Electrostatic precipitation as a method to control the emissions of particulate matter from small-scale combustion units

Journal Pre-proof Electrostatic Precipitation as a Method to Control the Emissions of Particulate Matter from Small-Scale Combustion Units

Molchanov Oleksandr, Krpec Kamil, Horák Jiří  PII:

S0959-6526(19)33892-2

DOI:

https://doi.org/10.1016/j.jclepro.2019.119022

Reference:

JCLP 119022

To appear in:

Journal of Cleaner Production

Received Date:

08 July 2019

Accepted Date:

22 October 2019

Please cite this article as: Molchanov Oleksandr, Krpec Kamil, Horák Jiří , Electrostatic Precipitation as a Method to Control the Emissions of Particulate Matter from Small-Scale Combustion Units, Journal of Cleaner Production (2019), https://doi.org/10.1016/j.jclepro.2019.119022

This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier.

Journal Pre-proof

1

Electrostatic Precipitation as a Method to Control the Emissions of Particulate Matter from Small-Scale Combustion Units Molchanov Oleksandr a, Krpec Kamil a, Horák Jiří a a Energy

Research Center, Technical University of Ostrava, 17. listopadu 2172/15, 708 00 Ostrava–Poruba, Czech Republic [email protected], [email protected], [email protected] Corresponding Author: [email protected]

Highlights -

An ESP was encased in a 15-kW automatic boiler’s body as an inseparable part The boiler thereby met the EU’s Ecodesign Directive emissions requirements An optimisation method was proposed to ensure the ESP´s required efficiency Two-type numeric concentration measurements verified this method The method is useful for assessing the structural potential of any ESP

Abstract The aim of this study was to investigate the use of an electrostatic precipitator (ESP) for controlling the emissions of particulate matter (PM) from small-scale heating units, specifically boilers that combust solid fuel with a heating power output of <300 kW. The equilibrium between the required precipitation efficiency and the structural parameters of the ESP in combination with the applied voltage were determined. This study presents a method of optimising the applied voltage alongside the specific ESP structural parameters while retaining the required precipitation efficiency. The ESP was designed, optimised, and integrated into a 15kW boiler without expanding its volume. The theoretically predicted voltages and resulting ESP efficiency (20 kV and 85% respectively) were verified by measuring the reduction in the particle number concentration. The measurements were based on different principles and simultaneously sampled. The PM concentrations were reduced below 40 mg/m3 (0 °C, 101.3 kPa; at reference O2 = 10 %vol.) by nominal and reducing the heat power, which allowed the boiler to meet the requirements of the EU's Ecodesign Directive. The presented optimisation method can be used in the practical engineering of any type of ESP.

Keywords electrostatic precipitator; small-scale boiler; solid fuel; particulate matter; pollution control

Symbols and constants Symbol A C Cc

Unit m2 mg/m3 #/m3 -

Description Collecting electrode area Particle concentration mass numerical 𝜆

(

Cunningham correction factor, 𝐶𝑐 = 1 + 𝑑 ∙ 2.514 + 0.8 ∙ 𝑒

𝜆

)

―0.55 ∙ 𝑑

Journal Pre-proof D

m2/s

dp e E 𝐅𝐂 𝐅𝐒 h I kb l L 𝑚𝑝 N Pa P Qp r0 t T V v 〈𝑣〉 w 𝑤𝑓 𝑤𝑝 ui U U0 β

m C V/m N N m mA J/K m m kg 1/m3 Pa kW C m °C K m3/s m/s m/s m/s m/s m/s m2/V*s V V -

0  𝜂𝐸𝑆𝑃  µ



F/m

2

𝑘𝑏 ∙ 𝑇 ∙ 𝑢𝑖

Diffusion coefficient, 𝐷 = 𝑒 Particle diameter Electron charge, e = 1.6·10−19 Electric field strength Electrostatic force vector Aerodynamic drag force vector Distance between the discharge and collecting electrodes Electrostatic precipitation current Boltzmann constant, 1.3806488(13) × 10−23 Interwire distance of the discharge electrodes Total electrode length Particle mass Number concentration of ions Standard pressure, Pa = 101325 Pa Boiler heat power Charge-on particle Discharge electrode radius Gas temperature Absolute gas temperature Flue gas volume flow rate Vector of particle velocity in the gaseous medium Mean thermal velocity of ions Vector of a medium velocity effective particle drift velocity theoretical particle drift velocity Ion mobility High voltage Onset critical corona voltage 𝑃𝑎 ± 𝑃𝑔𝑎𝑠

Relative gas density, 𝛽 = 1.013 ∙ 105 ∙

293 𝑇

-

Electric constant (vacuum permittivity), 0 = 8.85 × 10−12 Relative material permittivity

nm Pa*s sec

Total precipitation efficiency Mean molecule free pass Gaseous medium viscosity time

1. Introduction Atmospheric particulate matter (PM) emissions contribute to growing public-health problems, particularly respiratory diseases (Pope, 2000). This was confirmed by (Zelikoff et al., 2003) and reflected in further research (Chow, 2006). Small-scale installations (under 300 kW) are responsible for over 30% of atmospheric PM emissions in the Czech Republic pollution balance. A similar quotient has been reported in several other European countries. However, the location of the release of such emissions is unimportant because the atmosphere is not physically divided by state borders. Therefore, PM emissions are sources of global pollution.

Journal Pre-proof

3

Small-scale installations include heating units for combusting solid fuels at a heat power output of less than 300 kW. Such units include residential and commercial heating boilers that use solid fuels, particularly biomass and fossil fuels, and will continue to do so for a long time. This study focuses on the primary particles formed in the combustion zone, and does not consider the secondary particles formed in flue gas plumes with atmospheric interaction. To prevent atmospheric pollution by PM, some legislative limits of the PM concentrations in the flue gases of small-scale combustion units have been implemented. The EU’s Ecodesign Directive will be enacted in 2020 (2015/1189, 2015). This regulation sets a clear limit on the pollution emissions of small-scale boilers and limits the PM concentration to 40 mg/m3 (0 °C, 101.3 kPa; at reference O2 = 10 %vol.). Therefore, the producers of boilers must reduce boiler emissions. In some cases, these emissions can be reduced by optimising the combustion processes. Combusting biomass is less polluting than the combustion of fossil fuels in the same processes; it is shown in the study (Johansson et al., 2003) and confirmed in (Verma et al., 2011). Therefore, the fuel type selection and the optimisation of its combustion can reduce emissions, to a certain extent. However, such measures cannot be successful for every boiler, and approaches to reducing PM emissions are still unclear. For small-scale heating boilers, more polluting fossil fuels will still be used for economic reasons, particularly in Eastern Europe. Therefore, flue gases should be treated to reduce their PM concentration (Senior et al., 2000). Particle removal in small-scale combustion units is complicated as the characteristics of the flowing gases are unstable and related to the combustion conditions. This combustion instability is evidenced in the varying quantity, temperature, and composition of flue gases. Moreover, combustion conditions directly impact the particle formation mechanisms. Therefore, the particle characteristics are volatile, particularly their chemical composition, concentrations, and size distribution. Almost 80% of particles are within the size range of 40 nm to 1 µm. An example of the instability of the particle concentrations and size distributions is given in Figure 1, which was adapted from (Leskinen et al., 2014).

Figure 1. Numerical particle sizes distributions and their dependence on combustion condition

Some technologies for reducing the concentrations of particles in emitted gases are available, including mechanical separation, wet scrubbing, filtrating, and electrostatic precipitation. The apparatus involved in the mechanical treatment of flue gases operate by changing the velocity and/or direction, thereby removing particles by mechanical forces. The

Journal Pre-proof

4

impact of these forces is low owing to small size of PM. Therefore, such cleaning methods (Fritz and Kern, 1990) are insufficient for controlling particle emissions from small-scale boilers. The removal efficiency of fabric filters for particles in the size range mentioned above is approximately 99%. Fabric filters mainly filter flue gases by removing particles from them when they pass through the material. The particles then collect on the material and form a porous cake on its surface. This porous cake continuously grows over time, which causes the pressure drop, which adversely affects the combustion processes. Moreover, close to the dew point, the porous fabric tends to clog rapidly, which may result in severe consequences. Thus, fabric filtration should only be employed in industrial applications. In addition the specific energy consumption of fabric filters is approximately 1 Wh/m3 (Bianchini et al., 2016). Therefore, their maintenance cost is relatively high. The efficiency of wet scrubbing for removing fine particles is high and can exceed 95% (Bianchini et al., 2018). However, this process requires wastewater treatment. The collection efficiency strictly depends on the scrubbing energy requirements. For example, (Bianchini et al., 2016) mentioned that different types of scrubbers have a specific energy consumption about 10 Wh/m3. Electrostatic precipitators (ESPs) also have a high removal efficiency of over 95% for particles in the mentioned size range. The electrostatic dedusting of gases involves the charging of suspended particles in the electric field and, consequently, their removal from the gas flow under electrical forces. As ESPs mainly operate on the particles and not on the whole gas flow, the energy consumption of electrostatic precipitation is low, and can be below 0.3 Wh/m3 (Mihelcic and Julie, 2014). These data are in line with those of (Jin et al., 2018). ESPs are reliable, and their operation and maintenance processes are simple and low-cost. Complications related to the appliance of high voltage (HV) have simple engineering solutions. Therefore, HVs are widely used in households. Owing to the factors discussed above, the electrostatic precipitation is preferable to control mentioned PM emissions. Furthermore, their ability to suppress the emissions of submicron PM, particularly from small-scale boilers, has been widely reported (Poskas et al., 2017). The electrostatic precipitation of particles in small-scale boilers is not a new concept, and several engineering solutions have been developed. Some studies described a separate installation of EPS into chimneys (Schmatloch and Rauch, 2005), while most described the installation of an ESP as an addition to the bodies of biomass boiler (Berhardt et al., 2017) and multi-fuel boiler (Bologa et al., 2010). The installation of an ESP often requires the expansion of the boiler body, but this is not always possible or suitable for domestic boilers. To avoid or minimise boiler enlargement, the volume of ESPs must be as low as possible while still ensuring that the emissions of the boiler meet the required limits. The issue of minimising the volume of gas cleaning equipment is often observed in industrial engineering. In our case, a successful design will enable the integration of ESP into the available, unused space inside the boiler. Therefore, the ESP would be an inseparable part of this boiler. There is a great need to develop an analytical model in the design stage that not only analyses the efficiency of the ESP capacity usage, but also predicts the electrical ESP parameters required for sufficient precipitation. The objective of this study is to develop and employ such a method in the design of an ESP to predict the voltage required to ensuring the desired precipitation efficiency, and to verify this method by taking simultaneous measurements. The complex interactions of the large number of ESP operation parameters influence the precipitation efficiency. To evaluate the usage of the capacity of a precipitator, White (White, 1977) proposed a model that introduces a useful indicator, the specific corona power, that combines the total area of collecting electrodes, gas flow rate, and the ESP power consumption. Petersen further developed and verified this model by applying it to an industrial scenario (Petersen, 1986). However, the described approach also requires detailed emission

Journal Pre-proof

5

measurements, because the specific corona power must be registered in connection with the achieved efficiency. Therefore, this modelling method is limited and cannot be used in the design of new ESPs. Lagarias described another ESP prediction model (Lagarias, 1963), but did not sufficiently consider the dependency of the particle drift velocity on the charge value for predicting the precipitation of particles within the abovementioned size range. The novelty of this work lies in the consideration of the precipitation efficiency as a relationship between the power consumption of the ESP and its structural parameters, specifically its total collecting area. We introduce the theoretical background of the initial ESP designs and evaluate the application of a high voltage value, which is required to achieve the necessary efficiency for cleaning flue gases. This optimisation method is described and practically tested using a traditional ESP, i.e., it was not equipped with any specials elements. The required efficiency is ensured by only supplying the high voltage value determined by the proposed method. The suitability of this evaluation method is corroborated by results that were based on different suspended particles concentration measurements.

2. ESP background and design The principle of gas cleaning by an ESP lies in the separation of suspended particles in the electric field by electric forces. An electric field is typically formed by a combination of discharge and collecting electrodes with the application of a HV between them. An HV is generated by a supply unit and can be positively or negatively polarised. At the critical voltage, corona discharge occurs, which is reflected in gaseous ionisation and the directive motion of gaseous ions. The critical voltage U0 of corona discharge depends on the properties of the gas and the arrangement of electrodes. A further increase in the applied voltage proportionally intensifies the ionisation process, thereby increasing the concentration and moving velocity of ions. That is, an increase in the applied HV value results in an increase in the corona discharge current. The HV is limited by the electric durability of the gaseous medium. As the particles reach the ionised medium, they immediately begin to charge through the colliding with moving ions. The electrostatic field forces the charged particles to move directionally, as they drift to the corresponding electrode with the following emplacement. Therefore, the particles are separated, and the cleaned gas leaves the ESP and enters the atmosphere.

Figure 2. Particle in the electrode system

The proposed method of optimising the ESP with predetermined structural parameters is the determination of a suitable HV value based on the required total gas cleaning efficiency. With the knowledge of the existing 𝐶𝐸𝑥𝑖𝑠𝑡𝑖𝑛𝑔 and required 𝐶𝑅𝑒𝑞𝑢𝑖𝑟𝑒𝑑 numerical particle concentrations [#/m3], the total precipitation efficiency 𝜂𝐸𝑆𝑃 can be obtained as follows: 𝐶𝐸𝑥𝑖𝑠𝑡𝑖𝑛𝑔

𝜂𝐸𝑆𝑃 = 1 ― 𝐶𝑅𝑒𝑞𝑢𝑖𝑟𝑒𝑑

(1)

The particle removal efficiency depends on the parameters of the ESP, which are given by (Deutsch, 1922):

Journal Pre-proof

6

(

𝐴

𝜂𝐸𝑆𝑃 = 1 ― 𝑒𝑥𝑝 ― 𝑤𝑓𝑉

)

(2)

The efficiency of an ESP depends on its operating and design parameters. The design parameters are expressed as the ESP specific collecting area (SCA), which is the ratio of the total collecting area A to the flue gas flow rate V. The total collecting area A describes the collecting electrode surface involved in the process of particle precipitation and collection. Here, the area A was determined from the collecting plate sides, which face the electric field, and practically evaluated based on the available inner boiler volume. The effective particle drift velocity 𝑤𝑓 is the most important parameter in the operation of an ESP and not only depends on the particle characteristics, such as the particle sizes and their material properties, but also on some ESP operation characteristics (high voltage and electric current values), and the composition and temperature of the gaseous medium. To achieve the required precipitation efficiency, the effective drift velocity 𝑤𝑓 should be calculated (Petersen, 1986) from equation (2) as follows: 𝑉 1 (3) 𝑤𝑓 = 𝐴ln 1 ― 𝜂𝐸𝑆𝑃

(

)

The motion of charged particles in an electric field is described by the Newton equation: 𝑑(𝐯 ― 𝐰) (4) 𝑚𝑝 𝑑𝑡 = 𝐅𝐜 + 𝐅𝐬 Here, the gravitational force, and aerodynamic and electrodynamic turbulences are ignored for simplicity. The processes between the particles (such as coagulation, collision, and recharging) are also not considered for the same reason. The electrostatic force Fc is determined as follows: (5) 𝐅𝐂 = 𝑄𝑝𝐄 The charge values that particles could obtain in the electric field depend on their size. Any particle in the electric field becomes charged due to the adsorption of charges during collision between particles and ions generated under corona discharge. However, the methods by which ions reach the surface of particles varies depending on the particle diameters. Therefore, the charging mechanisms can be categorised as field and diffusion. The diffusion charging mechanism involves the collision of particles and ions, which are driven by their thermal energy. This mechanism is mainly involved in the charging of fine particles (smaller than 100 nm), and this charge can be evaluated as follows (White, 1951): 𝑑𝑝

(

𝑄𝑊 𝑑 = 2𝜋𝜀0 𝑒 𝑘𝑏𝑇𝑙𝑛 1 +

〈𝑣〉𝑒2𝑑𝑁 8𝜀0𝑘𝑏𝑇

)

(6)

From a practical engineering viewpoint, another formula proposed by Mirzabekan (Mirzabekan, 1969) is more suitable owing to its simplicity and accuracy, as follows: 𝐷 (7) 𝑄𝑑 = 2𝜋𝜀0𝑑𝑝 ∙ 𝑢𝑖 ∙ 𝐴(𝑛0𝜏) 𝑞𝑒

where 𝐴(𝑛0𝜏) = 2𝜋𝜀0𝑑𝑝𝑘𝑏𝑇 is a tabulated function. The field mechanism affects mainly particles with diameters larger than 2 µm. In this mechanism, the particle is impacted only by ions that are accelerated by the electric field with subsequent accumulation of these ions on the surface of particles. The field-charge value can be calculated as follows (Pauthenier and Moreaur-Hanot, 1932):

(

)

𝜀―1

𝑒𝑢𝑖𝑁

𝑄𝑓 = 3𝜋𝜀0 ∙ 1 + 2𝜀 + 2 𝐸𝑑2𝑝4𝜀0 + 𝑒𝑢𝑖𝑁

(8)

In practice, the total particle charge Qp can be calculated as the sum of the charges (Mirzabekan, 1969; Ughov and Valdberg, 1981; White, 1963) accumulated by the particle under both mechanisms. The aerodynamic drag force FS describes the resistance of a medium to the migration of a particle towards the collection electrodes. The value of this force also depends on the particle

Journal Pre-proof

7

diameter (Hinds, 1999). For fine particles with diameters smaller than 1 µm, particularly those with diameters comparable to the mean molecule free pass value , the gaseous medium is no longer homogeneous, the particle travels in intermolecular distances, thereby reducing the drag force FS. The drag force reduction is considered by Cunningham with the implementation of the slip correction factor. Therefore, the expression for evaluating the drag force FS takes the form of: 𝐅𝑺 =

3𝜋𝜇𝑑𝑝

(9)

(𝐯 ― 𝐰)

𝐶𝐶

Stabilised particle movement is characterised by equal drag and electrostatic forces. Therefore, the theoretical particle drift velocity 𝑤𝑝 value can be determined by: 𝐸𝑎𝑣𝑄𝑝

(10)

𝑤𝑝 = 3𝜋𝜇𝑑𝑝 ∙ 𝐶𝑐

As the theoretical particle velocity 𝑤𝑝 would be equal to the effective drift velocity 𝑤𝑓 calculated by equation (3), the required precipitation efficiency 𝜂𝐸𝑆𝑃 can be achieved. Assuming wf=wp, we can obtain an equation for optimising the structural and electrical ESP parameters. For example, the field strength value (and finally, consumed electric power) can be optimised to achieve the required efficiency of the designed precipitator applied to the existing boiler.

(

)=

𝑉 1 𝐴ln 1 ― 𝜂𝐸𝑆𝑃

𝐸𝑎𝑣𝑄𝑝 3𝜋𝜇𝑑𝑝

(11)

∙ 𝐶𝑐

Considering 𝑄𝑓 (8) and 𝑄𝑑 (10) for determining the cumulative particle charge Qp, equation (11) can be transformed:

(

)=

𝑉 1 𝐴ln 1 ― 𝜂𝐸𝑆𝑃

𝜀0𝐸𝑎𝑣 𝜇

(

∙

𝜀𝐸𝑎𝑣𝑑𝑝 𝜀+2

+

𝐴(𝑛0𝜏)𝐷 3𝑢𝑖

) ∙ 𝐶𝑐

(12)

The average field strength Eav can be obtained as the ratio of the applied voltage (U) to the length between the field-forming electrodes with opposite polarity (h) with satisfactory accuracy. Therefore, the optimal HV value U that would enable the precipitation of particles with the required efficiency can be determined for every ESP. The applied voltage values can vary from the critical voltage value of corona discharge (U0) to the voltage limited by electrical gaseous breakdown (Ulim). The critical voltage U0 required to achieve the desired negative corona discharge for the proposed electrode arrangement is calculated as follows:

(

𝑈0 = 3.04·105 ∙ 𝛽 1 +

0.298

)𝑟 (𝜋

𝛽 ∙ 𝑟0

0

ℎ 𝑙

( )) 𝑟0

― 𝑙𝑛 2𝜋 𝑙

(13)

The phenomenon of gaseous electrical breakdown has been explored in detail (Pedersen, 1967), and the mathematical expressions of the conditions required for the occurrence of this phenomenon were introduced by Pedersen, who also attempted to adapt them to meet the needs of engineering (Pedersen, 1989). McAllister (2002) presented another breakdown voltage evaluation method (McAllister, 2002), and (Chamnong et al., 2018) presented a breakdown voltage computation program. However, the most suitable method of calculating the breakdown voltage limit, from a practical engineering standpoint, was proposed by Levitov, as follows (Levitov, 1980): ℎ

𝑈𝑙𝑖𝑚 = 𝑈0 ∗

0,306 ∗ 𝑟 ― 0,0642 0

()

𝑙𝑛

ℎ 𝑟0

(14)

This ESP optimisation method was practically implemented to reduce (potentially up to 85%) the PM emissions from an automatic 15-kW boiler body. The ESP was encased in the empty region of the boiler. The structure and main dimensions of the ESP (in mm) are presented in Figure 3. The critical 𝑈0 for the specified ESP should be 6500 V, and value of 𝑈𝑙𝑖𝑚 is 45 kV.

Journal Pre-proof

8

Figure 3. Scheme of the ESP’s structure and experimental set-up.

To achieve the specified precipitation efficiency, the effective drift velocity of the particle 𝑤𝑓, according to equation (3), must be at least 0.055 m/s, which corresponds to the horizontal plane in Figure 4. The theoretical drift velocities 𝑤𝑝 were calculated considering the designed ESP for different particles diameters and applied voltages. The theoretical drift velocities formed the curved surface. The surface crossing points were matched to the conditions of the required particle precipitation efficiency of the given ESP. All particles could be precipitated with lower/higher efficiency depending on whether the theoretical drift velocity lies under/over this plane. For the ESP studied here, applying a voltage of 20 kV results in the precipitation of all particle fractions at an efficiency of at least 85%, which corresponds to particles of approximately 150 nm (see Figure 6).

Journal Pre-proof

9

Figure 4. Comparison of the effective and theoretical drift velocities as a function of the applied voltage

3. Experimental setup and evaluation The practical evaluation of the suppression of boiler PM emissions and the ESP optimisation method are described below. The experimental setup is shown in Figure 3, and allows the isokinetic evaluation of the dust concentration in the dilution tunnel, which complies with EPA Method 5G (EPA). The tests were conducted at nominal and reduced (30%) outputs under testing standard (EN303-5:2012). In the standardised measurement section located behind the boiler, the gaseous compounds (O2, CO, NOx, CO2) and flue gas temperature were measured using a flue gas analyser (ABB, NDIR, and paramagnetic principle) and a type-K thermocouple. The total particle concentration was measured using a condensation particle counter (CPC, 2018), which can optically count particles during their previous condensation growth. The particle number size distribution was measured using an electrical low-pressure impactor (ELPI®+, Dekati Ltd., Finland) (Dekati, 2019). The total dust mass concentration was measured gravimetrically using an isokinetic sampling system following standard (EN13284-1:2017). The requirements of isokinetic sampling defined in standard (EN13284-1:2017) were adopted following standard (EN3035:2012). Samples were obtained during ESP ON/OFF regimes for 30 min. The errors in the particle concentration measurements did not exceed 35%. Lignite was used as a fuel for the automatic boiler, with a granulometry of 5–25 mm. The fuel characteristics are presented in Table 1. Table 1 Characteristics of the analysed fuel Element Carbon Hydrogen Nitrogen Oxygen Sulphur Water content

Designation C H N O S W

Content [mass %] 53.75 4.32 0.63 13.50 0.57 15.00

Journal Pre-proof Ash Net calorific value

A Qi

10

12.23 21.00 MJ/kg

The ESP efficiency was evaluated following several methods. The common precipitation efficiency is calculated by comparing the PM concentrations obtained during the ESP ON/OFF regimes: 𝐶𝐸𝑆𝑃 𝑜𝑛

(15)

𝜂𝑁/𝑚𝑎𝑠𝑠 = 1 ― 𝐶𝐸𝑆𝑃 𝑜𝑓𝑓

These concentrations can be expressed as the mass or number concentration [mg/m3, #/m3]. The precipitation efficiency 𝜂𝑁 was expressed as the total numerical [#/m3] concentrations obtained with the CPC. The efficiency of the ESP 𝜂𝑚𝑎𝑠𝑠 was expressed as the mass concentrations [mg/m3], which was obtained by gravimetric measurements. As an ELPI+ was used to record the number concentrations for every fly ash fraction Fi, the ESP numerical fractional efficiency 𝜂𝐹𝑖 can be defined by comparing the changes in each numerical fraction concentration CFi [#/m3] for the ESP ON/OFF regimes: 𝐶𝐹𝑖 𝑜𝑛

(16)

𝜂𝐹𝑖 = 1 ― 𝐶𝐹𝑖 𝑜𝑓𝑓

The total efficiency of precipitation in the numerical fraction term 𝜂𝐹𝑁 can be obtained as follows: 𝑛 (17) 𝜂𝐹𝑁 = ∑𝑖 = 1𝐹𝑖 ∙ 𝜂𝐹𝑖 The excess air supply was regulated to keep the concentrations of gaseous emission as low as possible, and thus meet the Ecodesign level. ESP efficiency tests were conducted during stable boiler operation, when the CO concentration and flue gas temperatures were stable. During the tests, the HV on the ESP was set to 20 kV, which produced an electric current I approximately 0.75 mA. Twelve pairs of tests with alternating ESP on/off regimes were conducted. Information about the experiments and conditions is given in Table 2. The PM mass concentrations were obtained from gravimetric measurements. Table 2 Averaged experimental parameters and conditions Parameter

Unit

Boiler heat output, P kW Flue gas temperature, t °C Combustion gas flow rate, V** m3/h Particle residential time in ESP, t s Specific collecting area, SCA m2/(m3/s) ESP current density (calculated/measured), I mA O2 concentration %obj. Excess air ratio CO * mg/m3 NOX* mg/m3 SO2 * mg/m3 PM concentration (ESP OFF/ON), Cmass * mg/m3 ESP efficiency - mass * in dry flue gas (0 °C, 101.3 kPa); at reference O2 = 10 %vol.

Nominal heat output 15.3 170 41.4 < 0.6 15.7 0.75 /0.75 9.7 1.86 300 312 1589 86/38 0.56

Reduced heat output 4.5 100 17.6 < 1.3 34.7 0.75/0.75 14.5 3.22 315 340 1253 96/32 0.67

Journal Pre-proof

11

** effective value, valid for the flue gases at the given actual temperatures and actual atmospheric pressures. The flue gas compositions under the nominal and reduced boiler heat outputs are listed in Table 3. Table 3 Averaged flue gas composition Content of component, [vol %], Content of component, [vol %], nominal heat output reduced heat output N2 63.92 70.08 CO2 16.08 9.34 O2 9.7 14.5 H 2O 10.3 6.08 Averaged H2O concentrations were calculated with respect of the evaporation of humidity from fuel, moisture entering the combustion zone from the combustion air, and moisture produced during combustion as a result of a chemical reaction between the O2 and H2 in the fuel. The numerical particle concentration measurements indicated distinct concentration changes under the nominal heat output. These changes were observed for fractions of almost every diameter. The measurement results were validated by the parallel CPC measurements, which show a proportional increase in the total number concentration. The total number concentrations measured by ELPI+ and CPC differed by less than 20% under both the nominal and reduced heat output. Examples of the total number concentrations measured by ELPI+ in the ESP ON/OFF regime are shown in Figure 5. As the CPC method is based on a different principle, the influence of particle electric charge on the measured result was rejected. Sulphuric acid liquid particle nucleation has been accepted as a cause of such atypical changes in the number concentrations (Molchanov et al., 2018). This phenomenon requires detailed study and is not discussed here. Under the reduced boiler heat output, the changes in the concentrations due to the ESP were typical, excluding fractions 1.2–3 µm. However, as their numeric concentration was negligible, the total removal efficiency was not affected. The practical measurements show that the behaviour of particles smaller than 20 nm was also detected as distinctive due to the nucleation of sulfuric acid. The theoretical and measured ESP precipitation efficiencies are compared in Table 4. Component

Table 4 Theoretically calculated and measured ESP efficiencies ESP efficiency 𝜂𝐹𝑁 (calculated/measured) Measured 𝜂𝑁 Measured 𝜂𝑚𝑎𝑠𝑠 1) based on CPC data 2) based on ELPI+ data

Nominal heat output 0.75/0.56

Reduced heat output 0.95/0.92 2) 0.95 1) 0.67

Figure 5 shows the ESP particle precipitation efficiency calculated under the nominal (red curve) and reduced (blue curve) boiler heat outputs. The blue squares correspond to the fractional efficiency obtained from the reduced heat output measured with the ELPI+. All presented number concentrations were recalculated for dry flue gas (0 °C, 101.3 kPa; at reference O2 = 10 %vol). The theoretical efficiency curves indicate the lowest efficiency of particles ranging from approximately 100 to 300 nm for both nominal and reduced boiler operating regimes. This reflects the drag force over the electric field for these particles as they were weakly charged.

Journal Pre-proof

12

1.0E+08

1.0

1.0E+07

0.9

1.0E+06

0.8

1.0E+05

0.7

1.0E+04

0.6

1.0E+03

0.5

1.0E+02

0.4

1.0E+01

0.3

1.0E+00

0.2

1.0E-01

0.1

1.0E-02

ESP fractional efficiency [-]

Particles numerical concentration dN/dlog (Dp) [#/cm3]

Intensifying the charging process could resolve this problem and should be considered in further studies.

0.0 0.009

0.016

0.025

0.041

0.070

0.129

0.231 0.431 Dp [µm]

reduced heat otput, ESP off nominal heat otput, ESP off measured ESP efficiency, reduced heat otput (right Y-axis) theoretical ESP efficiency, nominal heat otput (right Y-axis)

0.733

1.218

2.007

3.007

4.433

7.288

reduced heat otput, ESP on nominal heat otput, ESP on theoretical ESP efficiency, reduced heat otput (right Y-axis)

Figure 5. ESP efficiency data

Both the measured fractional efficiency data and ESP SCA parameter were used to obtain the fractional effective drift velocities, which are shown as the empty squares in Figure 6. The results obtained for the 40–1000-nm particles meet the theoretical drift velocity due to a combination of measurement errors and unavoidable secondary processes with the particles suspended in the ionised gaseous medium. The latter acts on 18-nm particles in the form of nucleation, and particles larger than 7 µm in the form of coagulation.

Drift velocity [m/s]

1

0.1

Effective drift velocity Theoretical drift velosity Obtained drift velocity 0.01 0.01

0.1

1

10

Dp [µm]

Figure 6. Evaluation of ESP by comparing the required effective, calculated, and measured drift velocities

Journal Pre-proof

13

4. Conclusions The issue of reducing PM emissions from small-scale boilers was resolved by encasing the ESP inside the 15-kW automatic boiler, which was successfully tested and met the Ecodesign Directive that will be enacted by the EU. When the ESP was operating, the PM concentrations in the boiler emissions were 38 and 32 mg/m3 under nominal and reduced boiler heat outputs, respectively. These values satisfy the limit set by Ecodesign, which is 40 mg/m3. All of these concentrations were determined under the following conditions: 0 °C, 101.3 kPa; at reference O2 = 10 %vol. The proposed analytical optimisation method was based on the principles of electrostatic precipitation, and does not depend on the source and characteristics of emissions. Therefore, the optimisation method of ESP can be applied for any type of boiler, heat output level, and type of fuel. The introduced model can promote the implementation of ESPs and suppress emissions from small-scale boilers, which would aid in reducing global pollution. The designed method has low labour intensity and is not applicable for small-scale ESP, it also provides a useful perspective in the engineering of new industrial ESPs and optimising the efficiency of existing ESPs. The industrial application of the presented method allows us to indicate how the capacity of installed ESP is utilised to control emissions and evaluate the possibility of reducing power consumption. However, the calculated results did not always match the measured data. The particle drift velocity evaluation does not reflect the influence of real processes in ESP, such as electric wind and the turbulence of flowing gases; the processes between the particles and gaseous media must also be considered. The charging calculation method also does not consider the physics of the charging process of actual particles. Therefore, future research must focus on the following: - numerical modelling of electrostatic precipitation should be conducted considering the charging process of low-scale boiler particles with combined field and diffusion mechanisms, - accurately modelling particles precipitation by including the secondary processes in the ESP electric field.

Acknowledgements The authors gratefully acknowledge the financial support of the Ministry of Education, Youth and Sports in the framework of National Sustainability Programme I (project "Innovation for Efficiency and Environment – Growth", identification code LO1403) and the project "Research on the identification of combustion of unsuitable fuels and systems of self-diagnostics of boilers combusting solid fuels for domestic heating" (identification code CZ.02.1.01/0.0/0.0/18_069/0010049) with financial support from the European Regional Development Fund.

References 2015/1189, C.R.E., 2015. Commission Regulation (EU) 2015/1189 of 28 April 2015 implementing Directive 2009/125/EC of the European Parliament and of the Council with regard to ecodesign requirements for solid fuel boilers. Berhardt, A., Lezsovits, F., Gross, B., 2017. Integrated Electrostatic Precipitator for Small-Scaled Biomass Boilers. Chemical Engineering & Technology 40(2), 278-288.

Journal Pre-proof

14

Bianchini, A., Cento, F., Golfera, L., Pellegrini, M., Saccani, C., 2016. Performance analysis of different scrubber systems for removal of particulate emissions from a small size biomass boiler. Biomass and Bioenergy 92, 31-39. Bianchini, A., Pellegrini, M., Rossi, J., Saccani, C., 2018. Theoretical model and preliminary design of an innovative wet scrubber for the separation of fine particulate matter produced by biomass combustion in small size boilers. Biomass and Bioenergy 116, 60-71. Bologa, A., Paur, H.-R., Ulbricht, T., Woletz, K., 2010. Particle Emissions from Small Scale Wood Combustion Devices and their Control by Electrostatic Precipitation. Aaas10: Advanced Atmospheric Aerosol Symposium 22, 119-124. Chamnong, C., Chuayin, C., Nganpitak, T., Sontayananon, N., Kittiratsatcha, S., Pattanadech, N., 2018. Computation Program for Breakdown Voltages of a Gas Insulation under Different Pressures and Slightly Nonuniform Electric Fields, 2018 15th International Conference on Electrical Engineering/Electronics, Computer, Telecommunications and Information Technology (ECTI-CON). pp. 704-707. Chow, J.C., 2006. Introduction to the A&WMA 2006 Critical Review - Health effects of fine particulate air pollution: Lines that connect. Journal of the Air & Waste Management Association 56(6), 707-708. CPC, 2018. TSI 3775 Condensation Particle Counter. https://www.tsi.com/discontinuedproducts/condensation-particle-counter-3775/. 2018). Dekati, L., 2019. The Dekati® ELPI®+ (Electrical Low Pressure Impactor). https://www.dekati.com/products/elpi/. Deutsch, W., 1922. Bewegung und Ladung der Elektrizitätsträger im Zylinderkondensator. Ann. Der Physik, 68. EN303-5:2012, Heating boilers - Part 5: Heating boilers for solid fuels, manually and automatically stoked, nominal heat output of up to 500 kW - Terminology, requirements, testing and marking. EN13284-1:2017, Stationary source emissions – Determination of low range mass concentration of dust – Part 1: Manual gravimetric method. EPA, EPA Method 5G - Determination of particulate matter emissions from wood heaters (dilution tunnel sampling location). Fritz, H., Kern, W., 1990. Reinigung von Abgasen (Flue gas cleaning). Vogel Communications Group GmbH & Co. KG, Würzburg. Hinds, W.C., 1999. Aerosol Technology, Properties, Behaviour, and Measurement of Airborne Particles. John Wiley & Sons Inc., New York. Jin, H., Ling, L., Jianguo, L., Qidi, S., kuixu, C., Xiaoxin, Z., 2018. Research on the minimum allowable values of energy efficiency and standard of energy efficiency grades for precipitator in power industries. IOP Conference Series: Earth and Environmental Science 153, 042013. Johansson, L.S., Tullin, C., Leckner, B., Sjovall, P., 2003. Particle emissions from biomass combustion in small combustors. Biomass & Bioenergy 25(4), 435-446. Lagarias, J.S., 1963. Predicting Performance of Electrostatic Precipitators. Journal of the Air Pollution Control Association 13(12), 595-599. Leskinen, J., Tissari, J., Uski, O., Virén, A., Torvela, T., Kaivosoja, T., Lamberg, H., Nuutinen, I., Kettunen, T., Joutsensaari, J., Jalava, P.I., Sippula, O., Hirvonen, M.R., Jokiniemi, J., 2014. Fine particle emissions in three different combustion conditions of a wood chip-fired appliance – Particulate physico-chemical properties and induced cell death. Atmospheric Environment 86, 129-139. Levitov, V., 1980. Dymovyje elektrofiltry. Energija Moscow. McAllister, I.W., 2002. Electric fields and electrical insulation. IEEE Transactions on Dielectrics and Electrical Insulation 9(5), 672-696. Mihelcic, R.J., Julie, Z.B., 2014. Environmental Engineering: Fundamentals, Sustainability, Design, 2nd Edition ed. Wiley Global Education. Mirzabekan, G., 1969. Zaradka aerozolej v pole koronnogo razrada. Silnze elektricheskije pola v tehtologicheskyh processach. . Energija, Moscow. Molchanov, O., Krpec, K., Horák, J., Hopan, F., Ždímal, V., Schwarz, J., 2018. The Distinctive Changes of Particles´ Numeric Concentrations, are Caused by Electrostatic Flue Gases Cleaning, PROCEEDINGS OF 19TH ANNUAL CONFERENCE OF THE CZECH AEROSOL SOCIETY. Czech Aerosol Society, Piešťany.

Journal Pre-proof

15

Pauthenier, M.M., Moreaur-Hanot, M., 1932. La chargé les partikules sferiques dan sun champ ionise J. Phys. Radium 3(12). Pedersen, A., 1967. Calculation of Spark Breakdown or Corona Starting Voltages in Nonuniform Fields. IEEE Transactions on Power Apparatus and Systems PAS-86(2), 200-206. Pedersen, A., 1989. On the electrical breakdown of gaseous dielectrics-an engineering approach. IEEE Transactions on Electrical Insulation 24(5), 721-739. Petersen, H.H., 1986. PERFORMANCE OF ELECTROSTATIC PRECIPITATORS. Waste Management & Research 4(1), 23-33. Pope, C.A., 2000. Review: Epidemiological basis for particulate air pollution health standards. Aerosol Science and Technology 32(1), 4-14. Poskas, R., Sirvydas, A., Poskas, P., Simonis, V., Jankauskas, J., 2017. Investigation of the ESP Cleaning Efficiency of the Flue Gases in the Wide Range of Re numbers. Mechanika 23, 47-54. Schmatloch, V., Rauch, S., 2005. Design and characterisation of an electrostatic precipitator for small heating appliances. Journal of Electrostatics 63(2), 85-100. Senior, C.L., Panagiotou, T., Sarofim, A.F., Helble, J.J., 2000. Formation of ultra-fine particulate matter from pulverized coal combustion, American Chemical Society. Division of Fuel Chemistry. Ughov, V.N., Valdberg, A.U., 1981. Ochistka promyshlennych gazov ot pyli. Chimija Moscow. Verma, V.K., Bram, S., Gauthier, G., De Ruyck, J., 2011. Evaluation of the performance of a multi-fuel domestic boiler with respect to the existing European standard and quality labels: Part-1. Biomass & Bioenergy 35(1), 80-89. White, H.J., 1951. Particle Charging in Electrostatic Precipitation. Transactions of the American Institute of Electrical Engineers 70(2 ). White, H.J., 1963. Industrial Electrostatic Precipitation. Addison-Wesley Pub Co, U.S.A. White, H.J., 1977. Precipitator Design. Journal of the Air Pollution Control Association 27(3), 206-217. Zelikoff, J.T., Chen, L.C., Cohen, M.D., Fang, K.J., Gordon, T., Li, Y., Nadziejko, C., Schlesinger, R.B., 2003. Effects of inhaled ambient particulate matter on pulmonary antimicrobial immune defense. Inhalation Toxicology 15(2), 131-150.

Journal Pre-proof

Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: