OPET CHP/ · PDF fileOPET CHP/DHC Work Package 3 ... of a steam boiler and steam turbine, b) ... – zmanjšanje toplotnih izgub omrežja in s tem stroškov preskrbe s toploto,

European Commission (Directorate-General for Energy and Transport)Contract no. NNE5/2002/52: OPET CHP/DH Cluster

Project Title:

OPET CHP/DHC

Work Package 3Conversion to Biomass CHP and District heating

Deliverable No. 3.j.

TWO ARTICLES ON BIOMASS CHP/DHC INSLOVENIA

May 2004

The project "OPET CHP/DH Cluster" has obtained financial support from the European Commission(Directorate-General for Energy and Transport) under the contract no. NNE5/2002/52 forCommunity Activities in the Field of the specific programme for RTD and demonstration on "Energy,Environment and Sustainable Development - Part B: Energy programme"

The responsibility for the content on this publication lies solely with the authors. The content doesnot necessarily represent the opinion of the European Community and the Community is notresponsible for any use that might be made of data appearing herein.

“2 Articles on Biomass CHP/DHC in Slovenia”

Author(s): Stane MeršeOrganisation: Jozef Stefan Institute - Energy Efficiency Centre (EEC)Address: Jamova 39, SI-1000 Ljubljana, SloveniaTel.: +386 (0)1 588 52 10Fax: +386 (0)1 561 23 35E-mail: [email protected]: http://www.rcp.ijs.si/~eec/

mailto:[email protected]

http://www.rcp.ijs.si/~eec/

OPET CHP/DHC • Work Package 3 • 2 Articles on Biomass CHP/DHC in Slovenia Page 3 of 26

Contents

1. ARCTICLE 1: ŽELEZNIKI BIOMASS DISTRICT HEATING SYSTEM DEVELOPMENT

WITH COMBINE HEAT AND POWER ................................................................................................ 4

2. ARCTICLE 2: EXERGY ANALYSIS OF BIOMASS UTILISATION WITH

EXAMPLE OF BIOMASS COGENERATION IN SLOVENIA............................................................ 16


1. ARCTICLE 1: ŽELEZNIKI BIOMASS DISTRICT HEATING SYSTEMDEVELOPMENT WITH COMBINE HEAT AND POWER

Authors: -Stane Merše (JSI)

-Mihael G. Tomšič (JSI)

-Damir Staničič (JSI*)

*- Currently employed within the Agency for Efficient Use of Energy (GEF project)

Publication:

Collection of papers at 7th Expert meeting of Slovenian Society for District Heating,Portorož, March 14th-16th 2004.

Abstract:

Owners of an industrial and district heating plant in Železniki (Slovenia) faced adevelopment decision when the existing installations approached the end of their usefullifetime. In any case they had to increase the capacity of heat supply from the presentpeak power of 16 MWt to 22.4 MWt in the year 2009, but they were in doubt whether togo for co-generation of electric power. Based on technically proven alternatives, thechoice was between construction:

a) of a steam boiler and steam turbine,

b) of a steam boiler and steam motor, or

c) of a hot water boiler without cogeneration.

By technical and economic criteria option a) was found to be the best solution. Thearticle also deals with environmental issues of electricity production from biomass fromthe point of view of development of new technologies and their market penetration.

Article (Slovenian Language)

-See next page.


RAZVOJ SISTEMA DALJINSKEGA OGREVANJA NA LESNO BIOMASO VŽELEZNIKIH S SOPROIZVODNJO ELEKTRIČNE ENERGIJE

mag. Damir Staničić*, mag. Stane Merše, dr. Mihael G. Tomšič

Inštitut “Jožef Stefan” – Center za energetsko učinkovitost

* sedaj sodelavec Agencije RS za učinkovito rabo in obnovljive vire energije (projekt GEF)

POVZETEK

Lastniki industrijske in krajevne toplarne v Železnikih so se ob izteku uporabneživljenjske dobe nekaterih naprav soočili z razvojno odločitvijo, ki je v vseh primerihpredvidevala nadaljnji razvoj daljinskega ogrevanja, od sedanje priključne močiodjemalcev 16 MWt na 22,4 MWt leta 2009, vprašanje pa je bilo ali je smotrna tudisoproizvodnja električne energije. V okviru tehnično že preizkušenih rešitev je bilazastavljena izbira med: a) postavitvijo parnega kotla in parne turbine, b) parnega kotla inparnega motorja ali c) postavitvijo novega vročevodnega kotla brez soproizvodnjeelektrične energije. Tehnična in ekonomska analiza je pokazala prednost variante a). Vprispevku so predstavljeni tudi okoljski vplivi proizvedene električne energije in nekaterividiki soproizvodnje iz biomase z zrelišča razvoja novih tehnologij in njihovega tržnegaprodora.

Ključne besede: daljinsko ogrevanje, biomasa, soproizvodnja.

ŽELEZNIKI WOOD BIOMASS DISTRICT HEATING SYSTEM DEVELOPMENT WITHCOMBINED HEAT AND POWER PRODUCTION

ABSTRACT

Owners of an industrial and district heating plant in Železniki (Slovenia) faced adevelopment decision when the existing installations approached the end of their usefullifetime. In any case they had to increase the capacity of heat supply from the presentpeak power of 16 MWt to 22.4 MWt in the year 2009, but they were in doubt whether togo for co-generation of electric power. Remaining with technically proven alternatives,the choice was between: a) construction of a steam boiler and steam turbine, b) steamboiler and steam motor of c) hot water boiler without cogeneration. Option a) was foundto be the best solution by technical and economic criteria. The report addresses alsoenvironmental issues of electricity production from biomass from the point of view ofdevelopment of new technologies and their market penetration.


Key words: district heating, biomass, cogeneration.


1. UVOD

Toplarna Železniki že vrsto let uspešno izvaja program toplifikacije mesta Železniki,učinkovite energetske oskrbe ter zmanjševanja emisij snovi v zrak. V zadnjih letih sesooča z pomembnimi razvojnimi izzivi in vprašanji, ki jih postavljajo potrebe po prenovi,posodobitvi in razširitvi obstoječega omrežja daljinskega ogrevanja s priključevanjemnovih toplotnih odjemalcev. Za zagotavljanje dolgoročno zanesljive, cenene in okoljuprijazne proizvodnje in distribucije daljinske toplote na lesno biomaso je potrebnozagotoviti nove ekološko sprejemljive proizvodne kapacitete, saj se delu obstoječihvročevodnih kotlov izteka življenjska doba, njihove emisije pa presegajo z zakonomdovoljene vrednosti za emisije snovi v zrak iz velikih kurilnih naprav, povečanje toplotnihkapacitet pa zahteva tudi širitev omrežja oziroma naraščanje odjema toplote. Z novimiviri toplote želi Toplarna Železniki znižati stroške pridobivanja toplote z manjšo porabolesnih ostankov ter s popolnoma avtomatiziranim načinom obratovanja. Racionalizacijain zniževanje stroškov obratovanja je pogoj za ohranitev konkurenčnost daljinskegaogrevanja pred ostalimi individualnimi viri ogrevanja ter s tem sedanji interesodjemalcev za priklop na omrežje.

Poleg toplote je možna tudi soproizvodnja električne energije. Klasična tehnološkarešitev, kurjenje biomase v parnem kotlu in proizvodnja dela oziroma električne energijes parnim turbinskim agregatom ali parnim motorjem je še vedno edina v praksipreverjena možnost. Šibka stran te rešitve je majhen delež soproizvedene električneenergije v primerjavi s toploto in razmeroma visok investicijski strošek, posledično pamajhna ekonomska konkurenčnost tako proizvedene električne energije. V zadnjemčasu pa je v Sloveniji, tako kot v številnih drugih državah, bila uvedena prednostnaodkupna cena za električno energijo proizvedeno iz obnovljivih virov in soproizvodnje("zelena elektrika") [2]. Prednosti pri odkupu "zelene" električne energije niso tolikšne,da bi v vsakem primeru zagotavljale gospodarnost naložbe, zato je potrebna dovoljpodrobna predhodna ocena projekta.

Na ta vprašanja je skušala odgovoriti tudi »Študija izvedljivosti kogeneracije na lesnobiomaso v toplarni Železniki«[1]1, ki jo želimo kratko predstaviti v tem prispevku.

2. IZHODIŠČA NAČRTOVANJA

Glavna vprašanja, na katera je želela študija odgovoriti so bila:

– izhodiščne energetske potrebe (sedanja raba in napoved bodoče rabe toplote zširitvijo omrežja),

1 Študijo izvedljivosti je v letu 2002 izdelal Center za energetsko učinkovitost na Institutu »Jožef Stefan« obsofinanciranju Ministrstva za okolje in prostor (MOPE) oz. Agencije RS za učinkovito rabo energije (AURE).


– določitev najprimernejše vrste in velikosti novih proizvodnih kapacitet(soproizvodnje, kotlov) in njihov način obratovanja,

– ekonomski učinki novega postrojenja,

– vključitev novih naprav v obstoječi toplotni in elektroenergetski sistem,

– vpliv postavitve in obratovanja novega postrojenja na okolje.

Opis obstoječega stanja

Kotlovnica Toplarne Železnike oskrbuje z daljinsko toploto tehnološke in ogrevalnesisteme v tovarnah Alples, Domel in Niko ter ogrevalne sisteme neindustrijskihporabnikov (stanovanja, in ostali objekti). V primarnem krogu se uporablja vroča voda130/80°C, v sekundarnem pa topla voda 90/70°C.

V novi kotlovnici, ki je bila zgrajena leta 1979, sta vgrajena dva vročevodna kotla. Prvikotel je bil vgrajen leta 1979 in ima nazivno kapaciteto 10 MWt. Kurjen je lahko na lesnobiomaso in mazut. Drugi kotel je bil vgrajen leta 1998 in ima nazivno kapaciteto 6 MWt.Kurjen je lahko le na lesno biomaso.

Oba kotla obratujeta izključno na lesno biomaso oziroma predvsem na lesne ostanke.Kotel 10 MWt je star že 23 let in njegove emisije CO presegajo mejne vrednosti, tako daje potreben zamenjave z novim. Temu je treba prišteti še potrebo po razbremenitvinadzornega osebja z avtomatskim načinom obratovanja kurilnih naprav ter željo poracionalni rabi energije in znižanju stroškov.

V stari kotlovnici je instaliran (l. 1970) še vročevodni kotel na lesno biomaso in mazutkapacitete 5,81 MWt. Kotel je dotrajan, že vrsto let ni v obratovanju in nima ustrezneregulacije za obvladovanje zgorevanja v kurišču.

Skupna razpoložljiva nazivna toplotna moč na pragu nove kotlovnice torej znaša 16MWt toplote, dejansko razpoložljiva toplotna moč pa se spreminja v odvisnosti odkvalitete kuriva, predvsem pa v odvisnosti od vlažnosti različne lesne biomase, ki znaša10 – 60%.

Toplarna Železniki razpolaga z ustrezno velikimi kapacitetami za sprejem inshranjevanje sipkih in kosovnih lesnih ostankov2. Dimni plini se čistijo v elektrofiltrih terse s pomočjo ventilatorjev odvajajo preko dimnika višine 50 m v atmosfero.

Priključna moč porabnikov daljinske toplote za ogrevanje in pripravo sanitarne toplevode znaša 16,8 MWt. Načrtovano kratkoročno povečanje priključne moči porabnikovdaljinske toplote do leta 2009 znaša 4,42 MWt. Tako naj bi celotna priključna močtoplotnih porabnikov leta 2009 znašala 22,4 MWt. Povprečna letna poraba toplote v

2 Betonski silos 375 m3 (suhi lesni ostanki), kovinski silos 662 m3 (suhi lesni ostanki), kovinski silos 662 m3 (mešanilesni ostanki), kovinski silos 3000 m3 (suhi lesni ostanki) ter lopa za kosovne lesne ostanke 1.500 m3.


obdobju 1998 – 2001 je znašala 14,5 GWh (toplotna moč na pragu kotlovnice niha medpribližno 10,4 MWt in 3,2 MWt). Skupna dolžina primarnih (magistralnih) in sekundarnih(razdelilnih) vodov je 4.400 m..

Analiza možnosti znižanja porabe energije

Pred načrtovanjem novih proizvodnih kapacitet je bila izdelana podrobnejša analizasedanjega stanja in predlagane možnosti in ukrepi za znižanje porabe in stroškov zaenergijo. Poleg potrebne zamenjave in racionalizacije proizvodnih enot inorganizacijskih ukrepov je bil velik možni potencial ocenjen zlasti pri:

1. Zmanjšanju izgub omrežja

Za zmanjšanje izgub omrežja je potrebno izboljšati kvaliteto izolacije toplotnega omrežjain zmanjšati puščanje z nadaljevanjem prenove omrežja, ter z nadziranjem vročevodovin toplovodov z ustrezno merilno opremo. Tako bi bilo smotrno vgraditi ustrezenračunalniško podprt sistem spremljanja proizvodnje toplote v kotlovnici (po posameznihenotah ter na pragu kotlovnice), ki bi omogočal spremljanje učinkovitosti proizvodnjetoplote s posameznim kotlom ter spremljanje učinkovitosti distribucije toplote.

Analiza tehnoloških porabnikov toplote kaže na dejstvo, da bi bilo mogoče z ustreznimiukrepi na strani industrijskih porabnikov znižati temperaturni nivo v primarnem deluomrežja s 130°C na 100°C ali še nižje. Prednosti znižanega temperaturnega nivoja vsistemu daljinskega ogrevanja so predvsem:

– zmanjšanje toplotnih izgub omrežja in s tem stroškov preskrbe s toploto,

– možnost uporabe cenejših predhodno izoliranih cevi pri sanaciji omrežja,

– manjši temperaturni raztezki cevovodov/utrujenost materiala cevovodov,

– možnost za povečani obseg proizvodnje električne energije – obratovanjeturbine pri nižjem protitlaku,

– možnost uporabe in izkoriščanja atmosferskega hranilnika toplote zapovečanje učinkovitosti soproizvodnje in kotlovskih enot ter možnostoptimiranja potreb po vršni toplotni moči (shranjevanje toplote v hranilniku včasu manjše porabe toplote pri enakomerni proizvodnji električne energije teroddajanje toplote v omrežje v času koničnih obremenitev in izogibanje potrebipo vklopu vršnih kotlovskih enot).

Okvirno lahko rečemo, da znašajo zmanjšanje toplotnih izgub približno 1 do 1,5% zavsako 1 C znižano temperaturo omrežja. Znižanje temperature na dovodu je vednovečje kot znižanje temperature povratka, kar ima za posledico manjše povečanje


pretoka tople vode, ki pa v praksi ne predstavlja večjega povečanja rabe električneenergije za pogon črpalk.

2. Uvedbi centralnega vodenja sistema daljinskega ogrevanja in izboljšanje regulacijekotlovnice

Na osnovi analize urne dinamike proizvedene toplote na pragu kotlovnice (Slika 1 zateden v decembru leta 2001), izdelane na osnovi zapiskov grafičnega registratorjaproizvedene toplote, je mogoče sklepati, da so nihanja urne porabe toplote zelo visoka,kar nakazuje na potrebo centralnega vodenja sistema daljinskega ogrevanja terizboljšano regulacijo v kotlovnici. Ugotovitve so bile preverjene tudi s kasnejšimikontrolnimi računalniško podprtimi meritvami, ki so potrdile zelo neenakomernoobratovanje kotlovskih naprav.

Simulacija dinamike urne porabe toplote

Ker v sistemu ni bilo razpoložljivih podatkov o urnih obremenitvah je bila za leto 2000narejena simulacija celotne dinamike povprečne urne porabe toplote. Simulacija je bilanarejena na osnovi poteka urne dinamike zunanje temperature v letu 2000, projektnemoči toplotnih porabnikov (16,8 MWt), mesečnih podatkov o prodaji toplote ter porabilesne biomase, vzorčnimi dnevnimi podatki o porabi toplote, idr.

Pri simulaciji bodoče dinamike porabe toplote3, Slika 2, je bila upoštevana priključitevdodatnih porabnikov s priključno močjo 2,2 MWt (naselje Dašnica).

3 Iz urejenega diagrama porabe toplote je mogoče ugotoviti, da je povprečna urna poraba toplote v pasu v časukurilne sezone zelo nizka v primerjavi z inštalirano toplotno močjo kotlov in znaša le 3,2 MWt. Urejeni diagramporabe toplote, ki je osnova za dimenzioniranje soproizvodnje, ima tipičen potek s sezonsko izraženo konico(maksimalno 8,12 MWt) značilno za ogrevalno sezono. Povečanje priključne moči porabnikov toplote na lokacijiDašnica, nominalno 2,2 MWt, dejansko pa 0,9 MWt, če upoštevamo obstoječi faktor istočasnosti obremenitvesistema daljinskega ogrevanja, vpliva na povečanje porabe toplote na 18 MWht (v povprečju za 24%).


poraba toplote - december 2001 (9.12.2001)

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Slika 1: Urna poraba toplote na pragu kotlovnice dne 9.12.2001 (00 – 24 h)

poraba toplote z Dašnico Q=18.046,40 MWh

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Slika 2: Urejen diagram porabe toplote z upoštevanjem dodatne priključne moči

Variante razvoja

Na podlagi analiziranih potreb po toploti v sistemu daljinskega ogrevanja in uporabilesne biomase kot primarni energent so bile identificirane naslednje možne varianterazvoja:


1. Dve varianti soproizvodnje4:

– varianta I: vgradnja kotla na lesno biomaso 10 MWt (13,2 t/h, 36 bar, 450°C) inparne turbine električne moči 663 kWe s soproizvodnjo toplote 4.917 kWt ter4.346 kWt brez soproizvodnje (toplota oddana na toplotnih izmenjevalcihpara/vroča voda), skupaj 9.263 kWt toplote,

– varianta II: vgradnja kotla na lesno biomaso 10 MWt (14,1 t/h, 30 bar, 355°C) inbrezoljnega parnega motorja električne moči 700 kWe s soproizvodnjo paremoči 4.433 kWt ter 4.741 kWt brez soproizvodnje (toplota oddana na toplotnihizmenjevalcih para/vroča voda), skupaj 9.174 kWt toplote.

2. Vgradnja vročevodnega kotla na lesno biomaso 10 MWt

3. OPTIMIZACIJA IN EKONOMSKO VREDNOTENJE IZBRANIH VARIANT

Za vse izbrane možne variante razvoja sistema je bila izdelana tehno-ekonomskaoptimizacija obratovanja, ki je omogočila preizkus, ovrednotenje in primerjavoposameznih variant. Z izdelanim matematičnim modelom proizvodnih enot ternačrtovane porabe toplote sistema je bila izdelana simulacija urnega obratovanja(8760 ur) z upoštevanjem vseh glavnih vplivnih faktorjev obratovanja (cene goriva,toplote, električne energije, stroški vzdrževanja, idr.)5

Rezultati optimizacije obratovanja

Analiziranih je bilo več režimov obratovanja soproizvodnje (1. glede na potrebe potoploti; 2. pri polni moči) v obdobju kurilne sezone in preko celega leta ter več variantdimenzioniranja kapacitet za prisilno hlajenje odpadne toplote6.

Primer optimiranega obratovanja sistema v tednu v januarju prikazuje Slika 3. Toplotnakapaciteta parne turbine omogoča pokrivanje le okrog 50% potreb po toploti v časunižjih zimskih temperatur, zato v času vršnih obremenitev parni kotel obratuje pri polnimoči (toplotna izmenjevalca), potreben pa je tudi zagon vročevodnega kotla. Pri

4 Pri dimenzioniranju soproizvodnje smo izhajali iz urejenega diagrama porabe toplote, kjer je razmejitvena mejapovprečne porabe toplote v kurilni sezoni pri 3,2 MWt. Upoštevajoč razmerje med električno in toplotno močjoPel/Q=0,2 ter povprečno porabo toplote v kurilni sezoni smo se odločili za preučitev soproizvodnje z električnomočjo med 600 in 700 kWe z možnostjo zasilnega hlajenja omrežne vode (4,5 MWt) oziroma oddajanje toplote vzrak v času premajhne porabe toplote in ekonomsko upravičenega obratovanja soproizvodnje.5 Z iterativno simulacijo obratovanja celotnega sistema pri različnih vplivnih vhodnih parametrih se lahko dovoljdobro približamo iskanemu optimumu, ki je maksimiranje prihodkov obratovanja.6 Čeprav je ekonomsko učinkovito obratovanje soproizvodnje preko celega leta (ugodne odkupne cene električneenergije pri nizkih stroških lesnih ostankov) z visokim odstotkom odpadne toplote smo pri končni izbrani variantikonzervativno upoštevali le obratovanje soproizvodnje v obsegu 5000 ur letno, potrebe industrijske toplote vobdobju junij – september pa se pokrivajo z obratovanjem manjšega vročevodnega kotla.


zmernih zimskih temperaturah zadostuje obratovanje parnega kotla, saj se vršnepotrebe po toploti poleg izhodna toplote iz parne turbine pokrivajo s proizvodnjo vtoplotnih izmenjevalcih (v nočnem času je nekaj odpadne toplote). Kapaciteta prisilnegahlajenja (4,5 MWt) omogoča obratovanje parne turbine pri polni moči tudi pri višjihtemperaturah v času prehodnega obdobja kurilne sezone, minimalno zmanjšanje močije potrebno le ob koncu tedna, ko ni industrijskega toplotnega odjema.

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Slika 3: Obratovanje parne turbine: mrzel teden v januarju

PK Proizvodnja pare v parnem kotlu za potrebe parne turbine ter toplotnihizmenjevalcih (para/vroča voda) za pokrivanje vršnih obremenitev

PKtizm Proizvodnja vroče vode v toplotnem izmenjevalcu parnega kotlaVK Proizvodnja toplote v vročevodnem kotluQspteVH Vhodna moč pare za parno turbinoQspteIZH Koristna izraba izhodne toplote parne turbine za daljinsko ogrevanjeQdo Skupna poraba toplote v sistemu daljinskega ogrevanjaQodp Odpadna toplota soproizvodnje odvedena s sistemom prisilnega hlajenjaPel Proizvodnja električne energijeTz Zunanja temperatura

Z vidika obratovanja je parni motor nekoliko ugodnejši, saj zaradi nižje izhodne toplotnemoči motorja (nekoliko ugodnejše - višje razmerje električna energija/toplota), lahkoobratuje pri polni moči tudi pri višjih temperaturah, nekoliko pa je manjši tudi obsegodpadne toplote. Nižja izhodna toplotna moč zahteva višjo moč izmenjevalnikov toplotepri parnem kotlu pri vršnih obremenitvah sistema, obratovanje vršnega vročevodnegakotla pa je praktično enako kot pri parni turbini.

V primeru proizvodnje toplote le z vročevodnimi kotli je simulacija pokazala, da novi10 MWt kotel pokriva predvsem višje dnevne obremenitve v času nižjih temperatur,


obstoječi manjši vročevodni kotel pa zadošča za celotno proizvodnjo toplote pri višjihtemperaturah, v hladnejšem obdobju pa obratuje le ponoči.

Glede na ekonomske pokazatelje posameznih variant7 je bila kot najbolj primernatehnologija za izvedbo soproizvodnje v Toplarni Železniki izbrana parna turbina, ki vprimerjavi s parnim motorjem, za izbrani obratovalni režim, izkazuje boljše ekonomskeučinke. Boljši učinki parne turbine so posledica nižjih investicijskih stroškov ter stroškovvzdrževanja. Dodatni kriterij, ki prispeva k izbiri parne turbine, je nižja intenzivnostiposluževanja in vzdrževanja.

Ugodne cene za odkup električne energije [2] omogočajo ekonomsko prednostinvesticije v soproizvodnje pred investicijo v nov vročevodni kotel.

Podrobnejša ekonomska analiza in finančne ocene izbranega projekta je pokazalauspešnost naložbe v soproizvodnjo toplote in električne energije (545 mioSIT). Internastopnja donosnosti projekta znaša 17,3%. Doba vračanja vloženih sredstev je 6,2 let inje nekajkrat krajša od življenjske dobe projekta. Neto sedanja vrednost znaša pri 8%diskontni stopnji 354 mioSIT. Iz ekonomske analize izhaja, da je ugoden rezultatinvesticije predvsem rezultat povečanja prihodkov s prodajo električne energije, ki jihzagotavljajo cene za odkup električne energije od kvalificiranih proizvajalcev [2]. Analizaobčutljivosti je pokazala, da je uspešnost projekta najbolj odvisna predvsem od višineinvesticijskih stroškov.

Pozitivni ekološki učinek izvedbe investicije v soproizvodnjo je tudi ocenjeno zmanjšanjeemisij CO2 za 2.415 tCO2/leto.

4. REALIZACIJA IN TRENUTNO STANJE V TOPLARNI ŽELEZNIKI

V Toplarni Železniki poteka načrtovana širitev omrežja, preučeni pa so bili tudi predlogiza učinkovitejše delovanje sistema. Tako se trenutno že izvajajo aktivnosti za znižanjetemperaturnega nivoja sistema, kar bo pomembno vplivalo tudi na samo realizacijosoproizvodnje, za katero je bila v tem času že sprejeta odločitev o izvedbi (parnaturbina). Z predhodno optimizacijo sistema se nakazujejo možnosti nakupa manjšegakotla, ki bi obratoval pri nižjih parametrih pare, kar bi precej znižalo stroške investicije,povečalo možno proizvodnjo električne energije ter zmanjšalo obseg potrebnih dovoljenjza obratovanje.

7 Preprosti vračilni rok investicije v soproizvodnjo se giblje med 4 in 5 let in je nekoliko krajši pri parni turbini vprimerjavi s parnim motorjem.


5. ZAKLJUČKI

Učinkovitega delovanja sistemov daljinskega ogrevanja tako z vidika ekonomike kotemisij toplogrednih plinov brez soproizvodnje električne energije in toplote si je vprihodnje težko predstavljati. Tudi v primeru uporabe obnovljivih virov, kjer je vsaj gledetoplogrednih plinov situacija drugačna, soproizvodnja povečuje dodano vrednost kot tudiokoljske učinke izrabe obnovljivih virov (npr. lesne biomase). Analiza možnosti inodločitev za soproizvodnjo v Toplarni Železniki pomeni nov razvojni cikel sistemadaljinskega ogrevanja v Železnikih, ki bo omogočal ekonomičnost delovanja in širokosprejemljivost daljinskega ogrevanja med uporabniki tudi v prihodnje. Pri pripravirazvojnih projektov (nove proizvodnje kapacitete) je potrebno dovolj pozornosti namenititudi optimizaciji in ukrepom za učinkovitejše delovanje celotnega sistema, saj je urejenoizhodiščno stanje pogoj za uspešno načrtovanje in dimenzioniranje novih razvojnihrešitev.

REFERENCE

[1] Študija izvedljivosti kogeneracije na lesno biomaso v Toplarni Železniki, IJS,Center za energetsko učinkovitost, februar 2002.

[2] Sklep o cenah in premijah za odkup električne energije od kvalificiranihproizvajalcev električne energije, Ur.l. RS, 25/02 (nove cene Ur.l. RS, 8/04).


2. ARCTICLE 2: EXERGY ANALYSIS OF BIOMASS UTILISATIONWITH EXAMPLE OF BIOMASS COGENERATION IN SLOVENIA

Authors: -Stane Merše (JSI)

-Damir Staničič (JSI)

-Vincenc Butala (Faculty of Mechanical E. – University of Ljubljana)

-Uroš Stritih (Faculty of Mechanical E. – University of Ljubljana)

-G. Zupanc (Faculty of Mechanical E. – University of Ljubljana)

Publication:

Article presented and published* at 2nd World Biomass Conference, Rome, April 2004.

*-Official publication not yet available.

Article:

-See next page.


EXERGY ANALYSIS OF BIOMASS UTILISATION WITH ANEXAMPLE OF BIOMASS COGENERATION IN SLOVENIA

V. Butala*, D. Staničić**, U. Stritih*, S. Merše**, G. Zupan**University of Ljubljana, Faculty of Mechanical Engineering, Aškerčeva 6, 1000 Ljubljana, Slovenia

**Institute "Jožef Stefan", Energy Efficiency Centre, Jamova 39, 1000 Ljubljana, Slovenia**Tel: +386-1-4771-421, Fax: +386-1-2518-567, E-mail: [email protected]

It is known that combined heat and power generation (CHP) is a more efficient method of transforming theinternal energy of fuel than separate production. Since, in Europe, as in Slovenia, biomass is being ever moreused as a source of energy, the question of combined heat and power generation (CHP) must also beconsidered in this context. An energy and exergy analysis of the conversion of biomass into thermal andelectrical energy is given herein. In the second part of the article, the possibility of using combined heat andpower generation (CHP) at the Železniki Heating Plant in Slovenia is presented, as they already employremote heating using biomass. This study also shows the advantages of and savings from CHP comparedwith the production of thermal energy alone.

Key words: Combined heat and power generation (CHP), biomass conversion, district heating

1 INTRODUCTION

The energy supply for dwellings and industry is usually ensured by buyingelectrical energy from the public electrical energy system and by purchasing fuels fromfuel suppliers to provide one's own heating. Recently, an alternative method of coveringenergy needs has appeared called combined heat and power generation (CHP), orcogeneration for short. Combined heat and power generation has already proven itsreliability and technical efficiency, however its economic justification is dependent onmovements in energy prices and available energy sources. Under current conditions,the prices of electrical energy and fuels make combined heat and power generation oneof the most efficient and economical means of lowering energy costs [1].

With the rise in the prices of fossil fuels on world markets, increased environmentalconsciousness, and the introduction of new fuel combustion technologies, woodbiomass is becoming an important means of acquiring both thermal and electricalenergy. There are a number of arguments in favor of the use of wood biomass, forexample: wood biomass is a renewable source of energy, it is an unexploited andaccessible domestic source of energy, it does not use sulfur, etc. The exploitation ofwood biomass allows us to the simultaneously care for the forest, to contribute toequilibrating the CO2 balance and thus to decreasing the greenhouse effect, and toencourage the more ecologically friendly short-distance transportation of fuel. Theadded value derived from this type of fuel production would stay within the country,while a domestic source of energy represents a buffer to worldwide energy crises. Thisenergy producing potential has been badly or even improperly exploited up to now,despite the fact that it can contribute positively to primary energy sources and theenergy independence of individual countries, including Slovenia.


2 THE CONCEPTS OF EXERGY AND ANERGY

According to the first law of thermodynamics, nothing can be destroyed and nothing cananything be made from nothing. Energies can only change from one form into anotherand there are balanced equations for these transformations. However, what theseequations do not say is whether such a process is even possible. That is covered by thesecond law of thermodynamics which considers the direction in which thermodynamicprocesses proceed. In adiabatic systems, the only possible processes are those thatare not connected with decreasing the entropy of the adiabatic system. Therefore, thesecond law of thermodynamics limits the transformation of energy and states that not allenergy can be transformed into all other forms of energy.

The concepts of exergy and anergy:

Exergy is energy that can under given conditions transform completely into everyother form of energy.

Anergy is energy that cannot be transformed into exergy.

Limited transformations of energy are made up of a part which is transformable and apart which is untransformable. Therefore, it generally holds that energy is made up ofexergy and anergy. One of the two parts may also be equal to zero.

3 THE EXERGY OF BIOMASS

Heat released during combustion, reduced to the mass of the fuel, is called the superiorheat of combustion and is the difference in enthalpies of the participating materials priorto combustion h1 and after combustion h2. When combusting biomass we get twovalues, which are dependent on whether the humidity in fuel condenses or remains inthe gas phase. If the humidity in the fuel completely condenses we speak of superiorheat of combustion Hs, but if it doesn't condense then we speak of inferior heat ofcombustion Hi. The difference between the two is in the heat of condensation of watervapor:

x

s iH H w r= + ⋅ ,

where wx is water as the product of combustion remaining from 1 kg of fuel, and r is theheat of vaporization of water at the temperature of the surroundings (r ≈ 2.5 MJ/kg).

The exergy that would be available if the oxidation of combustible substances in thebiomass were reversible is called the exergy of biomass. This can be calculated usingthe same equation as for the calculation of the exergy of the internal energy of thematerial [2]:

( )1 2 1 2b ok

e h h T s s= − − ⋅ − ,


where h1 and s1 are the values for the enthalpy and entropy component of the fuel andthe oxygen prior to combustion at the temperature and pressure of the surroundings(Tok, pok), while h2 and s2 are the enthalpy and entropy products of combustion at Tokand pok. Since the materials are changed chemically during combustion, the calculationmust use absolute entropies.

The exergy of biomass only slightly differs from the inferior heat of combustion ofcombustible substances Hig, which is calculated using the following equation:

ig iH H r w= + ⋅ ,

where r is the heat of vaporization and w is the humidity of the raw fuel. Therefore, thespecific exergy of biomass is:

b ige H= .

Since the heat of vaporization is used up we have to use a negative value for the heatof vaporization r. Thus, the inferior heat of combustion of a combustible substance Hig isequal to the inferior heat of combustion of biomass Hi only in the case where the fuel isabsolutely dry. For all remaining values for the moisture content of the fuel, theflammability of the combustible substances is less than the inferior heat of combustionof biomass for the heat of vaporization of water.

4 LOSS OF EXERGY DURING COMBUSTION OF BIOMASS

In all irreversible processes, exergy changes into anergy. Since the reverse procedureis impossible, we denote the transformed part of exergy into anergy in irreversibleprocesses as loss of exergy. Thus, we can say that the loss of exergy is acharacteristic of irreversible processes and that it refers quantitatively to thermodynamiclosses due to irreversibility. Since losses of exergy due to the irreversibility of ourprocesses are actually lost work, it is important to recognize the reasons for theirreversibility and thus the reasons for the loss of exergy and to know how to calculatethe magnitude of these losses.

The losses of exergy due to the irreversibility of combustion are defined by the followingequation:

zg g tee e e∆ = − ,

where ete is the exergy of that quantity of the products of combustion which originatedfrom the unit for the quantity of fuel, calculated at the theoretical temperature ofcombustion. The exergetic yield from combustion is:

te

zg

g

e

eζ = .


The losses from exergy during combustion are considerable. The available exergy fromthe products of combustion is of course less than ete, because the exergy found in theproducts of combustion at the entry into the chimney must be subtracted out.

5 COMBINED HEAT AND POWER GENERATION (CHP)

By using combined heat and power generation, the same fuel can simultaneouslyproduce electrical and thermal energy, thus allowing for great savings in primary energyand lowering the costs of supplying energy without necessitating a change in productionprocesses. The purpose of building in a system of combined heat and power generationis to cover users needs for electrical energy and heat, although the needs for both formsof energy are only rarely ever able to be completely covered. Therefore, a cogenerationsystem is usually built in parallel with conventional energy installations such thatelectrical energy can be bought from the grid if there is a shortage and any surplusproduced by the grid can be utilized. (Figure 1) [3].

Figure 1: Comparison of energy supplies

6 EXERGY ANALYSIS OF COGENERATION

The following part seeks to show why the combined production of heat and electricalenergy is a suitable alternative to separate production [4].A remote heating system requires a defined amount of heat Qd at a temperature Td(Figure 2). In the T, s diagram the heat is shown as the area of the rectangle (1, 2, 3, 4).

GRID

BOILER

USER

ELECTRICAL ENERGY

FUEL

CONVENTIONAL INSTALLATION

GRID USER

FUEL COGENERATION INSTALLATION

BUY / SELL

ELECTRICAL ENERGY

HEAT

ADDITIONAL FUEL

COMBINED HEAT AND POWER GENERATION

BOILER


Figure 2: Example of Remote Heating.

Heat is available at temperatures Tpz (e.g. average temperature of the products ofcombustion), while the temperature of the surroundings is Tok. The simplest means ofremote heating is the direct removal of heat from the heat source and its transfer to amedium of remote heating. In this manner we use just as much heat energy Qpz fromthe source temperature Tpz, as is conveyed to the remote heating system:

pz dQ Q= .

In Figure 2 Qpz is equal to the area of the rectangle (7, 8, 9, 10), which is also equal tothe area (1, 2, 3, 4). From the energetic point of view there is no loss, however from theexergetic the situation is different. The heat Qpz has an exergy of Epz, which is equal tothe area of the rectangle (7, 8, 11, 12); however, the heat for the remote heating Qd hasan exergy of Ed, which is equal to the area of the rectangle (1, 2, 5, 6).

( )pz pz ok

pz

pz

Q T TE

T

⋅ −=

( )d d ok

d

d

Q T TE

T

⋅ −=

Since Qd = Qpz and Td < Tpz, then Ed < Epz. By this process, exergy decreases. Thereason for the loss of exergy is the irreversible conversion of heat from a source withinthe system at the final, and usually, large temperature difference Tpz – Td.

Let us examine an example of combined heat and power generation (Figure 3).

Figure 3: Combined heat and power generation.

The area of the rectangle (1, 2, 3, 4) represents the process of remote heating. Inaddition to this, heating process (7, 8, 9, 10) is performed at temperatures between Tpz

s

TTpz

Td

Tok

1 2

3 4

5 6

9 10

8 7

11 12

s

T

1 2

56

34

7 8

910

11 12

13 14 Tok Td

Tpz


and Tok. The width of this heating process is equal to the width of remote heatingprocess:

s9 – s10 = s3 – s4

The heating process removes energy from the heat source, which is equal to the area(7, 8, 9, 10) and has an exergy of (7, 8, 11, 12). By this process we remove energy (12,11, 9, 10) which has no exergy. One part of the exergy of this process will be used forthe remote energy process, while the remaining energy will be available to use asdesired. Exergy will be used for remote heating (13, 14, 11, 12), while the remainingexergy is equal to the area (7, 8, 14, 13). For the process of remote heating the exergyused (13, 14, 11, 12) is now in the diagram in the same place that as the exergy of theheat for remote heating (1, 2, 5, 6) – at equal temperatures and entropy differences.Heat with exergy (12, 11, 9, 10) can be directly transferred to the process of remoteheating.

The process of remote heating is thus carried out directly using energy (13, 14, 9, 10),which is equal to (1, 2, 3, 4). This is a combined process which yields work or electricalenergy and heat. Energy enters this process (7, 8, 9, 10) with an exergy of (7, 8, 11,12). For producing electrical energy we use an exergy of (7, 8, 14, 13), while for heatingthe exergy is (13, 14, 11, 12). Using this process, from high initial parameters we obtainelectrical energy with low initial parameters which is then used for remote heating.

7 DEVELOPMENT OF REMOTE HEATING IN ŽELEZNIKI USING WOOD BIOMASSWITH COGENERATION OF ELECTRICAL ENERGY

Železniki is a town which lies in northwestern Slovenia and has 4179 inhabitants. TheŽelezniki Heating Plant has had a heating program for the town of Železniki for anumber of years which has efficiently supplied power and lowered air emissions. In thepast few years, they have had to confront important development challenges andissues, such as the necessity to renovate, modernize, and widen the existing remoteheating grid due to an increase in new heating customers. However, in addition toheating, there is the possibility of cogeneration of electrical energy. The classicaltechnological solution, burning biomass in a steam boiler and producing work orelectrical energy with a steam turbine aggregate or steam motor is still the only tried andtested method of cogeneration in practice. The weak side of this solution is the smallpercentage of cogenerated electrical energy produced compared with heat and therelatively high investment costs, resulting in low economic competitiveness of electricalenergy produced in this way. Therefore, a feasibility study of combined heat and powergeneration was carried out and is presented [5].


7.1 Development Variations

On the basis of the analyzed needs for heat in a remote heating system and the use ofwood biomass as the primary energy source, the following were identified as possibledevelopment variations:

a) Two cogeneration variations:

-Variation I: built-in wood biomass boiler of 10 MWt (13,2 t/h, 36 bar, 450°C) and steamturbine with an electrical power of 663 kWe, with cogeneration of heat of 4.917 kWt or4.346 kWt without cogeneration (heat given out via heat exchangers as steam/hotwater), for a total of 9.263 kWt of heat.

-Variation II: built-in wood biomass boiler of 10 MWt (14,1 t/h, 30 bar, 355°C) and oil-free steam motor with an electrical power of 700 kWe with cogeneration of steam powerof 4.433 kWt or 4.741 kWt without cogeneration (heat given out via heat exchangers assteam/hot water), for a total of 9.174 kWt of heat.

b) Built-in wood biomass hot water boiler of 10 MWt.

7.2 Results of optimizing operations

Various regimens for cogeneration operations were analyzed (dependent on the needfor heat and full power) during the heating season and over the entire year, includingsome variations in the dimensions of the capacity for forced cooling of waste heat.

An example of the optimized operating system for one week in January is shown inFigure 4. The heat capacity of the steam turbine allows us to cover only around 50% ofheating needs during low winter temperatures, therefore at peak demand times thesteam boiler works at full power (heat exchanger), but the hot water boiler also has tobe used. At moderate winter temperatures, only the steam boiler need be used sincethe peak demand for heat, besides the emission of heat from the steam turbine, arecovered by production from the heat exchangers (there is some waste heat produced atnight). The capacity for forced cooling (4.5 MWt) allows the steam turbine to operate atfull power even at higher temperatures at the beginning and end of the heating season,although a minimal decrease in power is necessary at the end of the week when thereis no industrial heat requirement.


0123456789

10111213141516

1 7 13 19 25 31 37 43 49 55 61 67 73 79 85 91 97 103

109

115

121

127

133

139

145

151

157

163

Ure

Moc

[MW

]

-60-55-50-45-40-35-30-25-20-15-10-505101520

Zuna

nja

tem

pera

tura

[°C

]

PK PKtizm VK QspteIZH Qdo Qodp Pel TZ

Figure 4. Steam turbine operations: a cold week in January.

PK Production of steam in a steam boiler for a steam turbine PKtizm Production of hot water in the heat exchanger of a steam boiler

VK Production of heat in a hot water boiler

QspteIZH Beneficial exploitation of heat emission from a steam turbine for remote heating

Qdo Total use of heat in a remote heating system

Qodp Waste heat of cogeneration out of a forced cooling system

Pel Electrical Energy Production

Tz Outside temperature

From the point of view of operations, the steam motor is somewhat more favorable,since, due to the low output of the heating power of the motor (somewhat morefavorable - higher proportion of energy/heating), it can operate at full power even athigher temperatures and produces a somewhat lower amount of waste heat. The loweroutput of the heating power demands higher powered heat exchangers in the steamboiler at peak demand times on the system, while the operation of the hot water boilerat peak times is practically the same as for the steam turbine.

When heat is produced only by a hot water boiler the simulation showed that the new10 MWt boiler covers all of the higher daily demands at low temperatures, while theexisting smaller hot water boiler is enough for total heat production needs at highertemperatures - at colder temperatures it only operates at night.

Looking at the variations from an economic point of view, the most suitable technologyfor introduction into the Železniki Heat Plant was the steam turbine, which was the mosteconomically efficient in comparison with the steam motor for the selected operatingregimen. The higher efficiency of the steam turbine is the result of lower investmentcosts and lower upkeep costs. An additional criterion which led us to choose the steamturbine was the lower level of servicing and upkeep required.

The favorable prices for repurchasing electrical energy [5] means that there is aneconomic advantage to investing in cogeneration compared to investing in a new hotwater boiler.


A more detailed economic analysis and financial evaluation of the selected project hasshown the success of investing in combined heat and power generation (545 mio SIT).The internal level of profitability is 17.3%. The time period for the return on investment is6.2 years and is a few times shorter than the lifetime of the project. The net currentvalue is 354 mio SIT at an 8% amortization rate. From the economic analysis we canconclude that the favorable results from this kind of investment are mostly due to anincrease in income from the sale of electrical energy, which is ensured by the set priceof repurchasing electrical energy from qualified producers [6]. The analysis has shownthat the success of the project is mostly dependent on the level of investment costs.

A positive ecological outcome of investment in cogeneration is the estimated decreasein CO2 emissions by 2.415 tCO2/year.

8 CONCLUSION

Energy with exergy is necessary for all processes, including heating and the like.Energy without exergy is not useful. Exergy for heating can be pumped from the desiredsource and in the desired form, e.g. as electrical energy from a hydroelectric powerstation or from heating processes between the temperature of the products ofcombustion Tpz and the temperature of the environment Tok. With the help of thisexergy we acquire from the environment as much "worthless" internal energy as we stillneed for heating, and we raise it to a suitable temperature Td. With exergy we canmake this "worthless" energy into something. During heating and other similarprocesses, the measure of the value of energy is its exergy. In today's production ofelectrical energy only exergy is taken into consideration. Anergy is lost in this process.From both an economic and an emission of greenhouse gases point of view, theefficient functioning of a remote heating system without combined heat and powergeneration will be difficult to justify in the future. Even with the use of renewable sourcesof energy, where the greenhouse gas emissions issue is different, cogeneration willincrease both added value and the ecological effectiveness of using renewableresources (e.g. wood biomass) Analyzing the possibilities and making the decision touse cogeneration in the Železniki Heating Plant means that there is a new developmentcycle for the remote heating system in place in Železniki which will be more economicaland thus promote wider acceptance of remote heating among users in the future. Inpreparing development projects such as new energy production capacities, measures tooptimize and improve the effectiveness of the overall system must be stronglyencouraged, since a clear vision is a condition for the successful planning and shapingof new development solutions.


9 LITERATURE

[1] Femopet Slovenija: Pregled sistemov soproizvodnje toplote in električne energije zizbranimi primeri iz Evrope, Ljubljana 1998

[2] Miran Oprešnik: Termodinamika, Ljubljana 1987.

[3] FEMOPET Slovenija: Pregled sistemov soproizvodnje toplotne in električne energijez izbranimi primeri iz Evrope, Ljubljana 1998.

[4] Zoran Rant: Vrednost in obračunavanje energije, Strojniški vestnik, Ljubljana 1955.

[5] Študija izvedljivosti kogeneracije na lesno biomaso v Toplarni Železniki, IJS, Centerza energetsko učinkovitost, februar 2002.

[6] Sklep o cenah in premijah za odkup električne energije od kvalificiranihproizvajalcev električne energije, Ur.l. RS, 8/04.

Documents

OPET CHP/ · PDF fileOPET CHP/DHC Work Package 3 ... of a steam boiler and steam turbine, b) ... – zmanjšanje toplotnih izgub omrežja in s tem stroškov preskrbe s toploto,