High emissivity coating technology improves heater performance

Introduction
The increased demand and tightened specifications for gasoline and diesel throughout the world has placed an increasing burden on refineries. Efforts to increase capacity by adding fired heaters and related process equipment requires years of planning and large outlays of capital. In many instances, fired heaters are not run at optimal levels of efficiency, thus preventing the facility from capitalizing on the favorable market conditions that now exist for refined products. Furthermore, the cost of fuel to operate these heaters has increased to record levels in many areas, which further drives up the unit cost of production. Efforts to reduce harmful emissions through the use of low NOx burner technology has introduced a number of other processing problems, including carburization of process tubes and an increased propensity for unwanted coke formation in many high heat flux applications.

Cetek high emissivity coating technology has become a proven means of increasing fired heater efficiency, thereby reducing fuel consumption, increasing process severity and capacity where needed, without compromising reliability and safe operation, and improving process control in many types of fired heaters, around the world.

With over 200 units treated thus far in more than 18 countries, Cetek coating technology is becoming a standard means of economically improving heater efficiency by large and small refiners for a wide range of objectives.

Economic and Operational Implications
Productivity benefits of 5 to 10% are typical, which means that the applications provide a rapid payback. Furthermore, coatings can last more than eight years and continue to provide a high degree of benefit throughout the life of the coating. The coating’s longevity means that Cetek customers can expect to see a substantial return on their investment. Additionally, Cetek performs a full technical evaluation of the fired heater before each coating application to provide an estimate of the benefit that will be derived.

Cetek coating technology is a very versatile method of solving a broad range of fired heater limitation issues. This technology is commonly used to increase production rates as well as to reduce fuel consumption.

External process tube fouling makes accurate tube metal temperature measurements via infra-red thermography or pyrometry virtually impossible. By removing all of this scale of varying thickness and installing a thin and uniform layer of coating on the process tube surfaces, such inspections can more accurately determine the true tube metal temperature which will improve the safety and operational reliability of the fired heater.

Process Tube Coatings
Fired heater process tubes suffer from varying degrees of fouling while in operation. In gas-fired heaters this is in the form of oxidation and scale growth. Often the scale accumulates up to several millimeters thick, providing a very significant barrier to heat transfer to the process. In oil-fired heaters, the fouling is in the form of thick layers of corrosive combustion products mixed with oxidation. The thermal conductivity coefficient of a fouled tube is less than half that of a clean tube.

Once fouling materials appear on process tube surfaces, the heater has to be fired harder to overcome their insulating effects. This leads to high fuel consumption and excessively high flue gas temperatures in the bridge wall area and convection sections. This compromises the reliability of the heater, shortens
the life of structural components, and increases the demands for maintenance.

Cetek coatings for process tubes are dense and impermeable. The thin film layer provides excellent oxidation and dry corrosion resistance. Cetek offers a proven method of maximizing process tube thermal conductivity in fired heaters. By eliminating oxidation and corrosion and reducing fouling, there are significant advantages in absorbed duty, productivity, temperature uniformity, and tube life.

Process tube metallurgies that are commonly coated include carbon steel, and up to and including 9%Cr, 1%Mo (P9) steel alloys.

High Emissivity Coatings for Refractories
In gas-fired heaters, Cetek’s high emissivity coatings for refractories improve the radiant heat transfer efficiency, and reduce fuel consumption, bridge wall and stack temperatures at the same charge rate. The nature of the radiation being emitted from the refractory surfaces is transformed to near Black Body Radiation. This type of radiation is less easily re-absorbed by the flame and flue gases in the radiant section and can therefore penetrate more efficiently to the process tubes. The resulting effect is an increase in the usable heat flux from the refractory surfaces. This benefit can be used to increase heat flux on the wall side of the process tubes and to improve the circumferential heat flux distribution for single fired heaters which will further help reduce the fired side tube metal temperature.

Reduction of Emissions
A beneficial consequence of the reduction in flame and flue gas temperature is in reducing the “Thermal NOx” component of the NOx emission. This is the primary contributor to the overall NOx emission and is formed by the reaction of N2 from combustion air and O2. Historical data from Cetek’s clients shows that a reduction of 15-30% in NOx emission can be expected after the application of ceramic coatings. Applications in high temperature, high flux heaters provide the greater benefit.

Conclusion
Cetek high emissivity coating technology has been proven to provide significant, measurable fired heater efficiency improvements. Heater limitations in tube metal temperature, bridge wall temperature, firing rate, excess air, etc., can be partially or completely mitigated through an engineered coating solution, including the application of ceramic coatings to the process tubes, radiant section refractory, or both. The results are immediate and have been proven to last for years. In natural gas-fired heaters, the tube coatings may last 8 or more years, whereas 4 to 6 years can be expected in oil-fired heaters.

The technology is routinely applied in a wide range of heater types including continuous catalytic and semi-regenerative reformers, atmospheric and vacuum crude distillation heaters, Cokers, reboilers, hydrogen reformers, pyrolysis heaters, and others.

The application is fast and completed within required maintenance turnaround schedules, typically requiring only a few days to complete all aspects.