The challenges of effectively burning natural gas date back much further than one might think. Natural gas seeps were first discovered in China as early as 900 B.C. Surprisingly, natural gas was not discovered in the United States until around 1815 during the digging of a salt brine well in Charleston, W.V., and it was not until 1886 when natural gas was first discovered in the northeastern states.
Some of the early issues with burning natural gas for industrial use included difficulty in keeping the flame consistently lit and achieving the higher temperatures necessary for melting glass and metals. “As early as the 1800s a chemist named T.R. Bunsen invented a burner that produced a very hot — and practically non-luminous — flame by permitting air to enter at the base of the burner to mix with gas before igniting.”1 It was not until the early 1900s, however, that these principles of premixing gas and air were applied to industrial burners.
Even in those early days, it was understood that the precise mixture of gas and air had a major effect on the flame and its efficiency. For as long as natural gas has been used in industrial combustion, the same question is still being asked today: How should natural gas be burned to get the most efficiency out of a system?
Natural gas continues to gain a strong foothold globally as production techniques are increasing availability, lowering costs and making it more attractive for industrial combustion use. The industrial sector currently consumes 27 percent of the natural gas in the United States, with projected increases of 6.25 percent between 2014 and 2021. The largest use of natural gas by the industrial sector is 42 percent for process heating, and the second largest use is boilers at 22 percent.2 As a major cost factor in any industrial process, it only makes sense to carefully analyze how natural gas is being used and whether the system is running at peak efficiency.
Throughout the manufacturing process, energy is lost due to equipment inefficiency and mechanical and thermal limitations. Optimizing the efficiency of these systems can result in significant energy and cost savings and reduce carbon dioxide emissions. Understanding how energy is used and wasted — energy use and loss footprints — can help plants pinpoint areas of energy intensity and ways to improve efficiency.3
The U.S. Department of Energy’s Industrial Technologies Program (www.eere.energy.gov) has identified that improved burner control systems offer a significant opportunity for reducing energy operating costs, waste and environmental emissions. As plant managers seek to improve combustion performance and product quality, they must balance cost, time, fuel and energy-saving measures.4
What does it mean to optimize combustion performance? In industrial process heating, combustion performance can be optimized by reducing fuel usage, minimizing cost, improving burner productivity, lowering furnace operating costs and producing a better end product.5 All good things to consider, so the question becomes how can these all be achieved?
Numerous checks can be made to ensure a combustion system is operating at its highest potential efficiency before committing capital to upgrade existing equipment. They include:
- Inspect burners, regulators and control valves for wear or damage. In addition to the wear experienced in mechanical devices, burners can degrade as they operate at combustion temperatures up to 3400°F (1871°C). Periodically inspect burner internals for accumulated dirt and debris, wear, excessive oxidation or warping. Pay particular attention to gas nozzles, mixing plates and bluff bodies.
- Evaluate the furnace refractory, which must be maintained or face the prospect of overheated furnace casings, deterioration of casings or other furnace parts, increased maintenance costs and lost production time.
- Clean or replace air filters and fuel strainers. Any device that could impact burner air/fuel ratios should be given extra attention, including valves, linkages, regulators and regulator impulse lines. Make sure that bearing lubrication has been performed on a regular schedule.
- Inspect process heating equipment such as ovens, dryers or kilns to look for energy loss due to air leaks, improper damper settings or less than effective process control settings, programming or tuning.
- Inspect air/fuel control mechanisms to make sure that there is no excessive hysteresis or variability present. Coarse positioning resolution, lack of synchronization and nonlinear response make it harder to maintain process temperature and air/fuel ratio setpoints.
- Tune the control loop. If the system has temperature modulation, be certain the control loop is tuned correctly to limit hunting or overshoot. These actions produce wasted energy and add unnecessary wear on modulating actuators and control valves.
- Tune the burner. At times, combustion equipment appears to be operating properly while out of adjustment, contributing to inefficiency and poor emissions. Depending upon the specific application of the burner, tuning to a designed air/gas ratio often can reduce fuel consumption.
- Check the products of combustion with a combustion analyzer to verify that all the fuel is burning completely and cleanly. Telltale signs of incomplete combustion are aldehydes, high carbon monoxide and unburned hydrocarbons in the exhaust or process.
- Remember that burners only create heat. How the heat transfers to the end product is a function of the specific equipment design.
All of the issues listed can contribute to less-than-optimal use of fuel and increased operating costs. If the combustion system appears to be functioning properly and the maintenance checklist has been reviewed, but the system is still not able to hit the targets for temperature control, fuel efficiency or emissions control, there are several additional areas to be examined. These include excess combustion air, the air/fuel ratio control and burner selection.
Excess Combustion Air. Defined as any air that is not necessary to burn all of the fuel, excess combustion air can rob boiler and furnace systems of efficiency. 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.
Excess air can vary widely in all applications. High temperature is normally 5 to 15 percent excess air while low temperature applications like air heaters can be 5 to 100 percent. Excess air can increase NOX emissions in boiler and furnace applications because typically not enough excess air is available to reduce the flame temperature. Conversely, high excess air is actually used in air heating applications to lower NOX by cooling the flame.
Air/Fuel Ratio Control. A significant efficiency impact on burners is the ratio mix of the air and fuel. Essentially, there are two types of air/fuel burners: forced draft and natural draft. Forced-draft burners use blowers to provide pressurized air to oxidize the fuel and to produce different flame patterns. The blowers run continuously, increasing electrical usage, and require a means to proportion airflow to the rate of fuel flow. By contrast, with natural-draft burners, the air and gas flow are unforced and follow the natural convection patterns created by the mechanics of the combustion chamber and ducts. Blowers are not used with natural-draft burners.
By keeping tighter control of the air/fuel ratio, one can better control the combustion reaction and its efficiency. One method for doing this includes using a fixed air system — also called fuel-only control — where airflow is held constant and the burner output is controlled by trimming incoming gas via a control valve. Another option is to control air input with a variable-frequency drive (VFD), regulating the blower speed with a single gas valve controlling gas inputs. A third, and more desirable, option is to use flow sensors and control valves that monitor and continually adjust air and gas. This method is often known as a mass flow air/fuel ratio control system.
The system controls burner performance by metering the incoming air/gas flows and modulating the flows via precision actuators. This system automatically compensates for changes that affect combustion performance such as variations in air and fuel temperature, supply pressures and variable combustion chamber pressures. Mass flow air/fuel ratio control often is applied on low emissions applications.
Industrial Burner Selection
|See the related web exclusive, "How Reducing Excess Combustion Air Helped One Industrial Plant," to learn how better air management can lead to significant improvements. It did at a major car manufacturer in the Midwest.|
Many of the industrial burner manufacturers have product catalogs that measure nearly a foot thick. Why? The answer is that decades of gas-fired process heating applications have proven that specific burner designs can have dramatic effects on the heating efficiency of various types of equipment. Once all of the above checklist items have been exhausted and the desired performance targets still cannot be reached, it may be necessary to consider upgrading to a different burner design to gain the desired outcomes.
By varying characteristics such as discharge velocity, flame shape, flame radiance, control methods and flame stoichiometry, burner manufacturers can match the heat transfer characteristics of their burners to the specific needs of a process or application.
For optimal performance, select burners that are intended for the process or device to be heated. Consider how each burner actually burns fuel and transfers heat to the end product. The correct burner can have a significant effect on fuel bills. Similarly, incorrect burner sizing can have a negative effect on performance and efficiency. It is not uncommon to find burners installed that are too large for the actual demand of the process. When this oversizing occurs, the combustion air blower is less efficient. In addition, most process heating burners of metallic construction use higher proportions of excess air for cooling at lower firing rates. Therefore, in addition to reduced blower efficiency, process thermal efficiency may be sacrificed when burners are oversized.
It is typical for the operating cost of an industrial heating system to outweigh the initial capital expense. The best suggestion is to regularly maintain the system as recommended by the manufacturer to make sure it is running as efficiently as possible. It is not always necessary to consider a complete overhaul of the system. Tuneups and system adjustments usually will result in some improvements. If targets are still out of reach, consider upgrading the burner or the air/fuel control system. Only rarely does an overhaul require replacement of an oven or boiler structure. It is critical to spend the time, effort and capital to make sure that the combustion components are specified to match necessary operational requirements.
After that, it is imperative to closely monitor how well the new system is performing compared to initial targets and make ongoing adjustments as needed. In the long run, efficiencies and performance will be improved and profits are likely to increase.