U3AOS1 Topic 1: Fuels

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U3AOS1 Topic 1: Fuels

        Fuel definition:

A fuel is a substance with stored chemical energy that can be released relatively easily for use as heat or power (both chemical or nuclear)

Types of fuels:

Wood, Coal, LPG, Ethanol, Petrol are some common fuel types

Sometimes fuels are referred to as renewable, these are primarily fossil fuels

Renewable = "Must be replenished at equal or greater rate than production by natural processes"
Fossil Fuels

Organic matter formed from ancient organisms which are trapped under sediment for millions of years, retaining chemical energy. These fuels are non-renewable as they take millions of years to form.

Note: The process in which different fossil fuel derived molecules are separated is by fractional distillation whereby molecules are separated via different boiling temperatures of different sized hydrocarbons

Fossil fuels include:

• Coal

• Natural gas

• Crude Oil

Coal

Coal is comprised of many different molecules (mainly carbon) such as hydrogen, sulphur, oxygen and nitrogen

Formed from fossilisation of plants + animals under high pressure and heat underground

Coal types:

• Black coal (Anthracite)→ >80% carbon

• Brown coal (Lignite)→ 60-80% carbon

• Peat → <60% carbon

Note: peat is the precursor to coal

These differences in carbon percentage in the coal types is due to the amount of time taken for coal to form underground

As more time has elapsed, more heat and pressure reduces the moisture content in coal

Decreasing moisture content, increases carbon content which is used as fuel

Energy efficiency ranking:

1. Black coal

2. Brown coal

3. Peat coal

Water Content ranking:

1. Peat coal

2. Brown coal

3. Black coal

Explanation for energy efficiency ranking:

Water has high specific heat capacity (related to calorimetry) and absorbs large quantities of heat during coal combustion, leading too less net energy released

Water is also already fully oxidised (Redox, more on that later) and therefore cannot be combusted to release energy

Higher water content in coal means less carbon content for combustion and less energy per gram as energy absorbed by evaporation of water

Brown coal is processed and turned into black coal by removing water content via drying

Environmental concerns:

• Destroy habitats→bad for ecosystem→deforestation

• Release pollutants

Coal fired power stations

Steps in electricity production:

1. Chemical energy→thermal energy (heat)

2. Heat boils water therefore steam produced

3. Steam passed through turbine where thermal energy transformed into mechanical energy (kinetic energy) as turbine rotates

4. Electricity is produced at generator connected to turbine where mechanical energy is transformed into electrical energy

Crude Oil

Raw non-renewable fuel sourced from underground, primarily from remains of ancient marine organisms

Crude oil contains a mixture of hydrocarbons and produce many specific fuel types from the process fractional distillation such as:

• Natural gas (C1-C4)

• LPG (C3-C4)

• Petrol (C8-C12)

• Petrodiesel (C12-C20)

Note: Petrol is a type of fuel comprised mostly of octane therefore think of it synonymously unless told otherwise

Natural Gas

Natural gas is comprised of mainly methane (CH4), approx. 95%, the other 5% is made of ethane (CH3CH3), propane (CH3CH2CH3) and butane (CH3CH2CH2CH3) and extracted by fracking

Found as:

• Coal seam gas → adsorbed to coal

• Shale gas → adsorbed to shale rock

• Crude oil → within mixture

• Gas deposits → trapped between rocks

CH4(g) + 2O2(g)→CO2(g) + 2H2O(l)

Gas fired power station

Less energy transformations than coal fired power stations →higher energy efficiency as less steps for heat to be lost to external environment (no need to heat water to steam)

Therefore less fuel is required to generate same amount of electricity with gas fired power station compared to coal fired power station and less CO2 emissions per unit of energy released produced from gas fired power station compared to coal fired power station

Also less particulates produced compared to coal fired power stations as S and N are usually present as solids rather than gases

Steps in electricity production:

1. Chemical energy from hot natural gases expand air at high pressures therefore transforming into kinetic energy

2. Kinetic energy from moving particles rotates turbine therefore generating electricity
   
   

Biofuels

Definition: Renewable fuels that are derived from recently deceased organisms such as plants.

Biofuels include:

• Bioethanol

• Biogas

Bioethanol

Bioethanol is ethanol but produced via renewable processes such as fermentation, therefore chemical identical to normal ethanol

Formation:

Fermentation of glucose(C6H12O6) using yeast enzymes

The glucose obtained for bioethanol production is from fruits, vegetables and sugarcanes

Distillation:

Fermentation does not produce pure ethanol (about 10%) therefore distillation is required to purify the ethanol.

1. Heat up mixture until boiling point of water (100°C)

2. Ethanol collected at top of column

3. Collected ethanol further purified via micro-filtration and dehydrating agents

Environment:

• Bioethanol may result in conflict between using crops for fuel or food and when scaled for mass production results in habitat destruction through deforestation.

• Not carbon neutral as energy required in distillation process
  
  

C6H12O6(aq) →2C2H5OH(aq) + 2CO2(g)

Have you taken notice of the petrol stations and wondered what E10 is?

E10 consists of 10% bioethanol and 90% petrol by volume

This composition has no energy advantage, in fact an energy disadvantage as ethanol is partially oxidised (discussed earlier). However as bioethanol is renewable sourced, it is better for the environment as bioethanol production is theoretically carbon neutral

As glucose is produced via photosynthesis:

6CO2(g) + 6H2O(l) →C6H12O6(aq) + 6O2(g)

Combustion of bioethanol:

C2H5OH(l) + 3O2(g) →2CO2(g) + 3H2O(l)

As we can see there is a net zero CO2 production as CO2 absorbed by photosynthesis is released via combustion and fermentation therefore offset

However this carbon neutrality is only theoretical as it does not consider the CO2 required to cultivate the plants and carbon released via transportation ect

Biogas

Biogas composition:

• Methane (60%)

• Carbon dioxide (40%)

Biogas is a renewable produced fuel via anaerobic fermentation of bacteria when breaking down organic waste

Biogas mixture is slightly different to natural gas due to lower composition of methane content (80% compared to upwards of 95%) which can actually be used as fuel as remaining CO2 is fully oxidised thus cannot release energy upon combustion. 

However biogas and natural gas fundamentally consist of the same molecules, only different proportions from the different sourcing method

Environment:

Capture of methane gas from biogas is beneficial for the environment as methane contributes more to enhanced greenhouse effect than CO2 which is released after combustion. As without capturing methane, the natural breakdown of organic waste would release methane to the atmosphere

• Theoretically carbon neutral

• May lead to competition for land use as large quantities of crops required to produce biogas

Biodiesel

Biodiesel is a renewable sourced mixture of fatty acid methyl esters (FAME) produced via transesterification of triglycerides

Note: Biodiesel does not have to be FAME, it could be fatty acid ethyl ester ect (FAEE)

Transesterification steps:

1. Hydrolysis - triglyceride must be broken using water into constituent glycerol and three fatty acid chains

Triglyceride + 3H2O(l) →Glycerol + Fatty acid

[Insert picture below]

2. Condensation esterification - Each fatty acid reacts with methanol with NaOH or KOH catalyst to produce FAME and a water molecule

Fatty acid + methanol →FAME + H2O(l)

[Insert picture below]

Environment:

• Theoretically carbon neutral

• May lead to competition for land use as large quantities of crops required to produce biodiesel

Biodiesel vs petrodiesel:

Why does biodiesel have a higher boiling / melting point than petrodiesel?

Think back to intermolecular bonding

• Biodiesel has longer hydrocarbon chain therefore stronger dispersion forces than petrodiesel [reference dispersion forces]

• Biodiesel exhibits dipole-dipole interactions due to polar ester groups whereas petrodiesel only exhibits dispersion forces [reference dipole-dipole]

• Thereby biodiesel has stronger overall intermolecular bonding than petrodiesel, requiring greater kinetic energy to disrupt / break the intermolecular bonding, thus greater melting point / boiling point [refer to total intermolecular forces]

Why does biodiesel have greater cloudpoint than petrodiesel?

• Cloudpoint is the minimum temperature at which crystals form [definition]

• Biodiesel has longer hydrocarbon chain therefore stronger dispersion forces than petrodiesel [reference dispersion forces]

• Biodiesel exhibits dipole-dipole interactions due to polar ester groups whereas petrodiesel only exhibits dispersion forces [reference dipole-dipole]

• Therefore due to greater intermolecular forces, biodiesel requires more kinetic energy to disrupt crystals hence greater cloudpoint than petrodiesel [reference total intermolecular forces]

Author's special tip: If cloudpoint is difficult to conceptually understand, think of the trends as similar to melting point

Why does biodiesel have greater flashpoint than petrodiesel?

• Flashpoint is minimum temperature required for volatile fuel to produce sufficient combustible vapours for ignition in the presence of an ignition source [define]

• Biodiesel has longer hydrocarbon chain than petrodiesel thus greater dispersion forces [reference dispersion forces]

• Biodiesel has polar ester group thereby able to form dipole-dipole interactions whereas petrodiesel is non-polar hydrocarbon chain [reference dipole-dipole]

• Hence, due to greater intermolecular bonding biodiesel requires greater average kinetic energy to ignite flammable vapours compared to petrodiesel [reference total intermolecular forces]

Is biodiesel more viscous or petrodiesel?

• Biodiesel is more viscous than petrodiesel [state answer]

• Biodiesel has longer hydrocarbon chain than petrodiesel thus greater dispersion forces [reference dispersion forces]

• Biodiesel has polar ester group thereby able to form dipole-dipole interactions whereas petrodiesel is non-polar hydrocarbon chain [reference dipole-dipole]

• Hence, biodiesel has more resistance to flow compared to petrodiesel due to greater intermolecular forces thus more viscous than petrodiesel [reference total intermolecular forces]

Why is biodiesel hygroscopic?

• Biodiesel consists of polar ester group [reference polarity]

• The polar ester group forms hydrogen bonds with water vapour present in the air (as partial positive hydrogen from water bonds with partial negative oxygen) thus hygroscopic [answer question]

Fuel properties

Fuel properties are primarily related to intermolecular bonding of the fuel

Properties include:

• Cloudpoint

• Hygroscopicity

• Boiling point / melting point

• Flashpoint

• Viscosity 

Here are some commonly tested points of knowledge that I’ve noticed:

• Energy content order (kJg-1): methane (LPG), propane (LPG), butane, octane, ethanol

• Energy density order (kJL-1): same as energy content but opposite except ethanol remains last therefore octane, butane (liquified), propane (liquified), methane (liquified), ethanol

• Bioethanol produces similar CO2 as petrol due to low energy output

• Fracking release natural gas adsorbed in coal (coal seam gas) or rock (shale gas) 

• Three reactions with alkanes: cracking, substitution and combustion

• Glycerol is denser than biodiesel thus filtered out first

• Fuels are never aqueous (aq), always liquid, gas or solid

• C1 to C4 hydrocarbons are gas, C5 and above are liquid or solid

Source: (JavaLab - Vapor Pressure Lowering, https://javalab.org/en/vapor_pressure_lowering_en/)

Food for energy (Food chemistry introduction)

Cellular respiration

C6H12O6(aq) + 6O2(g) →6CO2(g) + 6H2O(l) H=-2803kJ/mol

Author's normal tip: Remember cellular respiration equation by remembering photosynthesis equation however swap the products and reactants and sign of H

Carbohydrates

Break down the name - carb for carbon, hydrate for water therefore contain H and O

General formula Cx(H2O)y 

Carbohydrate monomers (building block molecules) are called monosaccharides

Carbohydrate molecules that are formed from two of the same or different monosaccharides are called disaccharides

Large carbohydrate molecules that consists of many monosaccharides are called polysaccharides

Fats and Oils (lipids)

Fats and oils are triglycerides (3 long hydrocarbon chains and one glycerol backbone)

Stores large quantities of energy

Proteins

Polymer of amino acids joined by peptide bonds (amide links)

Can be used as energy source when undergone chemical reaction to produce ketones

Energy comparison

Energy value is the energy available to the human body, not total energy of the nutrient as the body is not 100% efficient at absorbing energy (energy loss via heat ect)

Also note: The unit of energy value is in kJ/g rather than kJ/mol as the nutrients are not a pure substance therefore do not have a corresponding molar mass to determine mole number

The same principle applies to the combustion of fuels

Lets say petrol is 80% efficient, this means for a desired energy quantity we need 10080 the amount required if petrol were 100% efficient, in this case we finding the amount required

Think logically

Petrol is less efficient → less energy is released than desired → more petrol required to compensate for the lower energy released

Don’t confuse this with the fact that 80% efficient means it produces 80100 the energy, in this case we are finding the energy output

I’ll run through a made up difficult example that will cover almost everything (May require U3AOS1 Topic 2 knowledge). Try it first and look at the solutions later:

An unknown mass (in milligrams) of E10 (90.0% octane, 10.0% ethanol by volume) is combusted knowing that the reaction is 70% efficient and produces 100J. Find the mass of E10 combusted. Assume density of octane is 0.70g/mL and density of ethanol is 0.79g/mL.

            1. Whenever a percentage composition is given, assume one unit total. In this case, 1L total volume

\begin{align*} V(\text{octane}) &= 1 \times 90\% = 0.9 \, \text{L} = 900 \, \text{mL} \\ V(\text{ethanol}) &= 1 \times 10\% = 0.1 \, \text{L} = 100 \, \text{mL} \end{align*}

2. Now find relative mass of octane and ethanol using density

\[ \begin{align*} m(\text{octane}) &= 900 \, \text{mL} \times 0.7 \, \text{g/mL} = 630 \, \text{g} \\ m(\text{ethanol}) &= 100 \, \text{mL} \times 0.79 \, \text{g/mL} = 79 \, \text{g} \end{align*} \]

3. To find the % of energy released coming from combustion of octane and from ethanol by first finding the relative energy released from each fuel type.

Databook shows the heat of combustion of:

\[ \begin{align*} \Delta H(\text{octane}) &= 5460 \, \text{kJ/mol} \quad \text{and} \quad \Delta H(\text{octane}) = 47.9 \, \text{kJ/g} \\ \Delta H(\text{ethanol}) &= 1360 \, \text{kJ/mol} \quad \text{and} \quad \Delta H(\text{ethanol}) = 29.6 \, \text{kJ/g} \end{align*} \]

You can use either the kJ/mol or kJ/g however I recommend stick with the kJ/g unit in this case as we already have quantities to do with mass from the previous step to save a step from conversions using relative molecular masses

\[ \begin{align*} E(\text{octane}_{\text{relative}}) &= 630 \, \text{g} \times 47.9 \, \text{kJ/g} \approx 3.02 \times 10^4 \, \text{kJ} \\ E(\text{ethanol}_{\text{relative}}) &= 79 \, \text{g} \times 29.6 \, \text{kJ/g} \approx 2.34 \times 10^3 \, \text{kJ} \end{align*} \]

4. Now add both relative energies released to find total energy from e10

\[ E(\text{octane}_{\text{relative}}) + E(\text{ethanol}_{\text{relative}}) = 3.02 \times 10^4 \, \text{kJ} + 2.34 \times 10^3 \, \text{kJ} \approx 3.25 \times 10^4 \, \text{kJ} \]

5. Now find the % composition for octane and ethanol respectively

\[ \begin{align*} \%E(\text{octane}) &= \left( \frac{3.02 \times 10^4 \, \text{kJ}}{3.25 \times 10^4 \, \text{kJ}} \right) \times 100 \approx 92.8\% \\ \%E(\text{ethanol}) &= \left( \frac{2.34 \times 10^3 \, \text{kJ}}{3.25 \times 10^4 \, \text{kJ}} \right) \times 100 \approx 7.2\% \end{align*} \]

Author's special tip: Whenever you find a % composition, rather than solving again, use the complement composition.

Eg. After finding %E(octane) to find %E(ethanol) just use the below:

\[ \begin{align*} \%E(\text{octane}) &= 100\% - \%E(\text{ethanol}) \approx 92.8\% \\ \%E(\text{ethanol}) &= 100\% - \%E(\text{octane}) \approx 7.2\% \end{align*} \]

6. Now we can find the exact amount of energy released from octane and ethanol respectively to produce 100J of energy when E10 is combusted

\[ \begin{align*} E(\text{octane}_{\text{actual}}) &= 100 \, \text{J} \times 92.8\% \approx 92.8 \, \text{J} \\ E(\text{ethanol}_{\text{actual}}) &= 100 \, \text{J} - E(\text{octane}_{\text{actual}}) \approx 100 \, \text{J} - 92.8 \, \text{J} \approx 7.2 \, \text{J} \end{align*} \]

7. Now find the actual mass used of octane and ethanol respectively within the E10 fuel

\[ \begin{align*} m(\text{octane}) &= \frac{\Delta E_{\text{octane}}}{\Delta H_{\text{octane}}} = \frac{92.8 \, \text{J}}{47.9 \, \text{kJ/g}} = \frac{92.8 \times 10^{-3} \, \text{kJ}}{47.9 \, \text{kJ/g}} \approx 1.94 \times 10^{-3} \, \text{g} \\ m(\text{ethanol}) &= \frac{\Delta E_{\text{ethanol}}}{\Delta H_{\text{ethanol}}} = \frac{7.2 \, \text{J}}{29.6 \, \text{kJ/g}} = \frac{7.2 \times 10^{-3} \, \text{kJ}}{29.6 \, \text{kJ/g}} \approx 2.43 \times 10^{-4} \, \text{g} \end{align*} \]

Author's special tip: To know what equation to use, in the above case \[ m = \frac{E}{\Delta H} \] simply look at the units

We used H which has units kJ/g which means kJ divide by g, so to find kJ - energy we must multiply by g - mass

\[ \Delta H \cdot m = \frac{E}{m} \cdot m \]

Author's normal tip: For calculations to work, always convert the units so its the same size eg. J to kJ or kJ to J

8. Now find the total mass of E10 by adding both mass of octane and mass of ethanol

\[ m(\text{E10}_{\text{theoretical}}) = m(\text{octane}) + m(\text{ethanol}) = 1.94 \times 10^{-3} \, \text{g} + 2.43 \times 10^{-4} \, \text{g} \approx 2.18 \times 10^{-3} \, \text{g} \]

9. We work backwards, we know 100J is produced. As reaction is 70% efficient, if it were theoretically 100% efficient, more energy would be produced for the same mass. Hence, more mass must be burned for 100J to compensate for the low efficiency.

\[ m(\text{E10}_{\text{actual}}) = \frac{2.18 \times 10^{-3} \, \text{g} \times \frac{100}{70}}{1} \approx \frac{3.12 \times 10^{-3} \, \text{g}}{1} \]

10. Oh wait you thought you were finished🤣? Read the question carefully, it asks for mass in milligrams therefore must convert from grams

\[ m(\text{E10}) = 3.12 \times 10^{-3} \, \text{g} \times 10^3 = 3.12 \, \text{mg} \]

11. You now think you are finished? Sorry but no! We have to check significant figures for every calculation question.

Look at the quantities given in the question

“90.0% octane” - 3sf

“10.0% ethanol” - 3sf

“70% efficient” - 2sf

“100J” - 3sf

“0.70g/mL” - 2sf

“0.79g/mL” - 2sf

The smallest amount of significant figures present in the question is 2, hence the final answer must be in 2sf (sf stands for significant figures).

Final answer: 3.1mg of E10 combusted

Authors note: I will be using (approximately) instead of = (equals) in some instances in the calculations as I have rounded from the exact solution given.

There are alternative and more efficient ways to solve the question above, however the method above applies most of the separator calculation techniques.

Extended knowledge:

Ever wondered why the molar heat of combustion (kJ/mol) increases as carbon chain length of the hydrocarbon increases?

With an increase in carbon chain, there are more atoms in the molecule thus more covalent bonds where each bond has a certain amount of bond energy. Thus greater potential to release thermal energy from chemical energy. 

Below is a table of bond energies:

It seems paradoxical that the energy content (kJ/g) decreases as length of the carbon chain increases. Why?

Longer carbon chain → lower ratio of C-H bonds to C-C bonds

For example methane (CH4) has 4 C-H bonds and 0 C-C, ethane (CH3CH3) has 6 C-H, 2 C-C, and it goes closer to 2:1)

C-H produce more energy than C-C due to greater electronegativity difference

Alternatively think that hydrogen is “light” therefore carbon to hydrogen bonds produces proportionately more thermal energy upon combustion than carbon to carbon bonds

How come ethanol has a greater energy content than ethane when both have the same number of carbon?

Ethanol (CH3CH2OH) is already partially oxidised due to the presence of O at hydroxyl group therefore cannot oxidise to the same extent as ethane which does not have O atom.

Partially oxidised fuels can also undergo complete combustion easier as there is less involvement to completely oxidise.

Fracking 

Process to extract natural gas or oil which involves creating fissures in underground 

Steps to fracking (extended knowledge):

1. Hole is drilled to rock layer (shale or coal)

2. Hole drilled horizontally increasing surface area in contact with shale rock or coal seams

3. Special drill inserted in hole to fracture shale rock or coal seams

4. Fracking fluid which contains water and chemical additives are injected in hole at high pressures cracking the shale rock or coal seams and creating fissures 

5. Clay or sand within fracking fluid keeps fissures open and allows gas to continuously desorb for capture

6. Natural gas desorbs from shale rock or coal seams and pumped to surface

Environmental impacts

• Kills potential underground wildlife → uninhabitable

• Pollution → aquifers are contaminated

• Can’t control size of fissures → more unstable ground