Conversion of agro-wastes into energy can be key to a circular-driven economy that could lead to models for sustainable production. Thermochemical processing is an interesting alternative for the upgrading of agro-wastes to energy. However, owing to the complex and largely unknown set of reactions occurring during thermal breakdown, to ensuring consistent quality of the final products is still a goal to achieve at industrial level. The present study investigates the evolution of solid products of pyrolysis, to gain some insights in these complexities. Chars derived from slow pyrolysis (200–650 °C) of citrus pulp in a horizontal reactor have been characterized by means of Fourier Transform Infrared spectroscopy (FT-IR), X-Ray Diffraction (XRD), Thermo Gravimetric Analysis (TGA) and Scanning Electron Microscopy (SEM). Results are discussed also in light of similarities with coal thermal breakdown. At temperatures below 300 °C, changes in solid matrix are mainly due to breaking of aliphatic compounds. Significant changes in char structure and behavior then occur between 300 °C and 500 °C mainly related to secondary char-tar reactions. Above 500 °C, changes appear to occur mainly due to recombination reactions within matrix, which thereby becomes progressively less reactive.