Instrumentation & control

Efficiency vs corrosion: sulphuric acid dewpoint monitors find the right balance

1 July 2007

Power plant operators need to steer a course between low flue gas temperatures that may cause H2SO4 deposition, with consequent corrosion, and high flue gas temperatures, which may avoid corrosion but increase heat losses and reduce efficiency. A sulphuric acid dewpoint monitor can provide continuous and clear guidance on what the optimal flue gas temperature should be.

The volume of SO3 in flue gas, an inevitable result of fossil fuel combustion, is dependent on factors such as the sulphur content of the fuel burned and whether or not selective catalytic reduction technology is used, but some SO3 will always be present and will always pose a serious problem for plant owners. It combines with moisture in the gas to form sulphuric acid, which, if flue gas temperatures fall below the acid dewpoint or if the gas comes into contact with a cold surface, will condense and cause damage by corroding the stack and equipment. Particular problem areas include the cold end, reheater surfaces in FGD systems and cold spots (due to insulation damage or other faults). This corrosion is costly in terms of maintenance and repairs. Preventing the condensation of H2SO4 can extend plant life.

Reducing the level of SO3 in flue gas is one way of tackling the problem. Preheaters have been shown to typically achieve a 10-50% reduction in SO3. The benefits of this effect are somewhat undermined by the increased risk that damaging H2SO4 will form due to lower outlet temperatures. Scrubbers also reduce levels of SO3, but by converting it to H2SO4 mist. However, as scrubbers are not able to remove sulphuric acid aerosols particularly effectively, this poses the risk of condensation at the cold end as well as acid emissions.

In coal fired boilers condensation of sulphuric acid is common in the preheater. Not only does this cause damage but it reduces the thermal efficiency of the process allowing extra heat energy to be lost.

The air preheater is a typical area vulnerable to corrosion from the build-up of sulphuric acid as it condenses out of the hot flue gas

Straightforward guide

A sulphuric acid dewpoint temperature (ADT) monitor can provide a straightforward guide to minimum operating flue gas temperatures by determining the temperature at which the acid in the gas will condense. As fuel types change and conditions in the boiler vary, an ADT monitor can also provide continuous monitoring to ensure that damage is being avoided at all times, constantly updating plant operators with fast results.

The ADT monitor can identify an ideal temperature zone, just above the dewpoint, which will prevent corrosion damage but also save energy costs and improve energy efficiency. Operating at higher flue gas temperatures than one needs to, with consequent wasting of heat and power, need not be adopted as a precautionary measure to lessen maintenance requirements.

A reduction of around 10ºC can produce a 1% increase in overall thermal efficiency.

An ADT monitor can provide detailed process information

Improving ESP efficiency

Another common corrosion issue arises from inefficient electrostatic precipitator (ESP) function. If the acid is allowed to come into contact with fly ash, acid smut is formed which may then initiate corrosion in places where it settles. It is important to ensure that the ESP is working efficiently in order to remove this acid smut, thereby preventing its emission to the atmosphere or collecting in the stack and causing additional corrosion.

In many ESPs SO3 is injected into the gas stream to improve the efficiency of the ESP. An acid dewpoint monitor can be used to monitor for SO3 slip caused by the injection of too much SO3.

No choice needed

A choice doesn't have to be made between thermal efficiency and corrosion avoidance, both can be achieved and maintained using sulphuric acid dewpoint temperature monitoring.

Sulphuric acid dewpoint temperature monitoring – how it works


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