Thermo-optical investigation of burning biomass pyrolysis oil droplets

An extensive investigation has been carried out on the thermal behavior of biomass pyrolysis oil derived from different feedstocks (mixed hardwood, pine, and poplar). Experiments were performed on streams of monodispersed droplets (50–100 μm) and on large suspended droplets (300–600 μm). The small droplets were generated by a single-droplet generator and injected in a drop-tube furnace, the temperature of which ranged from 300 to 850°C. Light-scattering methods have been used to follow the process undergone by the pyrolysis oil droplets. The solid material, residual of the droplet heating and vaporization processes, has been examined by means of optical and scanning electron microscopy. Two different morphologies of residual particles have been observed at the exit of the furnace: (a) more compact, mechanically resistant spheres, with typical diameter of 10–40 μm, and (b) fragile, glasslike cenospheres with thin walls and menisci, with diameter of 100–200 μm. The large droplets were suspended at the exit of the furnace, using both a quartz fiber and an exposed junction thermocouple. The furnace temperature was varied between 400 and 1200°C. The phenomena, occurring during the vaporization and burning of each droplet, were followed by coupling the thermocouple measurement to high-speed visualization. The imaging was performed by means of fast digital video recording (50–250 frames/s) and high-speed cinematography (400–1000 frames/s). Temperature-time curves of the burning droplets show a stepped behavior, with two zones at constant temperature, at ∼100 and ∼450°C. The first step is due to the vaporization of water. The second plateau corresponds to the heating of heavy compounds due to selective vaporization and liquid-phase pyrolysis. Swelling, shrinking, and microexplosions with ejection of matter characterize this phase. The combustion of the droplets starts with an enveloping blue flame. The flame then develops a yellow tail of increasing size. After the flame extinction, the remaining char particle burns without residual.