(255c) Uncovering the Effect of Mechanochemical Pretreatment on Biocrude Yields and Chemical Mechanism of Lignocellulosic HTL

Lignocellulosic wastes have emerged as a popular feedstock for hydrothermal conversion to biofuels. They can be preferable to food waste and sewage sludge due to negligible nitrogen which has proven challenging in biocrude upgrading. Additionally, research has shown lignocellulosic feedstocks to result in low biocrude yields typically below 25%¹, decreasing their overall commercial suitability due to increased production costs. Previous research has estimated ball-milling to consume 5.6 MJ/kg bamboo feed², whereas biocrude typically results in higher heating values (HHV) between 30 – 40 MJ/kg, indicating the potential for pre-treatment to be energy-favorable.³

Mechanical pre-treatment has the potential to improve biocrude yields and process economics from lignocellulose-fed hydrothermal liquefaction. This work explores the effect of ball milling as a mechanical pre-treatment for lignocellulosic waste from two sources with varying cellulose and lignin contents. Preliminary results reveal that ball milling increases biocrude production from lignocellulosic waste by 95% compared to HTL at identical conditions with un-modified lignocellulosic material as seen in Figure 1a.

Figure 1b begins to uncover the underlying reasoning for the increase in biocrude yield. Micrograph analysis reveals that ball-milling results in a significant decrease in green waste particle size from 0.16 ± 0.09 mm to 0.05 ± 0.04 mm, corresponding to a decrease of nearly 70%. This does not fully account for the increase in biocrude yield, wherein Figure 1b indicates that ball milling decreases lignocellulose crystallinity, which also plays a role in increasing biocrude yield. To determine the specific contribution of particle size and crystallinity, un-modified lignocellulosic waste will be sieved into particle size ranges that mimic those found in ball-milled green waste and biocrude yields compared. Cellulose and lignin will undergo HTL at identical sieve fractions with and without ball milling to identify the specific contribution of crystallinity on biocrude yield.

This work aims the advance the potential for lignocellulosic hydrothermal liquefaction at a commercial level through an understanding of particle size and crystalline properties of the feedstock beneficial to increased biocrude yields.