Heterogeneous and Homogeneous Components in Gas-Phase Pyrolysis of Hydrolytic Lignin

Pyrolysis of hydrolytic lignin (HL) in newly designed, gas phase continuous droplet evaporation (CDE) and continuous atomization (CA) reactors was studied. The product distribution was strongly dependent on the heterogeneous character of either delivery of lignin solution into the CDE reactor (in situ formation of solid phase) or sampling conditions using quartz wool in both CDE and CA reactors. The effect of residence time, initial concentration of HL solution, and injection temperature on product distribution in the CDE reactor was investigated and discussed in terms of mass and heat transfer limitation. The experimental data confirm that at low initial mass delivery rates of lignin (micrograms per second) and by increasing initial lignin concentration (up to 40 times), the formation of phenolics is slightly intensified (six times). However, the solid surface or any condensed phase that forms in situ during the reaction in the gas phase may largely govern the pyrolysis processes. The detailed experimental examination of homogeneous pyrolysis of lignin in both gas-phase reactors by implication of diverse analytical techniques (gas chromatography, gel permeation chromatography, laser desorption/ionization, Fourier transform infrared spectroscopy, electron paramagnetic resonance, nuclear magnetic resonance) revealed break down of HL macromolecules into oligomer fragments after pyrolysis at negligible amounts of phenolics detected. A mechanistic interpretation of primary steps for formation of dominant intermediate products, oligomers and oligomer stable radicals, is represented.