Parr Instrument 6200 - Manual (Page 66)

67
6200
Calorimeter Operating Instruction Manual
m
Spiking
Samples
Conversion
to Other
Moisture
Bases
Conversion
to Net
Heat of
Combustion
It is sometimes necessary to add a
spiking material to samples which are very
small, have a low heat of combustion, or
have a high moisture content to add
sufficient heat to drive the combustion to
completion. Benzoic acid is an excellent
material for spiking for all of the same
reasons it is a good standard material.
White oil is also an excellent material,
particularly for liquid samples. The 6200
Calorimeter can automatically compensate
for the addition of spiking materials to these
samples. The calculations are modified in
these cases as follows:
H
c
=
where Hcs
= Heat of combustion of
the spiking material
(cal/gm)
Ms = Mass of spiking material
This factor is added to the
calculations when Spike Controls, Use
Spiking is set to ON. Heat of Combustion of
Spike is entered as calories per gram. The
controller will prompt the user to enter the
weight of spiking material. Fixed spikes can
be used when, Use Fixed Spike is set to ON
and entering the mass of the spike on -
Weight of Fixed Spike.
The calculations described above give
the calorific value of the sample with
moisture as it existed when the sample was
weighed. For example, if an air-dried coal
sample was tested, the results will be in
terms of heat units per weight of air-dry
sample. This can be converted to a
moisture free or other basis by
determining the moisture content
of the air-dry sample and using
conversion formulae published in
ASTM Method D3180 and in other
references on fuel technology.
The calorific value obtained in a bomb
calorimeter test represents the gross heat of
combustion for the sample. This is the heat
produced when the sample burns, plus the
heat given up when the newly formed water
vapor condenses and cools to the
temperature of the bomb. In nearly all
industrial operations, this water vapor
escapes as steam in the flue gases and the
latent heat of vaporization, which it contains,
is not available for useful work. The net heat
of combustion obtained by subtracting the
latent heat from the gross calorific
value is therefore an important
figure in power plant calculations.
If the percentage of hydrogen H, in
the sample is known, the net heat
of combustion, Hnet Btu per pound
can be calculated as follows:
H
net
= 1.8Hc - 92.7H
(Solid fuels, ASTM D2015)
H
net
= 1.8Hc - 91.23H
(Liquid fuels, ASTM D240)
WT- e
1
- e
2
- e
3
- (Hcs)(M
s
)
B-7
APPENDIX B