Fuel 87 (2008) 3403–3408

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Fuel

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Properties of high ash coal-char particles derived from inertinite-rich coal:II. Gasification kinetics with carbon dioxide

Raymond C. Everson a,*, Hein W.J.P. Neomagus a, Rufaro Kaitano a, Rosemary Falcon b, Vivien M. du Cann c

a Separation Science and Technology Research Group, School of Chemical and Minerals Engineering, North-West University, Potchefstroom Campus, Private Bag X6001,Potchefstroom 2520, South Africab School of Process and Materials Engineering, University of the Witwatersrand, Johannesburg, P.O. Box Wits, Wits, South Africac Petrographics SA, Suite 155, Private Bag X025, Lynwood Ridge 0040, South Africa

a r t i c l e i n f o

Article history:Received 9 July 2007Received in revised form 9 May 2008Accepted 13 May 2008Available online 20 June 2008

Keywords:High ash charsDense charsCarbon dioxide-char reaction ratesRandom pore model

0016-2361/$ - see front matter � 2008 Elsevier Ltd. Adoi:10.1016/j.fuel.2008.05.019

* Corresponding author. Tel.: +27 18 299 1986; faxE-mail address: ray.everson@nwu.ac.za (R.C. Evers

a b s t r a c t

The reaction rate of carbon dioxide-nitrogen gas mixtures with a well-characterised high ash charderived from an inertinite-rich coal discard was investigated by experimentation and reaction rate mod-elling. Experimentation with a thermogravimetric analyzer at 87.5 kPa and 287.5 kPa between 850 �C and900 �C and with 1mm diameter particles, similar to operating conditions used for bubbling fluidised bedgasification, was carried out. The char consisted of a large proportion of dense char formed from the inert-inite in the parent coal, fine pores from the low concentration of reactive macerals and cracks formed as aresult of thermal deflagration. The effects of carbon dioxide concentration, temperature and pressure onthe carbon conversion with time were found to follow expected trends with long reaction times. The ran-dom pore model, which accounts for intraparticle structural changes, was examined to predict the overallreaction rate. For this evaluation, a new procedure was developed to determine the structural parameterwhich could not be calculated directly from initial characterisation results. This procedure consists ofdefining a reduced time parameter, which conveniently eliminates the effect of the intrinsic kineticswhen conversion versus the reduced time results is used. Thus, the structural parameter, which charac-terises the pore growth and coalescence, can be evaluated by a regression procedure using experimentalresults obtained at all temperatures and pressures. Intrinsic reaction parameters based on the power ratelaw were also calculated from carbon conversion versus real time results using a stepwise regression pro-cedure. It was found that the random pore model predictions for the carbon conversion with time usingthe determined parameters correlated very well with experimental results, thus confirming that the reac-tion rate is chemical-reaction controlled.

� 2008 Elsevier Ltd. All rights reserved.

1. Introduction

Coal gasification continues to receive attention in coal utiliza-tion with the development of fluidized bed gasifiers (FBG) andcombined cycle power plants (IGCC). The use of certain low gradecoal discards, characterized by a high ash content and rich ininertinites obtained from coal classification/separation plants, pre-sents a further challenge as a result of the presence of dense charsformed from the inert inertinites [1–3], and the char morphologyand catalytic effects due to the minerals in the ash [3]. A detailedunderstanding of reaction kinetics and diffusional mechanisms isthus essential, with special attention devoted to identifying reac-tion regimes [4].

The gasification of chars with carbon dioxide has been exam-ined extensively by many investigators [5-13] and results concern-ing the correlation of reactivity with char rank [9], char

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: +27 18 299 1535.on).

morphology, pore surface area [9] and catalytic activity of the min-erals present [10] have been published. Liu et al. [13] addressed theeffect of char rank, pressure, temperature and pore diffusion andreported that (1) the pressure effect on the reaction rate varieswith coal rank, and is more pronounced for high rank coals (2)the apparent reaction rate increases as char rank decrease (3) thetemperature has the greatest effect on the reactivity, with adecreasing activation energy as the coal-char rank decreases and,(4) for pulverised particles (<100 lm) at high temperatures(>1400 K) pore diffusion becomes important. Unfortunately, de-tailed characterisation (rank) of the coals referred to is unknownexcept for the high surface areas that were calculated. The validityof the Langmuir–Hinshelwood equation for describing the intrinsickinetics for gasification has been established [4,14-17] for low andhigh pressure operation and is used successfully for both carbondioxide and steam gasification as well as with gas mixtures andmany different chars. The main disadvantage of the Langmuir–Hin-shelwood equation is the evaluation of the many different adsorp-tion and kinetic parameters from experimental results when many

Nomenclature

E activation energy, kJ mole�1

kso reaction rate constant (rs = kspn), m s�1 Pa�n

k0so lumped pre-exponential factor, ksoSo/(1 � eo), s�1 Pa�n

Lo total pore length per unit volume, mm�3

n order of reactionp partial pressure of reacting gas (CO2), kParS reaction rate, m s�1

So initial surface area, m2 m�3

T temperature, �C and Kt time, s and min

tf time factor, rsSoð1�eoÞ, s�1

t0.9 time for fractional carbon conversion of 0.9, s and minX fractional conversion of carbon (ash free)

Greek symbolseo initial porositys dimensionless time, rsSot

ð1�eoÞs0.9 dimensionless time for fractional carbon conversion of

0.9w structural parameter, 4pLoð1�eoÞ

S2o

3404 R.C. Everson et al. / Fuel 87 (2008) 3403–3408

gases are present. The low pressure Langmuir–Hinshelwood equa-tion can however, be reduced to a power rate law [18] and hasbeen used successfully by many investigators [6,7,12,19]. Theintrinsic reactivity of a char is dependent on the surface properties,which is determined by the chemical and petrographic propertiesof the coal and the pyrolysis conditions. During the pyrolysis ofcoal at high temperatures annealing of the carbon structure occurswith an accompanying increase in reflectance and decreasing sur-face reactivity [5,20,21].

During the fast pyrolysis period when coal is gasified, structuralchanges, which include surface area and pore volume, also occurand the properties of chars formed are linked to the initial petro-graphic properties of the parent coal [22]. These structures havecharacteristic morphological features with different porosities,which can introduce limiting diffusional effects for certain reac-tions and reaction conditions. Different models incorporatingstructural effects in addition to the intrinsic kinetics have beenevaluated for predicting overall reaction rates, which include theshrinking core model (SCM), capillary/random pore models(RPM) and percolation models. The shrinking core model has astructural factor [23] depending only on the initial char properties(surface area and porosity) whereas the random pore model[24,25] accounts for surface area variations during reaction depen-dent on the initial structural properties. Percolation models ac-count for connectivity properties associated with the porebehaviour which include opening/closing of pores and disintegra-tion of the chars [26,27]. The random pore model has been usedby many investigators [6,12,13] with success and has been im-proved to account for non-uniform pore size distributions and forhigh ash discards [12].

The objective of this study was to determine the gasificationperformance of chars derived from a high ash, inertinite-rich coaldiscard with carbon dioxide in order to assess the effect on theoverall reaction rate model of the dense char structure formedand the pores/cracks formed during the pyrolysis period [28].The study consisted of the gasification of 1mm diameter char par-ticles prepared at 900 �C in a thermogravimetric analyzer in thetemperature range 850–900 �C at 87.5 kPa and 287.5 kPa, respec-tively, and the modeling of the reaction rate. These conditionsare very similar to those used in bubbling fluidized bed gasifiers.This paper accordingly presents (1) experimental results to showthe effects and trends of operating conditions, and (2) resultsdemonstrating the validity of a suitable reaction rate model,together with a new procedure developed for model evaluation.

2. Experimental

2.1. Char preparation and characterisation

The preparation procedure and properties of the char used havebeen reported in detail by Everson et al. [28]. The char was pre-

pared in an atmosphere of flowing nitrogen in a tubular reactorat a charring temperature of 900 �C for 60 min following an initialheating rate of 20 �C/min from room temperature. The char wasderived from a high ash coal sample (33.8 wt.%), rich in inertinite(59% vol) with a low vitrinite content (15% vol). The reflectance(rank) of the char is high (4.47%) and the final structure consistsof a large fraction of dense char (58%) with a low overall porosity(approximately 1%) and surface area (3.46 m2 g�1). The vitrinite-containing macerals generated fine pores and thermal stress frac-turing (‘‘passive deflagration”) occurred as result of the phase het-erogeneity of the carbon/mineral particles.

2.2. Reactivity measurements

The thermogravimetric analyzer (TGA) used was a Bergbau-Forshung GMBH7 model supplied by Deutsche Montan Technolo-gie (DMT), Germany, and can handle solid granules with diametersbetween 1mm and 5mm and sample masses up to 2 g. The TGAwas designed to operate as a differential reactor using a low gasconversion (per pass), thus ensuring that the effect of the combus-tion products on the kinetics is negligible. For this purpose, a min-iature basket is suspended from a very accurate microbalanceinside a heated tube with good temperature control. The reactorcan operate up to 10 MPa pressure and 1100 �C. Isothermal exper-iments were carried out with small sample batches in order toeliminate a packing effect and with a gas flowrate (1.80 m3/minSTP), high enough to ensure the absence of film diffusion. Thegas mixtures were prepared on-line and all experiments were ta-ken to complete conversion. The experimental procedure consistedof dropping the basket containing the char samples into a well-mixed region with flowing reactant gas at the desired temperatureand pressure. The initial mass of chars used was of the order of200 mg and the accuracy of the microbalance (during experimen-tation) was ±0.2 mg. The reactivity experiments were confined toa char prepared at 900 �C, to ensure that the reactions (<900 �C)did not include any devolatilisation.

3. Random pore model and evaluation

The random pore model initially proposed by Bhatia and Perl-mutter [24] and Gavalas [25] was found to be valid and is conse-quently summarized together with derivations and proceduresdeveloped for the evaluation. This model was chosen since the charwas characterised by the presence of fine pores and cracks [28],which can contribute to intraparticle gas penetration and subse-quent particle structural changes [24].

The overall reaction rate is [24]

dXdt¼ rsð1� XÞSo

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi1� w lnð1� XÞ

pð1� eoÞ

ð1Þ

Table 1Details of experiments used for model evaluation for gasification

Reaction variable Conditions

Temperatures (�C) 850–900Pressures (kPa) 87.5 and 287.5Gas composition mole% Carbon dioxide 20–100Coal-chars Prepared at 900 �CChar particle diameter (mm) 1

0.0

0.2

0.4

0.6

0.8

1.0

0 500 1000 1500 2000 Time,t (min)

100% mole

80% mole

60% mole

40% mole

20% mole

Con

vers

ion,

X (

-)

Fig. 1. Effect of CO2 concentration on gasification rate at 900 �C and 287.5 kPa.

R.C. Everson et al. / Fuel 87 (2008) 3403–3408 3405

w being the structural parameter characteristic of the initial charstructure and defined as

w ¼ 4pLoð1� eoÞS2

o

ð2Þ

Eq. (1) can be written in terms of the dimensionless parameter asfollows:

dXds¼ ð1� XÞ

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi1� w lnð1� XÞ

qð3Þ

With

s ¼ rsSotð1� eoÞ

ð4Þ

Relationships for the fractional carbon conversion X in terms of timet or dimensionless time s (implicit and explicit) obtained by inte-gration of the above equations are as follows:

In terms of time t

t ¼ 2ð1� eoÞrsSow

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi1� w lnð1� XÞ

q—1

� �ð5Þ

In terms of dimensionless time s

s ¼ 2w

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi1� w lnð1� XÞ

q—1

� �ð6Þ

and explicitly as

X ¼ 1� exp �s 1þ ws4

� �� �ð7Þ

Defining the time factor as

tf ¼rsSo

ð1� eoÞð8Þ

Eq. (7) in terms of time t becomes

X ¼ 1� exp �tf t 1þ wtf t4

� �� �ð9Þ

A reduced time t/tX with tX the time for a fractional conversionof X, being the upper limit for reliable experimental results, can bedefined, which is independent of the parameters appearing beforethe square root term (Eq. (5)) and only dependent on w as shown inEq. (10), where t0.9 is the time for 90% carbon conversion. Thus, allresults for a particular gasified char should be the same, which en-ables this property to be used for the determination of the struc-tural parameter

tt0:9¼

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi1� w lnð1� XÞ

p� 1ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi

1� w lnð1� 0:9Þp

� 1ð10Þ

It should be noted that

tt0:9¼ s

s0:9:

A power rate law (nth order) for the intrinsic reaction rate rs

given by Eq. (11) was used together with the Arrhenius equationfor the temperature dependence, the equation being

rs ¼ kso expð�E=RTÞpn ð11Þ

The evaluation of this model requires estimates of the structuralparameter w (Eq. (2)) and the time factor tf (Eq. (8)). The structuralparameter has been be determined by other investigators from BETresults and image analysis [11,24] and from experimental reactionrate results [11,12]. However, it has been found that with non-uni-form pore size distribution, the use of surface areas and pore vol-ume estimates from BET measurements and image analysis werenot accurate enough because of the approximations required to de-

scribe the non-uniformity of the pore sizes as well as the accuracyof the pore size estimates within the micro-pore range. Some inves-tigators [11,12] estimated this parameter from the maximum ofexperimental reaction rate curves obtained from conversion results.These estimates, however, depend upon the accuracy of the numer-ical estimation of the maxima and are limited to a very narrow car-bon conversion range. In this investigation, this problem wasovercome by fitting Eq. (10) by regression with the unknown struc-tural parameter to corresponding experimental results. Using anupper limit of 90% for the conversion, the uncertainty of the exper-imental conversion results, which are normally asymptotic near theend of the reaction, is eliminated. With a regressed value for thestructural parameter, a second regression calculation was carriedout involving Eq. (9) with real time conversion results with tf asthe unknown parameter. With tf values determined from experi-mental results, at different gas concentrations and temperaturesthe intrinsic reaction rate parameters were determined using thelogarithmic form (Eq. (12)) of Eq. (8) together with linear regressionprocedures to determine the order of reaction, activation energyand pre-exponential factor

ln tf ¼ �E=RT þ n ln pþ lnðk0soÞ ð12Þ

With

k0so ¼ ksoSo=ð1� eoÞ ð13Þ

4. Results and discussion

4.1. Experimental results

The experimental results used for the evaluation of the modeland subsequent determination of the parameters were obtainedunder reaction conditions given in Table 1. These conditions werechosen to correspond to an intermediate temperature range(<900 �C) and at atmospheric and an elevated pressure, corre-sponding to fluidised bed gasification operating conditions.

The results from the thermogravimetric analyzer, which in-cluded the residual minerals, were converted to fractional carbonconversion results on an ash free basis, based on the assumption

0.0

0.2

0.4

0.6

0.8

1.0

0 500 1000 1500 2000 2500 3000 3500

Time (min)

900ºC

875ºC

850ºC

Con

vers

ion,

X (

-)

Fig. 2. Effect of temperature on gasification rate at 287.5 kPa and 100% mole CO2.

0.0

0.2

0.4

0.6

0.8

1.0

0 1000 2000 3000 4000 5000

Time, t (min)

Con

vers

ion,

X (

-)

287.5 kPa

87.5 kPa

Fig. 3. Effect of pressure on gasification at 850 �C and 80% mole CO2.

Con

vers

ion,

X (

-)

0

0.2

0.4

0.6

0.8

1

0 0.2 0.4 0.6 0.8 1t/t 0.9

Model 80% CO2 850oC

100% CO2 863oC 80% CO2 863oC100% CO2 875oC 80% CO2 875 oC

60% CO2 875 oC 100% CO2 888oC

80% CO2 888oC 100% CO2 900oC80% CO2 900oC 60% CO2 900oC40% CO2 900oC

Fig. 4. Determination of the structural parameter at 87.5 kPa.

0

0.2

0.4

0.6

0.8

1

0 0.2 0.4 0.6 0.8 1t/t0.9

Model 100% CO2 850˚C

80% CO2 850˚C 60% CO2 850˚C

40% CO2 850˚C 20% CO2 850˚C

100% CO2 875˚C 80% CO2 875˚C60% CO2 875˚C 40% CO2 875˚C

20% CO2 875˚C 100% CO2 900˚C80% CO2 900˚C 60% CO2 900˚C

40% CO2 900˚C 20% CO2 900˚C

Con

vers

ion

(X)

Fig. 5. Determination of the structural parameter at 287.5 kPa.

3406 R.C. Everson et al. / Fuel 87 (2008) 3403–3408

that the ash decomposition was negligible [11]. Typical resultsshowing the influence of carbon dioxide concentration, tempera-ture and pressure on reactivity of the chars are shown in Figs. 1–3. These results demonstrate the relative effects of the variablesand the order of magnitude of the reaction times for the completegasification, and were also used to evaluate the model.

The effects of carbon dioxide concentration over the range 20–100% mole at 900 �C and at 287.5 kPa as shown in Fig. 1, of temper-ature over the range 850 �C to 900 �C with pure carbon dioxide at287.5 kPa as shown in Fig. 2 and of pressure with 80% mole carbondioxide at 850 �C as shown in Fig. 3, all show trends as expected[5], with very long reaction times characteristic of the slow car-bon–carbon dioxide reaction. Lu and Do [12] also reported verylong reaction times for the carbon dioxide reaction with a coaldiscard.

4.2. Modeling

The random pore model summarized in Section 3 was evaluatedagainst experimental results according to a new procedure (alsodescribed in Section 3), which consists of using plots of (1) carbonconversion versus the reduced time parameter, and (2) carbon con-version versus real time. Both these calculations involved regres-sion procedures with the structural parameter and reaction rateconstants as unknown parameters, respectively.

Carbon conversion versus reduced time results. The structuralparameter (w) was determined from the carbon conversion versusreduced time plots using Eq. (10), which involved carbon conver-

sion up to 90%. A comparison of the model results (random poremodel) and experimental results is shown in Figs. 4 and 5 at87.5 kPa and 287.5 kPa, respectively for a structural parameterw = 1.04 (±0.34). The structural parameter can be seen to be inde-pendent of pressure, gas composition and temperature as expected(Eq. (10) and corresponds to a reaction (w < 2) with negligible poregrowth and dominated by pore coalescence [24]. This result indi-cates that the initial fine pore network present in the char, whichwas established during the pyrolysis treatment [28], had experi-enced only pore growth during the pyrolysis period [24] and thatthe initial char structure before the gasification period was suchthat coalescence only occurred during the gasification. Kajitani etal. [6], Liu et al. [11], Lu and Do [12] also determined structuralparameters based on the random pore model. All these investiga-tors however, examined chars with porosities much greater thanthose used in this investigation and obtained structural parametersgreater than 2.0.

Table 2

R.C. Everson et al. / Fuel 87 (2008) 3403–3408 3407

Carbon conversion versus real time results. The intrinsic reactionrate parameters were determined using the overall reaction ran-dom pore model (Eq. (9)). With the structural parameter (w) deter-mined from the carbon conversion versus reduced time (w = 1.04),the intrinsic kinetic parameters were determined by a stepwiseregression procedure consisting of the calculation of (1) the timefactor parameter, tf, for each experiment using Eqs. (9) and (2)the calculation of the relevant intrinsic parameters, consisting ofthe order of reaction with respect to carbon dioxide partial pres-sure (n), the activation energy (E) and the pre-exponential factor(k0so), using Eq. (12). The plots for the determination of the time fac-tors (tf) are shown in Figs. 6 and 7 for experiments at 87.5 kPa and287.5 kPa at different temperatures with an 80% mole carbon diox-ide concentration. A complete set of time factors for different car-bon dioxide concentrations and different temperatures at 87.5 kPaand 287.5 kPa are given in Table 2.

The reaction order with respect to carbon dioxide partial pres-sure (n) was obtained with Eq. (12), by plotting ln(tf) as a functionof ln(p) for different temperatures and total pressures. The resultsobtained at total pressures of 87.5 and 287.5 are given in Fig. 8with fitted linear relationships. The partial pressure dependencewas obtained directly from the slope of the linear lines and the re-sults are given in Table 3. From this Table, it can be seen that theorder (average) with respect to carbon dioxide partial pressure is0.50 (±0.04). The effect of total pressure on the reaction order isseen to be insignificant over the relatively low pressure range used.The results reported in Table 3 are in agreement with the results

0.0

0.2

0.4

0.6

0.8

1.0

0 1000 2000 3000 4000 5000Time, t (min)

Con

vers

ion,

X (

-)

900ºC

875ºC

863ºC

850ºC

Model

Fig. 6. The random pore model for char gasification at 87.5 kPa and 80% mole CO2.

0.0

0.2

0.4

0.6

0.8

1.0

0 500 1000 1500 2000 2500 3000 3500

Time, t (min)

Con

vers

ion,

X (

-)

900ºC

875ºC

850ºC

Model

Fig. 7. The random pore model for char gasification at 287.5 kPa and 80% mole CO2.

obtained by Kajitani et al. [6] who reported values of 0.43–0.56,Kasaoka et al. [7] with values of 0.45–0.5 and Lu and Do [12] witha value of 0.53. These results were obtained at different pressuresand temperatures and with very different coal samples and corre-sponding chars. There is no consensus in the literature on the mag-nitude of the order and it is generally accepted that this is withinthe range 0.2–0.8 [29] . The activation energy was determined byusing Eq. (12), which consisted of plotting ln(tf) against the reci-procal of the absolute temperature (1/T), at constant partial pres-sures of carbon dioxide. These results are given in Fig. 9 for atotal pressure of 87.5 kPa only. The activation energy and thepre-exponential factor were determined by linear regression andthe results are given in Table 4. It can be seen that the average acti-vation energy is 235 (±37) kJ mole�1 (for carbon dioxide concentra-tions greater than 20% mole) and the pre-exponential factor beingbetween 8.69 (±0.9) 107 s�1 kPa�n and 1.00 (±0.10) 108 s�1 kPa�n forcarbon dioxide concentration between 40% to 100% mole. The acti-vation energies obtained compare favourably with published re-sults by Kajitani et al. [6], Kasaoka et al. [7] and Dutta et al. [19].However, it should be noted again that a meaningful comparisonof results in the literature with the results obtained in the presentstudy is very difficult, since detailed characterisation results of thechars used are not available. These kinetic parameters depend onthe properties of the char surface, which are determined by the

Values for time factor (tf) at 87.5 kPa and 287.5 kPa

Pressure 87.5 kPa Pressure 287.5 kPa

Temperature CO2 (%mole)

tf (min�1) Temperature CO2 (%mole)

tf (min�1)

850 �C 80 4.53 � 10�4 850 �C 20 5.37 � 10�4

863 �C 80 5.95 � 10�4 40 6.18 � 10�4

100 7.26 � 10�4 60 7.83 � 10�4

875 �C 20 4.20 � 10�4 80 8.79 � 10�4

40 5.09 � 10�4 100 9.59 � 10�4

60 6.57 � 10�4 875 �C 20 6.28 � 10�4

80 8.18 � 10�4 40 8.31 � 10�4

100 8.86 � 10�4 60 1.21 � 10�3

888 �C 80 9.34 � 10�4 80 1.31 � 10�3

100 1.09 � 10�3 100 1.49 � 10�3

900 �C 20 7.43 � 10�4 900 �C 20 1.30 � 10�3

40 8.72 � 10�4 40 1.83 � 10�3

60 1.10 � 10�3 60 2.17 � 10�3

80 1.38 � 10�3 80 2.75 � 10�3

100 1.84 � 10�3 100 2.97 � 10�3

-8

-7.5

-7

-6.5

-6

-5.5

2 3 4 5 6ln(p)

ln(t

f)

900˚C 287.5 kPa 900˚C 87.5 kPa 875˚C 87.5 kPa

875˚C 287.5 kPa 850˚C 87.5 kPa

Fig. 8. Determination of the order of reaction.

Table 3Order of reaction values at different temperatures and pressures

Temperature (�C) Pressure (kPa) Order of reaction, n

875 87.5 0.46900 87.5 0.54850 287.5 0.48875 287.5 0.51900 287.5 0.50

-8

-7.5

-7

-6.5

-6

-5.5

8.5 8.6 8.7 8.8 8.9 9

1/T·104 K-1

ln(t

f)

80% mole CO2

100% mole CO2

Fig. 9. Determination of intrinsic reaction parameters at 87.5 kPa.

Table 4Intrinsic reaction rate parameters at 287.5 kPa

CO2 (%mole)

Activation energy, E(kJ mol�1)

Pre-exponential factor, k0so(s�1 kPa�n)

100 247 1.00 � 108

80 234 9.39 � 107

60 223 9.05 � 107

40 237 8.69 � 107

20 192 1.03 � 108

3408 R.C. Everson et al. / Fuel 87 (2008) 3403–3408

characteristics of the parent coal, the charring conditions and thecatalytic effect of the minerals present [3].

5. Conclusions

The gasification reaction rate with carbon dioxide-nitrogenmixtures using a high ash char derived from an inertinite-rich coalwas investigated with experiments conducted with a thermogravi-metric analyzer under conditions similar to fluidised bed gasifica-tion (850–900 �C and 87.5 and 287.5 kPa).

The random pore model and the structural parameter characte-rising the pore behaviour during the gasification process wereevaluated by using a plot of carbon conversion versus a reducedtime parameter (real time/time for 90% conversion). This enabledthe estimation of the structural parameter only using all the resultsobtained at different temperatures, pressures and carbon dioxideconcentrations. A low value for the structural parameter was ob-tained (<2) which indicated that the reaction rate does not displaya maximum when plotted against carbon conversion and that porecoalescence was the main structural mechanism [24].

From calculations involving the overall random pore model itwas found that for the char prepared at 900 �C, with its very lowporosity (fine pores and cracks) and consisting of a large fractionof dense char [28], the overall reaction was chemical-reaction con-trolled, for reaction temperatures and pressures used in fluidisedbed gasification. Intrinsic reaction rate constants based on thepower law equation were also calculated and values for the reac-tion order and the activation energy are within the ranges (verywide) published in the literature. As a result of the limited informa-tion concerning char types (and minerals) used by other investiga-tors, it was not possible to deduce the effect of the uniqueproperties (high rank and high ash) of the char used in this studyon the intrinsic reaction rate constants.

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