Waste to biofuels through zero-energy hydrothermal solar plants: Process design

Hydrothermal processes enable an effective conversion of waste biomasses into fuels and carbonaceous materials. Covering the heat requirements with concentrated solar energy is a clever strategy to increase the plant efficiency and pursue the principles of circular economy. With the purpose of producing liquid and solid biofuels through zero-energy routes, this work presents two conceptual designs for integrating a concentration solar system (CSS) with a hydrothermal liquefaction (HTL) and a hydrothermal carbonization (HTC) plant. The solar configuration used to cover hydrothermal heat requirements consists of a set of parabolic trough collectors operating with molten salts, which are used as both thermal carrier fluid and thermal energy storage medium. Two different scenarios were modelled to continuously process wood and organic wastes. In the first scenario, the CSS is coupled with a continuous HTL reactor (operating at 400°C and 300 bar) followed by thermal cracking and hydroprocessing for upgrading bio-crude to a saleable liquid biofuel. The second scenario considers the CSS operating with a continuous HTC reactor (working at 220°C and 24 bar) for the conversion of organic wastes into a solid fuel (hydrochar). The CSS and both the hydrothermal plants were modelled based on experimental data. Energy consumptions and techno-economic aspects were investigated.