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• What are the combustion products produced when burning natural gas or LP gas in a heating appliance?
• What are the normal combustion products and what dangerous combustion products may be produced when a gas fired appliance is not working normally?
• How much carbon dioxide, carbon monoxide, sulphur dioxide, or nitrous oxide are produced when burning natural gas?
• How to find & report defects found in LP or natural gas controls, regulators, or storage tanks

This article lists the components in the flue gas or exhaust gas produced by the combustion of natural gas (and similarly propane gas or liquified natural gas (LNG) such as when gas is burned in a home heating appliance like a water heater or a heating boiler.

A Guide to LP or Natural Gas Fired Appliance Combustion Products found in Flue Gas Exhaust

Natural gas, currently supplying about 22% of energy in the U.S., has been used as a fuel since its development by the Chinese more than 2500 years ago. Today it is used in buildings for heating as well as for production of hot water and in some cases even for cooling. In industry it is used as a heating fuel for many processes. In the U.S. the first natural gas well was dug in Fredonia New York in 1821.

Perfect combustion of natural gas (Methane – CH4) produces only CO2 and water vapor The equation for the combustion of natural gas is CH4[g] + 2 O2[g] -> CO2[g] + 2 H2O[l] + 891 kJ

• [g]=gaseous form
• [l]=liquid form;
• the 891 is kilojuoules of energy released by the chemical reaction (combustion) process.
  

But this is a simplification since natural gas is not pure. Natural gas, as it is produced from a gas well, also contains ethane C₂H₆, propane C₃H₈, butaneC₄H10, carbon dioxideCO₂, nitrogen (N), helium (He), and hydrogen sulfide H₂S.

Before it is distributed to consumers, ethane, propane, butane are removed from natural gas. Small quantities of other molecules may be produced during natural gas combustion than those in the “pure” case we listed above.

In the table shown here we list the relative quantities of combustion products produced when burning natural gas. the numbers are pounds produced per million Btus of NG burned:

Table of Combustion & Flue Gas Products in Natural Gas

-- from various information sources on natural gas. We anticipate that the combustion products from burning liquefied natural gas LNG and propane (C₃H₈) will be similar. Note that one cubic foot of gas contains about 1031 BTUs. 1 million BTUs is provided in 970 cubic feet of natural gas. (27.4 cubic meters). MWh = million watt hours.

Normal natural gas combustion: In a practical sense in a home or office building if we are considering a small natural gas appliance such as a water heater, and provided that the equipment and its flue vent connector and chimney area all working correctly and that there is adequate combustion air, once the equipment has warmed up and draft is established, the system is producing CO₂ and H₂O (in the form of water vapor) and not much else that will be detected by the building occupants.

Imperfect (and unsafe) natural gas combustion, short on Oxygen from too little combustion air or from a chimney problem, will produce CO as well as nitrogen oxides (NOx), organic particulate material, and polynuclear aromatic hydrocarbons (PAH). Incomplete combustion of natural gas may also release un-burned methane CH₄ itself.

The reason for the initial versus stabilized- burn CO level spec is that until the appliance heats up combustion is incomplete and higher levels of CO are produced.

The percentage makeup in flue gas from a gas fired water heater will probably not be given as a general overall standard in many references and by most onsite HVAC technicians or inspectors except in theoretical combustion instances because of the wide variability in equipment, vents, and chimneys.

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• U.S. Energy Information Administration - eia.doe.gov/
• U.S. Environmental Protection Agency - epa.gov/solar/energy-and-you/affect/natural-gas.html
• At Natural Gas.Org www.naturalgas.org/environment/naturalgas.asp#emission you’ll find a table of combustion products
• At geocities.com/rainforest/6847/report1.html is an interesting and detailed though not “neutral” report on the components and contaminants in the combustion of natural gas. You’ll see a long long list of emissions products, but look again – most of the contaminant levels listed are in the picograms.
• apvgn.pt/documentacao/iangv_rep_part1.pdf lists the components in natural gas exhaust from vehicles
• The Need Project, Manassas, VA: need.org/needpdf/infobook_activities/SecInfo/NGasS.pdf
• Kroschwitz, Jacqueline I., and Mary Howe-Grant (eds.). "Gas, Natural." In Encyclopedia of Chemical Technology. 4th ed., vol. 12. New York: John Wiley and Sons, Inc., 1993.
• Tussing, Arlon R., & Bob Tippee. The Natural Gas Industry: Evolution, Structure, and Economics. 2nd ed. Tulsa, OK: PennWell Publishing, 1995.

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