Effect of carbon dioxide on thermal treatment of food waste as a sustainable disposal method
Introduction
Significant amounts of food waste are generated worldwide originating from various sectors such as residual waste of agricultural production, household, and restaurants [1]. According to Food and Agriculture Organization, food waste of 1.3 billion tons is generated annually in the US [2]. It was estimated that approximately 2.9 Gt CO2 per year can be mitigated by reducing food waste [3]. Food waste has been disposed by different ways such as landfill, composting, and anaerobic fermentation [4]. Food waste landfill releases landfill gas mainly consisting of methane (CH4) and carbon dioxide (CO2) that are potent greenhouse gases [5]. It also emits leachate and dusts that are harmful to the environment [6]. Composting food waste is a simple and well-established method to treat food waste; however, it requires a long reaction time to complete composting process [7] and needs additional transportation costs after making compost [8]. While anaerobic fermentation has recently received much attention to produce biogas from food waste [9], it needs high initial costs [10] and generates toxic compounds containing sulfur [11]. Hence, it is necessary to develop an effective clean method to dispose food waste.
Thermal treatment of food waste would be an option for the food waste disposal considering that it can ultimately reduce the volume of food waste [12]. Incineration is the most widely used thermal treatment process for various wastes [13] because it reduces the solid waste volume by over 90% [14] and produces direct heating and electrical energy to operate power generation or steam turbines [15]. However, incineration of food waste emits different air pollutants (e.g., particulate matters, dioxins, sulfur dioxide, nitrogen oxides, hydrogen fluoride, and hydrogen chloride) into the atmosphere [16]. Therefore, it is important to design a new class of thermal treatment technology to dispose food waste in an environmentally benign way.
Various thermochemical conversion methods including pyrolysis and gasification have recently been used to treat and process food waste. For example, orange peel was converted into an activated carbon used for catalyst support by microwave pyrolysis, yielding ∼70 wt.% activated carbon [17]. Co-pyrolysis of used frying oil and plastic waste was conducted to produce bio-oil (81 wt.% yield) [18]. To simultaneously reduce waste and recovery energy from used cooking oil and waste plastic, vacuum pyrolysis heated was carried out using a microwave for heating [19]. Clean liquid fuel was obtained with 84 wt.% yield. The production cost of the pyrolysis process was estimated at $0.25 L−1.
Recently, it has been tried to apply CO2 to thermochemical conversion of biomass [[20], [21], [22]]. For example, CO2 was used to enhance the production of hydrogen (H2) from pine sawdust via pyrolysis [23]. The use of CO2 at a pyrolysis medium improved thermal efficiency of algal biomass pyrolysis by 45% compared to pyrolysis conducted under typical inert condition [24]. It was also reported that CO2 helps thermal cracking of volatile species evolved during thermochemical process of different samples such as manure [25], algal biomass [26], and coal [27]. Thus, it was hypothesized that the application of CO2 to thermal treatment of food waste would be helpful to dispose food waste in a clean way.
In this context, we researched effects of the supply of CO2 on the products generated from thermal treatment of food waste. In this work, real food waste was obtained from a food waste treatment plant (a part of a domestic wastewater treatment facility) and used for experiments to make the process more realistic. To the best of the authors’ knowledge, many studies about food waste treatment use model feedstocks that simulate food waste. The food waste was first characterized prior to thermal treatment, and non-condensable gases and condensable species produced from the thermal treatment of food waste were collected at different temperatures. The thermal treatment products were then identified and quantified to investigate the effect of CO2 on the process. The role of CO2 in changing constituents of the thermal treatment products of food waste is also discussed.
Section snippets
Materials and chemicals
Food waste used as the feedstock in this work was obtained from a food waste treatment facility located in Seoul, Republic of Korea. The raw food waste just collected was dewatered using a screw press, followed by a separation of foreign materials using a magnetic separator. The dewatered food waste was dried at 90 °C for one day prior to any experiment. Phenol (purity: 99.5%) was purchased from Sigma-Aldrich (St. Louis, MO, USA). Dichloromethane (purity: 99.9%) was purchased from Daejung
Results and discussion
The composition of the food waste used in this work was analyzed according to the method previously reported elsewhere [28]. The food waste characterization results are presented in Table 1. The food waste had high contents of glucan (23.1 wt.%) and protein (21.5 wt.%). It also contained fats and oils (36.5 wt.%), ash (16.6 wt.%), and polysaccharides such as galactan, mannan, and xylan (2.3 wt.%.).
The TGA of food waste were conducted to characterize thermal behavior of the food waste in N2 and
Conclusions
In this work, real food waste obtained from a food waste treatment plant was thermally treated under N2 and CO2 atmospheres. The results indicate that the employment of CO2 decreased the contents of cyclic compounds such as benzene derivatives in the condensable compounds produced during the thermal treatment of food waste. The use of CO2 as a reaction medium did not affect the amount of solid residue after the thermal treatment of food waste. However, it affected the amount of non-condensable
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgement
This work was supported by a National Research Foundation of Korea (NRF) Grant funded by the Korean Government (Ministry of Education) (No. 2018R1D1A1A09082841).
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