Acid gas dew point corrosion phenomenon in incinerators

[Orga78]

Hi all,

We have a recurring problem of corrosion in our waste gas incinerator units, suspected to be due to the acid gas dew point issue.

The temperature at the bottom section of the incinerator shell showing below dew point of 126 degC (temperature reading 110 degC). Inspection was conducted and corrosion was evident at the "cold end" sections where maintenance team had been performing corrective measure work which is now proven as a "Band-Aid" repair work.

We are in the midst of investigation at the moment. Appreciate if you guys would be willing to share your experience on this problem, and methods how to tackle this problem either as preventive measures or eliminating it once and for all.

Look forward to hear your inputs and recommendations.

Thank you!

 

[MJCronin]

Can you offer us some pictures ? Close-up photographs of the corrosion area ?

Can you show us using drawings exactly where the corrosion occurs within the incinerator ?

Is the corrosion forming where the gas stream changes direction ?

Has the composition of the waste gas changes and are there more chlorides in the gas stream ???

Are you recovering heat using boiler tubes within the unit ???

MJCronin
Sr. Process Engineer

 

[Orga78]

Hi MJCronin,

Will post some drawings later (no pics unfortunately).

"Occurs on both top and bottom section of the incinerator.

Thanks for the good sets of questions as well, will take them up with the process guys for further discussion.

Will revert back to you once I have more information.

There is indication of "cold end" phenomenon, where the skin temperature within the incinerator dropped below the acid dew point (both top and bottom section) - therefore, allowing the flue gases to condense to form either sulfurous or sulfuric acid. Currently, we're looking at the operational modes of the incinerator during combustion. There could be some parameters are off, which led to the "cold-end", i.e flue gas in temp, combustion air temp, damper %, etc.

 

[chicopee]

Increase the combustion temperature if you suspect the dew point is too low. Also describe the type of fuel being used to incinerate the waste gas.

 

[chicopee]

Also do a search of the periodical Power Boilers as there are many hints to solve technical issues with incineration.

 

[Orga78]

The combustion temperature is set at 650 degC.

While measured skin temp ranged from 118degC to 260 degC. Acid Gas Dew point as per spec is 126 degC.

Fuel gas used is the Feed Gas (Methane).

Another information shared by the Operations folk is that, they "generally" will see this cold-end phenomenon during winter period (I am in the midst of gathering all historical process trends to back this claim).

 

[greenche]

Cold end corrosion is primarily dependent on the sulfur content of the fuel and the temperature of the cold surface. For example in boilers, raising combustion temperature doesn't mitigate it much since the temperature of the tube metal is primarily dependent on water temperature. So you should consider if you can keep the area warmer. If winter is causing a problem, is this because poor exterior insulation is allowing cold air to reach the shell? My rule of thumb was that you may start seeing cold end corrosion when the surface temperature drops 20degF (6.7degC) below the acid dew point, depending on sulfur content of the fuel.

 

[Orga78]

Thanks greenche

We are in the midst of our investigation, gathering as much data as we could to find the cause of the cold-end phenomenon.

This is a Tail Gas Incinerator Unit where it burns the tail gas (from CBA Reactor), with feed gas (methane) and air.

Detection of the corrosion was only known during shut-down, where the refractory lining of the incinerator shell has shown "thinning" and some erosion too especially at bottom of the incinerator, and also at top although not as severe as at bottom section. This led to investigation initiative.

Prelim checks were done, and found cold-end symptoms shown during combustion at top and bottom section of the incinerator, measured by thermocouples - to measure the skin temp. When this occurs (cold-end), the ops folks will fine tune the incineration parameters to keep the skin temp > acid gas dew point of 126 degC. Some instances, they managed to control, and some period, it is a challenge.

Indeed the sulfur content in the Tail gas plays huge role in this issue. But the ops mentioned they do not have problem getting the combustion temp within operating condition (650 degC), except for the skin temp drop below dew point limit.

 

[Orga78]

Hi all,

We have recently done investigation of the cold-end corrosion case on 4 (four) of our incinerators.

From the analysis, we have seen a correlation between the low skin temperature of the incinerator vs weather temperature --> where high low skin temp alert occurred especially during winter season with minimum outside temperature is < 10 degC. Acid gas dew point limit is 126 degC. During these instances, the skin temp of the incinerators would go < 126 degC; minimum at 116 degC.

Here are some other facts:

1. Corrosion is found and detected during unit shut-down periodic inspection.
2. Perforation and pitting seen on shell wall mainly at 6 o'clock and some at 12 o'clock.
3. Similar corrosion occurred on all 4 (four) units of incinerators.
4. Combustion temperature is always maintained at 650 degC.
5. Burners are all in good condition - no damage or chokage, oxidation observed during inspection for all 4 units.

what are the possible causes why outside weather could influence acid gas dew point corrosion in incinerator?

Could the insulation play some part in this phenomenon?

It seems there is a potential heat-transfer issue at the incinerator wall - where heat loss occurred through convection, causing the cold-end zones at incinerator bottom and top section.

 

[weldstan]

How often do you start up and shut down? Frequent start ups and shut downs can lead to the cool temps during same.