Page 102 - Biomass Gasification, Pyrolysis And Torrefaction Practical Design and Theory

80 Biomass Gasification, Pyrolysis and Torrefaction

The percentage of all constituents on any basis totals 100. For example:
C daf 1 H daf 1 O daf 1 N daf 1 S daf 5 100% (3.27)

3.6.5 Heating Value of Fuel
The heating value of biomass is relatively low, especially on a volume basis,
because its density is very low.

3.6.5.1 Higher Heating Value (HHV)
It is defined as the amount of heat released by the unit mass or volume of
fuel (initially at 25 C) once it is combusted and the products have returned

to a temperature of 25 C. It includes the latent heat of vaporization of water.

HHV is also called gross calorific value. In North America, the thermal effi-
ciency of a system is usually expressed in terms of HHV, so it is important
to know the HHV of the design fuel. Table 3.10 gives HHV of some
biomass.

3.6.5.2 Lower Heating Value (LHV)
The temperature of the exhaust flue gas of a boiler is generally in the range
120 180 C. The products of combustion are rarely cooled to the initial tem-

perature of the fuel, which is generally below the condensation temperature
of steam. So, the water vapor in the flue gas does not condense, and there-
fore the latent heat of vaporization of this component is not recovered.
The effective heat available for use in the boiler is a lower amount, which is
less than the chemical energy stored in the fuel.
The lower heating value (LHV), also known as the net calorific value, is
defined as the amount of heat released by fully combusting a specified quan-
tity less the heat of vaporization of the water in the combustion product.
The relationship between HHV and LHV is given by:

9H M

LHV 5 HHV 2 h g 2 (3.28)
100 100
where LHV, HHV, H, and M are lower heating value, higher heating value,
hydrogen percentage, and moisture percentage, respectively, on an “ar”
basis. Here, h g is the latent heat of steam in the same units as HHV. The
latent heat of vaporization when the reference temperature is 100 Cis

2260 kJ/kg.
Many European countries define the efficiency of a thermal system in
terms of LHV. Thus, an efficiency expressed in this way appears higher than
that expressed in HHV (as is the norm in many countries, including the
United States and Canada), unless the basis is specified.