Page 196 - Biomass Gasification, Pyrolysis And Torrefaction Practical Design and Theory
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Chapter | 5 Pyrolysis 173
A higher heating rate, a higher temperature, and a shorter residence time
maximize the gas yield.
A higher heating rate, an intermediate temperature, and a shorter residence
time maximize the liquid yield.
There is an optimum pyrolysis temperature for maximum liquid yield.
The yield is highest at 500 C and drops sharply above and below this tem-
perature (Boukis et al., 2007). The residence time is generally in the range
of 0.1 2.0 s. These values depend on several factors, including the type of
biomass (Klass, 1998). We can use a kinetic model for a reasonable assess-
ment of the yield. The one proposed by Liden et al. (1988) may be used for
predicting pyrolysis liquid yields over a wide range of conditions.
Heat transfer is a major consideration in the design of a pyrolyzer.
The heat balance for a typical pyrolyzer may be written as:
½Heat released by char combustion 1 ½Heat in incoming stream
(5.10)
5 ½Heat required for pyrolysis 1 ½Heat loss
Assessing heat loss accurately is difficult before the unit is designed. So,
for preliminary assessment, we can take this to be 10% of the heat in the
incoming stream (Boukis et al., 2007, p. 1377).
Fast, or flash, pyrolysis is especially suitable for pyrolytic liquefaction of
biomass. The product is a mixture of several hydrocarbons, which allows
production of fuel and chemicals through appropriate refining methods. The
heating value of the liquid produced is a little lower or in the same range
(13 18 MJ/kg) as that of the parent biomass. The pyrolytic liquid contains
several water-soluble sugars and polysaccharide-derivative compounds and
water-insoluble pyrolytic lignin.
Pyrolytic liquid contains a much higher amount of oxygen (B50%) than
does most fuel oil. It is also heavier (specific gravity B1.3) and more viscous.
Unlike fuel oil, pyrolytic oil increases in viscosity with time because of poly-
merization. This oil is not self-igniting like fuel oil, and as such it cannot be
blended with diesel for operating a diesel engine.
Pyrolytic oil is, however, a good source of some useful chemicals, like
natural food flavoring, that can be extracted, leaving the remaining product
for burning. Alternately, we can subject the pyrolytic oil to hydrocracking to
produce gasoline and diesel.
5.8 BIOCHAR
Charcoal, also known as biochar, is a preferred product of slow pyrolysis at
a moderate temperature. Thermodynamic equilibrium calculation shows that
the char yield of most biomass may not exceed 35%. Table 5.6 gives the
theoretical equilibrium yield of different products of biomass at different
