Page 254 - Biomass Gasification, Pyrolysis And Torrefaction Practical Design and Theory

230 Biomass Gasification, Pyrolysis and Torrefaction

7.4.2.2 Effect of Mineral Matter in Biomass
Inorganic materials in fuels can act as catalysts in the char oxygen reaction
(Zolin et al., 2001). In coal, inorganic materials reside as minerals, whereas
in biomass they generally remain as salts or are organically bound. Alkali
metals, potassium, and sodium are active catalysts in reactions with oxygen-
containing species. Dispersed alkali metals in biomass contribute to the high
catalytic activity of inorganic materials in biomass. In coal, CaO is also dis-
persed, but at high temperatures it sinters and vaporizes, blocking
micropores.
Inorganic matter also affects pyrolysis, giving char of varying morpho-
logical characteristics. Potassium and sodium catalyze the polymerization of
volatile matter, increasing the char yield; at the same time, they produce
solid materials that deposit on the char pores, blocking them. During subse-
quent oxidation of the char, the alkali metal catalyzes this process.
Polymerization of volatile matter dominates over the pore-blocking effect. A
high pyrolysis temperature may result in thermal annealing or loss of active
sites and thereby loss of char reactivity (Zolin et al., 2001).

7.4.2.3 Intrinsic Reaction Rate
Char gasification takes place on the surface of solid char particles, which is
generally taken to be the outer surface area of the particle. However, char
particles are highly porous, and the surface areas of the inner pore walls are
several orders of magnitude higher than the external surface area. For exam-
ple, the actual surface area (BET, named after Brunaeur, Emma, and Teller)
2
of an internal pore of a 1 mm diameter beech wood char is 660 cm , while
2
its outer surface is only 3.14 cm . Thus, if there is no physical restriction, the
reacting gas can potentially enter the pores and react on their walls, resulting
in a high overall char conversion rate. For this reason, two char particles
with the same external surface area (size) may have widely different reaction
rates because of their different internal structure.
From a scientific standpoint, it is wise to express the surface reaction rate
on the basis of the actual surface on which the reaction takes place rather
than the external surface area. The rate based on the actual pore wall surface
area is the intrinsic reaction rate; the rate based on the external surface area
of the char is the apparent reaction rate. The latter is difficult to measure,
so sometimes it is taken as the reactive surface area determined indirectly
from the reaction rate instead of the total pore surface area measured by the
physical adsorption of nitrogen. This is known as the BET area (Klose and
Wolki, 2005).

7.4.2.4 Mass-Transfer Control
For the gasification reaction to take place within the char’s pores, the react-
ing gas must enter the pores. If the availability of the gas is so limited that it