IKA C 7000 Grundausstattung Set 2 - Manual (Page 21)

IKA  -WERKE C 7000 Ver. 07 09.07
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To make a calorimetric measurement, a sample of a substance is burnt. In order to
ignite the sample, energy must be supplied from an external source. An ignition
wire, which is heated to glowing point by passing an electric current through it, per-
forms this function. If the measurement is to be accurate, it is essential that the
sample is completely burnt. For this reason, combustion takes place in an atmos-
phere of oxygen at a pressure of 30 bar.
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In a calorimeter combustion takes place under defined conditions. A weighed sam-
ple of a substance is placed in the decomposition vessel, the sample is ignited, and
the increase in temperature of the decomposition vessel is measured. The gross
calorific value of a sample is calculated from:
• temperature increase of the decomposition vessel
• heat capacity (C value) of the calorimeter system
• mass of the fuel sample
• heat energy that is released by burning the ignition aid and auxiliary fuel, and
also by the formation of sulphuric and nitric acids (external energy).
To optimise the combustion process, the decomposition vessel is filled with pure
oxygen (99.95%). The pressure of the oxygen atmosphere in the decomposition
vessel is 30 bar. The fuel sample is weighed to an accuracy of 0.1 mg using an
analytical balance.
Precise determination of the calorific value of a substance demands that combustion
takes place under precisely defined conditions. The relevant standards make the
following assumptions:
• any water contained in the fuel, and any water formed by the combustion of
compounds containing hydrogen in the fuel is present in a liquid state after
combustion;
• no oxidation of atmospheric nitrogen has taken place;
• gaseous products present after combustion consist of oxygen, nitrogen, carbon
dioxide and sulphur dioxide;
• solids can also be formed (e.g. ash).
Frequently, other combustion products, not foreseen by the standards, are formed.
In such cases, analyses of the sample material and the combustion products are
required to supply data for correction calculations. The standard gross calorific value
is then determined from the measured value and the analysis data.
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