FIELD OF INVENTION
The present invention relates to a method for improving the efficiency of
pressurized fluidized bed gasification (PFBG) based integrated gasification combined
cycle (IGCC) power plant with heat recovery from ash and flue gases and integrating
of recovered heat for condensate preheating. More particularly, the invention relates
to a method of improving IGCC power plant efficiency through heat recovery from
bottom and cyclone ashes generated in pressurized fluidized bed gasification of high
ash coals, and efficient heat recovery from flue gases. The heat recovered from ash
and flue gases is integrated with condensate preheating system for improving overall
. cycle efficiency of integrated gasification combined cycle power plant.
BACKGROUND OF THE INVENTION
Coal is most important and abundant fossil fuel used for power generation.
Coal continues to occupy a pivotal role in world energy scene. Conventional subcritical
pulverized coal fired power plants burn coal to generate steam and use generated
steam for power generation at an efficiency of around 30 - 35%. These power plants
emit large quantities of carbon dioxide, which causes global warming and oxides of
sulfur and nitrogen which are responsible for acid rains. Hence, it is essential to utilize
coal more efficiently to control gaseous emissions polluting environment. Various
clean coal technologies are at various stages of development to utilize coal more
efficiently.

One of the current concepts for utilizing coal more efficiently is Integrated
Gasification Combined Cycle (IGCC) technology. In IGCC, coal is gasified at high
pressure and high temperature to generate coal syngas (mixture of CO and H2). The
generated syngas is cleaned to remove various contaminants such as particulates,
H2S, NH3 and alkali vapors. The cleaned syngas drives gas turbine (GT) and generates
power. Heat energy present with GT exhaust is used to generate steam in heat
recovery steam generator (HRSG). The steam generated in HRSG in turn drives steam
turbine. Because of combined Brayton gas turbine cycle and Rankine steam cycle,
IGCC power plants have efficiencies of around 45-50%. Further coal gasification
process offers flexibility for polygeneration to generate various other products such as
hydrogen, liquid fuels, chemicals and fertilizers.
Various gasification technologies are available for gasification of different coals.
Pressurized fluidized bed gasification (PFBG) has been identified as suitable
technology for gasification of high ash content coals. In prior art, coal is gasified by
reacting it with air/oxygen and steam mixture at high pressure and high temperature
typically 30 bar pressure and 950°C - 1050°C temperature in PFBG, and in the process
coal gas along with elutriated fly ash and coarse coal ash also referred as bottom ash
are generated. PFBG reactor is main equipment of gasification system and this is a
pressure vessel internally lined with refractory and insulation bricks or castables in

layers to hold the heat and to maintain outer surface temperature within safe limits
(normally 60 - 70°C). Coal syngas along with fly ash leaving gasifier fed to two
refractory lined cyclone separators arranged in series to separate fly ash particulates
from fuel gas. A portion of the fly ash separated from cyclone 1 is fed back to gasifier
through a refractory lined stand pipe and loop seal system. Bottom, some portion of
cyclone 1 and cyclone 2 ashes are cooled in static ash cooler followed by screw ash
cooler by circulating cooling tower water before disposing ash through lock hopper
system. In prior art, the heat recovered from the ash cooling is not being used and
around 2 - 2.5% (depending upon coal ash content) of total thermal energy is being
wasted because of sensible heat energy present with ash.
Gas leaving from cyclone 2 is cooled in heat recovery boiler (HRB) to a
temperature level suitable for gas cleanup system (GCS). HRB consists of evaporator
and economizer sections, and generates high pressure saturated steam at pressure
level matched to the high pressure saturation steam generation in HRSG. Gas from
HRB is taken to GCS to remove residual particulates and trace contaminants such as
ammonia, hydrogen sulfide and alkali vapours from the gas and brings clean gas
purity to the fuel gas specification required for gas turbine. GCS consists of barrier
filter system for removal of fine dust particulates followed by gas cooling and
scrubbing systems for removal of other contaminants.

Cleaned syngas from GCS is taken to the gas turbine with integrated air
compressor. A small fraction of air from GT compressor is bleed at compressor exit
and sends to gasifier air supply system. Flue gas from GT exhaust is passed to HRSG.
In HRSG, steam is generated at two or three pressure levels by utilizing heat energy
present with flue gas. The HP saturated steam generated in HRB is exported and
mixed with the high pressure saturated steam generated in HP evaporator and super
heating of combined steam is carried in HP super heater. The LP and HP steams from
HRSG are taken to steam turbine generator for power generation. Steam turbine has
provision to extract steam at pressures slightly higher than the gasifier maximum
operating pressure. In prior art, flue gas from HRSG leaves at around 20°C higher
than the acid dew point of flue gas. At this temperature stack gas has around 11% of
total heat energy intake of gasifier. Overall efficiency of.IGCC plant can be improved
by heat recovery from ashes and maximum heat recovery from flue gas, and
integrating recovered heat with the Rankine cycle.
In prior art, EP0471055B1 patent discloses method for dry bottom ash
discharge and cooling of ash generated in combustion boilers. Heat present with
bottom ash, which is discharged through specially designed hopper and conveyer
system is exchanged with counter current flow of air. The preheated air is finally
introduced into the boiler.

US20100064950A1 patent considers the same above arrangement for cooling
of ash discharged from fossil fuel boilers. In this patent, during ash storing in hopper
step, distribution of air and some quantity of water through side walls of the hopper
was identified for ash cooling during storing step.
US20100294457A1 patent explains extraction and air/water cooling system and
energy recovery from large quantities of ash generated in boilers. This also considers
similar system as in EP0471055B1, but, extractor and conveyer system has been
separated into two sections to allow cooling of large quantities of ashes.
W02012098504A2 patent describes another method of recovering heat from
heavy ashes generated in combustion process using similar method as specified
above.
US20120276492A1 patent also discloses method of recovering heat from
bottom ash discharged from combustion furnace. Heat present with bottom ash is
recovered to bottom ash cooling water circuit. Heat gained to the cooling water circuit
is used for preheating the combustion air in a heat exchanger.
US4340572A patent discloses a method for recovering heat from flue gases to
a liquid medium in two steps. In 1st step cooled flue gases leaving 2nd stage and fresh

liquid flow counter currently in packed column and in 2nd stage which is a parallel
vertically oriented passages, warm liquid leaving 1st stage is further heated with initial
flue gas.
EP2644254A1 patent relates to another method of heat recovery from flues
gases and scrubbing and condensation of flue gases are. carried out in a single wet
scrubbing system.
The heat recovery systems proposed in prior art for recovering heat from ash
and flue gases can't be applied for heat recovery from ash generated in pressurized
fluidized bed gasification system and heat recovery from flue gases. Because PFBG
. operates at high pressures and ash is discharged at high pressures. Moreover,
gasification air achieves more than required temperature at GT compressor exit and
this air can't be used for cooling of ashes. Heat energy present with flue gases is
required to be exchanged with condensate through efficient way of indirect contact.
OBJECTS OF THE INVENTION
Therefore, it is an object of the invention to propose a method for improving
the efficiency of pressurized fluidized bed gasification (PFBG) based integrated
gasification combined cycle (IGCC) power plant with heat recovery from ash and flue

gases and integrating of recovered heat for condensate preheating which is capable of
cooling ash generated in pressurized fluidized bed gasification process.
Another object of the invention is to propose a method for improving the
efficiency of pressurized fluidized bed gasification (PFBG) based integrated gasification
combined cycle (IGCC) power plant with heat recovery from ash and flue gases and
integrating of recovered heat for condensate preheating which is able to utilize the
recovered heat from bottom and cyclone ashes by integrating the recovered heat with
IGCC process cycle.
A further object of the invention is to propose a method for improving the
efficiency of pressurized fluidized bed gasification (PFBG) based integrated gasification
combined cycle (IGCC) power plant with heat recovery from ash and flue gases and
integrating of recovered heat for condensate preheating which is capable of
recovering maximum heat from flue gas and integrate with steam cycle.
A still another object of the invention is to propose a method for improving the
efficiency of pressurized fluidized bed gasification (PFBG). based integrated gasification
combined cycle (IGCC) power plant with heat recovery from ash and flue gases and
integrating of recovered heat for condensate preheating which is able to improve

overall cycle efficiency of IGCC power plant by effectively utilizing the heat energy
present with ashes and flue gases.
SUMMARY OF THE INVENTION
Present invention is related to a method of recovering heat from ash generated
in gasification process and maximum heat recovery from flue gases, and integration of
recovered heat to the overall cycle for improving cycle efficiency of integrated
gasification combined cycle power plant. Bottom and cyclone ashes are generated
during gasification of coal, in which coal, air/oxygen and steam reacts together in
pressurized fluidized bed gasifier at high pressure and high temperature. Bottom ash
and cyclone ashes are cooled in ash coolers by exchanging heat with cooling water
circuit and discharged to ash disposal system through lock hopper system. Heat
present in cooling water circuit is used for preheating the condensate in a heat
exchanger. Heat energy present with stack flue gases is also recovered by reducing
H2S composition in synthesis gas which allows to decrease temperature of flue gas
leaving into stack and heating the condensate first in water preheater section of HRSG
to take advantage of increased temperature difference between flue gas and
condensate.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
In the following, the method and system identified in present invention will be
explained in more detail with reference to the following drawings.

Figure 1 is schematic representation of the integrated gasification combined
cycle (IGCC) process flow diagram of prior art.
Figure 2 is schematic representation of first preferred embodiment of present
invention in connection with IGCC process flow diagram with heat recovery from
bottom and cyclone ashes and integration of the recovered heat with steam cycle.
Figure 3 is schematic representation of second preferred embodiment of
present invention in connection with IGCC process flow diagram with heat recovery
from bottom and cyclones ashes and with maximum heat recovery from flue gas.
DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE
INVENTION
Figure 1 schematically illustrates an Integrated Gasification Combined Cycle
(IGCC) process scheme of prior art without heat recovery from ash. Coal (46) is
gasified by reacting with air (47) and steam (48) in pressurized fluidized bed gasifier
(PFBG) 11 operating at high pressure and high temperature. In gasification process,
coal gas along with elutriated fine particles leaves from top of the gasifier and bottom
ash is discharged from bottom of the gasifier. Ash particles present in syngas are
separated in 2 cyclone separators (12, 14) arranged in series. Total or some portion of
the ash collected in cyclone separator (12) is recycled back to the gasifier through
refractory lined stand pipe and loop seal arrangement (13). Syngas from cyclone

separators (12, 14) is taken to the heat recovery boiler (15) to cool the gas to the
temperature suitable for gas cleanup.system (16). Feed water from deaerator (22)
using high pressure pump (23) is sent to the Heat Recovery Boiler (HRB) (15). In
HRB, high pressure saturated steam is generated utilizing heat energy available with
syngas. Cooled gas leaving HRB (15) is fed to the Gas Cleanup System (GCS) (16)
where left over fine particulates, H2S, NH3 and alkalis are removed. Cleaned syngas
meeting gas turbine specifications is combusted in combustor (17) and then fed to the
gas turbine (19) which is integrated with compressor system (18). Air required for
gasification process is bleed from Gas Turbine (GT) compressor (18). Bleed air taken
from GT compressor (18) is cooled in bleed air cooler (29) and further compressed in
booster compressor (31) to meet required gasifier pressure. Exhaust from GT (19), is
fed to the HRSG (20). Feed water from deaerator (22) is pumped to the HRSG (20)
using low and high pressure feed water pumps (24, 25). High Pressure (HP) and Low
Pressure (LP) superheated steam is generated in HRSG (20) utilizing heat energy
available with GT exhaust flue gases. HP saturated steam generated in HRB (15) is
mixed with the HP saturated steam generated in HP evaporator. Flue gases leaving
Heat Recovery Steam Generator (HRSG) (20) is left into atmosphere through stack
(21). Some quantity of LP steam is taken for condensate preheating and deaerating of
feed water. Remaining LP and HP steams are fed to the steam turbine (26) to
generate power. ST (26) has provision to withdraw required quantity of steam at
conditions slightly higher than the maximum gasifier operating conditions. Exhaust
from ST (26) is condensed in condenser (27) by exchanging heat with circulating

cooling water from cooling tower (32) using pump (33). Condensate from condenser
is pumped using pump (28) to bleed air cooler (29), LP steam heater (30) and then to
water preheater of the HRSG for preheating the condensate. Preheated condensate is
taken to the deaerator (22) and deaerating of feed water is done using LP steam. Ash
from gasifier operating at high pressure is discharged to ash receiver through lock
hopper system (36, 37). Ash discharged from bottom of the gasifier is cooled in static
ash cooler (34) followed of screw ash cooler (35) using cooling tower water. Ash from
cyclones (12,14) is also discharged through lock hopper system and cooled in static
ash cooler (38, 42) followed by screw ash coolers (39, 43).
Figure 2 is first preferred embodiment of present inventions showing process
flow diagram of IGCC plant with heat recovery from bottom and cyclone ash and
integration of recovered heat with condensate preheating system. Heat present with
ash is exchanged with water circuit by circulating water to ash coolers. Ash discharged
from gasifier is cooled in tubular ash cooler (34) and then in screw ash coolers (35).
Ash discharged from cyclone separator. (12 & 14) are cooled in tubular ash coolers (38
& 42) followed by screw ash coolers (39 & 43) respectively. Tubular ash cooler is
jacketed vessel with horizontal tubes and screw cooler is jacketed cooler with water
circulation to the rotating shaft. Though in the envisaged scheme, tubular and screw
coolers are arranged in series with water entering screw cooler and leaving tubular
cooler, they can as well arrange in parallel with water distributing between both the
coolers. Ash from screw ash coolers is discharged through lock hopper system. Heat

recovered by the ash cooling water circuit (53, 54) is used in preheating the
condensate using heat exchanger (55). One more heat exchanger (56) in ash cooling
water circuit is provided if further cooling of circulating water is required.
Condensate from condenser is initially heated in bleed air cooled (29) and then
heated by the heat recovered from ash using heat exchanger (55). The heated
condensate from heat exchanger (55) is further heated in LP heater (30) and then
admitted into water preheater of HRSG (20). Because of utilization of heat energy
available with ashes to increase the temperature of condensate, the amount of LP
steam required in LP heater (30) which has been extracted from HRSG (20) to
increase condensate temperature has been reduced. The equivalent amount of steam
saved through the above said process is admitted into LP section of steam turbine
(26) to generate additional power which leads to efficiency improvement.
Figure 3 second preferred embodiment of present invention illustrating IGCC
process scheme with heat recovery from ash and flue gases and integration of
recovered heat for condensate preheating. As illustrated earlier, flue gas leaving
through stack (21) from HRSG (20) has high heat content and the maximum heat that
can be recovered is depended on the temperature of flue gas that can be left into
stack which is determined or fixed based on acid dew point. The acid dew point is
depended on amount of H2S in synthesis gas. By reducing the amount of H2S to the
minimum possible extent using the latest available technologies, the temperature of

synthesis gas leaving into stack can be further reduced. But, because of high
temperature of condensate water entering to the water preheater section of HRSG
(20), it is not possible to recover more heat using process scheme illustrated in prior
art. More heat available in flue gas by reduction in stack temperature can be
recovered by preheating of condensate (51) first in water preheater section HRSG
(20) by taking advantage of higher temperature differences between flue gas and
condensate. In this scheme, the condensate is first preheated in water preheater
which is the last portion of HRSG (20) and then preheated condensate (57) is heated
in Bleed Air Cooler (BAC) (29). Condensate leaving BAC (29) is taken to heat
exchanger (55) present in ash cooling water circuit. Condensate is further heated in
LP heater (30) and then taken to deaerator (22). This process leads to effective way
to recover more heat from flue gas and ashes which helps in reducing the amount of
steam required in LP heater (30) to increase the condensate temperature. The
amount of LP steam extracted from HRSG for LP heater through the above said
scheme in fig 3 is less than that of the scheme explained in fig 2 and by admitting
that much equivalent steam into LP section of steam turbine (26) to generate
additional power leads to overall cycle efficiency improvement.
In view of the above illustrated descriptions, only two of the most preferred
embodiments have been presented in the present invention. It has to be understood
that this invention is not limited to the attached embodiments and various other
embodiments which comes under the scope of the appended claims would also come
under this invention.

WE CLAIM
1. A method for improving the efficiency of pressurized fluidized bed gasification
(PFBG) based integrated gasification combined cycle (IGCC) power plant with heat
recovery from ash and flue gases and integrating of recovered heat for condensate
preheating, the method comprising;
cooling of discharged ash from gasifier (11) in tubular ash cooler (34) and then
in screw ash coolers (35);
cooling of discharged ash from cyclone separators (12,14) in tubular ash
coolers (38,42) and then in screw ash coolers (39,43) respectively;
characterized in that,
heat recovered by the ash cooling water circuit (53,54) arranged to preheat a
condensate by a heat exchanger (55) disposed in the process before LP heater (30)
wherein the condensate from condenser is heated initially in bleed air cooler (29) and
then heated by the heat recovered from ash aided by the heat exchanger (55) which
is further heated in LP heater (30) and admitted into water preheater of HRSG (20)
(gasifying coal (46) by reacting with air (47) and steam (48) in pressurized fluidized
bed gasifier (PFBG) (11) ) when utilization of heat energy available with ashes to
increase the temperature of condensate, the amount of LP steam required in LP
heater (30) is reduced when the equivalent amount of steam saved by this process is
admitted into LP section of steam turbine (26) to generate additional power leading to
efficiency improvement.

2. A method for improving the efficiency of pressurized fluidized bed gasification
(PFBG) based integrated gasification combined cycle (IGCC) power plant with heat
recovery from ash and flue gases and integrating of recovered heat for condensate
preheating, the method comprising;
cooling of discharged ash from gasifier (11) in tubular ash cooler (34) and then
in screw ash coolers (35);
cooling of discharged ash from cyclone separators (12,14) in tubular ash
coolers (38,42) and then in screw ash coolers (39,43) respectively;
removing H2S composition in synthesis gas to the possible extent to decrease
acid dew point of flue gas to reduce flue gas temperature leaving into stack (21);
characterized in that,
a condensate preheated in water preheater of HRSG (20) to extract more heat
because of large temperature difference between flue gas and condensate and the
preheated condensate (57) being heated in Bleed Air Cooler (29) enters a heat
exchanger (55) disposed in the ash cooling water circuit to raise the temperature of
the condensate (57) and is heated in LP heater (30) and is taken to deaerator (22)
and thus recovers more heat from flue gas and ashes reducing the amount of steam
required in LP heater (30) to increase the condensate temperature and by admitting
that much equivalent steam into LP section of steam turbine (26) generating
additional power leading to overall cycle efficiency improvement.