Fuels, combustion and energy storage notes — Unit 5

A fuel is a combustible substance containing carbon as the main constituent, which on proper burning in the presence of oxygen, produces a massive amount of heat energy.

⇒Classification of Fuels

⇒Characteristics of an Ideal Fuel

• High Calorific Value: Should produce maximum heat per unit mass.
• Moderate Ignition Temperature: If too low, it poses an extreme fire hazard. If too high, it is difficult to start the fire.
• Low Moisture & Ash Content: Moisture wastes heat (energy is used to evaporate it), and ash causes disposal and blockage issues.
• Safe: Easy to store, transport, and handle.
• Eco-Friendly: Should not release poisonous gases (like SOx, NOx) upon combustion.

The efficiency of any fuel is strictly measured by its Calorific Value. It is the total amount of heat liberated when a unit mass (or volume) of fuel is burnt completely.

⇒Gross vs Net Calorific Value

All fuels contain some Hydrogen. When burnt, this hydrogen reacts with oxygen to form Water Vapor (steam). This steam carries away a massive amount of latent heat.

The Bomb Calorimeter is a heavy laboratory apparatus used to experimentally determine the Gross Calorific Value (HCV) of solid and non-volatile liquid fuels.

⇒Construction

• The Bomb: A strong, sealed stainless steel pot capable of withstanding extreme explosive pressure. The fuel sample is placed in a crucible inside it.
• Oxygen Supply: The bomb is pressurized with pure Oxygen gas (at 25-30 atm) to ensure instantaneous, complete combustion.
• The Copper Calorimeter: The bomb is submerged in a highly insulated copper vessel containing a known mass of water.
• Stirrer and Thermometer: A mechanical stirrer keeps water temperature uniform. A Beckmann thermometer measures temperature changes up to 0.01°C accuracy.

⇒Working Principle

An exact known mass of fuel is ignited electrically inside the bomb. It explodes, releasing massive heat. The heat transfers through the steel bomb into the surrounding water. By measuring the rise in the water's temperature, we calculate the exact heat liberated.

Coal is a highly carbonaceous solid fuel formed by the slow, millions-of-years degradation of plant matter under extreme underground pressure and heat (Coalification).

⇒Ranks of Coal

As coal matures, its Carbon content increases while Moisture and Volatile matter decrease. Higher rank means a vastly better fuel.

⇒Analysis of Coal

To determine the quality and pricing of coal, laboratories perform two types of analyses: Proximate Analysis and Ultimate Analysis.

⇒Significance of Proximate Parameters

• Moisture: Bad. Reduces heating value, increases transport weight, consumes heat to evaporate.
• Volatile Matter: Bad in excess. Coal with high volatile matter burns with a long, smoky flame, wasting unburnt carbon as smoke. It implies low heating value.
• Ash: Very Bad. Non-combustible mineral residue. Lowers heating value, clogs furnace grates, and costs money to dispose of safely.
• Fixed Carbon: EXCELLENT. Higher fixed carbon means higher calorific value. Anthracite has the highest.

Crude petroleum is a dark, foul-smelling viscous liquid extracted from the earth. It is a complex mixture of hundreds of different hydrocarbons (paraffins, naphthenes, aromatics). In its raw form, it is practically useless.

⇒Fractional Distillation

The crude oil is heated in a furnace to ~400°C, vaporizing it. The vapors are pumped into the bottom of a massive, tall Fractionating Column. The column is hotter at the bottom and cooler at the top.

As vapors rise, they cool. Hydrocarbons with high boiling points (heavy molecules) condense back into liquids at the bottom. Hydrocarbons with low boiling points (light molecules) continue rising and condense at the top.

The market demand for Gasoline (Petrol) is massive due to millions of cars. However, fractional distillation only yields about 20% petrol from crude oil. To meet global demand, refineries use a process called Cracking.

⇒What is Cracking?

Cracking is the thermal or catalytic decomposition of large, heavy, high-boiling hydrocarbon molecules (like heavy oils) into smaller, lighter, low-boiling molecules (like petrol).

⇒Thermal vs Catalytic Cracking

In a Spark Ignition (SI) petrol engine, a mixture of petrol vapor and air is compressed by a piston, and then ignited perfectly by a spark plug. The resulting explosion smoothly pushes the piston down.

⇒The Problem of Knocking

If the petrol is of poor quality, the extreme compression causes the unburnt fuel mixture ahead of the spark flame to self-ignite instantaneously and violently. This premature, secondary explosion creates a massive shockwave that crashes into the cylinder walls.

This shockwave produces a sharp metallic rattling sound called 'Knocking'. Knocking drastically decreases engine efficiency, overheats the engine, and can physically shatter the piston.

⇒Molecular Shape dictates Knocking

• Straight-chain alkanes (e.g., n-heptane) ignite far too easily under pressure. They cause SEVERE knocking.
• Branched-chain alkanes (e.g., Iso-octane) and aromatics (Benzene) are highly resistant to self-ignition. They burn smoothly and PREVENT knocking.

To quantify the quality of petrol (its resistance to knocking), the industry established a scale called the Octane Number.

⇒The Scale Definitions

Octane Number: The percentage (by volume) of iso-octane in a mixture of iso-octane and n-heptane that perfectly matches the knocking behavior of the fuel being tested.

Example: If a fuel at the pump is '87 Octane', it means it knocks exactly as much as a laboratory mixture containing 87% iso-octane and 13% n-heptane.

⇒Anti-Knocking Agents

To improve cheap petrol, additives are used. Historically, Tetra Ethyl Lead (TEL) was added. It absorbed the free radicals to stop premature explosions. However, it released highly toxic Lead into the atmosphere and is now banned globally. Modern petrol uses safer oxygenated additives like MTBE (Methyl tert-butyl ether).

A Diesel engine works differently. There is no spark plug. Air is compressed to extreme pressures, making it blisteringly hot. Liquid diesel is then sprayed in, and it must self-ignite instantly due to the heat.

⇒Diesel Knock

In a diesel engine, knocking is caused by an Ignition Delay. If the fuel takes too long to self-ignite, a massive pool of unburnt fuel accumulates in the cylinder. When it finally ignites, the explosion is massive and uncontrolled, causing a harsh rattle.

Therefore, good diesel must self-ignite IMMEDIATELY. This is the exact OPPOSITE of petrol, where we wanted resistance to self-ignition!

⇒The Cetane Scale

Straight-chain molecules make good diesel (high Cetane) but terrible petrol (low Octane). Branched/Aromatic molecules make good petrol (high Octane) but terrible diesel (low Cetane).

Gaseous fuels are considered the cleanest and most efficient fuels because they mix instantaneously with air, ensuring complete combustion with zero ash and almost zero smoke.

⇒1. Natural Gas (CNG)

Found trapped above crude oil wells underground. It consists primarily of Methane (CH4) (70-90%) with traces of ethane and propane.

• When compressed to ~200 atm, it becomes Compressed Natural Gas (CNG), used heavily as an eco-friendly fuel in city buses and cars.
• High calorific value and produces extremely low greenhouse gas emissions compared to petrol.

⇒2. Liquefied Petroleum Gas (LPG)

Obtained as a byproduct during the fractional distillation of crude oil. It consists mostly of Butane and Isobutane.

• Under moderate pressure, butane becomes a liquid, making it incredibly easy to store in steel domestic cylinders.
• Because LPG has no smell, a foul-smelling chemical called Ethyl Mercaptan (Ethanethiol) is deliberately added so humans can instantly detect dangerous gas leaks.

Industrial processes require specific types of gases for massive furnaces and metallurgical extraction.

⇒Producer Gas

A mixture of Carbon Monoxide (CO) and Nitrogen (N2). It is produced by passing a limited supply of air over a bed of red-hot coke (carbon).

• It has a very low calorific value because 60% of it is inert Nitrogen gas, which doesn't burn.
• It is extremely cheap to make, so it is used in enormous volumes in steel and glass manufacturing furnaces.

⇒Water Gas (Syngas)

A mixture of Carbon Monoxide (CO) and Hydrogen (H2). Produced by blasting steam over red-hot coke.

• Both CO and H2 are highly combustible, so Water Gas has a much higher calorific value than Producer Gas.
• It burns with an intensely hot, non-luminous blue flame.

In industrial boilers, supplying the exact required amount of air is critical. Too little air causes incomplete combustion (producing toxic CO and wasting fuel). Too much air chills the furnace, as energy is wasted heating up the excess nitrogen.

⇒Calculating Air Required

The theoretical amount of Oxygen required to burn 1 kg of fuel is calculated based on its ultimate analysis (percentages of C, H, O, S).

The net oxygen needed is calculated. Since air is only 23% Oxygen by weight (and 21% by volume), the mass of air required is:

⇒End-of-Unit Verification

• Define a fuel and classify fuels based on their occurrence and physical state.
• Explain the exact difference between Gross Calorific Value (HCV) and Net Calorific Value (LCV) regarding latent heat.
• Draw a neat diagram of a Bomb Calorimeter and explain how it determines the calorific value of solid fuels.
• Distinguish between Proximate and Ultimate analysis of coal. State the significance of Volatile Matter and Ash.
• List the main fractions obtained during the fractional distillation of crude petroleum along with their boiling ranges.
• Explain Catalytic Cracking and why it is superior to Thermal Cracking for petrol production.
• Define 'Knocking' in spark-ignition engines and explain the chemical structure of molecules that prevent it.
• Define Octane Number and Cetane Number. Explain why a fuel with high Octane necessarily has a low Cetane number.
• State the composition and uses of Natural Gas (CNG), LPG, Producer Gas, and Water Gas.
• Solve a numerical problem to calculate the theoretical mass of air required for the complete combustion of 1kg of a given coal sample.