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Enhancing the circular economy: Resource recovery through thermochemical conversion processes of landfill waste and biomass

Time: Fri 2021-06-11 14.00

Location:, Stockholm (English)

Subject area: Materials Science and Engineering

Doctoral student: Ilman Nuran Zaini , Processer, Energy and Furnace Technology

Opponent: Professor Ashwani K. Gupta, University of Maryland

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

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Currently, the global economy looses a considerable amount of potential secondary raw materials from the disposed waste streams. Furthermore, the existing landfill sites that often do not have proper environmental protection technologies pose a long-lasting risk for the environment, which urge immediate actions for landfill remediations. At the same time, the energy recovery from waste through conventional incinerators has been criticized for its CO2 emissions. Alternatively, pyrolysis and gasification offer the potential to recover secondary resources from waste and biomass streams, which can increase the circularity of the material resources and limit the CO2 emissions.

This thesis aims to realize feasible thermochemical processes to enhance the material resources' circularity by treating landfill waste and biomass. Correspondingly, fundamental studies involving experimental works and process developments through lab-scale experiments and process simulations are carried out. The thesis is written based on the results from five different studies that cover the investigation regarding the effect of waste/biomass fuel properties on the performance of the pyrolysis and gasification processes, as well as the process development and improvement of thermochemical conversion processes of waste and biomass.

The first study investigates the primary fragmentation behaviour of waste fuel pellets during the pyrolysis stage of thermochemical conversion processes. This study shows that the fragmentation degree of waste pellets correlates well with their volatile matter contents. Meanwhile, there is no clear relation between the fragmentation degree and the pellets’ mechanical strength. Generally, due to the high volatile matter content from plastic, fuel pellets from waste tend to fragment into a high number of smaller particles than typical biomass or coal pellets during thermochemical processes. Hence, for some processes, improving the thermal stability of waste pellets is more relevant than improving their mechanical strength. 

Subsequently, the second study examines the reactivity and kinetics behaviour of waste-derived char during gasification. In general, it is found that the char reactivity is a function of the ash amount and the ratio of inorganic catalytic elements (K, Ca, Na, Mg, and Fe) to the inhibitor elements (Si, Al, and Cl). More importantly, the char gasification test results demonstrated the significance of the waste sorting processes' operating conditions on the thermal behaviour of the waste fuel, especially during the gasification process.

Meanwhile, the third study investigates the syngas and tar formations resulting from different interactions between plastic and paper fractions of solid waste. The results show that the interaction between plastic and paper significantly depends on the hydrocarbon chain structures of the plastic polymer. Specifically, the interactions of aliphatic-structured plastic polymers (represented by PE) and paper cause synergistic effects that reduce the tar and increase the syngas yields. Meanwhile, the synergistic effects tend to be less evident in the case of co-gasification between paper and an aromatic hydrocarbon polymer, represented by PS. 

Based on the results of the previous studies, a co-gasification process of waste with biomass or biochar is proposed in the fourth study. It is found that adding biochar during the gasification of waste could significantly increase the syngas and H2 production to become higher than that of when adding biomass. Synergistic effects are observed in the form of an extensive syngas yield increment and a tar yield reduction, due to the tar reforming reactions over biochar particles. In general, both biochar and biomass additions result in a higher energy yield ratio, suggesting that it could improve the efficiency of the waste gasification.

Finally, the fifth study focuses on process simulations and operational cost assessments of co-production of H2, biochar, and bio-oil from biomass. The process simulation study is carried out to evaluate different scenarios for producing biochar, bio-oil, and H2 based on a biomass pyrolysis process coupled with a steam reforming and a WGS process. Based on the calculations of the total operating cost and the potential revenue, it is found that the production of bio-oil is more economically beneficial than the production of H2. The estimated minimum selling price for biochar and bio-oil based on the operating cost alone is within the price ranges of related commodities in Sweden (i.e., charcoal, coal, coke and oil crude). Nevertheless, capital and operating costs for post-processing of bio-oil should also be considered in the future to obtain a more complete economic judgement.