Study On Synergetic Thermal Desorption Characteristics Of Diesel-based Drill Cuttings

Oil-based drill fluid has been used in shale gas drilling widely,and the disposal of large amount of oil-based drill cuttings has become an environmental problem.Thermal desorption technology can efficiently dispose solid waste containing oil,but there are still low heat transfer efficiency,high energy consumption and secondary pollution etc.In order to improve thermal desorption heat and mass transfer efficiency,reduce treatment energy consumption and pollution to the environment,this paper takes diesel-based drill cuttings as the research object to analyze its basic properties and pyrolysis characteristics,select the appropriate modification additives and the mixing proportion,explore the best working conditions of thermal desorption.By means of synergetic thermal desorption,the distribution and composition of products and energy consumption in the process of thermal desorption of oil-based drill cuttings with quick lime as modifier were studied.The synergistic effect and influencing mechanism of quick lime were discussed.After drying and centrifugation,the dry-base of drill cuttings contains less water and heavy metal elements,and the pollution components are mainly base oil of petroleum hydrocarbons,with the oil content of about 5%?6%.The base oil extracted from drill cuttings is mainly composed of C₁₁?C₂₀ long-chain normal alkane molecules,so it has low viscosity and good fluidity.The distillation range is 172.6°C?498.6°C and the distillation temperature of most aliphatic hydrocarbon molecules is within the range of 200°C?300°C.The oil-based drill cuttings pyrolysis process is divided into three main stages:desorption of water and light components(50°C?250°C),thermal decomposition of heavy components(250°C?650°C)and mineral decomposition(650°C?1000°C).The mixed pyrolysis characteristics of biomass sawdust,coal ash,quick lime with dry-based drill cuttings were compared.According to the thermogravimetric ratio and synergetic degree,quick lime was selected as the optimal modifier to determine the most significant synergetic effect with its mixing ratio of 5wt.%.At this point,the pyrolysis kinetic activation energy was the lowest,which was conducive to the desorption and removal of organic pollutants from drill cuttings.Based on the self-developed dynamic thermal desorption experiment platform for oil-containing solid waste,the optimal thermal desorption temperature of dry-based drill cuttings was determined at 350°C and the residence time was 30 min.The properties of thermal desorption products were analyzed.Non-condensable gas is mainly composed of H₂,CO₂,CH₄ and C₂?C₅ light hydrocarbon components.The viscosity of recovered oil is close to that of base oil but contains a small amount of ketones.The whole distillation range is shifted about 30°C?50°C in the direction of low temperature compared with that of base oil.The inorganic components of cuttings residue are mainly Si O₂,Ca CO₃,Al₂O₃ and Fe₂O₃.Through synergetic thermal desorption experiments of dry-based drill cuttings with quick lime,it is found that quick lime can effectively avoid the formation of extruded and cohesive groups in the process of drill cuttings stirring,and has certain promoting effect on the desorption of oil components,reducing energy consumption by 20%.The effect of the catalytic cracking mechanism of quick lime on the thermal desorption of oil-based drill cuttings is shown in:increased the yield of non-condensing gas and the components of H₂ and light hydrocarbon,so the heat value of the non-condensing gas increases;increased the yield of desorption recovered oil and the carbon molecular weight distribution tended to average;decreased the yield of residue and its oil content significantly,but excessive quick lime powder hinders the heat transfer and oil molecules escape,resulting in deep dehydrogenation and carbonization of hydrocarbons,so the fixed carbon content of residue increases.Meanwhile,Ca-centered polycondensation also accelerates process coking.Thus,the selection of appropriate mixing ratio is the key factor.In addition,the drying action of quick lime reduces the secondary pollution of oily wastewater in the condensate.