Fossil Fuels and Biofuels – Comprehensive Study Notes

Fossil Fuels

• Formed by anaerobic decomposition of dead organisms over millions of years.

  • Geological time-scale formation ⇒ classified as non-renewable.

  • Current human consumption rate ≫ natural formation rate ⇒ eventual depletion.

• Key practical benefit

  • High energy density → “energy efficient”.

  • Historically cheap & affordable compared with most alternatives.

Environmental & Health Implications of Fossil Fuels

• Greenhouse-gas emissions (mainly $CO_2$) → global warming.

• $CO<em>2$ dissolves in water forming carbonic acid ( $CO</em>2 + H<em>2O \rightarrow H</em>2CO_3$ ) ⇒ ocean acidification and acidification of other water bodies.

• Combustion releases particulates (soot) → respiratory damage, lung diseases.

Biofuels – General Overview

• Definition: Fuel derived from biomass (plant or animal material).

• Regrowth of biomass occurs on human time-scale ⇒ renewable & sustainable.

• Two mainstream types discussed

  • Bioethanol

  • Biodiesel

• Shared properties

  • Biodegradable: spills are metabolised by microorganisms, reducing persistent pollution.

  • Labeled “carbon-neutral” in theory: $CO<em>2$ released on combustion ≅ $CO</em>2$ taken up by new biomass via photosynthesis.

Production of Bioethanol (focus of this video)

• Prior lecture covered three synthetic routes: hydration of alkenes, substitution of halogenoalkanes, fermentation of glucose.

• This session focuses on fermentation because it starts from a biomass feedstock (glucose from crops).

  • Simplified equation: $C<em>6H</em>{12}O<em>6 \xrightarrow{yeast} 2 \, C</em>2H<em>5OH + 2 \, CO</em>2$.

  • Conditions: yeast enzyme, $\approx 30-35\,^{\circ}C$ (higher T denatures enzyme) → inherently slow rate.

  • Product mixture then needs fractional distillation to isolate ethanol, an energy-intensive step.

Production of Biodiesel

• Feedstock: vegetable or animal oil (triglycerides).

• Reaction: base-catalysed transesterification (ester hydrolysis) with an alcohol (commonly ethanol or methanol).

  • Will be explored in detail in the esters lecture.

• Outcome: mixture of fatty-acid esters (biodiesel) + glycerol by-product.

Advantages of Biofuels

• Renewable & sustainable supply chain compared with fossil fuels.

• Biodegradability reduces long-term environmental damage from spills.

• Lower particulate emissions → reduced incidence of soot-related lung disease.

• Intrinsic oxygen content of molecules → more complete combustion with less external $O_2$ required.

  • Example stoichiometry

  • Diesel: $C<em>{12}H</em>{23} + 27.5\,O<em>2 \rightarrow 12\,CO</em>2 + 11.5\,H_2O$.

  • Biodiesel (representative formula): $C<em>{19}H</em>{34}O<em>2 + 20.5\,O</em>2 \rightarrow 19\,CO<em>2 + 17\,H</em>2O$.

  • Lower $O_2$ demand ⇒ reduced risk of incomplete combustion → less soot & $CO$.

Energy & Combustion Data Comparison

• Molar enthalpy of combustion (ΔHc)

  • Octane (fossil diesel surrogate): $\Delta H_c = -5460\ \text{kJ mol}^{-1}$.

  • Ethanol: $\Delta H_c = -1370\ \text{kJ mol}^{-1}$.

• Gravimetric energy density

  • Diesel: $45\ \text{kJ g}^{-1}$.

  • Biodiesel: $42\ \text{kJ g}^{-1}$.

• Implication: Biofuels deliver less energy per unit mass or mole ⇒ lower fuel economy.

Practical Limitations of Biofuels in Engines

• Most internal-combustion engines incompatible with 100 % biofuel.

  • Typical retail blend: E10 ( $10\%$ ethanol + $90\%$ petrol by volume ).

  • 100 % ethanol fuels available in limited regions (e.g., Brazil).

Economic, Social & Environmental Drawbacks of Biofuels

• Slow production (fermentation) → higher cost; low-temperature enzymatic process cannot be speeded up significantly by heat.

• Energy-intensive distillation often powered by fossil fuels ⇒ erodes carbon-neutral claim.

• Land-use competition

  • Large areas required for energy crops.

  • Diverts arable land from food to fuel ⇒ drives up food prices.

• Agricultural inputs

  • High fertiliser demand.

  • Consequences: soil erosion, eutrophication, land & water pollution.

• Thus, while tail-pipe emissions decrease, upstream agriculture & processing introduce indirect environmental impacts.

Ethical & Philosophical Considerations

• Food vs Fuel dilemma: prioritising mobility for wealthier populations over global food security.

• True carbon neutrality questioned when cradle-to-grave life-cycle emissions (fertilisers, machinery, distillation energy) are included.

• Balance between technological readiness and sustainability imperatives remains contentious.

Connections to Other Lectures

• Will further expand on:

  • Ester chemistry to elucidate biodiesel transesterification mechanism.

  • Industrial ethanol synthesis (hydration, substitution), contrasting fossil-derived vs biomass-derived pathways.

• Builds on fundamental topics of combustion thermodynamics, acid–base equilibria (carbonic acid formation), and enzyme catalysis.

Key Take-Away Points

• Fossil fuels: high energy, cheap, non-renewable, significant GHG & health issues.

• Biofuels: renewable, potentially carbon-neutral, lower particulates, biodegradable but lower energy density and carry indirect environmental & socioeconomic costs.

• Mathematical/chemical data critical for exams:

  • $C<em>6H</em>{12}O<em>6 \rightarrow 2 \ C</em>2H<em>5OH + 2 \ CO</em>2$

  • Biodiesel vs diesel $O<em>2$ stoichiometry: $20.5$ vs $27.5$ mol $O</em>2$ per mol fuel.

  • Energy values: $-5460$ vs $-1370\ \text{kJ mol}^{-1}$; $45$ vs $42\ \text{kJ g}^{-1}$.

These notes capture every concept, detail, numerical figure, and implication highlighted in the transcript, making them a stand-alone study resource.