This work reports experimental results on the effects of temperature (25, 45, and 65 C at different relative humidity) on the scrubbing of charged submicron particles by means of cold (25 C) droplets charged with opposite polarity. The aim of the study is to experiment how the capture of particles is influenced by the simultaneous presence of electrostatic and phoretic forces related to the occurrence of thermal and water vapor gradients close to the droplet surface. This information plays an important role in the development of wet electrostatic scrubbing (WES), an emerging technology for submicron and ultrafine particle capture. Tests were performed in a lab-scale system in which the particle laden-gas was scrubbed by a train of identic droplets. Particles were charged by a corona source while droplets are generated by electrospraying. Experiments revealed that for particles larger than about 250-300 nm, there were higher removal efficiencies in nonisothermal conditions, with limited differences between 45 and 65 C tests. For particles finer than about 150 nm, we sometimes observed lower removal efficiencies for higher gas temperatures, probably due to the difficulties in controlling particle charging for these particles. The experiments were interpreted with a consolidated stochastic model that predicted successfully the data at isothermal conditions, but was less effective for tests at higher gas temperatures. In our opinion, this discrepancy relies on synergies among the fluid dynamic field induced by droplet evaporation/ condensation, the phoretic and the electrostatic forces, which are not considered in the model.
Energy generation by fossil fuels produces significant amount of pollutants. Among the most toxic of them, there are SO2 and particulate matter. The first is a toxic gas that is subjected to severe regulations, the second is only partially regulated since the most toxic fractions of particles, i.e. the ultrafine particles, are nor easily measured neither properly captured by conventional technologies available at commercial level. Electrification of water sprays provide a reliable way to improve both the SO2 mass transfer rates and the particle capture efficiency, thanks to the multiple effects of electric charges imposed on the sprayed droplets. In this paper, we report experimental findings on the use of electrified sprays of water to reduce SO2 and particulate matter form a model flue gas. Tests were performed both laboratory and pilot scale. The experiments are compared with the performances of the same spray operated without electrification. In the pilot scale unit, particle removal efficiency is negligible and SO2 removal is up to 97% with the uncharged spray, The use of induction charging and exposure to corona pre-charging allow achieving >93% reduction of particulate matter and to >99% SO2 reductions. Experiments at laboratory scale shed light on the mechanisms of particle and SO2 capture. In particular, the experimental results revealed that a stochastic scavenging model presented in our former works (data not shown) well described the particle capture and that for charged droplets, the absorption rate for SO2 improved by about 60% respect to uncharged droplets.
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