1. Principle
Gross energy is measured by combustion of a sample in an excess of oxygen in a bomb
calorimeter under standardized conditions. The gross calorific value is calculated from the
temperature rise of the water in the calorimeter vessel and the mean effective heat capacity
of the calorimeter.
2. Scope
This method is applicable to all feeds (see remark 9.1).
3. Responsibilities
Laboratory Analysts shall perform the analysis as per this method. It is the responsibility
of the Laboratory Analyst to ensure that all conditions laid down in the method are met
and strictly adhered to. Any deviations from the prescribed method shall be recorded and
supervisor notified.
4. Equipment
4.1 Balance, accurate to 0.1 g, capacity 5000 g.
4.2 Balance, analytical accurate to 0.1 mg.
4.3 Bomb calorimeter, automatic adiabatic.
4.4 Oxygen fill unit.
4.5 Titration unit (optional).
5. Reagents
5.1 Cotton thread.
5.2 Platinum wire.
5.3 Benzoic acid (combustion tablets or standard grade crystals).
5.4 Phenolphthalein indicator (0.1% in 95% ethanol).
5.5 Barium hydroxide, 0.1 N (17.13 g/litre).
5.6 Sodium carbonate, 0.1 N (14.2 g/litre).
5.7 HCl, 0.1 N (place 500 ml in a 1-L flask, add 8.73 ml concentrated HCl, dilute to 1 litre
with distilled water).
5.8 Methyl orange.
6. Procedure
Before starting the procedure also check the instructions of the manufacturer.
This procedure contains three parts. The first part (6.1) describes the determination of
the hydrothermal equivalent of the bomb calorimeter and has to be performed annually or
if something changes within the equipment, such as use of a new bomb vessel or of a vessel
after maintenance. The second part (6.2) describes the procedure to measure samples.
The third part (6.3) describes the determination of the acidity, which is a correction factor
for the observed values. The laboratory should decide to use this correction based on their
accuracy requirements (see remark 9.2).
6.1 Determine the hydrothermal equivalent (water value) of the bomb calorimeter
6.1.1 Use a sample of known calorific value (benzoic acid combustion tablet) to
determine the hydrothermal equivalent (Joules/degree rise in temperature) of
the bomb, bucket, and water. Calculate the temperature rise using the above
procedure. Make four determinations and calculate the mean value (this value
should not change unless parts of the bomb are replaced).
6.1.2 Dry benzoic acid (5.3) at 105 ± 2 °C overnight and cool in a desiccator. Weigh
1 g of dry benzoic acid crystals, make into a tablet and reweigh. Determine the
temperature rise from the combustion of benzoic acid in the bomb calorimeter.
6.1.3 For calculations required to determine the hydrothermal equivalent including
corrections for the cotton thread, platinum wire, and the heat liberated in the
formation of the acid (see 7. Calculation).
6.2 Measuring samples
6.2.1 Weigh approximately 1 g of sample to the nearest 0.1 mg (W) (see remark 9.1)
and place in the combustion cup.
6.2.2 Attach 10 cm platinum wire (5.2) between the electrodes of the bomb and set
the combustion crucible with the sample in place in the loop electrode.
6.2.3 Tie 6.5 cm of cotton thread (5.1) at the middle of the wire. Adjust the thread
so that it touches the sample.
6.2.4 Assemble the bomb, tighten the screw cap, close the pressure release valve and
fill with oxygen to 25–30 atmospheres.
6.2.5 Weigh 2000 g distilled water in the calorimeter bucket and place in the calorimeter.
Set the bomb in the bucket and attach the clip terminal.
6.2.6 Close the cover, lower the thermometers and start the water circulating motor.
Remove the cap from the jacket cover and fill the cover with water until it runs
out the drain hose.
6.2.7 Adjust the temperature of the water in the outer jacket to approximately equal
that of the calorimeter by adding hot or cold water, and allow 1 minute to
attain equilibrium.
6.2.8 Read and record the initial temperature to the nearest 0.002 °C and ignite the
sample. Wait until temperature rise is maximum, read and record final temperature.
This may also be done automatically by the apparatus.
6.2.9 Open the calorimeter, take the bomb from the bucket, release the residual pressure
of the bomb and open.
6.3 Determination of acidity (see remark 9.2)
6.3.1 After burning is complete, remove the bomb, release the pressure, and open.
Rinse all inner bomb surfaces with a stream of distilled water, collect all washings
in a clean beaker and make washings up to 100 ml.
6.3.2 Filter and boil to remove carbon dioxide.
6.3.3 Titrate hot filtrate to phenolphthalein (5.4) end point with 0.1 N barium
hydroxide (5.5) (A).
6.3.4 Add 20 ml of 0.1 N sodium carbonate (5.6) (B), filter the precipitate and wash
with distilled water.
6.3.5 Cool and titrate with 0.1 N HCl (5.7) (C) using methyl orange (5.8) as indicator.
7. Calculation
7.1 The formula used for calculation of hydrothermal equivalent (He) of the bomb is:
W x A – (L x C) –14
He =
Tf – Ti
where,
He = hydrothermal equivalent (J/˚C),
W = weight of benzoic acid sample (g),
A = joules per gram benzoic acid, i.e. 26442 J/g,
L = weight of cotton thread (g),
C = joules per g cotton, i.e. 17500 J/g,
14 = correction for acid formation (J),
Tf = final temperature, and
Ti = initial temperature.
For cotton thread, values given by suppliers can be used.
Correction for combustion of platinum wire is very small and can be neglected.
Value for acid formation is also small and is fixed to 14 J. This value can also be measured
by following procedure 6.3 with 1 ml of distilled water and calculate the correction
value as described in 7.3.
7.2 The formula used for calculation of the gross energy (GE) content of samples is:
(Tf – Ti) x He
GE (kJ/g) =
W
where,
Tf = final temperature (˚C),
Ti = initial temperature (˚C),
W = weight of sample (g), and
He = hydrothermal equivalent (J/˚C)
Express the results in kJ/g.
7.3 Calculation of the acid correction factor
Calculation of the acid factor is done by:
Nitric acid correction (J) = 6.0 (B – C)
Sulphuric acid correction (J/W) =
W
The corrections are 94.6 J for 0.01 g (1%) of sulphur in the fuel and 6 J/ml of 0.1 N nitric
acid formed.
This factor should be used instead of the fixed value of 14 J.
8. Quality Control
Benzoic acid and a control sample should be run with each day’s batch.
In each batch a control standard (QC sample) should be analysed. The laboratory QC
sample can be made from an animal feed/forage sample that is similar in nature to the samples
to be analysed. Take 3–4 kg of the chosen QC sample, grind to pass through a sieve
of 1 mm pore size and store in a cool, dry place. Analyse the QC sample 15–20 times, take
average and allow ± 2 SD as an acceptable range. Samples should be analysed in duplicate.
The difference between the values of two parallel determinations carried out on the same
sample shall be < 0.5% compared to the higher result.
9. Remarks
9.1 For liquid samples, approximately 5–10 g should be weighed into a bag and dried in
a vacuum oven or freeze dried before analysis.
9.2 Variation in acid formation has only a very small effect on the values found. Therefore,
most laboratories use a fixed value (i.e. 14 J) for acid correction.
10. References
Hill, W.H., Seals, J., and Montiegel, E. 1958. Destruction of animal and vegetable tissue by
combustion in a Parr oxygen bomb. Am. Ind. Hyg. J. 19: 378–81.
ISO 9831. 1998. Animal feeding stuffs, animal products, and faeces or urine – Determination of
gross calorific value – Bomb calorimeter method. Geneva, Switzerland.
Parr Manual 120. 1948. Oxygen bomb calorimetry and oxygen bomb combustion methods.
Parr Instrument Company, Moline, IL, USA.