Disintegration of Carbon Dioxide Molecules in a Microwave Plasma Torch
Hyoung S. Kwak1, Han S. Uhm1, Yong C. Hong2 & Eun H. Choi1
Scientific Reports 5, Article number: 18436 (2015)
doi:10.1038/srep18436
17. Dec. 2015
[
www.nature.com]
Abstract
A pure carbon dioxide torch is generated by making use of 2.45?GHz microwave. Carbon dioxide gas becomes the working gas and produces a stable carbon dioxide torch. The torch volume is almost linearly proportional to the microwave power. Temperature of the torch flame is measured by making use of optical spectroscopy and thermocouple. Two distinctive regions are exhibited, a bright, whitish region of high-temperature zone and a bluish, dimmer region of relatively low-temperature zone. Study of carbon dioxide disintegration and gas temperature effects on the molecular fraction characteristics in the carbon dioxide plasma of a microwave plasma torch under atmospheric pressure is carried out. An analytical investigation of carbon dioxide disintegration indicates that substantial fraction of carbon dioxide molecules disintegrate and form other compounds in the torch. For example, the normalized particle densities at center of plasma are given by nCO2/nN?=?6.12?×?10?3, nCO/nN?=?0.13, nC/nN?=?0.24, nO/nN?=?0.61, nC2/nN?=?8.32?×?10?7, nO2/nN?=?5.39?×?10?5, where nCO2, nCO, nC, nO, nC2, and nO2 are carbon dioxide, carbon monoxide, carbon and oxygen atom, carbon and oxygen molecule densities, respectively. nN is the neutral particle density. Emission profiles of the oxygen and carbon atom radicals and the carbon monoxide molecules confirm the theoretical predictions of carbon dioxide disintegration in the torch.
Introduction
One of the most difficult problems in mankind is the global warming phenomenon, caused by an increase in the carbon dioxide concentration in the atmosphere. The major source of the carbon dioxide (CO2) is the burning of hydrocarbon fuel. There is a project1,2,3,4,5 called CCS (carbon dioxide capture6,7,8 in a burning system and store9,10,11), but its cost is formidably high. Carbon dioxide may dissociate through a reaction12,13 with oxygen and nitrogen atoms in air, but the densities of these atoms are very low at room temperature. The ocean can take carbon dioxide, but apparently this uptake14 is likely insufficient. Carbon dioxide was dissociated by recently laser beams15 at room temperature, but laser energy needed for a substantial amount of CO2 dissociation is very high. Therefore, the most practical means of reducing carbon dioxide may be the thermal dissociation16,17,18,19,20,21,22 of carbon dioxide molecules. In this context, we propose a method of carbon dioxide dissociation associated with carbon dioxide capture and utilization (CCU)23,24.
In this article, we present a carbon dioxide torch which makes use of microwaves and investigate the dissociation properties of carbon dioxide molecules in a high-temperature torch. A carbon dioxide torch can contain highly active species, such as electrons, ions, and radicals, which serve to enhance the chemical reaction rate, eliminating the need for catalysts during the processing of materials. The dissociation of carbon dioxide molecules at a high temperature produces oxygen atoms abundantly, which are very reactive. A conventional torch operated by arc-discharge processes may not be appropriate due to electrode erosion caused by oxidation. Although an inductively coupled plasma (ICP) in the range of radio-frequency is recently used in thermal processing fields, it is not efficient. The typical energy efficiency of ICP into the plasma is less than 50% and drops markedly at high power (>100?kW)25. For these reasons, the best solution would be to generate a pure carbon dioxide torch operated by microwaves without electrodes. The present article presents an in-depth study of the pure carbon dioxide torch and discusses its characteristics, including its temperature profile and the CO2 disintegration properties in the torch. The carbon dioxide torch exhibits two distinctive regions: a bright, whitish region of a high-temperature zone and a bluish, dimmer region of a relatively low-temperature zone. The bright, whitish region is a typical torch based on plasma species and the bluish, dimmer region is carbon monoxide (CO) recombining with oxygen.
Results
Disintegration of carbon dioxide molecules at high temperatures
Carbon dioxide molecules pass through an extremely high temperature torch where a local thermodynamic equilibrium (LTE) is assumed for T?>?2000K. They then may disintegrate into various chemical compounds. However, we assume the disintegration of carbon dioxide molecules into carbon monoxide and oxygen atoms, i.e., CO2 ? CO?+?O for simplicity in the initial analytical attempt. The enthalpy and entropy changes due to this reaction are found from a table26 to be ?H?=?530?kJ mol?1 and ?S?=?147?J mol?1 degree?1, respectively. The Gibbs free energy of the spontaneous disintegration is given by G?=??H ? T?S; therefore, the disintegration temperature of the carbon dioxide molecules into carbon monoxide and oxygen atoms is calculated to be about T?=??H/?S?=?3600K.
In reality, carbon dioxide decomposition is far more complicated than the above analysis of the Gibbs free energy. The dominant species after the decomposition of carbon dioxide are C, O, CO, C2, O2 and CO2. Thus, there are 19 reactions expected in total.
Generation of the carbon-dioxide plasma torch
The magnetron in Fig. 6 receives electrical power from a power supply, generating 2.45?GHz microwaves, which propagate through a circulator, a power meter, a three-stub turner and a tapered waveguide, entering a discharge tube made of quartz. The gas feeder provides a swirl gas of carbon dioxide in the discharge tube, creating a vortex flow, stabilizing the torch flame in the center of the tube and protecting the discharge tube from the heat of the torch. Microwaves from the magnetron propagate through the waveguide, concentrating their power in the tube and generating a plasma torch with a temperature of 6000 K and a plasma density on the order of 1013 particles cm?3. Once the plasma torch is ignited, nearly 100% of the microwave power is absorbed by the torch plasma with a reflected wave power of less than 1%.
The quartz discharge tube sits on the gas feeder through a steel tapered waveguide. Carbon dioxide gas enters the discharge tube though the gas feeder before the ignition of the plasma. The CO2 plasma torch is very stable and can usually operate until the carbon dioxide in a gas tank is consumed.
Kwak, H. S. et al. Disintegration of Carbon Dioxide Molecules in a Microwave Plasma Torch. Sci. Rep. 5, 18436; doi: 10.1038/srep18436 (2015).
3 mal bearbeitet. Zuletzt am 04.01.2016 21:31 von Dr. Munzert.
