Conversion Technologies 77
3000
2500
2000
1500
1000
C + 4H + 4 O
C + 4H + 2 O 2
CO 2 + 4H + 2 O
CO 2 + 2 H 2 O
CH 4 + 2 O 2
Breaking
2×O=O
(2 × 494 kJ)
+ 988 kJ
Breaking
4 × C–H
(4 × 410 kJ)
+ 1640 kJ
Forming
2×C=O
(2 × 799 kJ)
–1598 kJ
Forming
4 × O–H
(4 × 460 kJ)
Net energy gain
500
Energy (kJ)
0
Figure 3.6Complete combustion of methane, overview of bond energy changes. Energy investment
phase in marked with upward arrows, energy payoff phase with downward arrows. The net energy
gain is the difference in energy between reactants and products.
as heat or converted into work and power. The energy change during combustion of
simplest hydrocarbon, methane, is presented in Figure 3.6.
There are many factors that contribute to the quantity of energy released during
oxidation process and the resultant products, but the most important one is the
availability of oxidant, in most cases oxygen. Based on this criterion, two types of
processes can be distinguished.
Complete oxidationtakes place in the abundance of oxygen. During oxidation, carbon
molecules are oxidised to carbon dioxide and hydrogen molecules to water. Complete
oxidation releases maximal amount of energy during the conversion process.
Incomplete oxidationtakes place when supply of the oxygen is limited. It produces less
energy and results in the formation of numerous products. The range of these products
is largely dependent on the conditions of the conversion, that is, oxidant availability,
temperature, pressure and so on.
Another important factor is a type of fuel and more specifically its elemental content
and types of chemical bonds that are broken during combustion. In general, the lower
the oxygen content of a particular fuel, the higher the energy content. During the pro-
cess of combustion, oxygen is the terminal electron acceptor and cannot be oxidised;
consequently oxygen atoms in the fuel produce no energy during conversion process.
The relative content of hydrogen and carbon is another important parameter. Higher
ratio of these two atoms is indicative of higher level of carbon–carbon bond saturation
and consequently higher energy content of such fuel and an indication of lower molar
CO 2 emissions per energy unit.
Biomass and biomass-derived fuels have lower energy content than fossil fuels.
First, biomass contains significant amount of bound water (moisture). Moisture
content of biomass has a significant impact on the heating values and possibilities
of biomass conversion. Thermochemical processes such as combustion, gasification
