(SEM V) THEORY EXAMINATION 2022-23 FUELS AND COMBUSTION

SECTION A – Short Answer Type Questions (2 Marks each)

(a) Define Primary and Secondary Fuels.

Primary Fuels:
These are naturally occurring fuels found in nature and used in raw form.
Examples:

Coal, wood, natural gas, petroleum.

Secondary Fuels:
These are artificial or derived fuels obtained from processing of primary fuels to improve their properties.
Examples:

Coke (from coal), Petrol, Diesel, Kerosene (from crude oil), Producer gas, Water gas (from coal gasification).

Difference:
Primary fuels occur naturally, whereas secondary fuels are man-made and refined for specific industrial uses.

(b) What is Coal Ranking?

Coal ranking is the process of classifying coal based on its degree of carbon content, moisture, volatile matter, and calorific value.
It reflects the maturity of coal formed through geological processes.

Order of Coal Ranks (lowest to highest):

Peat: Least carbon, highest moisture, lowest heating value.

Lignite: Soft brown coal with 25–35% carbon.

Bituminous Coal: Most commonly used; 60–80% carbon; high calorific value.

Anthracite: Hardest, 90%+ carbon, high luster and highest energy content.

Importance: Ranking helps determine coal’s suitability for power generation, coking, and industrial heating.

SECTION B – Long Answer Type Questions (10 Marks each)

(a) Differentiate between (i) Proximate and Ultimate Analysis of Fuel

(ii) Solid, Liquid, and Gaseous Fuels

(b) What is a Burner? Write the Features of a Good Burner and Explain Gas Burners.

Definition:
A burner is a device that mixes fuel and air in correct proportions and ignites the mixture to produce a stable flame for efficient combustion.

Features of a Good Burner:

Proper air-fuel ratio mixing.

Stable flame without pulsation or blow-off.

High combustion efficiency and complete fuel burning.

Safety features like flame monitoring and control.

Low maintenance and ability to handle variable loads.

Types of Gas Burners:

Bunsen Burner: Simple design; primary air mixed before combustion.

Surface Burner: Flame spreads over a perforated plate; used in domestic stoves.

Ribbon Burner: Long narrow flame for industrial heating.

Infrared Burner: Emits radiant heat; used for drying and heating processes.

Applications: Industrial furnaces, boilers, and IC engines.

SECTION C – Very Long Answer Type Questions (10 Marks each)

(a) Define Atmospheric Distillation. Explain the Construction and Working of Atmospheric Distillation Unit (ADU).

Definition:
Atmospheric distillation is the primary refining process where crude oil is separated into components (fractions) based on differences in their boiling points at atmospheric pressure.

Construction:

Distillation Column: Tall tower equipped with trays or packing.

Heater: Heats crude oil up to 350–400°C.

Condensers & Coolers: Cool vapors to collect liquid fractions.

Reflux System: Improves separation efficiency.

Working:

Crude oil is first heated in a furnace and fed into the distillation column.

The vaporized components rise through trays — each tray corresponds to a temperature zone.

Fractions condense at different levels according to their boiling points:

Top: LPG, gasoline.

Middle: Kerosene, diesel.

Bottom: Heavy oils, residue.

Residual fractions are sent to vacuum distillation for further separation.

Importance: ADU is the first step in crude oil refining, providing feedstock for conversion units like cracking and reforming.

(b) Discuss the Working of Pulverized Fuel Firing System with Diagram. Write Its Advantages and Disadvantages.

Definition:
In a Pulverized Fuel Firing (PFF) System, coal is finely ground into powder (pulverized) and then blown into the furnace with air for complete and efficient combustion.

Working Principle:

Coal Pulverizer (Mill): Crushes coal into fine powder (~70% below 75 microns).

Primary Air: Carries pulverized coal from mill to the burner.

Burner: Mixes coal dust with secondary air and ignites it.

Combustion Chamber: Coal burns in suspension, producing high-temperature flue gases.

Heat Transfer: The generated heat converts water into steam in the boiler tubes.

Advantages:

Very high combustion efficiency.                                Uniform heat distribution.

Easy control of flame and temperature.                        Suitable for large power plants.

Disadvantages:

High initial cost.                                                             Requires fine dust handling (explosion risk).

Greater maintenance and power consumption for pulverizers.