Page 328 - Air pollution and greenhouse gases from basic concepts to engineering applications for air emission control

10.6 Control of Volatile Organic Compounds 305

costs may become excessive. Gases with VOC concentrations above 25 % of the
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LFL can be processed with a wide range capacity of 0.05–10 m /s. Readers are
referred to literature (e.g., [11]) for the principles and designs of VOC condensers.
Control of VOCs can also be done with a liquid solvent by absorption in a wet
scrubber. It is based on the principle of absorption introduced above. The choice of
wet scrubber depends on the presence of particulates in the gas stream. Absorption
using liquid solvent is suitable for gas streams with high moisture contents
(RH > 50 %) and the concentrations of VOCs in the range of 500-5000 ppmv. It can
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function at a higher flow capacity of 1–50 m /s.

10.7 Control of Soot Particles

Ideally soot should be oxidized completely before leaving a combustion device
(e.g., engine or furnace). However, soot is a big problem for combustion of solid
fuel and diesel due to lack of knowledge about the formation and oxidative
destruction of soot. Recent studies indicate that a lot of soot is also generated from
bio-diesel fueled diesel engines.
A particulate control system can separate and collect some of the soot as part of
the total particulate matter. Other options for the control of soot after combustion
are thermal oxidation, catalytic oxidation and carbon filtration. In most of the cases,
they are integrated into one unit. An example of a popular soot control technology
is the continuously regenerating trap (CRT) system. Briefly, NO in the exhaust gas
is converted to NO 2 by catalytic oxidation right before the gas entering the filter.
Then the carbonaceous particles collected on a filter are oxidized at 250 °CbyNO 2 .
Meanwhile, 80–90 % of hydrocarbon and CO are also oxidized. The removal
efficiency of soot particles can be 90 % or higher.

10.8 Control of Trace Metals

As introduce above, trace elements that are found in fossil fuels can be grouped into
three classes, Class I, Class II (Pb, Cd, Sb, Ni), and Class III (Hg, Se, As). As
mentioned in the pre-combustion control technologies, some of these chemicals can
be removed from the fuels by fuel cleaning. Crushing and milling of coal particles
can also remove As, Se, FeS x , and significant amount of Hg from the fuel. The
elements related to sulfide minerals (pyrite, sphalerite) can be largely removed by
coal pre-cleaning: As, Cd, Co, Cu, Hg, Mo, Ni, Pb, Se, and Zn [20].
Class I and Class II elements are removed together with the particulates by the
dust control system such as ESP and bag-house filter at efficiencies of >99 and
90–99 %, respectively. Trace elements that penetrate through an ESP or bag-house
filter may be captured downstream by a wet or dry scrubber, likely for SO 2 control.
Wet scrubber, if any, also can capture some of the submicron particles downstream