Abstract: The operation modes of spraying layers in the large scale desulphurization tower with four spraying layers were investigated by computational fluid dynamics numerical simulation． The Euler-Lagrange approach was adopted and the inter-exchanges of momentum，mass and energy between the gas and liquid phase were fully studied; the effect of instantaneous turbulent velocity fluctuations on the particle trajectories was also taken into account through the use of stochastic methods． The simulation results were also validated via actual on-site data． The main results are as follow: ( 1) for all spraying modes，the area-weighted mean total velocity and its horizontal component of the gas phase on the horizontal cross-sections reach to their peak values inside the height of the flue gas inlet and then decline quickly，the horizontal component almost decreases to 0 when the flue gas comes to the bottom spraying layer． Meanwhile，the curves of area-weighted mean vertical velocity components of all spraying modes almost overlap completely along the absorption tower，and inside the height of the flue gas inlet，they show perfect monotonous linear increase． ( 2) Among all spraying modes，the most non-uniformity in the temperature field and water vapor field of the flue gas occurs when only bottom spraying layer operating． In addition，in such spraying mode， the area-weighted mean temperature on the flue gas outlet is much higher than the other spraying modes，and the area-weighted mean humidity on the flue gas outlet is not saturated． Whereas，the above-mentioned non-uniformity is alleviated obviously when only top spraying layer operating． ( 3) For all spraying modes except only bottom spraying layer operating，the curve extremums of area-weighted mean temperature and humidity of the gas phase on the horizontal cross-sections along the absorption tower are all located within the lower quarter height of the flue gas inlet． With the increase of spraying layers，the maximum values of the above-mentioned temperature curves decrease，while the minimum values of the humidity curves increase． ( 4) When only bottom spraying layer operates， the evaporation water quantity accounts for four fifths the value when all the four spraying layers operating． There is no significant difference in evaporation water quantity between only top spraying layer mode and other multiple layers modes． Among all the spraying modes，the highest evaporation ratio is found when only top spraying layer adopted．

Key words: wet flue gas desulfurization, spraying layers operation modes, gas-liquid two phases flow, gas phase velocity distribution, temperature field, numerical simulation