Three-Ring PAH Removal from Waste Hot Gas by Sorbents: Influence

Environ. Sci. Technol. 2002, 36, 1821-1826

Three-Ring PAH Removal from Waste Hot Gas by Sorbents: Influence of the Sorbent Characteristics A . M . M A S T R A L , * , † T . G A R C IÄ A , † M . S . C A L L EÄ N , † J . M . L O Ä PEZ,† M. V. NAVARRO,† R. MURILLO,† AND J . G A L B AÄ N ‡ Instituto de Carboquı´mica, CSIC, P.O. Box 589, 50080 Zaragoza, Spain, and Department of Analytical Chemistry, Zaragoza University, 50007 Zaragoza, Spain

Three-ring polycyclic aromatic hydrocarbons (PAH) are one of the most abundant PAH groups emitted during coal combustion. Four of themsacenaphthene (Ac), phenanthrene (Phe), fluorene (Fu), and anthracene (An)shave been listed by the U.S. EPA as priority pollutants. The aim in this paper is to study the abatement of this particular group of three-ring PAH from hot gas emissions during energy generation in coal combustion. The three-ring PAH adsorption capacities are related to the morphological and chemical properties of the 16 sorbents used in this work. Single and multiple linear regressionssprincipal component regression (PCR)swere applied in this study. The main conclusions reached are, first, that the micropore volume is the most determinant parameter for removal of these PAHs and, second, that the adsorption of three-ring PAH by sorbents from waste hot gas emissions is inversely proportional to their volatility: the lower the PAH volatility, the higher the adsorbent adsorption capacity. The adsorption isotherms show that Phe and An, both examples of PAH with three aromatic rings, behave similarly. However, their behavior differs from that of Ac and Fu, compounds in which only two of their three rings exhibit an aromatic nature.

Introduction Organic emissions, the harmful nature of which has become a cause of growing concern, constitute one of the most important sources of pollution (1). The organic emissions generated by the use of fossil fuels have been present in our society for a long time, but due to growing development their incidence is becoming higher and higher. However, few studies have been devoted to this type of emissions, probably because no legislation has been introduced on their use yet and because the amount of organic emissions is lower than that of inorganic emissions (COx, NOx, and SOx). Organic emissions include a wide variety of compounds. One of these groups of compounds, polycyclic aromatic hydrocarbons (PAH), shows special properties that make them interesting to study: some PAH are able to modify the normal metabolic functions of cells, promoting mutagenic and carcinogenic effects (2, 3). * Corresponding author phone: 34-976-733977; fax: 34-976733318; e-mail: [email protected]. † CSIC. ‡ Zaragoza University. 10.1021/es010095k CCC: $22.00 Published on Web 03/07/2002

 2002 American Chemical Society

PAH are mainly generated in the combustion processes of fossil and nonfossil fuels (4-7) through mechanisms (8-11) that can be classified into two processesspyrolysis and pyrosynthesis. On the other hand, PAH can also have their origin in the fuel structure because some small particles can be removed from the combustor as unburned material (7), but these PAH can be controlled by improving the efficiency of the combustion process (11). Due to the high volatility of PAH (12), they can be released both supported onto particulate matter (PM) and in the gas phase (8, 13). Whereas the most volatile compounds, compounds with two or three aromatic rings, will be mainly released in the gas phase, the compounds containing three or more aromatic rings in their structure will be associated with the PM emission (14). The PAH gas/solid partitioning is related to the liquid vapor pressure, the ambient temperature, the size, the chemical composition, and the surface area of the PM (15-17). These characteristics, together with the PAH volatile character, will determine the way in which they are emitted in the combustion process. The PAH supported on the PM can be trapped in cyclones, electrostatic precipitators, scrubbers, etc., whereas the most volatile compounds are released to the atmosphere through a chimney, making necessary the use of catalysts and sorbents to diminish their negative environmental impact. Few references have been found on this topic in the existing literature because very little information has been reported on PAH abatement from combustion hot gas. As opposed to our quantitative approach in this work, a qualitative study of PAH abatement by activated lignite cokes from waste material incineration showed that PAH were emitted below the detection limit (18). A second paper dealt with the removal of incomplete combustion compounds from different incinerators using carbon materials (19). The data reported in this case showed 90% control efficiencies, but this work was restricted only to the solid products from incomplete combustion and not to the volatile ones. In a previous work (20), the authors had already demonstrated that the phenanthrene (Phe) adsorption capacity of different carbon materials depended on the morphological characteristics of each material and showed a good correlation to the total micropore volume (VN2) from the N2 isotherm data calculated by the Dubinin-Radusckevich equation (DR). They showed (21) that the adsorption capacity of PAHs with two, three, and four aromatic rings in their structures, that is, naphthalene (Np), phenantrene (Phe), and pyrene (Py), respectively, depended on the sorbent characteristics. Furthermore, it was proved that (1) the narrow micropore volume and the diameter mean size were important for Np adsorption; (2) the total micropore volume and the micropore distribution were determinant for Phe adsorption; and, finally, (3) the total micropore volume and the volume contributed by 2-50 nm sized pores were the most relevant sorbent characteristics for Py adsorption. These results demonstrated that the higher the number of rings, the lower the influence of the adsorbent microporosity. Finally, it was demonstrated (21) that the higher the number of aromatic rings contained in the PAH molecule, the more favored were the adsorbateadsorbate interactions. Once the influence of the number of rings contained in the PAH molecule has been established, it would be interesting to focus on the abatement of different PAH with the same number of rings. Consequently, we examine in this paper the adsorption capacity onto different sorbents of four PAH with the same number of ringssacenaphthene (Ac), VOL. 36, NO. 8, 2002 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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TABLE 1. Carbon Material Morphological and Chemical Properties DR equation

DA equation

TPD

BJH method

sample

origin

VN2 (cm3/g)

VCO2 (cm3/g)

n

L (nm)

CO (µmol/g)

CO2 (µmol/g)

VBJH (cm3/g)

CA-1 CA-2 CA-3 CA-4 CA-5 CA-6 CA-7 CA-8 CA-9 CA-10 CA-11 CA-12 CA-13 CA-14 CA-15 CA-16

tire tire coal tire cherry grape coal apricot coal coal coconut coal coal apricot coal apricot

0.06 0.04 0.11 0.15 0.13 0.19 0.21 0.46 0.30 0.36 0.57 0.41 0.52 0.52 0.48 0.62

0.03 0.01 0.11 0.08 0.18 0.21 0.19 0.25 0.28 0.28 0.42 0.22 0.27 0.32 0.36 0.29

2.4 2.2 2.1 1.6 2.4 2.3 2.0 2.0 2.0 1.9 2.3 1.5 1.6 1.6 1.7 1.5

0.9 1.1 1.3 1.4 0.7 0.8 1.1 1.6 1.0 1.1 1.1 1.5 1.4 1.4 1.3 1.6

860 280 500 940 1900 1500 1400 840 670 680 680 1100 660 700 610 880

370 120 540 290 870 1000 580 590 350 390 460 340 360 160 320 480

0.13 0.39 0.11 0.39 0.004 0.072 0.068 0.22 0.11 0.26 0.10 0.25 0.43 0.57 0.14 0.51

TABLE 2. Ac, Fu, Phe, and An Molecular Parameters compd Ac Fu Phe An a

FIGURE 1. Ac, Fu, Phe, and An molecular structures. fluorene (Fu), Phe, and anthracene (An)sin the concentration interval normally emitted at energy generation. Although Ac, Fu, Phe, and An contain the same number of total rings, their aromaticities are different. Two of these three-ring PAH (Ac and Fu) have only two aromatic rings, whereas the other two PAH (An and Phe) have three aromatic rings (Figure 1). To determine the influence of the morphological and chemical characteristics of carbon materials on the adsorption of three-ring PAH, single- and multiplevariable analyses have been carried out to explain the cause of most of the data variance.

Experimental Section Sixteen carbon materials of different origins were used to study their Ac, Fu, Phe, and An adsorption capacities. All materials were provided to their direct application in PAH adsorption. CA-1, CA-2, and CA-4 were carbon black from tire pyrolysis. CA-1 and CA-2 were not subjected to any activation process, but CA-4 was activated to increase its micropore volume. CA-3 was a coke from German lignite. CA-5, CA-6, and CA-11 were active carbons from cherry stone, grape seeds, and coconut shells, respectively. CA-8, CA-14, and CA-16 were different active carbons from apricot stones. Finally, CA-7, CA-9, CA-10, CA-12, CA-13, and CA-15 were different chars from Spanish lignite. Because the basic issue in this paper was the influence of the morphological and chemical properties of these sorbents on PAH adsorption, and not their activation protocol, the activation procedures are not described. The active carbons used in this work were characterized through their morphological and chemical properties, and the obtained data are compiled in Table 1. The 16 porous carbons were characterized by N2 and CO2 adsorption at 77 1822

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Vw (Å3) σ (Å) bp (°C) 148 160 169 170

6.56 6.73 6.86 6.87

279 294 338 340

vapor pressurea (Pa)

π-orbital density (β units)

2750 1520 533 227

16.6 17.2 19.4 19.3

At 150 °C.

and 273 K, respectively, using an ASAP 2000 (Micromeritics). The experimental error due to sample heterogeneity was ∼5% depending on the sample. Their micropore volumes were obtained by applying the DR equation, and the mesopore volume, VBJH, was obtained by applying the BJH method (22) to the N2 isotherm data. Their corresponding micropore distribution and diameter mean size were calculated by applying the Dubinin-Astakhov (DA) equation to the CO2 isotherm data. Characterization of the surface oxygen groups was carried out on a Pulse Chemisorb 2700 (Micromeritics) by temperature-programmed desorption (TPD). The carrier flow rate was adjusted with an electronic mass flow controller. The gases evolved by thermal decomposition from the surface groups were measured by gas chromatography with Porapak N-120 and molecular sieve columns. The temperature program was the following: isotherm at 35 °C during 4 min, temperature increased to 180 °C at a rate of 35 °C/min, and isotherm held. The measured experimental error was ∼2%. Adsorbates. Ac, Fu, Phe, and An, the three-ring PAH designated priority pollutants by the U.S. EPA, were selected to carry out this study. The PAH physical and chemical properties are compiled in Table 2. In this table, the molecular volume, Vw (24), the molecular diameter, σ, calculated as if the molecules were perfect spheres, the boiling point, the vapor pressure at 150 °C, and the electronic density of the π-orbital (25) are shown. All PAH were provided by Supelco Inc. in analysis grade. Adsorption Capacities. Experiments were carried out using the laboratory scale rig elsewhere described (20). The adsorbing bed, composed of 15-50 mg of adsorbent (100200 µm average particle size diameter) depending on its surface, was mixed with 1.0 g of sand, inert material with the same particle size, to provide enough bed length (11 cm) to generate a uniform flow throughout the reactor situated inside a gas chromatograph furnace. Blank tests were carried out to check the inert material adsorption capacity. The experimental temperature inside the gas chromatograph furnace was 150 ( 1 °C.

TABLE 3. Mean Adsorption Capacity (w*/C0) and Relative Standard Deviation (RSD) for Ac, Fu, Phe, and An on Sample CA-3 compd

(w*/C0)mean (mL/mg)

RSD

Ac Fu Phe An

23 32 62 67

11 9 10 11

Before the start of each experiment, it was necessary to reach a constant and known concentration of the specific PAH studied in the bed reactor inlet stream, by passing the saturator outlet gas stream to the flame ionization detector (FID) until this concentration was obtained. Once this was reached, the gas stream was passed through the reactor starting the reaction time, which lasted until saturation was reached (C0 ) C). Adsorption capacities, w* (mg of PAH/mg of CA), were calculated as tbC0Q/W, where tb was the breakthrough time for a 2% penetration time (min), C0 was the Phe inlet stream concentration (mg/mL), Q was the flow (mL/min), and W was the adsorbent weight (mg). The PAH concentration (C) in the outlet gas stream was directly measured by a previously calibrated FID. The experimental error associated with the adsorption capacity measurement ranged between 9 and 11% due to the sample heterogeneity and the concentration variability in the experiments. The results obtained for five measurements of the adsorption capacity of sample CA-3 are shown in Table 3. Methodology. Principal Component Analysis (PCA). PCA is a frequently used multivariate technique that provides a powerful tool for data compression, exploration, and interpretation. PCA allows the investigation of the variance sources present in a multivariate data set using a reduced set of orthogonal variables or principal components (PCs), which are a linear combination of the original measured variables. The advantages of this linear transformation are that a few new variables explain the original data almost completely. The analysis was developed by applying SPSS software to the eight normalized variables shown in Table 1, VN2, VCO2, VBJH, n, L, CO, CO2, and O*. The correlation coefficients between the normalized variables and the PCs were calculated as the product of the square root eigenvalues and scores. However, the solutions for the PCs are pure mathematical solutions that do not correspond, in general, with physical solutions. In this way, an equimax rotation to the generated autovector space was carried out to determine the PCs physical meanings. Multiple Lineal Regression (PCR). The basic assumption in this study was that most of the observed data variance followed a linear model with a reduced number of components, PCs, where each component was obtained from the PCA. The following model and equation were assumed: N

(w*/C0)PAH ) A +

∑[B (PC )] i

i

(1)

i)1

In eq 1 (w*/C0)PAH is the adsorption capacity of each individual PAH, i is the number of components obtained in the PCA analysis, A is a constant, and Bi is the term associated with the dependent variable PCi. The PCR analysis was carried out using a stepwise procedure with SPSS software.

Results and Discussion Adsorbent Characterization. In Table 1, a morphological and chemical characterization of the 16 carbon materials used is shown. In this table, the total micropore volume, VN2

FIGURE 2. Breakthrough curves for the four polyaromatics for sample CA-3: (9) Ac; ([) Fu; (b) Phe; (2) An (Ta) 150 °C, C0 ≈ 2 ppm).

FIGURE 3. Ac, Fu, Phe, and An adsorption isotherms at 150 °C for sample CA-3: (9) Ac; ([) Fu; (b) Phe; (2) An. (pore size < 2 nm), was calculated from the DR equation for the N2 isotherm data, and the narrow micropore volume, VCO2 (pore size < 0.7 nm), was calculated from the DR equation for the CO2 isotherm data. The mesopore volume, VBJH (pore size ) 2-50 nm), was obtained from the BJH method for the N2 isotherm data, and the n exponent and L value were obtained by applying the DA equation to the CO2 isotherm data. For most of the carbon adsorbents, n ranged from 1 to 4. An n value >2 means a highly homogeneous, small micropore carbon adsorbent, whereas an n value 95% of the data variance. Table 8 shows the correlation coefficients between the normalized morphological and chemical properties and the PCs. It can be observed that in this new space generated, PC1 was positively

correlated to the total active sites, CO and CO2 desorbed groups, and the total number of active sites (CO + 2CO2), and therefore it was related to the hydrophilic character of the adsorbent chemical surface (28). PC2 was positively correlated to L and negatively correlated to n, showing the micropore distribution, whereas PC3 was strongly correlated to VN2 and VCO2. Therefore, PC3 showed the total volume microporosity and, finally, PC4 was positively correlated to the mesopore volume. The best results obtained for each PAH with eq 1 are shown in Table 9. In this table r is the correlation coefficient, which was used to compare the differently sized model fit qualities, sE is the standard deviation explained by the regression and sPAH is the standard deviation associated with the dependent variable. The numbers in parentheses following the independent variable coefficients represent their standard deviations. The Student t parameters, calculated from these standard deviations, associated with each variable coefficient were all >2, indicating a significant relationship between PCs and PAH adsorption capacities. It can be observed that the percentage of explained variance was higher in Phe and An adsorption than that obtained in the single-variable analysis and that the correlation diagram and analysis of residuals had improved. It can also be observed that the incorporation of new variables, PCR analysis, to Ac and Fu adsorption decreased the percentage of explained variance. From Table 9 and according to Figure 3, the different behaviors of PAH containing two and three aromatic rings can be observed. On the one hand, Ac and Fu, examples of PAH with two aromatic rings and one nonaromatic ring, showed a similar behavior, being positively correlated to PC2 and PC3. On the other hand, Phe and An, examples of PAH with three aromatic rings, showed a different behavior, both VOL. 36, NO. 8, 2002 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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TABLE 9. Equation Parameters of the PCR Regression Model [(w*/C0)PAHs ) A + B1 × PC1 + B2 × PC2 + B3 × PC3 + B4 × PC4] PCR parameters compd

A

Ac Fu Phe An

63 ((5) 92 ((7) 160 ((5) 164 ((7)

statistical parameters

B

C

D

-20 ((5) -17 ((7)

20 ((5) 28 ((7) 50 ((5) 49 ((7)

41 ((5) 53 ((7) 94 ((5) 93 ((7)

sE

sPAH

% var

17 ((5) 19 ((7)

0.94 0.92 0.99 0.97

18.4 26.6 19.4 29.5

48 64 110 110

88 85 98 95

Literature Cited

relative importance (%) variable

Ac

Fu

Phe

An

PC1 PC2 PC3 PC4

0.0 20.2 79.8 0.0

0.0 19.7 81.3 0.0

3.2 21.0 73.5 2.3

2.4 20.4 73.8 3.2

being negatively correlated to PC1 and positively correlated to PC2, PC3, and PC4. A study of the variables and the percentage of explained variance was carried out in order to determine the factors controlling the adsorption of the four PAH studied. Results in Table 10 show the relative importance of each variable, calculated from the r2 value change, when a new variable was introduced in the regression. From this table, it can be deduced that the micropore volume was the most important variable in the adsorption of the four PAH. The micropore distribution also contributed to the four PAH adsorption capacities. A wide distribution with a high mean micropore diameter favored these four three-ring PAH adsorptions, avoiding diffusion problems. Moreover, it was observed that the mesopore volume also had an influence on the Phe and An adsorption. The relative importance of this variable was higher for An, according to its lower volatility, and this could favor the capillary condensation process. Phe and An adsorption was also correlated to PC1, the total active sites, but their relative importance was very low. The sign of the coefficient was negative and, therefore, the adsorption could be favored by the hydrophobic character of the adsorbent, according to the hydrophobic nature of these PAH. From the single and multiple applied models, it can be observed that for Ac and Fu adsorption the best percentage of explained variance was obtained from a linear regression (LR). Concerning Phe and An adsorption, PCR analysis gave the best results. In this regression model, the influence of mesoporosity was detected. Although no evidence was deduced from the LR analysis, this could be expected due to the low Phe and An volatility at 150 °C, allowing the capillary condensation on the mesopores. Figure 4 shows the experimental results versus the predictable values calculated from the models (LR for Ac and Fu and PCR for Phe and An). In general, there is a good concordance between them. It is worth pointing out the difference in values for samples CA-1, CA-2, and CA-5. Whereas samples CA-1 and CA-2 showed low microporosity, CA-5 showed a very narrow microporosity. Therefore, these three samples should have been initially rejected because their morphological and chemical properties did not make them suitable for PAH adsorption. However, they were not eliminated, which could decrease the statistical quality of the model.

Acknowledgments The authors would like to thank the European Community (Contract Ref. 7220-Pr/067) and the Regional Government 9

r

of Arago´n (DGA) for their partial financial support of this project.

TABLE 10. Relative Importance of Each Variable in PCR Analysis

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Received for review April 2, 2001. Revised manuscript received November 6, 2001. Accepted December 21, 2001. ES010095K