A transient kientics study of the combustion reactivity of a coal char

Twenty-Fifth Symposium (International) on Combustion/The Combustion Institute, 1994/pp. 527~535

A TRANSIENT KINETICS STUDY OF THE COMBUSTION REACTIVITY O F A C O A L C H A R

P. SALATINO, F. ZIMBARDI AND M. PAULICELLI

Dipartimento di Ingegneria Chimica Unive~gsita" "Federico II"

Istituto di Ricerche sulla Combustione--C.N.R. P.le V. Tecchio, 80125 Napoli, Italy

Transient kinetics experiments have been used to characterize the combustion reactivity and the reactive surface area of char obtained from a bituminous coal. They consisted of isothermal-pulsed combustion of char samples with reiterated combustion and desorption stages. Experiments showed that the combustion rate passed through a sharp maximum at the beginning of each combustion stage, to decline afterwards as the carbon conversion degree increased. Combustion rate and reactive surface area have been deter- mined as functions of the carbon conversion degree and of the oxygen partial pressure during the combustion stage. Direct correlation between combustion rate and reactive surface area is satisfactory at low- oxygen partial pressure; it becomes poorer when data at higher pressure are included in the comparison.

A descriptive model based on a Monte Carlo simulation technique has been developed to shed light on some features of transient and steady behaviour observed in experiments. In particular, the relevance of nonuniform reactivity of the exposed surface to the combustion behaviour and to overall char reactivity is assessed on the basis of qualitative comparison between model calculations and experimental results. Simulation results suggest that a pronounced "'screening effect" of more stable surface oxides with respect to more reactive ones takes place. It should be responsible for the sharp maxima observed in the combustion rate vs time experimental relationship at the beginning of the combustion stages. The model predicts the steady combustion rate to be lower than that obtained by averaging the intrinsic reactivity of different classes of active sites on the basis of their concentrations in the unconverted solid.

Introduction

One of the challenging issues in combustion sci- ence is the problem of defining some "'intrinsic" reactivity index of carbons towards oxygen, by correcting the observed burning rates for the effects of intra- particle diffusion and of the chemical and morpho- logical heterogeneity of the solid. The first level approach to this problem is that of relating apparent combustion rates measured under conditions of chemical kinetic control (commonly indicated as regime I conditions) to the total surface area (TSA) of the carbon. This approach, although simple, presents significant drawbacks:

1. The TSA is measured by physical adsorption of gases different from oxygen at temperatures far below those at which combustion takes place; it is questionable whether the same surface is "seen" by oxygen molecules at temperatures of practical interest in combustion. This point has been addressed by several authors [1-5]; and

2. reference to TSA tout court hides the heteroge- neous character of the carbon surface, which is actually made up of regions of different reactivity depending on the local concentration and chem-

ical reactivity of active sites. The subject is exten- sively surveyed in review articles by Laurendean [6], Essenhigh [7], and more recently, by Walker [8].

The substantial inadequacy of this procedure is confirmed by the analysis of a body of data carried out by Smith [9], who observed large discrepancies between "intrinsic" reactivities (i.e., combustion rates normalized with respect to TSA) of carbons of various nature at the same temperature and oxygen .partial pressure.

In the attempt to overcome these drawbacks, some authors [10-13] have devised transient kinetics (TK) experimental techniques. These are based on the analysis of the dynamic response of a batch of carbon burning in an environment containing a time-dependent oxygen concentration, typically according to step or square pulse profiles. The amount of carbon desorbed as CO or COz after oxygen feed is suddenly interrupted can be related to the amount of surface oxide formed during combustion and unstable at the reaction temperature. This value is eventually worked out to yield the reactive surface area (RSA) of the carbon; RSA, unlike TSA, is a function of temperature and oxygen partial pressure in addition to

527

528 COAL AND ORGANIC SOLIDS COMBUSTION

TABLE 1 Properties of the South African coal

tion stages and helium of chromatographic grade during the desorption stages.

Net calorific value (kJ kg a) 26,300 Proximate analysis (%)

Moisture 2.3 Volatile matter 22.6 Fixed carbon 59.8 Ash 15.3

Ultimate analysis (%) on dry basis Carbon 68.0 Hydrogen 3.8 Sulfur 0.6 Nitrogen 1.2 Oxygen 10.9 Ash 15.5

Free-swelling index (ASTM D 720-67) 1-2

Random vitrinite reflectance (~) 0.72

the properties of the carbon. The importance of RSA as a normalizing surface for obtaining intrinsic combustion rates is widely discussed by Radovic and co- workers [10-12].

A study of the combustion reactivity of a coal char based on isothermal TK experiments is presented in the following. Experiments have been designed to investigate the influence of oxygen partial pressure and of carbon burnoff on the combustion rate and on the RSA. A descriptive model, based on a simple representation of the solid as a two-dimensional (2D) square lattice made up of cells of different reactivity, is used to simulate the observed behaviour and to shed light on the influence of the existence of sites of different reactivity on the overall combustion rate.

Experimental

Materials:

The carbon used in the experiments is char obtained from quick devolatilization of particles of South African bituminous coal. The properties of the coal are reported in Table 1. Coal devolatilization was carried out by feeding particles about 2-3 mm in size into a bed of silica sand (300-400/am in diameter) fluidized with nitrogen at a temperature of 1123 K for about 5 min. Particles were then retrieved, ground, and sieved to obtain samples in the size range of 75-125/~m. Ash content of the char is 20% by mass.

Gases used in TK experiments were oxygen-nitrogen mixtures of technical grade during the combus-

Apparatus and Technique:

The experimental apparatus consisted of a 15-ram i.d. vertical tubular quartz reactor. The lower section of the reactor, filled with quartz powder, served as a gas preheater. In the upper section, about 50 mg of char sample were introduced, sandwiched between two quartz wool plugs. The char sample was thoroughly mixed with about 1 g of quartz powder 100 /~m in size in order to minimize the possible occur- rence of hot spots under combustion conditions. The reactor was electrically heated in a tubular furnace equipped with a proportional-integral-differential (PID) temperature controller. The sample temperature was monitored by means of a type K shielded thernaocouple 1 mm in diameter placed inside the bed.

Samples were dried at 380 K overnight prior to the TK experiments. They were outgassed by rapid heating to the temperature of 1123 K for about 2 min under nitrogen atmosphere, followed by rapid cooling to the test temperature. The TK experiments were carried out isothermally at 693 K. Each run consisted of the iteration of a cycle of a combustion (C) and a desorption (D) stage pursued until complete conversion of the sample was achieved. During the C stage, oxygen-nitrogen mixture at the desired oxygen concentration was fed to the reactor at the flow rate of 60 NL/h. During the D stage, helium was fed at the same flow rate. The C stages lasted from a few minutes to about 1 h, depending on the oxygen partial pressure. The D stages lasted 20 rain under all the experimental conditions. On the whole, each TK run consisted of 5-7 complete C + D cycles. During the run, gases leaving the bed were con- tinuously drawn through a probe and analyzed by means o fa Hartmann Braun Urns 3E NDIR gas analyzer to monitor CO and CO2 molar fractions at the outlet, Xco, and Xco2, respectively.

The residence time distribution (RTD) of the transfer line and of the gas analyzer has been evaluated by means of separate pulse experiments using CO2 as a tracer. The RTD could be approximated satisfactorily by that of a sequence of three contin- uously stirred tank reactors (CSTR) with an overall space time of 2.5 s and a variance of 2.2 s 2. Although very limited, the influence of the transfer line and of the analyzer on the dynamic response was accounted for by deconvolution of the measured concentration signal.

Rates of carbon removal were obtained as F(Xco + Xcoz), where F is the total molar flow rate of gas through the bed. Results have been expressed in terms of the carbon conversion degree f = (m0 - m)/mo, where m 0 and m are, respectively, the initial

A TRANSIENT KINETICS STUDY OF THE COMBUSTION REACTIVITY OF A COAL CHAR 529

and the actual mass of carbon in the reactor and of the rate of change off, d f / d t = ( - 1 / m o ) ( d m / d t ) .

Theory

The TK experiments have been simulated by means of a descriptive model based on a Monte Carlo technique. In view of the descriptive character of the model, a number of simplifying assumptions have been made, which are not believed to significantly affect the qualitative comparison between experimental results and model calculations.

The solid is represented by means of a 2D square lattice (Fig. 1). Only the upper side of the lattice is exposed to the gaseous atmosphere. Periodic bound- arc conditions are imposed on its lateral sides. Each cell of the lattice represents a reactive site. It is assumed that two types of reactive sites are present, denoted with C1 and C2. Prior to starting the simulation, each cell of the lattice is marked at random as either C1 or C2 according to a fixed value of site concentration a = C1/(C1 + C2).

Many different combustion mechanisms have been proposed in the literature, either for noncatal- ytie [6,14] or for catalytic [15] oxidation of carbon. For the sake of simplicity, the simple combustion mechanism based on two steps, first-order nondis- soeiative oxygen adsorption and zero-order desorption, is assumed [16]. Oxygen chemisorbs irreversibly onto reactive sites, according to

1 C~ + ~O2~C~(O);

r a = k a . p ; = 1 ,2 (1 )

where ra expresses the frequency at which sites un- dergo adsorption, p is oxygen partial pressure, and k a is the adsorption constant. A single adsorption constant k~ is assumed regardless of the type i of the site. This simplifying assumption is consistent with the ob- servation that activation energies for oxygen chemisorption are much less dependent on the type of site than are desorption activation energies [17]. Desorp- tion occurs according to the following parallel paths:

Cl(O) ~ CO,CO2 rd, 1 = kd, t (2)

C2(O) --' CO,CO2 ra.2 = kd,2 (3)

w i t h kd. l > k d 2 , where rd, i = kd, i represents the turnover frequency of sites i and t, = 1/kd, i is the time after which a complex formed by oxygen chemisorption on an i site vanishes as either CO or COe. Sur- face migration of chemisorbed oxygen is not consid- ered.

The TK experiment is simulated by sequences of combustion and desorption stages.

Combustion stage The combustion stage consists of N c discrete time

steps. At each step, sites belonging to the frontier of the lattice, i.e., those which can be reached by oxygen, are flagged. Then N a frontier sites are marked at random as chemisorbed, with N a given by

N a = k a ' p N f r (4)

subject to the constraint k a .p ~< 1, where Nf~ is the number of frontier sites. Chemisorption on a site al- ready chemisorbed is ineffective. The time at which a site undergoes chemisorption designates the beginning of the site turnover interval: a counter associated with each chemisorbed site is updated at each step, and after h time iterations, the site is depleted. Nc is large enough to provide for steady combustion rate to be reached at the end of the combustion stage, before moving on to the desorption stage.

Desorption stage No oxygen chemisorption occurs during this stage,

and only desorption of previously chemisorbed sites takes place. At each time step, the counter associated with chemisorbed sites is updated, and sites are depleted when their turnover time t, expires. Desorp- tion is carried on until all chemisorbed sites have vanished.

Variables evaluated during the simulation procedure are the rate of site consumption, the total number of sites vanishing during the desorption stage (related to RSA), and the fraction of frontier sites of types 1 and 2. Input variables are the initial fraction a of sites of type 1, the product k a "p, and the desorption constants kd,i, where i = 1,2. Computations have been performed using a CONVEX C210 vector computer.

Results and Discussion

E x p e r i m e n t a l R e s u l t s :

Typical results of TK experiments are exemplified in Fig. 2, showing mole fractions of CO and CO2 at the reactor outlet as a function of time during iterated combustion and desorption cycles. In the same figure, the CO2/CO ratio is plotted as a function of time. Data in Fig. 2 were obtained at 0.21 atm oxygen partial pressure during the combustion stage. Exper- iments carried out at oxygen partial pressures of 0.1 and 1 atm yielded results qualitatively similar to those in Fig. 2. Apart from the initial spike at the beginning of each C stage, the COe/CO molar ratio in the exit gases was relatively constant during burnoff at the values of 2.9, 3.1, and 3.2 for 0.1, 0.21, and 1 atm oxygen partial pressure, respectively. The very limited change of the CO2/CO ratio over a 10-fold


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FIG. 1. 2D square lattice (24 • 25) representing the solid. Numbers in the cells are turnover times t, = 1/kd., associated with each site: (A) the lattice at the beginning of the computation (a = 0.5), (B) the lattice after 100 iteration steps in the combustion stage (ko 'p = 1; k,t.1 =

0.1; ka.2 = 0.01).


ppm co, co2 co2/co

800 4

i - i ! ............. 600 .. . . . . . . . . . . . ! .................. i ', ..... 2

CO2/CO

400 1 coz

200 .................

0 o 1,ooo 2,000 3,000 4,000 5,000

t, S

Fro. 2. Typical result of a TK experiment. Three cycles of C and D stages are represented: p = 0.21 arm during the combustion stage; T = 693 K; gas flow rate = 60 Nl/h.

change of O 2 concentration suggests that homoge- neous CO oxidation was negligible. Values of COJ CO during combustion are larger than predicted by Tognotti et al. [18] and should be related to extensive catalytic effects of the inorganic constituents of the c h a r .

The overall transient behaviour in the combustion stages appears to be the result of two combined effects: (1) a fast transient phenomenon occurring very early in the combustion stage and consisting of a sudden increase followed by rapid decay of the combustion rate as well as of the CO2/CO ratio and (2) a slower decline of combustion rate with an almost constant CO2/CO ratio as the carbon conversion degree increases. Notably, the maximum reaction rate at the beginning of each combustion stage is larger than that observed at the end of the previous combustion stage. Similar phenomena have been reported by Brown et al. [13] (in both oxygen and steam gasification of carbon) and previously by Tucker and Mulcahy [19], Bonnetain et al. [20], and Laine et al. [21]. Two mechanisms should be at work to explain the pulse of combustion rate at the beginning of the C stage. Tucker and Mulcahy suggested that the observed decay of reaction rate is the result of the desorption of unstable surface oxides followed by the building up of stable surface complexes, which limits further progress of reaction. In this respect, the desorption stage between two successive combustion stages would have a regenerating effect on the surface, providing enough time for more stable surface oxides to be desorbed and restoring a higher level of surface reactivity. Another factor to explain pulses of combustion rate might be possible: the

heating up of carbon particles due to oxygen chemisorption at the beginning of the C stage. Oxygen chemisorption has proved to be largely exothermic, integral heats of adsorption as large as 60 keaI/moI being reported in the literature [22,23]. Thermal effects during early oxygen chemisorption of coal chars in a DTA have also been reported by Brown et al. [13]. With reference to the present experiments, it can be evaluated that particle temperature might increase by as much as 15 K if chemisorption of an amount of oxygen equal to that desorbed during the D stages would occur adiabatically. It must be noted, however, that in the present experiments the char sample was thoroughly dispersed in a bed of quartz powder. The characteristic thermal diffusion time under unsteady heat generation conditions can be conservatively estimated as zh = d~/ah ~ 0.05 s, where ah is the relevant thermal diffusivity (ah 10 -4 m2/s for air, --10 -7 m2/s for quartz or char). It follows that transient combustion rate associated with nonisothermal behaviour, if any, should take place over a timescale much shorter than the decay time observed in experiments. In addition, the slight, though appreciable, increase of the COJCO ratio at the beginning of the C stage (observed also by Brown et al. [13]) further reinforces the idea that fast de- pletion of reactive sites (yielding preferentially CO2) and building up of less reactive surface oxides is relevant to the observed transient behaviour.

Values of CO and COz concentration measured after the fast decay at the beginning of the combustion stage have been worked out to obtain "steady" combustion rates df/dt as a function of carbon burnoff f and of oxygen partial pressure p. The ratio df/dt has been found to vary almost linearly with oxygen partial pressure, as is apparent from Fig. 3, where (df/dt)/p is reported as a function of carbon burnoff. The question regarding the reaction regime associated with combustion of the same char at temperatures around 690 K has been addressed in Refs. 1 and 2, where it was concluded that combustion essentially occurs on the surface of meso- and ma- cropores, whereas microporous domains are negligibly penetrated by oxygen. The persistence of dif- fusional resistances within micropores, in spite of the low temperature at which combustion is carried out, might partly explain the reaction order close to unity.

The CO2 and CO concentrations rapidly drop at the beginning of the D stage, and so does the CO2/ CO ratio, consistently with a mechanism that asso- ciates larger CQ/CO ratios to the desorption of more reactive sites. Data obtained in the desorption stage are worked out to yield the value of RSA, according to the following equation:

RSA = __1 1 td F(Xco + 2Xco2) dt (5 ) m 0 J0

where t d is the duration of the desorption stage. In


0.06

0.05

E "e 0.04 .E

0.03

~-~ 0.02

0,01

[ ]

0 i I 20

p = 0 . 1 atm

p =O.z~ atm

p = l ~ atrn

G ~ o

i I i 40

f , %

& O 0

I I 60 80 100

FIG. 3. Rate of change of carbon conversion degree per unit oxygen partial pressure (df/dt)/p vs f for different oxygen partial pressures in the combustion stage.

0.04

0.03

.c_ E

0.02 =

" 0

0.01

0 0

0

/ 0

/ 0

/

S 0

�9

0 ~

, , I , I 0.5 1 1.5

RSA, mmol/g

I i

2 2.5

FIc. 5. Correlation between df/dt and RSA.

2.5

1 . 5

E E <- f,/) 1 re

�9 \

O \

O \ \

O

~'O

0,5 a~A--A.. [] 13~ ~ - ~ z 2 ~ . . ~

ZS~ A 0 ~ I , I I I

20 40 60 80 100

f , %

FIG. 4. RSA vs f for different oxygen partial pressure during the combustion stage.

pressure. Correlation is rather good at the lower oxygen partial pressures but deteriorates when data at p = 1 atm are compared with those for the lower pressures. This implies that the goal to obtain, through RSA, a simple index to express the intrinsic reactivity of the char is only partially reached: the same value ofdf/dt (0.007) is observed, for instance, a tp = 0.21 atm, f = 0.1 and a tp = 1 a tm, f = 0.6, whereas values of RSA corresponding to the two conditions differ, being 0.6 mmol/g and 1.2 mmol/g, respectively.

Model Calculations:

A 100 X 1000 lattice made up of an equal number of C~ and C2 sites (a = 0.5) has been used to rep- resent microporous domains of the char not penetrated by oxygen [1,2].

Values of the product ka "p and of k,/,1 and kd,2 have been changed so as to compare results corresponding to the three cases:

k~ "p > kclA > kd,2

view of the difficulty of assigning a precise value to the surface occupied by a surface oxide, RSA has been expressed as mmoles of atomic oxygen desorbed per gram of carbon initially charged into the reactor. Carbon removal during the desorption stage contributed negligibly to burnoff. Values of RSA measured at different carbon conversion degrees and oxygen partial pressures are given in Fig. 4.

Figure 5 represents a cross plot of Figs. 3 and 4 with the purpose of directly correlating combustion rate df/dt to RSA at any burnoff and oxygen partial

(ka'p = 1; kd. 1 = 0.1; kd.2 = 0.01) (a)

kd, 1 > k a'19 > kd, 2

(ka" p = 0.1; kd,1 = 1; kd,2 = 0.01) (b)

kd, 1 > kd, 2 > ka. p

(ka'p = 0.01; k~, x = 1; ka,e = 0.1) (c)

The values of ka" p and ka.i, as well as of a, used in the computations have no direct relation with the


0.08 ~ a)

0.07 b)

0.06 ...................... C)

0.05

0.04

0.020.03 . . . . . . '

0.01 .... ~: ~ ~ ~I

0

0 100 200 300 400 500 600

time, arbitrary units

FIG. 6. Combustion rate vs time relationship from the Monte Carlo simulation model. Case (a): k s "p = 1; ka.1 = 0.1; ka.2 = 0.01. Case (b): k , ' p = 0.1; ka. 1 = 1; ka.2 = 0.01. Case (c): ko'p = 0.01; ka. ~ = 1; ka.2 = 0.1.

real experiment. They have been arbitrarily chosen with the only purpose of shedding light on the qualitative aspects of transient kinetics experiments. Re- sults are shown in Fig. 6 as profiles of the carbon combustion rate per unit char surface area vs time. Close similarity between curves for cases (a) and (b) and those obtained in experiments (Fig. 2) is observed. A sudden increase followed by a fast decay of the combustion rate occurs at the beginning of the combustion stage when combustion is fully or partially controlled by the rate of desorption. This is due to preferential consumption of the more reactive C1 sites and to the building up of the slowly reacting C2 sites on the exposed carbon surface. The latter act as a screen towards further combustion of C1 sites. This "screening effect" is expected to be markedly dependent on the topology of the spatial distribution of sites, expressed, among others, by the site coordina- tion number and by the lattice dimensionality. Dur- ing the desorption stage, enough time is allowed for complete desorption of both C1 and C2 sites. This regenerates the initial status of the surface (a = 0.5) and increases the average surface reactivity for the subsequent combustion stage.

The transient behaviour is different when the adsorption rate ks" p is smaller than the desorption rate of any site (case (c)). At the beginning of each combustion stage, there is an induction interval during which the reaction rate increases, as the surface cov- erage of sites by chemisorbed oxygen, wiped out during the previous desorption stage, is regenerated by the slow adsorption process. The relative abundance of surface sites of the two types (C1 and C2) stays

nearly constant and equal to the bulk value of a = 0.5.

Steady combustion rates r s per unit surface area of carbon are 0.036, 0.026, and 0.0092 for cases (a), (b), and (c), respectively (Fig. 6). If r s is calculated according to the Langmuir-Hinshelwood (LH) kinetic model applied to series-parallel paths (1), (2), and (3) and assuming additive contributions to the reaction rate of sites C1 and C2 weighed by their concentrations a and (1 - a), respectively, then the following expression is obtained:

o _oi) r~,LH =k~'p k~.p + 1 k~.p +

kd,1 kd,2

(6)

Equation (6) leads to rs,LH = 0.0504, Fs,LH = 0 . 0 5 0 ,

and rs,Ln = 0.0095 in the cases (a), (b), and (c), respectively. The finding that Fs,LH > r s in any case should warn about the assumption of additive contributions of sites having different desorption rates when evaluating carbon combustion rates from temperature programmed desorption (TPD) spectra [17]. The screening effect might increase the importance of the more stable active sites far beyond the level that would be expected on the basis of their concentration in the unconverted char. This is par- ticularly so under desorption-controlled combustion conditions.


Conclusions N~

Transient kinetics (TK) experiments with iterated Nf~ sequences of combustion and desorption have been directed to obtaining combustion rates and reactive p surface areas (RSA) of char of a bituminous coal as r~ a function of oxygen partial pressure and of the de- rd, i gree of carbon conversion. Attempts to correlate r s combustion rate directly to RSA at different oxygen r~,LH partial pressures and carbon burnoff are not in every case satisfactory. The difference between reactivities RSA of sites of different nature is the starting point to t explain the sharp maximum combustion rate ob- t~ served within the present work, and by previous au- ti thors, at the beginning of combustion stages during TSA TK experiments. Xco, Xco2

A descriptive model assumes that the solid is made up of two types of reactive sites having different de- a sorption rates, and it predicts sharp extremes in the ah reaction rate vs the time relationship when reaction rh is even partially controlled by the surface oxide desorption rate. The "screening effect" of less reactive sites with respect to more reactive ones is responsible for this behaviour and brings into the problem the effect of the topology of the spatial distribution of reactive sites in the solid matrix. No maximum in the combustion rate vs time relationship, but rather a smooth increase at the beginning of the combustion stage is predicted in the fully adsorption-controlled combustion of carbon. Because of the screening effect, steady combustion rates are smaller than those predicted according to the LH kinetic model with the assumption of additive contributions of sites having different reactivity.

Acknowledgments

The relentless guide of Professor Leopoldo Massimilla, who passed away during the preparation of this paper, is gratefully acknowledged. The authors are indebted to Mr. Sabato Masi for the precious assistance in the experimental work.

ddfld t

f F ka

mo,m

N~

Notation

char particle diameter rate of change of carbon conversion

degree carbon conversion degree total gas molar flow rate in the reactor adsorption rate constant desorption rate constant for sites of

type i initial and actual mass of carbon in

the reactor number of sites undergoing chemi-

sorption per iteration step

number of time steps in the combustion stage

number of sites belonging to the lattice frontier

oxygen partial pressure adsorption frequency /-site turnover frequency steady combustion rate steady combustion rate according to

the LH kinetic model reactive surface area time duration of the desorption stage /-site turnover time total surface area molar fractions of CO and CO2 in

outlet gases concentration of sites C 1 in the lattice thermal diffusivity characteristic thermal diffusion time

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