FIELD OF THE INVENTION
The invention relates to a system for removing contaminants from effluent gas produced in pressurized fluidized bed gasifiers. The invention depicts the arrangement and sequence of process operations (separation equipments) in order to remove all the contaminants to a level that is suitable for power plant application such as Gas Turbine of Integrated Gasification and Combined Cycle (IGCC) Plant.
BACKGROUND OF THE INVENTION
Gasification is a process that converts organic or fossil fuel such as coal into carbon fuel gas along with particulates and trace metals in presence of controlled amount of oxygen and steam in a reactor called Gasifier. The resulting gas mixture is called syngas (from synthesis gas or synthetic gas) or producer gas and is itself a fuel. Selection of gasification process is dictated by the type and characteristics of the fuel, its reactivity and the process requirements. Basic generic classification of the gasification processes, based primarily on the method of contact between fuel and gasification medium (air/oxygen and steam), are: moving bed, fluidized bed and entrained flow gasification.
Pressurized Fluidized Bed Coal Gasification is one of the preferred method to convert high ash coal into fuel gases for burning in a gas turbine combustor to generate electric power. The pressurized fluidized bed process is especially suited for conversion of high ash coals having high ash content, such as Indian coals containing ash content in the range of 25% to 50%. The thermal energy in the gas turbine exhaust is utilized in a bottoming steam cycle, for example a

Rankine cycle based heat recovery steam generator, to generate additional electric power.
The outlet fuel gas from the pressurized fluidized bed gasifier contains not only fuel gases like Hydrogen (H2), Carbon Monoxide (CO) and Methane (CH4) but also Nitrogen(N2), Water(H20), Ammonia (NH3), Hydrogen Sulphide (H2S), particulates and trace metals like Calcium(Ca), Magnesium(Mg), Sodium (Na), Potassium (K), Lead (Pb), Vanadium (V). There are limitations on NH3, H2S, Ca, Mg, Na, K, Pb, V, particulate content of fuel gas for firing in Gas Turbine combustor. In other words, the said components are called as contaminants for Gas Turbine combustor. The limitations on contaminants necessitate their removal before firing in Gas Turbine combustor. Hence a process, method and system is needed to remove the said contaminants from Synthesis Gas.
Fluidized bed gasification system is known, in which high ash coals such as Indian coals used as raw material to produce fuel gas. The fluidized bed gasifier uses Coal, Steam and Air as raw materials in a pressurized fluidized bed reactor (gasifier) which operates at high temperatures such as 1000° C and pressure about 30 bar. The outlet gas stream of Fluidized Bed Gasifier contains constituents viz Hydrogen (H2), Carbon Monoxide (CO), Methane (CH4), Carbon Dioxide (C02), Nitrogen (N2), Water (H20), Hydrogen Sulphide, Ammonia (NH3), trace metals like Calcium(Ca), Magnesium(Mg), Sodium (Na), Potassium (K), Lead (Pb), Vanadium (V) along with particulates(solid particles). The fuel gas can be used to produce power by using a Gas Turbine after cleaning the gas with respect to particulates and other contaminants.
The trace elements like Na, K, V and Pb needs to be removed from fuel gas before feeding into Gas Turbine for power generation. Ammonia (NH3) content

in the fuel gas resulting into formation of Nitrogen Oxides in the combustion chamber of Gas Turbine and ultimately resulting into emission of such Nitrogen Oxides into atmosphere. Hence NH3 is to be removed from fuel gas before applying in Gas Turbine. Similarly it is essential to remove sulphur containing components such as H2S before letting it in Gas Turbine for power generation in order to avoid pollution and corrosive problems associated with the sulphur at the gas turbine downstream pipe and equipment. In the combustion chamber of Gas Turbine, NH3 and H2S undergo combustion reaction in presence of air and forms combustion products such as Oxides of Nitrogen (NOx, where x= 1, 2, 3).) and Oxides of Sulphur (SOx, where x= 1, 2, 3). The NOx and SOx have an adverse effects such as pollution and corrosion problems on downstream pipes or equipments of Gas Turbine.
US patent No 5112586 describes about scrubbing of gas stream and resulting liquid stream stripping repeatedly to remove HCN, C02, NH3 and COS. Whereas present invention describes about removal of NH3, H2S, Ca, Mg, Na, K, Pb, V and particulates using unit operations viz spray tower, packed tower, stripper, filtration unit, water treatment unit, reactors, pumps and heat exchangers. In other words, present invention is different from US patent No 5112586 in terms of removing constituents, unit operations and sequence of unit operations.
US patent No 4088735 describes about removal of NH3, Mercaptan, H2S and COS using scrubbing with Water and Organic Solvent. Whereas present invention talks about simple scrubbing with water for not only removal of particulates but also constituents viz NH3, H2S, Ca, Mg, Na, K, Pb and V and also no Organic Solvent is used.

US patent No 4087258 describes about removal of particulates by using radial flow scrubber, venturi type scrubber and also Hydrocarbons are added to scrubbing water. Whereas present invention describes about only spray tower, packed tower in series for not only removal of particulates but also constituents viz NH3, H2S, Ca, Mg, Na, K, Pb and V and also no Hydrocarbon is added to scrubbing water.
US patent No 5906803 describes about removal of NH3 from gas by selective oxidation in the presence of solid catalyst, whereas present invention is not using any selective oxidation for NH3 removal.
US patent No 6090356 describes about removal of H2S, COS and C02 from gas by absorption with liquid solvent, whereas present invention is using only water, no other solvent is used.
US patent No 3994699 and US patent No 3793181 describes about effect of V, Pb, Na and K on Gas turbine blades and the necessity of removal before using in Gas Turbine.US patent No 5912198 describes about NH3 content in fuel gas resulting in Nitrogen Oxides formation in Gas Turbine and consequently its emission to atmosphere. US patent No 4765132 describes the need of removal of sulphur containing components such as H2S before putting in Gas Turbine for power in order to avoid pollution and corrosive problems associated with the sulphur at the gas turbine downstream pipe/equipment. The present invention is about removal of V, Pb, Na, K, NH3 and H2S all together from Synthesis Gas.

Indian patent No 240077 describes about Fluidized Bed Gasification system for High ash coals. This patent describes about only fluidized bed reactor called gasifier and its internals. But present invention is about the outlet gas of gasifier and its cleaning process, method and system.
Unlike the invention disclosed in the current art, none of the above mentioned prior art methods or processes or systems address all the contaminants removal from synthesis gas.
OBJECTS OF THE INVENTION
It is therefore an object of this invention is to propose a system for removal of particulates from fluidized bed gasifier outlet gas after heat recovery.
Another object of the invention is to propose a system for removal of particulates from fluidized bed gasifier outlet gas after heat recovery, which enables removal of H25 from fluidized bed gasifier which enables removal of H2S from fluidized bed gasifier outlet gas after heat recovery in order to avoid adverse effect of SOx on downstream pipe and equipments.
A still another objective is to propose a system for removal of particulates from fluidized bed gasifier outlet gas after heat recovery, which enables removal of NH3 from fluidized bed gasifier outlet gas after heat recovery in order to avoid adverse effect of NOx on downstream pipe and equipments.
Yet another object of the invention is to propose a system for removal of particulates from fluidized bed gasifier outlet gas after heat recovery, which enables removal of trace metals viz Calcium(Ca), Magnesium(Mg), Sodium (Na),

Potassium (K), Lead (Pb) and Vanadium (V) for meeting the Gas Turbine specifications (limitations imposed by Gas Turbine manufacturer) from fluidized bed gasifier outlet gas after heat recovery.
SUMMARY OF THE INVENTION
Accordingly, there is provided a system for removal of particulates from fluidized bed gasifier outlet gas after heat recovery. The system comprises a plurality of process equipments such as spray tower, packed tower, stripper, filtration unit, water treatment unit, reactors, pumps and heat exchangers. The sequential operations of these process equipments removes contaminants from synthesis gas of pressurized fluidized bed gasifier. The invented arrangement of process equipments and sequence of equipments have capability to utilize water stream as circulating liquid in an efficient way. Minimization of equipment size also a unique feature of the current invention. The ammonia contaminant of synthesis gas gets converted into a valuable product, Ammonium sulfate, a fertilizer. Overall, the invention adapts water recirculation process where water flows into towers viz spray tower and packed tower to pick up particulates and other contaminants, releases the particulates and other contaminants in the sequential equipments and recirculates water back to towers. Minimum amount of fresh water intake is another feature of the current invention. The system of the invention uses only the abundantly available water and avoids using costly hot gas filtration equipment, costly chemicals like hydrocarbons, liquid solvents, special chemical processes like selective oxidation, catalysts etc. from the pressurized fluidized bed gasification process island.

BRIEF DESCRIPTION OF THF ACCOMPANYING DRAWING
Figure 1 - shows the present invented process scheme of arranging process equipments to remove contaminants from synthesis gas.
DETAILED DESCRIPTION OF THE INVENTION
Prior art teaches processes for gasification of coal and similar carbon containing solid fuels into a combustible mixture of gaseous constituents such as carbon monoxide, hydrogen, methane and other relatively minor gaseous constituents commonly called as synthesis gas in a pressurized fluidized bed reactor which is commonly called Gasifier. Coal (1), steam(2) and air(3) are the feed materials to the Gasifier(4) which produces synthesis gas which is intended for a variety of end uses such as generation of electric power by combusting the gases in a gas turbine, conversion into liquid fuels, chemicals etc. The outlet fuel gas from the pressurized fluidized bed gasifier contains not only fuel gases such as Hydrogen (H2), Carbon Monoxide (CO) and Methane (CH4) but also contains Nitrogen(N2), Water(H20), Ammonia (NH3), Hydrogen Sulphide (H2S), particulates and trace metals like Calcium(Ca), Magnesium(Mg), Sodium (Na), Potassium (K), Lead (Pb), Vanadium (V). The total ash content of the coal converts into heavier ash due to gravity and lighter ash due to elutriation in the Gasifier. Heavier ash called bottom ash comes out of Gasifier as a bottom product (5) and lighter ash called fly ash exits along with synthesis gas from Gasifier. The synthesis gas from Gasifier outlet (6) at a higher temperature of around 1000° C passes through a Cyclone (7) in which major fly ash content gets removed from cyclone bottom(8). The resultant Synthesis gas (9) at high temperature moves to heat recovery section where Synthesis gas temperature gets reduced to a lower temperature of around 250° C. The Heat Recovery section consists of two

number of heat exchangers. In the first heat exchanger(10) of heat recovery section, synthesis gas gives heat to preheated boiler feed water(57) to form saturated steam(58). Synthesis gas temperature falls from high temperature to intermediate temperature in the first heat exchanger. The fly ash gets collected in the synthesis gas inlet side of first heat exchanger due to flow direction change as first heat exchanger fly ash product stream (11). First heat exchanger outlet synthesis gas (12) passes through the second heat exchanger (13) of heat recovery section where synthesis gas temperature further reduces to lower temperature of around 250° C by exchanging heat to boiler feed water (56) where it gets preheated before introducing into first heat exchanger. The outlet gas stream from second heat exchanger (15) at lower temperature enters the Wet Gas Clean up section where it gets cleaned up with respect to fly ash (particulates) and other contaminants (viz NH3, H2S, Ca, Mg, Na, K, Pb, V). The fly ash gets collected in the synthesis gas inlet side of second heat exchanger due to flow direction change as second heat exchanger fly ash product stream
(14).
Wet Gas clean up system consists of various unit operations (Separating equipments or units). Second heat exchanger outlet synthesis gas stream (15) enters into a spray tower (16) at bottom. Spray tower is an empty cyl.ndncal vessel with a liquid distributor at the top for uniform distribution of downward moving liquid stream. In the spray tower, the upward moving synthesis gas contacts with downward moving high amount of liquid and due to the contact between gas and liquid, particulates, Ca, Mg, Na, K, Pb and V gets removed completely, NHS gets removed partially and H2S gets removed at very m.n.mal level The downward moving liquid streams in the spray tower are recycled water stream (19) from packed tower bottom pump (24) and recycle water stream (18) from Stripper bottom air cooler (55).

The outlet synthesis gas stream (17) from spray tower enters into a packed tower (21) at bottom in which the synthesis gets intimate contact with downward moving liquid. Packed tower is a cylindrical vessel filled with packing material such as raschig rings or structural packing for enhancing mass transfer.
In the packed column, due to the enhanced contact between liquid and gas, NH3 gets removed completely and H2S gets removed at minimal. The downward moving water stream for packed tower is fresh water stream (22). The packed tower bottom liquid stream (23) basically contains water which flows back to spray tower (16) via a pumping unit (24) in which liquid pressure gets raised to a level that the liquid reaches the spray tower through a pump discharge line (19). The packed tower outlet synthesis gas(25) now free from NHS, Ca, Mg, Na, K, Pb v and particulates enters into a H2S removal unit(26). H2S removal unft basically a reactor with packed bed of Zinc Oxide (ZnO) material. On passing synthesis gas through H2S removal unit, H2S component in the synthesis gas gets removed from synthesis gas by reaction with Zinc Oxide (ZnO) bed of the reactor and converts packed bed materia, into Zinc Sulphide (ZnS). Over a period of time all the packed bed gets consumed and it is to be replenished with new ZnO bed material. The H2S removal unit outlet synthesis gas stream (27) is free from particulates, NH3 removed in (26), H2S removed in (21), Ca, Mg, Na, K,Pb and V removed in (16) and ready for use as fuel in Gas Turbine.
The spray tower bottom liquid stream (20) flows to an air cooler (28). In the air cooler, the liquid temperature is reduced by using air as cooling medium so that the air cooler outlet liquid stream (29) temperature to suit the filtration unit (30). in the filtration unit, all the particulates gets removed along with minimum water as solids stream (31). The filtration outlet liquid stream (32) further goes to a

water treatment unit (33). In the water treatment unit, all the contaminants viz Ca, Mg, Na, K, Pb and V gets removed along with minimum water as solids stream (34). The water treatment unit outlet liquid stream (35) further flows to a Stripper (39) after preheating in stripper preheater (36).
The stripper bottom liquid (40) splits into two parts in which first fraction flows to preheater as first fraction of stripper bottom liquid stream (37) and second fraction flows to reboiler as second fraction of stripper bottom liquid stream (41). The stripper preheater is a heat exchanger which preheats the water treatment unit outlet liquid stream by using heat content of first fraction (37) of stripper bottom liquid stream. The preheated liquid stream (38) from Stripper preheater flows to the stripper (39). The stripper is a column filled with either packing material or trays inside in order to provide intimate contact between downward moving liquid and upward flowing gas. The intimate contact further enhances stripping of gases from liquid stream. In the stripper, the main objective is stripping of Ammonia from preheated liquid stream. Gas stream required for the stripper gets generated by employing a reboiler (42) at the stripper bottom in which second fraction (41) of stripper bottom liquid stream (40) gets converted into mixed stream (43) of gas and liquid. Steam (59) which is a utility uses as heating medium for the reboiler. The stripper outlet gas (44) flows to a partial condenser (45) in order to separate gas stream into uncondensed gas stream (46) and condensed liquid stream (47). The condensed liquid stream flows back to stripper top portion as downward moving liquid flow which enhances sweeping of liquid droplets from the upward flowing gas. Cooling water (60) which is a utility acts as cooling medium in the partial condenser.

The uncondensed gas stream from partial condenser flows to an ammonium sulphate reactor (48). In the ammonium sulphate reactor, ammonia in the uncondensed gas stream mixes with sulphuric acid stream (49) and forms ammonium sulfate due to reaction. Ammonia sulfate stream (51) flows out of reactor as a valuable product and used as fertilizer. Outlet gas stream (50) from ammonium sulfate reactor flows to a vent stack or flare for disposal.
The outlet liquid stream from preheater (52), basically water, flows to a pump (53) where its pressure increased further to so that it overcomes pressure losses in pipeline, air cooler and spray tower elevation. The discharge stream (54) from the pump flows to an air cooler (55) where water gets cooled before introducing into spray tower through air cooler outlet stream (18).

WE CLAIM :
1. A system for removing contaminants of synthesis gas produced from solid fuels in a gasifier having a cyclone, a first heat exchanger, and a second heat exchanger, the system comprising :-
- a spray tower (16) in which synthesis gas from the outlet of said second
heat exchanger enters via bottom flows upward in the tower and contacts
with down coming water streams where Ca, Mg, Na, K, Pb, V and
particulates gets removed completely;
- a packed tower(21) in which synthesis gas from the spray tower top enters at bottom and flows upward in the tower and contacts with the down coming fresh water stream where NH3 gets removed upto desired levels;
- an air cooler(28) in which bottom water stream from the spray tower (16) gets cooled;

- a filtration unit (30) in which particulates from outlet of the air cooler (28) outlet water gets separated as filter cake by leaving particulates free water at outlet;
- a water treatment unit(33) in which Ca, Mg, Na, K, Pb and V gets removed from the liquid stream (32) delivered from the filtration unit (33) and leaving clean water at outlet;

- a preheater (36) in which water treatment outlet stream from a stripper (39) gets preheated and leaving hot water stream;
- a stripper (39) in which hot water stream (38) from the water treatment unit (33) delivering a first portion of the stream to the preheater (36) and a second portion to a reboiler (42); and
- a reactor(26) in which H2S content of packed tower outlet synthesis gas (25) reacts with Zinc Oxide (ZnO) bed and converts the bed into Zinc Sulphide, thereby leaving H2S free synthesis gas (27);

2. The system as claimed in claim 1, wherein said spray tower is a tower in which downward moving liquid (water) consists of two streams, one is from stripper bottom liquid (18) and other is from packed tower bottom liquid (19) after increasing its required pressures by respective pumps(53)(24).
3. The system as claimed in claim 1, wherein said stripper a stripper with a reboiler (42) at bottom which produces required gas stream by boiling fraction of stripper bottom liquid stream into liquid-gas mixer using steam(59) as the heating medium.
4. The system as claimed in claim 1, wherein said stripper comprises a partial condenser (45) in which upcoming gas stream (44) from top of the tower (16) gets condensed partially by cooling water (60) and condensed stream (47) goes back to the tower as washing liquid for upcoming gas stream. Uncondensed gas stream (46) flows to an ammonium sulphate reactor.

5. The system as claimed in claim 1, wherein said contaminants comprises particulates, NH3, H2S, Ca, Mg, Na, K, Pb and V.
6. The system as claimed in claim 1, comprising a pump (53) in which pressure of the stripper bottom recirculating water stream (52) gets increased after losing heat to water treatment outlet stream in the preheater.
7. The system as claimed in claim 1, comprising a recirculating water air cooler (55) in which recirculating water (54) temperature further reduces and flows back to spray tower.
8. The system as claimed in claim 1, comprising an Ammonium sulfate reactor
(48) in which stripper uncondensed ammonia (46) reacts with sulphuric acid
(49) and forms ammonium sulphate (51),