80 Introduction to Renewable Biomaterials
designed to maximise their production. In the first step of the conversion process,
drying, biomass loses moisture, that is, increases its heating value at the expense of
energy coming from external source. In the second step, pyrolysis, biomass is converted
into higher energy intermediates such as tars, other gaseous products and solid char.
Although these transformations are endothermic in nature, the gaseous products of
these transformations such as hydrogen (H 2 ), carbon monoxide (CO) or methane
(CH 4 ) can be combusted to recover the energy to power this process. Oxidation is the
next step of thermochemical transformation. Depending on the availability of oxygen
and temperature of oxidation, the transformation can either proceed to completion
or not. Complete oxidation, that is, combustion occurs when all the gaseous products
from previous steps are rapidly oxidised to release energy in an exothermic process.
In case of oxidant shortage, both exo- and endothermic transformations take place
in the process of gasification. These transformations between gaseous products and
solid chars as well as gaseous products themselves result in the production of syngas.
In all thermochemical processes, each of these reactions takes place to some extent
and the names of predominant conversion technologies are named according to the
most important step of the conversion process, that is, combustion, pyrolysis and
gasification. Figure 3.8 summarises major steps and transformations between various
compounds.
3.4.4 Biomass Combustion
Direct combustion of biomass is both the easiest and the most developed technology of
converting biomass into energy. From the historical perspective, the controlled usage of
Bio-oil
Volatiles collection
condensation and
processing
Volatiles
Particle
Drying Pyrolysis
Flame front
Volatiles + CO H 2 CO 2 CH 4 C 2 H 4
Flame combustion Char combustion
Core shrinking
Char Char
Increased
particle porosity
Particle
Gas-solid reactions Gas-gas reactions
Exothermic
½ O 2
CO 2
CO 2 CH 4 C 2 H 4
CO
CO
CO + H 2 OCO 2 + H 2
CO + 3 H 2 CH 4 + H 2 O
H 2 O
H 2 OCOO 2 H 2 O CO 2
2 O 2
H 2
CH2 H^2
H 2 4
CO
2 CO
Endothermic
Char
H 2 O
H 2 OCO 2
C
G
G
C
P
P
Figure 3.8Summary of thermochemical transformations in thermochemical conversion processes.
Four major transformation steps are annotated in grey boxes; major products from each
transformation are outlined in solid lines; common processes are in solid arrows, specific routes are in
dashed arrows, abbreviations: C, combustion; G, gasification; P, pyrolysis.
