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Energy and Exergy Analyss of the Dansh Industry Sector Faban Bühler * Department of Mechancal Engneerng Techncal Unversty of Denmark, Kgs. Lyngby, Denmark e mal: fabuhl@mek.dtu.dk Tuong-Van Nguyen, Bran Elmegaard Department of Mechancal Engneerng Techncal Unversty of Denmark, Kgs. Lyngby, Denmark e mal: tungu@mek.dtu.dk, be@mek.dtu.dk ABSTRACT A detaled analyss of the Dansh ndustry s presented n ths paper usng the energy, exergy and emboded exergy methods. The 22 most energy-ntensve process ndustres, whch represent about 80% of the total prmary energy use of the ndustry, were modelled and analysed n detals for the years 2006 and 2012. The energy and exergy losses, as well as the exergy destructon, were establshed, together wth the emboded ones, by ncludng the transformaton processes n the utlty sector. The energy and exergy effcences for each subsector were calculated n a fnal step and ranged from 12% to 56% n 2012. Industres wth hgh-temperature processes, such as the cement and metal producton sectors, present the hghest exergy effcences but the lowest energy ones. The opposte concluson s drawn for the food, paper and chemcal ndustres. The exergy losses, whch ndcate the potental for recoverng and valorsng heat, amounted to 3,800 TJ for the same year. Meanwhle, the emboded exergy losses, from the central producton of heat and power, exceeded 8,700 TJ. The comparson of the emboded energy effcences from 2006 to 2012 shows a clear ncrease of 4.2%-ponts, but ths trend s not seen wth the emboded exergy effcency, whch remans at around 29% for the Dansh ndustry. Ths analyss shows that there are stll large potentals to recover waste heat n most Dansh ndustral sectors and thus to ncrease ther effcences. KEYWORDS Exergy, Energy, Emboded exergy, Industral sector, Denmark. 1. Introducton Wth an ncreasng awareness of the envronmental mpacts and practcal lmtatons assocated wth the tradtonal fossl energy carrers, many countres am to ncrease the effcency of the processes usng energy, whle shftng to more sustanable energy sources. It s thus crucal to understand and analyse the systems where resources and energy are consumed and depleted, n order to plan and steer future developments. The ndustral sector s one of the systems consumng the largest quanttes of resources. These levels can be expected to further rse as the energy demand ncreases n par wth the global affluence and populaton. Denmark has a focus on energy effcency snce the frst ol crss n 1973 and the country has mplemented polces on the ndustral sector, partcularly at the begnnng of 1990. Currently, energy effcency oblgatons for the Dansh energy dstrbuton companes affect all end-consumer sectors, and, snce 2013, an nvestment subsdy scheme promotes the * Correspondng author 1
use of renewable energy and the mplementaton of energy effcency measures for ndustral processes [1]. The applcaton of energy-based methods s useful for trackng the energy flows wthn a gven system and vsualsng the converson from one form of energy to another. However, such tools present some nherent lmtatons, as they cannot be used for assessng the performance losses wthn a gven system. Unlke energy, exergy can be destroyed by thermodynamc rreversbltes and ths concept s used n ths work to account for the qualty of energy: t thereby better descrbes the neffcences and waste heat recovery potentals of the system. There have been a number of studes conducted analysng the energy and exergy effcency of a country. The most notable are the ones conducted for the Unted States [2], Canada [3], Sweden [4], Turkey [5] and Norway [6]. These works demonstrated the usefulness of thermodynamc methods for depctng opportuntes for better energy management, and they showed sgnfcant potentals for mprovements n both countres. A revew of the studes and methodologes was performed by Utlu and Hepbasl [7,8]. They suggested a formalsaton of the methods for modellng the sectoral energy and exergy utlsaton, startng from the lstng of all energy and exergy nputs and outputs, then wth a subgroupng of the sectors nto utlty, ndustral, commercal, resdental & transportaton, and a further splttng nto each end-user. The work of Dncer et al. [9] focuses on the resdental sector of Saud Araba, whle the one of Hammond et al. [10] deals wth the case of the utlty sector of Unted Kngdom. The studes that are the most relevant to the present work may be the ones of Al-Ghandoor et al. [11], who apply emboded energy and exergy methods, and Sanae et al. [12], both focusng on the ndustral sector of a country. The effcences for several ndustres are determned and compared for the cases of Unted States and Iran. These works, however, do not dstngush between the destroyed exergy due to rreversbltes and the exergy lost to the envronment. In addton, great dfferences n the level of detal, e.g. the number of consdered processes, exst amongst them. Ths paper presents a detaled analyss of the ndustry sector n Denmark, usng energy, exergy and emboded exergy methods, and s dvded as follows. Secton 2 presents the methods and approach of ths work. Twenty-two ndustral sectors, representng 79% of the energy used n the Dansh ndustry, are assessed n order to determne the energy and exergy effcences, as well as the destroyed and lost exergy. The effcences are calculated based on the scentfc lterature avalable for Denmark and on complementary assessments. Secton 3 descrbes the man results, whch () show where n the Dansh ndustry the lowest effcences and hghest losses occur, () document the changes n the ndustral sector over the last years, and () pnpont the ndustres wth potental for the recoverng energy and exergy. In a further step, Secton 4 dscusses the valdty and relevance of the results, whch are compared to smlar studes performed n ths feld, whle Secton 5 concludes the present study and fndngs. 2. Methodology 2.1 Case study 2.1.1 Industral Sector The ndustry sector n Denmark conssts of several subsectors, wthout beng domnated by sngle ndustres. The total energy nput to the ndustry sector, excludng the extracton of ol and gas resources, agrculture and the servce sector, accumulated to 112 PJ n 2012, whch s a 2
reducton of 12 % compared to 2006 [13]. In ths study, the 22 most energy ntense ndustres were selected, whch together represented 79 % of the energy consumpton of the ndustral sector n 2012. For each of these sectors, the energy nput from 16 dfferent fuel types (e.g. ol, natural gas, bogas), electrcty, dstrct heat and heat pumps s avalable. In addton, prevous publcatons by the Dansh Energy Agency [14,15] provde the dstrbuton of fuels and dstrct heatng amongst 12 process categores, such as dstllaton, heatng, evaporaton, dryng and converson and transmsson losses. The electrcty nput s dstrbuted between 12 fnal processes. In ths work, the end-consumers for transportaton wthn the ndustry sector are not consdered, reducng the process categores to 10 for the fuels. Losses Fuels Heat Electrcty Thermal Converson Processes Heat Pump Faclty Losses Fgure 1. Processes and energy flows wthn an ndustry sector 2.1.2 Utlty sector In ths study, the utlty sector s also taken nto account. In Denmark, electrcty from thermal power plants s almost exclusvely produced n combned heat and power plants (CHP), usng prmarly coal, natural gas and bomass. Furthermore, a share of 29 % of the net electrcty produced orgnated from wnd power and 15 % was from net mports n 2012 from the neghbourng countres (e.g. Germany, Sweden and Norway) [16]. Almost 74 % of the dstrct heat s produced n CHP unts and the remanng part n heatng unts. The data from the Dansh Energy Agency [16,17] also gves nformaton on the self-consumpton of the power plants, as well as on the dstrbuton and transmsson losses. Wnd Wnd Power Fuel CHP Transmsson Industry Fuel Heatng Losses Other Sectors Electrcty Heat Losses Fgure 2. Processes and energy flows wthn the utlty sector. 3
2.2 Theoretcal background 2.2.1 Energy balance As stated by the 1st law of thermodynamcs, energy may be stored, transformed from one form to another (e.g. from mechancal to electrcal), but can nether be created nor destroyed. For an open system, energy can be transferred n- and out of the system under study wth streams of matter, heat and work. The present work does not consder changes n knetc (veloctes) and potental (heghts) energes, whch mples that the energy balance n steady-state condtons, on a rate form, s as follows: H H + Q W = 0 (1) n n n out out k k h m h m + Q W = 0 (2) n n where: H denotes the energy assocated wth a stream of matter; h the specfc enthalpy of a materal stream; m the mass flowrate of the correspondng stream; the subscrpts n and out the n- and outflowng streams; Q and W the heat and work rates exchanged wth the surroundngs. out out The use of an energy analyss s relevant for trackng the energy flows and the transformaton of one form of energy to another across dfferent systems. 2.2.2 Exergy accountng Unlke energy, exergy can be destroyed and accounts for the use of addtonal prmary energy nduced by the systems mperfectons. It can be defned as `the maxmum useful work as the system s brought nto complete thermodynamc equlbrum wth the thermodynamc envronment, whle the system nteracts wth t only. A system n thermal and mechancal equlbrum (same temperature and pressure) wth the envronment s called n `envronmental state, whle t s n `dead state f also n chemcal equlbrum (same chemcal speces). Ths thermodynamc concept bulds on the frst and second laws of thermodynamcs, reflectng that all transformatons are rreversble n nature and generate entropy. The exergy destructon s defned as the dfference between the exergy nflowng and outflowng the system under study, and can thus be derved from the prevous relatons as: E n E = out E d (3) n out Q W enm n eoutm out + E k E = E d (4) n out k where: E denotes the exergy assocated wth a stream of matter, heat or work; e the specfc exergy of a materal stream; E and E Q k W E d the destroyed exergy. out the heat and work exergy rates exchanged wth the surroundngs; k k 4
2.2.3 Flow exergy The specfc exergy of a flowng stream of matter consst of physcal, chemcal, knetc and potental components. Excludng the knetc and potental components, the specfc exergy can be expressed as follows: e = [( h h0 ) T0 ( s s0 )] + ( µ 0 µ 00 ) x (5) The frst term of the formula descrbes the physcal exergy, whch s the maxmum useful work that can be extracted from the stream when brought to equlbrum wth the envronment. The second part, the chemcal exergy, s the maxmum avalable work that can be extracted from the stream when brought from the envronmental state (denoted wth the subscrpt 0) to the dead state (denoted wth the subscrpt 00). The chemcal exergy for the fuels used n the ndustral sector was calculated based on ther chemcal composton n Denmark, where applcable. For lqud and sold fuels, the approach by Szargut et al. [18] and for gaseous fuels by Bean et al. [19] was used. The rato of the specfc chemcal exergy e CH to the lower heatng value of the fuel LHV, φ s gven for the dfferent fuels n table 1 and can be calculated wth eq. (6). e CH = ϕ H (6) f f Table 1. Propertes of fuels used n the ndustry sector at reference condtons. Fuel LHV φ (MJ/kg) (-) Refnery Gas 52.00 1.161 LPG 46.00 1.056 Gasolne 43.80 1.071 Fuel Ol 42.70 1.067 Desel 42.70 1.068 Heavy Fuel Ol 40.65 1.066 Petroleum coke 31.40 1.048 Natural Gas 48.03 1.065 Coal 24.23 1.076 Coke 29.30 1.048 Waste 10.50 1.152 Wood Chps 9.30 1.193 Wood Pellets 17.50 1.072 Straw 14.90 1.084 Bogas 19.83 1.041 Bo Ol 36.69 1.114 The exergy assocated wth work s equal to ts energy, whlst the exergy transferred wth heat depends on the heat transfer and dead state temperatures (n ths case, above ambent condtons). Q T E 0 k = (1 ) Q k (7) Tk The dead state condtons are selected as a temperature of 15 C, a pressure of 1.013 bar, and wth the reference chemcal envronment of Szargut. The selecton of the envronmental 5
temperature refers to the average condtons n Denmark and has an mpact on the calculatons of the chemcal energy and exergy of fuels, whch can vary n a range of +/- 0.5 % per gradent of 10 C for the fuels nvestgated n ths study. A varyng dead state temperature n the range of 0 to 25 C showed no sgnfcant mpact on exergy effcences n a sectoral analyss [20]. 2.2.4 Energy and exergy effcency The energy (η ) and exergy (ψ ) effcency of the system s defned below, as the sum of energy or exergy n the product, dvded by the total energy or exergy nput to the system. energy n product η = (8) total energy nput exergy n product ψ = (9) total exergy nput 2.3 Applcaton In the followng, the appled technques are explaned for the case of the ndustral and utlty sectors of Denmark, and the sources of losses and exergy destructon are ponted out. 2.3.1 Industral sector Global approach Fgure 3 shows the overall approach for the determnaton of energy and exergy losses and the exergy destructons for the ndustry sector. For each of the 22 ndustry sectors, the fuel consumpton for all ndvdual process categores s dstrbuted amongst three temperature levels and for each level, the mean process temperature s determned. The process nformaton used to establsh ths dstrbuton and the mean temperatures orgnates from several sources, wth the man ones beng [14, 15, 21, 22, 23]. The energy losses derve from () the converson and transmsson losses, and () the drect use of fuels and electrcty. The frst ones, determned by the Dansh Energy Agency [14,15], take nto account the converson of fuels to a secondary energy carrer, whch s suppled to the processes. Transmsson losses occur prmarly n the bolers and n the steam and hot water dstrbuton systems. The magntude of these losses dffers from sector to sector: t s mpacted by the process type and the share of room heatng wthn the total heatng demand. The heat reected to the envronment (waste heat) has a temperature of up to 260 C, and does not exceed 150 C for about 50% (45% n 2006) of these sources, snce waste heat recovery equpment are nstalled [24,25]. The second type of energy losses result from the drect use of fuels and electrcty n the process and thermal losses of hgh-temperature processes. Examples of these processes are dryng of gravel n drect-fred dryers or meltng of metals n furnaces, where the energy used wthn the sector s drectly utlsed n the process. The effcency for drect process heatng s dependent on the process temperature and s presented n Table 2. The appled effcences are based on Rosen [3] and Dncer et al. [26] but are adusted to Denmark. For temperatures below 120 C, the fuel heatng effcency s 100 %, as ths heat s almost fully suppled by secondary energy carrers for whch the converson and transmsson losses were appled. The values of the waste heat temperatures for the losses n the drect converson and hgh temperature components are based on lterature data [25,27]. 6
Table 2. Energy effcency for heatng wth fuels and electrcty used n the ndustry sector. Drect Heatng Effcency Range Electrcal Fuel ( C) (%) (%) Low < 120 100 100 Medum 120-380 90 85 Hgh > 380 75 70 For electrcty use n machnery and the facltes (excl. process and room heatng), effcences for the converson were taken from [28], assumng large-scale unts wth an average load rate of between 70 to 80 %. The producton of combned heat and power wthn the ndustry s not taken nto account, as data on a sectoral level s not avalable. Energy nput (ndustry, fuels) Fuel and energy converson for heat needs Fuel and energy converson for electrcal needs Fuel nput (for thermal exergy) Exergy factor ϕ > 1 Fuel nput (for thermal energy) Fuel nput (for electrcal energy) Exergy factor ϕ = 1 Fuel nput (for electrcal exergy) Exergy losses Mean temperatures (waste heat) Converson and transmsson losses Mean electrc converson effcency Exergy losses Exergy destructon Mean room temperature Electrc energy dstrbuton faclty Electrc energy dstrbuton process Exergy destructon Mean process temperatures (low, medum, hgh) Thermal energy dstrbuton processes Thermal energy dstrbuton faclty Exergy products (heat to process and faclty) Energy products (heat to process and faclty) Energy products (electrcty to process and faclty) Exergy products (electrcty to process and faclty) Fgure 3. Flow chart of the methodology for the analyss of the ndustral sector wth the fuels and consdered processes. Process heat and room heatng Frst, the thermal energy used for the processes Q p s determned based on the energy dstrbuton for the dfferent processes. The losses for fuel converson are subtracted, and for the drect use of fuels and electrcty, the effcency s defned based on the temperatures as shown n Table 2. 7
The exergy n the product m LHV = Q + f,, ) p,,, temperature Tp and the thermal energy ( Q (10),, L,, E Q p s found wth equaton (11) based on the average process Q p of the product. The exergy losses Q E L are found n the same manner as a functon of the mean waste heat temperature Tw and the thermal energy loss Q L. The rate of exergy destructon E d of each process and fuel s found by subtractng the exergy n the product and losses from the total exergy nto the process. Q T E 0 p = (1 ) Q p (11) T p Q T E 0 L = (1 ) Q L (12) T w E d CH Q Q = e m f E p EL (13) Process and faclty electrc use The use of electrcty n processes and faclty s based on the electrc effcency of the unts η e. The useful work W retreved from the electrc energy n W e can be calculated usng Eq.(14). As work and electrc energy are equal to the exergy of work and electrcty, Eq.(14) also apples to the exergy calculatons. W = η ew e (14) Effcency of each ndustry sector For each sector, the process heatng effcency η pr, h s defned as the rato of the sum of the thermal energy n the products and the total energy nput to the thermal processes n the sector. where: p m f, Q p, η (15) pr, h m f, ( LHV ) = Q, denotes the heat transfer assocated wth the process ; the mass flowrate of the fuel ; ( LHV ) s the lower heatng value of the fuel. 8
Smlar to the energy effcency the exergy effcency for process heatng ψ pr, h s defned as: where: Q p ψ pr, h = m f, Q E p ϕ ( LHV) (16) E denotes the exergy transfer assocated wth heat transfer Q p, of the process ; ϕ s the fuel to exergy rato of the fuel. For the electrc heatng effcency, the sum of heat transfer for the processes s dvded by the electrc work nto the system. For the exergetc electrc heatng effcency, the exergy transfer assocated wth the heat transfer s used. The effcency for the use of mechancal work n the processes s derved wth the followng equaton, where the energy ( η pr, e ) and exergy ( ψ pr, e ) effcency are equal. W η pr, e = ψ = (17) pr, e We, where: W denotes the work of the process ; W e, s the electrcal work nto the process. For the facltes, the effcences are found by analogy to the process effcences, wth the energy ( η fa, h ) and exergy ( ψ fa, h ) effcency for the heatng processes wthn the faclty, as well as for the electrcty use ( η fa, e and ψ fa, e ). 2.3.2 Utlty sector Global approach In Fgure 4 the approach for the analyss of the utlty sector s shown. There are three sources of energy losses, namely converson, transmsson and self-consumpton. When consderng exergy, losses only occur n the form of waste heat from the power plants off-gases. The average temperature of the flue-gases s taken as 150 C [29] and s assumed constant, although t changes n practce wth the fuel used n the combuston process. The waste heat dscharged through the condenser of steam power plants s neglected as t s reected at low to very low temperatures (between 30 and 100 C). Exergy s destroyed n the converson of the fuels to electrcty and dstrct heat, the off-gases from the power plants, and wth the transmsson losses and self-consumpton. The transmsson losses of the dstrct heatng dstrbuton ppes are assumed to be close to the dead state temperature, mplyng that very lttle exergy can be recovered. In the case of electrcty from wnd energy, only the transmsson losses are taken nto account. The mport and export of electrc energy are not consdered n ths study. For each utlty system, the requred fuel nput for the generaton of one unt electrcty and dstrct heat s found. The fuel allocaton, n the case of combned heat and power producton, s done based on the product dstrbuton. The allocaton of the exergy destructon and losses 9
to the fnal exergy products delvered to the ndustry follows the same reasonng, wth a separaton between the destructon and losses. The am of the analyss of the utlty sector s to fnd the emboded energy and exergy loss, as well as the exergy destructon, for electrcty and dstrct heat. Fgure 4. Flow chart of the methodology for the analyss of the utlty sector wth the fuels and consdered processes. Electrcty and dstrct heat from the utlty sector For combned heat and power plants, the energy balance used s as follows: m f, LHV) + We, SC ( = W + Q + Q (18) e DH L The reformulaton of the energy balance s done for the losses smlar to the thermal processes wthn the ndustry. The exergy destructon wthn the power plant s found as the dfference between the products and losses exergy content and the exergy nto the system. By applyng Eq.13 to the losses, the exergy content of them can be found. Emboded energy and exergy effcency The emboded energy ( η ) and exergy ( ψ em pr,h em pr,h ) effcences account for the generaton and transmsson losses assocated wth the producton of electrcty and dstrct heat. They are defned as the sum of exergy or heat contaned n the product, dvded by the sum of the drect energy or exergy nput at the thermal ste and the ndrect nput at the utlty sector for the supply of dstrct heat and power. The effcences can be expressed as follows: Q p, em η = (19) pr, h m f ( LHV) + m, f, n ( LHV) n n 10
ψ em pr, h = m f, Q E p ϕ ( LHV) + m f, n ϕn ( LHV) n n (20) where: s the mass flowrate of the fuel n used to generate electrcty and dstrct heat for the processes; ϕ s the fuel to exergy rato of the fuel n; m f, n n ( LHV ) n s the lower heatng value of the fuel n. Wth the same approach, the effcences for the generaton of work and heat n the facltes can be found. 3. Results The results of the analyss are presented n the followng. Frst, the ndustral ste analyss s shown, followed by the emboded analyss and the quantfcaton of exergy losses. At the end, a comparson of the results wth data from 2006 s performed. Table 3. Total ste energy consumpton of the ndustres consdered n 2012 and 2006 (TJ). No. Industry Process Heatng Machne Drve Faclty 2012 2006 2012 2006 2012 2006 1 Gravel and stone 2,847 3,819 326 283 43 88 2 Refned ol 16,789 17,142 1,020 742 66 46 3 Meat 1,855 1,762 1,094 1,646 904 892 4 Dary products 3,394 3,332 1,298 1,001 776 595 5 Compound feed 1,158 1,460 658 839 221 288 6 Sugar 2,725 3,285 354 172 140 239 7 Other food products 2,403 3,530 999 1,355 444 693 8 Wood 2,706 2,082 585 962 718 1,060 9 Paper 1,770 2,183 559 986 284 373 10 Industral Gasses - - 399 447 60 69 11 Enzymes 1,026 1,191 875 1,028 195 292 12 Other chemcals 520 562 707 654 304 361 13 Pharmaceutcals 1,592 1,208 1,289 1,146 264 1,640 14 Plastc and rubber 897 1,737 965 1,189 913 1,770 15 Pant, soap etc. 3,065 734 1,006 807 353 930 16 Cement 9,116 14,734 1,038 1,703 52 85 17 Brcks 1,310 1,334 119 134 14 15 18 Asphalt 1,343 1,252 96 108 77 69 19 Rockwool 1,666 2,257 293 330 76 72 20 Concrete and brcks 2,273 1,956 275 309 270 281 21 Basc metals 2,187 2,807 386 728 421 780 22 Metal products 1,326 2,132 856 1,129 1,490 2,769 11
In Table 3, the total energy consumpton of the ndustral sectors s shown for the analysed years 2006 and 2012. The two ndustres wth the greatest process heatng demand are the ol refneres and the producton of cement. Despte the general trend that for most ndustres the energy nput decreased between 2006 and 2012, some sectors such as the wood ndustry have an ncrease. Ths s partly a result of producton changes and of a dfferent sectoral dstrbuton by Statstcs Denmark (.e. sector 15). In total, the energy consumpton was reduced by 16% between 2006 and 2012. Ste analyss of the ndustral sector The energy and exergy effcences for all thermal processes occurrng n the ndustral sectors n 2012 are shown n Fgures 5 and 6, respectvely. For heatng processes n the facltes, hgh energy effcences are acheved, where ndustres usng electrc and dstrct heat reach the hghest ones. In exergy terms, the effcency s the lowest for room heatng because of the low product temperatures. Fgure 5. Energy effcences for thermal heatng n processes and facltes wthn the ndustry. For the thermal use of energy wthn ndustral processes, energy effcences above 70% are found for all sectors. Sectors wth hgh-temperature operatons and the drect use of fuels for processes,.e. sectors wthn metal and buldng materal producton, have the lowest effcences. For those sectors, hgh exergy effcences are found, as the hgh temperature operatons ncrease the exergy content n the products. Only sector 20 has a comparable low exergy effcency, as t ncludes the producton of concrete elements and gypsum plates, where thermal energy s requred at lower temperatures. The overall exergy effcences range from 10 to 55% for thermal processes, excludng sector 10 (ndustral gases), where no thermal processes occur n the producton. The comparson of the energy and exergy effcences for process heatng shows that exergy can be more useful. The example of room heatng suggests that the process s already close to ts optmum, as very hgh energy effcences, between 85 and 100%, are retreved. However, the very low exergy effcency of room heatng, below 10 % for most ndustres, reveals that consderable mprovement potentals exst. Hgher exergy effcences can be acheved by usng low exergy sources for low temperature heatng processes. Ths could be for nstance dstrct heat or heat recovered from hgh temperature processes. Wth these measures not only the room heatng, but also the processes can be desgned more effcently. 12
Fgure 6. Exergy effcences for thermal heatng n processes and facltes wthn the ndustry. Emboded effcency of the ndustral sector The exergetc effcences, ncludng losses of dstrct heat and electrcty occurrng at the central power statons and durng transmsson, are shown for the total thermal and electrc use n Fgure 7. A comparson of the total ste and the total emboded exergy effcency s done n Fgure 8, where all heatng and mechancal processes are ncluded. The emboded exergy effcency for electrc processes s nearly constant over all the sectors, as t s a drect functon of the electrc energy effcency. However, the thermal exergy effcency s decreased for several ndustres consderably. For the metal processng ndustres, whch had the hghest thermal ste effcences, the emboded one s consderably reduced. Wthn the food and chemcal ndustry, no consderable reductons are found as most of the thermal energy orgnates from natural gas and other fuels. Fgure 7. Emboded exergy effcences for thermal and electrc processes and faclty wthn the ndustry sector (2012). 13
Fgure 8 shows a comparson of the total ste and total emboded exergy effcency, takng nto account all heatng and electrc processes. The producton of ndustral gases has the hghest ste effcency but the emboded effcency s only half, as ths ndustry uses prmarly electrc energy. Smlar dfferences n the effcency are found for food and metal ndustry, where the producton reles on electrcty and dstrct heat. In contrary, ndustres such as ol refnery, sugar, cement and brck producton have only small dfferences n the ste and emboded effcency. By usng the emboded exergy effcency and thereby extendng the system boundares, t s possble to account for all the losses occurrng n the ndustry. The emboded exergy effcency s an mportant ndcator for a system analyss. Also on a ste level, the emboded exergy can be used to determne the most optmal energy source for the producton. For some ndustres, e.g. producton of ndustral gasses, the possble actons are lmted as there s no alternatve to the use of electrcty n the processes. Fgure 8. Total ste and emboded exergy effcences for exergy use n processes and facltes wthn the dfferent ndustry sector (2012). Exergy loss and destructon The analyss of exergy loss and destructon shows the recovery potentals n the ndustres. Ths s possble as the exergy content of the stream descbes the maxmal work whch can be retreved. Fgure 9 presents the share of exergy loss and destructon of the total ste exergy nput for the thermal converson n the ndustry. The producton of buldng materals has the largest potental, wth the exergy loss beng up to 10% of the total thermal nput. But also n the food, wood, paper and chemcal ndustry potentals of above 5% are found. In Fgure 10 the exergy loss and emboded exergy loss for each ndustry s shown, for the thermal processes and machne drves. The ndustres wth the hghest energy nput, also have the hghest exergy loss on ste. However ndustres wth a hgh electrc energy consumpton, almost reach the same total emboded exergy loss, such as the producton of meat and dary products (sector 3 and 4). 14
Fgure 9. Dstrbuton of exergy for process and faclty heatng wthn the ndustry sector (2012). In total, approxmately 3,800 TJ of exergy are lost from thermal processes wthn the ndustry and an addtonal 200 TJ n the supply of room heatng. The producton of cement and the refnery of ol have together an accumulated exergy loss of 1,600 TJ from thermal processes. In these ndustres possbltes of more process ntegraton and the export of heat should be consdered, by mplementng heat recovery systems. For most ndustres, the maorty of the exergy loss s emboded n the electrcty use n machnes. Only the producton of metal and rubber (ndustry no. 14) has a consderable emboded exergy loss for thermal processes, due to the use of electrcty for heatng. Fgure 10. Exergy loss dvded by source for the dfferent ndustral sectors (2012). 15
LPG (399) Pet. Coke (7,035) Refnery Gas (18,182) Ol (7,516) Natural Gas (21,311) Bomass (5,898) Coal & Coke (6,070) Waste (2,586) Bofuels (291) Wnd (757) Fuel for Process Heat (63,316) Process Heat Industry (65,819) Gravel & Stone (3,058) Buldng Materal (16,762) Ol Refnery (18,921) Food (11,685) Wood & Paper (4,849) Chemcal (6,947) Fuel for Utlty (6,729) Metal (3,628) Dstrct Heat (426) Electrcty (2,079) Fgure 11. Exergy flows for process heatng wthn the ndustral sector n TJ (2012). Product (20,519) Destroyed (44,590) Loss (4,912) 16
The overall exergy flows for thermal processes n the ndustry are shown n Fgure 11 and confrm the prevous fndngs. Only a small fracton of the total exergy destructon results from the utlty sector. The maorty of the lost exergy orgnates from the producton of buldng materal and ol. In total, an exergy loss for thermal processes of almost 5,000 TJ s found when ncludng the emboded losses. The emboded losses can be reduced by ncreasng the share of wnd energy and the producton of dstrct heat. The exergy losses, as found n ths secton, descrbe the potental of explotng the energy assocated wth the streams currently dscharged nto the envronment. These losses can be reduced by further process ntegraton and waste heat recovery. For example, the mplementaton of heat pumps and organc Rankne cycles would result n the converson of low-temperature heat nto dstrct heatng and electrcty. Comparson of 2006 and 2012 A comparson of the change n effcency between 2006 and 2012 for the thermal ndustral processes shows that the sectors had dfferent developments. However, not all sectors are drectly comparable between these years, because of changes n the allocaton of ndustres. For the ndustres n meat, dary product and sugar producton a clear mprovement n both exergy and energy effcency can be found. Small mprovements can be found for ol refnery, gravel and stone processng, paper and metal producton. For these ndustres, the datasets are comparable and allow a drect comparson. Fgure 12. Absolute change n the ste energy, ste exergy and emboded exergy effcency for thermal use n the ndustry between 2006 and 2012. Consderng the overall effcences for the Dansh ndustry as a whole, a clear mprovement can be found from the frst law analyss for almost all effcences, as can be seen n Table 4. For the exergy analyss, the effcency of the thermal processes has decreased, whereby the total exergy effcency has ncreased slghtly. Ths ncrease s a result of the mproved use of electrcty n the facltes, whch has a strong weght on the result due to ts hgh exergetc value. 17
Table 4. Total ndustry effcency of the Dansh ndustral sector for 2012 and 2006. Effcency Ste Exergy Emboded Exergy Ste Energy Emboded Energy 2012 2006 2012 2006 2012 2006 2012 2006 Thermal Processes 31.2% 32.6% 29.3% 30.6% 80.3% 78.8% 77.8% 75.8% Thermal Faclty 3.6% 3.5% 2.7% 2.7% 90.9% 90.3% 78.1% 73.2% Electrc Processes 81.4% 81.6% 34.0% 32.4% 81.4% 81.6% 57.2% 47.3% Electrc Faclty 64.3% 60.3% 27.2% 23.9% 64.3% 60.3% 45.2% 34.9% Total 39.7% 39.7% 29.6% 28.8% 80.6% 79.7% 71.8% 66.4% 4. Dscusson Ths sectoral analyss s subect to some uncertantes n the used data and appled method, whch are dscussed n the followng. The dstrbuton of the fuels amongst the categores s based on the Dansh Energy Agency [14, 15], where detaled nformaton of the energy consumptons of the man companes of each sector was used. Where no nformaton was avalable, processes representng the sector and assumptons were undertaken. These dstrbutons are representatve for homogenous ndustry sectors, but for sectors such as (7.) other food products and (12.) other chemcals, assumptons and generalsatons had to be made. The same apples for the process temperatures and ther dstrbuton. In partcular, for the producton of pharmaceutcal products, enzymes and other chemcals, nsuffcent nformaton was present to create a precse end-use model. The mplcatons of the resultng uncertantes are small for the energy effcences, as the process temperatures n these ndustres are manly below 125 C, for whch the drect heatng effcency was chosen to be between 85 and 100 %. The exergy effcency however, s related to the process temperature and changes wth a varyng fuel dstrbuton amongst the process temperatures. For the most crtcal sectors, the temperatures are nevertheless n a smlar range of 50 to 125 C and do not nclude any hgh temperature processes. The data of 2006 and 2012 s not drectly comparable for all sectors and some assumptons had to be made. Danmark Statstk has reorgansed the ndustry classfcaton n 2008, and, as a result, some ndustres were allocated to new sectors. Furthermore, structural changes wthn n some sectors and dfferent economc developments were not taken nto account. The producton of combned heat and power wthn the ndustry s neglected n ths study, as nsuffcent data s avalable. The calculaton of the exergy losses s nonetheless not mpacted by these lmtatons, as the basc data does not nclude the fuels for heat and power producton on the ndustral ste. For the emboded energy n electrcty and dstrct heat, the allocaton of prmary energy was based on the product dstrbuton. As n the case of the frst law analyss, the value of the products s dentcal, the emboded fuel consumpton n the product s the same. For exergy, the allocaton of the nput to the utlty sector was dstrbuted based on the exergy content of the products. Ths results n a hgher allocaton of the nput to the electrcty producton, than n the energy analyss. However, as more exergy s destroyed n the producton of dstrct heat, the specfc exergy destructon per unt of exergy s hgher for dstrct heatng. The total process heatng effcency for the Dansh ndustry s n the same range as for other countres, amongst others Iran [12], Saud Araba [26] and South Afrca [30], where exergetc process heatng effcences of around 30% were found. The energy effcency for both process heatng and the total ste are however hgher n ths study, compared to values between 50% and 70% n the other studes. Ths s prmarly a result of the hgher drect process heatng 18
effcences chosen n ths study. The same apples on a sectoral level, where for comparable ndustres smlar exergetc effcences are found but hgher ones for energy. 5. Concluson Ths paper analyses the energy and exergy effcency, as well as the destroyed and lost exergy, of 22 ndustral sectors n Denmark for the years 2006 and 2012. By usng the dstrbuton of fuels and temperature levels for dfferent processes wthn the sectors, a detaled end-use model for the thermal energy use for ndvdual ndustres s created. The utlty sector s ncluded n a further approach to fnd the emboded exergy and energy flows, for electrcty and dstrct heat suppled to the ndustry. The share of lost exergy found n the thermal processes wthn the ndustry suggests that there are large potentals for waste heat recovery. The lost exergy from the central producton of heat and power s consderable hgher than the losses on-ste, as the use of electrc energy for machnes s ncluded n the losses. In 2012 for ndvdual ndustres, the thermal process effcences range from 12 to 56%, where ndustres wth hgh temperature processes such as cement and metal producton acheve the hghest effcences. The energy effcency s between 63 and 90%, the less effcent ndustres are charactersed by hgh-temperature processes, and the most effcent ones are namely the food, paper and chemcal ndustry. On an ndustry level, the total exergy effcency s approxmately 40% wth the emboded exergy beng around 10 % ponts lower. A comparson of the years 2006 and 2012 shows no remarkable mprovements on an exergetc level, but the energy effcency s consderably mproved. It s suggested that future actons towards energy effcency measures n the ndustry, target the hgh temperature processes, where large quanttes of energy are recoverable. Furthermore, the use of dstrct heat and heat pumps for low temperature processes would mprove the ste effcences. Although the share of dstrct heat and heat pumps has ncreased between 2006 and 2012, the mprovement s not notable n the total effcency. To reduce the emboded losses, contnuous efforts should be made to avod electrc heatng f the electrcty orgnates from other sources than wnd power. Moreover, ths paper gves a bass for future analyses of the ndustral sectors, and the applcaton of the method s descrbed n detals. Future work n ths area should be drected towards the mprovements of the exergy models for machne drves,.e. coolng, where also large quanttes of surplus heat and thus exergy losses can be found. NOMENCLATURE η energy effcency µ chemcal potental φ exergy to fuel rato ψ exergy effcency E exergy rate e specfc exergy H energy of stream of matter h specfc enthalpy LHV lower heatng value m mass flow rate Q s T W x heat rate specfc entropy temperature work rate mass fracton 19
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