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CU-Boulder MCEN 4131 - Nitrogen Oxide Control

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Module 6PreliminariesModule 5 Educational ObjectivesModule 6 Educational ObjectivesClicker Question?…question?Thermal NOx formationZeldovich MechanismQuiz Question?The equation is …Chemical EquilibriumNO formation kineticsReaction RateReaction rateNO formation depends on Time and TemperatureFuel NOFuel-Nitrogen ChemistryControlFlue Gas RecirculationOff-Stochiometric CombustionQuestion?Cost EffectivenessQuestions of the Davis bookQuestionNo free lunchModule 6Module 6Nitrogen Oxide ControlMCEN 4131/51312PreliminariesPreliminaries•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)MCEN 4131/51313Module 5 Educational Module 5 Educational ObjectivesObjectives•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 ReviewMCEN 4131/51314Module 6 Educational Module 6 Educational ObjectivesObjectives•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)LearningObjectivesfor TodayMCEN 4131/51315Clicker Question?Clicker Question?•What is the main source of nitrogen oxides in the US? a. vehiclesb. large industrial furnacesc. electric utility furnacesd. 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.LearningObjectivesSources and effectsThermal vs. Fuel NOxZeldovich MechanismFuel NOControlMCEN 4131/51316……question?question?•What is thermal NOx?a. NOx formed by reactions between nitrogen and oxygen in the air used for combustionb. NOx formed by the combustion of fuels that contain organic nitrogenLearningObjectivesSources and effectsThermal vs. Fuel NOxZeldovich MechanismFuel NOControlMCEN 4131/51317Thermal NOx Thermal NOx formationformation•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 NOxLearningObjectivesSources and effectsThermal vs. Fuel NOxZeldovich MechanismFuel NOControlMCEN 4131/51318Zeldovich MechanismZeldovich Mechanism•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)LearningObjectivesSources and effectsThermal vs. Fuel NOxZeldovich MechanismFuel NOControlMCEN 4131/51319Quiz Question?Quiz Question?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 + HLearningObjectivesSources and effectsThermal vs. Fuel NOxZeldovich MechanismFuel NOControlMCEN 4131/513110The equation is …The equation is …•NO can also be formed in the flame by the following reactionN + 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???LearningObjectivesSources and effectsThermal vs. Fuel NOxZeldovich MechanismFuel NOControlMCEN 4131/513111Chemical EquilibriumChemical Equilibrium•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)€ Kp=CPaCQbCRxCSyxR + yS -> aP + bQLearningObjectivesSources and effectsThermal vs. Fuel NOxZeldovich MechanismFuel NOControlMCEN 4131/513112NO formation kineticsNO formation kinetics•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 formationLearningObjectivesSources and effectsThermal vs. Fuel NOxZeldovich MechanismFuel NOControlMCEN 4131/513113Reaction RateReaction Rate• 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.RATE = SPEEDxR + yS -> aP + bQRate of generation of product P?LearningObjectivesSources and effectsThermal vs. Fuel NOxZeldovich MechanismFuel NOControlMCEN 4131/513114Reaction rateReaction rate•K is reaction rate constant (temperature dependent•CR, CS are concentrations or reactants, mol/L•x,y are exponents that often relate to stoichiometry€ rp= kCRxCSyxR + yS -> aP + bQLearningObjectivesSources and effectsThermal vs. Fuel NOxZeldovich MechanismFuel NOControlMCEN 4131/513115NO formation NO formation depends on Time and depends on Time and TemperatureTemperature•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?a.Fuel rich NOb.Flame NOc.Prompt NOd.Don’t knowLearningObjectivesSources and effectsThermal vs. Fuel NOxZeldovich MechanismFuel NOControlMCEN 4131/513116Fuel NOFuel NO•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


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CU-Boulder MCEN 4131 - Nitrogen Oxide Control

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