Rethink High-Temperature Materials Processing

Take advantage of opportunities to enhance energy efficiency.

By Tom Mroz and Robert Blackmon, Harper International

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Furnaces in which boats of material or trays of components move through on a carrier or car, such as a pusher, roller hearth or mesh belt conveyor, enable production of significant amounts of material but put the ancillaries through the heating and cooling cycle. Frequently, the reaction time or the need for a highly uniform process will mandate using such a system. In some cases, rails or some other form of conveyance can transport containers of material directly through the furnace, eliminating any heating and cooling of support materials; this is much more efficient.

Conveyance furnaces come in a wide variety of designs and materials of construction. The highest efficiency units are ones that recover heat or recirculate hot containers within the heating zone of the furnace, thereby limiting heating and cooling requirements almost exclusively to the process material. In more-advanced designs, heat recovery from product carriers can boost system efficiencies further. Sometimes, it's even possible to engineer material flows so cool incoming reactants get preheated by exiting material. However, these designs are less common and can pose design and operational difficulties. Nonetheless, they can provide the best efficiency for processes demanding container-supported heating.

Gaining much more energy efficiency requires eliminating the need for containered material conveyance altogether. One option is a rotary tube furnace. As the reactant bed moves through the tube, it's constantly stirred by the tube rotation. This stirring action boosts thermal transfer to the bed, improves removal of product gases, and increases solid/gas exchange in cases where the furnace gas also is a reactant. These enhancements often lead to product with better uniformity than materials processed in a static bed. Additionally, because only the reactant powder is heated and cooled, thermal efficiency is significantly better than that of pusher-style furnaces.

Vertical furnaces offer a reasonable alternative to rotary tube furnaces in cases where material movement in the rotary furnace is unsatisfactory or where other features such as very short or very long lead times, significant interaction with reaction atmospheres or completely contact-free reactions are required. Similar to rotary furnaces, energy use primarily is related to heating of the product and supporting necessary reactions and, thus, is relatively efficient. The design allows for minimal interaction with the furnace wall, making it a good choice where contamination is an issue. In some cases, this type of furnace is invaluable in combination processes, e.g., spray pyrolysis coupled with calcining.

Refractory. The selection and design of refractory can significantly impact thermal efficiency. In batch systems, highly efficient refractory designs may save energy and boost ramp speed during heatup and soaks but lead to extended cooling times and, thus, longer total cycle times. Less efficient designs may improve the cooling rate but pose shell-temperature or other limitations. Alternatively, water-cooled equipment may provide a reasonable solution for cooling rate and minimize space and weight requirements for refractories — but will dramatically decrease energy efficiency.

Continuous processes decouple the heating, soak and cooling portions of the process by distance. Therefore, using different refractory designs along the length of the furnace can optimize the thermal management and thermal efficiency requirements in each stage.

Atmosphere management. Heat losses via exhaust designs also significantly differ between batch and continuous equipment. The ability to utilize waste heat generated by batch furnace exhausts is limited. Because the waste heat in such equipment essentially is at the temperature of the load, there's no opportunity to insert the heat value directly into the process.

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