Renewable Energy Sources for the mCCHP-SE-RES Systems - Design for Micro-Combined Cooling, Heating and Power Systems - page 106

c value of fuels from
lignocellulosic materials and of various vegetable oils based on their chemical
composition. There are many attempts to correlate calori

There have been developed equations to estimate the calori

c value with composition.

The high calori c value (HHV) for dry, ash-free sample can be calculated based on

the chemical composition as follows:

: þ : = ð Þ

HHV ¼ 0 889 LC 16821 8kJ ½ kg 7

where:
LC content of lignin of dry, ash-free sample, %.

The biomass density depends on the presentation form (Fig. 10 ). Due to the
lower heating value (LHV) of biomass feedstock, it requires mechanical processing
in order to increase the energy density by compacting, pressing, and briquetting.
Therefore, the size of solid biofuels can vary from a few millimeters for sawdust up
to a maximum of 80 cm for wood logs or straw bales.
The pellets are produced by grinding the wood chips, shavings, bark, or straw/
stalks followed by the pressing of the obtained dust. The heat generated by friction
forces is suf

—

cient to soften the lignin. During the cooling of pellet or briquette, the
lignin becomes rigid and it acts as a binder. The pellets have cylindrical or spherical
diameter of less than 25 mm. The briquettes have rectangular or cylindrical form
and are obtained by pressing together the sawdust, wood chips, shavings, or bark in
a press with piston or screw. The energy content of the pellets and briquettes is
about 17 GJ/ton, and the moisture content is 10 %, and density of about

700 kg/m 3 .
The ash content and inorganic substances of solid biofuels depend on the species
of the plant and the contamination of the soil in which the plant has grown, ranging
from 0.4

-

600

- 0.5 % (dry basis) for wood to 22.3 % for rice husks. The ash can be

classi ed into:

bottom ash which is produced in the primary combustion zone and which falls
below the combustion grate in case of

￿

fixed bed or remains in bed, in case of

fluidized bed combustion, due to the high density and tendency to coalescence.

(a) (b)

Fig. 10 Compacted solid biofuels, a Pelletes, b Briquettes