Page 241 - Biomass Gasification, Pyrolysis And Torrefaction Practical Design and Theory

218 Biomass Gasification, Pyrolysis and Torrefaction

slurry. The gas velocity in the reactor is sufficiently high to fully entrain the
fuel particles. Slurry-fed gasifiers need additional reactor volume for evapo-
ration of the large amount of water mixed with the fuel. Furthermore, their
oxygen consumption is about 20% greater than that of a dry-feed system
owing to higher blast requirements (Higman and van der Burgt, 2008).
Entrained-flow gasifiers are of two types depending on how and where
the fuel is injected into the reactor. Chapter 8 discusses several types. In all
of these designs, oxygen enters the reactor and reacts rapidly with the vola-
tiles and char in exothermic reactions. These raise the reactor temperature
well above the melting point of ash, resulting in complete destruction of tar.
Such high temperatures should give a very high level of carbon conversion.
An entrained-flow gasifier may be viewed as a plug-flow reactor.
Although the gas is heated to the reactor temperature rapidly upon entering,
solids heat up less slowly along the reactor length because of the solid’s
larger thermal capacity and plug-flow nature, as shown in Figure 7.7.Some
entrained-flow reactors are modeled as stirred tank reactors because of the
rapid mixing of solids.

7.4 KINETICS OF GASIFICATION

Stoichiometric calculations (Section 3.6.6) can help determine the products
of a completed reaction. Not all reactions in gasifier are instantaneous and
completely convert reactants into products. Many of the chemical reactions
discussed in the preceding sections proceed at a finite rate and to a finite
extent.
To what extent a reaction progresses is determined by its equilibrium
state. Its kinetic rates, on the other hand, determine how fast the reaction
products are formed and whether the reaction completes within the gasifier
chamber. A review of the basics of chemical equilibrium may be useful
before discussing its results.

7.4.1 Chemical Equilibrium

Let us consider the reaction:
k for
nA 1 mB ! pC 1 qD (7.27)
where n, m, p, and q are stoichiometric coefficients. The rate of this reaction,
r 1 , depends on C A and C B , the concentration of the reactants A and B, respec-
tively as below:
n
r 1 5 k for C C m (7.28)
A B
The reaction can also move in the opposite direction:
k back
pC 1 qD ! nA 1 mB (7.29)