(Reference: Power Engineering Third Class Edition 2.5; Part B1; Chapter 6: Burner Designs and Supply Systems)
James R. Stringer
The parts to a natural gas burner are: main gas cock(s), pilot gas cock, gas pressure regulator(s), low pressure switch, main gas valves, vent valve, test cocks, high gas pressure switch, gas pilot pressure regulator and solenoid valve, gas butterfly valve.
The natural gas to a burner arrives through an isolation valve at the plant pressure and then is reduced in a pressure reducing valve or a pressure control valve. The total natural gas is measured by a flow meter. Next, the natural gas passes through two safety shut off valves (SSOVs) which also have an automatic vent valve between them, referred to commonly as a “double block and bleed” arrangement. During abnormal conditions, such as low gas pressure, the SSOVs will shut off and the vent will open. Downstream further is the flow control valve whose purpose is to modulate the gas flow to the burner.
Natural gas to the pilot is supplied before the first SSOV; this allows gas to flow to the pilot before the main burner. The gas pressure is reduced to approximately 20 kPa by a pressure reducing valve. The gas then continues through SSOVs to the pilot burner, passing through the automatic shut-off valve which opens and closes depending on the burner sequence.
Safety devices on the burner includes the high pressure and low pressure switches. If the natural gas goes above or below the allowable limits, unstable combustion and flame conditions can occur with the possibility of causing a furnace explosion. The high and low pressures switches will close shut-off valves to the gas preventing the limits to be exceeded.
Gas Pilot Pressure Regulator: reduces incoming header gas pressure and ensures a proper pilot flame by maintaining a steady and constant gas pressure.
Main Gas Pressure Regulator: controls the main gas at a higher pressure than the pilot regulator. Maintains flame and combustion efficiency.
Low Gas Pressure Switch: prevents burner operation when gas falls below the necessary pressure required for a stable flame. Contacts will open which then breaks a circuit, closing the main gas valve. Normally open with a manual reset.
High Gas Pressure Switch: prevents burner operation when gas exceeds the necessary pressure for a stable flame. Contacts will open which then breaks a circuit, closing the main gas valve. Normally open with a manual reset. Ensures stable combustion as well as the proper air-to-fuel ratio.
Main Gas Valves: On/off; normally closed; single seated valves. Electric motors open the valves when energized during the burner sequence determined by the boiler programming control.
Gas Butterfly Valves: controlled by the damper motor, it regulates the rate of gas flowing to the burner at different loads. The linkage to this valve is connected to the same motor that controls the air dampers to the burner which controls the amount of combustion air to the boiler.
Natural gas does not need to be atomized but the burner must be able to control the proper air-to-fuel mixture and cause turbulent mixing of the air and gas. It is most common for burners to have the gas enter the air stream at a right angle, promoting good mixing.
Nitrogen Oxides (NOx) are emissions formed during combustion that should be avoided. Though hydrocarbons reacting with nitrogen from the combustion air can produce NOx or nitrogen from the fuel with the oxygen in the air, the major sources would be thermal NOx.
Thermal NOx is produced within the furnace when nitrogen and oxygen from the combustion air react at high temperatures and rapidly occurs above 1535°C. This form of NOx receives the greatest amount of attention to control.
With pollution and high fuel prices, burners operating at the highest efficiency are paramount, with burners that require less excess air produce less NOx. Less excess air requires that there is thorough mixing of the fuel and air. This also means that the flame temperature in the furnace must be kept as low as possible, which can be done through flue gas recirculation that produces a slower burning and cooler flame or by air staging where air is fed through different stages, primary and secondary air. Low NOx burners generally have a longer, more orange flame.