Page 151 - Wastewater Solids Incineration Systems

118 Wastewater Solids Incineration Systems

of the fluid bed are 820 to 870°C (1500 to 1600°F), with excursions to 930°C (1700°F)
or higher. A typical temperature range of preheated air is 540 to 650°C (1000 to
1200°F); however, this can rise even higher during furnace temperature excursions.
Although the feed cake is dewatered before incineration, fluid bed furnace exhaust
flue gases still contain approximately 35 to 45% water vapor by volume.
Fluid bed incinerators must, by function, operate as an oxidizing environment
with excess air rates of 40% or greater; however, occasional upsets may create
reducing environments. This situation not only aggravates corrosion but also intro-
duces the risk of after-combustion in the heat exchanger and connecting ductwork.
Chlorides in the feed cake are another common source of corrosion attack. The typ-
ical composition of flue gases was discussed in section 2.3 of this chapter.
Many incinerators are run on a demand basis rather than continuously. A
common operating cycle follows the normal working week: 8 hours per day; 5 days
per week; or, in some cases, 24 hours per day, 5 days per week. Such cyclic operation
can be damaging, both from mechanical stress and materials wear. Repeated transi-
tions through the dewpoint range are particularly harmful because this can cause
corrosive condensation, deposits, and potential plugging problems. If lime is intro-
duced to the feed cake for sulfur control, then plugging can be further aggravated,
which can lead to corrosion, erosion, and mechanical problems.

4.1.2 Air Preheater Design
With an FGTT design, each tube is fitted with an expansion joint to allow differential
thermal movement between individual tubes. As shown in Figure 6.5, large main expan-
sion joints compensate for differential movement between the tube bundle and the cool,
refractory-lined shell. Most modern expansion joint bellows are made of Alloy 625.
A typical FGTT recuperator may have 91 tubes of type 304 stainless steel or alloy
20, arranged on a triangular pitch. Tube (pipe) size may be 9 m long 89 mm diam-
eter 3 mm wall thickness (30 ft 3.5 in 0.120 in). Units have been built with as
few as 19 and as many as 162 tubes. Pipe sizes range from 50 to 200 mm (2 to 8 in) in
diameter, illustrating the range of the preheater supply and capacity.
Tubes are welded directly to the tube sheet. Alloy or stainless steel is typically
specified for the upper tube sheet, which is exposed to the full temperature of the hot
air and supports the entire tube bundle. The shell, flue gas plenums, and lower tube
sheet are carbon steel. Insulating refractory linings are typically required for the
entire casing and hot face on the tube sheets. Dense, abrasion-resistant refractories
help avoid erosion by the particulates in the flue gas. Vapor barriers or coatings on
interior carbon steel surfaces minimize acid attack on flue gas plenums.