k 2 co 3 -catalyzed co 2 gasification of ash-free coal: kinetic study

K2CO3‑Catalyzed CO2 Gasification of Ash-Free Coal: Kinetic StudyJan Kopyscinski,† Rozita Habibi,† Charles A. Mims,‡ and Josephine M. Hill*,†

†Department of Chemical and Petroleum Engineering, University of Calgary, 2500 University Drive Northwest, Calgary, Alberta T2N1N4, Canada‡Department of Chemical Engineering and Applied Chemistry, University of Toronto, 200 College Street, Toronto, Ontario M5S3E5, Canada

*S Supporting Information

ABSTRACT: The kinetics of K2CO3-catalyzed CO2 gasification of ash-free coal was investigated with a thermogravimetricanalyzer and compared to raw coal and uncatalyzed ash-free coal. At 750 °C, the gasification of ash-free coal dry mixed with 20 wt% K2CO3 was approximately 3 and 60 times faster than the raw coal and ash-free coal without catalyst, respectively. Increasingthe amount of catalyst from 20 to 45 wt % increased the gasification rate 3-fold. The gasification rate of ash-free coal containingpotassium catalyst strongly depended upon the pretreatment (i.e., heating gas atmosphere and heating time) because it directlyaffected the degree of catalyst reduction. The catalytic gasification behavior could only be predicted with the extended randompore model, whereas the random pore model and integrated model were essentially equal for fitting the gasification rate for rawand ash-free coal. The activation energy for the catalyzed ash-free coal gasification was approximately 100 kJ mol−1 larger than forraw coal and the uncatalyzed ash-free coal. This increase might be due to the energy required for the potassium (i.e., catalyst)transfer to a new carbon site or caused by the pyrolysis process, because the formed char might have different properties.

1. INTRODUCTIONSince early 2000, research on catalytic gasification has againbecome prominent especially for coal,1−3 petroleum coke,4−7

biomass, and their mixtures8 for the production of hydrogen,methane, and/or synthesis gas. With the oil crisis in the 1970sand 1980s, much work has been done on catalytic coalgasification.9 A few pilot plants were constructed during thistime, but no commercial catalytic gasifier was ever build.10 Themain reasons might be of political−economic nature as the oilcrisis ended, but technical issues, such as catalyst deactivation,might also have played a role.Alkali (e.g., potassium and sodium) and alkali earth (e.g.,

magnesium and calcium) metals, nickel, iron, and other metalshave been used as catalysts to promote the gasification reactionof coal and other carbon sources.9 Today, special attention hasbeen given to co-feeding biomass species, such as switchgrass,which are rich in alkali and alkaline earth metals. Here,potassium naturally present in the switchgrass ash catalyzes thegasification of coal and/or petcoke.11 However, these catalysts,especially potassium and calcium, deactivate during the processas these components react with alumina- and silica-containingmineral matter from the coal ash to form stable potassium orcalcium aluminosilicates.11−13 Thus, when the ash content (<1wt % dry basis) of the coal is reduced prior to gasification, thisdeactivation can be reduced or even avoided.3,14 The activecatalyst would stay in the gasifier, while new “beneficiated coal”would be fed to the reactor. By doing so, the amount of catalystneeded could be reduced significantly. Several research projectsin Japan (hyper-coal),15 Australia (ultra-clean coal),16 andCanada [ash-free coal (AFC)]14,17,18 are underway to study theproduction of the beneficiated coal and its combustion andgasification behavior.During catalytic gasification, the catalyst undergoes an

oxygen transfer cycle, in which the catalyst is reduced and

oxidized.9,19,20 The catalyst, potassium in the present case, takesoxygen from the reaction gas (in this case, CO2) (eq 1) andtransfers it to the surface where oxygen reacts with carbon toform carbon monoxide (eq 2).

+ ↔ − +KC CO KC O CO(site) 2 (1)

− → +KC O K(s) CO (2)

+ →K(s) C KC(site) (3)

KC represents a generalized site with proper potassium−carboncontact, such as a −COK complex, with an unknownstoichiometry. The third step (eq 3) symbolizes siteregeneration, which requires a certain potassium mobility,designated here non-specifically as K(s). Despite numerousstudies in this area, the interaction between the catalyst andcarbon and the type of reactive surface intermediate are stilldebatable.20 Phenoxide type21 and K-oxide clusters22 are thetwo favored surface intermediates. Moulijn and Kapteijn22 andFreund23,24 suggested that the catalyst accelerates the gas-ification reaction by increasing the number of surface oxygenand active sites at the carbon surface without changing thekinetic network and the activation energy dramatically.The studies dealing with hyper- and ultra-clean coals focused

on gasification behavior only. No kinetic data for the catalyzedgasification of these coals have been published. Therefore, inthis study, we determined the CO2 gasification kinetics of ash-free coal with and without potassium catalyst and compared itto the corresponding raw coal. Besides the influence of thetemperature and catalyst loading, the influences of the heating

Received: March 28, 2013Revised: July 2, 2013Published: July 3, 2013

Article

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© 2013 American Chemical Society 4875 dx.doi.org/10.1021/ef400552q | Energy Fuels 2013, 27, 4875−4883

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protocol on the char conversion and kinetics were examined bythermogravimetric experiments and comprehensive modeling.Especially for catalytic gasification, the heating protocol (i.e.,heating gas and heating time prior to gasification) is importantas our previous study showed.14 The parameter estimation andmodel discrimination are based on a nonlinear least-squaresmethod and Akaike information criteria,25 respectively. Theresults of this investigation and estimated kinetics parametersmight serve as a baseline for further steam gasificationexperiments and help to improve the catalytic gasificationprocess of ash-free coals.

2. MATERIALS AND METHODS2.1. Experimental Section. Canadian sub-bituminous coal from

the Genesee coal mine (Alberta) was chosen for the experiments. Ash-free coal (GEN-AF) and ash-free coal mixed with K2CO3 were used inthis study. The GEN-AF samples were produced by solvent extractionas previously described.14 For comparison, the parent Genesee coal(GEN-raw) with a similar particle size to the ash-free coal (<90 μm)was used as well. The proximate and ultimate analyses of the GEN-rawand GEN-AF samples are summarized in Table 1. The GEN-raw had∼30 wt % ash, whereas less than 700 ppmw ash was measured byinductively coupled plasma mass spectrometry (ICP−MS) for GEN-AF. The GEN-AF sample had higher volatile and carbon contents andlower sulfur and oxygen contents compared to GEN-raw. More detailsabout GEN-AF are given in our previous study.14

The catalyst potassium carbonate anhydrous (K2CO3, FisherScientific, >99.0%, dP < 90 μm) was added to the GEN-AF sampleson a dry basis by solid mixing in a mortar for approximately 10 min atroom temperature, including 1 h of wait time to absorb moisture.Samples with 20, 33, and 45 wt % K2CO3 were produced and named,for example, as GEN-AF + 20 wt % (80 wt % ash-free Genesee coal +20 wt % K2CO3). The wet impregnation method could not be usedbecause the solvent-extracted ash-free coal samples were hydrophobic.Because the solids (catalyst and coal) were dry-mixed, a perfecthomogeneous mixture is not possible but does represent the realindustrial process better.The CO2 gasification experiments were carried out in a

thermogravimetric analyzer (TGA, Thermo Scientific, TGA Thermax500). The setup is described elsewhere.14 Typically, 10 mg of samplewas placed in the reactor and heated at a rate of 15 °C min−1 to thedesired temperature (i.e., 650−950 °C) under a N2 (300 mLN min−1,Praxair, 99.999%) atmosphere, while the mass change was monitored.After a further holding time in N2 at the isothermal temperature, thegas was switched to CO2 (300 mLN min−1, Praxair, 99.99%). At thispoint, the gasification time was defined as t = 0. To study the influenceof the heating protocol on the catalyzed gasification, the holding time(time at the isothermal temperature before CO2 is introduced) wasreduced to 10 and 0 min. In another experiment, the sample washeated with CO2, and in this experiment, the initial gasification time (t= 0) was defined when the sample reached the desired isothermaltemperature (e.g., 700 °C). Besides the heating and reaction gas, N2was introduced to purge the microbalance (330 mLN min−1) and topurge the furnace of the TGA setup (300 mLN min−1). The GEN-rawsamples were charred prior to the gasification experiments in a fixed-bed reactor. Here, the charring temperature was the same as thegasification temperature in the TGA setup, which produced charmorphology comparable to that in a fixed-bed gasifier. All gasificationexperiments were conducted with undiluted CO2 as the gasification

agent. For this study, neither the CO2 partial pressure was changed norwas CO, which has a strong inhibition effect,23 added.

Prior to calculating the char conversion and gasification rates, themeasured data (i.e., mass as a function of time) were smoothed toreduce the quantity of the data. During the experiment, the TGAsoftware recorded the mass every 2 s, which resulted in up to 150 000data points over a run. The locally weighted scatterplot smoothing(LOWESS)26 function was applied to smooth and reduce the numberof data points to 100. The char conversion was then defined as

=−

−X

m mm m

t0

0 end (4)

where m0 is the initial char mass at gasification time t = 0, mt is themass at gasification time t, and mend is the mass of the sample aftercomplete conversion. For K2CO3-catalyzed gasification, the mend wasup to 10 wt %relative smaller than the target catalyst loading, indicatingthat, after complete conversion, not all of the catalyst was oxidized toK2CO3 and/or some of the catalyst had evaporated. Because of thesmall sample weight used and the solid dry mixing process (i.e., non-homogeneous catalyst distribution), the results (conversion versustime) of repeated experiments for the same target loading variedwithin 10%. The variation between the same repeated experiments,however, was very small compared to the change in conversion fordifferent experimental conditions (i.e., temperature and catalystloading).

2.2. Kinetic Modeling. In the present experiments, the measuredmass change in a given time interval was differential compared to thetotal mass (i.e., 0.001−0.05 mg min−1 versus 10 mg) and CO2 was fedin excess (300 mLN min−1). Thus, the CO2 partial pressure did notchange significantly during the gasification experiment, and theproduced CO was on the order of a few parts per million by volume.For the current study, the inhibition effect of CO was neglected andthe reaction order with respect to CO2 was assumed to be first-order.On the basis of these assumptions, the equations for the volumetric,shrinking, integrated, random pore, and extended random pore modelscould be written as shown in eqs 5−8, respectively. The volumetricmodel (VM) assumes that the reaction takes place uniformlythroughout the whole volume of the particle. Thus, the gasificationrate is written as

= −Xt

k Xdd

(1 )j (5)

In contrast to the VM, the shrinking model considers that the reactionis taking place on the external surface of the particle.

= −Xt

k Xdd

(1 )jn

(6)

Assuming a sphere, the exponent n in eq 6 has the value of 2/3, whichis considered the shrinking particle model (SM). If the exponent isestimated, the corresponding model is referred to as the integratedmodel (IM).

On the basis of overlapping cylindrical pores, Bhatia andPerlmutter27 developed the random pore model (RPM) for a gas−solid reaction, including a structural parameter ψ, as shown in eq 7.

= − − Ψ −Xt

k X Xdd

(1 ) 1 ln(1 )j (7)

Zhang et al.28 modified the RPM by introducing empirical parametersc and p to better fit the experimental data obtained by catalytic

Table 1. Properties of Genesee Coal Samples (Data from Kopyscinski et al.14)a

proximate analysis (wt %, db) elemental analysis (wt %, daf)

VM FC ash C H N S Ob

GEN-raw 31.5 38.3 30.5 73.1 4.3 1.0 0.4 21.2GEN-AF 69.5 30.5 682 ppmc 87.2 5.3 3.4 0.1 4.3

aVM, volatile matter; FC, fixed carbon; db, dry basis; and daf, dry and ash-free. bOxygen content by difference. cDetermined by ICP−MS.

Energy & Fuels Article

dx.doi.org/10.1021/ef400552q | Energy Fuels 2013, 27, 4875−48834876

gasification of coal char and carbon by steam and CO2. The so-calledextended random pore model (eRPM) is shown in eq 8.

= − − Ψ − + −Xt

k X X c Xdd

(1 ) 1 ln(1 ) (1 (1 ) )jp

(8)

The parameters to be estimated were the pre-exponential factor,activation energy, exponent, structural factor, and two semi-empiricalparameters for the eRPM. The temperature dependency of thereaction constant kj can be described by the Arrhenius equation, asshown in eq 9. In the present study, the modified form based on afinite reference temperature (Tref = 1023 K for GEN-raw, 1073 K forGEN-AF, and 973 K for GEN-AF + 20 wt % K2CO3) was used toavoid large magnitudes of the pre-exponential factor because of theinfinite temperature. The reference temperature should be in thetemperature range of the conducted gasification experiments.

θ θ= −⎜ ⎟⎧⎨⎩

⎛⎝

⎞⎠⎫⎬⎭k

TT

exp{ }exp 1j k Eref

a (9)

The pre-exponential factor was determined as θk = ln(kTref) to ensure

non-negative values according to the chemical theory. Activationenergies was estimated by introducing dimensionless energy parameterθEa = Ea/(RTref). The use of these transformed parameters allows for abetter estimation. The number of parameters to be estimated for theVM, SM, IM, RPM, and eRPM were 2, 2, 3, 3, and 5, respectively. Atotal of 100 data points (i.e., dX/dt versus X) per temperature wereused. In the present study, all data points at all temperatures (i.e., 400)were evaluated simultaneously. Thus, the degrees of freedom (dof =experimental data points − number of parameters) for the VM, SM,IM, RPM, and eRPM were 398, 398, 397, 397, and 395, respectively.Moreover, it was assumed that the parameters n, ψ, c, and p wereconstant at all temperatures.The software package Athena Visual Studio, version 14.2, was used

for parameter estimation and model discrimination.29 Because themeasured mass change and, thus, the calculated gasification rate wasthe only response, the nonlinear least-squares method was applied toestimate the kinetic parameters of each model. The models describedabove were fit to the observed gasification rate (dX/dt) as a function ofchar conversion (X) by minimizing the residual sum of squares (RSS)assuming an experimental error of 10%. When the dX/dt values werecalculated, eqs 5−8 could be treated as algebraic equations, whichsimplified the parameter estimation. Thus, the differential equationsdid not need to be integrated. Fitting the data to the observedgasification rate versus char conversion with a constant ΔX intervalallows for a better fit, because the observed data points are equallydistributed with the char conversion. By doing so, the over-representation of data points in the high conversion range (>80%)can be avoided, which is common if a constant Δt interval is used.2.3. Model Discrimination. The model discrimination was based

on the Akaike information criterion (AIC),25 as shown in eq 10, withthe assumption of normally distributed errors

= + { }mn n

AIC2

ln1

RSS(10)

where m is the number of estimated parameters and n is the number ofobservations. The number of observations was 100 per temperature.The model with the lowest AIC number is the preferred model. Thedifferent models can be compared by calculating the Akaike probabilityshare (πAIC), which is given by

π =∑ =

L

Lk

ik

kAIC

1 (11)

where Lk is the relative likelihood of model k that is defined as

=−{ }L exp

AIC AIC2k

kmin

(12)

In the present paper, the RSS and R2 values were also calculated.

3. RESULTS AND DISCUSSION3.1. Experimental Results. 3.1.1. Influence of the

Gasification Temperature. The gasification temperature wasvaried between 700 and 950 °C for the GEN-raw, between 750and 900 °C for GEN-AF, and between 650 and 750 °C for theGEN-AF + 20 wt % K2CO3 samples. The reason for thedifferent temperatures applied was the different gasificationreactivities, as shown below. The gasification temperature forthe ash-free coal mixed with K2CO3 was less than 800 °C toavoid potassium evaporation.14 A typical mass loss profile wascomprised of three stages, as depicted in Figure 1 for GEN-AF

+ 20 wt % K2CO3. In stage A, the sample was heated at 15 °Cmin−1 to the desired temperature (e.g., 725 °C) in N2. Theweight decrease in this section was attributed to thedevolatilization (i.e., pyrolysis) process. In the next stage (B),the sample was kept at this temperature for 150 min in a N2atmosphere. Here, the mass loss was attributed to the release ofCO because of the reduction of the catalyst to form an activesurface intermediate as discussed in our previous study.14 ForGEN-AF samples without catalyst, the mass was constant instage B (not shown). The last stage (C) was the gasification ofthe char with CO2.Figure 2 depicts the char conversion for (a) GEN-raw, (b)

GEN-AF, and (c) GEN-AF + 20 wt % K2CO3 as a function ofthe gasification time for different temperatures. The symbolsrepresent the experimental results, and the lines represent thebest fit model, as discussed in section 3.2. As expected, thehigher the gasification temperature, the faster the charconversion for all samples. However, the gasification behaviorof the three samples differed significantly. For example, GEN-raw gasified at 850 °C required ∼8 h for complete conversion,while GEN-AF was only 60% converted after the same time(panels a and b of Figure 2). A 60% conversion was reachedafter approximately 2 h for the GEN-raw gasified at 850 °C.Ash-free coal exhibited very low gasification reactivity between750 and 900 °C. Adding potassium to the ash-free coal sampleimproved and accelerated the gasification significantly (Figure2c). GEN-AF + 20 wt % K2CO3 gasified at 750 °C completelyafter 8 h, which was the same time needed for the GEN-rawsample at 850 °C. Thus, a temperature decrease of 100 °C canbe achieved by removing the ash and adding a potassiumcatalyst.The reactivity index, i.e., inverse time to reach 5% (1/t5) and

50% (1/t50) char conversion, has been calculated to comparethe gasification rates at 750 °C for the three samples (Table 2).

Figure 1. Weight decrease profile of GEN-AF + 20 wt % K2CO3during heating with 15 °C min−1 to 725 °C in N2 (A), kept for 150min at 700 °C in N2 (B), and gasifying with CO2 (C).



Normalized to the GEN-AF sample, GEN-raw and GEN-AF +20 wt % K2CO3 had a 20 and 70 times faster initial gasificationrate (i.e., 1/t5), respectively. To reach 50% char conversion, thenormalized reactivity index (1/t50) for GEN-raw and GEN-AF+ 20 wt % K2CO3 decreased slightly but were still 16 and 64times higher compared to GEN-AF. The reason for the slowgasification of the GEN-AF sample was the negligible amountof ash minerals (e.g., Ca, Fe, and Na) that could accelerate thereaction. The ash of the GEN-raw sample, on the other hand,contained the following catalytically active components: 4.2 wt% Ca, 2.0 wt % Fe, 1.9 wt % Na, 0.8 wt % Mg, and 0.6 wt %K.14 On the basis of the experimental results, the gasification ofGEN-raw can be considered a catalyzed process.

3.1.2. Influence of the Potassium Concentration. Toinvestigate the influence of the potassium concentration onthe char conversion, all experiments were carried out at 700 °Cwith a holding time of 150 min prior to CO2 gasification toensure that the potassium catalyst was sufficiently reduced (i.e.,activated).14 The results showed that increasing the amount ofK2CO3 from 20 to 45 wt % enhanced the CO2 gasification(Figure 3; symbols represent observed data). The main

difference can be observed in the first 2 h with char conversionsof 34, 51, and 74% for catalyst loadings of 20, 33, and 45 wt %,respectively. After 2 h, the slope of catalyzed char conversiondecreased. The slower gasification rates might be explained bycollapsing the pore structure and/or the slow transfer ofpotassium to new carbon sites (eq 3), which required a certaindegree of mobility. On the basis of the amount of K2CO3 mixedwith the ash-free coal and the carbon content in the char, theinitial K/C ratios were 0.12, 0.25, and 0.43 for the GEN-AFsamples loaded with 20, 33, and 45 wt % K2CO3, respectively.

3.1.3. Influence of the Heating Protocol. The heatingprotocol influenced the char conversion for ash-free coal mixedwith K2CO3, as shown for GEN-AF + 45 wt % K2CO3 in Figure4 (symbols represent observed data). Here, sample (a) washeated with 15 °C min−1 in N2 to 700 °C, and after a further

Figure 2. Influence of the CO2 gasification temperature on the charconversion of (a) GEN-raw, (b) GEN-AF, and (c) GEN-AF + 20 wt %K2CO3. Symbols represent observed data, and lines represent the bestfit model (eRPM for GEN-raw and GEN-AF + 20 wt % K2CO3 andRPM for GEN-AF).

Table 2. Reactivity Index Based on the Time to Reach 5%(1/t5) and 50% (1/t50) Char Conversion at 750 °C for GEN-raw, GEN-AF, and GEN-AF + 20 wt % K2CO3

sample 1/t5 (min−1) rN

a 1/t50 (min−1) rNa

GEN-AF + 20 wt % K2CO3 2.7 × 10−1 70 2.3 × 10−2 64GEN-raw 7.7 × 10−2 20 5.9 × 10−3 17GEN-AF 3.8 × 10−3 1 3.6 × 10−4 1

aNormalized reactivity to that of GEN-AF.

Figure 3. Influence of K2CO3 loading on the CO2 gasification behaviorof GEN-AF at 700 °C. Symbols represent observed data, and linesrepresent the best fit model (eRPM).

Figure 4. Influence of the heating protocol of the CO2 gasificationbehavior of GEN-AF + 45 wt % K2CO3 at 700 °C: (a) 150 minholding time, (b) 10 min holding time, (c) 0 min holding time in N2 at700 °C before switched to CO2, and (d) CO2 heating. Symbolsrepresent observed data, and lines represent the best fit model(eRPM).



holding time of 150 min, the gas was switched to CO2. Samples(b) and (c) were heated with the same heating rate to 700 °C,but the holding time in N2 (time prior to CO2) was reduced to10 and 0 min, respectively. A 10 min holding time was chosenbecause the devolatilization of the ash-free coal at 700 °C wasfinished and the mass of the uncatalyzed sample did not changesignificantly after this time.14 Sample (d) was heated with CO2to 700 °C (15 °C min−1). The gasification time of zero wasdefined when the gas was switched to CO2 (cases a−c) or whenthe temperature of 700 °C was reached (case d). The samplewith the longest holding time (curve a in Figure 4) showed thefastest char conversion, while the sample heated in CO2 (curved) showed the slowest char conversion. The initial slopes of thechar conversion for samples (c) and (d) were comparable tosample (a). This result can be explained by an overlapping ofthe devolatilization step with the gasification reaction in the first10 min. Thereafter, the slope decreased, and the charconversion after 2 h differed significantly; sample (a) had aconversion of ∼75%, whereas samples (b), (c), and (d) hadchar conversions of only ∼53, ∼39, and ∼22%, respectively.After 5 h, the char conversion for samples (a−c) were >80%.The results indicate that a longer holding time in a N2atmosphere at a operating temperature prior to the gasificationlead to a higher degree of catalyst reduction and, thus, to afaster char gasification, as we have seen previously.14 Moreover,CO2 inhibited the catalyst reduction. At lower K2CO3 loadings,the effect of the heating protocol was less pronounced but thephenomenon was still observed (not shown).3.2. Modeling Results. 3.2.1. Influence of the Gas-

ification Temperature. For the present kinetic study, it wasassumed that the parameters n, ψ, c, and p from eqs 6−8 wereindependent of the temperature. In addition, all data at alltemperatures were fit simultaneously, and the pre-exponentialfactor follows the Arrhenius equation, as described above.The kinetic parameters with the 95% confidence interval for

the GEN-raw, GEN-AF, and GEN-AF + 20 wt % K2CO3experiments are reported in Table 3. In addition to the best fitvalues, the normalized parameter covariance matrix for eachmodel considered is shown in Tables S1−S3 of the SupportingInformation. The covariance matrix in kinetic studies is seldompublished, but it is an important criterion to evaluate the quality

of the parameters and the model. The elements in thecovariance matrix are limited to the interval (−1, 1), where avalue of 1 indicates a strong correlation between twoparameters and a value of −1 indicates a strong anti-correlation.VM, SM, IM, and RPM showed a very high anti-correlationbetween the pre-exponential factor (kTref

) and the activationenergy (Ea), whereas eRPM did not (see Tables S1−S3 of theSupporting Information). High anti-correlation values can beexplained by the fact that VM and SM are models with onlytwo parameters (i.e., Ea and kTref

), which are connected via anexponential function.The structural parameter ψ for RPM could not be estimated,

because its value always approached zero (lower boundary)during the parameter estimation. Thus, the second term in theRPM equation (eq 7) was always 1, collapsing this model to theVM. That is, the parameters for the RPM were the same as forthe VM (Table 3). All estimated parameters had a tight fitbecause the confidence interval was very small (within ±10%)compared to the best value. The activation energies estimatedwith the VM, RPM, and eRPM were essentially the same (131kJ mol−1), whereas the SM estimated a higher activation energy(195 kJ mol−1) and the IM estimated a lower activation energy(116 kJ mol−1). Table 4 summarizes the model discrimination

criteria and ranks all five considered models. In most kineticstudies, only R2 values are published. These values were all veryhigh (>0.95) for most of the investigated models. The eRPMfollowed by the IM were the best model fits according to theirlowest AIC values and highest relative likelihood andprobability shares, πAIC. That is, the eRPM had a likelihoodof 100%, whereas the IM had a likelihood of ∼78% and the VM

Table 3. Estimated Kinetic Parameters for All Considered Models for GEN-raw, GEN-AF, and GEN-AF + 20 wt % K2CO3a

model parameter GEN-raw GEN-AF GEN-AF + 20 wt % K2CO3

VM k1,Tref3.6 × 10−3 ± 2 × 10−4 6.5 × 10−4 ± 2 × 10−5 3.3 × 10−3 ± 3 × 10−4

Ea 131 ± 2 124 ± 3 265 ± 16SM k1,Tref

1.8 × 10−4 ± 3 × 10−5 1.4 × 10−5 ± 6 × 10−7 1.5 × 10−4 ± 3 × 10−5

Ea 195 ± 9 185 ± 5 396 ± 33IM k1,Tref

7.3 × 10−3 ± 6 × 10−4 1.3 × 10−4 ± 3 × 10−5 2 × 10−2 ± 4 × 10−3

Ea 116 ± 2 149 ± 6 185 ± 13n 1.14 ± 0.02 0.83 ± 0.03 1.4 ± 0.1

RPM k1,Tref3.6 × 10−3 ± 2 × 10−4 6.1 × 10−4 ± 2 × 10−5 3.3 × 10−3 ± 3 × 10−4

Ea 131 ± 2 124 ± 3 265 ± 16ψ 0 (lower bound) 0.45 ± 0.1 0 (lower bound)

eRPM k1,Tref7.5 × 10−4 ± 5 × 10−5 1.1 × 10−4 ± 1 × 10−5

Ea 131 ± 1 264 ± 4ψ 4.3 ± 0.6 64 ± 11c 3.5 ± 0.3 16 ± 2p 1.52 ± 0.14 2.7 ± 0.1

aVM, volumetric model; SM, shrinking model; IM, integrated model; RPM, random pore model; and eRPM, extended random pore model (only forGEN-raw and GEN-AF + 20 wt % K2CO3). Tref = 1023 K (GEN-raw), 1073 K (GEN-AF), and 973 K (GEN-AF + 20 wt % K2CO3).

Table 4. Model Discrimination Results for GEN-raw (700−950 °C), with Tref = 1023 K

model rank AIC Lk πAIC RSS R2

eRPM 1 −14.79 1.000 0.31 1.46 × 10−4 0.9977IM 2 −14.29 0.779 0.24 2.43 × 10−4 0.9962VM 3 −13.73 0.589 0.19 4.30 × 10−4 0.9933RPM 3 −13.73 0.589 0.19 4.30 × 10−4 0.9933SM 5 −11.79 0.223 0.07 2.99 × 10−3 0.9531



and RPM had likelihoods of only ∼60%. Thus, the probabilityof the IM is approximately 3/4 compared to the eRPM. Figure2a compares the observed and calculated char conversion forGEN-raw samples gasified from 700 to 950 °C. The agreementbetween the observed values and the eRPM calculated data isvery good. The IM and VM estimated the gasification ratesalmost as well as the eRPM, but the SM failed to predict thegasification, as shown in Figure S1 of the SupportingInformation.As mentioned above, the gasification rate of ash-free coal

(GEN-AF) was very slow. Thus, only the VM, SM, IM, andRPM were applied for the parameter estimation. The parameterresults, covariance matrix, and model discrimination criteria aresummarized in Table 3, Table S2 of the SupportingInformation, and Table 5, respectively. The latter showed

that all four models were statistically equal; the Akaikeinformation criteria marginally favored the RPM. Activationenergies of 124 and 131 kJ mol−1 were estimated by the RPMand VM, respectively, while the SM and IM estimated highervalues of 185 and 149 kJ mol−1, respectively. In comparison tothe GEN-raw, the R2 values for the GEN-AF model fits wereslightly lower (see Tables 4 and 5), which is also illustrated in

panels a and b of Figure 2. In detail, at 750 and 800 °C, themodel predicted a higher char conversion. Figure S2 of theSupporting Information depicts the observed and modeledgasification rates as a function of the char conversion. Especiallyat 850 and 900 °C, the rates were in good agreement over thewhole conversion range. Because the gasification rate of GEN-AF monotonically decreased with char conversion without aclear maximum, all four models were very close to each other.In contrast, the ash-free coal mixed with potassium catalyst

(GEN-AF + 20 wt % K2CO3) exhibited a clear maximum in therate at ∼15−20% conversion (Figure 5), which is common forcatalytic gasification. The VM, SM, and IM did not capture thisgasification behavior, as shown by the model discriminationcriteria summarized in Table 6. The eRPM has the lowest AIC

values and the highest probability share πAIC. For the GEN-rawand GEN-AF, the probability share of the eRPM and IM weremuch closer to each other (see Tables 4 and 5). For the GEN-AF + 20 wt % K2CO3, the difference between these two modelsincreased significantly, consistent with the eRPM fitting thedata better than the IM. More precisely, the eRPM had a 3times higher probability share than the other models (see Table

Table 5. Model Discrimination Results for GEN-AF (750−900 °C), with Tref = 1073 K


RPM 1 −18.81 1.000 0.272 2.69 × 10−6 0.978IM 2 −18.81 0.995 0.270 2.71 × 10−6 0.978VM 3 −18.49 0.852 0.231 3.71 × 10−6 0.970SM 4 −18.45 0.835 0.227 3.85 × 10−6 0.969

Figure 5. Observed (symbols) and calculated (lines) gasification rate as a function of char conversion for GEN-AF + 20 wt % K2CO3 gasified at (a)650 °C, (b) 700 °C, (c) 725 °C, and (d) 750 °C. VM, volumetric model; SM, shrinking model; IM, integrated model; and eRPM, extended randompore model. Note the different scale for the gasification rates.

Table 6. Model Discrimination Results for GEN-AF + 20 wt% K2CO3 (650−750 °C), with Tref = 973 K


eRPM 1 −15.84 1.000 0.47 5.12 × 10−5 0.9931IM 2 −13.69 0.341 0.16 4.43 × 10−4 0.9404VM 3 −13.32 0.284 0.13 6.45 × 10−4 0.9131RPM 3 −13.32 0.284 0.13 6.45 × 10−4 0.9131SM 5 −12.70 0.208 0.10 1.20 × 10−3 0.8385



6). The distinct maximum in the gasification rate could only bemodeled with the eRPM. If the char conversion is plotted as afunction of the gasification time (Figure 2c and Figure S3 of theSupporting Information), the distinction between the differentmodels especially up to 50% is difficult. Only above 50%conversion does the eRPM appear to fit the data the best. Thus,Figure 5, in which the rate as a function of char conversion isshown, better distinguishes the gasification behavior of thecatalyzed sample.For the catalyzed sample, an activation energy of 264 kJ

mol−1 was determined for the eRPM (Table 3 and Table S3 ofthe Supporting Information). The estimated activation energiesfor GEN-raw and GEN-AF were much lower (131 and 124 kJmol−1, respectively). A reason for the difference could bepotassium mobility, which is most likely higher at 750 °C thanat 650 °C. Thus, the observed activation energy might be thesum of the intrinsic activation energy plus the energy forpotassium transfer (Eobs = Ea + Etrans,K). The theory that theobserved activation energy is influenced by the mobility of thepotassium surface complex needs further study and is currentlyunder investigation.The activation energies have also been calculated by means

of the reactivity index (1/t50) data. The resulting Arrhenius plot(Figure 6) shows that the rates for the three samples increased

linearly with the inverse temperature, indicating no change inthe reaction regime. Thus, even at high temperatures, thegasification was reaction-controlled and not limited by masstransfer. The corresponding activation energies were 135, 155,and 241 kJ mol−1 for GEN-raw, GEN-AF, and GEN-AF + 20wt % K2CO3, respectively, which were similar to the activationenergies estimated with the best fit model.To our knowledge, no activation energies for catalyzed and

uncatalyzed CO2 gasification of ash-free coals have beenreported. For K2CO3-catalyzed gasification of activated carbonwith a low ash content (5 wt %), a similar high activationenergy of 244 kJ mol−1 was reported.30 However, thecorresponding activation energy for the uncatalyzed samplewas slightly higher (255 kJ mol−1). Huhn et al.31 reportedactivation energies of 160 and 140 kJ mol−1 for coal char andK2CO3-impregnated coal char gasified with CO2, respectively.Schumacher et al.32 observed an activation energy of 145 kJmol−1 for the CO2 gasification of coal char, which is similar to

our value of 131 kJ mol−1 (RPM in Table 3). When K2CO3 wasadded to the coal, Schumacher et al. determined a slightlyhigher activation energy of 164 kJ mol−1; however, noexplanation for the increase was given. The reported activationenergies of the catalyzed and uncatalyzed reactions varied onlyslightly in these studies, which is in agreement with thestatement that the catalyst does not change the kinetic networkfundamentally.22 The main difference with our study is that wedid not impregnate K2CO3; we physically mixed K2CO3 withdry ash-free coal. Thus, the dispersion and potassium−carboncontact might not be as good as those for impregnated samples.Freund25 also dry-mixed K2CO3 with a model carbon [i.e.,Spherocarb with low ash and volatile matter contents but highBrunauer−Emmett−Teller (BET) surface area of 950 m2 g−1]and obtained an activation energy of 242 kJ mol−1 for CO2gasification carried out between 600 and 800 °C in a TGA.However, coal char, activated carbon, and Spherocarb aredifferent carbon materials compared to our ash-free coal (i.e.,volatile matter of ∼70 wt % and a CO2 surface area of ∼10 m2

g−1).14 The pyrolysis process of the ash-free coal with andwithout catalyst differed as shown by Kopyscinski et al.14 Thus,the produced chars have different properties and gasificationbehavior.An increasing activation energy in the presence of a catalyst

does not seem logical. However, we have to keep in mind thatcatalytic gasification with a solid−solid contact is differentcompared to a heterogeneous catalytic process (i.e., gas−solidcontact). In the latter, the catalyst is not mobile, often bound toa support, and directly promotes the reaction between theadsorbed gas species, leading to a decrease in the activationenergy. In the gasification process, however, the solid carbonmust react with oxygen and/or hydrogen from the gas phase,which then results in the destruction of the solid matrix. Thus,the catalyst must (a) be able to promote the oxygen/hydrogentransfer from the gas phase onto the solid and (b) be mobileand move to a new carbon site.

3.2.2. Influence of the Potassium Concentration. Theinfluence of the potassium concentration was investigated at agasification temperature of 700 °C, as shown in section 3.1.2.For the kinetic parameter estimation, only the eRPM with theassumption of a constant parameter ψ (i.e., independent fromthe catalyst loading) was applied and the parameters c and pwere determined. The value of the structure parameter was ψ =63.8, estimated in the previous section. The physicalinterpretation of this parameter indicates that, during thegasification, a maximum surface area exists. The larger the ψvalue, the higher the ratio between initial and maximum surfaceareas.27 The increasing surface area might be promoted by theaccelerated potassium-catalyzed gasification. Figure 3 andFigure S4 of the Supporting Information illustrate the goodagreement between the observed and modeled gasificationbehavior for GEN-AF with 20, 33, and 45 wt % K2CO3. Whenthe amount of K2CO3 was increased in the ash-free coal, themaximum gasification rate increased by a factor of 3 and shiftedfrom 12 to 20% conversion as well (see Figure S4 of theSupporting Information). This behavior was reflected in theparameters c and p. Parameter p predominately influences theshape of the gasification curve and the position of the maximumrate (i.e., the higher the p value, the more the maximum rateshifts to a lower conversion value). Parameter c, on the otherhand, is correlated with a value of the maximum gasificationrate. Thus, a larger c value means a higher rate. The parameter cincreased linearly, while p decreased with a log function as the

Figure 6. Arrhenius plot based on the observed t50 value (time toreach 50% char conversion) for the CO2 gasification of GEN-raw,GEN-AF, and GEN-AF + 20 wt % K2CO3.



potassium loading increased (Figure 7). Zhang et al.33 found asimilar behavior of the two parameters for the catalytic CO2

gasification of activated carbon.

3.2.3. Influence of the Heating Protocol. As mentionedabove the heating protocol significantly influenced the catalyticgasification behavior. Figure 4 shows the observed and eRPM-calculated char conversions, which seem to be in goodagreement for all four different heating protocols. However,after plotting the gasification rate as a function of charconversion for both the experimental data and the eRPM-modeled data (see Figure S5 of the Supporting Information),the results, especially in the low conversion range, are verydifferent. Only protocol (a), holding time of 150 min in N2before introducing CO2, is predicted well with the eRPM. Thebehavior of the experiment with the other protocols (b−d,shorter holding times or heating in CO2) could not be fitsufficiently by the eRPM or the other models (not shown).The high initial rate and subsequent decline of the rate for

protocols (c) and (d) can be explained by the influence of thedevolatilization, which was not completed when the gasificationstarted. The devolatilization was completed after approximately10 min at 700 °C, as mentioned earlier. At around 10−15%conversion, the gasification rates of samples (b−d) increased asthe catalyst is reduced and undergoes the redox cycle.Nevertheless, the rates for sample (a) were still higher up to60% conversion, possibly because of the degree of catalystreduction. Under a N2 atmosphere and sufficient holding time,the catalyst was likely fully reduced. Decreasing the holdingtime reduced the degree of catalyst reduction and, hence, thegasification rate. Catalyst reduction was most hindered byheating in a CO2 environment.14

4. CONCLUSIONIn this work, the kinetic data for K2CO3-catalyzed CO2gasification of ash-free coal were collected at ambient pressurein a TGA. These data were compared to uncatalyzedgasification of ash-free coal (GEN-AF) and parent coal(GEN-raw). On the basis of the experimental data, the kineticparameters for each model were estimated with the nonlinearleast-squares method. In addition, the best model wasdetermined by applying the AIC.The main conclusions from this work are as follows: (1) At

750 °C, the CO2 gasification of GEN-AF + 20 wt % K2CO3 wasaround 3 and 60 times faster compared to GEN-raw and GEN-

AF samples, respectively. (2) Increasing the amount of K2CO3in the ash-free coal (from 20 to 45 wt %) increased themaximum gasification rate by a factor of 3. (3) The eRPM fitthe gasification behaviors of GEN-raw and GEN-AF + 20 wt %K2CO3 best. The RPM and IM were equally good to predictthe gasification of GEN-AF. (4) The calculated activationenergy for GEN-raw and GEN-AF had similar values (i.e., 133and 121 kJ mol−1). However, adding K2CO3 to ash-free coaldoubled the calculated activation energy (264 kJ mol−1). Thehigh activation energy might due to the energy required for thepotassium transfer or caused by the pyrolysis process, whichcreated a char with different properties. (5) The heatingprotocol, i.e., gas atmosphere and holding time prior to thegasification, influenced the gasification behavior and rate. Noneof the applied models could sufficiently fit the catalyticgasification behavior of the sample that was heated with CO2and/or had a short holding time in a N2 atmosphere as thepyrolysis and main catalyst reduction (i.e., K2CO3 to activepotassium−carbon surface intermediate) overlap with thegasification process. Catalyst reduction is inhibited under agasification atmosphere. In addition, complete reduction mightnot be possible in CO2, whereas it could be possible under N2.

■ ASSOCIATED CONTENT*S Supporting InformationEstimated kinetic parameters, normalized parameter covariancematrix, and degrees of freedom for each model and sample, asdescribed in the text (Tables S1−S3) and modeled andobserved gasification rates as a function of char conversion forall models and samples, as described in the text (Figures S1−S5). This material is available free of charge via the Internet athttp://pubs.acs.org.

■ AUTHOR INFORMATIONCorresponding Author*Telephone: +1-403-210-9488. E-mail: [email protected] authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThe authors acknowledge the financial support from CarbonManagement Canada (CMC).

■ NOMENCLATUREAIC = Akaike information criterion (see eq 10)c = empirical parameter of the eRPM (see eq 8)dof = degree of freedom (experimental data − number ofparameters)Ea = observed activation energy (kJ mol−1)f i = modeled value (i.e., rate)kj = rate constant (difference)kTref

= pre-exponential factor for rate constant kj (difference)LAIC = relative likelihood of model k (see eq 12)m = mass (kg)m = number of estimated parametersn = number of observations (data points)p = empirical parameter of the eRPM (see eq 8)R = universal gas constant (8.314 472 J mol−1 K−1)rN = normalized reactivity index (see Table 2)t = time (s or min)t5 and t50 = time to reach 5 and 50% char conversion,respectively (min)

Figure 7. Influence of K2CO3 loading on the parameters c and p fromthe eRPM. The parameters were estimated at 700 °C with a constantψ parameter (ψ = 63.87).



http://pubs.acs.org

mailto:[email protected]

T = temperature (K or °C)X = char conversionyi = experimental observations

Greek SymbolsπAIC = Akaike probability share (see eq 11)θk = parameter for the reaction constantθEa = dimensionless activation energy

AbbreviationseRPM = extended random pore modelFC = fixed carbon (see Table 1)IM = integrated modelRPM = random pore modelRSS = sum of squares of residualsSM = shrinking particle modelVM = volumetric modelVM = volatile matter (see Table 1)

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