Communications Electrostatic Charging of Submicron and Micrometer Particles at High Temperatures* By Annette Schiel**, Alfred Peter Weber, and Gerhard Kasper Gas cleaning at high temperatures, as it is required in new combined gas and steam turbine processes on the basis of coal still is one of the major open tasks. Here, the flue gas has to be cleaned at the operating temperature and pressure. Conven- tional separation techniques (granular bed filters) are still not able to achieve the limiting values of particle size and mass concentration for a secure operation of the turbines. There- fore, new separation techniques have to be consulted. The fact that at these temperatures the particles are highly charged due to thermionisation may enable the realization of a separation technique based on electrostatic precipitation at high tem- peratures. Therefore, the size dependent grade efficiencies of fine slag particles (Al-Si) had been determined. The results have shown, that separation of particles in the micron and submicron size range is possible. Furthermore, the separation efficiency was found to be virtually independent of the particle size. 1 Problem The policy in respect of climate interests aims at a CO 2 reduction, especially in the energy industry. By increasing the energy efficiency in particular of coal combustion processes it is possible to decrease the per capita emission of CO 2 .A promising method of the process management is presently tested by burning coal dust under pressure, whereby the flue gas is fed to a combined gas and steam turbine system at operating conditions [1]. Consequently, the flue gas has to be purified at a temperature of 1400 C, so that an almost particle free flue gas can be lead over the turbine shovels. Conven- tional separation techniques still exhibit insufficient separa- tion efficiencies to reach the limiting values for particle concentrations and particle sizes. Therefore, it is necessary to evaluate the possibility of separating the highly thermionically charged fly ash particles by means of electrostatic precipita- tion. 2 Measurement Technique and Experimental Setup The experimental setup is displayed in Fig. 1. The principal item of the arrangement was the high temperature furnace, capable of reaching a temperature of 1600 C. A ceramic tube (total length = 150 cm) was mounted within the furnace; about 30 cm of the tube were sticking out of the furnace at bottom and top. At the top, close to the tube, two parallel electrodes (10 cm length) were mounted forming a plate capacitor. To prevent current leakage through the ceramic, which becomes somewhat conductive at very high temperatures, the elec- trodes had to be mounted outside the high temperature zone at a temperature of approx. 1000 C. The aerosol was fed at the bottom of the tube and then passed the furnace. The particles were heated up and, due to thermal charging, were precipitated at the electrodes. legend: a: high tempeature furnace b: electodes, U max = 20 kV c: optical particle counter (OPC) d: heat exchanger e: gas purification: electrostatic precipitator max. 1600 °C HV exhaust air OPC aerosol a b c d e legend: a: high tempeature furnace b: electodes, U max = 20 kV c: optical particle counter (OPC) d: heat exchanger e: gas purification: electrostatic precipitator max. 1600 °C HV exhaust air OPC aerosol a b c d e Figure 1. Schematic drawing of the experimental setup. From the aerosol flow leaving the ceramic tube a sample flow was extracted, leading to an aerosol measurement device to determine particle size distributions. It is important to emphasize that the particle separation occurred in the high temperature region, although the particle size measurements took place at ambient temperatures. The rest of the aerosol leaving the furnace was cooled down to room temperature and particles were separated from the gas flow in an electrostatic precipitator. A scattered light particle counter (SPZ-A 10) detecting particle sizes ranging from 0.3±10 lm was utilized. The principle of this device is the detection of the scattered light of illuminated single particles in an optical array. The intensity of the scattered light gives information on the particle size [2]. Slag particles (Al-Si) as a well-suited substitute for fly ash, were generated from ground slag pellets originating from a granular bed filter of a coal combustion plant. These particles Chem. Eng. Technol. 25 (2002) 12, Ó 2002 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 0930-7516/02/1212-1149 $ 17.50+.50/0 1149 ± [*] Presented at the European Filtech Conference 2001, 16. Oktober 2001, Düsseldorf. [**] A. Schiel (e-mail: annette.schiel@mvm.uni-karlsruhe.de), A. P. Weber, G. Kasper, Institut für Mechanische Verfahrenstechnik und Mechanik, Universität Karlsruhe (TH), Kaiserstr. 12, D-76128 Karlsruhe. 0930-7516/02/1212-1149 $ 17.50+.50/0