Module 6

Module 6Module 6

Nitrogen Oxide Control

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PreliminariesPreliminaries• 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)

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Module 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

Review

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Module 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 Today

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Clicker 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.

LearningObjectives

Sources and effectsThermal vs. Fuel NOxZeldovich MechanismFuel NOControl

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……question?question?

• What is thermal NOx?a. NOx formed by reactions

between nitrogen and oxygen in the air used for combustion

b. NOx formed by the combustion of fuels that contain organic nitrogen

LearningObjectives


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Thermal 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 NOx

LearningObjectives


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Zeldovich 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)

LearningObjectives


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Quiz Question?Quiz Question?

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

LearningObjectives


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The 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???

LearningObjectives


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Chemical 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 =C

P

a CQ

b

CRx C

S

y

xR + yS -> aP + bQ

LearningObjectives


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NO 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 formation

LearningObjectives


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Reaction 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 = SPEED

xR + yS -> aP + bQRate of generation of product P?

LearningObjectives


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Reaction 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 = kCR

x CS

y

xR + yS -> aP + bQ

LearningObjectives


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NO 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 know

LearningObjectives


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Fuel 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

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.

LearningObjectives


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Fuel-Nitrogen Fuel-Nitrogen ChemistryChemistry

Fast reactions

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ControlControl

• 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

LearningObjectives


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Flue Gas RecirculationFlue Gas Recirculation

combustor

Portion of exhaust gas recirculated

useful workCooled exhaust

Fuel + air

GOAL: Reduce Temperature

Expensive to retrofit

LearningObjectives


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Off-Stochiometric Off-Stochiometric CombustionCombustion

GOAL: Reduce temperature and lower oxygen

Fuel lean

LearningObjectives


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Question?Question?

• In selective catalytic reduction, what is the most commonly used gas to reduce NO to N2?

– a. Ammonia– b. methane– C. carbon dioxide

LearningObjectives


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Cost EffectivenessCost Effectiveness

LearningObjectives


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Questions of the Questions of the Davis bookDavis 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

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QuestionQuestion

• When towns in poor black towns in the south became so polluted from industry leaks, emissions, etc what was done?

a. Clean up the contaminated siteb. Move residents to a new

locationc. Paid people that got sick

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No free lunchNo free lunch

• Is it better to err on the side of protecting public health, or on the side of promoting industrial growth?

a. Public healthb. Industrial growth

Module 6

Documents

fuel nox formation

effects of nox

thermal nox formation

combustion nox

effects thermal

fuel nop

nox control schemes

high nox concentrations