Glid ARC Plasma Conversion of Pyrolysis Gas Into Syngas

Pyrolysis gas or Pyrogas is a gas fuel formed primarily from the Pyrolysis or Gasification of Coal, Biomass or Municipal wastes. Gasification process of producing Pyrogas is usually an endothermic reaction forming a complex mixture of combustible gases such as CH4, CO, H2 and heavier hydrocarbons; Tar and noncombustible gas such as CO2. The presence of the heavy hydrocarbons and tar diminishes the quality of Pyrogas from the point of view of its usage for power generation or synthetic fuel production. This situation prompts the use of Non equilibrium Gliding arc Plasma for the chemical reforming of Pyrogas into Synthesis gas or Syngas. Syngas is a higher mass heating value gas comprising a varying quantity of Hydrogen (H2) and Carbon Monoxide (CO). Gliding Arc Plasma reforming is a homogenous process of fuel reforming which eliminates the need for catalysts¹.The reforming reactions considered are Steam reforming and Dry CO2 reforming, both of which are endothermic. Partial oxidation reaction will be inappropriate in this situation due to the presence of H2 in the Pyrogas mixture. Gliding Arc plasma serves as a resource for active species and radicals which are necessary to stimulate the desired chemical reactions.²

2. Experimental

The gliding arc plasma reformer is a specially designed low current reactor known as Gliding Arc Plasmatron. The plasmatron essentially consists of different component parts: High Voltage Stainless steel electrode and a ground electrode with swirl gas jets that creates a unique flow which propels the gliding discharge - formed in a 3mm gap between both electrodes. The high voltage electrode and Ground electrode are separated by a dielectric insulator; tangential air inlet at the flange assists in cooling flange and dielectric material.¹ Thermocouple insert ports in the post plasma zone for measuring Gas temperature at various points in the plasma reactor zone; an exhaust pipe designed to incorporate a heat exchanger for cooling product gases before subsequent analyses by the Gas Chromatographer. The plasmatron is designed to function at high reforming temperatures.

Pyrogas conversion to Synthesis gas (Syngas): Pyrogas gas composition is simulated in the laboratory by combining the various constituent gases together in a cylindrical container. The gases flow rates were controlled with the aid of mass flow controllers before directing the resulting Pyrogas mixture into the gliding arc plasma zone inside the plasmatron where reforming occurs. The resulting changes in gas molar concentrations are observed under different operating conditions such as enthalpy, H2O/C ratios (for steam reforming). The effects of gases preheating and plasmatron inner geometry in the reforming processes were also examined. Similar procedures were conducted for Dry CO2 reforming experiments.

3. Results

For the steam reforming reaction of Pyrogas, a steady increase in H2 and CO were observed when H2O/C ratio was lower than 1.5. At higher H2O/C ratios, the H2 and CO yields diminish due to water gas shift reaction which limits attainment of thermodynamic predictions. This phenomenon can be partially attributed to limitations in the steam temperature and possible heat losses. Conversely, Dry CO2 reforming results showed better Syngas yields at lower temperatures with concentrations of heavy hydrocarbons such as CH4, C2H6 and C3H8 reduced considerably with enthalpy increase.

References

[1] Michael J. Gallagher, Robert Geiger, Anatoliy Polevich, Alexander Rabinovich, Alexander Gutsol and Alexander Fridman. "On-board plasma-assisted conversion of heavy hydrocarbons into synthesis gas" Fuel 25 November 2009

[2] Alexander Fridman “Plasma Chemistry” Cambridge university Press, 2008.