Approximately one-third of the food produced globally is either lost or wasted. From both economical and environmental perspectives, sustainable management of food wastes is a global challenge. According to the 2018 Food Waste and Rescue Report (produced by Leket Israel and BDO), the volume of food loss in Israel amounted to 880,000 tons at a value of about NIS 7.9 billion. Decomposition of food waste in landfills produces greenhouse gases as well as other contaminates that leach to soil and water bodies. On the other hand, treating food waste as a resource for renewable energy can be an excellent approach as it has high caloric value. Although some of these waste streams are reused, mainly as animal feed, the majority of food processing wastes is not valorized and should be considered as an alternative feedstock for renewable energy production.
Animal food products provide 18% of the global calorie consumption and its demand is expected to increase with respect to population growth. Livestock manure conversion into energy products and services, such as fuel for heat, electric power and transportation may enhance sustainability in animal feeding operations. Most of "waste to energy" strategies for livestock manure and food waste are based on biological processes, which convert the biomass into biogas in anaerobic conditions. However, biological processes generate large volumes of a secondary liquid effluent that should be further treated to maximize its economic value and minimize the environmental footprint. Hydrothermal conversion of biomass using subcritical water represents a promising technology for energy recovery from waste streams with high water contents.
Hydrothermal biomass processing
The main advantage of hydrothermal processes is the use of water as the reaction media and therefore they offer opportunities for an energy efficient valorization of wet-waste streams, such as food waste and livestock manure. Hydrothermal processes enable fast hydrolysis, phase fractionation, and re-polymerization reactions of organic matter. Hydrothermal carbonization (HTC) and liquefaction (HTL) typically take place over a range of temperature, pressure and time conditions (200 to 380 °C, 2 to 30 MPa, and 10 to 120 min, respectively). These conditions allow the production of biocrude oil and solid phase (hydrochar), both with a higher energy content than the raw material. Substantial changes in the properties of water that occur near its critical point (Tc = 374°C; Pc = 22 MPa, Fig. 1) turn the water into an attractive media for chemical conversion. Near the critical point of water, an increased ionic product with decreased density and dielectric constant increases its reactivity for reforming organic compounds. All these properties affect the chemical reformation of biomass in the subcritical region to produce energy products that contains a higher carbon content and a lower oxygen content than present in the biomass feedstock.
Fig 1. Hydrothermal biomass processing zones based on the phase diagram of water.
Livestock manure as a case study
Reaction temperature affected the relative yields of biocrude oil and hydrochar from livestock manure. Looking at carbon distribution among reaction products at three different temperatures (200, 250 and 300°C) provides an additional insight into the reaction chemistry. For example, the majority of manure carbon (~58%) is recovered as hydrochar at the lowest temperature (200°C) while only 16% is recovered as biocrude oil. At the highest temperature (300°C), the majority of manure carbon (~54%) is recovered as biocrude oil, while 34% is recovered as hydrochar. For all three temperatures, the residual carbon ranges between 2 to 8%, probably resulting from a small production of CO2-rich gaseous phase.
Changes in elemental composition between the feedstock and the energy products are illustrated in Fig. 2. HTL is therefore a useful platform to generate an energy-rich biocrude oil with carbon content that is ~2 times higher, compared to manure. Moreover, the oxygen content in biocrude oil is lower by 22–42%, compared to manure. The characteristics of hydrochar, the secondary energy product, are highly influenced by the reaction temperature. For example, hydrochar generated at 200 contains 26% oxygen; while at 300°C it is only 14.7%, 40% lower than its content in manure. Higher temperature leads to an increase in the degree of aromatization and condensation of the hydrochar, thereby improving its the most ability.
Fig 2. Changes in elemental composition between the livestock manure and the generated biocrude oil and hydro char
Evaluating the quantity and quality of each of the energy products (i.e., biocrude oil and hydrochar) normalized to initial energy content of the feedstock, enables the comparison of different reaction temperatures from an energy recovery perspective. Fig. 3 illustrates the effect of process temperature (200, 250 and 300°C) on energy recovery using HTL. Higher process temperatures seem to benefit the overall energy recovery. The residual fraction at 300°C is only 5% of the original energy content in the feedstock, while at 200°C, 28% of the energy is not recovered. Overall, our simulation demonstrates high values of total energy recovery achieved by hydrothermal conversion: 95% for 300°C, 82% for 250°C and 72% for 200°C.
Theoretical calculation of the overall energy balance obtained by heating the feedstock up to the target temperature for the required reaction time. Our analysis also demonstrates the role of heat integration in hydrothermal systems, as the steam heat may offset a substantial amount of the required thermal energy. With the returned energy values as biocrude oil and hydrochar we could sum the total returned energy per day and by using the calculated values of invested energy we calculated the energy return on invested (EROI). All scenarios provide a positive energy balance with energy returned, which is 1.5 to 3 times higher than the energy invested. Those values are encouraging in terms of future implementation.
Fig 3. Simulated distribution of energy (in percentage) contained in manure (100% at left) and different HTL products (biocrude oil, hydrochar and process water) at different temperatures.
Clean technologies are required to generate renewable energy products using food-processing waste. From a circular economy perspective, our analyses demonstrate the capacity of HTL to recover multiple energy products from food processing waste and by that to increase the energy efficiency and sustainability of intensive agricultural systems. Altogether, the experimental data, the energy balance and sensitivity analysis may bridge the gap between the laboratory measurements and the 'real world' application.