The Thermal Power Industry in India has an important role in development of industry as well as development of rural India. But in the wake of its coming it also brings the dangers of pollution which unless controlled can have large-scale short-term and long-term effects. As time has proceeded along with improvement in efficiency of such plants, the development has also been on more sophisticated equipment for pollution control.
Air Pollution Prevention and Management in Thermal Power Station:
A Thermal Power Station involves large-scale combustion and the consequential flue gases are hazards. Thermal Power Stations use generally coal, fuel oil or gas for combustion. Presently produced boilers use heavy oil firing to start with. Newer boilers are being designed for direct coal firing. Use of oil is on the decline, due to obvious reasons.
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With the availability of more natural gas from indigenous sources attention has been on installation of gas-based power stations even though at present the population of gas-fired power stations is being increased only nominally. Still coal-fired ores are presently the mainstay.
The main objectionable constituents of flue gases from the boiler depend on quality of combustion and on the coal used. A typical coal analysis of Indian coal is typically made of carbon, ash, sulphur, volatile material, moisture.
(a) Dust Concentration:
The content of ash in coal is very high. This leads to very high dust burden even with high efficiency of ash collection. The limit for dust concentration at outlet of chimney has been brought down drastically by the Pollution Board. As an example at present in Andhra Pradesh it is fixed at 115 mg/NM3.
Dust concentration is reduced in the Thermal Power Station by using high efficiency electrostatic precipitators. Equipped with modern EPMS control system they are able to have an efficiency of as high as 99.9%.
The ESPs have to be designed and guaranteed for a value as prescribed by the concerned pollution board and also with an eye on future, as the allowable limits can be further brought down depending on the existing pollution levels in the area.
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When the ESP provided does not meet the required levels, additional fields have to be added to reduce the level. Spraying of sulphuric acid at high temperature has also been a method abroad to improve the efficiency of old ESPs.
However this introduction needs to be carefully studied with respect to coal characteristics as well as stack outlet temperatures and stack outlet flue gas analysis for sulphur.
EPMS (Electrostatic Precipitator Management System) introduced for a systematic and sequential operation of the fields has also given good results in decreasing the dust level at ESP outlet.
(b) Sulphur Dioxide:
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The content of SO2 in flue gases has been a very important factor abroad since the coal there has a sulphur content of 0.8 to 1.0%. But in India this factor is not yet vital considering that the sulphur content in coal is 0.4 – 0.5%.
However SO2 is a very critical ingredient considering the possibility of formation of H2SO4 vapours and subsequently getting condensed in atmosphere, now popularly known as acid rain.
Hence regulations are becoming very stringent and watch needs to be kept on this parameter. Desulphurisation plants are a solution to this problem but are highly capital intensive.
The flue gas outlet temperature at furnace also needs to be monitored as the corrosion can be very high with low ESP inlet temperatures.
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At present the sulphur dioxide emissions from power stations are not controlled but are rendered ‘harmless and inoffensive’ by the use of sufficiently tall stacks. Here the options for reducing sulphur dioxide emissions will be examined.
Sulphur dioxide emissions can be reduced either by removing the sulphur from the fuel before combustion (oil desulphurisation or coal cleaning) or by removing the sulphur dioxide from the flue gases (flue-gas desulphurisation – FGD).
Flue-Gas Desulphurisation Processes (FGD):
There are about one hundred different flue gas desulphurisation processes available in various degrees of development ranging from the small laboratory scale to full-scale commercial units.
Modern methods of FGD began with the construction of the various power plants. No further FGD plants have been built in Britain due to the proven success of tall stacks in dispersing the sulphur dioxide from power station effluents.
The early British work has been extended by the US, Japan and W. Germany such that there are now FGD systems installed on a total of about 105 MW of power plant in these countries.
However, due to differences between UK and other coals this experience may not necessarily be relevant to the UK. Thus, for example – the high chlorine content of UK coals means that for most processes a chloride removal step must be added.
The plants use aqueous scrubbers to remove the sulphur dioxide and a considerable amount of research was invested into dry methods of removing sulphur dioxide. These methods were of two types, the injection of an alkali into the combustion chamber and the use of solid adsorbents to take up the sulphur dioxide from the flue gas.
These dry methods have the big advantage that the flue gases are not cooled so that extra heat energy is not required to reheat the flue gases. This reheating, which consumes a large amount of energy, is necessary for all wet processes in order to prevent acid corrosion of ductwork and to provide adequate plume buoyancy. However, in general, dry methods were not commercially successful and wet methods were more fully developed.
Wet Processes:
All wet processes are capable of removing 90% of the sulphur dioxide but if the removal is limited to about 60%, by not treating about one third of the flue gases then considerable energy savings can be made by the elimination of the necessity of reheating the flue gases.
Once-Through Systems:
The first FGD units installed in the world at Battersea and Bankside were once-through processes which made use of the natural alkalinity of the river Thames. The only other process in this category suitable for use in the UK is that using sea-water as the scrubbing liquid. However, for a modern large power station the natural alkalinity of sea-water will not be sufficient and thus extra limestone will be required.
The dispersion of effluents from the process in estuaries and the sea and the effects of these effluents on marine life are problems which have not yet been fully elucidated. A considerable amount of work has been carried out by the CEGB on these and other aspects of sea-water washing. Much further work will be required before sea-water washing can be considered a fully viable process.
Throwaway Processes:
In these processes the absorbent liquor is re-circulated in a closed loop. The reaction products are separated off and fresh absorbent is added. The only water usage is to cover losses by evaporation and that entrained with the product. In most cases a pre-cooler is used to cool the gases to avoid concentration changes in the absorber due to evaporation.
Lime/Limestone Processes:
The vast majority of scrubbing systems installed, use lime or limestone as the alkali to neutralise the sulphur dioxide. The resulting sulphite/sulphate mixture is then disposed off in ponds or disused mines.
This mixture is very difficult to dewater and forms a thixotropic solid commonly known as ‘sludge’. Usually various fixation methods, including mixing with fly ash, cement, lime, soil etc. are used to stabilise the sludge.
However even with these treatments the resulting land will probably still be sterile and unsuitable for agricultural or amenity use. Because of environmental considerations these processes are considered to be unacceptable. In addition, with the advent of gypsum producing systems, there is little or no economic advantage in installing sludge producing systems.
Double-Alkali Processes:
Problems of scaling in the scrubber have led to the search for other methods. One such method is to absorb the sulphur dioxide in some soluble medium and then treat the absorber effluent with lime or limestone in a reactor outside the scrubber loop. However a calcium sulphite product is again produced and so these processes would not be generally suitable for use.
Gypsum Processes:
Since the disposal of sulphite/sulphate sludges is difficult there has been an incentive to produce a more amenable product. Gypsum is such a product which can be readily disposed off to landfill or if produced in a suitable form can be sold for the manufacture of plasterboards or as a cement setting retardant.
Oxidation is usually carried out at lower pH than that used for absorption. Intentional oxidation can also be carried out in the scrubber loop. Some of these processes produce gypsum of marketable quality and a survey has shown that there are adequate markets for gypsum for a strictly limited number of FGO plants only. Moreover, the gypsum product from any single station might have to be sent to several end usages.
Regenerative Processes:
In these processes the absorbent is chemically or thermally regenerated for reuse and a saleable product (liquefied SO2, sulphur or sulphuric acid) is produced. Ideally, there should be no disposal problems but all the processes produce some by-products which means that there is still a need for absorbent make-up and for by-product disposal.
Wellman-Lord Process:
The Wellman-Lord process is based on the absorption of sulphur dioxide in a concentrated solution of sodium sulphite and the regeneration of the resultant bisulphite (pyrosulphite) solution by steam stripping in an evaporator-crystalliser.
The process produces a pure concentrated sulphur dioxide gas stream which can be treated to produce sulphur or sulphuric acid. Sodium sulphate must be disposed off as a by-product. The Wellman-Lord process is very popular in Japan.
Magnesium Oxide Process:
In this process an aqueous slurry of magnesium hydroxide and magnesium sulphite is used to absorb sulphur dioxide. The resulting magnesium sulphite is then calcined to generate sulphur dioxide and to regenerate magnesium oxide.
The concentration of sulphur dioxide produced is suitable for conversion to either sulphuric acid or sulphur. Since the magnesium salts are separated from the absorber as a solid and solid MgO is regenerated it is possible to separate the absorption and regeneration sites. In certain circumstances, economic savings can be made by using one regeneration complex to serve several sites.
Dry Processes:
Alkali Injection into the Combustion Chamber:
It is well known that not all the sulphur present in the coal ends up as sulphur dioxide in the flue gases after combustion. A significant proportion (5-15% with coals) reacts with alkaline elements in the coal and the sulphur is then fixed and removed with the ash. The proportion of sulphur removed in this way depends on the amount and alkalinity of the ash present in the coal.
In the dry injection method alkali (usually limestone or slaked lime) is injected into the hot boiler gases where it reacts with the sulphur oxides and is then removed after reaction. The equipment for preparation of the alkali, injection into the gas and for removing it together with the products is required.
In pulverised fuel boilers because of the high flame temperature and low residence times only very low removal efficiencies are achieved (∼ 20%) even with a large excess of limestone. The process was thus never used on a large scale.
In both fluidised bed boilers and lignite-fired boilers the flame temperature is much lower and the residence time is much longer so that removal efficiencies of upto 90% can be achieved.
Interest has recently been revived in direct injection because of research into new types of burners for the reduction of nitrogen oxides (NOx) emissions. These burners give lower flame temperatures and, because of the staged introduction of air, part of the flame is reducing.
This leads to the possibility of increasing limestone utilisation but even so it is unlikely that the maximum removal efficiency will be greater than 50% with a limestone stoichiometry of 2. Some of the latest results suggest that a stoichiometry as high as 4 might be required.
This means that large amounts of limestone would be needed in a station and large quantities of product and calcined reagent would be required to be removed. This would create a difficult disposal problem.
Adsorption Processes:
Both the Shell copper oxide processes and the Bergbau-Forschung carbon adsorption processes can remove sulphur and nitrogen oxides. However neither of these processes has been fully developed to a large scale on coal-fired boilers.
Spray-Drying Processes:
During the past few years, the use of spray driers for the adsorption of sulphur dioxide has become very popular. Already these processes are being installed on 3500 MW of power plant in the US. In these processes, a spray drier replaces the absorber and the sulphur dioxide absorption takes place in the droplets created by the atomiser of the spray drier.
The absorbent used can be either lime or sodium carbonate or bicarbonate. A mixture of fully reacted and unreacted absorbent is produced (which is collected in an ESP or bag filter), some of which can be recycled to optimise the utilisation of the absorbent. The remainder is sent for disposal.
Several advantages are claimed for spray-dry absorption among which are the minimisation of reheat, no scaling or plugging, the production of a totally dry product, and low investment.
However, spray drying processes are most suitable for low to medium sulphur fuels. In addition large quantities of lime must be imported and large quantities of product and unused reagent are produced which are collected and intimately mixed with the fly-ash.
This mixture must then be disposed off in a satisfactory manner. On a 2% sulphur coal only about 50-70% removal will generally be achieved with a lime stoichiometric ratio of 2. At present spray drying FGD is not a proven process on medium to high sulphur coals but a considerable amount of work is being carried out to investigate the use of this process on higher sulphur coals.
An interesting concept in spray-drying technology is the use of magnesium oxide as absorbent. The resulting product can then be regenerated as in the magnesium oxide process. However the high chlorine levels of UK coals would present many problems in this process.
The Effect of Flue-Gas Desulphurisation on the Environment and Other Industries:
Two important aspects of FGD which have not been much examined are the effects on the environment caused by disposal of waste products from the processes and the effects that the vast import and export of materials from a large power station can have on other industries. These are problems which need very careful attention for each specific site.
In order to obtain system cost comparability between all the FGD systems it is necessary to assume a common commissioning date. In practice this is unlikely to be the outcome because all the FGD systems investigated would not be fitted to new power stations at the same time and the time-scales for any retrofit would be affected by different considerations from those affecting new plants.
Coal Cleaning:
A radical alternative to the direct processing of flue gases lies in the removal of sulphur from coal prior to burning. However, the primary aim of existing commercial coal preparation plants is to separate mineral matter from run-off of mine coal and to produce different grades of fuel with ash contents controlled according to market specifications.
Any separation of sulphur which occurs is incidental. Power station fuel with an average of 16-20% ash represents the lowest quality end of the production spectrum at the other end of which is, for example – coal destined for domestic use with less than 5% ash.
The beneficiation processes currently in use exploit the differences in physical properties, mainly those of density and ease of wetting, between the coal constituents and can therefore extract only that part of the sulphur which exists in mineral form.
Over 60% of all treated coal is typically processed in a jig which depends upon the stratification of various density fractions in a water-borne bed which is kept in motion by alternate upward and downward currents of water generated by pulses of compressed air.
The separation of the coal from the mineral matter is achieved principally during the upward current of water with clean coal transferred near to the surface and the dirt collecting at the bottom of the vessel.
Such a separation is, of course, not complete and part of the fuel ends up in the intermediate layer of water from which it can be extracted as a product with medium to high ash content, the so-called middlings.
Consequently, the heavy pyrites are redistributed among the final products. The total sulphur content of the clean coal (expressed in terms of sulphur content related to the heating value) would be substantially lower (about half) than that of the raw feed coal, with most of the pyrite appearing in the reject and middlings.
In order to make the process economic it is necessary to utilise the thermal value of the middlings while preventing the release of the associated sulphur into the atmosphere.
This can be achieved by further crushing and reprocessing before combustion or by directing the middlings into boilers fitted with gas washing plant. In effect, the coal cleaning in this case would be used to boost the effectiveness of the expensive gas washing units, by putting a greater proportion of sulphur through them than would be the case if flue gases from only untreated coal were being processed.
In most of coals about 0.8% of the total sulphur content is chemically bound to the organic coal matter, with only the excess of sulphur over this value being present in the form of pyritic particles which are amenable to extraction by physical cleaning. As the average sulphur content is about 1.6% coal cleaning could not, even theoretically, remove more than 50% of the sulphur.
Because of the inefficiency of any practical process industrial plants could more realistically achieve 20 to 30% removal. The higher limits of this range are associated only with the more highly sulphurous coals which have a correspondingly higher proportion of pyritic sulphur. In such cases where coal preparation would be combined with separate combustion of middlings the total sulphur retention could be raised to about 50%.
It must be emphasised that the ease of sulphur extraction, the achievable thermal yield and the resulting size composition of the product are all factors very specific for individual coals. The presently available experimental data cover only a small proportion of the production seams and a reliable technical and financial assessment of coal beneficiation potential would require much further investigation.
The applicability of pyrites removal is closely linked with the type of emission control regulations which might be enacted. If an overall reduction in current CEGB emissions in the 10 to 20% range were required, then it would probably be competitive with gas washing aimed at the same target.
However, the further development of improved coal preparation depends not only on its importance for emission control but also from its essential role in the production of substitute liquid fuels suitable for medium to large boilers.
The emerging technology of production of stable coal-water slurries with solid loadings typically in the 65-75% range could offer a fuel cheaper than oil while retaining the advantages of handling liquids.
The energy penalty associated with the increased water content is relatively small (only 2-3% reduction in the heating value of the original coal). The size of the potential market would be therefore to a great extent determined by the convenience of use of fuel slurries.
The design and the cost of the boiler with its auxiliary equipment is greatly affected by the level of the ash content (and its composition) which should ideally approach that encountered in heavy fuel oil (<0.5%).
Present coal preparation cannot generally result in clean coals with less than 3-5% ash and further substantial reduction would be achieved only on a finely ground product which would also ensure preliminary mechanical liberation of pyritic particles. Provision of very low ash coal slurries would therefore most probably also achieve the limits of physical desulphurisation of coal.
(c) Nitrous Oxides:
Nitrogen oxides are released into atmosphere when coal is burnt due to oxidation of nitrogenous compounds present in coal and sometimes by oxidation of atmospheric nitrogen during combustion process.
Here one way of controlling NOx would be through monitoring furnace temperature. Effect of NOx would also be similar to SOx. Combining with atmospheric moisture they tend to produce harmful acidic formations.
(d) Carbon Dioxide/Carbon Monoxide:
During combustion in boiler furnace formation of carbon dioxide/carbon monoxide takes place and are released to atmosphere. They tend to raise levels of CO2/CO in atmosphere. Massive green belts around power stations help in controlling effects of CO2 in the atmosphere.
Air Pollution Prevention Measures for Thermal Power Station:
The Pollution Board acts as the guide and watch dog in prevention of pollution in the country. They have set certain guidelines and limiting values of emissions which have to be adhered by the Power Stations and industries.
There are two monitoring factors for air pollution. Firstly measurement of pollutant factors at the stack outlet or each unit flue gas outlets are controlled. However apart from the concerned industry, the other units in the surroundings also contribute to atmospheric pollution. Hence the second controlling area would be ambient air monitoring in the surroundings and to keep the overall pollutant levels within limits.
The dust concentration level at stack outlet is at present limited to 115 mg/NM3 at stack outlet. This measurement is taken generally at Electrostatic Precipitator outlet. This level can be achieved mainly by efficiency in operation of electrostatic precipitators.
The measurement is done generally by manual samplers, two to four times every month depending on size of stations. These results need to be corrected with load and combustion conditions.
Efforts are also being made to establish other continuous measurement methods such as opacity monitors which reveal dust level by analysing optically the density of flue gases.
The ambient air dust concentration limits allowed depends on the location of power plant mainly considering urban and rural areas. Based on dispersion studies adequate monitoring stations where the pollutant levels are highest are identified.
The number of stations and frequency of testing are also fixed by Pollution Board regulations. Either permanent stations can be established in such locations or we can go for mobile stations.
However the fixed station has the benefit of recording the values simultaneously at all locations representing similar operating condition. There are many types of instruments available for use at these stations.
Measurements of SO2 and NOx are also done both at flue gas outlet sampling point and ambient air measurement point and should be within the limits prescribed by Pollution Board.
Water Pollution Prevention and Management in Thermal Power System:
In a Thermal Power Station water pollution goes on a more conventional system and norms are of general nature.
The sources of water effluent from a power station come from:
(i) Boiler blow down.
(ii) CW system blow down/CW discharge.
(iii) Water treatment plant discharge
(iv) Ash pond outlet
(v) Miscellaneous plant drains.
Boiler blow down will be of small quantity of the order of 25 to 30 Cu M per hour for a 500 MW unit and does not contain any components beyond allowed limits.
In a closed cycle CW system a certain percentage of water has to be regularly blown down in order to maintain the quality of water in closed cycle. The effluent is totally normal as per analysis and does not need any treatment.
This is often used in other processes in many plants. However if the power station adopts an open cycle system the entire water quality of the order of 25-30000 cubic metres for every 200 MW will be led into the water body at a temperature upto 5-7°C higher than ambient.
Depending on type of water body such as lake/river/sea etc., and the type of marine life available therein the impact needs to be studied and proper action taken in this regard.
In water treatment plant all effluents are collected in a neutralising pit, and after suitably treating to acceptable levels, same is led into drain. Thus it is not expected to contain any harmful ingredients if treated properly before discharge. Further the quantity is also very small to have any effect in the final effluent.
In ash pond normal system is to pump ash in a slurry form into the pond and by providing suitable settling and filter system the entire ash gets settled and water goes out. Here obviously the emphasis on analysis of outlet water will be on total suspended solids which bear testimony to the effectiveness of filtering system.
In addition to the same depending on the ash analysis the pH value is also likely to show alkalinity in nature. Hence a decision on treatment of effluents is necessary after air analysis of same.
Apart from all above there are many miscellaneous drains which individually though small finally add up to a fairly large quantity. The drains from coal handling area are likely to contain high level of suspended particulate matter for which if required we may have to provide filters.
The drains from fuel oil area should be ensured free from oil and grease. It is essential to provide oil traps for these drains before they join other drains.
The above explained points are various sources of plant drains. However ultimately the entire drain goes out of plant area after joining together in one or two outlets. Thus it is essential that weekly samples are taken regularly from these channels for testing and any abnormalities are immediately controlled from source which can be identified. Further regular observations are necessary for any visible abnormalities to take immediate corrective action.
In order to assess the impact of the total plant effluents to the surrounding water bodies sampling of surface water from the water body as well as ground water samples in the plant and ash pond surroundings are also necessary. This helps in identifying any adverse impacts of plant water effluents.
In general it can be said that water effluents from a thermal power station can be well controlled as not to cause any harmful effects on surroundings.
Ash Pond:
Fly ash is a major solid discharge from a thermal power station which is the source of a major worry for a thermal power station. The magnitude of the problem is very huge considering the high ash content of Indian coal, which is generally between 35% to 45%, and about 5000 MT of ash is produced by a 1000 MW station.
Generally the ash is converted to slurry form and pumped to a large pond. Here the ash is allowed to settle and water should filter out. The problems of water effluent are already covered under the water effluent.
While the ash settles in the pond it is important to design the system in such a way that a large water sheet is maintained on the surface of the pond. Without this, the dry areas of pond will create a dust nuisance to the atmosphere.
For maintenance of good water surface a continuous study of flow pattern of slurry in pond should be made and the slurry discharge points should be varied regularly such that ash build up in the pond is as uniform as possible.
The maintenance of water sheet is all the more important in the summer months, when heat evaporates the water content in wet areas and winds contribute to flying of ash particles.
A good method of preventing ash particles flying is to develop a thick green belt all around the ash dyke well in advance such that by the time the level of ash rises the protective barrier is formed. This assumes more importance with large ash ponds getting filled up without compartmentalisation.
After an ash pond has been filled up, it becomes a major source of nuisance creating dusty atmosphere over the whole area and the land is as well considered a waste land. However the recent experience of development of forest on ash ponds in A.P. and Maharashtra has given a hope in this direction. Trees such as casuarina, acacia and eucalyptus have successfully grown without any earth cover whatsoever.
The growth has been very healthy and by this means it is possible to convert these lands to dense forest lands. For a country lacking very much in forest lands this will come as a boon. By compartmentalising the ash pond filling, it is possible to progressively develop the forest lands, minimising at the same time the areas of semi-filled ponds.
Further it is possible to use the ash in various industrial products such as bricks, cement, road making etc. This has already been taken up with substasive support from Government. This will assist in the situation, though the amount of ash generated is too huge at present to be totally consumed. However this needs to be pursued at a larger scale.
Where feasible, filling of slurry in areas such as abandoned mines can be looked into.
Another source of regular problem is failure of slurry carrying pipe lines. If this is not noticed immediately this is likely to spoil useful land areas. Hence a constant vigil is necessary in this regard. There have also been cases of dyke failures, which needs maintenance.
Coal Handling:
Coal handling system consisting of mills, crushers, conveyors etc., is always a source of dust nuisance. Dust control in these areas can be done by spraying of water. This has to be controlled since wet coal is more difficult to handle and fire in the boiler. These buildings are also provided with dust extraction system to ensure a good working atmosphere.
Problems of dust also arise in coal yard where coal stacked creates dust problems due to winds. Hence a CHP plant and yard are located in such locations where wind blows away from the yard in opposite direction of main plant. Also water spray is used to control the dust. These also helps in controlling fires of instantaneous combustion in coal yard area.
How to Control Noise Pollution in Thermal Power Station:
Huge crushing equipment such as crushers and mill as well as large rotating equipment such as turbine, feed pumps and draft fans contribute to a great extent to noise pollution. Also flow of air and gas in boiler area at high range of velocities also contribute to noise pollution.
At the tendering stage itself the limit of noise levels as measured at various points near the equipment is identified as not more than 85 dba and the equipment is designed to meet these conditions.
During performance test of the equipment the noise levels are ensured to be within limits. Insulation also helps in limiting noise levels. Many cold air ducts are also insulated only from the view of limiting noise levels due to the flow of flue gases in ducts.
How to Control Heat Pollution in Thermal Power Station:
The major effect comes from cooling water discharge from a power station. In a station adopting open system of cooling, water at temperatures 5°C to 7°C higher than ambient temp, are led to an external source such as river, pond, sea etc.
In such cases study of ecological effects of this water discharge have to be studied before adopting the system. Sometimes cooling towers are adopted even in open cycle to limit the temperature of discharged water. However this problem is not encountered in a closed system.
The heat from equipment is controlled by effective insulation. Insulation helps in not only controlling heat generated but also in saving heat for the process. The insulation is designed and guaranteed for a surface temperature of 55°C-65°C.
This is also a safety necessity. In spite of such effective insulation the areas of working have to be properly ventilated to contain the temperatures in working areas within working limits.
The major heat input to atmosphere from a thermal power station comes mainly from the flue gases going out of chimney. Their minimum temperature is limited by the condensation temperature of sulphuric acid and nitric Acid formations from vapours.
Any temperature lower than this leads to corrosion of equipment on flue gas path and chimney walls. As a protection measure, the top one third of a chimney is lined with Acid resistant bricks. The heat led into atmosphere has a tendency to generally raise the temperature which can be controlled only by development of large green belts.
Green Belts:
As discussed above there are many factors which make it necessary on the part of power station authorities to develop green belt as a compensation to the nature.
To repeat they include:
(i) Protection from dust particles in the plant area.
(ii) Absorption/Protection against warmth generated by plant.
(iii) General make up for the lost green lands during the station construction.
Generally it is mandatory on the part of power station authorities to grow at least 4 times the main plant area as green belt area in and around the plant. Also under the ash pond management waste barren lands can be converted to green forest lands after dumping of ash has been completed.
Operation Control and Equipment Maintenance:
It is also necessary to highlight the importance of proper plant operation and maintenance. The quality of stack emission depends very much on the operating parameters of the plant. A proper balanced combustion is a must for less impure flue gases.
By improper combustion in boiler the unburnt carbon particles in the exhaust flue gases increase. Similarly oil support for combustion also gives a bad smoke with higher sulphur oxides content.
Similarly maintenance of electrostatic precipitators with all fields available for dust collection is an important factor. Maintenance of proper operating parameters in ESP as well as maintenance of collecting and rapping mechanisms is an important factor. Similarly maintenance of equipment in other areas also leads to limiting the harmful discharges from the plant.
Fuel handling area with its spillage is another potential polluting area. In this area maintenance and clearing oil traps regularly is very important to prevent oil leakage into plant effluents along with drain.
Retrofits:
With the emission limitations becoming more and more stringent day by day it becomes necessary to carry out modifications in old power stations which had been designed for generally higher emission values.
It is very common to introduce more efficient electrostatic precipitators in old plants. Apart from above, methods to increase collection efficiently of ESPs is also a practice. Improvement in instrumentation as a part of measurement and increase in operation efficiency are other steps. This aspect in old thermal power stations has to be reviewed thoroughly and new modifications have to be brought forth.