Properties of second order transitions in Argonne Premium coals

FuelVol. 75 No. l, pp. 85-88, 1996 Copyright © 1996ElsevierScienceLtd Printed in Great Britain. All rights reserved 0016-2361/96/$15.00+ 0.00

0016-2361(95)00200-6

ELSEVIER

Properties of second order transitions in Argonne Premium coals Alexander J. M a c k i n n o n and Peter J. Hall* Department of Pure and Applied Chemistry, University of Strathclyde, 295 Cathedral Street, Glasgow G1 1XL, Scotland, UK (Received 23 May 1994; revised 10 April 1995) Differential scanning calorimetry (DSC) has been used to investigate the thermal transitions occurring for a series of eight Argonne Premium coals. Each of the coals was subjected to different heating/cooling profiles in order to determine the effect of cooling at varying rates on the second order process and to determine the effect of temperature of exposure on the second order process. The variable cooling rate scans showed that the position of the transition increased with decreasing cooling rate of the previous scan. The sequential heating scans demonstrated that the rank of coal had a direct influence on the resistance of the second order process to exposure to heat. Low rank coals showed a greater degree of resistance and this was assumed to be related to the density of non-covalent bonds within the structure. (Keywords: coal; thermal transitions; DSC)

Coal is generally accepted to have a covalently crosslinked macromolecular network of aromatic and hydroaromatic clusters with cross-linking attributed to covalent bonds of etheric, methylinic and sulphides; hydrogen bonds or entanglements between macromolecules. Consequently, common polymeric characterisation techniques such as solvent swelling, N M R , differential scanning calorimetry (DSC) and dynamic mechanical thermal analysis (DMTA) have been used to investigate the nature of these networks. DSC has been used extensively in investigations of the macromolecular nature of coal 1-7 and specifically the glass transition process. Traditionally this process for coals has been observed at ,,~ 325°C r'2'4 but recent work by Mackinnon and Hall 7 has demonstrated that a distinct process exists at ~,,110°C. Further evidence for this transition has been highlighted by low frequency dielectric and conductivity measurements for Argonne Premium coals 8 in which the real conductivity of each coal shows a distinct break at a temperature consistent with the second order process. Oxidation studies on Beulah Zap lignite 9 demonstrate the second order process appears to enhance the oxidation process by increasing the accessibility of the coal structure at this critical point. In this paper we examine in two series of experiments the effect of altering the heating/cooling profiles on the position and intensity of the second order process. In the first series, the process is observed as a function of the cooling rate of a preceding scan. In the second series, a sample is sequentially heated with the final temperature of each heating scan progressively increasing up to the point where the onset of major pyrolysis is expected. * To whom correspondenceshould be addressed

EXPERIMENTAL Coal samples were selected from the APC sample bank l° and were used as received. Calorimetry measurements were conducted on a Mettler DSC 30 system which was used in conjunction with Mettler software TA12PS.2 for data acquisition and processing. The temperature and intensity of the second order transitions were derived using routines within the Mettler software. The sensor on the instrument consisted of a five-fold A u - N i thermopile mounted on a glass disc. The use of this type of sensor was believed to improve the recognition of low intensity transitions. Temperature calibration was by the melting points of indium, lead and zinc standards supplied by Mettler and temperatures are accurate to i0.5°C. Enthalpy calibration was by integration of the melting endotherm of an indium standard supplied by Mettler. It was estimated that enthalpies were accurate to + 0 . 0 5 J g -2. Standard aluminium crucibles with cold welded lids were used with two pin holes pierced through the lids to allow evaporation of water. The sample size was approximately 10 rag. A nitrogen carrier was used at a flow rate of 50 ml rain-'. Cooling of the furnace between consecutive runs was carried out using liquid nitrogen from a reservoir directly beneath the furnace. Steady cooling rates could be achieved using this arrangement. Two different series of scans were conducted on each of the APC samples. (1) A sample of each fresh, untreated coal was heated to ll0°C at 10°Cmin -1 and held for 30min, cooled at a nominal rate of 200°C min-I to 30°C, heated to 250°C at 10°Cmin -l, held for 10min and then cooled to 30°C at 200°Cmin -l. This series of scans is referred to as the pretreatment stage and was intended to fully dry the

Fuel 1996 Volume 75 Number 1

85

Second order transitions in coals: A. J. Mackinnon and P. J. Hall Table 1 Properties of the Argonne Premium coals

Heat flow (W/g)

Coal

Water (as received) %

MAF C%

MAF 0 %

Beulah-Zap Lignite Wyodak Anderson Illinois No. 6 Blind Canyon Pittsburgh No. 8 Lewis Stockton Upper Freeport Pocahontas No. 3

32.2 28.1 8.0 4.6 1.7 2.4 1.1 0.7

72.9 75.1 77.7 80.7 83.2 82.6 85.5 91.1

20.3 18.0 13.5 11.6 8.8 9.8 7.5 2.5

-0.25

,

~

N

5.0*C/min

1.0 * C / r n i ~ ~ k ' ~ 0 3 C/ram -0.3

sample. The sample was then alternately cooled and heated between 30 and 250°C, with successive cooling rates of 30.0, 20.0, 10.0, 5.0, 2.5, 1.0 and 0.3°Cmin -1 in between alternate heating scans of 10°Cmin -1. This sequence was designed to assess the effect of cooling rate on the possible breakage and reformation of crosslinks and hydrogen bonds and the subsequent effect on the second order transition. Similar scans were also conducted on a sample of polystyrene (PS) (Polymer Laboratories UK; number-average molecular weight, Mn 14,300) for comparison. The upper temperature limit was restricted to 150°C to avoid degradation but the same cooling and heating profiles were used as for the coal samples. (2) A sample of each fresh, untreated coal was heated to ll0°C at 10°Cmin -1 and held for 30min, cooled at a nominal rate of 200°C min -1 to 30°C, heated to 250°C at 10o Cmin- 1 , held for 10min and then cooled to 30 o C at 200 o C min- 1 . The sample was then heated from 30 to 250°C, cooled to 30°C and then sequentially heated to a temperature 25°C in excess of the previous temperature, up to 475°C. All heating and cooling scans were conducted at 10°C min -1. RESULTS AND DISCUSSION Variable cooling rate runs Each sample of pre-treated coal was successively cooled at various rates, with all intermediate heating scans at 10°Cmin -1. Each heating scan showed the familiar second order process, characteristic of a glass transition process 8A1. The position of the second order process from the heating scan showed a dependence on the cooling rate of the scan which the sample had previously encountered. This is illustrated by the example of Lewis Stockton in Figure 1. For simplicity, only four scans are illustrated

Table 2

q 2 0.0 * C/m i n , - - , , ~ , , ~ . ~

-0.35

I

0

50

I

=

100 150 Temperature (°C)

=

200

250

Figure 1 DSC (heating scans) of Lewis Stockton after being subjected to various cooling rates

in Figure 1. The results of the dependence are listed in Table 2 and clearly show that the position of the process increases with decreasing cooling rate of the previous scan. The cooling rate would therefore appear to have a profound effect on the structure which is formed after exposure to 250°C. We have already demonstrated that for a coal sample continually cycled from 30 to 250°C five times in succession 11, the position and intensity of the process remains virtually unaltered. Therefore the shift in the position of the process in the present work cannot simply be attributed to continuous cycling and prolonged exposure to heat. Suuberg 12 has proposed that on heating, hydrogen bonds could dissociate, effectively causing a decrease in the degree of crosslinking and resulting in an increase in macromolecular freedom of motion. This would cause a second order process in DSC. On cooling the hydrogen bonds could reform. Miura et a113 have demonstrated using DSC and FTIR that hydrogen bonds are disrupted between 150 and 200°C during heating of coal, although no indication is given as to whether they reform on cooling. The rate of cooling would therefore appear to affect the degree of crosslinking on reformation would occurs in the cooling sequences, and this is reflected in the position of the process. In the case of a polymer, a more highly crosslinked network would restrict the molecular motion at a given temperature and hence increase the glass transition process 14. Polystyrene was also studied by repeatedly cooling through its glass

Second order transitions of APC samples after variable cooling rate scans Transition temperatures (°C)

Cooling rate (°C min -1)

Beulah-Zap

30.0

101.8

93.0

95.0

104.9

106.5

99.2

103.9

88.4

20.0

97.9

93.7

93.3

101.9

109.5

99.7

98.8

90.3

10.0

94.1

99.5

97.1

105.2

103.5

102.0

103.2

94.7

5.0

94.0

102.9

98.6

116.1

108.9

109.0

104.9

95.0

2.5 1.0

102.1 109.7

105.1 111.0

104.3 112.7

121.2 123.4

109.5 120.0

117.6 118.1

103.0 106.8

103.9 104.5

0.30

130.5

140.7

127.9

N/A

124.0

131.4

N/A

N/A

86

Wyodak-Anderson

Fuel 1996 V o l u m e 75 N u m b e r 1

Illinois No. 6

Blind Canyon

Pittsburgh No. 8

Lewis Stockton

Upper Freeport

Pocahontas No. 3

Second order transitions in coals. A. J. Mackinnon and P. J. Hall H e a t f l o w (W/g)

H e a t f l o w (W/g)

-0.45

-0.65

i -0.5

~

-0.69

- ----

":,,, ,

-0.55

% "",

30-250 °C 30-300°C

---

30-350 °C

....

30-400°C 30-475 °C

-0.72

-0.76

-0.65

(Z3~C/min~ " /

-0.7

-0.8 90

95

100

105

Temperature

7

-

I 110

115

50

100

150

120

250

Temperature

(°C)

Figure 2 DSC (heating scans) of polystyrene after being subjected to

200

300

350

450

400

(°C)

Figure 3 Sequential DSC heating scans of Upper Freeport

various cooling rates

transition temperature using a range of cooling rates, as illustrated in Figure 2. The DSC traces clearly show that as the cooling rate decreases, the magnitude of the enthalpy relaxation peaks increases. This is most prominent with the lower cooling rates and is typical behaviour for an amorphous polymer. The position of the transition is unaltered during the complete sequence of scans. The behaviour of the coal samples is in complete contrast to that of polystyrene; they show a distinct shift in the position of the transition. In certain of the coal samples though, such as Figure 1, Lewis Stockton, it is possible to discern a small endotherm on the heating scan following the final cooling scan.

H e a t f l o w (W/g) -0.4 - 0 . 4 5 .~ " . . . . . E~-" :4

. ~4t

-0.5 ~'~:'~,'

- -

30-250 °C

------

30-300 °C 30-350 °C

----

30-400°C

1' . . . . . . ,/ " '" " . . . .

-0.55 C

- 0 . 7 ~-' 50

100

t 150

, 200

Samples of each of the Argonne Premium coals were sequentially heated from 30 to 475°C. Figures 3 and 4 illustrate the examples of Beulah-Zap lignite and Upper Freeport coals respectively which are characteristic of the entire suite of APC samples, and the characteristics for the entire suite of coals are listed in Table 3a-h. Data is only displayed in the tables for those scans which exhibit a second order process. In certain coals, the process disappeared at higher temperature of exposure. The first DSC trace of each sample clearly shows the distinct first order process which may be associated with an enthalpy relaxation. A previous paper has shown that an endotherm is observed for each of the Argonne coals immediately on scanning after undergoing the pretreatment process. It is deduced that this endotherm is a function of the cooling rate of the preceding scan and that slow cooling (10°C min -1) induces a second order process whereas faster cooling (200oC min- l ) induces a first order process. This first order process is completely induced after the first scan to 250°C. Second and subsequent scans show the distinctive second order process, as a consequence of cooling at 10°Cmin -1. Thermogravimetric analysis has demonstrated that neither the first or second order processes are assodated with weight loss u. In general, the intensity of the transition, as measured by the change in heat capacity, diminishes with increasing temperature of exposure. This could be due to pyrolytic depolymerisation of the structure coupled with nonreversible crosslinking reactions. Volatilisation of low molecular organic species could also result in a less mobile

Figure 4

4

3 0 - 4 7 5 oc . . . . . .. -'

, 250

Temperature

Effect of sequential heating scans

!

300

350

.

,

l 400

-4

450

(°C)

Sequential DSC heating scans of Beulah-Zap lignite

structure. In certain of the coals, particularly in the case of low rank coals such as Beulah-Zap lignite and WyodakAnderson, the second order process is partially resistant to exposure to 450°C while in other coals, particularly high rank coals such as Upper Freeport and Pocahontas No. 3, the transition is no longer evident in the scans to the higher temperatures. The remaining coals of intermediate rank (i.e. Illinois No. 6, Blind Canyon, Pittsburgh No. 8 and Lewis Stockton), display a variable degree of resistance. Illinois No. 6 appears to be the least resistant. It is clear from this data that the low rank coals display a process which is more resistant to temperature exposure than those of high rank coals. It has been suggested that low rank coals have relatively large proportions of hydrogen bonds compared to lower rank coals 17' 18 and indeed Larsen 19 suggests that the crosslinking density decreases with the rank of coal, up to about 86% C. If we expect the second order process and intensity to depend on the breakage of hydrogen bonds, then low rank coals would be expected to exhibit a larger intensity than high rank coals. Of course due to the highly complex nature of coal, it would be difficult to assume that one single factor is responsible for our observations. CONCLUSIONS The DSC traces clearly show that the glass transition process displays certain properties dependent on the

Fuel 1996 Volume 75 Number 1

87

Second order transitions in coals: A. J. Mackinnon and P. J. Hall Table 3a-h DSC characteristics of sequentially heated APC samples

Table 3 Continued

(a) Beulah-Zap

(e) Pittsburgh No. 8

Temp. range (°C)

Ttr (°C)

/kTtr (°C)

Acp (jg-1 K-l)

Temp. range (°C)

Ztr (°C)

ATtr (°C)

Acp (jg-1 K-l)

275 300 325 350 375 400 425 450

130.3 127.3 131.5 129.4 122.8 127.1 122.9 123.4

15.7 14.7 19.0 18.5 20.8 21.2 23.1 17.6

0.69 0.56 0.64 0.60 0.51 0.47 0.45 0.31

275 300 325 350

119.4 117.3 114.3 114,8

11.7 10.2 12.5 4.7

0.22 0.15 0.14 0.02

Temp. range (°C)

Ttr (°C)

ATtr (°C)

Acp (jg-I K-l)

275 300 325 350 375

117.1 121,9 120,3 119.5 116.9

10.3 12.2 13.5 8.6 11.4

0.20 0.12 0.11 0.07 0.10

Temp. range (°C)

Ttr (°C)

ATtr (°C)

Acp (jg-1 K-l)

275 300 325 350 375 400

123.7 124.0 122.9 124.2 124.0 123.1

7.2 9.7 10.4 12.4 16.6 16.9

0.10 0.15 0.12 0.13 0.11 0.09

Temp. range (°C)

Ttr (°C)

ATtr (°C)

ACp (jg-I K-l)

275 300 325 350 375 400 425

114.4 114.8 111.1 105.5 114.6 101.6 101.1

9.1 8.1 13.7 9.4 14.3 18.7 13.1

0.06 0.07 0.08 0.05 0.06 0.15 0.06

Co) Wyodak-Anderson Temp. range (°C)

Ttr (°C)

ATtr (°C)

Aep (jg-I K-l)

275 300 325 350 375 400 425 450 475

119.1 120.8 124.4 124.3 122.3 121.6 119.0 115.6 119.4

10.9 14.5 13.9 16.5 14.8 14.7 15.9 14.8 18.8

0.43 0.41 0.41 0.35 0.36 0.32 0.39 0.29 0.38

Temp. range (°C)

Ttr (°C)

ATtr (°C)

Acp (jg-I K-l)

275 300 325 350 375

113.0 119.4 121.6 117.7 123.8

11.3 7.8 10.3 16.3 9.8

0.24 0.12 0.15 0.19 0.06

Temp. range (°C)

Ttr (°C)

ATtr (°C)

Acp (jg-1 K-l)

275 300 325 350 375 400 425 450 475

124.4 129.2 119.7 129.2 121.7 118.5 118.4 116.8 120.6

13.6 13.6 14.4 9.4 16.9 10.0 10.0 14.1 2.7

0.19 0.12 0.11 0,07 0.16 0.06 0.05 0.05 0.01

(c) Illinois No. 6

(d) Blind Canyon

(f) Lewis Stockton

(g) Upper Freeport

(h) Pocahontas No. 3

REFERENCES 1 2

h e a t i n g / c o o l i n g regime to which the coal is exposed. C o o l i n g o f s a m p l e s o f coal at different rates induces changes in the p o s i t i o n o f the glass t r a n s i t i o n process which is exhibited o n s u b s e q u e n t h e a t i n g scans. T h e effect o f c o o l i n g rate is a s s u m e d to be a f u n c t i o n o f the rate at which h y d r o g e n b o n d s m a y r e f o r m u p o n cooling. Sequential s c a n n i n g to 450°C shows t h a t in certain o f the coals, the t r a n s i t i o n is p a r t i a l l y resistant to the t e m p e r a t u r e o f exposure. I n p a r t i c u l a r , the glass transition o f low r a n k coals such as B e u l a h - Z a p a n d W y o d a k A n d e r s o n are o b s e r v e d even after e x p o s u r e to 450°C w h e r e a s for high r a n k coals such as U p p e r F r e e p o r t a n d P o c a h o n t a s N o . 3, the t r a n s i t i o n d i s a p p e a r s quite readily. It is a s s u m e d this effect is in p a r t d u e to the relatively large n u m b e r o f h y d r o g e n b o n d s p r e s e n t in low r a n k coals in c o m p a r i s o n to higher r a n k coals. ACKNOWLEDGEM

ENTS

This w o r k was funded by S E R C grant n u m b e r G R / H 18821. T h a n k s are also due to D r M. J. R i c h a r d s o n o f the N a t i o n a l Physical L a b o r a t o r y , T e d d i n g t o n , U K for discussions.

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Fuel 1996 Volume 75 Number 1

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