Reactant dilution effects and reaction kinetics of methane dry reforming in dielectric barrier discharge reactor

Abstract

Methane dry reforming (MDR) is a promising way for fuel production due to the mitigation of carbon dioxide (CO2) and methane (CH4) emissions, as well as tackling global warming. Recently, dielectric barrier discharge (DBD) has received much attention for greenhouse-gas conversion. This study is divided into two main parts. In the first part, the feasibility of the main reactions in MDR as well as the key reactions generating solid carbon was investigated. A carbon-free MDR is practically possible by increasing the temperature higher than 1173 K at the atmospheric pressure, yielding a considerable amount of syngas with hydrogen to carbon monoxide ratio of unity (H2/CO=1) suitable for downstream Fischer–Tropsch synthesis. A thermodynamic analysis was also performed for oxidative MDR to identify the condition for syngas production with no carbon deposition, with the minimum loss of syngas and a higher reactant conversion at a lower temperature. In the second part of the work, extensive laboratory and modeling studies were conducted to identify the effects of influential parameters (discharge power, CO2/CH4 ratio, gap spacing, and reactant flow rate) on DBD MDR in terms of reactant conversion, product distribution, discharge characteristics (including the reduced electric field, breakdown voltage, dielectric and gas capacitances, electron density, electron energy distribution function and mean electron energy) and energy efficiency. In the present study, CO2/CH4 ratio of 1, the flow rate of 50 ml/min, discharge gap of 1 mm, discharge power of 30 W and frequency of 10 kHz have been justified to present acceptable values of reactant conversion and yields of CO and H2 as well as to maintain the H2/CO ratio of close to unity (suitable for liquid fuel production) while maximizing the energy efficiency, conversion ability and production ability of H2 and CO. Reactant dilution with coplasmagen gas, argon (Ar), facilitates the plasma generation due to their low breakdown voltage. Therefore, the effects of the diluent gas (Ar) on DBD MDR in terms of reactant conversion, product selectivity, discharge characteristics and energy efficiency were investigated. The results revealed that higher Ar dilution factor led to the greater performance and a further restriction of carbon deposition. To benchmark our model forecasts, we also presented an overview of reported conversions and energy efficiencies in literature, to show the potential for an enhancement in comparison with the state-of-the-art. However, adding Ar is not an economical approach to improve the efficiency of non-catalytic DBD MDR, due to increased energy consumption. Furthermore, a global kinetics model for Ar diluted DBD CH4/CO2 was proposed, and the kinetics behaviour was compared to the one for helium (He) diluted DBD MDR reported in the literature.