Why is coal sprayed with water

The application nozzles in mining engineering are flat fan, hollow cone and full cone. Flat fan nozzles use a fan spray pattern with a narrow-strip impact area.

They produce small-to-medium droplet sizes and are usually used in a narrow or enclosed space. Hollow cone nozzles have an annular impact area that can produce a medium-to-large diffusion angle. This type of nozzle is normally used to capture the dust in regions where particles have been scattered in the air. Full cone nozzles use a solid cone-shaped spray pattern with a round impact area.

They produce medium-to-large droplet sizes. When the distance between the nozzle and the target is large, full cone nozzles usually have a better dust removal effect than the other nozzles [ 13 ]. Choosing the proper spray pattern is essential for effective dust control.

However, an important problem of the treatment of a ring-shaped dust source generated by a longitudinal axis machine always exists. The best approach to suppressing the ring-shaped dust source is the use of an annular spray pattern.

The spatial morphology of water spray, which is produced by flat fan nozzles and full cone nozzles, does not correspond to the ring-shaped dust source. The hollow cone nozzle can produce an annular impact area. However, the nozzle can only be placed on one side of the dust source, and the droplets will not be able to reach the other side of the rotating head. Thus, a set of nozzles are usually arranged around the rotating head. As a result, a group of hollow cone nozzles produce a set of small ring dusting areas that do not match the single large ring-shaped dust source produced by the rotating head.

To solve this problem, a higher spray pressure is usually adopted to ensure that as the droplet size decreases, the spray volume increases, thereby improving the efficiency of dust reduction [ 14 , 15 ]. However, this method is energy intensive and water consuming. Moreover, a large number of droplets are not involved in the improvement of the working environment.

As the droplet size decreases, the droplets become more susceptible to airflow and other factors. Therefore, it is not the best choice. The arc jet nozzle used for foam spray was designed to improve the dust removal efficiency of foam [ 16 ]. But this nozzle is not suitable for water spray. In this paper, an arc fan nozzle used for water spray is proposed. The flow field generated by this nozzle is represented as a three-dimensional arc fan in the space.

The previous studies have reported that the flow field characteristics of nozzles have an important influence on the performance of dust removal [ 17 — 19 ]. So, we studied the flow field characteristics of this nozzle with numerical simulation.

The dimensionless formula for calculating the jet speed near the wall is obtained. The relationship between the geometric characteristics of flow field and the critical structural sizes of the arc fan nozzle is analysed. The effect of dust removal is tested in the field. As mentioned, regardless of which conventional nozzle is used, the spray pattern is not consistent with the geometric shape of the ring-shaped dust source.

For this reason, a water spray nozzle that can produce an arc-shaped spray pattern is developed to accurately and efficiently control a ring-shaped dust source. The water spray effect of full cone nozzle, hollow nozzle, flat fan nozzle and arc fan nozzle is respectively shown in Fig 2. A The water spray effect of full cone nozzle. B The water spray effect of hollow cone nozzle. C The water spray effect of flat fan nozzle.

D The water spray effect of arc fan nozzle. The ring-shaped dust source is divided into several segments of arc dust sources. Each arc dust source is suppressed by an arc fan flow, which is generated by an arc fan nozzle, as shown in Fig 3. In this manner, each arc fan flow can correspond highly to one arc dust source. The structure of the arc fan nozzle is illustrated in Fig 4. It is mainly composed of an inlet, a jet orifice, a pair of rectifying wings and a guiding object.

The spray characteristics of the nozzle are specifically influenced by the geometry of the spray holes. Thus, the arc fan nozzle used for water spray is designed with an arched jet orifice.

The fundamental parameters of the nozzle are the chord length of the nozzle orifice cl no , the height of the nozzle orifice h no , the length of the guiding object l g , the chord length of the guiding object cl g and the height of the guiding object h g.

The chord length of the nozzle orifice cl no and the height of the nozzle orifice h no determine the area of the jet orifice, which affects the mist flux. To meet the demand of field conditions in a coal mine, cl no was set as 10 mm, and h no was set as 5 mm. The length of the guiding object l g , the chord length of the guiding object cl g and the height of the guiding object h g are the fundamental dimensions to determine the geometrical characteristics of the arc fan flow.

They can affect the coverage performance of the water. When l g is greater than 35 mm, the nozzle is easily impacted by foreign objects and is likely to be damaged. When l g is less than 25 mm, forming the ideal arc spray is often difficult because the guide distance is too short.

Therefore, l g is set to 30 mm. The water ejected from the arched jet orifice impinges on the guiding object to form an arc-like flow. The guiding object is located outside the nozzle.

It is designed to control the spray pattern. The section of the guiding object is an arc, which is similar to the section of the nozzle orifice. This approach is conducive to forming the stable arc fan flow. Fig 5 shows the water spraying effect.

The main goal of this numerical research is to understand the flow formation process and resolve the spatial characteristics of the flow field. The key characteristic dimension of flow field is critical to understand the dedusting performance of arc fan nozzle. It helps in achieving the best design and performance of the nozzle. So, prediction of the phase interface between the water and the air is essential to the successful computation of the arc fan flow. Unlike many other methods developed to flow with the interface, the VOF method can model the interface and the coalescence without special treatment.

In the VOF model, a single set of momentum equations is solved for both fluids. By defining and using the VOF function to reconstruct the moving interface, the accurate position of the interface can be obtained [ 20 , 21 ]. In the numerical simulation of the flow field of the arc fan nozzle, the air is the first phase, the water is the second phase, and the flow interface is tracked by the change of the VOF function with time.

The basic governing equations are as follows. The term G k indicates the generation of turbulence kinetic energy corresponding to the mean velocity gradients. The term G b represents the generation of turbulence kinetic energy corresponding to the buoyancy.

The term Y M denotes the contribution of the fluctuating dilatation in compressible turbulence to the overall dissipation rate. The target distance is set to 1 m. The chord length of the nozzle orifice and the height of the nozzle orifice are 10 mm and 5 mm, respectively. The length of the guiding object is 30 mm. The chord length of the guiding object and the height of guiding object are 20 mm and 10 mm, respectively.

As the water transports gradually outward, water and gas are gradually mixed. In the practice of dust removal, both water and gas will interact with dust particles.

It is meaningful to study the velocity of the mixed phase. Take the central axis of arc fan flow as the monitoring line. The velocity of the mixed phase in the monitoring line can be obtained, as shown in Fig 6. After leaving the guiding object, the flow velocity is increased in seconds. With the interaction of water and gas, the flow velocity decreases gradually. Within 0. This zone is the impact zone. The numerical simulation results of the flow velocity in the impact zone are derived, and the non-dimensional analysis is conducted.

Next, the following formula is obtained. When L , u 0 , and d are known, the flow velocity near the wall can be calculated by using formula 9. Skip directly to site content. Section Navigation. Facebook Twitter LinkedIn Syndicate. Download PDF Document. Peer Reviewed Journal Article - July Links with this icon indicate that you are leaving the CDC website. The turbulence of the water droplets moving through the air creates a powerful air flow around them.

The dust fines move around the water droplet, as shown in the slip-stream image below. Turbulence of the water droplets moving through the air creates a powerful air flow around them, causing the dust fines to move around them and resulting in less effective dust control.

Wetter water. How can you make water wetter? The right blend of non-ionic and anionic surfactants blended with penetrants, dispersants, and binding agents will produce a wetter water capable of targeting dust fines and locking them down for weeks or even months.

As shown in the video, a small amount of the right surfactant chemistry will go a long way. A shot-glass of the right chemistry will make gallons of water up to X wetter by lowering the surface tension of the water, and changing the ionic charge to the opposite charge of coal fines.

The binding agents incorporated into the chemistry cause the fine to adhere to lump coal and other fines, preventing it from becoming airborne even after the treatment dries.

Wetter water allows for the production of smaller droplets, reducing the volume of water required. Wetter water also means every water droplet has X better odds of connecting with a dust fine. This means better dust control performance when applied directly into coal processes, and better dust control performance when sprayed into the air around a dusty process.

The image below demonstrates how a smaller, oppositely charged water droplet will attract dust fines, wetting them faster. Better results are achieved using a fraction of the volume of water. Smaller, oppositely charged water droplet will attract dust fines, wetting them faster and resulting in better dust control. Water conservation is a growing concern in all industries, but especially in the coal-fired power industry. The reduction in water consumption not only saves money, but also reduces or eliminates slippery conditions, creating a safer working environment.

The reduction in water also lowers or eliminates the impact on BTUs, allowing coal-fired power plants to get the maximum heating value from their coal.