Energetic and Exergetic Analysis in a Firewood Boiler

Transcription

1 Energetc and Exergetc Analyss n a Frewood Boler Análse Energétca e Exergétca de uma Caldera a Lenha RUBENS ALVES DIAS Unesp (Guaratnguetá, Brasl) [email protected] JOSÉ ANTÔNIO PERRELLA BALESTIERI Unesp (Guaratnguetá, Brasl) [email protected] ABSTRACT In ths paper, thermodynamcs frst and second laws were appled to a frewood boler n order to evaluate ts performance. Some other physcal parameters, such as the electrc ones, were taken nto account n the development of the analyss for ts performance evaluaton. The procedure based on thermodynamc laws s recommended as a general tool for the analyss of heat systems, amng at reachng some operatonal and desgn nformaton manly qualtatve results by means of the second law analyss, whch helps to estmate the crtcal ponts for a better energy conservaton. The analyss of a frewood boler n a small pasta factory s presented to llustrate the use of exergetc modelng. Keywords ENERGY CONSERVATION EXERGETIC ANALYSIS PERFORMANCE EVALUATION. RESUMO Neste artgo, a prmera e a segunda le da termodnâmca foram aplcadas a uma caldera a lenha para avalar seu desempenho. Alguns outros parâmetros físcos e elétrcos foram consderados para o desenvolvmento da análse para a avalação de desempenho. O procedmento baseado nas les da termodnâmca é recomendado como uma ferramenta geral para a análse de sstemas térmcos, vsando obter nformações operaconas e de projeto, em especal resultados qualtatvos, por meo da análse a partr da segunda le, que auxla na estmatva dos pontos crítcos para um melhor aprovetamento da energa. A análse de uma caldera a lenha de uma pequena fábrca de massas é apresentada para lustrar a potencaldade do uso da modelagem exergétca. Palavras-chave CONSERVAÇÃO DE ENERGIA ANÁLISE EXERGÉTICA AVALIAÇÃO DE DESEMPENHO. REVISTA DE CIÊNCIA & TECNOLOGIA V. 12, Nº 23 pp

2 NOMENCLATURE a specfc exergy [J/kg] c p specfc heat at constant pressure [J/kg K] CV control volume h specfc enthalpy [J/kg] HHV hgher heatng value [J/kg] İ rreversblty [W] LHV lower heatng value [J/kg] ṁ mass flow [kg/s] P power [W] Q. heat flow [W] s specfc entropy [J/kg K] T temperature [K] W. mechancal power [W] Underwrtten: eg exhaust gas el electrc nput mec mechanc o output SG steam generator; boler 0 reference condton (25 o C and 101kPa) Greek words: δ effcency defect (2 nd Law of Thermodynamcs) ε exergetc effcency (2 nd Law of Thermodynamcs) η energetc effcency (1 st Law of Thermodynamcs) ψ ratonal effcency (2 nd Law of Thermodynamcs) INTRODUCTION T he performance of an energy system, from the thermodynamc vewpont, can be evaluated by the frst (energy) and second (exergy) laws; the energy balance provdes a quanttatve nterpretaton of the thermodynamc analyss, whle the exergetc balance s assocated to qualtatve nformaton, descrbng the system n ts crtcal ponts by the rreversbltes (losses) occurred n the process. Frst and Second Thermodynamcs Law effcences can be useful for a decson makng process: ther results llustrate, respectvely, operatonal and desgn condtons for a thermal system under analyss. The values obtaned for energetc and exergetc effcency, as well as the evaluaton of the rreversbltes presented by the system, serve as a reference when comparng dfferent technologes accordng to the capacty of convertng the nput energy nto some useful forms of energy, and are ndcatve of some possble adjustment and modfcaton necessary to warranty a thermal system performance mprovement. In ths paper the frewood boler of a pasta factory s analyzed n ts energetc and exergetc effcency. Irreversbltes are calculated for the fresde and watersde streams, to dentfy ther behavor accordng to the varaton of the watersde stream temperature profle. EXERGETIC BALANCE For the techncal evaluaton of the proposed system, frst and second thermodynamc law equatons were used; for the control volume of fgure 1, energy conservaton frst law can be wrtten by Eq. (1). v Q 2 Ẇ ṁ h gz v 2 = ṁ h gz 2 2 Fg. 1. Control volume. (1) Ths formulaton lets one knows that the quantty of thermal (heat) and mechancal/electrc energy s transformed nternally nto knetc or potental energy, as well as the pressure energy. It s the bass for the most common effcency evaluaton rate, calculated by Eq. (2). 16 jan./jun. 2004

3 Q η SG SG ṁ ( h h ) = = water Q fuel ṁ fuel LHV fuel (2) For the evaluaton of the losses that occurred nto the steam generatng system, the exergetc balance rate can be appled to the thermal system, accordng to Eq. (3): İ = j Q Tj T 0 j Ẇ + ṁa T ṁa j for j=1,2,3,... (3) that expresses the rreversblty that are assocated to the process when the other parameters are known. Accordng to the evaluaton of rreversbltes and exergetc effcency t s possble to dentfy major losses ncurred n a thermal system to provde further desgn mprovements. The second law effcency may present dfferent defntons; n ths analyss t wll be consdered the followng expressons: the ratonal effcency, showed n Eq. (4) (Kotas, 1985): Ȧ out ṁ out a out ψ İ = = = (4) Ȧ ṁ ṁ n a n ṁ ṁ a ṁ whch represents the rato of the exergy transfer assocated wth the plant output to the exergy transfer rate assocated wth the correspondng exergy nput; for clearly dentfyng the fracton of the nput exergy whch s lost through rreversblty n a mult-component system, the effcency defect for a system of N components s presented n Eq. (5): N İ j j = 1 İ 1 ψ İ İ N = = = δ 1 + δ 2 + δ N (5) Ȧ n Ȧ n Ȧ n Ȧ n for whch the numerator s relatve to the whole plant. by solatng the watersde expresson of exergetc balance nto the numerator, the boler effcency can be stated by Eq. (6) (Moran and Shapro, 1995): ṁ ε water ( a a ) = ṁ wood a wood + ṁ ar a ar ṁ eg a eg (6) that evaluates the exergetc exchanges effcency from a system to another one. In ths case, from fresde to watersde. ENERGETIC AND EXERGETIC ANALYSIS A hot water generatng system of a small pasta factory s descrbed n ts components to llustrate the energetc and exergetc analyss performed n an old frewood boler. The confguraton presented n fgure 2 llustrates the system under analyss, whch s slghtly dfferent from the usually met n the pasta sector because ths one doesn t present a return tank to receve the ndustral process and the reposton water. The water system operates n a closed loop, and a pressure vessel, whch controls the water nlet when the pressure decreases n the man pump sucton, regulates water replacement. For the proposed analyss, an estmaton of the water flow n the boler man pump were necessary because of the lack of an adequate nstrumentaton for that; some data were then measured for an approxmaton of the hot water flow. The followng values were obtaned by drect nspecton of the exstng system: pressure pump nlet = 0.2 MPa pressure pump outlet = 0.35 MPa external dameter of the pump rotor = 210 mm nomnal rotaton = rpm REVISTA DE CIÊNCIA & TECNOLOGIA V. 12, Nº 23 pp

4 Fg. 2. Hot water generatng unt of the pasta factory. Legend: 1. return of hot water 8. fretube boler 2. hot water outlet 9. man pump 3. replacement water nlet 10. pressure vase 4. compressed ar nlet 11. replacement pump 5. fuel nlet 12. Retenton valve 6. ar nlet 13. manual valve 7. combuston gases outlet 14. pneumatc valve From the curve of total head versus flow for man pump (fgure 3), accordng to the manufacturer (Mark Peerless, 1998), the estmated water flow s 34 m 3 /h (9.4 kg/s). However, ths value could be consdered adequate for the desgn pont of the equpment, and the operatonal load may dffer substantally. By means of a wattmeter, the electrc power of the three-phase nducton motor of 15 cv (11 kw) that drves the pump was measured, stayng at a steady state regme whose average value was 3.59 kw. Fg. 3. Curve of total head and power versus flow for man pump 18 jan./jun. 2004

5 From the characterstc curve of the electrc motor (fgure 4), for the average electrc current of 17.4 A measured the electrc motor was operatng wth 75% of effcency, and so the mechanc power was determned accordng to Eq. (7). Consderng the mechanc power (converted nto cv) and the external dameter of the pump rotor, the characterstc curves of ths pump (see fgure 3) was consulted and a flow of approxmately 34 m 3 /h was the one that best ftted the crossng values of the mechancal power demanded and the external rotor dameter of the avalable pump, confrmng that stream flow as the operatonal data for ths analyss. P mec = η el P el = 2.69kW (7) From the collected data, the followng values are avalable for analyss (only LHV of wood was not measured and t was taken the mean value cted n BEESP, 1997): average amount of measured frewood: ṁ wood = kg/s average mass flow of water: ṁ water = 9.4kg/s frewood lower heatng value: LHV wood = kj/kg wth 25% of humdty average values of temperature and pressure accordng to table 1: Tab. 1. Measured values n the pasta factory boler. WATER AIR EXHAUST GASES TEMPERATURE ( O C) INPUT OUTPUT PRESSURE(MPA) TEMPERATURE ( O C) TEMPERATURE ( O C) Tab. 2. Pasta factory enthalpy values of water. TEMPERATURE ( O C) Fg. 4. Characterstc curve of the electrc motor 15cv 220/380V (WEG, 1998). ENTHALPY (KJ/KG) Input Output Source: WEG Table 2 presents the enthalpy of saturated steam and lqud water accordng to the measured values. The frst law boler effcency s calculated usng the Eq. (2) and resulted n η SG = 73,5%. Ths value s consdered adequate for the frewood boler, specally because t s an old and hghly used equpment. For the exergetc analyss, t s necessary to know the chemcal composton of the frewood; the average values n dry condton (Carvalho Junor et al., 1995) were theoretcally ncreased of a certan amount of REVISTA DE CIÊNCIA & TECNOLOGIA V. 12, Nº 23 pp

6 water to consder 25% of humdty because the reference data collected (BEESP, 1997; BEN, 1998) take nto account ths humdty percentage n ther analyss. Such values are showed n Table 3, and the stochometrc condton adopted n the process of frewood combuston (wth 25% of humdty) may be represented by the followng balance equaton: C 3 H 7.3 O O N 2 3CO H 2 O N Tab. 3. Frewood average chemcal composton. COMPOSITION IN MASS (%) CARBON HYDROGEN OXYGEN ASHES Dry % humdty For the specfc heat at constant pressure calculaton of the combuston gases and usng the specfc heat equatons of the carbon doxde, water steam and ntrogen (Perry, Chlton and Krkpatrck, 1963), an expresson can be proposed for representng the varaton of the specfc heat of the combuston gases of the frewood as a functon of the temperature, as shown below: c peg, ( T) = c 44 pco c ph2 O c 28 pn2 18 n whch the numerator expresses the product of the molars fractons of each chemcal substance by the respectve specfc heat n the molar bass, and the denomnator expresses the molecular masses. Consderng the specfc heat equatons of CO 2, H 2 O and N 2 : c peg, ( T) = T T ( T T 2 ) ( T) 28 Reducng the expresson and convertng t to the Internatonal System, Eq. 8 can be obtaned: c ep ( T) T = (8) T T 2 For the exhaust gas temperature measured n the boler stack c p,eg (393) = 1.2 kj/kgk. The specfc heat of the ar s calculated usng the Eq. 9 (Sheh and Fan, 1982) for a local temperature of 30 o C (303 K), resultng: c p, ar ( T) T (9) 10 7 T T = T 4 = ( 303) = kJ/kg.K c p, ar Equaton (10) s used for the exergy calculaton n the work flud, accordng to the C.V. showed n fgure 5 and assumng the temperature T 0 = 298 K (25 o C) as reference state. For the enthalpy and entropy values taken from thermodynamcs tables, h 0 = kj/kg and s 0 = kj/kg K. The portons of the knetc and potental energy were not consdered. Table 4 shows the enthalpy and entropy values of the water used n ths analyss. 20 jan./jun. 2004

7 Fg. 5. Control volume for the boler analyss. Tab. 4. Enthalpy and entropy values of water. TEMPERATURE ( O C) ENTHALPY (KJ/KG) ENTROPY (KJ/KG.K) Input Output In that way, t was obtaned the exergy n the C.V. boundary for water, at nlet and outlet condtons: a = (h h 0 ) T 0 (s s 0 ) a = kj/kg (10) a º = kj/kg The equaton proposed by Sheh and Fan (1982) for calculatng the exergy of a fuel (Eq. 11) s adjusted to the data presented n table 3. a º fuel = (C) (N) (H) (S) (O) (F) (Cl) (Br) (I) ( x s º asches x mº ashes ) (O){ (C) (H) (O) (S) (11) (F) (Cl) (Br) (I)} n whch: the values n parentheses are the percentage n mass of the carbon (C), ntrogen (N), hydrogen (H), sulfur (S), oxygen (O), fluorne (F), chlorne (Cl), bromne (Br) and odne (I), that can compose a fuel: s º asches s the entropy of formaton of the ashes: = 0.84 kj/kg K [8] m ashes s the mass of ashes n one klogram of fuel (kg). Substtutng the values for the frewood wth 25% of humdty: a º wood = (0.36) (0.073) (0.552) + ( x 0.84 x 0.015) 0.63 (0.55) { (0.36) (0.073) (0.552)} a º wood = kj/kg The exergy value of the frewood s very close to ts hgher heatng value (HHV wood = kj/kg wth 25% of humdty (BEESP, 1997)), representng a varaton of 5.2% among these. Kotas (1985) suggests that the rato (a o wood /HHV wood ) should stay between 1.15 and 1.30; n ths analyss, such value s 1.05 and ths dfference s specally attrbuted to the HHV consdered, that n our calculaton was not drectly measured but collected from natonal average data avalable. For the data presented n Table 1 and the calculated values of the specfc heats, Eq. (12) can calculate exhaust gas and ar exergy: REVISTA DE CIÊNCIA & TECNOLOGIA V. 12, Nº 23 pp

8 a eg c peg, ( T eg T 0 ) T 0 In T eg = = 15.0kJ/kg T 0 (12) a ar c p, ar ( T ar T 0 ) T 0 In T ar = = kJ/kg T 0 The mass flows that cross the control volume at fgure 5 s calculated from the molecular masses of the frewood and the present ar n the combuston process (calculated at stochometrc condton), resultng an ar/fuel relaton of The average consumpton of frewood n the furnace was accompaned and estmated to be kg/s, resultng: ṁ ar = 4.33x = 0.055kg/s ṁ eg = ṁ wood + ṁ ar = kg/s The values of boler rreversblty and effcency, for the control volume of fgure 5, are calculated, respectvely, by Eqs. (13) and (6); ths value of rreversblty ncludes the combuston process rreversblty. 0 = ( ṁ wood a wood + ṁ ar a ar + ṁ water a ) ( ṁ eg a eg + ṁ water a ) İ CV İ CV = kW ε = 9.54% The ratonal effcency s calculated by usng Eq. (4): ṁ ψ eg a eg + ṁ water a = = 61.80% ṁ wood a wood + ṁ ar a ar + ṁ water a Fgure 6 shows the prevous CV changed for the rreversblty calculaton of heat transfer components; t dscrmnates the contrbutons of watersde rreversblty, and s desgnated as CV 2. Hence, t s used the exergetc balance agan expressed by the Eq. (14), consderng the porton of the heat transfer at a gven temperature. Here t s nterestng to stand out, n supplement, that such procedure can be extended for the other parts of the equpment under analyss, that the necessary care about the CV locaton and the nstrumentaton that wll be needed for the correct data acquston. Fg. 6. Control volume modfed to consder only water flow. (13) T 0 = T j Q j + ṁ a ṁ a İ CV j T İ CV2 = Q + ṁ T water ( a a ) n whch s Q calculated startng from the balance energy through the Eq. (15). Q = ṁ water ( h h ) = 98.7kW Therefore, the rreversblty nsde of the new control volume t s gven for: 298 İ CV2 = T (14) (15) 22 jan./jun. 2004

9 Table 5 shows how the rreversblty vares nsde the control volume of fgure 6, together wth the combuston process rreversblty (l cv1 = l cv. l cv2 ), taken as a functon of the heat transfer temperature; such procedure was necessary because of the dffculty of settlng the exact temperature for ths process, servng as a range of values n a enlarged spectrum whch meets the sought value. Tab. 5. Irreversblty for the boler n pasta factory. TEMPERATURE IN THE VC 2 IRREVERSIBILITY (KW) O C K İ VC2 İ VC İ VC Note: negatves values despsed because they are unfeasble. An evaluaton of effcency defect s done for the temperature of 100ºC, consdered as close to the real watersde temperature, for the values of Table 5; accordng to Eq. (5), the effcency defect for fre and watersde streams are: İ δ CV CV1 = = = 37.65% ṁ wood a wood + ṁ ar a ar + ṁ water a İ δ CV CV2 = = = 0.55% ṁ wood a wood + ṁ ar a ar + ṁ water a CONCLUSIONS The frst law effcency for the boler under analyss resulted 73,5%; for a comparson, n a study appled to the frewood small bolers (Noguera and Peres, 1996) smlar to the one here presented, t was obtaned a value of 65.6%. The second law boler effcency was 9.54%, and due to ts qualtatve character, ths value should be nterpreted as an ndcatve that the process of obtanng hot water has a consderable rreversblty, beng most of ths located n the combuston process and n the fresde flow nsde the boler; ths nformaton may be of great mportance to the equpment manufacturer to decde how to act to reduce the losses ncurred n each stream flow. Ths fact can be confrmed by the effcency defects calculated for the fre and watersde sectons: the value relatve to the fresde s hgher than the one of watersde, and the summaton of these values s 38.20%, correspondng to the total of boler rreversbltes and the complement of the ratonal effcency, 61.80%. Based on the analyss of Thermodynamcs frst and second laws, three acton fronts may be prescrbed by the Energy Conservaton studes: the frst s destned to the effcency estmaton of components and/or the whole system, and consequently decdng whether or not t s necessary some nterventon; n the second acton, by knowng these results more elements are avalable for decsons about mplementng or expandng an ndustral plant; last, ths analyss can be vewed for the manufacturers as a tool for the performance study of equpment, through the locaton of crtcal ponts, manly by the use of exergetc analyss. REVISTA DE CIÊNCIA & TECNOLOGIA V. 12, Nº 23 pp

10 REFERENCES ACKNOWLEDGEMENT Ths research was supported by Fundação de Amparo à Pesqusa do Estado de São Paulo-FAPESP (Proc. 97/ and 98/ ; Proc. 99/ ). BEESP. São Paulo, Secretara de Estado de Energa. Balanço energétco estadual. São Paulo: CESP Sére Informações energétcas, 1997 (n Portuguese). BEN. Brasl, Mnstéro de Mnas e Energa. Balanço energétco naconal. Brasíla, 1998 (n Portuguese). CARVALHO JÚNIOR, J.A. et al. A tropcal ranforest clearng experment by bomass burnng n the Manaus regon, Atmospherc Envronment, Norwch, 29(17): , KOTAS, T.J. Exergy Method of Thermal Plant Analyss. Essex: Butterworths, MORAN, M.J. & SHAPIRO, H.N. Fundamentals of Engneerng Thermodynamcs. New York: John Wley & Sons, NOGUEIRA, L.A.H. & Peres, C.A. Effcency of small bolers fueled by bomass. In: European Conference on Boenergy, IX, Proceedngs... v.1; 1996: PEERLESS, M. Personal communcaton, PERRY, R.H. et al. Chemcal Engneers Handbook. 4.ª ed. New York: McGraw Hll Book Company, SHIEH, J.H. & FAN, L.T. Estmaton of energy (enthalpy) and exergy (avalablty) contents n structurally complcated materals, Energy Sources, Larame, 6, 1/2: WEG Electronc Catalogue. CD-rom verson 3.0, Dados dos autores RUBENS ALVES DIAS Doutor em engenhara mecânca pela Unesp. Professor assstente da UNESP (Campus de Guaratnguetá) e da Unversdade Metodsta de São Paulo (Campus de Guaratnguetá). JOSÉ ANTÔNIO PERRELLA BALESTIERI Lvre docente em Máqunas Térmcas da Unesp e professor adjunto (MS-5) da Unesp (Campus de Guaratnguetá). Recebmento do artgo: 27/mao/03 Consultora: 22/set./03 a 26/nov./03 Aprovado: 26/nov./03 24 jan./jun. 2004