Innovative belt drier for fermented residues
Belt drying is a well-known, established process for biomass. However, as Armin Vonplon and Werner Jenewein of the Austrian manufacturer Andritz report, the challenge was to find ways of developing proven sludge drying components in a newly adapted belt drying system.The techniques of closed air circuit, back-mixing, low-temperature drying and product cooling from Andritz's drying process, which had proved reliable and efficient over time, have been successfully remixed into a new concept. The first of these new type plants was commissioned in Altenrhein in Switzerland in July 2002. Within a year of the launch, other contracts were received and filled in Frohnleiten in Austria, Golbey in France, Mollerusa in Spain, Wohlen in the Czech Republic, and Böblingen in Germany.
The main strength of the belt dryer technique is its adaptability to different heat sources. Waste heat, that would otherwise be discarded, such as steam, hot water or flue gas can be used for heating alongside primary energy sources such as natural gas and oil. The process comes in three different variants.
- Direct heating by burner
- Indirect heating with waste heat via heat exchanger
- Indirect heating with primary energy via heat exchanger. While securing optimum thermal efficiency, closed-circulation air loops keep exhaust air to a minimum. Additionally, the process can be carried out with a controlled negative pressure, which prevents the emission of dust and high-odour materials.
The process also minimises water consumption and provides for the dryer's waste air to be cooled in a saturator before being returned to the biofilter for cleaning. In contrast to a condenser, the saturator is optimised for minimum water consumption and, therefore, environmental credit. The remaining process water consumption is limited to the automatic cleaning system for belt washing in a batch system and the tank rinsing system below the dryer.
Overall water consumption is very low, the drying system has practically no effluent and is consequently suitable for greenfield installations. In the drying process, the heated circulation air passes through the product layer and the belt from top down, taking up the moisture from the material.
The special flow pattern in the dryer acts to prevent air and material blending and to reduce the dust freight both in the circulation and exhaust air far below the stipulated limits. A part of the moist air is extracted from the circulating air loop and led to the exhaust air cleaning process.
The dried material is cooled while still on the belt in a separate cooling section, before leaving the dryer and being available for silo storage and further use. The virtually dustfree granulate has unlimited practical uses.
In developing the system, a high degree of automation and simple handling were essential goals: plant operators were to be able to run the system unmanned. Quick start and stop sequences, in addition to the normal starting and stopping modes, allow a simple plant oper-ation mode.