The existence of liquid water was found very important in incineration flue gas clean-up systems for enhancing the absorption of acid components contained. In a newly developed incineration flue gas clean-up tower, which works in a semi-dry mode, the water is injected in the form of spray to maximum its contact surface with the gas. The criteria for the design of the water nozzles would be high water concentration but no liquid impinging on the solid wall and complete evaporation inside the tower. In order to optimize the atomizer design, the effects of the spray type (hollow or solid cone), their initial droplet size distribution and water flow rate on the performance of the acid gas absorption were investigated. The liquid behaviour was studied with a fluid dynamic simulation code, and the overall performance was checked experimentally. This paper presents the use of a commercial CFD code, FLUENT, and some modifications made during such investigation. The modification includes the viscosity of the flue gas defined as a function of the temperature, and the initial mass fraction of different droplet size group described with an exponential distribution formula of Rosin-Rammler. The investigation results (the optimal spray parameters) were used to guide the water nozzle design. The general performance of the flue gas clean-up system measured during the plant operation complied with the design criteria.