Bio-FLASHCHAIN® theory for rapid devolatilization of biomass. 6. V.2.0 decomposition mechanism for mineral-free lignins

Abstract

According to lig-FC, lignin devolatilization entails depolymerization via monomer rupture, repolymerization via bimolecular recombination, a three-stage charring cascade, and flash distillation of oil precursors in the condensed phase. Simulation results from lig-FC are evaluated with measured product distributions including bio-oil molecular weight distributions (MWDs) from 16 mineral-free lignins for temperatures to 800 °C, heating rates from 60 to 6000°C/s, contact times after heatup to 10 s, and pressures from vacuum to 0.1 MPa. Lig-FC accurately depicts the impacts of lignin quality, temperature, heating rate, and pressure on the major products and oils MWDs. All raw lignins contain abundant oil precursors so that flash distillation ensures that oils are released as soon as a flow of noncondensables carries them into the free stream. In lig-FC the first gases form by monomer decomposition and rupture. The distinctive devolatilization behavior of different mineral-free lignins can be depicted from the lignin MWD, the elemental composition, and the average monomer weight. Oils yields diminish and char yields increase for progressively heavier MWDs and heavier mean monomer weights; and for smaller H/C and O/H and larger O/C. Bimolecular recombination is required to interpret the disparate ultimate oils yields for vacuum and 0.1 MPa. Faster recombination in conjunction with suppressed volatility at atmospheric pressure reduce the ultimate oils yield to only half that for vacuum.