Dr. Stefan Kern, A TEC Production and Services GmbH, details the conversion of the kiln at Lafarge Retznei and shows how an optimised calciner design allowed for 100% alternative fuel usage. "Minimising emissions, maximising alternative fuels". Article published on World Cement Magazine, edition June 2022.

1. Minimising
emissions,
maximising
alternative
fuels
Dr. Stefan Kern,
A TEC Production
and Services
GmbH, details
the conversion
of the kiln at
Lafarge Retznei
and shows how an
optimised calciner
design allowed for
100% alternative
fuel usage.
T
o minimise the
environmental impact
caused by the cement
manufacturing
process, central effort is given
to the reduction of emissions
alongside fuel flexibility, and
fuel economics.
Alternative fuel utilisation
plays a key role in the
reduction of CO2
emissions
caused by fossil fuels as well
as reducing dependency
on fossil fuels in general.
The utilisation of solid
alternative fuels, however,
poses several challenges the
clinker formation process,
such as through the impact
on the combustion process
itself, but also on through
trace components being
introduced, such as chlorine.
Due to the nature of solid
alternative fuels, the impact
on CO emissions can
generally be observed by
increasing the TSR (thermal
substitution rate), which
for the main burner often
leads to a limitation in their
usage or, for the calciner,
in sufficiently large systems
allow complete burnout of
the fuels.1
In addition to national
regulations, many cement
producers have also
committed themselves to a
reduction of NOx and CO
emissions (among other
emissions as well).
In the case of the Lafarge plant
at Retznei, the aim was to reach
NOx levels at the stack of less

2. than 200 mg/Nm3
and CO levels of less than 1000 mg/
Nm3
.
In-line calciners (ILC) are the preferred choice when
it comes to the utilisation of solid alternative fuels. Unlike
separate-line calciner (SLC) systems, where any heavy
particles of the fuel would fall in the tertiary air bend,
ILC systems can cope with larger fuel particles and any
heavy fraction (if limited in its extent to being a minor
component) can fall through the orifice and burn in the
kiln directly without affecting the operation.
It is well known that NOx emissions are caused by
the clinker burning process. Due to the required sinter
zone temperature in the kiln (a minimum of 1450˚C,
depending on the raw material composition), the
window of thermal NOx formation – where nitrogen
in the combustion air is oxidised – is entered and a
significant amount of NOx emissions are formed in the
kiln. The options for treating this kiln-NOx are via primary
measures in an in-line calciner, a reduction system like
SNCR, a SCR system, or a combination of primary and
secondary measures.
Only an ILC is capable of reducing the kiln-NOx
emissions with primary measures as, unlike in in a
SLC, the complete amount of kiln gas (except the gas
purged by the Cl-bypass) passes through the calciner
and can go through a staged calciner combustion. This
is typically realised with a split of the combustion air
(tertiary air), creating a reduction zone at the beginning
of the calciner where the calciner fuel is injected (Figure
1). In this zone, NOx molecules can be reduced by CO
and fuel radicals, represented by the following reactions:
f
f CO + NO = CO2
+ 0.5 N2
f
f CX
HY
+ z NO = x CO2
+ y/2 H2
O +z/2 N2
To maximise these reactions, the concentration of
CO/hydrocarbon radicals needs to be relevant,
the temperature must be high enough and the flow
pattern in the calciner needs to allow for good mixing
of the components. To reach even lower values these
calciners can be equipped with an SNCR system as
well.2
This NOx reduction strategy results in a discrepancy
with regard to CO emissions as they are needed for
NOx reduction. Moreover, CO is known to inhibit the
reaction of an SNCR ammonia/urea injection system.
Therefore, designing a calciner for such an emission
target, while fulfilling the capability of 100% alternative
fuels usage, requires a combination of many design
aspects.
In the applied case for the kiln line at Lafarge Retznei,
Austria, the system was completely converted from a
preheater kiln to an ILC kiln to make the kiln system a
benchmark for emissions and alternative fuel usage. The
calciner system was developed and installed by A TEC
Production & Services GmbH, in 2018/2019.
Clinker production at Lafarge Retznei and
situation before modification
Before modification, Lafarge operated the kiln line in
Retznei, Austria, as grey cement kiln with a production
Figure 1. Principle of staged combustion in a
kiln system with an in-line calciner.
Figure 2. Preheater before modification.
World Cement Reprinted from June 2022

3. capacity of 1400 tpd. The preheater is a four stage two-
string system without a calciner. Even without a calciner
and tertiary air duct at all, the plant has set a benchmark
for preheater kilns for alternative fuel usage and the plant
is internationally recognised for this. At the main burner
a mix of petcoke, SSW (solid shredded plastic waste),
waste oil, solvents, animal meal and tire fluff were
fired. With the addition of whole tires at the kiln inlet, a
remarkable TSR of 96% was already reached before
the modification. The SSW is produced directly next
to the plant by Thermoteam,3
which is a joint venture of
Lafarge and the company Saubermacher. A tube belt
conveyor brings the SSW directly from the preparation
plant to the main burner. Before modification, NOx
emissions were ~400 mg/Nm3
and the CO value was
at about 1800 mg/Nm3
on a yearly average basis. The
preheater prior to modification can be seen in Figure 2.
Project development and project
description
Due to the reasons explained in the previous section,
the plant was limited in its treatment of NOx and CO
emissions and was targeting 200 mg/Nm3
of NOx and
< 1000 mg/Nm3
of CO emissions at the stack.
At the same time, the approach was also to increase
the TSR rate even more and operate at 100% thermal
substitution on regular basis. This meant that the
approach was that the fuel mix was planned to be
shifted significantly towards SSW from approximately
39% to 77 % of the total fuel mix. Other fuels were
planned to be reduced, such as tires, petcoke and oil/
solvents. The result would be a TSR after modification
of close to 100% on a yearly basis.
The plant was already equipped with a Cl-Bypass
system from A TEC, installed in 2001 and in operation
since then.4
As the bypass purges gas from the kiln
inlet, where the NOx concentration is very high, this
fraction of the bypass gas cannot be treated in a
calciner or by an SNCR system. The bypass gas, mixed
with cooling air, after the bypass filter is merged with the
preheater flue gas. To reach NOx emission values of
below 200 mg/Nm3
at the stack, therefore the flue gas
from the preheater (calciner) has to reach even lower
values to compensate this effect and reach the target
value.
In the case of the Retznei plant, the system of kiln and
calciner must reach a NOx concentration of less than
180 mg/Nm3
. This presented an additional challenge
for the calciner and SNCR system to reach even lower
baseline values and achieve a target of 200 mg/Nm3
of
NOx emissions at the stack during full operation.
Summing up the situation at the development phase,
the main stages of the project were as follows:
f
f Installation of an inline calciner:
»
» Capable of firing SSW, solvents and
coal/petcoke, each up to 100%.
»
» SSW particle size up to 80 mm (2D).
f
f Complete new fuel transport and dosing systems
for SSW, solvents and petcoke/coal.
f
f Installation of a tertiary air take-off at the cooler
including kiln hood modification and a tertiary air
duct system.
f
f New kiln inlet chamber and kiln adaption.
f
f New bottom stage cyclones and optimised meal
pipe system.
f
f New SNCR injection system and conversion of the
SNCR system from urea to NH3
.
f
f Upgrade of the kiln drive to increase the kiln speed
from a maximum of 2.5 rpm to 3.8 rpm in order
to adjust the material residence time due to the
precalcination.
Figure 3. 3D Model of the new system with new
in-line calciner (the new/modified parts are
shown in blue).
Figure 4. Installation of the kiln hood at the
clinker cooler for TA take off.
Reprinted from June 2022 World Cement

4. To achieve these goals, the following implementation
process was used:
1.
The new in-line calciner could not be placed inside the
existing building; therefore it was placed in front of the
existing preheater tower in a new steel structure (Figure 3).
A challenge was, as in many retrofitted calciner systems,
to achieve in all inclinations certain minimum angles and
cross sections to maintain proper operating conditions and
to minimise meal recirculation and accumulation in certain
areas.
As the preheater system was a two-string system it was
decided that the calciner upstream would be designed as
a common calciner, which is split at the top in the A TEC
Post Combustion Chamber (PCC) into two symmetrical
downstream ducts, each entering one of the two bottom
stage cyclones.
To be able to connect the new calciner with the kiln
on its inlet side and with the preheater on its exit side,
the connection points in the preheater were completely
re-designed. The bottom stage cyclones were both newly
designed, with larger diameters to increase the process
efficiency due to possible higher gas flow caused by
increased SSW usage after modification. Moreover, the
inlets were turned 90˚ and the two downstream calciner
strings enter the two bottom stage cyclones from the left
and right side of the preheater tower rather than from the
front so as to maintain steep angles and avoid flat cyclone
entries.
For the connection of the calciner to the kiln, the kiln was
shortened by approximately 3 m. This allowed the kiln inlet
chamber to be moved to the front of the preheater building
and made a steep inclination of the connection part of the
calciner to the orifice section possible.
An overview of the calciner after realisation can be seen
in Figure 5.
2.
The fuel inlets of the three fuels (solvents, petcoke/coal
and SSW) were located at the area of the lower tertiary
air inlet to allow proper ignition. The second tertiary air inlet
was located some levels higher in the upstream part of
the calciner. In this area a reducing zone is formed, which
is after the second tertiary air inlet is oxidised but still
partly active for some seconds. To safely allow complete
combustion of any unburned fuel and CO, originating from
the NOx reduction zone, sufficient gas residence time in
the calciner is needed. The calciner design chosen here
presents a feature which avoids the need of very large
calciners, and provides a result that can be achieved with
a reasonable real residence time. This feature is called the
Post Combustion Chamber (PCC).5
The PCC’s design
features the following aspects: In the upward stream
of the PCC, the diameter is increased in order to increase
the residence time for particles that are not fully burned yet
(Figure 6). The entrance to the downstream calciner part
is designed eccentrically to create turbulence and a mixing
effect as shown in the flow pattern in Figure 7. A top view
Figure 5. New, completed, in-line calciner at
Retznei.
Figure 6. Scheme of the A TEC Post
Combustion Chamber (PCC).
Figure 7. Flow pattern modelled by CFD in the
PCC.
World Cement Reprinted from June 2022

5. of the PCC in the Retznei plant is shown in Figure
8. This leads to the effect that unburned matter and
gases (CO, etc.) are mixed effectively with unused
combustion air and complete their combustion
before entering the preheater cyclones. The overall
installed gas residence time in the calciner is
5.9 sec., much lower than conventional systems
would require to reach the required performance
parameters for burnout and NOx reduction.
3.
The existing Cl-bypass was originally designed for
a bypass rate of 5% when it was in operation for
the preheater kiln. The bypass was upgraded to a
bypass rate of approximately 10% in order to be
able to handle the increased Cl-input caused by the
higher SSW rate. As the specific kiln gas in the kiln
inlet is nearly half the amount for calciner operation
compared to preheater operation, it was possible
to make this increase by fitting an adapted take-off
hood and a new quenching stage. The existing filter
and bypass fan could be used as the absolute gas
amount as roughly the same as before modification.
4.
The fuel system for SSW posed the challenge
of not only to dosing the material to the system
in a constant way, but also transporting it from
the Thermoteam preparation facility, located
approximately 250 m away. A TEC chose a u-belt
conveyor for this task. This is similar to a tube belt
conveyor, but the section in which the material is
transported is open at the top (the belt is curved into
a u-shape) and covered separately to avoid spilling.
The return belt is closed completely to avoid spilling.
The partially closed belt offers operational benefits
as it is not sensitive to oversize material which would
otherwise cause a puncture. Figure 9 shows a part
of the u-belt conveyor and Figure 10 shows the
dosing bins to the conveyor.
Metering of the material is carried out at the
entrance to the u-belt by a pre-bin with agitator on
load cells. The u-belt can simultaneously be loaded
from the Thermoteam facility or from a new truck
unloading station to allow maximum flexibility.
5.
A completely new, customised system was installed
for transport and dosing of liquid alternative fuels
(LAF, solvents). As one of the boundary conditions
was that particles with a size of up to 5 mm can
be present, a special system was designed where
the volume flow rate is completely controlled
by a variable speed piston pump, avoiding
any equipment in the pressurised line to the nozzle.
As for the nozzle itself, a so-called ‘open chamber
nozzle’ was used which atomises the liquid with
pressurised air; a large nozzle opening can be seen
as a result.
Figure 8. Top view of the calciner (PCC) in Retznei.
Figure 9. Installation of the conveyor for SSW from
the Thermoteam building (preparation facility) to the
calciner feeding point.
Figure 10. Dosing bins for SSW to the conveyor.
Figure 11. CFD modelling of the geometry (stream
lines – colour represents velocity).
Reprinted from June 2022 World Cement

6. 6.
The SNCR System was a separate process focus.
As explained previously, it has to be designed to reach
maximum efficiency but it also must be placed in the
calciner in a way that allows:
f
f Minimised interference with the primary NOx reduction
by staged combustion and only to treat the residual
amount of NOx after the staged combustion.
f
f Treatment of both a low amount (SSW/LAF operation)
and high amount (petcoke) of base-line NOx.
f
f Low ammonia slip.
The concept chosen was to realise the injection only in the
downstream part of the calciner in two levels. One level
was right before the inlet of each stage 4 cyclone, the
second inlet one level above. In petcoke operation of the
calciner, the second level of nozzles can be activated.
The complete calciner design, including the nozzle
positioning, was checked by CFD analysis to validate the
calculation results (Figure 11).
A TEC Production and Services GmbH was the main
contractor for the project on a mechanical EPC basis.
The company was responsible for the complete design,
dismantling, manufacturing and erection works of steel
structure and equipment including the necessary site
works and services.
The site works for mechanical erection in Retznei started
in late summer 2018 with the erection of the steel structure
for the new calciner followed by the calciner installation.
Both were finished in December 2018.
During kiln stop 2019 the kiln inlet chamber and C4
cyclones were dismantled, the adaptions on the bottom
stage cyclones and the meal routing was carried out to
allow the connection of the calciner to the system. Also
the kiln was cut and a tertiary air take-off (kiln hood) was
fitted to the cooler. This was realised within a kiln stop of 8
weeks (Figures 12 – 13).
Results
Successful restart and commissioning were conducted
in April 2019 according to the project schedule. The
operational results confirmed the process values
calculated before and showed on all sides even further
improvements. Summing up, the observed differences
during the guarantee run were:
f
f Operation achieved with 100% alternative fuels both
at the calciner and the main burner.
f
f Clinker production of 1400 tpd.
f
f NOx emissions < 190 mg/Nm3
maintaining a NH3
slip
of approximately 22 mg/Nm3
.
f
f CO emissions < 560 mg/Nm3
.
As a conversion from a preheater kiln to a calciner kiln
leads to a significant change of the operation procedures
and parameters that were well known for this system, it
is even more remarkable that the team of operators and
engineers at Lafarge Retznei managed a smooth start-up
of all systems and the calciner within the heat-up period of
the system. It is particularly worth noting that the change
to 100% alternative fuels at the calciner was managed
right after the start of clinker production within around one
week. Additionally, the overall project was completely on
track and full production was achieved as planned.
References
1. RAHMAN, A., RASUL, M.G., KHAN, M.M.K., SHARMA,
S., ‘Recent development on the uses of alternative fuels in cement
manufacturing process’, Fuel, 2015, 145, pp. 84 – 99.
2. BODENDIEK, N., ‚NOx-Minderung durch gestufte
Verbrennung und deren Wechselwirkung mit dem SNCR-Verfahren
bei Vorcalcinieranlagen der Zementindustrie‘, VDZ, Schriftreihe der
Zementindustrie, 2005, 68.
3. ‘SRF production facility saves one million tons of CO2
’, ZKG –
Cement Lime Gypsum, 5 / 2018.
4. WEICHINGER, M., & SCHÖFFMANN, H., ‘Suitability of
different chlorine bypass systems for practical application’, Cement
International, 01, 2002.
5. KERN, S., ‘A New Calciner Design’, World Cement, 11, 2017,
pp. 95 – 101.
Figure 12. Installation of the last calciner piece
by end of 2018.
Figure 13. Installation works at the calciner
(SSW conveyor bridge connected to the new
calciner structure).
World Cement Reprinted from June 2022