Abstract
Approximately 15 million dry tons of food waste is produced annually in the United States (USA), and 92% of this waste is disposed of in landfills where it decomposes to produce greenhouse gases and water pollution. Hydrothermal liquefaction (HTL) is an attractive technology capable of converting a broad range of organic compounds, especially those with substantial water content, into energy products. The HTL process produces a bio-oil precursor that can be further upgraded to transportation fuels and an aqueous phase containing water-soluble organic impurities. Converting small oxygenated compounds that partition into the water phase into larger, hydrophobic compounds can reduce aqueous phase remediation costs and improve energy yields. HTL was investigated at 300 °C and a reaction time of 1 h for conversion of an institutional food waste to bio-oil, using either homogeneous Na2CO3 or heterogeneous CeZrOx to promote in situ conversion of water-soluble organic compounds into less oxygenated, oil-soluble products. Results with food waste indicate that CeZrOx improves both bio-oil higher heating value (HHV) and energy recovery when compared both to non-catalytic and Na2CO3-catalyzed HTL. The aqueous phase obtained using CeZrOx as an HTL catalyst contained approximately half the total organic carbon compared to that obtained using Na2CO3—suggesting reduced water treatment costs using the heterogeneous catalyst. Experiments with model compounds indicated that the primary mechanism of action was condensation of aldehydes, a reaction which simultaneously increases molecular weight and oxygen-to-carbon ratio—consistent with the improvements in bio-oil yield and HHV observed with institutional food waste. The catalyst was stable under hydrothermal conditions (≥16 h at 300 °C) and could be reused at least three times for conversion of model aldehydes to water insoluble products. Energy and economic analysis suggested favorable performance for the heterogeneous catalyst compared either to non-catalytic HTL or Na2CO3-catalyzed HTL, especially once catalyst lifetime differences were considered. The results of this study establish the potential of heterogeneous catalysts to improve HTL economics and energetics.
Highlights
A variety of sustainable energy solutions are being developed to displace the use of petroleum-derived fuels that contribute to increasing greenhouse gas levels in the atmosphere.the growing demand for transportation fuels has driven alternative energy research for conversion of biomass into fuels [1]
The feedstock used for Hydrothermal liquefaction (HTL) reactions was a mixture representative of institutional food waste and included seven commonly disposed food items
The list of solid ingredients used as the feedstock is included in Table 1, which includes nutrient data calculated using values for each individual food item found in the United States
Summary
The growing demand for transportation fuels has driven alternative energy research for conversion of biomass into fuels [1]. Fuel Standards (RFS) program targets the production of 36 billion gallons of renewable fuel by 2022 [2]. Food waste is an inexpensive, energy dense alternative to lignocellulosic biomass, with the potential to be converted into drop-in transportation fuels with thermochemical properties comparable to petroleum-derived fuels [4]. Repurposing food residues helps divert material from landfills and reduce life-cycle greenhouse gas emissions caused by the biodegradation of organic waste. Repurposing food waste for biofuel production would reduce the environmental impact from landfills and reduce global reliance on crude oil
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