The gas produced as a result of a variety of material conversion processes such as pyrolysis, gasification, plasma gasification and/or plasma melting may be utilized in a variety of downstream applications including power generation, industrial synthesis of chemicals and in the production of liquid fuels, stored for later use or flared off.
The product gas formed via the gasification of carbonaceous feedstock and in particular low quality feedstocks such as municipal solid waste (MSW) contains CO, H2, CO2, CH4, H2O, and N2 as well as organic and inorganic impurities and particulates. A wide range of organic molecules are present in the product gas and include low molecular weight hydrocarbons such as acetylene, ethylene as well as high molecular weight hydrocarbons including polycyclic aromatic hydrocarbons (PAH), such as naphthalene and pyrene. The low molecular weight hydrocarbons can be used as fuel in gas turbine or engine applications. In contrast, the high molecular weight hydrocarbons are problematic in downstream applications and may cause plugging or clogging of downstream equipment. High molecular weight hydrocarbons are generally referred to as “tars,” and can be defined as the downstream condensable hydrocarbon component in the product gas.
Tar components found in gasification product gas include benzene, toluene, naphthalene, pyrene and indene. Once formed, tars can be difficult to decompose with the thermal requirements for decomposition, varying depending upon process parameters and the individual molecule. Tar removal strategies are known in the art but can be very expensive and could exhibit low removal efficiency. If tar is removed as opposed to being converted to lower molecular weight components, there is a loss in potential heating value of the gas and could still pose a downstream problem.
Plasma reforming of tars, modeled primarily with surrogate tar molecules including naphthalene and/or toluene, has been studied in a variety of discharges including microwave, corona and gliding arc discharges. Detailed studies have been conducted in corona discharges for the conversion of toluene and naphthalene. Gliding arc has been studied for conversion of a variety of tars including benzene, toluene, methyl-naphthalene, fluorene, anthracene, and pyrene. Although these studies demonstrated high removal efficiencies, many factors were not realistic for industrial application including: high specific energy input, unrealistic input gas, low tar concentration, low flow rate, and low energy efficiency.
Plasma has been used as both a source of intense heat, and as a source of free electrons that can be used to initiate and drive many chemical processes requiring the dissociation of molecules into (reactive) dissociation fragments.
Based on the relative temperatures of the electrons, protons and neutrons, plasmas are classified as “thermal” or “non-thermal”. Thermal plasma atoms have electrons and nucleus particles at the same temperature and are said to be in thermal equilibrium. In other words electron temperature and gas temperature are in equilibrium with each other.
Non-thermal plasmas, also referred to as low enthalpy plasma, “cold plasma”, non-equilibrium plasma or “low-pressure” plasma, are characterized by a low gas temperature but much higher energy distribution of the electrons than that of atom nucleus. “Cold plasma” or “low-temperature plasma” or “low enthalpy plasma” is a partly ionized gas, which is generated in a high-voltage electric field in a low pressure.
Non-equilibrium, low-temperature plasmas are known in the art to destroy relatively low concentrations of volatile organic compounds at atmospheric pressure and are particularly attractive for treatment of low-level waste concentrations and for dealing with compounds that resist treatment by standard chemical means. These low-temperature plasma processing techniques generally involve either high energy electron-beam irradiation or electrical discharge methods, such as gliding arc, pulsed corona, dielectric barrier, capillary, hollow cathode, surface, and packed-bed corona discharge. All of these techniques rely upon the fact that electrical energy can produce electrons with much higher average kinetic energies than the surrounding gas-phase ions and molecules. These energetic electrons can interact with the background gas to produce highly reactive species (i.e., radicals, anions, cations, and secondary electrons) that will preferentially destroy pollutants.
Non-equilibrium plasma differs from thermal plasma with respect to their roles in plasma chemical kinetics. Thermal plasma is usually more powerful, whereas non-equilibrium plasma is more selective. Since concentrations of active species in non-equilibrium plasma can exceed those in thermal plasma by many orders of magnitude at the same gas temperature, the successful control of non-equilibrium plasma can permit chemical reactions to be manipulated in a desired direction and through an optimal pathway.
The well-known categories of non-equilibrium plasma are dielectric barrier discharge, gliding arc, reverse vortex gliding arc, direct-current (DC) and pulsed coronas, corona discharge, corona reactor, microwave plasma, electron-beam, micro-discharge and RF-driven discharge.
This background information is provided to reveal information believed by the applicant to be of possible relevance to the present invention. No admission is necessarily intended, nor should be construed, that any of the preceding information constitutes prior art against the present invention.