Combustion Products and Emission Controls

CHE 450

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

Combustion Pollutants

• Particulate Matter (PM)• SOx• NOx• CO-usually not an issue with large, well-

controlled power plants• CO2

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

PM

– Gravity Settler/Cyclone– Fabric Filter/Baghouse– Electrostatic Precipitator– Wet scrubbing

Fabric Filter

• Shaker

Fabric Filter

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

ESP

/1 wA Qe

collection efficiency

w drift velocity

A area of each plate

Q volumetric flowrate

/

,

/

adjustable constant

1 c

e

e c

kP Q

Effective drift velocity w

w kP A

k

e

ESP• Advantages

– High Efficiency– Low pressure drop– Low operating costs

• Disadvantages– High capital costs– Large – Not flexible to

changes in operating conditions

Sulfur

• SO2 (and some SO3) is emitted

• SO2 oxidized to SO3 in atmosphere

• SO3 + H2O = H2SO4 = ACID RAIN

Sulfur

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

-

• CaCO3(s)+2HSO3-+2Ca2+ =2CaSO3+CO2+H2O

SOx Scrubbing

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

Nitrogen

• O +N2 →NO+N

• N+O2 →NO+O

• N2+O2 →2NO

• K~10-3 @2500K

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

SCR

Carbon Capture and Storage

Capture

• Use temperature swing to absorb/desorb2

+ -2 3Amine(A) + CO AH , HCO , ACO

Storage

• Reduces delivered electricity by at least 25%

The Thermodynamic System

The System

Let U be the

iU PE KE E= + =åInternal Energy

Contains some energy, E

The Surroundings(i.e., the rest of the Universe…)

First Law – Conservation of Energy“energy can be converted from one form to another

but cannot be created nor destroyed”

2 1E E Q W

1 2

Q

W

“Enthalpy” - HEnthalpy 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

H U PV

PV and workIn general, work is given by PV

=

= ´ =

-2 3

-2 3

Work in ; Pressure in , Volume in J Nm Nm m

PV Nm m Nm

P

V

1

2

B

A

Question: is work done by path A same as by path B?

d=dW P VWork done depends on path taken!

d signifies an exact differential signifies an inexact differential

Second Law – Quality of Energy Decreases“energy flows from a higher potential (higher quality)

to lower potential”

Q

W

High T Low T

Q

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

“Entropy” - SEntropy 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

dæ ö÷ç= ÷ç ÷çè ørev

QdS

T

Reversible Systems

For a reversible process involving heat transfer at T0,

dæ ö÷ç= ÷ç ÷çè ørev

QdS

T

dæ ö÷çÞ D = - = ÷ç ÷çè øò2

2 11

rev

QS S S

T

D =0

revQS

TÛ = DrevQ T S

Irreversible SystemsClausius inequality:

d£ò 0Ñ

Q

T

d dæ ö÷ç+ £÷ç ÷çè øò ò2 1

1 2int rev

0Q Q

T T

T

S

1

2

dæ ö÷çÛ D ³ ÷ç ÷çè øò1

2int rev

QS

TÛ D ³ 0S

Heat Engine

• Want colder Tcold and hotterThot

1

0

hot cold

hot cold cold

hot hot hot

hot cold

hot cold

W Q Q

Q Q TW

Q Q T

Q QS

T T

Carnot Cycle

Isothermal Expansion

Isentropic Expansion

Isothermal Compression

Isentropic Compression

http://en.wikipedia.org/wiki/Carnot_cycle

Brayton Cycle

http://commons.wikimedia.org/wiki/File:Brayton_cycle.svg