Environ. Sci. Technol. 2008, 42, 5068–5073
Characteristics of Particulate Carbon Emissions from Real-World Chinese Coal Combustion Y U A N X U N Z H A N G , †,‡ J A M E S J A Y S C H A U E R , †,‡ YUANHANG ZHANG,† LIMIN ZENG,† YONGJIE WEI,† YUAN LIU,† AND M I N S H A O * ,† State Key Joint Laboratory of Environmental Simulation and Pollution Control, College of Environmental Sciences, Peking University, Beijing 100871, China, and Environmental Chemistry and Technology Program, University of WisconsinsMadison, 660 North Park Street, Madison, Wisconsin 53706
Received September 8, 2007. Revised manuscript received March 26, 2008. Accepted April 10, 2008.
Particulate matter emissions from a series of different Chinese coal combustion systems were collected and analyzed for elemental and organic carbon (EC, OC), and molecular markers. Emissions from both industrial boilers and residential stoves were investigated. The coal used in this study included anthracite, bituminite, and brown coal, as well as commonly used coal briquettes produced in China for residential coal combustion. Results show significant differences in the contribution of carbonaceous species to particulate mass emissions. Industrial boilers had much higher burn out of carbon yielding particulate matter emissions with much lower levels of OC, EC, and speciated organic compounds, while residential stoves had significantly higher emissions of carbonaceous particulate matter with emission rates of approximately 100 times higher than that of industrial boilers. Quantified organic compounds emitted from industrial boilers were dominated by oxygenated compounds, of which 46-68% were organic acids, whereas the dominate species quantified in the emissions from residential stoves were PAHs (38%) and n-alkanes (20%). An important observation was the fact that emission factors of PAHs and the distribution of hopanoids were different among the emissions from industrial and residential coal combustion even using the same coal for combustion. Although particulate matter emissions from industrial and residential combustion were different in many regards, picene was detected in all samples with detectable OC mass concentrations, which supports the use of this organic tracer for OC from all types of coal combustion. 17R(H),21β(H)-29-norhopane was the predominant hopanoid in coal combustion emissions, which is different from mobile source emissions and may be used to distinguish emissions from these different fossil fuel sources.
Introduction Coal combustion accounts for about a fourth of the total world energy consumption and is expected to have an annual * Corresponding author phone: (8610) 6275-7973; fax (8610) 62757973; e-mail:
[email protected]. † Peking University. ‡ University of WisconsinsMadison. 5068
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global consumption of 10.6 billon tons in 2030, twice that of 2003, with a current annual increase of 2-3% per year (1). Due to the expected growth and potential high emissions produced by coal combustion, there exists considerable concerns over regional and global impacts of expected future coal combustion (2). Much of the attention in the past over the environmental impacts of coal combustion have focused on gaseous pollutants such as CO, CO2, NOx and SO2 due to their impacts on climate changes, acid rain, and indoor environmental pollutions. However, particulate matter emitted from combustion contains toxic constituents such as heavy metals, arsenic, mercury, polycyclic aromatic hydrocarbons (PAHs), silica, fluorides, and black carbon (BC), as well as many other hazardous compounds that also have adverse impacts on environmental quality and human health (3–6). In recent years, there has been a significant increase in the concern of the carbonaceous material in fine particulate matter emissions from coal combustion, due to their impacts on climate changes and health effects (7–9). It is estimated that the world coal combustion contributed 1129 Tg BC and 877 Tg organic carbon (OC) in 1996, accounting for 14.2% and 2.6% of the global combustion related emissions of BC and OC, respectively (10). Such emissions by coal combustion are leading to the development and implementation of clean coal technologies (11); however, there still exists considerable high emissions coal combustion in many regions of the world. Emissions from coal combustion are very diverse due to the large variations in real-world coal combustion processes that include industrial and residential burnings and span wide ranges of combustion conditions, control technologies, and coal feedstocks (12, 13). Although there are only a few studies that have characterized the carbonaceous particulate matter emissions from coal combustion, the limited data shows large differences among the compositions of the particulate matter emisisons from different coal combustion systems (10). In some cases, variation in the BC emissions may be as large as a factor of 8 (14). Furthermore, organic compounds in the carbonaceous particulate matter emissions from real-world coal combustion can provide important source apportionment information, but have not been well quantified in the past. To this end, there is a great need to better characterize emissions from real-world coal combustion facilities to aid in emission inventory development and source apportionment studies. There is great interest in the emissions from coal combustion in China since coal combusiton makes up about 70% of China’s primary energy consumptions (15). Pollution is the leading cause of death in China, and coal combustion is regarded as an important air pollution source (2). In China, coal is burned in many different combustion systems, which utlize a range of different combustors and types of coal. In many developed mega-cities such as Beijing, coal combustion has been banned except for permitted high capacity boilers with effective control technologies, whereas in the rural areas and undeveloped regions, simple household stoves without emmissions controls are still widely used for coal combustion for heating and cooking (15). In this study, 23 coal combustion tests were conducted to characterize their carbonaceous particulate matter emissions. Particle-phase organic compounds were measured for each emission test to understand how emisisons vary with combustion processes and the types of coal burned. The results are expected to provide more information about particulate organic compounds from different types of coal 10.1021/es7022576 CCC: $40.75
2008 American Chemical Society
Published on Web 06/07/2008
TABLE 1. Emission Factors of PM2.5 Mass, Organic Carbon, And Elemental Carbon for Different Types of Chinese Coal Combustion (Units: mg kg-1 Coal, Mean ± Stdev) industrial PM2.5 EC OC
residential
bituminite (n ) 16)
brown (n ) 4)
mixed (n ) 13)
anthracite (n ) 8)
bituminite (n ) 9)
coal briquettea (n ) 4)
16 ( 4 0.062 ( 0.016 0.30 ( 0.07
100 ( 15 2.8 ( 0.4 17.1 ( 2.6
45 ( 26 0.7 ( 3.4 1.9 ( 0.7
1054 ( 450 28 ( 35 470 ( 249
7373 ( 2671 2750 ( 2044 2975 ( 1942
5242 ( 471 95 ( 34 2265 ( 387
a Emission factors of residential combustions were results of laboratorial tests (No. 18-23 of Table S1 in the Supporting Information) since the mass of field coal consumptions were not measured.
combustion to aid emission inventory development and source apportionment studies.
Materials and Methods Sampling. The 23 coal combustion experiments were conducted (see Table S1 in the Supporting Information) including 13 industrial boiler tests and 10 residential stove tests, which were chosen to represent dominate real-world coal combustion emissions in China. Different coals were combusted including anthracite, sub-bituminite, bituminite, and brown coal, which represent Chinese coals with maturity orders from high to low. Mixed coal, which consists of a mixture of bituminite and brown coal, is burned in some industrial boilers in China. Considering the fact that coal properties are not the same for different regions of China, coal from the main coal-mining regions in China were selected including Shanxi, Hebei, and Beijing. For the residential coal combustion, coal briquettes, which are made from bituminite and clay, as described by Chen et al. (16), were also burned. All the industrial coal combustors had pollution controls, including electrostatic dust collectors, cyclones, and a water film duster. The residential stoves, which were tested, did not have any emissions control technologies. The industrial coal combustors included pulverized coal combustors and stoker boilers, which are the most popular coal combustors used in China. Two residential stoves were tested: (1) a randomly selected brick stove in a house in a rural region of Beijing, which was fabricated with a brick chimney, and (2) a steel stove with refractory lined furnace chamber, which is widely used in the suburban areas of China for cooking and heating. The steel stove was operated in the laboratory. A schematic of the sampling equipment is presented in Figure S1 in the Supporting Information. Industrial samples were collected via two sets of dilution samplers constructed using the design presented by Hildemann et al. (17), which has been widely used for source testing in the past. Emissions emitted from the exhaust stacks were isokinetically withdrawn and mixed with ultra purified air in the dilution sampler to simulate the process of aerosol evolution in dilution in the ambient air. Samples were collected from the dilution sampler residence chamber. Emissions from the residential stove in laboratory tests were collected by a large iron exhaust hood and were mixed with ambient air and then sampled into the dilution system for further dilution, aging, and sample collection. The emissions from the residential brick stove were collected at the chimney outlet downwind in the plume. More details are presented in the Supporting Information. Mass and Chemical Analysis. Emission factors (EFs) were determined by the mass ratio of particulate matter emitted and the fuel consumptions. Coal mass was weighed by a platform balance. Filters were weighed by a microbalance (Mettler AE 204) in clean room maintained at 25 °C and 50% relative humidity after equalized for at least 48 h before and after sampling. The weight of every filter was taken as the average of three continuous weights if they agree within 60 µg. The organic and element carbon (OC/EC) concentrations
were measured using a thermal-optical method, Sunset Laboratory laboratory-based instrument (Sunset Laboratory Inc.) (18). Organic compounds collected on the quartz filters were quantified using the extraction-derivatization GC-MS method described by Zhang et al. (19) as a part of a series of Chinese combustion source tests. The analytical method was a modified version of method previously reported by Zheng et al. (20) and Schauer et al. (21, 22). The organic compound analysis involved the extraction, derivatization, and an instrumental analysis. Before the extraction, mixtures of isotopically labeled internal standards, consisting of 26 deuterated and 2 C13 labeled organic compounds, were quantitatively spiked on the filters. After spiking, the sample was extracted three times with 30 mL of a mixture of dichloromethane and methanol (3:1, v/v, pesticide grade, TEDIA) in thick wall sealed bottles using a mild ultrasonic bath maintained at room temperature. The extracts were filtered and then reduced to about 5 mL using a rotary vacuum evaporator. After the rotary evaporation, the extract was concentrated to 1 mL by nitrogen (ultra purified,>99.9%) blow down and then split into two aliquots. One was derivatized with BSTFA (BSTFA/TMCS, 99:1, Supelco) to substitute the active hydrogen with trimethylsilanized derivative. Both of the derivatized and underivatized fractions were analyzed using an Agilent GC-MS (6890plus-5973N) equipped with a DB-5MS column (60 m length, 0.25 mm diameter, 0.25 µm film thickness, Agilent). The oven temperature program was operated as follows: 60 °C hold for for 10 min followed by a heat to 300 °C for 38 min at 6 °C/min). Multipoint calibration method was conducted using authentic standards spiked with the same internal standard concentrations as used in the samples. Quantified compounds include monosaccharides, methoxylated phenols, polycyclic aromatic hydrocarbons (PAHs), n-alkanes, nalkanols, sterols, etc. (19). Laboratory blanks were included with each batch of samples. In this study, laboratory blanks were much lower than emissions samples for all target compounds. Spiked filters were analyzed to determine the overall efficiency and accuracy of the measurements. Average recoveries and deviations of the spike samples were 67.9-103.3 and 5.1-16.3%, respectively. Field blanks were analyzed and subtracted from all the samples. Coefficients of correlation (R2) of standard compounds calibration curves were 0.990 ( 0.017. Target compounds with the single to noise ratio (peak area) less than 10 were not quantified. Instrument detection limits for target compounds in this study was 0.1-4 ng µL-1 (1 µL splitless injection) (19).
Results and Discussion Emission Factors (EFs) of Mass and EC/OC. Emission factors of PM2.5 mass, EC, and OC for different Chinese coal combustion tests are listed in Table 1. Since the residential combustors did not contain after-treatment control devices, significant differences between industrial and residential combustion were observed. The EFs for residential stoves VOL. 42, NO. 14, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 1. Mass fractions of organic matter (OM) and element carbon (EC) in fine particulate matter emission from Chinese coal combustion. OM is estimated as 1.2 times of organic carbon (OC). See text for details. were approximately 2 orders of magnitude higher than of the emissions of the industrial boilers, even when using the same coal for combustion. Mass emission factors of residential coal combustion tests ranged from 1 to 7 g kg-1 coal, which close to the results by Streets et al. (14) and Bond et al. (23), which were 8 and 4.6-12 g kg-1 coal, respectively. For the industrial coal burnings, Bond et al. (10) summarized the EFs of particulate matter emissions, which spanned from 1.3 to 12 g kg-1 and from 17 to 33 g kg-1 for hard coal and brown coal combustion, respectively. The EFs for industrial coal combustion reported by Bond et al. (10) are much larger than the results of the current study. The results of Bond et al. (10) included literature data of emissions of total PM and PM10 from different studies that were more than 10 years old, which may be the reason for the differences in EFs. Comparison of EFs within the same group of industrial boilers or residential stoves showed that EFs were highly influenced by the coal maturities. Coals with low maturity usually have relative high volatile contents, which is the precursor material for particulate matters during combustion (23). Therefore, EFs are expected to increase as the volatile content of the coal increases. Both industrial and residential combustion showed such a tendency. However, results in Table 1 show that combustion category (industrial or residential) was the most important factor influencing the EFs. The one exception was the EFs of coal briquette combustion in residential stoves, which had lower EFs than raw bituminites, even though they use the same stove and have the same coal maturities. The contributions of carbonaceous matter, including EC and organic matters (OM, 1.2 × OC) (24), to particulate matter mass are shown in Figure 1. Compared with the residential stoves, mass fractions of EC and OM of industrial emissions were very low, especially for industrial bituminite combustion emissions, which were too low (0.004 ( 0.001 and 0.019 ( 0.005 for EC and OM, respectively) to be visible in Figure 1. Industrial brown coal combustion emitted more carbonaceous material than the combustion of bituminite in the same system. Mixed coal combustion has moderate levels of carbonaceous material in the emissions, between the levels present in bituminite and brown coal emissions. Previous studies showed that the primary emissions from industrial coal combustion were dominated with crustal material, with carbon levels typically below 1%. These results were close to 5070
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FIGURE 2. Mass fractions of organic compound classes in quantified organic compound in PM2.5 emission from Chinese coal combustion. the data summarized by Bond et al. (10). Figure S2 in Supporting Information shows the inorganic fractions of industrial emissions, which is predominantly sulfate and ammonium ions, along with other ions including phosphate and nitrate. Industrial combustion of coal typical yields more complete combustion of the carbon to carbon dioxide and thus inorganic ions make up much more of the particulate matter mass in the industrial emissions compared to the residential coal burning emissions. In contrast, the residential stoves have emissions dominated by carbonaceous material with 7.8-56.3% of the mass being OC and 1.8-37.3% of the mass being EC. However, the coal briquette was different from the other residential coal combustion emissions in that its particulate mater emissions had much lower amounts of carbonaceous material. The coal briquette emissions from the field tests had similar OC and EC contents as the emissions from the industrial coal combustion facilities, while the emissions from the laboratory burned coal briquette had particulate matter emissions with carbonaceous constituents more similar to the residential coal combustion units. Bulk Organic Matters. The mass emissions rates of about 200 organic compounds were quantified and are listed in Table S2 in the Supporting Information, including n-alkanes, aliphatic acids, aromatic acids, and PAHs, etc. The total speciated organic mass fractions are very small ranging from 0.001 to 0.121. Small amount of levoglucosan, the marker of biomass burning (19), were measured in the particulate emissions suggesting that up to 0.2% of OC mass emissions from the industrial coal combustion and up to 1.1% of OC mass emissions from the residential stoves (field tests) were contributed by biomass smoke since some biomass materials are usually added into those stoves as combustion-supporting agents (e.g., wood or paper as kindling). The contributions of different classes of organic compounds to OM are presented in Figure 2. Similarly to the previous discussion of EFs, characteristics of bulk OM can be classified into two primary groups: industrial and residential combustion. Quantified OM of industrial coal combustion were predominant comprised of aliphatic acids, both n-alkanoic acids and dicarboxylic acids. Aliphatic acids plus aromatic acids accounted for 46-68% of the quantified OM mass. For the residential combustion, the predominant organic compounds measured were PAHs and n-alkanes, which accounted for about 38 and 20% of total quantified OM mass, respectively.
FIGURE 3. OC normalized emission rate of PAHs in PM2.5 emission from Chinese coal combustion.
Such large differences are consistent with the previous discussion about the completeness of combustion. PAHs are regarded as the byproducts of incomplete combustion. In the residential stoves, a wide range of temperature regions exist during combustion. Smoldering (300 °C) conditions determine the distributions and ratios of the original and altered compounds present in coal smoke (13). Therefore, compounds in coal, dominated with aromatic derivatives, are largely released directly and condensed in the regions of low temperature, insufficient ventilation during the incomplete combustion processes which is commonly occurred in the residential stoves. By contrast, industrial boilers have high temperatures and controlled air ratio, leading to pulverized coal combustion operating at temperatures in the range of 1600-1800 °C, and stoker combustion operating in the range of 1200-1300 °C. Previous studies have shown that the emissions of PAHs in industrial boilers are primarily dependent on the combustion temperature and excess air ratio (25), thus the relatively complete combustion in industrial boilers tend to generate the compounds with high oxidant status such as organic acids. Figure 3 shows OC normalized emission rate of different PAHs. Obviously, the levels of PAHs in fine particulate organic matters of industrial boiler were much lower than that of residential stoves. Notably for the residential coal combustion, although coal briquette combustion has relative lower EFmass as discussed previously, levels of PAHs in the particulate matter emissions were similar to the other residential coal combustion tests. Coal briquettes can reduce the mass emissions but not significantly change their chemical compositions. Homologous Compound Series. Normal alkanes were detected from all the samples, making up 4.8-45.2% of total quantified OM mass. n-Alkanes are predominant in the emissions from many fossil fuel combustion systems including vehicle exhaust and oil burning. They are also present
FIGURE 4. Comparison of the emission rates of source profiles of hopanes normalized to 17 r(H),21β(H)-hopane between coal combustion (this study) and mobile source emissions. Note: L ) Laboratory test, F ) Field test, Gas(S) ) Gasoline powered vehicle exhaust tested by Schauer et al. (21), Gas(R) ) Gasoline powered vehicle exhaust tested by Rogge et al. (32), LDV Tunnel ) Gasoline powered light duty vehicle predominate tunnel tested by Phuleria et al. (31), HDV Tunnel ) Diesel powered heavy duty vehicle predominate tunnel tested by Phuleria et al. (31). Light and heavy duty diesel exhaust results were referenced from the works by Schauer et al. (22) and Rogge et al. (32), respectively. VOL. 42, NO. 14, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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in the emissions from nonfossil fuel sources including biomass burning, cooking emission, or vegetative detritus (26). Distributions of n-alkanes are different between anthropogenic (mostly fossil fuel combustion) and natural sources: the former typically has a Gaussian distribution of n-alkanes in terms of carbon number, while emissions from natural source tend to have odd-to-even carbon number predominance (27). In this study, carbon predominant indexes (CPI, ratio of summed odd-carbon number n-alkanes to even-carbon number n-alkanes) for industrial and residential coal combustion were 1.2-1.4 and 1.1-1.6, respectively, dominated with n-docosane, n-tricosane, and n-pentacosane, which is similar to the previous study by Oros and Simoneit (13). Normal alkanoic acids account for a large portion of total quantified OM in the emissions especially for industrial coal combustion. Oros and Simoneit (13) reported that the CPI values of the emissions will decrease with increasing coal maturities ranging from 0.9 to 4.3. In this study, CPI values ranged from 1.8 to 6.6 for residential combustion and 2.8 to 12.6 for industrial combustion with the predominant compounds n-hexadecanoic acid. No significant differences were observed among different coals, except individual comparisons within industrial bituminite (CPI ) 2.8) and brown coal (CPI ) 4.6), or residential anthracite (CPI ) 5.3) and bituminite (CPI ) 7.0). PAHs. PAHs are byproducts of incomplete combustion which can be classified into two processes, pyrolysis and pyrosysthensis. For example, benzo[a]pyrene and other PAHs are typically formed (pyrosysthensis) through the pyrolysis process of methane, acetylene, butadiene, and other compounds as precursors (28). Although the individual PAHs yield as a function of combustion temperature follows a random distribution, the total yield of PAHs in the emissions from the coal combustion tests had a consistent trend. Previous study by Davis et al. (29) showed a similar result that PAHs emission factors of residential combustion were 3 magnitudes higher than that of industrial combustion. Previous studies applied the ratio of indeno[1,2,3-cd]pyrene (IP) to benzo[ghi]pyrene (BghiP), expressed as IP/ (IP+BghiP), as an indicator of emission sources. Ratios of 0.18, 0.37, and 0.56 were reported for gasoline powered vehicles, diesels and coal combustion, respectively (30). In this study, values of IP/(IP+BghiP) were, on average, 0.50 and 0.57 for industrial and residential combustion, respectively, except coal briquette combustion which have average IP/(IP+BghiP) values of 0.35, similar with a previous study which showed a value 0.33 (16). However, a common PAHs compound, picene, was detected in all the samples with detectable OC mass concentrations in this study and is unique to the OC emissions from coal combustions. Hopanoid Hydrocarbons. Hopanoid hydrocarbons are a series of pentacyclic triterpenoids, which include many kinds of isomers regarded as organic tracers for fossil fuel combustion (26). The origin of hopanoids are thought to be derived from cell membranes of prokaryotes and cyanobacteria in sedimentary organic matter over geological time or from certain high plants such as ferns (13). During geological time periods, the original structures of hopanoid gradually transformed into more stable thermodynamic structures, such as the stereochemical changes at C17, C21 and C22. Hopanes with the structure of 17β(H),21β(H) are immature, 17β(H),21R(H) are moderately mature and 17R(H),21β(H) are fully mature (13). Therefore, the fingerprint distributions of hopanoid isomers can be indictors of maturities of coals and their emissions, i.e., hopanoid indexes applied in geochemistry studies, including Tm/Ts, ∑(C29+ C30)/∑(C27+C28), C31[S/(S+R)], HP30/HP31R, and HP29/ HP31R, etc (13). 5072
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Figure 4 shows the relative emission rates of eight quantified hopanes normalized to 17R(H),21β(H)-hopane (HP30). Some previous results of vehicle exhausts were also plotted (21, 22, 31, 32). From Figure 4, significant differences between coal combustion and vehicle exhausts are observed. Almost all the coal combustion (except industrial bituminite) emit 17R(H),21β(H)-30-norhopane (HP29, Oros and Simoneit (13) used a different nomenclature for HP29 as “17R(H),21β(H)29-norhopane) as the predominant hopane, while the dominate hopane in vehicle exhausts is HP30. In addition, 17R(H)-22,29,30-trisnorhopane (Tm) in the emissions from coal combustion are higher than that of vehicle exhaust. In the emissions from the presented tunnel test, concentrations of HP29 and HP30 are both high due to the fact that tunnel emissions are actually a mixed source, including vehicle exhausts, road dust, tire detritus, etc. Results of this study agreed with the previous study of residential coal combustion (13). The values of some hopanoid indexes are listed in Table S3 in the Supporting Information but do not follow any significant trends that associate with geochemical maturity. As an example, the homohopane index (C31[S/(S+R)]) should be increased with maturity increasing (13), but the most mature coal, anthracite, has a lower value (0.511) than industrial bituminite (0.870). However, when divided into two groups, Industrial or residential combustion, geochemical rules are generally followed. Clearly, the type of coal combustion has a large influence on all the emission characteristics. Therefore, the traditional method which tracking and explaining the combusting pollutants relying on the fuel characteristics should not only pay attention to the fuel components, but also to the combustion equipment changes induced reaction kinetic and thermodynamic differences, which may have larger effect than the other factors.
Acknowledgments We thank Nan Zhou and Xuena Yu for their assistance with samplings and analysis of inorganic compounds. This research was funded by China National Basic Research and Development Program-2002CB410801, Hi-Tech Research and Development Programs of China-2001AA641060 and 2003AA641040.
Supporting Information Available Source sampling schematics for emission test are shown in Figure S1; Percentages of ion in fine particulate matters from industrial coal combustion are shown in Figure S2; Summary of Chinese coal PM2.5 emission tests are shown in Table S1; Mass fractions of organic compounds in fine particles from coal combustion are listed in Table S2; Hopanoid index values are listed in Table S3. This material is available free of charge via the Internet at http://pubs.acs.org.
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