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 SO ₂ (sulphur dioxide and NO _x (including isl and 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 1 MW. For obtaining such a high power, four electron accelerators (26OkW, 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 SO2 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

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 N ₂ , 5% by volume of O ₂ , 13% by volume of CO ₂ , 10% by volume of H ₂ O, I OOOppm of SO ₂ and 350ppm of NO _x . The last two components are particularly harmful to the environment because, when released in the atmosphere, and under influence of UV rays, they produce SO ₃ , and NO ₂ , 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 (NH ₃ ). 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 SO ₂ and NO _x . 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

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. Helfhtch, P. L. Feldman,

Cottrell Environment Sciences. FLUE GAS SO2/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 SO ₂ and NO _x 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 μs, and a peak power of 1 MW. This accelerator therefore has a e-beam output power of only 2OkW. 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

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

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 -fuel led electric power plants, the disinfection of municipal waste water and the 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 (1 MW 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.

[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 SO ₂ / NO _x 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.

[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

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 = 100 cm 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%

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 TT1000 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

= 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 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. 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)

• 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.

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.