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Energy recovery of a rotary kiln system in a calcium oxide plant

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Abstract

The dominant source of calcium carbonate is limestone; the most common constituent of all rocks. It occurs in nature with clay, silica and other minerals, which may interfere in many applications. Synthetic calcium carbonate is produced on a large scale where a calcium chloride stream is treated with sodium or ammonium carbonate to produce a high grade of calcium carbonate. In turn, calcium carbonate is heated to 900–1000 degree centigrade in a horizontal lime kiln to produce calcium oxide (burned lime). Calcium oxide is used extensively in cement, iron and steel industries due to the low cost of the material and its accepted chemical properties. In this study, composition of raw meal, ultimate analysis of the fuel, dust contents in the exhaust gases, losses in ignation (COI) and exhaust gas composition in the preheated suspension are calculated. The heat losses from kiln exhaust are minimized. A secondary shell on the kiln surface is also investigated. In this work mathematical models for the calculations of inside heat transfer on the rotary are used, and the total energy utilized by burning fuel oil in the process of calcium oxide production is also calculated. The heat losses for kiln exhaust gas, hot air from the cooler stock losses and radiation losses from kiln surface are also minimized. A secondary shell on the kiln surface is studied in the present study, where 4% of total energy input is saved. This energy saving would result in a considerable reduction of fuel consumption in the kiln system. The overall efficiency would be improved by 5%. Keywords: rotary kiln reactor, radiant heat transfer, coating, combustion.
Energy recovery of a rotary kiln system in a
calcium oxide plant
M. Aldeib, A. Elalem & S. Elgezawi
Department of Chemical Engineering, Faculty of Engineering,
University of AlFateh Tripoli, Libya
Abstract
The dominant source of calcium carbonate is limestone; the most common
constituent of all rocks. It occurs in nature with clay, silica and other minerals,
which may interfere in many applications. Synthetic calcium carbonate is
produced on a large scale where a calcium chloride stream is treated with sodium
or ammonium carbonate to produce a high grade of calcium carbonate. In turn,
calcium carbonate is heated to 900–1000 degree centigrade in a horizontal lime
kiln to produce calcium oxide (burned lime). Calcium oxide is used extensively
in cement, iron and steel industries due to the low cost of the material and its
accepted chemical properties. In this study, composition of raw meal, ultimate
analysis of the fuel, dust contents in the exhaust gases, losses in ignation (COI)
and exhaust gas composition in the preheated suspension are calculated. The heat
losses from kiln exhaust are minimized. A secondary shell on the kiln surface is
also investigated. In this work mathematical models for the calculations of inside
heat transfer on the rotary are used, and the total energy utilized by burning fuel
oil in the process of calcium oxide production is also calculated. The heat losses
for kiln exhaust gas, hot air from the cooler stock losses and radiation losses
from kiln surface are also minimized. A secondary shell on the kiln surface is
studied in the present study, where 4% of total energy input is saved. This energy
saving would result in a considerable reduction of fuel consumption in the kiln
system. The overall efficiency would be improved by 5%.
Keywords: rotary kiln reactor, radiant heat transfer, coating, combustion.
Energy and Sustainability III 353
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WIT Transactions on Ecology and the Environment, Vol 143, ©2011 WIT Press
doi:10.2495/ESUS110301
1 Introduction
Radiant heat (mainly infra-red ray) is emitted from the burning flame at high
temperature in a rotary kiln reactor, some fraction of which arrives at the surface
of the rotating solids layer, and the other arrives at the inner refractory surface.
The rest is absorbed in the combustion gas around the flame. For predicting
radiant heat transfer, however, radiant heat transfer is simplified to apply to the
complex mechanism of heat transfer in a rotary kiln, without losing significant
fundamentals. Radiant heat absorbed in combustion gas around the flame is
converted to thermal energy, which should be emitted as infra-red ray from the
gas to the rotating solids and inner wall surface. Thus we can assume that all the
radiant heat emitted from the flame arrives at the surface of the rotating solids
and the inner wall surface.
2 Mathematical modeling of inside heat transfer on a rotary
kiln with radiant heat transfer from flame and combustion
gas
In the region where combustion takes place, radiant heat transfer from the
burning flame is controlling, whereas convectional heat transfer is one order of
magnitude less. For convenient application to practical design calculation, there
are simplified models to represent the heat transfer mechanism in this region [1].
Radiant heat absorbed in combustion gas around the flame is converted to
thermal energy, which should be emitted as infra-red rays from the gas to the
rotating solids and inner wall surface. Thus we can assume that all the radiant
heat emitted from the flame arrives at the surface of the rotating solids and the
inner wall surface. The radiant heat transfer coefficient, hrg, to two solid surfaces
is calculated by, [1]
=
.
 
 
 (1)
where df is the outer diameter of the flame, εf is the emissivity of the flame, εm is
the average emissivity of the solids layer surface and the inner wall surface, and
T is the average temperature of the above two surfaces. The emissivity of the
luminous flame was measured very precisely by members of an international
committee. The average diameter of flame df depends strongly on the
hydrodynamic feature of turbulent diffusion. By visual observation of the flame
in a practical rotary kiln, in which a burner is operated under similar flow
conditions to the one planned, we can estimate the approximate value of df/dti
The average temperature T is calculated using the following equation:
  (2)
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3 Radiant heat transfer from inner wall surface to rotating
solids layer
Radiant heat emitted from the wall surface per unit length of the reactor is given
by [1]
(4.88)
 (3)
The geometrical view (angle) factor from the inner surface πdti(1-χ) to the
rotating layer of solids is represented by FHC.
  
(4)
Since FCH = 1, we have FHC = χ / 1-χ.
Radiant heat, emitted from the inner surface, is mainly infrared rays. When it
passes through the flame and combustion gas, some part of the infrared ray is
absorbed by them.
Let us take εg* to be the average value of emissivity for the flame and
combustion gas. Thus, the rate of radiant heat transfer from the inner wall
surface to the rotating solids layer is calculated approximately with Eq. (5), in
the case where εH and εC are close to unity.

 
  (5)
Define the radiant heat transfer coefficient from the hot inner wall to the layer
of solids by (hrs)HC, on the basis of the surface area of the inner wall.

(6)
Combination of Eqs. (5) and 6) leads to the following equation
  (1-).
 
  (7)
4 Heat transfer coefficient by direct contacting of solids from
the hot wall surface
In a rotary kiln, solids are heated by direct contact with the hot wall surface. The
inner wall surface functions as a kind of regenerator, changing the surface
temperature periodically during the rotation. Theoretical calculation reveals that
the amplitude in the periodical change of surface temperature is not too much, as
long as the rotation is larger than 3 r.p.m. In this section, let us take the time
averaged temperature of the wall. For a packet of solids, which suddenly contact
the hot surface and then leave it after residing there for a time t, the following
equation calculate the time averaged value of the heat transfer coefficient due to
the above contact, on the basis of contacting surface area.
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0.5
   .
(8)
where ke is the effective thermal conductivity of a packet of solids,
is the bulk
density, and Cs is the specific heat of solid.
Introduce the following substitution:
(9)
Substitution of Eq. (9) in Eq. (8) gives
   .


. (10)
The temperature of the inner wall surface TH is determined by a given value
of the flame temperature Tf. On the basis of unit length, using the following
equation: 
(11)
Heat transfer from the inner wall to the rotating solids by direct contact can be
calculated using Eq. (12)
(12)
Heat loss to outside with coating is estimated from the following equation:
"
/
/
 /
 ……… (13)
5 Mathematical modeling of inside heat transfer on a rotary
kiln without coating
The same procedure done: from equation (1) to equation (12) but eliminate the
first term in the denominator from equation (13).
5.1 Investigation of the mathematical model:
Known data
Tc= 950oC,  .,.,.
dti=4.2 m (with coating), and equal 4.35 m (without coating), df =1.4 m.
Flame temperature (Tf) range in cement industry from 1200 to 2000oC.
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Results from Program
The results calculated are shown in the following tables:
With coating
Table 1 presents both internal and external temperatures of rotary kiln for the
burning zone; they are functions of flame temperature, and increase with the
flame temperature. Also, the heat loses are increased, due to increase in the
difference of temperature between the inside (flame) and outside (environment)
temperatures.
Table 1: Prediction of inner and outer shell surface temperatures with
coating.
Tf (oC) 1200 1400 1600 1800 2000
TH (oC) 1029.8 1133.6 1237.4 1341.2 1445
T
W
(oC) 267.85 297.85 327.85 357.85 387.85
Hrg 98.66 133.06 174.9 224.95 283.94
Hrs 158.68 179.76 203.14 228.95 257.35
Hp 561.89 561.89 561.89 561.89 561.89
Qlosses
(kcal/m2.hr)
5990.41 6570.621 7150.83 7731.04 8311.25
These behaviors are clarified in table 1 whereas increasing the flame
temperature with the industrial range (from 1200 to 2000oC) note that, both
internal and external wall surface temperatures are increased respectively.
Table 2 presents both internal and external temperatures of rotary kiln for the
burning zone; are function of flame temperature, and increasing with increase the
flame temperature. Also, the heat loses increase, due to increase in the difference
of temperatures between the inside (flame) and outside (environment)
temperatures. The optimum external wall surface temperature in industrial
processes ranged (from 250 to 340oC), Note that in Table 2, Tw reaches up to
379oC when the flame temperature greater than 1580oC. That will lead to many
Table 2: Predicted temperatures of inner wall surface and outer shell surface
without coating.
Tf (oC) 1200 1400 1600 1800 2000
TH (oC) 1029.8 1133.6 1237.4 1341.2 1445
TW (oC) 306.25 342.65 379.05 415.45 451.85
Hrg 87.097 117.64 154.4 198.58 283.94
Hrs 158.68 179.76 238.14 228.95 257.35
Hp 561.89 561.89 561.89 561.89 561.89
Qlosses
(
kcal/
m
2.hr
)
5921.224 6463.698 7242.03 8271.945 8802.627
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Figure 1: Hot spot formation on the rotary kiln shell surface.
troubles in the rotary kiln in this case the red spot (hot spot as shown in Figure 1
through the rotary kiln will occur, in which the coating will start to collapse that
leading to decrease in the thermal resistance that leads to increase in the external
wall surface temperature (Tw) reaching up to 450oC which is known as red spots.
Table 3 shows that the internal wall surface temperature predicted by model is
satisfied with practical data obtained from industrial process with acceptable
deviation.
Table 3: Prediction and practical temperatures of inner wall surface and
outer shell surface.
Tf (oC) 1200 1400 1600 1800 2000
TH (oC)
(actual)
1029.8 1133.6 1237.4 1341.2 1445
TH (oC)
(p
redicted
)
1021 1120 1249 1410 1501
TW (oC) 267.85 297.85 327.85 357.85 387.85
Hrg 98.66 133.06 174.9 224.95 283.94
Hrs 158.68 179.76 203.14 228.95 257.35
Hp 561.89 561.89 561.89 561.89 561.89
Qlosses
(
kcal/
m
2.hr
)
5990.41 6570.621 7150.83 7731.04 8311.25
Qlosses
(
kcal/
m
2.hr
)
5921.224 6463.698 7242.03 8271.945 8743.66
Hot Spot
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6 Conclusion
A detailed energy audit analysis, which can be directly applied to any dry kiln
system has been made in this study. The distribution of the input heat energy to
the system components showed good agreement between the total input and
output energy and give significant insight about the reasons for the low overall
system efficiency. According to the results obtained, the system efficiency is
50. The major heat loss sources have been determined as kiln exhaust with
20. Cooler exhaust heat loss is calculated at 5 of total input. The combined
radiative and convective heat transfer from kiln surface is at 4 of total input.
The simulation done on the process showed that the predicted external wall
surface temperature is ranged from 267 to 327 degree centigrade to avoid hot
spot formations on the rotary kiln surface.
References
[1] D. Kunii and T. Chisaki, Rotary Reactor Engineering, First edition, Jordan
Hill, 2008
[2] Deliang Shi, Watson Vargas, J. Mc Carthy, Heat Transfer in Rotary Kilns
with Interstitial gases, Chemical Engineering Science 63, 2008
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