GAS TECHNOLOGY: Boiler NOx Emissions

In some areas, operators are required to meet what are called “Ultra-Low NOx” (ULN) requirements, typically less than 10-12 ppm. It is widely believed that the ULN requirements will be broadened to more areas. In recent years, boiler and burner designers have focused on achieving ULN compliance without significant performance penalties.

July 20, 2010

In many areas, industrial boiler operators face stricter regulation of NOx emissions, even with the use of clean natural gas. In some areas, operators are required to meet what are called “Ultra-Low NOx” (ULN) requirements, typically less than 10-12 ppm. It is widely believed that the ULN requirements will be broadened to more areas. In recent years, boiler and burner designers have focused on achieving ULN compliance without significant performance penalties.

 Reduced Atmospheric NOx the Goal

Early on, it was discovered that the largest contributors to NOx in most regions are motor vehicles and non-stationary engines, followed by industrial, residential, agricultural and natural sources. Passage of the Clean Air Act in 1963, followed by major amendments in 1970, 1977 and 1990, established a framework for the ever-tightening regulation of NOx. The first NOx emission target was highway vehicles. In the 1970s major strides were made in reducing automotive NOx as well as other pollutants through the use of exhaust gas recirculation valves and catalytic exhaust gas treatment.

Regulators next turned to industrial emissions and to boilers in particular. Federal air quality laws and rules were applied to industrial boilers, and allowed states to set local standards more stringent than the federal standard to achieve acceptable ambient air quality. This was first done in California.

Changing to Natural Gas

For many operators of industrial boilers, an important first step was to change from coal or oil boiler fuels to natural gas, which is cleaner burning in many respects. However, NOx continues to be a target.  With natural gas, fuel based nitrogen is not the concern, but the use of air for combustion does provide the primary source for NOx. An efficient, clean-burning traditional gas burner in high temperature applications may still produce significant levels of NOx. 

Commonly used strategies for natural gas-fired boiler burners include staged combustion, flue gas recirculation (FGR), flame temperature adjustment, and combinations of these approaches. These reduce NOx emissions from previous levels of 80-150 ppm of NOx to less than 30 ppm. But in many parts of the country, such strategies alone may fall short of what is necessary for ULN compliance.

 Tougher Standards for Some Areas

In some areas, air quality compliance requires boiler emissions to be further reduced to below 30 ppm and in some cases to less than 10-12 ppm of NOx. According to Steve Connor from Cleaver Brooks, the current major markets for these ULN boilers are in California, Texas, Louisiana and New Jersey. In addition, Clark County, Nevada (Las Vegas) has a similar requirement. However, Chad Fletcher from Hurst Boiler & Welding Co. indicates that customers across the country are asking about boilers with ULN performance levels. He says, “Mainly, I think this is in fear of regulations changing in the future.”

In some cases existing boilers can be brought to ULN compliance with a burner replacement. Jon Backlund is Vice President of Sales & Marketing for ALZETA Corporation, a major manufacturer of ULN compliance burners. Backlund indicates that Alzeta burners use gas premix surface combustion designs. He says, “The burner surface can be built in many geometries and the flame supply conforms to the surface. This approach works very well in both firetube and watertube boilers, and also in small atmospheric boilers.” Backlund notes that his firm has supplied ULN burners for hundreds of boilers as well as other applications.

Burner Replacement is Practical

Chad Fletcher from Hurst indicates that retrofit ULN burners can be installed on all his company’s scotch (firetube) boilers over 125 hp. “What we run into with smaller sizes are furnace dimensions that are too small, so we make custom boilers of 125 hp and smaller for these specific applications.” Fletcher believes that burner technology is the most important determinant of NOx emissions. “There are areas like furnace dimensions that we must adjust but it is mainly burner technology that gets it done.”

Some providers focus on a complete ULN boiler design. Miura North America offers its LX Series boiler that provides ULN levels of 9 ppm and has plans to release a near-zero NOx boiler into the North American market. This unit uses a proprietary catalyzer to virtually eliminate NOx altogether. According to Jason Smith from Miura North America, it features a sophisticated pre-mix burner that produces a very low-temperature, “self-quenching” flame, thereby minimizing the endothermic reaction that causes oxygen and nitrogen to form NOx in the first place.

Smith adds, “Miura utilizes its own flue gas recirculation system to reduce NOx levels to 9 ppm, meeting California’s stringent air quality regulations for stationary equipment.” Smith points out that he sees a general trend of natural gas boilers replacing other fossil fuels. “It is the cleanest fuel when considering CO2, NOx and other greenhouse gas emissions per Btu of energy produced.”   

The FIR Solution from GTI

More recently, some advanced burner designs have included a concept called forced internal recirculation (FIR), which was developed by the Gas Technology Institute. The FIR burner can achieve ULN emissions of less than 10 ppm without the use of flue gas recirculation. The unique burner design provides excellent flame retention for stable combustion at sub-9 ppm NOx levels.

These low levels of NOx and CO will meet the stringent emission requirements found in California, Houston and other non-attainment zones. Two boilers using FIR burners are installed and running at Fullerton College, California. The boilers are used to supply steam heat to the campus buildings and utilities. These boilers operate at less than 9 ppm NOx while maintaining thermal efficiencies of over 85%.

According to Jon Backlund from ALZETA, even lower NOx emission standards of 7 ppm are already being promulgated in parts of California and these levels can be achieved with ALZETA technology. As mentioned above, Miura is also introducing boilers that can achieve very low NOx emissions. Whether these “extra-ultra-low” NOx rules will be more widely encountered is not yet known.

Another promising development is the C-RMB Burner developed by Todd Combustion, a division of John Zink Company. Information on this new solution was provided at a recent Energy Solutions Center Technology and Market Assessment Forum. According to the company, this offering uses a burner-only stable flame solution than can achieve emissions of less than 5 ppm. The key to this approach is the use of multiple smaller burners rather than a single larger unit.

The company believes that the need for this level of ULN performance is growing. They note that <9 ppm performance was once considered to be exclusive to California. Today, fewer than half of the company’s ULN jobs are in that state and interest in lower levels seems to be growing.

Owners Still Look for Performance

In any case, boiler operators ask for equipment that meets increasingly rigorous emission performance levels with little or no penalty in performance. Newer solutions are meeting the ULN challenge with minimal impact on operating efficiency. Continuing progress in boiler and burner design will undoubtedly follow to meet that need. If you have an expectation of tightening NOx standards in your area, remember to consider ULN boilers and burners before making a selection.