6-2 Fossil Fuels Pre-class Post

8/13/2019 6-2 Fossil Fuels Pre-class Post

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Fossil Fuels

Photosynthesis: Plants take in:energy from sunlight

matter from the soil and air

simplest form:

CO2(g)+ H2O(g,l) + energy Carbohydrate + O2(g) .

eg

6 CO2(g)+ 6 H2O(g,l) + energy C6H12O6+ 6 O2(g)

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Fossil Fuels

When plants decay in air:aerobic

decay:Rapid recycling of the carbon, hydrogen andoxygen in the biosphere via the action of aerobicbacteria

When plants decay in the absence of air: anaerobicdecay:

Slow decay via anaerobic bacteria

Sedimentation

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Fossil Fuels

Anaerobic decay (no oxygen) and sedimentation(high pressure and temperature):

The oxygen is slowly expelled from thecompounds, leaving behind hydrocarbons

Carbohydrates (C, H, O) Hydrocarbons (C, H)

CxHyremain: x = 1100s

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Fossil FuelsExtraction: Coal

Coal is simply Carbon, Cand the contaminantsfound with it.

Two types of extraction categories:

Surface extraction: Strip mining (< ~30m deep)

Subsurface extraction: Closed mining (> ~30mdeep)

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cheaper

safer for workers

greater environmental impact

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Strip off surface soil

Dig up overlying rock scoop up coal

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Strip off surface soil

No topsoil, no ecology! Living material isunlikely to survive the process.

Post-mining reclamation can restore this,but often doesnt restore the full diversity.

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Dig up overlying rock

Acid Leachates: Natural iron sulfide minerals in the

rock are exposed to air and water by the process:

eg.4 FeS(s)+ 4 H2O(l) + 9 02(g) 4 H2SO4(aq)+ 2 Fe2O3(s)

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Surface extraction: Strip mining (< ~30m deep)4 FeS(s)+ 4 H2O(l) + 9 02(g) 4 H2SO4(aq)+ 2 Fe2O3(s)

H2SO4 is sulfuric acidso mine run-off is generallyhighly acidic.

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This acid helps to dissolve other metals in the rock, and

can contribute to mercury, lead and other toxic metals

being carried into natural waters from mine run-off.

The metals (and the starting materials for the sulfuric

acid) were naturally present in the rock, but theexposure to the air is what makes them available in the

environment rather than staying bound in the rocks.

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Subsurface mining: Generally if the coal is more thanabout 30 metres deep.

A network of tunnels is used.

There is lessdisruption to topsoil

There is lessexposure of rock to air

There is much greater danger to mine workers!(accidents and deaths are both about 4X greater)

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Fossil FuelsUses: Coal

As a fuel:

Coal contains:

sulfur, S:which can produce sulfur dioxide, SO2,when the coal is burned. This can then form

sulfuric acid, H2SO4(g)when exposed to humid air.

mercury, Hg: combustion of coal is responsible for

about 1/3 of all mercury emissions world-wide.

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As a fuel:

Soot: Combustion is often incomplete, and

particles of soot (C) are produced. These are

serious respiratory irritants.

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Treatments:

I: Remove S and Hg before burning:

Wash the crushed coal before burning; coalfloats, while sulfur and mercury containing

minerals sink.

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Treatments:

I I : Remove S after burning:

Pass the product gas stream through limestone,

CaCO3(s) .

2 SO2(g)+ 2 CaCO3(s) + O2(g) 2 CaSO4(s) + 2 CO2(g)

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Resource reclamation:

Finding an economic use for the by-product of anindustrial process

CaSO4(s)is gypsum, used to manufacture wallboard andas a soil amendment

A major coal-fired power plant will produce enough inone year to cover 1 square mile 1 foot deep.

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Newer burning technique: Fluidized Bed

Crushed coal and limestone are mixed prior to

burning

O2(g) is passed over the combustion

Calcium carbonate forms, so the sulfur is dealt with.Lower burning temperatures can be used (less NOX)

More heat is released per kilogram of coal

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Fossil Fuels: Oil and Gas

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porous

rock

non-porous rock

Gas deposit

Oil deposit


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porous

rock

non-porous rock

Gas deposit

Oil deposit

Extractiongenerallyby geologic pressure

Extraction

pumping plusother

techniques


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Extraction techniques:

Gas: the naturally occurring pressure is enough

Oil: 1:Mechanical pumping.

2:pump water (liquid or steam down to displaceand warm the oil (reduce viscosity)

3:pump detergent down to disrupt surfacetension, or heat by burning on-site to reduceviscosity.

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Fossil Fuels: Oil refining

Gas separates out spontaneously:

Oil: separated based on mass (proportional to the

number of carbon atoms in the molecule)

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400 C

heat to 400 C

tar

Natural gas 1-5 C

Solvents 57 C

Gasoline 5 - 12 C

Kerosene 12 - 16 C


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Fossil Fuels: Burning oil/gas

Carbon dioxide: All of the carbon atoms in the fuelare converted to CO2during combustion.

You can see the relative amounts (in moles)directly from the formula of the fossil fuel:

C8H18(octane) will give 8 CO2

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Fossil Fuels: Burning oil/gas - smog

Smog:

Another case of what is present naturally in the

environment coming back to bite you

The combustion of fossil fuels

is generally carried out in metal containers

liberates heat

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Fossil Fuels: Burning oil/gassmog

Hot metal and air: The nitrogen and oxygen in theair interact in the presence of hot metal: Above

1100 C, the airburns!

combustion of nitrogen gas:

N2(g) + O2(g) 2 NO(g)

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ozone depleter


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N2(g) + O2(g) 2 NO(g)

then:

2 NO(g) + O2(g) 2 NO2 (g)

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ozone depleter

when unburned hydrocarbonsare present as catalysts

reddish-brown gasyou seeit as the orange haze of smog


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2 NO(g) + O2(g) 2 NO2 (g)

NO2 (g) + O2(g) NO(g) + O3 (g)

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sunlight as catalyst

Ozone: very welcome in thestratosphere, but NOT a good

thing to have at ground level.

Severe lung irritant

Damaging to rubber and plastics


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N2(g) + O2(g) 2 NO(g)

2 NO(g) + O2(g) 2 NO2 (g)

NO2 (g) + O2(g) NO(g) + O3 (g)

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hot metal as catalyst

unburned hydrocarbons as catalyst


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Catalytic converters in cars:

1. Help the hydrocarbon burn more fully

minimizing unburned hydrocarbon

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N2(g) + O2(g) 2 NO(g)

2 NO(g) + O2(g) 2 NO2 (g)

NO2 (g) NO(g) + O(g)

O(g) + O2(g) O3 (g)

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Catalytic converters in cars:

2. Reduction catalysts (less common) convert

the nitric oxide back to nitrogen gas andoxygen gas (i.e. air)

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F il F l B i il/


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Photochemical smog

NOx (g) + CnHy PAN

PAN stands for peroxyacyl nitrates, the name of afamily of compounds containing C, H, O, and N.

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sunlightas catalyst

powerful eye irritants

persistent : do not break down

easily

O

RCO ONO2

R stands for some

hydrocarbon attachedhere

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6-2 Fossil Fuels Pre-class Post