Methods to Minimize Low-Temperature Corrosion Problems
Methods to Minimize Low-Temperature Corrosion Problems
There are two methods to handle the corrosion problem. One is to allow condensation of acid vapor on the economizer but select proper materials to minimize the corrosion rate. This is discussed in Chapter 6. The other option is to avoid condensation itself by using a higher feed water temperature at the economizer inlet.
152 Steam Generators and Waste Heat Boilers: For Process and Plant Engineers
Peak corrosion
Dow point
Corrosion rate
Wall temperature, °C
FIGURE 3.39
Tube wall temperature and corrosion rate.
As mentioned earlier, by increasing the lowest tube wall or surface temperature to above the acid dew point, one can prevent deposition of acid vapor on the heating sur- faces. This will also ensure that water does not condense on the exchanger surface as the water dew point is much lower than the sulfuric acid dew point. However, this option increases the exit gas temperature from the boiler thus reducing the boiler efficiency.
Figure 3.39 shows the acid corrosion rate versus tube wall temperature based on tests conducted by Land Instruments. This is similar to Figure 1.10. It shows that the corrosion rate is higher not at the acid dew point but at about 10°C−15°C lower temperature. The reason for a higher corrosion rate at lower temperatures is that the acid deposition rate is highest in that area; the rate of condensation of sulfuric acid depends on its concentra- tion and temperature depression (the difference between the metal and acid dew point temperatures). As sulfuric acid condenses out of the flue gas stream, its concentration in the flue gas decreases; this results in maximum acid deposition rate. The general practice in the boiler industry is to design the economizer or air heater prone to low-temperature corrosion so that corrosion is never initiated. Hence, the lowest tube wall temperature is selected to be slightly above the acid dew point. However, this results in a higher boiler exit gas temperature and loss in efficiency. The curve shows that the tube wall temperature may be slightly lower than the dew point, and the increase in efficiency is worth the slight increase in corrosion rate. Note that every 22°C decrease in exit gas temperature results in 1% improvement in efficiency for oil and gaseous fuels.
If the incoming feed water temperature is significantly lower than the acid dew point temperature, the following options are available to the plant engineer.
• A steam-to-water heat exchanger may be used to raise the incoming feed water
temperature (Figure 3.40). • The water leaving the economizer may be used to heat the incoming feed water,
or portion of hot water may be recirculated to the economizer inlet as shown in Figure 3.41.
• A coil located inside the steam drum may be used to preheat the feed water to
near-acid dew point temperature (Figure 3.42). The sizing procedure for this coil is discussed in Chapter 6.
• In an air heater, a steam coil may be used to preheat the incoming air to increase
the minimum cold-end temperature.
Steam Generators 153
Steam–water
Steam for preheating
heat exchanger Feed water
To process
Economizer
Condensate To drum
FIGURE 3.40
Using steam for preheating feed water to minimize low end corrosion problem.
Feed water Water-to-water
Economizer heat exchanger
Gas
To drum
(a) Feed water
Economizer
To drum
(b)
Recirculating pump
FIGURE 3.41
Using hot water from economizer outlet for preheating incoming feed water or recirculating hot water to increase incoming water temperature. (a) Using a heat exchanger and (b) using a recirculation pump.
• Cold air may be bypassed around the air heater so that the tube wall temperature
is higher with the larger flue gas-to-air ratio. • Some even suggest a parallel-flow economizer where the gas temperature is higher
at the cold water end. However, one should perform calculations as shown earlier to ensure that the tube wall temperature is above the acid dew point at all operat- ing loads. A better solution is to raise the inlet water temperature.
154 Steam Generators and Waste Heat Boilers: For Process and Plant Engineers
Cold feed water to exchanger coil
Steam drum
Hotter water to economizer inlet
Economizer
To drum
Flue gas
FIGURE 3.42
Using drum coil exchanger to preheat the feed water.