of the Internet becomes critically present again on Usenet Web archives

. An interdiscourse that appears superficially as an aggregation of opinions, it actually reveals how human language interact with computer technologies in many technical and social mediations.

Among CMC genres, mailing-lists and newsgroups are not dependent on the Web, which explains why they have been ignored in the "appraisal process" (Niu, 2012) underlying institutional born-digital archiving, mostly focused on Web media. We think that they are, in fact, weaved into the Web in a way that reveals its multi-layered nature -at theoretical and practical levels. Their historical and heritage value, as well as their technical constraints when collected for building corpora, are arguably interdependent from Web archive sites -a first stage of "vernacular" (non institutional) archiving process. One specific example concentrates general issues raised by Web-archived CMC: the case of the Usenet archive, a text-based collection of group conversations gathered from the most prominent ancestor of digitally-based social networking. This paper is based on two studies we did on Usenet archives as an object of analytical and methodological research:

1/ a genealogy of its successive archival processes, with the archived collections ending up in the Google Groups web service, put in perspective with an analysis of the reception of this first heritage initiative by Usenet users themselves (Paloque-Berges, 2013); 2/ our own collection and analysis of a corpus of newsgroups' messages in order to research a chapter of the French history of computer communication networks, with a socio-technical approach inspired by the sociology of innovation refined with the perspectives of infrastructure and code studies; followed by an attempt at engineering the corpus for structuration, standardization and interoperability purposes (to be published) 3 .

Accounting for the historical and heritage value of CMC material: from Internet history to Internet heritage

Mailing-lists, a collective form of emailing processed through automated software (since the mid 1970's) and newsgroups, a specific form of collective emailing native to Usenet (since 1979), were defined in their heydays as "electronic conferences" testifying of the first social uses of computer networks [START_REF] Quarterman | The Matrix: Computer Networks and Conferencing Systems Worldwide[END_REF]. Both stakeholders, protagonists and witnesses [START_REF] Rheingold | Virtual Community -Homesteading on the Electronic Frontier[END_REF][START_REF] Hauben | Netizens: On the History and Impact of Usenet and the Internet[END_REF]Hafner and Lyons, 1998;[START_REF] Huitema | Et Dieu créa l'Internet[END_REF][START_REF] Mounier | Les maîtres du réseau[END_REF] and historians or sociologists of the field (Abbate, 2001;Flichy, 2001) have underlined their role in structuring and deploying the modes of cooperation in the history of computer networks has been 4 . We work at defining source value of the borndigital documents that were produced through these interactions, for the technical, social and cultural history of computer networks, but also for Internet studies more broadly (for instance: [START_REF] Paloque-Berges | La mémoire culturelle d'Internet : le folklore de Usenet[END_REF][START_REF] Paloque-Berges | Un patrimoine composite : le public Internet face à l'archivage de sa matière culturelle[END_REF].

Pioneer initiatives in archiving this material for heritage purposes are to be found within Internetcentric communities, especially when the Web started to offer a favorable environment for gathering and accessing mailing-lists and newsgroups archives5 . Prominent organizations in the technological development of the Internet (IETF, RIPE or W3C) have operated such web-based archiving, facilitated by automated archival processes by mailing-lists management systems like Listserv or Sympa, easily integrated within websites. IETF took one step further in 2012 by issuing a Request for Comment (RFC n°6778) for "Email List Archiving, Web-based Browsing and Search Tool Requirement", a project still under construction.

The case of newsgroups is slightly different from mailing-lists'. Because of its general access purpose, Usenet's newsgroups accommodate a global conversation expanding well beyond the themes of Internet historical cooperation (mostly gathered in the comp.* -for computers -and sc.* -for sciencegroups). Its massive and international scope might explain why no institution has tried to archive italthough the Library of Congress' decision to archive Twitter content since 2009 testifies that such an interest could technically occur. However, heritagization processes have started outside of the professional and institutional archivist field. In the first decade of Usenet, an implicit rule was to keep newsgroups archives on administrators' servers no longer than two weeks, for resource management limitations. Isolated initiatives, such as the collection gathered at the Zoology department at University of Toronto with material going back to 1981, offered the ground for the first archival file curated and provided by the Internet archive foundation6 in 2011. Before that, Déjà News Research Service (a Usenet newsgroup subscription-based provider for professionals) led a first Usenet massive archiving initiative in 1995, in order to experiment and offer its users a tool for searching in past conversation through an innovative Web interface.

The archive was bought by Google in 2001 and integrated in their new online collective discussion services, Google Groups. This initiative raised the interest on the part of Internet active users, especially early adopters and more particularly old-timers who had been using Usenet since the 1980s. Popular technology-oriented online media, like Wired or Slashdot, covered it, with enthusiastic focus on the Usenet timeline provided by Google as an entry point to the archive7 . Two referential levels structure this timeline: how global history events appeared and were discussed on Usenet (for instance, the 1984 Chernobyl catastrophe); how technical and cultural computer history landmarks emerged and were shaped within Usenet communities (for instance, the first call for participation in the Linux project in 1991). This convergence shows how Google was set to legitimize its position as a major player to be in Internet history by calling on early adopters' memory of Internet use -Usenet itself being a major meeting point for Internet players until the rise of the Web. This strategy, however, was scrutinized and criticized by users themselves, at the occasion of numerous debates in the Slashdot forums. On the one hand, it was praised; indeed, the archive was used for what we call "ego-searches" (finding one's contributions in the past conversations) and "alter-searches" (finding common references of events known to Usenet's community). These reflexive practices helped build the collective cultural memory of Usenet [START_REF] Paloque-Berges | La mémoire culturelle d'Internet : le folklore de Usenet[END_REF][START_REF] Paloque-Berges | Un patrimoine composite : le public Internet face à l'archivage de sa matière culturelle[END_REF]. One the other hand, it was criticized for unearthing and bringing out of the shadows information relative to individuals, a concern towards the risks associated with displaying personal data and opinions. Actually, Google soon provided a feature for the removal of messages, as an early example of experimental device to solve "right to be forgotten" issues. The ambivalent status of Usenet conversations, public in that newsgroups were open access, but private in the sense that the vast majority of Usenet was composed of small, confidential groups, was thus revealed. It is one of the first and most fundamental impediments to an archival in compliance with the rights of individuals, as it is in terms of Internet research ethics, that calls for a "contextual integrity" when facing the problem of publicity or privacy of network mediated discourse [START_REF] Latzko-Toth | Enjeux éthiques de la recherche sur le Web[END_REF].

Testing the potential and limitations of web-based newsgroup archives for building corpora

We experimented the practice of handling and exploiting Usenet archives for historical and heritage purposes, with the constraint of standardizing a corpus so it can be operated for uses beyond our own. Our research project tackled the subject of French computer networks history studied through the networksbased exchanges of their participants (researchers and engineers in the field of network computing). Our corpus was comprised of several academic mailing-lists as well as newsgroups from the fr.comp.*8 Usenet hierarchy. Our methodology for analysis was based in a socio-technical perspective borrowed from the sociology of innovation [START_REF] Paravel | Les reseaux, des objets relationnels non identifies ?[END_REF], which we adapted and refined according to the specificity of our material. We thus operated a multi-layered analysis for studying these born-digital and Web-based archives, embracing content, software infrastructures and encoding standards -following advice from Infrastructure studies and Critical Code studies. As such, our focus was the different layers of source material that can be found in electronic conversations, from content to header metadata. We argue they have value both in terms of discursive content analysis and in software and infrastructure analysis.

The Usenet archive is embedded within the Google Groups systems, with a forum-type threaded structure and search engine. These interface and infrastructure require critical analysis, as ambivalence lies at the core of the system. The Usenet archive is considered dynamic: it is incremented as more Usenet activity is produced, similarly to the Google Groups themselves. The embedded structure produces a mutualization between Google Groups and Usenet in terms of interface display and search functions, with no visual separation between the two services and their associated archives. For instance, searching for one keyword outputs results without discrimination in regards to their respective belonging. Thus, a prior knowledge of Usenet's complex hierarchy of thematic groups is crucial to finding relevant material. The same applies to search functions: the user should know of trick search queries such as "fr.*" in order to find all the groups falling under the francophone branch category. We found no detail manual of use, and no way to communicate with the administration through faulty contact forms and dead ends links. There is a general sense of under-maintenance (except from a few changes to the system through the years, resulting in disallowing the most important features9 ). Beyond these impediments, Google Groups do not allow the automated crawling, indexing or retrieving of the archive, blocking any attempts to do so, for obvious reasons regarding the commercial and security issues of Google information systems.

We retrieved by hand all the messages in a 2-year period of a particular group of our corpus, in order to: first, operate a qualitative analysis on all aspects pertaining to messages, both at the level of infrastructure and content; second, test quantitative analysis software10 ; third, experiment how to standardize the corpus for interoperable search and research within the academic community. One particular feature of the archive was relevant; that is, accessing "original messages", making the whole of the message visible (metadata and body content). Our socio-technical methodology required the study of the traces left by infrastructure, and metadata does offer a great view of these traces: along classical header data (sender, receiver, subject, date and time), information on format, protocols and routing are crucial to analyze how communication is materialized. Metadata also proved crucial for the two other tasks. Indeed, as we chose to use XML for structuring our test corpus, metadata elements fit right into the mark up structure, facilitating the automated process to do so. We thus experimented two ad hoc XML structuring: first, with the help of a given template in order to run the quantitative analysis software ("XML Forum"); second, through the frame of the OLAC11 standardized XML recommendations, as they aim at providing accessible online corpora fit for interoperability.

Eventually, we encountered a few difficulties preventing us to further the fulfillment of our experiment into effective, open, accessible and searchable corpora: anonymization (the names and personal information not being restricted to the metadata categories); noise in the content resulting from the original uses and circulation of messages through different mail readers with their own format parameters. Such a project, far from being an easy task even though we are handling plain text files, requires the expertise of professional digital archivists and programmers in order to be executed properly, even more if the goal is to put back the corpora online for sharing with the community.

Ultimately, Usenet Web archives sites (and web archived CMC in general) are themselves a layered archive of the digital past and present Internet, as we showed through the history of its archival on the Web from the mid-1990s. An object of digital cultural and technical memory, their values lie in interactions inscribed in discourse, but also the technical framing at the interface and infrastructure levels. They are weaved into the history of Web, if not entirely constrained by it. One could argue that a systematic archiving of all digital-born networked conversation is an impossible, if not trivial, task. However, we find critical value in studying the archival processes, whether vernacular or institutional, beyond our own interest in writing the history of communication computer networks. Indeed, the archival of CMC as a new form of recording the everyday online conversations inevitably questions the fear of general network surveillance and the binding of the informal word to digital inscription with resilient memory. Effectively, our relation to digitally archived network communication is marked by an unbalance between the difficulty to grasp the conditions of recording user documents and discourse online for heritage and historical purposes and an acute conscience of the economical and political stakes attached to the exploiting of digital data.

We joined this workgroup in

2012, for our post-doctoral research purposes (see note 3). 2 Comprised of communication history of computer networks, methodologies for studying born-digital archives. 3 This research was funded by a post-doctoral research contract (2012-2013) at the Laboratoire d'Excellence HASTEC (History and Anthropology of Knowledge, Technologies and Beliefs) and operated within the French community of universities Hautes Etudes Sorbonne Arts et métiers (Hésam) based in Paris.

Before that, they were stored on personal computers and exchanged through rudimentary technologies like File Transfer Protocol (FTP).

Internet Archive [http://archive.org/details/utzoo-wiseman-usenet-archive].

« Get started with Usenet on Google Groups -Groups Help » [https://support.google.com/groups/answer/6003482].

This notation shows that in the francophone hierarchy (fr.*), there is a number of subcategories, such as fr.comp*, which hosts groups dedicated to computer-related themes in the francophone branch (for instance fr.comp.infosystemes).

Such as an overview of quantity of messages per month and year, with links to the actual content categorized chronologically. The difficulty to navigate diachronically in the archive has increased over the years.

The online software suite Calico, developed a team of researchers in Education sciences in order to analyze web forum interactions. It requires the encoding of corpus into « XML-Forum », an XML structure native to Calico (Erté Calico, at UMR STEF [http://woops.crashdump.net/calicorss2/index.php].

The Open Language Archive Community (OLAC) is an international collaborative network working at standardizing natural language-based corpora for research purposes.


Vincent Repellin

Alexandre Govin

René Guyonnet

Thermo-gravimetric analysis as a tool for the optimisation of wood heat treatment parameters

Keywords: heat treatment, thermo-gravimetric analysis, wood pyrolysis, time temperature equivalence

Retification is a heat treatment that decreases the swelling of wood and increases its resistance to fungal attack. It consists in a mild pyrolysis of wood (180°C-260°C) that takes place in a non oxidative atmosphere (nitrogen). The industrial development of retification requires optimisation of the treatment temperature and duration. In order to enhance the homogeneity of temperature in the furnace, and to avoid exothermic reaction, low temperatures seem to be preferable to high temperature. On the contrary, duration and temperature of treatment have to be high enough to provide good biological resistance and stabilization to the wood. However, high temperatures lead to a loss of mechanical strength. A question arises from these previous observations: is there any equivalence between a treatment of short time carried out at high temperature and a treatment of longer time at lower temperature? Answering this question can help to optimise rétification temperature and duration. The purpose of this study is to evaluate the relevance of a "time temperature equivalence" (TTE) for wood pyrolysis in the temperature range of retification. The principle of TTE is adapted from the study of wood viscoelastic properties. In this study, it is applied to the rate of anhydrous weight loss during wood pyrolysis. Thermo-gravimetric analysis (TGA) were performed on maritime pine (Pinus pinaster Ait.-) and beech (Fagus sylvatica) wood powder. Isothermal degradations were carried out at different temperatures ranging from 160°C to 260°C. A specific data analyse was carried out on the TGA derivative (DTG) in order to assess the relevance of the TTE in the temperature range of retification. It gave interpretable results for maritime pine, but not for beech. It showed that for maritime pine wood the TTE is confirmed from 200°C to 220°C, and not confirmed for temperatures superior to 230°C. An optimization of the temperature and time of treatment is thus possible.

INTRODUCTION AND THEORETICAL CONSIDERATIONS

Short presentation of retification treatment

Retification [START_REF] Armines | Procédé de fabrication d'un matériau ligno-cellulosique par traitement thermique et matériau obtenu par ce procédé[END_REF]) is a heat treatment which consists in a mild pyrolysis of wood (180°C-260°C). It takes place in a non oxidative atmosphere (nitrogen). This treatment decreases the swelling of wood [START_REF] Repellin | Evaluation of heat-treated wood swelling by differential scanning calorimetry in relation to chemical composition[END_REF] and increases its resistance to fungal attack [START_REF] Kamdem | Durability of heat treated wood[END_REF][START_REF] Mouras | Propriétés physiques de bois peu durables soumis à un traitement de pyrolyse ménagée[END_REF] without the use of chemicals. However, the main drawback of wood heat treatment is an important decrease of mechanical strength [START_REF] Repellin | Evaluation of heat treated beech by nondestructive testing[END_REF].

As a consequence, the commercial and industrial development of the retified wood would be made easier if it were possible to minimize the loss of mechanical strength. The purpose of this study is to investigate the possibility to optimize heat treatment parameters (duration and temperature) in order to minimize the loss of mechanical strength.

Wood pyrolysis

Wood pyrolysis follows two pathway : slow and fast pyrolysis, which involve many different reactions and products [START_REF] Shafizadeh | The chemistry of pyrolysis and combustion[END_REF][START_REF] Faix | Thermal degradation of wood products : gas chromatographic separation and mass spectrometric characterization of polysaccharides derived products[END_REF]. For temperatures ranging between 200 and 280°C both slow and fast pyrolysis are present, but slow pyrolysis predominates. The main products of wood decomposition by slow pyrolysis are water, carbon dioxide, carbon monoxide, formic acid, acetic acid and furfural. At temperature superior to 300°C, fast pyrolysis is predominant. Wood constituents decompose rapidly in tar and volatiles of low molecular weight such as methane, formaldehyde, methanol, phenol etc. These decompositions are exothermic reactions. Retification occurs between 180°C and 260°C. Consequently, the gaseous products evolved during rétification are mainly those of slow pyrolysis. Degroot et al. [START_REF] Degroot | First chemical events in pyrolysis of wood[END_REF]) studied isothermal decomposition of poplar sapwood (Populus trichocarpa) at 250°C. [START_REF] Weiland | Analyse de la pyrolyse ménagée du bois par un couplage TG-DSC-IRTF[END_REF] studied the chemical modification of wood powder of chestnut tree (Castanea sativa), Norway pine (Pinus sylvestris) and maritime pine (Pinus pinaster Ait.-) from room temperature to 350°C. The relationship between the evolved gases and the corresponding degradation mechanism are presented in table 1, as proposed by these two authors.

It can be seen (Table 1) that the evolved gases come mainly from hemicelluloses and lignin degradation. Nevertheless, cellulose degradation is not excluded. It is known that gases released during cellulose degradation by pyrolysis are water, carbon monoxide, and carbon dioxide, and [START_REF] Browne | Theories of the combustion of wood and its control a survey of the litterature[END_REF], and these three gases [START_REF] Weiland | Analyse de la pyrolyse ménagée du bois par un couplage TG-DSC-IRTF[END_REF]) are indeed released during retification. Thus the degradation of the three constituents of wood (hemicelluloses, lignin and cellulose) may occurs during heat treatment of wood.

Cellulose is often considered as responsible for the mechanical strength of wood. Consequently, as soon as cellulose begins to decompose, loss of mechanical strength may take place. Nevertheless, hemicelluloses may also play an important part in wood strength. Some authors found a correlation between the decrease of mechanical strength and the degradation of hemicelluloses by brow-rot fungi [START_REF] Winandy | Relationship between incipient decay, strength, and chemical composition of douglas-fir heartwood[END_REF].

Hypothesis concerning pyrolysis reactions

It is postulated here that the reactions involved in wood pyrolysis may be classified into two groups. Both group induce an increase of stabilization and durability and a decrease of mechanical strength. However, the First group favours stabilization and durability whereas the second favours mechanical strength loss (Table 2). In the range of retification temperature a competition between these two groups is likely to happen. When the treatment temperature increases, fast pyrolysis reaction and cellulose destruction (second group) are enhanced.

According to these hypothesis, it would be profitable to carry out the heat treatment at low temperature. In the same time, the improvement of wood end use properties depends on the strength of the treatment. The material modification (that match to the anhydrous weight loss) must be high enough to provide good biological resistance and stabilization to the wood. Thus, a decrease in temperature of treatment should be associated with an increase in the treatment duration, since anhydrous weight loss increases with increasing temperature and duration.

Both reactions of the first and second group obviously result in anhydrous weight loss. By another way, anhydrous weight loss is the only mean to control the advancement and the rate of wood pyrolysis. The rate of anhydrous weight loss should depend on the type of reaction involved. In particular, reactions of the first groups and reactions of the second group should result in different values of the rate of anhydrous weight loss.

On these grounds, if the contribution of each group of reaction is the same over the range of temperature (180°C to 260°C), then, there should be an equivalence between a treatment of short time carried out at high temperature and a treatment of longer time at lower temperature as regards the rate of anhydrous weight loss.

The purpose of this study is thus to evaluate the relevance of a "time temperature equivalence" (TTE) for wood pyrolysis in the temperature range of rétification.

Principle of time temperature equivalence (TTE)

The principle of TTE is adapted from the study of wood viscoelastic properties [START_REF] Pluvinage | Viscoélasticité du bois[END_REF]). It may be expressed as follow. The value of a measurable quantity is the same at high temperature for short time as at low temperature for long time.

The TTE principle is observable for instance, when measuring the dynamic flexural vibration of wood beams. For a given range of frequency, the real part of the complex modulus of elasticity has the same value whether it is measured at high temperature for short time of exposure, or at low temperature for a long time of exposure.

Consider a quantity R, that can be measured on a wood sample at two different temperatures (T ref and T) that belong to a given range of temperature. The TTE is verified if: for any value of R, the time t (that takes to reach R at temperature T) and the time t Tref (that takes to reach R at temperature T ref ) are linked by Eqn. 1:

T Tréf T α t t = (1)
where α T is called the translation factor. It depends only on the temperature and it is constant versus time. The TTE principle can also be written (Eqn. 2):

R=R(t Tref ,T ref )=R(t T ,T)=R(t Tref /α T ,T) (2)
And Eqn. 1 leads to :

ln(t T )=ln(t Tref )-ln(α T ) (3)
According to Eqn. 2 and Eqn. 3, when R is plot against ln(t), isothermal curves have to superpose by a translation of ln(α T ) (Figure 1). Thus, an equivalence between the effect of treatment duration and the effect of treatment temperature can be found when any isothermal curve superposes on the isothermal curve obtained at T ref , by horizontal translation (Figure 1).

In order to check if the TTE principle is confirmed in the temperature range of retification, thermo-gravimetric analysis (TGA) were carried out. Isothermal DTG (TGA derivative) curves were plot and analysed, in order to assess the relevance of the TTE in the temperature range of rétification with regard to the rate of anhydrous weight loss. In this study, R is a given value of the DTG.

The purpose of this study

This study aims to test the TTE with regard to the rate of anhydrous weight loss, in the range of temperature of rétification (180-260°C).

If this TTE principle is confirmed over a given range of temperature, the modification of the material will be the same at any temperature in this particular range for a given anhydrous weight loss. In this temperature range, it will be possible to increase the temperature and reduce the duration without further degradation of wood constituents due to the second group of reactions.

On the contrary, if the temperature exceeds the temperature range where the TTE is verified, the second group of reactions will induce further degradation of some of the material constituents and increase the loss of strength of the material.

EXPERIMENTAL

Samples

Natural wood powder was obtained by grinding and sieving specimens between 150 and 212 µm. The samples were made of 10 mg of wood powder. Two species were studied : maritime pine (Pinus pinaster Ait.-) and beech (Fagus sylvatica). Before experiment, these samples were stored at room temperature and hygrometry.

Method

The device was a thermobalance NETSCH TG 209. The temperature program was set as follow:

-a ramp from 30°C to 150°C at 30°C/min, -an isotherm at 150°C during 20 min, in order to dry the sample until its anhydrous state, -a ramp from 150°C to the chosen temperature (from 160°C to 260°C), at 99°C/min -an isotherm at the chosen temperature, with a duration ranging from 3 hours to 8 hours depending on the temperature (Table 3).

The reproducibility was evaluated and each isothermal ATG curve was carried out three times.

RESULTS AND DISCUSSION

Presentation of ATG curves and choice of a reference temperature T ref

The ATG isotherm of beech at 180°C presents no weight loss during three hours (Figure 2). At 160°C and 180°C, the maritime pine isotherms present a loss of weight during the first 20 min. Above 20 min, their weight is constant. Moreover, isotherm at 160°C and 180°C are superposed. We suggest that the weight loss at the beginning of the isotherm corresponds to resin evaporation. Since the weight stabilizes, it is also suggested that no degradation of wood constituents occurs at this temperature. At 200°C both beech wood and pine wood loose weight. This loss of weight increases when the duration increases, and should thus be due to wood degradation by pyrolysis reactions. Since 200°C is the lowest temperature at which degradations seem to happen, we chose it as the reference temperature (T ref ).

TTE during the first 30 minutes

The DTG curves are presented versus the decimal logarithm of time (Figure 3), in order to check the TTE principle. On these two graphs (Figure 3), the TTE principle is verified when the DTG curves can be superposed on each other by an horizontal translation. The first 3 first minutes are instable are not discussed.

For maritime pine, it is obvious that TTE is not verified all over the temperature range studied. For a given rate of weight loss, the slope of the DTG curves varies with increasing temperature. This is well visible on maritime pine DTG curves, and less visible for beech wood.

The beech seem to be less resistant and stable than maritime pine at the beginning of the isotherm. Its rate of weight loss is higher than the rate of weight loss of maritime pine, and beech wood DTGs seem to be scattered in the first minutes of isotherm. Consequently, it is not easy to assess if the TTE principle is confirmed or not for beech.

Nevertheless, for beech and maritime pine, superposition of isotherms at 210 and 220°C on the isotherm at 200°C by horizontal translation seem to be possible. During the first half hour, for maritime pine as well as for beech, the TTE seem to be verified from 200°C to 220°C, and not verified for temperatures superior or equal to 230°C.

TTE for durations superior to 30 minutes

It is not possible to see on figure 3 if the TTE is confirmed after 30 minutes (Figure 3). That is why, we calculated t/t Tref (Eqn. 1). According to Eqn. 2 and Eqn. 3, when the TTE is verified ln(α T ) (and consequently t/t Tref ) is constant versus R, and should depend only on the temperature.

Figure 4 shows t/t Tref at different values of R and of the temperature of isotherm. when it is constant versus R, it can be concluded that the TTE principle is confirmed. When it is not constant, the TTE principle is not valid.

Concerning beech wood, it is impossible to give any conclusion concerning the TTE principle. The values of 1/α T , are too scattered to see if it is constant in function of R. We suggest that it is a consequence of hardwood thermal instability, which is known to be higher than thermal instability of softwood. Concerning maritime Pine, t/t Tref is almost constant versus R at 210°C and 220°C. It increases with R at any temperature superior or equal to 230°C. This increase is enhanced with temperature augmentation. This observation suggests that for maritime pine, TTE is almost confirmed between 200 and 220°C, and is not verified at temperatures superior to 230°C. Moreover, as the difference of temperature with reference temperature increases, the TTE principle becomes more and more wrong.

CONCLUSIONS

Time and duration of heat treatment are crucial parameters to obtain a material with good properties. In this study a methodology (the TTE principle) is adapted from the study of wood viscoelastic properties. It is applied to beech and maritime pine wood powder in order to assess the possibility to optimise treatment time and duration.

Isothermal DTG were analysed according to this TTE principle. It gave relevant results for maritime pine, but not for beech wood. We suggest that it is a consequence of the lower thermal stability of hardwood than thermal stability of softwood.

For temperatures ranging between 200°C and 220°C, the TTE principle seem to be confirmed for maritime pine. The temperature can be thus increased until 220°C without further loss of mechanical strength. This is not the case for temperatures superior to 230°C, at which reactions that favour mechanical strength loss should be enhanced.

An optimisation of heat treatment parameters is thus possible for maritime pine wood. At first, temperature of treatment should not exceed 220°C. Secondly, in the range of temperature from 200°C to 220°C, the duration has to be increased until the material reaches a certain anhydrous weight loss.

Figure 1 :

 1 Figure 1: Principle of time temperature equivalence

Figure 2 :

 2 Figure 2: TGA of Beech and Maritime Pine wood.

Figure 3 :

 3 Figure 3: DTG of beech and maritime pine wood

Figure 4 :

 4 Figure 4: 1/α T in function of temperature, for different values of R.

Table 1 : Relationship between low temperature pyrolysis gaseous products and chemical modifications Product of degradation Chemical reaction a Chemical reaction b

 1

	water	Hemicelluloses degradation	Drying and dehydration
	carbon dioxide	Decarboxylation of uronic acids of hemicelluloses and pectins	Hemicellulose and lignin decarboxylation
	methanol	Decomposition of 4-O-methyl groups of uronic acids Decomposition of lignin's syringyl and guaiacyl units	Degradation of hydroxymethyl groups of lignin and hemicelluloses
	acetic acid	hydrolysis of O-acetyl groups of xylan chains.	Degradation of acetyl groups of hemicelluloses
	formic acid	No explanation	No explanation
	furfural	No explanation	Pentosans degradation

a According to DeGroot et. al, b According to Weilland et al.

Table 2 : The postulated two groups of wood pyrolysis reactions. Properties modification First group Second group

 2

	Increase of stability	++	+
	Increase of durability	++	+
	Decrease of mechanical strength	+	++

At low temperature (200°C) only slow pyrolysis reaction (first group) may be taken into account. Other reactions can be neglected.

Table 3 : Duration of isotherm at different temperatures

 3

	Temperature (°C)	Isotherm Duration	Isotherm Duration
		(hours)	(hours)
		Maritime Pine	Beech
	160	24	-
	180	24	40
	200	16	12
	210	9	10
	220	5	5
	230	5	5
	240	3	3
	250	3	3
	260	3	3


Laure Meljac
email: meljac@emse.fr

Laurent Perier-Camby

Calorimetric study of the heat effects induced by the fixation of hydrogen sulphide on the surface of modified activated carbon fibres

Keywords: Calorimetry, activated carbon fibres, hydrogen sulphide

In order to improve their efficiency for the removal of H 2 S contained in polluted effluents, activated carbon fibres are impregnated by KOH aqueous solutions. In presence of water vapour, H 2 S is not simply physically adsorbed on the surface of the treated cloths but several reactions can occur. The fixation mechanism proposed has been validated and completed through the calorimetric study of the heat effects induced by the reactions that occur when the treated fibres are in contact with a gas mixture of H 2 O and H 2 S.

INTRODUCTION

Activated carbon fibres, as activated carbons, are known to be very good adsorbents. They are highly porous: their specific surface areas often exceed a BET value (conventional method) of 1200 m 2 .g -1 . Many chemical groups such as red-ox and acid-base couples resulting from the manufacturing process are distributed on their surface. Activated carbons come from the activation of any carbonaceous material. The activation treatment consists of two distinct steps: the carbonisation followed by the activation itself. During carbonisation, volatile organic components are eliminated, the carbons rearrange themselves into graphitic sheets laid out in a random way [START_REF] Pacault | Les Carbones tome II[END_REF] . During activation, an oxidising agent attacks the carbons located at the edges of the graphitic sheets, this step is responsible for the creation and development of the porosity and surface groups [START_REF] Roop | Active Carbon[END_REF][START_REF] Mattson | Activated Carbon[END_REF] . The application the fibres studied are devoted to is the filtering at room temperature (25°C) of gas mixtures containing very small amounts of hydrogen sulphide (few vpm). Under these conditions, hydrogen sulphide is weakly adsorbed by the activated fibres. An interesting route to improve these sorption capacities consists in impregnating the activated fibres [START_REF] Adib | [END_REF] with a basic solution in order to create basic functions which will favour reactions between H 2 S and the impregnate. The treatment chosen consists in an impregnation of the activated fibres by an aqueous solution of potassium hydroxide [5,6] . The impregnation treatment greatly increases the H 2 S fixation capacities of the activated carbon fibres. On the impregnated cloths, H 2 S molecules are not simply adsorbed but several reactions occur. The fixation mechanism has been clarified using characterisation techniques and measurements of the fibres capacities to retain H 2 S. All the reactions involved in the mechanism proposed present heat effects which can be detected and quantified by calorimetry. Thus, the study of the heat effects by this technique is a way to validate the H 2 S fixation mechanism and eventually to evidence unsuspected reactions.

MATERIAL AND METHODS

The activated carbons studied are commercial carbon fibres coming from the activation of rayon fibres. Their specific surface areas reach 1400 m 2 .g -1 and they are mainly microporous. In the laboratory these fibres are impregnated with potassium hydroxide aqueous solutions.

The impregnation protocol consists in immersing for 20 minutes the cloths into an aqueous solutions of potassium hydroxide. The cloths are then taken out of the solution and dried under air during 2 hours at 100°C. The treatment ends with a thermal treatment performed at 25°C under deep vacuum. The KOH molecules deposited on the fibres surface after immersion in the impregnation solution are carbonated onto KHCO 3 by CO 2 of air and then decomposed into K 2 CO 3 during the thermal treatment [7] . The non treated fibres will be called virgin fibres, the treated ones KOH C KOH , where C KOH is the concentration of the impregnated solution in g.L -1 (e.g. : KOH 10).

The apparatus for the measurement of the H 2 S fixation capacities of the virgin and the impregnated fibres is illustrated on figure 1. A sample of cloth is held in a reactor surrounded by a double walled vessel. The working temperature is controlled by the water circulating between the two walls. The reactor is crossed by a gas mixture containing nitrogen, water vapour and hydrogen sulphide [8] . Preliminary experiments have highlighted that the presence of water vapour in the gas mixture strongly enhances the H 2 S fixation capacities of the fibres. The working temperature is fixed at 25°C. The gas flow Φ is 30 L.h -1 , the volumic fraction θ of H 2 S is fixed to 50 vpm and the relative humidity to 30% at 25°C. A photo ionisation gas detector (PID) records the volumic fraction of H 2 S coming out of the reactor as a function of time. From the data collected, breakthrough curves can be plotted. The capacities of H 2 S fixation (expressed as the amount of H 2 S fixed per unit of mass of fibres: mol.g -1 ) of the cloths are calculated by integration of these curves. The apparatus for the measurements of heat flows is illustrated on figure 2. The heat effects are detected by a calorimeter SETARAM C80. The heat flows recorded are plotted as a function of time by the software: setsoft 2000 from SETARAM. The integration of the calorimetric peaks, which can be done by the software, corresponds to the specific enthalpy of the reactions that occur in the cells. The experimental protocol consists in introducing dry nitrogen in the cells of the calorimeter until the heat flow signal gets flat. Then half of the nitrogen flow is directed into a vessel filled with liquid water. The water partial pressure is fixed and controlled by the temperature of the water in the vessel. When the signal has been recovering the base line, the dry nitrogen flow is replaced by the H 2 S flow. When the base line is recovered, only dry nitrogen is introduced in the cells. All the thermal analyses are carried out under the following operating conditions: measured temperature 24.5°C, gas flow 3 L.h -1 , H 2 S volumic fraction 50 vpm and relative humidity at 24.5°C 30%. H 2 S fixation capacities of the carbon fibres are measured using the apparatus illustrated figure 1. Experiments evidenced that both impregnation treatment and that the presence of water vapour together with H 2 S greatly enhance the H 2 S fixation capacities of the fibres. After being in contact with H 2 S, the impregnated fibre surface is analysed by X-rays diffraction: the sulphur matter loaded is KHS (potassium hydrogen sulphide). Considering these main results, a mechanism has been proposed to describe the fixation, in presence of water vapour, of H 2 S gas on the surface of our modified activated fibres. The mechanism can be summarized as follows:

H 2 S gas H 2 S aq [1] solubility constant for H 2 S gas : K 1 =91. [5] equilibrium constant: K 5 =very low

The large amount of water adsorbed forms a water layer [8,[START_REF] Flood | The Solid-Gas Interface[END_REF] in which the impregnate and H 2 S g can dissolve and react together. This mechanism highlights the essential role played by water. Without water vapour, no surface solution can be formed and no reaction can occur.

Results of the calorimetry study

For all the samples the curves obtained present the same shape (figure 3): they exhibit three peaks corresponding to the three steps already described: introduction of water vapour in the calorimeter cells, addition of H 2 S gas and finally introduction of dry nitrogen only.

The first peak is attributed to the adsorption of water and the dissolution of the impregnate.

The second peak is due to the H 2 S fixation and the third peak to the desorption of water and the crystallisation of the remaining impregnate and KHS. This third peak will not be studied in this paper. A sample of virgin cloth has been introduced in the calorimeter cell. In such conditions, adsorption of water constituted the only contribution to the exothermal effect induced by the introduction of water vapour . The specific amount (per weigh of fibres) of water adsorbed, in the calorimeter operating conditions, has been determined by thermogravimetry, and the specific enthalpy of the reaction is deduced from the integration of the heat flow signal. These results leads to a molar enthalpy of -24 kJ.mol -1 (at 25°C) for the adsorption of water vapour. Water is linked to the surface of activated carbons by hydrogen bond established between the water molecules and the oxygenated surface groups or water molecules already adsorbed. The data found in the thermodynamic tables [START_REF]Handbook of Chemistry and Physics[END_REF] for the hydrogen bonds energy vary between -20 and -25 kJ.mol -1 (at 25°C). So, the enthalpy measured is in fair agreement with the literature data.

To determine the dissolution enthalpy of K 2 CO 3 in the water layer on activated fibres, calorimetric experiments on fibres impregnated in solution containing from 10 to 300 g.L -1 of KOH have been performed. The specific enthalpies corresponding to the first peak obtained for each impregnated fibre samples are plot as a function of the concentration of the impregnation solution (figure 4). The specific enthalpies increase until C KOH reaches 100 g.L -1 . The amount of water adsorbed remains constant whatever the concentration of the impregnation solution. The evolution of the thermal effect induced by the introduction of water vapour in the calorimeter cells is due only to the dissolution of K 2 CO 3 whose amount deposited increases with C KOH . The solubility limit of K 2 CO 3 is reached for KOH 100: so beyond C KOH =100 g.L -1 the enthalpy does not increase any more but remains constant.

In thermodynamic tables [START_REF]Handbook of Chemistry and Physics[END_REF] , the value given for the enthalpy of dissolution of K 2 CO 3 is equal at 25°C to -29 kJ.mol -1 . Experimentally, the same value has been obtained. This means that the aqueous surface solution present on the fibres surface behaves as a bulk solution.

All the experimental specific enthalpies of dissolutions for K 2 CO 3 obtained are reported in table 1 and compared with the theoretical values. One can notice that theoretical and experimental values seem to be in good agreement. KOH The enthalpy corresponding to the second peak is greater for the virgin fibres than for the sample KOH 10. Actually, the amount of H 2 S fixed, measured by integration of the breakthrough curves, is greater for the sample KOH 10 than for the virgin cloth. From this result one can conclude that the H 2 S fixation mechanism is different according as the fibres are impregnated or not.

H 2 S fixation on virgin fibres Without water vapour in the gas flow, the molar enthalpy for the fixation of H 2 S is equal to: -30 kJ.mol -1 . This value corresponds to a physical adsorption. Without water vapour, the virgin fibres adsorb a small amount of H 2 S: 3.2.10 -4 mol.g -1 . With water vapour in the gas flow (30% HR at 24.5°C), the amount of H 2 S fixed does not exceed 2.7.10 -5 mol.g -1 . One can conclude that H 2 S and H 2 O are adsorbed on the same sites. This result is not really surprising as they are both polar molecules which can be adsorbed only on polar oxygenated surface sites. As H 2 O is introduced in the calorimeter cells before H 2 S, the water molecules occupy the surface and leave no free site for H 2 S. Nevertheless, some H 2 S molecules are fixed on the surface of fibres because they are dissolved in the water layer formed on the fibre surface. The specific enthalpy for H 2 S fixation on virgin fibres in presence of water vapour is quite huge: -833 kJ.mol -1 (at 24.5°C). This value corresponds to enthalpies generally involved in chemisorption. The amount of H 2 S molecules which dissolve in the surface solution exceed the solubility of this gas in these operating conditions (H 2 S 50 vpm, 24.5°C) which is equal to 7.7.10 -8 mol.g -1 . It can be deduced that H 2 S dissolves in the surface solution and must be involved in some very exothermic reactions. These reactions could be oxidising reactions between H 2 S and the surface oxygen [START_REF] Adib | [END_REF] . As a reference the oxidation by gaseous oxygen yields two exothermic reactions:

H 2 S g + 3/2 O 2g SO 2g + H 2 O g Δ r H o = -520 kJ.mol -1 SO 2g + ½ O 2g +H 2 O liq 2 H aq + + SO 4aq 2- Δ r H o = -227 kJ.mol -1
The fixation mechanism of H 2 S on virgin fibres in presence of water vapour could now be described as follows: formation of a water layer on the surface of the fibres, dissolution of H 2 S gas in this layer, migration in solution towards the oxygenated surface groups and finally oxidation of H 2 S molecules. The analysis by ionic chromatography of a virgin fibres after being in contact with water vapour together with hydrogen sulphide evidenced the presence of sulphate ions on the surface of the fibres. This result confirms our hypothesis concerning the oxidation reactions that occur.

H 2 S fixation on impregnated fibres The amount of K 2 CO 3 deposited on the surface of the fibres and the H 2 S fixation capacities of the impregnated fibres are reported in table 2 together with the theoretical and experimental variation of standard enthalpies corresponding to the H 2 S fixation on the impregnated fibres. When the concentration of the impregnation solution becomes greater than 50 g.L -1 , the surface solution becomes completely saturated and the H 2 S g is weakly dissolved onto the surface solution. For this reason, the H 2 S fixation capacities start to decrease and the impregnate does not react entirely.

CONCLUSION

The study of the heat effects induced when the impregnated fibres are in contact with water vapour and then, water vapour and H 2 S, allowed us to validate the H 2 S fixation mechanism proposed. It also evidences oxidation reactions occurring when virgin fibres are in contact with the water vapour / H 2 S gas flow.

Figure 1 :

 1 Figure 1 : Apparatus for measuring the H 2 S sorption capacities of carbon fibres

Figure 2 :

 2 Figure 2 : Calorimetric apparatus

Figure 3 :

 3 Figure 3 : Typical curve obtained following the experimental calorimetric protocol

  / g.L -1 specific enthalpy / j.g-1

Figure 4 :

 4 Figure 4 : Evolution of the water adsorption specific enthalpy with the concentration of the impregnation solution

Table 1 :

 1 Experimental and theoretical enthalpies corresponding to the introduction of water vapour in the calorimeter cell containing impregnated cloth samples

		10	KOH 50	KOH 100	KOH 200
	ΔH K2CO3 dissolution J.g -1	20.2	53.7	57.7	61.6
	ΔH water adsorption J.g -1	240	336	312	333
	ΔH theoritical J.g -1	260	390	370	395
	ΔH experimental J.g -1	258	361	380	390
	variance	0.8%	7%	2.6%	1%

Table 2 :

 2 Data concerning H 2 S fixation on virgin and impregnated fibres The experimental specific enthalpies are obtained by integration of the calorimetric signals, the theoretical values are calculated. If n represents the amount of H 2 S fixed, the theoretical specific enthalpy Δ r H o th is given by: Δ r H o th =Δ r H o dissolution × n + Δ r H o reaction 4 × n/2 + Δ r H o reaction 5 × n/2 The reactions 4 and 5 corresponds respectively to the reactions between H 2 S aq and CO 3aq 2-and HCO 3aq -. Δ r H o dissolution = -19.2 kJ.mol -1 , Δ r H o reaction 4 = 7.8 kJ.mol -1 and Δ r H o reaction 5 = 13 kJ.mol -1 . The experimental and theoretical data are in fair agreement.