Effects of Particle Size of Dielectric and Operating Time through Nonthermal Plasma on Fuel Characteristics of Refined Fuel from Oily Sludge

The refining processes of oil refineries, waste lubricants replaced by maintenance plants of gasoline and diesel vehicles, and mechanical lubricants of various types of ships produce oily sludge with different complex components, including varying amounts of water, hydrocarbons, metal rust, solid residues, inorganic compounds, and other components. The high heavy content of the sludge results in its high kinematic viscosity, and it exists in a liquid state close to a solid state at ambient temperature. Although the composition of oily sludge is complex and difficult to separate, its heating value can still reach 9000 cal/g, which is about 80% of the heating value of gasoline or diesel. Therefore, it has a great recycling value. This study therefore intends to develop oily sludge refining technology and establish an economical treatment process by nonthermal equilibrium plasma. The pretreatment technology of solvent extraction combined with filter filtration was used to extract and separate the oily part of the sludge containing hydrocarbons from the oil sludge. Two types of nonthermal plasma, such as DC streamer discharge and dielectric plasma discharge, were considered for the refining processes. Under operating conditions, the oily sludge is cracked and intensified to produce liquid and gaseous fuel products. The gaseous products are condensed and collected into fuel oil by a vacuum rotary concentrator. The composition and fuel properties of the fuel refined under different operating conditions of nonthermal plasma, including dielectric particle size of Al₂O₃ and action time, were tested. The research results showed that the pretreatment procedure can effectively improve the properties of originally discarded sludge at the beginning of the oil treatment process. In the second part of the experiment, after comparing different types of plasma reactors, the direct treatment type of dielectric-barrier discharge (DBD) plasma was found to be superior because the plasma can directly contact the treated sludge and extended the plasma treatment range. When quartz glass beads were added to the sludge, a large number of raised areas were formed on the surface of the sludge, making it easier to form a tip discharge for plasma generation and increasing the plasma treatment area. When the weight ratio of dielectric to sludge, the operating time, and the particle size were 2/1, 8 min, and 100 μm, respectively, there is the highest heating value and the lowest residual carbon content. In addition, the smaller the dielectric particle size, the smaller the carbonaceous size of the refined oil after the plasma treatment. As the plasma treatment time increased, the surface of the oily sludge was carbonized, resulting in a reduction in the plasma operating range. This research converts waste sludge into a fuel of precious energy, thus providing an important contribution to the development and application of energy.