(25d) Hydrothermal Carbonization of Organic Fraction of Municipal Solid Waste and Corresponding Digestate Via Anaerobically Digestion

Steam autoclaving is a technology that has been around since the 80â??s and can efficiently divide the biogenic and non-biogenic fractions of Municipal Solid Waste (MSW) for best re-use and upgrading. Autoclaving eliminates hazardous hand sorting of raw garbage and in the process of separating the pulp fibers, sterilizes the output and allows for sequestration of pollutants that could otherwise leach into the groundwater or the atmosphere over time. The organic fraction is recovered by trommel screen (accepts <3/8â?) and is comprised of a moist pulp matrix embedded with ash and debris. The food waste from MSW is solubilized and absorbed onto the pulp matrix, creating a unique feedstock for further thermochemical processing (e.g., hydrothermal carbonization, HTC) biological processing (e.g., anaerobic digestion, AD), or a combination of both.

Hydrothermal carbonization (also called thermal hydrolysis, or wet torrefaction) is a treatment process which converts moist feedstocks into homogenized, carbon rich, and energy dense solid fuel, called hydrochar. One of the main advantages of HTC compared to other thermochemical treatment processes is the use of moisture as reaction medium and reactant, thereby precluding the need for drying prior to treatment. Thermodynamic properties of water change greatly in the subcritical region from 180-280 °C, and as a result, subcritical water behaves as a non-polar solvent and mild acid and base catalyst simultaneously. When subjected to HTC, biomass undergoes rapid hydrolysis and other reactions, releasing oxygen-containing volatiles and producing a highly hydrophobic hydrochar. Aqueous products typically include organic acids, sugars, and furans.

In this study, the autoclaved organic fraction of municipal solid waste pulp (OFMSW) and the digestate from OFMSW pulp after AD were processed by HTC at 200, 250, and 300 °C for 30 min and 2 h. The focus of this work was to evaluate the potential for producing an energy-dense, stable, solid hydrochar by HTC from either OFMSW or its digestate. The fate of inorganics, especially the heavy metals in hydrochar is discussed. In addition, the aqueous products were analyzed to identify the chemicals produced and the chemical changes during HTC. A carbon balance was performed from the solid and liquid phase analyses. Moreover, two conceptual MSW treatments consisting of autoclaving with or without AD, but including HTC were proposed and mass and energy balances were performed. The proximate, ultimate, FTIR, and fiber analyses show that the fuel value of the hydrochar increases with increasing HTC temperature. Inorganics have a strong propensity for the hydrochar matrix, and are found only in small amounts in the aqueous phase. The fuel content is as high as 44.7 MJ kg^-1 (dry ash-free basis) when HTC digestate was treated at 300 °C for 2h. A cascaded design including AD and HTC yields a higher net energy production than HTC or AD alone.