Page 162 - Synthetic Fuels Handbook

148 CHAPTER FIVE

In the entrained flow gasifier a dry pulverized solid, an atomized liquid fuel, or a fuel
slurry is gasified with oxygen (much less frequent: air) in cocurrent flow. The gasification
reactions take place in a dense cloud of very fine particles. Most coals are suitable for this
type of gasifier because of the high operating temperatures and because the coal particles
are well separated from one another. The high temperatures and pressures also mean that
a higher throughput can be achieved; however, thermal efficiency is somewhat lower
as the gas must be cooled before it can be cleaned with existing technology. The high
temperatures also mean that tar and methane are not present in the product gas; however
the oxygen requirement is higher than for the other types of gasifiers. All entrained flow
gasifiers remove the major part of the ash as a slag as the operating temperature is well
above the ash fusion temperature. A smaller fraction of the ash is produced either as a
very fine dry fly ash or as black colored fly ash slurry. Some fuels, in particular certain
types of biomasses, can form slag that is corrosive for ceramic inner walls that serve to
protect the gasifier’s outer wall. However some entrained bed type of gasifiers do not
possess a ceramic inner wall but have an inner water- or steam-cooled wall covered with
partially solidified slag.
These types of gasifiers do not suffer from corrosive slag. Some fuels have ashes
with very high ash fusion temperatures. In this case mostly limestone is mixed with the
fuel prior to gasification. Addition of a little limestone will usually suffice for lowering
the fusion temperatures. The fuel particles must be much smaller than for other types
of gasifiers. This means the fuel must be pulverized, which requires somewhat more
energy than for the other types of gasifiers. By far the most energy consumption related
to entrained bed gasification is not the milling of the fuel but the production of oxygen
used for the gasification.
In high temperature conditions (typically 1300–1400°C) high boiling oils and tars are
almost completely destroyed. This type of gasifier was developed for coal and limited
experience with biomass is available.
A more recent development is the open core gasifier design for gasification of small-
sized biomass with high ash content. However, the producer gas is not tar free.
In the open core gasifier the air is sucked over the whole cross section from the
top of the bed. This facilitates better oxygen distribution since the oxygen will be
consumed over the whole cross section, so that the solid bed temperature will not
reach the local extremes (hot spots) observed in the oxidation zone of conventional
gasifiers due to poor heat transfer. Moreover, the air nozzles in conventional gasifiers
generate caves and create obstacles that may obstruct solid flow especially for solids
of low bulk (e.g., rice husk). On the other hand, the entry of air through the top of
the bed creates a downward flow of the product gases thereby transporting the tar
products to the combustion zone.
Finally, air-blown reactors (of any of the above type) produce reaction heat by partial
oxidation inside the reactor and the product gas is diluted with nitrogen. The use of oxygen
(or oxygen-enriched air) results in more concentrated gases. Indirect heating of the reactor
is achieved by means of hot solids or through heat exchanger walls.

5.5.2 Gaseous Products
The products of coal gasification are varied insofar as the gas composition varies with
the system employed and the predetermined follow-up to the production of products
(Fig. 5.13). It is emphasized that the gas product must be first freed from any pollutants
such as particulate matter and sulfur compounds before further use, particularly when
the intended use is a water gas shift or methanation (Cusumano et al., 1978; Probstein
and Hicks, 1990).