Note: Descriptions are shown in the official language in which they were submitted.
The present invention relates to a process f~r the reduction of
the content of S02 and/or the nitrogen oxides N0 and N02
(referred to herein by the general term "N~Xn) in flue gases.
It has been proposed to remove N0x from flue gases by mixing the
gas with NH3 and irradiating the mixture with ultraviolet
light.
By this process, the NH3 is photolysed to yield amino radical
(NH2) in accordance with the equation
NH3 ~ NH2 + H
The amino radical reacts with N0x to yield the inert gas
nitrogen, water and N20 which is widely regarded as being inert
and harmless in the atmosphere, in accordance with the equations
NH2 ~ N0 ~ N2 + H20 (2)
NH2 + N2 N20 ~ H20 13)
It has now been found that increased efficiency of the
utilization of the ultraviolet light car. be obtained when
ult-aviolet light of a wavelength falling within a selected
range is employed. More specifically, the present invention
provides, in one aspect, a process for reduction o the content
of N0 and N02 in flue gas containing also substantial quantities
of H20 vapor comprising mixiny the flue gas with NH3 and
irradiating the mixture with ultraviolet radiation containing at
least one component of wavelength in the range about 190 to
about 220 nm, said radiation being substantially wholly free of
any component of wavelength below about 190 nm, and said proc~ss
taking place in the absence of a solid state catalyst.
-- , .
It will be appreciated that the efficiency of the utilization of
the ultraviolet radiation is of economic significance. The
process is of course usually to be applied to a flowing stream
of the gas and if the irradiation is conducted with low
efficiency, a prolonged exposure to the radiaton`is required.
This would require a bank of ultraviolet lamps of greatly
extended length for irradiation of a prolonged section of the
duct conveying the flue gas. The result would be that not only
would there be an increase in the operating costs of supplying
the energy required to energize the ultraviolet lamps, but also
the capital cost of supplying and installing the irradiation
apparatus would be considerably increased.
When employing wavelengths in the range about 190 to about 220
nmr the main NOx-removing reactions that occur are the above
reactions (1)g (2), and ~3), resulting in the formation of
N2O,H2O, and N2 in the waste gas stream. It is considered that
these products can be safely passed to the atmosphere. By
employing radiation free from any component below about 190 nm
undesirable side reactions are avoided. With radiation below
2G 190 nm there is considerable formation of OH radical due to
photolysis of water vapor which is usually present as a matter
of course in flue gas streams. Due to the high concentrations
of water vapor normally present, the predominating reaction is
thererore:
H20 --~ OH + H (4)
The CH radical reacts with NO2 to yield acidic species
OH + N02 --~ HNO3 (5
12:~3851
Normally, it would be desirable to remove these acidic species
by absorption in the presence of moisture, e.g. water vapor, by
an alkaline-reacting medium, e.g. by reaction with an excess of
ammonia gas to yield nitrate salts NH4N03. The nitrate salts
may form a particulate second phase and normally it is desirable
to remove the particulate salts from the gas stream before
passing it to the atmosphere. In this case, particulate salts
are obtained as a by-product. The particulate salts may be
recovered and may have a value e.g. as fertilizer. NH4N03 is,
however, explosive and presents handling difficulties. 8y
employing a radiation source substantially wholly free of any
component below 190 nm, the formation of an explosive
precipitate of NH4N03 is much reduced or is substantially wholly
avoided.
When the wavelength of the ultraviolet radiation employed in the
irradiation step is in the range about 190 to about 220 nm, the
NH3 gas present in the mixture absorbs the radiation strongly,
and a satisfactory removal of NOX is achieved as there is little
interference from undesired side reactions. Above about 220 nm
the efficiency of the process is impaired as the degree to which
NH3 absorbs radiation drops off sharply with wavelengths higher
than about 220 nm and therefore only negligibly small
concentrations of amino radical are generated, so that if the
wavelength of the radiation is increased much above 220 nm the
concentrations of ammonia that need to be employed, and the
intensity of the radiation that is required, in order to achieve
removal or NOX within reasonable times, rapidly become
impracticably large.
The absorption of the radiation by NH3 pea~s at about 195 nm and
drops off, as indicated in Table 1 toward wavelengths below
about 170 nm. At the same time, however, as indicated in Table
1, the radiation is quite strongly absorbed by the 2 and H2O
which are usually present as a matter of course.
Table 1 - Extinction coefficients
(1 mol~l Cm-l~
.____
Wavelength ~nm)
Reaction 184.9 193 213.9
.
NH3 + hv ~ NH2 H 1000 1500 100
2 + hv ->Z0 ( P) 4.2 0.3 0.002
H20 + hv -~ OH ~ ~ 14
The extinction coefficent E iS defined by
I = lo 10 cl
where Io = light intensity incident on a cell containing the
photolysable species
I = intensity emerging from the cell
c = concentration of the photolysable species
(mol dm 3)
1 = cell length tcm~
and ~ = extinction coefficient (1 mol 1 cm
The extinction coefficient therefore indicates how strongly
the photolysable species absorbs the radiation, the higher the
coefficient, the greater the degree of absorption.
-- 4 --
s~
It will be noted that above about 190 nm, the extinction
coefficient of water vapor is zero and therefore substantially
no OH radical is formed, so that reaction t5) does not occur to
any significant extent.
Although the coefficients f 2 and H2O are substantially lower
than that of NH3 in the range 170 to 220 nm, as may be perceived
by the above Table, such coefficients increase sharply toward
the lower end of this range and, moreover the concentrations of
2 and H2O present in the reaction mixture are normally
conside~ably higher than the concentration of NH3. Desirably,
in the process of the present inven~ion the concen~ration of NH3
present in the mixture that undergoQs photolysis is in the range
from about 5 x 1 o-6 to about 5 x 10-3 mol/l. With
concentrations of NH3 belsw about 5 x 10~~ mol/l, the efficiency
: of the removal of NO~ tends to be impaired and undesirably
prolonged exposure to the ultraviolet radiation is required to
achieve satisfactory degrees of removal of the undesired NOX
material. Concentrations of NH3 above about 5 x 10-3 mol/l
appear to be unnecessary and are undesirable as not only does
the maintenance of high NH3 concentrations in the photolysis
reaction zone greatly increase the consumption of NH3 and hence
also the operating costs of the proce~s, but also this may
result in emission of substantial quantities of unconsumed NH3
to the atmosphere. More preferably, the said concentration of
NH3 is in the range of about 1 x 10-5 to about 1 x 10~3 mol~l,
still more preferably about 1 x 10-5 to about 2 x 10~5 mol/l.
The quantities f 2 (moles) present in flue gases due to
incomplete consumption f 2 in the combustion air will however
typically be about 100 times the molar concentrations of NH3
30 which it is desired to maintain in the reaction mixture, and the
quantities of H20 vapor present as a product of combustion will
typically be of the order of about ~0 times the said NH3 molar
concentrations. Therefore, at wavelengths much below about 190
nm, absorptions by 2 and H2O compete significantly with the
absorption by NH3 as the concentration of 2 and ~2 is much
higher than that of NH3 and the extinction coeffi ients of 2
and H2O increase rapidly while the extinction coefficient of N~3
drops rapidly below about 190 nm, and therefore below 190 nm the
formation of NH2 radicals is greatly reduced.
As a result, when the ultraviolet radiation includes components
with a wavelength below about 190 nm the efficiency of the
utilization of the radiation energy is much reduced as a large
proportion of the radiation energy is directed to the production
of incompetent species.
There is some tendency during the present reaction for
combination of NH2 radicals to occur, yielding hydrazine
which is poisonous.
NH2 ~ N~2 N2H4
It is of course desirable to maintain the concentration of
hydrazine in the reaction mixture leaving the photolysis
reaction zone as low as possible, as it may otherwise be
necessary to take special steps to absorb hydrazine from the
reaction mixture.
Hydrazine is a strong absorber of radiation in the wavelength
range about 18C to about 270 nm, and dissociates to reform the
amino radical
N2H4 ~ hv ~~--~ 2NH2
By employing a svurce of ultraviolet light including one or more
components in the wavelength range 190 to 220 nm, the content of
hydrazine in the reaction mixture can be kept to acceptably low
level~ by its re-conversion to free radicals.
5~
In a further aspect, the present invention provides a process
for reacting a flue gas containing at least about 1 x 10-6 mol/l
S2 to convert said SO2 to a~ oxidized acidic species, in the
absence of a solid state catalyst, said gas containing also
substantial quantities oE H20 vapor and 2~ comprising
irradiating the gas with ultraviolet radiation containing at
least a component of wavelength below about 190 nm.
Preferably~ the radiation contains a component in the range
about 170 to about 190 nm. As noted above, photolysis of the
water vapor with radiation below 190 nm, preferably in the range
170 to 190 nm, efficiently results in the formation of large
quantities of OH radical through reaction (5~O
The hydroxyl radical reacts with SO2 to yield oxidized acidic
species.
OH + SO2 ~ HSO3 (63
These acidic species can be removed by absorption in the
presence of moisture e.g. by water or by an alkaline-reacting
medium e.g. by reaction with ammonia gas to yield sulfate salts,
or may be absorbed by conventional means such as scrubbers or
employed as a feed material to a sulfuric acid manufacturing
process.
Frequently, flue gases to be treated will contain both NOX and
SO2. In such cases it may be desirable to subject the gas
firstly to an NOx-removing procedure before subjecting it to the
S2 oxidation reaction described above, to avoid undesired
formation of ~NO3 acidic species in the latter reaction. In
accordance with a further aspect, the invention therefore
provides a process for the redu~tion of the content of NOX and
SO~ in flue gas containing also substantial quantities of H2O
30 vapor and 2 comprising mixing the flue gas with NH3 and
irradiating the mixture with ultraviolet radiation containing at
least one component of wavelength in the range about 190 to
-- 7 --
about 220 nm, said radiation being substantially wholly free of
any component with a wavelength below about 190 nm, to obtain a
gas with a reduced content of NOX and irradiating the gas having
a reduced content of NOX with ultraviolet radiation containing
at least a component of wavelength below about 190 nm, to
convert SO2 to oxidized acidic species, the process taking place
in the absence of a solid state catalyst.
An advantage of the present process is that as the NOX removal
reaction takes place in the absence of a solid state catalyst,
there is no need to remove SO2 and other compounds of sulfur,
which are catalyst poisons, from the flue gas prior to carrying
out the reaction.
The form of ultraviolet lamp to be employed will depend on the
desired wavelength range. As noted above, in one form, the lamp
may preferably be one which includes at least one component in
the range about 170 to 190 nm. One class of lamps which may be
employed comprises low pressure mercury arc lamps. These
provide a strong emission line at 184.9 nm, along with a s~rong
emission line a~ 253.7 nm, and weaker emissions at other
wavelengths. These lamps may therefore be employed for the SO2
oxidation process described above. A further class of lamps
which may be employed consists of high pressure mercury xenon
lamps. These provide an output which is a continuous spectrum
from 190 nm to above 300 nm. ~hey provide some emission at
wavelengths lower than 190 nm, but this may be readily screened
out with an appropriate filter. These lamps are therefore
particularly useful for photolytic removal of NOX from flue gas
streams by irradiation of an NH3-containing reaction mix~ure at
190 to 220 nm, even though a substantial proportion of their
output is not employed efficiently as it consists of radiation
with a wavelength above 220 nm.
It may be noted that the above-mentioned low pressure mercury
arc lamps and high pressure mercury xenon lamps are readily
available commercially. It is one advantage of the processes of
the present invention that they can be carried out employing
readily commercially-available ultra violet lamps.
Some examples of procèsses in accord~nce with the present
invention are illustrated in the accompanying drawings in which.
Fig. 1 shows in schematic form combustion apparatus e.g. a
boiler, and associated equipment for removal of NOx and/or S02
from the flue gases emitted ~y the boiler;
Fig. 1a shows a modification of the apparatus of Fig. 1;
Fig. 2 is a perspective view illustrating a longi$udinal section
through a portion of the wall of the flue gas duct of the above
apparatus; and
Fig. 3 shows a further modification of the apparatus.
Referring to the drawings, wherein like reference numerals
indicate like parts, Figure 1 shows a combustion apparatus e.g.
a boiler 1, which is supplied with combustion air through an
inlet line 2 which passes through an air preheater 3 from which
the heated air is supplied to the boiler through a line 4~ Flue
gases from the boiler pass direct to the preheater 3 through a
duct 6.
The boiler 1 may be in general any form of combustion apparatus
wherein atmospheric air is employed to sustain combustion and
the fuel contains sulfur, yielding S02 in the flue gas, and/or
the fuel is burnt at a temperature sufficiently high ~hat a
substantial quantity of NOx is formed. At high temperatures
nitrogen, present in the combustion air and, particularly in the
case of solid fuels, bound up in the fuel itself, reacts with
oxygen, present in the combustion air, to yield NO.
N2 ~ 2 ~~~ 2NO
_ g _
Some oxidation o N0 to N02 al80 occurs, so that the flue gase3
containing N0 mixed with som~ N02
2N0 ~ 2 > 2N02
The fuel may therefore be, for example, hydrogen gas, or a
primarily carbonaceous or hydrocarbon fuel, e.g. the so-called
fossil fuels such as oil, coal, and natural gas, or a fuel
derived from fossil fuels e.g. petroleum gas or coal gas~
Ammonia gas is injected into a duct 7 which conveys the cooled
flue gas from the air preheater 3 to the usual device for
separation of particulate material e.g. fly ash from the gas
stream, in this example an electrostatic precipitator 8. The
ammonia is injected through an inlet line 9 under the control of
a valve 10 which permits addition of the ammonia at a metered
rate. Frequently, the flue gas will contain quantities of S03
and HCl. These react with NH3 to yield particulate ammonium
salts, e.g. NH4HS04 and NH4C15 If the ammonia is added
downstream from the hoiler or other combustion apparatus 1 and
the air preheater 2, there is no risk of ammonium bisulfate and
ammonium chloride condensing on critical components such as the
boiler 1 and preheater 2 and causing corrosion and fouling. The
ammonia may be added as indicated upstream from the precipitator
8 so that the bisulfate salt ~an be removed to avoid exces~iv~
turbidity in the gas stream and loss of efficiency in the
subsequent photolysis step. The rate of addition of ammonia may
be calculated so that there is sufficient to react with the S03
and HCl and leave an excess of ~mmonia over in the concentration
required during the photolysis step i.e. generally in the
30 above-mentioned range of about 5 x 10-~ $o about 5 x 10~3
mol/l. Alternatively, and more desirably, the rate of addition
of NH3 may be controlled automatially in response to sensors
located in the duct 7 and in the stack 13, the former sensors
being responsive to the concentrations of S02 and/or N0x and
-- 10
5~
serving to increase the rate of addition of NH3 as th~
concentrations of SO2 and/or NOX increase and the latter sensors
being responsive to the presence of NH3 and serving to decreas~
the rate of addition when the concentration of unconsumed NH3 in
the stack gases rise above a predetermined limit.
Ammonium bisulfate and chloride mixture collected at the
precipitation may be separated from the fly ash and be
recovered. If, depending on the chemical composition of the
flue gas, the recovered mixture does not contain excessive
quantities of heavy metals or other toxic materials, it may be
utilizable as a valuable by-product.
As shown, the ammonia addition is desirably made at a point
upstream from the usual induced draft fan 11 which passes the
cleaned ~as from the precipitator 8 along a duct 12 to the stack
13 from which the flue gases are vented to the atmosphere.
Passage of the flue gas/NH3 mixture through the fan 11 ensures
that the NH3 is mixed uniformly in the gas stream. It is an
advantage of the process of the invention that the ammonia can
be introduced and the photolysis conducted at a region of the
flue gas ductwork adjacent the electrostatic precipita~ors where
the flue gases are at comparatively low temperatures e.g. up to
400UC, more typically 150 to 250C, as high temperatures are not
required for the photolytic generation of the reactive NH2 and
O~ radicals, which can proceed in the cold. These regions of
the flue gas ductwork are normally readily arcessible so the
fitting of the inlets, lamps etcO required for carrying out the
process can be readily carried out on existing combustion
plant. A lamp 14 is provided adjacent the duct 12 for
irradiating the gas stream passing through the duct 12. As
shown in Fig~ 2, the lamp may comprise a lamp body proper 16
housed within a metal reflector 17 and separa~ed from the
interior of the duct by an utraviolet-transmissiYe window 18
e~g. of quartz~ The window 18 may comprise a fil~er to screen
out undesired components of the radiation. Desirably, the space
i`~ within the reflecto~ 17 and window 18 comprisesa sealed unit
J~l
filled with an ultraviolet inert gas e.g. nitrogen to avoid
absorption losses to avoid or reduce generation of ozone in the
atmosphere adjacent the lamp. It may be desirable to mount the
lamp 14 external to the duct 12, as shown, because the flue gas
will normally contain significant amounts of particulate matter
even after p~ssage through the precipitator 8 and there may
therefore be a risk of fouling of the lamp s~ructure. Normally,
it will be desirable to equip the window 18 with automated
mechanical cleaners (not shown) to remove any fouling which may
build up on the face of the window adjacent the interior of the
duct 12.
In order to conduct a sequential NOX- and SO2-removing process
in which firstly an NH3-containing mixture is irradiated at 190
to 220 nm to remove NOX and the photolysed reaction mixture is
subsequently irradiated at 170 to 190 nm to oxidize SO2 to
acidic species, the lamp 14 may be constituted by two
longtudinally spaced ultraviolet sources emitting radiation in
the respectively desired wavelengths.
As noted above, the efficiency o~ the removal of the NOX and/or
S2 is strongly dependent on the wavelength of the ultraviolet
radiation emplvyed. It has been found~ however, that the
efficiency of the process when operated in the selected
wavelength ranges, in terms of the rate of remova~ of the
undesired species, is relatively insensitive to the
concentrations of other species present in the reaction
mixture. It is convenient to measure the effectlveness of the
process in removing NOX in terms of the percentage removal of
NO. Thus, for example when a reaction mix~ure consisting of a
given flue gas composition and NH3 is irradiated with
ultraviolet light of wavelength 193 nm, the rate of reduction of
NO content is approximately 2.5 times the rate that is achieved
when the irradiation is conducted at a wavelength of 213~9 nm.
These rate~ are relatively unaffected by the initial
concentration of NO, SO2 or NH3 (as long as a certain minimum
concentration of NH3 is present), or by variation in the
concentration of any other species normally present in the
- 12 -
reaction mixture, and given that the usual quantities of H2O
vapor (normally at least about 1 x 10~4 mol/l) are present in
the flue gas.
The rate of reduction of NOX and/or SO2 is also dependent on the
total quantity of radiant energy to which the reaction mixture
is subjected. Thus for example when the NH3 and flue gas
mixture is irradiated with ultraviolet light in the preferred
wavelength range of 190 to 220 nm, about 80% of the NOX is
removed when the mixture is irradiated continuously Eor 10
millisec at a light intensity of about 1018 photon/cm2/sec, or
is irradiated continuously for 100 millisec at an int~nsity of
about 1017 photon/cm2/sec, these light intensities being typical
of those achievable with the preferred forms of ultraviolet
lamps. Typically the flow rate of flue gases through the normal
uniform cross-section ductwork encountered in e.g. conventional
coal-fired power stations is of the order of 10 m/sec, thus
requiring that the irradiated lengths of ductwork should be of
the order of 20 cm in the case of the higher powered lamps or 2m
in the case of the lower powered lamps. The percentage
reductions of NOX that will be required in any given case will
of course depend on the initial concentrations of NOX present in
the flue gas and the levels of NOX conc~ntration that it is
desired to achieve in stack gas passed to the atmosphere but
usually percentage reductions of at least 80~ in the NOX
concentration will be called for. More generally, therefore it
will normally be desired to subject the reaction mixture to a
total quantity of radiant energy flux in range of about 1017 to
about 1018 photon/cm2 of the irradiated area of the flue gas
duct during the photolysis reaction.
The rate of removal of SO2, in the substantial absence of NO~,
is such that about 72% of the S02 is removed when the
S02-containing gas is irradiated at 175 nm for 10 millisec at an
intensity of 3 x 1 ol 8 photon/cm2/sec.
- 13 -
The embodiment illustrated in FigO 1 is particularly well sui~ed
for use when either it is desired to control only emission of
NOX, and the irradiation is conducted at a wavelength of 190 to
220 nm so that the gaseous photolysis products N2 and N2O may be
vented through the stack 13, or when the object is to control
emissions of solely SO2 or both SO2 and NOX and the irradiation
is conducted at a wavelength of 170 to 190 nm and it is
acceptable to vent particulate photolysis products e.~.
(NH4)2SO4 and NH4NO3 to the atmosphere.
In the embodiment of Fig. la, after irradiation at a dual lamp
arrangement 14 providing firstly reaction of NOX with
photolytically-generated NH~ radical and secondly oxidation of
502 to HSO3 species by irradiation with ultraviolet light in the
wavelength range 170 to 190 nm, the HSO3 species are
subsequently neutralized to form particulate (NH4)2S04 through
reaction with an excess of ammonia in the presence of moisture.
The reaction mixture is passed through a second solids separator
device e.g. a second electrostatic precipitator 19 wherein the
particulate products are removed. Depending on the chemical
composition of flue gases, and particularly if the flue gases
are free from toxic heavy metal materlals, the separated-out
sulfate salts may be recovered e.g. for use as agricultural
fertilizer. Instead of supplying an excess of ammonia to the
flue gas before the irradiation step it will normally be more
efficient to supply the additional ammonia required for
neutralization of the acidic bodies through an auxiliary ammonia
inlet $ubsequent to the lamp 14 and as indicated in broken lines
at 2~, under the control of the sensors in the stack 13 which
control the emission of unreacted ammonia to the atmosphere.
In Figure 3, a preferred arrangement for NOX or NOX and SO2
removal is shown wherein the ammonia is added through line 9
after the flue gas has passed through the precipitator 8. In
many cases, the flue gases emitted by the boiler 7 will contain
HCl and SO3. These will react with NH3 to form particulate
ammonium salts. Addition of the ammonia subsequent to the
- 14 -
precipitator 8 avoids recovery of these soluble salts with the
fly ash from the precipitator 8. In many cases, it is desired
to recover a fly ash which is relatively free from solubles, as
these may render the fly ash unsuitable for some purposes such
as for land fill where the solubles may cause problems owing to
their tendency to leach out when in contact with water. In some
cases it may be desirable to remove the particulate ammonium
salts produced from the addition of ammonia before exposure of
the flue gas to the ultraviolet light source. The lamp 14 may
be of the single NOx-removing type or may be the dual
arrangement providing for sequential removal of NOX and SO2 and
means, such as conventional scrubbers, may be provided between
the lamp 14 and ~he stack 13 for removing acidic species and
particulates.
The optimum conditions required for the photolysis reaction for
any given flue gas composition and, in particular the duration
of the exposure to the ultraviolet radiation and hence the
length of ductwork that needs to be irradiated for any given
source of ultraviolet radiati~n, can best be investigated by
conducting a computer simulation o the photolysis reaction when
conducted with a monochromatic source. Such computer simulation
requires the provision o a set of parameters that comprise the
input to the computer program. These comprise a set of initial
concentrations of all reactive chemlcal species present in the
flue gas, and the extinction coefficients applicable to the
wavelength under investigation for the photolysis reactions of
all photosysable species present in the gas mixture. For the
avoidance of doubt, these extinction coefficients are set out in
Table 2 below for certain selected wavelengths. Other
coefficients applicable to different wavelengths can be readily
obtained from standard texts~
- 15 -
TABLE 2
Photolysis Reactions
Reaction Extinctivn Coeff./l mol cm
184.9 nm193 nm 213.9nm
NH3 + hv ~NH2 + H1000 1500 100
S2 + hv ~SO + O (3P) 172 1000 150
H2O + nv~OH + H 14 0 0
O3 + hv ~2 + ('D) 156 100 150
2 + hv ~20 ( P) 4.2 0.3 0.002
NO2 + hv~NO + O ( P) 68 68 100
N2O + hv ~N2 ~ ('D) 36 35 ~ 1
N2H4 + hv ~2 NH2850 1000 600
- 16 -
51
Further, the parameters include the intensity of the
irradiation, and the rate constants of the significant chemical
reactions that occur during the photolysis reaction. As a
result of extensive study of the chemistry of the reaction, 52
chemical reactions have been identified as being significant
exclusively, and these are listed in Table 3 along with their
respective rate constants.
TABLE 3
Reactions Rate Constant
--1
(dm~ mol s
1. NH2 ~ NO = N2 + H20 1.3 x 10
2 NO2 N20 + H20 1.3 x 10
3. NH2 + NH2 = N2H~ 1.5 x 101
2 2 NO H20 1.2 x 10
5. NH~ + RH = NH3 ~ R 5.0 x 10
6. NH2 ~ H - NH3 1.0 x 10
. NH2 H2 NH3 ~ H 8.7 x 10
8. NH2 + = HNO + H OR HO + NH 2.1 x 10
9. NH2 + OH = HNO + H2 OR NH + H20 6.0 x 10
2 2 H2 2.0 x 10
11. NH3 + H = NH2 ~ H2 3.0 x 108
12. NH3 + OH = NH2 -~ H20 1.0 x 108
13. NH3 + 0 ~ NH2 + OH 4.0 x 105
NO2 2 + NO 5.6 x 109
15. + 2 a 03 8.7 x 10
16. 0 + NO = NO2 9.9 x 108
17- NO ~ 3 = NO2 ~- 1.0 x 10
18. 0('D) + H~0 = 2 OH 1.3 x 1011
19. 0('D) (+M) = 0(3P) (+M) 8.2 x 10 s at 1 Atm.
20. H + 2 = HO2 8.0 x 108
21. HO2 ~ NO = NO2 ~ OH 5.0 x 10
22. 0 + SO = SO2 5.0 x 107
23. 2 f SO = SO2 ~ 0 (3P) 5.0 x 104
24. 03 + SO = SO2 ~ 2 4.~ x 107
25. SO + SO = SO2 + S OR (SO~2 2.0 x 106
- 18 -
TABLE 3 (continued)
Reactions Rate Constant
26. OH + OH = H20 + 0 ( P) 1.1 x 10
27. OH + N02 = HNO3 3.8 x 10
28. + 3 = 22 5.7 x 6
29. OH + CO = CO2 ~ H 9.0 x 107
30- SO + NO2 = S2 ~ NO 8.5 x 10
31. H ~ HO2 = H2 ~ 2 8.4 x 10
= 20H 1.9 x 10
2 5.7 x 10
32. 0 + OH = 2 -~ H 2.3 x 10
33. OH + ~IO2 = H20 + 2 2.1 x 10
34. HO2 + HO2 = H22 + 2 1.4 x 109
35. 03 + OH = HO2 + 2 4.9 x 107
36. 03 + H~2 = OH = 2 1.2 x 10
37. OH + SO~ = HSO3 6.6 x 10
3 2 1.7 x 10
39. 0 H20 OH + 2 1.9 x 101
40. 0 + 0 = 0~ 5.9 x 107
41~ 0 ~ H = OH 2.9 x 108
42. H2 + = OH + H 4.0 x 103
43. S2 + = SO3 1.1 x 107
44. 0 + CO = CO2 3.0 x 104
45. N2H~ ~ H = N2H3 + H~ 1.1 x 108
46. OH ~ H = H20 7.2 x 109
47- OH + H2 = H20 + H 3.9 x 106
48. SO2 ~ HO2 = SO3 + OH 5.4 x 105
49. 0 + NO = N ~ 2 7.7 x 10
-- 19 --
TABLE 3 (continued)
Reactions Rate Constant
50. NO + H - HNO 8.3 x 10
51. NO2 + H = NO + OH 7.5 x 10
52. Free radical~ wall termination 7.5 x 10
Note 1: Many of these reactions are third oxder at
atmospheric pressure. A pseudo-second order rate
constant is quo-ted, and was obtained by
substituting one atmosphere pressure for the third
body.
2: In practice, wall termina~ion was applied only to
NH2 radicals to ensure a conservative result.
- 20 -
Certain of the rate constants and extinction coefficients
sp~cified in Tables 2 and 3 are subject to uncertainties owing
to conflicting reports in the literature, notably in the
extinction coefficients of NH3, 2~ H2O, and SO2 and in the rate
constants of reactions numbered (1) and (2) in Table 3. The
effect of th~se uncertainties can however be readily checked by
varying the input param~ters in ~he computer program and their
effect on the resultant rates of NO removal has been found to be
very small or negligible.
As will be apparent to those skilled in the art, the set of
fi~teen rate equations, one for each of the species NH2, NH3, O,
NO, O('D), H, HO2, ~ 2~ O3, SO, OH, SO2, N2H4, and (free
radical- -> termination) described by the 52 reactions of Table
3 constitute a stiff system of differential equations, because
the rate constants cover a wide range of values. These
equations may be integrated numerically using conventional
methods e~g. as described in "The automatic integration of
ordinary differential equations" Gear C.W., Commun ACM, 14, Pl76
(1971). It should be noted that computer time can be reduced,
and the stability of the solution improved, by using units of
micromoles and microseconds rather than moles and seconds. This
results in concentrations and rates whose numerical magnitudes
are such that round-off errors are less important than they
would be if more conventional units are employed.
As will be appreciated the solutions obtained from the procedure
provide the instantaneous concentrations of any selected species
at any selected time during the oDurse of the photolysis
reaction, and the analysis may be applied to all flue gases
obtained from conventional combustion processes including, as
mentioned above, the combustion at high temperatures in the
presence of air of fuels as diverse as hydrogen gas, fossil
fuels and fuels which are derivatiYes of fossil fuels, which
flue ga~es contain substantial quantities of SO2 and/or NO~,
typically 1 x 10-6 to 1 x 10-4 mol/1 SO2, more typically 1000 to
-- 21
2000 ppm SO2, 1 x 10~7 to 1 x 10-5 mol/1 NO2 and 1 x 1 o-6 to 1 x
10-4 mol/1 NO~
Usually the concentrations of the species present in such fuel
gases will be as indica~ed in Table 4.
-~ 22 -
TABLE 4
Species Concentration Range mol/l
~03 0 to 1 x 10 6
S2 0 to 1 x 10 4
NO2 0 to 1 x 10 5
N20 0 to 1 x 10 7
NO 0 to 1 x 10 4
HCl 0 to 1 x 10
OH 1 x 10 to 1 x 10 12
H20 1 x 10 to 1 x 10
2 1 x 10 to 1 x 10 2
RH ~hydrocarbons) 0 to 1 x 10 6
CO 1 x 10 to 1 x 10 5
C2 1 x 10 4 to 1 x 10 2
Others (mainly nitrogen) balance
2~ -
Merely by way of example a typical flue gas composition as
obtained from a coal-fired power station is given in Table 5
(reactive species only).
TABLE 5
Species Concentration
0~ 3 1/2 to 9% by volume
(full to part load3
H2O 10% by volume
NO 300 ~ 700 ppm (vol)
S2 1100 ~ 1600 ppm (vol)
HCl 100 ppm (vol)
so3 10 - 16 ppm (vol3
NO2 30 - 70 ppm Ivoll
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