Oxides Of Nitrogen And Oxides Of Sulphite

Oxides Of Nitrogen

The term NO_x is used to represent the composite atmospheric condition of gaseous nitric oxide(NO) and nitrogendioxide(NO₂).These two oxides of nitrogen in this family are the primary pollutants in the urban areas,However,there are four more oxides of nitrogen are known to exist.These are nitrousoxide(N₂O),nitrogen trioxide(N₂O₃),nitrogen tetraoxide(N₂O₄) and nitrogen pentaoxide(N₂O₅).The NO is emitted into the atmosphere in much larger quantities than NO₂.

As we know air is composed of 80% nitrogen and 20% oxygen,these gases react at high temperature(above 1200°C) to produce oxides of nitrogen according to the following equation.

$$N_2+O_2 \rightleftharpoons 2NO$$

$$2NO+O_2 \rightleftharpoons 2NO_2$$

Sources of NO_x

NO enters the atmosphere from natural(lightning and biological processes) and other pollutant sources.Side reaction of internal combustion engine is main source of NO.

The most important sources of NO₂ is vehicular exhaust.Power plants and other energy conversion system as well as variety of chemical processes emit NO₂ into the atmosphere.

Effects of Nitrogen Oxides (NO_x) on human beings

The nitrogen oxides are among the most toxic substance found in the atmosphere.NO and NO₂ are potential health hazards,however NO₂ is about four times toxiv than NO.

Nitric oxide(NO) is biochemically relatively inert and not very toxic.Like CO,it has the tendency to bind to haemoglobin and therefore can reduce the oxygen transport tendency,however the reaction is reversible and nitrosyl complex of haemoglobin dissociates to produce haemoglobin and NO back.

$$Hb+4NO \rightleftharpoons [Hb(NO)_4]$$

Nitrogendioxide(NO₂) is a deadly poison.The health effect of NO₂ varies with the exposure time and its concentration.A level of 50 to 100 ppm of NO₂ brings about inflammation of long tissue for a period of nearly two months.Exposure to 150-200 ppm of NO₂ brings about "bronchiolitus Fibrosa obliterans" a condition fatal in one month exposure.Death generally occurs,if one exposed to 500 ppm or more of NO₂ for nearly one week.Lung diseases due to inflammation,may result in Edema(accumulation of fluid) which is fatal.

Cigarette smoke contains 350 to 500 ppm,nitrogen oxides which are removed completely by inhalation,presumably by absorption in lumgs.Therefore,smokers are at greater risk for NO₂ poisioning.

Toxicity of dinitrogenpentoxide(N₂O₅) has been in dispute.not much information is available in literataure,as far as its effect are concerned in the gas phase.However,it is believed it has about the same acute toxicity NO₂.

Effect of Nitrogen Oxide(NO_x) on plants(vegetation)

NO₂ is highly injurious to the plants.Most of this damage probably comes from the secondary products of NO_x,such as PAN(Peroxyacyl nitrate) formed in smog.NO₂ suppresses growth when plants are exposed to 0.3 to 0.5 ppm for nearly 20 days.Exposuure of plants to 100 ppm or more of NO₂ for several hours bring about leaf spotting and breakdown of plant tissue.

Exposure of plant to 10 ppm of NO causes a reversible decrease in the rate of photosynthesis.

Effect Of Nitrogen Oxides on Materials

The exposure of NO_x between 1-2 ppm level on textiles can fade out their colours_.It can also affect the fiber(especially cotton and nylon fibre) strength.High levels of NO_x can also corrode nickel-brass wire springs in relays used bt the telecommunication companies.

No direct effect of NO_x have been noticed on building materials.However,the secondary pollutants such as HNO₃ produced from nitrogen oxides can damage the building materials.

Control Of NO_x Pollution

The use of catalytic converters for the control of automotive emission provides for the removal of NO_x in the first stage.

Power plants emit about 50 to 1000 ppm of NO_x.Such emission can be reduced by 90% by two stage combustion process.

a)At first stage,fuel(coal/oil/gas) is fixed at a relatively high temperature with a substoichiometric amount of air(90-95% of stoichiometric requirement).This way NO formation is limited and therefore control its emission.

b)At second stage,fuel burnt out is completed at relatively low temperatue in excess air.The low temperature prevents the formation of NO and therefore control its emission.

A possible approach of NO_x removal from stack gas in the chemical sorption process using H₂SO₄ solution or alkaline scrubbing solution containing Ca(OH)₂ and Mg(OH)₂.Simultaneously,SO₂ is also removed.NO is converted into N₂O₃ which is easily absorbed.This means to remove NO from combustion product,NO₂ is added to produce N₂O₃.

$$NO_2+NO \rightarrow N_2O_3$$

The recycling of NO₂ is needed in the above method and the process is referred as Tyco modified sulphuric acid scrubbing process.Four steps are involved in this scrubbing process.

i)Flue gas and NO₂ are introduced into an oxidiser where H₂SO₄ is formed along with NO.

ii)NO and NO₂ react to form N₂O₃,which is scrubbed by H₂SO₄ in a Scrubber.Then the cleaned flue gas is released into the atmosphere.

$$NO_2+NO \rightarrow N_2O_3$$

$$N_2O_3+2H_2SO_4 \rightarrow 2NOHSO_4+H_2O$$

iii)In decomposer NOHSO₄ is decomposed to produce H₂SO₄ and NO₂ again.H₂SO₄ can be recycled to the scrubber.

2NOHSO_4+\(\frac{1}{2}\)O_2+H_2O \rightarrow 2H_2SO_4+2NO_2$$

iv)The NO₂ react with H₂O to produce HNO₃ in the HNO₃ reactor.

$$3NO_2+H_2O \rightarrow 2HNO_3+NO$$

and excess NO and NO₂ are recirculated through oxidiser in step i).

Oxides of Sulphur(SO_x)

Mainly two oxides of sulphur namely sulphur dioxide(SO₂) and sulphur trioxide(SO₃) are important forms of primary pollutants in the atmosphere.Both the gases are colourless.SO₂ has pungent odour and does not burn in the air.SO₂ is an extremely reactive compound.The oxides of sulphur are moderately soluble in water,In presence of water or moisture SO₂ and SO₂ converted into sulphourous and sulphuric acid respectively.

$$SO_2+H_2O \rightarrow H_2SO_4$$

$$SO_3+H_2O \rightarrow H_2SO_4$$

The mixture of SO and SO₂ is denoted by SO_x.

Origin And Sources of SO_x

The combustion of any sulphur containing material will produce sulphur oxides(SO_x).The combustion of sulphur is a two step process.

$$S+O_2 \rightleftharpoons SO_2$$

$$2SO_2+O_2 \rightleftharpoons 2SO_3$$

Natural process eg. volcanoes,provide 67% of the SO_x pollution which is evenly distributed all over the globe.Manmade sources contribute about 33% of SO_x pollution which is however,localized in some urban areas.Among manmade sources,fuel combustion(coal) in stationary sources accounts for 74%,industries 22% and transportation 2% of the total SO_x emission.In other words,coal-fired power stations are the major culprit in this respect.

Effects of SO_x on Human beings and plants

SO₂ and the product of its reactions are devastating damaging to human health.SO₂ is intensely irritating to the eyes and respiraory tract.The exposure to SO₂ produces breathelessness and mucus secretion.

SO₂ has been known for many years to be major causes of injury to higher plants.

Control of SO_x pollution

There are four possible methods for the removal and control of SO_x emission.

i)Removal of SO_x from fuel gas.

ii)Removal of sulphur from fuel burning.

iii)Use of low sulphur fuels.

iv)Substitution of other energy sources foe fuel combustion.

i)SOx from fuel gases can be conveniently eliminated by using chemical scrubbers.The flue stack gases are passed through a slurry of limestone,CaCO₃,which absorbs SO₂ quite efficiently.

2CaCO_3+2SO_2+O_2 \rightarrow 2CaSO_4+CO_2$$

An alternative process is based on a reaction between HSO₃^- ions(from SO₂) and citrate ions.The flue gas is cooled to 50ºC or lower and freed from particulates and traces of H₂SO₄.It is then led into an absorption tower and brought into the contact with a solution containing citrate ions.(H₂Cit).

$$SO_2+H_2O \rightleftharpoons HSO_3^-+H^+$$

$$HSO_3^-+H_2Cit \rightleftharpoons (HSO_2.H_2Cit)^2-$$

The solution is next fed into a closed vessel in which H₂S is bubbled.Sulphur precipitates out and is later melted and removed from the solution.

$$(HSO_3.H_2Cit)^-2+H^++H_2S \rightarrow 2S↓+H_2Cit^-+2H_2O$$

The regenerated citrate solution is recirculated for further SO₂ removal,and part of the precipitated sulphur gets converted to the H₂S and circulated to the above system.By this method,99% SO₂ is removed from the flue gas and the recovered S is 99.99% pure.

ii)The removal of S from fuels before burning needs aa variety of approaches to handle the various types of fuels and forms of sulphur.Organically bound S in coal can be removed with difficulty through processes like carbonization,liquefaction or gasification.Pyritic 'S' in coal can be eliminated by physical techniques such as a grinding followed by washing.

iii)The use of low-sulphur coal to minimize SO_x pollution is,however,a more costly proportion than using high sulphur coal.

iv)Alternative energy sources such as hydroelectric plants and nuclear power plants are still not economically viable.

References

Poudel, Bhoj Raj Poudel and Dr.Megh Raj. A Text Book Of Inorganic Chemistry. Kathmandu: National Book Centre, 2012.

Sons, John Wiley and. J.D Lee Concise Inorganic Chemistry. 2007.