Note: Descriptions are shown in the official language in which they were submitted.
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ELECTRON ACCELERATOR AND DEVICE USING SAME
TECHNICAL FIELD
[0001] The invention relates to the field of electron beam
accelerators and to a device using said electron accelerator for treating
products or materials. The invention is more particularly directed to a
device for treating combustion exhaust gases for reducing SO2 (sulphur
dioxide and NOX(including NO2and NO nitrogen oxides) emissions.
DESCRIPTION OF RELATED ART
[0002] A description of radiation processing of flue gas and of the
installations therefore is given in "Radiation processing: environmental
application", International Atomic Energy Agency, 2007, available from
www-pub.iaea.org/MTCD/publications/PDF/RPEA-Web.pdf, and in A. G.
Chmielewski, J. Licki, A. Pawelec, B. Tyminski, Z. Zimek, Operational
experience of the industrial plant for electron beam flue gas treatment,
Radiatin Physics and Chemistry, Vol. 71, Issues 1-2, pp. 441-444, 2004.
The first report describes (pages 11 to 13) amongst others, the industrial
scale installation located at the Pomorzany electric power station in
Szczecin, Poland. This installation purifies flue gas from two boilers (65
MW(e), 100 MW(th) each). The maximum flow rate of the gases is 270 000
Nm3/h, and the total electron beam power exceeds 1MW. For obtaining
such a high power, four electron accelerators (260kW, 700 keV each) are
required. The irradiation of flue gases is performed in two cylindrical
process vessels, each 2.6m in diameter and 14m in length. The average
absorbed dose ranges between 7 and 12 kGy. The S02 removal efficiency
ranges from 80% to 90% (It was measured as 92.5% at 9.5 kGy dose) The
NOx removal efficiency ranges from 50% to 60% (It was measured as 65%
at 9.5 kGY dose). Of course, such an installation comprising four
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accelerators and two large process vessels will require more space,
ancillary equipment, and costs.
[0003] A typical flue gas exiting from coal or oil burned in an
electrical power plant contains about 72% by volume of N2, 5% by volume
of 02, 13% by volume of C02, 10% by volume of H20, 1000ppm of SO2
and 350ppm of NOX. The last two components are particularly harmful to
the environment because, when released in the atmosphere, and under
influence of UV rays, they produce SO3, and NO2, and ultimately, in
presence of water, sulphuric and nitric acid, which cause acid rains. Known
processes for neutralizing these acids comprise the addition of calcium
oxide, also known as quick lime (CaO), calcium hydroxide, also know as
slaked lime (Ca(OH)2) or ammonia (NH3). The mixture of flue gas with
dispersed lime and/or ammonia is treated with ionizing radiation. This
irradiation produces ions, electrons and free radicals, and increases the
reactivity of the harmful components. The final products are calcium
sulphate and calcium nitrate, with lime addition, and ammonium sulphate
and ammonium nitrate, with ammonia addition. These compounds are in
the form of fine particles, which can be removed from the flue gas stream
with conventional bag filters or electrostatic precipitators.
[0004] A method and apparatus for treating waste gases by
exposure to electron beams is known from documents US 4324759 and
US 5834722. In this method, a mixture of ammonia gas and air is mixed
with water and sprayed into a reactor containing the waste gases. The
waste gases are then exposed to an electron beam for removing SO2 and
NOX. These documents however give no details on the electron beam
used: no data are given on electron beam energy, electron beam power or
type of accelerator. The reaction vessel is only represented in a schematic
way, and no details are given on its structure and dimensions. No
information is given as to how to produce an industrial scale flue gas
processing plant. A pollution control by spray dryer and electron beam
treatment is known from document US 4372832. This document describes
the use of lime, limestone, sodium compounds, magnesium compounds or
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mixtures thereof as reagents. According to this document (Fig. 3) a
electron beam reactor using 11 MW of electricity power is needed for
providing a dosage of 0.5 to 2 Mrad (5 to 20 kGy). However, providing
such a large electron beam power would require a large number of
separate accelerators. The publication "D.J. Helfritch, P.L. Feldman,
Cottrell Environment Sciences. FLUE GAS S02/NOx control by
combination of dry scrubber and electron beam, Radiat. Phys. Chem. Vol.
24, N 1, pp 129-143, 1984" describes a solution for achieving this goal,
requiring four reactor channels, each utilizing three power supplies and
twelve accelerator/scanners, which is not a satisfactory solution with
regards to space and costs.
[0005] A process and apparatus for removing SO2 and NOX from flue
gases is known from WO 92/20433. According to this document, flue gas
is subjected to irradiation by electron beam and to the action of
microwaves. An electron accelerator produces pulses having an energy of
0.7 to 2 MeV, at a repetition rate of 20 ms and a pulse duration of 400 ps,
and a peak power of 1 MW. This accelerator therefore has a e-beam
output power of only 20kW. This power level is not sufficient for treating
the large quantity of flue gas of an industrial electrical power plant.
Electron accelerators used in the radiation processing industry can reach
energy levels of 700 keV, using relatively low cost iron core transformers
for producing the required high voltage. Producing higher energies, such
as 1 MeV or above, approaches the technological limit of these designs,
especially if high power is also required. At low energies, a flexible cable
can be used for transporting the energy from the power supply. At energies
above 800 keV, these cables could be damaged by sparking, which would
reduce the reliability of the device.
[0006] A different type of accelerator was designed for industrial
high power, high energy industrial applications. A Rhodotron , described
in document US5107221, comprises a cavity having two coaxial cylinders.
This type of accelerator increases the electron energy in discrete steps
while they pass repeatedly through the same resonant cavity. The patent
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describes a system consisting of a single electron injector and output
beam line with multiple passes of the electron beam through the resonant
cavity. The beam is turned around between passes with deflection
magnets located outside of the cavity. A unique feature of this design is the
production of high-energy electrons with a low energy gain per pass. This
minimizes the intensity of the electric field in the cavity and reduces the
loss of radio-frequency (rf) power in the walls of the cavity. Therefore, the
rf amplifier can be operated in the continuous-wave mode. This avoids the
need to pulse the rf power source to limit the cavity losses. As a result, the
electron beam is essentially continuous, except for bunching at the
resonant frequency. This is advantageous when scanning the beam across
a fast-moving product conveyor or continuous stream of fluid or granular
material. The use of a single cavity avoids the requirements of fine tuning
to synchronize the resonant frequencies of many cavities, as in a
microwave linear accelerator (linac). Consequently, a Rhodotron does
not need accurate temperature control, which would be required to
maintain a constant resonant frequency. The driver for the rf amplifier
detects the resonant frequency of the cavity and follows small variations in
frequency caused by changes in the cavity temperature. The present IBA
Rhodotrons produce electron energies in the range of 5 MeV to 10 MeV,
with electron beam currents up to 100 mA and beam powers up to 700 kW.
These capabilities are well suited for radiation processing applications,
such as sterilizing medical devices, crosslinking plastic products, curing
fiber-reinforced composite materials and preserving foods with high-energy
electrons or X-rays. The beam current and beam power limitations of the
present Rhodotrons are mainly due to the characteristics of the low-
energy electron beam that is injected into the resonant cavity. The first
pass is especially critical. After traversing the large radius of the cavity,
the
injected beam must have a small diameter and a low value of emittance in
order to pass through the small apertures in the inner conductor of the
resonant cavity. Even the most powerful Rhodotron , the Model TT1000,
can use only about half of the rf power that could be provided by its large
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amplifier. These accelerators are optimized for high power, high energy
applications. However, in some applications, such as the extraction of acid
forming compounds from the flue gases emitted by fossil-fuelled electric
power plants, the disinfection of municipal waste water and the
5 detoxification of industrial waste materials to reduce environmental
pollution, other beam characteristics are needed. Applications like these
could require more beam current and beam power than can be provided
with the known electron accelerators. No accelerator exists for the
treatment of flue gas, where very high power (1MW or more) in the mid-
energy (1 to 3 MeV) range is required.
SUMMARY OF THE INVENTION
[0007] According to a first aspect, the invention is directed to an
electron accelerator comprising a resonant cavity with an outer cylindrical
conductor and an inner cylindrical conductor having the same axis of
revolution, a high frequency power source coupled to the cavity and
supplying an electromagnetic field at a resonant frequency of the cavity, a
first electron source able to inject into the cavity a first electron beam in
a
first direction through a first inlet port made in the outer conductor, said
first
electron beam being injected along an electric field line of the resonant
field, in a plane perpendicular to the axis of the cavity where the radial
component of the rf electric field is at a maximum and the rf component of
the magnetic field is at a minimum. The accelerator of the invention
comprises at least one second electron source able to inject into the cavity
at least one second electron beam in a second direction through a second
inlet port made in the outer conductor, said second electron beam being
injected along an electric field line of the resonant field, said second
electron beam being injected in same plane as said first electron beam and
said first direction being angularly displaced from said second direction.
This accelerator can be obtained by a modification of a Rhodotron where
the multiple pass is replaced by a single pass, and where multiple electron
sources are installed and multiple electron beams are produced
simultaneously.
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[0008] Preferably, the accelerator of the invention comprises two to
ten, and preferably six to eight electron sources and corresponding
electron beams.
[0009] In a preferred embodiment, the accelerator comprises
deflecting magnets outside the cavity for redirecting the first electron beam
and/or the second electron beam into the cavity for additional acceleration.
In this embodiment, higher total beam energies can be obtained.
[0010] In a variation of the invention, the accelerator comprises a
second high frequency power source coupled to the cavity and supplying
additional electromagnetic power at a resonant frequency of the cavity.
Using this variation, a higher beam power can be obtained.
[0011] According to a second aspect, the invention is directed to a
device for irradiation of gases comprising an electron accelerator
according to the invention comprising a process vessel into which the
electron beams are directed, and the flue gases pass.
[0012] Preferably, said process vessel is cylindrical conical or
spherical and the electron beam is directed into the vessel along its axis of
revolution.
[0013] Alternatively, said process vessel is cylindrical conical or
spherical and the electron beam is directed into the vessel in a plane
perpendicular to its axis of revolution.
[0014] According to a third aspect, the invention relates to the
use of a device according to the invention for the reduction of SO2/
NOX contained in flue gases.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] FIG. 1 is a schematic side view of a section of an electron
accelerator according to a first embodiment of the invention.
[0016] FIG. 2 is a top view of an electron accelerator according to
said first embodiment.
[0017] FIG. 3 is a top view of an electron accelerator according to a
second embodiment of the invention.
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[0018] FIG. 4 is a side view of a device for irradiating gases
according to the invention.
[0019] FIG. 5 is a top view of another device for irradiating gases
according to the invention.
DETAILED DESCRIPTION OF THE INVENTION
[0020] Fig. 1 is a general schematic side view of a section of an
electron accelerator according to the invention. A resonant cavity 10
comprises an outer cylindrical conductor 20, and an inner cylindrical
conductor 30, connected by upper and lower flanges. A high frequency
power source 40 excites a resonant frequency of the cavity. An electron
source 50 directs an electron beam 60 along the median plane of the
cavity, towards the revolution axis of the cavity. The cavity 10, the power
source 40 and the electron source 50 may advantageously be taken as the
components of the existing Rhodotron models.
[0021] In contrast to the existing Rhodotrons , the accelerator of the
invention comprises two of more electrons sources 50, 52, directing
multiple beams into the cavity. FIG. 2 is a top view of an electron
accelerator comprising six electrons sources arranged around the outside
of the cavity, producing six electrons beams, each beam traversing the
cavity only once. By using existing components, six beams of 125mA
each, having energy of 1.25 MeV, for a total power of 937, 5 kW can be
obtained. The minimum angular space between the electron sources 50,
52 is determined by the physical size of the source. Using the existing
components, the different electron sources can be arranged around the
cavity at an angular space of 15 . The accelerated beams leave the cavity
at different angles, but they are directed to a common flue-gas duct, fluid
stream or product conveyor by external beam transport systems of
conventional design.
[0022] For applications requiring higher electron energies, up to 2.5
MeV, two passes through the cavity provide sufficient beam energy. In this
case, multiple electron injectors can be placed around the outside of the
cavity and all of the beams can be accelerated simultaneously. This
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method of increasing the total beam current and beam power permits the
use of existing designs for the electron sources. Fig. 3 illustrates a design
where two electron sources 50, 52, direct two electron beams 60, 62 into
the cavity. Said beams are redirected into the cavity for a second pass,
using deflecting magnets 80, in a known way. In this design, the bending
magnets 80 achieve achromatic bends. The beam current capability of the
present single-beam Rhodotrons, up to 125 mA, can be maintained for
each beam. The use of two injectors can provide a total beam current of
250 mA and a total beam power of 625 kW. With three injectors, the total
beam current would be 375 mA and the total beam power would be 937,5
kW.
[0023] Using a high-power, mid-energy accelerator as described
above, a device 90 for irradiating flue gases can be built. As illustrated on
Fig. 4, a process vessel 100 having generally a cylindrical shape, with
length L, and radius R, comprises a gas inlet 110 and a gas outlet 120. An
electron beam is directed axially into the cylinder. A foil window 105
separates the vacuum in the accelerator and the beam transport system
from the process vessel 100. Preferably, two metallic foil windows would
be used, one to maintain the vacuum in the accelerator and the other to
confine the flue gases to the process vessel. Monte Carlo simulations were
performed with 2.5 Mev electrons for 5 process vessels with the following
dimensions:
case 1: L= 500 cm; R= 100cm
case 2: L = 750 cm; R = 150 cm
case 3: L = 1000 cm; R = 200 cm
case 4: L = 1250 cm; R = 400 cm
case 5: L = 1250 cm, R = 600 cm.
It was determined that the total energy deposited by the beam in the
irradiation volume are:
case 1: 28.6%
case 2: 39.2%
case 3: 47.8%
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case 4: 71.8%
case 5: 86.8%.
Other Monte Carlo simulations were performed with 1.5 Mev electrons for
4 process vessels with the following dimensions:
case 6: L = 700 cm; R = 200 cm
case 7: L = 700 cm; R = 300 cm
case 8: L = 700 cm; R = 400 cm
case 9: L = 700 cm; R = 500 cm
It was determined that the total energy deposited by the beam in the
irradiation volume are:
case 6: 59.5%
case 7: 75.9%
case 8: 86.3%
case 9: 89.8%
Of course, a larger process vessel would allow a better utilization of beam
energy, but these results show that acceptable results can be obtained
with process vessels having a reasonable size. As an alternative, the
process vessel can have a conical or spherical shape.
[0024] The theoretical capability of a multiple-beam 1.5 MeV
Rhodotron for flue gas irradiation is evaluated as follows:
= The maximum electron beam power from a Rhodotron with 6 single-
pass beams, each 125 mA at 1.5 MeV would be equal to 6 x 125 x
1.5 = 1,125 kW.
= The power dissipated in the walls of the Rhodotron cavity at 1.5
MeV per pass would be about 180 kW. So, the total rf power
requirement would be about 1,125 + 180 = 1,305 kW. The
maximum power available from the rf amplifier of the TT 1000
Rhodotron is about 1,400 kW. Therefore, it a 1,125 kW beam can
be produced with one TT1 000 Rhodotron with 6 beams.
= The electron energy deposition through two 50 micron titanium
windows plus 15 cm of air between the windows with a 1.5 MeV
incident electron energy is evaluated as follows: Backscatter Losses
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= 2.12%, Window + Air Losses = 7.16 %, Transmitted Energy =
90.7%.
= The Monte Carlo calculation for the 1.5 MeV electron energy
deposition in a cylindrical vessel 500 cm in radius and 700 cm in
5 length shows that 89.8% of the transmitted energy would be
absorbed in the gas(case 9 above)
= Therefore, the overall electron energy deposition would be 0.907 x
0.898 = 0.814 or 81.4% and the energy deposition in the flue gas
would be 1,125 x 0.814 = 916 kW.
10 Assuming the same gas flow rate as in the Pomorzany facility and the
density of normal air, the average dose in the gas can be calculated as
follows:
= Mass Flow Rate = 270,000 x 1.2 / 3,600 = 90 kg/s.
= Average Absorbed Dose = 916 (kJ/s) / 90 (kg/s) = 10.2 (kJ/kg) or
kGy.
This analysis shows that the four accelerators at the Pomorzany facility
can be advantageously replaced with one TT1000 Rhodotron with six
single pass 1.5 MeV beams, as shown on Fig. 2
[0025] A similar calculation can be done for the two-pass, two-beam
Rhodotron of Fig. 3.
= The maximum beam power with the proposed two-pass
configuration would be equal to 2 x 125 x 2.5 = 625 kW.
= The electron energy depositions through two 50 micron titanium
windows plus 15 cm of air between the windows with a 2.5 MeV
incident electron energy is evaluated are as follows: Backscatter
Losses = 1.49%, Window + Air Losses = 3.75 %, Transmitted
Energy = 94.8%.
= The Monte Carlo calculation for the 2.5 MeV electron energy
deposition in a cylindrical vessel 600 cm in radius and 1250 cm in
length shows that 86.8% of the transmitted energy would be
absorbed in the gas (case 5 above)
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= Therefore, the overall electron energy deposition would be 0.948 x
0.868 = 0.823 or 82.3% and the energy deposition in the flue gas
would be 625 x 0.823 = 514 kW.
Assuming the same gas flow rate as in the Pomorzany facility and the
density of normal air, the average dose in the gas can be calculated as
follows:
= Mass Flow Rate = 270,000 x 1.2 / 3,600 = 90 kg/s.
= Average Absorbed Dose = 514 (kJ/s) / 90 (kg/s) = 5.71 (kJ/kg) or
kGy.
So, two TT1000 Rhodotrons , each with two 125 mA beams at 2.5 MeV
could deliver an average dose of 11.4 kGy and could advantageously
replace the four accelerators of the Pomorzany facility. Since the overall
electron beam power utilization would be nearly the same at 1.5 MeV as at
2.5 MeV, the Rhodotron with lower energy would be a more economical
choice.
[0026] An alternative design of a device for irradiation of gases is
illustrated on Fig. 5. In this design, the device 95 comprises an electron
accelerator comprising a resonant cavity, and six electron guns as
described above in relation to Fig. 2. A cylindrical process vessel 100 is
located next to the accelerator, the axes of the electron accelerator and of
the vessel being parallel. The six electron beams 60, 62 are directed
across the process vessel 100 in a plane perpendicular to the axis of said
vessel while the flue is circulated in the vessel. The electron beams 60, 62
may be straight, as illustrated on Fig. 2, or bent by dipole magnets for
being redirected more to the center of the vessel 100. In the design, the
fact that a plurality of beams is directed in the vessel along different paths
improves the conversion efficiency.
[0027] Known Rhodotrons use a single high frequency power
source 40, which is mounted coaxially on the top of the resonant cavity. In
the accelerators of the invention, in case more rf power is needed to
provide even more electron beam power, a second, identical, high
frequency power source 42 can be mounted on the bottom of the cavity.
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Both amplifiers are connected to the same rf driver to ensure that their
frequency and phase would be synchronized. The use of two smaller
power sources, such as the ones used on the Model TT300 Rhodotron ,
could increase the beam power of that model without requiring the use of
the larger amplifier, which is used on the Model TT1000. A configuration
having two high frequency power sources 40, 42 is illustrated in Fig. 1.
[0028] The electron accelerators described above can be
implemented with cavities, electron sources and high frequency power
sources which are subsystems on existing Rhodotrons . These
accelerators will be especially useful for applications such as flue gas
treatment, needing more electron beam current at lower energies than can
be provided with present Rhodotron designs.
