Bio-FLASHCHAIN® theory for rapid devolatilization of biomass. 4. V.2.0 decomposition mechanism for mineral-free cellulose

Abstract

According to cel-FC, the essential aspects of cellulose devolatilization are (1) Depolymerization reactions that disintegrate cellulose chains into a fragment distribution; (2) Probabilities that a given bond rupture produces a certain chain size; (3) A charring cascade that inhibits subsequent bond ruptures while bound glucose monomers decompose into progressively more refractory structures plus noncondensable gases; and (4) The Flash Distillation Analogy whereby volatile chains in the condensed phase vaporize into escaping noncondensable gases, depending on the molecular weight, temperature, and pressure. Predictions from cel-FC are evaluated with measured product distributions including bio-oil molecular weight distributions (MWDs) from mineral-free cellulose for temperatures to 900 °C, heating rates from 100 to 5000 °C/s, contact times to 30 s, and pressures from vacuum to 6.9 MPa. Cel-FC accurately depicts the impacts of temperature, heating rate, and pressure on the major products of primary cellulose devolatilization. Devolatilization during slow heating occurs at relatively cool temperatures where vaporization is limited for all but the lightest oil precursors. Unzipping circumvents this bottleneck by delivering a preponderance of monomer derivatives into the cumulative oil sample. Devolatilization at fast heating usually occurs at hotter temperatures where longer volatile chains can freely vaporize and produce an oil with relatively few monomers but abundant longer depolymerization fragments. As longer precursors vaporize, there are necessarily fewer monomers in a cumulative oil sample. This same observation explains why ultimate oil yields vary little across broad ranges of heating rate, temperature (given sufficient contact time at low temperatures), and pressure, and why oil MWDs shift toward heavier MWs for faster heating rates, hotter temperatures, and lower pressures. Cel-FC also interprets the optimal primary products - 94 wt% oil plus a mixture of light oxygenates, CO2, and CO in H2O – as the limit of depolymerization via scission and unzipping without any charring.