chemistry

1. 5. Experimentation:
5.1 Combustion:
We chose post Combustion prior to pre combustion and oxy-fuel combustion as it has:
Relatively good temperature control (due to N2 as coolant)
Reduction of NOx with staging
Post-combustion capture consists of treating exhaust gases on the output side. Technologies
based on chemical absorption appear to be best adapted to this separation. Other
technologies, adsorption, membranes and cryogen, are even less suitable for post-combustion
capture than for pre-combustion capture, mainly for the following two reasons:
1. a much lower partial pressure of CO2 in post-combustion exhaust gases than in synthetic
gas originating from a gasifier or a reformer
2. the presence of larger quantities of dust, impurities such as SOx and NOx and incondensable
gases, particularly oxygen (which do not exist or exist only in very small quantities in
synthetic gas)
The primary advantage of using this process is to reduce the amount of NOx in the exhaust
gases which react with oxygen and unburned hydrocarbons in the upper atmosphere to
produce smog, a dangerous atmospheric pollutant.

2. 5.2 HowNOx emissions are reduced?
Reduction of NOx is brought about by lowering the
temperature during combustion. The activation energy for the formation of nitrogen oxides is
very high. The result is that NOx is formed at very high temperatures. As temperatures reduce,
the NOx is “locked” in and cannot revert to elemental nitrogen and oxygen. This can be
achieved by use of a combustion catalyst to change the rate of combustion. This proven
technology reduces the time the systemis at combustion temperature.
Two approaches are being followed to reduce NOx formation:
I. The first approach is to reduce temperature by slowing the combustion reaction by a small
amount. An effective reduction of 400 oC will reduce nitric oxide formation by 50%.
II. The second approach is to modify burners to extend combustion and reduce residence time
at high temperatures. Fuel Induced Recirculating (FIR) burners have been used in boilers to
reduce NOx formation significantly.
The combination of these two approaches reduces the NOx formation upto 90%.
5.3 Types of NOx:
There are six types of NOx:
I. Nitric Oxide (NO)
II. Nitrogen dioxide (NO2)

3. III. Nitrous oxide (N2O)
IV. Di nitrogen trioxide (N2O3)
V. Di nitrogen tetra-oxide (N2O4)
VI. Di nitrogen penta-oxide (N2O5)
The latter three oxides are unstable and convert to NO or NO2. NO and NO2 are highly
toxic. Human limits in air are presented below with comparison to some other well-known toxic
materials.1
Toxic agent ppm
NO 25
NO2 5
CO 50
HCN 10 (skin)
1
R. C. Weast, ed., Handbook of Chemistry and Physics, CRC Press,Inc., Boco Raton, FL, 1984.
5.4 Mechanism ofNOx formationfrom Coal:
Although the complete mechanism is not fully understood, there are two primary paths of
formation. The first involves the oxidation of volatile nitrogen species during the initial stages of
combustion. During the release and prior to the oxidation of the volatiles, nitrogen reacts to
form several intermediaries which are then oxidized into NO. If the volatiles evolve into a
reducing atmosphere, the nitrogen evolved can be readily made to form nitrogen gas rather
than NOx. The second path involves the combustion of nitrogen contained in the char matrix
during the combustion of the char portion of the fuel. This reaction occurs much more slowly
than the volatile phase. Only around 20% of the char nitrogen is ultimately emitted as NOx,

4. since much of the NOx that forms during this process is reduced to nitrogen by the char, which
is nearly pure carbon.
5.5 Operating parameters affecting NOx formation:
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:
I. Excess air.
II. Fuel composition.
III. Furnace temperature –Air preheat temperature.
5.5.1 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
increases NOx.
Excess air also decreases the overall flame temperature and contributes to a loss in thermal
efficiency. Figure 7 shows the effect of excess air, expressed as percent excess oxygen, on NOx
emissions.

5. 2Fig. 1 NOx vs. Excess Oxygen for a Typical Diffusion Flame Burner
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.
5.5.2 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.
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6. Table 2 provides a list of the calculated adiabatic flame temperatures for a group of fuel gases.
The variation in flame temperature with composition is apparent from the table, ranging from
3334°F for methane to 3652°F 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.
3Table 2 Adiabatic Flame Temperature:
Component Temperature (oC)
Methane 1950
Ethane 1955
Propane 1980
Butane 1970
Hydrogen 2210
5.5.3 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. Figure 8 shows the effect
of air preheat temperature on NOx. Note that the NOx essentially follows an exponential
increase with increasing air preheat temperature. A reasonable good rule of thumb is that the
thermal NOx emissions will double as the combustion air temperature is increased from
ambient to about 500 to 600°F.

7. 3Fig. 2 NOx vs. Combustion Air Temperature
5.5.4 Furnace temperature:
The effect of furnace temperature on NOx is shown in Figure 9. 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.
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8. 4Fig. 3 NOx VS. Furnace Temperature
5.6 NOxControl:
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.
5.6.1 Staged Combustion:
NOx emissions can be reduced by introducing either the combustion air or the fuel into the
flame in stages. With air staging, a portion of the combustion air, typically about 60% to 75%, is
supplied to a primary combustion zone with all of the fuel. This produces a fuel rich flame zone.
NOx emissions in this flame zone are reduced due to the sub-stoichiometric combustion
conditions. The remainder of the air is injected downstream, forming a secondary flame zone

9. where combustion is completed. NOx formation in this secondary flame zone is reduced
because the inert byproducts from the primary flame zone reduce flame temperatures.
Fuel staging is the reverse of air staging. Generally 30 to 50% of the fuel is injected into the
combustion air to form a lean primary flame zone. Although excess oxygen is available, NOx is
minimized by the low flame temperatures that are generated due to the lean combustion
conditions. The remainder of the fuel is then injected downstream forming a secondary flame
zone where combustion is completed. NOx formation rates in this zone are low because the
inert from the primary flame zone lower the flame temperatures and local oxygen
concentrations.
Figure 10 shows a comparison of NOx emissions from a conventional, non‐low NOx burner, a
staged air burner, and a staged fuel burner under similar operating conditions. This data is for
burners firing natural gas with 15% excess air in a 1600°F firebox.
5Fig. 4 Comparison of Conventional and Low NOx Burners
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