Boilers are the closed tank in which liquid is heated to steam. There are different control systems which govern different process cycle associated with the boiler operation, such as air-fuel control, feed water system, Combustion and draught control, Steam-temperature control.
Here, we are looking at different types of fuel system, there are coal or solid fuel system and liquid fuel managing system and gas fuel management. Because fossil fuels that are burned in boilers can be used in solid, liquid or gaseous form, or a mixture of these. Each of these fuels requires specialised handling and treatment, and the control and instrumentation have to be appropriate to the fuel and the plant that processes it.
Coal firing (pulverised fuel):
Coal is burned in solid form on grates, before feed into the combustion chamber the coal is broken into pieces it is usual. It depends on the coal used, some coal is fine to use directly. But some other coals are hard and in size can be jammed in the propeller.
Coals are powdered and propelled to the burners by an air stream. Various types of the pulverised-fuel mill will be encountered, but two are most commonly used: the pressurised vertical-spindle ball mill and the horizontal-tube mill
Vertical-spindle ball mills:
Vertical-spindle uses ball mills, where coal is crushed through rotating balls. In this device, the coal that is discharged from the storage hoppers is fed down a central chute onto a table where it is crushed by rotating steel balls. Air is blown into the crushed coal and carries it, via adjustable classifier blades, to the PF pipes that transport it to the burners.
The air that passes through the PF pipes carries out the fine coal powder through the PF outlet to the burner.
Horizontal tube mills:
The coal is drawn to a cage that rotates in the horizontal axis, This cage contains a charge of forged-steel or cast alloy balls which are carried up the sides of the cage by the rotation, until they eventually cascade down to the bottom, only to be carried up again.
The coal is pulverised by a combination of the impact with these balls, attrition of adjacent particles and crushing between the balls and the cage and between one ball and another. The crushed mixture is drawn out of the cage by a fan, which is called an exhauster. Because of this configuration, the tube mill runs under a negative pressure, which prevents the fine coal dust from escaping (as it tends to do with a pressurised mill).
The oil-firing system is much simpler compared to the coal firing system what only required is a system to extract the oil from its storage tank and pump it to the burners.
The important thing is the proper ignition of the oil into the burner. Proper ignition of oil depends on the fuel being broken into small droplets (atomised) and mixed with air. The atomisation may be achieved by expelling the oil through a small nozzle (a ‘pressure jet’), or it may be achieved by the use of compressed air or steam.
The ingectors maintain the flow of fuel to the burner. Depending on the purposes the fuel can much viscous, there is clear chance of cooling fuel in the pipeline. To prevent cooling the fuel must be heated constantly.
With the heavier grades of oil, prewarming is necessary, and to prevent it cooling and thickening the fuel is continually circulated to the burners via a recirculation system. The latter process is sometimes referred to as ‘spill-back’. When a burner is not firing, the oil circulates through the pipework right up to the shut-off valve, which is mounted as close to the oil gun as possible.
Gas leakage is the important problem faced by the gas-firing system. Great care must therefore be taken to guard against leakage, for example, from flanges and through valves. All gases cannot have colour thus it is difficult to find. To sense gas leakage, the gas fuel used are given a specific odour.
It is also necessary to prevent gas from seeping into the combustion chamber through leaking valves. If gas does enter undetected into the furnace during a shut-down period, it could collect in sufficient quantities to be ignited either by an accidental spark or when a burner is ignited. Which lead to explosion.
Protection against leakage into the furnace through the fuel-supply valves is achieved by the use of ‘double-block-and-bleed’ valve assemblies which provide a secure seal between the gas inlet and the furnace.
The operation of this system is that before a burner is ignited both block valves are closed and the vent is open. This is to vent out the gas between the block valves and therefore never develop enough pressure to leak past the second block valve.
When start-up of the burner is required, a sequence of operations opens the block valves in such a way that gas is admitted to the burner and ignited safely.