Contents
Soot yield is the mass of soot produced per unit mass of fuel consumed or reacted, with the precise denominator defined by the measurement or model. It is commonly expressed in kilograms per kilogram. In fire calculations, the yield connects the burning rate to a particulate source term that can subsequently be transported through a building.[1][2][3]
The yield is conditional on the fuel and combustion environment. It is not a fixed conversion from heat release to smoke, nor does it directly specify local visibility. Ventilation, heating and combustion efficiency affect the source term, while transport and aerosol properties affect the resulting conditions away from the fire.[4]
Care is also needed with terminology. Soot denotes carbonaceous particles, whereas a measured smoke aerosol can include condensed droplets. Engineering sources sometimes group smoke and soot yields together. Their numerical values should therefore be interpreted with the original measurement definition and combustion conditions intact.[5]
Definition and mass basis
For a stated interval and a consistent fuel basis, the yield is the ratio of produced soot mass to consumed fuel mass. If that yield remains constant, the same ratio applies to the corresponding rates:[2][3]
| Symbol | Meaning | Unit |
|---|---|---|
| ys | Soot yield on the stated fuel basis | kg/kg |
| ms | Produced soot mass | kg |
| mf | Fuel mass consumed or reacted, as defined | kg |
| ṁs, ṁf | Corresponding mass production and fuel consumption rates | kg/s |
| Q̇ | Heat release rate consistent with the fuel consumption | kW |
| ΔHeff | Effective chemical heat of combustion | kJ/kg |
| Cs | Suspended soot concentration at the location considered | kg/m3 |
The denominator deserves explicit attention. A reaction model may define yield per mass of fuel reacted, while an experimental report may use specimen mass lost or material burnt. Those descriptions should not be silently exchanged. A calculation needs the measured or prescribed mass rate that corresponds to the stated yield.[2][3]
Yield and production rate answer different questions. A yield of 0.03 kg/kg means that the selected mass basis produces three percent as much soot mass. It does not describe how quickly that soot appears. The production rate also requires the rate at which fuel enters the relevant combustion process.[2]
For changing conditions, the total production follows the accumulation of the instantaneous source rate. A constant yield can be taken outside that accumulation only when the assumption is justified. If a fire changes from well-ventilated burning to oxygen-limited burning, a single average value can conceal different stages of particulate production.[4]
Formation, oxidation and combustion conditions
Flaming soot production reflects competing processes. Particles develop in fuel-rich regions, while oxidation can consume them where oxygen becomes available. The emitted quantity represents what survives these processes. Fuel chemistry influences formation, but the surrounding combustion environment also determines how much particulate material leaves the flame.[5]
Mulholland contrasts the relatively high smoke production of aromatic polystyrene with that of polypropylene, whose molecular structure differs. This comparison explains why fuel identity matters. It does not establish one yield for every product bearing a broad polymer name. Formulation, physical form and test conditions remain relevant when selecting a numerical input.[5]
The distinction between flaming and non-flaming aerosols is equally significant. Pyrolysis can release vapours that condense as droplets after cooling. Smouldering can sustain decomposition and produce an aerosol without an ordinary flame. Calling all these particles soot can obscure differences in composition and optical response.[5]
As an illustration, Mulholland reports flexible polyurethane smoke conversion factors of 0.07–0.15 for pyrolysis and less than 0.01–0.035 for flaming. These are results associated with small specimens and specified conditions. They describe the measured smoke aerosol; they should not be relabelled as universal elemental-carbon yields.[6]
Ventilation changes more than the total oxygen supplied to a room. It affects the conditions under which gases burn and particles oxidise. The ABCB data sheets describe soot yield as sensitive to both ventilation and imposed heating. Their treatment pairs increased yields under ventilation control with reduced effective heats of combustion.[4]
The equivalence ratio provides one way to describe the fuel-to-air balance relative to stoichiometric demand. The ABCB source presents correlation-derived yields for several values of this ratio. These data demonstrate a dependence on combustion conditions; they do not justify replacing a spatially varying fire environment with an unexplained universal multiplier.[4]
Suppression creates a related interpretation problem. Less complete combustion can increase the particulate yield per kilogram, while a reduced burning rate decreases the fuel consumed each second. Van Rossum explains that the absolute smoke production can consequently remain limited even when the yield increases. Both factors must be examined together.[7]
Converting a design fire into a particulate source
A design fire is often specified by heat release rate rather than fuel consumption. The conversion to particulate production then requires an effective heat of combustion consistent with the combustion conditions.[2]
With heat release in kilowatts and effective heat of combustion in kilojoules per kilogram, the resulting mass rates are kilograms per second. A value supplied in megajoules per kilogram must first be converted consistently. The equation describes a mass source, rather than the volume of hot smoke generated.[2]
The ABCB definition of effective chemical heat of combustion includes combustion efficiency. It is therefore distinct from the net heat available if the fuel burns completely. Using a complete-combustion value together with a yield representing inefficient burning can produce an inconsistent conversion from energy to mass.[2]
This coupling is especially important when heat release is held fixed during a sensitivity study. A lower effective heat of combustion implies that more fuel must be consumed to supply that prescribed heat release. If the yield also rises, both changes increase the calculated particulate production. Varying the yield alone misses part of this relationship.[4]
The same reasoning applies to a total released energy. For a constant yield and effective heat of combustion, the produced mass follows from energy divided by effective heat of combustion, multiplied by yield. Van Rossum uses this energy-based approach to calculate the particulate mass available to a smoke volume.[7]
A designer should therefore identify whether the starting input is fuel loss, energy release or a prescribed heat release curve. Each route can support a soot calculation, but the necessary conversion assumptions differ. Documenting those assumptions makes it possible to check that the mass and energy descriptions refer to the same fire.[2][7]
Worked comparison of paired inputs
The ABCB data sheets provide a useful illustration using red oak. Their correlation-derived data give a well-ventilated yield of 0.015 kg/kg and effective heat of combustion of 12.4 MJ/kg. At an equivalence ratio of three, the corresponding values are 0.034 kg/kg and 6.2 MJ/kg.[8]
For an illustrative prescribed heat release of 1000 kW, the well-ventilated input pair gives a fuel rate of 0.0806 kg/s. Multiplying by the yield gives a particulate source of approximately 0.00121 kg/s, or 1.21 g/s. These are calculated values using the source's paired inputs.[2][8]
The second input pair gives a fuel rate of 0.161 kg/s and a particulate source of approximately 0.00548 kg/s, or 5.48 g/s. The calculated soot rate is approximately 4.53 times the first value. The increase reflects both the higher yield and the lower effective heat of combustion.[8]
| Input condition | Yield, kg/kg | Effective heat, MJ/kg | Calculated fuel rate, kg/s | Calculated soot rate, g/s |
|---|---|---|---|---|
| Well ventilated | 0.015 | 12.4 | 0.0806 | 1.21 |
| Equivalence ratio three | 0.034 | 6.2 | 0.161 | 5.48 |
This is a controlled arithmetic comparison at fixed heat release. It is not a prediction that a real red-oak fire necessarily maintains 1000 kW as ventilation changes. Actual heat release can change with fuel supply, oxygen availability and the location of combustion. The example isolates the consequences of converting a prescribed energy source into a mass source.[2][4]
The underlying values also have a specific evidential status. The ABCB tables use correlations derived from bench-scale experiments to convert well-ventilated properties to other conditions. They are not a set of independent full-scale building measurements. Their value here is the demonstration of coupled inputs and the need to preserve their conditions of application.[8]
A complementary illustrative calculation holds fuel consumption explicit. At a yield of 0.03 and a fuel rate of 0.05 kg/s, soot production is 1.5 g/s. If the yield doubles while the fuel rate falls to 0.01 kg/s, production becomes 0.6 g/s. A larger yield can therefore accompany a smaller absolute source.[2][7]
Transport and optical consequences
The produced source must be distinguished from suspended concentration. Particles can leave a compartment with the gas flow, accumulate within a layer, or deposit on surfaces. The local concentration depends on those processes and the volume into which the source is distributed. Yield alone does not determine conditions at an occupied location.[9]
Under the restrictive assumption of complete retention and uniform mixing, concentration is soot mass divided by the receiving volume. Mulholland's room example converts 0.200 kg of fuel at a yield of 0.03 into 0.00600 kg of aerosol. Uniform distribution through 90.0 m3 gives 6.67 × 10−5 kg/m3.[10]
The source explicitly discusses why that calculation is approximate. Concentration can be higher near the ceiling, and the chosen generic foam yield may not represent the actual item. Earlier smouldering can produce a different aerosol. A uniform-room calculation should therefore retain these assumptions when its result is used elsewhere.[10]
Concentration is converted to optical density using an optical coefficient and a stated logarithm convention. A further empirical relationship is needed to estimate visibility in smoke. These steps introduce information beyond yield, including the optical properties of the aerosol and the type of object being viewed.[11]
The CFAST reference illustrates this separation by transporting soot as a gaslike species and deriving optical density from its concentration. Its optical parameter is independent of the fuel-to-soot mass conversion. Changing yield modifies the source; changing the optical coefficient modifies attenuation per suspended mass. The two changes should remain distinguishable in a sensitivity assessment.[12]
Aerosol properties can also change during transport through coagulation, evaporation, condensation and deposition. These processes explain why a source measurement does not contain every detail needed to predict detector response or visibility remotely. The assumed transport representation is part of the overall smoke prediction.[9]
Treatment in fire models
In computational fluid dynamics, a soot source may be prescribed through a reaction yield rather than obtained from a detailed particle-formation model. The supplied FDS technical reference describes this approach in its simple hydrocarbon chemistry. The user can specify soot and carbon monoxide yields, which are incorporated into the product composition.[3]
In that documented formulation, yield is based on mass of fuel reacted. The remaining reaction coefficients follow an atom balance. A prescribed soot yield is therefore an input to the chemical representation. A detailed gas-flow solution does not make that input an independently predicted material property.[3]
The supplied 2026 FDS reference states that soot and carbon monoxide yields are zero by default in this simple-chemistry formulation. Consequently, omission of a soot source in that formulation is not evidence that the represented fuel produces no smoke. The chosen reaction inputs must be checked against the intended fire description.[3]
This distinction also clarifies what model validation can establish. Agreement for a particular fire configuration supports the combined inputs and model treatment used for that comparison. It does not make one prescribed yield transferable to every ventilation state or fuel product. Source-data applicability remains a separate engineering question.[2][4]
The broader term smoke should likewise not be reduced to the prescribed soot species. Gaseous products and condensed aerosols have different roles in toxicity, optical attenuation and transport. A soot input serves a particular particulate calculation; it is not a complete chemical description of the fire atmosphere.[5][3]
Selecting and reporting a yield
Selection starts with the fuel description and the expected combustion regime. The test source should identify what was measured, the fuel mass basis, imposed heating and ventilation. Broad labels such as polyurethane or wood do not establish that two tabulated values describe equivalent materials or conditions.[5][4]
The ABCB guidance notes the limited evidence for transferring small-scale results directly to larger fires. It also cautions that general mixed-content assumptions can be inappropriate where the contents are not well mixed. A selected representative value should therefore be connected to the relevant burning contents and assessment purpose.[2][4]
Comparative assessments can reduce sensitivity when the same assumptions apply to both a proposed design and its reference building. That does not guarantee that the ranking is unchanged. The ABCB source recommends sensitivity analysis where an alternative yield or effective heat of combustion could change the comparison.[4]
A useful report records the selected yield, effective heat of combustion, fuel mass basis and combustion regime together. It then distinguishes produced particulate mass from transported concentration and optical consequences. This preserves the physical meaning of the source term throughout the calculation and makes the effects of alternative assumptions traceable.[2][4][12]
See also
Notes
- ↑ Mulholland (2002), pp. 2-258–2-259.
- ↑ 2.00 2.01 2.02 2.03 2.04 2.05 2.06 2.07 2.08 2.09 2.10 2.11 2.12 2.13 Australian Building Codes Board (2020), pp. 19–20.
- ↑ 3.0 3.1 3.2 3.3 3.4 3.5 3.6 McGrattan et al. (2026), pp. 52–54.
- ↑ 4.00 4.01 4.02 4.03 4.04 4.05 4.06 4.07 4.08 4.09 4.10 Australian Building Codes Board (2020), pp. 21–23.
- ↑ 5.0 5.1 5.2 5.3 5.4 5.5 Mulholland (2002), pp. 2-258–2-259.
- ↑ Mulholland (2002), p. 2-259, Table 2-13.1.
- ↑ 7.0 7.1 7.2 7.3 Van Rossum (2027), §§6.2–6.3.
- ↑ 8.0 8.1 8.2 8.3 Australian Building Codes Board (2020), p. 22, Tables 9–10.
- ↑ 9.0 9.1 Mulholland (2002), pp. 2-259, 2-263.
- ↑ 10.0 10.1 Mulholland (2002), pp. 2-265–2-266.
- ↑ Mulholland (2002), pp. 2-263–2-266.
- ↑ 12.0 12.1 Peacock et al. (2026), pp. 35–36.
References
- Australian Building Codes Board. (2020). Fire safety verification method data sheets: Handbook annex (Version 1.1).
- McGrattan, K., Hostikka, S., Floyd, J., McDermott, R., Vanella, M., Mueller, E., & Paul, C. (2026). Fire Dynamics Simulator technical reference guide. Volume 1: Mathematical model (6th ed., revision 6.11.1, NIST Special Publication 1018-1). National Institute of Standards and Technology.
- Mulholland, G. W. (2002). Smoke production and properties. In P. J. DiNenno (Ed.), SFPE handbook of fire protection engineering (3rd ed., pp. 2-258–2-268). National Fire Protection Association.
- Peacock, R. D., McGrattan, K. B., Forney, G. P., & Reneke, P. A. (2026). CFAST: Consolidated model of fire growth and smoke transport, version 7. Volume 1: Technical reference guide (NIST Technical Note 1889v1, revision 7.7.7). National Institute of Standards and Technology.
- Van Rossum, E. (2027). Introduction to Fire Safety Engineering. Fire Safety Academy.