Question

Give representative chemical formulas for the three principal types of fossil fuels.

Write a balanced chemical reaction converting each fuel to CO2 and water.

Using the equation calculate the grams of CO2 emitted per gram of fuel burned.

Answer 1

a) Crude oil mainly consists of the following compounds having the following percentage

Alkanes ( C_nH_2n+2 ) - 30%

The reaction is C_nH_2n+2 +(3n+1)/2 O₂ -----> nCO₂+ (n+1) H₂O

Napthanes ( C_nH_2n)-49%

The reaction is C_nH_2n+(3n/2)O2---->nCO₂+nH₂O

Aromatics( C_n H_2n-6 )- 15%

The reaction is C_nH_2n-6 +((3n-3)/2O₂---> nCO₂+(n-3)H₂O

asphaltics- 6%

They do not have a specific chemical formula

The percentage by weight of various elements is given by

Carbon-85%

Hydrogen-14%

Rest-1% Nitrogen, Oxygen sulfur

C+O₂ ---> CO₂

If 100 gs of fuel is burned, Carbon percentage in the fuel =85 g =85/12 g moles =7.0833 g moles

As per stoichiometry, 1 g moles of Carbon gives rise to 1 g mole of Oxygen

Hence, 7.0833 g moles of Carbon gives rise to 7.0833 g moles of Carbondioxide = 311.667 g of Carbondioxide

Hence, Carbondioxide emitted per gms of Fuel= 311.667/100 =3.11667gms/ gm of fuel

b) The representative composition of Coal in weight % is given by

Carbon : 80%

Oxygen : 10%

Hydrogen :5%

Nitrogen :2%

Sulfur : 3%

The reactions are

C+O₂---> CO₂

H₂+O₂---> H₂O

Basis : 100 gms of coal

Carbon in coal :80 gms =80/12 =6.667gmoles

As per the reaction of Carbon and Oxygen, 1 mole of Carbon gives rised to 1 moles of Carbondioxide and 6.667gmoles of Carbon gives rise to 6.667 gmoles of Carbondioxide

gms of Carbondioxide= 6.667* Molecuar weight of Carbondioxide= 6.667*44= 293.48 gms of Carbondioxide

Gms of Carbondioxide produced per gms of fuel = 293.48/100 =2.9348 gms/ gm of fuel

c) Natual gas mainly constists of methane (CH₄ -80 %) and ethane (C₂H₆ -20%) by volume

Moles in 100 gms of mixture can be calculated once the molecular weight of mixture is calculated.

Molecular weight of mixture =0.8*16+0.2*30=18.8

Moles=Mass/ Molecular weight= 100/18.8= 5.319 moles

Moles of methane= 5.319*0.8= 4.255gmoles

Moles of Ethane=5.319-4.255 =1.064 gmole

CH₄+2O₂--->CO₂+2H₂O

As per stoichiometry, 1 mole of methane produces 1 Moles of CO₂. Hence CO₂ produced from 4.255 moles= 4.255 moles= 4.255

Mass of CO₂ produced = Moles* Molecular weight of CO₂= 4.255*44=187.22gms

For the reaction

C₂H₆+3.5O₂---->2CO₂+3H₂O

Moles of CO2 produced= 2*1.064=2.128 gmoles= 2.128 gmoles

Mass of CO2 produced = 2.128*Molecular weight of CO₂= 2.128*44=93.632 gms

Total CO₂ produced=187.22+93.632=280.852gms

CO₂ produced per 100gms of fuel = 280.852/100 =2.80852gms/ gm of fuel

To conclude

Fuel                 gms of CO₂ produced/gm of fuel

Diesel                 3.1167

Coal                    2.9348

Natural gas 2.80852

The above data suggest that Natual gas produces less Carbondioxide than Crude oil or coal. Hence, it is more environment friendly

Fuel