Energy Sources - Comprehensive Notes

ENERGY SOURCES

  • Fuel contains mainly carbon ('C') which undergoes combustion, liberating large amounts of heat energy.

  • Fuel + Combustion -> Heat energy + Light energy + Combustion products

  • C + O2 \rightarrow CO2 + \text{heat}

  • Combustion is burning a substance (in the presence of O_2) or oxidation of a compound; it's exothermic

CLASSIFICATION OF FUELS

  • Primary or Natural Fuels
    • Solid: Wood, coal, lignite
    • Liquid: Crude oil
    • Gaseous: Natural gas
  • Secondary or Derived Fuels
    • Solid: Coke, charcoal, petroleum
    • Liquid: Petrol, kerosene, diesel
    • Gaseous: Coal gas, water gas, biogas

Characteristics of a good fuel

  • High Calorific Value
  • Moderate ignition temperature
  • Low moisture content
  • Low non-combustible matter
  • Combustion products should not be harmful
  • Low cost
  • Easy to transport
  • Should undergo spontaneous combustion
  • Should leave less carbon residue

SOLID FUELS

  • Coal: Found in the Earth's crust; formed from plants subjected to high temperature and pressure, and bacterial action.

  • Coal Classification: WOOD -> PEAT -> LIGNITE -> BITUMINOUS COAL -> ANTHRACITE COAL

  • Coalification: Process of conversion of wood to coal

  • Carbonification: Process of conversion of wood to charcoal

  • Charcoal: Used to absorb gases and for discoloration of sugar.

Ranking of Coal (Based on Carbon, Hydrogen, Moisture, Calorific Value)

  • Dry Wood

    • Carbon: 48-50 %
    • Oxygen: 42-44 %
    • Hydrogen: 5-6%
    • Traces of minerals
    • Calorific value: 4000-4500 Kcal /Kg

  • FUEL % of Carbon Calorific Value ( Kcal) Main application

  • Wood 50 4000-4500 Domestic Fuel

  • Peat 50-60 4125-5400 If deficiency of high rank coal is prevailing

  • Lignite 60-70 6500-7000 Steam generation in thermal power plant , production of producer gas

  • Bituminous 80-90 8000-8500 Making of coal gas , metallurgic coke , steam generation

  • Anthracite 90-98 8650-8700 Households , metallurgic where no smoke and high heat is required

Analysis of Coal

  • Helps in ranking, price fixation, commercial classification, industrial utilization, etc.
  • The quality of coal can be analyzed via proximate and ultimate analysis

Proximate Analysis

  • Determines moisture, volatile matter, ash, and fixed carbon.

  • Determination of Moisture Content

    • A known amount of powdered coal is taken in a crucible.
    • The crucible is placed in an electric hot air-oven at 105^o to 110^oC for 1 hour.
    • The crucible is then cooled and weighed.
    • The weight loss provides information about moisture.
    • \text{% of moisture} = \frac{\text{Weight loss due to moisture}}{\text{Weight of sample}} \times 100
    • Lesser moisture content indicates better fuel quality.

  • Determination of Volatile Matter

    • The moisture-free coal sample is taken in a crucible, covered with a lid, and placed in a muffle furnace at 950^oC for 7 minutes.
    • Then cooled and weighed again.
    • Loss in weight indicates the presence of volatile matter.
    • \text{% of volatile matter} = \frac{\text{Loss in weight due to volatile matter}}{\text{Weight of sample}} \times 100
    • Low quantity of volatile matter indicates better coal quality.

  • Determination of Ash Content

    • The residual coal sample is heated without a lid in a muffle furnace (in the presence of air) at 750^oC for an hour.
    • The process of heating, cooling, and weighing are repeated until a constant weight is obtained.
    • \text{% of ash} = \frac{\text{Weight of ash}}{\text{Weight of sample}} \times 100
    • Ash is a non-combustible substance that reduces the calorific value; low ash content is desirable.

  • Determination of Fixed Carbon: Determined indirectly.

    • \text{% of fixed carbon} = 100 - (\text{% of moisture} + \text{% of volatile matter} + \text{% of ash})
    • Greater calorific value corresponds to a higher percentage of fixed carbon.

Ultimate Analysis

  • Measures C, H, N, S, and O.
  • Carbon and Hydrogen: Coal sample is burnt in oxygen, converting C and H into CO2 and H2O respectively.
    • The gaseous products are absorbed in KOH and CaCl_2 tubes of known weights.
    • C + O2 \rightarrow CO2
    • 2KOH + CO2 \rightarrow K2CO3 + H2O
    • H2 + \frac{1}{2}O2 \rightarrow H_2O
    • CaCl2 + 7H2O \rightarrow CaCl2 \cdot 7H2O
    • \% \text{ of Carbon } = \frac{\text{Increase in weight of KOH tube}}{\text{Weight of coal sample}} \times \frac{12}{44} \times 100
    • \% \text{ of Hydrogen } = \frac{\text{Increase in weight of } CaCl_2 \text{ tube}}{\text{Weight of coal sample}} \times \frac{2}{18} \times 100
  • Determination of Nitrogen
    • Powdered coal is heated with concentrated H2SO4 and K2SO4 (catalyst) in Kjeldahl’s flask.
    • Nitrogen is converted into ammonium sulphate.
    • Ammonium sulphate is treated with excess NaOH to liberate ammonia.
    • The liberated ammonia is distilled and absorbed in a known volume of standard (N/10) H2SO4 solution.
    • N2 + H2SO4 \rightarrow (NH4)2SO4
    • NaOH + (NH4)2SO4 \rightarrow 2Na2SO4 + 2NH3 + 2H_2O
    • 2NH3 + H2SO4 \rightarrow (NH4)2SO4
    • The volume of unused H2SO4 is determined by titrating against standard NaOH solution (N/10).
  • Calculate amount of H2SO4 required to neutralize ammonia:
    • Amount of acid = N/10 V1 – N/10 V2 = 0.1 (V1 – V2) mili equivalents
    • Where V1 = Volume of standard H2SO4 (N/10) solution and V2 = Volume of standard NaOH (N/10) solution
    • \% \text{ of Nitrogen } = 0.1 (V1 – V2) \times \frac{14}{W}
  • Determination of Sulphur: Coal sample is burnt in a bomb calorimeter in the presence of oxygen.
    • Sulphur is converted into SO2 and SO3.
    • The ash is extracted with dil. HCl.
    • Acid extracts are treated with Barium chloride solution to precipitate sulphates as Barium sulphate.
    • Precipitate is filtered, washed, dried, and heated to constant weight.
    • S \rightarrow BaSO_4
    • Weight of sulphur = \frac{32}{233} \times \text{weight of } BaSO_4
    • \% \text{ of sulphur} = \frac{\text{Weight of sulphur}}{\text{Weight of coal sample}} \times 100
  • Determination of Ash: Percentage of ash is calculated by the method in proximate analysis.
  • Determination of Oxygen: Calculated by subtracting the sum of total % of carbon, hydrogen, nitrogen, sulphur, and ash from 100.
    • \% \text{ of Oxygen} = 100 – [\% \text{carbon} + \% \text{hydrogen} + \% \text{nitrogen} + \% \text{sulphur} + \% \text{ash}]

Liquid Fuels

  • Ex: Petroleum (Crude Oil)
  • Composition:
    • Carbon: 80-85%
    • Hydrogen: 11-15%
    • Sulphur: 0.1-3.5%
    • Nitrogen: 0.4-0.9%
    • Oxygen: 0.1-0.9%
  • Origin of Petroleum:
    • Carbide theory/Mendeleev’s theory (inorganic theory of petroleum)
      • Ca + 2C \rightarrow CaC_2 (high t)
      • 4Al + 3C \rightarrow Al4C3
      • CaC2 + 3H2O \rightarrow Ca(OH)2 + C2H_2
      • Al4C3 + 12H2O \rightarrow 4Al(OH)3 + 3CH_4
    • Engler’s theory: Organic matter, animals, vegetation, and marine animals died and accumulated in the sea, decomposing under high temperature to give petroleum.

Refining of Petroleum

  • Crude oil contains soluble and insoluble impurities that must be removed.

  • Refining is the process of separating crude oil into various useful fractions by fractional distillation and converting these into desired products.

  • The industry where refining occurs is called an oil refinery.

  • Stages Involved in Refining of Petroleum:

    • Removal of sulphur compounds: Treated with copper oxide to convert sulphur into insoluble copper sulphide, which is removed by filtration.
    • Removal of water (Cottrell’s process): Crude oil passed between two charged electrodes to destroy emulsion films, causing water droplets to coalesce and separate from the oil.
    • Fractional distillation: Heating crude oil to around 400^oC in an iron retort produces hot vapours passed through a fractionating column.
      • The column is a tall cylindrical tower with horizontal stainless trays at short distances with small chimneys.
      • Vapours cool gradually as they go up, causing fractional condensation.
      • Higher boiling fractions condense first later the lower boiling fractions.

  • Fraction Name, Boiling range, Approximate composition, Uses

    • Uncondensed gas Below 30^oC C1 to C4 A domestic or industrial fuel
    • Petroleum ether 30^oC - 70^oC C5-C7 As a solvent.
    • Gasoline or petrol 40^oC -120^oC C5-C9 As motor fuel, solvent, in dry cleaning
    • Naphtha or solvent spirit 120^oC -180^oC C9-C10 As solvent, in dry cleaning
    • Kerosene 180^oC - 250^oC C10-C16 As an illuminant ,Engine fuel
    • Diesel oil 250^oC - 320^oC C10-C18 Diesel engine fuel
    • Heavy oil 320^oC - 400^oC C17-C30 Gasoline by cracking
    • Lubricating oil As lubricant
    • Petroleum jelly As lubricant and in cosmetics and ointments
    • Paraffin wax In candles, boot polishes
    • Greases As lubricant
    • Residue (asphalt, petroleum coke) Used for making tar roads, water proof roof

Cracking

  • The process of breaking and converting higher molecular weight long chain hydrocarbons with high boiling points to lower molecular weight hydrocarbons with low boiling points.
  • Catalytic cracking:
    • Fixed bed catalytic cracking
    • Fluid bed catalytic cracking or moving bed catalytic cracking
  • Fluid bed catalytic Cracking
    • The heavy oil is heated first passed through a preheater upto 425-450^oC
    • The preheated oil vapours are then passed through finely powdered catalyst maintained a temperature of 500^oC.
    • Cracked oil Vapours are passed to fractionating column.
    • The vapors condenses heavy oil is separated.
    • This is then sent to dissloved gases
    • a cooler where where heavy oil is separated.