Exergoeconomic analysis of a rotary kiln used for plaster production as building materials

Applied Thermal Engineering 104 (2016) 486–496

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Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng

Research Paper

Exergoeconomic analysis of a rotary kiln used for plaster production as building materials Mert Gürtürk a, Hakan F. Oztop b,⇑ a b

Department of Energy Systems Engineering, Technology Faculty, Fırat University, 23100 Elazig, Turkey Department of Mechanical Engineering, Technology Faculty, Fırat University, 23100 Elazig, Turkey

h i g h l i g h t s
The exergoeconomic analysis of the rotary kiln used for plaster production was carried out by using actual operational data.
The exergy cost is 593.6 US$/h and the cost per unit exergy is 1502.4 US$/GJ of the plaster produced by the system.
The exergoeconomic factor of the rotary kiln is calculated as 70%.
The specific manufacturing cost of the CSH is found as 0.03 US$/kg.

a r t i c l e

i n f o

Article history: Received 23 November 2015 Revised 20 April 2016 Accepted 18 May 2016 Available online 19 May 2016 Keywords: Rotary kiln Exergoeconomics Plaster Gypsum

a b s t r a c t This study has been proposed to determine the production of the cost of the plaster production and present some solutions, techniques and processes especially for rotary kiln to reduce the cost of plaster production which is used as building material. The rotary kilns consume the highest energy in any processes. Thus, the exergoeconomic analysis of the rotary kiln used for plaster production was carried out by using actual operational data to see its effectiveness. The exergy cost is 593.6 US$/h and the cost per unit exergy is 1502.4 US$/GJ of the Calcium sulfate hemihydrate (CSH) or plaster produced by the system. The capital cost flow is very high due to the hourly levelized operating and maintenance cost. Especially, the transport cost of the raw materials or the CSD is very high due to be far away of distance between the gypsum quarry and the plaster plant. The exergoeconomic factor of the rotary kiln is calculated as 70%. The exergy loss and destruction should be considered for reducing production cost. The results of the exergoeconomic analysis are evaluated different perspectives for reducing the production cost. The specific manufacturing cost of the CSH is found as 0.03 US$/kg. Ó 2016 Elsevier Ltd. All rights reserved.

1. Introduction Energy consumption increases due to increasing of population and industrialization. This rapid increase in the world’s energy consumption also leads to an increase in the polluting effect and concepts such as energy, exergy efficiency and sustainability gain importance. In this perspective, important results are obtained from analysis of energy production and consumption systems are obtained by detailed analysis methods such as exergy analysis. Moreover, the production costs for carrying out the sustainable production must be taken into account. Companies have their own methods for determining economic parameters and calculating the cost of the main product. The operators of the plants want to know the exact cost [1]. Exergoeconomics is a very useful tool to ⇑ Corresponding author. E-mail address: [email protected] (H.F. Oztop). http://dx.doi.org/10.1016/j.applthermaleng.2016.05.106 1359-4311/Ó 2016 Elsevier Ltd. All rights reserved.

optimize of different industrial sectors and energy systems, and it is considered as a tool for determining cost of materials produced by different industrial sectors. The application fields of exergoeconomic analysis are the evaluation of utility cost for systems which can be applied the concept of exergy. Results obtained from exergoeconomic analysis can be discussed and obtained useful data according to economic and technical practicability investigations, evaluating the investment case, different alternatives encountered, considering modern technologies, techniques and operating conditions, determining cost of components of systems [2]. Also, results obtained from the exergoeconomic analysis of several combined cycle power plants have been used for comparison [3]. The exergoeconomic concept can be applied to the complex energy systems and some of them are discussed in following literature survey. In the literature, number of studies on the exergoeconomic analysis of rotary kiln is too few. Generally, the exergoeconomic analysis has been applied to energy and power

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487

Nomenclature C_ Z_ PW S J PWF ieff n p i AC CRF OM TCRM MR PEC CE c _ Ex T Q_

_ HCF _ m EUM X Ex

exergy cost (US$/h) capital cost flow (US$/h) present worth (US$) salvage value (US$) salvage value ratio present value factor effective discount rate life time number of interest compounding per year cost of money annual capital cost (US$) capital recovery factor cost of operating and maintenance (US$) transport cost of the raw materials (US$) cost of maintenance and repairing (US$) price of electricity consumption (US$) cost of employees (US$) unite exergy cost (US$/GJ) the exergy flow (GJ/h) temperature (K) heat transfer rate (kW) hourly cost of the fuel (US$/h) mass flowrate (kg/h) exergy of a unit mass of the fuel (kJ/kg) mass fraction of the components of the fuel exergy of a unit mass of the components of the fuel (kJ/kg)

plants in the literature. For example, the exergoeconomic methods have been used for evaluating and optimizing the complex energy systems [4]. In this context, Sahoo [5] performed exergoeconomic analysis and optimization of a cogeneration system having production capacity of 50 MW of electricity and 15 kg/s of steam. Specially, many the analysis of complex thermal systems having high capacity can be found in the literature. In the one of the these studies, Ahmadi and Dincer [6] studied the exergoenvironmental analysis and optimization of a cogeneration plant system having production capacity of 50 MW and 33.3 kg/s of steam. They used multimodal genetic algorithm for optimization. Important values and parameters about components of those complex thermal systems can be obtained by using the exergoeconomic analysis. For example, Balli et al. [7] studied the exergy cost balance for components of the system. Their results showed that the unit exergy cost of products of the combined heat and power system calculated as 18.5 US$/GW. The exergoeconomic analysis is used for optimization. Sayyaadi and Saffari [8] performed an optimization study of desalination system and they used thermoeconomic methodology for optimization. They presented a model based on the energy and exergy analysis. The authors developed an economic model which is total revenue requirement method. The exergoeconomic analysis can be used for comparison of two different systems. It is very useful tool for comparison analysis [9]. Some researchers developed different methods for exergoeconomic analysis and they applied to these techniques to complex energy systems. Among these studies, Kim et al. [10] proposed a combination of exergetics and economic analysis. Under this objective, the method obtained by them was applied to different energy systems. They derived a general cost balance equation applied to any component of a thermal system.

M MC f

molar mass (kg/kmol) specific manufacturing cost (US$/kg) exergoeconomic factor

Greek letters s total annual number of hours of the system operate (h) eo standard chemical exergy (kJ/kmol) / factor of operating and maintenance cost Sub- and superscripts CSH calcium sulfate hemi-hydrate or plaster k kth content q loss D destruction 0 reference environment s surface PH physical CH chemical OM operating and maintenance CI capital investment – molar unit Abbreviations CSD calcium sulfate di-hydrate or gypsum CSH calcium sulfate hemi-hydrate or plaster

Different methods can be used for comparison analysis and results obtained from these methods show important cost parameters. In this perspective, Kwon et al. [11] presented a thermodynamic study. The purpose of that study is the effect of the annualized cost of a component on the production cost in cogeneration plant. They also made a comparison among typical exergy costing methodologies. They found that the cost of products is depended on the change in the annualized cost of the component of the system. Kalinci et al. [12] studied on exergoeconomic analysis of biomass-based hydrogen production. The authors evaluated components and associated streams using a method which considers exergy, cost, energy and mass and called as EXCEM. They investigated how key parameters of the system affected from the unit of hydrogen cost. The results obtained by the authors show that the system produces the unit hydrogen costs between 5.3 US$/kg and 1.5 US$/kg. The authors considered both the exergoeconomic methodology and optimization methods, such as fuzzy mathematics, genetic algorithm and artificial intelligence [13]. Exergoeconomic studies for rotary kilns are extremely limited. In this context, Camdali et al. [14] made a discussion for these thermal systems. In the other study, Atmaca and Yumrutasß [15] studied on the energy, exergy and exergoeconomic analysis of a cement factory. Based on above literature survey and authors’ knowledge there is no work on exergoeconomic analysis for rotary kiln on plaster production. In this work, measured values from a plaster plant, which is installed in Turkey, is used. A brief description will be shared information about exergoeconomic analysis, description of the plaster and gypsum production. The exergoeconomic analysis can be considered as an economic feasibility study. This study has been proposed to determine the production of the cost of the plaster production and present some solutions, techniques and

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processes especially in rotary kiln to reduce the cost of plaster production as building material.

Raw Material Stock Area

Grinding

Raw Material Storage

Silo Feed Kiln

Rotary Kiln

Silo Feed Mill

Mill

Separator

Mixer

Manufactured Material Storage

Filling, Weighing and Packing

Loading Truck

2. About exergoeconomic analysis In the literature, some studies, which combine the exergy analysis and economic principles, have been called as thermoeconomics and exergoeconomics. However, thermoeconomics and exergoeconomics represent different concepts. Tsatsaronis [16] states that when the exergy cost methodology is not applied on thermal systems. Thus, researchers should use an alternative term such as thermoeconomics. The thermoeconomics term is a general term and characterizing any combination of a thermodynamic analysis with an economic one. The exergoeconomic analysis is an important for determining in finding ways to improve the performance of a complex thermal system. It considers both the exergy and economic analysis or advanced exergy analysis and economic analysis in the recent years [17]. This method provides a technique to evaluate the cost of inefficiencies or the costs of products [2]. Comparisons of different exergoeconomic approaches can be found in literature. For example, Cerqueira and Nebra, [18] were compared four thermoeconomical methodologies. One of the most preferred of these exergoeconomic analyses and approaches is the SPECO and final form of the SPECO was presented by Lazaretto and Tsatsaronis [19]. A brief description of the gypsum and the plaster production should be presented. The construction industry is developing with growing world population. The plaster is mostly preferred in construction application.

Fig. 1. Flow diagram of the plaster plant.

rotary kiln and the Sample B which is the Calcium Sulfate Hemihydrate was taken from output of the rotary kiln. Both samples are taken as 30 g. The CSD is the xanthic in color but the CSH is white in color. 2.2. Definition of the rotary kiln system used in plaster production

2.1. A brief description of the gypsum and the plaster production The gypsum is one of the important building materials and it is used for decorative objects, plates for civil construction, building material, molds for ceramic slip casting, health applications and so on. The use of plaster is based on very old times. The gypsum is available from the nature. Also, it is produced from various chemical processes [20]. The thermal systems that use coal as fuel emits a large amount of SO2. In these systems, large amount of SO2 is removed from flue gas by using flue gas desulfurization gypsum method [21]. The gypsum is a mineral and composed of calcium sulfate with 20% per weight of chemically combined water of crystallization. Chemical formula of the gypsum is given as CaSO42H2O (calcium sulfate di-hydrate) [22]. In this study, the gypsum is presented as the Calcium Sulfate Di-hydrate and the Calcium Sulfate Di-hydrate is abbreviated as the CSD. The plaster powder is obtained by heating and grinding of the CSD. The plaster, which is also called the plaster of Paris, has the Calcium Sulfate Hemihydrate, which is obtained by the dehydration of the CSD according to reaction in Eq. (1) as [23].

CaSO4  2H2 O ! CaSO4  1=2H2 O þ 3=2H2 O

ð1Þ

The Calcium Sulfate Hemi-hydrate is abbreviated as the CSH in this study. When the plaster is mixed with water, the inverse reaction is occurred as given in the Eq. (1). Thus, the calcium sulfate di-hydrate is obtained. The final product becomes rigid and it is possible to make solid bodies with different shapes for applications [24]. Recently, using areas of the plaster as a construction material is increasing for many countries like the USA, France, Germany, etc. produce these elements [25]. In this study, the flow diagram of the plaster plant is illustrated in Fig. 1. In this flow diagram, the most important process occurs in the rotary kiln. Photos of the CSD and CSH taken from inlet and outlet of the rotary kiln are illustrated in Fig. 2. The scale is cm in Fig. 2. The sample A which is Calcium Sulfate Di-hydrate was taken from input of the

In the literature, the energy and exergy analyses of the rotary kilns have been discussed [14,26,27]. The rotary kilns are used for various fields which are drying, clinker and farine production, heating and chemical reaction etc. in industry. Also, the rotary kilns are used for experimental studies [28]. Basically, the rotary kilns are a kind of heat exchangers. In this study, the rotary kiln is used for partial dehydration of the CSD. The photograph of the rotary kiln is illustrated in Fig. 3. In Fig. 3, the photograph on the left presents the rotary section and the right one is the combustion chamber of the rotary kiln. The energy transfer is provided from the hot gases of combustion to the bed material by heat transfer mechanisms in these systems [29]. The flow diagram of the rotary kiln considered in this study is illustrated in Fig. 4. The dehydration process of the CSD is obtained in between 155 °C and 160 °C but this situation may change due to different features of the gypsum in each region or design of the rotary kiln. The partial dehydration process of the CSD occurs in 160 °C and this temperature value is determined by laboratory experiments of the plaster plant. The plaster plant has production capacity of 22.8 tons plaster per hour. The rotary kiln which is considered in the plaster plant can be divided into two parts as rotary section and combustion chamber. The power of the rotary dual-fuel burner is 7.8 MW and the burner can burn all of the liquid and gas fuels in the combustion chamber. The main aim of this work is to make an exergoeconomic analysis of a rotary kiln used for plaster production. When the results obtained from the exergoeconomic analysis is considered for investment of same system, the analysis will provide important information for investment of a new system. In this perspective, this study can be considered as an economic feasibility study. As a novel work, this study has been proposed to determine the production of the cost of the plaster production and present some solutions, techniques and processes especially in rotary kiln to reduce the cost of plaster production as building material. Also, this study shows that effects on product cost of parameters such as exergy destruction and losses are very important and plant

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Fig. 2. (A) The CSD and (B) the CSH.

Fig. 3. Photographs of the studied rotary kiln, rotary kiln (on the left) and combustion chamber (on the right).

location, unused system components, design and operation conditions of the rotary kiln, selection wrong investment and preferences effect production costs throughout the life of the system. In addition, significant parameters were obtained from analysis. These parameters are energy saving potential, environmental effects and responsibilities, production cost, cost reducing possibilities, technical capacities and so on [30]. 3. Material and methods In this part of the study, the exergoeconomic analysis of the rotary kiln will be carried out. However, some economical data are needed to analysis and results obtained from the exergy analysis of the rotary kiln in a previous study should be presented. 3.1. Economic data The accuracy of the analysis depends on the amount and quality of the available information. Trust, communication and working

facilities among the staff of plant and research team should be provided in a very good way in a similar analysis by persons responsible for the analysis. All cost data which is used in an exergoeconomic analysis must be brought to the same reference year, thus, all economic data was discussed in the below for 2013. The plaster plant was founded in 2006 and since then, it has produced as different types of the plaster which are grouting mortar plaster, gypsum board gluing plaster, construction gypsum, the plaster, machine plaster with perlite, hand plaster with perlite and satin finishing gypsum plaster. The installation of the plaster plant was cost 7.5 million TL (Turkish Lira) in 2006 and exchange rate from TL to US$ was average 1.41 which is taken from the Central Bank of the Republic of Turkey [31]. The dollar-denominated investment cost was 5.3 million US$ in 2006. The exchange rate from TL to US$ was average 1.8 in 2013 [32]. The costs will be expressed in terms of US$ in this paper. The type of the fuel used is important in the processing of the CSD or the CSD dehydration. The white and color quality of the CSH is very important due to the consumption purposes which are decorative and health, etc.

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Fig. 4. The flow diagram of the Rotary kiln.

Therefore, the pollution created by waste fuel over the CSH is undesirable in the partial dehydration of the CSD. In this case, different fuels which are coal, biomass, diesel fuel and petrol etc. cannot be used for the rotary kiln by the plaster industry. The factory consumes 456 m3/h of the natural gas to produce 19,162.8 kg/h of the CSH. The price of the natural gas is 0.4 US$/m3 based on year 2013. The plaster plant is consumed 3,283,200 m3/year of the natural gas, thus the annual cost of fuel was 1,569,369.6 US$ in 2013. Maintenances and repair cost of the rotary kiln system become approximately 1851 US$/year. The system is running 24 h a day and it is stopped 2 month in a year. The natural gas is mainly used in Turkey to generate electricity and it imports the natural gas from other countries having rich natural gas resources [33]. The plaster plant’s electricity tariff varies depending on the time of day. As a result, when all situations are considered, the price of electricity consumption was determined as 0.1 US$/kW h in 2013. The amount of 250 kW h of electricity is consumed for the production of the CSH. In the plaster plant 68 people are employed in different position. Employees are paid minimum wage in the plant. Minimum wage is 540.4 US$/month in Turkey but different parameters effect the minimum wage [34]. These parameters are determined by Republic of Turkey Ministry of Labor and Social Security. The cost to the employer of a worker is approximately 629.6 US$/month (this value is valid in 2013 [34]) but salaries of employees in key positions are higher than others. As a results, the cost to the employer of all employees is determined as 44,860.4 US$/month. The large amounts of the CSD are transported to the factory from different regions by train and small amounts of the CSD are transported by truck. The factory was built in a remote location from the CSD mines, thus transportation cost of the CSD is high. Transportation cost of 1 ton of the CSD is 11.6 US$. The transportation cost of the CSD is calculated as 265.3 US$/h. There is no certain value of operating life of the rotary kiln, thus operating life of the rotary kiln is assumed as 30 years. The operating life of it is dependent on many parameters, such as working condition and regular periodic maintenance. 30 years was determined after the authors had interview with technical personnel of the plaster plant and manufacturer of the rotary kiln. The operating and maintenance cost includes different parameters which are transportation cost of the CSD, the cost of the all employee, the cost of electricity consumption and maintenances and repair cost of the rotary kiln system. The economic data of the analysis is shown in Table 1.

Table 1 Economic data of the analysis. Cost of the installation of the plant Exchange rate from TL to US$ (2006) Exchange rate from TL to US$ (2013) Volumetric flow rate of the natural gas Mass flow rate of the CSH Price of the natural gas (2013) The annual cost of fuel Maintenances and repair cost of the rotary kiln The price of electricity consumption Minimum wage The cost to the employer of a worker The cost to the employer of all employees Transportation cost of 1 ton of the CSD The transportation cost of the CSD Operating life of the rotary kiln

5.3 million US$ 1.4 1.8 456 m3/h 19,162.8 kg/h 0.4 US$/m3 1,569,369.6 US$ 1851 US$/year 0.1 US$/kW h 540.4 US$/month 629.6 US$/month 44,860.4 US$/month 11.6 US$ 265.3 US$/h 30 years

3.2. The exergy analysis of the rotary kiln The exergy analysis of the rotary kiln were performed and this study can be found in Ref. [35]. The some assumptions are performed in the calculations as follows
The pressures of the inputs and outputs are assumed as 1 atm.
For a control volume at steady state, the identity of the matter within the control volume changes continuously, but the total amount of the mass remains constant.
Air, natural gas and flue gas behave as ideal gas.
Electrical energy which is used for the rotary kiln to rotate is not considered in the analysis.
Kinetic and potential energies in the analysis are neglected.
Reference environment conditions are P0 = 1 atm and T0 = 27 °C. The assumptions in the exergy analysis are valid for the exergoeconomic analysis. The results obtained from the exergy analysis of the rotary kiln were illustrated in Tables 2 and 3. In the exergy analysis, input parameters to the control volume, which is the rotary kiln, are air, natural gas, the CSD and flue gas recirculation. The CSD enters under environmental conditions, thus exergy of the CSD was calculated as zero. The preheating application to the combustion air is not applied, but combustion air fan is very close to flue gas recycling line. Some parts of the flue gas recycling line is insufficient in terms of insulation, thus

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M. Gürtürk, H.F. Oztop / Applied Thermal Engineering 104 (2016) 486–496 Table 2 Input parameters, values used in the exergy balance and the obtained results [35]. Input parameters

Contents

T1 (K)

T (K)

Mass flow (kg/s)

Natural gas

CH4 C2H6 C3H8 C4H10 C5H12 N2 CO2

300 300 300 300 300 300 300

300 300 300 300 300 300 300

0.07 0.005 0.002 0.001 0.0004 0.003 0.001

Air

N2 O2 CO2 H2O (g)

300 300 300 300

302 302 302 302

2.80 0.85 0.001 0.04

Ds (kJ/kg K)

_ ph (kW) (total) Ex

 eo (kJ/kmol)

_ ch (kW) (total) Ex

0 0 0 0 0 0 0

0 0 0 0 0 0 0

0

836,510 1,504,360 2,163,190 2,818,930 3,477,050 – –

4403.5

2.13 2.05 2.25 3.98

0.006 0.006 0.006 0.012

1.1 –

–

Dh (kJ/kg)

Gypsum

CaSO42H2O

300

300

6.26

0

0

0

-

–

Moisture

H2O (l)

300

300

0.07

0

0

0

–

–

The flue gas recirculation input combustion chamber

N2 O2 CO2 H2O (g)

300 300 300 300

353 353 353 353

1.31 0.24 0.11 0.58

56.87 55.15 61.58 107

0.169 0.15 0.142 0.304

21.8

720 3,970 20,140 11,710

17.35

 eo (kJ/kmol) [24]

_ ph (kW) (total) Ex

_ ch (kW) (total) Ex

720 3970 20,140 11,710

554.73

54.18

–

109.77

–

Table 3 Outputs parameters, values used in the exergy balance and the obtained results [35]. Outputs parameters

Contents

T1 (K)

T (K)

Mass flow (kg/s)

Dh (kJ/kg)

The flue gas

N2 O2 CO2 H2O (g)

300 300 300 300

433 433 433 433

4.1 0.7 0.3 1.8

144.59 141.21 161.24 274.75

Plaster

CaSO41/2H2O

300

433

5.3

119.7

temperature of the combustion air is more 2–3 Kelvin than reference environmental temperature. The exergy of the fuel was calculated as 15.8 GJ/h [35]. The purpose of the flue gas recirculation is to ensure the combustion of unburnt gas and the exergy of the flue gas recirculation was calculated as 0.1 GJ/h [35]. The exiting parameters from the control volume are exergy loss, flue gas and the CSH. The exergy destruction is calculated as 10.6 GJ/h. The energy and exergy efficiencies are found as 69% and 16%, respectively. 3.3. Theory and methods In a conventional exergoeconomic analysis, a cost balance is usually formulated for overall system operating at steady state [1] as

X X C_ in þ C_ q þ Z_ ¼ C_ out þ C_ w in

ð2Þ

out

ð3Þ

nance. The hourly levelized costs of capital investment are calculated by using the hourly levelized cost method. Algorithm of this method is composed in six steps as [2,7]. The present worth of the investigated plaster plant ðPWÞ,

ð4Þ

In the Eq. (4), TCI indicates total capital investment (US$). The salvage value ðSÞ,

S ¼ TCI  J

99.07

where J is the salvage value ratio and it was taken as 10%. The present value factor ðPWFÞ

PWF ¼

1 n ð1 þ ieff Þ

ð6Þ

Also, the PWF is called the single payment present-worth factor or the single-payment discount factor [1]. Here n is the life time of the system and it is determined as 30 years. ieff is called effective discount rate.

 p i ieff ¼ 1 þ 1 p

ð7Þ

Here, p denotes number of interest compounding per year and i is cost of money. Annual capital cost ðACÞ as

AC ¼ PW  CRF ðieff ;nÞ

ð8Þ

n

In above equation, Z_ CI is the hourly levelized cost of capital investment. Z_ OM indicates hourly levelized cost of operating and mainte-

PW ¼ TCI  S  PWF ðieff ;nÞ

0.28 0.25 0.24 0.5

Capital recovery factor ðCRFÞ as

where C_ is the exergy cost rate and Z_ is the capital cost flow.

Z_ ¼ Z_ CI þ Z_ OM

Ds (kJ/kg K)

ð5Þ

CRF ¼

ieff ð1 þ ieff Þ n ð1 þ ieff Þ  1

ð9Þ

Annualized equipment cost of the rotary kiln system ðZ_ T Þ

Z_ T ¼

/AC 3600 ðs=hÞsðh=yearÞ

ð10Þ

where s is the total annual number of hours of the system operate and it is determined as 7200 h. / indicates the factor of operating and maintenance cost. In this study, the operating and maintenance cost are calculated separately, thus / is taken as 1. Hourly levelized capital investment cost of the plaster plant (Z_ CI ) is shown as

AC Z_ CI ¼

s

ð11Þ

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The operating and maintenance costs can be divided into fixed and variable costs. The fixed O&M costs are composed of cost for operating labor, maintenance labor, maintenance materials, overhead, administration and support, distribution and marketing, research and development, and so forth. The variable operating costs depend on the average annual system capacity factor, which determines the equivalent average number of hours of system operation per year at full load. The variable operating costs consist of the costs for operating supplies other than fuel costs, catalysts, chemicals, and disposing of waste material [1]. Considering in the economic data, Z_ OM which is operating and maintenance, was

where c1 is cost per unit of exergy of the natural gas, c5 is cost per unit of exergy of the flue gas and c4 indicates cost per unit of exergy of the flue gas recirculation.

calculated using Eqs. (12) and (13). The cost of operating and maintenance (OM) is given as

The combustion air is taken from environment, thus, the cost per unit of the combustion air becomes zero.

OM ¼ TCRM þ MR þ PEC þ CE

ð12Þ

where TCRM is transport cost of the raw materials or the CSD, MR is maintenance and repairing, PEC is the price of electricity consumption and CE is the cost of employees in 2013 (US$). Fuel costs and the CSD costs may be part of the operating and maintenance costs. However, the fuel costs are considered separately from the O&M costs due to the fact that these costs are needed as inputs in the exergoeconomics analysis. Exergy of the CSD was calculated as zero, thus the CSD costs are considered as part of the operating and maintenance costs. Hourly levelized operating and maintenance cost (Z_ OM ) as

OM Z_ OM ¼

s

ð13Þ

In this study, When Eq. (2) is considered for the rotary kiln, Eq. (14) is obtained as

C_ 1 þ C_ 2 þ C_ 3 þ C_ 4 þ Z_ ¼ C_ 5 þ C_ 6 þ C_ q

ð14Þ

In accordance with Fig. 2, heat loss occurs due to insufficient insulation of the rotary kiln and the associated cost rate appears on the right side of the cost balance. In Eq. (14), the exergy transfers, which are heat transfer and the flue gas, are regarded as losses. According to the exergy costing principle, the cost stream (C_ k ) associated with an exergy stream (E_ k ) is given by [19]

_ k C_ k ¼ ck  Ex

ð15Þ

where ck indicates the average cost associated with providing each _ k in the plant being considered for kth exergy unit of the stream Ex steam of the system. The exergy cost rates is expressed as input and output streams of matter with associated rates of exergy transfer, power and the exergy transfer associated with heat transfer, respectively [1].

_ in C_ in ¼ cin  Ex _ out C_ out ¼ cout  Ex

ð16Þ

_ C_ w ¼ cw  W _ q C_ q ¼ cq  Ex

ð18Þ

ð17Þ ð19Þ

However, there is no power generation from the rotary kiln, thus, the cost rate expressions of Eq. (14) can be written as

_ 1 þ c2 Ex _ 2 þ c3 Ex _ 3 þ c4 Ex _ 4 þ Z_ ¼ c5 Ex _ 5 þ c6 Ex _ 6 þ cq Ex _ q c1 Ex

ð20Þ

_ is exergy Here c denotes average costs per unit of exergy, and Ex rate which is calculated from the exergy analysis [35]. The exergy transfers associated with the heat transfer and flue gas can be considered as losses. Thus, auxiliary relations are shown as follows

C_ 1 C_ 5 C_ 4 ¼ ¼ ðc ¼ c5 ¼ c4 Þ _ 1 Ex _ 5 Ex _ 4 1 Ex

ð21Þ

C_ 1 C_ q ¼ ðc ¼ cq Þ _Ex1 Ex _ q 1

ð22Þ

where cq indicates cost per unit of exergy loss and it was calculated using Eq. (23) as given in Ref. [1].

  _ q ¼ c1 Q_ 1  T 0 c1  Ex Ts

ð23Þ

c2 ¼ 0

ð24Þ

The fan is used the combustion air for sending in the rotary kiln. The cost of consumption electricity of the fan is considered in the operating and maintenance. Fuel cost is part of the operating and maintenance costs. However, because of the importance of fuel costs in thermal systems, in this presentation fuel costs are considered separately from the O&M costs. In the literature, different approach and method is used for determining the cost per unit of the fuel. In this study, a different method offered by Gürtürk et al. [36] is used for calculating the cost per unit of the fuel. The cost per unit of the natural gas was calculated using Eq. (25) as

c1 ¼

_ HCF _  EUM  106 ðGJ=kJÞ m

ð25Þ

_ is hourly cost of the fuel (US$/h), m _ is the mass flow of where HCF the fuel (kg/h) and it is 323.1 kg/h [35]. EUM is exergy of a unit mass of the fuel (kJ/kg).

_ ¼ HCF

ACF

ð26Þ

s

where ACF is the annual cost of the fuel and it is 1,569,369.6 US$ in _ is hourly levelized cost of the fuel (US$/h), not hourly 2013. HCF fuel cost in the exergetic terms, because both the hourly levelized cost of the fuel and hourly fuel cost in the exergetic terms have same unit (US$/h). The EUM can be calculated using Eq. (27) [36].

EUM ¼ ¼

X X

 X  X k  ExCH þ X k  ExPH k k

Xk

eok Mk

þ

X

X k ExPH k

ð27Þ

where X denotes mass fraction of the components of the fuel, Ex is exergy of a unit mass of the components of the fuel (kJ/kg). eok and Mk are standard chemical exergy (kJ/kmol) and molar mass (kg/ kmol), respectively. The fuel enters to the rotary kiln under environmental conditions, thus the fuel has only the chemical exergy.

c6 ¼

_ 2 þ c3 Ex _ 3 þ Z_ _ 1 þ Ex _ 4 Þ  ðEx _ q þ Ex _ 5 Þ þ c2 Ex c1 ½ðEx _ 6 Ex

ð28Þ

_ 3 , which is the exergy flow of the CSD, was calculated as zero Ex because the CSD enters under reference environmental conditions [35]. The cost per unit of the CSH can be calculated using Eq. (29).

c6 ¼

_ 1 þ Ex _ 4 Þ  ðEx _ q þ Ex _ 5 Þ þ Z_ c1 ½ðEx _Ex6

ð29Þ

In the analysis, the cost equations in a matrix form is shown in Eq. (30).

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M. Gürtürk, H.F. Oztop / Applied Thermal Engineering 104 (2016) 486–496

2

1 60 6 6 60 6 6 60 6 61 6 6 41

0

0 1

1 0 0 1

0 0

0

0

1

0

0

0

1 1

1

1 0

0

0

3

2

3

2

3

0 C_ 1 7 6_ 7 6 0 7 7 7 6 C2 7 6 7 7 6 7 6 7 6 C_ 3 7 6 0 7 7 7 6 7 6 7 7 6_ 7 6 1 0 0 7  6 C4 7 ¼ 6 0 7 7 7 6 7 6 7 6_ 7 6 1 0 0 7 7 6 C5 7 6 0 7 7 6_ 7 6 _7 1 1 1 5 4 C 6 5 4 Z 5 0 0 0 1 C_ q 0

0

0

0 0

0 0

0 0

ð30Þ

The cost rate of the exergy destruction C_ D shows how much US$/h is destructed during processing partial dehydration of the CSD in the rotary kiln and defined as [2]

_ D C_ D ¼ c1 Ex

ð31Þ

where c1 is the cost per unite of exergy of the natural gas. The performance of the rotary kiln system should be known in regard to the exergoeconomic analysis. This is provided by the exergoeconomic factor defined for the rotary kiln which is defined as [1].

by using Eq. (13) and considering some parameters which effective on cost of operating and maintenance. These parameters are the transport cost of the raw materials or the CSD, maintenance and repairing, the price of electricity consumption and the cost of employees. The sum of levelized costs of these parameters was calculated as 363.3 US$/h, thus, the operating and maintenance cost was determined as 363.3 US$/h. When parameters in the economic data are considered, the transport cost of the raw materials or the CSD is found very high. The capital cost of flow was calculated as 441.1 US$/h using Eq. (3). The results obtained are illustrated in Table 4. The cost per unit of exergy of the natural gas was calculated _ value was calculated as using Eq. (25). In the Eq. (25), the HCF 217.9 US$/h in 2013 using Eq. (26). The EUM value was found 49,011.74 kJ/kg. The cost per unit of exergy of the natural gas, c1 , was found as 13.76 US$/GJ. The fuel exergy cost entering the rotary kiln, C_ 1 , was 218.1 US$/h. The C_ 2 value is zero because the cost per

ð32Þ

unit of the combustion air is zero. The exergy loss cost, C_ q , is calculated using Eq. (22) and it was 37.4 US$/h. The flue gas exergy cost, C_ 5 was 30.1 US$/h and the flue gas recirculation exergy cost, C_ 4 ,

The specific manufacturing cost of the CSH is calculated using Eq. (33) [15]

was 1.9 US$/h. The exergy cost was 593.6 US$/h and the cost per unit of exergy was 1502.4 US$/GJ of the CSH produced by the system. The cost rate of the exergy destruction, C_ D , was found as 147.1

f ¼

Z_ _Z þ c1 ðEx _ D þ Ex _ qÞ

MC CSH

C_ 6 ¼ _ CSH m

ð33Þ

4. Results and discussion The production of the cost of the plaster production and present some solutions, techniques and processes for the rotary kiln to reduce the cost of plaster production as building material in this study. All presented economic data is valid for year 2013. Firstly, the hourly levelized cost of capital investment was calculated. Then, the hourly levelized cost of operating and maintenance was determined using Eqs. (12) and (13). The Present Worth ðPWÞ of the investigated system was calculated as 5,279,569.4 US$. Based on this analysis, it observed that the effected dominant parameters on PW are the rotary dual-fuel burner, components of the rotary section and combustion chamber. Investment cost could be reduced by a well-prepared technical specifications. For example, the rotary dual-fuel burner burns both liquid and gas fuels but the burner is only used with gas fuel, which is the natural gas. Since the plant establishment, the burner have been not used with liquid fuels. The natural gas is used for producing plaster because other fuels pollute the plaster. Thus, investment cost and present worth values can be considered as very high. The Salvage Value ðSÞ indicates the price of a system which completes lifetime and the salvage value of the plaster plant which was calculated as 531,000 US$ using Eq. (5). The Present Value Factor ðPWFÞ was determined as 0.05. The effective discount rate ðieff Þ, was calculated as 10%. Annual Capital cost ðACÞ was determined as 560,051.4 US$ and Capital Recovery Factor ðCRFÞ was calculated as 0.1. The hourly levelized capital investment cost of the plaster plant ðZ_ CI Þ was found as 77.7 US$/h using hourly levelized cost method. This value could be reduced in the installation of the plaster plant. As mentioned earlier that the rotary dual-fuel burner is used but a burner that can be burned only for gaseous fuels could be purchased. Also, many plaster producers have been used the burner burning with natural gas. This situation shows that installation at the stage, technical and economic feasibility studies were not carried out. As a result, the hourly levelized capital investment cost of the plaster plant can be considered as having high value. The hourly levelized operating and maintenance was calculated

US$/h using Eq. (30). The results obtained by the exergoeconomic analysis of the rotary kiln are illustrated in Table 5. Also, percentage of the exergy costs are shown in Fig. 5. The performance of the rotary kiln system should be known in regards to the exergoeconomic analysis. The exergoeconomic factor defines the performance of the rotary kiln system. In the earlier part of this work, the authors stressed that the rotary kiln is a heat exchanger and the exergoeconomic factor depends on the component type. The exergoeconomic factor is typically lower than 55% for heat exchangers [1]. In this study, the exergoeconomic factor of the rotary kiln was calculated as 70% using Eq. (31). A high value of the exergoeconomic factor suggests a decrease in the investment cost of this component at the expense of its exergetic efficiency [1]. The specific manufacturing cost of the CSH, MC CSH is calculated using Eq. (32). The mass flow rate of the CSH was measured as 19,162.8 kg/h [35]. The specific manufacturing cost of the CSH was found as 0.03 US$/kg. In the studied system, the investment on devices is performed. In this context, a rotary dual-fuel burner is installed with 7.8 MW and the burner can burn all of the liquid and gas fuels in the combustion chamber. Since the factory was established in 2006, any liquid fuel has been not used since this date. Also, the capital cost of the flow is very high due to the hourly levelized operating and maintenance. Especially, the transport cost of the raw materials or the CSD is very high due to be far away of distance between the gypsum quarry and the plaster plant. The transportation cost of the CSD was calculated as 265.3 US$/h. The factories of some gypsum manufacturers were built in the next to gypsum quarry, thus the transportation cost of the CSD is minimized. The LNG

Table 4 Results of hourly levelized cost method. PW S PWF ieff AC CRF Z_ CI

5,279,569.4 (US$) 531,000 (US$) 0.05 %10 560,051.4 (US$) 0.1 77.7 (US$/h)

Z_ OM Z_

441.1 (US$/h)

363.3 (US$/h)

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Table 5 The results obtained by the exergoeconomic analysis of the rotary kiln. Parameters

The exergy cost

The cost per unit of exergy

The natural gas

C_ 1 ¼ 218:1 US$=h C_ 2 ¼ 0 US$=h C_ 3 ¼ 0 US$=h

c1 ¼ 13:7 US$=GJ

The combustion air The CSD (the gypsum) The flue gas recirculation The flue gas The CSH (the plaster) The exergy loss The exergy destruction

C_ 4 ¼ 1:9 US$=h C_ 5 ¼ 30:1 US$=h C_ 6 ¼ 593:6 US$=h

C_ q ¼ 37:4 US$=h C_ D ¼ 147:1 US$=h

c2 ¼ 0 US$=GJ c3 ¼ 0 US$=GJ c4 ¼ 13:7 US$=GJ c5 ¼ 13:7 US$=GJ c6 ¼ 1502:4 US$=GJ cq ¼ 13:7 US$=GJ c1 ¼ 13:7 US$=GJ

(Liquid Natural Gas) has been used as fuel in these factories. The transportation cost of the LNG will be cheaper than transportation cost of the CSD. The exergoeconomic factor is expected to be lower than 55% but the exergoeconomic factor was accounted as 70%. As a result, the exergoeconomic factor value is higher than the expected value. The wrong investment and preferences affect the production costs throughout the life of the system. These situations showed that the economic feasibility and the exergoeconomic studies are very important tools. In the exergoeconomic analysis of the rotary kiln, the exergy costs of the exergy destruction, the exergy loss, the flue gas and the flue gas recirculation should be considered. These parameters are effective on the production cost. The situation assessment can be made by considering the each of these parameters. The purpose of the flue gas recirculation is to ensure the combustion of unburnt gas, but high amount of water vapor is fed into the combustion chamber along with the flue gas. This situation may be affect negatively on the combustion efficiency. Also, the mass transfer can be decreased in the rotary kiln. If the flue gas recirculation application is stopped, it makes a positive effect on the exergy cost. Thus, the flue gas recirculation system can be considered unnecessary an investment. Additionally, the flue gas recirculation fan consumes unnecessary power. If the some parts of the system is insulated in a good-manner both energy and exergy efficiencies which were calculated as 69% and 16% [35], the cost of the CSH can be reduced. Also, this situation shows that the rotary kiln has a very high energy saving potential because energy and exergy losses are very high. In the analysis, the exergy loss cost was accounted as 37.4 US$/h. Capital cost flow extremely effects with hourly levelized operation and maintenance cost. Location of the plat increases cost of transporta-

tion of the CSD. Transportation cost of 1 ton of the CSD was 11.6 US $. The transportation cost of the CSD was calculated as 265.3 US$/h. The pre-heating is not applied to the CSD, the combustion air and the natural gas in the system. The temperature of inlet air to control volume is equal to environmental temperature. The heat recovery can be maintained through the pre-heating in many applications. Adopting this measure can lead to thermal energy savings of 0.08–0.1 GJ/ton and emission reduction of 8.4–9.3 kgCO2/t in the cement industry [37]. Also, the CSH production can be carried out more efficiency with respect to energy consumption by pre-heating application. The flue gas can be used as an energy source for pre-heating applications but water vapor within the flue gas is an important problem for this process. The non-contact preheating applications should be selected to solve this problem. The material used for preheating application should be stainless steel. These situations will increase the investment cost of preheating applications due to its high price. As a result, preheating applications for the CSH, the natural gas and the combustion air will increase the capital investment cost and the operating and maintenance costs. However, the factory management of the plaster plant can be set a course for preheating applications by the planned such as economic and technical feasibility studies. Effects of particle size have been frequently discussed in the literature. The smaller particles causes less erosion. Better mass and heat transfer occur between the smaller particles and combustion produces [38]. The particle size of the CSD should be minimized as much as possible. The operating and maintenance costs are reduced with the particle size minimized, but the main effect of the particle size minimized will be energy saving due to occur a good heat and mass transfer between the particle of the CSD and the combustion products. Turkey is importing the natural gas from the countries, which have rich natural gas resources, due to be limited the natural gas resources of Turkey. Thus, the price of the natural gas should be considered for the sustainable production. The fuel cost can be decreased by reducing the particle size of the CSD. Parameters occurring in a combustion process can be reason for the exergy destruction [39]. Also, the phase change is important parameters affecting the exergy destruction. In partial dehydration process of the CSD, phase change occurs in the rotary kiln, this situation increases the exergy destruction. As a result, the exergy destruction is very high due to parameters occurring in a combustion process. Tsatsaronis et al. [39] state that the most of the exergy destruction in a combustion system is unavoidable by

The Exergy Costs 14%

21%

4%

The natural gas

3%

The combustion air The CSD (the gypsum) The flue gas recirculation The flue gas

58%

The CSH (the plaster) The exergy loss The exergy destruction

Fig. 5. Percentage of the exergy cost.

M. Gürtürk, H.F. Oztop / Applied Thermal Engineering 104 (2016) 486–496

technical means, measures for improving the thermodynamic performance of a combustion process should concentrate on the avoidable endogenous exergy destruction and the avoidable exogenous exergy destruction. If the necessary precautions are taken, the exergy destruction can be reduced. Nowadays, automation units are very important tools. Actually, automation systems having good control strategy and monitoring important parameters of the systems are expected to perform many tasks. The automation units developed will allow the systems in higher efficient and less production costs [33]. The automation unit of the present system is not adequate, because the automation unit measure temperature from four-point and mass flow rate of the CSD. Also, some values which are dimensionless are entered for the volumetric flow of the fuel and automation operator cannot see the volumetric or mass flow rate of the natural gas consumed from the automation unit. The advanced automation units and applications, which are off-line or on-line, allow the system to operate efficiently [40,41]. Consequently, the production cost can be reduced by advanced automation units. These precautions and parameters discussed are avoidance of the exergy losses, setting the air–fuel mixture for a good combustion and applying preheating to the combustion air, the CSD and the natural gas. Especially, significant fuel saving can be provided with optimization of the combustion and prevention of exergy losses and the production cost can be reduced. These precautions, the obtained results from this study and other parameters must be considered by manufacturer of the rotary kiln. Both manufacturers of the rotary kiln and plaster producers must perform sustainable production. The manufacturers of the rotary kiln have to focus on research and development and the plaster producers have to consider well planned energy management program for reducing costs.

5. Conclusions The exergoeconomic analysis can be considered as an economic feasibility study. When the results obtained from the exergoeconomic analysis is considered for investment of same system, the analysis will provide important information for investment of a new system. Also, effect on product cost of parameters such as exergy destruction and losses can be understood better with the exergoeconomic analysis. From the study, the following conclusions can be drawn as:
The hourly levelized capital investment cost of the plaster plant was found as 77.7 US$/h. The hourly levelized operating and maintenance cost was 363.3 US$/h. The capital cost flow was 441.1 US$/h.
The capital cost flow is very high due to the hourly levelized operating and maintenance cost. Especially, the transport cost of the raw materials or the CSD is very high due to be far away of distance between the gypsum quarry and the plaster plant.
The exergy cost was calculated as 593.6 US$/h and the cost per unit was accounted as 1502.4 US$/GJ of exergy of the CSH produced by the system.
In the exergoeconomic analysis, the exergy costs of the exergy destruction, the exergy loss, the flue gas and the flue gas recirculation should be considered for better a production.
The production cost based exergy was found as 0.03 US$/kg and the exergoeconomic factor of the rotary kiln was calculated as 70%.
The results obtained are discussed from different perspectives. As a result, the factory has huge potential for reducing production cost.

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