Process Advantages of Direct Heat Drying
1) Rugged design is well suited for handling high tonnage inorganic materials such as aggregate, sand and foundry waste.
2) Due to the lifting design, DHD dryers can accommodate high moisture organic materials such as distillers’ dried grains, wood, and waste sludge.
3) The rotating system and temperature control allow for mixtures of inorganic and organic materials which are found in recycling systems such as paper and glass.
4) Capable of handling hotter inlet gas temperatures than competing technologies, which results in increased thermal efficiency and smaller air pollution control (APC) systems.
5) Greater ability to handle upset conditions occurring upstream.
Direct heat dryers typically work best when the burner system can operate with a minimal amount of excess air or near stoichiometric balance of the gas and air. This allows the dryer to minimize the air required to carry the thermal energy.
With heat-sensitive materials such as biomass and organic products, the approach temperature of the gas is limited to the temperature the material can withstand without either combusting inside the dryer or giving off excessive VOC from oils being driven off. For this application, either a direct heat dryer with an air heater to control the temperature or the steam tube dryer technology would be the best fit.
For practicality and in order to illustrate the difference in processing organic and biomass products, we chose to illustrate one specific process, which is fairly evenly split between direct heat and steam tube dryer technology: Dried Distillers Grain with Solubles (DDGS). Other material processes can be compared with this model.