Page 356 - Air Pollution Control Engineering

07_Chap_Wang.qxd 05/05/2004 4:19 pm Page 333
Flare Process 333

Table 2
Flare Gas Exit Velocities for 98% Destruction Efficiency
Flare gas heat constant a Maximum exit velocity
h (Btu/scf) U (ft/s)
flg max
<300 b
300 ≤ h <1000 3.28[10 (0.00118h flg + 0.908) ]
flg
>1000 400
a If no supplementary fuel is used :h flg = h .
e
b Based on studies by the US EPA, waste gases having heating values less than 300 Btu/scf are not
assured of achieving 98% destruction efficiency when they are flared in steam-assisted flares.

2.3. Flare Gas Exit Velocity and Destruction Efficiency
Table 2 presents maximum flare gas exit velocities (U ) necessary to achieve at
max
least 98% destruction efficiency in a steam-assisted flare system. These values are
based on studies conducted by EPA. Flare gas exit velocities are expressed as a func-
tion of flare gas heat content. The maximum allowable exit velocity can be determined
using the values in Table 2 (1,2,9). The information available on flare destruction effi-
ciency as a function of exit velocity does not allow for a precise determination of this
value. All that can be ascertained is whether the destruction efficiency is greater than or
less than 98%, based on the exit velocity.
If a flare is controlling an intermittent process stream (or streams), a continuous mon-
itoring system should be employed to ensure that the pilot light has a flame. If a flare is
controlling a continuous process stream, continuous monitoring of either the flare flame
or the pilot light is acceptable.
From the emission stream data (expected flow rate, temperature) and information on
flare diameter, the flare gas exit velocity (U ) may be calculated and compared with
flg
U . An engineer may use Eq. (3) to calculate U (6):
max flg
( 5.766 × 10 − 3 Q )( flg)( T + 460)
U flg = flg (3)
D 2
tip
where U is the exit velocity of flare gas (ft/s), Q is the flare gas flow rate (scfm),
flg flg
and D is the flare tip diameter (in.).
tip
If U is less than U , then the 98% destruction level can be achieved. However,
flg max
if U exceeds U , this destruction efficiency level may not be achieved. This indicates
flg max
that the existing flare diameter is too small for the emission stream under consideration
and may lead to reduced efficiency. Note, at very low flare gas exit velocities, flame
instability may occur, affecting destruction efficiency. In this text, the minimum flare
gas exit velocity for a stable flame is assumed as 0.03 ft/s. Thus, if U is below 0.03
max
ft/s, the desired destruction efficiency may not be achieved. In summary, U should fall
flg
in the range of 0.03 ft/s and U for a 98% destruction efficiency level.
max
In a permit review case, if U exceeds U , then the application is not acceptable.
flg max
If U is below U and exceeds 0.03 ft/s, then the proposed design is considered
flg max
acceptable, and the reviewer may proceed with the design or analysis calculations.