:: Journal of Agriculture & Life Science ::
Journal Search Engine
Search Advanced Search Adode Reader(link)
Download PDF Export Citaion korean bibliography PMC previewer
ISSN : 1598-5504(Print)
ISSN : 2383-8272(Online)
Journal of Agriculture & Life Science Vol.52 No.3 pp.103-110
DOI : https://doi.org/10.14397/jals.2018.52.3.103

Development of a Heated Air Drying Unit for Food Waste

Dae-Hong Jung2, Jong-Hyun Yoon2, Sung-Min Choi2, Ki-Hyeon Lim2, Dae-Bin Song1*
1Dept. of Bio-Industrial Machinery Eng., Gyeongsang National Univ.(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: +82-55-772-1899dbsong@gnu.ac.kr
August 4, 2017 November 2, 2017 January 10, 2018

Abstract


In Korea, the daily waste production in 2015(excluding specified waste) was 404,812 tons, of which household waste accounted for 12.7%(51,247 tons/day). Total household food and vegetable waste amounted to 1,120 tons/day; of this, 70% of was ultimately used as feed or fertilizer and 30% was buried. In this study, a drying unit was developed to enable the production of solid refuse fuel using high-moisture food waste, and its performance was examined through an experiment. Thus, a laboratory pyrolysis system with a drying capacity of 500 kg/hr was manufactured. Food wastes were collected from a company cafeteria and from Changwon City and used for experiment. The drying characteristics of the food waste were examined depending on the input temperature of the drying air. The results of the food waste drying experiment showed that the total required drying time was approximately 20 hours, and the drying speed was approximately 2.90 %/hr. The drying time was five hours longer than the research target value(15 hours per batch). However, the time was approximately 16 hours when the preheating and cooling times required for the input and output were excluded, which was close to the research target value. The drying time did not show a large difference depending on the temperature of the input drying air. Drying time was 21 hours at 155°C, and thus drying operation would be possible without the use of high-temperature air(more than 200°C) when waste heat is utilized in the future. It is thought that rather than the temperature of the input air, it is the contact area between the input air and the food waste that has a significant effect on reducing the drying time.



초록


    Introduction

    In Korea, daily waste production in 2015(excluding specified waste) was 404,812 tons, of which household waste accounted for 12.7%(51,247 tons/day). Of total household waste, the food and vegetable waste production was 1,120 tons/day; of this, 70% of was ultimately used as feed or fertilizer and 30% was buried(Me & keco, 2015). In Korea, the total production of tomatoes, green chillies, and paprika cultivated through controlled agriculture(vinyl and glass greenhouse) in 2015 amounted to 705,506 tons(MAFRA, 2016), and the related wastes(e.g., leaf and stem) is estimated to be about 30% of the yield(211,651 tons). Most of the produced waste is naturally dried in open land and is incinerated in the winter. If cost for collection and transportation is assumed to be about 50 won/kg, the total cost of this waste can be estimated as about 30 billion won per year.

    When the food waste and controlled agriculture waste that are simply incinerated and discarded are reused as fuel for combustion, a total electric power of 224,246,445 kwh per year can be produced: 144,000,000 kwh per year in the case of food waste(lower heating value 5,000 kcal/kg, moisture content 80%, 1 kwh = 2,110 kcal) and 80,246,445 kwh per year in the case of controlled agriculture waste(lower heating value 4,000 kcal/kg, moisture content 80%, 1 kwh = 2,110 kcal). If the price of electric power sales is assumed to be 100 won per 1 kwh, this corresponds to approximately 22.4 billion won per year. Therefore, an economic effect of about 52.4 billion won per ear is expected, along with a reduction in the operation costs of controlled agriculture farms, when controlled agriculture waste that is currently used for manufacturing feed/fertilizer or is simply buried/incinerated is reused as an energy source in the form of fuel.

    The proportion of heating cost of a controlled agriculture farm is very high from 19 to 58%(Ryoo et al., 2012). In particular, the burden of heating and cooling cost for controlled agriculture farms is expected to increase after 2023, when the support for the distribution cost of export items is discontinued based on the agreement on the cessation of the agricultural export support for developing countries by the World Trade Organization(WTO), and thus a relevant measure needs to be urgently prepared(Kim, 2017). This necessitates the reuse of controlled agriculture wastes, which are currently simply incinerated and discarded at significant cost, as fuel for combustion.

    In Korea, food waste is handled by intermediate treatment companies located throughout the country. In the extrusion process after grinding, moisture is physically removed, and the solid content is dried through an indirect drying method that uses vapor or oil as the heat source, after which it is utilized as feed or fertilizer(Kang et al., 2012). Kim et al.(1999) reported a vacuum drying method that significantly reduces the drying energy cost based on a low drying temperature when using food waste as feed. Choe(2010) analyzed the thermal characteristics of a high-performance vacuum dryer for the dry quality maintenance and high-speed drying of high moisture content agricultural/fishery products. The heat source of the high-moisture food waste drying unit that is currently used in Korea shows the steam using a boiler and the oil heated using an electric heater are used, and thus a significant cost is involved in drying.

    Currently, about 161 waste incineration facilities are operated in the rural settlement regions of nine local governments excluding large cities, and combustion gases with a temperature of more than 900℃ or 250℃ are discharged into the atmosphere. Accordingly, if the high-temperature combustion gases produced from the existing incineration plants of the rural settlement regions are applied to a food waste drying unit, food waste can be dried economically. However, as combustion gas experiences cubical expansion since it is in a gaseous state, unlike steam or oil, the heat exchange capacity with a drying material would be very low. Thus, when combustion gas is used as the heat source, a drying unit that can maintain the heat exchange capacity with a drying material is needed.

    Therefore, in this study, to reduce the energy required for the drying of high-moisture food waste, a heated-air passage type drying unit that uses the waste heat of an incineration plant was developed, and the characteristics of heated-air flow within the drying unit and drying depending on the input temperature of the drying air were investigated.

    Materials and Methods

    1 Analysis of the heat flow within the drying unit

    1.1 Analysis method

    To examine the heat exchange capacity of the heated-air passage type for food waste drying unit, a heat flow analysis was conducted for the drying air of the heated-air chamber between the inner and outer tanks of the drying unit. Using dedicated fluid flow analysis software(ANSYS 16.1 professional), the velocity and temperature distribution within the heated-air chamber were analyzed depending on the air flow rate and temperature of the input drying air. For the verification of the model, the temperatures at the front and back sides of the interior of the drying unit were measured, and the values were compared with the estimated values obtained using the model.

    1.2 Model establishment

    A mesh model was established that consists of three volumes(food waste filling part, internal air evaporating part, and heated-air flowing part) and has 225,099 nodes and 859,627 cells, as shown in Fig. 1. It has a bilaterally symmetrical structure based on the front side, except for the drying material outlet, and thus only 1/2 of the drying unit was established as a mesh model(Fig. 1).

    1.3 Model analysis

    The analysis was conducted using the Fluent program based on the established mesh model. Table 1 summarizes the conditions and variables used for the analysis(Table 1).

    2 Analysis of the drying characteristics

    2.1 Experiment device

    To examine the heated-air drying characteristics of the food waste, a drying unit with a drying capacity of 500 kg/hr was manufactured and used for the experiment, as shown in Fig. 2. The drying unit consisted of a food waste storage tank, a vapor cooling unit, a blast fan and a heated-air blower for the supply of heated air, and a control panel. The raw material put into the top part is stored in the inner tank, and drying is carried out as it is agitated by the auger. The heated air for drying performs heat exchange with the food waste of the inner tank as the air is supplied through the inlet on the right side, it fills the empty space between the inner and outer tanks, and is discharged through the outlet on the left side. The vapor produced through the progress of drying is circulated through the cooling unit, where it is condensed and discharged as condensate. Temperature sensors for measuring the temperatures of the drying air and drying material were installed at one spot of the inner tank and at one spot of the heated-air inlet and outlet, respectively. Fig. 3 shows a photograph of the experimental drying unit used for the experiment (Fig. 2, 3).

    2.2 Experiment material

    Food wastes collected from the cafeteria of NDT Engineering & Aerospace Co., LTD. and from Changwon were used as experimental material. Fig. 4 shows that the experiment material was put into the drying unit(Fig. 4).

    2.3 Experiment method

    The drying characteristics of the food waste were examined based on the input temperature of the drying air. The amount of wastes input for drying was 500 kg, and this was weighed using a scale. The moisture contents before and after the drying treatment were measured using the oven method (100℃-20 hours). For each specimen, measurements were taken three times, and the average value was used as the moisture content. Temperature sensors (pt-100Ω) were installed at the inlet and outlet of the input air and inside the drying unit. The temperature changes were measured throughout the entire process of the experiment.

    Results and Discussion

    1 Analysis of heat flow within the drying unit

    1.1 Model verification

    For the verification of the heat flow analysis model, a food waste drying unit with a drying capacity of 100 kg was manufactured, and the temperatures at the front and back sides of the interior of the drying unit were measured while performing drying operation, where the values were compared with the estimated values obtained by the model. Table 2 summarizes the results of an analysis of the estimated value and the measured value. In the experiments with 80 and 90 kg drying capacities, the temperatures at the front side were 1∼4℃ higher than the estimated value, and the temperatures at the back side were 5℃ lower than the estimated value(Table 2).

    1.2 Flow velocity and temperature distribution

    Figure 5 shows the flow velocity distribution within the heated-air chamber of the drying unit for the drying air heated by the heated-air blower. The drying air supplied through the bottom part moved to the back side and ascended, and was then discharged through the top part of the back side. The estimated maximum flow velocity at the inlet was about 21 m/s, and the estimated flow velocity at the outlet was about 15 m/s.

    Figure 6 shows the temperature distribution of the heated-air chamber depending on the flow of the drying air. The temperature distribution was similar to the flow of the air. From the middle part of the heated-air chamber to the outlet, when the average temperature of the wall of the inner tank was about 187℃, the internal temperature of the tank was about 64℃, indicating that heat exchange based on the heated-air passage method was possible. However, the thermal characteristics necessary for the development of a heated-air passage type drying unit, such as heat exchange capacity and thermal efficiency, need to be analyzed(Fig. 5, 6).

    2 Analysis of the drying characteristics

    Table 3 summarizes the results of the food waste drying experiment. The experiment showed that the total required drying time was approximately 20 hours and the drying speed was approximately 2.90 %/hr. The drying time was five hours longer than the research target value(15 hours per session). However, the time was approximately 16 hours when the preheating and cooling times required for the input and output were excluded, which was close to the research target value. The drying time did not show a large variation depending on the temperature of the input drying air. Significantly, the drying time was 21 hours at 155℃, and thus drying operation would be possible without the use of high-temperature air(more than 200℃) when waste heat is utilized in the future. It is thought that the contact area between the input air and the food waste, rather than the temperature of the input air, has a significant effect on the reduction of the drying time. This needs to be verified through an experiment in future research. Fig. 7 shows the temperature changes for the input drying air, the output air, and the food waste within the drying unit depending on the drying time during the drying process(Table 3, Fig. 7).

    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)

    Figure

    JALS-52-103_F1.gif

    Mesh model of the experimental food drying unit.

    JALS-52-103_F2.gif

    By-sectional 3D drawing of the experimental food drying unit.

    JALS-52-103_F3.gif

    Photography of the experimental drying unit. (Top: Front view, Bottom: Rear view)

    JALS-52-103_F4.gif

    Photography of the experimental material.

    JALS-52-103_F5.gif

    Velocity distribution of the heated air. (Top: Air inlet part, Middle: Middle part, Bottom: Air outlet part)

    JALS-52-103_F6.gif

    Temperature distribution of the heated air. (Top: Air inlet part, Middle: Middle part, Bottom: Air outlet part)

    JALS-52-103_F7.gif

    Temperature change according to drying time. (Top: 1st, Middle: 2nd, Bottom: 3rd)

    Table

    Analysis of heated air flow

    Temperature comparison between estimated and measured value

    Drying characteristics of the food waste

    *Treatment amount = Inlet amount/treatment time.
    *Drying speed = (Initial M. C. - Dried M. C.)/Drying time.

    Reference

    1. S.Y. Choe (2010) A study on the thermal characteristics of the low temperature vacuum dryer by the vacuum chamber temperature., Journal of the Korean Society for Power System Engineering., Vol.14 ; pp.23-28
    2. D.S. Kang , B.H. Choi , Y.I. Choi (2012) A study on the drying characteristics from food waste by using inner-cycle thermal air drying process., J. of Society of Environmental Technology., Vol.13 ; pp.132-138
    3. I.S. Kim (2017) Sea exporting of fresh vegetables., http://www.kookje.co.kr
    4. S.H. Kim , B.S. Shin , B.R. Hwang (1999) Operating conditions of vacuum dryer-treatment of food waste., Proceedings of the KSAM 1999 Winter Conference, ; pp.390-398
    5. Mafra (2016) 2015 The Status of Vegetable Grown in Facilities, Greenhouse and Vegetable Production.,
    6. Me keco (2015) The status of production and evaluation of the waste..,
    7. Y.S. Ryoo , H.J. Joo , J.W. Kim , M.L. Park (2012) Economic analysis of cooling-heating system using ground source heat in horticultural greenhouse., Journal of the Korean Solar Energy Society., Vol.32 ; pp.60-67
    오늘하루 팝업창 안보기 닫기