Gazéification des huiles de pyrolyse de bois

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THESE 2008

 Titre de la thèse

Vapogazéification non catalytique des huiles de pyrolyse de bois

 Doctorant

Younes Chhiti

 Université-Ecole doctorale

Ecole doctorale : Mécanique, Energétique, Génie civil et Procédés (MEGEP)

 Directeur de thèse

Sylvain Salvador, Responsable du groupe Energétique-Environnement, Professeur au centre RAPSODEE, Ecole des Mines d’Albi-Carmaux, Campus Jarlard, route de Teillet, 81013 Albi

 Laboratoire d’accueil

Ecole des Mines,  Albi-Carmaux

 Responsable de thèse

Professeur S. Salvador

 Durée

Trois ans
fin 2008 à fin 2011

Résumé

Pour la vapogazéification de la biomasse lignocellulosique, la pyrolyse rapide est envisagée comme une voie de pré-conditionnement, sous forme de charges liquides (bio-huiles) ou de slurries (mélange de bio-huiles avec le charbon co-produit).
Ce projet de thèse vise à combler le manque de connaissances concernant le déroulement des étapes de transformation de l’huile de pyrolyse en gaz de synthèse, utilisant la gazéification non catalytique dans des réacteurs à flux entraîné. Il s’agit d’un processus complexe qui met en œuvre un changement de phase lors de la vaporisation, accompagné de réactions de craquage pour certains composés avec formation d’un résidu solide. La gazéification du résidu solide carboné est une réaction hétérogène, influencée par la présence de matières inorganiques.
Nous proposons d’isoler la première phase d’évaporation/craquage, en travaillant sous atmosphère neutre. Une seconde série d’expériences sera réalisée en présence de vapeur d’eau, mettant en jeu les réactions de reformage des gaz et de gazéification du résidu solide.
Les matières inorganiques qui peuvent induire des problèmes technologiques dans un réacteur industriel, seront suivies au cours de ces transformations. L’influence des paramètres température et taille des gouttes sera analysée.
Les connaissances acquises seront intégrées dans un modèle décrivant l’ensemble du processus de conversion d’une goutte d’huile en gaz de synthèse.
La variabilité de la qualité des ressources de biomasse se traduit par une variabilité des huiles de pyrolyse obtenues. Pour caractériser cet impact, les expériences seront effectuées avec trois différentes huiles.

Résultats

Gasification of biomass is one of the leading near-term options for renewable energy production. When large scale units are considered, bio-oil shows lots of advantages compared to solid biomass. The combination of decentralized fast pyrolysis of biomass followed by transportation and gasification of bio-oil in bio-refinery has attracted great attention.
The overall purpose of this research was to investigate the feasibility of a whole bio-oil non catalytic steam gasification process for the production of high quality syngas in entrained flow reactor.
From a chemical point of view, bio-oil gasification process is quite complex and consists of the following main stages: vaporization, thermal cracking reactions with formation of gas, tars and two solid residues - char and soot – that are considered as undesirable products. This is followed by steam reforming of gas and tars, together with char and soot oxidation. To better understand the process, the first step of gasification (pyrolysis) and thereafter, the whole process (pyrolysis+gasification) were separately studied. The objectives of this work were identified as follows.

To better understand the pyrolysis step of bio-oil and investigate the effect of operating conditions. A temperature increase from 550°C to 1000°C greatly enhanced the gas yield, whilst solid and liquid yields decreased significantly in agreement with the literature. The heating rate of bio-oil has little impact on the gas yield, but plays a major role on the char yield. Hence the char yield decreases from 11 wt. % with a heating rate of 2°C.s-1 down to 1 wt.% for flash heating rate of 2000°C.s-1 at a final temperature of 1000°C. At very high heating rate, the final temperature has little influence on the char yield. These results show that for gasification under industrial EFR conditions, the quantity of char is very small. Thus the gasification process mainly consists in gas/tar reforming. Nevertheless, the production of clean syngas will require either complete gasification of char or its removal from the gas produced by the gasifier.

In steam gasification process, whole bio-oil was successfully steam gasified in EFR. An increase in the reaction temperature over a wide range from 1000°C to 1400°C implies higher hydrogen yield and higher solid carbon conversion. A thermodynamic equilibrium calculation showed that equilibrium was reached at 1400°C. At this temperature steam reforming of bio-oil leads to yield of equal 84 % of theoretical maximum.

The influence of ash on both bio-oil pyrolysis and gasification has been investigated. In the pyrolysis process, ash greatly increased the yield of solid products and decreased the yield of gaseous products. Liquid yield undergoes no dramatic change. Ash also clearly affects the gas composition. When 3 % of ash was added CH4 and CO yields decrease, while CO2 yield increases. In gasification process, when ash is added to bio-oil, a strong decrease can be observed in gas yield, although literature results on solid biomass predict an increase. Ash seems to favor polymerization reactions leading to the formation of char, and resulting therefore in a decrease in the gas yield.

The high temperature gasification of bio-oil in non catalytic processes leads to the formation of soot, which is an undesirable solid product. In the last part of this work, the soot formation and oxidation during bio-oil gasification have been investigated. The temperature of the reaction and the fraction of added steam were tested. Another parameter taken into account here is the amount of oxygen that is necessary when an autothermal process is envisaged. A model is proposed to describe soot formation and oxidation during gasification. It is based on the description of bio-oil heating, devolatilization, reforming of gases and conversion of both char and soot solids. Detailed chemistry is used in the gas phase. Soot production is described by a single reaction based upon C2H2 species concentration and one main heterogeneous reaction to describe soot oxidation. Three thermochemical situations were experimented and modeled: the lack of steam, large excess of steam (H2O/C = 8), and in the presence of oxygen in the range O/C = 0.075 to 0.5. The amount of the main gases is very accurately predicted by the model and the prediction of soot yield is correct over a wide range of temperature, water content and O2 content of the atmosphere. Note that a single set of identified parameters is used for all situations. Hence the model may be a useful tool to support the design of a large scale gasifier.
This study confirms the strong influence of temperature on the mechanisms of soot formation and oxidation. Emphasis was also made on the effect of soot oxidant agents during experiments. Water in excess causes an almost complete gasification of soot at 1300°C and 1400°C. In the partial oxidation situation, at very low concentrations of O2, the soot yield undergoes a slight decrease; an increase of O2 amount greatly reduces the soot yield.
The contribution of each reaction of soot oxidation was identified using the model. CO2 is shown to reduce only small quantities of soot. O2 has no contribution to soot oxidation because it is consumed before soot is formed. Nevertheless, O2 indirectly acts, by consuming C2H2 and therefore causes a decrease in the soot production. Only steam oxidizes directly the soot and causes an almost complete oxidation.

Livrables

Publications

  • Chhiti Y, Salvador S, Commandre JM, Broust F, Couhert C (2010) Wood bio-oil non catalytic gasification : influence of temperature, dilution by an alcohol and ash content. Energy and Fuels 2010

  • Chhiti Y, Salvador S, Commandre JM, Broust  F. Wood bio-oil pyrolysis : influence of temperature heating rate and ash content on char, gas and tar yield. Submitted to Fuel  in 2011

  • Soot  formation and oxidation during  bio-oil gasification : experiments and modeling. Submitted to Fuel

Contact

Sylvain Salvador
Professeur au centre RAPSODEE,
Ecole des Mines d’Albi-Carmaux, Campus Jarlard, route de Teillet, 81013 Albi
Tel : 05 63 49 30 26
salvador@enstimac.fr