How to clean static air preheater?

[ponderer]

The flue gas from our CDU(Crude distillation unit) heater is heat exchanged with the heater inlet air.

The air preheater is a static type with cast iron fin tube in the upper section and glass tube in the lower section.

There is a DeNOx facility(SCR) upstream of the air preheater.

We encounter serious plugging problem in the upper section of the air preheater.

How to clean the air preheater? By water jet? or
Do we need to dismantle the air preheater?

 

[SMF1964]

What is plugging the upper section? Corrosion product? Scale? Need more data.

 

[orenda1168]

Ponderer:

From your system description, though you don't indicate the heater fuel type or air preheater temperature regimes, it sounds as though the cold end pluggage is a result of flue gas SO3 condensation to sulfuric acid and subsequent laydown on the fin tubes. The condensed, sticky H2SO4 will not only corrosively degrade the mild steel tubes, but also act as a "fly paper" trap for flue gas particulates, with both leading to significant pluggage.

Two suggestions: 1) wash the plugged tube section with an elevated pH water - use a pH adjustment product available from the major refining additive companies such as Baker-Hughes, Nalco or GE Betz, and 2)as a preventative, either elevate the cold end metal surface temperature above the SO3 dew point, or use a magnesium-based additive in the fuel prior to firing to inhibit catalytic SO3 formation and neutralize residual H2SO4 in the cold end of the system.

Regards, Orenda

 

[25362]

As I am given to understand, newly-designed SCR units are supposed to catch most of the particles. Kindly answer to SMF1964 post. And please inform with what frequecy you are changing fuels and what types are used.

Is plugging the result of soot deposition ? Please note that because of the presence of oxygen when using ammonia in the SCR step, some conversion of SO2 to SO3 takes place and corrosion products may result. What are the
temperatures in the SCR and in the preheater ?

 

[SMF1964]

I disagree with orenda1168's statement that the sticky H2SO4 will not corrosively degrade the mild steel. Condensation of SO3 as sulfuric acid is the principle mechanism behind acid dewpoint corrosion in our power plants. If this is condensation is going on, you are going to be losing your mild steel rather rapidly.

 

[JEB66]

They could have ammonina slip from the SCR and form ammoninum bi sulfate.

The rotating air heaters at power plants are washed with hot water. It is important to monitor the pH of the wash water before and after the air heater to know when you get it clean.

 

[SMF1964]

We've had success on our air heater washing with soda ash in water. The high pH helps remove the sludge from the system quite nicely.

 

[orenda1168]

SMF1964:

If you'll reread my post regarding H2SO4 corrosion of mild steel tubes, you will find that I DID state that condensed sulfuric acid DOES result in fintube corrosion!

Orenda

 

[SMF1964]

That's what I get for speed-reading.
Upon further review, the call is reversed.

 

[rmw]

There are 'rake' type soot blowers that can clean the tubes on line, but unless you have lanes designed in the tube banks to accomodate them, all you could do is clean the first few rows of tubes that were exposed to the steam cleaning flow.

Is the deposit friable, or is it a hard deposit? We need to know this to give you any advice.

Some plate type heaters are water washed (off line, of course) but this is not for hard deposits, just dust and dirt. These type units used on solid fuel furnaces use the rake type soot blower.

rmw

 

[ponderer]

We fire refinery fuel gas and fuel oil(Sulfur content 1%) in our CDU heater.

We operate soot blower in the convection zone(with studded fin tube) of the heater every day.

We inject ammonia to the SCR and in order to prevent NH3 slipping to the downstream Air Preheater, we control the SCR outlet NOx to be above 20ppm.

We do not water wash the air preheater during normal operation as it may affect the heater draft pressure.

The deposited material from the APH are corrosion product and crystallized material.