Thermochemical processing of digestate from biogas plant for recycling dairy manure and biomass

This paper describes the experimental results of the thermal decomposition of the digestate obtained as a result of anaerobic digestion of dairy manure and dry biomass of Amaranthus retroflexus L. The high yield of Amaranth green mass in the main soil and climate zones of Russia is its advantage over other crops. The ratio of amaranth stems, amaranth leaves, and dairy manure for volatile solids was equal to 1:5:16. Amaranthus retroflexus L. is a weed; therefore, a thermochemical digestate processing is proposed to avoid the seed integrity. The higher heating value (HHV) of digestate was 18.6 MJ/kg. The HHV of the char residue was 19 MJ/kg. In the inorganic part of the digestate sample, the oxides CaO, SiO₂, and K₂O are prevailed. Thermal decomposition of digestate was studied in the temperature range 25–1000 °C in an inert atmosphere using thermogravimetric and differential scanning calorimetry (TG-DSC) at the heating rates of 5, 10, and 20 °C/min. Char residue mass was ranged from 39.8 to 41.08%. Consequently, the rest of the organic matter components pass into the pyrolysis gas and liquid phase. Additionally, studies of the digestate pyrolysis process were carried out in a laboratory setup (in an inert atmosphere) at the heating rate of 10 °C/min and the temperature of 550 °C. As a result, 31% of pyrolysis liquid, 28% of gas, and 41% of char residue were obtained. The HHV of the char residue increased on 4.8% compared with the initial value of digestate. The main components of the pyrolysis liquid are acetic acid (71.44%) and propionic acid (6.12%).