78 Introduction to Renewable Biomaterials
or pyrolysis require low content of moisture to yield high energy conversion rates.
Biochemical processes on the other hand can utilise biomass with high moisture.
Enzymatic reactions that are essential to this type of conversion are performed in
aqueous environment and therefore additional content of moisture in the biomass
does not impact their efficiency. Second reason for biomass and biomass-derived fuels
having lower energy content is their elemental composition. Biomass and bio-fuels
contain more oxygen than fossil fuels. The elemental composition of typical biomass
corresponds to the following formula CH1.4O0.66[18] compared to the typical formula
of coal that can be approximated to C 135 H 96 O 9 NS [19]. Comparing these two formulae
indicates that the ratio of energy yielding elements, that is, carbon and hydrogen versus
non-energy-yielding oxygen is much higher in fossil fuels than biomass. This is due to
the fossilisation process in which fossil fuels were created, that is, partial decomposition
of biomass in the absence of air and under high pressure and temperature. This process
resulted in the deoxygenation of these substances increasing their calorific value.
3.4.2 Overview of Biomass Conversion Technologies
As it was introduced earlier, biomass conversion methods can be broadly divided into
thermochemical and biochemical. Thermochemical conversion utilises temperature,
pressure and catalysts to transform biomass into various chemical compounds of high
utility such as fuels or chemicals. Biochemical conversion on the other hand utilises
microorganisms and their ability to transform carbohydrates, lipids and other biomass
components into an array of compounds including fuels and chemicals. Although
at the first glance these processes are very different and utilise completely different
approaches, closer look at them will reveal numerous parallels and similarities especially
at the energy level. It has been stated in the earlier that biomass has rather complex
chemical composition and various types, and even different fragments of biomass have
different chemical compositions. To address this complexity, a number of different
thermo- and biochemical routes of biomass conversion have been developed. The
choice of particular process of biomass conversion will largely depend on biomass avail-
ability and the type of the product that is desired: heat, electricity, fuels, chemicals, food
or feed components and so on. Routes that allow conversion of various types of biomass
into these respective products have been developed and are summarised in Figure 3.7.
3.4.3 Thermochemical Conversion of Biomass
Thermochemical conversion of any fuel including biomass is composed of four com-
mon steps, and all major technologies of biomass conversion take their names from
the most important step of a particular conversion process. The four thermochemical
transformations are as follows:
Drying– Drying is the first step of any thermochemical processes of biomass con-
version. During this endothermic process, water (moisture) is removed from biomass
particles. The process takes place above 100∘Candresultsintheincreaseoftheheating
value of biomass at the expense of energy. Although potentially any form of biomass may
be dried, it is only practicable if the initial content of moisture is less than 50%, preferably
in range of 10%. Otherwise, energy used for drying may significantly outweigh the energy
gain from the conversion itself. In case of feedstocks with high moisture content, bio-
chemical routes are more applicable than the thermochemical ones.
