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Principles of Combustion for Outdoor Gas Fire Pits

Gathering around a gas fire pit with family and friends, everyone will be enjoying not only the heat it provides but the ambiance it sets.  Both heat and ambiance are the results of proper combustion of the gas in your outdoor fire pit.  Whether you’re using natural gas, or propane, having an understanding of how these gases combust will help you when you design and install your gas fire pit.  This will ensure you have both efficient heat and the ambiance you desire (heat and light).

What is Combustion?

It’s actually pretty simple. If you’ve got fuel, oxygen, and a heat source that’s hot enough to combine them, you will get combustion. 

Natural gas and propane are made up of hydrogen and carbon molecules. When they meet up with oxygen molecules and sufficient heat is supplied, these molecules will speed up and ram into each other.  The force is sufficient to break old bonds and create new ones—carbon dioxide (CO2) and water vapor (H2O).

All that action becomes a heat source on its own, which can sustain further combustion. When these elements come together in the proper amounts, controlled combustion occurs and results in a steady flame.

It is not the actual spark or flame from your lighter that ignites the gas, it’s the heat they generate. Keep in mind, that propane will ignite at a lower temperature than natural gas.

What is Incomplete Combustion?

Incomplete combustion occurs when there isn’t enough oxygen present to pair with the fuel, and therefore, the fuel doesn’t burn completely. 

When the burn is incomplete, the appliance is less efficient, generates less heat, creates more soot, and may produce small amounts of carbon monoxide. 

Gas fireplaces and fire pits use two ways of getting oxygen to complete combustion. One is a primary source that mixes with the gas before it reaches the burner. The other is a secondary source, pulled from the air around the burner.

Propane requires more than twice the oxygen natural gas does for complete combustion. That’s why an air mixer is required for propane appliances. It increases the amount of oxygen in the gas line to sufficient levels before it reaches the burner.

Incomplete combustion can occur when the air supply has been reduced intentionally (to affect flame color), or by poor venting and/or maintenance.

Why Does the Flame Change Color? 

The yellow color is the incandescent light produced by the heating of the unburned carbon in the gas.  When a flame doesn’t completely burn the carbon in the gas there is some carbon left behind as very fine soot particles.  When these carbon particles are heated to a high temperature, they produce a yellow glow. It’s similar to an incandescent light bulb where the carbon filament is heated to produce a slightly yellow-colored light. 

How to Create a Yellow Flame with Natural Gas 

To get that yellow color, we need a less efficient fire so that there is unburned carbon to glow yellow in the flame. Gas fireplaces and fire pits have been engineered to burn yellow at an acceptable level of efficiency.

To create a yellow flame with natural gas, the primary source of air is eliminated, and the oxygen is pulled from the ambient air around the flame. 

While this helps to make the flame yellow, it is impossible to remove all of the blues because of the low carbon content of natural gas.  Yellow is created by the glow of the heated carbon, and there simply isn’t enough carbon to completely eliminate the blue in the flame.  

However, the blue is typically isolated at the bottom of the flame and the more visible parts of the flame are yellow.

How to Create a Yellow Flame with Propane Gas 

It’s easier to create a yellow flame with propane because of its higher carbon content.

In this case, the air coming into the air mixer is reduced so more ambient air will be used for combustion. 

While the flame will still burn steadily with this method, the level of oxygen will drop and not all of the carbon will burn. The unburned carbon casts a yellow light as a result of being heated.  

How ‘Bout That Heat

While fire pits can be mesmerizing, one of the main reasons people have them is to provide a bit of heat. Combustion is the source of that heat.

Heat value is measured in Btu (British thermal unit). Basically, it’s the amount of energy it takes to raise the temperature of one pound of water by one degree. 

Propane gas burns hotter. It generates more heat at 2,500 Btu per cubic foot compared to natural gas at 1,000 Btu per cubic foot.

Safety Matters

As convenient as a gas fire pit is compared to wood, never lose sight of the fact that gas is a volatile element. Improper handling of propane or natural gas can result in fire, explosion, or asphyxiation. And between the two, propane is considered more dangerous for several reasons.

Propane is heavier than air and drops downward upon release. It can pool in enclosed areas and cause a number of potential problems.

It also requires less fuel and a lower temperature for ignition. That means combustion can happen from a heat source that is smaller and farther away.

In contrast, natural gas is lighter than air, so it dissipates quickly when released. It takes more heat and more gas volume in order to get combustion.

Note: Carbon monoxide (CO) is a by-product of incomplete combustion. It is the incomplete combustion that produces a more yellow flame. Gas fire pits and fireplaces are engineered to safely burn this way, but they still create a small amount of CO. 

Therefore, it is vitally important to make sure you always use your fire pit in a well-ventilated area, both around the sides and above the structure so as not to trap the gases in any way. Even small amounts of CO can cause problems, especially in infants, the elderly, and those with compromised health.

Education is the key. As long as you understand the properties of the gas you are working with and take the proper precautions, you can operate your fire feature with confidence. 

Don’t Hesitate to Call

The Tech Team at Celestial Fire Glass wants you to feel completely comfortable using your fire feature, so don’t hesitate to call us at any point in your journey. We’re here every weekday to answer your questions and walk you through the process. 

Now, let’s invite the family over and get combusting! 

CARBON MONOXIDE (CO) is a stealthy opponent. It has no odor or color. It is toxic and combustible. Even small amounts can cause trouble over time including organ and brain damage. More concentrated amounts can lead to death.

The effects of CO depend on the exposure time and the general health and size of a person. CO poisoning can be treated if immediate attention is sought.

Symptoms to look for:
-Nausea/Stomach Pain
-Flu-like symptoms
-Shortness of breath
-Impaired Vision
-Loss of Consciousness

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