Color Indistinction Method for the Determination of Devolatilization Time of Large Fuel Particles in Chemical Looping Combustion

Abstract

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.

Chemical Looping for Combustion of Solid Biomass: A Review

“…Pragadeesh and co-workers , focused their attention on the evaluation of the devolatilization timeboth for coal and biomassunder CLC conditions. In particular, they proposed a novel technique, called the “color indistinction method” (CIM), for the determination of the devolatilization time (τ d ) in FB-CLC.…”

“…Specifically, the devolatilization time was defined as the time period between the introduction of the fresh fuel particle into the bed and the time of its disappearance (Figure ). The CIM method successfully determined the devolatilization time of different types of fuel particles, in a range of particle sizes at different bed temperatures (Figure ), as validated by comparison with other techniques …”

“…Photographs of a 15 mm wood particle at different residence times during devolatilization under CLC conditions at 875 °C [Reproduced from ref . Copyright 2019 American Chemical Society].…”

“…Devolatilization time of (a) coal and (b) biomass for different particle sizes at 800, 875, and 950 °C [Reproduced from ref . Copyright 2019 American Chemical Society].…”

“…In addition, fuel particle characteristics have an important role to control, to a certain measure, the amount of unburnt volatiles: for small particles and high volatile contents, carbon slip to AR is almost negligible, but at the expense of lower volatiles conversion. 69 Pragadeesh and co-workers 103,104 focused their attention on the evaluation of the devolatilization timeboth for coal and biomassunder CLC conditions. In particular, they proposed a novel technique, called the "color indistinction method" (CIM), for the determination of the devolatilization time (τ d ) in FB-CLC.…”