Numerical Study on Effect of Ambient Humidity Variation on Self-heating and Spontaneous Ignition of the Eucalyptus Bark Pile


The self-heating and spontaneous ignition process pose a fire risk for industrial biomass piles during storage. Most studies, from theoretical to numerical, pay more attention on the effect of pile size on self-heating and self-ignition, which in essence is due to chemical reactions. However, the effect of ambient humidity on the self-heating and spontaneous ignition process, which is due to physical process of water evaporation and vapor condensation, is not well understood. In fact, fire accidents and the related experimental studies have shown that the sudden increase of ambient humidity would cause the rapid increase of biomass pile temperature, leading to spontaneous ignition. In order to fill this knowledge gap, this work proposed a computational self-heating model, coupling heat and mass transfer process and both the microbial and chemical reactions. The processes of moisture evaporation, transportation and convective exchange of vapor were considered in this model to study the effect of humidity on self-heating process. The model was validated against the full-scale experiments of Zhanjiang Biomass Power Plant firstly. The numerical results show that the sudden increase of ambient humidity can lead to a quick increase of pile temperature due to the condensation process. Spontaneous ignition is highly dependent upon the heat generation evolution of the chemical reaction within the pile. The sudden increase of humidity could speed up the chemical reaction process, leading to a fire. This study helps understand the role of humidity on self-heating process during biomass storage.

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Data Availability

All datas generated or analysed during this study are available from the corresponding author on reasonable request.

Code Availability

The codes used or analysed during the current study are available from the corresponding author on reasonable request.


A 1 :

Pre-exponential factor (s−1)

A 2 :

Pre-exponential factor (–)

A 3 :

Pre-exponential factor (m3 kg−1 s−1)

a w :

Water activity (–)

c :

Concentration (kg m−3)

c p :

Specific heat capacity (J kg−1 K−1)

D c :

Capillary diffusivity (m2 s−1)

D eff,g :

Vapor diffusion coefficient (m2 s−1)

E :

Activation energy (J mol−1)

h c :

Convective heat transfer coefficient (W m−2 K−1)

h fg :

Latent heat of evaporation (J kg−1)

h m :

Convective mass transfer coefficient (m s−1)

k :

Intrinsic permeability (m2)

k r :

Relative permeability (–)

k th :

Thermal conductivity (W m−1 K−1)

M :

Molar mass (kg mol−1)

M db :

Moisture content (dry basis) (–)

M wb :

Moisture content (wet basis) (–)

n :

Mass flux (kg m−2 s−1)

p :

Pressure (Pa)

p v,a :

Ambient vapor pressure (Pa)

Q b :

Reaction heat of microbial activity (J kg−1)

Q c :

Reaction heat of chemical reaction (J kg−1)

R evap :

Evaporation rate (kg m−3 s−1)

S :

Saturation (–)

T :

Temperature (K)

V :

Volume (m3)

ρ :

Density (kg m−3)

σ :

Moisture content (wet basis) (–)

φ :

Porosity (–)

μ :

Viscosity (Pa s)

χ :

Molar fraction (–)


Promotion item of microbial activity


Inhibition item of microbial activity


Chemical reaction

a :


eff :


g :


o :


s :


w :

Liquid water

v :



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This work was sponsored by National Key R&D Program of China (No. 2016YFC0800100) and National Natural Science Foundation of China (No. 51576184). HX Chen was supported by Science and Technological Fund of Anhui Province for Outstanding Youth (No 1808085J21) and Fundamental Research Funds for the Central University (WK2320000036 and 2320000042).

Author information




S.F.: Conceptualization, Methodology, Formal analysis, Writing—review & editing, Writing—original draft. H.C.: Supervision, Formal analysis, Writing—review & editing. S.D.W.: Software, Writing—review & editing. H.S.S.: Conceptualization, Writing—review & editing. T.L.: Software, Writing—review & editing. Y.S.: Data Curation.

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Correspondence to Haixiang Chen.

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Fu, S., Chen, H., Watt, S.D. et al. Numerical Study on Effect of Ambient Humidity Variation on Self-heating and Spontaneous Ignition of the Eucalyptus Bark Pile. Fire Technol (2021).

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  • Ambient humidity variation
  • Self-heating
  • Spontaneous ignition
  • Eucalyptus bark
  • Moisture
  • Numerical model