Page 174 - Biomass Gasification, Pyrolysis And Torrefaction Practical Design and Theory

Chapter | 5 Pyrolysis 151

Cyclone
Noncondensable
gases

Biomass
Gas condenser

Oil

Char
Screw feeder collection Bio-oil
storage
Pyrolyzer

Gas burner

FIGURE 5.5 Simplified layout of a pyrolysis plant.

chamber as product gas, which is the product of interest. Similarly, the solid
char may be collected as a commercial product or burned in a separate
chamber to produce heat that is necessary for pyrolysis. As this gas is free
from oxygen, part of it may be recycled into the pyrolysis chamber as a heat
carrier or fluidizing medium. There are, of course, variations of the process,
which will be discussed later.

5.2.1 Pyrolysis Products

As mentioned earlier, pyrolysis involves a breakdown of large complex
molecules into several smaller molecules. Its product is classified into three
principal types:
1. Liquid (tars, heavier hydrocarbons, and water)
2. Solid (mostly char or carbon)
3. Gas (e.g., CO 2 ,H 2 O, CO, C 2 H 2 ,C 2 H 4 ,C 2 H 6 ,C 6 H 6 ).
The relative amounts of these products depend on several factors including
the heating rate and the final temperature reached by the biomass.
The pyrolysis product should not be confused with the “volatile matter”
of a fuel as determined by its proximate analysis. In proximate analysis, the
liquid and gas yields are often lumped together as “volatile matter,” and the
char yield as “fixed carbon.” Since the relative fraction of the pyrolysis
yields depends on many operating factors, determination of the volatile mat-
ter of a fuel requires the use of standard conditions as specified in test codes
such as ASTM D-3172 and D-3175. The procedure laid out in D-3175, for
example, involves heating a specified sample of the fuel in a furnace at

950 C for 7 min to measure its volatile matter.