Devolatilization and Combustion of Coarse-Sized Coal Particles in

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Devolatilization and combustion of coarse sized coal particles in oxy-fuel conditions: experimental and modelling studies Shyamal Bhunia, Anup Kumar Sadhukhan, Subhamay Haldar, Partha Pratim Mondal, Ashok Prabhakar, and Parthapratim Gupta Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b02539 • Publication Date (Web): 09 Dec 2017 Downloaded from http://pubs.acs.org on December 10, 2017

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Graphical abstract

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Devolatilization and combustion of coarse sized coal particles in oxy-fuel conditions: experimental and modelling studies Shyamal Bhunia, Anup Kumar Sadhukhan *, Subhamay Haldar, Partha Pratim Mondal, Ashok Prabhakar, Parthapratim Gupta

Department of Chemical Engineering, National Institute of Technology, Durgapur-713209, West Bengal, India

Abstract A generalized unsteady state kinetic model, coupled with all modes of heat transfer, was developed to describe the combined coal devolatilization and the subsequent combustion of the residual char under oxy-fuel condition in both O2 -CO 2 and O 2 -N2 environments. Experiments were conducted to validate the model which was also found to predict the experimental data published in the literature well. The effect of coal particle diameter, temperature of the reactor and oxygen concentration on devolatilization time was investigated. Peaks in devolatilizatio n and char combustion rates and particle center temperature were studied and the effect of differe nt parameters assessed. Higher reaction time was observed in O 2 -CO2 environment compared to that in O 2 -N 2 due to lower particle temperature resulting from endothermic gasification reaction and the difference in thermo-physical properties. Simulation studies were carried out to generate temperature, carbon, O 2 , CO, and CO 2 contours to understand the char combustion reaction mechanism. The reaction started at the external surface of the particle, following unreacted shrinking core model with two zones; the solid product layer and the unreacted shrinking core, separated by a thin reaction front. Gradually, the reaction front thickness increases, leading to Shrinking Reactive Core Model, consisting of three zones; completely reacted ash layer, partially burnt reacting char/reaction zone, and unreacted/very little reacted char core. At the outset, O2 cannot penetrate into the particle and CO produced near the surface diffuses out to the boundary layer, forming a thin flame. Subsequently, O 2 diffuses through porous ash layer into the char, and CO burns within the pores, with hardly any CO detected in the boundary layer as the particle temperature increases.

Keywords: Oxy-fuel combustion, devolatilization, coal, char, modelling, simulation 2 ACS Paragon Plus Environment

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* Corresponding author: E-mail address: [email protected] (A.K. Sadhukhan), Mobile: +91-9434788048


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1. Introduction Coal is most widely used in many applications including in thermal power plants for electric ity generation,

iron making,

cement industries and chemical synthesis processes. However,

combustion of coal results in the emissions of greenhouse gases (GHGs) like CO 2 , and harmful gases like SO x, NO x, etc. So the development of clean coal technologies (CCTs) is very important for the sustainable use of coal in a feasible manner

1

that use of non-conventional energy resources

such as nuclear power and renewable energy resources can predominantly mitigate the greenhouse gas emission during energy production. Our future energy demand is expected to be met mostly from renewable energy resources, but the development of proper technology in industrial scale for large-scale energy generation from renewable resources may take some time. The demand of energy is expected to be fulfilled by the conventional combustion route using fossil fuel in the near future. However, oxy-fuel combustion is a potential alternative route for utilization of coal instead of using the conventional air combustion as noted by organizations assessing energy policy 2 . The technology using oxy-fuel coal combustion route is considered to be an appropriate and feasible technology for clean utilization of coal. This has attracted considerable attention in the recent past 2–4 .

Oxy-coal combustion technology has been elaborately investigated

5–7

and reviewed

2–4

by

many researchers. In oxy-fuel combustion technology, oxygen from the air separation unit is fed to the boiler along with the recycled flue gas which ensures effluent gases with a higher percentage of CO 2 (~95 %), ready for sequestration 2 . The pure CO 2 can be sequestrated in desirable geologica l formations like depleted gas and oil reservoirs or under marine aquifers 8 . Coal combustion in industrial boilers is comprised of two successive processes, coal devolatilization and combustion of residual char. An elaborate discussion on various reactions undergone by a single coal particle within a combustor or gasifier was reported elsewhere

9.

Devolatilization of coal particles under oxidizing conditions is mostly independent of the composition of reacting environment around the particle

10 . However,

devolatilization in nitrogen

environment promotes the emission of sub-micrometer particles (PM0.5) and coarse particles (PM1-10), while the devolatilization in CO 2 restricts their emission

11 . During

devolatilization at

high temperature, fast-evolving highly inflammable volatiles form a protective shield around the coal particle. The oxygen diffusing from the surroundings towards the particle gets consumed by volatile combustion, and the volatile shield prevents the oxygen flow to the particle surface. Hence, the heterogeneous char combustion reaction hardly occurs during the devolatilization stage. 4 ACS Paragon Plus Environment

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Towards the later stages of devolatilization process, the volatile shield disappears, and oxygen starts diffusing to the devolatilized hot char surface and initiating the heterogeneous char combustion reaction. It may be noted that during the devolatilization stage, combustion of the volatiles near the particle external surface causes a rapid increase in the particle temperature. This ensures a high reaction rate during the residual char combustion, even though a small quantity of diffused oxygen is present at the hot char surface

5,9 .

Over the past decades, the research thrust shifted towards fluidized bed combustion and gasification of millimeter-sized low-rank coals in electricity generation. Borah et al.

10

carried out

experimental investigations on devolatilization characteristics of northeastern Indian coal of size 4-9.5 mm in argon, air and oxygen enriched environments. They reported that the reactor temperature and heating rate on devolatilization stage influenced the char reactivity and thereby affected the residence time of overall combustion. They further reported the mass loss patterns during the devolatilization of five different Indian coals, explored the influence of the particle diameter, reacting gas composition and the reactor temperature on the volatile evolution rate, and also correlated the devolatilization time with the dependent parameters. Devolatilization of a coal particle in an oxidizing environment is a very complex process involving simultaneous

chemical

reactions

with

transport

limitations.

devolatilization assumed a constant rate of volatiles evolution

Initial

12 , while

modelling

of coal

later models used a single

reaction kinetics with Arrhenius type of rate constant. Distributed Activation Energy (DAE) kinetics included multiple parallel chemical reactions with varying activation energy evolution

of various chemical compounds during devolatilization.

13

for the

Chemical Percolation

Devolatilization (CPD) model represented coal, consists of many functional groups, like aromatic rings, aliphatic chains, the bridges and groups carrying oxygen.

14 . The

model proposed that during

devolatilization, the coal undergoes a series of bridge-cleavage and cross-linking reactions with structural evolution and parameters like pore surface area, macro-porosity and tortuosity factor of the pores undergo a rapid dynamic change. Therefore, the reactivities of the fully devolatilized and partially devolatilized char differ widely during the combustion process. In reality, char gets formed after complete devolatilization since it is the product of cross-linking reactions that occur among the organic constituents of the coal during pyrolysis and devolatilization stage

15 .

Among the various models reported in the literature, the comprehensive coal devolatiliza tio n models incorporating unsteady state non-isothermal energy balance equation coupled with 5 ACS Paragon Plus Environment

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distributed activation energy reaction kinetics

9

Page 6 of 40

is most suitable in this context. In this work, the

fully transient Distributed Activation Energy (DAE) model is employed to predict the volatile release profile, devolatilization time and temperature profile during devolatilization as it is applicable over a wide range of operating conditions. A review of the literature on the oxy-coal combustion reveals that the research on combined devolatilization of coal and residual char combustion is primarily limited to pulverized coals The few work reported on the modelling of millimeter-sized coal combustion

6,7,27–29

16–26.

mostly deal

with devolatilized char. Combined studies on coal devolatilization, evolved volatiles combustion, and the subsequent residual char combustion for millimeter-sized coal particles have rarely been reported in the literature. The present study attempts to bridge this gap. Several experimental investigations on char combustion reported the effect of reacting gas mixture on the combustion behavior in the enriched O 2 environment in presence of CO 2 or N 2 . The lower diffusivity of O 2 in O 2 -CO 2 is the governing factor for longer char conversion time, lower char temperature and slower reaction rate in CO 2 environment compared to N2 environments for millimeter-sized coal char combustion 7 . The influence of endothermic CO 2 gasification reaction on combustion characteristics of coal char in O 2 -CO 2 environment was also investigated

6.

In

another work, the combustion behavior of a 5.2 mm coal char particle in air was modelled, and the importance of carbon–CO2 gasification reaction along with diffusion of CO 2 within the micro pores was explored

30 .

In another study, a detailed analysis on the combustion of raw coal in both the O 2 -CO2 and O 2-N2 atmospheres was carried out

16 . The

experiments were carried out with pulverized coal (90-106

µm) in an electrically heated entrained flow reactor at a furnace temperature of 900-1400 o C with O2 concentration ranging from 5 to 28 vol.% in CO 2 and N 2 environments. Results showed that devolatilization atmosphere hardly influenced the devolatilization characteristics in terms of char morphology, volatile yield, BET surface area etc. Therefore it was inferred that shift of air combustion to oxy-fuel mode would have negligible effect on the devolatilization stage of the coal conversion process. It was also observed that char conversion profiles showed similar behavior in both the atmospheres, while the combustion reaction was kinetically controlled at 900-1200 o C at above conditions. On the contrary, when the reaction was diffusion-controlled at higher temperatures, conversion profile in O 2 -CO2 showed lower conversion rate because of low molecular diffusion coefficient of oxygen in O 2 -CO2 than that in O 2 -N2 . Researchers did not find 6 ACS Paragon Plus Environment

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the influence of CO 2 gasification reaction in the conversion profiles at the applicable experimenta l conditions. In a detailed review on fluidized bed (FB) oxy-coal combustion, the effects of heat transfer on the char combustion and pollutant emissions were presented along with the modeling aspects of the combustor

31 . Oxy-fuel

fluidized bed combustion behavior of coarse coal char particles (5-6 mm)

in fluidized bed at a bed temperature of 880-925 o C was studied experimentally and theoretica lly using maximum O 2 concentration of 15% 27 . It was found that the particle temperature exceeds the bed temperature

even at an O2 concentration as low as of 5% and the temperature differe nce

increased up to 48 o C using 15% steam in oxy-fuel environment

27 .

It was identified through

modeling study that the oxygen diffusion through boundary layer was the main governing factor in fluidized bed oxy-coal combustion, and carbon-CO2 gasification rate had no significant effect on overall

oxy-combustion

behavior

under the experimental

conditions

employed

27.

Investigations on oxy-fuel combustion of large coal char (6-7 mm diameter) in fluidized bed at 850o C in low O 2 concentrations (1-8%) in CO 2 environment indicated that besides heterogeneous O2 -char reaction, CO 2 -char gasification and CO-O2 oxidation reaction also influenced the overall char conversion process significantly. The boundary layer oxygen diffusion was found to be the rate-controlling step under the experimental conditions

28,29 .

A large research gap still exists in the area of experimental and modeling investigation for direct utilization of millimeter sized coal particle using either combustion or gasification route. Therefore, the present study is intended to explore the experimental findings of oxy-coal combustion

characteristics

of millimeter-sized

coal (1-8 mm range)

particle

through

comprehensive modeling, incorporating both coal devolatilization, followed by residual char combustion. This work is an extension of the authors’ previous study on coarse size coal char combustion in oxygen enriched environment

32 .

It was reported that devolatilization was always the first stage in most of the coal utilization process like combustion, gasification, etc., and it had a major impact on the succeeding steps

10 . Hence,

an

effective oxy-coal combustor design needs the inclusion of coal devolatilization in a suitable combustion model. The DAE model is employed to model the devolatilization process coupled with multiple chemical reactions for char combustion model. Emphasis is given on estimation of coal devolatilization time and devolatilization rate and identification of important process conditions like particle size, bulk gas temperature, O 2 volume percentage in the O2 -CO2 7 ACS Paragon Plus Environment

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environment. The reacting char particle temperature is estimated through fully transient energy balance equation considering coal devolatilization along with char combustion reaction kinetics. A comprehensive single particle volume reaction model is employed to model the char conversion process. The model prediction is compared with the present experimental results and that reported by the other researchers.

2. Modeling Modeling of coal combustion is a complex exercise, involving a number of highly-coup led physical sub-processes, namely reaction kinetics for coal devolatilization and char combustion, species transport, temperature and composition dependent physical properties and heat transfer

32.

The resulting model equations are coupled and non-linear partial differential equations (PDEs). The solution of such PDEs consists in the discretization of the spherical char particle into a finite number of concentric shells or discrete elements

33 . The

resulting governing equations are solved

using the software COMSOL Multiphysics (version 5.2a). Two distinct phases are considered; the solid coal/char and a stagnant gas film surrounding the particle

34,35 . Exothermic

heterogeneous

char combustion reactions only inside the pore structure and the homogeneous gas phase combustion inside the pore structure as well as gas film are considered. The heat transfer model incorporates the heat transfer by conduction, convection and radiation. A 1-D unsteady state model in spherical coordinate considering various phenomena of coal devolatilization and char combustion is solved using finite element method. The results presented here are tested for mesh independence with the computations performed with finer mesh producing results having the maximum deviation less than 0.5%.

2.1. Modeling of coal devolatilization Composition, structure and physical properties of coal are very complex and vary widely depending on the origin and location resulting in diverse products mix due to devolatilizatio n. Therefore, the modeling of devolatilization needs a certain degree of approximations and lumping of products as gaseous volatiles, liquid tar and solid char. Most of the devolatilization models reported in literature deal with only kinetic model of coal fines considering isothermal conditions. However, in the present work, an involved devolatilization model for mm-sized coal particles is


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developed including two sub-models; (i) devolatilization reaction kinetics,

(ii) energy

conservation incorporating conduction, convection and radiation modes of heat transfer

36 .

2.1.1. Coal devolatilization reaction kinetics The Distributed Activation Energy (DAE) model

13

is applied for devolatilization kinetics. The

model considers a number of simultaneous chemical reactions having the same value of Frequency factor, k o, but differing in activation energy E. The activation energy is assumed to vary according to the Gaussian distribution function, f(E), with mean activation energy Eo and standard deviation σE . f(E).dE indicates the fractional volatile loss (V) corresponding to the activation energy in the range of E to E+dE

13 .

V* is the effective volatile content of the coal, which is released after a

long time. The differential equation for fractional volatile release at any instant of time may be presented as  E0  2 E  t  V * V   E      exp  k 0  exp dt f ( E )dE V*  RT    E0  2 E  0 

f E  

1

E

 E  E0 2  exp   2 E2  2 

(1)

(2)

The instantaneous volumetric volatile evolution rate is expressed as

Rdv  

 E0  2 E  0 s



 E0  2

 E  k0 exp   f ( E )  (V * V )dE RT   E

(3)

2.1.2. Energy conservation during coal devolatilization As the particle enters the hot isothermal mass loss apparatus, it is heated by radiative and convective heat transfer from its outer surface. Drying takes place up to 110 o C, and at about 350 o C, volatiles

start evolving, and subsequently the volatiles undergo homogeneous combustion near

the vicinity of the particle outer surface. A spherical volatile envelope encloses the particle and restricts the O 2 diffusion into the solid phase. The effect of radiation from the volatile flame becomes prominent and may heat up the particle to higher than the reactor temperature. The energy balance equation may be written as

(  s Ts )  1   T  1  e 2  r2 s   Rdv .(H dv ) t Cp s r  r r  Cp s

(4)

Initially, the particle is at the room temperature. At t  0 for 0 ≤ r ≤ r0 , T  T 0

(5) 9 ACS Paragon Plus Environment

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At r  0 , zero heat flux; i.e. for t  0 ,  e

Ts 0 r

Page 10 of 40

(6)

A fraction (ξ < 1) of the total heat liberated from volatile combustion is assumed to be effective in rising particle temperature during coal devolatilization and is incorporated through boundary conditions (Eq. 7). The rest of the heat released is carried away by the flowing gas 9 . Boundary condition at particle surface, for t  0 , at r  r0

 e

Ts r

 r  r0

Fv .( H vc ) 4  r0

2

  r .(Ts4  Tb4 )  hc .(Ts  Tb )

(7)

2.2.Residual char combustion model The coal particle undergoes various morphological changes during devolatilization which continues even in char combustion stage. The devolatilized char constitutes a complex porous structure with interconnected macro, meso and micropores

37 . The

heterogeneous char combustion

reactions occur within the internal pore surface of the char particle. The macropores act as the transportation channels for gaseous components. Hence, the chemical reactions proceed throughout the porous structure of the char, following Volume Reaction Model (VRM) or the Shrinking Reactive Core Model (SRCM) 9 . The model assumes that the spherical shape of the particle remains unchanged, while the reactive carbon reacts leaving behind the inorganic inert as ash. During the course of combustion reaction, the density and the morphological properties of the char particle changes continuously with carbon conversion. The widely used random pore model (1980) 38 is adopted in the present study. 2.2.1. Reaction scheme Various heterogeneous and homogeneous chemical reactions considered for char combustion process along with reaction stoichiometry and the rate equations are presented below. Further details on the reaction scheme are available elsewhere

32 .

Heterogeneous reactions:

1.

2

 1 2 2 k C  O2  CO  CO2 2 2 2 1

(Reaction 1)

39

η=70exp (-3070/T)


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  E1  Rs1  k s01 exp  pO2 T  RT 





moles of O 2 reacted /m2 .s

k2 C  CO2  2CO

2.

  E2  Rs 2  k s02 exp  pCO2 T  RT 





(Reaction 2)

40

(Reaction 3)

41

moles of CO 2 reacted /m2 .s

Homogeneous reaction: k3 2CO  O2  2CO2

3.

  E3  Rv3  k v03 exp CCOCO0.25 C H0.25O mole of CO consumed/s.m3 of gas  RT  2.2.2. Conservation equations in solid Gaseous species conservation:









 1  2  g , s Yk , s  2 r N tg , s M avYk , s t r r Yk , s  l 3 1     g , s Dk e, s r 2    Rvl  k l M k  2  r  l 1 r r 

(8)

Continuity equation/mole balance:

( ct ,s ) t

2 1 (r N tg,s ) 3 5  2   Rvl  kl r r l 1 k 1

(9)

Conservation of carbon in char:

  1  dWc   2 Rv1  Rv 2 M c dt   2 

(10)

Conservation of energy:







 1  2 c ps  s Ts  2 r N tg , s M av c pg, s Ts t r r T  3 1    2  e r 2 s    Rvl (H l ) r  l 1 r r 

 (11)

2.2.3. Conservation equations in gas film: Gaseous species conservation:


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  g Yk   12  r 2 N tg M avYk  r r t 1   2 Yk   2   g Dke r    k 3 Rv3 M k r r  r 

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(12)

Continuity equation/mole balance: 2 (ct ) 1 (r N tg ) 5  2    k 3 Rv3 t r r k 1

(13)

Conservation of energy:

 c pg  gTg   12  r 2 N tg M avc pgTg  t r r Tg  1    Rv3 (H 3 )  2   g r 2 r r  r 

(14)

2.2.4. Initial Conditions At t=0, for all values of r, Yk ,s  Yk0,s

(15)

Ts  Ts0

(16)

Yk  Yk0

(17)

Tg  Tg0

(18)

N tg  N tg,s  0

(19)

Wc  Wc0

(20)

2.2.5. Boundary conditions Solid char: At the particle centre (r=0):

Ntg ,s  0

(21)

 Dke, s  g ,s  e

Ts r

Yk ,s r

 N tg,s M av,sYk ,s r 0

 N tg ,s M av,s c pg ,sTs r 0

r 0

r 0

0

0

(22)

(23)


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At r = r0

Yk ,s  Yk

(24)

Ntg ,s  Ntg

(25)

e

Ts r

  g

 N tg , s M av , s c pg , sTs

r  r0

r  r0

r  r0

(26)

Tg r

  r (Ts 4  Tb4 )

 N tg M av c pgTg r  r0

r  r0

Gas film: At the particle surface (r = rs)

 Dke , s  g , s   Dke  g

Yk , s r Yk r

 N tg , s M av , sYk , s r  r0

 N tg M avYk r  r0

r  r0

r  r0

(27) Boundary condition at the other end of the gas film (r = rb ) may be written as

Yk  Yk ,b , Tg  Tb

(28)

2.2.6. Physico-chemical parameters The Chapman-Enskog equation is employed to calculate the binary diffusion coefficients, while the effect of the multicomponent gas mixture on diffusivities of various species is incorporated through Stefan-Maxwell equation. The gaseous species effective diffusivity within the char is estimated using porosity and tortuosity factor of the pores. Dke, s 

Dke  

 2 Dke

(29)

The volumetric pore surface area (S) is evaluated from the random pore model S S0

38 ,

 ( 1  ). 1  ψ ln ( 1  )

(30)

S depends on the local carbon conversion (ζ) obtained from the random pore model

38 .

The

parameter ψ is determined by fitting the experimental data of S(ζ) to Eq. (30) at various conversion levels. Other thermo-physical properties are available elsewhere

32 .


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Page 14 of 40

The proposed model is applicable for coal devolatilization and combustion of coal in both freeflowing gas and fluidized bed conditions. Ranz-Marshal equation

42

is employed to calculate the

gas boundary layer thickness in case of free-flowing gas. Dennis et al.

43

reported that there exists

a gas film surrounding the particle even for fluidized bed conditions. Its thickness depends on τbp, the tortuosity of diffusion path through the bed particles in the gas film surrounding the char. It also depends on a factor αcush , the fractional particle surface occupied by the gas cushion. The correlation (Eq. 31), developed by Palchonok

44

from experimental observations for fluidized bed

combustion process, was found to be most appropriate and has been used in the present study Finally, the gas boundary layer thickness is calculated from the Sherwood number, using the correlation (Eq. 32) proposed by Dennis et al.

Sh  Sh 

hm dbp Dke 2 mf

 bp

43 .

 0.09 Arbp0.5 Sc0.3   cush 

(31)

d char

(32)



The experimental findings from the oxygen enriched combustion of lignite char in fluidized bed carried out by Bu et al. 6 showed that the skeleton of reacting char particles remained intact during the course of combustion with no change in particle diameter. Anderi et al.

45

too observed that the

ash layer remained attached to partially burnt lignite coal char particles retrieved from the fluid ized bed combustor at 1023 and 1173 K for a superficial gas velocity of 3.2 times the minimum fluidization velocity at various time instants. This is particularly valid when the loading of the fuel into the bed is low. Hence, the particle size is assumed to be invariant in the present investigatio ns.

2.3. Numerical solution The model equations presented above form coupled non-linear PDEs and algebraic equations which are solved using finite element method by COMSOL Multiphysics software (version 5.2a). Around 3 minutes of CPU time is required for each simulation run using Intel® i5 quad-core processor (3.50 GHz).

3. Experimental procedure 3.1. Sample preparation 14 ACS Paragon Plus Environment

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Energy & Fuels

Coal sample from Jharia in Barakar region of West Bengal, India was used to for experiments. The proximate and ultimate analysis of the coal and char is presented in Table 1. The lump coals were ground in pestle to prepare millimeter-size coal particles. Some of the particles of cubical shape were filed in rotating grinder and then manually filed in sand papers to prepare the 5 mm and 8 mm spherical particles. Particle diameters were measured using vernier caliper in three mutua lly perpendicular directions, and the cube root of the products of the diameters was chosen as the equivalent diameter of the coal particles. Smaller particles were sieved and filed to make spherical particles of 1.1 and 3.1 mm. Coal particles were dried at 105 o C for 1 hour in hot air oven before the experiments.

Table 1 Proximate and elemental analysis of the raw coal and char Proximate analysis(dry basis)

Elemental (weight%)

analysis(dry ash

(weight%)

free basis) Raw coal

Raw coal

VM

20.0

C

91.04

A

26.0

H

4.62

FC

54.0

N

1.85

S

0.15

O (by difference)

2.34

Char VM

Char 3.5

C

93.20

A

32.0

H

0.63

FC

64.5

N

1.31

S

0.04

O (by difference)

4.82

Identical 5 mm and 8 mm spherical coal samples (Fig.1c) were used for experiments, while a few 8 mm coal particles were drilled up to its center using a 1 mm bit. The center temperature was 15 ACS Paragon Plus Environment

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Page 16 of 40

monitored using a fine k-type thermocouple inserted into the hole and sealed with iron-cement as sealant (Fig.1b).

(a)

(b)

(c)

Fig. 1. (a) Schematic of the experimental setup, (b) temperature measuring procedure, (c) images of raw coal in experiments (black color particles) and coal ash (whitish) after combustion. 16 ACS Paragon Plus Environment

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Energy & Fuels

3.2. Combustion of the coal particles The experimental investigations were carried out under isothermal condition in an experimenta l setup, which is a 1 m long and 60 mm ID Inconel alloy tubular reactor (Fig.1a). The Super-canthal wire wrapped around the reactor tube act as the heating element and it was insulated with thick mica sheet. The reactor tube was then insulated by wrapping with asbestos ropes, and finally it was covered by plaster of paris insulation coating. A PI controller with an accuracy ±4 o C was used for controlled heating. The mass of the sample was continuously measured with an electronic precision balance (make: A&D Company, Japan; accuracy: ± 0.01 mg). The microbalance, whose response time was 1 s, was connected to a computer with RS-232 data interfacing cable. The reactor was purged continuously with a reacting gas mixture of either O 2 -N2 or O 2 -CO2 at 5 L/min through a nozzle at the bottom of the reactor. Oxygen volume percentages were maintained at 10, 20, 30 and 40%. The velocity of the gas mixture was controlled at 0.029 m/s (950 o C), which was found not to affect the balance reading. The furnace temperature was raised to a set value, and kept constant for at least 10 min. The spherical coal particle(s) was placed in a basket made of 100 µm stainless steel wire and it was suspended from the precision balance via a thin nichrome wire. A single coal particle was used for 8 mm sample; two particles were used for 5 mm samples, while 3-4 particles were employed for 3.1 and 1.1 mm particles for mass loss experiments. For the measurement of center temperature, the coal particle with an embedded thermocouple was kept suspended in the reactor. The instantaneous mass and the particle center temperature data were logged continuously in a computer at an interval of 1 s during the experimentation. Each experimental run was repeated thrice, and the mean value was used for analysis.

4. Results and Discussion The model parameters, E0 , E, V* are estimated as 204624 J mol-1 , 29396 J mol-1 and 0.2 respectively by the experimental data fitting to the devolatilization model. The experimenta l observation on coal devolatilization reveals higher devolatilization rate with the rise in oxygen concentration within the reactor

10 .

The rate of devolatilization depends on several parameters

likely particle size, bulk gas temperature, bulk gas composition, coal type, the volatile content of coal, the velocity of the gas stream. The smaller particles get heated up faster and, as a result, release volatiles earlier than the larger particles

46 . Volatiles

released may burn near the vicinity of 17

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Page 18 of 40

the particles surface in the presence of oxygen. Generally, the composition of the volatiles released during devolatilization of coal depends on the composition of the parent coal, operating conditions like temperature, reactor type and particle size

47–49 . CH , CO, CO , H O, 4 2 2

H2 , C2 H6 and traces of

other higher hydrocarbons are the main gaseous constituents in the evolved volatiles

50 .

In the

present study, however, the individual components have not been considered, and the evolution of total volatiles is estimated at different time instants and with ΔHvc, the average value of heat of combustion for the volatiles as -44×106 J/kg 51 . A part of the heat released due to volatile burning raises the particle temperature flowing gas stream sweeps away the remaining heat.

51 ,

while the

The fraction of the heat of volatiles

combustion utilized for the particle heat up, the best fit value of ξ is found to be 0.52 using Levenbarg Marquardt method 9 .

4.1. Coal devolatilization 4.1.1. Devolatilization time Devolatilization time was estimated from the dry ash free (daf) basis conversion plots, as the time elapsed where the particle has released 99% of its volatile content (daf VM content ~ 27%). The effect of particle size on devolatilization time at the treactor temperatures of 850 o C and 950 o C is presented in Fig. 2a; Fig. 2b depicts the effect of reactor temperature for particle sizes 5 mm and 3.1 mm, while Figure 2c shows the effect of oxygen concentration on devolatilization time. Figure 2d portrays the fractional residue vs. time profile for 5 mm and 3.1 mm particles at 950 o C in the presence of 30% O 2 (O 2 -CO 2 mixture). The model predicted values replicates well the experimental observations on devolatilization time with the maximum relative mean error being 0.05.


Page 19 of 40

24

28

Experimental

24

Model prediction

16

Devolatilization time (s)


20

850 oC 30% O2-CO2

12 8

950 oC

4

a 0

2

4 Particle size (mm)

6

Experimental

5 mm

20

Model prediction

16 12 3.1 mm

8 4

0

b

0

8

700

25

800

900 1000 Temperature (oC)

1100

1 Experimental

Experimental 20

Model prediction

0.9

850 oC

Fractional residue (-)


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

15 950 oC 10

Model prediction

0.8

5 mm

3.1 mm

0.7

950 oC, 30% O2-CO2

c 5

0

10

20 30 O2 concentration (vol %)

40

50

d

0.6 0

5

10

15

Time (s)

Fig. 2. Devolatilization time profiles from the experiments and model predictions for (a) varying particle size (O 2 volume percentage 30% in O 2 -CO2 ), (b) varying bulk gas temperature (O 2 volume percentage 30% O 2 -CO 2 ), (c) varying O 2 volume percentage (particle size 5 mm). (d) Experime nta l and model predicted residual mass profiles for different particle sizes. (O 2 volume percentage 30%, bulk gas temperature 950 o C in O 2 -CO2 environment)

The devolatilization time extends with increasing particle sizes at a given reactor temperature, while it decreases with reactor temperature for the same particle size (Figs. 2a and 2b). As the particle size decreases, the effect of reactor temperature on devolatilization time decreases (Fig. 2a). It is observed from both experiments and the model that devolatilization takes place very fast for particles below 0.5 mm where the devolatilization time is independent of the reactor temperature (Fig. 2a). The experimentally observed devolatilization times at 750 o C for 5 mm and 3.1 mm coal particles are 25 s and 16 s, while at a reactor temperature of 1050 o C the devolatilization times are 8 s and 6 s respectively (Fig. 2b). Hence, at higher reactor temperature 19 ACS Paragon Plus Environment

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Page 20 of 40

the impact of particle size on devolatilization time decreases. The experimental findings are well predicted by the devolatilization model having mean relative errors of 0.046 and 0.05 for 5 mm and 3.1 mm particle respectively. The higher concentration of oxygen in the reacting O2 -CO 2 gas mixture reduces the devolatiliza tio n time (Fig. 2c) as it enhances the volatile combustion rate with the heat transfer from volatile flame around the particle heating up the particle quickly. The higher particle temperature causes higher volatile release rate, thereby lowering the devolatilization time. The initial particle heat up time is reflected in the initial constant mass period in fractional residue profile during devolatiliza tio n (Fig. 2d). Increasing the particle size extends the initial particle heat up time due to higher heat capacity and slower heat transfer rate. It is found to be 6 s and 4 s for 5 mm and 3.1 mm coal particles respectively under identical reaction conditions. At higher temperature, the heat transfer by radiation contributes significantly for heating up of the particle than that at lower reactor temperature. 4.1.2. Devolatilization rate Figure 3 presents the effect of O 2 concentrations on the temporal profiles of the devolatiliza tio n rate for 8 mm particle. The experimental rate values, which were calculated from the conversion profile using cubic spline technique in MATLAB, are in good agreement with the model predictions with the maximum relative mean error of 0.05. The rate profiles show a very slow reaction rate at the beginning of devolatilization due to low particle temperature (27 o C). As the devolatilization proceeds, the volatiles undergo homogeneous combustion reaction with O2 diffusing from the bulk phase, and the particle temperature starts rising leading to an increase in the devolatilization rate. The rate passes through a maximum and then it diminishes as the volatile content of the coal particle gets released and consumed by combustion. As the O 2 concentratio n increases, the devolatilization rate curve shifts towards left with a higher rate, signifying faster devolatilization, though the area under each curve remains the same owing to the same initial mass of volatiles present in the coal particle.


Page 21 of 40

4.0E-5 Experimental Model predicted

3.0E-5

Devolatilization rate (kg/s)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

40% 2.0E-5

30%

1.0E-5 20%

0.0E+0

5

10

15

20

25

30

Time (s)

Fig. 3. Temporal profiles of coal devolatilization rates obtained from the experiments and model predictions at varying O 2 percentage for 8 mm coal particle at reactor temperature of 950o C in O 2CO2 environment

Figure 4 demonstrates how the temporal profile of the devolatilization rate is influenced by the reactor temperature for 5 mm coal particle at 30% O 2 concentration. The profiles indicate that the particle heat up time is significantly lower at higher reactor temperature compared to that at lower reactor temperatures, leading to a shift to the left of the bell-shaped rate profile with a constant area under the curve, as observed in Fig. 3.


Energy & Fuels

1.5E-5

Experimental Devolatilization rate (kg/s)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 40

Model predicted

1.0E-5

1050 oC

950 oC

5.0E-6 850 oC 750 oC 0.0E+0

0

5

10

15

20

25

30

Time (s)

Fig. 4. Temporal profiles of coal devolatilization rate obtained from experiments and model predictions at varying bulk temperatures for 5 mm coal particle with O2 volume percentage of 30% in O 2 -CO 2 ambience

4.2. Combined coal devolatilization and residual char combustion Immediately after devolatilization is completed, the oxygen starts diffusing onto the char particle surface, and the char combustion starts. The activation energy and the frequency factor values for the char combustion (reaction 1) are estimated by fitting the experimental data fit into the proposed model (Table 2). Thermo-physical properties like specific heat, thermal conductivity, diffusivity, etc. are evaluated as functions of composition and temperature. Figure 5 compares the experimental and model predicted conversion profiles of dry ash-free 8 mm coal particle at a furnace temperature of 950 o C in varying O 2 concentration (20, 30, and 40%) in the O2 -CO2 atmosphere. Char combustion period is much longer than devolatilization time with both periods decreasing with increasing O2 concentration. Devolatilization combustion times are 20 s and 15 s, while char combustion times are 1180 s and 595 s for 20% and 40% O 2 respectively. The combined model predicts the trends well for all three O 2 concentrations having the mean relative errors of 0.051, 0.04 and 0.046 for 20%, 30% and 40% O 2 respectively. The experimental and modelling trends show that immediately after completion of devolatilization, there exists a constant mass


Page 23 of 40

period during which O 2 diffuses to the particle surface from the bulk phase which is followed by the heterogeneous char combustion.

1.0

0.8

Conversion (-)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

Experimental 40%

Predicted

0.6

30%

20% 0.4

0.2

0.0

10

100

1000

Time (s)

Fig. 5. Temporal profiles of carbon conversion profiles obtained from experiments and model predictions in varying O 2 concentration for 8 mm coal particle at the reactor temperature of 950oC in O 2 -CO 2 environment: coal devolatilization followed by char combustion.

Table 2 Kinetic parameters for the combustion reactions of coal char Reaction

Frequency factor

Activation energy

Source

Reaction 1

k s1 0 =127,080 mol E1 =181,400 J mol-1

Fitting the

(Heterogeneous)

m-2 atm-1 s-1

experiment results in the present work into the proposed model

Reaction 2

k s2 0 =5.301 × 107 E2 = 248,120 J mol-1

(Heterogeneous)

mol K m-2 atm-1 s-1

Reaction 3

k v3 0 = 1.3 × 108 m3 E3 = 125,400 J mol-1

(Homogeneous)

mol-1 s-1

Dutta et al.

40

Howard et al.

41


Energy & Fuels

Figure 6 illustrates similar profiles for 5 mm coal particle in O 2 -N 2 as reacting gas mixture. The experimental devolatilization time and the residual char combustion time for 5 mm coal particle in 20% O 2 are observed to be 13 s and 387 s respectively. The model computation shows that under identical reaction conditions in an O 2 -CO2 environment the devolatilization time and the residual char combustion time for 5 mm coal particle are predicted to be 14 s and 420 s respectively. Higher heat capacity and emissivity and lower thermal conductivity of CO 2 and lower molecular diffusivity of O2 32 , along with endothermic gasification reaction during char combustion, lead to lower particle temperature. This results in higher reaction time in O 2 -CO2 environment than that in O 2 -N2 . 1.0

0.8

Conversion (-)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 40

Experimental 40%

Predicted

0.6

30% 20% 0.4

0.2

0.0

1

10

100 Time (s)

Fig. 6. Temporal profile of carbon conversion profiles obtained from the experiments and model predictions in varying O 2 concentration for 5 mm coal particle at the reactor temperature of 950oC in O 2 -N2 environment.

The corresponding center temperature profile during the combined devolatilization of coal and combustion of the residual char for 8 mm coal particle in O 2 -CO 2 environment shows two peaks (Fig. 7). The first one, which appears during devolatilization due to volatile combustion, becomes prominent at higher O 2 concentrations 9 . The second peak which appears near the end of residual 24 ACS Paragon Plus Environment

Page 25 of 40

char combustion followed by extinction

52 ,

also becomes prominent at higher O 2 concentratio n.

The experimentally observed maximum temperature during devolatilization is found to be 1025 o C, while

that during char combustion is 1260 o C at 40% O 2 concentration and detected after 16 s

and 628 s respectively. The corresponding peak temperatures for char combustion are 1062 o C and 1174 o C respectively at 20% and 30% O 2 concentrations. The existence of two similar peaks in temperature profile is also predicted by the model. The relative mean errors computed at 20%, 30% and 40% O 2 concentration are 0.03, 0.037 and 0.043 respectively. 1300

1200 40% 1100 Temperature ( oC)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

30%

1000

20%

900

Experimental

Predicted

800

700 600 500

10

100

1000

10000

Time (s)

Fig. 7. Temporal profile of particle center temperature obtained from experiments and model predictions in varying O 2 concentration for 8 mm coal particle at the reactor temperature of 950oC in O 2 -CO 2 ambiance.

The effect of combustion environment on the particle centre temperature as measured experimentally is presented in Table 3. It is observed that the centre temperature in the O 2-N2 environment throughout the course of combustion is exceeded by that in the O 2 -CO2 environme nt at similar oxygen concentrations and reactor temperature. The difference is found to be as high as 86 o C at the O 2 concentration of 30% at 950 o C reactor temperature. The difference in the particle temperature in different environments is caused primarily by the difference in the gaseous properties like specific heat, thermal conductivity and emissivity. At higher reactor temperatures, 25 ACS Paragon Plus Environment

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Page 26 of 40

the endothermic gasification reaction by CO 2 plays an important role in reducing the particle temperature in O 2 -CO2 environment.

Table 3. Comparison of experimentally measured particle centre temperature during the combustion of coal in O 2 -N 2 and O 2 -CO 2 environments for 8 mm coal particle in 30% O2 concentration at 950 o C reactor temperature. Time (s)

Particle centre temperature (o C) O2 -N2 environment

O2 -CO2 environment

14

770

767

18

990

983

57

1097

1045

111

1101

1059

184

1134

1073

285

1162

1094

404

1200

1121

559

1239

1153

724

Fully converted

1177

4.2.1. Rates of combined coal devolatilization and residual char combustion Fig. 8 depicts the combined rate profiles of the devolatilization of coal followed by char combustion. The experimental char combustion rates are calculated from the experimental char conversion profiles incorporating similar cubic spline interpolation technique in MATLAB. The rate profiles have two distinct segments similar in nature; the first one being presented by dashed lines, corresponds to devolatilization and is already explained in section 4.1. The char combustion rate starts immediately after devolatilization is completed and increases with time due to pore evolution in the char. The active surface area of pores gradually increases causing the devolatilization rate attain a maximum. Finally, the rate starts falling as active surface area for reaction decreases, and the char particle is no longer able to sustain its higher temperature due to slow heterogeneous char combustion rate. During the early stages of char combustion, the 26 ACS Paragon Plus Environment

Page 27 of 40

heterogeneous chemical reactions are mostly confined near the particle surface. However, as the surface carbon gets consumed the further reactions proceed through intra-particle pore diffus io n of oxygen, and the reaction rate gradually falls. The experimental rate profiles follow the above trends with the maximum rate values of 4.27×10-7 , 6.655×10-7 , 9.36×10-7 kg/s at O 2 concentrations 20, 30, and 40% and the profiles predicted by the model replicates the experimental observations satisfactorily during the entire period of the char combustion process. At higher oxygen concentrations, the rate curves show higher peak values but lower char combustion duration. However, the area under all the curves for char combustion segment remain the same due to the same carbon content of the devolatilized char particle. 4.0E-5

1.0E-6 Experimental 40%

Model prediction for coal devolatilization Model prediction for char combustion

30%

6.0E-7

2.0E-5 4.0E-7 20% 1.0E-5

Char combustion rate (kg/s)

8.0E-7

3.0E-5


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

2.0E-7

0.0E+0

0.0E+0

10

100

1000

Time (s)

Fig. 8. Evolution of experimental and model predicted rate (combined coal devolatilization and char combustion) profiles in varying O 2 volume concentration for 8 mm coal particle at bulk gas temperature of 950 o C in O 2 -CO2 ambience

Figure 9 shows similar rate profiles for 5 mm particle at different reactor temperatures. As the reactor temperature decreases the char combustion reaction rate profiles becomes flatter, and last for a longer duration due to a lower rate of combustion reactions. It is found through experimentation and model prediction that at 750 o C reaction rate is mostly sluggish and unvarying due to kinetic control, and no peak is observed in the rate profile. 27 ACS Paragon Plus Environment

Energy & Fuels

6.0E-7

1.3E-5

1050 oC

1.0E-5 950 oC 7.5E-6

5.0E-6

Experimental

5.0E-7

Model prediction for coal devolatilization

4.0E-7

Model prediction for char combustion

3.0E-7

2.0E-7

850 oC

2.5E-6

Char combustion rate (kg/s)

1.5E-5


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 40

1.0E-7 750

oC

0.0E+0

0.0E+0

3

30

Time (s)

300

3000

Fig. 9. Evolution of experimental and model predicted rate (combined coal devolatilization and char combustion) profiles in varying bulk temperature for 5 mm coal particle at O 2 volume percentage of 30% in O 2 -CO2 ambiance

4.3. Validation of combined model with literature reported experimental data After validating the model with experimental results (Figs. 2-9), it is also endeavoured to validate it with the published experimental data. To this end, the centre temperature profile, as predicted by the combined devolatilization and char combustion model, is compared with the experimenta l results for the fluidized bed combustion of a single lignite char particle

6

under oxygen enrichme nt,

both in CO 2 and N 2 environments (Fig. 10 & 11). The boundary layer thickness is calculated using Eqs. 31 and 32. A good agreement is observed except during the initial particle heat up time when the model predicted temperature is much higher. This is due to the fact that the initial char particle was at room temperature for the char combustion experiments

6,

while in the present model char

combustion starts with a particle which is already heated up after devolatilization.


40% O2-N2

1400

40% O2-N2

1400

1200

Temperature (K)

1200

Experimental

1000

1000

Model Devolatilization

800

Char combustion

800

8.8 600 400

600 0

0

5 Time(s)

100

200

300

Time(s)

Fig. 10. The experimental

6

and model predicted center temperature profiles of 6 mm lignite coal

char particle at 40% O 2 (O2 -N 2 mixture) and 1088 K reactor temperature. [Experimental data 6 is for only char combustion while model prediction is for combined devolatilization and char combustion.]

1400

1400

1200

1200

1000

1000

Experimental Model Devolatlization

800

Char combustion

Temperature (K)

40% O2-CO2

40% O2-CO2

Temperature (K)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

Temperature (K)

Page 29 of 40

800

10.1 600

0

6 Time (s)

Fig. 11. The experimental

0

6

100

200

300 Time (s)

400

500

600 600

and model predicted center temperature profile of 6 mm lignite coal

char particle at 40% O 2 (O2 -CO2 mixture) and 1088 K reactor temperature. [Experimental data 6 is for only char combustion while model prediction is for combined devolatilization and char combustion.

4.4. Simulation studies 29 ACS Paragon Plus Environment

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Page 30 of 40

Simulation of the combustion behaviour of the devolatilized char particle is carried out at the end of devolatilization period for an 8 mm coal particle in an O2 -CO 2 environment with 30% O2 concentration at 950o C temperature. Fig. 12 shows the carbon content contours of a single char with the progress of combustion of the devolatilized char at various time instants. The char combustion starts at the external surface of the particle after 21 s of devolatilization (Fig. 12a). At the outset, the reaction almost follows unreacted shrinking core model with a thin reaction front and a layer of ash forming near the particle surface (Fig. 12b). The carbon content of the char after 35 s is 96% (Fig. 12b), which reduces to 64% after 157 s (Fig. 12c). Two predominant zones are observed; ash layer and unreacted char core separated by a sharp reaction interface moving inward to the center. The ash layer thickness increases and the radius of the unreacted char core decreases with time. After 360 s, when the carbon content of char reduces to 30%, it is observed that, due to increase in active surface area of pores in the char, the reaction zone thickness gradually increases. Three zones; completely reacted ash layer, partially burnt reacting char/reaction zone, and unreacted / very little reacted char core, are observed (Fig. 12d-12f). Such type of char combustion mechanism is known as Shrinking Reactive Core Model (SRCM)

53 . It

is observed that gradually

the reaction front extends to the entire volume of the core (Figs. 12e-12f). The carbon content of the particle continues to decrease, reaching 7.5% after 600 s (Fig. 12e) and complete conversion at 820 s (Fig. 12 g), leaving the ash residue.

a) After 21 s

b) After 35 s

c) After 157 s

d) After 360 s

e) After 600 s

f) After 790 s

g) After 820 s

Fig. 12. Contours of carbon content inside the coal particle (particle size 8 mm, combustion of coal in O 2 -CO 2 environment, 30% O 2 , 70% CO 2 at 950o C).

Figure 13 shows temperature contours for 8 mm coal particle and the boundary layer around it during the combustion of char at a reactor temperature of 950 o C and 30% O 2 in an O2 -CO2 30 ACS Paragon Plus Environment

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environment. After the devolatilization period of 21 s, that the devolatilized char particle reaches the reactor temperature 950 o C (Fig. 13a), while the narrow gaseous zone in the vicinity of particle surface reaches a higher temperature (~1025o C) due to volatile combustion. As the exothermic heterogeneous combustion commences near the char surface, the surface temperature starts rising, attaining 1060 o C after 35s (Fig. 13b). Gradually the carbon near the particle surface gets consumed by combustion, and the reaction front recedes towards the center, moving the temperature peak inward (Fig. 13c). The gas boundary layer temperature near the particle surface still remains higher than that of the reactor. After 360s the char combustion reaction front spreads within the particle, and the particle temperature shoots up to 1110 o C. However, the entire gas boundary layer starts attaining the reactor temperature (Fig. 13d). After 600 s the char particle reduces to a small reacting char core at about 1140 o C, surrounded by a thick ash layer. The particle surface temperature, along with the entire boundary layer, attains the reactor temperature owing to the high heat conduction resistance of the ash layer (Fig. 13e). The char core continues to burn at 1170 o C after 790s (Fig. 13f) and complete conversion is achieved after 820s, leaving behind the ash at 950o C (Fig. 13g).

a) After 21 s

b) After 35 s

c) After 157 s

d) After 360 s

e) After 600 s

f) After 790 s

g) After 820 s

Fig. 13. Temperature contours for 8 mm coal particle at 950 o C reactor temperature in 30% O 2CO2 environment (Temperature scale in o C).

When the combustion of the char starts after the completion of devolatilization period, O2 is hardly detected within the particle (Fig. 14a). The devolatilized char starts burning in the presence of O2 at the particle surface till 157s (Figs. 14b-14c). At 360 s it is observed that, due to the presence of porous ash layer, O 2 diffuses into the particle even as the reaction front propagates towards the 31 ACS Paragon Plus Environment

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particle interior, increasing its concentration to 12% (Fig. 14d). After 600 s the O 2 concentratio n at the particle surface reaches 20% and it penetrates well inside the particle due to increased pore surface area, indicating a wide reaction zone for the heterogeneous char combustion reaction as explained earlier (Fig. 14e). The increasing thickness of the ash layer and the presence of porous reacting char zone ensure the presence of O 2 over the entire particle at 790 s with a wide variatio n of O 2 concentration from within the reacting char core due to diffusion resistance. Within the ash layer, however, O2 concentration is almost same as that of the bulk gas concentration (30%) due to very low diffusion resistance (Fig. 14f). After complete conversion, O2 concentration equals the bulk gas concentration throughout the ash layer (Fig. 14g).

a) After 21 s

b) After 35 s

c) After 157 s

d) After 360 s

e) After 600 s

f) After 790 s

g) After 820 s

Fig. 14. Contour plots for O 2 mole percentage in the particle and the gas film around it (8 mm diameter, O 2 -CO2 environment, 30% O 2 , 70% CO 2 at 950o C).

Figures 15a-15g and Figs. 16a-16g portray the evolution of CO and CO2 profiles within the particle and the gas boundary with time. Immediately after devolatilization, the hot char particle at 1000 o C starts heterogeneous combustion reaction forming CO and CO 2 (η ~ 6.3; reaction 1). Consequently, the CO content in the particle is observed to be 15-20%, while CO 2 is 80-85%, as no O 2 has yet diffused into the particle (Figs. 14a, 15a, 16a). The heterogeneous char combustion reaction primarily forms CO and its concentration increases within the particle with the progress of combustion of the char reaction, even as the temperature of the particle continues to rise (Figs. 13, 15b). The CO 2 concentration in the thin reaction front at the particle surface is found to be 32 ACS Paragon Plus Environment

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85%, while it decreases towards the center (Fig. 16b). CO produced in the particle undergoes homogeneous combustion reaction with O 2 in a thin layer within the gas film surrounding the char particle, resulting in a high CO2 cover having a concentration of ~90% (Fig. 16b). No CO is detected in the boundary layer beyond the flame (Fig. 15b). With the progress of char combustion topochemically at increasing temperature, CO concentration in the core increases due to reaction 1, with a decreasing profile towards the particle surface (Figs. 15c-f). Higher temperatures and increased pore surface area effect homogeneous combustion of CO (Reaction 3) within the particle pores, forming high CO 2 envelop inside the particle (Figs. 16c-15f). The high CO core and the high CO 2 envelope move towards the particle center with time (Figs. 15b-15f, 16c-16f) as the reaction zone shrinks. After complete conversion, CO concentration drops to zero and CO 2 attains the bulk condition of 70% for the entire particle pore and boundary layer (Figs. 15g, 16g).

a) After 21 s

b) After 35 s

c) After 157 s

d) After 360 s

e) After 600 s

f) After 790 s

g) After 820 s

Fig. 15. Contour plots for CO mole percentage in the particle and the gas film around it (8 mm diameter, O 2 -CO2 environment, 30% O 2 , 70% CO 2 at 950o C).


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a) After 21 s

b) After 35 s

c) After 157 s

d) After 360 s

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e) After 600 s

f) After 790 s

g) After 820 s

Fig. 16. Contour plots for CO 2 mole percentage within the char and the gas film around it (8 mm diameter, O 2 -CO2 environment, 30% O 2 , 70% CO 2 at 950o C).

5. Conclusion 1. A fully transient kinetic-heat transfer model, developed for combined coal devolatiliza tio n and residual char combustion, is found to predict the results of oxy-fuel condition very well. The present experimental findings along-with that already reported in literature was used in order to access the comprehensive model described in the article. 2. The devolatilization time increases with particle size and decreases with reactor temperature with the impact being more prominent for larger particles at lower reactor temperatures. Higher oxygen concentration heats up the coal particle faster, reducing the devolatilization time. The devolatilization rate reaches a peak which increases and occurs earlier for higher O 2 concentrations due to faster particle heat up. 3. Higher heat capacity and emissivity and lower thermal conductivity of CO 2 and lower molecular diffusivity of O 2 in a CO2 environment, along with endothermic gasifica tio n reaction during char combustion lead to lower particle temperature. This results in higher reaction time in an O2 -CO2 environment than that in O 2 -N2 . 4. The particle temperature attains two peaks; one during coal devolatilization and the other during char combustion, more prominently at higher O2 concentration. The char combustion rate also shows a peak which increases and appears earlier with increasing O2 concentration, particularly at high reactor temperature. 5. Simulation studies show that the reactions start with unreacted shrinking core model, shifting thereafter to Shrinking Reactive Core Model. At the outset, O2 cannot penetrate 34 ACS Paragon Plus Environment

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into the particle, and CO forms a thin flame in the boundary layer. Subsequently, O2 diffuses through porous ash layer into the char, and CO burns almost entirely within the particle pores, as the particle temperature increases.

Acknowledgement The authors acknowledge the grant received under DST-FIST program, funded by Department of Science and Technology (DST), Government of India. The experiments were carried out in Combustion Engineering Laboratory, Department of Chemical Engineering, National Institute of Technology Durgapur, India.

Nomenclature: Notations Ar

Archimedes number

ck

Concentration of gaseous component k, mol m-3

Cp

Specific heat, J kg−1 K−1

ct

Total gaseous mixture concentration, mol m-3

D

Molecular diffusivity, m2 s-1

d

Diameter, m

Dp

Particle size, mm

E

Activation energy, J mol−1

E0

Mean activation energy of Gaussian distribution function f(E), J mol−1

Fv

Total volatile evolution rate, kg s-1

hc

Convective heat transfer coefficient, W m-2 K-1

hm

Mass transfer coefficient, m s-1

k0

Frequency factor for coal devolatilization, s-1

k s1 0 , k s2 0

Frequency factors for reaction 1 & 2, mol K m−2 atm−1 s−1

k v3 0

Frequency factor for reaction 3, m3 mol-1 s−1

M

Molecular weight, kg mol-1

Nt

Total gas mixture molar flux, mol m-2 s-1

pO2, pCO2

Partial pressure of O 2 & CO 2 .

r

Radial coordinate, m 35 ACS Paragon Plus Environment

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R

Gas constant, 8.314 J mol−1 K −1

r0

Particle radius, m

Rs1

Rate of combustion of O 2 , mol m−2 s−1

Rs2

Rate of consumption of CO 2 , mol m−2 s−1

Rv

Reaction rate per unit volume, mol m-3 s-1

Rv3 ’

CO combustion rate, mol m−3 s−1

Rdv

Instantaneous volumetric volatile evolution rate, kg m-3 s-1

S

Instantaneous available pore surface area per unit volume, m2 m−3

Sc

Schmidt number

Sh

Sherwood number

t

Time, s

T

Temperature, K

V

Instantaneous volatile release (fraction)

V*

Volatile released at infinite time (fraction)

Wc

Instantaneous active carbon content of solid char, kg m-3

Y

Mass fraction

Page 36 of 40

Greek letters γkl

Stoichiometric coefficient of the k-th component in the l-th reaction

δ

Thickness of boundary layer for mass transfer, m

ΔH

Enthalpy change of reaction, J mol−1

ΔHdv

Enthalpy change of coal devolatilization, J mol−1

ΔHvc

Enthalpy change of volatile combustion, J mol-1

ε

Porosity

εmf

Bed porosity at minimum fluidization

εr

Emissivity

ζ

Point conversion of carbon in solid char

η

CO/CO 2 molar ratio

λ

Thermal conductivity of char, J m−1 s−1 K−1

λe

Effective coal thermal conductivity, W m-1 K-1

λr

Gas thermal conductivity at reference temperature, W m-1 K -1

ξ

Fraction of enthalpy from volatile combustion utilized for particle heat up 36 ACS Paragon Plus Environment

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ρ

Density, Kg m-3

ρs

Particle bulk density, kg m-3

σ

Stephan–Boltzmann constant, 5.67×10-8 W m-2 K-4

σE

Standard deviation of Gaussian distribution function f(E), J mol−1

τ

Tortuosity factor of char

ψ

Pore parameter

Subscripts 0

Particle surface

av

Average value

b

Bulk value

bp

Bed particle

c

Carbon

char

Char particle

e

Effective property

k

Index of gaseous species

l

Index of reaction number

g

Gas phase

s

Solid particle phase

Superscripts 0

Initial value


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Devolatilization and Combustion of Coarse-Sized Coal Particles in

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