December 2022

Heat Transfer

Cut emissions and fuel while increasing capacity in fired heaters

According to a recent digital refining article,1 fired heaters are responsible for an estimated 400 MMtpy–500 MMtpy of CO2, with at least 73% of average refinery CO2 emissions coming from combustion.

According to a recent digital refining article,1 fired heaters are responsible for an estimated 400 MMtpy–500 MMtpy of carbon dioxide (CO2), with at least 73% of average refinery CO2 emissions coming from combustion. Refineries and petrochemical plants are under increasing pressure to reduce emissions while also looking for ways to reduce fuel consumption amid rising fuel prices.

Fired heaters have the greatest running cost in a refinery or petrochemical site. If any heater is inefficient by 1%–2%, it can consume an additional $1 MM/yr in fuel or result in millions of dollars in lost revenue. High fuel consumption also translates to increased emissions. Units of particular interest include catalytic reformers, steam methane reformers (SMRs) and ethylene furnaces.

This article will discuss solutions to issues in the radiant and convection sections of fired heaters that affect performance and profitability.


Process fluids are heated in the radiant sections of fired heaters. The fluids travel through steel alloy tubes, which are heated principally by radiant heat generated by burners within the refractory lined box. The configuration and condition of the process tubes and the refractory surfaces affect heat transfer efficiency in the radiant section.

How does process tube oxidation occur?

When used, steel alloy tubes oxidize and scale layers continuously grow on the external surfaces. The developing scale insulates the tube surface, hindering conductive heat transfer to the process and decreasing radiant section efficiency.

Extra heat is needed, and the firing rate is increased to overcome the insulating effect. This results in increased flue gas temperature (bridgewall temperature). Consequently, CO2 and nitrogen oxide (NOx) emissions are increased. As the scale grows and further increases in firing rate are required, boiler water treatment limitations are encountered and production rates are threatened.

Carburization is an industry trend used to operate fired heaters more efficiently—at lower excess oxygen levels—to save fuel and reduce CO2 emissions. The increased potential for the carburization of external surfaces of radiant section tubes leads to the grain boundary penetration of carbon, carbide formation, surface embrittlement, crack formation and metal loss. The result is a reduced service life of the radiant tubes.

The author’s company offers online tube descaling. This will improve the radiant section heat transfer efficiency and reduce the bridgewall temperature. However, the effect is temporary since oxidation and scale formation will continue.

High-emissivity ceramic coatings provide a protective thin-film layer on the outer surfaces of process tubes, which prevents metal oxidation, corrosion and carburization. This maintains the thermal conductivity coefficient close to new tube conditions. The coatings may be applied to existing tubes during a shutdown or to new tubes at a remote facility where the surface preparation, coating and curing occurs. The average benefit of catalytic reformer heaters is to increase the radiant section efficiency by 6.6%, with a corresponding 6.6% reduction in CO2 emissions and approximately 20% reduction in NOx emissions.

Refractory surface emissivity: Why does it matter?

A significant portion of the radiant energy interacts with the refractory surfaces, and the mechanism of this interaction has an appreciable effect on the overall efficiency of radiant heat transfer. A major factor in determining radiant efficiency is the emissivity of the refractory surface.

The ultimate radiant heat transfer efficiency is achieved where the enclosure is a black body, where all the surfaces have the maximum emissivity factor of 1.0.

Ceramic coatings with emissivity values > 0.9 have been designed to supplement the refractory surfaces’ radiation characteristics. Benefits of up to 5% in radiant section efficiency improvement with corresponding CO2 emissions reduction of up to 5% and NOx emissions reduction in ethylene and SMR up to 30% can be achieved.


How does fouling occur?

Fouling occurs when deposits accumulate on the tube or fin surfaces. The burner operates with one part per million (ppm) airborne particulates. This means that a single burner can pass 2.5 tpy of debris through a heater, some of which will deposit.

Airborne debris is drawn in from the surrounding environment.

One of the most significant effects of fouling is an increase in emissions. If fouling is left to worsen and the emissions become excessive, refineries may be liable to pay government fines (FIG. 1). Fouling requires fired heaters to burn more fuel to operate at the desired capacity—higher fuel consumption results in a higher stack temperature and reduced heat transfer. High operating temperatures cause heat stress on tubes, creating uneven expansion and irregular heat distribution, potentially leading to tube failure. These side effects can cost plants millions of dollars in lost revenue.

FIG. 1. Heavy fouling on a convection section.

Two commonly used offline cleaning methods are used to tackle the issue of fouling: chemicals and dry-ice blasting.

Chemical cleaning

Chemicals can be used when an asset is online; however, if the nature of the fouling is unknown, it is difficult to predict how it will react with certain chemicals. Therefore, chemicals can compound the issue of fouling, creating hotspots in areas that the chemical has not managed to penetrate or reach.

During a shutdown, the refractory lining of the convection section can become saturated with water-borne chemicals, which may change the refractory’s high-temperature properties, leading to premature failure.

Dry-ice blasting

Dry-ice blasting can be undertaken during the unit’s operation and is offered by the author’s company. It provides a limited benefit since accessibility reduces the ability to reach > 40% of the tube surface area. However, this is often sufficient to provide a significant improvement until a planned shutdown.

The author’s company offers a comprehensive solution that uses robotic cleaning systems to clean more than 90% of convection section tube fouling during planned turnarounds (FIG. 2).

FIG. 2. After–convection section robotically cleaned.


Fired heaters are a critical piece of equipment for refineries and facilities worldwide. With rising fuel costs and stringent emissions legislation, it is becoming increasingly important to ensure that mission-critical assets, particularly fired heaters, perform efficiently and at full capacity.

Maintaining these assets regularly and thoroughly will maintain output, increase asset life, reduce the likelihood of unplanned outages, reduce stack temperatures and emissions, and deliver a significant return on investment. HP


  1. XRG Technologies, Matthew, M.,, “Carbon dioxide emissions from fired heaters,” June 2021, online:

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