Chemical composition of biomass - Biomass as Energy Source: Resources, Systems and Applications - page 45

determined by subjecting dry biomass samples to complete combustion to CO 2 ,NO 2 ,SO 2 and
H 2 O in a bomb calorimeter under an excess of oxygen with a starting temperature of 25 ◦ C.
Thus, the heat released between the initial and final temperatures (which are both usually 25 ◦ C)
is recorded as the gross calorific value. The heat of vaporization of any water vapor produced
during combustion is therefore included in the GCV .
The gross calorific value ( GCV ) takes into account the latent heat of vaporization of water in
the combustion products, and is useful in calculating heating values for fuels where condensation
of the reaction products is practical (e.g., in CHP plants connected to a district heating system). In
other words, GCV assumes all that all of the water in the fuel will be in the liquid state at the end
of combustion (and will be mixed with the other combustion products).The net calorific value
( NCV ) does not include this heat of condensation of water vapor and is measured using a heating
cycle whose final temperature is above the boiling point of water, e.g. at 150 ◦ C for smoke gases.
It is also known as the net energy, the lower heating value, or the lower calorific value. Thus, the
NCV is the energy obtainable if the water produced during combustion remains in the form of
vapor, while the GCV is the energy obtainable if the water is in the liquid state. The relationship
between NCV and GCV is outlined in a CEN standard (EN 14918:2009):

NCV = ( GCV − 21 . 22 × H − 0 . 08 × (O + N)) × (1 − m ) − 2 . 443 × m

Here, the NCV and GCV are measured in kJ/g. H, O and N are the mass concentrations on a dry
basis (db) of hydrogen, oxygen and nitrogen in the dry biomass, and m is the mass concentration
of water (on a wet basis, w.b.) in the wet biomass.
Two of the major contributors to the variation in the energy value of different dry biomass
samples are their moisture and ash contents. There is little variation in the energy contents of
ash- and water-free biomass samples, so when the H content is known, the NCV can be estimated
according to Stockinger and Obernberger (1998) as follows:

NCV = GCV × (1 − m ) − 2 . 447 × m − 0 . 5 × H × 18 . 02 × 2 . 447 × (1 − m )

where GCV is the gross calorific value (kJ/g, dry basis), m is the mass concentration of water in
the wet biomass, and H is the mass concentration of hydrogen in the dry biomass.
The net calorific value ( NCV ) on a wet basis (wb) and a dry basis (db) can also be calculated
using the equations of van Loo and Koppejan (2008):

NCV wb = GCV × (1 − m ) − 2 . 444 × m − 2 . 444 × 8 . 936 × H × (1 − m )

m ) -1 H

where the constant 2.444 is the enthalpy difference between gaseous and liquid water at 25 ◦ C
in KJ/g (or MJ/kg) and the constant 8.936 is the ratio of the molar masses of water (H 2 O) and
hydrogen (H 2 ).
The variation in the calorific value of fresh biomass is highly dependent on the composition
of the material as well as its moisture and ash content. It is shown that these parameters can be
predicted accurately by using multivariate calibration modeling based on near-infrared spectra
from biomass samples (Lestander and Rhén, 2005). The same reference gives the information
that the energy contents of cellulose, lignin and extractives range from 17.5-19.5, 23.3-26.8 and
32.3-39.4 kJ g in wood, respectively, assuming that the dry matter contains 6% H.
If the mass concentrations of C, H, O, N, S and ash are known, the higher energy content of
biomass can be estimated using the expression developed by Gaur and Reed (1995):

NCV db = GCV − 2 . 444 × m × (1 − − 2 . 444 × 8 . 936 ×

P GCV = 0 . 3491 × C + 1 . 1783 × H − 0 . 1034 × O − 0 . 0151 × N + 0 . 1005 × S − 0 . 0211 × ash

Here, P GCV is the calculated gross calorific value in kJ/g and C, H, O, N, S and ash are the
mass concentrations of the corresponding substances in the dry biomass. This calculated heating