Question

Below are the line structures for four biofuels, as well as data for the energy content of ethanol and biodiesel. H3C OH Ethanol O OH OH HO HO HO Glucose (wood) H3C OH Butanol H3C O O CH3 Biodiesel

Fuel Energy Content (MJ/kg) Chemical Formula O:C ratio

Ethanol 27

Biodiesel 44

Butanol

Glucose

a. Interpret the structures to determine the chemical formula of each fuel and write it in the table above.

b. Use the chemical formula information to determine the oxygen to carbon (O:C) ratio of each fuel and fill in these values in the table above.

c. Based on the above data, predict the energy content of butanol and glucose. Would the energy content be below 27 MJ/kg, above 44 MJ/kg, or somewhere in between these two values? Indicate these predictions in the table above.

Answer 1

a) Chemical formula of the fuels

Ethanol(C₂H₆O)      CH₃CH₂OH

Butanol (C₄H₁₀O)   CH₃CH₂CH₂CH₂OH

Glucose C₆H₁₂O₆

Biodiesel:

A typical molecule of biodiesel looks like the structure below. Mostly it is a long chain of carbon atoms, with hydrogen atoms attached, and with an ester functional group at one end. The ester may be saturated or unsaturated

There are two types of biodiesel with C19 and the C20 chain lengths. These are used in the calculation of combustion data

C₁₉H₃₆O₂ It is an Unsaturated ester with 1 double bond

b) Oxygen to Carbon ratio    O : C

Ethanol (C₂H₆O)            O : C = 1:2

Butanol (C₄H₁₀ O)           O : C = 1:4

Glucose C₆H₁₂O₆           O : C = 6:6 = 1: 1

Biodiesel (C₁₉H₃₆O₂)      O : C = 2 : 19 = 1:9.5

c) The energy content increases with an increase in the O : C ratio ie. with an increase in C- content

For example the energy content of Methanol CH₃OH with O:C ratio of 1:1 is about 23 and that of Ethanol CH₃CH₂OH with O:C ratio of 1:2 is about 27

We expect Butanol with O:C ratio of 1:4 to have an energy content greater than that of ethanol (O:C ratio of 1:2) and less than that of Biodiesel (O:C ratio of 1:9.5)

In fact the energy content of Butanol is 36MJ/Kg which is in between the 27 MJ/Kg and 44MJ/Kg , the values corresponding to Ethanol and Biodiesel respectiely.

By similar reasoning we can predict that the energy content Glucose with O:C ratio of 1:1 will be les than that of both Butanol and Bodiesel