MULTI OBJECTIVE OPTIMIZATION OF WASTE HEAT RECOVERY IN CEMENT INDUSTRY (A CASE STUDY)

Cement plants have the potential points to waste heat recovery . The method studied in this paper is based on the use of air quenching chamber (AQC) and suspension preheater (SP) Boilers which are installed at the output of the clean cooler and preheating stage respectively in Cement Plant. Due to the low temperature of the existed gases, three different fluids, water, R123 and R245fa are used as the working fluids and are evaluated in a similar cycle in terms of energy, exergy and the optimum parameters selection based on genetic algorithm. The results of this study showed that fluid R123 with optimized parameters leads to a 4% increase the total exergy loss and also will increase the production power from 5 MW to 9 MW. That is while in the case of water production power is increased from 4.8 to 5 MW is optimal state. Also the Results showed that the cost of produced electricity and exergy efficiency are lower in the case of organic fluid application in comparison with water as working fluid.

regenerative Organic Rankine Cycles Using dry organic fluids to change wasted energy to product power from lowgrade heat sources [11]. Yari investigated a study of the arrangement of ORC in geothermal resources based on the exergy analysis to select the best formation [12]. This study was carried out based on the rate of exergy loss and efficiency of first and second law that the ORC and internal heat exchanger was determined as the best cycle based on the first law of thermodynamics while R123 is the working fluid. Marco Astolfi and et al. presented a binary ORC to use low temperature geothermal resources [13]. Thermodynamic optimization results demonstrate that in those configurations which are based on the supercritical cycle, practical fluids with a little lower critical temperature than the geothermal resources temperature, represent highest efficiency for most of the cases. J. Wang and et al. investigated recover the exhaust gas heat of preheater and clinker cooler in cement plant have used singlepressure cycle, Kalina cycle, double vapor pressure cycle and ORC [14]. In a same expression, the optimal parameters resulted by Genetic Algorithm optimization were considered in order to gain the highest exergy efficiency. The results displayed that the exergy losses in the turbine, condenser and heat recovery vapor generator are high, and the exergy loss of these components can promote the efficiency of cogeneration systems. The Kalina cycle can get the best efficiency in the cement plant through other systems. F. Campana and et al. presented the first general evaluation of ORC module based on an exact measure for operating plants, such as cement, steel, glass, and oil and gas industries in 27 Europe Union countries [15]. Evaluation of energy savings was based on the number of work life, the decrease of CO2 emissions and electricity generation and utilization. This study showed that more than 20000 GWh/year of heat energy can be saved and 7.6 ton of CO2 can be recovered with the use of ORC technology. Based on the Best Available Technique References (BREF) in the cement industry, the best cycles for heat recovery could be ORC and steam cycle technology [15]. Simin Anvari et al. reviewed regeneration ORC has been applied to recover gas turbine's heat using HRSG [16]. Results showed that the first and second thermodynamic efficiencies in cycle with regeneration instead of reheat cycle has raised 2.62% and 2.6%, respectively. Also, sensitivity analysis demonstration that as gas turbine and combustion chamber inlet temperatures become greater, energetic and exergetic efficiencies become increase. Moreover, once condenser and evaporator temperature enhance, a slight reduction in energetic and exergetic efficiency is attended. Dan Song and Bin Chen provided a primery frame for exergy analysis of the cement generation plan [17]. Compared with the offered scenarios, an optimized method of sustainable development of cement industry was expected to be found out in the future. The framework may develop on the minimizing environmental effect and optimizing energy efficiency for cement manufacture. Daniel L. Summerbell et al. considered a case study of a plant in the UK, operating a pre-calciner type kiln commissioned in 1986 [18]. The paper concludes that there is significant opportunity to decline the emissions from cement plants by operational means and that fuel mix and excess air ratio should be the concentrate on future research. Optimizing the factors affecting performance was predicted to decrease energy utilization by 8.5% and CO2 emissions by 19.5%. In another paper, Madlool performed a model of a promoted four-stage suspension cyclone preheater system [19]. This sample was used to study the effect of waste preheater gas and dust bypass systems on preheater efficiency and performance. The results display that the calcination degree is reciprocally proportional to the heat amount of waste preheater bypass gases. As long as increasing of bypass opening at constant the content of dust kiln gas will cause reducing of waste heat amount of kiln gases. Madlool et al. reviewed the exergy, exergoeconomic and exergoenvironmental analyses of the ORC cycle with examined working fluids [20]. The multi-objective optimization is considered to get the system optimal operating conditions. The optimization investigates the exergy efficiency, the cost and the environmental impact per exergy unit of the net generated power as the objective functions. N.A. Madlool et al. reviewed exergy balance and exergy analysis for cement plants [21]. It is found that the exergy efficiency for cement units ranges from 18% to 49%. Also the exergy destraction because of the irreversibility from kiln are higher than other plants in cement production units. In this paper, a thermo-economic modeling and analysis of ORC and Rankine cycle for waste heat recovery, this equipment is optimized to get the greatest annual cost and thermal efficiency. Applying genetic algorithms the optimal points that is recognized as optimal front pareto, can be introduced.
As a summary, main features in this paper are as follow: • 3-function analysis system that includes energy, efficiency, and economy is set for the system.
• The selection of the temperature difference pinch point, the evaporator pressure, flow rate of ORC working fluid, as design parameters. • Applying ORC multi-objective optimization with exergy efficiency and electricity prices as two objects.
• The assessment of parameters in the optimization mode of the design mode.

MATERIALS AND METHODS Energy Analysis
A schematic of the cement plant case study which is located in the city of Qeshm with Rankine cycle is shown in Figure 1. In Fig. 4, upon combining and grinding the primary raw material, it is entered to the preheater and after passing through the cyclones, its moisture is removed and the limestone is calcified. Then, it enters the kiln. The raw materials are cooked in the kiln and converted to a clinker prior to being transferred to a clinker cooler to cool down. There are fans installed at the bottom of the clinker cooler which absorb most of the hot clinker heat and exhaust it through its upper duct. Raw materials in multiple cyclones are preheated by the exhausted gases from the rotary kiln. Thermal energy of gases (317 °C and 311 °C) can be recovered by the suspension preheater boiler (SP boiler). After being baked at a temperature of 1200 °C in a rotary kiln, clinker needs to be cooled. The second source of heat is obtained by gases from the clinker cooler (300 °C) and is recovered by the quenching chamber boiler (AQC). Heat exchangers usually operate with diathermic Oil that the temperature is maintained at stable value. Then the heat is transferred from diathermic oil to organic fluid and in ORC unit the electricity is generated. The power plant has been modeled under the following assumptions:  Air as a mixture of gases (Table 1) and its thermo physical characteristics are defined as a function of temperature.  Fluid flow is assumed as steady and changes in kinetic and potential energy is assumed to be negligible. Equations and its details for different parts of the ORC and Heat Recovery Steam Generator (HRSG) units are as follow: -Orc Cycle: Compressor exergy efficiency, turbine efficiency and thermal retention factors were 80%, 75%, and 90%, respectively [22].

The mass balance equation:
Production or consumption power and disposed or absorbed heat by each of the cycle components are calculated using the first and second laws of thermodynamics. Energy balance equation is used as follow: Energy balances for the turbine cycle ( Figure 1) are as follows: -Evaporator: -Turbine: -Condenser: Pressure drop in the evaporator ( p ∆ ) were assumed as follows: -Pump:

Heat Recovery Steam Generator (HRSG)
HRSG design in the project is based on pinch and approach point which by considering these parameters as input, steam flow rate is calculated. Also in the HRSG design the steam and water temperature graph is determined and the inlet and outlet temperature of the heat exchanger is determined. Finally, given the prevailing limits such as the temperature of the exhausted gases and the lack of steam generation in economizer and etc. the design will be completed [23,24]. Both pinch points and approach points are considered in modeling. (1) Mass and energy balance equations for the economizer, evaporator and superheater are given below: -Economizer: -Evaporator: -Super heater:

Exergy Analysis
In this paper, exergy at each point is a combination of physical and chemical exergy which is calculated according to the following equation [25][26][27]: Chemical exergy are considered in calculations with respect to the components of exhausted gases from clinker cooler and combustion products and also clinker production processes in cement plant which are determined based on the data obtained from the studied plant. Air is considered as a mixture of gases presented in table 1. Also chemical exergy of each component is determined according to the table 2.
ph ch Ex Ex Ex = + Exergy loss rate for each component is calculated according to the following equation [28]: Also by defining the following parameter the rate of exergy loss to the total exergy loss can be achieved [28]: Exergy efficiency of the cycle is calculated as follows [28]: Ex  is the resulted exergy from the outlet of clinker coolers and inlet preheater to the designed plant.

Economic Analysis
The economic analysis of each component is determined according to the following references [14] and [29]. A steadied electricity price over the 20 years is calculated as follows: which In Equation 18 i is inflation percent which is considered to be of 20% and N is useful life of plant and OM is the costs of maintenance per KWh. The cost of operation is intended 2% of the cost of the equipment. The cost of operation is considered to be 2% of the equipment cost [28][29][30][31].

Case Study
Qeshm cement Plant is located at 3 km southeast of the city of Qeshm, northern Iran, with two gray cement production line and nominal capacity of 4,000 and 3,000 tons of clinker per day. It took two year to finish a project (15)   To fulfill that, steel preheater with the inlet air flow from the Clinker cooler fan blades is applied. To reach the desired temperature, the air flow is heated by a torch which is installed under the preheater. Line 1 of the plant is studied in this paper as a case study (Table 3). The temperature profile of line 1 (Figure 1) is given in Table 4.  The Optimization Method (Genetic Algorithm) Genetic Algorithm is a search method which is used for finding the exact or approximate solutions of the optimization problems. This method was first used by John Holland in 1960, and is now employed in many fields including engineering, chemistry, mathematics, physics, computational sciences, phylogenetic, etc. This algorithm is a special class of evolutionary algorithms, which have been inspired by evolutionary biology concepts such as inheritance, mutation, selection and crossover. The computational procedure in this algorithm includes the following steps [32,33]

RESULTS AND DISCUSSION
In the modeling, ORC and water cycle are suggested as upstream cycle to use the heat wasted from cement plant. Water, R123 and R245f as working, have been compared from energy and thermo-economic point of view. Also, considering the changes in parameters such as temperature, the main fluid pressure, condenser pressure and the outlet pressure of the flash, cycle optimization is discussed. Comparing the thermal balance of the steam cycle and ORC cycles, it can be concluded that organic fluids due to its unique condition and characteristics could lead to a higher fluid flow which lead to a higher heat energy absorption of the gas in recovery boilers.

Energy Analysis
According to the table 5, it can be concluded that cycle thermal efficiency is greatly increased by organic fluid; this is when the temperature and the rate of the gas flow in two cycles are specified and equal. The difference in thermal efficiency is due to of the cycle parameters and type of fluid. The generated power for steam turbine is 4.8 MW, for organic fluid (R123) is 5.06 MW and for R245fa is equal to 6.9 MW. By comparing the organic fluids it can be realized that R245fa has higher exergy efficiency. Due to the existed limitations in the cycle, it can be seen that the only factor in increasing the efficiency is the change of fluids. In comparison with water, SP1 boiler operating with Organic cycles (R123 and R245fa) show 72% and 71% increase in the amount of recycled energy respectively. And for AQC the increases were a53% and 244% respectively. Also R245fa shows a better heat recovery than other fluids which increase the heat transfer and power generation. Compared to the steam cycle, because of high specific volume and increasing the flow rate in organic cycle the pump powers is sharply increased. the required outlet temperatures is 200 °C, given that the SP boiler output is used to reduce the moisture content of the raw materials which in this analysis, the outlet temperature of both boilers is also designed to comply with these limitations. The use organic fluid R245fa increases the thermal efficiency to 12%. The methodology followed in order to estimate on large scale the waste heat recovery potential in cement industry is summarized in Fig. 2.

Exergy Analysis
In this section the results of exergy analysis and exergy loss of each component of the water cycle and organic fluid are reviewed. As it can be seen for the organic fluid AQC boiler has the most exergy loss. This increase in entropy is contributed to the temperature difference between the hot and cold stream. Total exergy loss for R123 is 4.9 MW while for water would be 3.9 MW. An organic fluid with a higher exergy loss but also a higher power generation in cycle and lower exergy loss has higher exergy efficiency (table 5). In the meantime it can be seen that the fluid R245fa has different exergy loss in comparing with R123 fluid (Fig 3). It can be related to the function of the R245fa fluid in various pressures and temperatures in comparison with R123. Among the studied fluid, this fluid has the higher exergy loss (8.9 MW).

Figure 3. Exergy loss between cycle components for water and R123
In this paper, according to the specific exergy function definition, heat transfer for each boiler is investigated. It should be noted that the total exergy loss for boilers is defined based on the heat transfer in each boiler. It can be concluded from the figure that heat transfer process in boilers for water shows a better function in comparison with organic fluid (Fig 4). According to the Table 5 (recovered heat transfer) this amount is higher for organic fluid than for water. However, specific exergy value of organic fluid is higher due to greater exergy loss in these components. Figure 4 shows the ratio of exergy loss of components to total exergy. As it can be seen exergy loss in organic fluids allocate a greater region in comparison with water which can be contributed to the low operating temperature of the organic fluids. The point to note in this graph is the ration of turbine exergy loss to the total exergy loss. The steam quality of the turbine must be higher than 88% and this limitation leads to higher exergy loss to total exergy ratio in steam turbine in comparison with organic turbine. One of the advantages of using of organic Fluids is due to the related thermodynamic properties that can be used at lower pressure and without limitation.

Economic Analysis Results
The produced electricity costs have been calculated based on 6073 hours of operation per year in cement plant. In the case of organic fluid, steam turbine and condenser and for water turbines and boilers have the highest costs in comparison with other components. In general, due to higher power productivity of the organic fluids, the end cost of electricity is lower than water and also R245fa has lower cost in comparison with R123.
The higher cost of organic fluid in the condenser can be contributed to the low surface tension when compared to water which considering the condensation, the use of fin and also the surface area extension (higher surface means higher cost) in these types of condensers are needed. On the other hand among the cycle components, pumps the least expensive. Low electricity costs reflect the efficient use of waste heat in the cycle which considering the use of heat exchangers and heat recovery boilers this energy can be used with minimum cost.

Optimization Results (Two-Objective Optimization)
Exergy efficiency of the cycle and electricity cost are considered to be the objective functions. Optimizing curve for a working fluid is shown Figure 6. 1. The followings are optimization limitation: 2. The outlet quality of organic fluid from the turbine shouldn't be less than 95%. 3. Output steam quality from flash is X = 1. 4. Turbine outlet temperatures shouldn't be lower than of the outlet temperature of water from the condenser. 5. SP boilers outlet temperatures shouldn't be lower than 215 °C. As it can be seen organic fluid R123 has more suitable pareto curve than other fluids. R245fa fluid has many points, but because of the scale increase and curve changes in a limited bound is shown with a dot. In the table 6 cycle parameters can be investigated based on the best spot of the pareto curve.  Figure 7 is plotted based on the optimal point of Figure 6. According to Figure 5, R123 has greater efficiency and less ultimate cost than other working fluids. As can be seen, the Genetic Algorithm determines optimum decision parameters (presented in Table 5) to improve the functions optimum. Also, some parameter was selected to improve the advantages of present cycle. In Figure 7 condenser pressure is greater than ambient pressure. At result of the need for systems such as ejector becomes comic. The fluid pressure in the water according to vacuum the condenser to the air outlet and condensate process improvement ejector system is needed. No need for the system to reduce costs. Also according to the temperature of the fluid lies R123 29 in the condenser to reduce costs as well as factories making cement be used to remote areas of Ab¬Hay level of air-cooled condenser. due to lower energy and cooling water flow rate of a fluid heat exchanger for Ypsh used R123. Due to space restrictions in cement factories operate to the advantage of energy recovery system at the factory.  Figure 6). According to Figure 5, the R123 has the highest efficiency and the lowest annual cost between fluids. As can be seen, the genetic algorithm leads to improve objective functions by determining the optimal decision points (presented in Table 5). Fig. 7 illustrates the final condenser pressure is higher than the ambient pressure (101 kPa). Also, using auxiliary systems such as ejector is ignored by choosing this pressure. While omitting of auxiliary condensate systems would be reduction in the amount of annual cost. Also, air cooled condenser is more practical, due to the temperature of R123 in the condenser is 29 ºC, as well as the location of cement factory is far away from the sea water. Moreover, the exhausted steam from turbine is still superheated which leads to a longer life span of the end rows of the turbine (Fig. 7). Due to the heat capacity of R123 is lower than water and with reducing difference between turbine inlet and outlet pressure, the number of turbine blades and its volume decreases. Therefore, this is an advantage to overcome the lack of space in cement factories.

CONCLUSION
In this paper, an energy recovery system for Qeshm cement plant was modeled and the following results were obtained: • The use of organic fluids can increase the thermal efficiency and power generation. In energy analysis it was found that due to the low yield of organic fluid, the obtained flow rate in boilers is greater than water. In organic cycle with R245fa and R123 as working fluids SP1 Boiler, compared to water shows a 72% and 71% increase in recycled energy and for AQC this increase is 153% and 244% respectively. Also in power generation R123 with 4% and R245fa with 28.8% increase, compared to water, benefit more from energy in a cycle with the same structure. • The exergy analysis of the results showed that water has higher exergy efficiency in comparison with other organic fluids. o Also turbine outlet steam quality limitation results in a higher exergy loss to total exergy ratio for steam (10%) than organic fluids (8%). o By defining the specific exergy it can be seen that boilers with operating with water as working fluid show better performance in comparison with organic fluids. • In the economic analysis because of the higher power generated by organic fluids during operation of the cement plant, electricity prices have fallen comparing with water. • The obtained results from the genetic algorithm optimization has showed that in the desired cycle the parameters can have a major role in changing the fluid performance and have a great impact on the electricity prices and exergy efficiency. Based on the best point on the fluid pareto curve, with increase in power generation and decrease in electricity price, R123 can be considered a suitable fluid for cycle and show a 4.1% decrease in total exergy loss of the system.