Characterization of granular electrostatics generation

Highlights

• The effects on granule electrostatics generation were investigated.

• Two kinds of granules were used: PVC and coal.

• Electrostatics increases with granule length-ratio, contact area and sliding velocity.

• Granule sliding orientation has obvious effect on electrostatics.

• Electrostatics increases with normal stress and independent of relative humidity.

Abstract

Granule electrostatics is widely found in most granule engineering areas. However, the working mechanism has never been fully understood due to its sensitivity to working conditions, such as granule geometrical shape (including length-ratio and front-facing angle), contact area, granule material, relative humidity, sliding velocity, sliding orientation, normal stress and so on. In this work, single granules were chosen to slide along a metal plate and the electrostatics charge generated were measured. The working factors above have been considered for two materials, i.e. PVC and coal. Regarding granule shape, it is found that granules with larger length-ratio tend to produce more electrostatics as well as the slender or sharper granules. Triangular granules generate more electrostatics than trapezoidal granules. In addition, electrostatics charge generation increases with decreasing granule front-facing angle and charges tend to gather at the granule tip. Regarding granule material, it is found that coal granule tends to generate more electrostatics in comparison with PVC granule. The sequence of the relative work function for the three materials used in this work from high to low is given as PVC, stainless steel and coal. Regarding surrounding factors, it is found that electrostatics increases with granule contact area and such trend increases with decreasing relative humidity. Electrostatics generated increases with decreasing relative humidity and sliding velocity. Granule sliding orientation does affect electrostatics. In this work, granules sliding with smaller front-facing angle as well as shorter side tend to generate more electrostatics. Electrostatics increases with normal stress.

Introduction

In the granule processing industry, the phenomenon of triboelectrification is non-negligible due to ceaseless collision between granules and pipe wall, which could be found widely in energy, chemical and pharmaceutical industries. Under the circumstances, the granules have a tendency to be charged and a series of safety problems may occur such as granules clustering, sparking hazard and even explosion due to electrostatic charge accumulated tool large [[1], [2], [3]]. In addition, electrostatic charge of granules will affect accuracy of the measuring system and cause high error to the measurement. Moreover, given the specific operational environment, electronic equipment may break down under the effect of electrostatic charges [4]. Expect of the shortcomings of electrostatic charge generation, electrostatics has been applied in many industries in recent years, for example, measurement technique and electrostatic precipitation. Electrostatic sensor array has been designed to measure physical properties of granules in pneumatic conveying system [5,6] as well as other two-phase flow measurements [7,8]. Meanwhile, considerable research works have been carried out to look into the working mechanisms for this design. In 1960s, the earliest electrostatic study was conducted by Cole [9], where electrostatic generation in a pneumatic transport system was found to damage system operation due to a large amount of charge accumulated [10]. Yao et al. [1] observed and defined three typical granule flow patterns in a pneumatic conveying system due to electrostatic effects on granules. Mobbs et al. [11] studied the effect of mass flow rate of solids on electrostatic charge generation, where they found that accumulated charges increase with mass flow rate of granules. In addition, some other factors have been found to affect electrostatic charge generation, such as physical properties of granules and external environment. Yao et al. [12,13] investigated the effects of granule properties on the electrification of PVC granules in pneumatic conveying systems, where they argued that granule size and shape make obvious effects on charge generation. Matsusaka and Masuda [14] investigated the impacting electrification by glass bead impacting on plates for particle-wall impact velocity, angle, and initial charge on the charge accumulation of granules.

In recent years, more and more researchers investigated granule electrostatics via experimental and numerical methods. Murata and Kittaka [16] proposed an electrification model involving granules contact with dissimilar materials by comparing between photoelectric and contact electrification experiments [16]. Matsuyama and Yamamoto [17,18] set up a charging relaxation model to explain the working mechanism of breakdown voltage leading to air ionization. In addition, Dwari and Rao [19] and Matsusaka [20] developed a condenser model based on the relative work function and applied it in subsequent practical electrostatic experiments, where a work function was summarized to explain the direction of electron transfer. Subsequently, Bunchatheeravate et al. [21] developed a collision model and used it to predict charge accumulation of granules in pneumatic conveying systems.

So far, three contacting ways between granules and wall have been found in pneumatic conveying systems and corresponding charging ways i.e. collision electrification [22,23], rolling and sliding triboelectrifications [1,12,13] have been considered for electrostatics generation. However, the principles of granule charging generation have not been fully understood. For example, as granules impact on a metal plate [24], the impact frequency and amplitude effect on electrostatic charge generation is little investigated. Matsusaka et al. [23] investigated the collision electrification of granules excluding the consideration of granule size, shape, surface roughness affecting electrostatics generation. Yao et al. [1,12,13] confirmed that particle-wall sliding is one of the main electrostatics charge generation in a granular pneumatic conveying system while the working mechanism of particle-wall sliding triboelectrification was not identified. In this paper, sliding triboelectrification of single granule along a metal plate is investigated in detail. Some sensitive factors affecting electrostatics generation are investigated, such as front-facing angle, particle-wall contact area, granule material, granule sliding velocity and orientation as well as external environment condition (relative humidity), granule geometries (shape and size), dynamic status (normal stress) and surface roughness. In addition, the working mechanism of electron transfer is explained by relative work function which is used to compare electrostatics capability of various granule materials. To this end, an integrated conclusion including all possible influencing factors effect on granule sliding triboelectrification is presented.