R Jaumann
email: ralf.jaumann@dlr.de.

N Schmitz

T.-M Ho

S E Schröder

K A Otto

K Stephan

S Elgner

K Krohn

F Preusker

F Scholten

J Biele

S Ulamec

C Krause

S Sugita

K.- D Matz

T Roatsch

R Parekh

S Mottola

M Grott

P Michel

F Trauthan

A Koncz

H Michaelis

C Lange

J T Grundmann

M Maibaum

K Sasaki

F Wolff

J Reill

A Moussi-Soffys

L Lorda

W Neumann

J.-B Vincent

R Wagner

S Kameda

H Yano

S Watanabe

M Yoshikawa

Y Tsuda

T Okada

T Yoshimitsu

Y Mimasu

T Saiki

H Yabuta

H Rauer

R Honda

T Morota

Y Yokota

T Kouyama

Images from the surface of asteroid Ryugu show rocks similar to carbonaceous chondrite meteorites

spectrally different inclusions implying that it did not experience extensive aqueous alteration.

The inclusions appear similar to those in carbonaceous chondrite meteorites.

One Sentence Summary: The presence of abundant multi-color, millimeter-sized inclusions in a Ryugu rock indicate strong similarities with carbonaceous chondrites.

Main text:

Japan Aerospace Exploration Agency's (JAXA) Hayabusa2 sample return mission (1), carrying the MASCOT lander, arrived at asteroid Ryugu on 2018 June 27 (2). Its initial measurements showed that that Ryugu has an equatorial radius of 502 ± 2 m, a bulk density of 1.19 ± 0.02 g cm -3 , and spectral properties typical for carbonaceous chondritic material of Cb-type (2).

Ryugu's visible geometric albedo is 4.5 ± 0.2%, making it one of the darkest objects in the Solar System (3). The German Aerospace Center (DLR) developed MASCOT in cooperation with the French Centre National d'Etudes Spatiales (CNES) (4). MASCOT was released from the Hayabusa2 mothership at 01:57:20 UTC on 2018 October 3, from an altitude of 41 m above the surface of Ryugu [START_REF]Materials and methods are available as supplementary materials[END_REF]. At 02:03:14 UTC, the lander made first contact with the surface (at contact point CP1), impacting the shadowed side of a large boulder. MASCOT then bounced backward and traveled a further 17 m over the surface to come to its first rest at settlement point 1 (SP1) in a local depression (Fig. 1A-C). During the six-minute descent toward the surface and the bouncing phase, the MASCam camera (6) took 20 images. In the same period, Hayabusa2's Optical Navigation Camera (ONC) (3,7) recorded both the shadow of MASCOT on the surface, and the lander itself when its bright, white top was visible. These images allowed us to reconstruct the descent and bouncing trajectory (Fig. 1) [START_REF]Materials and methods are available as supplementary materials[END_REF]. An up-raising maneuver (4,[START_REF]Materials and methods are available as supplementary materials[END_REF], at this measurement position (MP1) (Fig. 1B) left the lander upside down with most instruments looking into the sky. A sequence of 5 images acquired during the first night on the asteroid shows three moving objects, whose angular velocities coincide with Ryugu's rotation period of 7.63262 ± 0.00002 hours (2). We identified these as Jupiter, Saturn, and the 2.0 magnitude star σ Sgr. In the morning of the second day, MASCOT activated an internal mobility unit to reach a second location (MP2) (Fig. 1B), now properly orientated for its instruments to study the surface.

Here MASCam initiated a full day/night imaging cycle, using an onboard array of LEDs to illuminate the area in front of the camera (Fig. 2). On the morning of the third day, the lander slipped about 5 cm sideward by executing a mini-move to enable stereo imaging for photogrammetric analysis, reaching a third location (MP3) (Fig. 1B and2C) [START_REF]Materials and methods are available as supplementary materials[END_REF]. Additional imaging over that day (Fig. 2D) enabled us to observe the surface at changing illumination and viewing geometries [START_REF]Materials and methods are available as supplementary materials[END_REF]. ONC also imaged MASCOT close to the lander's end-of-mission, allowing us to constrain its final location after the 2 nd relocation to MP4 as 22.31 ± 0.05°S, 317.16 ± 0.05°E [START_REF]Materials and methods are available as supplementary materials[END_REF]. After 17h 7min of surface operations, MASCOT lost contact with Hayabusa2, shortly before its batteries ran out.

The MASCam images acquired during descent and bouncing reveal a surface covered with rocks and boulders of different lithologies. Four types of rock have been identified in ONC images (3).

Two of these types are readily identified in MASCam images (Fig. 3): dark and rough (type 1) and bright and smooth (type 2). Type 3 rocks (bright and mottled) were not observed. In the highest resolution MASCam images, the surfaces of rock type 1 have a cauliflower-like appearance with a very small-scale rough and crumbly surface (Figs 1,2 A,C). Type 2 rocks appear flat and possess elongated, non-dendritic fractures that divide the rock into planar slabs with smooth surfaces. MASCam observed rocks with sizes from decimeters to a few tens of meters. The distribution of both rock types is spread almost evenly over the surface in the areas imaged by MASCam. This observation is consistent with the interpretation that Ryugu originated either from two parent bodies after collision, break-up and re-accumulation resulting in two types of material or formed within the same parent body at different interior temperature and pressure conditions as a result of catastrophic disruption and redistribution also resulting in two types of material (3). Ryugu's bulk composition is expected to be similar to that of carbonaceous chondrites containing phyllosilicates and organics (CI or CM meteorites) (2,3,8,9). Exposure of such materials to the diurnal temperature changes of ~100 K on Ryugu (2, 10) may trigger thermal fatigue, leading to crack growth and potentially breakdown of consolidated material (11,12). Measurements by the MASCOT radiometer (MARA) instrument indicates that the rock in front of the lander has a bulk porosity of at least 28% and a low tensile strength (10). Type 1 rocks, may respond to thermal fatigue by crumbling, resulting in their cauliflower appearances at small scales. Type 2 rocks may instead split in planar slabs, indicating preferential fracturing along weaker dimensions.

At the second location, MASCam imaged a small rock in front of the lander. Night-time images were acquired with illumination provided by onboard LEDs [START_REF] Jaumann | [END_REF]. Because of the small phase angle (~5°), they reveal bright inclusions embedded in a dark matrix (Fig. 3). Most inclusions are smaller than one millimeter, but some are several millimeters across [START_REF]Materials and methods are available as supplementary materials[END_REF]. In one location (Fig. 3,E) the areal coverage of the inclusions is about 10%. The size of the inclusions is similar to those of chondrules (sub-mm range), calcium aluminum rich inclusions (CAIs), and other refractory inclusions in carbonaceous chondrite meteorites (8,9). Typical carbonaceous chondrites of subtypes CI2, CM, CO and CV possess refractory inclusions that appear similar to those in Ryugu's rock [START_REF]Materials and methods are available as supplementary materials[END_REF]. However, CI1 meteorites are not a good analog due to their very low abundance of inclusions (11,12). The destruction of inclusions in carbonaceous chondrites is thought to result from extensive aqueous alteration (13,14), so their preservation on Ryugu is consistent with the notion that the asteroid was only moderately hydrated in the past (3).

Ryugu's spectrum is similar to that of certain moderately heated CI and CM meteorites (3,15). The CI2 type meteorite Tagish Lake, which did not experience extensive heating, has a very low albedo and abundant inclusions, like Ryugu [START_REF]Materials and methods are available as supplementary materials[END_REF]16).

Judging by their color, we can distinguish two broad types of inclusions: blue and red, although inclusions of different colors are also abundant (Fig. 4). Blue inclusions feature a moderately negative spectral slope of about 10% over the visible wavelength range. The positive spectral slope of the red inclusions is much steeper, in the 20-50% range. Their reflectance in the near-IR (0.8 µm) is variable. Inclusions appear typically twice as bright as the rock matrix, although the smallest may be brighter as they are not spatially resolved. The spectral shape of unresolved inclusions is affected by the surrounding matrix to a minor degree, but because of their brightness they dominate the signal in each pixel. The inclusions in Fig. 4 B are well registered over the four color channels, and we expect no spectral artifacts as a result [START_REF]Materials and methods are available as supplementary materials[END_REF]. The very flat nature of the matrix reflectance spectrum is consistent with ground-based spectra of Ryugu (17,18,19) and the average ONC reflectance spectrum (3). The matrix reflectance has a slight excess at 0.63 µm that is responsible for the reddish appearance of the rock in the color images (Fig 3E -J), but it is within the calibration uncertainty. The visible colors of the inclusions provide compositional clues, but their interpretation is limited by the lack spectral data > 1m wavelength , where most diagnostic bands for mineralogy are located. However, we may infer the composition by comparing the Ryugu inclusions to those in carbonaceous chondrites. For example, the decreasing reflectance between 0.5 and 0.8 µm of the bright chondrules of the Murchison meteorite is due to olivine (9,20). The similar drop in reflectance towards 0.8 µm displayed by some Ryugu rock inclusions could also be due to olivine. The origin of the orange color of some of Murchison's larger inclusions is not known (20). The MASCam observation of abundant multi-color, millimeter-sized inclusions suggest that C-type asteroids are linked to carbonaceous chondrites.

The clearly visible inclusions suggest that the rock in front of MASCOT is not covered by microscopic (sub-mm) particles, which the MARA results confirm (10). MASCam observed no deposits of fine material during the descent. Such deposits were also absent from the surface of Itokawa (21), another rubble pile asteroid, whereas they are thought to be present on the surface of Eros (22). We expect dust to be formed continuously on the surface of Ryugu through exposure to the space environment (23,24,25). However, all boulder appears clean, and therefore dust particles have likely been removed, lost either to space or the porous interior (2).

The fate of fine particles is determined by a balance of forces. Cohesive forces are thought to dominate over electrostatic, gravitational, and solar radiation pressure forces for particles up to a decimeter in size, for a body as small as Ryugu (26). Although the cohesive barrier may be overcome by means of electrostatic levitation (25), the physical conditions leading to this process on small bodies are not well understood. On Ryugu, larger particles (>100 µm) are more likely to be removed from the surface than finer dust (27). Alternatively, cohesion may be overcome by means of physical force, such as micrometeorite impact, seismic shaking, boulder migration and collision, drag by sublimation of volatiles, and/or thermal fracturing (25). The absence of fine deposits on the surface of small rubble piles like Ryugu and Itokawa implies the existence of an efficient removal mechanism for small particles. Gravel-sized particles may collect into the interior under the influence of gravity, but the absence of dust is not easily explained because of the strong cohesive forces involved. All images and input data used in this paper and supplementary material are available at http://europlanet.dlr.de/MASCAM at least until they become available at the long term archive of PDS at https://pdssbn.astro.umd.edu/. Additional data from the mission will be delivered to the DARTS archive at https://www.darts.isas.jaxa.jp/planet/project/hayabusa2/, and higher-level data products will be available in the Small Bodies Node of the Planetary Data System https://pds-smallbodies.astro.umd.edu/ one year after landing on the asteroid.

Figures

Materials and Methods

The Landing Process.

It required about 4±1.5 sec. for MASCOT to leave the Mechanical and Electrical Support System (MESS), the structural interface to the Hayabusa2 mothership. The lander underwent a free-fall which lasted 6 min towards Ryugu before experiencing the first contact (CP1) on the asteroid followed by a bouncing phase of 15 min 37 sec. until it finally came to rest at the first settlement point (SP1). During the bouncing phase, the MASCOT Magnetometer (MASMag) instrument ( 28) detected up to three soft impacts. At SP1 the Guidance, Navigation and Control (GNC) sensors detected a false orientation of the lander, thus a self-raising maneuver was executed to turn the lander into the correct orientation for scientific measurements. Unfortunately, this was a wrong interpretation, due to a sub-threshold signal return from the dark surface or/and a cavity to the GNC sensors detecting ground proximity, and MASCOT started its 1 st science measurement cycle (MP1) upside down with the instruments looking into the sky. The measurement sequence was interrupted immediately after a link was established with Hayabusa2 and a relocation was commanded (1 st relocation) to correct for the orientation error. MASCOT jumped for 70cm (Fig. 1). Once it came to rest, its GNC detected the orientation correctly and the 2 nd science measurement cycle (MP2) was initiated. After its completion and a mini-move of 5 cm dedicated to stereo measurements (MP3), a second change of location was commanded (2 nd relocation). MASCOT reached its fourth and last measurement point (MP4) at an additional distance of 10 cm. However, at this time the anticipated operational lifetime of 16 hours was already exceeded. The remaining energy just allowed MASCOT to enter a predefined "end of life" status with priority on data transmission and only minimal scientific operations. During the descent and bouncing, MASCam performed a low cadence imaging (1/min) for 6 min, followed by a high cadence imaging (1 per 30s) for another 6.5 min. In total, MASCam acquired 20 descent and bouncing images, starting at 01:59:47UTC (about 2.5 min after separation from Hayabusa2), and ending at 02:12:27UTC (about 6.5 min before coming to rest in SP1). In addition, before, during and after the landing of MASCOT, the Optical Navigation Cameras (ONC) of Hayabusa2 took images from distances of hundreds down to tens of meters. A stereo-photogrammetric analysis (29)(30)(31)(32)of ONC images was used to derive precise orientation parameters (orbit position and attitude) for the landing site area and to reconstruct the descent and bouncing trajectory (Fig. 1). Based on the orientation data, together with image distortion models from star calibration, an initial digital terrain model (DTM) of the landing site (Fig. 1C) was computed. Rectified ONC image maps of the landing site were produced to aid in reconstructing the descent and bouncing trajectory. In the close-up images of ONC, MASCOT's white top radiator and/or its shadow can be detected on the ground. MASCOT was visible during descent in 7 ONC images and in another 10 ONC images during bouncing (Fig. 1C). At 02:03:14UTC, MASMag recorded a first contact (CP1) with the surface, and ONC images show this happened at the shadowed side of a boulder. After deflection by the boulder, MASCOT bounced backward  m across the surface before coming to rest in SP1(Fig. 1). The DTM reveals that, during bouncing, MASCOT traversed downwards into a local depression. At 02:14:04 UTC, ONC acquired its last image showing MASCOT during bouncing, almost 2 minutes after the last MASCam image had been taken. ONC also imaged MASCOT at its final position (Fig. S1) and confirmed the surface coordinates of 22.31±0.05°S/317.16±0.05°E, which allowed us to reconstruct the entire trajectory of MASCOT from its release to its final rest.

Depth Map and Scaling from MASCam Images.

A small, unintentional positional change of the MASCOT lander occurred at MP3. To differentiate between the positions before and after that movement we use the denominations MP3a (before, Fig. 2C) and MP3b (after, Fig. 2D). The movement appears to have been a mainly lateral repositioning of the lander to its right, effectively creating MASCam stereo image pairs of the landscape in front of the camera. Although the baseline and the stereo angle are very small and therefore not well suited for stereo-photogrammetric analysis, we could still use the disparities created by the slightly different viewing geometries. With an increasing distance of the objects to the image plane, the disparities decrease linearly allowing the differentiation between foreground and background objects. At this point we did not possess any knowledge about the sizes of the imaged objects nor the length of the stereo base. Therefore, no absolute values for the distances to the image plane could be given, only relations between individual regions. Fortunately, reflections from the MASCOT structure are visible in the MASCam image. These include the rectangular reflection caused by the alignment cube (Fig. 2C &D), a metallic cuboid at MASCOT's front used during the assembly of the lander for optical alignment. The reflection appears to be horizontally undistorted in the MP3a images, thus having the same width of 15 mm as the alignment cube's front side. With this information, we determined a distance of 33 cm between the reflection on the rock wall and the camera. The relative distances found previously were then scaled accordingly leading to a rough depth map of the area (Fig. S2). We also derived a base length of 5 mm between the positions of MP3a and MP3b. Due to different lighting conditions, the feature matching results are quite noisy and unreliable for background objects. Nevertheless, these estimates determine the sizes of foreground objects.

Supplementary Text

Analysis of Inclusions. MASCam was equipped with an array of LEDs in four different colors (blue (0.465 µm), green (0.523 µm), red (0.633 µm), and near-IR (0.812 µm)) to illuminate the surface in front of the lander at night [START_REF] Jaumann | [END_REF]. The last sets of images have the highest signal-to-noise ratio as the detector temperature had dropped during the night. During the first night on the asteroid, the camera was peering into empty space, but during the second night the lander was in the correct orientation to closely observe the surface. Despite applying a correction derived from pre-launch laboratory exposures of a uniform surface, several color artifacts remain unaffected, such as the green strip at the bottom of the frame and the reddish vertical stripes at left (Fig. 3 II in report). The color balance should be most accurate to 10% in the middle of the frame. In the image center, the rock appears gray with a reddish tint, and many small, predominantly bright inclusions are visible. The inclusions are only revealed due to small phase angle of the reflected LED light (<5°), and are not evident in images acquired during the asteroid day.

The MASCam optics display minor distortion with a slight dependence on the wavelength of light. Normally, one would apply a wavelength dependent distortion correction to images acquired with LEDs of different colors to construct a properly registered color composite. We characterized the wavelength dependence of the distortion in the laboratory before launch by imaging a checkerboard target [START_REF] Jaumann | [END_REF]. However, the wavelength dependence is so minor, that our attempts to correct for it actually introduced wavelength dependent artifacts. Therefore, we performed the color analysis on images that were not corrected for distortion. Moreover, Fig. 4B & C are located in the center of the image, where the wavelength dependent distortion should be the smallest. To illustrate the accuracy of the color registration in the inset in Fig. 4B, we enlarge the area around inclusion e in Fig. S4, preserving the shape of the original pixels. The brightest pixel is at the same location in all four images, with no perceptible color fringing around it. The apparent size of this inclusion is the same as that of the point spread function, which implies that inclusion e is probably not fully resolved. However, the accurate registration and lack of color fringing demonstrates that the inclusion is present at approximately the same location within the same pixel in all four images.

To estimate the percentage of inclusions, we manually mapped the bright inclusions in a slab of rock in front of MASCam using the night time image, illuminated by the red LED (Fig. 3). A total of 410 bright inclusions ranging between 0.03 to 4.56 mm² were identified assuming a constant pixel resolution of 0.2 mm at this site. This corresponds to an area of 10% to be covered by bright inclusions. CI, CM and CO meteorites include similar sized refractory materials as the rock observed on Ryugu. Table S2 shows a comparison of inclusions in meteorites with respect to albedo, inclusion abundance and size. Using the Delesse principle (33), which states that the areal density equals the volume density, these numbers are comparable to the areal abundance of inclusions of 10% observed for the rock on Ryugu.

  38. H.Y. McSween, Geochim et Cosmochim Acta 41, 477-491 (1977). 39. J.I. Simon, et al., Earth Planet Sci Lett 494, 69-82, (2018). Acknowledgments: The MASCOT lander on the Hayabusa2 Mission of JAXA is a DLR/CNES cooperation. MASCam was developed and built under the leadership of the DLR Institute of Planetary Research with contracted contributions of Astrium GmbH, and is operated by the DLR Institute of Planetary Research in Berlin in cooperation with the DLR Institute of Space Systems in Bremen and DLR-MUSC in Cologne. This study was supported by JSPS International Planetary Network. The authors thank H. Sadada, M. Yamada, E. Tatsumi, N. Sakatani, C. Honda, and K. Ogawa M. Hayakawa, H. Suzuki, Y. Cho, and M. Matsuoka for Hayabusa2 ONC development and science observations. We cordially thank Mike Zolensky for providing Figure S5. The Hayabusa2 Mission is operated by JAXA. Funding: The team acknowledges funding by DLR and CNES. Author contributions: R.J. coordinated and wrote the paper, R.J., N.S., S.E.S., A.K., F.T., K.D.M., and T.R. contributed to camera development, operations and data processing. R.J., N.S., S.E., F.W., F.P., F.S., K.A.O., S.K., H.J., S.E.S., S.W., M.Y., S.S. and H.R. contributed to the reconstruction of the trajectory, stereo processing, MASCOT location, comparison with ONC data and Hayabusa operations. R.J., N.S., K.A.O., K.K., R.P., J.B.V., R.W., S.S. and P.M., contributed to the geomorphological analysis and discussion of Ryugu's origin. R.J., S.E.S., K.S., N.S., S.M., J.P.B., M.G., K.A.O., and A.H., contributed to the spectral and photometric analysis. R.J., T.M.H., S.U., C.K., J.B., A.M. F.W., J.R., J.T.G., L.L., Y.T., T.O., T.Y., Y.M., and T.S. contributed to the MASCOT development, landing site selection and on surface operations. Competing interests: There are no competing interests to declare. Data and materials availability: All data is available in the main text and the supplementary materials.

Fig. 1 .

 1 Fig. 1. Reconstructed MASCOT trajectory. (A) Hayabusa2/ONC context image, with its

Fig. 2 .Fig. 3 .Fig. 4 .

 234 Fig. 2. Images of a type 1 rock at the second and third MASCOT locations (5). (A) Image

Fig. S1 .

 S1 Fig. S1.

Fig

  Fig. S3.

Fig. S5 .

 S5 Fig. S5.

Supplementary Materials for

Images from the surface of asteroid Ryugu show rocks similar to carbonaceous chondrite meteorites R. Jaumann 1,2,* , N. Schmitz 1 , T.-M. Ho 3 , S.E. Schröder 1 , K.A. Otto 1 , K. Stephan 1 , S. Elgner 1 , K. Krohn 1 , F. 11 Dept. of Physics, Rikkyo University, 3-34-1 Nishi-Ikebukuro, Toshima, Tokyo 171-8501, Japan 12 Inst. of Space and Astronautical Science, Japan Aerospace Exploration Agency, Sagamihara, Japan 13 Dept. of Earth and Planetary Sciences, Nagoya Univ. Furo-cho Chikusa-ku, Nagoya, Japan 14 Dept. of Earth and Planetary Systems Science, Hiroshima University, Hiroshima, Japan 15 Kochi University, Dept. of Information Science, Akebono, Kochi, Japan 16 Univ. of Tokyo, Dept. of Earth and Planetary Science, Hongo, Bunkyo, Tokyo, Japan 17 National Institute of Advanced Industrial Science and Technology, Aomi, Koto, Tokyo, Japan This PDF file includes:

Materials and Methods

The Landing Process. Depth Map and Scaling from MASCam Images. Supplementary Text Analysis of Inclusions. Figures S1 to S5, Tables S1 to S2, References (28)(29)(30)(31)(32)(33)(34)(35)(36)(37)(38)(39) Table S1. Landing timeline. The table summarizes the main events from separation until the end of the MASCOT operations on asteroid Ryugu as well as the day-night cycles at the measurement points (see also Fig. 1


Hassan Mohanna

Jean-Michel Commandre

Bruno Piriou

Gilles Vaitilingom

Benoit Taupin

David Honore

Shadowgraphy Investigation of the Combustion of Raw and Pre-treated Single Biomass Particles: Influence of Particle Size and Volatile Content

An experimental study of single particle combustion is performed in a high temperature particle reactor.

The particle degradation is simultaneously monitored by high magnification direct imaging and by shadowgraph imaging techniques giving access to the full behaviour of the particle even when enveloped by a flame. This allows tracing the time-resolved evolution of the particle shadow during its degradation as a function of its burnout. The method provides access to the whole process timeline, especially the onset of the heterogeneous oxidation even during the flame phase. It is observed to occur earlier for larger particles containing lower volatile matter. The latter occupies around 40% of the initial particle shadow, which decreases with devolatilisation progress following a power trend. The char burns at the surface until the reaction front penetrates the particle leaving an ash matrix behind. Effects of particle size and volatile matter on the different steps of particle combustion are discussed. Moreover, biomass behaviour is compared to that of coal. The whole results give new insights in the combustion of single biomass particle and are available for the development and validation of dedicated solid fuel combustion models.

Introduction

In the view of the polluting role of coal, incorporating a fraction of biomass into coal in pulverized burners can be an effective environmental strategy. However, this significantly modifies the functionality of the installations considering the different combustion behaviour of biomass at flame and particle level [START_REF] Riaza | Ignition and combustion of single particles of coal and biomass[END_REF]. However, unlike coal which has been extensively studied by the scientific community given its influential role in the industrial revolution, the integration of biomass in industry is more recent, and more research is required to improve the available technology.

Single particle reactors are successfully employed for coal in literature using heated grid reactor, batch type reactors, free fall and entrained flow reactors, etc. [START_REF] Riaza | Ignition and combustion of single particles of coal and biomass[END_REF]- [START_REF] Marek | Case Studies Investigating Single Coal Particle Ignition and Combustion[END_REF]. In general, the experimental rigs consist basically of a heated chamber in which a particle is introduced at high temperature to record its degradation.

The particle can be suspended on a fixed support or in a free stream according to the size. This kind of experiments is necessary to validate the theoretical concepts describing the phenomena and to examine the related parameters of the process conditions. However, fewer studies adapted these reactor to biomass [START_REF] Vorobiev | Determination of Char Combustion Kinetics of Torrefied Biomass by Use of Stereoscopic Pyrometry[END_REF]- [START_REF] Houshfar | Experimental Study of a Single Particle Reactor at Combustion and Pyrolysis Conditions[END_REF]. Lu et al. [START_REF] Lu | Effects of particle shape and size on devolatilization of biomass particle[END_REF] demonstrated in such experiment on biomass that shape and aspect ratio differences show measurable differences in pyrolysis rates. Schiemann et al. [START_REF] Vorobiev | Determination of Char Combustion Kinetics of Torrefied Biomass by Use of Stereoscopic Pyrometry[END_REF], [START_REF] Schiemann | Char burning kinetics from imaging pyrometry: Particle shape effects[END_REF] also concluded to the same result using a new stereoscopic pyrometer to measure the particle temperature and shape at the same time in a laminar flow reactor. Momeni et al. [START_REF] Momeni | Fundamental Study of Single Biomass Particle Combustion[END_REF] reported that the ignition delay of a stationary particle is clearly influenced by the oxygen concentration in the surrounding atmosphere. Similar aspects were assessed by Houshfar et al. [START_REF] Houshfar | Experimental Study of a Single Particle Reactor at Combustion and Pyrolysis Conditions[END_REF] using gas emissions of single particles. These results serve to translate the combustion stage into empirical expressions that can be integrated in physical models [START_REF] Lu | Effects of particle shape and size on devolatilization of biomass particle[END_REF], [START_REF] Mason | Single particle flame-combustion studies on solid biomass fuels[END_REF], [START_REF] Brix | Modeling char conversion under suspension fired conditions in O2/N2 and O2/CO2 atmospheres[END_REF]. Beside the combustion behaviour, some studies examined the impact of fuel pre-treatment. Lu et al. [START_REF] Lu | Influence of Torrefaction on Single Particle Combustion of Wood[END_REF] pointed out that torrefaction shortens the devolatilization time while increasing the char yield and density. Riaza et al. [START_REF] Riaza | Combustion of single biomass particles in air and in oxy-fuel conditions[END_REF] also reported that increasing oxygen mole fraction in ambient atmosphere reduces the surrounding flame luminosity.

The objective of this study is to provide additional insight to the combustion timeline of a single particle of biomass using imaging techniques. Direct visible imaging is performed to visualize particle combustion.

Moreover, shadowgraphy is employed to trace simultaneously the particle projection area and shape evolution. Incorporating both techniques with the temperature measurement provides fundamental data on the events corresponding to each involved phenomenon. The investigation examines the impact of torrefaction and particle size on the combustion behaviour.

Fuel composition and properties

The fuels are selected in the purpose of investigating the volatile matter (VM) content influence on the combustion process. Pine is chosen as a reference biomass and is torrefied at 280 °C for 25 min to produce pint and also partially pyrolyzed at 450 °C for 15 min to produce pinp. A longer residence time of 20 min at 450 °C released more volatiles and produced carbonized pine (denoted pinc). The thermal preparation of the received wood chips was done in a heated screw reactor under nitrogen inert atmosphere. Demolition wood (DW) is also used as representative of waste fuels. 2 shows the physical properties (shape and size) of the tested particles of each tested fuel. Fuels are grounded thanks to a knives mill with a 1.5 mm sieve. The size of the particles obtained after grinding is in the range 0.4 and 2 mm possessing different aspect ratios and their mass is up to maximum 2 mg. Pine particles are mostly needle-like while torrefied pine and coal are between cylindrical and spherical particles. Demolition wood particles are mostly thick flakes made of small compressed fragments of wood.

Experimental methods

Single particle combustion apparatus

An experimental apparatus is designed to monitor the shadow of a solid particle during its thermal conversion [START_REF] Andrzejewski | Procédé et dispositif de mesure de la vitesse d'évaporation et du délai d'inflammation d'un carburant liquide[END_REF]. The setup in Figure 1 consists of a 12 x 13 x 14 cm heated combustion chamber fixed on a horizontal bench alongside with two cameras and a LED lamp. The chamber is electrically heated to a set point of 800 °C maximum and maintained at the set temperature with the help of an isolation layer. Two optical windows are situated on opposite sides of the chamber. These 3cm-dia. windows beside the small sizes of the cameras were sufficient to place both of them on the same side at long distance from the chamber to limit non-paraxial effect. The monochrome camera (Sony XCD-SX90) for shadowgraphy imaging is equipped with Navitar precise eye lens, 0.25x lens attachment to increase the working distance, and 2.0x adapter producing suitable magnification (0.4 µm/pixel). The camera is adjusted to capture images (850x850 pixel²) at 37 fps with 0.2 ms exposure time. Another camera (MicroCapture) records at 30 fps visible color images (352x288 pixel²) with an optical zoom giving a magnification ratio of 4.16µm/pixel.

Both cameras are held at a fixed distance from the particle position to achieve a consistent optical magnification and similar line-of-sight. The background is lighted with a collimated illumination generated by a LED lamp on the same axis of the particle for shadowgraphy measurements. The advantage of this technique is that it can surpass the flame barrier during the combustion of volatiles and access the full particle shape and volume evolution with time that is not available with usual direct visible imaging techniques.

Methodology

The samples are dried for 24 hours before every experiment considering the high variation in moisture content Moisture: 13% pinp, 1.9% pint, which can hugely influence the particles behaviour and ignition delay. A significant number of particles are tested for each fuel to reduce the deviations and better determine the trends and average values (table 2).

Prior to an experiment, a particle is glued to the tip of a 250 µm thermocouple measuring the surface temperature at a temporal frequency of 12 Hz. The glue is a high-temperature nonorganic paste used in a very small quantity at the position of the particle and left to dry to ensure little interference with the occurring thermochemical phenomena. As the biomass is tested with glue, dedicated experiments have been performed to determine if an influence of glue can be noticed on reaction kinetics. This potential influence is evaluated by comparing the thermogravimetric analysis of a sample of particles covered from one side with the glue and held in a wide crucible, and a reference sample of pine without glue in Figure 2. Under inert atmosphere, it seems to have little effect on the pyrolysis behaviour. However, the combustion tests revealed that the glue shifts slightly the degradation peaks to a higher temperature and separate the char and volatile oxidation process. Little to negligible difference on the conversion peaks values, processes durations, and ignition temperature is observed in comparing both curves. So, the effect of the glue is limited and it allows the comparison of results obtained with different fuels using same methodology. Once the chamber attains and stabilizes at the experiment temperature, the particle is inserted quickly to the hot medium by the help of a guiding rod fixed to the external wall of the chamber. The particle position is adjusted to be always in the field of view of both cameras during the whole process.

Processing of simultaneous time-series of images

Shadowgraph monochrome images are binarised in order to facilitate the detection of the particle contours (Figure 3 -A1 and B1). The background is subtracted using the rolling ball algorithm creating black objects on a smooth continuous background in Figure 3 -A2 [START_REF] Sternberg | Biomedical Image Processing[END_REF]. In the progress of its combustion, the particle luminosity changes after the flame extinguishes due to char radiation. Therefore, it is necessary to apply a threshold method for the flame stage followed by another for the char combustion part. During the flame stage we used an iterative procedure based on the IsoData algorithm (Figure 3 -A3) [START_REF] Daho | Study of droplet vaporization of various vegetable oils and blends of domestic fuel oilecottonseed oil under different ambient temperature conditions[END_REF]. For the char combustion part, the external contour was more difficult to detect as the threshold depends strongly on the particle luminosity, which varies with time and particle nature. The particle luminosity was similar to that of the background, so the algorithm was highlighting it in white surrounded by the particle edges as shown in Figure 3 (B2). Indeed, the white inner zone here is the bright region of the particle where the heterogeneous reaction takes place, while the surrounding black region is the burnt part (ash). It is noticeable how the reaction zone during the char combustion separates from the external area of the particle and undergoes internal combustion. Hence, the IsoData method was repeated in this stage with additional treatment and the purpose this time was to follow the evolution of the reaction bright zone, which is directly related to the progress of the heterogeneous combustion. Colors were inverted to keep the detection zone in black for the contour detection later. Further treatment including automatic dilating and filling holes was performed to distinguish as much as possible the zone of interest (B3).

Moreover, the external area at this stage are extracted using the Mean method that fits best the data among seventeen tested methods [15]. The images are then processed by a contour detection program capable of generating the evolution curve of the projection area of the particle as a function of time [START_REF] Daho | Study of droplet vaporization of various vegetable oils and blends of domestic fuel oilecottonseed oil under different ambient temperature conditions[END_REF]. The code can detect the thermocouple and subtract it from the calculation. In order to subtract the glue area and eliminate the sensitivity of the thresholding process, the curves were then rectified by manually integrating several points on each curve and account for the error.

The two image series and the thermocouple data are zeroed at the insertion moment in order to have a synchronicity between the temporal data. Superposition of the data helps to identify the events associated to each phenomenon. Moreover, superposing both imaging techniques provides the moment of the onset of heterogeneous combustion even during the flame phase. In fact, the combustion sub-processes occur simultaneously at high heating rates, but the overlap of homogenous and heterogeneous combustion is difficult to evaluate experimentally. However, using shadowgraphy, the particle can be monitored throughout the whole combustion experiment. The moment oxygen attacks the char surface is associated with a slight brightness at that position. The luminosity is really clear after the flame is completely extinguished but it can be also traced back on shadow images to its onset during the flame period, while the flame still appears to radiate on the direct visible images at the same time. The curve in Figure 4 follows the intensity at the surface of a pine particle. The particle brightness is quite constant before frame 6 (yellow marker) where the intensity commences to increase afterwards till it becomes very bright after the flame extinguishes in frame 11. This increase in intensity at the char surface is then considered as the heterogeneous ignition moment. Once identified, the time difference until the flame extinguishes is defined as the homo-hetero overlap.

Visual observation of the combustion

A general trend of particle behaviour is observed for biomass. A typical size evolution of a biomass particle with time is presented as a series of images in Figure 5 and the corresponding time evolution of particle area in Figure 6. After its insertion to the hot zone, a series of events is triggered. The particle swells for a certain moment due to the increasing pressure of volatiles inside it. The swelling coefficient depends on the biomass type: it is longer for pine than for pint whereas it is negligible for pinp. in Figure 5). The reaction front continues to travel inside the particle (internal combustion) leaving a dark zone behind until the complete burnout. The heterogeneous reaction zone curve in Figure 6 follows the bright zone that separates at a certain moment from the external area curve. The result is a hollow particle of ash that swells sometimes at the end. Fragmentation of the particle is also sometimes observed especially for DW.

Temperature-time history of biomass particles

Similar to area evolution, a trend of temperature profile is observed for biomass during their burnout history.

Figure 7 presents the temperature and its derivative (dT/dt) with time which is the result of the energy balance of the particle. The insertion is at t = 0 sec in synchronicity with images recordings. We assume that the limitation of heat transfer through the glue is negligible, as well as the influence of radiation on the thermocouple, so the measured temperature of the thermocouple is representative of that of the particle.

The temperature derivative peaks at three events: upon insertion and rapid heating, maximum flame size, and when the flame is blown out. The flame was detected visually in the two image series, when the volatile cloud around the particle is lit, and by the sudden increase in the surface temperature profile giving the ignition temperature shown as event #1 on Figure 7. The second peak -event #2 -corresponds to the devolatilization peak where dT/dt decreases afterwards due to lower volatile emission rate. This is also obtained in TGA experiments by the DTG peak and oxygen consumption peak [START_REF] Saldarriaga | Fast characterization of biomass fuels by thermogravimetric analysis (TGA)[END_REF]. The peak value declines with increasing particle mass, which suggests that lighter particles undergo intense pyrolysis. dT/dt reduction arrives to a local minimum (event #3), which coincides with the heterogeneous ignition detected by the previously mentioned technique. The char combustion heats the particle without losing significant heat to the high temperature surrounding flame. Once the volatile flame quenches, the particle starts losing heat, whence the second chute. The peak of event #4 is then attributed to the flame extinguishment. Sometimes a shorter overlap between the volatile and char combustion makes this peak less pronounced. A significant difference between the wall (800°C) and particle temperatures reverses the heat flow direction after the particle reaches its maximum temperature. Subsequently, a balance is established between the energy production and loss until the complete burnout, which is characterized by a third decrease -event #5 -with no more energy production. The temperature then stabilizes at the chamber temperature (800°C).

Combustion durations

Ignition delay

The ignition of biomass particles is in general initiated in the gas phase. The combustion reaction is activated when the volatile cloud around the particle acquires sufficient energy and flammable concentration. The two entities are properties of the volatile gas composition and the heat capacity of the surrounding gas [START_REF] Riaza | Combustion of single biomass particles in air and in oxy-fuel conditions[END_REF]. The TGA curves showed that the devolatilisation commences early (at 221 °C for pine)

and it can be detected in the shadowgraph image time-series by the sudden decrease in the particle volume after building internal pressure. The delay between the release of volatiles and their ignition is so short that they appear simultaneously at 37 frame/sec recording. The ignition delay is defined by the time needed to have a rapid temperature rise after the insertion of the particle. TGA curves are usually used to identify the ignition temperature of biomass. Previous investigations pointed out that the ignition delay times increased with the increase of the initial particle density and diameter. Nevertheless, TGA conclusions cannot be extrapolated for high heating rates conditions. Li et al. [START_REF] Li | Studies of Ignition Behavior of Biomass Particles in a Down-Fire Reactor for Improving Co-firing Performance[END_REF] found that torrefied softwood was faster to ignite than raw softwood at high heating rates. The authors addressed that by the change in ignition mechanism from homogenous to heterogeneous ignition promoted by higher char reactivity of torrefied biomass. Moreover, at high heating rates, the volatiles are easily driven out of high porous particles with barely any swelling detected as in the case for pinp, while the swelling of raw particles implies that the volatile release is delayed until acquiring sufficient pressure to break the particle wall structure. The ignition temperatures measured in the present experiments are shown in Table 3. Raw pine ignites at 240 °C, which is close to the temperature reported in [START_REF] Gucho | Experimental Study on Dry Torrefaction of Beech Wood and Miscanthus[END_REF] for beech wood (235°C). Demolition wood ignites at around 258 °C with high variations (± 41°C) due to the presence of impurities and high particle-toparticle variations. The improved hydrophobicity of torrefied fuel generally reduces its ignition delay compared to high moisture raw counterpart [START_REF] Mason | Single particle flame-combustion studies on solid biomass fuels[END_REF], [START_REF] Akinrinola | Torrefaction and Combustion Properties of some Nigerian Biomass[END_REF]. In this study, the fuels were dried before the experiments, so the influence of moisture is eliminated and the conclusions are purely based on the particle properties. Pint flame appeared at 290 °C due to lower volatile concentration. Despite its poor VM content, pinp ignites even before raw pine at 220 °C where the ignition is dominated by the higher porosity and improved reactivity of pinp. The release of pyrolytic water during torrefaction and the significantly lower density may also have a role in improving the ignitability of the pinp volatiles. In all cases, the presence of volatiles facilitates the ignition at relatively low temperatures while for no flame is visible for the low VM carbonized pine and the heterogeneous ignition is delayed until the particle reaches 346 °C.

Fuel

Volatile flame duration

Figure 8,a plots the flame duration as a function of the particle mass. The quasi linear increase is logical since more volatiles require more time to burn [START_REF] Bridgeman | Torrefaction of reed canary grass, wheat straw and willow to enhance solid fuel qualities and combustion properties[END_REF]. Besides, bigger particle's temperature rises slower than finer particle one at the same heating conditions. This slows down the devolatilisation rate. However, little to no difference is detected between the flame durations of different fuel types. Only, pinp flame duration appears shorter than the other flames due to relatively low volatile content. Akinrinola et al. [START_REF] Akinrinola | Torrefaction and Combustion Properties of some Nigerian Biomass[END_REF] reported a similar result of close flame duration for raw and torrefied Nigerian biomass. It was however reported longer by 10-15% in the torrefied state for Miscanthus and Beech wood in [START_REF] Panahi | Direct observations on the combustion characteristics of Miscanthus and Beechwood biomass including fusion and spherodization[END_REF]. The authors attributed this effect to the increasing influence of the catalytic metals in the fuels. It is also possible that the new pores created during torrefaction facilitate the emission of volatiles. In this case, the flux of volatile emission is lower for the treated biomass, which elongates their emission time in comparison to time needed for same amount of volatiles in their raw counterparts.

Homo-hetero combustion overlap

The overlap of flame combustion and char combustion is introduced in Figure 8,b. Note that the points only represent the particles that their heterogeneous ignition observed on shadow images coincides with the local minimum in the temperature derivative curve. In the other case, the difference between the luminosity detection and the (dT/dt) local minimum for some particles may be addressed by the fact that the ignition may be triggered from the backward side of the particle so that the temperature derivative minimum appears earlier than predicted by the rear view.

The heterogeneous ignition occurs earlier for larger particles containing lower volatile content, and thus increases the overlap duration. Torrefaction of pine induces an increase of overlap duration. The maximum overlap among the pine-derived fuels was obtained for pinp with 0.4 sec for a 1.4 mg particle. On the other hand, a 0.1 mg pine particle had the minimum overlap of 0.076 sec. When the particle is surrounded by flame, the temperature is high enough for the heterogeneous ignition to happen. The heterogeneous ignition of carbonized pine starts at significantly lower temperatures. Nonetheless, the lack of oxygen at the surface induces a delay in the ignition. This may occur early for a particle in a free stream where the motion of the irregularly shaped particle exposes an edge to the hot stream out of the spherical flame envelope. Mock et al. [START_REF] Mock | Combustion Behavior of Relatively Large Pulverized Biomass Particles at Rapid Heating Rates[END_REF] in these conditions observed overlap taking place in the last 40% of the flame duration for coffee waste and sewage sludge particles (150-215 µm). On the other hand, the more spherical treated wood experienced a relatively short overlap (9% of flame duration). However, for a stationary particle in the current study, the char undergoes ignition while enveloped by a flame. Therefore, the ignition here is controlled by the diffusion of oxygen to the surface through the volatile cloud without being consumed or diverted away. Larger particles have larger surface area and therefore it is more probable for them to receive locally more oxygen than smaller ones. This explains why larger particles of the same fuel present char ignition earlier. The same analysis may apply in explaining why the overlap increases with the torrefaction degree. Torrefaction reduces the particle density so that a raw particle of the same mass as a torrefied one has larger size, and thus higher overlap. Moreover, the outward flow of volatile matter seems to obstruct the molecular diffusion of oxygen towards the particle surface. Results show that the volatile flow tends to be more intense with smaller particles and with more VM. Therefore, the probability of oxygen passing the volatile cloud to react with the char is lower for smaller particles and for higher VM content so that the probability of ignition is lower.

Burnout time

The higher overlap between the flame phase and char combustion phase shortens the total burnout duration.

Figure 8,c shows the total burnout duration of each biomass from the moment of ignition to the total consumption of the particle. An increasing tendency of combustion duration versus mass is measured for all fuels. Lower volatile content favours longer combustion duration since more fixed carbon is retained in the char for the torrefied biomass caused by the increase in C-C bonds during the decomposition of the lignocellulose component. Yet, the 45% reduction of VM for pinp does not lead to a significant difference with pine and pint as expected. This could be attributed to the higher interference between the two combustion phases and to the improved catalytic effect of ash in both torrefied fuels due to higher ash content. The role of volatile matter in accelerating the combustion is evident as the low volatile pinc takes up to three times more time to complete its oxidation. On top of rapid combustion of VM in the surrounding flame, volatile ejection deforms the particle and creates new pores in the char structure enhancing its reactivity.

Evolution of the projection area

The evolution of the projection area gives an indication of the particle size evolution assuming homogeneous and isotropic behaviour. In order to compare the area evolution of different particles, the projection area is normalized by the initial projection area (at t=0) versus the normalized combustion time t/t_burnout where t_burnout is the total combustion duration of each fuel type. On the other hand, the unburnt fraction (𝑈 1 𝑐𝑜𝑛𝑠𝑢𝑚𝑒𝑑 𝑚𝑎𝑠𝑠/𝑖𝑛𝑖𝑡𝑖𝑎𝑙 𝑚𝑎𝑠𝑠) is also traced versus the normalized combustion time. The unburnt fraction is obtained from combustion experiments of the samples performed in a horizontal batch reactor at the same temperature (800°C), where similar combustion events and durations are observed [START_REF] Piednoir | Comportement en combustion de résidus de biomasse : mise en évidence de synergies par mélange sous forme de granulés[END_REF]. Figure 9 shows the normalized area (A/A 0 ) of pine particles and their average associated with the unburnt fraction (U) of pine vs the corresponding combustion time. The procedure is repeated for every single particle and the result is the average of all curves. The coefficient of determination (R²) is higher than 0.9 for all fuel types. The variations between the particles are more pronounced during the char combustion. Nevertheless, high similarity is found during the flame phase. The average curves of each fuel are gathered in Figure 10. The rupture of each curve is the separation point of the flame and char phase (represented as dashed lines in the graph). 44%, 37%, 40% and 46% of the initial projected area are lost during the flame phase for pine, pint, pinp and DW respectively. The curves follow a power trend (𝐴 𝐴 𝑈 ) for the flame part where α is the power coefficient listed in Table 4 for each fuel. It is noticeable that it gets higher with lower VM content except for torrefied pine Pint.

The table also includes the shrinking core model for char combustion that assumes a constant particle density during char combustion. According to the model, char oxidation occurs in a diffusion controlled process. The model depicts well the evolution of the reaction zone rather than the particle volume. DW char is the closest to the model as it undergoes fragmentation during its oxidation and leaves little to no ash matrix after the complete burnout. However, the other fuel chars follow the carbonized pine profile given in Figure 11. The curve represents char oxidation in comparison with the shrinking particle model as a function of the unburnt char fraction Uc, defined as the ratio of the remaining carbon to the fixed carbon content. The model is fairly correct at the beginning of the char combustion but the data diverge later at the point of separation of the two curves in Figure 6 to an approximately constant volume as the combustion progresses. Carbon is reacting with small variation of the external volume that ends with 33% to 39% of the initial projection area after the complete burnout of all the fuel types except for DW with low ash residue. Higher ash content accounted for larger ash matrix at the end. The char was sometimes swelling in the course of its oxidation. This is obvious in the curve of carbonized pine that increases from Uc  0.3 and then shrinks again to reach 56% of its original shadow at the end. The swelling is probably caused by trapped volatiles in the ash matrix (most likely lignin volatiles) that are liberated at the end because of the internal pressure increase. Ejections of particle fragments are also observed at the end when the particle swells. The phenomenon is notable for pint and it can be spotted by a slight increase in the slope of torrefied pine size as U approaches zero. Assuming spherical particles and isotropic consumption, the particle volume can be directly evaluated from its projection area. Even with initial irregular particle shape which rapidly tends to disappear and becomes more rounded than the mother particle [START_REF] Panahi | Direct observations on the combustion characteristics of Miscanthus and Beechwood biomass including fusion and spherodization[END_REF], [START_REF] Meesri | Experimental and numerical analysis of sawdust-char combustion reactivity in a drop tube reactor[END_REF].

On the difference between biomass particle and coal particle combustion

The difference between the combustion of coal and biomass is obviously demonstrated by the classical characterization methods like TGA. These differences are the result of the physical and chemical variations including density, composition (proximate and ultimate analysis), structure and porosity. The huge difference in the volatile content is a prominent change that defines the flame characteristics and the char properties. The chemical composition of the fuels is also different and the reaction is slower on coal due to the difference of the intrinsic reactivity of the material as much as the surface area of the char [START_REF] Gil | Kinetic models for the oxy-fuel combustion of coal and coal/biomass blend chars obtained in N2 and CO2 atmospheres[END_REF]. Biomass flames are transparent and possess uniform shape (near spherical) regardless of the particle irregular shape, to the contrary of coal flames that are sooty and disturbed. The volatiles released from biomass consist mostly of light hydrocarbons. Tars and heavy hydrocarbons crack expediently into light gases and pyrolytic water before reacting with oxygen. Moreover, biomass volatiles have high oxygen proportion leading to the formation of CO and CO 2 . The latter inert gas dilutes the volatile cloud around the particle and reduces the flame luminosity. In contrast, coal volatiles contain mostly heavy hydrocarbons and tars that appeared to condense in a sooty contrail (Figure 12). Upon heating, the particle swells considerably indicating high internal pressure due to lower porosity compared to biomass, which does not permit the volatiles to flow out easily [START_REF] Gil | Oxy-fuel combustion kinetics and morphology of coal chars obtained in N2 and CO2 atmospheres in an entrained flow reactor[END_REF]. The pressure is relieved by brutal release of jets containing dense species that cast a shadow in the monochrome video. The dense trails are precursors of soot formation so they condense and agglomerate to develop a contrail of intense luminosity inside the flame. It can be distinguished from the flame by its high luminosity while entraining the flame in a large fluctuation. The condensed material extends outwards to a long distance forming a bright tail. The contrail sometimes bends back to the particle forming a closed loop (Figure 12) or extends long to the point that it cannot be held by the particle causing its far end to bend and break (Figure 13). Since more time is needed to oxidize tars especially in low oxygen environment like the flame, the attached condensed structure persists after the flame extinction and undergoes oxidation to disappear gradually in less than a second. The same phenomenon is reported for bituminous coal in [START_REF] Mclean | Direct observations of devolatilizing pulverized coal particles in a combustion environment[END_REF][29][2] [START_REF] Marek | Experimental studies of single particle combustion in air and different oxy-fuel atmospheres[END_REF]. The flame is followed by a bright particle that burns gradually.

The high magnification of the shadowgraph videos allows observing in details the combustion taking place on the surface of the particle. Note that the heterogeneous combustion of coal is maintained mostly at constant temperature. At the beginning of the char combustion, the particle appears barely compared to the soot contrail. As the burning time goes by, the oxidation reaction intensifies and the char glowing as well. Some chars burn with constant volume where oxygen penetrates the cenosphere interior through the pores and the combustion continues internally similar to the biomass char combustion. In this case, the particle glow fades slowly as the combustion proceeds internally. However, the particle volume shrinks most of the time, with small filaments appearing on the displaced surface (Figure 14). The formation of filaments has been also detected by Olsbye et al. on coke formed during methane-to-synthesis gas reactions [START_REF] Olsbye | An investigation of the coking properties of fixed and fluid bed reactors during methane-to-synthesis gas reactions[END_REF]. These filaments are distributed radially at the surface, and thereafter, separate from the char as fly ash. Due to lower reactivity and higher fixed carbon in coal, the biomass char burns up to five times faster than coal.

This also applies for biomass with comparable volatile matter and fixed carbon content as coal. For example, pinp with about 52% FC burns four times faster than the 61% FC coal. This is addressed by the faster char gasification reactions induced by the higher oxygen to carbon ratio in biomass char [START_REF] Matsumoto | Gasification reaction kinetics on biomass char obtained as a by-product of gasification in an entrained-flow gasifier with steam and oxygen at 900-1000[END_REF]. It is also attributed to enhanced catalytic role of alkali metals in biomass chars due to higher concentration [START_REF] Ollero | The CO2 gasification kinetics of olive residue[END_REF].

Conclusion

The combustion of single particles of raw and torrefied pine, demolition wood and coal is studied using simultaneous shadowgraphy and direct emission imaging techniques, allowing detailed investigation of the combustion events at the particle level. It gives insight to the phenomena occurring at the particle surface and around it during the whole process, including the onset of heterogeneous ignition and the volume and shape evolution. The results verify the important role of the VM content and particle size in determining the combustion intensity and durations. Higher VM particles undergo intense devolatilisation for comparable durations as less volatile containing fuel. The flame duration appears to be more influenced by the particle size than the particle volatile content. This leads to more intense combustion for higher VM particles that obstructs the oxygen diffusion to the particle and delays the heterogeneous ignition. On the other hand, the char left is more reactive and burn faster than the char resulting from particles with lower VM content. A char resulting from a particle with no initial VM burns three times slower than its raw counterpart. On the other hand, larger particles mean slower temperature rise before and after ignition and lower devolatilisation intensity. This, beside the larger surface area, increases the probability of earlier heterogeneous ignition and longer overlap.

The projection area evolution is traced versus the burnout using gas emission results. VM occupy around 40% of the initial particle shadow which decreases with devolatilisation progress following a power trend.

The power coefficient doubles with intense pre-treatment of the fuel. The latter leads to increase the char volume, which shrinks after the flame end following the shrinking core model. But it diverges later to a constant residual ash volume, which increases with more intense pre-treatment. An empirical model can be extracted from the curves assuming spherical particles. The volume evolution obtained by shadowgraphy will be integrated in a 1-D solid combustion model developed and validated for different compositions and preparations of biomass. A comparison between biomass and coal shows the visual differences of the gas phase composition and the char structure giving rise to differences in the flame and char combustion of both fuels.

LED

Figure 1 :

 1 Figure 1: Particle combustion shadowgraphy experimental apparatus

Figure 2 :

 2 Figure 2: TGA profiles showing the influence of glue on the combustion of pine

Figure 3 :Figure 4 :

 34 Figure 3: Image treatment for the contour detection of the external area of a particle during devolatilisation (left) and the heterogeneous reaction zone (right)

Figure 5 :Figure 6 :

 56 Figure 5: High magnification shadow images of the combustion of single particles of raw pine, torrefied pine, pyrolized pine and demolition wood. The displayed number for each image is the frame number recorded at 37 fps from the insertion moment

Figure 7 :

 7 Figure 7: Temperature variation of biomass particle during combustion. The corresponding events (1-5) are detailed in the text.

Figure 8 :

 8 Figure 8: Combustion durations: a) flame; b) overlap; c) burnout

Figure 9 :Figure 10 :Figure 11 :Table 4 :

 910114 Figure 9: Normalized area (A/A 0 ) and unburnt fraction (U) vs the normalized combustion time of pine

Figure 12 :Figure 13 :Figure 14 :

 121314 Figure 12: Soot contrail forming closed loop during coal particle combustion

Table 1 :

 1 Fuel composition analysis (%db)

		pine	pint	pinp	pinc		DW	Coal
	Ash	0.87	1.33	2.54	3.24		2.13	2.03
	VM	81.22	73.62	44.83	19.59	78.29	37.08
	FC	18.58	25.05	52.62	77.17	19.57	60.89
	C	51.68	55.10	67.77	79.87	48.51	82.14
	H	5.93	5.68	4.50	3.44		5.63	5.57
	N	0.22	0.29	0.32	0.39		4.39	1.54
	O	41.27	37.52	25.15	16.28	40.58	8.71
	LHV (MJ/kg)	18.45	19.744	25.30	28.33	17.70	32.34
		pine	pint	pinp			pinc	DW	Coal
	Particle shape	Needle-like	cylindrical cylindrical	Near-spherical	Flake-like	Near-spherical
	Aspect ratio	2.50±0.70 1.67±0.40 1.65±0.41 1.41±0.29 2.08±0.48 1.28±0.26
	Particle size (mm) (min-max)	0.68-1.57	0.42-1.15	0.49-1.25	0.49-1.02	0.50-1.74	0.48-0.93
	Mass (mg) (min-max)	0.1-1.8	0.1-1.3	0.1-1.1			0.1-1.1	0.5-2.1	0.6-1.6
	Number of tested particles	32	29	30			16	31	25

Table 2 :

 2 Physical appearance of the tested particlesIn addition, Venezuelan coal was tested aiming to show the different behaviour of combustion of biomass and a coal even when containing a high volatile content. The proximate and ultimate analyses of the fuel are listed in Table1obtained following the ASTM norms. Table

Table 3 :

 3 Ignition temperature of the tested fuels

		Pine	Pint	Pinp	Pinc	DW
	Ignition temperature (°C)	240 ± 11 290 ± 22 220 ± 26 346 ± 14 258 ± 41

Acknowledgments

The financial support of ANRT for H. Mohanna's PhD scholarship is acknowledged (Convention CIFRE -N° 2016/0523).