Introducere.............................. Introducere..................................................... .............................................. .............................................. ...........................................................1 ....................................1 Reglementări privind poluarea datorată autovehiculelor....................... autovehiculelor.............................................. ..........................................5 ...................5 Formarea emisiilor poluante............................ poluante................................................... .............................................. ...................................................... ................................14 .14 Tehnologii active de reducere a emisiilor poluante.............................. poluante..................................................... ...................................... ..................34 ...34 Tehnologii pasive de reducere a emisiilor poluante ........................................................ ..................................................................... .............59 59 Concluzii................................. Concluzii........................................................ .............................................. .............................................. .........................................................83 ..................................83 Bibliografie................................... Bibliografie.......................................................... .............................................. .............................................. .................................................. ............................86 .86
Introducere Înainte de a analiza impactul asupra mediului a autovehiculelor trebuie prezentată istoria autovehiculelor, autovehiculelor, istoria motorului. Primul motor a fost realizat în anul 1698 de către inginerul englez Thomas Saverz, şi era un motor cu abur ce avea ca scop pomparea apei în casele înalte din Londra.
Fig.1 Principiul de funcţionare a motorului cu abur realizat de Thomas Saverz
În anul 1712, inginerul Thomas Newcomen din Cornwall a realizat un motor performant care avea un braţ lung cu ajutorul căruia pompa apa cu o turaţie de 16 rot/min. Acest motor a fost optimizat în anul 1776 de constructorul scoţian James Watt.
1
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F i g.2 Mot or u l l u i Ne wc om en
Fi g.3 Mot o r ul l ui W at t
Primul care a utilizat un motor cu abur pentru propulsia unui vehicul a fost Nicolas Cugnot în anul 1769. Acest vehicul putea transporta patru persoane, dar el a fost utilizat pentru transportul armamentului. Viteza maximă pe care o putea atinge era de 5 km/h.
Fig.4 Vehiculul realizat de Nicolas Cugnot
Pe baza proiectului lui Lenoir în anul 1861 Nikolaus August Otto construieşte primul motor. Iar împreună cu industriaşul german Eugen Langen înfiinţează o companie lângă Koln, unde în anul 1867 construieşte primul motor. Acest motor era unul în doi timpi mult mai eficient decât motorul lui Lenoir deoarece înainte de ardere se realiza o comprimare a amestecului carburant. În anul 1876 cei doi au pus la punct un nou motor, un motor care funcţiona în patru timpi. Acest motor era silenţios şi eficient astfel că şi-a găsit repede locul în industrie, rămânând model pentru cele mai mai moderne motoare cu ardere ardere internă existente existente astăzi în lume.
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Eugene de Rochas. Chiar dacă acest lucru anula patentul lui Otto, motoarele lui au rămas singurele motoare cu ardere internă folosite pe scară largă.
Fig.6 Motorul lui Otto şi Langen Fig.5 Motorul lui Lenoir
În 1890, Wilhelm Maybach şi Gottlieb Daimler, doi dintre inginerii companiei lui Otto, şi-au deschis propria companie producătoare de automobile, propulsate de motoarele în patru timpi ale lui Otto. Ei au perfecţionat vechiul motor şi au reuşit să producă, în 1899, primul automobil Mercedes.
Fig.7 Primul autovehicul Mercedes
Motorul cu aprindere prin comprimare a fost inventat de Rudolf Diesel în 1892, şi a fost patentat la data de 23 februarie 1893. Acest motor a fost conceput ca o alternativă pentru întreprinzătorii particulari având în vedere dimensiunile variabile, costul scăzut al motorului şi carburantului în comparaţie cu celelalte soluţii existente. Lucrul la acest motor a pornit de la obsesia lui Diesel pentru a doua legea a termodinamicii şi maxima eficienţă a ciclului Carnot. Intenţia lui Diesel a fost ca motorul său să utilizeze o varietate largă de combustibili inclusiv praful de cărbune. Diesel şi-a prezentat invenţia funcţionând în 1900 la Expoziţia Universală (World's Fair) utilizând ulei de alune.
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Fig.8 Motorul Diesel
În acele acele vremu vremuri ri nu erau erau proble probleme me legate legate de poluar poluarea ea mediul mediului ui deoare deoarece ce num număru ărull autovehiculelor autovehiculelor era mic şi impactul asupra mediului era neglijabil. În cei peste o sută de ani de existenţă automobilele s-au dezvoltat dezvoltat şi perfecţionat fără însă a se produce modificări fundamentale asupra soluţiilor tehnice utilizate la primele modele. Restricţiile extrem de severe impuse şi cele ce urmează a fi adoptate în primul deceniu al noului mileniu, referitoare la protecţia mediului şi resursele naturale, conduc la transformări esenţi esenţiale ale ale autove autovehic hicule ulelor lor atât atât în conce concepţi pţiaa compon component entelo elorr acesto acestora ra cât şi în privin privinţa ţa consumului şi manevrării. În anul 2000 circulau în lume circa 800 milioane de autovehicule, dintre care 500 mili milioa oane ne erau erau auto autotu turi rism sme, e, iar iar rest restul ul auto autoca cami mioa oane ne,, auto autobu buze ze,, mo moto toci cicl clet etee şi scut scuter ere. e. Aproximativ 35% se aflau în Europa şi America de Nord, iar restul în Asia, America de Sud, Africa. Producţia mondială anuală de este de aproximativ 60 milioane de autovehicule. În Europa aproximativ 14 milioane de autovehicule autovehicule îşi încheie ciclul de viaţă, iar în SUA 10-11 milioane. Creşterea continuă a numărului de autovehicule este strâns legată de creşterea populaţiei globului şi de creşterea economică. Astfel conform previziunilor ONU populaţia globului va creşte până în 2050 la aproximativ 9 miliarde de locuitori, iar până în 2020 populaţia Europei va atinge valoarea de 450 milioane de locuitori. Această creştere nu este uniform distribuită, ea fiind mai accentuată în ţările din Asia, Africa şi America Latină. Ca rezultat al acestei tendinţe este de aşteptat ca numărul autovehiculelor să crească
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Reglementări privind poluarea datorată autovehiculelor Standardele de emisie sunt cerinţele care trebuie să stabilească limite specifice pentru poluanţii care pot fi eliberaţi în mediu. Multe standarde de emisii se axeze pe reglementarea poluanţilor eliberaţi de autovehicule, dar ele pot reglementa şi emisiile provenite din industrie, centrale electrice şi mici echipamente cum ar fi cositoarele pentru gazon sau generatoarele diesel. Politicile frecvente ca alternativă la standardele de emisii sunt standardele tehnologice (care reglementează emisiile de oxizi de azot, oxizi de sulf, pulberi în suspensie sau funingine, monoxid de carbon, hidrocarburi volatile). Un standard de performanţă privind emisiile reprezintă o limită care stabileşte pragurile peste care un alt tip de tehnologie de control al emisiilor ar putea fi necesare. În timp ce standardele de performanţă privind emisiile au fost folosite pentru a dicta limitele de poluanţi convenţionali, cum ar fi oxizii de azot şi oxizi de sulf, această tehnică de reglementare poate fi folosită pentru ţinerea sub control a gazelor cu efect de seră, în special de dioxid de carbon.
În Statele Unite, standardele de emisii sunt gestionate de către Agenţia de protecţie a Mediului (EPA). Statul California are dispensă specială de a promulga standarde mai stricte privind emisiile vehiculelor, iar celelalte state pot alege să urmeze fie standardele naţionale, fie cele emise de statul California. Standard Standardele ele de emisii din California sunt stabilite de către Consiliul Consiliul Rezervelo Rezervelorr de Aer California, cunoscut la nivel local prin acronimul "CARB". Având în vedere că piaţa auto din California este una dintre cele mai mari din lume, CARB are o influenţă mare asupra cerinţelor privind normele de emisii pe care trebuie să le îndeplinească îndeplinească majoritatea autovehiculelor autovehiculelor dacă doresc fie vândute pe această piaţă. În plus, alte câteva state din SUA aleg să urmeze standardele CARB, deci reglementării lor au implicaţii largi în SUA. De asemenea politica CARB are influenţă şi asupra standardelor privind emisiile din UE. Automobilele şi camioanele uşoare (SUV-urile, camionetele, microbuzele) sunt tratate în mod diferit în conformitate cu anumite standarde. California încearcă să reglementeze emisiile gazelor cu efect de seră provenite de la automobile, dar se confruntă confruntă cu probleme din partea partea instanţei guvernului guvernului federal. State încearcă de asemenea să oblige APE să reglementeze emisiile de gaze cu efect de seră, dar se lovesc de refuzul acesteia din anul 2007. În viitorul apropiat se prevede că APE va adopta în mare măsură standardele California privind emisiile de gaze gaze cu efect de seră.
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în suspensie (PM) sunt reglementate pentru majoritatea tipurilor de vehicule. Standarde nu se aplică navelor maritime sau avioanelor. Standardele europene de emisii definesc limitele acceptabile pentru gazele de evacuare produse de autovehiculele autovehiculele noi vândute în statele membre UE. Standardele de emisii sunt definite într-o serie de directive ale Uniunii Europene şi treptat se vor introduce treptat standarde din ce în ce mai stricte. Vehicule neconforme nu pot fi vândute în UE, dar noile standarde nu se aplică la vehiculele aflate deja pe şosele. Standardele de emisii diferă în funcţie de ciclul de testare utilizat: R49 ECE (vechi) şi CES (European Steady Cycle, din 2000). Parlamentul European a sugerat introducerea unor standarde obligatorii de emisii de CO 2 pentru a înlocui angajamentelor actuale de voluntariat de către producătorii auto. La sfârşitul anului 2005, Comisia Europeană a început să lucreze la o propunere pentru o nouă lege de limitare a emisiilor de CO 2 provenite de la autovehicule. Comisia Europeană a primit sprijinul Parlam Parlamen entulu tuluii Europe European an pentr pentruu propun propunere ereaa sa de promo promovar varee a introd introduc uceri eriii pe o piaţă piaţă a vehiculelor curate şi eficiente energetic prin intermediul achiziţiilor publice. În cadrul Uniunii Europene, transportul rutier este responsabil pentru aproximativ 20% din toate emisiile de CO 2, cu autoturismele care contribuie cu aproximativ 12%. Ţinta stabilită prin Protocolul de la Kyoto a fost o reducere cu 8% a emisiilor în toate sectoarele economiei, comparativ cu nivelurile din 1990 până în 2008-2012. Emisiilor de CO2 provenind din transporturi au crescut rapid în ultimii ani, de la 21% în 1990 la 28% în 2004. Emisiile de CO 2 datorate transportului rutier din UE au o pondere de aproximativ 3,5% din emisiile totale de CO2 la nivel mondial. În prezent sunt reglementaţi şapte poluanţi atmosferici: dioxidul de sulf SO 2, particulele în suspensie PM10, plumbul Pb, dioxidul de azot NO 2, ozonul O3, oxidul de carbon CO, benzenul C6H6, şi se poartă tratative pentru reglementarea următorilor poluanţi: hidrocarburi aromatice policiclice HPA, cadmiu Cd, arsenic As, nichel Ni, mercur Hg.
1. Diox Dioxid idul ul de sulf sulf Emisiile de SO2 provin din instalaţii care ard combustibil fosil. O lungă perioadă de timp autovehiculele dotate cu motoare diesel au contribuit la poluare cu SO 2, dar odată cu reducerea cantităţii de sulf din motorină s-a redus şi participaţia autovehiculelor autovehiculelor la poluare. Conţinutul de sulf din motorină e de 0,3…0,1% S, iar prin reglementări se doreşte să se ajungă la 0,05% S. Noile autovehicule, echipate cu sisteme de reducere a poluării, nu mai emit
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la formarea ploilor acide care afectează solul, produc deteriorarea clădirilor şi monumentelor, reduc vizibilitatea atmosferică.
2. Diox Dioxid idul ul de azot azot Mai mult din jumătatea emisiilor de NO 2 sunt datorate traficului rutier, iar restul datorită instalaţiilor industriale şi de încălzire. NO2 are efect negativ asupra sănătăţii umane deoarece afectează căile respiratorii. El se constituie ca precursor al formării ozonului de suprafaţă şi a acidului azotic ce are ca rezultat ploi acide, acidificarea solului.
3. Partic Particulel ulelee în suspensi suspensie,P e,PM10 M10 Particulele în suspensie reprezintă un amestec de particule solide şi picături de lichide care se găsesc în suspensie în atmosferă. O serie dintre ele au dimensiuni mari şi au o culoare închisă: fumul, funinginea, iar unele dintre ele sunt mici şi pot fi detectate doar cu microscopul. Aceste particule reprezintă particulele fine şi au dimensiuni sub 2,5 μm, PM2,5. Ele au ca origine arderea în motor, instalaţii energetice, industriale şi de încălzire. Particulele mai mici de 10 μm, PM10, sunt generate de traficul pe străzi nepavate, la manipularea unor materiale, operaţii de sfărmare şi măcinare, rafale de vânt. Acestea sunt emise direct de la sursă sau rezultă prin interacţiunea cu SO, SO 2, NOx sau compuşi organici volatili cu compuşi aflaţi în aer rezultând particule fine.
4. Ozonul Ozonul de suprafaţă este un poluant secundar care se formează prin reacţiile dintre NO x şi compuşii organici volatili în prezenţa căldurii şi radiaţiei solare. NOx sunt emişi de autovehicule, instalaţii energetice şi alte surse de ardere, iar compuşii volatili sunt emişi de autovehicule, fabrici chimice, rafinării şi alte surse industriale. Nivelul ridicat de O3 se găseşte în aglomeraţiile urbane şi la periferii. Expunerea pe terme termenn lung lung duce duce la schim schimbăr bării irever ireversib sibile ile la nivelu nivelull plămâ plămânil nilor, or, afecte afectează ază fauna fauna şi flora, flora, producţiile agricole scad. scad.
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Benzenul este un constituent de origine al produselor petroliere. El poate apărea în gazele de evacuare ale autovehiculelor chiar dacă nu se găseşte în compoziţia combustibilului utilizat. Are Are un cara caract cter er canc cancer erig igen en pute putern rnic ic,, dist distru ruge ge sist sistem emul ul imun imunita itar, r, neur neurol olog ogic ic şi reproducător. Efectele datorate expunerii la C 6H6 apar după o perioadă mare de timp.
7. Plumbul În trecu trecutt princi principa pala la sursă sursă de polua poluare re cu plumb plumb era reprez reprezent entată ată de autove autovehic hicule ulele le alimentate cu benzină aditivată cu compuşi ai plumbului. Plumbul era emis sub formă de aerosoli şi odată inhalat nu se mai elimină din organism. Se acumulează în sânge, carne, carne, oase şi afectează ficatul, sistemul nervos. Expunerea prelungită la plumb determină apariţia retardului mintal şi a sterilităţii. Concentraţiile medii anuale ale emisiilor de plumb sunt sub 0,5 μg/m3, iar zonele cu concentraţie ridicată sunt cele din apropierea turnătoriilor şi fabricilor ce produc baterii.
8. Ploile ac acide Ploi Ploile le acid acidee repr reprez ezin intă tă căde căderi ri de comp compuş uşii cu cara caract cter er acid acid rezu rezult ltaţ aţii din din reac reacţi ţiil ilee atmosferice ale SO2 şi NOx cu H 2O, O 2 şi compuşi oxidanţi. Aceşti compuşi se formează şi sunt transportaţi la distanţă de locul de formare, ei cad pe sol sau apă sub formă uscată sau umedă.
1.1 Reglem Reglementă entăril rilee Europen Europenee privind privind prot protecţi ecţiaa mediulu mediuluii - Normele Normele EURO EURO Pentru autoturismele şi autoutilitarele uşoare, sub 3,5t, verificarea se face pe standul cu rulouri pe baza ciclului Europa (urban) sau Europa NCCE (urban + interurban). Primele reglementări asupra emisiilor poluante au fost introduse în anul 1970, şi ele se refereau la emisiile de monoxid de carbon, hidrocarburi, oxizi de azot şi particule. În anul 1992 începe reducerea sistematică a poluării prin introducerea normelor EURO. Normele EURO 1 prevedeau tratarea post-combustie prin utilizarea catalizatorilor de oxidare pentru CO şi NO x pentru ciclul de conducere Europa (urban). Au fost introduse la 1 ianuarie 1992 pentru omologarea de tip şi la 1 ianuarie 1993 pentru livrări pe piaţă.
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Se introduc sistemele OBD (sisteme de diagnosticare la bord) şi se utilizează carburanţi superiori. Aceste norme au fost introduse la 1 ianuarie 2000 pentru omologări tip şi la 1 ianuarie 2001 pentru livrarea pe piaţă. Normele EURO 4 au fost fost intr introd odus usee la 1 ianu ianuar arie ie 2005 2005 pent pentru ru om omol olog ogăr ării tip, tip, 1 ianuarie2006 pentru livrarea pe piaţă. Ele prevăd o nouă reducere cu 55-77% a noxelor şi 75% a particulelor, utilizarea carburanţilor carburanţilor superiori. Normele EURO 5 presup presupun un noi reduce reduceri ri ale emisii emisiilor lor poluan poluante, te, în specia speciall asupra asupra emisiilor de particule, introducerea filtrelor pentru particulele ultra fine. Realizarea „vehiculelor prietenoase cu mediul”. Introducerea normelor s-a făcut la data de 1 septembrie 2009 pentru omologări tip şi 1 ianuarie 2010 pentru livrare. În anul 2014 se prevede introducerea unui nou tip de norme, EURO 6, care vor avea prevederi şi mai drastice drastice în privinţa poluării.
Tab.1 Evoluţia privind privind poluanţii a normelor normelor EURO
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Fig.10 Emisiile de NOx şi PM pentru motoarele cu aprindere prin scânteie
1.2 Cicl Ciclur uril ilee de de înc încer erca care re În Europa primul ciclu de testare a fost introdus în anul 1969 la recomandarea Comisiei Economice ONU. Acest ciclu simulează un parcurs prin Paris şi avea următoarele caracteristici: Lungimea 1,013km, viteza medie 18,7km/h, durata 195 secunde, viteza maximă 50km/h, ciclul se parcurge de patru ori.
Fig.11 Ciclul de încercare EUROPA
Ciclul european cu 13 moduri a fost elaborat pentru testarea motoarelor diesel pe stand, un ciclu similar este si pentru SUA.
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mult de 130km/h. Durata fazelor este de 300s, cu faze de mers în gol de 15s. Pentru parcurgerea ciclului se prevăd accelerări şi decelerări de 1,5m/s 2. Parcursul echivalent este de 30000km, cu opriri pentru alimentare şi reglaje. Ciclul european NCCE are o lungime de 11km, viteza medie este de 32,5km/h, viteza maximă 120km/h, iar durata este de 1200s.
Fig.12 Ciclul european de încercare NCCE
1.3 Echipamente Echipamente pentru pentru determina determinarea rea concentraţi concentraţiei ei poluanţi poluanţilor lor din gazele de evacua evacuare re Echipamentele de evaluare a emisiilor poluante depind de caracteristicile poluanţilor ale căror limite sunt legiferate. Determinarea concentraţiei gazelor de evacuare se poate face prin colectarea acestora în saci de plastic, urmând a fi supuse apoi analizei, sau prin măsurarea în mod continuu a acestora.
1.3. 1.3.11 Măsu Măsura rare reaa conc concen entr traţ aţie ieii de CO CO şi CO CO2 din gazele de evacuare Emisiile de CO şi CO2 pot fi determinate utilizând metoda absorbţiei energiei radiante nedispersive prin spectrul de radiaţii infraroşu, care se bazează pe principiul absorbţiei selective a energ energiei iei radian radiante te în spec spectru trull infrar infraroşu oşu la diferi diferite te lungim lungimii de und undăă carac caracteri teristi stice ce fiecă fiecărei rei substanţe. Metoda poate fi folosită şi pentru măsurarea concentraţiei de HC şi NO x. Radiaţia infraroşie este absorbită de CO la lungimea de undă de 4,7 μm, iar la CO2 la lungimea de undă de 4,3 μm. Analizorul NDIR prezentat în figura de mai jos dispune de două tuburi paralele cu pereţi
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Fig.13 Sistemul Non Dispersive Infrared Radiation
1.3. 1.3.22 Măsu Măsura rare reaa con conce cent ntra raţi ţiei ei NOx din gazele de evacuare Conc oncentr entraţ aţia ia oxiz xizilor ilor de azot este ste determ termin inat atăă cu ajuto jutoru rull anali nalizzorulu ruluii cu chemiluminiscenţă. Acest aparat funcţionează pe baza reacţiei dintre NO şi O 3 în urma căreia rezultă NO2 marcat electric, care prin descărcare produce o cuantă de lumină roşie. această emisie luminoasă poate fi măsurată cu precizie. Aparatul dispune de o cameră de reacţie în care se introduce gazul de analizat şi ozonul, unde se generează molecule de NO 2 încărcate electric şi unde are loc descărcarea. Senzorul fotoelectric transformă impulsurile luminoase în curent electric.
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Fig.15 Sistemul de determinare a conţinutului de hidrocarburi
1.3.4 1.3.4 Măsura Măsurarea rea conce concentr ntraţi aţiei ei de partic particule ule din gaz gazele ele de evacua evacuare re Prin metoda gravimetrică se determină cantitatea de particule. Astfel gazele de evacuare trec printr-un filtru, iar masa particulelor este determinată prin cântărirea filtrului gol şi cu particule. Filtrele sunt confecţionate din fibre de sticlă acoperite cu fluorcarbon sau teflon, sau sunt filtre cu membrană din fluorcarbon. Acestea pot influenţa măsurătorile prin reţinerea vaporilor de apă şi de hidrocarburi din gazele de evacuare. La o măsurătoare se utilizează mai mult filtre montate în serie, dispuse la o distanţă de maxim 100mm.
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Fig.17 Fummetrul Hartridge
Formarea emisiilor poluante În cazul emisiilor nocive datorate autovehiculelor, gradul de poluare are o dependenţă liniară de intensitatea traficului, exprimată prin numărul de autovehicule care trec printr-un punct de control în unitatea de timp. În figura de mai jos este prezentată dependenţa dintre numărul de autovehicule pe oră şi
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1.4 1.4
Form Fo rmar area ea oxiz oxizil ilor or de azot azot
1.4. 1.4.11 Fo Forma rmare reaa oxi oxidu dulu luii de de azo azott NO În cazul motoarelor cu ardere internă formarea oxidului de azot este datorată prezenţei azotului molecular în compoziţia aerului admis în motor, dar şi datorită micilor cantităţi de azot prezente in compoziţia compoziţia benzinei şi a motorinei. motorinei. Reacţiile de formare a oxidului de azot au loc în frontul flăcării şi în gazele care părăsesc flacăra, zone cu amestec stoichiometric. Ele sunt următoarele:
În frontul flăcării se formează o mică parte din cantitatea de NO deoarece arderea în motor are loc la presiune ridicată astfel că frontul flăcării are o grosime mică şi presiunea din interiorul camerei de ardere creşte pe parcursul procesului de ardere ceea ce determină creşterea temperaturii gazelor arse. Deci principala cantitate de NO se formează in gazele ce părăsesc flacăra.
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, iar NO2 este convertit în NO:
.
Funcţionarea motorului la sarcini reduse duce la creşterea concentraţiei de dioxid de azot deoarece există numeroase zone reci ce duc la inhibarea procesului de conversie a NO 2 în NO. S-a constatat că la turaţii scăzute NO 2 se formează în evacuare deoarece gazele arse rămân un timp mai lung în prezenţa oxigenului.
1.4. 1.4.33 Fo Forma rmare reaa prot protox oxid idul ului ui de de azot azot N2O La motoarele cu ardere internă emisiile de protoxid de azot ating o magnitudine de 3 până la 8 ppm. N2O rezultă din reacţia substanţelor intermediare cu NO. Mecanismul de formare este limitat de reacţia de oxidare:
Protoxidul de azot este distrus cu ajutorul hidrogenului atomic:
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1.4.4.1 1.4.4.1 Influenţa Influenţa dozajului dozajului Performanţele energetice ale motorului sunt influenţate de dozaj, ştiut fiind faptul că motorul dezvoltă puterea maximă la funcţionarea cu amestecuri bogate, deficit de aer. De asemenea consumul minim de carburant se obţine la funcţionarea motorului cu amestecuri sărace, surplus de aer. Temperatura maximă a gazelor arse se obţine la funcţionarea cu amestecuri bogate, λ≈0.9, moment în care concentraţia de NO x este redusă. Odată cu creşterea coeficientului de
exces de aer, efectul creşterii presiunii parţiale a oxigenului contrabalansează efectul reducerii temperaturii gazelor arse, care tinde să reducă formarea oxizilor de azot. Concentraţia maximă de oxizi de azot se obţine în zona amestecurilor uşor sărace, λ=1.1. Dacă excesul de aer creşte atunci temperatura flăcării se reduce şi rata de formare a NO x scade.
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1.4.4.3 Influenţa avansului avansului la aprindere aprindere Mărirea avansului la aprindere duce la creşterea emisiilor de oxizi de azot, pentru că o cantitate importantă din amestecul carburant arde înainte de ajungerea pistonului în punctul mort superior ceea ce duce la creşterea presiunii maxime a ciclului şi implicit creşterea temperaturii ciclului. Micşorarea avansului la aprindere produce scăderea temperaturii pe ciclu şi implicit reducerea emisiilor de oxizi de azot. Nivelul temperaturii gazelor arse evacuate creşte, este favorizată post-arderea hidrocarburilor, dra se înrăutăţeşte puterea specifică şi consumul de combustibil.
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1.4.5.1 1.4.5.1 Influenţa Influenţa dozajului dozajului Concentraţia de oxizi de azot prezenţi în gazele de evacuare creşte odată cu mărirea dozajului. Creşterea dozajului se realizează prin mărirea cantităţii de combustibil injectată, deci concentraţia de NOx este dependentă de cantitatea de combustibil injectată.
Fig.22 Influenţa dozajului asupra emisiilor la MAC
La motoarele cu injecţie directă formarea NO x are loc în camera separată, aprox. 65%, şi 35% în camera principală. Camera separata este caracterizată de un amestec neuniform, λ=0,8-1,
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Monoxidul de carbon rezultă în urma arderii incomplete a combustibilului datorită lipsei oxigenului. Astfel că la amestecurile sărace sunt mai mari decât la amestecurile bogate.
1.5.1 1.5.1 Formarea Formarea monoxi monoxidului dului de carbon carbon la la motorul motorul cu aprinde aprindere re prin scânteie scânteie Emisi Emisiile ile de CO sunt sunt ma mari ri în cazul cazul mo motoa toarel relor or cu ardere ardere intern internăă deoare deoarece ce acest acestea ea funcţionează la pornire şi la sarcini mari cu amestecuri bogate. Cantitatea de emisii poate fi redusă prin controlul cantităţii de combustibil introduse în cilindri. De asemenea cantitatea de monoxid de carbon poate fi redusă folosirea unor amestecuri având nivel de temperatură ce generează disocierea dioxidului de carbon. Nivelul monoxidului de carbon este mai mare în camera de ardere faţă de cel din gazele de evacuare.
1.5.2 1.5.2 Formarea Formarea monoxi monoxidului dului de carbon carbon la la motorul motorul cu aprinde aprindere re prin comprimare comprimare În cazul motoarelor diesel emisiile de monoxid de carbon sunt mai reduse faţă de motorul cu aprindere prin scânteie deoarece acestea funcţionează cu amestecuri sărace.
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Aldehide Olefinice Diolefinice Tri şi tetra alchil benzen Ciclo olefinice
10 1 00
Tab.4 Reactivitatea hidrocarburilor
Compoz Compoziţia iţia em emisi isiilor ilor organ organice ice şi nivelu nivelull acest acestora ora sunt sunt influe influenţa nţate te de compoz compoziţi iţiaa combu combusti stibil bilulu ului,i, astfel astfel combu combustib stibilii ilii ce au în comp compozi oziţie ţie o canti cantitat tatee ridica ridicată tă de substa substanţe nţe aromatice şi olefinice produc o concentraţie apreciabilă de hidrocarburi reactive. S-a constatat că în gazele de evacuare apar compuşi organici care nu se găsesc în compoziţia combustibilului ceea ce denotă faptul că în timpul arderii au loc reacţii de piroliză şi sinteză. Compuşii oxigenaţi sunt prezenţi în gazele de evacuare, ei participă la formarea smogului fotochimic, o parte din ei sunt urât mirositori şi iritanţi. Aldehidele volatile sunt iritante pentru ochi şi aparatul respirator, iar formaldehidele
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Radicalul RO˙format în timpul descompunerii termice a peroxidului de tipul ROOH de aldehidă în cazul radicalului secundar prin spargerea legăturii C-C şi prin rearanjare:
şi cazul radicalului primar RO˙, radicalul aldehidă poate reacţiona cu oxigenul
Formarea cetonelor Radicalul RO2˙ este implicat în formarea cetonelor:
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La motoarele cu aprindere prin scânteie distanţa de stingere a flăcării între două suprafeţe plate este de 0,2-1 mm şi ea reprezintă distanţa distanţa minimă în care flacăra se poate propaga. Distanţa Distanţa minimă de stingere a flăcării este de 0,04-0,2 mm. Fenomenul de stingere a flăcării se desfăşoară în două faze: 1. flacăra flacăra este stinsă la distanţă distanţă mică de perete, perete, peretele peretele rece preia căldura căldura de la zona caldă de reacţie şi se încălzeşte; 2. după stingerea flăcării are loc difuzia difuzia gazelor şi oxidarea oxidarea acestora. Hidrocarburile Hidrocarburile care
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În timpul curse de comprimare amestecul aer combustibil este forţat să ocupe volumul interstiţiilor, care sunt caracterizate de un raport ridicat dintre suprafaţă şi volum ceea ce duce la răcirea amestecului prin transferul de căldură spre pereţi. În timpul arderii presiunea din camera de ardere continuă să crească şi în volumul interstiţiilor continuă să intre hidrocarburi nearse. Amestecul reţinut în interstiţii poate fi ars în întregime sau parţial prin pătrunderea flăcării în volum, numai dacă intrarea e suficient de largă şi permite acest lucru, sau se produce stingerea flăcării la intrarea în aceste interstiţii.
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mai târziu în ciclul motor. Astfel ele pot scăpa de procesul de ardere completă fapt ce duce la creşterea emisiilor de hidrocarburi nearse. Reducerea participării uleiului la producerea emisiilor de hidrocarburi nearse poate fi obţinută prin utilizarea unor uleiuri sintetice cu efect de solvent scăzut. Reducerea emisiilor se poate obţine şi prin realizarea unei răciri direcţionate: capul pistonului să fie răcit normal, iar pereţii cilindrului sa fie răciţi la temperatura suportată de pelicula de ulei.
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gazele se află la o temperatură ridicată şi amestecarea hidrocarburilor nearse limitează reacţiile de oxidare mai puternic decât cinetica acestor reacţii. Prin studii experimentale s-a demonstrat că hidrocarburile reţinute în interstiţii pot fi oxidate complet prin amestecarea cu gazele arse numai dacă acestea au o temperatură mai mare de 1400K, astfel o parte din hidrocarburile din interstiţii şi cele desorbite de pelicula de ulei sunt supuse unei oxidări parţiale. Cantitatea de oxigen necesară reacţiei de oxidare a hidrocarburilor poate fi asigurată prin
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evacuare
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1. Amestecul Amestecul aer-co aer-combus mbustibil tibil poate poate fi fi prea sărac sărac pentru pentru a se autoa autoaprind prindee sau frontu frontull de
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- depăşirea dozei optime de combustibil pe ciclu duce la creşterea emisiilor;
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1.6.6 1.6.6 Influenţa Influenţa amestecurilo amestecurilorr superbog superbogate ate asupra emisiilor emisiilor de de hidroca hidrocarburi rburi
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1.6.7 1.6.7 Influenţa Influenţa răcirii răcirii la perete şi şi lipsa lipsa arderii arderii asupra asupra emisiilor emisiilor de hidroca hidrocarburi rburi
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Complexitatea constructivă, necesitatea prezenţei sursei de aer secundar antrenată de motor, dependenţa directă a eficienţei reactorului termic faţă de variaţiile calităţii amestecului aer-combustibil din cilindrii motorului, sunt elemente care au determinat limitarea utilizării acestuia, preferându-se folosirea convertoarelor catalitice, acestea având în plus şi avantajul diminuării concentraţiei compuşilor poluanţi din gazele de evacuare prin reacţii de reducere.
1.12.2 1.12.2 Conver Convertor torul ul catalitic catalitic Funcţionarea sistemelor de conversie catalitică se bazează pe acţiunea catalitică a unor metale preţioase: Pt, Rh,Pd, ce sunt capabile să diminueze energia chimică necesară reacţiilor de reducere şi oxidare, mărind implicit viteza de desfăşurare a acestor reacţii. Accelerarea catalitică a reacţiilor de oxidare şi/sau reducere are loc la o temperatură relativ scăzută a gazelor arse, evacuate la diversele regimuri de funcţionare ale motorului. Reacţiile ce au loc în convertorul catalitic: - oxidarea hidrocarburilor:
- oxidarea monoxidului de carbon:
Datorită polarităţii puternice, molecula de monoxid de carbon este absorbită în stratul de metal nobil al catalizatorului la o temperatură cuprinsă între 100 şi 200 °C. Prin absorbţia monoxidului de carbon locul locul rămas liber este ocupat de oxigen, oxigen, molecula de oxigen disociază disociază la nivelul stratului catalitic, şi astfel au loc reacţii de oxidare ale monoxidului de carbon, urmate de absorbţia dioxidului de carbon. - reducerea oxidului de azot:
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acestor produşi de disociere pe suprafaţa catalizatorului. Desorbţia oxigenului format limitează continuarea reacţiilor de reducere. Pentru restaurarea acţiunii catalitice este necesară ridicarea nivelului temperaturii gazelor de evacuare sau prezenţa unui agent chimic reducător. Agenţii redu reducă căto tori ri pot pot fi comp compuş uşii chim chimic icii care care înso însoţe ţesc sc NO în gaze gazele le de evac evacua uare re:: CO, CO, H2, şi hidrocarburi nearse. Hidrogenul molecular poate rezulta din reacţia CO cu vaporii de apă, în prezenţa catalizatorului: catalizatorului:
Reacţiile de reducere a NO:
Oxidarea agenţilor reducători CO, HC şi H2 are loc prin utilizarea oxigenului rezultat din disocierea NO sau a oxigenului molecular existent în gazele de evacuare. În cazul în care tensiunea parţială a oxigenului molecular depăşeşte substanţial presiunea parţiala a NO, rata de disociere a NO se diminuează considerabil. Acesta este motivul pentru care nu este posibilă reducerea NO din gazele de evacuare ale motoarelor care funcţionează cu exces de aer: MAS cu amestecuri sărace şi MAC. La motoarele care funcţionează cu amestecuri bogate, agenţii reducători se găsesc în cantităţi ridicate, iar azotul atomic rezultat din disocierea NO poate reacţiona cu aceşti compuşi. Deci în urma acestor reacţii poate rezulta amoniacul:
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1.12.2.1 1.12.2.1 Convertorul Convertorul catalitic catalitic cu două căi Dacă agenţii reducători sunt prezenţi în exces este necesară utilizarea a două reactoare legate în serie pentru limitarea emisiei de amoniac.
Fig.58 Sistem cu reactoare legate în serie
În primul reactor se produce o reducere a NO, rezultă cantităţi variabile de amoniac, după care amestecul de gaze intră în următorul reactor, unde au loc procesele de oxidare. Pentru dezvoltarea acestui tip de reacţii, se introduce aer adiţional în gazele ce pătrund în reactorul de oxidare. Amoniacul format în timpul reacţiilor de reducere ar trebui sa fie oxidat în al doilea reactor după reacţiile:
sau
Cantitatea de NO formată după ultima schemă de reacţie depinde de tipul catalizatorului folosit, de nivelul temperaturii şi de valoarea presiunii parţiale a oxigenului.
1.12.2.2 1.12.2.2 Convertorul Convertorul cataliti cataliticc cu trei căi Conver Convertor torul ul catal cataliti iticc cu trei trei căi căi poate poate reduce reduce simult simultan an trei trei compu compuşi şi din gazele gazele de evacuare: CO, HC, NO x. Acesta are randamentul maxim atunci când motorul funcţionează cu amestecuri stoichiometrice. Dacă în motor sunt arse amestecuri sărace are loc o scădere a eficienţei reducerii NO, iar dacă amestecul proaspăt este bogat atunci apare o reducere a eficienţei reacţiilor de oxidare a HC şi CO. Domeniul de variaţie a calităţii amestecului aer-combustibil, pentru care catalizatorul funcţionează eficient este extrem de restrâns, ceea ce impune utilizarea unui sistem electronic de control şi reglare a calităţii amestecului, pentru menţinerea acestuia în domeniul de eficienţă al sistemului catalitic. Senzorul care culege informaţia relativ la cantitatea de oxigen remanentă în
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unităţii electronice de comandă care după procesarea datelor determină ce acţiuni sunt necesare pentru obţinerea unui unui amestec stoichiometric. stoichiometric.
Fig.59 Eficienţa convertorului catalitic în funcţie de calitatea amestecului amestecului
Fig.60 Convertorul catalitic cu trei căi: poziţionare şi structură
Prin adăugarea de aditivi în compoziţia combustibilului se poate realiza o îmbunătăţire a funcţionării convertorului catalitic cu trei căi. Materialul catalitic utilizat la construcţia convertoarelor catalitice trebuie depus pe un suport, suport, suport suport ce trebuie trebuie să îndeplineasc îndeplineascăă o serie de cerinţe: cerinţe: să permită depunerea depunerea stratului stratului catalitic prin minimizarea contactului dintre acesta şi suport, fără ca elementul catalitic să sinterizeze cu materialul suportului, pentru a nu se diminua eficienţa reacţiilor catalitice ce au loc la nivelul atomilor din stratul de suprafaţă al depunerii de element catalitic; să inducă un nivel de contrapresiune cât mai redus pe traseul gazelor evacuate prin sistem, pentru a nu diminua semnificativ performanţele energetice ale motorului; să asigure o cât mai mare suprafaţă de conta contact ct între între gazele gazele arse arse şi stratu stratull de compus compus catal cataliti itic, c, în scopu scopull ma maxim ximiză izării rii eficie eficienţe nţeii sistemului; să răspundă cât mai bine solicitărilor mecanice şi termice. Astfel materialul suportului poate fi ceramic sau metalic, având o structură granulată sau monolitică. La început se utilizau suporturi granulate, dar acum sunt răspândite cele cu structură
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de pulb pulber eree după după care care aces aceste teaa sunt sunt înco încorp rpor orat atee întrîntr-un un agen agentt lian liantt (met (metil ilce celu lulo loza za sau sau carb carbox oxim imet etil ilce celu lulo loza za)) cu un lubr lubrif ifia iant nt (eti (etile leng ngli lico col) l) şi un agen agentt fond fondan antt (hid (hidro roxi xizi zi ai pământurilor alcaline) pentru a favoriza sinterizarea. Amestecul obţinut este sinterizat în forma finală. Structura obţinută este introdusă apoi în cuptorul de sinterizare; în faza de presinterizare sunt eliminaţi compuşii volatili, agenţii lianţi şi lubrifianţi. Secvenţele la temperaturi ridicate (între 800 şi 1300 la 1450 °C) determină nivelul de porozitate al materialului ceramic pe care se va aplica stratul de material catalitic. Monoliţii se extrudează ca volume cilindrice cu secţiunea circulară, eliptică sau “turf”. Canalele de trecere a fluxului de gaze evacuate de secţiune triunghiulară sau pătrată sunt aranjate sub formă de fagure. Volumul monolitului ia valori cuprinse între 0,2…0,3dm 3 pentru fiecare kW de putere efectivă a motorului. Secţiunea în formă de “turf” se utilizează la motoare cu puteri de până la 100kW, pentru motoare cu puteri mai mari se folosesc doi monoliţi identici montaţi în paralel. Valoarea diametrului suportului se constituie în factorul principal ce determină nivelul căderii de presiune pentru un volum dat al monolitului, comparativ cu lungimea canalelor şi numărul de canale pe unitatea de suprafaţă. Micşorarea grosimii pereţilor despărţitori reduce inerţia termică a suportului şi diminuează nivelul presiunii gazelor de evacuare din amontele sistemului catalitic. Caracteristicile principale ale monoliţilor ceramici sunt următoarele: - număr de canale pe cm 2 - 46…62; - grosime de perete - 0,1, 0,15 sau 0,3mm; - porozitate - ≅ 30%; - aria suprafeţei active a canalelor raportată la unitatea de volum - 2,19 sau 2,79m2/dm3; - densitatea materialului ceramic - 1,68kg/m3; - temperatura maximă de exploatare - 1100°C; - mărimea macroporilor - 7000…10000Å; 7000…10000Å; - mărimea microporilor - 70…90Å.
Suportul monolitic metalic este realizat din oţel inoxidabil cu conţinut de crom şi aluminiu, obţinându-se grosimi ale pereţilor canalelor de 40…50μm. Ca efect al conţinutului relativ ridicat de aluminiu (aproximativ 4%) la suprafaţa monolitului se formează un strat protector de alumină alumină rezultată prin oxidare la temperatură ridicată. Prin introducerea în compoziţia aliajului a 0,2÷0,3% zirconiu, rezistenţa monolitului la solici solicităr tării caract caracteri eristi stice ce oxidăr oxidării ii la temper temperatu aturi ri ridica ridicate te creşte creşte;; se împie împiedic dică, ă, de aseme asemenea nea
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ridicarea durabilităţii materialului supus solicitărilor date de gazele fierbinţi. Deoarece ytriul este un material scump, se utilizează în proporţie de maxim 0,05%, restul fiind înlocuit de un amestec conţinând pământuri rare şi ceriu. După recoacere la 1200 °C, folia de tablă este ondulată într-un laminor special pentru a i se asigura o configuraţie de suprafaţă sinusoidală sau trapezoidală. Asamblarea foliilor pentru obţinerea structurii principale se poate realiza fie prin sudură cu flux de electroni, fie prin brazare la o temperatură de maxim 1200°C, direct în carcasa suportului. Brazarea este recomandată deoarece previne deformarea structurii sub acţiunea şocurilor termice. Folia este răsucită în formă de spirală sau sub formă de "3", această ultimă aranjare oferind o mai bună anduranţă mecanică. Aria secţiunii transversale echivalente oferită pentru curgerea gazelor arse creşte de la 57%, în cazul monolitului ceramic, la 72%, în cazul monolitului metalic. Se reduce astfel, la utilizarea monolitului metalic, contrapresiunea din sistemul de evacuare al motorului, ceea ce determină o îmbunătăţire a performanţelor motorului. Spargerea curentului principal de gaze arse, la intrarea în sistemul catalitic cu suport metalic, creşte intensitatea turbulenţei curgerii, ceea ce conduce la un contact mai bun între gazele evacuate şi suprafaţa cu metal catalitic. Efectul Efectul final este o creştere creştere a eficienţe eficienţeii catalizato catalizatorului rului sau o posibilă posibilă diminuare diminuare a volumulu volumuluii şi greutăţii suportului. Acesta are avantajul că dimensiunea peretelui despărţitor este mai mică decât în cazul celui ceramic şi atunci pe aceeaşi suprafaţă a secţiunii transversale se pot amplasa mai multe canale, deci dimensiunea va fi mai mică, gabarit redus. Caracteristicile principale ale monolitului metalic sunt: - compoziţia materialului - 15÷20%Cr, 4÷5%Al, 0÷2%Si, 0,1÷0,3%Y, restul până la 100%Fe; - număr de canale pe cm 2 - 62 sau 93; - grosime de perete - 0,05mm;
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Ca şi în cazul suportului din material ceramic, suprafaţa canalelor se acoperă cu un strat de Y alumină, înainte de impregnarea cu elementul catalitic.
Materialul Materialul activ ce este este depus depus pe suport suportul ul catali cataliza zator torulu uluii este este alcătu alcătuit it din metale metale preţioase, deoarece acestea au o intensă reactivitate chimică atât la temperaturi reduse cât şi la temperaturi ridicate, ele rezistă foarte bine tendinţelor de sinterizare prin care se reduce aria suprafeţei specifice de activitate. Dacă s-ar utiliza oxizi metalici ca: CoO 4, CuO, Cr 2O3, ZnO, SiMnO2, singuri sau în combinaţie prin alţi oxizi, ar fi necesară o cantitate dublă sau chiar de trei ori mai mare pentru a realiza aceeaşi rată de conversie. Elementele active utilizate utilizate de convertorul catalitic catalitic sunt platina, Pt, şi paladiul, paladiul, Pd. Aceste elemente pot fi utilizate separat sau în combinaţie. Pentru oxidarea CO 2, hidrocarburilor olefine sau pentanului, activitatea paladiului este mai intensă decât cea a platinei, iar pentru oxidarea hidrocarburilor aromatice activitatea celor două metale preţioase este asemănătoare, în schimb, oxidarea hidrocarburilor parafinice este mai eficient realizată de platină. Reacţiile catalitice încep cu un stadiu de absorbţie prin care se impune dehidrogenarea hidrocarburilor saturate. Platina este foarte eficientă in acest proces. La temperaturi cuprinse între 600...900°C, într-un într-un mediu mediu oxidant, oxidant, platina tinde să sinterizez sinterizeze. e. Sinterizar Sinterizarea ea platinei platinei este mult diminuată într-un mediu reducător. Paladiul rezistă mai bine tendinţelor de sinterizare întrun mediu oxidant şi, combinat cu stratul intermediar, prezintă aceeaşi durabilitate cu perechea de metale platină-rodiu. Prez Prezen enţa ţa plum plumbu bulu luii sau sau a fosf fosfor orul ului ui bloc bloche heaz azăă irev irever ersi sibi bill supr supraf afaţ aţaa acti activă vă a catalizatorului, platina dezactivându-se dezactivându-se mult mai uşor decât paladiul. Rata de conversie a NO x şi CO este mult mai scăzută în cazul Pd, la regimuri de funcţiona funcţionare re caracteri caracterizate zate de amestecu amestecuri ri bogate. bogate. Tendinţa Tendinţa actuală, actuală, în tehnolog tehnologia ia sistemelo sistemelor r catalitice, este de a înlocui perechea Pt-Rh cu perechea Pd-Rh deoarece au eficienţe comparabile însă costul Pd este mai mic decât cel al Pt.
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de reducere a NO x la N2, însă el nu este utilizat deoarece formează un oxid volatil în mediul oxidant care nu a putut fi încă stabilizat într-un într- un amestec de oxizi. Prin adăugarea de aditivi în suportul intermediar, dar şi în stratul activ, se realizează accentu accentuarea area efectului efectului catalitic, catalitic, stabilizar stabilizarea ea chimică chimică a suportului suportului şi inhibarea inhibarea procesulu procesuluii de sinterizare a metalelor preţioase. Aceşti aditivi pot fi: Ni, Ce, La, Ba, Zr, Fe, şi Si. Prin introducerea nichelului se intensifică activitatea Pt şi Pd de reducere a NO. Efect similar are introducerea oţelului şi ceriului, ele având şi rolul de a preveni sinterizarea aluminei din stratul intermediar. Procesul de reducere în mediul reducător al NO poate fi îmbunătăţit prin adăugarea de lantariu, el previne desprinderea cristalelor de platină.
Fig.61 Structura convertorului catalitic
Aria suprafeţei suprafeţei canalelor canalelor oferite oferite atât de suportul suportul ceramic cât şi de suportul suportul metalic este insuficientă pentru aplicarea stratului activ de metale preţioase. Pentru a se mări suprafaţa de depunere a catalizatorului, pereţii canalelor sunt îmbrăcaţi cu un material intermediar care de cele mai multe ori este alumina. Masa stratului intermediar depus reprezintă 5…20% din masa suportului iar aria suprafeţei canalelor este amplificată cu un factor de aproximativ 100. Aplicarea stratului intermediar se face în trei etape: pregătirea lichidului precursor, un amestec de Al 2(OH)5Cl cu hidrosoli de alumină şi oxizi de siliciu sau zincroniu, pentru acoperire; imersia suportului în acest lichid; uscarea şi sinterizarea combinaţiei precursoare pe suport. Prin
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Stratul activ se depune prin imersie sau injecţie utilizând o soluţie de metale preţioase, H2PtCl6 sau PdCl2 sau Rh(NO3)3; soluţii pe bază de acetonă conţinând (n-butil 4 N) N)2Rh(CO)2Br 8 sau H2PtCl6-6H2O. Adâncimea de pătrundere a catalizatorului în suport este influenţată de aciditatea mediului de impregnare. Adâncimea trebuie să fie redusă pentru a se obţine o suprafaţă de spălare cu gaze arse a cristalelor de metal preţios cât mai mare, dar suficient de mare pentru a preveni desprinderea desprinderea cristalelor prin abraziune. abraziune. Unele tehnologii prevăd depunerea stratului intermediar şi a celui activ simultan sub formă de hidrosol amestecat cu precursori catalitici, însă o parte din cristalele active sunt acoperite de stratul intermediar, deci eficienţa conversiei este scăzută.
Fig.62 Montarea monolitului
Suportul impregnat cu cristale de metale preţioase este montat într-o carcasă metalică, din oţel inoxidabil refractar austenitic sau feritic, ce are rolul de înveliş protector. Carcasa este realizată din două semicarcase unite prin presare. Convertorul catalitic se poziţionează pe conducta de evacuare ţinând seama că suportul trebuie să se încălzească cât mai repede după pornire deoarece la temperatura mediului ambiant el nu acţionează asupra emisiilor poluante. Pentru a funcţiona el trebuie să atingă o temperatură de 200-300°C, deci el trebuie poziţionat cât mai aproape de motor dar la o distanţă sigură deoarece temperatura poate ajunge până la 900 °C. Timpul necesar convertorului catalitic pentru a atinge eficienţa de conversie depinde de:
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b) Suprafaţa Suprafaţa catalizato catalizatorului rului Depozitele de metale şi metaloizi ca Pb, P, S, Zn, au efect ireversibil. Astfel depozitele de impu impurit rităţ ăţii împr împreu eună nă cu efec efecte tele le term termic ice, e, sint sinter eriz izar are, e, dete determ rmină ină intra intrare reaa în func funcţi ţiun unee a materialului activ la o temperatură mai ridicată ceea ce duce la creşterea emisiilor poluante. c) Aditivii din materialul suportului Prin Prin adăuga adăugarea rea de aditiv aditivii la ma materi terialu alull suport suportulu uluii se inhibă inhibă sinter sinteriza izarea rea metale metalelor lor preţioase, se încetineşte încetineşte îmbătrânirea acestora acestora şi se coboară temperatura temperatura la care catalizatorul catalizatorul intră în funcţiune. Se utilizează ca aditivi: CeO 2, Fe2O3, oxizii pământurilor rare.
d) Poziţia catalizatorului în instalaţia de evacuare evacuare a gazelor gazelor arse Pentru ca temperatura gazelor de evacuare ce intră în catalizator să nu fie mai mare de 900°C, dar să nu fie nici prea mică pentru a se realiza oxidarea la pornirea la rece s-a adoptat o soluţie de compromis ce presupune separarea convertorului catalitic în două componente: un miniconvertor cu capacitate termică redusă care este amplasat cât mai aproape de colectorul de evacuare unde este încălzit rapid şi atunci se iniţiază oxidarea CO şi HC, convertor cu monolit metalic, şi un convertor catalitic principal amplasat la o distanţă sigură de funcţionare. Sau o altă soluţie combină un microconvertor ca monolit amplasat în colectorul de evacuare cu un monolit ceramic amplasat pe traiectul de evacuare pentru a se evita supraîncălzirea acestuia la sarcini mari. Pentru a aduce convertorul la temperatura de funcţionare se prevăd sisteme electrice de încălzire a acestuia înainte de pornirea motorului.
e) Compoziţia Compoziţia gazelor gazelor de evacuare evacuare La pornirea motorului conţinutul de CO, HC, O 2, NO este variabil, el depinzând de gradul de îmbogăţire a amestecului, temperatură şi presiunea mediului ambiant. Un conţinut ridicat de O2, o concentraţie scăzută de CO şi prezenţa HC în gazele de evacuare determină scăderea temperaturii de oxidare a catalizatorului.
f) Raport Raportul ul aer-co aer-combu mbusti stibil bil
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1.12.2.3 1.12.2.3 Catalizator Catalizatorii pentru tratarea tratarea gazelor net oxidante Dacă se utilizează catalizatorul cu trei căi pentru tratarea gazelor net oxidante se observă că apare o scădere a eficienţei reducerii NOx şi apare oxidarea echivalenţilor reduşi ai CO şi HC. În cazul motoarelor ce funcţionează cu amestecuri sărace se poate obţine o reducere a emisiilor poluante numai dacă se dezvoltă noi catalizatori pentru reducerea acestor gaze de evacuare net oxidante. a) Reducerea selectivă non-catalitică SNR Se utilizează în centralele termoelectrice şi agentul reducător este amoniacul sau ureea. Reacţiile de reducere în cazul utilizării amoniacului au loc la temperaturi de 800-1000 °C, iar în cazul ureei intervalul de temperaturi e între 800-1100 °C. Au fost încercări de reducere a emisiilor cu ajutorul acidului cianuric, dar datorită intervalului strâns în care are loc procesul de reducere şi datorită faptului că la temperaturi sub 310 °C a gazelor arse cresc emisiile de NO, această soluţie s-a abandonat. b) Reducerea catalitică non-selectivă NCR Reducerea catalitică non-selectivă presupune utilizarea catalizatorului cu trei căi şi a agenţilor reducători reprezentaţi de metale nenobile. În tabelul de mai jos este prezentată eficienţa conversiei NO a unor metale nenobile:
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Utilizarea zeolitului drept catalizator la MAS este limitată de creşterea vitezei gazelor prin catalizator, astfel pentru îmbunătăţirea funcţionării catalizatorului trebuie mărit volumul acestuia. La motoarele cu aprindere prin comprimare pentru a se putea utiliza acest catalizator pentru reducerea NOx este necesar să se introducă agentul de reducere. Cercetările experimentale au arătat ca prin introducerea a 4700 ppm etilenă rata de reducere s-a mărit cu 15%. Convertorul cu catalizator zeolitic este ineficient pentru reducerea NO x la motorul cu aprindere prin comprimare. c) Reducerea catalitică selectivă SCR Reducerea catalitică selectivă presupune utilizarea catalizatoarelor din metale nenobile bazate pe TiO2/V2O5, zeoliţi şi cărbune activ. Soluţiile cu cărbune activ nu se utilizează pentru reducerea emisiilor poluante în cazul autovehiculelor din cauza riscului incendiilor. Agentul reducător utilizat poate fi amoniacul. În convertor au loc o serie de reacţii paralele la care procentul depinde de temperatura gazelor, concentraţia de oxigen, viteza gazelor prin convertor şi selectivitatea catalizatorului utilizat. Catalizatorii din zeolit care utilizează amoniacul ca agent reducător realizează conversia NOx independent de concentraţia de oxigen pe întreaga gamă de temperatură. În plus, zeoliţii au o mare capacitate de a absorbi amoniacul şi în acest fel convertorul stochează o cantitate relativ ridicată de agent reducător. Stocarea unei cantităţi ridicate de amoniac duce la întârzierea intrării
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Pentru MAS şi MAC care au gaze de evacuare net oxidante reducerea catalitică selectivă rămâne deocamdată singura modalitate eficientă de reducere a NO x fără a afecta emisiile de CO şi HC. Reducerea catalitică selectivă a fost dezvoltată de Mazda pentru un autoturism echipat cu un mo motor tor turbo turbo diesel diesel care care oferă oferă „perfo „performa rmanţe nţe remarc remarcabi abile le în ceea ceea ce priveş priveşte te reduc reducere ereaa emisiilor” şi respectă normele Euro V. Sistemul SCR purifică gazele de evacuare prin pulverizarea cu AdBlue® o substanţă apoasă pe bază de uree dintr-un rezervor special amenajat sub portbagaj direct în fluxul de gaze din faţa convertorului catalitic. Acest proces determină o reacţie chimică prin care AdBlue® transformă aproximativ 40% din NOx în azot, gaz inofensiv. Datorită designului compact acest sistem nu ocupă din spaţiul destinat bagajelor. Consumul de AdBlue® se estimează ca fiind suficient pentru parcurgerea a 20.000 de km în condiţii normale de condus, astfel reumplerea rezervorului este necesară doar la reviziile programate.
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combinare a acestor specii este limitată de termodinamica reacţiilor şi de stabilitatea chimică a plumbului, oxidului de plumb şi a acizilor. acizilor. Transferul plumbului din camera de ardere spre convertorul catalitic începe cu oxidul de plumb care se află în stare de vapori. Vaporii condensează condensează rapid când gazele ating temperatura de 850-900°C, generându-se particule fine care intră în curentul de gaze sau se depun pe pereţii circuitului de evacuare. În timpul transferului particulele sau depozitele de oxid de plumb pot reacţiona cu oxizii sulfului sau fosforului. Efectul dăunător al plumbului poate fi datorat acoperirii catalizatorului sau înfundării porilor suportului. La o depunere moleculară mai mică de 1% cu PbO la oxidarea CO nu-şi manif ma nifest estăă efectu efectull dăună dăunător tor,, din contr contră, ă, poate poate duce duce la cobor coborâre âreaa temper temperatu aturii rii de intrar intraree în funcţiune a catalizatorului. Depăşirea unui singur strat molecular de plumb va duce la inhibarea reacţiilor de conversie a CO. În cazul concentraţiilor ridicate, peste 5% de PbO rata de conversie a CO este inhibată de înfundarea porilor. Efectul de otrăvire cu plumb este mai pronunţat la paladiu decât la platină în special la temperaturi reduse. Catalizatorul contaminat poate fi regenerat prin încălzire la aproximativ 500°C într-o atmosferă fără plumb. Deschiderea porilor înfundaţi de către depozite de sulfaţi de plumb se poate realiza realiza prin încălzire la o temperatură mai mare mare de 900°C. Convertoarele catalitice cu trei căi, bazate pe metale preţioase tind să reziste la prezenţa plumbului dacă gazele de evacuare au un nivel de temperatură mai mare de 500°C şi nu conţin compuşi halogenaţi.
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1.12 1.12.3 .3 Redu Reducer cerea ea nive nivelu lulu luii noxe noxelo lorr la porn pornir irea ea la rece rece a moto motoru rulu luii prin prin util utiliz izar area ea hidrogenului degajat în motor pentru arderea gazelor de evacuare Metoda prin care se poate realiza o reducere a emisiilor poluante la pornirea la rece a motorului se numeşte EGI - Exhaust Gas Ignition. Funcţionarea sistemului se bazează pe reacţia de postardere din sistemul de evacuare al autovehiculului. Această reacţie are rolul de a asigura încălzirea rapidă a catalizatorului ce are ca rezultat intrarea acestuia mai rapid în funcţionare. Arderea din sistemul de evacuare are două funcţii: - reducerea emisiilor poluante; - degajarea de căldură pentru a aduce catalizatorul în domeniul optim de funcţionare. Reacţiile au loc într-un timp relativ scurt: la 10 secunde de la pornirea motorului amestecul de HC şi CO generează elementele necesare arderii şi devine inflamabil. Sistemul EGI se bazează pe utilizarea unui exces mare de combustibil introdus în motor imediat după pornirea acestuia. Gazele de evacuare vor conţine cantităţi mari de HC nearse, NO x, CO şi H2, datorită arderii incomplete a amestecului carburant. Limita de ardere a gazelor este determinată de concentraţia elementelor prezente şi de nivelul de oxigen cu care funcţionează motorul. În figura de mai jos este prezentată zona în care trebuie să se situeze amestecul de gaze arse pentru a putea fi ars.
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atun atunci ci când când gaze gazele le de evac evacua uare re au o anum anumită ită temp temper erat atur ură, ă, devi devinn reac reacti tive ve.. Punc Punctu tull C caracterizează procesele ideale de ardere la temperaturi de regim constante şi amestecuri ideale aer-gaze de evacuare, fapt ce nu are loc în realitate. Punctul D caracterizează arderea reală în care hidrogenul depăşeşte 5% pentru a putea asigura o ardere eficientă şi sigură. Prin Prin utiliz utilizare areaa acestu acestuii sistem sistem se reduc reduc la minim minim em emisi isiile ile deoare deoarece ce se ard comple complett hidrocarburile nearse înainte de a ajunge în atmosferă.
1.12.4 1.12.4 Fil Filtru trull de parti particul culee Emisiile de particule pot fi diminuate prin amplasarea pe conducta de evacuare a filtrelor de particule. Filtrele de particule se clasifică după mai multe criterii: - după principiul de reţinere a particulelor: mecanică, electrostatică; - după materialul filtrant: ceramice, metalice; - după metoda de regenerare: termică: cu arzător cu flacără, cu încălzire electrică sau
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1.12.4.2 1.12.4.2 Filtrul Filtrul cu reţinere reţinere mecanică mecanică Funcţionarea filtrelor cu reţinere mecanică se bazează pe trecerea gazelor de evacuare prin interstiţiile materialului filtrant unde particulele de dimensiuni mai mari pot fi reţinute la intrarea în interstiţii, iar cele de dimensiuni reduse pot fi reţinute pe suprafaţa interstiţiilor sau în spaţiile oferite în interiorul materialului filtrant prin inerţie la schimbarea sensului de curgere al gazelor arse. Filtrele cu reţinere mecanică pot fi filtre ceramice şi filtre metalice. a) Filtrele metalice sunt confecţionate din aliaje de Ni-Cr pentru a rezista la acţiunea corozivă a gazelor de evacuare. Ele sunt alcătuite din mai mulţi cilindri concentrici din material poros. Mărimea porilor porilor influenţează performanţele performanţele filtrului. Aceste Aceste filtre filtre prezin prezintă tă o rezist rezistenţ enţăă sporit sporităă la şocuri şocuri termic termicee şi me meca canic nice, e, durata durata de funcţionare fără intervenţii asupra lor este mare. Filtrele metalice pot fi realizate şi din fibre metalice din oţel inoxidabil acoperite cu y-alumină. Aceste filtre au o eficienţă cuprinsă între 68-80%.
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Filtrele din spumă ceramică sunt realizate sub formă de monolit din SiC sau cordierit. Aceste filtre pot avea pori cu diametre de până la 500μm ce au formă de canale în adâncime. Eficienţa filtrului este destul de redusă 60-70%. Filtrele din fibre ceramice neţesute sunt alcătuite din fibre acoperite cu răşini. Structura oferă un număr mare de pori fini cu diametrul 10μm, distribuiţi pe o suprafaţă şi adâncime mare. Cartuşul filtrant se obţine prin înfăşurarea firelor realizate din fibre ceramice pe o conductă din material ceramic sau oţel prevăzută cu orificii pentru trecerea gazelor arse. Grosimea înfăşurării de fire ceramice este de 10-12mm. Gazele arse traversează înfăşurarea din fibre ceramice şi astfel particulele sunt reţinute în pori. Acest filtru rezistă mai bine la şocurile termice comparativ cu monolitul ceramic şi eficienţa lui este peste 80%. La construcţia elementului filtrant ţesut se utilizează fibre ceramice care folosesc ca materiale carbonul, carbonul activ, sticla, oxizii de aluminiu, carbura de siliciu şi materiale ceramice. Elementul filtrant se realizează prin ţesere sau tricotare, astfel rezultă o structură
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particulelor pe traiectul de evacuare. În realitate, datorită regimurilor tranzitorii la care funcţionează motorul, temperatura gazelor de evacuare variază în limite largi fapt ce duce la acumularea particulelor în filtre. Pentru a reda filtrului capacitatea de funcţionare, funinginea trebuie îndepărtată printr-un proces care poartă denumirea de regenerare a filtrului. Prin regenerare se asigură o funcţionare a filtrelor de până la 150000km, perioadă după care care acest acestea ea trebui trebuiee înloc înlocuit uitee datori datorită tă creşte creşterii rii irever ireversib sibile ile a căderi căderiii de presiu presiune ne datora datorată tă înfundării înfundării cu materiale materiale necombus necombustibile tibile,, cenuşă cenuşă metalică metalică datorată datorată aditivilor aditivilor din lubrifiant lubrifiant şi sulfului din combustibil. a) Regen Regenera erarea rea termic termicăă pentr pentruu îndepă îndepărta rtarea rea depozi depozitel telor or de funing funingine ine se realiz realizea ează ză în cazul în care care temperatura gazelor gazelor de evacuare evacuare este de 550-600 550-600 °C şi conţinutul conţinutul de oxigen este suficient. Conţinutul de oxigen determină temperatura la care are loc regenerarea, astfel pentru un conţinut redus de oxigen este necesară o temperatură ridicată. În timpul regenerării filtrului este necesară menţinerea motorului la un regim constant pentru a nu apărea modificări ale temperaturii şi debitului gazelor arse.
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- regenerarea utilizând pierderi în dielectric se bazează pe utilizarea microundelor pentru încălzirea selectivă a funinginii care le absoarbe cu un randament energetic cuprins între 60-70% la o frecvenţă de 2,45GHz. Prin încălzirea cu microunde materialul ceramic nu este afectat, de aseme asemenea nea carca carcasa sa metali metalică că const constitu ituie ie o barie barieră ră pentru pentru microu microunde nde deoar deoarece ece nu le permit permitee scăparea lor în exterior prin reflectarea lor către filtrul de particule. - regenerarea termică utilizând sistemul electrostatic. În acest caz monolitul ceramic este tubular iar canalele de trecere a gazelor sunt deschise la ambele capete ca în cazul monolitului convertorului cu trei căi. În centrul monolitului se montează electrozii cu încărcare electrostatică. Sistemul asigură oxidarea continuă a funinginii cu un randament de 90%. Tensiunea aplicată pe electrozi este de 20-25kV, iar puterea de 2W pentru fiecare kW putere motor. b) Regenerarea cu arzătoare cu flacără presupune utilizarea unor arzătoare ce au ca şi combustibil propanul sau motorina şi pot fi cu aprindere prin scânteie sau aprindere catalitică. Pentru regenerarea filtrelor pot fi utilizate aceleaşi arzătoare ca şi cele pentru condiţionarea aerului sau încălzirea lichidului de răcire.
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Sistemul cu un singur filtru de particule cu sistem de deviere a gazelor arse pe timpul regenerării va deversa în atmosferă gaze arse nefiltrate pe durata procesului. Se estimează că pe timpul regenerării filtrului, aproximativ 9 minute, emisiile instantanee de particule vor creşte cu 26% iar cele de CO cu 17%. c) Regenerarea catalitică poate fi realizată prin mai mai multe metode: metode: depunerea de de material catalitic pe suprafaţa materialului filtrant, aditivarea combustibilului, injectarea intermediară de material catalitic în gazele de evacuare în amonte de filtrul de particule. Monoliţii ceramici impregnaţi cu un amestec catalizator pe bază de Pt, V, Ag coboară
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Concluzii Evoluţia tehnologică a atras după sine şi dezvoltarea mijloacelor de transport, implicit dezvo dezvolta ltarea rea autov autovehi ehicul culelo elorr echip echipate ate cu motoar motoaree cu ardere ardere intern internăă care care au o contr contribu ibuţie ţie însemnată asupra poluării mediului înconjurător, afectând practic toate ecosistemele. Acest lucru a dus la înăsprirea normelor referitoare la cantitatea de emisii poluante acceptate la autovehiculele noi. Astfel constructorii de autovehicule sunt nevoiţi să dezvolte noi tehnologii pentru reducerea emisiilor datorate arderii combustibilului în motor. Reducerea emisiilor poluante se poate face începând cu optimizarea proceselor ce au loc
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Astfel au apărut reactoarele termice, dispozitive care menţineau gazele de evacuare la temperatură ridicată fapt ce favorizează oxidarea CO şi HC din gazele arse. Eficienţa reactorului termic depinde de nivelul de temperatură, volumul său şi oxigenul rezidual din gazele evacuate. Astfel temperatura atinsă în reactor este influenţată de temperatura gazelor la intrare, de pierderile de căldură şi de cantităţile de HC, CO şi H 2 care sunt arse în interiorul acestuia. Prin arderea unei cantităţi de CO reprezentând 1 % din volumul reactorului se produce o creştere creştere a temperaturii din reactor reactor cu 80°C. Prin utilizarea reactorului termic nu se obţine o oxidare totală a cantităţilor de HC şi CO,
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mecanică care duce la blocarea porilor catalizatorului împiedicând accesul gazelor la suprafeţele active ale catalizatorului. Contaminarea se datorează plumbului, fosforului şi sulfului. Emisiile de particule pot fi diminuate prin amplasarea pe conducta de evacuare a filtrelor de particule care se clasifică clasifică după mai multe criterii: criterii: după principiul principiul de reţinere reţinere a particulel particulelor, or, după materialul filtrant, după metoda de regenerare, după periodicitatea regenerării. Filtrele utilizate în cazul autovehiculelor trebuie să îndeplinească o serie de cerinţe: să aibă eficienţă ridicată, rezistenţă aerodinamică redusă, rezistenţă la solicitări mecanice şi vibraţii şi nu în ultimul rând să aibă cost redus.
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Bibliografie
[1] BOBESCU, BOBESCU, Gh. CHIRU, CHIRU, A. COFARU, COFARU, C. ş.a. (2000) (2000) Motoare pentru automobile şi tractoare, vol. III, Chişinău: Editura Tehnică-Info.
[2] BOBESCU, BOBESCU, Gh. RADU, RADU, Gh. Al. COFARU, COFARU, C. ş.a. (1989) (1989) Tehnici speciale pentru reducerea consumului de combustibil şi limitarea noxelor la autovehicule,
Reprografia Universităţii Transilvania.
Braşov: