Mercury Sampling and Measurement in Coal-Fired Power Plants - Mercury Emission and its Control in Chinese Coal-Fired Power Plants - page 12

spectrometry (CVAAS), cold vapor atomic fluorescence (CVAFS), atomic excita-
tion spectrometry (AES), new chemical sensors, and so on [2] .
The process of CEM is the same as that of other flue gas pollution emission
monitoring systems. First, the analyzed object is obtained from the flue gas using
the sampler. The potential interferential components in the sampling gas, such as
HCl, SO 2 , SO 3 and other acidic gases, are eliminated before they go through the
detector. The concentration and speciation of mercury is then detected by the de-
tector. However, the continuous monitoring of mercury is complex because of the
existence of different mercury speciation, which are generally divided into ele-
mental mercury (Hg 0 ), oxidized mercury (Hg 2+ ) and particle mercury (Hg p ). The
CEM detector only detects Hg 0 . All the non-Hg 0 and Hg 2+ should be transformed to
gaseous Hg 0 through a transformation system for measuring total mercury by CEM.
Except for mercury chloride (HgCl 2 ) as the main form of Hg 2+ , research indicated
that there are still other Hg 2+ forms [3] .
In theory, Hg p can be converted into gaseous Hg 0 . However, most mercury
CEMs only measure total Hg (g) (TGM). A negative deviation should be introduced
into the measurement of TGM because the Hg p cannot be measured in actual con-
ditions. Furthermore, the fly ash on a filter can capture Hg (g) during sampling, which
will enlarge the negative deviation. Therefore, it is very important to monitor Hg p
during the analysis by CEM. A similar problem also exists during the quantitative
transformation of HgCl 2 . The HgCl 2 is water-soluble and more active; thus, its
quantity might decrease due to the adsorption that occurs while sampling.
2.3 Process of Mercury Sampling
For the present study, the sampling and analyzing of flue gas mercury were mostly
conducted using the OH method. At the same time, the CEM was also used to
monitor Hg (g) in some testing. Coal, fly ash, slag, and slurry liquid of WFGD were
also analyzed during the testing. Solid and liquid mercury samples were collected at
the same time of sampling Hg (g) . Mercury in the collected samples was mostly
analyzed by the direct detection method with MA2000 instrument, or the AFS
detection method with pretreatment by microwave digestion.
Flue gas mercury concentrations at the inlet and outlet of existing pollution
control devices were tested by sampling or on-line analysis, which can study the
mercury control capabilities of different conventional pollution control devices. Six
different coal-fired boilers with different pollution control equipment were chosen
in this mercury emission research. The detailed sampling conditions of the typical
coal-fired boiler systems are shown in Table 2.2. The concentration and speciation
analyses of flue gas mercury from No. 1 and No. 2 coal-fired boilers were con-
ducted according to the OH method; those from No. 3 and No. 5 were conducted
using the CEM method; and those from No. 4 and No. 6 were conducted according
to both the OH and CEM methods. In addition, the 324 method was also adopted
during the test of the Nos. 4, 5, and 6 coal-fired boilers.