Page 90 - Wastewater Solids Incineration Systems

Combustion Technology 59

TABLE 5.2 Fundamental parameters of design.

Parameter Values
Bed material size range, μm (U.S. mesh) 1 680–177 (10–80)
U gas velocity in bed, m/s (ft/sec) 0.75–1 (2.5–3)
o
U gas velocity in freeboard, m/s (ft/sec) 0.76–0.64 (2.5–2.1)
f
Excess air, over stoichiometry 40%
Freeboard temperature, °C (°F) 843 (1 550)
Residence time in freeboard, sec 6.5 minimum
Combustion air temperature Depends on percent total solids

There are several fundamental parameters: excess air, size, bed material, bed gas
spatial velocity, freeboard gas spatial velocity, temperature in freeboard, and gas resi-
dence time. See Table 5.2.

4.1.1 Size of Bed Material and Gas Velocities
In designing the fluid bed system, the selection of bed material is critical. As shown in
Figure 5.5, solid particle size has a direct effect on the quality of fluidization. For incin-
eration of WWTP solids, sandlike material with a median size of 550 μm (30 mesh) is
typically used. At bed operating conditions, using equation 5.1, the minimum fluidiza-
tion gas velocity U of the 550 μm (30 mesh) particle equals 0.33 m/s (1 ft/sec).
mf
Because optimal bed fluidizing gas velocity is in the range of 2.5 to 3 U , U is
mf o
equal to 0.75 to 1 m/s (2.5 to 3 ft/sec) for the selected solid. Gas corrected to bed tem-
perature and pressure should be used in sizing of the bed section.
Because entrainment increases with freeboard gas velocity, U is maintained as
f
low as possible. It is typically in the range of 0.76 to 0.64 m/s (2.5 to 2.1 ft/sec).
4.1.2 Excess Air
If the ultimate analysis of the feed and feed rates is known, then combustion air can
be calculated from the oxidation reactions shown in Table 5.1, with an excess of 40%
greater than stoichiometry. Because the combustion of dewatered wastewater solids
is a two-phase process (evaporation followed by combustion), almost one-half the
volume of gas in the reactor is water vapor. Therefore, designing the system based on
an excess air of less than 40% presents the risk of having incomplete combustion.
Greater than 40% excess air is not efficient and can generate higher emission of
nitrogen oxides (Dangtran and Holst, 2001).