Page 103 - Biomass Gasification, Pyrolysis And Torrefaction Practical Design and Theory

Chapter | 3 Biomass Characteristics 81

3.6.5.3 Bases for Expressing Heating Values
Similar to fuel composition, heating value (HHV or LHV) may also be
expressed in any of the following bases:

“ar” basis
db, also known as moisture-free basis (mf)
daf, also known as moisture ash-free basis
If M f kg of fuel contains Q kJ of heat, M w kg of moisture, and M ash kg of
ash, HHV can be written in different bases as follows:
Q
HHV ar 5 kJ=kg
M f
Q
HHV db 5 kJ=kg
ðM f 2 M w Þ
Q
HHV daf 5 kJ=kg (3.29)
ðM f 2 M w 2 M ash Þ

3.6.5.4 Estimation of Biomass Heating Values
Experimental methods are the most reliable means of determining the heat-
ing value of biomass. If these are not possible, empirical correlations like the
Dulong Berthelot equation, originally developed for coal with modified
coefficients for biomass, may be used. Channiwala and Parikh (2002) devel-
oped the following unified correlation for HHV based on 15 existing correla-
tions and 50 fuels, including biomass, liquid, gas, and coal.
HHV 5 349:1C 1 1178:3H 1 100:5S 103:4O 15:1N 21:1ASH kJ=kg
(3.30)
where C, H, S, O, N, and ASH are percentages of carbon, hydrogen, sulfur,
oxygen, nitrogen, and ash as determined by ultimate analysis on a dry basis.
This correlation is valid within the range:

0 , C , 92%; 0.43 , H , 25%
0 , O , 50; 0 , N , 5.6%
0 , ASH , 71%; 4745 , HHV , 55,345 kJ/kg
Ultimate analysis is necessary with this correlation, but it is expensive
and time consuming. Zhu and Venderbosch (2005) developed an empirical
method to estimate HHV without ultimate analysis. This empirical relation-
ship between the stoichiometric ratio (SR) and the HHV is based on data for
28 fuels that include biomass, coal, liquid, and gases. The relation is useful
for preliminary design:
HHV 5 3220 3 SR kJ=kg (3.31)
where the SR is the theoretical mass of the air required to burn 1 kg fuel.