Fuels and Combustion Notes

Fuels and Combustion

Introduction

• A fuel is any substance used to produce heat or power through combustion.
• Combustion is any chemical process that produces light and heat, converting chemical energy into heat and light.
• Overall reaction: Fuel + Oxygen → Combustion products + Heat

Fuels

• Fuels are substances that undergo combustion in the presence of air to produce a large amount of heat for domestic and industrial purposes.
• Nuclear fuels are excluded from this definition because they are not easily used by the common man.
• Examples include wood, coal, kerosene, petrol, diesel, gasoline, coal gas, producer gas, water gas, and natural gas (LPG).

Calorific Values

• Calorific Value: The total quantity of heat liberated by complete combustion of a unit mass or volume of the fuel.
• Two types of calorific values:
  • High or Gross Calorific Value (H.C.V. or G.C.V.)
  • Low or Net Calorific Value (L.C.V. or N.C.V.)

Higher Calorific Value (H.C.V. or G.C.V.)

• The total amount of heat liberated when a unit mass/volume of the fuel is burnt completely and the combustion products are cooled to room temperature (15°C).

Lower Calorific Value (L.C.V. or N.C.V.)

• The net amount of heat liberated when a unit mass/volume of the fuel is burnt completely and the combustion products are allowed to escape.

Classifications of Fuels

Based on Physical State

• Solid fuel (e.g., wood, coal)
• Liquid fuel (e.g., crude petroleum, natural gasoline)
• Gaseous fuel (e.g., natural gas)

Based on Occurrence

• Primary or natural fuels (e.g., wood, coal)
• Secondary or prepared fuels (e.g., charcoal, petroleum coke)

Classifications by Origin and Physical State

• Natural or Primary:
  • Solid: Wood, peat, lignite, coal
  • Liquid: Crude oil, Vegetable oils
  • Gaseous: Natural gas
• Artificial or Secondary/Derived:
  • Solid: Semi coke, charcoal
  • Liquid: Petrol, kerosene, gas oil, coal tar, alcohol
  • Gaseous: Producer gas, coke-oven gas, water gas, blast furnace gas, compressed butane gas, LPG

Characteristics of Good Fuels

• High Calorific Value
• Moderate Ignition Temperature
• Low Moisture Content
• Low Ash Content
• Moderate Velocity of Combustion
• Should not produce harmful products
• Low Cost
• Easy Storage & Transportation
• Easily Controllable

Detailed Characteristics of a Good Fuel

1. Ignition Temperature: Should ignite easily. Moderate ignition temperature is ideal. The ignition temperature is the point at which the fuel starts and continues to burn without additional heat.
2. Specific Heat: Should give out a lot of heat, implying a high specific heat.
3. Combustion Products: Should have low smoke and combustible matter (ash) and should not give out harmful combustion products. This depends on the elements present in the fuel.
4. Availability and Cost: Should be inexpensive and readily available.
5. Storage and Transport: Should be easy to store and transport.
6. Ash Content: Should have low ash content. Ash reduces the calorific value, hinders air and heat flow, reduces specific heat, and leads to disposal problems.

Advantages & Disadvantages of Solid Fuels

Advantages

• Easy to transport.
• Convenient to store without risk of spontaneous explosion.
• Low production cost.
• Moderate ignition temperature.

Disadvantages

• High ash content.
• Large proportion of heat is wasted.
• Burn with clinker formation.
• Combustion operation cannot be easily controlled.
• High handling cost.

Liquid Fuels

• Fuels in liquid form, generally obtained from petroleum and its byproducts.
• Complex mixtures of hydrocarbons refined from crude petroleum oil.
• Examples: petrol, diesel, kerosene.

Advantages of Liquid Fuels

• Higher calorific value per unit mass compared to solid fuels.
• Burn without dust, ash, clinkers, etc.
• Easy to transport through pipes.
• Can be stored indefinitely without loss.

Disadvantages of Liquid Fuels

• Higher cost compared to solid fuels.
• Costly special storage tanks are required.
• Greater risk of fire hazards, especially with highly inflammable and volatile fuels.
• They give bad odour

Gaseous Fuels

• Fuels in gaseous phase, also hydrocarbons.
• Derived from petroleum reserves.
• Most common is natural gas (methane is the main component).
• May also be produced artificially from burning solid fuels (coal & water).
• Examples: Natural gas, Coal gas, Producer gas, etc.

Bomb Calorimeters

• Used to test the calorific value of solid and liquid fuels like coal and oil.
• Principle: A known mass of fuel is burnt completely, and the heat produced is absorbed in water; this determines the quantity of heat produced by burning a unit mass of the fuel.

Construction

• Stainless steel or nickel vessel (bomb) with a lid.
• The lid has two stainless steel electrodes and one oxygen inlet valve.
• Stainless steel or nickel crucible supported by the electrode.
• The bomb is placed in a copper calorimeter surrounded by an air jacket, water jacket, and insulating jacket to prevent heat loss.
• The calorimeter has a stirrer and thermometer to measure the temperature difference.

Working

• The fuel sample is placed inside the steel bomb.
• The bomb is placed inside a bucket filled with water, maintained at a constant temperature using a stirrer.
• Slots on top of the bomb for ignition wires and oxygen supply.
• An electric current passes through the ignition coil, initiating combustion.
• The heat released is absorbed by the water, increasing its temperature.
• The temperature is monitored with a thermometer for accurate readings.
• Heat losses are minimized by an air space between the bomb and an exterior insulating jacket.

Formula to Determine Calorific Value

• High calorific value (L) = \frac{(m1 + m2)(t2 - t1)}{M_f} cal / g
  • Where:
    • M_f = mass or weight of fuel sample
    • m_1 = mass or weight of water in calorimeter
    • m_2 = water equivalent of calorimeter, stirrer, thermometer, bomb etc.
    • t_1 = Initial temperature of water in calorimeter
    • t_2 = Final temperature of water in calorimeter

Analysis of Coal

• Proximate Analysis
  1. Moisture
  2. Ash
  3. Volatile matter
  4. Fixed Carbon
• Ultimate Analysis
  1. Carbon and hydrogen
  2. Nitrogen
  3. Sulphur
  4. Ash

Proximate vs Ultimate Analysis

Proximate Analysis Details

Moisture

• Determined by heating about 1 gram of finely powdered coal at 105°C to 110°C for an hour in an electric oven.
• The loss in weight is reported as moisture.
• Moisture content: Lesser the moisture content, better is the quality of coal.
• Formula: Moisture (%) = \frac{Loss \ in \ weight}{Weight \ of \ coal \ taken} \times 100
• Decreases calorific value of coal and takes away heat in the form of latent heat.

Volatile Matter

• A known weight of dried sample is heated in a crucible with a lid at 950°C ± 20°C for 7 minutes in a muffle furnace.
• The loss in weight is due to volatile matter.
• Lesser the volatile matter, better is the rank of coal.
• Formula: Volatile matter (%) = \frac{Loss \ in \ weight \ due \ to \ removal \ of \ volatile \ matter}{Weight \ of \ coal \ sample \ taken} \times 100
• Decreases calorific value and forms smoke, polluting the air.

Ash (Non-Combustible Matter)

• A known weight of sample is burnt completely at 700°C - 750°C in a muffle furnace until a constant weight is obtained.
• The residue left is the ash content.
• Lower the ash content, better is the quality of coal.
• Formula: Ash (%) = \frac{Weight \ of \ ash \ left}{Weight \ of \ coal \ taken} \times 100
• Reduces calorific value as it is the non-burning part, and ash disposal is a problem.

Fixed Carbon Content

• In a good sample of coal, moisture, ash, and volatile matter should be low, and fixed carbon should be high.
• Higher fixed carbon content, better is the quality of coal.
• Formula: Fixed carbon (%) = 100 - (Moisture + Volatile matter + Ash)

Ultimate Analysis Details

• Directly measures Carbon, Hydrogen, Nitrogen, and Sulphur in coal.
  1. Analysis of Carbon and hydrogen: