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Minimization of Hg and trace elements during coal combustion and gasification processes 77

Table 3.4 Trace element removal and electrostatic precipitator (%)
Element Removal efficiency Element Removal efficiency
Antimony 81.0 Copper 99.6
Arsenic 99.1 Manganese 99.6
Barium 99.8 Mercury <20.0
Beryllium 97.4 Molybdenum 96.0
Cadmium 99.2 Nickel 98.2
Chromium 99.2 Phosphorous 98.0
Cobalt 99.3 Vanadium 99.5

Data from Easom, B.H., Smolensky, L.A., Wysk, S.R., Altman, R.F., Olen, K.R., 1998. Electrocore separator for
particulate air emissions. In: Proceedings of the 23rd International Technical Conference on Coal Utilisation and
Fuel Systems, Clearwater, FL, USA, 9e13 Mar 1998. Coal and Slurry Technology Association, Washington, DC,
USA, 683e692.
surface area on an equal mass basis. ESPs have lower capture efficiencies for ultrafine
particles, as smaller particles are more difficult to charge to a sufficient level for elec-
trostatic collection. Table 3.4 shows the average removal efficiency for TEs across a
conventional ESP on an unspecified boiler (Easom et al., 1998; Sloss and Smith, 2000).
The distribution of TEs in the inlet and outlet of an ESP system burning a bitumi-
nous Pittsburgh coal has been investigated (Tumati and DeVito, 1993). The concentra-
tions of several TEs were measured in the coal, bottom ash, and ash sampled at the ESP
inlet and outlet. The ESP retention efficiency for most solid phase TEs (except Ni, Se,
and Cd) was close to the overall particle removal efficiency (99.54%) on a mass basis
(Table 3.5).
The effectiveness of ESP in removing Hg from flue gases is dependent on the tem-
perature of the gases. On average, cold-side ESP systems (downstream of the air pre-
heater, 135e175 C) capture around 30% of the mercury in the coal, the capture rate

ranging from 0 to over 60% depending on the coal. Hot-side ESP systems (upstream of

the air preheater, closer to the boiler, 300e400 C) show lower mercury capture
(average 3%) (Sloss, 2008). The longer residence time and cooler temperature in the
cold-side systems is more conducive to mercury adsorption onto fly ash than the
shorter, hotter conditions in the hot-side ESPs. High flue gas cooling rates between
the air preheater inlet and the air pollution control device inlet can enhance reaction
rates associated with oxidation by species such as chlorine (Kolker et al., 2006).
FFs (baghouses) are the main alternative to ESP for LCPs. Baghouses are often
considered superior to ESP for the capture of fine particles and would therefore be ex-
pected to have higher collection efficiencies for the associated TEs. Because baghouse
collection efficiency is not affected by the resistance of the ash, baghouses are consid-
ered superior for use with low-sulfur coals (Rentz et al., 1996). However, baghouses
can have problems with on-line cleaning of high air-to-cloth ratio bags due to redisper-
sion of collected particulates. The presence of unburnt carbon in ash enhances mercury
capture by adsorbing oxidized mercury. Studies at a Western Kentucky power plant