Ind. Eng. Chem. Res. 2003, 42, 6625-6637
6625
Flue-Gas Desulfurization in an Advanced in-Duct Desulfurization Process: An Empirical Model from an Experimental Pilot-Plant Study F. J. Gutie´ rrez Ortiz* and P. Ollero† Departamento de Ingenierı´a Quı´mica y Ambiental, Universidad de Sevilla, Camino de los Descubrimientos s/n, 41092 Seville, Spain
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. Background The in-duct desulfurization process involves the injection of a dry sorbent, typically hydrated lime, and the humidification of the flue gas by spraying water into the ductwork downstream from the air preheater but upstream from the particulate collection equipment. The main reaction binding the SO2 is a simple acid-base reaction:
SO2 + Ca(OH)2 f CaSO3‚1/2H2O + 1/2H2O Apart from hemihydrate calcium sulfite, different reaction products have been proposed.1 A small calcium sulfite fraction is oxidized to gypsum following the reaction
CaSO3‚1/2H2O + 1/2O2 + 3/2H2O f CaSO4‚2H2O The SO2 is removed by the sorbent particles entrained by the gas in the duct and by the sorbent accumulated in the particle collection equipment, an electrostatic precipitator (ESP) or a fabric filter (FF). Because of the short residence time in the duct (1-3 s), the sorbent must be kept active; that is, its moisture content must be kept high. The main operating variables of this simple in-duct sorbent injection (DSI) process are the Ca/S ratio and the approach to the adiabatic saturation temperature (AST). Increasing the Ca/S ratio leads to * To whom correspondence should be addressed. Tel.: 00 34 95 448 72 60/61/ 65/68. Fax: 00 34 95 446 17 75. E-mail:
[email protected]. † E-mail:
[email protected].
higher SO2 removal yields but at the expense of lower sorbent utilization. Furthermore, decreasing the approach to the saturation temperature has a strong positive effect on the SO2 removal efficiency and sorbent utilization. However, there is a minimum practical approach to temperature of around 8-10 °C because of the growing risk of solid deposits on the duct walls and on the internal parts of the process equipment and also because of the corrosion of the material. The advantages of the DSI process in its simplest form are low capital cost and low energy consumption, extremely simple installation, little space required, a short construction time, and no wastewater treatment plant. However, the SO2 removal efficiency of this simple process is in the range of 40-50% at Ca/S ratios of 1-2 and an 11-14 °C approach to the saturation temperature. This, in turn, means low sorbent utilization (
