Measurements of Temperature and Chemical Composition in a Thermal Plasma Reactor for Waste Gas Pyrolysis

Abstract

Summary form only given. For the cleaning treatment of hazardous waste gases by pyrolysis, thermal plasmas have been introduced taking advantage of their extremely high temperature and enthalpy. However, the high-temperature decomposition region of a thermal plasma jet, having steep temperature gradients both in the radial and axial directions, is so narrow that the waste gas injected into it has difficulty in mixing up uniformly with the fast hot thermal plasma in a pyrolysis reactor. In this experimental study, we have measured the temperature and chemical composition of thermal plasma in a pyrolysis system to find optimum conditions for uniform mixing of waste gases. An experimental setup includes a DC thermal plasma torch with hollow electrodes, a decomposition reactor containing mixing chamber, and an enthalpy probe system combined with quadrupole mass spectrometry for measurements of temperature and chemical composition. Diagnostics with the enthalpy probe svstem are made while operating the system under the following conditions: air plasma-forming gas flow rate of 100 slpm, nitrogen purging gas flow rate of 200 slpm, and torch input power of 60 kW. Axial and radial measurements are carried out to obtain temperature and chemical composition distributions depending on experimental conditions, such as diameters of nozzle electrodes and types of gas injection rings. By comparing radial uniformities of nitrogen mole fractions and temperature distributions, we can figure out the effects of design variables on mixing ratio in the reactor. The mixing effects are discussed in terms of turbulent flow characteristics concerned with the injection ring tvpe and the plasma jet column size determined from the nozzle diameter. We also examine the correlation between variances in radial uniformities and axial plasma characteristics. From the present experimental results, some thermodynamic characteristics of the plasma in the pyrolysis system will be revealed, and the decomposition efficiency is expected to be improved in the field of thermal plasma waste gas treatment.