FIELD OF THE INVENTION
The present invention generally relates to pressurized fluidized bed gasification based systems operable under any grade of coal or other solid fuels. More particularly, the invention relates to a system for removal of bottom ash from pressurized fluidized bed gasifier.
BACKGROUND OF THE INVENTION
In a typical pressurized fluidized bed gasifier, air or oxygen or oxygen/air+steam mixture or any other carrier gas along with oxygen/air/steam is passed through a distributor, which is provided with a plurality of bubble cap nozzle arrangement to fluidize the bed. During gasification process, syngas is generated at high temperature due to partial oxidation of the crushed coal; the syngas contains CO, H2, CH4, CO2 and other gases based on process conditions; as a bye-product mineral matter in the coal is split in the process of gasification & collected in two streams namely “Coarse ash or Bottom ash” and “Fly ash”. The heavier ash formed in the dense bed region of the gasifier is called “Coarse ash” or “Bed ash” or “Bottom ash” and is extracted through a plurality of pipes located at various points in the dense bed region of the gasifier; this extraction of ash helps in maintaining the bed height and ensures proper fluidization. To meet the downstream process requirement, the total gasification system is operated under high pressure. For pressurized systems, lock and receiver arrangement is generally used for feeding solids into the system and extracting solids out of the system. The coarse ash is cooled to a desired temperature in bottom ash cooler and extracted in a controlled way by using a rotary ash extractor. The bottom ash from the extractors are collected in a hopper via an ash receiver and an ash lock through an ash lock sequence. During operation, the ash lock and ash receiver are connected and they are maintained under system pressure. The ash is collected in the ash lock, once the ash level in the ash lock reaches at a high level, said ash lock sequence is initiated. At the start of the ash lock sequence,

the bottom ash lock is isolated from the ash receiver. The ash lock is depressurized to atmospheric pressure through depressurizing the vessel. As the ash lock pressure reaches atmospheric, pressure, the Ash lock is connected to the hopper to collect all the bottom ash in the hopper. After completion of the bottom ash discharge, the ash lock is isolated from the hopper. The ash lock is pressurized using either an inert gas or the recycled syngas. As the bottom ash lock reaches the system pressure, it is connected back to the ash receiver to complete the sequence.
The syngas along with fly ash exits from the top portion of the gasifier. The fly ash and the unburnt carbon particles in the syngas are separated by a serially arranged cyclones. The fly ash from the particulate removal device (cyclones) is disposed through a separate arrangement of ash lock and ash receiver, similar to said bottom ash removal arrangement. The syngas at the exit of the cyclones is cooled to a desired temperature in a heat recovery system that is called as a Heat Recovery Boiler (HRB) or syngas cooler. The syngas is further processed in a gas cleaning system to meet the end process requirement and the syngas can be utilized as fuel for the Gas Turbine (GT), for chemical synthesis, or as an alternate fuel.
During the gasification process, in order to maintain the bed height, the bottom ash is extracted from the dense bed region of the gasifier either continuously or periodically. Maintaining the gasifier bed height is vital to ensure proper fluidization, to achieve desired syngas composition and to maximize carbon conversion. The bottom ash vessels namely, the cooler, receiver, lock, hopper, extractor and valves handle the bottom ash dispersed in syngas, increasing the risk of syngas leakage to atmosphere or increasing the risk of explosion due to syngas leakage. The problem to be solved by the present invention is to reduce the capital and operating expenses of the plant by reducing the structural height of the gasifier, improving the availability & life of valves & extractors and

improving the plant safety by reducing the risk of syngas leakage in the bottom ash handling systems.
In a known Pressurized fluidized bed gasifier (01) (refer Fig.01) lined with refractory bricks (02), a fluidizing media is passed through a distributor (03) which is provided with a plurality of bubble cap nozzle (05) arrangement for fluidization to produce syngas with a desired composition and calorific value. The fluidizing media consists of air or oxygen or oxygen/air+steam mixture or any other carrier gas along with oxygen/air/steam.
The fluidization media is admitted into the refractory lined gasifier plenum (07) through an inlet pipe (04). In the gasification process, the bottom ash is extracted through; a plurality of refractory lined or water/air cooled pipes (06) located at the distributor (03) via an ash cooler (10) by using an ash extractor (13) at a desired rate and collected in a bottom ash hopper (23) via a plurality of ash receivers (14) and an ash lock (16) to maintain the bed height and to ensure proper fluidization. A number of cooling water inlet pipes (08) and cooling water outlet pipes (09) assist in effective circulation of cooling water for the bottom ash extraction pipe (06).
The ash cooler (10) can be a static cooler or screw cooler or any other type. A number of cooling water inlet pipes (11) and cooling water outlet pipes (12) assist in effective circulation of the cooling water for the ash cooler (10).
The bottom ash from the ash receiver (14) under high pressure is collected through a valve (15) in an ash lock (16), whose pressure is same as that of the system pressure. As the bottom ash level in the ash lock (16) reaches at high level, an ash lock sequence is initiated. After isolating the ash receiver (14) from the ash lock (16), the ash lock (16) is then depressurized to atmospheric pressure using a depressurizing vessel (20), the syngas or any inert gas at high pressure in the ash lock (16) is vented to atmosphere or sent to flare based on

safety considerations via a first pipe (19), the depressurizing vessel (20) and a second pipe (22). At times, the solids collected in the depressurising vessel (20) is removed through the outlet valve (21). Once the ash lock (16) reaches atmospheric pressure, the bottom ash from the ash lock (16) is collected through a valve (17) in a bottom ash hopper (23). As the ash lock (16) is emptied, the ash lock (16) is isolated from bottom ash hopper (23), and the ash lock (16) is pressurized using a suitable gas to reach the system pressure and connected back to the ash receiver (14), which completes the ash lock sequence. The ash receiver (14) is always under system pressure and the ash lock (16) undergoes periodic pressurization and depressurization cycle. In order to ensure a free flow of the ash under gravity, the ash cooler (10), ash extractor (13), ash receiver (14), ash lock (16) and the ash hopper (23) are disposed one above the other. After providing a clearance at the disposal point from the bottom ash hopper (23), the above mentioned equipment are positioned one above the other and the gasifier needs to be located at a substantially higher elevation. This leads to a tall structure which in turn leads to higher capital cost, higher operational cost and abnormal levels of coal silo/bunker. Handling of dry ash results in severe erosion of the valves and extractors, which results in reduced life of the components. There is a possibility of syngas leakage in flange fittings & other instrument fittings of the bottom ash removal system as the medium handled is bottom ash with syngas at high pressure which leads to safety concern. In order to reduce the overall structural height, to improve the availability and life of valves & extractors and to improve the plant safety “A Process and method for Removal of Bottom Ash from Pressurized Fluidized Bed Gasifier is invented”.
U.S. Patent No. 4790251 dated July 13, 1988, entitled “HIGH PRESSURE AND HIGH TEMPERATURE ASH DISCHARGE SYSTEM” discloses a system for discharging and cooling hot ash from a coal combustion unit, such as a coal combustor or an associated filter, where the hot ash at a temperature in excess of 700° C. and at superatmospheric pressure is charged to a jacketed, cooled screw conveyor and passed therethrough in contact with a countercurrent flow of

a purge gas. The ash is cooled in the screw conveyor to a temperature of below 320° C. and discharged to a collection hopper, under pressure, while the purge gas and gases evolved from the hot ash are returned to the combustion unit.
U.S. Patent No. 6082442 dated July 4, 2000, entitled “APPARATUS FOR REMOVAL AND COOLING OF ASH FROM THE BED OF A FLUIDISED BED FURNACE” teaches a device for extracting and cooling dry ash from a fluidized bed furnace. This system consists of a combustion chamber, a floor of combustion chamber is provided with a number of ash extraction pipes and an intake and an outlet for cooling medium, a housing provided with several floors, a chain conveyor, a motorized driving wheel and an ash discharge port. The chain conveyor is a drag conveyor and comprises two parallel and circulating chains connected together at intervals by scrapers. The semi-cooled floors ensures enough heat exchange surfaces-inside the conveyor’s housing to satisfactorily cool even large volumes of ash. The mutual displacement of the tables that constitute the floors keeps the pile of ash on each table fairly low even when a lot of ash is being handled and accordingly promotes cooling.
OBJECTS OF THE INVENTION
It is therefore, an object of the present invention to propose a system for removal of bottom ash from pressurized fluidized bed gasifier, which eliminates the drawbacks of the prior art.
Another object of the present invention is to propose a system for removal of bottom ash from pressurized fluidized bed gasifier, which reduces the structural height of the gasifier and improves availability/life of valves & extractors due to wet ash removal.

A still another object of the present invention is to process a system for removal of bottom ash from pressurized fluidized bed gasifier, which ensures plant safety by eliminating syngas leakage due to handling of wet ash instead of dry ash.
A further object of the present invention is to propose a system for removal of bottom ash from pressurized fluidized bed gasifier, which ensures increased availability of the system.
SUMMARY OF THE INVENTION
According to the present invention there is provided a system for removal of bottom ash from pressurized fluidized bed gasifier lined with refractory bricks or any cooling / heat recovery for producing syngas with desired composition and calorific value, by maintaining the height of high temperature fluidized bed at a desired level by allowing easy bottom ash removal. A water cooled flat plate distributor is positioned and supported in the refractory lined gasifier having diameter varying from and not limited to 300 mm to 5000 mm for an uniform distribution of a fluidizing medium such as air or oxygen or oxygen/air+steam mixture to maintain a uniform temperature across the bed which in turn enables to produce desired composition of syngas. A plurality of bubble cap nozzles enables the fluidizing medium to enter the gasifier, the distributor also holds the high temperature bed at a desired height by controlled bottom ash removal, bubble cap nozzles maintain uniform fluidization velocity and minimize formation of agglomerates by rapid and thorough mixing of the bed. A plurality of pipes located at the various points of the distributor enables removal of bottom ash from the gasifier at controlled rate, wherein the ash extraction pipes can be water/air cooled. A gasifier plenum section containing water bath for direct cooling of the bottom ash from the gasifier where the water level is monitored and controlled, wherein one each plenum inlet valve and plenum outlet valve control the water level inside the plenum section, wherein the plenum inlet valve is connected to a water source through a pump via a main inlet valve and water

inlet pipe. An ash extractor for removal of bottom ash from the outlet of the gasifier in a controlled manner, the water bath temperature is maintained at a desired temperature by controlling the water flow and ash extraction rate. An ash lock inlet valve connecting the extractor and the ash lock vessel, the ash lock vessel being filled with water for collecting the cooled bottom ash, the ash level in the ash lock vessel is monitored continuously, wherein a water inlet valve for filling and pressurizing the ash lock vessel using water during the ash discharge sequence including a vent valve for reducing the pressure of the ash lock vessel to a desired pressure or to near atmospheric, and wherein the outlet of the vent valve may be connected to a system via vent pipe for depressurization of the high pressure water. An outlet pipe is connected to a main outlet valve from the gasifier bottom via gasifier outlet valve and from the ash lock vessel via ash lock outlet valve, which is used to maintain the water level in the gasifier plenum water bath and also to drain the excess water in the ash lock vessel, wherein the ash lock outlet valve and the gasifier outlet valve is used for circulating the water back to the gasifier plenum water bath while dumping ash/solids to the ash lock vessel, and wherein the water from the outlet pipe can also be reused after necessary treatment. An ash discharge valve conveys the bottom ash and water to the conveyor where the water and ash is separated and ash is conveyed. The improved system allows eliminating ash cooler, ash receiver and ash hopper vessels, of prior art and reduces the risk of syngas leakage in the extractor, valves and flange fittings as water sealing is available. Erosion of valves is reduced, lifetime of extractor and valves are increased and energy can also be conserved by reusing the water from the gasifier and ash lock vessel after necessary pre-treatment.
In contrast to the AAPI, according to the present invention, ash is collected in the water bath of the plenum section, which results in direct cooling and collected in a water filled ash lock vessel for further cooling and disposal. In the present invention, cooling of ash is done by direct cooling using water bath.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Fig.1 – shows an elevational view of a system for removal of bottom ash from pressurized fluidized bed gasifier, according to the prior art.
Fig.2 – shows an elevational view of a system for removal of bottom ash from pressurized fluidized bed gasifier, according to the present invention.
DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE PRESENT INVENTION
Referring to figure 2, the pressurized fluidized bed gasifier (refer Fig.02) comprising, a gasifier (01) lined with refractory bricks (02) for producing syngas with desired composition and calorific value, by maintaining the height of high temperature bed at a desired level and allowing easy bottom ash removal, a water cooled flat plate distributor (03) positioned and supported in the refractory lined gasifier (01) having diameter varying from and not limited to 300 mm to 5000 mm for uniform distribution of fluidizing medium such as air or oxygen or oxygen/air+steam mixture to ensure an uniform temperature across the bed which in turn enables to produce desired composition of syngas; a plurality of bubble cap nozzles (05) enables the fluidizing medium to enter the gasifier, the distributor (03) also holding the high temperature bed at a desired height by a controlled bottom ash removal; a plurality of bubble cap nozzle (05) to maintain uniform fluidization velocity and minimize the formation of agglomerates by rapid and thorough mixing of the bed; a plurality of ash extraction pipes (06) located at various points of the distributor (03) enables removal of bottom ash from the gasifier (01) at controlled rate, wherein the ash extraction pipes (06) can be water/air cooled; a number of cooling water inlet pipe (08) and cooling water outlet pipe (09) to assist circulation of cooling water for ash extraction pipes (06), a gasifier plenum (07) section containing water bath for direct cooling of the bottom ash from the gasifier where the water level is monitored and controlled;

one each plenum inlet valve (14) and plenum outlet valve (10) to control the water level inside the plenum portion, the plenum inlet valve (14) being connected to a water source through a pump via a main inlet valve (21) and water inlet pipe (20); an ash extractor (13) for removal of bottom ash from the outlet of the gasifier in a controlled manner, the water bath temperature being maintained at a desired temperature by controlling the water flow and ash extraction rate; an ash lock inlet valve (15) connecting the extractor and the ash lock vessel; an ash lock vessel (16) filled with water for collecting the cooled bottom ash, the ash level in the ash lock vessel (16) being monitored continuously; a water inlet valve (22) for filling and pressurizing the ash lock vessel (16) using water during the ash discharge sequence; a vent valve (18) for reducing the pressure of the ash lock vessel to a desired pressure to near atmospheric, the outlet of the vent valve (18) being connected via the vent pipe (19) for depressurization of the high pressure water; an outlet pipe (25) connected to a main outlet valve (24) from the gasifier bottom via said gasifier outlet valve (10) and from the ash lock vessel via an ash lock outlet valve (23), which is used to maintain the water level in the gasifier plenum water bath and drain out the excess water in the ash lock vessel, the ash lock outlet valve (23) and the gasifier outlet valve (10) being used for circulating the water back to the gasifier plenum water bath while dumping ash/solids to the ash lock vessel, wherein the water from the outlet pipe (25) can also be reused after necessary treatment and an ash discharge valve (17) to convey the bottom ash and water to the conveyor where the water and ash is separated and ash is conveyed.

WE CLAIM
1. A system for removal of bottom ash from pressurized fluidized bed gasifier comprising:
a. a plurality of ash extraction pipes (06) located at various points of a distributor
(03) for removal of bottom ash from the Pressurized Fluidized Bed Gasifier (01)
at controlled rate, wherein the ash extraction pipes (06) can be water/air cooled,
and wherein a number of cooling water inlet pipe (08) and cooling water outlet
pipe (09) provided to assist circulation of cooling water for ash extraction pipes
(06);
b. gasifier plenum (07) section containing water bath for direct cooling of the bottom
ash from the gasifier wherein the water level is monitored and controlled by one
each plenum inlet valve (14) and plenum outlet valve (10) inside the plenum
portion, the plenum inlet valve (14) being connected to a water source through a
pump via a main inlet valve (21) and a water inlet pipe (20), and wherein the
water bath temperature is maintained at a desired temperature by controlling the
water flow and ash extraction rate;
c. an ash extractor (13) for removal of bottom ash from the outlet of the gasifier in a
controlled manner;
d. an ash lock inlet valve (15) connecting the ash extractor (13) and an ash lock
vessel (16), the ash lock vessel (16) filled with water for collecting the cooled
bottom ash, wherein the ash level in the ash lock vessel (16) is monitored
continuously, and wherein a water inlet valve (22) disposed for filling and
pressurizing the ash lock vessel (16) using water during the ash discharge
sequence;
e. a vent valve (18) for reducing the pressure of the ash lock vessel (16) to near
atmospheric, the outlet of the vent valve (18) being connected to a system via a
vent pipe (19) for depressurization of the high pressure water;

f. an outlet pipe (25) connected to a main outlet valve (24) from the gasifier bottom
via a gasifier outlet valve (10) and from the ash lock vessel (16) via an ash lock
outlet valve (23), which is used to maintain the water level in the gasifier plenum
water bath and also to drain-out the excess water in the ash lock vessel (16), the
ash lock outlet valve (23) and the gasifier outlet valve (10) being further used for
circulating the water back to the gasifier plenum water bath while dumping
ash/solids to the ash lock vessel (16), the water from said outlet pipe (25) can
also be reused after necessary treatment; and
g. an ash discharge valve (17) to convey the bottom ash and water to a conveyor
where the water and the ash is separated and ash is conveyed,
2. The system as claimed in claim 1, wherein the pressurized fluidized bed gasifier (01) is lined with refractory for producing syngas with suitable composition and required calorific value, by maintaining the high temperature bed to a desired height by allowing easy Bottom ash removal.
3. The system as claimed in claim 1 or 2, wherein the water cooled flat plate distributor (03) is provided for uniform flow distribution.
4. The system as claimed in one of claims 1 to 3, wherein a plurality of bubble cap nozzle (05) is provided to maintain uniform fluidization velocity and to minimize the formation of agglomerates.