- Email: [email protected]
Characterization of porous structure of coal char from a single devolatilized coal particle: Coal combustion in a fluidized bed
Fuel Processing Technology 90 (2009) 692–700
Contents lists available at ScienceDirect
Fuel Processing Technology 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 / f u p r o c
Characterization of porous structure of coal char from a single devolatilized coal particle: Coal combustion in a fluidized bed Anup Kumar Sadhukhan a,⁎, Parthapratim Gupta a, Ranajit Kumar Saha b a b
Chemical Engineering Department, National Institute of Technology, Durgapur - 713209, W.B., India Chemical Engineering Department, Indian Institute of Technology, Kharagpur - 721302, W.B., India
a r t i c l e
i n f o
Article history: Received 24 June 2008 Received in revised form 29 November 2008 Accepted 15 December 2008 Keywords: Coal char Pore structure Surface area Porosity Carbon matrix
a b s t r a c t The combustion characteristics of coal char are highly dependent on initial pore structure of devolatilized char as well as on the structural evolution during the combustion of char. The development of pore structure also throws light on the mechanism of the combustion process. In the present work evolution of pore structure of partially burnt coal char of Indian origin has been investigated experimentally in a batch-fluidized bed and analyzed. The BET surface area, micropore surface area and porosity of char at various levels of carbon burn-off have been determined. Experimental specific surface area has been found to agree well with theoretical prediction using random pore model. Modified random pore model is used to determine the active surface area. Char combustion mechanism based on shrinking unreacted core and shrinking reacted core models are delineated during the course of reaction at various bed temperatures. This is substantiated with the proportional representation of ash and carbon matrix in scanning electron microscope images. It is also concluded that in the present investigation the mean pore size is much smaller and hence the Knudsen diffusion predominates. Analysis based on similar experimental observations and models for pore structure evolution to investigate char combustion reaction regime has not been reported in literature. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Coal utilization processes such as combustion and gasification generally involve several steps: (i) the devolatilization of organic matter leaving behind char; (ii) homogeneous reactions of volatile matters with reactant gases and (iii) heterogeneous reactions of the residual char with reactant gases during which ash is formed. The devolatilization process and subsequent combustion of volatiles have profound influence on the physical structure of char. Besides, the char also undergoes significant changes during combustion. The char structure is highly heterogeneous and usually affects the char combustion reactions and ash formation. These phenomena have attracted widespread research interests over the last few decades. Studies on combustion behavior and the role of coal petrographic analysis on the morphology of char during pyrolysis and combustion have already been reported in literature [1–3]. A single particle analysis provides more detailed insight about the process [4–6] during combustion in fluidized bed. The present article provides a detailed study of the characterization of a typical sub-bituminous coal as it undergoes devolatilization and subsequent burning in a batch fluidized bed combustor. The study is expected to be useful in
⁎ Corresponding author. Tel.: +91 9933989609; fax: +91 343 254 7375. E-mail address: [email protected] (A.K. Sadhukhan). 0378-3820/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.fuproc.2008.12.010
understanding the mechanism of the combustion process and modeling single particle coal char combustion. 1.1. Structure of coal char In a fluidized bed combustor (FBC) a coal particle first undergoes devolatilization. The characteristic time of release of volatiles depends on the volatile content of original coal sample, particle diameter and bulk temperature of fluidized bed. A large number of models are available in literature to describe coal devolatilization kinetics. The model demonstrated by Anthony and Howard [7] has wide range of applicability. Development of comprehensive single particle coal char combustion model requires detailed study of the structural development of porous char particle during pyrolysis and subsequent combustion. Borghi et al. [8] proposed that the evolution of volatiles is often accompanied by their ignition in a laminar yellowish diffusion flame in the vicinity of the particle surface causing further increase in temperature and acceleration in the volatiles evolution process. The coal particle temperature and the oxygen concentration at the particle surface are highly influenced by the homogeneous gas phase combustion of the volatiles. This finally governs the residual char combustion involving an interaction of heterogeneous and homogeneous reactions with transport limitations. Under rapid devolatilization of spherical coal particles, cenospheres are formed leading to
A.K. Sadhukhan et al. / Fuel Processing Technology 90 (2009) 692–700
the generation of highly porous char. The internal surface area depends on the volatile matter content of coal and the nature of pore structure. Under slow devolatilization, less internal pores are evolved due to decreased yield of volatile mater. Young and Smith [9] reported that a typical burnt brown coal particle with volatile mater content of 40%, collected from pulverized fuel fired furnaces, has a porous structure with pore openings 1000 times the size of an oxygen molecule. The mean free path of oxygen molecules is in the order of 50 to 150 nm and its molecular radius is 0.2 nm only. Hence oxygen molecule can easily penetrate into the micropores and react with carbon internally and char combustion occurs through internal combustion. Su and Perlmutter [10] proposed that during coal char combustion the pore radius varies in the range of 2 to 100 nm. D'Amore et al. [11] classified pores of char particles during combustion as micropores, mesopores and macropores and determined the pore size distribution during burn-off of various char particles. IUPAC Manual of Symbols and Technology [12] classifies pores as: (i) micropores: pore width, wp b 2 nm (ii) mesopores: 2 b wp b 50 nm (iii) macropores: wp N 50 nm. Experimental studies by Hurt et al. [13] revealed that specific internal surface area monotonically increases with the carbon burn-off due to opening of more new pores. However the pore-collapsing phenomenon occurs during the later part of char combustion, causing decrease in specific surface area after a critical burn off. Experimental data with coal char by D'Amore et al. [11] showed that the maximum specific surface area occurred at about 35% carbon burn-off. Recently Daud and Ali [14] reported that for activated carbon produced from palm shell and coconut shell, the maximum micropore volume occurred at 38% and 48% burn-off respectively. Micropores typically constitute more than 80% of the internal surface area of char particles and the rest are due to mesopores and macropores. 1.2. Structural model Shrinking sphere model [4], shrinking unreacted core model [5] and volume reaction model [6] are widely used for coal char combustion in fluidized bed. Non-porous coal char with low ash content generally follows the shrinking sphere in fluidized bed due to attrition with bed particles, non-porous char with high ash content conforms to the shrinking unreacted core model and the porous char follows the volume reaction model. In actual practice, the char combustion reactions occur both at the outer surface and inside the pore surface, depending on the porosity of the particle. Hence both particle size and the density may change during the combustion process which has been studied by a number of researchers including Haldar and Saha [15] and Waters et al. [16]. Tomeczek and Mlionka [17] determined the existence of noncylindrical pores in coal char by means of experiments with mercury porosimetry. The most elaborate model on development of pore structure was described by Bhatia and Perlmutter [18] and Gavallas [19] who presented convenient mathematical relationships for pore structure evolution. The work was further extended by Gavallas and Wilks [20] into a form suitable for application to char gasification at chemically controlled rates. Su and Perlmutter [21] also presented a theoretical model to predict the evolution of pore volume distribution during gasification of coal char. The pore structure of coal char during combustion is usually characterized by its pore volume distribution and it can be determined experimentally by sorption analyzer and adsorption isotherms of nitrogen at the temperature of liquid nitrogen. Other integral properties of the pore structure, such as total microporosity, surface area, and total pore volume can also be determined from experimental findings using the standard method. Most of the existing literature on characterization of internal structure of coal is mainly applicable to coals with low ash content, typically of the order of 4–5% by weight. Remiarova et al. [2] studied the internal structure by measuring the internal surface area, pore
693
volume, microporosity and macroporosity of the particle at different carbon burn-off. In contrast, Indian coals have high ash content due to its drift origin, typically 14–30% after treatment in coal washery with the washery rejects containing ash as high as 30–50% by weight. 2. Background information on structural model According to Wang and Bhatia [22] the devolatilized char particle comprises microporous grains surrounded by mesopores and macropores. The heterogeneous gas–solid combustion reaction takes place in micropores, while the macropores serve as channel for transportation of gaseous reactants and products. However, Biggs and Agarwal [23] proposed that it is the accessible porosity of the char particle, which is responsible for internal transportation of reactants and products within the porous texture of the char particle during the course of combustion. As the carbon burn-off increases, both the micro-porosity and the accessible porosity gradually increase causing a decrease in transport resistance. The variation of specific char surface area with carbon burnoff can be measured experimentally. Several investigators including Kajitani et al. [24] and Adschiri and Fursawa [25] experimentally found that the internal surface area gradually increases, attains a maximum and then decreases with carbon burn-off. 2.1. Random pore model Random pore model developed by Bhatia Perlmutter [18] is the most widely accepted structural model to predict the development of pore surface area during the combustion and gasification of coal char. The carbon burn-off in partially burnt char sample is calculated as X=
ðW0 − Wvm − W Þ ðW0 − Wvm − Wa Þ
ð1Þ
where W0, Wvm, W and Wa are initial mass of coal sample, mass of volatiles in sample, mass of partially burnt char and mass of ash in sample respectively. The surface area per unit volume of char S may be estimated from experimentally measured surface area per unit mass, Sm using the following equations Sð X Þ = Sm ð X Þ · ρð X Þ
ð2Þ
ρ = ρ0 ½α a + ð1 − X Þ · ð1 − α a Þ
ð3Þ
where ρ0 is initial density, ρ(X) is density of the char particle at carbon burn off X and αa is the mass fraction of ash in devolatilized char. According to Bhatia and Perlmutter [18] the specific surface area S per unit volume of the char particle can be expressed by the following equation qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi Sð X Þ = S0 · ð1 − X Þ 1 − ψ lnð1 − X Þ
ð4Þ
where S0 is the surface area at zero burn-off and ψ is the dimensionless parameter of pore model indicating the nature of pore structure. The value of ψ may be calculated using pore length L0 and initial accessible porosity ε0 of char particle as follows 2
ψ = 4πL0 ð1 − e0 Þ = S0 :
ð5Þ
The parameter ψ may be treated as fitting parameter and may be obtained with the curve fitting of experimental values of S(X) at different carbon burn-off. Ash particles present in the char remain as inert during char combustion and the surface area provided by ash particles do not contribute for heterogeneous surface reaction. Lu and Do [26] further modified the random pore model in order to eliminate the ash surface area and to find out active surface area where the heterogeneous gas solid reaction takes place.
694
A.K. Sadhukhan et al. / Fuel Processing Technology 90 (2009) 692–700
2.2. The modified random pore model The modified random pore model assumes that the char particle contains separate ash and carbon matrix without overlapping each other. Between the two matrices there is void space contributing for macro porosity of the particle. The model further assumes that the carbon is present in the porous carbon matrix and ash matrix is totally nonporous. The gas solid reaction occurs only on the internal surface of the carbon matrix. The porous carbon matrix has an initial porosity εµ0 called micro-porosity. The experimental measurement of pore surface area with sorption analyzer accounts for both ash surface area and carbon surface area. The modified random pore model provides mathematical equation to estimate the carbon surface area, which is considered as active surface area during the course of combustion of coal char with significant quantity of ash content. According to this model the carbon matrix is considered to be an entity consisting of randomly distributed geometrical pore space with volume Vµi (i = 1, 2, … etc). If all pore voids grows by an infinitesimal amount drp at carbon burn off X in the direction normal to pore surface, the following differential equation can be written for pore surface area: deμ = Sc ð X Þ drp
ð6Þ
where Sc(X) is the carbon surface area per unit volume of carbon matrix. Applying the random pore model by Bhatia and Perlmutter [18] to the carbon matrix in the particle, Eq. (4) may be modified as qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi Sc ð X Þ = ð1 − X Þ 1 − ψμ lnð1 − X Þ: Sc ð0Þ
ð7Þ
ψµ is the structural parameter of pore in carbon matrix and carbon burn-off may be defined in terms of micro porosity of carbon matrix as X=
eμ − eμ0 : 1 − eμ0
ð8Þ
Considering the presence of ash in the particle with volume fraction αva the total surface area of the particle at zero carbon burn-off is: v v S0 = α a · Sa + 1 − α a Sc ð0Þ
ð9Þ
ð10Þ
where Sa is the surface area of ash per unit volume of ash matrix. The volume fraction of ash matrix in the char particle can be related to weight fraction of ash by the following equation: α a · 1 − eμ0 ρc α va = : ð11Þ ð1 − α va Þ ð1 − α a Þ · ρa The value of Sc(0) can be calculated from Eq. (9) by experimentally measuring the total surface area of the particle at zero burn-off and measuring the surface area of ash after complete carbon burn-off in the char. Hence γ, the ratio of ash surface area to initial carbon surface area in carbon matrix, is a measurable quantity and may be defined as γ=
Sa : Sc ð0Þ
ð12Þ
Sc(0) can also be expressed as: Sc ð0Þ =
S0 ½γ · α va + ð1 − α va Þ
Sc ðXÞ =
SðXÞ − α va · Sa : ð1 − α va Þ
ð14Þ
By substituting Eq. (13) in Eq. (7) the following equation may be written as:
Sc ðXÞ = S0 ·
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ð1 − XÞ 1 − ψμ lnð1 − XÞ ½γ · α va + ð1 − α va Þ
:
ð15Þ
Again the fitting parameter ψµ may be obtained by curve fitting of Sc(X) with X. It may be mentioned here that the pore parameter ψ is associated with total surface area including ash where ψµ does not count for ash surface area. The present study reports the development of porous structure of a typical sub-bituminous coal (ash 14.9% d.b.) of Indian origin in a fluidized bed combustor. Partially burnt char samples have been investigated for specific surface area, micropore volume, pore size distribution and macro porosity. Random pore model is applied to predict the experimentally obtained surface area of the partially burnt char samples. Further, the modified random pore model is used to estimate the active surface area per unit volume of carbon matrix. The effect of ash content on active carbon surface area is evaluated. Finally, scanning electron microscope (SEM) images of coal sample, devolatilized char and a few partially burnt char samples are presented to show the evidence and development of porous texture at different burn-off. 3. Experimental 3.1. Materials The sub-bituminous coal of Indian origin was used for experimentation in fluidized bed combustion. Spherical coal particles were prepared by rubbing and samples were selected with sphericity of 0.81 to 0.85 and with an average particle diameter 6.0 mm. Table 1 shows the proximate and ultimate analysis of the coal used for experimentation. 3.2. Experimental set up
the total surface area of the particle at any carbon burn-off is: v v Sð X Þ = α a · Sa + 1 − α a Sc ð X Þ
and active carbon surface area Sc(X) at any carbon burn-off may be estimated from experimentally measured values of S(X) by using the following expression.
ð13Þ
The fluidized bed combustor consists of a stainless steel column, 82 mm in diameter; fitted with a S.S. distributor, having 1.6 mm diameter holes, and 1.2% open area (Fig. 1). It is enclosed in an insulated refractory chamber. The annular region between the stainless steel column and the brickwork forms the heating chamber, provided with a ring-burner to burn LPG and air was blown in by an air-blower into the fluidized bed consisting of crushed refractories. A sufficiently long
Table 1 Proximate and ultimate analysis of coal sample. Analysis Proximate analysis (wt.%) Moisture Volatile (d.b.) Fixed carbon (d.b.) Ash (d.b.) Ultimate analysis (wt.%) Carbon Nitrogen Hydrogen Sulphur Oxygen (by diff.)
Coal
Char
5.40 27.0 58.1 14.9
0 2.56 76.44 21.00
86.46 1.82 4.51 0.15 7.06
89.24 1.42 0.51 0.03 8.80
A.K. Sadhukhan et al. / Fuel Processing Technology 90 (2009) 692–700
695
Fig. 2. TG and DTG profile from thermogravimetric analysis of coal.
used in each run which ensures that the oxygen concentration in the bulk gas around the particle remained almost constant at 21% during the entire period of experimentation which was corroborated by sample gas analysis inside the bed. Biggs and Agarwal [23] used similar operating conditions to study the burning characteristics of porous coal char in a fluidized bed. Fig. 1. Fluidized bed combustor: 1. Refractory brick wall; 2. Feeding pulley; 3. Mirror arrangement; 4. Thermo couple probe; 5. Distributor; 6 LPG entry; 7. Orifice meter; 8. Air control valve; 9. Screen supporting; Glass beds; 10. Blow air; 11. Digital multimeter; 12. Strip-chart recorder; 13. Bed material 14. Basket; 15. Char sample inside basket.
(around 200 mm) isothermal section was ensured, where the temperature could be maintained constant with a variation within ±5 °C. The bed temperature was measured by a chromel-alumel thermocouple and recorded continuously by strip-chart recorder. The combustor was heated to the desired temperature, stabilized for 10 min and was then used for experimentation. Table 2 presents the operating variables and different parameters for experimentation. Considering heterogeneous nature and complex structure of coal and coal char the experiments were repeated several times for each sample and the reproducibility is found to be within ±3%. The different experimental conditions are detailed in following section. 3.3. Combustion of coal samples in fluidized bed A single coal particle was weighed, put in a small basket made of S.S. wire mesh (200 mesh) and introduced inside the combustor. The particle was heated up, devolatilization started and a bright yellowish volatile flame was produced visible through a mirror arrangement. Immediately after the volatile flame disappeared, the partially burnt char sample was taken out after a stipulated time, quenched in liquid nitrogen immediately and weighed again. Only a single coal particle was
3.4. Experimentation with partially burnt char 3.4.1. Pore structure characterization A large number of partially burnt char samples under different operating bed temperatures were analyzed for pore structure characterization by measuring specific surface area, pore volume, pore size distribution and macro porosity. Nitrogen adsorption technique at −196 °C with automatic sorption analyzer (Autosorb-1, Model No. ASIC-9 by Quantachrome instruments) was used for this purpose. Mercury porosimeter (Pore-Master-GT Model No. PM-33-6) was used for determination of accessible porosity of partially burnt char at various carbon burn-off. Prior to the gas adsorption measurements char samples were degassed at 200 °C for a period of 2 h. Nitrogen adsorption isotherms were measured over a relative pressure (P/P0) range of 1.67× 10− 6 to 0.995. The specific surface area was determined by application of Brunauer–Eemmett–Teller (BET) analysis software available with the instrument. The pore size distribution of partially burnt char samples were estimated by Density Functional Theory (DFT) software available with the sorption analyzer. 3.4.2. Scanning electron microscope (SEM) images SEM images for original coal and several partially burnt char samples were obtained using a scanning electron microscope JEOL-
Table 2 Fluidized bed combustor. Material of construction
Stainless steel
Inside diameter (mm) Bed inert
82 Crushed refractory with a mean diameter of 0.43 mm. Density — 2100 kg/m3 Stainless steel plate, hole diameter — 1.6 mm, pen area — 1.12% Air 650, 750 and 900 °C 700 100 200 0.30
Distributor Fluidizing medium Operating bed temperature, Tb Total column height (mm) Static bed height (mm) Expanded bed height (mm) Superficial gas velocity (ms− 1)
Fig. 3. Internal structure of original coal particle [dp = 6 mm, Tb = 750 °C, ash (white), carbon matrix (grey)].
696
A.K. Sadhukhan et al. / Fuel Processing Technology 90 (2009) 692–700
Fig. 4. Internal structure of just devolatilized coal particle [dp = 6 mm, Tb = 750 °C, ash (white), carbon matrix (grey)].
Fig. 6. Internal structure of coal char particle at carbon burn off 0.67 [dp = 6 mm, Tb = 750 °C, ash (white), carbon matrix (grey)].
Japan Model No. JSM-5800 apparatus equipped with a TESCAN digital unit.
release about 3% volatiles, which might be due to large particle size or very small devolatilization time. Borghi et al. [8] found experimentally that the volatile release of millimeter sized coal particle during combustion in fluidized bed depends on the size of the particle and temperature of the bed. The residual volatiles in the char burns during the char combustion period and this small amount has negligible influence on solid char combustion process.
4. Results and discussion 4.1. Devolatilization of coal particle The observed devolatilization time for 6 mm coal particle at 650 °C was around 30 s and at 900 °C it was around 20 s including the heatup time (around 5 s) for the particle in the fluidized bed. The thermogravimetric analysis of coal sample in nitrogen atmosphere is presented in Fig. 2, which shows that the residual mass fraction of coal particle decreases with increasing temperature due to the release of volatiles. It is evident that maximum mass loss occurs within the temperature range of 400–700 °C. It stops at around 850 °C with a final residue of 0.68, indicating volatile release of 32.0%, which is more than the proximate volatiles (27%). An increase in temperature above 850 °C does not have a significant influence on particle mass loss. This observation suggests that it is required to heat up the coal particle above 850 °C in order to remove the volatiles completely. However in fluidized bed combustion of coal particles the released volatiles burn in gas phase near the particle surface and due to heat transfer from the flame to the particle surface, the particle temperature rises several hundred degrees above the bed temperature. Hence, the selection of our experimental temperatures (650, 750 and 900 °C) was appropriate. For 6 mm coal particles the estimated mass loss due to devolatilazation was 29.0%, indicating that the char sample was yet to
The scanning electron microscope (SEM) images of the coal particle used for our experiment are presented in Fig. 3. It shows the presence of plant vessels and tissues, which form the main skeleton of the coal particle. The visible grey portion rich in carbon, called the carbon matrix, contains the micropores and the ash is distributed randomly in the carbon matrix. Fig. 4 is the image of devolatilized char, where the contrast of grey scale has changed slightly indicating total devolatilization. There are a few small yet-to-be devolatilized islands in carbon matrix. According to Smith et al. [27] the backbone of coal structure is made of dense polycyclic aromatic hydrocarbons and linked by alternative single and double bonds and has extra resonance stability which is more resistant to thermal decomposition with higher bond energy of 1000 kJ/mol. The polycyclic aromatic hydrocarbons are transformed from the original plant tissues during prolonged decomposition under heat and bacteria. This may lead to highly heterogeneous structure of coal. The micro-islands in the carbon matrix are made of different group polycyclic aromatic
Fig. 5. Internal structure of coal char particle at carbon burn off 0.18 [dp = 6 mm, Tb = 750 °C, ash (white), carbon matrix (grey)].
Fig. 7. Internal structure of coal char particle at carbon burn off 0.92 [dp = 6 mm, Tb = 750 °C, ash (white), carbon matrix (grey)].
4.2. Study of internal structure by SEM images
A.K. Sadhukhan et al. / Fuel Processing Technology 90 (2009) 692–700
697
Table 3 Pore characteristics of partially burnt char at 750 °C.
Fig. 8. Carbon burn off for devolatilized char at different time (dp = 6 mm).
hydrocarbons, which decompose at higher temperature to release its volatile contents. This causes very slow mass loss which is observed from the thermogravimetric curve (Fig. 2), where the fractional residue decreases very slowly after 850 °C. The microstructures of the partially burnt char particle at different carbon burn-off are presented in Figs. 5–7. The structure of the ash that covers the carbon matrix gradually with increase in carbon burn-off is visible from these figures. The figures also show that the accessible porosity of the particle increases with carbon burn-off. The SEM images do not show appreciable effect of bed temperature for the same degree of carbon burn off. 4.3. Effect of carbon burn off on pore structure The sigmoidal temporal burn-off profile of devolatilized char (Fig. 8) is considerably influenced by the bed temperature (Fig. 8). The adsorption isotherms (Fig. 9) of partially burnt devolatilized char at 750 °C belong to a mixture of type-I and type-IV isotherms according to IUPAC classification. Type-I isotherm is associated with microporous structure while type-IV isotherm indicates a mixture of microporous and mesoporous char. Similar types of isotherms were obtained for other bed temperatures as well. Nitrogen adsorption at low relative pressure indicates filling of the micropores (wp b 2 nm) in the char samples. The linear portion of the curve represents multilayer adsorption of nitrogen on the internal surface of the char sample, and the concave upward portion of the curve represents filling up of mesopores (2 nmb wp b 50 nm) and macropores (wp N 50 nm). An entire isotherm is needed to calculate the pore size distribution of the char. However, for BET surface area evolution, data in the linear portion of the
Carbon burn off in char (%)
Total pore volume (m3/g of char)
BET surface area (m2/g of char)
Micropore surface area (m2/g of char)
0.00 0.18 0.49 0.67 0.78 0.92
0.0176 0.0382 0.0551 0.0692 0.0731 0.0883
17.56 22.80 36.12 33.99 31.56 22.90
16.80 19.10 24.80 18.70 12.00 8.32
curve has been used. The micropore surface area of partially burnt char sample is calculated using the Dubinin–Asthakov (DA) analysis technique. The BET surface area and the microporous surface area at different burn-off are presented in Table 3. The accessible porosity of original coal sample, devolatilized char and fully converted ash were measured with mercury porosimeter and is presented in Table 4. It has been observed that though the original coal sample contains little porosity, it increases to 0.22 immediately after devolatilization, almost independent of the bed temperature. The fully converted ash has the accessible porosity of as high as 0.53. The data on accessible porosity at different stages suggests a continuous change in pore structure during the course of the combustion process. Fig. 10 shows the development of pore structure at various carbon burn-off in char. As the char structure consists mostly of micropores at low carbon burn-off, reactions occur at the pore surface and are controlled by diffusion transport of oxygen due to availability of low mesopore and macropore surface area (Fig. 10). However, after a carbon burn-off of 0.40 the accessible porosity starts to increase. At a burn-off of 0.67 active carbon content of the particle drops and oxygen has to penetrate deeper into the particle for the reaction to occur according to shrinking reacted core model. The unburnt carbon is surrounded by a shell of ash with pore size ranging from 40 to 70 nm. 4.4. Effect of temperature on specific surface area Both the specific surface area and pore structure of char, that affect the transport of gaseous reactants and products and consequently the combustion reactions, vary during the course of combustion and hence need to be modeled accurately. Fig. 11 shows the variation of specific surface area Sm (per unit mass) with carbon burn-off at different fluidized bed temperatures. At low bed temperature the char combustion is controlled kinetically and oxygen has enough time to diffuse in creating more micropores which results in higher specific surface area. At higher bed temperatures the pore surface area developed inside the char particle is found to be lower and the char combustion proceeds through diffusion controlled regime. Oxygen gets consumed by the reaction before it can diffuse into the inner micropores and reaction occurs at fewer reaction sites giving rise to macropores which leads to smaller surface area. At all bed temperature the micropores get transformed to mesopores gradually and then to macropores towards the end of combustion due to pore-collapsing phenomenon leading to a decreases in total surface area. The contribution of micro and macropores to the surface area at carbon burn-offs 0.50 and 0.75 for different bed temperatures are presented in Fig. 12. At low conversion the contribution of
Table 4 Accessible porosity of coal, coal char and ash.
Fig. 9. Adsorption isotherm of for devolatilized char at different carbon burn off (dp = 6 mm, Tb = 750 °C).
Sample
Accessible porosity
Coal Devolatilized char Ash
0.08 0.22 0.53
698
A.K. Sadhukhan et al. / Fuel Processing Technology 90 (2009) 692–700
Fig. 10. Development of porous structure of char at different carbon burn off (dp = 6 mm, Tb = 750 °C).
micropore surface area is larger than that of meso and macropores, but at a burn-off of 0.75 the contribution of macropore surface area increases causing less total surface area. A similar trend is observed at all bed temperatures with varying relative contributions. So it may be concluded that at low bed temperature due to high pore surface area and lower reaction rate, the combustion proceeds by shrinking reacted core model. After the reaction starts throughout the volume of the core, carbon at the surface gets consumed completely, forming an ash layer around the char core. The SEM images of partially burnt char sample shows the evidence of reaction throughout the volume of particle by presence of randomly distributed ash inside the carbon matrix (Figs. 5–7). Gradually the volume of the char core diminishes with conversion. However, eventually as the pore-collapsing phenomenon takes place at the end, the surface area decreases and the combustion of unburnt carbon between the macropores follows shrinking unreacted core model. At higher bed temperature with lower pore surface area and higher reaction rate the combustion reaction proceeds by shrinking unreacted core model right from the beginning. 4.5. Application of the random pore model Fig. 11 shows that the internal surface area gradually increases with conversion, attains a peak, and then decreases as the combustion proceeds, which is corroborated by experimental observations of Kajitani et al. [24] and Adschiri and Fursawa [25]. The experimentally obtained specific surface area data per unit mass (Sm) of char is converted to surface area per unit volume (S) of char using Eqs. (2) and
Fig. 12. Effect of temperature on development of surface area at different carbon burn off for (dp = 6 mm).
(3) and presented in Table 5. The parameter ψ in Eq. (4) was obtained by fitting the experimental data of S and X. The estimated values of ψ for the char sample were 8, 6 and 4 at 650, 750 and 900 °C respectively. The greater the value of pore parameter, the greater is the porosity of the char. Larger pore surface area is obtained during char combustion at lower temperature giving rise to higher ψ value. An excellent agreement between the surface area predicted from random pore model with the estimated values from experiment is observed at different carbon burn-off for different bulk temperatures (Fig. 13). 4.6. Application of the modified random pore model The values of active carbon surface area Sc(X) estimated from the total surface area S using Eq. (15) are presented in Table 5. The active specific surface area of char is slightly smaller than the corresponding total specific surface area. Lu and Do [26] derived the reaction rate expression for char gasification by using the active carbon surface area based on carbon matrix, while Adschiri and Furusawa [25] argued for active char surface area based on char volume. The fitting parameter ψµ was re-estimated for active char surface area with the curve fitting of estimated values of Sc(X) and X. The new values are 6, 5 and 3.2 at 650, 750 and 900 °C respectively. Hence the modified random pore model predicts slightly lower char porosity. 4.6.1. Effect of ash in modified random pore model It may be concluded from the above discussion that the presence of ash significantly affects the specific surface area for carbon in the char sample. To examine the effect of ash in char further, a simulation is carried out considering modified random pore model with pore parameter ψ of 8 and initial surface area S0, 16 m2/cm3. According to Lu and Do [26] the initial total surface area of the char sample remains almost constant irrespective of its ash content. Fig. 14 shows that the higher the ash content, the lower is the specific surface area per unit volume. The predicted initial surface area also behaves in the same
Table 5 Pore surface area of partially burnt char at 750 °C.
Fig. 11. Change in surface area with burn off for devolatilized char during combustion (dp = 6 mm).
Carbon burn off (%)
BET surface area (m2/g of char)
Total surface area (m2/cm3 of char)
Active surface area (m2/cm3 of char)
0.00 0.18 0.49 0.67 0.78 0.92
17.56 22.80 36.12 33.99 31.56 22.90
17.41 19.36 24.08 15.62 11.80 6.02
16.77 18.72 23.45 14.99 11.16 5.38
A.K. Sadhukhan et al. / Fuel Processing Technology 90 (2009) 692–700
Fig. 13. Comparison of experimental surface area with prediction by random pore model (dp = 6 mm).
manner. All the surface area curves increase with increase in carbon burn-off and at a value of 0.35 it reaches a maxima known as characteristic carbon burn-off. Interestingly, though different ash fractions predict different initial surface area, the characteristic carbon burn-off is not affected by the ash content of the char sample. 4.7. Pore size distribution The structural heterogeneity of porous texture of coal char during combustion is generally characterized by its pore size distribution. According to Ismadji and Bhatia [28] pore size distribution assumes that equivalent set of non-overlapping and regular geometric shaped model pores represent the existing void space within the char particle. The kinetics and equilibrium properties of porous adsorbents (like activated carbon prepared from different sources) are highly dependent on its pore size distribution. Recently Sudaryanto et al. [29] discussed the pore size distribution of activated carbon prepared from cassava peel, to characterize its surface area. The pore size distribution of coal char at different carbon burn-off is presented in Figs. 15 and 16. The pore size distribution is determined using DFT software associated with the sorption analyzer. It is clearly observed that there is a significant change in pore sizes during the course of combustion. At the outset large number of micropores and quite a few mesopores and very few macropores are present in the devolatilized char. As carbon burn-off increases the micropores are converted to mesopores, which is evident from the lateral shifting of the distribution curves towards the right (Fig. 15). Beyond the carbon burn-off of 0.67, pores with pore width between 4 and 20 nm contribute to the porosity. The contribution of pores with pore radius 20–60 nm is negligibly small even at higher carbon burn-off (Fig. 16). The mean pore diameter is
Fig. 14. Effect of ash content on active carbon surface area per unit volume of char (prediction by modified random pore model).
699
Fig. 15. Pore size distribution at lower carbon burn off (dp = 6 mm, Tb = 750 °C).
found to be around 12 nm. So it can be concluded that the combustion of coal char in the present study starts with internal combustion on the micropore surface predominantly and on small mesopore surface and negligibly small macropore surface. The pore size affects the mode of diffusion in a porous matrix significantly with Knudsen diffusion dominating at smaller pore size and molecular diffusion dominating at larger pore size. It may be worked out that molecular and Knudsen diffusivities become comparable for a pore size of around 250 nm. In the present investigation the mean pore size is much smaller and hence it may be concluded that Knudsen diffusion predominates. 5. Conclusion In the present study the effect of the evolution of pore structure on the combustion of Indian sub-bituminous coal in fluidized bed has been examined. This will provide vital inputs to develop a comprehensive char combustion model for a single particle in a fluidized bed. Coal devolatilization is studied in fluidized bed as well as in thermogravimetric analyzer. The mechanism for combustion of porous coal char in fluidized bed has been delineated. The specific pore surface area, pore volume distribution, porosity of the char particle at different level of carbon combustion are experimentally determined. The applicability of random pore model is explored to fit the experimental surface area and pore parameter value is estimated. The possibility of influence of high ash in coal char on surface area measurement is corrected against the ash surface area and pore parameter is re-estimated using modified random pore model. At a low bed temperature the combustion proceeds by shrinking reacted core model which is corroborated by SEM images of partially
Fig. 16. Pore size distribution at higher carbon burn off (dp = 6 mm, Tb = 750 °C).
700
A.K. Sadhukhan et al. / Fuel Processing Technology 90 (2009) 692–700
burnt char sample. However, near the end pore-collapsing phenomenon decreases the surface area and the combustion follows shrinking unreacted core model. At a higher bed temperature the combustion reaction proceeds only by shrinking unreacted core model. It is also concluded that the mean pore size being very small Knudsen diffusion predominates. Acknowledgements The authors acknowledge the support extended by Prof. B. Adhikari and Ms Piyali Basak of Materials Science Department, Indian Institute of Technology, Kharagpur during the experiments using Mercury Porosimeter (Pore-Master-GT Model No. PM-33-6). References [1] S. Biswas, N. Choudhury, P. Sarkar, A. Mukherjee, S.G. Sahu, P. Boral, A. Choudhury, Studies on the combustion behaviour of blends of Indian coals by TGA and drop tube furnace, Fuel Processing Technology 87 (2006) 191–199. [2] B. Remiarova, J. Markos, R. Zajdlik, L. Jelemensky, Identification of the mechanism of coal char particle combustion by porous structure characterization, Fuel Processing Technology 85 (2004) 303–321. [3] G.Q. Lu, D.D. Do, Structure changes of coal reject char during pyrolysis at low heating rates, Fuel Processing Technology 28 (3) (1991) 247–258. [4] P. Gupta, A.K. Sadhukhan, R.K. Saha, Analysis of the combustion reaction of carbon and lignite char with ignition and extinction phenomena: shrinking sphere model, International Journal of Chemical Kinetics 39 (6) (2007) 307–319. [5] A.K. Sadhukhan, P. Gupta, R.K. Saha, Analysis of the dynamics of coal char combustion with ignition and extinction phenomena: shrinking core model, International Journal of Chemical Kinetics 40 (2008) 569–582. [6] P. Gupta, Combustion and pyrolysis of coal and biomass, PhD thesis (1999) Indian Institute of Technology, Kharagpur, India. [7] D.B. Anthony, J.B. Howard, Coal devolatilization and hydro-gasification, AIChE Journal 22 (4) (1976) 625–656. [8] G. Borghi, A.F. Sarofim, J.M. Beer, A model of coal devolatilization and combustion in fluidized beds, Combustion and Flame 61 (1985) 1–16. [9] B.C. Young, I.W. Smith, The combustion of Loy Yang brown coal char, Combustion and Flame 76 (1) (1989) 29–35. [10] J. Su, D.D. Perlmutter, Effect of pore structure on char oxidation kinetics, AIChE Journal 31 (6) (1985) 973–981.
[11] M. D'Amore, L. Tognotti, J.P. Longwell, A.F. Sarofim, Oxidation rate of a single char particle in an electrodynamic balance, Combustion an Flame 95 (4) (1993) 374–382. [12] IUPAC Manual of Symbols and Terminology, Appendix 2, 1, Colloid Surface Chem. Pure and Appl. Chem, 31 (1972) 587. [13] R.H. Hurt, D.R. Dudek, J.P. Longwell, A.F. Sarofim, The phenomena of gasification induced carbon densification and its influence on pore structure evolution, Carbon 26 (4) (1988) 433–449. [14] W.M.A.A. Daud, W.S.W. Ali, Comparison on pore development of activated carbon produced from palm shell and coconut shell, Bioresource Technology 93 (2004) 63–69. [15] S. Haldar, R.K. Saha, On structural changes of lignite char during fluidized bed combustion, Canadian Journal of Chemical Engineering 68 (1990) 337–339. [16] J.B. Waters, R.E. Mitchel, R.G. Squires, N.M. Laurendeau, Evidence for formation of CO2 in the vicinity of burning pulverized carbon particles, Combustion and Flame 74 (1988) 91–106. [17] J. Tomeczek, J. Mlionka, The parameters of a random pore network with spherical vesicles for coal structure modelling, Fuel 77 (1998) 1841–1844. [18] S.K. Bhatia, D.D. Perlmutter, A random pore model for fluid solid reactions: I. Kinetic control, AIChE Journal 26 (3) (1980) 379–386. [19] G.R. Gavallas, A random capillary model with application to char gasification at chemically controlled rates, AIChE Journal 26 (4) (1980) 577–585. [20] G.R. Gavallas, K.A. Wilks, Intraparticle mass transfer in coal pyrolysis, AIChE Journal 26 (1980) 201–207. [21] J. Su, D.D. Perlmutter, Evolution of pore volume distribution during gasification, AIChE Journal 30 (6) (1984) 967–973. [22] F.Y. Wang, S.K. Bhatia, A generalized dynamic model for coal char particle gasification with structure evolution and peripheral fragmentation, Chemical Engineering Science 56 (2001) 3683–3697. [23] M.J. Biggs, P.K. Agarwal, The CO/CO2 product ratio for a porous char particle within an incipiently fluidized bed: a numerical study, Chemical Engineering Science 52 (6) (1997) 941–952. [24] S. Kajitani, N. Suzuki, M. Ashizawa, S. Hara, CO2 gasification rate analysis of coal char in entrained flow coal gasifier, Fuel 85 (2006) 163–169. [25] T. Adschiri, T. Furuswa, Relation between CO2-reactivity of coal char and BET surface area, Fuel 65 (1986) 927–931. [26] G.Q. Lu, D.D. Do, A kinetic study of coal reject-derived char activation with CO2, H2O and air, Carbon 30 (1) (1992) 21–29. [27] L.H. Smith, L.D. Smoot, T.H. Fletcher, The Structure and Reaction Process of Coal, Plenum Press, New York, 1994. [28] S. Ismadji, S.K. Bhatia, Characterization of activated carbons using liquid phase adsorption, Carbon 39 (2001) 1237–1250. [29] Y. Sudaryanto, S.B. Hartono, Irawaty, H. Hindarso, S. Ismadji, High surface area activated carbon prepared from cassava peel by chemical activation, Bioresouce Technology 97 (2006) 734–739.
Contents lists available at ScienceDirect
Fuel Processing Technology 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 / f u p r o c
Characterization of porous structure of coal char from a single devolatilized coal particle: Coal combustion in a fluidized bed Anup Kumar Sadhukhan a,⁎, Parthapratim Gupta a, Ranajit Kumar Saha b a b
Chemical Engineering Department, National Institute of Technology, Durgapur - 713209, W.B., India Chemical Engineering Department, Indian Institute of Technology, Kharagpur - 721302, W.B., India
a r t i c l e
i n f o
Article history: Received 24 June 2008 Received in revised form 29 November 2008 Accepted 15 December 2008 Keywords: Coal char Pore structure Surface area Porosity Carbon matrix
a b s t r a c t The combustion characteristics of coal char are highly dependent on initial pore structure of devolatilized char as well as on the structural evolution during the combustion of char. The development of pore structure also throws light on the mechanism of the combustion process. In the present work evolution of pore structure of partially burnt coal char of Indian origin has been investigated experimentally in a batch-fluidized bed and analyzed. The BET surface area, micropore surface area and porosity of char at various levels of carbon burn-off have been determined. Experimental specific surface area has been found to agree well with theoretical prediction using random pore model. Modified random pore model is used to determine the active surface area. Char combustion mechanism based on shrinking unreacted core and shrinking reacted core models are delineated during the course of reaction at various bed temperatures. This is substantiated with the proportional representation of ash and carbon matrix in scanning electron microscope images. It is also concluded that in the present investigation the mean pore size is much smaller and hence the Knudsen diffusion predominates. Analysis based on similar experimental observations and models for pore structure evolution to investigate char combustion reaction regime has not been reported in literature. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Coal utilization processes such as combustion and gasification generally involve several steps: (i) the devolatilization of organic matter leaving behind char; (ii) homogeneous reactions of volatile matters with reactant gases and (iii) heterogeneous reactions of the residual char with reactant gases during which ash is formed. The devolatilization process and subsequent combustion of volatiles have profound influence on the physical structure of char. Besides, the char also undergoes significant changes during combustion. The char structure is highly heterogeneous and usually affects the char combustion reactions and ash formation. These phenomena have attracted widespread research interests over the last few decades. Studies on combustion behavior and the role of coal petrographic analysis on the morphology of char during pyrolysis and combustion have already been reported in literature [1–3]. A single particle analysis provides more detailed insight about the process [4–6] during combustion in fluidized bed. The present article provides a detailed study of the characterization of a typical sub-bituminous coal as it undergoes devolatilization and subsequent burning in a batch fluidized bed combustor. The study is expected to be useful in
⁎ Corresponding author. Tel.: +91 9933989609; fax: +91 343 254 7375. E-mail address: [email protected] (A.K. Sadhukhan). 0378-3820/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.fuproc.2008.12.010
understanding the mechanism of the combustion process and modeling single particle coal char combustion. 1.1. Structure of coal char In a fluidized bed combustor (FBC) a coal particle first undergoes devolatilization. The characteristic time of release of volatiles depends on the volatile content of original coal sample, particle diameter and bulk temperature of fluidized bed. A large number of models are available in literature to describe coal devolatilization kinetics. The model demonstrated by Anthony and Howard [7] has wide range of applicability. Development of comprehensive single particle coal char combustion model requires detailed study of the structural development of porous char particle during pyrolysis and subsequent combustion. Borghi et al. [8] proposed that the evolution of volatiles is often accompanied by their ignition in a laminar yellowish diffusion flame in the vicinity of the particle surface causing further increase in temperature and acceleration in the volatiles evolution process. The coal particle temperature and the oxygen concentration at the particle surface are highly influenced by the homogeneous gas phase combustion of the volatiles. This finally governs the residual char combustion involving an interaction of heterogeneous and homogeneous reactions with transport limitations. Under rapid devolatilization of spherical coal particles, cenospheres are formed leading to
A.K. Sadhukhan et al. / Fuel Processing Technology 90 (2009) 692–700
the generation of highly porous char. The internal surface area depends on the volatile matter content of coal and the nature of pore structure. Under slow devolatilization, less internal pores are evolved due to decreased yield of volatile mater. Young and Smith [9] reported that a typical burnt brown coal particle with volatile mater content of 40%, collected from pulverized fuel fired furnaces, has a porous structure with pore openings 1000 times the size of an oxygen molecule. The mean free path of oxygen molecules is in the order of 50 to 150 nm and its molecular radius is 0.2 nm only. Hence oxygen molecule can easily penetrate into the micropores and react with carbon internally and char combustion occurs through internal combustion. Su and Perlmutter [10] proposed that during coal char combustion the pore radius varies in the range of 2 to 100 nm. D'Amore et al. [11] classified pores of char particles during combustion as micropores, mesopores and macropores and determined the pore size distribution during burn-off of various char particles. IUPAC Manual of Symbols and Technology [12] classifies pores as: (i) micropores: pore width, wp b 2 nm (ii) mesopores: 2 b wp b 50 nm (iii) macropores: wp N 50 nm. Experimental studies by Hurt et al. [13] revealed that specific internal surface area monotonically increases with the carbon burn-off due to opening of more new pores. However the pore-collapsing phenomenon occurs during the later part of char combustion, causing decrease in specific surface area after a critical burn off. Experimental data with coal char by D'Amore et al. [11] showed that the maximum specific surface area occurred at about 35% carbon burn-off. Recently Daud and Ali [14] reported that for activated carbon produced from palm shell and coconut shell, the maximum micropore volume occurred at 38% and 48% burn-off respectively. Micropores typically constitute more than 80% of the internal surface area of char particles and the rest are due to mesopores and macropores. 1.2. Structural model Shrinking sphere model [4], shrinking unreacted core model [5] and volume reaction model [6] are widely used for coal char combustion in fluidized bed. Non-porous coal char with low ash content generally follows the shrinking sphere in fluidized bed due to attrition with bed particles, non-porous char with high ash content conforms to the shrinking unreacted core model and the porous char follows the volume reaction model. In actual practice, the char combustion reactions occur both at the outer surface and inside the pore surface, depending on the porosity of the particle. Hence both particle size and the density may change during the combustion process which has been studied by a number of researchers including Haldar and Saha [15] and Waters et al. [16]. Tomeczek and Mlionka [17] determined the existence of noncylindrical pores in coal char by means of experiments with mercury porosimetry. The most elaborate model on development of pore structure was described by Bhatia and Perlmutter [18] and Gavallas [19] who presented convenient mathematical relationships for pore structure evolution. The work was further extended by Gavallas and Wilks [20] into a form suitable for application to char gasification at chemically controlled rates. Su and Perlmutter [21] also presented a theoretical model to predict the evolution of pore volume distribution during gasification of coal char. The pore structure of coal char during combustion is usually characterized by its pore volume distribution and it can be determined experimentally by sorption analyzer and adsorption isotherms of nitrogen at the temperature of liquid nitrogen. Other integral properties of the pore structure, such as total microporosity, surface area, and total pore volume can also be determined from experimental findings using the standard method. Most of the existing literature on characterization of internal structure of coal is mainly applicable to coals with low ash content, typically of the order of 4–5% by weight. Remiarova et al. [2] studied the internal structure by measuring the internal surface area, pore
693
volume, microporosity and macroporosity of the particle at different carbon burn-off. In contrast, Indian coals have high ash content due to its drift origin, typically 14–30% after treatment in coal washery with the washery rejects containing ash as high as 30–50% by weight. 2. Background information on structural model According to Wang and Bhatia [22] the devolatilized char particle comprises microporous grains surrounded by mesopores and macropores. The heterogeneous gas–solid combustion reaction takes place in micropores, while the macropores serve as channel for transportation of gaseous reactants and products. However, Biggs and Agarwal [23] proposed that it is the accessible porosity of the char particle, which is responsible for internal transportation of reactants and products within the porous texture of the char particle during the course of combustion. As the carbon burn-off increases, both the micro-porosity and the accessible porosity gradually increase causing a decrease in transport resistance. The variation of specific char surface area with carbon burnoff can be measured experimentally. Several investigators including Kajitani et al. [24] and Adschiri and Fursawa [25] experimentally found that the internal surface area gradually increases, attains a maximum and then decreases with carbon burn-off. 2.1. Random pore model Random pore model developed by Bhatia Perlmutter [18] is the most widely accepted structural model to predict the development of pore surface area during the combustion and gasification of coal char. The carbon burn-off in partially burnt char sample is calculated as X=
ðW0 − Wvm − W Þ ðW0 − Wvm − Wa Þ
ð1Þ
where W0, Wvm, W and Wa are initial mass of coal sample, mass of volatiles in sample, mass of partially burnt char and mass of ash in sample respectively. The surface area per unit volume of char S may be estimated from experimentally measured surface area per unit mass, Sm using the following equations Sð X Þ = Sm ð X Þ · ρð X Þ
ð2Þ
ρ = ρ0 ½α a + ð1 − X Þ · ð1 − α a Þ
ð3Þ
where ρ0 is initial density, ρ(X) is density of the char particle at carbon burn off X and αa is the mass fraction of ash in devolatilized char. According to Bhatia and Perlmutter [18] the specific surface area S per unit volume of the char particle can be expressed by the following equation qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi Sð X Þ = S0 · ð1 − X Þ 1 − ψ lnð1 − X Þ
ð4Þ
where S0 is the surface area at zero burn-off and ψ is the dimensionless parameter of pore model indicating the nature of pore structure. The value of ψ may be calculated using pore length L0 and initial accessible porosity ε0 of char particle as follows 2
ψ = 4πL0 ð1 − e0 Þ = S0 :
ð5Þ
The parameter ψ may be treated as fitting parameter and may be obtained with the curve fitting of experimental values of S(X) at different carbon burn-off. Ash particles present in the char remain as inert during char combustion and the surface area provided by ash particles do not contribute for heterogeneous surface reaction. Lu and Do [26] further modified the random pore model in order to eliminate the ash surface area and to find out active surface area where the heterogeneous gas solid reaction takes place.
694
A.K. Sadhukhan et al. / Fuel Processing Technology 90 (2009) 692–700
2.2. The modified random pore model The modified random pore model assumes that the char particle contains separate ash and carbon matrix without overlapping each other. Between the two matrices there is void space contributing for macro porosity of the particle. The model further assumes that the carbon is present in the porous carbon matrix and ash matrix is totally nonporous. The gas solid reaction occurs only on the internal surface of the carbon matrix. The porous carbon matrix has an initial porosity εµ0 called micro-porosity. The experimental measurement of pore surface area with sorption analyzer accounts for both ash surface area and carbon surface area. The modified random pore model provides mathematical equation to estimate the carbon surface area, which is considered as active surface area during the course of combustion of coal char with significant quantity of ash content. According to this model the carbon matrix is considered to be an entity consisting of randomly distributed geometrical pore space with volume Vµi (i = 1, 2, … etc). If all pore voids grows by an infinitesimal amount drp at carbon burn off X in the direction normal to pore surface, the following differential equation can be written for pore surface area: deμ = Sc ð X Þ drp
ð6Þ
where Sc(X) is the carbon surface area per unit volume of carbon matrix. Applying the random pore model by Bhatia and Perlmutter [18] to the carbon matrix in the particle, Eq. (4) may be modified as qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi Sc ð X Þ = ð1 − X Þ 1 − ψμ lnð1 − X Þ: Sc ð0Þ
ð7Þ
ψµ is the structural parameter of pore in carbon matrix and carbon burn-off may be defined in terms of micro porosity of carbon matrix as X=
eμ − eμ0 : 1 − eμ0
ð8Þ
Considering the presence of ash in the particle with volume fraction αva the total surface area of the particle at zero carbon burn-off is: v v S0 = α a · Sa + 1 − α a Sc ð0Þ
ð9Þ
ð10Þ
where Sa is the surface area of ash per unit volume of ash matrix. The volume fraction of ash matrix in the char particle can be related to weight fraction of ash by the following equation: α a · 1 − eμ0 ρc α va = : ð11Þ ð1 − α va Þ ð1 − α a Þ · ρa The value of Sc(0) can be calculated from Eq. (9) by experimentally measuring the total surface area of the particle at zero burn-off and measuring the surface area of ash after complete carbon burn-off in the char. Hence γ, the ratio of ash surface area to initial carbon surface area in carbon matrix, is a measurable quantity and may be defined as γ=
Sa : Sc ð0Þ
ð12Þ
Sc(0) can also be expressed as: Sc ð0Þ =
S0 ½γ · α va + ð1 − α va Þ
Sc ðXÞ =
SðXÞ − α va · Sa : ð1 − α va Þ
ð14Þ
By substituting Eq. (13) in Eq. (7) the following equation may be written as:
Sc ðXÞ = S0 ·
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ð1 − XÞ 1 − ψμ lnð1 − XÞ ½γ · α va + ð1 − α va Þ
:
ð15Þ
Again the fitting parameter ψµ may be obtained by curve fitting of Sc(X) with X. It may be mentioned here that the pore parameter ψ is associated with total surface area including ash where ψµ does not count for ash surface area. The present study reports the development of porous structure of a typical sub-bituminous coal (ash 14.9% d.b.) of Indian origin in a fluidized bed combustor. Partially burnt char samples have been investigated for specific surface area, micropore volume, pore size distribution and macro porosity. Random pore model is applied to predict the experimentally obtained surface area of the partially burnt char samples. Further, the modified random pore model is used to estimate the active surface area per unit volume of carbon matrix. The effect of ash content on active carbon surface area is evaluated. Finally, scanning electron microscope (SEM) images of coal sample, devolatilized char and a few partially burnt char samples are presented to show the evidence and development of porous texture at different burn-off. 3. Experimental 3.1. Materials The sub-bituminous coal of Indian origin was used for experimentation in fluidized bed combustion. Spherical coal particles were prepared by rubbing and samples were selected with sphericity of 0.81 to 0.85 and with an average particle diameter 6.0 mm. Table 1 shows the proximate and ultimate analysis of the coal used for experimentation. 3.2. Experimental set up
the total surface area of the particle at any carbon burn-off is: v v Sð X Þ = α a · Sa + 1 − α a Sc ð X Þ
and active carbon surface area Sc(X) at any carbon burn-off may be estimated from experimentally measured values of S(X) by using the following expression.
ð13Þ
The fluidized bed combustor consists of a stainless steel column, 82 mm in diameter; fitted with a S.S. distributor, having 1.6 mm diameter holes, and 1.2% open area (Fig. 1). It is enclosed in an insulated refractory chamber. The annular region between the stainless steel column and the brickwork forms the heating chamber, provided with a ring-burner to burn LPG and air was blown in by an air-blower into the fluidized bed consisting of crushed refractories. A sufficiently long
Table 1 Proximate and ultimate analysis of coal sample. Analysis Proximate analysis (wt.%) Moisture Volatile (d.b.) Fixed carbon (d.b.) Ash (d.b.) Ultimate analysis (wt.%) Carbon Nitrogen Hydrogen Sulphur Oxygen (by diff.)
Coal
Char
5.40 27.0 58.1 14.9
0 2.56 76.44 21.00
86.46 1.82 4.51 0.15 7.06
89.24 1.42 0.51 0.03 8.80
A.K. Sadhukhan et al. / Fuel Processing Technology 90 (2009) 692–700
695
Fig. 2. TG and DTG profile from thermogravimetric analysis of coal.
used in each run which ensures that the oxygen concentration in the bulk gas around the particle remained almost constant at 21% during the entire period of experimentation which was corroborated by sample gas analysis inside the bed. Biggs and Agarwal [23] used similar operating conditions to study the burning characteristics of porous coal char in a fluidized bed. Fig. 1. Fluidized bed combustor: 1. Refractory brick wall; 2. Feeding pulley; 3. Mirror arrangement; 4. Thermo couple probe; 5. Distributor; 6 LPG entry; 7. Orifice meter; 8. Air control valve; 9. Screen supporting; Glass beds; 10. Blow air; 11. Digital multimeter; 12. Strip-chart recorder; 13. Bed material 14. Basket; 15. Char sample inside basket.
(around 200 mm) isothermal section was ensured, where the temperature could be maintained constant with a variation within ±5 °C. The bed temperature was measured by a chromel-alumel thermocouple and recorded continuously by strip-chart recorder. The combustor was heated to the desired temperature, stabilized for 10 min and was then used for experimentation. Table 2 presents the operating variables and different parameters for experimentation. Considering heterogeneous nature and complex structure of coal and coal char the experiments were repeated several times for each sample and the reproducibility is found to be within ±3%. The different experimental conditions are detailed in following section. 3.3. Combustion of coal samples in fluidized bed A single coal particle was weighed, put in a small basket made of S.S. wire mesh (200 mesh) and introduced inside the combustor. The particle was heated up, devolatilization started and a bright yellowish volatile flame was produced visible through a mirror arrangement. Immediately after the volatile flame disappeared, the partially burnt char sample was taken out after a stipulated time, quenched in liquid nitrogen immediately and weighed again. Only a single coal particle was
3.4. Experimentation with partially burnt char 3.4.1. Pore structure characterization A large number of partially burnt char samples under different operating bed temperatures were analyzed for pore structure characterization by measuring specific surface area, pore volume, pore size distribution and macro porosity. Nitrogen adsorption technique at −196 °C with automatic sorption analyzer (Autosorb-1, Model No. ASIC-9 by Quantachrome instruments) was used for this purpose. Mercury porosimeter (Pore-Master-GT Model No. PM-33-6) was used for determination of accessible porosity of partially burnt char at various carbon burn-off. Prior to the gas adsorption measurements char samples were degassed at 200 °C for a period of 2 h. Nitrogen adsorption isotherms were measured over a relative pressure (P/P0) range of 1.67× 10− 6 to 0.995. The specific surface area was determined by application of Brunauer–Eemmett–Teller (BET) analysis software available with the instrument. The pore size distribution of partially burnt char samples were estimated by Density Functional Theory (DFT) software available with the sorption analyzer. 3.4.2. Scanning electron microscope (SEM) images SEM images for original coal and several partially burnt char samples were obtained using a scanning electron microscope JEOL-
Table 2 Fluidized bed combustor. Material of construction
Stainless steel
Inside diameter (mm) Bed inert
82 Crushed refractory with a mean diameter of 0.43 mm. Density — 2100 kg/m3 Stainless steel plate, hole diameter — 1.6 mm, pen area — 1.12% Air 650, 750 and 900 °C 700 100 200 0.30
Distributor Fluidizing medium Operating bed temperature, Tb Total column height (mm) Static bed height (mm) Expanded bed height (mm) Superficial gas velocity (ms− 1)
Fig. 3. Internal structure of original coal particle [dp = 6 mm, Tb = 750 °C, ash (white), carbon matrix (grey)].
696
A.K. Sadhukhan et al. / Fuel Processing Technology 90 (2009) 692–700
Fig. 4. Internal structure of just devolatilized coal particle [dp = 6 mm, Tb = 750 °C, ash (white), carbon matrix (grey)].
Fig. 6. Internal structure of coal char particle at carbon burn off 0.67 [dp = 6 mm, Tb = 750 °C, ash (white), carbon matrix (grey)].
Japan Model No. JSM-5800 apparatus equipped with a TESCAN digital unit.
release about 3% volatiles, which might be due to large particle size or very small devolatilization time. Borghi et al. [8] found experimentally that the volatile release of millimeter sized coal particle during combustion in fluidized bed depends on the size of the particle and temperature of the bed. The residual volatiles in the char burns during the char combustion period and this small amount has negligible influence on solid char combustion process.
4. Results and discussion 4.1. Devolatilization of coal particle The observed devolatilization time for 6 mm coal particle at 650 °C was around 30 s and at 900 °C it was around 20 s including the heatup time (around 5 s) for the particle in the fluidized bed. The thermogravimetric analysis of coal sample in nitrogen atmosphere is presented in Fig. 2, which shows that the residual mass fraction of coal particle decreases with increasing temperature due to the release of volatiles. It is evident that maximum mass loss occurs within the temperature range of 400–700 °C. It stops at around 850 °C with a final residue of 0.68, indicating volatile release of 32.0%, which is more than the proximate volatiles (27%). An increase in temperature above 850 °C does not have a significant influence on particle mass loss. This observation suggests that it is required to heat up the coal particle above 850 °C in order to remove the volatiles completely. However in fluidized bed combustion of coal particles the released volatiles burn in gas phase near the particle surface and due to heat transfer from the flame to the particle surface, the particle temperature rises several hundred degrees above the bed temperature. Hence, the selection of our experimental temperatures (650, 750 and 900 °C) was appropriate. For 6 mm coal particles the estimated mass loss due to devolatilazation was 29.0%, indicating that the char sample was yet to
The scanning electron microscope (SEM) images of the coal particle used for our experiment are presented in Fig. 3. It shows the presence of plant vessels and tissues, which form the main skeleton of the coal particle. The visible grey portion rich in carbon, called the carbon matrix, contains the micropores and the ash is distributed randomly in the carbon matrix. Fig. 4 is the image of devolatilized char, where the contrast of grey scale has changed slightly indicating total devolatilization. There are a few small yet-to-be devolatilized islands in carbon matrix. According to Smith et al. [27] the backbone of coal structure is made of dense polycyclic aromatic hydrocarbons and linked by alternative single and double bonds and has extra resonance stability which is more resistant to thermal decomposition with higher bond energy of 1000 kJ/mol. The polycyclic aromatic hydrocarbons are transformed from the original plant tissues during prolonged decomposition under heat and bacteria. This may lead to highly heterogeneous structure of coal. The micro-islands in the carbon matrix are made of different group polycyclic aromatic
Fig. 5. Internal structure of coal char particle at carbon burn off 0.18 [dp = 6 mm, Tb = 750 °C, ash (white), carbon matrix (grey)].
Fig. 7. Internal structure of coal char particle at carbon burn off 0.92 [dp = 6 mm, Tb = 750 °C, ash (white), carbon matrix (grey)].
4.2. Study of internal structure by SEM images
A.K. Sadhukhan et al. / Fuel Processing Technology 90 (2009) 692–700
697
Table 3 Pore characteristics of partially burnt char at 750 °C.
Fig. 8. Carbon burn off for devolatilized char at different time (dp = 6 mm).
hydrocarbons, which decompose at higher temperature to release its volatile contents. This causes very slow mass loss which is observed from the thermogravimetric curve (Fig. 2), where the fractional residue decreases very slowly after 850 °C. The microstructures of the partially burnt char particle at different carbon burn-off are presented in Figs. 5–7. The structure of the ash that covers the carbon matrix gradually with increase in carbon burn-off is visible from these figures. The figures also show that the accessible porosity of the particle increases with carbon burn-off. The SEM images do not show appreciable effect of bed temperature for the same degree of carbon burn off. 4.3. Effect of carbon burn off on pore structure The sigmoidal temporal burn-off profile of devolatilized char (Fig. 8) is considerably influenced by the bed temperature (Fig. 8). The adsorption isotherms (Fig. 9) of partially burnt devolatilized char at 750 °C belong to a mixture of type-I and type-IV isotherms according to IUPAC classification. Type-I isotherm is associated with microporous structure while type-IV isotherm indicates a mixture of microporous and mesoporous char. Similar types of isotherms were obtained for other bed temperatures as well. Nitrogen adsorption at low relative pressure indicates filling of the micropores (wp b 2 nm) in the char samples. The linear portion of the curve represents multilayer adsorption of nitrogen on the internal surface of the char sample, and the concave upward portion of the curve represents filling up of mesopores (2 nmb wp b 50 nm) and macropores (wp N 50 nm). An entire isotherm is needed to calculate the pore size distribution of the char. However, for BET surface area evolution, data in the linear portion of the
Carbon burn off in char (%)
Total pore volume (m3/g of char)
BET surface area (m2/g of char)
Micropore surface area (m2/g of char)
0.00 0.18 0.49 0.67 0.78 0.92
0.0176 0.0382 0.0551 0.0692 0.0731 0.0883
17.56 22.80 36.12 33.99 31.56 22.90
16.80 19.10 24.80 18.70 12.00 8.32
curve has been used. The micropore surface area of partially burnt char sample is calculated using the Dubinin–Asthakov (DA) analysis technique. The BET surface area and the microporous surface area at different burn-off are presented in Table 3. The accessible porosity of original coal sample, devolatilized char and fully converted ash were measured with mercury porosimeter and is presented in Table 4. It has been observed that though the original coal sample contains little porosity, it increases to 0.22 immediately after devolatilization, almost independent of the bed temperature. The fully converted ash has the accessible porosity of as high as 0.53. The data on accessible porosity at different stages suggests a continuous change in pore structure during the course of the combustion process. Fig. 10 shows the development of pore structure at various carbon burn-off in char. As the char structure consists mostly of micropores at low carbon burn-off, reactions occur at the pore surface and are controlled by diffusion transport of oxygen due to availability of low mesopore and macropore surface area (Fig. 10). However, after a carbon burn-off of 0.40 the accessible porosity starts to increase. At a burn-off of 0.67 active carbon content of the particle drops and oxygen has to penetrate deeper into the particle for the reaction to occur according to shrinking reacted core model. The unburnt carbon is surrounded by a shell of ash with pore size ranging from 40 to 70 nm. 4.4. Effect of temperature on specific surface area Both the specific surface area and pore structure of char, that affect the transport of gaseous reactants and products and consequently the combustion reactions, vary during the course of combustion and hence need to be modeled accurately. Fig. 11 shows the variation of specific surface area Sm (per unit mass) with carbon burn-off at different fluidized bed temperatures. At low bed temperature the char combustion is controlled kinetically and oxygen has enough time to diffuse in creating more micropores which results in higher specific surface area. At higher bed temperatures the pore surface area developed inside the char particle is found to be lower and the char combustion proceeds through diffusion controlled regime. Oxygen gets consumed by the reaction before it can diffuse into the inner micropores and reaction occurs at fewer reaction sites giving rise to macropores which leads to smaller surface area. At all bed temperature the micropores get transformed to mesopores gradually and then to macropores towards the end of combustion due to pore-collapsing phenomenon leading to a decreases in total surface area. The contribution of micro and macropores to the surface area at carbon burn-offs 0.50 and 0.75 for different bed temperatures are presented in Fig. 12. At low conversion the contribution of
Table 4 Accessible porosity of coal, coal char and ash.
Fig. 9. Adsorption isotherm of for devolatilized char at different carbon burn off (dp = 6 mm, Tb = 750 °C).
Sample
Accessible porosity
Coal Devolatilized char Ash
0.08 0.22 0.53
698
A.K. Sadhukhan et al. / Fuel Processing Technology 90 (2009) 692–700
Fig. 10. Development of porous structure of char at different carbon burn off (dp = 6 mm, Tb = 750 °C).
micropore surface area is larger than that of meso and macropores, but at a burn-off of 0.75 the contribution of macropore surface area increases causing less total surface area. A similar trend is observed at all bed temperatures with varying relative contributions. So it may be concluded that at low bed temperature due to high pore surface area and lower reaction rate, the combustion proceeds by shrinking reacted core model. After the reaction starts throughout the volume of the core, carbon at the surface gets consumed completely, forming an ash layer around the char core. The SEM images of partially burnt char sample shows the evidence of reaction throughout the volume of particle by presence of randomly distributed ash inside the carbon matrix (Figs. 5–7). Gradually the volume of the char core diminishes with conversion. However, eventually as the pore-collapsing phenomenon takes place at the end, the surface area decreases and the combustion of unburnt carbon between the macropores follows shrinking unreacted core model. At higher bed temperature with lower pore surface area and higher reaction rate the combustion reaction proceeds by shrinking unreacted core model right from the beginning. 4.5. Application of the random pore model Fig. 11 shows that the internal surface area gradually increases with conversion, attains a peak, and then decreases as the combustion proceeds, which is corroborated by experimental observations of Kajitani et al. [24] and Adschiri and Fursawa [25]. The experimentally obtained specific surface area data per unit mass (Sm) of char is converted to surface area per unit volume (S) of char using Eqs. (2) and
Fig. 12. Effect of temperature on development of surface area at different carbon burn off for (dp = 6 mm).
(3) and presented in Table 5. The parameter ψ in Eq. (4) was obtained by fitting the experimental data of S and X. The estimated values of ψ for the char sample were 8, 6 and 4 at 650, 750 and 900 °C respectively. The greater the value of pore parameter, the greater is the porosity of the char. Larger pore surface area is obtained during char combustion at lower temperature giving rise to higher ψ value. An excellent agreement between the surface area predicted from random pore model with the estimated values from experiment is observed at different carbon burn-off for different bulk temperatures (Fig. 13). 4.6. Application of the modified random pore model The values of active carbon surface area Sc(X) estimated from the total surface area S using Eq. (15) are presented in Table 5. The active specific surface area of char is slightly smaller than the corresponding total specific surface area. Lu and Do [26] derived the reaction rate expression for char gasification by using the active carbon surface area based on carbon matrix, while Adschiri and Furusawa [25] argued for active char surface area based on char volume. The fitting parameter ψµ was re-estimated for active char surface area with the curve fitting of estimated values of Sc(X) and X. The new values are 6, 5 and 3.2 at 650, 750 and 900 °C respectively. Hence the modified random pore model predicts slightly lower char porosity. 4.6.1. Effect of ash in modified random pore model It may be concluded from the above discussion that the presence of ash significantly affects the specific surface area for carbon in the char sample. To examine the effect of ash in char further, a simulation is carried out considering modified random pore model with pore parameter ψ of 8 and initial surface area S0, 16 m2/cm3. According to Lu and Do [26] the initial total surface area of the char sample remains almost constant irrespective of its ash content. Fig. 14 shows that the higher the ash content, the lower is the specific surface area per unit volume. The predicted initial surface area also behaves in the same
Table 5 Pore surface area of partially burnt char at 750 °C.
Fig. 11. Change in surface area with burn off for devolatilized char during combustion (dp = 6 mm).
Carbon burn off (%)
BET surface area (m2/g of char)
Total surface area (m2/cm3 of char)
Active surface area (m2/cm3 of char)
0.00 0.18 0.49 0.67 0.78 0.92
17.56 22.80 36.12 33.99 31.56 22.90
17.41 19.36 24.08 15.62 11.80 6.02
16.77 18.72 23.45 14.99 11.16 5.38
A.K. Sadhukhan et al. / Fuel Processing Technology 90 (2009) 692–700
Fig. 13. Comparison of experimental surface area with prediction by random pore model (dp = 6 mm).
manner. All the surface area curves increase with increase in carbon burn-off and at a value of 0.35 it reaches a maxima known as characteristic carbon burn-off. Interestingly, though different ash fractions predict different initial surface area, the characteristic carbon burn-off is not affected by the ash content of the char sample. 4.7. Pore size distribution The structural heterogeneity of porous texture of coal char during combustion is generally characterized by its pore size distribution. According to Ismadji and Bhatia [28] pore size distribution assumes that equivalent set of non-overlapping and regular geometric shaped model pores represent the existing void space within the char particle. The kinetics and equilibrium properties of porous adsorbents (like activated carbon prepared from different sources) are highly dependent on its pore size distribution. Recently Sudaryanto et al. [29] discussed the pore size distribution of activated carbon prepared from cassava peel, to characterize its surface area. The pore size distribution of coal char at different carbon burn-off is presented in Figs. 15 and 16. The pore size distribution is determined using DFT software associated with the sorption analyzer. It is clearly observed that there is a significant change in pore sizes during the course of combustion. At the outset large number of micropores and quite a few mesopores and very few macropores are present in the devolatilized char. As carbon burn-off increases the micropores are converted to mesopores, which is evident from the lateral shifting of the distribution curves towards the right (Fig. 15). Beyond the carbon burn-off of 0.67, pores with pore width between 4 and 20 nm contribute to the porosity. The contribution of pores with pore radius 20–60 nm is negligibly small even at higher carbon burn-off (Fig. 16). The mean pore diameter is
Fig. 14. Effect of ash content on active carbon surface area per unit volume of char (prediction by modified random pore model).
699
Fig. 15. Pore size distribution at lower carbon burn off (dp = 6 mm, Tb = 750 °C).
found to be around 12 nm. So it can be concluded that the combustion of coal char in the present study starts with internal combustion on the micropore surface predominantly and on small mesopore surface and negligibly small macropore surface. The pore size affects the mode of diffusion in a porous matrix significantly with Knudsen diffusion dominating at smaller pore size and molecular diffusion dominating at larger pore size. It may be worked out that molecular and Knudsen diffusivities become comparable for a pore size of around 250 nm. In the present investigation the mean pore size is much smaller and hence it may be concluded that Knudsen diffusion predominates. 5. Conclusion In the present study the effect of the evolution of pore structure on the combustion of Indian sub-bituminous coal in fluidized bed has been examined. This will provide vital inputs to develop a comprehensive char combustion model for a single particle in a fluidized bed. Coal devolatilization is studied in fluidized bed as well as in thermogravimetric analyzer. The mechanism for combustion of porous coal char in fluidized bed has been delineated. The specific pore surface area, pore volume distribution, porosity of the char particle at different level of carbon combustion are experimentally determined. The applicability of random pore model is explored to fit the experimental surface area and pore parameter value is estimated. The possibility of influence of high ash in coal char on surface area measurement is corrected against the ash surface area and pore parameter is re-estimated using modified random pore model. At a low bed temperature the combustion proceeds by shrinking reacted core model which is corroborated by SEM images of partially
Fig. 16. Pore size distribution at higher carbon burn off (dp = 6 mm, Tb = 750 °C).
700
A.K. Sadhukhan et al. / Fuel Processing Technology 90 (2009) 692–700
burnt char sample. However, near the end pore-collapsing phenomenon decreases the surface area and the combustion follows shrinking unreacted core model. At a higher bed temperature the combustion reaction proceeds only by shrinking unreacted core model. It is also concluded that the mean pore size being very small Knudsen diffusion predominates. Acknowledgements The authors acknowledge the support extended by Prof. B. Adhikari and Ms Piyali Basak of Materials Science Department, Indian Institute of Technology, Kharagpur during the experiments using Mercury Porosimeter (Pore-Master-GT Model No. PM-33-6). References [1] S. Biswas, N. Choudhury, P. Sarkar, A. Mukherjee, S.G. Sahu, P. Boral, A. Choudhury, Studies on the combustion behaviour of blends of Indian coals by TGA and drop tube furnace, Fuel Processing Technology 87 (2006) 191–199. [2] B. Remiarova, J. Markos, R. Zajdlik, L. Jelemensky, Identification of the mechanism of coal char particle combustion by porous structure characterization, Fuel Processing Technology 85 (2004) 303–321. [3] G.Q. Lu, D.D. Do, Structure changes of coal reject char during pyrolysis at low heating rates, Fuel Processing Technology 28 (3) (1991) 247–258. [4] P. Gupta, A.K. Sadhukhan, R.K. Saha, Analysis of the combustion reaction of carbon and lignite char with ignition and extinction phenomena: shrinking sphere model, International Journal of Chemical Kinetics 39 (6) (2007) 307–319. [5] A.K. Sadhukhan, P. Gupta, R.K. Saha, Analysis of the dynamics of coal char combustion with ignition and extinction phenomena: shrinking core model, International Journal of Chemical Kinetics 40 (2008) 569–582. [6] P. Gupta, Combustion and pyrolysis of coal and biomass, PhD thesis (1999) Indian Institute of Technology, Kharagpur, India. [7] D.B. Anthony, J.B. Howard, Coal devolatilization and hydro-gasification, AIChE Journal 22 (4) (1976) 625–656. [8] G. Borghi, A.F. Sarofim, J.M. Beer, A model of coal devolatilization and combustion in fluidized beds, Combustion and Flame 61 (1985) 1–16. [9] B.C. Young, I.W. Smith, The combustion of Loy Yang brown coal char, Combustion and Flame 76 (1) (1989) 29–35. [10] J. Su, D.D. Perlmutter, Effect of pore structure on char oxidation kinetics, AIChE Journal 31 (6) (1985) 973–981.
[11] M. D'Amore, L. Tognotti, J.P. Longwell, A.F. Sarofim, Oxidation rate of a single char particle in an electrodynamic balance, Combustion an Flame 95 (4) (1993) 374–382. [12] IUPAC Manual of Symbols and Terminology, Appendix 2, 1, Colloid Surface Chem. Pure and Appl. Chem, 31 (1972) 587. [13] R.H. Hurt, D.R. Dudek, J.P. Longwell, A.F. Sarofim, The phenomena of gasification induced carbon densification and its influence on pore structure evolution, Carbon 26 (4) (1988) 433–449. [14] W.M.A.A. Daud, W.S.W. Ali, Comparison on pore development of activated carbon produced from palm shell and coconut shell, Bioresource Technology 93 (2004) 63–69. [15] S. Haldar, R.K. Saha, On structural changes of lignite char during fluidized bed combustion, Canadian Journal of Chemical Engineering 68 (1990) 337–339. [16] J.B. Waters, R.E. Mitchel, R.G. Squires, N.M. Laurendeau, Evidence for formation of CO2 in the vicinity of burning pulverized carbon particles, Combustion and Flame 74 (1988) 91–106. [17] J. Tomeczek, J. Mlionka, The parameters of a random pore network with spherical vesicles for coal structure modelling, Fuel 77 (1998) 1841–1844. [18] S.K. Bhatia, D.D. Perlmutter, A random pore model for fluid solid reactions: I. Kinetic control, AIChE Journal 26 (3) (1980) 379–386. [19] G.R. Gavallas, A random capillary model with application to char gasification at chemically controlled rates, AIChE Journal 26 (4) (1980) 577–585. [20] G.R. Gavallas, K.A. Wilks, Intraparticle mass transfer in coal pyrolysis, AIChE Journal 26 (1980) 201–207. [21] J. Su, D.D. Perlmutter, Evolution of pore volume distribution during gasification, AIChE Journal 30 (6) (1984) 967–973. [22] F.Y. Wang, S.K. Bhatia, A generalized dynamic model for coal char particle gasification with structure evolution and peripheral fragmentation, Chemical Engineering Science 56 (2001) 3683–3697. [23] M.J. Biggs, P.K. Agarwal, The CO/CO2 product ratio for a porous char particle within an incipiently fluidized bed: a numerical study, Chemical Engineering Science 52 (6) (1997) 941–952. [24] S. Kajitani, N. Suzuki, M. Ashizawa, S. Hara, CO2 gasification rate analysis of coal char in entrained flow coal gasifier, Fuel 85 (2006) 163–169. [25] T. Adschiri, T. Furuswa, Relation between CO2-reactivity of coal char and BET surface area, Fuel 65 (1986) 927–931. [26] G.Q. Lu, D.D. Do, A kinetic study of coal reject-derived char activation with CO2, H2O and air, Carbon 30 (1) (1992) 21–29. [27] L.H. Smith, L.D. Smoot, T.H. Fletcher, The Structure and Reaction Process of Coal, Plenum Press, New York, 1994. [28] S. Ismadji, S.K. Bhatia, Characterization of activated carbons using liquid phase adsorption, Carbon 39 (2001) 1237–1250. [29] Y. Sudaryanto, S.B. Hartono, Irawaty, H. Hindarso, S. Ismadji, High surface area activated carbon prepared from cassava peel by chemical activation, Bioresouce Technology 97 (2006) 734–739.