Production of alternative fuels by thermochemical conversion of waste materials
The ongoing combat against climate change and energy transition requires the utilisation of novel and sustainable technologies and energy sources. High penetration of renewable energy sources (RES) like wind and solar for electricity production is a significant step. Still, full decarbonisation requires additional alternative energy sources and fuels to fill the gap where electrification is not a sustainable solution. Alternative fuels may vary by their origin and production process, but the common for all of them is that they are produced through a sustainable and clean procedure without the additional emissions of Carbon dioxide (CO2). There are two main pathways for the synthesis of alternative fuels: direct utilisation of electricity surplus into the production of chemicals or thermo-chemical conversion of waste feedstock into liquid, gaseous and solid products that can be further refined and used where appropriate. Thermochemical conversion methods like pyrolysis and gasification have recently gained much attention since they allow the integration of waste management with power production systems. This integration could bring significant environmental benefits since derived products could be used as a substitute for fossil fuels, simultaneously preventing waste accumulation at landfills and irretrievable loss of valuable resources. Co-pyrolysis is a promising solution since it can process various waste materials, such as plastics, biomass, sewage sludge, etc., into valuable products. Nevertheless, the scaling-up process on a commercial level still faces different challenges. Besides economic viability, significant enhancements are required to better understand product yield and distribution due to feedstock origin and interaction. Furthermore, integration with renewables should be prioritised to reduce environmental burdens and increase process sustainability. Finally, the research focus should be narrowed to the most promising feedstocks to create a standardised procedure that would ensure results repeatability and yield of high-quality products with minimal after-treatment requirements.
The primary objective of this dissertation was to identify the most promising waste biomass and plastic feedstocks for the co-pyrolysis process with an aim to produce high-quality liquid fraction that could be further refined into alternative fuels compatible with existing standards for conventional fuels. This was done by conducting a detailed literature review of performed investigations, examining the feedstock properties from ultimate and proximate analysis, and running a series of experimental investigations. The results showed that sawdust (SD), polystyrene (PS), and polypropylene (PP) are among the most promising feedstock to be utilised in the co-pyrolysis process. This evaluation is based on product yield quantity and quality from an individual and mixture analysis.
The secondary objective was to determine the acceptable share of plastic content in the fuel mixture to enhance liquid quantity and quality. This was achieved by varying the plastic content in the investigated mixtures and observing the composition of the collected liquid fraction. Based on obtained results, it was concluded that plastic share in the mixture should be around 50% since this is sufficient to enhance pyrolysis oil properties compared to individual samples. At the same time, this amount of plastic in the mixture, allows achievement of chemical composition similar to conventional fuel requirements. Another important observation is that feedstock pre-treatment is inevitable before introducing the mixture to the process. This is especially important with plastic separation and biomass drying processes. The former allows better product yield prediction and selectivity toward preferred compounds, while the second increases product yield and quality.
Finally, the research brought deep insight into the environmental assessment of the proposed procedure to ensure process sustainability and compatibility with decarbonisation goals. Life cycle assessment (LCA) showed that most of the process burdens are associated with background processes related to electricity production, while utilisation of waste materials brings mostly credits. The overall process environmental performance depends on the type of end-of-life treatment methods from which waste flows are diverted. For most ecotoxicity impact categories, co-pyrolysis is a much better option than incineration and landfilling, which are currently used. Besides, appropriate after-treatment would allow using obtained products as a substitute for conventional fossil fuels, bringing significant environmental benefits due to the avoided mining and extraction operations. Finally, integration with renewables like photovoltaics or wind to power up the process greatly reduces global warming potential to almost neutral levels, making the process completely compatible with general decarbonisation goals.
