Silica ratio estimation from carbon dioxide content, ash yield and pyritic-sulphur content of australian bituminous coals

Silica ratio estimation from carbon dioxide content, ash yield and pyritic-sulphur content of Australian bituminous coals M. SHIBAOKA A new method is described for estimating the silica ratio of the ashes from bituminous coals. While for most Australian coals it involves only determinations of the carbon dioxide content and the ash yield, for other coals a pyritic sulphur determination is also necessary. The method requires less equipment and is thus quicker and more convenient than the conventional method. IN SELECTING a coal for combustion under slagging conditions it is essential to know both the fusion temperature of the ash and the flow properties of the fused slag. For measuring the viscosities of fused slags at high temperatures special equipment (a rotating-cylinder viscometer l-a) and experience are needed. Consequently, attention has been widely directed to the possibility of predicting the viscosity/temperature characteristics of coal-ash slags from their chemical composition. The silica ratio: I00 SiO2 SiOz + CaO + MgO + total iron as Fe2Os originally proposed by W. T. Reid and P. Cohen e, has proved to be a useful guide in selecting coals for slagging combustion, but it presupposes that the SiO2, CaO, MgO and Fe203 contents of the coal ashes are known. This is not always the case, and although they can be determined by the semi-quantatitive spectrographic method of M. C. Clark and D. J. Swaine 4 the new method of silica-ratio estimation is preferable because ash yield and carbon dioxide content are more readily determined, and pyritic sulphur content fairly readily determined, in most laboratories. C. R. Kent and A. O. Champion s early in their investigation found a weak but recognizable correlation between ash-fusion temperature and the ratio 100(CO~ in coal)/(ash yield of coal), which we shall denote by X; but later, when data on other bores became available, they had to admit that the correlation was too weak to form the basis of a reliable method. The present work is based on the hypothesis that the silica ratio can be predicted from the value of X combined with that of the ratio 100(pyritic sulphur in coal)/(ash yield of coal), which we shall denote by Y. METHODS AND RESULTS The carbon dioxide content of coals is roughly proportional to their mineral carbonate content (mainly iron, calcium, magnesium and manganese 6, 7). 431

M.

SHIBAOKA

Since the manganese content of coal is usually fairly low (less than 0 . 3 ~ Mn in most coal ashes, according to Clark and Swaine 8) X should be roughly proportional to the (Fe203 + C a O + MgO) content derived from the carbonates. Furthermore, if the total amount of non-carbonate iron, calcium and magnesium present is small and almost constant, X may also be expected to be roughly proportional to the total amount of (Fe.,O3 q- CaO q- MgO) in the coal ashes. There is in fact a high degree of correlation between these two parameters. Now the (FezO3 ÷ C a O + MgO) content is used for calculating the silica ratio, and it should therefore be possible to use X for the same purpose. This has in fact proved to be the case, except for coals which are rich in pyritic iron (Figure 1), such as those front the Greta Coal Measures [where I00(~o pyritic S ) / ( ~ ash) > 1.0]. With these coals there is no simple relation between X and the silica ratio (see Figure 2) and it is consequently necessary to take into consideration the

lOOt x

8

0

~

Z: 92.57-1.21X

to

Z :9

20

1'0

2b

3b

~b

s'0

100xCOl/Ash

do

~o

Figure 1 Relation between 100-: COs/ash value and silica ratio (Blair Athol coals)

Fe203 content of the ash which has resulted from decomposition and oxidation of the pyritic iron. The silica ratio may thus be expected to be proportional to X and Y. Computer programmes were used to determine best-fit curves from the available data and to establish regression equations between X and Z, where Z is the silica ratio (see Table 1 and Figure 1), and X and Y and Z (see Table

Table 1 Relation between 100 • CO.2/ash value and silica ratio Quadratic function Coal measures Newcastle Illawarra

Collinsville Baralaba Ipswich X ~

District and coal seam Northern Coalfield Southern Coalfield Bulli seam Other seams Blair Athol Moura West Moreton

Regression equation ~ 95.46

2.35X

Z = 96.30 Z = 99"85 Z = 95-75 Z == 90"51 Z - 96"88

Z

-- 2.51X -- 3.07X 1-75X -- 2.72X -- 1-58X

1 0 0 [ C O . . i n c o a l ( % ~ ) [ a s h y i e l d o f c o a l (",]}]; Z

=

Regression equation

+ 0.032X'*

0"96

Z -- 94"98

+ + -t+ --

0"98 0.98 0.99 0"93 0"92

Z - 95.30 Z 97.99 Z = 92-57 Z -: 89.82 Z -- 97-35 -

0.027X*" 0-058X'* 0'008X = 0 . 0 3 2 X ~0"042X z

Silica ratio

432

Linear fimction Corr. coeff:

Corr_ ¢'ot'ff~

- 2.02X

0.96

2.07X 2"20X 1-21X 2"18X 1.92X

0-97 0"97 0.99 0"90 0.92

SILICA RATIO ESTIMATION OF A U S T R A L I A N B I T U M I N O U S COALS 100

90 x

x

x

8O

% X

70

•

l x

.o_. 60

'~

X

SO

X

X x

X

40

30

0

2

4

6

8

10

12

100x COz/Ash

Figure 2 Relation between 100 • CO~/ash value and silica ratio (Greta coals) × , Maitland; 0 , Liddell

2 a n d Figure 3). T h e l i n e a r - f u n c t i o n e q u a t i o n (Table 1) p r o v e d to b e as a c c u r a t e as t h e q u a d r a t i c . A s i n d i c a t e d in Table 2, t h e r e g r e s s i o n e q u a t i o n s g i v i n g Z in t e r m s o f X a n d Y d i f f e r s l i g h t l y f o r t h e d i f f e r e n t c o a l m e a s u r e s . T h e ffoIlowi n g e q u a t i o n , w h i c h w a s c a l c u l a t e d o n t h e b a s i s o f 2 6 6 sets o f d a t a , a p p l i e s with a fair degree of accuracy to most Australian bituminous coals: Z :

95.79 - - 2 . 1 9 X - -

1.86 Y

Table 2 Relation between 100 . CO:~/ash value, 100 ~ pyritic-S/ash value, and silica ratio Coal measures

District

Regression equation

Corr. coe/.l-

Greta Greta Baralaba

Northern Coalfield (Maitland) Northern coalfield (Liddell) Moura

Z = 92-55 2.70X - 1.57 Y Z = 95-23 - 1 . 8 8 X - 1.53Y Z = 89.75 -- 1 . 9 9 X - 1.67 Y

0.95 0.97 0.93

X = 100[COe in coal ( '~/o)/ash yield of coal (/o)] °/ Y = 100[pyritic sulphur in coal (%)/ash yield of coal ( %)] Z = Silica ratio 433

M.

SHIBAOKA

100

./

90 x

x

/!

80

70 X

•

x •

60

t,C~

20

10 20

. 30

L 40

.

L

50 Predicted

L

60 70 silico ratio

.

.

80

90

100

F(eure 3 Comparison between predicted silica ratio values and actual values ,'<, Maitland;

@, Liddell

DISCUSSION The method described here is essentially statistical and the following factors should be borne in mind: (1)

X will be proportional to the total number of atoms of Fe, Ca and Mg in ashes derived from carbonate minerals; but it cannot be proportional to the percentage by weight of (Fe203 + CaO + MgO) in ashes derived from the carbonate minerals, because of the considerable differences in the molecular weights of these oxides.

(2)

Elements other than Fe, Ca and Mg may occur also as carbonates, although not in abundanceL

(3)

The amounts of Fe, Ca and Mg derived from non-carbonate material differ slightly from sample to sample.

(4)

The silica ratio depends not only on the amount of(Fe2Oa + CaO 4- MgO) in coal ashes but also on the amount of SiO2.

The above factors (2), (3) and (4) can all be sources of error. Some of the Moura coals, for instance, (Table 1) are rich in SiO2 and consequently deviate

434

SILICA RATIO ESTIMATION OF AUSTRALIAN BITUMINOUS COALS

from the curve for typical Moura coals (not reproduced here), i.e. the correlation coefficient is relatively low for these coals (0-93 and 0.90, Table 1). In Figure 1, the deviation of one of the points appears to be due to the high proportion of siderite compared with other carbonate minerals. The fact that in most cases there is a high correlation between X, Y, and the silica ratio is probably due to the factors cancelling each other out.

CONCLUSION

The method described gives reliable silica-ratio determinations for Australian bituminous coals and would probably also be applicable to bituminous coals from other parts of the world. It is not applicable to brown coals. Provided the coal is known to be low in pyritic sulphur, the calculation of silica ratio involves only the ash yield and the CO2 content of the coal. In other cases it involves the pyritic sulphur content also.

ACKNOWL EDGMENTS

The author is indebted to the many officers of CSIRO who collected the extensive data on Australian coals on which this work is based; to Dr H. David, Dr T. Miyazu, and Mr A. Watts, for the regression analyses; and to Mr. H. N. S. Schafer and Dr D. J. Swaine, for helpful suggestions.

CSIRO Division of Mineral Chemistry, P.O. Box 175, Chatswood, New South Wales, Australia 2067

(Received 23 October 1969) (Revised 16 January 1970)

REFERENCES Boow, J. J. Inst. Fuel 1965, 38, 3 Reid, W. T. and Cohen, P. Trans. Amer. Soc. Mech. Engrs 1944, 66, 83 Shaw, J. T., BCURA Information Circular No. 249, 1961 Clark, M. C. and Swaine, D. J. Fuel. Lond, 1963, 42, 315 Kent, C. R. and Champion, A. O. Proc. Syrup. Inorganic Constituents of Fuel (Melbourne, 1964), p 83; Inst. Fuel (Australian Membership) Collected Preprints 6 Pringle, W. J. S. and Bradburn, E. Fuel, Lond. 1958, 37, 166 7 Brown, H. R., Durie, R. A. and Schafer, H. N. S. Fuel, Lond. 1960, 39, 59 8 Clark, M. C. and Swaine, D. J., CSIRO Division of Coal Research, Technical Communication 45, 1962

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