WET ELECTROSTATIC SCRUBBER FOR GAS POLLUTANTS EMISSION CONTROL

In this study, we investigated the remove of combustion particles by means of wet electrostatic scrubbers. In particular, the role of droplet charging on particle capture was investigated. To this end, the experiments were performed to estimate the Droplet Charge to Mass Ratio (D-CMR) for two hollow cone spray nozzles. The experiments revealed that the D-CMR increased linearly with the potential with a slope β until to an optimum potential named Vopt. At different potential, the removal efficiency was estimated. It increased more than linearly with the voltage, reaching values higher than 80% for particles in the investigated size range. Introduction Recent studies highlight the inefficiency of conventional technologies in capturing ultrafine and nanometric particles, mostly produced by combustion and pyrolysis processes. Carbonaceous particles are classified in Elemental Carbon (EC), Organic Matter (OM) and Organic Carbon (OC). A constant exposure to fine particles of soot leads to diseases affecting human health. They are highly hazardous materials emitted by diesel engines [1], and, among them, particles are classified by the International Agency for Research on Cancer (IARC) as a human mutagen and carcinogenic substances [I]. A fraction of EC, the Black Carbon (BC), is able to absorb solar radiation in the visible spectrum range and it is considered as the second most important climate-warming agent after CO2 [2]. Our group is developing a new technology, named Wet Electrostatic Scrubber (WES) to capture ultrafine and nanometric particles from industrial [3] and engines [2,4] exhausts. A WES is a spray tower equipped with an electrified spray unit to generate charged water droplets and, optionally, a particle and gas corona precharging unit [4]. The electrostatic forces acting between droplets and particles lead to far more rapid and effective particle capture compared with conventional spray scrubbers [5-6]. Besides, wet electrostatic scrubbers inherit all the structural and process advantages of conventional scrubbers, as the low pressure drops, the simple design and operation and above all the ability to simultaneous removal soluble gases as SO2 and NOx, even with higher rate of absorption [7]. In previous studies [3, 8-10], our research group proved that, when submicron particles are scrubbed by uncharged droplets, negligible (< 1%) removal XXXIX Meeting of the Italian Section of the Combustion Institute IV9.2 efficiencies were obtained. However, when the system operated as a wet electrostatic scrubber, the removal efficiency was at least larger than 80% for all particles size. The electrified spray properties are among the most relevant variable affecting the efficiency of WES. In order to describe and predict the effect of spray properties on WES, a detailed experimental investigation is needed. Besides, once the spray properties are known, the particle removal efficiency can be estimated through the application of the scavenging model reported in former studies [5, 8, 9, 11]. The equations provided by D’Addio et al [8, 9] for particle charging results were adopted. In this paper, tests results on the characterization of two induction charging spray nozzles are reported. Their properties were used to simulate the particle capture efficiency in the same experimental facility and gas operating condition reported in Di Natale et al [3]. Materials and Methods This paragraph briefly summarizes the experiments and modelling conditions adopted in this study, providing references where they were described in details. The lab-scale experimental rig was the same employed in a previous work [12]. The experiments were performed with tap water. Two Lechler hydraulic nozzles (models 216.364 and 216.404) were used [II]. Hereafter they are indicated as Nozzle 1 and 2, respectively. The experimental tests aimed to measure the charge acquired from sprayed droplets. The current of droplets Idrop was measured at the Faraday cage and the Droplet Charge to Mass Ratio (D-CMR) was evaluated by the ratio of droplets current Idrop and the exerted flow rate QL, as shown in Equation 1: (1) As regards the mathematical model for particle capture, we referred to the work of Carotenuto et al. [5] and Di Natale et al. [13] applied to the capture of combustion particle from open gasoline flame in a gas stream of 150 Nm 3 /h at T=50°C treated in a WES with a diameter of 40 cm and high 3 m. Results and Discussion Figure 1 shows the current droplets Idrop and the D-CMR for both pressures and nozzles at different electric potentials. XXXIX Meeting of the Italian Section of the Combustion Institute IV9.3 V [kV] 0 2 4 6 8 10 12 14 16 18 I d ro p [ υ m ]