BH-F (Engineering) Ltd - Air Gas Combustion.

Air Gas Combustion

The air / gas ratio control and mixing unit is designed to deliver
gas and air at a constant ratio throughout the firing range.
This is achieved using a proportional mixer suitable for use with
gases such as propane, butane, LPG, LNG, natural or town gas,
with calorific values from 4,000 up to 28,000 kcal per cubic metre
(450 up to 3150 BTU per cubic foot).

The system is capable of a high turndown ratio
with burner manifold pressures up to 500mm (20 inches) water gauge.

The mixers can be vertically or horizontally mounted allowing them
to be positioned at the side of the forehearth, immediately under the
forehearth or on a combustion skid at machine floor level for instance.

The system allows for simple setting of the air / gas ratio.
The regulator is adjusted such that the gas entering the mixer is at the
same pressure as the exiting mixture.
Once done the ratio is held constant through the firing range.
Rotating the mixer barrel moves a venturi within the mixer and allows
the correct mixture ratio to be set.
Having completed this the firing output can be adjusted by varying the flow
of combustion air with the flow of gas being
automatically adjusted to maintain the ratio.

Required Air Gas ratio for Stoichiometric Combustion

The oxygen level in an unburnt air gas mixture for Stoichiometric combustion
depends on the composition of the fuel gas and air, for example:

1 volume of CO requires 0.5 volumes of Oxygen
1 volume of Hydrogen requires 0.5 volumes of Oxygen
1 volume of Methane requires 2.0 volumes of Oxygen
1 volume of Ethane requires 3.5 volumes of Oxygen
1 volume of propane requires 5.0 volumes of Oxygen
1 volume of n-Butane requires 6.5 volumes of Oxygen
1 volume of Pentane requires 8.0 volumes of Oxygen
1 volume of Hexane requires 9.5 volumes of Oxygen

The proportion of Oxygen by volume of the mixture also depends
on the composition of the air. Assuming air composed as follows:

Oxygen	20.99%
Nitrogen	78.03%
Argon	0.94%
Water	0.00%
Carbon Dioxide	0.03%
Hydrogen	0.01%

And gas composed as follows:

Carbon Dioxide	0.78%
Carbon Monoxide	0.00%
Hydrogen	0.00%
Methane	90.61%
Nitrogen	3.81%
Ethane	3.30%
propane	0.79%
n-Butane	0.38%
Pentane	0.15%
Hexane	0.18%

The Stoichiometric ratio is calculated as:

One Volume of Gas used		Stoichiometric ratio
	proportion	Volume of Oxygen	Volume of Air
Carbon Dioxide	0.78%	0.000	0.000
Carbon Monoxide	0.00%	0.000	0.000
Hydrogen	0.00%	0.000	0.000
Methane	90.61%	1.812	8.634
Nitrogen	3.81%	0.000	0.000
Ethane	3.30%	0.116	0.550
propane	0.79%	0.040	0.188
n-Butane	0.38%	0.025	0.118
Pentane	0.15%	0.012	0.057
Hexane	0.18%	0.017	0.081
		2.021	9.628

For Stoichiometric combustion the percentage of
oxygen in the premix is calculated as:

Volume of Oxygen/(Volume of Air + Volume of fuel gas) x 100

Which in this example is: 2.021/(9.628 +1) x 100 = 19.02%.

Possible Causes of Backfire

One or more of the burner nozzles becomes blocked to such an extent that the flow of gases through the nozzle is not sufficient to keep the nozzle below the temperature required to ignite the air / gas mixture in the burner pipe. The mixture is therefore ignited in the pipe. This in turn ignites the air / gas in the burner manifold and an explosion takes place. The pressure from an explosion is such that it opens the safety head, which is in effect a spring loaded valve. Opening the safety head relieves the pressure from the explosion reducing the possibility of damage to other parts of the combustion system.
The manifold pressure is too low. If the speed of the gas / air mixture coming out of the nozzle is less than the speed of the flame the flame will burn back into the burner pipe. Again it will ignite the fuel in the burner manifold and an explosion will take place.
The pressure inside the forehearth is too high. If the forehearth internal pressure is high it will slow down the speed of the gas / air mixture coming out of the burner nozzles. Again it will ignite the fuel in the burner manifold and an explosion will take place.