Air Pollution Control
Technologies
VOC Incinerators
VOCs
• Includes pure hydrocarbons and partially
oxidized compounds (organic acids,
aldehydes, ketones)
• There are hunderds of individual
compounds with each with its own
properties and chracteristics
• Combustion processes, various industrial
operations and solvent evaporation are the
main sources
VOC Incineration
• Vapor incinerators are also called thermal
oxidizers and afterburners
• They can be also used successfully for air
polluted with small particles of solids or
liquids.
• Incineration can be used for odor control,
to destroy a toxic compound, or to reduce
the quantity of photochemically reactive
VOCs released to the atmosphere
VOC Incineration
• VOC vapors might be in a concentrated
stream (such as emergency relief gases in
a petroleum refinery) or might be a dilute
mixture in air ( such as from a paint-drying
oven)
• In the case of a dilute fume in air two
methods are used
– Direct thermal incineration
– Catalytic oxidation
VOC Incineration
• In some sources (such as printing presses
or parts-painting stations), emissions need
to be captured by hoods and a common
duct system and routed to a thermal
oxidizer (TO)
• Total VOC reduction in this case depends
on both the destruction efficiency of TO
and capture efficiency
• Total efficiency will be equal to destruction
efficiency times capture efficiency
VOC Incineration
• The main disadvantage of the VOC
incineration is the high fuel cost
• Some of the products of combustion of
some VOCs are themselves pollutants
– When a chlorinated HC is burned HCl and Cl2
will be emitted and then additional control for
these pollutants might become neccessary
Alternatives to VOC Incineration
• Biofilter
• Wet scrubber
• Recovery of the vapors (recompression,
condensation, carbon adsroption or liquid
absorption)
If very stringent control objective exists, only
incineration can achieve that
e.g. Reduction in concentration from 10,000
ppm to 5 ppm for some malodorous organic
sulfur compounds.
Oxidation Chemistry
• Consider only the case of a premixed
dilute stream of a pure HC in air
• CxHy +
(b)O2+3.76(b)N2xCO2+(y/2)H2O+3.76(b)
N2
CxHy the general formula for any HC
b= x+(y/4)
3.76=the number of moles of N2 present in
air for every mole of O2
Oxidation Chemistry
• (b)O2+3.76(b)N2xCO2+(y/2)H2O+3.76(b)
N2
The actual detailed mechanisms of
combustion are complex and do not occur
in a single step as might be inferred from
above eqn.
The mechanism involves a branching
chain reaction and further complicated by
the larger number of possible
intermediates for higher order HCs.
Oxidation Chemistry
• (b)O2+3.76(b)N2xCO2+(y/2)H2O+3.76(b)
N2
The actual detailed mechanisms of
combustion are complex and do not occur
in a single step as might be inferred from
above eqn.
The mechanism involves a branching
chain reaction and further complicated by
the larger number of possible
intermediates for higher order HCs.
Major Reactions in CH4 Oxidation
CH4 +O2CH3·+HO2 Chain initation
CH3·+O2CH2O·+OH
CH4+OHCH3 ·+ H2O Chain propagation
CH2O+OHHCO ·+ H2O
CH2+O2HO2 ·+ HCO · Chain branching
HCO+O2CO+HO2 ·
Chain propagation
HO2+CH4H2O2+CH3 ·
HO2+CH2OH2O2+HCO ·
OH · wall
Chain termination
CH2O · wall
Developing Global Models
• Instead, global models ignores many of
the detailed steps and ties the kinetics to
the main stable reactants and products
• Since CO is a very stable intermediate, the
simplest two step model for HC oxidation
is:
CxHy+(x/2+y/4)O2xCO+(y/2)H2O 11.2
xCO+(x/2)O2xCO2
11.3
Global Model Kinetics
• Using Eqs 11.2 and 11.3 a global kinetic
model that is of the first order in each
reactant results in the rate equations:
rHC=-k1[HC][ O2]
rCO=xk1[HC][ O2] – k2[CO][ O2] and in the
presence of excess O2:
rHC=-k1[HC]
rCO=xk1[HC]– k2[CO]
A third equation for CO2:
rCO2=-k2[CO]
Global Model Kinetics
• Those 3 equations represent a special case
of a general set of consecutive first order
irreversible reactions:
k1
k2
ARS
Levenspiel (1962) presented solutions for the
concentration of all components as a
function of dimensionless reaction time for
various k2/k1 ratios.
The Three T’s
• Temperature
• Time
• Turbulence
For good destruction afterburners should be
designed for temperatures 650-1100 C, for
residence times of 0.2-2 sec, and for flow
velocities of 20-40 ft/sec
Afterburner is now a standart as part of a hazardous
waste incinerators requiring %99.99 destruction
and removal efficiency of the Principal Organic
Hazardous constituents
The Three T’s
• In mathematical sense 3Ts are related to 3
characteristic times:
tC=1/k
Chemical time
tr=V/Q=L/u
Residence time
tm=L2/De
Mixing time
V: Volume of the reaction zone, m3
Q: Flow rate (at T in the afterburner) , m3/s
L: Length of the reaction zone, m
u: gas velocity, m/s
De: effective (turbulent) diffusion coef. m2/s
The Three T’s
Peclet number:
Pe= Mixing time/residence time
Damkohler number:
Da=Residence time/chemical time
If Pe is large and Da is smallmixing is the
rate controlling process in the afterburner
If Pe is small and Da is largechemical
kinetics is the rate controlling process
Predicting VOC Kinetics
• Kinetics are surely important to the proper
desing of an afterburner but kinetic data
are scarce and difficult and costly to obtain
by pilot studies
• So past methods of determining the
desing or operating temperature of an
incinerator were very rough at best
Predicting VOC Kinetics
1. Ross’s Approach
• Ross (1997) summarized the older methods by suggesting Tdesign be set
several hunders degrees (F) above the VOC autoignition temperature
Autoignition T: the temperature at which cobmustible mixtures of the VOC in air
will ignite without an external source
Substance
Acetone
Autoignition T, Substance
(F)
1000
Acrolein
Autoignition T,
(F)
453
Ethanol
799
Hydrogen
Sulfide
500
Cyclohexane
514
Styrene
915
Phenol
1319
Toluene
1026
•Excessively high design Ts will result in very high estimated costs for purchasing
and operating a VOC incinerator
2. Lee and coworkers’ Approach
• A purely statistical model to predict the Ts required to give various levels
of destruction in an isothermal plug flow afterburner based on their
experimental studies
• T99.9= 594-12.2W1+117W2+71.6W3+80.2W4+0.592W5-20.2W6420.3W7+87.1W8-66.8W9+62.8W10-75.3W11
• T99= 577-10W1+110.2W2+67.1W3+72.6W4+0.586W5-23.4W6430.9W7+85.2W8-82.2W9+65.5W10-76.1W11
• T99.9= T for 99.9% destruction efficiency, F
• W1=number of carbon atoms
• W2= aromatic carbon flag (0=no,1=yes)
• W3= C C double bond flag (0=no,1=yes)
• W4= number of N atoms
• W5= autoignition Temperature, F
• W6= number of O atoms
• W7= number of S atoms
• W8= hydroge/carbon ratio
• W9= allyl (2-propenyl) compound flag (0=no,1=yes)
• W10= carbon-double-bond-chlorine interaction (0=no,1=yes)
• W11= natural log of residence time
Predicting VOC Kinetics
3. Cooper, Alley and Overcamp’s Approach
Combined collision theory with empirical data and proposed a
method to predict an effective first order rate constant k for HC
incineration over the range from 940 to 1140 K.
The method depends on MW and the type of the HC
Once k is found, Tdesign can be obtained
 E / RT
k  Ae
A
A: pre-exponential factor
Z ' Sy O 2 P
R'
Z’: collision rate factor (from Figure 11.5 for alkanes, alkenes, and aromatics)
S=steric factor: to account for the fact that some collisions are not effective in
producing reactions becasue of molecular geometry S=16/MW
yO2: mole fraction of O2 in the afterburner
P: absolute pressure, atm
R’: gas constant, 0.08206 L-atm/mol-K
Predicting VOC Kinetics
3. Cooper, Alley and Overcamp’s Approach
k  Ae
A
 E / RT
Z ' Sy O 2 P
R'
The activation energy E=-0.00966(MW)+46.1 (From figure 11.6)
Now k can be calculated for any desired temperature.
In an isothermal pluf flow reactor (PFR) the HC destruction efficiency,
the rate constant and the residence time are related as:
  1
[ HC ] out
[ HC ] in
 1 e
 kt r
Predicting Overall Kinetics
The destruction of VOC occur quickly relative to CO
destruction
The kinetics of CO destruction have been studied by many
researchers
Howard published the following expression for CO oxidation
(valid for the range of 840-2360 K)
Destruction rate of CO =
1.3(10)14e-30,000/RT{O2}1/2 {H2O}1/2{CO}
Where { }indicates concentration in mol/cm3
This equation can be combined with the VOC kinetic.
Example 11.1
• Estimate the temperature required in an
isothermal plug flow incinerator with a
residence time of 0.5 sec to give 99.5%
destruction of toluene by using 3 methods
given.
Method 1.
autoignition T + 300 F=1026+300 = 1326 F
Example 11.1
• Method 2. Lee et al.
• T99.9= 594-12.2(7)+117+0+0+0.592(1026)-00+87.1(11.4)-0+0-75.3ln(0.5)
• T99= 577-107+110.2+0+0+0.586(1026)-0-0+85.2(1.14)0+0-76.1ln(0.5)
T99.5 can be calculated by taking a linear
average:
T99.5=(T99.9+T99)/2=1378 F
Example 11.1
• Method 3. Cooper et al.
First calculate required value k
  1 e
e
 k 0 .5
 kt r
 0 . 995  1
k  0 . 5 ln( 0 . 005 )  10 . 6 s
1
E=-0.00966(MW)+46.1
S=16/MW=16/92=0.174
Z’=2.85(10)11
Then calculate A
11
Z ' Sy O 2 P
2 . 85 (10 ) ( 0 . 174 )( 0 . 15 )( 1)
10 1
A

 9 . 07 (10 ) s
R'
0 . 08205
Now find T
k  Ae
T 
 E / RT
E
1
R ln( k / A )

 45200
1 . 987
1
10
ln( 10 . 6 / 9 . 07 (10 ) )
 995 K  1331 F
Catalytic Oxidation
• Catalyst: is an element or a compound that
speeds up a reaction without undergoing
permananet change itself
• Gaseous molecules diffuse to and adsorb
onto the surface of the catalyst
• After reaction product gases desorb and
diffuse back into the bulk gas stream
• The detailed mechanisms of the reaction
are not known but the reaction proceeds
much faster and/or at much lower T
Design Considerations for
Thermal Oxidizers
• The process of a VOC thermal oxidizer
involves selection of
– Operation T
– Residence time
– Sizing the device to achieve those with the
proper flow velocity
Factors:
O2 content
Type of operation (continous or intermittent)
Concentration of VOCs
Design Considerations for
Thermal Oxidizers
• To minimize the cost, it is desirable to keep
the stream to be treated as low as possible
• i.e. Not dilute VOC streams with too much
air
• However most insurance regulations limit
the max VOC concentraiton to 25% of the
lower explosive limit (LEL) of the VOC
LEL and UEL Limits
LEL and UEL Limits
Design Considerations for
Thermal Oxidizers
Organic
LEL (% by volume in air)
Acetone
2.15
Benzene
1.4
Ethane
3.2
Ethanol
3.3
Methane
5
Propane
2.4
Toluene
1.3
Design Considerations for
Thermal Oxidizers
• Material and energy balances are performed
to the device to calculate the flow rate of fuel
gas required to raise the air T at a given flow
rate to the required T.
• 2 Methods
1. The traditional approach: assume an
isothermal plug flow reactor
2. The use of computer program to handle the
calculations and allows for non-isothermal
operation
Method 1. Traditional Approach
• Material and energy balance:
Fuel
Gas (G)
Polluted
air (PA)
Burner
air (BA)
Exhaust
gas (E)
Burner
Flame
mixing
chamber
Mass flow
rates (kg/min)
.
Material
Balance
0M
.


G
M
.
PA
M
.
0 M
Energy
Balance
.
PA
.
M
Reaction
chamber
h PA  M
.
BA
M
.
G
Net heat of
combustion
Lower heating
value kj/kg
hG  M
E
.
BA
h BA  M
VOC i (  H c ) VOC X i  q L
i
.
E
hE  M
G
(H c )G
h: Specific enthalpy kj/kg
Xi:Fractional conversion of VOCi
Method 1. Traditional Approach
• If we assume that enthalpy functions of all
streams similar to those for pure air and
consider all heat losses (qL)as a simple
fraction of the heat input (fL):
.
.
0 M
PA
hT PA  M
.
M

VOC
i
.
G
hTG  M
.
BA
hT BA  M
0M
.
M
G

M
.
PA
E
hT E  M
G
(  H c ) G (1  f L )
(  H c ) VOC i X i (1  f L )
.
.
.
( hT E  hT PA )  M
.
G
M
.
PA
M
.
BA
( hT E  hT BA )   M
VOC
i
.
BA
M
E
(  H c ) VOC i X i (1  f L )
(  H c ) G (1  f L )  ( hT E  hTG )
Method 1. Traditional Approach
• Fuel gas is mixed with the outside ambient air
in a preset ratio Rb, then MBA=RBMG, also T of
burner air equals to T of fuel gas then the
equation takes its final form:
.
.
M
G

M
.
PA
( hT E  hT PA )   M
VOC
i
(  H c ) VOC i X i (1  f L )
(  H c ) G (1  f L )  ( R B  1)( hT E  hT BA )
Example 11.2
• Calculate the mass flow rate of CH4 required for an afterburner
to treat 2645 acfm of polluted air. It is estimated that the burner
will bring in 200 scfm of ouside air. The fuel gas enters at 80 F
and the burner air enters at 80 F. The lower heating value
(LHV) of CH4 is 21560 Btu/lb. Assume 10% overall heat loss
and ignore any heat gained by the oxidation of the pollutants.
• SOLUTION
From Appendix Table B.2, densities of the inlet PA and BA are
0.060 and 0.074 lb/ft3.
MPA = 2465 acfm (0.060 lb/acf)=148 lb/min
MBA = 200 scfm (0.074 lb/acf)=14.8 lb/min
Appendix Table B.7 enthalpies:
hTE= 328 hTBA= 4.8 hTBA= 33.6 hTG= 4.8
Example 11.2
SOLUTION
MPA = 2465 acfm (0.060 lb/acf)=148 lb/min
MBA = 200 scfm (0.074 lb/acf)=14.8 lb/min
hTE= 328 hTBA= 4.8 hTBA= 33.6 hTG= 4.8 Btu/lb
fL:0.1
.
.
M
G

M
.
PA
( hT E  hT PA )  M
.
BA
( hT E  hT BA )   M
VOC
i
(  H c ) VOC i X i (1  f L )
(  H c ) G (1  f L )  ( hT E  hTG )
MG = 2.53 lb/min
Sizing the Device
• To ensure adequate mixing and to
approach the condition of plug flow a
linear velocity of 20-40 ft/sec
recommended
• Usually the residence time 0.4-0.9 sec is
enough
• For hazardous waste incinerator residence
time must be 2 sec or longer
• For biomedical waste incinerator
residence time must be 1 sec or longer
Sizing the Device
• Length of the reaction chamber (L) is given by
– L=utr
• The volumetric flow rate (Q)from the ideal gas
.
law:
M E RT E
QE 
P ( MW ) E
• The diameter of the chamber (D)
.
D 
4Q E
u
Example 11.3
• Specify the length and the diameter of the
afterburner of Example 11.2 given that the
design velocity in the main chamber is 15
ft/sec and the desired residence time is 1
sec
Solution
• Length of the reaction chamber
L=utr=15(1.0) = 15 ft
ME=148+14.8+2.5=165 lb/min
• The volumetric flow rate (Q)from the ideal gas
law:
.
QE 
M E RT E

(165 )( 0 . 73 )( 1810 )
P ( MW ) E
 7790 ft / min
1 . 0 ( 28 )
• The diameter of the chamber (D)
D 
.
.
4Q E
4 / 7790 )
u

 (15 )( 60 )
 40 in
3
General Approach
• Start with equation 11.21 and 22 for a small slice of the
afterburner
• Treat each small slice as a CSTR
• The fraction combusted depends on T, k and residence
time in the CSTR
• Assume an inlet T and kinetic model of 11.6 and 11.7 for
both VOC and CO
• Because of the heat losses and gains Texit and Tinlet
are different
• Use iterative appraoch to find outlet T and CO and VOC
concentrations
• Repeat the procedure for all small CSTRs
• If the outlet VOC and CO concentrations are not those
desired, start with a new assumed inlet T.
Design Considerations for
Catalytic Oxidizers
• Can reduce the required temperatures by
hunders of degrees and can save
considerable amount of space for
equipment as compared with thermal
oxidizers
• Catalysis is usually a noble metal such as
palladium or platinum deposited on an
alumina support in a configuration to give
miniumum pressure drop
• The pressure drop consideration is often
cirtical for incinerator design
Design Considerations for
Catalytic Oxidizers
• A honeycomb arrangement typically results
in a pressure drop of 0.05-0.5” H2O/inch of
bed depth
• Packed bed of 1/8” diameter pellets 1.0-10”
H2O/inch of bed depth
Design Considerations for
Catalytic Oxidizers
• Catalyst activity refers to the degree to
which a chemical reaction rate is increased
compared with the same reaction without
the catalyst. They can also be very
selective. Such activity and selectivity
translated into lower operating
temperatuerd required for the desired
percentage of destruction.
Design Considerations for
Catalytic Oxidizers
• Table 11.4 Some T used for catalytic
incineration
Compound
Reported Temperatures, C
H2
20, 121
CO
150, 260, 315
n-pentane
310
n-decane
260
benzene
227,302
toluene
230,302
MEK
282,340
Basic Design Equation
[VOC]L/[VOC]0=e-L/Lm
[VOC]L :concentration of the VOC at length L
[VOC]0= inlet concentration of the VOC
L=length of catalyst bed
Lm=length of one mass transfer unit
If Re<1000 Then
Lm=ud2/17.6D
u: linear velocity in the channel
d=effective diameter of the channel
D=diffusivity, m2/s
Basic Design Equation
[VOC]L/[VOC]0=e-L/Lm
[VOC]L :concentration of the VOC at length L
[VOC]0= inlet concentration of the VOC
L=length of catalyst bed
Lm=length of one mass transfer unit
If Re<1000 Then
Lm=ud2/17.6D
u: linear velocity in the channel
d=effective diameter of the channel
D=diffusivity, m2/s
Basic Design Equation
If the flow is turbulent then
Lm=2/fa(Sc)2/3
f: Fanning friction factor
a=surface area per unit voluem of bed, m-1
Sc=Schmidt number (m/pD)
The calculated length (usually 2-10” for 99% removal)should
be doubled for the safety of the design
Catalyst surface area 0.2-0.5 ft2 per waste gas