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Module 6

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Module 6

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  1. Module 6 Nitrogen Oxide Control

  2. 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. 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. MCEN 4131/5131 LearningObjectivesfor 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 LearningObjectives 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. MCEN 4131/5131 LearningObjectives …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 LearningObjectives 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 LearningObjectives 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 LearningObjectives 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 + O2b) NO + 1/2 O2  NO2c) OH + N  NO + H

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

  18. MCEN 4131/5131 LearningObjectives 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. MCEN 4131/5131 LearningObjectives 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. MCEN 4131/5131 LearningObjectives 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 LearningObjectives 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. MCEN 4131/5131 LearningObjectives Cost Effectiveness Sources and effects Thermal vs. Fuel NOx Zeldovich Mechanism Fuel NO Control

  23. 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