Sylvain Rodat
email: sylvain.rodat@promes.cnrs.fr

Stéphane Abanades

Gilles Flamant

CO-PRODUCTION OF HYDROGEN AND CARBON BLACK FROM SOLAR THERMAL METHANE SPLITTING IN A TUBULAR REACTOR PROTOTYPE

Keywords: Solar energy, methane cracking, carbon black, hydrogen, process economics. 1.Introduction

This study addresses the solar thermal decomposition of natural gas for the co-production of hydrogen and Carbon Black (CB) as a high-value nano-material with the bonus of zero CO2 emission. The work focused on the development of a medium-scale solar reactor (10 kW) based on the indirect heating concept. The solar reactor is composed of a cubic cavity receiver (20 cm-side), which absorbs concentrated solar irradiation through a quartz window by a 9 cm-diameter aperture. The reacting gas flows inside four graphite tubular reaction zones that are settled vertically inside the cavity. Experimental results in the temperature range 1740K -2070K are presented: acetylene (C2H2) was the most important by-product with a mole fraction of up to about 7%, depending on the gas residence time. C2H2 content in the off-gas affects drastically the carbon yield of the process. The effects of temperature and residence time are analyzed. A preliminary process study concerning a 55 MW solar chemical plant is proposed on the basis of a process flow sheet. Results show that 1.7 t/h of hydrogen and 5 t/h of CB could be produced with an hydrogen cost competitive to conventional steam methane reforming.

In order to counter the rising CO2 emissions, the decarbonisation of fossil fuels is required and the ultimate step is the hydrogen production. Hydrogen appears as one of the most promising energy carriers along with electricity. Natural gas (mainly composed of methane) may play a major role on the way towards a carbon free energy (DOE, 1999). It is the cleanest hydrocarbon fuel (highest H/C ratio) and it can benefit from transportation infrastructures. Nowadays, hydrogen is mainly produced via steam methane reforming (SMR) (Dahl et al., 2004a) while Carbon Black (CB) is principally produced from the furnace process [START_REF] Donnet | Carbon Black, Second Edition[END_REF]. Both processes release large amounts of CO2 in the atmosphere. It is estimated that steam methane reforming produces 11.9 kg CO2 eq. per kg of hydrogen produced and the furnace process about 5.7 kg CO2 eq. per kg of CB produced. Solar thermal methane cracking can be a solution for the clean co-production of hydrogen and carbon black. About 92% of the pollution associated with the two conventional processes can thus be eliminated [START_REF] Wyss | Rapid Solarthermal Decarbonization of Methane in a Fluid-wall Aerosol Flow Reactor -Fundamentals and Application[END_REF]. The overall reaction can be written as:

CH42H2+C

(1) H• = 75 kJ.mol -1 (216 kJ.mol -1 for CH4 at 298K and products at 2000K) Nevertheless, a more complex reaction scheme must be used to explain the production of by-products such as C2H2, C2H4, C2H6, and more complex molecules like polycyclic aromatic hydrocarbons (PAH). The kinetic mechanism can be described either as a simplified stepwise dehydrogenation: 2CH4C2H6C2H4C2H2C [START_REF] Back | Thermal decomposition and reactions of methane[END_REF] or as a more complex scheme including a free radical mechanism composed of 36 and 119 reactions as proposed by [START_REF] Olsvik | Pyrolysis of methane in the presence of hydrogen[END_REF] and [START_REF] Billaud | Thermal decomposition of pure methane at 1263 K. Experiments and mechanistic modelling[END_REF], respectively. A kinetic study of methane decomposition in a tubular solar reactor along with numerical simulations of the reactor were presented previously (Rodat et al., 2009a(Rodat et al., , 2009b)). At thermodynamic equilibrium, the methane dissociation is complete for temperatures above 1500K [START_REF] Hirsch | The solar thermal decarbonization of natural gas[END_REF].

Special interest was given to solar methane thermal cracking for hydrogen production during the last years.

Two possible reactor designs were investigated, the direct [START_REF] Kogan | Production of hydrogen and carbon by solar thermal methane splitting. I. The unseeded reactor[END_REF][START_REF] Trommer | Kinetic investigation of the thermal decomposition of CH4 by direct irradiation of a vortex-flow laden with carbon particles[END_REF][START_REF] Kogan | Production of hydrogen and carbon by solar thermal methane splitting. II. Room temperature simulation tests of seeded solar reactor[END_REF][START_REF] Hirsch | Solar hydrogen production by thermal decomposition of natural gas using a vortex-flow reactor[END_REF][START_REF] Abanades | Hydrogen production from solar thermal dissociation of methane in a hightemperature fluid-wall chemical reactor[END_REF] and the indirect heating concepts (Dahl et al., 2004a;[START_REF] Wyss | Rapid Solarthermal Decarbonization of Methane in a Fluid-wall Aerosol Flow Reactor -Fundamentals and Application[END_REF]Dahl et al., 2004b). In the indirect heating configuration, the solar irradiated zone is separated from the reacting flow thereby avoiding the particle deposition issue on the window, whereas the direct irradiation of reactants provides an efficient means of heat transfer directly to the reaction site but particle seeding is required for efficient gas heating. This work reports on the results obtained with a 10 kW indirectly heated reactor prototype featuring a cubic graphite cavity (behaving as a black body cavity) with 4 vertical inside tubular reaction zones. This design enables the control of the reactor tube temperature and is also suitable for scaling-up. The design and economic assessment of a commercial solar chemical plant are then outlined.

2.Experimental section

2.1.Experimental set-up

The experimental set-up, shown in Fig. 1, has already been described by Rodat et al. (2009c). It is composed of a 20 cm-side graphite cavity receiver (blackbody absorber of cubic shape). A 9 cm-diameter aperture lets concentrated solar radiation entering within the cavity through an hemispherical quartz window. Thus, the inside cavity swept by nitrogen is separated from ambient oxidizing atmosphere. The reaction takes place in the four tubular graphite zones settled in parallel and vertically in the solar absorber. Each reaction zone, fed independently by a mixture of Ar and CH4, is composed of two concentric graphite tubes (Fig. 2): an inner tube for gas inlet (12 mm O.D., 4 mm I.D., 345 mm length) and an outer tube for gas outlet (24 mm O.D., 18 mm I.D., 380 mm length, closed at the bottom). The gas enters the inner tube and flows out by the annular space between the outer and inner tubes. The graphite tubes are heated up by both direct solar radiation coming from the aperture and by IR radiation from the cavity walls. The heated tube length inserted in the graphite cavity is about 0.161 m, the remaining part (about 0.203 m-length) corresponds to the insulation zone. Three different insulating layers envelop the reactor cavity in order to limit conduction losses. In total it forms an insulation layer of 0.15 m (0.05 m for each insulating material). The surrounding outer shell of the reactor is made of stainless steel (535x535x373 mm). The reactor is designed for a nominal power of 10 kW and it is set-up at the focus of the 1 MW solar furnace of CNRS-PROMES laboratory. The furnace is composed of a field of 63 heliostats for full power (45 m² per heliostat) and of a parabolic concentrator (1830 m 2 ) delivering up to 9000 suns at the focal plane. During experiments at 10 kW scale, only a fraction of the parabola is used by limiting the number of the sun-tracking heliostats and by using a shutter and a small reactor cavity opening (9 cmdiameter aperture). A photograph of the experimental set-up including the solar reactor is displayed in Fig. 3.

The first experimental step was the heating of the reactor under an argon flow in the tubes. Once the desired temperature is reached, the mixture of argon and methane was injected with a controlled composition. Two mass-flow meters were dedicated to each tube to control accurately the Ar and CH4 flow-rates. The temperature was measured by a solar blind optical pyrometer (wavelength: 5.14 µm) pointing toward the outer wall of a graphite tube inside the cavity through a CaF2 window and by a Pt-Rh thermocouple in contact with the graphite cavity wall. At the exit of the 4 tubes, the exhaust gas-solid flows were collected and mixed together. The resulting gas temperature was about 373K, then the products were further cooled down and flowed through a filter bag to separate carbon particles. The pressure was monitored by pressure sensors placed at each tube entrance and was regulated with the use of a vacuum pump. Pressure monitoring allowed the detection of tube blocking mainly due to thermophoretic deposit at the tube outlet. The filtered gas was then analyzed to determine the gas composition. A continuous analyzer permitted to monitor the concentrations of H2 and CH4.

The methods used for H2 and CH4 analysis were thermal conductivity and non-dispersive infrared detection, respectively. A gas chromatograph also measured online the outlet concentration of CH4, C2H6, C2H4, C2H2, and H2. The chromatograph (Varian CP 4900) was equipped with 2 columns: MolSieve 5A PLOT for H2 and CH4, and PoraPLOT U for light hydrocarbons (C2Hy). The chromatography analysis was based on thermal conductivity detection and the carrier gas was argon, also used as buffer gas during methane cracking experiments.

2.2.Experimental results

For each experimental condition, the following parameters were calculated:

 The space time [START_REF] Nauman | Residence time theory[END_REF] of the gas species is calculated by dividing the volume Vr of the isothermal part of the tubes (part inserted in the graphite cavity) by the volumetric inlet flow-rate of argon and methane at the actual tube temperature and pressure (Q0). It gives a characteristic reaction time even if chemical expansion is not included.

0 r Q V   (2)
 The CH4 conversion gives the proportion of methane that has been transformed and it is defined as:

4 4 4 4 CH 0, CH CH 0, CH F F.y F X   (3)
 The H2 yield is the proportion of methane that has been converted into hydrogen and it is calculated from:

4 2 2 CH 0, H H 2.F F.y Y  (4)
 The C yield is the proportion of methane that has been converted into solid carbon and it is expressed as:

4 6 2 4 2 2 2 4 4 CH 0, H C H C H C CH CH 0, C F ) F.y . 2 F.y . 2 F.y . 2 -(F.y F Y     ( 5
)
where F is the total molar outlet flow-rate (including argon as buffer gas) obtained from:

6 2 4 2 2 2 2 4 H C H C H C H CH Ar F.y F.y F.y F.y F.y F F       (6)
Eq. ( 6) assumes that other species different from Ar, CH4, H2, C2H2, C2H4, C2H6 were not present in the offgas. This assumption was confirmed by gas chromatography analysis. The total molar flow-rate was also determined with a mass-balance on H, resulting in values about 5% higher than the ones obtained from Eq. ( 6).

Resulting errors on methane conversion and on H2 and C yields were thus estimated based on a differential calculus.

This set of indicators was the foundation of the discussion below.

2.3.Influence of the temperature

A maximum operation temperature of 2073K has been achieved. In Fig. 4, CH4 conversion and C2H2 mole fraction are plotted versus space time for different temperatures (1740K, 1823K, 1973K, and 2073K). The methane mole fraction in the inlet gas is kept constant at 20% (the rest being Ar). The total gas flow-rate is changed, which thereby determines the space time of the gas. The influence of temperature is noteworthy: the higher the temperature, the higher the conversion for a given space time. For a 11 ms space time, CH4 conversion is 62%, 79%, 93%, and 100% at 1740K, 1823K, 1973K, and 2073K, respectively. The CH4 conversion is complete for temperatures higher than 1823K and space times higher than 25 ms. Concerning C2H2 mole fraction in the off-gas, it seems to be more affected by space time than by temperature. The lowest C2H2 concentrations are reached for the highest space times.

2.4.Influence of the space time

The influence of space time was studied at a given temperature (1823K) for a constant CH4 mole fraction in the feed gas mixture (10%). Space times between 15 and 70 ms were considered (Fig. 5). In order to change the space time, flow-rates were modified. The first conclusion is that the CH4 conversion and the H2 yield are improved when increasing the space time. A more detailed analysis of the results also gives a scheme of the methane decomposition sequence. For space times between 15 and 18 ms, the H2 yield increases due to a better CH4 decomposition rate (the off-gas mole fraction of CH4 decreases), while the off-gas mole fraction of C2H2 slightly increases. It seems that methane is converted into acetylene, thus releasing 1.5 mole of H2 per mole of CH4 converted. For higher space times, the H2 yield is enhanced due to acetylene decomposition. Acetylene constitutes a reaction intermediate as discussed previously. Similar trends are observed in Fig. 4. For the lowest temperatures (1740K and 1823K), C2H2 off-gas concentrations present a maximum whereas C2H2 decreases quite monotonously for higher temperatures most probably because the maximum is reached for shorter space times. This suggests that C2H2 formation (CH4 decomposition) is faster at high temperatures. Carbon yield is also reported in Fig. 5 and it can be observed that the decrease in the C2H2 off-gas mole fraction corresponds closely to an increase in carbon yield. C2H2 is thus converted into carbon black and H2. Hydrogen and carbon yields are two interesting parameters that permit to follow and quantify the achievement of the methane dissociation process: namely the co-production of hydrogen and carbon black.

Error bars on YC resulting from the uncertainty on F (Eq. ( 6)) are reported in Fig. 5 and they obviously increase when the inlet methane flow-rate increases (i.e. the space time decreases). For clarity reasons, error bars for the other parameters are not plotted. Anyway, the highest relative uncertainties were observed for YC.

A typical carbon mass balance showed that about half of the initial carbon content in the feed was found in the form of C2H2 at the reactor exit, 2% was found as particles in the filter, 34% in the tubes (mainly thermophoretic carbon deposit), 3% in the non-converted CH4, the rest being attributed to other hydrocarbons and un-removed deposit. The CB showed primary particle sizes between 10 and 100 nm, which is comparable to commercial conductive carbon blacks (TIMCAL E250G, 20-40 nm). Up to 10% of thermal efficiency was reached at laboratory scale (Rodat et al., 2009c). These promising results led us to think about a potential industrial application. Consequently, a preliminary process study was performed.

3.Process analysis at 55 MW scale

3.1.Power transfer from the sun to the reactants by the solar concentrating system A process analysis was performed at 55 MWsolar scale (similar to the industrial solar tower power plant in Seville, PS10). This study is based on the work of [START_REF] Charvin | Analysis of solar chemical processes for hydrogen production from water splitting thermochemical cycles[END_REF] for the scaling up of water-splitting thermochemical cycles. Such cycles (e.g. ZnO/Zn cycle) show similar working temperatures to the solar thermal cracking process. The solar concentrating system is a central tower configuration, as the most suitable solution for large scale production. The receiver is located at the top of the tower where the solar beam, reflected by the heliostat field, is concentrated. It is supposed that a solar concentration ratio C of 3000 is reached at the reactor entrance (for a Direct Normal Irradiation (DNI) of 1 kW/m²). This high concentration ratio may require a secondary concentrator. The fraction of the incoming solar radiation reaching the receiver is supposed to be 80% (ηHeliostat field=0.8), due to atmospheric absorption, shadowing and cosine effects, and guidance errors.

Then, the fraction of solar power that enters the reactor amounts to 85.5% of the power reaching the receiver (ηReceiver=0.855) due to losses at the reactor aperture caused by concentration defects (10% of losses) and back reflection at the window (5% of losses). A sensitivity study is proposed in the following to quantify the importance of these assumptions. The reactor efficiency is determined from:

actor Conduction Radiation actor actor Q Q Q Q Re Re Re ) (    
with Qreactor, the power entering the reactor, Qradiation the power lost by IR-radiation through the window, and Qconduction the conduction losses through the reactor walls. The absorption efficiency of a perfectly insulated black-body cavity can be written as [START_REF] Steinfeld | Encyclopedia of physical science & technologie[END_REF]:

C DNI T T Q Q actor actor Radiation Absorption . ) ( 1 1 4 0 4 Re Re       
with σ, the Stephan Boltzmann constant (5.67.10 -8 W.m -2 .K -4 ), Treactor, the reactor temperature and T0=298.15K. Assuming 20% of conduction losses, the reactor efficiency can finally be written as: , that is to say, 39% of the incident solar radiation captured by the heliostat field is transferred to the reactants (for their heating and for the reaction). Fig. 6 illustrates these data for each components of the solar chemical plant including the heliostat field, the tower receiver and the solar reactor.

3.2.Hydrogen and carbon black production: process design

In the previous part, the power available for carrying out the reaction was estimated. Consequently, it is now possible to determine the production capacity of a first process design (Fig. 7). A 3 modules reactor at the top of a tower is envisaged with three different apertures and dedicated heliostats fields (North, East, and West fields). Secondary concentrators must be used. Each tower is expected to operate at about 11 MWsolar (divided in the three modules). A tubular receptor is being designed to work with turbulent flows (instead of laminar, as it was done experimentally). This will improve both heat transfer and CB transportation. In addition, the objective is to maintain a sufficiently high residence time (about 100 ms) to reach a satisfactory chemical conversion (see Fig. 5).

The feed is introduced in the reactor at 298K. It is a mixture of 50% methane (fresh and recycled) and 50% hydrogen (from PSA) pre-heated up to 1173K by the hot products. The CH4 conversion in the reactor at 1873K is assumed to be 90% and it is supposed that only hydrogen is produced (YH2/XCH4=1). Then in order to reach complete CH4 conversion the unconverted hydrocarbons must be recycled at the reactor exit, which requires a gas separation unit (after the gas-particle separation). Pressure swing adsorption (PSA) is preferred for separating hydrogen from the other gaseous species. The solar power available in the reactor (  reactor the in Available P 21.5 MW) permits to heat the reactant from 1173K to the reaction temperature and to provide the power needed for the endothermic cracking reaction. This determines the possible methane flowrate in the feed ( At the exit of the reactor, the products are cooled down by the reactants and further to enter the filtering device at 473K. The carbon black is separated and the gaseous products are compressed and cooled down before entering the PSA system. This system is supposed to produce pure hydrogen with 80% recovery (20% of the initial hydrogen content is released with the off-gases, that is to say, it is not separated). The PSA off-gases are recycled for reaching complete methane conversion, which permits to overcome kinetic limitations responsible for unconverted gaseous species. A production of 5 t/h of carbon black and 1.7 t/h of hydrogen is obtained (for 6.7 t/h of fed methane). An average operation time of 2000 h per year at nominal power is chosen. It was evaluated that the possible exergy recovery (from products cooling) can largely compensate the compression work (that represents the main exergy requirements). Consequently, the plant is supposed autonomous and thus, neither electricity purchases nor electricity sales are considered. An economic analysis was conducted from a spreadsheet edited by DOE (2004). The same assumptions as the ones used by [START_REF] Charvin | Analysis of solar chemical processes for hydrogen production from water splitting thermochemical cycles[END_REF] are made regarding the size and cost of the solar concentrating system. The total investment cost is supposed to be the same (16 M$). Fig. 8 provides the hydrogen production costs versus the carbon selling price (natural gas price of 0.24$/Nm 3 (6$/GJ (HHV)) for reference year 2005). The cost of hydrogen produced by SMR (Steam Methane Reforming) is also reported (1 $/kg [START_REF] Muradov | Green'' path from fossil-based to hydrogen economy: An overview of carbon-neutral technologies[END_REF]). The reference case is plotted for a 15% IRR (Internal Rate of Return) and 16 M$ of investment. It shows lower H2 production costs than the ones reported by Dahl et al. (2004a) for a solar installation about half the size of the present case. Thus, a second case is plotted with a 15% IRR and a doubled investment (32 M$). As a result, the H2 production costs are similar to the ones of Dahl et al. A sensitivity study was also achieved to highlight the influence of the heliostat field efficiency. If it decreases from 0.8 to 0.65, the hydrogen production drops to 1.4 t/h and the hydrogen production price rises from 0.84 to 1.22 $/kg for a carbon black selling price of 0.7 $/kg. The optimization of the concentrating system is thus a very strong improvement factor. For the reference case, solar hydrogen production becomes competitive with SMR at a carbon black selling price of 0.7 $/kg, which was a typical price of standard carbon black in 2007 [START_REF] Adams | Booming world carbon black demand but price rises fail to keep pace with rising gas & feedstock costs[END_REF]. Accordingly, the investment for such a solar plant should not exceed 16 M$ for being competitive with current non-renewable process.

4.Conclusion

Thermal methane dissociation was carried out successfully in a prototype-scale solar reactor operating in the temperature range 1740K -2070K. The influence of two main parameters, temperature and space time, on the reactor performances was stressed. Increasing the temperature permits to enhance methane conversion. Nevertheless, it is important to favour long residence times to lower the C2H2 off-gas content and to increase the carbon yield. In addition, a process analysis for a 55 MW solar tower plant was outlined and the production was estimated to be 1.7 t/h of hydrogen and 5 t/h of carbon black. An economic evaluation showed that the hydrogen production cost can be competitive to the conventional steam methane reforming if the CB is sold at the current market price for a 16 M$ investment. A 50 kW solar reactor is now developed to provide further information on the process at a larger scale, especially concerning the carbon black properties with respect to the commercial standards.

  (Eq. 1) at 1873K (J/mol).

  Figure captions

Acknowledgments

This study was funded by the European Project "Solhycarb" (FP6, Contract SES-CT2006-19770). The authors wish to thank J.L. Sans, M. Garrabos, and O. Prévost for their contribution to experimental campaigns.


Sylvain Rodat
email: sylvain.rodat@promes.cnrs.fr.

Stéphane Abanades

Julien Coulié

Gilles Flamant

Julien Coulie

Kinetic modelling of methane decomposition in a tubular solar reactor

Keywords: Methane cracking, Kinetic model, Plug-flow reactor, Hydrogen production, Solar reactor

à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

Special interest is given to the production of hydrogen as the future energy vector. Several H2 production processes have reached maturity for commercial exploitation such as steam methane reforming (SMR), partial oxidation of heavy oils, catalytic decomposition of natural gas, and coal gasification, while other non-commercial H2 sources are being developed such as methane pyrolysis, electrochemical processes, thermochemical water-splitting cycles, biomass gasification, and photobiological processes [START_REF] Momirlan | Current status of hydrogen energy[END_REF]. Currently, hydrogen is mainly produced from fossil fuels and especially natural gas (consisting primarily of methane). These conventional processes lead to important CO2 emissions. It was estimated that the global warming potential (GWP) of hydrogen production via the SMR process is 13.7 kg CO2 (equiv.) per kg of H2 produced (CO2 accounts for 77.6 % of the system's GWP) [START_REF] Spath | Life cycle assessment of hydrogen production via natural gas steam reforming[END_REF]. In order to avoid environmental impacts, new solutions are investigated. A promising one is the methane thermal dissociation [START_REF] Kogan | Production of hydrogen and carbon by solar thermal methane splitting. I. The unseeded reactor[END_REF][START_REF] Dahl | Solar-thermal dissociation of methane in a fluid-wall aerosol flow reactor[END_REF][START_REF] Hirsch | Solar hydrogen production by thermal decomposition of natural gas using a vortex-flow reactor[END_REF][START_REF] Trommer | Kinetic investigation of the thermal decomposition of CH4 by direct irradiation of a vortex-flow laden with carbon particles[END_REF][START_REF] Kogan | Production of hydrogen and carbon by solar thermal methane splitting. II. Room temperature simulation tests of seeded solar reactor[END_REF][START_REF] Dahl | Rapid solar-thermal dissociation of natural gas in an aerosol flow reactor[END_REF][START_REF] Zedtwitz | Hydrogen production via the solar thermal decarbonization of fossil fuels[END_REF][START_REF] Abanades | Hydrogen production from solar thermal dissociation of methane in a high-temperature fluid-wall chemical reactor[END_REF] into hydrogen and carbon black (CB), two valuable products. If solar thermal power is used as energy input, no CO2 emission occurs during the process.

The overall reaction can be described as:

CH4  2H2+C (1)
H° = 75 kJ mol -1 (216 kJ mol -1 for CH4 at 298 K and products at 2000 K)

Nevertheless, a more complex reaction scheme has to be used to explain the production of by-products such as C2H2, C2H4, and C2H6 and more complex molecules like polyaromatic hydrocarbons (PAH).

The kinetic mechanism can be described either as a simplified stepwise dehydrogenation:

2CH4C2H6C2H4C2H2C [START_REF] Back | Thermal decomposition and reactions of methane[END_REF] or as a more complex scheme including a free radical mechanisms composed of 36 and 119 reactions as proposed by Olsvik et al. [START_REF] Olsvik | Pyrolysis of methane in the presence of hydrogen[END_REF] and Billaud et al. [START_REF] Billaud | Thermal decomposition of pure methane at 1263 K. Experiments and mechanistic modelling[END_REF], respectively. A kinetic mechanism including only C2H2 as intermediate product was also reported [START_REF] Happel | Acetylene and hydrogen from the pyrolysis of methane[END_REF]. A complete bibliographic survey concerning works on thermal decomposition of methane after 1960 was published by Billaud et al. [START_REF] Billaud | Thermal decomposition of methane: bibliographic study and proposal of a mechanism[END_REF]. Usually, the overall methane decomposition was considered of first order [START_REF] Trommer | Kinetic investigation of the thermal decomposition of CH4 by direct irradiation of a vortex-flow laden with carbon particles[END_REF][START_REF] Gonzalez-Aguilar | Comparison of simple particle-radiation coupling models applied on a plasma black process[END_REF][START_REF] Holmen | High-temperature pyrolysis of hydrocarbons 1. Methane to acetylene[END_REF] but other orders were also reported. For example, [START_REF] Dahl | Intrinsic kinetics for rapid decomposition of methane in an aerosol flow reactor[END_REF] identified a reaction order of 4.4 [START_REF] Dahl | Intrinsic kinetics for rapid decomposition of methane in an aerosol flow reactor[END_REF][START_REF] Dahl | Sensitivity analysis of the rapid decomposition of methane in an aerosol flow reactor[END_REF]. This paper addresses a study on the kinetic scheme of methane decomposition reaction using the Dsmoke software and the comparison with experimental results obtained with a 10 kW tubular solar reactor. This reactor comprising a cavity-type receiver and four independent tubular reaction zones was developed to investigate natural gas cracking with a controlled temperature and with the possibility of scaling-up.

Description of the kinetic model

A kinetic software was used to predict the kinetics of natural gas cracking. The considered software called Dsmoke was developed by Faravelli and Ranzi at the Polytechnic University of Milan [START_REF]User's Manual Dsmoke Program Release 6[END_REF].

This software uses a detailed chemical kinetic scheme, with lumping procedures [START_REF] Ranzi | Low-temperature combustion: Automatic generation of primary oxidation reactions and lumping procedures[END_REF], for the modelling of alkane transformation for a wide composition range (from methane up to Diesel fuel). It includes a detailed kinetic modelling of PAHs (Poly Aromatic Hydrocarbons) and soot formation. The high temperature mechanism for large PAHs and soot particles formation and growth is based on homogeneous reactions between gas phase species. More than 240 species and 14 000 reactions are implemented in Dsmoke code.

This model has been previously tested with respect to a very large set of experimental data for conditions that do not involve transport and diffusion processes, including plug-flow and stirred reactors, batch reactors and shock tubes, rapid compression machines and motored engines. It has been finally validated for a wide variety of chemical engineering problems, such as combustion, oxidative pyrolysis and steam cracking of alkane mixtures for application to future hypersonic propulsion [START_REF] Bruno | CH4 fuel steam reforming for enhanced propulsion performance[END_REF] or for NOx prediction in glass melting furnaces [START_REF] Falcitelli | Modelling practical combustion systems and predicting NOx emissions with an integrated CFD based approach[END_REF].

Using a sectional approach [START_REF] Richter | Detailed modeling of PAH and soot formation in a laminar premixed benzene/oxygen/argon low-pressure flame[END_REF], PAHs composed of more than 20 carbon atoms and soot particles with diameters of up to 60 nm are divided into a limited number of classes covering different mass ranges. Each class is represented by a two or three lumped pseudo-species, called BIN, with a given number of carbon and hydrogen atoms [START_REF] Granata | Detailed kinetic of PAH and soot formation in combustion processes: analogies and similarities in reaction classes[END_REF]. The H/C ratio decreases with increasing molecular mass.

BIN1 has 20 carbon atoms, and includes three different H/C ratios: 0.3, 0.5, and 0.8, so the species included in the model are BIN1A (C20H16), BIN1B (C20H10), and BIN1C (C20H6). BIN(n+1) has twice the number of C atoms and mass range of BIN(n). This exponential sectional approach saves computation time while still maintaining a fair description of the low molecular weight components.

For the smallest BINs, three different H/C ratios are selected (0.8, 0.5, and 0.3), while for larger BINs the selected level of H/C decreases in order to better match the overall dehydrogenation trend. The H/C ratios assigned to the last BINs are 0.2 and 0.05.

For this study, all BINs are considered as soot particles. The model permits to save data concerning a limited number of species (up to 30) even if much more species are computed. Thus, a limited number of the most relevant species was carefully selected: H2, CH4, BINs, PAHs: C6H6, C8H6, C8H8, C9H8, C10H8, C12H8, C13H10, C14H10, C16H10; NMHCs (Non-Methanic Hydrocarbons): C2H2, C2H4, C2H6, C3H4, C3H6, C3H8, C4H2, C4H4 and free radicals: H, CH3, C3H3. The kinetic model accounting for particle nucleation, formation of radicals, growth mechanisms of particles, oxidation mechanisms is fully described in reference [START_REF] Richter | Detailed modeling of PAH and soot formation in a laminar premixed benzene/oxygen/argon low-pressure flame[END_REF]. The residence time is calculated for each iteration with the gas composition corresponding to the calculation step.

Kinetic analysis of the methane decomposition

Before modelling real experiments, a kinetic study of the methane decomposition was conducted. An isothermal plug-flow reactor at 101.325 kPa was assumed. Neither preheating of the reactants, nor cooling of the products was considered. Calculation results are presented as a function of residence time  in the reactor (main variable). As a parametric analysis, the effects of reactor temperature and fuel composition are studied.

Performance criteria are reported in terms of: F is the inlet molar flow-rate of CH4, yi is the mole fraction of species i, and F is the total outlet flow-rate (with argon as buffer gas) obtained from:

6 2 4 2 2 2 2 4 H C H C H C H CH Ar F.y F.y F.y F.y F.y F F       (4)

Kinetic mechanism in an ideal isothermal reactor

The evolution of products mass fractions, CH4 conversion, and H2 yield as a function of residence time in an isothermal plug-flow reactor at 1700 K is given on Fig. 1. A gas feed composed of pure methane is considered. Residence time varies from 10 -5 s to 100 s, which are extreme, non realistic values, but this range is chosen in order to understand the kinetic mechanisms that are involved in H2 and CB formation.

The total mass fraction is very close to 100 % except for residence times between 0.3 ms and 30 ms. A total mass fraction near 100 % means that the selected species are the only main compounds and that other side products are negligible. A total mass fraction lower than 100 % means that side products, different from the selected ones, are also generated. It can be either PAHs that were not selected or other non-selected intermediate hydrocarbons. In this case, the real composition of the products should have to be checked more precisely and accurately. The mass fraction of hydrogen can increase up to 25 %, which corresponds to the theoretical maximum value for complete dissociation of CH4.

Concerning CH4 conversion, it reaches 90 % for a residence time of about 15 ms. A residence time longer than 100 ms is required in order to reach complete conversion. H2 yield increases as soon as CH4 conversion starts. H2 yield strongly increases between 10 -3 s and 10 -1 s. A 90 % yield is reached for a 100 ms residence time. Then for longer residence times, H2 yield only slightly increases up to 100 %. Increasing the residence time results firstly in the formation of C2H6, then C2H4 and C2H2 are produced. This scheme agrees with the successive dehydrogenation of CH4 through C2H6, C2H4, and C2H2 proposed by Back and Back [START_REF] Back | Thermal decomposition and reactions of methane[END_REF]. During these decompositions, the released hydrogen atoms create H2. Next, C2H2 is used as the starting point for the growth of soots, and the release of H2 on the other hand. Nevertheless, the simulation shows that additional compounds are involved in this mechanism since up to 10 % of the total mass fraction is missing (between 3.10 -4 and 3.10 -2 s) when only considering the selected species. This missing mass fraction mentioned above can also be observed for the other temperatures, the higher the temperature, the earlier the appearance of this missing mass fraction. As this result is an indication of the formation of intermediate hydrocarbon (HC) compounds, this means that the off-gas is "dirtier" at low temperature than at high temperature. For example, at 1500 K, intermediate HC compounds are formed between 10 ms and 0.3 s. Despite CH4 conversion significantly begins at  = 10 -4 s, soot formation only appears at  > 2.10 -3 s.

Soot formation increases markedly up to 1 s and increases slightly for larger . The maximum BINs mass fraction (75 %, corresponding to the maximum carbon mass fraction that can be reached from complete CH4 dissociation) would be ideally reached after 100 s. Soot mean diameter increases with the residence time to stabilize around 60 nm (the limit of the model) at large residence times.

Parametric analysis

A parametric analysis was carried out on temperature and natural gas composition.

Figs. 2 and 3 plot the CH4 conversion and H2 yield as a function of residence time for different temperatures ranging between 1500 K and 2300 K. Concerning CH4 conversion, the higher the temperature, the lower the residence time required for the start of CH4 dissociation (Fig. 2). Indeed, CH4 dissociation starts from 5x10 -4 s at 1500 K whereas it starts from 3x10 -6 s at 1900 K. The same tendencies are observed on the H2 yield versus residence time profiles (Fig. 3). However, for a temperature higher than 1900 K, a plateau on the H2 yield profiles is observed between 75 % and 80 % where H2 yield grows up more slightly. This slow down in the H2 yield rise mainly corresponds to unconverted C2H2. At 1500 K, the formation of H2 is delayed and occurs for residence times longer than 1 ms; at this temperature, the final H2 yield is lower than 95 % because a complete conversion into H2 cannot be reached even for the largest residence time considered.

The peaks of formation of all considered light HC compounds (C2H6, C2H4, C2H2 species) are shifted towards low residence times when the temperature increases. Nevertheless, it also appears that C2H2 is still unconverted after long residence times and that the level of remaining C2H2 is the most important at high temperatures (4 % in mass after 10 s at 2300 K). As a result, increasing the temperature does not suppress the formation of secondary hydrocarbons.

BINs formation begins between 2.10 -4 s (0.2 ms) for the highest temperature (2300 K) and 2.10 -2 s (20 ms) for the coolest one (1500 K). For residence times higher than 20 ms, the evolutions of BINs mass fractions are very similar, except for the case at 1500 K where BINs formation begins later and BINs mass fraction does not reach 75 % even after 100 s.

Based on the results presented in Fig. 2, the kinetic parameters for the global reaction of methane dissociation were identified with a simple reactor model. Assuming a first order kinetic expression [START_REF] Trommer | Kinetic investigation of the thermal decomposition of CH4 by direct irradiation of a vortex-flow laden with carbon particles[END_REF][START_REF] Gonzalez-Aguilar | Comparison of simple particle-radiation coupling models applied on a plasma black process[END_REF][START_REF] Holmen | High-temperature pyrolysis of hydrocarbons 1. Methane to acetylene[END_REF] and an ideal plug-flow reactor model, the residence time τ can be calculated from [START_REF] Abanades | Hydrogen production from solar thermal dissociation of methane in a high-temperature fluid-wall chemical reactor[END_REF]:

1 ln( ) 1 ( CH CH X X k          (5)
where α is the chemical expansion factor, β is the physical dilatation factor, and k is the kinetic rate constant, following an Arrhenius law:

) / .exp(-E k k a 0 RT  (6)
where Ea denotes the activation energy (J/mol), k0 the pre-exponential factor (s -1 ), R the universal gas constant (8.314 J/mol K), and T the absolute temperature (K).

For each simulated temperature, it is possible to identify the kinetic rate constant k in Eq. ( 5) which best fits the Dsmoke simulation (Fig. 2). Then, from the plot ln(k)=f(1/T), a linear regression allows estimating a pre-exponential factor of 6.6x10 13 s -1 and an activation energy of 370 kJ/mol. The fair determination coefficient (0.9982) shows that the hypothesis of a first order reaction is adapted. These values of the kinetic parameters are in the range reported for an homogeneous (non-catalytic) methane decomposition reaction [START_REF] Holmen | Pyrolysis of natural gas: chemistry and process concepts[END_REF].

The model sensitivity on gas fuel composition at 1900 K is presented in Fig. 3 for three different natural gases and pure methane. Compositions of natural gases are given in Table 1. CH4 conversion and H2 yield show similar values whatever the gas composition for residence times higher than 1 ms.

For residence times lower than 1 ms, CH4 conversion and H2 yield are better for natural gas than for pure methane due to the presence of ethane. Special care has to be taken if the natural gas contains CO2 (NG3) since it is converted into CO, a highly toxic gas. For potential H2 use in a PEMFC (Polymer Electrolyte Membrane Fuel Cell), purification would be required to avoid the poisoning of the anode catalyst (usually based on platinum) by CO. The maximum tolerance on CO concentration for such application is 100 ppm [START_REF] Ledjeff-Hey | CO2-scrubbing and methanation as purification system for PEFC[END_REF].

Simulation of a tubular solar reactor

Reactor description

The reactor is presented in Fig. 4. It is composed of a 20 cm-side graphite cavity receiver (blackbody absorber of cubic shape). A 9 cm-diameter aperture lets concentrated solar radiation entering within the cavity through a quartz window. Thus, the inside cavity swept by nitrogen is separated from ambient oxidizing atmosphere. The reaction takes place in the four tubular graphite zones settled in parallel and vertically in the solar absorber. Each reaction zone, fed independently by a mixture of Ar and CH4, is composed of two concentric graphite tubes: an inner tube for gas inlet (12 mm O.D., 4 mm I.D.) and an outer tube for gas outlet (24 mm O.D., 18 mm I.D.). The gas enters the inner tube and flows out by the annular space between the outer and inner tubes. The graphite tubes are heated up by both direct solar radiation coming from the aperture and by IR radiation from the cavity walls. The heated tube length inserted in the graphite cavity is about 0.161 m, the remaining length (about 0.203 m) corresponds to the insulation zone (Fig. 5). Three different insulating layers envelop the reactor cavity to lower conduction losses. It forms an insulation layer of 0.15 m (0.05 m for each insulating material). The 3 insulating materials are graphite felt in contact with the cavity (=0.46 W.m -1 .K -1 ), an intermediate refractory ceramic fiber operating up to 1600°C (62% Al2O3, 30% SiO2, = 0.35 W.m - 1 .K -1 at 1400°C), and an outer microporous insulator operating up to 1000°C (20% ZrO2, 77.5% SiO2, 2.5% CaO, =0.044 W.m -1 .K -1 at 800°C). The surrounding outer shell of the reactor is made of stainless steel (535x535x373 mm). The reactor is designed for a nominal power of 10 kW and it is setup at the focus of the 1 MW solar furnace of CNRS-PROMES laboratory. The furnace is composed of a field of 63 heliostats for full power (45 m² per heliostat) and of a parabolic concentrator (1830 m 2 ) delivering up to 9000 suns at the focal plane. During experiments at 10 kW scale, only a fraction of the parabola is used by limiting the number of heliostats tracking the sun and by using a shutter.

Experimental procedure and results

The first experimental step was the heating of the reactor under an argon flow in the tubes. Once the desired temperature reached, the mixture of argon and methane was injected with a controlled composition. Two mass-flow meters were dedicated to each tube to control with accuracy the Ar and CH4 flow-rates. The temperature was measured by a solar blind optical pyrometer (wavelength: 5.14 µm) pointing toward a graphite tube outer wall inside the cavity through a CaF2 window and by a Pt-Rh thermocouple in contact with the graphite cavity wall. Since measurements showed that similar temperatures were reached by the tube wall and the graphite cavity wall (maximum discrepancy of 40 K), the temperature of the tubes inside the cavity was considered uniform [START_REF] Abanades | High-temperature solar chemical reactors for hydrogen production from natural gas cracking[END_REF].

At the exit of the 4 tubes, the exhaust gas-solid flows were collected and mixed together. The gas temperature was about 373 K, the products were cooled down and flowed through a filter bag to separate carbon particles. The pressure was monitored by pressure sensors placed at each tube entrance and was regulated thanks to the use of a vacuum pump. The filtered gas was then analysed to determine the gas composition. A continuous analyser permitted to monitor the concentration of H2 and CH4. The methods used for H2 and CH4 analysis were thermal conductivity and infrared detections, respectively.

A gas chromatograph also measured online the outlet concentration of CH4, C2H6, C2H4, C2H2, and H2.

The chromatograph (Varian CP 4900) was equipped with 2 columns: MolSieve 5A PLOT for H2 and CH4, and PoraPLOT U for light hydrocarbons (C2Hy). The chromatography analysis was based on thermal conductivity detection and the carrier gas was argon, also used as buffer gas during methane cracking experiments.

The main experimental conditions and corresponding results are listed in Table 2. 17 experimental tests were carried out. Temperatures varied between 1670 K and 1770 K and the CH4 flow-rate in the feed varied between 1.2 and 8 L/min (every volumetric gas flow-rates are given at normal conditions: 101.325 kPa and 273.15 K). CH4 mole fraction was between 10% and 33%. The gas residence time ranged from 12 ms to 35 ms and it was calculated by dividing the volume of the tubular reaction zones inserted in the graphite cavity (isothermal zone in Fig. 5) by the total volumetric inlet gas flow-rate calculated at the actual tube temperature and pressure. This residence time assessment did not include the gas flow-rate expansion due to chemical reaction.

The gas-flow in the tubes was laminar with a Reynolds number smaller than 2000. The highest Reynolds number (1900) was obtained at the tube entrance. Then, it decreased with temperature because the gas kinematic viscosity increased faster than the gas velocity (gas feed: 4 L/min Ar and 1 L/min CH4, pressure: 40 kPa, tube diameter: 4 mm, gas kinematic viscosity at 300 K: 3.87x10 -5 m 2 s -1 ).

The duration of an experiment was up to one hour. Results did not depend on the experiment duration since steady state was rapidly reached. However, when the tubes started to block because of carbon deposition, the chemical conversion rate increased because the residence time increased. The experiment was then stopped.

The ratio between the carbon quantity exiting the reactor (weighted mass of solid carbon from the filter and the tubes, acetylene and residual methane from GC analysis) to the carbon quantity introduced (methane) was about 93%. The missing mass of carbon may be the non-removed carbon deposit in the tubes and the other undetected secondary hydrocarbons.

Model of the experimental reactor

The four tubes are identical since they are in parallel in an isothermal cavity. Moreover, the gas feed in each tube is independent and the same gas flow-rates are injected. Experiments also confirm that the conversion rate remains unchanged when stopping the gas feeding in one tube. Therefore, only one tube is considered in the kinetic simulations. When flowing in the tube, gases pass through 3 different zones consisting of a pre-heating zone, an isothermal zone, and a cooling zone. According to the tubular design of the reaction zones and to the laminar flow regime inside the tubes, a plug-flow is the most suitable assumption for the reactor model. Each tube is thus modelled by three plug-flow reactors in series (Fig. 5):

-Pre-heating plug-flow reactor: the temperature increases linearly from 300 K to the measured reactor temperature during the test. It corresponds to the part of the inner tube in the insulating layers. Since graphite has a much higher thermal conductivity than the insulating materials, the temperature gradient along the tube wall is directly connected to the thermal conductivity of graphite according to Fourier's law. Therefore, a linear temperature profile was chosen. This profile did not require any more adjustment because it did not affect the results at all. Indeed, simulation results showed that no reaction occurred in this pre-heating zone because both residence time and temperature were not high enough. It just allowed to increase the gas temperature and it was thus neglected in the kinetic simulations.

It was assumed that the gas temperature reached the reactor wall temperature at the exit of the preheating zone. Two parameters favor the gas pre-heating: first, the argon thermal conductivity is multiplied by 3 between 300 K and 1700 K, second, methane absorbs IR radiations. Besides, the start of the thermally developed region in the entrance region of a tube with constant wall temperature can be determined from [START_REF] Eckert | Analysis of Heat and Mass transfer[END_REF]: From this correlation, xd is about 28 cm for a wall temperature of 300 K and 10 cm for a wall temperature of 1700 K. Thus, the mean magnitude order for xd corresponds roughly to the pre-heating zone length (20 cm).

-Isothermal plug-flow reactor: the temperature is set to the measured temperature of the reactor during the test. This zone of the tubes located in the cavity (thus receiving the solar irradiation) corresponds to the isothermal zone.

-Cooling plug-flow reactor: the temperature decreases from test temperature to 373 K. It corresponds to the annular space between the inner and the outer tube located in the insulating layers. This zone has been called "cooling zone" because it is not exposed to the solar irradiation and thus, the gas temperature decreases. Similarly to the pre-heating zone, the rate of temperature decrease in the cooling zone is linear.

For each simulated run, the experimental temperature and pressure used in the model are those specified in Table 2.

Simulation results and experimental validation

The kinetic model predictions concerning the gas composition as a function of residence time are given in Figs. 6 and7 for experimental conditions 2 (temperature: 1770 K, CH4 mole fraction: 20 %).

As expected, the isothermal part is the key zone of the reactor: the conversion of methane and the production of hydrogen mainly occur in this zone as well as the non-simultaneous formation of NMHCs, PAHs, and BINs. Likewise, CB formation occurs later than H2. These results are reported on Fig. 6 as a function of residence time, which was set to 0 at the entry of the isothermal zone. The residence time is about 17 ms in the isothermal zone and about 34 ms in the cooling zone. CH4 starts to dissociate after 1.10 -4 s. The formation of NMHCs (light hydrocarbons) is first observed followed by the formation of H2. Then, PAHs and BINs are formed in a short residence time period. Actually, the decomposition of PAHs and the formation of CB just begin toward the exit of the isothermal zone for  > 10 ms. Among NMHCs, C2H6 and C2H4 are the first species formed from CH4 dissociation, as shown in Fig. 7. Both species are then converted into C2H2 with a nearly complete conversion at the exit of the isothermal zone ( > 10 ms). In the cooling zone, the CB formation slowly continues while C2H4 and C2H6 slightly increase presumably from C2H2 due to the temperature decrease. Predicted mole fractions of CH4, H2, and C2H2 at the reactor exit are 0.014, 0.248, and 0.056 respectively, which corresponds closely to the experimental values given in Table 2, except for C2H2. Fig. 8 compares the CH4 conversions obtained from kinetic simulations with Dsmoke code and from experiments. For most of the tests, experimental CH4 conversion is higher than the simulated one but there is a reasonably good agreement between the two series (mean discrepancy of 12 %). The carbon particles formed in the tubes may catalyse further the reaction. This catalytic effect is not included in Dsmoke software (homogeneous reaction), which may explain why the chemical conversions predicted by Dsmoke are usually lower than the experimental ones. The statement of homogeneous reaction considered by Dsmoke is consistent with the high value of activation energy (370 kJ/mol) identified from the plug-flow reactor model. In few cases (tests N° 9, 15, 16), the predicted CH4 conversion is higher than the experimental one. This may be due to temperature measurement uncertainty. In some cases, especially at high CH4 flow-rate in the feed, the gas temperature must be lower than the wall temperature measured by the pyrometer, due to the endothermic reaction. If the gas temperature is over-estimated, then the predicted conversion from Dsmoke simulation becomes higher than the experimental one. The influence of temperature is also clear: the higher the temperature, the higher the conversion, as shown in Fig. 8. The reaction of methane decomposition is endothermic; as a consequence, a temperature increase results in the reaction rate increase. According to thermodynamics [START_REF] Fincke | Plasma pyrolysis of methane to hydrogen and carbon black[END_REF], CH4 decomposition is complete between 1273 K and 2273 K. Due to kinetic limitations, the experimental CH4 conversions are not complete, which was expected since the residence time is too short to reach thermodynamic equilibrium. This influence of residence time on reaction extent is strong according to Dsmoke kinetic calculations.

Concerning the comparison between experimental and simulated H2 yields, the mean discrepancy is 24 %, which is not as good as for the CH4 conversion. Experimental H2 yield is always higher than the predicted one. Finally, Dsmoke always predicts higher off-gas C2H2 concentrations than the experimental ones (mean discrepancy of 32 %), which explains the discrepancy on the H2 yield. This disagreement with experiments may be due to the catalytic properties of CB reported previously [START_REF] Muradov | Hydrogen via methane decomposition: an application for decarbonization of fossil fuels[END_REF] but that are not integrated in the Dsmoke model. An heterogeneous surface reaction may also occur on the tube walls, enhancing the dissociation reaction [START_REF] Abanades | Experimental study and modeling of a high-temperature solar chemical reactor for hydrogen production from methane cracking[END_REF]. Concerning C2H4 and C2H6, their quantities in the gas products are not significant enough to conclude quantitatively but their concentrations tend to decrease when increasing the temperature.

The time evolution of H2 and CH4 mole fractions at 1740 K obtained from the model are reported on Figs. 9 and 10 (lines) along with the experimental data corresponding to each experimental condition (marks). The slope breakage on lines takes place between the isothermal and the cooling zones and proves that a higher residence time in the isothermal zone (where the reaction rate is the highest) could lead to enhanced CH4 conversion. At this transition point, the kinetic model gives the residence time of the gas in the isothermal zone, which corresponds roughly to that calculated without taking into account the chemical expansion (values given in Table 2 for each experimental condition). Actually, the residence times obtained from the two different calculation methods (kinetic model and assessment from inlet gas-flow rate) are similar owing to the high dilution with argon. Moreover, simulated and experimental CH4 and H2 mole fractions are very comparable, which permits to validate the kinetic model applied to the solar process of methane cracking. On the basis of the results obtained with this 10 kW solar reactor, a pilot-scale solar reactor is being designed. To limit the carbon deposition issue, simple tubes will be preferred instead of concentric tubes. The graphite tubes could also be replaced by ceramic tubes since ceramics are successfully used in CB industrial processes [START_REF] Donnet | Carbon Black, Second Edition[END_REF]. The kinetic model presented in this paper gave consistent results that were validated from experiments. The kinetic code predicts the chemical conversion as a function of residence time for given conditions of temperature, pressure, and feed gas flow-rates. Thus, it can be used as a design tool during reactor scaling-up. For example, the required residence time to obtain a complete chemical conversion is a key parameter that determines the dimension of the reaction zone.

Conclusion

Kinetic simulation of methane dissociation in various experimental conditions was carried out with the Dsmoke software. The kinetic model predictions agreed with the general sequence of decomposition: a stepwise dehydrogenation with the successive intermediates C2H6, C2H4, and C2H2. Nevertheless, other compounds were also involved as chemical intermediates whatever the temperature. Simulations with various natural gas compositions showed small difference in CH4 conversion after 1 ms. A global kinetic expression for the overall dissociation reaction was identified from the reactor model (k0 = 6.6x10 13 s -1 and Ea = 370 kJ/mol), assuming a plug-flow and non-catalytic reaction.

The 10 kW solar experimental reactor was modelled by three plug-flow reactors in series to take into account the temperature profile in the different zones of the reactor tubes (pre-heating, isothermal, cooling). The model shows reasonably good agreement with experimental results, especially for CH4 conversion, and mole fractions of CH4 and H2. It also confirms the importance of residence time and temperature in order to reach high conversion. Moreover, the formation of CB occurs later than H2 formation and the production of PAHs as intermediate products is identified. Although the kinetic model is limited in terms of particle diameters (60 nm as the upper value), it proved to be a valuable and accurate calculation code in the field of thermal methane dissociation.

  the Reynolds number, Pr is the Prandtl number, xd is the start of the thermally developed region and d is the tube diameter.

Figure 1 :Figure 2 :

 12 Figure 1: Mass fractions, CH4 conversion, and H2 yield versus residence time at 1700 K

Figure 3 :Figure 4 :

 34 Figure 3: Hydrogen yield versus residence time for different temperatures and natural gas

Figure 6 :

 6 Figure 6: Gas composition versus residence time from kinetic simulation of experimental condition 2

Figure 7 :

 7 Figure 7: C2H6, C2H4 and C2H2 mass fractions versus residence time from kinetic simulation of

Figure 8 :Figure 9 :

 89 Figure 8: Experimental and predicted methane conversions from the kinetic model for each

Table 1 :

 1 Natural gas compositions

		Volume fractions (%)
		CH4 C2H6 C3H8 C4H10 CO2 N2
	Methane (REF-G1)	100
	Modified Algeria gas (NG1) 91.2 6.5 2.1	0.2
	Modified Groningen gas (NG2) 83.5 4.7 0.7	0.2	10.8
	North Sea gas (NG3)	88.2 5.4 1.2	0.4 1.4 3.2

Table 2 :

 2 Experimental conditions and corresponding results

		Ar(L/min) CH4(L/min) Pressure(Pa) Tpyrometer (K) Residence time (s)	Off-gas (mole fractions) H2 CH4 C2H2
	1	10.8	1.2	40000	1770	0.032	0.1625 0.0018 0.0202
	2	16	4	38000	1770	0.018	0.2518 0.0138 0.0392
	3	16	4	35000	1770	0.017	0.2555 0.0055 0.0417
	4	20	4	35000	1770	0.014	0.2170 0.0090 0.0302
	5	16	4	32000	1700	0.018	0.2056 0.0294 0.0398
	6	16	6	35000	1710	0.017	0.2925 0.0340 0.0530
	7	16	6	25000	1710	0.012	0.2278 0.0790 0.0361
	8	10.8	1.2	30000	1670	0.027	0.1033 0.0227 0.0207
	9	16	8	30000	1670	0.013	0.1230 0.2123 0.0179
	10	18	2	30000	1670	0.018	0.0795 0.0388 0.0160
	11	16	4	30000	1670	0.018	0.1480 0.0727 0.0282
	12	14	6	30000	1670	0.018	0.2270 0.1000 0.0426
	13	18	2	30000	1740	0.018	0.1177 0.0173 0.0258
	14	16	4	30000	1740	0.018	0.2173 0.0307 0.0448
	15	14	6	30000	1740	0.018	0.2943 0.0560 0.0536
	16	24	6	30000	1740	0.012	0.1617 0.0703 0.0264
	17	8	2	30000	1740	0.035	0.2523 0.0037 0.0187

4)

Acknowledgements

This study was funded by the European Project Solhycarb (FP6, Contract SES-CT2006-19770). The authors wish to thank J.L. Sans, M. Garrabos, and O. Prevost for their contribution to experimental results.


Antoine Pierart

Camille Dumat

Nathalie Séjalon-Delmas

Juan Carlos

Recycling coffee wastes for healthy urban agriculture: do spent

de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Open Archive TOULOUSE Archive Ouverte (OATAO)

OATAO is an open access repository that collects the work of Toulouse researchers and makes it freely available over the web where possible. This is an author-deposited version published in : http://oatao.univ-toulouse.fr/ Eprints ID : 18333

To cite this version : Pierart, Antoine and Dumat, Camille and Séjalon-Delmas, Nathalie and Sanchez-Hernandez, Juan Carlos Recycling coffee wastes for healthy urban agriculture: do spent coffee ground and its biochar reduce the impact of antimony on soil quality and plant growth? (2017) In: Sustainable Urban Agricultures (UA) : Vector for Ecological Transition, 6 June 2017 -9 June 2017 (Toulouse, France). (Unpublished) Any correspondence concerning this service should be sent to the repository administrator: staff-oatao@listes-diff.inp-toulouse.fr

Context

Challenge of soil quality for Urban Agriculture Concept of coupling remediation and amendment in one-shot: Revalorization of wastes is a matter of current concern! (fresh, composted, biochar…) Spent coffee ground (SCG) or its charred by-product (SCGc) showed intriguing outcomes: Suitable in remediation actions as 'buffer' amendments in soils receiving contaminants Direct interaction between biochar and rhizospheric microorganisms…

Objectives

To determine whether addition of SCG and SCG-derived biochar reduce the impact of Sb on selected extracellular enzyme activities and plant performance.

Results

SCG caused a phytotoxic effect on plant growth and pea yield SCGc enhanced plant performance and was colonized by microorganisms SCG and SCGc impacted soil enzymes activity (time & treatment-dependent) SCGc activated extracellular enzymes implied in C-and P-cycling SCGc decreased Sb toxicity at the end of the experiment Recycling coffee wastes for healthy urban agriculture: do spent coffee ground and its biochar reduce the impact of antimony on soil quality and plant growth?


Michel Lonjon

Ste ´phane Litrico

Anne-Marie Barbanel

Pascal Sabatier

Luc Bauchet

Hugues Chevassus

Nicolas Lonjon

Keywords: chronic subdural hematoma, corticosteroid, surgery

Chronic subdural hematoma (CSDH) is a common condition necessitating surgery; however, recurrence occurs in 15-25% of cases despite surgical management. The HEMACORT trial was a prospective randomized, double-blind, placebo-controlled, multi-centric study (NCT01380028). The aim of this trial was to determine the effect of corticosteroids as an adjuvant treatment to surgery on CSDH recurrence at 6 months. After surgery, participants were assigned by block-randomization to receive either placebo or oral prednisone at a dose of 1 mg/kg/day followed by weekly stepwise tapering in steps of 10 mg/day. The primary outcome was CSDH recurrence, defined by the need for reoperation and/or radiological progression of CSDH. Secondary outcomes were one-year death, radiological changes, safety, neurological status, and quality of life. The trial was discontinued at midpoint of expected inclusions: 78 participants received prednisone and 77 received placebo controls. In an intention-to-treat analysis, CSDH clinicoradiological recurrence was not different between prednisone and placebo groups (21.8% vs. 35.1%, respectively; hazard ratio 0.56; 95% confidence interval 0.30-1.02; p = 0.06), although post hoc analyses concluded to statistical significance ( p = 0.02). Earlier radiological resolution was observed after prednisone administration, but reoperation rates (reaching 5.8% overall) and functional outcomes were not different at 6 months. Among adverse events, sleep disorders occurred more often in the prednisone group (26.1% vs. 9.1%, p = 0.02). The HEMACORT trial data suggest that prednisone, as an adjuvant treatment to surgery, may reduce early radiological recurrence of CSDH, although clinical benefits are unclear. In view of these findings, the authors suggest that shorter treatment duration should be assessed for safety and efficacy in future trials.

Corticosteroids as an Adjuvant Treatment to Surgery in Chronic Subdural Hematomas: A Multi-Center Double-Blind Randomized Placebo-Controlled Trial Sam Ng, 1, * ,{ Julien Boetto, 1,{ He ´le ´na Huguet, [START_REF] Bourgeois | Chronic subdural hematoma in patients over 80 years of age[END_REF][START_REF] Balser | Actual and projected incidence rates for chronic subdural hematomas in United States Veterans Administration and civilian populations[END_REF] Pierre-Hugues Roche, [START_REF] Iorio-Morin | Chronic subdural hematoma: toward a new management paradigm for an increasingly complex population[END_REF] Ste ´phane Fuentes, Introduction Chronic subdural hematoma (CSDH) is one of the most frequent entities necessitating neurosurgery. The incidence is 15 per 100,000 person-years in the general population, increasing to 127.1 per 100,000 person-years in patients aged over 80 years. [START_REF] Karibe | Epidemiology of chronic subdural hematomas[END_REF][START_REF] Bourgeois | Chronic subdural hematoma in patients over 80 years of age[END_REF] Because the elderly population is growing, CSDH prevalence is constantly increasing, becoming a significant public health problem. [START_REF] Balser | Actual and projected incidence rates for chronic subdural hematomas in United States Veterans Administration and civilian populations[END_REF][START_REF] Iorio-Morin | Chronic subdural hematoma: toward a new management paradigm for an increasingly complex population[END_REF][START_REF] Blaauw | Prevalence of cognitive complaints and impairment in patients with chronic subdural hematoma and recovery after treatment: a systematic review[END_REF] In addition, results of conventional treatments including surgical evacuation are heterogeneous with 15% recurrence after one year increasing to 25% in high-risk patients. [START_REF] Schmidt | Recurrence of subdural haematoma in a population-based cohortrisks and predictive factors[END_REF][START_REF] Almenawer | Chronic subdural hematoma management: a systematic review and meta-analysis of 34,829 patients[END_REF][START_REF] Torihashi | Independent predictors for recurrence of chronic subdural hematoma: a review of 343 consecutive surgical cases[END_REF] Beyond its classic traumatic etiology, the development of CSDH involves complex mechanisms of inflammation leading to a self-sustaining process of neoangiogenesis and fibrinolysis that is mediated by proinflammatory cytokines. [START_REF] Edlmann | Pathophysiology of chronic subdural haematoma: inflammation, angiogenesis and implications for pharmacotherapy[END_REF][START_REF] Stanisic | Local and systemic pro-inflammatory and anti-inflammatory cytokine patterns in patients with chronic subdural hematoma: a prospective study[END_REF][START_REF] Vaquero | Vascular endothelial growthpermeability factor in granulation tissue of chronic subdural haematomas[END_REF] Surgical removal of the hematoma is effective in reducing the mass effect induced by the hematoma, [START_REF] Almenawer | Chronic subdural hematoma management: a systematic review and meta-analysis of 34,829 patients[END_REF][START_REF] Liu | Chronic subdural hematoma: a systematic review and meta-analysis of surgical procedures[END_REF][START_REF] Alcala ´-Cerra | Efficacy and safety of subdural drains after burr-hole evacuation of chronic subdural hematomas: systematic review and meta-analysis of randomized controlled trials[END_REF] but does not treat the underlying pathophysiological mechanisms. Consequently, adjuvant therapies to surgery are under evaluation, including atorvastatin, [START_REF] Jiang | Safety and efficacy of atorvastatin for chronic subdural hematoma in Chinese patients: a randomized clinicaltrial[END_REF] angiotensin converting enzyme inhibitors, [START_REF] Bartek | The role of angiotensin-converting enzyme inhibitors in patients with chronic subdural hematoma: a Scandinavian population-based multicenter study[END_REF][START_REF] Poulsen | Perindopril and residual chronic subdural hematoma volumes six weeks after burr hole surgery: a randomized trial[END_REF] middle meningeal artery embolization [START_REF] Ban | Middle meningeal artery embolization for chronic subdural hematoma[END_REF][START_REF] Ng | Middle meningeal artery embolization as an adjuvant treatment to surgery for symptomatic chronic subdural hematoma: a pilot study assessing hematoma volume resorption[END_REF] and corticosteroids. [START_REF] Henaux | Steroids in chronic subdural hematomas (SUCRE trial): study protocol for a randomized controlled trial[END_REF] Among these, corticosteroids are widely used as an adjuvant treatment or as an alternative to surgery in patients with mild neurological impairment, [START_REF] Miah | Dexamethasone therapy in symptomatic chronic subdural hematoma (DECSA -R): a retrospective evaluation of initial corticosteroid therapy versus primary surgery[END_REF] but there is no clear demonstration of their efficacy in CSDH, with only retrospective studies or preliminary studies supporting its efficacy. [START_REF] Dran | Effectiveness of adjuvant corticosteroid therapy for chronic subdural hematoma: a retrospective study of 198 cases[END_REF][START_REF] Berghauser Pont | The role of corticosteroids in the management of chronic subdural hematoma: a systematic review[END_REF][START_REF] Zarkou | The role of corticosteroids in the management of chronic subdural hematomas: a critically appraised topic[END_REF] Recently, a randomized trial even suggested that patients under corticosteroids may have fewer favorable outcomes and more adverse events, although patients experienced fewer reoperations. [START_REF] Hutchinson | Trial of dexamethasone for chronic subdural hematoma[END_REF] This study presents the results from HEMACORT, a trial assessing the efficacy of corticosteroids as an adjuvant treatment to surgery in CSDH.

Methods

Study design

HEMACORT was a multi-center, double-blind, placebocontrolled, randomized 1:1, parallel group trial. This study was conducted in five hospitals in France (Marseille Nord University Hospital, Marseille La Timone University Hospital, Perpignan Hospital, Nice Pasteur University Hospital, and Montpellier University Hospital). This trial was sponsored by Montpellier University Hospital (France, UF8545). The study was approved by the French institutional ethics committee ''Comite´ de Protection des Personnes Sud Me ´diterrane ´e IV Montpellier Saint-Eloi'' and conducted in accordance with the Declaration of Helsinki, International Conference on Harmonisation guidelines for Good Clinical Practice and French regulatory requirements. All patients gave written informed consent. The study was registered on the United States clinical trial database before the first patient inclusion. [START_REF]Interest of oral corticosteroids in the treatment of chronic subdural hematomas (hemacort)[END_REF]

Participants

Eligible patients were men and women over age 18, weighing less than 104 kg, who underwent surgery for a one-sided or bilateral CSDH in the previous 72 h. Preoperative radiological eligibility criteria included hypodense or isodense collection (in case of multiple component hematomas, the thickness of the low-density component [<50 Hounsfield units] had to represent >50% of the maximal thickness of the hematoma), and CSDH size measured greater than 3 mm (documented on pre-operative axial computed tomography [CT]). All radiological evaluations were performed by an independent radiologist.

Exclusion criteria were: a past history of CSDH, a past history of cranial surgery, Glasgow Coma Scale score under 9 at admission, ongoing infection, diabetes mellitus necessitating medical treatment, uncontrolled arterial hypertension, uncontrolled psychotic state, gastric ulcer, osteoporosis, organ failure, pregnancy, and breastfeeding.

Sample size and randomization

Using estimated recurrence rates of CSDH after surgical treatment of 10% in the corticosteroid group and 20% in the placebo group [START_REF] Dran | Effectiveness of adjuvant corticosteroid therapy for chronic subdural hematoma: a retrospective study of 198 cases[END_REF] for a = 5%, b = 20% and using a two-sided log-rank test, 148 participants were required in each group. Taking into account a probable dropout rate of 15%, a total of 340 participants were randomized. Sample size was determined for the primary outcome using nQuery Advisor, version 4.0 (Statistical Solutions, Boston, MA).

We stratified the randomization by center with a 1:1 allocation using block sizes of four patients. Eligible patients were randomized by the pharmacy department of their center in the first 72 h after surgery. Each pharmacy department had an independent randomization list produced with SAS software, version 9 (SAS Institute, Cary, NC).

Interventions

Subjects randomized into the corticosteroid group received oral prednisone at an initial dose of 1 mg/kg (rounded to the nearest 10 mg) once daily for seven days. The treatment was started the day after randomization. Of note, 5 mg of prednisone is an equivalent dose to 0.75 mg of dexamethasone and to 4 mg of methylprednisolone. Patients then underwent a decrease in the dose of 10 mg/day every seven days. After completing seven days at the dose of 10 mg/day, patients received 5 mg/day for seven days before treatment was stopped.

The treatment duration thus varied from one participant to another depending on participant weight. For instance, a patient weighting 80 kg was administered one week of treatment at the dose of 80 mg/day followed by a weekly taper (70 mg/day for one week, then 60 mg/day for one week, etc.) and finally 5 mg on its ninth and last week of treatment. The same treatment procedure was applied in the placebo group using nonpharmacologically active agents. Prednisone tablets were masked by administration in the same capsules used to prepare the placebo, all capsules being indistinguishable.

Study drug adherence was checked by the pharmacy centers at two end-points: (1) after 28 days of treatment (first outpatient period) and (2) at the end of the treatment (second outpatient period, depending on the treatment duration). Adherence was expressed as the ratio (%) between consumed and prescribed treatment units. Participants, investigators, and caregivers remained blinded throughout the study. Additional cholecalciferol (vitamin D3; 800 UI/day) and calcium (1 g/day) was given to all patients during the treatment.

Outcomes

The primary outcome was the clinicoradiological recurrence rate of CSDH within six months after surgery. The CSDH clinicoradiological recurrence was a composite criterion defined by the onset of one or more of the following clinical and/or radiological issues: recurrence of clinical symptoms of CSDH necessitating a second cranial surgery; any radiological bleeding (defined as an increase in Hounsfield densities) and/or any enlargement of the subdural space (defined as an increase of the maximum thickness compared with the immediate post-operative CT scan). The primary outcome was evaluated blindly by an independent team (one neurologist and one radiologist).

Secondary outcomes were: mortality rate one year after surgery; neurological status (assessed by the Markwalder Grading Scale [MGS]), [START_REF] Markwalder | The course of chronic subdural hematomas after burr-hole craniostomy and closed-system drainage[END_REF] quality of life (assessed by the Nottingham Health Profile [NHP] questionnaire), [START_REF] Hunt | A quantitative approach to perceived health status: a validation study[END_REF] autonomy (assessed by the Karnofsky scale), safety, and the percentage of decrease in hematoma size (based on maximal thickness) at one month and six months after the immediate post-operative assessment.

Study procedures

Briefly, after giving informed consent, subjects were assessed for eligibility before surgery by a full physical examination and a CT scan. Although the surgical technique was left to the discretion of each participating center, data for the use of subdural drain, duration of the drainage, and the use of a percutaneous/nonpercutaneous technique were collected.

Randomly assigned treatment was initiated in hospital within 72 h after the surgery, immediately after a baseline CT scan was performed. Three evaluation visits were planned at one month, three months, and six months (at least 150 days after randomization) after discharge. A blood sample was obtained at the one-month visit for investigation of electrolyte and glycemic impairments. Death at one year was assessed by telephone interview of relatives. The Time and Events schedule and safety checkpoints are detailed in Table 1.

Early discontinuation of the trial

Premature discontinuation of the trial was decided by the sponsor because of funding not matching costs incurred, mainly because of placebo supply problems. It was therefore decided to perform a final analysis at the midpoint of the patient inclusion.

Statistical methods

All analyses were conducted on data from the intention to-treat population. The study population is described using mean and standard deviation (SD) for quantitative variables and frequencies for qualitative variables. Continuous variable distributions were tested with the Shapiro-Wilk statistic. Quantitative variables were compared using the parametric Student t test, when the distribution was Gaussian or otherwise with the Mann-Whitney test. For qualitative variables, groups were compared using the chi-square test or Fisher test. The clinicoradiological recurrence analysis was pre-specified as a recurrence-free roids and 77 participants received a placebo. Figure 1 shows the participant flow. Baseline characteristics for the prednisone and placebo groups, including surgical technique features, are provided in Table 2.

=---< First visit (1 month) =---< Second visit (3 months) =---<
Patients randomized to the corticosteroid group were more likely to be receiving antiplatelet or anticoagulant therapy (52.6% vs. 40.3%). The post-surgical maximum thickness of the hematomas was lower in the prednisone group than in the placebo group (12.8 -5.1 mm vs. 14.9 -5.2 mm, p = 0.02, respectively) although the comparative evolution of thickness of the hematomas immediately after surgery from pre-operative CT was not significantly different. The total treatment duration never exceeded 10 weeks because no patients weighing more than 90 kg were included.

Adherence in the first outpatient treatment period was 80.5 -30.2% in the prednisone group and 79. survival analysis. Patients lost to follow-up or deceased from causes other than CSDH were considered as censored.

The recurrence-free survival rate was described by the Kaplan-Meier method and compared with a log-rank test. A post hoc multi-variate analysis was performed with the

Primary outcome results

Primary outcome results are detailed in Table 3. Overall, 44 (28.4%) patients experienced a clinicoradiological recurrence within the first six months after surgery, including 17 participants (21.8%) in the prednisone group, and 27 (35.1%) participants in the placebo group ( p = 0.06). The Kaplan Meier method estimate of the probability of recurrence-free survival at six months was 77% (95% confidence interval [CI]: 66% to 85%) in the prednisone group and 63% (95% CI: 51% to 73%) in the placebo group ( p = 0.054). Kaplan-Meier plots describing radiological progression and reoperation over time are displayed in Figure 2A, 2B, respectively. Post hoc analyses adjusting the recurrence-free survival at six months to antiplatelet and anticoagulant drug use and to post-surgical hematoma thickness concluded to a significantly higher recurrence-free survival in the prednisone group than in the placebo group ( p = 0.02).

Reoperation, a subcomponent of the primary outcome, occurred in four (5.1%) participants in the prednisone group and five (6.5%) participants in the placebo group ( p = 0.66). Note that all patients requiring a reoperation presented a concomitant radiological progression.

Radiological progression, the second subcomponent of the primary outcome, occurred in the same proportions as clinicoradiological recurrence.

Secondary outcome results

Results for death, hematoma size, and Karnofsky index are provided in Table 4.

Nine deaths occurred during the trial-five in the corticosteroid group and four in the placebo group. Among these, the monitoring board estimated that 1one in each group was related to CSDH recurrence. In the placebo group, the other causes of death were pneumonia (two participants), ischemic stroke, and gastrointestinal bleeding. In the corticosteroid group, the other causes of death were pneumonia (two participants), digestive cancer progression, and ischemic stroke.

Details regarding percentage of decrease in hematoma size are provided in Figure 3. The percentage decrease in hematoma size at one month after the immediate post-operative assessment was significantly higher in the corticosteroid group than in the placebo group (-35.5 -48.1% vs. -11.9 -49.0%; p = 0.001), although no significant difference was observed at six months. Complete data for NHP and MGS score are detailed in Table 5. The Sleep score from the NHP was significantly higher in the corticosteroid group at one month ( p = 0.01), although again no differences between groups were found at six months.

Safety

Table 6 summarizes adverse events recorded at the time of data lock. Thirty-one (20.0%) participants experienced infections, 11 (7.1%) participants experienced gastrointestinal disorders, and 13 (8.4%) participants experienced metabolic and general disorders during the trial, without any difference found between groups. Insomnia occurred more frequently in the prednisone group than in the placebo group (26.1% vs. 9.1%, p = 0.02).

Discussion

The results of this double-blind, randomized, placebocontrolled study suggest that prednisone as an adjuvant treatment to surgery may reduce early radiological progression of CSDH, although clinical outcomes (including reoperation rate, death, neurological and functional outcomes) did not seem to be modified by prednisone administration.

In contrast to the findings of Hutchinson and associates, [START_REF] Hutchinson | Trial of dexamethasone for chronic subdural hematoma[END_REF] we report no significant difference in functional outcomes and reoperation rates between patients who received corticosteroids and placebo at six months, although we observed quite close rates of reoperation in the whole population (5.8% vs. 1.7-7.1% in the study by Hutchinson and associates [START_REF] Hutchinson | Trial of dexamethasone for chronic subdural hematoma[END_REF] ). Despite the fact that we used a significantly longer period of corticosteroid treatment, we observed no effect of corticosteroids on the occurrence of metabolic diseases and infections. The therapeutic protocol used in our study was not harmless, however, with significant psychiatric comorbidities.

Surgery remains the treatment of choice for symptomatic CSDH, because it is effective in reducing intracranial hypertension. [START_REF] Weigel | Outcome of contemporary surgery for chronic subdural haematoma: evidence based review[END_REF][START_REF] Santarius | Use of drains versus no drains after burr-hole evacuation of chronic subdural haematoma: a randomised controlled trial[END_REF] Nevertheless, the risk of clinicoradiological recurrence after surgery is high and leads to high rates of post-surgical morbidity. Corticosteroids have been introduced in CSDH management because of the potential local inflammatory reactions leading to a self-sustaining process of neoangiogenesis and fibrinolysis. In practice, despite the lack of high-quality studies providing evidence of corticosteroids effectiveness, the use of corticosteroids currently ranges 13-55%, depending on the institution, [START_REF] Santarius | The management of primary chronic subdural haematoma: a questionnaire survey of practice in the United Kingdom and the Republic of Ireland[END_REF][START_REF] Berghauser Pont | Ambivalence among neurologists and neurosurgeons on the treatment of chronic subdural hematoma: a national survey[END_REF] with the aim of either avoiding surgical management or reducing CSDH recurrence after surgery.

Our results are more nuanced than previous nonrandomized studies that have suggested favorable results with corticosteroids in reducing CSDH recurrence rate as an adjunctive treatment to surgery [START_REF] Berghauser Pont | The role of corticosteroids in the management of chronic subdural hematoma: a systematic review[END_REF][START_REF] Holl | Corticosteroid treatment compared with surgery in chronic subdural hematoma: a systematic review and metaanalysis[END_REF] or as a peri-33 operative treatment. We were not able to confirm the SD, standard deviation; CT, computed tomography; KI, Karnofsky Index.

Patients with data available for analyses were (a) 73 vs. 71, (b) 64 vs. 61, (c) 72 vs. 70, (d) 63 vs. 60, (e) 73 vs. 67, (f) 66 vs. 62, (g) 65 vs. 60 in the prednisone group and placebo group, respectively. findings of Dran and coworkers, 21 a retrospective study in which patients who received corticosteroids as an adjuvant to surgery had a significantly higher rate of survival than patients without corticosteroid treatment, although the treatment protocol was not strictly comparable (prednisone was initiated at a dose of 0.5 mg/kg/day for one month followed by tapering).

The definition of CSDH recurrence is a key point for better estimation of success and failure rates in the management of CSDH. [START_REF] Chari | Core outcomes and common data elements in chronic subdural hematoma: a systematic review of the literature focusing on reported outcomes[END_REF] To better illustrate the resolution of the disease, we decided to use a composite criterion including surgical reoperation and any reaccumulation of CSDH on the basis of immediate post-operative imaging. We considered that the occurrence of a second surgery was insufficient as a single indicator of treatment failure.

In routine clinical practice, a conservative attitude is frequently observed in elderly patients presenting progressive regrowth of CSDH without symptoms or with mild neurological impairment, [START_REF] Almenawer | Chronic subdural hematoma management: a systematic review and meta-analysis of 34,829 patients[END_REF] with an unknown impact on the clinical management. At this stage, steroid therapy may be used to postpone a potential reoperation, [START_REF] Berghauser Pont | The role of corticosteroids in the management of chronic subdural hematoma: a systematic review[END_REF][START_REF] Delgado-Lo ´pez | Dexamethasone treatment in chronic subdural haematoma[END_REF] and it can be assumed that clinicians usually delay the resumption of concurrent anticoagulant therapy, leading to additional medical appointments, neuroimaging modalities, and health-related costs. Because there are no guidelines providing decision support in this situation, the therapeutic choice is based mainly on the surgeon's experience, institutional practices, patient age, and comorbidities, and CSDH radiological features. [START_REF] Berghauser Pont | Ambivalence among neurologists and neurosurgeons on the treatment of chronic subdural hematoma: a national survey[END_REF][START_REF] Miranda | Chronic subdural hematoma in the elderly: not a benign disease[END_REF] Finally, the reduction in clinicoradiological recurrence observed during prednisone administration was related mainly to a lower rate of radiological regrowth of residual CSDH during the early post-surgical period. These findings must be tempered, because no long-term clinical benefits were observed and there was a notable level of side effects, an observation already highlighted by the recent findings of Hutchinson and colleagues. [START_REF] Hutchinson | Trial of dexamethasone for chronic subdural hematoma[END_REF] Therefore, the authors suggest that a shorter treatment duration should be assessed for safety and efficacy in future trials.

Limitations

The main limitation of these results is that final data analysis was performed at the midpoint of patient inclu-sions because of early discontinuation of the trial. In addition, the surgical technique was left to the discretion to each participating center and was not a stratified out-come. Further, the choice of the perioperative manage-ment of antiplatelet and anticoagulant drugs was also left to each recruiting center, although this factor was considered in post hoc adjusted analyses. The ideal man-agement of these medications and their impact on CSDH recurrence would certainly require a specific additional study. [START_REF] Poon | Influence of antiplatelet and anticoagulant drug use on outcomes after chronic subdural hematoma drainage[END_REF] The authors also acknowledge that the safety of the therapeutic protocol used in this study was not formally demonstrated. Psychiatric disorders were more frequent after corticosteroid administration (n = 26, 37.7%), which is not insignificant in an elderly population. Among these patients, most reported insomnia (n = 18), and some patients had agitation (n = 11), although only insomnia occurred significantly more frequently in the corticoste-roid treatment group.

Nevertheless, these adverse events appeared reversible, as suggested by the comparable results for the NHP (sleep score) at six months between both groups.

No case of corticosteroid-induced diabetes mellitus was reported during the trial. Although patients had a weekly taper of their prednisone dose, however, glycemic disorders and adrenal insufficiency may have been underdiagnosed because biochemical assessment took place only during the initial hospital stay and at one month post-operatively.

Conclusion

The results of the HEMACORT trial suggest that prednisone as an adjuvant treatment to surgery may reduce early CSDH radiological recurrence, although clinical benefits remain unclear with similar reoperation rates, and similar neurological, functional, and survival outcomes. Additional data are also required to support the safety of this treatment, because a high rate of sleep disorders was transiently observed. In view of these findings, the authors suggest that a shorter treatment duration should be assessed for safety and efficacy in future trials.

FIG. 2 .

 2 FIG. 2. Time to chronic subdural hematoma recurrence from randomization. The shaded areas indicate 95% confidence interval. (A) Kaplan-Meier plot describing the time to radiological progression; (B) Kaplan-Meier plot describing the time to reoperation. Color image is available online.

FIG. 3 .

 3 FIG. 3. Percentage of evolution in hematoma size from post-operative assessment to 1 month (A) and 6 months (B) post-surgery. The horizontal line in the boxes indicates the median, the top and bottom of the box indicates the 25th and 75th percentiles, and I bars indicate the extreme upper and lower values. Color image is available online.

Table 1 .

 1 Time and Events Schedule

	Admission
	and surgery Hospital stay

Table 2 .

 2 Demographics and Main Characteristics at BaselineCox proportional-hazard regression model to calculate

	Third visit
	(6 months) 12 months

CT, computed tomography; MGS, Markwalder Grading Scale; NHP, Nottingham Health Profile. a In the first 72 h after surgery.

FIG. 1. Flowchart. CSDH, chronic subdural hematoma. a Overall, 17 patients had clinicoradiological recurrence, including patients with deaths related to CSDH recurrence. b Overall, 27 patients had clinicoradiological recurrence, including patients with deaths related to CSDH recurrence. adjusted

  8.5 -31.0%

	Percutaneous Nonpercutaneous Subdural drainage	20 (26.0) 57 (74.0)	13 (16.9) 0.24 64 (83.1)	in the placebo group ( p = 0.93). Adherence for the sec-ond outpatient treatment period (i.e., at the end of the
	Duration of subdural drainage
	1 day or less	26 (33.3)	31 (40.3) 0.65
	2 days	38 (48.7)	36 (46.7)
	3 days	9 (11.6)	8 (10.4)
	4 days or more	5 (6.4)	2 (2.6)
	Duration (days), mean -SD	1.92 -0.95	1.74 -0.77 0.28
	SD, standard deviation; CSDH, chronic subdural hematoma.; CT, com-
	puted tomography.
	Reported percentages may not add up to 100% because of rounding.

Table 3 .

 3 Primary Outcome Results

		Corticosteroid group N = 78	Placebo group N = 77	HR (95% CI)	p	HR	a	p a
	Primary outcome b	17 (21.8)	27 (35.1)	0.56 [0.30 ; 1.02]	0.06	0.48 [0.26 ; 0.89]	0.02
	Reoperation	4 (5.1)	5 (6.5)	0.75 [0.20 ; 2.77]	0.66	0.59 [0.16 ; 2.27]	0.45
	Radiological recurrence b	17 (21.8)	27 (35.1)	0.56 [0.30 ; 1.02]	0.06	0.48 [0.26 ; 0.89]	0.02

HR, hazard ratio; CI, confidence interval. a Comparisons were adjusted for histories of antiplatelet and anticoagulant drugs and for post-surgical hematoma thickness.

b Note that all participants with reoperation firstly experienced a radiological recurrence.

Table 4 .

 4 Mortality, Hematoma Size and Karnofsky Index Results

		Prednisone Placebo
	Outcomes	group	group	p

Table 5 .

 5 Markwalder Grading Scale and Nottingham Health Profile Scores at 1 and 6 Months

Table 6 .

 6 Main Complications Reported as Possible Adverse Events by Treatment

	Outcome	Prednisone group	Placebo group	p		Prednisone	Placebo
						group N = 69	group N = 66
	MGS, No. (%)				Complications	No. (%)	No. (%)	p
					disorders a
	MGS, Markwalder Grading Scale; NHP, Nottingham Health Profile; SD,	Gastritis	1 (1.5)	0 (0)	> 0.99
	standard deviation.				Gastric ulcer	1 (1.5)	0 (0)	> 0.99
	Each subcategory from Nottingham Health Profile is rated on a total of	Abdominal pain	3 (4.4)	1 (1.5)	0.62
	100 points. A greater score indicates a lower quality of life.		Abdominal pain upper	2 (2.9)	2 (3.0)	>0.99
	Patients with data available for analyses were (a) 72 vs. 66, (b) 63 vs. 60,	Esophageal hemorrhage	0 (0)	1 (1.5)	0.49
	(c) 69 vs. 63, (d) 60 vs. 69 in the prednisone group and placebo group,	Patients with metabolic	6 (8.7)	7 (10.6)	0.71
	respectively.				and general disorders a

Funding Information

This study was supported by a grant from the French Ministry of Health (HEMACORT, PHRC 2009-015524-28). It was promoted by Montpellier University Medical Center.

Authors' Contributions NL, HC, and LB designed the study protocol. The HEMACORT Study Group contributed to data collection. SN, HC, JB, HB and NL contributed to data analysis. HB and HC conducted statistical analysis. SN wrote the first draft of the article. All authors helped in interpreting the findings, contributed toward subsequent revisions and approved the submitted article.

HEMACORT Study Group


Editorial (R) evolution in spinal surgery

Solène Prost 1 Like in all fields of orthopedics, spinal surgery is in constant progression. However, after the first wave of technical achievements such as pedicle screws and modern vertebral instrumentation, the description and spread of vertebral osteotomies and the development of minimally invasive surgery, the pace of innovation has been slowing over the last few years. This has not been due to any decline in research: on the contrary, spinal surgery has reached maturity and recent technical developments have stimulated continuing research efforts. The present edition of OTSR thus testifies to an ongoing dynamic and constant progression in all the dimensions of spinal surgery.

Minimally invasive surgery has greatly advanced, and indications are now numerous. It plays an increasing role in the trend supported by the French health authorities toward enhanced recovery after surgery and day surgery [START_REF]du 26 janvier 2016 de modernisation de notre système de santé[END_REF]. The present edition provides an opportunity to set out the benefits of patient accompaniment programs and of certain recent innovations.

At the same time, extensive surgery, notably for spinal deformity, has also progressed. Analysis of sagittal balance is now central to planning in most spinal surgeries, and current studies are improving knowledge in this area, both in asymptomatic subjects and in patients with a variety of spinal pathologies. Made-to-measure and patient-specific instrumentation is under assessment and contributes perfectly to these developments [START_REF] Burnard | 3D-printed spine surgery implants: a systematic review of the efficacy and clinical safety profile of patient-specific and off-the-shelf devices[END_REF][START_REF] Prost | Patient-specific" rods in the management of adult spinal deformity. One-year radiographic results of a prospective study about 86 patients[END_REF].

Experimental research equally features in this special edition of OTSR. Spinal surgery in general is still a promising field of study, enhanced by digitization. Computerized solutions, shedding new light on spinal mechanics and also on variations in outcome, are of ever greater help in patient management.

And lastly, quality of life, rather than exclusively radiological data, is increasingly important in assessing postoperative results. It is now an essential aspect, assessed on patient-dedicated standardized clinical scales. This highlights a fundamental issue in functional surgery: that the prime aim is patient satisfaction.

In the coming decade, our work will further progress, with the spread of new technologies to every stage of the diagnostic process, surgical treatment and specific follow-up. Many health-care sectors are impacted by the digital revolution and advent of artificial intelligence, which is already playing an important role in spinal surgery [START_REF] Chang | The Role of Machine Learning in Spine Surgery: The Future Is Now[END_REF][START_REF] Joshi | Artificial intelligence for adult spinal deformity: Current state and future directions[END_REF]. Digital operative planning tools, navigation, robotics and gait analysis are becoming central, enabling "personalized" surgery, changing our understanding of complex mechanical phenomena, allowing "big data" analyses [START_REF] Bouyer | Le Système National des Données de Santé : description et application dans le champ de l'orthopédie[END_REF] and providing innovative solutions for training the spinal surgeons of tomorrow. This edition of OTSR, dedicated to spinal surgery, highlights all of these points and demonstrates the vigor of the teams working in the field. We can expect many good things to come!

  , Benjamin Bouyer 2 , Benjamin Blondel 1 1 Aix-Marseille Université, APHM, CNRS, ISM, CHU Timone, Unité de Chirurgie Rachidienne, 264 Rue Saint-Pierre, 13005 Marseille, France 2 Service d'Orthopédie, Unité Rachis, Hôpital Pellegrin, CHU de Bordeaux, 33000 Bordeaux, France

	Corresponding author: B. Blondel,
	Benjamin.BLONDEL@ap-hm.fr