Gas Emission Control - Mathematics and Statistics for the Environment - page 476

16.4.4.6 Contaminants
Particulate matter, entrained liquid droplets, and organic compounds that have high boiling points
can also reduce adsorber efficiency if present in the air stream. Any micron-sized particle of dust
or lint, which is not filtered, can cover the surface of the adsorbent. This greatly reduces the surface
area of the adsorbent available to the gas molecule for adsorption. Covering active adsorption sites
by an inert material is referred to as “blinding” or “deactivation.” To avoid this situation, almost all
industrial adsorption systems are equipped with some type of particulate matter removal device.
Entrained liquid droplets can also cause operational problems. Liquid droplets that are nonadsorb-
ing can act the same as particulate matter: The liquid covers the surface, blinding the bed. If the
liquid is the same as the adsorbate, high heats of adsorption occur. This is especially a problem in
activated carbon systems where liquid organic droplets carried over from the process can cause bed
fires from the released heat. When liquid droplets are present, some type of entrainment separator
may be required.
16.5 INCINERATION
Incineration (or combustion) is a major source of air pollution; however, if properly operated, it can
be a beneficial air pollution control system in which the object is to convert certain air contami-
nants (usually organic compounds classified as volatile organic compounds (VOCs) and/or air toxic
compounds (Spellman, 1999; USEPA, 1973). Incineration is a chemical process defined as rapid,
high-temperature gas-phase oxidation. The incineration equipment used to control air pollution
emissions is designed to push these oxidation reactions as close to complete incineration as possible,
leaving a minimum of unburned residue. Depending upon the contaminant being oxidized, equip-
ment used to control waste gases by combustion can be divided into three categories: direct-flame
incineration (or flaring), thermal incineration (afterburners), or catalytic incineration.
16.5.1 F aCtors a FFeCting i nCineration For e mission C ontrol
The operation of any incineration system used for emission control is governed by seven variables:
temperature, residence time, turbulence, oxygen, combustion limit, flame combustion, and heat.
For complete incineration to occur, oxygen must be available and put into contact with sufficient
temperature (turbulence) and held at this temperature for a sufficient time. These seven variables are
not independent—changing one affects the entire process.
16.5.1.1 Temperature
The rate at which a combustible compound is oxidized is greatly affected by temperature. The higher
the temperature, the faster the oxidation reaction will proceed. The chemical reactions involved in
the combination of a fuel and oxygen can occur even at room temperature, but very slowly. For
this reason, a pile of oily rags can be a fire hazard. Small amounts of heat are liberated by the slow
oxidation of the oils. This in turn raises the temperature of the rags and increases the oxidation rate,
liberating more heat. Eventually a fully engulfed fire can break out (USEPA, 1981, p. 3-2). Most
incinerators operate at higher temperature than the ignition temperature, which is a minimum tem-
perature. Thermal destruction of most organic compounds occurs between 590 and 650°F (1100 and
1200°F). However, most incinerators are operated at 700 to 820°C (1300 to 1500°F) to convert CO
to CO 2 , which occurs only at these higher temperatures.
16.5.1.2 Residence Time
Much in the same manner that higher temperature and pressure affect the volume of a gas, time
and temperature affect combustion. When one variable is increased, the other may be decreased
with the same end result. With a higher temperature, a shorter residence time can achieve the same