Sami Vapalahti (Rodbay Oy):
The Theory And Effects Of Surface Properties Altering Ceramic Coatings
On Heat Transfer And Combustion
In combustion processes surface coatings can be used for changing heat transfer properties of the
surfaces. This can be used for improving heat extraction or heating capability of the process. This
presentation is concentrating on possibilities to improve heating capability and the effect this has on
combustion and energy efficiency. Energy in a process originates from combustion and energy is
mainly transferred through radiation. Radiation is coming from combustion products that are carried
through the process by the flue gas. This radiation has specific distribution of wave lengths.
When radiation is interacting with walls it can be reflected or absorbed. After it has been absorbed
the energy can be re-emitted back in to the process. It is important to understand that materials can
be high absorbing or high reflecting but in both cases it can be also high emissivity. In order for the
coating to improve surface properties of conventional materials it has to be able to allow less heat to
be conducted through the wall and make it available as increased radiation in the process. To
understand the use of coatings and the benefits available through the use of these coatings, method
how the coating works has to be known.
High emissivity coatings re-emit the radiation back in the process. This will change the nature of
the radiation since typical distribution of wave lengths are transformed to a continuous band. Since
flue gas is only capable of absorbing wave lengths that it has, wave lengths between these specific
peaks are transparent and carried through the flue gas without interception. Heating of products is
then improved.
This was shown in a study made in Oulu University 2011.
High reflectivity will cause the same wave lengths to be reflected back and the flue gas will absorb
this reflected energy. This will lead to raise in the flue gas temperature and process pressure. In
pressure controlled processes it will cause higher flue gas losses unless adjustments to the process
are made. The reflected heat will allow the process power to be reduced and combustion to be made
with lower amount of excess oxygen. An extensive case study was made at Sulzer Pumps Finland
Kotka foundry. This study was funded by Tekniikan Edistämissäätiö, Sulzer, Ruukki, Outokumpu,
and Ovako. It shows that after using high reflecting coating a furnace could be run at 0.7 % excess
oxygen and 7 % energy saving was obtained only after the coating was made compared to
maximum optimized furnace before the coating.