An experimental investigation of the catalytic activity of natural calcium

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Article

An experimental investigation of the catalytic activity of natural calcium rich minerals and a novel dual supported CaO-Ca Al O /AlO catalyst for bio-tar steam reforming 12

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Fengkui Yin, Priscilla Tremain, Jianglong Yu, Elham Doroodchi, and Behdad Moghtaderi Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b03201 • Publication Date (Web): 02 Jan 2018 Downloaded from http://pubs.acs.org on January 2, 2018

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An experimental investigation of the catalytic activity of natural calcium rich minerals and a novel dual supported CaO-Ca12Al14O33/Al2O3 catalyst for biotar steam reforming Fengkui Yin, Priscilla Tremain, Jianglong Yu, Elham Doroodchi, Behdad Moghtaderi* Priority Research Centre for Frontier Energy Technologies & Utilisation, Discipline of Chemical Engineering, School of Engineering, Faculty of Engineering and Built Environment, The University of Newcastle, Callaghan, NSW 2308, Australia.

Abstract Tar removal plays a key role in the process efficiency and viability of biomass gasification for syngas production applications. Amongst currently available tar treatment technologies, catalytic cracking was found to be the most attractive due to minimal energy losses by avoiding cooling of the raw product gas. Naturally available calcium based catalysts, particularly stone dust and dolomite, have been proven to be effect for bio-tar cracking, however, they have poor resistance to attrition and undergo deactivation after a few carbonation/calcination cycles. As such, these characteristics play a critical role in determining the viability of their application at a large-scale. Hence to overcome the shortcomings previously stated, a novel dual supported calcium-based catalyst which includes a stable support with great mechanical strength (alumina; Al2O3 and mayenite; Ca12Al14O33) dosed with CaO nanoparticles was synthesised by wet impregnation of calcium on alumina particles with and without the assistance of ultrasonication, referred to as CA and CAU respectively. The synthesised catalysts, as well as the naturally occurring calcium rich minerals stone dust and dolomite, were physically and chemically characterised using a variety of analytical techniques. The synthesised catalysts showed superior mechanical strength up to 5 times greater than the natural minerals. Each of the natural and synthesised catalysts was then investigated in a fixed bed reactor for steam reforming of bio-tars. In these experiments, toluene was used as a model tar compound to assess the catalytic activity of each and determine the best option in

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terms of catalytic activity, cost and mechanical strength. The synthesized CA catalyst without ultrasonic treatment exhibited better tar cracking performance in comparison to stone dust and dolomite in the temperature range of 600 to 800 °C. The synthesised CA catalyst also had the greatest performance in terms of superior surface area and mechanical strength due to the core support of Al2O3. This makes it a potential bed material for further study of tar cracking in large-scale fluidized applications. Keywords: bio-tars, CaO-Ca12Al14O33/Al2O3 catalyst, mayenite, Al2O3 support core, mechanical strength 1. Introduction Biomass gasification is considered to have one of the widest ranges of utilizations such as in biofuels and chemicals among all the biomass thermal-chemical technologies.1-3 However, one hindrance in the utilisation of biomass is the formation of unexpected bio-tars during the gasification process, causing operational issues, including pipe and filter blockages, in downstream processes which increase operational costs. In addition, most gas applications also require removal of tars from syngas produced. Hence, the removal of tar plays a key role in the successful application of biomass gasification technologies. A series of techniques including mechanical methods, catalytic cracking and thermal treatment have been employed to remove tars from hot syngas.4-7 Among them, catalytic cracking is considered the most attractive due to minimal energy loss as cooling of the raw product gas is avoided 8. Moreover, tars can be cracked and reformed into light gaseous components through thermal catalytic cracking, which increases the overall gasification efficiency and improves the higher heating value (HHV) of syngas. Numerous different catalysts, such as iron oxide, limestone, dolomite, olivine, ilmenite, zeolites and transition-metal-based catalyst, especially nickel-based catalysts, have been studied as catalysts for tar cracking in biomass gasification processes.4,

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From these studies, calcium-based (Ca-based)

catalysts have mostly been studied in order to obtain H2-enriched syngas or sufficient H2 to meet future demands towards the H2 economy.3, 5 2 ACS Paragon Plus Environment

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Natural Ca-based catalysts such as limestone and dolomite are inexpensive and are active in tar-removal reactions including steam reforming, dry (CO2) reforming and steam and thermal cracking reactions. However, there are problems associated with their practical implementation.13 For example, they were much less active in the conversion of heavy hydrocarbons than those of metallic catalysts such as nickel oxide.4 Besides, the life of these natural catalysts were short, as limestone and dolomite particles have low mechanical strength and are non-resistant to attrition.14 Based on these properties, other studies have attempted to add other metals to prepare highly active Ca-based catalysts, such as nanosized CaO/MgO/NiO catalysts,8 Ni/dolomite4,

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and CeO2 promoted Ni/CaO-Al2O3,16 in order to

simultaneously resist attrition and produce H2 rich syngas.

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Obviously, the synthesised Ca-based

catalysts with the addition of transition metals performed well in catalysis for the cracking of bio-tars, but some drawbacks still pertained such as high cost, toxicity and susceptibility to sulphur poisoning. Therefore, it is essential to look for a new Ca-based catalyst to solve the aforementioned issues. Recently, Ca12Al14O33 (mayenite) has been considered as an active and stable support of CaO catalysts for steam tar reforming and CO2 capture.23, 24 Orecchini et al.

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reported that the Ni/CaO supported on

Ca12Al14O33 showed around 99% toluene conversion performance and superior stability for CO2 capture. Li et al.

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elucidated the mechanism of mayenite formation and proved its stability as a CaO support

through multiple carbonation-calcination cycles. They also confirmed that mayenite gave high conversion of CaO when the calcination temperature was below 1100 °C. However, the above CaO/ Ca12Al14O33 catalysts were prepared by mixing certain mole ratios of aluminium nitrate and calcium salts in solution, which is costly in terms of catalyst preparation. Furthermore, its mechanical strength after reactions has rarely been reported in literature, which can determine its suitability for application on large-scale chemical/calcium looping processes. In this study, a relatively low cost and high reactivity mayenite supported CaO catalyst with an Al2O3 support core, was prepared and evaluated in a fixed bed reactor for steam reforming of bio-tars using toluene as a model compound. Meanwhile, calcium in the form of natural minerals; stone dust and 3 ACS Paragon Plus Environment

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dolomite, were also used to compare the catalytic activity of CaO in different forms. Moreover, a crushing test was conducted to compare attrition resistance, which is critical information in developing and designing of new CaO catalysts for bio-tars cracking.

2. Experimental 2.1. Natural samples preparation Omya limestone (Queensland, Australia), referred to as stone dust, and a natural dolomite from Western Australia were utilized in this study. Both stone dust and dolomite were firstly crushed and sieved to obtain the two desired particle sizes of 150-212 µm and 100-150 µm, respectively. Each of natural samples, of two different particle sizes, were calcined in a muffle oven at 850 oC under N2 for 16 hours. These four samples were named as SD1, SD2, DM1 and DM2, respectively. 2.2. CaO-Ca12Al14O33/Al2O3 synthesis A wet impregnation method, with and without ultrasonic treatment, was employed to prepare the CaOCa12Al14O33/Al2O3 catalyst. For the catalyst preparation γ-Al2O3 (100 µm particle size) and calcium nitrate tetrahydrate (Ca(NO3)2.4H2O, 99% A.C.S. reagent) from Sigma-Aldrich were utilized as the support and calcium resource in this study, respectively. 100 g of Ca(NO3)2.4H2O was dissolved into 100 mL distilled water, and then 40 g of γ-Al2O3 powder was added into this solution and mixed at 60 °C. The mixture was stirred at the same temperature for 16 hours, and then the resulting mixture was heated to 110 °C and continually stirred until it became a slurry. The slurry was moved to alumina pan and dried in an oven at 110 °C. After drying, and the sample was move into a muffle oven and calcined at 500 °C for 4 hours, and finally calcined at 850 °C for 16 hours. The catalyst (briefly named as CA) obtained at this stage was milled and sieved to obtain the same particle size range as that of γ-Al2O3 (100-150 µm). The aforementioned procedure was similarly followed for the ultrasonically treated catalyst preparation method, with the exception that an ultrasonic instrument (Branson Sonifier 450) was used to treat the mixtures instead of stirring. The ultrasonic treatment lasted for 1 hour with 2 4 ACS Paragon Plus Environment

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seconds pulse intervals. The sample after calcination was simply assigned as the name CAU. Detailed information of all the samples utilised in this study are detailed in Table 1. Table 1. Summary of the samples used in this study. Sample name

Particle size (µm)

Description

SD1

150-212

Calcined stone dust

SD2 DM1 DM2 CA

100-150 150-212 100-150 100-150

CAU

100-150

Calcined stone dust Calcined dolomite Calcined dolomite Synthesised CaO-Ca12Al14O33/Al2O3 Synthesised CaO-Ca12Al14O33/Al2O3 with ultrasonic treatment

2.3. Catalyst characterization methods The elemental analysis of raw samples was carried out using x-ray fluorescence (XRF) analysis by ALS Minerals in Brisbane, Australia. The compositions of all samples are listed in Table 2. In addition, samples before and after reaction were analysed by x-ray diffraction (XRD) analysis on a Philips X’Pert MRD, at 2 theta angles ranging from 5 to 90 degrees to characterise any crystalline phase changes. Meanwhile, the morphology of samples before and after reaction were analysed by using scanning electron microscopy (SEM) on a ZEISS Sigma VP FESEM in the EMX-Ray Unit at the University of Newcastle. Moreover, the crushing strength of single particle for each catalyst was examined via a force gauge (Shimpo, FGE-10X) and the BET surface area was measured via nitrogen adsorption using a Micromeritics Gemini 2375 analyser. 2.4. Catalyst activity experiments Catalyst activity experiments were conducted in a fixed bed apparatus as shown in Figure 1. Toluene was used as the tar model in this study. Toluene and water were accurately injected into the quartz reactor by their individual syringe pumps, with flow rates of 1.7 mL/hr and 6.072 mL/hr, respectively, ensuring the mole ratio of steam to carbon (S/C) was 3. The composition of gases in the reactor were toluene vapour (2.8 mL/min), steam (50.2 mL/min) and N2 (27.5 mL/min). The total gas space velocity

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is 1000 h-1 and the volume of the loaded catalyst for each experiment was 4.8 mL (~ 2.5 g). The fixed bed experiments were carried out at constant temperatures ranging from 600 to 800 °C.

Figure 1. Schematic of fixed bed reactor apparatus.

3. Results and discussion 3.1. Characterisation of fresh natural and synthesised Ca-rich catalysts Elemental analysis of the raw samples was carried out using XRF and the composition of each sample is listed in Table 1. CaO was the major component for the SD sample while CaO, MgO and SiO2 were the major components for the DM sample. The main components for the synthesised samples (CA and CAU) were CaO and Al2O3, and the composition of CaO for both synthesised samples were similar. Figure 2 compares the XRD patterns of the natural and synthesised catalysts before the toluene steam reforming reaction. As illustrated in Figure 2, the active components in SD were CaO and Ca(OH)2, while the active components in DM were CaO, Ca(OH)2, CaMg2, MgO and Mg(OH)2, respectively. For CA and CAU, the main active components were CaO, Ca(OH)2 and Ca(AlO2)2, with CAU, having the additional phases of 5CaO·3Al2O3 formed.

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Table 2. XRF analysis on the samples used in this study.

Samples SD DM CA CAU

CaO 98.52 38.15 32.23 32.03

Al2O3 0.37 3.7 66.15 66.81

SiO2 1.06 31.67 0.08 0.06

Composition (%) MgO Fe2O3 MnO 0.81 0.31 0.03 22.55 1.87 0.04 0.14 0.01 SD2 > CAU > DM2, which was in accordance with the catalytic activity experiments presented in Section 3.3. Table 5. Estimates of the kinetic parameters for toluene steam reforming on the four different types of catalyst. Catalysts

Ea (kJ/mol)

A (h-1)

R2

SD2 DM2 CA CAU

49.8 52.4 48.1 50.2

1.27×105 1.29×105 1.17×105 1.26×105

0.9869 0.9703 0.9992 0.9912

5. Conclusions Two natural CaO based catalysts, stone dust and dolomite, were used to compare the influence of particle size on the catalytic activity for bio-tars steam cracking. The results showed that the smaller particle size for both natural catalysts investigated performed better in terms of reactivity. Moreover, stone dust showed better tar cracking performance than dolomite at the same particle size. A novel dual supported CaO catalyst (CaO-Ca12Al14O33/Al2O3) was prepared by two different methods and their catalytic activities for bio-tars steam cracking were tested. The novel CA catalyst exhibited better reactivity by comparison to stone dust (SD2) and dolomite (DM2) in the temperature range of 600 to 800 °C, while the CAU catalyst performed worst in terms of catalytic activity when the temperature was above 650 °C. The influence of the synthesised catalyst preparation method (with and without assistance of ultrasonic treatment) on the bio-tars steam cracking reactivity showed that the ultrasonic treatment can change the

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distribution of CaO on the pore surface of the Al2O3 support, which was not beneficial to the formation of Ca12Al14O33. The novel CA catalyst had the greatest mechanical strength of the catalysts examined due to the core support of Al2O3, which was more resistant to attrition than the SD and DM samples and further confirmed that CA was a potential catalyst that may be used in a chemical/calcium looping biomass gasification process. Corresponding author Prof. Behdad Moghtaderi, Email: [email protected]. Acknowledgements The authors would like to thank the funding that they received from The University of Newcastle. References 1. Moghtaderi, B., Effects of controlling parameters on production of hydrogen by catalytic steam gasification of biomass at low temperatures. Fuel 2007, 86, 2422-2430. 2. Moghtaderi, B., Review of the Recent Chemical Looping Process Developments for Novel Energy and Fuel Applications. Energy Fuels 2011, 26, 15-40. 3. Yin, F.; Shah, K.; Zhou, C.; Tremain, P.; Yu, J.; Doroodchi, E.; Moghtaderi, B., Novel CalciumLooping-Based Biomass-Integrated Gasification Combined Cycle: Thermodynamic Modeling and Experimental Study. Energy Fuels 2016, 30, 1730-1740. 4. Anis, S.; Zainal, Z. A., Tar reduction in biomass producer gas via mechanical, catalytic and thermal methods: A review. Renew. Sust. Energ. Rev. 2011, 15, 2355-2377. 5. Florin, N. H.; Harris, A. T., Enhanced hydrogen production from biomass with in situ carbon dioxide capture using calcium oxide sorbents. Chem. Eng. Sci. 2008, 63, 287-316. 6. Narváez, I.; Corella, J.; Orío, A., Fresh Tar (from a Biomass Gasifier) Elimination over a Commercial Steam-Reforming Catalyst. Kinetics and Effect of Different Variables of Operation. Ind. Eng. Chem. Res. 1997, 36, 317-327. 7. Devi, L.; Ptasinski, K. J.; Janssen, F. J. J. G., A review of the primary measures for tar elimination in biomass gasification processes. Biomass Bioenergy 2003, 24, 125-140. 8. Rownaghi, A. A.; Huhnke, R. L., Producing Hydrogen-Rich Gases by Steam Reforming of Syngas Tar over CaO/MgO/NiO Catalysts. ACS Sustainable Chem. Eng. 2013, 1, 80-86. 9. Abu El-Rub, Z.; Bramer, E. A.; Brem, G., Review of Catalysts for Tar Elimination in Biomass Gasification Processes. Ind. Eng. Chem. Res. 2004, 43, 6911-6919. 21 ACS Paragon Plus Environment

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