Publication venue
Publication date 01/01/2009
Field of study Full text link This paper describes a compound Poisson-based random effects structure for
modeling zero-inflated data. Data with large proportion of zeros are found in
many fields of applied statistics, for example in ecology when trying to model
and predict species counts (discrete data) or abundance distributions
(continuous data). Standard methods for modeling such data include mixture and
two-part conditional models. Conversely to these methods, the stochastic models
proposed here behave coherently with regards to a change of scale, since they
mimic the harvesting of a marked Poisson process in the modeling steps. Random
effects are used to account for inhomogeneity. In this paper, model design and
inference both rely on conditional thinking to understand the links between
various layers of quantities : parameters, latent variables including random
effects and zero-inflated observations. The potential of these parsimonious
hierarchical models for zero-inflated data is exemplified using two marine
macroinvertebrate abundance datasets from a large scale scientific bottom-trawl
survey. The EM algorithm with a Monte Carlo step based on importance sampling
is checked for this model structure on a simulated dataset : it proves to work
well for parameter estimation but parameter values matter when re-assessing the
actual coverage level of the confidence regions far from the asymptotic
conditions.Comment: 4
Publication venue
Publication date 01/12/2011
Field of study Get PDF Lâoptimisation multicritĂšres de la conception dâun procĂ©dĂ© permet dâen minimiser les Ă©missions polluantes et les coĂ»ts simultanĂ©ment. Cela permet de dĂ©terminer une sĂ©rie de configurations reprĂ©sentant le meilleur compromis possible entre ces deux objectifs, peu importe leur pondĂ©ration ultĂ©rieure. Lâespace de dĂ©cision peut comprendre des variables de conception, comme le choix de la taille ou de la tempĂ©rature dâopĂ©ration des Ă©quipements, et des variables dâapprovisionnement, comme le choix dâun fournisseur ayant de meilleures pratiques environÂŹnementales plutĂŽt quâun autre. Pour obtenir le meilleur compromis possible, il faut dâabord dĂ©finir les fonctions-objectif correctement, ce qui pourrait nĂ©cessiter de les considĂ©rer dans une perspective du cycle de vie. La conception rĂ©ellement optimale dâun procĂ©dĂ© pourrait donc dĂ©pendre des impacts environnementaux du cycle de vie de ses intrants.
LâhypothĂšse de recherche de cette thĂšse est quâune dĂ©cision de conception de procĂ©dĂ© prise post-optimisation avec analyse du cycle de vie (ACV), comparativement Ă la dĂ©cision prise sans ACV, apporte un bĂ©nĂ©fice Ă©conomique et environnemental Ă long terme dont lâespĂ©rance est mesurable, du moins lorsque les contraintes environnementales ne sont pas dĂ©jĂ internalisĂ©es dans le coĂ»t des intrants mais le seront plutĂŽt entre le moment de la conception et celui de la construction.
Lâobjectif gĂ©nĂ©ral de recherche de cette thĂšse est de faire une validation de principe de lâintĂ©gration de donnĂ©es dâACV dans un contexte dâoptimisation de la conception dâun procĂ©dĂ© par simulation informatique, et dâen mesurer le bĂ©nĂ©fice pour un cas dâĂ©tude. Ce faisant, il est souhaitĂ© de dĂ©velopper de nouvelles façons dâutiliser lâACV.
Le cas dâĂ©tude est la conception prĂ©liminaire dâun procĂ©dĂ© de capture de CO2 en postcombustion dans une centrale Ă©lectrique Ă cycle combinĂ© fonctionnant au gaz naturel. Le procĂ©dĂ© en boucle met la fumĂ©e refroidie en contact avec un absorbant chimique aqueux qui rĂ©agit avec le CO2, pour ensuite chauffer lâabsorbant et libĂ©rer le CO2 concentrĂ© qui peut ensuite ĂȘtre injectĂ© dans un aquifĂšre salin en haute mer. Les impacts environnementaux de ce procĂ©dĂ© viennent principaleÂŹment du fait que la dĂ©sorption consomme de la vapeur qui aurait autrement pu produire davantage dâĂ©lectricitĂ©. Des impacts supplĂ©mentaires sont associĂ©s au remplacement pĂ©riodique de lâabsorbant, aux fuites de sĂ©questration, ainsi quâaux infrastructures, Ă la machinerie et Ă lâĂ©nergie requises pour compresser, assĂ©cher, re-compresser, transporter et injecter le CO2.
La modĂ©lisation informatique du cas dâĂ©tude comprend un modĂšle de schĂ©ma dâĂ©coulement du procĂ©dĂ© incluant la cinĂ©tique de la capture, un modĂšle sĂ©parĂ© dâintĂ©gration thermique du procĂ©dĂ©, un algorithme gĂ©nĂ©tique dâoptimisation multicritĂšres, le calcul des fonctions-objectif, et une plate-forme sous-jacente fournie par le Laboratoire dâĂ©nergĂ©tique industrielle de lâĂcole polytechnique fĂ©dĂ©rale de Lausanne. Lâespace de dĂ©cision combine des variables de dĂ©cision continues (par exemple, la pression dâune colonne) et discrĂštes (par exemple, le branchement dâun Ă©coulement).
Une partie de lâoriginalitĂ© de la dĂ©marche est dâexplorer simultanĂ©ment de nombreuses configu-rations dâĂ©coulement et dâĂ©changeurs de chaleur grĂące Ă la plate-forme utilisĂ©e. Un objectif secondaire de recherche est donc de contribuer Ă lâĂ©tat de lâart de la conception des procĂ©dĂ©s de capture de CO2 en milieu aqueux, particuliĂšrement au niveau de lâintĂ©gration thermique.
La principale originalitĂ© de la dĂ©marche est toutefois de comparer la prise de dĂ©cisions en considĂ©rant toujours plusieurs façons de mesurer les impacts environnementaux avec ou sans lâACV. Cela permet de mesurer la contribution de lâACV elle-mĂȘme pour la prise de dĂ©cisions, mais aussi lâimportance des impacts environnementaux spĂ©cifiques Ă chaque intrant, quâil sâagisse de gaz naturel, dâacier, dâabsorbant, ou dâun service de transport et de sĂ©questration du CO2, ou encore spĂ©cifiques Ă chaque substance Ă©mise, quâil sâagisse du CO2 lui-mĂȘme, des autres gaz Ă effet de serre ou dâautres polluants.
Les principaux rĂ©sultats de ce travail sont que le coĂ»t de la capture du CO2, par unitĂ© de potentiel de rĂ©chauffement global Ă©vitĂ©, augmente dâenviron 3 % lorsquâon considĂšre les impacts dans une perspective du cycle de vie, et que câest le CO2 lui-mĂȘme, Ă©mis par les producteurs de gaz naturel et les transporteurs de CO2, qui contribue principalement Ă cette diffĂ©rence. Ainsi, lâACV peut mener Ă de meilleures dĂ©cisions dans plusieurs circonstances, en favorisant lâefficacitĂ© Ă©nergĂ©tique et la substitution de combustibles biogĂ©niques comme le gaz naturel synthĂ©tique du bois gazĂ©ifiĂ©, et en dĂ©cidant dâencourager les fournisseurs Ă rĂ©duire leurs propres Ă©missions. Dans le cas prĂ©cis oĂč une taxe anticipĂ©e sur le CO2 est tout juste suffisante pour donner lâimpression que la capture est rentable, alors quâune Ă©valuation dĂ©taillĂ©e de la mĂȘme taxe chez les fournisseurs indique quâelle nâest pas rentable, le recours Ă lâACV mĂšne alors Ă la dĂ©cision de payer la taxe plutĂŽt que de capturer le CO2, pour un gain dâenviron 0,64 / M W h a t t r i b u a b l e a Ë l â A C V , v a l i d a n t a i n s i l â h y p o t h e Ë s e d e r e c h e r c h e . L e s c o n c l u s i o n s t e c h n i q u e s , e Ë c o n o m i q u e s e t e n v i r o n n e m e n t a l e s d e Ë c o u l a n t d e c e s r e Ë s u l t a t s s o n t d e Ë t a i l l e Ë e s d a n s t r o i s a r t i c l e s s o u m i s a v e c c e t t e t h e Ë s e , r e s p e c t i v e m e n t . Q u e l q u e s o b s e r v a t i o n s o r i g i n a l e s s u r l e p l a n t e c h n i q u e r e Ë p o n d e n t a Ë l â o b j e c t i f s e c o n d a i r e d e r e c h e r c h e . E n p a r t i c u l i e r , i l p o u r r a i t e ^ t r e a v a n t a g e u x d â u t i l i s e r l a c h a l e u r d e l â a b s o r b a n t d e C O 2 a p p a u v r i p o u r g e Ë n e Ë r e r d e l a v a p e u r , c e q u i p e r m e t t r a i t d e s i m p l i f i e r l e d e Ë s o r b e u r . A u s s i , l e s r e Ë s u l t a t s d o n n e n t d e s v a l e u r s o p t i m a l e s r e l a t i v e m e n t e Ë l e v e Ë e s p o u r l a l a r g e u r d e l â a b s o r b e u r , l a c h a r g e n e t t e d e l â a b s o r b a n t e t l e t a u x d e c a p t u r e . C e l a p o r t e a Ë c r o i r e q u e l e s a u t e u r s p r e Ë c e Ë d e n t s n â o n t p a s p u a r r i v e r a Ë u n o p t i m u m g l o b a l p a r c e q u â i l s o n t u t i l i s e Ë u n n o m b r e i n s u f f i s a n t d e v a r i a b l e s d e d e Ë c i s i o n e t p a r c e q u e l e u r f o n c t i o n â o b j e c t i f , m i n i m i s e r l a c o n s o m m a t i o n d e v a p e u r , e s t i n a d e Ë q u a t e . C e p e n d a n t , c e s i d e Ë e s r e s t e n t a Ë v a l i d e r a v e c u n m o d e Ë l e p l u s d e Ë t a i l l e Ë . L e p r i n c i p a l a p p o r t a u x c o n n a i s s a n c e s d e c e t t e t h e Ë s e c o n s i s t e e n u n e n o u v e l l e m e Ë t h o d o l o g i e d â o p t i m i s a t i o n d u c y c l e d e v i e q u i c o m b i n e l â A C V e t l â a n a l y s e d e s c o u ^ t s d u c y c l e d e v i e . E l l e p e r m e t d â o p t i m i s e r l a c o n c e p t i o n d â u n p r o c e Ë d e Ë t o u t e n c o n s i d e Ë r a n t q u e l e s f o u r n i s s e u r s v o n t o p t i m i s e r e u x â m e ^ m e s l e u r s e Ë m i s s i o n s , s o u s l a p r e s s i o n s o i t d e n o u v e l l e s t a x e s , s o i t d â u n e p o l i t i q u e d â a p p r o v i s i o n n e m e n t a Ë d e Ë t e r m i n e r p l u s t a r d . S o n o r i g i n a l i t e Ë r e p o s e s u r u n e p o n d e Ë r a t i o n d e s e Ë m i s s i o n s d e s f o u r n i s s e u r s s e l o n l e u r c o u ^ t d â e Ë v i t e m e n t c o r r e s p o n d a n t a Ë u n m e Ë l a n g e o p t i m a l d e m e s u r e s d e p r e Ë v e n t i o n e t d e c o m p e n s a t i o n . C e t t e m e Ë t h o d o l o g i e e s t l a s e u l e p e r m e t t a n t u n e c o n c e p t i o n g l o b a l e m e n t o p t i m a l e , s e l o n l a d e Ë m o n s t r a t i o n f o u r n i e d a n s c e t t e t h e Ë s e , q u i s u g g e Ë r e e Ë g a l e m e n t q u e l a v a l i d i t e Ë d e l â a p p r o c h e s â e Ë t e n d a Ë t o u t e s l e s d e Ë c i s i o n s d e c o n c e p t i o n e n g e Ë n e Ë r a l . L â a n a l y s e d e s c o u ^ t s d â e Ë v i t e m e n t d e 3850 p r o c e s s u s e Ë l e Ë m e n t a i r e s d â u n e b a s e d e d o n n e Ë e s d â A C V d e Ë m o n t r e q u e p o u r u n e t r e Ë s l a r g e m a j o r i t e Ë d e p r o c e s s u s , q u a l i f i a b l e s d â e Ë n e r g i v o r e s , l e s c o u ^ t s d â e Ë v i t e m e n t d e s s u b s t a n c e s a u t r e s q u e l e C O 2 s o n t n e Ë g l i g e a b l e s p a r r a p p o r t a u x a u t r e s c o u ^ t s . C o m m e l e s c o u ^ t s n o n â e n v i r o n n e m e n t a u x r e p r e Ë s e n t e n t i n d i r e c t e m e n t d e s o p p o r t u n i t e Ë s d â e Ë v i t e m e n t d â i m p a c t s a i l l e u r s , i l e s t s o u v e n t p r e Ë f e Ë r a b l e , m e ^ m e d â u n p o i n t d e v u e s t r i c t e m e n t e n v i r o n n e ÂŹ m e n t a l , d â u t i l i s e r u n e c o n c e p t i o n d e f a i b l e c o u ^ t m a i s e n i n c i t a n t ( m o n e Ë t a i r e m e n t ) l e s f o u r n i s s e u r s a Ë p r e Ë v e n i r o u a Ë c o m p e n s e r l e u r s e Ë m i s s i o n s , p l u t o ^ t q u e d â u t i l i s e r l a c o n c e p t i o n d e f a i b l e s i m p a c t s s e l o n l â A C V s a n s e Ë g a r d a u x c o u ^ t s . E n f a i t , l o r s q u e l a t o t a l i t e Ë d e s i n t r a n t s d â u n p r o c e Ë d e Ë s o n t e Ë n e r g i v o r e s , l a c o n c e p t i o n o p t i m a l e e s t s i m p l e m e n t c e l l e q u i m i n i m i s e l e s c o u ^ t s d u c y c l e d e v i e , i n c l u a n t l e s f u t u r e s t a x e s i n d i r e c t e s s u r l e C O 2 q u e l â A C V p e r m e t d â e s t i m e r p o u r c h a q u e i n t r a n t . â â â â â â â â â â M u l t i â o b j e c t i v e p r o c e s s d e s i g n o p t i m i z a t i o n m a k e s i t p o s s i b l e t o s i m u l t a n e o u s l y m i n i m i z e t h e p o l l u t i n g e m i s s i o n s a n d t h e c o s t s o f a p r o c e s s , d e t e r m i n i n g a s e t o f c o n f i g u r a t i o n s t h a t r e p r e s e n t s t h e b e s t p o s s i b l e c o m p r o m i s e b e t w e e n t h e s e t w o o b j e c t i v e s , r e g a r d l e s s o f t h e i r f u t u r e w e i g h t i n g . T h e d e c i s i o n s p a c e m a y i n c l u d e d e s i g n v a r i a b l e s s u c h a s e q u i p m e n t s i z e o r o p e r a t i n g t e m p e r a t u r e a n d p r o c u r e m e n t v a r i a b l e s s u c h a s t h e c h o i c e o f a s u p p l i e r w i t h b e t t e r e n v i r o n m e n t a l p r a c t i c e s t h a n a n o t h e r . I n o r d e r t o o b t a i n t h e b e s t p o s s i b l e c o m p r o m i s e , t h e o b j e c t i v e f u n c t i o n s m u s t b e c o r r e c t l y d e f i n e d i n t h e f i r s t p l a c e â a p r o c e s s t h a t m a y r e q u i r e c o n s i d e r i n g t h e m i n a l i f e c y c l e p e r s p e c t i v e . T h e t r u l y o p t i m a l d e s i g n o f a p r o c e s s c o u l d t h u s d e p e n d o n t h e l i f e c y c l e e n v i r o n m e n t a l i m p a c t s o f a l l i t s i n p u t s . T h e r e s e a r c h h y p o t h e s i s o u t l i n e d i n t h i s d i s s e r t a t i o n a d v a n c e s t h a t a p r o c e s s d e s i g n d e c i s i o n t a k e n p o s t â o p t i m i z a t i o n w i t h l i f e c y c l e a s s e s s m e n t ( L C A ) , a s c o m p a r e d t o t h e d e c i s i o n t a k e n w i t h o u t L C A , b r i n g s a l o n g â t e r m e c o n o m i c a n d e n v i r o n m e n t a l b e n e f i t w i t h a m e a s u r a b l e e x p e c t a t i o n , a t l e a s t w h e n e n v i r o n m e n t a l c o n s t r a i n t s h a v e n o t b e e n i n t e r n a l i z e d i n i n p u t p r i c e s y e t , b u t w i l l b e i n t e r n a l i z e d b e t w e e n t h e d e s i g n p h a s e a n d t h e c o n s t r u c t i o n p h a s e . T h e g e n e r a l r e s e a r c h o b j e c t i v e i s t h e r e f o r e t o s e t o u t a p r o o f o f c o n c e p t o f L C A d a t a i n t e g r a t i o n i n t o a p r o c e s s d e s i g n o p t i m i z a t i o n c o n t e x t t h r o u g h c o m p u t e r s i m u l a t i o n a n d t h e n t o m e a s u r e t h e b e n e f i t s f o r a c a s e s t u d y , w i t h a v i e w t o d e v e l o p n e w w a y s o f u s i n g L C A . T h e c a s e s t u d y i n v o l v e s t h e p r e l i m i n a r y d e s i g n o f a p o s t â c o m b u s t i o n C O 2 c a p t u r e p r o c e s s i n a n a t u r a l g a s c o m b i n e d c y c l e p o w e r p l a n t . T h e c l o s e d â l o o p p r o c e s s p u t s c o l d f l u e g a s i n c o n t a c t w i t h a n a q u e o u s c h e m i c a l a b s o r b e n t t h a t r e a c t s w i t h t h e C O 2 , a f t e r w h i c h t h e a b s o r b e n t i s h e a t e d t o r e l e a s e c o n c e n t r a t e d C O 2 t h a t c a n l a t e r b e i n j e c t e d i n t o a s a l i n e a q u i f e r a t s e a . T h e e n v i r o n m e n t a l i m p a c t s o f t h e p r o c e s s s t e m i n m a j o r i t y f r o m t h e s t r i p p e r c o n s u m p t i o n o f s t e a m t h a t w o u l d o t h e r w i s e p r o d u c e m o r e e l e c t r i c i t y . A d d i t i o n a l i m p a c t s a r e g e n e r a t e d b y m a k e â u p a b s o r b e n t p r o d u c t i o n a n d b y s e q u e s t r a t i o n l e a k s , a s w e l l a s b y t h e i n f r a s t r u c t u r e , t h e m a c h i n e r y a n d t h e e n e r g y r e q u i r e d t o c o m p r e s s , d r y o u t , r e c o m p r e s s , t r a n s p o r t a n d i n j e c t t h e C O 2. C o m p u t e r m o d e l i n g o f t h e c a s e s t u d y i n c l u d e s a p r o c e s s f l o w â s h e e t i n g m o d e l t h a t a c c o u n t s f o r C O 2 c a p t u r e k i n e t i c s , a s e p a r a t e t h e r m a l i n t e g r a t i o n m o d e l , a g e n e t i c m u l t i â o b j e c t i v e o p t i m i ÂŹ z a t i o n a l g o r i t h m , t h e c a l c u l a t i o n o f o b j e c t i v e f u n c t i o n s a s w e l l a s a n u n d e r l y i n g p l a t f o r m p r o v i d e d b y t h e I n d u s t r i a l E n e r g y S y s t e m s L a b o r a t o r y o f E Ë c o l e p o l y t e c h n i q u e f e Ë d e Ë r a l e d e L a u s a n n e . T h e d e c i s i o n s p a c e c o m b i n e s c o n t i n u o u s ( e . g . a c o l u m n o p e r a t i n g p r e s s u r e ) a n d d i s c r e t e v a r i a b l e s ( e . g . t h e b r a n c h i n g o f a f l o w ) . P a r t o f t h e o r i g i n a l i t y o f t h e a p p r o a c h i s t h a t i t c o n c u r r e n t l y e x p l o r e s s e v e r a l a b s o r b e n t f l o w a n d h e a t e x c h a n g e r c o n f i g u r a t i o n s u s i n g t h e u n i q u e c a p a b i l i t i e s o f t h e p l a t f o r m . A s e c o n d a r y r e s e a r c h o b j e c t i v e i s t h e r e f o r e t o c o n t r i b u t e t o t h e s t a t e o f t h e a r t i n C O 2 c a p t u r e p r o c e s s d e s i g n , e s p e c i a l l y a s i t p e r t a i n s t o t h e r m a l i n t e g r a t i o n w i t h t h e p o w e r p l a n t s t e a m c y c l e . H o w e v e r , t h e o r i g i n a l i t y o f t h e a p p r o a c h i s m a i n l y d r i v e n b y t h e f a c t t h a t i t c o m p a r e s d e c i s i o n s m a d e b y c o n s i d e r i n g s e v e r a l w a y s o f m e a s u r i n g t h e e n v i r o n m e n t a l i m p a c t s , w i t h a n d w i t h o u t L C A , t h u s m a k i n g i t p o s s i b l e t o a s s e s s t h e c o n t r i b u t i o n o f L C A i t s e l f f o r d e c i s i o n â m a k i n g a s w e l l a s t h e s i g n i f i c a n c e o f t h e e n v i r o n m e n t a l i m p a c t s s p e c i f i c t o e a c h i n p u t ( e . g . n a t u r a l g a s , s t e e l , a b s o r b e n t , o r C O 2 t r a n s p o r t a n d s e q u e s t r a t i o n s e r v i c e s ) , o r s p e c i f i c t o e a c h s u b s t a n c e e m i t t e d ( e . g . C O 2 , o t h e r g r e e n h o u s e g a s e s , o r o t h e r p o l l u t a n t s ) . T h e m a i n r e s u l t s o f t h e r e s e a r c h a r e t h a t t h e C O 2 c a p t u r e c o s t s , p e r u n i t o f a v o i d e d g l o b a l w a r m i n g p o t e n t i a l , i n c r e a s e b y a p p r o x i m a t e l y 3 /MWh attribuable Ă lâACV, validant ainsi lâhypothĂšse de recherche.
Les conclusions techniques, économiques et environnementales découlant de ces résultats sont détaillées dans trois articles soumis avec cette thÚse, respectivement.
Quelques observations originales sur le plan technique rĂ©pondent Ă lâobjectif secondaire de recherche. En particulier, il pourrait ĂȘtre avantageux dâutiliser la chaleur de lâabsorbant de CO2 appauvri pour gĂ©nĂ©rer de la vapeur, ce qui permettrait de simplifier le dĂ©sorbeur. Aussi, les rĂ©sultats donnent des valeurs optimales relativement Ă©levĂ©es pour la largeur de lâabsorbeur, la charge nette de lâabsorbant et le taux de capture. Cela porte Ă croire que les auteurs prĂ©cĂ©dents nâont pas pu arriver Ă un optimum global parce quâils ont utilisĂ© un nombre insuffisant de variables de dĂ©cision et parce que leur fonction-objectif, minimiser la consommation de vapeur, est inadĂ©quate. Cependant, ces idĂ©es restent Ă valider avec un modĂšle plus dĂ©taillĂ©.
Le principal apport aux connaissances de cette thĂšse consiste en une nouvelle mĂ©thodologie dâoptimisation du cycle de vie qui combine lâACV et lâanalyse des coĂ»ts du cycle de vie. Elle permet dâoptimiser la conception dâun procĂ©dĂ© tout en considĂ©rant que les fournisseurs vont optimiser eux-mĂȘmes leurs Ă©missions, sous la pression soit de nouvelles taxes, soit dâune politique dâapprovisionnement Ă dĂ©terminer plus tard. Son originalitĂ© repose sur une pondĂ©ration des Ă©missions des fournisseurs selon leur coĂ»t dâĂ©vitement correspondant Ă un mĂ©lange optimal de mesures de prĂ©vention et de compensation. Cette mĂ©thodologie est la seule permettant une conception globalement optimale, selon la dĂ©monstration fournie dans cette thĂšse, qui suggĂšre Ă©galement que la validitĂ© de lâapproche sâĂ©tend Ă toutes les dĂ©cisions de conception en gĂ©nĂ©ral.
Lâanalyse des coĂ»ts dâĂ©vitement de 3850 processus Ă©lĂ©mentaires dâune base de donnĂ©es dâACV dĂ©montre que pour une trĂšs large majoritĂ© de processus, qualifiables dâĂ©nergivores, les coĂ»ts dâĂ©vitement des substances autres que le CO2 sont nĂ©gligeables par rapport aux autres coĂ»ts. Comme les coĂ»ts non-environnementaux reprĂ©sentent indirectement des opportunitĂ©s dâĂ©vitement dâimpacts ailleurs, il est souvent prĂ©fĂ©rable, mĂȘme dâun point de vue strictement environneÂŹmental, dâutiliser une conception de faible coĂ»t mais en incitant (monĂ©tairement) les fournisseurs Ă prĂ©venir ou Ă compenser leurs Ă©missions, plutĂŽt que dâutiliser la conception de faibles impacts selon lâACV sans Ă©gard aux coĂ»ts. En fait, lorsque la totalitĂ© des intrants dâun procĂ©dĂ© sont Ă©nergivores, la conception optimale est simplement celle qui minimise les coĂ»ts du cycle de vie, incluant les futures taxes indirectes sur le CO2 que lâACV permet dâestimer pour chaque intrant.
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Multi-objective process design optimization makes it possible to simultaneously minimize the polluting emissions and the costs of a process, determining a set of configurations that represents the best possible compromise between these two objectives, regardless of their future weighting. The decision space may include design variables such as equipment size or operating temperature and procurement variables such as the choice of a supplier with better environmental practices than another. In order to obtain the best possible compromise, the objective functions must be correctly defined in the first place â a process that may require considering them in a life cycle perspective. The truly optimal design of a process could thus depend on the life cycle environmental impacts of all its inputs.
The research hypothesis outlined in this dissertation advances that a process design decision taken post-optimization with life cycle assessment (LCA), as compared to the decision taken without LCA, brings a long-term economic and environmental benefit with a measurable expectation, at least when environmental constraints have not been internalized in input prices yet, but will be internalized between the design phase and the construction phase.
The general research objective is therefore to set out a proof of concept of LCA data integration into a process design optimization context through computer simulation and then to measure the benefits for a case study, with a view to develop new ways of using LCA.
The case study involves the preliminary design of a post-combustion CO2 capture process in a natural gas combined cycle power plant. The closed-loop process puts cold flue gas in contact with an aqueous chemical absorbent that reacts with the CO2, after which the absorbent is heated to release concentrated CO2 that can later be injected into a saline aquifer at sea. The environmental impacts of the process stem in majority from the stripper consumption of steam that would otherwise produce more electricity. Additional impacts are generated by make-up absorbent production and by sequestration leaks, as well as by the infrastructure, the machinery and the energy required to compress, dry out, recompress, transport and inject the CO2.
Computer modeling of the case study includes a process flow-sheeting model that accounts for CO2 capture kinetics, a separate thermal integration model, a genetic multi-objective optimiÂŹzation algorithm, the calculation of objective functions as well as an underlying platform provided by the Industrial Energy Systems Laboratory of Ăcole polytechnique fĂ©dĂ©rale de Lausanne. The decision space combines continuous (e.g. a column operating pressure) and discrete variables (e.g. the branching of a flow).
Part of the originality of the approach is that it concurrently explores several absorbent flow and heat exchanger configurations using the unique capabilities of the platform. A secondary research objective is therefore to contribute to the state of the art in CO2 capture process design, especially as it pertains to thermal integration with the power plant steam cycle.
However, the originality of the approach is mainly driven by the fact that it compares decisions made by considering several ways of measuring the environmental impacts, with and without LCA, thus making it possible to assess the contribution of LCA itself for decision-making as well as the significance of the environmental impacts specific to each input (e.g. natural gas, steel, absorbent, or CO2 transport and sequestration services), or specific to each substance emitted (e.g. CO2, other greenhouse gases, or other pollutants).
The main results of the research are that the CO2 capture costs, per unit of avoided global warming potential, increase by approximately 3% when considering impacts in a life cycle perspective and that it is the CO2 released by natural gas producers and CO2 transporters that largely contribute to the increase. LCA can therefore lead to better decision-making in several circumstances by fostering energy efficiency and the substitution of biogenic fuels such as synthetic natural gas from wood gasification as well as by choosing to incite suppliers to reduce their emissions. In the specific case in which an anticipated CO2 tax is just enough to give the impression that capture is profitable while a detailed assessment of the same tax as paid by suppliers indicates that it is not, LCA will support the decision to pay the tax rather than capture the CO2, for a net gain of some / M Wha tt r ib u ab l e a Ë l â A C V , v a l i d an t ain s i l â h y p o t h e Ë se d erec h erc h e . L esco n c l u s i o n s t ec hni q u es , e Ë co n o mi q u ese t e n v i ro nn e m e n t a l es d e Ë co u l an t d ecesr e Ë s u lt a t sso n t d e Ë t ai ll e Ë es d an s t ro i s a r t i c l esso u mi s a v ecce tt e t h e Ë se , res p ec t i v e m e n t . Q u e lq u eso b ser v a t i o n sor i g ina l ess u r l e pl an t ec hni q u er e Ë p o n d e n t a Ë l â o bj ec t i f seco n d ai re d erec h erc h e . E n p a r t i c u l i er , i lp o u rr ai t e ^ t re a v an t a g e ux d â u t i l i ser l a c ha l e u r d e l â ab sor ban t d e CO 2 a pp a uv r i p o u r g e Ë n e Ë rer d e l a v a p e u r , ce q u i p er m e tt r ai t d es im pl i f i er l e d e Ë sor b e u r . A u ss i , l esr e Ë s u lt a t s d o nn e n t d es v a l e u rso pt ima l esre l a t i v e m e n t e Ë l e v e Ë es p o u r l a l a r g e u r d e l â ab sor b e u r , l a c ha r g e n e tt e d e l â ab sor ban t e tl e t a ux d ec a pt u re . C e l a p or t e a Ë cro i re q u e l es a u t e u rs p r e Ë c e Ë d e n t s n â o n tp a s p u a rr i v er a Ë u n o pt im u m g l o ba lp a rce q u â i l so n t u t i l i s e Ë u nn o mb re in s u ff i s an t d e v a r iab l es d e d e Ë c i s i o n e tp a rce q u e l e u r f o n c t i o n â o bj ec t i f , minimi ser l a co n so mma t i o n d e v a p e u r , es t ina d e Ë q u a t e . C e p e n d an t , ces i d e Ë esres t e n t a Ë v a l i d er a v ec u nm o d e Ë l e pl u s d e Ë t ai ll e Ë . L e p r in c i p a l a pp or t a ux co nnai ss an ces d ece tt e t h e Ë seco n s i s t ee n u n e n o uv e ll e m e Ë t h o d o l o g i e d â o pt imi s a t i o n d u cyc l e d e v i e q u i co mbin e l â A C V e tl â ana l yse d esco u ^ t s d u cyc l e d e v i e . Ell e p er m e t d â o pt imi ser l a co n ce pt i o n d â u n p roc e Ë d e Ë t o u t e n co n s i d e Ë r an tq u e l es f o u r ni sse u rs v o n t o pt imi sere ux â m e ^ m es l e u rs e Ë mi ss i o n s , so u s l a p ress i o n so i t d e n o uv e ll es t a x es , so i t d â u n e p o l i t i q u e d â a pp ro v i s i o nn e m e n t a Ë d e Ë t er min er pl u s t a r d . S o n or i g ina l i t e Ë re p oses u r u n e p o n d e Ë r a t i o n d es e Ë mi ss i o n s d es f o u r ni sse u rsse l o n l e u rco u ^ t d â e Ë v i t e m e n t corres p o n d an t a Ë u nm e Ë l an g eo pt ima l d e m es u res d e p r e Ë v e n t i o n e t d eco m p e n s a t i o n . C e tt e m e Ë t h o d o l o g i ees tl a se u l e p er m e tt an t u n eco n ce pt i o n g l o ba l e m e n t o pt ima l e , se l o n l a d e Ë m o n s t r a t i o n f o u r ni e d an sce tt e t h e Ë se , q u i s ugg e Ë re e Ë g a l e m e n tq u e l a v a l i d i t e Ë d e l â a pp roc h es â e Ë t e n d a Ë t o u t es l es d e Ë c i s i o n s d eco n ce pt i o n e n g e Ë n e Ë r a l . L â ana l yse d esco u ^ t s d â e Ë v i t e m e n t d e 3850 p rocess u s e Ë l e Ë m e n t ai res d â u n e ba se d e d o nn e Ë es d â A C V d e Ë m o n t re q u e p o u r u n e t r e Ë s l a r g e maj or i t e Ë d e p rocess u s , q u a l i f iab l es d â e Ë n er g i v ores , l esco u ^ t s d â e Ë v i t e m e n t d ess u b s t an ces a u t res q u e l e CO 2 so n t n e Ë g l i g e ab l es p a rr a pp or t a ux a u t resco u ^ t s . C o mm e l esco u ^ t s n o n â e n v i ro nn e m e n t a ux re p r e Ë se n t e n t in d i rec t e m e n t d eso pp or t u ni t e Ë s d â e Ë v i t e m e n t d â im p a c t s ai ll e u rs , i l es t so uv e n tp r e Ë f e Ë r ab l e , m e ^ m e d â u n p o in t d e vu es t r i c t e m e n t e n v i ro nn e ÂŹ m e n t a l , d â u t i l i ser u n eco n ce pt i o n d e f aib l eco u ^ t mai se nin c i t an t ( m o n e Ë t ai re m e n t ) l es f o u r ni sse u rs a Ë p r e Ë v e ni ro u a Ë co m p e n ser l e u rs e Ë mi ss i o n s , pl u t o ^ tq u e d â u t i l i ser l a co n ce pt i o n d e f aib l es im p a c t sse l o n l â A C V s an s e Ë g a r d a ux co u ^ t s . E n f ai t , l ors q u e l a t o t a l i t e Ë d es in t r an t s d â u n p roc e Ë d e Ë so n t e Ë n er g i v ores , l a co n ce pt i o n o pt ima l ees t s im pl e m e n t ce ll e q u iminimi se l esco u ^ t s d u cyc l e d e v i e , in c l u an tl es f u t u res t a x es in d i rec t ess u r l e CO 2 q u e l â A C V p er m e t d â es t im er p o u rc ha q u e in t r an t . â â â â â â â â â â M u lt i â o bj ec t i v e p rocess d es i g n o pt imi z a t i o nmak es i tp oss ib l e t os im u lt an eo u s l y minimi ze t h e p o ll u t in g e mi ss i o n s an d t h ecos t so f a p rocess , d e t er minin g a se t o f co n f i gu r a t i o n s t ha t re p rese n t s t h e b es tp oss ib l eco m p ro mi se b e tw ee n t h ese tw oo bj ec t i v es , re g a r d l esso f t h e i r f u t u re w e i g h t in g . T h e d ec i s i o n s p a ce ma y in c l u d e d es i g n v a r iab l ess u c ha se q u i p m e n t s i zeoro p er a t in g t e m p er a t u re an d p roc u re m e n t v a r iab l ess u c ha s t h ec h o i ceo f a s u ppl i er w i t hb e tt ere n v i ro nm e n t a lp r a c t i ces t hanan o t h er . I n or d er t oo b t ain t h e b es tp oss ib l eco m p ro mi se , t h eo bj ec t i v e f u n c t i o n s m u s t b ecorrec tl y d e f in e d in t h e f i rs tpl a ce â a p rocess t ha t ma yre q u i reco n s i d er in g t h e mina l i f ecyc l e p ers p ec t i v e . T h e t r u l yo pt ima l d es i g n o f a p rocessco u l d t h u s d e p e n d o n t h e l i f ecyc l ee n v i ro nm e n t a l im p a c t so f a ll i t s in p u t s . T h erese a rc hh y p o t h es i so u tl in e d in t hi s d i sser t a t i o na d v an ces t ha t a p rocess d es i g n d ec i s i o n t ak e n p os t â o pt imi z a t i o n w i t h l i f ecyc l e a ssess m e n t ( L C A ) , a sco m p a re d t o t h e d ec i s i o n t ak e n w i t h o u t L C A , b r in g s a l o n g â t er m eco n o mi c an d e n v i ro nm e n t a l b e n e f i tw i t ham e a s u r ab l ee x p ec t a t i o n , a tl e a s tw h e n e n v i ro nm e n t a l co n s t r ain t s ha v e n o t b ee nin t er na l i ze d inin p u tp r i cesye t , b u tw i ll b e in t er na l i ze d b e tw ee n t h e d es i g n p ha se an d t h eco n s t r u c t i o n p ha se . T h e g e n er a l rese a rc h o bj ec t i v e i s t h ere f ore t ose t o u t a p roo f o f co n ce pt o f L C A d a t ain t e g r a t i o nin t o a p rocess d es i g n o pt imi z a t i o n co n t e x tt h ro ug h co m p u t ers im u l a t i o nan d t h e n t o m e a s u re t h e b e n e f i t s f or a c a ses t u d y , w i t ha v i e wt o d e v e l o p n e ww a yso f u s in gL C A . T h ec a ses t u d y in v o l v es t h e p re l imina ry d es i g n o f a p os t â co mb u s t i o n CO 2 c a pt u re p rocess inana t u r a l g a sco mbin e d cyc l e p o w er pl an t . T h ec l ose d â l oo pp rocess p u t sco l df l u e g a s in co n t a c tw i t hana q u eo u sc h e mi c a l ab sor b e n tt ha t re a c t s w i t h t h e CO 2 , a f t er w hi c h t h e ab sor b e n t i s h e a t e d t ore l e a seco n ce n t r a t e d CO 2 t ha t c an l a t er b e inj ec t e d in t o a s a l in e a q u i f er a t se a . T h ee n v i ro nm e n t a l im p a c t so f t h e p rocesss t e minmaj or i t y f ro m t h es t r i pp erco n s u m pt i o n o f s t e am t ha tw o u l d o t h er w i se p ro d u ce m oree l ec t r i c i t y . A dd i t i o na l im p a c t s a re g e n er a t e d b y mak e â u p ab sor b e n tp ro d u c t i o nan d b yse q u es t r a t i o n l e ak s , a s w e ll a s b y t h e in f r a s t r u c t u re , t h e ma c hin ery an d t h ee n er g yre q u i re d t oco m p ress , d ryo u t , reco m p ress , t r an s p or t an d inj ec tt h e CO 2. C o m p u t er m o d e l in g o f t h ec a ses t u d y in c l u d es a p rocess f l o w â s h ee t in g m o d e lt ha t a cco u n t s f or CO 2 c a pt u re kin e t i cs , a se p a r a t e t h er ma l in t e g r a t i o nm o d e l , a g e n e t i c m u lt i â o bj ec t i v eo pt imi ÂŹ z a t i o na l g or i t hm , t h ec a l c u l a t i o n o f o bj ec t i v e f u n c t i o n s a s w e ll a s an u n d er l y in g pl a t f or m p ro v i d e d b y t h e I n d u s t r ia lE n er g y S ys t e m s L ab or a t oryo f E Ë co l e p o l y t ec hni q u e f e Ë d e Ë r a l e d e L a u s ann e . T h e d ec i s i o n s p a ceco mbin esco n t in u o u s ( e . g . a co l u mn o p er a t in g p ress u re ) an dd i scre t e v a r iab l es ( e . g . t h e b r an c hin g o f a f l o w ) . P a r t o f t h eor i g ina l i t yo f t h e a pp ro a c hi s t ha t i t co n c u rre n tl ye x pl oresse v er a l ab sor b e n t f l o w an d h e a t e x c han g erco n f i gu r a t i o n s u s in g t h e u ni q u ec a p abi l i t i eso f t h e pl a t f or m . A seco n d a ryrese a rc h o bj ec t i v e i s t h ere f ore t oco n t r ib u t e t o t h es t a t eo f t h e a r t in CO 2 c a pt u re p rocess d es i g n , es p ec ia ll y a s i tp er t ain s t o t h er ma l in t e g r a t i o n w i t h t h e p o w er pl an t s t e am cyc l e . Ho w e v er , t h eor i g ina l i t yo f t h e a pp ro a c hi s main l y d r i v e nb y t h e f a c tt ha t i t co m p a res d ec i s i o n s ma d e b yco n s i d er in g se v er a lw a yso f m e a s u r in g t h ee n v i ro nm e n t a l im p a c t s , w i t han d w i t h o u t L C A , t h u s makin g i tp oss ib l e t o a ssess t h eco n t r ib u t i o n o f L C A i t se l ff or d ec i s i o n â makin g a s w e ll a s t h es i g ni f i c an ceo f t h ee n v i ro nm e n t a l im p a c t ss p ec i f i c t oe a c hin p u t ( e . g . na t u r a l g a s , s t ee l , ab sor b e n t , or CO 2 t r an s p or t an d se q u es t r a t i o n ser v i ces ) , ors p ec i f i c t oe a c h s u b s t an cee mi tt e d ( e . g . CO 2 , o t h er g ree nh o u se g a ses , oro t h er p o ll u t an t s ) . T h e main res u lt so f t h erese a rc ha re t ha tt h e CO 2 c a pt u recos t s , p er u ni t o f a v o i d e d g l o ba lw a r min g p o t e n t ia l , in cre a se b y a pp ro x ima t e l y 3 0.64/MWh attributable to the LCA, therefore validating the research hypothesis.
The technical, economic and environmental conclusions drawn from these results are detailed in the three articles submitted with this dissertation, respectively.
Certain novel technical observations meet the secondary research objective. More specifically, it may be advantageous to use the lean CO2 absorbent heat to generate steam, simplifying stripper design. Also, results include relatively high optimal values for the absorber width, the net absorbent loading and the overall capture rate, leading one to believe that other authors have not yet been able to reach a global optimum because they used too few decision variables and they relied on the inadequate objective function of minimizing steam consumption. However, these ideas must still be validated using a more detailed model including the individual cost of the main heat exchangers.
This dissertationâs main contribution to scientific knowledge consists in a new life cycle optimization methodology that combines life cycle assessment and life cycle costing, making it possible to optimize a process design while considering that suppliers will also optimize their emissions themselves because of future taxes or voluntarily through a procurement policy to be determined at a later date. Its originality is based on a method for weighting supply-chain emissions according to the avoidance cost of an optimal combination of prevention and compensation measures. According to the theoretical demonstration set out in the dissertation, this methodology is the only approach that makes it possible to determine a globally optimal design, and it is suggested that its validity extends to all design decision-making in general.
The avoidance costs of a batch of 3 850 elementary processes from an LCA database show that, for the vast majority of these processes defined as energy-intensive, the avoidance costs of non-CO2 substances are negligible as compared to all other costs. Since non-environmental costs indirectly represent avoidance opportunities elsewhere, it is often preferable â even from a strictly environmental perspective â to use a low-cost design and incite suppliers (using money) to prevent or compensate their emissions rather than rely on a design with low-impacts according to LCA but without considering costs. In fact, when all inputs of a process are energy-intensive, the optimal process design is simply the one that minimizes all life cycle costs, including indirect future CO2 taxes that LCA can estimate for each input
Publication venue 'IntechOpen'
Publication date 17/02/2012
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Publication venue 'Springer Science and Business Media LLC'
Publication date 01/01/2013
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Publication venue 'Springer Science and Business Media LLC'
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Publication venue Blackwell Publishing Inc
Publication date 01/01/2010
Field of study Get PDF Ecological speciation is the process by which barriers to gene flow between populations evolve due to adaptive divergence via natural selection. A relatively unexplored area in ecological speciation is the role of gene expression. Gene expression may be associated with ecologically important phenotypes not evident from morphology and play a role during colonization of new environments. Here we review two potential roles of gene expression in ecological speciation: (1) its indirect role in facilitating population persistence and (2) its direct role in contributing to genetically based reproductive isolation. We find indirect evidence that gene expression facilitates population persistence, but direct tests are lacking. We also find clear examples of gene expression having effects on phenotypic traits and adaptive genetic divergence, but links to the evolution of reproductive isolation itself remain indirect. Gene expression during adaptive divergence seems to often involve complex genetic architectures controlled by gene networks, regulatory regions, and âeQTL hotspots.â Nonetheless, we review how approaches for isolating the functional mutations contributing to adaptive divergence are proving to be successful. The study of gene expression has promise for increasing our understanding ecological speciation, particularly when integrative approaches are applied
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Publication date 01/01/2015
Field of study Get PDF The "sonic region" of the Sun corona remains extremely difficult to observe with spatial resolution and sensitivity sufficient to understand the fine scale phenomena that govern the quiescent solar corona, as well as phenomena that lead to coronal mass ejections (CMEs), which influence space weather. Improvement on this front requires eclipse-like conditions over long observation times. The space-borne coronagraphs flown so far provided a continuous coverage of the external parts of the corona but their over-occulting system did not permit to analyse the part of the white-light corona where the main coronal mass is concentrated. The proposed PROBA-3 Coronagraph System, also known as ASPIICS (Association of Spacecraft for Polarimetric and Imaging Investigation of the Corona of the Sun), with its novel design, will be the first space coronagraph to cover the range of radial distances between ~1.08 and 3 solar radii where the magnetic field plays a crucial role in the coronal dynamics, thus providing continuous observational conditions very close to those during a total solar eclipse. PROBA-3 is first a mission devoted to the in-orbit demonstration of precise formation flying techniques and technologies for future European missions, which will fly ASPIICS as primary payload. The instrument is distributed over two satellites flying in formation (approx. 150m apart) to form a giant coronagraph capable of producing a nearly perfect eclipse allowing observing the sun corona closer to the rim than ever before. The coronagraph instrument is developed by a large European consortium including about 20 partners from 7 countries under the auspices of the European Space Agency. This paper is reviewing the recent improvements and design updates of the ASPIICS instrument as it is stepping into the detailed design phase
Publication venue
Publication date 27/03/2008
Field of study No full text Multi-objective optimisation methodology is ideally suited to incorporate environmental objectives in the optimal design of energy conversion systems. It is also desirable to optimise life-cycle emissions rather than local emissions only. However, using LCA results directly as an objective function gives sub-optimal designs, since inputs whose life-cycle emissions are easy to reduce are underused and vice-versa. We develop a method integrating LCA results in an optimisation framework, so that the optimal designs found are logically consistent with the aim of minimising global emissions. We then apply the method to the optimisation of a NGCC power plant with CO2 capture, optimising column dimensions, solvent flow and heat exchange configuration with respect to two functions: cost and life cycle global warming potential. This method of using LCA results has an actual benefit since the cost of reducing local emissions cannot be lower than for global emissions