Calorimetric Measurements; Determining The Calorific Value; Corrections - IKA C 200 Operating Instructions Manual

Calorimetric measurements

5.1 Determining the calorific value

Combustion is carried out in a calorimeter under
specific conditions. The decomposition vessel is
filled with a weighed fuel sample, the fuel sample
is ignited and the temperature increase in the ca-
lorimeter system measured. The specific calorific
value of the sample is calculated as follows:
Ho = (C * DT - QExt1 - QExt2) / m
m
Weight of fuel sample
C
Heat capacity (C-value) of calorimeter system
DT
Calculated temperature increase of water in inner
vessel of measuring cell
QExt1
Correction value for the heat energy generated
by the cotton thread as ignition aid
QExt2
Correction value for the heat energy from other
burning aids
The decomposition vessel is filled with pure oxy-
gen (99.95 %) to optimise the combustion pro-
cess. The pressure of the oxygen atmosphere in
the decomposition vessel is max. 30 bar. Formula
(1) for the calorific value of a material requires
that combustion takes place under specifically
defined conditions. The relevant standards are
based on the following assumptions:
• The temperature of the fuel and its combustion
products is 25 °C.
• The water contained in the fuel before combus-
tion and the water formed whilst combusting
the hydrogenous compounds of the fuel is in
fluid form after combustion.
• The atmospheric nitrogen has not oxidised.
• The gaseous products after combustion consist
of oxygen, nitrogen, carbon dioxide and sulp-
hur dioxide.
• Solid materials may form (e.g. ashes).
In many cases, however, not just the combus-
tion products referred to in the standards are
produced. In such cases the fuel sample and the
combustion products must be analysed to pro-
vide data for a revised calculation. The standard
calorific value is then calculated from the measu-
red calorific value and the analysis data. The heat
value Hu is the same as the calorific value, minus
the condensation energy of the water contained
in the fuel and formed through combustion. The
(1)
heat value is the more important parameter from
a technical point of view because in all major,
technical applications only the heat value can be
evaluated in terms of energy.
The complete bases of calculation for the calori-
fic and heat value can be found in the relevant
standards (e.g.: DIN 51 900; ASTM D 240; ASTM
D 5865..). They are also contained in the CalWin
calorimeter software.

5.2 Corrections

Due to the nature of the system a combustion
test does not just produce the combustion heat
of the sample, but also heat from external energy
(QExternal).
This can fluctuate considerably in relation to the
heat quantity of the fuel sample.
The combustion heat of the cotton thread which
ignites the sample and the electric ignition energy
would distort the measurement. This influence is
taken into account in the calculation by way of a
correction value.
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