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Radiation Physics and Chemistry 77 (2008) 490–496

www.elsevier.com/locate/radphyschem

Polycyclic aromatic hydrocarbons in coal combustion flue gas underelectron beam irradiation

Anna Ostapczuka,�, Janusz Lickib, Andrzej G. Chmielewskia

aInstitute of Nuclear Chemistry and Technology, Dorodna 16, 03-195 Warsaw, PolandbInstitute of Atomic Energy, 05-400 Otwock-Swierk, Poland

Received 28 May 2007; accepted 19 August 2007

Abstract

The electron beam (EB) technology has been investigated as a one-stage multi-component purification technology. The initial

concentrations of SO2, NOx, and 16 polycyclic aromatic hydrocarbons (PAH) in flue gas have been reduced simultaneously by

over 60%, 50%, and 20%, respectively, in flue gas at the dose of 8 kGy. Determined PAH distribution in the by-product has shown

negligible role of adsorption in PAH removal. PAH-based overall toxicity of flue gas decreased remarkably in the range of 30–80% under

EB irradiation.

r 2007 Elsevier Ltd. All rights reserved.

Keywords: Flue gas treatment; Electron beam irradiation; Polycyclic aromatic hydrocarbons

1. Introduction

Electron beam (EB) processing is already applied inindustrial scale for the treatment of flue gas from coalcombustion: full-scale EB installation is in operation inPoland for simultaneous DeSOx and DeNOx (Chmielewskiet al., 2004). The technology is based on radiationchemistry: in the presence of active species (OH, O, HO2)created under EB in flue gas, SO2 and NOx are transformedinto H2SO4 and HNO3. When ammonia is added to thegas stream these acids are transformed into mixture ofNH4NO3 and (NH4)2SO4 which is used as a fertilizer(Chmielewski et al., 2000). EB technology for volatileorganic compounds removal from flue gases has beeninvestigated since the late 1980s in order to apply thistechnology in industry (Paur et al., 1991; Hakodaet al., 1998; Han et al., 2003; Kim et al., 2003). Pilotplant tests have been carried out in Japan with the purposeof decomposing/detoxifying a dioxin stream in waste-incineration flue gas (Hirota et al., 2003). The results haveshown 80% decomposition ratio for 10 kGy dose applied

ee front matter r 2007 Elsevier Ltd. All rights reserved.

dphyschem.2007.08.002

ing author. Tel.: +4822 504 1368; fax: +48 22 504 1277.

ess: aostap@ichtj.waw.pl (A. Ostapczuk).

and over 90% for 14 kGy for the initial concentration ofdioxin on the level of 1 ng/m3.The influence of EB irradiation on polycyclic aromatic

hydrocarbons (PAH) as organic pollutants has beenpreliminarily investigated in Poland and results showedremarkable reduction of PAH concentration under EBinfluence (Chmielewski et al., 2002). The observed decreaseof PAH concentration in flue gas can be caused bythree processes: (i) PAH destruction in the oxidationprocesses (Biermann et al., 1985; Atkinson and Arey,1994), (ii) adsorption on the surface of by-products,and (iii) new compound formation in radical process(Gerasimov, 2005).PAH molecules demonstrate a strong tendency to

adsorb on the dust surface and in EB flue gas treatmentwith ammonia addition powdered fertilizer is produced.The precipitation process is initiated in irradiationchamber and PAH present in flue gas may be adsorbedon the surface of aerosols there, causing PAH concentra-tion to decrease in the gas phase. The clarificationof PAH adsorption role in investigated process istaken as one objective of this research work. In thiswork, pilot plant experiments were focused also on16 PAH homologues to find the tendency of how

ARTICLE IN PRESSA. Ostapczuk et al. / Radiation Physics and Chemistry 77 (2008) 490–496 491

the PAH structure influences the change of its concentra-tion under EB and, in consequence, overall flue gastoxicity. The general aim of this study is to demonstrateEB technology as a possible multi-pollutants purificationmethod.

2. Experimental

2.1. Facility

The experimental work has been carried out at EB pilotplant, placed at coal-fired electro-power station, installedon a bypass of the main flue-gas stream from boilerWP-120 (Fig. 1). The EB pilot plant was equipped withhumidifier, ammonia injection system, reaction chamberwith two accelerators installed on its top (500–700 keV,50 kW each), process monitoring system and electrostaticprecipitator (ESP) for collecting by-product (Chmielewskiet al., 2000).

A stream up to 10,000Nm3/h of combustion gas,pretreated in ESP1, was humidified in spray cooler toreach optimum humidity H ¼ 6–6.5 vol% and temperatureT2 ¼ 65 1C. The humidified gas was then mixed withammonia at the inlet to the cylindrical irradiation vesselof 1.6m diameter and 7m length. The amount of ammoniato be added was calculated on the base of by-productformation reaction stoichiometry for the actual SO2 andNOx concentration (monitored during pilot plant opera-

NH3

A

H2O

INLET

CO2, CO

T1 T2

SP

RA

Y C

OO

LE

R

SO2, NO (NOx)

O2, H2O dust H2O

PAH SP1

BOILER ESP 1

Fig. 1. The scheme of EB pilot plant and the arr

tion). In the irradiation chamber flue gas was treated bytwo EBs (2-staged irradiation) with energies in the range500–700 keV. EB energy absorbed in the flue gas, defined asabsorbed dose, has been determined by PCV foil stripsplaced inside the irradiation vessel in advance and duringexperimental work kept on the level of 8 kGy as anoptimum value for simultaneous DeSOx and DeNOx

(Chmielewski et al., 2000). Solid particles of salts NH4NO3

and (NH4)2SO4 formed in EB process were separated inESP2.The PAHs investigated are listed in Table 1. There are

five PAH homologue groups with different number ofaromatic rings and their toxicity equivalent factors (TEF)proposed by US EPA are also included (Safe, 1998).

2.2. Sampling and analytical procedure for PAH

Flue gas sampling and PAH determination: the sampleprobes, made of stainless steel, were installed at the inletand outlet of irradiation chamber. The probes wereequipped with two ceramic coaxial filters to removeparticulate matter from gas sample and kept at 150 1Cto avoid water condensation. Filtered gas was flownthrough the trapping system consisting of dry-ice-cooledcondenser trap and three-stage adsorption tubes section(2 XAD-2 resin+1 activated carbon). Sampling flow rateof 2 dm3/min was controlled by a rotameter, the volume ofsampled flue gas was about 300 dm3. The samples were

ES

P 2

CH

IMN

EY

CCELERATORS ELV - 3A

OUTLET

NH3

T3

T4

SO2, NO (NOx)

O2, H2O, dust

SO2, NO (NOx)

O2, H2O

BP SP3

IRRADIATION

CHAMBER

CO2, CO

PAH SP2

CO2, CO

dust

angement of PAH sampling points (SPs1–3).

ARTICLE IN PRESS

Table 1

Polycyclic aromatic hydrocarbons investigated—characteristics

PAH name Abreviation Stucture EPA TEF Number of rings Homologue group

Naphthalene NL C10H8 NA 2 AR 2 R (two-ringed)

Acenaphthylene ACL C12H8 NA 2 AR+1PR

Acenaphthene AC C12H10 0.000

Fluorene FL C13H10 0.000

Antracene AN C14H10 NA 3 AR 3 R

Phenanthrene PHE C14H10 0.000

Fluoranthene FA C16H10 0.000 3 AR+1PR

Pyrene PY C16H10 0.100 4 AR 4 R

Chrysene CHR C18H12 0.000

Benzo(a)anthracene BaA C18H12 1.000

Benzo(b,k)fluoranthene BfkFA C20H12 0.100 4 AR+1PR

Benzo(a)pyrene BaP C20H12 1.000 5 AR 5 R

Benzo(e)pyrene BeP C20H12 NA

Perylene PE C20H12 NA

Dibenzo(ah)anthracene DBahA C20H14 1.0909

Benzo(ghi)perylene BghiP C22H12 0.000 6 AR 6 R

NA, not available; AR, aromatic ring; PR, pentacyclic ring.

0

5

10

15

PA

H c

on

ce

ntr

atio

n [

ug

/Nm

3]

1 3

series

inlet

outlet

2 4

Fig. 2. Total PAH concentration measured at the inlet and outlet of

irradiation vessel.

A. Ostapczuk et al. / Radiation Physics and Chemistry 77 (2008) 490–496492

analyzed within 48 h after sampling to prevent degradationof collected organic material. Two-step extraction bydiethyl ether and toluene was applied, then receivedmixture was concentrated in Kuderna-Danish evaporatorand analyzed by GC/FID technique (GC-8160 Fisons,capillary column DB-5 30mL/0.32mmID/0.25 mm). Twointernal standards 1, 2, 3-thriphenylmethane and 9,10-diphenyloanthrancene were used for quantitative PAHfraction analysis.

PAH determination in by-product: samples of by-product powder were collected at the bottom of ESP 2every 30min (Fig. 1). Samples referred to the samesampling experiment were mixed and flooded bymethylene chloride within 48 h after sampling. The solventextraction was realized in an ultrasonic bath, thenanhydrous sodium sulfate was added to remove water.The compounds were separated by the use of columnchromatography with activated SiO2 as an adsorbentand the mixture of n-pentane and methylene chlorideas an eluent (Kubica et al., 1998). The sample concentra-tion and GC/FID analytical methods applied for by-product were analogous to the procedures for flue gassamples.

The measurement of by-product concentration in flue

gas: the concentration of solid salts (CP) formedin EB process has been determined at the inlet of ESP 2(applied as by-product separator) by optical dustmonitor SICK RM-41. Additionally gravimetric periodicmeasurement as the reference was applied. The concentra-tion was unstable in continuous EB plant operationdue to its dependency on the other process parameters,like e.g., flue gas flow, SO2 concentration, etc. (Chmie-lewski et al., 2002). The registered momentary CP valueswere averaged.

3. Results and discussion

3.1. PAH homologues under EB

The total PAH concentration measured at the inlet ofirradiation chamber ranged from 6.2 to 14.9 mg/Nm3 andits values after EB treatment at a dose of 8 kGy were17–36% lower (Fig. 2). Simultaneously the initial SO2

concentration ranging 362–420 ppm was reduced remark-ably by over 60%; for NOx inlet concentration of105–122 ppm dropped down to 50% of this value at theoutlet of irradiation vessel.

ARTICLE IN PRESS

0.0

2.0

4.0

6.0

8.0

10.0

CO

NC

EN

TR

AT

ION

[ug/N

m3]

SERIE 2

INLET

OUTLET

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

CO

NC

EN

TR

AT

ION

[ug/N

m3]

2 R 3 R 4 R 5 R 6 R 2 R 3 R 4 R 5 R 6 R

2 R 3 R 4 R 5 R 6 R 2 R 3 R 4 R 5 R 6 R

SERIE1

INLET

OUTLET

0.0

1.0

2.0

3.0

4.0

5.0

CO

NC

EN

TR

AT

ION

[ug/N

m3]

SERIE 3

INLET

OUTLET

0.0

1.0

2.0

3.0

4.0

CO

NC

EN

TR

AT

ION

[ug/N

m3]

SERIE 4INLET

OUTLET

Fig. 3. PAH homologues distribution at inlet and outlet of irradiation chamber.

A. Ostapczuk et al. / Radiation Physics and Chemistry 77 (2008) 490–496 493

The differences in observed PAH removal efficienciescan be caused by various homologues distributions in fluegas before irradiation. The inlet homologues distribution,presented in Fig. 3, shows that in all series 4 R fraction wasdominating.

PAH concentration decreased regularly in the caseof high-ringed homologues 4–6R. Observed increaseof low-ringed PAH concentration was consequentlyfollowed by simultaneous decrease of higher-ringedPAH content in flue gas. This phenomenon suggeststhat low-ringed PAH may be formed as products ofheavier PAH fragmentation, but it is still probablethat they are formed in ongoing radical processes inirradiated flue gas. The results of pilot plant experimentsare sufficient only for general discussion about thepossible pathways of PAH decomposition. It is consideredthat some main pathways of PAH decomposition initi-alization under EB are analogical to the atmosphericchemistry: pollutants are transformed through reaction

with active species like OH radicals (Atkinson andArey, 1994) but in EB process other reactions are alsopossible, like e.g., charge transfer reactions (Midey et al.,2000):

PAHþMþ ! PAHþ þM,

where M+ can be N+, N2+, O+, O2

+, or NO+ formed inprimary ionization of flue gas molecules by EB.

3.2. EB processing influence on overall flue gas toxicity

The decrease in PAH concentration under EB wasdirectly related to the flue gas overall toxicity (TEQ). TEQvalue, based on 16 investigated PAHs, was calculatedaccording to Eq. (1) as the sum of concentration ci andtoxicity equivalence factor (TEFi) products for each PAHmeasured in experimental work (Safe, 1998):

TEQ ¼X

ciTEFi. (1)

ARTICLE IN PRESS

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

E T

EQ

1 3

EPA Chu & Chen Nisbet & LaGoy

2 4

series

Fig. 4. Relative decrease of overall PAH-related toxicity of flue gas after

EB treatment.

0.684

0.094

3.329

1.9252.029

0.677

0.00

0.50

1.00

1.50

2.00

2.50

3.00

3.50

Cp

[u

g/k

g]

PH

E

AN

FA

Ba

P

Be

P

PE

2.27

0.00

0.50

1.00

1.50

2.00

2.50

Cp

[u

g/k

g]

AC

L

0.095

1.035

0.821

0

0.00

0.20

0.40

0.60

0.80

1.00

1.20

1.40

1.60

1.80

Cp

[ug

/kg

]

AN

FA

CH

R

Serie 1

Fig. 5. PAHs distribution in by-product sam

A. Ostapczuk et al. / Radiation Physics and Chemistry 77 (2008) 490–496494

TEQ calculated on the basis of experimental data at theinlet (TEQi) and the oulet (TEQo) allows to estimaterelative overall toxicity decrease (ETEQ):

ETEQ ¼ ðTEQi� TEQoÞ=TEQi. (2)

The change of TEQ under EB has been evaluated basedon three TEF scales proposed by different research groups:US EPA, Chu&Chen, and Nisbet&LaGoy (EuropeanCommission, 2001).In all cases observed TEQ value of flue gas at the oulet of

irradiation chamber was lower than at the inlet (Fig. 4).The minimum relative decrease at the level of 30% wasobserved for series 1 when US EPA factor was applied, butconsidering the other two TEF scales, TEQ relativedecrease reached over 80%. The results for the otherexperimental series show that TEQ reduction calculated onthe basis of three TEF categorizations are close and in theranges of 0.35–0.55, 0.70–0.75, and 0.35–0.45 for series 2–4,

6

0.896

0.125

2.178

1.114

0.484 0.458

1.177

0.792

1.210

0.400

PH

E

AN

Ba

A

CH

R

Bb

kF

A

Ba

P

Be

P

PE

Db

ah

A

Bg

hiP

.084

1.092

0.711

0.534

0.092

1.678

Bb

kF

A

Ba

P

Be

P

PE

Db

ah

A

Bg

hiP

Serie 2

Serie 3

ples determined in three sampling series.

ARTICLE IN PRESS

Table 2

Experimental data and calculated PAH adsorptive removal coefficient Y in EB process

SERIE A CF (mgPAH/mg) CP (mg/Nm3) CA (mgPAH/Nm3) (Ci�Co) (mgPAH/Nm3) Y

1 8.738� 10�6 377.21 0.0033 1.265 0.0026

2 11.110� 10�6 264.71 0.0029 5.388 0.0005

3 6.142� 10�6 500.51 0.0031 1.996 0.0015

A. Ostapczuk et al. / Radiation Physics and Chemistry 77 (2008) 490–496 495

respectively. The observed TEQ decrease can be explainedas the consequence of high-ringed PAH concentrationdecrease. According to the results of bioassay these heavierPAH homologues are characterized as more toxic andhigher TEF values are dedicated for them (Safe, 1998). Dueto the fact that observed PAH-based TEQ decrease wasfollowed by simultaneous SO2 and NOx removal, EBtechnology can be considered as a benefit one-stage multi-pollutants flue gas cleaning technology.

3.3. The role of adsorptive mechanism in PAH removal

To estimate the share of PAH adsorptive removal in EBflue gas processing, a coefficient Y has been defined:

Y ¼ CA=ðCi � CoÞ, (3)

where CA—adsorptive PAH removal from flue gas [mgPAH/Nm3]; Ci—PAH concentration in flue gas at the inlet ofirradiation chamber [mgPAH/Nm3]; and Co—PAH concen-tration in flue gas at the outlet of irradiation chamber[mgPAH/Nm3].

Adsorptive PAH CA was determined by the use offollowing equation:

CA ¼ CPCF, (4)

where CP—by-product (fertilizer) concentration down-stream of irradiation chamber [mgF/Nm3] and CF—PAHconcentration in fertilizer [mgPAH/mgF].

PAH distribution in by-product samples (Fig. 5) showsthat compounds adsorbed on by-product belong mainly to5-ringed homologues (BaP, BeP, PE in 3 series, DbahA andBbkFA in series 2 and 3). The other homologues wereidentified in selected samples: 2-ringed ACL appeared onlyin series 2; 3-ringed AN and PHE in all 3 series, FA inseries 1; 4-ringed PAH were represented by CHR in series 2and 3, and BaA in series 2. The presence of heavier PAH inby-product can be explained as related to their very lowvapor pressure (below 10�5 Pa at 25 1C).

Total concentration of 16 PAH determined in by-product fertilizer (CF) samples was relatively low, rangedfrom 6.142 to 11.11 mg/kgp (Table 2). The adsorptive PAHremoval CA, calculated using Eq. (4), allows to establish Yvalue which appeared very low proving negligible impor-tance of adsorptive PAH removal from flue gas treated byEB (Table 2).

In consequence, it can be concluded that the reduction ofPAH concentration is caused mainly by oxidation processinitialized by irradiation. Some decomposition pathways

are analogous to the processes taking a place in theatmosphere but much more rapid and effective due tomuch higher concentration of active species such as OHformed in flue gas ionized by EB (Matzing, 1991). Howeverthe other, alternative decomposition pathways specific toEB application, e.g. PAH secondary ionization by primaryions like N+, Nþ2 , O

+, etc., or reaction with O radical mustbe also considered.

Acknowledgement

The work presented in this paper has been partiallysupported by International Atomic Energy Agency in theframe of POL-13136 project.

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