Page 477 - Air pollution and greenhouse gases from basic concepts to engineering applications for air emission control

458 15 Air Monitoring

In general, air emission monitoring is completed by gas sampling followed by
gas characterization. In addition, the properties of total gas flow from the source
must also be determined accurately before calculating the corresponding air
emission rates. If a gas sample with a sampling flow rate of Q s is taken from a
continuous emission source with a total flow rate of Q, typical value of Q s is much
less than Q, Q s ≪ Q. Characterization of the sample leads to the concentration of
certain air pollutant in the sample as C s . If this sample can be used to represent the
total gas, one can claim that the concentration of the corresponding air pollutant in
the main stream C 0 is the same as c s .
Now consider a simplified case as follows. The total gas flow rate discharged
3
from a stack to the atmosphere is Q (m /s), and a sample is taken with a volume
3
flow rate of Q s (m /s) for a period of time, t(s). Analysis of this sample, either in situ
or in a laboratory setting, shows that the mass of air pollutant of interest is m (kg).
With this information, one can calculate the mass concentration of the air pollutant
3
in the sample c ¼ m=Q s t (kg/m ). If the sample is representative, it can be used to
represent the concentration of the air pollutant in the stack: C = c. Then the mass
flow rate, _ m, of this air pollutant in the stack is determined as

Qm
_ m ¼ ðkg=sÞ ð15:16Þ
Q s t

The emission rate of the air pollutants can be calculated with the data obtained
from the samples. However, the samples taken from different stacks are different in
temperature, pressure, and moisture contents. The calculated results shall be cor-
rected to a standard temperature and pressure and expressed on a dry basis. This
standardized measure allows the source test results under different conditions to be
comparable to regulatory standards or the results of other similar tests.
A common standard for emission test report is to correct the concentrations
against 50 % excess air, 7 or 12 % of CO 2 ,or7%ofO 2 . Take 12 % of CO 2 as an
example, the calculated emission rate is converted using the equation that follows.
Actual procedure is much more complex, and readers are encouraged to seek local
guidelines for more information.
12 %
C std 12 % CO 2 Þ ¼ C s ð15:17Þ
ð
100 % y CO 2
is the mole fraction of CO 2 in the stack gas measured (e.g., following
where y CO 2
US EPA Method 3).