(328c) Techno-Economic Assessment of Sustainable Biolubricant Production from Lignin and Waste Cooking Oil

Lignin accounts for 15-30% of lignocellulosic biomass weight and 40% of the biomass heat energy. Today, lignin from lignocellulosic biorefineries is underutilized because of the absence of a high-value product from wet unhydrolyzed solids (UHS), a waste stream that has very little value. Finding ways to valorize this byproduct is crucial to enhance the economic viability and sustainability of biofuels production.

In this work, a process is developed to produce a sustainable biolubricant starting from waste cooking oil (WCO) and lignin from UHS. The development of sustainable and environmentally friendly lubricants has in fact become an important challenge, and their market has increased significantly in recent years.

The proposed process consists of three steps: (1) Hydrothermal liquefaction (HTL) of lignin, to depolymerize it into monomeric phenolic compounds, collected into a bio-oil; (2) Hydrodeoxygenation (HDO) of the bio-oil to replace the hydroxyl group of phenolic compounds with hydrogen, and thus produce aromatic hydrocarbons; (3) alkylation of unsaturated FAMEs (in particular from transesterification of WCO) with the aromatic hydrocarbons obtained from lignin degradation.

Experiments were first carried out to find out the optimal operating conditions that maximize the reaction yields of the three process steps. In particular, the HTL step was conducted using an extremely efficient induction heating system that allowed reaching the reaction temperature within 2-3 min. The optimal operating conditions that allowed to maximize the bio-oil yield were found to be 320 °C at 1 min residence time, which resulted in the highest selectivity of phenol (52 wt%).

The hydrodeoxygenation of a mixture of phenolic compounds was performed at 250°C, utilizing alkane solvents. To improve both yields and selectivity toward the formation of aromatic hydrocarbons, a multi-step approach was adopted. This strategy yielded a conversion rate of 81 wt%, with 58 wt% selectivity. In the final step, the alkylation of the aromatic hydrocarbons using unsaturated FAMEs occurred at 210°C for 4 hours, employing an acid-treated montmorillonite catalysts under argon gas. The resulting product contains 19 wt% alkylated fatty acid methyl esters.

Based on the experimental data collected, a techno-economic analysis was conducted to verify the scalability and economic viability of the proposed process when integrated in an already existing lignocellulosic biorefinery. In particular, capital and operating expenditures were evaluated to determine the minimum selling price of the bioethanol produced by the integrated biorefinery to that of the conventional one.