- Email: [email protected]
Low temperature char oxidation kinetics: effect of preparation method
Low temperature char oxidation effect of preparation method
kinetics:
Department of Chemical Engineering and Advanced Combustion Engineering Center (ACERC), Brigham Young University, Provo, UT 84602, USA (Received 1 February 1997; revised 30 July 7991)
Research
Chars derived from Beulah-Zap (lignite A) and Dietz (subbituminous B) coals were prepared by three different methods utilizing three different reactor systems. These included a high heating rate method achieved in a methane flat flame burner, a moderate heating rate method achieved in a drop tube reactor, and a slow heating rate method achieved in a muffle furnace. The flat flame char was produced in a flame environment, while the drop tube and muffle furnace chars were produced in inert environments. Low temperature oxidation rates and kinetic parameters were determined using isothermal thermogravimetric analysis at temperatures between 550 K and 950 K. Reactivities at different oxidation burn-out levels (l&75%) were compared on both an initial mass and an available mass basis. Using the available mass basis, rates in the intrinsic regime were found to be nearly identical for the different burn-out levels. It was also found that the lower burn-out levels are more highly influenced by diffusional effects. This was manifest by a decrease in the slope of the Arrhenius plot which began at a temperature of -750 K for the char at 10% burn-out compared with a temperature of nearly 900 K for the char at 75% burn-out. In comparing the chars produced by the three different methods, reactivities in the reaction control regime showed that, for both coals, the drop tube char was more reactive than either the flat flame or muffle furnace char.
Further tests indicated that the drop tube chars had a hydrogen to carbon ratio that was 2.5--5 times greater than the char from either of the other reactors and the devolatilization conversion was significantly less. The activation energies for all three Beulah-Zap chars, and for the Dietz muffle furnace and flat flame chars, were found to be 118 f 3 kJ mol- ‘. A comparison of the reactivities for the flat flame burner chars of the lignite and the subbituminous showed that the lignite chars were more reactive by a factor of two. This was consistent
over all burn-out
levels.
(Keywords: oxidation; kinetics; char)
Char reactivity has been shown to be dependent on a number of individual char preparation conditions. Conditions such as heating rate, peak temperature, gas environment and residence time are very important in determining the resulting char reactivity in that they affect its chemical and physical structure. In addition, since chars produced for academic and industrial studies are made in a variety of different apparatus or reactor types’-*, and since these apparatus usually use a combination of different preparation conditions, a good comparison of char reactivity results from one study to another is often very difficult. Thus, it is important to understand both the effects of individual preparation conditions and the effects of preparation reactor type on char reactivity. found that intrinsic Radovic et al.2 and Rybak3 reactivity increased with increasing heating rate. Solomon et a1.4 found that heating rate greatly affected the reactivity (critical temperature) of bituminous chars, but had little effect on lignite chars. Young and Smith’ found that heating rate had a significant effect on the global reactivity of the char because it affects its evolving pore structure during devolatilization. They also state that the preparation gas environment affects the pore structure of the resulting char and pyrolysis products.
00162361/92/03031945 1‘ 1992 Butterwor&Heinemann
Ltd
One reason for this is that swelling is greatly affected by the gas environment. The presence of oxygen decreases the amount of swelling in the char. Nelson et a1.6 prepared chars at gas temperatures of 813, 873 and 1073 K and found that the global reactivity of the chars decreased with increasing preparation temperature. The same result has been noted by Jenkins et a1.7, among other researchersz,8,9, relative to intrinsic reactivity. This effect was found to occur, however, only up to preparation temperatures of _ 1273 K5*6. The activation energy for char oxidation was found by Khan’ to be the same for chars prepared at different temperatures (from 773 to 1223 K) from the same coal, though Rastam-Abadi et al lo found that the activation energy varied with burn-out level. The overall objective of this work was to determine the varying effects of char preparation method, coal type and oxidation burn-out level on the intrinsic kinetics of coal char oxidation as determined by thermogravimetry. Knowing these effects may allow meaningful comparison of data obtained at different laboratories under different conditions. In addition, attempts were made to relate differences in char oxidation rates to the chemical and physical properties of the chars.
FUEL, 1992, Vol 71, March
319
Low temperature
char oxidation kinetics: K. M. McDonald
et al.
EXPERIMENTAL Char preparution The chars used in this study were prepared from Beulah-Zap lignite A and Dietz subbituminous B coals with sources, proximate analyses and ultimate analyses as shown in Tab/e 2 ll. Three different methods of producing char from these coals were used : a high heating rate method in a methane flame environment; a moderate heating rate method in an inert environment; and a slow heating rate in an inert environment. The conditions for these char preparation methods, including gas temperature, residence time and per cent devolatilization conversion, are shown in Table 2. The high heating rate was achieved in a flat flame burner (FFB) which provided a flame environment similar to that to which a coal particle is exposed during combustion. The details of the construction and operation of the FFB are reported elsewhere’2*‘3. The FFB was operated with air to facilitate the oxidation of the volatile material. The moderate heating rate was achieved in a drop tube reactor operated under conditions described by Wells14. The slow heating rate chars were prepared in a muffle furnace operated under conditions similar to those used by Leslie15. Conversions for all chars were determined using a Ti tracer technique described by Hyde’ 3. Reactivity meusuremrnts The reactivity of the chars was measured at temperatures of 550-950 K using standard thermogravimetric systems. These microbalances enable mass loss to be measured as a function of temperature and gas
Table 1 .~
Physical
properties
of the coals studied”
Rank and source of coals Coal name
Rank
Dietz Beulah-Zap
Subbituminous Lignite A
Proximate
analysis
B
Mine/location
Source
Bighorn Co., MT Mercer Co., ND
PETC ANL
of coals studied
(% as received) _
Dietz Beulah-Zap
Moisture
Ash
Volatile matter
Fixed carbon
23.1 32.2
4.1 6.6
32.0 30.5
40.3 30.7
Ultimate
Dietz Beulah-Zap
Table 2
analysis
of the coals studied
(% daf)
C
H
N
S
0
76.0 72.9
5.2 4.8
0.9 1.2
0.5 0.7
17.3 20.3
Devolatilization
conditions
~___
Preparation method
Gas temp. (K)
Residence % Devol. time (ms) conv. (daf)
Dietz (subbit)
Flat flame burner Drop tube Muffle furnace
1473 1300 1220
100 200 9 min
67.7 53.8 59.9
Beulah-Zap (lignite)
Flat flame burner Drop tube Muffle furnace
1473 1300 1220
130 200 9min
66.5 55.3
Parent
coal
_
320
FUEL,
1992,
Vol 71, March
0.0010
I 0.0012
I 0.0014
I 0.0016
0.0018
l/Temperature [KI Figure 1 Effect of oxidation burn-out level in measuring oxidation rates for Dietz FFB char (initial mass basis): 0, 10% burn-out: q i, 25% burn-out; A, 50% burn-out; 0, 75% burn-out
environment. For each run a 1.6-2.0 mg sample was placed in the platinum sample pan and heated to the desired temperature in nitrogen. Flow of oxygen was then initiated and the mixture was passed over the sample at a combined gas flow rate of 200 cm3 min- ’ (10% oxygen) at ambient pressure. A thermocouple inserted into the sample chamber allowed measurement of gas temperature simultaneously with the mass measurements recorded by the balance. The experiments covered a temperature range that was chosen to assure that a significant portion of the data was taken and all activation energies were measured under zone I (reaction controlled or intrinsic) conditions. Oxidation rates, i.e. the slope of the mass versus time curve, were determined at 10, 2.5, 50 and 75% burn-out (daf) levels. After doing experiments at several temperatures, an Arrhenius plot was then constructed to determine the activation energy for the sample.
Chemical and physical properties The CO, and N, surface areas of the chars were determined at this facility using standard flow adsorption techniques by White16 and were supplemented by measurements done at the Coors Analytical Laboratory. Apparent or particle densities were determined from measured helium and bulk densities as described by White16. Hydrogen and carbon contents were determined using an elemental analyser and calcium content was determined using ICP atomic adsorption14. Aromatic cluster sizes were determined using 13C n.m.r.17.
RESULTS
for char preparation _____ ~~
a
-12
AND DISCUSSION
Reactivit_p Effect of oxidation burn-out level. Figure 1 is an Arrhenius plot showing the char oxidation reactivity of the Dietz char prepared in the FFB. This plot shows oxidation rates for the char at four different burn-out levels and various temperatures between 573 K and 923 K. (Duplicate runs were made at several temperatures for verification. Many points overlie each other.) These rates were normalized to an initial mass basis (daf).
Low temperature
As seen in Figure 1, there appear to be two temperature regimes. In the lower temperature region, below - 800 K, several characteristics are significant: at any fixed temperature, reactivity appears to decrease as burn-out level increases; and at each individual burn-out level the anticipated Arrhenius behaviour is observed. In the higher temperature regime, the slope of the rate curve for each of the four burn-outs decreases and levels off. As the level of burn-out increased, the mass available for further reaction decreased, so it was decided to determine the rates on an available mass basis (daf). These data, presented in Figure 2, show several distinct differences from the data in Figure 1. In the lower temperature (intrinsic) region the data became essentially colinear. Thus, the reactivity of the char appears to be independent of the level of burn-out under these conditions. Activation energies were determined for each burn-out level. These values, shown in Table 3, indicate a possible increase in activation energy with increasing burn-out. In the higher temperature region a transition into what appears to be diffusion or mass transfer limited phenomena is observed. This transition, manifest by a decreased slope in Figure 2, occurs at a progressively higher temperature as the burn-out level increases. Thus, the 75% burned out char remains under chemical reaction control to a higher temperature than the other burn-out levels. The probable explanation for this is that the pores become more open and accessible to transport of gas phase reaction participants as the oxidation proceeds. In light of the clarity provided by normalizing to available mass, all reactivity data presented below will
char oxidation
o.lmo
kinetics:
0.0012
K. M. McDonald
0.0014
0.0016
et al.
0.0018
l/Temperature @Cl Figure 3 Effect of preparation method on Dietz subbituminous char oxidation rates at 25% burn-out: A, flat flame: 0, muffle furnace; 0. drop tube
1 -6 -
*A
0
0.0010
0.0012
0.0014
0.0016
0.0018
l/Temperature [K] Figure 4 oxidation
Effect of preparation rates at 25% burn-out.
method on Beulah-Zap lignite char Symbols as in Figure 3
be normalized to this basis. The choice of basis does not affect either the activation energy or the order of reaction obtained from the data. EfSects of char preparation technique and coal type. Figure 3 shows reactivities of chars derived from the
a -12
I
O.DolO
I
I
0.0014
0.0012
0.0016
0.0018
l/Temperature [K] Figure 2 Effect of oxidation burn-out level in measuring oxidation rates for Dietz FFB char (available mass basis). Symbols as in Figure I
Table 3
Activation for FFB chars
Source
coal
energies
Burn-out (%)
level
at different
levels of oxidation
(K)
Activation (kJ mol-‘)
Temp. range
Dietz (subbit.)
10 25 50 75
515-125 575-125 575-775 575-175
95 105 116 118
Beulah-Zap (lignite)
10 25 50 75
650-700 650-700 650-765 65G765
84 93 103 115
burn-out
energy
subbituminous coal by the three different preparation methods. Figure 4 is a similar plot for chars derived from the lignite. Similar trends were seen at all burn-out levels in both chars, but only the 25% burn-out levels are shown. It is clear that over the range of these data the muffle furnace and flat flame chars behaved quite similarly, while in the lower temperature region the drop tube chars were more reactive. This seems to be inconsistent with the studies referred to earlier which indicate that reactivity increases with increased heating rate and decreases with increased peak preparation temperatures up to 1273 K. The fact that the muffle furnace and flat flame chars show similar reactivities is not necessarily inconsistent with these studies, however, since the muffle furnace char was produced both at a lower peak temperature (higher reactivity) and a much lower heating rate (lower reactivity). On the other hand, based on these studies we would have expected the FFB char to be more reactive than the drop tube char instead of the observed opposite behaviour.
FUEL,
1992,
Vol 71, March
321
et al. Table 4 Parent Rank
Properties
of Dietz and Beulah-Zap ~~~_._~
chars _._
~
Preparation
% Ca carbons/cluster
__~____ DT ~-
FFB
DT
MF
0.011 301 217 0.41
0.028 305 7 1.10
0.010 374 42 1.22
0.006 301 140 0.69
0.030 273 6.5 1.31
0.008 _ _ _
0.999
0.635 16
0.760 22
2.176
2.049
-
FUEL, 1992, Vol 71, March
_ __
~--
Two possible explanations exist for the higher reactivity of the drop tube char. The temperatures reported in Table 2 for the different preparation methods are gas temperatures, whereas it is well known that the particle temperature is the significant variable. The devolatilization work of Fletcher18 showed that in a system similar to our drop tube apparatus the particle temperatures were 50-150 K less than the measured gas temperature. This suggests that the particle temperatures of the drop tube chars were significantly lower than the reported gas temperatures and probably much less than the particle temperatures of the flat flame chars (which were produced in a flame), leading to the possibility that the preparation temperature effect overrode the heating rate effect. Currently, efforts are being made to incorporate two-colour pyrometry into our drop tube system which will make particle temperature measurements possible”. Another explanation, or contributing factor to the above explanation, involves the disposition of the volatiles and tar cloud produced during devolatilization. The FFB provides an oxidizing environment and rapid conversion of volatiles and tar to CO, and H,O. The drop tube, on the other hand, provides no chemical pathway to convert the volatiles. At the end of the char’s stay in the drop tube, quenching occurs and the cloud of tar (shown photographically by McLean et ~1.~‘) and other volatiles surrounding the particle are probably recondensed back onto the particle. High hydrogen to carbon (H/C) ratios observed for the drop tube char (Table 4) are consistent with the presence of the more hydrogen-rich tar moieties21, and since higher H/C leads to higher reactivity, would tend to support this hypothesis. Also the fact that mass diffusion limitations are seen at lower temperatures for the drop tube chars than for the FFB chars (Figures3 and 4) supports the tar condensation hypothesis. Devolatilization conversion data in Table 2 confirm that the drop tube char is indeed less burned out than the other chars in the study. Figure 5 shows a comparison of the oxidation rates of chars produced from subbituminous and lignite parent coals in the FFB. In the intrinsic regime the data show the lignite to be somewhat more reactive than the subbituminous. In the mass transfer regime this difference decreased and became negligible. Similar trends were seen for the other preparation methods. Activation energies were determined for five of the chars of this study. Table 5 shows these values obtained at burn-outs of 75%. The activation energies obtained for the two coal types and various preparation techniques are quite similar, all being 118 f 3 kJ mol-’ (95% confidence).
322
__
MF
FFB ~_~ ~
method _____~~
Char properties WC CO, surface area (m2 g- ‘) N, surface area (m’ g-l) Apparent density (g cme3) Aromat.
Beulah-Zap (lignite) ____.~~
Dietz (subbit.)
coal
~
_
---
0
F0 0
0 0.0010
0.0012
0.0014 memperature
0.0016
O.cQlt?
[K]
Figure 5 burn-out;
Effect of coal type on oxidation rates for FFB chars at 25% 0, Beulah-Zap (lignite); 0, Dietz (subbituminous)
Table 5 oxidation
Activation burn-out
energies for different preparation
methods
Activation (kJ mol-‘)
Source coal
Preparation
Dietz
Flat flame Muffle furnace
118 120
Beulah-Zap
Flat flame Drop tube Muffle furnace
115 117 118
method
at 75%
energy
Correlations of physical and chemical properties A comparison of some of the physical and chemical properties for the six chars of this study is shown in Table 4. A few observations can be made regarding these in an attempt to account for observed reactivity trends. It appears that neither CO2 nor N, surface area can be correlated with reactivity. In fact, in this instance there is no distinct pattern between the two surface area methods. The CO, surface areas are all of the same magnitude, whereas the N2 surface areas differ by as much as two orders of magnitude, with the drop tube chars showing the smallest surface area. This is contrary to what would be expected from the observed oxidation rates discussed earlier. An attempt was made to correlate the reactivities of the chars with their calcium content, since calcium can be a catalyst for char oxidation under some conditionsz2. The higher calcium content of the Zap chars may explain their somewhat higher reaction rates compared with the Dietz. No trends were seen, however, to explain the
low
temperature
differences in reactivity as a function of preparation method. Although no correlation was seen, differences in the dispersion of the calcium due to the different preparation conditions may have contributed to the reactivity differences. The H/C ratio in the chars seems to show the strongest correlation with the observed reactivity trends. The drop tube chars from both source coals show an H/C ratio that is 2.885 times greater than their less reactive counterparts from the other preparation methods. This correlation of H/C with reactivity is also consistent with the observations of Serio et al.‘” who found an increase in reactivity with increasing hydrogen content for chars from five different coals. The higher H/C values of the drop tube chars are consistent with their lower devolatilization conversion (Table 2) as well as their purported retention of tar precursor moieties discussed previously. They are also consistent with n.m.r. data (Table 4) which show the drop tube char to have a smaller cluster size, and thus a lower degree of aromatization, than the muffle furnace char.
The results of this study of char oxidation at low temperature showed that intrinsic char reactivity is essentially constant over a range of l&75% burn-out for a given preparation method and parent coal. This was shown for both the Beulah-Zap and Dietz FFB chars by the colinearity of the reactivities of the four burn-out levels in the lower temperature regime. In the higher temperature region, the more burned-out chars showed a lesser level of diffusional influence as their pore structure became more open. Chars produced in the drop tube reactor were more reactive than either the flat flame chars or the muffle furnace chars. This greater reactivity was due to their higher H/C ratio rather than pore or surface effects. The higher H/C ratio was probably due to the recondensation of volatiles on the char particles in the drop tube. The chars derived from the lignite were somewhat more reactive than those derived from the subbituminous, possibly due to catalytic effects of the calcium in the char. The activation energy for the char oxidation reaction was found to be 118 + 3 kJ mol-’ and was independent of preparation method.
kinetics:
K. M. McDonald
al.
ACKNOWLEDGEMENTS This work was sponsored by the Advanced Combustion Engineering Research Center. Funds for this Center are received from the National Science Foundation, the State of Utah, 25 industrial participants and the US Department of Energy. The authors gratefully acknowledge the assistance of Mr Richard F. Cope and Dr Calvin H. Bartholomew. REFERENCES 1
2 3 4 5 6 7
SUMMARY
char oxidation
8 9 10 11 12
13 14 15
16 17 18 19 20
21 22 23
Suuberg, E. M., Wojtowicz, M. and Calo, J. M. Twenty-Second Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1988, pp. 79-87 Radovic. L. R.. Walker. P. L. and Jenkins. R. G. Fuel 1983.62, 849 Rybak, W. Fuel Proc. Technol. 1988, 19, 107 Solomon, P. R., Serio, M. A. and Heninger, S. G. Am. Chem. Sot., Die. Fuel Chem. Prepr. 1986, 31, 200 Young, B. C. and Smith, I. W. International Symposium on Coal Combustion, Peking, China, 1987 Nelson. P. F.. Smith. I. W.. Tvler. _ R. J. et ul. Enerav “, & Fuels 1988, 2, 391 Jenkins, R. G., Nandi, S. P. and Walker Jr., P. L. Fuel 1973, 52, 288 Khan, M. R. Fuel 1987, 66, 1626 Cumming, J. W. Thermochim. Acta 1989, 155, 151 Rastam-Abadi. M., Moran, D. L. and DeBarr, J. A. Am. Chem. Sot. Dia. Fuel Chem. Prepr. 1988, 33(4), 862 Smith, K. L. and Smoot, L. D. Pray. Energy Combust. Sci. 1990, 16, 1 Bartholomew, C. H., Hecker, W. C. and Smoot, L. D. Char Preparation Facility, ACERC-Char Report, Brigham Young University, 1987 Hyde, W. D. MS Thesis Brigham Young University, 1990 Wells, W. F. PhD Dissertation Brigham Young University, 1990 Leslie, I. H. Coal Char Structure and Reactivity, HTGL Report no. T-241, Mechanical Engineering Department, Stanford University, 1984 White, W. E. MS Thesis Brigham Young University, 1990 Solum, M. S., Pugmire, R. J. and Grant, D. M. Energ! & Fuels 1989, 3, 187 Fletcher, T. H. Combust. Flame 1989, 78, 223 Cope, R. F. PhD Disseruztion Brigham Young University, 1992 McLean, W. J., Hardesty, D. R. and Pohl, J. H. Eighteenth Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1981, pp. 1239- 1248 Freihaut, J. D., Proscia, W. M. and Seery, D. J. Energy & Fue1.y 1989, 3, 692 Levendis, Y. A., Nam, S. W., Lowenberg, M. et al. Energy & Fuels 1989, 3, 28 Serio, M. A., Solomon, P. R. and Suuberg, E. M. ‘1987 International Conference on Coal Science’, (Eds. J. A. Moulijn, K. A. Nater and H. A. G. Chermin), Elsevier, Amsterdam, 1987
FUEL, 1992, Vol 71, March
323
kinetics:
Department of Chemical Engineering and Advanced Combustion Engineering Center (ACERC), Brigham Young University, Provo, UT 84602, USA (Received 1 February 1997; revised 30 July 7991)
Research
Chars derived from Beulah-Zap (lignite A) and Dietz (subbituminous B) coals were prepared by three different methods utilizing three different reactor systems. These included a high heating rate method achieved in a methane flat flame burner, a moderate heating rate method achieved in a drop tube reactor, and a slow heating rate method achieved in a muffle furnace. The flat flame char was produced in a flame environment, while the drop tube and muffle furnace chars were produced in inert environments. Low temperature oxidation rates and kinetic parameters were determined using isothermal thermogravimetric analysis at temperatures between 550 K and 950 K. Reactivities at different oxidation burn-out levels (l&75%) were compared on both an initial mass and an available mass basis. Using the available mass basis, rates in the intrinsic regime were found to be nearly identical for the different burn-out levels. It was also found that the lower burn-out levels are more highly influenced by diffusional effects. This was manifest by a decrease in the slope of the Arrhenius plot which began at a temperature of -750 K for the char at 10% burn-out compared with a temperature of nearly 900 K for the char at 75% burn-out. In comparing the chars produced by the three different methods, reactivities in the reaction control regime showed that, for both coals, the drop tube char was more reactive than either the flat flame or muffle furnace char.
Further tests indicated that the drop tube chars had a hydrogen to carbon ratio that was 2.5--5 times greater than the char from either of the other reactors and the devolatilization conversion was significantly less. The activation energies for all three Beulah-Zap chars, and for the Dietz muffle furnace and flat flame chars, were found to be 118 f 3 kJ mol- ‘. A comparison of the reactivities for the flat flame burner chars of the lignite and the subbituminous showed that the lignite chars were more reactive by a factor of two. This was consistent
over all burn-out
levels.
(Keywords: oxidation; kinetics; char)
Char reactivity has been shown to be dependent on a number of individual char preparation conditions. Conditions such as heating rate, peak temperature, gas environment and residence time are very important in determining the resulting char reactivity in that they affect its chemical and physical structure. In addition, since chars produced for academic and industrial studies are made in a variety of different apparatus or reactor types’-*, and since these apparatus usually use a combination of different preparation conditions, a good comparison of char reactivity results from one study to another is often very difficult. Thus, it is important to understand both the effects of individual preparation conditions and the effects of preparation reactor type on char reactivity. found that intrinsic Radovic et al.2 and Rybak3 reactivity increased with increasing heating rate. Solomon et a1.4 found that heating rate greatly affected the reactivity (critical temperature) of bituminous chars, but had little effect on lignite chars. Young and Smith’ found that heating rate had a significant effect on the global reactivity of the char because it affects its evolving pore structure during devolatilization. They also state that the preparation gas environment affects the pore structure of the resulting char and pyrolysis products.
00162361/92/03031945 1‘ 1992 Butterwor&Heinemann
Ltd
One reason for this is that swelling is greatly affected by the gas environment. The presence of oxygen decreases the amount of swelling in the char. Nelson et a1.6 prepared chars at gas temperatures of 813, 873 and 1073 K and found that the global reactivity of the chars decreased with increasing preparation temperature. The same result has been noted by Jenkins et a1.7, among other researchersz,8,9, relative to intrinsic reactivity. This effect was found to occur, however, only up to preparation temperatures of _ 1273 K5*6. The activation energy for char oxidation was found by Khan’ to be the same for chars prepared at different temperatures (from 773 to 1223 K) from the same coal, though Rastam-Abadi et al lo found that the activation energy varied with burn-out level. The overall objective of this work was to determine the varying effects of char preparation method, coal type and oxidation burn-out level on the intrinsic kinetics of coal char oxidation as determined by thermogravimetry. Knowing these effects may allow meaningful comparison of data obtained at different laboratories under different conditions. In addition, attempts were made to relate differences in char oxidation rates to the chemical and physical properties of the chars.
FUEL, 1992, Vol 71, March
319
Low temperature
char oxidation kinetics: K. M. McDonald
et al.
EXPERIMENTAL Char preparution The chars used in this study were prepared from Beulah-Zap lignite A and Dietz subbituminous B coals with sources, proximate analyses and ultimate analyses as shown in Tab/e 2 ll. Three different methods of producing char from these coals were used : a high heating rate method in a methane flame environment; a moderate heating rate method in an inert environment; and a slow heating rate in an inert environment. The conditions for these char preparation methods, including gas temperature, residence time and per cent devolatilization conversion, are shown in Table 2. The high heating rate was achieved in a flat flame burner (FFB) which provided a flame environment similar to that to which a coal particle is exposed during combustion. The details of the construction and operation of the FFB are reported elsewhere’2*‘3. The FFB was operated with air to facilitate the oxidation of the volatile material. The moderate heating rate was achieved in a drop tube reactor operated under conditions described by Wells14. The slow heating rate chars were prepared in a muffle furnace operated under conditions similar to those used by Leslie15. Conversions for all chars were determined using a Ti tracer technique described by Hyde’ 3. Reactivity meusuremrnts The reactivity of the chars was measured at temperatures of 550-950 K using standard thermogravimetric systems. These microbalances enable mass loss to be measured as a function of temperature and gas
Table 1 .~
Physical
properties
of the coals studied”
Rank and source of coals Coal name
Rank
Dietz Beulah-Zap
Subbituminous Lignite A
Proximate
analysis
B
Mine/location
Source
Bighorn Co., MT Mercer Co., ND
PETC ANL
of coals studied
(% as received) _
Dietz Beulah-Zap
Moisture
Ash
Volatile matter
Fixed carbon
23.1 32.2
4.1 6.6
32.0 30.5
40.3 30.7
Ultimate
Dietz Beulah-Zap
Table 2
analysis
of the coals studied
(% daf)
C
H
N
S
0
76.0 72.9
5.2 4.8
0.9 1.2
0.5 0.7
17.3 20.3
Devolatilization
conditions
~___
Preparation method
Gas temp. (K)
Residence % Devol. time (ms) conv. (daf)
Dietz (subbit)
Flat flame burner Drop tube Muffle furnace
1473 1300 1220
100 200 9 min
67.7 53.8 59.9
Beulah-Zap (lignite)
Flat flame burner Drop tube Muffle furnace
1473 1300 1220
130 200 9min
66.5 55.3
Parent
coal
_
320
FUEL,
1992,
Vol 71, March
0.0010
I 0.0012
I 0.0014
I 0.0016
0.0018
l/Temperature [KI Figure 1 Effect of oxidation burn-out level in measuring oxidation rates for Dietz FFB char (initial mass basis): 0, 10% burn-out: q i, 25% burn-out; A, 50% burn-out; 0, 75% burn-out
environment. For each run a 1.6-2.0 mg sample was placed in the platinum sample pan and heated to the desired temperature in nitrogen. Flow of oxygen was then initiated and the mixture was passed over the sample at a combined gas flow rate of 200 cm3 min- ’ (10% oxygen) at ambient pressure. A thermocouple inserted into the sample chamber allowed measurement of gas temperature simultaneously with the mass measurements recorded by the balance. The experiments covered a temperature range that was chosen to assure that a significant portion of the data was taken and all activation energies were measured under zone I (reaction controlled or intrinsic) conditions. Oxidation rates, i.e. the slope of the mass versus time curve, were determined at 10, 2.5, 50 and 75% burn-out (daf) levels. After doing experiments at several temperatures, an Arrhenius plot was then constructed to determine the activation energy for the sample.
Chemical and physical properties The CO, and N, surface areas of the chars were determined at this facility using standard flow adsorption techniques by White16 and were supplemented by measurements done at the Coors Analytical Laboratory. Apparent or particle densities were determined from measured helium and bulk densities as described by White16. Hydrogen and carbon contents were determined using an elemental analyser and calcium content was determined using ICP atomic adsorption14. Aromatic cluster sizes were determined using 13C n.m.r.17.
RESULTS
for char preparation _____ ~~
a
-12
AND DISCUSSION
Reactivit_p Effect of oxidation burn-out level. Figure 1 is an Arrhenius plot showing the char oxidation reactivity of the Dietz char prepared in the FFB. This plot shows oxidation rates for the char at four different burn-out levels and various temperatures between 573 K and 923 K. (Duplicate runs were made at several temperatures for verification. Many points overlie each other.) These rates were normalized to an initial mass basis (daf).
Low temperature
As seen in Figure 1, there appear to be two temperature regimes. In the lower temperature region, below - 800 K, several characteristics are significant: at any fixed temperature, reactivity appears to decrease as burn-out level increases; and at each individual burn-out level the anticipated Arrhenius behaviour is observed. In the higher temperature regime, the slope of the rate curve for each of the four burn-outs decreases and levels off. As the level of burn-out increased, the mass available for further reaction decreased, so it was decided to determine the rates on an available mass basis (daf). These data, presented in Figure 2, show several distinct differences from the data in Figure 1. In the lower temperature (intrinsic) region the data became essentially colinear. Thus, the reactivity of the char appears to be independent of the level of burn-out under these conditions. Activation energies were determined for each burn-out level. These values, shown in Table 3, indicate a possible increase in activation energy with increasing burn-out. In the higher temperature region a transition into what appears to be diffusion or mass transfer limited phenomena is observed. This transition, manifest by a decreased slope in Figure 2, occurs at a progressively higher temperature as the burn-out level increases. Thus, the 75% burned out char remains under chemical reaction control to a higher temperature than the other burn-out levels. The probable explanation for this is that the pores become more open and accessible to transport of gas phase reaction participants as the oxidation proceeds. In light of the clarity provided by normalizing to available mass, all reactivity data presented below will
char oxidation
o.lmo
kinetics:
0.0012
K. M. McDonald
0.0014
0.0016
et al.
0.0018
l/Temperature @Cl Figure 3 Effect of preparation method on Dietz subbituminous char oxidation rates at 25% burn-out: A, flat flame: 0, muffle furnace; 0. drop tube
1 -6 -
*A
0
0.0010
0.0012
0.0014
0.0016
0.0018
l/Temperature [K] Figure 4 oxidation
Effect of preparation rates at 25% burn-out.
method on Beulah-Zap lignite char Symbols as in Figure 3
be normalized to this basis. The choice of basis does not affect either the activation energy or the order of reaction obtained from the data. EfSects of char preparation technique and coal type. Figure 3 shows reactivities of chars derived from the
a -12
I
O.DolO
I
I
0.0014
0.0012
0.0016
0.0018
l/Temperature [K] Figure 2 Effect of oxidation burn-out level in measuring oxidation rates for Dietz FFB char (available mass basis). Symbols as in Figure I
Table 3
Activation for FFB chars
Source
coal
energies
Burn-out (%)
level
at different
levels of oxidation
(K)
Activation (kJ mol-‘)
Temp. range
Dietz (subbit.)
10 25 50 75
515-125 575-125 575-775 575-175
95 105 116 118
Beulah-Zap (lignite)
10 25 50 75
650-700 650-700 650-765 65G765
84 93 103 115
burn-out
energy
subbituminous coal by the three different preparation methods. Figure 4 is a similar plot for chars derived from the lignite. Similar trends were seen at all burn-out levels in both chars, but only the 25% burn-out levels are shown. It is clear that over the range of these data the muffle furnace and flat flame chars behaved quite similarly, while in the lower temperature region the drop tube chars were more reactive. This seems to be inconsistent with the studies referred to earlier which indicate that reactivity increases with increased heating rate and decreases with increased peak preparation temperatures up to 1273 K. The fact that the muffle furnace and flat flame chars show similar reactivities is not necessarily inconsistent with these studies, however, since the muffle furnace char was produced both at a lower peak temperature (higher reactivity) and a much lower heating rate (lower reactivity). On the other hand, based on these studies we would have expected the FFB char to be more reactive than the drop tube char instead of the observed opposite behaviour.
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1992,
Vol 71, March
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et al. Table 4 Parent Rank
Properties
of Dietz and Beulah-Zap ~~~_._~
chars _._
~
Preparation
% Ca carbons/cluster
__~____ DT ~-
FFB
DT
MF
0.011 301 217 0.41
0.028 305 7 1.10
0.010 374 42 1.22
0.006 301 140 0.69
0.030 273 6.5 1.31
0.008 _ _ _
0.999
0.635 16
0.760 22
2.176
2.049
-
FUEL, 1992, Vol 71, March
_ __
~--
Two possible explanations exist for the higher reactivity of the drop tube char. The temperatures reported in Table 2 for the different preparation methods are gas temperatures, whereas it is well known that the particle temperature is the significant variable. The devolatilization work of Fletcher18 showed that in a system similar to our drop tube apparatus the particle temperatures were 50-150 K less than the measured gas temperature. This suggests that the particle temperatures of the drop tube chars were significantly lower than the reported gas temperatures and probably much less than the particle temperatures of the flat flame chars (which were produced in a flame), leading to the possibility that the preparation temperature effect overrode the heating rate effect. Currently, efforts are being made to incorporate two-colour pyrometry into our drop tube system which will make particle temperature measurements possible”. Another explanation, or contributing factor to the above explanation, involves the disposition of the volatiles and tar cloud produced during devolatilization. The FFB provides an oxidizing environment and rapid conversion of volatiles and tar to CO, and H,O. The drop tube, on the other hand, provides no chemical pathway to convert the volatiles. At the end of the char’s stay in the drop tube, quenching occurs and the cloud of tar (shown photographically by McLean et ~1.~‘) and other volatiles surrounding the particle are probably recondensed back onto the particle. High hydrogen to carbon (H/C) ratios observed for the drop tube char (Table 4) are consistent with the presence of the more hydrogen-rich tar moieties21, and since higher H/C leads to higher reactivity, would tend to support this hypothesis. Also the fact that mass diffusion limitations are seen at lower temperatures for the drop tube chars than for the FFB chars (Figures3 and 4) supports the tar condensation hypothesis. Devolatilization conversion data in Table 2 confirm that the drop tube char is indeed less burned out than the other chars in the study. Figure 5 shows a comparison of the oxidation rates of chars produced from subbituminous and lignite parent coals in the FFB. In the intrinsic regime the data show the lignite to be somewhat more reactive than the subbituminous. In the mass transfer regime this difference decreased and became negligible. Similar trends were seen for the other preparation methods. Activation energies were determined for five of the chars of this study. Table 5 shows these values obtained at burn-outs of 75%. The activation energies obtained for the two coal types and various preparation techniques are quite similar, all being 118 f 3 kJ mol-’ (95% confidence).
322
__
MF
FFB ~_~ ~
method _____~~
Char properties WC CO, surface area (m2 g- ‘) N, surface area (m’ g-l) Apparent density (g cme3) Aromat.
Beulah-Zap (lignite) ____.~~
Dietz (subbit.)
coal
~
_
---
0
F0 0
0 0.0010
0.0012
0.0014 memperature
0.0016
O.cQlt?
[K]
Figure 5 burn-out;
Effect of coal type on oxidation rates for FFB chars at 25% 0, Beulah-Zap (lignite); 0, Dietz (subbituminous)
Table 5 oxidation
Activation burn-out
energies for different preparation
methods
Activation (kJ mol-‘)
Source coal
Preparation
Dietz
Flat flame Muffle furnace
118 120
Beulah-Zap
Flat flame Drop tube Muffle furnace
115 117 118
method
at 75%
energy
Correlations of physical and chemical properties A comparison of some of the physical and chemical properties for the six chars of this study is shown in Table 4. A few observations can be made regarding these in an attempt to account for observed reactivity trends. It appears that neither CO2 nor N, surface area can be correlated with reactivity. In fact, in this instance there is no distinct pattern between the two surface area methods. The CO, surface areas are all of the same magnitude, whereas the N2 surface areas differ by as much as two orders of magnitude, with the drop tube chars showing the smallest surface area. This is contrary to what would be expected from the observed oxidation rates discussed earlier. An attempt was made to correlate the reactivities of the chars with their calcium content, since calcium can be a catalyst for char oxidation under some conditionsz2. The higher calcium content of the Zap chars may explain their somewhat higher reaction rates compared with the Dietz. No trends were seen, however, to explain the
low
temperature
differences in reactivity as a function of preparation method. Although no correlation was seen, differences in the dispersion of the calcium due to the different preparation conditions may have contributed to the reactivity differences. The H/C ratio in the chars seems to show the strongest correlation with the observed reactivity trends. The drop tube chars from both source coals show an H/C ratio that is 2.885 times greater than their less reactive counterparts from the other preparation methods. This correlation of H/C with reactivity is also consistent with the observations of Serio et al.‘” who found an increase in reactivity with increasing hydrogen content for chars from five different coals. The higher H/C values of the drop tube chars are consistent with their lower devolatilization conversion (Table 2) as well as their purported retention of tar precursor moieties discussed previously. They are also consistent with n.m.r. data (Table 4) which show the drop tube char to have a smaller cluster size, and thus a lower degree of aromatization, than the muffle furnace char.
The results of this study of char oxidation at low temperature showed that intrinsic char reactivity is essentially constant over a range of l&75% burn-out for a given preparation method and parent coal. This was shown for both the Beulah-Zap and Dietz FFB chars by the colinearity of the reactivities of the four burn-out levels in the lower temperature regime. In the higher temperature region, the more burned-out chars showed a lesser level of diffusional influence as their pore structure became more open. Chars produced in the drop tube reactor were more reactive than either the flat flame chars or the muffle furnace chars. This greater reactivity was due to their higher H/C ratio rather than pore or surface effects. The higher H/C ratio was probably due to the recondensation of volatiles on the char particles in the drop tube. The chars derived from the lignite were somewhat more reactive than those derived from the subbituminous, possibly due to catalytic effects of the calcium in the char. The activation energy for the char oxidation reaction was found to be 118 + 3 kJ mol-’ and was independent of preparation method.
kinetics:
K. M. McDonald
al.
ACKNOWLEDGEMENTS This work was sponsored by the Advanced Combustion Engineering Research Center. Funds for this Center are received from the National Science Foundation, the State of Utah, 25 industrial participants and the US Department of Energy. The authors gratefully acknowledge the assistance of Mr Richard F. Cope and Dr Calvin H. Bartholomew. REFERENCES 1
2 3 4 5 6 7
SUMMARY
char oxidation
8 9 10 11 12
13 14 15
16 17 18 19 20
21 22 23
Suuberg, E. M., Wojtowicz, M. and Calo, J. M. Twenty-Second Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1988, pp. 79-87 Radovic. L. R.. Walker. P. L. and Jenkins. R. G. Fuel 1983.62, 849 Rybak, W. Fuel Proc. Technol. 1988, 19, 107 Solomon, P. R., Serio, M. A. and Heninger, S. G. Am. Chem. Sot., Die. Fuel Chem. Prepr. 1986, 31, 200 Young, B. C. and Smith, I. W. International Symposium on Coal Combustion, Peking, China, 1987 Nelson. P. F.. Smith. I. W.. Tvler. _ R. J. et ul. Enerav “, & Fuels 1988, 2, 391 Jenkins, R. G., Nandi, S. P. and Walker Jr., P. L. Fuel 1973, 52, 288 Khan, M. R. Fuel 1987, 66, 1626 Cumming, J. W. Thermochim. Acta 1989, 155, 151 Rastam-Abadi. M., Moran, D. L. and DeBarr, J. A. Am. Chem. Sot. Dia. Fuel Chem. Prepr. 1988, 33(4), 862 Smith, K. L. and Smoot, L. D. Pray. Energy Combust. Sci. 1990, 16, 1 Bartholomew, C. H., Hecker, W. C. and Smoot, L. D. Char Preparation Facility, ACERC-Char Report, Brigham Young University, 1987 Hyde, W. D. MS Thesis Brigham Young University, 1990 Wells, W. F. PhD Dissertation Brigham Young University, 1990 Leslie, I. H. Coal Char Structure and Reactivity, HTGL Report no. T-241, Mechanical Engineering Department, Stanford University, 1984 White, W. E. MS Thesis Brigham Young University, 1990 Solum, M. S., Pugmire, R. J. and Grant, D. M. Energ! & Fuels 1989, 3, 187 Fletcher, T. H. Combust. Flame 1989, 78, 223 Cope, R. F. PhD Disseruztion Brigham Young University, 1992 McLean, W. J., Hardesty, D. R. and Pohl, J. H. Eighteenth Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1981, pp. 1239- 1248 Freihaut, J. D., Proscia, W. M. and Seery, D. J. Energy & Fue1.y 1989, 3, 692 Levendis, Y. A., Nam, S. W., Lowenberg, M. et al. Energy & Fuels 1989, 3, 28 Serio, M. A., Solomon, P. R. and Suuberg, E. M. ‘1987 International Conference on Coal Science’, (Eds. J. A. Moulijn, K. A. Nater and H. A. G. Chermin), Elsevier, Amsterdam, 1987
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