H Boucard
email: helene.boucard@mines-albi.fr

M Watanabe

S Takami

E Weiss-Hortala

R Barna

T Adschiri

Catalytic Conversion of Black Liquor under Sub-/Supercritical Conditions

Black liquor is an alkaline liquid residue of paper industry, containing ~80wt% of water and ~20wt% of organic matter and minerals. Our global project explores black liquor conversion by using properties of supercritical water. Exploratory conversion experiments have been made previously without catalyst and revealed the formation of three phases; especially an interesting gaseous phase with a high proportion of H 2 (~80% at 600°C). A solid residue is also observed and results from the polymerisation of phenolic compounds together with aldehyde crosslinks. Based on these results, this study is devoted to the catalytic conversion using CeO 2 nanocatalysts to improve gasification and H 2 production as well as to block coke formation. In sub-/supercritical conditions, water splitting occurs by CeO 2 catalysts, to produce active hydrogen and oxygen species. Thus these actives species react with molecules of the medium. Experiments are performed in sub-/supercritical conditions in batch reactor, during 15-60 min reaction time with and without catalyst. As expected, nanocatalyst of CeO 2 improved the conversion of black liquor at sub and supercritical conditions. Hydrogen production was not significantly affected by the catalyst while amounts of CO and CO 2 are reduced at short reaction time. The high basic pH of raw feedstock and remaining solutions impacts the CO 2 dissolution into carbonates. Supercritical water media and catalyst clearly affects the fragmentation of dissolved lignin compared to subcritical conditions.

INTRODUCTION

Black liquor comes from Kraft process in paper industry. It results from the step of cooking wood chips with white liquor (Na 2 S and NaOH mixture). Delignification of wood corresponds to lignin dissolution from the fibrous part (cellulose, hemicellulose) to kraft lignin recovered in the cooking juice (black liquor) [START_REF] Leask | Pulp and papermanufacture[END_REF]. As white liquor is a basic mixture, black liquor is also a basic aqueous solution (pH ~ 13), containing dissolved lignin, fragments of cellulose and hemicellulose and several minerals and salts (Na, K, Ca, S.... carbonates, sulfates, sulfides...). In industrial facilities of the Kraft process, a large space is devoted to the concentration of black liquor followed by the incineration of this concentrated solution for heat and white liquor recovery [START_REF] Demirbas | [END_REF]. Nevertheless an extra volume of black liquor is produced, and thus black liquor would be valorized. As a result, an alternative process is to convert black liquor by using properties of supercritical water. Supercritical water has been firstly used to achieve total oxidation of waste and more recently it is used to recycle waste by generating energetical gas, platform molecules or added-value solids. Indeed, the water in these conditions (T > 374°C and P > 22.1 MPa) has the distinction of being both a reactant and a solvent. In subcritical condition (T < 374°C and P < 22.1 MPa), water is a polar solvent in which salts are miscible and organic molecules are immiscible. In supercritical state, the properties of water change drastically. The water molecules form clusters [3] that results in a nonpolar solvent and thus free radical reactions are favored. Furthermore, supercritical water and gases are miscible and the diffusion of the molecules in this phase is then improved. Either platform molecules (cresol, guaiacol,…) or gases (H 2 , CO, CO 2 ... ) are produced depending on the temperature reaction. As black liquor is a wet biomass (~80wt%), this kind of hydrothermal process is suitable to use its high water content to convert its significant organic content (~ 140 gC.L -1 ). Preliminary batch conversion experiments have been made without catalyst and revealed the formation of three phases [4]: gas with a high proportion of H 2 (~80% at 600°C), a rich bouquet of phenolic compounds in the liquid phase and an non negligeable proportion of carbon converted into solid phase (~20wt%). Here, we propose the use of catalysts to convert black liquor more efficiently. Cerium oxide is an interesting nanocatalyst to improve hydrogen production and to reduce coke formation as mentioned in a study with bitumen [5] using cubic CeO 2 . Indeed, CeO 2 has a "redox cycle" allowing to fix and release easily the oxygen generated by the water in supercritical conditions. Different morphologies of CeO 2 exist (Cubic, octahedral….); however the catalytic activity of cubic CeO 2 is better than octahedral CeO 2 [5]. These both studies lead us to consider the catalytic conversion of black liquor using cubic CeO 2 to improve the amount of H 2 into the gas phase and to reduce coke formation as well as to form more building block molecules in the liquid phase. Thus, this study focuses on the conversion of Black liquor with and without catalyst.

MATERIALS AND METHODS

1-Reagents

Black liquor

Black liquor was obtained from Smurfit Kappa Cellulose du Pin in Facture, France. It was obtained from the digester after the recovery of tall oil in Kraft process. The fraction of dried components was 23 wt% (organic and mineral) while the other part was water (77 wt%). The organic compounds amounted to 65 wt% of the dry components, which equals to 140 g-C.L -1 . In this study, the original solution was diluted to 10 wt% and used in the following experiments.

Cerium oxide: CeO 2 CeO 2 nanoparticles were synthesized by Adschiri's group in Tohoku University, Japan [5], [6]. The average size of the nanoparticles was around 8 nm. The catalytic activity of CeO 2 arises from its redox activity via Ce 4+ /Ce 3+ cycle, which is accompanied with capturing and releasing oxygen. CeO 2 is known to have high oxygen storage capacity (OSC). The OSC of cubic CeO 2 is 340 µg-O/g-cat and that is 3.4 times higher than octahedral CeO 2 [5]. Due to this high OSC, cubic CeO 2 nanoparticles were used in this study. The ratio of catalyst to the reactants was calculated and shown as a parameter in the following study.

⁄

(1) Equation 1 : ratio of catalyst

2-Experimental protocol

Experiments were performed at either 350°C or 450°C in sub-or super-critical conditions using a pressure-resistant batch reactor with an inner volume of 5.0 mL. The diluted black liquor was introduced into the reactor with the cubic CeO 2 nanoparticles (R= 0, 5, 20), then the reactor was capped tightly. After adding N 2 gas into the reactor (~0.26 MPa), the reactor was placed in an electric oven whose temperature was set to either 350°C or 450°C. After the reaction time (15 or 60 min) passed, reactor was cooled in an iced water to stop the reaction immediately. After quenching, the gas inside the reactor was analyzed by µ-GC without exposing to air. Liquid and solid products were collected after gas analysis by rinsing the reactor with THF. The collected products were separated by filtration using PTFE membrane filter with pore diameter of 0.1 µm. The THF-insoluble fraction was called coke and the THFsoluble fraction form the liquid phase. The weight of coke was evaluated as the weight loss of the solid products during calcination until 600°C, because the solid products contained CeO 2 nanoparticles as well as coke.

3-Analysis of gas, liquid and solid phases

Gas, liquid and solid phases were analyzed and characterized after reaction.

Gas phase:

The gas products were analyzed by µ-Gas Chromatography (Agilent GC-3000) to quantify following gases: H 2 , O 2 , N 2 , CO, CO 2 , and hydrocarbones.

Liquid phase:

The composition of the liquid phase was analyzed by complementary techniques of Gel Permeation Chromatography (GPC) (HP1100) and Gas Chromatography coupled with Mass spectroscopy (GC-MS) (GC: Agilent 7890A; MS:Agilent 5975C).

Solid phase:

Morphology of solid residue (coke) was observed via transmission electron microscopy (TEM, Hitachi H7650).

RESULTS

1-Gaseous phase

During the hydrothermal process of black liquor, H 2 is expected to be produced. Demirbas reported that more than 10 mol of H 2 was produced from 1 mol of black liquor if the formula C 10 H 12.5 O 7 Na 2.4 S 0.36 is considered [START_REF] Demirbas | [END_REF]. The use of CeO 2 nanocatalysts possibly results in higher gas productions [7]. Figure 1 shows the results of gas phase analysis at both sub-(Fig. 1a) and supercritical (Fig. 1b) conditions. In both cases, carbon species were formed during reaction. However, gasification of carbon was enhanced under supercritical conditions. CeO 2 acts as a catalyst and splits water into actives species [5]. Active hydrogen species stabilize the reactive intermediate species and active oxygen species involves in oxidation reactions. Although radical reactions are mainly involved at supercritical conditions, they remain significant at subcritical conditions. These hydrogen active species can form directly H 2 or react with organic molecules to form smaller molecules. Simultaneously, alkenes are formed at subcritical conditions and release H 2 in gas phase by deshydrogenation reaction. The role of active oxygen species were mainly involved in oxidation reactions. The more the oxidation is achieved, the more CO 2 is produced. The concentration of these active species increases in the presence of CeO 2 catalyst and thus the carbon conversion was enhanced. Figure 1 shows that the amount of carbon (~2 or 3%) is lower at subcritical conditions than at supercritical conditions. This tendency was in accordance with the lower quantity of gaseous products (not shown). Proportion of carbon at 350°C can be also explained by the remaining basic pH of the solution that dissolves a large amount of CO 2 in the liquid phase. The gas mixtures are composed of the same gases: H 2 , CO, CO 2 , and light hydrocarbons. However the composition changes slightly in the presence of the catalyst. The main part of carbon in the gas phase was CO and CO 2 , which was produced from the oxidation and watergas shift reactions. The amount of CO 2 after quenching the reaction was low, because CO 2 could dissolve into basic aqueous solution to form carbonates. The amount of CO was also low due to either its consumption through the water gas shift reaction, which is promoted by alkaline salts ( [4], [8]), or the strong oxidation strength of the CeO 2 catalyst. Reaction time, as well as the catalysts, played also an important role in the carbon conversion. At 350°C, the influence was not significant. However, longer reaction time resulted in enhanced gasification at 450°C. Catalysts increased the conversion of carbon by promoting water gas-shift reaction. The quantification of gaseous phases by µ-GC confirmed this result. Indeed, at 450°C and 15 min, a low quantity of CO was detected while no CO was detected after the experiment with the catalysts. The simultaneous action of catalyst and short reaction time may explain that no CO was detected in the gaseous phase due to thermodynamic equilibrium of CO 2 and kinetics.

Figure 2 shows that the amount of produced H 2 significantly increased in the presence of the catalysts at 350°C and 60 min. The other experimental conditions demonstrate a moderate effect of the catalysts. As H 2 is mainly produced from water splitting, its amount would be higher with the catalyst. Active hydrogen species that was produced from water splitting would easily and continuously react in gas phase to form H 2 . H 2 was also prouduced by hydrogenation reaction from alkane. However the increase in active hydrogen species would simultaneously increase reactions with organic molecules. Thus, the amount of H 2 was related to both the production and consumption of hydrogen actives species. The slight effect of the catalyst seems to indicate an increase of both phenomena. Concerning the experiments carried out at 350°C and 60 min, the amount of H 2 was multiplied by 10 using CeO 2 as a catalyst. This means that H 2 consumption was higher at long reaction time without catalyst while H 2 production would be higher using catalyst. As the gas-liquid equilibrium was expected for these experiments due to sub-critical water conditions, limitative diffusion of hydrogen from gas to liquid phase could explain its low consumption. The amount of H 2 was not affected by the catalyst ratio (R= 5 or 20, 350°C and 60 min). As the amount of H 2 produced from water splitting would be higher, the mass transfer at the interface would be also enhanced by this high concentration. As a result, produced H 2 would be used for capping reaction in liquid phase to produce smaller molecules.

To conclude, carbon conversion to gas phase was more influenced by the use of the catalyst that will consequently affected the composition of the liquid phase.

2-Liquid phase

The orignal black liquor is a dark brown liquid with a basic pH (>12). The conversion of black liquor is partially related to colour evolution of the liquid products (fig 3). The deep color is due to the presence of phenolic compounds [9]. In subcritical conditions, at 350°C, the liquid was still brown and no colour variation was observed within the reaction time. However at 450°C, the liquid was yellow and almost transparent. The colour became lighter with increased reaction time. The observed colour variation was caused by the conversion of oligomer molecules to either smaller colourless molecules (such as acids, aldehydes, alcohols…) and/or solid residues. This observation was supported by gas permeation chromatography (GPC, fig 4). GPC separates molecules towards their molecular weight. This first global result was interesting to compare the degradation efficiency. Indeed, the curve at 450°C was shifted to low molecular weight compared to the curve at 350°C; that suggests molecules into liquid phase are smaller at high temperature.

(a) (b) (c) (d)
This assumption is in accordance with the increase of gasification efficiency at high temperature. The presence of catalysts during reaction promoted the degradation. As a result, the products showed a lighter colour as shown in Fig. 3 and had lower molecular weight distribution. At 450°C the coupled effect of reaction time and the catalysts created a new population of molecules whose the molecular weight is center at log M = 1.77 (M = 5.87 g/mol). This different composition can then explain the higher release of carbon to gas phase. At 350°C, the impact of the catalyst at a ratio of 5 is limited as regards to GPC results. The sharp peak at log M = 2 suggests one small species was selectively obtained in high quantity. Its disparition and the shift of the curve for R=20 means other populations of molecules were created. For this temperature, all curves are stackable to Log M = 2; so no smaller molecules are formed. This result was confirmed by GC-MS results and also by the change in the colour of the products. In the presence of catalyst, oxygenation of molecules has also to be considered, more oxidized molecules have been detected by GC-MS in the presence of catalyst.

Especially at 350°C and R=20 and R=5, the same alkenes are detected, so hydrogenation reaction should be in competition with capping reactions. This tendency can explain why the amount of H 2 remains the same. Slight amounts of small aliphatic molecules (such as aldehydes) are detected in the presence of catalyst, while a high amount of aromatic compound was obtained into liquid and less coke was formed. 3 Solid phase Without catalyst, polymerisation of aromatic compounds occurred since their production began during reaction. This polymerisation resulted from the reactions between aromatic molecules and small molecules such as aldehydes. Coke formation was the result of this polymerization and was formed at both temperatures. At 350°C, solid was micrometric particles of carbon (fig 6) whereas at 450°C, the solid was shapeless (fig 7). The composition was always the same: Carbon, oxygen and minerals, particularly sulfur (darkest spots on pictures) but minerals did not play a role on the surface morphology [10]. For both temperatures, coke was formed (table 1); its quantity was increased in the supercritical conditions; so at 450°C, the amount of coke was higher than at 350°C. The proportion of coke formed decreased when R of catalyst increases at 350°C. The efficiency of catalyst seems to be higher at low temperature (subcritical conditions). However, for R=20, the amount of coke formed seems to be important. A hypothesis was an antagonist effet of CeO 2 as catalyst and as carrier of coke formation.

CONCLUSION

The catalytic conversion of black liquor with cubic CeO 2 allows to split water molecules into active hydrogen and oxygen species. Active hydrogen species leads to H 2 molecules in gas phase, H 2 molecules can also be formed by hydrogenation reaction from alkane. A part of these active species can react also with liquid molecules by capping to form smaller molecules (less complex). GPC and color of liquid attested this degradation. Active oxygen species oxidized organic molecules in liquid phase, which was confirm by GC-MS results.

When oxidation was extreme, some CO 2 was released to gas mixture; CO 2 was also due to the consumption of CO by the water gas shift reaction; which was promoted by alkaline salts and CeO 2 catalyst. Formation of very small molecules like aldehyde seems to be blocked which limits the polymerisation of phenolic compound. Indeed coke formation was limited, the compounds detected by GC-MS was higher in the presence of catalyst and the carbon conversion into gas increased that goes to an improvement of conversion of the black liquor using CeO 2 .

Figure 1 :

 1 Figure 1: Proportion of carbon converted into gas phase at 350°C and 450°C

Figure 2 :

 2 Figure 2: H 2 produced into gas phase after reaction at 350°C and 450°C

Figure 3 :

 3 Figure 3: liquid obtained after 15min and 60 min without catalyst; (a) at 350°C (b) at 450°C, liquid obtained after 15min and 60 min with catalyst (c) at 350°C (d): 450°C, 15 and 60 min

Figure 4 :Figure 5 :

 45 Figure 4: influence of the temperature without catalyst on the molecular weight

Fig 6: Fig 7 Figure 6 : 7 :

 767 Fig 6: Fig 7 Figure 6: solid obtained after 350°C, 60 min of reaction time, without catalyst Figure 7: solid obtained after 450°C, 60 min of reaction time, without catalyst

Figure 8 :

 8 Figure 8: Cubic CeO 2 nanoatalyst obtained after reaction

Table 1 :

 1 Proportion of coke formed after reaction


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Hydrochar from EWC 19.12.12 as a substitute of carbon black

Abstract

Carbon black is a carbonaceous material (more than 97 wt.% of colloidal carbon) used for tires and rubber production as well as plastics, conductive packaging and inks. It must possess low ash content and, to improve adhesion, convenient amounts of unsaturated functions and graphitic defects. At present, the processes used for carbon black production exploit nonrenewable substrates (mainly aromatic hydrocarbons and natural gas) and produce nonnegligible amounts of hazardous air pollutants. On the contrary, hydrochar is recognized to have very low environmental impacts. Indeed, hydrochar is a carbon-based material, produced from organic residues or waste substrates during hydrothermal carbonization (HTC), a wet thermochemical process. During HTC, the feedstock undergoes mainly dehydration and carbonization that enhance the colloidal carbon particles formation. For this purpose, HTC was applied to a specific substrate, i.e. the residue classified by the European Waste Catalogue as EWC 19.12.12. This is a by-product of the municipal solid waste (MSW) treatment, supplied by Contarina S.p.A., a company which collects and treats the MSW in the province of Treviso in Italy. The non-recyclable and non-compostable residual fraction of the MSW is treated to produce refuse-derived fuel (RDF). However, a small percentage of this residual is an organic fraction deemed to be contaminated that cannot be used for composting purposes. Thus, this fraction (EWC 19.12.12) is removed and currently bio-stabilized, and then landfilled. The EWC 19.12.12 was therefore carbonized in a 50 mL batch reactor [START_REF] Basso | Hydrothermal carbonization of off-specification compost: a byproduct of the organic municipal solid waste treatment[END_REF] at different HTC conditions, namely three temperatures (180, 220 and 250 °C) and three residence times (1, 3 and 8 hours). The hydrochar yield resulted dependent on the severity of the HTC treatment and ranged from 50% (T=250°C, residence time=8 h) to 64% (T=180°C, residence time=1 h). The main characteristics of the hydrochar were investigated to get insights on the possibility to introduce it as a substitute of the common carbon black [START_REF] Titirici | Hydrothermal carbon from biomass: a comparison of the local structure from poly-to monosaccharides and pentoses/hexoses[END_REF]. Hence, the chemical composition (elemental analysis, inductively coupled plasma), the crystalline structure (X-ray diffraction), the surface area and the particle size of the hydrochar were characterized. The preliminary results seem to support the possibility to exploit the hydrochar from EWC 19.12.12 as a precursor or a substitute of carbon black.

  Daniele Basso, Francesco Patuzzi, Elsa Weiss-Hortala, Marco Baratieri, Paolo Conto, Luca Fiori To cite this version: Daniele Basso, Francesco Patuzzi, Elsa Weiss-Hortala, Marco Baratieri, Paolo Conto, et al.. Hydrochar from EWC 19.12.12 as a substitute of carbon black. WasteEng 2016 -6th International Conference on Engineering for Waste and Biomass Valorisation, May 2016, Albi, France. p.608-620. hal-01706811

HYDROCHAR FROM EWC 19.12.12 AS A SUBSTITUTE OF CARBON BLACK

  Université de Toulouse, Mines Albi, UMR CNRS 5302, Centre RAPSODEE, Albi, France. *Corresponding author: luca.fiori@unitn.it, phone: +39 0461 282692, Fax: +39 0461 282672

	D. BASSO 1 , F. PATUZZI 2 , E. WEISS-HORTALA 3 , M. BARATIERI 2 and L. FIORI 1,*
	1 University of Trento, Department of Civil, Environmental and Mechanical Engineering, via
	Mesiano, Trento, Italy.
	2 Free University of Bolzano, Faculty of Science and Technology, Bolzano, Italy.

3

L'


Hussein Zahr
email: hussein.zahr@ensam.eu

Mohamed Trabelsi
email: mohamed.trabelsi@ensam.eu

Member IEEE Eric Semail
email: eric.semail@ensam.eu

Member IEEE Ky Ngac

Nguyen

Five-Phase Bi-Harmonic PMSM Control under Voltage and Currents Limits

Keywords: Bi-harmonic machine, control sensibility, Maximum Torque Per Ampere, I

For a particular five-phase synchronous machine, this paper investigates the sensitivity of a vectorial control strategy on the required peak phase voltage whose value is fundamental for the choice of the DC bus voltage. The specificity of the machine is that the first and third harmonic components of the back electromotive force (back-emf) have the same amplitude. As a consequence, the torque can be produced by one of them or both with suitable currents. This degree of freedom is interesting for optimizing the efficiency and generating high transient torque. However, using two harmonics having the same amplitude leads to a necessity to analyze the constraints on the required phase machine voltage. Considering a Maximum Torque Per Ampere (MTPA) strategy, the paper examines the impact of some parameters such as the phase shift between currents and back-emfs or the ratio between the third and the first harmonic of current on the torque and maximum voltage value. Experimental tests with a limited DC bus voltage have been carried out and compared to the results obtained by a Finite Element Analysis.

INTRODUCTION

Nowadays, multiphase machines are widely used for their fault tolerance and high torque density [START_REF] Barrero | Recent advances in the design, modeling and control of multiphase machines-Part 1[END_REF] especially in critical applications, such as marine [START_REF] Thongam | Trends in naval ship propulsion drive motor technology[END_REF], aerospace [START_REF] Bojoi | Control of Shaft-Line-Embedded Multiphase Starter/Generator for Aero-Engine[END_REF] and automotive traction [START_REF] Parsa | Fault-Tolerant Interior-Permanent-Magnet Machines for Hybrid Electric Vehicle Applications[END_REF]. Thanks to the vector control, these machines have an ability to produce torque without pulsation even with nonsinusoidal back-EMFs and non-sinusoidal currents, in similar way to the classical three-phase machines [START_REF] Wang | Harmonic current effect on torque density of a multiphase permanent magnet machine[END_REF]. Moreover, in low voltage and/or high power applications, a high number of phases leads to a lower current per phase and consequently decreases the power of switches. Nevertheless, a high number of phases impact the cost since the number of drivers and current sensors increases, and the control is more complicated since many harmonics can interfere in torque production.

The work addressed in this paper deals with a five-phase PMSM that appears as a compromise when it used in critical applications such as automotive and aerospace applications. This machine allows creating torque from both 1 st and 3 rd harmonic components [START_REF] Scuiller | Multi-criteria based design approach of multiphase permanent magnet low-speed synchronous machines[END_REF]. The contribution of each harmonic component depends mainly on the amplitude of the harmonic content of the back-EMF [START_REF] Aslan | New 5-phase concentrated winding machine with bi-harmonic rotor for automotive application[END_REF]- [START_REF] Zahr | Comparison of Optimized Control Strategies of a High-Speed Traction Machine with Five Phases and Bi-harmonic Electromotive Force[END_REF]. Generally, the 3 rd harmonic is used to improve the torque, which comes essentially from the fundamental harmonic. Therefore, many researchers focus on modifying the control strategies, the design or both to achieve this goal. However, the torque production due to 3 rd harmonic appears always to be marginal [START_REF] Mengoni | High-Torque-Density Control of Multiphase Induction Motor Drives Operating Over a Wide Speed Range[END_REF]- [START_REF] Wang | Torque Improvement of Five-Phase Surface-Mounted Permanent Magnet Machine Using Third-Order Harmonic[END_REF].

In this paper, the designed machine has a torque which can be created, with an equal sharing ratio between two harmonics since they have the same amplitude. This kind of machines is called bi-harmonic machine. Furthermore, in the machine design, many requirements for traction applications are considered:

1) High transient torque for a boost can be achieved by using the 3 rd harmonic component. 2) Ability of control in wide speed range at constant power: for three-phase machine, the flux-weakening is appearing almost as the unique solution to increase the speed above the base speed when the maximum voltage imposed by the voltage source inverter is reached. For the bi-harmonic machine, many solutions exist for flux weakening operation since the 3 rd harmonic offer an additional degree of freedom. The total torque/speed characteristic is the sum of the characteristic associated to each harmonic component [8][13]. 3) Low losses associated to eddy currents when high frequency is required at high speed: the winding configuration is selected so that the harmonic content of the MMF is low. Consequently, low level of losses can appear in magnets. Therefore, the fractional slot concentrated winding with a number of slots per pole and per phase equal to 0.5 is used 0- [START_REF] Bianchi | Index of rotor losses in three-phase fractional-slot permanent magnet machines[END_REF]. The control strategies applied to multiphase machines are based on the Maximum Torque Per ampere MTPA [START_REF] Zahr | Comparison of Optimized Control Strategies of a High-Speed Traction Machine with Five Phases and Bi-harmonic Electromotive Force[END_REF]-0. Consequently, a ratio ρ between 1 st and 3 rd current harmonic components must be fixed to have collinear current and back-EMF (these assertions are detailed in section III). Practically, it could be appeared that the actual current differs from the optimal (reference) ones, leading to a variation (increase or decrease) of torque and required voltage.

The aim of this paper is to study the sensitivity of the MTPA strategy. Taking into account the several degrees of freedom in the case of the 5-phase bi-harmonic PMSM (5-Φ B-PMSM), there are several possibilities to modify the reference currents by acting on the phase shift (φ) between current and back-EMF and/or the ratio between the 1 st and 3 rd harmonic current components ( ). Consequently, in addition to the classical MTPA method, two new strategies will be investigated and analyzed with constraints on the voltage and currents.

This paper will be organized as follows: Section II presents the structure of the considered prototype. Section III presents the MTPA supply strategies applied to the 5-Φ B-PMSM machine and the other strategies derived from the MTPA but with the variation of the phase shift φ and the ratio ρ=I 3 /I 1 . The obtained characteristics of the studied machine under these conditions are analyzed, taking into account the voltage and current limits.

II. TOPOLOGY AND VECTOR CONTROL OF INVISTIGATED 5-Φ BI-HARMONIC PMSM

This section aims to present the prototype of the five phase bi-harmonic PMSM and the adopted control for operating characteristics investigation. The investigated machine can be used in traction applications with capability for developing high torque during transient operation and good efficiency at steady state [START_REF] Boldea | Automotive Electric Propulsion Systems With Reduced or No Permanent Magnets: An Overview[END_REF]- [START_REF] El-Refaie | Motors/generators for traction/propulsion applications: A review[END_REF].

A. 5-Φ Bi-Harmonic PMSM Topology

The prototype is depicted in Fig. 1. This prototype consists in a 5-Φ 40-slot/16-pole bi-harmonic PMSM [START_REF] Zahr | Comparison of Optimized Control Strategies of a High-Speed Traction Machine with Five Phases and Bi-harmonic Electromotive Force[END_REF]0. In order to decrease the cogging torque and other torque ripples, the stator is skewed by ½ slot step. In the next section, thanks to vector control, the ability to produce torques with low torque ripples even when two harmonics are injected is verified and approved.

The back-EMF harmonic content, obtained for a 500 rpm rotor speed, is depicted in Erreur ! Source du renvoi introuvable.Fig. 3. It comprises two main harmonic components (E 1 and E 3 ). Here, it should be noticed that because of a high value of winding factor, the 3 rd harmonic component of the back-EMF is greater than the fundamental harmonic, but both have comparable magnitude since they have a ratio = E 3 /E 1 =1.22. It should be noticed also that, for a 5-Φ 40-slot/16-pole, this ratio-value allows to maximize the torque in low-speed region and to improve the flux-weakening operation mode [START_REF] Aslan | New 5-phase concentrated winding machine with bi-harmonic rotor for automotive application[END_REF][8]0.

B. Modeling of the 5-Φ Bi-harmonic PMSM

In this part, the basic knowledge of the reference frame theory applied to a 5-Φ B-PMSM is introduced to highlight the potentialities of this concept used for vector control of multiphase machines. The basics of the vector control for multiphase machines [START_REF] Kestelyn | Vectorial modeling and control of multiphase machines with non-salient poles supplied by an inverter[END_REF]- [START_REF] Kestelyn | A Vectorial Approach for Generation of Optimal Current References for Multiphase Permanent-Magnet Synchronous Machines in Real Time[END_REF], leads to three decoupled subspaces, as shown in Fig. 4.

First sub-space: is associated mainly with the 1 st harmonic electrical components (voltage, current, back- For an easier control of the 5-Φ B-PMSM, the Park transformation given in ( 5)-( 6) is used. In such frames, the currents, the voltages and the flux are constant in healthy operations. Consequently, the control of the two-phase fictitious machines can be achieved independently with two PI-controllers. For motor modeling, we suppose that the magnetic saturation, the hysteresis and slot effects, and the iron losses are neglected. Considering these assumptions, the electrical equations that describe the 5-Φ Bi-harmonic PMSM, in dq-rotating frames, are defined by: for the fictitious main machine (MM) [START_REF] Aslan | New 5-phase concentrated winding machine with bi-harmonic rotor for automotive application[END_REF] for the fictitious Secondary machine (SM)

dI d 3 V R I L e 3 L I m q3 q3 d 3 d 3 d 3 d 3 dt dI q3 V RI L e 3 L I m q3 q3 q3 q3 d 3 d 3 dt (8)
where, ([v dq1 ], [i dq1 ], [e dq1 ]) and ([v dq3 ], [i dq3 ], [e dq3 ]) are the stator voltages, the phase currents and the back-EMFs linked to the fictitious MM and the fictitious SM, respectively. R is the phase resistance and (L d1 , L q1 ) and (L d3 , L q3 ) are the equivalent self-inductances linked to the MM and SM, respectively.

The electrical fundamental pulsation corresponding to the MM, in steady state, ω m , is given by ( 9), where p is the pair poles number, fs is the electrical supply frequency and θ m is the mechanical rotor position. On the contrary, the electrical pulsation, ω s , corresponding to the SM, in steady state, is three times higher than ω m , as given by ( 10): [START_REF] Abdelkhalik | Eleven-phase induction machine: steady-state analysis and performance evaluation with harmonic injection[END_REF] For the considered machine, the developed torque by the fivephase PMSM is the sum of the torque produced by each fictitious machine, which is given by: [START_REF] Zhao | Torque density improvement of fivephase PMSM drive for electric vehicles applications[END_REF] where (E 1 , E 3 ) and (I 1 , I 3 ) are the 1 st and the 3 rd harmonic amplitude of the back-emf and current respectively, φ 1 (φ 3 ) the phase shift between the 1 st (3 rd ) harmonic current and the 1 st (3 rd ) harmonic of the back-emf.

) ) 3 cos( 3 3 ) 1 cos( 1 1 ( . 2 5 s T I E m T I E em T

III. OPERATING CHARACTERISTICS OF 5-Φ BI-HARMONIC PMSM UNDER VOLTAGE AND CURRENT LIMITS

In this section, several supply strategies will be presented and examined in order to test the sensitivity of MTPA strategy applied to the 5-Φ Bi-Harmonic PMSM.

A. Maximum Torque Per Ampere strategy (MTPA).

For this control strategy, the maximum torque is achieved when the back-emf and the current vectors are collinear [START_REF] Kestelyn | A Vectorial Approach for Generation of Optimal Current References for Multiphase Permanent-Magnet Synchronous Machines in Real Time[END_REF]. The two following conditions guarantee the collinearity:

1) Based on equation [START_REF] Zhao | Torque density improvement of fivephase PMSM drive for electric vehicles applications[END_REF], the maximum torque is guaranteed by setting :

0 3 1 (12)
2) The ratio between the 1 st and the 3 rd harmonic current component must be equal to ρ given by : Fig. 5. Currrent and back-emf shape for φ≠0. According to Fig. 3, the ratio ρ must be equal to 1.22. Assuming that the machine is supplied with currents characterized by equation ( 14):

eff I I eff I I 1 2 2 3 1 2 1 2 1 (14)
This strategy can be used as long as the maximum voltage delivered by the voltage source inverter (VSI) is not reached. When the maximum voltage is reached, the speed can still be increased by varying the ratio ρ and/or the phase shifts φ 1 and φ 3 . This approach is different from the one used in classical three phase machines, in which flux-weakening appears as the only possible solution to increase speed when the voltage limit is reached. For the bi-harmonic machine, the determination of the current references in high speed zone requires the resolution of four variables non-linear optimization problem [START_REF] Zahr | Comparison of Optimized Control Strategies of a High-Speed Traction Machine with Five Phases and Bi-harmonic Electromotive Force[END_REF].

As mentioned in the introduction, the two strategies will be considered and compared with the MTPA:

The first strategy: the phase shift φ 1 and φ 3 fixed to zero, and the ratio ρ is variable.

The second strategy: The ratio ρ is fixed to its optimal value and the phase shifts φ 1 and φ 3 are varying.

For each control strategy, the required voltage is calculated and compared with the one obtained in the MTPA strategy. The results are mainly presented from Fig. 7 to Fig. 10.

B. Control with ρ and variable phase shifts φ.

In this control strategy, the value ρ is always equal to 1.22. However, the phase shifts φ 1 and φ 3 are changed so that the shape of the current remains the same as the one obtained with MTPA strategy. Therefore, we introduce a global phase shift between the current and the back-emf, noted φ as presented in Fig. 5. Consequently, the phase shift φ 1 and φ 3 are expressed in function of φ as follows: component. In addition, the machine should deliver the same torque as the one delivered when the MTPA strategy is applied. In these conditions, the toque is given by: [START_REF] Boldea | Automotive Electric Propulsion Systems With Reduced or No Permanent Magnets: An Overview[END_REF] In MTPA conditions, the ratio ρ MTPA =1.22, this results in: [START_REF] El-Refaie | Motors/generators for traction/propulsion applications: A review[END_REF] In Fig. 9, we present the ratio between the current density due to the variation of the ratio ρ and the current density of the MTPA strategy J/J MTPA . Results show that it is necessary to inject more current density in the machine in order to maintain the same torque as the MTPA approach. This results in more copper losses in the 5-Φ Bi-Harmonic PMSM machine. The variation of the required voltage value according to ρ is given in Fig. 10. These values are measured experimentally and compared to the ones obtained by FE simulation. We can conclude that they are quite similar. We show that when the variation of the ratio ρ is less 1.22 (MTPA ratio), the required voltage is generally lower. If ρ=0which means that there is no 3 rd harmonic current componentthe required voltage is 3% less than the MTPA case at 963rpm, 11% at 790 rpm and 12% at 640rpm. The lower is the contribution of the 3 rd harmonic in the total torque; the lower is the required voltage. This is due mainly to the fact that the 3 rd harmonic induces more voltage drop than the 1 st harmonic. Fig. 11 shows a comparison between the phase voltage obtained by FE simulation and the measured voltage obtained after elimination of PWM harmonics. Finally, by comparing the two approaches developed in section III.B. and III. C., it appears that the required voltage is less sensitive to the variation of ρ than the variation of φ. Finally, Table .III summarizes the advantages and drawbacks of each control strategy in terms of peak current, peak voltage, torque and copper losses.

MTPA T I E I E em T ) 1 3 1 1 ( 2 5
) ( ) ( 1 2 ) ( 2 / 5 ) ( 3 1 3 1 2 3 1 1 E E E E I E E T I MTPA MTPA eff MTPA

IV. CONCLUSION

In this paper, the sensitivity of MTPA strategy was investigated for a particular bi-harmonic machine whose the torque developed from the 1 st harmonic is equivalent to the one from the 3 rd harmonic. Given that the MTPA is achieved when ρ MTPA =I 3 /I 1 =E 3 /E 1 and φ=0, two sensitivity tests are performed: the first one concerns the phase shift φ (control parameter) between the current and the back-emf. The second one concerns the ratio ρ=I 3 /I 1 (design parameter) which is equal in MTPA to E 3 /E 1 . The results show that if the phase shift φ varies between -π/10 and π/10, the torque is thus reduced up to 35%. Therefore, in order to obtain the desired torque, more current density should be injected in the machine. Furthermore, the required voltage can increase up to 20% (φ<0) or decreases by 40% (φ >0) compared to MTPA values.

On the other hand, the variation on the ratio ρ can decrease slightly the required voltage between 3% and 12%, while keeping the same torque as the one provided in MTPA strategy. But for this kind of variation, the current density must be increased.

In conclusion, the sensitivity of the required voltage and current with respect to ρ and φ should be taken into account when sizing the VSI. The presented results allow to define the security margin to be considered when fixing the volt-ampere rating of the VSI. Consequently, the machine remains able to provide the same torque as the one delivered in MTPA when the values of ρ and φ are not precisely determined.

  (a) Stator (b) Rotor Fig. 1. Machine prototype (5-Φ 40-slot/16-pole/48-pole).

Fig. 2 .

 2 Fig. 2. Test bench used for experimental validation.

Fig. 3 .

 3 Waveforms and harmonic content of the back-EMF obtained by FE and experimentally at XX rpm EMF) corresponding to the Main fictitious Machine (MM) and is noted with index (αβ).Second sub-space: is associated with the 3 rd harmonic component corresponding to the Secondary fictitious Machine (SM) and is noted with index (xy). Third subspace: is associated with the fifth harmonic component. Because of the star connection isolated neutral of the PMSM, there is no path for the zerosequence component of current. The transformation which allows to obtain these machines is the generalized Concordia transform. The equivalent components in orthogonal sub-spaces αβ and xy are obtained by: ] denotes the variable in natural frame abcde (voltage, current, back-EMF, ...). Their equivalent components in orthogonal frames are denoted by [x αβ ] and [x xy ]. [x d1q1 ] and [x d3q3 ] are variables in rotating frame.

Fig. 4 .

 4 Fig.4. Configuration of the 5-Φ PMSMTABLE I ELECTRICAL PARAMETERS OF 5-Φ B-PMSM Rs = 0.0324 Ω; Lp =139 μH; Ls = 178 μH, p=8, From spectrum analysis at load for 500 rpm, amplitudes (RMS) of emf E1=10,2V, E3=13V

Fig. 6 .

 6 Fig. 6. Variation of torque according to the phase shift angle φ.

Fig. 7 .

 7 Fig. 7. Variation of torque according to the phase shift angle φ.

Fig. 8 .

 8 Fig. 8. Voltage waveform for three value of phase shift φ and a current density of 20A/mm 2 obtained by FE simulation.

TABLE III ADVANTAGES

 III AND DRAWBACKS OF THE TWO STUDIED STRATEGIES

		Variation of φ	Variation of ρ
		(ρ=1.22)	(φ=0)
		1-Less required voltage
	Advantages	than MTPA strategy if φ>0 2-Same current peak	1-Less required voltage than MTPA for the same torque
		value as MTPA
	Drawbacks	1-More required voltage than the MPTA strategy if φ<0 2-Generally, lower torque than the once in MTPA is obtained.	1-More copper losses than MTPA are obtained for the same torque. 2-Higher peak current than MTPA strategy

ACKNOWLEDGEMENT

This work has been achieved within the framework of CE2I project (Convertisseur d'Energie Intégré Intelligent). CE2I is co-financed by European Union with the financial support of European Regional Development Fund (ERDF), French State and the French Region of Hauts-de-France.

The allowed variation of the phase shift φ belongs to the following interval:-π/10 ≤ φ ≤ π/10. The FE simulation is performed for a current density of 2.5 A/mm 2 , 5A/mm 2 and 20A/mm 2 . Another operating point is considered when the current peak value is equal to 200A. Here, it should be noticed that the maximum value which can be measured by the current sensors reaches 200A.

Each current density is injected in the machine for different speed value so that for φ=0, a peak voltage value equal to the half of DC bus voltage is obtained, for all the considered current densities. In this work, the DC bus voltage is assumed to be 48V. Fig. 6 and Fig. 7 present the variation of torque and voltage respectively according to the variation of phase shift φ. The results are determined using the FE software ANSYS Maxwell 2D. As observed in Fig. 6, the required voltage is greater than 24V when the phase shift φ<0.

Regarding Table. II, it has been shown that the percentage of the additional required voltage is within the band [8%-20%].

On the contrary, the required voltage becomes less than 24 V when φ>0. The percentage of required voltage decrease is within [14%-40%] in comparison to the MTPA value (24V).

In Fig. 8, the impact of phase shift on the voltage waveform is presented. The FE simulation is performed for three of phase shift values (φ=-π/10, φ=0 and φ=π/10) with a current density equal to 20A/mm 2 . The results show that for φ=-π/10, the voltage exceeds for several instants the maximum limit of 24V. Consequently, an important distortion can appear in the voltage waveform in this case.

In addition, the torque can decrease significantly by 35.9% as depicted in table .II and Fig. 6 when the phase shift φ<0 or φ>0.07. In order to compensate this torque decrease, more current density must be injected which increases significantly the required voltage. Notice that, in all the studied cases, a slight improvement of the torque is possible when φ is within [0-0.07] (up to 1.5 % according to Table. II and to Fig. 4), due to the slight saliency effect of the machine.

C. Control with variable value of ρ and phase shift φ equal

to zero. In this control strategy, the phase shift φ 1 and φ 3 are fixed to zero. The ratio between the 1 st and the 3rd harmonic current ρ is variable. This means that the error on the ratio ρ (<1.22) implies that the torque is mainly produced by the 1 st harmonic


Elsa Weiss-Hortala

Andrea Kruse

Christina Ceccarelli

Radu Barna

Influence of phenol on glucose degradation during supercritical water gasification

Keywords: supercritical water gasification, phenol, glucose, hydrogen production

Biomass is the ideal alternative renewable energy that might help decrease CO 2 emissions. Super Critical Water Gasification (SCWG) is a recent treatment method which is still being developed as regards wet biomass. Above its critical point, water has specific properties and is able to convert wet biomass into gas, and especially into hydrogen. In order to propose a general reaction scheme of the SCWG as regards the lignocellulosic biomass conversion, the interactions between lignin and cellulose must be highlighted. Lignocellulosic biomass could be modelled with phenol (substitute for lignin) and glucose (substitute for cellulose).

In a continuous flow tubular reactor, the gasification of solutions containing phenol and glucose or either of the two pure compounds is more efficiently performed when in presence of an alkaline catalyst. The comparison of global parameters, such as the Total Organic Carbon content (TOC), the composition of the liquid product phase (glucose, phenol…), the volume and composition of the gas phase (H 2 , CO 2 , CH 4 …), showed that a small quantity of phenol in a glucose solution dramatically decreased the efficiency of the solution's conversion.

In a continuous tubular reactor, the gasification efficiency of solutions containing phenol, glucose or the two pure compounds are conducted in presence of an alkaline catalyst. The comparison of the gas yields showed that presence of phenol in a glucose solution decreased dramatically the conversion efficiency of the solution at the comparable operating conditions (pressure, temperature, flow rate, catalyst concentration).

Introduction

Nowadays, researchers are getting more involved in the field of renewable or supposedly renewable forms of energies: solar power, wind energy, biogas, synthetic gas… with view to replace fossil energies and decrease carbon dioxide emissions that are ensued from fossil compound processing operations. With consideration of these two points, biomass is the ideal alternative energy, with a carbon dioxide balance close to zero. Most of the currently used techniques are efficient with dry biomass (pyrolysis for example). On the contrary, Super Critical Water Gasification (SCWG) is a recent treatment method which is still being developed [START_REF] Kruse | Hydrothermal biomass gasification[END_REF], and that allows wet biomass to be used, even with a natural water content of up to 80-90 %.

The principle is to convert wet biomass into gas, using high temperatures and pressures.

Above the critical point (22.1 MPa and 374°C), water has specific properties and is able to convert carbon from the biomass into methane and/or carbon dioxide. During this process, hydrogen that was bonded to biomass could also be transformed into dihydrogen. At the present time, hydrogen seems to be the best alternative source of energy thanks to its high energy potential and eco-friendly properties, but it has to be produced in a sustainable way.

Different technologies for H 2 production are under development.

Above its critical point, water is a monophasic system [START_REF] Clifford | Reactions in supercritical water[END_REF], and its physico-chemical properties are no longer the same when it is liquid [START_REF] Cansell | Thermodynamic aspects of supercritical fluids processing: applications to polymers and wastes treatment[END_REF][START_REF] Shaw | Supercritical water-a medium for chemistry[END_REF]. Generally speaking, supercritical fluids are interesting because they form a unique phase; their diffusivity is close to that of gases and their density could be easily adjusted to the desired values [START_REF] Savage | Reactions at supercritical conditions: applications and fundamentals[END_REF]. SCW media are very reactive because they efficiently hydrolyse organic compounds [START_REF] Marshall | Ion product of water substance, 0-1000°C, 1-10,000 bar new international formulation and its background[END_REF][START_REF] Franck | Thermophysical properties of supercritical fluids with special consideration of aqueous systems[END_REF][START_REF] Kruse | Hot compressed water as reaction medium and reactant. Properties and synthesis reactions[END_REF]. At low densities, SCW is a poor solvent for ionic species like inorganic salts [START_REF] Tester | Chemical reactions and phase equilibria of model halocarbons and salts in sub-and supercritical water (200-300 bar, 100-600°C)[END_REF], but it is completely miscible with many organic compounds and gases [START_REF] Kruse | Hot compressed water as reaction medium and reactant. Properties and synthesis reactions[END_REF], allowing the precipitation of inorganic salts on the one hand, and homogenous reaction processes between gases and organic compounds on the other hand. Reactions in supercritical water could be catalysed by acid or base [START_REF] Kruse | Hot compressed water as reaction medium and reactant. Properties and synthesis reactions[END_REF][START_REF] Hunter | Recent advances in acid-and base-catalyzed organic synthesis in high-temperature liquid water[END_REF] or initiated by free radicals [START_REF] Kruse | Hot compressed water as reaction medium and reactant. 2. Degradation reactions[END_REF].

Biomass reacts with water and forms hydrogen and carbon dioxide in a steam reforming reaction [START_REF] Yu | Hydrogen production by stream reforming glucose in supercritical water[END_REF][START_REF] Matsumura | Biomass gasification in near-and super-critical water: status and prospects (review)[END_REF] following Eq. 1. Both Water-Gas Shift reaction (Eq. 2, WGS) and methanation (Eq. 3) are used to determine the composition of the gas.

  2 2 2 2 2 2 H x y CO O H y O CH y x             (1) 2 2 2 H CO O H CO    (2) O H CH H CO 2 4 2 3    (3)
Even water is very reactive, chemical kinetics is a limitative factor in the development of a technical process [START_REF] Kruse | Hydrothermal biomass gasification[END_REF]. Sometimes, char and/or tar could be produced during the process [START_REF] Matsumura | Biomass gasification in near-and super-critical water: status and prospects (review)[END_REF].

Catalysts are used to increase reaction rates and support some reaction pathways, such as the WGS by alkaline salts [START_REF] Matsumura | Biomass gasification in near-and super-critical water: status and prospects (review)[END_REF][START_REF] Sutton | Review of literature on catalysts for biomass gasification[END_REF][START_REF] Yanik | Biomass gasification in supercritical water: II. Effect of catalyst[END_REF]. After cooling and depressurising sequences, both liquid and gas phases are analysed.

Most studies are published on the SCWG of glucose [START_REF] Kruse | Hot compressed water as reaction medium and reactant. 2. Degradation reactions[END_REF][START_REF] Yu | Hydrogen production by stream reforming glucose in supercritical water[END_REF][START_REF] Matsumura | Biomass gasification in near-and super-critical water: status and prospects (review)[END_REF][START_REF] Lee | Gasification of glucose in supercritical water[END_REF][START_REF] Watanabe | Catalytic hydrogen generation from biomass (glucose and cellulose) with ZrO 2 in supercritical water[END_REF][START_REF] Aida | Dehydration of D-glucose in high temperature water at pressures up to 80 MPa[END_REF][START_REF] Sinağ | Key compounds of the hydropyrolysis of glucose in supercritical water in the presence of K 2 CO 3[END_REF][START_REF] Goodwin | Conversion of glucose to hydrogen-rich gas by supercritical water in a microchannel reactor[END_REF], which is an intermediate compound of cellulose decomposition [START_REF] Sasaki | Kinetics of cellulose conversion at 25 MPa in sub-and supercritical water[END_REF][START_REF] Resende | Noncatalytic gasification of cellulose in supercritical water[END_REF]. Some recent studies show that both the presence of lignin in the feedstock and operating conditions have an influence on the gasification efficiency [START_REF] Yoshida | Gasification of cellulose, xylan and lignin mixtures in supercritical water[END_REF][START_REF] Saisu | Conversion of lignin with supercritical water-phenol mixtures[END_REF][START_REF] Yoshida | Gasification of model compounds and real biomass in supercritical water[END_REF][START_REF] Resende | Noncatalytic gasification of lignin in supercritical water[END_REF] and that the lignin seems to modify the conversion efficiency of the carbohydrate conversion [START_REF] Yoshida | Gasification of cellulose, xylan and lignin mixtures in supercritical water[END_REF][START_REF] Yoshida | Gasification of model compounds and real biomass in supercritical water[END_REF][START_REF] Yanik | Biomass gasification in supercritical water: Part 1. Effect of the nature of biomass[END_REF]. In order to propose a general reaction scheme of the SCWG for lignocellulosic biomass, the interactions between these two compounds must be highlighted. In this paper, lignocellulosic biomass was modelled by glucose as substitute for cellulose and phenol for lignin. In our experimental approach, we used two representative molecules. Glucose is the hydrolysis product of cellulose. Phenol is easily obtained in SCW by the degradation of guaiacol [START_REF] Dileo | Supercritical water gasification of phenol and glycine as models for plant and protein biomass[END_REF] and can be considered as one of the structural building bricks of lignin. We have decided to use phenol, the simplest molecule presenting the Ar-OH structure, as a substitute for lignin.

The lignin molecule is a complex arrangement of aromatic rings. In the different molecular forms of lignin that have been proposed, guaiacol (o-methoxy-phenol) is one of the most abundant aromatic rings containing hydroxyl and methoxy substituent. Di Leo et al. [START_REF] Dileo | Gasification of guaiacol and phenol in supercritical water[END_REF] have noted that guaiacol quickly disappears during SCWG and can be substituted by its reaction products, e.g. phenol. During the hydrothermal degradation of lignin, phenol and different substituted phenols are formed as intermediate products of lignin gasification [START_REF] Saisu | Conversion of lignin with supercritical water-phenol mixtures[END_REF][START_REF] Kruse | Biomass conversion in water at 330-410°C and 30-50 MPa. Identification of key compounds for indicating different chemical reaction pathways[END_REF]. Phenol is also a widely encountered industrial pollutant (oil, painting, pesticides, colouring agents and pharmaceutical industries), which is difficult to eliminate from wastewater [START_REF] Weiss | A comparison of electrochemical degradation of phenol on boron doped diamond and lead dioxide electrodes[END_REF][START_REF] Busca | Technologies for the removal of phenol from fluid streams: A short review of recent developments[END_REF].

Phenol's oxidation in supercritical water (SCWO) is a very big issue in many publications.

SCWO of phenol was performed in batch or continuous flow tubular reactors [START_REF] Thornton | Phenol oxidation pathways in supercritical water[END_REF][START_REF] Gopalan | A reaction network for phenol oxidation in supercritical water[END_REF][START_REF] Krajnic | On the kinetics of phenol oxidation in supercritical water[END_REF][START_REF] Henrikson | Inhibition and acceleration of phenol oxidation by supercritical water[END_REF][START_REF] Henrikson | Potential explanation for the inhibition and acceleration of phenol SCWO by water[END_REF] in various experimental conditions: temperatures from 300-630°C, pressure range from 21-31

MPa, oxygen excess, various water densities and reaction times. Different global kinetic models have been proposed for phenol degradation, almost pseudo first order for phenol; in general, the oxidation kinetic is considered as an Arrhenius type process. Water density in the mixture seems to play a complex role. In general, with the decrease of water density the main mechanisms change from heterolytic (ionic) to homolytic (radical) as a result of the concomitant change of the dielectric constant of water, of ion product of water Kw, of phenol dissociation, of diffusion coefficients and rates of the diffusion controlled reactions. The experiments showed that water inhibits the reaction rate at low densities and high temperatures and it accelerates the rate at lower temperatures and higher water densities.

Different mechanisms for phenol SCWO have been published [START_REF] Thornton | Phenol oxidation pathways in supercritical water[END_REF][START_REF] Gopalan | A reaction network for phenol oxidation in supercritical water[END_REF][START_REF] Krajnic | On the kinetics of phenol oxidation in supercritical water[END_REF][START_REF] Onwudili | Reaction mechanisms for the decomposition of phenantrene and naphtalene under hydrothermal conditions[END_REF], and among the characterised products of the reaction, the main ones in the liquid phase are phenoxyphenols, biphenol, dibenzofuran, acids (oxalic, acetic, benzoic, salicylic…). The gasification of phenol in supercritical water was also studied, showing a relatively slow degradation, yet more efficient when using an Ni catalyst in a quartz reactor [START_REF] Dileo | Supercritical water gasification of phenol and glycine as models for plant and protein biomass[END_REF][START_REF] Dileo | Gasification of guaiacol and phenol in supercritical water[END_REF].

Alkali salts are used as substitute for inorganic ("ash") components of natural biomass. It was shown that both KHCO 3 and natural inorganic components act as catalysts in the water-gas shift reaction [START_REF] Kruse | Hydrothermal biomass gasification[END_REF][START_REF] Kruse | Hot compressed water as reaction medium and reactant. 2. Degradation reactions[END_REF]. The addition of the salt enables the model system and real biomass to be more efficiently compared.

Materials and methods

Continuous flow reactor and operating conditions

Experiments were performed in a continuous flow tubular reactor (Fig In all experiments, KHCO 3 (Roth Company, 99.5% purity) was used as catalyst at 0.2 wt %.

The pressure was kept constant at 25 MPa. The working temperatures were of 400, 450, 500 and 550 °C. The flow rate was set between 0.9 and 3. The experimental error, estimated by repeated experiments in the same experimental conditions, is in the range of ± 10 % of the given data.

Analyses

After cooling, gas and liquid phases were analysed. No solid phase was observed in these operating conditions. After 30 and 60 minutes of stable state of the reactor, gas and liquid samples were taken for analysis. The obtained results were an average of the analysed samples. To compare the efficiency of the conversion, the only comparison of gas volume or liquid mass is not pertinent, particularly in case of different flow rates. In the outflow, a normalised gas yield (N gas yield), calculated by the volume of gas and the weight of the liquid obtained after the 60 min steady state of the reactor, was defined:

1   kg L in weight liquid volume gas yield gas N (4)
The mass error is 0.1 g and the gas volume error is 10%.

Liquid phase

The efficiency of the organic compounds' conversion was estimated by measuring the Total Organic Carbon (DIMATOC ® 2000) with an error of 2%. To represent this efficiency, the TOC removal will be calculated as follows:

  100    initial final initial TOC TOC TOC removal TOC ( 5
)
Residual phenols (phenol and substituted phenols) representing synthesised and/or undecomposed phenol as well as glucose and fructose were also quantified in the liquid phase by using a colorimetric test from Hach-Lange (LCK 346 Phenols), a specific enzymatic test and a UV-method supplied by Roche. The error in the analysis of glucose/fructose concentration was 0.5% and 5% for phenol.

In addition, UV-Vis (DR 5000, Lange-Hach) measurements are conducted with selected samples. The results show the presence of phenols and other aromatic compounds with a single aromatic system. No adsorption at longer wavelength was detected, therefore no hint for polyaromatic systems is found.

After the phenol-glucose mixture reaction, the GC-MS 1 analysis of the selected samples not only shows phenol presence but also that of alkyl-phenols (mainly methylphenols) in the product mixture.

Gas phase

An HP-6890A Series Gas Chromatograph was used to analyse the gas, and the injected volume was 100 µL. Two columns (80/100 Hayesep and 60/80 Molesieve 5A) and two kinds of detectors were placed in series so as to determine the composition and quantification of the gas. The main gases to be quantified were H 2 , CO 2 , CH 4 , CO and some light hydrocarbons (C 2 H 6 , C 3 H 8 …).

Results and discussion

Sinağ et al. demonstrated the effect of alkaline salt on glucose gasification in this reactor [START_REF] Sinağ | Formation and degradation pathways of intermediate products formed during the hydropyrolysis of glucose as a model substance for wet biomass in a tubular reactor[END_REF], they concluded that KHCO 3 improved gas generation and decreased the amount of furfural in the liquid phase. The influence of the glucose concentration was studied in the range of 0.25-2 wt % at 25

Glucose solutions

MPa, 500°C and with a flow rate of 1.385 kg h -1 (residence time equals to 0.1624 min).

Glucose was totally removed from the solution (<0.003 mg L -1 , not shown), meaning that molecules completely react with supercritical water in the considered operating conditions.

Moreover, fructose was also quantified and the values were less than 0.01 mg L -1 . These experimental results are in accordance with the literature [START_REF] Yu | Hydrogen production by stream reforming glucose in supercritical water[END_REF][START_REF] Goodwin | Conversion of glucose to hydrogen-rich gas by supercritical water in a microchannel reactor[END_REF][START_REF] Kabyemela | Kinetics of glucose epimerization and decomposition in subcritical and supercritical water[END_REF]. During the continuous process, no solid particle was observed.

Fig. 2 shows that, in these operating conditions, the normalised gas yield (Eq. 4) linearly increases along with the glucose concentration in the solution. For a glucose concentration in the range of 0.25 -2 wt %, this linear behaviour means that catalyst concentration is not kinetically limitative. This linear behaviour could also indicate that the conversion's efficiency, i.e. mole distribution between the gas and the liquid phases, is nearly the same in all experiments. The evolution of TOC and residual phenol of the liquid phase as function of glucose concentration is also linear (not shown). An increasing linear profile for the TOC indicates that the mineralisation of organic compounds is independent from the concentration in the investigated range. As seen in the literature, phenol is an intermediate product of

glucose degradation [START_REF] Kruse | Hot compressed water as reaction medium and reactant. 2. Degradation reactions[END_REF][START_REF] Sinağ | Key compounds of the hydropyrolysis of glucose in supercritical water in the presence of K 2 CO 3[END_REF][START_REF] Goodwin | Conversion of glucose to hydrogen-rich gas by supercritical water in a microchannel reactor[END_REF][START_REF] Watanabe | Oil formation from glucose with formic acid and cobalt catalyst in hot-compressed water[END_REF]. The concentration of residual phenol in the liquid outflow linearly increases with glucose concentration, meaning that the ratio between the phenol concentration and the initial glucose concentration is the same in all experiments.

Phenol concentration is a balance between phenol formation from glucose and phenol conversion during SCWG. This linear profile could indicate that rates and reaction mechanisms are similar in all experiments. As regards values, TOC removal was close to 90%

(between 88 and 90%) in each experiment, indicating that mineralisation was efficient in these operating conditions. Fig. 3 represents the volume percentage of the main gas (H 2 , CO 2 , CH 4 and CO) in the total volume as function of glucose concentration. The composition of the gas is of about 50 vol.% H 2 , 40 vol.% CO 2 , 10 vol.% CH 4 and 1 vol.% CO. These values are in the same order of magnitude as those obtained in the literature under various operating conditions [START_REF] Yu | Hydrogen production by stream reforming glucose in supercritical water[END_REF][START_REF] Lee | Gasification of glucose in supercritical water[END_REF][START_REF] Goodwin | Conversion of glucose to hydrogen-rich gas by supercritical water in a microchannel reactor[END_REF].

Hydrogen is the main compound in the gas phase. Of course, the presence of the catalyst decreases the amount of CO, and therefore the WGS reaction is promoted. Fig. 3 shows that, when glucose concentration increases, the proportion of H 2 slightly decreases and the proportion of CO 2 slightly increases. This phenomenon could be due to a difference between the kinetics of the reactions. Taking the general and total reaction scheme following Eq.6 into account:

2 2 2 6 12 6 12 6 6 H CO O H O H C    (6)
the part of hydrogen should be doubled compared to carbon dioxide volume. The different ratios of the gases indicate that the reaction is not complete, even if glucose has totally disappeared, and even if the kinetics of degradation or other consecutive reactions seem to be different for the intermediate products. In other words: hydrogen reacts with the intermediate products by hydrogenation and this reaction has a higher reaction order than glucose degradation.

Phenol solutions

According to literature, the reaction rate of phenol gasification in a batch reactor is low without any Ni catalyst, and a temperature of 700°C leads to a complete reaction of phenol in a short time compared to that of 600°C [START_REF] Dileo | Gasification of guaiacol and phenol in supercritical water[END_REF].

Influence of flow rate

As regards phenol solutions, the influence of flow rate (or residence time) was studied in the range of 0.9-3.8 kg h -1 (residence time equals to 0.23-0.06 min) for a phenol concentration of 1 wt % (corresponding to 10 g L -1 ) at 25 MPa and 500°C. Fig. 4 shows that the conversion of phenol is not efficient (less than 50%). Contrary to glucose solutions, the reactivity of phenol in supercritical water is low in these operating conditions. The experiments performed by DiLeo et al. on phenol solutions with or without Ni catalyst [START_REF] Dileo | Supercritical water gasification of phenol and glycine as models for plant and protein biomass[END_REF][START_REF] Dileo | Gasification of guaiacol and phenol in supercritical water[END_REF] showed that the relation between water density and phenol concentration has an influence on the conversion's efficiency. It was concluded that an optimal water density (0.079 g mL -1 ) might exist for homogeneous SCWG of phenol at 600°C with an Ni catalyst [START_REF] Dileo | Supercritical water gasification of phenol and glycine as models for plant and protein biomass[END_REF]. It was also demonstrated that phenol removal was only of 25% without any Ni catalyst while the value reached 93%

with an Ni catalyst in the same period of 10 min [START_REF] Dileo | Gasification of guaiacol and phenol in supercritical water[END_REF]. For all parameters (TOC, phenol and relative gas yield) as function of residence time, the profiles of the curves show an abrupt increase or decrease between the lower residence time and the intermediate one, and the values of the next points are quite similar. According to literature, this low conversion is a consequence of the short reaction times in the experiments hereby presented. The composition of the gas phase is quite stable at the different flow rates, especially as regards H 2 (45%) and CO 2 (32%). With an Ni catalyst, the ratio between H 2 and CO 2 is close to two in optimised conditions [START_REF] Dileo | Supercritical water gasification of phenol and glycine as models for plant and protein biomass[END_REF].

Influence of temperature

To study the influence of temperature in the range of 400-500°C at constant pressure, it has to be considered that a change in the temperature not only influences the reaction rate with a continuous activation energy but also changes the water density [START_REF] Dileo | Supercritical water gasification of phenol and glycine as models for plant and protein biomass[END_REF]. As regards pure water, density equals to 0.166 g mL -1 at 400°C, 0.109 g mL -1 at 450°C and 0.089 g mL -1 at 500°C. Fig. 5 shows the residual phenol as function of the reactor's temperature, for the lowest residence time. The quantity of phenols decreases when the temperature increases. At 500°C, phenol removal reaches 52% compared to 12% at 450°C. This means that higher temperatures promote the reactivity of phenols in supercritical water. Temperature also favours TOC removal, increasing from 1% (400°C) to 9% (450°C) and up to 50% at 500°C (values not shown on Fig. 5).

Glucose and phenol solutions

The first objective is to check the influence of phenol on glucose degradation. The gas volume from the solutions containing only one compound (1 wt %) was compared to that of mixture solutions (1 wt % of each compound). With this kind of representation, the degradation process of both components will be independent if the values for the mixture are the sum of the other two values. The gas volume of the three solutions and the theoretical sum is shown in Fig. 6 with a flow rate of 1.385 kg h -1 , at T=500°C and observations are the same with the other flow rates. The theoretical sum of gas volumes is higher than the experimental gas volume obtained from the mixture solution: the simultaneous presence of the two compounds in the mixture decreases the production of gas. For example, the gas volume obtained from the mixture is 50% less voluminous than the gas yield observed with glucose. As a conclusion, phenol has an influence on glucose conversion and more precisely on the quantity of gas produced in these operating conditions. Phenol probably influences the intermediate products of glucose degradation because glucose is also totally removed from the mixture outflow.

Gas quantity is not the only parameter affected by the presence of phenol: TOC and residual phenol values obtained from the mixtures are higher than the sum of the values obtained from the solutions of each compound. Fig. 7 shows the values of TOC as function of the solution's composition at the same flow rate. For glucose solutions of 1 wt % the TOC is very low. On the contrary, for 1 wt % phenol solutions, the TOC value in the liquid phase is 10 times higher. If the TOC of the mixture was simply the sum of the other two values, the TOC would only be a little higher than the value of the phenol solution. As seen on Fig. 7 TOC value of the mixtures is higher, meaning that phenol interacts with the intermediate products of glucose degradation, therefore modifying the glucose degradation process. Similar results concerning TOC and residual phenols are obtained with the other two flow rates: at 0.982 kg h -1 , the theoretical values of TOC and residual phenol would respectively be 5.45 g L -1 and 6.14 g L -1 . The experimentally found values are higher: 6.96 g L -1 of TOC and 8.3 g L -1 of residual phenol. For the flow rate of 3.8 kg h -1 , the experimental values are 1.5 higher than the theoretical sum: 6.1 g L -1 for the experimental TOC and 7.3 g L -1 for the experimental phenol.

Other experiments were performed with various compositions of mixture solution considering a total weight of organic compounds equal to 2%. As it was expected and in accordance with Yoshida et al. [START_REF] Yoshida | Gasification of cellulose, xylan and lignin mixtures in supercritical water[END_REF] who used a cellulose-lignin mixture in their operating conditions, the comparison of normalised yield gas as function of phenol fraction showed a non-linear profile. The influence of temperature was studied. Fig. 8 shows the influence of temperature on the TOC value for three different compositions of 2 wt % feed solutions. The initial TOC of these solutions were 7.3 mg L -1 for glucose, 7.7 mg L -1 for 0.1 wt % phenol and 8.3 mg L -1 for 0.25 wt % phenol. As for previous experiments, glucose concentration in outflow liquid phase equals to zero.

As shown in Fig. 7 for a glucose solution (▲) the TOC is removed from the solution with a high efficiency regardless of the temperature. Glucose degradation depends on temperature:

TOC removal increases from 82 to 91% when increasing the temperature from 450 to 500°C.

For the mixture containing 0.1 wt % of phenol (■), TOC removal linearly increases from 75

to 94% with the temperature. In fact, TOC removal is only of 22% at 450°C but reaches 77% at 550°C. On the contrary, the values of TOC removal dramatically decrease with 0.25 wt % of phenol in the mixture (♦), unlike results with glucose. Fig. 8 shows that the temperature has an influence on the mineralisation's efficiency regardless of the composition, and that a small initial quantity of phenol changes the gasification behaviour of the solution. The residual phenol values also showed a profile close to the profiles observed with TOC values. The analytical method for residual phenol did not separate phenol molecules from their derivates (cresol, oligomers…). However, an estimation of the part of TOC represented by phenol (6 atoms of carbon) was calculated. The part of phenol in the experimental TOC value for the solution at 0.25 wt % of phenol (♦) is around 20% whereas it is more than 50% in the other cases.

When adding a small amount of phenol to glucose solution, residual TOC and residual phenol are significantly increased, confirming the influence of phenol on glucose SCWG conversion.

To understand the chemistry of phenol gasification, the reaction path from phenol to compounds with a higher molecular weight has to be considered; DiLeo et al. [START_REF] Dileo | Gasification of guaiacol and phenol in supercritical water[END_REF] report the formation of e.g. dibenzofuran and biphenyl from phenol. The compounds with a large conjugated electron system obtained in the SCWG process would form relatively stable free radicals and would act as a free radical scavenger. This would explain the reduced gasification of glucose in the presence of phenol. As above mentioned, no such compound could be found here. Still, in further experiments it has to be verified whether these compounds are to be found, formed or not. For example, it is possible that such compounds may stick to the reactor's wall.

On the other hand, studies on phenol oxidation mentioned in the introduction show that the free radical chemistry of phenols is complex. Also during oxidation, higher molecular weight compounds are found. The addition of an active free radical to phenol, as observed in SCWG and SCWO of phenol, should lead to a less reactive free radical, slowing down the free radical chain mechanism. Yet, this effect seems to be too low to explain the results presented here.

Further investigations are necessary to clarify this point.

Conclusion

SCWG of glucose and/or phenol in diluted solutions was studied in a continuous flow reactor (25 MPa and 400-550°C). The achieved comparison between solutions of only one compound and mixtures showed that the presence of phenol, even in a small amount in a glucose solution, decreases the conversion's efficiency. In our experiments, phenol is considered as an intermediate of lignin hydrolysis/gasification. The role played by phenol in glucose degradation can be correlated with the literature data related to the lignin influence on the SCWG of carbohydrates.

Phenol reduces hydrogen yield and particularly the total volume of gas from the conversion of glucose. Consequently, in the presence of phenol, the TOC and residual phenols of the liquid phase is higher. TOC removal dramatically decreases in presence of phenol in the mixture.

One possible explanation is that, similarly to studies on the influence of proteins [START_REF] Yanik | Biomass gasification in supercritical water: Part 1. Effect of the nature of biomass[END_REF][START_REF] Kruse | Influence of proteins on the hydrothermal gasification and liquefaction of biomass. 2. Model compounds[END_REF],

phenol works as a free radical scavenger, which has to be proven in further experiments.

Captions

Figure 1: Scheme of the apparatus with nitrogen supply (A), feed tank (B), high pressure pump (C), preheater (D), reactor (E), cooler (D), pressure control unit (E), phase separation (F), wet gas meter (G) and gas sampling (H). ) 0,25 0,1 0 250 bars, 0,2% KHCO3, % phenol variable mais 2% total. 2,19 kg/h (6 mm).

  . 1) made of Inconel 625. It is an 18m-long helix with an internal diameter of 1.6 mm and an external diameter of 6.35 mm. The volume of the reaction zone is 36.19 mL. The electric heating was controlled by a thermocouple on the external surface of the reactor. The GraphWorX32 module of the Genesis32 Software program supplied by ICONICS was used to control and monitor the temperatures. The solution was injected into the reactor with a membrane pump, supplied by LEWA (Type EL1). After cooling, the product mixture was expanded to ambient pressure by a back-pressure regulator.

Figure 2 :

 2 Figure 2: Influence of glucose concentration on the normalized gas yield during the SCWG at 500°C, 25 MPa and 1.385 kg h -1 , in presence of 0.2 wt % of KHCO 3 .

Figure 3 :

 3 Figure 3: Influence of glucose concentration on gas composition during SCWG at 500°C, 25 MPa and 1.385 kg h -1 , in presence of 0.2 wt % of KHCO 3 .

Figure 4 :

 4 Figure 4: Influence of residence time on the residual phenol during the SCWG at 500°C, 25

Figure 5 :

 5 Figure 5: Influence of temperature on the residual phenol for a residence time of 0.06 min

Figure 6 :

 6 Figure 6: Comparison of theoretical sum of gas volume obtained during the SCWG of

Figure 7 :

 7 Figure 7: Comparison of theoretical sum of TOC obtained during the SCWG of solutions

Figure 8 :

 8 Figure 8: Influence of temperature on TOC values for 3 compositions of mixture solutions during the SCWG at 25 MPa, 2.19 kg h -1 and in presence of 0.2 wt % of KHCO 3 . Solutions

Fig. 1 :Fig. 2 :

 12 Fig. 1:

  8 kg h -1 . Considering the reactor volume of 36.19 mL and the volumic flow rate at the temperature and pressure in the reactor, the corresponding residence times were of 0.25 down to 0.06 min. The concentration of glucose (C 6 H 12 O 6 , H 2 O Monohydrate, Merck) or phenol (C 6 H

[START_REF] Marshall | Ion product of water substance, 0-1000°C, 1-10,000 bar new international formulation and its background[END_REF]

O, Merck, 99% purity) varied from 0.25 to 2 wt %.