Waste Incineration and Heat Recovery
Plant heat-recovery units designed to defray rising fuel costs must also comply
with current and anticipated environmental regulations.
JoAnn E. Ward and Andrew P. Ting, T h e Heyward-Robinson Co., Inc., New York, N.Y. 10048
GENERAL CONSIDERATIONS
Components in the Waste Stream
A recent project includes the incineration of organic and
aqueous waste streams, Table 1. The waste streams represent a gross heating value of 26 million kcal/hr (100 million
Btuihr). A system designed to generate approximately
36,000 kg/hr (79,366lldhr) of 1138 kPa (165psig) steam for
plant use, would save $5.5 million a year.jf the steam
produced is valued at $2011000 kg ($9/1000 11)). T h e
presence of certain components, such as inorganic salts,
chlorinated and nitrogen-containing organic compounds,
sulfur compounds, and phospho-organics, is critical to the
design, and each deserves special considerations, as described below.
Inorganic Salts-The presence of salts, such as NaCI,
Na2S0,, CuSO,, and FeSO,, in the waste accelerates wear
on the refractory in the combustion zone. Upon evaporation and combustion, the salts form particulates of NaCl,
NaZSO,, Cu,O, and Fe,O,.
CuSO, decomposes at 805°C (1481°F) to CuO, which in
turn decomposes at 1030°C (1886°F) to Cu,O [ I ] .
Ignition of FeSO, yields Fe20,[ 2 ] .Cu,O is stable at high
temperatures, and the temperature of Fe,O, decomposition is beyond the expected temperature range of most
incinerator operations.
Molten salts and oxides solidify on the waste-heat boiler
tubing when cooled, causing reduced efficiency and possible plugging in the waste-heat boilers. This problem can
ISS~-02i8-~191-8~-5898-0010-5.7.00.
'The 4lirrricatr Iri\titute
(if
C1irniic.d Eiigi-
~ i r e r * 19%
.
be alleviated by cooling the incinerator exhaust gas below
the salt melting points before it enters the waste-heat
boiler, either by quenching with water, ambient air, or
recycle gas from the waste-heat boiler exhaust. Melting
points for some salts and oxides found in incinerators are
listed in Table 2.
In determining the temperature to which the incinerator
exhaust gases should be quenched, the individual salts
and oxides and possible eutectics must be considered.
Eutectic mixtures can contain complex combinations of
salts with melting points lower than the pure salts. For
example, NaCl and Na,SO, form a eutectic mixture with a
melting point as low as 623°C (1153°F) at 65 mole %
Na,SO,; NaCl and Na,CO, form a low-melting eutectic
system having a minimum melting point of 633°C (1172°F)
at 62 mole % Na,CO,. In addition, the tertiary Na,SO, Na2C0, - NaCl system may form a eutectic mixture with a
melting point of 612°C (1134°F).
Low-melting eutectic mixtures are especially critical in
fluidized-bed incineration. If the eutectics are allowed to
accumulate, the fluidized-bed may collapse [3]. Eutectic
mixtures can affect the overall economics of a waste-heat
recovery system. An investigation of the costs of operating
with a quench to eliminate solidification of particulates
before entering the waste-heat boiler is necessary. In some
systems, without quench, the amount of deposition in the
waste-heat boiler may b e tolerable, requiring only one to
two hours of weekly blowout to maintain efficient operation.
A final concern when incinerating salts is the federal
EPA regulation, which limits emissions of particulate mat-
Component, kg/hr
Stream
1
2
3
4
5
6
7
8
9
10
11
Total
kgihr
Page 30
c
H
208.8
203.4
42.6
1169.8
32.1
35.3
6.4
101.8
~
460.7
40.5
163.3
14.6
34.1
4.9
276.3
27.2
173.7
41.7
118.8
18.2
3.5.4
3.7
C
H
- - - - - 2886.1
326.4
c1
309.9
111.3
31.0
1.5
3.1
4.8
228.4
160.6
0
S
~
218.8
25.8
1.5
80.3
66.6
9.0
46.3
34.9
17.3
3.1
36.8
CI
N
46
0
N
S
_ _ - - - - - 690.0 617.8 78.0
50.6
February, 1982
Total
Salts, kgihr
P
H20
NO,.
Na2S0,
FeSO,
~
3.6
433 (Atomizing Steam)
5.7
120
2
11.8
3.6
CuSO,
NaCI
3.6
5.7
14
30.2
P
163.0
H,O
0.85(AIR-23971 kgihr Dry)
CnSO,
NO,, Na2S0, FeSO,
30.2
729.8
0.85
9.3
3.6
-
-___
3.6
550
350
80
1530
433
634
400
50
590
376
250
203
h'aC1
19.7
5446
Environmental Progress (Vol. 1, No. 1)
TABLE
2. MELTING
POlNTS OF SOME SALTS AND OXIDES
PRESENTIK INCINERATORS
Component,
"F
"C
Molc Fraction
PYOS
0.50NaCI - 0.26N%S04
- 0.24N%C03
-
0.65Na2S04 0.35NaCl
- 0.38NaC1
0.62Ni~~CO3
NaCI
h;a,S04
cu20
Fey%
Fe,O,
Reinarks
569 1 ) 1056
612
623
633
801
at34
1236
1462
1560
2)
2)
2)
3)
3)
3)
3)
4)
1134
1153
1172
1474
1623
2257
2664
tertiary eutectic
binary eutectic
hinary eutectic.
decomposition
decomposition
2840
I). Fabian, 11. W.. P.Heher, and M.Schoen, "How Bayer IncineratesWastes."Hydnp
curlmi P ~ I W S . & ~ .185 (April. IS7e).
2). Rrrgniaii. A. C., and A. K. Senientsoiva. "The Teniary Systems NaNCl, SO,. (Xbninl
W/Cl, SO,. CO,," B u r . h'eiwg. Khiw., 3 (2).388 (1958).
3). Dean, J. A.. editor, "Lunge's Hundhook of Chemistry." Ed. 12.WCraw-Hill Rnnk
Co., New York. N.Y.. pp. 4-48-113 (lS79).
4). Kirk and Othnicr, "Encyclopedia of Chemical Technology." Ed. 2. Vol. 12.1). 39,
Jchn Wile? and Sons, New York. N.Y. (1084).
ter from incinerators to 0.18 g/dscm* at 20°C (0.08 ddscf at
68°F) after correction to 12% C02 [4].
Chlorinuted Organic Compounds-These compounds
produce HCl and some C1, as combustion prtducts. Assuming that equilibrium is reached at the incinerator temperature, equilibrium data, developed from heat caacities, Fi ure 1, can be used to determine the relative
Po,.* rates o HC1 and C12in the incinerator exhaust gas [S,
P
61.
Figure 2. CO formation. Log K vs. temp. K = Pco2p~JPcl$ atm.
The formationof phosgene (COCI,) from CO and C1, is a
concern, because exposure to phosgene is a health hazard
and OSHA has strictly limited phosgene's Threshold
Limit Value (TLV) to 0.1 pm [7].
Equilibrium wnstants or formation of CO from CO,,
derived from specific heat and thermodynamic data [5,6],
are shown in Figure 2. The rate of COCl, forniation can in
turn be calculated from the equilibrium constants, Figure
3. The equilibrium constantvalues were obtained from the
free energy of the reaction, given by the etption:
P
AF" = -24,100
= grams per dry wtanbrd cubic meter
1.00L2
d.42
d.0012
4.ooe1
* ddscni
[a
+ 4TInT + 3.5T
d.0091
Figure 3. COCli formation. Equilibrium constant vs. temp. K =
PctSay, atm-'.
Figure 1. HCI and C12formation. Eiuilibrium constant vs. temp.
K, = (p"c3Ypd
(P"YO)PC,Y)
.a ohno.s
'
Environmental Progress (Vol. 1, No. 1)
Prorld
Formation of phosgene is favored by temperatures much
lower than those found in incinerators, about 350°C
(662°F)or less, and a phosgene concentration significantly
less than its TLV is expected.
Upon coolingin the waste-heat boiler there is a possibility of condensation of HCI with water vapor. The condensation temperature can be estimated using a graph of parus. temperature [9]. Figtial pressure product, pHcLx pH2,,
ure 4. The acid dewpoint of the incinerator exhaust gas is a
critical consideration in the selection of operating temperatures (particularly the waste-heat boiler outlet temperature) and in selection of materials of construction, because
of the highly corrosive effect of hydrochloric acid,
chlorine, SO,, SO,, etc.
February, 1982
Page 31
I
I
I
I
1
,
1
I
0
2700.F
24W.F
21OO.F
1800.F
150OoF
6
I
1
I
I
I
I
1
I
I
4
I
E
E
-c'
P'
c
n
z
Y
3
i1
n
01
v)
z
Y
0
8
0
0
I
I
I
I
I
W'C
W.C
I
-16
I
I
M1.
40%
I
70%
I
W.C
i
I
90%
i
I
100%
TEMPERATURE
Figure 4.
HCI acid formation. Log (partial pressure product of H20 &
E O O l
-I
m
3
0
W
HCI) vs. temperoture.
aoa
PP XXVi - X X V I I I
After incinerator gases pass through the waste-heat
boiler, the quantities of HCI and C1, can b e reduced to 1
ppm HCI [lo] and 0.1 ppm C1, [I]] by absorption with
water and neutralization with caustic solution in a tray
scrubber or packed tower. Although emissions of hydrogen chloride and chlorine are not covered by EPA
guidelines, exposure of workers is limited by OSHA regulation to 5 ppm and 1 ppm ceiling TLV, respectively, to
prevent possible respiratory damages [7].
Nitro Groups-Fuel NO,r is the combustion product of
organic compounds containing nitro groups. In addition,
thermal NO, formation by reaction of nitrogen and oxygen
in air is favored by the high temperature and long residence time needed to fully combust polychlorinated compounds also present in the wastes [12].T h e overall rate of
NO,,. formation is in the range of 2,000 to 15,000 ppmisec,
and, based on data from a mathematical model, NO, concentration reaches a plateau after about 0.1 sec [13].
T e m p e r a t u r e , t y p e s , a n d a m o u n t s of nitrogencontaining compounds in the file1 and the percent theoretical air are factors which wili affect the degree of conversion of fuel nitrogen to NO,.
Thermal NO, formation occurs primarily in the range
1540-1930°C (2800-3500°F). This suggests that NO, formation will occur in or near the flame. Also, the longer it takes
for the combustion gases to cool down to 1540°C (2800°F)
the greater the NO, concentration. Very little thermal NO,.
formation occurs below 1540°C (2800°F) [13].
At the high temperatures of incinerators, N O formation
dominates the N O - N 0 2 equilibrium, Figure 5 [5, 61. The
reaction kinetics [I41 for the formation of NO2 from N O
were examined in the system. Based on the theoretical
calculation, the amount of NO, formed once the combustion gases leave the incinerator is insignificant.
Experimental studies found that the fraction of fuel nitrogen converted to NO, will range from 0.20 to 0.70 [15,
161. A graph of fraction conversion as a function of fuel
nitrogen content is shown in Figure 6. The degree of conversion appears to be a function of both the nitrogen content of the fuel and the percent excess air present, At lower
fue1-nitrogen levels, more than 60% of the elemental N is
converted to NO,r.T h e fraction conversion is about 45% at
0.4% fuel nitrogen and decreases further to about 25% at
2% nitrogen. At higher excess air levels, these values for
fraction conversion are expected to be slightly greater.
T h e effect of airlfuel ratio on NO, concentration is
Page 32
February, 1982
'HANDBOOK OF CHEMISTRY AND
PHYSICS"ED 60, PP D67-78 CHEMICAL AND
0 000
TEMPERATURE
Figure 5 NO, formation Equilibrium constant vs temp
0
07-
I
(
,
1
FRACTION CONVERSION VS FUEL N CONTENT
100% THEORETICAL AIR
I
I
I
I
I
I
I
MARTIN,G.B. AND E.E. BERKAU. AIChE SYMPOSIUM
SERIES NO. 126,68 PP.45-54(1972)
0 TURNER, D.W, R.L. ANDREWS, C.W SIEGMUN
SYMPOSIUM SERIES N0.126.68 PF!55-65( 1972)
I
I-
Z" 6
05
20
25
FU;~ N I T R ~ G E N w0 BY WEIGHT)
30
35
Figure 6. Fraction conversion vs. fuel N content. 100% theoretical air.
shown in Figure 7. NO, concentration peaks at around
90% theoretical air, according to one study [17]. Other
studies indicate from 83% [I51 to 105% [13]as the amount
of theoretical air at which NO, concentration reaches a
maximum. The decrease in NO,. concentration at higher
excess air levels is deceptive, primarily due to a dilution
effect. T h e amount of NO, formed will increase upon addition ofexcess air, although not as fast as the total volume of
exhaust gas. NO, concentration, corrected to 12% CO,,
will increase upon addition of air, leveling off at around
140-150% theoretical air [15], Figure 8. In summary, the
fraction of fixed nitrogen in the liquid waste converted to
NO, upon combustion decreases with increasing fuel ni-
Environmental Progress (Val. 1, No. 1)
FRACTION CONVERSION VS. FUEL N.
~
120% THEORETICAL AIR -
W
Figure 9. Froction conversion vs. fuel N content.
REFERENCES
OTURNER, DW R L ANDREWS, CW SIEGMUNO
AIChE SYMPOSIUM SERIES NO. 126.68 PP
LANGE JR, t i 8 AlChE SYMPOSIUM SERIES NO 126,68
PP 1 7 - 2 7 ( 1 9 7 2 )
A BARTOK,W ET AL AIChE SYMPOSIUM SERIES NO 126,68
PP 3 0 - 3 8 ( 1 9 7 2 )
lo+
I
I
I
I
I
60
70
80
90
100
110
120
,
4
130
PERCENT THEORETICAL AIR
Figure
7 Effect of air-fuel ratio on NO, concentration (as measured).
trogen, Figure 9, and increases with increasing theoretical
air [16].
The fraction conversion as a function of fuel-nitrogen
120% theoreticol oir.
content and percent theoretical air can be approximated,
Figures 9 and 11. Figure 11 is derived from the data shown
in Figure 10. The fraction conversion factor at a given
percent theoretical air is the ratio of fraction conversion at
that percent air to the fraction conversion at 120%thearetical air at the same fuel-nitrogen content. The use of 120%
theoretical air as a basis should be more reliable than the
use of 100% theoretical air. At 100% theoretical air, the
system probably fluctuates between reducing and oxidizing atmospheres, .causing instability.
A 35% conversion to NO, can be estimated for a liquid
waste containing 1.4 weight percent N and combusted
with 200% theoretical air, Figures 9 and 11.This compares
with a calculated equilibrium conversion of 45%.
The equilibrium concentration of NO,, calculated using
fuel-composition data [IS],is shown in Figure 12. Equilibrium concentration decreases as percent excess air increases because of the reduction in flame temperature
which accompanies the addition of air. Measured NO,
concentrations divided by the corresponding equilibrium
concentration yield values for fraction equilibrium conversion, Figure 13. The values indicate the actual NO,
concentration to be in the range of 4 to 19% of the calculated equilibrium concentration.
Most ofthe NO, can be removed by scrubbing, although
NO is especially difficult to remove. At the high tempera-
I
I
I
KK)
I10
130
140
Is0
I60
170
180
P E ~ C E N TTHEORETICAL AIR
Figure 8. Effect of air-fuel rotio on N0,concentrotion (corrected to 12%
C 0 2 dry).
Environmental Progress (Vol. 1,
No. 1)
PERCENT THEORETICAL AIR
Figure 10. Effect of air-fuel rotio on fraction conversion
February, 1982
Page 33
I 1
--
8
F
( 6 j - L
o I X NITROOEN
CI 0 5 U WITRODEN
0 I % WITROQEN
2k-4
O Z
GO
[LE
W
z
$
0.16
a
w
> 0.16
z
El2
8:
0
%,,
*
o m
I--
2
[L
LL
0.20
EFFECT OF AIR-FUEL RATIO ON
:RACTION, EQUl,Ll6RlUM CwERSloru
A 2 % NITROOEN I W FUEL. BY WElDHT
82
5
0.22
EFFECT OF AIR-FUEL RATIO ON
FRACTION CONVERSION FACTOR
(BASED ON 120% THEORETICAL A 1 R ) A - I
08
'G
I
1
,20
B AND EE BERKAN. AIChE
SYMPOSIUM SERIES NO. 126.68 P P 4 5 - W I 9 7 2 1 ~
I30
140
I50
160
PERCENT THEORETICAL A I R
Figure 1 1 , Effect of air-fuel ratio on fraction conversion factor (based on
120% theoretical air).
0.14
I
2
a 0.12
-
m
1
=) 0.10
8
0.08
0
tures of incinerators, NO formation dominates the NO-NO,
ecpilibriuni, Figure 5 [5, 61. Hence, NO, is not removed as
efficiently a s HCl and SO2i n the incinerator exhaust gas. If
further reduction of NO,. emissions is desired, the process
depicted and described here niay require modificatioii.
Several &nitrification processes currently under iuvestigation may provide some relief for this problem.
Potential problems d u e to eniission of NO,. to the atmosphere include particulate and smog formation and acid
rain. In attainment areas, NO, emissions from incinerators
are not regulated if less than 90,700 kgiyr (100 short toiisiyr)
are emitted for a new construction or less than 36,300 kgiyr
(40 short tonsiyr) for a major rnotlification of an existing
plant [18].
In addition, the possibility of HCN fornlation must be
n
F
2
0.04
0.02
Figure 13. Effect of air-fuel rotio on fraction equilibrium conversion F.E.C.
= pprn NOJequilibrium ppm NO,r; corrected to 12% CO,, dry.
examined. Since it is such a hazardous substance, HCN has
a TLV of 10 ppin (1I mgim3) skin exposure set hy OSHA [7].
From a therniodynamic viewpoint, it is highly unlikely
that HCN will be formed in the oxidizing atmosphere of
the incinerator. However, in a reducing atmosphere, substantial amounts of HCN can form. A study of the reaction
behavior of nitrogenous pollutants indicates substantial
HCN emissions during the combustion of alkyl amines at
temperatures below 827°C (1521°F) [19].
Su@r compounds-The waste streams contain sulfur
compounds which combust to form SO, and SO:,. In order
to determine the relative flow rates of SO, and SO4, it is
assumed that equilibrium is attained at the incinerator
temperature, Figure 14 [20]. At the high temperatures of
incinerators, equilibrium favors formation of SO,.
As with HCI, the possibility ofconclensatioii o f sulfiiric
acid from combination of SO, with water vapor intist he
coiisidered becaiise of its corrosive effects. T h e sulfuricacid dewpoint can b e calculated, if volume percents of
water vapor and SO:3in the gas are known, from the following equation:
_
100o
_ _ -- 1.7842
T,,P
+ 0.0269 fogP,,,,
- 0.1029 logP,,,,,
+ 0.0329 logP,,,,
logP,,);, [Z]
wherc, pressures are i i I atinosphercs and t h e tle\vpoint is in
K . Other equations giviiig sulfuric acid dewpoitits have
I ~ e e ipii1)lishetl
i
[22]. These equations, however, ;ire direct
drrivations ofthe original Verhoff-Banchero equation [ 2 3 ] .
Curves calculited from Equation 21 are shown in Figure
PER CENT THEORETICAL AIR
Figure 12 Effect of air-fuel ratio on equilibrium NO, concentration
Page 34
February, 1982
15.
III adtlitioir, SO, may condense to sulfurous acid, the
dewpoilit for which can lie estiniatetl from partial-pressure
data [g], Figure 16.
hlost of the SO, and a small ;imout~tof SO:, can be allsorliecl a s solublr Na2S0,, and Na,SO, salts b y scrulhing
with a caustic soliition.
Environmental Progress (Vol. 1, No. 1)
l60oT
1000*F
I
10
I
I
I
2100Y
SOX FORMATION
I
I
~EOUILIBRIUYCONSTANT vs. t
b=p*-*
50°F
2400.F
150°F
IOO'F
1
i
2~.
004
I
I
I
I
1
HnSOa'
_ ~ _ _FORMATION
-
~~iij
250"F
~.
.
!
1
OVER AQUEOUS SO0 SOLUTIONS
Po,
E
E I X 10
aoI I
a
I
m
I
-
0.m
X I XI
0
I
I
N
c
d
~
I
I
t
I
I
I
I
-
I
1
I
I
I
I
I
I
I
46oC
S'O'C
8b.C
IOO'C
I
0°C
i0.C
liO°C
I&%
TEMPERATURE
TEMPERATURE
Figure 14. SO,,. formation. Equilibrium constant vs. temp. K , = Pse:/
P , ~ ~ atm-'.
P ~ ~ ,
I
Figure 16. HtSO, farmotion. Partial pressure product of H,O and SO, over
aqueous SO, solutions.
Phospho-Organics-The main combustion product of
phospho-organics is gaseous P,O,. Above 599°C (1110"F),
P20sreacts with deposits on waste-heat boiler surfaces and
with iron oxide on the boiler wall, resulting in difficult-toremove incrustations. This causes corrosion, an increased
pressure drop through the waste-heat boiler, and a reduction in boiler efficiency and life.
Gaseous P 2 0 , liquefies at 591°C (1096°F)and solidifies
at 569°C (1056°F).It is emitted to the atmosphere as particulate matter, for which the EPA has strong restrictions.
Because of the small temperature difference between the
boiling point and melting point, fine aerosols will form as
the incinerator exhaust gas cools causing potential removal
difficulties [24]. However, most of the P20, can be removed in the scrubber.
PROCESS DESIGN
On-Stream Time
A minimum 95% on-stream time is the specification for
the incinerator and waste-heat boiler system. This factor is
relatively high for two reasons. First, a major portion of the
steam used in plant operations is considered to be generated in the waste-heat boiler. Additionally, inability to
incinerate wastes may necessitate shutdown of plant operations. The system must be reliable if it is to be economically feasible.
Flexibility of Handling Capability
Figure 15. H,SO, formation, SO, concentrations vs. calculated dewpoint.
Environmental Progress (Vol. 1, No. 1)
In addition to special consideration of waste materials to
be incinerated and high on-stream time, a high degree of
flexibility is required in the system's handling capability.
The ability to accept rapidly varying flows and compositions is required, including turndown to two-thirds of de-
February, 1982
Page 35
sign rate and provision for future expansion with minimal
modification.
Process Design Description
A simplified process flow diagram for the incineration
and heat recovery is shown in Figure 17.
Wastes are fed to the incinerator through four feed burners. The waste streams are separated so that reactions between stream components before entering the incinerator
are avoided. Waste streams containing chlorinated hydrocarbons are combined and fed to one burner, with atomization b y compressed air. The two tar streams are each fed
through separate burners. The fourth burner handles all
other liquid wastes. T h e wastes to these three burners are
atomized by steam. An air stream containing some aromatic hydrocarbon vapors is fed to the incinerator, as are
combustion and excess air.
I n c i n e ra tor opera t i n g co 11d i t i on s are based o n the
minimum EPA regulation for polychlorinated biphenyls
(PCBs). EPA regulation requires a 2-second residence
time and minimum flame temperature of 1200°C (2192°F)
with 3% excess air [25].Bayer, in Germany, encountered
no detectable chlorinated compounds in the flue gas when
incinerating polychlorinated compounds at 1200°C and
1-second minimum residence time [24].T h e EPA regulation is a reasonably conservative basis for design and has
been verified by test burns at Rollins and Ensco facilities
in 1980 [ I I ] . An incinerator operating pressure of -51 mm
( - 2 in) WC is chosen for safety and environmental reasons.
Immediately preceding the incinerator exit, incinerator
exhaust gas is cooled from 1204°C (2200°F) to 760°C
(1400°F) by recycle of waste-heat boiler exhaust gases, in
order to solidify molten salt particles. Although both NaCl
and Na2S04 exist in the wastes, eutectic forniation is
minimized by intensive atomization.
Recycle Mixing Quench Chamber
Cooled incinerator exhaust gas passes through the
quench chamber before entering the waste-heat boiler.
Coarse solid particles entrained in the gases settle out in
the bottom of the quench chamber for disposal. T h e
ciuench chamber and lines from the incinerator to the
waste-heat boiler are brick-lined.
capacity, with provision for future addition of a water-tube
boiler at production rates beyond this range. Inlet temperature is 760°C (1400°F). At the 100% rate, the approximate
nutlet temperature and boiler pressure drop are 246°C
(475°F) and 203 nim (8 in) WC respectively, with generation of approximately 36,000 kg/hr (79,366 Ib/hr) of 1137
kPa (165 psig) steam.
Venturi Scrubber
In the Venturi scrubber the waste-heat boiler exhaust
gas is quenched from 246°C (475°F) to 45°C (113°F) with
salt-containing waste water. I n addition, some of the soluble entrained salts such as NaCl and Na,SO, are removed.
Efficient removal of sub-micron particles can be obtained by installing an EPA recognized best available control technology (BACT) high-energy, variable-throat Venturi. T h e variable throat provides pressure drops as high as
1219 mm (48 in.) WC and very high efficiency of removal
over a range of gas flow rates.
Scrubbing Column
Quenched waste-heat boiler effluent gas enters the bottom of the scrubbing column, and flows couutercurrent to
the scrubbing liquor. Most of the HCI, SO,, P,O,, NO,r,and
solids from the exhaust gas are absorbed with water and
neutralized to salts (NaCl, Na2S0,, Na,SO,, and Na,P04)
by a slightly caustic solution. T h e p H of the solution
should range between 6 and 9 for most efficient removal.
Below p H 6, the efficiency ofremoval is reduced, and at a
pH greater than 9, CO, will be absorbed from the gas,
causing problems with scale formation [26].
Most of the resulting salt solution is recirculated to the
top of the scrubber column. T h e remaining liquor is removed a n d purged to a wastewater sewer or wastetreatment plant. Recirculation minimizes water pollution
and the cost of wastewater treatment. The scrubbed gas
goes through a mist eliminator which removes entrained
liquor and particulates before entering the exhaust stack.
Scrubber Exhoust Stack
The exhaust gas passes through the stack, located on top
of the scrubbing column, to the atmosphere. The stack
emissions point is 80 feet above grade.
SAMPLE CALCULATIONS
Waste-Heat Boiler
A fire-tube, horizontal boiler is preferable to a watertube boiler. T h e higher gas-film temperature of fire-tube
boilers is advantageous because salt deposits and corrosion are minimized, and cleaning or replacement is easier
and less frequent.
The waste-heat boiler is designed to operate at 67-100%
The material balance starts with the incinerator. An
elemental analysis of the waste streams is shown in Table
1.
The combustion reactions assumed for calculation purposes are:
1. CI + H = H C I
2 . 4 H 0, = 2 H 2 0
+
+ 0,= co,
s + o,=so,
3. C
4.
+ 50, = 2P,O5
6. 2N (in the nitro group)
5. 4 P
TO STACU
r-
RECYCLE GAS
t
---
7,
LOX CAUSTIC
PACKE
L
I
I
a
LEGEND
~
I
l
\
i
RECIRCULATION
LIQUOR
YENTURI
scnvnesn
- T I
TEMPERATURE
BLOWDOWN
TO SEWER
Figure 17. lncinerotion ond waste heat-recovery flowsheet.
Page 36
February, 1982
+
1.5 0, = NO + NO,
In addition, at an incinerator temperature of 1204°C
(2200"F), salts will decompose:
2CUS0, = c u , o + 2 s 0 , + Y20,
ZFeSO, = Fe,O, + SO3 + SO,
T h e amount of stoichiometric air required for combustion is determined on the basis of the incoming streams.
Excess air needed to maintain an incinerator temperature
of 1204°C (2200°F) is found by heat balance, based on the
average heat capacity of the combustion gases, and gross
heating values for each stream.
Assumptions are that equilibrium is attained at 1204°C
(2200°F) in the iucinerator for all reactions except NO,.
formation; and that on cooling ofthe gases, changes in flow
rate and concentration are negligible.
Investigation of equilibrium for the following reactions
Environmental Progress (Vol. 1, No. 1)
is necessary:
(Figure 1)
1. H,O + C1, = 2HC1 + YzO,
(Figure 2)
2 . X O , = 2CO + 0 2
(Figure 3 )
3 . co + Cl,=COC12
(Figure 5 )
= NO,
4. Y2N2 + %02
(Figure 5)
5. N O + % 0 2 = N 0 2
(Figure 14)
6. 2SO2 + 0 2 = 2SO3
Once combustion and equilibrium reactions have been
studied, the incinerator exit gas analysis is known, Table 3.
The calculation of caustic feed flowrate to the scrubbers
is based on the NaOH requirement for the following neutralization reactions:
HCI + NaOH = NaCl + H 2 0
SO, + 2NaOH = Na, SO, + H 2 0
SO:, + 2NaOH = Na, SO, + HzO
P,O, + 6NaOH =2Na,PO, + 3 H 2 0
The scrubber exhaust-gas temperature is determined by
a heat balance for the scrubber, based on 126 m3/hr (550
gpm) wastewater at 13°C (55°F) for quenching and scrubbing. Trial-and-error calculation converges on a 45°C
(113°F) exit temperature. The partial pressure of water at
this temperature determines the amount of water vapor in
the scrubber exhaust gas. The final scrubber exhanst
analysis is listed in Table 4.
A water balance around the scrubbers gives the blowdown stream flow rates. This stream includes water, solTABLE3. INCINERATOR
EXIT GAS
95.8% Excess Air
kg mol
hr
Cotn-
ponent
M.W.
~
N, + .A
28.2
Mol%
Dry
kg Wt%
hr
Dry
~
Mol%
Wet
38.02
1147.06
40.95
0.28
8.59
44 240.51
0.64
36.5
19.44
0.69
71
0.018 GPPM GPPM
c1,
0.06
0.06
30
1.81
N0
46
0.12 43PPM 40PPM
NO,
0.06
0.06
1.664
SO,
64
80
0.026 9PPM 9PPM
SO,
46.09
49.64
AIR
29 1390.52
100.0
280 1.168
H,O
18_ _
216.06
7.16
-_
_--Total
29.2 3017.228
100.0
100.0
kg = 1.46 g/dscm
P205and Particulates: 98 hr
CO,
HCl
~
38.45
32,347
12.58
10.582
0.84
710
1.3 15PPM
0.06
54
5.7 68PPM
0.13
107
2 24PPM
47.93
40,325
100.0
84,134
3,889
88,023
100.0
uble salts, and particulates. T h e recirculation liquor
stream has the same composition as the blowdown stream,
with a 141 m3/hr (616 gpm) flowrate.
LITERATURE CITED
1. Kirk and Othmer, “Encyclopedia of Chemical Technology,”
Ed. 3, Vol. 7, pp. 106-107, John Wiley and Sons, New York
(1979).
2. Kirk and Othmer, “Encyclopedia of Chemical Technology,”
Ed. 2., Vol. 12, pp. 38-42, John Wiley and Sons, New York
(1964).
3. Wall, C. J., J . T. Craves, and E. J. Roberts, “How to Burn Salty
Sludges,” Chemical Engineerin , 77-82 (April 14, 1975).
4. Ennironnzentul Reporter, “Stanckrds of Performance for New
Stationary Sources, 121:1526 (September 7, 1979).
5. Houeen and Watson, “Chemical Process Principles,” Vol. 2,
p. 985, Appendix pp. xxvi-xxviii, John Wiley a d Sons, New
York (1959).
6. “Handbook of Chemistry and Physics,” Ed. 60, pp. D-67-78,
Chemical and Rubber Company (1979).
7. Occupational Safety and Heulth R e p o r t f r , “OSHA
Standards-Toxic and Hazardous Substances, p. 31:8303
(January 21, 1981).
8. Kirk and Othmer. “EncvcloDedia of Chemical Technoloev.”
Ed. 2, Vol. 4, p 432. ]oh, Wiley and Sons, New York (19x44).
9. Perry and Chilton, “Chemical Engineer’s Handbook,” 5th
Ed., pp. 3-62-63, McCraw-Hill, New York (1973).
10. Santoleri, J. J., “Chlorinated Hydrocarbon Waste Disposal
and Recovery Systems,” Chem. Eng. Progr., 69, No. 1, 68-74
(January, 1973).
11. EPA-Air and Hazardous Materials Division, “Incineration
of PCBs Summary of Approval Actions” (February 6, 1981).
12. Batha, H. D., “Review of NO, Abatement Technology,” 88th
National Meeting of AIChE
(June 12, 1980).
13. Lange Jr., H. B., “NO, Formation in Premixed Combustion: A
Kinetic Model and Experimental Data,” AIChE Symposium
Series No. 126, 68, pp. 17-27 (1972).
14. Kirk and Othmer, “Encyclopedia of Chemical Technology,”
2nd Ed., Vol. 13, p. 801, John Wiley and Sons, New York
(1964).
15. T u rn e r, D. W., R. L. Andrews, a n d C . W. Siegmund,
“Influence of Combustion Modification and Fuel Nitrogen
Content on Nitrogen Oxides Emissions from Fuel Oil Combustion,” AIChE Symposium Series No. 126, 68, pp. 55-65
(1972).
16. Martin, C. B., and E. E. Berkan, “An Investigation of the
Conversion ofvarious Fuel Nitrogen Compounds to Nitrogen
Oxides in Oil Combustion,” AIChE Symposium Series No.
126, 68, pp. 45-54 (1972).
TABLE
4. SCRUBBER
EXIT GASANALYSIS
T = 45°C 1113°F)
Component
% Removal
CO,
N, + A
AIR
NO
NOI
S0,
SO,
HCI
C1,
P,O,
Total, Dry
H,O
Total, Wet
0
0
0
65
65
99
10
99.3(1)
99 (1)
99
kgihr
kg-mol/hr
10,580
32,350
40,325
19
1.9
1.1
1.8
5.3
0.013
0.7
83,284.813
5,270
88,554.813
240.45
1147.16
1390.52
0.63
0.042
0.017
0.023
0.146
0.0002
0.0049
2778.9931
292.78
3071.7731
g‘dscm
127,034
388,426
484,182
228
23
13
22
64
02
8
86,524
412,797
500,368
227
15
6
8
53
0.07
2
158
484
604
0.28
0.028
0.016
0.027
0.079
0.00019
0.010
Corrected to
12% CO,
g/dscm
120,000
572,508
307,060
315
21
8
11
74
0.1
3
220
672
837
0.39
0.039
0.023
0.037
0.11
0.00027
0.015
I ) E P A Incinrr.ilion of PCB,. Suinmarp of Approval Actions 2-6-XI
2778.9931
X
22.4 1 1 1 ~ 273 + 2VK
___
X
tg
~
niol
273°K
=
66810 dscm
Environmental Progress (Vol. 1, No. 1)
February, 1982
Page 37
~
17. Bartok, W., V. S. Engleman, R. Goldstein, and E. G . del Valle,
“Basic Kinetic Studies and Modeling of Nitrogen Oxide Formation in Combustion Processes,” AJChE Symposium Series
No. 126, 68, pp. 30-38 (1972).
18. EPA, Final PSD regulations, 52.21(h) (August 7, 1%0).
19. Meier, H., and F. Weger, Staub Reinhault Luft, 1980, 40(b),
245-9 (Chem. Abstracts, VoI. 94, 1981-94: I9746K).
20. Kirk and Othmer, “Encyclopedia of Chemical Technology,”
Ed. 2, Vol. 19, p. 461, John Wiley and Sons, New York (1964).
21. Pierce, R. R., “Estimating Acid Dewpoints i n Stack Gases,”
Chemicd Engineering (April, 1977).
22. Kiang, Y.-H., “Predicting Dewpoints of Acid Gases.” Chemi-
cal E n ineering, p. 127 (February 9, 1981).
23. Verhok, F. H. K., and J. T. Banchero, “Predicting Dewpoints
of Flue Gases,” C h e m . Eng. Progr., 70, No. 8, 71 (August,
1974).
24. Fabian, H . W., P. Reher, and M. Schoen, “How Bayer Incinerates Wastes,” Hydrocurbon Processing, 183-192 (April,
1979).
25. EPA 40 Code of Federal Regulations, Part 761. Final Rule on
PCB’s, effective February, 1979.
26. Industrial Environmental Research Laboratory, “Sulfur
Oxides Control Technology Series: Flue Gas
Desulfurization-Dual Alkali Process,” Research Triangle
Park, N.C. (October, 1980).
Andrew Ting is a process manager at T h e
Heyward-Robinson Company, where he is responsible for all environmental matters related to
air, water and solids pollution control. He earned
his B.S. at Chekiang University, China, his h1.S. at
Montana State University and his Ph.D. at the
University of Missouri, all in Chemical Engineer-
JoAnn Ward is a process engineer with The
Heyward-Robinson Company. She received her
B.S.Ch.Efrom Tiifts University in 1980.
ing.
Prediction of Destruction Efficiencies
How big must a boiler be to destroy a given hazardous waste by incineration?
Here are recommended options for the practicing design engineer.
C. Dean Wolbach, Acurex Corp., Mountain View, Calif. 94042
destroy the compound. The converse is not true. That is, if
Regulations being proposed under the Resource Conservation and Recovery Act (RCRA) have prohibited or made
very expensive some traditional methods for disposing of
hazardous wastes. Rather than impounding andlor landfilling wastes, there are increasingly strong economic incentives to use methods to completely destroy these materials.
Such destruction methods for organic-containing wastes
can be classified as either chemical or thermal. Chemical
destruction methods are usually tailored to particular
properties of a waste, while thermal destruction has more
universal application. Because many organic wastes have a
high heat content, and because of rising fuel costs, destruction in boilers in which the waste is co-fired appears to be
an attractive answer to a real problem.
One question that must b e addressed when contemplating the destruction of a hazardous organic waste in a boiler
is: will the boiler destroy the waste to a sufficiently low
level that it no longer constitutes a significant environmental hazard? This paper proposes a means of answering that
question. It proposes a method whereby an estimate of a
boiler’s ability to achieve a given destruction efficiency
can be made.
The model discussed in this paper is the core of a more
extensive, and complex rnodel which contains verification
procedures for the assumptions of the core model, and
relaxes the conservative restrictions placed on it. Thus, the
core motlel is the more conservative. If the assumptions of
the core model are met, and the core model predicts that a
certain coinpound will b e destroyed to a given efficiency,
then it can be assumed that the boiler will satisfactorily
81491-82-57R1-003X-%2.00. “ T h e American Institute of Chemical Engi-
Page 38
February, 1982
the model predicts insufficient conditions for destruction,
the boiler may or may not be adequate.
APPROACH
The chemical and physical processes undergone by a
material passing through a boiler are varied. T h e detailed
reaction paths a r e c o m p l e x , e v e n for t h e s i m p l e s t
molecules [I]. For larger and structurally more complex
molecules, little if anything is known of the detailed
mechanism of destruction. However, by the use of judicious assumptions, a conservative estimate of boiler destruction efficiency can be obtained.
T h e core model first establishes a time and temperature
relationship required to reach a certain destruction efficiency for a given compound. It then develops analogous
time and temperature relationships for the bulk gases
within the boiler furnace. Finally, a graphical overlay procedure is used to determine ifthere is sufficient time above
a given temperature in the boiler furnace to achieve the
required destruction.
Destruction efficiency, for the purposes of this paper, is
considered to be based on the disappearance of the compound of interest. It is assumed that the kinetics of disappearance are experimentally expressable as pseudo-first
order [2]. During actual thermal destruction three types of
mechanisms occur simultaneously. A portion of the material is destroyed in the flame zone at high temperatures and
at very fast rates d u e to high free-radical concentrations.
The bulk of the material may bypass the flame but can be
destroyed at much slower rates by either oxidation or
Environmental Progress (Vol. 1, No. 1)