Polycyclic Aromatic Hydrocarbons and Organic Matter Associated to

Environ. Sci. Technol. 1999, 33, 3177-3184

Polycyclic Aromatic Hydrocarbons and Organic Matter Associated to Particulate Matter Emitted from Atmospheric Fluidized Bed Coal Combustion ANA M. MASTRAL,* M A R IÄ A S . C A L L EÄ N , A N D T O M AÄ S G A R C IÄ A Instituto de Carboquı´mica, (CSIC) Marıá de Luna 12, 50015 Zaragoza, Spain

The polycyclic aromatic hydrocarbons (PAH) and the organic matter (OM) content associated with particulate matter (PM) emissions from atmospheric fluidized bed coal combustion have been studied. The two main aims of the work have been (a) to study OM and PAH emissions as a function of the coal fluidized bed combustion (FBC) variables in solid phase and (b) to check if there is any correlation between OM and PAH contained in the PM. The combustion was carried out in a laboratory scale (Di ) 7 cm and H ) 76 cm) plant at different combustion conditions: temperature (650-950 °C), percentage of oxygen excess (5-40%), and total air flow (700-1100 L/h). PAH associated on the particulate matter have been analyzed by fluorescence spectroscopy in the synchronous mode (FS) after PM extraction by sonication with dimethylformamide (DMF). It can be concluded that there is not a direct relationship between the OM content and the PAH supported in the PM emitted. In addition, neither PM or OM show dependence between themselves.

Introduction For the last years, the main problems associated with power generation have been concerning work conditions: trying to obtain the maximum energy with the lowest cost and producing minimum pollutant emissions. Considering that everything at the exit of the chimney can be classified as emission, the emissions can be released in gas and in solid (particulate matter, PM) phase depending on the physical state in which they are emitted to the atmosphere. On the other hand, gas and solid emissions can be classified as a function of their nature as inorganic (COx, NOx, SOx) (1), trace elements (2), or organic emissions. Particulate matter (PM) can be both organic (3, 4) and inorganic (5, 6) or can show organic-inorganic composition. In fact, some volatile organic emissions, like PAH, and some inorganic emissions, like Hg, can be emitted supported on fly ash or as organomercuric volatile compounds (7). One of the main objectives in fluidized bed combustion (FBC) (8) has been to control the inorganic emissions associated with NOx, SOx, and COx (1). The burning conditions have been modified so that currently these emissions are abated. Concerning organic emissions, these emissions are produced in lower amounts in comparison to inorganic * Corresponding author e-mail: [email protected]; phone: 34 976 733977; fax: 34 976 733318. 10.1021/es990241a CCC: $18.00 Published on Web 08/11/1999

 1999 American Chemical Society

emissions, but it does not mean that organic emissions do not have an important environmental impact (9). From these organic emissions, the volatile organic compounds (VOC) (10) represent a wide group in which polycyclic aromatic compounds (PAC) have a special interest, specifically PAH, due to their carcinogenic character (11, 12) that changes them to strong pollutants of great concern to humans. The nature of fuel (13-15) influences the emissions nature, but it is not the only cause. The other cause of emissions from combustion is their own combustion process. Any combustion process has a previous pyrolytic process (16) involving organic and inorganic radical release that can generate new emissions. In this way, the emissions produced will be also a function of the combustion variables (pressure, temperature, airflow rate, % oxygen) and of the reactor characteristics. Concerning coal combustion for energy generation, three basic techniques can be distinguished: fixed bed (16), suspension or “entrainment” firing (pulverized coal) (17), and fluidized bed (18). In FBC systems operating at atmospheric (AFBC) or above atmospheric pressure (PFBC) (8) in bubbling (BFBC) or circulating (CFBC) beds, the utilization of lower temperatures than those of conventional pulverized coal fired boilers will produce more unburned material released as a consequence of these different combustion conditions. Independent of the PM composition, the PM emission is submitted to legislation (about 50 mg/m3) (19) as a function of the particle size. However, it is very important from an environmental point of view to know the PM impact from its composition point of view. This is the case of the polyaromatic compounds in general and, in particular, of the PAH that as organic compounds of high volatility can be released in gas phase or supported on the PM. The complexity and variety of these PAH are enormous, and from them 16 PAH have been listed as priority pollutants by the U.S. EPA in such way that most of the studies carried out have been concentrated on emissions of these compounds. Their priority is mainly due to their carcinogenic character (11, 12). Once released to the atmosphere, they can be introduced into the human body by inhalation and/or ingestion, undergoing a transformation to diol epoxides of the benzenic ring of strong electrophilic character, reacting with biological nucleophyles as proteins, and modifying their normal metabolic activity. In addition, due to the complexity of PAH mixtures and the differences in results obtained with varied analytical techniques, the sampling and analysis of PAH in emissions and the atmosphere have been linked to many problems (20). This link to the low concentrations at which these compounds are emitted makes it necessary to use very sensitive analysis methods. Up to now, various sampling systems (21), extracting ways (22), and analytical techniques have been used to study PAH emissions. In this paper, the organic matter content of the PM from coal AFBC has been calculated, and its PAH content has been analyzed in the framework of a wider work performed to assess the PAH formation and emission (23, 24).

Experimental Section A low rank coal [ultimate analysis: % C, 73.8 (daf); % H, 6.4 (daf); % S, 6.3 (db); % N, 0.9 (daf); immediate analysis: % moisture, 15.7 (ar); % ashes, 23.9 (ar); % volatiles, 15.0 (ar); % fixed carbon, 45.4 (ar); and calorific power, 17.3 MJ/kg] VOL. 33, NO. 18, 1999 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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TABLE 1. Combustion Efficiencies (%) Reached at AFBC Depending on the Combustion Variablesa temperature (°C) conditions of %O2 and Q: Q ) 860 L/h, 20% O2

650 99.0

700 98.7

750 98.8

percentage of excess oxygen conditions of T and Q: T ) 850 °C, Q ) 860 L/h airflow rate (L/h) conditions of T and %O2: T ) 850 °C, 10% O2 a

700 98.3

800 98.4

800 99.0

850 99.1

900 99.3

950 99.4

5 98.7

10 99.3

20 99.5

40 99.6

860 99.3

900 98.8

1000 98.5

1100 98.6

Low rank coal; sand as the fluidizing agent.

FIGURE 1. Main mechanisms implied in the combustion process concerning solid and gas emissions. was burned in a laboratory atmospheric fluidized bed combustion pilot plant in the bubbling regime (25). The laboratory plant was composed of a continuous feeder, which allowed working from 40 to 300 g/h. The reactor was made of Kanthal steel (Di ) 7 cm and H ) 76 cm), and air from a compressor, controlled by a gas flow controller after preheating, was passed through the distributor plate to fluidize the bed. A thermocouple situated in the middle of the reactor determined the combustion temperature ((10 °C). After passing the gas streamflow through a two-cyclone system (first and second cyclones), an aliquot of the total gas was forced to pass through a nylon filter (20 µm), a Teflon filter (0.5 µm), and finally an adsorbent XAD-2 resin. Two gas counters were disposed to measure the total combustion gases and the sampling volume. From each experience, five samples were taken: two from the cyclones, two from the filter systems, and one from the adsorbent. Those from the two cyclones and the nylon filter correspond to the PM studied here. The samples were extracted by sonication with dimethylformamide (DMF) for 15 min and filtered in a Millipore Teflon filter. PAH analyses were performed by fluorescence spectroscopy in the synchronous mode after establishing the analytical conditions of identification and quantification of each PAH (26) with the corresponding model compounds. Each spectrum was performed in duplicate, and the areas were calculated for further quantification. Because there are hundreds of PAH, this work has been performed taking into account only the 16 PAH listed by U.S. EPA as priority pollutants: naphthalene, fluoranthene, phenanthrene, fluorene, benzo[a]pyrene, pyrene, chrysene, anthracene, acenaph3178

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TABLE 2. Organic Matter Content in the Particulate Matter of the Cyclone as a Function of the Combustion Variables at AFBC low rank coal 860 L/h, 850 °C

excess O2 5 10 20 40

organic matter (cyclone, g/kg) 2.3 2.2 1.8 3.0 organic matter (cyclone, g/kg)

low rank coal

air flow rate (L/h)

850 °C, 10% O2

700 800 900 1000 1100

low rank coal

temperature (°C)

5.2 9.4 8.4 8.1 8.2 organic matter (cyclone, g/kg)

860 L/h, 20% O2

700 750 800 850 900 950

3.2 5.0 5.7 5.0 4.0 1.7

thene, acenaphthylene, benz[a]anthracene, dibenz[a,h]anthracene, perylene, benzo[g,h,i]perylene, indeno[1,2,3cd]pyrene, benzo[b]fluoranthene, and benzo[k]fluoranthene. We also studied coronene and perylene because of their important role in radicals stabilization. The combustion variables studied, using sand as the fluidizing agent, were combustion temperature (650, 700,

FIGURE 2. PM (a) and its corresponding contents in PAH (b) collected in the first cyclone, PM (c) and its corresponding contents in PAH (d) collected in the second cyclone, and PM (e) and its corresponding contents in PAH (f) collected on the nylon filter as a function of the combustion temperature (Samca coal, AFBC, 860 L/h, 20% excess oxygen). 750, 800, 850, 900, and 950 °C), the percentage of excess oxygen (5, 10, 20, and 40%), and the total airflow (700, 800, 860, 900, 1000, and 1100 L/h) (27-29). All experiments have been performed by maintaining two variables as constant and modifying the third one. The combustion efficiencies were calculated (Table 1) in order to assess unburned emissions. The organic matter content in solid phase (OM) was calculated using the following equation (Table 2):

OM )

(100 - cz)Mcol 100Cfeed

(1)

where cz is the percentage of ashes as dry basis for the particulate matter collected in the first cyclone and the ashes of the ash hopper, Mcol is the particulate matter collected in the cyclone (g), and Cfeed is the total amount of coal feed (kg).

Results and Discussion The low rank coal combustion at the AFBC experimental installation was carried out at the described combustion

variables. The combustion at the laboratory plant was optimized in order to reach high efficiencies (Table 1) and to minimize emissions, which would correspond to one of the two possible ways of particulate matter emissions PM I (Figure 1). The main mechanisms involved at coal combustion are compiled in Figure 1. As Figure 1 shows, PM can be emitted as PM I and PM II. While PM I (unburned) could be minimized by combustion optimization and will be mainly composed of organic and inorganic components of the fuel, PM II is more difficult to control and will be mainly composed of organic matter (soots). The efficiencies reached (Table 1) allow us to discard the influence of the unburned material emission and to determine variables incidence on combustion during the process because at these high efficiencies conditions, their own combustion process will generate most of the emissions. It is observed that, according to results shown in Table 1, the higher the temperature, the higher the combustion efficiency; that the higher the excess oxygen, the higher the VOL. 33, NO. 18, 1999 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 3. PM (a) and its corresponding contents in PAH (b) collected in the first cyclone, PM (c) and its corresponding contents in PAH (d) collected in the second cyclone, and PM (e) and its corresponding contents in PAH (f) collected on the nylon filter as a function of the percentage of excess oxygen (Samca coal, AFBC, 860 L/h, 850 °C). efficiency; and finally, that the highest efficiency is obtained at 860 L/h, flow corresponding to double the minimum fluidization velocity (0.236 m/s). This velocity was calculated experimentally measuring the pressure drop in the bed at different air total rates. The representation of the pressure drops (Pa) versus gas velocity (m/s) fits to a curve in which a maximum is obtained decreasing slightly the pressure drop from this value up to a constant value. The interpolation of the value corresponding to this maximum (bed weight per reactor cross-sectional area) gives as result the minimum fluidization velocity. The high combustion efficiencies reached for all experiences also agree with the organic matter content (Table 2) in PM collected in the first cyclone. It is observed that (a) an increase in combustion temperature produces a decrease in the organic matter content of the corresponding PM; (b) at increasing percentages of excess oxygen, which facilitate the oxidation reactions, the OM content decreases due to its conversion into CO2 and H2O; and (c) an increase of airflow rate produces an increase in the organic matter content 3180

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reaching a maximum at 800 L/h and being constant at higher flow rates. At high flows, the oxidation reactions are not promoted due mainly to the high velocity of the radicals at the exit of the reactor and the short residence times, increasing the OM content by the low oxidation. High flows have as a consequence an increase of the air bubble size, thus diminishing its surface area and not favoring the elimination by oxidation of the radicals released: the deformation in the bed is produced and slugging regime appears. Furthermore, the PM entrain, as a consequence of the high flow, is another important factor that increases the organic matter content. Influence of the Combustion Temperature. Despite keeping the airflow rate (860 L/h) and the percentage of excess oxygen (20%) constant, a change in the combustion temperature will produce a change in the reactor outlet flow: the higher the combustion temperature, the higher the flow at the reactor exit. Moreover, it will produce a decrease of the residence time of the radicals inside the reactor.

FIGURE 4. PM (a) and its corresponding contents in PAH (b) collected in the first cyclone, PM (c) and its corresponding contents in PAH (d) collected in the second cyclone, and PM (e) and its corresponding contents in PAH (f) collected on the nylon filter as a function of the air total flow (Samca coal, AFBC, 10% excess oxygen, 850 °C). The variation of the PM (panel a) and its PAH content trapped in the first cyclone (panel b) at different combustion temperatures are shown in Figure 2. In general, an increase in combustion temperature (Table 2 and Figure 2) means an increase in PM and PAH emissions till a maximum in the 750-850 °C interval for the material trapped in the first cyclone is reached. At higher temperatures, a decrease in the PM and PAH content are observed due to a better combustion. The maximum observed in the PAH content trapped in the first cyclone is due mainly to anthracene, a three-ring PAH. The PM (Figure 2c) and its PAH content trapped in the second cyclone (Figure 2d) have also been studied as shown in Figure 2. While PM is almost constant, its PAH content reaches a maximum at temperatures of 800 °C, mostly due to acenaphthene. But both amounts are lower in comparison to the ones collected in the first cyclone. The PM (Figure 2e) and its PAH content trapped on the nylon filter (Figure 2f) as a function of the combustion temperature have also been studied. For the PM, the

minimum variation at the different combustion temperatures is observed. The PAH content shows a maximum in the range between 700 and 800 °C where the PAH emissions are mainly due to compounds such as acenaphthene, anthracene, and fluorene, compounds of three-ring size, the most volatile compounds. As a function of the combustion temperature, PM is mostly collected in the first cyclone. On the contrary, the worthless PM trapped on the filter shows the higher PAH content. When increasing the combustion temperature, the smaller the PM size, the higher the PAH content. These results could be explained taking into account that despite the airflow rate being constant, with increasing combustion temperatures there is an increase in the outlet flows. These faster flows diminish the residence time and so the radical interactions become difficult. As a consequence, longer stays corresponding to lower temperatures promote higher molecular weight than coronene, detected as PM but not as PAH. Percentage of Excess Oxygen Influence. The influence of the percentage of excess oxygen (28) was studied keeping VOL. 33, NO. 18, 1999 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 5. Representation of (a) PAH/OM and (b) PM/OM for the combustion temperature, (c) PAH/OM and (d) PM/OM for the percentage of excess oxygen, and (e) PAH/OM and (f) PM/OM for the airflow rate. constant the combustion temperature (850 °C) and the total airflow (860 L/h). The PM (Figure 3a) and its PAH content collected in the first cyclone (Figure 3b) as a function of the percentage of excess oxygen show opposite trends. While PM emissions increase with the increasing percentage of excess oxygen, the PAH supported on this PM decrease dramatically from 5 to 10% excess oxygen. The important lowering can be explained taking into account the two possible reactions undergone by the radicals released at the devolatilization step (Figure 1). Once the radicals are released, there are two competitive reactions so that radicals can stabilize: oxidation, driving to its elimination as CO2 and H2O, and condensation, driving to PAH formation and emission. The study of the PM (Figure 3c) and its PAH content trapped in the second cyclone (Figure 3d) shows that the PM is almost constant. With respect to its PAH content, it is clear that an increase in the percentage of excess oxygen implies a better combustion. High oxygen contents favor the interaction and elimination of radicals released in the 3182

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combustion, decreasing the possibility of radicals interaction between themselves and consequently the PAH content in the PM. For the PM (Figure 3e) and its PAH content collected on nylon filter (Figure 3f), there are not significant variations between the different percentages of excess oxygen and the PM supported on the nylon filter, mainly due to the low amounts collected. Concerning to the PAH emitted supported on the nylon filter PM, it is observed that, in general, an increase in the percentage of excess oxygen decreases the PAH amount collected on the nylon filter. Therefore, the influence of the excess oxygen in the PAH formation and emissions is remarkable. In general, the combustion conditions that favor the oxidation reactions with the radicals elimination by oxidation will decrease the PAH emissions. Working at these conditions, an increase of the percentage of excess oxygen is not relevant to PM emission, but it is a determinant to abate PAH emissions supported on this PM. The PAH decrease is mainly due to 3-4 rings lowering formation.

Air Total Flow Influence. Another variable studied has been the total airflow influence (29) keeping constant the combustion temperature and the percentage of excess oxygen. The study of the PM (Figure 4a) and its PAH content trapped in the first cyclone as a function of airflow rate (Figure 4b) shows that an increase in the airflow rate produces an increase in the PM. This increase can be mainly due to the entrainment at high fluidization velocities. Figure 4a shows that good fluidization conditions allow the reduction in PM, but when the air total flow is higher than double the minimum fluidization, to 860 L/h, a higher PAH amount is emitted. This PM, below 860 L/h, is mostly composed of elutriated material entrainment (PM I) more than PM II. In fact, this elutriated material is mainly collected in the first cyclone and does not go to the second cyclone (Figure 4c). At higher airflow rates, the PM is composed both of PM I and PM II. When the PAH content trapped in the first cyclone (Figure 4b) at various airflow rates is studied, it is observed that these emissions decrease at good fluidization conditions (flows close to double the minimum fluidization velocity). From 700 to 860 L/h, the increase in the total airflow implies a more homogeneous bed (better bubbles-fuel mixture), decreasing the PAH emitted supported on the PM. In fact, till 860 L/h, the PAH content decreases due to a good fluidization bed as a consequence of the good contact between oxygen bubbles and released radicals. When the airflow rate is increased, the small air bubbles become bigger. It is known as a slugging regime. Most of the oxygen is lost by reduction of the surface area without interacting with radicals emitted, which will stabilize between themselves by condensation, increasing the PAH content. So at 900 L/h, a maximum mainly due to acenaphthene, fluorene, and pyrene (volatile compounds of 3-4 rings) is observed. It means that radical elimination by oxidation reaction is difficult at higher flows, promoting the PAH formation and the PM II (soots) formation. Figure 4, panels c and d, show the PM and PAH content trapped in the second cyclone as a function of the total airflow. At the slugging regime, an entrainment is produced with the increase in the PM showing a maximum at 1000 L/h. Anyway, the PM variations with the airflow rate are not very relevant in the second cyclone in comparison with those trapped in the first cyclone. Concerning the PAH content (Figure 4d), the bed slugging produced as a consequence of the high flows and high exit velocities of the radicals increases the PAH emitted supported on the PM of the second cyclone in a drastic way. At 900 L/h, the main compounds emitted are fluorene, pyrene, chrysene, and acenaphthene. At higher flows than slugging regime, the PAH content decreases, although it always shows higher values than those obtained at lower flows (good fluidization conditions). At these higher flows than 900 L/h, the material is swept to the nylon filter, and it corresponds to the increase in the PAH content on nylon filter (Figure 4e). The study of the PM on the nylon filter (Figure 4e) at different airflow rates shows that an increase in the total airflow produces an entrainment of the PM, especially at flows corresponding to the slugging regime and higher. Anyway, the variations are minimal. On the other hand, the influence of the total airflow on PAH content supported on the nylon filter (Figure 4f) shows that good fluidization conditions (until 860 L/h) favor the radicals elimination by oxidation, decreasing the PAH emissions. The higher the airflow rate, the higher the PAH emissions supported on PM. This has also been shown when

the total PAH emitted has been studied at different airflow rates (29). These results show that a good fluidized bed is essential in order to diminish the radical condensation and to avoid the PAH formation and emissions. On the contrary, the slugging regime promotes the PAH formation and emission. According to the results obtained, it can be deduced that, at coal AFBC, the bed regime is determined considering the PAH formation and emission supported on PM. High percentages of excess oxygen and high total airflows increase the OM content of the PM. Despite the fact that there is not a direct relationship between PM and the PAH emitted supported on it (see Figure 5), the good fluidization conditions are determined to reduce the PAH emitted and supported on PM. Concerning PAH emissions control, at fluidized beds, the combustion variables (temperature, percentage of excess oxygen, and total airflow) can favor the PAH abatement emitted containing PM by burning coal at: temperatures out of the 700-800 °C range, high percentages of excess oxygen, mainly higher than 10%, and flows close to double the minimum fluidization velocity.

Acknowledgments The authors thank the ECSC by financial support, Project Ref 7220/EC/089, and the CICYT, Project Ref AMB 98-1583.

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Received for review March 3, 1999. Revised manuscript received June 10, 1999. Accepted June 28, 1999. ES990241A

Polycyclic Aromatic Hydrocarbons and Organic Matter Associated to

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