Exhaust-Gas Treatment

Offgas/exhaust-gas treatment is a necessary stage for most plasma-processing plants. A promising method for application to gas-phase pollution abatement is non-thermal plasma (NTP) technology. NTPs, which generate highly reactive species such as free radicals, show particular promise for the treatment of gas-phase hazardous and toxic pollutants (e.g., volatile hydrocarbons and halocarbons, oxides of nitrogen and sulfur, and dioxins/furans) because the reaction rates of free radicals with many compounds can be orders of magnitude larger than strong oxidizers like O3 or H2O2. A non-thermal plasma (also called non-equilibrium plasma) is characterized by electrons which are not in thermal equilibrium with the other gas species. The electrons are hot (~ few - 10s eV temperature), while the ions and neutral gas species are cold (near-ambient temperature). Such plasmas are good sources of highly reactive oxidative and reductive species, e.g., O(3P), OH, N, H, NH, CH, CH3, HO2, O3, O2 (1Δ), and plasma electrons. Because radical-attack reaction rate constants are very large for many chemical species, entrained pollutants are readily decomposed by NTPs. Using these reactive species, one can direct electrical energy into favorable gas chemistry through energetic electrons, rather than using the energy to heat the gas. NTPs are commonly created by an electrical discharge in a gas or the injection of an energetic electron beam into a gas. Figure 1 shows a pilot module scheme for NTP exhaust-gas treatment based on a hybrid-mode, regenerative-absorber, NTP system.

Fig. 1: Regeneration-mode NTP-absorber architecture.
Economic and performance advantages may be gained by regenerating the absorbers off-line from
pollutant capture, but employing on-site, rather than off-site handling

Technology advantages for such a system are:

  • Multi-pollutant removal
  • Near-instantaneous adjustment of plasma power (to deal with variable pollutant concentrations and/or upset conditions in preceding stages)
  • Potential for CO2 sequestration by plasma-based conversion to useful end products (e.g., plastics).
Such large-scale plasma technology still requires considerable development. But that can have a high payoff in time because pollution regulations are becoming more and more stringent as time passes.

For further information on this topic, the point of contact is:
Dr. Louis Rosocha
Applied Physics Consulting
536 Central Ave., #4
Los Alamos, NM 87544 (USA)
Tel: +1 505-662-7123
E-mail: plasmamon@msn.com