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:
Radiation, which is the transfer of heat from a hot body to a cold body without a conveying medium
Convection, the transfer of heat by a conveying medium, such as air or water
III. Conduction, transfer of heat by actual physical contact, molecule to molecule
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.
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
The water supplied to the boiler that is converted into steam is called feed water. The two sources of feed water are:
Condensate or condensed steam returned from the processes and
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.
There are virtually infinite numbers of boiler designs but generally they fit into one of two categories:
1. Fire Tube Boilers:
Fire tubeor “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 (Figure 4.2). 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 of17.5 kg/cm2.
2 .Water Tube Boilers:
Water tubeor “water in tube” boilers are those in which the conditions are reversed with the water passing through the tubes and the hot gasses passing outside the tubes (Figure 4.3). 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.
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.
III. 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.
4. 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:
Chain-grate or traveling-grate 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 burnt carbon in the ash.
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.
5. 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 pre-heat air temperatures 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
6. FBC Boiler:
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.
The performance parameters of boiler, like efficiency and evaporation ratio reduces with time due to poor combustion, heat transfer surface fouling and poor operation and maintenance. Even for a new boiler, reasons such as deteriorating fuel quality, water quality etc. can result in poor boiler performance. Boiler efficiency tests help us to find out the deviation of boiler efficiency from the best efficiency and target problem area for corrective action.
Thermal efficiency of boiler is defined as the percentage of heat input that is effectively utilized to generate steam. There are two methods of assessing boiler efficiency.
a)The Direct Method: Where the energy gain of the working fluid (water and steam) is compared with the energy content of the boiler fuel.
b)The Indirect Method: Where the efficiency is the difference between the losses and the energy input.
This is also known as ‘input-output method’ due to the fact that it needs only the useful output (steam) and the heat input (i.e. fuel) for evaluating the efficiency. This efficiency can be evaluated using the formula
Parameters to be monitored for the calculation of boiler efficiency by direct method are :
Quantity of steam generated per hour (Q) in kg/hr.
Quantity of fuel used per hour (q) in kg/hr.
The working gauge pressure (in kg/cm2) and superheat temperature (°C), if any
The temperature of feed water (°C)
Type of fuel and gross calorific value of the fuel (GCV) in kCal/kg of fuel
Boiler efficiency = Q x (hg-hf) ×100 /(q×GCV)
hg – Enthalpy of saturated steam in kCal/kg of steam
hf – Enthalpy of feed water in kCal/kg of water
It should be noted that boiler may not generate 100% saturated dry steam, and there may be some amount of wetness in the steam.
Advantages of Direct Method:
Plant people can evaluate quickly the efficiency of boilers
Requires few parameters for computation
Needs few instruments for monitoring
Disadvantages of Direct Method:
Does not give clues to the operator as to why efficiency of system is lower
Does not calculate various losses accountable for various efficiency levels
There are reference standards for Boiler Testing at Site using indirect method namely British Standard, BS 845: 1987 and USA Standard is ASME PTC-4-1 Power Test Code Steam Generating Units’. Indirect method is also called as ‘’heat loss method’’. The efficiency can be arrived at, by subtracting the heat loss fractions from 100. The standards do not include blow down loss in the efficiency determination process. A brief procedure for calculating boiler efficiency by indirect method is given below.
The principle losses that occur in a boiler are:
Loss of heat due to dry flue gas
Loss of heat due to moisture in fuel and combustion air
Loss of heat due to combustion of hydrogen
Loss of heat due to radiation
Loss of heat due to unburnt
In the above, loss due to moisture in fuel and the loss due to combustion of hydrogen are dependent on the fuel, and cannot be controlled by design. The data required for calculation of boiler efficiency using indirect method are:
Ambient temperature in °C (Ta) & humidity of air in kg/kg of dry air
GCV of fuel in kCal/kg
Percentage combustible in ash (in case of solid fuels)
GCV of ash in kCal/kg (in case of solid fuels)
With the help of these parameters the boiler engineers find the losses using standard approaches as specified by ASME and other boiler OEMs. Finally losses can be subtracted from the heat added and hence efficiency can be found.