno, T.; Sonoyama, N.; Hayashi, J.-i.; Li, C.-Z.; Sathe, C.; Chiba, T., Primary Release of Alkali and Alkaline Earth Metallic Species during the Pyrolysis of Pulverized Biomass. Energy & Fuels 2005, 19, (5), 2164-2171.

19.

Zhou, B.; Dichiara, A.; Zhang, Y.; Zhang, Q.; Zhou, J., Tar formation and evolution during biomass gasification: An experimental and theoretical study. Fuel 2018, 234, 944-953.

20.

Krerkkaiwan, S.; Fushimi, C.; Yamamoto, H.; Tsutsumi, A.; Kuchonthara, P., Influences of heating rate during coal char preparation and AAEMs on volatile–char interaction with different sources of biomass volatile. Fuel Processing Technology 2014, 119, 10-18.

21.

Kajita, M.; Kimura, T.; Norinaga, K.; Li, C.-Z.; Hayashi, J.-i., Catalytic and Noncatalytic Mechanisms in Steam Gasification of Char from the Pyrolysis of Biomass. Energy & Fuels 2010, 24, (1), 108-116.

22.

Wu, H.; Yip, K.; Tian, F.; Xie, Z.; Li, C.-Z., Evolution of Char Structure during the Steam Gasification of Biochars Produced from the Pyrolysis of Various Mallee Biomass Components. Industrial & engineering chemistry research 2009, 48, (23), 10431-10438.

23.

Wu, Y.; Wang, J.; Wu, S.; Huang, S.; Gao, J., Potassium-catalyzed steam gasification of petroleum coke for H2 production: Reactivity, selectivity and gas release. Fuel Processing Technology 2011, 92, (3), 523-530.


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

24.

Page 26 of 26

Long, J.; Song, H.; Jun, X.; Sheng, S.; Lun-shi, S.; Kai, X.; Yao, Y., Release characteristics of alkali and alkaline earth metallic species during biomass pyrolysis and steam gasification process. Bioresource Technology 2012, 116, 278-284.

25.

Fushimi, C.; Wada, T.; Tsutsumi, A., Inhibition of steam gasification of biomass char by hydrogen and tar. Biomass and Bioenergy 2011, 35, (1), 179-185.

26.

Kantschewa, M.; Delannay, F.; Jeziorowski, H.; Delgado, E.; Eder, S.; Ertl, G.; Knözinger, H., Nature and properties of a potassium-promoted NiMoAl2O3 water gas shift catalyst. Journal of Catalysis 1984, 87, (2), 482-496.

27.

Amenomiya, Y.; Pleizier, G., Alkali-promoted alumina catalysts: II. Water-gas shift reaction. Journal of Catalysis 1982, 76, (2), 345-353.

28.

Keown, D. M.; Hayashi, J.-I.; Li, C.-Z., Drastic changes in biomass char structure and reactivity upon contact with steam. Fuel 2008, 87, (7), 1127-1132.

29.

Williams, P. T.; Reed, A. R., Development of activated carbon pore structure via physical and chemical activation of biomass fibre waste. Biomass and Bioenergy 2006, 30, (2), 144-152.

30.

Hosokai, S.; Kumabe, K.; Ohshita, M.; Norinaga, K.; Li, C.-Z.; Hayashi, J.-i., Mechanism of decomposition of aromatics over charcoal and necessary condition for maintaining its activity. Fuel 2008, 87, (13–14), 2914-2922.


Catalytic Steam Reforming of Biomass-Derived Tar over the Coal

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