BIOMASS COMPOSITION, PROPERTIES, AND CHARACTERIZATION - Biomass as a Sustainable Energy Source for the Future - page 53

increasing moisture content, since the heat of vaporization of the water is usually not
recovered during combustion.
To determine the VM content, a biomass sample is heated ( ) in a cov-

ered crucible to 550 C (for coals to 900 C) in an inert environment. Biomass may
contain appreciable amounts of volatile alkali species and also carbonates that can
decompose at temperatures higher than 600 C, which limits the temperature for bio-
mass. Under these conditions, the more instable fuel fractions are released as a com-
plex mixture of gases and vapors. What remains is called char. The weight difference
with the original sample is the VM content:

“ carbonized ”

Y d VM = weight loss
dry weight

ð Eq : 2 : 3 Þ

The ash content then is determined by controlled combustion of the char left in the
previous step at a temperature of 815 C (German Standard DIN 51719) (tinyurl.com/
26yz894). After this combustion step, the amount of residual ash is determined, and
the difference with the mass at the beginning of this combustion step is called the frac-
tion of FC:

ash = weight ash

Y db

ð Eq : 2 : 4 Þ

dry weight

Y d FC =1 Y d VM − Y db

ash

− ð Eq : 2 : 5 Þ

The ash content is not exactly the same as the content of mineral matter because ash
forms the residual part of the combustion process; i.e., not all inorganic elements are
present in oxidic form (Spliethoff, 2009). Still, the ash content is a fairly good indi-
cator of the ash yields that can be expected in industrial processes. As the exact bio-
mass composition varies, in particular its moisture content, it is common use to report
these analyses on a db and sometimes on a daf basis.
Using thermogravimetric analysis (TGA), one also obtains useful information
regarding a solid fuel in terms of the contents of moisture, volatiles, FC, and ash
of a biomass sample. The device consists of a temperature-controlled oven and bal-
ance, and it allows introduction of sweeping flow gases. Depending on the technique
used for analysis of the gas phase, one is able to determine which gases are evolved
upon controlled heating of the biomass sample. A schematic is presented in Figure 2.8.
A schematic overview of the TGA concept is shown in Figure 2.9. A small piece of
solid fuel or ground material (biomass) is suspended on one of two balance pans (cru-
cible) in a furnace. The temperature of the furnace is slowly increased at a known heat-
ing rate (e.g., ~20 C min −1 ).

The weight of the sample is measured as a function of time and therefore temper-
ature. The derivative of the resulting weight curve corresponds to the conversion rate
and usually shows several peaks corresponding to the different thermal decomposition
processes associated with the main constituents of the biomass. When the temperature
is raised, initially drying of the sample occurs. In fact, drying commences as soon as
the furnace is purged with nitrogen (or helium or argon) to create the required inert
atmosphere, since the used gas is dry.