14-A-i

APPENDIX 14-A. EMISSIONS FACTORS FOR FUEL COMBUSTION FROM 

NATURAL GAS, LPG, AND OIL-FIRED RESIDENTIAL APPLIANCES

TABLE OF CONTENTS

14-A.1 OVERVIEW ........................................................................................................14-A-1

14-A.2 EMISSIONS FACTORS BY FUEL TYPE.........................................................14-A-2

14-A.2.1 All Fuels: Carbon Dioxide...................................................................................14-A-2

14-A.3 NATURAL GAS .................................................................................................14-A-2

14-A.3.1 Oxides of Nitrogen...............................................................................................14-A-2

14-A.3.2 Sulfur Dioxide......................................................................................................14-A-3

14-A.4 FUEL OIL #2.......................................................................................................14-A-3

14-A.4.1 Oxides of Nitrogen...............................................................................................14-A-3

14-A.4.2 Sulfur Dioxide......................................................................................................14-A-3

14-A.5 LPG......................................................................................................................14-A-4

14-A.5.1 Oxides of Nitrogen...............................................................................................14-A-4

14-A.5.2 Sulfur Dioxide......................................................................................................14-A-4

14-A.6 SUMMARY.........................................................................................................14-A-5

LIST OF TABLES

Table 14-A.1.1 Estimated National Average Emissions Factors for Household Fuel 

Combustion of Natural Gas, Fuel Oil #2, and LPG.................................14-A-1
14-A-1

APPENDIX 14-A. EMISSIONS FACTORS FOR FUEL COMBUSTION FROM 

NATURAL GAS, LPG, AND OIL-FIRED RESIDENTIAL APPLIANCES

14-A.1 OVERVIEW

The modified version of the National Energy Modeling System (NEMS) used for the 

appliance energy-efficiency standards analysis, called NEMS-BT, comprehensively considers a 

wide range of aspects of energy use. However, this model does not consider household 

emissions from the combustion of natural gas, residential heating fuel oil #2 (i.e., high-sulfur 

distillate fuel oil), and LPG. Because an efficiency standard could result in changes to household 

emissions, the Department has performed some elementary research to determine appropriate 

emissions factors for CO2, NOx, and SO2 for each of the fuel types consumed by household 

appliances. This work focuses on emissions rates from the use of furnace and boilers and is used 

to calculate emissions savings for proposed appliance efficiency standards. Emissions factors for 

furnaces are presented over small-scale boilers whenever both are available.

This analysis attempts to verify that the emissions factors from the U.S. Environmental 

Protection Agency (EPA) for CO2, NOx, and SO2 from natural gas, fuel oil #2, and LPG 

combustion are representative of the U.S.1 The EPA cautions against the use of these emissions 

factors as representative of actual emissions. Rather, they are to be used, and are used here, as 

general approximations of national average emissions factors in order to calculate emissions 

savings that result from proposed appliance standards. This analysis compares the EPA’s 

emissions factors with those from other sources and with regional regulations.

Table 14-A.1.1 summarizes the emissions factor estimates for the three fuels of interest. 

The values presented here are based on the EPA’s assessment of combustion emissions (with the 

exception of SO2 from LPG, see discussion below). These emissions estimates represent an 

approximate emissions factor for the U.S. as a whole. They are used, together with estimated 

energy savings, to calculate end-use site emissions savings. The emissions factors discussed in 

this analysis are represented as mass (g or kg) of the specific emission of interest per gigajoule 

(GJ) of energy input to the furnace and boiler.*

Table 14-A.1.1 Estimated National Average Emissions Factors for Household Fuel 

Combustion of Natural Gas, Fuel Oil #2, and LPG

CO2

(kg/GJ)

NOx

(g/GJ)

SO2

(g/GJ)

Natural Gas 50.6 40 0

Fuel Oil #2 68.6 55 218

LPG 58.7 66 7

 *

 0.454 kg = 1 lb; 1.055 GJ = 1 MBtu
14-A-2

There are significant differences in state regulations, making it difficult to generalize 

about localized emissions. For residential fuel oil combustion, stricter state standards exist in the 

northeastern United States and southern California than elsewhere. State and local agencies like 

southern California’s South Coast Air Quality Management District (SCAQMD)2

, the New 

Jersey Department of Environmental Protection, and the New York Department of 

Environmental Conservation have established regulations for residential fuel combustion 

emissions. These regulations, however, are local to individual counties and do not apply to all 

regions in the Northeast or even all of California. 

14-A.2 EMISSIONS FACTORS BY FUEL TYPE

14-A.2.1 All Fuels: Carbon Dioxide 

NEMS-BT tracks CO2 well, but factors are included here for completeness. The CO2

estimates in Table 14-A.1.1 are based on EPA’s assessment of CO2 combustion from natural gas, 

fuel oil #2, and LPG use. Other sources of information include the U.S. DOE’s Energy 

Information Administration (EIA), Oregon State EPA, and the Gas Research Institute (GRI). All 

agree to within ± 2.0 percent of EPA’s assessed estimate of CO2 emissions. EPA’s CO2

emissions factors for natural gas, fuel oil #2, and LPG combustion are therefore a reasonable 

evaluation of nationally averaged CO2 emissions rates.

Carbon monoxide (CO) can be formed by incomplete carbon oxidization, causing a 

severe health hazard. Carbon monoxide emissions only result from poorly functioning 

appliances, and since NEMS-BT assumes that all appliances are functioning properly, it is 

consistent to assume that these emissions will not change.

14-A.3 NATURAL GAS

14-A.3.1 Oxides of Nitrogen

Emissions of NOx that result from the combustion of natural gas from appliances are also 

covered by regulations in some jurisdictions. The EPA’s estimated NOx emissions factor of 40 

g/GJ from gas-fired furnaces is roughly equivalent to the regulation set by the state of New York. 

The SCAQMD limits NOx emissions to 40 g/GW from gas-fired residential furnaces, equivalent 

to EPA’s AP-42 estimate. The San Luis Obispo Air Pollution Control District also mandates the 

same NOx limit from furnaces. The SCAQMD also provided performance test results from 

numerous commercially available residential gas furnaces. In general, the emissions rates ranged 

from 35-40 g/GJ NOx with some higher efficiency units emitting between 26-35 g/GJ NOx. New 

Jersey’s state regulation mandates no more than 43 g/GJ NOx emissions from natural gas 

combustion in small boilers and furnaces. The absence of regulations in other states suggests 

that most have emissions rates comparable to the EPA’s estimated value. 
14-A-3

14-A.3.2 Sulfur Dioxide

SO2 emissions from natural gas combustion are assumed to be zero.

14-A.4 FUEL OIL #2

14-A.4.1 Oxides of Nitrogen

NOx formation in combustion processes is significantly more complex than the formation 

of SO2 or CO2. NOx can result from oxidization of N2 in the fuel, but the larger source is N2 in 

the combustion intake. This process is sensitive to combustion chemistry, and therefore, quite 

variable. It also implies that appliance design can lower NOx emissions. An added difficulty is 

that the potential hazard of residential NOx emissions is hard to quantify. NOx is both an acid 

precipitation and ozone (O3) precursor, but because residential appliances are a relatively small 

and dispersed (i.e., hard to monitor) source, these emissions tend to play a small role in NOx

control strategies. The O3 hazards of NOx emissions are highly variable both spatially and over 

time, as weather conditions favor or disfavor O3 formation. One would, therefore, expect NOx

emissions rules to vary significantly across jurisdictions.

The value estimated by EPA of 55 g of NOx/GJ is for residential oil-fired furnaces. 

Additional research into specific state regulations reveals good matches with EPA’s emissions 

factor: New Hampshire limits NOx emissions to 60 g/GJ for non-utility boilers and New York 

and New Jersey regulations cap emissions at 52 g of NOx/GJ. These state limits confirm that 

EPA’s estimate can serve as a reasonable national average.

14-A.4.2 Sulfur Dioxide

Emissions of SO2 are a direct function of the sulfur content of the fuel. Assuming 

complete combustion of sulfur to SO2 makes it easy to assess and compare emissions across data 

sources. Individual state and county regulations limiting the amount of sulfur in residential fuel 

oil, however, reveal substantial variability in SO2 emissions factors from EPA’s estimate of 218g 

of SO2/GJ of fuel energy content for both residential furnaces and boilers. This is roughly 

equivalent to 0.5 percent sulfur content by weight using an emissions factor for fuel oil #2 of 

43,950 g/GJ weight per unit energy content. A straight multiplication of the fuel energy content 

and the percentage of sulfur content in the fuel provide the emissions factor. 

In the state of New York, SO2 emissions limits range from 88-659 g/GJ depending on the 

county. In New Hampshire, the emissions rate of SO2 from high-sulfur distillate fuel oil 

combustion cannot exceed 176 g/GJ. In Maine and Michigan, the regulations are higher than 

EPA’s assessment, limiting SO2 emissions from fuel oil to 500 g of SO2/GJ of heat input. Such 

broad ranges and varying limits in certain regions are a strong argument for using EPA’s 
14-A-4

averaged national value over a regionally weighted average incorporating the various state and 

county regulations.

Comparing the sulfur content of fuel oil from different refineries also demonstrates 

regional variability. Most sources were obtained through online websites detailing product 

specifications. Mobil Corporation’s residential heating fuel oil (#2 high-sulfur) for most states 

contains a maximum 0.5% sulfur by mass (219 g of SO2/GJ of heat input), which is very close to 

EPA’s emissions factor. Mobil also markets fuel with lower sulfur content to some regions in 

the Northeast. For example, Mobil’s lowest sulfur content residential heating fuel oil contains 

0.2 percent sulfur by weight (88 g/GJ), as required in New York City, Philadelphia, parts of 

Delaware, and most of New Jersey. Other regional regulations adhered to by Mobil are 132 g/GJ 

for most of Pennsylvania and parts of New Jersey, a limit of 127 g of SO2/GJ of heat input for 

the state of Illinois, and an SO2 emissions factor of 145 g/GJ in Massachusetts. The sulfur 

contents of distillate fuel oil from Chevron, Phillips 66, and the American Society for Testing 

and Materials (ASTM) exactly match EPA’s estimate, substantiating the use of the latter for the 

analysis. Considerable variation does exist among counties and states, especially in the 

Northeast. Nonetheless, EPA provides a reasonable estimate of emissions from residential oilfired furnaces and boilers that are representative of the national average. 

14-A.5 LPG

14-A.5.1 Oxides of Nitrogen

EPA estimates emissions for only propane and butane, fuels within the LPG family of 

gases. In the United States, the composition of LPG is typically 90 percent propane (C3H8) by 

liquid volume, 5 percent propylene (C3H6), and 2.5 percent butane (C4H10).3, 4 This analysis uses 

EPA’s propane emissions estimate to verify its comparability with LPG combustion or 

determines a more suitable estimate from other sources that best represents LPG emissions rates.

EPA estimates an emissions factor of 66 g of NOx/GJ of energy input from propane-fired 

furnaces and boilers, which is slightly higher than the natural gas estimate. The University of 

New Hampshire’s Energy Office estimates an emissions factor of 43 g/GJ from LPG combustion 

for the state of New Hampshire. In contrast, the state of New Jersey’s EPA establishes an 86 

g/GJ emissions factor. Both sources, therefore, support EPA’s national assessment of NOx

emissions from residential propane combustion.

14-A.5.2 Sulfur Dioxide

The sulfur content of LPG is very low5

, although a few regulations exist in certain parts 

of the country. EPA’s SO2 emissions estimate is derived from the sulfur content of the propane 

fuel. With 90% of LPG comprised of propane, the EPA’s propane emissions factor is a 

reasonable value for LPG emissions rates. Under the New Hampshire Department of 

Environmental Services (DES) regulation, the maximum sulfur content allowable is 13.3 g/GJ of 

SO2 from LPG combustion. The San Diego County Air Pollution Control District enforces a 
14-A-5

stricter limit of only 8.8 g of SO2/GJ of energy input using EPA’s recommended value. Both the 

ASTM and Santa Barbara County Air Pollution Control District specify a sulfur standard of 

0.0185 percent sulfur by weight, which approximates an emissions factor of 4.0 g of SO2/GJ of 

energy input. This calculation is based on an LPG weight per unit energy of 21,756 g/GJ. Even 

lower, Nett Technologies Inc. estimates a 0.012 percent sulfur content by mass LPG average for 

the United States as well as Canada, which is approximately 2.6 g of SO2/GJ of heat input.

 

The wide range of emissions factors for SO2 makes it difficult to determine a 

representative estimate for the emissions factor and the very small net effect of any energyefficiency standard argue against making a significant research effort. The obtained estimates 

range in magnitude from approximately 4 to 13 g/GJ of energy input. The statistical average or 

arithmetic mean of these four estimates is 7.2 g/GJ, the value provided in Table 14-A.1.1. The 

averaged emissions factor estimate of 7.2 g/GJ energy input is reasonable to explain three of the 

five sources presented and suitably accounts for the average among the two extreme estimates. 

Table 14-A.1.1 therefore adequately represents national SO2 emissions from residential LPG 

combustion.

14-A.6 SUMMARY

This investigation of emissions factors from natural gas-, fuel oil-, and LPG-fired 

residential furnaces and boilers indicates that using the values estimated by EPA’s AP-42 is a 

robust and credible basis for analysis with NES output. This analysis indicates that regulations at 

the state and county level, especially in the northeastern region of the United States, are 

generally not good estimators of the national average set forth by the EPA AP-42 report. This is 

especially true for SO2 emissions factors from fuel oil combustion where individual state and 

county regulations are highly variable. Thus, although it is worthwhile examining regional 

legislation to support the national average, this analysis demonstrates the complexity that would 

be required to establish a weighted regional calculation for each emission. The EPA’s own lack 

of data on the sulfur content of LPG made it difficult to derive a nationally representative 

emissions factor for the country as a whole. External sources were therefore incorporated in the 

Table 14-A.1.1 estimate for SO2 emissions from LPG combustion.
14-A-6

REFERENCES

1. U.S. Environmental Protection Agency, Compilation of Air Pollutant Emission Factors, 

AP-42, Fifth Edition, Volume I: Stationary Point and Area Sources, 1998. (Posted 

October) < http://www.epa.gov/ttn/chief/ap42/index.html >

2. South Coast Air Quality Management District, Rule 1121: Control of Nitrogen Oxides 

From Residential Type, Natural Gas-Fired Water Heaters, 1995. March. 

<http://www.aqmd.gov/>

3. U.S. Department of Energy, U.S. Hemispheric Clean Cities Program, 1994.

<http://www.eere.energy.gov/cleancities/>

4. New Jersey State Department of Environmental Protection, New Jersey Administrative 

Code, Title 7, Chapter 27, Subchapter 19: Control and Prohibition of Air Pollution from 

Oxides of Nitrogen; 7:27-19.7 Non-Utility Boilers and other indirect heat exchangers, 

September 1993. <http://www.state.nj.us/dep/aqm/2719968.htm#Boilers>

5. U.S. Environmental Protection Agency, Compilation of Air Pollutant Emission Factors, 

AP-42, Fifth Edition, Volume I: Stationary Point and Area Sources, Chapter 1, External 

Combustion Sources, 1998. (Posted October)

< http://www.epa.gov/ttn/chief/ap42/index.html >