• Energy Research

Valorisation of food waste offers an economical and environmental opportunity, which can reduce the problems of its conventional disposal. Food waste is commonly disposed of in landfills or incinerated, causing many environmental, social, and economic issues. Large amounts of food waste are produced in the food supply chain of agriculture: production, post-harvest, distribution (transport), processing, and consumption. Food waste can be valorised into a range of products, including biofertilisers, bioplastics, biofuels, chemicals, and nutraceuticals. Conversion of food waste into these products can reduce the demand of fossil-derived products, which have historically contributed to large amounts of pollution. The variety of food chain suppliers offers a wide range of feedstocks that can be physically, chemically, or biologically altered to form an array of biofertilisers and soil amendments. Composting and anaerobic digestion are the main large-scale conversion methods used today to valorise food waste products to biofertilisers and soil amendments. However, emerging conversion methods such as dehydration, biochar production, and chemical hydrolysis have promising characteristics, which can be utilised in agriculture as well as for soil remediation. Valorising food waste into biofertilisers and soil amendments has great potential to combat land degradation in agricultural areas. Biofertilisers are rich in nutrients that can reduce the dependability of using conventional mineral fertilisers. Food waste products, unlike mineral fertilisers, can also be used as soil amendments to improve productivity. These characteristics of food wastes assist in the remediation of contaminated soils. This paper reviews the volume of food waste within the food chain and types of food waste feedstocks that can be valorised into various products, including the conversion methods. Unintended consequences of the utilisation of food waste as biofertilisers and soil-amendment products resulting from their relatively low concentrations of trace element nutrients and presence of potentially toxic elements are also evaluated.

The limited fossil fuel supply toward carbon neutrality has driven tremendous efforts to replace fuel vehicles by electric ones. The recycling of retired power batteries, a core energy supply component of electric vehicles (EVs), is necessary for developing a sustainable EV industry. Here, we comprehensively review the current status and technical challenges of recycling lithium iron phosphate (LFP) batteries. The review focuses on: 1) environmental risks of LFP batteries, 2) cascade utilization, 3) separation of cathode material and aluminium foil, 4) lithium (Li) extraction technologies, and 5) regeneration and transformation of cathode materials. Detailed analyses are elaborated with case examples and technical challenges. Our critical analysis demonstrates that compared with retired lithium nickel cobalt manganese oxide (NCM) batteries, LFP batteries do not contain the high-value elements such as Co and Ni, so the economic drive for LFP recycling is compromised although future market prospects are substantial. It is of great practical significance to develop low-carbon and cost-effective Li extraction technologies and regeneration processes for cathode materials to ensure a sustainable and stable development of the LFP battery and EV industry.

Abstract Sustainable biorefinery depends on the development of efficient processes to convert locally abundant, energy-rich renewable biomass into fuels, chemicals, and materials. Hydrothermal processing has emerged as an attractive approach for wet biomass conversion with less environmental burden. Although considerable efforts have been made in sustainable biorefinery by unitizing innovative technologies at a laboratory scale, its scaling-up is still impeded by the biomass heterogeneity. This article critically reviews the recent advances in hydrothermal carbonization and liquefaction technologies for the sustainable production of hydrochar and aromatics from different biomass wastes. Three main aspects, including lignocellulose-/lignin-rich feedstock, operating conditions, and design of liquid/solid catalysts, are critically reviewed and discussed to understand the reaction mechanisms and system designs for increasing the yields of aromatics and improving the properties of hydrochar. The latest knowledge and technological advances demonstrate the importance of identifying the physical and chemical properties of feedstock. The science-informed design of hydrothermal technology and optimization of operational parameters with reference to the biomass properties are crucial for the selective production of value-added chemicals and multifunctional hydrochar. This review identifies current limitations and offers original perspectives for advancing hydrothermal processing of biomass towards carbon-efficient resource utilization and circular economy in future applications.

Lignocellulosic paper waste constitutes a major waste stream globally, which should be valorised for chemical production. However, paper properties (e.g., feedstock composition, cellulosic crystallinity, and thermal stability/degradability) vary with raw materials and pulping processes. This study investigated levulinic acid (LA), hydroxymethylfurfural (HMF), and furfural production by H2SO4 and FeCl3 catalysed conversion of nine types of paper wastes in a green solvent system (1:1 γ-valerolactone/water). At 160-180 °C for 1-20 min, ∼23-27 wt% LA yield was achieved from sanitary papers, tracing/parchment paper, and paper food box mainly containing crystalline cellulose, while a lower LA yield (∼10-20 wt%) was obtained from other paper wastes with high contents of ash and lignin. A higher selectivity towards HMF (∼12 mol%) was achieved in the presence of FeCl3. A furfural yield of ∼ 4-7.5 wt% was also obtained from the hemicellulose content. This study elucidates crucial factors and desirable characteristics of paper waste for catalytic valorisation.

Hydrothermal carbonization (HTC) provides a promising alternative to valorize food waste digestate (FWD) and avoid disposal issues. Although hydrochar derived from FWD alone had a low calorific content (HHV of 13.9 MJ kg-1), catalytic co-HTC of FWD with wet lignocellulosic biomass (e.g., wet yard waste; YW) and 0.5 M HCl exhibited overall superior attributes in terms of energy recovery (22.7 MJ kg-1), stable and comprehensive combustion behaviour, potential nutrient recovery from process water (2-fold higher N retention and 129-fold higher P extraction), and a high C utilization efficiency (only 2.4% C loss). In contrast, co-HTC with citric acid provided ∼3-fold higher autogenous pressure, resulting in a superior energy content of 25.0 MJ kg-1, but the high C loss (∼74%) compromised the overall environmental benefits. The results of this study established a foundation to fully utilize FWD and YW hydrochar for bioenergy application and resource recovery from the process water.

Anaerobic digestion (AD) is a widely used technology to valorise food waste for biogas production yet a considerable amount of digestate remains under-utilised. Sustainable management and recycling of the nutrient-rich food waste anaerobic digestate (FWD) is highly desirable for closing resource loop and actualising circular economy. This work reviews the distinct properties of FWD and the existing treatment technologies. FWD shows great prospects as a nutrient source for microalgal cultivation and biofuel production. Emerging technologies such as thermal conversion (e.g., pyrolysis and hydrothermal treatment) of FWD into value-added products such as functionalised biochar/hydrochar with diverse applications would be attractive and warrant further research investigation. Integrated AD with subsequent valorisation facilities is highly encouraged to achieve complete utilisation of resources and reduce carbon emissions.