Module 6

477 views
330 views

Published on

0 Comments
0 Likes
Statistics
Notes
  • Be the first to comment

  • Be the first to like this

No Downloads
Views
Total views
477
On SlideShare
0
From Embeds
0
Number of Embeds
4
Actions
Shares
0
Downloads
3
Comments
0
Likes
0
Embeds 0
No embeds

No notes for slide

Module 6

  1. 1. Module 6 Nitrogen Oxide Control
  2. 2. Preliminaries <ul><li>Quiz today - 10 exam points </li></ul><ul><li>Plant tour - sign up sheet coming around, only 30 spaces available </li></ul><ul><li>Reminder - office hours Wed I will work sample problems, yesterday I worked 2 plus ideal gas law examples </li></ul><ul><li>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) </li></ul>
  3. 3. Module 5 Educational Objectives <ul><li>Sources and effects of SO2 </li></ul><ul><li>Fuel desulfurization (p. 461) </li></ul><ul><ul><li>Catalytic reaction with hydrogen, for oil and gas products </li></ul></ul><ul><ul><li>For coal must process by washing, crushing (p. 462) </li></ul></ul><ul><li>Flue gas desulfurization </li></ul><ul><ul><li>Throwaway vs. regenerative </li></ul></ul><ul><ul><li>Wet vs. dry </li></ul></ul><ul><ul><li>Lime and limestone scrubbing </li></ul></ul><ul><ul><li>Wellman-Lord process </li></ul></ul>Review
  4. 4. Module 6 Educational Objectives <ul><li>Sources and effects of NOx (p. 493) </li></ul><ul><li>Thermal vs. Fuel NOx formation (p. 494) </li></ul><ul><li>Zeldovich Mechanism (p. 496) </li></ul><ul><li>Fuel NO(p. 503) </li></ul><ul><li>Control </li></ul><ul><ul><li>Combustion (p. 508) </li></ul></ul><ul><ul><li>Flue gas (p. 511) </li></ul></ul>Learning Objectives for Today
  5. 5. Clicker Question? <ul><li>What is the main source of nitrogen oxides in the US? </li></ul><ul><ul><li>vehicles </li></ul></ul><ul><ul><li>large industrial furnaces </li></ul></ul><ul><ul><li>electric utility furnaces </li></ul></ul><ul><ul><li>forest fires, wood-burning </li></ul></ul><ul><li>95% of all NOx from stationary combustion sources is emitted as NO </li></ul><ul><ul><li>This is what we will study in this chapter, mobile emission in next chapter. </li></ul></ul>Learning Objectives Sources and effects Thermal vs. Fuel NOx Zeldovich Mechanism Fuel NO Control
  6. 6. …question? <ul><li>What is thermal NOx? </li></ul><ul><ul><li>NOx formed by reactions between nitrogen and oxygen in the air used for combustion </li></ul></ul><ul><ul><li>NOx formed by the combustion of fuels that contain organic nitrogen </li></ul></ul>Learning Objectives Sources and effects Thermal vs. Fuel NOx Zeldovich Mechanism Fuel NO Control
  7. 7. Thermal NOx formation <ul><li>From chemical reactions that take place at high T when both N 2 and O 2 are present (such as in your car’s engine) </li></ul><ul><li>The formation of thermal NOx is very T sensitive </li></ul><ul><ul><li>small increases in temperature can lead to large increases in NOx </li></ul></ul><ul><li>This is why many NOx control schemes rely on lowering the combustion temperature to reduce the formation of thermal NOx </li></ul>Learning Objectives Sources and effects Thermal vs. Fuel NOx Zeldovich Mechanism Fuel NO Control
  8. 8. Zeldovich Mechanism <ul><li>The chemical reactions that lead to thermal NOx formation are: </li></ul><ul><li>N 2 + O  NO + N (1) </li></ul><ul><li>N + O 2  NO + O (2) </li></ul><ul><li>In the first reaction di-nitrogen is attacked by O to form NO and a nitrogen radical </li></ul><ul><li>The nitrogen radical then attacks O 2 to form another NO and regenerates the oxygen radical </li></ul><ul><li>The overall reaction is given by, </li></ul><ul><li>N 2 + O 2  2(NO) (2b) </li></ul>Learning Objectives Sources and effects Thermal vs. Fuel NOx Zeldovich Mechanism Fuel NO Control
  9. 9. Quiz Question? <ul><li>There is another chemical reaction involved in the Zeldovich mechanism. Identify this reaction from the 3 below: </li></ul><ul><li>a) O + NO 2  NO + O 2 b) NO + 1/2 O 2  NO 2 c) OH + N  NO + H </li></ul>Learning Objectives Sources and effects Thermal vs. Fuel NOx Zeldovich Mechanism Fuel NO Control
  10. 10. The equation is … <ul><li>NO can also be formed in the flame by the following reaction </li></ul><ul><li>N + OH  NO + H (3) </li></ul><ul><li>In addition to the formation of NO by the Zeldovich mechanism, the equation for NO 2 formation is NO + 1/2O 2  NO 2 </li></ul><ul><li>Based on equilibrium in the flame zone there should be very high NOx concentrations </li></ul><ul><li>Based on equilibrium in the flue gas there should be very low NOx concentrations and there is much more NO 2 </li></ul><ul><li>IN REALITY, flue gas concentrations are high and there is more NO than expected at equilibrium -- WHY??? </li></ul>Learning Objectives Sources and effects Thermal vs. Fuel NOx Zeldovich Mechanism Fuel NO Control
  11. 11. Chemical Equilibrium <ul><li>Many chemical reactions do not go to 100% completion </li></ul><ul><li>An equilibrium depends on temperature that is established between reactants, products </li></ul><ul><li>Described by equilibrium constant (p. 318) </li></ul>x R + y S -> a P + b Q Learning Objectives Sources and effects Thermal vs. Fuel NOx Zeldovich Mechanism Fuel NO Control
  12. 12. NO formation kinetics <ul><li>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 </li></ul>Learning Objectives Sources and effects Thermal vs. Fuel NOx Zeldovich Mechanism Fuel NO Control
  13. 13. Reaction Rate <ul><li>The RATE of a reaction is the SPEED at which a reaction happens (p. 314) </li></ul><ul><li>If a reaction has a low rate that means the molecules combine at a slower speed than a reaction with a high rate. </li></ul>RATE = SPEED x R + y S -> a P + b Q Rate of generation of product P? Learning Objectives Sources and effects Thermal vs. Fuel NOx Zeldovich Mechanism Fuel NO Control
  14. 14. Reaction rate <ul><li>K is reaction rate constant (temperature dependent </li></ul><ul><li>C R , C S are concentrations or reactants, mol/L </li></ul><ul><li>x,y are exponents that often relate to stoichiometry </li></ul>x R + y S -> a P + b Q Learning Objectives Sources and effects Thermal vs. Fuel NOx Zeldovich Mechanism Fuel NO Control
  15. 15. NO formation depends on Time and Temperature <ul><li>The higher the flame temperature, the more NO is formed </li></ul><ul><li>The longer the gases are in the combustion zone, the more NO is formed </li></ul><ul><li>more oxygen in high T zone, more NO formed (eq 16.13) </li></ul><ul><li>Interestingly, higher NO concentrations in fuel rich flame zone have been observed than could have been formed by Zeldovich </li></ul><ul><ul><li>QUIZ Q: What is this called? </li></ul></ul><ul><ul><ul><li>Fuel rich NO </li></ul></ul></ul><ul><ul><ul><li>Flame NO </li></ul></ul></ul><ul><ul><ul><li>Prompt NO </li></ul></ul></ul><ul><ul><ul><li>Don’t know </li></ul></ul></ul>Learning Objectives Sources and effects Thermal vs. Fuel NOx Zeldovich Mechanism Fuel NO Control
  16. 16. Fuel NO <ul><li>If a fuel has some organically bound N, then NO is produced from the oxidation of this N </li></ul><ul><li>Not all of the organic N is converted to NO </li></ul><ul><ul><li>The conversion is very sensitive to the ratio of fuel to air </li></ul></ul><ul><ul><li>If the system is fuel rich, it is (chemically) a reducing environment and this tends to push the fuel nitrogen to form either N 2 or NH 3 </li></ul></ul><ul><ul><li>If the system is fuel lean, it is an oxidizing environment and more NO is formed </li></ul></ul><ul><li>In general, fuel NO is not very temperature sensitive. </li></ul>Learning Objectives Sources and effects Thermal vs. Fuel NOx Zeldovich Mechanism Fuel NO Control
  17. 17. Fuel-Nitrogen Chemistry Fast reactions Learning Objectives Sources and effects Thermal vs. Fuel NOx Zeldovich Mechanism Fuel NO Control
  18. 18. Control <ul><li>Temperature : Higher temperatures lead to higher equilibrium concentrations of NO </li></ul><ul><li>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). </li></ul><ul><li>Oxygen concentration : more O 2 means more NO </li></ul>Learning Objectives Sources and effects Thermal vs. Fuel NOx Zeldovich Mechanism Fuel NO Control
  19. 19. Flue Gas Recirculation combustor Portion of exhaust gas recirculated useful work Cooled exhaust Fuel + air GOAL: Reduce Temperature Expensive to retrofit Learning Objectives Sources and effects Thermal vs. Fuel NOx Zeldovich Mechanism Fuel NO Control
  20. 20. Off-Stochiometric Combustion GOAL: Reduce temperature and lower oxygen Fuel lean Learning Objectives Sources and effects Thermal vs. Fuel NOx Zeldovich Mechanism Fuel NO Control
  21. 21. Question? <ul><li>In selective catalytic reduction, what is the most commonly used gas to reduce NO to N2? </li></ul><ul><ul><ul><ul><li>a. Ammonia </li></ul></ul></ul></ul><ul><ul><ul><ul><li>b. methane </li></ul></ul></ul></ul><ul><ul><ul><ul><li>C. carbon dioxide </li></ul></ul></ul></ul>Learning Objectives Sources and effects Thermal vs. Fuel NOx Zeldovich Mechanism Fuel NO Control
  22. 22. Cost Effectiveness Learning Objectives Sources and effects Thermal vs. Fuel NOx Zeldovich Mechanism Fuel NO Control
  23. 23. Questions of the Davis book <ul><li>In 1921, what percentage of people owned a car? </li></ul><ul><ul><li>10% </li></ul></ul><ul><ul><li>50% </li></ul></ul><ul><ul><li>95% </li></ul></ul><ul><li>What did Standard Oil’s president call “essential in our civilization…a gift of god.” </li></ul><ul><ul><li>Trichloroethylene </li></ul></ul><ul><ul><li>Sulfur dioxide </li></ul></ul><ul><ul><li>Tetraethyl lead </li></ul></ul>
  24. 24. Question <ul><li>When towns in poor black towns in the south became so polluted from industry leaks, emissions, etc what was done? </li></ul><ul><ul><li>Clean up the contaminated site </li></ul></ul><ul><ul><li>Move residents to a new location </li></ul></ul><ul><ul><li>Paid people that got sick </li></ul></ul>
  25. 25. No free lunch <ul><li>Is it better to err on the side of protecting public health, or on the side of promoting industrial growth? </li></ul><ul><ul><li>Public health </li></ul></ul><ul><ul><li>Industrial growth </li></ul></ul>

×