Page 175 - Biomass Gasification, Pyrolysis And Torrefaction Practical Design and Theory

152 Biomass Gasification, Pyrolysis and Torrefaction

5.2.1.1 Liquid
The liquid yield, known as tar, bio-oil, or biocrude, is a black tarry fluid
containing up to 20% water. It consists mainly of homologous phenolic com-
pounds. Bio-oil is a mixture of complex hydrocarbons with large amounts
of oxygen and water. While the parent biomass has a lower heating value
(LHV) in the range of 19.5 21 MJ/kg dry basis, its liquid yield has a lower
LHV in the range of 13 18 MJ/kg wet basis (Diebold et al., 1997).
Rapid and simultaneous depolymerization and fragmentation of the cellu-
lose, hemicellulose, and lignin components of biomass produce bio-oil. In a
typical operation, the biomass is subjected to a rapid increase in temperature
followed by an immediate quenching to “freeze” the intermediate pyrolysis
products. Rapid quenching is important, as it prevents further degradation,
cleavage, or reaction with other molecules (see Section 5.4.2 for more details).
Bio-oil is a microemulsion, in which the continuous phase is an aqueous
solution of the products of cellulose and hemicellulose decomposition, and
small molecules from lignin decomposition. The discontinuous phase is
largely composed of pyrolytic lignin macromolecules (Piskorz et al., 1988).
Bio-oil typically contains molecular fragments of cellulose, hemicellulose,
and lignin polymers that escaped the pyrolysis environment (Diebold and
Bridgwater, 1997). The molecular weight of the condensed bio-oil may
exceed 500 Daltons (Diebold and Bridgwater, 1997, p. 10). Compounds found
in bio-oil fall into the following five broad categories (Piskorz et al., 1988):
Hydroxyaldehydes
Hydroxyketones
Sugars and dehydrosugars
Carboxylic acids
Phenolic compounds.

5.2.1.2 Solid
Biochar is the solid yield of pyrolysis. It is primarily carbon (B85%), but it
can also contain some oxygen and hydrogen. Unlike fossil fuels, biomass
contains very little inorganic ash. The LHV of biomass char is about 32 MJ/kg
(Diebold and Bridgwater, 1997), which is substantially higher than that of
the parent biomass or its liquid product. It is characterized by large pore
surface area.

5.2.1.3 Gas
Primary decomposition of biomass produces both condensable gases (vapor)
and noncondensable gases (primary gas). The vapors, which are made
of heavier molecules, condense upon cooling, adding to the liquid yield of
pyrolysis. The noncondensable gas mixture contains lower-molecular-weight
gases like carbon dioxide, carbon monoxide, methane, ethane, and ethylene.