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Fine-particle emissions of energy production in Finland
Atmospheric Environment 34 (2000) 3701}3711
Fine-particle emissions of energy production in Finland Mikael O. OhlstroK m , Kari E.J. Lehtinen , Mikko Moisio, Jorma K. Jokiniemi * VTT Energy, Aerosol Technology Group, P.O. Box 1401, FIN-02044 VTT, Finland Aerosol Physics Laboratory, Tampere University of Technology, P.O. Box 692, FIN-33101 Tampere, Finland Received 1 June 1999; received in revised form 30 November 1999; accepted 19 December 1999
Abstract The aim of this study was to "nd out the "ne-particle emissions (PM ) of di!erent energy production processes in Finland. The main purpose was to compare the calculated emission factors between di!erent energy production concepts. The purpose was also to de"ne what is known about "ne-particle emissions and what should still be studied/measured. Only those energy production processes that produce a signi"cant amount of direct emissions of solid particles have been treated here, i.e. pulverized combustion and oil burners from burner combustion, #uidized bed combustion processes, grate boilers, recovery boilers and diesel power plants. Secondary particles from gaseous pollutants have not been considered within this study. In pulverized coal combustion the particle emission is composed mainly of particles smaller than 5 lm in aerodynamic diameter. Roughly half of the total mass of particle emission is composed of "ne particles (PM ). Depending on boiler size category and particle separation devices, the speci"c emission factor for "ne particles is 1}30 mg MJ\. For pulverized combustion of peat, ca. 20}25% of the total mass of particle emission is "ne particles, and then the speci"c emission factor is between 5 and 8 mg MJ\. For recovery boilers, the "ne particle portion of the total particle emission is 50}60% (by mass) and the speci"c emission factor for "ne particles varies considerably according to the boiler size category, being between 12 and 77 mg MJ\. For oil burners, grate boilers and #uidized bed combustion processes, the "ne-particle portion of the total particle emission could not be determined, because there were no applicable measurement results at hand. For these combustion techniques, more public measurements are needed in order to clarify the amount and composition of "ne particles with di!erent fuel varieties. Additionally, the chemical composition of "ne particles is a very important feature when human health e!ects caused by particulate matter are considered. 2000 Elsevier Science Ltd. All rights reserved. Keywords: PM ; PM ; Speci"c emissions; Health hazard; Air quality; Coal combustion
1. Introduction Suspended particulates are among the most noteworthy air quality problems in the urban areas of Finland. Concentrations rise especially in the spring as snow melts and the ground gets dry, while during winter grinded gritting sand (anti-skid treatment) raises dust by tra$c and wind. In addition to gritting sand the suspended particulates contain material from road surfaces, car tires and exhaust gases and from the emissions of energy
* Corresponding author. Tel.: #358-9-456-6158; Fax: #358-9-456-7021. E-mail address: [email protected]." (J.K. Jokiniemi).
production and industrial processes. The whole mass concentration of suspended particulates in the air is called total suspended particulates (TSP). Particles less than 10 lm (0.01 mm) in aerodynamic diameter are called respirable particulates or PM (PM"partic ulate matter), whereas PM denotes "ne particles less than 2.5 lm in aerodynamic diameter. In this paper, when talking about "ne particles, the PM is signi"ed. PM is composed of a mixture of particles emitted directly into the air and particles formed in the air from the chemical transformation of gaseous pollutants (secondary particles). The principal types of secondary particles are ammonium sulfate and nitrate formed from sulfur dioxide (SO ) and oxides of nitrogen (NO ), V
1352-2310/00/$ - see front matter 2000 Elsevier Science Ltd. All rights reserved. PII: S 1 3 5 2 - 2 3 1 0 ( 0 0 ) 0 0 0 7 6 - 5
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M.O. Ohlstro( m et al. / Atmospheric Environment 34 (2000) 3701}3711
reacting with ammonia (NH ). The main sources of SO and NO are the combustion of fossil fuels in boilers and V the mobile sources. The types of directly emitted particles are soil-related particles and particles from the combustion of fossil fuels and biomass materials. The main sources of soil-related particles are roads, construction and agriculture. The main sources of combustion-related particles are diesel motor vehicles, managed burning, open burning, residential wood combustion, utility and commercial boilers. The reason for this study was the detected e!ects on human health caused by "ne-particles (e.g. Dockery and Pope, 1994). The main purpose was to de"ne the "neparticle (PM ) emissions of energy production and to compare the calculated emission factors between di!erent energy production concepts. The purpose was also to de"ne what is known about "ne-particle emissions and what should still be studied/measured. Furthermore, only the direct particulate emissions of energy production processes are considered here and the gas-to-particle conversions of SO and NO are left out. It must be V considered, however, that over half of the "ne particle concentration in the atmosphere, which is caused by combustion processes, are secondary particles from gasto-particle conversions (IPCC, 1996). Secondary particles are mostly in the size range of 0.1}1 lm in diameter (Turbin and Huntzicker, 1995), so they remain in the atmosphere longer than coarser particles. The rate of the gas-to-particle conversion is about 1% h\ for SO and 5}10% h\ for NO (Seinfeld, 1986). The factors a!ectV ing the rate of conversion are temperature, the amount of sunlight, the relative humidity of air and the concentrations of other chemically reactive substances in the air (e.g. ozone). The conversion is fastest during warm, bright and humid weather. According to present knowledge the health hazards of "ne particles depend on their chemical composition, deposition in lungs and behavior in respiratory organs. Thus, the smallest particles would be the most dangerous because they can penetrate all the way to the alveoli and further to blood circulation (Seaton, 1998). Additionally, their chemical compositions di!er from the coarser natural particles, because the "nest particles are formed in combustion processes (energy production, industrial processes, vehicles). The speci"c surface area of "ne particles is large due to their high number concentration and the non-spherical shape of agglomerates. Therefore, they typically contain large amounts of condensed heavy metals vaporized from fuels in boilers. Accumulation of this foreign matter in human organs may cause symptoms and illnesses that occur either acutely (e.g. symptoms of asthma, heart failures) or after a long-time exposure (e.g. cancers, pulmonary diseases). This has been veri"ed by epidemiological studies done by e.g. Dockery et al. (1992), Dockery and Pope (1994), Burnett et al. (1995) and Schwartz (1993, 1994). Accordingly,
Schwartz et al. (1996) reported that increased concentrations of PM , PM and sulfate particles in the big cities of U.S. East Coast are signi"cantly related to increased daily mortality of these cities. PM particles had the strongest correlation to the mortality "gures (#1.5% increase in mortality per 10 lg m\ rise in two day mean PM concentration of ambient air). Even greater e!ects on pulmonary diseases (#3.3%) and heart diseases (#2.1%) were obtained. World Health Organization (WHO) has estimated the reduction of life expectancy associated with a certain di!erence in long-term exposure to PM by combining the results of two cohort studies, i.e. Dockery et al. (1993) and Pope et al. (1995). A 10 lg m\ di!erence in longterm exposure to PM results in an estimated relative risk of 1.10 (WHO, 1994). This relative risk estimate has been applied to the 1992 life table for Dutch men for which the estimated e!ect on life expectancy is 1.1 yr. The calculation was restricted to ages 25}90, as the cohort studies have not gone beyond the age of 90. In Finland, the National Public Health Institute has presently several research projects studying the health e!ects caused by "ne particles. Because of di!erent climate conditions, the international studies cannot be directly adapted to national purposes. For instance, the cold winter creates an extensive need of heating energy. Also, the health e!ects of "ne particles in frosty weather may be di!erent from those in warm weather. Pekkanen et al. (1997) studied the exposure of asthmatic children to PM in Kuopio, Finland. The occurrences of coughing or wheezing attacks, recorded in a questionnaire, correlated with the ambient concentrations of PM , PM , and black smoke measured at a single air quality monitoring site in the area. The main sources of "ne particles were tra$c, a peat-"red power station, and a corrugated cardboard mill. Also, wood burning during the winter period was common in 25% of the homes in the area. In addition to these health hazards the "ne particles also have other, mostly injurious, e!ects. They reduce visibility, the surfaces get dirty and su!er material damages (corrosion). Fine particles also re#ect solar radiation back to the outer space, cooling the atmosphere and slowing down the greenhouse e!ect. Several countries and a number of regulatory bodies have set or are just considering the ambient air quality standards for PM and PM . Current standards in Finland were set on 1 September 1996. Earlier there was only a standard for TSP, but now a standard for PM also exists. The guideline value is 70 lg m\ as the second highest 24 h value per month. No standard for PM exists. The current guideline value for TSP is 120 lg m\ as the 98th percentile of 24 h values in a year and 50 lg m\ as annual arithmetic mean. WHO guidelines for PM have been set at 70 lg m\ for 24 h average concentrations (Wilson and Spengler, 1996), quite similar to Finland's
M.O. Ohlstro( m et al. / Atmospheric Environment 34 (2000) 3701}3711
standards. The European Commission (EC) has proposed that the existing limit for total particulate matter is removed and a new limit set for PM . The proposed new air quality standards for PM would be enforced as a two-stage process. At the "rst stage (1 January 2005), the annual average of 30 lg m\ is to be proposed (EC, 1997). At the second stage (1 January 2010) the value of annual average should be below (or equal) 20 lg m\. The values for 24 h averages should be 50 lg m\ not to be exceeded by more than 25 times per year (by 2005), and not more than 7 times per year (by 2010) (EC, 1997). The new national air quality standards (NAAQS) were promulgated in the USA in July 1997 by United States Environmental Protection Agency (US EPA). The new particulate matter NAAQS include standards also for PM particles. Both 15 lg m\ as annual 3 yr average and spatial averaging and 65 lg m\ as the 98th percentile of 24 h values in a year at highest monitor averaged over 3 yr were set (US EPA, 1997). The problems with PM standards, however, are the di$culties in measuring and thus controlling the concentrations of "ne particles. There are many sampling and analysis systems and numerous di!erent measuring instruments available. The results of di!erent measuring methods are not yet very comparable with each other. Inaccuracies can be of the order of 50%. The ambient air quality networks are also insu$cient for decent regional monitoring. In addition, there are regional areas where even the background concentrations of "ne particles alone may be higher than the new schemes of PM standards. There are some recent studies about PM and PM emissions. Smith and Sloss (1998) produced a quite comprehensive report about PM /PM emissions and ef fects concerning the member countries of International Energy Agency (IEA). The report includes legislation, sampling networks, emissions of primary particles, formation of secondary particles, concentrations in the atmosphere and health e!ects. They emphasize that the statistical epidemiological studies have only demonstrated the link between particulate matter and health. More data on the chemical characterization of particulate emissions and their biological e!ects are thus needed to prove the causality between ambient air concentrations of "ne particles and human health e!ects. Berdowski et al. (1996) have studied primary particles from anthropogenic sources. Fine particulate matter emissions have been calculated for each country in Europe based on statistical data and emission factors and particle size distributions based on literature data.
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cesses in Finland. This was done by calculating speci"c emission factors (mg MJ\ ) from powerplants' annual total particulate matter emissions (ty\, 1 t"1000 kg) and by estimating the fraction of "ne particles from the total emissions with the help of existing (publicly available) measurement results. The data containing e.g. the total particulate matter emissions and fuel energy contents of all the di!erent energy production subprocesses were obtained from the Finnish Environmental Institute (VAHTI, 1998). The reference year of data was 1995. Only those energy production processes that produce a signi"cant amount of direct emissions of solid particles have been treated here, i.e. pulverized combustion and oil burners from burner combustion, #uidized bed combustion processes, grate boilers, recovery boilers, and diesel power plants (see Fig. 1 for the primary energy consumption palette by energy sources in Finland). Secondary particles from gaseous pollutants have not been considered within this study. The energy production processes have been classi"ed according to boiler type, size category, main fuel and also according to dust separation devices. To be able to compare di!erent energy production processes, a shared speci"c emission factor has been calculated for the similar subprocesses. The energy production processes have been divided according to fuel power (P) into the following size categories: 0(P)50 MW, 50(P)300 MW, 300(P) 500 MW and P'500 MW. For oil burners and grate boilers, however, the smallest group has been additionally divided into two subgroups (0(P)5 MW and 5(P)50 MW) because of the great amount of small boilers using these combustion techniques. Each size category has been further divided according to combustion technique and dust separation devices. The dust separation devices include mechanical/inertial collectors (e.g. cyclones/multicyclones), electrostatic precipitators
2. Methods The aim of this study was to "nd out the "ne-particle emissions (PM ) of di!erent energy production pro
Fig. 1. Total consumption of primary energy (1285,2 PJ) by energy sources at 1995 in Finland (Statistics Finland, 1996).
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(ESP), fabric "lters (bag houses) and wet particulate scrubbers. These devices are not described in this paper, but an interested reader can check e.g. Flagan and Seinfeld (1988) or Soud and Mitchell (1997) for more information. Because of the limited availability of measured size distribution data, each combustion technique has been treated with a case study of one power plant subprocess. However, only one of them, namely the pulverized coal combustion, will be presented in this paper in detail, whereas the results of the other case studies have only been used when writing the summary tables. Extra caution should be taken when generalizing the result of only one subprocess even if all the other subprocesses in the speci"c group are exactly identical. There are always divergences e.g. in the function of dust separation devices in separate subprocesses. However, in this work the speci"c emission factors calculated from the case studies represent the whole groups of similar subprocesses in each size category.
3. Results The speci"c emission factors of the total particle emission have been calculated for each energy production subprocess. The similar subprocesses have then been combined in the same group and the speci"c emission factor of the whole group has been calculated. The subprocesses are weighted by operation hours to avoid distortions by subprocesses used only a few hours per year. The speci"c emission factor for "ne particles has then been evaluated from the speci"c emission factor of the total particle emission by using the available mass size distribution measurements (i.e. the results of case studies). This was possible only for pulverized coal combustion, pulverized peat combustion, and recovery boilers. For the other combustion techniques, i.e. oil burners, #uidized bed combustion processes, grate boilers, and diesel power plants, appropriate measurements were not available. This was one important conclusion of this work, or in other words, more public measurements of these energy production processes in Finland are needed in order to clarify the amount and composition of "neparticle emissions with di!erent fuel varieties. 3.1. Pulverized coal combustion Many "eld and laboratory studies indicate that pulverized coal combustion aerosol size distributions are bimodal (Kauppinen and Pakkanen, 1990; Moisio, 1997). These "ne-particle modes are around 0.05}0.1 and 2 lm (aerodynamic diameter) when measured after the ESP. The average logarithmic and cumulative mass size distributions measured from a pulverized coal combustion power plant (after ESP) are shown in Fig. 2. Measured
size distributions show that the size classi"cation limit of 2.5 lm includes not only the "ne particulates, but also a large amount (mass) of coarse-mode particles. Additionally, as the mass size distribution has very steep increase in the range from 1 to 5 lm, even a small error in the cut size can cause signi"cant change in PM . The concentration levels for pulverized coal combustion (58.8 mg m\, Moisio, 1997) shown in Fig. 2 are measured after the ESP. As ESP is often the only particle separation device in pulverized coal combustion plants, the measurements give typical PM emissions for these types of plants. However, in this speci"c plant there was also a bag house after the ESP, which further reduces the PM emissions. Real-time measurement results show that the process is very stable and the size distributions vary only a little. The boiler soot blowing and the a$nation rappings of the ESP are the main reasons for concentration variations and the changes in the size distributions of particle emission. Usually, at least an ESP is used when separating #y ash particles from the #ue gas in a pulverized coal combustion power plant. Thus, the concentrations measured after the ESP represent well the emissions of the pulverized coal combustion processes where only ESP is used. As the ESP collection e$ciency for the coarsemode particles is close to 100% only the "ne-mode particles are able to penetrate the ESP. The bag house will decrease the "ne-particle emissions further down. According to the measurements by Moisio (1997) almost 100% of total particle number and 69}95% of total particle mass after ESP are particles below 5 lm in aerodynamic diameter. 3.1.1. A case study of pulverized coal combustion power plant in Finland The speci"c particulate emissions (in the year 1995) for subprocesses of Salmisaari combined heat and power plant in Helsinki are presented in Table 1. The speci"c emission factors have been calculated including the support fuel (heavy fuel oil), because only the total emissions of each subprocess are known and no separation between fuels has been made. Later, especially in the cases of multi-fueled boilers as grate boilers and #uidized bed combustion, the calculated speci"c emissions have been aligned for the main fuel (50% or more of the total fuel assortment). The main process is marked as p1 in Table 1. Relatively new technology and very e$cient dust separation devices (ESP#fabric "lter) result in a low particulate emission level (circa 3 mg MJ\ in 1995). The subprocess p7 represents old technology and the speci"c emission factor is clearly greater (15 mg MJ\ ). The other sub processes in Table 1 are oil burners mainly in reserve without any dust separation devices. The combined heat and power production of the whole power plant in 1995 was 3291 GWh. Total particle
M.O. Ohlstro( m et al. / Atmospheric Environment 34 (2000) 3701}3711
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Fig. 2. The logarithmic and cumulative mass and number size distributions (D % . (8 lm) from a pulverized coal combustion power plant (in Finland) after ESP, measured with ELPI (ELPI"electrical low pressure impactor) (Moisio, 1997).
emissions of 53 t yr\ lead thus to an emission level of 16 mg kWh\. This can be assigned to coal because of the minor amounts of oil used compared to coal used. Roughly half of the total particle emission (by mass) is PM particles. Measurements from Naantali pulverized coal combustion power plant (located near Turku, Finland) showed that 45% of particle mass after the dust separation devices are "ne particles (Moisio,
1997). Thus the speci"c emission factor for PM in the main process of Salmisaari power plant is roughly 1.3 mg MJ\ . That is a very low emission level, as can be seen later when compared to the "ne-particle emission levels in the summary table. A scanning electron microscope (SEM) picture of an agglomerate under 1 lm formed in pulverized coal combustion of South African and Colombian coals is presented in Fig. 3. It is composed of several tiny, almost
0.3
19
52.719
p6
p7
SUM
125
12454
Oil burner 2278 with pressurized atomizer
Other burner combustion (oil burner) Jet burner
35.5
26.0
1.4
25.6
Data from Finnish Environmental Institute (VAHTI-database system). The estimate is based on literature and measurements (e.g. Moisio, 1997). The estimate is based on size distribution measurements (Moisio, 1997). (*)No separation device(s). HFO0.8"heavy fuel oil with 0.8% sulfur.
187.0
133.0
8.7
2246
0.419
13342.3
24.4 1274
12.30
18.70
11.983 29.90
p5
89.1
7805
Proportioning mixer
33
p1
510.0
Operation Utilization Fuel conhours ratio (%) sumption (h yr\) (TJ yr\)
Symbol of Emission Fuel power Combustion subprocess (t yr\) (MW) technique
HFO0.8 COAL0.8
HFO0.8
HFO0.8
COAL0.8 HFO0.8
Fuel
4.0
14.6
24.4
22.4
2.7
Collection e$ciency
ESP, half-dry desulphurization, fabric "lter
*
*
'99%
'99% of total mass
*
*
ESP, half-dry '99% of desulfurization, total mass fabric "lter
Speci"c Separation emission factor device(s) (mg MJ\ )
Table 1 The particle emissions of Salmisaari pulverized coal power plant (located in Helsinki, Finland) in 1995 (VAHTI, 1998)
ca. 50%("ca. 16 t, 1.3 mg MJ\)
PM (% of mass, estimation
98% (51 t) (4 mg MJ\)
50% (26 t) (2 mg MJ\)
No measurement results available No measurement results available 98}100% ca. 50% ("ca. 19 t, ("ca. 9 t, 14.6 mg MJ\) 7 mg MJ\)
98}100% ("ca. 32 t; 2,7 mg MJ\)
PM (% of total mass, estimation)
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M.O. Ohlstro( m et al. / Atmospheric Environment 34 (2000) 3701}3711
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categories as described earlier. In this table, however, the classi"cation by fuel used has not been made and therefore the speci"c emission values for #uidized bed processes and grate boilers are only best estimate values because of the fuel variety of these processes. There is clearly a common feature for each combustion technique: the speci"c emission factors decrease when size categories enlarge. This is the consequence of several reasons. Firstly, the combustion conditions in the larger boilers are better controlled and monitored than in the smaller boilers. This means that the process is more stable and the combustion is more complete. Secondly, in the greatest size categories the dust separation devices are more e$cient and diversi"ed. This is because of decreased relative investment costs in greater sizes. 3.3. Summary of the specixc xne-particle emission factors (D )2.5 lm, mg MJ\ ) in Finland
Fig. 3. A scanning electron microscope (SEM) picture of an agglomerate formed in pulverized coal combustion (Kauppinen et al., 1996).
spherical primary particles under 0.05 lm (i.e. (50 nm) in size. Because of high temperatures, oxidizing conditions and long enough residence times in modern boilers there is typically no elemental or organic carbon in "ne particles in pulverized coal combustion. The "ne particles are composed of ash and condensed volatile matter. The "ne particles consist of several trace elements such as antimony, arsenic, beryllium, cadmium, chromium, cobalt, lead, manganese, mercury, nickel and selenium (Helble et al., 1996). Some of these trace elements are enriched in the "ne-mode particles (e.g. heavy metals Cd and Pb, Kauppinen and Pakkanen, 1990). The formation of "ne particles is not described here, but can be found e.g. in Flagan and Seinfeld (1988), Quann et al. (1990), Clarke (1993) and Kauppinen et al. (1996). 3.2. Summary of the specixc emission factors (total particulate matter, mg MJ\ ) in Finland All the energy production processes and the speci"c emission factors calculated from the total particulate matter emission data (from year 1995) are shown in Table 2. The processes have been classi"ed by size
For oil burners, grate boilers and #uidized bed combustion processes, the "ne-particle portion of the total particle emission could not be determined, because there were no applicable measurement results at hand. For these combustion techniques, more public measurements are needed in order to clarify the amount and composition of "ne particles with di!erent fuel varieties. However, the "ne-particle emission factors for pulverized coal combustion, pulverized peat combustion and recovery boilers were evaluated. These values are shown in Table 3. In pulverized coal combustion, typically at least an electrostatic precipitator is used as a #y ash collector and therefore almost 100% of total particle number and 69}95% of total particle mass are particles below 5 lm in aerodynamic diameter (Moisio, 1997). Roughly half of the total mass of particle emission is "ne particles (PM ) as described in Section 3.1.1. Depending on boiler size category and particle separation devices, the speci"c emission factor for "ne particles is 1}30 mg MJ\ (the speci"c emission factors in Table 2 divided by two). According to the measurements for pulverized combustion of peat by Moisio (1997) at Naistenlahti power plant, about 96}100% of the total mass of particle emission is PM particles and 23% of the mass composed of particles under 8 lm in diameter is "ne particles. Thus, ca. 20}25% of the total mass of particle emission is "ne particles, and the speci"c emission factor is 5}8 mg MJ\. For recovery boilers, the "ne-particle portion of the total mass of particle emission is ca. 50}60% (57% according to the measurements from one recovery boiler by Moisio, 1997). The speci"c emission factor for "ne particles varies considerably depending on the boiler size category, being between 12 and 77 mg MJ\. It must be noted that data concerning dust separation devices at
M.O. Ohlstro( m et al. / Atmospheric Environment 34 (2000) 3701}3711
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Table 2 Summary table of speci"c particle emissions (mg MJ\, weighted by operation hours) of di!erent boiler types in Finland Size category (MW)
Separation device/separation devices
0(P(5 MW
(*) No separation device Cyclone/multicyclone
0(P(50 MW (5(P(50 MW)
(*) No separation device Cyclone/multicyclone Cyclone/multicyclone and fabric "lter Scrubber ESP
Shared speci"c emission factor (mg MJ\) weighted by operation hours Pulverized Pulverized Oil burners coal com- peat com- with pressbustion bustion urized atomizer
Oil burners with rotary bowl
Grate boilers
59
75
*
*
*
168
Bubbling Circulating #uidized #uidized bed (BFB) bed (CFB)
*
*
29
33
135
*
*
*
*
*
*
134
713
36
*
*
*
*
*
129
87
* *
* *
* *
* *
142 18
* 24
* 68
50(P(300 (*) No separation * MW device Cyclone/multi* cyclone ESP 61 ESP, fabric "lter 8 and half-dry desulphurization
*
30
16
*
*
*
*
*
*
102
9
*
33 *
* *
* *
45 *
27 *
15 *
300(P( 500 MW
23
24
*
*
*
*
4
6
*
*
*
*
*
*
-
*
*
*
*
*
6
*
*
*
*
*
*
3
*
*
*
*
*
*
2
*
*
*
*
*
*
ESP #cyclone/ multicyclone) ESP and wet desulfurization ESP, fabric "lter and half-dry desulfurization
P'500 MW ESP and wet desulfurization ESP, fabric "lter and half-dry desulfurization ESP, SCR and wet desulfurization
11
Recovery boilers
129
37
24
*
Oil burners and grate boilers only. Oil burners and grate boilers 5(P(50 MW. Recovery boilers have not been classi"ed by separation devices, but only by size category. (*) Not any process in Finland.
recovery boilers were not available, and thereby the emission factors are not sorted according to these devices. However, in practice, all the recovery boilers have at least electrostatic precipitators and many of them also have other devices such as scrubbers.
4. Discussion and conclusions Fine-particle emissions (PM ) of di!erent energy production processes in Finland were studied. There were no statistical distributions of di!erent energy
M.O. Ohlstro( m et al. / Atmospheric Environment 34 (2000) 3701}3711
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Table 3 Summary table of speci"c "ne-particle emission factors in Finland (mg MJ\, D )2.5 lm) evaluated from speci"c (total) particle N emission factors (from Table 2) Size category (MW)
0(P(50 MW
50(P(300 MW
300(P(500 MW
P'500 MW
Separation device/separation devices
Shared speci"c emission factor (mg MJ\) weighted by operation hours Pulverized coal combustion
Pulverized peat combustion
(*) No separation device Cyclone/multicyclone Cyclone/multicyclone and fabric "lter Scrubber ESP
* * *
* * *
* *
* *
ESP ESP, fabric "lter and half-dry desulfurization
30 4
8 *
ESP #cyclone/multicyclone) ESP and wet desulfurization ESP, fabric "lter and half-dry desulfurization
12 3 6
6 * *
6 1
* *
1
*
ESP and wet desulfurization ESP, fabric "lter and half-dry desulfurization ESP, SCR and wet desulfurization
Recovery boilers
77
22
14
*
Recovery boilers have not been classi"ed by separation devices, but only by size category. (*) Not any process in Finland.
production techniques available. However, it can be seen directly from Fig. 1 that e.g. recovery boilers (i.e. combustion of the waste sludge from pulp industry) have a share of 8.8% of total primary energy consumption in Finland. Correspondingly, the pulverized coal combustion is a dominating technique in the slice of coal (10.3%). It can also be seen that 2/3 of total primary energy consumption is covered by energy sources producing either primary and secondary (oil, coal, peat, black liquor, biomass) or only secondary particles (natural gas). The "ne-particle emissions depend strongly on the boiler size category and dust separation devices used. Fuel type or combustion technique does not seem to have as big an in#uence. However, this cannot be veri"ed for all combustion techniques due to the lack of measurements, especially for the #uidized bed combustion processes. They are becoming more general in Finland by replacing mainly grate boilers and pulverized peat combustion. In pulverized coal combustion typically at least an electrostatic precipitator is used as a #y ash collector and therefore particle emissions are composed mainly of particles under 5 lm in aerodynamic diameter. About half of
the total mass of the particle emissions is "ne particles (PM ). Depending on boiler size category and particle separation devices, the speci"c emission factor for "ne particles is 1}30 mg MJ\. The most e$cient way to separate the "ne particles formed in pulverized coal combustion is to use also a bag house and a desulphurization scrubber along with the ESP. Then the speci"c emission factors for "ne particles are only 1}6 mg MJ\. This simultaneous use of several dust separation devices is, in practice, possible only at large boiler size categories because of the high investment costs of these devices. For pulverized combustion of peat, ca. 20}25% of the total mass of particle emissions is "ne particles, and then the speci"c emission factor is between 5 and 8 mg MJ\. However, the problem in the pulverized peat combustion is large emissions of nitrogen oxides. Therefore, some old pulverized peat boilers have already been replaced by more pro-environmental #uidized bed combustion techniques (e.g. at Rauhalahti power plant, JyvaK skylaK ). These #uidized bed boilers have, however, another disadvantage. They produce nitrous oxide (N O), which is a greenhouse gas.
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For recovery boilers, the "ne-particle portion of the total particle emission is 50}60% (by mass) and the speci"c emission factor for "ne particles varies considerably according to the boiler size category, being between 12 and 77 mg MJ\. The particles emitted from recovery boilers are mainly salt particles. Fly ash consists mainly of sodium sulfate (Na SO ). It also contains lesser amounts of sodium carbonate (Na CO ), chlorine, po tassium and sul"de. Not many harmful trace elements such as heavy metals are found from the particles emitted by recovery boilers. For oil burners, grate boilers and #uidized bed combustion processes, the "ne-particle portion of the total particle emission could not be determined, because there were no applicable measurement results at hand. For these combustion techniques, more public measurements are needed in order to clarify the amount and composition of "ne-particles with di!erent fuel varieties. Also, small}scale combustion should be studied in detail, because the "ne-particle exposure which it causes can be signi"cant due to the low emission height and the absence of dust separation devices. This is especially important in densely populated areas, where the dominant heating form is individual wood, oil, peat or coal burning. Additionally, small-scale combustion is often incomplete, whereupon carcinogenic hydrocarbon compounds are formed. There were no large diesel power plants in Finland in 1995 (reference year), but "ne-particle emissions of marine diesel engines in big cruisers and cargo ships should be de"ned because their harbors are often in the city center, as in Helsinki. Due to the quite low emission height, the ships are a signi"cant "ne-particle emission source of concern to the ambient air of coastal cities. Fuel type is often heavy oil that produces particles rich in vanadium and nickel (LyyraK nen et al., 1999). As heavy metals, they are harmful to the environment. Because of the uncertain nature of statistical initial data, the results are di$cult to evaluate quantitatively. Additionally, the "ne-particle fractions of total particulate matter were based on single measurements due to the lack of emission side (i.e. after dust separators) measurements. More measurements have been done before ESP but they are not applicable since the size distributions change in the separation devices. It is important that enough measurements are made in the future in order to evaluate "ne-particle emissions of each combustion technique as for pulverized coal combustion in this paper.
Acknowledgements The authors are grateful to Marko Ekqvist from the Finnish Environmental Institute for the total particle emission data in VAHTI-database system.
References Berdowski, J.J.M., Mulder, W., Veldt, C., Visschedijk, A.J.H., Zandveld, P.Y.J., 1996. Particulate matter emissions (PM }PM }PM ) in Europe in 1990 and 1993. TNO Report, TNO Institute of Environmental Sciences, Energy Research and Process Innovation, Apeldoorn, The Netherlands, 90pp. Burnett, R.T., Dales, R., Krewski, D., Vincent, R., Dann, T., Brook, J.R., 1995. Associations between ambient particulate sulfate and admissions to Ontario hospitals for cardiac and respiratory diseases. American Journal of Epidemiology 142, 15}22. Clarke, L.B., 1993. The fate of trace elements during coal combustion and gasi"cation: an overview. Fuel 72 (6), 731}736. Dockery, D.W., Pope III, C.A., 1994. Acute respiratory e!ects of particulate air pollution. Annual Review of Public Health 15, 107}132. Dockery, D.W., Schwartz, J., Spengler, J.D., 1992. Air pollution and daily mortality: associations with particulates and acid aerosols. Environmental Research 59, 362}373. Dockery, D.W., Pope III, C.A., Xu, X., Spengler, J.D., Ware, J.H., Fay, M.E., Ferris Jr., B.G., Speizer, F.E., 1993. An association between air pollution and mortality in six U.S. cities. New England Journal of Medicine 329, 1753}1759. EC, 1997. Commission proposes new air quality limit values. Luxembourg, European Commission, 5pp. Flagan, R.C., Seinfeld, J., 1988. In: Fundamentals of Air Pollution Engineering. Prentice-Hall, Inc., New Jersey, pp. 542. Helble, J.J., Mojtahedi, W., LyyraK nen, J., Jokiniemi, J., Kauppinen, E., 1996. Trace element partitioning during coal gasi"cation. Fuel 75 (8), 931}939. IPCC, 1996. Climate change. In: Houghton, J.T., Meira Filho L.G., Callander, B.A., Harris, N., Kattenberg, A., Maskell, K. (Eds.), The Science of Climate Change. Cambridge University Press, Cambridge, UK, 584pp. Kauppinen, E., Pakkanen, T., 1990. Coal combustion aerosols: a "eld study. Environmental Science and Technology (ES&T) 24 (12), 1811}1818. Kauppinen, E.I., Lind, T.M., Valmari, T., YlaK talo, S., Jokiniemi, J.K., Powell, Q., Gurav, A.S., Kodas, T.T., Mohr, M., 1996. The structure of submicron ash from combustion of pulverized South African and Colombian coals. In: Baxter, L., DeSollar, R. (Eds.), Applications of Advanced Technology to Ash-related Problems in Boilers. Plenum Press, New York, pp. 471}484. LyyraK nen, J., Jokiniemi, J., Kauppinen, E., Joutsensaari, J., 1999. Aerosol characterisation in medium-speed diesel engines operating with heavy fuel oils. Journal of Aerosol Science 30 (6), 771}784. Moisio, M., 1997. Combustion aerosol size distribution measurements using Electrical Low Pressure Impactor. Licentiatum Thesis, Tampere University of Technology, Department of Electrical Engineering (Physics), Tampere, Finland, 181pp (in Finnish, with English abstract). Pekkanen, J., Timonen, K.L., Ruuskanen, J., Reponen, A., Mirme, A., 1997. E!ects of ultra"ne and "ne particles in urban air on peak expiratory #ow among children with asthmatic symptoms. Environmental Research 74, 24}33.
M.O. Ohlstro( m et al. / Atmospheric Environment 34 (2000) 3701}3711 Pope, C.A. III, Thun, M.J., Namboodiri, M.M., Dockery, D.W., Evans, J.S., Speizer, F.E., Heath Jr., C.W., 1995. Particulate air pollution as a predictor of mortality in a prospective study of U.S. adults. American Journal of Respiration Critical Care and Medicine 151, 669}674. Quann, R.J., Neville, M., Saro"rm, A.F., 1990. Laboratory study of the e!ect of coal selection and composition of combustion generated submicron particles. Combustion Science and Technology 74, 245}256. Schwartz, J., 1993. Particulate air pollution and daily mortality in Birmingham. Alabama. American Journal of Epidemiology 137, 1136}1147. Schwartz, J., 1994. Air pollution and hospital admissions for the elderly in Birmingham, Alabama. American Journal of Epidemiology 139, 589}590. Schwartz, J., Dockery, D.W., Neas, L.M., 1996. Is daily mortality associated speci"cally with "ne particles? Journal of Air Waste Management Association 10, 927}939. Seaton, A., 1998. The health e!ects of urban particulate air pollution. Invited lecture, in the Fifth International Aerosol Conference in Edinburgh. Seinfeld, J.H., 1986. Atmospheric Chemistry and Physics of Air Pollution. Wiley, New York, 738pp. Smith, I.M., Sloss, L.L., 1998. PM /PM } emissions and e!ects. IEA Coal Research, The Clean Coal Centre, London, UK, 75pp.
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Soud, H.N., Mitchell, S.C., 1997. Particulate control handbook for coal-"red plants. IEACR/65, London, UK, IEA Coal Research, 201pp. Statistics Finland, 1996. Energy statistics 1995. Helsinki, 126pp. Turbin, B.J., Huntzicker, J.J., 1995. Identi"cation of secondary organic aerosol episodes and quantitation of primary and secondary organic aerosol concentrations during SCAQS. Atmospheric Environment 23, 3527}3544. US EPA (the U.S. Environmental Protection Agency), 1997. EPA's revised particulate matter standards. http://ttnwww. rtpnc.epa.gov/naaqs"n/pmfact.htm. AltaVista 21.8.1998. VAHTI, 1998. A database system containing emission data of energy production and industrial processes in Finland. Maintained by the Finnish Environmental Institute. The total particulate emission data got from Marko Ekqvist 15.5.1998. WHO, 1994. Update and revision of the air quality guidelines for Europe. WHO, Regional O$ce for Europe. Meeting of the Working Group `Classicala Air Pollutants, Bilthoven, The Netherlands, 11}14 October 1994. Wilson, R., Spengler, J., 1996. Conclusion: policy implications: the national dilemma. In: Wilson, R., Spengler, J. (Eds.), Particles in our Air:: Concentrations and Health E!ects. Harvard University Press, Cambridge MA USA, pp. 205}216.
Fine-particle emissions of energy production in Finland Mikael O. OhlstroK m , Kari E.J. Lehtinen , Mikko Moisio, Jorma K. Jokiniemi * VTT Energy, Aerosol Technology Group, P.O. Box 1401, FIN-02044 VTT, Finland Aerosol Physics Laboratory, Tampere University of Technology, P.O. Box 692, FIN-33101 Tampere, Finland Received 1 June 1999; received in revised form 30 November 1999; accepted 19 December 1999
Abstract The aim of this study was to "nd out the "ne-particle emissions (PM ) of di!erent energy production processes in Finland. The main purpose was to compare the calculated emission factors between di!erent energy production concepts. The purpose was also to de"ne what is known about "ne-particle emissions and what should still be studied/measured. Only those energy production processes that produce a signi"cant amount of direct emissions of solid particles have been treated here, i.e. pulverized combustion and oil burners from burner combustion, #uidized bed combustion processes, grate boilers, recovery boilers and diesel power plants. Secondary particles from gaseous pollutants have not been considered within this study. In pulverized coal combustion the particle emission is composed mainly of particles smaller than 5 lm in aerodynamic diameter. Roughly half of the total mass of particle emission is composed of "ne particles (PM ). Depending on boiler size category and particle separation devices, the speci"c emission factor for "ne particles is 1}30 mg MJ\. For pulverized combustion of peat, ca. 20}25% of the total mass of particle emission is "ne particles, and then the speci"c emission factor is between 5 and 8 mg MJ\. For recovery boilers, the "ne particle portion of the total particle emission is 50}60% (by mass) and the speci"c emission factor for "ne particles varies considerably according to the boiler size category, being between 12 and 77 mg MJ\. For oil burners, grate boilers and #uidized bed combustion processes, the "ne-particle portion of the total particle emission could not be determined, because there were no applicable measurement results at hand. For these combustion techniques, more public measurements are needed in order to clarify the amount and composition of "ne particles with di!erent fuel varieties. Additionally, the chemical composition of "ne particles is a very important feature when human health e!ects caused by particulate matter are considered. 2000 Elsevier Science Ltd. All rights reserved. Keywords: PM ; PM ; Speci"c emissions; Health hazard; Air quality; Coal combustion
1. Introduction Suspended particulates are among the most noteworthy air quality problems in the urban areas of Finland. Concentrations rise especially in the spring as snow melts and the ground gets dry, while during winter grinded gritting sand (anti-skid treatment) raises dust by tra$c and wind. In addition to gritting sand the suspended particulates contain material from road surfaces, car tires and exhaust gases and from the emissions of energy
* Corresponding author. Tel.: #358-9-456-6158; Fax: #358-9-456-7021. E-mail address: [email protected]." (J.K. Jokiniemi).
production and industrial processes. The whole mass concentration of suspended particulates in the air is called total suspended particulates (TSP). Particles less than 10 lm (0.01 mm) in aerodynamic diameter are called respirable particulates or PM (PM"partic ulate matter), whereas PM denotes "ne particles less than 2.5 lm in aerodynamic diameter. In this paper, when talking about "ne particles, the PM is signi"ed. PM is composed of a mixture of particles emitted directly into the air and particles formed in the air from the chemical transformation of gaseous pollutants (secondary particles). The principal types of secondary particles are ammonium sulfate and nitrate formed from sulfur dioxide (SO ) and oxides of nitrogen (NO ), V
1352-2310/00/$ - see front matter 2000 Elsevier Science Ltd. All rights reserved. PII: S 1 3 5 2 - 2 3 1 0 ( 0 0 ) 0 0 0 7 6 - 5
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reacting with ammonia (NH ). The main sources of SO and NO are the combustion of fossil fuels in boilers and V the mobile sources. The types of directly emitted particles are soil-related particles and particles from the combustion of fossil fuels and biomass materials. The main sources of soil-related particles are roads, construction and agriculture. The main sources of combustion-related particles are diesel motor vehicles, managed burning, open burning, residential wood combustion, utility and commercial boilers. The reason for this study was the detected e!ects on human health caused by "ne-particles (e.g. Dockery and Pope, 1994). The main purpose was to de"ne the "neparticle (PM ) emissions of energy production and to compare the calculated emission factors between di!erent energy production concepts. The purpose was also to de"ne what is known about "ne-particle emissions and what should still be studied/measured. Furthermore, only the direct particulate emissions of energy production processes are considered here and the gas-to-particle conversions of SO and NO are left out. It must be V considered, however, that over half of the "ne particle concentration in the atmosphere, which is caused by combustion processes, are secondary particles from gasto-particle conversions (IPCC, 1996). Secondary particles are mostly in the size range of 0.1}1 lm in diameter (Turbin and Huntzicker, 1995), so they remain in the atmosphere longer than coarser particles. The rate of the gas-to-particle conversion is about 1% h\ for SO and 5}10% h\ for NO (Seinfeld, 1986). The factors a!ectV ing the rate of conversion are temperature, the amount of sunlight, the relative humidity of air and the concentrations of other chemically reactive substances in the air (e.g. ozone). The conversion is fastest during warm, bright and humid weather. According to present knowledge the health hazards of "ne particles depend on their chemical composition, deposition in lungs and behavior in respiratory organs. Thus, the smallest particles would be the most dangerous because they can penetrate all the way to the alveoli and further to blood circulation (Seaton, 1998). Additionally, their chemical compositions di!er from the coarser natural particles, because the "nest particles are formed in combustion processes (energy production, industrial processes, vehicles). The speci"c surface area of "ne particles is large due to their high number concentration and the non-spherical shape of agglomerates. Therefore, they typically contain large amounts of condensed heavy metals vaporized from fuels in boilers. Accumulation of this foreign matter in human organs may cause symptoms and illnesses that occur either acutely (e.g. symptoms of asthma, heart failures) or after a long-time exposure (e.g. cancers, pulmonary diseases). This has been veri"ed by epidemiological studies done by e.g. Dockery et al. (1992), Dockery and Pope (1994), Burnett et al. (1995) and Schwartz (1993, 1994). Accordingly,
Schwartz et al. (1996) reported that increased concentrations of PM , PM and sulfate particles in the big cities of U.S. East Coast are signi"cantly related to increased daily mortality of these cities. PM particles had the strongest correlation to the mortality "gures (#1.5% increase in mortality per 10 lg m\ rise in two day mean PM concentration of ambient air). Even greater e!ects on pulmonary diseases (#3.3%) and heart diseases (#2.1%) were obtained. World Health Organization (WHO) has estimated the reduction of life expectancy associated with a certain di!erence in long-term exposure to PM by combining the results of two cohort studies, i.e. Dockery et al. (1993) and Pope et al. (1995). A 10 lg m\ di!erence in longterm exposure to PM results in an estimated relative risk of 1.10 (WHO, 1994). This relative risk estimate has been applied to the 1992 life table for Dutch men for which the estimated e!ect on life expectancy is 1.1 yr. The calculation was restricted to ages 25}90, as the cohort studies have not gone beyond the age of 90. In Finland, the National Public Health Institute has presently several research projects studying the health e!ects caused by "ne particles. Because of di!erent climate conditions, the international studies cannot be directly adapted to national purposes. For instance, the cold winter creates an extensive need of heating energy. Also, the health e!ects of "ne particles in frosty weather may be di!erent from those in warm weather. Pekkanen et al. (1997) studied the exposure of asthmatic children to PM in Kuopio, Finland. The occurrences of coughing or wheezing attacks, recorded in a questionnaire, correlated with the ambient concentrations of PM , PM , and black smoke measured at a single air quality monitoring site in the area. The main sources of "ne particles were tra$c, a peat-"red power station, and a corrugated cardboard mill. Also, wood burning during the winter period was common in 25% of the homes in the area. In addition to these health hazards the "ne particles also have other, mostly injurious, e!ects. They reduce visibility, the surfaces get dirty and su!er material damages (corrosion). Fine particles also re#ect solar radiation back to the outer space, cooling the atmosphere and slowing down the greenhouse e!ect. Several countries and a number of regulatory bodies have set or are just considering the ambient air quality standards for PM and PM . Current standards in Finland were set on 1 September 1996. Earlier there was only a standard for TSP, but now a standard for PM also exists. The guideline value is 70 lg m\ as the second highest 24 h value per month. No standard for PM exists. The current guideline value for TSP is 120 lg m\ as the 98th percentile of 24 h values in a year and 50 lg m\ as annual arithmetic mean. WHO guidelines for PM have been set at 70 lg m\ for 24 h average concentrations (Wilson and Spengler, 1996), quite similar to Finland's
M.O. Ohlstro( m et al. / Atmospheric Environment 34 (2000) 3701}3711
standards. The European Commission (EC) has proposed that the existing limit for total particulate matter is removed and a new limit set for PM . The proposed new air quality standards for PM would be enforced as a two-stage process. At the "rst stage (1 January 2005), the annual average of 30 lg m\ is to be proposed (EC, 1997). At the second stage (1 January 2010) the value of annual average should be below (or equal) 20 lg m\. The values for 24 h averages should be 50 lg m\ not to be exceeded by more than 25 times per year (by 2005), and not more than 7 times per year (by 2010) (EC, 1997). The new national air quality standards (NAAQS) were promulgated in the USA in July 1997 by United States Environmental Protection Agency (US EPA). The new particulate matter NAAQS include standards also for PM particles. Both 15 lg m\ as annual 3 yr average and spatial averaging and 65 lg m\ as the 98th percentile of 24 h values in a year at highest monitor averaged over 3 yr were set (US EPA, 1997). The problems with PM standards, however, are the di$culties in measuring and thus controlling the concentrations of "ne particles. There are many sampling and analysis systems and numerous di!erent measuring instruments available. The results of di!erent measuring methods are not yet very comparable with each other. Inaccuracies can be of the order of 50%. The ambient air quality networks are also insu$cient for decent regional monitoring. In addition, there are regional areas where even the background concentrations of "ne particles alone may be higher than the new schemes of PM standards. There are some recent studies about PM and PM emissions. Smith and Sloss (1998) produced a quite comprehensive report about PM /PM emissions and ef fects concerning the member countries of International Energy Agency (IEA). The report includes legislation, sampling networks, emissions of primary particles, formation of secondary particles, concentrations in the atmosphere and health e!ects. They emphasize that the statistical epidemiological studies have only demonstrated the link between particulate matter and health. More data on the chemical characterization of particulate emissions and their biological e!ects are thus needed to prove the causality between ambient air concentrations of "ne particles and human health e!ects. Berdowski et al. (1996) have studied primary particles from anthropogenic sources. Fine particulate matter emissions have been calculated for each country in Europe based on statistical data and emission factors and particle size distributions based on literature data.
3703
cesses in Finland. This was done by calculating speci"c emission factors (mg MJ\ ) from powerplants' annual total particulate matter emissions (ty\, 1 t"1000 kg) and by estimating the fraction of "ne particles from the total emissions with the help of existing (publicly available) measurement results. The data containing e.g. the total particulate matter emissions and fuel energy contents of all the di!erent energy production subprocesses were obtained from the Finnish Environmental Institute (VAHTI, 1998). The reference year of data was 1995. Only those energy production processes that produce a signi"cant amount of direct emissions of solid particles have been treated here, i.e. pulverized combustion and oil burners from burner combustion, #uidized bed combustion processes, grate boilers, recovery boilers, and diesel power plants (see Fig. 1 for the primary energy consumption palette by energy sources in Finland). Secondary particles from gaseous pollutants have not been considered within this study. The energy production processes have been classi"ed according to boiler type, size category, main fuel and also according to dust separation devices. To be able to compare di!erent energy production processes, a shared speci"c emission factor has been calculated for the similar subprocesses. The energy production processes have been divided according to fuel power (P) into the following size categories: 0(P)50 MW, 50(P)300 MW, 300(P) 500 MW and P'500 MW. For oil burners and grate boilers, however, the smallest group has been additionally divided into two subgroups (0(P)5 MW and 5(P)50 MW) because of the great amount of small boilers using these combustion techniques. Each size category has been further divided according to combustion technique and dust separation devices. The dust separation devices include mechanical/inertial collectors (e.g. cyclones/multicyclones), electrostatic precipitators
2. Methods The aim of this study was to "nd out the "ne-particle emissions (PM ) of di!erent energy production pro
Fig. 1. Total consumption of primary energy (1285,2 PJ) by energy sources at 1995 in Finland (Statistics Finland, 1996).
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(ESP), fabric "lters (bag houses) and wet particulate scrubbers. These devices are not described in this paper, but an interested reader can check e.g. Flagan and Seinfeld (1988) or Soud and Mitchell (1997) for more information. Because of the limited availability of measured size distribution data, each combustion technique has been treated with a case study of one power plant subprocess. However, only one of them, namely the pulverized coal combustion, will be presented in this paper in detail, whereas the results of the other case studies have only been used when writing the summary tables. Extra caution should be taken when generalizing the result of only one subprocess even if all the other subprocesses in the speci"c group are exactly identical. There are always divergences e.g. in the function of dust separation devices in separate subprocesses. However, in this work the speci"c emission factors calculated from the case studies represent the whole groups of similar subprocesses in each size category.
3. Results The speci"c emission factors of the total particle emission have been calculated for each energy production subprocess. The similar subprocesses have then been combined in the same group and the speci"c emission factor of the whole group has been calculated. The subprocesses are weighted by operation hours to avoid distortions by subprocesses used only a few hours per year. The speci"c emission factor for "ne particles has then been evaluated from the speci"c emission factor of the total particle emission by using the available mass size distribution measurements (i.e. the results of case studies). This was possible only for pulverized coal combustion, pulverized peat combustion, and recovery boilers. For the other combustion techniques, i.e. oil burners, #uidized bed combustion processes, grate boilers, and diesel power plants, appropriate measurements were not available. This was one important conclusion of this work, or in other words, more public measurements of these energy production processes in Finland are needed in order to clarify the amount and composition of "neparticle emissions with di!erent fuel varieties. 3.1. Pulverized coal combustion Many "eld and laboratory studies indicate that pulverized coal combustion aerosol size distributions are bimodal (Kauppinen and Pakkanen, 1990; Moisio, 1997). These "ne-particle modes are around 0.05}0.1 and 2 lm (aerodynamic diameter) when measured after the ESP. The average logarithmic and cumulative mass size distributions measured from a pulverized coal combustion power plant (after ESP) are shown in Fig. 2. Measured
size distributions show that the size classi"cation limit of 2.5 lm includes not only the "ne particulates, but also a large amount (mass) of coarse-mode particles. Additionally, as the mass size distribution has very steep increase in the range from 1 to 5 lm, even a small error in the cut size can cause signi"cant change in PM . The concentration levels for pulverized coal combustion (58.8 mg m\, Moisio, 1997) shown in Fig. 2 are measured after the ESP. As ESP is often the only particle separation device in pulverized coal combustion plants, the measurements give typical PM emissions for these types of plants. However, in this speci"c plant there was also a bag house after the ESP, which further reduces the PM emissions. Real-time measurement results show that the process is very stable and the size distributions vary only a little. The boiler soot blowing and the a$nation rappings of the ESP are the main reasons for concentration variations and the changes in the size distributions of particle emission. Usually, at least an ESP is used when separating #y ash particles from the #ue gas in a pulverized coal combustion power plant. Thus, the concentrations measured after the ESP represent well the emissions of the pulverized coal combustion processes where only ESP is used. As the ESP collection e$ciency for the coarsemode particles is close to 100% only the "ne-mode particles are able to penetrate the ESP. The bag house will decrease the "ne-particle emissions further down. According to the measurements by Moisio (1997) almost 100% of total particle number and 69}95% of total particle mass after ESP are particles below 5 lm in aerodynamic diameter. 3.1.1. A case study of pulverized coal combustion power plant in Finland The speci"c particulate emissions (in the year 1995) for subprocesses of Salmisaari combined heat and power plant in Helsinki are presented in Table 1. The speci"c emission factors have been calculated including the support fuel (heavy fuel oil), because only the total emissions of each subprocess are known and no separation between fuels has been made. Later, especially in the cases of multi-fueled boilers as grate boilers and #uidized bed combustion, the calculated speci"c emissions have been aligned for the main fuel (50% or more of the total fuel assortment). The main process is marked as p1 in Table 1. Relatively new technology and very e$cient dust separation devices (ESP#fabric "lter) result in a low particulate emission level (circa 3 mg MJ\ in 1995). The subprocess p7 represents old technology and the speci"c emission factor is clearly greater (15 mg MJ\ ). The other sub processes in Table 1 are oil burners mainly in reserve without any dust separation devices. The combined heat and power production of the whole power plant in 1995 was 3291 GWh. Total particle
M.O. Ohlstro( m et al. / Atmospheric Environment 34 (2000) 3701}3711
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Fig. 2. The logarithmic and cumulative mass and number size distributions (D % . (8 lm) from a pulverized coal combustion power plant (in Finland) after ESP, measured with ELPI (ELPI"electrical low pressure impactor) (Moisio, 1997).
emissions of 53 t yr\ lead thus to an emission level of 16 mg kWh\. This can be assigned to coal because of the minor amounts of oil used compared to coal used. Roughly half of the total particle emission (by mass) is PM particles. Measurements from Naantali pulverized coal combustion power plant (located near Turku, Finland) showed that 45% of particle mass after the dust separation devices are "ne particles (Moisio,
1997). Thus the speci"c emission factor for PM in the main process of Salmisaari power plant is roughly 1.3 mg MJ\ . That is a very low emission level, as can be seen later when compared to the "ne-particle emission levels in the summary table. A scanning electron microscope (SEM) picture of an agglomerate under 1 lm formed in pulverized coal combustion of South African and Colombian coals is presented in Fig. 3. It is composed of several tiny, almost
0.3
19
52.719
p6
p7
SUM
125
12454
Oil burner 2278 with pressurized atomizer
Other burner combustion (oil burner) Jet burner
35.5
26.0
1.4
25.6
Data from Finnish Environmental Institute (VAHTI-database system). The estimate is based on literature and measurements (e.g. Moisio, 1997). The estimate is based on size distribution measurements (Moisio, 1997). (*)No separation device(s). HFO0.8"heavy fuel oil with 0.8% sulfur.
187.0
133.0
8.7
2246
0.419
13342.3
24.4 1274
12.30
18.70
11.983 29.90
p5
89.1
7805
Proportioning mixer
33
p1
510.0
Operation Utilization Fuel conhours ratio (%) sumption (h yr\) (TJ yr\)
Symbol of Emission Fuel power Combustion subprocess (t yr\) (MW) technique
HFO0.8 COAL0.8
HFO0.8
HFO0.8
COAL0.8 HFO0.8
Fuel
4.0
14.6
24.4
22.4
2.7
Collection e$ciency
ESP, half-dry desulphurization, fabric "lter
*
*
'99%
'99% of total mass
*
*
ESP, half-dry '99% of desulfurization, total mass fabric "lter
Speci"c Separation emission factor device(s) (mg MJ\ )
Table 1 The particle emissions of Salmisaari pulverized coal power plant (located in Helsinki, Finland) in 1995 (VAHTI, 1998)
ca. 50%("ca. 16 t, 1.3 mg MJ\)
PM (% of mass, estimation
98% (51 t) (4 mg MJ\)
50% (26 t) (2 mg MJ\)
No measurement results available No measurement results available 98}100% ca. 50% ("ca. 19 t, ("ca. 9 t, 14.6 mg MJ\) 7 mg MJ\)
98}100% ("ca. 32 t; 2,7 mg MJ\)
PM (% of total mass, estimation)
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M.O. Ohlstro( m et al. / Atmospheric Environment 34 (2000) 3701}3711
3707
categories as described earlier. In this table, however, the classi"cation by fuel used has not been made and therefore the speci"c emission values for #uidized bed processes and grate boilers are only best estimate values because of the fuel variety of these processes. There is clearly a common feature for each combustion technique: the speci"c emission factors decrease when size categories enlarge. This is the consequence of several reasons. Firstly, the combustion conditions in the larger boilers are better controlled and monitored than in the smaller boilers. This means that the process is more stable and the combustion is more complete. Secondly, in the greatest size categories the dust separation devices are more e$cient and diversi"ed. This is because of decreased relative investment costs in greater sizes. 3.3. Summary of the specixc xne-particle emission factors (D )2.5 lm, mg MJ\ ) in Finland
Fig. 3. A scanning electron microscope (SEM) picture of an agglomerate formed in pulverized coal combustion (Kauppinen et al., 1996).
spherical primary particles under 0.05 lm (i.e. (50 nm) in size. Because of high temperatures, oxidizing conditions and long enough residence times in modern boilers there is typically no elemental or organic carbon in "ne particles in pulverized coal combustion. The "ne particles are composed of ash and condensed volatile matter. The "ne particles consist of several trace elements such as antimony, arsenic, beryllium, cadmium, chromium, cobalt, lead, manganese, mercury, nickel and selenium (Helble et al., 1996). Some of these trace elements are enriched in the "ne-mode particles (e.g. heavy metals Cd and Pb, Kauppinen and Pakkanen, 1990). The formation of "ne particles is not described here, but can be found e.g. in Flagan and Seinfeld (1988), Quann et al. (1990), Clarke (1993) and Kauppinen et al. (1996). 3.2. Summary of the specixc emission factors (total particulate matter, mg MJ\ ) in Finland All the energy production processes and the speci"c emission factors calculated from the total particulate matter emission data (from year 1995) are shown in Table 2. The processes have been classi"ed by size
For oil burners, grate boilers and #uidized bed combustion processes, the "ne-particle portion of the total particle emission could not be determined, because there were no applicable measurement results at hand. For these combustion techniques, more public measurements are needed in order to clarify the amount and composition of "ne particles with di!erent fuel varieties. However, the "ne-particle emission factors for pulverized coal combustion, pulverized peat combustion and recovery boilers were evaluated. These values are shown in Table 3. In pulverized coal combustion, typically at least an electrostatic precipitator is used as a #y ash collector and therefore almost 100% of total particle number and 69}95% of total particle mass are particles below 5 lm in aerodynamic diameter (Moisio, 1997). Roughly half of the total mass of particle emission is "ne particles (PM ) as described in Section 3.1.1. Depending on boiler size category and particle separation devices, the speci"c emission factor for "ne particles is 1}30 mg MJ\ (the speci"c emission factors in Table 2 divided by two). According to the measurements for pulverized combustion of peat by Moisio (1997) at Naistenlahti power plant, about 96}100% of the total mass of particle emission is PM particles and 23% of the mass composed of particles under 8 lm in diameter is "ne particles. Thus, ca. 20}25% of the total mass of particle emission is "ne particles, and the speci"c emission factor is 5}8 mg MJ\. For recovery boilers, the "ne-particle portion of the total mass of particle emission is ca. 50}60% (57% according to the measurements from one recovery boiler by Moisio, 1997). The speci"c emission factor for "ne particles varies considerably depending on the boiler size category, being between 12 and 77 mg MJ\. It must be noted that data concerning dust separation devices at
M.O. Ohlstro( m et al. / Atmospheric Environment 34 (2000) 3701}3711
3708
Table 2 Summary table of speci"c particle emissions (mg MJ\, weighted by operation hours) of di!erent boiler types in Finland Size category (MW)
Separation device/separation devices
0(P(5 MW
(*) No separation device Cyclone/multicyclone
0(P(50 MW (5(P(50 MW)
(*) No separation device Cyclone/multicyclone Cyclone/multicyclone and fabric "lter Scrubber ESP
Shared speci"c emission factor (mg MJ\) weighted by operation hours Pulverized Pulverized Oil burners coal com- peat com- with pressbustion bustion urized atomizer
Oil burners with rotary bowl
Grate boilers
59
75
*
*
*
168
Bubbling Circulating #uidized #uidized bed (BFB) bed (CFB)
*
*
29
33
135
*
*
*
*
*
*
134
713
36
*
*
*
*
*
129
87
* *
* *
* *
* *
142 18
* 24
* 68
50(P(300 (*) No separation * MW device Cyclone/multi* cyclone ESP 61 ESP, fabric "lter 8 and half-dry desulphurization
*
30
16
*
*
*
*
*
*
102
9
*
33 *
* *
* *
45 *
27 *
15 *
300(P( 500 MW
23
24
*
*
*
*
4
6
*
*
*
*
*
*
-
*
*
*
*
*
6
*
*
*
*
*
*
3
*
*
*
*
*
*
2
*
*
*
*
*
*
ESP #cyclone/ multicyclone) ESP and wet desulfurization ESP, fabric "lter and half-dry desulfurization
P'500 MW ESP and wet desulfurization ESP, fabric "lter and half-dry desulfurization ESP, SCR and wet desulfurization
11
Recovery boilers
129
37
24
*
Oil burners and grate boilers only. Oil burners and grate boilers 5(P(50 MW. Recovery boilers have not been classi"ed by separation devices, but only by size category. (*) Not any process in Finland.
recovery boilers were not available, and thereby the emission factors are not sorted according to these devices. However, in practice, all the recovery boilers have at least electrostatic precipitators and many of them also have other devices such as scrubbers.
4. Discussion and conclusions Fine-particle emissions (PM ) of di!erent energy production processes in Finland were studied. There were no statistical distributions of di!erent energy
M.O. Ohlstro( m et al. / Atmospheric Environment 34 (2000) 3701}3711
3709
Table 3 Summary table of speci"c "ne-particle emission factors in Finland (mg MJ\, D )2.5 lm) evaluated from speci"c (total) particle N emission factors (from Table 2) Size category (MW)
0(P(50 MW
50(P(300 MW
300(P(500 MW
P'500 MW
Separation device/separation devices
Shared speci"c emission factor (mg MJ\) weighted by operation hours Pulverized coal combustion
Pulverized peat combustion
(*) No separation device Cyclone/multicyclone Cyclone/multicyclone and fabric "lter Scrubber ESP
* * *
* * *
* *
* *
ESP ESP, fabric "lter and half-dry desulfurization
30 4
8 *
ESP #cyclone/multicyclone) ESP and wet desulfurization ESP, fabric "lter and half-dry desulfurization
12 3 6
6 * *
6 1
* *
1
*
ESP and wet desulfurization ESP, fabric "lter and half-dry desulfurization ESP, SCR and wet desulfurization
Recovery boilers
77
22
14
*
Recovery boilers have not been classi"ed by separation devices, but only by size category. (*) Not any process in Finland.
production techniques available. However, it can be seen directly from Fig. 1 that e.g. recovery boilers (i.e. combustion of the waste sludge from pulp industry) have a share of 8.8% of total primary energy consumption in Finland. Correspondingly, the pulverized coal combustion is a dominating technique in the slice of coal (10.3%). It can also be seen that 2/3 of total primary energy consumption is covered by energy sources producing either primary and secondary (oil, coal, peat, black liquor, biomass) or only secondary particles (natural gas). The "ne-particle emissions depend strongly on the boiler size category and dust separation devices used. Fuel type or combustion technique does not seem to have as big an in#uence. However, this cannot be veri"ed for all combustion techniques due to the lack of measurements, especially for the #uidized bed combustion processes. They are becoming more general in Finland by replacing mainly grate boilers and pulverized peat combustion. In pulverized coal combustion typically at least an electrostatic precipitator is used as a #y ash collector and therefore particle emissions are composed mainly of particles under 5 lm in aerodynamic diameter. About half of
the total mass of the particle emissions is "ne particles (PM ). Depending on boiler size category and particle separation devices, the speci"c emission factor for "ne particles is 1}30 mg MJ\. The most e$cient way to separate the "ne particles formed in pulverized coal combustion is to use also a bag house and a desulphurization scrubber along with the ESP. Then the speci"c emission factors for "ne particles are only 1}6 mg MJ\. This simultaneous use of several dust separation devices is, in practice, possible only at large boiler size categories because of the high investment costs of these devices. For pulverized combustion of peat, ca. 20}25% of the total mass of particle emissions is "ne particles, and then the speci"c emission factor is between 5 and 8 mg MJ\. However, the problem in the pulverized peat combustion is large emissions of nitrogen oxides. Therefore, some old pulverized peat boilers have already been replaced by more pro-environmental #uidized bed combustion techniques (e.g. at Rauhalahti power plant, JyvaK skylaK ). These #uidized bed boilers have, however, another disadvantage. They produce nitrous oxide (N O), which is a greenhouse gas.
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M.O. Ohlstro( m et al. / Atmospheric Environment 34 (2000) 3701}3711
For recovery boilers, the "ne-particle portion of the total particle emission is 50}60% (by mass) and the speci"c emission factor for "ne particles varies considerably according to the boiler size category, being between 12 and 77 mg MJ\. The particles emitted from recovery boilers are mainly salt particles. Fly ash consists mainly of sodium sulfate (Na SO ). It also contains lesser amounts of sodium carbonate (Na CO ), chlorine, po tassium and sul"de. Not many harmful trace elements such as heavy metals are found from the particles emitted by recovery boilers. For oil burners, grate boilers and #uidized bed combustion processes, the "ne-particle portion of the total particle emission could not be determined, because there were no applicable measurement results at hand. For these combustion techniques, more public measurements are needed in order to clarify the amount and composition of "ne-particles with di!erent fuel varieties. Also, small}scale combustion should be studied in detail, because the "ne-particle exposure which it causes can be signi"cant due to the low emission height and the absence of dust separation devices. This is especially important in densely populated areas, where the dominant heating form is individual wood, oil, peat or coal burning. Additionally, small-scale combustion is often incomplete, whereupon carcinogenic hydrocarbon compounds are formed. There were no large diesel power plants in Finland in 1995 (reference year), but "ne-particle emissions of marine diesel engines in big cruisers and cargo ships should be de"ned because their harbors are often in the city center, as in Helsinki. Due to the quite low emission height, the ships are a signi"cant "ne-particle emission source of concern to the ambient air of coastal cities. Fuel type is often heavy oil that produces particles rich in vanadium and nickel (LyyraK nen et al., 1999). As heavy metals, they are harmful to the environment. Because of the uncertain nature of statistical initial data, the results are di$cult to evaluate quantitatively. Additionally, the "ne-particle fractions of total particulate matter were based on single measurements due to the lack of emission side (i.e. after dust separators) measurements. More measurements have been done before ESP but they are not applicable since the size distributions change in the separation devices. It is important that enough measurements are made in the future in order to evaluate "ne-particle emissions of each combustion technique as for pulverized coal combustion in this paper.
Acknowledgements The authors are grateful to Marko Ekqvist from the Finnish Environmental Institute for the total particle emission data in VAHTI-database system.
References Berdowski, J.J.M., Mulder, W., Veldt, C., Visschedijk, A.J.H., Zandveld, P.Y.J., 1996. Particulate matter emissions (PM }PM }PM ) in Europe in 1990 and 1993. TNO Report, TNO Institute of Environmental Sciences, Energy Research and Process Innovation, Apeldoorn, The Netherlands, 90pp. Burnett, R.T., Dales, R., Krewski, D., Vincent, R., Dann, T., Brook, J.R., 1995. Associations between ambient particulate sulfate and admissions to Ontario hospitals for cardiac and respiratory diseases. American Journal of Epidemiology 142, 15}22. Clarke, L.B., 1993. The fate of trace elements during coal combustion and gasi"cation: an overview. Fuel 72 (6), 731}736. Dockery, D.W., Pope III, C.A., 1994. Acute respiratory e!ects of particulate air pollution. Annual Review of Public Health 15, 107}132. Dockery, D.W., Schwartz, J., Spengler, J.D., 1992. Air pollution and daily mortality: associations with particulates and acid aerosols. Environmental Research 59, 362}373. Dockery, D.W., Pope III, C.A., Xu, X., Spengler, J.D., Ware, J.H., Fay, M.E., Ferris Jr., B.G., Speizer, F.E., 1993. An association between air pollution and mortality in six U.S. cities. New England Journal of Medicine 329, 1753}1759. EC, 1997. Commission proposes new air quality limit values. Luxembourg, European Commission, 5pp. Flagan, R.C., Seinfeld, J., 1988. In: Fundamentals of Air Pollution Engineering. Prentice-Hall, Inc., New Jersey, pp. 542. Helble, J.J., Mojtahedi, W., LyyraK nen, J., Jokiniemi, J., Kauppinen, E., 1996. Trace element partitioning during coal gasi"cation. Fuel 75 (8), 931}939. IPCC, 1996. Climate change. In: Houghton, J.T., Meira Filho L.G., Callander, B.A., Harris, N., Kattenberg, A., Maskell, K. (Eds.), The Science of Climate Change. Cambridge University Press, Cambridge, UK, 584pp. Kauppinen, E., Pakkanen, T., 1990. Coal combustion aerosols: a "eld study. Environmental Science and Technology (ES&T) 24 (12), 1811}1818. Kauppinen, E.I., Lind, T.M., Valmari, T., YlaK talo, S., Jokiniemi, J.K., Powell, Q., Gurav, A.S., Kodas, T.T., Mohr, M., 1996. The structure of submicron ash from combustion of pulverized South African and Colombian coals. In: Baxter, L., DeSollar, R. (Eds.), Applications of Advanced Technology to Ash-related Problems in Boilers. Plenum Press, New York, pp. 471}484. LyyraK nen, J., Jokiniemi, J., Kauppinen, E., Joutsensaari, J., 1999. Aerosol characterisation in medium-speed diesel engines operating with heavy fuel oils. Journal of Aerosol Science 30 (6), 771}784. Moisio, M., 1997. Combustion aerosol size distribution measurements using Electrical Low Pressure Impactor. Licentiatum Thesis, Tampere University of Technology, Department of Electrical Engineering (Physics), Tampere, Finland, 181pp (in Finnish, with English abstract). Pekkanen, J., Timonen, K.L., Ruuskanen, J., Reponen, A., Mirme, A., 1997. E!ects of ultra"ne and "ne particles in urban air on peak expiratory #ow among children with asthmatic symptoms. Environmental Research 74, 24}33.
M.O. Ohlstro( m et al. / Atmospheric Environment 34 (2000) 3701}3711 Pope, C.A. III, Thun, M.J., Namboodiri, M.M., Dockery, D.W., Evans, J.S., Speizer, F.E., Heath Jr., C.W., 1995. Particulate air pollution as a predictor of mortality in a prospective study of U.S. adults. American Journal of Respiration Critical Care and Medicine 151, 669}674. Quann, R.J., Neville, M., Saro"rm, A.F., 1990. Laboratory study of the e!ect of coal selection and composition of combustion generated submicron particles. Combustion Science and Technology 74, 245}256. Schwartz, J., 1993. Particulate air pollution and daily mortality in Birmingham. Alabama. American Journal of Epidemiology 137, 1136}1147. Schwartz, J., 1994. Air pollution and hospital admissions for the elderly in Birmingham, Alabama. American Journal of Epidemiology 139, 589}590. Schwartz, J., Dockery, D.W., Neas, L.M., 1996. Is daily mortality associated speci"cally with "ne particles? Journal of Air Waste Management Association 10, 927}939. Seaton, A., 1998. The health e!ects of urban particulate air pollution. Invited lecture, in the Fifth International Aerosol Conference in Edinburgh. Seinfeld, J.H., 1986. Atmospheric Chemistry and Physics of Air Pollution. Wiley, New York, 738pp. Smith, I.M., Sloss, L.L., 1998. PM /PM } emissions and e!ects. IEA Coal Research, The Clean Coal Centre, London, UK, 75pp.
3711
Soud, H.N., Mitchell, S.C., 1997. Particulate control handbook for coal-"red plants. IEACR/65, London, UK, IEA Coal Research, 201pp. Statistics Finland, 1996. Energy statistics 1995. Helsinki, 126pp. Turbin, B.J., Huntzicker, J.J., 1995. Identi"cation of secondary organic aerosol episodes and quantitation of primary and secondary organic aerosol concentrations during SCAQS. Atmospheric Environment 23, 3527}3544. US EPA (the U.S. Environmental Protection Agency), 1997. EPA's revised particulate matter standards. http://ttnwww. rtpnc.epa.gov/naaqs"n/pmfact.htm. AltaVista 21.8.1998. VAHTI, 1998. A database system containing emission data of energy production and industrial processes in Finland. Maintained by the Finnish Environmental Institute. The total particulate emission data got from Marko Ekqvist 15.5.1998. WHO, 1994. Update and revision of the air quality guidelines for Europe. WHO, Regional O$ce for Europe. Meeting of the Working Group `Classicala Air Pollutants, Bilthoven, The Netherlands, 11}14 October 1994. Wilson, R., Spengler, J., 1996. Conclusion: policy implications: the national dilemma. In: Wilson, R., Spengler, J. (Eds.), Particles in our Air:: Concentrations and Health E!ects. Harvard University Press, Cambridge MA USA, pp. 205}216.