Feasibility of high-temperature electrostatic precipitation for the removal of nanoparticles: A case study on iron oxide separation at up to 800 °C

The removal of nanoparticles from hot gas streams is a challenging task. However, there is a huge potential for heat recovery from waste gas incineration, glass furnaces, ceramic, metallurgical, pyrolytic and many other high temperature processes. A prototype application is the separation of nanoparticles formed by condensation in thermal post-combustion processes, in order to achieve efficient heat recovery at high temperatures.This case study evaluates the performance of a high-temperature electrostatic precipitator (HT-ESP) between 400 and 800 degrees C for both discharge polarities. The tube-type ESP with 150 mm diameter and 1500 mm length is operated isothermally. Two electrode designs are tested by separating flame-generated iron oxide nanoparticles from a flue gas atmosphere. The total number concentration in the raw gas is around 2.107 cm-3 with a temperature-dependent mode diameter of 20-40 nm.Between 400 and 600 degrees C very high separation efficiencies around 99.998% (number-based) were found with just 1.5 s of residence time and negative polarity, using a wire discharge electrode. In fact, the separation is more efficient than at room temperature which is explained by particle charging with free electrons leading to an exceptionally high particle charge. At 700 degrees C and 800 degrees C, thermionic emissions become more relevant for ESP operation. In this range the best separation efficiencies of 99.96% and 99.5% respectively were obtained using a rod discharge electrode operated below the corona onset voltage. The specific input of energy required for 99% separation efficiency at any temperature is less than 250 J/m3.These findings clearly prove that HT-ESPs are a feasible and highly performing alternative for nanoparticle removal from hot gases at up to 800 degrees C.