Semiwet Scrubbing: Design and Operational Experience of a State-of-the-Art Unit Tom Spooren, a Ann Raveel, a Bart Adams, a Gerrit Du Toit, b and Peter Waller c a Seghers Keppel Technology Group (SKG) NV, 2830 Willebroek, Belgium; Bart_Adams@keppelseghers.com (for correspondence) b SITA Waste (Isle of Man) Ltd., Energy from Waste Facility, Richmond Hill, Douglas, Isle of Man IM4 1JH, United Kingdom c Aker Kvaerner Engineering Services, Stockton-on-Tees TS18 3RE, United Kingdom Published online 7 April 2006 in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/ep.10138 Semiwet scrubbers are used for the removal of acid components from flue gases in a variety of thermal processes. The focus of this study is application of the technology in the recently commissioned municipal waste incinerator of the Isle of Man, characterized by considerable variations in the pollutant concentrations and the gas flow rate. The concept and principles, the design procedure, and the performance of the scrubber are discussed. The latter is illustrated by means of detailed operational data over extended periods. Spe- cial design features include adjustable guiding vanes in the diffuser, advanced temperature control for opti- mum removal efficiency, and advanced lime-dosing control for minimal reagent consumption. Long-term results indicate that the scrubber allows continuous operation below the current EU emission limit values for acid gas components (averages of 8 mg HCl per Nm 3 and 14 mg SO 2 per Nm 3 ). Furthermore, operation and maintenance experience indicates opti- mal spray pattern control, illustrated by low moisture content of the residues (average of 2%), no handling or transport problems, and the total absence of lime cak- ing on the reactor walls. Finally, the influence of op- erational parameters such as the reaction temperature and the inlet pollutant level are shown by means of industrial-scale measurements in the reference plant. © 2006 American Institute of Chemical Engineers Environ Prog, 25: 201–207, 2006 Keywords: atomizer; acid removal; lime slurry; flue gas cleaning INTRODUCTION Most industrial thermal processes generate one or more acid gas components such as SO x (SO 2 and SO 3 ), HCl, HF, or NO x (NO, NO 2 , and N 2 O) as by-products. Typical examples of these processes include coal-fired power plants; glass, steel, and cement industries; and waste incinerators. Unregulated emission of acid com- ponents will have a direct negative impact on human health and will also lead to acid deposition in the environment with damage to flora, fauna, buildings, and monuments as a result. The first legislation limiting the release of acid gas components dates back to the mid-1980s to early 1990s [1]. In general, the production of acid components can- not be avoided and an adequate removal technique is essential. Although concentrations of these components may vary widely depending on the type of process, similar removal technologies are used in different industries. Therefore, even though the remainder of this article focuses on semiwet scrubbing technology as applied in municipal waste incineration, the conclusions and con- siderations are also valid for other industries. Figure 1 provides an overview of the main compo- nents of a typical waste-to-power (WtP) plant. The waste is transported by trucks or trains to the waste bunker, in which it is stored and mixed. From the bunker, the mixture is transferred into the furnace. Here and also in the lower part of the boiler, combus- tion air is supplied and the actual combustion takes place. The ashes that are left after combustion contain the metal and inert fraction of the waste and are ex- tracted from the furnace for further treatment. The energy of the combustion process is recovered as heat © 2006 American Institute of Chemical Engineers Environmental Progress (Vol.25, No.3) October 2006 201