Hydrothermal Carbonization of the Wet Fraction from Mixed Municipal Solid Waste: A Fuel and Structural Analysis of Hydrochars

Abstract

One of the by-products of a mechanical-biological waste treatment plant is the under-sieve fraction, which requires separation prior to further processing of municipal mixed waste. This stream usually follows the fate of landfilling. Instead, it could be introduced to the hydrothermal carbonization (HTC) process to improve its fuel properties and become solid recovered fuel. The organic fraction and high moisture content (approximately 26%) of under-sieve fraction are favorable properties for the HTC process. In this study, hydrochars, the solid product of HTC, were produced at 200 and 220 °C with residence times of 1, 4, and 8 h. The main aim of this investigation was to establish the influence of different process parameters on hydrochars’ fuel properties. Thermogravimetric analysis (TGA), Fourier-transform infrared spectroscopy (FTIR), and scanning electron microscopy (SEM) were employed in the analyses. The results confirmed the positive effects of hydrothermal carbonization on the under-sieve fraction of municipal mixed waste properties. The ignition temperature increased from 247 °C to 288 °C and burnout temperature decreased to 443 °C from 489 °C after hydrothermal carbonization, causing a shorter combustion process. The determined key combustion parameters were: S = 12.4 × 10−8%·min−2·°C−3, Hf = 1174.9 °C and Di = 0.0075%·min−3, which in comparison to USF decreased by 44%, increased by 33%, and decreased by 29%, respectively, and became closer to those of coal. Furthermore, the identified structural changes indicate that hydrochars could be successfully used in energy production. The most promising results were found for hydrochar produced at 220 °C for 1 h, leading to a better combustion performance and providing a more stable and a less violent flame.

Hydrothermal Carbonization of Digestate Produced in the Biogas Production Process

“…A significant aspect is energy recovery from the by-products of the HTC process. Both hydrochar and process water can be used for energy supply [52]. Process water contains various kinds of sugar, volatile fatty acids, and aromatic organic compounds.…”

“…The highest percentage of carbon in hydrochar was obtained for a residence time of 8 h [54]. Śliz et al [52] researched the HTC wet fraction from mixed municipal solid waste. The effect of temperature and residence time on hydrochar was investigated.…”

“…The effect of temperature and residence time on hydrochar was investigated. Temperatures of 200 • C and 220 • C were implemented, as well as residence times of 1, 4 and 8 h. The best hydrochar parameters were obtained at 220 • C for 1 h [52]. The HTC process can be used to increase biomethane production.…”

Thermal Disposal of Post-processing Water Derived from the Hydrothermal Carbonization Process of Sewage Sludge

“…HTC reduces the volume of waste and enables the recovery of valuable components. In addition, the process improves the dewatering of waste as well as the fuel properties of solid products [9,10].…”

Influence of Hydrothermal Carbonization on Catalytic Fast Pyrolysis of Agricultural Biomass

“…Moreover, the process of hydrothermal carbonization also removes a part of the inorganic fraction of biomass [54,55]. Hydrothermal carbonization also has an influence on the morphology of the hydrochars [56,57], and, as a consequence, makes the latter a good precursor for the production of activated carbon [58,59] and also for direct application to soil or compost [60,61]. Furthermore, positive influence of the HTC process on biomass grindability should not be overlooked, as particle size is an important aspect of pyrolysis [62][63][64].…”