Conversion Technologies 81
fire to burn biomass was probably one of the first milestones that allowed the develop-
ment of human civilisation. Early humans burned biomass to generate heat, cook food
and provide protection from wild animals. Fast forward to present day and combustion
of various fuels still generates about 95% of global primary energy supply worldwide
[20]. Vast majority of economies from highly developed to those of low development
still rely on combustion to provide heat, power and electricity. The success of combus-
tion process over other methods of generating usable energy can be accredited to its
relative simplicity, high portability and energy return per unit mass of fuel.
The process of combustion comprises several phases that release energy from a fuel,
fossil or renewable alike. Combustion process starts with the supply of heat from the
external source to initiate endothermic processes of drying and pyrolysis. Depending on
the type of the fuel, these processes may take significant amount of time and energy or
be very brief and will not require significant energy inputs. In general, solid fuels require
more time and energy for these initial endothermic steps than liquid fuels do and much
more than gaseous fuels, which are already similar to the products of pyrolytic trans-
formations and as such can be easily combusted. If we take solid biomass as our fuel of
choice, the combustion process starts with drying, that is, removal of water molecules
bound in the biomass. When heat is applied to biomass its temperature increases to
about 100∘C, then water changes its state from liquid to gas and evaporates from the
biomass making it accessible for subsequent thermochemical transformations. Removal
of this moisture is a prerequisite for further steps of conversion and therefore the lower
initial content of moisture of biomass the less energy is needed in the drying step and
the higher energy return from biomass can be achieved. Once the moisture is removed
pyrolysis of biomass takes place. Biomass decomposes into volatile products such as
gasses and tars and solid particles of char. The ratio of these products is dependent on
the conditions of combustion such as temperature, oxygen availability, heating rate, par-
ticle size and effects of catalysts [21]. At the end of pyrolysis stage, two of the products
enter oxidation phase and combust to yield energy: volatile gasses (flame combustion)
and char (char combustion). Gaseous products of pyrolysis: hydrogen, methane and
lower hydrocarbons burn rapidly in bright yellow flame releasing energy, CO 2 and H 2 O
as products. Simultaneously, carbon contained in the solid product of pyrolysis, char,
combusts to yield energy and carbon dioxide in a process. The combustion takes place
in the surface of char particles and results in gradual decrease of the particle volume [21].
These processes are summarised in the combustion section of Figure 3.8. For the efficient
conversion of the fuel to energy through the process of combustion, the proper adjust-
ment of three ‘Ts’ is essential, which include temperature, turbulence and time [21].
It is critical to maintain these parameters optimal to prevent the formation of incom-
pletely oxidised products such as carbon monoxide, polycyclic hydrocarbons and soot
[21]. Appropriate adjustments of these parameters will limit incomplete combustion
and release maximal amount of energy stored in the fuel.
3.4.5 Gasification
Gasification is a process that converts fuel into gasses through partial oxidation. The dif-
ference in oxidant content is the major difference between combustion and gasification.
Partial oxidation of biomass results in the formation of higher energy compounds such
as hydrogen (H 2 ), carbon monoxide (CO), methane (CH 4 ) and to lesser extent carbon
