The in-duct sorbent injection desulfurization process is of great interest in regards to power plant retrofitting because of the small amount of capital required and the moderate operating costs involved. However, it is handicapped by its low sulfur removal capacity. Nevertheless, there are some technical options for improving the desulfurization efficiency without significantly increasing the process costs. An extensive experimental program has been performed in a 3-MWe equivalent pilot plant to assess the effects of the main operating variables (the Ca/S ratio, the approach to the adiabatic saturation temperature, and the recirculation ratio) on the sulfur removal efficiency, as well as the efficiency improvements that can be achieved using activation of the recirculated sorbent and precollection of fly ash and seawater for flue gas humidification. The experimental results show that the inlet SO2 concentration and the gas residence time have a negligible effect on the desulfurization efficiency. On the other hand, the activation of the partially converted sorbent through the addition of water in a pugmill turned out to be an efficient option for improving the desulfurization process without greatly increasing its complexity or cost. A semiempirical model has been developed to relate the desulfurization efficiency to the main operating variables. The model, based on the number of overall mass-transfer units, was fitted following a multiple linear regression by verifying the main tests of statistical significance.