Front Matter

 

76 Introduction to Renewable Biomaterials

these molecules. The energy content of fuels or in other words chemical energy available

per unit mass is described by heating values, which are corresponding to net enthalpy

released during the reaction of a particular fuel with oxygen under isothermal condi-

tions. Heating value is usually expressed in MJ kg−^1 or related units of energy per unit

of mass. The chemical energy stored in these fuels can be released through a process

of oxidation. Some typical bond energies are summarised in Table 3.1, and changes of

energy related to the combustion of typical fuels are presented in Figure 3.5.

Release of the energy from fuels is divided into two phases: an endothermic step

(energy investment) and exothermic step (energy pay off). First, the supply of energy in

form of heat is required to break the bonds of the fuel molecule (endothermic reaction),

and then in the second phase of the reaction the formation of new bonds results in the

release of energy (exothermic reaction). The products of this reaction have lower energy

than substrates of the reaction had, and this difference can be collected and utilised

Table 3.1Typical bond energies present in energy carriers and products.

Bond

Bond energy

(kJ mol−^1 )Bond

Bond energy

(kJ mol−^1 )

H=H 432 O=O 494

C=C 347 C=O 799

C=C 611 O=H 460

C=C (aromatic) 519 C=O 360

C=H 410 N=O 623

120 Hydrogen

Methane

Crude oil

Energy (MJ kg

-^1 )

CO 2

H 2 OH 2 OH 2 OH 2 O H 2 O H 2 OH 2 OH 2 O

CO 2 CO 2 CO 2 CO 2 CO 2 CO 2 CO 2

Biodiesel

Ethanol Coal*

Biomass**

Glucose Carbon

monoxide

60

50

40

30

20

10

0

Δ = –120.2

Δ = –50.0

Δ = –42.7Δ = –37.5

Δ = –27.0 Δ = –26.1

Δ = –17.2 Δ = –14.2

Δ = –10.1

Figure 3.5Energy released (ΔE) from complete combustion of typical fuels and their major

combustion products. Note: discontinuous y axis; * bituminous coal; ** herbaceous biomass. Values

from GREET, The Greenhouse Gases, Regulated Emissions, and Energy Use In Transportation Model. US

DOE.