Oxidation Kinetics of Eucalyptus Chars Produced at Low and High

2084

Energy & Fuels 2008, 22, 2084–2090

Oxidation Kinetics of Eucalyptus Chars Produced at Low and High Heating Rates Marta Guerrero,* M. Pilar Ruiz, Ángela Millera, María U. Alzueta, and Rafael Bilbao Aragón Institute of Engineering Research, Department of Chemical and EnVironmental Engineering, C/ María de Luna 3 (Torres QueVedo Building). UniVersity of Zaragoza, 50018 Zaragoza, Spain ReceiVed October 30, 2007. ReVised Manuscript ReceiVed January 11, 2008

An experimental and kinetic study of the oxidation of eucalyptus chars obtained by pyrolysis at 900 °C with low and high heating rates (LHR and HHR, respectively) has been performed. Oxidation experiments were carried out in a quartz reactor with an inlet oxygen concentration ranging from 100 to 1000 ppmv in a nitrogen flow, a temperature of 900 °C, and a flow rate of 1000 mL min-1 (STP). In order to analyze the temperature influence on the process and to obtain the process activation energy, char oxidation tests were also performed in the 650–900 °C temperature range, for a given oxygen concentration of 500 ppmv. Kinetic parameters were derived from the oxidation reaction data, which fitted properly the macroscopic model of type I in regime II. The reaction orders with respect to the oxygen concentration were found to be 0.58 and 0.66 for LHR and HHR chars, respectively. In both cases, it was observed a “break” in the Arrhenius plot. At low temperatures (650–800 °C), the activation energy was similar for the oxidation of both chars, in the 35–45 kJ mol-1 range. However, in the high temperature range (800–900 °C), the activation energy was 100 kJ mol-1 for the LHR char oxidation and 60 kJ mol-1 for the HHR char oxidation. The differences between the oxidative reactivities of the LHR and HHR chars are further discussed in terms of the chemical and physical char properties.

1. Introduction The use of biomass materials in thermochemical processes has different attractive environmental benefits since it can substantially reduce SO2, NOx, and heavy metals emissions.1 In combustion applications, coal–biomass cofiring is one of the most potential short-term options for the use of this renewable fuel with a positive impact on both the environment and the economics of power generation.2–4 Short-rotation woody crops such as willow, oak, and eucalyptus have turned out to be the biomass materials with the highest energy potential.2–5 Its renewability can be as short as 3–5 years in the case of eucalyptus trees cultivated in energy plantations, with results very interesting from an energy point of view. In order to improve the performance of combustors, it is necessary to study the subprocesses that coal or biomass particles experience when they are injected into the combustor. Concerning this, attention must be focused, among other things, on the kinetics of char oxidation, which is generally slower than the decomposition and combustion of volatile matter.6 The coal char oxidation process has been widely studied. Its reaction mechanism is a complex heterogeneous process involv* Corresponding author. Phone: +34-976-761150. Fax: +34-976-761879. E-mail: [email protected]. (1) Biagini, E.; Pintus, S.; Tognotti, L. Proc. Combust. Inst. 2005, 30, 2205–2212. (2) Harding, N. S.; Adams, B. R. Biomass Bioenergy 2000, 19, 429– 445. (3) Tillman, D. A. Biomass Bioenergy 2000, 19, 363–364. (4) Werther, J.; Saenger, M.; Hartge, E. U.; Ogada, T.; Siagi, Z. Prog. Energy Combust. Sci. 2000, 26, 1–27. (5) Sami, M.; Annamalai, K.; Wooldridge, M. Prog. Energy Combust. Sci. 2001, 27, 171–214. (6) Lu, L.; Kong, C.; Sahajwalla, V.; Harris, D. Fuel 2002, 81, 1215– 1225.

ing various physical and chemical steps.7–9 These include mass and heat transfer to and from char particle surface, diffusion processes through the pores of the char, surface area evolution with char conversion, char fragmentation, and heterogeneous reaction of gas molecules with sites and chemisorbed species on the surfaces of the char particles. The results arising from the theoretical and experimental researches into the carbon–oxygen heterogeneous mechanism, mainly for coal char, have made possible to establish the certainty of the fundamental aspects occurring during the char oxidation process.10 Reactions r.1, r.2, and r.3 are considered to describe the generic carbon–oxygen reaction mechanism.9,11,12 -C + –C + O2 f –C(O) + –C(O)

(r.1)

-C + –C(O) f CO + –C –C(O) + –C(O) f CO2 + –C

(r.2) (r.3)

The first step considered is a dissociative chemisorption of O2 on two adjacent free carbon sites, –C, to form two monooxide surface complexes, –C(O), (r.1). Generally, the higher the surface energy the more favored adsorption of gas species.10 (7) Chan, M. L.; Jones, J. M.; Pourkashanian, M.; Williams, A. Fuel 1999, 78, 1539–1552. (8) Backreedy, R. I.; Habib, R.; Jones, J. M.; Pourkashanian, M.; Williams, A. Fuel 1999, 78, 1745–1754. (9) Williams, A.; Pourkashanian, M.; Jones, J. M. Prog. Energy Combust. Sci. 2001, 27, 587–610. (10) Campbell, P. A.“Investigation into the roles of surface oxide complexes and their distributions in the carbon-oxygen heterogeneous reaction mechanism”. Ph.D. Thesis, Standford University, Palo Alto, CA, 2005. (11) Skokova, K.; Radovic, L. R. American Carbon Society, 23rd Biennial Conference on Carbon, Penn State University, State College, PA, July 13–18, 1997. (12) Di Blasi, C.; Buonanno, F.; Branca, C. Carbon 1999, 37, 1227– 1238.

10.1021/ef700643p CCC: $40.75  2008 American Chemical Society Published on Web 03/01/2008

Oxidation Kinetics of Eucalyptus Chars

Energy & Fuels, Vol. 22, No. 3, 2008 2085

Carbon sites located in high surface energy regions of a char surface are more vulnerable to be attacked by gas-phase species. Consequently, functional groups are more likely to be formed and found on these carbons. The –C(O) complexes are precursors to the formation of not only CO (r.2) but also CO2 (r.3). Reaction r.3 was suggested by Walker et al.13 as the most plausible reaction for the formation of CO2 from surface complexes. According to the proposed mechanism described, the CO/CO2 ratio is given by the eq 1, where k2 and k3 represent the kinetic constant of the reactions r.2 and r.3, respectively. k2 CO ) CO2 k3[-C(O)]

(1)

While several studies have been developed for coal char oxidation, as mentioned above, the studies related to the oxidation of chars formed from biomass pyrolysis are not as extensive as those for coal chars, particularly for chars produced at high heating rates.9,12,14–17 Kinetic parameters of char oxidation differ depending on the fuel type, experimental apparatus, operating conditions, and the reaction models used. According to this, a large spread of reaction orders with respect to oxygen partial pressure from 0.25 to 1 has been found in the literature with activation energies varying between 100 and 240 kJ mol-1. Taking into account all these issues, the aim of the present work is to carry out an experimental and kinetic study of the oxidation of biomass chars. For this purpose, eucalyptus has been chosen as the starting material to obtain the chars, since this biomass material has very fast growth rate and can therefore be used as a regular supply of fuel, as well as to be low in nitrogen and ash content. Due to the strong relationship between heat treatment and char reactivity, eucalyptus chars have been formed under low (LHR) and high heating rate (HHR) conditions. Therefore, the influence of heating rate on char reactivity toward oxygen is also analyzed. 2. Experimental Approach 2.1. Material. The biomass chars selected in order to study the heterogeneous carbon–oxygen reaction arise from the pyrolysis in nitrogen atmosphere of eucalyptus (Eucalyptus Globulus Labill from a forest station located in Cedeira, La Coruña, Spain) subjected to low (LHR) and high heating rates (HHR) and a final temperature of 900 °C. The experimental facilities employed for preparing the LHR and HHR chars along with the experimental approaches followed and the char characteristics have been reported elsewhere.18,19 Pyrolysis experiments at low heating rate (∼10 °C min-1) were carried out in a fixed bed reactor while a fluidized bed reactor was employed for preparing chars at high heating rate. Table 1 shows the results relating to the ultimate analysis and oxygen functional group content (O*) of the LHR and HHR chars, and the values of their surface area determined by CO2 adsorption (13) Walker, P. L.; Vastola, F. J.; Hart, P. J. Oxygen-18 tracer studies on the carbon-oxygen reaction. In Fundamentals of Gas-Surface Interactions; Saltsburg, H., Smith, J. N., Rogers, M., Eds.; Academic Press: New York, 1967; 307–317. . (14) Adánez, J.; De Diego, L. F.; García-Labiano, F.; Abad, A.; Abanades, J. C. Ind. Eng. Chem. Res. 2001, 40, 4317–4323. (15) Campbell, P. A.; Mitchell, R. E.; Ma, L. Proc. Combust. Inst. 2002, 29, 519–526. (16) Yu, Y. H.; Kim, S. D.; Lee, J. M.; Lee, K. H. Energy 2002, 27, 457–469. (17) Dennis, J. S.; Lambert, R. J.; Milne, A. J.; Scott, S. A.; Hayhurst, A. N. Fuel 2005, 84, 117–126. (18) Guerrero, M.; Ruiz, M. P.; Alzueta, M. U.; Bilbao, R.; Millera, A. J. Anal. Appl. Pyrolysis 2005, 74, 307–314. (19) Guerrero, M.; Ruiz, M. P.; Millera, A.; Alzueta, M. U.; Bilbao, R. Energy Fuels 2007, 22 1275–1284.

Table 1. Elemental Composition (wt %, dry basis), H/C Ratio (Molar Basis), O*/C Ratio (Molar Basis), and Surface Area Determined by CO2 Adsorption of the LHR and HHR Eucalyptus Chars Obtained at 900 °C C H N S H/C ratio (molar basis) O*/C ratio (molar basis) surface area (m2 g-1)

LHR chara

HHR charb

89.54 0.52 0.95 0.02 0.0697 0.0628 362

81.22 1.25 0.64 – 0.1847 0.1057 539

a Char obtained in a fixed bed reactor at 10 °C min-1. obtained in a fluidized bed reactor.

b

Char

Figure 1. Experimental setup used for the char/O2 reaction tests: (1) N2 and O2 cylinders; (2) mass flow meters; (3) control unit; (4) bubble flow meters; (5) quartz reactor; (6) electric furnace; (7) temperature controller; (8) compressor; (9) condenser; (10) particle filter; (11) NO analyzer; (12) CO/CO2 analyzer; (13) vent.

at 0 °C using the Dubinin–Radushkevich method. As shown, the H/C and O*/C ratios as well as the surface area values corresponding to HHR chars are higher than those from LHR chars. Besides, it is worth noting that, with respect to the char structural characteristics, the HHR chars have a more disordered structure than the LHR chars, as was found from the Raman spectroscopy analyses.19 2.2. Oxidation Experiments. The experimental conditions used for the study of the char oxidative reactivity comprise O2 inlet concentrations ranging from 100 to 1000 ppmv in a nitrogen flow, a temperature of 900 °C, and a flow rate of 1000 mL min-1 (STP). In addition, char oxidation tests in the 650–900 °C temperature range, with an oxygen concentration of 500 ppmv, have also been made to extend the study of temperature influence. The experimental setup used (Figure 1) and the experimental procedure followed for the study of biomass char oxidation have been described before,18 and only a brief explanation is given here. For each experiment, the amount of char put into the reactor was approximately 27 mg and was always previously mixed with 500 mg of silica sand (150 µm). The sand is necessary to facilitate the introduction of the sample into the reactor and to prevent agglomeration of the char particles. The mixture was located on a quartz wool plug placed in a bottleneck in the middle of a quartz reactor of 550 mm in length and 15 mm internal diameter, resulting in a very thin layer. An inert flow of N2 was fed while the sample was heated up to the reaction temperature. Once the desired temperature was reached, it was held for 30 min in nitrogen flow before the char sample was exposed to the reactant gas mixture (O2/N2). The experimental setup was equipped with a real-time analyzer (Uras14/IR CO/CO2) to allow the measurement of the CO and CO2 concentrations, key variables during the course of a char

2086 Energy & Fuels, Vol. 22, No. 3, 2008

Guerrero et al.

oxidation experiment. Several experiments were repeated to account for repeatability, and it was seen to be fairly good.

3. Results and Discussion The analysis of the char oxidation results is structured in two parts. The influence of the operating conditions used and the pyrolysis heating rate on the concentrations of the oxidation main products, CO and CO2, and the CO/CO2 ratio has been analyzed. Besides, the reaction order and activation energy of the char oxidation process have been determined. 3.1. Influence of the Operating Conditions and Char Characteristics. During char oxidation, carbon is mainly evolved from the particles in the form of CO and CO2. The carbon weight in the reactor at any time (WC) is calculated from the measured time variation of CO and CO2 concentrations in ppmv (CCO and CCO2, respectively) of the outlet gas. The equations needed to determine WC have already been reported in a previous work.18 The concentrations of CO and CO2 as well as the CO/CO2 ratio principally depend on the oxygen concentration, temperature, and thermal history of the char preparation. Figure 2 illustrates the influence of the inlet oxygen concentration on the evolution of the CO and CO2 concentrations and the CO/ CO2 ratio versus the remaining carbon weight in the reactor at any time (WC) for the LHR char oxidation tests at a given temperature of 900 °C. As shown, a general increase of CO and CO2 concentrations and a decrease of the CO/CO2 ratio can be observed when increasing oxygen concentration. Similar findings have been observed for the HHR char oxidation. These trends are expected and consistent with those reported in the literature.11,20 As pointed out above, the carbon–oxygen reaction mechanism goes through the dissociative chemisorption of the oxygen molecule, mainly in free carbon sites, resulting in the formation of surface oxygen complexes, –C(O), precursors of both CO and CO2. As the inlet oxygen concentration increases, the surface coverage with oxygen complexes increases and therefore, an increase in the CO and CO2 products occurs (eqs r.2 and r.3). On the other hand, the enhancement of the surface covered with oxygen complexes favors the reaction between two adjacent –C(O) complexes leading to the CO2 formation (r.3) and hence the CO/CO2 ratio decreases, eq 1. Figure 3 shows the effect of the temperature on the evolution of the CO and CO2 concentrations and the CO/CO2 ratio as a function of the remaining carbon weight for the HHR char. The CO/CO2 ratio increases with the temperature, mainly due to the fact that the concentration of CO increases significantly when the temperature increases (Figure 3a). Similar trends with temperature have been observed for the LHR chars. This is expected and explained because CO is primarily formed from the desorption of carbonyl and/or ether type complexes occurring at temperatures above 700 °C, since these are stable carbon– oxygen complexes. These results are in agreement with those observed by other researchers.21–26 As an example, Figure 4 shows a comparison of the oxidation behavior between the LHR and HHR chars for an inlet oxygen (20) Chen, W. Y.; Tang, L. AIChE J. 2001, 47, 2781–2797. (21) Tognotti, L.; Longwell, J. P.; Sarofim, A. F. Proc. Combust. Inst. 1990, 23, 1207–1213. (22) Du, Z.; Sarofim, A. F.; Longwell, J. P.; Mims, C. A. Energy Fuels 1991, 5, 214–221. (23) Wang, W.; Brown, S. D.; Hindmarsch, C. J.; Thomas, K. M. Fuel 1994, 73, 1381–1388. (24) Zhuang, Q. L.; Kyotani, T.; Tomita, A. Energy Fuels 1994, 8, 714– 718. (25) Haynes, B. S. Combust. Flame 2001, 126, 1421–1432. (26) Li, C.; Brown, T. C. Carbon 2001, 39, 725–732.

Figure 2. Evolution of CO, CO2, and the CO/CO2 ratio as a function of carbon weight (WC) in the reactor at any time, for the LHR char. Influence of oxygen concentration at 900 °C.

concentration of 1000 ppmv and a temperature of 900 °C. As can be observed, the HHR char oxidation leads to a higher CO formation, whereas there are not significant differences with respect to the CO2 concentration between both types of chars. Consequently, the CO/CO2 ratio is higher for the HHR char, which has a higher degree of structural disorder19 and higher oxygen content (Table 1). The higher CO formation and thus the higher CO/CO2 ratio for the HHR char can be explained on the basis of the study performed by De la Puente et al.27 These (27) De la Puente, G.; Fuente, E.; Pis, J. J. J. Anal. Appl. Pyrolysis 2000, 53, 81–93.



Figure 4. Evolution of CO, CO2, and the CO/CO2 ratio as a function of carbon weight (WC) in the reactor at any time, for a given oxygen concentration of 1000 ppmv at 900 °C. Comparison between the LHR and HHR chars.

Figure 3. Evolution of CO, CO2, and the CO/CO2 ratio as a function of carbon weight (WC) in the reactor at any time, for the HHR char. Influence of temperature for an inlet oxygen concentration of 500 ppmv.

authors indicate that the release of CO and CO2, during the heating of the char sample in an inert atmosphere until the oxidation reaction starts, is related to the active sites available for oxygen adsorption and to the type of oxygen-containing functional groups formed in the char during the oxidation. Figure 5 illustrates the CO and CO2 desorption profiles in time for the LHR and HHR char heating in an inert atmosphere of N2 until the oxygen is injected into the reactor. As can be seen, the CO desorption is significantly higher for the HHR char than for the LHR char. The CO2 desorption is also higher for the HHR char, but in this case the differences between both chars are not so important. From the results shown in Figure 5, it can be concluded that the active sites available for oxygen chemisorp-

Figure 5. Devolatilization of the LHR and HHR char samples during the heating until the oxidation reaction starts (WCr represents the accumulated carbon weight released in the form of CO and CO2).

tion are probably higher in the HHR char, and higher CO concentrations can be expected for its oxidation compared to the LHR char oxidation. 3.2. Kinetic Parameters. Several reaction models have been proposed to obtain the kinetic parameters describing the reaction of a porous solid with a gas. The first condition the reaction model has to meet is to describe the observed behavior of the particle during the reaction. Besides, it is perceived that the characteristics of the model allow it to be applied without an


Guerrero et al.

excessive mathematical complexity. Therefore, an involvement solution between the mathematical complexity and the realistic performance of the process should be chosen. In this study, experimental data of the LHR and HHR char oxidation tests are analyzed to calculate the kinetic parameters using the macroscopic model of type I in regime II. To apply this model, the reaction must be controlled by kinetic and pore diffusion (regime II), which is accomplished when the effectiveness factor (η) is lower than 0.7 according to Schönenbeck et al.28 In this work, this reaction model has been used as being suitable for fitting the experimental data and an effectiveness factor below 0.7 is satisfied in all conditions studied. This fact is checked later. A detailed description of the macroscopic model of type I in regime II has been reported by Szekely et al.29 For an irreversible reaction and considering the pseudostationary state approximation, the mass balance within the particle gives the following equation: De∇ CA - kSVCA ) 0

(2)

where De is the effective diffusivity of the gaseous reactant in the porous solid (m2 g-1), CA is the reactant gas concentration (mol m-3), n is the reaction order with respect to the gas reactant, k is the kinetic constant (mol1-n s-1 m3n-4) and SV represents the surface area per unit volume in the reaction zone (m-1). Two assumptions are made in this work: (i) a constant average effective diffusivity and specific surface area is used within the thin reaction zone; (ii) the structure of the reaction zone is maintained constant as it moves toward the solid particle interior. To solve the eq 2, Szekely et al.29 assume that the reaction zone may be considered a flat plate except in the final reaction stage. The overall rate of reaction per unit area of external surface, rs (mol s-1 m-2), is given by the expression: rs )

( n +2 1 ks D )

1⁄ 2

v

e

CAsn+1

)⁄2

(

(3)

The major feature showing the macroscopic model of type I in regime II is that the reaction rate is directly proportional to (kDe)1/2, the apparent activation energy for the reaction is onehalf the intrinsic activation energy, and the reaction order changes from n to (n + 1)/2. Therefore, the eq 3 can be expressed as rs ) -De

( ) dCA dx

ext.surf.

n′ ) k ′ C As

(4)

where the apparent kinetic parameters, k′ (mol(1-n)/2 s-1 m(3n-1)/2) and n′, are given by the eqs 5 and 6: k′ )

( n +2 1 kS D )

1⁄ 2

V

rs ) -

1 dNC ) k ′ COn′2S 4πRc2b dt

e

(5)

n+1 (6) 2 To deduce the equation connecting the fractional solid conversion and time (t), the following noncatalytic gas–solid reaction is considered: n′ )

O2 + bC f 2(b - 1)CO + (2 - b)CO2

(r.4)

(28) Schönenbeck, C.; Gadiou, R.; Schwartz, D. Fuel 2004, 83, 443– 450. (29) Szekely, J.; Evans, J. W.; Sohn, H. Y. Gas-solid reactions; Academic Press: New York, 1976.

(7)

where NC represents the moles of carbon (which is considered in this work as representative of reacting char) and Rc is the particle radius (in m) at time t. 4 -dNC ) -FCdVp ) -FCd πRc3 ) -4πFCRc2dRc 3

(

)

(8)

where FC is the solid molar density (mol m-3) and Vp is the particle volume (m3). Combining the eqs 8 and 7, and then integrating between t ) 0 and a given time t, the following expression can be obtained: t)

n

2

The overall rate of reaction per unit area of external surface, rs, assuming spherical particles, is given by the eq 7:30

FC bk ′ COn′2S

(R - Rc)

(9)

where R is the initial value of the particle radius (m). From the above equation, the particle complete conversion time can be deduced, which is obtained assuming Rc ) 0, τ (s): τ)

FCR bk ′ COn′2S

(10)

Therefore, the ratio between t and τ is given by the eq 11, that can be expressed as a function of the solid fractional conversion (XC), eq 12: Rc t )1τ R

(11)

t ) 1 - (1 - XC)1⁄ 3 (12) τ The carbon conversion at any time t, XC, is calculated from the equation XC )

WC0 - WC WC0

(13)

where WC0 represents the initial weight of carbon at the beginning of the experiment. As can be seen, eq 10 is formally equivalent to the relationship developed for nonporous solids from the shrinking core model, but involving apparent kinetic parameters (k′ and n′). That is the reason why even the analysis of the results in the literature from experiments on a porous solid has often based on the shrinking unreacted-core model.31–37 3.2.1. EValuation of Reaction Order and ActiVation Energy. The reaction order is determined from the experimental data of the char oxidation tests at 900 °C with different oxygen concentrations (100–1000 ppmv). From eq 12, using the carbon conversion values at any time, the τ values and their corre(30) Levenspiel, O. Chemical reaction engineering; John Wiley & Sons. Inc.: New York, 1999. (31) Flynn, J. H. Thermochim. Acta 1980, 37, 225–238. (32) Kwon, T. W.; Kim, S. D.; Fung, D. P. C. Fuel 1988, 67, 530–535. (33) Smith, F. G.; Pendarvis, R. W.; Rice, R. W. Combust. Flame 1992, 88, 61–73. (34) Sorensen, L. H.; Saastomoinen, J.; Hustad, J. E. Fuel 1996, 75, 1294–1300. (35) Liliedahl, T.; Sjöström, K. Fuel 1997, 76, 29–37. (36) Winter, F.; Prah, M. E.; Hofbauer, H. Combust. Flame 1997, 108, 302–314. (37) Lee, J. M.; Kim, Y. J.; Lee, W. J.; Kim, S. D. Energy 1998, 23, 475–488.



Table 2. Carbon Complete Oxidation Time (τ) and Average Values of b in the Experiments of Char Oxidation at 900 °C with Different Oxygen Concentrations LHR chara [O2] (ppmv)

τ (s)

R2

100 250 350 500 1025

22012 10580 9574 6263 4084

0.9975 0.9988 0.9993 0.9969 0.9987

Table 4. Carbon Complete Oxidation Time (τ) and Average Values of b in the Experiments of Char Oxidation at Different Temperatures with an Inlet Oxygen Concentration of 500 ppmv

HHR charb b 1.37 1.28 1.28 1.23 1.22

LHR chara

τ (s)

R2

b

T (°C)

19855 10378 8152 5170 3388

0.9981 0.9996 0.9979 0.9999 0.9990

1.39 1.35 1.32 1.32 1.27

650 700 750 800 850 900


b

Char

HHR charb

τ (s)

R2

b

τ (s)

R2

b

14614 13301 11995 10366 8113 6263

0.9999 0.9999 0.9985 0.9966 0.9955 0.9969

1.03 1.05 1.06 1.09 1.14 1.23

10485 9213 8270 7422 6364 5170

0.9980 0.9948 0.9926 0.9938 0.9987 0.9999

1.02 1.05 1.08 1.13 1.22 1.32


b

Char

Table 3. Values of n′ and n for the Experiments of Char Oxidation n′ R2 n

LHR chara

HHR charb

0.79 0.9929 0.58

0.83 0.9968 0.66


b

Char

sponding regression coefficients (R2) have been calculated (Table 2). These τ values are inversely related to char reactivity. As can be observed in Table 2, the more the oxygen concentration the lower the τ values. On the other hand, it is observed that τ values are lower for the HHR chars, indicating that they are more reactive toward oxygen than the LHR chars. This behavior is consistent with the higher H/C and O*/C ratios and surface areas values of the HHR chars with respect to the LHR chars (Table 1). Once the τ values have been calculated, the following step is to determine the value of the stoichiometric coefficient b. For that, the stoichiometric reaction describing the global oxidation process has been considered (r.4), and b can be calculated as b)

1 + rp 1 + 0.5rp

(14)

where rp represents the CO/CO2 ratio. For each oxygen concentration analyzed at 900 °C, an average value of b is calculated in the carbon conversion range from 20 to 95%, where the CO/CO2 ratio can be considered as constant. These results are also presented in Table 2. As shown, the value of b decreases when increasing the oxygen concentration. This is expected due to the fact that as the oxygen concentration increases, the CO/CO2 ratio decreases (Figure 3), and thus the b value decreases. The b values for the HHR char oxidation are slightly higher than the LHR char oxidation, in accordance with the higher CO/CO2 ratio values obtained from the oxidation of the HHR char (Figure 4b). From τ and b values previously obtained and the inlet oxygen concentration, the apparent reaction order (n′) can be determined from the logarithmic plot of eq 10, i.e. eq 15, and thus the reaction order (n) can be calculated based on eq 6.

( τb1 ) ) log( Fk′R ) + n ′ log(C

log

C

O2S)

(15)

Table 3 shows the values of n′ obtained in the experimental data fitting to eq 15), with the regression coefficient (R2) and the values of n obtained for the LHR and HHR char oxidation. Fractional reaction orders with respect to oxygen have been obtained for the oxidation process of both chars (0.58 and 0.66 for the LHR and HHR char, respectively). These results are in

Figure 6. Arrhenius plot of the char oxidation process.

agreement with those reported in the literature.14,38 Zimbardi38 found a fractional order of 0.5 for the oxidation of different types of lignocellulosic chars, and Adánez et al.14 obtained a reaction order of 0.9 for the eucalyptus char oxidation. The apparent activation energies (E′a) for the LHR and HHR char oxidation processes are determined using data of k′/FCR obtained in the experiments performed through a range of temperatures (650–900 °C) with a given oxygen concentration of 500 ppmv. For these char/O2 tests, the τ values and their corresponding regression coefficient (R2) at each temperature are calculated from eq 12, and summarized in Table 4. As can be seen, the particle complete conversion time (τ) decreases when the temperature increases. Besides, it can be observed that the τ values in the temperature range considered are lower for the HHR char oxidation, indicating again that it is more reactive toward O2 than the LHR char. The values of the stoichiometric coefficient b are also shown in Table 4. As can be expected for both chars, the b value increases with temperature according to the increase in the value of the CO/CO2 ratio (Figure 3c). Once τ and b values have been determined, the k′/FCR values are obtained at each temperature from eq 10, and treated by the Arrhenius plot. The Arrhenius plots of the LHR and HHR chars and the corresponding regression coefficients (R2) are presented in Figure 6. For each char, it is worth noting that two subregimes can be distinguished, in which higher activation energies are found in the high temperature regime. This fact, denoted as “breakdown of Arrhenius plot”, could indicate a change in the reaction mechanism or in the contribution of the different parallel reaction steps.30 For the char/O2 reaction, several authors have proposed a variation in the contribution of the different reaction steps with temperature.10,39,40 (38) Zimbardi, F. Combust. Sci. Technol. 2000, 156, 251–269. (39) Zhuang, Q.; Kyotani, T.; Tomita, A. Energy Fuels 1996, 10, 169– 172. (40) Haynes, B. S.; Newbury, T. G. Proc. Combust. Inst. 2000, 28, 2197– 2204.


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Table 5. Activation Energy Values of the LHR and HHR Char Oxidation Processes Ea (kJ mol-1) T (°C)

LHR chara

HHR charb

650–800 800–900

45 100

35 60

a Char obtained in a fixed bed reactor at 10 °C min-1. b Char obtained in a fluidized bed reactor.

Table 6. kSv, Thiele Modulus, and Effectiveness Factor Values in the Experiments of Char Oxidation at Different Temperatures with an Inlet Oxygen Concentration of 500 ppmv LHR chara R (µm) ) 154.5 FC (mol m-3) ) 32891 kSV (mol0.42 s-1 m-3.26)

φ

η

kSV (mol0.34 s-1 m-3.02)

φ

η

650 700 750 800 850 900

116.44 157.80 212.42 281.18 461.24 761.24

1.00 1.20 1.43 1.67 2.19 2.88

0.66 0.60 0.54 0.48 0.39 0.31

305.29 388.94 510.11 620.13 844.37 1151.91

1.38 1.58 1.85 2.08 2.48 2.94

0.55 0.50 0.44 0.40 0.35 0.30


b

Char

From the Arrhenius plot, the apparent activation energy (E′a) for the reaction is obtained. As has been mentioned, the apparent activation energy for the reaction is one-half the intrinsic activation energy. Therefore, the intrinsic activation energy (Ea) for the LHR and HHR oxidation processes can be calculated. These values are shown in Table 5. In the low-temperature range (650–800 °C), the activation energy values are similar for the oxidation of both chars, around 35–45 kJ mol-1. However, in the high-temperature range (800–900 °C), this value is 100 kJ mol-1 for the LHR char oxidation and 60 kJ mol-1 for the HHR char oxidation. In the literature regarding the reaction between biomass char and oxygen, the values of activation energy varies from 100 to 240 kJ mol-1,9,12,14–17 but these results cannot be compared directly with those reported in this study due to the fact that many factors influence the reactivity toward O2 (e.g., type of carbon solid, pyrolysis conditions, operating conditions in the char/O2 reaction). In order to check whether the char oxidation process in the studied conditions occurs in regime II, Thiele modulus (φ), eq 16, and effectiveness factor (η), eq 17, have been calculated considering the values of k′/FCR obtained above. The values of kSV at each temperature can be calculated from the eq 18 and are shown in Table 6. Vp Ap

(

n-1 n + 1 kSVCAs >3 2 De

)

(16)

where Vp and Ap are the volume and area of a solid particle, respectively. η)

1 1 1 φ tanh 3φ 3φ

(

)

(17)

(k ′ )2 (18) 2 De n+1 The effective diffusivity, De, at each temperature is determined from eq 19: kSV )

(

)

(41) Fogler, H. S. Elements of chemical reaction engineering; Prentice Hall Inc.: Englewood Cliffs, NJ, 1986.

D(O2-N2)p Fτ

(19)

where D(O2–N2) is the diffusivity of O2 in N2 (m2 s-1) (D(O2–N2) ) 1.81 × 10–5 m2 s-1 at T ) 0 °C),41 p is the particle porosity (it is assumed p ) 0.6),42 and Fτ is the tortuosity factor (it is assumed Fτ ) 3).19 In order to deduce the apparent kinetic constant, k′, the values of R and FC for both chars have been estimated. The initial average value of particle radius is obtained from the particle size distribution determined by sieve analysis of the char samples. The molar density values, FC, are calculated by means of eq 20.

HHR charb R (µm) ) 155.0 FC (mol m-3) ) 28224

T (°C)

φ≡

De )

FC )

mol C Vchar

(20)

where Vchar represents the volume occupied for the char (m3) and is calculated according to eq 21: Vchar )

(1 - bed of char) mchar Fbed of char

(21)

where bed of char represents the porosity of the bed of char. This value is assumed to be 0.5;41 mchar is the mass of char (kg); Fbed of char is the density of the bed of char (kg m-3) (Fbed of LHR char ) 220.4 kg m-3 and Fbed of HHR char ) 208.5 kg m-3). The values of R and FC are illustrated in Table 6, which also shows that the effectiveness factor is ranging between 0.30 at 900 °C and 0.66 at 650 °C. An effectiveness factor below 0.7 means that the reaction is controlled by kinetics and pore diffusion.28 Therefore, it can be concluded that the kinetic model used is suitable for evaluating the reaction order and activation energy of the char oxidation process under the conditions studied. 4. Conclusions Experimental and kinetic research on the oxidation of eucalyptus chars obtained from low (LHR) and high heating rate (HHR) devolatilization at 900 °C has been performed. The HHR char exhibited a higher reactivity toward oxygen under the conditions studied. This is attributed to the higher surface area, higher oxygen and hydrogen contents, and thus a higher availability of active sites. In order to evaluate the reaction order and activation energy of the char oxidation process, the macroscopic model of type I in regime II has successfully been applied. Fractional reaction orders respect to the oxygen concentration have been found for the LHR and HHR chars, 0.58 and 0.66, respectively. The activation energy values are dependent on the temperature range since a “breakdown in the Arrhenius plot” was observed. In the low-temperature range (650–800 °C), the activation energy value is about 35–45 kJ mol-1 for both chars. In the high-temperature range (800–900 °C), an activation energy of 60 kJ mol-1 is obtained for the HHR char oxidation process, whereas the LHR char oxidation is characterized by higher activation energy, 100 kJ mol-1. Acknowledgment. The authors express their gratitude to MCYT (Projects PPQ2000-1207 and PPQ2003-02394) for financial support and the ICB-CSIC for allowing us the use of the fast pyrolysis facility. M.G. acknowledges DGA for the predoctoral grant awarded (B179/2003). M.P.R. acknowledges the Spanish Ministry of Science and Education (MEC) for the award of a predoctoral grant (BES-2005-6898). EF700643P (42) Garijo, E. G.; Jensen, A. D.; Glarborg, P. Energy Fuels 2003, 17, 1429–1436.

Oxidation Kinetics of Eucalyptus Chars Produced at Low and High

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