Combustion of Propane vs Incomplete Combustion: Formulas & Equations
Proper combustion of LPG – propane – is important.
Complete LPG combustion saves you money on gas.
Incomplete combustion produces carbon monoxide, which is a poisonous gas.
Find out what you need to do…
Complete and Incomplete Combustion of LPG – Propane
Propane – LPG – burns within its limits of flammability.
The lower and upper limits of flammability are the percentages of LPG that must be present in an LPG/air mixture.
This means that between 2.15% and 9.6% of the total LPG/air mixture must be LPG in order for it to be combustible.
However, the optimal mixture is 4% LPG/air.
So, 4 parts LPG (propane) to 96 parts air.
With complete combustion, the burner produces a blue flame.
So, richer mixtures, those closer to 9.6%, are likely to suffer from incomplete combustion.
A yellow flame, soot and excessive condensation are three physical signs of incomplete combustion.
The natural gas – methane – limits of flammability are different, at 5.4% to 17%.
The optimal combustion mixture for methane is also different, at approximately 10.42%.
LPG - Propane Combustion Formula
In the presence of enough oxygen, propane burns to form water vapour and carbon dioxide, as well as heat.
So, in both words and chemical formulas:
Propane + Oxygen → Carbon Dioxide + Water + Heat
C3H8 + 5 O2 → 3 CO2 + 4 H2O + Heat
What's the Equation for Incomplete Combustion of Propane - LPG?
If not enough oxygen is present for complete combustion, incomplete combustion occurs.
The result of incomplete combustion is, once again, water vapour, carbon dioxide and heat.
But it also produces carbon monoxide.
Gas + Oxygen = Water + Carbon Dioxide + Carbon Monoxide + Heat
2 C3H8 + 9 O2 → 4 CO2 + 2 CO + 8 H2O + Heat
Incomplete Combustion Produces Carbon Monoxide
Carbon monoxide – CO – is a toxic gas.
Either natural gas or propane (LPG) appliances can produce carbon monoxide, with incomplete combustion.
Properly functioning gas appliances produce little, if any, carbon monoxide.
Combustion and Carbon Monoxide (CO)
All gas appliances, domestic and industrial, produce water vapour, Carbon Dioxide and heat, and usually very small amounts of Carbon Monoxide.
If installed and maintained correctly, the operation of the gas appliance provides quick and efficient heating, cooking, hot water and more, and the products of combustion do not create any hazardous situations.
If an appliance is not correctly installed and maintained or has been modified, the products of combustion might change, and become hazardous to the people around the appliance.
Something as simple as a ventilation change (getting fresh air to the appliance to sustain complete combustion) may cause a gas appliance to malfunction, and create a hazardous situation for the people around.
Sometimes it is obvious when a gas appliance malfunctions.
Sooty smoke, red or yellow flames or poor performance are indicators, but sometimes no indicators are obvious.
If Carbon Monoxide (CO) is produced and escapes the appliance into the surrounding air, it will not be obvious (no smell and no taste) but will be very dangerous.
It is essential that gas appliances are correctly installed and serviced every two years, to maintain good combustion and safe, efficient operation.
What about the Ethyl Mercaptan?
Ethyl Mercaptan is the odourant added to LPG, for safety.
When Ethyl Mercaptan burns in air, the products are CO2, H20 and SO2.
The SO2 is an irritating compound that is unpleasant in most concentrations. It is part of city smog.
Other sulphides are unlikely to form.
There are minute amounts of Ethyl Mercaptan in the LPG vapour on combustion.
So, the amounts of these products formed are tiny when compared to the total combustion products.
Gas Appliances Maintenance
Good maintenance helps ensure proper combustion and minimal CO generation.
This can vary by manufacturer and appliance model.
It is important that your gas appliances are only service by qualified technicians.
In most cases, this means a licensed gas fitter or a factory technician.
Key warning signs that your gas appliance requires servicing are a yellow or red flame, a flame with a yellow burning tip.
Yellow/brown soot, pilot lights that blow out, acrid smells and eye irritation are also indications.
The exceptions to this are gas fireplaces and gas log fires that have yellow flames by design.
The above signs may be indications of incomplete combustion.
The result is that you could be wasting gas and/or generating carbon monoxide.
The latter is a serious safety problem, if it occurs with an indoor appliance.
If you observe any of these warning signs, you should schedule a service ASAP.
Using Approved Appliances and Gas Fitters
Australia has some of the most stringent standards on gas appliance combustion emissions.
This makes our indoor gas heaters very safe to use.
The gases of primary concern are CO (carbon monoxide) and NOX (nitric oxide and nitrogen dioxide).
These gases can be produced during combustion.
Indoor gas heaters must meet or surpass these strict standards, for Australian certification.
Only buy or use Australian tested, certified and labelled gas appliances.
Other countries have their own certification systems.
Make sure a licensed gas fitter installs your gas appliances and that you have adequate ventilation.
Finally, when you use your gas appliances, always read and follow the manufacturer’s instructions.
Don't Use Outdoor Gas Heaters Indoors
The same requirements are not applicable to outdoor heaters and appliances.
They are intended for outdoor use, with unrestricted ventilation. So, they are not required to meet the indoor heater emission standards.
This includes heaters, BBQs, pizza ovens and all other outdoor appliances.
If a gas appliance is marked for outdoor use only, do not use it indoors. This includes semi-enclosed areas without adequate ventilation.
Detecting Carbon Monoxide
Carbon monoxide is colourless, odourless and tasteless.
CO is a bit less dense than air, so it rises.
It is lethal in 30 minutes at 4,000ppm. It drops to 5 minutes at 5,000ppm.
Symptoms of CO poisoning include:
Shortness of breath
Light-headedness and loss of consciousness
Stop using the gas appliance immediately, if you experience any of these symptoms.
Move to an area where you can breathe fresh air and seek medical attention.
If you smell gas, it is not carbon monoxide, it is the odourant added to natural gas or LPG.
You should immediately deal with the leaking gas.
Turn off the gas and ventilate the room, if safe to do so.
The best way to detect carbon monoxide is with a CO detector.
You can buy these from hardware stores, as well as from online merchants.
At the time of this writing, at least one major hardware chain and various online merchants had a choice of models, for under $50 each.
When used and maintained according to the manufacturer's guidelines, gas appliances are very safe.
So, make sure you observe the maintenance schedule, as recommended by the appliance manufacturer.
Also remember to never use outdoor gas appliances indoors.
There is nothing like the warmth you get from a gas heater, the fun of cooking with gas or the beautiful warmth of a gas fireplace. Enjoy!
The information in this article is derived from various sources and is believed to be correct at the time of publication. However, the information may not be error free and may not be applicable in all circumstances.