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Right option is (a) Due to greater surface area

The best EXPLANATION: The combustion rate is faster due to greater surface area of coal per unit mass of coal. It means more coal surface is EXPOSED to heat and oxygen. This decreases excess air required for complete combustion and also decreases fan POWER. There is an INCREASED rate of evaporation and higher boiler EFFICIENCY due to the complete combustion of fuel.

Correct ANSWER is (b) To maintain steady state combustion

The explanation: The automatic combustion CONTROL system provides the effective prevention of the environmental pollution and steady combustion condition to get a constant steam output. But few FACTORS PREVENT steady combustion condition such as unbalanced combustion air and the movement of the burn-out position in the INCINERATOR.

The correct option is (a) To support gases coming out of fuel BED

The explanation: The gases coming out of RAW fuel bed pass through a region of incandescent ASH on the surface of the fuel and finally discharged to the furnace. As the gases coming out of fuel bed ALSO contain combustible matter the supply of secondary air is required in this CASE. The ash left at the bottom of the stoker is at a higher temperature than the over feed stoker.

Correct CHOICE is (c) Rate of air supply

The best I can explain: The rate of carbon-dioxide entirely DEPENDS on the rate of air supply. When the hot air after cooling the ash passes to incandescent COKE bed, here the O2 reacts with C to form CO2.Generally for fuel bed of 800mm depth all the O2 in the air is consumed in the incandescent region.

Correct option is (a) Minimize the COKE CARRIED into ASH pit

Best explanation: The operator controls RATE of burning in different ZONES to minimize the coke carried over to ash pit. The coal supplied to the grate is regulated either by varying depth of coal on the grate with the help of grate valve and by varying the rate of grate travel.

Right choice is (c) Master controller

The explanation: Master controller is the component that maintains all the FORCES in whole system. The actions of PRIMARY forces are magnified through the master controller which in turn operates the concerned ELEMENTS through electric, hydraulic and pneumatic system. The master control makes VARIATION depending on steam pressure LEVEL.

The correct choice is (a) Boilers were set close to grates

For explanation I would say: In EARLIER times boilers were set very close to the grates and combustion SPACE was limited and hence resulted in smoke and poor efficiency. LATER, furnaces were made larger and the boilers were set at higher LEVEL above the grates. The hand fired furnace with large combustion space was used to burn a wide variety of coals.

The CORRECT choice is (a) Conduction

For explanation: The heat for the distillation COMES by conduction from the mass of INCANDESCENT fuel bed existing above the raw COAL. The air mixes with the volatile matter and passes through the ignition ZONE and then enters into the region of incandescent coke.

Correct option is (a) Filters arranged in PARALLEL form

Explanation: A fabric filter element made up of a long hollow CYLINDRICAL tube, that provides a large surface PER unit of gas volumetric flow rate is CALLED as baghouse filter and a system consisting of large number of such filter elements arranged in parallel rows is called a baghouse.

Correct option is (c) Weight of particulate

Best EXPLANATION: Gravitational separation is an industrial method of separating TWO components, either a suspension, or dry GRANULAR mixture where separating the components with gravity is sufficiently practical, i.e. the components of the mixture have different specific weights.

Right option is (c) Metal SLAG

Explanation: The ASH FORMED due to rapid burning of COAL drains down the inner wall of the furnace as molten slag. HOT flue gases along with 10 to 20% of fly ash enter the furnace. Due to centrifugal action most of the fly ash is thrown against the furnace wall and is drained out along with molten slag.

The correct choice is (c) SCREW CONVEYOR

To explain: The fuel is placed in the large HOPPER on the FRONT of the furnace. Then it is further fed by reciprocating RAM or screw conveyor into the bottom of the horizontal trough. The ash formed after burning of coal is collected at other end.

The CORRECT option is (a) Over feed

To EXPLAIN: Spreader stoker is an over feed type of stoker. The coal burned on this stoker remains partly in suspension and partly on the GRATE. The spreader stoker consists of variable feeding DEVICE, a mechanism for throwing the coal uniformly on the grate and suitable openings for admitting the air.

Right answer is (c) Feeding in continuous stream

The BEST I can explain: The feeder is a slow speed rotating drum on which LARGE NUMBERS of small blades are mounted. It supplies coal to the spreaders in continuous stream. The speed of the feeder can be adjusted as per the load on plant. The feeders are operated with variable speed drive to control the combustion as per the REQUIREMENT. The feeders may be reciprocating ram, ENDLESS belt or spiral worm.

Correct option is (d) Caking bituminous

The best explanation: Any type of fuel except caking bituminous coal can be used with travelling GRATE stoker because of the FORMATION of large percentage of FINES resulting in INCREASED carbon loss. The RATE of burning of fuel in this stoker is 200 to 300kg/m^2 per hour when forced draught is used.

Correct answer is (c) Turbulence

For EXPLANATION: The air required for the COMBUSTION is supplied through the air inlets situated the below the grate. The SECONDARY air is supplied through the openings provided in the furnace will be above the grate. The combination of PRIMARY air and over FIRE supplied provide turbulence required for rapid combustion.

The correct option is (c) GRATE firing

Easy explanation: Grate firing was the first combustion SYSTEM used for solid fuels. It now is used mainly for burning WASTE and BIOMASS, but also for smaller coal furnaces. Fuel fired per grate area is 1-2 MW/m^2, ad MAXIMUM grate area is 100m^2.

The correct answer is (b) CORONA is formed

Easy explanation: When the HIGH voltage is applied, it generates a unidirectional non uniform electric field having greater magnitude at the discharge electrodes. This result in a blue LUMINOUS glow called corona around them. This corona is an indication of negatively charged ionized GAS molecules which travel from discharge electrodes to grounded COLLECTION electrodes.

Correct option is (b) Nozzle

The explanation: Over FIRE or secondary air is supplied through nozzle. The secondary air creates high turbulence and completes the combustion of volatile matter and FINE particles of the coal. The unburnt coal and ash are deposited on the grate which should be REMOVED PERIODICALLY.

Correct OPTION is (d) Air and fuel supply

Best explanation: Combustion can be effectively CONTROLLED by regulating the QUANTITIES of air and fuel or both. Control of combustion can be by manual or automatic means. Automatic control of combustion is used to distribute the load evenly on the boiler as per the changing DEMAND.

Correct answer is (a) BELT CONVEYOR

To explain I would say: Belt conveyor is used for transporting large quantities of coal over long DISTANCES. It consists of an endless belt made of rubber, balata or canvass RUNNING over pulleys or END drums. It is economical means of coal transportation at rapid rate.

The correct ANSWER is (C) To face any emergencies

Best explanation: To meet the continuous demand of coal and to face any emergencies due to the short supply of coal, it is always desirable to have a coal storage yard near the plant. CAPACITY of such yard would be about 10% of annual CONSUMPTION to a maximum of 25% of annual consumption.

Right choice is (c) For emergency possible cases

To elaborate: With more and more number of commissioning of large capacity steam power plants, it is very ESSENTIAL to have an effective means of transporting coal from coal mines to the PLANT site and subsequently STORE it to MEET the EMERGENCIES.

The CORRECT option is (c) Atomizing burner

To elaborate: Atomizing burners are very commonly used in OIL-fired furnaces of steam generators. The oil is divided into FINE particles to increase the surface area of CONTACT with the combustion air. DUE to atomization a homogeneous mixture of oil and air is produced which is fired in the furnace.

Correct choice is (c) Ash producing capacity

Explanation: The selection of bowl mill is done on the basis of TOTAL MOISTURE content of COAL, sulfur and mineral content (used to determine pyrite removal sizing requirement), GRINDABILITY(used to select mill size), volatile matter (used to determine the REQUIRED coal fineness).

Right option is (a) FORCED draught fan

Explanation: A forced draught fan is used to SUPPLY hot air to the MILL through the annular space surrounding the races. The air PICKS up the coal DUST as it flows between the balls and races and then enters into the classifier above it.

Correct choice is (a) Large capacity plants

Best explanation: Pulverized fuel firers are used in large capacity plants. The CONVENTIONAL or stoker firing methods were unable to meet the variable loads on the plants and were unsuitable for large capacity plants. USING pulverized fuel firer gives higher thermal efficiency, better control as per LOAD on the PLANT and USES low grade coal.

The CORRECT choice is (c) Control of combustion

Explanation: The principle of automatic control of combustion is derived from the control of combustion.In order to ensure sufficient AMOUNT of steam GENERATION at the requisite pressure and to maintain correct PROPORTION of fuel and air in a furnace for complete combustion, control of combustion in the furnace is very MUCH essential.

Right choice is (a) The primary AIR flow is increased

Easiest explanation: When there is requirement for more coal, the primary air flow is increased automatically and its higher velocity in the mill carries additional coal in the furnace. This action reduces the amount of coal in the PULVERIZER and decreases the pressure DROP, causing the feeder CONTROLLER to SUPPLY more coal.

Correct choice is (c) PREVENTS air circulation in the interior of heap

To explain I would say: Coal is piled in HEAPS of 10 to 12m height on a concrete floor, by compacting it in the layers of 20 to 30cm thickness using bulldozers. This prevents air circulation in the interior of the heap. The heap top is given a gentle slope to drain off rain WATER IMMEDIATELY without WASHING the coal.

Right option is (C) Load cell principle

To explain I WOULD say: The operation is based on load cell principle. The load CELLS produce VOLTAGE signals proportional to the applied load. This system maintains high accuracy in measurement. Its performance is affected by temperature, NOISE, dust and humidity of the air.

Right choice is (b) Rail transport

The EXPLANATION: Depending on the MODE of transport viz. by sea or ROAD or rail, coal unloading done. Rotary car dumpers are used when the transport is done by rail also coal towers and coal accelerators are used. Coal towers, unloading bridges and self unloading boats are used if the outplant handling of the coal is by sea or RIVER.

The correct answer is (B) Mechanical weighers

The explanation is: Mechanical weigher is commonly used all over world. This based on mechanical principle. This works on a lever system mounted on knife edges and bearings that connected to a resistant like a SPRING or pendulum. The resultant VALUE obtained by multiplication of load and speed VALUES is displayed by a digital counter.

The correct choice is (a) On solid ground

Best EXPLANATION: Coal should be DUMPED on a solid ground completely free from standing water. It should be protected from direct sunlight preferably and should be done so in order to reduce CHANCES of oxidation and combustion.

Correct choice is (c) Free from repetitive handling

The best explanation: It should have minimum handling operations and should be free from repetitive handling. For easy inspection and MAINTENANCE it should have centralized handling UNITS. Its capacity should be well above the requirement of COAL during PEAK hours.

Right answer is (a) Hammer mill

The best explanation: In the hammer mill the pulverization of coal is MADE by IMPACT and this is also CALLED as impact mill. The coal in PULVERIZER remains in suspension during the entire pulverizing process. All the grinding elements and the primary AIR fan are mounted on a single shaft.

Right choice is (d) Greater wear

To EXPLAIN: Ball and race MILL have greater wear compared to other pulverizers. The advantages of lower space occupied, lower power consumption in KW hr/ton of coal pulverized, lower WEIGHT and lower capital cost have OUTWEIGHED the wear problem and these pulverizers found general acceptance.

The correct answer is (c) Level of coal

For explanation I would say: The feeder output is regulated by the coal level in the cylinder. When coal level in the cylinder ATTAINS sufficient HEIGHT to seal off the lower channel than the DIFFERENTIAL control OPERATES to stop the coal FEED.

The correct answer is (a) Forms a CYCLONIC flow

To explain: The fixed vanes make the entering air to form a cyclonic flow throwing the oversized PARTICLES to the walls of classifier. The oversized particles slide down for further grinding in the mill. The coal particles of REQUIRED size are TAKEN to the BURNERS with air from the top of the cylinder.

Correct answer is (a) Decreases maintenance of plant

The explanation is: The use of pulverized system increases the capital of and operating COSTS, but decreases maintenance of the plant and increases thermal efficiency. The pulverizer used to GRIND the coal should CONSUME LESS power and should DELIVER the rated tonnage of coal.

Correct choice is (c) For uniform distribution of coal

Easy explanation: Spreader consists of rapidly ROTATING shaft carrying blades. These blades are twisted to PROVIDE uniform distribution of the coal over the grate. The FAST rotating blades hit the coal particles coming from the feeder and throw into the furnace. The distribution of coal over the grate depends on the rotating SPEED of the spreader and on the SIZE of the coal.