ICR Final draft
Breaking the Hg cycle
38
MERCURY ABATEMENT
The cement industry has been identified as the second-largest gaseous emission source
of mercury after coal combustion in thermal power plants. Following the Minamata
Convention on Mercury in 2013, there has been a growing interest in abatement systems,
leading to the search for alternative approaches to traditional emission control methods.
n by Scheuch GmbH, Austria
M
ercury (Hg) is introduced into the
clinker production process by the
types of raw materials used and/or fuels
(especially alternative fuels1-3).
Hg emissions are a complex topic due
to the wide range of compounds that can
be formed in the process. Beside elemental
Hg, it can be released from the combustion
process as particle-bound (adsorbed) Hg
or oxidised as Hg2+. The formation depends
mainly of the operating conditions and the
compounds present in the pyroprocess
as well as the cooling step (quenching
speed and time).4 Examples of mercury
compounds and their related temperatures
of melting, boiling and sublimation are
shown in Table 1.
Mercury affects the internal as well as
the external cycle, as shown in Figure 1. It
evaporates in the pyrosection and then
condenses again in the raw mill or kiln
filter. Therefore, the highest concentration
of Hg can be found in the cycle between
Figure 1: mercury pathways in the cement production process
kiln meal silo and the pyroprocess (see
Figure 2). If the dust cycle is highly loaded
with Hg, or if the temperature prior to
the main filter is too high, Hg cannot be
adsorbed by or condensed on the dust
sufficiently, resulting in Hg emissions.
Table 1: mercury compounds with melting, boiling and decomposition/
sublimation points
Hg compound
Melting point (˚C)
Boiling point (˚C)
Decomposition/
sublimation point (˚C)
Hg(0)
-39
357
na
Hg2Cl2
525
na
383
HgCl2
277
302
na
Hg2SO4
na
na
na
HgS
na
446-583
580
HgO
na
356
500
Hg2Br2
405
na
340-350
HgBr2
237
322
na
Hg2I2
na
na
140
HgI2
259
350
na
Hg2F2
na
na
570
HgF2
645
650
645
Hg2(NO3)2
na
na
70
Hg(NO3)2
79
na
na
ICR JANUARY 2016
Traditional Hg
abatement methods
Reducing Hg emissions by attempting to
change the raw materials used is almost
impossible. In most cases, cement plants
are located near the quarry, and opting
to transport different raw materials over
a long distance is not always viable.
Switching the types of fuel used is also
not usually a possibility, particularly given
that the use of alternative fuels is often
“The disadvantage of
such more traditional
methods of Hg
abatement is that huge
amounts of dust have
to be removed from
the system, leading to
drawbacks of missing
raw materials and a
logistic problem to
dispose of the dust.”
MERCURY ABATEMENT
Figure 2: results of the thermal treatment tests of filter dust focussing on the evaporation and
therefore, reduction of Hg
39
The ExMercury system
To minimise or even eliminate the dust
issue, Austria-based A TEC Production &
Services (pyroprocessing experts), Scheuch
(filter specialists) and W&P Zement (local
cement producer) have teamed up to
develop the ExMercury system, which
unloads the Hg cycle in the system and
further removes Hg from the dust, allowing
kiln dust to return to the pyroprocess.
Basic principle
preferred due to the associated reduced
costs. Therefore, flue gas treatment is often
the only viable option.
Besides methods such as downstream
activated carbon filters and activated
carbon injection prior to the main filter
(both of which use large amounts of
activated carbon), dust shuttling (or
‘bleeding’) to unload the cycle has proven
to be a suitable method. However, the
downside to most of these more traditional
Hg abatement methods is that huge
amounts of dust must be removed from
the system, leading to drawbacks such as
losing valuable raw materials and having
to address disposal of the dust.
The ExMercury system was designed as a
‘split preheater’. This abatement methods
is based on the following basic principle
(see Figure 3): the kiln filter dust is heated
by hot combustion flue gas, which is
extracted from the lowest cyclone stage.
In this second preheater line, the dust is
heated to an adequate temperature to
evaporate the Hg. Through several cyclone
stages, the hot dust is separated and
returned to the preheater tower. After the
cyclones, using ceramic filter elements
a hot gas filter removes the remaining
dust which is also then returned into
the preheater while the mercury is still
gaseous.
Figure 3: the ExMercury split preheater system implemented in the existing system
JANUARY 2016 ICR
40
MERCURY ABATEMENT
The particle-free Hg-loaded gas stream
is rapidly quenched by water injection
in the quenching reactor. Afterwards
activated carbon or any other sorbent (eg,
brown coal coke) is injected and captures
the mercury. A bag filter then separates
the sorbent. The sorbent can be recycled
in the system until the Hg load is too high.
This allows for maximum utilisation of the
sorbent and leads to minimised sorbent
consumption. The Hg-free gas is added to
the main gas stream prior to the raw mill.
The ExMercury system installed at W&P
Zement’s cement plant in Wietersdorf,
Austria, was commissioned at the
beginning of 2015. It started operation with
the system’s design values of mass and
volume flow rates and temperatures, ie a
filter dust input of 5tph and heating up to
a maximum temperature of 400˚C. The
extracted gas volume from preheater stage
five represents around 3-5 per cent of the
total gas flow of the kiln.
Analysis of Hg content of the dust
particles shows the function of the system.
In the first phase of operation, filter dust
with a concentration of ~10ppm Hg (ie, 100
per cent of Hg input into the ExMercury
system) was used. Measurements showed
that after the separation of dust ~90-95 per
cent of the Hg load is still in gaseous form.
The particles separated in the cyclones
and the hot gas filter and returned to the
preheater at the elevated temperature
contained the rest of the mercury load.
Following the hot gas filter, the dust-free
gas stream is cooled to 100-120˚C by water
injection in the cooling reactor. In this
section the sorbent for Hg adsorption is
injected. The ExMercury system is designed
to handle organic as well as inorganic
sorbents. At present, the Wietersdorf Hg
abatement system uses brown coal char
as it is more cost effective compared to
activated carbon or bromined activated
carbon. Moreover, the brown coal char can
be loaded with Hg to an extraordinarily
high level without any issues arising. To
maintain these very high Hg loading levels,
the sorbent is continuously replaced at a
rate of ~50-100kg/day.
Even the consumption of fresh additives
is low. The removal efficiency of the
ExMercury system is more than 90 per cent
and the Hg emissions at stack could be
reduced by more than 80 per cent.
Energy and heat consumption
In terms of the energy and heat
ICR JANUARY 2016
© Scheuch GmbH
First operational results
Figure 4: the ExMercury split preheater system implemented at W&P Zement’s production
facility in Wietersdorf, Austria
consumption of the Hg abatement system,
it soon became clear that the removed gas
and heat from preheater stage five did not
result in increased heat consumption of
the overall system. As the dust is heated
and returned hot to the pyroprocess, it
is just a different way of feeding the kiln
dust into the system. Therefore, the overall
heat balance of the system remains almost
unchanged.
The fan that drives the Hg abatement
system and the small drives of the dust
conveying system is the only source of the
entire system’s operating costs.
Exploring long-term Hg
reduction potential
Following the short-term operation of the
system, the project’s main objective is now
to gain long-term experience relating to
the Hg reduction potential. Operational
testing campaigns are expected to provide
additional insights into the impact of single
parameters on the abatement system. n
References
1
Hills, LM and Stevenson, RW (2006) Mercury
and lead content in raw materials. PCA R&D
Serial No. 2888.
2
Sprung, S and Rechenberg, W (1998)
‘Levels of heavy metals in clinker and cement’
in: ZKG, 47, p183.
3
Fytili, D and Zabaniotou, A (2008)
‘Utilization of sewage sludge in EU application
of old and new methods – a review’ in:
Renewable and Sustainable Energy Reviews, 12,
p116-140.
4
Zheng, Y, Jensen, AD, Windelin, C
ANDJensen, F (2012) ‘Review of technologies
for mercury removal from flue gas from cement
production processes’ in: Progress in Energy an
Combustion Science, 38, p599-629.
5
Åmand, LE AND Leckner, B (2004) ‘Metal
emissions from co-combustion of sewage
sludge and coal/wood in fluidized bed’ in: Fuel,
83, p1803-1821.
6
Perry, RH AND Green, DW AND Maloney,
JO (EDS) (1997) Perry’s Chemical Engineers’
Handbook. Seventh edition. New York, USA: The
McGraw-Hill Companies Inc.