Complete Combustion Reaction & Incomplete Combustion Reaction – Carbon Monoxide
Complete combustion reaction includes the inputs and outputs of formula for combustion equation.
LPG complete combustion chemistry equation is: C3H8 + 5 O2 → 3 CO2 + 4 H2O + Heat.
LPG incomplete combustion chemistry equation is: 2 C3H8 + 9 O2 → 4 CO2 + 2 CO + 8 H2O + Heat.
The complete combustion equation in words is: Gas + Oxygen → Carbon Dioxide + Water + Heat.
Incomplete combustion equation in words is: Gas + Oxygen → Carbon Dioxide + Water + Heat + Carbon Monoxide.
Complete combustion reaction of LPG – propane – contains about 25 MJ/litre or 49 MJ/kg of energy.
As shown, incomplete combustion reaction can produce carbon monoxide, which could cause carbon monoxide poisoning.
The complete combustion equation involves propane and oxygen as fuel input, and carbon dioxide, water and heat as the outputs.
Carbon monoxide is a possible output with incomplete combustion.
Complete combustion of LPG saves you money on gas.
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Combustion Reaction Defined
Combustion reaction is an exothermic chemical reaction between a fuel and an oxidizer.
In this case, LPG (the fuel) reacts with oxygen (the oxidizer) in a combustion reaction (it burns) that generates heat (exothermic).
Visually, a combustion reaction may include flames and smoke.
Water vapour may also be produced, as is the case with the combustion of hydrogen and oxygen, as well as LPG.
It will also produce non-visible gaseous by-products like carbon dioxide and carbon monoxide
Depending on the fuel, it may also produce other gases.
Carbon Monoxide from Incomplete Combustion Reaction
Natural gas or propane (LPG) appliances can produce carbon monoxide when they burn with an incomplete combustion reaction.
The result could be carbon monoxide poisoning which is why you should consider a carbon monoxide detector.
Properly functioning gas appliances, with a complete combustion reaction produce little, if any, carbon monoxide.
Incomplete Combustions and Complete Combustion Reaction of 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 a complete combustion reaction of propane, the burner produces a blue flame yielding about 25 MJ/litre or 49 MJ/kg of heat.
So, richer mixtures, those closer to 9.6%, are likely to suffer from an incomplete combustion reaction.
A yellow flame, soot and excessive condensation are three physical signs of an incomplete combustion reaction.
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%.
Complete Combustion Equation – Burning Equation
In the LPG complete combustion chemistry equation (burning equation), in the presence of enough oxygen, propane burns to form water vapour and carbon dioxide, as well as releasing about 25 MJ/litre or 49 MJ/kg of energy. A complete combustion reaction yields a blue flame.
So, this is the complete combustion chemistry equation (burning equation) in both words and chemical formulas:
Propane + Oxygen → Carbon Dioxide + Water + Heat (about 25 MJ/litre or 49 MJ/kg)
C3H8 + 5 O2 → 3 CO2 + 4 H2O + Heat
Incomplete Combustion Equation – Carbon Monoxide Formula
The incomplete combustion chemistry equation (burning equation) or carbon monoxide formula for propane is: 2 C3H8 + 9 O2 → 4 CO2 + 2 CO + 8 H2O + Heat. An incomplete combustion reaction yields a yellow or reddish flame.
If not enough oxygen is present for complete combustion, incomplete combustion occurs with carbon monoxide.
The result of incomplete combustion is, once again, water vapour, carbon dioxide, heat and carbon monoxide.
Incomplete Combustion Equation of LPG (Propane) in Words & Symbols:
Incomplete Combustion Equation: Gas + Oxygen = Water + Carbon Dioxide + Carbon Monoxide + Heat
Incomplete Combustion, Carbon Monoxide Natural Gas & LPG
Either natural gas or propane (LPG) appliances can produce carbon monoxide, with an incomplete combustion reaction.
Properly functioning gas appliances produce little, if any, carbon monoxide.
Preventing Carbon Monoxide (CO) from Incomplete Combustion
Proper maintenance and installation are the key ways to prevent an incomplete combustion reaction and carbon monoxide.
The products are formed when LPG is burnt in gas appliances or industrial applications are water vapour, Carbon Dioxide, 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 (blocking the fresh air to the appliance required for complete combustion) may cause a gas appliance to malfunction and create a hazardous situation.
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.
This is why a carbon monoxide detector is a wise precaution. Even if it never activates it provides peace of mind.
It is essential that gas appliances are correctly installed and serviced every two years, or as indicated by the manufacturer, 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 minimal when compared to the total combustion products.
Gas Appliances Maintenance
Good maintenance helps ensure proper combustion and minimal CO generation.
Most gas appliance manufacturers recommend routine maintenance every 12 or 24 months.
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 combustion 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 an incomplete combustion reaction.
The result is that you could be wasting gas and/or generating carbon monoxide.
If you observe any of the aforementioned 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 combustion 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.
Never use an outdoor gas appliance indoors.
Detecting Carbon Monoxide & Symptoms of Carbon Monoxide Poisoning Symptoms
Carbon monoxide is colourless, odourless and tasteless.
CO is a bit less dense than air, so it rises.
It is toxic when encountered in concentrations above about 35 ppm.
It is lethal in 30 minutes at 4,000ppm. It drops to 5 minutes at 5,000ppm.
Carbon monoxide poisoning symptoms include:
- Headaches
- Nausea
- Dizziness
- 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 carbon monoxide 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.
Final Thoughts
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!
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Eric Hahn has over a decade of experience in the LPG industry and has become a key figure in shaping how LPG is marketed and communicated. As the creator of one of the world’s first LPG blogs, Eric has authored over 200 technical posts, helping educate and inform the industry and consumers.
Eric has served on multiple committees for Gas Energy Australia and the World LPG Association (WLPGA), contributing to important discussions and advancements within the global LPG sector. He has presented at WLPGA Annual Conferences, sharing his expertise as a speaker, panellist, and panel chair.
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Steve Reynolds
Technical Consultant
Steve Reynolds is a leading expert in the LPG industry with over 22 years of experience. As part of the national management team at ELGAS, Steve ensures the safe and efficient storage, handling, and transportation of LPG. He serves as the lead investigator for incidents and collaborates with authorities on industry developments.
Steve is a technical advisor to Standards Australia and Gas Energy Australia (GEA), and an active member of the World LPG Association (WLPGA), contributing to global standards and technical reviews. He holds a BSc. (Hons) in Industrial Chemistry from UNSW and has held senior safety and technical roles at ELGAS, making him a trusted authority in LPG safety and standards.