Biochar as a Fuel: 3. Mechanistic Understanding on Biochar

Bourke , J.; Manley-Harris , M.; Fushimi , C.; Dowaki , K.; Nunoura , T.; Antal , M. J. , Jr. Ind. Eng. Chem. Res. 2007, 46, 5954– 5967. [ACS Full T...
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Energy Fuels 2011, 25, 406–414 Published on Web 12/30/2010

: DOI:10.1021/ef101472f

Biochar as a Fuel: 3. Mechanistic Understanding on Biochar Thermal Annealing at Mild Temperatures and Its Effect on Biochar Reactivity Kongvui Yip,† Minghou Xu,‡ Chun-Zhu Li,† San Ping Jiang,† and Hongwei Wu*,† †

Curtin Centre for Advanced Energy Science and Engineering, Department of Chemical Engineering, Curtin University, GPO Box U1987, Perth, Western Australia 6845, Australia, and ‡State Key Laboratory of Coal Combustion, Huazhong University of Science and Technology, 1037 Luoyu Road, Wuhan, Hubei 430074, China Received November 1, 2010. Revised Manuscript Received December 3, 2010

This study reports a mechanistic investigation on the thermal annealing process at mild temperatures (750 and 900 °C) and its effect on the reactivity of biochar prepared from the pyrolysis of a Western Australia mallee wood. A range of analyses were carried out, including biochar oxidation reactivity, inorganic species, oxygen and hydrogen contents in the biochars, release of heteroatoms in biochar as the gaseous product, and biochar structural evolution during thermal annealing. Extensive thermal annealing (up to 600 min) of biochars at 750 and 900 °C leads to little loss of inorganic species from the biochars. Fourier transform (FT)-Raman spectroscopic analysis further shows that thermal annealing induces a progressively more ordered carbonaceous structure with an increase in the temperature and thermal annealing time. The process is coupled with the loss of heteroatoms, released as dominantly H2 and to a less extent CO. The effect of thermal annealing is drastic during the initial period up to 60 min and levels off with further holding at the annealing temperatures. As thermal annealing progresses, a carbon structural transformation clearly takes place and condenses at least part of the reactive and amorphous structures into larger and more inert ring systems, although little graphitization of biochar carbon structure is evidenced. As a result, thermal annealing leads to a significant change in the biochar reactivity. In the absence of catalytic species, the reduction in biochar reactivity is due to the ordering of the carbon structure induced by thermal annealing. In the presence of catalytic species, the changes in biochar reactivity are results of changes in both the carbon structure and catalytic activity. The changes in the catalytic activity appear to suggest a change in the form and dispersion of the catalytic species within the biochars, as results of the loss of heteroatoms and carbon structure condensation.

that the biochar produced from low-temperature pyrolysis of a mallee biomass has good fuel properties, excellent grindability, and high energy density. The second part of this series of papers (10.1021/ef901435f)9 has demonstrated the differences in fuel and ash properties of biochars from different components of the mallee biomass. Gasification of biochar is also possible to produce a high-quality syngas product that contains little methane.10 In applications of biochar in gasification, because the biochar resides in a practical gasifier (e.g., fluidized-bed gasifiers), biochar particles are continually subject to heat treatment at the gasification temperature, concurrent with the occurrence of the char gasification reactions. This effect of heat treatment is also referred to as thermal annealing, a process that can involve a series of transformations in the char structure and the inorganic constituents in the chars.11,12 Biochars are low-rank fuels with a very heterogeneous char structure13-17 and contain abundant inorganic species that can be catalytically active during gasification.10,15,18,19 It is

1. Introduction Biomass is a key renewable energy source for the world’s future energy security and sustainable development.1,2 In Australia, mallee biomass, as a byproduct of dryland salinity management with a potential of large-scale production,3,4 is regarded as a true second-generation feedstock because it does not compete with but complements food production. Moreover, mallee biomass production is economic, has a small carbon footprint, and achieves excellent energy performance.5-7 Pyrolysis is an attractive technology to produce highenergy-density fuels, such as biochar and/or bio-oil, from biomass, such as mallee, addressing the key problems hindering the use of raw biomass as a direct fuel because of its bulky nature, high moisture content, and poor grindability. The first part of this series of papers (10.1021/ef900494t)8 has shown *To whom correspondence should be addressed. Telephone: þ61-892667592. Fax: þ61-8-92662681. E-mail: [email protected]. (1) International Energy Agency (IEA). World Energy Outlook 2009; IEA: Paris, France, 2009. (2) Varma, A.; Behera, B. Green Energy: Biomass Processing and Technology; Capital Publishing Company: New Delhi, India, 2003. (3) Bartle, J.; Olsen, G.; Don, C.; Trevor, H. Int. J. Global Energy Issues 2007, 27, 115–137. (4) Bartle, J. R.; Abadi, A. Energy Fuels 2010, 24, 2–9. (5) Wu, H.; Fu, Q.; Giles, R.; Bartle, J. Energy Fuels 2008, 22, 190– 198. (6) Yu, Y.; Wu, H. Energy Fuels, Energy Fuels 2010, 24, 5660–5668. (7) Yu, Y.; Bartle, J.; Li, C.-Z.; Wu, H. Energy Fuels 2009, 23, 3290– 3299. (8) Abdullah, H.; Wu, H. Energy Fuels 2009, 23, 4174–4181. r 2010 American Chemical Society

(9) Abdullah, H.; Mediaswanti, K. A.; Wu, H. Energy Fuels 2010, 24, 1972–1979. (10) Yip, K.; Tian, F.; Hayashi, J.; Wu, H. Energy Fuels 2010, 24, 173– 181. (11) Oberlin, A. Carbon 1984, 22, 521–541. (12) Senneca, O.; Russo, P.; Salatino, P.; Masi, S. Carbon 1997, 35, 141–151. (13) Kweon, D. M.; Hayashi, J. I.; Li, C.-Z. Fuel 2008, 87, 1127–1132. (14) Kweon, D. M.; Li, X.; Hayashi, J. I.; Li, C.-Z. Fuel Process. Technol. 2008, 89, 1429–1435.

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Table 1. Properties of the Mallee Wood Used in the Present Study proximate analysis (%, db)a moisture (%) 5.3 a

b

c

ash

VM

0.45

80.7

ultimate analysis (%, daf)b d

c

FC

C

H

N

S

Oe

caloric value (MJ/kg)

18.9

49.0

6.7

0.19

0.02

44.1

19.4

d

e

db = dry basis. daf = dry and ash-free. VM = volatile matter. FC = fixed carbon. by difference.

widely accepted that char gasification reactivity is dominantly governed by char structure and/or catalytic species actions.20,21 For example, our recent studies on biochar gasification clearly revealed the significant effect of these two factors on biochar reactivity.10,14,15 Because of the heterogeneous and highly disordered carbon structure of biochar, the thermal annealing process may have profound influences on the biochar gasification behavior, even under mild heat treatment conditions (750-900 °C). There have been various studies on the thermal annealing of different chars, including modeling the thermal annealing process,12,22-28 investigating thermal annealing based on inferences from char reactivity,12,23-25,29-33 and probing the thermal annealing process using various microscopic techniques qualitatively.34-38 However, most of the past studies were focused on thermal annealing of coal chars at high temperatures (mostly over 1300 °C), and studies on thermal annealing of biochar39,40 are scarce. Additionally, little work has been performed to quantify and reveal the evolution of various carbon structures within the heterogeneous and highly disordered biochars during thermal annealing. Clearly, a systematic study

is required to gain a mechanistic understanding on the effect of thermal annealing on the structural evolution and reactivity of biochar because the structure of biochar is susceptible to changes upon thermal annealing. Therefore, it is the objective of the present study, which is part 3 of this series of papers, to investigate mechanistically the thermal annealing of biochar and to elucidate the evolution of the biochar structure and its reactivity at mild temperatures (750-900 °C), which are typically encountered in a practical fluidized-bed gasifier. To examine the effect of inorganic species on thermal annealing, both the raw and acid-washed biomass samples were used for biochar preparation. For a mechanistic understanding, this study carried out a range of biochar characterizations, including biochar gasification reactivity, inorganic species contents in biochars, carbon, oxygen, and hydrogen in biochars, release of heteroatoms as gaseous products and evolution of biochar carbon structure during thermal annealing, as directly evidenced by Fourier transform (FT)-Raman spectroscopy. 2. Experimental Section 2.1. Biomass Samples. The wood sample (150-250 μm) used in this study was prepared from the green mallee trees that were harvested from mallee farms in Narrogin, Western Australia. The details of sample preparation can be found elsewhere.10 The proximate and ultimate analyses, as well as the caloric value, of the wood sample used in the present study are shown in Table 1. Note that the moisture content shown is the reabsorbed equilibrium moisture content in the samples after drying. To study the roles of inorganic species on thermal annealing, acid-washed wood was prepared. Briefly, the raw wood was treated with 0.2 M HCl at 35 °C for 24 h. The wood sample was filtered, and the acid treatment process was repeated once. Then, the wood sample was filtered and washed repeatedly with ultrapure water (Milli-Q water) until no Clions were detected in the filtrate upon the addition of silver nitrate. Through this process, the ash yield of the acid-washed wood was reduced to 0.007% on a dry basis (db). In summary, the biomass samples used in the present study include the raw and acid-washed wood. 2.2. Biochar Preparation and Thermal Annealing. Pyrolysis to prepare the biochars was carried out using a specially designed fixed-bed quartz reactor, with an internal diameter of approximately 28 mm, housed vertically in an electrically heated furnace, under well-controlled conditions. The details of the reactor configuration can be found elsewhere.10 The biomass sample (∼2 g) loaded into the quartz reactor and held on the quartz frit was heated at a slow-heating rate of 10 K min-1 to a desired temperature (750 or 900 °C) and then held for various times (0-600 min) under ∼200 mL min-1 ultrahigh-purity (UHP) argon (purity >99.999%) for the thermal annealing study. At the end of the experiment, the reactor was lifted out of the furnace immediately to cool naturally to room temperature with a continuous flow of argon through the reactor. The biochar was then recovered for further characterization. During thermal annealing, the evolution of gases was also analyzed from time 0 to 600 min. The gaseous products (H2, CO, CO2, and CH4) was analyzed at sufficiently short sampling intervals throughout the thermal annealing process, using two Perkin-Elmer gas chromatographs (GCs) installed with dual

(15) Wu, H.; Yip, K.; Tian, F.; Xie, Z.; Li, C.-Z. Ind. Eng. Chem. Res. 2009, 48, 10431–10438. (16) Bourke, J.; Manley-Harris, M.; Fushimi, C.; Dowaki, K.; Nunoura, T.; Antal, M. J., Jr. Ind. Eng. Chem. Res. 2007, 46, 5954–5967. (17) Meszaros, E.; Jakab, E.; Varhegyi, G.; Bourke, J.; ManleyHarris, M.; Nunoura, T.; Antal, M. J., Jr. Ind. Eng. Chem. Res. 2007, 46, 5943–5953. (18) Encinar, J. M.; Gonzalez, J. F.; Rodriguez, J. J.; Ramiro, M. J. Fuel 2001, 80, 2025–2036. (19) Garca, L.; Salvador, M. L.; Arauzo, J.; Bilbao, R. Energy Fuels 1999, 13, 851–859. (20) Miura, K.; Hashimoto, K.; Silveston, L. Fuel 1989, 68, 1461– 1475. (21) Tsai, S. Fundamentals of Coal Beneficiation and Utilization; Elsevier Science Publishing Company: Amsterdam, The Netherlands, 1982. (22) Hurt, R.; Sun, J.-k.; Lunden, M. Combust. Flame 1998, 113, 181– 197. (23) Senneca, O.; Salatino, P. Proc. Combust. Inst. 2002, 29, 485–493. (24) Zolin, A.; Jensen, A.; Dam-Johansen, K. Proc. Combust. Inst. 2000, 28, 2181–2188. (25) Zolina, A.; Jensen, A.; Dam-Johansena, K.; Jensenb, L. S. Fuel 2001, 80, 1029–1032. (26) Mitchell, R. E. Proc. Combust. Inst. 2000, 28, 2261–2270. (27) Murphy, J. J.; Shaddix, C. R. Combust. Flame 2010, 157, 535– 539. (28) Zolin, A.; Jensen, A.; Dam-Johansen, K. Combust. Flame 2001, 125, 1341–1360. (29) Hurt, R. H.; Gibbins, J. R. Fuel 1995, 74, 471–480. (30) Russel, N. V.; Gibbins, J. R.; Man, C. K.; Williamson, J. Energy Fuels 2000, 14, 883–888. (31) Senneca, O.; Salatino, P. Combust. Flame 2006, 144, 578–591. (32) Senneca, O.; Salatino, P.; Menghini, D. Proc. Combust. Inst. 2007, 31, 1889–1895. (33) Zolin, A.; Jensen, A. D.; Jensen, P. A.; Dam-Johansen, K. Fuel 2002, 81, 1065–1075. (34) Davis, K. A.; Hurt, R. H.; Yang, N. Y. C.; Headley, T. J. Combust. Flame 1995, 100, 31–40. (35) Feng, B.; Bhatia, S. K.; Barry, J. C. Carbon 2002, 40, 481–496. (36) Kasaoka, S.; Sakata, Y.; Shimada, M. Fuel 1987, 66, 697–701. (37) Lu, L.; Kong, C.; Sahajwalla, V.; David, H. Fuel 2002, 81, 1215– 1225. (38) Xu, X.; Chen, Q.; Fan, H. Fuel 2003, 82, 853–858. (39) Fu, P.; Hu, S.; Sun, L.; Xiang, J.; Yang, T.; Zhang, A.; Zhang, J. Bioresour. Technol. 2009, 100, 4877–4883. (40) Wornat, M. J.; Hurt, R. H.; Yang, N. Y. C.; Headley, T. J. Combust. Flame 1995, 100, 131–143.

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columns (molecular sieve column and Porapak-N column). Higher hydrocarbons (e.g., C2, C3, and C4) were not detected in the gaseous products. The gas evolution profile was plotted for the whole course of thermal annealing. 2.3. Biochar Reactivity Measurement. The biochar oxidation reactivity was measured using a thermogravimetric analyzer (TGA) operating under isothermal conditions and adopted as an indicator to probe the effect of thermal annealing on biochar reactivity. The tests were conducted in an atmosphere of 5% O2 in nitrogen, instead of air, to minimize the effect of chemisorption of oxygen on the reactivity measurement.41 The reaction temperature chosen, 450 and 400 °C, for the biochars from acidwashed wood and the biochars from raw wood, respectively, was sufficiently low, so that the reactivity was measured under a chemical-kinetic-controlled regime. Such a temperature is also well-below the thermal annealing temperature, so that little thermal annealing takes place during the oxygen gasification itself. The specific reactivity (R) of a biochar at any instant was calculated from the differential mass loss data (dW/dt) according to R = -1/W(dW/dt), where W is the mass [dry and ash-free (daf)] basis of the biochar at any time t. 2.4. Quantification of Inorganic Species in Biomass and Biochars. Na, K, Mg, Ca, Fe, Al, Si, P, Ba, Sr, and Ti in the sample were analyzed by first ashing the sample, followed by borate fusion and then analysis using inductively coupled plasmaatomic emission spectroscopy (ICP-AES). The detailed procedure has been reported elsewhere.10 Briefly, a sufficient amount of biochar sample was charged into a large platinum (Pt) crucible and then ashed in a furnace under air conditions, following a specially designed ashing program for biochar. The resultant ash was decomposed by fusion with the X-ray flux (35.3% lithium tetraborate and 64.7% lithium metraborate), and finally, the fusion bead was dissolved in dilute nitric acid (high-purity redistilled grade). The prepared samples were then subject to ICP-AES analysis for the quantification of the above inorganic species. 2.5. Quantification of Carbon, Oxygen, and Hydrogen Contents in Biochars. Upon selected biochar samples after thermal annealing treatment, carbon, oxygen, and hydrogen contents were determined. These results were obtained through a range of analytical techniques. C, H, and N were analyzed using an elemental analyzer according to Australian Standard AS1038.6.4.42 The total S and Cl contents were determined by ICP-AES, following combustion of the samples under Eschka’s mixture and acid digestion, based on the method outlined in Australian Standards AS1038.6.3.143 and AS1038.8.1.44 Therefore, the C and H contents can be obtained directly from the elemental analysis, while the O content was determined by a difference from the C, H, N, S, and Cl contents of the sample, on a daf basis. 2.6. Characterization of the Biochar Carbon Structure. The carbon structure of various biochars after thermal annealing was characterized using FT-Raman spectroscopy, which was previously used to characterize carbon structures of various coal chars45,46 and biochars.13-15 The detailed procedure for the analysis, FT-Raman peak/band assignment, data processing,

Figure 1. Biochar yield (daf) as a function of the thermal annealing time at 750 and 900 °C: (a) percentage of biomass mass and (b) percentage of initial biochar mass.

and typical examples of curve deconvolution can be found in those studies. Briefly, an InGaAs detector operated at room temperature was used to collect Raman scattering using a backscattering configuration. The excitation Nd:YAG laser wavelength was 1064 nm. The Raman spectra in the range between 800 and 1800 cm-1 were curve-fitted using the GRAMS/32 AI software (version 6.00) into 10 Gaussian bands. Each band represents a specific type of carbon structure. The five major bands found for the samples in the present study are the G, Gr, Vl,, Vr, and D bands. The G band (at the band position of 1590 cm-1) mainly represents aromatic ring quadrant breathing and the graphite E22g vibrations. The D (1300 cm-1) band represents “defect” structures in the highly ordered carbonaceous materials and, more importantly, aromatics with no less than six 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 structures, especially smaller aromatic ring systems, semi-circle breathing of aromatic rings, and methylene or the methyl group. A high number (200) of scans is used for all of the biochar samples.

3. Results and Discussion 3.1. Effect of Thermal Annealing on the Biochar Yield and Reactivity. Figure 1a shows the biochar yield after thermal annealing for various times at 750 and 900 °C. It can be seen that the biochar yield following pyrolysis of the acid-washed wood is slightly lower than that of the raw wood, which is known to be as a result of the presence of the metallic species in the raw sample during pyrolysis, making the release of volatiles more difficult.47-49 Also, the biochar yield at 900 °C is slightly lower than that at 750 °C as expected. At a given temperature, the biochar yield decreases with thermal annealing

(41) Feng, B. Reactivity of coal chars and carbons—Effects of chemisorption and structure. Ph.D. Thesis, University of Queensland, Brisbane, Queensland, Australia, 2002. (42) Standards Australia. AS1038.6.4-2005, Coal and Coke—Analysis and Testing—Higher Rank Coal and Coke—Ultimate Analysis—Carbon, Hydrogen and Nitrogen—Instrumental Method; Standards Australia: Sydney, Australia, 2005. (43) Standards Australia. AS1038.6.3.1-1997, Coal and Coke—Analysis and Testing—Higher Rank Coal and Coke—Ultimate Analysis—Total Sulfur—Eschka Method; Standards Australia: Sydney, Australia, 1997. (44) Standards Australia. AS1038.8.1-1999, Coal and Coke—Analysis and Testing—Coal and Coke—Chlorine—Eschka Method; Standards Australia: Sydney, Australia, 1999. (45) Li, X.; Hayashi, J.; Li, C.-Z. Fuel 2006, 85, 1509–1517. (46) Yip, K.; Ng, E.; Li, C.-Z.; Hayashi, J.-i.; Wu, H. Proc. Combust. Inst., DOI: 10.1016/j.proci.2010.07.073.

(47) Shibaoka, M.; Ohtsuka, Y.; Wornatt, M. J.; Thomas, C. G.; Bennett, A. J. R. Fuel 1995, 74, 1648–1653. (48) Hayashi, J.; Takahashi, H.; Doi, S.; Kumagai, H.; Chiba, T.; Yoshida, T.; Tustsumi, A. Energy Fuels 2000, 14, 400–408.

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Figure 2. Specific reactivity, R, of biochars from acid-washed wood, as a function of the thermal annealing time at (a) 750 °C and (b) 900 °C. Reactivity was measured at 450 °C.

Figure 3. Specific reactivity, R, of biochars from raw wood as function of the thermal annealing time at (a) 750 °C and (b) 900 °C. Reactivity was measured at 400 °C. The reactivity of the initial biochar from acid-washed wood at 750 °C is also plotted for comparison.

time, particularly during the initial 60 min. The reduction in biochar yield during the initial thermal annealing time is clearly demonstrated in Figure 1b, which shows the biochar yield, with respect to the initial biochar, as a function of the thermal annealing time. Such a pattern of biochar yield during thermal annealing is similar for all samples studied under the current experimental conditions. Figures 2 and 3 present the data on the specific reactivity of the biochars from both acid-washed and raw wood, after thermal annealing for various times at 750 and 900 °C, respectively. It is important to note that the specific reactivity is plotted against biomass conversion, instead of biochar conversion. This enables comparisons between biochars of different temperatures and thermal annealing times, because part of the biochar will have been converted as the temperature and thermal annealing time increase. Such a plot of reactivity versus biomass conversion takes into account any changes of the biochar properties, which may result in variations in biochar reactivity. As aforementioned, the reactivity measurement for the biochars from acid-washed wood was carried out at 450 °C (Figure 2), while that for the biochars from raw wood was carried out at 400 °C (Figure 3), all having been verified to be under the chemical-kineticcontrolled regime. Different temperatures were adopted for the two sets of biochars because, at 400 °C, the reaction of the biochars from acid-washed wood was so slow that an adequately high conversion level for these biochars was not practically achievable during measurements. A limited conversion of biochar from acid-washed wood prepared at 750 °C and 0 time is shown in Figure 3a, and it is clear that its reactivity is significantly lower than that of the biochar from raw wood prepared under the same conditions, apparently because of the catalytic effect of the inorganic species in the biochar on biochar reactivity. Generally, the specific reactivity of the biochars from acid-washed wood (Figure 2) increases with conversion probably because of an increase in surface area.15,20 On the other hand, for the biochars from raw wood, the increase of specific reactivity with conversion (Figure 3a) can be due to the increase in both surface area and catalytic activity as the catalyst concentration in the biochar increases15,20 and the occurrence of a maximum point

(Figure 3b) may be related to the changes in the char structure and agglomeration and deactivation of the catalysts.20,45,50,51 For the biochars from acid-washed wood, panels a and b of Figure 2 show that the reactivity decreases with the temperature and thermal annealing time. Biochar reactivity decreases significantly during the initial thermal annealing time (0-60 min at 750 °C and 0-180 min at 900 °C) and then levels off with further thermal annealing. However, the effect of thermal annealing on the reactivity of biochars from the raw wood (as shown in Figure 3) is more complicated. A similar trend in biochar reactivity is observed in comparison to that of the biochars from acid-washed wood. The reactivity of biochars from raw wood initially decreases with thermal annealing time (0-420 min at 750 °C and 0-180 min at 900 °C) and then levels off afterward. However, it is interesting to note that the reactivity of biochars from raw wood after thermal annealing at 900 °C is not necessarily less reactive than those at 750 °C, as opposed to the common perception that chars prepared at higher temperatures are less reactive than chars at lower temperatures.20,52,53 For instance, the biochar from raw wood at 900 °C with 0 min holding time has considerably higher reactivity than the corresponding biochar at 750 °C. Clearly, the reactivity data in Figures 2 and 3 suggest that thermal annealing does influence biochar reactivity, most likely because of a combination of the inherent catalytic species in biochar and the biochar carbon structure. Further experiments were then carried out to investigate these aspects. 3.2. Retention of Inorganic Species in Biochars during Thermal Annealing. Table 2 shows the contents of various inorganic species in the raw wood sample. The acid-washed wood sample contains a negligible amount of inorganic species following the acid-washing process. For the raw wood, the alkali and alkaline earth metallic (AAEM) species, Na, K, Mg, and Ca, are dominant (∼90%) species. Other inorganic species, such as Si and Al, are of very low concentrations. Figure 4 shows the retention of AAEM species in the biochars from raw wood after thermal annealing at 750 and 900 °C for different times. The loss/volatilization of AAEM species during pyrolysis is not significant, as already shown in our previous study.10 The data in Figure 4 clearly show that extensive thermal annealing at 750 and 900 °C (even for

(49) Wornat, M. J.; Sakurovs, R. Fuel 1996, 75, 867–871. (50) Radovic, L.; Walker, P. L.; Jenkins, R. G. J. Catal. 1983, 82, 382–394. (51) Radovic, L. R.; Walker, P. L.; Jenkins, R. G. Fuel 1983, 62, 209–212.

(52) Johnson, J. L. Fundamentals of coal gasification. In Chemistry of Coal Utilisation; Elliot, M. A., Ed.; John Wiley and Sons: New York, 1981; Vol. 2. (53) Laurendeau, N. M. Prog. Energy Combust. Sci. 1978, 4, 221–270.

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Table 2. Contents (wt %, db) of Inorganic Species in the Raw Wood Na

K

Mg

Ca

Si

Al

Ba

Fe

P

Sr

Ti

0.0212

0.0744

0.0364

0.1236

0.0026

0.0025

0.0002

0.0001

0.0182

0.0021

nda

a

nd = not detected.

Figure 4. Retention of AAEM species in the biochars from raw wood as a function of the thermal annealing time: (a) 750 °C and (b) 900 °C.

600 min) leads to little loss/volatilization of the inherent AAEM species. This is not surprising because, during thermal annealing under the fixed-bed conditions in the present study, there are minimal interactions between any volatiles released and the biochar. Significant volatile-char interactions could, otherwise, induce breakage of bonds between AAEM species and the char matrix,54-56 leading to a more significant loss of AAEM species. Nevertheless, extensive thermal annealing for 600 min of the biochar from raw wood at 900 °C does lead to slight volatilization (∼10%) of Na, suggesting that, at this temperature, a small proportion of Na in biochar was thermally unstable. A similar observation was reported for brown coal chars under similar conditions.54,55 Changes/deactivation of catalytic effects on char reactivity is known to take place during thermal annealing of coal chars.12,40 Possible mechanisms leading to such changes/ deactivation of catalytic effects in coal chars include (a) the loss/volatilization of catalytic species54,57 during thermal annealing, (b) reactions of the catalytic species with other mineral matter (particularly quartz and clay materials or Si and Al organically bound to the carbon matrix) to form nonvolatile composite oxides or other catalytically inactive species (such as silicates, aluminates, and/or aluminosilicates),20,58,59 and (c) the condensation of the char carbon structure, leading to interactions between catalytic species

Figure 5. Carbon, oxygen (by difference), and hydrogen contents in biochars as a function of the thermal annealing time, from elemental analysis: (a-c) 750 °C and (d-f) 900 °C.

with the char carbon structure.15,45,60 In the presence study, any changes/deactivation of catalytic effects on the biochar reactivity is less likely to be due to the factor a because the loss/volatilization of AAEM species is minimal (see Figure 4). Factor b is also not expected to be important because the raw wood contains dominantly AAEM species and only very low contents of Si and Al. This means that any reactions of the AAEM species to form catalytically inactive species with Si and Al during thermal annealing would be insignificant. Therefore, it is most likely that the factor c plays the most important role in biochar deactivation, which can only occur (if any) when there are changes in the biochar structure during thermal annealing. Therefore, a further investigation was carried out to study the evolution of the biochar carbon structure during thermal annealing via probing biochar oxygen and hydrogen contents and the biochar carbon structure using FT-Raman spectroscopic analysis. 3.3. Oxygen and Hydrogen Contents in Biochars and Release of Gaseous Products during Thermal Annealing. Figure 5 presents the data on carbon, oxygen, and hydrogen

(54) Li, X.; Wu, H.; Hayashi, J.; Li, C.-Z. Fuel 2004, 83, 1273–1279. (55) Wu, H.; Quyn, D. M.; Li, C.-Z. Fuel 2002, 81, 1033–1039. (56) Wu, H.; Li, X.; Hayashi, J.; Chiba, T.; Li, C.-Z. Fuel 2005, 84, 1221–1228. (57) Quyn, D. M.; Wu, H.; Li, C.-Z. Fuel 2002, 81, 143–149. (58) Bayarsaikhan, B.; Hayashi, J.; Shinada, T.; Sathe, C.; Li, C.-Z.; Tsutsumi, A.; Chiba, T. Fuel 2005, 84, 1612–1621. (59) Wigmans, T.; Goebel, J. C.; Moulijn, J. A. Carbon 1983, 21, 295–301.

(60) Wu, H.; Hayashi, J.-i.; Chiba, T.; Takarada, T.; Li, C.-Z. Fuel 2004, 83, 23–30.

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Figure 6. Evolution of gaseous products during biochar thermal annealing at 750 °C: (a) CH4, (b) CO, (c) CO2, and (d) H2.

Figure 7. Evolution of gaseous products during biochar thermal annealing at 900 °C: (a) CH4, (b) CO, (c) CO2, and (d) H2.

contents of various biochars as a function of the thermal annealing time. Clearly, the oxygen and hydrogen contents decrease with the progress of thermal annealing, coupled with an increase in the carbon content. At a given thermal annealing time, the oxygen and hydrogen contents of biochar

decrease with the temperature as expected. The decrease in the oxygen and hydrogen contents in biochar is coupled with the release of gaseous products (H2, CO, CO2, and CH4) during thermal annealing. Figures 6 and 7 present the gas evolution profiles for thermal annealing at 750 and 900 °C, 411

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Figure 8. Total Raman intensity for biochars as a function of the thermal annealing time at 750 and 900 °C.

expressed as moles of gas evolved per mole of carbon. The total gas formation over the 600 min thermal annealing time is approximately 0.07 and 0.025 mol of gas (mol of C)-1 at 750 and 900 °C, respectively. It can be seen that the patterns of oxygen and hydrogen losses from the biochars as shown in Figures 6 and 7 are also in accordance with the profiles of biochar weight loss and reactivity as a function of the thermal annealing time. The gaseous products are dominantly H2 and to a much less extent CO, plus trace amounts of CO2 and CH4. As also shown in Figure 5, at both 750 and 900 °C, after extensive thermal annealing for 600 min, the contents of oxygen and hydrogen in the biochar become very low. The leveling-off amounts of oxygen and hydrogen at 900 °C are lower than those at 750 °C. The loss of oxygen and hydrogen in biochar is a clear indication of ordering/condensation of the biochar structure, via elimination of functional groups, elimination of carbon edges and defects, and/ or rearrangement/coalescence of aromatic rings.11,12,33 To provide direct evidence on the change in the biochar carbon structure, FT-Raman spectroscopic analysis was then carried out on various biochars to probe the carbon structural evolution during thermal annealing. 3.4. Evolution of the Biochar Carbon Structure during Thermal Annealing. Figures 8-10 show the results from FT-Raman spectroscopic analysis on various biochars. Figure 8 shows the Raman intensity (or total peak area). For biochars from both the acid-washed and raw wood, the Raman intensity is lower at 900 °C than 750 °C and, at a particular temperature, decreases with thermal annealing time, especially at 750 °C. The observed Raman intensity is affected by the biochar properties, namely, the Raman scattering properties as well as the light absorbing capacity of the biochar for both the excitation laser and Raman intensity.61 The decrease in Raman intensity most likely reflects the condensation/growth of aromatic ring systems.45,61 Figure 9 shows the (Gr þ Vl þ Vr)/D ratio and the band area of G out of the total peak area. D, (Gr þ Vl þ Vr), and G are major bands for the various biochars studied here. The D band represents “defect” structures in the highly ordered carbonaceous materials and, more importantly, aromatics with no less than six rings. The (Gr þ Vl þ Vr) band represents the amorphous structure with small aromatic ring systems. The G band represents aromatic ring quadrant breathing and the graphite E22g vibration (if any). From Figure 9a, for all biochars, the (Gr þ Vl þ Vr)/D ratio is lower at 900 °C than at

Figure 9. Peak areas from FT-Raman spectroscopy for biochars as a function of the thermal annealing time at 750 and 900 °C: (a) ratio of the (Gr þ Vl þ Vr) band area over the D band area and (b) fraction of the G band area out of the total peak area.

Figure 10. Ratio of the carbon structure population in the biochar at any thermal annealing time = t over the carbon structure population in the initial biochar (at thermal annealing time = 0): (a) (Gr þ Vl þ Vr), (b) D, and (b) G.

(61) Li, X.; Hayashi, J.; Li, C.-Z. Fuel 2006, 85, 1700–1707.

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750 °C and, at a given temperature, decreases with the thermal annealing time, showing that the proportion of the larger aromatic ring systems in the biochar becomes increasingly higher relative to the smaller aromatic ring systems. This has provided direct evidence that the biochar structure becomes increasingly ordered/condensed with the progress of thermal annealing. Meanwhile, from Figure 9b, the G band does not change for all of the biochars, indicating that the graphitization process is not important during thermal annealing under the current conditions. It can be further noted that, from Figures 8 and 9, perhaps except for the biochars before thermal annealing at 750 °C (i.e., at thermal annealing time = 0 min), there is no significant difference in the biochar carbon structure evolution between the biochars from acid-washed and raw wood. The discrepancy for the mentioned biochars at 750 °C with 0 thermal annealing time, especially in the Raman intensity, is probably due to the enhancement on the bond-breaking and bond-forming reactions during pyrolysis (during heating) by the catalytic species, leading to enhanced ring condensation.61 However, the discrepancy diminishes with the progress of subsequent thermal annealing. This, together with the similar trend of the biochar yield for acid-washed and raw wood (Figure 1b), shows that, under the current conditions, the inherent catalytic species in the biochars from raw wood have negligible effects on the thermal annealing process at these mild temperatures (750 and 900 °C). Another important observation is that, from Figures 8 and 9, the Raman intensity and (Gr þ Vl þ Vr)/D ratio change drastically at the early thermal annealing time up to ∼60 min and level off. This is similar to the trends of the biochar yield (Figure 1), loss of heteroatoms (Figure 5), and evolution of various gaseous productions (Figures 6 and 7) as a function of the thermal annealing time. Further insight into the evolution of the biochar carbon structure was obtained from the ratio of the population of a particular carbon structure in the biochar at any thermal annealing time t over its population in the initial biochar, i.e., biochar at 0 thermal annealing time. This ratio for a particular Raman band, calculated from the Raman band area and the biochar yield data, is given by

(Figure 1), loss of heteroatoms (Figure 5), and evolution of gaseous products (Figures 6 and 7) as a function of the thermal annealing time. It is also interesting to note that no appreciable transformation into the G band structure (G band) was evidenced from Figure 10c. This is not surprising because little graphitization is expected to take place under the heat treatment conditions. It is also worthwhile to note that, in our recent study15 on steam gasification of similar biochars, the biochars were produced from pyrolysis at 750 °C and further held at the temperature for 15 min before steam was introduced into the reactor for commencing biochar steam gasification. The ordering/condensation of the carbon structure during subsequent biochar steam gasification at the same temperature (750 °C)15 was substantially more significant than that because of thermal annealing itself observed here. As shown in Figure 9, the extensive holding at 750 °C from 15 to 600 min during thermal anneal leads to a reduction in the (Gr þ Vl þ Vr)/D ratio from 1.6 to 1.4. However, an increase in biochar conversion from 0 to 80% during steam gasification (Figure 4 in ref 15) leads to a significant reduction in the (Gr þ Vl þ Vr)/D ratio from 1.6 to 0.7. The data clearly demonstrate that, during biochar steam gasification at 750 °C, although thermal annealing does take place and contributes to a small part of the induced change in the biochar carbon structure, the dominant factor dictating the significant change in the biochar carbon structure during conversion is steam gasification itself. In the case of the study in the reference, the key mechanism is indeed “selective gasification” under the biochar gasification conditions.15 3.5. Further Discussion. Taking the biochar yield (section 3.1), oxygen and hydrogen contents (section 3.3) and results from FT-Raman spectroscopy (section 3.4) together, the mechanism of biochar thermal annealing at mild temperatures (750 and 900 °C) can be deduced. Thermal annealing occurs through the loss of heteroatoms, such as oxygen and hydrogen, via the release of gases that results in a reduction in the biochar yield. The thermal annealing process reduces the heterogeneity of the carbon structure and induces a more ordered/condensed carbonaceous structure, not only via the loss of part of the amorphous structure during gas release but also via the transformation of the amorphous structure into larger aromatic ring systems. Thermal annealing is drastic at the initial thermal annealing time and gradually levels off with little further release of heteroatoms from the biochar carbon matrix. The catalytic species appears not to affect the thermal annealing process under the current conditions. With the above mechanistic understanding, the effect of thermal annealing on biochar reactivity (section 3.1) will be discussed for the biochars from acidwashed wood (in the absence of catalytic effects) followed by the biochars from raw wood (in the presence of catalytic effects). In the absence of catalytic effects, the changes in biochar reactivity coincide with the changes in the biochar carbon structure induced by thermal annealing. For instance, the significant reduction of biochar reactivity at the early stage of thermal annealing is consistent with the drastic ordering/condensation of the carbon structure observed from FT-Raman during the initial period (section 3.4). This is because the more ordered/condensed carbon structure would have a higher activation energy to overcome for the oxidation reaction to occur.46,56 Thus, the change of the biochar carbon structure induced by thermal annealing is the

ðRaman bandtime ¼ t Þ=ðRaman bandtime ¼ 0 Þ ¼ ðfraction of band area out of total area for biochar at time ¼ t  biochar yield at time ¼ tÞ=ðfraction of band area out of total area for biochar at time ¼ 0  biochar yield at time ¼ 0Þ

The results are shown in Figure 10. This provides an indication on the extent of transformation of the carbon structure during thermal annealing. If there were no such transformation, the ratio would remain unchanged as 1 with the progress of thermal annealing, shown as dashed lines in Figure 10. However, Figure 10a shows that this ratio for the (Gr þ Vl þ Vr) band decreases and the ratio for the D band increases with the thermal annealing time. The data clearly demonstrate that, apart from the loss of part of amorphous carbon structures via gaseous product release during thermal annealing, a carbon structural transformation also took place and condensed at least part of the amorphous structures (Gr þ Vl þ Vr band, which is reactive) into the larger ring systems (D band that is more inert and less reactive) with the progress of thermal annealing. The trend of such a structural transformation is also similar to those of the biochar yield 413

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dominant factor controlling the subsequent reactivity of the biochars from acid-washed wood. In the presence of catalytic effects, the biochar reactivity is a more delicate function of both the biochar carbon structure and catalytic effects. Thermal annealing induces a change in the biochar carbon structure, which can in turn affect the catalytic activity during subsequent biochar oxidation. Thermal annealing appears to affect biochar reactivity in two broad pathways: (1) resulting in ordering/condensation of the carbon structure. This leads to a less reactive carbon structure, similar to the explanation for the case of biochars from acid-washed wood. (2) leading to changes in catalytic activity. Comparing the initial biochars (i.e., biochars at 0 thermal annealing time) at 750 and 900 °C (panels a and b of Figure 3), the latter generally has a higher reactivity. This is probably due to the fact that the loss of O-containing functional groups from 750 to 900 °C cause the catalytic species (originally bonded to the functional group) to be attached directly to the carbon matrix, which renders easier migration of the catalytic species to the biochar surface to form catalytically active compounds upon subsequent oxidation.62 This seems to be supported by the decrease of O content in the biochar from 750 to 900 °C (section 3.3) found in the present study. With further progress of thermal annealing, the ordering of the carbon structure with thermal annealing time can possibly enhance the sintering/ agglomeration of catalyst particles, leading to reduced dispersion of the catalysts and deactivation of the catalysts,20,45,50,51 as reflected in, for instance, the significant reduction in the reactivity up to 180 min thermal annealing time (Figure 3b), as well as the shift of the maximum reactivity point for biochars from raw wood (Figure 3b) to earlier conversion with increasing thermal annealing time. The present findings have important implications in the use of the biochar as a bioenergy source. The thermal annealing process can induce a significant change in the biochar carbon structure and can influence the biochar reactivity considerably, even under the current mild-temperature conditions. Therefore, in applications, such as biochar gasification in a fluidized bed, where the fuel particles typically

experience extensive residence time, it would be expected that the thermal annealing has significant effects on the gasification behavior as the biochar resides in the gasifier. In the modeling of biochar gasification kinetics as well as the development of energy use technologies of the biochar, the thermal annealing effect on the biochar reactivity and gaseous products evolved during thermal annealing thus need to be given important considerations. 4. Conclusions The present study has provided mechanistic understandings on the thermal annealing process of biochar. For biochars from both the acid-washed and raw wood, as directly evidenced from FT-Raman spectroscopy, the biochar carbon structure becomes increasingly ordered and condensed with the enrichment of larger aromatic ring systems, induced by thermal annealing. Thermal annealing is coupled with the loss of heteroatoms, released dominantly as H2. Such a process is drastic at the early stage of thermal annealing and levels off with further thermal annealing. Under these conditions, the inorganic species in biochar have negligible effects on the thermal annealing process. Overall, thermal annealing affects the biochar reactivity significantly. In the absence of catalytic effects, the change in biochar reactivity is controlled by the change in the biochar carbon structure induced by thermal annealing. In the presence of catalytic effects, the biochar reactivity is affected by changes in the carbon structure and catalytic activity. Under the current experimental conditions, using a biomass with minimal Si and Al contents, the loss/ volatilization of AAEM species during thermal annealing is insignificant. Therefore, the changes in the catalytic activity are most likely related to a change in the form/bonding of the catalytic species as well as a change in the dispersion of the catalysts within the biochar carbon matrix. Acknowledgment. The authors are grateful for the support from the Australian Commonwealth Government as part of the Asia-Pacific Partnership on Clean Development and Climate, as well as the support from the Curtin-Huazhong University of Science and Technology (HUST) Joint Research Laboratory for Coal and Biomass Utilisation. M. Xu also acknowledges partial support from the National Natural Science Foundation of China (50720145604).

(62) Quyn, D. M.; Wu, H.; Hayashi, J.-i.; Li, C.-Z. Fuel 2003, 82, 587–593.

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