INVENTION
'This invention relates to integrated gasification based power generation; in particular pressurized fluidized bed gasification based systems capable of using any grade of coals. More particularly; the invention relates to an improved water type evaporator for cooling and recovering heat from the syngas in pressurized
fluidized bed gasification systems.
BACKGROUND OF INVENTION
Pressurised Fluidised Bed gasification is increasingly being used in combine.cycle plants to generate electricity through integrated gasification and for other applications. Such combined cycle plants are capable of using various grades of low calorific value and high ash content coals.
In a refractory lined pressurized fluidized bed gasifier, a mixture of air and steam is introduced through a distributor located at a bottom of the gasifier. Coal is continuously injected to the gasifier through ejector using compressed air. Coal: gas also called 'fuel gas' is produced due to the chemical reaction of coal with air and steam. The fly ash and unbrurit carbon particles in fuel gas are separated by a serially arranged cyclones. The coal gas at the exit of the cyclone is cooled in a Heat Recovery Boiler (HRB). The gas is further cooled in an Economiser. Part of the steam is utilized for gasification process and the rest expands in the steam turbine to generate electricity. The gas is further processed in a gas cleaning system and admitted to a combustion chamber of a gas turbine to generate electricity.

One of the existing clean coal technology options is the Integrated Gasification Combined Cycle (IGCC) method of power generation.
In such plants, the syngas from the gasifier leaving the cyclones at around 950 -1000°C is cooled by heat recovery units before the syngas is sent to the gas clean up system and then to the gas turbine or for other uses. Generally an evaporator (boiler) is arranged to face the hot gas, followed by an economizer. The existing arrangement adopts a shell type heat exchanger with gas flow through tubes and water /steam mixture on the shell side. With the increase in size of the gasifier, the shell size normally increase. With the increase in pressure of the steam / water mixture (as proposed for larger plants to improve efficiency), the shell thickness and hence weight increases. In some cases it may lead to multiple units in parallel. The thickness of tube sheets also, increases considerably, leading to complexity in design. Depending on the nature of ash, plugging of tube and shut down for cleaning are also experienced. Even when water tube arrangement is adopted the tubes are arranged in a circle and gas flow is allowed parallel to the tubes.
US Patent No. 5253703 dated 19.10.1993, titled WASTE HEAT EXCHANGER' discloses a heat exchanger particularly adapted to handle dusty hot waste gases and is of the shell and tube type. This heat exchanger includes a vertically oriented shell through which the waste gases flow longitudinally with shell containing a tube assembly of vertical tubes having external, longitudinally extending heat transfer fins. Thus, particles may get deposited on the heat transfer surfaces due to fins.

OBJECTS OF INVENTION
An object of this invention is to propose an improved water type evaporator for cooling and recovering heat from the syngas in pressurized fluidized bed gasification systems.
Another object of this invention is to propose an improved water type evaporator for cooling and recovering heat from the syngas in pressurized fluidized bed gasification systems which prevents the ash accumulation on heat transfer surfaces.
A still another object of the invention is to propose an improved water type evaporator for cooling and recovering heat from the syngas in pressurized fluidized bed gasification systems which generates saturated steam for admission into the gasifler and generation of coal gas, the water/ steam mixture flowing" inside the tube and the gas flowing over the tubes;
A further object of the invention is to propose an improved water type evaporator for cooling and recovering heat from the syngas in pressurized fluidized bed gasification systems, which is reliable and easy, to operate and . maintain.
SUMMARY OF INVENTION
Accordingly, there is provided an improved water type evaporator for cooling and recovering heat from the syngas in pressurized fluidized bed gasification

systems, the evaporator comprising a plurality of smaller diameter water tubes arranged as a single bundle, enclosed by a pressure vessel which is having gas inlet and outlet ports and a number of man holes for utility services, a plurality of water inlet headers and supply tubes, raiser headers and raiser tubes capable of cooling the 'syngas' coming out of a pressurized fluidized bed gasifier and generating saturated steam.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figs. i(a), 1(b), 1(c) - shows respectively an elevation, plan and side view of the improved water tube type evaporator according to the invention.
DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION
- Figures 1(a), 1(b), 1(c) shows the assembled elevation, plan and side view of an improved water tube type evaporator for integrated gasification and combined cycle plant> comprising a tube bundle (01) having a plurality of smaller diameter tubes arranged vertically with two rows connected to an inlet header (14) and an outlet header (07). The inlet headers (14) are connected to downcomer pipes (05) (supplying water from a drum) by a set of supply tubes (06). The steam water mixture from the outlet headers (07) are taken to the drum (not shown)

by a set of riser pipes (08). The tubes bundle (01) with the headers (07,14) and supply tubes (06) and the riser pipes (08) are housed inside a shell (02A) having openings for the pipe connections including a gas inlet (09), a gas outlet (10) a, hopper (12) for ash collection, and a flanged end cover (02) with man hole (11).
To protect the shell (02A) from direct contact with the hot gas, an enclosures is formed with the first and last row tubes (03,04) in the gas flow direction, with fins (13) between the tubes. The side enclosures are formed with a separate row of tubes (15,16) with the inlet and outlet headers (07,14) the supply and riser pipes (06,08) provided with fins between the tubes which forms. By connecting the panels to the first and last row of tubes (03, 04) in the gas flow direction a gas tight enclosure is formed. However the space between the outlet headers (07) is left for easy removal of the tubes for maintenance. This space is also used for continuity of pressure on the top side of the shell (02A) and hence pressure equalization. As a safety measure a thin layer of special refractory lining. on the inner side of the shell is also provided.
To direct the gas flow across the tube bundle (1), a plurality of fins baffles (13) are provided. The arrangement does a not require any piping connection to be removed for access inside for inspection. For maintenance, the required module (consisting of inlet / outlet headers in two rows of tubes connecting them) can be removed after removing the flanged top end cover.
The pressure vessel (02A) for enclosing the water tubes having a number of enclosure tubes (03,04) for guiding / housing the desired gas flow path, a plurality of inlet headers (05) for water entry, a number of supply tubes (06) for

supplying the water to evaporator tubes (01), a number of raiser headers and raiser tubes (07, 08) respectively, are used for raising the generated steam from the evaporator to a steam drum, an inlet pipe (09) for admitting the syngas to the evaporator, an outlet pipe (10) for exiting the cooled syngas, a number of man holes (11) for erection, inspection, maintenance and repair work, a hopper vessel (12) for disposing the fly ash collected at the bottom of the evaporator, a number of baffle fins (13) arranged in such a manner so that they can guide the desired flow path of syngas.
The water tube bundle (01) is capable of generating saturated steam and can withstand very high pressure.
The pressure vessel (02A) is used as an enclosure for the evaporator tubes and capabl.es. of withstanding high pressure in the ordej of 80 bar.
The inlet headers (05) and supply tubes (06) are capable of supplying / admitting high pressure water / water and steam mixture to the evaporator
tubes.
• ■ i' ■ •■ . •
The raiser headers (07) and raiser tubes (8) are capable raising and sending the steam to a steam drum.
The baffle fins (13) are capable of diverting / guiding the syngas flow in desired path with suitable velocity and pressure, also prevents the ash accumulation.

WE CLAIM
1. An improved water tube type evaporator for cooling and recovering heat from syn gas in pressurized fluidized bed gasification system comprising:-
a plurality of smaller diameter tubes (01) arranged vertically to form at least two rows, a first end and a second end of the tubes respectively connected to larger diameter pipes known as inlet headers (14) and outlet headers (07),
the inlet headers (14) comprises plurality of pipes welded at the top with plurality of evaporator tubes (01) and welded at the bottom with plurality of supply tubes (06) to feed boiler water in the inlet header (14);
the outlet headers (07) comprises plurality of pipes welded at the bottom with plurality of evaporator tubes (01) through which boiler water rises up, convert into water/stream mixture, and collected in the outlet headers (07);
a plurality of riser pipes (08) welded at the bottom end with outlet headers (07) and welded at the top end with the drum;
a pressure vessel (2A) accommodating the tubes (01) including the header (07, 14) and the supply and riser pipes (06, 08) and having a gas inlet (09) for admitting syn gas to the evaporator, a gas outlet (10) for exiting the cooled syn gas, a hopper (12) for collecting ash from the evaporator and a manhole (11); wherein, a first panal is formed with the first and second row (03, 04) of the tubes in the gas flow direction, and a second panel formed with a separate row (15, 16) of the tubes including the inlet headers (07), the outlet headers (14), the supply tubes (06), and the riser pipes (08), the first and the second panel being connected to form a gas

-tight enclosure, and wherein a plurality of fins (13) are provided between the tubes (03, 04, 15, 16) such that the fins (13) can guide the syn gas in a desired flow-path with matching velocity and pressure.
2. The evaporator as claimed in claim 1, wherein the pressure vessel (2A) is provided with a flanged end cover (02).

ABSTRACT

WATER TUBE TYPE EVAPORATOR (BOILER) FOR COOLING SYNGAS IN PRESSURISED FLUIDIZED BED GASIFIER
The invention relates to water tube type heat recovery boiler for pressurized fluidized bed gasifier/ integrated gasification and combined cycle plant comprising, a tube bundle (01), a pressure vessel (02 A), a plurality of enclosure tubes (03, 04), a plurality of water inlet headers (05), a plurality of water supply tubes (06), a number of raiser tubes (08), an inlet and outlet headers (14, 07), a gas inlet (09), a gas outlet (10), a number of manholes (1), an ash hopper vessel (12), a few number of baffle fins (13), in which the system is capable of cooling and recovering heat from the hot syn gas coming from pressurized fluidized bed gasifier.