Nox Emissions and their Impact

Emissions of oxides of nitrogen, commonly referred collectively as NOx, are regulated because of their adverse effects on health and the environment. They play an important role in acid rain, the formation of harmful ozone and photochemical smog in the lower atmosphere and the depletion of the beneficial ozone in the upper atmosphere.

The most environmentally important oxides of nitrogen are:

NO, NO2, and, more recently, N2O.

N2O is a recent concern because it is a "greenhouse" gas which contributes to global warming and because it can aid in destroying the upper atmosphere ozone layer which protects us from ultraviolet radiation. Fortunately, very little N2O is emitted from the flame of a typical burner.

Over 90% of the NOx from a typical flame is in the form of NO and the remainder is NO2. However, since NO is eventually converted to NO2 in the atmosphere, most regulations treat all of the NOx as NO2.

NOx emissions from combustion sources are due to the oxidation of atmospheric N2 and the oxidation of nitrogen chemically bound in fuel molecules. Mechanisms for forming NO include the fuel NOx, prompt NOx and thermal NOx mechanisms. Some of these mechanisms are well understood, while others are still under investigation.

Although NO and NO2 molecules last only a matter of days in the atmosphere, N2O is a very stable species that can last 100 to 200 years in the lower atmosphere. Because of its long life span some N2O eventually reaches the upper atmosphere where it decomposes under ultra violet light and it's reaction products efficiently remove ozone from the upper atmosphere.

Fuel NOx and Prompt NOx

Fuel NOx is produced if nitrogen is chemically bound in the fuel molecule and is primarily a concern with heavy oils and solid fuels. Some gaseous fuels, however, can contain NH3, HCN or amine carry-over as potential sources of fuel bound nitrogen. For fuels with organically bound

nitrogen, the fuel NOx mechanism begins with the decomposition of the organic molecule in the flame zone:

CxHyN → Cx-1Hy-1 + HCN

or CxHyN → CxHy-1 + NH,

depending on the nature of the carbon/hydrogen/nitrogen bonds. The HCN or NH reacts further and may be oxidized to NO.

Prompt NOx is the NOx formed from N2 in the very early portion of the flame zone where the fuel and air are first reacting. It is formed in a part of the flame where little, if any thermal NOx should be formed and, by definition, is that NOx formed from molecular nitrogen which is in

excess of the NOx predicted by the thermal NOx mechanism. There are several reaction paths postulated for forming prompt NOx. One of the more important involves the reaction of molecular nitrogen with hydrocarbon radicals formed during the decomposition of the fuel in the

initial reaction zone. The major reactions are:

CH + N2 ↔ HCN + N,

and C + N2 ↔ CN + N.

The fuel NOx and prompt NOx mechanisms proceed identically after these initiation reactions.

NOx CONTROL

The major contributors to NOx emissions are thermal NOx and, if fuel bound nitrogen is present, fuel NOx. Most refinery process heaters in the US are fueled by refinery fuel gas and, thus, thermal NOx is the primary concern. As noted previously, thermal NOx is strongly influenced by peak flame temperatures and the key to controlling thermal NOx is to moderate peak flame temperatures.

Historically, thermal NOx control techniques have included excess air control, air or fuel combustion staging and flue gas recirculation. Low excess air operation provides only limited benefit. However, it is compatible with, and can be used together with, most of the other NOx control techniques. Combustion staging and flue gas recirculation have proven to be more beneficial. The combination of fuel staging and flue gas recirculation has proven to be the most beneficial combination until recent developments. Low NOx industrial burners have been developed utilizing these NOx control techniques

OPERATING PARAMETERS AFFECTING NOx

 Thermal NOx is the major source of NOx from the combustion of gaseous fuels. The parameters that influence the oxygen concentration in the flame zone or the temperatures achieved in the flame zone will affect thermal NOx emissions. The most important parameters are:

• Excess Air

• Fuel Composition

• Air Preheat Temperature

• Furnace Temperature

Excess Air

Excess air provides for additional oxygen beyond the stoichiometric air requirement and is generally required to minimize the emissions of CO and unburned hydrocarbons. It accomplishes this, however, by increasing the concentration of oxygen in the flame zone, which tends to increase NOx. Excess air also decreases the overall flame temperature and contributes to a loss in thermal efficiency. Figure 5 shows the effect of excess air, expressed as percent excess oxygen, on NOx emissions.

As the excess air is steadily increased the reduction in NOx due to the reduction in flame temperature finally overcomes the increase in NOx due to oxygen concentration and the NOx emissions peak. Further increases in excess air then reduce NOx emissions.

Fuel Composition

Fuel composition influences thermal NOx because of its direct effect on flame temperature. Different fuels are capable of achieving different flame temperatures and the maximum potential flame temperature for a fuel is best defined by the adiabatic flame temperature. The adiabatic flame temperature is the theoretical temperature attained when a fuel/air mixture is burned to completion and all of the sensible and chemical energy of the reactants is transferred to the products of combustion.

The variation in flame temperature with composition is apparent from the table, ranging from 3334oF for methane to 3652oF for hydrogen. Although practical flames transfer heat away from the flame zone, the adiabatic flame temperature provides a good method for evaluating the potential effect of fuel gas composition on flame temperatures and, therefore, the potential effect on thermal NOx emissions.

Air Preheat Temperature

Air preheat affects thermal NOx by its direct effect on flame temperature. Preheating the combustion air adds sensible heat to the flame reactants which increases the heat in the products of combustion and, thus, increases the flame temperature. Note that the NOx essentially follows an exponential increase with increasing air preheat temperature. A reasonably good rule of thumb from industrial experience is that the thermal NOx emissions will double as the combustion air temperature is increased from ambient to about 500 to 600 oF.

Furnace Temperature

Furnace temperature affects thermal NOx emissions by its effect on the rate of heat transfer from the flame and, as a result, it influences the actual temperatures attained within the flame zone. The lower the furnace temperature, the higher the heat transfer rate from the flame and the  lower the actual peak flame temperatures within the flame zone. Lower peak flame temperatures mean lower thermal NOx emissions.

CO2 emissions through time 1950 - 2022

The global CO2 emissions continuously increased over time and on e of the significant contribution is from China due to intense manufacturing. 

Every effort should be in place to reduce the same.