Environmental Engineering Reference
In-Depth Information
thermochemical conversions such as combustion. One can distinguish the HHV (also
called gross calorific value (GCV)) and the lower heating value (LHV) (also named
net calorific value (NCV)). The HHV assumes the water originally present as moisture
but also the water chemically formed from the fuel-bound H content to be in the
liquid form. In contrast, the LHV assumes that both types of water remain in the vapor
phase. Thus, the difference is the latent heat of vaporization of water at the standard
temperature of 25
C (~2.4 MJ
kg
−1
) of both water types released in the combustion
process. The LHV can be calculated from HHV in two steps. First, one corrects for the
hydrogen content in the dry (0 wt% moisture) fuel:
LHV
db
=HHV
kg
−1
−
2
:
4×8
:
9Y
H
MJ
ð
Eq
:
2
:
6
Þ
kg
−1
] is the
stoichiometric water to H ratio, being kg water formed per kg hydrogen bound in the
fuel structure. For most types of biomass, the hydrogen content is approximately 6 wt
%(Y
H
~ 0.06). The second step is to correct for the wet fuel
kg
−1
is the latent heat of vaporization of water and 8.9 [kg
Here, 2.4 MJ
'
s moisture content as
follows:
LHV
wb
= LHV
db
1
:
:
:
ð
−
Y
moisture
Þ−
2
4Y
moisture
ð
Eq
2
7
Þ
Often, the heat of condensation (the reverse of vaporization) is not utilized in
energy conversion processes, and therefore, usually, the LHV is used in calculations.
The HHV is experimentally determined using a so-called bomb calorimeter, a con-
stant volume calorimeter, in which in a closed vessel a fuel portion is oxidized using
pure oxygen. In this device, the heat transferred to a precisely known amount of water
is measured by its temperature increase. The sample is ignited electrically. This is a
standard method (e.g., DIN 51,900).
The HHV (db) can also be determined once one knows the biomass ultimate anal-
ysis, using empirical correlations; one comparatively accurate equation is given below
(Gaur and Reed, 1995):
:
:
:
:
:
:
:
:
HHV= 34
91Y
C
+ 117
83Y
H
+10
05Y
S
−
1
51Y
N
−
10
34Y
O
−
2
11Y
ash
ð
Eq
2
8
Þ
in which Y
i
is the mass fraction of element i on a dry fuel basis. As can be seen from
this relation, the contents of C, H, and S contribute positively to the HHV, while the
contents of N, O, and ash contribute negatively. Most biomass fuels have an HHV of
between 18 and 22 MJ
kg
−1
(db); herbaceous biomass shows slightly lower LHV
values than woody biomass.
Example 2.2 HHV calculation based on ultimate analysis data
Khan et al. (2008) have reported on the combustion of pepper plant residue
from greenhouse cultivation in a fluidized bed. The measured HHV value (bomb
calorimetry) was 16.9 MJ
kg
−1
(db). Compare this value with Equation (2.8) given
the ultimate analysis data presented in Table 2.3.
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