Note: Descriptions are shown in the official language in which they were submitted.
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K 5661
PROCESS FOR THE PARTIAL ccMæusTIoN OF
SOLID FUEL WITH FLY ASH RECYCLE
The present invention relates to a pr w ess for the partial
combustion of a finely divided solid fuel, and more particularly
Jo such a process wherein the generated fly ash is recirculated
to the combustion space.
In a well knswn process for the partial combustion - also
called gasification of solid fuel, such as coal and similar
carbonaceous substances, finely divided solid fuel is passed
into a gasifier at a relatively high pressure. In the gasifier a
hot flame is maintained in which the solid fuel reacts with
oxygen, supplied as pure oxygen or an oxygen-containing gas,
such as air. The solid fuel contains as useful components mainly
carbon and hydrogen, which react with the oxygen to orm a
product gas mainly consisting of coal monoxide and hydrogen.
Apart from carbon and hydrogen, mineral fuel such as coal
always contains certain quantities of inorganic, incombustible
matter, which is in the combustion process partly collected in
the bottom part of the gasifier as slag and partly entrained with
the product gas. Depending on the type of fuel and the conditions
during the combustion process the product gas may further
contain particulates consisting of unconverted coal. The total
mass of incombustible matter and unconverted coal entrained with
the product gas is normally indicated with the expression fly
ash.
For working up or usage of the product gas obtained with
the partial combustion of solid fuel, the presence of fly ash in
the product gas forms a disadvantage. It is therefore necessary
to separate the fly ash from the product gas prior to worklng up
or usage thereof. Various types of equipment are kncwn for
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separating the fly ash from the product gas, to obtain clean
product gas and solid fly ash. The obtained separated solid fly
ash, hcwever, has some unfavourable properties. Firstly, fly ash
consists of very fine, porous particles - with sizes in the
range of 5-60.10 6 _ having a low bulk density, making storage
thereof very inefficient. Secondly, fly ash normally contains
salts of metals, which may leach from the porous fly ash
particles, if contacted with water. This aspect makes it
inappropriate to dump fly ash on a refuse dumping ground, as the
bottom thereof might be inadmissably polluted due to such
leaching. For storing fly ash it is thereof necessary to use
specially constructed, expensive storage spaces. It should
further be noted that fly ash normally contains a valuable
portion in the form of unconverted coal, which is in fact throwr.
away if fly ash is merely dumped.
The object of the present invention is to overcome the
above disadvantages associated with the handling of fly ash, and
to make use of the valuable portion in the fly ash in an effective
manner.
The process for the partial combustion of a finely divided
solid fuel with fly ash recycle comprises according to the invention
the steps of contacting finely divided solid fuel with oxygen in a
hot flame in a reactor for partial combustion thereof, withdrawing
product gas containing fly ash particles frcm the reactor,
separating the fly ash particles from the product gas, mixing
the separated fly ash particles with finely divided solid fuel,
forming a fluidized mass of fly ash particles and the fresh solid
fuel by in-troducing a gaseous medium therein and transporting the
fluidized mass with the gaseous medium to the reactor for
partial combustion thereof.
The above process according to the invention offers a
plurality of advantages over known techniques for fly ash
handling. Firstly, by recirculating the fly ash to the reactor,
the valuable portion thereof, i.e. unconverted coal, can be
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converted into product gas, resulting in a better gasification
efficiency. Secondly, recirculation of the fly ash to the
reactor has a further effect in that at least a portion of the
fly ash will be converted into environmentally more acceptable
slag, which can be withdrawn from the bottcm part of the reactor
Thirdly, the fly ash is recycled to the reactor together
with solid fuel requiring only a relatively small quantity of
carrier gas. Pneumatic transport of merely fly ash through a
pipeline system is rather difficult, for two major reasons. The
very light and small particles forming the fly ash as well as
the rather sticky nature of fly ash easily cause clogging of
the valves and other parts of a pipeline system. Pneumatic
transport of fly ash is therefore only feasible when extremely
large amounts of carrier gas are used. The introduction of such
huge amounts of carrier gas into the reactor, will have an
adverse influence on the ccmbustion process in the reactor,
resulting in a less valuable gasification product and in a less
effective use of the solid fuel. It has now been found that the
total quantity of carrier gas, necessary for transporting the
fly ash to the reactor can be considerably reduced by intensively
mixing the fly ash with fresh solid fuel prior to further
transport to the reactor. It has further been found that, if the
fly ash forms about 30 per cent by weight at most of the mlxture
of fly ash and solid fuel, the total required amount of carrier
gas for transporting a mixture of fly ash and solid fuel can be
kept substantially equal to the amount of carrier gas necessary
for transporting solid fuel only.
m e invention will now be described by way of example only
m more detail with reference to the accompanying drawings, in
which
Figure 1 shows a first flcw scheme for a process for
partial ccmbustion of solid fuel with fly ash recycle according
to the invention; and
Figure 2 shows an alternative of the flow scheme shown in
Figure 1.
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It will be appreciated that identical elements shown in the
drawings have been indicated with the same reference numeral.
Reference is now made to the first flow scheme shown in
Figure 1. For the generation of product gas by partial combustion
of a carbonaceous solid fuel, finely divided solids of the feed
material employed is passed together with carrier gas through
line 2 towards a reactox 1. The solid fuel is intxoduced into
the reactor 1 via a plurality of burners (not shown), through
which simultaneously combustion air or another oxygen source is
supplied to the fuel for partial combustion thereof. In the
reactor 1 the finely divided solids are converted into product
gas, ash and slag. The slag is collected in the bottcm part of
the reactor vessel and can be withdrawn therefrom via kncwn, not
shown, withdrawal systems. The product gas with entrained fly
ash is recovered over the top of the reactor 1 and is caused to
flow through line 3 towards separating means for removing the
fly ash frcm the product gas. In the shown process scheme these
separating means consists of a cyclone 4, into which product gas
is tangentially introduced to bring the gas into a swirling
motion. miS swirl motion causes a separation of product gas,
leaving the cyclone over the top via line 5 and fly ash, removed
from the bottcm part of the cyclone through line 6.
The fly ash is subsequently collected in a storage vessel 7
arranged below cyclone 4, and fram there sluiced out to a
further vessel 8. This vessel 8 is provided with (not shown)
means, for (de)pressurization and for separating entrained gas
from the fly ash, for example by aerating or fluidizing the mass
of fly ash. The gas separation is essential in order to wake
sure that no waxer vapour, which might cause corrosion problems,
will condense. The vessel 8 is further provided with a cooling
jacket for cooling dcwn the fly ash. When the desired tempera-
ture and pressure have been reached and entrained gas has been
sufficiently removed from the fly ash, the fly ash is sluiced
out from vessel 8 via line 10 to a mlxing vessel 9, provided
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with mixing means, such as a stirring device. In the mixing
vessel 9 the fly ash particles are mixed with solid fuel
introduced via line 11. The so formed mixture of fly ash
particles and solid fuel is subsequently stored in a storage
vessel 12. From said storage vessel 12 the mixture is introduced
into a further vessel 13 provided with (de)pressurizing means.
After the mixture has been brought at the reactor pressure,
the solid fuel/fly ash mass is sluiced out from said vessel 13
and introduced into a fluidization vessel 14. For further
transport of the solid fuel and fly ash, the mixture is fluidized
in said fluidization vessel 14 by introducing a gaseous medium
in the bottom part of the vessel via injection line 15. The
fluidized mixture of solid fuel and fly ash is subsequently
allowed to flow with the gaseous mcdium to the reactor 1 via
line 2 for partial combustion thereof. Tests have been carried
out to investigate how fly ash-coal muxtures would fluidize at
several fly ash/coal ratios. It has been found that depending on
the type of solid fuel sufficient fluidization of the solid fuel
with fly ash can be obtained at a quantity of fly ash of about
30 per cent by weight of the total amount of solid fuel and fly
ash in the fluidization vessel 14. In these tests the solid fuel
consisted of particles hav mg an average size of about 50.10 6
m, i.e. the normal size of solids for gasification in a reactor
by means of solid fuel burners.
The fluidization of pure fly ash is hardly possible due to
the sticky nature of fly ash. When gas is introduced into a bed
consisting merely of fly ash, channels will be formed in the fly
ash bed, through which the gas will escape without causing a
significant suspension of the fly ash particles in the gas flow.
This phenomenon can be explained from the fact that fly ash
particles easily stick together so that they could only be
brought into a suspended condition at excessive gas supplies.
When the fly ash particles are mixed with fuel solids, the risk
of sticking together of the fly ash particles is considerably
reduced, especially if a weight ratio fly ash:fuel solids of
maximal 30:70 is chosen. In this case the fly ash particles will
substantially behave like the fresh fuel solids, and as a
consequence thereof the mixture of fly ash and solid fuel can be
easily fluidized. The quantity of gas necessary for fluidizing a
mixture of fly ash and solid fuel, is substantially the same as
the quantity of gas necessary for fluidizing pure solid fuel.
The so formed homogeneous mass of solid fuel and fly ash is
subsequently allcwed to flow via line 2 towards the reactor 1.
In the reactor the solid fuel as well as the fly ash are
contacted with oxygen in a hot flame causing a conversion of the
solid fuel in valuable product gas, fly ash and slag and a
conversion of a least part of the fly ash in product gas and
slag. As a result thereof the total amount of fly ash which
circulates in the process with the shown flow scheme will remain
substantially constant. The reactivity of a solid fuel/fly ash
mixture is less than that of pure solid fuel, since such a
mixture contains fewer volatiles, less oxygen and more ash. This
drawback is, however, balanced by the reduced heat loss through
the reactor wall when operating on solid fuel and fly ash.
Experiments have shown that the gasification performance of a
coal/fly ash mixture, in terms of carbon conversion, thermal
efficiency, oxygen requirement for gas production was very much
comparable to that of pure coal. Since the recycled fly ash
contains unconverted carbon, which is at least partly converted
into product gas, the proposed process results in a lower total
solid fuel consumption. A reduction of the solid fuel consumption
of about 5 per cent by weight for the same gas production can be
attained.
During gasification of a solid fuel/fly ash mixture, more
slag will be formed than when gasifying pure solid fuel. Care
should therefore be taken that the system for withdrawing the
slag from the reactor bottom should be adapted to such a greater
slag formation.
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The amount of fly ash produced in the reactor 1 depends on
the type of fuel which is gasified and on the operating conditions
in the reactor. When using coal as solid fuel, the amount of fly
ash prodùced will be normally far below 30 per cent by weight of
the coal, or in other words far belaw the upper limit for
preparing a sufficiently fluidized mixture of coal and fly ash in
the fluidization vessel 14.
If solid fuels generating more than 30 per cent by weight
fly ash are to be processed, steps are to be taken to reduce the
amount of fly ash in the fluidization vessel 14 to maintain a
proper fly ash/solid fuel ratio necessary for fluidization of
the solid fuel/fly ash mixture. A possible solution might be for
example increasing the throughput of fresh solid fuel either or
not in ccmbination with intermittently processing solid fuel
with a low fly ash production for diminishing the surplus of fly
ash obtained from the solid fuel with high fly ash production.
It should be no'.ed that the composition of the formed fly
ash can be examined for ex~,~le by sampling the flcw through
line lO, to determine the amount of oxygen necessary for an
optimal gasification process in reactor 1.
Reference is now made to Figure 2 shcwing a second flow
scheme according to the invention. In this further scheme the
mixing vessel 9 has been replaced by a direct transport system
of fly ash to the fluidization vessel 14. Fly ash, sep3rated
fram product gas and brought in vessel 8 at the operating
pressure of reactor 1 is pneumatically transported from vessel 8
to a further vessel 20 via transport line 21. For this transport
a carrier gas, such as for example nitrogen is injected into
line 21 via an injector 22. Vessel 20 is formed by a cyclone,
into which the fly ash and carrier gas are tangentially
introduced. The carrier gas separated from the fly ash in
cyclone 20 is withdrawn through line 23. For mixing the fly ash
with fresh solid fuel the fly ash, collected in the bottom part
of vessel 20, is subsequently dosed to fluidization vessel 14
via a rotary valve 24 positioned in a connecting line 25.
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To enable the flow of fly ash through line 25 the
fluidization vessel 14 is arranged at a lcwer level than cyclone
vessel 20 and at a rather acute angle with respect to the
vertical. The angle of inclination with the vertical of line 25
is preferably not greater than about 20 degrees. Fresh solid
fuel from storage vessel 26, brought at the required reactor
operating pressure m vessel 27, is introduced via transfer line
28 into the fluidization vessel 14. Carrier gas, withdrawn frcm
line 23, is transported via line 29 into tile bottom part of
vessel 14. In this manner the solid fuel and fly ash particles
are fluidized, while these components are simultaneously muxed
with one another to form a substantially homogeneous mLxture of
solid fuel particles and recirculated fly ash particles. The so
formed mixture of solid and fly ash particles is subsequently
allowed to flow with the gaseous medium to the reactor 1 via
line 2.
It should be noted that the amount of fly ash supplied into
fluidization vessel 14 can be controlled by regulating rotation
of rotary valve 24. Further, the fly ash can be continuously or
intermittently dosed to vessel 14. Rotary valve 24 can be
replaced by another suitable dosing system, such as a set of
sluicing valves, wherein an appropriate fly ash sluicing volume
is available between said valves.
The fluidization vessel 14 is suitably provided with a
level control system for regulating the level of solids in said
vessel 14. When the mim mum solids level is reached due to for
example stagnation of the fly ash recycle or due to a sudden
lower fly ash production a signal is given to increase the
supply of fresh solid fuel from the solid fuel storage vessels,
Lo maintain a stable reactor operation. Pressure losses occuring
during the transport of fly ash from the reactor 1 to the
fluidization vessel 14 can be overcome by pressurizing the fly
ash in one or more of the fly ash vessels in the systen, for
example by injecting gas into said vessel(s). m e shown system
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for fly ash recycle can be further provided with means, not
shown, for sluicing out minor amounts of fly ash, which might be
necessary when processing solid fuel with an excessive high fly
ash production.
m e gaseous medium for the transport of fly ash and the
formation transport of the solid fuel fly ash mixture frcm
fluidization vessel 14 to the reactor may be for example a
sultable inert gas, such as nitrogen, or cooled product gas
produced in the reactor.
Finally it is r OE ked that the reactor 1 may be provided
with a plurality of burners for solid fuel, wherein a part of
these burners is used m the above described recycling process.
