DETERMINATION OF ELEMENTAL CARBON EMISSION WOJCIECH MNISZEK, JACEK WYPYCH and URSZULA ZIELONKA Institute for Ecology of lndustrial Areas, ul. Kossutha 6, 40- 844 Katowice, Poland. (Received: June 1993)

Abstract. The studies on elemental carbon content in the atmospheric air, performed at the air monitoring station in Katowice (Poland), have revealed violations of allowable maximum average annual and diurnal concentrations. Elemental carbon is introduced into the atmosphere mainly as soot generated from combustion processes. This work presents the determination of elemental carbon in emission generated from coal combustion processes.

1. Introduction Solid fuel combustion is one of the major sources of atmospheric air pollution by dust as well as by gaseous substances. Dust originating from solid fuel combusion processes is a mixture of inorganic volatile dust and organic substances absorbed on the surface of the dust particles. Elemental carbon (soot) can be classified as inorganic dust constituent. Soot is one of the allotropic carbon varieties, occurring in the form of fine black powder composed of graphite crystals. Among numerous organic compounds and their isomers contained in the dust, polycyclic aromatic hydrocarbons (PAH) are the most common group. Other compounds usually present in the dust are oxidized polycyclic aromatic hydrocarbons (OXPAH). The amount of soot introduced into the atmosphere, together with dust and flue gases after fuel combustion, depends on many factors, such as, among others: - fuel variety as well as its degree of pulverization, ash content, sinterability, volatile particle content, etc.; - combustion conditions, type of grate used, excess air coefficient, method of slag discharge, etc.; - ellectiveness of the control equipment. The problem of elemental carbon (soot) emission from solid fuel combustion processes has been hardly recognized and described so far, which is mainly the result of poorly developed methods for the determination of elemental carbon emission. Although elemental carbon is not a toxic substance, its allowable concentration in ambient air is strictly regulated (Polish Standard, 1990); in Poland the standard value of 50 #g/m 3 (24-h sampling) is obligatory, the yearly average Environmental Monitoring and Assessment 29" 41-52, 1994. (~) 1994 Kluwer Academic Publishers. Printed in the Netherlands.

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WOJCIECH MNISZEK ET AL.

TABLEI Concentration of elemental carbon in ambient air in Katowice (Poland) Gag/m3). month

1991 min.

1991 1991 1992 1992 1992 m a x . average m i n. m a x . average

01 02 03 04 05 06 07 08 09 10 11 12 average

44.0 48.0 18.0 14.0 28.0 17.0 12.0 13.0 8.0 6.0 5.0 9.0

70.0 75.0 51.0 54.0 61.0 70.0 45.0 42.0 28.0 38.0 38.0 27.0

55.6 58.0 33.3 29.5 45.7 35.3 29.4 25.2 17.0 24.6 20.4 15.6 32.5

37.0 41.0 7.0 13.0 7.0 7.0 8.0 9.0 7.0 37.0 32.0 10.0

65.0 72.0 38.0 40.0 40.0 55.0 52.0 39.0 33.0 72.0 69.0 62.0

49.2 50.6 17.3 29.0 20.7 25.5 24.4 22.4 17.0 48.5 42.0 45.0 32.7

concentration of 8 #g/m 3 is limited. Elemental carbon is a compound of high sorption capacity, and it is a carrier of many organic substances, as mentioned above. Due to its optical properties, especially its ability to absorb a wide sunlight spectrum, elemental carbon plays a negative role in the atmosphere, particularly in highly industrialized regions, where its concentrations are usually higher than other suspended dust constituents. Studies on elemental carbon content in the atmospheric air performed at the air monitoring station in Katowice have revealed violations of allowable maximum average annual and diurnal concentrations (Table I). In a previous paper (Mniszek and Kolarczyk, 1990) several well-known methods for determining elemental carbon in ambient air were described, and the best method was chosen by the authors. The present paper presents a method for the determination of elemental carbon emission which is based on the previously described method, adjusted to emission conditions.

2. Experimental With the purpose of allowing the development of the analytical method, dust sampled from several coal-fired power plants in the Katowice area was mixed and the average sample was used as a model dust. The dust mixing and sampling was carried out in compliance with a standard procedure (Polish Standard, 1987).

DETERMINATION OF ELEMENTAL CARBON EMISSION

43

Elemental carbon in the model dust was determined according to the following stages: - dust sampling at the emitter; - extraction of organic compounds from the dust; - decomposition of inorganic carbon compounds in the dust; - combustion of the remaining dust in a closed oxygen cycle; - chromatographic C02 determination; - calculation of elemental carbon concentration in the dust. The procedure of dust sampling at the emitter has not been described in the present paper, as only standard methods were applied. 2.1. EXTRACTIONOF ORGANIC CARBON COMPOUNDS The organic compounds contained in the dust were removed by solvent extraction, using a Soxhlet apparatus. The Soxhlet method consists in continuous extraction using a closed solvent cycle, with the extract subjected to constant condensation and the apparatus operating automatically. The studies on extraction of organic compounds from the dust were aimed at determining the best solvent as well as the process conditions. The dust samples for extraction were placed in tight extraction thimbles made of hard filtration paper, type Filtrak 390. The hard filtration paper had been cut into rectangular sheets, size 6 x 4 cm, which were next wound up into tubes of 0.5 cm diameter, with the bottom part of the tubes closed by squeezing and braking. The extraction thimbles prepared in such a way were then dried under vacuum at ambient temperature until they achieved constant mass. The dried and weighed extraction thimbles were filled with about 0.7 g of the examined dust, carefully tamped with a glass rod, and after that the upper sections of the tubes were squeezed and closed by braking the paper. In order to prevent accidental opening of the thimbles during the extraction process, the tubes were wound with cotton thread. The thread was weighed and its mass added to the mass of an empty thimble. The tightly closed extraction thimbles were dried under vacuum at ambient temperature until they achieved constant weight. Based on the difference between the filled and empty thimbles, the dust mass was calculated (tad). The thimbles containing the dust samples were placed in the upper part of the Soxhlet apparatus, while each time about 90 mL of a suitable solvent was placed in the bottom part. The apparatus was warmed up on an electric sand bath, with cold water being passed through the reflux condenser of the apparatus. After the required extraction time, the process was interrupted, after which the solvent extract was evaporated to dryness. The mass of organic compounds separated from the dust (raorg) was calculated from the mass difference. In order to find the best solvent for extraction of

44

WOJCIECH MNISZEK ET AL.

organic compounds from the dust coming from extraction processes, the following substances were investigated: benzene, toluene, benzene + methanol 3 : 1 v/v, methanol + chloroform 1 : 2 v/v, cyclohexane, dimethylformamide, pyridine. For each of these solvents, the mass of extracted organic compounds was determined at hourly intervals. The ratio between the mass of the extracted organic compounds and the mass of the input dust (rnorg/rnd) served as an indicator of the effectiveness of the extraction at a given time. For each solvent, the progress of the extraction with time was determined as shown in Figure 1. The results presented show that N, N-dimethylformamide and a mixture of methanol and chloroform (1 : 2 v/v) were the most effective solvents for the organic compounds contained in the model dust. Considering the above results, and the reports (Mniszek and Kolarczyk, 1990) suggesting the selective activity of each solvent in relation to particular group of compounds, spectrophotometric analyses of the extracts were performed, i.e. analyses of individual solvents, together with the dissolved organic compounds. The recorded absorption spectra, in the wavelength range between 190 and 430 nm, excluding the solvent spectra, are presented in Figure 2. The spectra obtained confirm that each of the solvent extracts different organic compounds from the dust; the identification of those substances is beyond the scope of the present paper. It turned out that the widest spectrum of substances is extracted in N, Ndimethylformamide; the spectrum of this extract covers the spectra of all other solvents, which confirms its maximum usefulness. The best effect of extraction with that solvent has also been illustrated in Figure 1. Observation of the spectra in Figure 2 allows the conclusion to be drawn that the spectrum of chloroform-methanol is beyond the spectrum of the previously mentioned solvent, which suggests that it is used for extraction of other substances, besides N, N-dimethylformamide. Simultaneously, Figure 1 also shows that the above mixture exerts less intensive extraction effects than N, N-dimethylformamide; however, the effects are more intensive in comparison with other solvents. In order to ensure the removal of all organic substances from the dust, the extractions were always performed successively in two solvents; as a result, it was confirmed that the choice of the two solvents (namely: N, N-dimethylformamide and chloroform-methanol mixture) is the optimum one. Table II shows the results of the successive extraction technique. 2.2. DECOMPOSITIONOF INORGANICCARBONCOMPOUNDS Inorganic carbon compounds were removed from the model dust by decomposition with nitric acid. The studies were aimed at determining the process conditions. After the extraction of organic compounds and drying of the thimbles until they achieved constant mass, they were put into a crystallizer filled with the nitric acid solution. The processes in the acid at concentrations of 5, 10 and 20% were carried out during 0.5, 1.0 and 24 hours' time, after which the thimbles were rinsed in redistilled water and dried to constant mass. The effectiveness of such a procedure was recognized

45

DETERMINATION OF ELEMENTAL CARBON EMISSION

4,8

1 2

3.6

3

2.4

5

1.2

6

2

4

6

8

i. N , N - d i m e t h y l f o z m a m i d e 2. M e t h a n o l

+ chloroform

5. B e n z e n e !:i V / V

3. P y r i d i n e 4. B e n z e n e

h

6. T o l u e n e 7. C y c l o h e x a n e

+ methanol

3:1 v/v

Fig. 1. Indicator of effectiveness of extraction of organic compounds from the dust as a function of time.

after carbon determination in the samples subjected to the nitric acid effects, as well as in those for which the acid was not applied. It was found that neither the duration of the nitric acid effects nor its concentration have any marked impact upon the effectiveness of decomposition of inorganic carbon compounds contained in the dust. The contents of those compounds in the model dust did exceed 0.4%; however, in relation to about 4% of the organic compounds contained in the dust, this is quite a significant value. In view of the above findings, it has been accepted that the elimination of the inorganic carbon compounds should be included in the analysis cycle. The inorganic compounds

46

WOJCIECH MNISZEK ET AL.

.

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.

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3.11

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4---,---

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1.11

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3G~

4Ofl

1, :~, ~ - cl.t.me Cb.yl f oz]t~amiae 2 , iler, l~_m3_l ÷ cl"d.ozofo~t

Fig. 2. Absorption spectraof the organiccompoundsextractedin the differentsolvents.

should be subjected to decomposition in nitric acid at a concentration of 10%, during 1 h at room temperature. After the procedure, the thimbles should be rinsed in redistilled water and dried under vacuum. 2.3. SAMPLE COMBUSTION After extraction of the organic substances and elimination of the inorganic carbon compounds, the dust samples were burned in a closed oxygen installation, after which the carbon dioxide content was determined in the generated gases. This stage of the experiment was aimed at determining the process conditions. The diagram of the installation used for the sample combustion is presented in Figure 3. The installation was flushed with oxygen from a cylinder each time before the major combustion started. After the flushing procedrue, the system was filled with oxygen to an amount allowing for total sample combustion. The duration of a single combustion cycle can change, depending on the sample weight, furnace temperature, circulating pump efficiency as well as on total capacity of the installation. Special attention should be paid to complete combustion of the cellulose thimble; the carbon dioxide generated from the combustion process should be subtracted

DETERMINATIONOF ELEMENTALCARBONEMISSION

47

TABLE II Indication of the effectiveness of successive extractions in the different organic solvents (%) as a function of time. solvents

duration of the extraction (h) 1

2

3

4

N, N-dimethylformamide Chloroform + methanol

1.4

2.4

3.2

3.6

N, N-dimethylformamide Pyridine

1.4

Chloroform + methanol N, N-dimethylformamide

1.2

Chloroform + methanol Pyridine

1.2

Pyridine iV, N-dimethylformamide

0.9

Pyridine Chloroform + methanol

0.9

2.4 2.3 2.3 1.6 1.6

3.2 2.9 2.9 2.2 2.2

5

6

7

8

4.3

4.5

4.6

4.7

4.1

4.3

4.4

4.4

4.0

4.2

4.4

4.5

3.8

4.0

4.1

4.1

3.6

4.0

4.1

4.2

3.4

3.8

4.0

4.0

3.6 3.4 3.4 2.5 2.5

from the basic result, after having performed a blank test with an empty thimble. After the combustion process had been completed, gas mixing was continued until the carbon dioxide concentration was completely average. After having sampled the gas with a gas syringe from container No. 4, the chromatographic analysis was performed. The following combusion parameters were determined as a result of the experiment: ® combustion temperature: 700°C; ® closed installation capacity for the sample weight o f about 0.7 g: 0.8 dm3; duration o f preliminary flushing o f the installation with cylinder oxygen: 4 min at o x y g e n flow o f 2 dm3/min; during that time, ten gas volume exchange cycles could be obtained; ® duration o f secondary o x y g e n rinsing: 1 min; ® duration o f sample combustion: 15 min; ® duration o f gas mixing after combustion: 3 min.

48

WOJCIECH MNISZEK ET AL.

220V

&

4

5 /-

6

i. Q u a r t z

tube

5. W a s h i n g b o t t l e

2. F u r n a c e

3. C i r c u l a t i n g

4. Gas sampling

6. Gas flowmeter

pump

7. Cylindez

with oxygen

8. Sample

Fig. 3. Installation for sample combustion.

The duration of individual operations was determined on the basis of multiple analyses of carbon dioxide concentrations in the gas mixture filling the installation. The CO2 chromatographic analyses were repeated every 30 sec.

DETERMINATIONOF ELEMENTALCARBONEMISSION

49

2.4. CHROMATOGRAPHIC DETERMINATION OF CARBON DIOXIDE For chromatographic carbon dioxide determination in the gas mixture generated from sample combustion in oxygen, the Fraktowap 1400 Carlo Erba gas chromatograph, equipped with a thermoconductometric detector, was used. The studies were aimed at specifying the conditions necessary for the chromatographic determination of carbon dioxide found, together with oxygen and other trace elements, in the gas mixture generated from the sample combustion in oxygen The determination of carbon dioxide corresponds to the procedure for the determination of elemental carbon under the conditions described in the present paper. The following parameters of the chromatographic determination were specified on the basis of the experiments: ® carrier gas: hydrogen; • carrier gas flow: 0.03 dm3/min; • a steel column, 2 m long, with an internal diameter of 4 mm; . Porapak column packing, Q 50-80 mesh; ® column working temperature: 70°C; ® catharometer voltage: 9 V; ® gas sample volume: 2 cm3; ® detector sensitivity: 32; ® rate of the recorder paper travel: 0.2 minis. The determination of the carbon content in the studied sample was carded out on the basis of a standard diagram, namely, the relationship between the height of the chromatographic peaks corresponding to carbon dioxide concentration in the gas mixture, and carbon mass subjected to combustion. In order to determine the calibration curve, the weighed portions of active carbon were burned with the application of identical mass and conditions as those determined for the samples. Active carbon (high purity) was used. Prior to combustion, the portions were wanrmed up to a temperature of 105°C to achieve constant mass. The calibration curve is shown in Figure 4. The following observations were made while studying the carbon content in the hard filtration paper used for thimble production: * the filtration paper did not contain the organic compounds subjected to extraction with the solvents used; ® it did not contain any inorganic carbon compounds;

50

WOJCIECH MNISZEK ET AL.

peak

mm

240

200

160

...........

2_. . . . . . . . . . . . .

120

80

40

40

80

120

200

carbon mass /rag/

Fig. 4. Calibrationcurve for chromatographicdeterminationof carbon mass,

• the carbon content which undergoes combustion to CO2 amounts to 37% w/w. The carbon concentration in the dust was calculated from the following formula:

Cc--

~c

-- ~b fr~d

where: Ce = elemental carbon concentration in the dust (mg/g); mc = carbon mass read from the standard diagram (rag); m b = carbon mass in the thimble (rag); and m d = dust sample mass (g).

3. Determination of Elemental Carbon Concentration in the Dust Emitted from the Power Station Measurements were carried out in the Elektrownia Laziska power station, one of the power stations situated in the Katowice region. Two types of boiler are installed in this power station; OP 380 - 100 MW of power and OP 650 - 200 MW of power. The coal consumption in the OP 380 boiler is about 45 t/h and in the OP 650 boiler about 90 t/h. The method of elemental carbon emission determination was based

DETERMINATIONOF ELEMENTALCARBONEMISSION

51

TABLE III Elemental carbon concentration in the dust emitted from Elektrownia Laziska power station. Mass of the dust sampled (g)

Dust concentration in the gas emitted (g/Nm3)

Elemental carbon concentration in the dust (mg/g)

OP 380 boiler 1 2 3 4 5

0.933 !.603 1.393 1.205 1.435

0.11 0.14 0.17 O.12 0.13

31.0 47.0 59.0 35.0 39.0

OP 650 boiler 1 2 3 4 5

1.516 2.075 1.355 1.280 1.615

0.11 0.15

41.0 48.0 32.0 30.0 28.0

No. of sample

0.12 0.10 0.11

on the method described in this paper. Each sampling was repeated three times and the result presented in Table III are average values.

4 .

C o n c l u s i o n s

The main goal of the present experiment was the elaboration of a method for elemental carbon emission. The method consists of the following elements: -

-

-

-

-

-

-

sampling of the emitted dust, according to the standard; determination of the sampled mass; extraction of the organic compounds from the dust; decomposition of the inorganic carbon compounds; combustion of the dust residue in a closed system; chromatographic CO2 determination; calculation of the carbon concentration in the emitted dust.

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WOJCIECHMNISZEKET AL.

The most important stage of the method is the extraction of organic compounds from the dust. To perform that, the successive extraction in N , N-dimethylformamide followed by a mixture o f chloroform-methanol (2 : 1 v/v) has been proposed.

References Polish Standard: 1990, Rozporz~dzenie Ministra Ochrony Srodowiska i Zasob6w Naturalnych z dnia 19.02.1990. Polish Standard: 1987, PN 87 M/34 129. Mniszek, W., Kolarczyk, J. and Hanusik, A.: 1990, 'Methods for Determination of Elemental Carbon in Ambient Air', Environmental Monitoring and Assessment 14, 1-7.

Determination of elemental carbon emission.

The studies on elemental carbon content in the atmospheric air, performed at the air monitoring station in Katowice (Poland), have revealed violations...
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