PhD Nataliia Limanska
email: limanska@onu.edu.ua

Andrii Merlich

Maryna Zlatohurska

Mykola Galkin

Nadiia Korotaieva

Tetiana Ivanytsia

Yvan Choiset

Volodymyr Ivanytsia

Thomas Haertlé

BACTERIOCIN ASSOCIATED GENES IN FRENCH AND UKRAINIAN FERMENTED VEGETABLE ISOLATES OF LACTIPLANTIBACILLUS PLANTARUM

Keywords: Lactiplantibacillus plantarum, plantaricin, pln loci, diversity, geographical areas

INTRODUCTION

MATERIAL AND METHODS

Bacterial strains

L. plantarum originated from grape collected in France (13 isolates) and from grape and pickles collected in Ukraine (17 isolates, Tab 1) were studied for the presence of pln genes.

RESULTS AND DISCUSSION

Our results are in agreement with the data indicating that pln loci are widespread in oenological L. plantarum [START_REF] Sáenz | Genetic diversity of the pln locus among oenological Lactobacillus plantarum strains[END_REF]Knoll et al., 2008). However, French and Ukrainian isolates shared none of the identical combinations of genes from plantaricin regulon indicating the diversity of pln loci in L. plantarum strains of fermented plant origin from distant geographical regions (Tab 2).

Lactiplantibacillus plantarum species is known for the high diversity of the loci encoding bacteriocin (plantaricin) production. The aim of this work was to study the distribution of pln genes in French and Ukrainian isolates of L. plantarum originated from fermented plant material. PCR with the primers to plnA, plnB, plnC, plnD, plnEF, plnI, plnK, plnJ, plnG, plnN, plnW genes was performed to find out the peculiarities of L. plantarum strains. French and Ukrainian isolates shared none of the identical combinations of genes from plantaricin regulon indicating the diversity of pln loci in L. plantarum of fermented plant origin from the distant geographical regions. Certain combinations of genes from plantaricin regulon could be proposed as indicators marking the geographical origin of L. plantarum strains.

Among the studied isolates, some resembled the described in the literature L. plantarum strains, and others contained unique combinations of pln genes.

CONCLUSION

French and Ukrainian strains of L. plantarum of plant origin shared no identical combinations of pln genes showing the high diversity of pln loci in isolates from distant geographical regions. Certain combinations of genes from plantaricin regulon could be proposed as indicators marking the geographical origin of L. plantarum strains.

Among the studied isolates, some resembled those described in the literature for L. plantarum strains, while others encoded the unique combinations of pln genes.

  Lactiplantibacillus plantarum species is characterized by the high diversity of the strains (Martino et al., 2016; Choi et al., 2018; Araujo et al., 2020) including genetic organization of pln loci encoding bacteriocin production (Basa et al., 2020; Choi et al., 2021; Wu et al., 2021). As the result, bacteriocins belong to the most variable sub-categories of secretome (Martino et al., 2016). All pln loci are similar in the presence of a two-component regulatory system consisting of a histidine protein kinase and response regulators, in encoding inducible peptides, proteins of an ABC transport systems and some other products (Saenz et al., 2009; Diep et al., 2009). Synthesis of plantaricins is mediated by quorum-sensing (Kwak et al., 2016). Phylogenetic analysis of 54 L. plantarum strains from different geographical zones including 22 strains of plant origin, showed the presence of three groups encompassing strains with various combinations of pln genes (Choi et al., 2021). Martino et al. (2016) analyzed draft and complete genomes of 54 L. plantarum strains and found that the strains with the same geographical origin could share similar orthologous groups in their genomes as it was described for L. plantarum isolated from Vietnamese fermented fruits, vegetables and pickled meat products. At the same time, no correlation between the habitat origin of L. plantarum strains (animal or plant organisms, products) and genetic specificity was found (Martino et al., 2016; Choi et al., 2018). We have previously described the absence of association between geographical origin of the L. plantarum strains isolated from grape must and pickles in France and Ukraine by means of their RAPD-profiles (Limanska et al., 2019). The aim of this work was to study the distribution of pln genes in French and Ukrainian isolates of L. plantarum.

Table 1

 1 Origin of L.

plantarum strains Strain Source of isolation Country of origin L. plantarum ONU 351, ONU 352, ONU 353, ONU 354, ONU 355, ONU 356, ONU 357, ONU 359, ONU 362, ONU 363, ONU 364, ONU 365, ONU 471

		Grape must	France
	L. plantarum ONU 12, ONU 311, ONU
	312, ONU 313, ONU 333, ONU 335,
	ONU 337, ONU 339, ONU 340, ONU	Grape must	Ukraine
	342, ONU 345, ONU 348, ONU 349,
	ONU 350
	L. plantarum ONU 472, ONU 475	Sauerkraut	Ukraine
	L. plantarum ONU 476	Fermented pickled tomatoes	Ukraine
	PCR

amplification of the genes from pln loci

	DNA from overnight cultures of L. plantarum was isolated using Qiagen DNA
	purification kit (Germany). PCR was carried out with the primers to plnA, plnB,

plnC, plnD, plnEF, plnI, plnK, plnJ, plnG, plnN, plnW

genes as described and reviewed in Ben Omar et al. (2008) according to the wide distribution of these genes in fermented vegetable material (

Ben Omar et al., 2008; Rojo-Bezares et al., 2008; Sáenz et al., 2009; Diep et al., 2009; Choi et al., 2021).

Table 2

 2 Presence of pln genes in genomes of the French and Ukrainian isolates of L. plantarum Thus, 16 unique combinations of pln genes were found among French and Ukrainian isolates -8 in each geographical group. Within each geographical group, 2-5 strains could share identical set of pln genes; however, no combinations were identical in L. plantarum from distant geographical areas. The complete plnABCD locus was detected in 15.4% of the French isolates, and in 36.3% of Ukrainian L. plantarum (Fig 1). Pln E/F was found in 30.8% of the French strains and in 94.1% of Ukrainian isolates. Complete operon pln EFI was detected in 30.8% of the French isolates and 53% of the Ukrainian strains. plnK was detected in 100% of French strains, and in 65% of Ukrainian isolates. Strains ONU 12, ONU 311, ONU 312 and ONU 313 could resemble the described L. plantarum J23 strain (

	Strains and their origin	plnA	plnB	plnC	plnD	plnEF	plnG	plnI	plnJ	plnK	plnN	plnW
	France
	351, 352, 357, 471	-	+	-	+	-	-	+	-	+	+	-
	353, 354	-	+	-	+	-	+	+	-	+	+	-
	355	+	+	+	+	-	+	+	-	+	+	-
	356, 365	-	+	+	+	-	+	+	-	+	+	-
	359	-	+	-	+	+	+	+	-	+	+	-
	362	+	+	-	+	+	+	+	-	+	+	-
	363	-	+	+	+	+	+	+	-	+	+	-
	364	+	+	+	+	+	+	+	-	+	+	-
	Ukraine
	12, 311, 313	-	-	-	+	+	+	+	+	-	+	-
	312	-	-	-	+	+	+	+	+	+	+	-
	333	+	+	+	+	+	-	-	-	+	+	-
	335	-	+	+	-	+	-	-	-	+	+	-
	337	-	+	+	-	+	-	-	-	+	-	-
	339	-	+	+	-	-	-	-	-	+	+	-
	340, 472, 475, 476	-	+	+	+	+	-	-	-	+	+	-
	342, 345, 348, 349, 350	+	+	+	+	+	+	+	+	+	+	-

A B Figure 1 Percentage of L. plantarum strains originated from France (a) and Ukraine (b) The regulatory operon plnABCD encodes an inducing pheromone peptide PlnA, a histidine protein kinase PlnB and the response regulators PlnC and PlnD (Anderssen et al., 1998; Diep et al., 2009; Havarstein et al., 1995; Kleerebezem et al., 2003). Our strain L. plantarum ONU 362 containing plnA, plnB, plnD genes but missing plnC by the combination of the studied genes was identical to L. plantarum EC80 strain from fermented maize described by Ben Omar et al. (2008). Gene plnN indicating the presence of plnMNOP operon was found in 100% of French, and in 94.1% of Ukrainian strains (Fig 1). Strains L. plantarum ONU 355 and ONU 364 from France and ONU 333, 340, 345, 348, 349 and ONU 350 from Ukraine contained the complete plnABCD operon and the gene plnN which allowed to characterize them as C11-and WCFS1like strains. Strains WCFS1 and C11 with an exception of two genes are similar in pln loci (Diep et al., 2009; Kleerebezem et al., 2003). Sáenz et al. (2009) investigated strains isolated in Spain from must and wines for the presence of 27 pln genes and clustered them in groups with well described strains C11, WCFS1, J51, J23 and NC8. The group of the strains resembling C11 and WCFS1 in composition of pln regulon was the largest -60,6%. Strain L. plantarum ONU 337 with the absent plnN gene could resemble the strain L. plantarum J51 described in the literature (Diep et al., 2009; Navarro et al., 2008).

Rojo-Bezares et al., 2008) by

  absence of plnABCD and plnK. Gene plnJ was present in 53% of Ukrainian isolates, among them both genes of plnJKLR operon -plnJ and plnKwere found in 35.3%. In French isolates plnJ genes were absent (Fig1). Loss of the sequences of some genes resulting in negative PCR result could probably happen due to integration of mobile genetic elements found in the pln clusters (

Evanovich et al., 2019; Choi et al., 2021).

  The absence of plnJ in ONU 333, 355 and 364 was unusual for the WCFS1 and C11 types of plantaricin regulons. The similar situation was described in Sáenz et al. (2009) for an oenological L. plantarum strain missing plnJ and moreover, missing plnD but with the similarity > 89% ranged in one group with C11 and WCFS1. Two strains from fermented vegetables isolated in Korea were also characterized by the presence of plnK and absence of plnJ (

Choi et al., 2021).

  Operons plnEFI and plnJKLR encode two-peptide bacteriocins, PlnEF and PlnJK and their immunity proteins (

Anderssen et al., 1998; Diep et al., 2009; Havarstein et al., 1995; Kleerebezem et al., 2003).

  Among Ukrainian strains, 76.4% carried genes encoding two bacteriocins -plnEF and plnJK. Strains with the potential synthesis of two types of bacteriocins are more adaptable to the environment and indeed, they are more abundant among the strains of fermented food origin[START_REF] Choi | Distribution of bacteriocin genes in the lineages of Lactiplantibacillus plantarum[END_REF]. Use of such strains is the most perspective for probiotic and food production. plnG from plnGHSTUVW operon was found in 69.2% of the French strains, and in 53% of Ukrainian isolates (Fig1). Five from all the tested L. plantarumstrains -ONU 342, 345, 348, 349 and 350 (of Ukrainian origin) carried all the tested genes except the plantaricin W (Tab 2). our study describing various combinations of pln genes confirm the data of the previous investigators about the high genetic diversity of L. plantarum species (

	The results of
		100%	plnN
			plnJK
		100%	plnK
			plnJ
	69,2%		plnG
	30,8%		plnEFI
	30,8%		plnEF
	15,4%		plnABCD
			plnN
	35,3%	94,1%	plnJK
	65%		plnK
	53%		plnJ
	53%		plnG
	53%		plnEFI
			plnEF
	36,3%	94,1%	plnABCD

None of the studied French and Ukrainian strains encoded the plnW. The plantaricin W structural gene encoding two-peptide bacteriocin PLW was initially described for the strain L. plantarum LMG 2379 isolated from fermenting wine in the USA (Holo et al., 2001). Our results coincide with the previously reported rare occurrence of this gene in L. plantarum (Diep et al., 2009; Ben Omar et al., 2008), although strains encoding plnW were also described (Sogandi et al., 2019).

Saenz et al., 2009; Diep et al., 2009; Ben Omar et al., 2008; Sogandi et al., 2019).

  Although we could not find differences between L. plantarum originated from distant geographical areas in our previous work based on RAPD-analysis (Limanska et al., 2019), study of distribution of pln genes allowed us to distinguish the unique genetic combinations in French and Ukrainian isolates.

Acknowledgments: This work was supported by a bilateral French-Ukrainian project "Dnipro" granted by the Ministry of Foreign and European Affairs of France and the Ministry of Education and Science of Ukraine, and by the individual grant for Andrii Merlich (Campus-France) supported by the French Embassy in Ukraine.


P Atten G2elab
email: pierre.atten@grenoble.cnrs.fr

Collection of fine particles in electrostatic precipitators : influence of EHD induced agitation and of particles disintegration

Keywords: electrostatic precipitation, fine particles, electrohydrodynamics, EHD turbulence, electrical disintegration of aggregates

The paper focuses on two particular phenomena which contribute to the poor collection efficiency of fine particles (from ~ 0.1 µm to ~ 2 µm) by electrostatic precipitators (ESPs). After considerations on the influence of turbulence which tends to decrease the collection rate, a conjecture is recalled on the action of the charged fine particles which is expected to give the main contribution in the generation of small eddies in ESPs.

Results are presented of an experimental study of the collection efficiency η of cigarette smoke. The efficiency is found to decrease as the dust concentration is increased and the efficiency decrease is attributed to the increase in eddy diffusivity. The second phenomenon observed in the study is the disintegration of agglomerates in the ESP. A marked agglomeration process occurs because the smoke is kept in a big balloon during a few minutes; this agglomeration is more and more marked as the smoke concentration is raised. When charged in the ESP, some of the agglomerates disintegrate under the Coulomb repulsion, which increases the apparent collection efficiency η of the bigger particles (> ~ 0.6 µm) but clearly decreases η for the smaller ones (< ~0.5 µm).

Introduction

Industrial one-stage electrostatic precipitators (ESPs) typically consist in a series of grounded parallel plates through which the gaseous suspensions to be treated are flowing. Equally spaced ionising electrodes raised at high negative potential are located midway between the plates. The collection of particles involves the ionisation of the gas, the charging of particles, their migration towards the collecting electrodes and the removal of the dust layers from the collection surfaces. Most industrial ESPs treating very large fluxes of flue gases have collection efficiency in terms of mass higher than 99 or 99.9 % [START_REF] Parker | Electrostatic precipitation[END_REF]. However the non-captured part consists mainly of fine particles as can be seen on Fig. 1 which shows the penetration, i.e. the proportion of particles issuing from the ESP. These fine particles are hazardous for health and more stringent regulations concerning air pollution control make it necessary to substantially increase their removal.

Figure 2 -Normalised size distributions in mass and Figure 1 -Measurement on industrial sites of in number of ashes collected in the last (4 th ) field of penetration versus the particle diameter. (From [START_REF] Plaks | VI ICESP[END_REF]).

the ESP of a coal-fired power plant. (From [START_REF] Blanchard | [END_REF]). In order to improve the efficiency of ESPs, one has to first characterise the various phenomena which contribute to make the collection efficiency η f so low and to estimate their relative influence.

The first and well known reason for the low collection efficiency of fine particles is the small value of their drift velocity with respect to the gas. The second factor which influences η f is the degree of turbulence of the gas flow; although the influence of this factor is recognised, the origin of the "turbulence" is not so clear and authors retaining the electrical forces usually generally consider the action of the field on the ionic space charge only. Here, we examine the effect of the space charge associated with fine particles. The third phenomenon which might play a non negligible role, at least in the case of flue gases issuing from coal-fired power plants, is the disintegration of agglomerates.

Particles charging and drift velocity

The particles in flue gases of coal-fired power plants have a size extending over four decades, from typically 20 nm to 200 µm, with the largest mass corresponding to particles of diameter d p ranging from 10 to 100 µm [START_REF] Chenfeng | [END_REF]. The size distribution is characterised by several modes depending on the coal properties. Generally there is a mode of typical size on the order of tens or hundreds of micrometers [START_REF] Chenfeng | [END_REF]; these large particles originate from the ash mineral inclusions in the coal and are practically totally collected by the ESPs. Most often another mode has d p values in the range of micrometers as can be seen on Fig. 2 relative to the ashes collected in the last field of the ESP of a coal-fired power plant in France (the peak observed in the number distribution curve of collected particles is at a d p value -≅ 0.7 µm -different from the one characterising the fine ashes at the inlet of the ESP which collected the sample). The submicron particles in the flue gas have a size distribution centred at about 0.1 µm and they are dendritic clusters consisting of partly sintered primary ultra-fine particles of diameter ~ 20-30 nm [5]; the generation of ultra-fine primary particles is attributed to complex chemical processes giving volatilised suboxides or metals vapours which then reoxidise and lead to particles through homogeneous nucleation [5].

The low efficiency of ESPs for fine particles (with d p ranging from 0.1 to 2 µm) is mainly due to the rather low drift velocity w E of these particles with respect to the suspending gas. Two mechanisms are usually distinguished for particle charging. The first one is the field charging where the unipolar gaseous ions are assumed to follow the field lines and impact the particle. The saturation charge q p,sat acquired by the particle is proportional to the field and to the square of the particle diameter d p [6]. This picture is valid when the mean free path of ions is fully negligible compared with the sphere radius. For small enough particle size, the charging mechanism by diffusion of ions is dominant. The simple theoretical approach of White [START_REF] White | Industrial electrostatic precipitation[END_REF] leads to a charge value q p proportional to d p and having a logarithmic dependence on the time. In the diameter range 0.1 to 2 µm, the two mechanisms interplay and the sum of both contributions gives an upper bound for the particle charge (Fig. 3). In practice, in this size range, as a first approximation many authors retain the empirical formula of Cochet [START_REF] Cochet | Proc. Colloque Intern. n° 102 "La physique des forces électrostatiques et leurs applications[END_REF] : Under a field E the particles with charge q p experience the force F e = q p E which tends to move them toward the collecting electrodes. Neglecting the inertial effects (typical time ~ µs), the drift velocity with respect to the gas is determined by the viscous force F v compensating for F e ; as the molecular mean free path λ g is not much smaller than the particles radius, the friction force is smaller than in a continuum and the Stokes expression has to be divided by the Cunningham factor Cu [START_REF] White | Industrial electrostatic precipitation[END_REF]. The particle drift velocity w E then is proportional to the field E and has a non trivial dependence on the diameter d p as can be seen on Fig. 4. In typical conditions for ESPs, w E for fine particles (d p from 0.1 to 2 µm) ranges between 4 and about 10 cm/s. These values are much lower than the axial mean velocity of the gas flow and this is the main reason why it is difficult to collect the fine particles (there is a clear correlation between Figs. 1 and4).

Influence of gas turbulence

The flow of the flue gas in the ducts is always turbulent because the Reynolds number Re ~ 10 4 is higher than the critical value Re crit ≅ 2000. But the turbulent rate measured in ESPs is clearly higher than for the pure flow itself [START_REF] Leonard | [END_REF]. This is not surprising : there are electrical forces acting on the space charge and we will see in §4 how these forces can generate a strong agitation of the gas.

The fine particles convected by the mean flow are very sensitive to the velocity fluctuations of the gas. Indeed, for particles with w E < 10 cm/s, the fluctuations of the gas velocity which are high enough can entrain them in the direction opposite to the electric force at some places. We are facing an intricate problem which can be simplified by using the concept of eddy diffusion of the particles [10] characterised by the coefficient D t . The "mixing" property of turbulence results in a tendency to make the distribution of particles concentration more uniform, in particular in the z direction normal to the collecting plates. The Deutsch model considering a uniform distribution c in the z direction is equivalent to an infinite value of the z-component of turbulent diffusivity (Fig. 5). The model developed by Leonard et al. [10] is based on finite and uniform value of the eddy diffusivity further assumed to be isotropic. In this case, the z distribution of particles concentration c becomes more and more uniform as D t is increased (see the qualitative illustration of Fig. 5).

In the turbulent models of particles collection, a boundary layer is considered where the diffusivity of particles steeply decreases down to zero (gas velocity = 0 on the plates). Assuming that each particle reaching the plate is captured, the flux of collected particles is proportional to the product c(x,z=d) E(x,z=d) where c(x,z=d) is the particles concentration on the plate. From Fig. 5 it is clear that higher turbulence intensity will result in lower collection efficiency. In practice, the collection efficiency also depends on the magnitude of the electric field. The pertinent parameter controlling the particles collection rate is the electric Peclet number Pe defined by [10] : Peclet number.

t p E p D d d w d Pe ) ( ) ( = (1)
D t → ∞ d z c(x 0 , z) ∧ ∧ ∧ ∧ (D t ) 2 > (D t ) 1 (D t ) 1 O
Pe reflects the effect of eddy diffusivity which tends to counteract the action of the field inducing the drift velocity w E (and driving the particles towards the collecting plates). Increasing the field E and, therefore, the drift velocity of particles, is equivalent to decreasing the eddy diffusivity D t . The influence of Pe is qualitatively illustrated by Fig. 6.

Turbulence generation by fine particles

The secondary flow induced by the electric force density distribution associated with the ionic space charge density (in the absence of charged particles) is noticeable and clearly contributes to the turbulence generation by producing eddies of typical size ~ d (d : half width of the ducts), except in the case of negative wires due to the tuft corona inducing smaller size eddies. However this fact is not taken into account in most numerical simulations of ESPs which retain only the k-ε scheme for turbulence distribution and evolution. Now, the charge density of particles very likely plays the primary role in ESPs by generating eddies of small size.

Generally an intense movement of the fluid is induced by the action of the field E on the space charge of density ρ resulting from corona effect in gases and from ions injection by one of the electrodes in liquids [11]. The primary electrohydrodynamic (EHD) phenomenon in this case of injected space charge is the strong positive coupling between fluctuations u′ of the fluid velocity field u and perturbations of the space charge density ρ [12]. This coupling leads to a potentially unstable situation for a planar fluid layer with uniform unipolar injection. The motion sets in when the action of the electric force of density ρ E overcomes the viscous damping, which leads to the following relation for the first fundamental non dimensional EHD parameter T [12] :

c appl T K V T > = µ ε (2)
here ε denotes the fluid permittivity, V appl the applied voltage, K the mobility of charge carriers, µ the dynamic viscosity and T c is a critical value. For ions in air at ambient temperature, typically (V appl ) c ≅ 40 kV but for charged fine particles, we would have (V appl ) c ~ a few hundreds of volts [12,13]. It is therefore clear that the presence of charged fine particles can easily induce instabilities and, in practice, can create eddies of rather small size.

The fluid agitation contributes to the transport of the charge carriers; its effect is important when the typical fluctuations of fluid velocity u′ are high enough compared with the drift velocity of charge carriers w E = KE. The second basic non dimensional EHD number M defined by :

/ K M f ρ ε = (3)
(here ρ f stands for the fluid density) is the pertinent parameter to compare the velocities u′ and KE [11,12]. For M <~ 1 (u′ << KE), the trajectories of the charge carriers are only very slightly perturbed by the fluid motion which has no significant influence on the distribution of ρ and on the passage of electric current (case of ions in air : w E ~ 100 m/s >> electric wind ~ 1 m/s). If, conversely, M >~ 3 (KE < u′), the charge carriers are entrained by the fluid flow that they contribute to create and there is a significant increase of mean current density. Injected ions in dielectric liquids give the typical example of this situation [11,12]. The transition between the two regimes occurs for M = 3 [17]. The two parameters T and M are also pertinent for more complex EHD flows as obtained in asymmetric electrode configurations [13].

In the typical conditions of ESPs, the estimate of the charge acquired by the particles and of their mobility leads to estimates of the mobility parameter M, relative to the particles, which of course depend on the particles size [13]. For fine particles charged under a typical mean field E ~ 5 kV/cm, M > 3 [13]; therefore charged fine particles in gas have a behaviour analogous to that of ions in insulating liquids and a turbulent mixing which partly redistributes the particles charge density can be expected. The intensity of the generated turbulence will also depend on the concentration of charge carriers and this gives a way to test the pertinence of the proposed picture of the turbulence being mainly generated by the charged fine particles.

The above considerations are very general and concern problems with only one type of charge carriers. In ESPs there are two types of charge carriers : ions and charged dust particles which both contribute to the space charge and therefore have distributions which are inter-related. For sustained corona discharges in gases, from Kaptzov hypothesis, the field E wire on the ionising wire(s) is equal to the value it takes at the inception voltage V th of corona discharge. For a clean gas (no dust), the global charge density total ρ ~ affecting the field distribution is due to ions only and we have :

( ) ( ) ( ) th appl 0 ions ions clean total V V - ∝ = = ~ ~ ~ρ ρ ρ (4)
where ions ρ ~ is the global charge density associated with the ions. For a dusty gas, the field E wire on the wire remains the same and is not affected by the dust concentration c 0 at the ESP inlet; we thus obtain:

E j E j , p p p ions ions ions K K ρ ρ = = (6)
where K ion , ρ ion and K p , ρ p are the ions and particles mobilities and charge densities respectively. As the ratio K p /K ion ~ 10 -3 , for a dusty gas we have :

( )

p 0 ions ions ions I I ρ ρ ρ ~ ~ ~ - ≅ ∝ ≈ (7)
and the current should decrease with the dust concentration c 0 . In practice the relative current decrease should give an estimate of the mean space charge associated with the charged particles :

I I I 0 ions p - ≅ ~ρ ρ (8)
For fine particles, except just around the wires, the particles drift velocity is lower than the gas velocity so that the distribution of ρ p is controlled mainly by the flow which, itself, is determined by the total space charge distribution ρ ion + ρ p . When ρ p becomes comparable to ρ ion a marked change in the mean gas flow pattern is expected along with a significant rate of turbulence induced by ρ p .

Influence of particles concentration on collection efficiency

Experimental set-up

The study of fine particles collection was carried out using a laboratory ESP characterised by a duct of width 2d = 9 cm and height h = 29 cm. The test cell of total length L = 100 cm is divided in two sections of 50 cm in length. Initially a set of 9 equidistant ionising wire electrodes was used to create the space charge in the first section. In the second field, a plate can be introduced and polarized to create a uniform field of collection E c in the two half-ducts. The efficiency of particles collection is determined from concentrations measurements with and without applied field, using an optical counter (TOPAS LAP320) on scales ranging from 0.3 µm to 20 µm.

The study has been performed with cigarette smoke filling a balloon of about 2.5 m in diameter (~ 8 m 3 ). Starting with the empty balloon, the air pushed by a fan flows through a paper filter, then through a box where some cigarettes are burning and progressively fills the balloon. Once the balloon is full, the accumulated smoke is directed to the test cell. As the optical counter has a lower bound for the measured diameter of 0.31 µm, French cigarettes (trademark : Gauloises) were used which generate particles of not too small size (99% of particles with d p < about 1.05 µm). Fig. 7 shows the size distributions for different numbers of burnt cigarettes. For 8 and 16 burnt cigarettes, the curves exhibit a maximum for d p ≅ 0.4 µm and 0.55 µm respectively. The increase of mean size of particles results from an agglomeration of smaller particles.

Results

With the set of 9 vertical equidistant ionising wires (φ = 1 mm) perpendicular to the horizontal air flow of mean velocity U 0 = 0.4 m/s (E 2 = 0 in the second section), the measurements were performed by filling the balloon and counting the particles for successive sequences with and without the voltage applied on the wires (typical results are shown in Fig. 8). The fractional efficiency of collection of particles η f as a function of their size d p has been determined from the distributions as shown in Fig. 8 for five smoke concentrations. The major interesting fact indicated by Fig. 9 is the clear tendency of η f to decrease as c 0 is increased. Note that the concentration influence is most marked in the range 0.3 to 0.5 µm and that the low η f values obtained for the smoke generated by burning 16 cigarettes partly arise from the disintegration of charged agglomerates under the influence of Coulomb repulsion. function of c 0 (9 ionising wires)..

The efficiency decrease with the particles concentration is presumably due to the increase in secondary flow and turbulence intensities. The results appear to be consistent with those obtained by Podlinski et al. [18]. In particular a marked decrease in discharge current was obtained as shown in Fig. 10. This indicates that for the highest smoke concentrations, the space charge associated with the charged particles represents 30 to 40% of the total space charge.

Particles disintegration

In order to have η f measurements in conditions comparable to those of ref.

[18], the set of 9 wires was replaced by a unique ionising wire (φ = 0.5 mm) located at x 0 = 21 cm from the inlet. The measurements of size distributions without and with applied voltage were performed for a mean velocity of the gas flow U 0 = 0.4 m/s, V appl = 23.5 kV and various smoke concentrations. The curves of fractional collection efficiency η f (Fig. 11) clearly show that, in the range 0.3 to 0.5 µm, η f tends to decrease when the particles concentration c 0 is increased; conversely it tends to increase for d p > ~ 0.6 µm. An interesting observation is that for high concentrations (8 and 16 burnt cigarettes), negative values of η f are obtained, which means that, at the outlet, there are more -very fine -particles than at the inlet of the test cell : as can be seen on Fig; 9, for 16 burnt cigarettes the concentrations of particles of diameter ranging from 0.3 µm to about 0.4 µm are higher after the action of the corona discharge (V appl = 23.5 kV) than at the ESP inlet. Fig. 7 clearly indicates that the mean size of the particles increases as the concentration of the smoke is increased. This implies a noticeable agglomeration of the fine particles in suspension. Now, when an aggregate captures electric charges (here positive ions), these charges distribute over the different elements constituting the aggregate which then experiences a Coulomb repulsion so that it can disintegrate. This process of disintegration of aggregates explains why there are so many small particles issuing from the ESP and also why the relative number of the biggest particles at the outlet decreases when the smoke concentration is increased (this corresponds to an increase in effective fractional collection efficiency η f ). The process of dispersion of aggregates was also present in the first series of experiments with the set of 9 wires ( §5.2). But the addition of the influences of the successive wires partly damped the effect clearly exhibited by the unique wire experiment. Nevertheless, there remains a noticeable effect of the dispersion of aggregates in the range d p = 0.3 to 0.4 µm as can be seen on Fig. 9. c 0 values (applied uniform field E 2 = 0.5 kV/cm).

In order to eliminate the influence of disintegration of aggregates when determining the fractional collection efficiency, we performed measurements in the second section of the test cell where the field is uniform, where there is no particle charging (no ionic space charge) so that, presumably, no disintegration of aggregates occurs. A voltage V appl = 20 kV was applied on the wire in the first section of the ESP and the mean air velocity was again U 0 = 0.4 m/s. In the second section of the ESP a field E 2 was applied and the efficiency was determined from the size distributions without and with field E 2 . The results shown on Fig. 12 for E 2 = 0.5 kV/cm confirm that the efficiency decreases with the smoke concentration for the whole diameter range in which the measurements are significant. The difference of these results with those relative to the first section of the test cell ( §5.2) is clear and can be ascribed to the absence of disintegration of aggregates in the second section. For all diameter values d p < ~ 1 µm, the efficiency η f decreases as the particles concentration is increased. This is consistent with the picture of the turbulence intensity increasing with the mean charge density associated to the charged fine particles.

Conclusions

This study performed on cigarette smoke shows that there is an important effect of agglomeration of fine particles when the smoke concentration is high enough. The mean size of aggregates increases with c 0 , which supports the picture of agglomeration of slightly sticky particles through collisions due to their Brownian motion. The rather unexpected phenomenon we observed is the disintegration accompanying the charging of the aggregates by the ions created by corona discharges. Such a disintegration process implies rather limited adhesion forces between the particles constituting the aggregates. Extrapolation of the existence of such a disintegration process in industrial ESPs is not possible because of the very large variety of the particles collected in these ESPs. We can just suggest that this process might play a role in electrostatic precipitation and might partly explain the difficulties in collecting the fine particles.

By taking into account the influence of disintegration of aggregates occurring in the fist section of our ESP, the presented measurements show that the fractional collection efficiency η f of the particles decreases as their concentration c 0 is increased. From the general considerations on the effect of turbulence on collection efficiency ( §3) illustrated by Fig. 6, the decrease of η f with c 0 implies that the corresponding electric Peclet number Pe also decreases. The results concerning the influence of dust concentration on collection efficiency appear fully consistent with the observations and measurements of gas flow which show a modification of the mean flow pattern and an increase in turbulence intensity [ ]. In addition to their interest concerning the collection of fine particles in ESPs, the results on efficiency decreasing as the dust concentration is increased provide an indirect proof of the increase of the turbulence rate and give a support to the conjecture that an important source of turbulence in ESPs is "intrinsic", through the action of the space charge associated with the fine particles (characterised by a low mobility).

  diameter d p (µm) mass number

Figure 3 -

 3 Figure 3 -Charge of spherical particles (in elementary Figure 4 -Drift velocity of spherical particles charges e) as a function of their diameter (ε r = 4, T = as a function of their diameter (ε r = 4, T = 403 K (130 °C), E = 4 kV/cm, t charging = 1 s). 403 K (130 °C), E = 4 kV/cm, t charging = 1 s).

Figure 5 -

 5 Figure 5 -Concentration of charged particles between the Figure 6 -Variations of the fractional collection symmetry plane and the collecting plate (at distance x 0 efficiency as a function of the electrical from the inlet) for different D t values.Peclet number.

ρ

  ~being the global space charge density of particles. The total current I also has two components, I ion and I p ( I = I ion + I p ); the current densities write :

Figure 7 -

 7 Figure 7 -Size distributions (in number) of particles of Figure 8 -Size distributions of smoke particles "Gauloises" cigarettes smoke for different numbers (16 burnt cigarettes) for two applied voltages of burnt cigarettes.(9 ionising wires in the first section).

Figure 9 -Figure 10 -

 910 Figure 9 -Fractional efficiency of collection η f versus Figure 10 -Ratio of the currents I for a flow of particles diameter for various smoke concentrations smoke and I 0 for filtered air (U 0 = 0.4 m/s) as a c 0 (9 ionising wires, V appl = 26 kV).function of c 0 (9 ionising wires)..

Figure 11 -Figure 12 -

 1112 Figure 11 -Fractional efficiency of collection η f (d p ) in