82 Introduction to Renewable Biomaterials
dioxide (CO 2 )andwater(H 2 O), which are typical of combustion. Due to differences
in oxygen content, the products of gasification process have higher bond energy and
therefore higher utility than final products of combustion. Consequently, due to partial
oxidation gasification releases less energy than combustion does. These higher energy
compounds produced in a gasification reaction can be therefore used as both fuel and
chemical feedstocks for the production of other materials. The second product of gasi-
fication is a solid called char, which is made of mineral matter and unreacted carbon.
Char has numerous potential applications including solid fuel, activated carbon or soil
conditioner. The ratio of solid and gaseous products depends on the reaction condi-
tions of a particular gasifier. Two initial endothermic transformations, that is, drying
and pyrolysis are analogous to combustion process. Pyrolytic products such as hydro-
gen, methane, volatile lower hydrocarbons, tars and solid char enter partial oxidation
in gasification reaction. Depending on the conditions of operation of a gasifier, a series
of chemical transformations between gasses themselves and between gasses and char
take place. These reactions are both endothermic and exothermic and their sequence
and the composition of final product (syngas or producer gas) depend on the content
of oxygen, steam, residence time, temperature and design of a gasifier. Because gasi-
fication is the process in which both endo- and exothermic reactions take place it is
possible to manipulate the conditions of the process so the exothermic reactions pro-
vide energy for the endothermic changes and formation of desired products. Although
each process is specific to the conditions of operation, in general, low temperatures of
operation favour production of methane, whereas high temperatures yield hydrogen for-
mation [21]. Another parameter that has major effect on gas composition is gasification
medium, which can include air, oxygen, steam, nitrogen or hydrogen among others [22].
The process of gasification and chemical transformations between gaseous products and
solids and gasses alike are presented in the gasification section of Figure 3.8.
Syngas produced in the process of gasification is composed of high-energy com-
pounds such as hydrogen, carbon monoxide or methane. It is therefore possible to
further utilise these compounds as fuels or feedstocks for the production of mate-
rials. The most straightforward application of this producer gas is its utilisation as
low-heating-value fuel. Syngas can be combusted directly to generate steam for heat
and power generation via a steam turbine. This application is mostly suitable for low
calorific value syngas obtained though gasification of biomass in the presence of air.
High content of inert nitrogen present in the air significantly dilutes the producer
gas and limits its applications for the synthesis of higher value compounds and direct
applications in gas engines [22, 23]. Other designs of gasification reactor and utilisation
of other gasification medium such as oxygen, steam or hydrogen results in higher
quality producer gas of higher calorific value. Provided that this syngas is cleaned from
residual tars and dust particles, it can become a very valuable fuel for gas engines
or gas turbines that are more efficient generators of power then steam turbines are.
Most importantly, however, such high-quality syngas is applicable for the synthesis
of chemicals. An array of compounds that are formed in gasification process opens
numerous possibilities for the synthesis of platform chemicals and subsequently high
value compounds from these intermediates. The most typical reactions that can provide
these platform chemicals from syngas intermediates include the following:
Hydrogen enrichment– Hydrogen is one of the products of biomass gasification. Due
to high utility of hydrogen for chemical synthesis, gasification process can be tweaked to
