Combustion Products and Emission Controls

1. Combustion Products and Emission Controls CHE 450

2. Combustion • CxHy+(x+0.25y)*(O2+3.76N2)=xCO2+0.5yH2O+3.76(x+0.25y)N2 • Ex: CH4 + 2(O2 + 3.76N2) = CO2 + 2H2O + 7.52 N2

3. Combustion Pollutants • Particulate Matter (PM) • SOx • NOx • CO-usually not an issue with large, well-controlled power plants • CO2

5. PM • Particulate Matter-very small diameter solids or liquids suspending in gases. (ash/soot/gas conversion). • PM 10, PM 2.5 • >10 um-nasal passage/mucus • <0.1 um-bronchial tree/mucus • 0.1<10 um-respiratory problems

6. PM • Gravity Settler/Cyclone • Fabric Filter/Baghouse • Electrostatic Precipitator • Wet scrubbing

7. Fabric Filter • Shaker

8. Fabric Filter

9. ESP • Use electric field to attract PM • “Negative Corona” • Strong E-field generates high energy electrons • Electrons create ionized gas species • Ions are absorbed onto particles • Particles absorb onto plates

10. ESP

11. ESP • Advantages • High Efficiency • Low pressure drop • Low operating costs • Disadvantages • High capital costs • Large • Not flexible to changes in operating conditions

12. Sulfur • SO2 (and some SO3) is emitted • SO2 oxidized to SO3 in atmosphere • SO3 + H2O = H2SO4 = ACID RAIN

13. Sulfur • Pre-combustion or post combustion • Most common: limestone scrubbing • CaCO3(s)+H2O+2SO2= Ca2+ +CO2+ 2HSO3- • CaCO3(s)+2HSO3-+2Ca2+ =2CaSO3+CO2+H2O

14. SOx Scrubbing

15. Nitrogen • High Temps: NO, NO2 formed • 95% of stationary source NOx is NO • Thermal NOx, Fuel NOx • Brownish color • In combination with VOCs, form O3

16. Nitrogen • O +N2 →NO+N • N+O2 →NO+O • N2+O2 →2NO • K~10-3 @2500K

17. Nitrogen • SCR=Selective Catalytic Reduction • 300-400 ˚C • ~80% effective • 4NO + 4NH3 + O2 → 4N2 + 6H2O • 2NO2 + 4NH3 + O2 → 3N2 + 6H2O • NO + NO2 + 2NH3 → 2N2 + 3H2O

18. SCR

19. Carbon Capture and Storage

20. Capture • Use temperature swing to absorb/desorb

21. Storage • Reduces delivered electricity by at least 25%

23. The Thermodynamic System The Surroundings (i.e., the rest of the Universe…) The System Contains some energy, E

24. First Law – Conservation of Energy “energy can be converted from one form to another but cannot be created nor destroyed” W 1 2 Q

25. “Enthalpy” - H Enthalpy measures total energy of the system. • Internal energy (energy required to create system) • Energy required to make room for it by displacing its environment: function of volume and pressure

26. PV and work In general, work is given by PV Question: is work done by path A same as by path B? 1 B P Work done depends on path taken! A 2 d signifies an exact differential d signifies an inexact differential V

27. Second Law – Quality of Energy Decreases “energy flows from a higher potential (higher quality) to lower potential” Q Low T High T Q W A couple other statements of this: Clausius: No process is possible whose sole result is the transfer of heat from a body of lower temperature to a body of higher temperature Kelvin: No process is possible in which the sole result is the absorption of heat from a reservoir and its complete conversion into work

28. “Entropy” - S Entropy measures “disorder” of the system. (“quality”) For a “reversible process” “reversible process” ≡ process that can be carried out and reversed without leaving traces on the surroundings

29. Reversible Systems For a reversible process involving heat transfer at T0,

30. Irreversible Systems Clausius inequality: 1 T 2 S

31. Heat Engine • Want colder Tcold and hotterThot

32. Carnot Cycle Isothermal Expansion Isentropic Compression Isentropic Expansion Isothermal Compression http://en.wikipedia.org/wiki/Carnot_cycle

33. Brayton Cycle http://commons.wikimedia.org/wiki/File:Brayton_cycle.svg