Page 251 - Synthetic Fuels Handbook

FUELS FROM BIOMASS 237

producing a wide range of products. Thus, direct thermochemical conversion processes
including pyrolysis to produce fuels from biomass are also possible resulting in the produc-
tion of gaseous products (gaseous fuels), liquid products (liquid fuels), and charcoal (solid
fuels) (Kavalov and Peteves, 2005; Demirbas, 2007).
The pyrolysis of biomass is a thermal treatment that results in the production of char-
coal, liquid, and gaseous products. Thus, biomass is heated in the absence of air and
breaks down into a complex mixture of liquids, gases, and a residual char. If wood is
used as the feedstock, the residual char is what is commonly known as charcoal. With
more modern technologies, pyrolysis can be carried out under a variety of conditions to
capture all the components, and to maximize the output of the desired product be it char,
liquid, or gas.
The liquid fraction of the pyrolysis products consists of two phases: an aqueous phase
containing a wide variety of organooxygen compounds of low molecular weight and a
nonaqueous phase containing insoluble organics of high molecular weight. This phase is
called tar and is the product of greatest interest. The ratios of acetic acid, methanol, and
acetone of the aqueous phase were higher than those of the nonaqueous phase. The point
where the cost of producing energy from fossil fuels exceeds the cost of biomass fuels has
been reached. With few exceptions, energy from fossil fuels will cost more money than the
same amount of energy supplied through biomass conversion (Demirbas, 2007).
Wood contains 80 percent or more of volatile organic matter that may be recovered as a
gas and tar on pyrolysis. Pyrolysis of wood was at one time used for obtaining creosote oils
as well as acetic acid (wood vinegar) and some methanol (wood spirits). The gases evolved
(hydrogen, methane, carbon monoxide, and carbon dioxide) had no value for illumination
purposes and were used to supply the heat for the pyrolysis. Gasification of wood may be
used to produce either a low- or a medium-heat content gas if a gaseous fuel is desired, or
a synthesis gas that can be converted to liquid fuels using one of the indirect liquefaction
processes. Gasification followed by a synthesis step is expected to yield a higher-quality
liquid than can be obtained by pyrolysis.
The relative quantity of char, tars, and gases evolved on pyrolysis is strongly dependent
on the feedstock then on the rate of heating and the final temperature attained. Slow-heating,
low temperature pyrolysis favors char yields. After the initial decomposition, subsequent
coking and cracking reactions can result in a char that contains oxygen and hydrogen in
addition to the carbon.
Apart from char and water, some tars and gases are always produced. The watery distillate
evolved from wood at 160 to 175°C is called pyroligneous acid and contains 5 to 10 percent
acetic acid and 1.5 to 3 percent methanol. This used to be the source of methanol when it was
produced by the destructive distillation (pyrolysis) of wood.
The higher-boiling tar fraction produced at higher temperatures is more important as a source
of liquid fuels. It contains aromatic hydrocarbons and creosote oil evolved from the lignin, as
well as aliphatic compounds. The creosote oil, which consists of high-molecular-weight
phenols, used to be considered the most valuable constituent of the tar fraction, in part due to
its potential for resin manufacture. As a whole, the tar fraction is a viscous, highly oxygenated
oil that is unstable and corrosive.
The pyrolysis option for biomass is attractive because solid biomass and wastes can be
readily converted into liquid products. These liquids, as crude bio oil or slurry of charcoal
of water or oil, have advantages in transport, storage, combustion, retrofitting, and flexibility
in production and marketing.
The mechanism of the pyrolysis of biomass has been studied by many investigators
(Domburg et al., 1974; Adjaye et al., 1992; Demirbas, 2000) and is proposed to involve heat
variations, associated with the thermal degradation reactions, which affect the pyrolysis
route. Several endothermic and/or exothermic peaks for biomass pyrolysis are indicated
(Stamm and Harris, 1953; Shafizadeh et al., 1976). Accordingly, cellulose pyrolysis is