<p>Decarbonization of the energy sector is currently attracting much attention due to the limited availability of fossil fuels and the harmful emissions associated with their use. Another pressing issue today is waste management, with most of the waste being plant biomass. Therefore, it is becoming increasingly important to improve the efficiency of biomass processing. At present, the most feasible processing method is thermal treatment of waste by slow conductive pyrolysis followed by the conversion of carbonization products into adsorbents. Pyrolysis is characterized by high heat consumption, but this consumption can be significantly reduced at properly calculated thermal energy parameters of pyrolysis. This paper presents methods and results of studying the thermal decomposition of certain types of plant waste. The mass balance for slow conductive pyrolysis was determined using an experimental setup. The thermophysical parameters of the thermal processing products, such as heat capacity and specific heat of combustion, were found, and the heat balance for the thermal processing of plant waste was calculated using these parameters. A thermogravimetric analysis was conducted to identify the most favorable temperature conditions for maximizing the yield of solid residue while also maximizing the exothermic effect. This significantly reduces the heat required for thermal decomposition of waste. The greatest influence on the formation of carbonaceous residues was found to be exerted by lignin. Waste with a high lignin content in its structural composition requires high heat inputs due to the high thermal stability of lignin. The exothermic effect is maximized in the temperature range of 370–400°C at a thermal energy release of 75–156 kJ/kg. Heat balance calculations showed that the thermal energy generated by non-condensable products of pyrolysis of plant raw materials may be sufficient to cover the heat inputs for slow conductive pyrolysis, and an excess thermal energy may be generated.</p>