High-Temperature Processing with Calciners
Rotary calciners can provide efficient high-temperature processing and high heat transfer rates for bulk solids materials.
Used to heat materials that are fine and dusty, sensitive to oxidation, combustible, explosive, potentially contaminating or thermally sensitive, rotary calciners
Indirect Heat Source
Small Off-Gas System
isolate the materials to be processed in a high-temperature chamber. The calciner’s indirect-heat-source design ensures complete separation between the heat source and the product during processing. In applications from catalyst manufacturing, hydrocarbon processing and wood biomass torrefaction to chemical-making and precious metal recovery, as companies look for materials that have finer particle sizes and uniform quality, rotary calciners deliver the performance needed.
A rotary calciner consists of an alloy steel cylinder that rotates inside an electrically or gas-heated furnace. A material feeder continuously introduces the product inside the cylinder, which is turned by an external drive mechanism. As the cylinder rotates, the material moves into the heated furnace, within which the atmosphere inside is kept at a constantly controlled, predetermined condition. To close off the ends of the cylinder, stationary housings are used. The upstream housing allows manufacturers to hook up product-feeding equipment, and the downstream housing provides a connection for off-gas cleaning equipment to collect and process vapor emitted from the product. After exiting the furnace, the product can enter a cooling section if needed. Finally, it discharges from the calciner through the downstream housing.
A calciner performs all high-temperature processing — in the range of 1,000 to 2,100°F (538 to 1,149°C) — within its controlled atmosphere. It is capable of controlling the chamber to the exact atmosphere requirements of any type of material. With this capability, the calciner can provide enhanced characteristics to the finished product.
The heat source for a calciner is either electric resistive wires or fuel-fired burners. The heat source heats the surface of the cylinder. In turn, the cylinder heats up the material by radiant heat transfer, indirect heat transfer, and direct contact of the product and the hot surface of the cylinder.
Generally, in a calciner, the furnace heat is operated in zones, or groups. Each zone operates independently and controls one stage of the heating process. The stages include:
• Drying stage, during which the product is dried of its free moisture.
• Calcining stage, during which the product is elevated to the production temperature.
• Soaking stage, during with the temperature is maintained for a preset time to complete the process.
• Cooling stage, during which the product is cooled to a safe temperature for handling once it has been processed.
This zonal arrangement allows each processing stage to act independently without adversely affecting the following stage. As an example, the product moisture may fluctuate, requiring more drying before the calcining stage. The increased moisture level signals the furnace to raise the heat input in the drying zone. The moisture level of the material exiting the drying stage is equalized to its design condition for the calcining zone rather than allowing fluctuating moisture levels to change the calcining zone’s operating efficiency.
Calciners incorporate features such as temperature profiling and indirect heating to achieve effective processing. Another feature is a small off-gas system.
Temperature Profiling. Because the calciner has multiple heat inputs, the heat input from the furnace can be matched with the heat demand of the process. This results in a high log mean temperature, which is the kinetic driving force in the heating process. Maintaining a high log mean temperature reduces the process time and increases thermal efficiency.
Indirect Heat Source. Because the heat source is removed from the process stream, the material is subject to ideal conditions. This provides a more uniform quality product. In addition, atmosphere and pressure can be controlled throughout the various stages.
Small Off-Gas System. On calciners, the off-gas treatment equipment is simplified. It is smaller because it is sized only for the volume of the process gas. The waste heat from the heat source is vented directly to the environment because the airstream from the heat source remains clean and uncontaminated by the process.
In conclusion, a rotary calciner provides efficient high-temperature processing, high heat transfer rates, minimized equipment size, improved product quality due to the control of the processing atmosphere and reduced environmental risks due to the low, treatable off-gas volumes. With its efficiency and versatility, a calciner is well suited for many specialty applications.