N° d’ordre : 2005-ISAL-0089
Année 2005
Thèse
Traitement des boues par friture :
Des mécanismes physiques à l’éco-conception d’un procédé
Présentée devant
L’institut national des sciences appliquées de Lyon Pour obtenir
Le grade de docteur Formation doctorale Sciences de l’Environnement Industriel et Urbain École doctorale de Chimie de Lyon Par
Carlos-Alberto PEREGRINA-CAMBERO (Ingénieur) Soutenue le 01 décembre 2005 devant la Commission d’examen Jury MM.
P. ARLABOSSE R. GOURDON Rapporteur T. KUDRA D. LECOMTE V. RUDOLPH Rapporteur G. TRYSTRAM
Maître Assistant (EMAC) Professeur (INSA de Lyon) Chercheur Scientifique Senior (CANMET) Professeur (EMAC) Professeur (University of Queensland) Professeur (ENSIA)
N° d’ordre : 2005-ISAL-0089 Année 2005
Thèse
Traitement des boues par friture : Des mécanismes physiques à l’éco-conception d’un procédé
Présentée devant
L’institut national des sciences appliquées de Lyon Pour obtenir
Le grade de docteur Formation doctorale Sciences et Techniques du Déchet École doctorale École doctorale de Chimie de Lyon Par
Carlos-Alberto PEREGRINA-CAMBERO Soutenue le 01 décembre 2005 devant la Commission d’examen Jury MM.
P. ARLABOSSE R. GOURDON Rapporteur T. KUDRA D. LECOMTE V. RUDOLPH Rapporteur G. TRYSTRAM
Maître Assistant (EMAC) Professeur (INSA de Lyon) Chercheur Scientifique Senior (CANMET) Professeur (EMAC) Professeur (University of Queensland) Professeur (ENSIA)
Fry-drying of sewage sludge : From the physical mechanisms to the process eco-design
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To Cécilia…
“The gods did not reveal, from the beginning all things to us; but in the course of time, through seeking, men find that which is the better…” Xenophanes
Fry-drying of sewage sludge : From the physical mechanisms to the process eco-design
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ACKNOWLEDGEMENTS A mixture of feelings came to me when I started writing this section. On the one hand, it is always sad to say “good bye” to all the people who made my stay at LGPSD throughout the last three years so pleasant. On the other, there is a great satisfaction to end a great project and start new adventures in life. I would like to begin my acknowledgements by expressing my thankfulness to my principal advisor -and big FRIEND- Professor Didier LECOMTE, who gave me the chance to work in this amazing project and always stood by me. All your advice at both working and personal levels will be with me for a lifetime. I am also thankful to my co-advisor Patricia ARLABOSSE. Thanks for all your lessons and criticism, which sometimes were difficult to take but they always challenged me and made me think. Thanks to Professor Victor RUDOLPH who assisted me throughout my stay in Australia. Without your guidance this project would not have been the same. I want to express my gratitude to all the technical staff of the "Epi ENER"at LGPSD: JeanMarie, Jean-Claude, Dénis, Ludivine and especially to Bernard(o!), who designed the first fry-dryer for sewage sludge ever built. Thanks you all for your help and your time, feel sure that this Mexican is going to miss you! Thanks to Jean-Michel MEOT, Philippe BOHUON and Henri BAILLERES from the CIRAD. Thanks for sharing your knowledge with me in a so fine manner. Be sure that I will never see frying in the same way. I am grateful for the immeasurable help received from Professor Gilles TRYSTRAM and Senior Research Scientist Tadeusz KUDRA who accepted the responsible task of checking and assessing this thesis. I want to show my gratitude to Sylvie PADILLA and Marlène DRESCH not only because of the very important financial support received from the ADEME, but also for all the exchanges and discussions at the different stages of this thesis. Thanks a lot to all the friends and colleagues from the École des Mines d’Albi Carmaux who I shared a lot of good moments with, specially Máximo, Ana, Karim, Daniela, Naly and Anwar. I want to particularly thank Sofía (mi gran amiga Venezolana y mi colega de oficina). Thanks for all those moments when we started to talk about sewage sludge and finished discussing the sadness of being far away from home. Many thanks also to Carmen (mi Carmela) and Miguel(ón), my deepest Mexico-Albigeois friends. I will never forget all your help and support in the good, but above all, in the worst moments that I lived in Albi. In addition, to be part of other Research groups than LGPSD was an extremely enriching experience. First, during my meetings at the CIRAD and then during my stay at UQ, I met wonderful people who explained to me how amazing interdisciplinary work could be. I want to thank David, Aracely, Yanine and Juan in Montpellier as well as Adrian, Stephano, Federico, Rossana, Hein, Dino, Manu, Wally, Brama in Brisbane and especially Bradley LADEWIG: Thanks a lot for everything,…mate!!!. I wish I could drink a beer again with all you guys!!
Fry-drying of sewage sludge : From the physical mechanisms to the process eco-design
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Doing a PhD thesis overseas is not an every-day funny task. That is why I want to thank to all those friends that in despite of the distance have known how to remain close to me: Quetzalcoatl, Angel, Robert, Jesús, León, Solecillo, Mariachi, Ale, Olivier, Janette, Lola, Susana, Raymundo, Dr. Nungaray and padrino Michel! I want also express gratitude to my French family who made my stay in France a lovely experience. Thanks to Monsieur Alain, Madame Marie-France, Emilie and Amaël because when you opened me the doors of your house, you did the same with your heart’s. Furthermore, I want to show endless gratitude to my PADRE SANTO and my MADRE SANTA who just gave me life and the knowledge to live it… without YOU this could have never been done! Thank you from the deepest of my heart (Los adoro con toda mi alma canijos!). Thanks to my grand mother Lucrecia, I hope to see you soon!! Thanks to my cousin -almost brother- el Giorgio!. All my love and admiration go to my brothers and sisters: Cristina (Titina), Adriana (Adrianation), Miguel (Miguelito) and Vidal (Shélélé)…you have been every day -and especially THOSE days- in my mind and close to my heart from the beginning of this experience until now. Finally I want to thank Cécilia, mi güera adorada, because of everything. How could I imagine these three years without you? …and even less the rest of my life?! Thanks for being the perfect woman to me…celle qui m’invite à rêver en me posant bien les pieds sur terre! Thank you all.
Fry-drying of sewage sludge : From the physical mechanisms to the process eco-design
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Traitement des boues par friture : Des mécanismes physiques à l’éco-conception d’un procédé Résumé Le procédé de séchage par friture consiste à mettre en contact une phase solide humide divisée (la boue d'épuration) et une phase liquide non miscible (une huile alimentaire usagée), chauffée entre 120 et 180°C, pour obtenir un solide granulaire stable, hygiénisé et valorisable notamment comme combustible. Une étude expérimentale à l’échelle du laboratoire a permis d’identifier les différents mécanismes de transfert de chaleur et de masse mis en jeu lors de l’opération de friture de boue et d’optimiser les paramètres opératoires. Aux temps courts, les phénomènes limitants sont d’origine thermique. Aux temps longs, la limitation des transferts provient du transport d’huile au sein de la matrice poreuse puis du transfert de matière en phase vapeur. Une Analyse de Cycle de Vie (ACV) a été mise en œuvre pour évaluer les performances environnementales d’une filière thermique « séchage + incinération » de valorisation des boues. Le scénario de référence fait appel à un séchage par contact avec agitation tandis que le scénario alternatif prévoit un séchage par friture. Parmi les quatre catégories d’impact retenues, le séchage par friture s’avère extrêmement performant en terme de consommation des ressources non renouvelables et d’impact sur le changement climatique. Enfin, la simulation d’un procédé continu, fonctionnant sur la base d’une production d’une tonne par heure de boues auto-combustibles, avec différents systèmes de récupération de l’énergie contenue dans les buées a été réalisée à l’aide d’un logiciel du commerce. Ce dimensionnement a servi de base à une évaluation économique des coûts d’investissement et de fonctionnement de l’installation. Mots-Clés: séchage - friture – boues – huiles usagées alimentaires – transferts de chaleur et de matière – analyse du cycle de vie – analyse économique
Fry-drying of sewage sludge : From the physical mechanisms to the process eco-design
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Fry-drying of sewage sludge : From the physical mechanisms to the process eco-design Abstract Fry-drying of sewage sludge consists in bringing into contact the wet solid with a heated oil (120°C 95 % in about 600 seconds. Moreover, due to the oil impregnation, the lower heating value (LHV) of fry-dried sludge reached 24 MJ·kg-1 which is significantly higher to that of the same air-dried sludge (i.e. 14MJ·kg-1). Fry-drying principle gathers some characteristics of direct and indirect dryers reducing the number of technical problems found in the thermal drying of sewage sludge. In principle, the configuration of the frying process looks as simple as direct drying, where the product is directly contacted with the heating mobile phase (i.e. frying oil) without needing any frictional device, avoiding thus the plastic phase related problems [37, 38]. Moreover, the water is removed by a boiling mechanism giving small amounts of exhaust vapors highly concentrated in water vapor. Hence, fry-drying is suitable to be equipped with an emission management system to treat the exhaust emission and recover the vapor latent energy.
Fry-drying of sewage sludge : From the physical mechanisms to the process eco-design
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Introduction
Then, oil uptake offers several benefits concerning the conditioning of the dried sludge. It increases the energy value of the sludge [35]. Large size particles of the fried sludge could be obtained due to the agglomeration of the dry solids particles that are coated by a final oily layer. Finally, it seems that the several physico-chemical reactions taking place during fry-drying [39, 40] may stabilize the final product enabling its storage and/or transportation. However, the oil impregnation does not allow agricultural land spreading. As a result, incineration will remain the only valorization disposal route of the fried sewage sludge. This apparent major constraint is not very limiting in the European context since most countries tend to limit sewage sludge land filling and land spreading [3, 4, 17]. In addition, new incineration technologies as well as pyrolysis and gasification processes applied to the sewage sludge [17], which belong to the group of Energy from Waste Incineration (EfWI) [41], claim their place as natural companion of the practicable recycling in a truly integrated waste management hierarchy. Another key issue is that the application of this process requires the use of a co-product, the frying oil, and thus its availability. In order to improve the environmental and economic performance of the proposed operation, it was decided to use the recycled cooking oil (RCO) as the frying oil in this study. RCO is the generic name for the oily phase resulting after several stages of purification of waste vegetable oils and greasy wastewater collected in the grease-traps of restaurants, agricultural and food industries outlets [42]. Due to the food safety problems in 1999 in Europe, the market for recycled cooking oils (RCO) has considerably decreased– previously animal feed accounted for 85% of the oil collected in France [43]– so much that finding new ways of disposing RCO has become a major concern for the European food industry. As a consequence these food industry by-products are becoming available as an oil resource with good chemical and physical stability [43]. In France, the yearly production of this waste is estimated in 30000 tons and as a consequence new methods for the economic disposal of RCO are required [44]. Co-valorization of RCO and sewage sludge to formulate a derived bio-fuel provides such an opportunity.
Fry-drying of sewage sludge : From the physical mechanisms to the process eco-design
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Introduction
Although there seems to be significant advantages of this new drying process the feasibility of any new idea or a new process should be evaluated by comparing its performance with other equivalent or competing processes, usually assessed on a basis of technical, economical and environmental criteria [18]. The calculation of technical and economic performances can be made using extensive values (e.g. monetary values, energy contents) whereas the environmental impact needs a different approach. Environmental assessment uses a set of variables that are highly dependent on the assumptions made by the evaluators [45], a subjective input, which raises difficulties when using any environmental assessment tool [46].
The main goal of this study is to determine the feasibility of the immersion frying operation applied to the thermal drying of sewage sludge. The broad scope of the goal as well as the innovative characteristics of the subject, required an original procedure based on the following four specific goals: 1. Identification of the heat and mass transfer mechanisms involved in the operation; 2. Analysis of the effect of selected operating conditions on the fry-drying kinetics; 3. Extrapolation of the experimental results to simulate a continuous fry-dryer; 4. Assessment of the economic and environmental performance of the process
Table 1—1, summarizes the procedures derived of the specific goals adopted in this study.
Before the properly development of the objectives, this documents presents a Chapter 2, which is devoted to give the characteristics of the materials used in this study and provide the definitions that are used throughout the text. Afterwards, specific goals 1 and 2, which concern the fry-drying kinetics, are developed in Chapter 3. The environmental assessment of the process is treated in Chapter 4 and finally, the economic aspects are discussed in Chapter 5.
Fry-drying of sewage sludge : From the physical mechanisms to the process eco-design
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Introduction
Table 1—1 Synthesis of the specific goals adopted in this study. Specific goal
Procedure
1.Identification of the mechanisms involved in the fry-drying of sewage sludge
• Build up and validation of an experimental setup • Obtaining the fry-drying kinetics • Quantification of the thermal resistances
2.Study of the effect of some selected operating conditions on the fry-drying kinetics
• Obtaining the fry-drying curves varying the size and initial moisture content of the sample and the frying temperature
3.Economic assessment of the fry-drying as a intermediate step in the disposal of sewage sludge by incineration
• Proposition of a commercial scale fry-dryer • Computer simulation • Capital and operating costs estimation
4.Comparison of the environmental impacts for the disposal of sewage sludge by incineration using : -a conventional dryer -a fry-dryer
• Definition of a disposal scenario • Build up of an experimental set up to characterize the exhaust vapors • Development of a life cycle assessment (LCA)
Fry-drying of sewage sludge : From the physical mechanisms to the process eco-design
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Materials and Preliminary Definitions
2
MATERIALS AND PRELIMINARY DEFINITIONS
Figure 2-1 schematizes the sewage sludge fry-drying operation. It consists in bringing into contact the two raw materials (i.e. the mechanically dewatered sewage sludge and the recycled cooking oil), by immersing the wet solid into a heated deep-fat frying bath. At the end, the frydried sludge is produced, which is a granular solid composed of the dried indigenous sewage solid and the impregnated oil. In addition, an exhaust vapor stream is obtained.
Figure 2-1 Black box diagram of the fry-drying of sewage sludge. The aim of this Chapter is to describe the characteristics of the streams of materials involved in the sewage sludge fry-drying and provide some physical properties that are required for the development of this thesis.
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Materials and preliminary definitions
2.1
2.1.1
Raw Materials
Sewage sludge
The study was performed with the municipal sewage sludge coming from the waste water treatment plant (WWTP) of Albi (France). In that WWTP, a primary sedimentation is carried out followed by a biological secondary treatment. The primary and the activated sludges are mixed and sent to a mesophilic anaerobic digester1 where a fraction of the organic matter is decomposed into biogas. The sludge is thus stabilized to reduce pathogens, eliminate offensive odors and lower the potential for putrefaction. Finally, the digested sludge is mechanically dewatered with a belt filter press to obtain a pasty sludge with a final moisture content between 4.0 and 6.0 kg water·kg-1 total dry solids. Moisture content denotes the quantity of water per unit of mass of either wet or dry product. For most of the drying and dewatering applications, the composition of a sludge is usually described according to the volatility of its components [47]. Thus, from a macroscopic point of view, the composition is often presented as water mass ( m W ), which is the removed matter after drying of the wet sample at 105°C for 24h and total dry solids mass ( m TS ), which is the remaining matter. Consequently, the moisture content wet basis is defined as: ξ W (w.b.) =
mW m W + m TS
( 2-1 )
and the moisture content dry basis as: ξ W (d.b.) =
mW m TS
( 2-2 )
1
Currently, anaerobic digestion is not the most widely practiced treatment for sludge in France. However, since 1998, new policies regarding the practices of land spreading and landfill of sewage sludge, combined with recent technological progress in
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Materials and preliminary definitions
A more detailed description of the sludge, from the macroscopic point of view [37], is provided by the proximate and ultimate analyses. The proximate analysis is a thermal gravimetric analysis that describes the total solids content (TS), total volatile solids content (TVS) and total fixed solids content (TFS) in a sample. TS constitutes the remaining residue after drying of the wet sample at 105°C, TVS those solids that can be volatilized and burned off when TS are ignited at 550°C and TFS is the residue that remains after combustion. The ultimate analysis gives the elemental (C, H, O, N, S) compositions of the total solids matter using a self-integrated and microprocessor controlled elemental analyzer (mod: NA 2100 Protein, CE Instruments, Italy) according to classic organic elemental analysis techniques. Table 2—1 provides the composition of the sludge used in this study.
Table 2—1 Proximate and ultimate analysis of sewage sludge from the WWTP of Albi (France). Composition
[TS] (%)
[TVS] (%TS)
[TFS] (%TS)
[C] (%TS)
[H] (%TS)
[O] (%TS)
[N] (%TS
Sewage sludge
19±3
67±3
33±3
36.4±3
5.5±0.2 18.8±1.5 5.7±0.2
[S] (%TS) 1.0±0.1
For process design and management issues, this sludge is representative of the wastewater sludges produced in France[48]. Furthermore, the micro-pollutants contents of the Albi WWTP are close to average values from more than 500 French WWTP sludge samples (See Table 2—2), provided by Huyard et al. [49].
biogas production and valorization (in particular in Northern Europe and USA) give new impetus to these processes in the French context[28].
Fry-drying of sewage sludge : From the physical mechanisms to the process eco-design
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Materials and preliminary definitions
Table 2—2 Comparison of the micro-pollutant contents of the sewage sludge from the WWTP in Albi and those of the French average municipal sewage sludge[49]. Sewage sludge from Average French Micro-pollutant the WWTP in Albi sewage sludge[31] (mg·kg-1 total solids) (mg·kg-1 total solids) 7 polychlorinated biphenyls 0.10 0.19 (PCB’s) Fluoranthene
0.49
0.54
Benzo(b)fluoranthene
0.23
0.34
Benzo(a)pyrene
0.18
0.32
Cd
2.2
