TYPE OF HEAT LOSS IN BOILER AND THEIR PREVENTION
in this article we discussed about the Type of heat loss in boiler and their prevention. in power sector each boiler is not 100% efficient it has some losses so that the boiler efficiency remains up-to 75-80% in generally operating conditions and remaining 20-25 % are considered into loss . these losses reducing the performance of boiler as well as its also maximize the operating coast of the boiler . in this article we discussed the key factors of the heat loss in boiler and their prevention in detail way. these key factors are very useful for the minimize heat loss from the boiler and maximize the production rate of steam .
Types of heat loss in boilers in power plant:
1. Loss due to dry flue gas
2. Loss due to hydrogen in fuel
3. Loss due to moisture in fuel
4. Loss due to moisture in air
5. Loss due to carbon monoxide
6. Loss due to surface radiation
7. Unburnt losses in fly ash
8. Unburnt losses in bottom ash
9. Unaccountable losses (blow down losses, leakage etc.)
Prevention to reduced heat losses from boilers
1 . Stack temperature:
the stack temperature should be as low as possible. However it should not be so low that water vapor in the exhaust condenses on the stack walls. This is important in fuels containing significant sulfur as low temperature can lead to sulphur dew point corrosion. Stack temperature greater than 200 deg. C indicates potential for recovery equipment and hence the urgency of talking an early shutdown for water / flue side cleaning.
2. Feed water preheating using economizer
Typically the flue gases leaving a modern 3 pass shell boiler are at temperature of 200 to 300 deg. C .thus there is a potential to recovery heat from these gases . The flue gas exit temperature from a boiler is usually maintained at a minimum of 200 deg. C so that the sulphur oxides in the fuel gas do not condense and causes corrosion in heat transfer surfaces.
3. Combustion air preheater
Combustion air preheating is an alternative to feed water heating. In order to improve thermal efficiency by 1 % the combustion air temperature must be raised by 20 deg. C. Most gas and oil burners used in a boiler plant are not designed for high airs preheat temperatures.
Incomplete combustion can arise from a shortage of air or surplus of fuel or poor distribution of fuel . It is usually obvious from the color or smoke and must be corrected immediately. With coal firing unburned carbon can comprises a big loss. It occurs as grit carryover or carbon in ash and may amount to more than 2% of the heat supplied to the boiler. Non uniform fuel size could be one of the reasons foe incomplete combustion. In chain grate stokers, large lumps will not burn out completely, while small pieces and fines may block the air passage thus causing poor air distribution. In sprinkle stoker, Stoker grate condition, fuel distributor, wind box air regulation and over firs system can effect carbon loss. Increase in the fines in pulverized coal also increase carbon loss.
5. Excess air control
Excess air is required in all practical cases to ensure complete combustion, to allow for the normal variations in combination and to ensure satisfactory stacks condition for same fuels. The optimum excess air level for maximum boiler efficiency occurs when the sum of the losses due to incomplete combustion and loss due to heat in flue gases is minimum. This level varies with furnace design type of burners fuel and process variable. It can be determine by conducting tests with different air fuel ratios.
Methods for control the excess air:
>Portable oxygen analyzers and draft gauges can be used to make continuous readings to suggested the operator to manually adjust the flow of air according to the fuel supply for achieving optimum combustion for optimum operation. Excess air reduction up to 20% is feasible.
>Now these days oxygen analyzer with a local readout mounted draft gauge is used for monitoring the excess air continuous, with the help of this operator can adjust air flow according to demand . A further reduction of 10-15% can be achieved over the previous system.
6. Radiation and convection heat loss
The external surfaces of a shell boiler are hotter than the surrounding. The surface thus lose heat to the surroundings depending on the surface area and the difference in temperature between the surface and the surrounding the heat loss from the boiler shell is normally a fixed energy loss irrespective of the boiler output with modern boilers designs this may represent only 1.5 % on the gross calorific value at full rating but will increase to around 6 % , if the boiler operates at only 25 % output. Repairing or augmenting insulation can reduced heat loss through boilerwallsandpiping
7.Automatic blow down control
Uncontrolled conditions blowdown is very wasteful automatic blowdown control can installed that sense and respond to boiler water conductivity and ph . A 10% blow down in a 15 kg/cm2 boiler results in 3% efficiency loss.
8 reduction of scaling and soot losses
In oil and coal fired boiler soot built up on tubes acts as an insulator against heat transfer any such deposits should be removed on a regularly basis . Elevated stack temperature may indicates excessive soot buildup also same result will occur due to scaling on the water side.
9.Reduction of boiler steam pressure
This is an effective means of reducing fuel consumption, if permissible by as much as 1 to 2 % . Lower steam pressure gives a lower saturated steam temperature and without stack heat recovery a similar reduction in the temperature of the fuel gas temperature results.
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