Page 1
Proceedings of The South African Sugar Technologists' Association 
June 1977
CALORIFIC VALUES OF SOUTH AFRICAN BAGASSE *
By C. E. DON and P. MELLET
Sugar Milling Research Institute
B. D. RAVNO
Ifuletts Sugar, Ltd.
and R. BODGER
Department o f Mechanical Engineering, University of Natal
Abstract
When gross calorific values (GCV's) on a moisturefree,
brixfree and ashfree basis were determined on varieties of
S.A. bagasse, no significant differences were found when age,
time of harvest, source, fibre, pith, cane stalk or cane tops
were considered. Only cane leaves gave higher values. Since
ash exerts a significant influence, a formula including ash %
sample as an independent variable was developed to predict
the GCV. Using a hydrogen content of $91 % (dry basis) for
bagasse, an equation to predict net calorific value (NCV) of
bagasse was developed which also includes ash % sample as
an independent variablle. The equation predicts NCV defined
at 20°C.
Introduction
Accurate assessment of the calorific value of bagasse is of
economic importance, yet the only methods of assessment
presently available are direct laboratory analysis of calorific
value or the use of calculation formulae derived for specific
conditions elsewhere in the world.
In spite of considerable differences in appearance between
varieties of cane, the gross calorific value (GCV) of dry
bagasse varies only about two percent for bagasse from
different countries, and a universal value, as quoted by Hugotl
(based on values from s,everal different authors) has therefore
been adopted for the GCV of dry bagasse, i.e. GCV =
19 256 kJ.kgl.
Many formulae1*
the calorific value of wet bagasse; among the better known are
those of von Pritzelwitz van der Horst2 of Java and Hessey3
of Australia.
The ultimate analysis of bagasse is used to calculate net
calorific values (NCV's) from GCV'slj
equations used therefore depend on the percentage of net
hydrogen in bagasse which varies from country to country,
and the general value of NCV for dry bagasse being used, viz.
17 791 kJ.kgl, is not very precise.
The effects of various parameters on the calorific value of
South African bagasse have been tested including variety of
cane, age at harvesting, source, moisture content, sucrose and
nonsucrose contents. The variations of calorific values between
cane leaves, cane stalk, cane tops, pith and fibre were measured.
The ultimate analysis of bagasse was also experimentally
determined.
E(xyerimenta1
Preparcltion o f samples
Most samples were obtained from the Mount Edgecombe
Experiment Station ancl it was ensured that all samples fairly
represented the particular cane being tested. Details of all
procedures and results are given in the original worklo.
Normally about eight to ten sticks of cane were collected from
the field for each sample. The cane was sorted separately into
cane leaves, cane tops and cane stalks and the follbwing
procedurel1 was applied to all groups and samples.
* Part of a thesis submitted for the degree of M.Sc. (Mech.Eng.) Uni
versity of Natal by C. E. Don.
2 9 3 9 4 s
5 9 have been proposed to determine
' 1
s y
9. The NCV
The cane was cut into pieces 30 to 60 mm long and placed
in a shredding machine to reduce the particle size so as to be
suitable for direct analysis. If the samples were required free
of brix, they were treated in a cold digester bowl.
Four different South African varieties were tested, two with
average pith, N 551805 and NCo 376, one high pith, N 50121 1,
and a low pith variety NCo 310. All four varieties tested were
grown on dolorite soil and were twentyfour months old when
cut. The varieties were chosen to obtain an average calorific
value for some commonly grown South African canes and also,
with the increasing demand for fibre as opposed to pith by
the paper and particle board industries, to see if there was a
significant difference between varieties with high and low pith
contents. Samples were tested brixfree. Four samples of the
N 551805 variety which were of different ages when harvested
were chosen to test the effect of age on calorific values. Samples
of cane stalk which were 8, 10, 18 and 24 months old were
prepared and tested brixfree.
Samples of N 551805 stalk which were of the same age but
which were grown on two different types of soil were tested.
Samples grown on heavy soil (middle Ecca) and sandy soil
(Clansthal sand) were chosen and tested brixfree and samples
of pith and fibre were obtained using a similar apparatus to that
used by Snow12. Samples of leaves, tops and stalk were pre
pared in the standard way.
Bagasse samples with up to 60" brix by mass in the sample
were prepared and tested by partial extraction of brix and pol
in the cold digester bowl. Ash determinations were done on all
samples so that calorific values could be presented on an
ashfree, brixfree and moisturefree basis.
To test the validity of the developed formulae and to
compare the calorific values so obtained with actual de
termined values as well as with values obtained by existing
formulae, five samples of final mill bagasse were obtained
from Mount Edgecombe mill. The retention time of the
tandem of mills was calculated so that the variety of final mill
bagasse actually sampled could be identified with the aid of
the mill yard operator. A subsample of about 300 g was
taken from the sample collected by the Central Board for
analysis and was placed immediately in a moisture teller and
dried. This was done to minimise the breakdown of the sugars
from the time the sample was collected to when it was actually
tested in the bomb calorimeter. The subsample was milled
finely and then the gross calorific value, moisture content and
ash content were determined. The brix and pol percentages
and moisture percentage of the bagasse were obtained from
the Central Board laboratory. The five samples chosen were
from the varieties NCo 310 and NCo 376 with low ash content,
NCo 376 with a high ash content and NCo 376 fermented.
Bomb calorimeter
The gross calorific values of all the bagasse samples were
measured using a Baird & Tatlock bomb calorimeter (1 520
kPa of oxygen pressure, 6 V circuit voltage, ca. 0,7 g of
bagasse sample). Generally fresh bagasse samples were found
Page 2
170
Proceedings of The South African Sugar Technologists' Association June 1977
to contain a negligible amount of sulphur. A correction was
made for the formation of nitric acid during experiments.
Ultimate analyses
Nine samples were sent to the Butterworth Microanalytical
Consultancy Limited in Middlesex, United Kingdom for
analysis of C, H and N. They consisted of 5 samples of cane
stalk, two samples of cane tops and a sample each of cane
leaves and cane pith.
Results and discussion
Gross calorijic values (GCV) on a dry basis
The relatively unknown effects of many different parameters
on the GCV of South African bagasse were tested in this
study by experimentation under controlled conditions. The
GCV's were broken up into various groups which would
isolate one or two controlled variables and a factorial analysis
of variance (Fratio test) was applied to the samples within
the groups, corrected to a moisturefree and brixfree basis.
Linear regression analysis was applied to calorific values versus
moisture content and calorific values versus brix and pol
content because the data suggested an obvious correlation
between the dependent and independent variables. Results of
Fratio tests are given in Table 1.
TABLE 1
Summary of statistical tests on gross calorific values
Calculated Tabulated
Source of Variance
/ F value I F value
1. Different varieties on non ashfree basis .
2. Different varieties on ashfree basis.
3. Analysis of pith and fibre of (pith/fibrej
varieties on non ashfree basis
4. Analysis of pith and fibre of (pith/fibre)
varieties on ashfree basis
5. Analysis of leaves, stalk (leaves, stalks, tops)
and tops of different varieties
on non ashfree basis
6. Analysis of leaves, stalk (leaves, stalks, tops)
and tops of different varieties
on ashfree basis
7. Analysis of leaves and stalk on ashfree basis
8. Analysis of stalk and tops on ashfree basis
9. Analysis of leaves and tops on ashfree basis
10. Different sources on non ashfree basis. .
11. Different sources on ashfree basis . . .
12. Age at which cane is cut on ashfree basis. 1
The only variables that produced a significant Fratio test
on a brixfree and moisturefree basis, but not ashfree, were
the analyses of leaves, stalk and tops on cane. These data were
further broken down into three groups and the Fratio test
applied between each pair of independent variables. The tests
showed that the GCV's of cane tops and cane stalk were not
significantly different on a brixfree, moisturefree and ashfree
basis, but the GCV's of leaves were significantly higher than
those of stalk and tops when also expressed on a brixfree,
moisturefree and ashfree basis.
It can therefore be concluded that there is no significant
difference of GCV's on a moisturefree, brixfree and ashfree
basis for the different varieties tested, different ages of cane,
different sources and between fibre, pith, cane stalk and cane
tops. An average of all the nonsignificant GCV's (i.e. exclud
ing leaves) on a brixfree, ashfree and moisturefree basis,
gives :
GCV = 19 605 kJ.kgl
The average of the GCV's of all the leaf samples tested on a
brixfree, ashfree and moisturefree basis yields a value of:
GCV = 19 953 kJ.kgl
Gross calorific values are normally quoted on a non ashfree
basis and the above value, excluding leaves, on a brixfree,
moisturefree but non ashfree basis gave:
GCV = 19 424 kJ.kgl (ash % = 1,28).
(varieties)
.
(varieties)
(varieties)
(varieties)
0;76
1
2;78
The equivalent value for leaves is:
GCV  19 244 kJ.kgI (ash % = 335).
Thus the significant difference between leaves and stalk or
tops is only partially caused by the ash content of the sample.
Various GCV's of bagasse on a dry basis, but with varying
ash contents, have been reported1 and some of them differ by
up to 2 % from the universally accepted value of 19 256 kJ.kgI.
The value determined in the present study is only 0,87%
different from this universal value. This produces further
evidence that the universal value for GCV of dry bagasse can
be used with an error of scarely more than 2 %.
Linear regression analysis using GCV on an ashfree, moisture
free basis as the dependent variable and brix, pol and nonpol
as independent variables yielded the following equations:
GCV = 19 52342,23 nonpol20,78 pol kJ.kgI (1)
GCV = 19 523  42,23 brix + 21,45 pol kJ.kgI
GCV = 19 419  38,8 pol kJ.kgl
GCV = 19 514  28,66 brix k.J.kgl
Statistical analyses of the above equations show that it is
better to use the equation of GCV versus brix alone rather
than use the independent variables brix and pol or nonpol
and pol. Brix should also be preferred to pol as a term in the
equation for GCV's of bagasse for two other reasons, viz. brix
is the easier laboratory analysis and the brix coefficient is
less sensitive to variation in residuan purity and combustible
content of the polfree brix materiall4I. The "pooled" GCV for
bagasse (excluding leaves) on a brixfree, ashfree and moisture
free basis was 19 605 kJ.kgel. When the data is force fitted
through the point 19 605 kJ.kgl, a "method of least squares"
fit gives an equation to predict GCV:
GCV = (19 605  31,14 brix) kJ.kgl
Statistical evaluation of equations (4) and (5) showed that
the two equations are not different on the 95 % level of signifi
cance, thus the coefficients from equation (5) can be used to
predict the full GCV equation:
(2)
(3)
(4)
(5)
GCV predicted =
0 /
/o
r
moisture
o/
/o
ash*
) (I   )
(19 605  31,14 (brix % sample)
100 100
I
= 119 605  (31,14 brix % sample)

(196'05 moisture % sample)
(196,05 ash % sample?*)] kJ.kgml
*ash % dry sample; **ash % sample
Therefore, the value predicted by this equation for a brix
free, moisturefree and ashfree sample is 19 605 kJ.kgl.
(6)
Gross calorijic values of Jnal mill run bagasse
The GCV formula as derived in this study (Equation 6) as
well as formulae for GCV of bagasse as predicted by von
Pritzelwitz2 and Hessey3 were applied to predict the GCV of
some final mill run bagasse samples collected from the Mount
Edgecombe Mill. Results are given in Table 2. In his experi
mental work von Pritzelwitz2 assumed an average ash content
of bagasse in Java to be 3% so his formula was used in the
form :
GCV predicted = [ 19 046  30,98 (brix % sample)
 (190,46 moisture % sample)] kJ.kgl
Hessey3 assumed an average ash content of dry bagasse in
Queensland to be 2,7 % so his formula was used in the form :
GCV predicted = [ 19 406 
 (194,06 moisture % sample)] kJ.kgl
(7)
(34,12 brix % sample)
(8)
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Proceedings of The South A,Trican Sugar Technologists' Association June 1977
171
TABLE 2
M e a a ~ d 1 p e e n 1
Compariison of GCV values of final mill bagasse, predicted by various formulae to actual measured values (kJ.kgl)
1
sample
% sample sample
(a)
(6)
GCV
(0b)
(4
(ac) (dl (a4
Ash %
dry
Sample
Bx %
Moisture d z GCV 1 F 1
equa. (9)
GCV
equa. (8)
DiK
%
formu. (6)
TABLE 3
Comparison of calculated GCV values (kJ.kg') of final mill bagasse to actual measured values

(a)
Ash %
BX %
dry ~ ; ; . d
sample
% sample sample
. . . . . .
. . . . . .
. . . . . .
NCo376 . . . . . .
5,00
NCo376
. . . . . ,
4,16
3,65
10,76
NCO 376
NC0310
NCO 376
NCo 376
NCo376
. . . . . .
. . . . . .
. . . , . .
. . . . . .
. . . . . . 
(b)
GCV
(0b) ( 4 (ac)
(4
(ad)
Sample
1
~oisture 1
present 1
formu. (6)
d g 1
GCV
equa. (9)
DF I GCV
"if
%
equa. (10)
6,29
6,47
5,62
5,00
4,16
When these formulae are used in practice they do not take
account of the wide fluctuations in ash and extraneous matter
in bagasse. When the bagasse contained an ash and extraneous
matter higher than 3,O and 2,7% respectively, as can be seen
from the results in Table 2, the von Pritzelwitz's and Hessey's
formulae overpredicted GCV's by an amount depending on the
ash and extraneous matter percent.
However, if von Prit2:elwitz's formula is modified to include
ash % sample as an independent variable it becomes:
GCV predicted
= [19 632 (30,98 brix ;/,sample)(196,32moisture
 (196,32 ash % sample)] kJ.kgl
If Hessey's formula is modified to include ash % sample as
an independent variable: it becomes :
GCV predicted
= [19 946 (34,12 brix %sample) (199,46moisture %sample)

(199,46 ash % saimple)] kJ.kgI
The recalculated values of GCV obtained using the above
formulae for the five final mill bagasse samples are compared
to the experimentally mea.sured values and also to the values
obtained using the prese:nt developed formula. The results are
given in Table 3. It can be seen that the values obtained by
von Pritzelwitz's and Hessey's modified formulae are very close
to those found by the newly developed formula and all three
formulae predicted values that are within about 2% of the
actual determined values. Thus, the importance of including
ash as an independent variable in the equation is obvious.
Net calorific value
An average of the ultimate analyses of the cane stalk bagasse
samples tested has been calculated and is compared with
values from other sources in Table 4.
NCO 376
NCo 310
NCO 376
%sample)
(9)
4,25
3,78
3,59
6,66
3,65
(10)
6,24
6,47
5,62
2,69
2,94
14,29
3,15
10,76
The value that is of particular interest to calculate NCV
equations is the net % hydrogen. Statistical analysis of the
hydrogen values for all the bagasse samples tested shows that
there is no significant difference between stalk, leaves and pith
using a confidence level of 95 %. The overall average mean for
all nine samples of 5,91% net hydrogen in bagasse was thus
used to derive the NCV equations. There is statistically no
difference when this value is compared with values derived by
Magasiner8, Tromp7 and Paturaug but it is significantly different
from the value used by Hugotl.
4,25
3,78
339
6,66
It must be noted that predicted NCV equations differ from
those reported in the original textlo since further testing has
been done on ultimate analyses of bagasse samples.
18 375
18497
16209
17 546
16750
The bagasse tested had an average hydrogen content of
$91 % (dry basis) so the NCV for dry bagasse at 20°C (on a
non ashfree but brixfree basis) can be taken as:
NCV = 19 424  21 936 x 0,0591
2,69
2,94
14,29
3,15
18071
18 108
16025
16 727 /
This value differs less than 2 % from the universal value1 of
17 791 kJ.kgml due mainly to the lower hydrogen content
determined in the present work. In developing an equation to
predict NCV, ash % sample will have the same influence on
NCV as on GCV, therefore the NCV equation developed
contains ash as an independent variable. This predicted
equation is:
I
moisture % sample
 (1 296 (1 
18 375
17546 1
16750
18 071
18108
16025
17567
16727
18497 /
16209
NCV moist =
1,65
2,lO
1,13
+0,12
(GCV) predicted  (24,53 moisture % sample)
TABLE 4
100
Values of the ultimate analvsis of bagasse
17 567
0 , 1 4
1,65
2,10
1,13
+0,12
0,14
18042
18 126
18 188
17622
18 222 +8,79
18 098
18134
16048
17592,
16 751
1,81
2,Ol
+12,21
+0,43
Constituents
Tromp7 . . . . .
Magasiners . . . .
Hugotl . . . . .
Paturau8 . . . . .
This work . . . .
18 555
1,50
1,96
0,99
+0,26
00,OO
N e t % H
6 0
6,o
6,s
$88
5,91
O//C
,45,0
35,O
37.0
46,47
48,12
+10,78
18 366
18 452
18 518
17 942
0,05
0,24
+14,25
+2,26
18 A69
18407
16290
17861
17 008
Net %
O + N
47,O
46,O
44,O
44,71
44,91
0,03
0,40
+0,50
+1,80
+1,54
= [(I9 605  (196,05 moisture % sample)  (196,05 ash %sample)
 31,14 (brix % sample)  (24,53 moisture "/, sample)  1 296
' + (12,96 moisture % sample)]
%Ash
2,o
3,o
2 3
2,94
1,04
= [ 18 309  (31,14 brix % sample)  (207,6 moisture % sample)
 (196,05 ash % sample)] kJ.kgl
(1 1)
Page 4
172
Proceedings of The South African Sugar Technologists' Association June 1977
TABLE 6
TABLE 5
Compaiison of NCV values of final mill bagasse predicted by various formulae (kJ.kg')
Comparison of NCV values of final mill bagasse predicted by various formulae (k.l.kgl)
1
sample sample formula (1 I)
(a) (b)
NCV
(4
NCV
(0:~)
D l .
%
Moisture %
Ash % dry NCV, present Sample
8 x 5 .
sample
1
$!
equation (14)
equation (15)
(u:c)
D~ff.
%
+2,2
+2,5
+20,6
+2,8
+14,1
NCo 376 . . . 1
NCo 310 . . .
NCo 376 . . .
NCo 376 . . .
NCo 376 . . .
3.00
3;16
2,77
3,OO
2,31
( 4
NCV
equation (13)
6 840
7 105
7 227
7 106
7 510
Von Pritzelwitz's2 and Hessey3 original equations to predict
NCV values at 30°C are respectively:
Sample
NCo 376 . . .
NCo 310 . . .
NCo 376 . . .
NCo 376 . . .
NCo 376 . . .
NCV moist = [ 17 791  (30,98 brix % sample)
 (200,9 moisture % sample)] kJ.kgl
(12)
(0b)
Diff.
%
+1,3
+1,6
+19,4
+1,8
+13,0
(0)
NCV, present
formula (1 1)
6 691
6 931
5 992
6 912
6 578
Bx %
sample
3,OO
3,16
2,77
3,OO
2,31
Moisture %
sample
54,35
53,03
52,50
53,05
51,20
and
(b)
NCV
equation (12)
6 779
7 039
7 157
7 040
7 433
Ash % dry
sample
2,69
2,94
14,29
3,15
10,76
NCV moist = [ 18 100  (34,12 brix % sample)
 (205,3 moisture % sample)] kJ.kgl
(13)
The above formulae for NCV were applied to the five final
mill bagasse samples collected from Mount Edgecombe factory.
The results are given in Table 5 and show that the predicted
values by all formulae compare well for those samples with
low ash contents, as was the case with GCV values. However
for samples containing high ash contents the values obtained
by the present formula differed significantly from values as
predicted by von Pritzelwitz's and Hessey's formulae.
If the latter two formulae are changed to include ash as
an independant variable, they become respectively for von
Pritzelwitz and Hessey :
NCV moist = [ 18 335  (30,98 brix % sample)
 (207,6 moisture "/,ample)  (183,35 ash % sample)] kJ.kgl
and
(14)
NCV moist = [ 18 603  (34,12 brix % sample)
 (210,3 moisture % sample)  (186,03 ash % sample)] kJ.kgl
(15)
The recalculated NCV values based on the above modified
equations for the five mill bagasse samples, together with the
values as predicted by the present formula are given in Table
6. It is now found that these values agree very well and that
von Pritzelwitz's modified equation gave NCV values which
differ by less than 2% and Hessey's modified equation gave
values which differ by less than 3,5 % from the NCV values as
predicted by the present formula.
Summary and conclusions
It was found that there were no significant differences of
GCV's on a moisturefree, brixfree and ashfree basis for
bagasse from different cane varieties tested, from cane cut at
different ages and from different sources, and between fibre,
pith, cane stalk and cane tops. The data from these tests were
therefore pooled into a single set. The GCV's of cane leaves
were found to be significantly higher than those of fibre, pith,
stalk and tops, on an ashfree, moisturefree and brixfree
basis.
The final GCV equation predicted for clean cane (excluding
leaves) was :
GCV predicted = [I9 605  (196,05 moisture % sample)
 (196,05 ash % sample)  (31,14 brix % sample)] kJ.kgl
The formulae by von Pritzelwitz van der Horst and by
Hessey agree well with the values obtained by the present
formula only if the ash content of the sample is low. As the
ash content of bagasse increases the values derived by the von
Pritzelwitz van der Horst and Hessey formulae move further
and further away from the true values. The present developed
formula, containing ash as an independent variable, is therefore
preferred.
The ultimate analyses of some of the bagasse samples gave
slightly higher carbon contents and lower hydrogen contents
when compared to values used by other sources. The hydrogen
content for dry bagasse gave an average value of 5,91%.
This value is slightly lower than values quoted by other
sources1> '
NCV's but the significant difference in hydrogen content
measured has a very small effect on final NCV's of bagasse.
9 8 * 9. The hydrogen content is used to calculate
The NCV equation predicted was found by using the GCV
predicted, the hydrogen content of the bagasse and the latent
heat of vaporisation of water at 20°C. The equation thus
predicts the NCV defined at 20°C. The equation predicted
using H = 5,91 % (dry basis) was:
NCV moist = [ 18 309  (3 1,14 brix % sample)
 (207,6 moisture % sample) 
(196,05 ash % sample)] kJ.kgl
Page 5
Proceedings of The South AJkican Sugar Technologists' Association June 1977
173
Acknowledgements
Thanks are due to va~rious people at the S.A. Sugar Associa
tion Experiment Station for supplying cane samples.
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Proc XV, 1169/1174.
13. Anon. Methods of Analysis of Coal and Coke, Survey paper 44,
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