## 1 Introduction  
Coal has plenty of reserves, with several coal types evenly distributed throughout the world. It is a highly economical energy source. However, the rise in the use of fossil fuels has resulted in a global climate crisis. Hence, the world is reducing its dependence on coal. The use of green energy (hydrogen, solar, and nuclear) has increased, reducing greenhouse emissions and air pollution. However, coal-fired power generation is still used in many places. This energy transition will take a long time to be achieved, prompting many countries to strengthen their energy competitiveness considering its relevance to climate change.  
A good example is the investment in coal-fired power plants. Many power plants build and operate coal storage sheds to reduce the dust generated when storing coal. Coal stored outdoors generates coal dust because of the wind around it. Constructing an indoor coal storage facility reduces fine dust and provides a stable coal supply. Coal storage sheds (CSS) are classified into shed, dome, and silo types. The most common is the shed-type CSS, which is used in many power plants because it is economical and easy to construct. However, the major challenge with indoor coal storage is the spontaneous combustion of coal [1]. Factors of spontaneous combustion of coal include volatilization characteristics, storage period, size of coal particles, moisture, and impurities [2, 3]. Upon exposure to air, coal reacts slowly with oxygen and generates heat [4]. The stages of spontaneous combustion of coal are as follows: the temperature rises in the presence of oxygen, [5, 6, 7] leading to the initial ignition stage and increasing the temperature of the coal pile. Compounds in coal decompose, producing high amounts of harmful gases. Additionally, oxygen absorption and heat generation in the coal pile increase, and the temperature rises quickly to approximately 84 \({}^{\circ}\)C. Coal ignites when it enters the ignition stage, generating a large amount of harmful gas and smoke. Finally, at the late firing stage, the firing spreads [8; 9]. The ignition temperature of sub-bituminous coal is approximately 160-170 \({}^{\circ}\)C, while the ignition temperature of anthracite is approximately 185 \({}^{\circ}\)C. Thermal power plants prefer low-grade coal because it is economical. Low-grade coal has a moisture content of 15% or more and a calorific value of 23,848 kJ/kg, accounting for 47% of global coal reserves, and is cheaper. Therefore, it is the most preferred coal type for thermal power plants. However, low-grade coal causes a decrease in dust collection performance, increases heat loss, and shortens the lifespan of power generation facilities, while its spontaneous combustion remains its most important challenge. The high porosity of low-grade coal, coupled with its moisture content of 30-50%, enables it to react well with oxygen. If spontaneous combustion occurs in the CSS, less carbon monoxide is emitted, posing a significant risk to the safety of field workers [10; 11; 12]. The safest concentration of carbon monoxide required inside an indoor storage facility is 30 ppm. However, when spontaneous combustion occurs, the concentration of carbon monoxide exceeds 30 ppm. Researchers have studied CSS internal flow characteristics and spontaneous combustion using computational fluid dynamics (CFD). However, large-scale facilities find it challenging to determine CFD values through field experiments. Therefore, the internal flow characteristics of CSS must be analyzed comprehensively.  
This study analyzes the internal flow field of CSS using CFD, determines the flow information using laser flow visualization, and presents the methods employed.