Fly ash in flue gas can clog and cause corrosion in the air preheater. Therefore, the flue gas channel at the outlet of the power plant needs to be designed to accommodate fly ash using a hopper. The function of the hopper is to separate the flow of flue gas from fly ash by collecting the fly ash. The purpose of this research is to determine the effect of hopper dimensions on fly ash and flue gas and to find the optimal hopper dimensions for the power plant. In this study, the variations used are without a hopper and hoppers with depths of 3, 4, and 5 m. Deeper hoppers result in an increase in flue gas velocity at the outlet and heat transfer coefficient values towards the air preheater. However, the changes in flue gas velocity at the outlet without a hopper and with a hopper are very small, so they do not significantly affect the air preheater. Deeper hopper depths result in increased pressure drop of flue gas and Reynolds number. Increasing the hopper depth leads to a decrease in outlet temperature and heat transfer value towards the air preheater. Deeper hoppers also lead to a decrease in the percentage of fly ash and mass of fly ash passing through to the air preheater, while the mass of fly ash trapped in the hopper increases. The 5 m hopper dimension is the best in reducing fly ash escape, with only 18.53% of fly ash parcels escaping, 86.75 kg of fly ash trapped in the hopper, and only 0.9 kg of fly ash mass escaping.