Non-equilibrium Microwave Plasma for Efficient High Temperature Chemistry

The overall goal of this experiment is to convert or activate stable molecules such as carbon dioxide, nitrogen, methane or water with a microwave plasma reactor. This method can be used to measure the energy and convergent efficiency of the microwave plasma reactor. These measurements can be used to optimize the reactor conditions and improve the efficiency.

The main advantage of the plasma flowing reactor is that the startup times for driving continuous chemical processes are in the seconds time scale. Thermal equilibrium nature of the micro reactor enables efficiencies above the thermodynamic equilibrium limit of 55%which is the highest efficiencies achievable with standard thermal conversion. To begin setting up the microwave plasma system, connect a one kilowatt magnetron to a power isolator with an attached water load.

Connect the isolator to a three stub tuner. Attach a microwave applicator to the tuner, fit a sliding short toward the 24 millimeter aperture to the end of the wave guide. Insert a cord tube into the applicator and connect the tube to kf and vacuum flanges and a ten gentle gas inlet.

Connect to the tube, a throttle valve in series with a vacuum pump. Connect a shortcut valve in parallel to the throttle valve to switch between low and high pressures. The shortcut valve can be used to go to low pressure for plasma ignition without changing the throttle setting.

Connect the carbon dioxide source to the gas inlet via a mass flow controller. Then, turn on the magnetron water cooling system. Ensure that the radiation monitor and the gas detector are positioned around the microwave plasma assembly.

Verify that the reactor conditions are suitable for plasma ignition, then, manually increase the source power level until the plasma ignites. Adjust the three stub tuner to minimize the reflected power. Then, turn off the plasma.

Next, mount a 532 nanometer laser with the beam directed along the cord’s tube. Mount brewster or anti-reflection windows coated for 532 nanometers at the entrance and exit of the tube. Align the laser and construct the beam dump at the end of the vacuum set up.

Focus the laser to the center of the wave guide, then, pressurize the reactor with CO2 gas to a higher pressure than will be used in the measurement. If laser breakdown occurs, decrease the laser power. Install baffles in the vacuum tubes, mount a lens in line with a 24 millimeter aperture in the sliding short and focus the collected light onto a 400 micrometer diameter optical fiber.

Use the fiber to direct the light to the entrance slit of a custom spectrometer and measure the scattered light intensity. Repeat the measurement over a series of decreasing pressures and verify that the pressure intensity relationship is linear. Adjust the spectrometer to maximize the measured intensities.

Connect the cell in series with the gas exhaust and mount the calcium fluoride window on either end of the cell. Place the cell in a sample chamber of an FTIR spectrometer with the calcium fluoride windows in line with the IR beam. Use a sufficient length of tubing to ensure that the gas in the cell will be in chemical equilibrium.

Adjust the FTIR signal gain, until the intensity is as close as possible to the maximum without exceeding it. Preview the interferic gram, then, turn off the gas flow and evacuate the cell. Acquire a background spectrum.

Then, pressurize the system with CO2 gas to about one millibar and ignite the plasma. Acquire a spectrum over a range of 2, 400 to 2, 000 reciprocal centimeters using 10 averages. Determine the carbon monoxide volume fraction, by using the HITRAN database to fit the measured CL lines.

When fitting spectra, take care that the instrument function is applied to be transmission spectrum, not the absorbent spectrum. Include pressure instance of CO, broline by CO2, including only self-broadening can result in errors as large as 20%To perform NC2 FTIR measurements, fit a sapphire tube into a wave guide with a vertically aligned vacuum system. Position an FTIR spectrometer so that the wave guide sets in the sample chamber and cover the chamber walls with microwave absorbent material.

Set the FTIR to use at least 100 averages to mitigate fluctuations in the plasma. Acquire a spectra over a range of 2, 400 to 1, 800 in verse centimeters. Correct the spectra for the temperature dependent absorption of sapphire before analysis.

The CO conversion was found to increase linearly with power input over a range of pressures at a fixed flow rate of 14 SLM. The linear relationship tapered off at higher pressures, the plasma was found to convert CO2 to CO with an energy efficiency of up to 49%which is comparable to the maximal thermodynamic efficiency. CO conversion increased linearly with specific energy to about 2.2 electron volts per molecule.

CO concentration was measured by IR absorptions spectroscopy of the gas exhaust. A least square is fit was applied to the FTIR data, resulting in a calculated 14.7%conversion at 299.36 Kelvin, the gas temperature of the plasma center, was determined from NC2 Rayleigh’s scattering measurements, taken at various powers and pressures. The effect of Thomson’s gathering was calculated to be negligible based on the differential cross sections for Rayleigh and Thomson’s gathering and the predicted ionization degree.

The NC2 FTIR measurements were taken under conditions in which CO production was negligible and reused to investigate the CO2 plasma dynamics, a model based on the high trend implication programming interface, was applied to determine the vibrational temperatures corresponding to the vibrational normal modes. After watching this video, you should have a good understanding of how to operate a flowing microreactor and how to analyze the products that were formed. The flowing plasma reactor is ideal for processing stable molecules.

By following this procedure, other gases can be processed, like water, nitrogen and methane. For truly efficient, non-equilibrium conversion, the temperature must be kept as low as possible, to prevent quenching of vibrationally excited molecules.