(9c) High Emissivity Coatings:  Increasing Production and Efficiency in VCM and Ethylene Furnaces

Emissivity is a property that is routinely ignored in furnace operation. Emissivity is the dimensionless number representing the ratio of a material’s ability to radiate thermal energy to that of a blackbody / perfect emitter. One would assume that understanding and utilizing this property would be absolutely critical within furnace operation, however it is routinely ignored despite the ability to influence the emissivity of coils, burners and refractory to increase the efficiency of the furnace by up to 10%+. For fuel gas firing limited furnaces, the ability to increase conversion results in increased production. What is being seen is that the conversion increases and run lengths are greatly increased. This goes against typical logic but explanation by furnace modeling experts is that typical fuel gas results in “banded radiation” that causes faster coking but if coils have a high emissivity (greater uniform absorption) then the radiation bands are diminished into a uniform range. While the ultimate temperature is not changed, the radiation is not banded and the resulting coking is less. The understanding of how to appropriately utilize emissivity to increase run length has not been considered by most companies. This has now started to change. Companies are starting to utilize their knowledge of emissivity to increase the efficiency, productivity and reduced downtime of their furnaces to better compete. This paper goes through the basics of emissivity to enable process engineers to have a working knowledge of emissivity and understand how to better apply their knowledge to increase the productivity and efficiency of their fired heaters and cracking furnaces. Additionally the paper describes when and when not to consider emissivity coatings (i.e. let’s heal the sick but not try to raise the dead) and what is the performance specification for any high emissivity coating on refractory, ceramic fiber, burners or process coils.