Effects of different reaction conditions on the characteristics of hydrothermal carbonization products from black liquor

Black liquor, a primary waste product from pulping and paper making, contains substantial amounts of inorganic components such as free alkali and sulfides, along with organic substances like lignin and hemicellulose. As a result, black liquor presents both significant environmental challenges and considerable potential for resource recovery. Hydrothermal carbonization (HTC) offers an effective route to overcome the concentration limitations of traditional alkali recovery combustion technology, enabling the solidification of dissolved organic matter in black liquor and in-situ upgrading to produce high-value solid carbon materials. By simultaneously concentrating carbon components and controlling the migration of inorganic species from black liquor, HTC provides a promising pathway for the coupled resource valorization and pollution mitigation of black liquor. Accordingly, this study investigates the application of HTC for black liquor treatment and carbon recovery. The effects of hydrothermal temperature (240-320 ℃), holding time (0.5-8 h), black liquor concentration (5%-57.24%), initial pH (1-8.8), and co-HTC (co-hydrothermal carbonization) with corn straw on product characteristics were systematically investigated. The characteristics of the collected hydrochar (yield, proximate analysis, C content, and HHV) and the aqueous phase (SCOD) were thoroughly analyzed. And based on these measurements, quantitative calculations of carbon recovery rate, energy recovery rate, and SCOD degradation rate were performed to comprehensively evaluate the dual benefits in achieving both resource recovery and harmless disposal of black liquor. Results showed that the characteristics of the hydrothermal carbonization products were significantly affected by changes in hydrothermal temperature, time, black liquor concentration, initial pH, and co-HTC with corn straw. At different hydrothermal temperatures, the yield of hydrochar did not vary significantly and remains generally low (around 15%), but increasing hydrothermal temperature effectively improved the fuel quality of hydrochar. When the hrdrothermal temperature reached 320 ℃, the carbon content and HHV of hydrochar increased from 63.11% and 25.87 MJ/kg at 240 ℃ to 83.85% and 31.26 MJ/kg, respectively. The effect of holding time was similar to that of hydrothermal temperature, but to a lesser extent. Black liquor concentration was positively correlated with hydrochar yield. However, excessively increasing the black liquor concentration lead to an accumulation of ash in the hydrochar (7.96%-17.44%) and an increase in the organic load of the aqueous phase (33.39-320.10 g/L). Lowering the initial pH to 1 significantly increased the hydrochar yield to 22.72% and reduced the ash content to 1.43%, representing an ash removal efficiency of 99.36%. Furthermore, the maximum carbon recovery rate (65.71%), energy recovery rate (54.66%), and SCOD degradation rate (56.06%) were achieved at a relatively low temperature (240 ℃), which mitigates the energy consumption issues associated with high-temperature hydrothermal treatment. In addition, co-HTC with corn straw effectively addressed the low yield issue of hydrochar derived from black liquor. When the black liquor concentration exceeded 10%, the process exhibited significant synergistic effects in terms of hydrochar yield, carbon recovery, energy recovery, and SCOD degradation rate. These results indicate that HTC enables simultaneous resource utilization and pollution mitigation for black liquor. Given that 40%-60% of carbon remains in the hydrothermal liquor, coupling HTC with anaerobic digestion (AD) in a closed-loop configuration represents a viable pathway for achieving deeper contaminant removal and diversified resource recovery from black liquor.