Unit6 - Subjective Questions

Calorific Value:
It is defined as the total quantity of heat liberated when a unit mass (or volume) of a fuel is burnt completely in the presence of sufficient air or oxygen.

Distinction between GCV and NCV:

Relationship Formula:
Since hydrogen in fuel converts to water (), 1 part by mass of Hydrogen produces 9 parts by mass of water.

Where is the percentage of Hydrogen in the fuel.

Principle:
A known mass of fuel is burnt in a sealed steel bomb in the presence of excess oxygen. The heat liberated is absorbed by a known mass of water and the calorimeter. From the rise in temperature, the calorific value is calculated.

Construction:

• Bomb: A strong stainless steel pot capable of withstanding high pressure (25-30 atm). It has a lid with two electrodes.
• Calorimeter: A copper vessel containing a known mass of water in which the bomb is immersed.
• Accessories: A stirrer for uniform heat distribution, a Beckmann thermometer for precise temperature reading, and an oxygen inlet valve.

Working:

1. A weighed quantity of fuel () is placed in the crucible.
2. The bomb is filled with Oxygen at 25-30 atm.
3. The fuel is ignited electrically.
4. Heat is released, raising the temperature of water from to .

Calculation:
Let:

• = mass of fuel (g)
• = mass of water in calorimeter (g)
• = water equivalent of calorimeter (g)
• = initial temperature ()
• = final temperature ()

Proximate Analysis involves the determination of moisture, volatile matter, ash, and fixed carbon in coal. It provides data for the practical utility of coal.

1. Moisture Content ():

• Procedure: 1g of powdered coal is heated in a silica crucible at 105-110C for 1 hour.
• Significance: High moisture reduces calorific value.

2. Volatile Matter ():

• Procedure: The moisture-free coal is heated in a covered crucible at 925C for 7 minutes.
• Significance: High VM implies a long flame and more smoke.

3. Ash Content ():

• Procedure: Residual coal is heated without a lid at 700-750C for 30 minutes until constant weight (complete combustion).
• Significance: Ash is non-combustible impurity; it reduces calorific value and causes disposal issues.

4. Fixed Carbon ():

• Calculated by difference.
• Significance: Higher FC indicates higher calorific value and better quality coal.

Differences between Primary and Secondary Cells:

Construction:

• Anode: Spongy Lead ().
• Cathode: Lead Dioxide () supported on a lead grid.
• Electrolyte: Dilute Sulphuric Acid () (approx. 38% by weight, density 1.30 g/mL).

Discharging (Cell acts as Galvanic Cell):
When the battery supplies current:

• At Anode:
• At Cathode:
• Net Reaction:

Charging (Cell acts as Electrolytic Cell):
When DC current is passed:

• Reaction:
• The density of acid increases back to the initial value.

Applications:

• Automobiles (SLI batteries).
• UPS (Uninterruptible Power Supplies).
• Telecommunication systems.

Construction:

• Anode: Cadmium ().
• Cathode: Nickel(III) oxide-hydroxide ().
• Electrolyte: Aqueous Potassium Hydroxide ().

Cell Reactions (Discharging):

• Anode:
• At Cathode:
• Overall:

Disadvantages:

1. Memory Effect: If partly discharged and then recharged, it 'forgets' its full capacity.
2. Toxicity: Cadmium is a heavy metal and environmentally hazardous.
3. Self-discharge: Higher self-discharge rate compared to other rechargeable batteries.

Principle (Intercalation):
Li-ion batteries do not rely on chemical breakdown of electrodes but on the movement of Lithium ions () between the cathode and anode during charge/discharge cycles. This process of insertion and removal of ions into the crystal lattice is called Intercalation.

Construction:

• Anode: Graphite ().
• Cathode: Lithium Metal Oxide (e.g., ).
• Electrolyte: Lithium salt (e.g., ) in organic solvent.

Reactions:

• Discharging:
  • Anode:
  • Cathode:
• Charging: The reverse occurs.

Advantages over Ni-Cd:

1. High Energy Density: Store more energy per unit weight.
2. No Memory Effect: Can be charged at any state of discharge.
3. Higher Voltage: Approx 3.7V compared to 1.2V for Ni-Cd.
4. Low Self-Discharge: Retains charge longer when not in use.

Ni-MH Battery:
It is a rechargeable battery similar to Ni-Cd but uses a hydrogen-absorbing alloy instead of Cadmium at the negative electrode.

Construction:

• Anode: Metal Hydride (), typically an alloy of rare-earth metals (e.g., LaNi).
• Cathode: Nickel oxyhydroxide ().
• Electrolyte: Alkaline solution ().

Reaction (Discharging):

Comparison with Ni-Cd:

1. Environmental Impact: Ni-MH is more eco-friendly as it eliminates toxic Cadmium.
2. Capacity: Ni-MH typically has 2-3 times higher capacity than an equivalent size Ni-Cd.
3. Memory Effect: Significantly reduced in Ni-MH compared to Ni-Cd.

Concept:
A Li-Air battery uses the oxidation of lithium at the anode and the reduction of oxygen (from the surrounding air) at the cathode to induce a current.

Working Principle:

• Anode: Lithium metal ().
• Cathode: Porous carbon (Air electrode).
• Reaction: (Lithium Peroxide).

Significance:

1. High Energy Density: Theoretically, Li-Air batteries have an energy density comparable to gasoline (approx. 12 kWh/kg), which is 5-10 times higher than current Li-ion batteries.
2. Weight: Since oxygen is not stored in the battery but taken from air, the battery is much lighter.
3. Future Application: They are considered the "Holy Grail" for electric vehicles to achieve driving ranges comparable to petrol cars.

Definition:
A fuel cell is an electrochemical device that converts the chemical energy of a fuel (like hydrogen, methanol) and an oxidant (like oxygen) directly into electrical energy continuously as long as reactants are supplied.

Hydrogen-Oxygen Fuel Cell:

• Electrodes: Porous compressed carbon containing catalysts (Pt/Pd).
• Electrolyte: Hot concentrated aqueous KOH or a proton exchange membrane.

Working:

1. Hydrogen is bubbled through the anode compartment.
2. Oxygen is bubbled through the cathode compartment.

Reactions (Alkaline Electrolyte):

• Anode (Oxidation):
• Cathode (Reduction):
• Net Reaction:

Output: produces drinking water as a byproduct and electricity.

Advantages:

1. High Efficiency: They are not limited by the Carnot cycle efficiency (unlike heat engines). Efficiencies can reach 60-70%.
2. Pollution Free: The byproduct is usually water (in cells), making them eco-friendly.
3. Silent Operation: No moving parts implies no noise pollution.
4. Continuous Power: Generates power as long as fuel is supplied (unlike batteries that need recharging).

Disadvantages:

1. High Cost: Catalysts like Platinum are very expensive.
2. Storage: Storing and transporting Hydrogen is difficult and dangerous (high pressure/cryogenic requirements).
3. Sensitivity: Catalysts are easily poisoned by impurities like CO and Sulphur.
4. Durability: Long-term durability of membranes and electrodes is still a challenge.

Hydrogen as a Clean Energy Carrier:
Hydrogen is considered a future fuel because its combustion produces only water (), resulting in zero carbon emissions at the point of use. It has the highest calorific value per unit mass (150 kJ/g).

Production by Steam Methane Reforming (SMR):
This is the most common method of producing hydrogen industrially.

Process:

1. Reforming: Methane (Natural Gas) reacts with steam at high temperature (700-1000C) over a nickel catalyst.
   
2. Water-Gas Shift Reaction: The Carbon Monoxide produced is reacted with more steam to produce more hydrogen.
   

The result is hydrogen gas and carbon dioxide (which must be captured for the process to be truly 'clean', i.e., Blue Hydrogen).

Hydrogen Storage Methods:

1. Physical Storage (Gas): Stored as high-pressure gas (350-700 bar) in carbon-fiber reinforced composite tanks.
2. Physical Storage (Liquid): Cryogenic storage at -253C. High energy density but requires significant energy for cooling.
3. Chemical Storage (Solid State):
   • Metal Hydrides: Hydrogen bonds with metals (e.g., ) and is released upon heating.
   • Chemical hydrides: Using carriers like ammonia or methanol.

Safety Aspects:

1. Flammability: Hydrogen has a very wide flammability range (4% - 75% in air).
2. Leakage: molecules are tiny and can leak through seals or embrittle metals (Hydrogen Embrittlement).
3. Detection: Hydrogen flames are nearly invisible in daylight, making visual detection of fires difficult. Sensors are mandatory.

Principle of Nuclear Fission:
Nuclear fission is the process where the nucleus of a heavy atom splits into two or more smaller nuclei, releasing a tremendous amount of energy. This energy release is due to the mass defect (), calculated by Einstein's equation .

Reaction:
When a slow-moving neutron strikes a Uranium-235 nucleus, it becomes unstable and splits:

Role of Uranium-235:

• is the fissile isotope.
• It captures the neutron to initiate the reaction.
• The reaction releases 2 to 3 excess neutrons, which can strike other nuclei, creating a Chain Reaction. Controlled chain reactions are the basis of nuclear power plants.

Sustainable Nuclear Energy:
It refers to using nuclear power in a way that ensures fuel availability for the long term while minimizing waste and environmental harm. Current Once-Through cycles generate significant high-level waste.

Breeder Reactors:

• Concept: A breeder reactor produces more fissile material than it consumes.
• Mechanism: It uses Fast Neutrons to convert fertile material (non-fissile or ) into fissile material ( or ).

Reaction (Uranium-Plutonium Cycle):

Contribution to Sustainability:

1. Increases the energy potential of natural uranium by ~60 times (utilizing the abundant ).
2. Reduces the volume and radiotoxicity of long-lived nuclear waste.

Spintronics (Spin Transport Electronics):
It is an emerging field of nanoscale electronics involving the detection and manipulation of the electron spin (intrinsic angular momentum) in addition to its charge.

Difference from Conventional Electronics:

1. Giant Magnetoresistance (GMR):

• Principle: A significant change in electrical resistance occurs in thin-film alternating ferromagnetic and non-magnetic layers depending on whether the magnetization of the magnetic layers is parallel or anti-parallel.
• Application: Hard Disk Drive (HDD) Read Heads. GMR sensors allow for much higher storage density on hard disks by detecting smaller magnetic domains with high sensitivity.

2. Magnetoresistive Random Access Memory (MRAM):

• Principle: Uses magnetic tunnel junctions (MTJ) to store data as spin states rather than electric charge.
• Application: Universal Memory. It combines the speed of SRAM, the density of DRAM, and the non-volatility of Flash memory.

3. Other Applications:

• Spin Transistors: For faster, lower-power logic gates.
• Quantum Computing: Electron spin is a candidate for Qubits.

Direct Methanol Fuel Cell (DMFC):
DMFCs are a subcategory of Proton Exchange Membrane (PEM) fuel cells where liquid methanol () is used as the fuel directly without prior reforming.

Working:
Liquid methanol is oxidized at the anode in the presence of water to produce , releasing protons () and electrons. Protons pass through the membrane, electrons pass through the external circuit.

Reactions:

• Anode:
• Cathode:
• Overall:

Advantages: High energy density of liquid fuel, easy storage/transport compared to Hydrogen. Used in portable electronic devices.

Given:

• Mass of fuel () = 0.95 g
• Mass of water () = 1500 g
• Water equivalent () = 500 g
• Rise in temp () = 2.8C
• Hydrogen () = 5%
• Latent heat () = 587 cal/g

1. Calculate Gross Calorific Value (GCV):
Formula:

2. Calculate Net Calorific Value (NCV):
Formula:

Answer:

• GCV = 5894.74 cal/g
• NCV = 5630.59 cal/g

Classification of Fuels:
Fuels are classified based on:

1. Occurrence: Primary (Natural) vs. Secondary (Artificial).
2. Physical State: Solid, Liquid, Gaseous.

Comparison: