Page 375 - Air Pollution Control Engineering

08_chap_wang.qxd 05/05/2004 4:25 pm Page 351
Thermal Oxidation 351

Emissions occur from the surface of open-area sources, and high percentages of the
volatiles are lost as emissions in these sources. For enclosed sources, the displacement
of vapor containing volatiles from the enclosed air space is the emission mechanism.
For both types of source, heating or aeration increases emissions. Emissions also occur
from the evaporation of leaks and spills (8–11).
For emission control, open-area sources and containers can be covered or enclosed.
Control devices can be installed to collect and remove organics from vented vapors,
which is especially important if the sources are heated or aerated. Destruction of organic
vapor by incineration is one of the emission control options.
The applicability of thermal incineration depends on the concentration of oxygen and
contaminants in the waste gas. The waste gas composition will determine the auxiliary
air and fuel requirements. These requirements, in turn, will have a strong influence on
whether thermal incineration is an economical approach for controlling air emissions
(13–22). Thermal incineration is best suited to applications where the gas stream has a
consistent flow rate and concentration.
For most remediation technologies used at Superfund sites, the off-gases that require
control are dilute mixtures of VOCs and air. The VOC concentration of these gases tends
to be very low, whereas their oxygen content is high. In this case, auxiliary fuel is required
but no auxiliary air is needed. However, if the waste has VOC content greater than 25%
of its lower explosive limit (LEL), auxiliary air must be used to dilute the emission
stream to below 25% of its LEL prior to incineration. The LEL for a flammable vapor
is defined as the minimum concentration in air or oxygen at and above which the vapor
burns upon contact with an ignition source and the flame spreads through the flammable
gas mixture. Emission streams from some soil vapor extraction-based cleanups may
contain VOCs greater than 25% of the LEL.
If the remediation activity generates an off-gas that has low oxygen content (below
13–16%), ambient air must be used to raise the oxygen level to ensure the burner flame
stability. In the rare case when the waste gas is very rich in VOCs, using it directly as a
fuel may be possible.
Information is presented in Table 2 for determining the suitability of a waste gas for
incineration and establishing its auxiliary fuel and oxygen requirements. This same
information is shown in Fig. 3 in an alternative format.
2. PRETREATMENT AND ENGINEERING CONSIDERATIONS
2.1. Air Dilution
In hazardous air pollutant (HAP) emission streams containing oxygen/air and
flammable vapors, the concentration of flammable vapors is generally limited to less
than 25% of the LEL. Insurance companies require that if the emission stream is pre-
heated, the VOC concentration must be maintained below 25% of the LEL to minimize
the potential for explosion hazards. In some cases, flammable vapor concentrations up
to 40–50% of the LEL are permitted if on-line monitoring of VOC concentrations and
automatic process control and shutdown are provided. The LELs of some common
organic compounds are provided in Chapter 9 of this book.
In general, emission streams from waste-management facilities and Superfund sites
are dilute mixtures of VOC and air and typically do not require further dilution. For