(465f) A Method for Identifying Different Forms of Trace Metal Emissions From Coal-Fired Power Plants

A method for identifying
different forms of trace metal emissions from coal-fired power plants  

Shuang Deng^1,2*, Fan Zhang ¹, Chen zhang¹_,

1. Chinese Research Academy of
Environmental Science, 2. State Key Laboratory for  Environmental
Criteria and Risk Assessment,Beijing 100012,China

Trance metals(TM) such as
Hg, Pb, Mn and Cd present in
coal receives increasing concern for public health, and most of these trace
metals evaporate during coal combustion. Coal consumption was over 3 billion tons in China in 2010, among which 1.578 billion tons were used in coal-fired power plants. Coal-fired power plants have produced several
hundred tons of TM emissions into environment each year in China, so TM emissions from coal-fired power plants
are an important issue for China.

In flue gas, trace metals exist primarily in
three forms, element metals, oxidized metals, and particle-associated metals, which
can be removed by different methods from flue gas due to their different
properties. For example, most of oxidized metals are water-soluble and can be
removed by wet flue gas desulfurization (Wet-FGD). Since the content of atmospheric
TM in flue gas is typically very low (smaller than 10 µg/m³),
it is challenging to accurately identify different forms of TM emissions. Therefore,
an accurate and reliable analytical method is highly desired for measuring and
controlling different forms of TM emissions from coal-fired power plants.  

The objective this study is to develop
a method for the identification of different forms of TM emissions from
coal-fired power plants. A new method has been developed to determine different
forms TM emissions including Hg,
Pb, Mn, and Cd in the flue gas
based on EPA method-29 and Ontario Hydro Method, which are recommended by the
United States (US) Environmental Protection Agency (EPA) to determinate of TM emissions.
For the new method (as shown
in Figure 1), particle-bound TM are collected in the front half of the sampling
train. Eight ice-bath-chilled impingers are connected in a series with
leak-free ground glass fittings for collecting gaseous emissions. Because
oxidized metals such as Hg²⁺, Mn²⁺ and Cd²⁺ are water-soluble, the first three impingers
(starting from the left) all contain 100 ml of KCl solutionfor absorbing and
retaining oxidized-state metals in the gaseous phase. The fourth and fifth
impingers, which contain 100 ml of 5%(v/v) HNO₃/10%(v/v) H₂O₂,
absorbs and retains elemental state of TM in the gaseous phase. The sixth and seventh
impingers contain 100 ml of 4%(v/v) KMnO₄/10%(v/v) H₂SO₄
for element Hg capture. The eighth impinger contains 200-300 g of dried and pre-weighed silica gel and is used to determine the moisture gain and to prevent moisture condensation in the pump and the metering device. Samples are recovered,
digested, and finally analyzed for TM using hydride generation inductively
coupled plasma atom emission spectrometry (HG-ICP-AES) or inductively coupled
plasma mass spectrometry (ICP-MS).

Figure 1 Schematic of Trace metals-Sampling
Train.

Three methods, i.e., OHM, M-29, and our
proposed new method, were used to measure different forms of TM emissions from a 300 MW power plant in China. The
consistence and reproducibility of raw data from three methods has been
investigated, and the results clearly show that our method can accurately, reliably
and reproducibly measure the
different forms TMs in flue gas.