Page 157 - Wastewater Solids Incineration Systems

124 Wastewater Solids Incineration Systems

the heater can generate as much as 1.47 to 5.9 MW (5 to 20 mil. Btu/hr) of 206°C
(500°F) oil, while cooling the flue gas to approximately 200 to 260°C (400 to 500°F).
The thermal fluid heater unit is generally an all-convective tubular type, but
either watertube (as with economizers) or firetube types can be used. With the water-
tube approach, the design is similar in concept to a serpentine tube economizer. The
hot exhaust gas exiting the primary air preheater flows vertically across the hori-
zontal tubes contained within the casing. Hot oil is circulated within multiple passes
in the tubes. Because of the potential for fouling, it is important that the tubes are
arranged on a rectangular pitch to accommodate the use of online cleaning equip-
ment. For the same reason, bare tubes (no finning) are used. To avoid erosive attack,
15 m/s (50 ft/sec) is considered a conservative inlet velocity for the flue gas. Internal
insulation of the casing is provided. Tube sheets at both ends support the tubes.
With the firetube approach, the design is similar in concept to the air preheater.
The hot exhaust gases exiting the primary air preheater flow vertically up and
through the vertical tubes. Hot oil is circulated within the shell, over the carbon steel
tubes. As with the air preheater, flue gas velocity within the tubes is high enough that
the unit is self-cleaning. An auxiliary sootblower cleaning system is not required. The
tube bundle is contained within a carbon steel shell. External insulation of the shell is
required. Top and bottom tube sheets at both ends support the tubes. Tubes are con-
nected by rolling, seal welding, and lightly rerolling into the tube sheets.
With any design operating at pressures greater than 103 kPa (15 psig), the bundle
will be designed, fabricated, tested, and stamped in accordance with the latest ver-
sion of the ASME Code, Section VIII, Division 1.

4.5 Waste Heat Recovery Boilers
A waste heat recovery boiler for WWTP incineration service falls into a narrow, spe-
cialized category in the overall family of boilers, which includes small boilers for res-
idential heating and power boilers for large utilities.
The usual inlet temperature of waste gases is between 540 and 980°C F (1000 and
1800°F), depending on the type of incinerator, the presence of an afterburner, and
whether a combustion air preheater is incorporated to the system. The expected dust
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loading for a fluid bed system can be as great as 45 to 70 g/m (20 to 30 gr/dry sq ft).
The boiler exit temperature must be maintained at greater than the acid dewpoint of
the flue gas, which can be as high as 120 to 180°C (250 to 350°F). The actual acid dew-
point depends on how much water vapor and acid gases, such as SO , SO , and HCl,
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