In this article we will discuss about the methods and procedures used to treat and control waste water from coal based thermal plants. Learn about:- 1. Introduction to Thermal Plants 2. Waste-Water Management at Power Plant 3. Conclusion.
Introduction to Thermal Plants:
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In India, Thermal power constitutes about 2/3rd of total installed capacity, mostly in the form of coal based thermal power plants. A coal based thermal power plant utilises a huge quantity of natural water resources to generate steam, to cool condenser and to dispose of ash produced by the coal burning.
A typical 210 MW unit boiler which produces about 700 T of steam from demineralized (DM) water to run turbine with 1.5 per cent make-up, requires 30,000 T of cooling water and 500 T of ash transport water in an hour. In the water-steam cycle of a unit, different chemicals are added at different stages of cycle and blow-downs are performed to maintain chemical regime in view of preventing the costly components and equipment of the plants from corrosion, erosion, scale formation, etc.
Waste-water streams produced in water-stream cycle are: boiler blow down, circulatory water blow down, cooling tower blow down and condenser blow down, etc. A large quantity of water is also used to maintain moisture to avoid fugitive emissions at different stages of coal handling and to transport huge quantity of ash in wet disposal system.
These two systems generate coal handling effluents and ash pond overflow. In addition, different run-offs of the plants at oil handling systems, etc. meet together forming a main plant effluent.
This article describes characteristics of each effluent measured adopting standard sampling and analytical techniques for the key parameters such as pH, TSS, BOD, TDS, CI, SO4, O and G and heavy metals, etc. discussing the importance of right choice of sampling and analytical methods.
Keeping in view the typical characteristics of waste-water streams of a power plant, various reuse and recycle schemes are conceptualised. Visualised approaches are to manage the wastewater of a coal fired thermal power plant in order to achieve optimum water conservation and to meet the Minimum National Standards (MINAS).
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In coal fired thermal power stations, to maintain the chemistry of water-stream cycle, chemicals are fed at various stages and blow downs are carried out at other stages resulting in a number of waste streams.
In order to meet regulatory restrictions and to manage waste-water, meaningful physico-chemical measurements need to be carried out. This physico-chemical analysis involves: Collection and preservation of the samples; preparation and pre-treatment of samples; analytical evaluation and presentation of the data in meaningful form.
Waste-Water Management at Power Plant:
Waste-water management at a power plant is necessary for both in water-short regions and from water conservation point of view to minimise the discharges meeting environmental regulations.
Presently no specific water treatments are in use except the following:
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1. DM plant wastes are equalised in two tanks namely neutralisation pits where necessary pH corrections are made before letting into the drains.
2. Ash slurries are transported to ash ponds/dykes where after settling of solids, water is allowed to seep through ash to the ground to evaporate and any overflow is discharged to receiving water without treatments.
3. Hot water drainage from the once through cooling system are allowed to pass through a canal before discharge to surface water bodies, i.e. river or lake, etc.
4. In recirculating cooling system, cooling water is chlorinated. In addition it is treated with corrosion and scale inhibitors.
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It has been observed that most of the waste-water streams are produced intermittently on a batch basis except the ash transportation slurries and cooling water discharges. These intermittent wastewater streams can be collected in ‘equalisation reservoirs’ in batches as and where generated.
From these equalisation reservoirs, waste-water can be discharged in a continuous stream eliminating flow surges. This process may help in smoothening the pollutants loading at the discharge ruling out the possibility of upsets in the quality of waste stream due to sudden variation in quality and quantity of the influent intermittent waste-stream.
The waste-water management system may consist of either several individual specified treatment of each waste-water stream at or near its source or an integrated system treating all waste-water streams at a single location using common equipment for various treatments or combining all waste-water streams with reuse and recycling possibilities of most voluminous stream while low volume waste streams can be handled as guided by USEPA (Table 20.1).
However, at present authorities are insisting for recycling of ash transport water for further sluicing of ash, treatment of any part of the ash transport water discharged onto land surface water and reuse to meet the prevailing standards set by ‘Pollution Control Boards’.
Recycling and treatment of wastewater for reuse can be complementary approaches to waste-water management and pollution control as recycling reduces the volume of water released to the water course and amount of treatment is minimised.
Reuse and Recycle Possibilities:
There are various possible ways to reuse ash transport water within power plants. The possibilities of water reuse at a particular plant will depend on the presence of the systems in which overflow could be reused, such as limestone or lime scrubbers for removal of SO2 emissions or cooling towers for reducing waste heat discharges and the need to reuse the transport water for fly ash, bottom ash or both to meet the stipulated limits and to conserve water. In additions to possible reuse of sluice water, there are also the possibilities of recycling the used water in a closed loop ash sluicing system.
Assuming that major sources of waste (ash ponds receiving bottom ash or fly-ash or both) can be combined with anyone of the reuse or recycling possibilities, a number of combinations can be drawn.
Three schemes which can be of help in Indian context are discussed below:
1. Complete Reuse of Bottom Ash Sluice Water:
All small waste streams, such as chemical cleaning waste, floor drains and treated sanitary waste would be discharged into the closed loop bottom ash sluicing system. Also cooling tower blow down could be used, without treatment as the main source of water for replenishing evaporation losses and other losses of water from the ash sluicing system.
In the closed loop system, scaling problems could occur after a few cycles because of the super-saturation of carbonate and non-carbonate hardness. Treatment of side stream by some processes such as lime soda softening, ion-exchange or reverse osmosis would be required to control scaling.
Lime soda process appeal to be most economical for removing suspended solid, hardness and silica. The effluent from the side-stream treatment process would be reused to dilute the recycled water stream to help avoid exceeding the solubility limits within the ash sluicing system.
The sludge from the first stage of the lime soda treatment, the excess lime softening process is primarily calcium carbonate, magnesium hydroxide, calcium sulphate and other solids. The sludge from the second stage, the soda ash process is calcium carbonate. Both of these sludges would be dewatered for recovery or ultimate disposal. These sludges would contain trace metals that could affect decisions on their ultimate disposal.
The proportion of side-stream to total flow from the ash pond that would need treatment to avoid exceeding the solubility limits can be determined with the equation:
Qs/Qt= {[(l+ ki)(CiT-CiL)]/[I+ ki) (CiT- OS)]}
Where,
Qs = side-stream flow rate through softening.
Qt = bottom ash pond overflow rate.
ki = concentration increase factor of species in the pipe after each ash sluicing.
CiT = concentration of species in the ash pond effluent.
CiS = concentration of species i in the sidestream.
CiL = concentration of species i in the sluice water just below the solubility limit.
The control limits of, Ci are given in Table 20.2.
2. Complete Reuse of Ash Sluice Water:
It would involve closed loop recycling of the ash transport water for both bottom and fly ash. It would be applicable where the effluent limitations guidelines could not be achieved with once through settling, where there is a shortage of water or where trace metal concentration are at undesirable levels.
The scheme is similar to previous except that fly ash sluice water is also recycled.
Effluent from the side-stream treatment process possibly could be used for boiler make up water and in house service purposes. This effluent could also be used as make up for the cooling tower and blow-down from the cooling tower could be used as make up water for ash transport.
Recycle of Combined Ash Sluicing Water with Treatment of Blow-down for Discharge:
In this blow-down would be treated to meet applicable effluent guidelines before discharge to a natural water way. This scheme would be useful when there is a shortage of water, where the once through ash sluicing system does not meet effluent guidelines for suspended solids and possibly pH, particularly for acidic effluents.
Treatment of only the blow-down would reduce scaling in the treatment facility. Concentrations of trace metal would increase in the blow-down in proportion to the number of times the sluicing water is recycled, however, the total mass load of metals discharged to the receiving stream should not increase and therefore the concentration of these metals in the receiving stream should remain essentially unchanged (assuming the blow-down is discharged to the stream from which makeup water is taken).
In this partial closed loop ash sluicing system, scaling problems can be avoided by determining the relationship among the blow-down rate, the make-up water rate and ash sluicing rate in the equation:
{(QT- QB)/QR} {[CiL- (l+ ki)CiR]}/{[(l+ ki)CiT – CiL]}
Where,
QB = ash pond blow down rate.
QR = make up water rate in the sluicing loop.
CiR = concentration of species in the makeup water and other variables.
These applications for reuse and recycling of ash pond overflow require detailed evaluation. Bench scale studies, demonstration projects and economic studies may provide a feasible answer.
Once the scheme is reasonably worked out, it is necessary to finalise the water balance and determine the qualities of effluents discharged to satisfy regulatory limits. It should be noted that during plant operation, flow quantities and qualities of all the streams keep on changing depending on the plant load, made of operation, climatic conditions, fuel characteristics, etc.
There being hundreds of variables in the process, it is difficult to define the right combinations to serve as design parameters. The best course of action would be the simulation of the operation of the entire plant water system along with the related major equipment hour by hour for at least for a period of one year.
Mathematical models can be developed for various subsystems and processes and integrated into a comprehensive mode. The simulation can be performed by using representative plant load curves, meteorological data and fuel characteristics as a function of time. The results of simulation will give simulated range of values of quantities and qualities of all the streams which can be used for design and checking compliance with regulatory limits.
The most common method of final waste-water treatment may be evaporation ponds and mechanical evaporation. In India, high solar radiation and net evaporation rates may favour application of evaporation ponds in most of the areas. In fact evaporation ponds may be used wherever practical for final disposal of waste streams is necessary. The evaporation ponds may need to be lined by natural clay or synthetic material to provide an impervious barrier for the concentrated salts.
Conclusion:
A systematic measurement of all the waste-water streams in operating units of various sizes is required to generate realistic data on the quality and quantity of waste-stream. This may also include experience record of various methods adopted.
Characterisation of various waste streams should be carried out with the aims:
1. To find out various possible ways to reuse and recycle the waste streams within coal fired thermal power plant and to evaluate all the possible ways depending on-
(a) Characteristics of ash pond overflow and other waste-streams such as cooling tower blow downs, coal pile drainage, clariflocculator sludge, etc.
(b) Bench scale studies pertaining to-
(i) Identification of concentrations of heavy trace metals resulting from present system and repeated contact cycles of re-circulated sluice water with ash, and
(ii) Determination of solubility limits of alkaline compounds.
2. To study the applicability of the various schemes in view of-
(a) Water quality problem.
(b) Water flow and mass balance between the waste-water-streams and other power plant operations which require water.
(c) The number of allowable cycle of sluicing water without causing problems with plant operation.
(d) The economics of waste-water reuse and recycling by carrying out further pilot plant studies.