A pulse-modulated microwave plasma for n2 / h2 reaction system

Abstract

The application provides a pulse modulation microwave plasma N2 / H2 reaction system, a needle electrode is arranged in a cylindrical cavity combined by an upper connecting piece and a lower connecting piece in a reactor; microwave power is connected to the needle electrode covered by a brass hollow rod through an SMA radio frequency interface; a vent hole is reserved on the inner wall of the upper connecting piece, and a quartz tube is connected to the bottom of the lower connecting piece; a high-pressure gas cylinder is sequentially connected with a mass flow controller, a hydrogen generator and the reactor; a fiber probe is arranged outside the quartz tube, and the axis position is opposite to a discharge area; a product component analysis part comprises a gas chromatograph, and qualitative and quantitative detection of reactants and products is realized according to an output online spectrum. The application provides an N2 / H2 conversion experimental device under atmospheric pressure and low temperature conditions, which realizes low power, high conversion and high selectivity, and realizes integration of conversion, diagnosis and product detection.

Description

TECHNICAL FIELD

[0001] The application belongs to the field of plasma technology, and particularly relates to a pulse-modulated microwave plasma N2 / H2 reaction system. TECHNICAL BACKGROUND

[0002] The conventional NH3 synthesis adopts Haber-Bosch process. The process was commercialized in 1913, and is currently operated at a temperature of more than 400 DEG C and a pressure of more than 200 bar, with a catalyst composed of magnetite (Fe3O4) and some oxides (K2O, Al2O3 and CaO). The Haber-Bosch process is energy-intensive and seriously pollutes the environment, because the process consumes 2% of the global annual primary energy supply, and emits millions of tons of carbon dioxide every year. This means that the increase in NH3 consumption leads to an increase in CO2 emissions and energy consumption. Possible sustainable methods for converting N2 into NH3, including biological alternatives in addition to thermal, photo and electrocatalytic processes, have been studied. The current ammonia synthesis technology generally has problems such as harsh reaction conditions, high energy consumption, low ammonia conversion rate and selectivity, and serious environmental pollution of products.

[0003] Non-thermal plasma-assisted nitrogen fixation is a very promising technology, which has many advantages, such as being able to carry out reactions under milder conditions, being driven by renewable energy, and being suitable for small-scale operation. The use of catalysts provides new opportunities for improving the performance of plasma reactors for synthesizing chemicals including NH3. Most of the literature on plasma catalysis focuses on dielectric barrier discharge (DBD) reactors, and reports conversion, NH3 yield and energy consumption under different filler conditions. The best energy consumption reported so far is 2.04 MJ / mol, which is 4.3 times lower than the Haber-Bosch energy consumption of 0.48 MJ / mol.

[0004] Therefore, in view of the problems of high energy consumption, low ammonia conversion rate and serious environmental pollution of products in the existing plasma ammonia synthesis technology, the application uses a pulse-modulated microwave plasma source to achieve low energy consumption, high ammonia conversion rate and selectivity at atmospheric pressure. SUMMARY

[0005] In order to solve the problems of low nitrogen conversion rate, low ammonia generation rate and low selectivity in the ammonia synthesis technology under low energy consumption, the application provides a device for N2 / H2 reaction by pulse modulation microwave plasma, which can realize high selectivity and energy efficiency of products under low power, and the optical diagnosis technology is used to study the micro mechanism of the reforming reaction, thereby providing necessary theoretical basis for regulating the reaction process and product selection.

[0006] A device for N2 / H2 reaction by pulse modulation microwave plasma, comprising a reactor, an input part, an optical diagnosis part and a product component analysis part.

[0007] In the reactor, the needle electrode is fixed on the upper connector through the screw thread on the brass hollow rod and can be adjusted up and down through the screw thread; the needle electrode is arranged in the cylindrical chamber combined by the upper connector and the lower connector.

[0008] The needle electrode is a stainless steel needle with a length of 35-50 mm and a diameter of 1.5-3 mm.

[0009] The upper connector and the lower connector are screwed together to form a chamber with a length of 31 mm, i.e. 1 / 4 of the wavelength, and an inner diameter of 12-16 mm. The upper connector and the lower connector are made of brass.

[0010] The microwave power is connected to the needle electrode covered by the brass hollow rod through the SMA radio frequency interface.

[0011] There are 2-6 equally spaced air holes on the inner wall of the upper connector, and the air holes have a diameter of 1-2 mm. The air holes are inclined clockwise, and the inclined air hole structure can ensure that the entering gas enters the chamber to form a vortex in the same direction. The air holes are connected to the air branch pipe.

[0012] Meanwhile, a quartz tube is connected to the bottom of the lower connector to facilitate the collection and detection of the subsequent tail gas. The quartz tube has an inner diameter of 14-18 mm, an outer diameter of 16-24 mm and a length of 80-180 mm.

[0013] The lower part of the lower connector is provided with an inverted circular stair to enhance the local electric field, so that the plasma is generated at the needle tip of the needle electrode.

[0014] One side of the lower connector is provided with a hollow branch pipe with an inner diameter of 4-6 mm, and a quartz observation window is arranged above the hollow branch pipe to collect the spectrum and observe the discharge state.

[0015] The pulse modulation microwave power source drives the reactor.

[0016] The input part comprises a high-pressure gas cylinder, and the high-pressure gas cylinder is sequentially connected to a mass flow controller, a hydrogen generator and the reactor.

[0017] The optical diagnosis part comprises a fiber probe, an ICCD, a spectrometer and a computer connected in sequence; the fiber probe is placed outside the quartz tube with the axis position facing the discharge area, the optical signal is transmitted to the spectrometer and the ICCD through the fiber, the optical signal is converted into an electrical signal, and then the spectral image is output by the computer; the relative light intensity of the particles in the discharge process is detected by using the emission spectrum method to explore the micro mechanism of the reaction process;

[0018] The product component analysis part comprises a gas chromatograph, and the reactants and products are qualitatively and quantitatively detected according to the output online spectrum.

[0019] The present application mainly designs and builds a set of pulse modulation microwave plasma N2 / H2 reaction device, which can use the optical diagnosis part to study the micro mechanism of the N2 / H2 reaction process, and can detect the calibration and quantification of the products through the product analysis part. The high selectivity of nitrogen and the high yield of ammonia under the condition of atmospheric pressure and low temperature are realized. The present application realizes high product yield and selectivity under low power, and diagnoses the particles in the system through optical diagnosis technology.

[0020] The present application has the following beneficial effects: compared with the traditional dielectric barrier discharge, sliding arc discharge and other experimental devices, the present application provides an N2 / H2 conversion experimental device under the condition of atmospheric pressure and low temperature, which realizes low power, high conversion and high selectivity. A set of system for realizing conversion, diagnosis and product detection of N2 / H2 reaction through pulse modulation microwave plasma is built, and the micro mechanism of the conversion process is studied by optical diagnosis, which provides necessary theoretical basis for regulating the reaction process and the selection of products. BRIEF DESCRIPTION OF DRAWINGS

[0021] Figure 1 It is a schematic diagram of the pulse modulation microwave plasma N2 / H2 reaction system experimental device in the present application.

[0022] Figure 2 It is a schematic diagram of the N2 / H2 reactor in the present application.

[0023] Figure 3 It is a schematic diagram of the N2 / H2 reactor in the present application.

[0024] Figure 4 It is a schematic diagram of the N2 / H2 reactor in the present application. DETAILED DESCRIPTION

[0025] The specific embodiments of the present application are further illustrated in combination with the drawings and technical solutions.

[0026] A pulse modulation microwave plasma N2 / H2 reaction system comprises a reactor 5, an input part, an optical diagnosis part and a product component analysis part.

[0027] Inside reactor 5, needle electrode 11 is fixed to upper connector 13 by threads on brass hollow rod 12, and its position can be adjusted up and down by the threads; needle electrode 11 is placed in a cylindrical cavity composed of upper connector 13 and lower connector 15.

[0028] In this embodiment, the needle electrode 11 is 40mm long and 2mm in diameter; the brass hollow rod 12 has an inner diameter of 2mm.

[0029] The upper connector 13 and the lower connector 15 are screwed together by threads, forming a chamber with a length of 31mm, which is 1 / 4 of the wavelength, and an inner diameter of 12-16mm. Both the upper connector 13 and the lower connector 15 are made of brass.

[0030] Microwave power is connected to the needle electrode 11 covered by the brass hollow rod 12 via the SMA radio frequency interface 16;

[0031] Five equally spaced vent holes, 1-2 mm in diameter, are provided on the inner wall of the upper connector 13. These vent holes are inclined clockwise, employing an inclined swirling air structure to ensure that the incoming gas enters the chamber in the same direction, forming a vortex. The vent holes connect to a vent branch pipe 14; the vent branch pipe 14 has an inner diameter of 2 mm and an outer diameter of 4 mm, and contains five vent holes.

[0032] Meanwhile, a quartz tube 17 is attached to the bottom of the lower connector 15 to facilitate the subsequent collection and detection of exhaust gas; the inner diameter of the quartz tube 17 is 20mm and the outer diameter is 24mm.

[0033] The lower part of the lower connector 15 is provided with an inverted circular trapezoid to enhance the local electric field, so that plasma is generated at the tip of the needle electrode 11.

[0034] The lower connector 15 has a hollow branch pipe with an inner diameter of 4-6mm on one side, and a quartz observation window 19 on the top for collecting spectra and observing the discharge state.

[0035] Pulse-modulated microwave power supply 4 drives reactor 5;

[0036] like Figure 2 As shown, plasma is generated at the tip of stainless steel electrode 11.

[0037] The input section includes a high-pressure gas cylinder 2, which is connected in sequence to a mass flow controller 1, a hydrogen generator 3, and a reactor 5. The N2 (purity 99.999%) required for the experiment flows out of the high-pressure gas cylinder 2, and after the flow rate is controlled by the mass flow controller 1, it mixes with the H2 generated by the hydrogen generator 3 and then enters the reactor 5.

[0038] The air inlet adopts an inclined swirling structure. The mixed gas of N2 and H2 enters the reactor 5 through 2-6 inclined ventilation branches 14 to ensure that the airflow forms a vortex within the reactor 5 and is not blown away. It is driven by a pulse-modulated microwave power supply with a frequency of 2.45GHz (maximum power of about 300W), and the microwave power supply is grounded.

[0039] The input section provides the necessary energy input, experimental gas, and detection gas for the entire platform. The gas flow rate is regulated by a mass flow controller (HoribaMetron S4932 / MT). The energy input is achieved using a solid-state pulse-modulated microwave power supply. The reaction gases used in the experiment are nitrogen (99.999%) and hydrogen (99.999%).

[0040] The optical diagnostic section includes a fiber optic probe 7, an ICCD 8, a spectrometer 9, and a computer 10 connected in sequence; the ICCD 8 is an Andor DH334T, and the spectrometer 9 is an Andor SR750i 2400 l / m, 1200 l / m. The fiber optic probe 7 is placed outside the quartz tube 17, with its axis facing the discharge area. The optical signal is transmitted through the optical fiber to the spectrometer 9 and ICCD 8, where it is converted into an electrical signal, and then the computer 10 outputs a spectral image. The relative luminescence intensity of the particles during the discharge process is detected using emission spectroscopy to explore the microscopic mechanism of the reaction process.

[0041] The product composition analysis section includes a gas chromatograph 6 (GC6), which performs qualitative and quantitative detection of reactants and products based on the output online chromatograms. The GC6 is equipped with a flame ionization detector (FID) and a thermal conductivity detector (TCD), using argon as the carrier gas. Qualitative and quantitative detection of reactants and products can be performed based on the output online chromatograms. Each sample needs to be calibrated before quantitative detection.

[0042] The parameters selected in this embodiment are as follows: nitrogen flow rate is 1.48 L / min, microwave power incident power is 285.61 W, reflected power is 54.45 W, and the actual power consumed by the reactor is 231.14 W. The microwave power supply pulse frequency is 25000 Hz, and the pulse duty cycle is 50%. Each result was detected online using a gas chromatograph 6. The following experimental results are the average of three measurements.

[0043] N2 / H2 reaction experimental results:

[0044] When the actual power consumed by the reactor was 231.14W, the nitrogen conversion rate was 41.97% and the NH3 yield was 58.28%.

[0045] When the selected parameters are as follows: the incident power of the microwave power supply is 245.02W, the corresponding reflected power is 42.36W, and the actual power consumed by the reactor is 202.66W.

[0046] N2 / H2 reaction experiment results:

[0047] When the actual power consumed by the reactor is 202.66W, the conversion rate of nitrogen is 33.48%, and the yield of NH3 is 46.21%.

Claims

1. A pulsed microwave plasma for N2 / H2 reaction system, comprising: The reactor (5), the input part, the optical diagnosis part, the product component analysis part; In the reactor (5), the needle electrode (11) is fixed on the upper connector (13) through the screw thread on the brass hollow rod (12) and can be adjusted up and down through the screw thread; the needle electrode (11) is placed in the cylindrical chamber combined by the upper connector (13) and the lower connector (15); The microwave power is connected to the needle electrode (11) covered by the brass hollow rod (12) through the SMA radio frequency interface (16); At the same time, the quartz tube (17) is connected to the bottom of the lower connector (15) to facilitate the collection and detection of the subsequent tail gas; The pulse modulation microwave power source (4) drives the reactor (5); The input part includes a high-pressure gas cylinder (2), which is connected in turn with a mass flow controller (1), a hydrogen generator (3) and a reactor (5); The optical diagnosis part includes an optical fiber probe (7), an ICCD (8), a spectrometer (9) and a computer (10) connected in turn; the optical fiber probe (7) is placed outside the quartz tube (17) with the axis position directly opposite the discharge area; the optical signal is transmitted to the spectrometer (9) and the ICCD (8) through the optical fiber; the optical signal is converted into an electrical signal, and then the spectral image is output by the computer (10); the relative light intensity of the particles in the discharge process is detected by using the emission spectrum method to explore the micro-mechanism of the reaction process; The product component analysis part includes a gas chromatograph (6), which is used for qualitative and quantitative detection of the reactants and products according to the output online spectrum.

2. The pulsed microwave plasma for N2 / H2 reaction system according to claim 1, characterized in that: There are 2-6 equidistant air holes on the inner wall of the upper connector (13), which are inclined clockwise and adopt an inclined air hole structure; the air holes are connected with the air branch pipe (14).

3. The pulsed microwave plasma for N2 / H2 reaction system according to claim 1, characterized in that: The lower part of the lower connector (15) is provided with an inverted circular stair to enhance the local electric field, so that the plasma is generated at the needle tip of the needle electrode (11).

4. The pulsed microwave plasma for N2 / H2 reaction system of claim 1, wherein: The hollow branch pipe port is provided on one side of the lower connector (15), and the quartz observation window (19) is arranged thereon to collect the spectrum and observe the discharge state.

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