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Complete and Incomplete LPG combustion

19 Apr, 2024 | ELGAS Updates, Residential LPG Blogs

In this article:

LPG combustion - what do complete and incomplete combustion mean for your hip pocket and best practices to get the most from your LPG
LPG Blue Flam

Incomplete & Complete Combustion of Propane – Propane Equation & Formula

Complete combustion of propane (LPG) is important. Complete LPG combustion saves you money on gas.
The propane equation for complete combustion of propane involves propane and oxygen as fuel input, and carbon dioxide, water, heat and possible carbon monoxide as the outputs. Complete combustion of LPG – propane – yields about 25 MJ/litre or 49 MJ/kg of heat.
Incomplete combustion produces carbon monoxide, which is a poisonous gas.
Find out what you need to do…

Does Burning LPG Produce Carbon Monoxide – Does LPG Produce Carbon Monoxide

Natural gas or propane (LPG) appliances can produce carbon monoxide when they burn with incomplete combustion. Properly functioning gas appliances produce little, if any, carbon monoxide.

Incomplete and Complete Combustion 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 complete combustion 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 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%.

Complete Combustion of Propane Equation – Combustion Formula for LPG

In the  complete combustion of propane equation, in the presence of enough oxygen, propane burns to form water vapour and carbon dioxide, as well as yielding about 25 MJ/litre or 49 MJ/kg of heat. So, this is the complete combustion of propane 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

Equation for Incomplete Combustion of Propane-LPG

The equation for incomplete combustion of propane is: 2 C3H8 + 9 O2 → 4 CO2 + 2 CO + 8 H2O + Heat.

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.

Equation for Incomplete Combustion of Propane-LPG in Words & Symbols

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) – Products are Formed When LPG is Burnt

The products are formed when LPG is burnt in gas appliances, domestic and industrial, are 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.

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 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.

Never use an outdoor gas appliance indoors.

Detecting Carbon Monoxide

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.

Symptoms of CO poisoning 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 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.

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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