Reducing fuel consumption and CO2 emissions in fired heaters

A global inventory of carbon dioxide (CO₂) emissions recently published by One Earth suggests that global cumulative emissions could be reduced by up to 10% between 2020-2030 if refineries adopted low-carbon measures. The list of measures includes refineries improving their efficiency and upgrading heavy oil processing technologies.

Integrated Global Services

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Article Summary

With various global carbon tax schemes either underway or being deliberated, refineries are under increasing pressure to reduce CO₂ emissions and fuel consumption. But where should they start? A critical focus area should be on fuel combustion as this accounts for almost 80% of refinery carbon emissions.

Fired heaters are the largest consumers of fuel in a refinery emitting an estimated 400 to 500 million tons of carbon dioxide (CO₂) every year, so even minor improvements in the efficiency of fired heaters can lead to significant savings. To put it into perspective, if any fired heater is 1% or 2% inefficient it can consume an additional $1m in fuel over a year.

Having identified the greatest source of refinery carbon emissions and the piece of equipment which consumes the most fuel, we can now begin to explore mitigation strategies to reduce fuel consumption and CO₂ emissions in fired heaters.

Figure 1 outlines a broader spectrum of CO₂ emissions reduction strategies, and this article will focus on process optimisation solutions and more specifically, ways to improve energy efficiency in the radiant and convection sections of fired heaters.

Improving radiant section efficiency
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 contained within the refractory lined box. The configuration and condition of the process tubes and the refractory surfaces affect the efficiency of heat transfer in the radiant section.

A common problem affecting radiant section efficiency is the oxidation of radiant tubes.

How does process tube oxidation occur?
Severe scaling due to oxidation on steel alloy tubes is expected in fired heaters due to high operating temperatures. Above approximately 500°C, the 9Cr-1Mo materials tend to oxidise and may scale very rapidly as temperatures increase. The corrosion rate based on material and temperature can be as high as 0.25mm/year. Layers of oxidation scale grow continuously on the tube surfaces and create a significant insulating effect, reducing conductive heat transfer efficiency to the process.

What are the effects of process tube oxidation?
To overcome the insulating effect, extra heat is needed and firing rate is increased. This results in increased flue gas temperature (bridgewall temperature (BWT)). Consequently, CO₂ and NOx emissions are increased. As the scale grows, further increases in firing rate are required, BWT limitations are encountered, and production rates are threatened.  

An industry trend to operate fired heaters more efficiently, at lower excess oxygen levels, to save fuel and reduce CO₂ emissions has increased the potential for carburisation of external surfaces of radiant section tubes.

This leads to grain boundary penetration of carbon, carbide formation, embrittlement of the surface, crack formation and loss of metal. The result is reduced service life of the radiant tubes.

How can carburisation and oxidation be mitigated?
Hot-tek online solutions
Online tube descaling is offered by Integrated Global Services’ (IGS) Hot-tek service. This will immediately improve the radiant section heat transfer efficiency and reduce the bridgewall temperature. This a great short-term fix, however, the improvement is only temporary as oxidation and scale formation will continue.

Cetek ceramic coatings for process tubes
Cetek ceramic coatings provide a durable, protective, thin-film layer on the outer surfaces of process tubes, which prevents oxidation, corrosion, and carburisation of the metal and maintains the tube 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 surface preparation, coating and curing of the coating takes place.

What are the expected results?
The average benefit in catalytic reformer heaters is to increase radiant section efficiency by 6.6%, with a corresponding reduction of 6.6% in CO₂ emission and approximately 20% reduction in NOx emission.

Refractory surface emissivity: why does it matter?
A significant portion of the radiant energy interacts with the refractory surfaces. The mechanism of this interaction has a considerable 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.

Refractory ceramic coating solutions
Cetek ceramic coatings with emissivity values of above 0.9, have been designed specifically to supplement the radiation characteristics of the refractory surfaces. In use, benefits have been realized of up to 5% in radiant section efficiency improvement, with a corresponding CO₂ emission reduction of up to 5% and NOx emission reduction in Ethylene, CRU and SMR units of up to 30%. Figure 2 illustrates the average expected fuel savings, and CO₂ and NOx reduction of various types of heaters.

Convection section efficiency
A common cause for a drop in fired heater performance is fouling build-up on convection section tubes/fins. Symptoms of convection section fouling include high convection pressure drop due to accumulation of fouling, an increase in flue gas stack temperature, and a decrease in process crossover temperature.
How does fouling occur?

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