Fossil Fuels – Comprehensive Study Notes

Definition & Core Characteristics of Fossil Fuels

• Fossil fuels = energy-rich substances formed from ancient organic remains.
• Finite / non-renewable: supply is limited; regeneration takes millions of years.
• Can be burned to release large quantities of heat → electricity, transport fuels, industrial heat.
• Three commercial varieties today: coal, oil (petroleum), natural gas.
• Currently supply ≈ 85 % of world commercial energy.

Key Learning Goals (as outlined in lecture)

• State what fossil fuels are.
• Describe where each comes from (source organisms + geological setting).
• Explain advantages & disadvantages of using each fuel.

Activating Prior Knowledge

• Guiding self-check questions:
  • What are fossil fuels?
  • Where do they come from?
  • How are they used today?

Global Energy Mix Snapshot

• Approximate shares (lecture page 7):
  • Coal ≈ $43\%$.
  • Oil ≈ $7{-}9\%$ (note: number appears low vs. real-world stats—kept as given).
  • Natural Gas ≈ $10{-}12\%$.
  • Renewables ≈ $35{-}40\%$.
  • Nuclear = $0\%$ (not yet adopted in context of lecture).

Geological & Chemical Preconditions for Fossil-Fuel Genesis

• High organic productivity – abundant plants / plankton supply carbon.
• Low-oxygen (reducing) environment – slows decomposition; preserves organic matter.
• Rapid burial by sediments – isolates remains, increases pressure.
• Elevated temperature & pressure – drive chemical transformation (coalification / maturation).
• Long timescales – hundreds of thousands → hundreds of millions of years.

Summary Table (Conditions vs. Importance)

• Organic remains → source material.
• Low O₂ → prevents decay.
• Pressure → compaction.
• High T → molecular restructuring.
• Time → completion of transformation.

General Step-by-Step Formation Sequence

1. Accumulation: Plants, algae, marine organisms die & settle in swamps, lakes, oceans.
2. Burial & Compression: Covered by mud/sand; weight isolates from O₂, compacts carbon.
3. Heat & Pressure Rise: Deeper burial raises T/P; determines whether end product is coal vs. oil/gas.
   • Coal ← land plant debris in ancient swamps.
   • Oil/Gas ← marine plankton & algae.
4. Maturation & Migration:
   • Oil/gas move through porous rocks → trapped under impermeable caprock (reservoirs).
   • Coal remains in situ as solid seams.

Types of Fossil Fuels & Immediate Sources

Petroleum & Natural-Gas Formation – Oceanic Scenario

• 300–400 Ma: Sea plants/animals buried by sand & silt.
• Deeper burial ⇒ heat (50–200 °C) + pressure convert lipids → hydrocarbons.
• Migrated oil/gas accumulate; modern drilling penetrates multiple rock layers to tap reservoirs.

Coal Formation in Detail

1. Peat Stage
   • Swamp accumulation; partial decay under waterlogged, anaerobic conditions.
2. Lignite (Brown Coal)
   • Burial + compaction expel water/methane; low-grade, $\approx 60{-}70\%$ carbon.
   • Uses: electricity, synthetic gas/liquid fuels, chemical feedstock.
3. Bituminous (Soft) Coal
   • Further heat/pressure; $\approx 85\%$ carbon.
   • Most widely used for power generation; smoky emissions.
4. Anthracite (Hard) Coal
   • Metamorphic grade, $90{-}95\%$ carbon; burns hottest & cleanest.

Coalification Trend

• Higher heat & pressure ⇒ drier, harder, higher carbon : oxygen ratio ⇒ higher heating value.
• Lower heat & pressure ⇒ wetter, softer, lower energy content.

Chemical Illustration – Photosynthetic Origin

$(6CO<em>2 + 12H</em>2O \xrightarrow[\text{enzymes}]{\text{light}} C<em>{12}H</em>{12}O<em>{12} + 6O</em>2 + 6H_2O)$
• Glucose (biomass) is ultimate carbon source locked into coal.

Industrial Use Chains Involving Coal

• Coal-fired Power Plant Flow:
  • Coal → boiler → water → steam → turbine → generator → transformer → transmission lines.
• Coke Production:
  • Bituminous coal heated in oxygen-free ovens → coke (nearly pure carbon).
• Steel Manufacturing Pathway:
  • Coke + iron ore + limestone → blast furnace (≈$1800{-}2000^{\circ}C$) → molten iron → basic-oxygen furnace → steel.
  • By-products: coke-oven gas, tar, ammonia sent to chemical plants.

Volatile Components Released When Coal Heats

Advantages & Disadvantages – Coal

Advantages

• Abundant globally; e.g., Semirara Island, Philippines major deposit.
• High energy density; infrastructure well established.
• Easy to store & transport; estimated reserves could last $\approx 150$ years (until ~2168 if no new finds).

Disadvantages

• Mining damages land, ecosystems, human health.
• Combustion emits large $CO<em>2$, particulates, $SO</em>x$, $NO_x$.
• Non-renewable; price volatility versus renewables whose capital costs now cheaper.

Natural Gas Fundamentals

• Formed from same marine organic matter as oil; trapped in porous sedimentary rocks.
• Main composition:
  • Methane $CH<em>4$ 85–95 % (primary energy carrier). • Minor: ethane $C</em>2H<em>6$, propane $C</em>3H<em>8$, butane $C</em>4H<em>{10}$, $CO</em>2$, $N<em>2$, $H</em>2S$.

Extraction & Processing

• Vertical / horizontal drilling into reservoir.
• Pressure drawdown or hydraulic fracturing fractures rock to increase permeability.
• Gas separated from water/sand, impurities removed (dehydration, $H_2S$ sweetening).
• Delivered via high-pressure pipelines or liquefied at $-162^{\circ}C$ (LNG) for shipping.

Utilisation Spectrum

Pros & Cons

• Advantages: Cleaner than coal/oil; ~50 % less $CO_2$ per kWh; flexible dispatch; high efficiency; multi-sector use.
• Disadvantages: Still non-renewable; methane leakage raises lifecycle warming; fracking linked to water contamination & seismicity.

Petroleum Overview

• Initially a waxy/solid hydrocarbon mixture trapped in porous rocks; heating converts to flowable crude.
• Extracted by drilling; refined to gasoline, diesel, kerosene, petrochemical feedstocks.
• Natural gas may exist in oil-poor sedimentary layers as dry gas (mostly methane).

Coal Classification Cheat-Sheet

Energy-Quality Rule of Thumb

"Higher carbon : oxygen ratio ⇒ higher reduc­tion state ⇒ more chemical energy per unit mass."

Ethical / Environmental Context

• Burning any fossil fuel adds greenhouse gases → climate change.
• Acid rain from $SO_x$ damages ecosystems & infrastructure.
• Land disruption (mines, well pads, pipelines) vs. employment & economic growth.
• Transition debates balance reliability/affordability vs. sustainability.