| Large-scale coal gasification technology is a promising efficient clean utilization technology of coal. As one of the key techniques in gasification, high-pressure dense-phase pneumatic conveying technology is widely applied in various industries. Feeding hopper is one of the most important components of high-pressure conveying system, and its conveying characteristics will affect the stable operation of the gasifier and the gasification process performances significantly. During powder handling operations such as powder conveying into hoppers or powder discharging from hoppers, the powder invariably becomes electrostatically charged. The accumulation of electrostatic charges in the feeding hopper will cause severe problems, such as agglomerations, adhesions on the wall, electrostatic discharges leading to sparks, fire and even explosions. Investigation on electrostatic charges in the feeding hopper is few reported. In our work, experimental study and theoretical analysis are utilized to investigate the characteristics of powder electrostatic charges in the feeding hopper. This paper focuses on examining the effects of different operating parameters and material properties on electrostatic characteristics and conveying stability. It provides a more in-depth understanding for the design, operation and control of dense-phase pneumatic conveying at high pressure.Experiments were conducted to investigate the electrostatic characteristics of powders in the feeding hopper. The effects of the fluidizing time, fluidizing gas velocity and the height of sampling ports on static charge were studied. The results indicate that ratios of powders charge to mass increase with the fluidizing time and eventually reach electrostatics saturated state. The charge density of powders increases as the fluidizing gas velocity increases as well. The higher the sampling port is, the charge density of the sampling powders is larger and more sensitive to the fluidizing gas velocity.The particle size, the material type and the moisture content were varied in order to study the influence of material properties on static charge of powders. Results show that the ratios of charge to mass of fine anthracite particles are larger than those of the coarse anthracite particles. However, the ratios of charge to surface area of fine anthracite particles are smaller than those of the coarse anthracite particles. The ratios of powders charge to mass decrease significantly with the increase in the moisture content. Biomass powders and glass beads are always charged positively, whereas anthracite becomes negatively charged in each experiment. Empirical correlations of biomass powders and anthracite powders charge density using dimensional analysis were derived from experimental data. The predicted values show generally good agreement with experimental data.The standard deviation coefficient of material mass flow rate signals were used to analyze the conveying stability of the feeding hopper. The effects of the operating conditions and powder physical properties on conveying stability were carried out. As the fluidizing gas flow rate increases respectively, the conveying stability of the feeding hopper first increases then declines. The conveying stability of the coarse material are worse than those of the fine material under the same conveying conditions. Conveying capacity and flow stability of the anthracite powders are better than those of the biomass powders under the same conveying pressure and pressure differential There exists an optimum external moisture content corresponding to the minimum coeffic ient of variation. When the powder external moisture content exceeds its limit value for pneumatic conveying, unstable conveying phenomenon will happen inside the feeding hopper.Experimental investigations were performed on the feeding hopper conveying test facility to explore the effect of electrostatic charges on conveying stability by producing different levels of charging of particles inside the hopper. The influence of the operating conditions, powder properties and static charges on conveying stability were compared. The results indicate that the conveying stability become worse with the increase in electrostatic charges. As the fluidizing gas flow rate increases, the effect of electrostatic charges on conveying stability will be increased. It is seen that, the effect of electrostatic charges on conveying stability to the biomass powder, is more obvious than that to the anthracite powders. Electrostatic charges is the certain effect on conveying stability as the external moisture content is low, whereas when powder have a high external moisture content, the effect of the powder electrostatic charge on conveying stability weakens while the external moisture content turns to be the major factor on conveying stability. |
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