Stoichiometric Combustion

Bill_3

I was reading a post of the wall that mention Stoichiometric Combustion. What does it mean?

Jim Davis

It means that the fuel being burned is burning at its maximum temperature and BTU value, with Zero Excess Air or at zero O2(oxygen in the flue gas)

Bill_3

Perfect Combustion

Does this mean near perfect combustion, and no CO

Steve Minnich_1

stoichiometry

I kind of remember that from college chemistry. It was actually a little fun. Demented hah?

"Too much wine and too much song, wonder how I got along".

Its the combining of two or more elements or compounds based on the conservation of mass and energy and the law of combining weights and volumes. So if you end up with a balanced equation, you will have perfect combustion. I think? I also think that it is impossible in practical terms.

Steve




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Unknown

I was always taught

that it was really not possible to reach the Stoichiometric level of any fuel. It was also something that if you did because of the zero oxygen it would be dangerous to leave.

Jim is it possible with your techniques to come close to that point or can you actually reach it? Is it safe to leave it at that point?

I have run a Carlin G3A gas conversion burner all the way down to about O2 of 1 % with very low CO and 11% CO2 with a 350 degree net stack temperature. Could not leave it at that level as the gas company had a rule of minimum 4% O2.

Jerry Boulanger

Read ASHRAE 2001 Fundamentals

Handbook, Page 18.1, for an excellent explanation of not only stoichometric combustion, but combustion in general.

Glen

Yes -

Yes to all above - absolutely zero oxygen left - which means zero CO. Some industrial burners - if measured at the extreme edge of the flame are considered perfect - but as air is entrained by the combustion velocities - soon are not and lower carbon dioxide and higher monoxide levels can be measured. If our combustion analysers were able to completely eliminate water vapour from the test sample - and we measured 12% carbon dioxide in a nat gas flame - it would be a close to stoichiometric as you could get outside of a laboratory. This is interesting stuff - perfect combustion is the process of combining chemically exact amounts (based on molecular weight) of fuel and oxygen so that both components are totally consumed - with no combustibles or uncombined oxygen in the flue gases. One could - by calculating various amounts of excess air - predict quite accurately the carbon dioxide percentage long before we throw the match at the appliance.

LKE

stoichiometric combustion

stoichiometry: a branch of science that deals with the application of the laws of definite proportions and of the conservation of matter and energy to chemical activity. It is also defined as those calculations used to find the quantities of reactants and products in a chemical reaction. In combustion simple numerical relationships exist between the reactants and products, and in the resulting energy changes, for the overall combustion reaction from its intitial to final state. For example, 12.01 lb. of carbon reacts with 32 lb. of gaseous oxygen to form 44.01 lb. of gaseous co2, and owing to the enthalpy change, releases 169,044 Btu. That is the exact amount of oxygen needed for complete reaction. Air is considered to consist of 21% oxygen by volume and 79% by volume of atmospheric nitrogen. The amount of air that contains this weight of oxygen is called the theoretical weight of air. In practice it is necessary to provide more than the theoretical weight in order to attain complete combustion. The excess air is expressed either in percentage of the theoretical air or as the total air divided by the theoretical air. For natural gas some engineering handbooks suggest 5-10% of excess air. Stoichiometric combustion reaction equations tell us the amount of theoretical air and the theoretical amount of energy we should obtain. Measurements by sampling or by instruments tell us what our actual results are. I hope this helps in understanding this technical process.

Bill_3

Enthalpy Change

The last post mention Enthalpy change,what is this?

mike glass

nitrogen 79% ?????????

Not trying to argue but I was always taught that nitrogen content was 72% with the balance all other gases in natural form??

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mike glass

nitrogen 79% ?????????

Not trying to argue but I was always taught that nitrogen content was 72% with the balance all other gases in natural form??

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Glen

heat -

content per unit mass of a substance

Glen

I'll stick with 79

for gas purposes we round things up/down and would consider 10 cf of air to be 8 cf of nitrogen and two cf of oxygen. All the others gases floating about are much less than 1%. So when we add 10 cf of air to one cf of nat gas (for complete combustion) the flue gases should have one cf of carbon dioxide, 2 cf of water vapour, 8 cf of nitrogen, and of course heat. This gets us back to the idea that perfect mixing of fuel gases with oxygen is theorictically possible but - not safely achievable in the field. Always add excess air.

Jim Davis

Timmie,
When looking for numbers like this it is important to know what it does at .9% Oxygen. We usually teach that there should be at least a .5% oxygen safety factor in all final adjustments minimum and then evaluate how often the equipment is guaranteed to be maintained and go from there if more safety is needed.