Chemical composition of biomass - Biomass as Energy Source: Resources, Systems and Applications - page 44

Table 2.3. Initial basic and acid oxides in primary ash formation.

Initial basic oxides Initial acid oxides

KOH (l, g) (K 2 O) P 2 O 5 (g)

NaOH (l, g) (Na 2 O) SO 2 (g)/SO 3 (g)

CaO (s) SiO 2 (s)

MgO (s) HCl (g) (Cl 2 )

H 2 O (g) CO 2 (g)

H 2 O (g)

The states within parenthesis indicate whether the primary oxide is
refractory or volatile. For example, K 2 O is a solid but is assumed to
combine with water to form volatile KOH (l, g).

The reaction zones will also influence the outcome of the secondary reactions in ash formation.
If for example the feedstock consists of two different biomasses then there may be three sets of ash
reactions proceeding at once (one for each of the two biomass types and one arising where the two
interact) if they are not sufficiently well-separated to prevent interaction. For the gaseous species
in Table 2.3, such separation is probably impossible and so gaseous species can be assumed to
always interact with every biomass type present. The model is further complicated by the differing
availability of various species for participation in secondary reactions. For example, CaO (s) is a
refractory initial oxide that may form nano- or sub-micron particles whereas KOH (g) is gaseous
and is thereforemuchmore available for reactionwith other initial oxides. Volatile compoundsmay
therefore have higher effective concentrations than refractory ones even if their true abundance is
equal or lower. Furthermore, the volatile gases and particles of the refractory initial oxides may
be fractionated by the movement of the flue gases, depending on their speed. This fractionation
could prevent some expected chemical reactions from taking place. The kinetics of the reactions
involved will of course also influence the outcome of the ash transformation processes. These
classifications and pathways of chemical reactions will be important in predicting operational
problems.
For more complex and realistic situations, Boström et al . (2012) write: “To transfer these gen-
eral concepts to a realistic situation, the physical characteristics of the specific energy conversion
facility, such as process temperature, residence time, air supply, and flue gas velocities, have to
be taken in account. Thus, the practical consequences of the ash transformation reactions for a
certain fuel may be quite different depending upon if the fuel is used in a fluidized bed, on a grate,
or within a powder burner.”

2.5 ENERGY CONTENT

The most important characteristic of a biofuel is its energy content. The energy value or heating
value of biomass is the amount of heat released per unit mass during complete combustion.
The energy released during combustion is that tied up in the covalent bonds for C, H, O, N and S
atoms and their valence numbers in the biomass. The calorific value of different covalent bonds in
gases increases in the following order: O-H < C-O < C = O < N-H < C-N < C-C < C-H < S-

H < C C (Eberson, 1969).

Heating values for biomass-based fuels are measured in units of energy per unit mass of fuel,
i.e. kJ/g (or alternatively, MJ/kg). The energy content of a dry substance is expressed in terms
of the gross calorific value ( GCV ), which is also known as the higher heating value, the gross
energy, the upper heating value, or the higher calorific value. By definition, the GCV is the
quotient of the thermal energy of complete combustion and the mass of solid fuel, whereby the
water formed during combustion is liquid and the temperature of the fuel (before combustion)
and the combustion products are of the same specified value. This thermal energy is commonly

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