Evolution of Char Structure during the Steam Gasification of Biochars

Ind. Eng. Chem. Res. 2009, 48, 10431–10438

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Evolution of Char Structure during the Steam Gasification of Biochars Produced from the Pyrolysis of Various Mallee Biomass Components Hongwei Wu,*,† Kongvui Yip,† Fujun Tian,† Zongli Xie,‡ and Chun-Zhu Li† Curtin Centre for AdVanced Energy Science and Engineering, Department of Chemical Engineering, Curtin UniVersity of Technology, GPO Box U1987, Perth, Western Australia 6845, Australia, and CSIRO Materials Science & Engineering, Gate 5, Normany Rd, Clayton, Victoria 3168, Australia

This study reports the evolution of char structure during the steam gasification of biochars under chemicalreaction-controlled conditions. Partially gasified samples were collected at various conversion levels during the steam gasification of both the raw biochars and the acid-treated biochars that had been prepared via acid-washing of the raw biochars. Results from FT-Raman spectroscopy show that the biochars have highly heterogeneous and disordered structures, which are selectively consumed with progress of steam gasification, leading to enrichment of larger aromatic ring systems, hence the so-called “selective gasification”. Selective gasification of biochar can be significantly influenced by the inherent alkali and alkaline earth metallic (AAEM) species in the biochars. The abundant catalysts present in the raw biochars can alter the gasification reaction pathway, but such an alteration appears to have little effect on the evolution of pore surface area, which increases significantly with conversion. While the wood biochar has too low a content of AAEM species to have an apparent effect on selective gasification, for the raw leaf and bark biochars with high contents of AAEM species, selective gasification is considerably less significant in comparison with the respective acidtreated biochars. For acid-treated biochars, gasification seems to take place slowly throughout the biochar on carbon active sites to consume the smaller rings selectively; the reactivity is controlled by the biochar carbon structure. However, for the raw leaf and bark biochars, gasification would be more focused or localized on the catalytic sites so that the activity of carbon active sites becomes less important. The catalytic effect of the inherent AAEM species seems to in turn depend on the carbon structure that probably affects the catalyst dispersion. 1. Introduction The world is facing significant challenges on energy security and sustainable development.1,2 Biomass is recognized as one of the most important renewable energy resources and is considered to be a key alternative to fossil fuels.2 In Australia, through the so-called “alley-farming”, cultivation of perennial mallee trees is a key strategy for managing dryland salinity to prevent the loss of fertile agriculture land,3,4 leading to a potential large-scale biomass production.3 Therefore, mallee biomass is a byproduct of dryland salinity management, so its production does not compete with but complements food production. Recent studies5,6 have clearly shown that mallee biomass production in Australia is close to carbon neutral, economically competitive, and of superb energy performance. Biomass gasification is a promising technology for power generation and production of chemicals.7,8 However, biomass as a fuel is of bulky nature and typically contains high moisture. Direct gasification of biomass would be inefficient and less economically feasible, in comparison to other solid fuels such as coals. Pyrolysis9-11 is a well-known technology for producing bio-oil and/or biochar from biomass. It is therefore possible to utilize biochar and/or bio-oil as high-energy-density fuels for gasification applications. A recent study12 has demonstrated that biochars have good/desired fuel properties and exhibit excellent grindability. Low-rank fuels such as biomass also contain abundant inherent alkali and alkaline earth metallic (AAEM) species, which can significant influence gasification reactions. Previous studies on * To whom correspondence should be addressed. Tel.: +61-892667592. Fax: +61-8-92662681. E-mail: [email protected]. † Curtin University of Technology. ‡ CSIRO Materials Science & Engineering.

brown coal chars13-19 showed that the char reactivity during oxidation is greatly affected by the inherent AAEM species in the chars. The catalytic activities of AAEM species are also affected by the char carbon structure as a result of interactions between the catalysts and the carbon structure.16,18,19 Meanwhile, other studies20-23 also showed that the inherent moisture in lowrank fuels and the pyrolytic water formed during pyrolysis of these fuels can lead to significant in situ steam gasification of the char produced during pyrolysis and also lead to changes in the char structure, which can affect the char reactivity considerably.22-24 In the case of biochars, a related study25 of this group has shown that after pyrolysis under slow-heating conditions, the majority of AAEM species in mallee biomass can be retained in biochars, and can significantly influence biochar steam gasification reactivity. Therefore, it is possible that both carbon structure and inherent AAEM species may have significant effects on biochar gasification behavior. At present, biomass-based gasification processes are largely based on the experience on coal gasification.26-28 Understanding the relation between char carbon structure and reactivity29-32 is largely based on the studies of coal chars and/or highly ordered carbon materials. However, derived from biomass, biochar may have a highly heterogeneous and disordered structure, which can be prone to changes during the course of char oxidation or gasification. For example, a recent study33 has shown the drastic structural change of the char produced from cane trash upon short contact (20 s) with steam using lowtemperature char oxidation reactivity as an indicator. A further related study34 shows the direct evidence from FT-Raman spectroscopy on the evolution of the structure of the similar chars during low-temperature oxidation at 400 °C in air. Obviously, the knowledge of coal chars may not be applicable

10.1021/ie901025d CCC: $40.75  2009 American Chemical Society Published on Web 10/02/2009

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Ind. Eng. Chem. Res., Vol. 48, No. 23, 2009

to biochar and furthermore, oxidation may follow a different reaction pathway for steam gasification.13 Our related study25 shows that steam gasification of biochars can produce high-quality syngas products and the inherent AAEM species in biochars can significantly influence biochar gasification reactivity. However, little is known on the evolution of biochar structure during the course of biochar steam gasification and the influence of the inherent AAEM species on such evolution. Furthermore, investigations to study, altogether, the effects of the AAEM species, the biochar carbon structure discussed above, and the biochar pore surface area on the biochar reactivity have not been reported in the literature. These are all important for a mechanistic understanding of the biochar steam gasification behaviors, acquisition of biochar steam gasification reactivity, and the design of gasification systems using biochars as feedstock. The present study aims to investigate the evolution of char structure during the steam gasification of biochars produced from various components (wood, leaf, and bark) of mallee biomass in Australia. Both raw biochars and acid-treated biochars are considered, focusing on the effect of inherent AAEM species on the evolution of biochar structure and reaction pathways throughout the course of biochar steam gasification. This study considers the evolution of both biochar pore surface area, investigated by physical adsorption method, and biochar carbon structure, characterized by FT-Raman spectroscopy.

Table 1. Properties of Mallee Leaf, Wood, and Bark Biomass Samples

2. Experimental Section

a By difference; n.d., not detected; ad, air-dried; db, dry basis; daf, dry ash free; IC, ion chromatography; ICP-AES, inductively coupled plasma atomic emission spectroscopy.

2.1. Biomass, Biochar, and Partially Gasified Biochar Samples. Preparation of biomass, biochar, and partially gasified biochar samples was detailed elsewhere.25 Briefly, wood, leaf, and bark samples were fractioned from the green mallee trees (E. loxophleba lissophloia), air-dried at 40 °C, cut, and sieved to prepare the size fractions of 150-250 µm for experiments. The properties of the biomass samples are shown in Table 1. Pyrolysis experiments were carried out to prepare raw biochars using a 42 mm-ID quartz fixed-bed reactor at 750 °C (heating rate, 10 K min-1; holding time, 15 min; atmosphere, UHP argon, 99.999%). After each experiment, the reactor was lifted out of the furnace and cooled naturally to room temperature with continuous flow of argon through the reactor. Acid-treated biochars were prepared from the raw biochars for investigating the effect of the inherent AAEM species on the biochar steam gasification reaction pathway. Briefly, a raw biochar sample was treated with 5 M HCl at 50 °C for 24 h, then filtered and washed with copious amounts of ultra pure water (Milli-Q water) until no Cl- ions were detected in the filtrate on addition of silver nitrate. This removes significant proportions of AAEM species from the biochars, as shown in Table 2. It is important to point out that this is different from the common approach using acid treatment of raw biomass followed by pyrolysis of the acid-washed biomass to prepare biochars that contain little AAEM species, an approach during which the inherent AAEM species in biomass is known to affect the char characteristics considerably during pyrolysis.35-37 Biochar steam gasification experiments were carried out in the same fixed-bed reactor system used for pyrolysis. In each experiment, ∼0.1 g biochar sample was used. The sample was first heated to the reaction temperature in argon; gasification reactions commence when a stream of steam, mixed with the argon gas, was introduced into the reactor. The concentration of steam in the feed gas was 8.2% vol, corresponding to a steam flow rate of 0.0111 mol min-1. As demonstrated in our related study,25 it is important to minimize the steam consumption to

wood moisture, % (ad) proximate analysis, % db ash volatile matter fixed carbon ultimate analysis, % daf C H N S Oa contents of inorganic species in biomass, wt% db AAEM species (analyzed by IC) Na K Mg Ca other inorganic species (analyzed by ICP-AES) Si Al Ba Fe P S Sr Ti

leaf

bark

5.3

8.3

4.9

0.4 80.7 18.9

3.7 74.6 21.7

4.1 68.7 27.2

48.4 6.1 0.10 0.07 45.33

59.3 6.8 1.33 0.19 32.38

50.4 5.6 0.28 0.03 43.69

0.030 0.060 0.036 0.107

0.544 0.335 0.154 0.668

0.223 0.104 0.098 1.517

0.0041 0.0025 0.0002 0.0001 0.0159 0.0061 0.0021 n.d.

0.0498 0.0192 0.0010 0.0142 0.0939 0.0680 0.0078 0.0008

0.0508 0.0028 0.0012 0.0019 0.0204 0.0137 0.0223 0.0002

Table 2. Contents of AAEM Species (wt % db) in the Raw and Acid-Treated Biochars acidacidacidraw wood treated raw treated raw bark treated char wood char leaf char leaf char char bark char Na K Mg Ca

0.134 0.320 0.122 0.685

0.009 0.009 0.048 0.450

2.212 1.392 0.618 2.679

0.684 0.075 0.516 0.540

0.697 0.346 0.242 4.837

0.131 0.035 0.263 0.653

maintain a reasonably constant steam partial pressure through the biochar bed during biochar steam gasification. Following the approach by Miura and co-workers,38 our experiments also verified that, in all experiments, steam gasification reactions occur under chemical-reaction-controlled conditions. Therefore, the reactor can be regarded as a differential reactor with respect to the gasification agent. As a thin char bed is used and the gasification products are rapidly swept out of the reactor, the contact, hence interaction, of the gas products with the reacting char, and also any inhibition of the biochar gasification by these products,39 is also minimized. To collect partially gasified biochar samples, once the reaction time is reached, steam feeding is stopped, and the reactor was lifted out of the furnace and cooled naturally to room temperature continuous flow of argon through the reactor. 2.2. Characterization of Biochar and Partially Gasified Biochar Samples. The pore surface area was analyzed by N2 adsorption (Micromeritics Tristar II model 3020) based on the BET (Brunauer-Emmett-Teller) adsorption isotherm.40 The carbon structure of samples was characterized using FT-Raman spectroscopy (Perkin-Elmer Spectrum GX FT-IR/Raman spectrometer) following a procedure detailed elsewhere.41 An InGaAs detector operated at room temperature was used to collect Raman scattering using a back scattering configuration. The excitation Nd:YAG laser wavelength was 1064 nm. A laser


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18,19,34,41

Table 3. Summary of Raman Peak/Band Assignment Representing the Typical Structures Found in Chars from Low-Rank Fuels band name

band position (cm-1)

Gl G Gr Vl

1700 1590 1540 1465

Vr

1380

D

1300

Sl S

1230 1185

Sr R

1060 960-800

description

bond type

carbonyl group CdO graphite E22g; aromatic ring quadrant breathing; alkene CdC aromatics with 3-5 rings; amorphous carbon structures methylene or methyl; semicircle breathing of aromatic rings; amorphous carbon structures methyl group; semicircle breathing of aromatic rings; amorphous carbon structures D band on highly ordered carbonaceous materials; C-C between aromatic rings and aromatics with not less than 6 rings aryl-alkyl ether; para-aromatics Caromatic-Calkyl; aromatic (aliphatic) ethers; C-C on hydroaromatic rings; hexagonal diamond carbon sp3; C-H on aromatic rings C-H on aromatic rings; benzene (ortho-disubstituted) ring C-C on alkanes and cyclic alkanes; C-H on aromatic rings

sp2 sp2 sp2 sp2, sp3

power of 248 mW was used. Each spectrum represents the average of 40 scans. The spectral resolution was 4 cm-1. A curved baseline was considered for each spectrum, and the baseline correction was carried out with the software provided by Perkin-Elmer with the spectrometer. To reduce the heat up of the biochar sample, the biochar was mixed with spectroscopic grade KBr and ground manually. All spectra to be reported here were recorded with a biochar concentration of 0.5 wt % in the biochar-KBr mixture. This concentration was chosen to avoid the complications associated with the heating of biochar samples by the excitation laser while allowing sufficiently strong signals to be recorded.34 2.3. Data Processing and Analysis. The gas products after gasification were analyzed using two Perkin-Elmer GCs equipped with dual columns (molecular sieve column and Porapak-N column). The total carbon in the biochar was determined by the combustion method; that is, the biochar was combusted, and the combustion gas product was analyzed by the GCs to determine the sum of total carbon in CO and CO2. The specific gasification reactivity (R) was calculated on the basis of the rate of carbon consumption (a sum of total carbon in the gas products) per unit carbon remaining in the reacting biochar, using R ) -(1/C)(dC/dt) in which C is the amount (mol) of carbon in the remaining biochar at any time t. For FT-Raman analysis, the Raman spectra in the range between 800 and 1800 cm-1 were curve-fitted using the GRAMS/32 AI software (version 6.00) with 10 Gaussian bands (in Table 3) representing the typical structures found in chars from low-rank fuels such as brown coal and biomass. A detailed discussion on the band assignment and typical Raman spectra can be found elsewhere.18,19,34,41 The six main bands, the largest in terms of areas, are the G, Gr, Vl,, Vr, D, and S. The G band at 1590 cm-1 mainly represents aromatic ring quadrant breathing and the graphite E22g vibration so that the observed G band is mainly due to the aromatic ring systems. The D (1300 cm-1) band represents defect structures in the highly ordered carbonaceous materials and, more importantly, aromatics with not less than 6 rings. The overlap between the D and G bands has been deconvoluted into three bands: Gr (1540 cm-1), Vl (1465 cm-1), and Vr (1380 cm-1). These bands represent typical structures in amorphous carbon (especially smaller aromatic ring systems) as well as the semicircle breathing of aromatic rings. The S (1185 cm-1) band mainly represents Caromatic-Calkyl, aromatic (aliphatic) ethers, C-C on hydroaromatic rings, hexagonal diamond carbon sp3, and C-H on aromatic rings.

sp2, sp3 sp2 sp2, sp3 sp2, sp3 sp2 sp2, sp3

related study.25 As expected, the removal of AAEM species as results of acid treatment (see Table 2) leads to a significant reduction in biochar reactivity. With the progress of biochar conversion, the reactivity of all acid-treated biochars remains relatively unchanged, while that of the raw biochars generally increases with conversion. Figure 2 shows the pore surface areas of the various biochars as a function of gasification conversion, expressed on dry-ashfree basis. There are three key observations. First, for all samples, the surface area increases with conversion, suggesting the formation of new pores and/or opening of closed pores as results of steam activation during gasification. Second, there is little difference in the evolution of pore surface area between the raw biochars and the acid-treated biochars, suggesting that the effect of inherent AAEM species in biochars on surface area development during gasification is insignificant. It was shown previously42 that the surface area during catalyzed char gasification may either remain unchanged, decrease, or increase, relative to the uncatalyzed gasification, depending on the types of catalysis and/or the actions of catalysts during gasification, such as pitting, channelling, and intercalating. It appears that for the biochar samples in this study, the overall combination of these actions leads to a rather unchanged surface area evolution profile during the course of biochar steam gasification. Last, it is also interesting to see that biochars have high surface area (600-1400 m2 g-1, comparable to those of the typical commercial activated carbon from coals43), suggesting a highly porous nature of these biochars.

3. Results and Discussion 3.1. Evolution of Biochar Specific Reactivity and Surface Area during Steam Gasification. Figure 1 shows the evolution of specific gasification reactivity of both raw and acidtreated biochars during steam gasification, as reported in a

Figure 1. Specific gasification reactivity (data from a related study25) as a function of carbon conversion for various biochars, gasification at 750 °C with an intended steam concentration of 8.2 vol %: (a) acid-treated biochars; (b) raw biochars.

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Figure 2. Pore (specific) surface area (on daf basis) of various biochars, as labeled in the graphs, as a function of carbon conversion during gasification (“0% conversion” refers to the biochar from pyrolysis).

3.2. Evolution of Biochar Intrinsic Gasification Reactivity during Steam Gasification. The specific reactivity presented in Figure 1 is the “overall” reactivity of all active sites, obtained under chemical-reaction-controlled conditions. Measurement of active sites in char is not an easy task; however, for highly disordered carbon materials, it has been found that the total pore surface area can be a close approximation to the surface area of active sites (or the number of active sites).44-47 Therefore, this study attempts to investigate the importance of the carbon structure on the biochar gasification reactivity by normalizing the “overall” reactivity (Figure 1) by total surface area (in Figure 2), expressed as specific reactivity per unit pore surface area (g min-1 m-2),48-52 plotted in Figure 3. The specific reactivity per unit pore surface area may be considered as an indication of the average intrinsic reactivity, or the “quality”; that is, the average energy level of active sites27,53 in the reacting biochar. It should be noted that the N2 adsorption technique for surface area analysis may result in molecular-sieve effects so that some micropores may not be taken into account especially for the initial chars from pyrolysis,27 but such an effect usually diminishes and becomes minimal as gasification proceeds.27,54 Therefore, Figure 3 only includes data of carbon conversion level from 15% to ∼80%, at a carbon conversion above which it becomes difficult to produce a sufficient amount of samples. Therefore, Figure 3 may provide insights into the evolution of the average intrinsic reactivity of active sites with the progress of conversion. Figure 3a shows that for the acid-treated biochars, the reactivity per unit pore surface area decreases with increasing conversion. As carbon active sites dominate the gasification reactions for these biochars, the data indicate that with the progression of conversion, the average intrinsic reactivity of carbon active sites in the reacting biochars decreases with steam gasification, leading to a continuous evolution of carbon structure during conversion. Such observation is different from many high-rank coal chars or highly ordered graphite whose reactivity per unit surface area remains rather constant with the gasification conversion,27 suggesting that the carbon structure remains rather unchanged throughout conversion. The data in Figure 3a have two implications. One is that “selective gasification” seems to have occurred during steam gasification of the

Figure 3. Reactivity per unit pore surface area of various biochars: (a) acidtreated biochars; (b) raw biochars; and (c) retention of AAEM species (Na, K, and Ca, as labeled in the graph, data from a related study25).

acid-treated biochars. In other words, during steam gasification, the more reactive components in the acid-treated biochars are gasified earlier at low conversions, leaving those more inert ones at higher conversions. The other is that these biochars seem to have highly heterogeneous carbon structures, which make the observed “selective gasification” possible. On the other hand, for all raw biochars (especially raw leaf and bark chars), Figure 3b clearly shows that the reactivity per unit pore surface area increases with conversion, exhibiting a completely different manner as compared to those for the acidtreated biochars in Figure 3a. As shown in a related study25 (data replotted in Figure 3c), the majority of AAEM species in the raw biochars remains in the reacting biochars as results of insignificant volatilization of those catalytic species throughout the gasification process. Therefore, with carbon being gasified, the molar ratios between catalysts and remaining carbon in the reacting biochars continue to increase with the progress of biochar gasification, leading to an increasing biochar intrinsic reactivity with increasing conversion. The data seem to suggest that reactions occur on those catalytic active sites, which dominate during steam gasification of the raw biochars, or at least the noncatalytic reactions on carbon active sites are less important as compared to those in the case of acid-treated biochars. To clarify the significant differences observed in the evolution of the reactivity per unit pore surface area between the raw and acid-treated biochars, partially gasified biochar samples were then collected. Characterization of these samples was then carried out using FT-Raman spectropy to provide direct evidence on the evolution of biochar carbon structure during the steam gasification and also reveal the effect of AAEM species in biochars on such evolution. 3.3. Evolution of Biochar Carbon Structure during Steam Gasification Evidenced Directly by FT-Raman Spectrocopic Analysis. Figure 4 shows the D and (Gr + Vl + Vr) band area as a fraction out of the total area (total intensity), and their ratios, as a function of gasification conversion for all biochars. As shown in Table 3, the D band mainly represents


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Figure 4. Peak areas of D band and (Gr + Vl + Vr) band, and ratio of (Gr + Vl + Vr)/D, as a function of carbon conversion, for various biochars.

the large aromatics ring systems with no less than 6 fused benzene rings in the biochar in addition to defects in carbon graphitic structures, whereas (Gr + Vl + Vr) bands broadly represent amorphous carbon structure with smaller aromatic ring systems of 3-5 fused benzene rings. The initial values of D and (Gr + Vl + Vr) band area are also similar for all biochars. Two important observations can be made in Figure 4. One is that for acid-treated biochars steam gasification, during which it is dominated by reactions of carbon active sites (i.e., noncatalytic gasification reactions), D and (Gr + Vl + Vr) band areas change significantly with the steam gasification conversion; that is, the biochar carbon structure changes considerably during the course of gasification. D band area clearly increases with conversion (see Figure 4a-c), demonstrating that the carbon structure becomes increasingly enriched with highly ordered aromatic structure with the progress of steam gasification. Meanwhile, (Gr + Vl + Vr) band area decreases with conversion (Figure 4d-f), demonstrating that the amorphous structure is selectively consumed with the progress of steam gasification. The ratio of (Gr + Vl + Vr) to D bands (see Figure 4g-i), the ratio of the smaller ring system to the large ring system, decreases significantly with conversion. The data in Figure 4 provide direct evidence on the occurrence of selective gasification during acid-treated biochar steam gasification, that is, noncatalytic steam gasification. Such direct evidence explains the observed continuing decrease in the reactivity per unit pore surface area (i.e., average intrinsic biochar reactivity of carbon active sites) with increasing conversion during steam gasification, as shown in Figure 3a. The selective consumption of carbon structure has also been also reported in an oxidation study of a brown coal char.18 The other is that, from Figure 4, selective gasification still occurs during the steam gasification of raw biochars, but the pathway of selective gasification is clearly influenced by the inherent AAEM species present in the biochars. For the leaf

chars (Figure 4b, e and h) and bark chars (Figure 4c, f and i), the raw biochars have higher (Gr + Vl + Vr) band area and (Gr + Vl + Vr)/D ratio than do the acid-treated biochars at all conversion levels, but a lower D band area than the acid-treated biochars. This indicates that for these chars with high contents of AAEM species (see Table 2), after acid treatment to remove a significant amount of AAEM species, preferential consumption of smaller aromatic ring systems is more significant than that in the presence of abundant catalysts (for the raw biochars). In order words, as results of catalysis of inherent AAEM species during catalytic steam gasification in cases of raw leaf and bark chars, catalytic carbon activity seems to be less sensitive to the size of the aromatic ring systems (or carbon structure). This is reasonable because for the acid-treated biochars, gasification takes place slowly throughout the biochar (on carbon active sites) to consume the smaller rings selectively, while for the raw biochars, the gasification would be more focused or localized on the catalytic sites so the activity of carbon active sites becomes less important. It is also interesting to see in Figure 4g that for the wood chars, the difference in the D and (Gr + Vl + Vr) band areas as well as the (Gr + Vl + Vr)/D ratio between the raw and acid-treated biochars is not noticeable. Because of the low content of AAEM species in the wood char, the catalytic activity during the raw wood char gasification is not as significant as that for the raw leaf and bark chars, in accordance with the reactivity trends in Figure 1b. Figure 5 shows the peak area of S band, another major band in the biochars. As there would be little long-chain aliphatics and hydroaromatics (five- or six-membered) structures in biochar, the S band mainly represents such sp3-rich structures as alkyl-aryl C-C structures and methyl carbon dangling to an aromatic ring. During steam gasification, some ether structures are also possibly present as intermediates.19 Therefore, S band can be taken as a brief measure of cross-linking density in the biochars.18,19 Figure 5 shows that there are no appreciable

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Figure 5. Peak area of S band as a function of carbon conversion.

Figure 6. Peak area of G band, and ratio of G/D as a function of carbon conversion.

differences in the S band areas for various biochars and for different conversion levels. The proportion of this structure remains unchanged by conversion, and the presence of a significant amount of catalysts does not alter the evolution of this structure remarkably during the steam gasification. G band represents the graphite structure, which can be abundantly prevalent in some high-rank coals/chars and other highly ordered carbon materials.29-32,41 Figure 6 shows the peak area of G band and G/D ratio. Again, there appears to be no appreciable distinction between the various biochars. G band represents about 12-15% of the biochar carbon structure and only increases marginally with conversion, whereas the G/D ratio decreases with conversion. This suggests that the development/enrichment of graphite structure is relatively lower than the development/enrichment of the large aromatic ring systems represented by D bands (see Figure 4a-c). This is in opposition to the gasification of many other coals chars at higher temperatures, where G/D ratio increases remarkably with gasification conversion.30,31,55 Figures 4-6 have provided clear insights into the evolution of the biochar carbons structure during steam gasification. The above results depict the significance of various structures represented by G, D, Gr, Vl, Vr, and S bands during the gasification. Table 4 shows the relative proportion/importance of each of these bands, or groups of bands, in the biochar carbon structure, in comparison with the results from the previous studies on char structure evolution during steam gasification.

Taking Figures 4-6 and Table 4 together, the key findings on the biochar structure evolution can be summarized below: (A) The biochars are indeed heterogeneous carbon materials: the biochars contain a distribution of carbon structures ranging from graphite structure to large and small aromatic ring systems to cross-linking structures. (B) The biochars are also of highly disordered nature: the smaller aromatic rings or amorphous carbon structure (represented by Gr + Vl + Vr band) and cross-linking structure (represented by S band) constitute a great proportion (about or over 50%) of the biochar structure, as opposed to many coal chars whose carbon structures mainly consist of those represented by G and D bands.29-32 (C) It appears that the overall trend of the carbon structural evolution of a brown coal char during steam gasification in the literature (see Table 4) is rather similar to that found in the present study. Both biochars and brown coal chars clearly exhibit the phenomenon of selective gasification. (D) On the other hand, even sub-bituminous coal chars (Table 4) seem to contain more higher-ordered graphitic structure than the biochars, and graphitization (inferred from the increasing G/D ratio) occurs in the sub-bituminous coal chars with the progress of steam gasification conversion, but this is not obvious in the biochars in this study. (E) The above results have clearly illustrated the “selective steam gasification” of biochar carbon structure with progress of gasification conversion. Overall, the less ordered carbon structures are selectively consumed, with respect to the highly ordered carbon structures, with progress of gasification, for all cases. The abundance of catalysts (as for the raw biochars) does alter the reaction path for steam gasification, at least to a certain degree, making the gasification less selective as compared to the acid-treated biochars. The G and S bands, on the other hand, seem not to be appreciably affected by the presence of significant amount of catalysts. 3.4. Factors Affecting Biochar Specific Reactivity. The data present in this study provide important insights into the key factors affecting biochar specific reactivity during steam gasification under chemical-reaction-controlled conditions. For the acid-treated biochars (see Figure 3a), the reactivity per unit pore surface area decreases appreciably with increasing conversion, in accordance with the trend of enrichment of larger aromatic ring systems (the structure with a higher energy level to be overcome for the gasification reaction to take place17). Thus, when the catalytic effect of AAEM species is insignificant, the reactivity per unit pore surface area is mainly controlled by the carbon structure, which determines the carbon active sites on which the noncatalytic reaction occurs. For the raw leaf char and the raw bark char, the carbon structure of biochar still becomes increasingly ordered due to selective gasification as observed under section 3.3, and these carbon active sites are expected to be less reactive with progress of conversion. However, as shown in Figure 3b, the reactivity per unit pore surface area increases throughout the course of gasification conversion, indicating catalytic char reactions dominate the overall steam gasification. As for the raw wood char that contains low AAEM species (see Table 2), at conversion levels below 55-60%, the catalytic and noncatalytic gasification reactions seem to be rather comparable. At conversion level around 55-60%, the catalytic effect of AAEM species seems to reach a “critical” level,16 and again dominates the overall steam gasification reactions. Therefore, catalytic and noncatalytic gasification reactions proceed in parallel during biochar steam gasification; such a


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Table 4. Range of Peak Area of G, D, (Gr + Vl + Vr), and S, as well as G/D and (Gr + Vl + Vr)/D Ratios, of the Biochars in the Present Study, in Comparison with Results from Steam Gasification of Other Chars Reported in the Literature band or band ratio

G

(Gr + Vl + Vr)

D a

present study (mallee wood, leaf, and bark chars) 0.12-0.15 (v ) 0.22-0.33 (v) brown coal char19 0.2-0.31 (v) sub-bituminous coal char55 a

b

0.22-0.36 (V ) 0.17-0.37 (V)

S

G/D c

0.17-0.21 (s ) 0.45-0.58 (V) 0.18-0.23 (s) 1.9-2.6 (v)

(Gr + Vl + Vr)/D 0.7-1.6 (V) 0.6-1.8 (V)

“v”: increases with conversion. b “V”: decreases with conversion. c “s”: remains rather constant with conversion.

phenomenon was also reported in low-rank coal chars gasification studies.39,56 Meanwhile, the catalytic effects, in turn, may also depend on the carbon structure. The raw leaf char shows a much higher reactivity per unit pore surface area than do the raw wood char and the raw bark char. From Figure 4, the raw leaf chars have a lower D band area but a higher (Gr + Vl + Vr) band area and a higher (Gr + Vl + Vr)/D ratio than the raw bark and wood chars. This difference is possibly an important factor contributing to the much higher reactivity of the raw leaf char, apart from the sole factor of a higher concentration of catalysts in the raw leaf char as compared to the raw bark and wood chars. It is plausible that the less ordered carbon structure of leaf chars may have greatly enhanced the dispersion of the catalysts, leading to a much higher catalytic effects.18,19 Hence, despite the fact that the noncatalytic reaction on carbon active sites is relatively less important for the raw biochars, the carbon structure may still play an important role in the overall reactivity through interactions with the catalytic species. Such understanding has important implications on the modeling of reaction kinetics during biochar steam gasification. It is obviously not adequate to model biochar steam gasification using oversimplified (yet commonly used) models such as the homogeneous model57 and shrinking-core model,57 as well as random capillary model58 and random pore model59 that predict the char reactivity based on the pore surface area. Reasonable representation of biochar gasification kinetics would need to consider both catalytic and noncatalytic reactions in parallel, such as the one recently developed for modeling the steam gasification of brown coal chars.39,60 4. Conclusions The present work has provided direct evidence on the evolution of biochar carbon structure by adopting the FT-Raman spectroscopic technique. Pore surface areas of biochars increase significantly with gasification conversion and are similar for both the raw and the acid-treated biochars. The FT-Raman data indicate that the carbon structure of biochars is highly heterogeneous and disordered. With the progress of steam gasification of all biochars, the more reactive (less ordered) carbon structures are selectively consumed first, leading to the enrichment of the less reactive (more inert) structures. In the presence of abundant catalysts in the biochars, the gasification of carbon structure becomes less selective as compared to the acid-treated biochars, suggesting that gasification is more localized on the catalytic sites. Therefore, the presence of catalysts has altered the steam gasification reaction pathway. For gasification of the acid-treated biochars, the reactivity is controlled by the biochar carbon structure (where noncatalytic reaction occurs on the carbon active sites). For gasification of the raw biochars, the reactivity appears to be affected by the catalyst concentration, the carbon structure, as well as the interactions between the carbon structure and the catalysts. The catalytic effect seems also dependent on the biochar carbon structure that can probably affect the catalyst dispersion. The results suggest that biochar gasification kinetics

modeling should consider both catalytic and noncatalytic reactions in parallel. Acknowledgment This research is supported by the Australian Commonwealth Government as part of the Asia-Pacific Partnership on Clean Development and Climate. Partial support from Australia’s Department of Innovation Industry, Science and Research through the Australia-China Special Fund for S&T Cooperation (CH070008) is also acknowledged. Literature Cited (1) Varma, A.; Behera, B. Green Energy: Biomass Processing and Technology; Capital Publishing Co.: New Delhi, 2003. (2) Ture, S.; Uzun, D.; Ture, I. E. The Potential Use of Sweet Sorghum as a Non-polluting Source of Energy. Energy 1997, 22, 17–19. (3) Cooper, D.; Olsen, G.; Bartle, J. Capture of Agricultural Surplus Water Determines the Productivity and Scale of New Low-Rainfall Woody Crop Industries. Aust. J. Exp. Agric. 2005, 45, 1369. (4) Bartle, J.; Olsen, G.; Don, C.; Trevor, H. Scale of Biomass Production from New Woody Crops for Salinity Control in Dryland Agriculture in Australia. Int. J. Global Energy Issues 2007, 27, 115–137. (5) Wu, H.; Fu, Q.; Giles, R.; Bartle, J. Production of Mallee Biomass in Western Australia: Energy Balance Analysis. Energy Fuels 2008, 22, 190–198. (6) Yu, Y.; Bartle, J.; Li, C.-Z.; Wu, H. Mallee Biomass as a Key Bioenergy Source in Western Australia: Importance fo Biomass Supply Chain. Energy Fuels 2009, 23, 3290–3299. (7) Huber, G. W.; Iborra, S.; Corma, A. Synthesis of Transportation Fuels from Biomass: Chemistry, Catalysts, and Engineering. Chem. ReV. 2006, 106, 4044–4098. (8) Yung, M. M.; Jablonski, W. S.; Magrini-Bair, K. A. Review of Catalytic Conditioning of Biomass-Derived Syngas. Energy Fuels 2009, 23, 1874–1887. (9) Das, P.; Ganesh, A.; Wangikar, P. Influence of Pretreatment for Deashing of Sugarcane Bagasse on Pyrolyisis Products. Biomass Bioenergy 2004, 27, 445–457. (10) Bridgwater, A. V. Biomass Fast Pyrolysis. Therm. Sci. 2004, 8, 21–49. (11) Shen, D. K.; Gu, S.; Luo, K. H.; Bridgwater, A. V. Analysis of Wood Structural Changes under Thermal Radiation. Energy Fuels 2009, 23, 1081–1088. (12) Abdullah, H.; Wu, H. Biochar as a Fuel: 1. Properties and Grindability of Biochars Produced from the Pyrolysis of Mallee Wood under Slow-Heating Conditions. Energy Fuels 2009, 23, 4174–4181. (13) Li, C.-Z. Some Recent Advances in the Understanding of the Pyrolysis and Gasification Behaviour of Victorian Brown Coal. Fuel 2007, 86, 1664–1683. (14) Li, X.; Wu, H.; 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 VI. Further Investigations into the Effects of Volatile-char Interactions. Fuel 2004, 83, 1273–1279. (15) Quyn, D. M.; Wu, H.; Hayashi, J.-i.; Li, C.-Z. Volatilisation and Catalytic Effects of Alkali and Alkaline Earth Metallic Species during the Pyrolysis and Gasification of Victorian Brown Coal. Part IV. Catalytic Effects of NaCl and ion-exchangeable Na in Coal on Char Reactivity. Fuel 2003, 82, 587–593. (16) Wu, H.; Hayashi, J.-i.; Chiba, T.; Takarada, T.; Li, C.-Z. Volatilisation and Catalytic Effects of Alkali and Alkaline Earth Metallic Species during the Pyrolysis and Gasification of Victorian Brown Coal. Part V. Combined Effects of Na concentration and Char Structure on Char Reactivity. Fuel 2004, 83, 23–30.

10438


(17) Wu, H.; Li, X.; Hayashi, J.; Chiba, T.; Li, C.-Z. Effects of Volatilechar Interactions on the Reactivity of Chars from NaCl-loaded Loy Yang Brown Coal. Fuel 2005, 84, 1221–1228. (18) 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. (19) 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. (20) Hayashi, J.; Takahashi, H.; Iwatsuki, M.; Essaki, K.; Tsutsumi, A.; Chiba, T. Rapid Conversion of Tar and Char from Pyrolysis of a Brown Coal by Reactions with Steam in a Drop-tube Reactor. Fuel 2000, 79, 439– 447. (21) Hayashi, J.; Iwatsuki, M.; Morishita, K.; Tsutsumi, A.; Li, C.-Z.; Chiba, T. Roles of Inherent Metallic Species in Secondary Reactions of Tar and Char during Rapid Pyrolysis of Brown Coal in a Drop-tube Reactor. Fuel 2002, 81, 1977–1987. (22) Yip, K.; Wu, H.; Zhang, D.-k. Effect of Inherent Moisture in Collie Coal during Pyrolysis Due to in-situ Steam Gasification. Energy Fuels 2007, 21, 2883–2891. (23) Yip, K.; Wu, H.; Zhang, D.-k. Mathematical Modelling of Collie Coal Pyrolysis Considering the Effect of Steam Produced in situ from Coal Inherent Moisture and Pyrolytic Water. Proc. Combust. Inst. 2009, 32, 2675– 2683. (24) Yip, K.; Wu, H.; Zhang, D.-k. Pyrolysis of Collie Coal Briquettes to Produce Char as a Metallurgical Reductant. Energy Fuels 2007, 21, 419– 425. (25) Yip, K.; Tian, F.; Hayashi, J.; Wu, H. Effect of Alkali and Alkaline Earth Metallic Species on Biochar Reactivity and Syngas Compositions during Steam Gasification. Energy Fuels, DOI: 10.1021/ef900534n, in press. (26) Johnson, J. L. In Fundamentals of Coal Gasification, in Chemistry of Coal Utilisation (2nd Supplementary Volume); Elliot, M. A., Ed.; John Wiley & Sons: New York, 1981. (27) Laurendeau, N. M. Heterogeneous Kinetics of Coal and Char Gasification and Combustion. Prog. Energy Combust. Sci. 1978, 4, 221– 270. (28) Blasi, C. D. Combustion and Gasification Rates of Lignocellulosic Chars. Prog. Energy Combust. Sci. 2009, 35, 121–140. (29) Doorn, J. V.; Vuurman, M. A.; Tromp, P. J. J.; Stufkens, D. J.; Moulijn, J. A. Correlation between Raman Spectroscopy Data and the Temperature-Programmed Oxidation Reactivity of Coals and Carbons. Fuel Process. Technol. 1990, 24, 407–413. (30) Livneh, T.; Bar-Ziv, E.; Senneca, O.; Salatino, P. Evolution of Reactivity of Highly Porous Chars from Raman Microspopy. Combust. Sci. Technol. 2000, 153, 65–82. (31) Sheng, C. Char Structure Characterised by Raman Spectroscopy and Its Correlations with Combustion Rectivity. Fuel 2007, 86, 2316–2324. (32) Zaida, A.; Bar-Ziv, E.; Radoviv, L. R.; Lee, Y.-J. Further Development of Raman Microprobe Spectroscopy for Characterisation of Char Reactivity. Proc. Combust. Inst. 2007, 31, 1881–1887. (33) Kweon, 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. (34) Kweon, D. M.; Li, X.; Hayashi, J. I.; Li, C. Z. Evolution of Biomass Char Structure during Oxidation in O2 as Revealed with FT-Raman Spectroscopy. Fuel Process. Technol. 2008, 89, 1429–1435. (35) Quyn, D. M.; Wu, H.; Li, C.-Z. Volatilisation and Catalytic Effects of Alkali and Alkaline Earth Metallic Species during the Pyrolysis and Gasification of Victorian Brown Coal. Part I. Volatilisation of Na and Cl from a set of NaCl-loaded samples. Fuel 2002, 81, 143–149. (36) Quyn, D. M.; Wu, H.; Li, C.-Z. Volatilisation and Catalytic Effects of Alkali and Alkaline Earth Metallic Species during the Pyrolysis and Gasification of Victorian Brown Coal. Part II. Effects of Chemical Form and Valence. Fuel 2002, 81, 151–158. (37) Wu, H.; Quyn, D. M.; Li, C.-Z. Volatilisation and Catalytic Effects of Alkali and Alkaline Earth Metallic Species during the Pyrolysis and Gasification of Victorian Brown Coal. Part III. The Importance of the Interactions between Volatiles and Char at High Temperature. Fuel 2002, 81, 1033–1039.

(38) Miura, K.; Aimi, M.; Naito, T.; Hashimoto, K. Steam Gasification of Carbon, Effect of Several Metals on the Rate of Gasification and the Rates of CO and CO2 Formation. Fuel 1986, 65, 407–411. (39) Bayarsaikhan, B.; Hayashi, J.; Shinada, T.; Sathe, C.; Li, C.-Z.; Tsutsumi, A.; Chiba, T. Kinetics of Steam Gasification of Nascent Char from Rapid Pyrolysis of a Victorian Brown Coal. Fuel 2005, 84, 1612– 1621. (40) Brunauer, S. The Adsorption of Gases and Vapours; Oxford University Press: Oxford, UK, 1945. (41) Li, X.; Hayashi, J.; Li, C.-Z. FT-Raman Spectroscopy Study of the Evolution of Char Structure during the Pyrolysis of a Victorian Brown Coal. Fuel 2006, 85, 1700–1707. (42) Wigmans, T.; Hoogland, A.; Tromp, P.; Moulijn, J. A. The Influence of Potassium Carbonate on Surface Area Development and Reactivity during Gasification of Activated Carbon by Carbon Dioxide. Carbon 1983, 21, 13–22. (43) Juntgen, H.; Klein, J.; Knoblauch, K.; Schroter, H.-J.; Schulze, J. Conversion of Coal and Gases Produced from Coal into Fuels, Chemicals, and Other Products. In Chemistry of Coal Utilization (2nd Supplementary Volume); Elliot, M. A., Ed.; John Wiley & Sons: New York, 1981. (44) Jones, L. E.; Thrower, P. A.; Walker, P. L. Reactivity and Related Microstructure of 3D Carbon/Carbon Composites. Carbon 1986, 24, 51– 59. (45) Lizzio, A. A.; Jiang, H.; Radovic, L. R. On the Kinetics of Carbon (Char) Gasification: Reconciling Models with Experiments. Carbon 1990, 28, 7–19. (46) Radovic, L.; Walker, P. L.; Jenkins, R. G. Importance of Carbon Active Sites in the Gasification of Coal Chars. Fuel 1983, 62, 849–856. (47) Walker, P. L.; Taylor, R. L.; Ranish, J. M. An Update on the Carbon-Oxygen Reaction. Carbon 1991, 29, 411–421. (48) Roberts, D. G.; Harris, D. J. Total Pressure Effects on Chemical Reaction Rates of Chars with O2, CO2 and H2O. Fuel 2000, 79, 1997– 1998. (49) Roberts, D. G.; Harris, D. J. Char Gasification in Mixtures of CO2 and H2O: Competition and Inhibition. Fuel 2007, 86, 2672–2678. (50) Roberts, D. G.; Harris, D. J. Char Gasification with O2, CO2, and H2O: Effects of Pressure on Intrinsic Reaction Kinetics. Energy Fuels 2000, 14, 483–489. (51) Roberts, D. G.; Harris, D. J. On the Effects of High Pressure and Heating Rate during Coal Pyrolysis on Char Gasification Reactivity. Energy Fuels 2003, 17, 887–895. (52) Roberts, D. G.; Harris, D. J. A Kinetic Analysis of Coal Char Gasification Reactions at High Pressures. Energy Fuels 2006, 20, 2314– 2320. (53) Fundamentals Issues in Control of Carbon Gasification ReactiVity; NATO ASI Series, Series E: Applied Sciences; Lahaye, J., Ehrburger, P., Eds.; Kluwer Academic Publishers: The Netherlands, 1991; Vol. 192. (54) Kamishita, M.; Mahajan, O. P.; Walker, P. L. Effect of Carbon Deposition on Porosity and Reactivity of a Lignite Char. Fuel 1977, 56, 444–450. (55) Sekine, Y.; Ishikawa, K.; Kikichi, E.; Matsukata, M.; Akimoto, A. Reactivity and Structural Change of Coal Char during Steam Gasification. Fuel 2006, 85, 122–126. (56) Miura, K.; Hashimoto, K.; Silveston, L. Factors Affecting the Reactivity of Coal Chars during Gasification, and Indices Representing Reactivity. Fuel 1989, 68, 1461–1475. (57) Levenspiel, O. Chemical Reactions Engineering; Wiley: New York, 1972. (58) Gavalas, G. R. A Random Capillary Model with Application to Char Gasification at Chemically Controlled Rates. AIChE J. 1980, 26, 577– 585. (59) Bhatia, S. K.; Perlmutter, D. D. A Random Pore Model for FluidSolid Reactions: 1. Isothermal, Kinetic Control. AIChE J. 1980, 26, 379– 386. (60) Kitsuka, T.; Bayarsaikhan, B.; Sonoyama, N.; Hosokai, S.; Li, C.Z.; Norinaga, K.; Hayashi, J. Behaviour of Inherent Metallic Species as a Crucial Factor for Kinetics of Steam Gasification of Char from Coal Pyrolysis. Energy Fuels 2007, 21, 387–394.

ReceiVed for reView June 24, 2009 ReVised manuscript receiVed September 13, 2009 Accepted September 15, 2009 IE901025D

Evolution of Char Structure during the Steam Gasification of Biochars

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