Stoichiometric Combustion: Perfect Fuel Combustion for Gas or Oil Burners InspectAPedia® -
What is the definition of Stoichiometric Combustion
Theoretical Fuel to Air Ratios for Complete Combustion (Stoichiometric Combustion)
Oil Burners: characteristics of perfect combustion of fuel
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Here, courtesy of aerospace engineer Herman Vogel, we provide a definition of Stoichiometric Combustion and we explain and give formulas for Theoretical Fuel to Air Ratios for Complete Combustion (Stoichiometric Combustion). Stoichiometric Combustion discusses theoretical fuel to air ratios for hydrocarbon fuels (kerosene, jet fuel, heating oil, LP gas, etc) in which a fuel is burned completely, producing only carbon dioxide and water, with no other byproducts. Our sketch (page top) shows how an oil burner gun atomizes and sprays heating oil into the combustion chamber - Audel Oil Burner Guide
This website answers most questions about central heating and water heating systems to aid in troubleshooting, inspection, diagnosis, and repairs. Contact us to suggest text changes and additions and, if you wish, to receive online listing and credit for that contribution.
In brief: Stoichiometric combustion is by thermodynamic definition the theoretical combustion of every drop of fuel when mixed with the correct amount of air (oxygen) to yield exhaust products of only CO2 and H2O.
Stoichiometric combustion is by thermodynamic definition the theoretical combustion of every drop of fuel when mixed with the correct amount of air (a basic mixture of oxygen and nitrogen gases) to yield exhaust products of only CO2 and H2O. However, such combustion is ideal and in reality doesn't occur since the burning in furnaces, automobiles and jet-engines is always incomplete and less than 100% due to engineering design limitations.
Therefore while our two ideal exhaust products are relatively benign, non-stoichiometric combustion rules in the real world. This results in burning rich, as no matter how much extra air we cleverly add, it is never able to chemically react with all the fuel. Hence the resultant unburned fuel gets exposed to the high combustion temperatures and chemically reacts to form additional exhaust products of CO (Carbon Monoxide) and NO (Nitric Oxide), which can be dangerous exhaust products to both people and the environment.
Stoichiometric combustion has a chemical reaction equation attached to it:
For the above chemical reaction to be complete, both sides of the equation (about the arrow) must have its chemical elements matched. So, if the left-side has 12 Carbon molecules, then the right-side must also have 12.
Note that we are using air which contains a weighted ratio of N2/O2 = 2x37 / (37/2) = 4 (100% air = 80% N2 + 20% O2), or four times as much Nitrogen exists in a given volume of air as does Oxygen.
Also, the above chemical reaction equation represents kerosene fuel which is considered a Dodecanese (liquid) hydrocarbon having the chemical formula C12H26.
That means it contains a molecular composition of 12-carbon atoms and 26-hydrogen atoms and is a liquid at room temperature. Kerosene is the main constituent of Jet Propulsion (JP) fuels, where other additives include various blends of differently distilled crude oils.
The fuel to air ratio by chemical weight of the above relationship is:
FAR = [mf_C12H26 / ma_O2+N2] = (12*12.011+26*1.008)/(37/2*32+2*37*28.014) = 0.064 or 6.4%
This says that for complete combustion of typical hydrocarbon fuels, and in particular kerosene, we need 6.4 lbs. of fuel for every 100 lbs. of air that we burn. Clearly, on a relative basis, we need a lot of air to completely burn our small amount of fuel.
The beauty of the above chemical equation is that not only does it define the required fuel-to-air ratio, but it also provides us with:
The expectant adiabatic (pure, no losses in heat) flame temperature released by this chemical reaction as based on chemical heats of formation (4,310 R), and
The heating value released per pound of kerosene (18,950 BTU/lbs.) as based on its chemical formulation plus heat of formation.
Technical Note: The temperature above is in units of R (Rankin) not (F). If it were F (farenheit) we have 4,310R - 460R = 3,850F.
What Does Complete Combustion (Stoichiometric Combustion) Mean to the Heating System Designer?
The above three pieces of information helps the furnace designer to properly size combustion chambers and their air-fans, and to burn the correct amount of fuel to keep a home comfortable. It also helps the designer to choose the appropriate materials to avoid furnace melt-down. Note that the theoretical adiabatic flame temperature is very high.
While some of this temperature is reduced due to furnace heat losses, flame temperatures are generally controlled by intentionally burning lean. This dramatically reduces the theoretical flame temperature based on the lean chemistry of combustion, plus additional temperature reductions are realized by diluting high temperatures using the extra cold air entering the combustion chamber.
So, the furnace designer has a whole arsenal of possibilities to work with in designing today's reliable fuel oil furnaces.
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Thanks to aerospace engineer Herman Vogel, July 2010,
for providing this explanation of complete combustion of fossil fuels and the details as well as the significance (to non-engineers) of Stoichiometric Combustion
Audels Oil Burner Guide, Installation, Servicing, Repairing, Frank D. Graham, 1940's edition (obsolete). Updated versions of this guide are available in various editions, 1947, 1950, 1955, 1958, 1959, 1962, 1965, 1967, and at prices from around $3.00 to nearly $70.00 - useful for simple, clear, but not current, explanation of how heating equipment works. The original retail price was $1.00. Used copies are available at Amazon.com
Books & Articles on Building & Environmental Inspection, Testing, Diagnosis, & Repair
Our recommended books about building & mechanical systems design, inspection, problem diagnosis, and repair, and about indoor environment and IAQ testing, diagnosis, and cleanup are at the InspectAPedia Bookstore. Also see our Book Reviews - InspectAPedia.
The Home Reference Book - the Encyclopedia of Homes, Carson Dunlop & Associates, Toronto, Ontario, 2010, $69.00 U.S., is available from Carson Dunlop. The Home Reference Book is a bound volume of more than 450 illustrated pages that assist home inspectors and home owners in the inspection and detection of problems on buildings. The text is intended as a reference guide to help building owners operate and maintain their home effectively. InspectAPedia.com ® author/editor Daniel Friedman is a contributing author. Field inspection worksheets are included at the back of the volume.
Domestic and Commercial Oil Burners, Charles H. Burkhardt, McGraw Hill Book Company, New York 3rd Ed 1969.
National Fuel Gas Code (Z223.1) $16.00 and National Fuel Gas Code Handbook (Z223.2) $47.00 American Gas Association (A.G.A.), 1515 Wilson Boulevard, Arlington, VA 22209 also available from National Fire Protection Association, Batterymarch Park, Quincy, MA 02269. Fundamentals of Gas Appliance Venting and Ventilation, 1985, American Gas Association Laboratories, Engineering Services Department. American Gas Association, 1515 Wilson Boulevard, Arlington, VA 22209. Catalog #XHO585. Reprinted 1989.
The Steam Book, 1984, Training and Education Department, Fluid Handling Division, ITT [probably out of print, possibly available from several home inspection supply companies] Fuel Oil and Oil Heat Magazine, October 1990, offers an update,
Principles of Steam Heating, $13.25 includes postage. Fuel oil & Oil Heat Magazine, 389 Passaic Ave., Fairfield, NJ 07004.
The Lost Art of Steam Heating, Dan Holohan, 516-579-3046 FAX
Principles of Steam Heating, Dan Holohan, technical editor of Fuel Oil and Oil Heat magazine, 389 Passaic Ave., Fairfield, NJ 07004 ($12.+1.25 postage/handling).
"Residential Hydronic (circulating hot water) Heating Systems", Instructional Technologies Institute, Inc., 145 "D" Grassy Plain St., Bethel, CT 06801 800/227-1663 [home inspection training material] 1987
"Warm Air Heating Systems". Instructional Technologies Institute, Inc., 145 "D" Grassy Plain St., Bethel, CT 06801 800/227-1663 [home inspection training material] 1987
Heating, Ventilating, and Air Conditioning Volume I, Heating Fundamentals,
Boilers, Boiler Conversions, James E. Brumbaugh, ISBN 0-672-23389-4 (v. 1) Volume II, Oil, Gas, and Coal Burners, Controls, Ducts, Piping, Valves, James E. Brumbaugh, ISBN 0-672-23390-7 (v. 2) Volume III, Radiant Heating, Water Heaters, Ventilation, Air Conditioning, Heat Pumps, Air Cleaners, James E. Brumbaugh, ISBN 0-672-23383-5 (v. 3) or ISBN 0-672-23380-0 (set) Special Sales Director, Macmillan Publishing Co., 866 Third Ave., New York, NY 10022. Macmillan Publishing Co., NY
Installation Guide for Residential Hydronic Heating Systems
Installation Guide #200, The Hydronics Institute, 35 Russo Place, Berkeley Heights, NJ 07922
The ABC's of Retention Head Oil Burners, National Association of Oil Heat Service Managers, TM 115, National Old Timers' Association of the Energy Industry, PO Box 168, Mineola, NY 11501. (Excellent tips on spotting problems on oil-fired heating equipment. Booklet.)
Links to our list of additional information on heating system inspection, repair, maintenance
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