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Energy benchmarking of cement industry, based on Process Integration concepts
Accepted Manuscript Energy Benchmarking of Cement Industry, based on Process Integration Concepts
M.Amin Mirzakhani, Nassim Tahouni, M.Hassan Panjeshahi PII:
S0360-5442(17)30651-5
DOI:
10.1016/j.energy.2017.04.085
Reference:
EGY 10725
To appear in:
Energy
Received Date:
22 September 2016
Revised Date:
24 March 2017
Accepted Date:
15 April 2017
Please cite this article as: M.Amin Mirzakhani, Nassim Tahouni, M.Hassan Panjeshahi, Energy Benchmarking of Cement Industry, based on Process Integration Concepts, Energy (2017), doi: 10.1016/j.energy.2017.04.085
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ACCEPTED MANUSCRIPT Highlights: Applying pinch analysis modifies the pyroprocess to decrease the energy consumption. Pinch analysis provides a conceptual model for benchmarking the energy consumption. The developed mathematical models target the energy demand of the retrofitted process. The benchmark models use a few process parameters with no statistical data.
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Energy Benchmarking of Cement Industry, based on Process Integration Concepts M. Amin Mirzakhani1, Nassim Tahouni1,*, M. Hassan Panjeshahi1,2 1
School of Chemical Engineering, College of Engineering, University of Tehran, Tehran, Iran
2
Department of Chemical and Petroleum Engineering, University of Calgary, Calgary, Alberta, Canada
Abstract Cement industry is considered as an energy intensive industry, owing to the huge amount of heat consumed in pyro-process unit. There are many conventional methods around, which can be applied to carry out energy targeting in a cement production plant, including conceptual, mathematical or even hybrid. However, full application of these approaches would be timeconsuming and costly. So, in this research, our attempt has been focused on development of a rapid approach for benchmarking of an existing plant with respect to energy consumption, and subsequently estimating an achievable scope for energy saving. To realize this goal, five different pyro-process units were simulated and then targeted using Pinch Analysis approach. Having done this conceptual analysis, the obtained results were mathematically correlated to shape a benchmarking model. The resulting model was also validated, in terms of accuracy, and was applied to other cement plants and showed an energy saving potential of up to 24%.
Keywords: cement production process, simulation, Heat Integration, Pinch Analysis, energy benchmarking, CO2 reduction
* Corresponding author. Email: [email protected]
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1. Introduction The new technology of cement production process (dry process with preheater and pre-calciner) consumes an energy intensity of about 3.42 GJ ∕ t Clinker [1]. This remarkable energy use intensity requires energy management and saving measures. The studies in the field of energy consumption reduction have been done in the scope of energy auditing, waste heat recovery systems, Computational Fluid Dynamics (CFD) modeling and Heat Integration of pyro-process units. Followings are concise review on each area of these researches. Some of the energy auditing measures have been proposed in the literature: Replacing the obsolete or low-efficiency equipment with efficient ones, e.g. replacing ball mill with roller mill [2]; Optimizing the process control system [2]; Reducing the heat loss from shell of kiln, raw mill and cyclones by application of kiln secondary shell [3] and insulation layer as well as substitution of old refractory bricks with new ones inside kiln [4] and raw mill [5]; Use of alternative fuels such as waste-derived fuels [1]; Changing product and feedstock (e.g. decreasing the ratio of clinker to cement additives) [6]; Combustion system improvements in pyro-process unit [7]; Updating the pyro-process unit to dry process with preheater and precalciner/increasing the preheater stages (cyclones) with lower pressure drop [2]. Madlool et al. [8] have reviewed the energy measures in different units of cement industry. Two types of recuperator for waste heat recovery from the hot section of kiln shell have been proposed in literature and a mathematical modeling has been developed in each one: a heat recovery exchanger with axial tubes arrangement for heating the water which flows through the tubes [9] and an exchanger for primary air preheating by forming an annular duct around the kiln [10]. Preheater exhaust gas and clinker cooler discharged air are two heat sources for power generation by applying the thermodynamic cycles such as common Rankine cycle [11]. Also, several applications of numerical modeling have been investigated in the cement industry for prediction of the physical, geometric and process parameters and finding the useful measures for optimizing the equipment size and improving the energy consumption [12]. Some examples of this research are reported here. Mujumdar et al. [13] have modeled preheater, calciner, kiln and cooler, individually. They have also presented a new method for energy reduction in pyro-process unit by coupling these 2
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parameters together. They have validated the model through observations in cement industry. Then, they have shown how adding a new preheater, higher solids feed rate, lower rotation and slope of kiln and lower grate speed can reduce the energy consumption. In addition, conversion percentage of calcination reaction inside the calciner has been optimized. This optimum value is about 70% with a given data set. In recent years, Mikulcic et al. have numerically analyzed the operating conditions of cement calciner. Through the study [14], the effects of different amounts of fuel and flow rate of the tertiary air on the calcination and coal burning rate have been studied for a real calciner. Using this model, flow turbulence, particle motion path, temperature distribution, component mass fraction and interaction between the solid and gas phases have been simulated. Mikulcic et al. have done a similar research on a distinct real calciner, which the results agreed with the measured data [15]. In addition to the investigated parameters in the previous study, residence time distribution of the particles has been predicted such that plant operators can regulate the operating conditions for a more efficient calciner. Authors have emphasized that the proposed simulations could be used to optimize the energy consumption. However, they haven't presented any guidelines to improve the operating calciner. Using flue gas recirculation (FGR) technology with oxy-fuel combustion in the rotary kilns has been an area of research to enhance energy efficiency in the cement industry. An example of these researches is the work of Granados et al. [16] in which the effect of flame length and heat transfer on materials have been studied using CFD model for variation of FGR in a range of 3085%. Their simulation has shown that utilizing the oxy-fuel combustion can increase the energy flux of gases to solid phase by 2-4.5 times compared to the conventional air combustion case. It can also decrease the flame length to 30-65% shorter than the air-based system. Then, the kiln can be designed shorter or in other words the capacity of kiln can be increased with the same fuel consumption. However, implementation of FGR and oxy-fuel technology is not expected to be feasible before 2030 [17]. Mian et al. [18] have studied the Energy Integration of pyro-process unit of a cement plant. In order to obtain the accurate enthalpy-temperature profiles of the streams, the process units have been discretized to the separated cells and then have modelled. Consequently, 30% of hot utility demand has been moderated by utilizing Pinch Analysis in this process. Nevertheless, there are some obvious defects in this work such as ignoring specific process limitations and lack of 3
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providing a retrofit solution. Another Energy Integration study has been performed into a cement plant in Croatia [19]. Progression of this study related to Ref. [18] is proposition of a retrofit solution and a methodological application for Heat Exchanger Network (HEN) generation with respect to specific process limitations (e.g. infeasible direct heat transfer between the kiln feed and hot clinker) which has resulted in an energy saving of 30%. But the case study was not an updated cement plant including a preheater tower without calciner and a low efficiency grate cooler which doesn't provide a proper heat recovery from the hot clinker. Hence, this study has only yielded a heat recovery from discharged gases to preheat oil, coal dust and combustion air streams. Flue gas, fuel and combustion air preheating have been considered as process streams in both cited references, whereas such streams usually act as utility streams in Pinch Analysis. Besides, no solution has been presented to improve the clinkering energy consumption by exchanging heat between the kiln feed and hot clinker streams. So, the scientific and technical gaps indicate that more studies are needed to apply Pinch Analysis in the cement industry. Energy benchmarking has been recommended for energy management and identification of the energy improvement potentials. Most benchmarking studies are based on the method of comparing a given specification of a facility to other facilities by statistical data. Saygin et al. [20] have studied the energy use in 17 energy-intensive industries and have provided worldwide energy benchmark curves (indexed energy use versus normalized cumulative production). Besides, energy efficiency indices have been computed so that their energy improvement potentials are specified. Specific thermal energy consumption of best practice technology and least energy-efficient plant and total energy saving potential have been reported 3 and 6.6 GJ ∕ t Clinker and 2.2 EJ ∕ y, respectively. The cement (clinkering) benchmark curve shows that updating 10% of most energy intensive cement plants can moderate the worldwide clinkering benchmark curve with a maximum energy use index of 1.47 (equivalent to 4.4 GJ ∕ t Clinker). Hasanbeigi et al. [21] have examined the 23 electricity and 6 fuel efficiency measures in 16 Chines cement plants by a benchmarking and energy saving tool to estimate the energy improvement potential. Furthermore, using the Conservation Supply Curve (CSC) which is constructed by calculating the Cost of Conserved Energy (CCE), cost-effective measures have been determined. Similar researches have investigated the energy benchmarking, improvement potentials and economical measures of cement plants in Ethiopia [22] and India's cement, iron, and steel industries [23]. CSCs represent some common outcomes such as: all fuel measures are 4
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cost-effective in which production of blended cement has been reported as the most costeffective measure and using roller mill for coal preparation is the most cost-effective among the electricity measures in China and Ethiopia. Ke et al. [24] have presented a general system framework for benchmarking the industrial energy, based on statistical data of process. This resulting framework has been implemented as a prototype in the cement industry. The authors have estimated the key benchmarks of different units of the cement plant with the aim of detecting inefficient sectors and finding the energy saving potentials. Afterwards, this reference has compared the process-based and product-based energy benchmarking so that evaluate them. Energy benchmarking studies mentioned above are based on statistical data of energy auditing measures. Some of the other approaches, which have been recommended in the literature, are as follows: A mathematical modeling approach based on mass and energy balance relations has been presented for benchmarking of energy-intensive processes [25]. There are a few key parameters in this model that their variation can significantly affect the simulation results. Therefore, carrying out a sensitivity analysis over these parameters would be of great importance, if maximum energy efficiency were desired. The model of a glass furnace in India has been developed to illustrate this proposed approach; then, it has been verified by field data. Utilizing the Pinch Analysis is another approach for evaluating the energy efficiency and benchmarking the energy use in plants [26]. This approach has been used in a Kraft pulping mill in Canada to improve its energy consumption. After gathering the required data from the plant, the energy demand has been compared with average practice of the industry. Then, the base case has been analyzed to assess the inefficiencies, quantify the possible heat recovery and determine the minimum energy demand by shifting the Composite Curves to get a minimum temperature difference of 10ºC. Finally, the guidelines and opportunities for improving the process have been synthesized. Evaluating the Composite Curves indicates that the potential of Pinch Analysis is limited for energy saving in the Kraft process; therefore, it seems that improving the operating conditions and upgrading the energy systems are inevitable and necessary to get the energy benchmark value before applying the Pinch Analysis. Process simulation-based approach has been suggested as a new and systematic approach for the energy benchmarking of chemical process systems [27]. This benchmarking approach consists of modeling the process and subprocesses by simulation software, recognizing the energy consumption indicators and indices, 5
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optimizing the operational parameters of the basic system to achieve the benchmark process, applying Pinch Analysis to optimize the energy consumption, and computing the specific energy consumption of the process to evaluate the energy efficiency. This approach has been used in a natural gas refinery as a prototype to prove the approach applicability. In current study, a modern cement plant has been simulated and analyzed using Energy Integration concepts. Due to the difference between process and utility streams in data extraction stage, a new method for retrofitting of pyro-process unit has been developed. Also, reviewing energy benchmarking studies showed that a Process Integration-based benchmarking in the cement industry was missing. Five different cement case studies have been investigated using proposed application of Pinch Analysis. Based on the results achieved, a new approach has been provided for the energy benchmarking in cement industries. In this approach, the amount of heat and fuel was targeted for each case study using Pinch Analysis. Then, by regression of hot utility and required fuel in terms of impact parameters, a conceptual-mathematical model has been presented to show the energy benchmarking. Afterwards, another cement case study has been investigated to validate the model. The developed model can easily be applied to benchmark the similar processes and propose a primary estimation for scope of improvement.
2. Case study The studied case, pyro-process unit (i.e. clinker production process), is the main and most energy-intensive department of cement plant. The technology of case studies is 5-stage in-line calciner preheater system developed by FLSmidth Company [28]. 2.1. Process description As shown in the schematic of in-line calciner preheater system in Fig. 1, raw meal is fed to top of the preheater tower and is entered to cyclone 1 along with cyclone 2 outlet gases. Solid-gas phases, after heat transfer, are separated due to centrifugal force. The gas phase is discharged from preheater and led to cooling tower. Simultaneously, solid materials are entered to cyclone 2 along with cyclone 3 outlet gases. This procedure goes forward until the solid materials get to calciner where the calcination reaction is progressed to a high conversion by the heat of calciner combustion gas merged with kiln flue gas. Calciner is a fluidized bed; so, two immiscible phases 6
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are conducted to a cyclone (cyclone 5) for separation. Then, solid materials are decanted to kiln for progression of further reactions through its different temperature zones. Table 1 presents the reactions of clinkering process [29]. Finally, clinkers leave the kiln with temperature of 13001500ºC and are cooled to 60-250ºC in a grate cooler where some blowers supply cold air for cooling the hot clinker. First part of heated air with the temperature of 600-1100ºC is conducted to kiln and calciner to provide the combustion required air which is known as secondary and tertiary air. The second part of heated air is vented to ambient with temperature of 200-300ºC. Grate cooler technology has made opportunity of heat recovery from hot clinker. By using this cooler, combustion air is preheated and the fuel demand is decreased. The total amount of required air for combustion (i.e. summation of secondary and tertiary air) is about 0.9-1.1 Nm3 ∕ kg Clinker. When the major percentage of heated air is discharged to kiln and calciner, less amount of air is needed. So, the air is heated up to 1100ºC; however, when the lower percentage of the heated air is discharged through the duct of secondary and tertiary air, more amount of air is needed to provide the required air for combustion. Consequently, the temperature of air is diminished. Hence, in the case with higher temperature of secondary air, more heat is recovered from the clinker. 2.2. Process simulation In order to find flow rate and composition of solid and gas phases in any location of unit, the pyro-process has been simulated by common simulator software. Physical and chemical properties of solid phase components and reactions have been extracted from related references [30, 31, 32] and defined in software data bank. The operational data for increasing the accuracy of pyro-process simulation has been gathered from control rooms of five cement plants in Iran. These required data are as follows:
Mass fraction of components in feed and product.
Mole fraction of components in fuel, which is natural gas.
Mole fraction of oxygen in the entrance of kiln and the stack of preheater tower.
Volume flow rate of fuel consumption in the kiln and calciner.
Volume flow rate of supplied air to grate cooler and primary air.
Temperature of both solid and gas phases in any location of unit (input and output of cyclones, kiln, calciner, cooler, secondary and tertiary air). 7
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The pyroprocess unit of every five cement plants has been simulated based on the process flow diagram in Fig. 1 and by assuming the constant atmospheric pressure in all streams and using the gathered data (Fig. 2). Since calcination is begun in middle stages of preheater tower, cyclones have been considered as reactors. Also, the reactions, which happen in the kiln, are classified to three parts or temperature zones. So, the kiln is divided to three reactors. These multiplicities of solid phase reactors enhance the accuracy of data extraction in order that the temperature-enthalpy diagram will be constructed precisely for applying the Pinch Analysis. A considerable energy of fuel combustion is wasted through the shell of equipment. The heat loss should be considered in simulation and data extraction. From the top of preheater tower toward the kiln, the shell heat loss is increased. In order to consider this fact, the conversion of calcination in reactors is adjusted by using the temperatures, which have been monitored, in control room and setting the coolers which have been named qloss in Fig. 2. For computing the secondary and tertiary air flow rates, the mole fractions of oxygen have been utilized.
3. Methodology Pinch Analysis is a thermodynamic methodology to integrate the energy for improving the energy efficiency of energy-intensive processes [33]. This method is based on simple powerful and graphical representation of the process via such simple diagrams as Composite Curves and Grand Composite Curve [34]. The first step of this analysis is stream data extraction. The energy of flue gas in pyro-process unit is spent to heat the solid materials and progress the endothermic reactions. In data extraction, it should also be noted that a part of flue gas energy is wasted through the shell of equipment. Our simulation has covered these points for data extraction to be precise. Pyro-process unit has three process streams:
Cold stream: solid materials stream from preheater inlet to kiln outlet
Hot stream: hot clinker in the grate cooler
Hot stream: carbon dioxide generated as a product of calcination reaction
The cold stream consists of 7 segments (solid materials entering to 7 reactors). For better accuracy, heat losses from the shells are also considered as part of heat requirement. The carbon dioxide generated due to calcination reaction in each stage of process is added to the gas phase 8
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and is cooled to temperature of effluent gas of the plant. So, it can be considered as a hot stream consisting of some segments. The Composite Curves of a cement plant (case study 1) have been shown in Fig. 3(a), regardless of CO2 streams. As mentioned in process description section, the first part of heated air of grate cooler with higher temperature is utilized as combustion air in the kiln and calciner. In other words, in this technology of clinker cooler, the energy of hot clinker stream is used to preheat the combustion air. Because of preheating the combustion air, the fuel demand is less than that of old technologies. As combustion air is regarded as a cold utility stream, hot clinker stream transfers heat to a cold utility stream of air. This cold utility stream has been demonstrated in Fig. 3(b). Despite the current status of the process, Pinch Analysis propose that heat recovery between process streams should be maximized and afterward, excess required energy is secured by utility streams. So, to obtain the ideal status of the process, the Composite Curves of hot clinker and cold streams should be shifted towards each other to achieve a given heat recovery and temperature difference (Fig. 4). Considering the hot clinker stream in extracted data and Pinch Analysis calculations means that two solid streams directly exchange heat; however, doing so is impossible. Inevitably, it should be considered a gaseous heat transfer fluid for indirect heat exchange between the two solid streams. This medium fluid is equivalent to clinker stream and can be used in calculations of Pinch Analysis. By the current technology of grate cooler, the first part of heated gas/air can be utilized as medium fluid and the second part is vented to ambient. Fig. 5 represents the concluded Composite Curves of this new proposition based on extracted data for case study 1 in Table 2 Stream data of other five case studies have been given in annex. A. The best practice of grate cooler for maximum heat recovery from hot clinker is the case with secondary air temperature of 1100ºC. However, examining the case studies indicated that the secondary air temperature has a range of 600-1100ºC. The Pinch Analysis has been implemented in each case study by considering two practice of grate cooler. At first time, stream data include the hot gas/air stream by assuming the present practice of existing cooler and existing minimum temperature differences (∆Tmin) between solid-gas streams in each case study in order to illustrate the performance of Pinch Analysis in energy efficiency improvement. At the next time, it has been assumed that the existing cooler of each plant has been substituted with the best practice 9
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and then, energy targeting has been carried out in some ∆Tmin to achieve the model-based energy benchmarking. The targeting of hot utility and fuel consumption has been performed by Composite Curves and Grand Composite Curve tools. Grand Composite Curve specifies the hot utility demand in the calciner and the kiln and the temperature of leaving flue gas, which can be used for preheating the combustion air. So, the temperature of the preheated air is determined by taking a given temperature difference between the leaving flue gas and combustion air. By simultaneous utilization of simulation and utility selection on Grand Composite Curve, the fuel requirements in the calciner and kiln have been computed. An illustrative Grand Composite Curve with utility selection is shown in Fig. 6. Using the results of energy targeting for five case studies and detecting the impact parameters, the mathematical models for estimating the hot utility and fuel consumption have been achieved by regression techniques. Another case has also been analyzed to validate the models.
4. Result and discussion 4.1. Heat Integration Investigating the pyroprocess unit by Pinch Analysis has formed the balanced Composite Curves of clinkering process like the Fig. 7 for case study 1 and a modified process flow diagram like Fig. 8. As shown in Fig. 8, the heated gas/air stream recovers the heat of clinker and transfers it to cold solid materials. This stream is joined to the combustion gases with similar temperature. The mixed flue gas exchanges heat with solid materials and then, a part of flue gas with the flow rate equal to the combustion gases leaves the preheater tower which is cooled by exchanging heat with the required combustion air stream, and is utilized for cooling the clinker. Another part of flue gas is discharged through the stack of preheater tower. The Heat Integration has been applied to five case studies: (1)-by considering the existing practice of cooler in each case and (2)-by assuming the best practice of cooler. The results of fuel reduction have been illustrated in Table 3, assuming existing ∆Tmin for each case study. However, this energy targeting has been calculated for different minimum temperature approaches (∆tmin) of gas-air heat exchanger, as shown in Fig. 8. The second case study has the best practice of cooler; so, both assumptions resulted in the same percentage of fuel reduction. As observed, by increasing the combustion air temperature or decreasing the ∆tmin, more fuel can be saved. These 10
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results indicate that applying the Pinch Analysis can efficiently abate the clinkering energy consumption. 4.2. Energy benchmarking To attain a model for energy benchmarking based on Heat Integration, the grate cooler of each case study has been assumed the same cooler as in the second case. The first step to develop the model is to identify the effective parameters. It has been noticed that specific thermal energy consumption of clinker production (STEC, defined as summation of specific heat of clinker formation and specific shell heat loss) and ∆Tmin affect on specific hot utility (QH) targeting depicted in Table 4 Using the regression techniques, Eq. (1) is well fitted to the results of hot utility targeting with the r-square of 0.9991. Fig. 9 shows the plot of this equation. QH = 9,196 + 0.0728 ∆Tmin – 3.7007 STEC + 4.1927 × 10-4 ∆Tmin STEC + 4.9412 × 10-4 STEC 2
(1)
Simulation and Grand Composite Curve tools have been used simultaneously for computing the fuel requirements in the calciner and kiln. In addition to STEC and ∆Tmin, temperature of combustion air affects on fuel consumption. However, we have calculated the fuel consumption in terms of STEC, ∆Tmin and ∆tmin instead of air temperature. The fuel targeting results are well fitted by Eq. (2) with the r-square of 0.9998: Vfuel = 579.425 – 0.2013 ∆Tmin – 0.0168 ∆tmin – 0.3131 STEC + 4.2696 × 10-5 ∆Tmin ∆tmin + 1.0401 × 10-4 ∆Tmin STEC + 1.0304 × 10-5 ∆tmin STEC + 5.9032 × 10-5 STEC 2 – 1.0032 × 10-8 ∆tmin STEC 2 – 3.2323 × 10-9 STEC 3
(2)
Where specific hot utility (QH) and STEC have the unit of kJ ∕ kg Clinker, the unit of specific volume flow rate of fuel (Vfuel) is Nm3 ∕ t Clinker, ∆Tmin and ∆tmin are in degree Celsius. Also, the sensitivity analysis has been applied to both models to compare the dependency of energy consumption on input variables, i.e., ∆Tmin, ∆tmin and STEC. Figure 10 shows the results of this analysis. As shown in the figure, the values on horizontal axis are normalized between “0” 11
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and “1” indicating the magnitude of the effects (dependencies). It means that “0” shows that the related variable has no effect on energy consumption, while “1” is indication of maximum effect for the corresponding variable. The results of this analysis suggest that energy consumption highly depends on STEC, while ∆Tmin and ∆tmin have much less effect on energy usage. Therefore, we don’t have to set a small ∆tmin and invest on large and expensive gas-air heat
exchangers. To verify the accuracy of the models, all the simulation and Heat Integration steps have been taken for sixth case study with STEC of 4,636.1 kJ ∕ kg Clinker. The results showed specific hot utility and fuel demand to be 3,346.3 kJ ∕ kg Clinker and 114.2 Nm3 ∕ t Clinker, respectively. By assuming the ∆Tmin equal to its existing value (61ºC), setting ∆tmin to 26ºC and also, using Pinch Analysis, the specific hot utility and volume flow rate of fuel have been calculated 2,800 kJ ∕ kg Clinker and 80.04 Nm3 ∕ t Clinker, while these parameters are estimated 2,782.6 kJ ∕ kg Clinker and 79.52 Nm3 ∕ t Clinker by using the equations (1) and (2). Proximity of the results achieved by Pinch Analysis and benchmarking models validates the reliability of the models.
5. Conclusion Applying grate cooler in the cement industry has provided the opportunity of heat recovery from hot clinker. In the current technology of clinker production processes, the thermal energy of clinker is used to preheat the combustion air, which is based on heuristic rules. However, in this study, the clinker production process (pyro-process unit) has been investigated by Pinch Analysis to systematically modify the process and improve thermal energy consumption. This new design proposes an alternative method for recovering some heat from the clinker and preheats the materials in the preheater tower. The results indicate that an increase in STEC reduces the energy saving potential. In other words, when STEC is low, energy can be saved up to 24%, which is equivalent to reduction of 47.84 kg CO2 ∕ t Clinker. Whereas in the case of high STEC, retrofit of pyro-process unit tends to show a negligible energy saving. Comparing the reported benchmark value of 3 GJ ∕ t Clinker with an achieved value of 2.41 GJ ∕ t Clinker in this study highlights the capability of proposed retrofit solution to moderate the energy consumption of modern cement plants. As STEC is summation of specific heat of clinker formation and shell heat loss, the plants, which are located at a situation with warm climatic condition, have the 12
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lower shell heat loss and STEC. So, the retrofit of these plants results in more energy saving. In addition, applying the best practice of grate cooler (the cooler with secondary air temperature of 1100ºC) can significantly enhance the energy saving. This retrofit solution is practical, but it needs a detailed design, which can be carried out by CFD models. In this study, a novel approach for benchmarking of cement processes has been developed, which is based on Pinch concepts and does not need any statistical data. This approach enables us to benchmark any process with no need to undertake a full retrofit study. This can save both time and money. Also, the clinker production process can be mathematically modeled as a function of STEC, ∆Tmin and ∆tmin. Sensitivity analysis suggested that STEC has the highest contribution in hot utility consumption and hence fuel requirements. The hot utility benchmark model can be easily used to understand the improvement potential in any cement plant with a given STEC. Multiplying the fuel benchmark model by the capacity of clinker production and fuel price provides an energy cost model as a function of ∆Tmin and ∆tmin. This model can then be used in capital-energy trade-off for primary estimation of optimum minimum temperature difference, if a capital cost model were developed.
Nomenclature: ∆Tmin
minimum temperature difference between the gas-solid phases, ºC
∆tmin
minimum approach temperature of air-gas heat exchanger, ºC
STEC
specific thermal energy consumption of clinker production, kJ ∕ kg Clinker
QH
specific hot utility, kJ ∕ kg Clinker
Vfuel
volume flow rate of fuel, Nm3 ∕ t Clinker
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References [1] Benhelal E., Zahedi G., Shamsaei E., Bahadori A., Global strategies and potentials to curb CO2 emissions in cement industry, Journal of Cleaner Production 2013, 51: 142-161. [2] Hasanbeigi A., Morrow W., Masanet E., Sathaye J., Xu T., Energy efficiency improvement and CO2 emission reduction opportunities in the cement industry in China, Energy Policy 2013, 57: 287–297. [3] Engin, T., Ari. V., Energy auditing and recovery for dry type cement rotary kiln systems–A case study, Energy Conversion and Management 2005, 46(4): 551–562. [4] Atmaca, A., Yumrutas, R., Analysis of the parameters affecting energy consumption of a rotary kiln in cement industry, Applied Thermal Engineering 2014, 66(1-2): 435-444. [5] Atmaca, A., Kanoglu, M., Reducing energy consumption of a raw mill in cement industry, Energy 2012, 42(1): 261-269. [6] Kabir, G., Abubakar, A.I., Nafaty, U.A., Energy audit and conservation opportunities for pyroprocessing unit of a typical dry process cement plant, Energy 2010, 35(3): 1237–1243. [7] Ishak S.A., Hashim H., Low carbon measures for cement plant – a review, Journal of Cleaner Production 2015, 103: 260–274. [8] Madlool N.A., Saidura R., Rahim N.A., Kamalisarvestani M., An overview of energy savings measures for cement industries, Renewable and Sustainable Energy Reviews 2013, 19: 18–29. [9] Caputo A.C., Pelagagge P.M., Salini P., Performance modeling of radiant heat recovery exchangers for rotary kilns, Applied Thermal Engineering 2011, 31(14-15): 2578-2589. [10] Karamarkovi V., Marasevic M., Karamarkovic R., Karamarkovi M., Recuperator for waste heat recovery from rotary kilns, Applied Thermal Engineering 2013, 54(2): 470-480. [11] Khurana S., Banerjee R., Gaitonde U., Energy balance and cogeneration for a cement plant, Applied Thermal Engineering 2002, 22(5): 485–494. [12] Mikulčić H., Klemeš J. J., Vujanović M., Urbaniec K., Duić N., Reducing greenhouse gasses emissions by fostering the deployment of alternative raw materials and energy sources in the cleaner cement manufacturing process, Journal of Cleaner Production 2016, 136: 119–132. [13] Mujumdar K.S., Ganesh K.V., Kulkarni S.B., Ranade V.V., Rotary Cement Kiln Simulator (RoCKS): Integrated modeling of pre-heater, calciner, kiln and clinker cooler 2007, 62(9): 2590– 2607. 14
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[14] Mikulčić H., Berg E.V., Vujanović M., Priesching P., Tatschl R., Duić N., Numerical analysis of cement calciner fuel efficiency and pollutant emissions, Clean Technologies and Environmental Policy 2013, 15(3): 489–499. [15] Mikulčić H., Vujanović M., Duić N., Improving the sustainability of cement production by using numerical simulation of limestone thermal degradation and pulverized coal combustion in a cement calciner, Journal of Cleaner Production 2015, 88: 262–271. [16] Granados D.A., Chejne F., Mejía J.M., Gómez C.A., Berrío A., Jurado W.J., Effect of flue gas recirculation during oxy-fuel combustion in a rotary cement kiln, Energy 2014, 64 : 615-625. [17] Schneider M., Process technology for efficient and sustainable cement production, Cement and Concrete Research 2015, 78: 14–23. [18] Mian A., Bendig M., Piazzesi G., Manente G., Lazzaretto A., Maréchal F., Energy integration in the cement industry, Computer Aided Chemical Engineering 2013, 32(1): 349-354. [19] Boldyryev S., Mikulčić H., Mohorović Z., Vujanović M., Krajačić G., Duić N., The improved heat integration of cement production under limited process conditions: A case study for Croatia, Applied Thermal Engineering 2016, 105: 839–848. [20] Saygin D., Worrell E., Patel M.K., Gielen D.J., Benchmarking the energy use of energyintensive industries in industrialized and in developing countries, Energy 2011, 36(11): 66616673. [21] Hasanbeigi A., Price L., Lu H., Lan W., Analysis of energy-efficiency opportunities for the cement industry in Shandong Province, China: A case study of 16 cement plants, Energy 2010, 35(8): 3461-3473. [22] Tesema G., Worrell E., Energy efficiency improvement potentials for the cement industry in Ethiopia, Energy 2015, 93(2): 2042-2052. [23] William R.M., Hasanbeigi A., Sathaye J., Xu T., Assessment of energy efficiency improvement and CO2 emission reduction potentials in India’s cement and iron & steel industries, Journal of Cleaner Production 2013, 65: 1-11. [24] Ke J., Price L., McNeil M., Khanna N.Z., Zhou N., Analysis and practices of energy benchmarking for industry from the perspective of systems engineering, Energy 2013, 54: 32-44. [25] Sardeshpande V., Gaitonde U.N., Banerjee R., Model based energy benchmarking for glass furnace, Energy Conversion and Management 2007, 48(10): 2718–2738.
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[26] Espejel, E.M., Savulescu, L., Maréchal, F., Paris, J., Base case process development for energy efficiency improvement, application to a Kraft pulping mill. PartII: Benchmarking analysis, Chemical Engineering Research and Design 2011, 89(6): 729–741. [27] Yang F., Liu Y., Liu G., A process simulation based benchmarking approach for evaluating energy consumption of a chemical process system, Journal of Cleaner Production 2016, 112(4): 2730-2743. [28] FLSmidth-One source supplier of systems and services to the cement and minerals industries, www.flsmidth.com, Last Access: 2017/02/08. [29] Lea F.M., Hewlett P.C., Lea's Chemistry of Cement and Concrete, 4th Edition, Wiley, Arnold, United Kingdom, 1998. [30] Allaboun H., Al-Otoom A.Y., Energy requirements of using oil shale in the production of ordinary Portland clinker, Oil Shale 2008, 25(3): 301–309. [31] Balonis, M., Glasser, F.P., The density of cement phases, Cement and Concrete Research, Cement and Concrete Research 2009, 39(9): 733–739. [32] Peray, K.E., Cement Manufacturer's Handbook, Chemical Publishing Co., New York, 1979. [33]. Azadi, M., Tahouni, N., Panjeshahi, M.H., Energy conservation in methanol plant using CHP system, Applied Thermal Engineering 2016, 107: 1324–1333. [34] Smith R. Chemical Process Design and Integration. 2nd Edition, New York, USA: Jon Wiley & Sons; 2016.
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List of Figures: Fig. 1. In-line calciner-preheater system of pyroprocess unit Fig. 2. Simulation of pyroprocess unit Fig. 3. Composite Curves of case study 1 (a) and showing the cold utility stream of combustion air (b) Fig. 4. Ideal status of Composite Curves of case study 1 Fig. 5. Modified Composite Curves of case study 1 Fig. 6. Utility selection on Grand Composite Curve of case study 1 Fig. 7. Modified balanced Composite Curves of case study 1 Fig. 8. Schematic of modified pyroprocess unit Fig. 9. Plot of equation 1 Fig. 10. Sensitivity analysis of equations 1 (a) and 2 (b)
List of Tables: Table 1. The reactions of clinker production Table 2. Stream data extraction from simulation of case study 1 Table 3. Percentage of fuel reduction at existing ∆Tmin of each case study Table 4. Hot utility targeting of each case study for different ∆Tmin Table A.1. Extracted stream data
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Table 1 The reactions of clinker production [25].
Enthalpy of
Temperature
reaction (kJ ∕ mol)
range (ºC)
CaCO3 → CaO + CO2
179.4
500-900
Preheater, Calciner, Kiln
2CaO + SiO2 → C2S*
-127.6
900-1,250
Kiln
3CaO + Al2O3 → C3A*
21.8
900-1,250
Kiln
4CaO + Al2O3 + Fe2O3 → C4AF*
-41.3
900-1,250
Kiln
16
1,250-1,450
Kiln
Reaction
C2S + CaO → C3S*
Location
* C2S ≡ (CaO)2SiO2, C3S ≡ (CaO)3SiO2, C3A ≡ (CaO)3Al2O3, C4AF ≡ (CaO)4Al2O3Fe2O3
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Table 2 Stream data extraction from simulation of case study 1.
Hot (H)
Supply
Target
Heat load
Heat capacity flow
Cold (C)
temp. (ºC)
temp. (ºC)
(kW)
rate (kW ∕ ºC)
Cyclone 1, 2, 3 solids
C
50
685
50,910
80.2
Cyclone 4 solids
C
685
785
10,980
109.8
Calciner solids
C
785
820
73,650
2,104.3
Cyclone 5 solids
C
820
875
10,720
194.9
Kiln sec.1 solids
C
875
900
7,758
310.3
Kiln sec.2 solids
C
900
1,250
18,440
52.7
Kiln sec.3 solids
C
1,250
1,450
19,250
96.3
Heated gas/air
H
1,100
150
57,636.5
60.7
Calciner outlet CO2
H
990
150
15,279.6
18.2
Cyclone 5 outlet CO2
H
895
150
1,276.2
1.7
Cyclone 4 outlet CO2
H
798
150
337
0.5
Cyclone 2 outlet CO2
H
539
150
450.5
1.2
Stream
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Table 3 Percentage of fuel reduction at existing ∆Tmin of each case study.
Case study
1
2
3
4
5
∆Tmin (ºC)
110
124
70
144
103
∆tmin (ºC)
Percentage of fuel reduction by considering the existing ∕ best practice of cooler
15
4.76 ∕ 8.19
2
7.2 ∕ 24.04
4.89 ∕ 12.36
5.03 ∕ 16.14
30
4.13 ∕ 7.59
1.37
6.62 ∕ 23.56
4.27 ∕ 11.8
4.43 ∕ 15.61
45
3.51 ∕ 6.99
0.72
6.04 ∕ 23.09
3.66 ∕ 11.24
3.82 ∕ 15.07
60
2.88 ∕ 6.39
0.07
5.45 ∕ 22.6
3.04 ∕ 10.67
3.21 ∕ 14.53
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Table 4 Hot utility targeting of each case study for different ∆Tmin.
Case study
1
2
3
4
5
STEC (kJ ∕ kg Clinker)
4,601
4,831.9
4,074.3
4,486.8
4,345.9
∆Tmin (ºC)
kg Clinker) ∕ QH (kJ
50
2,729
2,954.3
2,411.8
2,647
2,529.2
75
2,777.7
3,007.4
2,456.3
2,695.9
2,578.2
100
2,826.3
3,060.4
2,500.3
2,744.8
2,627.2
125
2,874.8
3,113.6
2,544.3
2,793.7
2,675.9
150
2,923.2
3,166.4
2,588.3
2,841.8
2,724.5
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Annex. A Table A.1. Extracted stream data Stream Case study Cyclone 1, 2, 3 solids Cyclone 4 solids Calciner solids Cyclone 5 solids Kiln sec.1 solids Kiln sec.2 solids Kiln sec.3 solids Heated gas/air Calciner outlet CO2 Cyclone 5 outlet CO2 Cyclone 4 outlet CO2 Cyclone 2 outlet CO2
Hot (H) Cold (C)
Supply temp.(ºC)
C C C C C C C H H H H H
50 685 785 820 875 900 1,250 1,100 990 895 798
Target temp.(ºC) 2 685 785 820 875 900 1,250 1,450 130 130 130 130
72,380 15,880 110,100 14,450 19,570 19,550 27,250 86,960.5 23,691 1,681.6 1,527.6 286.2
47,130 17,460 73,880 14,410 9,068 14,590 20,500 60,630 1,376 15,781 2,911
Case study Cyclone 1, 2, 3 solids Cyclone 4 solids Calciner solids Cyclone 5 solids Kiln sec.1 solids Kiln sec.2 solids Kiln sec.3 solids Heated gas/air Calciner outlet CO2 Cyclone 5 outlet CO2 Cyclone 4 outlet CO2 Cyclone 2 outlet CO2
C C C C C C C H H H H H
50 670 720 820 880 900 1,250 1,100 990 909 800 550
4 670 720 820 880 900 1,250 1,400 130 130 130 130 130
Case study Cyclone 1, 2, 3 solids Cyclone 4 solids Calciner solids Cyclone 5 solids Kiln sec.1 solids Kiln sec.2 solids Kiln sec.3 solids Heated gas/air Calciner outlet CO2 Cyclone 5 outlet CO2 Cyclone 4 outlet CO2
C C C C C C C H H H H
50 640 757 830 875 900 1,250 1,100 716 990 870
6 640 757 830 875 900 1,250 1,430 130 130 130 130
22
Heat load (kW)
Supply temp.(ºC)
42,070 9,903 72,170 8,559 15,470 13,050 24,000 59,111.8 17,662.4 1,343.8 262.1
60 695 732 820 869 900 1,250 1,100 980 900 886 765
Target temp.(ºC) 3 695 732 820 869 900 1,250 1,300 130 130 130 130 130
50 710 804 820 851 900 1,250 1,100 990 909 815 572
5 710 804 820 851 900 1,250 1,400 130 130 130 130 130
Heat load (kW) 32,280 9,214 38,610 7,177 16,100 7,638 6,684 35,133.4 8,233.1 1,925 1,015.3 1,205.9
46,140 9,645 60,700 8,204 14,480 10,770 15,450 53,582.8 14,728.9 1,302 358 141.5
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Fig. 1. In-line calciner-preheater system of pyroprocess unit.
23
B: Bottom Stream C: Kiln/Cyclone/Calciner E: Exchanger Mix: Mixer q: heat R: Recycle S: Set
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Fig. 2. Simulation of pyroprocess unit.
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(a) Cold stream of solid materials
(b) Cold stream of combustion air
Hot stream of clinker
Fig. 3. Composite Curves of case study 1 (a) and showing the cold utility stream of combustion air (b).
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Fig. 4. Ideal status of Composite Curves of case study 1.
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Fig. 5. Modified Composite Curves of case study 1.
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Fig. 6. Utility selection on Grand Composite Curve of case study 1.
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Fig. 7. Modified balanced Composite Curves of case study 1.
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Fig. 8. Schematic of modified pyroprocess unit.
30
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Fig. 9. Plot of equation 1.
31
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(a)
(b) Fig. 10. Sensitivity analysis of equations 1 (a) and 2 (b).
32
M.Amin Mirzakhani, Nassim Tahouni, M.Hassan Panjeshahi PII:
S0360-5442(17)30651-5
DOI:
10.1016/j.energy.2017.04.085
Reference:
EGY 10725
To appear in:
Energy
Received Date:
22 September 2016
Revised Date:
24 March 2017
Accepted Date:
15 April 2017
Please cite this article as: M.Amin Mirzakhani, Nassim Tahouni, M.Hassan Panjeshahi, Energy Benchmarking of Cement Industry, based on Process Integration Concepts, Energy (2017), doi: 10.1016/j.energy.2017.04.085
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT Highlights: Applying pinch analysis modifies the pyroprocess to decrease the energy consumption. Pinch analysis provides a conceptual model for benchmarking the energy consumption. The developed mathematical models target the energy demand of the retrofitted process. The benchmark models use a few process parameters with no statistical data.
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Energy Benchmarking of Cement Industry, based on Process Integration Concepts M. Amin Mirzakhani1, Nassim Tahouni1,*, M. Hassan Panjeshahi1,2 1
School of Chemical Engineering, College of Engineering, University of Tehran, Tehran, Iran
2
Department of Chemical and Petroleum Engineering, University of Calgary, Calgary, Alberta, Canada
Abstract Cement industry is considered as an energy intensive industry, owing to the huge amount of heat consumed in pyro-process unit. There are many conventional methods around, which can be applied to carry out energy targeting in a cement production plant, including conceptual, mathematical or even hybrid. However, full application of these approaches would be timeconsuming and costly. So, in this research, our attempt has been focused on development of a rapid approach for benchmarking of an existing plant with respect to energy consumption, and subsequently estimating an achievable scope for energy saving. To realize this goal, five different pyro-process units were simulated and then targeted using Pinch Analysis approach. Having done this conceptual analysis, the obtained results were mathematically correlated to shape a benchmarking model. The resulting model was also validated, in terms of accuracy, and was applied to other cement plants and showed an energy saving potential of up to 24%.
Keywords: cement production process, simulation, Heat Integration, Pinch Analysis, energy benchmarking, CO2 reduction
* Corresponding author. Email: [email protected]
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1. Introduction The new technology of cement production process (dry process with preheater and pre-calciner) consumes an energy intensity of about 3.42 GJ ∕ t Clinker [1]. This remarkable energy use intensity requires energy management and saving measures. The studies in the field of energy consumption reduction have been done in the scope of energy auditing, waste heat recovery systems, Computational Fluid Dynamics (CFD) modeling and Heat Integration of pyro-process units. Followings are concise review on each area of these researches. Some of the energy auditing measures have been proposed in the literature: Replacing the obsolete or low-efficiency equipment with efficient ones, e.g. replacing ball mill with roller mill [2]; Optimizing the process control system [2]; Reducing the heat loss from shell of kiln, raw mill and cyclones by application of kiln secondary shell [3] and insulation layer as well as substitution of old refractory bricks with new ones inside kiln [4] and raw mill [5]; Use of alternative fuels such as waste-derived fuels [1]; Changing product and feedstock (e.g. decreasing the ratio of clinker to cement additives) [6]; Combustion system improvements in pyro-process unit [7]; Updating the pyro-process unit to dry process with preheater and precalciner/increasing the preheater stages (cyclones) with lower pressure drop [2]. Madlool et al. [8] have reviewed the energy measures in different units of cement industry. Two types of recuperator for waste heat recovery from the hot section of kiln shell have been proposed in literature and a mathematical modeling has been developed in each one: a heat recovery exchanger with axial tubes arrangement for heating the water which flows through the tubes [9] and an exchanger for primary air preheating by forming an annular duct around the kiln [10]. Preheater exhaust gas and clinker cooler discharged air are two heat sources for power generation by applying the thermodynamic cycles such as common Rankine cycle [11]. Also, several applications of numerical modeling have been investigated in the cement industry for prediction of the physical, geometric and process parameters and finding the useful measures for optimizing the equipment size and improving the energy consumption [12]. Some examples of this research are reported here. Mujumdar et al. [13] have modeled preheater, calciner, kiln and cooler, individually. They have also presented a new method for energy reduction in pyro-process unit by coupling these 2
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parameters together. They have validated the model through observations in cement industry. Then, they have shown how adding a new preheater, higher solids feed rate, lower rotation and slope of kiln and lower grate speed can reduce the energy consumption. In addition, conversion percentage of calcination reaction inside the calciner has been optimized. This optimum value is about 70% with a given data set. In recent years, Mikulcic et al. have numerically analyzed the operating conditions of cement calciner. Through the study [14], the effects of different amounts of fuel and flow rate of the tertiary air on the calcination and coal burning rate have been studied for a real calciner. Using this model, flow turbulence, particle motion path, temperature distribution, component mass fraction and interaction between the solid and gas phases have been simulated. Mikulcic et al. have done a similar research on a distinct real calciner, which the results agreed with the measured data [15]. In addition to the investigated parameters in the previous study, residence time distribution of the particles has been predicted such that plant operators can regulate the operating conditions for a more efficient calciner. Authors have emphasized that the proposed simulations could be used to optimize the energy consumption. However, they haven't presented any guidelines to improve the operating calciner. Using flue gas recirculation (FGR) technology with oxy-fuel combustion in the rotary kilns has been an area of research to enhance energy efficiency in the cement industry. An example of these researches is the work of Granados et al. [16] in which the effect of flame length and heat transfer on materials have been studied using CFD model for variation of FGR in a range of 3085%. Their simulation has shown that utilizing the oxy-fuel combustion can increase the energy flux of gases to solid phase by 2-4.5 times compared to the conventional air combustion case. It can also decrease the flame length to 30-65% shorter than the air-based system. Then, the kiln can be designed shorter or in other words the capacity of kiln can be increased with the same fuel consumption. However, implementation of FGR and oxy-fuel technology is not expected to be feasible before 2030 [17]. Mian et al. [18] have studied the Energy Integration of pyro-process unit of a cement plant. In order to obtain the accurate enthalpy-temperature profiles of the streams, the process units have been discretized to the separated cells and then have modelled. Consequently, 30% of hot utility demand has been moderated by utilizing Pinch Analysis in this process. Nevertheless, there are some obvious defects in this work such as ignoring specific process limitations and lack of 3
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providing a retrofit solution. Another Energy Integration study has been performed into a cement plant in Croatia [19]. Progression of this study related to Ref. [18] is proposition of a retrofit solution and a methodological application for Heat Exchanger Network (HEN) generation with respect to specific process limitations (e.g. infeasible direct heat transfer between the kiln feed and hot clinker) which has resulted in an energy saving of 30%. But the case study was not an updated cement plant including a preheater tower without calciner and a low efficiency grate cooler which doesn't provide a proper heat recovery from the hot clinker. Hence, this study has only yielded a heat recovery from discharged gases to preheat oil, coal dust and combustion air streams. Flue gas, fuel and combustion air preheating have been considered as process streams in both cited references, whereas such streams usually act as utility streams in Pinch Analysis. Besides, no solution has been presented to improve the clinkering energy consumption by exchanging heat between the kiln feed and hot clinker streams. So, the scientific and technical gaps indicate that more studies are needed to apply Pinch Analysis in the cement industry. Energy benchmarking has been recommended for energy management and identification of the energy improvement potentials. Most benchmarking studies are based on the method of comparing a given specification of a facility to other facilities by statistical data. Saygin et al. [20] have studied the energy use in 17 energy-intensive industries and have provided worldwide energy benchmark curves (indexed energy use versus normalized cumulative production). Besides, energy efficiency indices have been computed so that their energy improvement potentials are specified. Specific thermal energy consumption of best practice technology and least energy-efficient plant and total energy saving potential have been reported 3 and 6.6 GJ ∕ t Clinker and 2.2 EJ ∕ y, respectively. The cement (clinkering) benchmark curve shows that updating 10% of most energy intensive cement plants can moderate the worldwide clinkering benchmark curve with a maximum energy use index of 1.47 (equivalent to 4.4 GJ ∕ t Clinker). Hasanbeigi et al. [21] have examined the 23 electricity and 6 fuel efficiency measures in 16 Chines cement plants by a benchmarking and energy saving tool to estimate the energy improvement potential. Furthermore, using the Conservation Supply Curve (CSC) which is constructed by calculating the Cost of Conserved Energy (CCE), cost-effective measures have been determined. Similar researches have investigated the energy benchmarking, improvement potentials and economical measures of cement plants in Ethiopia [22] and India's cement, iron, and steel industries [23]. CSCs represent some common outcomes such as: all fuel measures are 4
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cost-effective in which production of blended cement has been reported as the most costeffective measure and using roller mill for coal preparation is the most cost-effective among the electricity measures in China and Ethiopia. Ke et al. [24] have presented a general system framework for benchmarking the industrial energy, based on statistical data of process. This resulting framework has been implemented as a prototype in the cement industry. The authors have estimated the key benchmarks of different units of the cement plant with the aim of detecting inefficient sectors and finding the energy saving potentials. Afterwards, this reference has compared the process-based and product-based energy benchmarking so that evaluate them. Energy benchmarking studies mentioned above are based on statistical data of energy auditing measures. Some of the other approaches, which have been recommended in the literature, are as follows: A mathematical modeling approach based on mass and energy balance relations has been presented for benchmarking of energy-intensive processes [25]. There are a few key parameters in this model that their variation can significantly affect the simulation results. Therefore, carrying out a sensitivity analysis over these parameters would be of great importance, if maximum energy efficiency were desired. The model of a glass furnace in India has been developed to illustrate this proposed approach; then, it has been verified by field data. Utilizing the Pinch Analysis is another approach for evaluating the energy efficiency and benchmarking the energy use in plants [26]. This approach has been used in a Kraft pulping mill in Canada to improve its energy consumption. After gathering the required data from the plant, the energy demand has been compared with average practice of the industry. Then, the base case has been analyzed to assess the inefficiencies, quantify the possible heat recovery and determine the minimum energy demand by shifting the Composite Curves to get a minimum temperature difference of 10ºC. Finally, the guidelines and opportunities for improving the process have been synthesized. Evaluating the Composite Curves indicates that the potential of Pinch Analysis is limited for energy saving in the Kraft process; therefore, it seems that improving the operating conditions and upgrading the energy systems are inevitable and necessary to get the energy benchmark value before applying the Pinch Analysis. Process simulation-based approach has been suggested as a new and systematic approach for the energy benchmarking of chemical process systems [27]. This benchmarking approach consists of modeling the process and subprocesses by simulation software, recognizing the energy consumption indicators and indices, 5
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optimizing the operational parameters of the basic system to achieve the benchmark process, applying Pinch Analysis to optimize the energy consumption, and computing the specific energy consumption of the process to evaluate the energy efficiency. This approach has been used in a natural gas refinery as a prototype to prove the approach applicability. In current study, a modern cement plant has been simulated and analyzed using Energy Integration concepts. Due to the difference between process and utility streams in data extraction stage, a new method for retrofitting of pyro-process unit has been developed. Also, reviewing energy benchmarking studies showed that a Process Integration-based benchmarking in the cement industry was missing. Five different cement case studies have been investigated using proposed application of Pinch Analysis. Based on the results achieved, a new approach has been provided for the energy benchmarking in cement industries. In this approach, the amount of heat and fuel was targeted for each case study using Pinch Analysis. Then, by regression of hot utility and required fuel in terms of impact parameters, a conceptual-mathematical model has been presented to show the energy benchmarking. Afterwards, another cement case study has been investigated to validate the model. The developed model can easily be applied to benchmark the similar processes and propose a primary estimation for scope of improvement.
2. Case study The studied case, pyro-process unit (i.e. clinker production process), is the main and most energy-intensive department of cement plant. The technology of case studies is 5-stage in-line calciner preheater system developed by FLSmidth Company [28]. 2.1. Process description As shown in the schematic of in-line calciner preheater system in Fig. 1, raw meal is fed to top of the preheater tower and is entered to cyclone 1 along with cyclone 2 outlet gases. Solid-gas phases, after heat transfer, are separated due to centrifugal force. The gas phase is discharged from preheater and led to cooling tower. Simultaneously, solid materials are entered to cyclone 2 along with cyclone 3 outlet gases. This procedure goes forward until the solid materials get to calciner where the calcination reaction is progressed to a high conversion by the heat of calciner combustion gas merged with kiln flue gas. Calciner is a fluidized bed; so, two immiscible phases 6
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are conducted to a cyclone (cyclone 5) for separation. Then, solid materials are decanted to kiln for progression of further reactions through its different temperature zones. Table 1 presents the reactions of clinkering process [29]. Finally, clinkers leave the kiln with temperature of 13001500ºC and are cooled to 60-250ºC in a grate cooler where some blowers supply cold air for cooling the hot clinker. First part of heated air with the temperature of 600-1100ºC is conducted to kiln and calciner to provide the combustion required air which is known as secondary and tertiary air. The second part of heated air is vented to ambient with temperature of 200-300ºC. Grate cooler technology has made opportunity of heat recovery from hot clinker. By using this cooler, combustion air is preheated and the fuel demand is decreased. The total amount of required air for combustion (i.e. summation of secondary and tertiary air) is about 0.9-1.1 Nm3 ∕ kg Clinker. When the major percentage of heated air is discharged to kiln and calciner, less amount of air is needed. So, the air is heated up to 1100ºC; however, when the lower percentage of the heated air is discharged through the duct of secondary and tertiary air, more amount of air is needed to provide the required air for combustion. Consequently, the temperature of air is diminished. Hence, in the case with higher temperature of secondary air, more heat is recovered from the clinker. 2.2. Process simulation In order to find flow rate and composition of solid and gas phases in any location of unit, the pyro-process has been simulated by common simulator software. Physical and chemical properties of solid phase components and reactions have been extracted from related references [30, 31, 32] and defined in software data bank. The operational data for increasing the accuracy of pyro-process simulation has been gathered from control rooms of five cement plants in Iran. These required data are as follows:
Mass fraction of components in feed and product.
Mole fraction of components in fuel, which is natural gas.
Mole fraction of oxygen in the entrance of kiln and the stack of preheater tower.
Volume flow rate of fuel consumption in the kiln and calciner.
Volume flow rate of supplied air to grate cooler and primary air.
Temperature of both solid and gas phases in any location of unit (input and output of cyclones, kiln, calciner, cooler, secondary and tertiary air). 7
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The pyroprocess unit of every five cement plants has been simulated based on the process flow diagram in Fig. 1 and by assuming the constant atmospheric pressure in all streams and using the gathered data (Fig. 2). Since calcination is begun in middle stages of preheater tower, cyclones have been considered as reactors. Also, the reactions, which happen in the kiln, are classified to three parts or temperature zones. So, the kiln is divided to three reactors. These multiplicities of solid phase reactors enhance the accuracy of data extraction in order that the temperature-enthalpy diagram will be constructed precisely for applying the Pinch Analysis. A considerable energy of fuel combustion is wasted through the shell of equipment. The heat loss should be considered in simulation and data extraction. From the top of preheater tower toward the kiln, the shell heat loss is increased. In order to consider this fact, the conversion of calcination in reactors is adjusted by using the temperatures, which have been monitored, in control room and setting the coolers which have been named qloss in Fig. 2. For computing the secondary and tertiary air flow rates, the mole fractions of oxygen have been utilized.
3. Methodology Pinch Analysis is a thermodynamic methodology to integrate the energy for improving the energy efficiency of energy-intensive processes [33]. This method is based on simple powerful and graphical representation of the process via such simple diagrams as Composite Curves and Grand Composite Curve [34]. The first step of this analysis is stream data extraction. The energy of flue gas in pyro-process unit is spent to heat the solid materials and progress the endothermic reactions. In data extraction, it should also be noted that a part of flue gas energy is wasted through the shell of equipment. Our simulation has covered these points for data extraction to be precise. Pyro-process unit has three process streams:
Cold stream: solid materials stream from preheater inlet to kiln outlet
Hot stream: hot clinker in the grate cooler
Hot stream: carbon dioxide generated as a product of calcination reaction
The cold stream consists of 7 segments (solid materials entering to 7 reactors). For better accuracy, heat losses from the shells are also considered as part of heat requirement. The carbon dioxide generated due to calcination reaction in each stage of process is added to the gas phase 8
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and is cooled to temperature of effluent gas of the plant. So, it can be considered as a hot stream consisting of some segments. The Composite Curves of a cement plant (case study 1) have been shown in Fig. 3(a), regardless of CO2 streams. As mentioned in process description section, the first part of heated air of grate cooler with higher temperature is utilized as combustion air in the kiln and calciner. In other words, in this technology of clinker cooler, the energy of hot clinker stream is used to preheat the combustion air. Because of preheating the combustion air, the fuel demand is less than that of old technologies. As combustion air is regarded as a cold utility stream, hot clinker stream transfers heat to a cold utility stream of air. This cold utility stream has been demonstrated in Fig. 3(b). Despite the current status of the process, Pinch Analysis propose that heat recovery between process streams should be maximized and afterward, excess required energy is secured by utility streams. So, to obtain the ideal status of the process, the Composite Curves of hot clinker and cold streams should be shifted towards each other to achieve a given heat recovery and temperature difference (Fig. 4). Considering the hot clinker stream in extracted data and Pinch Analysis calculations means that two solid streams directly exchange heat; however, doing so is impossible. Inevitably, it should be considered a gaseous heat transfer fluid for indirect heat exchange between the two solid streams. This medium fluid is equivalent to clinker stream and can be used in calculations of Pinch Analysis. By the current technology of grate cooler, the first part of heated gas/air can be utilized as medium fluid and the second part is vented to ambient. Fig. 5 represents the concluded Composite Curves of this new proposition based on extracted data for case study 1 in Table 2 Stream data of other five case studies have been given in annex. A. The best practice of grate cooler for maximum heat recovery from hot clinker is the case with secondary air temperature of 1100ºC. However, examining the case studies indicated that the secondary air temperature has a range of 600-1100ºC. The Pinch Analysis has been implemented in each case study by considering two practice of grate cooler. At first time, stream data include the hot gas/air stream by assuming the present practice of existing cooler and existing minimum temperature differences (∆Tmin) between solid-gas streams in each case study in order to illustrate the performance of Pinch Analysis in energy efficiency improvement. At the next time, it has been assumed that the existing cooler of each plant has been substituted with the best practice 9
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and then, energy targeting has been carried out in some ∆Tmin to achieve the model-based energy benchmarking. The targeting of hot utility and fuel consumption has been performed by Composite Curves and Grand Composite Curve tools. Grand Composite Curve specifies the hot utility demand in the calciner and the kiln and the temperature of leaving flue gas, which can be used for preheating the combustion air. So, the temperature of the preheated air is determined by taking a given temperature difference between the leaving flue gas and combustion air. By simultaneous utilization of simulation and utility selection on Grand Composite Curve, the fuel requirements in the calciner and kiln have been computed. An illustrative Grand Composite Curve with utility selection is shown in Fig. 6. Using the results of energy targeting for five case studies and detecting the impact parameters, the mathematical models for estimating the hot utility and fuel consumption have been achieved by regression techniques. Another case has also been analyzed to validate the models.
4. Result and discussion 4.1. Heat Integration Investigating the pyroprocess unit by Pinch Analysis has formed the balanced Composite Curves of clinkering process like the Fig. 7 for case study 1 and a modified process flow diagram like Fig. 8. As shown in Fig. 8, the heated gas/air stream recovers the heat of clinker and transfers it to cold solid materials. This stream is joined to the combustion gases with similar temperature. The mixed flue gas exchanges heat with solid materials and then, a part of flue gas with the flow rate equal to the combustion gases leaves the preheater tower which is cooled by exchanging heat with the required combustion air stream, and is utilized for cooling the clinker. Another part of flue gas is discharged through the stack of preheater tower. The Heat Integration has been applied to five case studies: (1)-by considering the existing practice of cooler in each case and (2)-by assuming the best practice of cooler. The results of fuel reduction have been illustrated in Table 3, assuming existing ∆Tmin for each case study. However, this energy targeting has been calculated for different minimum temperature approaches (∆tmin) of gas-air heat exchanger, as shown in Fig. 8. The second case study has the best practice of cooler; so, both assumptions resulted in the same percentage of fuel reduction. As observed, by increasing the combustion air temperature or decreasing the ∆tmin, more fuel can be saved. These 10
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results indicate that applying the Pinch Analysis can efficiently abate the clinkering energy consumption. 4.2. Energy benchmarking To attain a model for energy benchmarking based on Heat Integration, the grate cooler of each case study has been assumed the same cooler as in the second case. The first step to develop the model is to identify the effective parameters. It has been noticed that specific thermal energy consumption of clinker production (STEC, defined as summation of specific heat of clinker formation and specific shell heat loss) and ∆Tmin affect on specific hot utility (QH) targeting depicted in Table 4 Using the regression techniques, Eq. (1) is well fitted to the results of hot utility targeting with the r-square of 0.9991. Fig. 9 shows the plot of this equation. QH = 9,196 + 0.0728 ∆Tmin – 3.7007 STEC + 4.1927 × 10-4 ∆Tmin STEC + 4.9412 × 10-4 STEC 2
(1)
Simulation and Grand Composite Curve tools have been used simultaneously for computing the fuel requirements in the calciner and kiln. In addition to STEC and ∆Tmin, temperature of combustion air affects on fuel consumption. However, we have calculated the fuel consumption in terms of STEC, ∆Tmin and ∆tmin instead of air temperature. The fuel targeting results are well fitted by Eq. (2) with the r-square of 0.9998: Vfuel = 579.425 – 0.2013 ∆Tmin – 0.0168 ∆tmin – 0.3131 STEC + 4.2696 × 10-5 ∆Tmin ∆tmin + 1.0401 × 10-4 ∆Tmin STEC + 1.0304 × 10-5 ∆tmin STEC + 5.9032 × 10-5 STEC 2 – 1.0032 × 10-8 ∆tmin STEC 2 – 3.2323 × 10-9 STEC 3
(2)
Where specific hot utility (QH) and STEC have the unit of kJ ∕ kg Clinker, the unit of specific volume flow rate of fuel (Vfuel) is Nm3 ∕ t Clinker, ∆Tmin and ∆tmin are in degree Celsius. Also, the sensitivity analysis has been applied to both models to compare the dependency of energy consumption on input variables, i.e., ∆Tmin, ∆tmin and STEC. Figure 10 shows the results of this analysis. As shown in the figure, the values on horizontal axis are normalized between “0” 11
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and “1” indicating the magnitude of the effects (dependencies). It means that “0” shows that the related variable has no effect on energy consumption, while “1” is indication of maximum effect for the corresponding variable. The results of this analysis suggest that energy consumption highly depends on STEC, while ∆Tmin and ∆tmin have much less effect on energy usage. Therefore, we don’t have to set a small ∆tmin and invest on large and expensive gas-air heat
exchangers. To verify the accuracy of the models, all the simulation and Heat Integration steps have been taken for sixth case study with STEC of 4,636.1 kJ ∕ kg Clinker. The results showed specific hot utility and fuel demand to be 3,346.3 kJ ∕ kg Clinker and 114.2 Nm3 ∕ t Clinker, respectively. By assuming the ∆Tmin equal to its existing value (61ºC), setting ∆tmin to 26ºC and also, using Pinch Analysis, the specific hot utility and volume flow rate of fuel have been calculated 2,800 kJ ∕ kg Clinker and 80.04 Nm3 ∕ t Clinker, while these parameters are estimated 2,782.6 kJ ∕ kg Clinker and 79.52 Nm3 ∕ t Clinker by using the equations (1) and (2). Proximity of the results achieved by Pinch Analysis and benchmarking models validates the reliability of the models.
5. Conclusion Applying grate cooler in the cement industry has provided the opportunity of heat recovery from hot clinker. In the current technology of clinker production processes, the thermal energy of clinker is used to preheat the combustion air, which is based on heuristic rules. However, in this study, the clinker production process (pyro-process unit) has been investigated by Pinch Analysis to systematically modify the process and improve thermal energy consumption. This new design proposes an alternative method for recovering some heat from the clinker and preheats the materials in the preheater tower. The results indicate that an increase in STEC reduces the energy saving potential. In other words, when STEC is low, energy can be saved up to 24%, which is equivalent to reduction of 47.84 kg CO2 ∕ t Clinker. Whereas in the case of high STEC, retrofit of pyro-process unit tends to show a negligible energy saving. Comparing the reported benchmark value of 3 GJ ∕ t Clinker with an achieved value of 2.41 GJ ∕ t Clinker in this study highlights the capability of proposed retrofit solution to moderate the energy consumption of modern cement plants. As STEC is summation of specific heat of clinker formation and shell heat loss, the plants, which are located at a situation with warm climatic condition, have the 12
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lower shell heat loss and STEC. So, the retrofit of these plants results in more energy saving. In addition, applying the best practice of grate cooler (the cooler with secondary air temperature of 1100ºC) can significantly enhance the energy saving. This retrofit solution is practical, but it needs a detailed design, which can be carried out by CFD models. In this study, a novel approach for benchmarking of cement processes has been developed, which is based on Pinch concepts and does not need any statistical data. This approach enables us to benchmark any process with no need to undertake a full retrofit study. This can save both time and money. Also, the clinker production process can be mathematically modeled as a function of STEC, ∆Tmin and ∆tmin. Sensitivity analysis suggested that STEC has the highest contribution in hot utility consumption and hence fuel requirements. The hot utility benchmark model can be easily used to understand the improvement potential in any cement plant with a given STEC. Multiplying the fuel benchmark model by the capacity of clinker production and fuel price provides an energy cost model as a function of ∆Tmin and ∆tmin. This model can then be used in capital-energy trade-off for primary estimation of optimum minimum temperature difference, if a capital cost model were developed.
Nomenclature: ∆Tmin
minimum temperature difference between the gas-solid phases, ºC
∆tmin
minimum approach temperature of air-gas heat exchanger, ºC
STEC
specific thermal energy consumption of clinker production, kJ ∕ kg Clinker
QH
specific hot utility, kJ ∕ kg Clinker
Vfuel
volume flow rate of fuel, Nm3 ∕ t Clinker
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References [1] Benhelal E., Zahedi G., Shamsaei E., Bahadori A., Global strategies and potentials to curb CO2 emissions in cement industry, Journal of Cleaner Production 2013, 51: 142-161. [2] Hasanbeigi A., Morrow W., Masanet E., Sathaye J., Xu T., Energy efficiency improvement and CO2 emission reduction opportunities in the cement industry in China, Energy Policy 2013, 57: 287–297. [3] Engin, T., Ari. V., Energy auditing and recovery for dry type cement rotary kiln systems–A case study, Energy Conversion and Management 2005, 46(4): 551–562. [4] Atmaca, A., Yumrutas, R., Analysis of the parameters affecting energy consumption of a rotary kiln in cement industry, Applied Thermal Engineering 2014, 66(1-2): 435-444. [5] Atmaca, A., Kanoglu, M., Reducing energy consumption of a raw mill in cement industry, Energy 2012, 42(1): 261-269. [6] Kabir, G., Abubakar, A.I., Nafaty, U.A., Energy audit and conservation opportunities for pyroprocessing unit of a typical dry process cement plant, Energy 2010, 35(3): 1237–1243. [7] Ishak S.A., Hashim H., Low carbon measures for cement plant – a review, Journal of Cleaner Production 2015, 103: 260–274. [8] Madlool N.A., Saidura R., Rahim N.A., Kamalisarvestani M., An overview of energy savings measures for cement industries, Renewable and Sustainable Energy Reviews 2013, 19: 18–29. [9] Caputo A.C., Pelagagge P.M., Salini P., Performance modeling of radiant heat recovery exchangers for rotary kilns, Applied Thermal Engineering 2011, 31(14-15): 2578-2589. [10] Karamarkovi V., Marasevic M., Karamarkovic R., Karamarkovi M., Recuperator for waste heat recovery from rotary kilns, Applied Thermal Engineering 2013, 54(2): 470-480. [11] Khurana S., Banerjee R., Gaitonde U., Energy balance and cogeneration for a cement plant, Applied Thermal Engineering 2002, 22(5): 485–494. [12] Mikulčić H., Klemeš J. J., Vujanović M., Urbaniec K., Duić N., Reducing greenhouse gasses emissions by fostering the deployment of alternative raw materials and energy sources in the cleaner cement manufacturing process, Journal of Cleaner Production 2016, 136: 119–132. [13] Mujumdar K.S., Ganesh K.V., Kulkarni S.B., Ranade V.V., Rotary Cement Kiln Simulator (RoCKS): Integrated modeling of pre-heater, calciner, kiln and clinker cooler 2007, 62(9): 2590– 2607. 14
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[14] Mikulčić H., Berg E.V., Vujanović M., Priesching P., Tatschl R., Duić N., Numerical analysis of cement calciner fuel efficiency and pollutant emissions, Clean Technologies and Environmental Policy 2013, 15(3): 489–499. [15] Mikulčić H., Vujanović M., Duić N., Improving the sustainability of cement production by using numerical simulation of limestone thermal degradation and pulverized coal combustion in a cement calciner, Journal of Cleaner Production 2015, 88: 262–271. [16] Granados D.A., Chejne F., Mejía J.M., Gómez C.A., Berrío A., Jurado W.J., Effect of flue gas recirculation during oxy-fuel combustion in a rotary cement kiln, Energy 2014, 64 : 615-625. [17] Schneider M., Process technology for efficient and sustainable cement production, Cement and Concrete Research 2015, 78: 14–23. [18] Mian A., Bendig M., Piazzesi G., Manente G., Lazzaretto A., Maréchal F., Energy integration in the cement industry, Computer Aided Chemical Engineering 2013, 32(1): 349-354. [19] Boldyryev S., Mikulčić H., Mohorović Z., Vujanović M., Krajačić G., Duić N., The improved heat integration of cement production under limited process conditions: A case study for Croatia, Applied Thermal Engineering 2016, 105: 839–848. [20] Saygin D., Worrell E., Patel M.K., Gielen D.J., Benchmarking the energy use of energyintensive industries in industrialized and in developing countries, Energy 2011, 36(11): 66616673. [21] Hasanbeigi A., Price L., Lu H., Lan W., Analysis of energy-efficiency opportunities for the cement industry in Shandong Province, China: A case study of 16 cement plants, Energy 2010, 35(8): 3461-3473. [22] Tesema G., Worrell E., Energy efficiency improvement potentials for the cement industry in Ethiopia, Energy 2015, 93(2): 2042-2052. [23] William R.M., Hasanbeigi A., Sathaye J., Xu T., Assessment of energy efficiency improvement and CO2 emission reduction potentials in India’s cement and iron & steel industries, Journal of Cleaner Production 2013, 65: 1-11. [24] Ke J., Price L., McNeil M., Khanna N.Z., Zhou N., Analysis and practices of energy benchmarking for industry from the perspective of systems engineering, Energy 2013, 54: 32-44. [25] Sardeshpande V., Gaitonde U.N., Banerjee R., Model based energy benchmarking for glass furnace, Energy Conversion and Management 2007, 48(10): 2718–2738.
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ACCEPTED MANUSCRIPT
[26] Espejel, E.M., Savulescu, L., Maréchal, F., Paris, J., Base case process development for energy efficiency improvement, application to a Kraft pulping mill. PartII: Benchmarking analysis, Chemical Engineering Research and Design 2011, 89(6): 729–741. [27] Yang F., Liu Y., Liu G., A process simulation based benchmarking approach for evaluating energy consumption of a chemical process system, Journal of Cleaner Production 2016, 112(4): 2730-2743. [28] FLSmidth-One source supplier of systems and services to the cement and minerals industries, www.flsmidth.com, Last Access: 2017/02/08. [29] Lea F.M., Hewlett P.C., Lea's Chemistry of Cement and Concrete, 4th Edition, Wiley, Arnold, United Kingdom, 1998. [30] Allaboun H., Al-Otoom A.Y., Energy requirements of using oil shale in the production of ordinary Portland clinker, Oil Shale 2008, 25(3): 301–309. [31] Balonis, M., Glasser, F.P., The density of cement phases, Cement and Concrete Research, Cement and Concrete Research 2009, 39(9): 733–739. [32] Peray, K.E., Cement Manufacturer's Handbook, Chemical Publishing Co., New York, 1979. [33]. Azadi, M., Tahouni, N., Panjeshahi, M.H., Energy conservation in methanol plant using CHP system, Applied Thermal Engineering 2016, 107: 1324–1333. [34] Smith R. Chemical Process Design and Integration. 2nd Edition, New York, USA: Jon Wiley & Sons; 2016.
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List of Figures: Fig. 1. In-line calciner-preheater system of pyroprocess unit Fig. 2. Simulation of pyroprocess unit Fig. 3. Composite Curves of case study 1 (a) and showing the cold utility stream of combustion air (b) Fig. 4. Ideal status of Composite Curves of case study 1 Fig. 5. Modified Composite Curves of case study 1 Fig. 6. Utility selection on Grand Composite Curve of case study 1 Fig. 7. Modified balanced Composite Curves of case study 1 Fig. 8. Schematic of modified pyroprocess unit Fig. 9. Plot of equation 1 Fig. 10. Sensitivity analysis of equations 1 (a) and 2 (b)
List of Tables: Table 1. The reactions of clinker production Table 2. Stream data extraction from simulation of case study 1 Table 3. Percentage of fuel reduction at existing ∆Tmin of each case study Table 4. Hot utility targeting of each case study for different ∆Tmin Table A.1. Extracted stream data
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Table 1 The reactions of clinker production [25].
Enthalpy of
Temperature
reaction (kJ ∕ mol)
range (ºC)
CaCO3 → CaO + CO2
179.4
500-900
Preheater, Calciner, Kiln
2CaO + SiO2 → C2S*
-127.6
900-1,250
Kiln
3CaO + Al2O3 → C3A*
21.8
900-1,250
Kiln
4CaO + Al2O3 + Fe2O3 → C4AF*
-41.3
900-1,250
Kiln
16
1,250-1,450
Kiln
Reaction
C2S + CaO → C3S*
Location
* C2S ≡ (CaO)2SiO2, C3S ≡ (CaO)3SiO2, C3A ≡ (CaO)3Al2O3, C4AF ≡ (CaO)4Al2O3Fe2O3
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Table 2 Stream data extraction from simulation of case study 1.
Hot (H)
Supply
Target
Heat load
Heat capacity flow
Cold (C)
temp. (ºC)
temp. (ºC)
(kW)
rate (kW ∕ ºC)
Cyclone 1, 2, 3 solids
C
50
685
50,910
80.2
Cyclone 4 solids
C
685
785
10,980
109.8
Calciner solids
C
785
820
73,650
2,104.3
Cyclone 5 solids
C
820
875
10,720
194.9
Kiln sec.1 solids
C
875
900
7,758
310.3
Kiln sec.2 solids
C
900
1,250
18,440
52.7
Kiln sec.3 solids
C
1,250
1,450
19,250
96.3
Heated gas/air
H
1,100
150
57,636.5
60.7
Calciner outlet CO2
H
990
150
15,279.6
18.2
Cyclone 5 outlet CO2
H
895
150
1,276.2
1.7
Cyclone 4 outlet CO2
H
798
150
337
0.5
Cyclone 2 outlet CO2
H
539
150
450.5
1.2
Stream
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Table 3 Percentage of fuel reduction at existing ∆Tmin of each case study.
Case study
1
2
3
4
5
∆Tmin (ºC)
110
124
70
144
103
∆tmin (ºC)
Percentage of fuel reduction by considering the existing ∕ best practice of cooler
15
4.76 ∕ 8.19
2
7.2 ∕ 24.04
4.89 ∕ 12.36
5.03 ∕ 16.14
30
4.13 ∕ 7.59
1.37
6.62 ∕ 23.56
4.27 ∕ 11.8
4.43 ∕ 15.61
45
3.51 ∕ 6.99
0.72
6.04 ∕ 23.09
3.66 ∕ 11.24
3.82 ∕ 15.07
60
2.88 ∕ 6.39
0.07
5.45 ∕ 22.6
3.04 ∕ 10.67
3.21 ∕ 14.53
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Table 4 Hot utility targeting of each case study for different ∆Tmin.
Case study
1
2
3
4
5
STEC (kJ ∕ kg Clinker)
4,601
4,831.9
4,074.3
4,486.8
4,345.9
∆Tmin (ºC)
kg Clinker) ∕ QH (kJ
50
2,729
2,954.3
2,411.8
2,647
2,529.2
75
2,777.7
3,007.4
2,456.3
2,695.9
2,578.2
100
2,826.3
3,060.4
2,500.3
2,744.8
2,627.2
125
2,874.8
3,113.6
2,544.3
2,793.7
2,675.9
150
2,923.2
3,166.4
2,588.3
2,841.8
2,724.5
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Annex. A Table A.1. Extracted stream data Stream Case study Cyclone 1, 2, 3 solids Cyclone 4 solids Calciner solids Cyclone 5 solids Kiln sec.1 solids Kiln sec.2 solids Kiln sec.3 solids Heated gas/air Calciner outlet CO2 Cyclone 5 outlet CO2 Cyclone 4 outlet CO2 Cyclone 2 outlet CO2
Hot (H) Cold (C)
Supply temp.(ºC)
C C C C C C C H H H H H
50 685 785 820 875 900 1,250 1,100 990 895 798
Target temp.(ºC) 2 685 785 820 875 900 1,250 1,450 130 130 130 130
72,380 15,880 110,100 14,450 19,570 19,550 27,250 86,960.5 23,691 1,681.6 1,527.6 286.2
47,130 17,460 73,880 14,410 9,068 14,590 20,500 60,630 1,376 15,781 2,911
Case study Cyclone 1, 2, 3 solids Cyclone 4 solids Calciner solids Cyclone 5 solids Kiln sec.1 solids Kiln sec.2 solids Kiln sec.3 solids Heated gas/air Calciner outlet CO2 Cyclone 5 outlet CO2 Cyclone 4 outlet CO2 Cyclone 2 outlet CO2
C C C C C C C H H H H H
50 670 720 820 880 900 1,250 1,100 990 909 800 550
4 670 720 820 880 900 1,250 1,400 130 130 130 130 130
Case study Cyclone 1, 2, 3 solids Cyclone 4 solids Calciner solids Cyclone 5 solids Kiln sec.1 solids Kiln sec.2 solids Kiln sec.3 solids Heated gas/air Calciner outlet CO2 Cyclone 5 outlet CO2 Cyclone 4 outlet CO2
C C C C C C C H H H H
50 640 757 830 875 900 1,250 1,100 716 990 870
6 640 757 830 875 900 1,250 1,430 130 130 130 130
22
Heat load (kW)
Supply temp.(ºC)
42,070 9,903 72,170 8,559 15,470 13,050 24,000 59,111.8 17,662.4 1,343.8 262.1
60 695 732 820 869 900 1,250 1,100 980 900 886 765
Target temp.(ºC) 3 695 732 820 869 900 1,250 1,300 130 130 130 130 130
50 710 804 820 851 900 1,250 1,100 990 909 815 572
5 710 804 820 851 900 1,250 1,400 130 130 130 130 130
Heat load (kW) 32,280 9,214 38,610 7,177 16,100 7,638 6,684 35,133.4 8,233.1 1,925 1,015.3 1,205.9
46,140 9,645 60,700 8,204 14,480 10,770 15,450 53,582.8 14,728.9 1,302 358 141.5
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Fig. 1. In-line calciner-preheater system of pyroprocess unit.
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B: Bottom Stream C: Kiln/Cyclone/Calciner E: Exchanger Mix: Mixer q: heat R: Recycle S: Set
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Fig. 2. Simulation of pyroprocess unit.
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(a) Cold stream of solid materials
(b) Cold stream of combustion air
Hot stream of clinker
Fig. 3. Composite Curves of case study 1 (a) and showing the cold utility stream of combustion air (b).
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Fig. 4. Ideal status of Composite Curves of case study 1.
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Fig. 5. Modified Composite Curves of case study 1.
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Fig. 6. Utility selection on Grand Composite Curve of case study 1.
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Fig. 7. Modified balanced Composite Curves of case study 1.
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Fig. 8. Schematic of modified pyroprocess unit.
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Fig. 9. Plot of equation 1.
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(a)
(b) Fig. 10. Sensitivity analysis of equations 1 (a) and 2 (b).
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