Electrostatic precipitator and modifications

An electrostatic precipitator (ESP) is a flue gas cleaning device that uses electrostatic forces and consists of discharge wires and collecting plates. A high voltage is applied to the dis-charge wires to induce an electric field between the wires and the collecting plates, which also ionizes the gas around the discharge wires to supply ions. An ion collides with a particle and sticks to it, and the particle obtains its charge. Particles can be charged via two mecha-nisms, depending on their sizes: diffusion charging and field charging. The charging of par-ticles in unipolar gaseous ion density is referred to as diffusion charging (which is based on the effect of thermal agitation for small particles (<0.2 m) and weak applied electric fields).

Field charging is the mechanism for larger particles (> 0.5 m) and stronger electric fields.

Whenever a flue gas with fine particles passes between the collecting plates and discharge wires, particles in the gas are charged with ions. The Coulomb force that is generated by the electric field causes the charged particles to adhere to the collecting plates. Furthermore, the particles on the collecting plates can be removed via three methods: 1- rapping the collecting plates, 2-scraping off with a brush, or 3-washing off with water (wet type) and removing from the hopper for disposal.

The collection efficiency of the ESP depends strongly on the electrical properties of the particulate matter (electrical resistivity) that is being collected (Nussbaumer et al., 2010), the design parameters of the ESP (long electrodes and a large collection surface) and the combustion process (Nussbaumer et al., 2016). The collection efficiency of the PM in small-scale combustion and low-emission fuel might be at least 70% during normal operation (Carrol et al., 2017), and for large-scale appliances, it might exceed 90% (REF). In addition,

27

efficiency losses might occur due to re-entrainment, sneakage and back-corona. Re-entrain-ment is caused by rapping, and 10-15% of particles move back into the gas stream. Sneakage refers to the movement of a small portion of the gas flow (5-10%) around the charging zones while remaining uncharged due to design restrictions. Back-corona occurs if the electric field becomes sufficiently large to cause an electrical breakdown, which reduces the charge on the particles.

Figure 7: Operating principle of the parallel-plate electrostatic precipitator (Jaworek et al.

2007). HV – high voltage.

Various ESP types are widely used in large-scale industrial processes. All ESPs are divided into four main categories: plate-wire, flat-plate, tubular and wet precipitator. In large-scale appliances, in which large volumes of gas must be cleaned, the most suitable is the parallel-plate-type ESP design. However, for smaller gas-flow residential furnaces, the most suitable is the tubular-type electrostatic precipitator due to easier installation in or on top of the chimney (Report FutureBioTec, 2012).

Small-scale electrostatic precipitators can be divided according to the installation principle (Bologa et al., 2012); in a residential furnace, the precipitator might be placed between the furnace and the chimney, whereas for a chimney stove, it can be located next to the stove, in the flue gas duct or on the top of the chimney. ESPs can be used in automated wood chip/pellet boilers or in logwood boilers or stoves (Brunner et al., 2018).

Laitinen et al. (2016) introduced a new ESP technology, namely, a sonic jet charger, in which particles were removed based on diffusion charging and reached removal efficiencies of 80% for sub-micron particles. Moreover, in comparison to wire-type discharge elec-trodes, sonic jet-type chargers have the advantage of being shielded from the flue gas, which renders them suitable for systems in which the use of unshielded electrodes is not possible due to the flue gas properties. In addition, this type of ESP can be used if periodic cleaning

28

of the electrode would cause a problem, as the purged air protects it from contamination and corrosion. In comparison to wire-type electrodes, the sonic jet charger is more expensive.

These two types of ESPs (chimney top with manual cleaning and in-line with automated cleaning) were studied and evaluated by Carroll et al. (2017). It was found that removal efficiencies that exceed 70% are possible with an inline system with automated cleaning.

Moreover, for emissions from wood, which is regarded as a low-emission fuel, the precipi-tation efficiency could be maintained over a long exploiprecipi-tation time. A research study with a 50-kW wood pellet boiler and a 100-kW gasifier that was equipped with an ESP was con-ducted by Poškas et al. (2018). It was concluded that two these technologies produce differ-ent particle concdiffer-entrations, and therefore, a high collection efficiency of 98-99% was real-ized with a system with flue gas (a pellet boiler). Due to the challenging operating conditions in the tests with a gasifier, a lower removal efficiency of ~75% was obtained.

According to a study on seven heating plants with tube-type and plate-type ESPs by Nuss-baumer et al. (2016), a removal efficiency that exceeds 95% is realizable for new plants with optimal ESP operation and low PM concentrations. Moreover, the control of the fuel input and the primary and secondary air is important for the realization of high PM precip-itation efficiencies. Bäfer et al. (2012) investigated the efficiency of an ESP (installed in the chimney) in both efficient and poor combustion conditions. The removal efficiencies with respect to the mass of the particles were 87% and 93% for efficient and poor combustion, respectively. The higher efficiency in the poor combustion was attributed to the lower flue gas temperature, which induced a longer particle residence time in the ESP.

In the study of Dastoori et al. (2013), a CFD model was developed for analysing the trajec-tories of the particles in a small-scale biomass appliance with an ESP. It was concluded that the PM collection efficiency depends on the flue gas velocity, the applied voltage and the particulate size. Moreover, the removal efficiency can be increased by changing the geom-etry of the system; for example, the maximum efficiency can be realized by increasing the length of the chimney.

Several negative issues are encountered in the use of the ESP technology: the back-corona effect (solution: decrease the dust resistivity); in submicron particle removal, re-entrainment from the collector electrode back into the flowing gas; and contamination of the discharge electrode, among others (Jaworek et al., 2007).

In summary, these flue gas cleaning devices realize high PM collection efficiencies that range from 50 to 98% (Bologa et al., 2012, Brunner et al., 2018, Laitinen et al., 2016 Migliavacca et al., 2014; Meiller et al., 2015). However, regardless of these examples, small-scale ESP versions remain under development (Obernberger & Mandl et al., 2011).

29

4 Methods

The work that is described here was conducted at the Fine Particle and Aerosol Technology laboratory at the University of Eastern Finland. In Papers I and II, the novel fine particle emission precipitation technology was utilized. Paper III focused on particle physicochem-ical properties and their differences in continuous versus batch combustions. In Paper IV, the time-resolved emissions of batch combustion were analysed.

Combining all these studies, the gas and particle emissions and subsequent physicochemical characterizations in various combustion conditions were examined. The objective of Papers I and II was to investigate the efficiency of emission cleaning technology in small-scale combustion. Schematic diagrams of the measurement systems are presented in Figure 8 and Figure 9.

Figure 8: Experimental setup with (A) a condensing heat exchanger and (B) a reference boiler (Paper I).

30

Figure 9: Experimental setup with a combined condensing heat exchanger and shielded co-rona charger system (Paper II).