Incineration

1
Incineration
Reading Chap 11

• Combustion Kinetics Parameters
• Incinerator Design
• Mass balance
• Energy balance
• Required fuel
• Dimension
• Catalytic Incinerator
• Hazardous Waste Incinerator

2
Effects of Temperature and Time
3
Combustion Kinetics (Global models)
CA concentration of pollutant A k kinetic rate
constant n reaction order
If pollutant concentration is much less than O2
concentration, n can be assumed to be 1.
Arrhenius Equation for rate constant
Eact activation energy (cal/mol) R gas constant
(1.987 cal/gmole K)
4
Determination of A and Eact
5
Z collision rate factor
S Steric factor (ineffective collision) yO2
mole fraction of O2 in the afterburner P
pressure (atm) R gas constant (0.08205
l-atm/mol oK)
(Kcal/mol)
Q How long does it take to incinerate 99
toluene (MW 96) at 1000 K (O2 mole fraction
0.15)?
6
Thermal Incinerator Design Isothermal Plug
Flow Reactor
Knowns?
Unknowns?
mwaste gas mburner air mfuel T1
mexhaust T2
7
Mass Balance
(Stoichiometry)
Energy Balance
.
qh heat rate required Q gas volume flow
rate rg gas density H enthalpy (Table B.7) Cp
specific heat T0 reference temperature (25 oC or
60 oF)
8
Steady State Energy Balance
X fractional conversion of VOC
Assume (1) the enthalpy functions of all streams
are similar to that for air (2)
the heat loss is a fraction of the heat input
Combining mass balance and heat loss relationship
with energy balance
9
Then group items by each material flow
Heat of Combustion
1 BTU/lb 2.326 KJ/kg
Higher Heating Value (HHV) Lower Heating Value
(LHV) Latent Heat of Water (Table B.6)
10
Sizing the Incinerator
Incinerator diameter
Qexhaust exhaust flow rate vT gas velocity in
the incinerator (20 40 ft/s)
Q What will happen if velocity is too low?
Length of Incinerator
tr residence time (0.2 2.0 s)
Length/diameter 2 3
Q Stoichiometric air? Excess air? Effects?
A waste gas stream of 2465 acfm enters the
afterburner at 200 F. The desired exhaust
temperature is 1350 F. It is estimated that the
burner will bring in 200 acfm of outside air.
Both the burner air and the fuel gas enter at 80
F. Assuming 10 overall heat loss and ignoring
any benefits of the oxidation of the pollutants,
calculate the required mass flow rate of methane.

11
Catalytic Oxidizers

• Advantages of Catalytic Oxidation
• Lower fuel requirements
• Lower NOx, CO and CO2 emissions
• Disadvantages
• Subject to deactivation thermal aging
  (sintering of support and deposited catalyst)
• Presence of poisons and suppressants problematic
  "poison" is irreversible, fast-acting P, Bi,
  As, Hg, Pb
• slow-acting Fe, Sn, Si
• Suppressants halogens, S
• Masking covering or blocking access to sites -
  PM
• Erosion e.g. particles in gas stream

Q How can it do that?
Q How does it look like?
12
Reduces activation energy

• Heat required to raise temperature of gases is
  lower

13
Examples of Catalyst Supports
14
Catalytic Converter (automobiles)
15
Catalytic Oxidizers continued

• Types of Catalysts
• Precious Metal Pt, Pd, Rh more prevalent with
  lower operating T and shorter residence time
• Base Metal Cu, Cr, Mn, Co in some cases
  designed for more resistant to certain
  contaminants, Cl
• Operating temperature
• Too low, get products of incomplete combustion
  (PICs)
• Too high, increases thermal aging (operate lt 1200
  oF to avoid "sintering")
• Normally when getting high destruction
  efficiency,
• kchemical rxn gtgt kmass transfer

Q Pollutant concentration as a function of
distance from the catalyst surface (1) reaction
on catalyst surface is slow (2) reaction on
catalyst surface is very fast?
Q What design parameters are we looking for?
16
Mass transfer limited design

• For mass transfer limited case gas phase
  resistance controls rate of destruction use
  mass transfer coeff. km

17
Mass transfer model Retallick ref.

• Material balance

QdC-aAcdxkmC

• Integrate from 0ltxlt L

• Define mass transfer unit length, Lm, N number
  of transfer unit

? N L/Lm hence C(x) Coe-N
Q Physical meating of Lm?
18
Laminar Flow Solution

• For laminar flow conditions, molecular
  diffusivity governs and Nusselt number (Num) is
  available in the literature 4.4 circular holes,
  4.1 parallel plates.

• Hence substituting for km

• For a given destruction efficiency (penetration)
  N can be determined to yield bed length L.

19
Q A honeycomb catalytic converter has an
effective hole diameter of 0.059 inch and 72
open area. The flow contains toluene
(diffusivity 0.084 cm2/s) and its velocity is 20
ft/s. Determine the length of the incinerator in
order to have 99 destruction efficiency.

• Total amount of catalyst surface area is critical
  in assuring effective destruction
• Catalyst surface area/gas volume flow rate 0.2
  0.5 ft2/scfm
• Exhaust flow rate/volume of catalyst (space
  velocity) 50,000 100,000 hr-1

Q What is the inverse of space velocity?
20
Hazardous Waste Incineration

• 40 CFR, Part 261, Subpart O
• For each principal organic hazardous constituent
  (POHC) in the waste stream, there must be at
  least 99.99 (four 9s) destruction and removal
  efficiency (DRE).
• At least 99 of the hydrogen chloride must be
  removed if the emissions are more than 1.8 kg/hr.
• Particulate emissions must not exceed 180 mg/m3
  corrected to 7 oxygen in the stack gas
• Waste containing chlorinated dioxins, chlorinated
  dibenzofurans and chlorinated phenols (RCRD codes
  F020-F028) require a 99.9999 (six 9s) DRE.

21
Chlorinated Hydrocarbon
Q Problems associated with chlorinated
hydrocarbon
Stoichiometry
If H/Cl ratio smaller than 5,
Also other oxychlorinated compounds
Q Treatment of HCl, Cl2 and other oxychlorinated
compounds?
22
Kp
Stable flame is difficult to maintain if the heat
of combustion is lt 4000 Kcal/kg
Q How to enhance HCl formation?
23
Quick Reflection