Investigation of mechanisms in the plasma-assisted ignition of dispersed coal particle streams

The plasma-assisted ignition enhancement of pulverized lignite particles is intensively studied in a laminar, upward Hencken flat-flame burner. Under the conditions of different oxygen mole fractions, the discharge mechanisms and the reductions of ignition delay time are examined to distinguish the chemical and thermal effects on the ignition enhancement. This work provides a preliminary understanding of plasma-assisted ignition mechanisms on dispersed coal particles that may be applied in the practical combustion system.

Publication：Zhao FX, Li SQ, Ren YH, Yao Q, Yuan Y. Investigation of mechanisms in plasma-assisted ignition of dispersed coal particle streams. Fuel,  186: 518-524 (2016).

Combustion behaviors, PM formation, and ash deposition during coal combustion

This figure illustrates our fundamental researches on coal combustion, as a function of coal particle residence time in high-temperature ambiances. Combined with advanced in-situ diagnostics as well as standard off-line measurements, aspects like particle ignition, surface temperature, fragmentation properties, minerals release, particulates formation, and ash deposition were intensively investigated. Predictions were also made by developed models after revealing the dominant mechanisms in these processes.

Absorption-ablation-excitation mechanism of laser-cluster interactions in a nanoaerosol system

The absorption-ablation-excitation mechanism in laser-cluster interactions is investigated by measuring Rayleigh scattering of aerosol clusters along with atomic emission from phase-selective laser-induced breakdown spectroscopy. For 532 nm excitation, as the laser intensity increases beyond 0.16 GW/cm², the scattering cross section of TiO₂ clusters begins to decrease, concurrent with the onset of atomic emission of Ti, indicating a scattering-to-ablation transition and the formation of nanoplasmas. With 1064 nm laser excitation, the atomic emissions are more than one order of magnitude weaker than that at 532 nm, indicating that the thermal effect is not the main mechanism. To better clarify the process, time-resolved measurements of scattering signals are examined for different excitation laser intensities. For increasing laser intensity, the cross-section of clusters decreases during a single pulse, evincing the shorter ablation delay time and larger ratios of ablation clusters. Assessment of the electron energy distribution during the ablation process is conducted by nondimensionalizing the Fokker-Planck equation, with analogous Strouhal Sl_E, Peclet Pe_E, and Damköhler Da_E numbers defined to characterize the laser-induced aerothermochemical environment. For conditions where Sl_E ≫ 1, Pe_E ≫ 1,and Da_E ≪ 1, the electrons are excited to the conduction band by two-photon absorption, then relax to the bottomof the conduction band by electron energy loss to the lattice, and finally serve as the energy transfer media between laser field and lattice. The relationship between delay time and excitation intensity is well correlated by this simplified model with quasi-steady assumption.

Publication: Ren YH, Li SQ*, Zhang YY, Tse SD, Long MB. Absorption-ablation-excitation mechanism of laser-cluster interactions in a nanoaerosol system. Physical review letters, 114(9), p.093401 (2015).