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Water for Energy and Fuel Production
solids in both the loop seal and the chute are fluidized with steam, which effectively
prevent gas leakage between gasification and combustion zone, and also allow high
solid throughput. The temperature difference between the combustion and the gasification reactor is determined by the energy needed for gasification as well as bed
material circulation rate. The system is inherently auto-stabilizing since a decrease in
the gasification temperature leads to a higher amount of residual char, which results
in more fuel for the combustion reactor. This, in turn, transports more energy into
the gasification zone and therefore stabilizes the temperature. Both the gasifier and
the combustor operate at atmospheric pressure. The process yields two separate gas
streams: a high-quality producer gas and a conventional flue gas at high temperatures.
The high-quality producer gas contains low amounts of tars and nitrogen, and high
concentration of hydrogen. For practical use, olivine, a natural mineral, has proven
to be a suitable bed material with enough resistance to attrition and moderate tarcracking activity [72–84]. This concept was proven to be more efficient for converting
the primary fuel energy into producer gas than conventional dual fluidized bed steam
gasification because of the lower operating temperature.
4.6.1.2.3 Entrained Bed Reactor
This type of reactor uses fine solids and high rate of gas flow to provide uniform
temperature distribution and low residence time within the reactor [5]. The reactor is
generally operated at high temperatures so that the tar and methane concentrations
in the product gases are very low. The oxygen requirement in this type of gasifier
is higher than those in other types of gasifiers. All entrained bed gasifiers remove
the major part of the ash as a slag as the operating temperature is well above the ash
fusion temperature. These types of gasifiers do not suffer from corrosive slags and
can better handle biomass that can generate corrosive slag.
For processing fuels with very high ash fusion temperatures, some limestone is
mixed with fuel, which lowers the ash fusion temperature. The need for fine solids
requires the fuel pulverization process before gasification. The reactor needs more
energy due to fuel pulverization and the production of oxygen that is used for
gasification.
4.6.1.3 Plasma and Free radical Gasifiers
Both of these types of gasifiers use either thermolytic, photolytic, or high-voltage
torch to supply heat for the gasification process. These are high energy intensive reactors and mostly produce clean syngas. They are not often used for steam gasification.
4.6.1.4 molten salt steam Gasification reactors
There are at least four different designs that use molten salt media to gasify coal in the
presence of steam [1,3]. Two of these four designs, namely, Rockwell molten salt gasifier and Rummel–Otto single-shaft gasifier, are graphically illustrated in Figure 4.3a
and b, respectively. Here, we briefly describe the remaining two, namely, Kellogg–
Pullman molten salt process and Atgas molten iron coal gasification process.
4.6.1.4.1 Kellogg–Pullman Molten Salt Process
In this process, the coal is gasified in a bath of molten sodium carbonate through
which steam is passed. The process offers the following advantages [1,3]: