Page 120 - New Trends In Coal Conversion

Minimization of Hg and trace elements during coal combustion and gasification processes 83

3.6 Concluding remarks

TEs even when present in parts per million levels in coal can result in emission of
several tons of these pollutants into the environment because of the huge amount of
coal burned. Most TEs reaching the boiler will be released and redistributed into bot-
tom ash, fly ash, fine fly ash, and the gaseous phase. Legislation is becoming more
stringent, and the reduction on these emissions is necessary. An effective set option
it to take advantage of existing APCDs. Particle removal systems are effective to
capture most of the TE condensed on fly ash particles (80%e99%). Only the smallest
particles escape from these systems being able to be captured in FGD systems
(75%e99%). Most volatile elements, especially mercury, selenium, and to lesser
extent arsenic, escape from these systems reaching the stack. DeNO x systems
(60%e90%) may enhance TE oxidation, making easier their capture. Specific technol-
ogies, mainly based on sorbent injection, have been proved with good results. The
option receiving the most attention is AC injection for mercury removal. Modified
ACs are a cost-effective and promising alternative for its removal.

3.7 Future work

Coal is and will remain an important global energy source, so the possible ways for
producing safe, efficient, and clean energy from coal is necessary. To produce clean
energy from coal, toxic emissions need to be controlled. TE emissions have grown
in concern, and current international and national legislation point to set limits for their
emissions, especially for mercury. The Convention of Minamata discussions and the
LCPD BREF debate on mercury controls noted that emission levels can be achieved
3
(and have been achieved since 2010 in EU reference plants) < 1 mg/Nm (N: normal-
ized at 6% O 2 concentration) on a yearly average, based on hourly averages. Until now
TE emissions have been significantly reduced. This reduction was possible as a result
of the cobenefit effects of existing devices for particulate, SO 2 and NO x control.
TE emission inventories depend on analytical techniques employed for the
measurement. More research focused on the development of continuous emission
monitoring and for TE speciation should be carried out. To understand the behavior
of TEs during coal conversion processes, in addition to protect human health and envi-
ronment, would facilitate the development of technologies to minimize their impact.
Moreover, it would assist in the prediction of possible corrosion and fouling by TE
deposition, making easier the development on new technologies.
Coal combustion produces large amounts of residue, which is sometimes stored in
ponds or often used in other industries, such as construction or insulation material or
for soil treatment. The potential leaching of TEs to water and/or soil is of major
concern to authorities throughout the world, and special attention should be paid to
TE mobility during by-product disposal and the treatment of wastewater coming
from power generation plants.