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Energy and material recovery from high-ash waste through pyrolysis

Time: Thu 2021-09-16 10.00

Location:, Stockholm (English)

Subject area: Materials Science and Engineering

Doctoral student: Katarzyna Jagodzińska , Materialvetenskap

Opponent: Prof. Tobias Richards, University of Borås

Supervisor: Docent Weihong Yang, Materialvetenskap, Tillämpad termodynamik och kylteknik, Processer; Professor Pär Jönsson, Processer

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Undoubtedly, the past practices of profit maximisation by all means, which fuelled swift industrialisation and urbanisation, left behind a legacy of non-sanitary old landfills polluting the environment. These, together with active landfills supplied with tremendous streams of waste produced annually, are a loose cannon calling for action. Specifically, this action is the transition to resource- and energy-efficient economy model – the so-called circular economy.

The circular economy approach includes the closing of material loops by both: i)  the recycling of pre- and post-consumer residues and ii) the mining of existing landfills, treated as material stocks. Following this, the possibility of energy and material recovery from two types of high-ash waste was investigated within this thesis. The first type of waste is a fine residue from the shredding of the mixture of industrial and municipal metallic waste with end-of-life vehicles (the so-called 'shredder fines'), which represents the aforementioned pre-and post-consumer residues. The second type of waste is excavated waste from an old landfill, and its recovery falls within the latter way of closing the material loops mentioned above.

For the purpose of energy and material recovery, the aforementioned waste was subjected to thermochemical processes, namely pyrolysis and pyrolysis with in-line catalytic decomposition of the produced pyrovapours (volatile pyrolysis products). This thesis consists of four studies on that matter.

The research within this thesis started with the characterisation of excavated waste fractions to preliminarily assess their potential further applications. The fractions are characterised by significant contamination with heavy metals (mainly Hg and Pb) and chlorine. Furthermore, they are characterised by highly heterogeneous compositions, reflected in a high complexity of the formed pyrovapours. However, in order to maximise the fractions' utilisation ratio, their collective pyrolysis in the form of refuse-derived fuel (RDF) was proposed for further studies.

Following the above, the second study performed within the thesis aimed at the characterisation of the products from excavated RDF pyrolysis. The pyrovapours obtained at 500°C and 600°C showed potential for further catalytic upgrading to higher-quality products. 

Given the above, a study on the in-line catalytic decomposition of the pyrovapours from excavated RDF pyrolysis was performed. The process was aimed at the production of a H2-rich gas along with carbon nanotubes (CNTs). The study investigated the influence of the catalyst composition, its synthesis method, and catalytic bed temperature on the H2 and CNTs yields. Eventually, the bimetallic Fe-Ni/Al2O3 catalyst prepared using the sol-gel method showed the best performance as it tripled the H2 conversion rate (in comparison to the case without using a catalyst) and yielded 76 mg/gsample_daf CNTs of promising characteristics. Nevertheless, further research on this process is necessary to optimise it and to assess its feasibility subsequently.

The final study included in the thesis focused on enhancing metals recovery from shredder fines by subjecting them to torrefaction (low-temperature pyrolysis). The process results in partial decomposition of the so-called 'fluff' (textiles fibres with plastic, wood and rubber particles in which metal particles are entangled), which interferes with the sorting techniques. Therefore, torrefaction seems to be a promising way of liberating metal particles from shredder fines so that metals can be recycled.