Corrosion rate of sulphuric acid 1

[semarang]

Hi all, I have some difficulties to count the corrosion rate of sulphuric acid from the Boiler's flue gas.
As we know, in the fossil fuel power generating system, we've to maintain the flue gas temperature above to the dew point of sulphur. Its dew point is depend on the sulphur content in its fuel oil. So, if I set the flue gas temperature (in its cold end section of air heater), below the dew point of sulphur (at least 110oC), so there will be corrosion happens in the air heater's elements. The question is, can we measure, theoritically, the corrosion rate that happens in air heater's element? The material of the element is CRAS (corrosion resistant low alloy steel).

 

[metengr]

The creation of sulfuric acid as a result of ash deposits on the air heater tube surface coming in contact with moisture at the dew point of the flue gas can vary from a pH of 4 down to 1, depending on operating conditions. There are a lot of variables that can affect actual pH. The controlling variable is operating time under actual dew point conditions.

In lieu of trying to theoretically predict a corrosion rate, I would recommend installing some corrosion coupons of similar tube material and monitoring the corrosion rate under various operating conditions. I have found that obtaining actual corrosion rates using test coupons works best.

 

[rmw]

When you speak of air heater elements, please define. I take the term air heater elements as baskets in a regenerative Ljunstrom type APH, but Metengr comments on air heater tubes, indicating a tubular type. Each has its own particular set of pecularities, and problems. There are also welded plate types, less common, but very efficient, but not without problems.

If it is Ljunstrom, you have to remember that a brand new Ljunstrom in the "box" has something like a guaranteed 8% leakage rate, so that your APH flue gas outlet temperatures have to be high enough so that combination with this dilution of leakage air from the FD fans doesn't corrode the rest of the downstream system, ductwork, stack, etc.

The leakage rate gets worse with age, as seals wear, so that has to be accounted for as well.

By the time you put enough safety factor in all these leakage potentials, you have to have an actual APH outlet temp high enough that you lose a lot of heat recovery potential.

If it is a tubular APH, all of them have a cold corner where the 'cold' FD air initially enters the unit. This can be reduced as a problem for corrosion purposes by putting a steam coil air heater before the APH, but that takes steam that could be used elsewhere, (like producing electricity), and reduces the effectiveness of the APH by reducing the LMTD.

Plate types also have the potential for a 'cold corner' but they can have porcelein plates for the first bank of plates that see the coldest air.

rmw

 

[semarang]

Thank metengr and rmw,
That's right, I use the ljungstorm type. It made 20 years ago, and i'm not sure aboout the 8 % guaranteed leakage rate from its manufacturer. In the past 20 years, we use 114oC of cold end temp as our setting (dew point temperature), as the optimal temperature between the dew point of H2SO4 and plant efficiency. But now, the condition is different. The quality of Fuel Oil, as our main fuel,is getting worst (sulphur content is higher). Means that the 114oC setting has to be changed. In the other hand, if i raise the setting, the plant efficiency will drop. Now the question is, is there any the formula to find the corrosion rate of air heater element (from h2so4)? With this rate, i try to find the optimum relationship between the plant effeciency and the dew point temperature setting.