Page 353 - Air Pollution Control Engineering

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Flare Process 331

the recent regulatory requirements of 98% destruction efficiency for flares. The calcu-
lation procedure will be illustrated for emission stream 3 described in Table 1 using a
steam-assisted flare system. Note that flares often serve more than one process unit
and the total flow rate to the flare needs to be determined before the following calcu-
lation procedure can be applied. A number of flare sizing software packages have
been developed. One example is the Pegasus algorithm, described elsewhere (8). Related
hazardous-waste-treatment technologies and HAP emission control technologies can
be found from two United Nations Industrial Development Organization (UNIDO)
technical reports (12,13).

2. PRETREATMENT AND ENGINEERING CONSIDERATIONS
2.1. Supplementary Fuel Requirements
Based on studies conducted by the EPA, relief gases having heating values less than
300 Btu/scf are not ensured of achieving 98% destruction efficiency when they are
flared in steam-assisted or air-assisted flares. Therefore, the first step in the evaluation
procedure is to check the heat content of the emission stream and determine if additional
fuel is needed (1–3).
In a permit review case, if the heating value of the emission stream is less than 300
Btu/scf and no supplementary fuel has been added, the application is considered
unacceptable. The reviewer may then wish to follow the following calculations. If the
reported value for the emission stream heat content is above 300 Btu/scf, the reviewer
should skip to Section 2.3.
If the emission stream heating value is less than the 300 Btu/scf required to achieve
a destruction level of 98%, it is assumed that natural gas will be added to the emission
stream to bring its heat content to 300 Btu/scf. Calculate the required natural gas
requirements using
Q = ( [ 300 − ) ] 582 (1)
hQ
e
e
f
where Q is the emission stream flow rate (scfm), Q is the natural gas flow rate (scfm),
e f
h is the emission stream heating content or value (Btu/scf), and 582 = 882−300; 882 is
e
the lower heating content or value of natural gas (Btu/scf). If the emission stream heating
value is greater than or equal to 300 Btu/scf, then Q = 0.
f
2.2. Flare Gas Flow Rate and Heat Content
The flare gas flow rate is determined from the flow rates of the emission stream and
natural gas using
Q = Q + Q (2)
flg e f
where Q is the flare gas flow rate (scfm). Note that if Q = 0, then Q = Q .
flg f flg e
The heating value of the flare gas (h ) is dependent on whether supplementary fuel
flg
is added to the emission stream. When h is greater than or equal to 300 Btu/scf, then
e
h = h . If h is less than 300 Btu/scf, supplementary fuel is added to increase h to 300
flg e e e
Btu/scf, and h = 300 Btu/scf.
flg