Q.4(a): How to ensure efficient combustion of fuel in boilers? What are the parameters that can be confirmed by instrument for complete combustion of fuel in the boiler?
Answer: Complete combustion of fuel: For optimum combustion, the real amount of combustion air must be greater than that required theoretically. Optimization is most important in the view of complete combustion. It can be monitored by residual oxygen content in the flue gas. Theoretically, no oxygen content should be in flue gas because it is leading to more heat loss from the furnace. But practically it is not possible to maintain zero excess oxygen content because there must be few contents of oxygen to ensure complete combustion. If we try to maintain higher oxygen content in flue gas then the possibility of complete combustion is more the other part is that heat loss will be proportionally higher, Hence to ensure complete combustion oxygen & CO level in flue gas should be within the limit. Orsat apparatus is used for oxygen, Carbon dioxide & Carbon mono-oxide monitoring. Excess air is nothing but a higher content of the atm. more than the calculated theoretical air. Higher excess air means it is leading to heat loss but the lowering excess air is leading to poor combustion.
Hence optimization is the most important factor from a combustion point of view. With coal firing, unburned carbon can comprise a big loss. It occurs as grit carry-over 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 for 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 sprinkler stokers, stoker grate condition, fuel distributors, wind box air regulation, and over-fire systems can affect carbon loss. An increase in the fines in pulverized coal also increases carbon loss.
Q.4(b): Why excess air is required for the combustion of fuel in the boiler? How much excess air is normally kept in a modern FBC boiler? What is the advantage of the FBC system over the mechanical chain grate stoker?
Answer: Need for Excess Air to ensure complete combustion of fuel in Boiler: Excess air is required in all practical cases to ensure complete combustion, to allow for the normal variations in combustion, and to ensure satisfactory stack conditions for some 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 burner, fuel, and process variables. It can be determined by conducting tests with different air-fuel ratios. Normally excess air is about 20% to 40% in modern FBC Boiler. Excess air is 40% at lower load & 20% at higher load.
Advantage of FBC system over mechanical chain grate stoker :
1. High Efficiency: FBC boilers can burn fuel with a combustion efficiency of over 95% irrespective of ash content. FBC boilers can operate with an overall efficiency of 84% (plus or minus 2%).
2. Reduction in Boiler Size: High heat transfer rate over a small heat transfer area immersed in the bed result in an overall size reduction of the boiler.
3. Ability to Burn Fines: Coal containing fines below 6 mm can be burnt efficiently in an FBC boiler, which is very difficult to achieve in a conventional firing system.
4. Pollution Control: S02 formation can be greatly minimized by the addition of limestone or dolomite for high Sulphur coals. 3% limestone is required for every 1% sulfur in the coal feed, Low combustion temperature eliminates NOx formation.
5. Easier Ash Removal: Ash removal is easier as the ash flows like liquid from the combustion chamber. Hence less manpower is required for ash handling.
6.No Clinker Formation: Since the temperature of the furnace is in the range of 750 — 900 Deg. C in FBC boilers, even coal of low ash fusion temperature can be burnt without clinker formation.
7. Fast Response to Load Fluctuations: Inherent high thermal storage characteristics can easily absorb fluctuation in fuel feed rates
8. Provisions of Automatic Coal and Ash Handling System: Automatic systems for coal and ash handling can be incorporated, making the plant easy to operate comparable to oil or gas-fired installation.
9. Provision of Automatic Ignition System: Control systems using microprocessors and automatic ignition equipment give excellent control with minimum manual supervision.
10. High Reliability: The absence of moving parts in the combustion zone results In a high degree of reliability and low maintenance costs.
11. Reduced Maintenance: Routine overhauls are infrequent and high efficiency is maintained for long periods.
Q.5: What is the boiler flue gas pollution control equipment and measuring devices? Explain the function and working principle of bag filter and ESP with a sketch. Mention limits of boiler flue gas pollutant discharge through the chimney as per pollution control board.
Answer: Gas cleaning devices are mainly two types:
1. Mechanical type gas cleaning device
(ii) Cyclone separator
[a] Dry-type (i) Gravitational separator
(iii) Impingement type
[b] Wet type (i) Spray type (ii) Packaged type
2. Electrical type gas cleaning device :
[a] Rod type [b] Plate type => ESP(Electrostatic precipitator)
Principle of ESP(Electrostatic Precipitator) operation:
– High DC -ve potential is applied to discharge electrode with respect to grounded collecting plate.
– Above dielectric strength, the break-down of medium takes place.
– Discharge of free electrons from the emitting electrodes which move towards collecting plates.
– Ionize gas molecule & charge dust particle negatively.
– These negatively charged dust particles moves towards the collecting plate, get collected & lose their charge.
Working principle of bag filters: Dust-contained gas will enter into the upper wind path by ash hopper. In the wind screen board action of air flow up, reduce the flow velocity, the part of the big particle inertia force due to the role of dust be separated into ash hopper. Dust-contained gas into the filter bag in the filtration purification, dust is resistance in the outer surface of the filter bag, purified air filter into the case on the opening, the discharge outlet. As the filter bag dust surface increases, filter import and export With differential pressure rise. When the dust catcher was resistant to the setting data, a control system out dust instructions, and the clear grey system began to work.
First, an electromagnetic valve opened immediately after receiving the signal. It makes little patches in the gas chamber of the compressed air emissions. Because small diaphragm on both ends of the force change, make be small-diaphragm closed exhaust passage open, big diaphragm upper air chamber of the compressed air which channel eduction, big diaphragm ends stress change, make big diaphragm action. It will be closed the output open, airbags of compressed air through the output control, and spray wind.
Chimney emission permissible Limit from GPCB :
1 .SPM(Suspended Particulate matter) – 100mg/NM3
2. sox – 100 ppm
3. NOx – 50 ppm
Q.6(a) : How you will prepare your boiler for annual inspection? What are the ways of cleaning the boiler, and economizer tube externally and internally if required?
Answer: Procedure for Annual inspection of Boiler: The preparation for annual statutory inspection is mainly consisting of gradual cooling and opening up of the boiler and thorough cleaning of the fireside and waterside surfaces. If possible the boiler is allowed to cool down naturally and the water is removed only after the brickwork is sufficiently cooled to avoid any damage to the boiler due to temperature differentials. If the Boiler under inspection is working in battery with other boilers, it must be effectively disconnected from all steam and hot water communications with other boilers under steam.
Effective disconnection shall be made either by removal of the boiler stop valve or of a length of piping or by the insertion of substantial blind flanges between the boiler stop valves and piping. Remarks mentioned in the certificate of the Boiler issued by the Government Inspector should be carefully perused and if there are any requirements for removal of lagging, brickwork, etc. are mentioned, then the same should be complied with. Water must be drained through a blow-down system. After the boiler has cooled manhole covers of the main drum must be opened first. If the bottom manhole covers are opened first there is the likelihood of injury to personnel due to escaping steam or hot air. All doors of man, mud and sight holes, cleaning plugs, all caps of headers and mud drums, fire bars and their bearers, fire bridge arches, oil or gas fuel burners, mechanical stoker attachment, etc. is to be removed, and then the cleaning attended to.
-Any horizontal tube from which water can be drained should be cleaned of water. All tubes are cleaned internally by brushing or cleaned tools.
-All boiler components must be cleaned thoroughly.
– if the scale from the boiler components can not be removed, by mechanical cleaning, chemical cleaning must be restored, which must be attended to by experts.
– Before entering a boiler, it must be ascertained that it is isolated from other boilers under steaming conditions.
– All steam drum internals are to be removed for cleaning.
– After the boiler is thoroughly cleaned and attended to, the boiler must be offered for inspection
– A powerful torch or if available, a portable LED light of 24 volts or less should be made available for internal inspection.
External tubes cleaning :
1. Through air cleaning of tube external surface.
2. Through water pressure cleaning of tube external surface.
3. External tubes cleaned by rope.
4. Eternal tubes cleaning through wire brush.
5. External tubes cleaning through Soot Blowing.
6. External tubes cleaning through hammering.
Internal tubes cleaning :
1. Rinsing through water header(through drains).
2. Flushing of Ring header(through drains).
3. Blow down of CBD/lBD/ring header drains(during normal operation).
4. Chemical scale cleaning during the shutdown. (if required/decision to be taken after detailed analysis only)
5. Alkali blow-out cleaning during commissioning (at the time of commissioning of boiler only).
Q.6(b) : Why feed water treatment is a critical aspect of a boiler? Explain DM water treatment plant used for high-pressure boiler, starting from raw water up to boiler feed water tank.
Answer: Requirement of feed water treatment: Deposits in boiler tubes can reduce circulation through tubes. This may enhance further deposit formation due to the reduction of the cleansing effect of circulating water on solids concentrating at heat transfer surfaces. Since deposits are poor conductors of heat, they retard heat transfer from combustion gases. As heat transfer is further retarded, boiler tube metal temperatures increase. The approximate softening temperature of boiler tube metal is about 482.2 Deg.C If heat retardation of boiler deposits causes this temperature to be reached, tube softening and rupture will occur.
However, even when deposit build-up may not be sufficient to cause tube failure, their insulating effect may still result in reduced boiler operation efficiency and energy wastage by allowing excessive heat to exit the boiler with the stack gas. Deposits may also lead to differential corrosion cells beneath their surfaces. The result is localized corrosion or pitting. If such corrosion is severe, boiler metal can become thinned and weakened, resulting in ruptures due to internal boiler pressure.
Demineralization Water Treatment Plant: Demineralization is the removal of dissolved
ionic impurities that are present in water. Demineralized water is commonly produced by
one or a combination of the following processes:
• Membrane desalination
• Thermal desalination
The method selected to produce demineralized water depends on the quality of the effluent water, the required quality of the effluent water, the availability of resources such as regenerant chemicals, and wastewater treatment and disposal requirements. The economics of the processes that produce acceptable effluent quality must be evaluated to determine the most cost-effective method for a specific application. Inorganic removal is accomplished through the adsorption of contaminant ions into a resin exchange medium. One ion is substituted for another on the charged surface of the medium which is usually a plastic resin.
This type of surface is designed as either catatonic or anionic-negatively charged. The medium is saturated with exchangeable ions before the treatment operations. The contaminant ions during ion exchange, replace the regeneration ions because they are preferred by the exchange medium. When no ions are left to take the place of the contaminant ions, the medium is regenerated with a suitable solution that saturates the medium with the appropriate ions. Since there is a required downtime, the regeneration cycles are done only once per day. For resin exchange, capacity is expressed in terms of weight per unit volume of the resin used. Calculation of the breakthrough time for an ion exchange unit requires knowing the resin exchange capacity, effluent contaminant concentration, and the desired effluent quality.
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