Page 478 - Air pollution and greenhouse gases from basic concepts to engineering applications for air emission control

15.5 Continuous Emission Monitoring … 459

15.5 Continuous Emission Monitoring and Opacity
Measurement

Manual source test is a time-consuming and labor-intensive process, and there has
been a growing interest in a more efficient, automatic approach. Advances in
instrumental analyzers have enabled continuous emission monitoring (CEM) with
modern instruments that can quickly identify many gas compounds at very low
concentrations. CEM also allows monitoring of peak emissions.
There are many types of CEM instruments and systems for continuous moni-
toring of air emissions from stationary sources. They can be classified into
extractive and In situ systems. Extractive systems can be further categorized as
either dilution or direct. In situ analyzers directly measure source-level concen-
trations by changing the original gas properties by removal of particulate or
interfering vapors or gases.
There are two types of in situ analyzers: path and point measurements. Either
single or double path CEM instruments perform measurement across the entire duct
or stack at certain elevation, whereas point measurement is completed by a sensor at
a specific point or over a path of only a few centimeters distance. Typical in situ
CEM systems can tolerate harsh environment at most locations and require fewer
accessories such as probe heaters, gas conditioning systems, tubing and pumps than
their extractive predecessors [7].
The 1970s saw advancements in German optical systems, which have shown
great promise for measuring gas opacity, an indicator of the air emission rate [7].
The opacity of the stack gas is a function of light transmission through the plume
and is determined by

OP ¼ð1 I=I 0 Þ 100 ð15:18Þ

where OP = percent opacity, I = light flux leaving the plume, and I 0 = incident light
flux.
Opacity is more lenient than the corresponding mass emission standards. The
USEPA developed Method 9 to quantify the opacities of the plumes emitted
from stationary point sources (http://www.epa.gov/ttn/emc/promgate/m-09.pdf).
Recently, Du et al. [3] reported a method to quantify the plume opacity by digital
photography and Du et al. [4] published a paper for fugitive fume detection by opacity.
Most air pollutants can now be continuously monitored by different instruments.
The selection of the instruments is important to the design of the monitoring sys-
tem. The choice of instrumentation for an air monitoring depends on type of pol-
lutants and expected concentrations, accuracy required by local air quality
standards, budget and personnel, etc. In the USA, CEM is required by the Clean Air
Act Amendments to monitor SO 2 ,NO x , CO, CO 2 , opacity, total hydrocarbons, and
total reduced sulfur (TRS). All the instruments are required for calibration before
being used on site.