Page 138 - Wastewater Solids Incineration Systems

Heat Recovery and Reuse 105

FIGURE 6.1 Flue gas energy versus temperature (0.5556 [°F 32] °C).

gas on a molar or volume basis. Because of high moisture content, a large fraction of
the furnace exhaust energy is the latent heat of vaporization of this water, which is
generally not recoverable in this type of application. Figure 6.1 shows a typical graph
of the fraction of total flue gas energy versus temperature, with 100% occurring at
870°C (1600°F) and 0% occurring at 16°C (60°F).
The steep segment of the line occurs at 100°C (212°F) and represents the latent
heat of vaporization of moisture in the flue gas. In a real-world application, conden-
sation would occur over a range of different temperatures, corresponding to
changing flue gas saturation properties. This simplified example, however, is for
illustration purposes only. In a typical system, the latent heat of water vaporization
in the flue gas would equate to approximately 25% of total flue gas energy.
In WWTPs, few, if any, heat recovery systems are designed to recover energy from
flue gas less than approximately 175°C (350°F). High acid gas content of typical flue
gases makes acid dewpoint corrosion a significant concern at lower temperatures and
makes low-temperature heat recovery problematic. Allowing for a reasonable safety