Proceedings of The South African Sugar Technologists' Association -
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
When gross calorific values (GCV's) on a moisture-free,
brix-free and ash-free 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
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.kg-l.
the calorific value of wet bagasse; among the better known are
those of von Pritzelwitz van der Horst2 of Java and Hessey3
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.kg-l, 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
non-sucrose 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
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
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 twenty-four 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 brix-free. 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 brix-free.
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 brix-free 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
ash-free, brix-free and moisture-free 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 sub-sample 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 sub-sample 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.
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