Investigation of hydrothermal carbonization of wastes and determination of environmental effects

This study mainly aims to understand the potential of producing fuels with high energy content from waste biomass, which is difficult to treat and dispose, through a hydrothermal carbonization method and to determine the environmental impacts of its post-process products. Kitchen waste generated in every living area, municipal solid wastes that cause an increase in sanitary landfills, and sewage sludge, which is one of the most important problems of treatment plants, were chosen as feedstocks for the HTC process. The effects of process parameters such as different temperature (180 °C, 200 °C, 220 °C, 230 °C, 240 °C, 260 °C, 280 °C, 290 °C, 300 °C), holding time (30 min, 1, 4, 6, 8, 12, 16, 24 hours) and waste/water ratio (10%, 25%, 30%, 50%, 75%) on hydrochar quality were investigated. By examining the characteristics of the obtained hydrochars, the contribution of hydrothermal carbonization conditions to dewaterability, carbon content and energy value was investigated. Gas products containing 70%-80% CO2 were formed during the hydrothermal carbonization process. It was observed that the pH values of the liquid product decreased, while the conductivity values increased. Hydrothermal carbonization process water contains an average of 30,000-36,000 mg/lt of chemical oxygen demand. The highest energy content was found to be 31.63 ± 0.19 MJ/kg at 290 °C for 24 hours at a rate of 75% solid in the study conducted with kitchen waste. While the highest energy value in municipal solid waste is 23.49 ± 2.01 MJ/kg in hydrochar obtained at 260 °C for 12 hours at a rate of 30% solids, the highest energy value for urban sewage sludge is 20.93 ± 0.23 MJ/ kg, 50% solid ratio was obtained in hydrochar formed at 220 °C for 12 hours. The increases in energy values were supported by the results of elemental analysis and hydrocarbons with higher carbon content were obtained compared to lignite coal. In the hydrothermal carbonization process, while the carbon contents of all samples increased compared to the input material, it was also observed that the oxygen content decreased. The changes in the waste with the hydrothermal carbonization reaction coincide with the results of FTIR, SEM, XRD analysis. A life cycle analysis has been carried out, emphasizing the need to review the environmental impacts of hydrothermal carbonization. LCA results showed that the combination of HTC and anaerobic digestion processes have a less environmental impact than incineration due to their energy recovery potential.