Visualization of physicochemical phenomena during biomass pyrolysis in an optically accessible reactor

•A method for monitoring product evolution during pyrolysis was demonstrated.•Results provide direct visual evidence of dominant transport mechanisms.•Laser scattering images monitor in situ volatile product evolution and condensation.•Vapor-phase transport of products dominates in whole biomass and cellulose.•Lignin coalescence after liquefaction leads to thermal ejection of droplets. The thermochemical conversion of biomass via fast pyrolysis requires detailed descriptions of both the kinetic and heat and mass transport rates, which are often in direct competition. To investigate the evolution of products, whole biomass and biomass constituents (e.g. cellulose and lignin) are pyrolyzed in a novel optically accessible reactor. This enables real-time, in situ observation of the temporal evolution of light-oxygenates, volatile sugars, and phenolic compounds during melting, agglomeration, ejection, and volatilization of biomass under realistic heating rates (∼100 K/s). Both cellulose and lignin underwent liquefaction, but liquid coalescence in lignin limits vapor transport. This is overcome by dispersing extracted lignin in an inert matrix, and confirms the predominant mass transport of pyrolysis products from whole biomass, cellulose, and lignin occurs via devolatilization. These results differ from prior work on single-particle pyrolysis and reveal thermochemical mechanisms that are relevant for typical large-scale pyrolysis processes with transport limitations.