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Ambient conditions impact CO and NOx emissions: part II

Effects of air temperature and humidity on burner NOx emissions are discussed, including considerations for factoring in these effects

Wesley R Bussman and Charles E Baukal Jr
John Zink
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Article Summary
The effect of ambient air temperature and humidity on NOx emissions from gasoline and diesel-fuelled combustion engines has been known for many years. A method for correcting the NOx for air temperature and humidity can be found in the US Code of Federal Regulations.1 The effect of air temperature and humidity on burner NOx emissions has also been recognised for many years in the petrochemical and hydrocarbon industries. However, a general method for making these corrections has not been reported. Perhaps this is due, in part, to the wide variety of burner 
and heater designs and operating conditions. Part I of this two-part 
article, published in the Q2 2009 issue of PTQ, showed how ambient air conditions impact NOx and CO emissions from process heaters. Part II addresses the significance of ambient air temperature and humidity on burner NOx emissions.

Air humidity effects
Historical data shows that industrial burners emit lower NOx when the air humidity is higher and is clearly stated in API 535,2 as follows: “Some research (though not duplicated by others) has noted a tendency on NOx concentrations to decline as moisture in the combustion air increases. This change is theorized to occur by the following mechanisms: (a) the inert water in the air reduces the flame temperature, etc”

API 535 suggests that the presence of water vapour in the combustion air appears to reduce the effective temperature of the flame, resulting in lower NOx emissions. This mechanism of NOx production is attributed to a distinct chemical kinetic process referred to as thermal NOx.3 Thermal NOx is produced by the reaction of atmospheric oxygen and nitrogen at elevated temperatures, and is considered to be the dominant mechanism in most gas-fired combustion processes that burn clean fuels. High temperature zones within the flame cause nitrogen molecules in the air to separate, allowing them to combine with oxygen, which leads to the formation of NOx. The amount of thermal NOx formed depends largely on the temperature of the flame and residence time — the higher the flame temperature and the longer the residence time, the more NOx production.

Water vapour in the combustion air tends to act as ballast to absorb heat from the flame, lowering the temperature and reducing thermal NOx. Figure 1 shows the theoretical effects of combustion air humidity on the adiabatic flame temperature firing methane with 2% excess O2. The red line denotes zero flue gas mixed with the fuel prior to combustion, which is representative of a high NOx burner. As the humidity of the combustion air increases, the temperature of the flame decreases. For example, the temperature of the flame decreases by approximately 2.3% when the humidity increases from 0 to 120 grains. Although the change in flame temperature is small, it can have a significant impact on the thermal NOx production. The figure also shows the effects of humidity when 10lb (4.5kg) of recycled flue gas are mixed with 1lb (0.5kg) of fuel prior to combustion, which is representative of a low NOx burner. Again, notice that an increase in humidity corresponds to a reduction in flame temperature. For this case, the flame temperature decreases by approximately 1.7% when the humidity increases from 0 to 120 grains. These results suggest that the effect of air humidity on flame temperature is less sensitive when more flue gas is mixed with the fuel prior to combustion. This occurs because the flue gas also acts as a ballast to absorb heat from the flame, which reduces the effects of water vapour.4 Based on these results, it is expected that a low NOx burner will not see as a dramatic reduction in NOx with an increase in air humidity as a high NOx burner would.

Figure 2 demonstrates the effects of air humidity on NOx emissions for two burner styles. The circular symbols represent data from a large industrial heater full of premix, radiant wall burners (data for both high and low NOx burners).5 The triangular symbols represent experimental data firing a single, diffusion-style wall burner. All of the data are normalised to a NOx value corresponding to zero humidity. The NOx value, at zero humidity, is obtained by extrapolating a best-fit exponential curve through the experimental data. Experimental data clearly show that for both the premix and diffusion-style burners, NOx decreases with an increase in air humidity. For example, a variation in air humidity from 0 to 120 grains corresponds to a 45% change in NOx for the high NOx burner (red symbols). As previously discussed, this represents 
a change in the theoretical flame temperature of only about 2.3%. 
The data also show the percentage 
NOx reduction is more pronounced 
for the high NOx burner than the 
low NOx burner. Based on theoretical results previously discussed, the 
flame temperature from a low NOx burner is less sensitive to variations in air humidity and, hence, changes in NOx.

Figure 2 also shows results of a simplified thermal NOx model. Assuming NOx production is 
dominated by the thermal mechanism, the ratio of NOx production with zero air humidity to that at a given humidity can be written as follows:

where  χNO, at zero RH and χNO, at given RH is the NOx, in terms of parts per million, at zero air humidity and at a given humidity, respectively, while terms Tat zero RH  and Tat given RH represent the adiabatic flame temperature at zero air humidity and at a given humidity, respectively. Model results displayed in Figure 2 are based on firing methane with an ambient air temperature of 90°F (32°C) and a flue gas temperature of 2000°F (1100°C). The model results show that as the humidity increases, NOx decreases, as expected. The model predicts that as the excess O2 increases, the NOx becomes more sensitive to the air humidity. The model also shows that NOx becomes more sensitive to 
air humidity as less flue gas is mixed with the fuel prior to combustion. 
In other words, the model captures 
the trend, showing that NOx emissions from high NOx burners are more sensitive to air humidity than low 
NOx burners.

Air temperature effects
Historical data show that industrial burners emit more NOx when the combustion air temperature is higher and is recognised in API 535:2 “NOx production is favored by high temperatures. Local flame temperatures and NOx concentration will increase as the temperature of the combustion air increases.”

Figure 3 is a plot from API 535, showing the effects of air preheat temperature on NOx production. The broad range in data may be due to various types of burners operating at different fuel compositions, furnace temperatures, excess O2 and so on. Notice the plot does not give 
information on how the NOx varies for ambient air temperature range from about 32–110°F (0–43°C). More data need to be gathered to address 
ambient temperature effects on NOx production.
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