Hydrothermal carbonization vs. anaerobic digestion to valorize fruit and vegetable waste: A comparative technical and energy assessment

Abstract

Herein, the valorization of vegetable and fruit waste was assessed via hydrothermal carbonization (HTC) and anaerobic digestion (AD) in terms of product characterization and energy requirements. HTC was conducted at reaction temperatures between 150 ºC and 190 ºC, and residence times between 20 min and 40 min. The increase in the process severity resulted in hydrochars with higher carbon contents and higher energy densification ratios. AD was performed in two different ways. i.e., batch and semi-continuous reactions. From the batch experiments a methane yield of 300 L CH4/kg VS was obtained, while for the semi-continuous, the average specific methane production estimated (for HRTs from 75 to 50 days) was 213 ± 32 L CH4/kg VS. To estimate the energy requirements, mass and energy balances were performed considering the basic stages of each process to obtain a suitable biofuel material. In this sense, it was concluded that for this specific waste, AD was a more suitable process with a positive energy net balance. On the contrary, HTC presented a negative energy net balance being required 1.29 MJ/kg of fresh food waste. A combined HTC-AD treatment may be an efficient method to take advantage of both technologies leading to higher energy efficiencies and other valuable products.