Boiler Definition

A boiler is defined as "a closed vessel in which water or other liquid is heated, steam or vapor is generated, steam is superheated, or any combination thereof, under pressure or vacuum, for use external to itself, by the direct application of energy from the combustion of fuels, from electricity or nuclear energy."

Principles of Steam Generation

Kettle

The applied heat raises the water temperature. Eventually, for the given pressure, the boiling (saturation) temperature is reached ad bubbles begin to form. As heat continues to be applied, the temperature remains constant, and steam escapes from water surface. Evaporation continues till all of the water gets evaporated. At this point further addition of heat increases the temperature of steam.

The above is achievable by relatively small diameter tubes containing a continuous flow of water. Water inside the tube is heated to boiling point, buubles are formed, and wet steam is generated. Then this steam water mixture tube flows into a steam drum, where steam is separated from water. The remaining water is then mixed with feedwater (make up) and goes back to the tube.

Steam Boiler Overview

Boiler costs almost 20-30% of the total cost for a power plant. Boiler termed Steam Generator is one of the most important part of a thermal power plant. The steam generated rotates a turbines prime mover coupled to a generator which produces electricity.

Boiler with turbine

Introduction:

First the boiler burns fuel i.e. coal or oil to generate heat. This heats water up to boiling point in economiser. Then it generates steam in the evaporator. Then the steam is super-heated in the super-heater.

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Sensible Heat

When water is heated it’s temperature increases which can be sensed through a mercury glass thermometer. Hence the heat added to water in this condition is called sensible heat.

Boiling

After addition of sensible heat at certain point water starts boiling. At this condition steam bubbles are formed or vapour formation starts.

Latent Heat

As the water attends it’s boiling point further heat addition does not increase it’s temperature. Hence this heat addition is called Latent Heat which is used to change the state of water from liquid to gaseous stage( vapour / steam).

EL0001-M -1-5-1 - THERMODYNAMIC CYCLE.

Types of boiler

The boiler can be of the following types

If the pressure of air blanket above water is increased, more heat energy has to be added to the water to reach it’s boiling point, saturation temperature. Then comparatively less heat needs to be added for formation of dry steam. Thus as the sensible heat added increases the latent heat addition is reduced. Then a point is reached called critical point where no latent heat addition is needed and the water converts straight into steam. Based on the critical point of steam pressure The boilers are categorised as sub critical or super critical boiler.

EL0001-M -1-5-2 - SUBCRITICAL TECHNOLOGY.

a. Sub-critical or Drum Boileri. Single Drumii. Bi-drum Boiler

2.

3. Single drum boiler has the advantage that they can be designed for very high pressure, flow, and temperature. The down comers are welded to the drum and are pipes which are so selected in size and numbers two ensure the circulation in the furnace wall tubes. The single drum boilers are adopted for both non-reheat and reheat boilers. Only single drum boiler design can be adopted for high pressure natural circulation and forced circulation boilers. The single drum boilers are normally designed as top

supported only. 4. Bi-drum Boiler  are used both for power generation and process steam generators. For power

generation this type of boiler is used up to, say, 100 MW. These are essentially non-reheat units which are getting wiped out due to high heat rate of the plant. In process steam generating plants, this type of bi-drum boiler is preferred mainly because it can accommodate high load fluctuation and respond to load changes much faster than the single drum boiler.

EL0002-M -2-6-0 - THE FM.EL0002-M -2-6-1 - PFI BOILER.EL0002-M -2-6-5 - VU 40 Boiler.

a. Super-critical or once-through boilerInsert video-about sub-critical and super critical

5. EL0001- M - 1- 2.1 - TYPES OF BOILING.Based on types of furnace for steam generationFire tube boiler

A fire-tube boiler is a type of boiler in which hot gases from a fire pass through one or more tubes running through a sealed container of water. The heat energy from the gases passes through the sides of the tubes by thermal conduction, heating the water and ultimately creating steam.The fire-tube boiler developed as the third of the four major historical types of boilers: low-pressure tank or "haystack" boilers, flued boilers with one or two large flues, fire-tube boilers with many small tubes, and high-pressure water-tube boilers. Their advantage over flued boilers with a single large flue is that the many small tubes offer far greater heating surface area for the same overall boiler volume. The general construction is as a tank of water perforated by tubes that carry the hot flue gases from the fire. The tank is usually cylindrical for the most part – being the strongest practical shape for a pressurized container – and this cylindrical tank may be either horizontal or vertical.Insert animation:

EL0001-M -1-2.6 -WATER TUBE BOILER .

Natural Circulation Boiler with horizontal convection pass and vertical pendant heat transfer surfaces

For Coals with low volatile matter content, particularly anthracites, that are difficult to ignite and burn.

Tower Type employs all horizontal convection surfaces and Pulverized coal, natural gas, oil as fuel.

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a. Fluidized Bed Boiler

Based on fluidising

i. AFBC- Atmospheric Fluidise Bed Combustion, fluidising velocity is approximately 4 m/s and The atmospheric air, which acts as both the fluidisation air and combustion air, is delivered at a pressure and flows through the bed.

ii. CFBC- Circulating Fludised Bed Combustion, fluidising velocity is 9-10 m/s. CFBC technology utilizes the fluidised bed principle in which crushed (6-12 mm size) fuel and limestone are injected into the furnace or combustor. The particles are suspended in a stream of upwardly flowing air (60-70 per cent of the total air), which enters the bottom of the furnace through air distribution nozzles. The fluidizing velocity in circulating beds ranges from 3.7 to 9 m per second. The balance of the combustion air is admitted above the bottom.

iii. PFBC- Pressurized Fluidised Bed Combustion is a variation of fluid bed technology that is meant for large-scale coal burning applications. In PFBC, the bed vessel is operated at pressure up to 16 ata (16 kg per cm2). The off-gas from the fluidized bed combustor drives the gas turbine. The steam turbine is driven by steam raised in tubes immersed in the fluidized bed. The condensate from the steam turbine is preheated using waste heat from gas turbine exhaust and is then taken as feed water for stem generation. The PFBC system can be used for cogeneration or combined cycle power generation. By combining the gas and steam turbines in this way, electricity is generated more efficiently than in the conventional system. The overall conversion efficiency is higher by 5 to 8 per cent.

b. Multi-Fuel Firing Boilers

6. Based on the number of steam turbinesa. Reheat Boiler( set of high pressure and low pressure turbines)b. Non-reheat Boiler (Single Turbine with exhausts into condenser)

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7. Based on the super heated steam pressure and the corresponding density difference between steam and water

a. Natural Circulation Boilerb. Forced Circulation Boiler

Insert: EL0001- M - 1- 2.2 - NATURAL OR THERMAL CIRCULATION SYSTEM.8. Based on type of furnace

a. Water Tube

9.10. Radiant Boiler : Density difference between water in the down comers and the

steam/water mixture in the waterwall tubes provide natural circulation.11. EL0001- M - 1- 2.2 - NATURAL OR THERMAL CIRCULATION SYSTEM.12. Lower two paragraphs on positive circulation and forced circulation may be

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a. Fire Tube

13. EL0002-M -2-6-4 - simple combined cycle system14. Based on the heat energy in flue gas

a. Heat Recovery Steam Generator- HRSGEL0002-M -2-6-2 - HEAT RECOVERY STEAM GENERATOR.

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EL0002-M -2-6-2 - HEAT RECOVERY STEAM GENERATOR.

EL0002-M -2-6-3 - GENERAL WASTE HEAT BOILER.

Fuel: As-received, unprocessed municipal solid waste (MSW) and refuse-derived fuel (RDF) for new boilers; and RDF for retrofit applications; auxiliary gas and oil.

A chemical and heat recovery boiler uses pulp mill black liquor as fuel.

15. Based on the type of fuela. Coalb. Oilc. Gas

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EL0001-M -1-5-5 - BOILER LAYOUT.