Hydrothermal processing
| Technology | |
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| Technology details | |
| Name: | Hydrothermal processing |
| Category: | Primary processing |
| Feedstock: | Feedstocks with high moisture content. Food waste, Garden and park waste |
| Product: | Bio-oil, biogas, bio-char |
Hydrothermal processing, also known as Hydrothermal Upgrading (HTU), is a thermochemical conversion process that is used to convert biomass into valuable products or biofuel. The process is usually performed in water at 250-374°C under pressures of 4-22 MPa. The biomass is degraded into small components in water. Based on the target products, which are bio-oil, biogas or bio-char, the process conditions (e.g., temperature, pressure and residence time) are chosen. One of the most important advantages of hydrothermal processing is that it can use biomass with high moisture content withouth the need for pre-drying. Hydrothermal processing can be divided into three separate processes, depending on the severity of the operating conditions. These include hydrothermal carbonisation (HTC), hydrothermal liquefaction (HTL), and hydrothermal gasification (HTG).
Feedstock
Origin and composition
Feedstock with high moisture content are particularly suitable for hydrothermal processing and include feedstocks such as anaerobic digestion digestate, manures, sewage sludge, DDGS, food waste, municipal wastes, and aquatic biomass such as micro- and macroalgae. Hydrothermal processing routes can typically feed slurries up to 30 wt.% solids.
Process and technologies
A hydrothermal process is usually performed in water at 250-374°C under a pressure of 4-22 MPa. The process can also be carried out under self-generated pressure. The hydrothermal process is divided into two reaction conditions, namely subcritical and supercritical water conditions. These two conditions are determined by the critical point of water (i.e., 374°C and 22.1 MPa). Subcritical water is classified below the critical point at a 100-374°C temperature range and under sufficient pressure to remain liquid. Supercritical water occurs when the temperature is above 374°C and the pressure is above 22.1 MPa. The decomposition steps of biomass during the hydrothermal process can be summerized as follows: at approximately 100°C, the water-soluble portion of the biomsas disperses into water, and hydrolysis takes place above 150°C. Meanwhile, biomass polymers (i.e., cellulose and hemicellulose) disintegrate into heir monomeric chains. At approximately 300°C and 10 MPa, liquefaction occurs and bio-oil is obtained.