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A computational fluid dynamic simulation was carried out to study the effects of the flue gases velocity in the last stage modules of a HRSG preheater. The results allows us to visualize a map of temperature in a preheater module, hence, to better understand how energy is distributed. In addition, it provides valuable information to predict points of low efficiency and propose estrategies for optimization.

Image 1 represents the cross section top view of a HRSG preheater module It also shows how temperatures vary at the back-end tubes. The increase of velocity in the flue gas would produce gradients of pressure between the two opposite sides of the tube. In consequence, this would eventually create a vortex effect and turbulences in the backside of the tubes. This is shown in image 2.

Turbulence would drop velocity down to zero near the surface, which would decrease the rate of energy transfer from the flue gas to the condensate. By the same token, the lack of flue gas circulation around the tubes would bring temperatures down below the dew point and end up creating droplets of condensation. Image 3 shows areas of high probability for this phenomenon to occur. In that event, a layer of condensation would develop all along the surface of the tubes, while drastically altering the heat convection and thermal conduction rate across the wall.

Back-end corrosion occurs when the gas turbine’s exhaust temperature falls below the dew point of any combustion product. Subsequently, high corrosive liquid acid would form in the presence of moisture. When natural gas contains sulfur in its composition, the reaction products derived from combustion will have, in addition to carbon dioxide and water, an equivalent proportion of sulfur dioxide. This compound will oxidize to sulfur trioxide and create sulfuric gas as it combines with the humidity of the flue gas.

If the temperature of the flue gas falls below the dew point of the sulfuric gas, liquid acid will form on the surface of such regions.

The aggressiveness of this attack depends on the concentration of acid in the condensate, which depends on the equilibrium H2O – H2SO4.

This simulation allows us to predict points of high probability where condensation droplets might form in the preheater module, so satisfactory mitigation strategies can be implemented during the HRSG design.

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