Modeling and Experimental Investigations on the Pyrolysis of Large

ARTICLE pubs.acs.org/EF

Modeling and Experimental Investigations on the Pyrolysis of Large Coal Particles Anup Kumar Sadhukhan,† Parthapratim Gupta,*,† and Ranajit Kumar Saha‡ † ‡

Department of Chemical Engineering, National Institute of Technology, Durgapur 713209, West Bengal, India Department of Chemical Engineering, Indian Institute of Technology, Kharagpur 721302, West Bengal, India ABSTRACT: A fully transient and coupled kinetic, heat-transfer model is proposed to predict the pyrolysis behavior of a large coal particle. The model incorporates kinetics, internal convection because of volatile flow, conduction, external convection and radiation, variation in porosity and thermophysical properties, and changing particle size. The implicit Euler method is used to solve the kinetic model, while an implicit finite volume method (FVM) with a tridiagonal matrix algorithm (TDMA) is employed to solve the heat-transfer model equation. A general-purpose Fortran program is developed to solve the model equations. Experimental studies on mass loss and temperature profiles during pyrolysis are carried out for large coal particles in an isothermal mass-loss apparatus in the presence of nitrogen. Scanning electron microscope (SEM) images are used to explore the evolution of the structure of the coal/char. The swelling history is also investigated. The model predictions for coal temperature and fractional volatile loss are found in very fair agreement with the experimental results of the present authors, Fu et al. (Fu, W.; Zhang, Y.; Han, H.; Duan, Y. A study on devolatilization of large coal particles. Combust. Flame 1987, 70, 253266), and Adesanys and Pham [Adesanya, B. A.; Pham, H. N. Mathematical modelling of devolatilization of a large coal particle in a convective environment. Fuel 1995, 74 (6), 896902]. Finally, the effects of the temperature, particle swelling and shrinkage on the pyrolysis time, and loss of volatiles are analyzed through model simulation.

1. INTRODUCTION The energy demand of the modern age is still mostly met by fossil fuels. With quicker depletion of resources, such as oil and natural gases, coal is expected to play a dominant role for energy supply in the foreseeable future. Hence, combustion of coal still attracts a considerable amount of theoretical and experimental investigations. Most of the existing coal-fired power generation systems all over the world are based on the combustion of pulverized fuel. The process suffers from the disadvantages of coal preparation and handling of coal fines. Hence, use of medium-sized (35 mm) coal particles in combustion and gasification is gaining renewed interest. Coal, when put inside a furnace at a temperature in excess of about 350 °C undergoes a rapid thermal decomposition called pyrolysis, during which a fraction of the coal, about 3050% by weight, is released in the form of volatiles depending upon the volatile content of the original coal sample. Pyrolysis always precedes combustion and gasification. The composition of these volatiles varies with the type and origin of coal. Typically, the major constituents are methane, carbon monoxide, carbon dioxide, chemically bound water, hydrogen, ethane, and traces of other higher hydrocarbons. Howard1 proposed that chemically bound water and oxides of carbon are released at early stages of pyrolysis, while hydrogen and hydrocarbons are released for relatively longer periods. The characteristic time for volatile release from a coal particle depends upon the volatile content of the original coal sample, the particle diameter, and the operating temperature. Pilot-plant studies in fluidized-bed combustors by Gibbs and Beer2 with different types and sizes of coal particles revealed that the major source for CO emission during combustion is contributed by the volatiles produced during the pyrolysis stage. r 2011 American Chemical Society

Zhu et al.3 investigated the effect of the particle size on the yield of residual char during coal pyrolysis and proposed a secondary reaction of volatiles to take place inside the hot char for pyrolysis of large coal particles. Andrei et al.4 postulated that the evolution of volatiles is of prime consideration for designing an efficient coal injection system in a fluidized-bed combustor. Successful furnace simulation requires reliable fuel transformation submodels, including devolatilization/pyrolysis behavior. Singleparticle analysis plays the most significant role, incorporating detailed chemical kinetics, intra- and extra-particle mass, and heat transfer. During pyrolysis, the coal particles undergo structural change, which may influence the char combustion process greatly. Pallares et al.5 modeled the pyrolysis of three different coal types using the network pyrolysis model consisting of functional-group depolymerization, vaporization, and cross-linking (FG-DVC) and solved the model equations by commercial computational fluid dynamics (CFD) package Fluent. Knowledge of coal pyrolysis is very important for the proper design and efficient operation of the coal combustor and gasifier system. Most of the published work on coal pyrolysis dealt with different kinetic expressions and the reaction mechanism in the presence of nitrogen as the inert medium.69 An investigation by Duan et al.10 on coal pyrolysis in the presence of CO2 revealed that replacing N2 with CO2 does not influence the starting temperature of volatile release but seems to enhance the volatile releasing rate. Received: August 1, 2011 Revised: October 22, 2011 Published: October 27, 2011 5573

dx.doi.org/10.1021/ef201162c | Energy Fuels 2011, 25, 5573–5583

Energy & Fuels Badzioch and Hawksley6 presented a very simple kinetic model for coal pyrolysis, where the kinetic parameters E and k0 depend upon the type of coal. The model was not suitable for the non-isothermal process, thus limiting its applicability drastically. Kobayashi et al.7 proposed an improved model consisting of two competing reactions. This model was applicable for the nonisothermal case. Solomon et al.8 suggested that, during pyrolysis, different functional groups are released at different temperature ranges involving separate kinetic parameters in each range with an Arrhenius type of temperature dependency. Although the kinetic parameters were independent of the type of coal, the tar yield parameter was still determined experimentally for each coal type. The model proposed by Anthony and Howard9 has a wider range of applicability. The model assumed that pyrolysis of coal occurs as a result of a large number of simultaneous, independent, and irreversible first-order reactions, by which various organic species present in the coal are converted to volatiles. The activation energy was assumed to be a Gaussian distribution function. Sadhukhan et al.11 explored the applicability of this model to predict the experimental mass-loss profiles obtained from thermogravimetrydifferential thermal analysis (TGDTA) for lignite coal fines of Indian origin. However, only the kinetic model may not be adequate to predict the pyrolysis behavior of large coal particles because of the additional heat- and mass-transfer resistance to transportation of volatiles through the reacting coal particle. Fu et al.12 presented a different kinetic model coupled with heat-transfer resistance for large coal particles, considering external convective and radiative heat transfer for five different coal samples. They determined the kinetic parameters from the experimental mass loss profile obtained from argon plasma heating of coal particles in the temperature range of 7001727 °C for particle diameters ranging between 3 and 9 mm. However, the temperature history of the particle was not tested with experimental investigations. Adesanya and Pham13 further presented a mathematical model for the pyrolysis of a single large coal particle in a moving-bed gasifier in the temperature range of 350600 °C. They did not consider the radiative heat transfer between the hot particle surface and the surrounding medium. To eliminate the effect of radiative heat transfer during the experiments, the coal sample was enclosed in an insulating tube. Internal convection through the pores associated with pyrolysis was also not considered by Fu et al.12 and Adesanya and Pham.13 Moreover, the swelling and shrinkage of the coal particle during the course of pyrolysis was not included in the model. Strezov et al.14 measured the swelling pressure developed during slow pyrolysis because of volume expansion and detected important parameters, such as heating rate, final temperature, volatile composition, and volatile content of the original coal. Fu et al.15 found experimentally that, in the temperature range of 200450 °C, the coal undergoes swelling by 2045%. However, further heating of coal in the temperature range of 4501000 °C causes shrinkage by 3238%. According to Yu et al.,16 during the heating of the coal particle in the temperature range of 200450 °C, tiny bubbles are formed first and the neighboring bubbles coalesce to form bigger bubbles and an internal pressure is developed, causing the particle to swell. However, above 450 °C, the bubble pressure increases so much that the bubbles become ruptured. The volatiles are transported through the porous matrix of coal and released from the outer surface of the particle, causing particle shrinkage. The thermoplastic properties, such as viscosity and surface tension, play a key role in the coal particle swelling

ARTICLE

behavior. Low viscosity and low surface tension will result in more frequent bubble ruptures, because volatile gases are more difficult to be trapped inside the liquid particles. On the other hand, high surface tension and high viscosity will lead to a high swelling and less frequent bubble rupture. The overall change in the particle size depends upon the volatile contents, heating rate, and final temperature. Adesanya and Pham13 observed experimentally that the particle volume before and after the pyrolysis remained nearly the same. In the present study, a comprehensive pyrolysis model for a large coal particle is presented considering the following submodels: (i) kinetic model for pyrolysis, (ii) heat-transfer model consisting of convective, conductive, and radiative modes of heat transfer to and from the reacting coal particle, (iii) particle swelling and shrinkage with pyrolysis, (iv) convective transport of volatiles through the porous matrix of the coal particle in the radial direction, and (v) estimation of physical properties as a function of the temperature. This is one of the most comprehensive models on coal pyrolysis proposed thus far. The structural changes are experimentally observed using scanning electron microscope (SEM) images, and porosity and swelling ratio are experimentally correlated with fractional volatile loss and pyrolysis temperature, respectively. The profiles for volatile evolution, particle surface, and center temperatures predicted by the model are tested with the experimental findings of the present authors and that published in the literature. Finally, the effects of swelling/shrinkage and pyrolyzing temperature on the pyrolysis behavior of coal are investigated through simulation.

2. MATHEMATICAL MODEL Coal is extremely complex in nature, and the products of pyrolysis are even more complex. Hence, modeling of the process is always associated with a large degree of approximation and lumping of the components to ensure manageability. In this section, the submodels for pyrolysis kinetics and energy balance including internal convective flow of volatiles are described for a spherical coal particle. 2.1. Kinetic Model for Pyrolysis. The distributed activation energy model by Anthony and Howard9 is chosen for pyrolysis of coal for its wide applicability and compactness. The activation energy E is assumed to be a Gaussian distribution function, with a mean value of E0 and standard deviation of σE. The fractional volatile loss at any time instant, V, is calculated from the following equation: ðV V Þ 1 Z pffiffiffiffiffiffi ¼ V σ E 2π Z t

ðE0 þ 2σ E Þ ðE0 2σ E Þ

" (

ðE E0 Þ2 exp½k0 f expðE=Rg TÞdtgexp 0 2σE 2

)# dE

ð1Þ The prediction of the temporal profile of fractional volatile loss requires the values of k0, E0 and σE, and V*. During devolatilization of coal, CO, CO2, CH4, H2, and tar evolve in the form of volatiles. The tar consisting of high viscous polynuclear aromatic compounds diffuses slowly through the pores of the pyrolyzing coal particle. For large coal particles, the tar produced at an inner depth cannot leave the particle quickly because of transport limitations and starts to condense even at high temperatures. Eventually by the condensation reaction, 5574

dx.doi.org/10.1021/ef201162c |Energy Fuels 2011, 25, 5573–5583

Energy & Fuels

ARTICLE

Table 1. V* Value for Different Coals lignite coal (LAHVL) V* (%)

fuel type

sub-bituminous coal (HALVSB) V* (%)

sub-bituminous coal (MAMVSB) V* (%)

dp = 3 mm

dp = 8 mm

dp = 3 mm

dp = 8 mm

dp = 3 mm

dp = 8 mm

650

32.30

26.35

17.34

13.25

23.46

18.36

850

38.12

33.45

21.22

17.34

27.75

23.37

temperature (°C)

Table 2. Values/Correlations for Properties Used in the Model parameter

values/equations

1

source

5.07 10 , 194413, and 39395 10

k0 (s ), E (J/mol), and σE (J/mol) (LAHVL) k0 (s1), E (J/mol), and

6.00 1011, 206200, and 42300

parameters estimated by fitting the experimental

σE (J/mol) (HALVSB)

results to the model (present work)

k0 (s1), E (J/mol), and

8.00 1010, 199200, and 40300

σE (J/mol) (MAMVSB) V*

Table 1

experimental (present work)

Fs (kg/m3)

935 (LAHVL), 1190 (HALVSB), and

experimental (present work)

1122 (MAMVSB) kc

eq 12

Badzioch and Hawksley6

λe

eq 13

Sotirchos and Amundson18

cps

eq 14

hc

λg/r0 [1 + 0.9(Pr)

ε εr

Table 4 0.9

experimental (present work) Sadhukhan et al.20,21

1.0 1011

Casal et al.19

1050

Borghi et al.17

30

Borghi et al.17

K (m2) 1

cpv (J kg

1

K )

Mv (g/mol)

Badzioch and Hawksley6 0.333

0.5

it forms solid coke/char. Hence, V* of large coal particles is substantially lower than the proximate volatile content of the coal.8 However, as the particle size decreases, the chance of tar release from the particle surface increases, leading to a higher value of V*. Hence, the final fractional volatile loss V* increases with a decreasing particle size and increasing temperature. Similar observations were made by Borghi et al.,17 Anthony and Howard,9 Andrei et al.,4 Fu et al.,12 and Adesanya and Pham.13 V* is determined experimentally for a time period of 120 min (Table 1), and the values of k0, E0, and σE are obtained by fitting the experimental results to the model (Table 2). The volatile evolution is not uniform because of the temperature gradient in the radial direction. The instantaneous volatile evolution rate at any radial location is estimated as Rv ¼ Fs0 pffiffiffiffiffiffi σ E 2π

Z ðE þ 2σ Þ 0 E ðE0 2σE Þ

" ( ½k0 expðE=Rg TÞexp

ðE E0 Þ 2σ E 2

2

Adesanya and Pham13

(Re) ]

that the volatiles and the solid are in thermal equilibrium, the energy balance equation may be written as cpv 1 ∂ 2 ∂ðFs Ts Þ þ ðr uFv Ts Þ ∂t cps r 2 ∂r λe 1 ∂ 2 ∂Ts 1 ðF Ts Þ ∂v r ¼ Rv ðΔHÞ s þ 2 cps v ∂t cps r ∂r ∂r

The convective term in eq 4 arises because of transportation of the volatiles through the pores of the reacting coal particles. The initial and boundary conditions are at t ¼ 0 for 0 e r e r0 , for t > 0, at r ¼ 0, λe

)# ðV V ÞdE

The variation in particle bulk density is calculated by eq 3, where the second term arises because of the volume change of the coal particle. ð3Þ

T ¼ T0

∂Ts þ uFv cpv Ts ¼ 0 ∂r

for t > 0, at r ¼ r0 , λe

ð2Þ

∂Fs F ∂v ¼ Rv s ∂t v ∂t

ð4Þ

ð5Þ ð6Þ

∂Ts þ uFv cpv Ts ∂r

¼ σεr ðTs 4 Tf 4 Þ þ hc ðTs Tf Þ

ð7Þ

2.3. Convective Transport of Volatiles within the Solid Coal Particle. The equation of continuity for volatiles through

the porous structure of pyrolyzing coal may be written as ∂ðεFv Þ ∂ðFv uÞ εF ∂v þ ¼ Rv v ∂t ∂x v ∂t

2.2. Heat-Transfer Model. Using an infinitesimal control volume in the radial direction of the solid particle and assuming 5575

ð8Þ


Energy & Fuels

ARTICLE

Table 3. Proximate and Ultimate Analyses of Coal Samples lignite fuel type

sub-bituminous

sub-bituminous

(LAHVL) coal (HALVSB) coal (MAMVSB) Proximate Analysis (db)

volatile matter (%)

37.5

20.12

fixed carbon (%)

57.5

44.35

27.0 58.1

ash (%)

5.0

35.53

14.9

Ultimate Analysis (daf) carbon (%)

78.53

78.34

86.46

nitrogen (%)

1.12

1.94

1.82

hydrogen (%) sulfur (%)

5.26 0.35

4.26 1.76

4.51 0.51

oxygen (%) (by difference)

14.74

13.70

7.06

lower heating value (kJ/kg)

28925

22768

29376

The velocity field in the radial direction may be obtained using Darcy’s equation applicable for porous solid K ∂p u¼ μv ∂r p¼

Fv RT Mv

ð9Þ ð10Þ

where K and μv are the permeability of the porous char and viscosity of volatiles, respectively. Constant ambient pressure at the particle surface leads to at r ¼ r0 , p ¼ p0

ð11Þ

2.4. Physicochemical Properties. The thermal conductivity of coal is assumed to vary with the temperature, and it is calculated using the following expression given by Badzioch and Hawksley.6 ( 0:23 for T e 673 K kc ¼ 0:23 þ 2:24 105 ðT 673Þ1:8 for T > 673 K

ð12Þ The effective thermal conductivity of pyrolyzing solid coal, which appears in the heat balance (eq 4), accounts for the heat conduction through the solid and conduction through the entrapped volatiles within the pores. Therefore, the effective thermal conductivity of porous solid depends upon not only kc but also the thermal conductivity of volatiles kv and is calculated using the following equation from Sotirchos and Amundson:18 λe ¼ kc ð1 εÞ2 þ kv ε2

ð13Þ

The volumetric heat capacity also varies with the temperature and is estimated from Badzioch and Hawksley.6 (

Fs cps ¼

1:92 106 1:92 106 2:92 103 ðT 623Þ

for T e 623 K for T > 623 K

ð14Þ The porosity ε of the pyrolyzing coal particle undergoes continuous change because of possible structural transformation during pyrolysis. The experimentally measured values are used in the present study. The permeability of coal, K, is taken as

1.0 1011 m2, as reported by Casal et al.,19 while the Lucas correlation as discussed by Reid et al.20 is used to estimate the viscosity of volatiles. The heat-transfer coefficient hc is modeled using the correlation by Adesanya and Pham.13 The pyrolysis of coal is endothermic, and the average value of ΔH is taken as +300 kJ/kg from Adesanya and Pham.13 The values/correlations for the properties of coal employed in the study are presented in Table 2. 2.5. Numerical Solution. The coupled conservation equations (eqs 1, 4, and 8) along with the initial and boundary conditions (eqs 57) can be solved by an implicit integral finite volume method (FVM), as discussed by Patankar.21 The merits of the FVM in handling such problems with further details are available in the previous studies by the authors.2227 After integration of the governing equations along with the boundary conditions after multiplying with the volume of the control volume, the discretized equations can be presented in the form of a set of linear equations AP TP ¼ AW TW þ AE TE þ Q P

ð15Þ

where P is the central node and E and W are the neighboring node points. The resultant equations are solved using a tridiagonal matrix algorithm. A Fortran code is developed by the authors to solve the model equations. The convergence criterion was selected as 104 for normalized temperature.

3. EXPERIMENTAL SECTION Three coal samples, low-ash, high-volatile lignite (LAHVL) coal, high-ash, low-volatile sub-bituminous (HALVSB) coal, and mediumash, medium-volatile sub-bituminous (MAMVSB) coal of Indian origin are used for experimentation. The coal lumps are crushed and sieved for different size fractions. Spherical samples are obtained by grinding larger particles and repeated sieving. Subsequently, individual particles are separated and manually filed to remove the sharp edges and make it as spherical as possible (sphericity = 0.810.8523). The proximate and ultimate analyses are presented in the Table 3. Four different particle sizes (average particle diameters of 3.0, 5.0, 8.0, and 16 mm) are selected for pyrolysis experiments in an isothermal mass-loss apparatus with nitrogen gas purge. The 8 and 16 mm coal particle is also selected for monitoring the center temperature history during pyrolysis. A fine hole is drilled with a 2 mm bit up to the center of the coal particle (8 and 16 mm diameters), and the central point temperature is measured continuously by a fine sheathed ChromelAlumel thermocouple. After the insertion of the thermocouple into the hole, it is plugged by iron cement. The sample particles are air-dried in crucibles in an oven at a temperature of 110 °C for 2 h. The resulting coal samples are finally cooled to room temperature and stored in a closed container under a nitrogen atmosphere. The isothermal mass-loss apparatus consists of a tubular reactor (60 mm inside diameter and 1 m length) made of inconel. The tube is insulated by a sheet of mica, around which supercanthal heating wire is wound. It is then wrapped with asbestos ropes and insulated by a thick layer of plaster of paris casting to reduce heat losses. A resistance variac controls the temperature inside the reactor, and a ChromelAlumel thermocouple is inserted into the reactor to measure its temperature. An electronic microbalance is provided right above the reactor to monitor the sample mass continuously. The reactor is flushed continuously with nitrogen gas at a rate of 3.6 L/min inside the reactor to ensure an inert environment suitable for the pyrolysis reaction. The flow rate of nitrogen is not high enough to interfere with the mass-loss measurements. Further details are available elsewhere.24 The furnace is first heated to the desired temperature and kept at that level for at least 10 min. The air-dried coal samples (34 particles for 5576


Energy & Fuels

ARTICLE

Table 4. Variation of Porosity with the Progress of Pyrolysis (750 °C) lignite coal

sub-bituminous coal sub-bituminous coal

fuel type

(LAHVL)

(HALVSB)

(MAMVSB)

original coal 50% pyrolyzed

0.07 0.08

0.05 0.07

0.08 0.10

fully pyrolyzed

0.15

0.12

0.22

Figure 2. SEM images for HALVSB coal and fully pyrolyzed char: (a) coal sample and (b) fully pyrolyzed char at 750 °C.

Figure 1. SEM images for LAHVL coal and fully pyrolyzed char: (a) coal sample and (b) fully pyrolyzed char at 750 °C. 3 and 5 mm and 12 particles for 8 and 16 mm) are taken in a small basket made of stainless-steel wire mesh (200 mesh) and hung from the electronic microbalance by a very fine nichrome wire. The mass loss because of the pyrolysis of coal is monitored continuously until a constant mass is achieved. The experiments are repeated at 650, 750, and 850 °C. The instantaneous residual mass fraction and fractional volatile loss are calculated as follows: W ¼

m ; V ¼ 1W m0

ð16Þ

where m and m0 are the instantaneous and initial mass of the sample. A few partially pyrolyzed coal particles are quickly withdrawn from the reactor, quenched with liquid nitrogen, and preserved for SEM images and porosity measurement. The experiments are repeated 3 times for each sample, and the experimental error is found to be (4%.

4. RESULTS AND DISCUSSION 4.1. Change of the Pore Structure. The accessible porosity of the original coal sample, coal at 50% pyrolysis, and fully pyrolyzed char is measured with a mercury porosimeter (PoreMaster-GT, model PM 33-6) and is presented in Table 4 for different types of coals. It is observed that, although the original coal sample has little porosity, it increases gradually with the degree of pyrolysis. The data on accessible porosity at different stages suggest a continuous change in the pore structure during the course of the combustion process. Further details are available elsewhere.25 The progress of porosity is correlated with the degree of pyrolysis from the experimental results and incorporated in the model. 4.2. SEM Images of Original Coal Samples and Pyrolyzed Char. SEM images (JEOL, model JSM-5800) of the internal structure of the original coal and the resulting chars are shown in Figures 13. The images are analyzed for the presence of ash and carbon matrixes. Further details of SEM images are available

Figure 3. SEM images for MAMVSB coal and fully pyrolyzed char: (a) coal sample and (b) fully pyrolyzed char at 750 °C.

elsewhere.25 The formation of bubbles because of volatile evolution and, finally, large blow holes or passage because of bubble rupture is evident from the images. Figure 1 shows that the internal structure of lignite coal is somewhat crystalline, which breaks into smaller pieces upon pyrolysis with spacing because of bubble rupture phenomena. However, the high-ash coal looks fibrous in nature, and the structural change is evident in the pyrolyzed char with greater spacing between the parallel fiber layers. The MAMVSB coal shows a uniform carbon matrix, which, upon pyrolysis, becomes converted to the crystalline structure because of bubble rupture phenomena. 4.3. Swelling Behavior of Coal. During pyrolysis experiments, the swelling phenomenon of the coal particle is observed visually. Yu et al.16 suggested that swelling of coal particles is due to the pressure of bubbles inside the particle. Hence, the temperature is the main controlling factor for the swelling of coal under heat treatment. Temporal profiles of both the center and surface temperatures of an 8 mm coal particle are experimentally recorded by two fine-sheathed ChromelAlumel thermocouples. One is inserted inside the particle center, and the other is placed on a particle surface and sealed properly with iron cement. The particle temperature is taken to be the arithmetic mean of the particle surface and center temperature. A second set of experiments is carried out with similar coal samples at the same temperature, where 5577


Energy & Fuels partially pyrolyzed coal samples are withdrawn after the stipulated time interval, and the image of the particle is taken by a digital camera (Sony, DSC-HX1). The images are accessed by ACDSee software,15 and from the number of pixels in horizontal and vertical directions, the dimensions are estimated. The diameter is taken to be the geometric mean of the vertical and horizontal dimensions. The temperature versus particle swelling ratio (Figure 4) curves show that, in the temperature range of 440460 °C, the swelling is maximum when the bubble formation persists. Beyond this, the bubble rupturing starts, leading to particle shrinkage. However, even at a particle temperature of 900 °C, the original particle size is not fully regained with 24% of residual swelling for different types of coals. The LAHVL coal sample shows a peak swelling ratio of 1.13, while the HALVSB coal sample shows a peak swelling ratio of 1.10. However, qualitative trends of all coal samples are similar. The swelling ratio is then incorporated in the model to calculate the particle size. Figure 5 shows the frame photographs taken by a Sony DSC-HX1 digital camera during pyrolysis of LAHVL at various stages of conversions. 4.4. Testing of the Pyrolysis Model. The proposed comprehensive coal pyrolysis model with varying thermal conductivity, specific heat, porosity, particle size, and internal convection is tested with the experimental findings of the present authors. The fractional volatile loss and the center and surface temperature profiles are compared to experimental findings and are discussed below. 4.4.1. Fractional Volatile-Loss Profiles. Three kinetic parameters for the pyrolysis model, σE, k0, and E0, for different types of coals are determined by fitting to the experimental results and presented in Table 2. An analysis carried out to assess the effect of the convective term in the energy equation (eq 4) showed that t90, the 90% volatile loss time, decreases by 11% if the convective term is neglected, which is quite significant. This underlines the

Figure 4. Variation of the swelling ratio with the temperature for various coal samples during pyrolysis (dp = 8 mm, T 0 = 23 °C, and Tf = 850 °C).

ARTICLE

need of incorporating the convective term in the governing energy equation. The profiles of predicted and experimental fractional volatile loss of LAHVL samples for different particle sizes at pyrolysis temperatures of 650 and 850 °C are presented in Figures 6 and 7. In each case, the experimental results follow the predicted trend and, at a given temperature, the fractional volatile loss of smaller particles is steeper than that of larger particles. The 3 mm LAHVL coal sample (37% proximate volatile matter) loses about 29% volatile matter in 5 min at 650 °C but loses about 28% volatile matter within 1.2 min at 850 °C, and fractional volatile loss is almost instantaneous at even higher temperatures. A similar trend is observed for all other particle sizes. The maximum relative mean errors for 8 mm particles are found to be 0.034 and 0.038 at 650 and 850 °C, respectively. The model is further tested with the experimental coal with different ash and volatile matter content. Figure 8 compares the fractional volatileloss profiles for HALVSB coal with 35.53% ash and 20.12% proximate volatile content at two different temperatures. In both cases, the model predicts the experimental results well, with

Figure 6. Comparison of the experimental fractional volatile loss with model prediction for LAHVL (T0 = 23 °C, and Tf = 650 °C).

Figure 7. Comparison of the experimental fractional volatile loss with model prediction for LAHVL (T0 = 23 °C, and Tf = 850 °C).

Figure 5. Frame photographs of LAHVL coal at various stages of pyrolysis (dp = 8 mm, T0 = 23 °C, and Tf = 850 °C). 5578


Energy & Fuels

ARTICLE

Figure 8. Comparison of the experimental fractional volatile loss with model prediction for HALVSB (dp = 8 mm, and T0 = 23 °C).

Figure 10. Comparison of the experimental fractional volatile loss by Adesanya and Pham13 with model prediction (dp = 16 mm, T0 = 20 °C, and Tf = 650 °C).

Figure 9. Comparison of the experimental fractional volatile loss with model prediction for MAMVSB (dp = 8 mm, and T0 = 23 °C).

Figure 11. Comparison of the experimental fractional volatile loss by Fu et al.12 with model prediction (bituminous coal, volatile matter (daf) = 36.5%, T0 = 20 °C, and Tf = 900 °C).

relative mean errors of 0.032 and 0.034 at 650 and 850 °C respectively. Figure 9 compares the fractional volatile-loss profiles for MAMVSB coal with 14.9% ash and 27.0% proximate volatile content, with relative mean errors of 0.031 and 0.036 at 650 and 850 °C, respectively. The experimental fractional volatile-loss profile for 16 mm coal particle at 650 °C reported by Adesanys and Pham13 is compared to the prediction by the present model and is shown in Figure 10. A fair agreement is obtained with a relative mean error of 0.023. The experimentally observed fractional volatile-loss profiles of Fu et al.12 for Chinese bituminous coal samples of 25.62% proximate volatile content at argon plasma jet temperatures of 900 and 1460 °C are compared to the present model predictions (Figures 11 and 12). From the figures, it is evident that, at a given temperature, the smaller particle loses the volatiles faster (steeper curve) than the corresponding larger size particles. This may be attributed to the fact that the small particles are heated rapidly, causing rapid pyrolysis. At both temperatures, the model predicts the experimental results for five different coal particle sizes well, with the estimated relative mean errors being 0.019 at 900 °C and 0.021 at 1460 °C, respectively. 4.4.2. Heat-Transfer Study and Particle Temperature Profiles. The pyrolysis rate of a coal particle is influenced significantly by the particle temperature, which, in turn, depends upon intra- and extra-particle convective and radiative heat-transfer processes. The heat-transfer model is coupled with a kinetic model for

Figure 12. Comparison of the experimental fractional volatile loss by Fu et al.12 with model prediction (bituminous coal, volatile matter (daf) = 36.5%, T0 = 20 °C, and Tf = 1460 °C).

volatile evolution. The experimentally measured center temperature profiles for 8 and 16 mm coal particles by the present authors are presented in Figure 13 along with the model prediction, which exhibit reasonably good agreement. Figure 14 shows the surface and center temperature profiles predicted by the model along with the experimental data by Adesanys and Pham,13 with a relative mean error of 0.032. 5579


Energy & Fuels

Figure 13. Comparison of the experimental center temperature profiles to model prediction for LAHVL (T0 = 23 °C, and Tf = 650 °C).

Figure 14. Comparison of the experimental center and surface temperature profiles by Adesanya and Pham13 to model prediction (dp = 16 mm, T0 = 23 °C, and Tf = 650 °C).

Figure 15. Comparison of the experimental center temperature profiles by Adesanya and Pham13 to model prediction (T0 = 23 °C, and Tf = 650 °C).

The model is also tested with the measured experimental13 center temperature profile for coal particles of different size during pyrolysis (Figure 15). Both experimental data and model predictions show (Figure 15) that, at any time instance, a smaller size particle has a higher center temperature than the corresponding larger particle, with all other pyrolysis conditions remaining the same. This is due to the higher conductive resistance for larger particles. The relative mean errors are 0.029, 0.021, and 0.024 for 16, 13.5, and 10.2 mm particles, respectively.

ARTICLE

Figure 16. Model-predicted temperature profile: comparison between constant and variable size to convection and radiation for LAHVL (dp = 16 mm, T0 = 23 °C, and Tf = 650 °C).

Figure 17. Model-predicted radial volatile-loss profiles at different time instances for LAHVL (dp = 16 mm, T0 = 23 °C, and Tf = 550 °C).

4.5. Simulation Studies. The model is then used to simulate the pyrolysis characteristics of a single coal particle under different process conditions. The effects of various process parameters are assessed from the simulation studies suitable for tubular reactor conditions. 4.5.1. Effect of the Volume Change during Pyrolysis. Figure 16 shows the effect of the volume change of a coal particle during pyrolysis. It is assumed from Fu et al.15 that, up to 400 °C, the particle expands linearly with the temperature with a final volume expansion of 15%, beyond which the volatile evolution almost stops and the particle starts shrinking with a constant mass to its original size. As the particle expands because of the internal pressure of volatile evolution, the surface area per unit volume (3/r) decreases and the conduction path length increases, resulting in a slower heating of the particle (Figure 16). The difference is more pronounced in the center temperature profile because of the larger conductive resistance at the particle center. 4.5.2. Radial Profiles of the Temperature and Fractional Volatile Loss. The fractional volatile-loss profiles (Figures 17 and 18) are predominantly dictated by the radial temperature profiles (Figures 19 and 20) within the solid particle. The fractional volatile-loss and temperature profiles at 550 °C (Figures 17 and 19) show a slow release of volatiles because of gradual heating of the coal particle. A significant time lag 5580


Energy & Fuels

Figure 18. Model-predicted radial volatile-loss profiles at different time instances for LAHVL (dp = 16 mm, T0 = 23 °C, and Tf = 900 °C).

ARTICLE

Figure 20. Model-predicted radial temperature profile at different time instances for LAHVL (dp = 16 mm, T0 = 23 °C, and Tf = 900 °C).

from the volume reaction at low temperatures to the shrinking core at higher temperatures. Another interesting point to be noted from Figures 19 and 20 is that, at a low temperature (550 °C), the final temperature is reached in 420 s, while at a higher temperature (900 °C), the same particle reaches the final temperature within 180 s. This is due to the fact that the thermal conductivity of pyrolyzing char increases with the temperature (eq 12), while the volumetric heat capacity decreases with the temperature (eq 14), in addition to larger ambient temperatures.

Figure 19. Model-predicted radial temperature profile at different time instances for LAHVL (dp = 16 mm, T0 = 23 °C, and Tf = 550 °C).

exists for the reaction to start even at the surface of the particle at a low pyrolysis temperature of 550 °C (Figure 17). The initial heating of the surface causes a steep temperature gradient near the surface of the particle and higher volatile evolution at the surface. The fractional volatile-loss profile is, however, nearly flat at the completion of the reaction. It is interesting to note that, although the final temperature is attained by the entire particle within 420 s (Figure 19), pyrolysis continues beyond 600 s for a 16 mm particle. However, the particle releases only 15% of the volatiles after 420 s. At a higher temperature (900 °C), there is hardly any time lag for the onset of the reaction near the surface of the spherical particle (Figure 18). After about 30 s, the particle is clearly divided into three zones. (i) In the outermost zone, the particle has already pyrolyzed by more than 95% and the char is formed. (ii) In the second zone (between radial locations 0.6 and 0.9), the pyrolysis reaction is in progress. (iii) In the inner core (radial location beyond 0.6 to the center), the coal remains almost virgin and the pyrolysis is yet to start. In its limit, the pyrolysis phenomenon approaches the shrinking core mechanism at higher temperatures, while the reaction takes place throughout the volume of the coal particle (volume reaction mechanism) at low temperatures. Therefore, there is shift of the reaction mechanism

5. CONCLUSION (1) An involved fully transient model is proposed to predict the pyrolysis behavior of a large coal particle, coupling the kinetic model with heat transfer. This is one of the most comprehensive models on the pyrolysis of a large coal particle thus far, incorporating kinetics, internal convection because of volatile flow, conduction, external convection and radiation, changing particle size, and variation in porosity and properties. (2) The SEM images of coal and char corroborate the formation of bubbles, leading to expansion and bubble rupture, causing contraction. The experimentally observed swelling history with the temperature confirms that all three coals undergo swelling, followed by contraction with a small final residual swelling. (3) The model is found to predict the pyrolysis characteristics of large coal particles well, as experimentally observed by the present authors for pyrolysis of Indian lignite coal, medium- and high-ash sub-bituminous coals, and experimental results by Fu et al. 12 and Adesanys and Pham.13 Both the profiles of fractional volatile loss and center temperature are compared, unlike most of the previous works, where only the fractional volatile loss is compared. The model may be incorporated in the overall modeling of reactors for pyrolysis, gasification, and combustion of coal with different ash and volatile contents. (4) The effect of incorporating the convective term and the variation of the particle size is found to be quite significant, underlining the need of incorporating these in the model. The effect of the volume change on the pyrolysis behavior is found to be more pronounced at the center temperature compared to the surface. (5) The simulated temperature profiles show that the temperature at the surface attains the bulk temperature after a significantly long period of time at low pyrolysis 5581


Energy & Fuels temperatures, thus emphasizing the importance of considering the external film heat-transfer resistance in the model. The loss of volatiles increases with an increase in the pyrolysis temperature and a decrease in the particle size. (6) At low pyrolysis temperatures, the reaction follows the volume reaction mechanism, while at high temperatures, it shifts to the shrinking core mechanism.

’ AUTHOR INFORMATION Corresponding Author

*Telephone: +91-943-4788028. Fax: +91-343-2547375. E-mail: [email protected].

’ ACKNOWLEDGMENT Most of the work has been carried out in the Combustion Engineering Laboratory at the Department of Chemical Engineering of the National Institute of Technology, Durgapur, India. The authors acknowledge the financial support received from the Department of Science and Technology (DST) of the Government of India under its Fund for Improvement of S&T Infrastructure (FIST) Program to set up the laboratory. ’ NOMENCLATURE AP, AW, and AE = coefficients of the discretized equation cps = heat capacity of coal/char (J kg1 K1) cpv = heat capacity of volatiles (J kg1 K1) dp = initial particle diameter (mm) Ei = activation energy (J/mol) hc = convective heat-transfer coefficient (W m2 K1) k0 = pre-exponential factor (s1) Q P = constant term in the discretized equation r = radial coordinate (m) r0 = instantaneous particle radius (m) Rg = universal gas constant (J mol1 K1) Rv = rate of the devolatilization reaction (kg m3 s1) t = time (s) T = temperature (K) Tf = bulk temperature (K) T0 = initial temperature (K) V = fractional volatile loss at any time instant V* = final fractional volatile loss v = volume of the particle (m3) Greek Letters

ΔH = heat of reaction (J/kg) ε = porosity εr = emissivity λe = effective thermal conductivity of coal (W m1 K1) μv = viscosity of volatiles (kg m1 s1) Fs = coal bulk density (kg/m3) Fs0 = initial coal bulk density (kg/m3) Fv = volatile density (kg/m3) σ = StefanBoltzmann constant (W m2 K4) Subscripts

e = effective property

’ REFERENCES (1) Howard, J. B. Chemistry of Coal Utilization; Elliott, M. A., Ed.; Wiley: New York, 1981; Vol. 2, pp 665672.

ARTICLE

(2) Gibbs, B. M.; Beer, J. M. A pilot plant study of fluidized bed coal combustion. Int. Chem. Eng. Symp. Ser. 1975, No. 43, 1–11. (3) Zhu, W.; Song, W.; Lin, W. Effect of the coal particle size on pyrolysis and char reactivity for two types of coal and demineralized coal. Energy Fuels 2008, 22 (4), 2482–2487. (4) Andrei, A.; Beer, J. M.; Sarofim, A. F. Time-resolved burnout of coal particles in a fluidized bed. Combust. Flame 1985, 61, 17–27. (5) Pallares, J.; Arauzo, I.; Williams, A. Integration of CFD codes and advanced combustion models for quantitative burnout determination. Fuel 2007, 86, 2283–2290. (6) Badzioch, S.; Hawksley, P. G. W. Kinetics of thermal decomposition of pulversized coal particles. Ind. Eng. Chem. Process Des. Dev. 1970, 9, 521–527. (7) Kobayashi, H.; Howard, J. B.; Sarofim, A. F. Proceedings of the 16th International Symposium on Combustion; The Combustion Institute of Pittsburgh, Pittsburgh, PA, 1977; pp 411420. (8) Solomon, P. R.; Hamblen, D. G.; Carangelo, R. M.; Serio, M. A.; Deshpande, G. V. General model of coal devolatilization. Energy Fuels 1988, 2, 405–422. (9) Anthony, D. B.; Howard, J. B. Coal devolatilization and hydrogasification. AIChE J. 1976, 22 (4), 625–656. (10) Duan, L.; Zhao, C.; Zhou, W.; Qu, C.; Chen, X. Investigation on coal pyrolysis in CO2 atmosphere. Energy Fuels 2009, 23 (7), 3826–3830. (11) Sadhukhan, A. K.; Gupta, P.; Goyal, T.; Saha, R. K. Modelling of pyrolysis of coalbiomass blend using thermogravimetric analysis. Bioresour. Technol. 2008, 99, 8022–8026. (12) Fu, W.; Zhang, Y.; Han, H.; Duan, Y. A study on devolatilization of large coal particles. Combust. Flame 1987, 70, 253–266. (13) Adesanya, B. A.; Pham, H. N. Mathematical modelling of devolatilization of a large coal particle in a convective environment. Fuel 1995, 74 (6), 896–902. (14) Strezov, V.; Lucas, J. A.; Strezov, L. Investigation of the swelling pressure developed during slow pyrolysis of thermoplastic coal. J. Anal. Appl. Pyrolysis 2005, 74 (12), 88–95. (15) Fu, Z.; Guo, Z.; Yuan, Z.; Wang, Z. Swelling and shrinkage behavior of raw and processed coal during pyrolysis. Fuel 2007, 86, 418–425. (16) Yu, J.; Strezov, V.; Lucas, J.; Wall, T. Swelling behaviour of individual coal particles in the single particle reactor. Fuel 2003, 82, 1977–1987. (17) Borghi, G.; Sarofim, A. F.; Beer, J. M. A model of coal devolatilization and combustion in fluidized beds. Combust. Flame 1985, 61, 1–16. (18) Sotirchos, S. V.; Amundson, N. R. Dynamic behaviour of a porous char particle burning in an oxygen-containing environment, part-I constant particle radius. AIChE J. 1984, 30, 537–549. (19) Casal, M. D.; Diez-Faes, E.; Alvarez, R.; Diez, M. A.; Barriocanal, C. Influence of the permeability of the coal plastic layer on coking pressure. Fuel 2006, 85, 281–288. (20) Reid, R. C.; Prausnitz, J. M.; Poling, B. E. The Properties of Liquids and Gases; McGraw-Hill: New York, 1987. (21) Patankar, S. V. Numerical Heat Transfer and Fluid Flow; Hemisphere: New York, 1980. (22) Sadhukhan, A. K.; Gupta, P.; Saha, R. K. Modelling and experimental studies on pyrolysis of biomass particles. J. Anal. Appl. Pyrolysis 2008, 81, 183–192. (23) Sadhukhan, A. K.; Gupta, P.; Saha, R. K. Modelling and experimental studies on combustion characteristics of porous coal char: Volume reaction model. Int. J. Chem. Kinet. 2010, 42 (5), 299–315. (24) Sadhukhan, A. K.; Gupta, P.; Saha, R. K. Analysis of the combustion reaction of carbon and lignite char with ignition and extinction phenomena: Shrinking sphere model. Int. J. Chem. Kinet. 2007, 39 (6), 307–319. (25) Gupta, P.; Saha, R. K. Generalized mathematical modeling of fluidsolid non-catalytic reactions using finite volume method: Nonisothermal analysis. J. Chem. Eng. Jpn. 2003, 36 (11), 1298–1307. 5582


Energy & Fuels

ARTICLE

(26) Gupta, P.; Saha, R. K. Analysis of gassolid non-catalytic reactions in porous particles: Finite volume method. Int. J. Chem. Kinet. 2004, 36 (1), 1–11. (27) Gupta, P.; Saha, R. K. Generalized mathematical modeling of fluidsolid non-catalytic reactions using finite volume method: Multiple reactions. Can. J. Chem. Eng. 2004, 82 (5), 1096–1103.

5583


Modeling and Experimental Investigations on the Pyrolysis of Large

Recommend Documents