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In the industrial, commercial and institutional fuel markets, it is not unusual for plant managers to claim that their boilers operate more efficiently on #6 oil than on natural gas. However, what is generally not said and often not understood, is that their statements are purely based on the combustion efficiency difference between the two fuels and do not include other losses or costs associated with oil firing.

Due to its chemical composition, the combustion process of #6 oil produces less moisture than that of natural gas. As a result, #6 enjoys an approximate 2.0-2.5% combustion efficiency advantage over natural gas. However, once all of the other losses associated with burning #6 oil are considered, natural gas provides greater overall system efficiency. These other losses or costs include:

Oil storage
Heating of stored oil
Fuel oil additives
Oil pumping
Fuel oil atomization
Soot blowing of boiler tubes
Additional boiler makeup water
Additional boiler water treatment chemicals
Heating of additional makeup water
Additional maintenance

Although these additional losses or costs have been known for some time, little information existed as to what the additional costs were in terms of dollars. That is why National Fuel, in cooperation with the American Gas Association and other gas utilities throughout the country, conducted a comprehensive field test to identify and quantify these costs.

Dr. Herbert M. Eckerlin from the “Center of Boiler Studies” was hired to conduct the study and provide complete reports on his findings. Dr. Eckerlin tested 26 boilers across the country over a two year period. During that time, he was able to identify the following additional costs which are related to the combustion of #6 oil. These factors were then used to determine the “bottom line” efficiencies of both #6 oil and natural gas.


Oil Storage

Since #6 oil is typically used in large commercial and industrial boilers, the storage capability needs to be adequate to maintain a reasonable supply of oil to the boiler. This generally means large storage tanks which are kept as full as possible to minimize the accumulation of moisture build-up from condensate. Maintaining this inventory ties up monies which could be used for other business opportunities.

Oil Heating

Residual oil, as #6 is often called, is thick fluid which needs to be heated before its viscosity is reduced to the point where it can be pumped. This is generally accomplished through a steam to oil heat exchanger or electric heating elements within the storage tank. In the case of the heat exchanger, the steam condensate is generally dumped to drain rather than returned to the boiler. This is done to prevent contamination of the water in the boiler system should the heat exchanger develop a leak.

Oil Pumping

Residual oil needs to be pumped not only from the storage tank to the burner, but is also continually circulated within the tank in order to maintain a consistent viscosity and keep the additives mixed. Electricity is used to operate this pump.

Oil Additives

Residual oil contains sulfur and vanadium, both of which create numerous problems for boiler operation, primarily corrosion. Corrosion can occur in the storage tank and on boiler fireside surfaces. Oil treatment chemicals are used to reduce harmful effects of the oil on the equipment.

Oil Atomization

Residual oil cannot be burned in the form that it is delivered to the burner. It must be atomized into tiny droplets and sprayed out to form a flame pattern. Atomization can be accomplished through the use of either steam or compressed air.

Soot Blowing

When #6 oil is burned it produces a certain amount of carbon ash which is deposited on the fireside surface of the boiler. This soot acts as an insulation and reduces the efficiency of heat transfer from the flame to the boiler water. Unless the soot is removed, the overall efficiency of the boiler will continue to deteriorate. Soot blowing, which is nothing more than steam cleaning or blowing the soot from the surfaces with compressed air, is required on a daily basis in order to keep the fireside surfaces relatively clean. Some of the smaller residual oil fired boilers which are not equipped with soot blowers actually switch over to natural gas to help burn off some of the soot before going back to oil. This does not eliminate the need to manually brush out the boiler tubes, as boiler efficiencies continue to degrade when burning oil.

Additional Makeup Water

When steam is used for oil tank heating, atomization and soot blowing, it is lost to drain of atmosphere. Fresh makeup water must then be introduced into the boiler to maintain a safe water level.

Water Treatment Chemicals

When fresh makeup water is introduced to a boiler, water treatment chemicals have to be added to maintain desired water conditions.

Heating Additional Makeup Water

There are two reasons for heating makeup water. The first and most obvious is that the water needs to be heated near the temperature of the boiler to eliminate the possibility of boiler damage from thermal shock when the water is introduced. The other reason is to remove the oxygen in the water to prevent corrosion within the boiler.

Additional Maintenance

Perhaps the single highest additional cost experienced when burning residual oil is the cost of additional maintenance. Since oil requires considerable support equipment such as storage tanks, pumps, soot blowers, and heat exchangers, there naturally is more effort required to maintain these systems. A boiler plant which uses residual oil generally will require additional personnel to operate that plant. Even if the number of boiler plant personnel is identical, the personnel in the plant which uses residual oil are kept busy maintaining the oil firing support systems. As a result, less attention is paid to routine boiler plant maintenance such as repairing or replacing steam traps. This neglect tends to reduce the overall operating efficiency of the boiler plant and further raises the cost of burning #6 oil.

In Conclusion

Operating procedures vary between boiler plants even when the same fuels are being used. The additional requirements identified here, however, are typical of what is found in the field. Any variation in procedures would require an independent analysis to determine "bottom line" efficiencies.

Dr. Eckerlin's analysis determined that the "bottom line" efficiency of natural gas fired boilers is 2.7% higher than residual oil boilers. Therefore, this average can be used as a guide to help get a better understanding of the true cost of burning residual oil.

*This figure does not include the energy lost through storage tank heat loss. Dr. Eckerlin will provide this data in future reports. However, this is a significant cost which will increase the overall advantage for natural gas. The same is true for maintenance costs.

**This figure includes the cost of makeup water, treatment and heating.

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