Pyrolytic conversion of cellulosic pulps from “lignin-first” biomass fractionation

Authors: Mullen, Charles; Ellison, Candice; Elkasabi, Yaseen

Submitted to: Energies
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/31/2023
Publication Date: 4/4/2023
Citation: Mullen, C.A., Ellison, C.R., Elkasabi, Y.M. 2023. Pyrolytic conversion of cellulosic pulps from “lignin-first” biomass fractionation. Energies. https://doi.org/10.3390/en16073236.
DOI: https://doi.org/10.3390/en16073236

Interpretive Summary: The largest available renewable resource to produce liquid fuels, chemicals, and materials from which to replace those made from fossil resources is lignocellulosic biomass. This biomass consists of non-edible plant materials such as crop residues (straws, stover, bagasse), grasses and woody biomass and they are made up largely of carbohydrate polymers (cellulose and hemicellulose) and lignin. Lignin is the glue that holds plants together and gives them rigidity to stand, but this stability makes it difficult to convert to useful products. In most biorefining concepts, carbohydrates are broken down into sugars which can be converted to ethanol or other materials by fermentation or other processes, and lignin is a byproduct that is often just burned for heat. Lignin is more chemically favorable to convert in its native form than after isolation and therefore new biorefining concepts (called “lignin-first”) have looked at converting lignin to chemicals while removing it from cellulose and then taking the cellulose and converting it in a second step. This is usually done by releasing sugars to be converted to ethanol. In this study rather than converting the cellulose byproduct to sugars and ethanol we used in thermal processes called fast pyrolysis and catalytic pyrolysis to convert the cellulose from the described process to a chemical called levoglucosan and a mixture of benzene, toluene, ethyl benzene and xylenes (BTEX), an important petrochemical product. Our study looked at two biomasses switchgrass and oak wood. We found that the yield of levoglucosan produced via this process was nearly five times greater than processing of raw biomass. But for production of BTEX, there was no advantage to first removing lignin than compared with processing of the raw biomass. This information will be useful to those considering lignocellulosic biorefineries with different product possibilities.

Technical Abstract: Efficient conversion of lignin has been one of the most difficult hurdles to overcome for biorefineries converting lignocellulosic biomass. To overcome this, “lignin first” biomass fractionation and conversion schemes have received increasing attention in recent years. In standard biorefining concepts carbohydrate portions of the biomass are separated and depolymerized into their sugar components leaving a recalcitrant and relatively chemically stable lignin rich by-product. In contrast, in lignin-first concepts, a one-pot fractionation and lignin depolymerization is performed leading to a phenolic oil and a cellulosic pulp. Most considering lignin-first processes have treated the pulp as a fermentation feedstock to produce ethanol. Herein are results of a study comparing various cellulosic pulps for their potential to produce valuable products via pyrolysis processes, accessed via analytical pyrolysis (py-GC). Pulps derived from herbaceous (switchgrass) and woody biomass (oak) from acid catalyzed lignin-first processes are compared with those from supported metal catalyzed reductive depolymerization. The pulp composition can vary in important ways that effect their pyrolysis behavior including the fraction of lignin and hemicellulose remaining, the ash content, and other chemical modification. Fast pyrolysis of the pulps produced levoglucosan in yields of up to about 35 wt%. When normalized for the amount of biomass entering the entire process, performing the lignin-first reductive depolymerization resulted in 4.0-4.6 times the yield of levoglucosan than pyrolysis of raw biomass. Pulps derived from switchgrass were better feedstocks for levoglucosan production compared with pulps from oak and pulps produced from metal on carbon catalyzed depolymerization produced more levoglucosan than those from acid catalyzed depolymerization. Catalytic pyrolysis over HZSM-5 produced aromatic hydrocarbons from the pulps. In this case, the yields were similar from both feedstocks and catalyst types, suggesting that there is no advantage to lignin fractionation prior to zeolite catalyzed catalytic pyrolysis for hydrocarbons.