Environmental Engineering Reference
In-Depth Information
9.2 FUNDAMENTAL CONVERSION PROCESSES
9.2.1 Drying
Biomass particles may contain a large amount of water in different forms, either
adhered to the particle surface, inside the particle pores, or also chemically bound
to the solid matrix. When a fresh biomass particle enters a combustion system, it will
first heat up by conduction, convection, and radiation. When the temperature of the
particle rises above 100
C, the water it contains starts to evaporate, leaving the bio-
mass particle by convection and diffusion through the pores of the solid matrix.
A flexible combustion system should be capable of drying solid fuels with different
moisture contents. In grate or fluidized bed firing systems, the reactor can be fed with
moisture-containing fuels without further treatment. The fuel in the case of a pulver-
ized fuel firing is predried in order to ensure a fast combustion process within the
available short residence time. A maximum moisture content is acceptable for com-
bustion to be self-sustainable. This implies that the heat produced by combustion
should be larger than the heat losses plus the heat needed for water evaporation.
9.2.2 Pyrolysis
The thermal decomposition of biomass and the formation of gaseous products during
heating of the biomass particles are termed devolatilization or pyrolysis. Pyrolysis is
an endothermic process by definition occurring in the absence of oxygen. Devolati-
lization of the solid biomass starts at temperatures above 200
C, by the thermal
cracking of compounds. Formation of tars, liquids, and gaseous products occurs at
temperatures of up to 600
C. The mixture of volatile gases is composed of carbon
monoxide (CO), carbon dioxide (CO
2
), methane (CH
4
), and other light hydrocarbons
such as C
2
H
6
,C
2
H
4
, and C
2
H
2
. Tars are complex hydrocarbon compounds with an
organic structure similar to that of the base fuel. These tars evaporate from the biomass
at temperatures between about 400 and 600
C. Tars condense at temperatures below
200
C forming a sticky layer at the walls of the furnace and other equipment.
Further heating up to temperatures of more than 600
C results in the conversion of
the solid intermediate into char. This leads to a split-off of carbonmonoxide and hydro-
gen. With increasing temperature, secondary cracking reactions cause the formation of
light gas components such as hydrogen and carbonmonoxide and also soot from the tar
compounds. The fraction of the individual volatile components and the history of their
release depend on the fuel type, the final temperature, and the heating rate.With increas-
ing heating rate, the devolatilization maxima of the components shift toward relative
later moments and thus higher temperatures. Higher end temperatures lead to a larger
amount of volatile matter. The content of volatilematter determined at the high temper-
ature and heating rate of an entrained flow reactor may amount to 1.1
1.8 times the
amount detected in a proximate analysis at a relatively low temperature and heating rate.
The volatile matter content of biomass is in general much higher than that of coal.
Therefore, during the combustion of biomass, the pyrolysis stage lasts a relatively
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