Pyrolysis - Biomass Gasification, Pyrolysis and Torrefaction - page 158

300 C under pressure;
in the second stage, the produced hydrocarbon is cracked into lighter hydro-
carbon at a temperature of around 500 C (Appel et al., 2004). High oxygen
content is an important shortcoming of bio-oil. Hydropyrolysis can produce
bio-oil with reduced oxygen.

process, the first stage takes place in water at 200

5.3 PYROLYSIS PRODUCT YIELD
The product of pyrolysis depends on the design of the pyrolyzer, the physical
and chemical characteristics of the biomass, and important operating para-
meters such as:

Heating rate
Final temperature (pyrolysis temperature)
Residence time in the reaction zone.

Besides these, the tar and the yields of other products depend on (i) pres-
sure, (ii) ambient gas composition, and (iii) presence of mineral catalysts
(Shafizadeh, 1984).
By changing the final temperature and the heating rate, it is possible
to change the relative yields of the solid, liquid, and gaseous products of
pyrolysis. Rapid heating yields higher volatiles and more reactive char than
those produced by a slower heating process; slower heating rate and longer
residence time result in secondary char produced from a reaction between
the primary char and the volatiles.

5.3.1 Effect of Biomass Composition
The composition of the biomass, especially its hydrogen-to-carbon (H/C)
ratio, has an important bearing on the pyrolysis yield. Each of the three
major constituents of a lignocellulosic biomass has its preferred temperature
range of decomposition. Analysis of data from thermogravimetric apparatus
differential thermogravimetry on some selected biomass suggests the follow-
ing temperature ranges for initiation of pyrolysis (Kumar and Pratt, 1996):

350 C

Hemicellulose: 150

350 C Cellulose: 275

500 C. Lignin: 250

The individual constituents undergo pyrolysis differently, making varying
contributions to yields. For example, cellulose and hemicellulose are the
main sources of volatiles in lignocellulose biomass. Of these, cellulose is a
primary source of condensable vapor. Hemicellulose, on the other hand,
yields more noncondensable gases and less tar than released by cellulose
(Reed, 2002, p. II-109). Owing to its aromatic content,

lignin degrades

slowly, making a major contribution to the char yield.