Exploring the factors that affect the exhaust gas temperature of the air preheater

Burner operation mode

The burner operation mode primarily concerns the distribution of load and the method of starting and stopping the burners.

The burner operation mode affects exhaust gas temperature primarily by shifting the flame center upward within the furnace, leading to an increase in the boiler outlet flue gas temperature. When the heat absorption on the boiler’s convective heating surface remains constant, this upward shift results in a higher exhaust gas temperature at the boiler outlet.

Boiler Air Supply

The change in boiler air supply primarily affects the exhaust gas temperature by influencing combustion and heat exchange. When the air volume to the boiler increases, the proportion of radiation and convective heat transfer shifts. With the total heat entering the furnace remaining constant, radiation heat decreases while convective heat increases, transferring more heat from the furnace to the convective flue, which raises the exhaust gas temperature. On the other hand, as the air volume through the preheater’s heating surface increases, the heat transfer on the preheater’s heating surface also increases, leading to a decrease in the boiler exhaust gas temperature.

The change in boiler exhaust gas temperature is the result of the combined effect of these two factors.

Boiler Air Leakage

Boiler air leakage primarily includes leaks from the boiler body, the pulverizing system, and the air preheater. Leakage from the boiler body and the pulverizing system affects the excess air coefficient at the furnace outlet, which has a certain impact on both boiler combustion and exhaust gas temperature.

When the excess air at the furnace outlet remains constant, air leakage in the boiler body introduces cold air into the furnace, replacing hot air. This leads to delayed combustion, causes the flame center to shift upward, and increases the flue gas temperature at the boiler outlet. If the heat absorption by the boiler’s convective heating surface remains constant, this will result in a higher exhaust gas temperature.

Air preheater leakage primarily refers to air entering the flue gas side. Considering the static pressure distribution at the preheater’s air inlet and outlet, more air leaks into the flue gas side at the preheater outlet than at the inlet, leading to air short-circuiting into the boiler’s flue gas stream. As the leakage rate of the air preheater increases, the exhaust gas temperature of the boiler decreases

Heating Surface Contamination

Slagging on the water-cooled walls decreases the amount of radiative heat transfer within the furnace and the heat absorption by the water-cooled walls, leading to higher exhaust gas temperatures at the furnace outlet and increased boiler exhaust gas temperatures.

Ash accumulation on the convective heating surfaces increases thermal resistance, reducing heat transfer and causing the exhaust gas temperatures at various stages to rise, thereby elevating the boiler exhaust gas temperature. Ash blockage on the low-temperature convective heating surfaces further decreases heat transfer, resulting in higher exhaust gas temperatures across all stages and an increase in the boiler exhaust gas temperature.

At the same time, the increased resistance on the flue gas side of each convective heating surface leads to higher power consumption by the induced draft fan.

Sootblower Equipment Utilization Rate

A high utilization rate of sootblower equipment reduces slagging (or ash accumulation) on the furnace water-cooled walls, thereby increasing heat absorption, lowering the furnace outlet temperature, and decreasing the boiler exhaust gas temperature. Clean convective heating surfaces reduce thermal resistance and enhance heat transfer, further contributing to a lower boiler exhaust gas temperature.

Flue Gas Dew Point Temperature

When the flue gas dew point temperature is high, it is necessary to prevent low-temperature corrosion of the air preheater’s heat transfer surfaces. This can be achieved by maintaining the wall temperature of the preheater’s outlet heating surface slightly above the flue gas dew point temperature. To do so, both the boiler inlet air temperature and the exhaust gas temperature need to be increased. The flue gas dew point temperature is primarily influenced by the sulfur content of the coal. Higher sulfur content results in a higher flue gas dew point temperature.

Convective Heating Surface Area

In some units, measures to block tubes as a remedy for “four-tube leakage” can reduce the heat transfer area of the superheater, reheater, and economizer. This reduction can lead to an increase in the boiler exhaust gas temperature.

As a component responsible for recovering flue gas waste heat in boiler systems, excessively high exhaust gas temperatures in the air preheater indicate a reduction in the boiler’s heat exchange efficiency. Over time, this can lead to decreased economic performance of the boiler operation, a common issue in many boiler units. To resolve this problem, Bokk Tech has launched the elliptical pipe air preheater, which is a departure from traditional circular pipe designs. After installation and comparative testing, the elliptical pipes have demonstrated superior external flow characteristics, a more compact heat exchanger structure, and significantly improved heat exchange efficiency.

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