Jean-Claude Michel

Pierre Suquet
email: suquet@lma.cnrs-mrs.fr

Model-reduction in micromechanics of polycrystalline materials

The present study is devoted to the generalization of the Nonuniform Transformation Field Analysis (NTFA), a model-reduction approach introduced by the authors. First, the local fields of internal variables are decomposed on a reduced basis of modes. Second, the effective (average) dissipation potential of the phases is replaced by accurate approximations. The reduced evolution equations of the models, in other words the homogenized constitutive relations, can be entirely expressed explicitly in terms of quantities which are pre-computed "off-line". The example of creep of polycrystalline ice is used to assess the accuracy of the models. Their predictions, both the overall response and the local response, are shown to be in good agreement with full-field simulations with a significant speed-up.

1 Reduced-order models A common engineering practice in the analysis of composite (or polycrystalline) structures is to use effective or homogenized material properties instead of taking into account all details of the individual phase properties. Unfortunately when the individual constituents are nonlinear, the exact description of the effective constitutive relations requires the determination of the local fields (at the microscopic scale). For structural computations, the consequence of this theoretical result is that the two levels of computation, the level of the structure and the level of the unit-cell, remain intimately coupled. The nested resolution of these coupled problems (known as FE 2 analysis) is so far limited by their formidable size.

It is therefore quite natural to resort to model-reduction techniques achieving a compromise between analytical approaches, which are costless but often very limited by nonlinearity, and full-field simulations which resolve all complex details of the exact solutions, but come at a very high cost. This is the aim of reduced-order modelling (ROM) in general ( [1,2]). However, ROM is often understood 1. Experiments and numerical simulations evidence the patterning of the local fields induced by the presence of a microstructure. This patterning reveals the existence of a few microscopic mechanisms by which the composite deforms.

2. When the mechanical response of the individual constituents is governed by two potentials, the free-energy potential corresponding to reversible effects and the dissipation potential corresponding to dissipative effects, the overall response of the composite is also governed by two effective potentials which are the average of the local potentials. However deriving an exact expression for these two potential is a difficult task, which cannot be achieved in a closed form when the constituents are nonlinear.

3.

Recognizing that the problem of finding an averaged potential is similar to that encountered in homogenization of nonlinear composites with a single potential ( [10]), it is quite natural to transpose some of the techniques of nonlinear homogenization to the present context of ROM.

Individual constituents

The composite (or polycrystalline) materials considered in this study are comprised of individual constituents undergoing partly reversible and partly irreversible transformations modelled by a finite number of internal variables α. It is further assumed that the stress derives from a free-energy function w(ε, α) and that the evolution of the variables α is governed by the driving forces associated with α.

It is often the case (but not always) that this last relation can be expressed with the help of a dissipation potential ϕ and the constitutive relations take the compact form,

σ = ∂w ∂ε (ε, α), ∂w ∂α (ε, α) + ∂ϕ ∂ α ( α) = 0. (1)

Effective potentials

A representative volume element (r.v.e.) V of the material is comprised of phases occupying domains V (r) . The spatial averagings over V and V (r) are respectively denoted by . and . (r) . Each individual phase is governed by the above differential equation with potentials w (r) and ϕ (r) in V (r) . The r.v.e. V is subjected to a path of macroscopic strain ε(t) and periodicity conditions on ∂V . The local problem to be solved to determine the local fields σ(x, t), ε(x, t) and α(x, t) consists of a generalized thermoelastic problem (2), in which the field of internal variables α(x) is fixed,

σ(x, t) = ∂w ∂ε (x, ε(x, t), α(x, t)), div σ(x, t) = 0, ε(x, t) = ε(t) + 1 2 (∇u * (x, t) + ∇u * T (x, t)), u * periodic on ∂V, σ.n anti-periodic on ∂V,        (2)
coupled with the differential equation ( 12 ) at every point x in the volume element. The homogenized (or effective) response of the composite along the path {ε(t), t ∈ [0, T ]} is the history of average stress {σ(t), t ∈ [0, T ]} where σ(t) = σ(x, t) . It can be shown that the overall response of the composite is governed by two potentials

σ = ∂ w ∂ε (ε, α), ∂ w ∂α (ε, α) + ∂ ϕ ∂ α ( α) = 0, w(ε, α) = Inf ε =ε w(ε, α) , ϕ( α) = ϕ( α) , (3)
where unfortunately α is a field (infinitely many internal variables).

Reduced variables and reduced potentials

In order to reduce the number of macroscopic internal variables, and inspired by numerical simulations and experiments which clearly show the patterning of the plastic strain field in the volume element, the Nonuniform Transformation Field Analysis (NTFA) proposed by Michel and Suquet [3] is based on a decomposition of the fields of internal variables on a finite set of pre-determined shape functions (NTFA decomposition)

α(x, t) = M k=1 ξ (k) (t) µ (k) (x), (4)
where the fields µ (k) (x) are the modes and the ξ (k) are the reduced variables. The modes have the same tensorial character as the internal variables α. To avoid a possible indeterminacy in the definition of the reduced variables ξ (k) , it is further assumed that the modes µ (k) are linearly independent fields. How the modes are generated is recalled in section 1.4. Using the decomposition (4) into the variational definition (3) of the two effective potentials, an approximation of the evolution equations for the reduced variables ξ is given by

σ = ∂ w ∂ε (ε, ξ), ∂ w ∂ξ (ε, ξ) + ∂ ϕ ∂ ξ ξ = 0, w(ε, ξ) = Inf ε =ε w(ε, α(ξ)) , ϕ( ξ) = ϕ( α( ξ)) , (5)
where α(ξ) is given by the NTFA decomposition (4). In many cases of interest, the effective freeenergy w can be expressed in terms of pre-computed quantities and the difficulty lies in the evaluation of the effective dissipation potential ϕ in terms of ξ without computing "on-line" the local fields α.

Snapshot POD

The modes µ (k) (x) are generated by the snapshot Proper Orthogonal Decomposition (POD) method along some training loading paths, whose choice depends heavily on the application that the user has in mind ( [6]). Along a given training path, T snapshots of the appropriate internal variable fields computed by the full-field method are stored at different time steps. Then, following the classical POD procedure, the correlation matrix g of these snapshots is formed and its eigenvalues λ (k) and eigenvectors v (k) are computed. The actual modes are expressed as a combination of the eigenvectors as

g (kℓ) = θ (k) : θ (ℓ) , µ (k) (x) = T ℓ=1 v (k) ℓ θ (ℓ) (x). (6)
A main interest in the snapshot POD method lies in the fact that the quantity of relevant information (its correlation with the set of snapshots) contained in an eigenvector v (k) is expressed by the magnitude of the corresponding eigenvalue λ (k) . This property is used to truncate the set of modes, retaining the most relevant ones ( [6]).

Approximate effective potentials and nonlinear homogenization

The "reduced kinetic" equations of the model consist of (5). In order to arrive at a closed form expression of these equations one has to compute explicitly the effective potentials w and ϕ. This is the most difficult part of the procedure which requires additional physical and mathematical simplifications. The first simplification is that the free-energy function w of the individual constituents is assumed to be quadratic with respect to ε and α (when this is not the case a work-around has been proposed in [7] and [8]). Then the thermoelastic problem (2) becomes linear, its solution can be expressed by superposition as

σ(ε, ξ, x) = L(x) : A(x) : ε + M k=1 ρ (k) (x) ξ (k) , ( 7
)
where A is the elastic strain localization tensor and ρ (k) are elastic fields generated by the modes µ (k) . Using the decomposition (7), the effective free-energy w can be expressed explicitly in terms of ε and ξ (see [7,8]).

Expressing the effective dissipation potential ϕ is an even more difficult task. Fritzen and Leuschner [4] have proposed an hybrid formulation making use of the dual potential ψ = ϕ * . Their definition of ψ ψ(ε, ξ) = ψ(σ(ε, ξ) , σ given by ( 7), (8) is not rigorously equivalent to the exact definition by duality (as shown in [7]), but is a fairly accurate approximation of it. But still the cost of evaluating ψ(σ) is very high, since it involves the evaluation of the local field σ(x) from ( 7), followed by the evaluation of ψ (σ(x)) which is a nonlinear local function of σ, which is then averaged. An additional (and essential) step is achieved by approximating the average of the force potential ψ. For the sake of simplicity, attention is restricted here to polycrystals where the phases V (r) are the grains with characteristic function χ (r) (x), volume fraction c (r) and with slip systems m (r) s . The potential ψ(σ) can be expressed as

ψ(σ, x) = N r=1 χ (r) (x) S s=1 ψ (r) s (τ (r) s (x)), τ (r) s (x) = σ(x) : m (r) s (x). (9)
Two approximations of ψ(σ) have been investigated so far. The first approximation consists in replacing ψ by its tangent second-order expansion (TSO) in each individual phase ( [7,8]):

ψ(ε, ξ) ≃ ψ T SO (ε, ξ) = N r=1 c (r) S s=1 ψ (r) s (τ (r) s ) + 1 2 ∂ 2 ψ (r) s ∂τ 2 (τ (r) s ) C (r) (τ (r) s ) , τ (r) s = τ (r) s (r) , (10)
where

C (r) (τ (r) s ) = (τ (r) s -τ (r) s ) 2 (r)
denotes the quadratic fluctuations of τ (r) s over grain (r). The second approximation was proposed in [9], based on numerical integration and following and extending on Ponte Castañeda "fully optimized" method [11]. In its simplest version it consists in replacing ψ by

ψ F O (ε, ξ) = N r=1 c (r) S s=1 1 2 ψ (r) s (τ (r) s ) + ψ (r) s (τ (r) s ) , τ (r) s = τ (r) s (r) -C (r) (τ (r) s ), τ (r) s = τ (r) s (r)
+ C (r) (τ (r) s ). (11)
2 Sample example: creep of polycrystalline ice

Ice, as a single crystal, is an HCP material with 12 slip systems (3 basal, 3 prismatic and 6 pyramidal).

The single crystal plasticity model used in the present example is that proposed in [12]. This model accounts for both kinematic and isotropic hardening. A creep test on polycrystalline ice is simulated.

The representative polycrystalline aggregate used in the full-field simulations is shown in Fig. 2a. Three full-field simulations were performed with different material data. The first data set is that identified in [12]. The other two data sets correspond to different values of the latent hardening parameters and have been chosen to highlight the sensitivity of the response to the hardening parameters ( [8]).

The predictions of the two NTFA models for the overall response of the polycrystalline aggregate with these three different data sets are compared in Figs 1a and 1b with full-field simulations. The agreement between the NTFA predictions and the full-field simulations is seen to be slightly better in Fig. 1a than in Fig. 1b. This is due to the fact that the modes used in the NTFA simulations of Fig. 1b are those determined with the first set of material parameters. However, the plots in Fig. 1b show that the modes determined with the first set of material parameters can be used with other parameters and still capture correctly the influence of these parameters. The activity of the basal slip systems, the most active ones, as found from the full-field simulation with the first set of material paramaters and as obtained by post-processing the results of the NTFA models are compared in Figs 2b-d at the end of the creep test. Regarding the gain in computational time, the acceleration provided by the NTFA models is of the order of 10 2 for both models. The NTFA-FO model is however more appropriate for porous materials (Michel and Suquet [9]).

Figure 1 :Figure 2 :

 12 Figure 1: Creep of polycrystalline ice. Comparison between the full-field simulations (black solid line) and the predictions of the NTFA-TSO model (dotted-dashed blue line) and of the NTFA-FO model (dashed red line) for the response of the polycrystalline aggregate. a Material parameters identified in [12]. b Influence of the latent hardening parameters.


Weiss-Hortala

• Y Soudais

Keywords: Carbon fibers, Recycling, Thermoplastic resin, Supercritical water

Impact of Solvolysis Process on Both Depolymerization Kinetics of Nylon 6 and Recycling Carbon Fibers from Waste Composite

C. Chaabani 1 • E

Introduction

Regardless their good resistance and high rigidity, composite materials present the advantage of being lightweight, compared to steel and aluminium. Thus, they are employed in many industrial fields such as automotive, aeronautics and sports. The worldwide demand for composites reached approximately 59,000 tons in 2011 and expected to be 208,000 tons by 2020 [START_REF] Bernhard | The global CRP market: carbon composites[END_REF]. Especially, carbon fibers reinforced polymers (CFRP) materials have the largest share of the market (40%), followed by glass fibers (31%) [START_REF] Pickering | Recycling technologies for thermoset composite materials-current status[END_REF]. With the extensive use area of CFRP, the global demand of carbon fibers is expected to rise from 46,000 tons in 2011 to 140,000 tons by 2020 [START_REF] Pickering | Recycling technologies for thermoset composite materials-current status[END_REF]. Thus, the amount of carbon fibers based composite residues is going to increase. Since thermoplastics and thermosetting polymers are nonbiodegradable, nowadays, the main treatments are landfill and incineration. However, these two options contradict sustainable development. Thus, the European directives (1999/31/EC) and (2000/76/EC) have been adopted in order to minimize as much as possible the amount of waste destined to landfill and incineration. Therefore recycling options should be developed.

Abstract Carbon fibers reinforced polymers (CFRP) are extensively used. Regarding the high cost of production, the carbon fibers reuse is an important challenge. Chemical recycling using sub and supercritical fluids shows good prospects for recycling carbon fibers. Thus, solvolysis of CFRP waste was investigated to determine suitable operating conditions for preserving mechanical properties of carbon fibers, and to evaluate the kinetics parameters as well as the main reaction scheme. The experiments of decomposition of nylon 6 were carried out at temperatures in subcritical (280-350 °C) and in supercritical (400-600 °C) regions under the estimated pressure of 25 MPa for reaction times of 1-120 min. The microstructure of the recovered carbon fibers was observed using environmental scanning electron microscopy (ESEM) and Raman spectroscopy. The amount of carbon, measured by Total organic carbon (TOC), in the liquid phase highlights that the depolymerization was enhanced compared to a complete degradation into CO 2 . In subcritical region, the resin removal reached 98.95 wt% at 280 °C in 30 min while, in supercritical region, it has already reached 97.18 wt% at 400 °C in only 15 min. The monomer of the resin and other molecules were identified by gas chromatography coupled to mass spectroscopy (GC-MS) in liquid phases recovered from both regions. Assuming a pseudo-first order reaction, the activation energy was evaluated to be 77.79 and 78.51 kJ/ mol in the subcritical and supercritical regions respectively. These kinetics parameters highlight that the reaction mechanism pathways are similar in both regions. ESEM The criterion of carbon fibers recycling is related to the preservation of the mechanical properties of the recovered fibers in order to be reused in a composite [START_REF] Pickering | Recycling technologies for thermoset composite materials-current status[END_REF]. Currently, the recovery of carbon fibers from waste composites could be done through mechanical, pyrolysis, steam thermolysis or solvolysis processes. Mechanical process was first applied for composite residues reinforced thermosetting matrix glass fibers [START_REF] Oliveux | Chemical recycling of glass fibre reinforced composites using subcritical water[END_REF][START_REF] Garcia | Mechanical recycling of GFRP waste as short-fiber reinforcements in microconcrete[END_REF][START_REF] Ana | An integrated recycling approach for GFRP pultrusion wastes: recycling and reuse assessment into new composite materials using Fuzzy Boolean Nets[END_REF]. This method was then used to recycle carbon fiber from composites with thermoplastic or thermosetting resins [START_REF] Kouparitsas | Recycling of the fibrous fraction of reinforced thermoset composites[END_REF][START_REF] Ogi | Mechanical properties of ABS resin reinforced with recycled CFRP[END_REF]. This process allows the preservation of pieces of 4 to 15 mm of fibers. However the pieces of fibers are still impregnated with the polymer that limits the possibilities offered for recycling. Typical applications for these recyclates is the use in construction industry such us artificial woods. In France, this process is applied by Mixt Composites Recyclables. In the pyrolysis process, CFRP materials are pyrolysed in a reactor at 450-800 °C in the absence of oxygen. The organic material from the polymer matrix is degraded and volatilised into lighter products: hydrogen, carbon dioxide, methane and other hydrocarbons. The recovered solid phase consists of fibers, with or without resin depending on operating conditions. However, pyrolysis process could result in char deposit, which should be oxidized. This post treatment could influence negatively the mechanical properties of recovered carbon fibers. The pyrolysis technique is already used by different companies worldwide such as ELG Carbon Fibre Ltd. In United Kingdom, MIT-RCF in the United States, CFK Valley Stade Recycling GmbH & Co. KG in Germany and Karborek Recycling Carbon fibers in Italy. Recovered products by pyrolysis and mechanical process present low-value applications since mechanical performance are limited: mechanical treatment does not separate resin from fibers and pyrolysis should cope with charring issues (loss of tensile strength of recovered carbon fibers about 30% at 600 °C [START_REF] Ushikosha | Recycling of CFRP by pyrolysis method[END_REF]). Thus steam thermolysis and solvolysis were recently deeper investigated. On the one hand, steam thermolysis has shown two main advantages compared to pyrolysis process [START_REF] Ye | Parameter optimization of the steam thermolysis: a process to recover carbon fibers from polymer-matrix composites[END_REF]: the resin elimination is improved using the same temperature conditions and the steam partially inhibits char deposit at the carbon fibers surface. On the other hand, solvolysis in supercritical fluids has shown good results to recover carbon fibers that can be reused [10]. In the solvolysis process, CFRP materials are processed in a reactor containing a suitable solvent at high temperature and/or pressure. Recently, Morin et al. [10] reviewed these methods for the recycling of carbon fibers reinforced polymers and concluded that chemical recycling is a favourable process to recover carbon fibers with good surface and mechanical properties. For this purpose, solvolysis of carbon fibers reinforced composites has been extensively investigated using batch or semi-continuous reactors in near-or supercritical conditions of water [START_REF] Pinero-Hernanz | Chemical recycling of carbon fibre reinforced composites in nearcritical and supercritical water[END_REF][START_REF] Bai | Chemical recycling of carbon fibers reinforced epoxy resin composites in oxygen in supercritical water[END_REF][START_REF] Onwudili Jude | Catalytic hydrothermal degradation of carbon reinforced plastic wastes for carbon fibre and chemical feedstock recovery[END_REF],

in supercritical alcohols [START_REF] Pinero-Hernanz | Chemical recycling of carbon fibre composites using alcohols under subcritical and supercritical conditions[END_REF][START_REF] Jiang | Characterisation of carbon fibres recycled from carbon fibre/epoxy resin composites using supercritical n-propanol[END_REF] or in mixtures [START_REF] Jiang | Characterisation of carbon fibres recycled from carbon fibre/epoxy resin composites using supercritical n-propanol[END_REF]. Since the recycling depends strongly on mechanical properties, it is important to evaluate the tensile strength of recovered carbon fibers. Okajima et al. [START_REF] Okajima | Chemical recycling of carbon fibers reinforced plastic using supercritical methanol[END_REF] studied the recycling of CFRP in supercritical methanol, and clean carbon fibers were recovered at 380 °C, 25 MPa. The tensile strength of the fibers was 9% less than virgin ones. Liu et al. [START_REF] Liu | Chemical recycling of carbon fibers reinforced epoxy resin composites in subcritical water : synergistic effect of phenol and KOH on the decomposition efficiency[END_REF] confirmed that solvolysis is efficient to recover carbon fibers with a tensile strength between 2 and 10%. In addition, Pinero-Hernanz et al. [START_REF] Pinero-Hernanz | Chemical recycling of carbon fibre composites using alcohols under subcritical and supercritical conditions[END_REF] has investigated chemical recycling of CFRP using sub and supercritical alcohols. The process was able to depolymerize all the epoxy resin and produce fibers that retain 85-99% of the tensile strength of virgin carbon fibers. It has been shown that the use of alkali catalysts could decrease the tensile strength. Guozhan and Pickering [START_REF] Jiang | Stucture-property relationship of recycled carbon fibers revealed by pyrolysis recycling process[END_REF] have shown that the effect of recycling process on recovered carbon fibers properties can be attributed to two factors: oxidative effect and thermal effect. In solvolysis process, only the thermal effect can be considered, while, in pyrolysis process followed by the char oxidation, both are involved. According to the properties of recycled carbon fibers from pyrolysis and solvolysis recycling, it can be concluded that the thermal effect does not induce impacts on mechanical properties. However, it results in a reduction in the ratio Oxygen/Carbon at fibers surface. Thus, the reduction in tensile strength may be attributed to oxidative effect.

Although the added value material is clearly the recovered carbon fiber, the authors are recently paying more attention to the resin removal, which may also provide interesting compounds (gaseous phase or useful organics). Ghaemy et al. [START_REF] Ghaemy | Depolymerisation of poly(ethylene terephthalate) fibre wastes using ethylene glycol[END_REF] studied the decomposition of polyethylene terephthalate resin in ethylene glycol using metal acetate as a catalyst. They assumed a depolymerisation reaction since the glycolysis products mainly consisted of bis(hydroxyethyl) terephthalate (more than 75%) in the liquid phase. Onwudili et al. [START_REF] Onwudili Jude | Catalytic hydrothermal degradation of carbon reinforced plastic wastes for carbon fibre and chemical feedstock recovery[END_REF] also reported the dissolving of up to 81 wt% of the epoxy resin in water in the presence of NaOH or KOH at short reaction time at 400 °C. Platform molecules such as phenol and aniline which represent a potential monomer recovery were significantly recovered. Moreover, the decomposition of polybenzoxazine resin in ethylene glycol/water revealed the presence of aniline, methylaniline, quinolone and phenyloxazine at 400 °C [START_REF] Yildirir | Recovery of carbon fibres and production of high quality fuel gas from the chemical recycling of carbon fibre reinforced plastic wastes[END_REF]. They were also interested in the degradation of this liquid phase into gaseous molecule by supercritical water gasification. The main products were H 2 , CO, CO 2 , CH 4 and C 2 H 4 . Thus, the decomposition of resin in nearcritical conditions is expected to be a depolymerisation reaction rather than a complete degradation. The decomposition reaction depends on operating conditions and on the chemical nature of resin. Oliveux et al. [START_REF] Oliveux | Degradation of a model epoxy resin by solvolysis routes[END_REF][START_REF] Oliveux | Current status of recycling of fibre reinforced polymers: review of technologies, reuse and resulting properties[END_REF] and Morin et al. [10] have reviewed the treatment of composites waste based on several thermoset and thermoplastic resins. Strong resins such as epoxydes could be decomposed in the range of temperature 370-460 °C within 15 min and needs even catalysts to be decomposed easily. However, most of thermoplastic can depolymerize easily except polyetheretherketone and polyphenylene sulphide which requires high temperature and strong acidic and alkaline conditions. Thermoplastic polyurethane and polybutylene terephthalate needs about 300-370 °C to be decomposed. Among the thermoplastic polymers, Polyamides are of great interest as they are an important engineering plastic (high fatigue strength, resistance to chemical products…) widely used in different fields (automotive industry, fibers, clothing…). Their consumption is increasing year by year. Thus, recycling of waste polyamides by depolymerization (monomerization) becomes more urgent. Patil et al. [START_REF] Patil | Kinetics and thermodynamic studies of depolymerization of nylon waste by hydrolysis reaction[END_REF] studied the depolymerization of nylon 6,6 using acid chloride. The yield of depolymerization was 72.20% for two hours of reaction time at 80 °C. While Kamimura et al. [START_REF] Kamimura | Direct conversion of polyamides to ω-hydroxyalcanoic acid derivatives by using supercritical MeOH[END_REF] were interested to the depolymerization of nylon-12 in supercritical Methanol in the presence of glycolic acid to ω-hydroxyalcanoic acids. The glycolic acid was used to enhance the chemoselectivity towards producing acid function. Then, the decomposition reaction produced 85% of ω-hydroxy-dodecanoate acid. In addition, depolymerization of PA6 sheets in sub and supercritical water [START_REF] Goto | Reactions of polymers in supercritical fluids for chemical recycling of waste plastics[END_REF][START_REF] Iwaya | Kinetic analysis for hydrothermal depolymerization of nylon 6[END_REF] and in supercritical alcohols [START_REF] Kaweetirawatt | Nylon 6 depolymerization in supercritical alcohols studied by the QM/MC/FEP method[END_REF][START_REF] Kamimura | Supercritical secondary alcohols as useful media to convert polyamide into monomeric lactams[END_REF] has been well studied previously. Hydrolysis produces 6-aminocaproic acid and ε-caprolactam (monomer of PA6) in the liquid phase formed. Wang et al. [START_REF] Wang | Hydrolytic degradation of monomer casting nylon in subcritical water[END_REF] studied the hydrolysis of Monomer Casting nylon (MC nylon), which is a special kind of PA6, in subcritical water. They found that MC nylon decomposed completely within 45 min, 345 °C and 9 MPa. The reaction of decomposition produces 6-aminocaproic acid as an intermediate product and proceeds to reach 89% of the goal product which is the monomer ε-caprolactam within 75 min. However, alcoolysis generates aminocaproic ester as the major product which could be transformed into ε-caprolactam by further reaction (such as N-alkylation using methanol). Kamimura et al. [START_REF] Kamimura | Efficient conversion of polyamides to ω-hydroxyalcanoic acids: a new method for chemical recycling of waste plastics[END_REF] studied the conversion of polyamides especially PA6 into valuable chemical materials (hydroxycaproic acid derivatives) using supercritical methanol. According to their results, the reaction of decomposition of pure PA6 and waste PA6 produced the same reaction profile of these products: caprolactam, N-Methylcaprolactam, methyl 6-(N,N-dimethylamino) capronate, methyl 6-hydroxycapronate, methyl 6-hexanoate and methyl 6-methoxycapronate. This means that solvolysis process using supercritical methanol was efficient to convert waste nylon-6. To our knowledge, no work has been already done on PA6 from CFRP. Therefore, recycling of carbon fibers composite based PA6 resin is investigated in this paper. In our previous work [START_REF] Chaabani | Resin removal from carbon-based composites by wet thermochemical process[END_REF], the efficiency of different solvent (water, ethanol and acetone) was studied. Results showed that in a viewpoint of reactivity, water and ethanol were more efficient compared to acetone. However, water takes advantages in the ecological point of view and is the solvent selected for deeper investigations.

Our present study is focused on the solvolysis efficiency in order to reuse carbon fibers and to valorize the resin. Our aim is to study the mechanical, surface and structural properties of the recovered carbon fibers in order to support further optimisation of recycling processes in largescale operations. In addition, a better understanding of the reactions involved in the resin removal (depolymerisation, oxidation, charring effects) should help to understand both the mechanical properties of the recovered fibers and the resin valorization. Indeed, resin valorization may be done through energetic synthetic gas or with platform molecules recovery.

Materials and Methods

Materials

Cut-off Carbostamp UD Tape composites (Torayca T700SC M0E carbon fibers and PA6 resin) were used as the raw material. The size of the composite (length/ Width) is 35 mm/5.5 mm. PA6 resin represents 39-40% in the cutoff strips. Strips of composite are provided by Toray with a thickness of 0.28 mm.

The amount of the resin in the composite was experimentally determined (TGA and normalized oxidation procedure: EN 2564:1998) and compared to the company data. The amount of resin is equal to 39.0 wt%. The resin contains in majority organic materials. Minerals represent only 0.02 wt% of the mass of the resin. In addition, the amount of carbon was also evaluated (Total organic carbon using a solid module) in the resin and was about 78.1 wt%. Both values are then used to provide efficiency parameters.

Solvolysis Process

Figure 1 shows the principle of the experimental setup to determine the optimum conditions to decompose the PA6 resin in sub and supercritical water.

The 5 mL batch reactor was composed of 316 stainless steel. A given weight of the composite (about 0.1 g) was put in the reactor and then filled with ultrapure water. The volume of liquid calculated at room temperature depends on its density at the desired temperature and pressure. Thermodynamics properties were determined by Water&Steam software (Springer). An oven (chromatographic or muffle oven Nabertherm) was preheated at the desired temperature (280-350 °C) or (400-600 °C). Then, the reactors were introduced in the oven. The reaction time started as the reactor reached the desired temperature. At the end of the experiments, the reactor was cooled down to room temperature and the three phases (solid, liquid, and gas) were separated. The volume of gas and the mass of the solid were measured. The results are provided in terms of resin removal rather than total mass loss calculated as follows:

Liquid Phase Analysis

The Total organic carbon in the aqueous phase was measured using a TOC analyzer (Shimadzu TOC-VCSH). The amount of carbon in the liquid phase was then calculated assuming a constant volume of water after reaction. Then the percentage of carbon from the resin and recovered in the liquid phase is calculated following Eq. 2.

The molecules present in the liquid phase were analyzed by GC-MS (GC-MS-TQ8030, Shimadzu). The capillary column (ZB-5MS) used was 95% nonpolar and 5% polar, 30 m of length and 0.025 mm of inner diameter. The carrier gas was helium at a constant flow rate 1.56 mL/min. The GC-injector was at 250 °C. The furnace temperature was at 40 °C held for 2 min and ramped to 280 °C at a rate of 10°/min and then held at 280 °C for 4 min. The interface temperature was 280 °C. Spectral searches on the installed NIST11 and NIST11s Libraries were used to qualitatively identify the major unknown compounds in the liquid phase.

Carbon amount (%) = weight of carbon in the liquid phase weight of carbon in the resin × 100

Solid Phase Analysis

The fibers recovered were analyzed by environmental scanning electron microscopy (ESEM Philips XL30 FEG) with a Back-scattering electron detector in order to observe their physical appearance. The microstructure parameters of the virgin carbon fibers was analysed by Confocal Raman WITEC Alpha 300AR microscope. A CCD (charge-coupled device) camera detector (175-4000 cm -1 ) was used to collect the Raman spectra. The analysis was performed at room temperature. Laser with 532 nm wavelength was used as an illumination source. For each spectrum, five scans were collected and co-added in the instrument control computer. Spectra were compared and an average spectrum was used to represent each sample.

The tensile strengths (INSTRON 5942) of the virgin and recovered fibers were measured. The fibers length used was 35 mm. The tensile testing was performed on a testing frame with 5 N load cell and a crosshead speed of 1 mm/ min. The obtained values were processed on the one hand by an analysis based on normal distribution (mean value, standard deviation) conform to the standard norm ISO 11566. The reported tensile strengths were the mean value of 40 measurements. On the other hand, Weibull analysis [START_REF] Weibull | Statistical distributions function of wide applicability[END_REF], a widely used method for the analysis of fragile materials as carbon fibers [START_REF] Jiang | Characterisation of carbon fibres recycled from carbon fibre/epoxy resin composites using supercritical n-propanol[END_REF][START_REF] Deng | Evaluation of fibre tensile strength and fibre/ matrix adhesion using single fibre fragmentation tests[END_REF] was also utilized to evaluate the results. 40 specimens in each condition were tested. In Weibull-weakest link model, the fiber is supposed to be constituted of different links having each one its tear resistance. The break occurs when the weakest link is broken. For each link, it is attributed a failure probability. The distribution function of the two-parameter Weibull used to describe the fiber strength parameters is in the equation below [START_REF] Deng | Evaluation of fibre tensile strength and fibre/ matrix adhesion using single fibre fragmentation tests[END_REF]:

where P is the cumulative failure probability, σ the applied stress, m is the Weibull shape parameter and σ 0 is the Weibull scale parameter.

This cumulative failure probability, under a particular stress is approximated by: where i is the number of fiber that have broken at a stress level and N is the total number of fibers tested. After

Kinetics Parameters

Kinetics of PA6 can be treated as a pseudo-first-order reaction since water concentration is expected to be constant. Decomposition kinetics of resin is described using the following equations:

where (PA6) is the concentration of PA6, ∝ is thepartial order of PA6, is the partial order of H 2 O, m 0 is the initial mass of PA6 and m PA6 is the residual mass of PA6 at t, k is the rate constant and t is the reaction time.

The activation energy is calculated by the Arrhenius equation where K is the rate constant, A is the pre-exponential factor, E a is the activation energy, R is the gas constant and T is the temperature.

Results and Discussion

Solvolysis was performed to remove the PA6 resin from the composite and to recover carbon fibers. Using this process, resin could be dissolved in the liquid phase (depolymerization), remained in the solid phase (not efficient or charring issues) or converted in volatile molecules in gaseous phase (degradation). Several experiments in subcritical (280-350 °C) and supercritical water (400-600 °C) were carried out in order to evaluate the resin decomposition as well as the properties of recovered carbon fibers over reaction time (1-120 min).

As mentioned previously, this study is focused on two key points. On the one hand, a preliminary study is made in subcritical and supercritical water in order to understand the conversion of the reaction of resin ( 5)

ln (-ln (1 -P)) = m ln ( ) -m ln( 0 ) (6) -d[PA6] dt = k[PA6] ∝ H 2 O (7) -d[PA6] dt = k app [PA6] ∝ aveck app = kH 2 O (8) -dm PA6 dt = k app m PA6 avec ∝= 1 (9) Ln m 0 m PA6 = k app t (10) k = Ae -E a RT
decomposition. Then, a kinetic study was realized in order to compare the kinetic parameters in sub and supercritical regions. On the other hand, the properties of recovered carbon fibers are measured and compared to the virgin ones since the main issue regarding the reuse of recovered carbon fibers is their mechanical properties.

Efficiency of Solvolysis Treatment

This section is dedicated to the evaluation of solvolysis efficiency. Efficiency is first evaluated using the resin removal (Eq. 1) over the solvolysis treatment.

Conversion using subcritical water

Figure 2 shows the results of the decomposition efficiency (resin removal) as a function of reaction time at each reaction temperature: 280, 310 and 350 °C. The amount of resin eliminated from the solid residue increased rapidly with reaction time using the three reaction temperatures considered. The maximal rate reached almost 100% after 15 to 30 min of reaction time, and then is quite stabilized to the maximal value. This observation is confirmed by ESEM pictures, in the Fig. 3, in which the fibers are well-separated from balls of resin deposited, even if some resin deposits are locally still observed. We also noted that there are no visible damages on carbon fibers surface using ESEM resolution. Figure 2 shows that the process efficiency (resin removal) increased linearly during the first moments of reaction. This linear domain was (5-30min) at 280 and 310 °C, and decreased to (5-15min) at 350 °C. Thus, the kinetics of the reaction is faster as the temperature increased.

Conversion Using Supercritical Water

Figure 4 presents the resin removal using supercritical conditions. Reaction temperature was set at 400, 425, 450 and 500 °C. The resin removal increased rapidly at short reaction time, for the different temperatures used. The maximal value close to 100% was reached for less than 15 min of reaction time. Figure 5 presents ESEM pictures of some remaining solids and shows that the surface of fibers recovered are quite close to that of the virgin carbon fibers. No particular damages were highlighted. Otherwise, small white particles are observed on the fibers surface. EDS micro-analysis confirmed that these particles are attributed to titanium. These particles may be due to the impurities present on the precursor of PAN [START_REF] Dupupet | Fibres de carbone[END_REF] and/or from the surface treatment of carbon fibers surface in order to improve the chemical compatibility of fibers and resin [START_REF] Zhang | Different surface treatments of carbon fibers and their influence on the interfacial properties of carbon fibers/epoxy composites[END_REF].

We noted that at 500 °C, gaseous phase was generated. It was reported that high temperatures (≥500 °C) promote gasification, while low temperatures favor liquefaction [START_REF] Kruse | Hot compressed water as reaction medium and reactant properties and synthesis reactions[END_REF][START_REF] Sinag | Key compounds of the hydropyrolysis of glucose in supercritical water in the presence of K 2 CO 3[END_REF]. Regarding literature, the reactions involved at sub and supercritical conditions should be different [START_REF] Kruse | Hot compressed water as reaction medium and reactant properties and synthesis reactions[END_REF]. Ionic reactions are expected to be improved at subcritical conditions while radical reactions are enhanced in supercritical water. As for the reaction in subcritical conditions, the evolving is quite linear during the first reaction times and then reached a plateau. The time required to achieve the resin elimination is lower than using subcritical conditions meaning that the kinetics has been improved with the temperature and the state of water. Therefore, kinetics in sub and supercritical conditions are studied in the next section.

Kinetics Study in Sub and Supercritical Water

In this section, kinetics in sub and supercritical regions is discussed. Figure 6 shows ln (m 0 /m) versus reaction time at 280, 310 and 350 °C, and at 400, 425, 450 and 500 °C. In Fig. 3 ESEM micrographs of virgin carbon fibers and remaining composite after solvolysis using ultrapure water at 30 min Fig. 4 Resin removal versus reaction time at 400, 425, 450 and 500°C Fig. 5 ESEM micrographs of remaining composite after solvolysis using ultrapure water at 15 min addition ln(k) versus (1/T) is also plotted in order to determine the activation energies.

Figure 6a, c show that the pseudo-first order describes properly the experimental results both in subcritical conditions (350, 310 and 280 °C) and in supercritical conditions (400-500 °C). Using these data, the objective is to determine the activation energy (Fig. 6b,d). At subcritical condition, the three points are clearly giving a straight line, thus activation energy was equal to 77.79 kJ/mol. In the supercritical domain, the four points are not aligned in a straight line. However, using data from 400 to 450 °C a straight line is drawn giving activation energy of 78.51 kJ/ mol. As mentioned previously, the experiments carried out at 500 °C showed quantitative gas production. In this context, the mechanism pathway should be different and could explain the non-linearity of the Arrhenius plot in Fig. 6. Both values are similar meaning that the global reactions involved should be the same in both regions. This activation energy corresponds to the depolymerization of the resin into different lighter molecules (oligomers and monomers). Iwaya et al. [START_REF] Iwaya | Kinetic analysis for hydrothermal depolymerization of nylon 6[END_REF] experimentally determined activation energy of 97.1 kJ/mol in the range of temperature 300-360 °C. However, the authors studied precisely the reaction from pure PA6 sheet to 6-aminocaproic acid, while the present study only follows the disappearance of PA6 mass. Therefore the experimental value should mean that the global reaction is not so far from a depolymerization.

Suggested Reaction Schemes

In order to provide a reaction scheme, the first question is to determine whether the resin removal is a depolymerization or degradation pathway. As seen in the previous section, the activation energy value would direct to a depolymerization phenomenon.

Figure 7 shows the resin removal versus the carbon amount recovered in the liquid phase. At the beginning (less than 30 min at subcritical conditions) the curves follow a linear trend. In addition, the slope is very close to 1. This informs that the resin is totally decomposed in lighter molecules dissolved in the liquid phase and means that the polyamide 6 is depolymerized. At supercritical conditions, a linear trend is obtained up to 450 °C but the carbon amount should be underestimated compared to the rate of resin eliminated since the slope is not equal to 1. This gap between carbon amount and the resin removal could be due to the suspension materials remained in liquid phase. From 500 °C, the trend of curves changes and follows a polynomial trend which suggests a different mechanism of decomposition of the resin. Thus, the reaction involved in both subcritical and supercritical water near to critical region is a depolymerization rather than a degradation since no gaseous phase was recovered in these conditions. In supercritical conditions above 500 °C, the two reactions types are supposed to occur due to gas production.

At the atomic scale in the domain of linearity, the reaction of decomposition should begin with the shrinking of oligomers and monomers from the resin. GC-MS analysis revealed the presence of ε-caprolactam (monomer of PA6) and other molecules in sub and supercritical conditions. At short reaction times (5 and 10 min depending on temperature), only the molecules of ε-caprolactam were identified. Heavier and lower molecules than ε-caprolactam were also identified at longer reaction times: 1-8-Diazacyclotetradecane-2,9,-dione, 1-Methylcaprolactam, 2-oxepanone, 7-hexyl, trans-2-hexene, 6-Hydroxyhexanoic acid, 1-cyclopentene-1-carboxylic acid and cyclopentanone. In the presence of nucleophile, the weakness is situated on the peptide bond which is hydrolyzed from 280 °C and also on the alkyl-amid bonds which undergo homolytic scission at temperature greater than 500 °C [START_REF] Levchik | Thermal decomposition of aliphatic nylons[END_REF]. As reported in the literature [START_REF] Pinero-Hernanz | Chemical recycling of carbon fibre reinforced composites in nearcritical and supercritical water[END_REF] the depolymerization of PA6 into ε-caprolactam could produce an intermediate which is the 6-aminocaproic acid as presented in the Fig. 8. In the range of temperature 200-300 °C, ionic product of water increases up to around 10 -11 [START_REF] Kruse | Hot compressed water as reaction medium and reactant properties and synthesis reactions[END_REF]. To form 6-aminocaproïc acid, the decomposition reaction should be initiated on the carbon nitrogen bond (C-N). In the subcritical water medium, H + ions could react with the amide functional group to form ammonium functional group. This center of positive charges induces the hydroxylation of the carbon in the amide functional group. The carbonyl group is thus transformed into a carboxylic acid group. The amine and the carboxylic acid are functional groups existing in both sides of the molecule. The resulting molecule is 6-aminocaproic acid. However, this intermediate was not detected by GC-MS. The reaction is likely very fast and does not let us identify it. Finally, 6-aminocaproic acid quickly dehydrates and cyclizes to produce ε-caprolactam.

To summarize, the reaction scheme of PA6 depolymerization would be the hydrolysis of amides to form 6-aminocaproic acid followed by the cyclodehydration of 6-aminocaproic acid to ε-caprolactam [START_REF] Ritz | Ullman's Caprolactam[END_REF][START_REF] Chen | Catalytic hydrothermal depolymerization of nylon 6[END_REF]. This type of chemical scission is supported by Chen et al. [START_REF] Chen | Catalytic hydrothermal depolymerization of nylon 6[END_REF] and Iwaya et al. [START_REF] Iwaya | Kinetic analysis for hydrothermal depolymerization of nylon 6[END_REF].

A scheme reaction is proposed in Fig. 9: ε-caprolactam could be transformed into 1-Methylcaprolactam or could react with another identical molecule to produce 1-8-Diazacyclotetradecane-2,9,-dione. This latter could be transformed into 2-oxepanone, 7-hexyl. The trans-2-hexene should come from the degradation of 2-oxepanone, 7-hexyl. However, 6-hydroxyhexanoic acid could come either from the degradation of 2-oxepanone, 7-hexyl or from the ring cleavage of caprolactone [START_REF] Ritz | Ullman's Caprolactam[END_REF]. 6-hydroxyhexanoic acid could react with ammonia in order to form 6-hydroxyhexanamide which could undergo an intramolecular dehydration to form ε-caprolactam.

The detection of heavier molecules than caprolactam suggests a repolymerization phenomenon which could occur as reported in literature [START_REF] Kruse | Hot compressed water as reaction medium and reactant properties and synthesis reactions[END_REF]. In fact, the decomposition of phenols due to poly-condensation reactions leads to the formation of different short chain polymers. From these Fig. 7 Resin removal versus amount of carbon present in liquid phase in sub (a) and supercritical conditions (b) Fig. 8 Depolymerization of PA6 to ε-caprolactam [START_REF] Ritz | Ullman's Caprolactam[END_REF] intermediates, gases and coke formation occurs. In addition, Yong and Matsumura [START_REF] Yong | Reaction pathways of phenol and benzene decomposition in supercritical water gasification[END_REF] stated that the decomposition of phenol in water at near-critical conditions leads to a competition between gas and char formation. Repolymerization of degradation products could produce a layer that remained on the recovered carbon fibers surface after cooling. In order to confirm or deny this hypothesis, molecules contained in liquid phase should be analyzed and quantified at longer reaction time.

The efficiency of the process was validated. It remains to complete the characterization of recovered carbon fibers and especially to determine their mechanical properties as it is the most important data for recycling in order to select the best conditions able to provide good quality carbon fibers.

Properties of Recovered Carbon Fibers

Recovered carbon fibers are especially characterized regarding their mechanical properties for further use.

Mechanical Properties

The fibers obtained in sub-and supercritical conditions were subjected to tensile strength tests. All the recovered fibers were analyzed. As mentioned in Figs. 2 and4, reaction times of 15 and 30 min are sufficient to provide maximal resin elimination. Therefore, Table 1 only lists the mechanical properties measured for the "optimal" reaction time at each temperature. Tensile properties should be compared to that of the virgin carbon fibers (4931.29 ± 530 MPa). Results were studied by normal distribution based on the mean value and standard deviation, the method commonly used in order to determine the tensile strength of carbon fibers and by Weibull distribution which evaluate distribution of the default size in carbon fibers.

As shown in Table 1, the tensile strength of recovered carbon fibers is not affected by solvolysis treatment in both regions. The values of tensile strength of recovered carbon fibers are situated in the 10% of the margin of error of the tensile strength of virgin carbon fibers. It means that the process is suitable for obtaining fibers with good mechanical properties. Besides, the values of standard deviation of recovered carbon fibers are higher than that of virgin carbon fibers, meaning that the tensile strength values are more dispersed over the selected fibers. This observation would be a key item since this deviation indicates that the sizes of defaults originating the rupture are more heterogeneous. These defaults could be due to physical or chemical treatment. The physical treatment improves surface roughness which could serve as mechanical anchorings [START_REF] Su | Influence of reprocessing on the mechanical properties and structure of polyamide 6[END_REF][START_REF] Lozano-Gonzalez | Physical-mechanical properties and morphological study on nylon-6 recycling by injection molding[END_REF].

The chemical treatment adds some oxygenated functions at the surface. Bai et al. [START_REF] Bai | Chemical recycling of carbon fibers reinforced epoxy resin composites in oxygen in supercritical water[END_REF] studied the chemical recycling of carbon fibers reinforced epoxy resin using supercritical water oxidation. They concluded that oxygenated groups on carbon fibers microstructure impacted their mechanical properties, since they observed a loss of 38% in tensile strength for a decomposition efficiency of 100.2 wt%. However, in our case, the impact of chemical treatment is not as strong as in the last cited study since tensile strength values of treated fibers are closed to virgin fibers values.

In order to evaluate the distribution of defaults, Weibull parameters were also studied.

Table 2 shows the Weibull parameters obtained on the fibers treated at sub and supercritical conditions. These parameters are Weibull shape parameter, m, and the scale parameter, σ 0 . The latter reflects the average stress cracks activation causing the fiber break. The Weibull parameter m translated meanwhile, the dispersion of defects in the fiber. As m is big, the greater the defect distribution is fine. Weibull parameters seem to be coherent with the analysis based on normal distribution. In fact, the same observations are faithfully transcript compared to normal distribution considering the scale parameter σ 0 . Otherwise, the Weibull shape parameters are coherent with the standard deviations. As the standard deviation increases, the heterogeneous is the breakage and the lower is the Weibull parameter. Table 2 shows that treated fibers in subcritical water present Weibull parameter lower than the one of virgin carbon fibers at 280 °C. This is probably due to resin impurities still deposited on carbon surface (seen on ESEM picture Fig. 3b). However, at 310 and 350 °C, the Weibull parameters of treated fibers are higher than that of the virgin fibers. This could be due probably to an artefact in measurements. In supercritical water, Weibull parameters values are lower than that of virgin carbon fibers except at 450 °C. Thus, the distribution of defect seems to be larger in supercritical region than in subcritical region. These defects could be structural defects in the graphitic structure. In order to understand these eventual changes in carbon structure, structural properties are studied in the next section.

Structural Properties

Mechanical properties and ESEM observations informed that the treatment applied is not so severe to oxidize or to create visible cracks on the recovered carbon fibers. However the results from the previous section highlight that the carbon fibers are modified in their micro or nanostructure. For this purpose, Raman spectroscopy has been used since it informs on bond vibration mode in a molecule, and therefore gives information on carbon hybridization. Basically, the G and D bands are the broad bands related to the "pure" graphitic structures and to the defects in the graphene sheets respectively. In addition, the height of the valley (D and G bands overlapping) corresponds to the amorphous content. Fig. 10 presents some Raman spectra, namely that obtained for virgin carbon fibers and for the recovered carbon fibers in "optimal conditions" 350 °C, 30 min and 400 °C, 15 min. The Raman spectrum shows in the first order region (800-2000 cm -1 ) two main broad and overlapping peaks with maximum intensities at 1350 and 1590 cm -1 . These two peaks are attributed to the D and G bands respectively. The G band corresponds to the sp 2 carbon atoms in the graphitic structure (aromatic rings) [START_REF] Mcdonald-Wharry | Carbonization of biomass-derived chars and the thermal reduction of a graphene oxide sample studied using Raman spectroscopy[END_REF]. The D band, whose origin had long been debated by Tunistra and Koenig [START_REF] Tunistra | Raman spectrum of graphite[END_REF] and Ferrari et al. [START_REF] Ferrari | Interpretation of Raman spectra of disordered and amorphous carbon[END_REF] corresponds to the defects in the graphitic structure [START_REF] Mcdonald-Wharry | Carbonization of biomass-derived chars and the thermal reduction of a graphene oxide sample studied using Raman spectroscopy[END_REF]. It can be seen that the G band intensity is higher than that of D band for the virgin carbon fiber. On the contrary, the intensity of the D band becomes higher for recovered carbon fibers. This first observation indicates that the "pure graphitic" structures are consumed in favor to disordered graphitic structures since the height of the valley is kept constant. Overall, it could be deduce that defects induced in the graphitic nanostructures could come from "fullerene-like" structures, carbon nanotubes arranged in onion-like structures, pointdefects (edges), nanocrystallites reorganization and so on [START_REF] Terrones | The Geometry of hypothetical curved graphite structures[END_REF][50][START_REF] Ijima | Helical microtubules of graphitic carbon[END_REF][START_REF] Ugarte | Curling and closure of graphitic networks under electron-beam irradiation[END_REF]. In addition, Fig. 10 shows that G and D bands are

Conclusions

The aim of this work is to study the solvolysis of PA6 based composite in order to recover simultaneously carbon fibers and to valorize resin. The ability of water as solvent-reagent for chemical recycling of carbon fibers reinforced PA6 has been investigated in sub and supercritical conditions. On the one hand, experiments were performed at 280, 310, and 350 °C and 25 MPa for reaction times ranging from 5 to 120 min. On the other hand, experiments were performed at 400, 425, 450, and 500 °C and 25 MPa for reaction times ranging from 1 to 15 min. The resin decomposition was accelerated in supercritical water than in subcritical water. The resin removal reached 98.95 wt% at 280 °C in 30 min while, It has already reached 97.18 wt% at 400 °C in only 15 min. Kinetics was studied as a pseudo-first-order reaction. The activation energy was 77.99 and 78.51 kJ/ mol in sub and supercritical water respectively. Thus, the reaction mechanism of the decomposition of the resin is the same in both regions. In addition, the same molecules were identified in liquid phases recovered from both conditions. In fact, resin was completely decomposed into ε-caprolactam and other molecules. Only ε-caprolactam was detected in liquid phase recovered in short reaction times (10-30min). Above 15-30min (depending on the temperature), heavier molecules were detected. Thus, a repolymerization phenomenon was supposed to occur. Tensile strength of recovered fibers was very close to that of virgin fibers. Weibull parameters show that the distribution of defects seems to be lager in subcritical water than in supercritical water. In microscopic view, Raman analysis show slight modifications at carbon structure on fibers surface. These microstructural changes could be due to repolymerization phenomenon and may be correlated with the large distribution of defects. Further studies are in progress to investigate the repolymerization phenomenon and its impact on mechanical and structural properties of carbon fibers.

( 1 )

 1 Resin removal (%) = weight of compositeweight of solid residue Weight of resin in composite × 100

Fig. 1

 1 Fig. 1 Principle of the experimental setup

Fig. 2

 2 Fig. 2 Resin removal versus reaction time at 280, 310, and 350°C

Fig. 6 ln m 0 m

 0 Fig. 6 ln m 0 m versus reaction time and ln(k) versus (1/T) in subcritical water (a, b) and supercritical water (c, d)

Fig. 9

 9 Fig. 9 Reaction scheme (framed molecules are detected by GC-MS)

  slightly red-shifted from virgin carbon fibers to recovered fibers. D band maximum is shifted from 1370 to 1340 and 1350 cm -1 for experiments carried out at 350 and 400 °C respectively. The G band maximum is shifted from 1590 to 1580 cm -1 . McDonald-Warry et al. concluded that a red shift in the D and G bands position would mean that the graphitic structure is reorganized and would contain a large part of defects, and curved graphene fringes which are greatly organized (or distributed). Although the D band intensity could be related to the size of nanocrystallites (meaning size of defects), the current results are not precise enough to led to this conclusion. However it can be concluded that nanostructural changes occurred during the solvolysis. These changes seem to slightly modify the mechanical properties. Deepest investigations should be developed to better understand the relation between mechanical properties and nanostructure.

Table 1

 1 Mechanical properties of recovered carbon fibers in subcritical and supercritical water

	Water state	Reaction	Tempera-	Tensile strength	Standard
		time (min)	ture (°C)	(MPa)	deviation
	Subcritical	30	280	4240.93	906
		30	310	4233.56	635
		30	350	4128.58	572
	Supercritical 15	400	4575.03	940
		15	425	4061.97	788
		15	450	4161.87	449
		15	500	4239.45

Table 2

 2 Weibull mechanical properties of virgin carbon fibers (VCF) and recovered carbon fibers (RCF) in subcritical and supercritical water

	Water state	Samples	Weibull	Scaling
			parameter m	factor σ 0 (MPa)
	-	VCF	7.90	5126.36
	Subcritical water	RCF (280°C, 30min) 5.60	4596.50
		RCF (310°C, 30min) 8.08	4493.52
		RCF (350°C, 30min) 8.71	4367.81
	Supercritical water RCF (400°C, 15min) 5.65	4953.52
		RCF (425°C, 15min) 6.03	4382.34
		RCF (450°C, 15min) 9.37	4387.29
		RCF (500°C, 15min) 7.45	4519.75

Acknowledgements

The authors acklowledge Christine Rolland, Laurène Haurie and Jean-Marie Sabathier for their assistance with analysis.


Jun Dong

Yong Chi

Yuanjun Tang

Mingjiang Ni

Ange Nzihou

Elsa Weiss-Hortala

Qunxing Huang

Effect of Operating Parameters and Moisture Content on Municipal Solid Waste Pyrolysis and Gasification

Pyrolysis/gasification-based waste-to-energy (WtE) techniques, comprising partial oxidation of waste and subsequent syngas combustion, show potential benefits over direct incineration. To facilitate their development under the specific conditions of China, pyrolysis and gasification of typical municipal solid waste (MSW) are investigated in a fluidized bed reactor. The effect of the equivalence ratio (ER), reaction temperature, and moisture content on MSW conversion is studied. A rising ER increases the syngas yield but decreases the syngas heating value. The combustible gas yield is strengthened at lower ERs and later drops when the ER exceeds 0.4 as a result of the continuously enhanced oxidation reactions. A higher temperature favors pyrolysis reactions but causes an evident decrease in the syngas heating value during gasification. When the ER is at 0.4 and the temperature is at 650 °C, an optimum operating performance is obtained under the specific input simulated MSW (S-MSW) and test conditions, with an energy conversion efficiency of 68.5%. Under such a circumstance, the further increase of the MSW moisture content is effective for stimulating H 2 production; nevertheless, the quality of syngas degrades, and the energy conversion efficiency declines. The appropriate MSW moisture content is found to be lower than 20-25%. Besides, emiessions, such as heavy metals and dioxins, are also compared to conventional incineration to verify the environmental feasibility of gasification.

INTRODUCTION

With a booming social economy, the rapid generation of municipal solid waste (MSW) and its proper treatment have become one of the most critical challenges for both the government and the general public. It is well-recognized that MSW contains a high proportion of combustible components, and more attention has been paid to use MSW as a biomass for energy recovery. Over the past few decades, incineration has played a gradually rising role in waste management as a result of the advantages of significant waste volume reduction, complete disinfection, and energy recovery. 1 However, its broader spread is still restricted, owing to the harmful emissions, especially those of dioxins and acidic gases (SO 2 , NO x , HCl, etc.). 2 Besides, excessive corrosion by HCl has limited the maximum steam superheater temperature and, thus, led to relatively low energy efficiency. With the realization of these facts, a more energy-efficient and environmentally sound MSW thermal technique is in urgent demand.

As a novel waste-to-energy (WtE) process, MSW pyrolysis or gasification exhibits several potential benefits over traditional incineration and has received significant attention in recent years. 3 It is defined as the thermochemical conversion of MSW to combustible products in the absence of oxygen (pyrolysis) or in partial combustion conditions (gasification). In comparison to "single-step" incineration, a pyrolysis/gasification-based WtE technique, i.e., "two-step oxidation", makes it possible to decrease dioxins and NO x emission as a result of the reducing atmosphere 4 and the gaseous products could be cleaned to remove contaminants prior to the subsequent combustion. Moreover, the homogeneous gas-gas oxidation taking place in the combustor is much easier to control, thus allowing for the reduction of excess air and increasing the overall plant efficiency. 5 Basically, this second step has been well-developed; nevertheless, the pyrolysis or gasification step remains as a core challenge of this technique. Obviously, the quality of the products coming to the downstream combustion chamber depends upon a good operation of the gasifier. 6 In this context, a deeper understanding of an energy-efficient pyrolysis/ gasification conversion process becomes essential to better develop the "two-step" WtE technique.

However, the operating experience of MSW pyrolysis and gasification plants is quite limited worldwide to date; its industrial application in China has not yet been achieved. It is proven that the pyrolysis and gasification processes are crucially affected by the oprational variables and feedstock properties. For instance, a high MSW moisture content might obviously reduce the reaction temperature and degrade the gasification performance, [START_REF] Kaewluan | Gasification of high moisture rubber woodchip with rubber waste in a bubbling fluidized bed[END_REF] while the equivalence ratio (ER) and temperature are important factors that define the tar yield and composition of syngas. 6 In China, MSW is dominated by high organic and moisture contents and a low calorific value. Because the MSW characteristics differ significantly from western countries, it is meaningful to investigate the performance of MSW pyrolysis and gasification under the local conditions of China. Especially, the key operating parameters, such as the temperature, ER, and moisture content, should be optimized, and the environmental emissions need to be examined to facilitate a good and feasible gasification process that is adaptable to China.

Over the past few years, increasing studies on MSW pyrolysis and gasification emerged using different types of reactors, including a fixed bed, 2,8,9 a fluidized bed, [START_REF] Arena | Gasification of a solid recovered fuel in a pilot scale fluidized bed reactor[END_REF][START_REF] Kim | Air-blown gasification of woody biomass in a bubbling fluidized bed gasifier[END_REF][START_REF] Gregorio | Fluidized bed gasification of a packaging derived fuel: energetic, environmental and economic performances comparison for waste-to-energy plants[END_REF] a rotary kiln, [START_REF] Donatelli | High energy syngas production by waste tyres steam gasification in a rotary kiln pilot plant[END_REF] an entrained flow, [START_REF] Hernańdez | Gasification of biomass wastes in an entrained flow gasifier: Effect of the particle size and the residence time[END_REF] and a plasma furnace. [START_REF] Zhang | Gasification of municipal solid waste in the Plasma Gasification Melting process[END_REF] Among these facilities, a fluidized bed is particularly attractive for MSW characteristics in China. Because of the uniform temperature distribution and intense mixing of gas and solids, [START_REF] Li | Biomass gasification in a circulating fluidized bed[END_REF] a fluidized bed is effective to handle highly heterogeneous or wet fuels. Using a fluidized bed, Xiao et al. [START_REF] Xiao | Air gasification of polypropylene plastic waste in fluidized bed gasifier[END_REF] investigated the effect of the ER, bed height, and fluidization velocity on the gasification of plastic, observing that the ER has the most pronounced effect on syngas production. Ni et al. [START_REF] Ni | Study on pyrolysis and gasification of wood in MSW[END_REF] revealed the reactive courses and mechanism for the pyrolysis and gasification of wood waste. Guo et al. [START_REF] Guo | An innovative example of herb residues recycling by gasification in a fluidized bed[END_REF] designed a utilization approach for the gasification of wet biomass and determined the optimal condition to maximize the syngas heating value and efficiency. Nevertheless, most of these studies are mainly focused on the use of a single component of MSW; research on mixed components from MSW is still quite lacking. [START_REF] Niu | Simulation of syngas production from municipal solid waste gasification in a bubbling fluidized bed using Aspen Plus[END_REF] On the other hand, to clarify the performance of mixed MSW, previous works are mostly based on theoretical simulation; [START_REF] Niu | Simulation of syngas production from municipal solid waste gasification in a bubbling fluidized bed using Aspen Plus[END_REF][START_REF] Xiao | Gasification characteristics of MSW and an ANN prediction model[END_REF][START_REF] Chen | Simulation of municipal solid waste gasification in two different types of fixed bed reactors[END_REF] experimental studies are rarely assessed. In this sense, pyrolysis and gasification of the most typical mixed MSW are of great significance.

Accordingly, in the present study, pyrolysis and gasification of mixed MSW are experimentally studied in a fluidized bed. The influence of the ER, reaction temperature, and MSW moisture content on products distribution, syngas properties, and energy conversion efficiency is studied. The principle aim is to identify an energy-efficient condition of pyrolysis and gasification under the specific situation of China. Besides, emissions are also compared to conventional incineration to further test the environmental performance of gasification. This knowledge would assist in assessing the potential of pyrolysis/ gasification as a WtE pathway for energy recovery from MSW and provide scientific experience to facilitate the future developing energy-efficient and technically feasible MSW thermal technique.

EXPERIMENTAL SECTION

2.1. MSW Characteristics. To ensure the homogeneity of the feedstock in each test, a synthetic, simulated MSW (S-MSW) is used. The S-MSW is prepared in accordance with the annual statistical waste composition in Hangzhou, China. [START_REF] Dong | Life cycle and economic assessment of source-separated MSW collection with regard to greenhouse gas emissions: a case study in China[END_REF] It is composed of food waste (29 wt % rice and 29 wt % cabbage), cardboard (13 wt %), plastic (17 wt % polyethylene and 3 wt % polyvinyl chloride), textile (2 wt %), timber (5 wt %), and rubber (2 wt %). The major combustible components are considered, and the mineral fraction is represented by the silica sand used as bed material in the furnace. The S-MSW is crushed and sieved into sizes of approximately 2 mm and then dried at 85 °C for 24 h. The characteristics of the S-MSW are shown in Table 1. The moisture content of the S-MSW is 9.2%. To further investigate the effect of moisture, distilled water is then added to the sample to obtain the required moisture content in the S-MSW.

2.2. Test Facilities and Procedures. A schematic diagram of the experimental apparatus is depicted in Figure 1. It is a lab-scale continuously fed fluidized bed reactor, equipped with a cyclone separator as a flue gas cleaning unit. The effective height of the reactor is 1100 mm with an inner diameter of 60 mm. A three-stage alloy resistance wire is used to externally heat the furnace. Three K-type thermocouples are employed to monitor the temperature profile at the top, middle, and bottom of the furnace, respectively. To prevent tar condensing from the flue gas, a heating tape is installed in front of the sampling point to maintain the temperature above 300 °C. Silica sand, sieved to a size fraction of 0.28-0.45 mm, is used as bed material. The S-MSW is continuously fed into the furnace by a speed-adjustable screw feeder. In each test, the fluidization velocity (U m ) is kept constant at 0.18 Nm s -1 , which is 1.5 times the minimum fluidization velocity (U mf ). Air and N 2 act as gasifying agents for gasification and pyrolysis processes, respectively; they are controlled by valves from the bottom of the furnace. The total inlet gas in each test is kept the same at 2 Nm 3 h -1 , so that the S-MSW residence time in the furnace is approximately 6 s. The S-MSW feeding rate is set as a variable and calibrated beforehand by a range of screw speed tests.

The effect of the ER, temperature, and S-MSW moisture content is investigated. The ER is defined as the actual air supply to the stoichiometric air required for complete combustion. A temperature series of 550, 650, 750, and 850 °C is chosen, and ER under each temperature varies from 0 to 0.8 (ER = 0 refers to pyrolysis condition). The moisture level is varied in a range of 9.2-51.7% to represent the changes between the dried waste and realistic raw MSW. [START_REF] Dong | Life cycle and economic assessment of source-separated MSW collection with regard to greenhouse gas emissions: a case study in China[END_REF] At the start of the test, 400 g of bed material is placed in the reactor and the electric heater is turned on. The S-MSW begins to be supplied once the desired temperature is reached. After the system achieves stability, gas sampling is started. Samples are taken isokinetically, with the gas flow rate ranging from 0.6 to 1.0 L min -1 . Liquid products (including tar and water) are trapped in two serpentine condensers, and noncondensable gas is collected using gas bags. After each experiment, the electric heater is turned off but N 2 flow is still maintained until the furnace reaches room temperature to avoid the oxidation of the char remaining in the bed. The char is collected from the bottom of the furnace, separated from bed material, and weighted. To ensure the reliability of test data, each experiment is repeated 3 times under identical conditions, and the data reported are the average values recorded.

2.3. Analysis Method. Each experiment lasts for approximately 45-60 min after the system achieves stability, and the dry, clean, and cool syngas is collected by a gas bag every 10 min. Gas composition is analyzed using gas chromatography (micro GC-490 analyzer, Agilent). The gas mainly contains N 2 , CO, CO 2 , CH 4 , H 2 , C 2 H 2 , C 2 H 4 , and C 2 H 6 . The total gas yield (Nm 3 kg -1 of S-MSW) is quantified on the basis of the N 2 balance, according to the molar ratio of N 2 in the input and produced gas. The lower heating value (LHV) of the syngas is estimated by means of eq 1. [START_REF] He | Hydrogen-rich gas from catalytic steam gasification of municipal solid waste (MSW): Influence of catalyst and temperature on yield and product composition[END_REF] For each test, 3-5 gas samples are taken and the data are validated when the measuring variability is within 5%.

To quantify the tar yield, the condensers and connection pipes are flushed by ethanol to avoid tar adhesion. The washing liquors are then heated at 60 °C for ethanol evaporation and weighted. The moisture content of the liquid products is measured by the Karl Fischer titration method, and its LHV is determined following the National Standard GB 384-81. The solid products include fly ash and char; their LHV is determined according to the National Standard GB/T 213-2008. On the basis of the data, product mass distribution of the syngas, condensates, and char is calculated, which represents the mass ratio of the corresponding output product to the input S-MSW and agent gas. The mass balance closure might deviate from 100% probably as a result of several possibilities. Some tars may be condensed and depositing on the furnace wall, leading to the underestimation of the tar produced. The measurement uncertainties may lead to the inaccurate estimation of the syngas yield. Therefore, to ensure the test quality, the criterion of mass balance data acceptance is set at 85-105%. To obtain optimum gasification conditions, it is evident that the quantity of the energy escaping the furnace should be maximized to facilitate the subsequent combustion step. The energy conversion efficiency, defined as the total energy contained in the syngas and tar at the gasifier exit divided by the LHV of the S-MSW, is selected as the indicator evaluating the performance of gasification, as shown in eq 2 2. The ER is the most important operating parameter in pyrolysis and gasification. 6 Results reveal that a higher ER enlarges the mass of product gas and diminishes simultaneously the tar and char yields. For example, at 650 °C, the syngas yield rapidly increases by approximately 39% as ER increases from 0 to 0.8, while the amount of tar reduces sharply from 11.0 to 0.6%. This phenomenon is well in accordance with the mechanism of gasification reactions. 1,[START_REF] Knoef | Handbook Biomass Gasification[END_REF] In general, gasification processes can be divided into two main subcourses. The first step is primary pyrolysis, involving the thermal decomposition of MSW that produces volatiles, tar, and char. The second step is the hetero-and homogeneous reactions under a reducing atmosphere, including reactions of O 2 , H 2 O, H 2 , and CO 2 with hydrocarbon gases and the remaining carbonaceous residues. The main reactions involved are summarized in Table 3. The partial oxidation (reactions R1-R4) will be strengthened at higher values of ER, thereby leading to a decrease in char and tar production and an increase in syngas yield.

= + + + + + - LHV (MJ
The temperature also plays an important role in S-MSW pyrolysis and gasification by affecting all chemical reactions as well as their chemical equilibrium. Table 2 shows that the syngas yield rises from 60.1% (ER = 0 and 550 °C) to 89.7% (ER = 0.8 and 850 °C), accompanied by a gradual char decrease of 31-57%. With an increasing temperature, the tar yield under ERs of 0 and 0.2 first increases to its maximum point at 650 °C and then drops. As for an ER higher than 0.4, the tar yield decreases continuously when the temperature is raised from 550 to 850 °C, in a range of 45-66%. The Waterloo concept is widely accepted to account for the phenomenon: [START_REF] Van De Velden | Fundamentals, kinetics and endothermicity of the biomass pyrolysis reaction[END_REF] during biomass decomposition, char, gas, and tar are first formed, followed by the secondary reactions that convert part of the tar into an additional amount of gas and char. With regard to the present study, a higher temperature provides more favorable conditions for primary decomposition of char, which is converted into tar and gas vapors. Subsequently, the secondary cracking of tar (reaction R12) becomes strengthened as the temperature further increases, leading to a decrease in tar production to form more gas.

To ensure a good performance of the pyrolysis/gasification process, the conversion of MSW should be guaranteed in a high level. Syngas production can be viewed as one of the indicators reflecting this reaction degree. Figure 2 plots the mass distribution of syngas as a function of the ER and temperature. With an increasing ER, the syngas distribution can be divided into two separate linear-fitted curves. The slope of the curves at lower ERs is pretty large, indicating that the syngas is produced rapidly and ER is the main factor controlling the reaction. The increase of syngas generation becomes slower at higher ER values. The fitted curves are crossed at an ER of around 0.4, which demonstrates that relatively high conversion has already been achieved at such an ER value. Nevertheless, this phenomenon is improper for a temperature of 550 °C; the syngas production still keeps rapid growth for further shifts of the ER value from 0.4 to 0.6. It could be concluded that a low temperature and ER are not suitable for the sufficient conversion of MSW.

3.1.2. Syngas Characteristics. The effect of the ER on syngas composition is exhibited in Figure 3. With an increasing ER, the concentration of CO 2 rises significantly, while combustible species, such as CO, H 2 , and hydrocarbons, show an inverse relation. For the entire range of ERs, the CO 2 content increases from 5.2 to 16.3%. The concentration of C 2 H 4 and CH 4 is decreased by 61 and 72% when the ER is increased from 0 to 0.8, respectively. CO and H 2 contents start to rise at lower ERs and later drop when the ER exceeds 0.2. In comparison to pyrolysis conditions (ER = 0), the growth of CO and H 2 observed at an ER of 0.2 can be attributed to the watergas reaction (reaction R5) and carbon partial oxidation (reaction R1), together with secondary cracking of tar (reaction R12), enhanced by the heat supply from char oxidation. However, as ER further increases, oxidation reactions dominate and CO and H 2 contents are quite linearly decreasing.

The variation of syngas composition as a function of the temperature is shown in higher temperature. Therefore, the increase in H 2 can be attributed to the water-gas reaction (reaction R5), steam reforming reaction (reaction R7), and dry reforming reaction (reaction R11), while the enhanced Boudouard reaction (reaction R10) and reversed water-gas shift reaction (reaction R6) are responsible for the increase in the CO concentration. Meanwhile, higher temperatures also accelerate CO 2 production. The result follows the same trend as the study by Li et al. [START_REF] Li | Experimental study on catalytic steam gasification of municipal solid waste for bioenergy production in a combined fixed bed reactor[END_REF] but is different from the study by Narvaéz et al. 6 The content of CO 2 is mainly controlled by the water-gas shift reaction (reaction R6) and cracking of tar (reaction R12). A higher temperature accelerates the reaction rate to produce more CO 2 ; at the same time, CO 2 is consumed as a result of an endothermic reversed water-gas shift reaction. The final concentration of CO 2 could be viewed as the competition between these two reactions.

With regard to the content of CH 4 and C 2 hydrocarbons, pyrolysis and gasification processes behave oppositely. For pyrolysis, CH 4 and C 2 hydrocarbons keep an upward trend with an increasing temperature, whereas under an ER of 0.4, they start to decrease at a temperature higher than 650 °C. CH 4 is mainly recognized as a product of pyrolysis. [START_REF] Goḿez-Barea | Modeling of biomass gasification in fluidized bed[END_REF] A higher temperature sharply impacts the heating rate and reaction intensity of the pyrolysis process, allowing for a greater production of light hydrocarbon gases. As ER increases, the heat supplied by oxidation promotes endothermic reforming reactions of hydrocarbon (reactions R7 and R11), which becomes more dominant at a higher temperature and results in the decrease of CH 4 and C 2 hydrocarbons.

Figure 5 dipicts the syngas LHV and combustible gas yield as a function of the ER and temperature. As presented in Figure 5a, the combustible gas yield first rises to its maximum point at an ER of 0.4 and then drops, indicating that increasing the ER to a high value produces syngas with a low quality. The LHV of syngas diminishes crucially with an increasing ER as a result of not only the oxidization of combustible gas but also the enlarging dilution effect of N 2 . The syngas LHV ranges from 1.9 to 6.0 MJ Nm -3 . Previous studies 6,29 have demonstrated that the limit gas quality required for downstream combustion is around 4.0 MJ Nm -3 . Therefore, ER values should not exceed ca. 0.5 to guarantee efficient subsequent energy utilization. With respect to the effect of the temperature in Figure 5b, a higher temperature enhances the syngas LHV and combustible gas yield and leads to better a gas quality for an ER of 0 (pyrolysis only). Nevertheless, at an ER of 0.4, the gas LHV drops when the temperature exceeds 650 °C, mainly because of the sustained decrease in CH 4 and C 2 hydrocarbon contents. It also indicates that an ER of 0.4 is effective to produce more combustible gases, but the syngas LHV is diminished simultaneously compared to pyrolysis conditions.

3.1.3. Energy Conversion Efficiency. Figure 6 presents the energy conversion efficiency as a function of the tested ER and temperature. Results demonstrate that ER = 0.4 and temperature = 650 °C are the preferred conditions for gasification and the highest efficiency of 68.5% could be obtained. ER = 0.2 and temperature = 750 °C also lead to good performance (65.8% efficiency). The energy losses during gasification may be attributed to several reasons. Unconverted carbon contained in the char makes up a non-negligible proportion of the total energy. On the other hand, heat losses, mainly by conducting and converting, are strengthened in a lab-scale experimental apparatus. The energy conversion efficiency drops if ER is too high because the thermochemical reactions inside the furnace gradually convert from gasification to combustion. [START_REF] Werle | Impact of feedstock properties and operating conditions on sewage sludge gasification in a fixed bed gasifier[END_REF] The result is in accordance with recent works. Arena 1 indicated that ER values around 0.25-0.35 appear to maximize char conversion. Niu et al. [START_REF] Niu | Simulation of syngas production from municipal solid waste gasification in a bubbling fluidized bed using Aspen Plus[END_REF] simulated the MSW gasification in a bubbling fluidized bed and observed that the optimum ER was 0.35. On the other hand, it is also found that the energy conversion efficiency of gasification decreases greatly when the temperature exceeds 650-750 °C. However, an increasing temperature has a positive effect on the pyrolysis process. At 850 °C, the energy conversion efficiency reaches a high level that is comparable to the optimal conditions aforementioned. Nevertheless, it needs to be emphasized that pyrolysis is an endothermic process; i.e., additional thermal energy is requested to maintain the reaction, whereas autothermal gasification could be achieved at an ER of around 0.3-0.4. [START_REF] Zhang | Thermodynamic evaluation of biomass gasification with air in autothermal gasifiers[END_REF] Research has shown that the energy input for pyrolysis at 500 °C equals 9% of the MSW high heating value. [START_REF] Baggio | Energy and environmental analysis of an innovative system based on municipal solid waste (MSW) pyrolysis and combined cycle[END_REF] It could be concluded that, under the test conditions, an ER of around 0.4 and a temperature of around 650 °C are the most suitable for the efficient conversion of MSW for a pyrolysis/ gasification-based WtE process.

3.2. Effect of the S-MSW Moisture Content. The effect of the S-MSW moisture content has been studied at ER = 0.4 and temperature = 650 °C. In viewing the syngas composition in Figure 7, it is seen that a 61% increase in the H 2 content is achieved on raising the moisture content to 51.7%, because the water-gas shift reaction (reaction R6) is favored at a higher steam partial pressure. CO is consumed producing CO 2 and H 2 , causing a decrease in CO and an increase in CO 2 accordingly. The contents of CH 4 and C 2 hydrocarbons decrease straightforward with an increasing water quantity as a result of enhanced reforming reactions.

The gasification performance for different S-MSW moisture contents is presented in Figure 8. When the moisture content is increased, the syngas LHV declines continuously from 4.5 to 3.2 MJ Nm -3 , while the energy conversion efficiency decreases from 68.5 to 44.9%. This indicates that the gain in H 2 of the syngas could not compensate for the loss of energy as a result of the reduced CO, CH 4 and C 2 hydrocarbon generation. Doherty et al. [START_REF] Doherty | The effect of air preheating in a biomass CFB gasifier using ASPEN Plus simulation[END_REF] reported a similar result that rising feedstock moisture degrades the gasifier performance. Because the moisture content of the raw MSW in China is relatively high (normally more than 40-50%), [START_REF] Huang | The current situation of solid waste management in China[END_REF] the input MSW should be predried prior to gasification. Nevertheless, evaporation of a high content of water consumes additional energy; therefore, it is necessary to adapt the S-MSW moisture content to a suitable level. McKendry 35 pointed out that fuel with a moisture content above 30% makes ignition difficult. With regard to the results in Figure 8, the syngas LHV drops rapidly if the moisture content is higher than 22.3%. Considering the balance between syngas quality and energy conversion efficiency, the moisture content of the feedstock should be lowered to at least ca. 20-25% to guarantee the gasification performance.

Comparison of Environmental Emissions between Gasification and Incineration.

For the overall feasibility of a MSW thermal technique, the environmental performance is also a crucial factor. Therefore, an analysis is conducted comparing the emissions from gasification and conventional incineration. The pyrolysis/gasification-based WtE process provides a potential benefit to realize premixed flames and reduce the excess air of the plant. Premixed flames can yield substantial reduction of NO x , 5 while lower excess air contributes to make air pollution control less costly and complex. Arena 1 has reviewed some certified emissions from several existing waste gasification plants. The data indicate that gasification is able to meet the regulatory constraints of both the European Community and Japanese standards. Moreover, this process also allows for raw syngas treatment between the gasifier and the combustor. Especially, HCl could be removed prior to combustion, thus offering opportunities to increase the steam superheater temperature or to use the syngas in higher efficiency applications, such as a gas turbine or reciprocating engine.

Toxic emissions, such as heavy metals and dioxins, are regarded as the major resistance from MSW thermal techniques. The partitioning of heavy metals during MSW gasification and incineration was compared by a series of tests in our previous study, using the same reactor and feedstock. [START_REF] Dong | Partitioning of Heavy Metals in Municipal Solid Waste Pyrolysis, Gasification, and Incineration[END_REF] It is found that gasification retains the volatilization of Cu, Ni, and Cr but favors Zn evaporation, because metals are mainly reduced to their elemental form or sulfide. Cd and Pb are active during both gasification and incineration processes; thus, effective pollution control devices should be equipped to minimize their environmental impact. Besides, the dioxin emission from waste gasification and incineration was also compared in our previous work. [START_REF] Dai | PCDD/Fs and PAHs emission in sewage sludge thermal treatment[END_REF] Results indicate that gasification could strongly inhibit the formation of dioxins, which could be served as powerful evidence to consider a pyrolysis/gasification-based WtE technique a competitor of the current MSW incineration.

CONCLUSION

Pyrolysis and gasification of MSW are investigated in a fluidized bed reactor. The effect of the ER, temperature, and moisture content has been studied. Results reveal that ER is the dominant factor determining both product distribution and characteristics. Increasing the ER boosts oxidation reactions, thus ultimately improving the syngas yield but decreasing its LHV. The combustible gas yield is strengthened at lower ERs as a result of the increase in CO and H 2 contents but later drops when the ER exceeds 0.4. The temperature is found to have a significant influence on the decomposition reactions. A higher temperature accelerates CH 4 and C 2 hydrocarbon production from pyrolysis but causes an evident decrease in syngas LHV during gasification. The optimal process performance under the specific input S-MSW and test conditions is found at an ER of 0.4 and a temperature of 650 °C, and the energy conversion efficiency, defined as the ratio of energy at the gasifier exit, is attained at 68.5%. Under such conditions, the increase of the MSW moisture content facilitates steam reforming reactions to increase H 2 production, yet the syngas LHV declines, lowering the energy conversion efficiency. To guarantee gasification performance, MSW should be predried to a moisture content of ca. 20-25%. An environmental analysis verifies that gasification is able to meet the emission limitations. In comparison to conventional incineration, the reductive atmosphere constrains the metal oxidation for potential heavy metal recycling and also the formation of dioxins is inhibited.

In conclusion, the present study assists in developing a pyrolysis/gasification-based WtE technique under the local conditions of China. It is expected that the data obtained can serve as a guideline for identifying more efficient and feasible MSW processes, and the accumulated experiences could render alternative MSW thermal techniques in China more competitive.

Figure 1 .

 1 Figure 1. Schematic diagram of the furnace: (1) screw feeder, (2) reaction chamber, (3) thermocouples, (4) controller, (5) cyclone separator, (6) heating tape, (7) temperature measurement, (8) induced draft fan, (9) serpentine condenser, (10) rotor flowmeter, (11) vacuum pump, and (12) accumulative flowmeter.

  3. RESULTS AND DISCUSSION3.1. Effect of the ER and Temperature. 3.1.1. Product Distribution. The product distribution of syngas, char, and tar is listed in Table

Figure 4 .

 4 Although different ERs are tested under specified temperature ranges, only cases of ER = 0 and 0.4 are discussed here because they are representative of reflecting the effect of the temperature. Results show that the contents of CO and H 2 are enhanced as the temperature increased from 550 to 850 °C. Similar tendencies have been found by Narvaéz et al. and Luo et al. 6,8 According to Le Chatelier's principle, the endothermic reactions are favored at a

Figure 2 .

 2 Figure 2. Effect of the ER and temperature on syngas mass distribution during S-MSW pyrolysis and gasification.

Figure 3 .

 3 Figure 3. Effect of the ER on syngas composition during S-MSW pyrolysis and gasification (temperature = 650 °C; the result of N 2 is not presented). The error bars represent the standard deviation of three independent duplicate tests.

Figure 4 .

 4 Figure 4. Effect of the temperature on syngas composition during S-MSW pyrolysis and gasification (the result of N 2 is not presented): (a) ER = 0 and (b) ER = 0.4. The error bars represent the standard deviation of three independent duplicate tests.

Figure 5 .

 5 Figure 5. Combustible gas yield and syngas LHV during S-MSW pyrolysis and gasification: (a) effect of the ER (temperature = 650 °C) and (b) effect of the temperature (ER = 0 and 0.4). The error bars represent the standard deviation of three independent duplicate tests.

Figure 6 .

 6 Figure 6. Effect of the ER and temperature on energy conversion efficiency during S-MSW pyrolysis and gasification. The error bars represent the standard deviation of three independent duplicate tests.

Figure 7 .

 7 Figure 7. Effect of the moisture content on syngas composition during S-MSW pyrolysis and gasification (studied at ER = 0.4 and temperature = 650 °C; the result of N 2 is not presented). The error bars represent the standard deviation of three independent duplicate tests.

Table 1 .

 1 Characteristics of S-MSW Samples

	proximate analysis (wt %, air-dried basis)
	moisture	9.2
	volatile	73.8
	fixed carbon	12.8
	ash	4.2
	lower heating value (MJ kg -1 )	22.5
	ultimate analysis (wt %, air-dried basis)
	C	48.4
	H	6.3
	O	29.2
	N	0.8
	S	0.3
	Cl	1.6

Table 2 .

 2 Effect of the ER and Temperature on Product Distribution a

	product mass distribution (wt %)	syngas yield (Nm 3 kg -1 of S-MSW)

a The error bars represent the standard deviation of three independent duplicate tests. b The moisture content is excluded. c For N 2 -free syngas yield, uncertainty values of 0.0 are presented under all experimental runs as a result of the significant digit limitation.

Table 3 .

 3 Main Reactions Involved during Waste Gasification 1

	Oxidation Reactions

  CH 4 , H 2 , C 2 H 2 , C 2 H 4 , and C 2 H 6 are the molar fractions of the components in the syngas and E syngas and E tar are the summations of chemical energy and sensible energy contained in the syngas and tar, respectively.

	syngas	Nm ) 3
	0.126CO 0.358CH 4	0.108H	2	0.560C H 2 2
	0.590C H 2 4	0.638C H 2 6		(1)
	energy conversion efficiency (%)	=	+ -S MSW E LHV E syngas tar	×	100%	(2)
	where CO,

■ AUTHOR INFORMATION

Corresponding Author *Telephone: +86-0571-87952687. Fax: +86-0571-87952438. Email: chiyong@zju.edu.cn.

Notes

The authors declare no competing financial interest.

■ ACKNOWLEDGMENTS

This project is supported by the National Natural Science Foundation of China (51276168) and the Program of Introducing Talents of Discipline to University (B08026).

■ REFERENCES

(1) Arena, U. Process and technological aspects of municipal solid waste gasification. A review. Waste Manage. 2012, 32 (4), 625-639.

( (3) Bridgwater, A. V. Review of fast pyrolysis of biomass and product upgrading. Biomass Bioenergy 2012, 38, 68-94.

(4) Noma, T.; Ide, K.; Yoshikawa, J.; Kojo, K.; Matsui, H.; Nakajima, R.; Imai, K. Development of waste gasification and gas reforming system for municipal solid waste (MSW). J. Mater. Cycles Waste Manage. 2012, 14 (3), 153-161.

(5) Consonni, S.; Vigano, F. Waste gasification vs. conventional Waste-To-Energy: A comparative evaluation of two commercial technologies. Waste Manage. 2012, 32, 653-666.

(6) Narvaéz, I.; Orío, A.; Aznar, M. P.; Corella, J. Biomass gasification with air in an atmospheric bubbling fluidized bed. Effect of six operational variables on the quality of the produced raw gas. Ind. Eng. Chem. Res. 1996, 35 (7), 2110-2120.