Energy and Exergy Assessment and Heat Recovery on Rotary Kiln of Cement Plant for Cooling Effect Production by Using Vapor Absorption Refrigeration System

The main aim of this study is to use waste heat for cooling effect production from cement rotary kiln shell by applying vapor absorption refrigeration system. The plant has performance to manufacture 2000 tons of clinker per day. Energy and exergy analysis has been performed to assess first and second law efficiencies and rotary kiln is used as a control volume on dry type cement plant. Result shows that about 4.3MW energy is lost from kiln shell. From the analysis, 31.13% total exergy is wasted to the surrounding in case of pre-calcining and pre-heating of raw material. The overall result for exergy analysis of kiln indicates 59.46% of irreversibility and also the first and the second law efficiency of the rotary kiln is 53.39% and 40.54%, respectively. By using convective mode of heat transfer about 11% of energy is extracted by the generator for production of cooling effect which is wasted from kiln shell. About 300kW cooling effect is produced in the evaporator by applying absorption cycle with system performance 0.67 and exergetic efficiency 87%. From heat recovery there is direct savings by reducing fuel consumption and indirect savings by decreasing environmental impact. Hence, use of waste heat results in reduction of thermal pollution and energy consumption in auxiliary equipment.


INTRODUCTION 1
Manufacturing cement is the most energy consuming industries in the world. It is used about 10-15% of industrial energy use. Generally, energy attains 35-45% of production cost in the cement industry. So it is interesting to reduce production cost. Due to energy analysis fails to show the transformation and location of energy degradation, exergy analysis is needed to determine quality and quantity of energy [1].
Exergy analysis is very important to show and analyze thermodynamic imperfections quantitatively and qualitatively which is caused by thermal and chemical processes. The first and the second laws of thermodynamics is used to analyze exergy, while the energy analysis is used the first law only. Exergy is expressed as the maximum available work which is getting from a system when it comes to equilibrium with a reference environment. It has also a feature to show the assessment of energy degradation quantitatively [1,2].
Firstly, the rotary kiln system mass balance is determined. Then, by using the first law of thermodynamics enthalpies entering into and leaving the rotary kiln are assessed. Furthermore, according to the second law of thermodynamics *Corresponding Author Email: diyakonbenget27@gmail.com (B. G. Emyat) exergy is calculated. Finally, efficiencies based on the first and second laws are compared and also shown by using Grassman diagrams then by using vapor absorption refrigeration system some of the wasted energy from the rotary kiln shell is recovered for cooling effect production.

PROCESS DESCRIPTION
The main component of cement is limestone (CaCO3) which is extracted from quarry. These large sized particles are crushed in to small units. These units are mixed with appropriate additives such as laterite and sweeteners, which makes important mixture and going into raw mill at which it changes into a powdered form called raw meal. This powdered form becomes preheated in the preheater before going into rotary kiln. The key process of cement manufacturing is clinkerisation which it takes place in the rotary kiln during combustion process. The clinker leaving from the kiln after cooling process in the cooler vent will sum up with some necessary amount of gypsum and fly ash, and changes into fine powder in the cement mill. Finally, it goes for packaging and shipping [3]. Figure 1 shows a flow diagram for cement manufacturing process. Also, the overall method for the assessment of rotary kiln is shown in Figure 2.

THERMODYNAMIC ANALYSIS OF ROTARY KILN Assumptions
It is appropriate to use the following assumptions for thermodynamic analysis rotary kiln: a) The system is steady state and open. b) Neglect change of kinetic and potential energies. c) Environmental temperature is constant i.e. T∞ = 297K. d) The velocity of air is < 3 m/s. e) Consider the gases as an ideal gas.

Mass balance
The mass is conserved in rate form and can be defined as follows:

∑̇= ∑̇
The mass balance of the kiln system is summarized in Table  1 and Figure 3.

Energy balance
The total energy analysis for steady state process in rate form is described as follows:

∑̇= ∑̇
where, ̇ = ̇Cp ΔT All entering and leaving energies is determined by using the above equation. The energy of clinker formation is calculated by using Zur-Strassen equation [ There is also heat transfer between rotary kiln shell and the surrounding due to the temperature variation between outer and inner surface of rotary kiln. This is because of three modes of heat transfer, i.e. conduction, convection and radiation. This heat transfer can be calculated by the following equation [6]: Therefore, the efficiency of the first law of thermodynamics is given by the ratio of energy output to input energy: The detailed assessment of energy is shown below in Table 2.

Exergy balance
Exergy is expressed as the maximum shaft work which has been done by the composite of the system and a given reference surrounding. Exergy is not only a thermodynamic characteristic, but rather it is the characteristic of a system and reference surrounding. It has a property which is conserved simply when all processes held on a system and the surrounding are reversible [8].
The exergy balance is used to evaluate the locations, kinds, and actual magnitudes of waste energy resource, so it plays a significant role for effective use of fuel. Exergy is destroyed when the process is irreversible. Thermodynamic imperfections are also considered as exergy destructions, which indicate energy quality lost [7].   Dust and ash 1.00

Figure 3. Mass balance in the kiln system
Exergy efficiency is used to measure ideality approach (reversibility). It is not true for energy efficiency, because of misleading. Furthermore, exergy is expressed as the measurement of minimum work which is used to produce goods, assessment of energy conversion, and utilization [9].

Reference environment
The environment has to be considered a natural reference datum and in thermodynamic equilibrium without utilized energy when exergy of various systems is assessed. By considering any kind of process and environment which is netting from irreversibilities, environmental intensive property is not significantly varied [10].

Dead state
When the thermodynamic state (i.e. pressure, temperature, composition, velocity or elevation) of a system is deviate from the surrounding, there is a probability to enhance work. By changing the phase of the system with respect to the environment, the probability reduced, it doesn't exist when the two, at rest towards one another, are in equilibrium. This system phase is known as dead state. The four categories of exergy [11] are Physical exergy, Chemical exergy, Potential exergy and Kinetic exergy.

Potential and kinetic exergy
Kinetic energy and potential energy of various streams are forms of energy and have a full potential for converting to work. Consequently, when assessed towards with the surrounding reference frame, which are equal to kinetic and potential exergy respectively. Hence:

Physical exergy
Physical exergy (EPH) is given by the maximum work available when the stream of substance is brought from its starting phase to the final phase of the surrounding described by P0 and T0, by physical processes including simply thermal interaction with the surrounding. The physical exergy of a closed system at a given state is expressed as follows: Chemical exergy Chemical exergy is expressed as the maximum work found by considering the substance which is brought from the surrounding state to the dead state by processes including transfer of heat and exchange of substances only with the surrounding. The exergy balance of the rotary kiln is given by the following expression: where Q is the heat transfer rate from rotary kiln surface. By reducing exergy loss or irreversibility, the exergy efficiency of a process can be improved. Good exergy efficiency allows a convenient relation of energy sources and utilization. Δh = Δu + v ΔP By neglecting change of pressure, the enthalpy change becomes; Δh = Δu = CP ΔT The entropy change for ideal gases is given below: The detailed exergy assessment is given in Table 2. From Messebo Cement Plant line 1 large number of data have been collected for a long time and different measurements are used in 12 months and averaged and interpolated values are used in this study.

Exergy analysis of producing 'cooling effect' by using heat exhaust from kiln shell by means of 'absorption cycle' Absorption cycle
Absorption cycle is operated by heat for obtaining cooling and this heat replaces mechanical work. In this system, mechanical compression is substituted by a thermal compression by thermal compressor which is a combination of an absorber, a generator, a pump, and an expansion valve. Pure water (refrigerant vapor) from the evaporator is extracted by a lithium bromide (strong solution) in the absorber to form weak solution. This solution is pressurized by pump and fed to the generator where the vapor refrigerant (pure water) is generated again using a waste heat from rotary kiln by convective modes of heat transfer. The strong solution (Li-Br) is return back to the absorber via an expansion valve [13]. Figure 4 illustrates a schematic view of Vapor absorption (H2O-LiBr) refrigeration system. Process 1-2: high temperature pure refrigerant vapor is condensed and heat of condensation (Qc) is rejected to the external heat sink. where; ̇= ̇+ ̇ The system coefficient of performance and exergetic efficiency of vapor absorption refrigeration system is given as follows: Consider the operating temperatures of generator, evaporator, condenser and absorber are 110℃, 7℃ and 40℃, respectively with effectiveness of heat exchanger = 0.94 and the heat that supplied from the rotary kiln shell to the generator is 450kW by using convective mode of heat transfer.
The properties for H2O-LiBr solution and their operating conditions are listed in Table 3. The results of energy and exergy analysis for each component are summarized in Table 4.

RESULT AND DISCUSSION
By using the above equations, the first and the second law efficiency of the rotary kiln is 53.39% and 40.54% respectively. In addition to that the energy and exergy analysis are represented by Grassman diagram in Figures 5 and 6.
From the diagram 41.66% energy is wasted while 0.443MW is unaccounted loss. But the energy which is transferred through the kiln shell is recoverable for production of cooling effect by using convective mode of heat transfer. In this study the generator uses 450kW from the rotary kiln shell for 300kW (85.31TOR) cooling effect production in the   evaporator in the vapor absorption refrigeration system with system performance 0.67 and exergetic efficiency 87%. Also rotary kiln irreversibility is accounted 59.46% this indicates destruction of exergy into the system while 31.13% exergy is leaving with the hot gas that can be retrieved by arranging some auxiliary components. The calculated results are presented in tabulated form in Table 5.

CONCLUSION
The objective of this study is to evaluate and assess exergy utilization, exergy balance and their irreversibility in Messebo cement factory. Mass, energy and exergy analysis of the whole process and recovery process has been performed using the actual operational data of the plant. Exergy analysis is an important tool, which is used to carefully analyze the design and assess the performance of the system that related with energy. By recovering heat the volume of exhaust gas and emissions to the atmosphere can be reduced. From heat recovery there is direct savings in case of reducing fuel consumption and indirect savings in case of decreasing environmental impact. It also facilitates smaller and more economic furnaces. In this study the kiln shell is used as a furnace to supply heat to the generator by natural means (convective mode of heat transfer) in vapor absorption refrigeration system for cooling effect production in the evaporator and by using duct system the cooling effect can be distributed to the rooms which requires cooling effect or conditioned air.
It is recommended for various cement industry to use technical ways to reduce energy and exergy loss and also exergy destruction using by pass. It should be given great concern to increase thermal efficiency of the system by considering different affecting operation factors and recovery of waste heat from the cement plant.