1. Nitrogen Oxide Control
Module 6
Nitrogen Oxide Control

2. Preliminaries Quiz today - 10 exam points
MCEN 4131/5131
Preliminaries
Quiz today - 10 exam points
Plant tour - sign up sheet coming around, only 30 spaces available
Reminder - office hours Wed I will work sample problems, yesterday I worked 2 plus ideal gas law examples
Conference on World Affairs - Apr 10 to 14 - attend a session and write a paragraph for 5 HW points (can turn in up to 2)

3. Module 5 Educational Objectives
MCEN 4131/5131
Review
Module 5 Educational Objectives
Sources and effects of SO2
Fuel desulfurization (p. 461)
Catalytic reaction with hydrogen, for oil and gas products
For coal must process by washing, crushing (p. 462)
Flue gas desulfurization
Throwaway vs. regenerative
Wet vs. dry
Lime and limestone scrubbing
Wellman-Lord process

4. Module 6 Educational Objectives
MCEN 4131/5131
Learning Objectives for Today
Module 6 Educational Objectives
Sources and effects of NOx (p. 493)
Thermal vs. Fuel NOx formation (p. 494)
Zeldovich Mechanism (p. 496)
Fuel NO(p. 503)
Control
Combustion (p. 508)
Flue gas (p. 511)

5. MCEN 4131/5131
Learning Objectives
Clicker Question?
Sources and effects
Thermal vs. Fuel NOx
Zeldovich Mechanism
Fuel NO
Control
What is the main source of nitrogen oxides in the US?
vehicles
large industrial furnaces
electric utility furnaces
forest fires, wood-burning
95% of all NOx from stationary combustion sources is emitted as NO
This is what we will study in this chapter, mobile emission in next chapter.

6. …question? What is thermal NOx?
MCEN 4131/5131
Learning Objectives
…question?
Sources and effects
Thermal vs. Fuel NOx
Zeldovich Mechanism
Fuel NO
Control
What is thermal NOx?
NOx formed by reactions between nitrogen and oxygen in the air used for combustion
NOx formed by the combustion of fuels that contain organic nitrogen

7. MCEN 4131/5131
Learning Objectives
Thermal NOx formation
Sources and effects
Thermal vs. Fuel NOx
Zeldovich Mechanism
Fuel NO
Control
From chemical reactions that take place at high T when both N2 and O2 are present (such as in your car’s engine)
The formation of thermal NOx is very T sensitive
small increases in temperature can lead to large increases in NOx
This is why many NOx control schemes rely on lowering the combustion temperature to reduce the formation of thermal NOx

8. MCEN 4131/5131
Learning Objectives
Zeldovich Mechanism
Sources and effects
Thermal vs. Fuel NOx
Zeldovich Mechanism
Fuel NO
Control
The chemical reactions that lead to thermal NOx formation are:
N2 + O  NO + N (1)
N + O2  NO + O (2)
In the first reaction di-nitrogen is attacked by O to form NO and a nitrogen radical
The nitrogen radical then attacks O2 to form another NO and regenerates the oxygen radical
The overall reaction is given by,
N2 + O2  2(NO) (2b)

9. MCEN 4131/5131
Learning Objectives
Quiz Question?
Sources and effects
Thermal vs. Fuel NOx
Zeldovich Mechanism
Fuel NO
Control
There is another chemical reaction involved in the Zeldovich mechanism. Identify this reaction from the 3 below:
a) O + NO2  NO + O2 b) NO + 1/2 O2  NO2 c) OH + N  NO + H

10. MCEN 4131/5131
Learning Objectives
The equation is …
Sources and effects
Thermal vs. Fuel NOx
Zeldovich Mechanism
Fuel NO
Control
NO can also be formed in the flame by the following reaction
N + OH  NO + H (3)
In addition to the formation of NO by the Zeldovich mechanism, the equation for NO2 formation is NO + 1/2O2  NO2
Based on equilibrium in the flame zone there should be very high NOx concentrations
Based on equilibrium in the flue gas there should be very low NOx concentrations and there is much more NO2
IN REALITY, flue gas concentrations are high and there is more NO than expected at equilibrium -- WHY???

11. Chemical Equilibrium xR + yS -> aP + bQ
MCEN 4131/5131
Learning Objectives
Chemical Equilibrium
Sources and effects
Thermal vs. Fuel NOx
Zeldovich Mechanism
Fuel NO
Control
Many chemical reactions do not go to 100% completion
An equilibrium depends on temperature that is established between reactants, products
Described by equilibrium constant (p. 318)
xR + yS -> aP + bQ

12. MCEN 4131/5131
Learning Objectives
NO formation kinetics
Sources and effects
Thermal vs. Fuel NOx
Zeldovich Mechanism
Fuel NO
Control
during a significant period of time, the system is not at equilibrium, and we need to use the rate equations to obtain a more accurate value of NO formation

13. RATE = SPEED Reaction Rate
MCEN 4131/5131
Learning Objectives
Reaction Rate
Sources and effects
Thermal vs. Fuel NOx
Zeldovich Mechanism
Fuel NO
Control
RATE = SPEED
The RATE of a reaction is the SPEED at which a reaction happens (p. 314)
If a reaction has a low rate that means the molecules combine at a slower speed than a reaction with a high rate.
xR + yS -> aP + bQ
Rate of generation of product P?

14. Reaction rate xR + yS -> aP + bQ
MCEN 4131/5131
Learning Objectives
Reaction rate
Sources and effects
Thermal vs. Fuel NOx
Zeldovich Mechanism
Fuel NO
Control
xR + yS -> aP + bQ
K is reaction rate constant (temperature dependent
CR, CS are concentrations or reactants, mol/L
x,y are exponents that often relate to stoichiometry

15. NO formation depends on Time and Temperature
MCEN 4131/5131
Learning Objectives
NO formation depends on Time and Temperature
Sources and effects
Thermal vs. Fuel NOx
Zeldovich Mechanism
Fuel NO
Control
The higher the flame temperature, the more NO is formed
The longer the gases are in the combustion zone, the more NO is formed
more oxygen in high T zone, more NO formed (eq 16.13)
Interestingly, higher NO concentrations in fuel rich flame zone have been observed than could have been formed by Zeldovich
QUIZ Q: What is this called?
Fuel rich NO
Flame NO
Prompt NO
Don’t know

16. MCEN 4131/5131
Learning Objectives
Fuel NO
Sources and effects
Thermal vs. Fuel NOx
Zeldovich Mechanism
Fuel NO
Control
If a fuel has some organically bound N, then NO is produced from the oxidation of this N
Not all of the organic N is converted to NO
The conversion is very sensitive to the ratio of fuel to air
If the system is fuel rich, it is (chemically) a reducing environment and this tends to push the fuel nitrogen to form either N2 or NH3
If the system is fuel lean, it is an oxidizing environment and more NO is formed
In general, fuel NO is not very temperature sensitive.

17. Fuel-Nitrogen Chemistry
MCEN 4131/5131
Learning Objectives
Fuel-Nitrogen Chemistry
Sources and effects
Thermal vs. Fuel NOx
Zeldovich Mechanism
Fuel NO
Control
Fast reactions

18. MCEN 4131/5131
Learning Objectives
Control
Sources and effects
Thermal vs. Fuel NOx
Zeldovich Mechanism
Fuel NO
Control
Temperature: Higher temperatures lead to higher equilibrium concentrations of NO
Time: The longer the gases spend in the combustion zone, the closer the components are to equilibrium, and the higher the NO concentration (up to the equilibrium limit).
Oxygen concentration: more O2 means more NO

19. Flue Gas Recirculation
MCEN 4131/5131
Learning Objectives
Flue Gas Recirculation
Sources and effects
Thermal vs. Fuel NOx
Zeldovich Mechanism
Fuel NO
Control
GOAL: Reduce Temperature
useful
work
Fuel + air
combustor
Cooled exhaust
Portion of exhaust gas recirculated
Expensive to retrofit

20. Off-Stochiometric Combustion
MCEN 4131/5131
Learning Objectives
Off-Stochiometric Combustion
Sources and effects
Thermal vs. Fuel NOx
Zeldovich Mechanism
Fuel NO
Control
GOAL: Reduce temperature and lower oxygen
Fuel lean

21. MCEN 4131/5131
Learning Objectives
Question?
Sources and effects
Thermal vs. Fuel NOx
Zeldovich Mechanism
Fuel NO
Control
In selective catalytic reduction, what is the most commonly used gas to reduce NO to N2?
a. Ammonia
b. methane
C. carbon dioxide

22. Cost Effectiveness Learning Objectives MCEN 4131/5131
Sources and effects
Thermal vs. Fuel NOx
Zeldovich Mechanism
Fuel NO
Control

23. Questions of the Davis book
MCEN 4131/5131
Questions of the Davis book
In 1921, what percentage of people owned a car?
10%
50%
95%
What did Standard Oil’s president call “essential in our civilization…a gift of god.”
Trichloroethylene
Sulfur dioxide
Tetraethyl lead

24. MCEN 4131/5131
Question
When towns in poor black towns in the south became so polluted from industry leaks, emissions, etc what was done?
Clean up the contaminated site
Move residents to a new location
Paid people that got sick

25. MCEN 4131/5131
No free lunch
Is it better to err on the side of protecting public health, or on the side of promoting industrial growth?
Public health
Industrial growth