LOWERING
JÜRGEN LAUER,
BWF ENVIROTEC, GERMANY,
REVEALS A NEW APPROACH
TO DUST AND NOX CONTROL
IN THE CEMENT INDUSTRY,
AS EMISSIONS LIMITS
CONTINUE TO TIGHTEN.
THE LIMIT
Introduction
It is a fact that emission limits in the cement industry have become –
and will continue to get – more stringent. Although there are some
differences, depending on what part of the world one looks at, the limits
are undoubtedly heading downwards. A direct implication of this is that the
dust control activities of cement manufacturers will have to be improved
globally.
compared to a baghouse, for example. On the other
hand, the space requirements for an ESP filter are
huge and, if one wants to lower the dust emissions
even further, the filter becomes very large, and the
consumption of electricity very extensive. The extension
of an ESP to meet new emission limits could imply
higher fixed costs and increased operating costs. In
most cases, the option of expanding the ESP filter is not
available, as space is often the limiting factor in existing
facilities. However, depending on the desired lower
emission targets, ESP technology may not even be able
to reach those new levels.
Figure 1. Pyrotex® KE elements in a baghouse.
Figure 2. Pyrotex® KE filter elements.
Figure 3. Urea or ammonia injection for SCR with
Pyrotex® KE.
Dust control technologies
In cement plants across the world, cyclones, ESPs,
and baghouses, either alone or in combination, are
widely used dust control technologies, each with its
own benefits. The well-known and established ESP
technology is capable of handling dust emission limits
of 5 – 10 mg/Nm3. ESPs in the cement industry can be
operated up to a service temperature of around 450˚C,
so there is no cooling of the gas needed for kiln exit
gases and those of the clinker cooler. In the case of
bypass filters, depending on the kiln exit temperature,
cooling of the gases with air or water may be required.
One of the most compelling arguments for an ESP
installation is that ESPs are very easy to operate.
Moreover, maintenance is relatively simple and the
cost reasonable, due to fewer components involved as
Cement Plants of the Future
World Cement
Finding other options
Time and again, changes in regulations and lower
emission limits have forced the industry to look for
other solutions. Lower emissions, in the range of
about 3 – 5 mg/Nm3, can be achieved with baghouses,
which also have a much smaller footprint than ESPs.
No wonder that one of the industry’s responses to
the challenge was the conversion of an existing ESP
installation, either in full or in part, into a baghouse
with a fabric filter. This type of filter conversion has
become common practice in the cement industry and,
with new limits in place, many more retrofits are
expected worldwide in the coming years. However, the
right implementation is crucial to reaping the benefits
of such a retrofit. Some of the problems encountered
can be traced back to fundamental differences between
the two technologies. Two of these basic differences
are the direction of flow of the flue gases and the
operating temperature.
An ESP filter requires a horizontal flow of the
flue gases going through the collecting plates. In a
baghouse, the flue gases go through the vertically
hanging bags. Therefore, in a baghouse filter, the gas
flow should be vertical.
An ESP installation can be operated at
approximately 450˚C whereas, in the case of a
baghouse, the temperature is limited by the kind of
filter media used. Maximum continuous operating
temperatures for fabric filters are 250 – 260˚C; cooling
of the flue gases is therefore required. The actual filter
media may be a fabric cloth made of either a needle felt
or a woven fiber glass. Both fabrics could be equipped
with an expanded polytetrafluorethylene (ePTFE)
membrane material. Due to the very small pore size of
the membrane (1 – 2 µm), lower emission rates of about
3 – 5 mg/Nm3 can be achieved. The crystallite melting
point of PTFE material is 327˚C and a potential active
continuous service temperature of 288˚C seems possible.
However, practical continuous filtration operating
temperatures are between a maximum of 250 – 260˚C.
In order to protect the fabric filtration media, valuable
heat energy has to be wasted due to the cooling of the
flue gas. In many cases, where cooling is done by air,
about 30 – 50% of the air going through a fabric filter
baghouse is the air required for cooling the flue gas to
a desired temperature.
Figure 4. Various catalytic systems for Pyrotex® KE.
Cooling of flue gas can be avoided, if the filter medium
can withstand higher temperatures, presenting a number
of opportunities:
ll
ll
ll
ll
The volume of air can be reduced, which saves
electricity costs on the fan motor.
Increased production capacity may become possible
without having to scale up the ID fan capacity.
The clean gases are higher in temperature and
therefore do not need to be heated for potential
SCR NOX reduction treatment. This will save on fuel
consumption and therefore on cost.
The thermal energy from the clean hot gases can be
reused as thermal energy for drying raw material
or coal. Those clean hot gases could also be used to
generate electricity.
Rigid filter elements
Well-established in glass manufacturing, but new to the
cement industry, are rigid filter elements. BWF Envirotec’s
tradename for this product is Pyrotex® KE. Very low dust
emissions of less than 1 mg/Nm3 are achievable. These filter
elements are made out of calcium-magnesium-silicate
fibres, which are non-carcinogenic and bio-soluble. Those
fibres are safe for human health.
The materials used for the manufacture of Pyrotex® KE
filter elements can withstand high temperatures, making
the elements thermally stable up to a continuous
operating temperature of 850˚C. No cooling of flue gases
is required and no thermal heat energy will be wasted.
About 1 MJ heat energy could be saved per 10 t of clinker
when replacing an ESP kiln filter with a Pyrotex® KE filter.
The BWF Envirotec Pyrotex® KE filter elements are
of a low density, which manifests itself in a relatively
lightweight construction. Due to this low‑density
construction, the air permeability of Pyrotex® KE filter
elements is similar to that of fibre glass with membrane
material. A low differential pressure goes along with the
high air permeability.
The first successful commercial installation of Pyrotex®
KE elements in a clinker cooler filter, with operating
performance far exceeding the design parameters,
especially a highly favourable differential pressure, has
paved the way for more to come. In this application,
the cleaning pressure is about 2.0 – 2.5 bar with a
resulting differential pressure over the entire filter of
10 – 12 mbar. Due to its excellent air permeability, the
plant is experiencing only one full cleaning cycle per day.
Furthermore, significant savings in compressed air can be
achieved and an extended life of the Pyrotex® KE elements
can be expected.
The Pyrotex® KE elements are available in 60 and
150 mm dia. The top collar design can be built as V or
T-shape (Figure 2). Traditionally, the V-shaped collar has
been established in the glass industry. However, more
and more T-shaped designs are being requested. This will
make a replacement of regular filter bags much easier.
The elements are available in various lengths, with the
longest element available as a single 4.5 m piece. All
elements longer than 4.5 m are designed modularly and
put together onsite. 8 m elements are in the testing phase,
while 6 m elements are already available for commercial
installations.
In addition to a dust control performance of
≤1 mg/Nm3, the filter elements can be equipped with a
catalytic converter material for SCR NOX abatement. When
compared to a traditional SNCR, SCR NOX reduction can
take place at a lower temperature of 200 – 450˚C, due to
the use of the catalyst.
BWF Envirotec offers four different catalysts (Figure 4)
that will work within that temperature window. When
selecting a specific catalyst, the characteristics of the
process gases will have to be reviewed and considered.
The Pyrotex® KE elements provide a single solution for
dust emissions below 1 mg/Nm3. Combined with a catalytic
converter, Pyrotex® KE elements can also take care of other
gaseous emissions, especially NOX. Due to the ability of
operating at elevated temperatures, even SOX reduction
with calcium hydroxide (Ca(OH)2) can be optimised. The
optimum temperature for SOX reduction with Ca(OH)2 is
350˚C, which is above the temperature that fabric filter
media can handle; Pyrotex® KE elements, however, can.
Since dust collection and gaseous emissions control steps
can now be handled with a single dust control unit, the
investment and operating cost for this type of filter will
reach a new economical efficiency.
It is BWF Envirotec’s experience that each process
requires a tailored solution, especially when it comes to
NOX reduction with SCR catalytic systems. The savings in
heat energy will also be reviewed on an individual basis.
In some cases, it is beneficial to eliminate the cooling
of the flue gases as the clean, hot gases are often used
for material drying. When applying the Pyrotex® KE
technology to hot cement clinker filtration, the clean, hot
gases are used as combustion air for a different thermal
process. An OEM specialising in cogeneration of electricity
in a cement plant claims that the most economical
approach doing this will start with a kiln size of at least
5000 tpd of clinker.
It seems obvious that with more and more stringent
emission requirements different new technologies and
solutions will enter the market.
Cement Plants of the Future
World Cement
ealth
ss to H
Harmle oluble
Bio-s
BWF Envirotec
BWF Tec GmbH & Co. KG
Bahnhofstr. 20
89362 Offingen
Germany
Phone +49 8224 71-0
E-Mail: [email protected]