Flexible and low grade energy use

The patented Andritz 3Sys AG Belt Drying System (BDS) is characterized by its flexibility and ability to use a wide range of energy sources, particularly low-grade waste energy. BDS can be used for drying a broad range of products, such as industrial and municipal sludge, cow manure, biomass, as well as pre-treated municipal solid waste, and so on.

The strengths of the system

• Plant performance up to 10 t/h water evaporation per drying line >> fewer lines; less maintenance, lower costs.
• For capacities over 3 t/h, concrete dryer casing >> robustness; no corrosion; no extra building required
• Low cost and easy maintenance due the technical concept and easy access to all elements
• Simple operation with quick start and quick stop features
• Plant in full compliance with ATEX Safety regulation

BDS Plant in Nufri, Spain

• Optimum conditions for waste heat utilization
• Product cooling integrated into drying process
• High thermal efficiency due to air recycling rate for drying air
• Easily adaptable to different sludge types coming from various sewage plants and variable back mixing

The following additional advantages are achieved by using the waste heat recovery option:

• No thermal energy cost, significant reduction of operating costs
• Sustainable solution with no influence of possible future energy price rises, conservation of primary energy sources
• Environmental friendly concept,considerable reduction of green-house effect
• Optimum conditions for partial or complete integration of recycled exhaust heat, such as flue gas, steam, hot water, and so on, from cogeneration plants, waste incineration plants, power plants and other processes

View from the final product discharge end during normal operation

Product path

Mechanically dewatered sludge and already dried sludge are fed to a feed / mixing screw conveyor. The mixed sludge (approx. 60% DS) is distributed over the entire surface of the drying belt by means of a dosing/distribution screw conveyor. A homogenous layer thickness (4-15 cm, adjustable over the belt width) is achieved by this screw conveyor, as well as by a height-adjustable winder.

The mixed sludge is heated and dried by the hot air recycled from the dryer.

After passing through the drying zone, the product has a dry solids content of over 90% and is cooled down in a cooling zone. At the end of the belt, the dried product is discharged via a product screw conveyor and transferred for further use or backmixing.

BDS Plant in Wohlen, Switzerland

Drying air

The drying air is heated to an inlet temperature of 120-150°C. The air flows through the sludge layer, absorbing the moisture from the product. To achieve optimum thermal efficiency, the dryer is operated with a high recirculation rate. Thus, a large amount of the drying air is recycled to the heat generating and heated again. One part of the circulating air is transferred to the condenser and a chemical scrubber, where acidic and basic pollutants are removed. All dryer components are operated under low pressure, avoiding dust and odour emissions.

Heat generation

The dryer air is either heated directly by the hot gases produced by combustion of natural gas or indirectly by heat exchangers utilizing waste energy from various processes, such as exhaust gases from cement kilns, Waste-to-energy plants, gas turbines or gas motors for power plants, as well as hot water from gas motors. In a combustion chamber the hot gases from the gas burner are mixed directly  with the cooled dryer air and routed back to the dryer or in case of the waste heat the dryer air is heated indirectly by a heat exchanger, placed usually at the top of the dryer. The dryer air passes through the wet sludge layer and cools down again. The surplus vapours, produced by the evaporation process, are cleaned with the condenser and scrubber system.

Due to the arrangement of the fans after the drying belt, all dryer components and ancillary facilities in contact with sludge or product to be dried are maintained at a slight negative pressure. As a result no dust or odor can be emitted to the environment.