Hydrothermal vs. dilute acid pre-treatments: comparison of the biomass properties, distribution of pyrolysis products, and bio-oil characteristics

In this study, hydrothermal and acid pre-treatments were applied to improve the raw material properties of the bean pod. To evaluate the biomass as a fuel, it is superior to have low moisture content, high volatile matter, and carbon content. As acid concentration changed in acid washing and impregnation, different trends were observed in these contents. The optimum working concentration to meet all these properties was chosen as 1 M acid. In hydrothermal pre-treatment, as the temperature increased with constant reaction time, the moisture content of the biomass decreased while the amount of volatile matter was raised. According to the SEM, FT-IR, TGA, and preliminary and elemental analysis characterization results, the samples with high C and low O content after evaluating in hydrothermal pre-treatment at 190 °C for 20 min and hydrothermal pre-treatment at 150 °C for 20 min, washing with 1 M H₃PO₄, and impregnating with 1 M H₃PO₄ were subjected to pyrolysis process. When the tar yields obtained from impregnating with 1 M H₃PO₄ and washing with 1 M H₃PO₄ samples were compared, it was seen that the washing process gave a higher (19.56%) tar yield. In the pyrolysis experiments, the highest tar yield (22.18%) was achieved by using the hydrothermal pre-treatment at 190 °C for 20 min sample with a volatile matter content of 71.59%. Total volatile matter yields of pyrolysis were listed as (hydrothermal pre-treatment at 190 °C for 20 min) > (washing with 1 M H₃PO₄) > (impregnating with 1 M H₃PO₄) > (hydrothermal pre-treatment at 150 °C for 20 min). As a result, it can be concluded that the processes performed as hydrothermal method or acidic pre-treatment compared to raw biomass have an important role in obtaining the desired product distribution and properties in biofuel production.