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DTG combustion behaviour of coal
DTG combustion Correlations
with
behaviour
proximate
of coal
and ultimate
analysis
data
Paolo Ghetti ENEL, Thermal and Nuclear Research Center, 56100 Pisa, Italy (Received 5 August 7985; revised 15 October 7985)
Solid fuel samples were characterized by their proximate and ultimate analyses and then subjected to heat in a thermobalance, both under air and nitrogen. Trends in weight loss were recorded by thermogravimetry (TG) and derivative thermogravimetry (DTG). By examining the data obtained, it was possible to show the existence of good correlations between the VM/FC, C/H and (C + H)/O ratios and both rate of weight loss data and the temperatures deduced from the burning and volatile release profiles. Methods for evaluating the reactivity of coal are reported showing that these substantially agree with the results obtained. (Keywords: combustion of coal; tbermogravimetry;
devolatilization)
Derivative thermogravimetry (DTG) has been widely used for several years to study the behaviour of coal during combustion and devolatilization. The profiles obtained are considered to be characteristic of a given solid fuel’ - 3. Recently, a correlation between the rate of weight loss, deduced from the burning profiles of coal, and surface area data obtained both on unaltered and thermally treated samples4 has been demonstrated; Kopp and Harris’ established well-defined correlations between the initiation temperature of volatilization of a coal and its volatile matter content. This shows that trends in the burning and volatile release profiles are closely related to the characteristics of the fuel being considered. The present paper explores other possible connections that might exist between the burning and volatile release profiles of coals and their proximate and ultimate analyses. EXPERIMENTAL Each fuel was ground using a RETSCH SR 3 rotor beater mill. A part of this sample was then further ground in a RETSCH S 2 ball mill to give a particle size of 200 mesh (74pm). The sample was spread in a thin layer and exposed to air for several days to equilibrate with the atmospheric moisture. The proximate analysis on each sample was carried out by automatic LECO MAC 400 instrument and the ultimate analysis by a LECO CHN 600 analyser for carbon, hydrogen and nitrogen, and a LECO SC 32 for sulphur. The thermogravimetric curves were obtained using the METTLER TA 2000 C thermoanalyser equipped with a corrosive gas furnace. The apparatus gave automatic recording of the rate of weight loss (DTG). The weight of sample used was such that in all cases the amount of reactive matter was equal to 18 mg. To obtain the burning profiles the content of the combustible part, volatile matter and fixed carbon (VM + FC), was taken into account and for the volatile release profiles the content of volatile matter alone was considered. This means that the profiles are normalized and their heights are directly comparable. 0016-2361/86/050636~04$3.00 ‘Q 1986 Butterworth & Co. (Publishers)
636
Ltd.
FUEL, 1986, Vol 65, May
The samples were placed in cylindrical platinum crucibles, and heated from room temperature to 1000°C at 15°C min-’ using a 70 ml min-’ flow of air or nitrogen while continuously recording the TG and DTG traces. RESULTS The samples used and their analyses are listed in Table 1. Values of the volatile matter/fixed carbon (VM/FC) ratio, indicative of fuel rank, are also give. Table 2 gives the ultimate analyses, on a dry ash free (daf) basis, with oxygen calculated by difference. This table also gives the values of the C/H and (C + H)/O ratios (by weight) which, as reported by Hensel‘j, may be considered to be indicative of certain properties of coal. The initiation and peak temperatures and the peak heights were deduced from the burning and volatile release profiles using methods described in the literature2,4,7. It is important to note that, when determining the maximum rate of weight loss, secondary peaks possibly present around the maximum were also included. The maximum rate of weight loss (mgmin-‘) was calculated by adding the height of the main peak, H, to the value of the segment h, which was obtained by drawing the perpendicular to the baseline from the maximum of the secondary peak until it intersects the hypothetical trend of the main peak (see Figure I). The results are given in Table 3.
DISCUSSION Correlations can be established from the proximate and ultimate analysis data. For example, plotting the C/H and (C + H)/O ratios versus VM/FC, gives the curves shown in Figures 2 and 3, respectively. As the VM/FC ratio is considered to be indicative of coal rank, the C/H ratio is related to the aromaticity and the (C + H)/O ratio gives an indication of the extent of oxidation of the fuel, the curves obtained confirm the fact that when the rank decreases, the aromaticity also decreases and the extent of oxidation of the coal increases. These curves also show that the data are good enough to be used in further correlations.
DTG Table 1
Proximate
analysis
of the samples
I 2 3 4 5 6 1 8 9 10 11 12 13 14 15 16 11 18 19 20 21 22
Table 2
Type
Moisture
Graphite Anthracite Pet. coke Pet. coke Pet. coke Low vol. bituminous Bituminous Bituminous Bituminous Bituminous Bituminous Bituminous Bituminous Bituminous Bituminous Bituminous Bituminous High vol. bituminous Subbituminous Subbituminous Subbituminous Lignite
0.6 3.6 1.8 1.5 0.9 1.3 3.0 4.0 2.3 2.1 2.9 3.2 3.4 3.0 3.5 4.6 6.2 5.5 1.1 6.4 6.8 7.9
Ultimate
analysis
behaviour
of coal: P. Ghetti
investigated Volatile matter
Sam.ple no.
combustion
and (C + H)/O, C/H ratios
(wt 76)
Fixed carbon
(wt %)
Ash (wt 7;)
(wt “/,)
As rec.
Dry
As rec.
Dry
As rec.
Dry
VM/FC
3.3 7.5 11.3 11.8 13.2 19.7 24.1 25.7 28.8 28.1 21.6 30.6 32.0 33.1 35.8 35.5 33.9 40.1 40.4 39.4 42.5 39.3
3.3 7.8 11.5 12.0 13.3 19.9 24.8 26.8 29.5 28.9 28.4 31.6 33.1 34.1 37.1 37.2 36.1 42.4 43.5 42.1 45.6 42.1
78.4 86.5 86.4 77.4 85.4 73.6 51.1 56.6 62.6 56.5 51.6 56.5 55.7 55.3 53.8 50.3 46.5 53.1 40.8 36.6 39.3 23.7
78.9 89.7 88.0 78.6 86.2 74.6 58.9 58.9 64.1 58.1 53.2 58.4 57.7 57.0 55.8 52.7 49.6 56.2 43.9 39.1 42.2 25.7
17.7 2.4 0.5 11.0 0.5 5.4 15.8 13.7 6.3 12.7 11.9 9.1 8.9 8.6 6.9 9.6 13.4 1.3 11.1 17.6 11.4 29.1
17.8 2.5 0.5 11.2 0.5 5.5 16.3 14.3 6.4 13.0 18.4 10.0 9.2 8.9 1.2 10.1 14.3 1.4 12.6 18.8 12.2 31.6
0.042 0.087 0.131 0.152 0.154 0.261 0.42 1 0.455 0.460 0.497 0.534 0.541 0.514 0.598 0.665 0.106 0.128 0.154 0.99 1 1.077 1.081 1.661
for the samples
listed in Table I Ultimate analysis (wt %, dry ash free basis) Sample no. 1 2 3 4 5 6 1 8
9 10 11 12 13 14 15 16 11 18 19 20 21 22
C
H
N
98.4 94.6 91.0 89.5 89.6 89.9 83.7 82.7
0.4 2.3 4.0 3.9 4.2 4.7 5.0 4.2
0.4 1.2 2.7 2.4 2.3 1.3 1.1 1.9
S 0.7 0.6 1.9 4.0 4.4 0.9 0.7
0 (by diff.) 0.1 1.3 0.4 0.2 8.6 8.9
1.1 10.0 84.8 5.5 1.6 1.1 7.0 82.6 5.0 1.4 1.1 9.9 81.8 4.8 1.6 1.4 10.4 81.7 5.1 1.4 0.8 10.4 82.4 5.7 1.7 0.5 9.7 83.4 5.1 1.6 1.4 8.6 85.4 5.8 1.7 1.5 5.6 15.9 5.3 1.5 4.7 12.6 81.8 5.6 2.4 0.4 9.8 18.8 5.5 1.2 0.6 13.9 68.5 5.8 1.5 7.6 16.6 66.6 5.0 1.6 8.0 18.8 68.3 5.8 1.6 10.5 13.8 61.3 5.4 1.1 1.4 30.8
(C + H)/O
988.00 74.54 231.50 467.00 _ 11.00 9.97 8.69 12.90 8.85 8.33 8.40 9.08 10.29 16.28 6.44 8.92 6.06 4.48 3.81 5.37 2.17
C/H
246.00 41.13 22.75 22.95 21.33 19.13 16.14 19.69 15.42 16.52 17.04 14.33 14.46 16.35 14.72 14.32 14.61 14.33 11.81 13.32 11.78 11.35
Using the burning profiles, the initiation temperature of combustion (7;,) uersus the VM/FC ratio can be plotted (Figure 4). As expected, this shows a decrease in the initiation temperature of combustion when the VM/FC ratio increases. The peak temperature of combustion (T,,) tiersus VM/FC also decreases (Figure 5). The total rate of combustion (R,) increases as the VM/FC ratio increases (see Figure 6). Other correlations may be established for the volatile release profiles. For example, the initiation temperature of volatilization (IT;,),decreases as the VM/FC ratio increases (Figure 7). The peak temperature of volatilization (T’,) also decreases when the VM/FC ratio increases (Figure 8).
_I TEMPERATURE
Figure 1 maximum
Schematic representation rate of weight loss
of the method
of evaluating
the
As already reported5, it is not possible to detect particular trends with regard to the maximum rate of volatilization R, (Figure
COAL
9).
REACTIVITY
From these results it may be deduced that low-rank coals react at lower temperatures in both combustion and pyrolysis than do higher rank coals, and they show a higher rate of combustion. It may be asserted that, as the rank decreases the organic part of the coal is increasingly more reactive towards combustion and pyrolysis. When considering the measurement ofcoal reactivity, it is useful to mention the various criteria which may be thermogravimetric and kinetic used, i.e., chemical, considerations. In particular, chemical considerations6*8T9 include the oxygen content, or the degree of oxidation of the coal. More reactive coal is
~~~~,1986,Vol65
May
637
DTG combustion behaviour of coal: P. Ghetti Table 3
Data deduced
Sample No.
L Tpc ( ‘C) (“C) ._ 460 790
I 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 I8 19 20 21 22
430 360 365 365 335 305 298 305 300 295 290 285 300 305 275 265 265 250 245 253 185
” T,, = initiation combustion; temperature release; R, = b Two maxima
from burning
$g
min-‘)
and volatile
(%)
release profiles” R,’ (mg min _ ‘)
(%)
I .24
595 541 545 520 533 540 510 490 485 470 490 470 490 490 480 465 480 435 387427” 432450” 260-373’
4013-
_
2.08 1.95 1.90 1.96 1.87 1.88 1.75 2.06 1.94 2.0 1 2. I6 1.86 2.06
1.98 1.88 1.86 2.14 I .96 3.32 2.44 4.54
440 370 310 350 310 280 250 250 290 250 290 260 250 270 255 250 270 210 210 210 190
910 640 630 620 520 470 470 480 470 470 470 460 460 460 460-510* 460 460 450 450 450 350
1.06 0.78 0.66 0.68 0.96
G e 30( 3” h
I .oo 0.94 1.21 0.96
1.07 0.92
20( I-
I .oo 1.30 1.22 1.20 0.92 1.10 0.94 0.82 I.12 1.04
, 05
I
I
1.0
15
VMlFC
Figure 4 Trend VM/FC ratio
of initiation
temperature
of combustion
(T,) wrsus
temperature of combustion; T c = peak temperature of R, = maximum rate of corn YJustlon; 7;,, = initiation of volatile release; T,,,=peak temperature of volatile maximum rate of volatile release are present
C/H
20
15
10
05
15
VM IFC
Figure 5 Trend VMiFC ratio
L
10 0.0
temperature
of combustion
(T,,)
cersus
._
1Ll
0.5
of peak
1.5 VMlFC
Figure 2
Trend of C/H ~crsus VM/FC
ratio
.
. 1.0
0 0.5
0.0
Figure 3
638
1.0
Trend of (C + H)/O LWSIISVM/FC
FUEL, 1986, Vol 65, May
VMlFC
ratio
I
05
1
I
1.0
15
VM/FC
1.5
Figure 6 ratio
Trend of maximum
rate of combustion
(R,) wrsus VM/FC
DTG combustion
behaviour
of coal: P. Ghetti
1.5
. . . .
l*
. . . . 0. . .
. . . .
. t 0.5
I
05
I
I
10
15
1.0
VMlFC
Figure 7 Trend of initiation VM/FC ratio
temperatureofdevolatilization
(7;,) oersus
Figure 9 Trend VM/FC ratio
of maximum
rate
VM/FC
of devolatihzation
1.5
(R,)
~‘crsus
DTG peak height will indicate the relative reactivities of the coals. A more accurate estimate of reactivity may be achieved only by developing appropriate kinetic calculations. These would give the activation energy during the various stages of the combustion process’ 4P 16. A method recently proposed by Cumming7 gives a weighted mean value of the activation energy which is representative of the whole combustion process and it may therefore give a fairly reliable evaluation of coal reactivity.
. II 800 -
z 2 >600L?-
REFERENCES 1 400 t I
I
0.5
10
,
.
15
VM/FC
Figure 8 Trend of the peak temperature VM/FC ratio
of devolatilization
(T,,) versus
considered to be more oxidized or with a lower (C + H)/O ratio. Semi-qualitative evaluations have been made using the DTG of coal, taking into account both the position of the peak in the burning profile and its height. A coal is considered to be more reactive if the peak ofcombustion is shifted more towards low temperatures’. The height of the peak lo-l3 is related to a coal’s reactivity for kinetic reason based on the following equation:
where: R = reactivity; kl$ = initial weight of daf coal (mg); and (dw/dt),,, = maximum rate of weight loss (mg min I). Therefore, if the daf weights of the coals are equal, the
7 8 9
IO
II 12 13 14 15 16
Bryers, R. W., Biswas, B. K. and Taylor, T. E., ‘Fuel evaluation using differential thermal techniques’, paper presented at ‘Coal Technology ‘78. The international coal utilization conference and exhibition’, Houston, TX, 17-19 October 1978 Cumming, J. W. and McLaughlin, J. J. Thermochirn. Acta 1982, 57,253 Elder, J. P. and Harris, M. B. Fuel 1984, 63, 262 Ghetti, P., De Robertis, U., D’antone, S., Villani, M. and Chiellini, E. Fuel 1985, 64, 950 Kopp, 0. C. and Harris, L. A. Int. J. Cod Gad. 1984, 3, 333 Hensel, R. P., ‘Coal Classitication, chemistry and combustion’, paper presented at ‘Coal-tired industrial boilers workshop’, Kaleigh, NC, 10-I 1 December 1980 Cumming, J. W. Fuel 1984,63, 1436 ‘Combustion’, Combustion Engineering Inc., 3rd edition, 1981, p, 2-13 Frolova, N. V., Smutkina, Z. S., Klinkova, V. V., Erogova, T. F., Ekaterinina, L. N., Dolatova, A. G. and Skripchenko, G. B., Khimiya fierdogo Topha 1980, 14, 5, 34 Mahajan, 0. P. and Walker, P. L. Jr., ‘Analytical Methods for coal and coal products’, Vol. II, (Ed.C. Karr Jr.) Academic Press. Inc. New York, 1978, Ch. 32, p. 465 Knight, A. T. and Sergeant, G. D. Fuel 1982,61, 145 Frazier, G. C., Mason, C. and Badin, E. J. Fur! 1984, 63, 499 Fung, D. P. C. and Kim, S. D. Furl 1984.63, 1197 Smith, S. E., Neavel, R. C., Hippo, E. J. and Miller, R. N. Furl 198 1,60,458 Serageldin, M. A. and Pan, W. P. J. Thermochim. Actu 1983,71,1 Seradeldin, M. A. and Pan, W. P. J. Thermochim. Actu 1984,76, 145
FUEL, 1986, Vol 65, May
639
with
behaviour
proximate
of coal
and ultimate
analysis
data
Paolo Ghetti ENEL, Thermal and Nuclear Research Center, 56100 Pisa, Italy (Received 5 August 7985; revised 15 October 7985)
Solid fuel samples were characterized by their proximate and ultimate analyses and then subjected to heat in a thermobalance, both under air and nitrogen. Trends in weight loss were recorded by thermogravimetry (TG) and derivative thermogravimetry (DTG). By examining the data obtained, it was possible to show the existence of good correlations between the VM/FC, C/H and (C + H)/O ratios and both rate of weight loss data and the temperatures deduced from the burning and volatile release profiles. Methods for evaluating the reactivity of coal are reported showing that these substantially agree with the results obtained. (Keywords: combustion of coal; tbermogravimetry;
devolatilization)
Derivative thermogravimetry (DTG) has been widely used for several years to study the behaviour of coal during combustion and devolatilization. The profiles obtained are considered to be characteristic of a given solid fuel’ - 3. Recently, a correlation between the rate of weight loss, deduced from the burning profiles of coal, and surface area data obtained both on unaltered and thermally treated samples4 has been demonstrated; Kopp and Harris’ established well-defined correlations between the initiation temperature of volatilization of a coal and its volatile matter content. This shows that trends in the burning and volatile release profiles are closely related to the characteristics of the fuel being considered. The present paper explores other possible connections that might exist between the burning and volatile release profiles of coals and their proximate and ultimate analyses. EXPERIMENTAL Each fuel was ground using a RETSCH SR 3 rotor beater mill. A part of this sample was then further ground in a RETSCH S 2 ball mill to give a particle size of 200 mesh (74pm). The sample was spread in a thin layer and exposed to air for several days to equilibrate with the atmospheric moisture. The proximate analysis on each sample was carried out by automatic LECO MAC 400 instrument and the ultimate analysis by a LECO CHN 600 analyser for carbon, hydrogen and nitrogen, and a LECO SC 32 for sulphur. The thermogravimetric curves were obtained using the METTLER TA 2000 C thermoanalyser equipped with a corrosive gas furnace. The apparatus gave automatic recording of the rate of weight loss (DTG). The weight of sample used was such that in all cases the amount of reactive matter was equal to 18 mg. To obtain the burning profiles the content of the combustible part, volatile matter and fixed carbon (VM + FC), was taken into account and for the volatile release profiles the content of volatile matter alone was considered. This means that the profiles are normalized and their heights are directly comparable. 0016-2361/86/050636~04$3.00 ‘Q 1986 Butterworth & Co. (Publishers)
636
Ltd.
FUEL, 1986, Vol 65, May
The samples were placed in cylindrical platinum crucibles, and heated from room temperature to 1000°C at 15°C min-’ using a 70 ml min-’ flow of air or nitrogen while continuously recording the TG and DTG traces. RESULTS The samples used and their analyses are listed in Table 1. Values of the volatile matter/fixed carbon (VM/FC) ratio, indicative of fuel rank, are also give. Table 2 gives the ultimate analyses, on a dry ash free (daf) basis, with oxygen calculated by difference. This table also gives the values of the C/H and (C + H)/O ratios (by weight) which, as reported by Hensel‘j, may be considered to be indicative of certain properties of coal. The initiation and peak temperatures and the peak heights were deduced from the burning and volatile release profiles using methods described in the literature2,4,7. It is important to note that, when determining the maximum rate of weight loss, secondary peaks possibly present around the maximum were also included. The maximum rate of weight loss (mgmin-‘) was calculated by adding the height of the main peak, H, to the value of the segment h, which was obtained by drawing the perpendicular to the baseline from the maximum of the secondary peak until it intersects the hypothetical trend of the main peak (see Figure I). The results are given in Table 3.
DISCUSSION Correlations can be established from the proximate and ultimate analysis data. For example, plotting the C/H and (C + H)/O ratios versus VM/FC, gives the curves shown in Figures 2 and 3, respectively. As the VM/FC ratio is considered to be indicative of coal rank, the C/H ratio is related to the aromaticity and the (C + H)/O ratio gives an indication of the extent of oxidation of the fuel, the curves obtained confirm the fact that when the rank decreases, the aromaticity also decreases and the extent of oxidation of the coal increases. These curves also show that the data are good enough to be used in further correlations.
DTG Table 1
Proximate
analysis
of the samples
I 2 3 4 5 6 1 8 9 10 11 12 13 14 15 16 11 18 19 20 21 22
Table 2
Type
Moisture
Graphite Anthracite Pet. coke Pet. coke Pet. coke Low vol. bituminous Bituminous Bituminous Bituminous Bituminous Bituminous Bituminous Bituminous Bituminous Bituminous Bituminous Bituminous High vol. bituminous Subbituminous Subbituminous Subbituminous Lignite
0.6 3.6 1.8 1.5 0.9 1.3 3.0 4.0 2.3 2.1 2.9 3.2 3.4 3.0 3.5 4.6 6.2 5.5 1.1 6.4 6.8 7.9
Ultimate
analysis
behaviour
of coal: P. Ghetti
investigated Volatile matter
Sam.ple no.
combustion
and (C + H)/O, C/H ratios
(wt 76)
Fixed carbon
(wt %)
Ash (wt 7;)
(wt “/,)
As rec.
Dry
As rec.
Dry
As rec.
Dry
VM/FC
3.3 7.5 11.3 11.8 13.2 19.7 24.1 25.7 28.8 28.1 21.6 30.6 32.0 33.1 35.8 35.5 33.9 40.1 40.4 39.4 42.5 39.3
3.3 7.8 11.5 12.0 13.3 19.9 24.8 26.8 29.5 28.9 28.4 31.6 33.1 34.1 37.1 37.2 36.1 42.4 43.5 42.1 45.6 42.1
78.4 86.5 86.4 77.4 85.4 73.6 51.1 56.6 62.6 56.5 51.6 56.5 55.7 55.3 53.8 50.3 46.5 53.1 40.8 36.6 39.3 23.7
78.9 89.7 88.0 78.6 86.2 74.6 58.9 58.9 64.1 58.1 53.2 58.4 57.7 57.0 55.8 52.7 49.6 56.2 43.9 39.1 42.2 25.7
17.7 2.4 0.5 11.0 0.5 5.4 15.8 13.7 6.3 12.7 11.9 9.1 8.9 8.6 6.9 9.6 13.4 1.3 11.1 17.6 11.4 29.1
17.8 2.5 0.5 11.2 0.5 5.5 16.3 14.3 6.4 13.0 18.4 10.0 9.2 8.9 1.2 10.1 14.3 1.4 12.6 18.8 12.2 31.6
0.042 0.087 0.131 0.152 0.154 0.261 0.42 1 0.455 0.460 0.497 0.534 0.541 0.514 0.598 0.665 0.106 0.128 0.154 0.99 1 1.077 1.081 1.661
for the samples
listed in Table I Ultimate analysis (wt %, dry ash free basis) Sample no. 1 2 3 4 5 6 1 8
9 10 11 12 13 14 15 16 11 18 19 20 21 22
C
H
N
98.4 94.6 91.0 89.5 89.6 89.9 83.7 82.7
0.4 2.3 4.0 3.9 4.2 4.7 5.0 4.2
0.4 1.2 2.7 2.4 2.3 1.3 1.1 1.9
S 0.7 0.6 1.9 4.0 4.4 0.9 0.7
0 (by diff.) 0.1 1.3 0.4 0.2 8.6 8.9
1.1 10.0 84.8 5.5 1.6 1.1 7.0 82.6 5.0 1.4 1.1 9.9 81.8 4.8 1.6 1.4 10.4 81.7 5.1 1.4 0.8 10.4 82.4 5.7 1.7 0.5 9.7 83.4 5.1 1.6 1.4 8.6 85.4 5.8 1.7 1.5 5.6 15.9 5.3 1.5 4.7 12.6 81.8 5.6 2.4 0.4 9.8 18.8 5.5 1.2 0.6 13.9 68.5 5.8 1.5 7.6 16.6 66.6 5.0 1.6 8.0 18.8 68.3 5.8 1.6 10.5 13.8 61.3 5.4 1.1 1.4 30.8
(C + H)/O
988.00 74.54 231.50 467.00 _ 11.00 9.97 8.69 12.90 8.85 8.33 8.40 9.08 10.29 16.28 6.44 8.92 6.06 4.48 3.81 5.37 2.17
C/H
246.00 41.13 22.75 22.95 21.33 19.13 16.14 19.69 15.42 16.52 17.04 14.33 14.46 16.35 14.72 14.32 14.61 14.33 11.81 13.32 11.78 11.35
Using the burning profiles, the initiation temperature of combustion (7;,) uersus the VM/FC ratio can be plotted (Figure 4). As expected, this shows a decrease in the initiation temperature of combustion when the VM/FC ratio increases. The peak temperature of combustion (T,,) tiersus VM/FC also decreases (Figure 5). The total rate of combustion (R,) increases as the VM/FC ratio increases (see Figure 6). Other correlations may be established for the volatile release profiles. For example, the initiation temperature of volatilization (IT;,),decreases as the VM/FC ratio increases (Figure 7). The peak temperature of volatilization (T’,) also decreases when the VM/FC ratio increases (Figure 8).
_I TEMPERATURE
Figure 1 maximum
Schematic representation rate of weight loss
of the method
of evaluating
the
As already reported5, it is not possible to detect particular trends with regard to the maximum rate of volatilization R, (Figure
COAL
9).
REACTIVITY
From these results it may be deduced that low-rank coals react at lower temperatures in both combustion and pyrolysis than do higher rank coals, and they show a higher rate of combustion. It may be asserted that, as the rank decreases the organic part of the coal is increasingly more reactive towards combustion and pyrolysis. When considering the measurement ofcoal reactivity, it is useful to mention the various criteria which may be thermogravimetric and kinetic used, i.e., chemical, considerations. In particular, chemical considerations6*8T9 include the oxygen content, or the degree of oxidation of the coal. More reactive coal is
~~~~,1986,Vol65
May
637
DTG combustion behaviour of coal: P. Ghetti Table 3
Data deduced
Sample No.
L Tpc ( ‘C) (“C) ._ 460 790
I 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 I8 19 20 21 22
430 360 365 365 335 305 298 305 300 295 290 285 300 305 275 265 265 250 245 253 185
” T,, = initiation combustion; temperature release; R, = b Two maxima
from burning
$g
min-‘)
and volatile
(%)
release profiles” R,’ (mg min _ ‘)
(%)
I .24
595 541 545 520 533 540 510 490 485 470 490 470 490 490 480 465 480 435 387427” 432450” 260-373’
4013-
_
2.08 1.95 1.90 1.96 1.87 1.88 1.75 2.06 1.94 2.0 1 2. I6 1.86 2.06
1.98 1.88 1.86 2.14 I .96 3.32 2.44 4.54
440 370 310 350 310 280 250 250 290 250 290 260 250 270 255 250 270 210 210 210 190
910 640 630 620 520 470 470 480 470 470 470 460 460 460 460-510* 460 460 450 450 450 350
1.06 0.78 0.66 0.68 0.96
G e 30( 3” h
I .oo 0.94 1.21 0.96
1.07 0.92
20( I-
I .oo 1.30 1.22 1.20 0.92 1.10 0.94 0.82 I.12 1.04
, 05
I
I
1.0
15
VMlFC
Figure 4 Trend VM/FC ratio
of initiation
temperature
of combustion
(T,) wrsus
temperature of combustion; T c = peak temperature of R, = maximum rate of corn YJustlon; 7;,, = initiation of volatile release; T,,,=peak temperature of volatile maximum rate of volatile release are present
C/H
20
15
10
05
15
VM IFC
Figure 5 Trend VMiFC ratio
L
10 0.0
temperature
of combustion
(T,,)
cersus
._
1Ll
0.5
of peak
1.5 VMlFC
Figure 2
Trend of C/H ~crsus VM/FC
ratio
.
. 1.0
0 0.5
0.0
Figure 3
638
1.0
Trend of (C + H)/O LWSIISVM/FC
FUEL, 1986, Vol 65, May
VMlFC
ratio
I
05
1
I
1.0
15
VM/FC
1.5
Figure 6 ratio
Trend of maximum
rate of combustion
(R,) wrsus VM/FC
DTG combustion
behaviour
of coal: P. Ghetti
1.5
. . . .
l*
. . . . 0. . .
. . . .
. t 0.5
I
05
I
I
10
15
1.0
VMlFC
Figure 7 Trend of initiation VM/FC ratio
temperatureofdevolatilization
(7;,) oersus
Figure 9 Trend VM/FC ratio
of maximum
rate
VM/FC
of devolatihzation
1.5
(R,)
~‘crsus
DTG peak height will indicate the relative reactivities of the coals. A more accurate estimate of reactivity may be achieved only by developing appropriate kinetic calculations. These would give the activation energy during the various stages of the combustion process’ 4P 16. A method recently proposed by Cumming7 gives a weighted mean value of the activation energy which is representative of the whole combustion process and it may therefore give a fairly reliable evaluation of coal reactivity.
. II 800 -
z 2 >600L?-
REFERENCES 1 400 t I
I
0.5
10
,
.
15
VM/FC
Figure 8 Trend of the peak temperature VM/FC ratio
of devolatilization
(T,,) versus
considered to be more oxidized or with a lower (C + H)/O ratio. Semi-qualitative evaluations have been made using the DTG of coal, taking into account both the position of the peak in the burning profile and its height. A coal is considered to be more reactive if the peak ofcombustion is shifted more towards low temperatures’. The height of the peak lo-l3 is related to a coal’s reactivity for kinetic reason based on the following equation:
where: R = reactivity; kl$ = initial weight of daf coal (mg); and (dw/dt),,, = maximum rate of weight loss (mg min I). Therefore, if the daf weights of the coals are equal, the
7 8 9
IO
II 12 13 14 15 16
Bryers, R. W., Biswas, B. K. and Taylor, T. E., ‘Fuel evaluation using differential thermal techniques’, paper presented at ‘Coal Technology ‘78. The international coal utilization conference and exhibition’, Houston, TX, 17-19 October 1978 Cumming, J. W. and McLaughlin, J. J. Thermochirn. Acta 1982, 57,253 Elder, J. P. and Harris, M. B. Fuel 1984, 63, 262 Ghetti, P., De Robertis, U., D’antone, S., Villani, M. and Chiellini, E. Fuel 1985, 64, 950 Kopp, 0. C. and Harris, L. A. Int. J. Cod Gad. 1984, 3, 333 Hensel, R. P., ‘Coal Classitication, chemistry and combustion’, paper presented at ‘Coal-tired industrial boilers workshop’, Kaleigh, NC, 10-I 1 December 1980 Cumming, J. W. Fuel 1984,63, 1436 ‘Combustion’, Combustion Engineering Inc., 3rd edition, 1981, p, 2-13 Frolova, N. V., Smutkina, Z. S., Klinkova, V. V., Erogova, T. F., Ekaterinina, L. N., Dolatova, A. G. and Skripchenko, G. B., Khimiya fierdogo Topha 1980, 14, 5, 34 Mahajan, 0. P. and Walker, P. L. Jr., ‘Analytical Methods for coal and coal products’, Vol. II, (Ed.C. Karr Jr.) Academic Press. Inc. New York, 1978, Ch. 32, p. 465 Knight, A. T. and Sergeant, G. D. Fuel 1982,61, 145 Frazier, G. C., Mason, C. and Badin, E. J. Fur! 1984, 63, 499 Fung, D. P. C. and Kim, S. D. Furl 1984.63, 1197 Smith, S. E., Neavel, R. C., Hippo, E. J. and Miller, R. N. Furl 198 1,60,458 Serageldin, M. A. and Pan, W. P. J. Thermochim. Actu 1983,71,1 Seradeldin, M. A. and Pan, W. P. J. Thermochim. Actu 1984,76, 145
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