obtaining energy from fuels - fuel choices PDF

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Summary

Obtaining energy from fuels
• the definition of a fuel, including the distinction between fossil fuels and biofuels with reference to origin and
renewability (ability of a resource to be replaced by natural processes within a relatively short period of time)
• combustion of fuels a...

Description

FUELS FOSSIL FUELS: What is fuel?  fuel: a substance with stored energy that can be released relatively easily for use as heat or power  the stored energy can be in the form of chemical (e.g. coal) or nuclear (e.g. uranium) energy  measured in joules (J) What are fossil fuels?  fossil fuel: fuel that is formed from plant or animal remains being compressed underground for millions of years  originates from the fossilisation of organic matter over millions of years  e.g. coal, petrodiesel and natural gas (coal seam gas)  non-renewable energy source  the resource cannot be replenished as quickly as it is being consumed  rate of consumption > rate of replenishment  contains chemical energy that can be converted to electrical, heat or kinetic energy  the world’s primary source of energy  they have high carbon content which makes them easier to burn How is coal developed?  coal: a combustible fossil fuel formed by the partial decay of plant matter  it is formed from the natural breakdown of vegetation  vegetation dies to form peat  peat is compressed between sediment to form brown coal  it is further compressed to form black coal  as the layers of coal progress, the water content decreases which results in an increased energy content C(s) + O2(g)  CO2(g)

ΔH = – 33 kJ mol-1

How does the energy content increase when there is lower water content in the coal?  since less water needs to vaporise, the amount of heat released is greater than if the water content in the coal was greater  less chemical energy is lost as heat in steam; more of the energy content is converted to electricity  the higher the carbon content, the more heat that is released when it is burned  a lower water content = a greater carbon content  therefore, the energy content of the fuel increases with increasing carbon content

What is crude oil?  crude oil (petroleum): a mixture of hydrocarbon molecules of different chain lengths that is separated through the process of fractional distillation  the individual alkanes are separated during fractional distillation and are then used as a fuel or other products  e.g. petrodiesel, kerosene, gas  fractional distillation is the process of separating the crude oil within a tower based on their boiling point  petrodiesel: a liquid hydrocarbon fossil fuel obtained from crude oil  composed purely of hydrocarbons (about 8 to 21 carbon atoms)  often is contaminated by H2S and other sulfur and nitrogen compounds  burning petrodiesel has a negative effect on the environment because burning sulfur produces sulfuric acid and nitric oxide  diesel is a substance that has a high energy content  lower fuel consumption compared to regular fuels  liquid petroleum gas (LPG): a fuel composed of propane gas  produced during fractional distillation or extracted during the natural gas production process What is natural gas?  natural gas (coal seam gas): a fossil fuel that consists of mainly methane and small amounts of ethane and propane  found in underground coal deposits or trapped in shale rocks  it is extracted by drilling and a process called fracking  fracking: the process of injecting liquid at high pressure into coal to force open cracks in the rocks to extract the gas  the materials used for fracking have harmful impacts on the environment and water supplies  mainly used for cooking and heating CH4(g) + 2O2(g)  CO2(g) + 2H2O(l)

ΔH = – 890 kJ mol-1

What are the disadvantages of using fossil fuels?  they are non-renewable resources  this is an issue because fossil fuels are humans’ main source of energy and the demand of fuel is increasing due to the increased dependence on electricity & fueldriven technology  the burning of fossil fuels produces a high amount of emissions  SO2 emissions from burning coal and contaminated petrodiesel contribute to acid rain  electric power stations emit a large amount of carbon dioxide which accumulates in the atmosphere  the accumulations of gases in the atmosphere contribute to the greenhouse effect  greenhouse effect: process of the warming of the Earth’s lower atmosphere due to the increased quantities of gases (e.g. CO2, CH4, water vapour) in the air  the gases enable thermal energy to be trapped in the atmosphere  invasive processes such as mining and drilling destroy the natural landscape  oil drills in the ocean are risky because spills are very difficult to clean up BIOFUELS: What are biofuels?  biofuels: a broad category of renewable fuels sourced directly from organic matter  a renewable energy source that is sourced from plant or animal matter such as sugarcane, wheat, barley or vegetable oils  the three types of biofuels are biogas, bioethanol and biodiesel What is biogas?  biogas: a gaseous fuel produced by the anaerobic breakdown of organic waste e.g. rubbish or decomposing plant material

 gas is produced from the breakdown of organic matter by bacteria  mostly produces methane gas but it also contains a large proportion of carbon dioxide and trace amounts of N2, H2S, H2O and O2 What is bioethanol?  bioethanol: fuel produced by the fermentation of glucose in organic matter (sugar-based crops)  yeast is used to ferment the organic matter  the glucose undergoes hydrolysis in which the complex polysaccharides are broken down into simple sugars, then, the sugar is fermented to produce liquid ethanol  bioethanol is used for vehicle transport; E10 is a fuel composed of 10% bioethanol and 90% petrol  relatively carbon neutral – the production of CO2 when combusting bioethanol is balanced by the absorption of CO2 from plants during photosynthesis  carbon neutral: fuel which does not result in a net production of carbon dioxide from sourcing or consumption  not entirely carbon neutral because the process of producing bioethanol still requires energy yeast yeast

What is biodiesel?  biodiesel: fuel produced by the esterification of fats and oils in organic matter  extracts fats and oils from animal fats and plant oils that contain triglycerides (e.g. sunflower, canola, palm oil)  transesterification reaction: the chemical reaction to produce biodiesel from triglycerides  we react a triglyceride with methanol and use KOH as a catalyst  the reaction relocates the ester groups from the triglycerides into the biodiesel molecules  over 90% efficiency  the presence of an ester group makes biodiesel molecules polar

What are the advantages of using biofuels?  they are non-finite, renewable resources  they can be replenished by natural processes at a rate faster or equal to the rate it is being consumed  rate of replenishment ≥ rate of consumption  they are considered relatively carbon neutral because it is theoretically assumed that the amount of carbon dioxide produced by burning biofuel is the same as the amount of carbon dioxide absorbed by the plant during photosynthesis  burns more cleanly than fossil fuels and produces fewer toxic chemicals  easy to source material for biofuel production; can be produced from crop waste

COMPARISON OF FOSSIL FUELS AND BIOFUELS Fossil fuels  produced from the Origin fossilisation of organic matter over millions of years

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Biofuels produced from classic agricultural food crops

Energy content

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higher energy content than biofuels

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lower energy content than fossil fuels

Renewability

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non-renewable – forms over millions of years

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renewable – is produced at an equal or faster rate at which it is being consumed

Environmental impacts related to sourcing and combustion

sourcing:  extraction is extremely damaging to the environment i.e. land degradation, erosion  extraction of fossil fuels results in excess volumes of rock and soil left over which is dumped into adjacent streams and rivers, resulting in water pollution combustion:  emits a large amount of CO2 and other toxic chemicals (sulfur and nitrogen oxides)

More environmentally friendly sourcing:  locally sourced  farming requires a large amount of land which results in land clearing or deforestation  negative impact on the ecosystem and regrowth of native vegetation resulting in land degradation  high water usage to grow crops combustion:  burns more cleanly than fossil fuels  less CO2 and toxic chemical emissions  closer to being carbon neutral than fossil fuels  the CO2 released during combustion is somewhat offset by the plants that are grown to make biofuel absorbing CO2

COMPARISON OF PETRODIESEL AND BIODIESEL What is the chemical structure of petrodiesel?  contains a large percentage of straight chain alkanes which are non-polar molecules  petrodiesel molecules are held together by weak dispersion forces  less viscous  its polarity prevents the non-polar petrodiesel molecules from forming bonds with water molecules  petrodiesel can be stored for longer without compromising the quality of the fuel due to the absorbed water  less hygroscopic What is the chemical structure of biodiesel?  contains an ester functional group which makes its molecules polar  biodiesel molecules are held together by permanent dipole-dipole bonds  more viscous

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its polarity enables the polar biodiesel molecules to form hydrogen bonds with water molecules  the presence of water in fuel decreases the quality of fuel and lowers the efficiency of combustion reactions  more hygroscopic

Which fuel is more suitable for colder climates?  viscosity of a substance increases at lower temperatures so petrodiesel is more suitable because it is less viscous and therefore, is able to flow more easily through fuel lines

Source

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Chemical structures

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Energy content

Combustion products

Flow along fuel lines (viscosity and hygroscopicity)

Environmental impacts associated with their extraction and production

Petrodiesel drilling of crude oil (either on land or ocean)

smaller non-polar molecules; alkanes bonded by weak dispersion forces shorter hydrocarbon chain

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produces more energy per gram  produces up to 10% more energy than biodiesel 2C12H26(l) + 37O2(g)  24CO2(g) + 26H2O(g)  can produce other pollutants (SO2)

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lower viscosity  easier to flow through fuel lines

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less hygroscopic  quality of fuel isn’t affected by water absorption

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Biodiesel fats and oils from organic matter which can be grown/harvested on farmland typically have longer chains and contain two oxygen atoms; fatty acid methyl esters polar molecules bonded by stronger dipole-dipole forces higher melting point produces less energy per gram

2C17H34O2(l) + 49O2(g)  34CO2(g) + 34H2O(g)  the carbon dioxide produced is absorbed back into the plants/animals used to produce the fuel; reducing the net CO2 production  higher viscosity  harder to flow through fuel lines

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more hygroscopic  quality of fuel can be affected by water absorption; less efficient combustion

extraction:  crude oil can be mined on land or sea (land/ocean degradation)  non-renewable  drilling can result in oil spills and ecological damage  oil is a flammable liquid that poses fire risks

extraction:  consumes a lot of water and land which may result in land degradation  renewable  high-quality farmland is used for growing fuel instead of food  releases less particulate matter (SOx and NOx)

production:  produces more particulate matter from

production:  contains up to 20% less particulate matter than

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combustion (sulfur oxides and nitrogen oxides) greater net emission of greenhouse gases due to the burning of excess hydrocarbons energy is required to operate the refinery

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petrodiesel lower net emission of greenhouse gases due to partial carbon neutrality energy is required to operate the plant...