Experimental Investigation On Vitrification And Pyrolysis Of Oil-Based Drill Cuttings Using Thermal Plasma Technology

Oil-based drill cuttings（OBDC）are hazardous wastes associated with the process of shale gas extraction,and mineral oil,heavy metals and alkaline salts are the three major hazardous substances in OBDC.The efficient and environmentally friendly treatment of OBDC has been the focus of researchers and commercial technology innovation in the industry.The current treatment principle for OBDC is the extraction or decomposition of mineral oil,such as thermal desorption.Subject to local geology and drilling fluid category,thermal desorption method still suffers from difficult to control operating temperature and ensure the quality of extracted oil phase,residue and harmful gas during treatment indicate that the risk of environmental pollution from OBDC has not been fully minimized.In contrast,thermal plasma technology is widely used for solid waste treatment due to its extremely high temperature and reactivity.Under the action of high-temperature thermal plasma active particles,the chemical bonds of large organic matter in solid waste are broken into small molecules of clean gas,and the residue is transformed into an inert glassy slag by high-temperature melting,which encapsulates heavy metals and other harmful substances.There is still a lack of understanding of the mechanism of thermal plasma melting and cracking reaction of OBDC,and insufficient research on the regulation and characterization of vitrification and cracking products,resource utilization and evaluation of energy consumption.Therefore,this paper introduces the thermal plasma technology and carries out the experimental study of OBDC based on the self-designed and built 30 k W thermal plasma melting and pyrolysis autocontrol system.The physical and chemical properties of different OBDCs,the thermal plasma melting mechanism of OBDC,the chemical composition and heavy metal migration behavior,the design of response surface method（RSM）with the specific energy consumption of glassy slag as the response value and the thermal plasma cracking mechanism and product composition of pyrolytic gas are comprehensively explored.At the same time,the thermal plasma flame temperature diagnosis and melting temperature correction method of this experimental system is proposed,a semi-empirical model of the energy consumption of the final product is established,and an assessment of the economic and social benefits of the thermal plasma application of OBDC is made to provide a theoretical basis and data support for the industrialization path of thermal plasma for OBDC and other complex hazardous wastes.In this paper,the physicochemical characteristics of OBDC were firstly studied.OBDC is a high ash,low moisture mixed hazardous waste,rich in silica,barium sulfate and carbonate,and the oil phase content is 10%～20%,resulting in its low calorific value.The thermal decomposition process of OBDC can be divided into four stages:volatilization of gas attached to the surface of OBDC and a small amount of light oil and water（35～247℃）,gradual decomposition of residual light organic matter and heavy organic matter（247～628℃）,massive decomposition of heavy organic matter（628～730℃）and decomposition of residual heavy organic matter and mineral salts（730～1000℃）.The relatively high weight loss rates in the first and fourth stages indicate that the mineral oil contained in the OBDC is mainly light oil,while the drilling cuttings contain a large amount of carbonate and sulfate.A 30 k W non-transfer arc thermal plasma melting and cracking experimental unit was designed and built by ourselves.The experimental system mainly includes the following units:thermal plasma generation unit,thermal plasma furnace,exhaust gas treatment unit,cooling system and control unit.Compared with the conventional plasma furnace,the present thermal plasma furnace can automatically control the lifting height of the thermal plasma torch,so as to change the thermal plasma treatment distance online and make full use of the high temperature and high energy density characteristics of the thermal plasma flame.Based on this,thermal plasma flame temperature diagnosis and melt temperature correction method were proposed to clarify the temperature distribution in the inner flame zone,middle flame zone and outer flame zone of the thermal plasma flame,and to achieve accurate prediction of melt temperature.The experimental investigation of OBDC thermal plasma melting characteristics and vitrification mechanism was carried out.The effects of thermal plasma melting temperature,melting time and cooling form on the changes of melt appearance,crystalline phase characteristics and melt composition were investigated.When the thermal plasma melting temperature exceeded the flow temperature of the OBDC,the OBDC was gradually agglomerated into a pine green transparent glassy slag with a very dense and smooth surface and a maximum glass phase content close to 100%.Volatile substances such as SO₃ and P₂O₅ were very easy to be discharged with the exhaust gas under the high temperature environment of thermal plasma,while components such as Mg O,Ca O and K₂O will escape from the vitreous slag by chlorination reaction with chlorine elements in the OBDC.The vitreous matrix is a[Si-O]tetrahedral network structure formed by silica,which can effectively wrap heavy metal components such as Al,Fe,Mn,Ba,etc.and meet the criteria for resource utilization.When the oxygen-silica ratio was near 3.65～4.30,the OBDC can be transformed into completely homogenized glassy slag by the high temperature of thermal plasma.RSM design experiments and comprehensive performance characterization of the glassy slag based on the thermal plasma melting process were carried out.The regression equation of the specific energy consumption of the glassy slag with the thermal plasma melting time,input power and treatment distance were obtained.The heavy metal leaching toxicity procedure indicates that the risk of heavy metal environmental contamination of vitreous slag was greatly reduced.The loss ratio on acid dissolution index was introduced to examine the chemical environmental stability of the glassy slag,and the lowest loss ratio on acid dissolution of 5.05%was obtained experimentally.In the thermal plasma medium flame zone,increasing the treatment time and input power was beneficial to reduce the loss ratio on acid dissolution of glassy slag.The RSM regression model accurately calculated the optimal operating parameters in the range of experimental conditions as 6 min of melting time,14 k W of input power,60 mm of treatment distance,and a minimum specific energy consumption of 0.672 k W·h·g^-1 for the glassy slag.The thermal plasma pyrolysis characteristics of OBDC and the reaction mechanism were experimentally investigated.A batch of OBDC models were prepared based on different moisture ratio,oil ratio and moisture-oil ratio to investigate the effects of different factors on the composition of OBDC thermal plasma cracking gas and gas selectivity.The results showed that the thermal plasma can crack the mineral oil phase in OBDC into common small molecule gases,such as hydrogen,carbon monoxide,carbon dioxide,methane,acetylene,ethane,acetaldehyde and dimethyl ether.Among them,the volume share of hydrogen and carbon monoxide was relatively high.The higher the oil content,the higher the hydrogen yield;the increase of water content was beneficial to enhance the oxidation environment and promote the intermediate gas reaction of mineral oil;while too much water would absorb heat and affect the thermal plasma temperature distribution,which made the energy in the reaction zone decrease and adversely affected the reaction process.The gas selectivity analysis showed that the thermal plasma decomposition efficiency of the oil phase was high,and the selectivity of C₁～C₂ gases was as high as 97%,reflecting the absolute advantage of thermal plasma pyrolysis technology.A semi-empirical model of product energy consumption during thermal plasma treatment of OBDC was developed.When the W/O ratio was 1:4,the experimental system had the highest energy utilization,the thermal plasma melting and cracking reaction process of OBDC were the most intense,and the molar energy consumption of H₂ was the lowest,while the molar energy consumption of CO was the lowest when the W/O ratio was 1:7.This study discussed the advantages of thermal plasma treatment technology for OBDC and performed the evaluation of commercial application prospects and economic and social benefits analysis.The work in this paper will lay a solid theoretical foundation and data support for the industrial application of thermal plasma treatment of OBDC.