Nitrogen Oxide Control Module 6 Nitrogen Oxide Control

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)

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

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)

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.

…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

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

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)

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

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???

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

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

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?

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

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

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.

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

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

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

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

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

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

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

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

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