In biogas production, the drying of biomass is one of the most energy-intensive stages. The Pressurised Superheated Steam Drying (PSSD) method, developed by Research Institutes of Sweden (RISE), enables efficient energy use, a closed-loop process and heat recovery.
RISE has installed a mobile PSSD dryer in the LignoDemo facility at Nordic Paper’s pulp and paper mill in Bäckhammar, Värmland, less than 100 kilometres from the Norwegian border.
LignoDemo serves as a piloting and demonstration environment for technologies related to biorefining and bio-based materials. Its services include, amongst other things, scaling up the production of pure kraft lignin and testing new process technologies for industrial deployment.
As part of these activities, a mobile superheated steam dryer developed by Swedish Exergy AB has been installed at the site. Its purpose is to offer companies the opportunity to assess the performance of the drying technology under real industrial conditions prior to investment decisions and commercial deployment.
Heating above the boiling point
In superheated steam drying, water is heated at a specific pressure to its vapourisation point, forming saturated steam. When the steam is heated further beyond the boiling point corresponding to that pressure, it becomes superheated steam. This superheated steam is used as the actual drying medium.
Moisture evaporating from the material being dried is incorporated into the steam flow. As the process is closed-loop, the water vapour produced can be removed, condensed and utilised as a heat source in other processes. In this way, a significant proportion of the energy used in drying can be recovered and reused.
The mobile dryer installed at LignoDemo has the following specifications:
· Water evaporation capacity: up to 250 kg/h.
· Minimum feed rate: 10–15 kg/h.
· Operating pressure: 0.5–3 barg.
· Operating temperature: 110–145 °C.
· Housed in a standard 40-foot container, enabling mobile operation at various sites.
PSSD technology is suitable, for example, for drying the following materials:
· sludge
· timber
· peat
· coal
· paper and tissue products
· pulp
The dryer can also be integrated into a wider energy system. The recovered heat can be utilised, for example, in a district heating network, in electricity generation or in other industrial processes.
Key benefits
Energy efficiency
The heat contained in the evaporated moisture can be recovered and reused, which reduces energy consumption compared with traditional drying methods.
Closed-loop process
The PSSD operates in a closed loop and produces no direct emissions into the environment. No odour emissions or emissions of volatile organic compounds (VOCs) are generated during the process.
Flexibility regarding energy sources
The dryer can utilise a variety of heat sources, enabling the technology to be integrated into different industrial operating environments and existing energy systems.
Safety
Thanks to the enclosed steam environment, the drying process does not involve the fire or explosion risks typical of conventional hot-air dryers. This is particularly important when handling biomass and dusty materials.
Improved product quality
The use of superheated steam can improve the quality of the end product by reducing oxidation and other undesirable reactions during the drying process.
Challenges
The implementation of PSSD requires careful planning. Integrating the system into existing production facilities may require modifications to the process, automation and heat recovery systems. Furthermore, the investment costs may be higher than for traditional drying solutions. However, these challenges can be mitigated through trials conducted in pilot and demonstration environments.