Ultra-Low-NOx Burner Retrofits for Aluminum Furnaces
Upgrading burners is often an effective way to achieve process improvements such as increasing productivity, decreasing fuel use and/or minimizing NOx and CO2 formation. One of the most economical ways to perform an upgrade is by retrofitting an existing burner with new technology. Often, the modifications to the burner are comparatively minor and sometimes simply require changes to the internal components without the need for external flue-gas recirculation (FGR) or selective catalytic reduction (SCR).
Burner retrofits on aluminum melting furnaces can provide better efficiency and improved melt rates. The other major driver for burner upgrades (and thus retrofits) is the increasing importance of NOx and CO2 reduction. As regulations become more stringent, the aluminum industry will continue to require improved technology.
The most common way to upgrade a cold-air burner is to do a conversion to hot air (usually with an accompanying reduction in NOx due to technology improvements). It is, of course, necessary to evaluate the entire combustion system (for flow, pressure, temperature, etc.) when making any kind of changes to the burners.
Cold-Air to Hot-Air Ultra-Low NOx
The primary benefit achievable from a cold-air to hot-air conversion is a reduction in fuel use by the increase in combustion efficiency. These kinds of conversions could require significant modifications to the burners (although modifications could also be very minor). Recent developments in retrofitting technology have generally minimized the amount of work required to modify the burners.
Because combustion efficiency increases with combustion-air preheat temperatures, the fuel input required for the same process requirements decreases (unless productivity increases are part of the upgrade as well), thus necessitating an evaluation of the whole combustion system.
Figure 1 illustrates a typical conversion from a cold-air to a hot-air ultra-low-NOx burner. Some modifications to the burner are required. Please reference Figure 1 and corresponding numbers for the following descriptions.
- Refractory lining for the burner is required if the new air temperature exceeds the design temperature of the existing burner.
- The gas nozzle may need to be replaced for a few reasons. First, it would require replacement if the new ultra-low-NOx fuel nozzle requires higher pressure or a different spray angle/type. Second, the fuel-nozzle length may need to be modified to accommodate changes to the air nozzle. The materials of construction of the fuel nozzle must also be evaluated for a higher combustion-air temperature.
- The air nozzle/baffle would need to be replaced in ALL types of ultra-low-NOx conversions. Air/gas staging and air-nozzle design are critical to minimizing peak flame temperatures and NOx emissions.
- The gas connection may also need to be relocated at the rear of the burner. Nonsymmetrical combustion and gas staging are two reasons why the connection location may need to be modified, thus requiring a new end plate.
- In some cases, the port diameter and port angle may require modification. Port density (Btu/in2 [of the port area]), the maximum amount of heat load/capacity in a given burner size and port diameter, of the design may be the limiting factor in a potential conversion.
- Changes to the port may necessitate expansion of the opening in the furnace wall. Such modifications are more often required when air staging occurs in the port block itself – generally in smaller-capacity burners.
- In some cases, a completely new burner body may also be required depending on the requirements of the internal burner design and existing burner configuration.
Table 1 summarizes the expected emissions for both a cold-air burner and hot-air ultra-low-NOx burner with an expected reduction in emissions shown in both pound/MMBtu and pound/hour basis. Because the required installed capacity for a hot-air system is less than a cold-air system (for the same available-heat requirement), there is a more significant benefit on a pound/hour basis.
The following case study in the aluminum industry required the conversion of cold-air burners to ultra-low-NOx burners in three unique applications. The best-available burner technology was utilized to meet the strict requirements.
- One aluminum melter, one holder and two homogenizers
- Ultra-low-NOx requirement in California
- Ultra-low NOx required at all operating points
- Emissions guarantee mandated the use of incremental primary air control as furnace temperature increased
- Ultra-low-NOx hot-air burners with internal flapper valve for melter
- Small-capacity ultra-low-NOx burners for homogenizer and holder
- Field tuned to maximize flame signal and NOx reductions at all operating points
- Technology delivered BACT (best available control technology) NOx and exceeded customer expectations in all cases
- Public permit limit NOx not to exceed 37 ppmvd@3% O2 (RECLAIM concentration limit) in California SCAQMD agency. The low-NOx burner is considered achieved in practice.
The most significant production and fuel savings benefits can be gained through converting existing direct-fired burners to regenerative burners. With developments in regenerative technology, in many cases NOx emissions can also be reduced despite the increase in the combustion-air temperature.
Conventional to Regenerative Ultra-Low NOx
When compared to a cold-air system, a regenerative system reduces air and gas line/equipment sizes while requiring the addition of a complete exhaust system. Flue-system restrictions often limit burner capacity increases. Regenerative systems can often provide an efficient solution to this problem. If a cold-air system is being considered for retrofit to regenerative, approximately 90% of the air and gas equipment may be reuseable provided it is in acceptable condition. Additional cycle valves are required for air, gas and exhaust. Cooling-air equipment, start-up air equipment and exhaust equipment (including an exhaust blower) are also required.
Modifying a furnace from a conventional system to a regenerative system provides significant thermal and/or production enhancements. Because of extremely high air preheat temperatures and regenerative devices that are required for each burner, existing direct-fired burners cannot be reused. Space constraints must also be analyzed to accommodate the addition of a regenerative media box. Options such as roof-mounted media cases and dual-head regenerative burners are sometimes needed for successful installations (Fig. 2).
Despite a significant increase in air preheat temperature and thermal efficiency, expected NOx emissions on a pound/MMBtu and pound/hour basis can be improved (Table 2).
Sidewell-charged continuous-type aluminum melters represent an excellent application of regenerative burners in the aluminum industry. Ultra-low-NOx regenerative burners can be justified for all new sidewell-charged aluminum melters. Retrofits will be economically justified in nearly all cases on these furnaces due to the dramatic efficiency advantage for regenerative systems on continuous high-temperature processes.
The following case study on a sidewell melting furnace demonstrates the capability to retrofit regenerative burners on a furnace designed originally for cold combustion-air operation with inherent space limitations.
- Sidewell melting furnace
- Increase melt rate and fuel savings
- Long and narrow bath area
- Existing flue location made it impossible for burner heads to straddle the flue
- Space limitations
- Dual-head burner design
- Customer expectations met
- Predicted production increase of 5,612 tons/year, $400,000/year fuel savings and 1.1 tons/year NOx reduction
The solution was to provide dual-head regenerative burners (one regenerative media box plus two burner heads). This provided superior bath coverage and short flame length compared to a standard burner with identical input (Fig. 3).
Retrofitting existing combustion equipment can be a very cost-effective way to enhance production, save fuel and reduce emissions. Advancements in burner and retrofitting technology have paved the way for improvement projects in a variety of applications and existing furnace/burner configurations.
For more information: Contact Matt Valancius, Manager – Marketing & Strategy, Bloom Engineering Company, Inc., 5460 Horning Rd., Pittsburgh, PA 15236; tel: 412-693-4202; e-mail: email@example.com; web: www.bloomeng.com/USA