Cinetica de gasificacion

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Kinetic Modeling for the Combined Pyrolysis and Steam

Gasification of Petroleum Coke and Experimental Determination of

the Rate Constants by Dynamic Thermogravimetry in the 500-1520

K Range

D. Trommer and A. Steinfeld*,,

Department of Mechanical and Process Engineering, ETH-Swiss Federal Institute of Technology Zurich,8092 Zurich, Switzerland, and Solar Technology Laboratory, Paul Scherrer Institute,

5232 Villigen, Switzerland

ReceiVed September 7, 2005. ReVised Manuscript ReceiVed February 9, 2006

The detailed kinetic mechanisms for the combined endothermic pyrolysis and steam gasification of petroleumcoke are formulated using elementary chemisorption and reaction steps. The pyrolysis model is based on alinear combination of first-order decomposition rates of pseudo components. The gasification model is basedon the oxygen-exchange mechanism describing reversible O-transfer surface reactions followed by aunidirectional gasification step, and on an extended mechanism describing, in addition, reversible steam sorptionas OH/H groups and irreversible surface chemistry. The kinetic rate laws of Langmuir-Hinshelwood type are

derived by assuming sorption equilibrium and by considering inhibition due to recombination of adsorbedoxygen and competition for active sites. The Arrhenius parameters of the rate constants for the pyrolysis andgasification of Flexicoke and Petrozuata Delayed coke are experimentally determined by dynamic thermo-gravimetric analyses with Ar, H2O, CO2, and H2O-CO2 mixtures in the range 500-1520 K.

Introduction

Hybrid solar/fossil endothermic processes, in which fossilfuels are used exclusively as the chemical source for H2production and concentrated solar power is used exclusively asthe energy source of process heat, offer a viable route for fossilfuel decarbonization and create a transition path toward solarhydrogen.1 An important example of such hybridization is the

endothermic steam gasification of petroleum coke (petcoke) tosynthesis gas (syngas). The advantages of supplying solar energyfor process heat are 4-fold: (1) the calorific value of thefeedstock is upgraded, (2) the gaseous products are notcontaminated by the byproducts of combustion, (3) the dischargeof pollutants to the environment is reduced, and (4) the needfor energy-intensive processing of pure oxygen is eliminated.A 2nd-Law analysis for generating electricity using the solargasification products indicates the potential of doubling thespecific electrical output and, consequently, halving the specificCO2 emissions, vis-a-vis conventional petcoke-fired powerplants.2 The overall chemical process can be represented by thesimplified net reaction:

where x and y are the elemental molar ratios of H/C and O/Cin petcoke, respectively. Sulfur compounds and other impurities

contained in the raw materials are omitted from eq 1; these areof course important, but their exclusion does not affect the mainconclusions. The moisture content is also not accounted for ineq 1, but the stoichiometric addition of water can be adjustedaccordingly. The approximate main elemental chemical com-position, the low heating value (LHV), and elemental molarratios of H/C and O/C are listed in the Appendix for twodifferent types of petcoke: Flexicoke and Petrozuata Delayed

(PD) coke. The chemical thermodynamics indicate that the netprocess is endothermic by about 50% of the feedstocks LHVand proceeds at above 1300 K to produce, in equilibrium, anequimolar mixture of H2 and CO and less than 5% CO2. Thesolar chemical reactor technology, featuring a continuous vortexflow of steam laden with petcoke particles and directly exposedto concentrated solar radiation, was demonstrated at a powerlevel of 5 kW in a high-flux solar furnace.3

The steam gasification of petcoke is characterized by thesimultaneous occurrence of pyrolysis (i.e., thermal decomposi-tion in the absence of a gasifying agent) and a combined H2O-CO2 gasification. Previous kinetic models describing pyrolysisinclude chemical and physical processes among carbonaceousstructures,4 irreversible and first-order decomposition reactions

with continuously distributed activation energies,5-7 and a linearcombination of parallel, independent decompositions of pseudocomponents.8,9 Previous kinetic models describing gasification

* Corresponding author. E-mail: [email protected]. ETH-Swiss Federal Institute of Technology Zurich. Paul Scherrer Institute.(1) v. Zedtwitz, P.; Steinfeld, A. Energy (Amsterdam, Neth.) 2003, 28,

441-456.(2) Trommer, D.; Noembrini, F.; Fasciana, M.; Rodriguez, D.; Morales,

A.; Romero, M.; Steinfeld, A. Int. J. Hydrogen Energy 2005, 30, 605-618.

(3) ZGraggen, A.; Haueter, P.; Trommer, D.; Romero, M.; de Jesus, J.C.; Steinfeld, A. Int. J. Hydrogen Energy, in press.

(4) Migliavacca, G.; Parodi, E.; Bonfanti, L.; Faravelli, T.; Pierucci, S.;Ranzi, E. Energy (Amsterdam, Neth.) 2005, 30, 1453-1468.

(5) Ulloa, C.; Gordon, A. L.; Garca, X. J. Anal. Appl. Pyrolysis 2004,71, 465-483.

(6) Pitt, G. J. Fuel 1962, 41, 267-274.(7) Anthony, D. B.; Howard, J. B. AIChE J. 1967, 22, 625-656.(8) Meszaros, E.; Varhegyi, G.; Jakab, E.; Marosvolgyiet, B. Energy

Fuels 2003, 18, 497-507.

CHxOy + (1 - y)H2O ) (x

2 + 1 - y)H2 + CO (1)

1250 Energy & Fuels 2006, 20, 1250-1258

10.1021/ef050290a CCC: $33.50 2006 American Chemical SocietyPublished on Web 03/28/2006


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include those based on the oxygen-exchange mechanism forgasification with H2O, CO2, and H2O-CO2 mixtures at belowor near atmospheric pressure,10-16 and those that, in addition,consider reversible sorption mechanisms.2,17-21 This articlepresents the formulation of kinetic models for the pyrolysis andsteam gasification of petcoke in the absence of O 2, the detailedderivation of the rate laws, and the experimental determinationof the rate constants by thermogravimetry.

Kinetic Models

Pyrolysis. The rate of pyrolysis is calculated as a linearcombination of first-order decomposition rates of pseudocomponents:

where

and the temperature dependence of the rate constant is givenby the Arrhenius law:

Gasification. Two mechanisms for the gasification of carbonwith H2O and CO2 are considered: the oxygen-exchange andthe extended mechanisms. The rate constants of the oxygenexchange mechanism are denoted ki, and the rate constants ofthe extended mechanism are denoted ki. The oxygen-exchangemechanism postulates reversible oxygen-exchange surface reac-tions

followed by the unidirectional gasification step at highertemperatures through combination of oxygen and carbon atoms

from the solid to form CO(g):

Inhibition occurs by the recombination of adsorbed oxygen witheither CO or H2. Equation 6 typically controls the movementtoward equilibrium, while eqs 5 and 6 provide for the water-gas shift equilibrium. Assuming elementary reactions, the ratelaws for the formation/consumption of the gas species H2O, CO2,

CO, and H2 are obtained as functions of their partial pressuresin the bulk phase and the fraction of surface covered withadsorbed oxygen, O:

ri, with i ) H2O, CO2, CO, and H2, is expressed in units ofmoles of i transformed per time per mass of solid, mol/(g s).Further assuming sorption equilibrium yields:

Substituting O in eqs 8-11 gives the rate laws in terms of thepartial pressures and temperature:

with the species-dependent numerators Ni being

The rate for the carbon consumption is obtained by applyingcarbon mass conservation:

Since the gaseous products CO and H2 are continuouslywithdrawn from the reaction zone, the reverse reactions of eqs

(9) Porada, S. Fuel 2004, 83, 1191-1196.(10) Laurendeau, N. M. Prog. Energy Combust. Sci. 1978, 4, 221-270.(11) Fredersdorff, C. G. v.; Elliott, M. A. Coal Gasification. In Chemistry

of coal utilization; Lowry, H. H., Ed.; Wiley: New York, 1963; pp 892-1022.

(12) Gadsby, J.; Long, F. J.; Sleightholm, P.; Sykes, K. W. Proc. R.Soc. London 1948, A193, 357-376.

(13) Reif, A. E. J. Phys. Chem. 1952, 56, 785-788.

(14) Ergun, S. Kinetics of the reactions of carbon dioxide and steamwith coke; Bulletin 598; U.S. Bureau of Mines: Washington, DC, 1962;pp 1-38.

(15) Mentser, M.; Ergun, S. A study of the carbon dioxide-carbonreaction by oxygen exchange; Bulletin 664; U.S. Bureau of Mines:Washington, DC, 1973; pp 1-42.

(16) Gadsby, J.; Hinshelwood, C. N.; Sykes, K. W. Proc. R. Soc. London1946, A187, 129-151.

(17) Muller, R.; Zedtwitz, P. v.; Wokaun, A.; Steinfeld, A. Chem. Eng.Sci. 2003, 58, 5111-5119.

(18) Long, F. J.; Sykes, K. W. Proc. R. Soc. London 1948, A193, 377-399.

(19) Blackwood, J. D.; McGrory, F. Aust. J. Chem. 1957, 11, 16-33.(20) Muhlen, H. J. Zum Einfluss der Produktgase auf die Kinetik der

WasserdampfVergasung in Abhangigkeit Von Druck und Temperatur;Universitat Essen (D), 1983.

(21) Muhlen, H. J.; Heek, K. H. v.; Juntgen, H. Fuel 1985, 64, 944-949.

CO98k3

CO + nC* (7)

rH2O) k

-2pH2O - k2pH2O(1 - O) (8)

rCO2) k

-1pCOO - k1pCO2(1 - O) (9)

rH2) k2pH2O

(1 - O) - k-2pH2O (10)

rCO ) k1pCO2(1 - O) - (k-1pCO - k3)O (11)

where

ri )

1

mpetcoke

dni

dt (12)

O )k1pCO2

+ k2pH2O

k1pCO2+ k

-1pCO + k2pH2O+ k

-2pH2+ k3

(13)

ri

)Ni

1 + 1k3

(k1pCO2+ k

-1pCO + k2pH2O+ k

-2pH2) (14)

NH2O) -k2pH2O

-k2k-1

k3pH2O

pCO +k1k-2

k3pH2

pCO2(15)

NH2) -NH2O

(16)

NCO2) -k1pCO2

+k2k-1

k3pH2O

pCO -k1k-2

k3pH2

pCO2(17)

NCO ) 2k1pCO2 + k2pH2O -

k2k-1

k3 pH2OpCO +

k1k-2

k3 pH2pCO2

(18)

rC ) -rCO - rCO2) -k3O )

-

k1pCO2+ k2pH2O

1 +1

k3(k1pCO2

+ k-1pCO + k2pH2O

+ k-2pH2

)

(19)

dXP

dt)

i

ciri ) i

ci

dRi

dt(2)

ri )dRi

dT ) ki(1 - Ri) (3)

ki ) k0,i exp(- EA,i

RT) (4)

CO2 + C* {

\

}

k1

k-1 CO + CO (5)

H2O + C* {\}k2

k-2H2 + CO (6)

Combined Pyrolysis and Steam Gasification of Petcoke Energy & Fuels, Vol. 20, No. 3, 2006 1251


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5 and 6 are neglected (k-1 ) k-2 ) 0), leading to

The extended mechanism assumes that H2O undergoesdissociation at the carbon surface into hydroxyl radical and

hydrogen atom groups, both being rapidly chemisorbed atadjacent active sites, followed by a hydrogen transfer to theadsorbed hydrogen,18-21 and that CO2 reacts by oxygenexchange with the solid surface according to the previousmechanism. Inhibition occurs by recombination of CO withadsorbed oxygen and by H2/CO adsorption on active sites.Considering reversible sorption of gaseous species onto thecarbon surface and irreversible reactions among adsorbed speciesand with molecules in the gas phase, we get

Equations 21-23 are pertinent to CO2 gasification, whereas eqs23-26 are pertinent to H2O gasification, with eq 23 describingdesorption of the surface oxygen complex common to both H2O

and CO2 gasification. Gadsby et al.12 proposed the system ofeqs 21 (forward), 22, and 23, in which CO inhibition occursdue to adsorption of CO on the carbon surface, competing withCO2 for active sites. Ergun and co-workers14,15 proposed thesystem of eqs 21 and 23, in which CO inhibition occurs whensurface oxygen is detached by CO from the gas phase (eq 21reverse). Both mechanisms lead to the same form of the ratelaw for carbon consumption by gasification using only CO2 asgasifying agent:

with a1, a2, and a3 being lumped rate constants. Reif13 analyzedin detail the importance of the two CO inhibition paths describedabove and found experimentally that the recombination of COand CO is considerably faster than CO adsorption. For gasifica-tion at elevated pressures, Blackwood and Ingeme22 and Mulleret al.17 extended Erguns mechanism by adding

In the mathematical treatment that follows, eqs 28 and 29are omitted from consideration since the reduced systemrepresents data reasonably well in the 0-1 bar pressure rangeused in this study. CO inhibition can occur both by recombina-tion with an adsorbed oxygen (eq 21 reverse) and by competitionfor active sites (eq 22). Further assuming sorption equilibriumfor CO, O, H2, and the intermediate component OH-Hrepresenting the adsorbed water,

with

Assuming elementary reactions, the rate laws for the formationand consumption of the gas species H2O, H2, CO, and CO2 are

obtained as functions of their partial pressures and i:

The system of eqs 30-34 is solved analytically for the unknowni, and the solution inserted in eqs 35-38 to yield the rateexpressions of Langmuir-Hinshelwood type is

with the following numerator terms:

Equation 39 represents the complete solution of the reactionsdescribed by the system of eqs 21-26. Neglecting the mixedand squared terms in the denominator since they have negligibleeffect in the low-pressure range and replacing the elementary

(22) Blackwood, J. D.; Ingeme, A. J. Aust. J. Chem. 1960, 13, 194-209.

rC ) -k1pCO2

+ k2pH2O

1 +1

k3(k1pCO2

+ k2pH2O)

(20)

CO2 + C* {\}k1

k-1CO + CO (21)

CO + C* {\}k2

k-2CCO (22)

CO98k3

CO + nC* (23)

H2O + C* {\}k4

k-4COH + CH (24)

COH + CH98k5

CO + CH2 (25)

H2 + C* {\}k6

k-6CH2 (26)

rC ) -a1pCO2

1 + a2pCO2+ a3pCO

(27)

CO2 + CCO98k7

2CO + CO (28)

CO + CCO98k8

CO2 + 2C* (29)

OH,H: k4pH2Ov ) (k-4 + k5)OH,H (30)

CO: k2pCOv ) k-2CO (31)

H2: k6pH2v + k5OH,H ) k-6H2 (32)

O: k5OH,H + k1pCO2v ) k-1pCOO + k3O (33)

v ) 1 - (OH,H + CO + H2 + O) (34)

rH2O) k

-4OH,H - k4pH2Ov (35)

rH2) k

-6H2 - k6pH2v (36)

rCO ) k1pCO2v - k-1pCOO + k-2CO - k2pCOv + k3O

(37)

rCO2) k

-1pCOO - k1pCO2v (38)

ri ) {Ni}/{1 +k1

k3pCO2

+k4(k5k-6 + k3k-6 + k3k5)

k3k-6(k-4 + k5)pH2O

+

k6

k-6

pH2+ ( k2k

-2

+k

-1

k3)pCO + k-1k2k

-2k3pCO

2+

k-1k4

k3k-6

k5 + k-6

k-4 + k5

pH2OpCO +

k-1k6

k3k-6pH2

pCO} (39)

NH2O) -

k4k5

k-4 + k5

pH2O-

k-1

k3

k4k5

k-4 + k5

pH2OpCO (40)

NH2) -NH2O

(41)

NCO ) 2k1pCO2+

k4k5

k-4 + k5

pH2O-

k-1

k3

k4k5

k-4 + k5

pH2OpCO

(42)

NCO2) -k1pCO2

+k

-1

k3

k4k5

k-4 + k5

pH2OpCO (43)

1252 Energy & Fuels, Vol. 20, No. 3, 2006 Trommer and Steinfeld


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rate constants by lumped complex constants yields

where the Ki values are defined as

The rate of carbon consumption is calculated in analogy to the

oxygen-exchange mechanism by applying carbon mass conser-vation:

Further neglecting the inhibition effect of the product gases ineqs 44-47 and 49, the final expression for the rate of carbonconversion becomes

Note that eq 20 obtained by applying the oxygen-transfermechanism is a special case of eq 50, with the additionalboundary conditions

reducing the number of independent parameters from four ineq 50 to only three in eq 20. Note also that eq 39 does notconsider the dual-site mechanism or the two-center adsorption.23

The temperature dependence of the rate constants derived bythe oxygen-transfer mechanism (eq 20) and by the extendedmechanism (eq 50) is given by the Arrhenius law (eq 4).

Active Surfaces. Being a heterogeneous process, the rate ofthe reaction is strongly affected by the availability of active

surface, which in turn depends on, among other variables, theparticle size, porosity, temperature, reaction extent, and impuri-ties contained in the feedstock. The following proportionalityis used:

where a is the specific active surface per unit mass of petcoke,and n is determined experimentally for each type of petcoke.Limiting cases are n ) 1 for spherical particles, with the reactiontaking place only on the outer surface, and n ) 0 for particles,with the reaction taking place in the entire volume.

Thermogravimetric Analysis

Experimental Setup. Experimentation was carried out on athermogravimeter system (TG, Netzsch STA 409 CD) equippedwith two furnaces: a conventional high-temperature electricfurnace with a maximum working temperature of 1823 K andsuitable for reactive atmospheres having a dew point below roomtemperature, and a special electric furnace with a maximumworking temperature of 1523 K and suitable for reactiveatmospheres containing up to 100% steam at 1 bar total pressure.The latter is shown schematically in Figure 1, as used for thegasification runs with H2O-CO2-Ar mixtures. The reactive gasentered the furnace chamber from the top and flowed downwardpast a thin layer of petcoke sample mounted on a 17-mm

diameter flat sample holder. This arrangement avoided stagnantgas around the sample and minimized mass/heat transferresistances between the coke sample and the bulk gas.24,25 Thesteam furnace was coupled with a steam generator unit(Bronkhorst Hitec CEM) via a transfer line heated to 473 K,for avoiding steam condensation. The mass flow rates andthe steam concentration in the reactive gas were adjustedby mechanical flow controllers for Ar and CO2 (VogtlinQ-FLOW) and electronic flow controllers for water (BronkhorstLIQUIFLOW). Original petcoke was milled and sieved for

(23) Walas, S. M. Reaction Kinetics for Chemical Engineers; Butter-worths: Boston, 1989.

(24) Ollero, P.; Serrera, A.; Arjona, R.; Alcantarilla, S. Fuel 2002, 81,1899-2017.

(25) Gomez-Barea, A.; Ollero, P.; Arjona, R. Fuel 2005, 84, 1695-1704.

Figure 1. Schematic of the thermogravimeters steam furnace forexperiments up to 1523 K with reactive gas containing up to 100%steam.

rC a ( 1dP)n

(52)

rH2O)

-K2pH2O- K2K3pH2O

pCO

1 + K4pCO2+ K5pH2O

+ K6pH2+ K7pCO

(44)

rH2) -rH2O

(45)

rCO )2K1pCO2

+ K2pH2O- K2K3pH2O

pCO

1 + K4pCO2

+ K5pH2O + K6pH

2

+ K7pCO(46)

rCO2)

-K1pCO2+ K2K3pH2O

pCO

1 + K4pCO2+ K5pH2O

+ K6pH2+ K7pCO

(47)

K1 ) k1

K2 )k4k5

k-4 + k5

K3 )k

-1

k3

K4 )k1

k3(48)

K5 )k4(k5k-6 + k3k-6 + k3k5)

k3k-6(k-4 + k5)

K6 )k6

k-6

K7 )k2

k-2

+k

-1

k3

rC ) -rCO - rCO2) -k3O )

-

K1pCO2+ K2pH2O

1 + K4pCO2+ K5pH2O

+ K6pH2+ K7pCO

(49)

rC ) -K1pCO2

+ K2pH2O

1 + K4pCO2+ K5pH2O

(50)

K4 )K1

a4and K5 )

K2

a4(51)



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obtaining samples of different particle diameters using differentgrinding techniques. A cutting mill was used to obtain particlesin the 80-1000-m range, which were sieved through a towerwith 80-, 160-, 200-, 250-, 355-, and 500-m sieves. A balland jet mills were used to get powder with mean particlediameters of 3.8 and 1.3 m, respectively. Samples of 40 mgof petcoke were placed as thin layers on the flat sample holder.The TG furnace was first evacuated and purged with Ar threetimes to obtain an air-free atmosphere. Thereafter, the samplewas heated to the desired temperature at a rate of 10 or 20K/min, under a flow of the reactive gas H2O, CO2, or H2O-CO2 mixture, at a total pressure of 1 bar. Product gascomposition at the furnace exit was analyzed by gas chroma-tography (two-channel Varian Micro GC, equipped with aMolsieve-5 and a Poraplot-U columns for the detection of H2,CO, CO2, CH4, C2H2, C2H4, C2H6, H2S, and COS).

Experimental Results. On the basis of proximate analysis,we subdivided the coke in two fractions: volatiles and fixedcarbon. Both are assumed to participate in the gasificationreaction, but only the volatiles are subjected to pyrolysis. Thetotal weight loss of the coke sample is defined as the sum of

the pyrolysis and gasification weight loss:

where

XP and XG are the conversions due to pyrolysis and H2O-CO2gasification, respectively (0 e XG e 1, 0 e XP e ci).Adjustment due to the ash content is not necessary because it

represents only 0.33% weight for PD coke and 0.40% weightfor Flexicoke. Thus, the measured rate of gasification is basedon the weight loss data recorded by the TG:

Since pyrolysis and H2O-CO2 gasification occur simulta-neously, a set of experiments using only inert gas was

performed, and the resulting weight loss curve was subtractedfrom that obtained using reactive gases H2O/CO2 for calculatingthe rate of H2O/CO2 gasification.

The TG and DTG curves obtained for the pyrolysis ofFlexicoke and PD coke are shown in Figure 2. The experimentalruns were performed with 250-355-m particles, subjected toan Ar flow of 210 mL/min and a linear heating rate of 10 Kmin-1 in the 500-1520 K temperature interval. Both curvesindicate pyrolysis in a two-step process: (1) at 956 and 860 K,producing a weight loss of 1.7 and 8.4% for Flexicoke and PDcoke, respectively, and (2) at above 1090 and 1210 K, producinga weight loss of 6.7 and 4% for Flexicoke and PD coke,respectively. Total weight loss during pyrolysis up to 1520 Kis 8.4% for Flexicoke and 12.4% for PD coke. Gases evolved

during the first step of the pyrolysis of Flexicoke, shown inFigure 3a, include CO2 (with a peak about twice as much asthe one for PD coke) and small amounts of H2. In contrast,gases evolved during the first step of the pyrolysis of PD coke,shown in Figure 3b, include a C2H4 peak at 773 K, C2H6 at796 K, CH4 at 846 K, CO2 at 839 K, and H2 at 1007 K. Attemperatures above 1250 K, both types of petcoke released CO,CO2, and H2. The results for PD coke are similar to thosetypically observed during pyrolysis of coal and other types ofcoke.26,27

The weight loss and product gas composition as a functionof temperature for the steam gasification of Flexicoke and PDcoke are shown in Figure 4. The experimental runs wereperformed with 250-355-m particles subjected to a 60%

H2O-Ar mixture and a linear heating rate of 10 K min -1 inthe 500-1520 K interval. The curves represent the superpositionof pyrolysis, taking place above 650 K, and steam gasification,starting at about 1000 K. As expected for Flexicoke, pyrolysisis negligible compared to steam gasification. In contrast, forPD coke, it affects both the weight loss and gas composition.Further, the gasification of PD coke appears to proceedfollowing two regimes, a faster one up to 1310 K, and a slowerone afterward due to a thermally induced deactivation. Inaddition to the pyrolysis gas products already considered inFigure 3, the main gasification products are H2, CO, and CO2.

Figure 2. TG and DTG data for the pyrolysis of Flexicoke and PDcoke. dp ) 250-355 m, ) 10 K min-1.

Figure 3. Weight loss and product gas composition as a function of temperature for the pyrolysis of (a) Flexicoke and (b) PD coke. dp ) 250-355m, ) 10 K min-1.

-rC )1

1 - XG

dXG

dT (55)

X) XG(1 - XP) + XP (53)

X(T) ) 1 -m(T)

m0(54)



6/9

At above 1210 K, steam transforms the sulfur content of petcokeinto H2S. CH4 concentration is less than 0.013% for PD cokeand less than 0.02% for Flexicoke.

The effect of steam and CO2 concentrations is shown inFigure 5a,b for Flexicoke and PD coke, respectively, obtainedfrom a series of TG runs with 250-355-m particles subjectedto binary H2O-Ar and CO2-Ar mixtures in the 0-100% H2O/CO2 concentration range, and a linear heating rate of 20 K min-1

in the 500-1520 K temperature range. The simultaneousoccurrence of pyrolysis clearly affects the H2O/CO2 gasificationof PD coke. It affects as well the gasification of Flexicoke, but

in a manner less than that of PD coke. For PD coke, CO2gasification starts at higher temperatures than H2O gasification.

At higher concentration of the reactive gas, the gasificationexhibits a saturation behavior typical for heterogonous gas-solid reactions. A thermally induced deactivation at above1310 K is observed for H2O gasification of PD coke.

The effect of particle size is shown in Figure 6a,b forFlexicoke and PD coke, respectively, obtained with petcokeparticles subjected to 60% H2O-Ar mixtures and a linearheating rate of 10 K min-1 in the 500-1520 K range. ForFlexicoke, the reaction rate is not considerably affected byvarying the mean particle size. In contrast, for PD coke, thereaction rate increases with decreasing mean particle size.Pyrolysis starts at somewhat lower temperatures (about 600 K)

(26) Karcz, A.; Porada, S. Fuel 1995, 76, 806-809.(27) Kocaefe, D.; Charette, A.; Castonguay, L. Fuel 1995, 74, 791-

799.

Figure 4. Weight loss and product gas composition as a function of temperature for the steam-gasification of (a) Flexicoke and (b) PD coke. pH2O) 0.6 bar, dp ) 250-355 m, ) 10 K min-1.

Figure 5. Weight loss as a function of temperature for the gasification of (a) Flexicoke, and (b) PD coke in binary H 2O-Ar and CO2-Ar mixtures.dp ) 250-355 m, ) 20 K min-1.

Figure 6. Weight loss as a function of temperature for the steam-gasification of (a) Flexicoke and (b) PD coke. The parameter is the mean particlesize. pH2O ) 0.6, ) 10 K min

-1.



7/9

and has a higher reaction extent for powder size fractionsobtained by jet and ball mill. For particles bigger than 200-250 m, the reaction does not go to completion before reaching1310 K, and its rate undergoes attenuation by thermal deactiva-tion. A residuum of 3.6 and 1.1% weight was obtained forFlexicoke and PD coke, respectively.

Determination of the Rate Constants

Pyrolysis. Figure 7 shows the measured (data points) andmodeled (curves) conversion rate dXP/dt obtained for thepyrolysis of (a) Flexicoke and (b) PD coke. Smoothing algorithmbased on least-squares quadratic polynomial fitting was appliedto the measured TG curve. The kinetic parameters k0, EA, and

the maximum conversion ci for two pseudo-components, definedby eqs 2-4, were determined by the least-squares method. Theroot-mean-square error (RMS) between the experimentallymeasured and the theoretically modeled reaction rate dXP/dTwas minimized with an algorithm of MATLAB.28 MATLABscommand fminsearch finds the minimum of a scalar functionof several variables with the Nelder-Mead nonlinear minimiza-tion algorithm, generally referred to as unconstrained nonlinearoptimization.28 The kinetic parameters are listed in Table 1 forFlexicoke and PD coke. The weight losses of Flexicoke for thefirst and the second reactions were 1.36 and 2.07%, respectively,giving a total of 3.43%. The weight losses of PD coke for thefirst and the second reactions were 7.94 and 3.93%, respec-tively, giving a total mass loss of 11.87%. The RMS of the

absolute error between the measured and the modeled petcokepyrolysis was 8.40 10-7 for Flexicoke and 2.24 10-6 forPD coke.

Gasification. The Arrhenius parameters of the rate constantsdefined by eq 20 (derived by applying the oxygen-transfermechanism) and eq 50 (derived by applying extendedmechanism) were determined numerically using the samealgorithm by MATLAB28 for minimizing the RMS errorbetween measured and modeled data. The numerical procedurewas carried out simultaneously for all temperatures and allcombinations of partial pressures (H2O-CO2-Ar and CO2-Ar mixtures) applied in the experiments. Table 2 lists the best-

fit kinetic parameters of the rate constants derived for theoxygen-exchange mechanism of the steam gasification of PDcoke. The results are in agreement with data presented ingasification studies with pure H2O or C O2.10 Further, thegasification with H2O-CO2 mixtures can be described usingthe rate constants obtained from experiments with pure gases,

as claimed previously.14

As far as the steam gasification of Flexicoke is concerned,the oxygen-exchange mechanism is not capable of describingwith reasonable accuracy the reaction rate. As will be shownin the following figures, the addition of CO2 to a reacting gascontaining more than 60% H2O results in a decrease of thereaction rate, which is not predicted by eq 20. Presumably, masstransfer is limited by slow diffusion of the reactive gas into theinterior of the particles through their pores.14

Table 3 lists the best-fit kinetic parameters derived for theextended mechanism of the H2O-CO2 gasification of Flexicokeand PD coke. Figure 8 shows the comparison of the experi-mentally measured (data points) and the theoretically modeled

(curves) rate of the reaction rC as a function of temperature forvarious H2O-CO2 mixtures. The parameter is the compositionof the gaseous reactant, expressed in %H2O-%CO2 at 1 bartotal pressure. Note that, for the measured data, the weight lossby pyrolysis in Ar has been subtracted from the weight loss bygasification in H2O-CO2 mixtures. It is shown that the extendedmodel is closely obeyed over the 900-1310 K temperaturerange considered and over the complete range of H2O/CO2concentration. The RMS of the absolute error between experi-mental and modeled rate of reaction is 3.02 10-5 for Flexicokeand 2.75 10-5 for PD coke. Figure 8 further shows that therate of H2O gasification is comparable for both types of cokes,whereas the rate of CO2 gasification is considerably faster forFlexicoke than that for PD coke.(28) MATLAB R14; The MathWorks, Inc.: Natick, MA, 2005.

Figure 7. Experimentally measured (data points) and model simulated (curves) conversion rates obtained from pyrolysis of (a) Flexicoke and (b)PD coke.

Table 1. Kinetic Parameters for the Pyrolysis of Petcoke

Flexicoke PD coke

k0 EA ci k0 EA ci

r1 4.152 10-2 5.157 104 0.0136 1.905 10-1 5.087 104 0.0794r2 1.881 100 1.210 105 0.0207 7.528 10-3 4.626 104 0.0393

Table 2. Arrhenius Parameters of the Rate Constants Derived byApplying the Oxygen-Exchange Mechanism (Eq 20) for the

H2O-CO2 Gasification of PD Cokea

k1 (s-1 Pa-1) k2 (s-1 Pa-1) k3

k0 (ki) 2.229 102 1.602 10-1 8.021 107

EA (J/mol) 2.651 105 1.618 105 2.416 105

a RMS ) 2.66 10-5.



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Figure 9 is a cross-plot of the previous figure at 1273 K and

shows a contour of reaction rate rC, in s-1, as a function of theH2O and CO2 partial pressures, for (a) Flexicoke and (b) PDcoke. Despite the differences in the values of the kineticparameters K9 and K10 associated with the H2O terms of eq 50,the reaction rate with 100% H2O is similar for both types ofcoke, about 2.7 10-3 s-1. In contrast, the reaction rate with100% CO2 is 7 10-4 s-1 for Flexicoke, more than twice the

value observed for PD coke. Interestingly, adding CO2 to a

H2O-CO2 reactive gas at higher steam partial pressures resultsin a decrease of the reaction rate for Flexicoke and an increaseof the reaction rate for PD coke.

Figure 10 shows the reaction rate (log scale) as a function ofthe mean particle size for (a) Flexicoke and (b) PD coke andfor various reaction temperatures. The data points correspondto the experimentally measured values. The curves correspond

Table 3. Arrhenius Parameters of the Rate Constants Derived by Applying the Extended Mechanism (Eq 50) for the H2O-CO2 Gasification of

Flexicoke and PD Coke

K1 (Pa-1 s-1) K2 (Pa-1 s-1) K4 (Pa-1) K5 (Pa-1)

Flexicoke k0 (Ki) 1.445 101 6.472 102 1.155 10-5 5.592 10-4

RMS ) 3.02 10-5 EA (J/mol) 2.214 105 2.508 105 6.137 103 5.801 104

PD coke k0 (Ki) 2.251 101 9.771 10-1 4.683 10-8 6.640 10-6

RMS ) 2.75 10-5 EA (J/mol) 2.408 105 1.812 105 7.148 102 7.239 103

Figure 8. Experimentally measured (data points) and the theoretically modeled (curves) reaction rate rC as a function of temperature for thegasification of (a) Flexicoke and (b) PD coke. The parameter is the different composition of the reacting H 2O-CO2 mixture, expressed in % H2O- % CO2 at 1 bar.

Figure 9. Reaction rate rC, in s-1, as a function of the H2O and CO2 partial pressures, at 1273 K for (a) Flexicoke and (b) PD coke. Total pressureis 1 bar.

Figure 10. Experimentally measured (data points) and the theoretically modeled (curves) reaction rate rC as a function of the mean particle sizefor the gasification of (a) Flexicoke and (b) PD coke. The parameter is the reactor temperature.



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to the least-squares fit to the power proportionality of eq 52,with n ) 0.234 (RMS ) 0.0445) for Flexicoke, and with n )0.504 (RMS ) 0.0227) for PD coke.

Summary and Conclusions

We have derived kinetic rate laws for the combined pyrolysisgasification of petcoke and determined experimentally theArrhenius parameters by dynamic thermogravimetric analysesof the Ar pyrolysis and H2O/CO2 gasification of two types ofVenezuelan petcokes (Flexicoke and PD coke). The pyrolysisis well described by applying a two-pseudo-component decom-position mechanism. The gasification kinetics is modeled byapplying two mechanisms: an oxygen-transfer mechanism basedon reversible oxygen-transfer surface reactions and irreversiblecarbon gasification, and an extended mechanism that considers,in addition, OH/H sorption and hydrogen transfer. The former,simpler, mechanism fails to describe the Flexicoke gasification,presumably because of intraparticular mass transfer limitations.The latter mechanism is able to simulate, with good experimentalagreement, the H2O/CO2 gasification of both types of petcoke.For 250-355-m particles, the rate of gasification with pureH

2O, about 2.7 10-3 s-1, is comparable for both petcokes,

while the rate of gasification with pure CO2, about 7 10-4

s-1, is twice as fast for Flexicoke than that for PD coke. Mainevolved gases during pyrolysis include C2H6, C2H4, CH4, CO2,CO, and H2 for PD coke, and CO, CO2, and H2 for Flexicoke.In addition to the pyrolytic gas products, the main gasificationproducts are H2, CO, CO2, and H2S. The dependence of thereaction rate on the active site availability was empirically foundto be well described by a power proportionality to the meanparticle size:

where n is determined experimentally for each type ofpetcoke.

Acknowledgment. This study was conducted within theframework of a joint PDVSA-CIEMAT-ETH project aimed atdeveloping the chemical reactor technology for the solar gasifica-tion of petcoke. We thank A. Morales, D. Rodriguez, E. Lima, andJ. C. de Jesus from PDVSA, M. Romero and A. Vidal fromCIEMAT, and A. ZGraggen and P. Haueter from ETH for helpfuldiscussions.

Appendix

Table A1 shows the approximate main elemental

chemical composition in wt % (ultimate analysis), LHV,and elemental molar ratios of H/C and O/C for the two typesof petcoke.

Nomenclature

Ri ) conversion of pseudocomponent (0 e Ri e 1)a ) active surface, m2/g ) linear heating rate (ramp) of the thermobalance, K/sci ) maximum conversion of a pseudo component with respect tothe overall sample massC* ) carbon site on the coke surface

EA ) activation energy, J/molki ) elementary rate constants of the oxygen exchange mechanism,units according to rate lawki ) elementary rate constants of the extended mechanism, units

according to rate lawKi, ai ) lumped rate constants, units according rate lawk0 ) pre-exponential factor, same units as rate constantm ) mass of coke sample, gni ) mole number, moln ) constant

Ni ) species-dependent numerator of a rate expressionpi ) partial pressure, PaR ) ideal gas constant, J/(mol K)ri ) rate of reaction, s-1

i ) fractional surface coverage (0 e i e 1)T ) absolute temperature, Kt ) time, s

X ) overall coke conversion (0 e Xe 1)XP ) coke conversion due to pyrolysis (0 e XP e ci)XG ) coke conversion due to H2O-CO2 gasification (0 eXG e 1)x ) H/C elemental molar ratioy ) O/C elemental molar ratio

Subscripts

G ) gasificationi ) chemical species or number of reactionP ) pyrolysis0 ) initial

AbbreViations

BET ) Brunauer-Emmet-TellerDTG ) differential thermogravimetryGC ) gas chromatographLHV ) low heating valuePD ) Petrozuata DelayedRMS ) root-mean-square (error)TG ) thermogravimeter

EF050290A

rC ( 1dP)n

Table A1. Approximate Main Elemental Chemical Composition in

wt % (Ultimate Analysis), Low Heating Value (LHV), andElemental Molar Ratios of H/C and O/C for the Two Types of

Petcoke

Flexicoke PD coke

C 92.70 88.21H 0.67 4.14O 0.92 1.46N 0.90 2.28S 1.98 4.16

LHV (kJ/kg) 32983 35876H/C (mol/mol) 0.0859 0.5581O/C (mol/mol) 0.0074 0.0124


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