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

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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 gas 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.

Frequently Asked Questions (FAQs) about about gas fuels, methane gas leaks, odors, and hazards, and about gas fired heating appliances.

Question: can an electric water heater produce methane gas?

Hello, I was reading your website about possible odors associated with systems in the home. Is it possible for an electric water heater to produce a methane gas? I have recently verified readings from a hot water line that had methane readings in the flammable ranges. Any input or information would be appreciated. Thanks, R.T.

Reply: an electric water heater does not naturally produce methane gas but there can be other methane gas sources in a building water supply and other odor sources in a water heater or in water supply

A competent onsite inspection by an expert usually finds additional clues that help accurately diagnose a problem with the water heater or with gas piping in the building, but none of these ought to involve methane induced into the water heater tank interior from the appliance or its fuel piping. And simply heating water does not innately produce methane gas.

An electric, or oil fired water heater does not produce methane gas (CH4). A gas fired water heater indeed uses a fuel gas that includes methane plus an odorant. However a gas-fired water heater might leak LP or natural gas into the air but as there is no under-water gas piping at a conventional water heater I'm doubtful that the fuel gas would be likely to leak directly into the water supply or hot water tank from the heater itself or its gas piping.

Watch out: in some public and private well water supplies methane gas from mining, natural gas drilling and removal from the earth, or other sources may result in high levels of methane gas entering well water.

We have moved the details of this discussion to METHANE GAS SOURCES

Watch out: If you do have methane gas in your water supply it could be dangerous, presenting a possible explosion hazard. But if methane is in your water supply, it ought to be present in both hot and cold water, though due to the temperature differences it might be more obvious in one than the other.

Questions & Answers regarding this article

Questions & answers about gas fuels, methane gas leaks, odors, and hazards, and about gas fired heating appliances.

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GAS PIPING, VALVES, CONTROLS
  Gas BTUH & Cubic Feet
  Gas Conversion LP Natural Gas
  Gas Flame & Noise Defects
  Gas Igniter Defects, Repairs
  Gas Leak Detection
  Gas Lighting Pipes & Fixtures
  Gas Meters
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  Gas Regulators for Appliances
  Gas Regulators for LP Tanks
  Gas Regulators, Two Stage
  Gas Shutoff Valves
  LP Gas Tanks
  LP & Natural Gas Safety Hazards
  LP & Natural Gas Pressures
  METHANE GAS SOURCES
  Natural Gas Combustion Products
  SPILL SWITCHES - Flue Gas Detection
  Types of Fuel Gas Source

• 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.
• Thanks to careful reader (prefers to be Anonymous) for correcting our data table from erroneous Pounds per Million BTUs Burned to correct Pounds per Billion BTUs Burned, and including a second corroborating resource given just below
• NaturalGas.org - a website developed and is maintained by the Natural Gas Supply Association. This site has been created to serve as an informational resource on the many aspects of natural gas. http://www.naturalgas.org/environment/naturalgas.asp#emission

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