Off the wall - energy savings in refractories
Steven Lloyd from Emi-coat Ltd, and Dr Robert Tucker from Zerontec, show how wall coverings can achieve energy savings in refractories.
Over the last three decades there has been growing interest in high temperature and high emissivity coatings on refractory walls of fuel-fired furnaces for energy savings. These coatings raise the surface’s emissivity, improve radiative heat transfer and, therefore, theprocess’s thermal efficiency. Many system variations have been applied with different levels of success. Early failures were commonplace due to binder adhesion technology, coating thickness, particle size or misunderstanding the substrate. Having the ability to study and overcome these issues has been key to the success of high emissivity coating systems.
Significant energy savings on high temperature furnaces are claimed by Emi-coat, from Emi-coat Ltd, part of the Lloyd RIS Group, Staffordshire, UK. Application of Emi-coat ona large batch steel reheating furnace at Sheffield Forgemasters, Sheffield, UK, has delivered savings of seven per cent and £312,000pa. Another application on a gas-fired brick kiln produced savings of almost nine per cent.
However, apart from these two cases, energy saving claims resulting from these coatings are often difficult to verify because of a lack of understanding of the role of emissivity in furnaces. This has been a major barrier to their wider exploitation. Furthermore, the potential savings vary widely depending on the plant design and operation. It is tricky to forecast savings on different applications because most thermal design models for heat transfer in furnaces fail to show any influence of wall emissivity. This is due to the model’s limitations rather than the coating’sfailure.
If the combustion products are assumed to be grey – emissivity not varying with wavelength of radiation – then changing the wall emissivity in an enclosed furnace has no effect on overall predicted heat transfer. This is because, in general, for a well-insulated furnace, these simple models do not distinguish between radiation that is absorbed and re-radiated, and radiation that is reflected at the wall surface. By introducing a non-grey gas to more realistically represent the furnace combustion products, improvements have been noted when emissivity is raised.
Changing the wall emissivity affects the radiation in a furnace. Typically, the combustion gases have a low total emissivity, around 0.2, emitting 20% of the energy compared to a black body at the same temperature. The gas is also a ‘selective’ emitter in that it gives out and absorbs energy at discrete wavelengths. Hot combustion gases radiate because of the presence of carbon dioxide and water, the same gases found in our atmosphere that are responsible for maintaining climate temperatures. Radiation emission from these hot combustion products occurs at the following spectral wavelengths –
CO2 – 15,10.4, 9.4, 4.3, 2.7 and 2.0µm
H2O – 6.3, 2.7,1.87 and 1.38µm
The combustion gases have an equal and opposite affinity to both emit and absorb radiation at these wavelengths. The radiating and absorbing power of these ‘bands’ is compared to that of a black surface at 1,200ºC and to a grey surface of emissivity 0.8 at the same temperature. Those regions, where the gas neither emits nor absorbs energy, are known as spectral or waveband windows. This has a bearing on heat transfer efficiency in a furnace.
An un-coated furnace wall has a relatively low surface emissivity – a low ability to absorb radiation, and therefore a high reflectivity. At high temperature, emissivity can be as low as 0.5, meaning that 50% of the incident radiation is absorbed and 50% is reflected. Energy reflected off the furnace wall, which is at the same wavelength as originally emitted by the gas, is then easily re-absorbed by the gas, and cannot pass through to the load surface.
On the other hand, energy absorbed by the furnace wall is re-radiated over all wavelengths and is more likely to pass through a window in the gases to heat the load, because it is distributed through the wavelength spectrum. In this way, the wall assists in achieving more efficient energy transfer from the hot gases to the load. The higher the refractory emissivity, the greater the amount of radiation coming off the wall that can pass un-attenuated through the gases. The furnace can then achieve the same heat flux to the load, but with a lower combustion gas temperature, and therefore an improved thermal efficiency. Alternatively, it can be run at the same temperature, but with a higher heat flux to the load, and therefore a higher productivity.
In many furnace models, non-grey gas behaviour is ignored. More advanced furnace mathematical models, developed by Zerontec Ltd of Stratford-upon-Avon, UK, in collaboration with the University of Glamorgan, also in the UK, allow for these spectral effects and are able to predict efficiency and performance improvements when the surfaces are coated.Models for both continuous and dynamic performance prediction are available, which can simulate continuous and batch heating processes.
Further information: Emi-coat