🔥 Gross and net calorific value of solid biofuels — bomb calorimeter
In short
Combustion of a test portion of biofuel in oxygen at a pressure of 3 MPa in a bomb calorimeter calibrated with benzoic acid; the gross calorific value at constant volume at 25 °C, the net calorific value by calculation (moisture, hydrogen). The test is performed according to PN-EN ISO 18125:2017-07; The GIG range: 8055–37581 kJ/kg (the calorimetric method). The procedure comprises 8 steps; it is used for: Settlement of deliveries of energy biomass by energy (pellets, wood chips, straw, wood waste), Classification of pellets and briquettes according to the specifications for biofuels (the net calorific value on the as-received basis), Energy balances of boilers and biomass installations.
At a glance
- Standard: PN-EN ISO 18125:2017-07
- Category: Petrochemistry
- Procedure steps: 8
- Reference temperature: 25 °C (the international reference temperature in thermochemistry)
- Oxygen: A pressure in the bomb of 3 MPa, purity ≥ 99.5 % by volume, free of combustible substances — for that reason electrolytic oxygen, which may contain up to 4 % H2, is not used
- Calibrant: Certified benzoic acid
Overview
The standard specifies a method of determining the gross calorific value of a solid biofuel at constant volume and at the reference temperature of 25 °C in a bomb calorimeter calibrated by the combustion of certified benzoic acid. The result is the gross calorific value of the analysis sample at constant volume, with all the water of the products of combustion in the liquid state. In practice biofuels are burned at constant atmospheric pressure, and the water either is not condensed or is condensed — in both cases the useful heat is the net calorific value at constant pressure; the standard gives formulae for the net calorific value at constant volume and at constant pressure. The general principles of calibration and of the experiments are given in the main text, and the procedures for the individual types of calorimeter in Annexes A (adiabatic), B (isoperibol and constant-jacket) and C (automatic); Annex D contains checklists, E examples of calculations and G default values for typical biofuels.
The gross and the net calorific value are the basis for assessing the quality of a fuel as an energy raw material; they depend strongly on the type of fuel and on the moisture (biomass 8–17 MJ/kg, hard coals 16.7–29.3 MJ/kg according to the Wrocław University of Science and Technology). To calculate the net calorific value at constant volume the moisture and hydrogen contents of the analysis sample are needed, and at constant pressure in principle the oxygen and nitrogen contents as well. The dependence of the gross calorific value on temperature is small (below 1 J/(g·K)).
The bomb may serve at the same time for the digestion of the sample for the determination of sulfur and chlorine (ISO 16994). At GIG the gross calorific value of biofuels is determined according to PN-EN ISO 18125:2017-07 by the calorimetric method over a range of 8055–37581 kJ/kg, and the net calorific value by calculation; isoperibol calorimeters carry out the measurement in a few minutes.
Method principle
A test portion of the analysis sample is burned in oxygen at high pressure in a bomb calorimeter under prescribed conditions. The effective heat capacity of the calorimeter is determined in calibration experiments by burning certified benzoic acid under similar conditions. The corrected temperature rise is established from observations of the temperature before, during and after the combustion reaction (the time and frequency of the observations depend on the type of calorimeter). Water is added to the bomb at the start, so as to obtain a saturated vapour phase and to treat all the water formed (from the hydrogen and from the moisture of the sample) as liquid. The gross calorific value is calculated from the corrected temperature rise and the effective heat capacity, taking into account the energy of ignition, the combustion of the wire or thread and the thermal effects of the side reactions (the formation of nitric acid) as well as a correction for the difference of energy between aqueous sulfuric acid and gaseous SO2; the effect of hydrochloric acid may be disregarded for biofuels.
Applications
- Settlement of deliveries of energy biomass by energy (pellets, wood chips, straw, wood waste)
- Classification of pellets and briquettes according to the specifications for biofuels (the net calorific value on the as-received basis)
- Energy balances of boilers and biomass installations
- Digestion of the sample for the determination of sulfur and chlorine (ISO 16994, method A)
- Proficiency testing of solid fuel laboratories
Key parameters
| Parameter | Value |
|---|---|
| Reference temperature | 25 °C (the international reference temperature in thermochemistry) |
| Oxygen | A pressure in the bomb of 3 MPa, purity ≥ 99.5 % by volume, free of combustible substances — for that reason electrolytic oxygen, which may contain up to 4 % H2, is not used |
| Calibrant | Certified benzoic acid |
| Ignition wire | Ni-Cr 0.16–0.20 mm or platinum 0.05–0.10 mm; optionally a cotton thread |
| Result | The gross calorific value at constant volume; the net calorific value at constant volume and at constant pressure by calculation |
| The GIG range | 8055–37581 kJ/kg (the calorimetric method) |
Standard
- Standard number
- PN-EN ISO 18125:2017-07
- Title (PL)
- Biopaliwa stałe — Oznaczanie wartości opałowej
- Title (EN)
- Solid biofuels — Determination of calorific value
Step-by-step procedure
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Calibration
Determine the effective heat capacity ε of the calorimeter by burning certified benzoic acid under conditions similar to those of the experiments with the fuel, taking into account the conditions of the certificate (clause 9: the calibrant, the working range of ε, the auxiliary contributions, the precision of ε, redetermination).
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Preparation of the sample
Prepare the analysis sample according to ISO 14780 (clause 7) and determine the moisture at the same time according to ISO 18134-3; if need be press a pellet or use a combustion aid (capsule, bag, benzoic acid).
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Preparation of the bomb
Weigh a test portion into the crucible, fix the ignition wire (Ni-Cr 0.16–0.20 mm or Pt 0.05–0.10 mm) and, if need be, a cotton thread, add water to the bomb to saturate the vapour phase, close it and fill it with oxygen to 3 MPa (8.2).
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Assembly of the calorimeter
Place the bomb in the calorimeter vessel with a measured amount of water (the basis: a constant mass of calorimeter water or a constant total mass) and assemble the calorimeter (8.3).
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Combustion and measurement of the temperature
Ignite the sample and record the temperature before, during and after the combustion according to the procedure for the type of calorimeter concerned (8.4; Annexes A–C).
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Corrected temperature rise
Calculate the observed temperature rise and correct it for the exchange of heat, the stirring power and so on (8.6: isoperibol and constant-jacket calorimeters, adiabatic ones, thermometer corrections); refer it to a temperature of 25 °C (8.7).
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Analysis of the products of combustion (when required)
Determine the nitric and sulfuric acid in the bomb washings with standardised solutions (8.5) or take the default values for typical biofuels (Annex G).
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Calculation
The gross calorific value at constant volume from θ and ε with corrections for the energy of ignition, the wire/thread, the nitric acid and the sulfur (clause 10); the net calorific value at constant volume from the moisture and the hydrogen, and at constant pressure additionally from the oxygen and the nitrogen (clause 12); convert to the bases required (10.5).
Required equipment and apparatus
| Equipment | Example | Indicative price |
|---|---|---|
| Bomb calorimeter | Adiabatic (Annex A), isoperibol or constant-jacket (Annex B) or automatic (Annex C); a bomb for an oxygen pressure of 3 MPa | — |
| Calorimetric thermometers | According to ISO 651 / ISO 652 or platinum sensors, thermistors, quartz resonators with an established dependence on temperature | — |
| Analytical balance | For weighing out the sample, the calibrant and the combustion aids | — |
| Pellet press (optional) | For forming pellets of the biofuel or of benzoic acid | — |
| Apparatus for the analysis of the bomb washings (when required) | A burette, standardised solutions and an indicator for the determination of nitric and sulfuric acid | — |
Reagents, media and consumables
| Reagent | CAS | Details |
|---|---|---|
| Oxygen | — | At a pressure sufficient to fill the bomb to 3 MPa, purity ≥ 99.5 % by volume, free of combustible substances |
| Certified benzoic acid | 65-85-0 | The calibrant, with a certified gross calorific value; also a combustion aid |
| Combustion aids | — | Of known gross calorific value, composition and purity: benzoic acid, n-dodecane, paraffin oil, combustion bags or capsules |
| Standardised solutions and indicator (only for the analysis of the bomb washings) | — | Ba(OH)2 0.05 mol/l, Na2CO3 0.05 mol/l, NaOH 0.1 mol/l, HCl 0.1 mol/l; the indicator: methyl orange 0.25 g + xylene cyanol FF 0.15 g in 50 ml of 95 % ethanol, made up to 250 ml with water |
Health and safety (OHS)
- A bomb under an oxygen pressure of 3 MPa — use only bombs that have been pressure-tested, do not exceed the permissible mass of sample, oxygen free of combustible substances, no greases in the oxygen fittings
- The ignition of samples of unknown reactivity — the shield of the calorimeter, do not lean over the bomb during the ignition
- The bomb washings contain nitric and sulfuric acid — gloves and goggles when opening and rinsing the bomb
Frequently asked questions
Which standard describes this test?
The test is performed according to PN-EN ISO 18125:2017-07 — “Solid biofuels — Determination of calorific value”.
How does this method work?
A test portion of the analysis sample is burned in oxygen at high pressure in a bomb calorimeter under prescribed conditions. The effective heat capacity of the calorimeter is determined in calibration experiments by burning certified benzoic acid under similar conditions.
What is the measuring range and accuracy?
Reference temperature: 25 °C (the international reference temperature in thermochemistry); Oxygen: A pressure in the bomb of 3 MPa, purity ≥ 99.5 % by volume, free of combustible substances — for that reason electrolytic oxygen, which may contain up to 4 % H2, is not used; Calibrant: Certified benzoic acid; Ignition wire: Ni-Cr 0.16–0.20 mm or platinum 0.05–0.10 mm; optionally a cotton thread.
How long does the test take?
The procedure comprises 8 steps. The standard does not give a duration for every stage — the laboratory's own procedure decides.
What equipment is required?
Bomb calorimeter, Calorimetric thermometers, Analytical balance, Pellet press (optional), Apparatus for the analysis of the bomb washings (when required).
Where is this test used?
Settlement of deliveries of energy biomass by energy (pellets, wood chips, straw, wood waste); Classification of pellets and briquettes according to the specifications for biofuels (the net calorific value on the as-received basis); Energy balances of boilers and biomass installations; Digestion of the sample for the determination of sulfur and chlorine (ISO 16994, method A); Proficiency testing of solid fuel laboratories.
What safety precautions apply?
A bomb under an oxygen pressure of 3 MPa — use only bombs that have been pressure-tested, do not exceed the permissible mass of sample, oxygen free of combustible substances, no greases in the oxygen fittings; The ignition of samples of unknown reactivity — the shield of the calorimeter, do not lean over the bomb during the ignition; The bomb washings contain nitric and sulfuric acid — gloves and goggles when opening and rinsing the bomb.
Which laboratory can perform this test?
The test is performed by laboratories accredited to ISO/IEC 17025. On LabCoda you can find them by the standard number PN-EN ISO 18125.