Effective heat of combustion

Effective heat of combustion is the heat released by a burning material per unit of mass lost during the same burning process. It connects the amount of material consumed with the rate at which the fire produces heat. Unlike an assumed constant material value, it can change during a test as the material and its combustion change.

The quantity is expressed as energy per mass, commonly in kilojoules per kilogram or megajoules per kilogram. It differs from the complete heat of combustion, which represents complete oxidation and release of the material's chemical energy. An effective value describes the combustion actually represented by the measurement or calculation, including conditions in which combustion is incomplete.

The distinction matters when converting a measured mass loss into heat release rate. The same mass loss can correspond to different heat outputs if the effective heat of combustion changes. Test conditions, the stage of burning and the fuel construction therefore belong with the reported value.

Energy per mass and energy per second

Effective heat of combustion describes an energy yield, whereas heat release rate describes power. A kilogram of fuel and a kilogram per second of fuel represent different information. The first is an amount of material; the second describes how quickly material is being consumed. A value for energy per kilogram becomes a power only when combined with a corresponding mass loss rate.

The term burning rate can cause confusion because it is sometimes used for heat release and sometimes for mass loss. Babrauskas distinguishes the two explicitly: heat release rate has units of power, while mass loss rate has units of mass per time. Stating the quantity and its units prevents a numerical burning rate from being interpreted in the wrong way.

The basic relationship multiplies the mass lost each second by the heat released for each kilogram lost. With mass loss in kilograms per second and effective heat of combustion in kilojoules per kilogram, the result is kilowatts. The relationship can also be rearranged to obtain the effective heat of combustion from matching heat-release and mass-loss measurements.

The relationship between mass loss rate, effective heat of combustion and heat release rate. Based on Iqbal and Salley (2004), p. 3-3.

Q̇ = ṁ ΔH_c,eff

ΔH_c,eff = Q̇ / ṁ

Symbol	Description	Unit
Q̇	Heat release rate	kW
ṁ	Mass loss rate during the corresponding burning period	kg/s
ΔHc,eff	Effective heat of combustion	kJ/kg

The unit choice affects the numerical answer. Multiplying kilograms per second by megajoules per kilogram produces megajoules per second, or megawatts. Conversion to kilowatts then requires multiplication by 1,000. This is a unit conversion within the same physical relationship, rather than a different combustion model.

An energy inventory answers another question. Van Rossum describes fire-load energy as the energy associated with the combustible contents and construction. For constant power, energy equals power multiplied by duration. The effective heat of combustion helps connect fuel consumption to power, but an energy inventory alone does not specify how rapidly a fire develops.

Complete combustion and changing combustion conditions

Complete heat of combustion describes a limiting condition in which combustion releases all the relevant chemical energy. Effective heat of combustion is more appropriate when the burning process leaves residue or does not fully oxidise the released fuel. Treating these quantities as interchangeable can therefore assign a heat output that the represented combustion process does not produce.

Incomplete combustion becomes particularly relevant when an enclosure restricts the supply of oxygen. The fuel can continue releasing combustible gases while the available oxygen limits their combustion inside the compartment. Iqbal and Salley describe gases leaving an opening and burning after mixing with outside air. Mass leaving the original fuel and heat released inside the room are then affected by different parts of the process.

The enclosure also changes the heating received by the fuel. Hot gases accumulate beneath the ceiling and heat the compartment boundaries. Radiation from those gases and surfaces can increase the fuel's mass loss. Restricted oxygen supply can simultaneously reduce combustion within the room, so increased heating and reduced combustion need not act in the same direction.

These effects explain why a free-burning test does not automatically describe the same item burning inside a compartment. Babrauskas distinguishes open calorimeter measurements from room tests and discusses the additional heat feedback created by the enclosure. The relevant question is how closely the measured burning conditions represent the intended calculation, rather than whether a material name appears in a table.

Moisture introduces a separate issue when interpreting energy values. Van Rossum explains that evaporation consumes part of the available energy and that dry-material values differ from values already allowing for moisture. Applying a further moisture correction to a value that already includes it would mix different reporting bases. The stated basis of the value therefore matters alongside its numerical magnitude.

Variation during a burning test

A single specimen can have a changing effective heat of combustion. Babrauskas presents a test on Western red cedar with a thickness of 17 mm. The specimen was exposed to an irradiance of 65 kW/m², meaning the incident radiant power per unit surface area. The reported effective heat of combustion changes substantially during the test.

During the earlier part of that test, the effective heat of combustion is approximately 12 MJ/kg. During the final charring period it rises to about 30 MJ/kg. These values describe stages of this particular exposure and specimen. They are not a universal minimum and maximum for all timber, nor a substitute for a complete measured history.

Approximate values at two stages of one cedar test; they are not universal timber properties. Based on Babrauskas (2002), pp. 3-1–3-2.

The change has a direct mathematical consequence. If the same mass loss rate were multiplied by the later value instead of the earlier value, the calculated heat release would be 2.5 times larger. This ratio follows from dividing 30 by 12. It isolates the effect of the energy yield; it does not establish that the measured mass loss remained constant.

Consequently, a constant effective value can conceal changes within a burning process. Babrauskas notes that experiments commonly measure heat release directly when effective heat of combustion is not constant. A fixed conversion factor can still be a model assumption, but it does not reproduce the changing cedar result merely because the specimen is described as wood.

The example also distinguishes a representative value from a complete description. A number in megajoules per kilogram summarises a particular relationship between heat and consumed mass. It does not independently describe the onset of burning, the mass-loss history or the influence of surrounding surfaces. Those features require the additional measurements or modelling assumptions discussed in the heat-release chapter.

Measurement and the meaning of test data

Mass loss can be measured by weighing the fuel package as it burns. A load cell records the changing load beneath the specimen or fuel assembly. The rate of change supplies the mass-loss quantity used in the heat-release relationship. Calculating heat release from that measurement requires a corresponding effective heat of combustion.

Heat release can instead be determined through oxygen-consumption calorimetry. Combustion products are collected beneath a hood and drawn through an exhaust duct. Measurements of exhaust flow and gas composition establish the oxygen consumed. The method relates that oxygen consumption to the heat released by the fire, rather than relying solely on the specimen's changing mass.

These measurements serve complementary purposes. Heat-release measurement describes the thermal output, while weighing describes the rate at which material is lost. Their relationship provides information about effective heat of combustion for the corresponding burning conditions. A measured heat-release curve therefore contains information that cannot always be recovered from mass loss using one constant multiplier.

A cone calorimeter supplies heat-release data from a small specimen under a specified exposure. Babrauskas describes specimens nominally 100 mm by 100 mm and identifies the cone as a common bench-scale method. The resulting measurement describes that specimen and exposure. Additional analysis is needed before small-scale results become a description of a larger fuel assembly.

Intermediate-scale calorimetry provides another way of representing a fuel specimen. Babrauskas describes an apparatus accommodating specimens 1 m by 1 m, allowing more complex or non-uniform constructions to be tested. A larger specimen can therefore address aspects of the construction that a small sample does not represent. However, the chapter still distinguishes this measurement from a full-scale fire and states that further analysis is needed for modelling applications.

Full-scale measurement reduces some questions about how the fuel package has been represented, but does not remove questions about the surroundings. Open-burning data may need allowance for enclosure heat feedback. Room-test data may describe ventilation conditions different from those in the intended application. Babrauskas therefore treats the relationship between the test enclosure and the modelled enclosure as part of interpreting the data.

Room testing is particularly relevant when enclosure effects are expected to be strong or a more precise heat-release estimate is required. Its results nevertheless remain associated with the room used for the test. Babrauskas notes that heat release measured in a room cannot simply be extrapolated to rooms having larger ventilation openings. Open-burning data can have a different range of application because their combustion was not constrained by the same test enclosure.

The choice between these methods affects the information available for an effective-heat calculation. A mass-loss measurement and a heat-release measurement refer to the represented specimen, its surroundings and its stage of burning. Increasing the specimen size alone does not remove the need to identify those conditions. The underlying calculation remains a relationship between corresponding quantities, while the test method determines which physical situation those quantities describe.

Worked example using a published cable entry

NUREG-1805 was prepared for the United States Nuclear Regulatory Commission's fire-protection inspection programme. Its examples and material tables address fire hazards in nuclear power plants. Table 3-4 includes a polyethylene/polyvinylchloride cable entry, abbreviated PE/PVC, with a mass burning rate of 0.0044 kg/(m² s) and an effective heat of combustion of 25,100 kJ/kg.

For a calculation illustrating the conversion, take an assumed burning area of 1 m². The area is an explicit example assumption, rather than a reported cable-test dimension. Multiplying the tabulated mass burning rate by this area gives a mass loss rate of 0.0044 kg/s. Multiplying that result by the tabulated energy yield gives the corresponding calculated heat release.

A calculated example using the PE/PVC cable-material entry and an assumed burning area of 1 m². Based on Iqbal and Salley (2004), pp. 3-3 and 3-6.

ṁ = 0.0044 kg/(m² s) × 1 m² = 0.0044 kg/s

Q̇ = 0.0044 kg/s × 25,100 kJ/kg = 110.44 kW ≈ 110 kW

The result is about 110 kW after rounding. The intermediate product retains its digits so that rounding occurs only at the end. This is a worked calculation using a published material entry, not a measurement of a complete installed cable system. Its usefulness is in showing how the mass-loss quantity, the area assumption and the energy yield combine.

The same table shows why the cable description matters. Its crosslinked-polyethylene/neoprene entry has an effective heat of combustion of 10,300 kJ/kg, compared with 25,100 kJ/kg for PE/PVC. Those entries refer to different cable-material combinations. Selecting a value solely because both entries describe cable insulation would discard a distinction made by the source.

The table also lists 44,400 kJ/kg for diesel oil and a range of 13,000–15,000 kJ/kg for wood. These figures are examples from that report, with its stated application and material descriptions. They illustrate the range of values used in its calculations. They do not establish a single effective heat of combustion for all oils, all timber products or all burning conditions.

Application limits and interpretation

The multiplication relationship is useful only when the mass-loss rate and effective heat of combustion describe compatible burning conditions. Using a measured mass-loss history with an unrelated energy yield does not preserve the original heat-release measurement. The cedar example demonstrates one reason for incompatibility: the energy released per unit mass changes during the test itself.

Fuel construction is another limitation. Babrauskas stresses that even full-scale data must represent the specific fuel source, rather than an item that merely appears similar. The enclosure and its ventilation introduce further differences. Material identity, assembly and exposure are therefore part of interpreting the value, rather than optional background details accompanying an otherwise universal number.

An energy value also cannot establish the severity of every fire effect by itself. The heat-release relationship still requires a mass-loss rate, while a duration calculation requires an energy inventory and a power history. Van Rossum separates these quantities in the treatment of fire load and fire power. Confusing them can turn an energy yield into an unsupported statement about fire size or duration.

Finally, the provenance of tabulated values limits their interpretation. NUREG-1805 supplies engineering methods and examples for a particular United States inspection programme. Its cable and liquid entries provide useful calculation inputs with identified sources and conditions. They are not universal regulatory requirements, and the heat-release calculation does not itself demonstrate compliance with a building regulation.

References

• Babrauskas, V. (2002). Heat release rates. In P. J. DiNenno (Ed.), SFPE handbook of fire protection engineering (3rd ed., chapter 3-1, pp. 3-1–3-37). National Fire Protection Association.
• Iqbal, N., & Salley, M. H. (2004). Fire dynamics tools (FDTs): Quantitative fire hazard analysis methods for the U.S. Nuclear Regulatory Commission fire protection inspection program (NUREG-1805). U.S. Nuclear Regulatory Commission.
• Van Rossum, E. (2027). Introduction to Fire Safety Engineering. Fire Safety Academy.