Integrated conversion of municipal solid waste residues into fuels and energy

Municipal solid waste (MSW) residues are the leftover fractions after recycling activities have been carried out on household or similar wastes. These waste residues are typically incinerated or sent to landfill, however environmental concerns around these two treatment options highlight the need for more resource-efficient technologies to valorise the wastes.

One of the challenges with valorisation of MSW residues is the lack of sufficient methods for quantifying their composition to facilitate the selection of suitable conversion technologies. Therefore, in this project, a new and easy-to-implement framework for quantification of the key components of MSW residues was developed using two commercially available MSW residues. The developed framework employed three analysis techniques – Fourier transform infrared (FTIR) spectroscopy, acid hydrolysis and thermogravimetric analysis (TGA) – for identification and quantification of the waste residues. FTIR was purely qualitative and enabled the identification of the components present. Acid hydrolysis was then used for quantification of the cellulose content of the wastes as well as other volatile components while the lignin and plastics fractions of the wastes were quantified by TGA. One challenge identified with the use of TGA is the inability to quantify the composition of specific plastic types e.g. polyethylene, polyethylene terephthalate (PET) etc due to similar decomposition temperatures. Nonetheless, this novel waste characterisation framework resulted in good mass balance closure (~100%) for the two MSW residues and provides information on the relative contributions of organic and fossil-based components respectively, within the wastes. This would aid the choice of suitable technologies for valorisation of these wastes.

Subsequently, this project then sought to develop a novel integrated approach to recover the energy and carbon content of these MSW residues by combining hydrothermal liquefaction (HTL) and anaerobic digestion (AD) to valorise the wastes. MSW typically have a high moisture content, therefore, HTL was explored for conversion of the MSW residues to produce a fuel oil while eliminating the need for drying. In addition, although AD is an established technology, MSW residues can contain hard-to-degrade materials such as plastics which are unsuitable for AD and therefore inclusion of HTL plants alongside AD could increase recovery from the wastes. Therefore, in this work, residual waste materials from an existing industrialscale AD process were used as feed for HTL. The results showed that the oil yields were highly dependent on the compositions of the waste residues while the HTL operating conditions did not significantly impact the yields. The waste residue with high cellulose content resulted in higher oil yield during HTL while a lower cellulose content was correlated with lower oil yield. In addition, the plastic and lignin fractions of the wastes were recovered in the solid residue during HTL indicating the HTL temperature was insufficient to hydrolyse these two components.

During HTL of the MSW residues, a significant amount of the initial carbon content of the waste residues was recovered in the HTL aqueous phase (HTL-AP) making and the organic concentration of the HTL-AP too high for safe discharge on an industrial scale. Analysis of the HTL-APs showed that they were mostly composed of organic compounds such as carboxylic acids, alcohols and phenolic compounds. In order to valorise the HTL-AP and increase the carbon recovered from the MSW residues, the HTL-AP was further treated via anaerobic digestion (AD). The majority of studies on AD of HTL-AP have reported inhibition of the anaerobic microorganism due to presence of toxic compounds such as phenols and nitrogenated aromatics in the HTLAP. This is further complicated by the fact that higher HTL temperatures result in higher oil yields but also higher recovery of toxic phenols in the HTL-AP. Consequently, this study employed a new approach by co-digesting the HTL-AP with other feedstock to dilute the toxic compounds present in the HTL-AP. The results from this study showed that while using the same feed ratio, addition of HTL-AP resulted in lower biogas yield from AD for one waste residue (compared to the co-feed alone) while the relative biogas yield was higher for the other waste residue. This highlights that during anaerobic co-digestion of HTL-AP, the nature of organic compounds present in the HTL-AP is a more crucial factor than concentration of HTL-AP in the mixed feed.

Finally, the net energy recovered from the original method for integration of HTL and AD proposed in this PhD research was evaluated. To achieve this, the integrated process was simulated in Aspen Plus software using the experimental data and employing model compounds to represent the complex feeds and intermediate products present throughout the process. The energy required for heating up the feed during HTL, separating the HTL products, upgrading the oil and maintaining the AD temperatures were compared to the energy content of the produced fuels (HTL oil and biogas). The results showed a positive net energy recovery from conversion of the MSW residues however further economic analysis is required to confirm the viability of the process.