Unit 6: Energy sciences - Subjective Questions

Calorific value is the total amount of heat liberated when a unit mass (or volume) of a fuel is completely burnt in the presence of oxygen. It is expressed in (solid/liquid fuels) or (gaseous fuels).

Gross Calorific Value (GCV) / Higher Calorific Value (HCV):

• The total heat generated when a unit quantity of fuel is completely burnt and the combustion products are cooled to room temperature.
• Includes the latent heat of condensation of water vapour formed during combustion.

Net Calorific Value (NCV) / Lower Calorific Value (LCV):

• The heat produced when the combustion products are not cooled, i.e., the water vapour escapes with the flue gases.
• Latent heat of steam is not recovered.

Relationship:


where = percentage of hydrogen in the fuel and is the latent heat of steam.

Principle: A known mass of fuel is burnt completely in excess oxygen inside a closed vessel (bomb). The heat liberated is absorbed by a surrounding known mass of water, and the rise in temperature is used to calculate the calorific value.

Construction:

• A stainless steel bomb capable of withstanding high pressure.
• A crucible to hold the fuel sample, connected to electrodes with a fuse wire.
• The bomb is immersed in a known mass of water inside a copper calorimeter.
• Surrounded by an air jacket and water jacket to prevent heat loss.
• Fitted with a Beckmann thermometer and an electric stirrer.

Working:

• A known mass of fuel is placed in the crucible and the bomb is filled with oxygen at ~25 atm.
• The fuel is ignited electrically; heat released raises the water temperature.
• Initial and final temperatures are noted.

Calculation:
Let:

• = mass of fuel (g)
• = mass of water in calorimeter
• = water equivalent of apparatus
• = initial and final temperatures

Net Calorific Value:

where is the percentage of hydrogen in the fuel.

Proximate analysis determines the practical composition of coal by measuring four parameters. It is useful for assessing the commercial quality of coal.

1. Moisture content:

• Coal is heated at for 1 hour; loss in weight gives moisture.
• Significance: High moisture reduces calorific value and wastes heat during evaporation. Lower moisture is desirable.

2. Volatile matter:

• The dried coal is heated at for 7 minutes in absence of air; loss in weight (excluding moisture) gives volatile matter.
• Significance: High volatile matter causes long flames, more smoke and lower calorific value. Lower volatile matter is preferred.

3. Ash content:

• The residue after burning coal at gives ash.
• Significance: Ash reduces calorific value, increases handling/disposal costs and causes clinker formation. Lower ash is desirable.

4. Fixed carbon:

• Significance: Higher fixed carbon means higher calorific value and better quality coal.

Ultimate analysis determines the elemental composition of coal (C, H, N, S, O) and is used to establish the chemical formula and calculate theoretical calorific value.

Elements determined:

• Carbon and Hydrogen: Determined by burning a known mass of coal in oxygen. and produced are absorbed in KOH and respectively.
  
  
• Nitrogen: Determined by Kjeldahl's method.
• Sulphur: Determined from washings of bomb calorimeter as .
• Oxygen: Estimated by difference.
  

Difference from proximate analysis:

Primary Cells:

• Cells in which the cell reaction is irreversible.
• Cannot be recharged; used once and discarded.
• The chemical energy is converted to electrical energy until the reactants are exhausted.
• Examples: Dry cell (Leclanché cell), mercury cell, alkaline battery.

Secondary Cells:

• Cells in which the cell reaction is reversible.
• Can be recharged by passing current in the opposite direction.
• Act as energy storage devices (storage cells).
• Examples: Lead-acid battery, Ni-Cd battery, Li-ion battery.

Comparison Table:

The lead-acid battery is a widely used secondary (rechargeable) battery.

Construction:

• Anode: Spongy lead () plates.
• Cathode: Lead dioxide () plates.
• Electrolyte: Dilute sulphuric acid (, ~38%).
• Several cells (each ~2V) connected in series give a 6V or 12V battery.

Discharging reactions:

At anode (oxidation):

At cathode (reduction):

Overall:

Charging reactions: The reactions reverse; is converted back to and :

Applications:

• Automobiles (car batteries) for starting, lighting and ignition.
• Inverters and UPS systems.
• Backup power for telephone exchanges and power stations.

The Ni-Cd battery is a rechargeable secondary battery.

Construction:

• Anode: Cadmium ().
• Cathode: Nickel oxide-hydroxide ().
• Electrolyte: Alkaline solution of .

Electrode reactions (discharge):

At anode:

At cathode:

Overall:

Cell voltage is about 1.2 V.

Advantages:

• Long service life and many charge/discharge cycles.
• Good performance at low temperatures.
• Can be recharged rapidly.

Disadvantages:

• Cadmium is toxic and causes environmental pollution.
• Suffers from memory effect (loss of capacity if repeatedly recharged before full discharge).
• Lower energy density than Li-ion.

The Ni-MH battery is a rechargeable battery that uses a hydrogen-absorbing alloy instead of cadmium.

Construction:

• Anode (negative): Hydrogen-absorbing metal alloy (, where = LaNi type alloy).
• Cathode (positive): Nickel oxide-hydroxide .
• Electrolyte: (alkaline).

Electrode reactions (discharge):

At anode:

At cathode:

Overall:

Cell voltage ~1.2 V.

Comparison with Ni-Cd:

Applications: Hybrid vehicles, cameras, portable electronics.

The Li-ion battery works on the principle of intercalation — the reversible insertion and removal of lithium ions between electrodes.

Construction:

• Anode: Graphite (carbon) which intercalates lithium.
• Cathode: Lithium metal oxide, e.g., .
• Electrolyte: Lithium salt (e.g., ) in an organic solvent.
• A separator prevents short circuit while allowing ion flow.

Electrode reactions:

At cathode (discharge):

At anode (discharge):

During charging, ions move from cathode to anode; during discharging, they move back. Cell voltage is ~3.7 V.

Why preferred in portable electronics:

• High energy density (light and compact).
• No memory effect.
• Low self-discharge rate.
• High cell voltage and long cycle life.
• Used in mobile phones, laptops, EVs.

The Li-air battery is a metal-air battery that uses oxygen from the atmosphere as the cathode reactant, giving it a very high theoretical energy density.

Principle: Lithium metal at the anode reacts with oxygen (drawn from air) at a porous carbon cathode to form lithium peroxide.

Electrode reactions (discharge):

At anode:

At cathode:

Overall:

Advantages:

• Extremely high theoretical energy density (comparable to gasoline), since oxygen is not stored but taken from air.
• Lightweight; potential for long-range electric vehicles.

Challenges / Disadvantages:

• Poor cycle life due to unstable discharge products.
• Sensitivity to moisture and in air.
• Sluggish reaction kinetics needing catalysts.
• Safety concerns with reactive lithium metal.
• Still largely in research stage.

A fuel cell is an electrochemical device that continuously converts the chemical energy of a fuel and an oxidant directly into electrical energy, as long as the reactants are supplied.

Principle: Unlike a battery, a fuel cell does not store energy; it generates electricity by the controlled oxidation of a fuel (like ) at the anode and reduction of oxygen at the cathode.

Hydrogen-Oxygen Fuel Cell:

• Anode: Porous carbon electrode fed with hydrogen.
• Cathode: Porous carbon electrode fed with oxygen.
• Electrolyte: Aqueous .

Electrode reactions:

At anode (oxidation):

At cathode (reduction):

Overall:

The only by-product is water, making it clean.

Applications: Space vehicles (Apollo missions), submarines, backup power, and electric vehicles.

Advantages of Fuel Cells:

• High efficiency (60-80%) compared to conventional combustion engines.
• Low pollution — hydrogen fuel cells produce only water as by-product.
• No moving parts — silent operation and low maintenance.
• Continuous power supply as long as fuel is provided.
• Modular and scalable for various power requirements.
• No electrical recharging required.

Disadvantages of Fuel Cells:

• High initial cost due to expensive catalysts (platinum).
• Storage and handling of hydrogen is difficult and hazardous.
• Electrodes and catalysts are prone to poisoning by impurities.
• Limited infrastructure for hydrogen supply.
• Lower power density compared to some batteries.

Applications:

• Spacecraft, submarines, stationary power generation, and fuel cell vehicles.

A fuel is a combustible substance that, on burning in the presence of oxygen, produces a large amount of heat that can be used economically for domestic and industrial purposes.

Classification based on occurrence:

1. Primary (Natural) fuels: Found in nature as such.

• Solid: Wood, coal, peat.
• Liquid: Crude petroleum.
• Gas: Natural gas.

2. Secondary (Derived) fuels: Obtained from primary fuels by processing.

• Solid: Coke, charcoal.
• Liquid: Petrol, diesel, kerosene.
• Gas: Coal gas, water gas, LPG.

Classification based on physical state:

Characteristics of a good fuel: High calorific value, low moisture and ash, moderate ignition temperature, low cost, and easy availability.

Hydrogen is a clean energy carrier and can be produced by several methods:

1. Steam reforming of natural gas (Methane): Most common industrial method.

2. Electrolysis of water: Passing electric current through water splits it into hydrogen and oxygen. Produces green hydrogen if renewable electricity is used.

3. Coal gasification: Coal reacts with steam to give water gas.

4. Thermochemical / Photolytic methods: Water is split using heat from solar/nuclear sources or by photocatalysis.

5. Biological methods: Using algae and bacteria (bio-hydrogen).

Colour coding:

• Green hydrogen — from renewable electrolysis.
• Grey hydrogen — from fossil fuels.
• Blue hydrogen — fossil fuels with carbon capture.

Hydrogen storage is a major challenge because of its low density and high flammability.

Methods of storage:

1. Compressed gas storage: Hydrogen stored in high-pressure cylinders (350-700 bar). Simple but requires strong, heavy tanks.

2. Liquid hydrogen storage: Hydrogen liquefied at and stored in cryogenic tanks. High energy density but expensive due to cooling and boil-off losses.

3. Metal hydride storage: Hydrogen absorbed into metal alloys (e.g., , ) forming hydrides, released on heating. Safe and compact.

4. Adsorption storage: Hydrogen adsorbed on porous materials like carbon nanotubes, MOFs, zeolites.

Safety aspects:

• Hydrogen is highly flammable with a wide flammability range (4-75%).
• Colourless, odourless flame — hard to detect.
• Small molecule size causes leakage through seals.
• Requires good ventilation, leak detectors, and flame arrestors.
• Storage tanks must resist hydrogen embrittlement of metals.

Hydrogen is considered the fuel of the future due to the following reasons:

1. Clean combustion:

• On burning, hydrogen produces only water as a by-product with no , , or particulate emissions.
  

2. High calorific value:

• Hydrogen has the highest calorific value per unit mass (~), about three times that of gasoline.

3. Renewable and sustainable:

• Can be produced from water using renewable energy (solar, wind) — inexhaustible source.

4. Versatile use:

• Used in fuel cells, internal combustion engines, and industrial processes.

5. Energy carrier:

• Stores and transports energy generated from intermittent renewable sources.

Significance for sustainability:

• Reduces dependence on fossil fuels.
• Helps achieve carbon neutrality and combat climate change.
• Key element of the emerging hydrogen economy.

Nuclear energy is the energy released from the nucleus of an atom during nuclear reactions. It arises from the conversion of a small amount of mass into a large amount of energy, according to Einstein's equation:

Nuclear Fission:

• The splitting of a heavy nucleus (e.g., ) into two smaller nuclei on bombardment with neutrons, releasing energy and more neutrons (chain reaction).
  

Nuclear Fusion:

• The combining of two light nuclei (e.g., hydrogen isotopes) to form a heavier nucleus, releasing enormous energy. Requires very high temperature.
  

Comparison:

Nuclear energy is produced in nuclear reactors through controlled fission chain reactions. The heat generated produces steam that drives turbines to generate electricity.

Components of a nuclear reactor:

• Fuel: or .
• Moderator: Slows down neutrons (graphite, heavy water).
• Control rods: Absorb excess neutrons (cadmium, boron) to control reaction rate.
• Coolant: Removes heat (water, liquid sodium).
• Shielding: Protects against radiation.

Sustainability aspects:

• Produces large energy from a small amount of fuel.
• Low greenhouse gas emissions during operation.
• Reliable base-load power independent of weather.

Advantages:

• High energy density; small fuel quantity.
• Low carbon footprint.
• Continuous and reliable power.

Disadvantages:

• Radioactive waste disposal problem.
• Risk of nuclear accidents (Chernobyl, Fukushima).
• High capital cost and long construction time.
• Risk of misuse for weapons.

Spintronics (spin transport electronics) is a branch of electronics that exploits the intrinsic spin of electrons and its associated magnetic moment, in addition to their charge, to store and process information.

Basic Principle:

• Conventional electronics uses only the charge of electrons.
• Spintronics uses the electron's spin state (spin-up or spin-down), which can represent binary data (0 and 1).
• A key phenomenon is Giant Magnetoresistance (GMR) — the large change in electrical resistance depending on the relative alignment of magnetic layers.

Engineering Applications:

• Magnetic Read Heads in hard disk drives (based on GMR) for high-density data storage.
• Magnetic Random Access Memory (MRAM) — non-volatile, fast memory that retains data without power.
• Spin transistors and logic devices for low-power computing.
• Magnetic sensors for automotive and industrial use.
• Potential use in quantum computing (spin qubits).

Advantages:

• Lower power consumption, non-volatility, higher speed and higher data density than conventional electronics.

Dulong's formula for Gross Calorific Value (GCV):

Given: , , , .

Step 1 — Calculate available hydrogen:

Step 2 — Substitute into Dulong's formula:

Step 3 — Calculate Net Calorific Value (NCV):

Result:

• GCV = 8189 kcal/kg
• NCV = 7872.02 kcal/kg