Abstract
The resources demand is growing exponentially, while some raw materials are becoming less and less easy to find. According to some studies in the next 40 years the demand of energy and resources will not be satisfied by the supply [1].Besides every resource spoiled is inevitably lost because it is not possible to regenerate it. In this context it is clear that the studies about energy savings are becoming more and more important and new disciplines are born to try to understand the causes of the increasing consumptions and to find a solution to lower them. One of those disciplines is for sure the thermoeconomy which is a powerful tool to analyze an energy system from a point of view that is collocated in the middle of a second principle analysis and an economic one. This theory indeed takes some concepts from the economic world (the concept of cost, the Input-Output theory,etc..) and some others from the second principle balance (balance of entrophy,balance of exergy,irreversibilities,etc..) and uses them to look more exhaustively for the inefficiencies of a system. The economic bases of this model are: the introduction of an equivalent cost that is not measured in monetary unit but in exergy, and from the same field thermoeconomy takes all the mathematical structure needed to develop a coherent model. Thermodynamic instead furnishes to the theory the fundamental parameter, that is the exergy, with which the “costs” will be calculated.
The focal attention in the last years has been concentrated on the improvement of the processes’ efficiencies to reduce the above mentioned resources that are finite. Moreover the amelioration of efficiencies will clearly lead to a reduced raw materials consumption and to a reduction of the polluting agents. It is in fact of a capital importance to reduce emissions and our impact on the planet to preserve it.
The thermoeconomic theory is born on these requirements and it has been structured to firstly study the energy system and then to give to the analyst some tools to amend it in the most efficient way.
My work has consisted in applying this model to a Spanish ceramic industry trying to find the most unproductive parts of the plant and the causes that have generated those malfunctions. Found those causes it has been possible to control from the thermoeconomic point of view if some modifications of the plant could improve the overall situation. The results of the analysis have been very interesting because have made us understand on which parameter to focus to get a better reduction of consumption and residues.
Acknowledgements
I would like to thank professor Umberto Desideri for the opportunity that has given to me, professor Alicia Valero Delgado and all the people working in C.I.R.C.E., in particular Luis, Marianela, Ana, Daniel, Simon and Bea, for their support and help to develop this work and to live a wonderful experience. A special thanks goes to professor Cesar Torres Cuadra which has helped me so much and so many time to understand the thermoeconomic world.
It is with sincere thankfulness that I want to state my appreciation for the help given to me by Gres Aragon S.A. and in particular to the engineer Josè Manuel Grao, and to the employees Juan Carlos Orta and Carlos Saranda whose help has been fundamental to find the data I needed.
I want to sincerely thank my parents: Mizia and Francesco that have supported me so much during this period and have allowed me to complete my university career and my grandmother which always has a good word for me.
I would like to thank my grandmother Elisabetta and my grandfathers Antonio and Domenico which would have been very proud and happy to share this milestone with me.
And last but not the least I want to thank all my friends that have shared some of their experiences with me during this strange travel called life. I should thank each one of you talking about how important you are for me but there is not enough paper in this world.
Index
Abstract ... 1 Acknowledgements ... 2 Index of figures ... 5 Index of tables ... 7 Nomenclature ... 8 Subscripts ... 9 1 Introduction... 102 Description of the plant ... 12
2.1 Grinding stage ... 13 2.2 Extrusion stage ... 15 2.3 Drying stage ... 15 2.3.1 Drying Tunnel... 16 2.3.2 Drying chamber ... 17 2.3.3 Heating Chamber ... 18 2.4 Glazing stage ... 19 2.5 Baking stage ... 20
2.6 Tunnel kiln description... 23
3 Energy Analysis of Araklinker plant ... 25
3.1 Tunnel dryer ... 26 3.2 Chamber dryer ... 27 3.3 Heating Chamber ... 27 3.4 Baking stage ... 28 3.4.1 Pre-firing ... 29 3.4.2 Firing stage... 30 3.4.3 Cooling stage ... 31
4 Energy and Exergy results ... 32
4.1 Tunnel dryer ... 32
4.2 Chamber dryer ... 32
4.4 Pre-firing ... 33
4.5 Firing stage ... 33
4.6 Cooling stage ... 34
5 Thermoeconomic Analisys ... 35
5.1 Introduction ... 35
5.2 Thermoeconomic Cost Accounting ... 37
5.3 Cost Assessment Rules ... 38
6 Thermoeconomic Diagnosis ... 40
6.1 Technical Exergy Saving and Fuel Impact ... 40
6.2 Malfunction and Dysfunction Analysis ... 41
6.3 Closure ... 44 7 Improvements ... 46 Conclusions ... 56 Appendix A ... 58 Appendix B ... 71 Appendix C ... 84 Appendix D ... 99 Appendix E ... 110 Appendix F ... 111 Appendix G ... 117 Appendix H ... 132 Bibliography ... 135
Index of figures
Figure 1 :Araklinker geographical position ... 12
Figure 2: Araklinker position in Grès de Aragon S.A. complex ... 12
Figure 3: Plant of Araklinker ... 13
Figure 4: Alimentation of raw materials and detail of a silo ... 13
Figure 5: Detail of a control panel in the grinding process ... 14
Figure 6: Dosage of raw materials before of extrusion ... 14
Figure 7: Detail of the extrude mold and manufacturing of ingot clay for special orders ... 15
Figure 8: Detail of the Drying Tunnel ... 16
Figure 9: Detail of the drying chamber from the outside ... 17
Figure 10: Program for natural tiles 25x25 cm ... 18
Figure 11: Detail of a Heating Chamber ... 18
Figure 12: Preparation zone for the glazing machine ... 19
Figure 13: View of the glazing zone ... 19
Figure 14: Schematic view of a tunnel kiln ... 20
Figure 15: Baking oven seen from the outside ... 21
Figure 16: Entry of the oven ... 22
Figure 17: Control panel of the oven ... 23
Figure 18: Temperature evolution in the oven ... 24
Figure 19: Cooling parts of the oven ... 24
Figure 20: Schematic of drying tunnel with input and output terms ... 26
Figure 21: Schematic of the Drying Chamber ... 27
Figure 22: Schematic of the heating chamber ... 28
Figure 23: Schematic of the pre-firing part of the kiln tunnel ... 29
Figure 24: Schematic of the firing stage ... 30
Figure 25: Schematic of the cooling stage of the kiln oven ... 31
Figure 26:Productive structure resolved with the TAESS ... 34
Figure 27: Logical chain of thermoeconomic concepts ... 36
Figure 28:Malfunction cost and exergy saving ... 42
Figure 29: Irreversibility coefficients for operation state ... 42
Figure 30: Irreversibility increase analysis... 43
Figure 31:Malfunction cost of operational parameters ... 44
Figure 32: Scheme of the plant without recirculation of combustion gases ... 46
Figure 33: Scheme of the ceramic plant with recirculation ... 48
Figure 34:Irreversibility comparison between base case and case with better coating of the oven ... 49
Figure 35:Irreversibility comparison between base case and case with better coating of
the oven ... 50
Figure 36:Irreversibility comparison between base case and case with reduction of water quantity ... 51
Figure 37:Irreversibility comparison between base case and case with reduction of water quantity ... 52
Figure 38:Irreversibility comparison between base case and case with reduction of tiles thickness ... 53
Figure 39:Irreversibility comparison between base case and case with reduction of tiles thickness ... 54
Figure 40: Physical Diagram of a simple cogeneration plant ... 62
Figure 41: Energy vs. Exergy Cost of Cogenerative Steam ... 63
Figure 42: Irreversibility-Cost Diagram of Cogeneration Plant ... 65
Figure 43: Sequential processes system diagram ... 66
Figure 44: Fuel Impact and Technical Saving... 70
Figure 45: Block diagram of a HRSG plant ... 76
Figure 46: Productive structure of the cogeneration plant ... 76
Figure 47: Physical Diagram of TGAS Plant ... 80
Figure 48: Stream–Process Model ... 88
Figure 49: Fuel and Product streams modelling ... 88
Figure 50 : Definition of Junction Parameters ... 92
Figure 51: Residue Cost model for a generic Productive-Dissipative component ... 94
Figure 52:Malfunction cost and exergy saving ... 102
Figure 53: Irreversibility coefficients for operation state ... 103
Figure 54: Irreversibility increase analysis... 105
Figure 55:Malfunction cost of operational parameters ... 108
Figure 56: Matrix representation of the above seen linear system ... 113
Index of tables
Table 1: Nomenclature table ... 8
Table 2: Subscripts table ... 9
Table 3: Input values of the Drying Chamber ... 26
Table 4:Input values of the drying chamber ... 27
Table 5: Input values of the Heating Chamber ... 28
Table 6: Algerian natural gas composition [14] ... 29
Table 7: Composition of exhaust wet gases entering the pre-firing stage ... 30
Table 8: Fluxes entering in the cooling stage ... 31
Table 9: Fluxes exiting from the cooling stage ... 31
Table 10: Energetic and exergetic results of the Tunnel dryer ... 32
Table 11: Energetic and exergetic results of the Chamber dryer ... 32
Table 12: Energetic and exergetic results of the Chamber dryer ... 33
Table 13: Energetic and exergetic results of the Pre-firing ... 33
Table 14: Energetic and exergetic results of the Firing stage ... 33
Table 15: Energetic and exergetic results of the Cooling stage ... 34
Table 16: Comparison between exergy costs of the improvements ... 54
Table 17: Exergy and Exergoeconomic costs for the cogeneration plant flows ... 65
Table 18: Fuel-Product Table ... 85
Table 19: Extended Fuel-Product Table of TGAS Plant ... 89
Table 20: Generic extended input-output table partition ... 89
Table 21:Model Operation Variables ... 100
Table 22: Design and operation values ... 101
Table 23:Input-Output table ... 111
Table 24:Fuel-Product Table ... 115
Table 25:Analogy between I-O Analysis and Thermoeconomic model ... 116
Table 26: Components of the red body ... 118
Table 27: Algerian natural gas composition part 2 [14] ... 122
Table 28: Composition of exhaust gases ... 124
Table 29: Values in the pre-firing part of the oven ... 124
Table 30: Composition of exhaust wet gases entering the pre-firing stage ... 125
Table 31: Wet air model composition of exhaust dry gases entering the pre-firing zone ... 126
Table 32: Wet air model composition of exhausted dry gases exiting the pre-firing zone ... 126
Nomenclature
Symbol Name Unit of measurement
𝐶𝑝 Specific heat at constant
pressure [
𝑘𝐽 𝑘𝑔𝐾]
b Specific exergy [𝑘𝐽
𝑘𝑔]
𝐵̇ Rate of exergy flow [W]
H Specific enthalpy [𝑘𝐽
𝑘𝑔]
𝑚̇ Mass flow rate [𝑘𝑔
𝑠]
ϕ Percent relative humidity [𝑚𝑚𝑣
𝑣,𝑠𝑎𝑡∙100]
p Total pressure [kPa]
pv Partial vapor pressure [kPa]
psat Saturation pressure of
water
[kPa]
R Universal gas constant 8,314472 [ 𝑘𝐽
𝑚𝑜𝑙∗𝐾]
R.H. Relative humidity of the
paste [ 𝑘𝑔𝑤 𝑘𝑔𝑤𝑝] s Specific entropy [ 𝑘𝐽 𝑘𝑔𝐾] T Temperature [K] or [°C]
𝑥𝑖 Mass fraction of vapor in wet air in the i-th state
[𝑘𝑔𝑣
𝑘𝑔𝑤𝑎]
𝑥𝑖 Molar fraction 𝑛𝑛𝑖 [𝑚𝑜𝑙
𝑚𝑜𝑙]
𝑦𝑖 Mass fraction [kg/kg] [kg/kg]
𝑄̇ Rate of heat transfer [W]
𝜂 I Law efficiency [-]
ψ II Law efficiency [-]
Subscripts
Symbol Name 0 Dead state r Reference conditions a Air da Dry air wa Wet air i i’th State ev Evaporation L Loss av Average dp Dry paste wp Wet productq Heat transfer related
w Water
1
Introduction
The purpose of this thesis is to apply energetic and exergetic parameters to an industrial process and then to study it from a thermoeconomic point of view. In particular it will be studied the industrial facility Araklinker, that is part of a ceramic group called Grès Aragon S.A. part of the group SAMCA. This group is a family owned company located in Aragon (Spain) whose activities are focused on the sectors of mining, agriculture, energy, plastics, synthetic fibers and real estate promotion. Overall, approximately 3,500 people are employed in the SAMCA group, with an annual turnover of nearly 850 million euros. It has installations distributed throughout Spain as well as other countries such as France and Italy. The finite products realized in
the plant are ceramic tiles. The crystallinity of ceramic materials ranges from highly oriented to semi-crystalline, and often completely amorphous (e.g., glasses). Varying crystallinity and electron consumption in the ionic and covalent bonds cause most ceramic materials to be good thermal and electrical insulators and extensively researched in ceramic engineering. Nevertheless, with such a large range of possible options for the composition/structure of a ceramic (e.g. nearly all of the elements, nearly all types of bonding, and all levels of crystallinity), the broadness of the subject is vast, and identifiable attributes (e.g. hardness, toughness, electrical conductivity, etc.) are hard to specify for the group as a whole. However, general features such as high melting temperature, high hardness, poor conductivity, high module of elasticity, chemical resistance and low ductility are the standard, with known exceptions to each of these rules (e.g. piezoelectric ceramics, glass transition temperature, superconductive ceramics, etc.). Many composites, such as fiberglass and carbon fiber, while containing ceramic materials, are not considered to be part of the ceramic family.
The word "ceramic" comes from the Greek word κεραμικός (keramikos), "of pottery" or "for pottery",[2] from κέραμος (keramos), "potter's clay, tile, pottery".The earliest known mention of the root "ceram-" is the Mycenaean Greek ke-ra-me-we, "workers of ceramics", written in Linear B syllabic script. The word "ceramic" may be used as an adjective to describe a material, a product or a process, or it may be used as a noun, either singular, or, more commonly, as the plural noun "ceramics" [2].
The earliest ceramics made by humans were pottery objects, including 27,000 years old figurines, made from clay or mixed with other materials like silica sintered in fire. Later ceramics were glazed and fired to create smooth, colored surfaces, decreasing porosity through the use of glassy, amorphous ceramic coatings on top of the crystalline ceramic substrates. Ceramics now include domestic, industrial and building products, as well as a wide range of ceramic art. In the 20th century, new ceramic
materials were developed for use in advanced ceramic engineering, such as in semiconductors.
By using ceramic it is possible to produce a lot of products: tiles, dishes, decorative objects, building materials. For each one of those uses the ceramic has to undergo into an adequate processes that improves the properties needed for its specific use.
The colour of the ceramic depends on the presence of cromofor oxides like iron oxides, titanium oxides, etc...Its standard components are usually: clay, feldspar, silicium sand, iron oxides, alumina and quartz. Such an articulated composition results in a flattened form of the chemical structure that, when diluted with water, makes the ceramic more plastic and easier to manipulate.
In particular our attention on the ceramic production of Grès Aragon will be focused on tiles production. Those elements are made of ceramic, stone, metal or even glass. However, their fundamental characteristic to reach is a good structural resistance and to improve this feature the tile has to be baked at high temperature, from 1200° to 1300°C depending on the composition of the ceramic, to make it solidify with an higher molecular density. Later the tile has to be worked to obtain some particular features that depend on its destination of use.
2
Description of the plant
Grès Aragon S.A. is situated in 𝐴𝑙𝑐𝑎𝑛̃𝑖𝑧 ,part of the Teruel province, that is located at the south-east of Zaragoza , in the North-Est of Spain. The plant produces ceramic grès for tiles.
The plants of the company are two and they are very close one to another. One is called Araklinker, the one that was analyzed, and the other one 𝐶𝑎𝑛̃𝑎𝑑𝑎. The first one produces natural stoneware and glazed tiles with a multitude of possible formats. It consume clays (red body) to produce ceramic products to be applied on walls and floors. The average daily production is 3,600 m2, using a tunnel oven that operates 24 hours a day. At present time it concentrates almost all the production of Grès Aragón, there is in fact another plant with similar capacity located in Alcorisa but is out of work due to lack of demand.
Figure 1 : Araklinker geographical position Figure 1 :Araklinker geographical position
The plant of Araklinker is illustrated in figure 3:
Figure 3: Plant of Araklinker
In the following part of the work will be analyzed the different steps which transform the ceramic grès into a finite product:
2.1 Grinding stage
In this first part of Araklinker , physically separated from the rest of the plant located in Cañada building, are located the silos where the raw materials are stored: chamotte, clay and rubble. Through the conveyor belts those materials are transported to the plant where is added water and is homogeneously mixed in a mill in fixed proportion.
Each silo has its own feeder to the main belt to collect grinding. To get a better product ,sulfate and gypsum are added in the mixture, among other materials; in the Glazed case it is necessary to add Barium carbonate ( BaCO3 ) to favor the migration of salts to the surface. The dosing is controlled automatically, as shown in the following
Moreover, the dosing feeder divides continuously the raw materials, according to the demand requested, in two parts for the two extrusion lines in which the stoneware pieces will be physically assembled from raw ceramic.
Figure 5: Detail of a control panel in the grinding process
2.2 Extrusion stage
The plant has two identical machines that perform, first using suitable molds and later with a string , cuts to product specified products as requested by the production order. Extrusion occurs with an average percent moisture of 16% (by mass),which is then minimized in drying processes before the baking process in the oven. The following figure shows a detail of an extruder in Araklinker .
Figure 7: Detail of the extrude mold and manufacturing of ingot clay for special orders After having manufactured the extruded forms it is possible to obtain the ingots of red body, to later produce the final products, pressed and shaped using their respective molds. The extruded is divided into rows of 20/24 pieces and these parts are loaded onto a 4-5 tray rows based on the dimensions of extruded format. The trays ,filled with rows, are placed on carriages that will go to the drying part. These cars have a capacity of approximately 7-9 trays, depending on the height format (usually 25x25 or 33x33 cm) .
2.3 Drying stage
This stage allows to reduce the red body (that is the name of the ceramic paste before the baking process) humidity before the baking process ,where the tiles will receive the final ,and inalterable, texture and the colour, both decided on the basis of the stylistic trends of the moment and of the production orders. Water is present in the ceramic paste, because it is necessary to mix raw materials with other additives ,that
are fundamental to get some physical and thermophysical property requested to the product. A homogeneous drying process is fundamental for the life of the product and for the coherence of its shape. After a certain drying time the clay reaches a state in which it can be engraved and decorated, the paste in fact is already hardened but it can still be modified. In Araklinker there are 3 drying rooms which will be described in the following paragraphs:
2.3.1 Drying Tunnel
In this section of the plant the red body passes for 1 or 2 hours, depending on the dimension of the piece to dry, in a tunnel in which hot air enters at 40°C for the 30x30 format and at 50°C for the 25x25 format.
With the glazed product the Barium Carbonate (BaCO3) has to be added to eliminate the salt located on the surface of the red body, in fact presence of salt can spoil the adherence of the glaze resulting in a product to be rejected.
The tunnel is structured like a normal drying equipment: the hot air transfers its sensible heat to the red paste making evaporate the water in it present, and consequently the air removes the vaporized water. The hot air is pushed inside the chamber using industrial fans. In this section because of the low humidity reduction, from 15,9% to 15%, is used only environmental air previously heated.
After the process the dried red body is conveyed to the next drying section using 5 trays.
2.3.2 Drying chamber
In these chambers the humidity ratio of the red body is lowered from 15% to 1% using recirculating air coming from the cooling part of the oven. Sometimes, if the size of the piece to work is very small, it is possible to activate burners that utilize up to 500 kW of natural gas, to achieve the necessary standards of temperature and humidity to realize a good manufacture.
The loading in this chamber is like the one of the previous drying part: it has to be waited for the chamber to be completely full and later it is possible to start the drying process. Every tray has a charge capacity of 150 m2 and the capacity of the chamber is of 10 trays, with an overall capacity of 1500 m2 independently from the size of the pieces. The tray for the movement is made of metallic material with refractory properties. The daily production of the plant is 3600 m2 so it is necessary to operate always with 2 chambers out of the total 6.
Figure 9: Detail of the drying chamber from the outside
The operating procedure of the chamber is similar to the one described in the previous drying part. The control of air humidity and temperature is complex and effectuated by a software that, given the working conditions and the size of the paste to dry, decides to mix: environmental air, recirculating air that comes from the very drying chamber ( when the temperature is growing too much the software decides to extract some air to reduce the temperature), heated air from the oven and the heat produced by the burners (Figure 10). For example the maximum temperature that has to be obtained is 120°C for the smallest pieces 15x15 and it is obtained utilizing the burners. It is clear that is quite complicated to follow instantaneously the working conditions of the oven. In order to analyze the chamber from an energetic point of view, and consequently from the exergetic, and to perform a stationary energy balance, were applied some simplifications that will be described in the energy section of the thesis.
The natural gas consumptions , furnished by the ceramic company, are a daily average value of the 𝑁𝑚3 consumed, therefore that value counts for every format of ceramic product while the analysis was focused on a single format that is the 25x25 cm natural form.
2.3.3 Heating Chamber
In this chamber the relative humidity is reduced from 1% to 0.5-0.6 %. The cycle lasts from 2 to 10 hours, so it is more or less like an hot storage in which is possible to store 12 trays ( 1800 m2).
Natural gas is used in this drying process so there is no hot air coming from the oven.
2.4 Glazing stage
The pieces ,after having gone out from the drying chamber, go in a zone in which the red dried body gets prepared: the blocks of ceramic paste are removed from the trays and then moved in a single line to enter in the glazing machine.
There are three different lines for glazing: two of them are for different colors and another one is for the natural product ( the red body that does not get any colour treatment) that goes directly into the oven.
To colour the red paste it is possible to use different techniques, in the Arakliner plant it is performed using the slippery (ingobbio), whose origin is clavey (clay-like) and to be attached to the dried red body it is necessary to use 230 liters of water for every ton of paste; some of this water can be later recuperated.
2.5 Baking stage
The baking stage is fundamental, because it controls many important properties of the finished ware: mechanical strength, abrasion resistance, dimension stability, resistance to water, chemicals and fire. Obviously this is the part of the plant in which there is the largest energy consumption.
The baking oven present in Araklinker is a Klink Tunnel. These are essentially refractory tunnels in which kiln-cars move along rail track. The kiln-cars have refractory decks on which dried ware is set in defined stable patterns. The cars are pushed through the kilns at set intervals counter-currently to a flow of air drawn by fans. The oven is gas-fired, with a maximum temperature in the firing zone near the center of the kiln. Incoming ware is preheated in the pre-firing zone by hot gases from the firing zone, whilst incoming air cools the fired ware and is itself preheated for its combustion role. A portion of the air coming from the cooling zone is usually drawn off to the adjacent dryers, giving significant fuel savings. The oven is 105 m long, 1.75 m high and 4.77 m wide.
The most important changes related to the development of ceramic properties involve the breakdown of the lattice structure of the original clay minerals, followed by the formation of new crystalline compounds and glassy phases. The maximum temperatures reached in the oven are 1300°C depending on the composition of the raw material to bake.
The firing chamber and kiln-cars are usually sealed against secondary air with a sand seal.
In the oven there are forty burners BIC 80 (105 kW each), in the first part they are disposed in a zigzag distribution composed of 5 groups of 8 to create turbulence and are positioned in the lower zone of the device. They are regulated with a two position
regulation: all open or all closed, so the burners work intermittently accordingly to the heat demand. In the upper part of the machine there are air vents from which enters the cooling air and exits the hot air. Moreover there are 80 others burners BIC 65(70 kW each) that are located in the maximum temperature zone divided in 10 groups in the lower and in the upper part. Their regulation is continuous and the quantity of fuel to burn is determined according to the oven temperature.
Figure 15: Baking oven seen from the outside
The quantity of natural gas burned and of the air to utilize is decide by a software for every product line (figure 17). There is a set point temperature in every zone of the device but it is only a reference value, because the heat to furnish depends on the composition of the red clay to bake, on its dimension and on its format. For this reason it is recommendable to bake only pieces of similar size and composition ,not to modify the optimal temperature profiles that are inside the oven. To avoid this undesired effect, the collocation of the pieces is done by machines that put the pieces well balanced on 3 lines.
The working hours of the oven are 24 hours per day for 11 months in a year, so it is very important to elaborate a well-designed production plan because the other stages of the manufacturing process are not continuous: they have their characteristic times. The next figure shows the control system of the kiln tunnel. In it is possible to see the three parts in which the oven is divided and the parameters controlled in its functioning: temperature, pressure, frequency, etc.
2.6 Tunnel kiln description
Tunnel kiln consists of three main zones: Preheating zone, Firing zone and Cooling zone.
• Preheating zone: This is the first zone in the tunnel kiln, whereas the green products enter the kiln on the kiln cars. The temperature of these green products increases gradually, due to the contact with the flue gases coming from the firing zone. It could be then considered as countercurrent flow heat exchanger.
• Firing zone: The firing zone (sometimes called sintering zone) is the main zone in the kiln, in which the green products are subjected to the heat produced by the combustion. This zone is responsible of heating up the green products to the desired temperature to produce the finite products. Furthermore they proceed to the cooling zone in a continuous way.
• Cooling zone: The cooling zone plays an important role in the kiln to cool down the products’ temperature. Air comes from a blower at the end of the kiln tunnel, doing the function of a countercurrent flow heat exchanger. The cooling process has three main stages:
Static cooling: to avoid cracks in the products
Final cooling: to increase the production speed and improve the productivity.
Figure 18: Temperature evolution in the oven
Later the finite product goes on an automatized line to the packing zone, where all the products are put inside of boxes on which the data of production, plant name, shift number and type of product are printed, and then they are ready to be shipped all over the world.
3
Energy Analysis of Araklinker plant
The energy analysis performed on the Araklinker plant is based on the data available from the life cycle analysis of the plant [3] and from the data provided by the Grès Aragon company even if the lack of most of the significant ones has led to some assumptions and simplifications that will be explained in the appropriate sections. Moreover all the balances are calculated in steady state, and the dependence of the physical properties from temperature and time are simplified considering average values.
We should have considered the electrical consumption, but it has been estimated ,using the data given by the company, that the electrical contribution (77kWh/ton [6]) is smaller (almost 25% of the total energy consumption of the plant) than the thermal one, and the electrical equipments are very automatized so it is not so easy to modify them to save energy. For these reasons in every balance of this work they will be considered as negligible except for the electrical work consumed by the industrial fans. To solve the balance equations a commercial software called EES (Engineering Equation Software)was used, it is very useful because allows to rapidly resolve thermodynamic calculations with its large variety of thermodynamic functions (enthalpy,entropy,ecc..) implemented in it. A description of the softwares used in this work is present in the appendix C while the calculations performed for the balances are in the appendix G; the results will be presented in the following chapters.
All the balances realized in this chapter are based on the following assumptions:
steady state balance
ideal gas behavior of the gases
model for exhausted gases like the one for wet air: dry gases+vapour
steadiness of Cp with temperature
The analysis will start following the above mentioned order:
3.1 Tunnel dryer
Due to the lack of data, the air ,entering in this equipment, was assumed to be a mixture of air at ambient condition and hot air coming from the cooling part of the oven, plus it has been considered the energy supplied by the natural gas burners 𝑄𝑏̇ =
98.3 kW and the exhausted gases that mix with the air in the dryer .For this reason the schematic of the device looks like:
The data utilized for the analysis, taken in part from [3] and in part from the company, are summarized in the following table:
Table 3: Input values of the Drying Chamber
State ϕ [%] T [°C] X [ 𝒌𝒈𝒗 𝒌𝒈𝒘𝒂] h [ 𝒌𝑱 𝒌𝒈𝒅𝒂] H.R. [𝒌𝒈𝒘 𝒌𝒈𝒘𝒑] 𝒎̇ [𝒌𝒈𝒘𝒑 𝒔 ] 𝒎̇ [ 𝒌𝒈𝒘𝒂 𝒔 ] pi [bar] 0 50 20 0.007262 38.56 1.013 1 50 50 0.007262 69.17 1.073 2 1.013 3 21 15.9% 3.522 4 30 15% 5 230 233.1 0.0015 1.66 6 T2 0.0015 p3
3.2 Chamber dryer
This chamber has the same goal of the previous one but in this case has to bring down the humidity of the material from the 15% to 1%. Because of this bigger reduction of the quantity of water present in the red body , it’s necessary to use the hot recirculating air coming from the cooling part of the oven plus the heat provided from the burners 𝑄𝑏̇ = 251.3 𝑘𝑊
The input values used in the energy analysis are summarized in the next table:
Table 4:Input values of the drying chamber
3.3 Heating Chamber
In this last drying chamber the humidity is brought down to 0.6% that will be the humidity with which the red body will enter in the kiln oven. For this process it is utilized an external source of heat produced by the combustion of a natural gas burner that produces 𝑄𝑏̇ = 350 kW. The quantity of wet air utilized to dry is 15000 𝑁𝑚3ℎ ,that
State ϕ [%] T [°C] X [𝒌𝒈𝒗 𝒌𝒈𝒘𝒂] h [ 𝒌𝑱 𝒌𝒈] H.R. [𝒌𝒈𝒌𝒈𝒘 𝒘𝒑] 𝒎̇ [𝒌𝒈𝒘𝒑 𝒔 ] 𝒎̇ [ 𝒌𝒈𝒘𝒂 𝒔 ] 0 50 20 0.007262 38.56 1 20 1.84E-5 141.2 12.35 2 35.2 3 80 7.7 15% 0.44 4 100 61.6 1% 5 230 0.09713 6 T2
corresponds to 3.99 𝑘𝑔𝑠𝑤𝑎 having considered as value of air density in normal conditions (T=0°C e p=1 atm) ρ=1.197 𝑚3kg.
As done in all the previous cases the heat losses have been considered, based on the suggestions of Grès Aragon S.A., as the 15% of the 𝑄𝑏̇ .
The input values used are depicted in the following table:
Table 5: Input values of the Heating Chamber
3.4 Baking stage
As described above the baking stage is made up of 3 sub-stages: pre-firing, firing and cooling. As far as concerning the heat loss of every component of the oven, it was used the value provided by the company, that counted the losses of all the oven. That loss has been split up in all the parts of the oven, using ,as weight, the temperatures. So the biggest loss has been considered in the central part, the firing one, in which the temperature is higher than in the other zones. So we have obtained that the total heat loss 𝑄𝐿̇ =413,33 kW has been divided with the following shares: 1/4 for the pre-firing
and the cooling parts and ½ for the firing part.
State ϕ [%] T [°C] X [ 𝒌𝒈𝒗 𝒌𝒈𝒘𝒂] h [ 𝒌𝑱 𝒌𝒈𝒅𝒂] R.H. [ 𝒌𝒈𝒘 𝒌𝒈𝒘𝒑] 𝒎̇ [𝒌𝒈𝒘𝒑 𝒔 ] 𝒎̇ [ 𝒌𝒈𝒘𝒂 𝒔 ] 0 50 20 0.007262 38.54 1 80 45.6 1% 3.48 2 100 60.8 0.6% 3.47 3 20 0.007262 38.54 3.99 4 5 235 6 T2
3.4.1 Pre-firing
In this sub-stage the exhaust gases coming from the burners of the firing-stage are used to pre-heat and to dry the paste till to reach R.H.=0%. We only know the volume of the fuel burned 𝑉𝑛𝑔̇ = 8600 𝑁𝑚
3
ℎ but we don’t know, because of lack of data, the
composition of it, so it has been necessary to hypothesize the composition of the natural gas. It has been chosen the Algerian one because Spain is one of the biggest importer of this gas and because its LHV was very similar with the one furnished by the company. Lately through simplified chemical reactions it has been possible to calculate the mass rate and the composition of the exhausted gases (see appendix G). The analysis could have been more detailed: considering all the mass losses in the oven, the temperature dependence of the thermodynamic parameters, a non-stationary model, but the main aim of the thesis is to calculate the exergy fluxes involved in the process in an input-output vision, so it has been chosen an appropriate level of detail considering the objectives.
Table 6: Algerian natural gas composition [14]
Calculated this value it is possible to start with the analysis of this part of the oven:
Algerian Natural Gas % V M.W. [g/mol] ρ [g/Nl]
CH4 85.70 16.03 0.7167
C2H6 7.10 30.05 1.3567
CO 0.36 28.00 1.2501
N2 4.27 28.02 1.2507
He 0.15 4.0 0.1769
All the values know for this part of the oven will be stated in the following table:
Table 7: Composition of exhaust wet gases entering the pre-firing stage
3.4.2 Firing stage
This is the core of all the plant, in it we have the fundamental process that can strengthen the molecular aggregation of the red body, allowing it to become harder and more resistant. On this assume we can say that the chemical exergy of the paste will be constant in the process because, as already stated above, we have no chemical reaction for the ceramic, we simply have a re-arrangement of the molecular structure that doesn’t affect the chemical exergy parameters. In fact the heat given by the burners is only necessary to modify the reticular structure of the ceramic, to modify it in a commercial product. State ϕ [%] T [°C] X [𝒌𝒈𝒗 𝒌𝒈𝒘𝒈] h [ 𝒌𝑱 𝒌𝒈𝒅𝒈] R.H. [ 𝒌𝒈𝒘 𝒌𝒈𝒘𝒑] 𝒎̇ [ 𝒌𝒈𝒘𝒑 𝒔 ] 𝒎̇ [ 𝒌𝒈𝒘𝒈 𝒔 ] 0 50 20 0.007262 38.54 1 50 1% 2 400 0% 7.008 3 612 0.007262 38.54 4.89 4 25
3.4.3 Cooling stage
This is the last stage of the oven, in the productive structure it has been modelled as a single equipment while it is made up by three parts. However, as already stated, we don’t need to perfectly describe the three parts, because our aim is to find the exergy fluxes entering and exiting from the single equipments using the approach of the input-output theory ( see Appendix F) .
The parameters used in this section are summarized in these tables:
Components ni [mol] pi [atm] T [3] [°C] mi
[kg/day] yi
H2Ov 0.00990034 0.01049436 20 9769.11177 0.006177 Rest 0.99009966 1.04950564 20 1571638.89 0.993823
Tot 1 1.06 1581408 1
Table 8: Fluxes entering in the cooling stage
Components T4 [°C] xi=yi∙PMm/Pmi pi=xi∙p[kPa]
H2Ov 200 1.8425E-05 1.8315E-05
Resto 473.15 1.0 0.9940131
1.0000 0.99403142
Table 9: Fluxes exiting from the cooling stage
Figure 25: Schematic of the cooling stage of the kiln oven
4
Energy and Exergy results
The results of the above seen analysis are presented in this section using simple tables, while the whole calculations have been dealt in appendix G.
4.1 Tunnel dryer
Table 10:Energetic and exergetic results of the Tunnel dryer with 𝐼1̇ =22.39 kW
4.2 Chamber dryer
Table 11: Energetic and exergetic results of the Chamber dryer 𝐼2̇ = 177 kW State T [°C] 𝑯̇ [𝒌𝑾] 𝑺̇ [ 𝒌𝑾 𝑲] 𝑩̇ [kW] 0 20 0 5.747 0 1 50 225.9 5.83 21.12 2 35.2 200 5.874 0.16 3 21 38.38 0.31 0.005 4 30 66.02 0.46 0.56 5 230 0.349 0.35 0.19 6 35.2 0.02 0.42 0.007 State T [°C] 𝑯̇ [kW] 𝑺̇ [𝒌𝑾 𝑲] 𝑩̇ [kW] 0 20 38.56 74.19 0 1 35.2 2015 78.34 301 2 118.9 2139 79.18 179.9 3 30 22.19 0.02 0.187 4 100 49 0.13 5.601 5 230 23.46 0.037 12.57 6 118.9 11.06 0.04 4.88
4.3 Heating Chamber
Table 12: Energetic and exergetic results of the Chamber dryer 𝐼3̇ =29.1 kW
4.4 Pre-firing
Table 13: Energetic and exergetic results of the Pre-firing 𝐼4̇ =2.04 MW
4.5 Firing stage
Table 14: Energetic and exergetic results of the Firing stage
State T [°C] 𝑯̇ [kW] 𝑺̇ [𝒌𝑾𝑲 ] 𝑩̇ [kW] 0 20 0 22.77 0 1 80 171.3 0.49 15.9 2 100 221.6 0.63 25.45 3 20 152.6 22.7 19.4 4 32.2 238 23.06 0.023 5 230 4.918 0.007 2.7 6 32.2 0.28 0.008 3.1 State T [°C] 𝑯̇ [kW] 𝑺̇ [ 𝒌𝑾 𝑲] 𝑩̇ [kW] 0 20 0 0.1985 0 1 100 55.18 0.161 7.95 2 400 253.7 0.55 91.02 3 195 785.8 2.156 157.1 4 65 561.8 2.16 78.52 State T [°C] 𝑯̇ [kW] 𝑺̇ [ 𝒌𝑾 𝑲] 𝑩̇ [kW] 0 20 0 1.25 0 1 400 328.2 0.718 117.7 2 1300 1106 1.45 680 3 230 5260 13.88 1214 4 205 5257 14.29 1089
𝐼5̇ =3.4 MW
4.6 Cooling stage
Table 15: Energetic and exergetic results of the Cooling stage 𝐼6̇ =1.1 MW
With the availability of all the exergy fluxes flowing in the plant, it is possible to realize the fuel-product table, built through the choice of a productive structure. Then it is possible to insert our structure and the exergy values of the fluxes into the TAESS, to let it calculate all the thermoeconomic parameters that will be analyzed in the next chapter. State T [°C] 𝑯̇ [kW] 𝑺̇ [ 𝒌𝑾 𝑲] 𝑩̇ [kW] 0 20 0 104 0 1 1300 859.9 1.129 529 2 40 13.44 0.044 0.43 3 20 704.9 104.9 89.64 4 160 3328 112.5 480.4
5
Thermoeconomic Analisys
5.1 Introduction
The body of the thermoeconomic analisys has been taken from the Thermoeconomic Analisys Course of professors C. Torres and A. Valero, in particular from the notes taken in the academic course and from the notes written by the above mentioned professors [4].
Thermoeconomic analysis combines economic, in particular the Input-Output theory explained in appendix F, and thermodynamic analysis, II principle efficiency and exergy, applying the concept of cost, originally an economic property, to exergy. Many analysts agree that exergy is an adequate thermodynamic property with which it is possible to allocate costs, because it accounts for energy quality [5] while energy does not account for that. The exergy of a thermodynamic flow is the minimum amount of work needed for its production from the reference environment. Once the reference environment is defined, exergy is a thermodynamic function of state which makes it possible to formulate the equivalence between different energy and/or matter flow streams of a plant. Two flows are thermodynamically equivalent if it is theoretically possible to obtain one from the other without additional consumption of energy resources, and if ,and only if, they have the same exergy. An exergy analysis locates and quantifies the second principle losses that represent energy irremediably lost, those losses are called irreversibilities.
Thermoeconomy in fact has its physical root in the second principle of thermodynamics, that is not sufficient to perfectly describe an energy system, because in those systems is not so easy to identify which flux ,of the all flowing in the system, is the desired product. In all the analyzable systems it is possible to find more fluxes entering and exiting from its boundaries, but it is not so immediate to understand which one of those is our objective flux. For this reason the only second principle does not tell us any detailed information on the productive structure. We need to define a purpose for every part of our process and with this definition we can apply the II principle, and in particular the definition of efficiency which strictly depends on the aim of each equipment. This is a conceptual leap of the utmost importance, because allows to tail thermodynamics to every single case making the exergy analysis more exhaustive. Applying the definition of efficiency:
it is possible to describe the behavior of every component. Thermoeconomy surpasses this fundamental parameter simply inverting numerator and denominator. This operation leads to the definition of a basic concept for the whole theory: the unit exergy cost
𝑘 =𝑅𝑒𝑠𝑜𝑢𝑟𝑐𝑒𝑠 𝑃𝑟𝑜𝑑𝑢𝑐𝑡
With this definition it is possible to calculate how many resources have been used to manufacture a unit of product.
This conceptual path can be synthetized in a single image:
Figure 27: Logical chain of thermoeconomic concepts
Besides the economic cost allows the assessment of the cost of the consumed resources, using as a weight the exergy produced by the single equipment. This helps to find out how the irreversibilities affect the parts of the system, and how those additive costs, deriving from them, modify the production and the resources’ consumption. Those features make it possible that thermoeconomy could be used for: • Rational prices assessment of plant products based on physical criteria
• Optimization of specific process unit variables to minimize the final product cost,i.e. global and local optimization.
• Detection of inefficiencies and calculation of their economic effects in operating plants,i.e. plant operation thermoeconomic diagnosis.
• Evaluation of various alternative designs or improvements on profitability • Energy audits
5.2 Thermoeconomic Cost Accounting
The question now is how to assess those exergetic costs to the components of a process.
The exergy costing principle affirm that exergy is the only rational based criteria for assigning monetary values to the interactions of an energy system with its surroundings, and with the thermodynamic inefficiencies inside of it. Mass, energy or entropy should not be used for assigning the above mentioned monetary values because their exclusive use results in misleading conclusions.
Cost accounting consists of procedures for determining, or better for estimating, the total cost of production per unit of output for each product of a thermal system (e.g., for the electricity, steam, hot water, chilled water,. . . ). All of the capital and operating costs, which are incurred to operate a thermal system, must be allocated to the final products. Thus, for each product there are direct costs, which are clearly attributable to the product such as resources and materials used specifically for its production, but there are other indirect costs too. A main challenge to the cost accountant is to assign each indirect cost in an equitable manner. The purpose of cost accounting could be stated in broad terms as:
• Determining the actual cost of products.
• Providing a rational basis for pricing products and/or evaluating their profitability. • Providing means for controlling costs.
• Forming a basis for operating decisions and their evaluation.
Then the building up of costs for each final product could be traced through the energy system. If each cost of the internal streams of the system is assessed, they could be used, by comparing them with standard or reference cost values, to control and to avoid excessive resources consumption. This is the purpose of thermoeconomic diagnosis.
The determination of all costs of the streams is also useful to make trade-off analysis of the subsystem components economy. In an existing plant, such analysis can be used, for example, for maintenance and retrofit decisions, as well as for developing and implementing operation and control strategies. Likewise, it can be used for the discovery of improvements on system concept and optimizing the design of a particular component and or the system as a whole.
5.3 Cost Assessment Rules
To allocate the above mentioned costs it is necessary to use rules that allows the formulation of cost balance equations.
The cost, the economic one and the exergetic one too, is a conservative property so for each component of a system it is possible to write:
∑ 𝑐𝑖 ∗ 𝐸𝑖̇ 𝐼𝑛 𝑖 = ∑ 𝑐𝑗∗ 𝐸𝑗̇ 𝑂𝑢𝑡 𝑗
where 𝐸𝑖̇ represents the exergy of the inputs flows of the system/component and 𝐸𝑗̇
the exergy of the output flows, 𝑐𝑖 represent the unit costs of the flows, that are known
for the inputs, and 𝑐𝑗 must be determined for the outputs. In the case of a single fuel
and a single product plant we have one equation with one unknown value, supposing that the 𝑐𝑖 is known, that is the 𝑐𝑗. In the case of a multi-product plant this balance
equation is not sufficient because, supposing that the unknown 𝑐𝑗 are n, we have one
equation with n unknown values to calculate.
To overcome this obstacle it has been necessary to define some methods that make it possible to resolve that equation:
Equality method: the generation of the two products is considered to have the same priority
Extraction method: the cost of the product entering the equipment is the same of the product exiting from it
By-product method: the cost of one of the outputs is assumed to be known The body of rules that have to be followed to resolve the cost balance equations is the following:
P1 Rule: The exergy cost is relative to the resource flows. In the absence of external
assessment, the exergy cost of the flows entering the plant equals their exergy. In other words, the unit exergy cost of resources is one.
P2 Rule: The exergy cost is a conservative property. For each component of a system
the sum of the exergy costs of the inlet flows is equal to the sum of the exergy costs of the exiting flows.
P3 Rule: If an output flow of a unit is a part of the fuel of this unit (non-exhausted
fuel), the unit exergy cost is the same as that of the input flow from which the output flow comes.
P4 Rule: If a unit has a product composed of several flows with the same
thermodynamic quality, then the same unit exergy cost will be assigned to all of them. Even if two or more products can be identified in the same unit, their formation process is the same, and therefore we assign them a cost proportional to the exergy they have.
Using these four rules it is possible to resolve the cost accounting problem and to assess the irreversibilities and their related augmentation of cost to the parts of the system that are responsible of it.
6
Thermoeconomic Diagnosis
The thermoeconomic theory not only allows to assess the irreversibility costs to the component that has produced them, moreover is used to perform diagnosis of the whole system. To describe this part of the thermoeconomic model it will be followed the order used in [4].
The diagnosis is very useful because can predict failures, analyzes the state of the system, detects anomalies etc..It consists into comparing the operating state of a generic process with a reference state, which normally is the design condition. This comparison highlights the inefficiencies of the operating state, allowing to quantify this variation using as unit of measurements the exergy. This exergetic deviation clearly affects the cost of the single equipment and of the products, consenting to detect the inefficiencies’ causes [4].
6.1 Technical Exergy Saving and Fuel Impact
Thermoeconomic diagnosis is a Second Law based technique oriented to operation state analysis. The exergy balance of an installation allows us to allocate and calculate irreversibilities in the production process and to identify the equipments which affect the overall efficiency, and the reasons thereof. This information, although useful, has proved not to be enough. In practice, when attempting to achieve energy savings in an installation, we must consider that not all the irreversibilities can be avoided. The potential exergy saving is limited by technical and economic constraints. Thus, the technical possibilities for exergy savings, which is called technical exergy saving, are always lower than the theoretical limit of thermodynamic exergy losses. Therefore, the additional fuel consumption, called Fuel Impact, can be expressed as the difference between the resources consumption of the plant in operation and the resources consumption of a reference or a design condition with the same total product:
𝛥𝐹𝑇 = 𝐹𝑇(𝑥) − 𝐹𝑇(𝑥0) (6.1)
Clearly we can express the previous relation in terms of irreversibility:
Δ𝐹𝑇 = Δ𝐼𝑇 = ∑𝑁𝑗=1(𝐼𝑗(𝑥) −𝐼𝑗(𝑥0)) = ∑𝑁𝑗=1𝛥𝐼𝑗 (6.2)
However the local exergy savings, that can be achieved in different units or processes of an installation, are not all equivalent. The same decrease in the local irreversibility of two different components leads, in general, to different variations of the total plant
energy consumption. This issue is shown in the fuel impact formula, that expresses the increase of resources consumption in the plant as a function of the marginal exergy consumption of each individual component of the plant:
Δ𝐹𝑇 = ∑𝑖=1𝑁 (∑𝑁𝑗=0𝑘𝑃,𝑗∗ (𝑥)𝛥𝑘𝑗,𝑖)𝑃𝑖(𝑥0) + 𝑘𝑃,𝑖∗ 𝛥𝑃𝑠,𝑖 = ∑𝑁𝑖=1𝛥𝐼𝑖
(6.3)
The variation of the marginal exergy consumption of each component increases its resources consumption, and then its irreversibilities, of a quantity 𝛥𝑘𝑗,𝑖𝑃𝑖(𝑥0), which is
called: malfunction. It implies an additional consumption of the external resources, given by 𝑘𝑃,𝑗∗ (𝑥)𝛥𝑘𝑗,𝑖𝑃𝑖(𝑥0), which is called the malfunction cost. Therefore the total
fuel impact can be written as the sum of the malfunction cost and the dysfunction cost of each component:
𝛥𝐼𝑖 = 𝑀𝐹𝑖+ ∑ 𝐷𝐹𝑖,𝑗 𝑁
𝑗=1
In order to analyze the impact on resource consumption of a plant, we need to know the design and operation values of the irreversibilities, product, unit exergy cost for design and operation, and the increase of the marginal exergy consumption of each component of the plant.
6.2 Malfunction and Dysfunction Analysis
In this section will be analyzed the causes of the irreversibilities increase, and the relation with the malfunction costs. We have shown that there is no direct relationship between the increase of the irreversibilities and its fuel impact. The more advanced the production process is, the greater is the cost of the irreversibility malfunction and, as a consequence, the greater its fuel impact is.
Furthermore, the degradation of a component forces the other components to adapt their operating conditions in order to maintain their production rate, and so their consumption is modified because they have to produce the same amount of product having a different efficiency. Figure 28, shows how an increase of the unit consumption of a component leads to a rise, not only of the irreversibilities of such component, but also of the irreversibilities of the previous components.
Figure 28:Malfunction cost and exergy saving
The irreversibility increase of a generic system’s component can be expressed as: 𝜟𝑰 = 𝜟𝑲𝐷𝑷𝟎+ (𝑲
𝐷− 𝑼𝐷)𝜟𝑷 (6.4)
From the expression above, we can distinguish two types of irreversibilities:
1. Endogenous irreversibility or malfunction produced by an increase of the unit consumption of the component itself:
𝑀𝐹𝑗,𝑖 = 𝛥𝑘𝑗,𝑖𝑃𝑖0 (6.5)
2. Exogenous irreversibility or dysfunction induced in the component by the malfunction of other subsystems, which forces it to consume more local resources in order to obtain the additional production ,to sustain the same demand, required by the other components:
𝐷𝐹𝑖 = (𝑘𝑖− 1)𝛥𝑃𝑖 (6.6)
The malfunction only affects the behavior of the components; the dysfunction arises in how the components adapt themselves to maintain the total production.
Another way to get to the same definition is to start from the II Principle and from the definition of exergy cost:
{𝐼𝑖𝐹= 𝐹𝑖 − 𝑃𝑖
𝑖 = 𝑘𝑖𝑃𝑖
𝑦𝑖𝑒𝑙𝑑𝑠
→ 𝐼𝑖 = 𝑘𝑖𝑃𝑖 − 𝑃𝑖 = 𝑃𝑖(𝑘𝑖− 1) = 𝑓(𝑘𝑖; 𝑃𝑖) Deriving the equation above seen it is possible to obtain: 𝛥𝐼𝑖 ≃ 𝛥𝑘𝑖𝑃𝑖 + (𝑘𝑖 − 1)𝛥𝑃𝑖
from which we obtain the same definitions used before.
Now will studied the causes and effects of the system irreversibilities and introduced a new method to compute the fuel impact of a malfunction and its effect or, in other words, to compute also the dysfunction on the rest of the system components. The total irreversibility of a component can be written as the sum of its malfunction and the dysfunction generated by other components of the system:
Δ𝐼𝑖 = 𝑀𝐹𝑖 + ∑𝑁 𝐷𝐹𝑖,𝑗
𝑗=1 i=1,…..,N (6.7)
and the fuel impact can be written in terms of malfunction and dysfunction as follows: Δ𝐹𝑇 = ∑𝑁𝑖=1Δ𝐼𝑖 = ∑𝑁𝑖=1(𝑀𝐹𝑖 + ∑𝑵𝒋=𝟏𝐷𝐹𝑖,𝑗) (6.8)
The total malfunction cost can be expressed as the sum of all the malfunction costs of the plant:
𝑀𝐹𝑖∗ = ∑𝑵 𝑀𝐹𝑗,𝑖∗
It is possible to demonstrate how the total fuel impact can be written as:
Δ𝐹𝑇 = ∑𝑁𝑖=1𝑀𝐹𝑖∗ (6.10)
and so we can write: 𝑀𝐹𝑖∗ = 𝑀𝐹𝑖 + ∑𝑵 𝐷𝐹𝑖,𝑗
𝒋=𝟏 (6.11)
It means that the malfunction cost of each component is given by the sum of the malfunction and the dysfunction generated on this component. The dysfunction generated by a component, just as the fuel impact, depends on the malfunction and the position of the component in the productive process.
6.3 Closure
Thermoeconomic diagnosis is a good tool to analyze the behavior of energy systems. The fundamental feature of this method consists in a detailed analysis of the equipment interactions and its productive structure. Once reliable input data have been provided, malfunction quantification and their causes can be determined comparing the steady state obtained from a performance test and a reference state given by a model simulator.
Figure 31:Malfunction cost of operational parameters
It is possible to predict exactly the total additional fuel consumed when a plant malfunctions or inefficiencies occur. However the most important fact is not to predict the extra fuel consumed, which can be also calculated by a simulator model of the plant, but to explain component by component where the additional fuel consumption
has occurred and to assess for each one of those its corresponding responsibility or malfunction cost (as figure 31 shows). This information cannot be obtained with a conventional simulator but only applying the thermoeconomic analysis.
An operation diagnosis methodology requires both a thermoeconomic analysis and a model simulator, in order to identify and quantify the origin of the irreversibility increase at the components level, and then to take the correct decisions to reduce the energy introduced in the system.
However, the thermoeconomic diagnosis technique presented in this work is not completed. For example when the working conditions change due to partial load operation or to a modification of the control system of the plant the efficiencies of the components change and, based on the definition given of malfunction, the thermoeconomic analysis will detect this variation as a malfunction while it was not the case. So it is necessary to study properly and in a detailed way the results of the analysis to recognize the real causes of the malfunctions.
The complete theory of the thermoeconomic diagnosis will be better explained in appendix D while the softwares used for the thermoeconomic analysis will be presented in appendix E.
7
Improvements
The optimization of the drying process, with respect to the utilization of waste heat of the furnace, is very complex and deals with restrictions of the demand production, characteristic drying curves for each format, along with a small storage capacity. The energy balance in the drying process also shows that the use of burners is needed for initial drying ramps, especially for the natural form product, so that it is not possible to avoid consuming natural gas in specific periods. For this reasons in this work have been analyzed three possible improvements, applying them in the first place to the normal scheme of the plant and in the second place considering to recirculate the combustion gases of the oven in the pre-firing zone in order to save energy. The exergy of the air entering the burner will be neglected, because it has been considered that it is at ambient conditions. All these improvements will be analyzed from the thermoeconomic point. The figure shows the components chosen to describe the productive structure of the plant. In this first scheme have been modelled all the parts of the plant but it has been also necessary to create some virtual nodes, to help the software in the calculations and to get a better understanding of the whole process.
In particular the 𝑉6 is the industrial fan that draws the air into the first air mixer, in
which the external air and the combustion gases are mixed, represented as the 𝑀7
block. Then it is possible to see a number of blocks named with the B letter that are the equivalent blocks that represent all the burners present in the different parts of the plant : 𝐵8, 𝐵9, 𝐵10 and 𝐵11. From those block it is possible to see one exergy entry,
that is the combustible exergy because we have not considered the exergy of air, and 2 exits which represent: the combustion gases recirculated in the dryers and the heat given by the reaction that is a thermal exergy flux. The other blocks have more or less the same functions as the ones just described. A Particular block is the 𝐶19 which is a
dissipative one, it is in fact a chimney. Dissipative elements are those equipment that do not form any product but only dissipate the exergy entering in them. As far as concerning the virtual nodes they are: 𝑁14 and 𝑁17. These are not a node physically
presents in the plant, they are just introduced for particular purposes. The first one is defined to separate the combustion gases exiting from the oven ,they are directed in part to the mixer and in part to the drying chamber, while the second has been introduced to easily find the exergy cost of the final product and so to compare the various solutions proposed. Summarizing the first one represents a physical division of a flux, while the second one has been introduced only to make clearer the interpretation of the results.
The exergy fluxes depicted in the figure more or less follow always the same scheme: for the burner there is the chemical exergy of the fuel entering in it and exiting from it there are the exergy of the combustion gases and the exergy associated with a heat transfer; for the 6 processes of the plant considered there are the physical exergy fluxes of the ceramic entering and exiting from them and the physical exergy of the air that follow the same path; for the fans there are as inputs the electrical exergy ,which is equal to the electrical work, and the physical exergy of the air or of the combustion gases while as an exit there is an air or a gas flux with a bigger exergy than the one with which it entered.
The exergy fluxes used for the analysis of each case have been represented in appendix H.
As said the first modification to the base scheme is the recirculation of the combustion gases in the pre-firing zone of the oven, converting the model of the plant already seen in a new one:
In this second scheme more or less the situation is the same as described above but it has been necessary to introduce another virtual node: 𝑁13 and another dissipative
element: 𝐶21 ,that is another chimney. The first realized to separate the combustion
gases exiting from the firing zone and the second to represent their draft. Clearly the exergy cost of the final product in the second scheme will be lower than the cost of the first scheme because using the combustion gases we are saving fuel and, most of all,
we are evitating to dump exhausted gases to a high temperature, reducing the exergetic losses of the plant.
These two schemes have been used in all the three improvements considered, for every one of them first have been calculated the exergy fluxes using the EES software, and lately they have been analyzed with the TAESS software (see appendix E).
The thermoeconomic results obtained are described in the following paragraphs divided by groups.
1)Better isolation of the oven
This is an improvement that consists in replacing the old insulating coating of the firing part of the oven with one of new generation. For the calculation the insulator considered is the TECH Slab HT 6.1 produced by the Isover S.p.A. and which sizes are: length of 1 m, width of 0.60 m and thickness of 50 mm with a cost of 16.95 €/m^2 and a thermal conductivity at 700°C of 0.172 W/mK. With this improvement it has been calculated a primary energy saving of 7% in the firing part of the oven that have obviously led to a reduction of the exergy flows of the schemes seen above.
a)without recirculation
