In this work, an experimental study was done in an autoclave reactor to evaluate the gasification characteristics of food waste in supercritical water. The effects of reaction temperature (550–700 °C), residence time (0–30 min), feedstock concentration (5 wt%-9 wt.%), catalyst type (K₂CO₃, Na₂CO₃, and Raney-Ni), and catalyst loading (Catalyst/dry feedstock 0.5–2) on gas production and liquid products were investigated. The results indicated that higher reaction temperature and longer residence time positively promoted food waste gasification. The organic compound species in liquid products decreased quickly to form gas products with the increased temperature, and the aromatic compounds were the key organic matter for the complete gasification of food waste. The addition of catalysts could significantly convert more liquid intermediates into gaseous products, and improve the gasification performance of food waste. The catalytic performance of catalysts can be ranked as K₂CO₃> Raney-Ni > Na₂CO₃. H₂ yield and carbon gasification efficiency increased with the increase of K₂CO₃ loading, reaching the highest values of 38.29 mol kg⁻¹ and 95.84% with the addition of 14 wt% K₂CO₃, respectively. This work indicated that food waste could be well treated and utilized as an energy resource to produce H₂ by SCWG technology.

The sustainability of food production systems is inherently linked with energy, water and food (EWF) resources directly and in-directly throughout their lifecycle. The understanding of the interdependencies between the three resource sectors in the context of food production can provide a measurable account for resource requirements, while meeting food security objectives. The energy, water and food Nexus tool developed by the authors has been designed to model the inter-dependency between energy, water and food resources, whilst conducting an environmental assessment of product systems. With emphasis on the inter-linkages between EWF resources, the tool quantifies material flows, natural resource and energy consumption at component unit process level. This work integrates greenhouse gas control and waste to power technologies within the energy, water and food Nexus tool and evaluates the environmental impact of a hypothetical food product system designed to deliver a perceived level of food self-sufficiency (40%) for the State of Qatar. Multiple system configurations, representative of different pathways for the delivery of consistent food products are evaluated, transforming a once linear product system into a circular design. The sub-systems added consist of a biomass integrated gasification combined cycle which recycles solid waste into useful forms of energy that can be re-used within the nexus. In addition, a carbon capture sub-system is integrated to capture and recycle CO2 from both the fossil fuel powered and the biomass integrated gasification combined cycle energy sub-systems. The integration of carbon capture with the biomass integrated gasification combined cycle transforms the carbon neutral biomass integrated gasification combined cycle process to a negative greenhouse gas emission technology known as bio-energy with carbon capture and storage. For the different scenarios and sub-system configurations considered, the global warming potential can be theoretically balanced (reduced by ∼98%) through the integration of photovoltaics, biomass integrated gasification combined cycle and carbon capture technologies. The peak global warming potential, i.e. a fully fossil fuel dependent system, is recorded at 1.73 × 10⁹ kg CO2 eq./year whilst the lowest achievable global warming potential is 2.18 × 10⁷ kg CO2 eq./year when utilising a combination of photovoltaics, carbon capture integrated with combined cycle gas turbine in addition to the integrated negative emission achieving system. The natural gas consumption is reduced by 7.8 × 10⁷ kg/year in the best case configuration, achieving a credit. In the same scenario, the photovoltaics land footprint required is calculated to a maximum of 660 ha. The maximum theoretically achievable negative emission is 1.09 × 10⁹ kg CO2/year.

The gasification of biomass in supercritical water is a promising technology for hydrogen production and the paper reports a thermodynamic analysis, based on minimization of Gibbs free energy, of the gasification with supercritical water of different biomass and agro-food residues: almond shells, digestate from wastewater treatment, algae and manure sludge. Numerical simulations were performed in order to assess the effect of temperature, pressure and biomass-to-water ratio on gas-phase yield and composition.A partial energy integration was also discussed, by considering the energy recovery from a turbine expansion of the gas-phase stream leaving the gasifier. The proposed thermodynamic approach allows predicting not only gasification efficiency of gasifier but also energy balance on the entire gasification process. Results showed that the dry substrates (almond shells and algae more than digestate and sludge) tend to form more carbon monoxide. Besides, data comparison revealed that the produced hydrogen comes from biomass and water for high process temperature, while when temperature decreases, the thermodynamic path tends to promote water formation from the hydrogen of the dry biomass.

Largely due to manure management, intensive livestock production is known to negatively impact air, water, and soil quality. Excessive manure is often applied to soil as fertilizer or stored in lagoon. However, some thermo-chemical methods, such as gasification and pyrolysis, can transform manure from waste into a valuable resource. The closed-loop dairy concept employs these methods to create biochar derived from cow manure for use as a soil amendment and a water filtration medium. This closed-loop concept has the potential to produce syngas and bio-oil for production of electricity, and to reduce excessive nutrients in liquid manure irrigation by filtering manure slurry stored in lagoons. It replaces solid manure with biochar in land applications to further reduce nutrient runoff and increase soil resilience against erosion. In this study, a Water-Energy-Food-Waste nexus-based analysis and resource allocation tool was developed to evaluate the economic, environmental, and social feasibility of the closed-loop dairy system. The tool utilizes several levers to simulate a user-specified dairy operation, such as number of livestock, acres farmed, quantity of effluent irrigation, distribution of manure and biochar products, and type of biomass conversions. Financial estimates from central Texas in 2018 were used to evaluate the profitability of these practices against the costs of a dairy and hay operation. The study showed that the closed-loop dairy system, while case dependent, could be profitable and, based on operational costs, a small dairy of approximately 200 cows could break even. Results also indicate that the benefits of biomass conversions to produce energy byproducts should increase with scale. This study can help many dairy farms that are considering the economic and environmental sustainability of the industry, which has been under scrutiny. Copyright © 2022 Muell, Mohtar, Kan, Assi and Pappa.

In China, waste sorting practice is not strictly followed, plastics, especially food packaging, are commonly mixed in food waste. Supercritical water gasification (SCWG) of unsorted food waste was conducted in this study, using model unsorted food waste by mixture of pure food waste and plastic. Different operating parameters including reaction temperature, residence time, and feedstock concentration were investigated. Moreover, the effect of three representative food additives namely NaCl, NaHCO₃ and Na₂CO₃ were tested in this work. Finally, comparative analysis about SCWG of unsorted food waste, pure food waste, and plastic was studied. It was found that higher reaction temperature, longer residence time and lower feedstock concentration were advantageous for SCWG of unsorted food waste. Within the range of operating parameters in this study, when the feedstock concentration was 5 wt%, the highest H₂ yield (7.69 mol/kg), H₂ selectivity (82.11%), total gas yield (17.05 mol/kg), and efficiencies of SCWG (cold gas efficiency, gasification efficiency, carbon gasification efficiency, and hydrogen gasification efficiency) were obtained at 480 °C for 75 min. Also, the addition of food additives with Na⁺ promoted the SCWG of unsorted food waste. The Na₂CO₃ showed the best catalytic performance on enhancement of H₂ and syngas production. This research demonstrated the positive effect of waste sorting on the SCWG of food waste, and provided novel results and information that help to overcome the problems in the process of food waste treatment and accelerate the industrial application of SCWG technology in the future.

The gasification of some selected components of food wastes using H₂O₂ as the oxidant and in the presence of NaOH has been investigated under subcritical water conditions. Hydrogen production was enhanced when both NaOH and H₂O₂ were used compared to when either NaOH or H₂O₂ alone was used or in their absence. Results indicated that the H₂O₂ acted to partially oxidize the samples while NaOH significantly increased hydrogen gas yields by promoting the water-gas shift reaction with subsequent CO₂ capture. In the presence of NaOH, the main components were Na₂CO₃, CH₃COONa and CH₃COONa·3H₂O. Char and tar production were suppressed in the presence of NaOH.

Supercritical water gasification (SCWG) can efficiently convert food waste to hydrogen-rich syngas without pre-dewatering. NaCl salt is the most common food additive and therefore inevitably accumulated in food waste. Though, the role of NaCl in SCWG of food waste is still unexplored. In this study, the influences of NaCl salt in food waste during SCWG on syngas production and corrosion of reactor material were investigated. The model food waste, representing food waste in China, with different NaCl contents (0–6 wt% dry basis) was gasified in supercritical water at 500 °C and autogenous pressure of ca. 28 MPa. The highest total gas yield of 21.70 mol/kg and H₂ yield of 6.89 mol/kg were obtained in the case of 3 wt% NaCl. The corrosion test on Hastelloy C276 and stainless steel SS316L materials was conducted in supercritical water for 24 h with different NaCl concentrations. Hastelloy C276 showed better resistance to corrosion in supercritical water than stainless steel 316L. However, when NaCl was involved in the reaction, large oxides grains of NiFe₂O₄, FeCr₂O₄, NiO, and Fe₃O₄ were observed on the surface of both materials.

The effluent of food waste (FWE) is generated during food waste treatment process. It contains high organic matter content and is difficult to be efficiently treated. In this study, the sample was collected from a 200 t/d food waste treatment center in Hangzhou, China. Subcritical and supercritical water gasification were employed to decompose and convert FWE into energy. The effects of reaction temperature (300–500 °C), residence time (20–70 min) and activated carbon loading (0.5–3.5 wt%) on syngas production and the remaining pollutants in liquid residue were investigated. It was found that higher reaction temperature and longer residence time favored gasification and pollutant decomposition, resulting in higher H₂ production and gasification efficiencies. It is noteworthy that the NH₃-N was difficult to be converted and removed under current experimental conditions. The addition of activated carbon was found to increase the gasification efficiency. The highest total gas yield, H₂ yield, carbon conversion efficiency, gasification efficiency, total organic carbon removal efficiency and chemical oxygen demand removal efficiency were obtained from gasification at 500 °C for 70 min with 3.5 wt% activated carbon.

Securing the growing populations' demand for food energy and water whilst adapting to climate change is extremely challenging. In this regard, bioenergy coupled with carbon capture and storage or utilisation (BECCS/U) is an attractive solution for meeting both the population demand, and offsetting CO₂ emissions. The purpose of this study is to evaluate the effectiveness of BECCS/U pathways utilising CO₂ for agricultural enrichment in enhancing food systems and reducing GHG emissions within the energy, water and food nexus concept. The study bridges negative emissions with CO₂ fertilisation within an integrated system. It consists of a source of CO₂ represented by a biomass-based integrated gasification combined cycle with carbon capture, a CO₂ network for a sustainable CO₂ supply, and a CO₂ sink characterised by agricultural greenhouses. A techno-economic and environmental analysis of each of these subsystems is conducted, feeding to an overall performance analysis of the integrated BECCS/U pathway. Results reveal synergetic opportunities between the energy, water and food subsectors, whereby CO₂ is captured from an energy sub-system and is efficiently utilised to enhance food sub-systems by improving productivity and reducing crop water requirements. Thus, the proposed integrated BECCS/U system is able to improve food availability by enhancing the food system, increasing the yield by 13.8%, whilst reducing crop water requirements by 28%. System outputs resulted in a levelised cost of 0.35 $/kg of agricultural produce when the system is scaled-up, and an abatement of the related environmental burdens throughout the supply chain by achieving negative CO₂ emissions of 24.6 kg/m².year of cultivated land.