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Title: Waste derived carbons for NOx control or syngas tar removal
Author: Al-Rahbi, Amal Salim Said
Awarding Body: University of Leeds
Current Institution: University of Leeds
Date of Award: 2017
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The utilisation of waste materials as precursors for generating low-cost and effective carbonaceous materials, which can be used on a large scale, is very attractive and would help to solve the issues associated with waste disposal. In this work, scrap tyre, municipal solid waste in the form of refuse derived fuel (RDF) and date stones were selected for char and activated carbon production. The produced carbons were investigated as valuable adsorbent materials for control of (i) a problematic industrial nitric oxide (NO) gaseous pollutant or alternatively (ii) as a low-cost catalyst for tar cracking in relation to cleaning up the syngas produced from the gasification of biomass. The investigated carbonaceous materials were prepared using a fixed bed reactor. (i) The use of waste derived activated carbons as an adsorbent for NO removal under different test conditions at a low temperature of 50 ºC was investigated using a fixed bed reactor. The activated carbons were synthesised through carbonisation of the precursor at a temperature of 600 ºC, followed by subsequent physical activation with steam at 900 ºC. The NO removal efficiency of the waste-derived activated carbons was compared with different commercial carbons with varied porous texture and surface chemistry. Date stones activated carbon exhibited the highest NO removal efficiency of 40%, whereas a lower NO removal efficiency of 23% and 21% was obtained with RDF and tyre activated carbons respectively at 120 minutes time on stream. Commercially produced activated carbons had NO removal efficiencies of between 40% and 60%. The lower NO sorption of waste tyre and RDF activated carbons compared to those of commercial activated carbons or date stones was because of the difference in porous texture. Considering the Kinetic diameter of NO is 0.317 nm, effective adsorbents should exhibit a large volume of micropores. It was shown that the pores of the commercial activated carbons and date stones are mostly located in the micropore range of 1-2 nm (micropores), whereas RDF and waste tyre derived activated carbons have a much greater number of pores with diameters in the range of 2-10 nm (mesopores). Chemical activation can greatly alter the pore size and characteristics of the produced carbon. Therefore, surface modification of waste tyre was investigated via chemical activation to develop the porous texture and thereby enhance the NO adsorption capacity of tyre derived activated carbon. Thus, the influence of chemical activation of the waste tyre with KOH, K2CO3, NaOH and Na2CO3 on porosity development and the corresponding NO adsorption was investigated. It was shown that the activation of waste tyre with KOH favored the production of activated carbon with high micropore volume, which has been considered a key feature affecting NO adsorption at low temperature. Therefore, waste tyre activated with KOH at a char: KOH ratio of 1:3, with a total micropore volume of 0.437 cm3 g-1 and surface area of 621 m2 g-1 gave the highest NO adsorption capacity (17.23 mg g-1), which was double that of the physically activated tyre derived activated carbon. The results obtained in this study have shown that the adsorption capacity of carbonaceous sorbents relies greatly on the porous texture, in particular, the micropore structure of the carbon, as well as on the method of activation, but to a lesser extent on the BET surface area and acid-base surface groups on the carbon surface. (ii) Char materials derived from the pyrolysis of scrap tyre, RDF and date stones were also investigated in term of their use as a catalyst for the catalytic cracking of biomass pyrolysis gases during the two-stage pyrolysis/gasification of biomass. Biomass was used to generate a range of hydrocarbon gases typically found in biomass gasification tars through the pyrolysis of biomass. Among the investigated chars for bio-oil/tar decomposition, at a char cracking temperature of 800 ºC, tyre-derived pyrolysis char presented the highest activity resulting in a 70% reduction in bio-oil/tar yield compared to the non-char catalytic experiments. The results suggest that tar decomposition by char materials is mainly ascribed to the catalytic conversion of tar species, as the decrease of the hydrocarbon tar yields was accompanied with a consequent increase in total gas yield. Analysis of the tar composition showed the presence of naphthalene, fluorene and phenanthrene as the major polyaromatic hydrocarbon (PAH) components at the higher cracking temperature. To understand the tar decomposition mechanism and to further investigate the influence of porous texture and oxygen functional groups of char on tar decomposition process, the catalytic cracking of tar model (furfural, phenol, toluene, methylnaphthalene) over tyre char was investigated. The most reactive compound was furfural, followed by phenol and toluene, whereas methylnaphthalene presented the lowest reactivity. The results also indicated that both the porous texture and the oxygen functional groups of the carbonaceous materials had a marginal effect on tar decomposition. Additionally, tyre char was used as a sacrificial catalyst for the reforming/gasification of tars from the gasification of biomass to produce a hydrogen-rich syngas and also to contribute to the yield of biomass syngas through tyre char gasification reactions. The influence of tyre ash metals, catalyst bed temperature, steam to biomass ratio and reaction time were investigated. The metallic mineral content of tyre char has been shown to contribute significantly to the tar degradation. The maximum H2 content of the product syngas of 56 vol.% was obtained at a reforming temperature of 900 ºC and with steam to biomass ratio of 6 g g-1. Tyre char was also subjected to steam gasification during the process, whereby the tyre pyrolysis char catalyst is sacrificed to produce hydrogen and carbon monoxide to enhance the yield of the syngas. Overall, this research work shows that waste derived carbonaceous materials are low-cost promising adsorbents for NO control and tar removal from the syngas produced from biomass gasification.
Supervisor: Williams, Paul. T. Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available