Acid and alkali pretreatment of agro by-products: Evaluating torrefaction efficiency and dechlorination

Utilisation of crop residues is limited due to their low energy and high ash content. This study aimed to develop a method for enabling the use of residual agricultural biomass through acid or alkali pretreatment followed by torrefaction. Pepper stems were treated with hydrochloric acid or sodium hydroxide for a brief period of time and rinsed with distilled water. Untreated and acid-pretreated samples had cellulose contents of 43.85% and 36.57%, respectively. The lignin content in the alkali-pretreated samples (22.74%) was lower than that in untreated samples (27.40%). Mass yield decreased with torrefaction temperature, ranging from 86.19% to 49.89%. However, alkali-treated samples showed smaller changes in mass yield (70.31–60.43%) than other samples. Regarding calorific value, untreated samples exhibited the highest increase (26.37%), whereas alkali-treated samples showed a modest increase of 10.62%. Chlorine content was higher in acid-compared with alkali-treated samples but decreased with temperature. Our study demonstrated that agricultural by-products can be used as energy sources through acid or alkali pretreatment prior to torrefaction. Future studies should explore a variety of agricultural by-products and consider pretreatment with different acids or alkalis, such as sulfuric acid and potassium hydroxide.