Page 68 - AJWEP-22-6

Chen, et al.

L25 (4×5 ) orthogonal experimental design with four densified biofuels. When the impact resistance of
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4
factors and mixed levels was adopted. densified pellets is ≥95%, the pellets are considered
good quality. It demonstrates good performance and
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2.2. Quality evaluation indicators for densified market potential in practical applications, meeting the
biofuel combustion needs of high-temperature equipment such
For enhanced transportability and storage efficiency as industrial boilers and kilns, and ensuring that the
of densified biofuel, this study employed three key densified biofuel maintains high combustion efficiency
physical properties as quality evaluation metrics: and stability during the burning process. The formula
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Relaxed density (H1), relaxation ratio (H2), and impact is presented in Equation III:
resistance (H3).
Relaxed density refers to the density of biomass I M f 100 % (III)
pellets after they reach a stable state following expansion M b
or contraction post-forming. A higher relaxed density
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indicates a greater density of the densified pellets, which Where I represents the impact resistance of the
typically corresponds to better forming quality and densified pellets (%), M is the weight of the densified
f
higher combustion efficiency. The formula is presented pellets before the drop test, without sieving (g), and M
b
in Equation I: is the weight of the densified pellets after the drop test,
after sieving (g).
4m
ρ = (I) 2.3. Temperature field and temperature MSD
2
π dh Temperature represents the thermal energy state of
Where ρ represents the relaxed density (g/cm ), m is an object’s interior or surrounding environment. The
3
the pellet weight (g), d is the pellet diameter (cm), and h spatial distribution of temperature values within a
is the pellet length (cm). material system constitutes its temperature field. 30
The relaxation ratio of formed pellets is the ratio of In biomass densification, the temperature field
the initial relaxation density to the stabilized relaxation significantly influences pyrolysis, drying, and
density. A high relaxation ratio indicates more internal compaction processes, consequently affecting key
voids, making the pellets prone to moisture absorption fuel properties including heating value, stability,
and breakage, while a low relaxation ratio suggests durability, chemical composition, and energy density.
minimal shrinkage after forming, resulting in a compact The temperature MSD serves as a critical parameter
structure conducive to long-term storage. The lower the for characterizing temperature field uniformity, where
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relaxation ratio of the pellets, the higher their stability, lower values indicate more homogeneous distributions
which is beneficial for subsequent transportation and and higher values reflect greater heterogeneity. The
storage. The formula is presented in Equation II: MSD is calculated following Equation IV: 31

ρ N
λ = i ρ (II) i
T
T
r Kt i 1 (IV)
N
Where λ represents the relaxation ratio, ρ is the where K represents the temperature MSD, T is the
i
initial density (i.e., the density measured immediately average temperature within the observed region, T is
t
after pellet formation) (g/cm ), and ρ is the relaxed the temperature of the i-th pixel in the observed region,
i
3
r
density (g/cm ). and N is the total number of pixels in the observed
3
Impact resistance measures the ability of densified region.
pellets to maintain their original shape after multiple The formula for the average temperature T within
drops and collisions. A higher impact resistance the region is presented in Equation V:
indicates better mechanical strength and higher quality
of the pellets. According to the forestry industry T dxdy
standard LY/T 2552-2015, the impact resistance T = i (V)
of bamboo-based densified biofuel is considered A
compliant if it exceeds 90%. Furthermore, the energy Where A represents the area of the region.
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standard NB/T 34024-2015 classifies densified biofuels The temperature MSD serves as a quantitative metric
with impact resistance greater than 95% as Grade III for assessing temperature field uniformity, reflecting

Volume 22 Issue 6 (2025) 62 doi: 10.36922/AJWEP025240195