Bruna Rego De Vasconcelos

Doan Pham

P Sharrock

A Nzihou

Bruna Rêgo De Vasconcelos

Regeneration study of Ni/hydroxyapatite spent catalyst from dry reforming

Keywords:

In the present work, the regeneration of a spent nickel hydroxyapatite-based catalyst (Ni/Ca-HA1_S) used in dry reforming of methane (DRM) was investigated. Three successive cycles of DRM/regeneration were performed. Two different gasifying agents were tested for the regeneration step: air and carbon dioxide (21%CO 2 /N 2 ). The aim was to evaluate the ability of regeneration of the catalyst under different atmospheres. The regeneration was performed in situ at 700 °C under 70 mL/min of gasifying agent. Reproducible results were obtained between each cycle for the two atmospheres tested and only a small irreversible deactivation was noticed. Several characterizations (SEM, TEM, TPR, XRD) of the fresh and spent catalysts allowed proving that the irreversible deactivation was provoked by densification of nickel particles and core-shell carbon formation. Despite the slight decrease in reactants' conversions, selectivity to syngas was around 80-90% during the three cycles of DRM/ regeneration with both atmospheres and no changes in the selectivity were observed. The results proved that the Ni/Ca-HA1_S catalyst could be easily regenerated under different atmospheres and make it competitive for dry reforming of methane.

Introduction

Carbon dioxide (CO 2 ) reforming of methane (CH 4 ) (Eq. ( 1)) has gained increasing attention due to the conversion of two main greenhouse gases into syngas (mixture of hydrogen and carbon monoxide), which can further be used in the production of energy and of high-value added chemicals [START_REF] Kathiraser | Kinetic and mechanistic aspects for CO 2 reforming of methane over Ni based catalysts[END_REF][START_REF] Moradi | Syngas production with dry reforming of methane over Ni/ZSM-5 catalysts[END_REF][START_REF] Farniaei | Syngas production in a novel methane dry reformer by utilizing of tri-reforming process for energy supplying: modeling and simulation[END_REF]. Nickel-based catalysts have been extensively investigated for this process due to its high catalytic activity in this reaction. However, they undergo severe deactivation due to catalyst sintering under the high temperatures required for the reaction and mostly due to coke deposition over the active sites of the catalyst [START_REF] Lin | Carbon dioxide reforming of methane over Ni catalysts prepared from Ni-Mg-Al layered double hydroxides: influence of Ni loadings[END_REF][START_REF] Aw | Insights into durable NiCo catalysts on (-SiC/CeZrO 2 and (-Al2O 3 /CeZrO 2 advanced supports prepared from facile methods for CH 4 -CO 2 dry reforming[END_REF]. In fact, many parallel reactions occur along with the dry reforming reaction (DRM) (Eq. ( 2)-( 5)) and coke can originate from many of these reactions (Eq. ( 2)-( 4)). (5)

+ ⇄ + = °: C CO
So, in the past few years, many studies have focused on the improvement of the physico-chemical properties of the catalysts and on the optimization of the reaction conditions in order to obtain high catalytic conversion and stability during long periods of time [START_REF] Abdelsadek | In-situ hydrogasification/regeneration of NiAlhydrotalcite derived catalyst in the reaction of CO2 reforming of methane: a versatile approach to catalyst recycling[END_REF]. They include the use of basic promoters, of supports with oxygen storage capacity, the combination of different transitions metals, the optimization of preparation pathways of the catalysts etc [START_REF] Muraza | A review on coke management during dry reforming of methane[END_REF][START_REF] Zhang | Coke-resistant Ni@SiO 2 catalyst for dry reforming of methane[END_REF][START_REF] Wang | Tuning the metal-support interaction in catalysts for highly efficient methane dry reforming reaction[END_REF][START_REF] Liu | Progresses in the preparation of coke resistant Nibased catalyst for steam and CO 2 reforming of methane[END_REF][START_REF] Coronel | Pd based membrane reactor for ultra pure hydrogen production through dry reforming of methane, Experimental and modeling studies[END_REF][START_REF] Ballarini | Platinum supported on alkaline and alkaline earth metal-doped alumina as catalysts for dry reforming and partial oxidation of methane[END_REF][START_REF] Xu | Biogas reforming for hydrogen production over nickel and cobalt bimetallic catalysts[END_REF][START_REF] Hou | Production of synthesis gas via methane reforming with CO 2 on noble metals and small amount of noble-(Rh-) promoted Ni catalysts[END_REF][START_REF] Wu | Bimetallic Rh-Ni/BN catalyst for methane reforming with CO 2[END_REF][START_REF] García-Diéguez | Characterization of alumina-supported Pt, Ni and PtNi alloy catalysts for the dry reforming of methane[END_REF][START_REF] Estephane | CO2 reforming of methane over Ni-Co/ZSM5 catalysts, Aging and carbon deposition study[END_REF][START_REF] Klaigaew | Effect of preparation methods on activation of cobalt catalyst supported on silica fiber for Fischer-Tropsch synthesis[END_REF][START_REF] Gálvez | Enhanced catalytic stability through nonconventional synthesis of Ni/SBA-15 for methane dry reforming at low temperatures[END_REF]. Also, temperature, pressure, reactants ratio and type of reactor have been investigated [START_REF] Liu | Progresses in the preparation of coke resistant Nibased catalyst for steam and CO 2 reforming of methane[END_REF][START_REF] Usman | Dry reforming of methane: Influence of process parameters-A review[END_REF][START_REF] Lavoie | Review on dry reforming of methane, a potentially more environmentally-friendly approach to the increasing natural gas exploitation[END_REF].

Nevertheless, the capacity of regeneration of the catalysts is also an important parameter. The regeneration process allows cleaning the coke deposit from the surface and the pores of the catalyst without destroying or modifying the structure of the catalyst [START_REF] Muraza | A review on coke management during dry reforming of methane[END_REF]. This aspect has economic and environmental impact. In fact, the costs of regeneration should remain lower than the costs of obtaining fresh catalysts [START_REF] Muraza | A review on coke management during dry reforming of methane[END_REF]. Moreover, regenerating the catalyst instead of disposing it as solid waste is an environmentally friendly option since the spent catalyst can form toxic metal compounds in the environment [START_REF] Haghlesan | Reactivation of an industrial spent catalyst as an environmental waste by ultrasound assisted technique for using in styrene production[END_REF]. So, regeneration studies of spent catalysts have also been conducted aiming to re-use the deactivated catalysts.

Vicerich et al. [START_REF] Vicerich | Influence of time and temperature on the regeneration of PtReIn/Al 2 O 3 naphtha reforming catalysts[END_REF] investigated the regeneration with oxygen diluted in nitrogen at 450-550 °C of the PtReIn/Al 2 O 3 catalyst used in cyclohexane dehydrogenation and cyclopentane hydrogenolysis. They showed that the regeneration process could fully recover the activity of the spent catalyst. Wu et al. [START_REF] Wu | Bimetallic Rh-Ni/BN catalyst for methane reforming with CO 2[END_REF] showed that the regeneration at 300 °C of the Rh 0.1 Ni 10 /BN catalyst could remove the carbon deposit and reactivate the catalyst, but a small decrease in the activity was observed. Simson et al. [START_REF] Simson | Steam reforming of methanol/gasoline mixtures: deactivation, regeneration and stable performance[END_REF] investigated the regeneration of a Pt/Rh catalyst after steam reforming of an ethanol/gasoline mixture. They showed that thermal treatment under air could fully restore the performance of the catalyst. However, after regeneration, their catalyst deactivated more quickly than the fresh one. Similarly, Sanchez et al. [START_REF] Sanchez | Hydrogen by glycerol steam reforming on a nickel-alumina catalyst: deactivation process and regeneration[END_REF] reported that the catalytic performance of their reforming catalyst could be recovered by air combustion of the coke deposits but its selectivity to H 2 quickly decreased.

Most of the literature reports on catalyst regeneration are related to the combustion of coke with air due to its availability. Also, the mechanism of carbon species oxidation by O 2 has been widely investigated. There are many references in the literature to carbon bulk diffusion through the metal particle, oxygen spillover and redox mechanism [START_REF] Theofanidis | Carbon gasification from Fe-Ni catalysts after methane dry reforming[END_REF]. However, coke deposits can be eliminated not only by O 2 but also by different gasifying agents, such as steam and CO 2 [START_REF] Rostrup-Nielsen | Catalytic steam reforming[END_REF]. There are only few studies that investigated the effect of other gasifying agents such as CO 2 on the regeneration of catalysts [START_REF] Bednarczuk | H 2 -production from CO 2 -assisted ethanol steam reforming: the regeneration of Ni-based catalysts[END_REF] and the mechanism of carbon removal by these gases is not yet well known [START_REF] Theofanidis | Carbon gasification from Fe-Ni catalysts after methane dry reforming[END_REF].

Hydroxyapatite has been investigated as catalyst support in many different reactions such as water gas shift [START_REF] Venugopal | Hydroxyapatite as a novel support for gold and ruthenium catalysts Behaviour in the water gas shift reaction[END_REF], partial oxidation of methane [START_REF] Jun | Nickel-calcium phosphate/hydroxyapatite catalysts for partial oxidation of methane to syngas: characterization and activation[END_REF], steam reforming of glycerol [START_REF] Yaakob | Hydroxyapatite supported Nickel catalyst for hydrogen production[END_REF] etc. In fact, hydroxyapatite can undergo substitutions in its crystal lattice, which may favor the incorporation of an active phase to the catalyst [START_REF] Narasaraju | Review Some physico-chemical aspects of hydroxylapatite[END_REF][START_REF] Boanini | Ionic substitutions in calcium phosphates synthesized at low temperature[END_REF]. Also, this material present high chemical and thermal stability, it has very low solubility in water and it does not sinter at temperatures lower than 700 °C [START_REF] Ababou | Etude expérimentale et théorique du préfrittage et du frittage de l'hydroxyapatite[END_REF][START_REF] Bernache-Assollant | Sintering of calcium phosphate hydroxyapatite Ca10(PO4)6(OH)2 I. Calcination and particle growth[END_REF]. Moreover, it presents basic sites that can reversibly adsorb carbon dioxide and help coke removal [START_REF] Boukha | Methane dry reforming on Ni loaded hydroxyapatite and fluoroapatite[END_REF]. Despite all these advantages, only few works has been published using hydroxyapatite as catalyst for dry reforming of methane reaction. We have previously demonstrated that hydroxyapatite-based catalysts were performing for dry reforming of methane reaction [START_REF] Rêgo De Vasconcelos | Catalytic transformation of carbon dioxide and methane into syngas over ruthenium and platinum supported hydroxyapatites[END_REF][START_REF] De Vasconcelos | Phosphates-based catalysts for synthetic gas (syngas) production using CO2 and CH 4[END_REF]. So, in the present work, Ni/ hydroxyapatite catalyst was investigated in CO 2 reforming of CH 4 reaction, with a focus on the regeneration process of the deactivated catalyst. The effect of gasifying agent on the re-use of the catalyst was also investigated by using two different gasifying agents: air and carbon dioxide. CO 2 was used in this work because it is one of the reactants of DRM and the reaction between CO 2 and solid carbon (Boudouard reaction, Eq. (2)) produces CO, which is one of the products of DRM. Also, the basic properties of the hydroxyapatite could favor the CO 2 adsorption and consequently, solid carbon (C (s) ) gasification. The objective of this work was to evaluate the reproducibility of the performance of the Ni/hydroxyapatite catalyst under several reaction/regeneration cycles and under different regeneration atmospheres.

Materials and methods

Catalyst preparation

The hydroxyapatite (Ca 10 (PO 4 ) 6 (OH) 2 ) support used in this work was provided by PRAYON. Besides coke deposition, one of the reasons for catalyst deactivation is the support sintering [START_REF] Muraza | A review on coke management during dry reforming of methane[END_REF]. So, as in this work the DRM reaction is performed at mild temperature (700 °C), the hydroxyapatite support was previously sintered at 1000 °C for 5 h at 10 °C/min to ensure support stability during the catalytic tests and to avoid later catalyst deactivation by support sintering. The catalyst was prepared by incipient wetness impregnation method by impregnating the support with an aqueous solution of Ni(NO 3 ) 2 •6H 2 O (Fisher Scientific, > 98 wt%). The amount of nickel on the catalyst was 5.7 wt.%. Then the catalyst was dried at 105 °C overnight. The catalyst was referred to as Ni/Ca-HA1_S. In order to obtain more precise characterization results of nickel particles on the catalyst during the reaction, a calcination step under air at 500 °C was performed for two hours before XRD, SEM and TEM analyses.

Dry reforming of methane

The dry reforming of methane was carried out in a fixed-bed tubular reactor with inner diameter of 8 mm 300 mg of the catalyst was diluted 2 times with inert alumina powder to improve heat exchange inside the catalyst bed. The same alumina was used to place the catalyst at the center of the reactor. The pressure drop in the reactor was 1.6 bar. The temperature of the reaction was controlled by a thermocouple placed at the center of the catalyst bed. The catalyst was reduced in situ at 700 °C for 2 h under 4%H 2 /N 2 flow (70 mL/min). Then, DRM was carried out for 30 h at 700 °C with a WHSV of 15882 mL h -1 g cat -1 with a synthetic gas mixture containing 20% of CH 4 , 20% of CO 2 and 60% of N 2 (volume percentage). A silica gel tube was used as water trap at the reactor outlet. Quantification of the water produced during the reaction was made by mass difference of this water trap. Also, a gas counter was used for the measurement of the total gas flow rate at the reactor outlet. The measure of the total gas flow rate at the reactor outlet allowed taking into account the change in the total number of moles between reactants and products to calculate the composition at the reactor outlet. Gas products were analyzed by a μ-GC A3000 (Agilent) equipped with a thermal conductivity detector (TCD).

Methane and carbon dioxide conversion and products selectivity were calculated as follows:

= - x CH conversion(%) (Q ˙Q ˙)/(Q ) 100 4 (CH ) (CH ) . (CH ) 4 in 4 out 4 in (6) = - CO conversion(%) (Q Q ˙)/(Q ˙)x100 2 . (CO ) (CO ) (CO ) 2 in 2 out 2 in (7) = H Oselectivity(%) (Q ˙)/(Q ˙)x100 2 O(H2Oproduced) O(CO2consummed) (8)
= COselectivity(%) (Q ˙)/(Q ˙)x100 O(COproduced) O(CO2consummed) (9)
= H selectivity(%) (Q ˙)/(Q ˙)x100 2 H(H2produced) H(CH4consumed) (10)
= + C selectivity(%) (Q ˙)/(Q ˙Q ˙)x100 (s) Cs(out) C(CH4consumed) C(CO2consumed) (11)
where Q ˙(CH ) 4 in and Q ˙(CH ) 4 out : methane molar flow rate (mmol h -1 ) at the reactor inlet and outlet; Q ˙(CO ) 2 in and Q ˙(CO ) 2 out : carbon dioxide flow rate (mmol h -1 ) at the reactor inlet and outlet; Q ˙O(H2Oproduced) : atomic oxygen flow rate (mmol h -1 ) at the reactor outlet (under H 2 O form); Q ˙O(CO2consummed) : atomic oxygen flow rate (mmol h -1 ) (under form of CO 2 consumed); Q ˙O(COproduced) : atomic oxygen flow rate (mmol h -1 ) at the reactor outlet (under CO form); Q ˙H(H2produced) : atomic hydrogen flow rate (mmol h -1 ) at the reactor outlet (under H 2 form); Q ˙H(CH4consumed) : atomic hydrogen flow rate (mmol h -1 ) (under form of CH 4 consumed); Q ˙Cs(out) : solid carbon flow rate at the outlet of the reactor deduced from mass balance.

The reaction/regeneration cycles were carried out as follows: a) Activation of the catalyst at 700 °C under 4%H 2 /N 2 was performed for 2 h. The dry reforming reaction was carried out at 700 °C for 30 h as previously explained. After the reaction, the reactive gases were switched with air. The temperature was kept at 700 °C and 70 mL/min of air flowed through the catalyst for 1h30 min. During the regeneration process, the concentration of CO 2 resulting from the oxidation of coke was monitored by a gas chromatography.

b) Activation of the catalyst at 700 °C under 4%H 2 /N 2 was performed for 2 h. The dry reforming reaction was carried out at 700 °C for 30 h as previously explained. After the reaction, the reactive gases were switched to a gas mixture containing 21% of CO 2 and 79% of N 2 (volume percentage). The temperature was kept at 700 °C and 70 mL/min of the gas mixture flowed through the catalyst for 1h30 min. During the regeneration process, the concentration of CO resulting from the oxidation of coke was monitored by gas chromatography.

Three cycles of DRM/regeneration for both gasifying agents were performed to test the regeneration capacity of the hydroxyapatite catalyst under different atmospheres.

Characterization

N 2 adsorption isotherms were recorded with a Micromeritics Tristar II 3020 to evaluate the specific surface area and the pore volume of the support and of the fresh catalyst. BET surface area was calculated from the nitrogen adsorption isotherms. Prior to the analyses, the samples were outgassed at 105 °C for 24 h at low pressure (< 100 mbar).

Carbon dioxide temperature-programmed desorption (CO 2 -TPD) was performed on a Micromeritics AutoChem 2920 Analyzer equipped with a thermal conductivity detector (TCD) to evaluate the basicity of the hydroxyapatite support. 100 mg of the support was pretreated at 800 °C (10 °C/min) under helium flow and then it was cooled to 50 °C at 10 °C/min. Then, the sample was saturated with 0.5%CO 2 /N 2 (15 mL/ min) for 30 min. Afterwards, 25 mL/min of He were flushed for 30 min to eliminate physisorbed CO 2 molecules. TPD was finally recorded from 50 to 800 °C at a heating rate of 10 °C/min. X-ray diffraction (XRD) was carried out using a Phillips Panalytical X'Pert MPD diffractometer operating with Cu Kα radiation (λ = 1.543 Å) over a 2θ range of 10-75°and scan step size of 0.042°s -1 . Prior to the analyses, the catalyst was calcined at 500 °C under air.

Inductively coupled plasma-optical emission spectroscopy (ICP-OES) analyses were performed with a HORIBA Jobin Yvon Ultima 2 apparatus to evaluate the Ni content in the catalyst sample. Prior to the measurements, 100 mg of the hydroxyapatite-based catalysts were dissolved in aqua regia (33vol% HNO 3 + 67vol% HCl) and heated at 90 °C for 1 h.

Environmental Scanning Electron Microscopy (ESEM) analysis was performed of a Philips XL30 ESEM FEG to characterize the shape and size of support and metal particles.

Transmission electron microscopy (TEM) images of fresh and spent catalyst were recorded on a MET JEOL ARM200F. Prior to the analyses, the catalysts were calcined at 500 °C under air atmosphere.

Temperature-programmed reduction (TPR) profiles of the dried catalysts were recorded with a Micromeritics AutoChem 2920 Analyzer in the temperature range of 25-850 °C. 100 mg of the sample was placed in a quartz reactor and reduced by a 5%H 2 /N 2 gas mixture with a heating rate of 10 °C/min. Prior to the analysis, the sample was purged under pure helium at 100 °C for 30 min to clean the surface of the sample.

Results and discussion

Characterization of samples

Physico-chemical properties of support and catalyst

Table 1 depicts the specific surface area (S BET ), pore volume (V p ) and nickel amount of the Ca-HA1_S support and of the Ni/Ca-HA1_S catalyst. For the purpose of comparison, the S BET and the V p of the hydroxyapatite support before thermal treatment (Ca-HA1) is also shown. The initial hydroxyapatite presented 7 m 2 /g of specific surface area and negligible pore volume. As expected, the hydroxyapatite support (Ca-HA1_S) and the Ni/Ca-HA1_S catalyst presented very low specific surface area and pore volume. In fact, sintering took place during thermal treatment at high temperature, which resulted in the loss of surface area and porosity. The addition of nickel to the support did not modify its specific surface area. The nickel loss was related to the fixation of the metal on the flask wall during the impregnation step.

CO 2 -Temperature programmed desorption (CO 2 -TPD)

CO 2 -TPD was conducted to evaluate the density and the strength of basic sites of the Ca-HA1_S support in order to evaluate the influence of the hydroxyapatite support on the performance of the catalyst. The density of basic sites is represented by the area under the curves and the strength of the sites is represented by the temperature of desorption. High temperatures represent strong basic sites. For comparison, the hydroxyapatite before thermal treatment (Ca-HA1) was also evaluated. Fig. 1 presents the CO 2 -TPD curves of this support.

The hydroxyapatite before thermal treatment (Ca-HA1) presented a small peak centered at about 150 °C and a broad peak between 200 and 600 °C. The desorption peaks of this material covered a wide range of temperature, which indicated the presence of weak, medium and strong basic sites. The thermal treatment completely changed the basicity of the initial hydroxyapatite. No desorption peaks were observed for the Ca-HA1_S support, which indicated that the basicity of this support was negligible after thermal treatment. In fact, the thermal treatment led to a full densification of the hydroxyapatite and destroyed its surface functions.

XRD analysis

XRD patterns of support and catalyst is shown in Fig. 2. Both support and catalyst showed the presence of the hydroxyapatite phase (Ca 10 (PO 4 ) 6 (OH) 2 ) with the main peaks at 31.9°, 32.3°, 33.0°and 34°(pattern number: COD 96-230-0274). Comparison between XRD patterns of the support before (Fig. 2a) and after (Fig. 2b) thermal treatment indicated that the thermal treatment of the hydroxyapatite did not change its structure, only its crystallinity. Besides the hydroxyapatite phase, the Ni/Ca-HA1_S catalyst also presented a NiO phase with main diffraction peaks at 37°, 43°and 63°. No peaks of Niexchanged hydroxyapatite were detected, which suggested the nickel was mainly located on the surface of the hydroxyapatite and not in its structure.

SEM and TEM analyses

Fig. 3 shows the SEM images of the Ca-HA1_S support and of the Ni/ Ca-HA1_S fresh catalyst.

Fig. 3a shows the SEM image of the hydroxyapatite support after the thermal treatment. A coalescence of the hydroxyapatite particles could be observed, indicating the sintering of this material. Fig. 3b shows the SEM image of the Ni/Ca-HA1_S catalyst. Large nickel particles of about 100-200 nm could be observed. However, SEM analysis is not precise enough to indicate if each particle observed is only one nickel particle or an agglomerate.

To confirm the SEM results, TEM analysis was performed. Fig. 4 illustrates the TEM images of the fresh Ni/Ca-HA1_S catalyst.

TEM analysis showed the presence of large nickel particles with sizes between 100 and 200 nm. Also, the nickel particles observed are porous and have square shape (2D), which corresponded to the cubic structure of NiO [START_REF] Moradi | Syngas production with dry reforming of methane over Ni/ZSM-5 catalysts[END_REF][START_REF] Gálvez | Enhanced catalytic stability through nonconventional synthesis of Ni/SBA-15 for methane dry reforming at low temperatures[END_REF][START_REF] Courson | Grafted NiO on natural olivine for dry reforming of methane[END_REF].

TPR analysis

TPR analysis was performed to evaluate the reducibility of the Ni/ Ca-HA1_S catalyst. Fig. 5 shows the TPR curve of this catalyst.

Ni/Ca-HA1_S catalyst presented two reduction peaks centered at 395 and 402 °C. According to Ahmed et al. [START_REF] Ahmed | Effect of textural properties of alumina support on the catalytic performance of Ni/Al 2 O 3 catalysts for hydrogen production via methane decomposition[END_REF] and Wu et al. [START_REF] Wu | Steam reforming of methane to hydrogen over Ni-based metal monolith catalysts[END_REF], these peaks correspond to the reduction of NiO species with different oxidation degrees located at the external surface of the support and weakly interacting with the support. This result confirms the XRD analysis.

Catalyst behavior under DRM/regeneration cycles

Catalytic performance under DRM/Air-regeneration cycles

Fig. 6 shows the evolution with time on stream (TOS) of the CH 4 and CO 2 conversions as well as of the H 2 , CO, H 2 O and solid carbon (C (s) ) selectivities for the Ni/Ca-HA1_S catalyst under three successive cycles of DRM/Air regeneration.

In the first cycle (blue dots), the Ni/Ca-HA1_S catalyst showed high CH 4 and CO 2 conversion around 70% that decreased to 52% and 61%, respectively during 30 h oh TOS. H 2 and CO selectivity were constant at around 90%. As consequence, low H 2 O and C (s) were obtained. H 2 O selectivity was constant at around 10% and C (s) was lower than 10%. The deactivation of the catalyst was probably related to coke deposition (Eq. 2-4) and/or catalyst sintering. The catalyst presented initially good activity and stability on DRM for 30 h of TOS, showing that the hydroxyapatite-based catalyst was performing for this reaction. This proves that the physical properties (low S BET and V p , Table 1) are compensated by its chemistry.

After in situ regeneration by coke combustion under air flow and reactivation under 4%H 2 /N 2 flow, the catalyst exhibited similar performance as in the first cycle, showing only a small decrease in CH 4 and CO 2 conversions (red dots). This suggested that irreversible deactivation caused by catalyst sintering took place. CH 4 and CO 2 conversions were initially around 60 and 70% and decreased to 47 and 56%, respectively during 30 h of TOS. However, H 2 and CO selectivities were constant at about 90% during the test, as in the first cycle. H 2 O and C (s) selectivities were also similar at around 10%.

Subsequent to another step of in situ regeneration and reactivation, the catalyst exhibited similar performance as in the second cycle, but also with slight decrease in CH 4 and CO 2 conversions. The slight deactivation in this cycle can also be explained by catalyst sintering. CH 4 and CO 2 conversions were initially at 55 at 61% and decreased to 44 and 54% during the test. H 2 and CO selectivity were constant at 80%. Also, H 2 O and C (s) were kept around 10% during 30 h of test.

TEM images of the spent catalyst after the three cycles of DRM/ regeneration confirms the catalyst deactivation by sintering. Fig. 7 Fig. 2. XRD patterns of (a) Ca-HA1 support before thermal treatment and of (b) Ca-HA1_S support (after thermal treatment) and of Ni/Ca-HA1_S catalyst. presents the TEM images of the spent catalyst after three cycles of DRM/Air regeneration.

Fig. 7a shows the formation of carbon deposit in the form of carbon nanofibers (CNFs) and core-shell. The formation of carbon was possible due to the relatively low metal-support interaction (Fig. 5) and to the presence of large nickel particles (Fig. 4). The CNFs play minor role on the catalyst deactivation because they mostly grow away from the catalyst surface and do not cover the active sites [START_REF] Ay | Dry reforming of methane over CeO 2 supported Ni, Co and Ni-Co catalysts[END_REF][START_REF] Wang | The properties of individual carbon residuals and their influence on the deactivation of Ni-CaO-ZrO 2 catalysts in CH 4 dry reforming[END_REF][START_REF] Rêgo De Vasconcelos | Synthesis of carbon nanotubes/hydroxyapatite composites using catalytic methane cracking[END_REF]. Also, they can push nickel particles from the supports and in some cases, they do not encapsulate them. In many cases, the nickel particles are on the tip of the CNFs and thus still available to the reaction [START_REF] Ay | Dry reforming of methane over CeO 2 supported Ni, Co and Ni-Co catalysts[END_REF][START_REF] Rêgo De Vasconcelos | Synthesis of carbon nanotubes/hydroxyapatite composites using catalytic methane cracking[END_REF][START_REF] Amin | Review of methane catalytic cracking for hydrogen production[END_REF][START_REF] Amin | Highly stable ytterbium promoted Ni/(-Al 2 O 3 catalysts for carbon dioxide reforming of methane[END_REF]. On the other hand, the core-shell structure covers completely the active site of the catalyst and is hardly removed, leading to catalyst deactivation [START_REF] Wang | The properties of individual carbon residuals and their influence on the deactivation of Ni-CaO-ZrO 2 catalysts in CH 4 dry reforming[END_REF][START_REF] Wang | Coking and deactivation of a mesoporous Ni-CaO-ZrO 2 catalyst in dry reforming of methane: a study under different feeding compositions[END_REF]. So, the small decrease in the performance of the catalyst between each cycle could be partially related to the formation of coreshell carbon.

Fig. 7b andc illustrates nickel particles over the spent catalyst after three cycles of DRM/Air regeneration. The nickel particles on the spent catalyst have a completely different shape and structure than the nickel particles on the fresh catalyst (Fig. 4). They have round shape, size about 10-50 nm and are not porous. The nickel particles suffered densification during the reaction, losing their surface and porosity, which could also explain the small irreversible catalyst deactivation between cycles. However, it is worth noting that these nickel particles of round shape and small sizes (10-50 nm) are not poisoned by coke. In fact, it is widely accepted that the formation of coke on small nickel particles is not energetically favored [START_REF] Zhang | Coke-resistant Ni@SiO 2 catalyst for dry reforming of methane[END_REF]. Moreover, it is the metallic step edges that act as growth sites for the carbon growth [START_REF] Helveg | Atomic-scale imaging of carbon nanobre growth[END_REF][START_REF] Helveg | Atomic-scale studies of metallic nanocluster catalysts by in situ high-resolution transmission electron microscopy[END_REF]. So, despite the small loss of activity due the densification of the nickel particles, it might have prevented the deposition of coke over these particles and thus stabilizing the catalyst. This also suggests that coke deposition occurred mainly in the first hours of the reaction while the nickel particles still had large size (100-200 nm) and square shape.

Catalytic performance under DRM/CO 2 -regeneration cycles

Fig. 8 shows the evolution with time on stream (TOS) of the CH 4 and CO 2 conversions as well as of the H 2 , CO, H 2 O and C (s) selectivities for the Ni/Ca-HA1_S catalyst under three successive cycles of DRM/CO 2 regeneration. The results obtained with DRM/CO 2 regeneration cycles are very similar to those obtained with the DRM/Air regeneration cycles.

In the first cycle (blue dots), the Ni/Ca-HA1_S catalyst showed initial CH 4 and CO 2 conversion around 70 and 76%, respectively and then decreased to 46% and 57%, respectively during 30 h of TOS. As for the previous results, this deactivation could be related to carbon deposition and/or catalyst sintering. H 2 and CO selectivity were kept constant around 90% during the test. Also, H 2 O and C (s) were stable around 10% during the test.

Subsequent to an in situ regeneration under 21%CO 2 /N 2 flow and reactivation under 4%H 2 /N 2 , a small decrease in the CH 4 and CO 2 was observed compared to the first cycle, indicating the presence of irreversible catalytic deactivation. Although, high syngas yield was obtained. H 2 and CO selectivities were stable and around 90% during the test. H 2 O and C (s) were also stable around 10% during the test.

The third cycle of DRM/CO 2 regeneration (green dots) showed similar results to the second cycle (red dots), but a small decrease in the initial CH 4 and CO 2 conversions was also observed. The slight deactivation in this cycle could also be explained by catalyst sintering. CH 4 and CO 2 conversions decreased from around 50 and 60% to 41 and 52%. However, as for the previous cycles, high H 2 and CO selectivities were obtained. H 2 and CO selectivities were constant at around 80% during 30 h of test. Consequently, low selectivity to byproducts was obtained. H 2 O and C (s) were constant at 10% during the test. Fig. 9 presents the TEM images of the spent catalyst after three cycles of DRM/CO 2 regeneration. Fig. 9a shows the presence of carbon deposit under CNFs and coreshell forms. These were already observed for the Ni/Ca-HA1_S catalyst after 3 cycles of DRM/Air regeneration. The irreversible deactivation of this catalyst between the cycles was related to the formation of the coreshell carbon, which is hardly removed. Moreover, this catalyst also presented non-porous nickel particles with round shape and with size around 10-50 nm (Fig. 9b), indicating the occurrence of densification during the reaction. As previously explained, the densification of nickel particles provoked irreversible deactivation, which led to a small decrease in the performance of the catalyst. However, it also improved the stability of the catalyst since the formation of small nickel particles prevented the growth of coke.

Fig. 10 presents a summary of the results, comparing the values of CH 4 conversion and H 2 selectivity between regeneration under air and under CO 2 . The "fresh" and "spent" values correspond to the performance of the catalyst at the beginning and at the end of the first cycle, respectively. The values of "Regenerated (2nd cycle)" and "Regenerated (3rd cycle)" correspond to the values of performance at the begging of 2nd and 3rd cycles after the regeneration step, respectively.

The results obtained with air regeneration are quite similar than those obtained with CO 2 /N 2 regeneration. The catalyst after regeneration steps with both atmospheres always showed higher CH 4 conversion than the spent catalyst (red column). This indicates that both atmospheres used could successfully regenerate the catalyst surface by eliminating the carbon deposit.

As previously presented, the values of CH 4 conversion after the regeneration steps are slightly lower than the CH 4 conversion values of the fresh catalyst (dark blue column) due to irreversible deactivation provoked by core-shell carbon formation and nickel particles densification. Nevertheless, the values of H 2 selectivity after regeneration steps under air and CO 2 were relatively stable and comparable to the H 2 selectivity of the fresh catalyst. This suggests that the occurrence of irreversible deactivation did not affect the selectivity of the catalyst to syngas.

These results showed that the Ni/Ca-HA1_S catalyst can be easily regenerated under different atmospheres. An optimization of the regeneration step including changing the temperature and the duration of the regeneration are still needed to remove the core-shell carbon of the catalyst surface and obtain higher reactants' conversion.

Conclusions

The regeneration of a hydroxyapatite-based catalyst (Ni/Ca-HA1_S) was investigated during three successive cycles of DRM/regeneration. The in situ regeneration was carried out at 700 °C for 1h30 after a cycle of DRM at 700 °C for 30 h. The capacity of regeneration of the catalyst was tested under two different atmospheres: air and 21%CO 2 /N 2 . The catalyst presented good reproducibility between cycles for both atmospheres tested and only a small deactivation between cycles was observed. The irreversible deactivation between cycles was provoked by the formation of core-shell carbon and by densification of the nickel particles during the reaction. However, after densification the nickel particles had round shape and small size (10-50 nm), which helped to prevent further carbon deposition. Despite the small loss in the reactants' conversion, the syngas selectivity was constant between cycles, which indicated that the deactivation of the catalyst did not affect its selectivity. These results showed the Ni/Ca-HA1_S catalyst could be easily in situ regenerated under different atmospheres and make it competitive for dry reforming of methane. Further tests are needed to optimize the regeneration step and eliminate the core-shell carbon deposits.

Fig. 1 .

 1 Fig. 1. CO 2 -TPD of hydroxyapatite support before (e Ca-HA1) and after thermal treatment (e Ca-HA1_S).

Fig. 3 .

 3 Fig. 3. SEM images of (a) Ca-HA1 support and (b) Ni/Ca-HA1_S catalyst; → Ni particle.

Fig. 4 .Fig. 5 .Fig. 6 .

 456 Fig. 4. TEM of Ni/Ca-HA1_S fresh catalyst; → Ni particle, → hydroxyapatite.

Fig. 7 .

 7 Fig. 7. TEM of Ni/Ca-HA1_S spent catalyst after three cycles of DRM/Air regeneration; → Ni particle, → carbon.

Fig. 8 .

 8 Fig. 8. Catalytic performance of Ni/Ca-HA1_S catalyst under 3 cycles of DRM/CO 2 -regeneration. Reaction conditions: T=700 °C, WHSV = 15882 mL h -1 g cat -1 , CH 4 /CO 2 = 1, TOS=30 h. Regeneration conditions: 1h30 at 700 °C with 21%CO 2 /N 2 flow.

Fig. 9 .Fig. 10 .

 910 Fig. 9. TEM of Ni/Ca-HA1_S spent catalyst after three cycles of DRM/CO 2 regeneration; → Ni particle, → carbon.

Table 1

 1 Physico-chemical properties of support and catalyst.

	Sample	Textural characterization		ICP-OES
		S BET (m 2 /g)	V p (cm 3 /g)	Ni wt.%
	Ca-HA1	7	nd	-
	Ca-HA1_S	< 3	≈0	-
	Ni/Ca-HA1_S	< 3	≈0	4

Acknowledgments

The authors gratefully acknowledge PRAYON for their financial support to this study. The technical help of colleagues at RAPSODEE research center is also acknowledged.


Augustina Ephraim
email: augustinaephraimaug.ephraim@gmail.com

Linhdan Ngo

Doan Pham

Damien Lebonnois

Carlos Peregrina

Patrick Sharrock

Ange Nzihou

Valorization of Waste-Derived Inorganic Sorbents for the Removal of HCl in Syngas

Keywords: Syngas, Hydrogen chloride, Inorganic waste sorbents, Dry adsorption

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Introduction

Syngas produced from the pyro-gasification of waste biomass has many industrial applications including electricity and heat production, as well as the synthesis of transportation fuels, methanol and ammonia [START_REF] Boerrigter | Syngas production and utilisation[END_REF][START_REF] Milne | Biomass gasifier "tars": their nature, formation, and conversion[END_REF]. Nonetheless, waste biomass may have significant concentrations of chlorine, which may lead to the formation of hydrogen chloride (HCl) in syngas produced by the pyro-gasification process. Several problems may occur due to the presence of HCl gas, including the corrosion of metal equipments, health problems and environmental issues [START_REF] Liu | A thermodynamic study of hot syngas impurities in steel reheating furnaces-corrosion and interaction with oxide scales[END_REF][START_REF] Woolcock | A review of cleaning technologies for biomass-derived syngas[END_REF], which therefore necessitates the removal of HCl from syngas before use. The level of syngas cleaning required is governed by the quality requirements of the end-use applications and environmental emissions regulations [START_REF] Arena | Gasification: an alternative solution for waste treatment with energy recovery[END_REF]. As we can see in Table 1, the concentrations of HCl typically found in waste-derived syngas are significantly higher than those required by various syngas applications and the emission standards of areas like the European Union (EU) and Japan [START_REF] Woolcock | A review of cleaning technologies for biomass-derived syngas[END_REF][START_REF] Arena | Process and technological aspects of municipal solid waste gasification. A review[END_REF][START_REF] Evans | Evaluation of opportunities for converting indigenous UK wastes to fuels and energy[END_REF]. Thus, it is imperative to develop HCl removal methods that are cost-effective, efficient and environmentally friendly.

Dry adsorption is as a promising technique for eliminating HCl in syngas, which is based on chemical reactions involving HCl and metal-based solid particles (sorbents) [START_REF] Abdoulmoumine | A review on biomass gasification syngas cleanup[END_REF][START_REF] Asadullah | Biomass gasification gas cleaning for downstream applications: a comparative critical review[END_REF][START_REF] Sharma | Recent developments in dry hot syngas cleaning processes[END_REF][START_REF] Stemmler | Chemical hot gas cleaning concept for the CHRISGAS? process[END_REF][START_REF] Turk | Novel technologies for gaseous contaminants control[END_REF][START_REF] Baek | Cleaning of gaseous hydrogen chloride in a syngas by spray-dried potassium-based solid sorbents[END_REF][START_REF] Kameda | Removal of hydrogen chloride from gaseous streams using magnesiumaluminum oxide[END_REF][START_REF] Mondal | Syngas production through gasification and cleanup for downstream applications-recent developments[END_REF][START_REF] Sharma | Design and performance evaluation of dry cleaning process for syngas[END_REF]. A potential benefit of using this technique is that the sorbents can be regenerated using methods such as heating, which can be economically and environmentally interesting when compared to other methods, such as wet scrubbing, that produce wastewater which is costly to treat. Another advantage of the dry adsorption method is that it opens up opportunities to valorize metal-based solid wastes from industrial processes. In literature, various sorbents for HCl removal have been studied, and can be classified into three groups: alkali-based sorbents, alkaline-earth based sorbents and other metal based sorbents.

-Alkali-based sorbents studied by researchers are mostly sodium and potassium compounds due to their strong HCl binding capabilities [START_REF] Verdone | Reaction kinetics of hydrogen chloride with sodium carbonate[END_REF][START_REF] Verdone | Thermodynamic behaviour of sodium and calcium based sorbents in the emission control of waste incinerators[END_REF][START_REF] Hartman | Reaction of hydrogen chloride gas with sodium carbonate and its deep removal in a fixed-bed reactor[END_REF][START_REF] Fellows | HCl sorption by dry NaHCO3 for incinerator emissions control[END_REF]. For example, Krishnan et al. [START_REF] Krishnan | Removal of hydrogen chloride from high temperature coal gases[END_REF] have shown that Nahcolite (NaHCO 3 ) can remove HCl from a gas mixture to concentrations as low as 1 ppm in the temperature range of 525-650 °C. -Although the performance of alkaline earth-based sorbents is lower than that of alkali-based sorbents, they are significantly cheaper, and thus have attracted the attention of many researchers [START_REF] Dou | Single and combined removal of HCl and alkali metal vapor from hightemperature gas by solid sorbents[END_REF][START_REF] Dou | High-temperature HCl removal with sorbents in a fixed-bed reactor[END_REF][START_REF] Dou | HCl removal and chlorine distribution in the mass transfer zone of a fixed-bed reactor at high temperature[END_REF][START_REF] Chyang | Study of HCl absorption by CaO at high temperature[END_REF][START_REF] Wang | The kinetics of the reaction of hydrogen chloride with fresh and spent Ca-based desulfurization sorbents[END_REF]. Slaked lime (Ca(OH) 2 ), limestone (CaCO 3 ) and quicklime (CaO) are the most commonly studied, which have been shown to strongly bind to HCl in temperature range of 500-600 °C [START_REF] Daoudi | The reaction of HCl gas with calcined commercial limestone particles: the effect of particle size[END_REF][START_REF] Sun | Experimental study of HCl capture using CaO sorbents: activation, deactivation, reactivation, and ionic transfer mechanism[END_REF][START_REF] Tan | Laboratory study on hightemperature adsorption of HCl by dry-injection of Ca(OH)2 in a dual-layer granular bed filter[END_REF][START_REF] Mura | Reaction kinetics of gas hydrogen chloride and limestone[END_REF].

-Other metal based sorbents containing aluminium, iron, and zinc have also received considerable research interest [START_REF] Dou | Single and combined removal of HCl and alkali metal vapor from hightemperature gas by solid sorbents[END_REF][START_REF] Cao | Treatment of hydrochloric acid in flue gas from municipal solid waste incineration with CaMgAl mixed oxides at medium high temperatures[END_REF][START_REF] Bhaskar | Laboratory evaluation of calcium-, iron-, and potassium-based carbon composite sorbents for capture of hydrogen chloride gas[END_REF]. As an example, studies by Ohtsuka et al. [START_REF] Ohtsuka | Recent progress in Japan on hot gas cleanup of hydrogen chloride, hydrogen sulfide and ammonia in coal-derived fuel gas[END_REF] have shown that the addition of Al 2 O 3 to Na 2 CO 3 can reduce HCl in syngas to less than 1 ppm at 400 °C.

Although many literature works exist on the removal of HCl from flue gas using commercial sorbents, there lack studies on the cleaning of waste-derived syngas derived from waste biomass using solid waste sorbents. Hence, this paper evaluates and compares the performance of the solid waste sorbent, CCW-S, to the performance of the commercial sorbent, Bicar. CCW-S is a calcium carbonate residue recovered from an industrial process for producing sodium carbonate, that has been carbonated naturally by atmospheric carbon dioxide for a long duration. Bicar, on the other hand, is a commercial sorbent used in industry for flue gas treatment of acid gases. The experiments conducted with these sorbents will involve the adsorption of HCl in nitrogen and synthetic syngas atmospheres at ambient conditions, in order to study the influence of gas matrix on the sorbent performance.

Various physico-chemical characterisation methods will be conducted on the sorbents before and after the tests to enable a better understanding of the sorbent adsorption behaviour.

Experimental Methods

Sample Characterization

The physico-chemical properties of CCW-S and Bicar were analyzed before and after the tests using various techniques. Their mineral content was determined using an inductively coupled plasma with an atomic emissions spectra (ICP-AES, HORIBA Jobin-Yvon Ultima 2) and their molecular bonds were identified using fourier transform infrared spectrometry (FTIR, 8400S-Shimadzu). Furthermore BET analyses were performed to determine the specific surface area of the particles, using Micromeritics 3Flex analyzer. The crystalline structures were studied with X-ray diffraction (XRD, Philips Panalytical X'pert Pro MPD diffractometer) and environmental scanning electron microscopy (ESEM, XL30 ESEM aparatus-FEI Company) in order to obtain information related to the particles' morphology. A Transition electron microscope coupled to an energy dispersive X-ray (TEM-EDX) was employed to perform imaging and elemental mapping of chemical species in the samples. Before characterization and tests, the two sorbents were sieved to obtain samples with particle sizes below 315 μm, and then dried at ambient temperature for 48 h. For analyses with FTIR, reference samples of calcium carbonate (CaCO 3 -calcite, from Fisher Scientific) and sodium bicarbonate (NaHCO 3 , from Acros Organics) were used, with each sample having more than 98% purity.

Experiments with HCl in Nitrogen

Gas Matrix and Reactor Operating Conditions

The first set of experiments were performed with a 500 ppmv mixture of hydrogen chloride in nitrogen (HCl/N 2 ) under ambient conditions (25 °C, 1 atm). Figure 1 gives a schematic representation of the experimental setup for the tests. A glass fixed-bed reactor (height = 120 mm, internal diameter = 10 mm), was equipped with a porous glass disc to hold the sorbent particles and allow the passage of gas through the sorbent bed. The bed height was kept at 7.6 mm for all sorbents by adjusting the mass of sorbent used for the experiments (0.3-0.5 g) according to their bulk densities. After placing the sorbents in the reactor, glass beads (4.2 g, 1 mm size) were added as inert particles to maintain the bed height. The 500 ppmv HCl/N 2 gas passed through the sorbent bed in a downward flow at a flowrate of 50 mL/min, which was controlled by a mass flow meter. Thus, the gas residence time in the sorbent bed was maintained at 0.714 s.

Gas Sampling and Analysis

Gas was sampled at the reactor outlet with a gas cell, and then analyzed for its HCl content using FTIR. The gas cell had internal dimensions of 29 mm × 135 mm and was equipped with windows made of calcium fluoride (CaF 2 ). Figure 2 gives an example of the infrared spectrum of HCl in 500 ppmv HCl/N 2 gas mixture, which appears as a series of regular peaks between 2650 and 3100 cm -1 . The residual HCl gas was scrubbed in an aqueous solution of KOH as shown in Fig. 1.

Experiments with HCl in Syngas

Gas Matrix and Reactor Operating Conditions

The second set of experiments involved the use of 200 ppmv HCl in a simulated syngas matrix (HCl/syngas) under ambient conditions (25 °C, 1 atm), whose composition is shown in Table 2. These experiments were conducted with an online HCl analyzer named ProCeas, which will be described in the next section. In order to minimize the response time of this analyzer, a higher gas flowrate of 500 Balance Balance mL/min was employed. Thus, to maintain the gas residence time in the sorbent bed at 0.714 s, a larger reactor (height = 168 mm, internal diameter = 14 mm) was used and a bed height of 39 mm was fixed. For these experiments, the sorbent mass of 4-5 g was in the range, depending on the sorbent bulk density.

Gas Sampling and Analysis

Results and Discussion

Characteristics of Sorbents Before Experiments

Table 3 displays the bulk density, surface area and particle diameter of the sorbents before the experiments were performed. We can observe that Bicar has about twice as much surface area as CCW-S, which is favorable for HCl adsorption.

The inorganic element composition of the sorbents before the tests are displayed in Table 4. We observe that CCW-S mainly contains Ca, whereas majority of Bicar is composed of Na. CCW-S also has a significant concentration of contaminants (Al, Fe and Mg) due to its waste origin.

Figure 4 shows the infrared spectra of CCW-S and Bicar as well as the reference spectra of pure CaCO 3 and NaHCO 3 . Compared to pure CaCO 3 , CCW-S shows similar peaks at 1404, 873 and 713 cm -1 , which correspond to C-O bonds [START_REF] Galvn-Ruiz | Characterization of calcium carbonate, calcium oxide, and calcium hydroxide as starting point to the improvement of lime for their use in construction[END_REF]. This suggests that calcium carbonate (calcite) is a major compound in this industrial waste. Furthermore, the spectrum of Bicar is almost identical to that of pure NaHCO 3 , with peaks appearing between 600 and 1926 cm -1 .

XRD analysis was performed in order to identify the crystalline structure of the inorganic compounds in the sorbents. The results are displayed in Table 5 and Fig. 5. For CCW-S, the XRD analysis shows that calcium carbonate is the main component of this material, which has a rhombohedral structure that is characteristic of calcite. In contrast, Bicar is observed to be composed of only NaHCO 3 with a monoclinic crystalline structure. These results are in agreement with the observations made using the FTIR technique. However,

Adsorption of HCl in Nitrogen

Breakthrough Curves

Figure 6 displays the breakthrough curves obtained for the tests with 500 ppmv HCl/N 2 gas, which shows the evolution of the exit HCl concentration with time. Henceforth, we will define the breakthrough time t b as the duration for which the exit HCl concentration is zero. The corresponding t b value for each sorbent is displayed in Table 6. The tests, which were conducted at least three times for each sorbent, were repeatable with a deviation in t b of 2.4 and 6.5% from the mean value for Bicar and CCW-S respectively. It is obvious from Fig. 6 and Table 6 that Bicar has a long breakthrough time of 64 h and a high HCl adsorption capacity of 26.8 wt%, where the HCl adsorption capacity was calculated by dividing the mass of adsorbed HCl by the initial sorbent mass. CCW-S, however, has a significantly lower performance. These results are coherent with those of several authors who have found calcium-based sorbents to be less reactive with HCl than sodium-based sorbents [18-21, 33, 35]. An explanation for the relatively slow reactions of CaCO 3 in CCW-S with HCl gas has been linked by Duo et al. [START_REF] Duo | Formation of product layers in solid-gas reactions for removal of acid gases[END_REF] to the reaction's chemical free energy ( ΔG c ). They show that for a reaction involving CaCO 3 and HCl in an atmosphere containing trace amounts (500 ppm) of H 2 O and CO 2 , -ΔG c is only 50 kJ/mol at 25 °C, which provides a very low driving force for the reaction to occur. However, -ΔG c can increase to 80 kJ/mol if the temperature rises to 600 °C under the same atmosphere, thereby possibly leading to a significant increase in the adsorption rate of HCl by CCW-S. Another reason for the lower performance of CCW-S compared to Bicar might be its relatively small surface area (Table 3), which lowers the chances of collision with HCl molecules, and thus lowers the reaction rate. These proposed explanations suggest that the adsorption efficiency of CCW-S can be significantly improved by operating at higher temperatures and increasing the particle surface area by thermal activation to generate pores. In Fig. 6, a flat region can be observed on the graphs at an HCl outlet concentration of about 200 and 250 ppm for the adsorption tests with CCW-S and Bicar respectively. This may be due to the occurence of monolayer adsorption on the particle surface, which deserves further investigation by performing adsorption isotherm experiments.

Effect of Sorbent Mineral Elements

TEM-EDX analysis was performed on CCW-S and Bicar particles to qualitatively investigate the effect of the major elements-Ca, C, O, Na-and inorganic elements-Ca, Na, Al, Fe and Mg-on the formation of new products at the sorbent surface. Figure 7 displays cartographic pictures that showcase the distribution of inorganic elements on the surface of a CCW-S particle after the test with HCl/N 2 gas. The particle in this figure clearly shows Cl (HCl) fixed on its surface. In addition, the cartographs of Ca and Cl are almost identical, which implies that a chlorinated product is formed on the particle. The elements, O and C, also have a similar distribution on the particle surface as Ca, indicating that not all of CaCO 3 in the particle is converted. Furthermore, the impurities-Al, Fe and Mg-appear to also adsorb HCl although in relatively small amounts because of their low content in the CCW-S sample.

Figure 8 displays the TEM-EDX cartograph of a Bicar particle. As seen by the brightness of the cartograph, Cl (HCl) is strongly fixed onto a portion of the Bicar particle surface to form a chlorinated product. It is also obvious that not all of NaHCO 3 is converted because the elements, O and C, are present and have a similar distribution as Na on the particle. As a result of the high purity of Bicar (Table 4), a small scatter of Al, Fe and Mg can be seen on the particle surface, in the same region as Cl, which indicates that these impurities also adsorb HCl just as they do for CCW-S. The adsorption of HCl by Mg and Al has been well documented in literature [START_REF] Cao | Treatment of hydrochloric acid in flue gas from municipal solid waste incineration with CaMgAl mixed oxides at medium high temperatures[END_REF][START_REF] Kameda | Treatment of gaseous hydrogen chloride using MgAl layered double hydroxide intercalated with carbonate ion[END_REF][START_REF] Kameda | Removal of HCl, SO2, and NO by treatment of acid gas with MgAl oxide slurry[END_REF][START_REF] Blachman | Process for removing HCI from fluids with novel adsorbent[END_REF].

Effect on Sorbent Crystalline Structure

The crystalline structures of the sorbents after tests with HCl/N 2 gas are displayed in Table 7 and Fig. 9. The results show that CCW-S largely contains the same CaCO 3 crystal structures before and after the tests (Table 5), and therefore, the content of any chlorine-containing crystal in the sorbent is below the detection limit of the XRD analyzer. This is expected due to the low reactivity of CCW-S observed in Table 6, implying that the CCW-S particle analysed using the TEM-EDX technique was one of the few that actually adsorbed HCl to form chlorinated products via the following possible reactions.

For Bicar, NaCl crystals of cubic structure were detected alongside NaHCO 3 . This indicates that Bicar adsorbed HCl via the chemical reaction:

(1) 5 and7, showing that little change in the crystal system occured. Bicar particles, after HCl adsorption, are seen to have deposits of cubic crystals on their surface which are not present on the original particles. These cubic crystals correspond to the NaCl crystals detected using XRD (Table 7). These results as well as those of the XRD analysis, confirm the formation of a chlorinated-product layer on particles of Bicar as a result of chemisorption of HCl on the particles.

CaCO 3 + 2HCl ⇋ CaCl 2 + H 2 O + CO 2 (2) CaCO 3 + HCl ⇋ CaClOH + CO 2 (3) NaHCO 3 + HCl ⇋ NaCl + CO 2 + H 2 O

Adsorption of HCl in Syngas

For experiments using 200 ppmv HCl/Syngas mixture, the breakthrough curves and sorbent performance results are shown in Fig. 11 and Table 8. We can see that the breakthrough times, t b , for CCW-S and Bicar are 2.7 h and 48.2 h respectively, which in comparison to their corresponding values under the HCl/N 2 gas mixture (Table 6), is a significant decrease of 5.3 and 16.5 h respectively. Furthermore, the HCl adsorption capacities of the two sorbents dramatically decrease in the presence of syngas.

To verify that this decrease in performance of CCW-S and Bicar was not due to reactor scale-up and change of HCl analyzer, an adsorption test was repeated with CCW-S in the 'large' reactor configuration using 500 ppm HCl/N 2 gas. The results were then compared with those obtained with the 'small' reactor set-up under the same atmosphere. Figure 12 shows that, despite the difference in reactor scale and HCl analyzer, the value of t b remained 8 h, and thus, the breakthrough time was unaffected by the reactor and HCl analyzer used. We can therefore attribute the reduced performance of CCW-S and Bicar to competitive interactions that occured between HCl and the syngas components at the active sites of the sorbents. In literature, several authors have studied the influence of gas atmosphere composition on HCl adsorption capacity of carbon-based and sodium-based sorbents [START_REF] Mura | Reaction kinetics of gas hydrogen chloride and limestone[END_REF][START_REF] Duo | Kinetics of HCl reactions with calcium and sodium sorbents for IGCC fuel gas cleaning[END_REF][START_REF] Partanen | Part I: the effects of temperature, gas atmosphere and absorbent quality[END_REF][START_REF] Karlsson | Adsorption of hydrochloric acid on solid slaked lime for flue gas clean up[END_REF]. All of these authors agree that the presence of CO

Conclusion

In this paper, the dry adsorption potential of a solid waste sorbent, CCW-S, is compared to that of a commercial sorbent, Bicar. Various physico-chemical analyses were performed on each sorbent before and after the tests, including ICP, FTIR, XRD and TEM-EDX. The analysis results revealed that CCW-S was largely composed of CaCO 3 , and Bicar was purely NaHCO 3 . The impurities-Al, Fe and Mg-were also detected in CCW-S.

The first set of adsorption tests were performed using a gas mixture of 500 ppm HCl in nitrogen (HCl/N 2 ) at ambient conditions (25 °C, 1 atm). The results revealed that Bicar was the better performing sorbent with an average breakthrough time and HCl adsorption capacity of 66 h and 26.8 wt%, whereas the performance of CCW-S was lower (7.8 h and 3.9 wt%). Furthermore, TEM-EDX images of CCW-S particles revealed the participation of the impurities-Al, Fe, and Mg-in HCl capture. In addition, XRD and ESEM analyses showed the formation of new crystals on the surface of Bicar particles, although no new crystals were observed on the particles of CCW-S.

The second set of adsorption experiments were conducted with CCW-S and Bicar sorbents, using a simulated HCl/ Syngas atmosphere at ambient conditions. The breakthrough times and HCl adsorption capacities of the sorbents were, respectively, 48.2 h and 6.7 wt% for Bicar, and, 2.7 h and 0.5 wt% for CCW-S. This significant decrease in the sorbent performance in the presence of syngas highlights important interactions between HCl and syngas components, which deserves further investigation.

Future experiments on CCW-S will aim to improve its performance by determining the optimum adsorption temperature, and performing thermal activation and regeneration tests. Furthermore, an investigation of the effect of different partial pressures of CO 2 (in syngas) on the performance of CCW-S will be performed.

Fig. 1 Fig. 2

 12 Fig. 1 Experimental setup

FTIR

  technique could not be used to analyze the concentration of HCl in the syngas matrix because the infrared spectrum of methane, CH 4 , overlapped that of HCl as shown in Fig.3. Hence, a new HCl analyzer called ProCeas was employed, which was manufactured by AP2E. ProCeas uses an enhanced IR laser technology called Optical Feedback Cavity Enhanced Absorption Spectroscopy (OFCEAS), which enables simultaneous online analysis of HCl and CH 4 . During the experiments, the analyzer was directly connected to the reactor outlet and the HCl gas concentration was recorded at 1-s intervals.

Fig. 3

 3 Fig. 3 Zoom of infrared spectrums of 500 ppmv HCl/N 2 and 200 ppmv HCl/syngas (25 °C, 1 atm)

Fig. 4

 4 Fig. 4 Infrared spectra of sorbents and reference compounds before tests

Fig. 5

 5 Fig. 5 XDR results of sorbents before tests. Filled triangle: calcium carbonate (CaCO 3 )rhombohedral, filled diamond: sodium hydrogen carbonate (NaHCO 3 )-monoclinic. a CCW-S. b Bicar

Fig. 6 8 Fig. 7 Fig. 8

 6878 Fig. 6 Breakthrough curves for tests under HCl/N 2 gas conditions (25 °C, 1 atm). a CCW-S. b Bicar

  2 and H 2 O significantly lowers the performance of the sorbents. They attribute this to the shift in position of equilibrium to the left of the reversible reactions in Eqs. (1)-(3) due to the increase

Fig. 10 Fig. 11

 1011 Fig. 10 ESEM photos of sorbents before and after tests under HCl/N 2 gas conditions (25 °C, 1 atm). a CCW-S, before. b CCW-S, after. c Bicar, before. d Bicar, after

Table 1

 1

	Typical HCl concentrations in waste-derived syngas and target levels of associated syngas applications [4, 6, 7, 9]
	Contaminant (mg/Nm 3 ) Waste gasification Gas engine Gas turbine Methanol synthesis Fischer-Tropsch syn-thesis	EU/Japanese standards a
	HCl	0-300	< 1	< 1	< 0.1	< 0.01	10/90
	a At 11% O 2

Table 2

 2 Composition of syngas mixture used in experiments

	Components	Nominal value Analytical value
	Hydrogen chloride, HCl (ppmv) 200	232
	Methane, CH 4 (vol%) Carbon dioxide, CO 2 (vol%) Hydrogen, H 2 (vol%) Carbon monoxide, CO (vol%)	5 25 30	4.96 24.9 30.6

Table 3

 3 Physical characteristics of sorbents before tests

	Sorbent	Bulk density (kg/ m 3 )	Surface area (m 2 /g)	Particle diameter, d 50 (μm)
	CCW-S	840	5	113
	Bicar	986	13	136

Table 4

 4 ICP results of sorbent materials before tests Sorbent Ca (g/kg) Na (g/kg) Al (g/kg) Fe (g/kg) Mg (g/kg)

	CCW-S 454.8	-	1.8	3.4	2.7
	Bicar	1.6	192.9	0.2	0.1	0.1

Table 5 XDR

 5

	results of sorbents before tests	Sorbent	Reference code	Compound name	Chemical formula	Crystal system
		CCW-S Bicar	00-047-1743 00-021-1119	Calcium carbonate Sodium hydrogen carbonate	CaCO 3 NaHCO 3	Rhombohedral Monoclinic

Table 7

 7 XRD results of crystalline structures present in sorbents after tests with HCl/N 2

	Sorbent	Reference code	Compound name	Chemical formula	Crystal system
	CCW-S Bicar	00-047-1743 00-021-1119 96-900-8679	Calcium carbonate Sodium hydrogen carbonate Halite	CaCO 3 NaHCO 3 NaCl	Rhombohedral Monoclinic Cubic
	Fig. 9 XDR results of sorbents after tests under HCl/N 2 gas conditions (25 °C, 1 atm). Filled triangle: calcium carbonate (CaCO 3 )-rhombohedral, filled diamond: sodium hydrogen car-bonate (NaHCO 3 )-monoclinic, filled inverted triangle: halite (NaCl)-cubic. a CCW-S. b Bicar

Table 8

 8 Breakthrough time and HCl adsorption capacities of CCW-S and Bicar in HCl/ syngas atmosphere (25 °C, 1 atm) Fig. 12 Comparing breakthrough curves for adsorption tests with CCW-S in small and large reactor, under 500 ppmv HCl/N 2 conditions (25 °C, 1 atm)in the partial pressures of CO 2 and/or H 2 O. However, CO 2 has much more influence on this shift in position of equilibrium than H 2 O, and hence, the mechanism is called sorbent recarbonation.

	Sorbent	t b (h)	HCl capacity (wt %)
	CCW-S	2.7	0.5
	Bicar	48.2	6.7

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