WMR0010.1177/0734242X17722207Waste Management & ResearchZhang et al. 722207research-article2017 Original Article Waste Management & Research 2017, Vol. 35(10) 1045 –1054 Influence of silica–alumina support ratio © The Author(s) 2017 on H2 production and catalyst carbon Reprints and permissions: sagepub.co.uk/journalsPermissions.nav deposition from the Ni-catalytic pyrolysis/ https://doi.org/10.1177/0734242X17722207DOI: 10.1177/0734242X17722207 reforming of waste tyres journals.sagepub.com/home/wmr Yeshui Zhang1, Yongwen Tao2, Jun Huang2 and Paul Williams1 Abstract The influence of catalyst support alumina–silica in terms of different Al2O3 to SiO2 mole ratios containing 20 wt.% Ni on the production of hydrogen and catalyst coke formation from the pyrolysis-catalysis of waste tyres is reported. A two-stage reactor system was used with pyrolysis of the tyres followed by catalytic reaction. There was only a small difference in the total gas yield and hydrogen yield by changing the Al2O3 to SiO2 mole ratios in the Ni-Al2O3/SiO2 catalyst. The 1:1 ratio of Al2O3:SiO2 ratio produced -1 the highest gas yield of 27.3 wt.% and a hydrogen production of 14.0 mmol g tyre. Catalyst coke formation decreased from 19.0 to 13.0 wt.% as the Al2O3:SiO2 ratio was changed from 1:1 to 2:1, with more than 95% of the coke being filamentous-type carbon, a large proportion of which was multi-walled carbon nanotubes. Further experiments introduced steam to the second-stage reactor to investigate hydrogen production for the pyrolysis-catalytic steam reforming of the waste tyres using the 1:1 Al2O3/SiO2 nickel catalyst. The introduction of steam produced a marked increase in total gas yield from ~27 wt. % to ~58 wt.%; in addition, hydrogen production was increased to 34.5 mmol g-1 and there was a reduction in catalyst coke formation to 4.6 wt.%. Keywords Waste, tyres, pyrolysis, catalyst, hydrogen, carbon nanotubes Received 30th June 2016, accepted 16th June 2017 by Associate Editor Rodrigo Navia. Introduction Waste tyre disposal represents a major worldwide problem. In hydrogen economy. Whilst hydrogen is currently mostly pro- 2014, 4.67 million tonnes of tyres were produced in Europe duced from fossil fuels, production of waste materials such as (ETRMA, 2015), eventually leading to a similar number of waste waste tyres would represent a novel route to recycle waste tyres. tyres arising annually to be managed. The current main treatment Alternative feedstocks for hydrogen production have included routes for waste tyres include fuel for cement kilns as a replace- biomass, waste plastics and municipal solid waste (Wang et al., ment for fossil fuels, recycling as rubber crumb for sports fields 2012; Wu and Williams, 2009a). or playgrounds, retreading of part worn tyres, civil engineering Catalysts are extensively used for increasing hydrogen yield applications and export (Williams, 2013). However, recently, from the steam reforming of hydrocarbon sources (Sutton et al., there has been much interest in advanced thermal treatment tech- 2001; Wu and Williams, 2009a). Catalysts containing metals nologies such as pyrolysis and gasification as a means of produc- such as Ru, Pt and Rh have been shown to be effective for hydro- ing higher-value products from waste tyres. For example, the gen production; however, nickel is a preferred metal due to its solid char product from pyrolysis of the tyres can be upgraded to low cost (Sehested, 2006; Srinakruang et al., 2005). The type of produce high surface area activated carbons (Cunliffe and support used to support the nickel will also influence the catalytic Williams, 1998; Nunes et al., 2011; Zabaniotou et al., 2004). In addition, the product oil and gas can be upgraded for the produc- 1 School of Chemical & Process Engineering, University of Leeds, tion of high-grade liquid fuels, hydrogen or aromatic chemicals Leeds, UK (Leung and Wang, 2003; Williams, 2013; Williams and Brindle, 2 Australian Institute for Nanoscale Science and Technology, School of Chemical & Biomolecular Engineering, The University of Sydney, 2003; Xiao et al., 2008; Zhang et al., 2015). Sydney, Australia The production of hydrogen from waste tyres has been inves- tigated by only a few research groups (Elbaba and Williams, Corresponding authors: Paul Williams, School of Chemical & Process Engineering, University 2012; Lerner et al., 2012). However, there is predicted to be high of Leeds, Leeds LS2 9JT, UK demand for hydrogen in the future to support the proposed Email: [email protected] 1046 Waste Management & Research 35(10) activity of the catalyst through the characteristics of surface area, Material and methods porous structure and mechanical strength, and chemical interac- tion between the metal and catalyst. For example, Miyazawa Materials et al. (2006) investigated the performance of nickel-based cata- The waste tyre sample was prepared from shredded truck tyres lysts on various supports for the steam reforming of tars from by removing the metal and cutting into particles of ∼6 mm. A biomass pyrolysis. The catalyst supports tested were Al2O3, ZrO2, Carlo Erba Flash EA1112 elemental analyser was used to deter- TiO2, CeO2 and Ni/MgO. It was reported that the production of mine the elemental composition, which was 81.2 wt.% carbon, hydrogen was influenced by the support material, with Ni/Al2O3, 7.2 wt.% hydrogen, 0.8 wt.% nitrogen, and 2.1 wt.% sulphur Ni/ZrO2 and Ni/TiO2 catalysts producing the largest hydrogen (Zhang et al., 2015). yields. The interaction between the nickel and the support was The catalysts used in the experiments were 20 wt.% Ni/Al2O3/ shown to influence the metal dispersion and the size of the metal SiO2 catalysts with four different Al2O3 to SiO2 mole ratios (3:5, particles, which in turn affected catalyst activity. Wu and 1:1, 3:2, 2:1). The catalysts were synthesized by an incipient wet- Williams (2009a) reported on the use of Ni/Al2O3, Ni/MgO, Ni/ ness method, where 20 wt.% of Ni was impregnated onto the CeO2 and Ni/ZSM-5 catalysts with 10 wt.% nickel for the pyrol- alumina–silica mixture support that was prepared by a sol-gel ysis-catalytic steam reforming of waste plastics. The Ni/MgO method. Silica and aluminium isopropoxide were purchased catalyst produced the lowest yield of hydrogen and the Ni/ZSM-5 from Sigma-Aldrich and were the precursors for the silica and catalyst the highest yield, which could be linked with the surface alumina. The calculated amounts of SiO2 powder and aluminium area of the catalysts. In addition, Ni/Al 2O3 and Ni/CeO2 catalysts isopropoxide powder were mixed with distilled water, followed also showed significant amounts of carbon deposition. Inaba by filtering the solution with distilled water. The obtained et al. (2006) investigated Ni/SiO2, Ni/ZrO2, Ni/CeO2 and a series Al(OH)3-SiO2 mixtures were aged in air overnight and kept in an of zeolites for use as catalysts in hydrogen production from the oven at 40°C for 1 day. Finally, the dry solids were ground into gasification of cellulose. The highest hydrogen yield was with fine powder as catalyst support for the impregnation preparation the Ni/SiO catalyst. Srinakruang et al. (2005) investigated the 2 step. The 20 wt.% nickel was impregnated on the Al O and SiO gasification of tar using nickel catalysts supported on SiO – 2 3 2 2 matrix support via dissolution of the calculated amount of nickel Al O , Al O and dolomite. The nickel catalysts on SiO –Al O 2 3 2 3 2 2 3 nitrate in ethanol. The Al O and SiO catalyst support was then and Al O produced larger amounts of carbon deposition than the 2 3 2 2 3 added with continuous stirring until the mixture became a slurry. Ni-dolomite catalyst, which led to deactivation. The impregnation process ended with drying the slurry in an Alumina (Al O ) is the most commonly used catalyst support 2 3 oven overnight to evaporate all of the moisture. The last step was for nickel for investigations of hydrogen production because of to calcine the dry solids at 750°C in an air atmosphere with a its effectiveness in hydrogen production and its chemical and heating rate of 2°C min-1 and holding time of 3 hours. physical stability and mechanical strength (Simell et al., 1997). However, the deactivation of the Ni/Al O catalyst by carbon 2 3 Experimental system (coke) deposition and sintering are problematic (Sehested, 2006). Nickel catalysts with a SiO2 support have also shown to be effec- The reactor for the pyrolysis-reforming of waste tyres consisted tive for hydrogen production (Blanco et al., 2014; Inaba et al., of a two-stage fixed bed reactor. Figure 1 shows the schematic 2006; Sutton et al., 2001) and are also low cost. Mixed alumina– diagram of the experimental system. After being pyrolysed in the silica supports would generate surface acid groups, which could first pyrolysis stage, the gas products from the tyres are passed enhance the activity of supported Ni catalysts due to the bifunc- directly to the second stage where either catalysis or catalytic tional property. It is therefore interesting to compare Ni-based reforming takes place (Elbaba and Williams, 2013; Elbaba et al., catalysts with different Al2O3/SiO2 ratios for their effectiveness 2010; Zhang et al., 2015). as a nickel catalyst support for hydrogen production and also in The reactors for both stages were constructed of stainless steel terms of detrimental catalyst coke deposition. Using waste tyres with a diameter of 2.2 cm and a height of 16 cm. The two stages as feedstock for hydrogen production also extends the knowledge are heated separately by two external electrical heaters, which in relation to this lesser researched feedstock for hydrogen controlled the heating rate and the target temperature for the production.