FIELD OF THE INVENTION
The present invention relates to coal gasification/combustion systems. More particularly,
the present invention related to an improved method for recycling, extracting and
disposing of fly ash formed and carried by the syngas in pressurized fluidized bed boilers
and gasifiers.
SUMMARY OF THE INVENTION
The present invention provides a process for the fluidized combustion / gasification of
coal, lignite or other solid fuels, in which the crushed coal is continuously injected into a
furnace or a refractory lined pressurized fluidized bed gasifier, air or a mixture of air and
steam is introduced through a distributor located at the bottom of the gasifier, from
which a stream of high-pressure and high-temperature raw synthesis gas is generated
and discharged together with fly ash and unburnt carbon or char. The invention allows
recycling of fly ash and extraction and disposal of fly ash in desired proportions in
fluidized bed boilers and gasifiers.
As opposed to the prior art, according to the present invention, the gas is produced by
pressurized fluidized bed gasification process, the fly ash formed is non-sticky, so
quench vessel and quench gas are not employed to remove sticky particles from the fly
ash.
BACKGROUND OF THE INVENTION
In a typical pressurized fluidized bed gasifier (01) (refer Fig. 01), air or air+steam
mixture is passed through a distributor, which is provided with a plurality of bubble cap
nozzles arrangement to fluidize the bed. During gasification process, by the partial
combustion of carbonaceous materials such as crushed coal, petroleum or biomass is
converted into carbon monoxide (CO) and hydrogen (H2) by reacting the raw material at
high temperatures with a controlled amount of steam, oxygen and/or air, the resulting
gas mixture is called synthesis gas or ^syngag and it is a fuel. The mineral matter in the
coal splits into two streams namely "Bottom ash" and "Fly ash". The heavier ash formed
in the bottom of the distributor is called "Bottom ash" and is extracted through a pipe
located at the bottom of the distributor, this helps in maintaining the bed height and
ensuring proper fluidization. The lightweight particles and the unburnt carbon particles
called "fly ash" formed are carried away by the stream of synthesis gas through the exit
(02) at the top end of the gasifier. The product synthesis gas and fly ash with unburnt
carbon or char are passed through a cyclone (03) or a group of cyclones (03) arranged
in series at high pressure and temperature, thereby separating the fly ash with unburnt
carbon or char. The clean gas is passed through an exit pipe (04) to the next processing
unit.
The known fly ash extraction and disposal system comprises; a cyclone (03) (refer
Fig.Ol) to separate the fly ash and other particles from the syngas coming from the
gasifier (01) through a first gas exit pipe (02), a second gas exit pipe (04) to stream line
the cleaned syngas to next unit, an ash stand pipe (05) to transfer the fly ash to a non-
mechanical valve (06) (siphon seal) to recycle the fly ash back to the gasifier.
Currently the fly ash is either recycled or removed where there is diversion, in
Circulating Fluidized Bed Combustion (CFBC) it is through a mechanical device (spices
valve), systems without this do not have control over the proportion.
PRIOR ART
U.S. 4838898 discloses a process for the partial combustion of finely-divided coal at high
temperatures and pressures to make synthesis gas having entrained particles of fly ash
which are separated from the gas at high pressure. The fly ash and a minor amount of
entrained gas are handled in a batch-wise manner to isolate a batch in a lock hopper,
depressurize the batch, strip the synthesis gas from the fly ash and cool the fly ash prior
to disposal. According to the cited invention, the fly ash produced by the entrained
gasification is usually sticky. To remove the sticky particles in the fly ash a long straight
quench section of pipe is provided in the first section of the discharge duct from the
reactor. A stream of cooled product gas is recycled from, a selected point in the process
and injected into the upstream end of the quench section. By thorough mixing the cool
quench gas with the hot synthesis product gas along with sticky particles and flowing
the mixture through the straight quench section of sufficient length allowing the molten
or sticky particles of fly ash to "freeze" to the extent that they do not stick to the walls
of any down-stream equipment or piping.
OBJECTS OF THE INVENTION
It is therefore an object of the present invention to propose a process for recirculation,
extraction and disposal of fly ash in high pressure fluidized bed gasification system,
which ensures recirculation of fly ash (return back to the gasifier) having considerable
amount of char or unburnt carbon in the fly ash stream.
Another object of the present invention is to propose a process for recirculation,
extraction and disposal of fly ash in high pressure fluidized bed gasification system,
which ensures easy disposal of fly ash from the gas line.
A still another object of the present invention is to propose a process for recirculation,
extraction and disposal of fly ash in high pressure fluidized bed gasification system,
which enables the gasifier system to increase carbon conversion efficiency by recycling
the fly ash with high amount of char.
A further object of the present invention is to propose a process for recirculation,
extraction and disposal of fly ash in high pressure fluidized bed gasification system,
which ensures environmentally friendly operation of the gasifier through efficient
operation of fly ash recycling and disposal system.
A still further object of the present invention is to propose a device for recirculation,
extraction and disposal of fly ash in high pressure fluidized bed gasification system
which increases the carbon conversion efficiency of the system by recirculation of fly
ash.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Fig.l - shows an elevational view of a device for Recirculation of Fly ash In High
Pressure Fluidized Bed Gasification System according to the prior art.
Fig.2 - shows an elevational view of a Device for Recirculation, Extraction & Disposal of
Fly ash In High Pressure Fluidized Bed Gasification System according to the present
invention.
DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE PRESENT
INVENTION
As shown in figure 2, the fly ash extraction and disposal system comprising; a gasifier
(01) to produce syngas from high ash coal, a first gas exit pipe (02) to discharge the
syngas from the gasifier (01) to a cyclone (03) to separate the fly ash and other heavier
particles from the syngas stream, an inverted T piece (05) having two branches of
pipes, the first branch to divert a major portion of the fly ash to a recycle path and the
second branch acts as a fly ash extraction path. The hot fly ash coming from the
extraction path is cooled in an ash cooler (06), and an extractor (07) extracts the fly
ash in a controlled fashion from the ash cooler (06). The cooled fly ash is de-pressurized
and collected in a fly ash hopper, and disposed to a silo which is located far away from
the hopper through a fly ash disposal system, a second gas exit pipe (04) stream lines
the cleaned syngas, extracts and disposes the dry fly ash to a load carrier or any other
means of transportation as and when required. The system for fly ash recirculation
comprising; a distributor (08) having a plurality of bubble cap nozzles for fluidizing the
fly ash with unburnt carbon particles coming from the inverted T piece (05). When the
controlled quantity of clean coal gas/inert gas/air with slightly more than the system
pressure is admitted through gas inlet pipes (09) to a twin gas chamber (10) two
different fluidization velocities are generated in both left and right vertical refractory
lined limbs (11&12). The fly ash with unburnt carbon or char in both the limbs start
fluidizing and rising to upward direction, the ash and char particles collected in the
cyclone (03)
flow through the inverted T piece (05) and a first inclined refractory lined pipe (13),
cause the particles in the other limb (11) to over flow into the gasifier (01) through a
second inclined refractory lined pipe (14). An auxiliary fresh coal injection pipe (15)
enables fuel supplementing, a plurality of inspection ports (16) allows checking and
cleaning the ash+char handling pipes (11,12,13 &14) during maintenance. A plurality of
pressure & temperature measuring ports (17) is provided to install pressure and
temperature probes.
The fly ash and char recycling system with fluidizing condition acts as a seal, allowing
the material one way to gasifier.
WE CLAIM:
1. A device for recirculation, extraction, and disposal of Fly ash In High Pressure
Fluidized Bed Gasification System comprising :
means for fly ash extraction and disposal; and
means for recirculating fly ash containing unburnt carbon.
2. The device as claimed in claim 1, wherein means for fly ash extraction and
disposal comprises:
at least one cyclone (03) to separate the ash and heavier particles from the
syngas stream;
an inverted XY' piece (05) having two branches of pipes one is to divert major
portion of the fly ash with char to a recycle path and the other branch to send
the fly ash to an ash cooler (05);
an ash cooler (06) to cool the hot fly ash coming from the cyclone (03);
an extractor (07) for controlled extraction of fly ash from the ash cooler (06);
a gas exit pipe (04) located in the syngas downstream side of the cyclone (03) to
admit the cleaned syngas to the next system, and being enabled to extract and
dispose the dry fly ash to a load carrier or any other carrier for transportation.
3. The device as claimed in claim 1, wherein means for recirculating fly ash
containing unburnt carbon comprises :
a distributor (08) having a plurality of bubble cap nozzles for fluidizing the fly ash
with unburnt carbon particles coming from the inverted T piece (05);
a plurality of gas inlet pipes (09) to admit predetermined quantity of clean coal
gas/inert gas/air at a pressure higher than the system pressure to a dual gas
chamber (10);
the twin/dual gas chamber (10) generating two different fluidization velocities in
two refractory lined vertical limbs (11&12) to fluidize and raise up the fly ash with
unburnt carbon or char in both the limbs;
a first refractory lined inclined pipe (13) to stream line the fly ash and char
particles to the vertical limbs (11&12);
a second refractory lined inclined pipe (14) to cause the fly ash and char particles
in the left limb (11) to over flow into the gasifier (01); and
an auxiliary fresh coal injection pipe (15) for fuel supplementing.
4. The device as claimed in claim 1 or 3, wherein means for recirculating fly ash
containing unburnt carbon further comprising :'
a plurality of inspection ports (16) to check and clean the ash+char handling
pipes during maintenance; and
a plurality of pressure & temperature measuring ports (17) to install the pressure
and temperature probes.
5. A Method for recirculation, extraction and disposal of fly ash in high pressure
fluidized bed gasification system, comprising the steps of:
extraction and suppling of fly ash from the syngas stream in High Pressure
Fluidized Bed Gasification System; and recirculating fly ash having sizable
amount of unburnt carbon in the syngas stream to the High Pressure Fluidized
Bed Gasification System.
6. A method as claimed in claim 5, wherein the method allows the fly ash
recirculation means to operate under fluidizing condition so as to function as a
one way seal allowing material flow only towards the gasifier and preventing
material flow in the opposite direction.
7. A device for recirculation, extraction, and disposal of Fly ash In High Pressure
Fluidized Bed Gasification System as substantially described and illustrated
herein with reference to the accompanying drawings.

The invention relates to an device for recirculation, extraction, and disposal of Fly ash In
High Pressure Fluidized Bed Gasification System comprising : means for fly ash
extraction and disposal; and means for recirculating fly ash containing unburnt carbon.