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Flue gas cleaning by the electron-beam-process (I): Optimization of removal efficiency and energy consumption at the ITS-facility
Radiat. Phys. Chem. Vol. 35, Nos 1-3, pp. Int. J. Radiat. Appl. lnstrum., Part C
0146-5724/90 $3.00 + 0.00 Pergamon Press plc
422-426, 1990
Printed in Great Britain
FLUE GAS CLEANING BY T H E ELECTRON-BEAM-PROCESS (I): OPTIMIZATION OF REMOVAL EFFICIENCY AND ENERGY CONSUMPTION AT THE ITS-FACILITY U. Willibald, K.-H. Platzer, S. Wittig Lehrstuhl und Institut ffir Thermische Str6mungsmaschinen Universit~t Kadsruhe (TH), Kaiserstr. 12, 7500 Karlsruhe 1 Federal Republic of Germany
ABSTRACT Electron beam irradiation of flue gases is a method for simultaneous SO2 and NOx control. The energy requirement for N O removal is determined only by the initialN O concentration and increases linearly with it. In analyzing the total NOx removal efficiencythe production of NO2 and N20 has to be considered. For the SO2 removal, the irradiation dose is not the single determining factor. The NH3 mole ratio, the flue gas temperature and humidity are of predominant importance. The influence of the dose rate on the conversion efficiencyhas been investigated at the ITS for the firsttime by altering the irradiation conditions systematically. KEYWORDS Flue gas cleaning, electron beam irradiation, SO2 removal, NOffi removal, kinetic reaction model, phenomenological model, coupling of radiation and flow field.
INTRODUCTION At the Institute for Thermal Turbomachinery (ITS), University of Karlsruhe, the Electron Beam Process for simultaneous removal of NOx and SO2 has been under detailed theoretical and experimental analysis since early 1984 using a laboratory facilitywith a fine gas capacity of 1000 m~/h (Wittig et a/., 1988a, b). In the presence of ammonia, the chemical reactions induced by high energy electron beams lead to the formation of fine ammonium salt particles which can be precipitated. A kinetic reaction model has been built up, allowing the identification and understanding of physical and chemical processes involved in the removal of the pollutants. However, the description of the by-product particleformation is difficult,because there is a significantinfluence of heterogeneous reactions. Therefore, only experimental investigationsproduce complete and realisticinformation on the process performance. The results of a considerable number of removal experiments have been concentrated in an empirical model describing the process phenomenologically. Furthermore, basic research has been performed concerning the coupling of spatial non-uniform radiation and flow field. All the results influence the pilot plant project at the Badenwerk coal fired 550 MWe, unit R D K 7 of the Rheinhafen-Damplkraftwerk, Karlsruhe (Fuchs eta]., 1988) that will be discussed in detail in the second part of this paper (Platzer eta/., to be published). REACTION
EXCITATION
BY MEANS
OF ELECTRON
BEAM
IRRADIATION
Interaction of high energy electrons with atomic electrons causes ionization and excitation of the main flue gas components (N2, H20, CO2 und 02). If a sufficient amount of energy is transferred, excited molecules can decompose forming reactive radicals. Therefore, the radiation induced excitation of neutral chemical reactions is determined by the energy transferred to the target. Gas phase reactions with activated species, namely
422
7th International Meeting on Radiation Processing
423
radicals such as OH*, O2H', N', O', H', lead to the conversion of NOx and SO2. However, the formation of by-product-aerosols is primarily governed by heterogeneous reactions• The production rate of active species is determined by the dose rate. These species are consumed in chemical reactions with ground state flue gas molecules, a part of them leading to the desired conversion of the pollutants. At low and medium dose rates, the reactions consuming those radicals proceed fast enough to balance the radiation production• Therefore, the equilibrium concentrations of the radicals are low. However, at dose rates of several hundreds of kGy/s or more, the radical concentrations are much higher and radical recombination reactions may no longer be neglected, leading to a decreasing efficiency of the SOz/NO= removal. To investigate the influence of the dose rate in the technical range, special experiments have been performed at the ITS. Some of the results are presented in the last section of this paper. SO= REMOVAL Heterogeneous reactions are especially important for the SO~ removal. Therefore, in addition to the radiation dose, the NHs mole ratio as well as the flue gas temperature and humidity must be considered as important parameters governing the SO: removal.
•/•"eo~o~°~" 0
' 15 H20
"
"
' 12
"
"
concentrotion
' 9
"
"
' 6
[Vol.-
b
" ~]
Figure 1: Influence of flue gas humidity and temperature on the SO2 removal efficiency (RNHs----0.6) A set of experiments at the ITS laboratory facility with systematic variation of the relevant parameters indicates, that a considerable amount of the initial SO= content can be converted even without irradiation (Fig. 1). Irradiation leads with several intermediate reaction steps to (NH4)2SO4 as by-product. For technical applications there are other points of view to be regarded, such as the performance of the precipitator, byproduct deposition in the flue gas channel or NHs slip control, which lead to a SO2 removal characteristic as shown in Fig. 2.
I do
do~e
[kGy]
Figure 2: Influence of dose and NH: mole ratio for SO2 removal (T = 70 °C, H20 = 8 Vol.-~)
RPC3~I-3~CC
424
U. WILLIBALD et al.
NOz REMOVAL Homogeneous gas phase reactions with radicals dominate the NOx conversion. For NOx compositions typical for flue gases ( ~ _ >> 1) there are two competitive reaction mechanisms: In the lower dose range, the oxidation of NO to NO2, HNO2 and HNOs predominates. At higher doses, when the NO concentration is low, the produced NO2 is decomposing under the formation of HNO2, HNOs and N20. In the presence of ammonia, the acids are neutralized forming NH4NO2 and NH4NOs. NO removal Both the reaction kinetic studies and the experiments have identified the initial NO content [NO]0 and the irradiation dose as the essential parameters for the NO removal. As dose is related to the initial NO concentration, the graph for different [NOl0 values is identical as shown in Fig. 3. In other words, the energy requirement increases linearly with [NOl0 and for that, economical applications of the Electron Beam Process are limited to flue gas with low initial NO concentration. 1.00 ® 0
~,
o o
0
e ee o.7s Z
ee e e 0 o
B
o
O.SO
0
0 Z
O.2S o
.
.
.
.
,
o.ooo
.
.
.
0,025
.
,
.
O.QSO D,/[NO]0
dose
.
.
.
,
.
,
0,075
0.100
[kGy/vprn]
Figure 3: NO removal efficiency as function of dose related to [NO]o NO= production As mentioned earlier, the NO2 concentration increases with dose because of the NO decomposition and is reduced itself by radiation induced reactions. The NO2 production is not only influenced by the initial NO concentration and the irradiation dose, but also by the SO2 content and the added amount of NHs. As shown in Fig. 4, for typical process parameters and desulfurized flue gas, the NO2 production is in the order of 30 % of the initial NO concentration. The corresponding values for flue gases with SO2 are in the range of 10 %. 0.5
o 0.4 e
c
z
O
o O
0
e
e
o 0., o ~ e o
NO2
~
o Z
o
A
&
6
A
~ Q
o.o
0.000
0.0@5
ao-.
0.084)
O/[NOlo
O.O?S
[kOy/~nq
Figure 4:NO2 and N20 production as a function of dose related to [NO]0
7th International Meeting on Radiation Processing
425
N20 production The radiation induced NO~ decomposition leads to the formation of nitrous oxide, which is enhanced in the presence of ammonia during irradiation. The N20 concentration increases almost linearly with dose to a level of 10 % of the initial NO content for process parameters which are of technical interest (Fig. 4). As N20 is nearly inert in the lower atmosphere, it is tranported vertically to the stratosphere where it contributes to reduce the ozone concentration. In addition it is counted as a greenhouse effect gas. Thus, even without any legislative regulations it seems justified to take this component into account for the NOffi emissions. NO~ removal The overall NOx removal efficiency is, therefore, given by the superposition of NO removaland the production rates of NO2 and N20. It is not possible to achieve NOx removal rates beyond 70 % only by applying large doses because of the increasing N20 formation (Fig. 5). 1.00
o O Z
0.75
O o
oo
o
o.5o a O
7
o~e o
0°o0 0.000
o°
o
.
.
.
.
, . 0.025
.
.
.
don
, 0.050
0/[NOlo
.
.
.
.
, 0.07~
. 0.1OO
[kC~/vpm]
Figure 5: Dose dependence of NOx removal
INFLUENCE OF DOSE RATE As the removal of nitric oxides is considerably determined by radiation induced reactions, the influence of dose rate has been investigated using flue gases without SO2 (Wittig e t a / . , 1988b). Nevertheless, the results are also valid for simultaneous removal. To guarantee an excitation as uniform as possible for the whole gas flow, the flow field has been matched to the radiation field. As shown in Fig. 6, the maximum velocity corresponds to high values of local dose rate in the region of the electron beam window.
/
~ 2 ~ ' - ' a ~
~
-..
shape
of
"..reactor walt
to~
,lo
[mini
/ Figure 6: Flow field in the irradiation zone A set of experiments has been performed with reduction of irradiation time I" from 0.4 s to 0.02 s by variation of mass flow and the length of the irradiation zone. By that, the mean dose rate has been extended to a range
426
U. WILLIBALDet al.
which is more than of technical interest. A total dose of 10 kGy for example has applied in with a mean dose rate between 25 kGy/s and 480 kGy/s. The results shown in Fig. 7 illustrate that the dose rate does not affect the removal and production rate of nitric oxides in the referred range. Small differences of NO~ values are in the range of the measurement accurac~ X
o
O.lql
./
Z 0
irradiation
0
E 0 Z
time
X
"7" =
0.412
s
(D
T
0.213
s
=
+
T
=
0.043
s
O
T
=
0.041
s
A
7"
=
0.021
s
0.00 0.7 q
0 Z
i r
X, X 0 • I~
i Z
O.!W
X
I
I u
ek
0.2S
i
t
Z
0.00
~22S
0 Z X Z 0.184 I
~ o
X
X~
O.07S (3.
Z
o.~j
~r~,~~~ p~' . . . . .
'~
o.~16 dose
"
"
O/[NOlo
0.20
[kCy/,,p~]
Figure 7: Influence of dose rate on NOx-conversion rates REFERENCES
Fuchs, F., B. Roth, U. Schwing, H. Angele, J. Gottstein (1988). Removal of NOx and SOs by the electron beam process. In: Rad. Phys. and Chem., 31, 45 - 56. Platzer, K.-H., U. Willibald, J. Gottstein, A. Tremmel, H.-J. Angele, K. Zellner (to be published). Flue gas cleaning by the electron-beam-process (II): Recent activities at the RDK-? pilot plant, Karlsruhe. In: Proceedings of the 7th International Meeting on Radiation Processing, April 23rd to 28th, 1989, Noordwijkerhout, the Netherlands. Wittig, S., G. Spiegel, K.-H. Platzer, U. Willibald (1988a). The performance characteristics of the electronbeam-technique: Detailled studies at the (ITS) flue gas facility. In: Rad. Phys. and Chem., 31, 83 94. Wittig, S., G. Spiegel, K.-H. Platzer, U. Willibald (1988b). Simultane Rauchgasreinigung (Entschwefelung, Denitrierung) dutch Elektronenstrald. KfK-PEF 45, Kernforscliungszentrum Karlsruhe
0146-5724/90 $3.00 + 0.00 Pergamon Press plc
422-426, 1990
Printed in Great Britain
FLUE GAS CLEANING BY T H E ELECTRON-BEAM-PROCESS (I): OPTIMIZATION OF REMOVAL EFFICIENCY AND ENERGY CONSUMPTION AT THE ITS-FACILITY U. Willibald, K.-H. Platzer, S. Wittig Lehrstuhl und Institut ffir Thermische Str6mungsmaschinen Universit~t Kadsruhe (TH), Kaiserstr. 12, 7500 Karlsruhe 1 Federal Republic of Germany
ABSTRACT Electron beam irradiation of flue gases is a method for simultaneous SO2 and NOx control. The energy requirement for N O removal is determined only by the initialN O concentration and increases linearly with it. In analyzing the total NOx removal efficiencythe production of NO2 and N20 has to be considered. For the SO2 removal, the irradiation dose is not the single determining factor. The NH3 mole ratio, the flue gas temperature and humidity are of predominant importance. The influence of the dose rate on the conversion efficiencyhas been investigated at the ITS for the firsttime by altering the irradiation conditions systematically. KEYWORDS Flue gas cleaning, electron beam irradiation, SO2 removal, NOffi removal, kinetic reaction model, phenomenological model, coupling of radiation and flow field.
INTRODUCTION At the Institute for Thermal Turbomachinery (ITS), University of Karlsruhe, the Electron Beam Process for simultaneous removal of NOx and SO2 has been under detailed theoretical and experimental analysis since early 1984 using a laboratory facilitywith a fine gas capacity of 1000 m~/h (Wittig et a/., 1988a, b). In the presence of ammonia, the chemical reactions induced by high energy electron beams lead to the formation of fine ammonium salt particles which can be precipitated. A kinetic reaction model has been built up, allowing the identification and understanding of physical and chemical processes involved in the removal of the pollutants. However, the description of the by-product particleformation is difficult,because there is a significantinfluence of heterogeneous reactions. Therefore, only experimental investigationsproduce complete and realisticinformation on the process performance. The results of a considerable number of removal experiments have been concentrated in an empirical model describing the process phenomenologically. Furthermore, basic research has been performed concerning the coupling of spatial non-uniform radiation and flow field. All the results influence the pilot plant project at the Badenwerk coal fired 550 MWe, unit R D K 7 of the Rheinhafen-Damplkraftwerk, Karlsruhe (Fuchs eta]., 1988) that will be discussed in detail in the second part of this paper (Platzer eta/., to be published). REACTION
EXCITATION
BY MEANS
OF ELECTRON
BEAM
IRRADIATION
Interaction of high energy electrons with atomic electrons causes ionization and excitation of the main flue gas components (N2, H20, CO2 und 02). If a sufficient amount of energy is transferred, excited molecules can decompose forming reactive radicals. Therefore, the radiation induced excitation of neutral chemical reactions is determined by the energy transferred to the target. Gas phase reactions with activated species, namely
422
7th International Meeting on Radiation Processing
423
radicals such as OH*, O2H', N', O', H', lead to the conversion of NOx and SO2. However, the formation of by-product-aerosols is primarily governed by heterogeneous reactions• The production rate of active species is determined by the dose rate. These species are consumed in chemical reactions with ground state flue gas molecules, a part of them leading to the desired conversion of the pollutants. At low and medium dose rates, the reactions consuming those radicals proceed fast enough to balance the radiation production• Therefore, the equilibrium concentrations of the radicals are low. However, at dose rates of several hundreds of kGy/s or more, the radical concentrations are much higher and radical recombination reactions may no longer be neglected, leading to a decreasing efficiency of the SOz/NO= removal. To investigate the influence of the dose rate in the technical range, special experiments have been performed at the ITS. Some of the results are presented in the last section of this paper. SO= REMOVAL Heterogeneous reactions are especially important for the SO~ removal. Therefore, in addition to the radiation dose, the NHs mole ratio as well as the flue gas temperature and humidity must be considered as important parameters governing the SO: removal.
•/•"eo~o~°~" 0
' 15 H20
"
"
' 12
"
"
concentrotion
' 9
"
"
' 6
[Vol.-
b
" ~]
Figure 1: Influence of flue gas humidity and temperature on the SO2 removal efficiency (RNHs----0.6) A set of experiments at the ITS laboratory facility with systematic variation of the relevant parameters indicates, that a considerable amount of the initial SO= content can be converted even without irradiation (Fig. 1). Irradiation leads with several intermediate reaction steps to (NH4)2SO4 as by-product. For technical applications there are other points of view to be regarded, such as the performance of the precipitator, byproduct deposition in the flue gas channel or NHs slip control, which lead to a SO2 removal characteristic as shown in Fig. 2.
I do
do~e
[kGy]
Figure 2: Influence of dose and NH: mole ratio for SO2 removal (T = 70 °C, H20 = 8 Vol.-~)
RPC3~I-3~CC
424
U. WILLIBALD et al.
NOz REMOVAL Homogeneous gas phase reactions with radicals dominate the NOx conversion. For NOx compositions typical for flue gases ( ~ _ >> 1) there are two competitive reaction mechanisms: In the lower dose range, the oxidation of NO to NO2, HNO2 and HNOs predominates. At higher doses, when the NO concentration is low, the produced NO2 is decomposing under the formation of HNO2, HNOs and N20. In the presence of ammonia, the acids are neutralized forming NH4NO2 and NH4NOs. NO removal Both the reaction kinetic studies and the experiments have identified the initial NO content [NO]0 and the irradiation dose as the essential parameters for the NO removal. As dose is related to the initial NO concentration, the graph for different [NOl0 values is identical as shown in Fig. 3. In other words, the energy requirement increases linearly with [NOl0 and for that, economical applications of the Electron Beam Process are limited to flue gas with low initial NO concentration. 1.00 ® 0
~,
o o
0
e ee o.7s Z
ee e e 0 o
B
o
O.SO
0
0 Z
O.2S o
.
.
.
.
,
o.ooo
.
.
.
0,025
.
,
.
O.QSO D,/[NO]0
dose
.
.
.
,
.
,
0,075
0.100
[kGy/vprn]
Figure 3: NO removal efficiency as function of dose related to [NO]o NO= production As mentioned earlier, the NO2 concentration increases with dose because of the NO decomposition and is reduced itself by radiation induced reactions. The NO2 production is not only influenced by the initial NO concentration and the irradiation dose, but also by the SO2 content and the added amount of NHs. As shown in Fig. 4, for typical process parameters and desulfurized flue gas, the NO2 production is in the order of 30 % of the initial NO concentration. The corresponding values for flue gases with SO2 are in the range of 10 %. 0.5
o 0.4 e
c
z
O
o O
0
e
e
o 0., o ~ e o
NO2
~
o Z
o
A
&
6
A
~ Q
o.o
0.000
0.0@5
ao-.
0.084)
O/[NOlo
O.O?S
[kOy/~nq
Figure 4:NO2 and N20 production as a function of dose related to [NO]0
7th International Meeting on Radiation Processing
425
N20 production The radiation induced NO~ decomposition leads to the formation of nitrous oxide, which is enhanced in the presence of ammonia during irradiation. The N20 concentration increases almost linearly with dose to a level of 10 % of the initial NO content for process parameters which are of technical interest (Fig. 4). As N20 is nearly inert in the lower atmosphere, it is tranported vertically to the stratosphere where it contributes to reduce the ozone concentration. In addition it is counted as a greenhouse effect gas. Thus, even without any legislative regulations it seems justified to take this component into account for the NOffi emissions. NO~ removal The overall NOx removal efficiency is, therefore, given by the superposition of NO removaland the production rates of NO2 and N20. It is not possible to achieve NOx removal rates beyond 70 % only by applying large doses because of the increasing N20 formation (Fig. 5). 1.00
o O Z
0.75
O o
oo
o
o.5o a O
7
o~e o
0°o0 0.000
o°
o
.
.
.
.
, . 0.025
.
.
.
don
, 0.050
0/[NOlo
.
.
.
.
, 0.07~
. 0.1OO
[kC~/vpm]
Figure 5: Dose dependence of NOx removal
INFLUENCE OF DOSE RATE As the removal of nitric oxides is considerably determined by radiation induced reactions, the influence of dose rate has been investigated using flue gases without SO2 (Wittig e t a / . , 1988b). Nevertheless, the results are also valid for simultaneous removal. To guarantee an excitation as uniform as possible for the whole gas flow, the flow field has been matched to the radiation field. As shown in Fig. 6, the maximum velocity corresponds to high values of local dose rate in the region of the electron beam window.
/
~ 2 ~ ' - ' a ~
~
-..
shape
of
"..reactor walt
to~
,lo
[mini
/ Figure 6: Flow field in the irradiation zone A set of experiments has been performed with reduction of irradiation time I" from 0.4 s to 0.02 s by variation of mass flow and the length of the irradiation zone. By that, the mean dose rate has been extended to a range
426
U. WILLIBALDet al.
which is more than of technical interest. A total dose of 10 kGy for example has applied in with a mean dose rate between 25 kGy/s and 480 kGy/s. The results shown in Fig. 7 illustrate that the dose rate does not affect the removal and production rate of nitric oxides in the referred range. Small differences of NO~ values are in the range of the measurement accurac~ X
o
O.lql
./
Z 0
irradiation
0
E 0 Z
time
X
"7" =
0.412
s
(D
T
0.213
s
=
+
T
=
0.043
s
O
T
=
0.041
s
A
7"
=
0.021
s
0.00 0.7 q
0 Z
i r
X, X 0 • I~
i Z
O.!W
X
I
I u
ek
0.2S
i
t
Z
0.00
~22S
0 Z X Z 0.184 I
~ o
X
X~
O.07S (3.
Z
o.~j
~r~,~~~ p~' . . . . .
'~
o.~16 dose
"
"
O/[NOlo
0.20
[kCy/,,p~]
Figure 7: Influence of dose rate on NOx-conversion rates REFERENCES
Fuchs, F., B. Roth, U. Schwing, H. Angele, J. Gottstein (1988). Removal of NOx and SOs by the electron beam process. In: Rad. Phys. and Chem., 31, 45 - 56. Platzer, K.-H., U. Willibald, J. Gottstein, A. Tremmel, H.-J. Angele, K. Zellner (to be published). Flue gas cleaning by the electron-beam-process (II): Recent activities at the RDK-? pilot plant, Karlsruhe. In: Proceedings of the 7th International Meeting on Radiation Processing, April 23rd to 28th, 1989, Noordwijkerhout, the Netherlands. Wittig, S., G. Spiegel, K.-H. Platzer, U. Willibald (1988a). The performance characteristics of the electronbeam-technique: Detailled studies at the (ITS) flue gas facility. In: Rad. Phys. and Chem., 31, 83 94. Wittig, S., G. Spiegel, K.-H. Platzer, U. Willibald (1988b). Simultane Rauchgasreinigung (Entschwefelung, Denitrierung) dutch Elektronenstrald. KfK-PEF 45, Kernforscliungszentrum Karlsruhe