Dry Dust removal equipment

Lecture # 11.

Air pollution. Air pollution control equipment.

Air pollution is due to natural and anthropogenic sources that are to man's activities. The world Health organization of the united Nations defines air pollution and pollutants in the following way; Air is polluted when one or several pollutants are present in the atmosphere at such a concentration and for so long a time that they are harmful to man, animals, plants and material property, cause harm or reduce well-being or disturb appreciably its application.

As there is no possibility for a quantative definition of pure air, the composition of dry air for usual conditions at sea level is accepted as a standard for pure air. This composition is given in Table 1. It includes certain elements like carbon monoxide and nitrogen oxides, which are dangerous pollutants when present in the air beyond a defined concentration- the dose-that makes a pollutant.

Table 1. Composition of dry air at sea level conditions

Component	Concentration ppm=cm^3|m^3 air
Nitrogen	789,900
Oxygen	209,400
Argon	9,300
Carbon monooxide
Neon
Helium	5.2
Methane	1.0-1.2
Crypton	1.0
Nitrogen Oxide	0.5
Hydrogen	0.1-0.5
Xenon	0.08
Nitrogen dioxide	0.02
Ozone	0.01-0.04
Ammonia	0.01

Dust emission abatement

Removal of particulate pollutants.

Particulate pollutants may be either dust or drops. It has been mentioned that only the physical properties of particulate pollutants are important for the selection of the type of equipment used for their removal. The diameter of the particles varies over five decades, from 0.01 µm to 1,000µm. Particles with a diameter larger than 5 or 10 µm are sometimes called coarse particles, those smaller than 5 or 10 µm fine particles. It may be assumed that the fine particles will penetrate the respiration system, particularly the lung of man. Furthermore, the dust may absorb gaseous pollutants and carry them into the alveoli at a high local concentration. The fine dust is in every respect, i.e. in medical, biological, and technical respect, the harmful fraction of the particulate matter.

Removal of particulate pollutants.

The dust removal equipment available may be divided into two large groups: Dry dust removal equipment and wet dust removal equipment. For both groups: A few representative examples will be discussed.

Dry Dust removal equipment

In dry dust removal equipment dust separation from the carrier gas is achieved directly i.e. in the dry state, without the use of a special dust collection agent like water drops. The collected dry dust can be directly disposed of. The use of dry dust removal equipment involves however the danger of dust explosion. Therefore, special measures have to be taken to prevent dust explosion and pressure release in case of explosion.

The separation of dust particles from a gas is mainly due too the action of three forces: inertia force, surface or adhesion force, and electrical force. In most of the dry dust separators, one force dominate over the other thus determines the separation process. According to the prevailing force there are three groups of the dry dust removal equipment:

1. Mass force separator.

2. Adhesion force separator.

3. Electrical force separator.

A typical mass force separator is the cyclone. Under the action centrifugal forces, the dust particles are separated from the gas phase. Conventional adhesion force separators are bag and pocket filters. A typical electrical force separator is the plate precipitator. The three types of dry dust separators are schematically illustrated in Fig. 1.

The cyclone consists of entrance chamber, vortex chamber, dust collection chamber and gas exit tube, the letter extending into the interior of the cyclone. The gas is introduced tangentially so that a rotational movement of gas and dust particles is obtained. Due to this rotational motion, centrifugal forces result which carry the dust particles toward the wall entrance and vortex chamber. The cyclone separates the dust continuously.

A conventional electrical force separator is the plate precipitator. Charging wire electrodes and dust collection plate electrodes and arranged in a casing. The plates form straight passages for the gas. In the central plane of each passage a wire net charging electrode is installed. The dust layer that builds up during the collection period is removed by a mechanical shaking system. The electrical precipitator works continuously.

Дата добавления: 2015-10-26