Thermogravimetric analysis and carbon stability of chars produced from slow pyrolysis and hydrothermal carbonization of manure waste

Large amounts of manure waste are produced worldwide. Land application of manure waste is the most common approach to valorization. This practice has several environmental concerns, including pathogen spreading, emissions of odorous chemicals (ammonia or methane) and the pollution of waterbodies with nitrogen and phosphorous compounds. The pyrolysis of manure for biochar production has been tested as a useful treatment to avoid these concerns but the high moisture of some wastes hinders its thermo-chemical valorisation in the absence of a pre-drying step. Recently, hydrothermal carbonization (HTC) of wet biomass has been identified as a cost-effective valorisation method of biomass without the need for energy-intensive drying processes. The objective of this study is to compare the properties of pyrochars and hydrochars prepared from thermal treatment of rabbit manure (MW). Pyrochars were prepared at 450 degrees C (BMW450) and 600 degrees C (BMW600) and hydrochars were obtained using a rabbit manure solution (solid: water ratio 30: 70) that was heated at 190 degrees C (HMW190) and 240 degrees C (HMW240). The final temperature was maintained during 1 h for pyrochars and 6 h for hydrochars. Results showed that HTC generated more micro-, meso-and macro-porosity than pyrolysis. Pyrochar produced at 600 degrees C possessed a high thermal, chemical and biological stability, probably due to its highly aromatic structure. Hydrochars (HMW190 and HMW240) were predominantly aliphatic, and this was associated to a lower chemical and thermal stability than pyrochars. The germination index values indicated that the pyrochar BMW450 and the two hydrochars were highly phytotoxic while pyrochar MWB600 presented a moderate phytotoxicity, which was lower than the manure waste. Finally, CO2 emissions of different materials after 60 days followed the order: MW > HMW190 > HMW240 > BMW450 > BMW600. Specifically, pyrolysis evolved CO2 was reduced between 97.8-88.7% with respect to raw material after pyrolysis. The HTC treatment diminished evolved CO2 between 68.8-59.0%, with respect to the manure. These results indicate that both processes can be considered as carbon fixation technologies.