Combustion Behavior

DETECTOR (2.4 x2.5mm)

3.3 Combustion Behavior

In a previous study of the combustion of monosized glassy carbon spheres, consid- erable particle-to-particle variation in the ignition was observed[ll]. Modifications were made to the particle drying tower (reactor) to ensure that all particles were subjected to similar environments during this critical part of their evolution. The chars produced in the present study showed substantially less particle-to-particle variation than previously observed. For all furnace temperatures, the particles burned slowly at approximately the wall temperature when the oxidizing gas was air. Upon enriching the gas with oxygen a fraction of the particles burned fast emit- ting luminous radiation. The fraction of particles igniting increased with increased

oxygen content. Beyond a certain value of oxygen concentration, around 35-45%

depending on the kind of the char, all particles ignited and burned with bright flashes. At even higher oxygen partial pressures the combustion got increasingly fast and bright.

3.3 .1 Partial Combustion

Partial combustion experiments were conducted in air at furnace wall temperatures ranging from 1300 to 1600 K. The residence time in the hot zone was set to 2 s for all temperatures. The gas flows in the furnace and the injector were tuned to be approximately isokinetic. Particle residence time calculations took into account sedimentation (slip).

Microscopic observations m conjunction with density measurements indicate that the particles were burning in a shrinking core mode. Two SEM micrographs, one depicting a partially burned particle of plain polymer (PFA) char and the other partially burned particles formed from 17% tannic acid and PF A are shown in Fig. 7.

The copolymer particles appear to have suffered the highest weight loss. Since the oxidizing conditions in the two cases were similar, the particles containing tannic acid seem to be more reactive.

Apparent oxidation rates for the partially burned particles were inferred from two different methods: (a) mass reduction measurements, and (b) particle size re- duction and density changes. The mass reduction measurements were performed by monitoring the loss in mass of a known quantity of char as it passed through the combustion chamber and by correcting for the small losses in the injection lines. The particle size reduction was observed by optical and SEM microscopy. An average density was used. Additional data points, corresponding to lower temperatures, were obtained from thermogravimetric experiments (DuPont model 920 thermo- gra\·irnctric analyzer) on plain polymer at an oxygen partial pressure of 0.17 atm.

3.3.2 Pyrometry

Particles burning to completion in pure oxygen and 50% OrN2 mixtures at wall temperatures ranging from 1300 to 1500 K were monitored by the two-color opti- cal pyrometer. The ratios of the two intensity signals were substituted in Wien 's approximation of Planck's law, assuming gray body emission. Particle temperature- time histories were thus obtained for the entire combustion period. The uncertainty in these measurements is estimated to be ±40 K. The observed temperatures ranged from 1600 to 2850 K. The combustion times varied between 7 and 35 msec. In most cases the temperature increased in the first few milliseconds of combustion and then remained approximately constant to the end of burn-off. In general, the par- ticle temperatures increased with both gas and wall furnace temperatures. Three groups of traces of particles containing 25% carbon black burning in pure oxygen are shown in Fig. 8. The associated furnace wall temperatures ranged between 1350 and 1500 K and the gas temperatures between 1150 and 1300 K, since the particles burn to extinction only a few millimeters below the tip of the injector where the gas temperatures are generally lower than the average furnace temperatures. The average measured particle temperatures for this material ranged from 1900 to 2850 K and were higher than the temperatures exhibited by the other chars. The high combustion temperatures of the carbon black containing chars are attributed to the external surface area roughness, as well as to the large total pore volume of these chars and the availability of a large network of transitional pores. These pores serve as feeder pores[5] for the micropore matrix, thereby enhancing reaction by allowing oxygen to penetrate into the char volume. Close observation of Fig. 8 reveals that, while the particle temperature increased from 2000 K (case a, 1300K wall temper- ature) to 2600 K (case b, 1450 K wall temperature), the reaction time decreased from 20 to 14 ms. However, further increase of particle temperature to 2850 K (case c, 1500 J\ wall temperature) caused the reaction time to increase frorn 14 to

16 ms. Thus, the trend of the reaction rate increasing with temperature ceases once a certain temperature region is reached beyond which the rate decreases with tem- perature. The existence of this inversion region in carbon kinetics has been observed by Nagle and Strickland-Constable[l] as well as Walls and Strickland-Constable[24]

for oxidation of pyrolytic graphite rods. Park and Appleton[25J, also observed this behavior in the oxidation of submicron carbon black particles in a shock tube. The similarity of the the results obtained with graphite rods and particles, however, cannot be quantified because of the different time scales involved in the two types of experiments.

4 Oxidation Kinetics

Before analyzing the data to obtain the intrinsic oxidation rates of the synthetic chars it was necessary to verify that the combustion rate was not strongly controlled by film diffusion. A comparison of the experimentally observed, burnout times to the times required for combustion under film diffusion control will assess the importance of the role of the chemical kinetics. The burning time under diffusion control can be estimated using an analysis similar to that of Field et al.[26], assuming first order kinetics. Including the convective flow away from the char surface (Stefan flow),·

the oxygen conservation equation can be written:

(2)

where

m

02 is the flux of oxygen (g/cm²s),

m

101 is the net mass flux (g/cm² s), Db is the bulk diffusion coefficient, ^<Y₉ is the average gas density in the film, ^r is the distance from the center of the particle (cm) and Yo2 is the oxygen mass fraction.

Assuming the product of combustion to be only CO, the following mass balance holds:

(3) Integrating between the particle surface, r = a and infinity, r = oo and making use of the mass balance at the particle surface:

Oa

(~:)a

⁼ -mtot,s .

the following equation is obtained:

a -da dt

(4)

(5) Integrating between the initial and final radii and setting the particle surface oxygen concentration Yo₂^s equal to zero, the diffusion-limited combustion time tB can be obtained:

( ^•)

Oa

a; -

) IU'r,1 1

----·~---- (6)

56

where ai and a1 are the initial and final particle radii, R is the universal gas constant, and Tm is the mean temperature in the film in (K). The experimentally observed combustion times were about 3 times longer than tB for the largest particles burning in pure 0 2 at elevated temperatures, over 2000 K (worst case), and 2 orders of magnitude longer for the medium temperature experiments in air. Therefore, the combustion of the synthetic char particles was not controlled by external mass transfer.

As another estimate of diffusion limitations, oxygen concentrations at the par- ticle surface Yo₂^s for the initial radius

(r

=

a

0 ) were calculated from the integral of Eq. (5) by substituting for the observed combustion time tB,obs· This corresponds to the worse-case calculation since diffusional resistance is proportional to the particle size. The resulting Yo₂s values never fell below 50% of the free stream values.

Using the diffusion equation to calculate the oxygen concentration at the par- tide surface, thereby correcting for the external diffusion resistance, the observed combustion rate can be related to the apparent reaction rate constant. Combining

(7)

where Re is the chemical rate coefficient and the reaction order has been assumed to be equal to unity. Yo₂^s can be eliminated for the above equation by substituting:

Yo₂s = Yo200(l -1/;) and approximating the logarithmic term as

(8)

Substituting this expression in Equation (7), expressions for 'ljJ and rh101 ,, as a func- tion of Re can be obtained. The expression for

m

^101,s is then used to integrate Eq. (4) and obtain an expression for Re. From this expression of Re an average integrated value (between the initial and final radii) of r'ntot,s can be obtained f rorn

Equation (7) agalll. V\le call this average value the apparent rate f!a· An Arrhenius

plot of the apparent rate vs temperature is shown in Fig. 9. In general the reaction rate increases with temperature. The material containing carbon black appears to be the most reactive. Since this char is the only one that contains a transitional pore network superimposed on the microporous matrix, the higher reactivity can be attributed to enhanced oxygen diffusion inside the particles. In addition, the micropores themselves seem to be open to the oxidizer from the very beginning of combustion as suggested from the high initial total area of this material. The combustion rates differ from one char to another by as much as a factor of four.

Among the microporous chars the most reactive appear to be those formed from high percentages of tannic acid, the one formed from 18% PEG and the one formed from 35% glycerol and 7% triton X-100. The differences in reactivity between the chars could result from differences in the porous microstructure or from chemical composition and structure differences.

The intrinsic rate is the rate per unit total surface area that would be observed if there were no diffusional resistances. However, because of the diffusional resis- tances, not all of the internal surface area is equally available for reaction. A model describing the coupled diffusion and reaction of oxygen is needed if the intrinsic reaction rate is to be deduced from experiments in which there are appreciable diffusional resistances. An effectiveness factor 17 can be defined as the ratio of the apparent rate (that has been corrected for external diffusion resistance) to the rate which would be attained if there were no resistance to diffusion in the pores[4]. The effectiveness factor is a function of the Thiele modulus</> and the effective diffusivity

ry<f}(m

+

_{l)/ 2} = 1Pa(m

+

1),

2DeCs (9)

where C., is the oxygen concentration at the surface, m 1s the true reaction order related to the apparent reaction order, n, by the relation: n = ( m

+

1) /2, and 1 is the characteristic dimension of the particle defined as the ratio of the particle volume to the external area[32]. The Thiele modulus is given by

,./.. = 1(A a R cm-1 n-1)1/2

'I' G a • s e ' (10)

where Ac is the specific total surface area of the char. Previous workers have used the area determined prior to combustion[ 4], or the area measured after the com- pletion of a test run[33] to characterize the char. Since the surface area varies dramatically during combustion, as shown in Fig. 5, we used the specific BET sur- face area determined by averaging over the combustion process, from the beginning to the point of the final burnoff. The right hand side of Eq. (9) is a function of the reaction order and known quantities and, therefore, can be evaluated explicitly.

Mehta and Aris[34] present a theoretical analysis of the relationship between the effectiveness factor rJ and the group ry<J;²(m+ 1)/2. Following the approach of Smith and Tyler[4] with the major modification of using the average surface area, we use the plot of these variables presented by Mehta and Aris to determine r7.

If the pore diffusion coefficient is Dp the effective diffusivity is[27~,

(11)

2This equation incorporates Jamaludin's correction to Smith's Equation 1.5 of Ref. , {seep. ]()'ill foot.not.e). The correct implementation of this correction resnlt.s in a factor of 2 in the deno1ni11:11<>1 of Eq. 12 above as suggested in [50] not a factor of 4 that appeared in our previous paperil'I'.. Sm1tli had nsed a factor of 8 therefore his reported values are lower than what :d1011ld be, by ^11p to .1 Lier• ·I

of 4 when pore diffusion Lis a :otrong effect. This difference should be kept in 111incl when Pxa111i11111·.:

Fig. 10 oft he present wurk.

where E is the porosity and r is the tortuosity. With the exception of the carbon black containing char, all the chars under consideration are microporous, so the pore sizes are small compared to the mean-free-path of the gas molecules. Hence, the pore diffusion coefficient is that for Knudsen diffusion, i.e.,

cm² /s, (12)

where Tm is the mean temperature (K) in the film, and Mis the molecular weight of the diffusing gas (oxygen), and fp is the mean pore radius (cm). SAXS measure- ments were used to determine fp when such data were available. The carbon black containing chars have a bimodal pore size distribution and, therefore, require a spe- cial treatment. Making the smooth field approximation, the effective diffusivity for a network of micro- and transitional pores is[30]

D _K,micro Emicro , ( _{-+- -}1

₊

1

)-l _{- -}

^Etrans

T Db Dx,trans T

cm²/sec, ( 13) where the pore size and volume for the two pore classes was determined from porosymetry data. Finally, the intrinsic rate can be written as:

Pi= Pa

rJ/Ac<Ja (14)

Average areas and densities were used. The apparent order n was measured to be around 0.85 from partial combustion experiments of plain polymer char at 1500 I\

and oxygen partial pressures ranging from 0.05 to 0.3 atm. In this range of o~

concentrations, the measured particle temperatures were between the gas and the combustor wall temperatures.

The intrinsic rates plotted in Arrhenius form are shown in Fig. 10. All of the data presented are converted to an oxygen pressure of 101 kPa using the relationship:

g/cm s, 2 (LS)

where R; is a chemical rate coefficient. The intrinsic oxidation rates fall the best-fit line derived for a variety of coal chars[3J. The average a.ctivation energy for the chars at temperatures below 2000 l\ is 40 kcal,/rnole.

This difference in rates is most pronounced (up to an order of magnitude) at the low and medium particle temperatures (800-1700 K). Purified carbons have been reported to exhibit lower reaction rates than coal chars in the temperature region (400-800 K)[3]. The fact that purified chars exhibit lower reactivities suggests that the oxidation of carbon at low temperatures is very sensitive to catalysis by mineral impurities[S,35].

The discrepancy between the present rates and Smith's best-fit line decreases as the temperature is increased, the accord becoming fair at about 2000 K.³ This behavior suggests that the chars tend to a common chemical structure at high tem- peratures and that the importance of impurities diminishes. Previous experimental studies confirm that the catalytic effects become less pronounced with increasing temperature[35,36].

The reaction rates of the glassy carbon chars are, on average, one order of mag- nitude smaller than the N-S-Ckinetics for pyrolytic graphite at medium-to-low tem- peratures. The agreement between the reactivities of the two materials improves as the temperature increases. At temperatures around 1800 to 2000 K the agreement is very good indicating that the present chars approach a common graphitic struc- ture at elevated temperatures. Partial graphitization has been observed in x-ray diffraction measurements of glassy carbons that have undergone partial oxidation at medium temperatures (see Fig. 6 and TEM micrographs in Ref. 12). If 02-

catalyzed contraction of the interlayer spacing and two dimensional ordering of the amorphous glassy carbon matrix occured in the temperature range of 1300-1600 K, these changes would be accelerated with increasing temperature. This behavior is further enhanced by the availability of the oxidizer, since all high-temperature experiments were conducted in pure oxygen. It has been shown[37] that graphitiza-

3The reader should be reminded that Smith ':3 be:3t fit line would have :3hiftecl to higher values of rate by up to a factor of four had it been detennined :3imilarly to the pre:3ent work. Therefore, the difference between the rate:3 obtained herein alld S111itli'o line would have bee!l more pronounced.