In many locations throughout North America, nitrogen oxide (NOx) emissions of gas-heated furnaces are limited by law or code. This is typically the case in urban areas or in places where special topographical characteristics intensify the negative local and regional effects of NOx emissions.

While the approach of limiting hazardous emissions is for the benefit of the environment and society, the method used to define the limitation can significantly alter its outcome (e.g., if it does not account for the full extent of influencing factors). In particular, this is the case for the combustion efficiency in gas-heated industrial furnaces. 

This article presents a simple example showing how a one-dimensional emissions-limitation standard can lead to adverse effects, preventing a well-intended initiative from achieving its goals or even leading to the exact opposite. It further outlines an approach for a very simple correction of the current one-dimensional formula in order to properly reflect combustion efficiency and, therefore, actually achieve its intended goals. 

NOx Formation in Gas-Heated Industrial Furnaces

Nitrogen oxides are highly undesirable emissions because they can be directly harmful to our health and contribute to ozone formation and acid rain.^[1] NOx can be generated in combustion processes, even if “clean fuels” such as hydrogen or natural gas are being used. The main source of NOx in gas-heated industrial furnaces is called thermal NO. There are two other modes of formation: prompt NO and fuel-bound NO. These are, however, both negligible in industrial furnaces.

The formation of thermal NO is defined by the “extended Zeldovich Mechanism.”^[5,6]

N₂ + O → NO + N

N + O₂ → NO + O

N + OH → NO + H

As implied by the name “thermal NO,” the formation is temperature-driven and can be described with an exponential function. Peak temperatures, therefore, contribute greatly to NOx emissions. The peak temperature in a combustion process is typically found in the reaction zone (conventionally this is the “flame”). Traditionally, this has led to a trade-off between burner efficiency and NOx emissions, because preheating combustion air (being the most effective efficiency technology) also increases the flame temperature and, thus, leads to higher NOx emissions. 

Lowering peak temperatures while still maintaining complete combustion is the goal of NOx-reduction techniques, which has been a main focus in combustion research over the last several decades. The most common methods currently used in industrial furnaces are air/gas staging, flue-gas recirculation and flameless-combustion techniques such as FLOX® (Fig. 1).^[2] 

Example

A company is planning to build a new forge furnace in the Los Angeles, Calif., area. Per design, the furnace needs to be operated up to 2282°F (1250°C) and requires a net heat input of 10 MMBTU/hour (3,000 kW LHV) for the intended application. The furnace is going to be operated 6,000 hours per year with an average load factor of 75%. Since electricity is very expensive and also not desirable for heating industrial furnaces for environmental reasons, the company’s preferred technology is natural gas heating. 

Due to the location of the furnace, the South Coast Air Quality Management District (SCAQMD) has set a strict limit of 0.06 pound/MMBTU (50 ppm) NOx @3% O₂ for the combustion process at the highest furnace temperature. This limit is based on a one-dimensional approach and, as such, does not consider adjustments for high-efficiency technology. The limit is also not defined on an absolute scale such as pounds per year, but it is given as a concentration in the exhaust gas. This implies that all available technologies emit the same total volume of exhaust gases. This assumption is false, however, due to different combustion efficiencies of the various available combustion technologies.

In this example, the high furnace temperature and the operating time per year make efficiency measures highly desirable. Compared to a cold-air burner system (i.e., no heat recovery), a modern regenerative-burner system will achieve fuel savings on the order of 53%. While a cold-air burner system will achieve the required NOx concentration of 0.06 pounds/MMBTU (50 ppm) @3% O₂, the high air preheat of a regenerative-burner system would lead to an increase in its specific NOx emissions (i.e., concentration in the exhaust). 

Due to the use of appropriate NOx-reduction techniques such as FLOX combustion, the regenerative system can still achieve 0.08 pounds/MMBTU (70 ppm) @3% O₂. Based on the limits defined by the SCAQMD, however, the company is being forced to use the inefficient cold-air burner system since the highly efficient regenerative burners lead to a slightly higher NOx concentration than the specified limit of 0.06 pounds/MMBTU (50 ppm) @3% O2. 

The one-dimensional standard defined by the authorities, therefore, prevents the use of the best-available technology since the limitation of a concentration (pounds/MMBTU or parts per million pollutant in exhaust at a specified oxygen content) does not consider the total amount of exhaust gas produced by each alternative combustion setup. The reference cold-air burner system only achieves an efficiency of approximately 37% LHV at the specified furnace temperature. Hence, an equivalent of roughly two-thirds of the natural gas combusted is wasted through the stack of the furnace. At the same time, the alternative regenerative-burner solution achieves a combustion efficiency of 78% LHV and a significantly decreased use of natural gas along with proportionally less exhaust gases (Fig. 2). 

The resulting annual energy savings compared to the cold-air burner system equates to 19 GWh, which equals the annual natural gas use of 851 average U.S. households.^[4] Even though the specific NOx emissions of the regenerative-burner system are slightly higher than the reference system, the absolute amount of NOx emitted is, in fact, significantly lower. 

The inefficient cold-air burner system emits 8,172 pounds (3,707 kg) of NOx per year, whereas the high-efficiency regenerative system only emits 5,419 pounds (2,458 kg). This equates to 33% lower actual NOx emissions in the same time span (values derived from energy-based emission factors for natural gas H).^[2] 

At the same time, a CO₂ reduction of approximately 4,244 (3,850 metric) tons per year can be achieved due to the increased fuel efficiency (based on CO₂-emission factor 0.126 pounds/ft³ (2.02 kg/m³) for natural gas H). For just one furnace, this difference is equivalent to removing 820 passenger cars from the road.^[3]

To summarize, the way the standard is currently defined favors the inefficient and higher-emission technology rather than a far more advantageous combustion technology using less energy with significantly lower overall emissions. The next section introduces a simple correction method to account for the differences in efficiency. This method can be used to correct the emissions limit in order to easily equate two or more alternative technologies.

Improved Formula for Emissions-Limit Calculation

In general, there are two possible methods to better define the emissions limitation in order to make sure its actual goal will be achieved.

1. The overall absolute emissions of a pollutant can be limited. For example, the limit could be given in “pounds per year.”
2. The limitation of an emissions concentration in flue gases can be corrected with an efficiency factor.

While option 1 seems to be straightforward at first, it can be difficult to verify compliance in real-world applications. There is typically no emissions-monitoring device permanently installed to prove the true absolute amounts of a pollutant emitted per year. It would be possible, however, to use extrapolated values based on actual measurements to prove compliance with the limitation defined as an absolute value.

Option 2 could be used in a very similar way to today’s standard approach. Currently, the limitation of the concentration of a pollutant in the exhaust is spot-checked over the course of a typical operation cycle. In the case of NOx emissions, an exhaust-gas analyzer probe is inserted into the exhaust system and several NOx readings are taken over a certain period of time. These values are then averaged in order to compare them to the limit previously set for the audited furnace. 

At the same time, exhaust-gas analyzers typically also determine the efficiency of a combustion system based on the CO₂ content and the temperature of the exhaust. These efficiency readings or the published/guaranteed efficiency from the manufacturer can then be used to adjust the emissions limitation for increased efficiency (or vice versa, penalize the inefficient technology with the same factor).  

Assume that the reference system (ref) is defined as the most efficient combustion technology that still achieves the emissions concentration limit EB as defined by the authorities. If the reference technology is able to achieve lower specific emissions, EB should be set according to this lower value. Furthermore, assume that there is a more fuel-efficient technology available (eff) that does not meet the limitation for specific emissions. 

The corrected emissions concentration limit E_N shall serve as the new limit for the high-efficiency technology because it represents the value at which the total absolute emissions of the more efficient system are equal to those emitted by the less efficient system. If the high-efficiency alternative stays below this new emissions limit E_N, it is the environmentally favorable solution, because not only is the total amount of pollutant emitted lower than for the reference system but also the total fuel consumption.

For the example defined in the previous section, the corrected specific emissions limit E_N for NOx computes to:

The regenerative-burner system emitting 0.08 pounds/MMBTU (70 ppm) @3% O₂ stays well below the corrected limit E_N of approximately 0.14 pounds/MMBTU (105 ppm) @3% O₂, thus saving 33% NOx emissions per year compared to the option that would have had to be selected previously.

Figure 3 illustrates a broader comparison of cold-air and regenerative-burner systems over a range of operating temperatures. The graphs show that the more efficient combustion system consistently offers lower overall NOx emissions and also leads to significant energy savings. In addition, the energy-savings effect increases with the operating temperature, making the overall environmental benefits especially important for high-temperature applications.

Conclusions 

This article shows that the method currently used to define the limitation of pollutant concentration in the exhaust of a combustion system often does not achieve its intended goal. Without proper quantification and integration of the process inputs (fuel), the standard method can even act counterproductively to its intentions. 

An undifferentiated limitation of volume-based pollutant concentration (NOx) is only possible if all available technologies use the exact same amount of fuel to heat a furnace. Since this is rarely the case, we propose using an emissions limit corrected for combustion efficiency. Its compliance can be checked just as easily as the current, typically undifferentiated concentration limit since exhaust-gas analyzers not only reveal emission concentration but also the efficiency of a combustion system. As an alternative, authorities could also easily define classes of combustion equipment based on existing literature showing typical combustion efficiencies for different available technologies.^[2,7]

The example discussed represents a typical furnace application. Due to the disincentive set by the current approach for defining the emissions limitation, less efficient technology has to be selected. For only one exemplary furnace, this leads to 19 GWh of additional energy use, 4,244 (3,850 metric) tons of additional CO₂ emissions and 33% higher NOx emissions than what could be achieved with a more sophisticated combustion system. This equates to potential savings equivalent to the average annual natural gas use of 851 U.S. households and NOx reductions equivalent to removing 820 passenger cars from the road. 

Extrapolated to hundreds of furnaces, the effects quickly become highly relevant to the public and society. A simple correction to the standards set by the authorities will therefore lead to significant positive effects for both the environment and the economy. 

For more information:  Contact Dr. Martin Schönfelder, VP technical sales; WS Thermal Process Technology Inc., 8301 West Erie Avenue, Lorain, OH 44053; tel: 440-385-6829; fax: 440-960-5454; e-mail: wsinc@flox.com; web: www.flox.com

References        

1. United States Environmental Protection Agency (EPA), Office of Air Quality Planning and Standards: NOx – How nitrogen oxides affect the way we live and breathe, Triangle Park, NC, 1998
2. J. G. Wünning, A. Milani: Handbook of Burner Technology for Industrial Furnaces, 2nd Edition, Vulkan Verlag, Essen, 2015
3. United States Environmental Protection Agency (EPA), Office of Transportation and Air Quality: Greenhouse Gas Emissions from a Typical Passenger Vehicle, Ann Arbor, Michigan, 2014
4. U.S. Energy Information Administration: Residential Energy Consumption Survey, 2009
5. Y. B. Zel’dovich; The Oxidation of Nitrogen in Combustion Explosions, Acta Physicochimica 21: 577–628, U.S.S.R., 1946
6. G. A. Lavoie, J. B. Heywood, J. C. Keck; Experimental and Theoretical Study of Nitric Oxide Formation in Internal Combustion Engines, Combust. Sci. Tech. 1 (4): 313–326, 1970
7. R. J. Reed; North American combustion handbook. North American Mfg. Co., 2014