Investigation of the performance of CF3I/c-C4F8/N2 and CF3I/c-C4F8/CO2 gas mixtures from electron transport parameters

CF3I gas mixtures have attracted considerable attention as potential environmentally-friendly alternatives to SF6 gas,owing to their excellent insulating performance.This paper attempts to study the CF3I ternary gas mixtures with c-C4F8 and buffer gases N2 and CO2 by considering dielectric strength from electron transport parameters based on the Boltzmann method and synergistic effect analysis,compared with SF6 gas mixtures.The results confirm that the critical electric field strength of CF3I/c-C4F8/70%CO2 is greater than that of 30%SF6/70%CO2 when the CF3I content is greater than 17%.Moreover,a higher content of c-C4F8 decreases the sensitivity of gas mixtures to an electric field,and this phenomenon is more obvious in CF3I/c-C4F8/CO2 gas mixtures.The synergistic effects for CF3I/c-C4F8/70%N2 were most obvious when the c-C4F8 content was approximately 20%,and for CF3I/c-C4F8/70%CO2 when the c-C4F8 content was approximately 10%.On the basis of this research,CF3I/c-C4F8/70%N2 shows better insulation performance when the c-C4F8 content is in the15%–20%range.For CF3I/c-C4F8/70%CO2,when the c-C4F8 content is in the 10%–15%range,the gas mixtures have excellent performance.Hence,these gas systems might be used as alternative gas mixtures to SF6 in high-voltage equipment.

Investigation of the hydrate formation process in fine sediments by a binary CO2/N2 gas mixture

To obtain the fundamental data of CO2/N2 gas mixture hydrate formation kinetics and CO2 separation and sequestration mechanisms,the gas hydrate formation process by a binary CO2/N2 gas mixture(50:50)in fine sediments(150–250μm)was investigated in a semibatch vessel at variable temperatures(273,275,and 277 K)and pressures(5.8–7.8 MPa).During the gas hydrate reaction process,the changes in the gaseous phase composition were determined by gas chromatography.The results indicate that the gas hydrate formation process of the binary CO2/N2 gas mixture in fine sediments can be reduced to two stages.Firstly,the dissolved gas containing a large amount of CO2 formed gas hydrates,and then gaseous N2 participated in the gas hydrate formation.In the second stage,all the dissolved gas was consumed.Thus,both gaseous CO2 and N2 diffused into sediment.The first stage in different experiments lasted for 5–15 h,and>60%of the gas was consumed in this period.The gas consumption rate was greater in the first stage than in the second stage.After the completion of gas hydrate formation,the CO2 content in the gas hydrate was more than that in the gas phase.This indicates that CO2 formed hydrate easily than N2 in the binary mixture.Higher operating pressures and lower temperatures increased the gas consumption rate of the binary gas mixture in gas hydrate formation.

The Effect of Pressure on the Dissociation of H_2/CH_4Gas Mixture during Diamond Films Growth via Chemical Vapor Deposition

Monte Carlo simulations are adopted to study the electron motion in the mixture of H2 and CH4 during diamond synthesis via Glow Plasma-assisted Chemical Vapor Deposition (GPCVD). The non-uniform electric field is used and the avalanche of electrons is taken into account in this simulation. The average energy distribution of electrons and the space distribution of effective species such as CH3, CH+3, CH+ and H at various gas pressures are given in this paper, and optimum experimental conditions are inferred from these results.

INFLUENCE OF Al ON HIGH TEMPERATURE SULFIDATION OF AN Fe-25Cr ALLOY IN H_2S-H_2 GAS MIXTURES

The effects of 5 wt.-% Al and 10 wt.-% Al on the sulfidation behaviour of the Fe-25Cralloy in H2S-H2 gas mixtures in the temperature range of 700～900 C has been studied.Thesulfidation resistance of Fe-25Cr was improved and the sulfide scale morphologies,compositionand structure of Fe-25Cr changed remarkably.The sulfidation kinetics of the alloys with alu-minium obeyed basically the parabolic law after an initial period of reaction time.The sulfida-tion mechanism of the Fe-Cr-Al alloy has been proposed.

Breakdown Electric Field of Hot 30% CF_3I/CO_2 Mixtures at Temperature of 300–3500 K During Arc Extinction Process

We calculated the uniform dielectric breakdown field strength of residual 30% CF3I/CO2 gas mixtures during the arc extinction process over the temperature range 300-3500 K at 0.1 MPa. The limiting reduced field strengths are decided by a balance of electron generation and loss based on chemical reactions estimated by the electron energy distribution function （EEDF）, which employs the Boltzmann equation method with two-term expanding approximation in the steady-state Townsend （SST） condition. During the insulation recovery phase, the hot CF3I/CO2 gas mixtures have maximum dielectric strength at a temperature of about 1500 K. At room temperature 300 K, the electric strength after arc extinction （90.3 Td, 1 Td=10-21 V.m2） is only 38% of the original value before arc （234.9 Td）. The adverse insulation recovery ability of CF3I/CO2 gas mixtures in arc extinction hinders its application in electric circuit breakers and other switchgears as an arc quenching and insulating medium.

Numerical Study on Comparison of Negative and Positive Surface Discharge in c-C_(4)F_(8)/CF_(3)I/CO_(2) Gas Mixture

The dynamics of negative surface discharges in c-C_(4)F_(8)/CF_(3)I/CO_(2) gas mixture is investigated here with a 2D fuid model.The distributions of ion concentration,electric field strength and photon flux during the propagation of the streamer are obtained by solving the drift-diffusion equations of particles and Poisson's equation,and the photon flux variation function during the propagation is also fitted.It is found that the streamer branches occur when the streamer transitions from the upper surface of the insulator to the side surface,and then when the streamer approaches the plane electrode,the photon flux will increase significantly.On this basis,the positive and negative surface discharge models are compared in terms of streamer characteristics,particle characteristics and streamer branches.It is found that the streamer has a higher electron concentration and electric field in the positive model.The streamer develops“floating”in the positive surface discharge,while it is close to the surface of the insulator in the negative model.In addition,the negative streamer branch has a wider width and develops further.

Current status and technical challenges of C_(2)storage in coal seams and enhanced coalbed methane recovery:an overview

In the past two decades,research on C_(2)storage in coal seams and simultaneously enhanced coalbed methane recovery(ECBM)has attracted a lot of attention due to its win–win effect between greenhouse gas(C_(2))emission reduction and coalbed methane recovery enhancement.This paper presents an overview on the current status of research on C_(2)-ECBM in the past two decades,which involves C_(2)storage capacity evaluations,laboratory investigations,modelings and pilot tests.The current status shows that we have made great progress in the ECBM technology study,especially in the understanding of the ECBM mechanisms.However,there still have many technical challenges,such as the definition of unmineable coal seams for C_(2)storage capacity evaluation and storage site characterization,methods for C_(2)injectivity enhancement,etc.The low injectivity of coal seams and injectivity loss with C_(2)injection are the major technique challenges of ECBM.We also search several ways to promote the advancement of ECBM technology in the present stage,such as integrating ECBM with hydraulic fracturing,using a gas mixture instead of pure C_(2)for injection into coal seams and the application of ECBM to underground coal mines.