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Porosity-driven biomass combustion

(i) Pyrolysis stage in low-density regions (diffusion- (iii) Char combustion stage in high-density regions
dominated combustion): In the initial stage of (smoldering-dominated combustion): As the fibrous
deflagration, the 3D fibrous network of cotton network undergoes carbonization, its porosity
floc, characterized by high porosity (>80%), forms decreases to <40%, leading to severe oxygen
an open structure where the oxygen diffusion diffusion constraints and the formation of a dense
rate exceeds the pyrolysis gas generation rate. char layer. At this stage, combustion transitions into
The high-temperature electric arc from the spark a heterogeneous surface oxidation regime, where
initiates the thermal decomposition of cellulose the residual char (fixed carbon content >75%)
and hemicellulose, producing volatile species such reacts with oxygen through the Knudsen diffusion
as CO and CH , which dissipate rapidly through mechanism, with reaction kinetics dictated by oxygen
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the porous matrix. At this stage, combustion is penetration depth within nanoscale pores (<50 nm).
primarily diffusion-driven, exhibiting progressive Macroscopically, this stage exhibits smoldering
surface carbonization (volume shrinkage rate characteristics, with a surface glowing temperature
>60%) without visible flames (Figure 5). The high of ~550°C and an ash residue rate exceeding 90%.
permeability of the fiber network delays combustible The process ultimately self-extinguishes due to
gas accumulation, establishing a characteristic pore closure, with this smoldering phase persisting
“surface pyrolysis-gas diffusion” coupling mode. 3 – 5 times longer than the initial deflagration stage,
(ii) Oxidation stage in critical-density regions underscoring the dominant role of mass transfer
(deflagration transition): As pyrolysis progresses, limitations in combustion kinetics at high densities
the fibrous network contracts under thermal stress, (Figure 7).
reducing porosity to a critical range (55 – 65%)
and forming a semi-enclosed gas cavity. Within 3.3. Structural fuel theory of cotton floc
this confined structure, volatile gases accumulate deflagration
and mix with oxygen, reaching a pre-mixed While classical dust deflagration theory adequately
concentration of 12 – 15%, which falls within the describes the fundamental combustion characteristics of
flammability limits, thereby triggering chain radical cotton floc, its turbulence-mixing-dominated combustion
reactions. This transition marks a combustion mode mechanism fails to fully account for experimentally
shift from diffusion-controlled burning to pre-mixed observed anomalies. Under confined conditions, the
deflagration, characterized by flame propagation flame propagation velocity of cotton floc combustion
speeds of 1.2 – 1.8 m/s and peak temperatures (1.538 m/s) significantly exceeds that in loosely packed
exceeding 800°C. High-speed camera imaging configurations (0.833 m/s), with combustion efficiency
reveals that turbulence effects induced by the exhibiting local maxima at specific bulk densities.
critical porosity structure drive a fractal expansion These findings suggest that cotton floc combustion is
pattern of the flame front (Figure 6). not solely governed by turbulent fuel-oxidizer mixing

Figure 5. The pyrolysis stage in the deflagration Figure 6. Oxidation stage in the deflagration process
process of cotton floc of cotton floc

Volume 22 Issue 4 (2025) 211 doi: 10.36922/AJWEP025240193