Control of Particles from a Lime Kiln using an ESP The kiln described previously in Example 9.7. The volumetric flow rate of the exhaust gas is 50,000 SCFM, and a collection efficiency of 99.% is sought. In the previous example problem, the actual volumetric flow rate was computed to be 73,100 ACFM

Question

Control of Particles from a Lime Kiln using an ESP
The kiln described previously in Example 9.7. The volumetric flow rate of the exhaust gas is 50,000 SCFM, and a collection efficiency of 99.% is sought. In the previous example problem, the actual volumetric flow rate was computed to be 73,100 ACFM.
To do: Estimate the total collecting surface area of an ESP that can be used to capture lime dust from the kiln.

Accepted Answer

From Table 9.3, the precipitation parameter (w) associated with lime dust is found to be 1.5 cm/s (about 0.0492 ft/s). The magnitude of the area of the collecting plates can be estimated from the Deutsch equation, Eq. (9-76),

Table 9.3 Typical precipitation parameters or migration velocities, w (in units of cm/s) for plate-wire electrostatic precipitators (abstracted from Vatavuk, EPA Report 1990).
process (ESP temperature)	design efficiency (%)
process (ESP temperature)	95.0	99.0	99.5	99.9
bituminous coal fly ash (300 °F)	12.6	10.1	9.3	8.2
sub-bituminous coal fly ash in a tangentially-fired boiler (300 °F)	17	11.8	10.3	8.8
cement kiln (600 °F)	1.5	1.5	1.8	1.8
glass plant (500 °F)	1.6	1.6	1.5	1.5
incinerator fly ash (250 °F)	15.3	11.4	10.6	9.4
iron/steel sinter plant dust with mechanical precollector (300 °F)	6.8	6.2	6.6	6.3
lime dust	1.5	1.5	1.5	1.5

[latex]\eta (D_p)=1-\frac{c_{out}(D_p)}{c_{in}(D_p)} =1-\frac{c_L(D_p)}{c_0(D_p)} =1-exp(-\frac{wA_s}{Q} )[/latex] (9-76)

[latex]\eta _{overall} =1-exp(-\frac{wA_s}{Q} )[/latex]

where Q refers to the actual volumetric flow rate. Solving for the plate surface area,

[latex]A_s=-\frac{Q}{w} \ln (1-\eta _{overall})[/latex]

which yields

[latex]A_s=-\frac{73,100\frac{ft^3}{min} }{0.0492\frac{ft}{s} } (\frac{min}{60 s} )\ln (1-0.99)\cong 110,000 ft^2[/latex]

The value is reported to two significant digits because of the uncertainty in the value of w. Note that although the Deutsch equation was derived for a single passageway between two parallel plates, and [latex]A_s[/latex] was defined as the surface area of one collecting plate, the equation is also valid for the case of many collecting plates, as in Figure 9.39. Here, [latex]A_s[/latex] represents the total surface area of all the collecting plates combined.

Discussion: While not asked for in the problem, readers should realize that a dust concentration of 10 g/m³ is huge, but is nevertheless what can be expected in the exhaust gases from a lime kiln.
For this reason, it would be wise to consider using a cyclone collector upstream of the ESP to remove the large particles that are certainly expected to be present. Without removing the large particles, upstream modules of the ESP would be overloaded and performance would suffer.

Question 9.8: Control of Particles from a Lime Kiln using an ESP The kiln ...

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