Effluent Reuse in Power Plants

7/29/2019 Effluent Reuse in Power Plants

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Effluent Recycling, Reuse and Zero liquiddischarge systems


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Major consumers of water in power station

0

500

1000

1500

2000


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Continued..

Cooling tower make up : 1725 m3/h

Ash disposal : 650* m3/h

DM water make up : 60 m3/h

Potable & service water : 125 m3/h Clarifier sludge etc. : 55 m3/h

Coal dust suppression : 35 m3/h

Total : 2000 m3/h for a typical 500MW Power Plant


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Sources of effluents

Clarifier sludge

Filter back wash

CT blow down

Regeneration waste of DM plant & CPU Boiler blow down


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Factors influencing wastewater reuse

Quality, quantity and cost of raw water available

Quality and quantity of water needed for various

processes;

Possibility of recycling wastewater streams to otherprocesses, either directly or after suitable treatment;

Wastewater treatment technologies available; capital,

operational, maintenance and labour costs and space

requirements;

Environmental restrictions on the quantity and quality of

any wastewater that may be discharged


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Opportunities for Water Reuse

Ash handling system

- Wet slurry disposal system

- Recirculating ash pond water

- High Concentration Slurry Disposal(HCSD)- Using CT blowdown for makeup


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Ash handling system

As per MOE&Fs notification dated 3.11.2009, all new

coal based power stations are required to progressivelyachieve 100% utilization of fly ash by fourth year from date

of commissioning of the project


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Cooling water system

- Cycles of concentration

- Makeup water addition- Using Boiler blowdown water for makeup

Cycles of concentration Make up waterrequirement

3 2.5% of CW flow

5 2.1% of CW flow


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Clarifier system wastes

- Initially clarifier sludges were disposed off

- Recover water from sludge using treatment

- Recycle it back to clarifier inlet along with washwater from filters of DM plant


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Centralised Monitoring Basin

Storage vessel having influents from

Unused CT blowdown

Boiler Blowdown Treated plant drains and filter backwash


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Zero liquid discharge system


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Zero liquid discharge

By definition, reduced or zero liquid discharge (ZLD)

processes treat significant volumes of low quality

wastewater, such that the waste stream is greatly

reduced, or eliminated, and the bulk of the wastewater

becomes reusable

Minimise site water usage and maximise water

recycling


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Components of ZLD system

Processes that have been used for ZLD systems and

cooling tower blowdown treatment, individually or in

combination, include the following:

Evaporation basins;

Brine concentrators;

Crystallisers;

Membrane processes


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Brine concentrators

Capable of recovering greater than 95% of a wastewater

flow as high purity distillate (< 10ppm TDS)

The distillate is generally reused as either cooling towermake up or as feedwater for demineralisation plant

The concentrated brine slurry produced (> 150,000ppm

TDS) can be reduced to dry solids in a crystalliser orspray drier, or sent to an evaporation pond.


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Brine concentrator How does it work??


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Brine concentrator How does it work??

As the brine falls back down the tubes into the sump due

to gravity, a small portion is evaporated and drawn

through mist eliminators to a compressor.

The compressed vapour then flows over the outer tube

surfaces, where heat is transferred to the cooler brine

within the tubes.

This causes a small amount of internally circulating

brine to evaporate and condenses the external vapour

as distilled water.

The distillate is pumped back through a heat exchanger

to pre-heat the incoming wastewater.


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Brine concentrators - Disadvantages

High Capital cost: Use of titanium or stainless steel

alloys for heat exchanger surfaces to tolerate the

extremely corrosive conditions experienced when

treating saline wastewaters.

High operating cost: Large power consumption, typically

between 80 to 100kWh per 1000 USgallons of

wastewater treated

To minimise the size of brine concentrator required,

reverse osmosis is used frequently for wastewatervolume reduction beforehand


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Need for Crystallizers

For most wastewaters containing 1% to 5% TDS by

weight, it is relatively easy to remove75% to 95% of

water by falling film evaporator

As water is evaporated from a solution, the concentration

and ionic strength of soluble salts increase, as does the

boiling point of the solution

This increase in boiling point is causing difficulties in

evaporation and needs further processing


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Steam-driven crystallizer


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Steam-driven crystallizer

Slurry from the evaporator (waste brine) is pumped to

the crystallizer shell and tube heat exchanger (heater)

The tubes are flooded, the slurry is under pressure and

will not boil

After it is heated, the slurry enters the crystallizer vapor

body at an angle, where it swirls in a vortex

As water is evaporated from the brine, crystals form.

Most of the brine is recirculated back to the heater

A small stream from the recirculating brine is sent to a

centrifuge or filter to separate the remaining water from

the crystals.


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Steam-driven crystallizer - Disadvantages

The use of steam for heating simplifies the design, but

does result in significant steam and cooling water

requirement

Vapor recompression crystallisers eliminate steam and

cooling water requirements, but add electrical demand

and are very susceptible to operational upsets, such as

foaming.

Crystallisers require highly alloyed materials, which

again incurs a high capital cost.


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Crystallizer with Vapor compressor


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Spray dryers

Spray driers are alternatives to crystallisers for reducing

concentrated brine solutions to solids for disposal.

The spray drier consists of an atomising wheel spinning

at approximately 17,000rpm that sprays the brine slurry

into a hot, gas-filled chamber

Water instantly evaporates from the brine droplets and

the solids are drawn into bag filters.


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Spray dryers


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Reverse Osmosis Principle


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RO Membrane Cutaway view


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Reverse Osmosis - Advantages

Least costly method of wastewater volume reduction

It can be used to concentrate wastewaters containing

high levels of dissolved salts, silica and organic matter.

The purified permeate water is of suitable quality for

reuse in most plant areas.

The concentrated reject stream is either processed

further, for example in a brine concentrator and/or

crystalliser, or reused in low quality water applications

Clarifier, Microfiltration and Ultrafiltration are the

pretreatment process before RO


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High Efficiency Reverse Osmosis(HERO)

Need for HERO:

Membrane fouling due to silica

solubility

Increasing pH to take advantageof higher silica solubility

Advantages:

Greater rejection of weakly

ionised species

Higherpermeate recovery rates ( 90%)

Prevention of biological fouling.


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ZLD Cycle

Documents

Effluent Reuse in Power Plants