Chemical looping
combustion (CLC) is one of the promising fuel
conversion technologies for carbon capture with low energy penalty.
Devolatilization is an important physical phenomenon occurring during
solid fuel CLC. Devolatilization behavior influences fragmentation,
combustion rate, emission, and particulates generation in fluidized
bed CLC (FB-CLC), thus a critical input for its design. Existing visual
techniques for determining devolatilization time cannot be applied
in CLC conditions because of its flameless combustion nature. In the
present study, a new, simple, and quick technique called “color
indistinction method” (CIM) is proposed for the determination
of devolatilization time (τd) in FB-CLC, where the
end of devolatilization is inferred from the disappearance of fuel
particle in a hot fluidized bed. Single-particle devolatilization
studies in FB-CLC are conducted to determine the devolatilization
time using CIM for two types of fuels, viz., coal and biomass (Casuarina equisetifolia wood), of size range 8–25
and 10–20 mm, respectively, at three different fuel reactor
bed temperatures (800, 875, and 950 °C) and one fluidization
velocity. The proposed technique is validated in three ways: (i) the
measurement of residual volatiles present in char by thermogravimetric
analysis; (ii) mass loss history of the fuel during devolatilization;
and (iii) diagnostics using particle center temperature measurements.
The results of CIM experiments, in terms of degree of error involved,
are compared with an established flame extinction technique (FET)
and a more accurate particle center temperature (PCT) method. The
amount of volatiles released during devolatilization, as determined
by CIM, is 91.3% for coal and 98.9% for biomass. These values compare
very well with the results of the established FET, in which the volatile
release is 90.7% for coal and 99.1% for biomass samples. The devolatilization
times determined using CIM are in line with particle center temperature
measurements with an acceptable error range of −7.57 to +3.70%.
The proposed CIM is successful in establishing the devolatilization
time of different fuels in CLC conditions and can also be applied
in other flameless combustion conditions.
Devolatilization
and fragmentation are important physical phenomena
occurring during solid fuel chemical looping combustion (CLC). Primary
fragmentation during devolatilization strongly affects the rate of
fuel conversion, emissions, and fine particulates generation in a
fuel reactor of a fluidized bed CLC unit, thus forming a critical
design input. The present study focuses on investigating the primary
fragmentation behavior of large coal and biomass (wood) particles
during the devolatilization phase of CLC. Three types of coals (two
Indian coals, one Indonesian coal) and one type of Casuarina wood
of three sizes in the range of 8–25 mm, at different fuel reactor
bed temperatures (800, 875, and 950 °C) are studied for primary
fragmentation. Iron ore with 64% Fe is used as the oxygen carrier
bed material, with steam as the fluidizing medium in the fuel reactor.
The fragmentation behavior is expressed in terms of the number of
fragments, fragmentation index, frequency of fragmentation, and particle
size distribution of fragments at different residence times of coal
during devolatilization in the fuel reactor. Under the conditions
of study, the number of fragments increases with an increase in particle
size and temperature, for all fuels studied. Also, it is found that
the number of fragments increases with the decrease in compressive
strength of both coal and biomass particles. The Indian coals are
found to fragment in the earlier stages of devolatilization, while
the Indonesian coal and the biomass particles begin to fragment in
the later stages of devolatilization. The maximum fragmentation index
is found with Indian coal - IC1, which has the highest fixed carbon
content among the fuels studied, and the least value is observed in
biomass. Different modes of fragmentation exhibited by each fuel type
is discussed. Indian coals do not show any volumetric changes as such,
whereas Indonesian coal indicates some degree of volumetric expansion.
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