Fluidized-Bed Combustion of Selected Wood Chars from the Semi

Article pubs.acs.org/EF

Fluidized-Bed Combustion of Selected Wood Chars from the Semiarid Northeastern Region of Brazil Marcelo Ramos,† Nelson Rangel,‡ and Carlos Pinho*,‡ †

Universidade Federal Rural de Pernambuco, Rua Dom Manoel de Medeiros, s/n, CEP 52171-900 Recife, Pernambuco (PE), Brazil Centro de Estudos de Fenómenos de Transporte (CEFT), Departamento de Engenharia Mecânica (DEMec), Faculdade de Engenharia, Universidade do Porto, Rua Dr. Roberto Frias, s/n, 4200-465 Porto, Portugal

‡

ABSTRACT: We herein present some results obtained from the burning of chars derived from wood originated in the semi-arid region of northeastern Brazil (Cariri Paraibano) in a fluidized-bed reactor. A series of tests was performed in air and 10:90% (v/ v) mixtures of oxygen and nitrogen. The average diameter of the burning particles ranged from 1.8 to 3.6 mm for bed temperatures of both 750 and 850 °C. The temperature of the burning particles was determined using an energy balance at the surface of the particle. The maximum difference in the temperature between the particles and the bed was 82 °C, when the particles were burned in air. This difference decreased to 41 °C when the particles were burnt in a 10:90% (v/v) mixture of oxygen and nitrogen. Both kinetic and diffusive data were collected, in response to the general lack of this type of data in the literature relating to the design of fluidized-bed combustors.

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Komatina et al.6 found that the type of char or coke, the batch size, and the oxygen concentration all had a strong influence on the evolution of the particle temperature during the combustion process. We herein present the calculated temperature of char particles using a semi-analytical model that considers the energy balance at the surface of an individual burning particle. We later discuss how this balance accounts for the heat released during combustion, as well as the heat lost from the particle to the surrounding fluidized bed. The reactivities of eight chars obtained from Brazilian species of wood were measured during experiments on fluidized-bed combustion, in both air and a 10:90% (v/v) mixture of oxygen (O2) and nitrogen (N2). We used a 10% concentration of oxygen to minimize the difference between the temperature of the particles and that of the fluidized bed in which they were burning. Another reason for this selection of the concentration was the fact that, when operating under normal steady-state conditions, fluidized-bed furnaces use an oxygen concentration that is well below the 21% (v/v) typical of batch laboratory combustion experiments in air. We present data for the heterogeneous phase reaction rate constant for the char particles tested, for three different fractions of burned mass.

INTRODUCTION Biomass represents about 14% of the world’s primary consumption of energy, and countries with emerging economies, such as Brazil, are the major users. In these countries, 40% of the energy consumed is generated by biomass, as compared to 4% in the U.S.A., 14% in Sweden, and 10% in Australia.1 However, in such emerging economies, the exploitation and use of biomass as an energy source has generally been neither efficient nor sustainable. Because of its particular composition, especially in terms of its ash content, the burning of biomass requires adequate furnace facilities, where the temperature of the combustion can be closely monitored; to this end, fluidized-bed reactors are often used. The tar released during the pyrolysis or combustion of wood also requires proper disposal, something that can be achieved in a reactor of this type. The design and construction of energy conversion systems of this type requires a good knowledge of the oxidation rates of fuels.2 However, there is a lack of experimental data on the chars obtained from biomass, particularly in comparison to similar data for cokes (i.e., chars obtained from coal);3 this absence is even more important for those countries that depend upon biomass as an energy source. In the case of biomass of a Brazilian origin, data are particularly scarce as far as the combustion kinetics and diffusive data are concerned. The reactivity of chars depends upon the temperature of the burning particles, which varies according to the conditions of the combustion, and can therefore differ from the temperature of the bed to a greater or lesser degree. The temperature of the particles can be measured either using optical techniques or introducing fine thermocouples in the particles themselves; however, the difficulty and lack of reliability of these experimental procedures4 has led researchers to seek alternative approaches. Among the authors who have considered this problem recently, Khraisha5 determined that the particles generally burned at the same temperature as the bed, while © 2011 American Chemical Society

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EXPERIMENTAL SECTION

This study focused on eight species of wood obtained from the semiarid region of the Brazilian northeast known as the Cariri Paraibano. The samples were collected in the municipality of Soledad, which is located in the semi-arid mesoregion of the northeastern state of Paraiba in Brazil, where there is a strong dependence upon the energy derived from burning wood.7,8 The native vegetation of this region is of the caatinga type, which is characterized by shrubs and stunted trees. Received: September 8, 2011 Revised: November 24, 2011 Published: November 29, 2011 400

dx.doi.org/10.1021/ef201354f | Energy Fuels 2012, 26, 400−406

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Table 1. Denominations, Botanic Families, and Frequency of Use of the Studied Species of Wood

a

mnemonic

scientific name

common denominationa

author

botanic family

frequency of use (%)

ALG ARO BAR CAT IMB JUR MAR PER

Prosopis juliflora Myracrodruon urundeuva Schinopsis brasiliensis Poincianella pyramidalis Commiphora leptophloeos Mimosa tenuiflora Croton blanchetianus Aspidosperma pyrifolium

Algaroba Aroeira Baraúna Catingueira Imburana Jurema-preta Marmeleiro Pereiro

(Swartz) D. C. All. Engl. (Tul.) L. P. Queiroz (Mart) J. B. Gillett (Willd.) Poir Baill. Mart.

Mimosaceae Anacardiaceae Anacardiaceae Caesalpiniaceae Burseraceae Mimosaceae Euphorbiaceae Apocynaceae

72.7 77.3 50.0 100.0 72.7 81.8 86.4 95.5

In Portuguese.

Table 2. Char Yields wood

ALG

ARO

BAR

CAT

IMB

JUR

MAR

PER

char yield (wt %)

25.2

21.6

24.1

23.1

23.7

26.5

18.8

22.1

Table 3. Proximate Analysis and Particle Densities of the Chars ALG

ARO

BAR

CAT

IMB

JUR

MAR

PER

520.6 particle density (kg/m3)a proximate analysis (%, w/w, as received) moisture 5.4 ash 6.1 volatile matter 5.6 fixed carbon 82.9

wood char

485.1

613.4

698.0

474.2

623.0

521.0

798.3

5.0 6.2 7.1 81.7

4.3 9.2 10.8 75.7

2.9 10.8 10.6 75.7

7.9 7.1 7.3 77.7

4.4 5.2 2.9 87.5

4.8 6.6 5.1 83.5

4.1 8.1 7.3 80.5

a

Obtained by mercury intrusion porosimetry. and bark,11 can act as catalysts for combustion12 and can also promote the agglomeration of the inert sand particles in the fluidized bed.11,13,14 The combustion experiments were carried out on the resulting chars in a bubbling fluidized-bed reactor, which was operated at 1.5Umf, at bed temperatures of 750 and 850 °C; under such conditions, the corresponding fluidization flows were 8.5 and 7.3 L/min (NPT). The initial diameter of the particles was controlled using sieving, which yielded particles in the range of 1.8−3.6 mm. The fluidized bed had a static height of 150 mm, and the reactor had an internal diameter of 80.8 mm. The bed material was silica sand with a particle size of 250− 315 μm. The bed was heated electrically, using a Kanthal wire coil that was wrapped around the bed and covered with a Kawool ceramic blanket. The bed temperature was measured using a K-type thermocouple and was controlled using a proportional−integral− derivative (PID) temperature controller (Eurotherm, model 2116). The char was burned in batches of 1.5 g, and throughout their combustion, the evolution of CO2 in the exhaust gases was monitored using a Signal Instruments NDIR CO2 analyzer (model 7000FM), while the evolution of O2 was monitored using a paramagnetic Signal Instruments analyzer (model 8000M). We sampled the combustion gases using a stainless-steel probe with an internal diameter of 4 mm, which was placed 0.5 m above the free surface of the bubbling bed. The sampled flow rate was 0.7 L/min (measured using the flow meters of the analyzers), and the gases were passed through a system of filters before they were sent to the analyzers. Further details of the experimental procedure are provided elsewhere.15

The selection of the species to be tested was based on the results of ethnobotanical research. This research was carried out by questioning residents of the rural communities of Barrocas and Cachoeiras (located in the municipality of Soledad) to assess which species were most commonly used by them as sources of energy. This investigation resulted in the identification of a total of 22 species, from which the 8 most commonly used species were selected for laboratory combustion tests (see Table 1). For each species, branches or trunks with dimensions of about 1 m in length and diameters that ranged from 35 to 50 mm were chosen randomly from living adult specimens. All of the samples were collected in a native part of the forest located in the vicinity of the communities, with the exception of the exotic species Prosopis juliflora, which was collected from a specially cultivated zone. Samples of the eight species of wood were dried under ambient air conditions, and after they were reduced in size, all of the samples were then carbonized under laboratory conditions. Small batches consisting of 60−100 g of biomass were placed inside a refractory steel crucible, which was positioned in a 2 L/min [normal pressure and temperature (NPT)] flow of nitrogen and subjected to a heating rate of 0.5 °C/s. The batch stage (performed at a pyrolysis temperature of 850 °C) lasted 15 min. The char yields were consistent with those generally found in the literature for these carbonizing conditions;9,10 the results are shown in Table 2. As previously mentioned, the samples of wood that were used to produce chars were all small (35−50 mm in diameter) and the bark was not removed from the wood prior to carbonization. The properties of the chars obtained are given in Table 3. The density of the char particles was determined using mercury porosimetry using a Quantachrome PoreMaster porosimeter. Because of the presence of bark in the original wood, the resulting chars had a high ash content, which had a strong influence on the experimental results obtained. The mechanism of combustion is affected by the ash content, and the maximum possible operating temperature of the bed is limited by the agglomeration of sand in the bed, which is influenced by the presence of ash. The presence of alkaline metals, which are available in large quantities in small branches

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RESULTS AND DISCUSSION Kinetic and Diffusive Data. The experimental data were analyzed using a mathematical model for combustion of carbon particles in a bubbling fluidized-bed reactor, in which it was assumed that the particles burned at a constant density. This model was based on the two-phase theory of fluidization,16 wherein the overall resistance to combustion was the sum of (1) the oxygen diffusion from the dense phase of the bed toward the particle surface and (2) the heterogeneous phase reaction resistance at the surface of the burning particle. We 401


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Figure 1. Overall combustion resistance for 25, 50, and 75% burned mass fractions. Bed temperatures of 750 and 850 °C. Combustion in air and mixtures of oxygen and nitrogen [10% (v/v) O2].

assumed that the reaction C + 1/2O2 → CO took place only at the external surface of each particle and the effects related to the porosity of the char particle (e.g., the intrapore diffusion and the influence of the internal area of the pore on the kinetics) were discarded. As a result of the quenching effect of the inert sand particles of the bed, the subsequent oxidation of CO takes place away from the burning char particle,17,18 either inside the bubble phase or above the free surface of the bed, according to the reaction CO + 1/2O2 → CO2. The resistance associated with the transfer of oxygen from the bubble to the particulate phase could also be neglected for the operating conditions used here.19 According to the proposed mathematical model, the overall resistance to combustion may be defined as

1 d 2 = + K ShDG kc

Figure 1 shows a plot of eq 1 for the different chars, for bed temperatures of 750 and 850 °C. In this figure, the instantaneous values of the overall combustion resistance are represented for the eight tested types of char, at fractions of mass burned of 25, 50, and 75%, for both combustion in air and combustion in an O2/N2 mixture with 10% (v/v) O2. These fractions were determined by integrating the curves of the CO2 concentration for each batch combustion experiment. This figure shows that it is possible to compare the overall resistance to combustion for the chars obtained from the eight species used in the present study, for different test conditions. The usefulness of such data can be illustrated if the overall resistance to combustion is divided into its diffusive and kinetic components, which we achieved by plotting the corresponding linear trend line, and comparing the slope and intersect to that predicted by the theoretical equation (eq 1) in each case. The slope of the line represents the Sherwood number Sh, and its intercept represents the kinetic constant kc. Their average values are shown in Table 4 (the slopes and intercepts obtained from the linear correlations for all species and burned fractions; the correlations are shown in panels a−d of Figure 1). Table 4 also shows the Sherwood numbers obtained in the present work compared to those determined using the equation proposed by Guedes de Carvalho et al.18

(1)

where d is the instantaneous diameter of the burning carbon particle, Sh is the Sherwood number, DG is the diffusivity of oxygen in air, and kc is the heterogeneous phase reaction rate constant for the reaction that takes place at the surface of the burning particle. The overall resistance (1/K) was calculated as described elsewhere.15 Equation 1 thus represents the combination of the diffusive and kinetic terms. 402


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Table 4. Average Values of Sh and kc

On the other end, the kinetic constant depends upon the type of fuel used. Because of the heterogeneities of the physical and chemical characteristics of the char particles considered here (specifically in their inorganic components, which can be critical for the development of the combustion process21), it was important to evaluate the rates of reaction of the char particles using a large number of samples and to consider the kinetic behavior of each. In other studies of this type, kinetic data are generally determined using a thermogravimetric process. However, this methodology imposes the use of low-temperature experiments, to avoid the influence of diffusive behavior on the kinetic data obtained. This is because this experimental technique is based on the assumption that all resistance to combustion is uniquely kinetic, which is clearly not true for tests carried out at temperatures above 700 °C.22 Using the mathematical model adopted in the present work (which distinguishes between the diffusive and kinetic effects), it is possible to obtain a reliable set of data for any temperature range. Values of Sh and kc are shown in Table 5 for the chars obtained from each species of wood, determined using the linear trend lines obtained by considering all of the testing conditions and burned mass fractions (see Figure 1). When the temperature of the reactor is increased, the effects of the kinetic resistance diminish until a pure diffusion control regime is obtained; this is the case for the data shown in Figure 1b for combustion in air using a bed temperature of 850 °C. In such circumstances, it is impossible to obtain a value for the kinetic constant of the heterogeneous phase reaction rate; alternatively, it could be said that this constant has an infinite value (see Tables 4 and 5). In other words, the combustion reaction is controlled by the diffusion in such cases.

Sh 750 750 850 850

°C, °C, °C, °C,

this work

eq 2

eq 3

kc (m/s)

0.93 1.26 0.94 0.96

1.56 1.56 1.48 1.49

1.41 1.42 1.38 1.38

0.721 0.174 ∞ 0.480

air 10% O2 air 10% O2

⎡ ⎛ dUmf ⎞0.78 Sh′ ⎢ = 4 + 0.576⎜ + 1.28 ⎟ ⎢⎣ εmf ⎝ D′m εmf ⎠ 1/2

⎛ d p ⎞⎛ dU ⎞2 ⎤ ⎛ dUmf ⎞ mf ⎟ ⎥ ⎜ ⎟ + 0.141⎜ ⎟⎜ ⎝ D′m εmf ⎠ ⎝ d ⎠⎝ D′m εmf ⎠ ⎥⎦

(2)

and those obtained using the modified equation of Jung and La Nauze20

⎛ Re p,mf ⎞1/2 1/3 Sh = 2εmf + 0.69⎜ ⎟ Sc ⎝ εmf ⎠

(3)

The values obtained were consistent with those determined previously in studies of the combustion of coke in a bubbling fluidized bed.18,20 This result was not surprising, because that part of the overall resistance to combustion that depends upon the oxygen diffusion to the surface of the burning particle is mainly a characteristic of the reactor used. This was also true in the studies that formed the bases for eqs 2 and 3, and the effect is well-known.

Table 5. Values of Sh, kc, and Tp for Burned Mass Fractions of 25, 50, and 75% 750 °C, air char ALG

ARO

BAR

CAT

IMB

JUR

MAR

PER a

750 °C, 10% O2

850 °C, air

850 °C, 10% O2

f

Sh

kc (m/s)

Tp (°C)

Sh

kc (m/s)

Tp (°C)

Sh

kc (m/s)

Tp (°C)

Sh

kc (m/s)

Tp (°C)

0.25 0.50 0.75 0.25 0.50 0.75 0.25 0.50 0.75 0.25 0.50 0.75 0.25 0.50 0.75 0.25 0.50 0.75 0.25 0.50 0.75 0.25 0.50 0.75

0.97 0.96 0.78 0.78 0.82 0.81 0.78 0.80 0.78 1.12 1.08 0.75 1.36 1.26 1.35 0.81 0.85 0.87 0.83 0.74 0.90 1.12 1.02 1.21

0.344 0.389 0.824 0.345 0.334 0.366 1.538 0.976 1.092 0.400 0.454 3.224 0.462 0.727 0.699 ∞ 5.186 1.693 ∞ ∞ 1.277 2.399 6.426 0.649

791.9 798.9 810.4 787.2 793.6 804.5 794.9 802.1 815.2 801.5 808.9 811.8 796.3 805.0 822.2 798.4 806.6 821.0 794.4 801.0 816.5 810.8 818.8 834.4

1.68 1.46 2.09 0.80 0.88 0.98 1.09 1.44 2.20 a a a 1.69 1.65 1.91 0.96 0.97 1.04 a a a a a a

0.137 0.152 0.120 0.202 0.179 0.150 0.203 0.148 0.123 a a a 0.199 0.202 0.161 0.396 0.349 0.287 a a a a a a

770.2 772.7 776.1 767.1 769.7 773.2 771.1 773.8 778.0 a a a 771.2 774.3 778.3 772.5 775.5 780.9 a a a a a a

0.88 1.06 0.85 0.84 0.83 0.76 0.77 0.79 0.87 0.75 0.77 0.74 1.06 1.12 1.12 0.82 0.74 0.79 0.64 0.62 0.62 1.06 1.01 0.98

∞ 1.431 ∞ 2.478 ∞ ∞ ∞ ∞ ∞ ∞ ∞ ∞ ∞ ∞ ∞ ∞ ∞ ∞ ∞ ∞ ∞ ∞ ∞ ∞

885.2 892.3 903.9 881.7 888.2 900.7 887.8 894.9 909.8 891.6 898.6 910.7 885.6 892.8 907.8 887.5 894.1 908.1 888.0 895.2 908.1 899.4 908.0 923.0

0.76 0.71 0.60 0.84 0.89 0.95 1.04 0.87 1.01 0.74 0.83 0.90 1.50 1.56 1.32 0.87 0.82 0.90 0.72 0.79 0.72 1.36 1.51 2.13

0.496 0.590 0.551 0.368 0.333 0.283 0.324 0.495 0.326 ∞ ∞ 1.300 0.243 0.272 0.305 0.560 0.656 0.446 ∞ ∞ ∞ 1.181 0.658 0.389

864.3 866.5 869.3 864.0 866.5 870.7 866.7 869.0 873.9 869.1 872.6 878.7 865.5 868.7 873.2 866.7 869.3 874.2 867.0 870.6 875.8 873.8 877.8 885.5

Cases not studied. 403


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Figure 2. Comparison between the calculated (tcal) and measured (texp) burning time.

under analysis.

The likely explanation for such high wood char reactivity is connected to the fact that some components of the ash that had a high catalytic activity were present during the combustion process. The chars obtained in the present study had high ash contents (see Table 3). It was impossible to carry out experiments at a bed temperature of 950 °C, because of the effects of sand agglomeration and the subsequent blockage of the bed; this outcome is typical when the wood and char ashes contain components that have a low melting temperature, thereby intensifying any problems because of the agglomeration of the bed.11,13,14,23 The high ash content found in our experiments could be explained by the fact that the biomass available for carbonization and subsequent experiments mainly consisted of smalldiameter branches. The presence of the bark is therefore significant (a high proportion of the volume of each branch is bark), and the bark was not removed prior to carbonization. The values obtained for the burning particle temperature Tp, using the semi-analytical mathematical model discussed in a later section, are also shown in Table 5. Burning Times for the Batches of Char Particles. The calculated and measured burning times are compared in Figure 2 for several batches of char particles. To calculate the burning time for a batch of char particles, we used an expression (eq 4) based on Sh and kc.19 This equation made it possible to calculate the burning time for the three different burned mass fractions f (25, 50, and 75%) of a batch of char particles, given the corresponding values of Sh and kc for the eight species

ρ d i 2[1 − (1 − f )2/3 ] tf = c 96c0ShDG +

2ρcd i[1 − (1 − f )1/3 ] 48kcc0

+

mc f 12c0A t U

(4)

Although the different burned mass fractions are not shown in Figure 2, shorter burning times corresponded to lower burned mass fractions. This representation of the results has the advantage that it allows for the qualitative evaluation of the obtained kinetic and diffusive results and also permits a comparison to be made between the burning times for the batches of the eight wood chars, where either air or a 10% mixture of O2 in N2 was used. When a comparison is made between these results, it can be seen that, for combustion in air, the calculated burning times lay within a band that extended from −35 to +15% of the average measured time, while for combustion with 10% O2 in N2, the band was narrower, with values in the range of ±15% of the average measured time. The longest observed burning time for combustion in air was 240 s, while for combustion in 10% O2, it was 480 s; both times refer to the time taken for 75% of the mass fraction to burn (see Figure 2). The actual combustion time of the batch depends upon the transient phenomena that occur in the initial phase of the batch combustion process, when the particles are heating and the rate 404


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Figure 3. Difference between particle and bed temperatures, for combustion in air (ΔTair) and combustion in 10% O2 (ΔT10% O2). Bed temperatures (Tb): (a) 750 °C and (b) 850 °C.

of reaction is therefore lower than it is later on when the particles are hotter. This implies that measured combustion times are likely to be longer than those obtained by calculation, which assumes that the char particles reach the bed temperature instantaneously. The influence of the initial transient combustion is stronger for shorter combustion times, which explains why the difference was particularly significant in the present study, especially for combustion in air, when the difference was more marked. Temperature of the Burning Particles. Throughout the combustion process, brighter char particles could be seen moving up and down inside the bed, indicating that the fuel particles were burning at a temperature greater than the average temperature of the inert sand particles, of which the bed was composed. Although this was empirically evident when the particles were at the surface of the bed, it is not clear that this was true while they were burning deep within it, and it is furthermore doubtful that such observations represented the behavior of all of the particles in a given batch. Many different authors17,18,22 have shown that the reaction that occurs at the surface of the particle is the oxidation of carbon to CO, which, in broad terms, releases one-third of all of the energy available from the oxidation of carbon to CO2. This implies that the temperature at which the particle burns cannot rise much higher than the temperature of the bed. On the other hand, when a burning char particle reaches the bed surface, it is expected that the oxidation of CO to CO2 begins to occur at the particle surface, because the quenching effect of the inert bed particles on the homogeneous phase reaction is now smaller, leading to a rise in the surface temperature of the burning particle. This is the reason why brighter char particles may be observed at the surface of the fluidized-bed reactor. Again, when they move toward the deeper regions of the bed, the quenching effect of the homogeneous phase reaction imposes the same constraints as above. As a result of these considerations (which were also mentioned previously elsewhere15,19), the temperature of the burning particles was calculated here using an energy balance for a single particle; in this approach, it was considered that the conversion of carbon to CO occurred at the surface, while the resulting CO burned remote from the particle.

ṁ cΔh′ + Q̇ s−b + Q̇ rad = 0

dT Q̇ s−b = −k tg πd 2 dr

d /2

Q̇ rad = ε pπd 2 σ(Tp 4 − Tb 4)

(6) (7)

In eqs 5−7, ṁ c is the mass of carbon consumed for each particle per unit time, as determined from the experimental data, Δh′ is the enthalpy of combustion for the heterogeneous phase reaction C + 1/2O2 → CO, −9211 kJ/kg at 25 °C, and Q̇ s−b and Q̇ rad are the energies transferred by conduction and radiation toward the surrounding bed, per particle, per unit time, respectively.24 The thermal conductivity of the gas that interacts with the particle (ktg) was assumed to be equal to that of air, and εp is the emissivity of the burning particle. The convective effects were not considered. A similar approach was also used for the combustion model, where it was assumed that any mass transfer was only due to the diffusion of O2 to the particle surface, because the relative velocity between the particles and the interacting gas was small. Although, on the basis of different assumptions, mainly in terms of the heat released during combustion, this approach has been used by several authors.2,17,22,25 The convective heat transfer of the bed particles is negligible, on the order of 0.5−1.7% of the total heat-transfer coefficient in the combustion of a fuel particle in a fluidized-bed boiler.26 Figure 3 shows a comparison between the particle temperatures and the bed temperature, both for combustion in air and for combustion in the 10% O2 mixture in N2, for burned mass fractions of 25, 50, and 75% (although the distinction between the different burned mass fractions is not made in the figure). The temperature difference between the particles and the bed ranged from a minimum of 12 °C for combustion at a bed temperature of 850 °C with 10% O2 to a maximum of 82 °C for combustion in air at 750 °C.

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CONCLUSION

In a series of experiments on the batch combustion of carbon particles in a laboratory-scale bubbling fluidized-bed combustor, we obtained kinetic and diffusive data on the combustion of eight types of chars derived from species of wood found in northeastern Brazil. Because the wood was sourced from ligneous species of shrubs, the sizes of the elements used for the production of chars increased the relative composition of the bark and

(5) 405


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Dimensionless Numbers Rep,mf = particle Reynolds number at minimum fluidization conditions (=ρUmfd/μ) Sc = Schmidt number (μ/ρDG) Sh = Sherwood number of a char particle (=kGd/DG) Sh′ = modified Sherwood number (=kGd/Dm′ = Shτ) Acronyms NPT = normal pressure and temperature (1 atm and 20 °C)

influenced the mechanism by which the combustion was controlled. The minerals in the bark acted as catalysts, such that, for combustion in air at 850 °C, the reaction was controlled entirely by diffusion. When attempts were made to perform experiments at a higher bed temperature (950 °C), the effects of agglomeration brought about by the presence of these ash components prevented the successful completion of the combustion. A maximum calculated temperature difference of 82 °C was found between the particle and the bed for combustion in air, whereas the maximum differential found for combustion in a 10:90% mixture of O2/N2 was only 41 °C.

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AUTHOR INFORMATION

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ACKNOWLEDGMENTS

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REFERENCES

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Corresponding Author *Telephone: +351-225081747. Fax: +351-225081440. E-mail: [email protected].

The present study was supported by a postdoctoral grant (SFRH/BPD/49250/2008) from Fundaçaõ para a Ciência e a Tecnologia (Portugal) and was carried out in the laboratory facilities of INEGI. The authors also thank the Capes Foundation, Ministry of Education of Brazil, for the financial support provided to Marcelo Ramos (Process BEX 5427/09-6).

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NOMENCLATURE At = cross-sectional area of the fluidized bed (m2) c0 = molar concentration of O2 at the bed inlet (kmol m−3) d = diameter of a char particle (m) D′m = effective diffusivity considering the tortuosity of the bed (=DG/τ, m2 s−1) DG = diffusivity of oxygen in air (m2 s−1) di = initial diameter of char particles (m) dp = average diameter of bed sand particles (m) f = burned mass fraction Δh′ = enthalpy of combustion for the reaction C + 1/2O2 → CO (kJ kg−1) K = overall combustion constant (m s−1) kc = heterogeneous phase reaction rate constant (m s−1) ktg = thermal conductivity of a gas interacting with a particle (W m−1 K−1) mc = mass of carbon in a batch of char particles (kg) ṁ c = mass rate of carbon consumption per particle (kg s−1) Q̇ s−b = rate of thermal energy transferred by conduction, per particle, toward the bed (J s−1) Q̇ rad = rate of thermal energy transferred by radiation, per particle, toward the bed (J s−1) tf = burning time for mass fraction f (s) Tp = temperature of the char/carbon particle (°C or K) U = fluidization velocity (m s−1) Umf = minimum fluidization velocity (m s−1) Greek Symbols εmf = bed porosity at minimum fluidization conditions εp = emissivity of the burning particle μ = viscosity of fluidizing air (Pa s) ρ = density of fluidizing air (kg m−3) ρc = mass of carbon per unit volume of particle (kg m−3) σ = Stefan−Boltzmann constant (J s−1 m−2 K−4) τ = bed tortuosity 406


Fluidized-Bed Combustion of Selected Wood Chars from the Semi

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