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European Association for the
Development of Renewable Energies,
Environment and Power Quality (EA4EPQ)
International Conference on Renewable Energies and Power Quality
(ICREPQ’11)
Las Palmas de Gran Canaria (Spain), 13th to 15th April, 2011
Meat and Bone Meal as a Renewable Energy Source in Cement Kilns:
Investigation of Optimum Feeding Rate
W.K.H. Ariyaratne1, M.C. Melaaen1, K. Eine2 and L.A. Tokheim1
1Department of Process, Energy & Environmental Technology
Telemark University College
Kjølnes Ring 56, P.O. Box 203, N-3901, Porsgrunn (Norway)
Phone number:+47 35 57 51 35, Fax number:+47 35 57 54 01, e-mail: hiromi.ariyaratne@hit.no, Lars.A.Tokheim@hit.no
2Norcem AS Brevik, P.O. Box 98, N-3991, Brevik (Norway)
Phone number:+47 35 57 20 00, Fax number:+47 35 57 17 47, e-mail: kristin.eine@norcem.no
Abstract. Meat and Bone Meal (MBM) is a CO2 neutral fuel,
and hence is a good candidate for substituing fossil fuels like
pulverized coal in rotary kiln burners used in cement kiln
systems. MBM is used in several cement plants, but the optimum
substitution rate has apparently not yet been fully investigated.
The present study aims to find the maximum possible
replacement of coal by MBM, without negatively affecting the
product quality, emissions and overall operation of the process.
A full-scale experiment was carried out in the rotary kiln burner
of a cement plant by varying the MBM substitution rate from 0
to 7 t/hr. Clinker quality, emissions and other relevant
operational data from the experiment were analysed.
Additionally, coal and MBM were compared by laboratory
experiments. The results revealed that MBM could safely replace
more than 40% of the coal energy without giving negative
effects. The limiting factor is the free lime content of the clinker.
Possible explanations to the free lime increase are given. If 40%
of the coal in the rotary kiln burner was replaced by MBM on a
long-term basis, the total annual CO2 emissions of the plant
could be reduced by 10%.
Key words
Meat and bone meal, Alternative fuel, Carbon dioxide,
Rotary kiln, Free lime
1. Introduction
The cement production process is highly energy-intensive
and generates a world average CO2 emission of 0.81 kg
per kg cement produced [1]. The calcination of carbonates
in the raw materials accounts for roughly 60% of the CO2
emitted, while the remaining carbon dioxide results from
combustion of fuels in the kiln system [2]. Although coal,
petroleum coke and other fossil fuels traditionally have
been burnt in cement kilns, many cement plants have
turned to energy-rich alternative fuels due to economical
and environmental benefits. The replacement of coal by
carbon dioxide neutral fuels will reduce net carbon
dioxide emissions to the atmosphere, while letting the
manufacturer gain economic advantages by reducing fuel
costs and possibly earning CO2 allowances under an
Emissions Trading scheme [3].
In modern precalciner cement kilns typically 60 % of the
fuel energy is supplied in the precalciner, whereas the
remaining 40 % is supplied via the rotary kiln burner. The
operating temperature in the calciner is around 900 °C as
the decarbonation of calcium carbonate occurs at this
temperature. In contrast, the flame temperature in the
rotary kiln burner typically should be around 2000 °C to
ensure sufficient formation of melt in the solid materials
being processed. Traditionally, solid alternative fuels are
fed to the calciner rather than to the kiln burner, partly
because of the lower temperature, partly because
gravitation-facilitated feeding can be implemented in the
calciner. This also allows for feeding of lumpy fuels.
However, in the kiln main burner, solid fuels have to be
ground into a fine meal and then fed pneumatically into
the rotary kiln. There are examples of plants replacing as
much as 90 % of the coal energy in the calciner by solid
waste fuels [4], [5], provided the feeding and processing
of the alternative fuels are done properly [2]. When the
replacement potential in the calciner is fully utilized, one
may turn to the kiln burner to increase the overall fossil
energy replacement ratio even more.
The range of alternative fuels is very wide. Meat and bone
meal - MBM (also called animal meal) is used in several
cement kilns, in particular in Western Europe. The fuel is
prepared by post-treating (grinding and sterilizing) the
waste materials associated with slaughtering operations.
Since it contains only biogenic materials it can be
categorised as 100% biomass fuel which gives no net
carbon dioxide emissions during the combustion process.
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Compared to partly CO2 neutral fuels, another advantage
of using a 100% biofuel, is that one does not have to
determine the fossil fraction of a mixed fuel, which can be
a challenge [6].
Although MBM is already widely used as an alternative
fuel in cement kiln main burners, it is hard to find
scientific investigations of optimum usage of MBM in
cement kilns. However, one study has been carried out to
find out the effect of MBM ash addition on the clinkering
process and hydraulic reactivity of clinker [7]. In another
study [8] the air demand and energy input in the rotary
kiln burner were investigated through an ASPEN PLUS
model by partly replacing primary fuel by MBM.
Furthermore, numerous studies on MBM characteristics
and co-combustion in fluidised beds have been published.
Several proximate and ultimate analyses of MBM have
been found, some including additional elemental analyses
as well [8]-[13]. MBM residues are mainly due to bones,
which have high amounts of calcium (30.7%) and
phosphate (56.3%) [14]. Not only pure MBM [9], [14],
[15] but also different coal-MBM blends [10] were
analysed in Thermo Gravimetric Analysis (TGA) in
numerous studies to investigate thermal behaviour under
reducing and oxidizing conditions. The chemical kinetics
for devolatilization of MBM was also studied [16], [17].
Furthermore, different aspects of fluidised bed and power
plant co-combustion of MBM with other fuels were
investigated by several authors [11]-[13], [18]-[21].
The main objective of the present study was to find the
maximum possible replacement of coal by MBM without
negatively affecting the product quality, emissions and
overall operation of the process. A full-scale experiment
was carried out in a cement plant which annually produces
1.3 million tons of cement. The bottleneck of using higher
amounts of MBM in the rotary kiln burner was
investigated, and possible reasons were analysed by
evaluating process and quality data as well as by
comparing physiochemical properties of coal and MBM.
Finally, the optimum feed level was concluded for that
specific kiln system considering long term operation, and
the CO2 emission reduction was documented.
2. Method
Full-scale experiments were carried out to investigate the
impact of MBM feeding rate on the cement kiln process
and the product quality. Additionally, coal and MBM
were characterised through laboratory experiments,
including determination of proximate analysis, major
oxides, heating value, bulk density and particle size
distribution.
A. Fuel characterization
Laboratory analyses were carried out for characterization
of the coal and the MBM used in the full-scale
experiments. The proximate analysis was carried out using
a Perkin-Elmer TGA instrument (model: TGA 7) by
applying different temperature steps up to 750 °C. The
heating value was determined by a Leco AC-350
automatic calorimeter. The bulk density was measured by
weighing a known freely distributed volume. The particle
size was analysed by a Helos KF-Magic laser diffraction
instrument and by mechanical sieving.
Additional chemical analyses of the fuels were available
from the plant laboratory.
B. Full-scale experiments
A full-scale test was carried out in a modern 4-stage, 2-
string precalciner cement kiln system at a Norwegian
cement plant with several years of experience in burning
MBM at relatively low rates as a coal replacement in the
rotary kiln burner. The kiln is 68 m long and 4.4 m in
diameter. It is equipped with a modified KHD Pyrojet
burner at the kiln outlet (where the hot clinker is
discharged from the kiln), with three different tubular fuel
inlets for solid and liquid alternative fuels and one annular
inlet for coal, see Figure 1. The solid alternative fuel is fed
via a Pfister feeder system designed to control mass
feeding of solid secondary fuels up to 7 t/hr. When
material passes through the rotor weighfeeder, the
momentum is measured by load cells, and the rotor
angular speed is calculated according to the mass flow rate
set point [22].
Fig. 1. Kiln burner
The test was carried out for 12 hrs by varying the MBM
feeding rate in particular time intervals. MBM was fed via
the Pfister system described above. Ordinary portland
cement (OPC) clinker was produced. The raw meal
feeding rate was kept at 220 t/hr; however, the feeding
rate had to be reduced by 10 t/hr at the final stage of the
experiment to keep the clinker quality within the desired
range.
The conditions in the precalciner and in the kiln were kept
stable throughout the experiment. The raw material
composition and feeding rate, the conventional and
alternative fuel supply rates and the bypass stream were
maintained at constant levels in order to keep the degree
of calcination of hot meal and the thermal energy
consumption constant during the test. The ratio of thermal
energy consumption between main burner and calciner
was kept at 42:58. The MBM feeding rate was gradually
increased by reducing coal supply in order to keep the
thermal energy consumption of the kiln at almost constant
level.
1) Experimental plan. Table I shows the
experimental schedule and thermal energy
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replacement by MBM in the main burner. The
lower heating values of MBM and coal were
4423 kcal/kg and 6689 kcal/kg respectively
(Table II).
Table I. – Experimental schedule
Time interval MBM
feeding
rate (t/hr)
Coal
feeding
rate (t/hr)
Energy
substitution
at main
burner (%)
9.00-11.00 0 7.8 0.0
11.00-13.00 2 6.5 16.9
13.00-15.00 4 5.0-5.5 33.5
15.00-17.00 5 4.5 42.4
17.00-19.00 6 3.9 50.4
19.00-21.00 7 3.0-3.4 59.1
2) Data collection. The average process data
(emissions, operation of Pfister feeder) were
collected through a plant database system in one
minute intervals during the whole experiment.
Important clinker properties were analysed in 1
or 2 hr time intervals. The clinker samples were
collected from the apron conveyor after the
cooler. Clinker samples were collected just
before the changes in the MBM feeding rates
were made. The oxides were analysed with a
Philips PW 2404 x-ray spectrometer using the x-
ray fluorescence (XRF) method, and the SO3
content was analysed by means of an ELTRA CS
800 carbon-sulphur analyzer. The ethylglycol
method was used for analysis of free lime
(uncombined CaO) in the clinker.
3. Results & Discussion
This section presents the results from laboratory and full-
scale experiments. First the fuel characteristics determined
in the lab are given. Then impacts on the clinker quality
and on the process are discussed. Finally, CO2 savings
attributed to replacement of coal by MBM is presented.
A. Fuel characterization
The properties of coal and MBM are given in Table II.
Table II. – Properties of coal and MBM
Property Coal MBM
Moisture (wt.%) 1.0 4.0
Volatiles (wt.%) 31.6 60.8
Fixed carbon (wt.%) 56.3 8.0
Ash (wt.%) 11.1 27.2
P2O5 (wt.%) 0.07 13.0
CaO (wt.%) 0.20 13.3
Lower heating value (kcal/kg) 6689 4423
Freely settled density (kg/m3) 640 720
The cumulative distributions of coal and MBM particle
sizes are shown below. The particle size will affect the
burnout time of the particles in the kiln; this is further
discussed in Subsection 3B.
0
20
40
60
80
100
1 10 100 1000 10000
Particle size (µm)
Cumulative distribution
(%)
Coal MBM
Fig. 2. Cumulative particle size distribution
B. Clinker quality
Figure 3 represents the main oxides of clinker. These were
all at desirable levels to maintain an acceptable clinker
quality throughout the experiment.
0
10
20
30
40
50
60
70
10.00 (0) 12.00 (2) 14.00 (4) 16.00 (5) 18.00 (6) 20.00 (7)
Time (MBM fe ed rate;t/hr )
SiO
2
, CaO, Al
2
O
3
, Fe
2
O
3
(%)
0,0
0,5
1,0
1,5
2,0
2,5
3,0
3,5
MgO, K
2
O, Na
2
O(%)
SiO2 CaO Al2O3 Fe2O3 MgO K2O Na2O
Fig. 3. Main oxides of clinker
Among the results, emphasis was put on the content of
free lime, phosphorus and sulphur content in the clinker,
as these parameters might be impacted by feeding high
rates of MBM. A high level of free lime (CaO) would
indicate that part of the CaO had not combined properly
with the main acidic oxides SiO2, Al2O3 and Fe2O3 in the
raw materials, and a poor clinker quality would be the
result. Accordingly, the free lime level is the main quality
parameter monitored by the kiln operators. The level
should not be higher than 2.5% for OPC production.
Though there is no absolute recommended limitation for
phosphorus (measured as P2O5) in the clinker, the typical
maximum limit is taken as 2%, as higher values may give
concrete expansions when the product is used. Some
cement plants have even lower internal maximum limits to
be on the safe side. Typically MBM contains 13% CaO
and 13% P2O5 respectively (Table II). MBM does not
have high concentrations of sulphur (measured as SO3),
but changes in the SO3 content in clinker might be due to
process-disturbing changes in the internal sulphur cycles
in the kiln system [2], potentially induced by changes in
the combustion conditions in the main burner as a result of
the MBM feeding. The results are shown in Figure 4.
MBM has a high phosphorus content compared to other
fuel types, and consequently the P2O5 content in the
clinker increases with increasing feeding rates of MBM.
However, the P2O5 in the clinker is at an acceptable level
even at the maximum feeding rate of 7 t/hr.
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0,0
0,5
1,0
1,5
2,0
2,5
3,0
3,5
09:00 11:24 13:48 16:12 18:36 21:00
Time
Free lime, SO
3
and P
2
O
5
content(%)
0,0
1,0
2,0
3,0
4,0
5,0
6,0
7,0
8,0
MBM feed rate(t/hr)
Free lime content SO3 content P2O5 content MBM feed rate
Fig. 4. Free lime, SO3 and P2O5 content of the clinker
The SO3 content is also in an acceptable range (less than
1.5%) for the standard clinker, and the level appears not to
be impacted by the MBM feeding.
The main issue indicated here is the increase in free lime
content with increments of MBM feeding. When the
MBM feeding rate is 6 t/hr, the free lime content is
slightly lower than the maximum allowable limit, 2.5%. It
is even higher (3%) when the MBM feeding rate is 7 t/hr.
The final slight decrement of free lime shown in the
Figure 4 (although still above the recommended limit) is
likely due to a reduction in the raw meal feeding rate by
10 t/hr (i.e. total raw meal feeding rate of 210 t/hr) in the
last hour of the test. Therefore, it can be concluded that
even with low clinker production rate, 7 t/hr MBM
feeding is not appreciable. The high free lime content
reveals improper burning of the raw meal and poorer
quality of the clinker.
One possible reason for the increase in free lime content
with MBM feeding is the introduction of extra calcium at
the kiln outlet via a high calcium phosphate content in
MBM. The extra calcium, being fed from the outlet side
of the kiln, might not have sufficient residence time in the
rotary kiln and hence add to the clinker without proper
combination with the other oxides.
Figure 5 indicates the maximum theoretical free lime
content that could be added to the clinker via the MBM
feed, conservatively assuming that all the calcium which
enters with MBM, will end up as free lime. For these
theoretical calculations, the ash content of MBM and the
free lime percentage at the reference conditions (when
feeding only coal) are 27.2% (Table II) and 1.27% (Figure
4), respectively. The CaO content of MBM ash is 48.9%
(Table II).
The results shown in Figure 5 indicate that when the
MBM feeding rate is high, the experimental free lime
content is considerably higher than the maximum
theoretical values which were estimated based on the
stoichiometry of the clinker reactions. Consequently,
additional feeding of calcium could explain only 54% of
the increase in free lime content due to MBM feeding at 7
t/hr. Therefore, there must be other phenomena as well,
along with above mentioned reason, which cause the
138% increase in free lime content at 7 t/hr MBM feeding.
0,0
0,5
1,0
1,5
2,0
2,5
3,0
3,5
01234567
MBM feed rate (t/h)
Free lime (%)
Theoretical Experimental
Fig. 5. Resulting free lime in clinker by feeding MBM
(assuming all CaO from MBM ends up as free lime in clinker)
It has been shown [7], [23]-[27] that a high level of
phosphorous stabilizes belite and hence inhibits the
reaction between CaO and belite to form alite. Due to this
reason the free lime content can be increased.
Still another potential reason for an increase in free lime
content is reduced flame temperature: If the temperature is
not sufficiently high, part of the CaO in the raw materials
may not combine with other oxides, and the free lime
level increases. Reduced flame temperatures could be due
to lower heating value (Table II) or larger particle size
(Figure 2), as larger particles mean increased burnout
time, which will most likely reduce the maximum flame
temperature and hence the local heat flux from the flame
to the material bed in the kiln. For a given percentage, the
particle size of MBM is more than 10 times bigger than
that of coal. While the maximum size of the coal particles
is around 200 m, the MBM distributes up to 5000 m.
However, even if the burnout time of large particles is
higher, this may be partly offset by a higher content of
volatiles in MBM (Table II) via faster fuel ignition.
C. Process data
The typical O2 concentration maintained in the kiln inlet
(where the precalcined meal is fed) is 3-4% (dry basis).
The sudden peaks observed in the graph of O2 at kiln inlet
in Figure 6 are mainly due to automatic air cleaning of the
sensor probe at given intervals.
Typically, the CO concentration should be 0% at the kiln
inlet, but up to 0.1% is acceptable. It is observed that a
high O2 concentration at the kiln inlet causes less CO
production, which is generally an obvious fact. However,
more importantly, the result shown in Figure 6 also
reveals that the CO concentration in the kiln inlet was in
an acceptable range throughout the experiment; it did not
increase as a result of MBM feeding.
It is observed in Figure 7 that the NOx level at the kiln
inlet was not affected by MBM feeding, indicating that the
sum of thermal NOx formation and fuel NOx formation in
the kiln was more or less constant.
The kiln drive current (Figure 7) is in a typical range for
smooth operation, ie 400-500 A for OPC clinker
production. A drop in the kiln drive current would be an
indication of reduced melt formation in the kiln, which
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would give a poorer clinker quality. However, the kiln
drive current appears not to be impacted by the feeding of
MBM. This suggests that the thermal NOx formation in
the kiln was not considerably changed, which indicates
that also the fuel NOx formation stayed at the same level.
This suggests that reduced flame temperture, as discussed
in Subsection 3B, was actually not contributing
significantly to the increase in free lime content.
0
5
10
15
20
25
09:00 11:24 13:48 16:12 18:36 21:00
Time
O2
0,0
0,1
0,2
0,3
0,4
0,5
0,6
0,7
0,8
0,9
1,0
CO
O2, kiln inlet (%, dry) CO, kiln inlet (%, dry)
Fig. 6. O2 and CO concentrations at kiln inlet
0
200
400
600
800
1000
1200
09:00 11:24 13:48 16:12 18:36 21:00
Time
NO
x
200
250
300
350
400
450
500
550
Kiln drive current
NOx, kiln inlet (ppm) NOx, string 1 at 10% O2, dry (mg/Nm3)
NOx, string 2 at 10% O2, dry (mg/Nm3) Current kiln drive (A)
Fig. 7. NOx concentrations and kiln drive current
As also shown in Figure 7, the average NOx concentration
in the stack (average of two strings) is around 500
mg/Nm3 (dry, 10% O2), and hence considerably lower
than the emission limit of 800 mg/Nm3 (dry, 10% O2).
And the NOx emission seems to be unaffected by the
MBM feeding. The CO concentration in the stack (not
shown) was 0% during the entire test; any CO in the kiln
inlet (Figure 6) will be completely oxidized in the
precalciner part of the kiln system.
The above figures confirm that the emissions were
considerably lower than the emission limits during the
test, and not negatively affected by the feeding of MBM.
Figure 8 and 9 demonstrate the smooth operation of
Pfister feeder system which was used for MBM feeding.
The pipeline pressure between the Pfister outlet and the
kiln burner increased proportionally to the MBM feeding
rate, indicating a smooth flow of the MBM. The speed of
the weigh feeder motor was in a range of 0-3200 rpm. The
motor operated around 15% of full capacity, even with
maximum MBM feeding rate. The mass in the hopper was
smoothly controlled between 300 kg and 1500 kg. No
matter the discharge rate, the material level inside the
weigh feeder was always kept at 100% for simple and
accurate operation.
0
1
2
3
4
5
6
7
8
9
09:00 11:24 13:48 16:12 18:36 21:00
Time
Feed rate, Current
0,0
0,1
0,1
0,2
0,2
0,3
0,3
Pressure
Feed rate of MBM (t/hr) Current of Pfister weighfeeder motor (A)
Pressure after rotary valve in Pfister system (bar)
Fig. 8. The effects of feeding rate increment on current of
Pfister weigh feeder and pressure after rotary valve
0
50
100
150
200
250
300
350
400
450
500
09:00 11:24 13:48 16:12 18:36 21:00
Time
Speed, Level
0,0
0,2
0,4
0,6
0,8
1,0
1,2
1,4
1,6
1,8
Mass
Speed of Pfister motor (rpm) Level in Pfister weighfeeder (%) Mass in Pfister hopper (t)
Fig. 9. Level variation of Pfister hopper, weigh feeder and speed
of Pfister motor
Hence, it can be concluded that the Pfister feeder system
operated smoothly during the whole test period, hence
MBM is a material which is well suited for such a feeding
system. Especially the material density (Table II) which
should to be between 50 and 1500 kg/m3, the particle size
distribution and the moisture content of MBM were all
very well fit with the feeding system requirements.
It should also be noted that, not only the feeding system,
but also the entire kiln system, including precalciner,
cyclones and bypass system, were operated smoothly
throughout the test period, even though all operational
data are not presented here.
D. CO2 savings
The above mentioned results showed that 6 t/hr of MBM
feeding appears to be acceptable with respect to product
quality, emissions and operation of the whole kiln system.
From Table I it is seen that 6 t/hr feeding of MBM exactly
halves the coal feeding rate compared to the reference
conditions (i.e. when only coal is fed), resulting in 50%
reduction in net CO2 emissions from the kiln burner
combustion. However, to be on the safe side, and also
taking into account inherent delay effects in the kiln
system, one should probably set the maximum limit
somewhat lower; for example at 40% replacement. This is
equivalent to an annual net CO2 emission reduction of
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about 90,000 tons for the given plant, which corresponds
to 10% of the total annual CO2 emissions of the plant.
4. Conclusion
The test results reveal the possibility of significant
replacement of coal by MBM in the rotary kiln main
burner without negatively affecting product quality,
production rate, emissions or overall operation. No
significant impacts on emissions or operation of the kiln
system were observed, regardless of MBM feeding rate.
The P2O5 content of the clinker increased with MBM
feeding, but was still within the internal quality limit. The
most important impact of the MBM feeding was on the
free lime content of the clinker: When replacing more than
50% of the coal (6 t/hr of MBM feeding) the free lime
level increased to more than 3%, which is not acceptable
from a product quality point of view. To be on the safe
side, considering long-term operation, the replacement
ratio should not exceed 40%. Promisingly, if 40% of the
kiln burner coal is substituted by MBM, around 10% of
the total annual CO2 emissions from the plant can be
avoided.
Acknowledgement
The authors would like to thank MSc students Øyvind
Hasli, Liu Li and Rafid Al-Hawani for doing part of the
laboratory work.
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