Energy demand and comparison of current defrosting technologies of frozen raw materials in...

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Energy demand and comparison of current defrosting technologies of frozen raw materials in defrosting tunnels Marek Bezovsky a, * , Michal Stricik b , Maria Prascakova c a Slovenske Elektrarne, a.s., Power Plant Vojany, 076 73 Vojany, Slovak Republic b The Faculty of Business Economics with seat in Kosice of University of Economics in Bratislava, Slovak Republic c Institute of Geotechnics, Slovak Academy of Sciences, Kosice, Slovak Republic article info Article history: Received 16 June 2009 Received in revised form 24 February 2010 Accepted 28 February 2010 Available online 8 April 2010 Keywords: Defrosting of raw materials Defrosting tunnel Convective heating method Radiant heating method abstract The optimization process of coal defrosting in defrosting tunnels is solved in this article. Individual tech- nical solutions of defrosting tunnels, as well as energy demands, are dealt with in this report. Defrosting tunnels are used for the defrosting of deep-frozen substrates like coal, ore or powdery substances. There are two different ways of defrosting. The first one is based on convective heating and the second one on radiant heating. Nowadays, convective heating is used much more than radiant heating. However, theory and practice show that the radiant heating is much more efficient. The aim of this article is to describe the design and construction of a multifunctional method of a defrosting tunnel. In the second stage we make experimental measurements of convective and radiant methods of defrosting on a built model. Its third aim is to make energy and economy assessments of defrosting process on the model. Finally, to imple- ment the acquired knowledge in the practice, determining the conditions, in which the change of pres- ent-day convective defrosting technology to the new radiant technology becomes effective. Ó 2010 Elsevier Ltd. All rights reserved. 1. Introduction The pace of scientific and technological progress and intensifi- cation of social production guarantee long-term need for energy and raw materials. For each economically developed country with advanced industry, it is necessary to dispose of sufficient quantities of these. Individual landlocked countries transport a major part of their primary energy sources and raw materials by rail. In the win- ter season imported raw materials undergo adverse effects of atmospheric conditions. During the days when the outdoor tem- perature drops below freezing, the materials carried in railway cars get frozen. That is why we encounter the disruption of the continu- ity of transport, unloading, and use of the materials in plants. To enable unloading in the winter season, the process of defrosting must be included in the chain of handling with the material. Mostly the defrosting of coal is concerned, as it serves as fuel for thermal power plants. We encounter the defrosting pro- cess in the metallurgical industry, concerning the carriage and use of ores, as well as in the construction industry in the carriage of blast furnace slag. Generally it is possible to say that the defrosting process is carried out on powdery materials in devices called defrosting tunnels. In the individual plants, defrosting processes should be carried out in the highest possible quality, with high safety, reliability, and if possible at the smallest cost [1]. Defrosting tunnels are devices, the task of which is to defrost frozen substrate – coal, ore, or powdery materials (see Fig. 1). The substrate is stored in railway cars, and the defrosting process enables the substrate to be defrosted and the subsequent possibil- ity of its unloading. Operating defrosting tunnels is seasonal in its nature. The length of the defrosting season varies according to the climatic conditions. In central and northern temperate zones it usually lasts approximately 60 days a year. It is a period during which the defrosting tunnels are kept in standby operation. Now- adays, defrosting technologies use two methods of thermal heat- ing, namely convective and radiant. In convective heating, the heat is transferred through the flow- ing carrier medium (air, or combustion of natural gas, respec- tively). When using radiant heating, the heat is transferred through electromagnetic waves [2]. After arriving at their target plants, the railway cars carrying the frozen raw material are driven into a defrosting tunnel. The depth of the fuel freezing is verified by the substrate temperature mea- surement, and then the time for defrosting is determined on the basis of defrosting processes experience in the individual plants. After doors closing on the defrosting tunnel, the heating devices used for frozen materials heating are turned on. Once specified time length for the defrosting has been completed, the system turns off. The set of railway cars carrying the defrosted materials 0306-2619/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.apenergy.2010.02.031 * Corresponding author. Tel.: +421 910 673697; fax: +421 55 7922604. E-mail address: [email protected] (M. Bezovsky). Applied Energy 87 (2010) 2447–2454 Contents lists available at ScienceDirect Applied Energy journal homepage: www.elsevier.com/locate/apenergy

Transcript of Energy demand and comparison of current defrosting technologies of frozen raw materials in...

Page 1: Energy demand and comparison of current defrosting technologies of frozen raw materials in defrosting tunnels

Applied Energy 87 (2010) 2447–2454

Contents lists available at ScienceDirect

Applied Energy

journal homepage: www.elsevier .com/locate /apenergy

Energy demand and comparison of current defrosting technologies of frozenraw materials in defrosting tunnels

Marek Bezovsky a,*, Michal Stricik b, Maria Prascakova c

a Slovenske Elektrarne, a.s., Power Plant Vojany, 076 73 Vojany, Slovak Republicb The Faculty of Business Economics with seat in Kosice of University of Economics in Bratislava, Slovak Republicc Institute of Geotechnics, Slovak Academy of Sciences, Kosice, Slovak Republic

a r t i c l e i n f o a b s t r a c t

Article history:Received 16 June 2009Received in revised form 24 February 2010Accepted 28 February 2010Available online 8 April 2010

Keywords:Defrosting of raw materialsDefrosting tunnelConvective heating methodRadiant heating method

0306-2619/$ - see front matter � 2010 Elsevier Ltd. Adoi:10.1016/j.apenergy.2010.02.031

* Corresponding author. Tel.: +421 910 673697; faxE-mail address: [email protected] (M. Be

The optimization process of coal defrosting in defrosting tunnels is solved in this article. Individual tech-nical solutions of defrosting tunnels, as well as energy demands, are dealt with in this report. Defrostingtunnels are used for the defrosting of deep-frozen substrates like coal, ore or powdery substances. Thereare two different ways of defrosting. The first one is based on convective heating and the second one onradiant heating. Nowadays, convective heating is used much more than radiant heating. However, theoryand practice show that the radiant heating is much more efficient. The aim of this article is to describe thedesign and construction of a multifunctional method of a defrosting tunnel. In the second stage we makeexperimental measurements of convective and radiant methods of defrosting on a built model. Its thirdaim is to make energy and economy assessments of defrosting process on the model. Finally, to imple-ment the acquired knowledge in the practice, determining the conditions, in which the change of pres-ent-day convective defrosting technology to the new radiant technology becomes effective.

� 2010 Elsevier Ltd. All rights reserved.

1. Introduction

The pace of scientific and technological progress and intensifi-cation of social production guarantee long-term need for energyand raw materials. For each economically developed country withadvanced industry, it is necessary to dispose of sufficient quantitiesof these. Individual landlocked countries transport a major part oftheir primary energy sources and raw materials by rail. In the win-ter season imported raw materials undergo adverse effects ofatmospheric conditions. During the days when the outdoor tem-perature drops below freezing, the materials carried in railway carsget frozen. That is why we encounter the disruption of the continu-ity of transport, unloading, and use of the materials in plants.

To enable unloading in the winter season, the process ofdefrosting must be included in the chain of handling with thematerial. Mostly the defrosting of coal is concerned, as it servesas fuel for thermal power plants. We encounter the defrosting pro-cess in the metallurgical industry, concerning the carriage and useof ores, as well as in the construction industry in the carriage ofblast furnace slag. Generally it is possible to say that the defrostingprocess is carried out on powdery materials in devices calleddefrosting tunnels. In the individual plants, defrosting processes

ll rights reserved.

: +421 55 7922604.zovsky).

should be carried out in the highest possible quality, with highsafety, reliability, and if possible at the smallest cost [1].

Defrosting tunnels are devices, the task of which is to defrostfrozen substrate – coal, ore, or powdery materials (see Fig. 1).The substrate is stored in railway cars, and the defrosting processenables the substrate to be defrosted and the subsequent possibil-ity of its unloading. Operating defrosting tunnels is seasonal in itsnature. The length of the defrosting season varies according to theclimatic conditions. In central and northern temperate zones itusually lasts approximately 60 days a year. It is a period duringwhich the defrosting tunnels are kept in standby operation. Now-adays, defrosting technologies use two methods of thermal heat-ing, namely convective and radiant.

In convective heating, the heat is transferred through the flow-ing carrier medium (air, or combustion of natural gas, respec-tively). When using radiant heating, the heat is transferredthrough electromagnetic waves [2].

After arriving at their target plants, the railway cars carrying thefrozen raw material are driven into a defrosting tunnel. The depthof the fuel freezing is verified by the substrate temperature mea-surement, and then the time for defrosting is determined on thebasis of defrosting processes experience in the individual plants.After doors closing on the defrosting tunnel, the heating devicesused for frozen materials heating are turned on. Once specifiedtime length for the defrosting has been completed, the systemturns off. The set of railway cars carrying the defrosted materials

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Fig. 1. View of a defrosting tunnel.

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is moved from the area of the defrosting tunnel into the rotationtipple or above an underground reservoir, where the cars areemptied.

The speed of materials heating in defrosting process depends onthe physical and structural properties of the material. The watercontent in the raw material, which is an undesirable element,causes its freezing in the winter season during the transportation.Water fills blank spaces among the particles and porous structuredisplacing the air. The infiltration of water into the material vol-ume occurs. Thermo-physical properties of materials describingthe defrosting speed during the defrosting processes are; specificheat capacity cp (J/kg K), which expresses the ability of materialsvarious types to bind heat, and thermal conductivity k (W/m K),which reflects the coal ability to heat [3,4,2].

2. Existing defrosting tunnel technologies description

2.1. Defrosting tunnels using convective defrosting method

For the defrosting process the method of convective thermalheating is mostly used. In general terms, the heat transferred fromthe heat source to the defrosting object (railway car) by means ofdefrosting media (air, or natural or blast furnace gas combustion)is concerned. When using convective heating, the heat source firstwarms up the interior environment of the defrosting tunnel, andthen the defrosting of the object (frozen raw material in the car)occurs. The defrosting effect as a result of the surrounding environ-ment tempering does not, therefore, happen immediately, whichleads to energy losses. If the consumption of natural gas (coal) toproduce heat energy in traditional boilers (combustion chambers)represents 100%, then after minimum losses deduction, the finalthermal potential use of natural gas (coal) for fuel defrosting inthe defrosting tunnels is achieved at 45–65%. The remaining 35–55% are the heat losses to the surroundings (see Fig. 2) which de-picts the effectiveness of convective heating [5,6].

Fig. 2. Thermal balance in convective heating method [5].

In practice we encounter several types of defrosting tunnelsusing the convective heating method. They differ in their designsolutions, kind of defrosting medium, flow system and circulationof the defrosting medium through the defrosting tunnel. The men-tioned characteristics are used in various combinations in practice.

The first type design using the convective heating method withlower and lateral blowing and the lower extraction is a defrostingtunnel consisting of mutually connected thin-walled shells, whichare embedded into a concrete foundations canal. This technologyuses the heating of air with a fresh air inlet. As a source of hotair, a steam–air heat exchanger is used. Hot air is sucked by themiddle-pressure fan use with a circulating blade. Consequently,it is dispersed into distribution channels, which run along the cen-tre of the defrosting tunnel railway and along its sides. Side jetsconnected to the side channels disperse hot air diagonally up thecar walls. Jets connected to the channel in the centre of the railwaydisperse air to the bottoms of the cars. After the transfer of its heatpotential, the heating medium is dispersed through the exhaustchannel in the centre of the railway track and after being heatedto the necessary temperature, it is retransferred to the defrostingtunnel through the jets. This type of tunnel is outlined in Fig. 3.As well as the heated air, chilled combusted gas can also circulatein this cycle [6].

The second design type using convective heating method withlateral blowing and the upper extraction is a defrosting tunnelusing the thermal steam potential (see Fig. 4). In the air-condition-ing engine room the heat exchanger is connected to a steam pipe-line. From the exchanger the condensate is transported to the heatexchanger (condensate–air). It serves as a circulating air pre-hea-ter. The suction of the air from the circulation tunnel is providedthrough the holes in the top part of the defrosting tunnel and leadsinto the fan intake. From the fan the air flows first to the water hea-ter and then to the steam heater. The water heater has the functionof the condensate cooler, while it also pre-heats and directs air intothe steam heater. The heated air is transported through the pipe-line into an underground channel, through which the air flows intothe defrosting tunnel. From the horizontal section of each channelthe air flows into the tunnel by two standpipes. These are con-nected to the side pipes with jets. Similarly to the first design typeof the defrosting tunnel, this tunnel also consists of mutually con-nected thin-walled shells.

In the convective method of heating it is very important to en-sure good thermal insulation of the defrosting tunnel, because inthis heating method, temperature tempering of the defrosting tun-nel area occurs first. If the tunnel peripheral walls are not providedwith the necessary insulation, there is a leakage of heat into thesurroundings. The defrosting tunnel has a low capacity to accumu-late heat. This type of defrosting is highly effective as a loweramount of heat is lost through the peripheral tunnel walls to thesurroundings. Therefore, in the construction process, it is veryimportant to comply with thorough insulation of the structures(see Fig. 4). On the basis of the calculated heat loss through theperipheral walls, the required insulation thickness is determined.As a thermal insulation, mineral wool covered with aluminiumor galvanized sheet is most commonly used [7,8].

The third type of defrosting tunnel is designed using the con-vective heating method with upper blowing and lower extraction,over which the engine room of the air-conditioning is placed (seeFig. 5). The peripheral walls of the tunnel are built of cinder block,the porous structure of which ensures good thermal insulationproperties. Natural gas or blast furnace gas serves as a source ofthe defrosting process. It is burned in a combustion chamber lo-cated above the defrosting tunnel in the engine room air-condi-tioning. Hot exhausts are mixed in the intake of the circulatingfan with reversed combustion and inhaled exterior air and are ad-justed to the temperature of 120–140 �C. This heated combustion

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Fig. 3. Defrosting tunnel using convective heating method with lower and lateral blowing and lower extraction; 1 – peripheral tunnel wall, 2 – car, 3 – jet blowers, 4 –exhaust channel.

Fig. 4. Defrosting tunnel using convective heating method with lateral blowing and the upper extraction; 1 – peripheral wall of the tunnel, 2 – car, 3 – jet blowers, 4 – exhaustchannel, 5 – air-conditioning engine room.

Fig. 5. Defrosting tunnel using convective heating method with upper blowing and lower extraction; 1 – cinder block wall, 2 – railcar, 3 – jet blowers, 4 – exhaust duct, 5 –air-conditioning unit.

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Fig. 7. Defrosting tunnel using the radiant heating method.

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is channelled through the exhaust fan into the air distributionducts of the defrosting tunnel. Jets in the distribution ducts dis-charge it with a speed between 15 and 18 m/s into the defrostingtunnel area directly onto the frozen material loaded in the railwaycars. The combustion, thus, transmits its heat to the frozen rawmaterial, heats up the cars steel construction and recovers the heatlosses occurring through the peripheral walls of the defrosting tun-nel. The combustion cooled to the temperature of 60–70 �C issucked into the collecting ducts located between the railway trackand they are channelled through the pipeline back into the intakepart of the circulating fan. The cycle is repeated. In this defrostingprocess, it is necessary to keep the temperature below 80 �C, whichis the safe temperature limit for the car’s bearings.

The technology of this second design solution of the defrostingtunnel uses heated air as the defrosting medium. In case of thisalternative technology, the air is preheated in a heat exchanger(hot condensate–air). Preheated air is sucked into the main heatexchanger (steam–air), where the air is heated to the required tem-perature (120–140 �C) and it is consequently blown by the maincirculating fan into the defrosting tunnel. Condensate is channelledfrom the heater to the heat exchanger, where the residual heat istransmitted to the air sucked into the intake duct between therails. The cycle of air flow and heating is repeated.

2.2. Defrosting tunnels using radiant defrosting method

Radiant defrosting method is carried out by the infrared heatingthrough gas infra-red heaters. For heating by infra-red heaters, aprinciple, which has existed in nature for millenniums, is used.An example is given: ‘‘When we are standing on a snowy mountainand the sunlight comes down on us, we feel the heat, despite theambient temperature is low.” This is because the atmospheric airis considered transparent to infrared radiation so that radiationincident on a body propagates through the air with no attenuation.Energy absorbed by the body is converted into its internal energy.Cold objects within the sun reach (infra-red heater) heats withoutthe necessity to heat up the ambient air [9,10].

Infrared radiation coming out of a metal pipe or ceramic platesof a gas infra-red heater corresponds to the thermal radiation ofthe Sun. The effect of the infra-red heaters is almost instantaneous.There is no need in producing energy for heating of the air, whichin turn heats its surroundings. Infrared radiation is spread straight-forwardly, which in practice means, that infra-red heaters can heatspecifically designated areas and zones. The efficiency of heatingby infra-red heaters reaches 90% (see Fig. 6) [5,6,9].

Fig. 7 represents a defrosting tunnel that uses the radiantmethod to defrost frozen raw material in the cars using gas in-fra-red heaters. These are installed on both defrosting tunnelside walls. Their thermal performance and number correspondto the required heat for defrosting quantity. Start-up and shut-

Fig. 6. Heat defrosting balance in radiant heating method [5].

down system is flexible, with infra-red heaters not heating upair, but directly the car construction. Consequently, the sur-rounding environment of the tunnel interior is then heated fromthe car construction.

To intensify the defrosting process, the technical solutions ofthree-positional heating may also be used. In this solution the sideinfra-red heaters are complemented by a series of infra-red heatershanging from the defrosting tunnel ceiling, adding radiant heat di-rectly to the surface of the loaded raw material. A restricting crite-rion of this technical solution can only be the investor input costs,resulting from the installation of an additional infra-red heatersseries.

The advantage of this heating method is its high start-up andshut-down flexibility of the system, settings flexibility of the re-quired heat output, very low sensitivity to thermal losses, whichare high in convective heating systems, environmental friendli-ness, the immediate heat effect, and the sectional heating possibil-ity of the defrosting tunnel in the case of defrosting fewer carscarrying frozen raw materials.

The disadvantage compared to the method of convective heat-ing is higher financial costs for the unit heat production from nat-ural gas. The price per heating unit produced by steam inconvective heating is generally lower. The disadvantage of convec-tive heating is, however, in comparison with infrared heating itsinflexibility. Convective heating systems using steam heat as asource are necessary to be tempered throughout the defrostingseason, without distinction as to whether the defrosting processis being carried out in the tunnel or not. This creates inefficienciesin the cost of steam production in the days when no defrostingtakes place.

3. Method description

Based on the analysis of the existing defrosting tunnels technol-ogies, an idea to optimize defrosting processes arose. To meet theobjective of optimizing the defrosting process, a defrosting tunnelmodel (MDT), the scheme of which is shown in Fig. 8, was designedand constructed.

Fig. 8 shows that the proposed MDT system considers the pos-sibility of defrosting frozen coal samples by the radiant heatingmethod using infra-red heaters (IH), which are installed on theMDT side walls. Infra-red heaters are connected to the system ofregulation (R) and metering of the electric power consumptionby an electrometer (E).

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Fig. 8. Schematic representation of the defrosting tunnel model; MRT – defrosting tunnel model, O – piping air-heater, V – piping ventilator, IZ – infra-red heater,R – regulation, E – electrometer, t1–9 – temperature sensor, v – humidity sensor.

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The second functional MDT system is a system of convectiveheating technology. The piping heater (H), the piping ventilator(V), and pipeline distribution – marked by red1 and black linesand arrows – are placed in its chain. Just as in the radiant system,in the convective heating system as well, the technology is con-nected to the circuits of regulation and metering of the electricpower consumption.

In both bottom interior environment corners of the MDT, airchannels – float-chambers are located, just as they are located inexisting operating tunnels. Side float-chambers have slot outletsdirected obliquely upwards, so that airflow is directed towardsthe railcars side walls. In contrast to the side float-chambers, theupper and lower float-chambers are connected to the MDT fromthe outside. Just like the inner side float-chambers, the upperand the lower outer float-chambers are connected to the intakepipe air-conditioning system. For this experimental test the coalsample weight of 26 kg was used.

Before the defrosting process in the MDT a model of a car withcoal and thermocouples was inserted into the pre-lit freezer box.The freezer box was closed in the manner of allowing the thermo-couples in the coal to be plugged in and led out to an analogue re-corder, which recorded the temperatures during the course offreezing the coal samples.

After freezing the sample to the desired temperature, the carmodel was moved from the freezer to the MDT and the MDT wassubsequently closed. After the closing of the MDT the humidityand temperature of the nine thermocouples installed was re-corded. In the convective heating method, the piping ventilatorand piping heater were turned on. In the case of the radiant heatingmethod, the infra-red heaters were switched on. During thedefrosting course, temperatures and humidity were recorded.

Fig. 8 also shows the proposed layout of the thermocouples in-side the MDT and in the volume of the frozen coal samples. Themeasurements of the air humidity around the car model duringthe defrosting were carried out by a hygrometer.

The entire structural design of the MDT was executed in such away, so that the car model size fits into the freezer box, and freez-ing sample is sufficiently large. This was done so that the defrost-ing process and transfer of the heat through the frozen sample wasobserved during a sufficiently long time, and the ongoing processeswere available to be analyzed as accurately as possible.

1 For interpretation of color in Figs. 1–9, the reader is referred to the web version ofthis article.

The parameters of the thermal performance of convective andradiant heat source were reduced to match the given sample vol-umes. Following the harmonization of various aspects affectingthe structure and MDT size, the defrosting tunnel model was con-structed, being a reduction in the scale 1:16.

4. Method application

For the monitoring purpose he convective and radiant heatingmethod of frozen raw material in the defrosting process in the de-signed and constructed defrosting tunnel model, a series of exper-imental measurements were carried out.

Individual measurements of convective and radiant coal heatingmethod in the MDT were held at a constant value of heat outputfrom the air-heaters, infra-red heaters, and the constant air flowat the outlet from the pipeline fan. In the convective coal heatingmethod links of the air-conditioning distribution were changedto study the blowing on the defrosting coal by heated air from var-ious MDT sides [9].

Depending on the convective method type of heating the indi-vidual routes of the HVAC distribution were linked as follows:

� Heated air was blown by side and lower slots and the extractionwas carried out through the lower circular hole.� Heated air was blown by side slots and the extraction was car-

ried out through the lower circular hole.� Heated air was blown by side slots and extraction was carried

out through the top slot.� Heated air was blown by the upper slot and the extraction was

carried out through the lower circular hole.

Flowing air was directed into the MDT and extracted throughthe return pipeline. The return pipelines were connected in theair circulation system to the piping fan and this was subsequentlylinked to the coiled piping air-heater. The pipeline heater outletwas routed to the head distributor for the purpose of directingthe output air heated by the heater to the side or top or bottomfloat-chamber, respectively. After entering the interior of theMDT through the slots in the float-chambers, the heated air trans-fers its energy by forced convection to the frozen ample in the carmodel and the environment air. The air then went to the outlet ofthe MDT and through the reverse pipeline and the piping fan backinto the pipeline heater for the subsequent heating. The heated airrecovers, of course, on its route not only the tempering losses of theMDT interior, but also the losses of the whole air distribution sys-

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Table 1The length and effectiveness of defrosting in the MDT with the given electricity consumption.

Defrosting method Sample defrostingtime (min)

Electrical energyconsumption (kWh)

Defrosting temperatures order Defrostingefficiency (%)

1 2 3 4 5 6 7

Lower and lateral blow – lower extraction 170 1.26 t1 t5 t6 t3 t4 t7 t2 95Lateral blow – lower extraction 200 1.48 t3 t5 t1 t6 t4 t2 t7 111Lateral blow – upper extraction 265 1.96 t3 t4 t1 t6 t5 t7 t2 147Upper blow – lower extraction 270 1.99 t1 t3 t6 t4 t5 t7 t2 150Radiant heating by infra-red heaters 180 1.33 t4 t1 t5 t3 t6 t7 t2 100

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tem. Unused air distributors were closed in the convective defrost-ing method.

� In the case of radiant heating, as a thermal energy source fourceramic electric infra-red heaters mounted on the MDT sidewalls were used.

The measurement was completed when the last negative valueof the temperature recorded from the thermocouple reached 0 �C.

Upon the completion of the measurement and removal of thecar model with the defrosted coal sample, the interior of the tunnelmodel and air distribution pipes were ventilated by turning on thepipeline fan. This was done in order to avoid the residual humidityfor the next measurement.

Thermocouples layout – seven thermocouples were placed intothe coal volume, so that it is possible to observe the temperaturechanges in the coal. Two thermocouples were placed into the tun-nel interior environment according to the existing operation lay-out. The first thermocouple was located in the altitude of the carbox centre, the second in the altitude of the car wheels centre, asillustrated in the diagram in Fig. 8: t1 – rear upper point of infra-red diagonal heater, t2 – mid-point of infra-red diagonal heater,t3 – the lower point of infra-red diagonal heater, t4 – middle ofthe infra-red heater at the car wall, t5 – bottom in the centre belowthe infra-red heater, t6 – centre of the car at the car wall, t7 – thecar centre, t8 – thermocouple located in the MDT interior – hoppercar centre, t9 – thermocouple located in the MDT – car wheelscentre.

5. Results and discussion

The previous chapter of the article described the measurementson a defrosting tunnel model, which are in this chapter evaluatedfrom the point of energy and economy view and the obtained re-sults are applicable in practice.

On the basis of differences in prices for heat produced from dif-ferent energy sources – electricity, gas, steam, there is a need todeal with the calculations of the costs and brought savings fromthe radiant defrosting method for defrosting tunnels.

For the energy and economic calculation it is necessary to takeinto account the defrosting cost and defrosting tunnels temperingduring the defrosting season.

The results of the experimental measurements of coal defrost-ing in the MDT by convective and radiant heating are listed inTable 1.

5.1. Findings coming out of the measured values

The value of radiant heating in the fifth method was chosen asthe benchmark at 100% of the defrosting efficiency. According to itwe can say which defrosting method in the MDT lasted longer thanradiant heating, and which lasted shorter than radiant heating.

Convective defrosting methods with lateral – lower extraction,upper blowing – upper extraction, upper blowing – lower extrac-

tion lasted longer in comparison with the radiant defrostingmethods.

Convective defrosting method with the lower and lateral blow-ing – lower extraction lasted shorter than radiant defrostingmethod.

The values in Table 1 reflect the time and energy demands ofvarious heating methods.

Defrosting temperatures order shows that the last points in thefrozen sample were defrosted points at the positions of thermo-couples t2 and t7. These are the thermocouples located in the vol-ume core of the frozen samples.

The defrosting costs by radiant heating method in the MDT atthe same price of the heat produced from electricity are, thus, fromthe point of view of energy and finances, the most effective convec-tive heating method, having the lowest cost.

In order to put the results measured at the MDT into practice, itis necessary to consider the fact that the sources of the power sup-ply to the equipment of convective and radiant heating are differ-ent in practice. It is necessary to consider the divergence in pricesfor heating produced in the devices powered by electricity, gas andsteam. Defrosting processes cost at the MDT expressed in percent-age at different prices per heat unit produced from electricity,steam and natural gas are reflected in graphical representationsin Fig. 9.

5.2. Findings coming out of the stated values

Defrosting costs by the convective heating method in the MDT,at a price of heat supplied in the steam form are by one third lowerin comparison with the price of heat produced by the electrical airheating.

Defrosting costs by the radiant heating method in the MDT, at aprice of heat supplied in the gas form are almost identical in com-parison with the heat price produced by the electrical air heating.

Defrosting costs by the radiant heating method in the MDT, at aprice of heat supplied in the gas form are from by a half to two-thirds higher in comparison with the price of heat supplied inthe steam form.

When using steam as a heat source for the defrosting processes,we encounter with the arising of inefficient costs at the temperingtime. It is a time period when, during the defrosting season nodefrosting processes take place in tunnels. Tempering tunnels isnecessary for the protection against condensate freezing in pipesand heat exchanger.

The percentage representation of the defrosting cost using dif-ferent energy sources in Fig. 9 shows that when using radiant heat-ing with gas infra-red heaters, the defrosting processes costs is by ahalf to two-thirds higher than when using steam as a heat source.However, this system is flexible as to its rapid start-up and shut-down. The foregoing facts show that there is a breakpoint in thedefrosting season (number of days) when the costs of inefficienttempering outweigh the costs of the more expensive but flexibleradiant heating method. Then it pays to invest into the changesin technology.

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Fig. 9. The percentage defrosting costs representation using different energy sources; energy prices are as follows: electric energy 0.06 € per kW/h, steam 0.02 € per kW/h,natural gas 0.06 € per kW/h.

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For each technology it is possible to determine the savings fromthe use of various defrosting technologies.

6. Economic return calculation on new investments

Investing means placing capital in the individual asset com-ponents. In our case it is the investment in new technology.The investment process involves converting the financialsources into the property. Investment in technology renovationof defrosting tunnel consists of two phases. In the first phaseit is the expenditure of funds and new assets acquisition. Thesecond phase is associated with reward and a gradual invest-ment return. The aim of the investment policy is the prepara-tion, selection and implementation of those investmentprojects which contribute to the growth of the market valueof a company.

For an effective investment, it is necessary to take into accountsuch relevant factors before its implementation such as the deploy-ment of the tunnel into operation during the defrosting season,defrosting season length, energy consumption and startup andshut-down flexibility of the existing and the new system, thenecessity of spending the inefficient tempering cost when nodefrosting takes place in the tunnel.

In the case of the defrosting tunnel in the power plant Vojany(Slovakia), the defrosting season length is 120 days, with onedefrosting cycle lasting 12 h. The average use of the defrosting tun-nel has been 17.1% over the last eight defrosting seasons.

The original convective heating method used for the defrostingprocess steam produced in power plant, where the energy price inthe steam per a 12-h-defrosting cycle is 1938 €. During the defrost-ing season the defrosting processes take 17.1% of the time with thefinancial demands of 79,542 €. During the rest of the season it isnecessary to temper the defrosting tunnel in order to protect sys-tems against freezing, with the financial consumption for energyin steam valued at 261,749 €.

The total financial expenses per defrosting season means: en-ergy for defrosting + energy for tempering. Then total expenses forthe financial year are Sconvection = 341,291 €.

The new radiant heating method used for the defrosting pro-cesses energy of infra-red heaters burning natural gas. The energyprice of natural gas for a 12-h-defrosting cycle represents 5076 €.In the defrosting season the defrosting processes take 17.1% ofthe time with financial demands of Sradiation = 213,205 €, duringthe remainder of the season, it is not necessary to temper or other-wise maintain the defrosting tunnel.

Total investment costs for the supply and installation of gas-fired infra-red heaters are 268,560 €.

In both systems the costs of operating personnel and mainte-nance are comparable, therefore they were not considered in thecalculation.

Savings per season are calculated as followed: Sconvection �Sradiation = 341,291 € � 213,205 € = 128,086 €.

Return on investment in new technology was calculated asinvestment cost/savings for season, so: 268,560 €/128,086 € = 2seasons.

The calculation shows that investing in the defrosting technol-ogy for the plant, considering its 17.1% operation usage isprofitable.

7. Conclusion

The stated facts show that the resources of energy supply tofacilities of convective and radiant heating are different in practice.To determine the economic balance for the potential optimizing ofexisting defrosting processes in plants, it is necessary to considerthe divergence in prices for heating produced by the devicesfuelled by natural gas, blast furnace gas or technological steamproduced in the boilers of thermal power plants.

Calculating the economy of operating various defrosting tunnelstypes must be based on flexibility and frequency of defrosting pro-cess operations in the defrosting tunnel during defrosting season,the duration of the defrosting season, the energy losses associatedwith the heat losses to the surroundings, the inefficient costs oftempering tunnels using steam heating when no defrosting takesplace, the financial difficulties of the operation of various heatingsystems of the defrosting tunnels.

On the basis of the current prices of natural gas per cubic meterand prices for energy contained in the steam, the result is that thecosts of the steam production are by about a third lower than thecosts of the same quantity of energy generated by burning naturalgas. However, the radiant heating method using infra-red heatersensures flexible deployment of the defrosting tunnels in operation,without incurring the inefficient costs of tempering during thedays when no defrosting takes place. Consequently, it is most cur-rent to deal with the economy of operating the defrosting tunnelsin individual plants. Possible optimization of the defrosting pro-cesses brings financial savings and, thus, company costs reducing.

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