Characterization and modelling of a lime production kiln
Agostinho, J.
Characterization and modelling of a lime production kiln
João Emanuel Viegas Agostinho
Instituto Superior Técnico – Departamento de Engenharia Mecânica
Avenida Rovisco Pais, 1096-001 Lisboa, Portugal
[email protected],
Abstract: Energy demand has increased very rapidly because of a huge industrial development and due to a progressive
population growth, which will end up in a very delicate situation for the planet earth in terms of lack of natural resources,
including fossil fuels, energy available and pollution. Industries are the great responsible for the abusive energy consumption
in the world, and people have become more conscientious of the need to preserve those natural resources in order to assure
a better quality of life in the future. GALP 20-20-20 program supports these ideas, and has the purpose to make an energy
consumption analysis to a certain industry in order to improve its consumptions. This work was initially developed within that
program, in a company named Lusical with the purpose to make lime kilns more efficient in order to save some natural gas
provided to combustion. Further, were created 2 description models of the process of heat transfer inside the kilns, one
referring to the combustion gases and the second one referring to the lime stones. The given results from these models are
informative in one hand, because they give information about calcination process as temperatures behavior, and in the other
hand they are predictive because they are useful to study the process when we change some flow/material
parameters/values.
Keywords: Energy consumption; lime kilns; description models.
lime, so in order to improve this abusive consumption,
and by the GALP 20-20-20 program, this work were
based on trying to find natural gas saving strategies.
Lime is the final product of the limestone when it
is taken up to 1000ºC for the carbon dioxide can leave the
stones. For this process be completed, the stones must
absorb a certain quantity of energy which is about 840
Kcal/Kg (≈1 KWh). That calcination process can be
indicated in the following expression:
1. Introduction
Many industrial facilities present very high energy
consumption due to the high demand of their own specific
process. In many cases, that energy need is satisfied
through burning fossil fuels to convert chemical energy in
thermal energy (heat). For example, food industries,
refineries, cement and, in this specific case, limestone
industries are main examples of those which energy
demand is very high.
Limestone must be exposed within a flow which
temperature must be rounding 1000-1200ºC in order to
calcinations process be completed through the complete
liberation of carbon dioxide from inside each limestone.
Lime is a very important product in our lives, and
we can find it everywhere nowadays in so many
distinctive working areas such as in civil construction, in
agriculture, in food, paper and pharmaceutical industries
and in gas and water treatment stations.
In Lusical facilities, they are needed about 50
tons of natural gas for a production of about 800 tons of
+
→
+
(1)
With that necessary amount of heat, 1 Kg of
limestone expel from the inside 0,44 Kg of carbon dioxide
and gives only 0,56 Kg of lime. All the process takes
about 20 hours where the stones are firstly pre heated,
then burned, and finally cooled to be stored in containers
[1].
In this work, the 5 lime kilns in the factory are
analysed in detail in order to understand which measures
1/7
Characterization and modelling of a lime production kiln
Agostinho, J.
can be applied and in which sections of the kiln, where
the energy spent in the process is more significant.
For that study, this work is divided in 3 different
analyses:
1.
2.
3.
Company (Lusical) and the process of
calcinations.
Construction of a descriptive model for the
combustion zone of the kiln which provides
some detailed results in order to understand the
calcination temperatures, gases mass flow, and
other influent parameters.
Results given by the model and its variation in
order to improve it and so to improve the real
calcination process.
Figure 1: Scheme of a working kiln [2]
2.
Description of the company and of the
productive process
In figure 1, the left column is in combustion
process while right column is getting exhaust gases to
pre-heat the stones. The combustion zone, represented
by the red arrows, is where the limestone is transformed.
The fuel enters the kiln through the represented steel
spears (Lanças) and each combustion cycle lasts about
10 minutes. After each cycle, the process is inverted and
get started in the other column and so on.
The stone (raw material) is obtained by exploding
the stone fields surrounding Lusical facilities. Once the
stone is obtained it is taken by trucks to hammers to
obtain a smaller stone size to be transported to the
factory. The stone, before enter the kilns is taken to
sieves in order to get the exact size demanded by the
costumers.
Once the stones arrive into the kilns, the
calcination process gets started at temperatures above
1000ºC. There are 3 types of fuels used in Lusical which
are pet coke, slops (obtained from residual petroleum)
and natural gas. The last one it is the most used in the
factory and so its consumption requires a special
attention.
At the end of the process, the lime is taken to
conveyors in order to get stored in containers before it is
sold to the costumers.
There is still a different process which is called
hydration. The lime is taken inside a container with a
controlled amount of water at a certain temperature. The
lime, which reacts strongly with water within an
exothermal reaction, creates a kind of a dry powder at the
end of the process called hydrated lime.
There are 5 lime kilns (numbers 1, 2, 4, 5 and 7)
used by Lusical, and they are provided by Maerz and they
are called parallel flow kilns because they are constituted
by two parallel columns (with the shape of an “H”). When
a column is providing combustion, the other one is
receiving the exhaust gases to pre-heat the stones. The
figure 1 shows a scheme of how the process is done.
2.1. Energy indexes of Lusical
The calcination process requires 840 Kcal/Kg of
lime and it is independent of the stone size. It is the size
of the kiln itself that matters. In January, the factory
produced 26.501 tons of lime and for that, used 1.832.400
3
m of natural gas. This represents a thermal energy
consumption of 21.332.816 KWh. In February the factory
3
produced 27.543 tons of lime using 1.987.870 m of
natural gas. For this month, the thermal energy
consumption was 23.040.610 KWh. Considering electrical
and the other fuels consumption, for each month the
factory requires energy consumption for about 30 GWh.
2.2. Energy and mass balance
In order to understand in a detailed way the
process of calcination, it was studied the mass of fuel and
air that enters and exits the kiln and consequently the
energy inputs and outputs associated. The figure 2 shows
2/7
Characterization and modelling of a lime production kiln
Agostinho, J.
where those indicators are represented in the kiln
The results of this equation, allow understanding
the values of the 3 different energy losses in the kilns
during the calcination process and they can be seen in
figures 3 to 5. Considering just the kiln number 1, in
January of 2014,
Figure 3: Energy losses through the walls
Figure 4: Energy losses through exhaust gases
Figure 2: Energy/mass balance to the kiln [2]
Through the picture of figure 2, it is possible to
elaborate an equation which describes an energy balance
+
+
,
+
,
,
,
,
+
,
,
! °
,
,
,
+
(2)
Figure 5: Energy losses at the exit of the kiln
# + $%
The losses through the walls are the highest and
directly proportional to the lime production while the
Because of the working of these kilns take the
losses at the exit are lower because the lime is at a very
exhaust gases from the column which is in combustion
low temperature comparing with all the process behind.
process, this balance is not correct because the limestone
is already at calcination temperature right before it gets
2.3. Different zones of the kilns
submitted to the combustion.
In calcination process, the kiln can be divided in
Then, the correct equation is given by:
+
+
+
,
,
,
,
,
,
,
3 different zones:
! °
,
+
1.
,
# + $%
Pre-heating zone, where the stones use exhaust
gases from the opposite column to get to 835ºC;
(3)
2.
Combustion zone, where all the process is done
at very high temperatures (about 1200ºC);
3.
Cooling zone, where with atmospheric air, the
lime is cooled down until 100ºC.
3/7
Characterization and modelling of a lime production kiln
Agostinho, J.
In figure 6, it is possible to see these 3 zones:
5.
56% of combustion heat is used to calcination
process;
Energy losses associated to the process are
10% of total energy spent for the process.
6.
Figure 7, shows a scheme of how lime spheres
are distributed along the kiln.
Figure 7: a) Scheme of distribution of lime spheres in the
Figure 6: Temperatures distribution inside a kiln [2]
kiln and b) Section layer and flow direction
2.4. Hydrated lime – Electrical consumption and
In order to study the process, there is the given
improvement suggestions
equation below [3]:
This section of the factory has a production per
month of 2.500 tons of hydrated lime. This means, for a
12 ton production per hour, that the machines work 208
hours per month. Given the electricity prices for the
different periods of time it was possible to calculate the
costs for January of 2014. That value was 4.705,62 €. In
order to improve these costs, it was made a study for the
best periods of time for the machinery to work. Those
periods are the night periods because it is when electricity
is cheaper. Then, with the ideal solution, the costs will be
2.273,74 €. This represents a reduction of almost 50%3.
documentation from Lusical, while others were obtained
Modelling of calcination process
by calculation of non-constant values through correlations
'
%( )
*+,
−
*+,./
0 + 1$
−6 7
234
*+,
− :%
Some
of
these
− 5 1$
+
2
*+,./
234 #
−
9
(4)
0
values
were
taken
from
and formulas [3].
There were created 2 different models, one
considering the temperature profile of the combustion
gases and the other one considering the profile
temperature of the limestone.
For the first model was assumed that:
1. Temperature gradient is only in axial direction;
2. The kiln height is divided by n layers with ∆z
(with ∆z equal to the diameter of the lime
spheres);
3. Temperature is constant in time for each layer
∆z;
4. Thermal properties of the combustion gases are
constant;
For the second model, the equation is given by:
<
%,
=
=
=
1 =
@A
? =?
?
=
=?
And it is assumed that:
1.
4/7
Limestone have spherical shape;
B
(5)
Characterization and modelling of a lime production kiln
2.
Agostinho, J.
Temperature variation inside the spheres are
With the goal to improve the calcination process,
it is necessary to make some changes in the model to
watch its behaviour when some conditions are changed.
negligible while calcination is not complete;
3.
There is no heat generation inside the sphere;
4.
Limestone properties remain constant during the
The first parameter to change is natural gas
mass flow because natural gas is the main parameter in
this process. The energy required for a combustion cycle
to these kilns is about 5,3 MW. Then, the minimum value
of natural gas mass flow possible is 0,13 Kg/s. However,
this value is physically impossible because for this mass
flow the process requires almost all the total energy
available to complete the calcination. Then, the maximum
value that is possible (the border value) is 0,195 Kg/s.
Initially, the model were considering 0,2 Kg/s because it is
process.
By analysis to the Bi and Fo numbers, it is not
possible to use the global capacitance method to solve
the equation (5). Then, by equation (6), assuming r*=1
(only considering limestone surface) it was possible to get
a temperature profile.
4. Presentation of results
C∗
In order to solve the model equations, they were
assumed some border conditions, based on values taken
on control room of the factory.
The two models were calculated and with similar
results. To converge the two models in one, in order to
get a more precise and unique model, the temperatures
of gases and spheres were constantly substituted on both
models until the results converge.
Figure 8 shows the profile temperatures for
gases and for sphere after the convergence.
E exp)−IE
JK0
1
LMN IE ? ∗ #
IE ? ∗
(6)
the nominal value that is used in Lusical. So this gives a
low reduction of natural gas consumption although it
provides a significant reduction of gas consumption at the
end of the month of 10,8 tons.
The second parameter is the excess of air
coefficient. Initially the model was projected for a
coefficient of 1,05 because it is the average value that is
used in the factory. They constantly try to keep that value
more close to 1 as possible because it gives better
conditions for the process as it can be seen in figure 9.
Figure 8: Final model for gases and spheres
It is possible to see in the graphic of figure 8 that
Figure 9: Calcination temperatures according to excesso air
the gases reach a temperature of 1200ºC as predicted
coefficient [1]
and limestone remains at 900ºC during the calcination.
After the process is completed it rises its temperature
So how much more close as possible of a coefficient of 1,
the higher the temperature is. With a coefficient very high
it would be necessary to introduce more energy in the
kilns to compensate the loss of calcination temperature.
because of the high temperatures from the combustion
gases and then they both low their temperature at the end
of the combustion zone, where there is no more
combustion.
The third parameter is the size of the stone inside
the kilns. As it was said previously, energy consumption
does not depend on the size of the lime spheres. It is
4.1. Parameters variation – Improvement suggestions
5/7
Characterization and modelling of a lime production kiln
Agostinho, J.
possible to observe that changing the model because it
only affects the grid (∆z) and nothing else.
The fourth parameter is the convection
coefficient. This coefficient is very important to modelling
the process because it depends on many factors such as
Biot number, Reynolds number, Nusselt number, material
properties coefficients, etc. So this convection coefficient
must be dimensioned carefully. Basically, in order to
change convection coefficient, it can be changed the gas
mass flow (as it was done previously) and the excess air
coefficient. These changes result in a direct variation of
convection coefficient. As these values are already
optimized, it makes the convection coefficient optimized
2
too. It takes the value of 31,15 W/m K.
Figure 11: Final results
5. Conclusions
The fifth and last parameter that was changed is
the energy losses associated to the process. Of course if
we consider 0% losses the model would be perfect as it
can be seen in figure 10
This work had its main goal of suggesting
improvement measures to save energy consumption on
the 5 lime kilns of Lusical. It had also the purpose of save
some electricity consumption in machinery of hydrated
lime section. It was developed an optimized working plan
for this section which requires that the machinery work
only on night periods because of the cheaper cost of
electricity. With this new plan it is possible to reduce 48%
on the electricity costs per month. Unfortunately, natural
gas savings were impossible to suggest because of the
highly efficient calcination process provided by parallel
flow kilns.
To study the process with more detail, it was
developed a description model. After a variation on some
flow parameters it was possible to arrive to some useful
conclusions. With a reduction of 2,5% of natural gas mass
flow, it represents a reduction of 240 KW per combustion
cycle. This means at the end of the month, per kiln, there
is a reduction of 144 MWh which represents a reduction
of 1.728 MWh annually for each kiln which means an
energy saving of 2,67%. With this measure, the factory
can save 10 tons on each kiln of natural gas per month.
At the end of an year this represents a financial saving of
about 75.000 €.
Figure 10: Temperature profile with 0% energy losses
It is possible to see that gases energy are above
1200ºC and the distance between the two curves is
bigger which means that more energy is spent to
complete the calcination process. Gases temperature
would be higher than 1250ºC and the lime spheres would
receive more energy assuming the same conditions.
Because this is physically impossible, the value
of 10% admitted before will remain and the final and
optimized model is presented on figure 11.
6/7
Characterization and modelling of a lime production kiln
Agostinho, J.
References
[1] S, Bruno. 2007. Estudo da produção de óxido de
cálcio por calcinação do calcário: caracterização dos
sólidos, decomposição térmica e otimização paramétrica.
Dissertação de Mestrado, Universidade Federal da
Uberlândia. Brasil.
[2] Maerz. Fornos de Escoamento Paralelo. Disponível
em: https://www.maerz.com, Março de 2014.
[3] Incorpera, F. P. e Dewitt, D. P. e Bergman, T. L. e
Lavine, A. S., 2007. Fundamentals of heat and mass
transfer. 6th edition. John Wiley & Sons, Inc.
7/7