SALT-ASSISTED HYDROTHERMAL CARBONIZATION OF OLIVE TREE LEAVES: INSIGHTS INTO THE EFFECTS OF NaCl AND Na₂CO₃

Olive tree leaves are renewable sources and the main byproducts of olive oil production industry having high production quantity. Hydrothermal carbonization (HTC) is one of the commonly used and promising methods to produce chemical, material and energy. However, carbonization stage of HTC process is complicated and uncomprehended and different methods have been proposed to enhance this stage. One of the current and/or green methods for this is salt addition during the HTC process. Therefore, in this study, the effects of two different Na-based salts (NaCl and Na2CO3) on HTC process of olive tree leaves were revealed based on both characteristics of hydrochars and process liquid products. HTC experiments were carried out at 180°C and 200°C, 5 hours and 1/7 (w/w) mixing ratio. Additionally, Na-based salts were added to the reactor at 10% and 20% of the weight of olive tree leaves and HTC experiments were carried out at the same experimental conditions. The 10% and 20% salt addition resulted lower hydrochar yields as comparing to the without salt addition conditions for both HTC temperatures. Carboxyl groups and intermolecular bonded -OH groups in hydrochars decreased with the addition of 20% Na2CO3 salt, indicating increment of the dehydration and decarboxylation reactions with salt addition. Process liquids composed of mostly organic acid compounds, such as acetic acid and benzeneacetic acid 3,4-dihydroxy. The total area percentages of organic acids decreased with the addition of Na-based salts, which indicating enhancement of decarboxylation reactions. Consequently, NaCl and Na2CO3 can successfully be used in HTC process of lignocellulosic biomasses to change and/or enhance byproducts of HTC process quantity and quality.