Particle Morphology Evolution and Its Enhancement for Lignite Gasification in Supercritical Water

Supercritical water gasification is a promising technology
for
the clean and efficient conversion of lignite particles. The particle
morphology evolution has not yet been investigated for its impact
on gasification. We monitor the evolution of the sphericity (Cs) and local surface sharpness (σp*) of three-dimensional
(3D) irregular lignite particles during gasification by 3D optical
measurement. The particle morphology is found to mainly shift from
structures with high sharpness (σp* > 0.10) to low sharpness (σp* < 0.10). High-sharpness
structure decreases in the pyrolysis reaction, regenerates in the
gasification reaction, and consumes at the end of the reaction. Local
structure with high sharpness promotes particle conversion up to 86.21%
due to the enhanced heat and mass transfer. The optimum particle morphology
of Cs < 0.74 and size D0 < 1.5 mm are identified, which yield the average
mass conversion of around 79.12% without a catalyst. We obtain the
reaction activation energy Ea = 17.20
± 6.45 kJ·mol–1 under 550–650 °C
while 162.70 ± 8.42 kJ·mol–1 under 650–750
°C from the particle mass evolution.