Hydrothermal carbonation (HTC) is an energy efficient process, where for example wet sewage sludge can be upgraded to hydrochar and separated from the water after the process. RISE has expertise and equipment around this both in Stockholm (lab scale) and Örnsköldsvik (pilot scale at RISE Processum).
Hydrothermal carbonisation is a technique by which a moist biomass can be processed to a more valuable material. The process involves heating the biomass for several hours in an aqueous solution to 200-250°C under high pressure to avoid boiling. Chemical additives, such as acid or alkali for pH-adjustment, or catalysts, can be added. HTC has gained a wide interest as a method for upgrading various low-value streams, e.g. industrial waste of biological origin, to an attractive biofuel material. Not only does the HTC technique lead to increased carbon content and heating value of the stream; it also allows the user to improve its composition by controlling the distribution of some unwanted non-process elements. This way, the biofuel material can be adapted to a particular combustion unit.
The most interesting application of HTC technique in a pulp and paper mill is treatment of biosludge from the effluent treatment plant. Although rich in combustible material, biosludge often has high moisture content and poor dewatering properties. After HTC, sludge becomes more compact and easier to dewater; the content of chlorine and alkali metals decreases making it more suitable for combustion in both bark and recovery boilers. Other potential application areas include treatment of bark for other uses than bark boiler fuel and upgrading wood yard residues to valuable solid fuels.