Boiler burners are the central functional element of effective combustion system design, which includes air-fuel distribution, fuel preparation, combustion control, and furnace design. The evolution of industrial gas and boiler burners has seen numerous changes over the last 50 years, leading to more efficient and sustainable boiler systems.
Through new boiler technology, significant progress has been made to achieve multiple fuel flexibility, lower emissions, higher turndown, longer lifespan, and stable burner system operation in modern industrial boilers.
Advancing technology in industrial boiler controls with proven combustion performance has greatly improved boiler efficiency and enhanced industrial operations and applications. Read on to discover how boiler burner systems have changed in the last 50 years.
The Basic Register Burners: 1960 – 1970
From the very basic burners of the 1960s and 1970s to the more modern and highly efficient low NOx burners of today, a lot has changed in the boiler industry. Despite the changes in burner design over the years, achieving maximum efficiency and safety remained key design components, even as new boiler technology was adapted. One area that has seen significantly changed is in the use of register burners.
What is a Register Burner?
Register burners were used by factory service engineers to regulate and supply the correct quantity of air for efficient combustion in industrial boiler systems. Basic register burners of the past came with one or two sets of adjustable register louvers or vanes to optimize or control the spin of combustion air before it was mixed with fuel.
Too little air would lead to incomplete combustion, soot deposits, smoking, and flame instability. Too much air would reduce overall combustion efficiency by removing heat from the furnace space which may promote sulfurous deposits and “white smoke.”
Maintenance engineers also used register burners as a convenient tool for flame shaping or to adjust flame width. These burners were widely used for oil and gas fuels. For fuel gas, register burners came with multiple lances or even a center gun or a gas ring element. For fuel oil, a gun-type burner or atomizer would have to be inserted in the center of the register burner.
Changes in Burner Design
Over the years, changes in burner design saw more efficient burners developed that didn’t employ register louvers like the Venturi type burner. Despite the advances in new boiler technology, most burners still required a flame-stabilizing device, such as a burner spinner or diffuser.
Today, modern register burners are used in industrial boilers for precise control and distribution of high volume combustion air and are also designed with safety and efficiency in mind.
NOx Emission Reduction: 1980s
Until the early 1980s, most boilers had open flues in order to vent unwanted products of combustion in industrial facilities. Boiler burner emissions were one of the biggest safety concerns at the time. This necessitated the need for NOx emission reductions through various ways, including making minor design changes that would influence thermal NOx levels like the use of special gas element drill patterns. This influenced the development of low NOx burners.
Industrial Boiler Controls – NOx Control
NOx emission reduction involves various process changes such as making modifications to the burner combustion process, implementing new boiler technology or using various NOx control strategies, such as:
- Fuel switching
- Staged combustion
- Flue gas recirculation (FGR)
- Installation of low NOx burners
- Selective catalytic reduction (SCR) and others
FGR (Flue gas recirculation)
While there were other ways available to reduce and control NOx emissions in industrial gas burners, FGR soon became the main focus in the mid to late 80s when the South Coast Air Quality Management District in California mandated NOx emissions industrial boilers that used fuel gas at <30 PPM Nox. In most cases, induced FGR was used to achieve this. In the coming years, new burner designs helped achieve even lower NOx levels as stringent low NOx emission requirements became the norm.
Although flue gas recirculation (FGR) helped drastically reduce NOx emissions, it had its shortcomings. Increased FGR rates often led to increased mass flow, which then starved off the burner of the needed oxygen, leading to unstable combustion.
This is where boiler suppliers started developing new boiler technology and more complex burner control systems to cope with high FGR rates. In states like California, where single digit low NOx levels have been mandated for many years, low-NOx gas burner systems have always been used.
However, in most of the U.S, extreme NOx reductions aren’t required. Perhaps the biggest challenge faced in the reduction of NOx levels in the combustion process is prompt NOx. This is a common problem in all conventional burners. Since it forms instantaneously, the use of FGR cannot help reduce prompt NOx. This led to the adoption of ultra-low NOx burners that used higher FGR rates to solve both thermal and prompt NOx emission problems.
Ultra-Low NOx Burners
The main function of ultra-low NOx burners is to prevent thermal NOx or prompt NOx. Today, there are different types of designs for ultra-low NOx burners in the U.S, both for small commercial boilers like firetube boilers and large industrial watertube boilers. Using new boiler technology, these burners thoroughly pre-mix fuel and air before combustion, therefore eliminating most fuel-rich zones that lead to prompt NOx formation.
Advantages of Using Ultra-Low NOx Burners
- They help solve prompt NOx problems while also reducing thermal NOx.
- Reduce major operation problems caused by uncontrolled NOx levels.
- Can be integrated with complex industrial boiler controls to achieve lower NOx levels.
- Significant reduction in NOx emissions.
Design Changes and Their Impact on NOx Levels & Emissions
Over the years, different ultra-low NOx burner design modifications have been tested to ensure better control of NOx levels in the combustion process. These burners included new boiler technology such as complex combustion controls, larger combustion fans to induce larger amounts of FGR, use of an extended mesh tube, combustion air filters, and others. These design changes ensured that both small and large ultra-low NOx burners inhibited prompt NOx and thermal NOx effectively.
Back-End Cleanup of Stack Gases
While a lot has been done over the last 50 years in the reduction of NOx, the back-end cleanup of stack gases and its role in reducing NOx emissions has largely been ignored. Fortunately, there has been significant progress in the use of selective catalytic reduction (SCR) designs that help efficiently achieve single-digit NOx levels in stack gases from large industrial gas-fired boilers, with no need to rely on single-digit low NOx burners and control systems.
NOx Reduction using SCR
Selective catalytic reduction (SCR) is a post-formation NOx control technology that helps reduce NOx levels using specific catalyst materials that facilitate a chemical reaction between NOx and ammonia to produce water and nitrogen.
While there are questions about the costs of installing packaged SCRs and other new boiler technology, the service life of the catalyst materials used and maintenance costs, large industrial boiler users are adapting SCRs as a more efficient NOx reduction strategy with up to 95% reductions.
States like California, in particular, have been keeping a close eye on the high power costs associated with using ultra-low NOx burner systems when compared to burners that use SCR control technology.
The Bottom Line
Burner manufacturers are still developing more efficient combustion concepts and industrial boiler controls to significantly and efficiently reduce NOx emissions. This will continue as long as gas burners are powering all types of industrial boilers and furnaces. Burners that use heavy oil and diesel fuel pose a huge challenge in NOx reduction. The high levels of bound nitrogen in heavy fuel oils make it challenging to achieve any viable reduction in NOx levels in modern packaged boilers.
Continuing Improvements in Technology
With improving new boiler technology over the years, burner management and combustion control systems have helped reduce NOx levels and boiler operation problems. This has led to better boiler efficiency, sustainability, ease of operation, and maintenance.
Reach out to Manley’s Boiler, Inc. today to see how we can help you find the best boiler system for your business. Our expert staff can service, maintain, and repair your boilers, at your convenience.