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376 Lawrence K. Wang et al.

Typically, the concentration of flammable vapors in HAP emission streams contain-
ing air is limited to less than 25% of the lower explosive limit (LEL) (corresponding to
a heat content of 176 Btu/lb or 13 Btu/scf) for safety requirements. To convert from
Btu/lb to Btu/scf, multiply Btu/lb by the density of the emission stream at standard con-
ditions (0.0739 lb/scf). Table 3 contains a list of LEL and upper explosive limits (UEL)
for common organic compounds. In order to meet the safety requirement and to pre-
vent damage to the catalyst bed, it is assumed in this handbook that catalytic incinera-
tion is directly applicable if the heat content of the emission stream (air and VOC) is
less than or equal to 10 Btu/scf. For emission streams that are mixtures of inert gases
and VOC (i.e., containing no oxygen), it is assumed that catalytic incineration is
directly applicable if the heat content of the emission stream is less than or equal to 15
Btu/scf. Otherwise, dilution air will be required to reduce the heat content to levels
below these cutoff values (i.e., 10 and 15 Btu/scf). For emission streams that cannot be
characterized as air and VOC or inert gas and VOC mixtures, apply the more conser-
vative 10 Btu/scf cutoff value for determining dilution air requirements. The dilution air
requirements can be calculated from Eq. (1); note that the dilution air will change the
emission stream parameters:

Q = [(h /h )−1]Q (1)
d e d e
where Q is the dilution air retirement (scfm), h is the heat content of the emission
d e
stream (Btu/scf), h is the desired heat content of the emission stream (Btu/scf), and Q
d e
is the emission stream flow rate (scfm).
2.2. Design Variables

Most catalytic incinerators currently sold are designed to achieve an efficiency of
95% (10). Table 4 presents suggested values and limits for the design variables of a
fixed bed catalytic incinerator system to achieve 95% destruction efficiency. In selected
instances, catalytic incinerators can achieve efficiencies on the order of 98–99%, but
general guidelines for space velocities at these efficiencies could not be found. For
specific applications, other temperatures and space velocities may be appropriate
depending on the type of catalyst employed and the emission stream characteristics (i.e.,
composition and concentration). For example, the temperature of the flue gas leaving the
catalyst bed may be lower than 1000ºF for emission streams containing easily oxidized
compounds and still achieve the desired destruction efficiency (4,5).
The destruction efficiency (DE) for a given compound may vary depending on
whether the compound is the only VOC in the emission stream or part of a mixture of
VOCs (5). The DE for a given compound in different VOC mixtures may also vary with
mixture composition. Table 4 can be used to determine the ranges for temperature at the
catalyst bed inlet (T ), temperature at the catalyst bed outlet (T ), and SV for different
ci co
catalysts based on the required DE.
When the catalyst bed inlet temperature is controlled at a proper level, the performance
of a catalytic incinerator system depends greatly on both the temperature and pressure
differential across the catalyst bed. The temperature differential or rise across the cata-
lyst bed is the fundamental performance indicator for a catalytic incinerator system, as
it indicates VOC oxidation efficiency. The pressure differential across the catalyst bed