Pollution associated with coal washeries for beneficiation of coal:

Pollution associated with coal washeries for beneficiation of coal:

Coal seam in some of the countries has drift origin, resulted in intimate mixing of mineral matter with coal, giving rise to more ash content. Coal washeries are to reduce ash content in coal. For coking coal and non-coking / thermal coal quality improvement, coal washeries are used. Coal having drift origin, coal-ash distribution in the coal matrix is so interwoven that coal is essentially required to crush to smaller sizes for better liberation of coal and ash particles. Coal washing is a process of separation mainly based on difference in specific gravity of coal and associated impurities like shale, sand and stones etc so that we get relatively pure marketable coal without changing the physical properties of the coal. Coal beneficiation largely depends on gravity difference of coal and ash particles after passing through sizing.

Pollution and its control: During various operations in coal washeries, a lot of particulate matters and gaseous pollutants are generated causing a serious air pollution problem in the area. Besides, coal washeries release very large amount of solid and liquid waste causing serious environmental problems. The washeries reduce the ash content of coal to 17.5% or less. This process consumes clear water in the range of 0.2 to 0.25 m3/tonne of raw coal input. The washeries are operated in the closed water circuit system but still about 12-18% of raw water is discharged as effluent. Though, effluent is treated in settling tank but sometimes overloading results into failure of closed circuit system. The rejects (15 to 25% of input) are dumped near available land without caring for stability of dump, as a result dumps near the river bank cause erosion of coal particles during rainy season and accumulation of fine coal particles on the bottom of river / lake bed.

In general, typical coal washery effluents characteristics are:

1. Physical appearance - Blackish brown to deep black in colour

2. pH – 7.1 to 7.8

3. Suspended solids – 800 to 4000 mg/L

4. Particle size – 0 to 150, 150 to 200, 200 to 300 micron

5. Dissolved solids – 300 to 1500 mg/L

6. Oil & grease – 15 to 200 mg/L

Though many of the washeries have adequate settling tanks but disposal of solid waste required to be accelerated, specially in developing countries to check with the pollution. Proper the reject dump management is required to minimise soil erosion.

Another very useful process of generating electricity with the help of washery rejects by using eco-friendly Fludised Bed Combustion (FBC) technology. By adopting technology which uses washery rejects, the disposal of fines and rejects can be utilized properly and economically; as coal of drift origin require to grind coal before beneficiation and thereby generation of fines are more.

Dust control systems in coal handling plant

Dust control systems in coal handling plant:
Thermal power plants (coal-fired power plants) use coal as their fuel. To handle the coal, each power station is equipped with a coal handling plant. The coal has to be sized, processed, and handled which should be done effectively and efficiently. The major factor which reduces the staff efficiency in operation of coal handling plant is the working environment i.e. a dusty atmosphere and condition. Lots of care is always needed to reduce dust emission. In developing countries, all most all systems used in power station coal handling plants are wet dust suppression systems.

A. After dust is formed, control systems are used to reduce dust emissions. Although installing a dust control system does not assure total prevention of dust emissions, a well-designed dust control system can protect workers and often provide other benefits, such as

(a) Preventing or reducing risk of dust explosion or fire;

(b) Increasing visibility and reducing probability of accidents;

(c) Preventing unpleasant odors;

(d) Reducing cleanup and maintenance costs;

(e) Reducing equipment wear, especially for components such as bearings and pulleys on which fine dust can cause a "grinding" effect and increase wear or abrasion rates;

(f) Increasing worker morale and productivity;

(g) Assuring continuous compliance with existing health regulations. In addition, proper planning, design, installation, operation, and maintenance are essential for an efficient, cost-effective, and reliable dust control system.

B. There are two basic types of dust control systems currently used in minerals processing operations are:
(a) Dust collection system - Dust collection systems use ventilation principles to capture the dust-filled air-stream and carry it away from the source through ductwork to the collector. A typical dust collection system consists of four major components, such as (1) An exhaust hood to capture dust emissions at the source; (2) Ductwork to transport the captured dust to a dust collector; (3) A dust collector to remove the dust from the air; (4) A fan and motor to provide the necessary exhaust volume and energy.
(b) Wet dust suppression system - Wet dust suppression techniques use water sprays to wet the material so that it generates less dust. There are two different types of wet dust suppressions: (i) wets the dust before it is airborne (surface wetting) and (ii) wets the dust after it becomes airborne. In many cases surfactants or chemical foams are often added to the water into these systems in order to improve performance. A water spray with surfactant means that a surfactant has been added to the water in order to lower the surface tension of the water droplets and allow these droplets to spread further over the material and also to allow deeper penetration into the material.
i. Surface wetting system: The principle behind surface wetting is the idea that dust will not even be given a chance to form and become airborne. With this method, effective wetting of the material can take place by static spreading (wetting material while it is stationary) and dynamic spreading (wetting material while it is moving). For static wetting, more effective dust suppression arises by increasing the surface coverage by either reducing the droplet diameter or its contact angle. For dynamic spreading, more factors come into play such as the surface tension of the liquid, the droplet diameter, the size of the material being suppressed, and the droplet impact velocity.
ii. Airborne dust capture system - Airborne dust capture systems may also use a water-spray technique; however, airborne dust particles are sprayed with atomized water. When the dust particles collide with the water droplets, agglomerates are formed. These agglomerates become too heavy to remain airborne and settle. Airborne dust wet suppression systems work on the principle of spraying very small water droplets into airborne dust. When the small droplets collide with the airborne dust particles, they stick to each other and fall out of the air to the ground. This collision between the particles occurs due to three factors involving both the water and the dust particles. As a dust particle and water particle approach each other, the airflow could move the particle around the droplet, have a direct hit on the droplet, or barely graze the droplet. It is this factor that leads us to the second factor, which is that droplets and particles that are of similar sizes have the best chance of a collision. If a droplet is smaller than the dust particle or vice versa, then they may never collide and instead just be swept around each other. The last factor is the dependence of an electrostatic force on a droplet and how the path is affected by this force. Just like with magnets, similarly charged particles repel each others. Thus it is advantageous to have the particles either both neutrally charged (so that they neither repel nor attract one another) or oppositely charged (so that they attract one another) in order to increase the likelihood of a water and particle collision.

C. System Efficiency: Over the years, water sprays has established the following facts: (1) For a given spray nozzle, the collection efficiency for small dust particles increases as the pressure increases; (2) At a given pressure, the efficiency increases as the nozzle design is changed so as to produce smaller droplets. The efficiency of spray dust capture increases by increasing the number of smaller sized spray droplets per unit volume of water utilized and by optimizing the energy transfer of spray droplets with the dust-laden air.

D. Sophisticated system like ‘Ultrasonic Dust Suppression’ systems uses water and compressed air to produce micron sized droplets that are able to suppress respirable dust without adding any detectable moisture to the process. Ideal for spray curtains to contain dust within hoppers. The advantages of using Ultrasonic Atomizing Systems for dust suppression can therefore be summarized as:

(a) reduced health hazards;

(b) decrease in atmospheric pollution;

(c) improved working conditions;

(d) efficient operation with minimum use of water.