Mercury Control and Mercury Stability in Byproducts - Mercury Emission and its Control in Chinese Coal-Fired Power Plants - page 138

5.5.2 Mercury Stability in Actived Carbon Surface
The injection of AC upstream by either an ESP or an FF is a retrofit control tech-
nology that can be potentially applied to all coal-fired power plants, since it can
control almost all kinds of mercury. The stability of mercury captured on the AC is
very important to avoid possible mercury reemission.
Coal combustion byproducts (e.g., fly ash, bottom ash and slag, fluidized
combustion bed materials, and FGD residues) are either disposed of or utilized.
These byproducts are often used as cement replacement in concrete/grout, structural
fill, stability components for waste and soil, road base, mineral filler, concrete
block, blasting grit, wallboard, etc. Currently, the amount of mercury leached from
coal combustion byproducts is extremely low and does not appear to present a
serious hazard to humans. However, only limited information has been reported on
the stability of mercury captured in sorbents. This section aims to investigate the
released properties of mercury captured in AC under conditions of disposal and
reuse using leaching and thermal desorption techniques.
The stability of mercury in AC with respect to its potential disposal and reuse
conditions was studied using two techniques: leaching and thermal desorption.
Leaching was carried out using a leaching protocol called the toxicity characteristic
leaching procedure (TCLP) and a column test with five different leaching solutions.
TCLP is an accelerated experimental method designed to simulate conditions in an
anaerobic landfill; therefore, it is supposed to represent the worst-case leaching
conditions in a landfill where organic wastes are co-disposed.
The TCLP is a batch test and involves leaching a mass of waste material with an
acetic acid solution, using a 20:1 liquid-to-solid (L/S) mass ratio, and rotating the
mixture for (18±2) h at 30 rpm. TCLP pollutant concentrations are also used to
determine whether hazardous waste complies with land disposal restrictions and
whether waste may be disposed of in an unlined landfill that contains co-disposed
organic wastes.
Mercury released from AC was studied at air temperatures of 60 and 90 °C. An
aqueous thermostat was used to maintain the desired temperature. The results of
thermal desorption experiments can verify whether fly ash containing AC, which
has adsorbed mercury on its surface, is suitable for disposition in some thermal
conditions.
5.5.2.1 Mercury Stability in Three Activated Carbon Sorbents with TCLP
Three types of AC were used in the experiments. The first was the commercially
available untreated AC prepared from wood by the zinc chloride method [AC(MZ)]
and supplied by Shanghai Activated Carbon Co., Ltd. The other two were modified
from as-received AC through impregnation with MnO 2 solution [AC(MnO 2 -MZ)]
and FeCl 3 solution [AC(FeCl 3 -MZ)], respectively. The treated ACs had stronger
oxidability than untreated AC.