BOILERS
Introduction
A boiler is
an enclosed vessel that provides a means for combustion heat to be transferred
into water until it becomes heated
water or steam. The hot water or
steam under pressure is then usable for transferring the heat to a process.
Water is a useful and cheap medium for transferring heat to a process. When water is boiled into steam its volume increases
about 1,600 times, producing a force that is almost as
explosive as gunpowder. This causes
the boiler to be extremely dangerous
equipment that must be treated with utmost care.
The process of heating a liquid until it reaches its
gaseous state is called evaporation. Heat is
transferred from one body to another by means of
(1) Radiation,
which is the transfer of heat from a
hot body to a cold body without a conveying medium,
(2) Convection, the transfer of heat by a conveying medium, such as air or
water and
(3) Conduction, transfer of heat by actual physical contact, molecule to molecule.
Boiler Specification
The heating surface is any part of the boiler metal that
has hot gases of combustion on one side and water
on the other. Any part of the boiler metal that actually
contributes to making steam
is heating surface. The amount of heating surface
of a boiler is expressed in square
meters. The larger the heating surface a boiler has, the more efficient it becomes. The quantity of the
steam produced is indicated in tons of water evaporated to steam per hour.
Maximum continuous rating is the hourly evaporation that can be maintained for 24 hours. F & A means the amount of steam generated from water at 100 °C to saturated
steam at 100 °C.
|
Typical Boiler Specification |
|
|
Boiler Make & Year |
XYZ & 2003 MCR(Maximum Continuous Rating) : 10TPH (F & A 100°C) |
|
Rated Working Pressure |
10.54 kg/cm2(g) |
|
Type of Boiler |
3 Pass Fire tube |
|
Fuel Fired |
Fuel Oil |
Indian Boiler Regulation
The Indian Boilers Act was
enacted to consolidate and amend the law relating to steam boilers. Indian Boilers Regulation (IBR) was
created in exercise of the powers conferred by section 28 & 29 of the Indian Boilers Act.
IBR
Steam Boilers means any closed vessel exceeding 22.75 liters in
capacity and which is used
expressively for generating steam under pressure and includes any mounting or other fitting attached to such vessel, which is
wholly or partly under pressure when the steam is shut off.
IBR
Steam Pipe means any pipe through which steam passes from a
boiler to a prime mover or other user
or both, if pressure at which steam passes through such pipes exceeds 3.5 kg/cm2 above atmospheric pressure or such pipe
exceeds 254 mm in internal diameter and includes in either case any connected fitting of a steam pipe.
Boiler Systems
The boiler system comprises of: feed water system,
steam system and fuel system.
The feed water system provides water to the
boiler and regulates it automatically to meet the steam demand. Various valves provide access for maintenance and
repair.
The steam
system collects and controls
the steam produced in the boiler. Steam is directed through a piping system to
the point of use. Throughout the system, steam pressure is regulated using valves and checked with steam
pressure gauges.
The fuel
system includes all equipment used to provide fuel to generate the
necessary heat. The equipment
required in the fuel system depends on the type of fuel used in the system. A typical boiler room schematic
is shown in Figure 2.1.
The water supplied to the boiler that is converted
into steam is called feed water.
The two sources of feed water are:
(1) Condensate
or condensed steam returned from the processes
and
(2) Makeup water (treated raw water) which must come from outside
the boiler room and plant processes.
For higher boiler efficiencies, the feed water is preheated by economizer,
using the waste heat in the flue gas.
Boiler Types and Classifications
There are virtually infinite numbers of boiler
designs but generally they fit into one of two categories:
(1)
Fire
tube
or “fire in tube” boilers; contain long steel tubes through which the hot gasses
from a furnace pass and around which the water to be converted to steam
circulates. Fire tube boilers, typically have a lower initial cost, are more
fuel efficient and easier to operate, but they are limited generally to capacities
of 25 tons/hr and pressures of 17.5 kg/cm2.
(2) Water tube or “water in tube” boilers in which the conditions are reversed with the water passing through the tubes and the hot gasses passing outside the tubes these boilers can be of single- or multiple drum type. These boilers can be built to any steam capacities and pressures, and have higher efficiencies than fire tube boilers.
Packaged Boiler: The packaged boiler is so called because it comes as a complete package. Once delivered to site, it requires only the steam, water pipe work, fuel supply and electrical connections to be made for it to become operational. Package boilers are generally of shell type with fire tube design so as to achieve high heat transfer rates by both radiation and convection.
The features of package
boilers are:
✓
Small
combustion space and high heat release
rate resulting in faster evaporation.
✓
Large number
of small diameter tubes leading to good convective
heat transfer.
✓
Forced or induced draft systems resulting
in good combustion efficiency.
✓
Number of passes resulting
in better overall heat transfer.
✓
Higher thermal
efficiency levels compared
with other boilers.
These boilers are classified based on the number of
passes – the number of times the hot combustion gases pass through the boiler. The
combustion chamber is taken, as the first pass after which there may be one,
two or three sets of fire-tubes. The most common boiler of this class is a
three-pass unit with two sets of fire-tubes and with the exhaust gases exiting
through the rear of the boiler.
Stoker Fired Boiler:
Stokers
are classified according to the method of feeding fuel to the furnace and by
the type of grate. The main classifications are:
1. Chain-grate or traveling-grate stoker
2. Spreader stoker
Chain-Grate
or Traveling-Grate Stoker Boiler
Coal is fed onto one end of a moving steel chain
grate. As grate moves along the length of the
furnace, the coal burns before dropping off at the end as ash. Some
degree of skill is required, particularly
when setting up the grate, air dampers and baffles, to ensure clean combustion leaving minimum of un-burn carbon in the ash.
Chain Grate Stoker
The
coal-feed hopper runs along the entire coal-feed end of the furnace. A
coal grate is used to control the
rate at which coal is fed into the furnace, and to control the thickness of the
coal bed and speed of the grate. Coal
must be uniform in size, as large lumps will not burn out completely by the
time they reach the end of the grate. As the bed thickness decreases from coal feed end to rear end, different amounts of
air are required- more quantity at coal-feed end and less at rear end.
Spreader Stoker
Boiler
Spreader stokers utilize
a combination of suspension burning
and grate burning.
The coal is continually fed into the furnace above a burning bed of
coal. The coal fines are burned in
suspension; the larger particles fall to the grate, where they are burned in a
thin, fast- burning coal bed. This method of firing provides good
flexibility to meet load fluctuations, since
ignition is almost instantaneous when firing rate is increased. Hence, the spreader stoker is favored over other types of stokers in many industrial applications.
Spreader Stoker
Pulverized Fuel Boiler
Most coal-fired power station boilers use pulverized
coal, and many of the larger industrial water-tube
boilers also use this pulverized fuel. This technology is well developed, and
there are thousands of units around the world, accounting for well over 90% of coal-fired capacity.
The coal is ground (pulverized) to a fine powder, so
that less than 2% is +300 micro meter (µm) and 70-75% is below 75 microns, for a
bituminous coal. It should be noted that too fine a powder is wasteful of
grinding mill power. On the other hand, too coarse a powder does not burn completely
in the combustion chamber and results in higher un-burnt losses.
The pulverized coal is blown with part of the
combustion air into the boiler plant through a series of burner nozzles. Secondary
and tertiary air may also be added. Combustion takes place at temperatures from
1300-1700°C, depending largely on coal grade. Particle residence time in the
boiler is typically 2 to 5 seconds, and the particles must be small enough for complete
combustion to have taken place during this time.
This system has many advantages such as ability to
fire varying quality of coal; quick responses to changes in load, use of high
preheat air tempera ture etc.
One of the most popular systems for firing pulverized
coal is the tangential firing using four burners corner to corner to create a
fireball at the center of the furnace.
When an evenly distributed air or gas is passed upward
through a finely divided bed of solid particles such as sand supported on a
fine mesh, the particles are undisturbed at low velocity. As air velocity is gradually
increased, a stage is reached when the individual particles are suspended in
the air stream. Further, increase in velocity gives rise to bubble formation,
vigorous turbulence and rapid mixing and the bed is said to be fluidized.
If the sand in a fluidized state is heated to the ignition
temperature of the coal and the coal is injected continuously in to the bed,
the coal will burn rapidly, and the bed attains a uniform temperature due to effective
mixing. Proper air distribution is vital for maintaining uniform fluidization across
the bed. Ash is disposed by dry and wet ash disposal systems.
Fluidized bed combustion has significant advantages
over conventional firing systems and offers multiple benefits namely fuel
flexibility, reduced emission of noxious pollutants such as SOx and NOx,
compact boiler design and higher combustion efficiency.
Types of Boiler Bed
Traveling Grate Stoker Boiler |  | ||
 | Bubbling Fluidized Bed and Circulating Fluidized Bed Boiler | ||
Pulverized Fired Bed Boiler |  | ||
Comparisons between different types of Boiler are given in below table:
|
| Boiler Type | |||
| Characteristics | Stoker | Bubbling | Circulating | Pulverized |
1 | Type of bed | Packed | Fluidized bed | Fast bed | Pneumatic bed |
2 | Meal particle dia. (mm) | < 300 | 0.03 - 3 | 0.05 – 0.5 | 0.02 – 0.08 |
3 | Gas velocity through combustion zone (M/sec) | 1 - 3 | 0.5 - 3 | 3 - 12 | 15 - 30 |
4 | Typical U/Ut | 0.01 | 0.3 | 2 | 40 |
5 | Gas motion | Up | Up | Up | Up |
6 | Gas mixing | Near plug flow | Complex two phase | Dispersed plug flow | Near plug flow |
7 | Solid motion | Static | Up & down | Mostly up, some down | Up |
8 | Solid-Solid mixing | Negligible | Usually near perfect mixing | Near perfect mixing | Near plug flow |
9 | Overall voidage | 0.4 –0.5 | 0.5 – 0.85 | 0.85 – 0.99 | 0.98 – 0.998 |
10 | Temperature gradient | Large | Very small | Small | May be significant |
11 | Typical bed to surface heat transfer coefficient (W/m2K) | 50 - 150 | 200 - 500 | 100 - 250 | 50 - 100 |
12 | Attrition | Little | Some | Some | Considerable |
13 | Agglomeration | Considerable | Some | No problem | No problem |
14 | Height of bed or fuel burning zone (M) | 0.2 | 1 - 2 | 15 - 40 | 27 - 45 |
15 | Superficial velocity | 1.2 | 1.5 – 2.5 | 4 – 8 | 4 – 6 |
16 | Excess air % | 20 - 30 | 20 - 25 | 10 - 20 | 15 - 30 |
17 | Grate heat release rate (NW/m2) | 0.5 – 1.5 | 0.5 – 1.5 | 3 – 5 | 4 – 6 |
18 | Coal size (mm) | 6 - 36 | 0 - 6 | 0 - 6 | < 0.0001 |
19 | Turn down ratio | 4 : 1 | 3 :1 | 5 :1 |
|
20 | Combustion efficiency % | 80 - 85 | 90 - 96 | 95 - 98 | 99 |
21 | NOx emission (ppm) | 400 - 600 | 300 - 400 | 50 - 200 | 400 - 600 |
22 | SO2 capture in furnace | None | 80 -90 % | 80 -90 % | Small |
|
|
|
|
|
|
Comments
Post a Comment