Hydrothermal carbonisation of mixed agri-food waste for sustainable bioenergy production

The current trend in global energy supply shows high dependence on fossil fuels, posing significant risk of rapid depletion of these finite resources. This reliance has substantial environmental consequences, with over 75% of global greenhouse gas emissions attributed to burning fossil fuels. Besides, the security of fossil fuel is threatened by global crises and instabilities, evidenced by the impact of the ongoing Russia-Ukraine war. Therefore, meeting the ever-increasing energy demand more sustainably and securely highlights the need to develop alternative energy sources such as bioenergy. Nevertheless, bioenergy production can have adverse social and environmental impacts if biomass resources are sourced unsustainably. One promising, yet underexploited bioenergy resource for sustainable bioenergy production is crop residues, referred to herein as agri-food waste. However, the inherent agri-food waste properties such as hydrophilic nature, high moisture and inorganic content, low bulk density and calorific value limit their use in direct combustion applications. In this doctoral study, mixed agri-food waste (MAFW), with specific reference to those generated in sub-Saharan Africa, was upgraded into high-quality solid biofuel via hydrothermal carbonisation (HTC), demonstrating the potential to convert low-quality biowaste into valuable bioenergy resource.

This doctoral study addressed some of the research gaps towards the potential adoption of HTC as a pre-treatment technology for efficient energy generation from MAFW. First, a new theoretical approach was proposed to generate a recipe for simulating the MAFW test material, building on agri-food waste estimation approach. Subsequently, extensive HTC experiments of the MAFW simulant were conducted according to experimental conditions defined by a central composite face-centred experimental design. The influence of key HTC operating parameters, reaction temperature (190-230 ℃), residence time (1-5 hrs) and solid loading (5-20%), on the HTC products (hydrochar and process water) properties was investigated by employing multivariate analytical techniques to provide insight into the conversion mechanism of organic and inorganic components of the MAFW during the process and determine the optimum values of operating parameters that yields optimum energetic characteristics in the hydrochar. Furthermore, recovered solid biofuel (i.e., hydrochar) compliance assessment with ISO 17225-8 standard and performance assessment in a supercritical power plant (simulated in Aspen Plus software) was conducted.

The findings indicate that the physicochemical properties of the MAFW simulant offer a reliable approximation of the properties of MAFW obtained in practical scenarios, with specific reference to those generated in open-air agri-food markets. The holistic analysis of the recovered hydrochar and process water characteristics at different HTC operating conditions revealed that dehydration, decarboxylation, aromatisation and Maillard reaction constitute primary reaction mechanisms influencing the HTC conversion of MAFW. A multistage process involving the combination of wet oxidation, anaerobic digestion and nutrient recovery was proposed to efficiently valorise the MAFW process water recovered at operating conditions favourable for high-quality solid biofuel recovery. The response surface analysis of the hydrochar yield, higher heating value (HHV) and ash content revealed significant interaction effects, indicating that the effect of the three HTC operating parameters should not be analysed independently. The process optimisation resulted in hydrochar yield, HHV and ash content of 52.25%, 24.56 MJ kg-1 and 6.20%, respectively, at optimum operating condition of ⁓212 ℃, 5 hr and ⁓7.8% solid loading. The experimental validation of the predictive models at the optimum operating condition showed that the developed models can predict responses within a 3% error at any given experimental condition within the investigated design space. Findings on the speciation of inorganic elements in the MAFW in water-soluble and/or organically-bound forms revealed that they are removed significantly under HTC conditions. In contrast, those in acid-soluble and/or non-leached forms have more limited removal. In addition, solid loading had the most significant impact on the fate of inorganic elements and the leaching of inorganic elements is favoured at low solid loading and medium reaction severity. Finally, the recovered hydrochar complied with ISO 17225-8 standard specifications and had a better power plant performance than the raw MAFW.

The findings presented in this study provide valuable insight into the effectiveness of HTC in enhancing the bioenergy application of MAFW and useful referential information for steering the development of a sustainable and optimum approach for beneficially exploiting the inherent energy potential on a large scale.