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                            Energy auditing and recovery for dry type cement rotary kiln systems--A case study
	Process description and data gathering
	Energy auditing and heat recovery
		Mass balance
		Energy balance
		Heat recovery from the kiln system
			The use of waste heat recovery steam generator (WHRSG)
			Use of waste heat to pre-heat the raw material
			Heat recovery from kiln surface
Document Text Contents
Page 1

Energy Conversion and Management 46 (2005) 551–562
Energy auditing and recovery for dry type cement
rotary kiln systems––A case study

Tahsin Engin *, Vedat Ari

Department of Mechanical Engineering, University of Sakarya, Esentepe Campus, 54187 Sakarya, Turkey

Received 30 January 2004; accepted 29 April 2004

Available online 2 July 2004


Cement production has been one of the most energy intensive industries in the world. In order to produce

clinker, rotary kilns are widely used in cement plants. This paper deals with the energy audit analysis of a

dry type rotary kiln system working in a cement plant in Turkey. The kiln has a capacity of 600 ton-clinker

per day. It was found that about 40% of the total input energy was being lost through hot flue gas (19.15%),
cooler stack (5.61%) and kiln shell (15.11% convection plus radiation). Some possible ways to recover the

heat losses are also introduced and discussed. Findings showed that approximately 15.6% of the total input

energy (4 MW) could be recovered.

� 2004 Elsevier Ltd. All rights reserved.

Keywords: Cement plant; Rotary kiln; Energy audit; Heat balance; Heat recovery

1. Introduction

Cement production is an energy intensive process, consuming about 4 GJ per ton of cement
product. Theoretically, producing one ton of clinker requires a minimum 1.6 GJ heat [1]. How-
ever, in fact, the average specific energy consumption is about 2.95 GJ per ton of cement produced
for well-equipped advanced kilns, while in some countries, the consumption exceeds 5 GJ/ton. For
example, Chinese key plants produce clinker at an average energy consumption of 5.4 GJ/ton [2].

The energy audit has emerged as one of the most effective procedures for a successful energy
management program [3]. The main aim of energy audits is to provide an accurate account of
energy consumption and energy use analysis of different components and to reveal the detailed
* Corresponding author. Tel.: +90-264-346-0353; fax: +90-264-346-0351.

E-mail addresses: [email protected], [email protected] (T. Engin).

0196-8904/$ - see front matter � 2004 Elsevier Ltd. All rights reserved.

mail to: [email protected],

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552 T. Engin, V. Ari / Energy Conversion and Management 46 (2005) 551–562
information needed for determining the possible opportunities for energy conservation. Waste
heat recovery from hot gases [4] and hot kiln surfaces [5] in a kiln system are known as potential
ways to improve overall kiln efficiency. However, it is still fairly difficult to find a detailed thermal
analysis of rotary kiln systems in the open literature. This paper focuses on the energy audit of a
horizontal rotary kiln system, which has been using in the Van Cement Plant in Turkey. A de-
tailed thermodynamic analysis of the kiln system is first given, and then, possible approaches of
heat recovery from some major heat loss sources are discussed.
2. Process description and data gathering

Rotary kilns are refractory lined tubes with a diameter up to 6 m. They are generally inclined at
an angle of 3–3.5�, and their rotational speeds lie within 1–2 rpm. Cyclone type pre-heaters are
widely used to pre-heat the raw material before it enters the kiln intake. In a typical dry rotary kiln
system, pre-calcination gets started in the pre-heaters, and approximately one third of the raw
material would be pre-calcined at the end of pre-heating. The temperature of the pre-heated
material would be of the order of 850 �C. The raw material passes through the rotary kiln towards
the flame. In the calcination zone (700–900 �C), calcinations, as well as an initial combination of
alumina, ferric oxide and silica with lime, take place. Between 900 and 1200 �C, the clinker
component, 2CaO ÆSiO2, forms. Then, the other component, 3CaO Æ SiO2, forms in a subsequent
zone in which the temperature rises to 1250 �C. During the cooling stage, the molten phase,
3CaO ÆAl2O3, forms, and if the cooling is slow, alite may dissolve back into the liquid phase and
appear as secondary belite [6]. Fast cooling of the product (clinker) enables heat recovery from the
clinker and improves the product quality.

The data taken from the Van Cement Plant have been collected over a long period of time when
the first author was a process engineer in that plant. The plant uses a dry process with a series of
cyclone type pre-heaters and an incline-kiln. The kiln is 3.60 m in diameter and 50 m long. The
average daily production capacity is 600 ton of clinker, and the specific energy consumption has
been estimated to be 3.68 GJ/ton-clinker. A large number of measurements have been taken
during 2 yr, and averaged values are employed in this paper. The raw material and clinker
compositions are given in Table 1. The rotary kiln system considered for the energy audit is
schematically shown in Fig. 1. The control volume for the system includes the pre-heaters group,
rotary kiln and cooler. The streams to and from the control volume and all measurements are
indicated in the same figure.
3. Energy auditing and heat recovery

3.1. Mass balance

The average compositions for dried coal and pre-heater exhaust gas are shown in Fig. 2. Based
on the coal composition, the net heat value has been found to be 30,600 kJ/kg-coal.

It is usually more convenient to define mass/energy data per kg clinker produced per unit time.
The mass balance of the kiln system is summarized in Fig. 3. All gas streams are assumed to be
ideal gases at the given temperatures.

Page 6

Table 2 (continued)

Description Equations used Data Result (kJ/kg-clinker)

Natural convection from

pre-heater surface

Q15 ¼ hnconAphðTs � T1Þ, hncon ¼ kairLph Nu,
Nu ¼ 0:1ðRaÞ1=3

Pre-heater is modeled as a vertical cylinder

with D ¼ 4 m, Lph ¼ 12 m, Ra ¼ 0:649� 1013,
Nu ¼ 1865 (Ref. [8]), at Tf ¼ 45 �C (film temp.)

5 (0.14%)

Radiation from cooler


Q16 ¼ reAcðT 4s � T 41Þ=ð1000 _mcliÞ r ¼ 5:67� 10�8 W/m2 K4, e ¼ 0:78 (oxidized
surface, Ref. [8]), Ac ¼ 64 m2, Ts ¼ 353 K,
T1 ¼ 328 K, _mclinker ¼ 6:944 kg/s

4 (0.11%)

Natural convection from

cooler surface

Q17 ¼ hnconAcðTs � T1Þ, hncon ¼ kairLc Nu,
Nu ¼ 0:1ðRaÞ1=3

Cooler surface is modeled as a vertical plate

8 m · 8 m, Ra ¼ 0:7277� 1012, Nu ¼ 899 (Ref.
[8]), at Tf ¼ 70 �C (film temp.)

9 (0.24%)

Unaccounted losses 273 (7.41%)

Total heat output 3686 (100%)





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T. Engin, V. Ari / Energy Conversion and Management 46 (2005) 551–562 557
of hot water. There are a few major heat loss sources that would be considered for heat recovery.
These are heat losses by: (1) kiln exhaust gas (19.15%), (2) hot air from cooler stack (5.61%) and
(3) radiation from kiln surfaces (10.47%). In the following section, we discuss some possible ways
for recovering this wasted heat energy.
3.3.1. The use of waste heat recovery steam generator (WHRSG)
There are opportunities that exist within the plant to capture the heat that would otherwise be

wasted to the environment and utilize this heat to generate electricity. The most accessible and, in
turn, the most cost effective waste heat losses available are the clinker cooler discharge and the
kiln exhaust gas. The exhaust gas from the kilns is, on average, 315 �C, and the temperature of the
air discharged from the cooler stack is 215 �C. Both streams would be directed through a waste
heat recovery steam generator (WHRSG), and the available energy is transferred to water via the
WHRSG. The schematic of a typical WHRSG cycle is shown in Fig. 4. The available waste energy
is such that steam would be generated. This steam would then be used to power a steam turbine
driven electrical generator. The electricity generated would offset a portion of the purchased
electricity, thereby reducing the electrical demand.

In order to determine the size of the generator, the available energy from the gas streams must
be found. Once this is determined, an approximation of the steaming rate for a specified pressure
can be found. The steaming rate and pressure will determine the size of the generator. The fol-
lowing calculations were used to find the size of the generator.
QWHRSG ¼ Qavailableg
where g is the WHRSG efficiency. Because of various losses and inefficiencies inherent in the
transfer of energy from the gas stream to the water circulating within the WHRSG, not all of the







Feed pump


Fig. 4. Process schema of a typical WHRSG application.

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T. Engin, V. Ari / Energy Conversion and Management 46 (2005) 551–562 561
Assuming a temperature difference of DTins ¼ 250 �C (which means an outer surface temperature
of 58 �C), Rins can be determined:
DTins ¼ Q� ðresistance of insulationÞ

250 �C ¼ 147; 000�
We found Rins ¼ 4:108 m, and the thickness of insulation would be

e ¼ Rins � Rshell ¼ 0:108 m
11 cm
It should be noted that when the secondary shell is added onto the kiln surface, the convective
heat transfer would presumably become insignificant. This is because of the fact that the tem-
perature gradient in the gap would be relatively very low, e.g., 0.45 �C/cm. Therefore, the total
energy savings due to the secondary shell would be
ð386kJ=kg-clinkerÞ � ð6:944kg-clinker=sÞ � 147 ¼ 2533 kW

from the radiation heat transfer and
ð171 kJ=kg-clinkerÞ � ð6:944 kg-clinker=sÞ ¼ 1187 kW

from the convective heat transfer. Therefore, we can safely conclude that the use of a secondary
shell on the current kiln surface would save at least 3 MW, which is 11.7% of the total input
energy. This energy saving would result in a considerable reduction of fuel consumption (almost
12%) of the kiln system, and the overall system efficiency would increase by approximately 5–6%.
4. Conclusions

A detailed energy audit analysis, which can be directly applied to any dry kiln system, has been
made for a specific key cement plant. The distribution of the input heat energy to the system
components showed good agreement between the total input and output energy and gave sig-
nificant insights about the reasons for the low overall system efficiency. According to the results
obtained, the system efficiency is 48.7%. The major heat loss sources have been determined as kiln
exhaust (19.15% of total input), cooler exhaust (5.61% of total input) and combined radiative and
convective heat transfer from kiln surfaces (15.11% of total input). For the first two losses, a
conventional WHRSG system is proposed. Calculations showed that 1 MW of energy could be
recovered. For the kiln surface, a secondary shell system has been proposed and designed. It is
believed that the use of this system would lead to 3 MW of energy saving from the kiln surface.
Hence, the total saving for the whole system has been estimated to be nearly 4 MW, which
indicates an energy recovery of 15.6% of the total input energy. The pay back period for the two
systems is expected to be less than 1.5 yr.

[1] Liu F, Ross M, Wang S. Energy efficiency of China’s cement industry. Energy 1995;20(7):669–81.

[2] Khurana S, Banerjee R, Gaitonde U. Energy balance and cogeneration for cement plant. Appl Therm Eng


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[3] Pahuja A. Energy auditing and monitoring in the cement industry. Energy Conservation and Environmental Control

in Cement Industry, vol. 2, Part 2, Published by Akademia Books International, ISBN: 81-85522-05-7, 1996. p. 670–


[4] Kamal K. Energy efficiency improvement in the cement industry. Seminar on Energy Efficiency, organized by

ASSOCHAM-India and RMA-USA, January 1997.

[5] Engin T. Thermal analysis of rotary kilns used in cement plants. First Mechanical Engineering Congress

(MAMKON’97), Istanbul Technical University, TURKIYE, 4–6 June 1997, p. 29–35 [in Turkish].

[6] Kaantee U, Zevenhoven R, Backman R, Hupa M. Cement manufacturing using alternative fuels and advantages of

process modelling. Presented at R’2002 Recovery, Recycling, Re-Integration, Geneva, Switzerland, Available from:, 12–15 February 2002.

[7] Peray KE. Cement manufacturer’s handbook. New York, NY: Chemical Publishing Co., Inc.; 1979.

[8] Cengel YA. Heat transfer––A practical approach. 2nd ed. New York, NY: McGraw-Hill; 2003.

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