Methods and means of weeding of atmospheric emissions

Its self-cleaning ability is one of features of atmosphere. Free air self-cleaning occurs owing to dry and wet fallout of admixtures, absorption by their terrestrial surface, absorption by plants, processings by bacteria, microorganisms and in other paths. Landing of trees and bushes promotes weeding of air from a dust, white damps, dioxides of sulphur and other substances. Rather to sulphur dioxide the poplar, a linden, an ash have the best absorption properties. One adult tree of a linden can accumulate throughout time of ten kgs of dioxide of sulphur, transforming it in harmless substance. The big role in free air weeding belongs to soil bacteria and microorganisms. At temperature 15... 35 ºС microorganisms recycle on 1 м2 to 81 т for the period of oxides and carbon dioxides. However possibilities of the nature concerning self-cleaning have limitation, that a trace to consider by working out of specifications BAC.

Degree of their weeding from harmful substances is one of the main indexes of weeding of blowouts К_оч:

, (2.8)

where – Mass of harmful substances which are trapped in the cleaning device;

– Lump of harmful substances in blowouts.

Weeding degree should be defined on each pollutant. Weeding degree shares on design and actual, and on level - on maximum and operational.

For an estimation of security of the enterprises weeding in time uses factor of technological processes of weeding of gas:

, (2.9)

where – Process equipment operating time;

– Operating time of installations of weeding of gas.

For adverse meteorological conditions when blowouts with pollution can be unhealthy the population, the enterprises should reduce blowouts of harmful substances at the expense of means or a full (partial) stop of polluters.

Modern requirements concerning quality and degree of weeding of blowouts high enough. For their observance it is necessary to use technological processes and the equipment which reduce or completely exclude blowouts of harmful substances in atmosphere, and also provide neutralisation of the formed harmful substances; to operate industrial and power the equipment which evolves a minimum quantity of harmful substances; to close small boiler-houses and to connect consumers to HPP; to apply antitoxic front gardens; to translate heat power installations from solid fuel on gas.

Modes of weeding of atmospheric emissions from harmful substances can be aggregated in such groups:

· Weeding of blowouts from a dust and aerosols of harmful substances;

· Weeding of blowouts from gaseous harmful substances;

· Drop of air contamination by exhaust gases from combustion engines of vehicles and stationary installations;

· Air contamination drop at transportation, loading and unloading of free-flowing loads..

For clarification of emissions from harmful substances the mechanical, physical, chemical, physico-chemical and combined methods are used.

Mechanical methods are based on use of forces of weight (gravitation), forces of inertia, centrifugal forces, ways separations, diffusions, grab ит.п.

Physical methods are based on use of electric and electrostatic fields, refrigeration, condensation, crystallisation, absorption.

In chemical methods reactions of oxidising, neutralisation, restoration, catalisation, thermooxidizing are used.

Physico-chemical methods are based on principles of sorbations (occlusions, adsorptions, chemical absorption), coagulations and flotations.

Gravitational chambers of clarification of a dust work by a principle of decrease in speed of movement of gases to level when a dust and liquid parts settle out under the influence of forces of weight. An overall performance of clarification of a dust of chambers

, (2.10)

where – diameter of parts which precipitate; – Kinematic viscosity of gas; –height of camera; – speed of gas flowing; – Gravitational acceleration; – length of camera; – Density of parts; – Gas density.

Gravitational chambers of subsidence of a dust is perivisceral with regiments or a box from a sheet steel with the bunker for dust collecting. Length of a box

, (2.11)

де – height of chamber;

– speed of flow of gas;

– Speed from which the dust drops out in the bunker.

With decrease in height of the chamber clarification process improves, therefore emptiness of the chamber divide regiments which are projected under a corner or with regulation possibility. Gravitational chambers of dust precipitation suitable for sedimentation of parts of a dust in diameter over 50 microns. Hydraulic resistance of gravitational chambers lays within 50... 150 Pases. Speed of gas - 0,2... 1,5 km/s. Chambers provide degree of clarification of no more than 50 %, therefore them use as the previous degree of dust separation.

Inertial separators work on a principle of sharp change of a direction of a stream of gases. In places of change of a direction occurs subsidence of hard parts of pollutants. Separators allow to rebuff parts in diameter 25... 30 microns. Inertial gas purifiers have productivity from 45 to 582 m3/ch. To this type it is possible to carry and jalousie dust extractors which have hydraulic resistance 100... 400 Pases, suppose temperature of cleared gas to 450º With, speed on the approach to a lattice - 15... 25 km/s.

Cyclone separators work by a principle of use of centrifugal effect. Isolation of hard parts in them occurs under the influence of centrifugal forces:

, (2.12)

де – Constant dimensionless factor;

– Density of parts;

d – diameter of parts;

–tangent compound speeds of movement of parts;

– radius of parts;

– radius of device;

n – constant, which depends on radius of device and working temperature;

–height of cyclone.

Such types of cyclone separators are practically used::

q horizontal dust extractors which work by a principle of granting to gases whirlwind type circular movement by means of a revolving object with system of not declined blades;

q vertical separators which work by a principle of serve gas from above through horizontally established ring spinner which gives to gas of rotating movement; hard parts settle out at the bottom, and purified gas is taken away through the central pipe;;

q vertical separators with tangently had entrance part. In this separator the detained gas arrives sideways or from below and gets tangent movement which takes out hard parts to walls, and then to dust collectors;

q rotational stream dust extractors are species of the centrifugal cyclone separator, in which вихреподобность gas movements strenuously tangent an airflow. In them the dust accumulates in air mediuims and under the influence of gravitational forces falls on a bottom of dust collector.

Devices of wet clarification of gases from a dust work by a principle of washing of gases. These kinds of cleaning devices are applied on sites of dyeing of products, drawings of polymeric coverings, in closed systems of using by air. Such devices allow to clear gases of shallow mechanical pollution. There is a considerable quantity of devices of wet clarification of gases. Simple water curtains through which the polluted air flows are passed are applied also.

By a principle of work devices of wet clarification of gases share on cavernous and attachment; bubble-type and foamy; with great dispatch - inertial; the centrifugal; dynamic and turbulent washers.

Cavernous and attachment devices - scrubbers work by a principle of the admission of gases through a stream of the water sprayed sprayed or expiring after nozzles. Speed of a stream of gases does not exceed 1-1,2 km/s, hydraulic resistance of devices does not exceed 250 Pases. The water expense introduces to 10 м3 on 1 m of the device. Most full scrubbers delete particles in the size over 10 microns. A lack of scrubbers is frequent hammering holes of atomizers.

At work bubble-type and foamy devices the polluted gases pass through a liquid or foam layer. Devices have the big hydraulic resistance (to 2000 Pases). They allow to catch particles in the size to 2 microns. Productivity of devices of design ЛТІ - from 2 to 45 thousand m3/ch., speed of passage of gases - to 2 km/s, clarification degree - to 99 %.

Devices of with great dispatch - inertial type work by a principle inertial reining in mechanical pollution during change of a direction of a gas stream over a liquid surface. The biggest application have got static dust extractors of type ПВМ, ротоклони and scrubbers of shock action. Productivity of with great dispatch - inertial devices-2500... 90000 m3/ch. Speed of a stream of gas - to 56 km/s, clarification degree - to 98 %. A water expense - 0,8... 4 m3/ch. On 1000 м3 gas.

Centrifugal devices of wet clarification of gases work by a principle of a turbulence of gases as special shovels or for the account tangent leading of gas with simultaneous irrigating from atomizers. Them use for clarification of flue gases with the big contents of sulfuric gases, providing degree of clarification to 90 %. Are used also dynamic and turbulent washers.

At work of electrostatic installations cleared gases pass through an electrostatic field of a high voltage (to 50 кВ), created by special electrodes. At passage through part electric field get a negative charge and are drawn to the electrodes connected to land, therefore have a positive charge concerning parts. For clarification electrodes are provided special mechanical system. The electrostatic method of clarification of gases allows to catch parts in the size to 0,1 microns. Initial expenses on creation of electrostatic filters are the higher, than for devices of other types, however working costs the lowest. Power consumption by these devices makes 0,3... 0,6 kw on 10000 м3 gas.

In bank filters the polluted gases pass through a fabric, cloth, felt, synthetic materials (nitron, lavsan, chlorin), metal grids, gravel etc. These filters provide high quality of clarification. Their basic lack - decrease in a crush of gas after filtering, high cost of exploitation, frequent replacement of filtering elements.

The most widespread devices for clarification of gases from mechanical particles are sleeve filters which basic element is sleeve bag, used on a tubular frame. At passage of gases through a bag dust particles remain on a fabric. Removal of a dust from bags is carried out mechanical shaking out, its purge in the opposite direction, clearing by flows of air, use of low-frequency acoustic generators for branch of hard particles from a bag.

Granular filters, including made from ceramet, and also fabric rolled filters which provide high quality of clearing are used. However their lack is low capacity and fast contamination.

In technological ventilating and power emissions at the enterprises most often there are dioxide sulfurs, oxides nitrogen, oxides and dioxides of carbon, mineral substances from manufacture of building materials, connections of metals, phenols, synthetic materials, paint and varnish materials and so forth.

Methods of clarification of emissions from gaseous substances behind character of physical and chemical processes with cleared environments are divided in such a way:

q Washing of emissions by solvents which do not combine with загрязнителями (an absorption method);

q Washing of emissions by solutions which enter a chemical compound with pollutants (a method of hemosorbtion);

q Absorption gaseous pollutants with hard active substances (an adsorption method);

q Absorption and use of catalysts;

q Thermal processing of emissions;

q Sedimentation in electric and magnetic fields;

q Freezing out.

Absorption method (an armour. absorptio - absorption of substances from solutions or mixtures of gases by a hard body or a liquid) unlike absorption adsorption occurs in all volume of absorbent. Is based on division of an air-gas mix into components by absorption of harmful components by absorbent.

As absorbents select the liquids, capable to absorb harmful impurity. For an exception of emissions of ammonia, chloride and fluoric hydrogen water is used. One kg of water is capable to dissolve hundreds grammes of chloride hydrogen and ammonia. Sulphurous gases in water are dissolved badly, therefore water expenses in this case too big. Sulfuric acid is applied to an exception of emissions of aromatic carbohydrates, water steam and other substances. Tubular devices(absorbers) are applied to realisation of process of clarification of gas emissions by an absorption method. A volume expense of a liquid

, (2.13)

Where m - mass of impurity which are subject to an exception;

, -begining and final concentration of a harmful impurity in a liquid.

The area of contact of gas with a liquid

, (2.14)

Where - absorption factor (масопередач і);

- Average motive power of absorption.

Value is above at counter movement of gas and a liquid, than at one-way traffic. Absorption process should provide by all means application десорбции - regenerations of a liquid for the purpose of an exception of the dissolved impurity.

The method of hemosorbtion is based on absorption of gases and steams liquid and hard absorbers with formation of chemical compounds. This method is used at clarification of emissions through ventilation of galvanic sites. Thus solvent for clarification of emissions from chloride hydrogen are 3 % solution of caustic natra. This method is used also for clarification of emissions from oxides of nitrogen.

Adsorption method (an armour. ad - on, in, at + sorbere - to absorb) - absorption of substances from solutions or mixtures of gases with a blanket of liquids or hard bodies. Is based on a selective exception of gas mixes of harmful impurity by means of hard adsorbents. Most widely as adsorbent is the activated coal, ion-changed pitches and so forth is applied.

Necessary weight of adsorbent

, (2.15)

Where - volume expense of gas which is cleared;

- Concentration of an impurity which is withdrawn;

- Time of course of adsorption;

- Absorbing ability of adsorbent.

Geometrical parametres of absorbent are selected and considered for nomogramms or analytical dependences.

The Katalitichesky method is based on transducing of toxic components of emissions on less toxic or harmless catalysts at the expense of use. Speed of katalystic reactions can be defined with the equation

, (2.16)

Where -constant of katalystic reactions;

- Concentration of substances which enter reaction;

а, б, в, n - reaction order behind a corresponding component.

As the catalyst use platinum, metals of a platinum number, oxides of copper, manganese dioxide, 5-oxide of vanadium and so forth.

The Katalitichesky method is used for clarification of emissions from oxide carbon at the expense of its oxidation to carbon dioxide.

The thermal method is based on reburning and thermal neutralization of harmful substances in emissions. This method is used when harmful impurity in emissions give in to burning. The thermal method is effective in case of clarification of emissions from paint and varnish sites. Systems of thermal and fire neutralisation provide efficiency of clarification to 99 %.

As a whole the sequence of a choice of type of cleaning devices and filters is the such:

q Revealing of characteristics of emissions (temperature, humidity, a kind and concentration of impurity, toxicity, dispersion and so forth);

q Definition of type of the cleaning device or filter behind an expense of the gas, necessary degree of clearing, possibilities of manufacture and other factors;

q Search of working speed of gases;

q The technical and economic analysis of possible variants of clearing;

q Calculation of parametres of the cleaning device;

q Designing and choice of the cleaning device or filter.

At a choice of means of clarification of emissions to atmosphere it is necessary to use such recommendations:

q Dry mechanical ways and devices are inefficient at an exception smalldispersible and a sticky dust;

q Wet methods are inefficient at clarification of emissions in which substances which badly stick together contain and form a clod;

q Electric sedimentators are inefficient in case of an exception of pollution with small specific resistance which are badly charged by an electricity;

q Sleeve filters are inefficient for clarification of emissions with the sticky and humidified pollution;

q Wet scrubbers cannot be applied to work out of premises in winter conditions.

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