Understanding and Mitigating Stickiness in Biochar Produced Through Acid Hydrolysis and Dehydration

Lignocellulosic woody biomass presents an excellent alternative for producing specialty chemicals amid the global energy crisis and the pursuit of transition to renewable energy sources. Among the various conversion pathways for producing levulinic acid (LA) from biomass, acid hydrolysis and dehydration reactions has commercial promise. However, scaling the process for commercial use brings its challenges. Lower yields of products and higher production of sticky char add to process inefficiencies and operational downtimes, respectively. Mechanical and chemical pre-processing strategies have proven to be an effective solution in addressing and mitigating such problems. Chemical preconditioning (CPS) using steam and dilute sulfuric acid in batch reactors before the main acid hydrolysis and dehydration reaction (AHDH) at various temperatures (170, 180, and 200 °C) for 30 minutes has proven to be an effective method for reducing the xylan content and altering the crystallinity of cellulosic polysaccharides. Reduced xylan content can potentially help mitigate the char stickiness that builds inside the reactor vessel and downstream plugging of piping. However, a more intense chemical preconditioning at elevated temperature doesn’t necessarily dictate higher product yields for levulinic and formic acid (FA). Careful consideration needs to be given to pre-processing strategies to account for rigorous energy demands, higher yields of products (levulinic acid and formic acid), and reduced production of sticky char, a byproduct. Levulinic acid is a platform chemical with significant potential for conversion into a wide range of bio-based chemicals and fuels. A common process for producing levulinic acid from lignocellulosic feedstocks involves AHDH, where hexose polymers are hydrolyzed into monomeric sugars and subsequently dehydrated to levulinic acid and formic acid in the presence of dilute sulfuric acid. However, scaling up the AHDH process is challenging because of the formation of by-products such as sticky biochar, which accumulates in continuous-flow reactors, reducing effective reaction volume and increasing process downtime. This study investigates the effect of a chemical preconditioning step on mitigating sticky biochar formation. Woody biomass was preconditioned at 170 °C with 0.26 wt% sulfuric acid for 30 minutes, resulting in substantial removal of hemicellulose and acid-soluble lignin. AHDH of these preconditioned solids produced biochar that did not adhere to reactor surfaces. ATR-FTIR (Attenuated Total Reflectance Fourier Transform Infrared Spectroscopy) and TGA(Thermogravimetric Analysis) confirmed that the chemical preconditioning step minimized interactions between hemicellulose-derived degradation products and lignin side chains, reducing sticky char formation. Additionally, the study observed a 6% higher yield of organic acids from softwood species compared to hardwoods, with bark content shown to negatively impact yield. These findings suggest that targeted preconditioning of lignocellulosic biomass can enhance reactor operability and improve organic acid production efficiency in AHDH processes.