SUPERCRITICAL BOILER

Supercritical Boiler

 1.        Operating Pressure >  225 Kg/Cm2,  It Is Called A Supercritical Boiler.

2.           The Most Techno-Economical Parameters For Supercritical Plants Envisaged Are:

Main steam Pressure	@ SH outlet  is 250 Kg/Cm
540 Deg.C	Main Steam Temperature At Sh O/L.
566 Deg.C	Hrh Temperature At Rh O/L.

3.          Improvement In Cycle Efficiency for above Parameters Is Of The Order Of 3%.

4.       There Are Two Types Of Water Walls  Arrangements In The Case Of  Once Through Boiler:

A)             Spiral Wound WW.

B)              Vertical Ribbed WW.

5.               Most Of The Once Through Boilers In The World Are With Spiral Wound WW.  This Arrangement Has The Advantage Of Equal Heat Absorption By All The Steam Generating Tubes.

     The Disadvantage Of This Arrangement Is That The Tubes Are Not Self-Supporting.  Hence, Extra Supports Are Provided To WW For This Arrangement.

  Typical Materials Used For Super critical Boiler

Elements Of Boiler	Material
Economizer	Carbon Steel
Water Wall	Low Alloy Ferritic Steel
Pressure Vessels (Separator, Drain Tank)	Low Alloy Ferritic Steel
Sh Tubes (Heating Area)	Austenitic Steel (Tp347h / Tp321h)
Sh Tubes (Non-Heating Area)	Medium Alloy Steel (T91)
Main Steam Pipe	Medium Alloy Steel (P91)
HRH Pipe	Medium Alloy Steel (Scmv28)

START UP IN THERMAL PLANTS

COLD START

1)           After Capital overhaul.

2)      After Annual overhaul.

3)     After any major rectification work like H2 leakage / T-G Bearing High Vibrations / Stator winding fault / Rotor earth fault.

WARM START

1)              After LTSH, SH Re heater tube leak rectifications.

2)              After H2  leakage - Rectification, After Auxiliaries like PA Fan / CW pump motor rewinding.

3)              Reserve shutdown due to High frequency, CW Sump level (River level) problem.

4)              Electrical faults like Surge Capacitor failure / Rotor earth fault / Stator Earth fault / GT Buchholzs.

5)              Coal shortage.

HOT START

1)              Economizer tube failure.

2)              Water wall tube failure.

3)              Turbine impulse line leakage.

4)              MS-Valves heavy gland leak.

5)              Generator X-r bushing oil leaks.

6)              Gland cooler gasket failure (leakages in condensate flow path) , Turbine lube oil cooler leaks.

7)              Cable flash over of Unit bushes / UAT etc.

8)              Small maintenance works on feed water line, de-Aerator etc. / small fire hazard

9)              Start up after fictions tripping (Governing System trouble / I&C trouble.

10)          Start up after trip out due to Mal-operation human error / Instrument error or relay / Protection, Momentary drum level / draft / vacuum.

The Health Risks of Burning Coal for Energy

Nitrogen oxides (NOx):

Nitrogen oxide plays a major role in the formation of ground-level ozone (or smog) in summer and contributes to fine particulate matter (or soot). Both smog and soot are linked to a host of serious health effects. Nitrogen oxide also harms the environment, contributing to acidification of lakes and streams (acid rain) and the haze that often shrouds our national parks and scenic vistas. 

Mercury (HG): 

 Mercury can cause severe nervous system problems in humans and wildlife. Especially vulnerable are developing fetuses, babies and children. Eating fish is one of the primary ways people ingest mercury, which accumulates in the tissues of fish and other animals. Texas is home to five of the nation's top 10 mercury emitting power plants.

Sulfur dioxide (SO₂): 

 Sulfur dioxide contributes to the formation of microscopic particles (particulate pollution or soot) that can be inhaled deep into the lungs and aggravate respiratory conditions such as asthma and chronic bronchitis, increasing cough and mucous secretion.

Carbon dioxide (CO₂) and Global Warming:

 Carbon dioxide does not directly impair human health but is the most significant greenhouse gas that contributes to global warming. The dangers of global warming include disruption of global weather patterns and ecosystems, flooding, severe storms and droughts. A warming climate will also extend the range of infectious diseases.

DUST REDUCTION METHODS IN THERMAL PLANTS

 The major methods used in coal mines and thermal plants to control dust pollution are  

•      Ventilation
•      Water sprays
•      Dust collectors

 I). Ventilation:

                 The ventilation methods provide the best use of air in the vicinity of workers and in the vicinity of dust sources. There are different methods

1. Dilution Ventilation:

 In this method it is used to provide more air and dilute the dust. Most of the time the dust is reduced roughly in proportion to the increase in air flow, but not always.

2. Displacement Ventilation :

In this method it is used to use the airflow in a way that confines the dust source and keeps it away from workers by putting dust downward of the workers.

II).Water Sprays :

There are 3 methods 

1. Wetting :

          By wetting of the coal adequate dust pollution can be controlled. Due to wetting the dust particles stay attached with surface of coal material. 

2. High pressure sprays:

This method improves spray by raising the water pressure which may further causes to raise the efficiency per unit use of water. In this case there is a chance to get reduced 30 to 40% dust pollution?

3. Foam:

  For dust control foam may be better than water. It provides dust reductions upto 20% to 60% compared to water. Foam also can produce similar results at lower water use that is the amount of water needed to make the foam is less than the equivalent water spray. Only drawback is high cost. 

III).Dust Collectors :

                  The dust collectors play a vital role in dust reduction in mining and coal plants of thermal power plants. Dust collectors range from low-volume filtration to high volume wet collectors is used in various locations such as conveyor areas, tunnels, crusher areas and bunker areas. For dust collectors properly designed to trap respirable dust, the filtration efficiency is usually quite high in the range of 90-95%. 

 

       Methods generally implemented in thermal plants

 

Location	Method implemented
Coal transportation	Wagons covered with tarpaulins
Wagon tippling	Spraying water
Conveyor streams	Dust collectors and sprinkling water
Crusher houses	Dust collectors
Bunkers	Dust collectors
Coal yards	Sprinkling water

COMPARISON OF VARIOUS SYSTEMS:

DUST CONTROL METHOD	Effectiveness (Low is10-30%, Moderate is 30-50%, High is 50-75%)	Cost and draw backs
Dilution Ventilation	Moderate	High
Displacement ventilation	Moderate to high	Moderate
Wetting	Moderate	Low
Foam	Moderate	High
Water high pressure sprays	Moderate	Moderate
Dust collectors	Moderate to high	Moderate to high