Alternative Fuels

Getting rid of the stereotypes

Over the years, a number of misconceptions have arisen surrounding alternative fuels. Common alternative fuels include propylene, propane and natural gas, as well as proprietary gas blends. Without going into too many technical details, there are some important differences in the fuel gases (See Figure 1). First, alternative fuels have a tendency to “float” up or down. Natural gas is lighter than air, while propane and propylene are heavier. Acetylene is very close to that of air, so it won’t float as much. Because of these differences, operators need to learn different techniques for lighting a torch using alternative fuels.

Alternative Fuel Tips

End-users must understand that some gases require specific tips. Recessed tips for propylene (left) and propane (right) optimize flame attachment when using fuel gases with slower burning velocities.

Second, different gases have different burning rates at which the gas burns back toward the torch tip. (While a flame may look as if it’s burning away from the torch, the opposite is actually happening.) Alternative fuels have slower rates than acetylene. Achieving proper flame attachment, or the standoff between the end of the tip and ignition point of the flame, requires using tips specifically engineered to accommodate the particular burning rate of the fuel gas.

Heating characteristic of the gases is another difference. Acetylene releases a majority of its heat (BTUs) in the primary flame, which is why acetylene has a concentrated heat output. Alternative fuels release more of their heat in the secondary flame and create more of a “soaking” or “quenching” type heat. As an example, when heating a 5 in. x 15 in. plate with alternative fuels, you can easily heat the whole plate cherry red. With acetylene, the starting point will have darkened by the time the torch reaches the end of the plate.

Basic Characteristics of Fuel Gases

Fuel Gas




Natural Gas

Burning Velocity

22.7 fps

15.0 fps

12.2 fps

15.2 fps

Specific Gravity (air=1)





Neutral Flame Temp.

5,720º F

5,240º F

4,579º F

4,600º F

Total BTUs/ft3





Primary Flame BTU





Secondary Flame BTU






Alternative Fuel Heat

Alternative fuels create a soaking or quenching heat that works very well when heating a large workpiece.

Common Misconceptions
One big misconception is that alternative fuels cannot be used for welding steel because the flame is not hot enough. Considering that carbon steel alloys melt between 2,500o F and 2,800o F, this is quite inaccurate. The real reason is that alternative fuels don’t create any shielding gas. Conversely, a proper mixture of oxygen and acetylene with a neutral flame produces CO2, which in turn shields the molten weld puddle from atmospheric contamination. Realistically speaking, this is not much of a drawback, as most industries have substituted other processes for oxyfuel welding.

Another misconception is that because alternative fuels are not quite as hot, they don’t cut as well. When cutting, the oxygen stream does the real work. Oxyfuel cutting is nothing more than high-speed oxidation, or the common rusting of steel. All of the commercially available fuel gases have a high enough flame temperature to achieve the desired results when using the correct equipment and proper techniques.

Required Changes
To use alternative fuels, it is necessary to switch to the appropriate regulator, a Grade T hose and cutting and heating tips designed to optimize performance with alternative fuels. There is no need to change the torch as long as it uses a “universal” or “spiral” type gas mixer, and gas delivery is above 2 psi. Some alternative fuel tips have recommended pressure settings that are different from acetylene.

The proper use of alternative fuels and equipment will allow you to offer additional solutions to your customers.

Gases and Welding Distributors Association

John Henderson Meet the Author
John Henderson is senior product manager at Thermadyne Industries, headquartered in St. Louis, Missouri, and at