Characteristics of high heating rate biomass chars prepared under N2 and CO2 atmospheres

International Journal of Coal Geology 77 (2009) 409–415

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International Journal of Coal Geology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / i j c o a l g e o

Characteristics of high heating rate biomass chars prepared under N2 and CO2 atmospheres A.G. Borrego a,⁎, L. Garavaglia b, W.D. Kalkreuth b a b

Instituto Nacional del Carbón, CSIC. Ap. 73. 33080 Oviedo, Spain Instituto de Geociências, UFRGS, Av. Bento Gonçalves 9500, 91501-970 Porto Alegre, Brazil

A R T I C L E

I N F O

Article history: Received 21 January 2008 Received in revised form 6 June 2008 Accepted 30 June 2008 Available online 4 July 2008 Keywords: Biomass chars Biomass reactivity Biomass pyrolysis High heating rate chars Oxy-fuel

A B S T R A C T Partial substitution of coal by biomass in combustion systems in conjunction with advanced technologies for CO2 capture and storage may result in a significant reduction of greenhouse gases emissions. This study investigates three biomass chars produced from rice husk, forest residuals and wood chips under N2 and CO2 atmospheres using a drop tube furnace (DTF) heated at 950 °C. The char constitutes an unburned residue which has been devolatilized under conditions resembling in thermal history those in full scale boilers. Higher weight losses were achieved under N2 than under CO2 for each type of biomass, and the highest weight loss was that of wood chips biomass, followed by forest residuals and then rice husk. The results indicate significant morphological differences between the biomass chars produced. The wood chips yielded thick-walled chars with a cenospheric shape very similar to those of low-rank vitrinite. The forest residual chars were angular in shape and often had a tenuinetwork structure, while the rice husk chars retained their vegetal structure. Overall, the studied biomass chars can be described as microporous solids. However, in the case of the rice husk, the silica associated to the char walls was essentially mesoporous, increasing the adsorption capacity of the rice husk chars. The atmosphere in the DTF affects the development of porosity in the chars. The pore volumes of the rice husk and forest residual chars prepared under a CO2 atmosphere were higher than those of chars prepared under a N2 atmosphere, whereas the opposite was the case with the wood chip chars. The chars that experienced the most drastic devolatilization were those with the lowest intrinsic reactivity. This indicates a more efficient reorganization of the chemical structure that reduces the number of active sites available for oxygen attack. Overall a similar morphology, optical texture, specific surface area and reactivity were found for the biomass chars generated under N2 and CO2, which is a similar result to that obtained for coal chars. © 2008 Elsevier B.V. All rights reserved.

1. Introduction The growing concern over global warming and its effect on climate change has prompted the search for new, cleaner methods of electricity production preferably from renewable energy sources. Biomass fuels, such as wood, herbaceous materials and agricultural by-products are of significant interest because of their CO2 neutrality. Nowadays coal and biomass co-firing is routinely performed in many coal fired boilers (www.eubionet.net). Furthermore, the combination of biomass combustion with carbon capture and storage technologies would lead to the net removal of CO2 from the atmosphere and to the concept of negative CO2 emissions (Azar et al., 2006). Oxy-fuel combustion technology is one of the main methods being considered for carbon capture from large coal fired power plants. This technology has developed rapidly in recent years (Farzan et al., 2007)

⁎ Corresponding author. Tel.: +34 985119090. E-mail address: [email protected] (A.G. Borrego). 0166-5162/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.coal.2008.06.004

and at present pilot plants for pulverized coal oxy-firing are under construction in many countries (i.e., Germany, Australia, and Spain). Boilers using fluidized bed technology with an oxy-fuel atmosphere will also be operating in the near future (Erikson et al., 2007). Oxy-fuel technology essentially consists in burning the fuel in a mixture of recycled CO2 and O2, producing, as consequence, a flue gas mainly composed of water that is easily condensable and highly concentrated CO2 quasi-ready for storage (Anheden et al., 2005). As combustion is generally accepted to proceed through fast devolatilization followed by the combustion of the char, the structure, porosity and reactivity of the chars is critical for the overall efficiency of the process (Essenhigh et al., 1989). Despite the large amount of literature devoted to biomass combustion in recent years (Demirbas, 2004; 2005; Bourke et al., 2007), there have been few studies on the characteristics of high heating rate chars such as those generated when biomass is co-fired with coal in industrial boilers (Cetin et al., 2004; Boateng et al., 2007; Guerrero et al., 2008). Neither is there any reference to the characteristics of biomass chars generated under oxyfuel combustion atmosphere. The previous studies on oxy-fuel coal

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chars have shown similar morphology, reactivity and microporous surface area (Alvarez et al., 2005; Borrego et al., 2007a) to coal chars produced by conventional combustion, but different mesoporosity development (Elliot et al., 2005; Borrego and Alvarez 2007). In the present work, the characteristics of biomass chars generated at a high heating rate have been studied, including chars generated under inert (N2) and CO2 atmospheres. The behaviour under CO2 could be relevant if biomass is to be co-fired with coal in oxy-fuel combustion boilers. 2. Experimental 2.1. Fuel characterization The samples selected are widely used in Brazil for different applications and consist of Forest residues (FR), rice husk (RH), and wood chips (WC). Chemical analyses were performed using a LECO TGA-601 for proximate analysis, LECO AC-300 for calorific value determination and LECO CHN-2000 and S-144DR for the ultimate analysis. The reactor is normally fed with samples sieved to a narrow particle size interval (i.e. 36–75 μm) to ensure homogeneous environment for the particles. The peculiar morphology of the ground biomass particles, related to their botanical origin, and their low density make the sieving difficult. Considering the particle size relevance to the particle devolatilization (Zhang et al., 1992), a particle size analysis was performed using a Coulter Particle Size Analyser LS-13320. Additional fuel characterization included pyrolysis and combustion tests at programmed temperature using a thermogravimetric analyser (the sample quantity: 13 mg, the flow rate 35 mL min− 1, the temperature range: 35–900 °C, and the heating rate: 20 °C min− 1). Air was used as the reacting gas for the combustion profiles and N2 for the pyrolysis profiles. In the pyrolysis experiments the N2 was replaced by air at 900 °C to isothermally record the burning rate of the carbonaceous residues generated during devolatilization at this temperature. 2.2. Char preparation The biomass chars were prepared in the Drop Tube Furnace (DTF) at a temperature of 950 °C, under N2 and CO2 atmospheres. The reactor is a furnace which surrounds two concentric alumina tubes (with an inner diameter of 70 and 50 mm, and length of 1.3 and 1.0 m, respectively). The reacting gas (900 L h− 1) was injected at the bottom of the outer cylinder and was preheated while flowing upwards. Once at the top of the outer cylinder, the gas was forced into the inner tube through a flow straightener, and the gases flowed downwards and left the reactor through a water-cooled collection probe. The fuel particles were entrained by a jet of non-preheated gas of the same composition as the reacting gas (300 L h− 1) to a water-cooled injection probe placed on top of the inner tube. The feeding rate was around 6 cm3 min− 1 and the estimated residence time of the particles in the reactor was 0.3 s. The chars left the reactor through the collection probe, and an extra nitrogen flow was added to the exhaust gases in order to quench the reaction and improve collection efficiency in the cyclone. Weight loss was calculated with the ash tracer assuming that mineral matter undergoes similar transformations in the reactor to those during the determination of the ash content in proximate analysis. This is the most widely accepted procedure although it is well known that for such low ash fuels errors may be high (Carpenter and Skorupska, 1993). The expression for the ash tracer can be written as     Ashbiomass 100−Ashchar Weight loss ðkÞ ¼ 1−  100 100−Ashbiomass Ashchar

2.3. Char characterization Two widely used methods for determining the pore surface area of carbon from the gas adsorption isotherms were applied to the chars, using CO2 at 0 °C and N2 at −196 °C as adsorptives. The equipment used was a Micromeritics ASAP 2020 instrument. Chars were outgassed under vacuum prior to the gas adsorption experiments in order to eliminate moisture or any condensed volatiles which could prevent the adsorbate from gaining access. The heating rate used was 5 °C min− 1 with holding temperatures of 90 °C (1 h) and 350 °C (4 h). The latter is well below the char preparation temperature and is therefore unlikely to modify the structure of the char. During the degassing process, some condensed volatiles were released as shown by the deposition of dark tars in the sample tube. The Dubinin and Radushkevich (1947) equation (D–R) was applied to the CO2 adsorption data and the Brunauer–Emmett–Teller theory (BET; Brunauer et al., 1938) was applied to the N2 data. These two methods can be regarded as complementary, given the difficulties that CO2 has in filling the large micropores and the slow diffusion of N2 through the small micropores (Jagiello and Thommes, 2004). The t-plot method (De Boer et al., 1966) was applied to the N2 isotherms in order to split the pore volume assigned to the micro- and mesopores. The surface area and pore volume are expressed on an ash-free basis considering negligible the porosity of the inorganic material for the chars WC and FR. This is similar to coal char ashes with typical surface area values around 0.8 m2 g− 1. The large ash content of the RH chars (around 75%) and the fact that their mineral matter is essentially amorphous silica for which surface area values ranging between 6 and 150 m2 g− 1 (Chandrasekhar et al., 2005) have been reported led us to make a separate assessment of the RH ash textural properties. Therefore the RH char was ashed at 550 °C to minimize the transformation of amorphous silica into cristobalite, which is known to start at 700 °C (Chandrasekhar and Pramada, 2006). Then N2 and CO2 isotherms were performed on the ashed sample in order to determine the surface area of the silica from the rice husk. The apparent reactivity of the chars was measured in a thermogravimetric apparatus. A small quantity of char (13 mg) was homogeneously spread over the bottom of the platinum crucible and then heated up to 550 °C under a nitrogen flow (35 mL min−1) at a heating rate of 20 °C min− 1. After weight stabilisation, nitrogen was replaced by air at the same flow rate and the temperature was maintained until combustion was completed. At this low temperature and with such small sample sizes, bed effects in the thermobalance can be ruled out and the kinetic control of the reaction is ensured. The reactivity was calculated as R=m−1 o (dm/dt) where mo is the initial ash-free mass of the coal. The samples were embedded in polyester resin and polished for petrographic examination under incident polarized light. They were also examined under the scanning electron microscope (SEM). 3. Results and discussion 3.1. Characteristics of the biomass samples The proximate and ultimate analyses of the studied biomass samples are shown in Table 1. Samples FR and WC have very low ash

Table 1 Proximate and ultimate analyses of the individual fuels Fuel

ð1Þ

where Ashbiomass and Ashchar are the weight percent of ash in the feed biomass and in the collected char, respectively.

RH WC FR

CV

Ash

kcal kg− 1 db

db %

3440 4580 4815

29.06 0.62 3.12

VM

C

H

N

O

S

0.52 0.12 0.79

44.96 45.38 45.76

0.11 0.02 0.08

daf % 83.39 85.40 81.77

47.53 47.97 47.25

6.88 6.51 6.12

CV=calorific value, VM=volatile matter; db=dry basis; daf=dry-ash-free basis, RH=rice husk, WC=wood chips, FR=forest residuals.

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Fig. 1. Particle size distribution of the biomass samples fed into the drop tube furnace. Av=mean value.

contents as is common for biomass. The RH sample has a higher ash content, which mainly consists of amorphous silica. This silica has a structural function in the vegetal (Takahashi et al., 2006) and has been investigated as a source of materials for catalyst carriers and pozzolanic applications (Chandrasekhar et al., 2003). The calorific value is lower for sample RH due to its high ash content but the composition of the organic fraction expressed on an ash-free-basis is very similar for the three biomass samples. The biomass samples have lower carbon and sulphur contents and higher volatile matter and oxygen contents than typical lignite coals (C = 70–74%, S = 0.4–0.5%, volatile matter = 62–50%, O = 23.5–19.6%; van Krevelen, 1993). The shape of the particle size distributions (Fig. 1) showed that RH is the sample with the narrowest distribution, indicating a rather homogeneous particle size with a mean value of 259 μm. The sample with the widest distribution and the smallest average size (200 μm) was WC. The largest mean particle size was measured for FR (284 μm). The differences in particle size between the samples were not large and all the biomass samples had larger size than the typical pulverized fuel size (80% of sample under 75 μm). A comparison of the pyrolysis and combustion profiles provides information about the way that volatiles are released in the two processes. Pure cellulose, hemicellulose and lignin (Worasuwannarak et al., 2007), which are the main components of biomass in different proportions, display different patterns of devolatilization in the thermobalance. Yang et al. (2007) showed that during the pyrolysis experiments at programmed temperature lignin has the lowest volatile yield but the largest temperature decomposition interval, starting at 160 °C and ending at 900 °C. Both hemicellulose and

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cellulose decompose in a single step with the peak temperature decreasing from hemicellulose to cellulose. The decomposition peaks were clearly separated in physical blends of pure compounds but they significantly overlapped in natural biomass palm oil samples containing different relative proportion of the three constituent components (Yang et al., 2006). The higher proportion of hemicellulose in the sample resulted in the higher overlapping. Fig. 2 shows the different shapes of the decomposition curves of each biomass type. Both RH and FR have a main decomposition peak attributed to cellulose breakdown and a shoulder at lower temperatures assignable to hemicellulose decomposition. The hemicellulose and cellulose decomposition overlap in sample WC which shows only one main decomposition peak. This could be attributed to a higher relative amount of hemicellulose in WC sample (Yang et al., 2006). After the main devolatilization, there is a continuous and slight weight loss that is attributed to the decomposition of heavy chemical structures generated during the main devolatilization phase (Biagini et al., 2006). The combustion profiles share some distinguishing features that differ from the pyrolysis profiles (Fig. 2): i) they all have a main peak that is more symmetrical than in the pyrolysis profiles; ii) they undergo significant weight loss at temperatures over 350 °C corresponding to the combustion of a refractory carbonaceous residue formed during the first combustion stages, and iii) the main peaks of the combustion profiles occur at lower temperatures than in the pyrolysis profiles (Fig. 2). For the three samples the combustion and pyrolysis profiles follow the same trend at low temperature, at which the dominant process is the release of labile bonded compounds. This indicates that the desorption of volatiles is independent of the reacting atmosphere. At slightly higher temperatures combustion losses are greater than pyrolysis losses (Fig. 2). This can be attributed to heterogeneous combustion (gas–solid) or to the loss of species formed in the presence of oxygen as the temperature in the thermobalance rises. These species may be easier to remove than those forming the biomass structure and may be responsible for the shift to lower temperatures of the combustion profiles compared to the pyrolysis profiles. A similar situation is observed for low-rank coals (Alonso et al., 2001a). The temperature at which the combustion and the pyrolysis profiles separate is roughly coincident with the ignition temperature (Tg), at which combustion weight loss accelerates. The relevant parameters extracted from the combustion and pyrolysis profiles are shown in Table 2. The sample RH exhibits a higher reactivity and reacts at lower temperatures than both the FR and the WC. The faster reaction rate of RH could be related to its homogeneous particle size distribution. This can be controlled by the botanical morphology which can cause the preferential disarticulation or breakdown through fragile parts upon grinding. The influence of other parameters such as differences in intrinsic reactivity cannot be ruled out. In contrast, the reactivity of RH coke generated after N2-devolatilization at 900 °C was the lowest,

Fig. 2. Pyrolysis (py) and combustion profiles (cb) recorded at increasing temperature in a thermobalance for a) rice husk (RH), b) wood chips (WC) and c) forest residuals (FR).

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Table 2 Relevant parameters extracted from the biomass combustion (air atmosphere) and pyrolysis (N2 atmosphere) experiments performed at programmed temperature (from 35 to 900 °C at 20 °C min− 1) in a thermobalance (RH=rice husk, WC=wood chips, FR=forest residuals) Combustion profiles

RH WC FR

Pyrolysis profiles 3

3

Rcoke 900 °C (×103)

Tg

Tp

Rmax (×10 )

Tf

Tp

Rmax (×10 )

Tf

°C

°C

mg mg− 1 s− 1

°C

°C

mg mg− 1 s− 1

°C

mg mg− 1 s− 1

223 251 232

274 309 290

5.74 4.58 4.60

540 545 509

326 344 330

3.67 3.51 3.33

324 355 341

3.11 5.10 4.69

Tg=ignition temperature, Tf=final temperature Tp=peak temperature, Rmax=maximum reactivity, Rcoke=reactivity to air at 900 °C of the coke generated during the pyrolysis experiment.

which is indicative of strong reorganization of the structure upon heating at slow heating rate. At this temperature both the intrinsic reactivity of the material and its accessibility affects the combustion rate. 3.2. Characteristics of the DTF chars Chars were generated in the DTF under two different atmospheres. The weight losses due to volatile release are shown in Fig. 3 along with the results of the proximate volatiles obtained at low heating rate. The weight losses in the DTF, in which particles undergo high heating rates, were higher than those of the proximate volatiles. In addition, the weight losses under N2 were slightly higher than under CO2 for all three samples, indicating enhanced devolatilization under nitrogen (Fig. 3). A similar result has been obtained for coals (Borrego and Alvarez 2007), although the difference between conversion under N2 and CO2 was larger than for the biomass samples. This has been attributed to the participation of CO2 in crosslinking reactions at the surface of the devolatilizing particle, which would prevent a more drastic devolatilization. The highest weight loss was achieved by WC and the lowest by RH. The RH sample was the least sensitive to the devolatilization rate because the differences between proximate volatile matter content (low rate) and DTF weight loss (high rate) were the smallest. If these results are compared to the pyrolysis thermograms, a clear trend towards higher DTF weight loss for samples with a higher pyrolysis peak temperature can be observed. After passing through the reactor the particles had a swollen and devolatilized appearance (Fig. 4). The RH chars still retain the morphology of the botanic precursor, although certain parts have generated devolatilization voids. The particles typically show an important reflectance gradient with the core having a higher reflectance than the external part (Fig. 4a and d). This could be attributed to differences in the degree of transformation. The silica of the RH outer epidermal cells is not liberated during the rapid heating and remains associated with the walls of the devolatilized char (Fig. 4g). The morphology of the FR and WC chars was markedly different. Both samples generated high reflectance isotropic chars which indicate a carbon-rich disordered structure and had an appearance very similar to that of coal chars (Fig. 4b–c,e–f, and h–i). The WC char consisted mainly of thick-walled cenospheric (a single central void) particles resembling the typical vitrinite-derived particles (Fig. 4e) from lignites and subbituminous coals (Alonso et al., 2001b). The FR char gave rise to a variety of morphologies ranging from thin-walled networks to cenospheres, with a clear predominance of multivacuolated structures (Fig. 4f). Similar to coals, the diffusion of the volatiles would lead to the coalescence of the bubbles and the ultimate structure of the char would depend on when the coal particles resolidify (Yu et al., 2007). The differences in the morphology of the WC and the FR chars may reflect their differences in plasticity. The volatiles in WC evolved to form a single central bubble which grew, causing the swelling of the surrounding

carbonaceous matter (Fig. 4e). The pressure exerted by the volatiles in FR was not enough to cause further coalescence of the bubbles. This limitation generated a network structure and particles with an angular external shape, indicative of the lower swelling (Fig. 4f). The sample with the highest volatile content was WC. The difference between proximate volatiles, released when the sample was heated slowly, and the DTF volatiles released at high heating rates was the largest in WC (Fig. 3), indicating a very efficient volatile release. There were no major morphological differences between the N2-prepared chars and the CO2-prepared counterparts. The reactivity of the chars to oxygen at low temperature (550 °C), at which diffusional constraints are not expected to be significant, depends on the specific surface areas and the chemical structure. The values recorded in the thermobalance will be named “apparent” reactivity, as opposed to “intrinsic” reactivity which is reserved for the data once corrected for the specific surface area. The highest apparent reactivities were those of the RH chars; followed by the FR chars and the WC chars (Table 3). The reactivities of the N2-chars were consistently lower than those of the CO2-chars. In the case of the WC and FR chars the differences were small but the difference was larger for the RH chars. Information about the pore size and surface area of carbonaceous materials is always more complete if both N2 and CO2 isotherms are available. Both FR and WC chars can be described as essentially microporous solids, whereas the RH chars contain both micro and mesopores (Borrego et al., 2007b). As mentioned in the experimental section an adsorption isotherm was obtained for the ashed rice husk sample in order to determine how much of the porosity recorded could be attributed to the amorphous silica and how much to the organic material. The adsorption isotherms of the RH chars and the RH silica remaining after combustion at 550 °C are plotted in Fig. 5. The isotherms were of Type II on the BDDT classification (Brunauer–Deming–Deming– Teller, Brunauer et al., 1940), which corresponds to solids with a mixture of micro- and mesoporosity. In the case of the RH silica the isotherm indicates a mesoporous solid. At low relative pressures the volume of N2 adsorbed by micropores is higher for the RH chars (around 25% organic and 75% silica) than for the pure silica. The opposite occurs at higher relative pressures (adsorption on mesopores). The presence of microporous organic matter intimately associated to the silica (Fig. 4g) could prevent the N2 access to some of the inorganic mesopores. The isotherms in Fig. 5 show that the inorganic fraction of the chars is mainly responsible for the N2-adsorption on the mesopores. Although the RH isotherm could not be used to correct the RH char adsorption data on an ash-free basis, it shows that fast heated silica to 950 °C for some hundred milliseconds retain high specific surface areas (SBET = 116 m2g− 1). The t-plot method allows the micro- and mesoporosity to be split on the basis of the N2 adsorption data. Fig. 6 shows the

Fig. 3. Weight loss of the biomass samples in the drop tube furnace (DTF) under CO2 and N2 atmospheres compared to the proximate volatile yields expressed on an ash-free basis (VM daf).

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Fig. 4. Appearance of the biomass chars under the optical microscope (top and middle; a–f) and under the scanning electron microscope (SEM; bottom; g–i) showing the typical features of the biomass chars. Rice husk (RH) chars retained a botanical structure (a, d) and show the silica (brighter in the SEM image) associated to the char walls (g). Wood chip (WC) chars were cenospheric (b, e, h) and forest residual (FR) chars had mostly network-like structure (c, f). Optical microscope images were taken under incident polarized light with a 1-λ retarder plate.

distribution of the volume adsorbed into micro and mesopores for the various chars. In the case of the RH chars only the micropore volumes, which no doubt correspond to the organic matter, are expressed on an ash-free basis, whereas the mesopore volumes are plotted as measured. In the case of the WC and the FR, both the meso- and micropore volumes are considered as part of the organic matter and are therefore corrected for the ashes. The wood chips and forest residuals are somewhat similar materials, although the material that forms the wood chips is restricted to the woody parts of the plants, whereas the forest residuals contain more heterogeneous materials. The N2- and CO2-pore volumes were larger in the WC chars than in the FR chars (Fig. 6). In general, the CO2-micropore volume was larger than the N2-micropore volume and the difference indicates the importance of narrow microporosity in the char. Narrow microporosity was also present in the RH chars, although it was relatively less abundant than in the FR chars. In the case of the WC chars, narrow microporosity was only present in the char prepared under a CO2 atmosphere. The effect of the char preparation atmosphere was also different for the three biomass samples (Fig. 6). For the RH and FR chars, the pore volume was higher in the pyrolysis char than in the CO2 counterpart, whereas the opposite was the case for the WC chars. The WC char behaved like bituminous coal chars, in which the CO2 in the devolatilizing atmosphere was thought to participate in crosslinking reactions, thereby reducing the extent of devolatilization and preventing the coalescence and stacking in the structure, with the subsequent annihilation of the narrowest porosity (Borrego and Alvarez, 2007). This mechanism would be supported by the distribu-

tion of pore size that was larger in the N2 char (lower total pore volume) indicating inaccessibility of the narrowest porosity. The pore size distribution of the N2 chars was shifted to lower sizes compared to CO2 chars for the RH and the FR. If the same mechanism is to be considered, the observed distribution of pore size in RH and FR could be explained as consequence of the lower plasticity of the parent material. Rearrangement of the carbonaceous structure would be equally enhanced under N2 compared to CO2, but the lower plasticity of the parent material may possibly prevent the more drastic rearrangement of the structure and therefore destruction of the narrow microporosity.

Table 3 Relevant textural and reactivity parameters of the drop tube furnace biomass chars (RH=rice husk, WC=wood chips, FR=forest residuals) Sample

RH

Char preparation atmosphere

N2

SBET (m2 g− 1) ash-free-basis⁎ SD–R (m2 g− 1) ash-free-basis Rap × 103 (mg mg− 1 s−1) Rin × 106 (Rap/SD–R; g m− 2 s− 1) Rin × 106 (Rap/SBET; g m− 2 s− 1)

254 552 2.7 5.0 10.8

WC

FR

RH

WC

FR

331 636 1.9 3.0 5.7

158 446 1.5 3.3 9.3

CO2 277 493 2.0 4.1 7.3

225 490 1.6 3.2 6.9

208 522 2.3 4.4 11.0

SBET=specific surface area determined on N2 isotherms by the BET method. ⁎The SBET ash-free for RH has been calculated using the t-micropore ash-free surface area, plus the external surface area as measured. SD–R=specific surface area determined on CO2 isotherms by the Dubinin–Radushkevich method). R ap =apparent reactivity, Rin=intrinsic reactivity.

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Table 3 shows the specific surface areas expressed on an ash-free basis for the various chars. As the SBET of RH chars would comprise the surface area of both silica and carbonaceous material, only the micropore surface area extracted from the t-plot was corrected for the ashes. For all the chars, the SD–R was higher than SBET indicating the existence of the narrow porosity. The specific surfaces areas (Table 3) followed a similar trend to that observed for pore volumes. The WC char prepared in CO2 had larger SBET and SD–R values than the pyrolysis char, whereas the opposite was the case for the RH and FR chars. In addition, for each char series, WC exhibited the largest surface areas followed by RH and FR. The apparent reactivities can be corrected for surface area in order to obtain the intrinsic reactivity of the carbonaceous material. Regardless of the value used to correct the apparent reactivity data (SD–R or SBET), the highest intrinsic reactivity corresponded to the RH chars, which underwent the lowest weight losses. The lowest intrinsic reactivities corresponded to the WC chars. Overall, there is an inverse relationship between the weight loss in the DTF and the intrinsic reactivity of the generated char. 4. Conclusions The thermal behaviour of various biomass materials has been studied with special emphasis on the characteristics of the chars generated at a high heating rate in a drop tube furnace (DTF) under inert (N2) and CO2 atmospheres. The weight loss achieved at a high heating rate followed a linear trend with the peak temperature of the slow heating rate pyrolysis profiles indicating that these profiles can be used to predict the extent of devolatilization of the biomass at high heating rates (wood chips: WC N forest residuals: FR N rice husk: RH). For the three biomass chars, the DTF weight losses under CO2 were slightly lower than under N2 and both were higher than proximate volatile yields. The WC and FR samples generated a highly devolatilized char with an isotropic disordered structure which resembled the morphologies of lignite and subbituminous chars. The RH still retained the morphology of its botanical precursor with abundant silica on the char walls. Overall, biomass chars can be described as microporous solids although, in the case of the RH, the silica ash was essentially mesoporous affecting the volume adsorbed by the rice husk chars to an undetermined extent. Attempts to separate the amount adsorbed by the organic fraction and the silica were not effective because the silica adsorbed more when free of organic matter than when associated with it. This indicates that the organic-mineral association affects the adsorption properties of both components.

Fig. 6. Micro- and mesopore volumes of the drop tube furnace (DTF) biomass chars (RH=rice husk; WC=wood chips; FR=forest residuals) obtained under N2 and CO2 preparation atmospheres. Micropore CO2 was determined from CO2 isotherms at 0 °C. Micro- and mesopore volumes accessible to N2 was determined from N2 adsorption isotherms at −196 °C applying the t-plot method. Pore volumes are expressed on an ash-free basis, except for RH mesoporous volume that is plotted as measured because it was not possible to establish the contribution of silica and organic matter to the mesoporosity (Fig. 5).

The atmosphere of char preparation (CO2 vs. N2) affected the porosity development of the chars in different ways. A higher pore volume was observed for the RH and the FR chars generated under CO2 than under N2, whereas the opposite was the case for the WC chars. The biomass undergoing the highest weight loss in the DTF (wood chips) generated the chars with the lowest intrinsic reactivity, indicating a more efficient organization of the carbonaceous structure, as a result of which the number of active sites vulnerable to oxygen attack was reduced. The wood chip chars also exhibited the largest specific surface areas. A poor correlation was found between the peak temperature of the combustion profiles and the intrinsic reactivity of the drop tube furnace chars. Overall, similar characteristics were observed for morphology, optical texture, specific surface area and reactivity of the chars generated under the two atmospheres tested (N2 and CO2). A similar result was found for the coal chars generated under N2 and CO2 atmospheres. As far as the characteristics of the chars are concerned, no specific difficulties are expected from the replacement of air by O2/CO2 atmosphere in biomass/coal co-combustion boilers. Acknowledgements

Fig. 5. N2 adsorption isotherms of the rice husk (RH) chars prepared in the drop tube furnace (DTF) under N2 and CO2 atmospheres and after removal of the organic material at mild temperature (RH-ash). The isotherm indicates that RH silica is a mesoporous solid that might be responsible for at least some of the mesoporosity of the RH chars.

The assistance of Diego Alvarez from INCAR-CSIC in the operation of the drop tube furnace, the preparation of the chars and the SEM study is especially acknowledged. The support of CSIC and CNPq through a bilateral co-operation project is gratefully acknowledged (CSIC 2005BR0054/CNPq 690100/02-7). The Spanish team thanks the Ministry for Education for its financial support through the project PSE2-2005. This study is part of a major research project on the co-combustion of coal and biomass in small and local power genaration generation plants financed in Brazil by CNPq (CNPq 552113/2004-6). W. Kalkreuth thanks CNPq for a personal research grant (CNPq 304991/2003-1). The Guest Editor (Hamed Sanei) and two anonymous referees are thanked for their suggestions to improve the manuscript.

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