4.2 S TATIONARY C OMBUSTION S OURCES
4.2.2 Liquid Fuels and Natural Gas
Particulate emissions from burning of liquid fuels depend, similarly to solid fuels, on combustion conditions and fuel type, e.g., the ash content of the fuel. The ash content is, however, much lower than that of solid fuels, which makes the share of inorganic particles smaller. The major categories of fuel oil are distillate oils and residual oils. Residual oil is also known as heavy fuel oil and it is produced from the residue remaining after the lighter fractions like distillate oil or light fuel oil have been removed from the crude oil in the refining process (US EPA 1998; Lighty et al., 2000).
4.2.2.1 Heavy Fuel Oil
For heavy fuel oil, particulate emissions are influenced by the ash and sulfur content of the oil. The emissions are composed of three basic types of particles, i.e., submicron carbonaceous particles, ash residues and residues of the fuel spray droplets, called cenospheres (Goldstein & Siegmund, 1976). Ash and the carbon-rich cenospheric particles are coarse and may account for a major part of the mass emitted. However, owing to their relatively short atmospheric lifetime, they are not of primary interest for this analysis. They are both highly dependent upon fuel composition. Especially small boilers burning residual oil can have a large mass fraction in the coarse mode, composed largely of cenospheres. In bigger boilers, combustion is more efficient, so that the carbon content is lower and almost all emitted particles are in the fine or submicron mode (Goldstein & Siegmund, 1976; Lighty et al., 2000). For heavy fuel oil, the emission factors and the shares of black carbon, organic carbon and submicron fractions found in the literature are shown in Table 4.45, Table 4.46 and Table 4.47.
Table 4.45: Summary of the PM1 emission factors for heavy fuel oil combustion [mg/MJ]
Source PM1 Remarks
3.92 public power generation 2.10 industrial combustion
small plant (2MW), residential
(normal, warmup, overall emission factor)
9.46 50 hp package boiler, low sulfur (0.5%S) fuel Goldstein & Siegmund, 1976 a)
14.2 high sulfur (2.2%S) fuel 10.1*A b) utility boilers
9.20*A industrial boilers US EPA, 1998
3.58*A commercial boilers
a) original values 30 and 45 mg/SCM (mg/standard cubic meter), respectively; b) No 4 oil: A=0.84, No 5 oil:
A=1.2, No 6 oil: A=1.12*S+0.37, where S is the weight% of sulfur in the oil
Table 4.46: Summary of the PM1 shares in total particulate matter for heavy fuel oil combustion [%]
Ehrlich et al., 2001 49.9-64.3 10 MW boiler (operating 5-8.5 MW), abated with additive
~60 50 hp package boiler low sulfur (0.5%S) fuel Goldstein & Siegmund, 1976
~25 high sulfur (2.2%S) fuel 39 Utility boilers
36 Industrial boilers US EPA, 1998
14 Commercial boilers
Submicron shares for heavy fuel oil burned in power plants, industry and residential sector were derived based on the PM1 fractions found in the literature (Table 4.46). The resulting numbers are 40, 30 and 15 percent in TSP, respectively.
Table 4.47: Summary of the shares of BC and OC in total particulate matter for heavy fuel oil combustion. (HF) [%]
Source BC OC Remarks Method
Goldstein & Siegmund, 1976 Total carbon 60% of TSP Unknown Henry & Knapp 1980 23.8
12.5
<0.1 Power plants, (average of 6 fly ash samples, average without highest value)
T Lighty et al., 2000 TC >75% of TSP, (from Miller et al.,
1998), small boilers Unknown
7.7 2.7 % in PM2.5 (fine mass 93% of PM10), 395 MW power plant, in stack measurements, after ESP 21 Not
detected
% in coarse particles (2.5<PM<10)
8.6 2.5 % in PM10
0.22 0.75 % in PM2.5 (fine mass 96% of PM10), 660 MW power plant (operating 460 MW), duct measurements, after
Wolff et al., 1981 11.5 24.5 Utility and industrial boilers,
(see references in Wolff et al. 1981) Unknown
a) Power plants burning No. 6 fuel oil; b) Not including profiles that refer to Henry & Knapp, 1980
The emission shares of BC and OC for heavy fuel oil burning in industry and power plants were derived by taking the averages of all available measurements for fine fraction and converting them to refer to TSP using the current PM2.5 share in RAINS (60 percent). The shares introduced in RAINS are 4.3 percent for BC and 1.9 percent for OC. There were no studies available reporting measurements representing residential burning of heavy fuel oil.
Therefore, the OC value (0.375 mg/MJ) used by Bond et al. (2004) and the current RAINS PM2.5 emission factor of 10 mg/MJ were used to derive the share of OC. Combining these values leads to an estimated one percent of OC in TSP. In order to derive BC shares and emission factors, information available in the SPECIATE 3.2 database (US EPA, 2002) and Bond et al. (2004) was used. An average share of 3.5 percent was derived, which leads to a BC emission factor of about 1.3 mg/MJ, which is slightly higher than that reported by Bond et al. (2004), i.e., about 1 mg/MJ.
4.2.2.2 Light Fuel Oil
Particles that are formed during combustion of light fuel oil are found in fine and coarse modes. They are mainly carbonaceous, but may contain inorganic components, e.g., sulfates (Sabbioni & Zappia, 1992; Lighty et al., 2000). The ash content of the distillate fractions is, however, much smaller than that of residual oil. Distillate oils are often used for small-scale domestic heating, where emissions are usually uncontrolled. The emission factors of black carbon, organic carbon and submicron fractions in fine particles are shown in Table 4.48 and Table 4.50.
Table 4.48: Summary of the emission factors of PM1, BC and OC for combustion of light fuel oil [mg/MJ]
Source BC OC PM1 Remarks Method
1.26 public power generation
0.85 industrial combustion
APEG, 1999;
a) original values 0.0204 and 0.09 kg/1000 l, respectively
Table 4.49: Summary of PM1 shares in total PM for the combustion of light fuel oil [%]
Source PM1 Remarks
Based on the shares and absolute numbers in Table 4.48 and Table 4.49 (mainly from US EPA 1998), the PM1 share in TSP emissions is estimated at 10 percent for power plants and industry and at approximately 40 percent for the domestic sector. To derive the BC and OC shares for power plants and industry, results of Hildemann et al. (1991) were used. Results from Hangebrauck et al. (1964) were not included, since their reported emission factors for TSP are very high, which is probably influenced by the different quality of fuel oil in the US 40 years ago and the very different combustion technologies. The numbers from Hildemann et
al. (1991) (see Table 4.50) were converted to refer to TSP using the current RAINS PM2.5 shares. With this, the estimated shares are five percent for BC and 0.8 percent for OC in TSP.
For the domestic sector, the BC and OC shares were derived assuming that 40 percent of total particles is carbonaceous (TC). A BC to OC ratio of approximately four, as reported by Ålander (2000), was applied to the TC share, which results in 32.4 percent of BC and 8.1 percent of OC for burning of light fuel oil in domestic sector.
Table 4.50: Summary of the BC and OC shares in total particulate matter for the combustion of light fuel oil [%]
Wolff et al., 1981 16 Utility and industrial boilers
(based on Watson 1979) Unknown
Ålander, 2000 85-70 15-30 % of TC, small domestic boilers TO
4.2.2.3 Natural Gas
Over 85 percent of natural gas is methane, with small amounts of other gaseous substances (US EPA, 1998). Because it is a gaseous fuel without solid ash, particulate emissions are typically very low. Emissions are influenced by the quality of combustion and consist mainly of unburned organic carbon, practically totally found in the submicron mode (Hangebrauck et al., 1964; Hildemann et al., 1991; US EPA, 1998; APEG, 1999).
Table 4.51: Summary of the PM1, BC and OC emission factors for combustion of natural gas [mg/MJ].
Source BC OC PM1 Remarks Method
Bond, 2000 a) 0.067 Small plant (2MW), residential 0.99 Process heating (firetube boiler) 1.12 Hospital heating
(scotch-marine boiler) Hangebrauck et al., 1964
0.69-2.11
Home heating (hot air furnace, double shell boiler & wall space heater)
US EPA, 1998 0.82 Filterable PM, boilers and
furnaces -
a) original value 0.003 g/kg
Table 4.52: Summary of the shares of PM1, BC and OC in fine particulate matter for the combustion of natural gas [%].
Source BC OC PM1 Remarks Method
APEG, 1999 100 Industrial combustion, power generation and small combustion
US EPA, 1998 100
11 Process heating (firetube boiler) 8 Hospital heating (scotch-marine boiler) Hangebrauck et al., 1964
19-33 Home heating
SE
Hildemann et al., 1991 6.7 84.9 % of PM2, residential heater TO 4 8 Laboratory (10 tests)
Residential heater, methane gas, normal operating conditions.
8 40 Residential chimney (1 test) methane gas
89 5 Laboratory (6 tests)
Residential heater, propane gas, fuel rich operating conditions
Muhlbaier & Williams, 1982
46 33 Residential chimney (1 test) methane gas
T (mod.)
Based mainly on the information from US EPA (1998) it was assumed that all PM is emitted as PM1. For the domestic sector, the BC and OC numbers for RAINS were derived from the study by Hildemann et al. (1991), who also measured fine particle emissions in the range currently in RAINS. The reported shares of BC and OC were 6.7 percent and 84.9 percent, respectively. The OC share, however, is very uncertain. Hildemann et al. (1991) estimated that up to 61.7 percent may result from gas-phase adsorption. We relied on the BC share reported by Hildemann et al. (1991), but scaled down the OC share to 75 percent so that the BC+OM adds up to about 97 percent. Applying these shares results in emission factors of about 7 and 75 µg/MJ for BC and OC, respectively.
Table 4.53: Shares and ranges of BC, OC and PM1 emission factors for liquid and gaseous fuels as used in the RAINS model
Share in TSP [%] Emission factors [mg/MJ]
BC OC PM1 BC OC PM1
Heavy fuel oil
Power plants 0.2-10 (4.3) 0-2 (1.9) 40 0.74-0.84 0.33-0.37 6.9-7.8 Industry 0.2-10 (4.3) 0-2 (1.9) 30 0.67 0.29 4.7
Residential 3 1 15 1.3 0.38 5.7
Light fuel oil
Power plants 5 0.8 10 0.11-0.18 0.018-0.029 0.22-0.36
Industry 5 0.8 10 0.11 0.018 0.22
Residential 32.4 8.1 40 0.53 0.13 0.66
Natural gas 7 75 100 0.007 0.075 0.1