John Pachón-Morales

Julien Colin

Floran Pierre

François Puel

Patrick Perré
email: patrick.perre@centralesupelec.fr

Effect of torrefaction intensity on the flow properties of lignocellulosic biomass powders

Keywords: BtL, torrefaction, particles size and shape, lignocellulosic biomass, flowability

Flowability characterization of milled lignocellulosic biomass is essential for developing viable conveying, storing and handling solutions for gasification processes. This study investigated the effect of torrefaction on particles size and shape obtained after grinding and on flow properties of pulverized wood.

Spruce and poplar samples with six torrefaction intensities were knife-milled to obtain biomass powders.

Particles size and shape distribution were assessed using a morphological particle size analyser and flowability parameters were determined with a ring shear tester. A more intense treatment produces finer, rounder and more regular particles. Simultaneously, a gradual shifting was observed from a cohesive behaviour for native biomass to a nearly free flowing behaviour for the most intensively treated samples. The trends in flowability cannot be explained by the size reduction nor the increase of distribution width. Instead, the explanation lies in the reduction of shape factor and the sharpness of particle surfaces for treated samples. However, all observations are consistent with the loss of resilience of treated wood. From our results, it is clear that torrefaction, in addition to its interest on reduction of energy consumption of grinding, should also be considered as a pre-treatment step allowing to modify the flow behaviour of biomass powders.

INTRODUCTION

Effective use of biomass gains worldwide attention as a feedstock for production of biofuels likely to reduce greenhouse gas emissions and dependence from petroleum-based energy resources. After the first generation of biofuels, that consumes edible products, a second generation using the lignocellulosic part of plants -such as wood -started to be developed at the industrial scale for lignocellulosic ethanol, but still needs development to produce biomass-to-liquid (BtL) fuels. One thermo-chemical BtL route includes a gasification process followed by a Fischer-Tropsch synthesis [START_REF] Mckendry | Energy production from biomass (part 3): Gasification technologies[END_REF]. This thermochemical process requires a supply of ground biomass in the form of powder for three main reasons: i) to ease the conveying of the raw material along the processing chain, ii) to optimize its injection into the gasifier and iii) to increase the chemical reactivity of the biomass [START_REF] Mani | Grinding performance and physical properties of wheat and barley straws, corn stover and switchgrass[END_REF]. Grinding is then essential to obtain biomass powders meeting optimal conditions for gasification.

However, due to its fibrous and resilient structure, grinding of native lignocellulosic biomass is an energyintensive process. Therefore, a pre-treatment step, such as torrefaction, is needed to improve the process.

Torrefaction is a thermochemical treatment operating at temperatures in the range 200 to 300 °C under atmospheric pressure and in absence of oxygen. The process is traditionally characterised by low heating rates (<50 °C.min -1 ) and by a relatively long reactor residence time (typically higher than 30 minutes) [START_REF] Bergman | Torrefaction for biomass co-firing in existing coal-fired power stations[END_REF]. The resulting product is intermediate between wood and charcoal and exhibits several advantages when compared to the original material such as an increased energy density [START_REF] Almeida | Alterations in energy properties of eucalyptus wood and bark subjected to torrefaction: The potential of mass loss as a synthetic indicator[END_REF], a decreased hygroscopicity, a dimensional stabilization [START_REF] Almeida | Alterations in energy properties of eucalyptus wood and bark subjected to torrefaction: The potential of mass loss as a synthetic indicator[END_REF], an increased resistance to biological decay [START_REF] Hosseinpourpia | Mode of action of brown rot decay resistance of thermally modified wood: Resistance to Fenton's reagent[END_REF], a loss of mechanical resilience [START_REF] Almeida | Mild pyrolysis of fast-growing wood species (Caribbean pine and Rose gum): Dimensional changes predicted by the global mass loss[END_REF], etc. Several studies have shown the efficiency of heat treatment on easing biomass grinding. A reduction of 80-90 % of the power consumption needed for grinding torrefied biomass can be attained in comparison with raw biomass [START_REF] Bergman | Torrefaction for biomass co-firing in existing coal-fired power stations[END_REF]. This means a power consumption similar to that of coal. Previous research found a linear inverse correlation between the average particle diameter after grinding and the torrefaction temperature for wood chips and logging residues [START_REF] Phanphanich | Impact of torrefaction on the grindability and fuel characteristics of forest biomass[END_REF]. Pierre et al. developed an original impact device to evaluate the grindability of torrefied biomass [START_REF] Pierre | An original impact device for biomass characterisation: Results obtained for spruce and poplar at different moisture contents[END_REF]. The authors showed an energy consumption reduction of 86 % for oak torrefied to 38 % of mass loss and a reduction of 99 % for pine torrefied to 45 % of mass loss.

Currently, transport and handling of biomass powders are a key obstacle for the cost-competitive production of second-generation biofuels. Indeed, creating a steady flow of the biomass particles out of the storage equipment is a significant issue for biomass processing and conversion. From industrial experience, the most common cause of failure in continuous biomass valorisation facilities is feeding problems into the reactors [START_REF] Dai | Biomass feeding for thermochemical reactors[END_REF].

Notwithstanding the frequent occurrence of handling problems, there is still a relatively poor knowledge of the flow properties of biomass materials [START_REF] Miccio | Flow properties and arching behavior of biomass particulate solids[END_REF]. More research is therefore needed in this field to ensure the sustainability of the supply chain [START_REF] Mez | Research needs on biomass characterization to prevent handling problems and hazards in industry[END_REF]. This includes a better understanding of the combined effect of torrefaction and grinding parameters and the effects of particles shape and size distributions on powder flow properties. Flowability is often characterised through the unconfined yield strength (σ c ) which indicates the tendency of a bulk solid to form a cohesive arch in a hopper and to create ratholes in process equipment [START_REF] Johanson | Effect of particle shape on unconfined yield strength[END_REF].

Following the procedure described in Section 2.5, unconfined yield strength is obtained from the Mohr analysis of the yield locus (failure strength against load stress) of a granular material.

Regarding particle size, studies for coal-biomass systems have shown that bigger particles tend to reduce the bulk solids yield strength compared to finer particles [START_REF] Zulfiqar | Flow properties of biomass and coal blends[END_REF]. Several models have been proposed to relate the particle size properties to the bulk unconfined yield strength. So far, only inverse relationships between the particle size and the unconfined yield strength, whichever the mechanism considered, are proposed (Van der Waals Forces, capillary forces, elastic fracture, plastic-elastic fracture) [START_REF] Molerus | The role of science in particle technology[END_REF][15][START_REF] Specht | Caking of Granular Materials: an Experimental and Theoretical Study[END_REF]. These models have typically the following general expression [START_REF] Johanson | Effect of particle shape on unconfined yield strength[END_REF]:

n p C d K   (1)
Where σ c represents the unconfined yield strength, K and n stand for constants dependent on the material properties and d p is a particle size descriptor. Experiments conducted on flame retardant powders [START_REF] Abdullah | The use of bulk density measurments as a flowability[END_REF] showed that samples cohesion decrease with increase of particle size. Similar conclusions for black soybean powders, where fine fractions were reported as cohesive, were made by [START_REF] Lee | Flow behavior and hopper design for black soybean powders by particle size[END_REF]. Rohilla [START_REF] Rohilla | An experimental investigation on the effect of particle size into the flowability of fly ash[END_REF] concluded that there exists a particular particle size after which intermolecular forces become larger than weight forces, which triggered a decrease on flowability of fly ash samples.

The lack of suitable standard methods for measuring and classifying particle shapes, the heterogeneity of shapes and the fibrous nature of biomass particles makes it difficult to characterize the effects of these parameters on the flow properties [START_REF] Dai | Biomass feeding for thermochemical reactors[END_REF]. Yet, several studies assessed the effects of shape on unconfined yield strength and found that irregular shapes tend to increase the cohesive strength between particles. According to [START_REF] Johanson | Effect of particle shape on unconfined yield strength[END_REF], two key parameters influence the strength of a bulk powder system: the number of contacts per adjacent particles and the direction of these contacts relative to the centroid axis between adjacent particles. In the case of irregular particles, more effective contacts exist between particles and this situation can therefore increase yield locus values, resulting in a more cohesive behaviour. Several studies have shown that bridging properties of chipped solid biofuels are mainly controlled by the shape and the size of the particles [START_REF] Mattsson | Method and apparatus for measuring the tendency of solid biofuels to bridge over openings[END_REF]. A high proportion of hooked or long and thin particles increased the bridging tendency. Similar results were obtained by [START_REF] Paulrud | Particle and handling characteristics of wood fuel powder: Effects of different mills[END_REF] for ground pine and spruce. Hann [START_REF] Hann | Influence of Particle Size Distribution, Moisture Content, and Particle Shape on the Flow Properties of Bulk Solids[END_REF] found that powders with narrow particle size distributions had better flowability and that bulk solids with rounded edges resulted in a greater unconfined yield strength. Particle shape is also essential when studying the specific surface area that governs mass and heat transfer phenomena during gasification [START_REF] Lu | Effects of particle shape and size on devolatilization of biomass particle[END_REF][START_REF] Thunman | Combustion of wood particles -A particle model for Eulerian calculations[END_REF][START_REF] Jalan | 02102 Studies on pyrolysis of a single biomass cylindrical pellet kinetic and heat transfer effects[END_REF].

Even though particle size and shape are likely to have the greatest impact on powder flow properties, few studies have evaluated powder flow in relation to both, particle size and shape, especially regarding biomass powders, and even less knowledge is available about the effect of heat treatment on particles properties.

Previous works have reported the effect of torrefaction intensity as measured by the global mass loss (ML) on the flowability of biomass powders [START_REF] Pachón-Morales | Flowability of lignocellusic biomass powders: Influence of torrefaction intensity[END_REF]: a direct relationship between the ML and the flowability factor was observed. The current work investigates the effect of different torrefaction intensities on the flow properties of knife-milled biomass (spruce and poplar), and assesses the relations between:

-The torrefaction mass loss and the particle size and shape obtained after grinding -The particle size and shape and the granular material flow behaviour.

The results of this research are summarized as expressions between torrefaction operating conditions and physical properties and flowability of biomass particles. They are very useful in optimizing the BtL chain as they allow the compromise between energy loss due to torrefaction, grinding energy and flowability to be clearly assessed.

MATERIALS AND METHODS

Biomass preparation

A hardwood and a softwood species have been considered for this study. Poplar was chosen as hardwood (Populus euro-americana Koster) as it is a promising forest energy crop, namely due to its fast growth in temperate climates. The development of poplar genotypes with improved yield, higher pest resistance, increased site adaptability and easy vegetative propagation has made poplar a commercially valuable energy crop [START_REF] Jung | Evaluation of a transgenic poplar as a potential biomass crop for biofuel production[END_REF][START_REF] Dickmann | Poplar culture in North America[END_REF]. The poplar tree selected for the present study came from a forest located in La Suippe valley in Auménancourt-le-Petit (France). Spruce (Picea abies), a softwood species, is another potential candidate to be used in BtL chains, mainly because of its intensive current use in the forestry industry. A spruce tree coming from a plantation in the Le Châtaignier forest in Riotord (France) was selected. Both trees were chopped and cut in boards that were subsequently dried.

The sample size was chosen to fulfil several constraints such as the amount required for flowability tests (30 cm 3 of powder needed), the grinding capacity of the grinder (maximum length) and the homogeneity of the heat treatment (small thickness). The size of each individual wood chip was thus fixed to 20x20x3 mm 3 . 1 consists of 32 chips of this size. The basic densities (oven-dry mass over saturated volume) are 311 ± 9 kg/m 3 and 330 ± 10 kg/m 3 , for untreated poplar and spruce respectively. The respective growth ring widths are 14 mm and 2.85 mm. As it is well-known that wood properties present little changes in the longitudinal direction, all samples were collected along a single board to reduce the intra-tree variability. To further average the remaining variability and obtain twin samples, the 32 wood chips of each sample were selected following a recurrent stepped scheme along the board [START_REF] Pachón-Morales | Flowability of lignocellusic biomass powders: Influence of torrefaction intensity[END_REF]. In agreement with previous findings of [START_REF] Almeida | Mild pyrolysis of fast-growing wood species (Caribbean pine and Rose gum): Dimensional changes predicted by the global mass loss[END_REF][START_REF] Kamdem | Durability of heat-treated wood[END_REF][START_REF] Avat | Contribution à l ' étude des traitements thermiques du bois jusqu ' à 300[END_REF], the mass loss values confirm the lower thermal resilience of poplar compared to spruce (Table 1). This is mainly explained by the lower resistance of hardwood hemicelluloses and lignins to heat treatment [START_REF] Repellin | Modelling anhydrous weight loss of wood chips during torrefaction in a pilot kiln[END_REF][START_REF] Prins | Torrefaction of wood. Part 2. Analysis of products[END_REF].

Each sample of Table

Torrefaction

A batch torrefaction furnace specifically developed in the laboratory was used to treat the samples [START_REF] Colin | Séchage en continu du bois énergie comme moyen de conditionnement en vue de sa conservation thermochimique : approches expérimentale et numérique[END_REF]. The atmosphere of a hermetically closed Memmert UFP400 chamber is controlled by sweeping a nitrogen flow (5 L.min -1 ) to reduce the oxygen content, thereby avoiding oxidation and ignition. In all cases, the oxygen level, as measured at the gas outlet, remained below 1.5 %. A powerful fan inside the chamber ensures an efficient heat transfer either to heat-up the sample or to limit thermal overshot due to exothermic reactions.

The gas temperature was measured nearby the samples using a K-thermocouple.

A total of six treatments were performed at 220, 250 and 280 °C for 1 or 5 hours with the following protocol: (i) heating from room temperature at a rate of 5 °C min -1 up to 100 °C (ii) plateau at 100 °C for 30 minutes to remove the residual bound water (iii) heating at a rate of 5 °C min -1 until the treatment temperature.

(iv) plateau at the treatment temperature for the desired duration and (v) cooling down thanks to thermal losses and an increased nitrogen flow entering the reactor. Preliminary simulations were performed using a comprehensive computational code to ensure that the treatment duration together with the sample thickness ensures the treatment to be quasi-uniform within the sample [START_REF] Perré | A comprehensive dual-scale wood torrefaction model: Application to the analysis of thermal run-away in industrial heat treatment processes[END_REF].

After treatment, samples were weighed to determine the mass loss (ML) due to heat treatment as follows:

100 (%) 0 0    m m m ML t (2)
Where, m 0 and m t are the oven-dry mass before and after torrefaction, respectively. Mass loss is known to be a good indicator of the treatment intensity [START_REF] Almeida | Alterations in energy properties of eucalyptus wood and bark subjected to torrefaction: The potential of mass loss as a synthetic indicator[END_REF] and has been successfully correlated to several properties of the treated biomass such as energy properties [START_REF] Pierre | An original impact device for biomass characterisation: Results obtained for spruce and poplar at different moisture contents[END_REF] or dimensional changes [START_REF] Almeida | Mild pyrolysis of fast-growing wood species (Caribbean pine and Rose gum): Dimensional changes predicted by the global mass loss[END_REF].

Grinding procedure

Heat-treated samples were ground in a M20 IKA laboratory 550W knife miller. An automatic control system was implemented to guarantee the repeatability of the grinding procedure. The rotational speed of the knives (stainless steel cuter M21) was 20 000 rpm. To avoid any additive heat treatment due to thermal dissipation, several grinding and resting cycles were carried out, up to a total grinding time of 95 seconds. A circulation of cold water inside the grinder jacket further reduced sample heating.

Particle size and shape analysis

A Sympatec-QICPIC morphological particle size analyser with a M7 lens (measurement ranges from 10 to 10240 µm) with a GRADIS dispersion and VIBRI feeding units was used in this work [35]. To conduct the test, about 5 g of wood powder was loaded onto the hopper of the instrument. The size and shape of powders were recorded and analysed using the manufacturer software.

The mean value of the Feret diameters over all orientations of the particle) is used as the magnitude characterizing particles size. The 50 th centile of the cumulative volume distributions (x 50 ) was taken as a mean size descriptor of each size distribution. The particle shape was characterised through the circularity, ψ (from 0 to 1), which is defined as the ratio of the perimeter of a circle having the same area as the projected area of a particle to its actual perimeter. A perfect sphere would give ψ =1 and the smaller the value of ψ, the more irregular is the shape of the particle. Mean values of circularity ψ 50 were calculated as the 50 th centile of the cumulative circularity distributions.

The flow of a polydispersed bulk solid primarily depends on flow properties of the fines fraction [START_REF] Zulfiqar | Flow properties of biomass and coal blends[END_REF],

the size analysis of particles having a size under 100 µm was made for all samples. Hence the sieve fraction of the samples with a size below 100 µm was analysed. This fraction was obtained by using a vibratory sieve shaker

Retsch AS 200 at a frequency of 60 Hz for 20 minutes. Particles size and shape were analysed using a M4 lens allowing a better resolution for finer particles in the range from <1µm to 1700 µm. The use of this lens needs samples to be fed as a liquid dispersion. Biomass powders were first coloured by means of a 24-hour soaking in a Safranin-O -10 % ethanol solution to avoid agglomeration, then diluted in 500 mL of water. This solution was pumped into the LIXELL wet dispersion system. Results presented are the combination of three repeatable tests.

Flowability tests

A RST-XS Schulze ring shear tester was used to assess the flow properties of biomass powders. The ring shear tester is a widely-used device to measure flow properties of powders, including angle of internal friction, wall friction and bulk density. The standard procedure leads to results with low variability [START_REF]Standard Test Method for Bulk Solids Using Schulze Ring Shear Tester 1[END_REF].

According to this procedure, once the shear cell with the powder sample has been prepared, the maximum normal preshear stress, σ pre , is applied on the cell lid. A typical evolution of the shear stress obtained from torque measurements is represented in Figure 1a. The pre-shearing step is carried out up to the attainment of a steady state value of shear stress in which the frictional forces between particles are maximum with respect to the applied normal load. At this point the powder attains a well-defined and reproducible state of consolidation corresponding to the top point of the yield locus represented in Figure 1b. This steady state often occurs when the sample volume and thus its bulk density, ρ b , reaches a steady state value at the end of compaction [START_REF] Schulze | Flow properties of powders and bulk solids (fundamentals)[END_REF]. After the preshearing, the direction of shear is reversed, i. e. the shear stress is reduced to zero. Then, the normal stress is reduced to a value σ sh,1 < σ pre and the specimen is sheared until a peak value of shear stress, τ sh is reached (incipient flow or failure) [START_REF] Miccio | Flow properties and arching behavior of biomass particulate solids[END_REF]. The sequence of two steps is repeated with the same σ pre and increasing σ sh . Finally, a verification point at σ sh,1 is made. The (σ, τ) couples obtained at failure are drawn to represent the yield locus corresponding to the consolidation applied as shown in Figure 1b).

A set of three pre-consolidation stresses (σ pre ), considered as representative of the stress range for industrial applications, were tested: 2, 5 and 10 kPa [START_REF] Schulze | Powders and Bulk Solids[END_REF]. Three shear points at 25, 50 and 75 % of σ pre for each pre-consolidation stress were used to determine the yield locus of each sample. The yield locus curves were regressed from experimental points by a linear regression.

As represented in Figure 1 b), the Mohr circle drawn through steady state point and tangent to the yield locus locates the major principal stress σ 1 , corresponding to the consolidation applied. The unconfined yield strength is given by the major principal stress passing through the origin and tangent to the yield locus. In Figure 1b), the dashed line passing the origin and tangent to the larger Mohr circle is the effective yield locus. Its angle of inclination with respect to the σ axis is the effective angle of internal friction ϕ e . Yield locus for a non-cohesive granular material passes through the origin, so cohesion, C, corresponds to the value of the shear stress where the yield locus intersects with the τ axis, i.e., at the normal stress σ = 0. By measuring yield loci for different preconsolidation stresses a flow function (σ c vs. σ 1 ) can be drawn.

Unconfined yield strength governs the stress holding the material together on a free surface. It is the major principle stress that acts in a direction parallel to the free surface which supports the external forces tending to tear the surface apart [START_REF] Jenike | Gravity flow of bulk solids[END_REF]. The bigger its value, the higher the stress required to fail or fracture the granular material to initialize the flow. The unconfined yield strength indicates the tendency of a bulk solid to form a cohesive arch in a hopper and form ratholes in process equipment [START_REF] Johanson | Effect of particle shape on unconfined yield strength[END_REF].

The ratio FFC (flow function coefficient) of consolidation stress to unconfined stress (σ 1 /σ c ) is used to quantify the flowability. The larger FFC, the better a bulk solid flows. All properties obtained from the Schulze ring shear tester are usually reported as a function of the major principal stress of consolidation.

RESULTS

Effect of torrefaction intensity on particle size and shape distributions

A surface representation (Figure 2) was chosen to represent both the shape factor and the particle size distributions for three degrees of torrefaction: untreated, mildly treated (250 °C, 1 hour) and intensively treated (280 °C, 5 hours). A gradual shifting from large particles with relatively low circularity towards finer particles and higher values of circularity becomes evident when increasing the torrefaction intensity. This reduction of particle size is a direct effect of the loss of resilience [START_REF] Pierre | Reduction of biomass resilience by torrefaction: apparent stiffness during failure (ASF) and specific failure energy (SFE) assessed by a custom impact device[END_REF] . This is also the primarily reason of using torrefaction as pretreatment on the BtL chain: to obtain small and reactive particles at a lower grinding energy cost. These 2D graphs deserve a deeper analysis of the concomitant changes of size, circularity and peak intensity. For poplar powders, the peak value for circularity changed from ψ = 0.6 to ψ = 0.75 (increase of 25%) from untreated to the most treated samples. For spruce, circularity peak values increased from ψ = 0.55 to ψ = 0.77 (increase of 40%). This increase of circularity with torrefaction was already observed for pine [START_REF] Phanphanich | Impact of torrefaction on the grindability and fuel characteristics of forest biomass[END_REF].

From the colour scale, it is also noticeable that, for both species, a spreading of the distributions occurs at increased heat treatment intensity. For poplar, the initial, sharp, peak at 2 mm evolved progressively towards a wider peak spreading towards smaller and more spherical particles. Spruce presents a different behaviour as two populations gradually appear. The initial peak is slightly shifted towards smaller particles (1800 µm, 1500 µm and 600 µm, from untreated to 7 % and 38% of ML) and progressively disappears. Another peak is already visible at ML = 7.1% at a mean particle size of ca. 300 µm. For the most severe treatment (ML = 38.1 %) the initial peak almost disappeared and the second peak represents a large part of the power volume, with smaller (ca. 100 µm) and round particles (ψ = 0.77).

Figure 2. Influence of torrefaction intensity on circularity and size distributions for spruce and poplar powders. The colour scale indicates the volume fraction of each size and shape range and varies between the same values regardless of the torrefaction intensity

for each species. This synthetic representation allows the size reduction at increasing heat treatment intensities to be quantified.

The trends reported in this figure confirm literature data [START_REF] Mani | Grinding performance and physical properties of wheat and barley straws, corn stover and switchgrass[END_REF][START_REF] Phanphanich | Impact of torrefaction on the grindability and fuel characteristics of forest biomass[END_REF][START_REF] Himmel | Comminution of biomass: hammer and knife mills[END_REF]. In the case of poplar, for the highest values of mass loss, the cumulative distributions tend to overlap. This indicates that a plateau in particle size seems to be reached after a certain treatment intensity. Again, spruce presents a different behaviour, as a significant shifting of the particle size distribution towards finer particles is still observed for the most severe treatments.

These trends are confirmed by the analysis of the fines fraction (Figure 3, bottom). For poplar, the fines fraction presents a plateau at ca. 7 % of the total particle volume and the three most severe treatments (21% to 47% of mass loss) are very close. On the contrary, a large increase of the fines fraction is observed for ML = 38%: almost 40% of the particle volume. It is also obvious that the particle size distribution continues to shift towards small particles. were also fitted from these experimental data. The fitted parameters are presented in Table 2. These models imply an asymptotic behaviour whose limit at "infinite torrefaction intensity" equals 427 µm for poplar and 226 µm for spruce. The general trend clearly confirms the loss of resilience at increasing treatment intensity. A similar behaviour is observed for the two species. Yet, larger particles are obtained for poplar compared to spruce at similar torrefaction intensity. For spruce, the particles size is reduced by 88% for untreated to severely treated wood (ML = 38%). The reduction is only by 68% for poplar. A higher effect of thermal treatment on the particle size reduction is therefore evident for spruce, even though the ML presents an opposite trend (lower values for spruce than for poplar for given torrefaction conditions). This result is quite surprising as the alteration of lignins and hemicelluloses by heat treatment explains the increase of cell wall brittleness. The native lignins of conifers are essentially constituted of guaiacyl (G) units together with a low proportion of p-hydroxyphenyl (H) units (except in compression wood rich in H units).

Along with the G units and traces of H units, the lignins of deciduous trees are constituted of syringyl (S) units.

The proportion of resistant bonds in native lignins increases with the frequency of G and H units whereas S units are mostly involved in weaker β-O-4 linkages. From this molecular point of view, poplar resilience should therefore be more deeply altered by heat treatment than spruce.

As the observation regarding PSD depicts an opposite trend, the explanation is likely to rely on the difference in anatomical structure. Poplar is a pore-diffuse hardwood species which presents two levels of porosity: vessels and cell lumen (fibers and parenchyma cells). This heterogeneity creates various possible fracture pathways of the tissue that permits large particles to be formed. On the contrary, spruce is made of tracheids for 95% of its volume. Consequently, the formation of particles necessarily required fractures at this unique pore scale. This produces smaller particles, namely single tracheids or fractions of tracheids and a significant proportion of fines particle even at low treatment intensity. Obviously, this trend is stronger at higher ML levels, due to its effect in weakening the cell wall. The consequence on the proportion of fines (particle < 100 µm) is impressive: whereas both species depict a linear increase with mass loss, the proportion of fines increases up to 40% for spruce, against 8 % for poplar.

Finally, two synthetic indicators, in addition to the 50 th centile values, are proposed to quantify the spreading of the distributions, the span criteria: S X for size and S ψ for circularity, calculated from the respective 90 th and 10 th centiles of cumulative distributions as follows:

S         (4)
Where 90

x and 10 x stand for the 90 th and 10 th centiles of the PSD and 90  and 10  represent the 90 th and 10 th centiles of circularity distributions.

These criteria are reported in Table 3. The span of the particle size distribution tends to increase with the increase of torrefaction intensity. This trend is clearer for poplar than for spruce. However, for both species, an important increase from torrefied to untreated samples is observed (+50% for spruce and +19% for poplar).

Powders of torrefied wood exhibit a more polydispersed population with smaller sizes than powders of native wood. The span of circularity distributions did not depict any clear tendency.

Effect of torrefaction intensity on flow properties of biomass powders.

Flow functions presented in Figure 5 give information about the different flowability regimes by considering the ratio between the unconfined yield strength and the major consolidation stress (Jenike, 1964).

A FFC value below 1 indicates a not flowing behaviour, while a value of 4 represents the transition from cohesive to easy-flowing. Above 10, the powder is considered as free-flowing. The graphs obtained for our samples depict a clear effect of the heat treatment intensity on the evolution of these flow functions, either for spruce or for poplar.

For poplar, the curve obtained for the powder of native biomass is close to the line FFC = 4. It is therefore at the transition between cohesive and easy-flowing behaviour. Starting from this value, a gradual and regular shift of the curves towards higher values of FFC is evident. The most intense treatment lies close to FFC = 10, which is the transition between easy-flowing and free-flowing.

Spruce samples depict the same trend, but over a larger amplitude. Untreated biomass is at the middle of the cohesive zone, whereas the curve obtained for the most intense treatment (ML = 38 %) the FFC = 10

boundary. One has also to notice that the progression observed for spruce is not as regular as for poplar.

Figure 5. Effect of torrefaction intensity (ML) on flow properties for a. poplar and b. spruce powders (σ pre = 5 kPa).

These trends encouraged us to use the mass loss due to heat treatment (ML) as indicator of the flowability of biomass powder. As stated in this introduction, this parameter already proved to be an excellent synthetic indicator [START_REF] Almeida | Alterations in energy properties of eucalyptus wood and bark subjected to torrefaction: The potential of mass loss as a synthetic indicator[END_REF] for other biomass energy and mechanic properties. To that purpose, the value of FFC obtained with a pre-consolidation stress of 5 kPa was plotted as a function of ML for both species (Figure 6).

Except one point for spruce, the quasi-perfect linear relationship obtained is remarkable. Similar trends were found for the other consolidation stresses studied (see the Annexe). Both species showed a quantitatively similar trend. Flowability factor increased by 105% and 118% for poplar and spruce, respectively, from the untreated to the most intensively treated samples. Mass loss could then be used in industry for practically assessing the powder flowability of biomass powder under consolidation, since its ease of obtaining. In addition, both species are on the same line. By removing the singular point found for spruce at ML = 14.4 %, a unique, linear, expression can be proposed to predict the effect of mass loss on flowability, valid for all the consolidation stresses studied (r 2 =0.80): In this expression, that should be considered has one of the major outcomes of the present work, FFC untreated represents the flowability factor (dimensionless) of powder from native biomass and ML is the mass loss due to heat-treatment (kg/kg). As a conclusion from this trend it can be seen that, in contrast with raw biomass, torrefied wood particles have higher flowability, which will ease the injection of pulverized biomass into boilers in gasification units. The previous results told us that:

1 2.81 untreated FFC ML FFC   (5)
1. The treatment intensity allows smaller and more spherical particles to be produced by grinding, 2. The flowability of particles obtained from heat-treated wood increases significantly and linearly with mass loss.

It is therefore likely to go further in the explanation and to investigate if the evolution of the flowability can be explained by the particle size and shape. To that purpose, Figure 7 represent some of flow properties as a function of particle size and circularity. Either for poplar or for spruce, the unconfined yield strength increases with particle size and decreases with circularity. Similar trends were found for the other consolidations stresses studied and presented in Annexe. The third plot (Figure 7c) represents the value of FFC obtained with a preconsolidation stress of 5 kPa in a 3D plot, which depicts the concomitant change of size and circularity. This last plot summarises the effect of heat-treatment on flowability:

-Heat-treatment reduces the mean particle size and increases the circularity, -The cumulated of these two trends increases the powder flowability.

As stated in previous works [START_REF] Schulze | Flow properties of powders and bulk solids (fundamentals)[END_REF][START_REF] Lu | Effect of particle size on flow mode and flow characteristics of pulverized coal[END_REF][START_REF] Liu | An investigation of the effect of particle size on the flow behavior of pulverized coal[END_REF][START_REF] Fu | Effect of particle shape and size on flow properties of lactose powders[END_REF], for a given shape, flow properties tend to improve with mean particle size. As particles become finer, the van der Waals forces of attraction between them increase relatively because of their larger specific surface area (m 2 /g). During shear cell testing, there is a larger number of particleparticle contacts on the shear failure plane for a powder comprised of smaller particles (Hou & Calvin, 2008).

Consequently, the total area of contact is larger for smaller particles. At the same time, particles with less mass are subject to lower gravitational forces.

The net result of these effects is the shifting of the yield locus towards higher stresses and thus, a larger value unconfined yield strength and smaller flowability, which is opposite to the clear trends observed in the present work. Other effects are therefore likely to counterbalance the sole effect of particle size. In addition to the mean size, the width of size distribution might be involved. As reported in [START_REF] Schulze | Powders and Bulk Solids[END_REF], qualitatively speaking, flowability of bulk solids having the same median, x 50 , increases with decreasing width of the particle distribution. Again, this observation would produce an effect opposite to our observations: Table 2 tells us that the intensity of heat-treatment increases the width of the particle distribution, yet the flowability increases.

Polydispersity effects were also studied by [START_REF] Schulze | Flow properties of powders and bulk solids (fundamentals)[END_REF] for a mixture of coarse and fine lactose particles. The larger the percentage of the finer component, the more the yield locus was shifted towards higher stresses and the smaller the flowability. Interestingly, this trend was not observed for torrefied biomass powders.

Additional explanations have therefore to be found. The flowability improvement could then be triggered by the change in the shape factor of the particles. Generally, when considering coarse particles, smooth, spherical particles flow better than rough, sharp-edged, non-spherical particles. This has been demonstrated for different kinds of granular materials: [START_REF] Lewandowski | A Preliminary Investigation Concerning The Effect Of Particle Shape On a Powder's Flow Properties[END_REF] found that the higher the mean circularity the lower the cohesion for glass beads samples. Deviation of particle shape from sphericity led to higher angles of repose for microcrystalline cellulose [START_REF] Horio | Effect of particle shape on powder flowability of microcrystalline cellulose as determined using the vibration shear tube method[END_REF] and higher compressibility in lactose powders [START_REF] Fu | Effect of particle shape and size on flow properties of lactose powders[END_REF]. [START_REF] Danjo | Effect of particle shape on the compaction and flow properties of powders[END_REF] also concluded that there is an increase in the flowability of bulk solids when there is an increase in the shape factor of the particles. For untreated biomass powders, the steric repulsion and frictional forces that are a consequence of the particle geometry lead to a greater extent of inter-particulate friction and locking. Hence, yield loci are shifted to bigger values when compared with torrefied samples, resulting in a decreased flowability. Similar findings have been reported previously by [START_REF] Goh | Comparative evaluation of powder fl ow parameters with reference to particle size and shape[END_REF] for pharmaceutical powders, where some flowability properties such as the internal angle of friction where primarily influenced by particle shape and relatively independent of particle size.

Another effect to take into consideration, regarding polydisperse systems such as biomass powders, is the lubricant effect of fine particles on coarser ones. Flow agents prevent particle surface from direct contact, reducing inter-particle adhesion forces and improving flowability. In the case of intensively torrefied powders, where very fine-grained powders were obtained and therefore the adhesive force between the biggest particles would be reduced due to the increased inter-particle distance caused by very small covering particles acting as lubricant. However, flow agent particles must be sufficiently small in order to limit the adhesive forces of the flow agent particles themselves. The bigger shifting of flow functions from cohesive to free-flowing behaviour shown in Figure 5 for spruce powders could be seen as a result of the higher amount of very fine particles compared to poplar samples. Since flow agents are often based on nanosized particles, a granulometric analysis at nanometric resolution would be useful for further validation of this hypothesis.

Besides size and shape modifications, torrefaction is likely to alter the surface properties of biomass powders as well. The hydrophobic nature of torrefied samples could affect flow properties trough the electrostatic forces in two ways. Firstly, as the presence of bound water lowers the electrical resistivity, raw samples (which tend to capture moisture more easily) will tend to dissipate the electrostatic charges better.

Secondly, as indicated in previous research [START_REF] Biegaj | Surface Chemistry and Humidity in Powder Electrostatics: A Comparative Study between Tribocharging and Corona Discharge[END_REF], the electrostatic properties of the particles are directly related to the surface functional group chemistry: hydrophobic groups accumulate greater quantities of charge than hydrophilic groups. All of this would mean that, all other parameters being the same, torrefied samples are more likely to present flow problems linked to charge build-up. However, flow problems induced by accumulation of electrostatic charges are mainly to be expected in free surface flows of fine powders or when the powder is in contact with non-conductive surfaces. In closed systems, as the ring shear tester, the effect of electric charges could thus be mainly neglected. As the samples were oven-dried prior to the flowability measurements, the formation of liquid bridges at the particle scale (which could increase cohesion) was also neglected.

SEM images of particles were grabbed for spruce and poplar for different treatment intensities using a FEI Quanta electron microscope (Figure 8). When the treatment intensity increases, a decrease of particles length for a relatively similar width can be observed. This means a reduction of particles elongation which generally goes hand-in-hand with a circularity increase, as measured by the granulometric analysis made. The changes of circularity are very gradual, especially at low treatment intensities (Table 3). They become significant for intense treatments (for poplar, average circularity increased by +26 % compared to the raw biomass). SEM images are rather intended to show tearing, pulling-out and breaking profiles, that are affected by torrefaction, in addition to the shape changes (that are better evaluated statistically by the granulometric analysis). For untreated samples, the fibrous and resilient behaviour of wood produces irregular needle-like particles, with tearing and pull-out profiles. Pulled-out fibres together with rather elongated particles certainly promote particle agglomeration, which is likely to reduce powder flowability. On the contrary, sharp breaking surfaces are observed for torrefied samples. These observations confirm the conclusions of both shape and size analysis and are in good agreement with those done by [START_REF] Almeida | Caractérisation mécanique du produit traité et mise en relation des taux de dégradations avec les propriétés mécaniques[END_REF][START_REF] Arias | Influence of torrefaction on the grindability and reactivity of woody biomass[END_REF]. The effects of loss of resilience on the particle shape becomes thus evident, from native to severely treated biomass. As a conclusion of this discussion, we proved that the increase in flowability of wood powder with heat-treatment intensity is in complete opposition with the reduction of mean particle size or the increase in distribution width. Therefore, different effects should be involved to counterbalance the effect of particle size and to explain the clear improvement of flowability with heat treatment. Two major effects were pointed out.

Both are tied to the loss of resilience of the fibrous behaviour of native biomass: i) the reduction of the shape factor (higher particle circularity) and ii) the much higher regular and convex shape of particles.

CONCLUSIONS

The study, focused on two wood species, poplar and spruce, investigated the effect of torrefaction on the particles size and shape obtained after grinding and on the flow properties of milled wood. Overall, a gradual shifting of the particle size and shape distributions towards finer and rounder particles was observed when increasing treatment intensity. At the same time, a significant improvement of the flowability with torrefaction intensity was also clearly exhibited. Milled biomass gradually shifted from a cohesive character for the native samples to a nearly free flowing behaviour for the most intensively treated samples. Considering these observations, a unique linear expression between torrefaction intensity and flowability factor was proposed for the two-species studied here.

Finally, we proved that the concomitant evolution of particle sizes and distribution width and flowability parameters are not consistent as smaller particles should exhibit lower flowability. Indeed, the increase of flowability is mostly due to rounder particles with sharp surfaces of treated biomass compared to long and irregular needle-like particles obtained for native biomass. Both consequences, flowability improvement and size/shape factor reduction, are therefore explained by the same effect: the loss of resilience of the fibrous structure of raw wood by heat-treatment.

APPENDIX: Additional flow properties: angles of friction, cohesion and bulk density.

The friction angles are parameters related to the inter-particle friction of bulk solids and are one of the most used parameters to characterize their handling behaviour. The angle of internal friction ϕ lin is obtained from the slope of the linearized yield locus and the slope of the effective yield locus represents the effective angle of friction ϕ e [START_REF] Schulze | Powders and Bulk Solids[END_REF].

Cohesion characterizes the shear stress at yield under zero normal stress, i.e. the intersection of the yield locus with the ordinate. Bulk density is defined as the ratio of the mass of an amount of bulk solid to its volume. It is usually function of the consolidation stress. In the Schulze ring shear tester, the bulk density is calculated from the total charged mass of sample and the volume considering the lid displacement for a given consolidation stress [START_REF] Schulze | Powders and Bulk Solids[END_REF].

Erreur ! Source du renvoi introuvable. presents the values of the linearized internal angle of friction and the effective angle of internal friction resulting from the Schulze shear cell tests. There is not much difference between the angles for poplar and spruce showing that there is a similar interparticle friction when the two species are compared. Variations of angles between the different degrees of torrefaction intensity are not very clear and are often below the standard variation of the set of measurements (around 1°). These results are comparable with those obtained by [START_REF] Wu | Physical properties of solid biomass[END_REF] for poplar chips (φ lin ≈ 46°).

Regarding cohesion, it follows the same trend than the unconfined yield strength: a more cohesive behaviour is observed for the untreated powders then a gradual decrease follows until the minimal value for the most intensively torrefied samples. Like the other flow properties, cohesion is strongly dependent on the consolidation stress: with increasing consolidation stresses the yield locus skewed towards greater shear stresses because of the greater shear stress necessary to initiate flow. When the two-wood species are compared, relatively more cohesive behaviour was found for the spruce samples than for poplar powders. Bulk density was found to be directly dependent on the torrefaction intensity of the samples and the consolidation stress applied. Most intensively treated samples trigger denser powders after grinding that untreated ones. This is due to the reduction of both inter-and intra-particle voids generated after grinding with the reduction of particle size. This behaviour agrees with the observations by Mani et al [START_REF] Mani | Grinding performance and physical properties of wheat and barley straws, corn stover and switchgrass[END_REF]. The increase of bulk density with torrefaction intensity is important: an average of 43% is gained for the most intensively torrefied poplar sample with respect to the untreated sample. For spruce an average increase in bulk density of 58% was observed.

Figure 1 .

 1 Figure 1. Flow properties determination (Untreated poplar sample, σ pre = 5 kPa). a). Experimental course of shear stress; b) yield locus construction and Mohr stress circles defining unconfined yield strength and consolidation stress.

Figure 3 (

 3 Figure 3 (top) presents the cumulative volume particle size distribution (PSD) for poplar and spruce.

Figure 3 .Figure 4 .

 34 Figure 3. Influence of torrefaction intensity (Mass Loss ML) on PSD: whole sample (top) and fines fraction (bottom)

Figure 6 .

 6 Figure 6. Effect of the torrefaction intensity on the flowability factor of poplar and spruce powders (for clarity purposes only values at σ pre = 5 kPa are presented).

4 .

 4 DISCUSSION ON THE COMBINED EFFECTS OF PARTICLE SIZE/SHAPE AND TREATMENT INTENSITY.

Figure 7 .

 7 Figure 7. Effect of particle size and shape on flow properties for poplar and spruce powders: a. b. Unconfined yield strength, c. flowability factor (σ pre = 5 kPa).

Figure 8 .

 8 Figure 8. Scanning Electronic Microscopic images for untreated and torrefied spruce and poplar powders.

Table 1 .

 1 Torrefaction conditions and global mass loss for the 14 samples studied.

Torrefaction temperature (°C) Duration of torrefaction (h) Global mass loss, ML (%) Poplar Spruce

	Untreated		0	0
	220	1	2.8	2.0
	220	5	8.0	4.7
	250	1	12.0	7.1
	250	5	21.3	14.4
	280	1	25.2	18.7
	280	5	47.3	38.1

Table 2 .

 2 Parameters of the exponential regressions of the form

	50 x	 a b   	exp(	) c ML  		for the mean particle size after

Table 3 .

 3 Ground samples properties: Global mass loss, mean particle size, mean circularity and span values.

	Torrefaction temperature (°C)	Torrefaction duration (h)	ML (%)	x 50 (µm)	Poplar S x	ψ 50	S ψ	ML (%)	Spruce x 50 (µm) S x	ψ 50	S ψ
	Untreated	0	1504	0.56 0.57 0.24	0	1611	0.67 0.52 0.26
	220	1	2.8	1261	0.69 0.60 0.22	2.0	1153	0.86 0.53 0.29
	220	5	8.0	1006	0.73 0.62 0.20	4.7	881	0.87 0.55 0.28
	250	1	12.0	975	0.79 0.63 0.21	7.1	940	0.90 0.54 0.29
	250	5	21.3	606	0.84 0.70 0.19	14.4	520	0.86 0.59 0.28
	280	1	25.2	628	0.82 0.68 0.20	18.7	504	0.88 0.58 0.30
	280	5	47.3	475	0.84 0.72 0.21	38.1	192	0.80 0.72 0.21

Table 1 .

 1 Flow properties for milled poplar and spruce at different torrefaction intensities.

					POPLAR						SPRUCE
						ML (%)							ML (%)
		σpre (kPa)	0.0	2.8	8.0	12.0	21.3	25.2	47.3	0.0	11.4	14.7	17.4	25.4	30.9	46.5
	Unconfined yield strength (kPa)	2 5 10	1060 3393 6992	1148 3310 6635	1375 3190 7607	1207 3011 6171	906 2554 4610	983 1926 5267	582 1430 3293	1299 4717 7729	1508 4042 8277	1151 3172 6619	1174 2979 6925	884 1945 4570	988 2426 5006	912 1557 2370

Acknowledgements

The support of the Ministère de l'Enseignement Supérieur et de la Recherche (France) is gratefully acknowledged.


Imed Laaridh
email: imed.laaridh@alumni.univ-avignon.fr

Christine Meunier
email: christine.meunier@lpl-aix.fr

Corinne Fredouille
email: corinne.fredouille@univ-avignon.fr

Dysarthric speech evaluation: automatic and perceptual approaches

Keywords: Dysarthria, speech disorders, automatic speech processing, perceptual evaluation

Perceptual evaluation is still the most common method in clinical practice for the diagnosis and monitoring of the condition progression of people suffering from dysarthria (or speech disorders more generally). Such evaluations are frequently described as non-trivial, subjective and highly time-consuming (depending on the evaluation level). Clinicians have, therefore, expressed their need for new objective evaluation tools more adapted to longitudinal studies or rehabilitation context. We proposed earlier an automatic approach for the anomaly detection at the phone level for dysarthric speech. The system behavior was studied and validated on different corpora and speech styles. Nonetheless, the lack of annotated French dysarthric speech corpora has limited our capacity to analyze some aspects of its behavior, and notably,its severity (more anomalies detected automatically compared with human expert). To overcome this limitation, we proposed an original perceptual evaluation protocol applied to a limited set of decisions made by the automatic system, related to the presence of anomalies. This evaluation was carried out by a jury of 29 non-naive individuals. In addition to interesting information related to the system behavior, the evaluation protocol highlighted the difficulty for a human, even expert, to apprehend and detect deviations at the word level in dysarthric speech.

Introduction

Dysarthria is a motor speech disorder that is a consequence of neurological damage located in the central or/and the peripheral nervous system. This may result in disturbances in any of the components involved in speech production and may be reflected by weakness, spasticity, incoordination, involuntary movements, or abnormal muscle tone depending on the location of the neurological damage [START_REF] Duffy | Motor speech disorders: substrates, differential diagnosis and management[END_REF]. Perceptual evaluation by a set of listeners is the most common paradigm used to evaluate the characteristics and severity of impairment in speech pathologies. Furthermore, the current classification of dysarthria into six categories is based on a perceptual description of deviant speech dimensions. [START_REF] Darley | Differential diagnostic patterns of dysarthria[END_REF]Darley et al., 1969a;[START_REF] Darley | Motor speech disorders[END_REF]. The clinical evaluation of patients is also based on several batteries of tests in which the production of dysarthric speakers is coded perceptually by clinicians. These batteries evaluate the vocal quality, phonetic realizations, prosody, respiration and intelligibility. The BECD (Batterie d'Evaluation Clinique de la Dysarthrie) [START_REF] Auzou | Batterie d'évaluation clinique de la dysarthrie[END_REF] is the most commonly used test by clinicians for French speech. This test differentiates 35 items in order to characterize dysarthria. Consequently, the use of perception for the evaluation of dysarthric speech is frequent and well documented. And the clinicians evaluating the speech of patients are very well trained to the phonetic characteristics associated with the physiopathology of dysarthria. However, a frequent criticism to perceptual evaluation is the subjectivity of the listeners (both naive and expert). In some previous work, the authors have proposed an automatic phone-based anomaly detection approach (Laaridh et al., 2015a). By detecting and localizing anomalies in speech production, this approach aims at enhancing the manual investigation of human experts and, at the same time, reducing the extent of their intervention by scrutinizing the speech signal. Indeed, this automatic process should permit treating a larger amount of speech production while guiding human experts to focus on specific parts of the speech, considered as atypical. This process is notably interesting for speech productions of people with mild to moderate dysarthria, for which speech impairment may be scattered along the speech signal. Moreover, this automatic detection and localization of abnormal acoustic phenomena can have applications in clinical practice. For example, the evaluation of dysarthria by clinicians could be partially helped by a visual display of abnormal phenomena localized in the speech production of dysarthric speakers, like a map. In the same manner, maps could be relevant to compare the speech productions of a dysarthric speaker over time, in clinical treatment or rehabilitation for instance. Finally, this automatic process could be extended to other kinds of speech disorders resulting in acoustic alterations in the speech signal, such as larynx or head and neck cancers.

In this paper, the authors investigate on the behavior of the system, and typically on its potential quality or shortcoming to over-detect anomalies compared to a human expert. The larger research question this work tries to handle could be also that of the relationship between the human perception of alterations in speech and their modeling by automatic speech processing systems. In this context, the objective of this work is to propose an original perceptual evaluation protocol, suitable for evaluating the performance of the automatic system. This evaluation protocol is based on the comparison of the output decisions yielded by the system relating to the presence of anomalies, to those of a jury composed of a large set of expert listeners (in order to minimize the effect of individual subjectivity) on a selection of speech sequences produced by a large number of dysarthric patients representing four different pathologies, and control speakers. The rest of this article is organized as follows. In section 2, the automatic anomaly detection approach and the motivations of this research work are presented. The experimental protocol and corpora used for the perceptual evaluation are presented in section 3. In section 4, the evaluation results are presented and discussed according to different aspects. Finally, section 5 provides a conclusion and directions for future work.

Motivations

Automatic anomaly detection approach

The automatic phone-based anomaly detection system relies on two steps: a text-constrained phone alignment to obtain the phone segmentation and a classification of speech segments into normal and abnormal phones (anomalies). The automatic phone segmentation of the speech utterances into phones is carried out thanks to an automatic textconstrained phone alignment tool [START_REF] Laaridh | Automatic speech processing for dysarthria: A study of interpathology variability[END_REF]. This tool takes as input the parameterization of the speech signal produced by a given speaker, gender-dependent acoustic models of French phones, the sequence of words pronounced by the speaker in each utterance and a phonetized phonologically-varied lexicon of words based on a set of 37 French phones. The sequence of words comes from a manual orthographic transcription performed by a human listener following some annotation rules. Then, a set of features considered as relevant for the anomaly detection task are extracted over each segment y p associated with the phone p. The list of the features used can be found in (Laaridh et al., 2015a). These phone features are then fed into a 2-class automatic classification system based on Support Vector Machines (SVM) [START_REF] Vapnik | The Nature of Statistical Learning Theory[END_REF][START_REF] Scholkopf | Learning with Kernels: Support Vector Machines, Regularization, Optimization, and Beyond[END_REF]. Figure 1 describes the automatic anomaly detection process. In (Laaridh et al., 2015a), the system was evaluated on a very limited corpus of dysarthric speech (4 female and 4 male dysarthric patients, suffering from the same pathology, and 6 control speakers) annotated by one human expert. This annotation was made especially for the system development and evaluation, by labeling each phone as deviant or not from an acoustic point of view. On this corpus, the system obtained a quite high averaged recall measure1 of 0.81 (0.72 on male patients and 0.89 on female patients) and a less convincing averaged precision measure2 of 0.63 (0.61 on male patients and 0.65 on female patients). Still in this work, the automatic system was applied on a non annotated corpus, implying a larger number of speakers (118 dysarthric patients and normal speakers) and different pathologies like Parkinson's Disease, Amyotrophic Lateral Sclerosis, and Cerebellar Ataxia. Since no labeled data regarding anomalies was available, evaluation was carried out by observing the relationship between the amount of speech anomalies reported by the automatic system and the perceptual rates given by an expert jury on the global severity degree of dysarthria, the global degree of intelligibility and of articulation impairment, and finally, the speech rate of speakers. Analysis of the results pointed out very interesting behavior of the automatic system, which exhibits very relevant correlations with the majority of the perceptual criteria (e.g. between 0.8 and 0.9 for almost all of the pathologies for the global severity degree). In another work [START_REF] Laaridh | Automatic anomaly detection for dysarthria across two speech styles: Read vs spontaneous speech[END_REF], the application of the automatic anomaly detection on read and spontaneous speech still highlighted the interest of such an approach.

Proposed protocol features

The aim of the work reported in this paper is to cope with the lack of annotated corpora suitable for evaluating the automatic detection of anomalies in speech produced by patients suffering from speech disorders, compared with normal speech. Generally, the annotation of corpora is long, costly and time-consuming. In our context, difficulties increase by the fact that the automatic detection of anomalies is carried out at the phone level. Previous unpublished work we did demonstrated that the perceptual evaluation of the presence of anomalies in speech production by human at the phone level is a very complex task, leading to very heterogeneous decisions, even while involving a large number of listeners. Based on these observations, we propose an original perceptual evaluation protocol of the outputs of the automatic system. The task of listeners in this protocol is still to determine the presence of speech deviance (anomalies), in terms of articulatory realization. The first feature of this protocol is to transpose the decision of the automatic system, initially at the phone level, to the word level, to facilitate the perceptual evaluation done by humans. In this way, each monosyllabic word including, at least, one phone detected as an anomaly by the automatic system is considered as abnormal. In parallel, the presence of two phones, at least, detected as an anomaly in a polysyllabic word makes it abnormal. The second feature of this protocol is the set of speech sequences used for the perceptual evaluation task. Due to the cost of such tasks mentioned above, the entire corpora automatically annotated by the system can not be used. The concentration level and cognitive effort required for each participant for the evaluation task has also to be taken into account. For these different reasons, the set of speech sequences has to be limited in size to make the task feasible and efficient while relevant for the assessment of the quality of the automatic system decisions. First, records presenting low signal quality, noise or other artifacts are excluded from the study. Then, on the basis of the automatic annotation, speech sequences are selected on the expertise of two human annotators (distinct from the jury used later). Each speech sequence contains from one to several words targeted for the perceptual evaluation. For example, in the sequence "il mange tout seul bien tristement" (he eats very sadly alone), the words "mange" (eats) and "tristement" (sadly) are targeted for the evaluation; the other words of the sequence were considered to be normally produced by the system and both annotators. Speech sequences are chosen to fit one of the following 4 predefined categories, regarding uniquely the target word(s) (as noted above, the rest of the words in the speech sequences is considered as normal, independently of the categories) :

• 12.5% of the test sequences are referred to as "obvious segments". Here, both annotators agree with the system annotation considering the target word(s) as abnormal. This category is rather limited in size, compared to the others since the authors were more interested by the potentially wrong behavior of the automatic system;

• 37.5% are referred to as "ambiguous segments". Here, the human annotators disagree with each other and are not able to decide whether the automatic annotation of the target word(s) as abnormal is correct or not;

• 25% are referred to as "false negatives". Here, both annotators consider that the system fails to detect the presence of a true anomaly on the target word(s);

• 25% are referred to as "false positives". Here, both annotators consider that the system falsely labels the target word(s) as abnormal.

Other factors shape the set of the speech sequences. First of all, efforts have been concentrated on selecting speech produced by the largest number of patients, and representing the four pathologies available on our corpora. Second, efforts are made to balance the selected sequences and targeted words in order to vary their nature (grammatical, and lexical words), their length (long, and short words) and their position in the sequence (start, middle, and end).

To respond to these different constraints, a total of 98 speech sequences produced by 40 speakers, included 33 dysarthric patients and 7 healthy control speakers, are finally selected for the perceptual evaluation task.

The last feature of the protocol relies on the choice of the participant listeners and their degree of expertise to evaluate the presence or not of abnormal words in the speech sequences. Let's recall that this perceptual evaluation protocol aims at evaluating the quality of the outputs of an automatic system, considered itself as an "expert" -its goal is to bring some objective "expertise" to clinicians or phoneticians in their analysis of speech disorders. It seems natural to demand that listeners, qualified in evaluating such speech disorders, participate in this evaluation protocol. A jury of expert listeners are, therefore, selected.

Experimental protocol

Corpora

All the selected speech sequences are extracted from French read speech recordings of the fairy tale "Tic Tac" (The elves and the shoemaker). In total, 98 sequence produced by 40 speakers from dysarthric speech corpora VML and TypALoc [START_REF] Meunier | The TYPALOC corpus: A collection of various dysarthric speech recordings in read and spontaneous styles[END_REF] are selected. Four pathologies are represented in these corpora : Parkinson's Disease (PD), Cerebellar Ataxia (CA), Amyotrophic Lateral Sclerosis (ALS), and lysosomal diseases (LYS). Table 1 details the number of patients and sequences for each pathology and their dysarthric class. The selected segments were extracted from the recordings using Praat (Boersma and Weenink, ) and artificial silences of 400 and 200ms were added to each at the beginning and the end respectively in order to avoid abrupt signal cuts for the perceptual evaluation process.

Jury and experimental design

The selected jury includes 29 experts aged between 22 and 58 (average age of 33). They all have French as their mother tongue and have no prior audition or learning disorders. The jury is composed of 10 speech therapists, 18 final-year speech therapy students and 1 Ear, Nose and Throat (ENT) specialist, and speech pathologist. The proposed perceptual evaluation task is then computerized using the Perceval platform [START_REF] Ghio | Perceval: une station automatisée de tests de perception et d'évaluation auditive et visuelle[END_REF].

The evaluations last between 25 and 40 minutes and are performed in quite rooms as follows:

(1) participants are presented with an instruction list on the screen explaining the test procedure;

(2) an oral instruction indicating to focus only on articulatory realization and not to take prosodic or vocal aspects into account is then given to all participants;

(3) a training phase of 4 sequences is proposed in order to get participants familiarized with the task and the use of Perceval platform; (4) when the evaluation starts, an orthographic transcription of the sequence appears on the screen. Under each word, the expert has to check one of two boxes to label the word as "normal" or "abnormal". Figure 2 shows an example screen shot of the experiment. The sequences are presented in a totally randomized order for each participant and can be played up to 3 times before making an evaluation.

Evaluation methodology

To analyze the perceptual evaluation results, 3 System-Jury agreement rates are computed:

• AG targetAnomaly rate, measures the System-Jury agreement rate on the targeted abnormal words (target words automatically labeled as abnormal) for the "obvious segments", "ambiguous segments" and "false positives" categories. This rate measures the capacity of the automatic processing in detecting present abnormal zones and how much the jury agrees with it on the detected segments. The closer to 100 the rate is, the better the automatic system detects the abnormal zones;

• AG targetN ormal rate, measures the System-Jury agreement rate on the targeted normal words (target words automatically labeled as normal) for the "false negatives" category. This rate reflects the system inability to detect potential present anomalies (according to the couple of annotators). The closer to 100 the rate is, the better the automatic approach is in distinguishing anomalies from normal words and not labeling them as abnormal;

• AG nonT argetN ormal rate, measures the System-Jury agreement rate on the non target words (automatically labeled as normal and considered as such during the sequence selection by both the annotators) for all test categories ("obvious segments", "ambiguous segments", "false positives" and "false negatives"). This rate will measure the system precision and capacity to distinguish between normal and abnormal words. The closer to 100 the rate is, the better the automatic approach is in not labeling normal words as anomalies. Considering the AG targetAnomaly measure, we observe a strong heterogeneity in the results depending on the test category, reaching 78%, 58%, and 13% for "obvious segments", "ambiguous segments" and "false positives" categories respectively. The high AG targetAnomaly rate on "obvious segments" confirms the capacity of the automatic approach to detect highly distorted segments. This capacity has been already highlighted in (Laaridh et al., 2015a) with 81% of phone-based anomalies annotated by an expert well detected by the system. In contrast, the low AG targetAnomaly rate of 13% observed on "false positives" reveals the limitations of the proposed approach and its somehow approximate judgment when facing more subtle anomalies. This result calls for a more in-depth acoustical analysis of these segments in order to better comprehend the automatic system behavior and whether they could be related to acoustic distortions of non-pathological nature (noise, breaths, etc.). Other hypotheses can be advanced to explain this behavior such as the presence of true anomalies not detected by the human experts in these segments or the presence of erroneous data in the system training corpora (errors in the human annotation used as reference). Nonetheless, we support that this observation has to be balanced. Indeed, the System-Jury agreement rate on the non target normal words AG nonT argetN ormal, aggregated over all the test categories, reaches 88% and confirms that the behavior of the automatic approach is far from being arbitrary. Therefore, the system behavior on the "false positives" category being finally restrained to a small amount of speech segments, and judged as marginal, it would be more appropriate to describe the behavior of the system as more severe than the human experts.

Results and discussions

Results according to test sequence categories

Considering the two remaining categories, "ambiguous segments" and "false negatives", the System-Jury agreement rates confirm the difficulty and the non trivial nature of the perceptual evaluation of dysarthric speech task even when performed by experts. Here, more than half (58%) and nearly third (30%) of the jury decisions agree with the system on both categories respectively, which demonstrates a strong heterogeneity in the decisions given by the experts themselves.

Inter-population variability

Table 2 details the System-Jury agreement rates per population and test category. We can note that the best AG targetAnomaly rate is computed over LYS patients reaching 98.3% and 68.1% on "obvious segments" and "ambiguous segments" respectively. This behavior can be expected considering that this population is involved in the modeling of the abnormal phones in our system and is consistent with previous results in (Laaridh et al., 2015a). This does also highlight the importance of the training phase in our automatic approach and suggests that the use of more data associated with different pathologies and dysarthric classes should improve the system performance, already very promising given the results reported earlier.

Considering the other populations, notable differences are observed between the different pathologies. This is highly important considering that the instructions given to the jury explicitly restrict the evaluation task to the articulatory production of speakers. This behavior is particularly evident on ALS patients on whom the jury annotated the most anomalies compared to other populations and where the AG targetAnomaly rate reaches 19.6% on the "false positives" category. In contrast, an opposite behavior is observed on CTRL speakers and PD patients for whom an overall good quality of the speech is usually observed and the computed AG targetAnomaly rate over the "ambiguous segments" reaches 15.2% and 42.7% respectively.

Figure 4: Anomaly rate (%) per jury member (perceptual, blue bars) and for the system (red bar).

Jury responses

Even though the experimental protocol proposed here aim at studying the behavior of an automatic anomaly detection approach, the computed results highlighted the difficulty for a human, even expert, to apprehend and detect deviations at the word level in dysarthric speech and revealed some evaluation tendencies depending on patients' pathologies and dysarthria severity. Figure 4 depicts the perceptual anomaly rate (%) per jury member. This tends to show that listeners may be influenced by the contextual information. If speech sounds pathological (ALS patients for instance) then anomalies are more often detected in words. We should therefore ask if listeners are able to perform the same task as the system; the system is able to focus on short units to detect anomalies (phones, syllables, words) while subjects perform a contextual task to take their decision. The variability may be interpreted as a consequence of the difficulty of the task proposed to the jury. Indeed, listeners were asked to focus their attention on a single word which may be produced with or without an anomaly. This is not the way clinicians usually evaluate their patients. And this is also not the way we perceive speech. The process of speech perception requires a large context of speech in order to evaluate if it is distorted or not. The need to focus on a specific item is a very difficult task for listeners.

Conclusions

This paper investigates the results of a perceptual evaluation of the annotation performed by an automatic anomaly detection system on dysarthric speech. The results confirm the capacity and relevance of the automatic approach in detecting the presence of anomalies in dysarthric speech (high AG targetAnomaly rates on "obvious segments"). Moreover, and even on the more nuanced anomalies ("ambiguous segments"), the jury agrees 58% of the time with the automatic approach decisions. However, the low AG targetAnomaly rate computed over the "false positives" category confirms the approach tendency to be more severe than the human experts. Consider-ing the limits of the perceptual evaluation recognized in the literature [START_REF] Zyski | Identification of dysarthria types based on perceptual analysis[END_REF][START_REF] Fex | Perceptual evaluation[END_REF], we suggest that a more primitive question must be raised: should an automatic approach replicate the behavior of a human expert and what place should be envisaged for future investigations between supervised (relying on human annotations) and semi-or unsupervised approach ?

Figure 1 :

 1 Figure 1: Automatic approach for a phone-level anomaly detection.

Figure 2 :

 2 Figure 2: Screen shot from the Perceval platform used in the perceptual evaluation.

Figure 3

 3 Figure 3 depicts the distribution of the different agreement rates when computed on each test category.Considering the AG targetAnomaly measure, we observe a strong heterogeneity in the results depending on the test category, reaching 78%, 58%, and 13% for "obvious segments", "ambiguous segments" and "false positives" categories respectively. The high AG targetAnomaly rate on "obvious segments" confirms the capacity of the automatic approach to detect highly distorted segments. This capacity has been already highlighted in(Laaridh et al., 2015a) with 81% of phone-based anomalies annotated by an expert well detected by the system. In contrast, the low AG targetAnomaly rate of 13% observed on "false positives" reveals the limitations of the proposed approach and its somehow approximate judgment when facing more subtle anomalies. This result calls for a more in-depth acoustical analysis of these segments in order to better comprehend the automatic system behavior and whether they could be related to acoustic distortions

Figure 3 :

 3 Figure 3: System-Jury agreement rates (%) per test sequence category.

Table 1 :

 1 Information related to the corpora used for the perceptual evaluation task including the different populations and dysarthria class -control speakers and patients suffering from Parkinson's disease, cerebellar ataxia, amyotrophic lateral sclerosis, and lysosomal diseases, the number of speakers and of speech sequences per population.

	Population	Corpora	Dysarthria	# of	# of
			class	speakers sequences
	Control speakers	TypALoc	-	7	15
	Parkinson's disease	TypALoc Hypokinetic	6	15
	Cerebellar ataxia	TypALoc	Ataxic	8	22
	Amyotrophic Lateral Sclerosis TypALoc	Mixed	11	28
	Lysosomal storage disease	VML	Mixed	8	18
	Total	-	-	40	98

Table 2 :

 2 System-Jury agreement rates (%) on automatically detected abnormal (AG targetAnomaly) and normal (AG targetN ormal and AG nonT argetnormal) words per population and test sequence category.

		Obvious segments	Ambiguous segments	False negatives	False positives
	Population	AG target AG nonTarget	AG target AG nonTarget	AG target AG nonTarget	AG target AG nonTarget
		Anomaly	normal	Anomaly	normal	Normal	normal	Anomaly	normal
	CTRL	81.0	99.1	15.2	99.7	50.6	97.5	1.4	100.0
	CA	71.3	92.3	59.8	86.4	24.9	86.6	9.8	94.6
	PD	78.2	89.1	42.7	93.7	64.4	91.2	8.6	97.3
	ALS	74.6	52.9	79.0	77.1	25.6	75.3	19.6	98.0
	LYS	98.3	81.9	68.1	86.6	8.6	72.9	15.5	85.6

The ratio between the number of phones correctly detected as anomalies by the automatic approach and the number of zones labeled as abnormal in the reference.

The ratio between the number of phones correctly detected as anomalies by the automatic approach and the total number of anomalies reported by the automatic processing (truly or falsely).

Acknowledgments

This work has been carried out thanks to the support of the BLRI Labex (ANR-11-LABEX-0036) and the A*MIDEX project (ANR-11-IDEX-0001-02) funded by the French government "Investissements d'Avenir" program managed by the ANR, and thanks to the French ANR projet Typaloc (ANR-12-BSH2-0003-03). We would also like to thank Laura Restivo and Laura Pianelli, who were in charge of the initial speech sequence annotation (the two annotators mentioned in the paper), of recruiting the expert jury and of organizing the perceptual evaluation-based experiments.