2.1 Steam Condenser | unit 2 steam condenser and environmental impacts of thermal power plant

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ENERGY

Unit 2

Steam Condenser and Environmental Impacts of Thermal Power Plant

2.1 Steam Condenser

Steam Condenser is a mechanical device which converts the low pressure exhaust steam from the turbine into water.

It condenses exhaust steam of the turbine into water.

It is done with the help of cooling water circulated into it from the cooling tower.

Objectives

It maintains low pressure at the outlet of the steam turbine so that maximum possible energy can be obtained.

2. It supplies pure feed water to the hot well and from hot well the water is again pumped to the boiler with the help of boiler feed pump.

2.2 Necessity of steam condenser, elements of steam condensing plant

1. Condenser: It is a closed vessel used to condense the steam. The low pressure steam gives off its heat to the coolant (here water from cooling tower) and gets converted into water during the process of condensation.

2. Condensate Extraction Pump: It is a pump which is installed in between the condenser and hot well. It transfers the condensate from the condenser to the hot well.

3. Hot Well: It is a sump that lies in between the condenser and boiler. It receives the condensate from the condenser by condensate pump. The feed water is transferred from the hot well to the boiler.

4. Boiler Feed Pump: It is a pump installed in between the hot well and boiler. It pumps the feed water from the hot well to the boiler. And this is done by increasing the pressure of condensate above boiler pressure.

5. Air Extraction Pump: It is a pump used to extracts or removes the air from the steam condenser.

6. Cooling Tower: It is a tower which contains the cold water and this water is made to circulate within the condenser for cooling of steam.

7. Cooling Water Pump: It is a pump lies in between the cooling tower and condenser. It circulates the cooling water through the condenser.

2.3 Classification

The steam condenser is classified as

Jet condensers or mixing type condenser

2. Surface condenser or non-mixing type condenser

Jet Condenser

Jet condenser is a condenser in which the condensate gets mixed with the cooling water. That’s why it is also called as mixing type condenser.

This type of condenser is used sometime because it lost some of the condensate and requires high power for the pump during the process of condensation.

In jet condenser, as the condensate is not free from the salt, so it cannot be used as feed water for the boiler. It can be used at the place where sufficient amount of good quality water is available.

Types of Jet Condenser

(i) Parallel Flow Jet Condenser

Parallel flow and Counter Flow Jet Condenser

In parallel flow jet condenser, the steam and water enters into the condenser at the top and leaves at the bottom.

The cooling water and steam enters at the top. As both steam and cooling water mix with each other, the steam gets condense. The condensate, cooling water and air moves downward and it is removed by two separate pumps known as air extraction pump and condensate extraction pump. The condensate pump transfers the condensate to the hot well and from there the extra water is made to flow in cooling water tank or pond through overflow pipe.

(ii) Counter Flow or Low Level Jet Condenser

In counter Flow or low level jet condensers, the steam enters at the bottom and the cooling water at the top. The steam flows upward and meets the cooling water coming downward.

In these types of steam Condensers, the air pump is located at the top. Air pump creates vacuum and this vacuum draws water from the cooling tower. The cooling water enter into the condenser and falls on the perforated conical plate. The perforated conical plates convert the cooling water into a large number of jets as shown in the figure. The falling jet of water caught in the trays and from there it escapes out in second series of jets and meets the exhaust steam entering at the bottom. As the steam mix with the water, it gets condense. The condensate and cooling water moves down through a vertical pipe to the condensate pump. And finally the pump delivers it to the hot well.

(iii) Barometric or High Level Jet Condenser

Barometric or High Level Jet Condenser

Barometric or high level jet condensers are provided at high level with a long vertical discharge tube or tailpipe. It does not have condensate extraction pump and the condensate and cooling water flows in the hot well because of the gravity. An injector pump is used to flow cooling water at the top of the condenser.

These types of jet condensers are used at a high level with a vertical discharge pipe. In this condenser, the steam enters at the bottom and flows in upward direction and meets with the down coming cooling water. Its working is similar as the low level jet condenser. The vacuum is created at the top of the condenser shell. With the help of vacuum and injector pump, the cooling water is moved to the top of the condenser. The condensate and cooling water comes down in the hot well through a long vertical discharge pipe. And finally the extra hot water flows to the cooling tank or cooling pond by an overflow pipe

(iv) Ejector Condenser

Ejector Condenser

In ejector condensers, it has a non-return valve through which exhaust steam enters, hollow truncated cones, and diverging cone.

In these condensers, the cooling water is injected at the top. The steam enters into the condenser through a non-return valve. The steam and water mixes with each other while passing through series of hollow truncated metal cones and steam changes into water. At the end of the metal cones a diverging cone is present. When the condensate passes through diverging cone, its kinetic energy is partly transformed into pressure energy.
The condensate and cooling water is then discharged to the hot well.

Surface Condensers

Surface condenser is a type of steam condenser in which the steam and cooling water do not mix with each other. And because of this, the whole condensate can be used as boiler feed water. It is also called as non-mixing types condenser.

The figure above shows the longitudinal section of a two pass surface condenser. It consists of a horizontal cylindrical vessel made of cast iron and packed with tubes. The cooling water flows through these tubes. The ends of the condensers are cut off by the perforated type plates. The tubes are fixed into these perforated type plates. It is fixed in such a manner that any leakage of water into the center of condensing space is prevented. The water tubes are passed horizontally through the main condensing space. The exhaust steam from the turbine or engine enters at the top and forced to move downward due to the suction of the air extraction pump. In this steam condenser, the cooling water enters into boiler through lower half of the tubes in one direction and returns in opposite direction through the upper half as shown in the figure above.

This type of condenser is used in ships as it can carry only a limited quantity of water for the boiler. It is also widely used for the land installation where there is a scarcity of good quality of water.

Types of Surface Condensers

The surface condenser on the basis of direction of flow of condensate, the arrangement of the tubing system and the position of the extraction pump are classified as

(i) Down Flow

Types of Surface Condenser ( down flow and Central Flow)

In Down flow surface condenser, the steam enters at the top of the condenser and flows downwards over the tubes due to the gravity and air extraction pumps. The condensate gets collected at the bottom and then pumped with the help of condensate extraction pump. The pipe of dry air extraction pump is provided near the bottom and it is covered by baffle plates so as to prevent the entry of the condensate into it.

The steam in down flow condenser flows perpendicular to the direction of flow of cooling water, so it is also called as cross-surface condenser.

(ii) Central Flow

In central flow condenser, the steam enters at the top of the condenser and flows in downward direction. In this the suction pipe of the air extraction pump is provided in the center of the tube nest as shown in the figure. Due to this placement of the suction pipe in the center of the tube nest, the exhaust steam flows radially inward over the tubes towards the suction pipe. The condensate is collected at the bottom of the condenser and pumped to the hot well.

(iii) Regenerative

In regenerative surface condensers, the condensate is heated by the use of regenerative method. In that the condensate is passed through the exhaust steam coming out from the turbine or engine. This raises its temperature and it is used as the feed water for the boiler.

(iv) Evaporative

Evaporative Surface Condenser

In evaporative surface condensers, the steam enters at the top of the condenser in a series of pipes over which a film of cold water is falling. At the same time, current of air is made to circulate over the film of water. As the air circulates over the water film, it evaporates some of the cooling water. As a result of this rapid evaporation, the steam circulating inside the series of pipes gets condensed. Remaining cooling water that left is collected at an increased temperature and reused. It is brought to the original temperature by adding required quantity of cold water.

Advantages of Steam Condenser

It increases the efficiency of the plant.

It reduces the back pressure of the steam and as a result of this, more work can be done.

It reduces the temperature of the exhaust steam and this allows to obtain more work.

It allows the reuse of condensate for the feed water and hence reduces the cost of power generation.

The temperature of the condensate is more than the feed water. This reduces the supply of heat per kg of steam.

Comparison of Jet and Surface Condenser

S.no	Jet Condenser	Surface Condenser
1.	Exhaust steam and cooling water mixed with each other.	Exhaust steam and cooling water are not mixed with each other.
2.	It is less suitable for high capacity plants.	It is more suitable for high capacity plants.
3.	The condensing plant using this type of steam condenser is simple and economical.	The condensing plant using surface condenser is costly and complicated.
4.	Condensate is wasted and cannot be reused.	The condensate is reused.
5.	Less quantity of circulating water is required.	Large quantity of circulating water is required.
6.	It has low maintenance cost.	It has high maintenance cost.
7.	In jet condenser, more power is required for the air pump.	In surface condenser, less power is required for the air pump.
8.	High power is required for water pumping.	Less power is required for water pumping.

2.4 Cooling water requirements

1. INLET TEMPERATURE

2. TERMINAL DIFFERENCE

It is the difference in temperature between the steam saturation temperature (ts) with respect to condenser pressure and outlet cooling temperature (t2).

3. TEMPERATURE RISE

It is the difference between inlet and outlet cooling water temperatures. It ranges between 100F to 250F.

2.5 Condenser efficiency

The condenser efficiency may be defined as ''the ratio of the difference between the outlet and inlet temperature of cooling water to the difference between the temperature corresponding to the vacuum in the condenser and the inlet temperature of the cooling water."

Then, it is given by,

2.6 Vacuum efficiency (Numerical Treatment)

It may be defined as the ratio of actual vacuum as recorded by the vacuum gauge to the ideal vacuum.

It is given by,

Numericals:

During a trial a condenser following readings were taken:

Barometer reading – 766 mm of mercury

Actual vaccum in condenser - 710 mm of mercury

Temp of hot well – 290C

Inlet temp of cooling water - 180C

Outlet temp of cooling water - 270C

Determine:

A) Vaccum corrected to a saturated barometer of 760 mm of mercury

B) Vaccum efficiency

C) Under cooling of condensate

D) Coefficient of performance

Solution:

Absolute vaccum = 766 – 710 = 56 mm of mercury

Corrected vaccum = 760-56 = 704 mm of mercury

Vaccum efficiency = (Actual vaccum)/ Ideal vaccum

= (766 – 56)/ (766- 30.43)

= 710/735.57

= 0.9652 = 96.52%

Under cooling of condensate = 39-29 =100C

Coefficient of performance = (t2 – t1)/(ts – t1)

= (27-18)/(29-18)

= 0.8181 = 81.81%

2.7 Cooling towers

It is a heat rejection device that rejects waste heat to the atmosphere by cooling the water stream.

They use the evaporation of water to remove process heat and cool the working fluid or, in the case of closed circuit, rely solely on air to cool the working fluid.

Application:

In oil refineries, petrochemical and other chemical plants

In thermal and nuclear power stations

HVAC systems for cooling buildings.

Function:

It cools the cooling water of the condenser, by the current of air flowing in the opposite direction.

A large amount of cooling water is required for condensation in large capacity power plants. If the water is freely available either from the river or lake, then the water can be directly pumped from the river to the condenser as shown in Fig.

If the water is not freely available, then a cooling tower has to be used for cooling the hot water of condenser. The cooled water can be used again as cooling water for the condenser.

Figure below shows the hyperbolic cooling tower. It is usually made of steel Reinforced cement concrete. The hot water from the condenser is supplied to the ring troughs which are placed at 8-10 m above the ground level. The nozzles are provided on the bottom side of troughs to break up water into sprays.

The air rises up from the pond in the opposite direction of water flow and absorbs heat from the falling water spray. The cooled water is collected into a pond built below the tower. This type of cooling tower is generally used since it is very efficient, however, it needs about 3-5 % of makeup water for compensating the evaporation losses.

Types:

(i) Natural Draught Towers:

In this type the circulation of air is obtained by virtue of pressure difference of the air inside and outside the tower. Here no fan is required.

(ii) Forced Draught Tower:

In this case, the circulation of air is obtained by means of fans provided at the bottom of the tower.

(iii) Induced Draught Tower:

In this case, the circulation of air is obtained by providing a fan at the top of the tower.

2.8 Air leakage and its effects on condenser performance

Air is used to condense the steam coming out of turbine.

If the air leakage happens in Condenser then it will results in pressure increase in Condenser since it operates at vacuum.

At lower pressure saturation temperature is less hence we can extract more work from steam in turbine.

Due to air leakage pressure will increase due to which condensation of steam starts at higher temperature and in worst case condensation happens on turbine blades which results in pitting and blade damage.

It also results in increased back pressure on turbine.

Higher pressure results in sub cooling of steam and hence decreases thermal efficiency.

It also results in turbine damage, lower efficiency, corrosion in heaters and frequent maintenance.

2.9 Air pumps (Numerical Treatment for Air Pump capacity)

Air pump is a machine which is used in the condenser to remove condensate air from the condenser. Basically, Air pump creates a vacuum in the condenser as nearly as possible, corresponding to exhaust steam temperature. It is used in the condenser to remove both air and condensate from the condenser.

There are two types of air pump available in the market.

Dry air pump

Wet Air pump

The pump which extracts both condensate and air is called wet air pump and a pump which extracts only moist air is known as dry air pump. Rotary type air pump is also present in the market but it is not so popular.

2.10 Environmental impact of thermal power plants

The thermal power plant has serious impacts on land, soil, air and water.

It also has various social impacts and are also said to emit large amount of mercury and generate large quantity of fly ash which destroys the surrounding environment.

Among thermal based power generation, coal based power plants are highest in: Air pollution, Waste generation, Water consumption, Emission of mercury ,Greenhouse emission.

2.11 Different pollutants from thermal power plants, their effects on human health and vegetation

Impact on Air, Water, Soil and People:

Causes respiratory ailments

Affects historic structures

Causes climate change

Affects water quality and thus reduces quantity available for human consumption

Affects fishing as hot water let into sea kills or causes migration of marine species

Limits crop cultivation due to increase in alkalinity of soil

Limits crop cultivation as land available for agriculture reduced

Affects plant growth

Affects livelihood for farmers and fishermen

Increases risk of accidents due to hazardous working conditions

Health Impacts

Chemical Pollutant	Health Impact
Sulphur Dioxide	Affects respiratory system and lung functions Causes asthma and chronic bronchitis Causes eye irritation Causes cardiac disease
Nitrous Oxides	Causes asthma Causes Chronic Obstructive Pulmonary Disease Stunts lung growth Causes cardiac disease
Particulate Matter(PM): Coarse Particulates(PM10), Fine Particulates(PM2.5)	Causes asthma Causes Chronic Obstructive Pulmonary Disease Stunts lung growth Causes lung cancer Causes cardiac disease Causes congestive heart failure
Ammonia	Causes respiratory problems Causes skin and eye burns
Hydrogen Chloride and Fluoride	Causes irritation to skin, eyes, nose, throat, breathing passages
Dioxins and Furan	Probable cause of stomach cancer Affects reproductive, endocrine and immune systems
Polycyclic Aromatic Hydrocarbons	Adversely affects the liver, kidney and testes May damage sperm cells and impair reproduction May attach to small particulate matter and be deposited in the lungs
Mercury	Damages brain, nervous system, kidneys, liver Causes neurological and birth defects
Lead	Damages nervous system of children Adversely affects learning, memory and behaviour of children Damages kidneys Causes cardiovascular disease Causes anaemia
Antimony, Arsenic, Beryllium, Cadmium, Nickel, Selenium, Manganese	Probable effects of carcinogens (lungs, bladder, kidney, skin cancers) Adversely affects nervous, cardiovascular, skin, respiratory and immune systems
Radium	Probable carcinogens (lung and bone cancers) Causes anaemia Causes brain swelling
Uranium	Probable carcinogens (lungs and lymphatic system) Causes kidney disease

2.12 Methods to control pollutants such as particulate matter

Air pollution control – Point source

For boiler stacks – ESP/Bag house

Coal crusher – Bag filter

Coal mill – Bag filter

2.13 Oxides of sulphur and nitrogen

2.14 Dust handling systems

Fugitive Dust Control

Covered storage yard for coal

Closed unloading of coal with adequate dust suction device

Closed conveyor belt for transportation of raw

2.15 ESP

Electrostatic precipitation is a commonly used method for removing fine particulates from airstreams.

In this, particles suspended in the airstream are given an electric charge as they enter the unit and are then removed by the influence of an electric field.

It comprises of baffles for distributing airflow, discharge and collection electrodes, a dust clean-out system, and collection hoppers.

A high voltage of direct current (DC), as much as 100,000 volts, is applied to the discharge electrodes to charge the particles, which then are attracted to oppositely charged collection electrodes, on which they become trapped.

It consists of a collection of electrodes which are large rectangular metal plates suspended vertically and parallel to each other inside a boxlike structure.

There are often hundreds of plates having a combined surface area of tens of thousands of square metres. Rows of discharge electrode wires hang between the collection plates. The wires are given a negative electric charge, whereas the plates are grounded and thus become positively charged.

Particles that stick to the collection plates are removed periodically when the plates are shaken, or “rapped.”

Rapping is a mechanical technique for separating the trapped particles from the plates, which typically become covered with a 6-mm (0.2-inch) layer of dust. Rappers are either of the impulse (single-blow) or vibrating type. The dislodged particles are collected in a hopper at the bottom of the unit and removed for disposal.

An electrostatic precipitator can remove particulates as small as 1 μm with an efficiency exceeding 99 percent. The effectiveness of electrostatic precipitators in removing fly ash from the combustion gases of fossil-fuel furnaces accounts for their high frequency of use at power stations.

2.16 Scrubbers

Devices called wet scrubbers trap suspended particles by direct contact with a spray of water or other liquid. In effect, a scrubber washes the particulates out of the dirty airstream as they collide with and are entrained by the countless tiny droplets in the spray.

Several configurations of wet scrubbers are in use. In a spray-tower scrubber, an upward-flowing airstream is washed by water sprayed downward from a series of nozzles. The water is recirculated after it is sufficiently cleaned to prevent clogging of the nozzles. Spray-tower scrubbers can remove 90 percent of particulates larger than about 8 μm.

In orifice scrubbers and wet-impingement scrubbers, the air-and-droplet mixture collides with a solid surface. Collision with a surface atomizes the droplets, reducing droplet size and thereby increasing total surface contact area. These devices have the advantage of lower water-recirculation rates, and they offer removal efficiencies of about 90 percent for particles larger than 2 μm.

2.17 Water pollution

Sources of water pollution:

Cooling Tower Blow Down

Boiler Blow Down, Demineralisation (DM)

Plant Effluent

Coal Handling Plant Dust Suppression

Ash handling (Leachate of heavy metal (ash pond) contaminate groundwater)

Effluent from oil handling and transformer areas

Power House and Turbine Area Effluent

Domestic waste water

2.18 Thermal pollution

The term thermal pollution has been used to indicate the detrimental effects of heated effluent discharge by various power plants. It denotes the impairment of quality and deterioration of aquatic and terrestrial environment by various industrial plants like thermal, atomic, nuclear, coal-fired plants, oil field generators, factories, and mills.

Sources:

1. Nuclear Power Plant

2. Coal-fired power Plant

3. Industrial Effluents

4. Domestic Sewage

5. Hydro-electric power

6. Thermal Power Plant

Effects:

The harmful effects of the thermal pollution are discussed below:

1. Reduction in dissolved Oxygen

The pollutant from various industrial plants are heated decreases the concentration of oxygen with an increase in the temperature of water.

2. Change in water properties

The decrease in density, viscosity and solubility of gases in water increases the setting speed of suspended particles which seriously affect the food supplies of aquatic organism.

3. Increase in toxicity

The concentrated pollutant causes the rise in the temperature of water which increases the toxicity of the poison present in water. The toxicity in water will increase the death rate in marine life.

4. Disruption of Biological activities

Temperature changes disrupt the entire marine ecosystem because changes in temperature causes change in physiology, metabolism and biological process like respiration rate, digestion, excretion and development of an aquatic organism.

2.19 Noise pollution from TPP and its control

The unwanted sound that can have deleterious effects on human health and environmental quality is known as Noise Pollution.

It is generated inside many industrial facilities and some other workplaces, but it also comes from highway, railway, and airplane traffic and from outdoor construction activities.

Frequency of a sound wave is expressed in cycles per second (cps), but hertz (Hz) is more commonly used (1 cps = 1 Hz). The human eardrum is a very sensitive organ with a large dynamic range, being able to detect sounds at frequencies as low as 20 Hz (a very low pitch) up to about 20,000 Hz (a very high pitch). The pitch of a human voice in normal conversation occurs at frequencies between 250 Hz and 2,000 Hz.

High Noisy Area Section of TPP

Turbine Floor

Turbine Basement

Boiler Feed Pump

Compressor Area

Firing Floor

Coal Feeder

Coal Mill

PA Fan FD Fan

ID Fan

AHP Compressor Area

AHP Pump

GS Pump

House CW

Pump House

Bunker House

Control:

Design of equipments, Acoustic enclosures / barrier shields ,Construction of sound barrier in the form structure , Personal protective equipment i.e. ear plug & ear muffs ,More than 33 % of total area under green cover

References:

1. E.I.Wakil, Power Plant Engineering‖, McGraw Hill Publications New Delhi

2. P.K.Nag, Power Plant Engineering‖, McGraw Hill Publications New Delhi.

3. R.Yadav , Steam and Gas Turbines‖ ,Central Publishing House, Allahabad.

4. G.D.Rai, Non-Conventional Energy Sources, Khanna Publishers,Delhi

5. S.P.Sukhatme, Solar Energy‖ Tata McGraw-Hill Publications, New Delhi

6. G R Nagpal Power Plant Engineering , Khanna Publication

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