Modeling of coal char gasification in coexistence of CO2 and H2O considering sharing

Modeling of coal char gasi?cation in coexistence of CO 2and H 2O considering sharing of active sites

Satoshi Umemoto ?,Shiro Kajitani,Saburo Hara

Energy Engineering Research Laboratory,Central Research Institute of Electric Power Industry (CRIEPI),2-6-1Nagasaka,Yokosuka-City,Kanagawa 240-0196,Japan

a r t i c l e i n f o Article history:

Received 20December 2010

Received in revised form 10November 2011Accepted 12November 2011

Available online 24November 2011Keywords:

Coal gasi?cation

Char gasi?cation reaction rate

Langmuir–Hinshelwood type model

a b s t r a c t

In a coal gasi?er,CO 2gasi?cation and H 2O gasi?cation occur at the same time.Many researchers have constructed CO 2gasi?cation model and H 2O gasi?cation model separately.Only a few models explain the competition between CO 2gasi?cation and H 2O gasi?cation.These models are divided into two types according to the properties of the active sites.Some models assume that CO 2gasi?cation and H 2O gasi-?cation occur at separate active sites,whereas others assume that all of the active sites are shared and are common to both CO 2and H 2O gasi?cation.There are some coals whose gasi?cation reaction rates,with CO 2and H 2O in coexistence,cannot be described by the conventional models.In this study,a Langmuir–Hinshelwood type gasi?cation model was modi?ed to explain the char gasi?cation by CO 2and H 2O.In the proposed model,CO 2gasi?cation and H 2O gasi?cation partially share active sites.Coal chars,which were prepared through pyrolysis using a drop tube furnace at ambient pressure and 1673K,were gasi?ed with CO 2and H 2O using a pressurized drop tube furnace (PDTF)and a thermogravimetric analyzer (TGA)at various temperatures.The proposed model was found to describe the gasi?cation reaction rates of the experiments more accurately than the conventional models.

1.Introduction

Integrated coal gasi?cation combined cycle (IGCC)is being developed worldwide to use coal more ef?ciently and cleanly.There are many types of gasi?ers.The oxygen-blown entrained ?ow type gasi?er is the most popular type.In Japan,an air-blown entrained ?ow gasi?er has been developed by CRIEPI and Mitsubi-shi Heavy Industries,Ltd.[1].Two-hundred and ?fty megawatt IGCC power plant using an air-blown gasi?er was constructed and is in operation [2].Furthermore,CRIEPI has proposed an O 2–CO 2blown gasi?er for an IGCC system with CO 2capture [3].In any type of gasi?er,three types of char gasi?cation reaction occur:O 2gasi?cation (combustion),CO 2gasi?cation and H 2O gasi?cation.The O 2gasi?cation rate is so rapid that the latter two gasi?cations are regarded as the rate-determining processes.Many researchers have constructed CO 2gasi?cation model and H 2O gasi?cation model respectively,for coal or biomass chars [4–6].They have used simple Langmuir–Hinshelwood (L–H)types of expressions derived based on the absorption and desorption theories.Typical expres-sions are the following equations:

r ?

k 11P CO 2

1tk 12P CO 2tk 13P CO

e1Tr ?

k 21P H 2O

1tk 22P H 2O tk 23P H 2

e2T

where

k ij ?A ij e àE ij

ei ?1;2;j ?1—3Te3T

These L–H types of expressions can describe two phenomena.The ?rst phenomenon is that the gasi?cation reaction rate at a high partial pressure of the reactant gas (CO 2or H 2O)becomes too low to extrapolate from the gasi?cation reaction rates at lower partial pressures of the reactant gas,because many active sites are occu-pied by the reactant molecules.Parameters k 12and k 22represent this phenomenon.The temperature dependences of these parame-ters are so low that parameters E 12and E 22can be smaller than E 11and E 21.The second phenomenon is that the product gas (CO or H 2)can inhibit gasi?cation because the product molecules can also oc-cupy the active sites.Parameters k 13and k 23represent this phe-nomenon.These parameters become lower as the temperature decreases and parameters E 13and E 23are negative.However,these equations cannot describe the competition between the CO 2gasi?-cation and H 2O gasi?cation,although these gasi?cations occur at the same time in coal gasi?ers.Only a few researchers have pro-posed L–H type gasi?cation models that consider the coexistence of CO 2and H 2O [7–10].These are divided into two types according

Corresponding author.Tel.:+81468562121;fax:+81468563346.

E-mail address:umemoto@criepi.denken.or.jp (S.Umemoto).

to properties of the active sites.One type assumes that the CO2gas-i?cation and H2O gasi?cation occur at separate active sites without sharing active sites or interacting with each other(designated as model1)[7,8],while the other assumes that all of the active sites are shared and common to CO2gasi?cation and H2O gasi?cation (designated as model2)[9,10].The experimental data for some coals agree with model1,while the experimental data for some coals agree with the model2.One of the differences between these experiments is the partial pressure of the gasifying agents.The partial pressure of the gasifying agents is known to affect the degree of surface saturation[11].Therefore,at a higher partial pressure,the difference between the gasi?cation rates calculated based on model1and model2becomes larger.In this study,a new assumption about the properties of the active sites is pro-posed to describe the gasi?cation rates for all coal based on gasi?-cation tests using thermogravimetric analyzer(TGA).Furthermore, using a pressurized drop tube furnace(PDTF),the availability of the proposed model for higher temperatures is examined.

2.Experimental

2.1.Char preparation

Test chars were prepared prior to the gasi?cation test.Three bituminous coals were used(SF coal from the USA,MN coal from Indonesia and DT coal from China).The properties of these coals and the prepared chars are shown in Table1.The pyrolysis condi-tions have to be carefully controlled,because they have an effect on the reactivity of the char.The pyrolysis temperature and heat-ing rate have a greater in?uence than the pyrolysis pressure on the char gasi?cation reactivity[12].Therefore,the test chars were prepared by rapid pyrolysis in an inert gas(nitrogen)using a drop tube furnace at a constant temperature(1673K)and atmospheric pressure.The residence time was set at about3s.All of the produced char,including soot particulates produced by gas phase carbon deposition reactions,was collected in the pyrolysis tests. Soot particulates smaller than1l m were removed using a preci-sion classi?er,and test char with an average particle diameter of about40l m was prepared for each coal.2.2.Char gasi?cation test

A thermogravimetric analyzer(TGA)was used for the char gas-i?cation tests with the isothermal reaction technique at low tem-peratures,with the reaction rate of the coal char controlled by the surface chemical reaction.In all cases,5mg of the char sample were placed in an alumina pan and heated at15°C/min to the prescribed temperature(1123–1273K)under a continuous argon ?ow of400–450cm3/min.The isothermal gasi?cation of the char was initiated by switching to the gasifying agents.Argon was used to control the concentration of the gasifying agents.The char conversions(x[–])and gasi?cation reaction rates(r[sà1])were calculated by:

m0àm t

0ash

e4Tr?

e5T

where m0[mg]denotes the sample mass at the start of the gasi?ca-tion,m t[mg]is the sample mass at reaction time t[s],and m ash[mg] is the mass of the ash.

The pressurized drop tube furnace(PDTF)facility shown in Fig.1was used for the high temperature region char gasi?cation tests where the reaction rate of the coal char was controlled by pore diffusion,the same as in entrained?ow gasi?ers.Three test chars were gasi?ed with several partial pressures of CO2and/or H2O at1673K or below.The residence time was controlled by vertically traversing a water-cooled sampling probe inserted up-ward from the bottom of the furnace.The feed rate of the char ranged from8to26g/h.The diameter of the reaction tube was 5cm.

3.Results and discussion

3.1.Proposal of new assumption for properties of active site

An example of the char gasi?cation mechanisms is given in the following steps[4]:

Nomenclature

a parameter for the proposed L–H model,dimensionless A ij(i=1,2,j=1–3)corresponding to frequency factor for L–H

equation,sà1MPaà1(j=1),MPaà1(j=2,3)

b parameter for the proposed L–H model,dimensionless

c ratio of the number of vacant active site for CO2gasi?-

cation to the number of vacant active site for H2O gasi-

?cation expressed,dimensionless

C As mol concentration of gasifying agent at the surface of

the char particle,mol/m3

d e average pore diameter,m

D eA effective diffusion coef?cient of the char particle,m2/s D N diffusion coef?cient that includes molecular diffusion

and Knudsen diffusion,m2/s

E ij(i=1,2,j=1–3)corresponding to activation energy for L–H

equation,kJ/mol

f c correction function,dimensionless

k ij(i=1,2,j=1–3)reaction rate coef?cient for L–H equation, sà1MPaà1(j=1),MPaà1(j=2,3)

k j,m(j=1,2)reaction rate coef?cient for L–H equation per volume of char particles,sà1MPaà1mà3(j=1),MPaà1mà3

(j=2)L P characteristic length of the char particle,m

m0sample mass at the start of the gasi?cation,mg

m t sample mass at gasi?cation time t,mg

n share number of shared active sites for CO2gasi?cation and H2O gasi?cation,kgà1

n tC total number of active sites for CO2gasi?cation,kgà1 n tH total number of active sites for H2O gasi?cation,kgà1 P CO

partial pressure of CO2,MPa

P H

partial pressure of H2O,MPa

r gasi?cation reaction rate,sà1

R gas constant,kJ/(K mol)

t reaction time,s

T temperature,K

x conversion,dimensionless

e porosity o

f the char particle,dimensionless

g effectiveness factor,dimensionless

s the tortuosity factor of the pore,dimensionless

/Thiele number,dimensionless

W i(i=1,2)parameter for random pore model,dimensionless

S.Umemoto et al./Fuel103(2013)14–2115

reactant molecules[13–15].Therefore,the active sites for CO2gas-

i?cation and the active sites for H2O gasi?cation should be the same.However,some active sites may be present in pores smaller than the larger reactant(in this case,CO2)and are thus active only for the smaller reactant(in this case,H2O).In addition,CO2is adsorbed molecularly,while H2O can be dissociatively chemisorbed to form OH species[16].Furthermore,the relation between the CO2 gasi?cation rate and the amount of catalytic materials is different from the relation between the H2O gasi?cation rate and the amount

Table1

Properties of coals and prepared chars.(Test chars were prepared by rapid pyrolysis in nitrogen using atmospheric DTF at1673K).

SF coal SF char MN coal MN char DT coal DT char

Proximate analysis[wt%,dry basis]

Ash9.318.468.3818.0911.1817.45 Volatile matter41.840.6527.28

Fixed carbon48.950.9761.54

Fuel ratio(=FC/VM) 1.2 1.3 2.3

HHV(MJ/kg)29.030.829.9

Median diameter(l m)37.942.237.344.821.441.4

Ultimate analysis[wt%,dry ash-free basis]

C78.597.381.198.883.297.7

H 5.50.3 5.70.1 4.30.1

N 1.2 1.1 2.0 1.10.30.7 S0.20.20.6

Fig.1.Schematic of PDTF facility.

16S.Umemoto et al./Fuel103(2013)14–21

of catalytic materials [17].This indicates that the catalysts behave differently in CO 2and H 2O atmospheres.These effects must depend heavily on the raw coal type and pyrolysis condition.For these reasons,a new assumption about the properties of the active site is proposed,as shown in Fig.2.In the proposed model,two new parameters,a and b,are used.These are expressed as:

a ?n share =n tH e14T

b ?n share =n tC

e15T

where n share is the total number of shared active sites for CO 2gasi-?cation and H 2O gasi?cation,n tH is the total number of active sites for H 2O gasi?cation,and n tC is the total number of active sites for CO 2gasi?https://www.360docs.net/doc/0115489483.html,ing these constants,the gasi?cation reaction rate is obtained as:

r ?

k 11P CO 2

1tk 12P CO 2tk 13P CO ta k 22P H 2O ta k 23P H 2

k 21P H 2O

12CO 2

13CO 22H 2O 23H 2

e16T

where c is the ratio of the number of vacant active sites for CO 2gas-i?cation to the number of vacant active sites for H 2O gasi?cation ex-pressed as:

c ?

a 1te1à

b Tk 22P

H 2O te1àb Tk 23P H 2

12CO 213CO

e17T

3.2.Application of proposed model to coal char gasi?cation

3.2.1.Determination of kinetic parameters

First,CO 2gasi?cation and H 2O gasi?cation tests were conducted separately using the TGA and PDTF.Figs.3and 4show Arrhenius plots of the initial gasi?cation rates for CO 2gasi?cation and H 2O gasi?cation,respectively.The experiments on the low temperature region,where gasi?cation rates were below 10-2s -1,were con-ducted using the TGA,while those in the higher temperature re-gion were conducted using the PDTF.Because continuous gasi?cation rates were obtained with the PDTF and TGA,the gasi-?cation rate in the high-temperature region,called zone II,where the reaction rate was controlled by pore diffusion,could be ana-lyzed using the effectiveness factor,as in the previous paper [6].The effectiveness factor,g (g 1for CO 2gasi?cation and g 2for H 2O gasi?cation),has a correlation with the modi?ed Thiele number,

/,as shown in Eq.(18).The modi?ed Thiele number and the cor-rection function,f c ,of Hong et al.[18]were used.

g ?f c 1/1tanh e3/Tà

13/ e18Tf c ?1t????????

1=2

p 2/2

t1=e2/2T

!0:5e1àn obs T2e19T

S.Umemoto et al./Fuel 103(2013)14–2117

For the L–H type reaction rate equation,/and n obs are ex-pressed as:

/?L p

?????????????

v0k1m

2D eA

k2m C As

1tk2m C As

???????????????????????????????????????????????????

k2m C Asàlne1tk2m C AsT

e20T

n obs?

1tk2m C As

e21T

where L P is the characteristic length of the char particle,correlated with the char particle diameter,D P(L P=D P/3);D eA is the effective diffusion coef?cient of the char particle;C As is the mol concentra-tion of gasifying agent at the surface of the char particle;and k m, k1m and k2m are the intrinsic reaction rate coef?cients per volume of the char particle.The parallel pore model is used to express D eA in this paper:

D eA?e

s D Ne22T

where e is the porosity of the char particle and s is the tortuosity factor of the pore.D N is the diffusion coef?cient,which includes the molecular diffusion and the Knudsen diffusion,and the Knudsen diffusion coef?cient is proportional to the average pore diameter,d e. The parameter d e/s|1(for CO2gasi?cation)and d e/s|2(for CO2 gasi?cation)are determined by?tting.

The kinetic parameters obtained from the reaction rate analyses are shown in Table2.Some of the kinetic parameters for inhibition reaction by CO and H2(k13and k23)are not shown,because they were not used to con?rm the competition between the CO2gasi?-cation and H2O gasi?cation and to determine the parameters in Eq.

(16),a and b.As mentioned in the introduction,parameters E12and E22for any coal are negative or smaller than E11and E21.In addi-tion,parameters E13and E23are negative.Table2also lists param-eters,W i,a and b which are discussed later.The difference between d e/s|1and d e/s|2indicates the difference between the diffusion properties of CO2and H2O.

3.2.2.Discussion of char gasi?cation in coexistence of CO2and H2O

Fig.5a–c shows the gasi?cation reaction rates at1173K of SF char,MN char,and DT char,respectively.The open circles are experimental data of the gasi?cation reaction rate for the coexis-tence of CO2and H2O.The sum of the partial pressures of CO2 and H2O is0.1MPa.The cross marks show the sum of the gasi?ca-tion rate for CO2without H2O(squares)and the gasi?cation rate for H2O without CO2(triangles).The lines show the gasi?cation reac-tion rates predicted by model1(Eq.(12)),model2(Eq.(13)),the proposed model(Eq.(16)),the model for CO2(Eq.(1)),and the model for H2O(Eq.(2)).For any chars,the experimental data of the gasi?cation reaction rate for the coexistence of CO2and H2O are lower than the prediction by model1and higher than the prediction by model2.Therefore,the proposed model was used. In Fig.5,parameters[a,b]are equal to[0.50,0.62],[0.42,1.0]and [0.47,1.0]for the SF char,MN char,and DT char,https://www.360docs.net/doc/0115489483.html,ing these parameters,the proposed model describes the experimental data exactly.Parameter b for the MN char and DT char was equal to 1.0.This indicates that H2O can use any active sites,which is natural from the point of view of molecular size.However,param-eter b for the SF char was not equal to1.0.One of the reasons may be the contents of the catalytically active metals(e.g.,Fe,Ca,K,and Na),which are larger in SF coal compared to the other coals.

https://www.360docs.net/doc/0115489483.html,bination of proposed L–H model and the random pore model

In the previous section,the gasi?cation rate at x=0.5was used to discuss the applicability of the proposed L–H model.Another model is needed to describe the change in the gasi?cation reaction rate caused by a change in the char surface area[6,12,19–22]. The random pore model has been used by many researchers [6,12,21,22].In this section,the proposed L–H model and the random pore model are combined.The gasi?cation reaction rate equation is written as:

k11P CO

e1àxT

???????????????????????????????????

1àW1lne1àxT

1tk12P CO

tk13P COta

k22P H

k23P H

k21P H

e1àxT

???????????????????????????????????

1àW2lne1àxT

12CO213CO22H2O23H2

e23T

where W i(i=1,2)is a dimensionless parameter that indicates the initial pore structure.Some examples of the gasi?cation test results using MN char are shown in Fig.6a and b.Fig.6a shows the CO2gas-i?cation reaction rate(CO2=50%)and H2O gasi?cation reaction rate (H2O=50%).From these results,the dimensionless parameter for CO2gasi?cation,W1is equal to1,and that for H2O gasi?cation, W2,is equal to4for the MN char.The difference between W1and W2indicates the difference between the active sites for the CO2 gasi?cation and the active sites for the H2O gasi?cation.Fig.6b shows the gasi?cation reaction rate for the coexistence of CO2and H2O(CO2=50%,H2O=50%).The dotted lines show the experiment results.The three lines are the calculation results using model1, model2,and the proposed model.The proposed model is better at describing the experimental data than the other models.

3.2.

4.Temperature dependence of proposed model

If parameters a and b do not depend on the temperature,just a few experiments are needed to determine the parameters for each char.Fig.7a–c shows the gasi?cation reaction rates at1223K of the SF char,MN char,and DT char,respectively.The proposed mod-el using the parameters used at1173K describes the experimental data more exactly than model1and model2.

Table2

Kinetic parameters for char gasi?cation.

i=1,2,j=1–3SF char MN char DT char

A ij E ij(kJ/mol)A ij E ij(kJ/mol)A ij E ij(kJ/mol)

k11(sà1MPaà1) 2.66?109256 1.33?1012332 3.39?1011327 k12(MPaà1)0.987à35.390.713.576.18.38 k13(MPaà1)––0.05à72.7 1.72?10à2à96.5 k21(sà1MPaà1) 1.00?109231 2.87?1011304 1.40?108226 k22(MPaà1) 1.87-30.2 4.85?10354.80.121à52.2 k23(MPaà1)––––0.162à68

SF char MN char DT char SF char MN char DT char

d e/s|1(nm)0.340.550.04W1(–)6127

d e/s|2(nm)9.100.67 3.14W2(–)0.543

a(–)0.500.420.47b(–)0.62 1.00 1.00

18S.Umemoto et al./Fuel103(2013)14–21

Furthermore,PDTF gasi?cation tests for the coexistence of CO2 and H2O in the diffusion control zone(zone II)were conducted. Fig.8a and b shows the results of the gasi?cation tests for the SF char and MN char,respectively.The conditions for the SF char gas-i?cation were0.5MPa,10vol.%H2O,40vol.%CO2,and1473K or 1673K.The conditions for the MN char gasi?cation were 1.0MPa,5vol.%H2O,20vol.%CO2,and1373K.The temperature distributions of the gas in the height direction that were measured before the experiments were considered for the calculation of the models.The variation in the gas concentration was also considered. The effectiveness factors were used for considering diffusion con-trol.The start point(reaction time=0)is the upper side of the heater.The conversions,x of the experiments were always smaller than the conversions predicted by model1.For the MN char,under these conditions the prediction of model2was almost the same as the prediction of the proposed model because the interference be-tween the CO2gasi?cation and H2O gasi?cation was large.In any of the chars or under any conditions,the proposed model was better at describing the experimental data than the other models,even if the temperature was in the diffusion control zone.

As seen above,the proposed model is helpful at describing the competition between the CO2gasi?cation and H2O gasi?cation, although the physical implications of a and b need to be considered in the future work.Some sets of a and b might be determined to?t

S.Umemoto et al./Fuel103(2013)14–2119

the calculation to the data.The physical implications help to deter-mine these parameters.An example of the hypotheses is that b is larger than a because the molecular size of CO2is larger than that of H2O,and CO2molecules cannot enter the small pores where H2O can enter.

4.Conclusions

Coal chars were gasi?ed with CO2and H2O using a TGA and a PDTF.The conventional gasi?cation models could not exactly describe the competition between the CO2gasi?cation and H2O gasi?cation.The Langmuir–Hinshelwood(L–H)gasi?cation model was modi?ed to explain the interference between CO2and H2O. The modi?ed model proposed that CO2gasi?cation and H2O gasi?cation share partially active sites.The proposed model could explain the gasi?cation rate of coal chars in the presence of CO2and H2O which was not explained by conventional models. Furthermore,the combination of the random pore model and pro-posed L–H model could describe the change in the gasi?cation reaction rate as the reaction progressed.The parameters used in the proposed model need not be changed as the temperature rises. Acknowledgments

A part of the presented work was supported by the New Energy and Industrial Technology Development Organization(NEDO)pro-gram‘‘Innovative zero-emission coal gasi?cation power generation project’’,P08020.

20S.Umemoto et al./Fuel103(2013)14–21

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