Page 178 - Biomass Gasification, Pyrolysis And Torrefaction Practical Design and Theory

Chapter | 5 Pyrolysis 155

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Hydrogen Carbon dioxide
Carbon monoxide Hydrocarbons

FIGURE 5.6 Release of gases during dry distillation of wood. Source: Drawn based on the
data of Nikitin (1966).

Conventional pyrolysis involves all three types of pyrolysis product
(gas, liquid, and char). As such, it heats the biomass at a moderate rate to a

moderate temperature (B600 C). The product residence time is on the order
of minutes.
5.2.2.2 Fast Pyrolysis
The primary goal of fast pyrolysis is to maximize the production of liquid or
bio-oil. The biomass is heated so rapidly that it reaches the peak (pyrolysis)
temperature before it decomposes. The heating rate can be as high as
1000 10,000 C/s, but the peak temperature should be below 650 Cif

bio-oil is the product of interest. However, the peak temperature can be up to

1000 C if the production of gas is of primary interest. Fluidized beds similar
to the one shown in Figures 5.5 and 5.7A,B may be used for fast pyrolysis.
Four important features of the fast pyrolysis process that help increase
the liquid yield are (i) very high heating rate, (ii) reaction temperature within
the range of 425 600 C, (iii) short residence time (,3 s) of vapor in the

reactor, and (iv) rapid quenching of the product gas.
5.2.2.3 Flash Pyrolysis
In flash pyrolysis, biomass is heated rapidly in the absence of oxygen to a rela-

tively modest temperature range of 450 600 C. The product, containing con-
densable and noncondensable gas, leaves the pyrolyzer within a short residence
time of 30 1500 ms (Bridgwater, 1999). Upon cooling, the condensable vapor
is then condensed into a liquid fuel known as “bio-oil.” Such an operation
increases the liquid yield while reducing the char production. A typical yield
of bio-oil in flash pyrolysis is 70 75% of the total pyrolysis product.