Fuel 119 (2014) 38–46

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Experimental and modeling study of pinewood char gasificationwith CO2

0016-2361/$ - see front matter � 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.fuel.2013.11.026

⇑ Corresponding author. Tel.: +1 774 242 5489; fax: +1 765 494 0530.E-mail addresses: isircar@purdue.edu (I. Sircar), asane@purdue.edu (A. Sane),

wang1301@purdue.edu (W. Wang), gore@purdue.edu (J.P. Gore).

Indraneel Sircar a,⇑, Anup Sane a, Weichao Wang a, Jay P. Gore b

a Purdue University, School of Mechanical Engineering, Maurice J. Zucrow Laboratories, 500 Allison Road, Chaffee Hall, West Lafayette, IN 47907-2014, USAb Reilly University Chair Professor of Engineering, School of Mechanical Engineering with courtesy appointments in the School of Aeronautics and Astronautics and in the Schoolof Chemical Engineering, Purdue University, West Lafayette, IN 47907-2014, USA

h i g h l i g h t s

� Apparent rate parameters for the C–CO2 reaction are measured for a low-ash pinewood char.� Measured activation energy for the large and porous char agree with those for smaller char particles.� Surface area increases significantly with char conversion and confirms random pore model assumptions.� Effects of increasing surface area on apparent mass loss weaken with decreasing gasification temperatures.� A random pore model with W = 0–16.5 at 1000–1170 K shows best agreement with experimental data.

a r t i c l e i n f o

Article history:Received 8 April 2013Received in revised form 8 November 2013Accepted 11 November 2013Available online 25 November 2013

Keywords:BiomassGasificationRenewable energyCO2 recycling

a b s t r a c t

The objective of this study is to measure apparent rate parameters for pinewood char gasification withCO2 using large particles with relevance to practical gasifiers. The novel features of this work include:(1) char gasification for CO2 recycling, which is studied less in the past than char gasification in otherenvironments, (2) independent measurements involving gravimetric analyses and product gas composi-tion gas chromatography, (3) detailed uncertainty analyses of both methods to report resulting uncer-tainties in kinetic rate constants, and (4) investigation of the char structure development and its rolein gasification. A fixed-bed reactor providing a slip-velocity of 0.12 m/s gasifier and control of the bedtemperature to within ±10 K at 1000–1170 K is used. A low-ash (<0.01 wt.%) pinewood char is selectedto minimize the catalytic effects of ash on mass-loss rates. The char is prepared by heating pinewoodsawdust in an electrical furnace to a temperature of 1100 K. Gravimetric and product gas compositiondata are interpreted using the volumetric, non-reactive core and random pore models. The results showthat the activation energies corresponding to these models are 217 ± 6, 186 ± 13, 125 ± 30 kJ/mol respec-tively. The random pore model shows the closest agreement with the experimental data, despite theuncertainties in the measured activation energies. The estimated random pore model structure parame-ter W increases from 0 to 16.5 with increases in the gasification temperature. Measurements of BET sur-face area show significant increase with char conversion. The results of this study show that the apparentgasification rate parameters for relatively large particles of practical relevance are comparable to thoseobtained from laboratory studies with much smaller particles.

� 2013 Elsevier Ltd. All rights reserved.

1. Introduction

Gasification is an important thermo-chemical conversion pro-cess in the production of liquid fuels, chemicals and power frombiomass or coal. Gasification is the conversion of biomass or coalto a gaseous fuel mixture containing carbon-monoxide (CO) andhydrogen (H2) in the presence of mixtures of carbon-dioxide

(CO2), water vapor (H2O) and oxygen (O2). Gasification reactionswith CO2 and H2O are endothermic. In industrial gasifiers operat-ing on auto-thermal cycle, combustion of a part of the feedstockwith O2 is used for establishing a high temperature environmentto support gasification. The gasification process may produce largequantities of CO2, which lower gasifier efficiency and add to green-house-gas emissions. CO2 recirculation into biomass gasifiers is apotential CO2 control and renewable energy generation technology[1].

The biomass-CO2 gasification process is represented by threeglobal reaction steps. The first step involves heating and drying

Nomenclature

_x production rate, (kg/s)y mass fraction, (kg/kg)x mole fraction, (mol/mol)_m rate of mass flow, (kg/s)

MW molecular weight, (kg/kmol)X conversion fractionm mass, (kg)t time, (s)k apparent char mass-loss rate constant, (1/s)W structure parameter for random pore modele porosity (%)S surface area per volume (m2/m3)q density (g/cm3)Ea activation energy (kJ/mol)

Ru ideal gas constant (8.314 kJ/kmol K)A pre-exponential constant (1/s)T temperature (K)N number of data points

Subscriptv volumetric reaction modelnrc non-reactive core modelrpm random pore modelco relating to carbon-monoxideapp apparent0 initial (X = 0)

I. Sircar et al. / Fuel 119 (2014) 38–46 39

of the biomass. The second step involves pyrolysis of the dried bio-mass to release volatile organic matter (CO2, CH4, CO, H2, tar) andcreate char. The third step involves oxidation of the char by CO2 toproduce CO. Char gasification with CO2 is slow and energy inten-sive. Char gasification involves complex heat and mass transferprocesses inside a porous solid [2]. These processes are coupledwith heterogeneous chemistry [3] at the internal and external sur-faces. Rates for the heterogeneous carbon gasification reactions areaffected by inorganic content of the char [4,5]. Furthermore, porestructure of the char develops with conversion and influencesapparent gasification rates [6,7]. In practical gasifier design calcu-lations, the gasification process is simulated using mathematicalmodels [8,9] supported by empirical reaction constants [10]. Thesedesign calculations depend on models of gasification rates withreliable apparent rate parameters.

Measurement of apparent rate parameters for the global gasifi-cation reaction is the objective of many biomass gasification stud-ies. A common procedure for measuring gasification kinetics forbiomass involves: (1) drying and pyrolysis of the biomass to pre-pare char, (2) grinding of the char to a fine pulverized powder,(3) collecting mass loss data for the pulverized char as it undergoesoxidation in a prescribed gaseous environment, and (4) interpreta-tion of the mass loss data with empirical reaction models [10] toreport apparent rate parameters. Butterman and Castaldi [11] mea-sured apparent reaction rates for pinewood and other biomasswith a non-isothermal thermogravimetric analyzer (TGA) operatedup to 980 �C. Pinewood samples were obtained from core drillingwithin wooden planks. The pinewood samples were not pre-treated and used directly in gasification experiments. The authorsreported activation energy of 141 kJ/mol. Yuan et al. [12] measuredthe CO2 gasification rates for pinewood char in an isothermal TGAat 850–1050 �C. Char was prepared from sawdust in a high-heatingrate (�1000 K/s) furnace to simulate fast pyrolysis. Three charsamples were prepared at different pyrolysis temperatures of800, 1000 and 1200 �C. The char was crushed to a particle size be-tween 56 and 180 lm. Activation energies of 249, 237, and 232 kJ/mol corresponding to the char samples prepared at 800, 1000, and1200 �C respectively were reported. Seo et al. [13] measured appar-ent gasification rates for ground pinewood with particle sizes inthe range 250–300 lm in a fixed-bed reactor with product gasanalysis at temperatures between 850 and 1050 �C. The sampleswere pyrolyzed in the fixed-bed reactor during heating of the reac-tor for the gasification experiments. The authors reported activa-tion energies between 134 and 172 kJ/mol for the char CO2

gasification reaction based on multiple reaction models. Fermosoet al. [14] measured apparent gasification rates for pinewood charin an isothermal TGA operated between 800 and 950 �C. The charwas prepared by grinding the pinewood to a sieve size between

75 and 106 lm and pyrolyzing at two different temperatures of1000 �C and 1400 �C in a drop-tube furnace. The authors reportedmeasured activation energy of �184 kJ/mol for the char samplespyrolyzed at 1000 �C and �245 kJ/mol for the char samples pyro-lyzed at 1400 �C. Khalil et al. [15] measured apparent reaction ratesfor pinewood char with CO2 in a non-isothermal TGA operated to1000 �C. Pinewood was pyrolyzed in a furnace at 500 �C for dura-tion of 150 minutes to prepare the char. The char was milled andsieved to 45–63 lm. The authors reported activation energy of263 kJ/mol for the global char-CO2 reaction. Cetin et al. [16]measured apparent gasification rates for pinewood char at800–1000 �C in two different reactors; a tubular continuous flowreactor and an isothermal TGA. Pinewood was pyrolyzed in thetubular reactor at 950 �C to prepare char for both experiments.The char was ground to particle sizes between 120 and 180 lm.Activation energies of 224 kJ/mol and 238 kJ/mol were reportedfor the gasification reaction from measurements with the tubularreactor and the TGA respectively. The authors concluded that thereactor configuration did not influence the gasification reactionin the kinetic control regime, observed up to 900 �C in their studies.Reported activation energies for pinewood char-CO2 reaction fromthese studies vary between 141 kJ/mol and 263 kJ/mol. Compre-hensive reviews of biomass gasification studies by Buttermanand Castaldi [11] and Di Blasi [17] report a similarly large rangeof variation of kinetic rate parameters for the global char-CO2 reac-tion for other types of biomass. It is recognized that char prepara-tion procedures, biomass ash content, and experimental methodsaffect char mass-loss rates and may explain the variations in mea-sured kinetic rate parameters.

Fewer studies have investigated the char CO2 reaction using rel-atively large biomass char particles with relevance to non-slagginggasification processes such as fluidized-bed gasification [18–20].Ahmed and Gupta [21] analyzed product gases from the gasifica-tion of pine char in a fixed-bed apparatus at 900 �C. Char was pre-pared from pinewood sawdust by heating to 900 �C for 1 h. Twochar samples were prepared; one with regular sized wood chipsand the second with finer sized wood chips. The authors reportedsimilar conversion rate histories for the two char samples and con-cluded that the experiments were not influenced by internal massdiffusion control. Other CO2 gasification studies [22–28] usinglarge char particles derived from various biomass report inconsis-tent conclusions on the influence of internal mass diffusion to theapparent mass loss.

The objective of this study is to investigate char gasification forCO2 recycling for relatively large particles with relevance to non-slagging gasification applications. Apparent rate parameters forthe global char-CO2 reaction are reported for a char with negligibleash content. The study included: (1) a well-defined procedure for

Fig. 1. Schematic of the fixed-bed reactor arrangement and instrumentation.Diagram components: 1. pressure regulator, 2. pressure transducer, 3. on-off valve,4. thermocouple, 5. check valve, 6. mass-flow controlling orifice, 7. fixed-bedreactor, 8. filter, 9. back-pressure controller, and 10. flow-rate meter.

40 I. Sircar et al. / Fuel 119 (2014) 38–46

char preparation from a low-ash pinewood sawdust, (2) gravimet-ric analyses and product gas composition gas chromatography astwo independent methods of measuring gasification rates in a

Table 1Composition and physical properties of the pinewood sawdust and cha

Proximate analysis (air-dried at 35 �C) (wt.%)MoistureVolatile matterFixed carbonAsh

Ultimate analysis (air-dried at 35 �C) (wt.%)CarbonHydrogenNitrogenOxygenSulfur

Moisture free gross calorific value (kJ/kg)HHV

Sieve size (lm)

Cumulative particle size distribution by sieving<150<425<500<1168<1680<3350

Physical properties

True density (g/cm3)Mesopore + micropore volume (cm3/g)Total pore volume (cm3/g)Porosity (%)BET surface area (m2/)

tubular reactor, (3) detailed uncertainty analyses of the kinetic rateparameters based on the new experimental data and existingempirical reaction models, and (4) investigation of char structuredevelopment with measurements of specific surface area, pore vol-ume and porosity with char conversion. The results of this studyshow that the apparent gasification rate parameters for relativelylarge particles of practical relevance are comparable to those ob-tained from laboratory studies with much smaller particles.

2. Experimental method

2.1. Reactor Apparatus

Fig. 1 shows a schematic diagram of the tubular fixed-bed reac-tor and instrumentation. The reactor is 60 cm tall and has an innerdiameter of 7.5 cm. The topmost section contains an inlet for thereactant gases and a transducer to measure the reactor pressureduring the experiments. The central spool section houses thefixed-bed. The bed support is constructed by welding two layersof stainless steel wire meshes onto which biomass char is dis-persed. The reactant gases flow across the bed with a uniformflow-profile resulting from a honeycomb flow straightener that islocated between the gas inlet and the bed. The reactor sits concen-trically within surrounding electric radiant heaters as shown inFig. 1. Two thermocouples are inserted into the bed and providetemperature measurements. The operating conditions of the reac-tor are remotely monitored and automated. The average heating-rate of the reactor is approximately 3 K/min to minimize thermallag and allow the bed to attain a uniform temperature. Productgases exit at the bottom and are cooled and filtered before gas sam-ple collection. Gas samples are analyzed using a Shimadzu GC-8AITgas chromatograph (GC) for species composition.

2.2. Char preparation and characterization

Char is prepared by inert heating of a large quantity of low-ashpinewood sawdust in the reactor. The reactor, filled with the

r.

7.2477.3515.41<0.01

53.476.320.1740.04<0.005

20,204

Fraction within bin (wt.%)

1329426279100

Sawdust Char

0.561 1.7431.370E�3 5.321E�21.43 4.2645 880.780 218

I. Sircar et al. / Fuel 119 (2014) 38–46 41

pinewood sawdust, is heated to 1100 ± 6 K at atmospheric pres-sure with a continuous flow of N2, at rate of 0.30 ± 0.01 g/s. Oncethe reactor reaches 1100 K, the heaters are switched off and thereactor is allowed to cool. N2 flows through the reactor duringthe cooling process at 0.15 ± 0.01 g/s. The inert N2 gas is usedto avoid continued reaction of the char. At the end of the coolingcycle, the char is removed from the reactor and stored in an inertand moisture-free chamber. Energy-dispersive-spectroscopy(EDS) measurements of the resultant char showed fixed-carboncontent exceeding 99% by weight. Total porosity and pore volumeare calculated from measurements of true density and envelopedensity using an AccuPyc II 1340 pycnometer and a GeoPyc1360 envelope density analyzer respectively. Brunauer–Emmett–Teller (BET) [29] surface area and mesopore-and-micropore vol-ume are measured on a Micromeritics Tristar analyzer with N2

adsorption. The particle size distribution is obtained using avibratory sieve shaker. Large char particles (>150 lm) are slab-shaped while small particles resemble spherical grains. Thicknessof the large particles (>150 lm) varies widely with a mean of427 lm (r = 261 lm) and a median of 229 lm. Table 1 presentsthe composition and physical properties of the pinewood sawdustand char. True density and total pore volume of the char areapproximately three times that of the wood. The porosity of thechar is approximately two times that of the wood. The char hashigh macroporosity indicated by the large value for the total porevolume (4.26 cm3/g) relative to the volume occupied by mesop-ores and micropores (5.321E�2 cm3/g). The SEM images of thesawdust and the char using a Philips XL40 FESEM are presentedin Fig. 2. The sawdust contains smooth surfaces divided by paral-lel walls. In contrast to the sawdust, the char surface appearsrough due to randomly oriented fibers that exhibit significantwrinkling. Numerous wide tunnels protruding into the char parti-cles are observed. These tunnels may provide pathways for bulktransport of CO2 into the particle. These tunnels occupy signifi-cant volume within the char consistent with the large measuredvalue for the total pore volume.

Fig. 2. SEM images of the pinewood sawdust and char acquired at 1000�magnification.

2.3. Gasification of pinewood char with CO2

A fixed mass of 4.50 ± 0.02 g of char is loaded into the reactorfor each of the experiments. Air and moisture in the reactor areremoved by flowing N2 for 15 min before the start of the exper-iment. The char is heated to the experiment temperature with acontinuous N2 purge at a flow-rate of 0.30 ± 0.01 g/s. The gasifi-cation experiments are conducted at 1000, 1050, 1080, 1100 and1170 K with a maximum measured uncertainty of less than±10 K. The N2 flow is turned off and the CO2 flow is started oncethe reactor has reached the experiment temperature. The CO2

flow rate is established to be 0.30 ± 0.01 g/s marking the startof the gasification process. The velocity of the CO2 flow acrossthe bed is estimated to be 0.12 ± 0.02 m/s. The duration of thegasification process is �60 min for the experiments at 1000,1100 and 1170 K and �90 min for the experiments at 1050 Kand 1080 K. Gas samples are collected at one-minute separationfor the first 15 min and at three-minute separation over the next45 min of the gasification experiment. Five samples are collectedduring the additional 30 min for the 1050 and 1080 K experi-ments. CO2 is replaced with N2 and the heaters are switchedoff to end the gasification process. The N2 flow serves to purgethe CO2 and to cool down the reactor. The reactor is thendisassembled and the residual mass is collected and weighedto calculate the mass conversion. The BET surface area measure-ments, porosity measurements and SEM images of the residualchar are acquired.

3. Interpretation of experimental data

The global C–CO2 reaction is shown by Eq. (1). Measured valuesof CO concentration in the product gas along with product gasflow-rates are used to quantify the apparent char conversion ratesin the context of this global reaction. Based on mass conservation,the production rates for CO and char are related by Eq. (2):

CðcharÞ þ CO2 ! 2CO; DH ¼ 172:67 kJ=mol C ð1Þ

_x ¼ yCO _mout ¼�2 _xcharMWCO

MWcharð2Þ

The evolution of char mass with time and char conversion, X,are determined from CO composition measurements as describedby Eq. (3). Independent final char conversions are obtained by massmeasurements of the post-gasification feedstock using a micro-balance scale and compared with final conversion based on thegas composition analysis.

mchar;t ¼ mchar;t¼0 � MWchar2MWCO

R t0

_xCOdt

X ¼ 1� mchar;tmchar;t¼0

ð3Þ

The volumetric (VM) [30,31], non-reactive core (NRC) [32] andrandom pore (RPM) [33] reaction models are used to interpret charconversion. These models are used frequently to model char gasi-fication reactions since they provide three simple expressions for

Fig. 3. CO mole-fraction in product gas samples. Error bars represent 95%confidence intervals.

Fig. 4. Char conversion histories. Error bars represent 95% confidence intervals.

Table 2Final char conversion from mass balance and product gas analysis.

Temperature(K)

Final conversion from massmeasurement (%)

Final conversion from COconcentration analysis (%)

1000 12 ± 1 10 ± 11050 47 ± 1 (after 90 min) 35 ± 4 (60 min), 48 ± 5

(90 min)1080 71 ± 1 (after 90 min) 54 ± 5 (60 min), 70 ± 6

(90 min)1100 85 ± 1 82 ± 71170 98 ± 1 100 ± 9

Fig. 5. CO mole-fraction histories versus char-conversion. Error bars represent 95%confidence intervals.

42 I. Sircar et al. / Fuel 119 (2014) 38–46

the change in apparent gasification rates with conversion [10]. TheVM assumes that the reaction rate is proportional to the instanta-neous char mass:

X ¼ 1� expð�kmtÞ ð4Þ

The NRC assumes that the reaction initiates on the external sur-face of the particle and gradually progresses inwards:

X ¼ 1� ð1� knrctÞ3 ð5Þ

The RPM assumes that pores form on the solid material surfaceand that existing pores enlarge when reactions begin. The specificsurface area increases with conversion until a maximum is at-tained. Following the peak, the pores coalesce and the specific sur-face area decreases with further conversion:

X ¼ 1� exp �krpmt 1þW4

krpmt� �� �

ð6Þ

The models are used to regenerate char conversion (Xmodel)histories. A measure of agreement between the Xmodel and theexperimental conversion histories (Xexp) is quantified by thequality-of-fit (QoF), defined by following equation:

QoF ð%Þ ¼ 100 1�

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi1N

XN

i¼1

Xmodel � Xexp

Xexp

� �2vuut

0@

1A ð7Þ

The apparent reaction rate constants, km, knrc, and krpm, that ap-pear in Eqs. (4)–(6) are described by the Arrhenius rate law, givenby following equation:

ki ¼ Ai expð�Ea;i=ðRuTÞÞ; i ¼ m;nrc; rpm ð8Þ

Values for apparent rate constants at each temperature aredetermined from empirical fitting of models to experimental data.Fitting of the empirical models required linearization of Eqs. (4)–(6) versus time [13,34]. The pore structure parameter, W, andkrpm are implicitly related. Bhatia and Perlmutter [33] definedW as a function of volumetric pore surface area So, volumetricpore length Lo and the solid porosity eo for a particle, shown byEq. (9). In practice, W is calculated empirically from experimentalgasification data [35]. In the present study, W is treated as a fit-ting parameter, and values for W for each temperature are calcu-lated iteratively until the QoF is maximized. The pre-exponentialfactor, Ai, and the activation energy, Ea,i, for each kinetic ratemodel are determined from Arrhenius plots. Uncertainties forthe calculated values are estimated following the procedures de-scribed by Moffat [36]:

W ¼ 4pLoð1� eoÞ=S2o

� �ð9Þ

4. Results and discussion

Measurements of CO mole-fractions in the product gas are pre-sented in Fig. 3. The CO mole fractions with time at lower temper-atures (1000, 1050 and 1080 K) are flatter than the correspondingtrends at higher temperatures (1100 and 1170 K). CO productionincreases as the temperature increases during the initial periodof char mass-loss. This observation confirms the high sensitivity

Table 3Activation energies, Ea, and pre-exponential factors, A, for CO2 gasification of pine char.

Study Nominal Particle Size (lm) Reaction Model Ea (kJ/mol) lnA (1/s) W

This study 63350 VM 217 ± 6 15.4 ± 0.5 –NRC 186 ± 13 10.7 ± 1.0 –RPM 125 ± 30 5.0 ± 1.8 5.5 (average)

Yuan et al. 2011 [12] 56–180 RPM 232.4, 236.5, 248.5 16.3, 17.0, 18.3 50.0

Seo et al. 2010 [13] 250–300 VM 172 9.6 –NRC 142 6.4 –RPM 134 5.5 4.2

Fermoso et al. 2009 [14] 75–106 VM 183.9 ± 14, 245.3 ± 35 14.4, 19.8 –NRC 184.5 ± 14, 246.2 ± 35 14.2, 19.7 –RPM 184.3 ± 14, 246.1 ± 35 14.2, 19.6 0.7, 0.9

Khalil et al. 2008 [15] 45–63 VM 263 21.4 –

Cetin et al. 2005 [16] 120–180 VM 224, 238 17.0, 17.6 –

Fig. 6. Experimental and computed conversion histories. The symbols represent theexperimental data and the lines represent the model values.

Table 4The quality-of-fit (QoF) between the experimental and computed conversion history.

Temperature (K) Quality-of-Fit (%)

VM NRC RPM

1000 71 72 751050 83 79 841080 88 92 931100 58 80 911170 �14 36 92

I. Sircar et al. / Fuel 119 (2014) 38–46 43

of the chemical kinetics processes within the narrow range of tem-peratures captured by the present experiments. Small periodicoscillations in CO mole-fractions for the 1050, 1080 and 1100 Kexperiments occur due to temperature fluctuations. The char con-version histories are shown in Fig. 4. The char conversion rate andthe final char conversion increase significantly with increases inthe reaction temperature. The final char conversions from the massbalance and the product gas analysis are in close agreement as pre-sented in Table 2. The conversion of the feedstock increases from�10% at 1000 K to �100% at 1170 K.

The CO mole-fractions against char conversion for the gasifica-tion experiments are presented in Fig. 5. The CO mole-fraction his-tory is divided into regions I, II, and III. Region I for the two highertemperature experiments is characterized by a gradual decrease inapparent reaction rate. Region II represents an increase in produc-tion of CO. The transition from region I to region II occurs atapproximately 10% char conversion for the two higher tempera-tures. In region II, CO mole-fractions for 1100 and 1170 K increaseto reach their peak values of approximately 0.03 and 0.06 respec-tively. The peak values occur at around 50% conversion for bothtemperatures. Kajitani et al. [37] reported a similar period ofincreasing char conversion rate with a peak near 40% conversion.Their study showed that the increasing conversion rate was relatedto growth of the specific surface area of the char. In the presentstudy, the CO concentration decreases with further increases inconversion represented by region III. The end of region III marksthe end of the gasification process. For the low temperature exper-iments (<1100 K), the CO mole-fractions decrease continuouslyand do not transition to region II.

The empirical reaction models are fitted to the experimentalchar conversion data to obtain apparent rate constants. Valuesfor W are calculated in the process. Values for W that provide accu-rate fits for the RPM to the experimental conversion histories are 0,0, 0.5, 10.6 and 16.5 at 1000, 1050, 1080, 1100 and 1170 K respec-tively. Lahijani et al. [38] reported an increase for values of W from5.69 to 10.11 at 1070 to 1170 K in their study on CO2 gasification ofoil palm shell char. They interpreted the larger values for W as an

Table 5Values for physical properties of the char at different conversions.

X SSA (m2/g) e (%) (Meso + micropore volume) � 100 (cm3/g) Total pore volume (cm3/g) q (g/cm3)

0 218 88.1 5.321 4.26 1.7430.10 391 87.3 18.739 3.73 2.0470.48 559 87.8 30.299 3.43 2.1000.70 651 88.6 36.549 3.62 2.2580.82 672 90.0 38.358 3.79 2.374

Fig. 7. Change in surface area as a function of char conversion.

Fig. 8. SEM images of char after gasification a

44 I. Sircar et al. / Fuel 119 (2014) 38–46

indication of greater involvement of the pore structure in the charmass-loss. Values for W are sensitive to the char preparation con-dition and char properties. Struis et al. [24] reported values rangingbetween 3.9 and 8.1 for W for CO2 gasification of fir chars that werepyrolyzed at different temperatures. They reported W = 19.2 for firchar that had been washed by hydrochloric acid. Recent biomassgasification studies have reported values as high as 50 [12] and450 [39] for W. The average value for W from this study was 5.5,which was close to values reported in the literature [13,24].

Values for A and Ea for the gasification reaction based on thethree models from this study and prior literature are presentedin Table 3. The pre-exponential factor, A, and the activation energy,Ea, are determined from an Arrhenius plot. Values for Ea decreasein the following order based on the reaction models:VM > NRC > RPM. This trend is consistent with measurements bySeo et al. [13]. Values for activation energy from the present studyshow good agreement with activation energies reported in the

t multiple temperatures and conversion.

I. Sircar et al. / Fuel 119 (2014) 38–46 45

literature. Good agreement between the measured and literaturevalues for activation energy indicate that char mass loss is chemi-cally controlled in the present study. When char mass loss is diffu-sion controlled, Roberts et al. [40] have shown that values for theactivation energy should be significantly lower than the true acti-vation energy. Values for A show greater variation, but agree with-in the scatter of measurements reported from other studies. TheVM produces lowest uncertainties for Ea and A. Maximum relativeuncertainties for Ea and A are estimated for the RPM, despite theRPM considered to be most effective at representing char conver-sion amongst the three models [10]. Few studies [14,24,26] havereported uncertainties for Ea and A. Absolute uncertainties for Ea

and A estimated from this investigation are comparable with thosestudies.

Comparison between experimental and computed conversionhistories based on the empirical reaction models is presented inFig. 6. The Quality of Fit (QoF) between the conversion curvesbased on the models and those based on experiments are pre-sented in Table 4. The QoF based on the VM improves from 71%to 88% as the gasification temperature increases from 1000 K to1080 K. However, the QoF declines to 58% at 1100 K and reachesan irrelevant value of �14% at 1170 K. The QoF based on the NRCimproves from 72% to 92% as the temperature increases from1000 K to 1080 K and then declines to 80% as the temperature in-creases to 1100 K and to 36% as the temperature further increasesto 1170 K. The NRC shows significantly better agreement with theexperimental data at 1100 and 1170 K than the VM. The RPMshows best agreement with the data at all temperatures eventhough uncertainties for the rate parameters based on the RPMwere largest. The RPM shows small improvements for the QoF at1000–1080 K. The values for the QoF based on the RPM exceed90% for temperatures above 1080 K showing distinctly betteragreement with the experimental data at high temperatures.

Measurements of BET surface area and pore volume for the charare presented in Table 5. The values for SSA and porosity indicatehigh internal surface area. The specific surface area (SSA) of thechar increases continuously with char conversion along with thevolume contained within the meso-and-micropores. The total porevolume decreases following the start of gasification, reaches a min-imum value around X � 0.50, and increases with further char con-version. The measurements indicate that a significant proportion ofthe total pore volume is contained within macropores, althoughmicropores are reported to contain the majority of the internal sur-face area [7]. The relative change in meso-and-micropore volumewith conversion is much greater than the relative change in totalpore volume. This indicates that char mass loss due to the surfacegasification reactions occurs within the smaller pores, leading topore widening or new pore inception that are consistent with theRPM assumptions. The RPM models the change in surface area withchar mass loss by Eq. (10) where S is the surface area per volume.Measurements of SSA are converted to surface area per volume bymultiplying with the product of (1 � X) and char density (densityincreased with conversion). Fig. 7 presents the comparison of mea-sured and RPM fitted values for change in surface area as a functionof char conversion. A value of 15.8 for W provides the best fit withthe measurements and confirms the close agreement between theRPM and char conversion data at 1100 and 1170 K. Char mass lossrates at T 6 1080 K continuously decrease, despite significant SSAdevelopment. The char gasification process involves three steps:(1) adsorption of gas-phase species to the char surface, (2) surfacereactions and (3) desorption of gasification products from the sur-face. Studies have reported [14,16,41] the desorption step to berate-limiting for char-CO2 reactions. If the desorption step is slow,then an increase in surface area may have little impact on theapparent char mass loss rates at lower temperatures.

S ¼ S0ð1� XÞffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi1�W lnð1� XÞ

qð10Þ

Fig. 8 shows six randomly selected SEM images of the char.Images (a) and (b) present two different structures observed forthe char after 10% conversion. Most char samples have smoothsurfaces separated by channels represented by image (a). Fewsamples contain tunnels leading into the char particle repre-sented by image (b). The number of visible tunnels on the charprior to gasification (Fig. 2) has decreased. This observationagrees with the reduction in total pore volume reported inTable 5. An increase in SSA points toward rapid development ofmicropores, although they could not be observed on the SEMimages, and supports the measured increase in meso-and-micro-pore volume. The large tunnels occupy most of the pore volumeand provide the crucial pathways for bulk diffusion of gases intothe char; a reduction in their numbers would inhibit this trans-port process. The decreasing CO production that was observedup to 10% char conversion for all experiments may be related tothis decrease in total pore volume. Images (c) and (d) show struc-ture for the chars at �80% conversion. Image (c) shows reappear-ance of channels on the char surface and wrinkling of the biomassstrands. The surface of the char has roughened from mass lossnear pore openings resulting in significant sub-micron poredevelopment shown in image (d). The wide distribution of porediameters and shapes in image (d) is emphasized. These observa-tions support the maximum measured values for the SSA and thepore volume following the onset of gasification. Images (e) and (f)present the structure for char particles near complete conversion(X � 0.97). Image (e) shows a significant change in particle sizesdue to fragmentation resulting from weakening of the connectingstructure with mass-loss. Image (f) shows a magnified view of aparticle containing several micro-cracks on the surface. Frag-mented particles provide high surface area-to-volume ratio whilecracks within the particle provide pathways for internal diffusionof reactant gases. Sufficient mass for BET and porosity measure-ments from gasification at 1170 K was unavailable. Fig. 8 supportsthe BET and porosity measurements and shows that the structureof bio-chars is complex. The structure of bio-chars can be de-scribed by a wide range of length-scales, each of which play animportant role in internal mass transport and apparent mass-lossand motivate future studies.

5. Summary and conclusions

Gasification of a low-ash pinewood char with CO2 was investi-gated in a fixed-bed gasifier with product gas composition analysisin a temperature range of 1000–1170 K (±<10 K). Apparent rateparameters for the char-CO2 reaction were measured for the volu-metric, non-reactive core and random pore models. Uncertaintiesfor the rate parameters were quantified for model validation. BETsurface area and porosity measurements provided evidence forstructure development with conversion. The following conclusionswere made from the study:

(a) Activation energy for the pinewood char-CO2 reaction basedon the volumetric, non-reactive core, and random pore mod-els were 217 ± 6, 183 ± 13 and 125 ± 30 kJ/mol respectively.Relative uncertainties for Ea and A for the RPM were highestdespite best fit with experimental data. Measured values forthe activation energy agree well with reported values. Theresults show that the apparent gasification rate parametersfor relatively large particles of practical relevance are com-parable to those obtained from laboratory studies withmuch smaller particles.

46 I. Sircar et al. / Fuel 119 (2014) 38–46

(b) The volumetric reaction model showed best agreement withexperimental data at temperatures below 1100 K. The ran-dom pore model showed best agreement with data at 1100and 1170 K. The values for W at 1080, 1100 and 1170 K were0.5, 10.6 and 16.5 respectively.

(c) The specific surface area for the char increased rapidly withchar conversion. The biomass char was highly porous withporosity near 88%. Most of the pore volume for the charswas contained within macropores. W = 15.8 was calculatedfrom measurements of specific surface area with conversion.The effect of increasing surface area on apparent char massloss rates weakened with decreasing gasificationtemperatures.

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