Predicting the rapid devolatilization of mineral-free cellulose

Abstract

This paper extends the validation of a reaction mechanism for the devolatilization of mineral-free cellulose called “cel-FC.” A single set of model parameters accurately interprets the partitioning of raw cellulose into oil, noncondensables, and solid residue and oil molecular weight distributions (MWDs) for temperatures to 600 °C, heating rates from 60 to 6000°C/s, contact times to 120 s, and pressures of vacuum and 0.1 MPa. The analysis incorporates depolymerization via random scission and unzipping from chain ends; charring via monomer decomposition, spontaneous charring, and thermal annealing; fragmentation statistics, and the flash distillation analogy. Random depolymerization is responsible for the induction of primary cellulose devolatilization and eventually generates chains short enough to vaporize. The accompanying surge in the relative concentrations of chain ends activates the unzipping process. The fastest oil production rates are driven by scission plus unzipping and mediated by flash distillation. Whenever vaporization of longer oil precursors is limited, unzipping delivers monomer derivatives into the oil. Whenever longer volatile chains can freely vaporize they are no longer subject to unzipping or scission, so there are necessarily fewer monomers in oil. This is why ultimate oil yields are essentially the same across broad ranges of heating rate, temperature (given sufficient contact time at low temperatures), and pressure, and why oil MWDs shift toward heavier molecular weights for progressively faster heating rates, hotter temperatures, and lower pressures. Cel-FC 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 reactions. Monomer decomposition and spontaneous charring produce hardly any bio-char from mineral-free cellulose but are major sources of noncondensable gases and responsible for greater gas yields for slower heating rates, hotter temperatures, and higher pressures.