Alkali pretreatment is one of the chemical pretreatment technologies that has been examined on various types of lignocellulosic biomass. To gain a better insight into the effects of a potassium-based catalyst on pyrolysis behavior with different materials, potassium bicarbonate (KHCO3) and potassium nitrate (KNO3) were used as additives in this study. The experimental parameters which included particle size, heating rate, and additive loading were investigated. The results showed that adding potassium for both KHCO3 and KNO3 to feedstocks led to increase in biochar. A model-free method, Flynn–Wall–Ozawa (FWO), was implemented in this study to determine the activation energy values for untreated and potassium-treated feedstocks. A reduction in apparent activation energy values of treated biomass was observed. This indicates that adding potassium salt to biomass influenced the structures of the main components and promoted the catalytic effect of pyrolysis. Activation energies of treated pine range from 250 to 308 kJ/mol, and energies of wheat straw range from 277 to 402 kJ/mol.