Page 93 - Biomass Gasification, Pyrolysis And Torrefaction Practical Design and Theory
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Chapter | 3 Biomass Characteristics 71
TABLE 3.8 Heat of Formation of Some Important Compounds
Compound H 2 O CO 2 CO CH 4 O 2 CaCO 3 NH 3
Heat of formation at 25 C 241.5 393.5 110.6 74.8 0 1211.8 82.5
(kJ/mole)
Source: Data collected from Perry and Green (1997, pp. 2 186).
For example:
(3.15)
ΔH comb 5 2ΔH H 2 O 1 ΔH CO 2 2 ΔH CH 4 2 2ΔH O 2
The ΔH comb for a fuel is also defined as the enthalpy change for the
combustion reaction when balanced:
Fuel 1 O 2 -H 2 O 1 CO 2 2 HR (3.16)
Example 3.1
Find the heat of formation of sawdust, the heating value of which is given as
476 kJ/mol. Assume its chemical formula to be CH 1.35 O 0.617 .
Solution
Using stoichiometry, the conversion reaction of SW can be written in the
simplest terms as:
CH 1:35 O 0:617 1 1:029O 2 -CO 2 1 0:675H 2 O 476 kJ=mol sawdust
Similar to Eq. (3.14), we can write:
Heat of reaction 5 ½HF CO 2 1 0:675HF H 2 O 2 ½HF sw 1 1:029HF O 2
Taking values of HF of CO 2 ,O 2 , and H 2 O (g) from Table 3.8, we get:
HR sw 5 ½ 393:5 1 0:675 3 ð 241:5Þ 2 ½HF sw 1 1:029 3 0 5 556:5 HF sw
The HR for the above combustion reaction is 2476 kJ/mol. So
HF sw 52556.5 (2476) 5280.5 kJ/mol.
3.5.2.5 Heating Value
The heating value of biomass is the amount of energy biomass releases when
it is completely burnt in adequate oxygen. It is one of the most important
properties of biomass as far as energy conversion is concerned. Compared to
most fossil fuels, the heating value of biomass is low, especially on a volume
basis, because its density is very low and it is high oxygen containing fuel.
Section 3.6.5 discusses this in more detail.
