Biosludges and MWW sludges are known to be extremely tricky to dry because of their tendency to form glue phase. Municipal Waste Water Sludge is generally an extremely tough application with severe wear and tear on the drying equipment, but the Fjell Turbo Disc Dryer has proven to resist the extreme conditions since the first unit was installed at I.V.A.R – Stavanger in 2001
Fjell Technology Group offers following dryer solutions for sludge:
- Part drying in Fjell Turbo Disc Dryer
- Full drying in Fjell Turbo Disc Dryer
- Full drying in TMD friction dryer
- Part drying in Fjell Turbo Disc Dryer followed by full drying in TMD friction dryer
Part drying in Fjell Turbo Disc Dryer
In part drying, or scalping, the challenge is often the content of chlorides and acids, which can cause severe corrosion problems with the rotor. A particularly annoying problem in the industry has been stress corrosion cracking in welds. This is avoided in the Fjell Turbo Disc Dryer because of the unique design and manufacture:
- Welds not exposed to external environment in dryer, thereby risk for stress corrosion cracking is eliminated.
- Welds do not seal between pressurised and non-pressurised side. Thereby risk for steam leakage is eliminated.
- Claws act as reinforcement beams in radial direction, thereby increasing the lateral bending strength of the discs, which reduces the risk for stress corrosion cracking in foot welds between discs.
Full drying in Fjell Turbo Disc Dryer
A classical full drying process with recycling of dry solids is shown in the flowsheet below. We have delivered such processes to plants in Norway, Italy, Saudi-Arabia and Japan.
Full drying in TMD friction dryer
Full drying in the TMD friction drier is the most robust, compact and simple way to dry “difficult to dry” sludges. Biosludge in any form is known to be extremely difficult, because of the formation of strong glue phase. This is avoided in the TMD dryer.
The characteristics of the process are:
- Extremely compact process
- Minimal generation of exhaust gas
- Inert atmosphere in the process
- Low maximum occurring temperatures (<110°C)
- Fundamentally hazard free process
- Minimal risk for self-ignition in dried material
- Meets US Class-A sludge requirements
- High thermal efficiency (>95 %)
- No recycling of dried material required.
- Low sensitivity to material properties such as glue phase and dry solids content
- No heating medium like steam, hot oil or hot air required
- Minor man-hour requirements during operation
- Very fast start up and shut down
The only disadvantage with the process is the fact that the energy of vaporisation of water needs to be supplied in the form of mechanical energy. That means either electrical power or a gas or diesel engine. However, in a waste water treatment plant excess amounts of bio gas are normally produced, and this can be used as energy source for the TMD via gas engine or turbine generators. Moreover, the vapour out of the TMD can be pure water vapour, which means valuable waste heat can be recovered and reused in other energy demanding systems of the waste water treatment plant.
Part drying in Fjell Turbo Disc Dryer followed by full drying in TMD friction dryer
An optimum configuration of the dryer technology we offer is a process where the Fjell Turbo Disc Dryer is used for scalping of the sludge to a state right below the glue phase. The still wet sludge is then pumped to the TMD where the drying process is completed. With this configuration the waste heat from the TMD can be recovered and used in the disc dryer. This is possible by operating the disc dryer under a pressure of 0.3 bar absolute or so, allowing boiling to occur at approximately 60°C.
In this process heat is used twice, so that energy usage is reduced to 0.3 kWh per kg water evaporated.
Another important advantage is that the disc drier has a dust free low pressure water vapour atmosphere inside. There is absolutely zero risk for fire or dust explosion in under such conditions.
Same for the TMD process. This operates below 105°C in a 90% water vapour atmosphere. Also here the risk for fire or explosion is practically zero, but if as a result of an extremely rare event this should happen, the TMD process mill is so compact and rigid that an ignition inside will not at all disturb the operation.