Multi-scale characterisation of chars mineral species for tar cracking

Abstract

International audienceSyngas from thermochemical conversion of waste or biomass is a renewable energy carrier that may contain pollutants – such as tar – that should be removed before further syngas utilisation. Chars have proved to be promising catalysts for tar cracking, but the influence of the physico-chemical properties on their reactivity is still unclear. This work aimed to better understand the structure and the composition of the mineral species of pyrolysis char, as well as their catalytic role in tar cracking. For this purpose, a characterisation of the minerals has been performed at bulk, surface (studied at micro and nano-scale) and crystallite scale. Pyrolysis chars were produced from wastes generated on cruise ships – namely used wood pallets (UWP), food waste (FW) and coagulation flocculation sludge (CFS) – having different mineral amount and content. Ethylbenzene was used as surrogate of light aromatic hydrocarbons in a tar cracking process. The results showed that ethylbenzene was converted into lighter gases meaning that the chars were efficient for this. Ethylbenzene conversion at 650 °C was found to be significantly higher with the char from a mixture of sludge and food waste (c.FW/CFS) compared to that of wood-based char (c.UWP): 71 wt.% against 45 wt.%, respectively. The combination of multi-scale and complementary techniques has highlighted that the higher catalytic activity of this char was mainly attributed to the mineral content. Well dispersed mineral particles with various morphologies and natures were observed on the surface of c.FW/CFS using Scanning and Transmission Electron Microscopy (SEM and TEM). Especially, Ca, Al and P were the main mineral species identified using XRFS and SEM. These mineral species in form of oxides and hydroxyapatite were considered to be the main active mineral components for tar cracking. Oxides were identified using EDX-analysis. XRD analysis highlighted the presence of crystalised particles of hydroxyapatite (Ca5(PO4)3(OH)), while Raman spectroscopy revealed that these particles were embedded in the carbon matrix

H2S removal from syngas using wastes pyrolysis chars

“…The peak observed at 960 cm −1 was also present on the spectrum of ac.FW/CFS before the tests. This peak matched with the hydroxyapatite phase (Ca 5 (PO 4 ) 3 (OH)), as detailed elsewhere [47]. The other peaks testified to the presence of an anhydrous calcium sulphate named Anhydrite (CaSO 4 ).…”

“…The ashes of UWP-based chars were rich in Ca, K and Fe, while the ashes of FW/CFS-based chars had high contents of Ca, P, Al and Cl. A precise multi-scale characterization of the ashes of FW/CFSbased chars has been published in a previous article [47]. Table 2 shows that pyrolysis chars had specific surface area (S BET ) lower than 10 m 2 .g −1 .…”

“…4-A). Calcium was the main mineral specie in FW/CFS-based chars and was proved to be present in the form of hydroxyapatite and small calcium oxide particles (1-10 µm) which were well dispersed on the char surface [47]. Original results obtained with Raman spectroscopy showed that the calcium-containing particles contributed to the high H 2 S removal capacity of ac.FW/CFS with the formation of calcium sulphate CaSO 4 ( Fig.…”

Advanced characterization unravels the structure and reactivity of wood-based chars

“…A better understanding of biochar structure and its evolution depending on the fabrication process is therefore required to tailor and optimize biochar properties depending on the targeted application. In the above example (biochar as a catalyst), the key parameters to consider include surface area, porosity, texture, surface functionality (including specifically acidic-basic sites), metals nature and distribution [10,[14][15][16][17]21,23,24]. High surface area promotes mineral distribution and favors pollutant diffusion through the porous network to the active sites [25].…”

“…Besides, they do not provide information on the chemical composition of the carbonaceous matrix and the various types of carbon structures that may be present in it and affect char reactivity. Finally, chars are also characterized by the total amount and the surface distribution of mineral and metal species [24]. It has been shown that mineral clusters could clog the pores at the surface [17,22,27,32,33], which would impact the interconnected porosity in the resulting chars.…”