Existing Combustion Installations and Energy Use
A focus on the ongoing maintenance of existing combustion installations for gasfired ovens and furnaces can result in reducing energy consumption and lowering energy costs.
In a gas-fired oven combustion system, heat is generated through an exothermic reaction (a reaction where heat is given off) between hydrogen, carbon atoms and oxygen. Too much oxygen can make heating inefficient and can create NOX pollutants. Too little oxygen results in incomplete combustion, forming dangerous levels of carbon monoxide.
According to the August 2014 Process Heating article, “The Impact of Combustion Controls,” written by K. Huller, “When a burner is operating at the desired air/fuel ratio, it is said to be tuned. This is typically the point where the equipment operates at near peak efficiency, with the best possible emissions for the given burner, and where all of the fuel is consumed. …If not tested and maintained, combustion controls can cause a system to drift off-ratio.”
And, according to the U.S. Department of Energy’s “Process Heating Tip Sheet 2c,” “Periodic checking and resetting of air-fuel ratios for burners is one of the simplest ways to get maximum efficiency out of fuel-fired process heating equipment such as furnaces, ovens, heaters and boilers. Most high temperature, direct-fired furnaces, radiant tubes and boilers operate with about 10 to 20 percent excess combustion air at high fire to prevent the formation of dangerous carbon monoxide and soot deposits on heat transfer surfaces and inside radiant tubes.”
Further, as stated in the February 2015 Process Heating article, “How Reducing Excess Combustion Air Helped One Industrial Plant,” written by M. Lampe, “Excess combustion air is defined as any air that is not necessary to burn all of the fuel. Excess air can rob boiler and furnace systems of efficiency because it takes energy to heat the excess air not utilized in the combustion process, and most of this heat is lost up the stack. Better air management can lead to significant improvements.”
The straightforward advice from the U.S. Department of Energy to set and check excess air levels on burner systems is well known. However, the regular activity of checking and tuning burners still remains an opportunity for improvement across a sizeable chunk of industry.
The following advice from Jeff Ryan, president of H.E.A.T. Combustion Solutions LLC (a Eurotherm by Schneider Electric Partner organization) describes the steps needed to undertake burner tuning. His practical advice for tuning natural- gas-fired burners includes:
- When tuning burners on natural gas, it generally is advisable to adjust the combustion air at 10 percent excess air (XSA). The reasoning for this is to ensure there is enough free oxygen to burn the carbon completely, thereby avoiding an excessive amount of CO. • High CO readings are both a health and equipment hazard.
- When adjusting nozzle-mix burners, refer to the manufacturer’s data chart and begin with setting the combustion airflow at high fire. This is accomplished by measuring a static pressure at the combustion air inlet of the burner or a differential pressure across the appropriate pressure taps as designated by the manufacturer. The air is adjusted according to the readings on the manometer.
- After the high fire combustion air has been adjusted properly, the high fire gas flow is adjusted to obtain the proper air/ fuel ratio. Use a manometer to measure the differential pressure across the metering orifice and adjusting the flow accordingly. • Once the burner or burners are properly adjusted to the correct high fire air/fuel ratio, the pressure readings are recorded for future reference.
- Using the same procedures as performed to adjust high fire, the low fire for the burner or burners is adjusted according to the manufacturer’s specifications.
- Once low fire has been satisfactorily achieved, the settings are recorded for future reference.
- Once this is accomplished, the temperature controller is placed in the manual operating mode. Then, the burners are increased to 25, 50 and 75 percent output to ensure that the proper air/fuel ratio is maintained throughout the firing range.
Historically, all this information was captured in a technician’s logbook (basic pen/paper) and then entered into an excel sheet to create historical record, Ryan noted. Some combustion controls suppliers are developing an online solution to digitize the entire process. The goal of an online solution is to be able to accurately predict the next best date to tune the system in order to optimize energy usage.
An online solution such as this is part of the Industrial Internet of Things (IIoT). IIoT technology can help control excess combustion air to desired levels. So why IIoT and burner systems?
