Abstract
Dairy manure is rich in nutrients but presents significant management challenges due to its potential for environmental contamination. Hydrothermal carbonization (HTC) is emerging as a promising alternative for sustainable dairy manure treatment, attracting significant research interest for its ability to produce value-added products such as hydrochar. Hydrochar is a partially carbonized material produced under moderate temperatures and pressure. It contains a substantial amount of macro- and micro-nutrients, including a high phosphorus (P) content, which can be utilized as an organic fertilizer for nutrient cycling back to the soil. The objective of this research work is to develop a conceptual continuous HTC process for converting dairy manure into hydrochar and explore the economic feasibility of the system as an alternative technology for sustainable manure management. Numerous runs were performed using a 300-mL batch reactor to produce hydrochar as part of the preliminary experiments. The hydrochar produced at five temperatures (195-255℃) were evaluated on its physiochemical properties relating to those for land applications. The surface of hydrochar was enriched with oxygen-containing functional groups, beneficial for holding moisture and nutrients. P in manure remains largely in the hydrochar after HTC along with major nutrients. Applying hydrochar to cropland can enhance soil nutrient concentrations, positively influencing plant physiological and biochemical processes.
Following the experiments, a conceptual process was developed capable of handling dairy manure slurry with 10% solids produced on a medium size farm with 1000-head of lactating cows which equates to approx. 38,000 tons of manure per year. The process design includes a detailed flow diagram, equipment selection and sizing, and integration of unit operations such as feed preparation, working media heating, HTC reaction, solid-liquid separation, product de-watering and/or drying, etc. Techno-economic assessment (TEA) was performed to evaluate the economic feasibility of the continuous process. The minimum selling price (MSP) of the hydrochar product was found to be $1.10/kg to reach breakeven for a 20-year project life. This cost could be reduced to $0.85/kg if existing manure management expenses are excluded. An increase of 25% to 50% in hydrochar price can reduce the payback period (PBP) from 8 to 5 years and raise NPV from $2.50 to $4.91 million. Results of the Monte Carlo simulations show product price has the greatest impact on profitability.
This work demonstrates that continuous dairy manure management via HTC is technically feasible, commercially viable, and scalable, while recovering value-added resources in the form of hydrochar. Although the technology is not economically feasible yet comparing to the current market scenario, HTC process remains a promising option with stricter environmental regulations, growing demands for time- and space-effective manure management technologies, and soil amendments for organic farming.