Improving hydrothermal carbonization (HTC) processes by hydrochar gasification

Abstract

Hydrochar is produced by means of hydrothermal carbonization at relatively low temperatures (180-260°C) in sub-critical water. While, in many respects similar to biochar, its physical and chemical properties differ significantly (Basu, 2018). Of particular interest for this work are the higher energy density and lower ash content, that make hydrochar a possible feedstock for gasification plants with energy generation purposes. A gasification step is used instead of direct combustion to convert solid fuels in energy to achieve cleaner combustion and higher efficiency (Wang and Stiegel, 2016). In this work a 400 kg/h hydrochar gasification plant was modelled to identify optimal conditions and energetic yield of hydrochar obtained from municipal sewage. The fixed bed, updraft, gasification reactor was modeled in detail using a multi-scale, multiphase methodology already widely tested on biomass (Corbetta et al., 2015; Ranzi et al., 2014). The gas solid kinetic model was coupled with a detailed gas-phase kinetic scheme with over 200 species, including reaction intermediates, and 2000 reactions for reliable product yield prediction. Using Visual Basic Application as an interface, the predictions from the detailed simulation package were delivered to a commercial simulation package to model the energy generation section of the plant. Aspen HYSYS V10 was used for this purpose for the simplicity of integration and its widespread use in similar industrial plants. The gasification was carried out with air, air enriched in oxygen to 28%, air enriched in oxygen to 35% and pure oxygen with different amounts of steam to control the temperature in the chamber and at different values of equivalence ratio. The gasification performance was evaluated in terms of lower heating value of the generated fuel gas while the H2S formation was accounted for only in a superficial manner using rules of thumb derived from previous experimental experience.