Environmental Engineering Reference
In-Depth Information
Ash-related problems such as agglomeration may occur and exchange of the sand
bed may be required after some time [39]. One way to overcome some problems
with agglomeration is to use a mixture of biomass and peat in order to increase the
melting point of the raw material in the furnace. It is also possible to use lime
(which contains CaO and other Ca salts) instead of quartz in the fluidized bed
reactor to increase the agglomeration temperature of some biomass types under
the gasification conditions [40].
In CFB systems, biomass particles are placed on a bed of hot sand and all of the
materials including the gas, biomass particles, and sand move together. In CFB
reactors, char is ignited externally and separated using cyclones, and its residence
time is almost the same as that of gas. In addition, heated solids are returned to the
gasifier after the removal of flue gasses [41]. The main advantages of CFB reac-
tors over alternative designs are their flexibility with respect to feed biomass, high
biomass flow rate, scalability, and low amount of tar production.
Entrained flow reactors are based on a simple technology whereby fine solid
particles are injected into a high-velocity oxygen stream and undergo rapid gasi-
fication at high temperature (up to 1,400°C). These reactors have not been very
successful due to their poor rates of heat transfer between gases and solids. In
addition, they suffer from high sample preparation costs due to the very small
particle sizes required (100 µm to 1 mm) and their need for substrates with very
low moisture contents [41].
7.4.2
Gasification
Gasification involves the conversion of carbon from biomass into gases (CO,
CO 2 ) in the presence of controlled quantities of air at high temperatures (above
800°C). The resulting CO 2 reacts with hydrogen and is directly converted into
methane. Certain other chemicals can also be formed in the presence of appropri-
ate catalysts including diesels and 1-alkenes [42]. Notably, such processes are
used in the production of Fischer-Tropsch (FT) diesel.
7.4.2.1
Fischer-Tropsch Diesel
In 1923, two German scientists, Franz Fischer and Hans Tropsch, investigated
the conversion of syngas (CO, CO 2 , and H 2 ) into a number of different useful
organic compounds. FT diesel can be produced by the gasification of coal or
biomass. Coal or carbohydrates of different molecular size from biomass react
with oxygen and steam to generate CO and H 2 via the Lurgi process. The result-
ing mixture is called synthetic gas (or syngas or synthesis gas) and can be
directly used in gas turbines for power generation. Alternatively, in the presence
of different catalysts and under different process conditions, it can be converted
into different fuels such as methanol, DME, methane, or FT diesel. The compo-
sition of the products is determined by process parameters such as the
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