Effect of Pressure on Pyrolysis of Milled Wood Lignin and Acid-Washed Hybrid Poplar Wood

Abstract

Thin films (∼115 μm thick) of milled wood lignin from hybrid poplar and acid-washed hybrid poplar were pyrolyzed at 500 °C and ∼55 °C/s at five pressures (4, 250, 500, 750, and 1000 mbar) to determine the impact of secondary liquid intermediate reactions on the product distribution. For both milled wood lignin extracted from poplar and acid-washed hybrid poplar wood, pressure had a significant effect on the product distribution for thin film pyrolysis between 4 and 1000 mbar. For lignin, lowering the pressure from 1000 mbar to 4 mbar reduced the char yield from 36 to 23% and enhanced production of large cluster pyrolytic lignin. However, the pressure did not dramatically impact the gas yield (CO 2 , CO, methane, H 2 , ethane, propane, and butane), nor did it significantly impact the release of monomeric phenolic compounds. ICR-MS shows limited changes in the composition of lignin oligomers. The increase in the production of large lignin oligomers observed by UV fluorescence and the reduction of char yield with vacuum confirm the importance of oligomeric combination reactions to form large polyaromatic structures in the liquid intermediate. For hybrid poplar, lowering the pressure from 1000 mbar to 4 mbar decreased the char yield from 19 to 7% and enhanced production of heavy sugars (cellobiosan and cellotriosan). ICR-MS results clearly show the importance of dehydration reactions in the liquid intermediate. Lowering the pressure also enhanced production of CO, CO 2 , and methane due to heterogeneous catalysis by residual alkali and alkaline earth metals in the solid wood matrix. However, it also decreased production of levoglucosan from 10 to 6.1 wt %. The yields of levoglucosan and cellobiosan obtained for hybrid poplar were higher and lower, respectively, compared with those expected if the pyrolysis products were the result of the additive contribution of hybrid poplar constituents. This result could be explained by the tendency of lignin liquid intermediate to bubble vigorously, contributing in this way to the removal of cellulose oligomers from the liquid intermediate.

Progress in understanding the four dominant intra-particle phenomena of lignocellulose pyrolysis: chemical reactions, heat transfer, mass transfer, and phase change

“…It has been suggested that these compounds are generated by thermal ejection caused by intense bubbling of the liquid intermediates. 117,206,207 These aerosols can be trapped inside the solid matrix after ejection and the collision probability will increase with particle size. 106,207,208 Heat transfer Effect of particle size and pore structure on heat transfer…”

“…Using a fast speed camera the formation of intermediate liquid phase was observed in 3 stages: complete lignin melting, foaming and swelling of liquid phase by bubbles, and shrinking by evaporation and bubble collapse. Pecha et al 52,82,206 published a series of manuscripts in 2017 detailing experiments which studied the effect of vacuum on pyrolysis of cellulose, lignin, and xylan. These works illustrated, through high speed photography and pyrolysis product analysis, that the liquid intermediate is made up of partially pyrolyzed oligomers which will either break down into monomers or turn into char at atmospheric pressure.…”

“…52 Similar trends were observed for lignin and poplar, where the char yield increases as pressure increases due to evaporation of compounds at lower pressures. 206 Some work has also been done to characterize evaporation of pyrolysis oil droplets, which may be used to model heat of vaporization in simulations. 241,274 Lumped reaction schemes include vaporization rate and enthalpy in the step for conversion of biopolymer to "oil", but because of this they may not be extendible to significantly different pressures.…”

“…52 Similar results have also been reported for lignin and wood. 206 It is well known that char largely retains the tissue structure of its native lignocellulosic feedstock. 280,281 However, if pyrolysis proceeds through a melt phase it will undergo alterations to its pore structure at some scales.…”

Advancing catalytic fast pyrolysis through integrated multiscale modeling and experimentation: Challenges, progress, and perspectives

“…Between 150 and 420 °C, lignin fragments primarily at the propyl chains, β‐O‐4, α‐O‐4, and C α C β bond bonds (Elder & Beste, ). These reactions form lignin oligomers, which go into the liquid intermediate phase and are thermally ejected, further split to monomers, or cross‐link into char (Montoya, Pecha, Chejne‐Janna, & Garcia‐Perez, ; B. Pecha et al, ; Smith, Pecha, Helms, Scudiero, & Garcia‐Perez, ). Above 380 °C, functional constituents of the aromatic ring detach and the lignin fragmentation product form clusters that combine into polyaromatic rings through a series of reactions that release CO, H 2 , CH 4 , and H 2 O (Huang, Wei, Qiu, Yin, & Wu, ; Smith et al, ).…”