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ISSN : 1598-5504(Print)
ISSN : 2383-8272(Online)
Journal of Agriculture & Life Science Vol.54 No.3 pp.105-109
DOI : https://doi.org/10.14397/jals.2020.54.3.105

Drying Characteristics and Energy Analysis of Food Waste Dryer using Steam from Incineration Plant

Dae-Bin Song1*, Ki-Hyeon Lim2, Dae-Hong Jung2, Jong-Hyun Yoon2
1Department of Bio-Industrial Machinery Engineering, Gyeongsang National University (Institute of Agric. & Life Sci.), Jinju, 52828, Korea
2NDT Engineering Co., Ltd., Changwon, 51343, Korea
*Corresponding author: Dae-Bin Song Tel: +82-55-772-1895 Fax: +82-55-772-1899 E-mail: dbsong@gnu.ac.kr
February 26, 2020 ; April 4, 2020 ; April 6, 2020

Abstract


This study was conducted to develop a high-moisture food waste dryer that uses steam as a direct heat source to improve the drying speed. Another objective was to verify its performance through experiments. A dryer with a drying capacity of 10,000 kg/hr, which uses steam from an incineration plant as a drying heat source, was fabricated. The performance and applicability of the dryer were verified through drying experiments, in which the food waste collected from large restaurants near the incineration plant was used as the experimental material. The drying experiment results showed that the input steam temperature increased by 21°C from approximately 145°C to 166°C compared to the case in which drying was performed by converting steam into heated air. The drying speed increased by 1.5 times from approximately 0.63 to 0.94 %/hr, and drying up to approximately 20%(wb) moisture content was possible. The drying energy rate, which represents the ratio of the energy consumed for drying to the input energy, increased by approximately ten times from 7.17% to 70.87%. The total drying time still remained approximately 100 hr due to the re-condensation of moisture. When steam was directly used as a drying heat source to improve the drying speed of food waste containing high moisture, the drying speed, water content after drying, and drying energy rate were clearly improved compared to the case in which steam was converted into heated air for use. Therefore, it was deemed necessary to develop a dryer that directly uses steam from an incineration plant for drying. To shorten the total drying time, it is necessary to develop a device that solves the problem of moisture condensation in the dryer.



초록


    Introduction

    The food and vegetable waste generated in 2018 amounted to approximately 1,743 tons/day (Ministry of Environment & Korea Environment Corporation, 2018), and regulations have specified that more than 70% of the generated food waste should be used as raw material for recycled products, such as food for animals and compost (Act, 2016). Food waste was converted into feed and used as swine diet. The recycling of food waste as swine diet, however, was completely prohibited based on the allegation that the African swine fever that broke out in 2019 was caused by feed made of food waste (Act, 2019). As a result, wastewater from food waste that amounts to approximately a few thousand tons per day cannot be treated, which may lead to social problems.

    The moisture content in food waste is higher than 90%. Thus, for the recycling of food waste, appropriate drying capability for reducing the moisture content to 20% or less is required (Song et al., 2018b). In the case of the commonly used heated-air drying, the heat loss is high and it is not possible to dry materials with high water content because the heated air cannot penetrate through such materials. Considering that there should be no wastewater and odor as well as the amount of generated food water, it is necessary to develop a dryer capable of treating more than ten tons per hour (Song et al., 2018a).

    Kim (2001) introduced food waste drying technology that utilized far-infrared drums and screw dryers, which used auxiliary heat sources. Jang et al. (2007) performed food waste drying experiments using microwaves to investigate the drying characteristics of food waste. When the 700 W microwave was used, it took 20 min to dry a 50 g food waste sample with 73.0% initial moisture content to 8.5% moisture content, resulting in a drying speed of approximately 189.7 %/hr. Lee et al. (2017) used oil-drying to address the problem of heated-air drying that leads to a large heat loss during the drying of sludge with more than 90% moisture content, and examined its applicability. In the experiment results, it took 25 minutes to dry a 30 g sample with 80% initial water content to 10% water content, resulting in a drying speed of approximately 205.9 %/hr. The microwave and oil-drying methods showed dramatically improved results as compared to the heated-air drying method in terms of drying speed. However, it cannot be guaranteed that they will produce the same results in case of large-scale treatment of more than ten tons because their drying treatment quantities were extremely low (50 g or less).

    Choe (2010) analyzed the thermal characteristics of a highperformance vacuum dryer for the drying quality maintenance and high-speed drying of agricultural and marine products with high water content. Jung et al. (2018) performed drying experiments using a small scale high-moisture food waste dryer for laboratories, which used heated air from a hot air heater. Food waste with 80% (wb) initial moisture content could be dried to 20% (wb) moisture content in approximately 20 hours in a stable manner. Song et al. (2019) developed a large-capacity food waste vacuum dryer with a drying capacity of ten tons, and performed drying experiments by converting steam from an incineration plant into heated air. They found that a drying time of more than 80 hours was required due to the heat loss that occurred during the heat exchange between steam and air.

    Therefore, this study aimed to verify the drying characteristics of a large-capacity vacuum dryer, which directly uses steam from an incineration plant as a drying heat source, through experiments.

    Materials and Methods

    1. Drying unit & system

    The process of the food waste drying system installed at the Geoje Resource Recovery Facility is shown in Fig. 1. Steam from the incineration plant (1 ton/hr, 600,000 Pa, 160℃) was supplied to the heating jacket installed at the bottom of the dryer. The steam was condensed after drying the material and then collected in a condensate tank.

    The dryer used in the experiments mainly consisted of a dryer tank, pressure reducing device, condenser, cooling water circulator, and food waste input and discharge devices. The dryer tank was fabricated as a circular tank with an input capacity of 10,000 kg using corrosion-resistance stainless steel. At the bottom of the dryer tank, a heating jacket was installed through which the steam used for drying, passed. Drying was performed through heat exchange with the material to be dried. The injected food waste was stirred vertically and horizontally by the auger installed inside the dryer tank to promote heat exchange in the heating jacket.

    The food waste injected into the dryer tank was sealed and dried at a saturation temperature of approximately 89℃ while its pressure was reduced to 60,000 Pa by a vacuum pump. The water vapor generated during drying was transported to an external condenser by the vacuum pump, and was temporarily stored in a condensate tank after being converted into condensate. The condensate stored in the tank passed through a filter device to remove odor and was collected in a condensate tank. A cooling tower was used to collect water vapor in the condenser as condensate. For the input of food waste, an input device composed of an input hopper and a screw conveyor was constructed. To separate foreign substances (e.g. vinyl and metals) from the dried solids, a discharge device composed of a transport device and a sorter was constructed. Fig. 2 shows the drying system installed at Geoje Resource Recovery Facility.

    2. Experimental materials

    Food waste from the restaurant in Samsung Heavy Industries was used as experimental material. The food waste was discharged from a food waste transport vehicle (loading capacity: approx. 3.5 tons) to the input hopper, and then delivered into the dryer using a fork crane and the screw conveyor. The dried materials were sorted using the screw conveyor and screening device and then stored in bags (Fig. 3).

    3. Drying experiment

    Steam from the incineration plant was directly used as a drying heat source, and the drying characteristics (changes in drying time, drying speed, and moisture content) according to the food waste input amount were investigated.

    The moisture content of the input sample was converted into the input amounts of moisture and solids, and the collected sample was measured using the oven method (100℃-20 hr). In the oven method, measurements were repeated five times for each sample, and the average value was set as the moisture content. The moisture content of the dried material was obtained by measuring its weight and converting.

    The temperature of the material inside the dryer and that of the upper water vapor were measured using thermometers (pt-100 Ω, NDT Engineering, Changwon, Korea). The temperature of the input steam was measured by installing thermocouples at the input and discharge part.

    To measure the changes in drying energy efficiency and moisture content, the steam temperature and pressure, input amount of steam, cooling water input and discharge temperatures, and the discharge amount of cooling water were measured by installing thermometers, pressure gauges (SS-3040, WOOJIN Instrument, Incheon, Korea), and flowmeters (KV050W, flotron, Seoul, Korea).

    Results and Discussion

    1. Drying characteristics

    Table 1 shows the food waste drying characteristics when the input amounts were 11,000, 13,000, and 7,000 kg. The input steam temperatures were 163℃, 170℃, and 165℃, respectively and the average was 166℃.

    The input moisture contents were 93.29%, 65.64%, and 60.00% while the moisture contents after drying were 12.87%, 23.49%, and 5.55%, respectively. The drying speeds were found to be 0.99, 0.36, and 1.47 %/hr and the drying capacities were found to be 135.8, 112.1, and 189.2 kg/hr, respectively.

    The average drying speed was about 0.94 %/hr, which was approximately 1.5 times higher compared to the case in which steam was converted into heated air for drying (Song et al., 2019). In particular, it was found that drying up to an approximately 20% moisture content was possible. This appears to be because the dryer input steam temperature (166℃) when steam was used was approximately 21℃ higher than the average temperature of the dry air (145℃) when steam was converted into heated air through a heat exchanger.

    Fig. 4 shows the moisture content and temperature inside the dryer according to the drying time for each experiment. It can be observed that the moisture content rapidly decreased between 10 and 20 hours, and then showed a tendency to slowly decrease afterwards. This tendency was clear in the first experiment in which the input moisture content was high (93.29%), but was relatively slow in the second experiment in which the input moisture content was 65.64%.

    Additionally, the temperature of the upper water vapor remained at approximately 90℃ throughout the entire drying duration, indicating that moisture continuously evaporated from the material to be dried because the temperature was higher than 89℃, which was the evaporation temperature at a reduced pressure of 60,000 Pa. Therefore, it appears that a drying delay occurred because the water vapor generated from the material returned again after being condensed in the upper section of the dryer instead of escaping outside. This condensation phenomenon inside the dryer is more likely to occur with increase in the total amount of moisture inside it. The total amount of moisture inside the dryer for each experiment were 10,262, 8,533, and 4,200 kg. The drying delay was clearly observed in the first and second experiments in which the amount of moisture was relatively large. In the third experiment with the lowest total amount of moisture, however, the water content continuously decreased without a drying delay.

    In particular, in the case of the third experiment in which no condensation occurred, the drying speed was 1.47 %/hr and the moisture content after drying was 5.5%, indicating that a device for discharging the evaporated moisture to the outside is essential for a large-capacity dryer that handles high water content.

    2. Heat exchange characteristics and efficiency

    Fig. 5 shows the drying energy rate, which represents the ratio of the energy consumed for drying to the input energy, according to the drying time for each experiment. In the first experiment, the drying energy rate increased to 90% or higher until 12 hours of drying, but it showed a tendency to rapidly decrease to approximately 60%. This implies that rapid drying occurred until 12 hours, but drying was delayed or slowed down afterwards. This appears to be because the evaporated moisture was condensed inside the dryer and returned to the material in the form of condensate. In the second experiment, the drying energy rate remained low from the beginning of drying, but showed a tendency to rapidly increase at the end of drying. This appears to be due to the moisture condensation that occurred from the beginning of drying. In the third experiment, the drying energy rate reached 70% after five hours of drying and then continuously increased without reduction. Hence, drying was performed well without moisture condensation in the third experiment.

    When the food waste was dried by directly inputting steam from the incineration facility to the dryer, the drying energy rates were found to be 64.8%, 70.7%, and 77.0% for each experiment, respectively, which were approximately ten times higher compared to the case in which steam was converted into heated air for drying (Song et al., 2019). Therefore, directly using steam as a drying heat source is judged to be effective when steam from an incineration plant is used as a drying energy source. The drying energy rate was obtained using the following equation.

    D r y i n g e n e r g y r a t e = c o n s u m e d d r y i n g e n e r g y s t e a m e n e r g y × 100 ( % )

    where steam energy: steam energy used in the dryer consumed drying energy: energy required for drying.

    Acknowledgement

    This work was supported by the Korea Institute of Energy Technology Evaluation and Planning (KETEP), and the Ministry of Trade, Industry & Energy (MOTIE) of the Republic of Korea (No. 20153030101110).

    Figures

    JALS-54-3-105_F1.gif

    Process diagram of the drying system installed in Geoje Resource Recovery Facility.

    JALS-54-3-105_F2.gif

    Photographs of the drying unit installed in Geoje Resource Recovery Facility.

    JALS-54-3-105_F3.gif

    Photograph of the experimental materials.

    JALS-54-3-105_F4.gif

    Drying characteristics of the drying experiments. This study was conducted three times. First experiment data (upper), Second experiment data (middle), and Third experiment data (bottom).

    JALS-54-3-105_F5.gif

    Energy efficiency of the drying experiments. This study was conducted three times. First experiment data (upper), Second experiment data (middle), and Third experiment data (bottom).

    Tables

    Drying performance of the drying experiments

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