# The Corrosion Failure Mechanism of a Peak Load Boiler in a District Heating System
## 3 Results
### Simulation on Gas Temperature Distribution
Figure 9: Simulated temperature distribution of the combustion gas entering into the lower end plate of the convection tube, (**a**) temperature distribution of the inlet flowing into the convection tube, and (**b**) temperature variation from center to wall (edge) direction. Figure 10 illustrates the temperature distribution of combustion gas along the tube height, comparing a tube located at the center with one near the boiler wall. The x-axis represents the inner diameter of the tube, while the y-axis denotes the gas temperature at different heights from the lower end plate. The simulation results indicate that, in both cases, the gas temperature decreases progressively as it ascends through the tube, reflecting heat exchange with the boiler water. A radial temperature gradient was observed, where the gas at the tube centerline remained at a higher temperature than the gas near the tube wall due to convective heat transfer effects. Comparing the two tube locations, the tube near the boiler wall exhibited consistently lower gas temperatures at equivalent heights compared to the centrally located tube. This difference is attributed to two main factors: (1) reduced gas inflow near the boiler wall, which limits the available thermal energy, and (2) enhanced heat dissipation due to increased heat transfer between the tube and the surrounding boiler water. These combined effects resulted in a more significant temperature drop near the wall region. Figure 10: Temperature change in the gas as a function of height from the lower end plate located (**a**) at the center and (**b**) at the wall of the lower plate (Figure 8b). X-axis denotes an inner diameter of tube. Figure 11 shows how the gas temperature changes along the inner tube surface at different heights, comparing the center and wall-side tubes. The temperature of both tubes gradually decreases as the gas moves upward. However, the wall-side tube consistently exhibits lower temperatures than the center tube, with a temperature difference of 70 \({}^{\circ}\)C (343 K) at a height of 6 m from the lower end plate. The temperature drop is particularly pronounced at 4 m, where the turbulator is installed. Notably, in this region, the wall-side tube temperature falls below 60 \({}^{\circ}\)C (333 K). This temperature difference is likely influenced by the proximity to the boiler water supply and the surrounding tube arrangement. The lower surface temperature of the wall-side tube suggests a higher likelihood of condensation, as moisture in the air can accumulate on surfaces below 100 \({}^{\circ}\)C (373 K). Additionally, the presence of acidic ions such as NO3\({}^{-}\), Cl\({}^{-}\), and SO4\({}^{2-}\) can further promote dew formation even at elevated temperatures, as described by the following equations [23, 27]. The partial pressure of moisture, chloride, and acid ions directly affects the dew point temperature [3, 8, 26, 27, 28]. \[1000/(\mathrm{T_{d}}+273.15) =\{2.9882-0.1376\times\mathrm{ln}(\mathrm{P_{H2O}})-0.2674\times \mathrm{ln}(\mathrm{P_{SO3}})+0.03287\times\mathrm{ln}(\mathrm{P_{H2O}}\times \mathrm{P_{SO3}})\} \tag{1}\] \[1000/(\mathrm{T_{d}}+273.15) =\{3.9526-0.1863\times\mathrm{ln}(\mathrm{P_{H2O}})+0.000867\times \mathrm{ln}(\mathrm{P_{SO2}})-0.00091\times\mathrm{ln}(\mathrm{P_{H2O}}\times \mathrm{P_{SO2}})\}\] (2) \[1000/(\mathrm{T_{d}}+273.15) =\{3.6614-0.1446\times\mathrm{ln}(\mathrm{P_{H2O}})-0.0827\times \mathrm{ln}(\mathrm{P_{HNO3}})+0.00756\times\mathrm{ln}(\mathrm{P_{H2O}}\times \mathrm{P_{HNO3}})\} \tag{3}\]  
where Td is dew point temperature, PHO is the partial pressure of water vapor, PSO3 is sulfur trioxide, PSO2 is sulfur dioxide, P HNO3 is nitric acid. Dew containing acid ions creates an acidic environment on the surface of tube and causes corrosion of carbon steel. Irregular operating conditions of the PLB cause the accumulation of acid ions and create a stronger acidic environment, thus accelerating the corrosion of carbon steel tubes leading to corrosion failure.