Published at : 25 Nov 2019
Volume : IJtech Vol 10, No 6 (2019)
DOI : https://doi.org/10.14716/ijtech.v10i6.3620
|Ricky Putro Satrio Wicaksono||Department of Mechanical Engineering, Faculty of Engineering University of Indonesia, Depok 16424, Indonesia|
|Sofi Hesti Fathia||Department of Mechanical Engineering, Faculty of Engineering University of Indonesia, Depok 16424, Indonesia|
|Inkasandra Faranisa Kolang||Department of Mechanical Engineering, Faculty of Engineering University of Indonesia, Depok 16424, Indonesia|
|Achmad Riadi||Department of Mechanical Engineering, Faculty of Engineering University of Indonesia, Depok 16424, Indonesia|
|Wahyu Nirbito||Department of Mechanical Engineering, Faculty of Engineering University of Indonesia, Depok 16424, Indonesia|
|Yuswan Muharam||Department of Chemical Engineering, Faculty of Engineering, Universitas Indonesia, Kampus UI Depok, Depok 16424,Indonesia.|
|Yulianto Sulistyo Nugroho||Department of Mechanical Engineering, Faculty of Engineering University of Indonesia, Depok 16424, Indonesia|
The increased use of coal for power generation has increased the demand for low-rank coal, such as lignite and sub-bituminous coal, and during its supply, it may need to be stored for long periods. Because low-quality coal is more susceptible to spontaneous combustion than high-quality coal, its storage could potentially cause work-related accidents. One method being developed to control the temperature of stored coal to prevent spontaneous combustion is the immersion of heat exchangers in coal piles. This method can be used to control the temperature during both the storage and transportation processes. The purpose of this study was to test this method and, in particular, study the effect of changes in the heat-exchange surface area on the effectiveness of temperature control. An experiment was set up to control the temperature of a laboratory-scale coal pile using a heat exchanger made from copper tubes. Coal samples were placed in a cylindrical container with a spiral-shaped heat exchanger, placed in the center of the cylindrical container, and cooled with ~27o seawater. Tests were carried out using several configurations of heat exchanger dimensions to determine the effect of changing the ratio of heat-exchange surface area to volume of combustible material. The test results showed that greater heat-exchange surface area produced a greater amount of cooling load and temperature difference.
Coal; Heat exchanger; Heat transfer; Spontaneous combustion; Surface area ratio
is still used as a significant source of energy in most parts of the world
(Benalcazar et al., 2017), especially for electricity generation. The high demand
for coal worldwide has caused a shortage in the supply of high-quality coal.
This has resulted in the increased use of low-quality coals, such as
sub-bituminous coal and lignite, for combustion and gasification (Tristantini
et al., 2015). In the coal supply chain process, some regions of the world
still experience long storage times, whereby coal can remain stored in a ship for long
periods. Especially for low-rank coal, this tends to
lead to self-heating that can lead to spontaneous combustion, which has the
potential to cause accidents (Nugroho et al., 2000; Singh, 2012; Onifade &
Genc, 2018). Several methods have been used to reduce the spontaneous
combustion of coal, including the compaction of coal piles, direct spraying of
certain liquids onto the
coal, periodic temperature checks, volcano traps, and trenching (Wan-xing et
Different approaches have also been explored using laboratory-scale experiments, such as the use of water-sprays, mists, injections, showers, and gases (Tuomisaari & Baroudi, 1998; Goransson & Husted, 2007; Hadden & Rein, 2011). Each of these methods has advantages and disadvantages in their practice; so further research is required to find other methods that can more effectively prevent spontaneous combustion.
One such method under development for the indirect cooling of coal is the immersion of heat exchangers in coal piles (Mikalsen et al., 2018; Zhafira et al., 2018; Nugroho et al., 2019). This method can be used to control temperatures during both the storage and transportation processes. The purpose of this study was to test this method and determine the effect of the ratio of the heat-transfer surface area to the volume of the combustible material for the effectiveness of temperature control to prevent spontaneous combustion. Laboratory-scale tests were carried out using a heat exchanger made of copper pipe. Coal samples were placed in a cylindrical container and heated to certain temperatures in an oven. After the coal reached a certain temperature, saltwater was then flowed through the heat exchanger to maintain the temperature of the coal below a critical temperature. Previous experiments have used this identical method except for the method of water flow; Zhafira et al. (2018) and Nugroho et al. (2019) manually used a water bag while in the current experiment, a pump was used to automatically maintain a constant flow rate. Another similar experiment to study spontaneous combustion used a hotplate heater that was then cooled by flowing water after the sample reached a certain temperature (Mikalsen et al., 2018). Previous research determined the characteristics of spontaneous combustion by modeling heat distribution in heated coal (Saleh et al., 2017; Nugroho et al., 2019).
The current research explored this method because the transportation of coal by barges introduces new problems mainly due to the frequent occurrence of spontaneous combustion from coal being transported and the size of a heat exchanger impacts ship design and stability; additions to ships greatly affect the load, which in turn affects the stability of a vessel. Therefore, laboratory-scale experiments to determine the optimum dimensions of a heat exchanger were undertaken to ensure its effective use.
While the effect of increasing a heat-exchanger surface area on heat transfer has been generally examined, this study carried out an in-depth exploration of a heat exchanger immersion method to control coal pile temperatures to avoid spontaneous combustion. For the sub-bituminous coal used in this experiment, the critical temperature was determined to be 122.5±5oC, which was lower than the work undertaken by Nugroho et al. (2019) because of the different coal samples used in the experiments. The spiral-shaped heat exchanger used in this experiment was quite effective because of its small impact on the coal loading volume compared with the additional heat-exchange surface area. This eliminates reducing the volume of coal that can be loaded onto barges that use heat exchangers, one of the drawbacks of using immersion heat exchangers on coal barges. Spiral-shaped heat exchangers can reduce coal temperatures to below critical spontaneous combustion temperatures. The experimental results showed that additional heat-exchange surface area increased the heat loss and reduced the temperature difference. All other similar, previous work has reported the same results; immersed heat exchangers can effectively prevent the spontaneous combustion of coal (Zhafira et al., 2018; Nugroho et al., 2019). However, the current study did not determine an optimal value for the surface area ratio for this method; it ranged between 0.118 to 0.205. The results of this research can be combined with the distribution of coal temperatures (Nugroho et al., 2019) to identify critical points of spontaneous heating before continuing research on the implementation of this method for coal barges. Therefore, further research related to this method must be undertaken to discover other possibilities for optimal configurations.
The authors would like to thank the Ministry of Research, Technology and Higher Education, Republic of Indonesia, for financial assistance through the 2019 Penelitian Dasar Unggulan Perguruan Tinggi (PDUPT) funding scheme with contract numbers 1/E1/KP.PTNBH/2019, 234/PKS/R/UI/2019 and NKB-1672/UN2.R3.1/HKP.05.00/2019.
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