Low-Temperature Hydrothermal Carbonization of Fresh Pig Manure: Effects of Temperature on Characteristics of Hydrochars

The impacts of temperature on the characteristics of hydrochars from pig manure (PM) following hydrothermal carbonization (HTC) at 140°C–220°C for 1 h were investigated. The total organic carbon (TOC) content was 35.7%–39.1% and the total contents of nitrogen (N), phosphorus (P), and potassium (K) were 7.9%–8.1% in the hydrochars with yields of 97.4%–53.5%. The hydrochars had high nutrient recovery rates, and the contents of TOC, N, P, ash, and fixed carbon increased whereas those of K and volatiles decreased with increasing temperature. Fourier transform infrared (FTIR) spectroscopy and mass ratio of hydrogen to carbon (H/C) showed continuing aromatization of the hydrochars with increasing temperature. The characteristics of the hydrochars and the derivative thermogravimetry curves confirm that the composition of the hydrochars changed dramatically at 160°C–180°C, which was found to be a suitable temperature for low-temperature hydrothermal carbonation for fresh PM. However, the hydrochars were weakly acidic and the concentrations of Cu and Zn exceeded the standard limits for organic fertilizers.