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Table 4. Range and median emission factors (lb/MMBtu) from data sources collected herein including air permits, stack test data, and industry research reports as compared to EPA’s AP-42 values, disaggregated by fuel type.
Emission Factors (lb/MMBtu)
Wood and Wood Residues [a]
Sludge [a]
Biomass Waste [a]
Wood Residues and Sludge
EPA’s AP-42 (lb/ MMBtu)
Range [b] 0.33–0.56 0.30–0.36 0.25–0.43
Median Point Estimate [c]
Range [b] 0.40–4.36 0.40–4.36 0.40–4.36 0.018–0.06 0.02–0.51 0.12–0.67 0.02–0.39
Range [b] 0.73–7.5 6.0–7.0 5.8–6.0
Pollutant
Median
Median
PM f
0.56
0.40 0.36 0.30 0.13 0.79 0.72 0.009 0.004
0.26
0.41 0.40 0.38 0.04 0.02 0.37 0.10
7.39 6.89
PM 10 PM 2.5 PM c
0.5
n/a
0.43
0.19
6.0
0.017 0.025
0.009
0.006
0.005–0.02 0.49–2.65 0.08–0.76 0.002–0.22 0.013–0.019
0.017
0.13 0.79 0.72
n/a n/a n/a n/a n/a n/a n/a
2.35 0.70 0.22
SO 2 NO x
0.22
CO
0.6
VOC
0.017
0.004
0.003
0.017 0.002
0.017 0.002
0.017
H 2 SO 4
n/a n/a 195
n/a n/a n/a
n/a n/a n/a
0.10–0.16
0.15
NH 3 CO 2
n/a
n/a 218 n/a
0.013
0.013
210–225
216
216
Acetaldehyde Formaldehyde
8.3E-04 4.4E-03 1.9E-02
2.7E–04–2.8E-04 2.7E-04 1.0E–03–4.4E-03 2.5E-03 1.6E–03–1.2E-02 6.9E-03
n/a
8.3E–04–2.0E-03 1.3E-03
1.3E-04 6.7E-02 6.4E-04
3.0E–05–5.8E-04 1.3E-04 2.0E–04–4.4E-03 2.3E-03
HCl
1.5E–03–2.7E-01 1.6E-02 8.5E–04–1.0E-03 9.2E-04
2.6E-01 3.0E-04
2.6E-01 3.0E-04
HF
n/a
1.4E-04
1.4E-04
lb/MMBtu = pounds per million British Thermal Unit, PM f = particulate matter (filterable), PM 10 = PM with diameter less than or equal to 10 micrometers, PM 2.5 = PM with diameter less than or equal to 2.5 micrometers, SO 2 = sulfur dioxide, NO x = nitrogen oxides, CO = carbon monoxide, VOC = volatile organic compounds, H 2 SO 4 = sulfuric acid, NH 3 = ammonia, CO 2 = carbon dioxide, HCl = hydrogen chloride, HF = hydrogen fluoride, n-a = not available. [a] Wood and wood residues include bark, dry/wet wood, logging residue, and wood waste, whereas sludge refers to wastewater treatment sludge. Biomass waste includes biomass, biogas, digester sludge, biomass fines, and lignin stillage. [b] For several pollutants, the range is not reported due to lack of emission factor data from multiple sources. In this case, only a point estimate is included. [c] A point estimate is included where only one data source is available.
emission factors for PM from wood residues are similar to AP-42, whereas burning sludge produces 54% less emissions as compared to AP-42. When combusting biomass waste, the emission factors are at least 23% higher than the values listed in AP-42. Overall, the emission factors for PM fractions (filter- able PM 10 and PM 2.5 ) range from 0.2–7.5 lb/MMBtu, as compared to 0.5 and 0.43 lb/MMBtu for PM 10 and PM 2.5 from AP-42, respectively, whereas the conden- sable fraction of PM is expected to be relatively small, ranging from 0.005–0.06 lb/MMBtu as com- pared to 0.017 lb/MMBtu from AP-42. The large difference in variability of emission factors for PM may be attributed to composition of biomass fuel which is not always available in the literature. For example, if the moisture content in the fuel is high, the emission factors for reported PM are likely high as well. Filterable PM 10 and PM 2.5 have a large coef- ficient of variation (~180% for both) compared to condensable PM, which has a coefficient of variation of 80%. This is consistent with EPA’s finding that emissions of the condensable fraction of PM were independent of fuel type (EPA 2009). Although the emission factors for SO 2 are closely correlated with the amount of sulfur in the fuel mixture, the reported SO 2 emissions vary considerably, ranging from 0.13 lb/MMBtu for wood residues, 0.02–0.51 lb/ MMBtu for a mixture of wood residues and sludge, and 0.49–2.65 lb/MMBtu for biomass waste. The variation in
emission factors for SO 2 from different sources is mainly due to the amount of sulfur present in the fuel. The value reported in AP-42 for SO 2 is at the low end of the range at 0.025 lb/MMBtu. Most sulfur in the fuel is expected to be emitted as SO 2 , though a small fraction could end up in ash. Thus, we recommend utilizing a mass balance approach to estimate the emission factor of SO 2 , as it is closely tied to the amount of sulfur in the fuel. The reported AP-42 emission factor for NO x is 0.22 lb/MMBtu, which falls in the range from the surveyed data (0.02–0.79 lb/MMBtu). The emission factor of NO x is 3.6 times higher than in AP-42 for wood resi- dues, whereas it could be lower by 45% or higher by 300% as compared to AP-42 when burning a mixture of wood residues and sludge. Similarly, for biomass waste, the emission factor of NO x can be up to 3.5 times higher compared to AP-42. The variability in emission factors for NO x is mainly a factor of combustion char- acteristics (complete vs incomplete combustion), boiler type, and composition of fuel. Fuel-bound nitrogen plays an important role in the amount of NO x to be formed and emitted. However, boiler manufacturers’ experience appears to indicate that a well-designed and properly tuned boiler should be able to reduce NO x emissions to 0.18–0.28 lb/MMBtu for stoker boilers and 0.15–0.24 lb/MMBtu for bubbling fluidized bed boilers (Bowman et al. 2009). CO, which is the result of incomplete combustion, and which is inversely proportional to NO x
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