Hydrothermal Gasification of Biomass - Biomass Gasification, Pyrolysis and Torrefaction - page 338

For synthetic natural gas (SNG) production, high methane, and low
hydrogen are required; therefore, we can choose a reaction temperature of
350 500 C, but catalysts are necessary for a reasonable yield. With cata-

lysts, methane-rich gas may be produced even just below the critical temper-
ature ( 350 C) (Mozaffarian et al., 2004).

B

9.7.2 Catalyst Selection
The choice of catalyst influences reactor temperature, product distribution, and
plugging potential. Section 9.4.2 discussed the catalysts used in SCW gasifica-
tion. They are selected on the basis of the desired product. Catalyst deactivation
is an issue assigned to most catalyzed reactions because the deactivated cata-
lysts must be regenerated. If they are deactivated because of carbon deposits, as
happens in a fluid catalytic cracker (FCC), they can be combusted by adding
oxygen, preferably in a separate chamber. The combustion reaction reactivates
the catalysts and can additionally provide enough heat for preheating the feed.

9.7.3 Reactor Size
Consider a simple reactor receiving W f of feed while producing W p of prod-
uct per unit of time. The product comprises a number of hydrocarbon com-
ponents represented by species i. The total carbon in the product gas is sum
of carbon in the individual gaseous hydrocarbons:

X W p C i α i kmol

Total carbon production in the product gas = s (9.9)

5

where α i is the number of carbon atoms in component i in the gas product;

C i is the mole fraction of i in the gas product; and W p is the product gas
flow-rate (kmol/s). The amount of carbon in the feed is known from the feed
rate, W f (kg/s), and its carbon fraction, F c . The carbon gasification yield, Y,
is defined as the ratio of gasified carbon to the carbon in the feed:

P i 12W p α i C i
W f F c

Y (9.10)

5

where 12 is the carbon's molecular weight (kg/kmol).
From Eq. (9.8) the reaction rate is given in terms of conversion as:

ln ð 1 X c Þ

2

k g 5 (9.11)

τ

where is the residence time in a reactor of volume V.

For a continuous stirred-tank reactor,

τ

V
Volume flow-rate of feed at reactor condition s

τ 5 (9.12)

Thus, for a known reaction rate, k g , and a desired conversion, X c , we can
estimate the reactor volume required for gasification.