Photograph of  this gas flame which gives a clue that there may be an operating problem and an unsafe gas furnace in this buildingLP or Natural Gas Fired Appliance Combustion Products
in Flue Gas Exhaust

  • NATURAL GAS COMBUSTION PRODUCTS - CONTENTS: 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? Methane gas hazards and hot water tanks - can a hot water heater tank produce methane gas? Methane gas production by methanogens & methanogenesis in water heaters?
  • POST a QUESTION or READ FAQs about gas fuels, methane gas leaks, odors, and hazards, and about gas fired heating appliances
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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

LP gas tank (C) Daniel Friedman

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

But this is a simplification since natural gas is not pure. Natural gas, principally methane (CH4) 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

Combustion & Flue Gas Products in Natural Gas CH4

Combustion Product Pounds per Billion BTUs Burned Ratio to CO2 Power Plant Emissions Burning CH4
Carbon dioxide CO2
1135 lbs/MWh
Carbon monoxide CO
Nitrous oxide NO
1.7 lbs/MWh
Sulphur dioxide SO2
0.1 lbs/MWh
Mercury Hg

-- 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

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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