Chapter 15: Fuels and Crude Oil - Comprehensive Study Guide

Overview of Singapore’s Electricity Consumption and Energy Sources

• Electrical Consumption Trends: Singapore's total electricity consumption has shown a general upward trend since 2013.

• Measurement Units:

  • Gigawatt hours (GWh): A unit of energy representing one billion ($10^9$) watt hours, equivalent to one million ($10^6$) kilowatt hours (kWh).

  • Kilowatt hour (kWh): Represents a steady power of one kilowatt ($1000\,W$) running for one hour.

  • Joule Equivalence: $1\,kWh = 3.6 \times 10^6\,J$ (or $3.6\,megajoules$).

• Consumption Sectors (2017–2022): Electricity is consumed across various sectors including industries, commerce and services, households, transport, and others.

• Primary Fuel Source: More than $95\%$ of Singapore's electricity is produced from natural gas, which is a non-renewable fossil fuel.

Fundamental Concepts and Prior Knowledge

• Fractional Distillation: A separation technique used for mixtures like air and crude oil (petroleum) based on differences in boiling points.

• Kinetic Particle Theory:

  • Impure substances (mixtures) do not have fixed melting or boiling points; they melt and boil over a range of temperatures.

  • Physical states (solid, liquid, gas) are defined by the arrangement and movement of particles.

• Classification of Mixtures: Air, crude oil, and aqueous solutions are classified as impure substances.

• Stoichiometry: The percentage mass of an element in a compound is calculated using the formula:     $\text{Percentage mass of an element} = \frac{\text{Ar of element} \times \text{number of atoms}}{\text{Mr of compound}} \times 100\%$

Definitions: Organic Compounds and Hydrocarbons

• Organic Compounds: Pure substances containing carbon atoms covalently bonded to atoms of other elements (e.g., hydrogen, oxygen, or chlorine).

  • Examples: Methane ($CH_4$), ethanoic acid ($CH_3COOH$), and ethanol ($C_2H_5OH$).

• Hydrocarbons: A sub-category of organic compounds made up only of carbon and hydrogen atoms covalently bonded together.

  • Example: Methane is the simplest hydrocarbon ($CH_4$).

Classification and Formation of Fossil Fuels

• Definition: Mixtures consisting mainly of organic compounds that are hydrocarbons.

• Formation: Fossil fuels are formed from the remains of decayed plants and animals that lived millions of years ago.

• Common Forms: Coal, crude oil (petroleum), and natural gas.

• Non-renewable Nature: These fuels are considered finite resources because an extremely long period of time (millions of years) is required to replace them. They cannot be replaced quickly enough to match current rapid human consumption.

Natural Gas: Characteristics and Uses

• Description: A colourless and odourless gaseous fossil fuel.

• Composition: Primarily methane ($CH_4$), ranging from $\sim 70\%$ to $90\%$. It also contains short-chain alkanes like ethane ($\sim 0-20\%$), propane, and butane.

• Chemical Combustion: Methane undergoes complete combustion in excess oxygen:     $CH_4(g) + 2O_2(g) \rightarrow CO_2(g) + 2H_2O(g)$

• Carbon Content: Natural gas contains the lowest percentage of carbon by mass compared to other fossil fuels.

• Environmental Impact: It is considered the "cleanest" fossil fuel because burning methane results in lower emissions of air pollutants (like sulfur dioxide, $SO_2$) and carbon dioxide ($CO_2$) compared to coal or crude oil.

• Liquefied Natural Gas (LNG):

  • Transportation: Natural gas is cooled and liquefied to reduce its volume by $600$ times for easier transport and storage.

  • Safety: LNG does not burn as easily as gaseous natural gas, making it safer for consumer handling.

  • Safety Additive: Because natural gas is naturally odourless, tetrahydrothiophene (THT) is added to give it a pungent smell to alert users of gas leaks.

Crude Oil (Petroleum): Characteristics and Processing

• Physical State: A dark, sticky liquid at room temperature and pressure.

• Composition: A complex mixture of many different types of hydrocarbons with varying properties (containing $1$ to $70$ carbon atoms per molecule).

• Processing: It is separated into useful fractions via fractional distillation.

Comparison of Fossil Fuels: Crude Oil vs. Natural Gas

• Crude Oil (Petroleum):

  • State: Liquid.

  • Advantages: Distilled into many useful fractions; easily transported and stored in tanks; economical and convenient.

  • Disadvantages: Finite resource; high carbon content and sulfur presence leads to soot, carbon monoxide ($CO$), and $SO_2$ (air pollutants); produces $CO_2$ (greenhouse gas).

• Natural Gas (Methane):

  • State: Gas.

  • Advantages: Cleanest fossil fuel (no soot or $SO_2$); lower $CO_2$ emissions per unit of energy; burns more efficiently (more heat for the same amount of fuel); easily stored once liquefied.

  • Disadvantages: Finite resource; produces $CO_2$.

Fractional Distillation of Crude Oil

• Laboratory Scale: Uses a fractionating column containing glass beads to increase surface area for condensation and boiling.

• Industrial Scale (Oil Refinery):

  1. Crude oil is heated in a furnace until boiling occurs.

  2. Hot vapour is passed into a fractionating column.

  3. Heavier Fractions: Consist of larger hydrocarbons with more carbon atoms ($C_{atoms} \uparrow$). They have higher boiling point ranges ($B.P. \uparrow$), condense at higher temperatures, and are collected at the hot lower levels of the column.

  4. Lighter Fractions: Consist of smaller hydrocarbons with fewer carbon atoms ($C_{atoms} \downarrow$). They have lower boiling point ranges ($B.P. \downarrow$), rise further up the column, and condense at the cooler higher levels.

• Key Distinctions:

  • Petroleum and petrol are NOT the same; petrol is a specific fraction of petroleum.

  • Petroleum gas and natural gas are NOT the same; petroleum gas is a fraction of petroleum.

Properties and Trends of Hydrocarbon Fractions

• Uses of Lighter Fractions: $90\%$ used as fuels (cooking, heating, cars, electricity); $10\%$ used as petrochemical feedstock (detergents, medicines, fertilizers, pesticides, synthetic rubber).

• Uses of Heavier Fractions: Lubricating oil for machines, road surfacing (bitumen).

• Supply/Demand Solution: Catalytic cracking is used to break down heavier fractions (low demand) into lighter fractions (high demand).

Biofuels and Environmental Sustainability

• Renewable Alternatives: Biofuels are derived from plant and animal matter (e.g., bioethanol from sugarcane).

• Sustainability Definition: The ability to be maintained at a steady level without exhausting natural resources or causing ecological damage.

• Types of Biofuels:

  • Bioethanol: Produced from fermentation of sugars in sugarcane, sweet corn, sugar beet, and sorghum.

  • Biodiesel: Derived from vegetable oils and animal fats.

  • Green Diesel: Derived from algae and other plant sources.

  • Biogas: Methane from decomposed organic waste.

• Carbon Neutrality: Biofuels are considered more sustainable because they are roughly carbon neutral.

  • Process: During growth, plants absorb $CO_2$ from the atmosphere for photosynthesis:         $6H_2O(l) + 6CO_2(g) \rightarrow C_6H_{12}O_6(aq) + 6O_2(g)$

  • Combustion: When burned, the fuel releases $CO_2$:         $C_2H_5OH(l) + 3O_2(g) \rightarrow 2CO_2(g) + 3H_2O(g)$

  • Offset: The $CO_2$ emitted during burning is offset by the $CO_2$ taken in during the plant's growth, resulting in lower net carbon emissions compared to fossil fuels.

Practice and Discussion

• Multiple Choice Question: Which sentence best explains why bioethanol from sugarcane is considered renewable?

  • Response: (A) Sugarcane plants grow fast and can be collected after one year. (Note: This rapid replacement cycle distinguishes it from fossil fuels which take millions of years to form).

• Instructional Tasks: Students are directed to complete assignments on the Student Learning Space (SLS) for "Crude Oil and Fuels" and "Biofuels."