🔥 Calorific value (heat of combustion)
Determination of gross heat of combustion and net calorific value of solid, liquid and gaseous fuels by combustion of sample in a bomb calorimeter under oxygen pressure atmosphere.
Overview
Gross calorific value (gross calorific value — GCV) is the total amount of energy released during complete combustion of a unit mass of fuel in pure oxygen atmosphere. It includes the heat of condensation of water vapor formed from hydrogen combustion in the fuel. Net calorific value (NCV) is lower than gross calorific value by the heat of water vaporization and represents the actual amount of energy recoverable under technical conditions.
Determination of heat of combustion by bomb calorimetry is a fundamental test in energy industry, petrochemistry and fuel trade. Results are used for fuel pricing (price per GJ), energy balancing of boiler rooms and power plants, coal quality control, biomass, pellets, briquettes, fuel oils, coke and alternative fuels. ISO 1928 standard is applied to solid fuels, while parallel ASTM D240 standard is used for liquid fuels.
Bomb calorimetry is the only reference method accepted by customs authorities, commodity exchanges (ARA, Richards Bay) and accredited laboratories for fuel trading purposes. Measurement precision is typically 0.1–0.3% CV, corresponding to uncertainty of 50–100 kJ/kg at value ~30 MJ/kg.
The method is also used in fire reaction testing of materials (EN ISO 1716), analysis of municipal waste as alternative fuels (RDF/SRF) and assessment of solid biofuels according to EN ISO 18125.
Method principle
A weighed fuel sample (~1 g) is placed in a calorimetric crucible inside a steel pressure bomb (decomposition vessel). The bomb is filled with pure oxygen to 30 bar pressure, then placed in a calorimetric vessel filled with measured amount of water. The sample is ignited electrically (Ni-Cr ignition wire or cotton thread). Heat released during combustion raises the temperature of water in the calorimetric vessel. Based on temperature rise and known calorific constant of the apparatus, gross heat of combustion is calculated. Calibration is performed using certified benzoic acid (heat of combustion 26,454 J/g). Net calorific value is calculated by subtracting heat of water condensation (2,442 kJ/kg H₂O at 25°C).
Applications
- Pricing and trading of solid fuels (coal, coke, briquettes)
- Quality control of biomass, pellets and wood briquettes
- Energy balancing of boiler rooms and combined heat and power plants
- Assessment of alternative fuels from waste (RDF/SRF)
- Testing of fuel oils and liquid fuels
- Certification of solid biofuels (EN ISO 18125)
- Material testing for fire reaction (EN ISO 1716)
- Analysis of municipal waste for energy purposes
Key parameters
| Parameter | Value |
|---|---|
| Measurement range | 5–45 MJ/kg (typical) |
| Precision | ±0.1–0.3% CV (repeatability) |
| Sample mass | 0.8–1.2 g (solid fuels), 0.5–0.7 g (liquid) |
| Oxygen pressure | 30 bar (3 MPa) |
| Reference temperature | 25°C |
| Measurement time | 15–25 min (isoperibol mode), 8–12 min (dynamic) |
Standard
- Standard number
- PN-EN ISO 1928:2020
- Title (PL)
- Paliwa stałe — Oznaczanie ciepła spalania metodą spalania w bombie kalorymetrycznej i obliczanie wartości opałowej
- Title (EN)
- Solid mineral fuels — Determination of gross calorific value by the bomb calorimetric method and calculation of net calorific value
Step-by-step procedure
1. Sample preparation
Grind fuel sample to granulation <0.2 mm. Determine analytical moisture (drying at 105°C). Press ~1 g of sample into pellet or place in capsule.
2. Sample weighing
Weigh pellet/capsule on analytical balance with accuracy of 0.1 mg. Record mass (typically 0.8–1.2 g).
3. Bomb assembly
Place crucible with sample in bomb holder. Attach ignition wire between electrodes, touching the sample. Add 1 mL water to bomb bottom (HNO₃ absorption). Close bomb.
4. Oxygen filling
Flush bomb with oxygen (2× fill to 10 bar and release). Fill to 30 bar pressure. Check tightness (immersion in water — no bubbles).
5. Placement in calorimeter
Place bomb in calorimetric vessel. Fill with measured amount of water (2,000 mL ±0.5 mL). Connect ignition electrodes. Close calorimeter.
6. Pre-period
Start stirrer. Record temperature every 1 min for 5 minutes (thermal drift stabilization). Reading must show constant trend.
7. Ignition and main period
Trigger electrical ignition. Record temperature every 15–30 s. Note maximum temperature rise (typically 2–3°C). Main period continues until new drift stabilizes.
8. Post-period
After reaching maximum continue recording for 5 min (final drift stabilization).
9. Bomb disassembly
Remove bomb, slowly release gases (>1 min). Open bomb. Check complete combustion (no residues). Rinse interior with distilled water.
10. Acid correction
Titrate bomb washings with Na₂CO₃ 0.1 mol/L (phenolphthalein). Calculate correction for HNO₃ and H₂SO₄ (correction ~50–100 J).
11. Calculations
Qv = [(ε × ΔT) − e₁ − e₂ − e₃] / m, where ε = calorific constant, ΔT = corrected temperature rise, e₁ = wire correction, e₂ = acid correction, e₃ = sulfur correction, m = sample mass.
12. Net calorific value calculation
Qi = Qs − 24.42 × (9H + W), where Qs = gross calorific value [kJ/kg], H = hydrogen content [%], W = moisture [%]. Result in MJ/kg.
Required equipment and apparatus
| Equipment | Example | Indicative price |
|---|---|---|
| Isoperibol bomb calorimeter | IKA C 6000, Parr 6400, LECO AC-600 | 60,000–180,000 PLN |
| Calorimetric bomb (decomposition vessel) | IKA C 7012, Parr 1108 (Cr-Ni steel, acid resistant) | 15,000–30,000 PLN |
| Oxygen cylinder (purity ≥99.5%) | Medical/technical oxygen with reducer to 40 bar | 500–1,500 PLN (cylinder + rental) |
| Analytical balance (0.1 mg) | Mettler Toledo ME204, Sartorius Entris II BCE | 5,000–15,000 PLN |
| Pellet press | IKA C 21 — for pressing powdered samples | 3,000–8,000 PLN |
| Laboratory dryer (105°C) | Binder FD 56, Memmert UF110 | 5,000–15,000 PLN |
| Calorimeter chiller | IKA KV 600 or built-in Peltier cooling system | 8,000–20,000 PLN |
Reagents, media and consumables
| Reagent | CAS | Details |
|---|---|---|
| Certified benzoic acid (CRM) | 65-85-0 | Calibration standard, heat of combustion 26,454 ± 3 J/g, traceable to NIST SRM 39j, 1 g tablets |
| Compressed oxygen ≥99.5% | 7782-44-7 | Purity min. 99.5%, free of organic contaminants, working pressure 30 bar |
| Ni-Cr ignition wire | — | Nickel-chromium ø 0.2 mm or cotton ignition thread with known heat of combustion (~50 J) |
| Na₂CO₃ solution 0.1 mol/L | 497-19-8 | For titration of bomb washings (acid correction HNO₃ and H₂SO₄) |
| Distilled water | 7732-18-5 | For filling calorimetric vessel (2,000–2,500 mL) and bomb rinsing |
Health and safety (OHS)
- Bomb under 30 bar pressure — explosion risk in case of leakage, technical condition inspection mandatory
- Compressed oxygen — strong oxidizer, no contact with oils and greases
- Combustion — do not open bomb before complete cooling and gas release
- Post-combustion gases contain NOₓ and SO₂ — release only under fume hood
- Benzoic acid — irritant, use nitrile gloves and safety goggles