Biomedical Engineering Reference
In-Depth Information
Therefore, the gasification of biomass is similar to the gasification of coal (which also contains
small fraction of H) in CO:H 2 ratio. Therefore, the energetics in gasification of coal and ligno-
cellulosic biomass are similar on the same carbon mass basis. Another feedstock of interest is
the black liquor in a chemical pulp mill. Currently, the black liquor is being turned to recover
the pulping chemicals and thermal energy that is needed in the pulp and paper making
processes. The black liquor gasification is therefore not going to make a significant contribu-
tion to our needs in liquid fuel or hydrocarbon, but it can improve the pulp mill energy
efficiency.
In gasification, O 2 is usually added for energetics reasons. The gasification reactions
without oxygen are indothermic, requiring energy to complete. Partially burning H 2 to
H 2 OorCOtoCO 2 can produce the heat needed for the reactions to complete. To increase
the availability of H 2 , the steam shift reaction,
CO
þ
H 2 O
/
CO 2 þ
H 2
(5.28)
can be carried out.
Gasidfication is employed today, for example, in ammonia production. Ammonia has
wide applications. It is a precursor for urea, a commonly used fertilizer in agriculture. The
ammonia synthesis reactions will be discussed in Chapter 9 (section Chemical Reactions
on Nonideal Surfaces Based on Distribution of Interaction Energy). The hydrogen gas is
provided by gasification.
This FT process together with gasification may one day be the dominant method by which
we will obtain some of our liquid fuels from renewable biomass, and coal. This technology is
capable of supplying at least 200 years of liquid hydrocarbons at current consumption rates
from known proven reserves of coal. The biomass as renewable resources, on the other hand,
can supply our needs for liquid fuel “indefinitely.”
Two types of reactors will be discussed, a straight-through transport reactor , which is also
referred to as a riser or circulating fluidized bed , and a PBR, which is also referred to as
a fixed-bed reactor.
Riser . Because the catalyst used in the process decays rapidly at high temperatures
(e.g. 350 C), a straight-through transport reactor is ideal. This type of reactor is also called
a riser and/or a circulating bed. A schematic diagram is shown in Fig. 5.6 . Here the catalyst
particles are fed to the bottom of the reactor and are shot up through the reactor together
with the entering reactant gas mixture and then separated from the gas in a settling hopper.
The volumetric gas feed rate is high in order to suspend and transport the particles upward
against the gravity. In Sasol plant, South Africa, the gas feed rate is 3
10 5 m 3 /h.
A schematic of an industrial straight-through transport reactor used at Sasol are shown in
Fig. 5.7 together with the composition of the feed and product streams. The products that
are condensed out of the product stream before the stream is recycled include Synoil
(a synthetic crude), water, methyl ethyl ketone, alcohols, acids, and aldehydes. The reactor
is operated at 25 atm and 350 C and at anyone time contains 150 tons of catalyst. The
catalyst feed rate is 6 e 9.5 tons/s, and the gas recycle ratio is 2:1.
Packed-Bed . Transport of solid particles upward requires high fluid velocity and thus
energy consumption. To reduce the energy consumption in the reactor, the solid catalyst
can be fixed in the reactor. The packed-bed reactor used at the Sasol plant, South Africa to
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