Thermal Concentration of Kraft Black Liquor and Lignin Production: Evolution in Odor Profile

Kraft black liquor is a crucial byproduct of the wood treatment process for paper production. Its thermal treatment for concentration plays a pivotal role, particularly before acid precipitation for lignin recovery. This process induces significant structural changes, which must be studied to optimize efficiency. Additionally, its impact on the odor profile is critical for industrial operations. This study examines the evolution of black liquor when exposed to moderately high temperatures (120–150 °C) under an almost inert atmosphere, simulating evaporator conditions and their effect on lignin transformation. Results show that organic acids and aromatic compounds, especially phenolics like 2-methoxyphenol, are dominant odor contributors, consistent with lignin’s aromatic nature. Thermal hydrolysis at 120 °C significantly increased odorous emissions, while higher temperatures (135–150 °C) reduced the release of compounds like 3-methylbutanal. Thermal treatment intensified lignin’s odor and increased VOC complexity, with new aldehydes and organic acids appearing in treated samples. Moderate temperatures and shorter treatment times led to higher VOC emissions, peaking at 120 °C. However, above 135 °C, odor-active compounds decreased, resulting in a less pronounced odor profile. These conditions also caused a slight reduction in lignin yield and substantial changes in its composition and appearance. This study provides new insight into how the thermal concentration of Kraft black liquor affects both the recovery and the odor characteristics of lignin. While previous research has focused mainly on process efficiency or chemical yield, this work connects the operating temperature with changes in volatile organic compounds responsible for odor. Understanding this relationship helps to improve lignin recovery while minimizing pollutant emissions during industrial processing, contributing to cleaner and more sustainable pulp and paper production.