Mass and energy equilibrium analysis on co-hydrothermal carbonization coupled with a combined flash-Organic Rankine Cycle system for low-energy upgrading organic wastes

Co-hydrothermal carbonization (co-HTC) of sewage sludge (SS) with corn stalk (CS), as well as process heat recovering method, was proposed by this study for converting the biomass wastes to alternative solid fuel in an economical manner. The mass and energy equilibrium characteristics of the whole process were focused. Results indicated that the higher heating value (HHV) of hydrochar increased by increasing HTC temperature, CS mixing ratio, and reaction time. However, the energy consumption of HTC process would increase with increasing HTC temperature. A flash-organic Rankine Cycle (FSPG-ORC) system was coupled after HTC to recover the heat contained in the HTC slurry product, so as to improve the energy efficiency of HTC process. Slurry product (190 degrees C-280 degrees C) was first flashed and 5.62%-20.5% of moisture in slurry product was evaporated to drive a generator, while the remaining slurry was used as a heat source to evaporate organic working fluid which then drive another generator to generate power. After treating 1 ton/batch of SS-CS mixture under a typical HTC condition (HTC at 220. C for 2 h, and SS:CS = 1:1), 125.1 kg of hydrochar with 15% moisture content was produced, and 114.59 MJ of waste heat contained in HTC slurry product was recovered by FSPG-ORC system. Ultimately, the energy ratio of the whole HTC-FSPG-ORC process reached 24.11%. HTC-FSPG-ORC coupled process converted organic wastes into lignite-like solid fuel in a low-energy manner due to the waste heat was stepwise recovered, and therefore showed a good application prospect.

Automated summary

The study proposed a co-hydrothermal carbonization (co-HTC) method for converting sewage sludge and corn stalk into solid fuel, focusing on the mass and energy equilibrium characteristics. The energy efficiency was improved by coupling a flash-organic Rankine Cycle (FSPG-ORC) system to recover the waste heat. The HTC-FSPG-ORC coupled process showed a good application prospect by converting organic wastes into lignite-like solid fuel in a low-energy manner.