24-09-2014, 02:11 PM
ABSTRACT /GIST OF THE PROJECT DONE In this project efficiency of boiler and cooling tower which is assigned by company to me. Initially I used to measure the boiler efficiency by using some standards or by some methods likewise direct or in direct methods in direct method there are two methods of finding are heat input and heat output . after that I m going to calculate the weakness of boiler means loses of boiler. Loses of boilers I find are Dry Flue Gas Loss, Loss due to Evaporation of H2O formed Loss ,due to Moisture present in Air Loss due to Incomplete Combustion ,Radiation & Convection Loss, Loss due to Unburnt Fly Ash , Loss due to Unburnt Bottom Ash . By taking these precautions analysis I done some calculations about loses and minimize the loses by some methods mathematically and in last by some analysis I reached at that point what which I controlled the situation and loses as well as and concluded As seen from the Output sheet above, by far the greatest heat loss is that carried away by the flue gases, a reduction of the flue gas temperature of 20°
abstract /Gist of the project done
In this project efficiency of boiler and cooling tower which is assigned by company to me. Initially I used to measure the boiler efficiency by using some standards or by some methods likewise direct or in direct methods in direct method there are two methods of finding are heat input and heat output . after that I m going to calculate the weakness of boiler means loses of boiler. Loses of boilers I find are Dry Flue Gas Loss, Loss due to Evaporation of H2O formed Loss ,due to Moisture present in Air Loss due to Incomplete Combustion ,Radiation & Convection Loss, Loss due to Unburnt Fly Ash , Loss due to Unburnt Bottom Ash . By taking these precautions analysis I done some calculations about loses and minimize the loses by some methods mathematically and in last by some analysis I reached at that point what which I controlled the situation and loses as well as and concluded As seen from the Output sheet above, by far the greatest heat loss is that carried away by the flue gases, a reduction of the flue gas temperature of 20°C is equivalent to an increase in thermal efficiency of approximately 1%, example an increase of efficiency from 85% to 86%. When a boiler is fitted with economisers or air heaters which are designed to give the selected flue gas temperature, the boiler working pressure has no influence upon the thermal efficiency.
In the event of the required boiler efficiency being specified in an enquiry to the boiler maker, the design engineer has to determine the flue gas temperature required to give this efficiency before the amount of heating surface in the heat recovery equipment can be determined.
As the output from a boiler reduces below the design of 100% the values of the losses vary and will result in a change of boiler efficiency. The effects upon the losses of reducing boiler output are as follows:
a) Unless action is taken to prevent it for corrosion reasons, the flue gas temperature will fall, giving a reduction of the dry flue gas and moisture losses.
b) The excess air in the combustion chamber, and hence oxygen content in the flue gases, will increase, thus increasing the dry gas and moisture in air losses.
c) The radiation loss will increase as a percentage of the heat input.
d) The unburnt combustible loss may change depending upon the fuel and firing system. The cumulative effect will vary with boiler size, fuel and final gas temperature. The trend will be for the efficiency to fall with reducing load, with small units due to the relatively high radiation loss, whereas with large boilers the efficiency may peak at a load below 100% maximum continuous rating (MCR).
INTRODUCTION
IMPROVING THE EFFICIENCY OF BOILER
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.
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.
A boiler is an important part of any thermal power plant. The boiler used in the Nv industry Power Plant is the water tube boiler. The Nv power Plant works on the modified form of the Rankine Cycle. Boiling and subsequent steam generation is the basic function of the boiler. The proper functioning of the boiler is possible with the help of a furnace, boiler drum, water walls, super heaters, reheater, economiser, down comers, draft system & necessary control valves. Steam generator is radiant reheat, wet bottom, natural circulation, single drum, direct corner fired, tilting burners, balanced draft, top supported type boiler.
Rankine Cycle
Process 1-2: Boiler Heat added at constant pressure.
Process 2-3: Turbine Isentropic expansion
Process 3-4: Condenser Heat rejected at constant pressure.
Process 4-1: Pump Isentropic compression
The role of efficiency monitoring lies in maximizing generation from the thermal power plants. It enhances energy efficiency of the power plant. In order to keep maximum output from a given input, the units must run at the maximum possible efficiency. Power plant performance at various steps helps in improving the power generation capacity.
It is usual for the Boiler manufacturer to prove to the purchaser that the Boiler, after commissioning, can achieve its rated output and stated efficiency; this is an ‘acceptance’ test and is carried out very formally as legal matters may be involved if the Boiler does not meet, within stated limits, its guaranteed output and efficiency. Once the Boiler has been accepted, its efficiency (and output) may be influenced by various factors such as maladjustment of the controls, fouling, or a change from specified operating conditions. Both maximum output capability and efficiency should therefore continue to be measured during its working life, or at least those factors which affect efficiency, mainly exit gas composition and temperature, should be. It is the purpose of this project to explain how efficiency can be measured.
BOILER EFFICIENCY is defined as the heat added to work in fluid expressed as a percentage of heat in the fuel being burnt. The thermal efficiency of a boiler is the ratio of usual energy output to the energy input. By far the greatest component of the latter is the energy in the fuel; that supplied as power to the auxiliaries being negligible in comparison. Efficiency is expressed as the percentage of the energy input which appears as useful output, and is always less than 100%; 80-90%, based on the gross calorific value (GCV), is typical of modern plant. Boiler efficiency depends solely on the boilers ability to burn the fuel and transfer the resulting heat to water and steam.
The difference between energy input and output is the sum of the various energy losses from the boiler. These are:
1) Dry flue gas loss.
2) Evaporation of water formed due to hydrogen in fuel.
3) Moisture present in fuel.
4) Moisture present in air.
5) Incomplete combustion.
6) Radiation and Convection losses.
7) Loss due to unburnt fly ash.
8) Loss due to unburnt bottom ash.
The efficiency of a boiler can therefore be expressed alternatively as 100% minus the sum of the losses expressed as a percentage of the input energy.
THE MEASUREMENT OF BOILER EFFICIENCY : STANDARDS
The two definitions of efficiency given above lead to two methods of measuring it:
1) By measuring input and output (this is called the “Direct Method”)
2) By measuring individual losses, totaling them and deducting the sum from 100%. (this is called the “Indirect Method”, but the terms ‘losses method’ and ‘efficiency by difference’ are also used)
Ideally, both methods should be used, one as a check upon the other, and a complete heat balance obtained. There will be a difference between the results obtained from each, this difference being due to human and instrumental errors. Testing by both methods is rarely done, however, the costs of doing so being prohibitive compared with the benefits obtained. Having obtained two values for efficiency, the question would arise which one should be accepted in case of dispute.
It is now generally accepted that the indirect method is the simpler to carry out and yields the more accurate results. It also identifies the loss areas which need attention should a shortfall in efficiency below the expected value occur. The reason for greater accuracy of the indirect method is really quite simple. Whereas the direct method needs to measure the output, which is around 80% of the input, the indirect method only measures 20% of the input. A 1% error in the direct method therefore gives an error of 0.8 efficiency points, whereas a similar error in the indirect method only gives an error of 0.2 efficiency points. The full analysis of the probability of errors occurring is much more complex than this, but the basic principle is similar.
Where thermal testing is conducted for contractual purposes, it is essential that the agreed standard be implemented rigorously and properly witnessed; where routine tests are involved the requirements are less stringent but in his own interests the boiler user is advised to ensure that accurate instruments in good working conditions are used in accordance with the method described in the Standards.
Heat input:-
Both heat input and heat output must be measured. The measurement of heat input requires knowledge of the calorific value of the fuel and its flow rate in terms of mass or volume, according to the nature of the fuel. With natural gas the process is simple: a gas meter of the type approved for the sale of gas is used and the measured volume is corrected for temperature and pressure. A sample of gas can be obtained for calorific value determination, but it is usually acceptable to use the calorific value declared by the gas suppliers. It is strongly advised that approved gas meters be permanently installed in every boiler house using this fuel.
Oil can be dealt with in the same way, but it is rather more difficult. Heavy fuel oil is very viscous, and this property varies sharply with temperature. The meter, which is usually installed on the combustion appliance, should be regarded as a rough indicator only and, for test purposes, a meter calibrated for the particular oil to be used and over a realistic range of temperature should be installed. Even better is the use of an accurately calibrated day tank.
The accurate measurement of the flow of coal or other solid fuel is very difficult, there is no short cut, measurement must be by mass, which means that bulky apparatus must be set up on the boiler-house floor, and the coal manhandled. Samples must be taken and bagged throughout the test, the bags sealed and sent to the laboratory for the analysis and calorific value determination. In some more recent boiler houses, the problem has been alleviated by mounting the hoppers over the boilers on calibrated load cells, but these are as yet uncommon.
Continuous coal flow measurement for the assessment of fuel consumption and boiler efficiency during normal operation of stoker-fired boilers is achieved with a reasonable degree of accuracy by continuous measurement of fuel bed depth and grate speed. These factors, together with the known width of the grate, enable the volume of the coal used to be continuously calculated by a microprocessor. The fuel bulk density is input to the microprocessor using information obtained from measurements of the density of samples taken at intervals. The microprocessor can then indicate and record the ongoing fuel consumption in suitable units.
Heat output:-
There are several methods which can be used for measuring heat output. With steam boilers, an installed steam meter can be used to measure flow rate, but this must be correct for temperature and pressure. In earlier years, this approach was not favoured due to the change in accuracy of orifice or venturi meters with flow rate. It is now more viable with modern flow meters of the variable- orifice or vortex- shedding
types. It is not usually easy to install a meter specially for a test, as bends in pipes can affect its accuracy. The alternative with small boilers is to measure feed water, and this can be done by previously calibrating the feed tank using weighed increments of water to fill the tank from a marked low level to a marked high level, and operating the tank between these limits. The numbers of fills are counted and, finally, the intermediate position is interpolated when the test ends. Where feedwater is measured, however, it is important to allow for boiler blowdown and, with saturated steam boilers, steam wetness. Normally, this latter should not exceed about 2% of the output, but it can be much more than this if water conditions are liable to cause foaming.
With hot-water boilers the heat output is measured by an installed water meter, preferably of the variable-orifice tank. The temperatures of the water entering and leaving the boiler are also required. With low temperature hot-water systems, of which there are many, the difference between flow and return water temperatures can be as little as 20°C,in which case an error of only 1°C in the measurement of this differential is equivalent to an error of 5% in the measurement of heat output.
According to this method the Boiler efficiency is expressed as,
n boiler direct = steam flow divided by fuel burnt
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
THE INDIRECT METHOD
The efficiency can be measured quickly and easily by measuring the losses using the principles to be described. Where, as in an acceptance test, the output is needed, this must be measured as described by the ‘Heat Output’ of Direct Method or by multiplying input, measured as described by the ‘Heat Input’ of Direct Method, by the efficiency. In some cases, e.g. with waste-heat boilers the input can be difficult to measure, in which case the output must be used and its possible inaccuracies accepted.
Dividing this by the efficiency makes an estimate of input possible. This can be very useful in itself, for instance in determining the calorific value of a waste fuel. The important part of the indirect method, however, is the measurement of the losses.
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’.
According to this method the Boiler efficiency is expressed as
n Boiler, Indirect = 100 -Total % of Boiler losses
Types of losses:-
1) Dry flue gas loss.
2) Loss due to evaporation of water formed due to hydrogen in fuel.
3) Moisture present in fuel.
4) Moisture present in air.
5) Incomplete combustion.
6) Radiation and Convection losses.
7) Loss due to unburnt fly ash.
8) Loss due to unburnt bottom ash.
1) Dry flue gas loss :~
This is the heat loss from the boiler in the dry component of gases to the stack. This loss in a typical example can be of the order of 4.5%.
The Dry Flue Gas Loss Depends Upon Two Factors:-
1. EXCESS AIR
2. AIR HEATER OUTLET GAS TEMPERATURE
heat lost to the flue gases depends upon the quantity of products of combustion and the temperature of the gas leaving the heat recovery equipment. The quantity of excess air to be employed to ensure complete and satisfactory combustion is determined from knowledge of the combustion characteristics of the fuel and the type of combustion equipment to be used; previous experience plays an important role in this. Typical values for industrial water tube boilers where variable loads are experienced are:
Natural gas 10%
Heavy fuel oil 10%
Coal, stoker fired 35-40%
Lower values can be used, but they require good maintenance and operation on the firing equipment for them to be sustained. When selecting the flue gas temperature that is to be achieved, the following factors must be considered.
Boiler availability, i.e. the percentage of the annual working hours for which the boiler is available for use.
Fuel costs
Type and composition of fuel
Feedwater temperature available
The effects of these are interrelated to some extent. For instance, a boiler giving high thermal efficiency does not necessarily give a high availability if a low flue gas temperature is used which results in rapid fouling of the heat recovery equipment.
Low-temperature corrosion has a significant influence upon the choice of final gas temperature when heat recovery equipment is included. A combination of a gas outlet and cold fluid inlet temperatures must be selected to give the desired minimum tube surface temperature within the heat-recovery equipment.
If low-temperature corrosion is to be avoided and a long life is desired for the heat-recovery equipment, it is recommended that the flue gas temperature should be less than 180°C for sulphur bearing oils and 160°C for sulphur bearing coals. When using economisers it is rarely economical to reduce the flue gas temperature to a value less than 30°C above the inlet temperature of the feed water.
Loss due to Evaporation of Water formed
due to Hydrogen in Fuel :~
Coal contains hydrogen, which burns to form water. This loss is the latent heat removed in flue gases by the water. This loss also accounts a lot in calculation of boiler efficiency, which cannot be simply ignored. This loss depends on the specific heat of superheated steam.
Loss due to Moisture present in Fuel :~
This is the loss of heat from the boiler in the flue gases due to water vapour which was present initially as moisture in the coal burnt. This is such type of loss which cannot be altered as the moisture which is inherent inside the coal cannot be released by simply heating it by hot air from PA fan in the pulveriser
Loss due to air present:-
This is the loss of heat from the boiler in the flue gases due to water vapour which was present initially as moisture in the actual air supplied for combustion of fuel. This loss cannot be ignored as the air supplied by PA and FD fan, even after getting heated in the air pre-heater still has certain amount of moisture contained in it which accounts for this type of loss. Thus it can be said that this type of loss depends upon the humidity factor of the supplied air.
Loss due to Incomplete Combustion :~
This is because of incomplete combustion of carbon, i.e. C to CO only. Allowance is rarely made in the design for losses due to incomplete combustion when firing natural gas and fuel oils unless the latter have high ash content. With solid fuels unburnt carbon is contained in the residuals discharged from ash and grit hoppers and the chimney. The value depends upon the combustion system used and is determined from experience with the operation of boilers under similar conditions. The value
Radiation and Convection Loss :~
For a given output a coal-fired boiler will be larger than one fired with gas or oil, and hence the radiation loss will tend to be higher with coal firing. Because the temperature of the outer surface of a boiler enclosure does not reduce significantly as the steam demand upon it reduces, the heat lost from the surface will be substantially the same over the whole load range of the boiler. As a percentage of the heat input to the boiler, the radiation loss will therefore increase as the load on the boiler reduces and is usually taken as inversely proportional to load. For example, the percentage heat loss by radiation at 50% boiler output will be approximately twice that at 100% load.
For a given fuel and method of firing the heat lost to radiation as a percentage of the heat supplied in the fuel reduces as the design output, and hence physical size of a boiler, increases. This gives a marginally higher efficiency at high design outputs, all other factors being equal. This is explained by the fact that the output capability of a boiler is approximately proportional to its volume, but the surface area of the boiler increases at a lower percentage rate than its volume, and hence the quantity of heat lost increases at a lower rate than its output.
7) Loss due to Unburnt Fly Ash :~
During firing of fuel, i.e. coal in the furnace there may be certain reasons for incomplete combustion of carbon present in coal. This may be due to high value of draught created, lack of sufficient air for combustion, improper ignition temperature, very less time of exposure to flame. So some of the carbon particles present in supplied coal gets carried away with the flue gas without being properly burnt. This results in the presence of traces of carbon particles in the exit flue gas. Due to this carbon composition in flue gas there is loss of calorific value of these carbon particles which would have been fully utilised if their complete combustion would have occurred ideally in the furnace. But practically this does not happen which accounts for the loss in boiler efficiency due to unburnt fly ash.
Loss due to Unburnt Bottom Ash :~
When the pulverised fuel (coal) is fired in the furnace, all of it does not undergo simultaneous combustion. Some of the coal particles partially burn and due to insufficient time, temperature and turbulence these settle down in the hopper below the furnace as bottom ash which contains a considerable amount of unburnt carbon particles, which is actually a loss in the utilization of the calorific value of the fuel (coal) supplied. Greater will be the favourable ideal conditions of combustion, lesser will be
the carbon particle’s content in the bottom ash and hence greater will be the boiler efficiency.
CONCLUSION
As seen from the Output sheet above, by far the greatest heat loss is that carried away by the flue gases, a reduction of the flue gas temperature of 20°C is equivalent to an increase in thermal efficiency of approximately 1%, example an increase of efficiency from 85% to 86%. The quantity of flue gases should be maintained at the minimum possible by good combustion control and elimination of unwanted air infiltration through the boiler enclosure by good maintenance. When a boiler is fitted with economisers or air heaters which are designed to give the selected flue gas temperature, the boiler working pressure has no influence upon the thermal efficiency. When no heat recovery is included, for a given heating surface and heat input to steam, a boiler operating at a high pressure will give a high temperature of gas leaving the convection heating surfaces than if it is operating at a low pressure. This is due to the higher saturation temperature of the water in the boiler tubes giving a lower LMTD and hence lower heat transfers within the convection surfaces.
The heat loss to unburnt combustibles in the ash and grit may be high due to either a high as content or a very high carryover of particulates of high carbon content into the boiler, as experienced with spreader stokers and fluidised beds. Some reduction of this loss, and hence improvement of efficiency, can be obtained by refiring or reinjection of the grits from the boiler hoppers into the furnace to burn off some of the remaining carbon. Two-stage dust collectors (in series on the gas side) are often included in such cases, and the coarse material from the first stage only is refired.
Complete refiring of all the grits should not be carried out as this will increase the quantity of grit flowing across the boiler heating surfaces significantly, hence increasing the probability of erosion of the tubes. Also, with refiring the grits are reduced in size with the consequence that a more efficient dust collector is required or the chimney particulate emission will increase.
In the event of the required boiler efficiency being specified in an enquiry to the boiler maker, the design engineer has to determine the flue gas temperature required to give this efficiency before the amount of heating surface in the heat recovery equipment can be determined.
As the output from a boiler reduces below the design of 100% the values of the losses vary and will result in a change of boiler efficiency. The effects upon the losses of reducing boiler output are as follows:
a) Unless action is taken to prevent it for corrosion reasons, the flue gas temperature will fall, giving a reduction of the dry flue gas and moisture losses.
b) The excess air in the combustion chamber, and hence oxygen content in the flue gases, will increase, thus increasing the dry gas and moisture in air losses.
c) The radiation loss will increase as a percentage of the heat input.
d) The unburnt combustible loss may change depending upon the fuel and firing system.
The cumulative effect will vary with boiler size, fuel and final gas temperature. The trend will be for the efficiency to fall with reducing load, with small units due to the relatively high radiation loss, whereas with large boilers the efficiency may peak at a load below 100% maximum continuous rating (MCR).