At a basic level, burners are devices used to mix fuel and air (or oxygen) to achieve controlled combustion while producing a specific flame and heat-release pattern. The heat generated is used to control the temperature within a combustion chamber to the process setpoints.
When selecting the correct burner for an application, the first question to ask is, “What am I trying to achieve with my new or retrofit project?” It is important to know whether you are trying to reduce fuel usage, increase production, improve reliability, limit maintenance, reduce dross formation or mitigate emissions.t a basic level, burners are devices used to mix fuel and air (or oxygen) to achieve controlled combustion while producing a specific flame and heat-release pattern. The heat generated is used to control the temperature within a combustion chamber to the process setpoints.
An industrial burner is a bit like a tire for a vehicle in that it provides a basic function with a simple concept. All tires are round and provide stability and traction for vehicles on the road. Likewise, most burners are cylindrical and mix a fuel and oxidant to produce a flame and heat. Despite the basic principles, a burner and tire must both be applied appropriately to produce desired results. For example, you would not put racing slicks on your grandmother’s station wagon any more than you would put regenerative burners on an air heater.
Burners generally include the following types:
- Cold air (ambient air).
- Hot air (recuperative).
Most people are familiar with cold-air burners. Among these are the burners found on residential devices such as gas furnaces and ranges.
Hot-air burners use a central heat-exchanger device to preheat incoming combustion air.
Regenerative burners utilize a pair of burners that alternate between firing and exhausting. Higher air preheats are obtained by preheating an integral regenerative media box/heat exchanger.
Oxy-fuel firing uses pure oxygen instead of combustion air to burn with the fuel.
Table 1 shows a relative comparison of some factors you may be considering when selecting a burner and combustion system. There are potential trade offs when selecting one alternative over another. For example, oxy-fuel firing has high thermal efficiency and corresponding low fuel usage and CO2 emissions, but the operating costs are typically higher due to cost of supplying oxygen.
Different types of technologies and options are available within each of the four categories described in table 1. Good burner solutions will mitigate the negative aspects inherent to the four options. Environmental requirements, cost restrictions, demand for production increases and reliability requirements have forced burner manufacturers to develop new technologies. Regenerative technology, for example, has improved dramatically. Ultra-low-NOX designs were developed to meet industry requirements. Modifications to the regenerative media boxes have made it easier to clean and maintain the ceramic heat storage media. Your burner supplier should be able to match the burner type and technology that meets your specific requirements.
Increased production (melting aluminum, for example) often is cited as a reason for upgrading or selecting new burners on a furnace. Regenerative and oxy-fuel technologies increase the amount of available heat to the load by reducing the amount of heat lost through the flue. However, many other factors influence the proper heating and melting of metal:
- The number of burners is selected to provide the maximum heat transfer to the metal.
- The position of the burners relative to the flue and load location is important to achieve maximum heat transfer and lowest fuel use. One example of computational fluid dynamics (CFD) analysis illustrates staggered firing with flames that were well contained and had good direction (figure 1).
- Determining the proper downward angle of the burner is important to direct heat toward the load while not creating hot spots. Cold spots can be minimized by examining the bath surface heat flux through CFD analysis and varying the burner orientation (figure 2).
- Selecting a burner with the correct flame length and shape will provide complete heating coverage while not impinging on the furnace walls or bath surface.
- The luminosity of the flame also affects production and heating rates. A more luminous flame will provide the added benefit of increased radiation heat transfer from the flame to the load.
- The flame velocity, or flame speed, also affects the transfer of heat from the flame to the load.
High velocity burners produce shorter, more intense flames with smaller flue-gas-recirculating zones. Lower velocity burners have longer, lazier flames. Medium-velocity flames are preferred in a melting application, driving heat to the load when required while still providing a uniform flame profile and large enough products-of-combustion (POC) recirculation zone.
Selecting a burner for low-NOX operation is another major consideration. The potential to emit NOX increases with higher air preheat temperatures and higher furnace temperatures. Most burners use some type of air or gas staging (figure 3), creating regions of partial combustion in the flame. This reduces the peak flame temperature, which leads to a decrease in thermal NOX emissions.
Flame characteristics and burner stability also are considerations for burner designers when considering the application of a low NOX burner. Lower NOX burners have the potential to be less stable with lazier flames, particularly at lower furnace temperatures. A good burner design and selection will provide a stable flame that effectively transfers heat to the product at all operating points. Sometimes, this requires two modes of operation: a low-NOX mode at cold furnace temperatures and an ultra-low-NOX operation at higher temperatures.
The right burner selection, along with proper furnace design and combustion controls, will maximize the benefits of your project. Keep in mind that production increases can be realized through higher efficiency designs and strategic burner arrangements. Fortunately, the latest burner technologies can meet nearly any NOX requirements. Burner suppliers who understand your process and application will be able to help select the best possible option.