Avoiding Boiler Chimney Corrosion

Corrosion can substantially bring down the life of boiler chimney. Hence, it is important to understand boiler chimney corrosion and how to avoid it.

Exhaust gas quality plays an important role in corrosion of the boiler chimney. Many a times, it is observed that the chimney starts corroding within a short time after commissioning of the boiler. This chimney corrosion typically happens when the flue gas temperature falls below the dew point of sulphur which enables so2 and so3 to combine with water vapour to form sulphuric acid.

Dew point of sulphur depends on the sulphur percentage in the fuel.
Fuel Sulphur Content (% by weight) Dew Point (Deg. C. )
0 43
1 129
2 135
3 138
4 143
5 149

  There are various reasons why flue gas temperature may reach sulphur dew point. Following are the factors which support the dew point corrosion.

1. Air leakage

Cold air can get into the ducting through various places such as gaps between chimney bottom flange and foundation, joints in flue gas path or holes/openings in ducting or the chimney. This cold air mixes with flue gases and cools down the flue gases. If the temperature falls below the dew point temperature, the chimney will get corroded. 

2. Low exit velocity

Capacity of the chimney depends on the top diameter of the chimney. The top diameter is designed for 12 m/s of flue gas exist velocity. If the boiler is not running at it full load, the exist velocity of flue gases drops.

Many times, a single, oversized chimney is connected to multiple boilers. If all the connected boilers are not operating, the exit velocity of the flue gases will go down. In such situations, ambient air can rush into the chimney. This will bring down the flue gas temperature and might lead to corrosion.

3. Low flue gas temperature at inlet
Most of the boilers are designed for flue gas exit temperature of 250 Deg. C. By the time these gases reach the chimney top, temperature drops by 25-30 Deg. C.

If boiler is designed for much lower exit temperature or if any heat recovery unit like economiser or air pre-heater is installed between the boiler and the chimney, the flue gas temperature at the outlet might fall below the dew point resulting in the corrosion of chimney.

4. Ambient temperature
Ambient air temperature plays an important role in facilitating the corrosion. In cold climates, outer surfaces of chimney drops down which leads to dew point condensation.

5. Ambient wind velocity

In windy region, the outer surfaces of the chimney are cooled by constant flow of air which results in dew point condensation.

6. Sulphur in fuel
As seen in the table, as the sulphur content of the fuel increases, the chances of corrosion increase. If the sulphur content of the fuel is high, more precautions will need to be taken to avoid dew point condensation.

7. Moisture in fuel
For fuels containing more than 1% sulphur, it is essential that the moisture content does not exceed 1%. It is advisable to keep moisture content as minimum as possible.

8. Relative humidity
During rainy season, ambient humidity increases causing flue gas humidity to approach saturation levels. Corrosion rate also increases during such conditions.

In order to minimize the chimney corrosion, following steps should be taken:

1.    Keep flue gas temperature at boiler exit well above the dew point limit for fuel used- preferably 50 Deg. C. more than the dew point limit.
2.    Eliminate any points from where air can seep into the flue gas path.
3.    Insulate ducting and chimney outer surface.
4.    Operate boiler/s connected to the chimney at maximum possible loads. This will not only reduce the corrosion rate, but will also improve the boiler efficiency.
5.    Do not oversize the chimney
6.    Use fuels with lower sulphur content.