Methane Slip Reduction From Low-Pressure Marine Gas Engines By Using Hybrid Non-Thermal Plasma-Catalytic System

With the widespread application of lean-burn low-pressure gas engines in the marine sector,the problem of methane slip has received increasing attention because the global warming potential of methane is 28 times higher than that of carbon dioxide.Using after-treatment technology to control methane slip is a big challenge,as methane requires a high temperature above 600? to break the C-H bonds and to oxidize the molecule,which cannot be achieved in lean-burn Otto cycle gas engines due to their exhaust gas temperatures being less than 400?.Oxidation catalysts could be a solution but there are still unsolved problems related to catalyst degradation and the methane conversion ratio at low exhaust gas temperatures.In order to solve the problem of conversion of methane at low exhaust gas temperatures,this study uses a combination of the heterogeneous non-thermal plasma dielectric barrier discharge(DBD)technology with catalysts to enable the low-temperature activation of methane.The plasma reactor is fixed inside a tubular furnace and two different kinds of catalysts palladium-supported catalysts and platinum-supported catalysts in beads form having different precious metal contents are placed individually inside the reactor of the plasma discharge zone(post-plasma catalyst).The two studied cases are the plasma alone condition and the plasma-catalyst condition,respectively.Throughout the plasma alone experiment,the influence of input power,the gas mixture flow rate,the gases mixture temperature,the methane initial concentration,the reaction time,the frequency,the oxygen initial concentration,the input voltage,and the input current on the methane conversion efficiency and the selectivity of carbon monoxide and carbon dioxide are thoroughly investigated,which helps to understand the methane conversion inside the BDB reactor.The results showed that the input power greatly influences the methane conversion by increasing the methane conversion efficiency and the selectivity of CO2 to 97.87%and 58.01%,respectively,and decreasing the CO selectivity up to 37.98%even at a low temperature of 220?.Conversely,increasing the gas mixture flow rate and the methane's initial concentration decreased the methane conversion efficiency.In addition,increasing the oxygen concentration from 5%to 16%had a weak effect on the conversion efficiency and the selectivity of by-products.Additionally,in the plasma-catalyst experiment,the influence of the catalyst type,the different precious metal contents,and the space velocity are also considered.By collating the results,the best condition giving the highest methane conversion efficiency is eventually determined.Compared to the plasma alone condition,the decomposition efficiency of methane improved after the catalyst was added,which also helped to decrease the selectivity of CO to less than 1%and increase the selectivity of CO2 to more than 96%.Besides,the increase of precious metal contents in the catalyst enhanced the conversion efficiency of methane and the performance of the platinum-supported catalyst surpassed that of the palladium-supported catalyst.Furthermore,a deep understanding of methane conversion inside the plasma reactor is obtained,which consists of two stages.High-energy electrons collide and induce the dissociation of methane and oxygen into different reactive species like free radicals and ions during the first stage.After that,the reactive species continue to react with methane to generate carbon monoxide,carbon dioxide,and water during the second stage.Eventually,the presence of the catalyst improved the overall methane conversion efficiency and enhanced the oxidation of carbon monoxide to carbon dioxide on the active sites of its surface.Finally,this research work is the first of its kind to use non-thermal plasma technology to address methane slip problem from the lean-burn low-pressure marine gas engines,which makes it a reference and guidance for other researchers.