Carbon Sequestration Potential of Manure-Derived Hydrochar Aided by Secondary Stabilization

Hydrothermal carbonization (HTC) is a process that produces a carbon-rich solid from wet organic materials through the application of heat and pressure. Carbonized solids, previously correlated to long-term soil stability, may be considered for carbon sequestration through incorporation into soil. Chars produced by pyrolysis are known for exceptional stability in soil, but pyrolysis is expensive when applied to wet biomass, such as manure. Chars produced from manure by HTC show considerably improved potential for carbon sequestration relative to untreated manure, although not as great as that of chars produced by pyrolysis. This study focuses on producing and evaluating chars by HTC paired with pyrolysis and different methods of chemical oxidation for long-term carbon sequestration in soil. It is shown that a two-step process of pyrolysis following HTC produces a char that outperforms those produced by either individual process (HTC or pyrolysis) in carbon yield, carbon content, and, more importantly, soil carbon sequestration potential. It was found that acid-catalyzed HTC followed by pyrolysis resulted in a char with a 13% increase in carbon yield, a 51% increase in carbon content, and an atomic O/C ratio 64% smaller than the char produced by conventional pyrolysis.