I. INTRODUCTION

Power plants are one of the most important emitters of atmospheric contaminants. It refers equally well to both conventional coal-fired and biomass co-firing plants. Recently, biomass is added to coal in order to decrease additional emission of carbon dioxide, assuming that the same amount of CO2 will re-circulate between boiler - atmospheric air - and plants. To biomass belong all organic products originating from agriculture, forestry or food industry, which are suitable for burning. With co-fired biomass, the chemical composition and physical properties of fly ash significantly change that generates new operational and maintenance problems with cleaning flue gases by electrostatic precipitators. These problems have not been sufficiently extensively dealt in the scientific publications. Some of the reports indicate that short-term operational properties of electrostatic precipitators do not change significantly when a few percents of sawdust is co-fired with bituminous coal [1], however, a long-term effect on the electrostatic

precipitator has not been identified yet. In the last decade, several new constructions of electrostatic precipitators, including collection and discharge electrodes, have been proposed in order to increase overall collection efficiency, increase fractional efficiency in submicron size range or prevent back discharge [2-5], however, with only minor or no attention to the cleaning flue gases from biomass firing. To our opinion, in this decade the research will be aimed at increasing the energy efficiency of electrostatic precipitators, removal of submicron particles (PM1) including heavy metals, and remedy measures for detrimental effects caused by biomass co-firing. One of such solutions that answer these questions is the combined removal of fly ash and abatement of noxious gases, proposed over twenty years ago [3, 5, 6].

The purpose of this paper is to outline some new aspects of the effect of biomass co-firing on the short- and long-term operational properties and maintenance of electrostatic precipitator. The considerations are based on preliminary reports met in the literature and some experimental results of the authors.

Biomass Co-firing -New Challenge for Electrostatic Precipitators-

A. Jaworek1, 3, M. Jędrusik2, A. Świerczok2, T. Czech3, A. T. Sobczyk3, and M. Lackowski3

1Institute of Physics, Pomeranian Academy in Słupsk, Poland 2Institute of Heat Engineering and Fluid Mechanics, Wrocław University of Technology, Poland

3Institute of Fluid Flow Machinery, Polish Academy of Sciences, Poland

Abstract—Power plants are the most important sources of atmospheric pollutants. It refers equally well to

conventional, coal-fired, and to biomass co-firing plants. The composition of flue gas from biomass co-firing differs from that emitted by coal fired boilers. Particularly, there are an increased number of particles in submicron size range and the number of particles larger than 100 m, and increased emission of CO and tar leaving the boiler. These new after-burning products can decrease the overall collection efficiency of electrostatic precipitator (ESP), increase emission of submicron particles, and cause degradation of the construction elements of precipitator. The construction elements are degraded due, for example, to chlorine or sulfur corrosion of metal elements, or their contamination with tar. Tar present in flue gases after biomass burning can, after longer time, be deposited onto HV insulators, changing their electrical properties, leading to increasing leakage current or even to the surface breakdown. It was reported in the literature that increased amount of CO in flue gases, or increased amount of unburned coal could result in fires in ESP. The long-term effects of biomass co-firing on the electrostatic precipitator are, however, not known. Although there are an increasing number of publications on troubles encountered in boilers co-firing biomass, there is still low number of papers on experimental results regarding the changes in performances of ESPs cleaning flue gases from biomass burning. These new problems arising in electrostatic precipitators result from the fact that the precipitators were designed for cleaning flue gases from coal burning boilers. Different chemical composition of biomass results in different chemical composition and physical properties of flue gases and fly ash to be removed.

The positive and negative effects of biomass co-firing on the operation of electrostatic precipitators are discussed in the paper. The paper indicates future directions in the investigation of operational properties of electrostatic precipitators cleaning flue gases after co-firing biomass with coal or firing various kinds of biomass in boilers. It is concluded that further research should be aimed at investigating the collection efficiency of electrostatic precipitator, maintenance of the precipitators, emission of submicron particles, heavy metals and tar as well as contamination and degradation of electrodes and insulators due to increased amount of tar, chlorine and sulfur compounds. The goal of this paper is to turn an attention of scientists and engineers to these specific problems.

Keywords—Electrostatic precipitator, biomass, co-firing, gas cleaning

Corresponding author: Anatol Jaworek e-mail address: jaworek@imp.gda.pl Presented at the 12th International Conference of Electrostatic Precipitation, ICESP, in May 2011 (Nuremberg, Germany)

161Jaworek et al.

II. FLY ASH PROPERTIES

The main components of fly ash emitted from coal-

fired boilers are SiO2 (roughly 1/2), Al2O3 (1/4) and Fe2O3 (1/8) [8]. Other elements, which content is larger than 1 mg/g each, are Ca, Na, Mg, K, Ti, S [9-11]. The particles can exist in the form of oxides or chlorides. The ratio of these elements can vary depending on the mine the coal was taken from.

In chemical composition of fly ash from biomass co-fired boilers, the percentage of the SiO2, Al2O3, and Fe2O3 components decreases because the alkaline compounds (CaO, MgO, Na2O, K2O, P2O5) originating from co-fired biomass appear [12-15]. The minerals comprise about 8 wt.% in coal, but about 6 wt.% in straw, 2.8 wt.% in willow, and 0.3 wt.% in beech [16]. The biomass ash components, mainly salts of Ca, Na, Mg, K, Ti cause boiler corrosion [17]. It can be expected that they will also cause degradation of construction elements of ESP, for example, chlorine or sulfur corrosion of metal elements, degradation and contamination of HV insulators, etc.

The elemental composition of fly ash can be determined, for example, by Energy Dispersive X-ray spectroscopy (EDX) method. An example of EDX spectra of fly ash from bituminous coal and coal co-fired with biomass are shown in Fig. 1a and Fig. 1b, respectively. The height of each peak is proportional to the abundance of the element. The investigated samples were taken from pulverized-fuel boiler in Gdansk Power Plant EC-2 (Poland). The atomic concentration in

percents of elements in fly ash from bituminous coal and coal co-fired with biomass is shown in Figs. 2a and 2b, respectively. The main elements, of weight percentage larger than 0.1%, found in the fly ash from bituminous coal are Oxygen, Silicon, Aluminum, Carbon, Potassium, Calcium, Sodium, Magnesium, Chromium, Titanium, Sulfur, Phosphorus, and Iron. Similar results are obtained for the fly ash from coal fired boiler and coal co-fired with biomass, but of different proportions (Fig. 1b). The most noticeable are the increased level of carbon, which can be a result of unburned biomass, and decreased percentage of silicon and aluminum. Trace elements, like heavy metals (Mercury, Lead, Thallium) of excitation energy larger than 8 eV, although detected in fly ash at lower levels, are not shown in this spectrum.

The size of fly ash particles from the coal-fired boilers is in the range of 20 nm to 200 m [18]. Two distinctive ranges in particle size can be distinguished: smaller than 1 m, which constitutes only 1% of total mass but 99% in number size distribution, and larger than 1 m [19]. The submicron particles are mainly formed from the compounds of Ca, Fe, Al, Mg, Si [13, 20], and small amount of compounds of K, S, Mn, Cl [15]. Submicron particles are formed as a result of bursting of larger ones due to explosion of gas present within the

(a)

(b)

Fig. 1. EDX spectrum of fly ash from bituminous coal (a) and coal co-fired with biomass (b). Pulverized-fuel boiler in Gdansk Power

Plant EC-2, year 2002 (a), year 2009 (b).

(a)

0

10

20

30

40

50

60

70

Carbon

Oxygen

Sodium

Aluminium

Silicon

Phosphorus

Sulfur

Potassium

Calcium

Titanium

Iron

Atomic concentration [%]

(b)

0

10

20

30

40

50

60

70

Carbon

Oxygen

Sodium

Aluminium

Silicon

Magnesium

Sulfur

Potassium

Calcium

Titanium

Iron

Atomic concentration [%]

Fig. 2. Elemental composition of fly ash from bituminous coal (a) and the coal co-fired with biomass (b). Pulverized-fuel boiler in

Gdansk Power Plant EC-2, year 2002 (a), year 2009 (b).

162 International Journal of Plasma Environmental Science & Technology, Vol.5, No.2, SEPTEMBER 2011

mineral components of the fuel. These particles absorb volatile elements before condensing and bear most of the metals released from the fuel [10].

The mean mass density of fly ash is about 2.45103 kg/m3 [8, 21], and is close to the density of the main component of fly ash - SiO2 (2.2103 kg/m3). The total emission of particulate matter decreases after the addition of biomass, but, at the same time, increases the number of submicron particles, which are difficult to remove by electrostatic precipitators [20, 22-24].

The differences in fly ash morphology from burned bituminous coal and the coal co-fired with biomass are compared in Figs. 3 and 4. Scanning electron microscope (SEM) micrographs of fly ash from bituminous coal taken from the third stage of electrostatic precipitator in Gdansk Power Plant EC-2 show that particles larger than 1 m are spherical (Fig. 3). Particles smaller than 0.1 m are irregular in shape and firmly adhere to the larger ones. The larger particles are formed from fused silica with alumina. In the fly ash from bituminous coal co-fired with biomass the particles are irregular (Fig. 4a) or basket-like (Fig. 4b), of the size of 100 m or larger, which are probably formed from unburned biomass. These baskets are frequently filled with smaller particles, which are the mineral components of coal. The aerodynamic density of the basket-like particles is much lower than the mineral components, mainly SiO2, and these particles are difficult to remove in the first and second stages of ESP.

The equilibrium of Stokes drag force and electrostatics force on a fly ash particle of the radius r charged to the Pauthenier limit in the electric field E gives the terminal velocity of the particle:

22 2

0

r

rErw

(1)

where is the dynamic viscosity of the gas, 0 is the permittivity of the free space, and r is the dielectric constant of the particle. From this equation results that the particle velocity is proportional to the particle radius. That should facilitate the migration of larger fly ash particles towards the collection electrode and increase the collection efficiency compared to the smaller ones. However, equation (1) is valid for solid spherical particle or undistorted droplet. Particles emitted after biomass firing are not smooth, have many voids, and for this reason it is difficult to determine the drag force for such particles theoretically. The process of charging of such irregular particles has also not been recognized. The charge of biomass-burned particles would also be changed because of different dielectric constant of the material. It is therefore evident that biomass generated particles are not easy to remove in the first and second stages, and they pose a real challenge for ESPs.

It is, therefore, evident that charging and deposition in ESP the particles from biomass, is a complex process, which for a long time will be difficult to modeling. It seems that experimental methods will be the only way to

learn about the particle removal within electrostatic precipitator. One of such method, of principal significance, is PIV measurement of particles velocity and visualization of their trajectories during the deposition onto collection electrodes [25, 26].

The emission of particles from burned biomass decreases when the moisture content in the fuel is in the range of 15-25% (sawdust) [27]. However, too high moisture content causes a decrease in the temperature in boiler that results in not complete burning of biomass and higher emission of fly ash and tar. On the other hand, lower water content causes volatilization of lighter hydrocarbons, which are exhausted to the atmosphere [27]. Moisture originating from biomass would, therefore, load electrostatic precipitator with increased level of fly ash and tar.

Gaseous chlorides are predicted to be decreased and gaseous hydroxides increased with the increasing moisture content [14]. With an increase in moisture in fuel, decreases the content NaCl and increases HCl due the hydrogen provided from H2O, which reacts with chlorine [28]. The increased content of HCl can be dangerous for the construction elements of ESP. In boilers co-firing biomass, Na and K released during burning are volatilized at higher temperatures and are deposited onto boiler inner surfaces that results in faster corrosion of the boiler parts [28]. An effect of Na, K and Cl on the corrosion in electrostatic precipitators has not been investigated yet.

III. HEAVY METALS EMISSION The emission of heavy metals is another issue in coal

burning. The heavy metals detected in bituminous coal are Se, As, Cd, Hg, Ni, Pb, Cr, Sr, Be, V and U. They are mainly in the form of submicron particles, which are particularly difficult to remove [9, 11, 21, 29-31]. The heavy metal concentration in the particles smaller than < 100 nm, can be even 50 times higher than in the large particles [32].

Biomass also contains various trace elements, for example, Zn, Cd, Cu, Co, Cr, Pb, and Hg, which nucleate to form particles concentrated in the size range < 100 nm [32]. The content of these elements depends on the biomass type and source. In the case of biomass co-firing, heavy metals are chemically bounded in the bottom ash at a temperature of 700oC or higher. For example, only 0.8-4% of Cd has been volatilized (at 830oC) while the rest was bounded within the fly ash particles. Pb and Cu were bounded in the bottom ash (Cu: 28-30%, Pb: 14-16%). Volatilized Zn was at the level of 0.1-0.3% [33]. The results of investigation the effect of biomass co-firing on heavy metals emission are not conclusive: an addition of biomass can increase as well as decrease the emitted heavy metals, depending on many circumstances, such as the content of moisture, chlorine and sulfur in the biomass. For example, heavy metals can react with chlorine during the combustion process, leading to the formation of metal chlorides (Co, Cu, Fe, Mn, Ni, Pb,

163Jaworek et al.

Zn), which are more volatile than the elemental metals or metal oxides [14, 32]. Laboratory experiments showed that the addition of chlorine increased the content of heavy metal salts and enhanced their transfer into the gas phase [32].

Hg and Cd are principally in the gaseous elemental form at all temperatures in boiler and for all moisture levels. About 2% of Hg is in the HgO form. Cd after leaving the boiler condensates entirely onto fly ash

particles, which can be removed by ESP, however, gaseous Hg is not captured [28].

It can be concluded from this section that the problem of heavy metal removal is similar for coal-fired and biomass co-fired exhausts. An increase of moisture when biomass is co-fired can, under certain conditions, decrease the emission of heavy metals with fly ash. Alkali metals like Ca, Na and K have a stronger affinity to chlorine than heavy metals that can result in forming alkaline salts and volatilization of heavy metals [32].

IV. GASEOUS COMPOUNDS EMISSION

In pulverized-fuel boilers, nitrogen is mainly emitted as NOx from nitrogen originating from the air and fuel. Additionally HCN is emitted after coal burning, but after biomass burning, much higher level of NH3 than HCN is exhausted [33, 34]. The emission of NOx and SO2 decreases after the addition of biomass to the coal due to lower sulfur and nitrogen contents in biomass and high content of alkali metals, such as Ca, K, Na, which are easier oxidized than N and S [1]. In the case of co-firing of coal and biomass, the conversion of sulfur to SO2 is inversely proportional to biomass percentage. The emission of SO2 decreases for coarser biomass particles because part of sulfur remains in the bottom ash [33].

In electrostatic precipitators, flue gas conditioning is usually required at too low concentration of SO3 and SO2, which are reduced by CaO and MgO, in order to increase fly ash conductivity. At too low conductivity of fly ash, the collection efficiency decreases due to back discharge. Back discharge is a type of electrical discharge that occurs at plate electrode covered with fly ash of resistivity, higher than about 1010 m [35-40]. Back discharge generates ions of polarity opposite to those emitted from the discharge electrode that decrease the charge magnitude on fly ash particles, and decrease the collection efficiency of ESP. Experiments carried out by [41] on a test electrostatic precipitator designed for cleaning flue gas from biomass combustor have shown that fly ash from straw pellets combustion contains salt particles, which are less conductive and after deposition onto collection electrode can provoke back discharge. This effect has not been noticed for wood ash. This experiment indicates that operational properties of ESP significantly depend on the kind of biomass burned. The effect of biomass on the CO emission, and the role of CO on the collection efficiency of electrostatic precipitators are still controversial. The results presented in. [33] indicate a decrease in the CO emission in boilers co-firing forest-wood waste, while reported in [42] that biomass causes an increase of CO concentration. Probably the differences depend on the kind of biomass used. Fires within the ESP due to too high concentration of CO have also been reported. The effect of CO concentration on the collection efficiency and operational parameters of ESP still require further investigations.

Fig. 3. SEM micrograph of fly ash from bituminous coal. Pulverized-fuel boiler in Gdansk Power Plant EC-2, year 2002.

(a)

(b)

Fig. 4. SEM micrographs of fly ash from bituminous coal co-fired with biomass. Pulverized-fuel boiler in Gdansk Power Plant EC-2,

year 2009.

164 International Journal of Plasma Environmental Science & Technology, Vol.5, No.2, SEPTEMBER 2011

V. TAR EMISSION

Tar is a class of organic compounds (mixture of

condensable hydrocarbons) of molecular weight larger than molecular weight of benzene [43, 44]. Biomass burning is a severe source of light hydrocarbons and tar, which can influence the operation of electrostatic precipitator. Electric-discharge generated plasma can be favorable to the condensation of light hydrocarbons to heavier ones. Such heavy hydrocarbons can form a coating on the insulators and electrodes, which is difficult to remove. Results of experimental investigations indicated that the collection efficiency of an electrostatic precipitator cleaning flue gas after wood combustion decreased from about 80% to below 20% after 5 hours of operation due to high content of tar [45]. Incomplete burning of wood formed a difficult-to-remove deposit on insulators and electrodes. Most of the particles were deposited just under the discharge electrodes as a result of highest electric field and discharge current density at this place. Metal corrosion caused by tar present in flue gases has also been reported [44].

It is therefore evident that novel inventions solving the problem of tar deposition without degrading operational properties of ESP are an urgent need. One of such solutions is using wet electrostatic precipitator or scrubbing exhaust gases with water. However, experiments showed that only about 30% of gravimetric tar is removed by water scrubber. More than two times higher removal efficiency was obtained by scrubbing with vegetable oil, which has lower contact angle to tar [46].

Van Paasen et al. [47] reported on the results of operation of a wet electrostatic precipitator cleaning a biogas, and concluded that the contamination of electrodes was insignificant after 200 h of operation. The concentration of tar at the inlet of precipitator was from 0.9 to 2.2 g/m3, and it dropped to 0.4-0.8 g/m3 at the outlet. It can be supposed that more than half of tar was precipitated within the ESP but it could also contaminate the insulators.

Besides several case studies, there lack systematic investigations providing sufficient experimental data on the effect of biomass-originating tar loading on the operational properties and maintenance of electrostatic precipitators.

VI. AN EFFECT OF BIOMASS CO-FIRING ON THE COLLECTION EFFICIENCY OF ELECTROSTATIC

PRECIPITATORS

Preliminary laboratory tests on the effect of biomass

co-firing on the collection efficiency of a model electrostatic precipitator have been carried out by Jędrusik [48]. Fly ash from a coal-fired fluidized boiler (sample C), fly ash from the same coal with 10% of co-fired biomass (sample B), and 50% of biomass (sample W) were used in the experiments. The size distributions of fly ash particles and their chemical composition do not

differed much in each case, with only increased percentage of K2O (by about 1 wt.%) and SiO2 (by about 4 wt.%) in the fly ash from co-fired biomass. Measurements of the collection efficiency of laboratory-scale electrostatic precipitator with double-spike discharge electrode (Fig. 5b) are presented in Fig. 5a. The collection efficiency depends on the discharge voltage and the biomass percentage. The collection efficiency increases with an increase of the voltage supplying the discharge electrodes, but it is saturated for a certain voltage magnitude, in this specific case of about 50 kV. Further increase in the voltage causes a slight decrease in the collection efficiency. It was concluded that small addition of biomass (10%) to bituminous coal causes an increase in the collection efficiency, whereas for higher content of biomass, 50% or larger, the collection efficiency decreases. These preliminary results indicate that further research on the effect of co-fired biomass content on the collection efficiency is required in order to optimize the operational parameters of ESP.

20 30 40 50 60

Applied voltage, kV

0.7

0.8

0.9

1

Pre

cip

itatio

n e

ffic

ien

cy

ash B

ash C

ash W

10

42

12 a=95

Fig. 5. a) Collection efficiency of laboratory-scale ESP vs. voltage of discharge electrode for various fly ash sources: C fly ash from

burned bituminous coal, B fly ash from bituminous coal with 10% of co-fired biomass, W fly ash from bituminous coal with 50% of co-

fired biomass; b) Scheme of double-spike discharge electrode.

(a)

(b)

165Jaworek et al.

Similar experiments carried out by Savolainen [1] showed that no significant changes in the electrostatic precipitator performances during sawdust co-combustion can be noticed. The fly ash emissions were lower during the co-combustion of coal and sawdust than during the coal combustion. The author supposes that lower fly ash emission is probably an effect of low concentration of ash in the sawdust [1]. These ambiguous result show that further systematic research on the effect biomass addition to bituminous coal on the collection efficiency of electrostatic precipitator is still required.

VII. CONCLUSION

The real, known from various reports, and potential effects of biomass co-fired with coal on the operational parameters and maintenance of electrostatic precipitators have been discussed in the paper. This brief overview indicates that there still lack sufficient experimental data to answer the question which properties of flue gas from co-fired biomass are decreasing the collection efficiency of electrostatic precipitator, and which help to improve the quality of exhaust gases. Another issue is the effect of chemical composition of flue gas, different from that from coal fired boilers, on the contamination and degradation of the construction elements of electrostatic precipitator, including electrodes and insulators. The research in this field is also poor. The paper shows further directions for future investigations into the operation properties and maintenance of electrostatic precipitators used for cleaning flue gases from biomass fired or co-fired boilers.

The positive and negative effects of biomass co-firing on electrostatic precipitator operation are as follows:

Positive effects 1. Reduced emission of SO2 and NOx (due to Ca, K,

Na content and lower content of N and S in biomass). 2. Increased collection efficiency (larger particles,

ease of agglomeration). 3. Reduced emission of mineral particles (due to

lower content of minerals in biomass). 4. Reduced emission of heavy metals due to

increased content of moisture. Negative effects 1. Chlorine and sulfur corrosion (due to increased

acidity of gases). 2. Reduced collection efficiency (due to changes in

morphology and resistivity of fly ash particles). 3. Electrodes and insulators contamination (due to

increased tar emission). 4. Back-discharge ignition (due to increased content

of salts and reduced content of SO3). 5. Possible fires in ESP (due to increased emission of

CO and unburned carbon). 6. Increased emission of PM1 (due to higher

concentration of submicron particles).

The analysis was based on incomplete data presented in various papers on this subject and on the experience of the authors of this paper, but potential dangers are inferred from the publications on the effect of biomass co-firing on the maintenance of boilers and on the physical properties and chemical composition of exhaust gases. It should be noted that despite rich literature on the effect of biomass co-firing on changes in fly and bottom ash composition in boilers there still is only scarce research on this co-combustion on the collection efficiency and long-term performances of electrostatic precipitators. The further research should be aimed at the effect of biomass co-firing on the collection efficiency of electrostatic precipitator, maintenance of the precipitators, emission of submicron particles, heavy metals and tar as well as contamination and degradation of electrodes and insulators due to increased production of tar, chlorine and sulfur compounds. The research should be carried out both in laboratory as well as industrial scale. The goal of this paper was to turn an attention of scientists and engineers on these specific problems.

ACKNOWLEDGMENT

The research was supported by Polish Ministry of Science and Higher Education within the projects No. NR06-0014-06 and No. 3180/B/T02/2010/38. The authors are grateful to the Gdansk Power Plant EC-2 for delivering fly ash samples for research purposes.

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