The desire to increase the role of renewable biomass resources in the energy sector sets the task of finding promising areas for its resource-efficient use. Pyrolytic conversion (pyrolysis) of biomass can be considered as one of such directions. The efficiency of pyrolysis depends on the possibility of its implementation in the autothermal mode. In this regard, the purpose of this work is to study the thermal conversion of biomass in the process of slow low-temperature pyrolysis in relation to its implementation in a fixed bed reactor. Physical experiment methods, differential thermal analysis and electron scanning microscopy were used in the work. As a result of the study, it was shown that in the process of straw and peat low-temperature pyrolysis (heating rate of 10 °C/min) a thermal exothermic decomposition effect was observed when the reactor was heated to 500 °C. This effect led to an increase in the rate of heating of the biomass bed. Moreover, in the case of straw pyrolysis, the temperature in the bed began to exceed the temperature of the reactor wall (up to 55–60 °C) when heated above 303 °C. The total values of the exothermic effect of straw and peat pyrolysis in the temperature range of 170–600 °C were 1,475 kJ/kg and 862 kJ/kg, respectively (based on the dry mass of the feedstock). The scanning microscopy method shows the change in the biomass structure during the pyrolytic decomposition process.