Stoichiometry and coal-type effects on homogeneous vs. Heterogeneous combustion in pulverized-coal flames

C O M B U S T I O N A N D F L A M E 64:253-266 (1986)

253

Stoichiometry and Coal-Type Effects on Homogeneous vs. Heterogeneous Combustion in Pulverized-Coal Flames K. C. MIDKIFF, R. A. ALTENKIRCH, and R. E. PECK* Department o f Mechanical Engineering, University of Kentucky, Lexington, K Y 40506

Results of experiments to determine the extent of heterogeneous combustion in the rapid-devolatihzatton regime of fuel-rich bituminous and subbituminous coal-dust flames are reported. Major gas concentrations, gas and particle temperatures, and solids proximate and elemental compositions were measured as functions of residence time in pulverized-coal/O2/Ar flames stabilized on a flat-flame burner. Proximate fixed carbon and volatile matter measurements are corrected to account for devolatihzation exceeding that predicted by ASTM proximate analysis. The corrected proximate data are used to determine the fraction of volatile matter consumed heterogeneously in situ and the fraction released to the gas phase by pyrolysis. Seven to thirty-five percent of the initial corrected volatile matter is removed heterogeneously. A mass transfer analysis is applied to predict a time dependent critical particle size such that the volatiles flux emerging from larger particles is sufficient to prevent surface oxidation. Critical particle radii as large as 39 #m are calculated. Results of the volatde matter partitioning and critical radius calculations indicate that heterogeneous combustion is more important m the leaner flames and for the subbituminous coal.

INTRODUCTION The proposed use of staged combustion for reduced NOx emissions from pulverized-coal (p.c.) combustion points to the need for an improved understanding of the fuel-rich combustion that occurs in the primary zone of a staged combustor. Whether ignition and early combustion occur homogeneously in the gas phase or heterogeneously via in situ oxidation of the solid fuel matrix influences the composition of the combustion products. For the small particles considered here, i.e., with particle radius, rp, less than 40 #m, the mode of ignition and early combustion is the subject of considerable disagreement. The Faraday mechanism, characterized by this sequence--0) particle * Present address: Department of Mechanical and Aerospace Engineering, Arizona State University, Tempe, AZ 85287. Copyright © 1986 by The Combustion Institute Published by Elsevier Science Publishing Co., Inc. 52 Vanderbilt Avenue, New York, NY 10017

heatup and pyrolysis, (ii) homogeneous ignition and combustion of volatiles for the remainder of the period of vigorous devolatilization, and (iii) heterogeneous oxidation of the char--was unchallenged in years past [1] and continues to enjoy wide acceptance [2-5]. Furthermore, most proposed mechanisms of fuel-NOx formation during the early, rapid-devolatilization phase of p.c. combustion are implicitly based on the Faraday mechanism in that they rely exclusively on homogeneous NOx formation chemistry following the devolatilization of pollutant precursors [6-9]. However, model revision to include heterogeneous volatile matter consumption and fuel-NOx formation seems warranted in circumstances where significant surface burning in the rapid-devolatilization regime is evident. The amount of fixed carbon combustion and the relative degree of heterogeneous versus homogeneous removal of volatile matter can be estimated from proximate analyses (p.a.) of

0010-2180/86/$03.50

254 partially burned solids removed after various flame residence times. Such an analysis of bituminous coal combustion by Howard and Essenhigh [10, 11] appears to show substantial early heterogeneous combustion. Using a model based on droplet burning theory, Howard and Essenhigh computed a critical particle radius, re, below which some surface burning occurs and above which the emerging volatiles flux shields the particle from oxidation. During the early period of rapid weight loss, re was found to be about 30 #m. Because about 70% by weight of the coal was smaller than the critical size, this calculation provided additional support for the occurrence of significant surface burning. More recent experimental and analytical studies also found that heterogeneous ignition and/or early combustion may occur for sufficiently small particles [12-14]. In contrast, some investigations of p.c. devolatilization infer that the results of Howard and Essenhigh may be due simply to volatiles evolution in excess of that predicted by the ASTM p.a. test rather than to heterogeneous combustion [15, 16]. The purpose of the present study is to determine the importance of heterogeneous combustion at short residence times in fuel-rich flames for different particle loadings and coal types. Proximate and ultimate analyses of partially burned solids and measurements of gas and particle temperatures and major gas species concentrations were obtained as functions of residence time in both bituminous and subbituminous coal-dust/oxidizer flames. After correction for underprediction of the volatile content by p.a., the experimental results are used to partition the volatile matter lost from the particles into two fractions, one homogeneously evolved volatiles, and the other potential volatile matter consumed heterogeneously in situ. Finally, the results are used to calculate timedependent critical radii, similar to the re of Howard and Essenhigh, to estimate the significance of heterogeneous combustion. EXPERIMENTAL

The 40 cm 2 Meker-type burner described in previous work [7, 17, 18] was used in the

K . C . MIDKIFF ET AL. present experiments. Pulverized coal, supplied by a pressurized, fluidized-bed-type feeder [19], and oxidizer are premixed and fed vertically upward through the burner. The laminar, freeburning, coal-dust flames are attached to the uppermost screen of a set of four cooled, stainless-steel, smoothing screens at the burner outlet. For the subbituminous coal experiments, it was necessary to insert a 0.16 cm thick copper plate perforated with 0.32 cm diameter holes between the upper and lower pairs of screens in order to prevent flashback. Vigorous vibration of the burner with a small, compressed-airdriven vibrator prevented dust accumulation and improved flame smoothness. Flames are shielded from room drafts by placing the burner within an aluminum chimney. The burner is mounted on a screw-driven translator so that its vertical location may be varied with respect to a probe attached to the chimney wall. Gas samples were extracted at various heights along the burner's vertical centerline using a 0.46 cm i.d. water-cooled suction probe. The filtered and dried gas samples were analyzed online by NDIR for CO and CO2, by polarographic analysis for O2, and by gas chromatography for H2. Solids samples were extracted, during separate experiments, from the same flame positions using a 0.6 cm i.d. water-quench probe. The quenched samples were fed to two water-filled 250 ml impingers connected in series to separate condensed matter from the gas. Captured solids were filtered from the impinger liquid, dried, and later subjected to proximate analysis by automated thermogravimetry and ultimate analysis using a Carlo-Erba 1106 CHN Analyzer and a Fisher 470 Sulfur Analyzer. Additional details of the gas- and solids-sampling procedures are given in Ref. [20]. Gas and particle temperatures were separately measured by making infrared emission and transmission measurements using a specially designed, four-wavelength pyrometer that detects radiant intensity at three particle wavelengths (1.6, 2.3, and 3.8 #m) and at the fundamental CO2 band (4.4 #m). Gas- and solidphase temperatures were calculated by solving the radiative transfer equation written along the pyrometer line of sight assuming gray particle

HOMOGENEOUS VS. HETEROGENEOUS COAL COMBUSTION behavior and that the radiant intensity about any point along the line of sight within the flame is isotropic. More sophisticated radiation modeling that includes more accurately the effects of scattering and nongray behavior show that this simplified approach is adequate [21]. For the measured temperatures, an uncertainty analysis and an estimate of errors due to edge cooling of the flame and due to the presence of soot have been reported elsewhere [17, 22, 23]. RESULTS Two pulverized coals, an Eastern Kentucky high volatile A coal and a Wyoming Power River Basin subbituminous A coal, were burned in 285 and 470 mg/1 coal-dust/23.4% 02/76.6% Ar flames. The cold gas speed through the burner was maintained at 13.8 cm/s, and the cooling water flow rates were 2, 0.5, and 5 cm3/s to the screen and to the water-quench and gas-sampiing probes, respectively. Prior to burning, both coals were pulverized, mixed with 1% by weight Aerosil (SiO2) to prevent caking, and sieved to < 7 4 /~m. Proximate and ultimate analyses, size distributions determined by sieving, and fuel equivalence ratios for the processed coals and given flame conditions are shown in Table 1. The fuel equivalence ratio calculated with respect to the ASTM volatile matter is computed by assuming that the volatiles fuel consists of the volatiles carbon, which is determined as the difference between the ultimate and fixed carbon, and all of the hydrogen, oxygen, and sulfur. The temperature profiles from the infrared pyrometer measurements, which have been published previously [17, 18] but are reproduced here for completeness, are shown in Fig. 1. Distance above the burner screen was converted to residence time using the measured gas temperature profiles and the one-dimensional continuity equation, assuming constant mole number and negligible particle slip velocities. The particle temperature (Tp) peaks ahead of the gas temperature (Tg) in all four flames, and the peak heights are higher and later in the leaner 285 mg/l flames. Initial heating rates of approximately 105-106K/s are observed for all condi-

255

tions. Early particle and gas temperatures differ little in the two bituminous coal flames, but the particle temperature overshoots the gas temperature by nearly 400K early in the subbituminous flames, possibly because of heterogeneous oxidation or measurement interferences [23, 24] caused by the presence of soot. After the particle temperatures begin to decline, gas temperatures peak at 100-375K hotter than the particles, and both temperatures decline thereafter due to solid-phase radiant heat loss and/or the occurrence of endothermic processes such as the heterogeneous reduction of CO2 to CO. The measured dry gas mole fractions for the bituminous and subbituminous coals are shown in Figs. 2 and 3, respectively. Oxygen is quickly depleted in the two richer flames, but remains in significant amounts for about 15-20 ms in the leaner 285 mg/1 flames. After an early burst of rapid formation, CO2 levels remain virtually constant in the 285 mg/l flames but gradually diminish in the richer flames as CO2 is reduced to CO. The formation of H2 and CO is considerably more protracted than that of CO2, and it is more extensive in the richer flames and for the subbituminous coal. Assuming that elemental hydrogen released from the solids appears as either H2 or water, the H20 mole fractions reach about 10 and 15% in the 285 and 470 mg/1 flames, respectively, for both coals burned. The proximate analyses of the partially burned solids were used to determine the mass fraction of the initial or unburned, dry, ash-free coal (d.a.f.c.) remaining as ASTM fixed carbon, Cp, and volatile matter, Pp, at various residence times by using the ash content as a tracer. In addition to the limited sample quantity available for analysis, three other possible sources of error in determining the true remaining fixed carbon and volatile matter fractions are ash vaporization [25, 26], collection of soot and tar along with the partially burned solids, and devolatilization in excess of that predicted by p.a. Ash begins to vaporize only after most volatiles evolution has occurred and continues through char burnout [27]. Because ash vaporization is expected to be kinetically limited [25], and because it is only the early, rapid-devolatili-

256

K.C.

MIDKIFF

ET AL,

TABLE 1

Coal Analysis and Size Distribution

Proximate Analysis (as received)

Bituminous Weight (percent)

Subbttummous Weight (percent)

1.6 31.6 47.7 19.1

100.0

Moisture Volatile matter Fixed carbon Ash

Total

Ultimate Analysis

Bituminous Weight (percent)

Subbituminous Weight (percent)

7.8 41.2 41.1 10.0

C H S N O Ash

68.58 4.61 1 53 1.63 4.24 19.41

62.20 4.66 0.53 0.88 20.88 10.85

100.0

Total

100.00

100.00

Bituminous heating value 26,540 kJ/kg Subbituminous heating value 22,472 kJ/kg Size Distribution by Sieving

Range 40-74 30-40 20-30 10-20 0-20

t~m t~m #m #m #m

Bituminous Weight (percent)

Bituminous Number (percent)

Subbitummous Weight (percent)

Subbitummous Number (percent)

16.9 13.5 19.8 38.1 11.7

0.1 0.3 12 10.6 87.8

18.3 18.3 25.3 25.4 12.7

0.1 0.4 1.5 6.8 91.3

Fuel Equivalence Ratios (4~) Flame Condition 285 470 285 470

mg/I mg/l mg/I mg/I

bituminous bituminous subbituminous subbitummous

~bw r t Whole Coal

w.r.t. ASTM Volatile Matter

1.99

0.80

3.30 I 57 2.59

0.55 091

1.34

HOMOGENEOUS VS. HETEROGENEOUS COAL COMBUSTION 2500

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zation phase that is of interest here, errors due to the alteration of ash content should be minimal. In fact, Quann and Sarofim [28] found, for p.c. burned in a 20% 02/80% N2 oxidizer, that less than 2.0 % of the ash of several bituminous coals and about 1.1% of the ash of a subbituminous coal similar to ours were vaporized. Because their particle temperatures were comparable to ours and their residence times were sufficiently

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long ( - 3 0 0 ms) for char burnout to occur, the ash losses reported by Quann and Sarofim should set upper limits on the ash vaporization occurring in our flames. Using ash as a tracer, the relative error in C o or Pp due to these low levels of ash vaporization is approximately equal to the fraction of vaporized ash. Consequently, calculated values of Cp or Pp are no more than 2 % greater than they would be in the absence of ash vaporization. The collection of soot and tar along with the partially burned solids is, as mentioned above, another possible source of error. Assuming that p.a. primarily accounts for soot as fixed carbon, the value of Cp will be overestimated not only because the proximate fixed carbon will appear larger, but also because the proximate ash will appear smaller. Similarly, if p.a. accounts for tar as mostly volatile matter, the value of Pp will be overestimated because the volatile matter fraction will appear larger and the ash fraction smaller. Lacking quantitative knowledge of the fraction of volatiles converted to soot or appearing as tar, the soot or tar capture efficiency of the solids-sampling probe water filtration system, or the relative proportion of the soot or tar accounted for as fixed carbon or volatile matter, we are unable to evaluate the magnitude of this

258 error. However, because the rate constant for tar pyrolysis is much greater than the rate constant for tar evolution in our relatively hot flames, the tar concentration should be quite low. In contrast, because of the fuel-rich character of our flames, it is likely that the soot formed will persist. Thus is seems probable that the error resulting from the collection of soot with the solids considerably exceeds that resulting from the collection of tar. A final source of error in interpreting the fixed carbon and volatile matter results is that the extent of devolatilization probably exceeds that predicted by p.a. Numerous experimental studies have shown that the ASTM p.a. underestimates the amount of volatile matter released at p.c. combustion temperatures and heating rates [15, 16, 25, 29-33]. A quantity Q has been introduced that may be interpreted as the ratio of the true weight loss by devolatilization to the weight loss of ASTM proximate volatile matter. Thus, devolatilization only to the extent predicted by ASTM p.a. yields a Q of unity, and additional devolatilization results in Q > 1.0. For a particular coal at constant thermal conditions, Q is approximately constant, i.e, Q is not dependent on extent of devolatilization [ 15, 31 ], and some studies have found Q to be independent of heating rate [14, 30, 32]. The quantity Q clearly increases with increasing particle temperature [14-16, 25, 33]. Correlations of Q with coal rank and other coal properties have been reported [16, 33], although the results obtained often conflict with the results of other devolatilization studies. In order to correct the p.a. of the partially burned solids for Q > 1.0, both the functional form of Q as well as its effect on the apparent volatile matter and fixed carbon contents must be determined. To approximate the effect of Q on the proximate quantities, the definition of Q used here is consistent with its usage in the previously mentioned devolatilization studies, i.e., Q is the instantaneous ratio of the true rate of mass loss by devolatilization, dW/dt, to the rate of loss of ASTM proximate volatile matter,

K . C . MIDKIFF ET AL.

dPp/dt, by devolatilization; thus, dW/dt Q= [ ~ ]

devolatd,zat,on"

(1)

The definition of Q given by Eq. (1) can be applied unambiguously to pure devolatilization experiments, but some error would arise in calculating the actual volatile matter loss in our experiments by integrating dW/dt as given by Eq. (1) because our measured proximate volatile matter profiles, Pp(t), reflect heterogeneous as well as homogeneous volatile matter removal. Consequently, the effect of Q > 1.0 is approximated by determining a corrected volatile matter fraction of the initial d.a.f.c, remaining in the solids, P, that includes both the ASTM proximate volatile matter and the proximate fixed carbon that, in reality, is devolatilizable at the flame conditions. The fraction of initial d.a.f.c. remaining in the solids as proximate fixed carbon that is not devolatilizable, designated as C, is the corrected fixed carbon. Expressions for P and C in our flames can be developed by noting that, in terms of the ASTM proximate quantities, the ratio of the time rate of change of the corrected volatile matter fraction, dP/dt, to the rate of change of the proximate volatile matter fraction, dPp/dt, is

dP/dt

(dPvv/dt) + (dPps/dt) + (dCpJdt)

dPpldt

(dPpv/dt) + (dPps/dt)

(2) Here dPpv/dt and dPpJdt are the rates of removal of Pp by devolatilization and by heterogenerous combustion, respectively, and dCp+/dt is the rate of removal of proximate fixed carbon, C v, by devolatilization. For the case of simultaneous heterogeneous and homogeneous combustion, Eq. (1), which defines Q, can also be written in terms of the above proximate quantities as

( dPpv/dt ) + ( dCpv/dt ) Q =

dPpJdt

(3)

HOMOGENEOUS VS. HETEROGENEOUS COAL COMBUSTION The corrected volatile matter fraction can be calculated by solving Eq. (2) for P, once all remaining terms are replaced by known quantities. Determining the amount of proximate volatile matter that is removed heterogeneously, Pps, is analogous to the partitioning of volatile matter loss into heterogeneously and homogeneously removed fractions that was performed by Howard and Essenhigh [1 l]. The proximate volatile matter proportioning procedure is based on four considerations: (i) pyrolysis of p.c. particles appears to be a volumetric process [10], (ii) temperature gradients within the small particles considered here are negligible, (iii) all particles collected at a particular residence time have a similar temperature history, and (iv) heterogeneous combustion removes both corrected fixed carbon and undecomposed proximate volatile matter in proportion to the relative abundance of each. Taking these factors into account leads to the conclusion that all particles, regardless of size, of a particular solids sample should have the same ratio of proximate volatile matter to corrected fixed carbon, namely, Pp/C. Applying the above results, for a solids sample extracted at a given residence time, the rate of proximate volatile matter removal by heterogeneous com-

dP (It

dPp Q dt

-

259

bustion is the product of the rate of corrected fixed carbon removal, dC/dt, which is directly proportional to the overall rate of mass loss by heterogeneous combustion, and the ratio of proximate volatile matter to corrected fixed carbon; thus

dPpJdt - Pp dC C dt

(4)

The rate of proximate volatile matter removal by devolatilization is calculated by balancing the proximate volatile matter loss rate, i.e.,

dPpv/dt=dPp dt

dPp~ dt

Finally, the unknown corrected fixed carbon fraction, C, that appears in Eq. (4) can be expressed in terms of P and the known proximate quantities by a mass balance on the fraction of initial d.a.f.c, remaining in the solids:

C= Pp + Cv- P.

-

The behavior of Eq. (7) is easily determined for the limiting cases of unity Q and pure devolatilization. Clearly, for Q = 1.0, Eq. (7) yields dP/ dt = dPp/dt. Also, in the limit of mass loss exclusively by devolatilization, where d W / d t = dCp/dt + dPp/dt, the definition of Q given by Eq. (1) applies, yielding

~pp/~

(6)

The following differential equation for determining P as a function of residence time is obtained by substituting Eqs. (3)-(6) into Eq. (2):

( Q - l)[Pp/( Cp + lap P)]{[(dCp/dt) + (dPp/dt)l/(dPp/dt)} l - ( Q - l)[Pp/(Cp+Pp-P)]

(dCp/dt) + (dPp/dt)

(5)

] _] devolatlhzatlon~--Q"

(8)

Equation (8) may be substituted into Eq. (7) to show that (dP/dt)/(dPp/dt) = Q for the case of no heterogeneous combustion. An initial value of the corrected volatile matter fraction, P0,

(7)

which is unknown, is needed to determine P(t) from Eq. (7). This quantity can be approximated by exploiting the fact that (P0 - P(t))/(Ppo Pp(t)) = Po/Ppo as t --* oo. Applying this relation at some large experimental time, tx, where Pp is nearly zero, gives that P0 = Ppo(P(tx)/Pp(tO). The value of P0 is then chosen so that the P(t) profile obtained from Eq. (7) satisfies this latter condition. The final step in calculating C and P is the determination of Q. An upper bound on Q, Q . . . . may be estimated by assuming that all weight loss of d.a.f.c, at a particular instant is caused only by devolatilization, even though some proximate fixed carbon loss apparently

260 occurs. Under this assumption, Qmaxis identical to the Q given by Eq. (8). Selection of the lowest computed values resulting from the application of Eq. (8), in the residence time range considered, to analytical, least-square-error fits to the experimental Cp and Pp data yielded estimates of Q .... of 1.25 for the bituminous coal and 1.39 for the subbituminous coal. No devolatilization studies of our bituminous coal have been published, but Rau and Robertson [29] determined that Q = 1.17 at about 1450K for a Wyoming subbituminous B coal, which sets a probable minimum for our subbituminous coal. In order to estimate Q for this study, note has been taken of the previously cited studies showing that Q is independent of heating rate and extent of devolatilization; thus it is assumed that Q is a function only of coal type and particle temperature. Values of Q as a function of particle temperature were calculated for each coal as straight-line, least-square-error fits to Q values at 1600 and 2400K computed as Q(Tp) = AW(Tp)/App from experimental, isothermal devolatilization data reported by Neoh and Gannon [33], and to the point Q = 1.0 at the ASTM p.a. temperature of 1223K. An expression for Q(Tp) for our subbituminous coal was estimated by a fit to devolatilization data reported for a Rosebud subbituminous B coal, and devolatilization data for a Cedar Grove HVA bituminous coal similar to our own were fit to approximate Q for our bituminous coal. The resulting values of Q are Q = 2.98 × 10 4Tp + 0.638 a n d Q = 3.66 × 10 4Tp + 0.569 for the bituminous and subbituminous coals, respectively, for particle temperature in K. The correlation coefficients, r, obtained for both straightline fits yielded r 2 > 0.99. Using the above values of Q and the analytical, least-square-error fits to the experimental proximate data, the corrected volatile matter and fixed carbon fractions, P and C, were calculated as functions of residence time by first solving Eq. (7) and then applying Eq. (6). The computed P and C profiles are shown as solid lines in Figs. 4 and 5 for the bituminous and subbituminous coals, respectively. Corrected experimental volatile matter and fixed carbon fractions of

K. C. MIDKIFF ET AL. o

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Fig. 4. Expemmentalsolids proximate compositzonfor 285 mg/I and 470 mg/l E. Ky. bituminous coal-dust/23% 02/ 77% Ar flames, corrected for Q = 2.98 × 10-4Tp + 0.638.

the unburned d.a.f.c., Pexp and C e x p , w e r e then calculated from the measured proximate data, Ppexp and Cpexp, using the relations

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(9)

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HOMOGENEOUS VS. HETEROGENEOUS COAL COMBUSTION

where Ppflt is the least-square-error fit to the measured proximate volatile matter, and P is the estimate of the corrected volatile matter fraction calculated using Eq. (7). The corrected experimental data, Cexp and Pexp, are also shown in Figs. 4 and 5. The corrected fixed carbon and volatile matter results in Figs. 4 and 5 show that volatile matter is rapidly and nearly completely removed at all conditions. Ninety percent of the total volatile matter loss occurs in approximately 20 ms in both 285 mg/1 flames and 13 ms in both 470 mg/1 flames, which corresponds to about 1.9 and 1.0 cm above the burner screen for the two respective particle loadings. A greater fraction of fixed carbon is lost at the leaner stoichiometry and in the subbituminous coal flames. Fixed carbon removal occurs at the onset and throughout the period of rapid volatiles evolution in all four flames, which strongly suggests the occurrence of some heterogeneous combustion. Compared with the bituminous coal-dust flames studied by Howard and Essenhigh [10, 11], the extent of volatiles removal in our flames is similar but considerably more rapid, and the (corrected) fixed carbon removal in our fuel-rich flames is not as great except in the 285 mg/1 subbituminous coal flame. Fixed carbon and volatile matter loss in our flames is more complete than in the bituminous p.c. flames supported on a downward-fired, fiatflame burner studied by Smoot et al. [2].

261

removal of ASTM proximate volatile matter, dPps/dt, can be applied to derive an analogous

expression for the rate of heterogeneous removal of corrected volatile matter: P dC

dVs/dt -

(11)

Cdt

Values of Vs for the four flames were calculated by numerically integrating Eq. (11) using the previously calculated C and P profiles. Values of Vv, the pyrolyzed volatile matter, were then calculated by closing the volatile matter balance, i.e., Vv = P0 - P - Vs. The volatile matter proportioning results appear in Fig. 6, which shows Vv and Vs as functions of residence time. Heterogeneous volatile matter consumption ranges from more than 3% of the initial d.a.f.c, in the 470 mg/1 bituminous flame up to nearly 21% of the initial d.a.f.c, in the 285 mg/1 subbituminous flame. Heterogeneous combustion is more extensive for the leaner stoichiometry and is clearly more important in the subbituminous flame, as evidenced by larger Vs fractions in the 470 mg/1 subbituminous flame than in the leaner 285 mg/l bituminous flame. The opposite trend is observed for Vv with respect to stoichiometry; i.e., the fraction of volatile matter removed by

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ANALYSIS AND DISCUSSION

Partitioning of the Volatile Matter Loss Using the corrected fixed carbon and volatile matter data, the volatile matter loss, P0 - P, may be divided into two components [11]. The first component, Vv, is the fraction of the initial d.a.f.c, that is pyrolyzed volatile matter evolved to the gas phase as " v o l a t i l e s . " The second component, Vs, is the fraction of the initial d.a.f.c, that has been removed from the solids by the heterogeneous combustion of undecomposed volatile matter. The same analysis that was used to determine the rate of heterogeneous

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3.66 × 10-4Tp + 0.569.

262

K. C. MIDKIFF ET AL.

pyrolysis is larger for the richer 470 mg/1 flames. Because the initial volatile matter fraction, Po, is considerably larger in the subbituminous than in the bituminous coal, similar fractions of the initial d.a.f.c, are removed by devolatilization for the two coals. However, the fraction of the initial volatile matter removed by devolatilization, Vv/Po, is larger in both bituminous flames than it is in either subbituminous flame. It should be noted that the overestimation of Cp caused by the collection of soot along with the solids sample results in lower calculated values of Vs; i.e., the inability to separate soot from the unburned solids causes the extent of heterogeneous combustion to be underpredicted. In order to determine its sensitivity to the magnitude of Q, the volatile matter proportioning calculation was performed for the 285 mg/1 subbituminous flame after using Eq. (6) and (7) to calculate the corrected fixed carbon and volatile matter profiles for various values of Q. The results of this calculation are shown in Fig. 7 for the Q values listed in the figure caption. These linear relations for Q(To) all yield Q = 1.0 at Tp = 1223K and Q1-Q6 = 1.0, 1.1, 1.2, 1.3, 1.4, and 1.5, respectively, at Tp = 2000K. The Q value used for the subbituminous volatile matter proportioning shown in Fig. 6 is Q4, o

60

I 285

I mglf

50

I SIBA Wyo

I

,&c,~

c "6 E

40

~_ 30 d o d 20

"6

~ °o

I0

15

Residence

5

Time

20

25

30

35

m MiIhseconds

Fig. 7. Effect of Q on loss o f v o l a t d e m a t t e r by heterogeneous c o m b u s t i o n and by d e v o l a t i l i z a t i o n for a 285 m g / l W y o . s u b b i t u m m o u s c o a l - d u s t / 2 3 % 0 2 / 7 7 % Ar flame, c a l c u l a t e d w i t h Q l = 1.00, Q2 = 1.29 × 10 4Tp + 0 . 8 4 3 , Q3 = 2.57 x 10-4Tp + 0 . 6 8 5 , Q , = 3.66 × 10-4Tp +

0.569, Q5 = 5.15 x 10-*Tp + 0.370, and Q6 10-4Tp 4- 0 . 2 1 2 .

=

6.44 x

which gives Q = 1.3 at Tp = 2000K. The volatile matter proportioning is fairly sensitive to the value of Q, with the apparent extent of heterogeneous combustion diminishing with increased Q. But even for Q6, which is significantly larger than the Qmax estimated from the experimental data, about 19% of the initial volatile matter is consumed heterogeneously. Using the relationships described above, a rate constant for volatile matter pyrolysis may be calculated that accounts for Q > 1.0 and parallel heterogeneous volatile matter removal. Assuming a first-order devolatilization process, the rate of volatiles release is proportional to the amount of volatile matter remaining, i.e., dVv/dt = kP. Here k is an Arrhenius rate constant, k = A e x p [ - TA/Tp], where A is the preexponential factor in reciprocal seconds, and TA is the activation temperature in K. Using kP for dVv/ dt and the right-hand side of Eq. (11) for dVs/dt in the differentiated form of the volatile matter closure equation gives, after integrating, In

=

s k(r) dr.

(12)

0

A standard, nonlinear, Levenberg-Marquardt least-square-error algorithm was used to fit the integrated rate expression of (12) to the corrected experimental Cexp and Pexp data, shown in Figs. 4 and 5, and to the experimental Tp data, shown in Fig. 1. The integrated rate expression was fit to the combined 285 and 470 mg/1 data for each coal, which resulted in k = 1030 exp[ - 4742/Tp] s- i for the bituminous coal and k = 2040 e x p [ 6703/Tp] s-I for the subbituminous coal, with temperature in K. At temperatures near 2000K, the rates reported here are very similar to those measured by Khobayashi et al. [25] for bituminous and lignite coals, although their activation temperatures are higher than ours. Extrapolated to the lower temperatures at which other devolatilization studies have been reported, our rates seem typical, somewhat higher, for example, than those reported by Howard and Essenhigh [10], or Anthony et al. [30], but lower than those reported by Badzioch and Hawksley [16].

HOMOGENEOUS VS. HETEROGENEOUS COAL COMBUSTION Though these rate constants are quite similar to those reported by others and may be quite useful for calculational purposes, no great physical significance should be attached to them, because, as many studies have shown, the process of coal devolatilization is too complex to be adequately modeled from a mechanistic standpoint by a simple, first-order mechanism.

Calculation o f the Critical Particle Radius A mass transfer analysis may be used to determine a critical particle radius, rp = re, as a function of residence time such that the volatiles flux emerging from particles with rp > re is sufficient to drive the flame away from the particle surface, but particles having rp < rc burn at the particle surface. To derive an expression for re, homogeneous combustion is assumed, and equations for the conservation of molecular oxygen and volatiles in the gas surrounding a particle are combined by assuming equal mass diffusion coefficients and applying a Schwab-Zeldovich transformation. The resulting equation is solved for the flame-standoff radius, rf, using a Spalding B-number mass transfer analysis. The value of the critical radius, similar to that obtained by Howard and Essenhigh [11], but incorporating the details of the assumptions made here, is obtained as the particle radius at which re = rp: rJ =

3OgD In[1 - fvmo~o- mvo~] pc Ydaf dVddt

(13)

Here Oc is the coal density, Ydaf is the mass fraction of the unburned coal that is dry and ash free, both of which are constants, pg is the gas density, D is the mass diffusion coefficient of 02 in Ar, fv is the stoichiometric fuel/oxygen mass ratio for 'the combustion of volatiles, and mo~ and mv~ are, respectively, the mass fractions of 02 and volatiles far from the particle surface, all of which are calculated as functions of residence time. Equation (13) was solved to determine the critical radius as a function of residence time for each of the four flames studied. A coal density

263

of Oc = 1250 kg/m 3 was used for both coals. The values of Ydaf were 0.793 for the bituminous coal and 0.822 for the subbituminous coal. The mass diffusion coefficient, D, was approximated by the Fuller, Schettler, and Giddings correlation, presented in Ref. [34], for the binary diffusion of 02 in Ar. The volatiles release rate, dVv/dt, was determined by numerically differentiating the results obtained for V~. In order to calculate fv, it was necessary to estimate the instantaneous elemental composition of the volatiles flux. The analysis used to establish the volatiles proportioning procedure may be extended to show that

Yv, =

at

~j.~'

(14)

where Yv, is the mass fraction of element i in the instantaneously evolved volatiles, and P, is the fraction of the initial d.a.f.c, appearing as element i in the volatile matter. The elemental composition of the volatile matter was estimated from a least-square-error fit to the solidssamples ultimate analysis data by assuming that all H, S, N, and O atoms are volatile matter and that the volatile carbon is the difference between the ultimate and fixed carbon. Values o f f v as a function of residence time were calculated for the complete conversion of volatiles having the composition given by Eq. (14) to N2, CO2, H20, and SO2. This procedure' yields fv values of around 0.33 for both bituminous coal flames and values of about 0.70 for the two subbituminous coal flames. This difference in fv is largely due to the greater oxygen content of the subbituminous volatiles. The specification of mooo, mv~, and Og requires a knowledge of the ambient gas composition. In order to obtain an element balance between the measured major gas mole fractions and the input mass fed as oxidizer or released from the solids, and to allow a continuous distribution of re as a function of time to be obtained, the gas measurements were adjusted so that C, H, O, and Ar atoms were conserved. Assuming that CO, CO2, H20, H2, 02, and Ar were present, the two additional conditions used

264

K . C . MIDKIFF ET AL.

to determine the six mole fractions were that both the experimentally measured H2 mole fraction and CO/CO2 ratio be matched. The mechanical moisture was assumed to be released from the fuel at zero residence time, and the release of C, H, and O was estimated from the previously mentioned fits to the solids ultimate analyses. Using the gas mixture resulting from this adjusting procedure, mo~ was set equal to the 02 mass fraction, and the ambient volatiles mass fraction, mv~, was computed as the sum of the H2 and the CO mass fractions. The gas density, pg, was calculated assuming ideal gas behavior and a pressure of 1 atm using the measured gas temperatures and the average molecular weight of the mixture determined by the above procedure. The critical radii calculated using Eq. (13) are shown in Fig. 8 as functions of residence time. Critical radii in all four flames are initially large compared with the initial mass-mean radius of 12 #m but decline to zero because the logarithmic term of Eq. (13) goes to zero as mo~o decreases and mvoo increases. However, for flames considerably more fuel lean than ours [11], the critical radii predicted by Eq. (13) would increase with residence time because the devolatilization rate, dVv/dt, would become small while the logarithmic term remained finite. As was the case for the extent of heterogeneous volatile matter removal, re is larger and in excess of zero for a longer period of time in the leaner 285 mg/l flames and for the subbituminous coal. The stoichiometry effect, of course,

~ ~o _~ 2 o

u o

0

I0

15

is primarily due to the larger ambient 02 mass fractions observed in the leaner flames. Larger critical radii are calculated for the subbituminous coal because its flames are somewhat leaner for a given particle loading, have lower rates of volatiles evolution, and have higher values of fv. The fact that fv is about twice as large in the subbituminous flames as it is in the bituminous flames is by far the most important cause of the coal-type differences in the size of rc, and, as mentioned earlier, is a result of the greater oxygen content of the subbituminous coal volatiles. Comparing computed rc values with either the maximum particle radius of 37/zm or the massmean radius, it is apparent that some heterogeneous combustion is occurring for a large fraction of the particles in the early, rapiddevolatilization-and-combustion period. Although the period for which rc > 0 is brief, more than 40% and up to 70% of the total mass loss occurs during this time. The critical radii calculated for our bituminous coal are smaller than the rc of 33 /~m for a Pittsburgh Seam bituminous coal reported by Howard and Essenhigh [11], thus implying less heterogeneous conversion of volatile matter in our flames. However, it must be remembered that our flames, unlike those of Howard and Essenhigh, are fuel rich. In order to gauge the influence of Q, r~ was calculated for the 285 rag/1 subbituminous flame using fixed carbon, volatile matter, and volatile matter proportioning results corrected for various values of Q. Resulting values of r~ as a function of residence time are shown in Fig. 9 for the same values of Q listed in the caption of Fig. 7. The critical radius calculation, like the volatile matter proportioning, is fairly sensitive to variations in Q. But even for the large Q6, the bulk of the coal mass is comprised of particles smaller than the computed critical radius of about 25 t~m, so heterogeneous combustion is not insignificant.

20

Residence T i m e in MiIhseconds

Fig. 8. Critical particle radii for E. Ky. bituminous and Wyo. subbitummous coal-dust/23% 02/77% Ar flames, using Q values listed m the Fig. 6 caption.

CONCLUSIONS The analysis method used to partition the volatile matter into heterogeneously and homogene-

HOMOGENEOUS VS. HETEROGENEOUS COAL COMBUSTION

2 8 5 mg/ll Wyo SBA

QI50 ~ & ' "

-

'

I

E o b 40 =_

50

o E

20

265

by the American Electric Power Company. Material analysis services were provided by the Institute f o r Mining and Minerals Research at the University o f Kentucky. K C M was supported in part by a U.S. Department o f Energy (DOE) Grant No. DE-FGO582ER75039. However, any opinions, findings, conclusions, or recommendations expressed herein are those o f the authors and do not necessarily reflect the views o f the DOE.

~0

REFERENCES 0

0

5

I0

15

20

1.

Residence Time ir~ Milliseconds

Fig. 9. Effect of Q on critical particle radii for a 285 mg/l Wyo. subbituminous coal-dust/23% 02/77% Ar flame, calculated using Q values listed m the Fig. 7 caption.

ously consumed portions and to compute the critical particle radius, as developed by Howard and Essenhigh [11], and extended here to account for Q > 1.0, has proved to be useful for investigating the importance of heterogeneous combustion at early residence times in coal-dust flames. The experimental results reported here and their analysis show that heterogeneous combustion can be quite important in the rapiddevolatilization phase of pulverized-coal flames, even at fuel-rich conditions. The volatile matter proportioning and critical radius calculations indicate that heterogeneous removal of undecomposed volatile matter is more extensive in the leaner flames and for the lowerrank subbituminous coal. A high coal oxygen content has been identified as one major contributing factor to the increased occurrence of heterogeneous combustion in the subbituminous coal-dust flames. The results reported here point to the necessity of including heterogeneous processes in models of pollutant formation during the early, rapid-combustion period of pulverized-coal f l a m e s .

We thank the National Science Foundation for providing partial support for this work under Grant CPE-7926312. Bituminous coal was provided by the R A C Mining Corporation, and subbituminous coal was provided

2.

3.

4.

5.

6.

7. 8.

9.

10. 11.

12. 13.

Essenhigh, R. H., Sixteenth Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1977, pp. 353-374. Smoot, L. D., Horton, M. D., and Williams, G., Sixteenth Symposium (lnternattonal) on Combustion, The Combustion Institute, Pittsburgh, 1977, pp. 375-388. Hertzberg, M., Cashdollar, K. L., and Lazzara, C. P., Eighteenth Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1981, pp. 717-730. Smith, P. J., Fletcher, T. H., and Smoot, L. D., Eighteenth Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1981, pp. 1185-1202. Hertzberg, M., Cashdollar, K. L., Ng, D. L., and Conti, R. S., Nineteenth Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1982, pp. 1169-1180. Pohl, J. H., and Sarofim, A. F., Sixteenth Symposium (International) on Combustion, The Combustion Inst]tute, Pittsburgh. 1977, pp. 491-502. Altenkirch, R. A., Peck, R. E., and Chen, S. L., Combust. Sci. Technol. 20:49-58 (1979). Rees, D. P.. Smoot, L. D., and Hedman, P. O., Eighteenth Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1981, pp. 1305-1312. Smith. P. J., Hill, S. H., and Smoot, L. D., Nineteenth Symposium (lnternattonal) on Combustion, The Combustion Institute, Pittsburgh, 1982, pp. 1263-1270. Howard, J. B., and Essenhigh, R. H., Ind. Eng. Chem., Process Des. Dev. 6:74-84 (1967). Howard, J. B., and Essenhigh, R. H., Eleventh Symposium (International) on Combustton, The Combustion Institute, Pittsburgh, 1967, pp. 399-408. Annamalal, K., and Durbetaki, P., Combust. Flame 29:193-208 (1977). Seeker, W. R., Wegener. D. C., Lester, T. W., and Merklin, J. F., Seventeenth Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1979, pp. 155-168.

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Received 28 March 1985; revised 10 December 1985