Environmental Engineering Reference
In-Depth Information
λ
stoichiometric oxygen ratio (=1 for complete combustion)
[
-
]
m
−1
K
−1
]
λ
thermal conductivity
[W
m
−3
]
ρ
density
[kg
τ
p
tortuosity of a particle
[
-
]
thiele modulus for n
th
-order reaction, def.: Equation (10.19)
ϕ
n
[
-
]
s
−1
]
φ
m
mass flow
[kg
Subscripts
0
initial
b
reverse reaction (rate coefficient) or in the bulk phase (concentration)
f
forward reaction (rate coefficient)
g
grain
HE
heat exchanger
n
of an nth-order reaction
p
particle
s
at the surface
t
at time t
vap
vaporization
after infinitely long time
∞
10.1 WHAT IS GASIFICATION? A CHEMICAL AND
ENGINEERING BACKGROUND
10.1.1
Introduction
Gasification is a thermochemical fuel conversion technology carried out at high tem-
perature using a gaseous agent (mostly oxidizing, sometimes also reducing) to convert
a liquid or solid fuel into a combustible product gas. When in this process air is used as
the oxidizing gas, a product gas is generated that contains H
2
, CO, CO
2
,CH
4
, and
other higher molecular weight hydrocarbons together with H
2
O and a bulk N
2
concentration. This gas is also called
(Knoef, 2012; Milne et al.,
1998). Other oxidizing gases that can be used to gasify a carbon-based resource
are steam, (more or less pure) oxygen, CO
2
, or steam/oxygen mixtures. The product
gas mixture then generated is also known as (raw, uncleaned) biosyngas.
The technology can be integrated with many end uses of the produced gas. Gas-
ification can be used for (sole) heat generation, (combined heat and) power produc-
tion, as well as biofuel and chemical production (e.g., methanol, diesel). Figure 10.1
shows a generic schematic regarding the technology chain. This chapter deals with
gasification fundamentals and reactor technologies.
“
producer gas
”
10.1.2 Chemical Reactions
The process of biomass gasification is characterized by several different subprocesses
(steps), namely, particle drying, pyrolysis, and gasification. Pyrolysis
—
also called
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