air pollution control-sox

Air Pollution Control (Sox, Nox)

Presented byDhaval. N. YadavLecturer in Chemical

Engineering

SOx includes six different gaseous compounds namely:SO – Sulfur MonoxideSO2 – Sulfur DioxideSO3 – Sulfur TrioxideSO4 – Sulfur TetroxideS2O5 – Sulfur SesquioxideS2O6 – Sulfur Heptoxide

SOx Emissions

Most Significa

nt

SO2 is Colorless Non Flammable Non Explosive Suffocating Odor (Threshold value 0.5 ppm) Highly Soluble in water (70% Acid rain) Reacts photo-chemically to form SO3, H2SO4

SO2 + H2O → H2SO3 (Sulphurous Acid)SO3 + H2O → H2SO4 (Sulphuric Acid)

SOx Emissions

Natural sources of sulphur dioxide include volcanoes and hot springs.

Sulphur dioxide is also formed by the oxidation of hydrogen sulphide (H2S), a toxic gas that smells like rotten eggs. Hydrogen sulphide is frequently found with natural gas.

Man-made sources of sulphur dioxide include sour gas processing, oil production, coal combustion, ore refining, chemical manufacturing and other fossil fuel processing and burning.

Sources of SOx Emissions

There are six procedures for controlling of SOx emissions.

Natural dispersion by dilution. Using Alternate fuels. Removal of Sulfur by desulphurization. Control of SOx in the combustion process. Treatment of flue gas→ Dry Methods &

Wet Methods

Control of SOx Emissions

The control method is based on natural dispersion at high elevation so that the ground level concentration are acceptable.

In India, minimum stack height recommended is 30m.

Height of Chimney H = 14*Q0.3 ;for Q kg /hr of SOx emission.

Obsolete method/very less used. People have now become more conscious

about the atmosphere, environment.

Natural Dispersion by Dilution

A switch to natural gas from the conventional high sulfur fuels like coal & petroleum reduces SOx emissions.

LNG/PNG are also quiet effective. Low sulfur containing coals is also an alternative.

Use Alternate Fuel

Use Alternate FuelSubstance % Sulfur Calorific

ValueAnthracite 1.0 34.0Bitumren 1.6 25.3Coal Tar 0.5 38.3Coke 0.5 29.0Crude Oil 0.5 – 3.0 VariesFuel Oil 2 – 4 43.5Lignite 1.5 23.0Natural Gas Traces 34.0Peat 1.0 18Wood 0 18

Removal of sulphur from fuels prior to combustion is theoretically an attractive route.

Sulphur in coal is present in both inorganic and organic forms. All inorganic sulphur is present in coal as Iron Disulphide (FeS2).

Washing can reduce the pyritic sulphur content which also results in the loss of combustible material and may add 20% to the cost of coal. Organic sulphur is present in forms of cystin, thiols, sulphides which is uniformly bound with most of the carbonaceous material and can be removed only by chemical processing.

Removal of Sulfur from Fuels

The term hydro treating is used to describe the process of the removal of sulphur, nitrogen and metal impurities in the feedstock by hydrogen in the presence of a catalyst.

Objectives: Removing impurities, such as sulphur, nitrogen and

oxygen for the control of a final product specification or for the preparation of feed for further processing.

Removal of metals, usually in a separate guard catalytic reactor when the organo-metallic compounds are hydrogenated and decomposed, resulting in metal deposition on the catalyst pores (e.g. atmospheric residue desulphurization (ARDS) guard reactor).

Hydro Treating

Flow Diagram of Any Hydro Treating Process

Process Explanation The liquid feed is mixed with hydrogen and

fed into a heater and the mixture is brought to the reaction temperature in a furnace and then fed into a fixed bed catalytic reactor.

The effluent is cooled and hydrogen-rich gas is separated from the liquid using a high pressure separator.

Before the hydrogen is recycled, hydrogen sulphide can be removed using an Amine Scrubber.

Some of the recycle gas is also purged to prevent the accumulation of light hydrocarbons (C1–C4).

The liquid effluent for the reactor is introduced to a fractionators for product separation.

This process can remove both the organic & inorganic sulfur from coal.

Steps : Preparation of Slurry (Oil + Anthracene

Coal) Heating at high temperature (450°C) Filtration of organic & inorganic form

from ash. Evaporation followed by Distillation Solidification

Hydro-desulphurization of Coal (Solvent Extraction)

Flow Diagram

In this process slurry of anthracene oil with finely grounded coal is prepared to which a small amount of hydrogen is added to prevent repolymerization.

The slurry is then heated at high temperature of 450°C in order to dissolve coal.

The ash residue consisting of pyritic sulphur and other minerals is eliminated by pressure filtration.

The coal solution filtrate is sent to flash evaporator where the light fraction is removed which is followed by distillation operation to recover solvent.

Process Explanation

The hot liquid evaporator residue is cooled to a brittle solid fuel which can be pulverized.

The product can be liquefied at 250°C which has a higher heating value than raw coal and contains less than 1% sulphur.

Process Explanation

Flue gas desulfurization aims to reduce sulfur oxides emissions from the stack gases of the refineries and power plants to 150-1500 ppm range, for pollution control.

Classified as Dry Process and Wet Process.

Wet scrubbing processes are effective but the cost of equipment is very high and difficult to install at various plants.

Thus Dry Process are much preferred.

Desulphurization of Flue Gas

Adsorption of SO2 by metal oxides to from stable sulphites or sulphates with subsequent regeneration.

Alkalized Alumina Process Manganese Oxide Process

Adsorption on activated carbon followed by regeneration and conversion of concentrated SO2 to sulphuric acid or elemental sulphur.

The Reinluft Process

Dry Process

Also called as Cyclic Adsorption Process.

Developed by U.S Bureau of Mines.

Adsorbent : Sodium Aluminate (Na2O.Al2O3)

This process uses Sodium Aluminate

(Na2O.Al2O3) to remove SO2 in fluidized bed

at 315°C.Na2O.Al2O3 + SO2 + ½ O2 → Na2SO4 + Al2SO3

Alkalized Alumina Process

Alkalized Alumina Process

Sodium Sodium AluminateAluminate

The product of above reaction is then

contacted with a reducing gas such as H2 in a

regenerator at 680°C to produce H2S.

Na2SO4 + Al2O3 + 4H2 → Na2O.Al2O3 + H2S +

3H2O

Sodium Aluminate is recycled back and H2S is

sent to Claus Process for producing Sulphur.

Developed by Mitsuibishi Heavy Industries, Japan.

The powdered adsorbent is fed into the reactor wherein the Manganese Oxide reacts with SO2 to produce Manganese Sulphate (MnSO4) :

MnOx.yH2O + SO2 + ½ (2-x) O2 → MnSO4 + yH2O

Where x ranges from 1.5 to 1.8, and y ranges from 0.1 to 1.0.

Manganese Oxide Process

The spent adsorbent is regenerated by reacting with Manganese Sulphate with Air and Ammonia to produce Ammonium Sulphate:

MnSO4 + 2NH3 + 2H2O + (y-1) H2O + ½ (x-1) O2 →

(NH4)2SO4 + MnOx.yH2O

The activated manganese oxide is filtered from

the Ammonium Sulphate (NH4)2SO4 Solution

and the filtrate is passed through a crystallizer

where Ammonium Sulphate (NH4)2SO4 is

Recovered.

Manganese Oxide Process

MnOx.yHMnOx.yH22OO

MnSOMnSO4 4 with with impuritiesimpurities

Reinluft Process (activated charcoal)

Uses cheap semicoke of peat, carbonized under vacuum at 600°C as a adsorbent.

The flue gas enters the lower section of the adsorber at 150-200°C and after partial adsorption of the SO2 gas it is withdrawn and then passed through the cooler where it is cooled to about 110°C. After cooling it is returned to the upper section of the adsorber and leaves the top of adsorber at temperature 100°C.

The spent carbon is dropped into a regenerator where it is heated upto 380-450°C by an inert gas stream which removes the adsorbed acidic materials and produces SO2 of 10 – 15% concentration which can be used as a feed gas for contact acid plant.

Absorption of SO2 by to from stable sulphites or sulphates with subsequent regeneration.

Lime Stone Scrubbing Process (Calsox Process)

Magnesium Oxide Scrubbing Process (Chemico Process)

Welman Lord Process (Single Alkali Process)

Wet Process

Lime Stone Scrubbing (Calsox)

Uses slurry of Lime or Lime stone as scrubbing liquid.

Waste gases are first made free from any objectionable materials like ash in a ash collector and then introduced into Scrubber.

The flue gas is scrubbed within 5 to 15% slurry of Lime and Limes stone through which SO2 is absorbed forming Calcium Sulphate & Sulphite. (CaSO4 , CaSO3)

The solids are continuously separated from the slurry and are discharges into settling pond.

The remaining liquor of pH 6 to 8 is recycled back to the scrubbing tower after fresh lime or limestone has been added.


Removal Efficiency:Generally can remove 80-90% of SO2 . However if designed properly can remove upto 99% of SO2.

Removal Efficiency CaO + H2O → Ca (OH)2

Ca (OH)2 + CO2 → CaCO3 + H2OCaCO3 + CO2 + H2O → Ca (HCO3)2

Ca (HCO3)2 + SO2 + H2O → CaSO3.2H2O + 2CO2

CaSO3.2H2O + ½ O2 → CaSO4.2H2O2CaCO3 + 2SO2 + ½ O2 → CaSO3 + CaSO4+ 2CO2

2CaOH2 + 2SO2 + ½ O2 → CaSO3 + CaSO4+ 2H2O

Mg Oxide Process (Chemico)

Also known as Chemico Process. Uses Slurry of Magnesium Oxide to scrub the flue

gas in a Venturi Scrubber. The flue gas is scrubbed with a slurry of

magnesium oxide in venture scrubber which absorbs SO2 and yields Magnesium Sulphite and Sulphate according to the following reaction:

MgO + SO2 → MgSO3

MgSO3 + SO2 + H2O → Mg (HSO3)2Mg (HSO3)2 + MgO → 2MgSO3 + H2O

MgSO3 + ½ O2 → MgSO4


The mixed sulphite/sulphate slurry along with unreacted MgO is separated from the liquid phase in a Centrifuge and the mother liquor is recycled to the absorber.

The centrifuged wet cake is dried and regenerated in a calciner.

Carbon is added in the calcining step to reduce any MgSO4 to MgO and SO2:

MgSO4 + ½ C → MgO + SO2 + ½ CO2

MgO is returned to the process, while the concentrated SO2 can be reprocessed into H2SO4 or elemental sulphur.

Welman Lord (Single Alkali)

Also known as Single Alkali Process. Uses Slurry of aqueous solution of

sodium sulphite to scrub the flue gas in a Absorber.

In this process sulphite is converted into bisulphite as the SO2 from the flue gases is absorbed by saturated solution of sodium sulphite according to the following reaction:

Na2SO3 + SO2 + H2O → 2NaHSO3


The sodium bisulphite formed is led to a double effect evaporator cum crystallizer where it decomposed into sodium sulphite at a temperature 100°C. This results in ejection of SO2 and steam.

2NaHSO3 → 100°C → Na2SO3 + SO2 + H2O Fly ash is removed before the SO2 scrubbing to

keep the rate of its accumulation in the scrubbing liquid low. SO2 and water vapour released from the evaporators are passed into a condenser and the product goes to the dissolving tank for dissolution of Na2SO3 crystals and the rich SO2 gas is processed.


Sodium sulphate is produced in this reaction which is removed and substituted by an equivalent amount of NaOH.

Clear solutions of either sodium or ammonia are excellent absorbers of SO2.

The regeneration step can be carried out a low temperature in liquid system.

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