FIELD OF THE INVENTION
The present invention relates to steam generated boilers which provide carbon
dioxide rich flue gas at the exhaust for easy carbon dioxide capture. More
particularly, the present invention relates to a method for retrofitting oxy-fuel
combustion technology in existing air-carbonaceous fuel fired systems.
BACKGROUND OF THE INVENTION
Rapid increase in Carbon dioxide (CO2) concentration in the atmosphere and
consequent rise in atmospheric temperature forcing industrialists, policymakers,
scientists and environmentalists to invent new methods and technologies which
can reduce anthropogenic CO2 emissions into atmosphere. In this regard, power
production by non-conventional methods such as solar power, wind power, hydro
power, nuclear power and tidal, etc is under constant focus as the future
resources. However, none of these technologies are seen as immediate
alternative to the carbonaceous fuel fired systems due to its lack of commercial
maturity (high cost) or environmental related issues. Therefore, energy
production by carbonaceous fuels is likely to continue for at least few decades to
come. Under these circumstances, it is an urgent need to develop technologies
which can reduce CO2 emissions from the carbonaceous fuel firing systems.
Since, the maximum amount of anthropogenic emissions is contributed by
industries such as power plant, metallurgical industries, cement production,
refineries, glass making industries, efforts have been concentrated on these
industries to cut down CO2 emissions along with other pollutants emissions.
Among these industries, thermal power plants have received particular attention

because of availability of flue gas a at one point therefore any CO2 capture
technologies can be implemented easily. At this juncture, it was identified that
Carbon Capture and Storage (CCS) is one of the best methodologies to restrict
anthropogenic CO2 emissions. This can be best achieved by implementing oxy-
fuel combustion technology. In this technology, the combustion takes place with
pure oxygen instead of air, hence the flue gas mainly consists of C02 and water
vapour. The CO2 can be captured easily by condensing all the water vapour by
cooling the flue gas to a low temperature, which is then sent for compression
and storage or further utilization such as Enhanced Oil Recovery (EOR), coolant,
urea manufacturing etc. However, because of absence of nitrogen during the
combustion, the flame temperature would be very high with pure oxy-fuel
combustion. In power plants, such a high temperatures are not necessary. To
moderate high temperatures in the furnace, a part of the exhaust flue gas is
recycled back and mixed with pure oxygen then, sent to the furnace through a
burner.
Considering the large capacities of installed carbonaceous fuel based thermal
power plants across the world, if implementation of oxy-fuel combustion
technology is only limited to new power plants, it requires high capital cost and
CO2 reduction targets may delay. Therefore retrofitting existing thermal power
plants with oxy-fuel combustion technology offers many advantages such as it
would be economical, capable of reducing NOx formation, facilitate easy and
economical CO2 capture. In this regard, the inventors noted that there are many
innovative methodologies have been proposed in the past to convert existing
boiler to oxy-fuel boiler.

DE 10356703 teaches a methodology for oxy-fuel combustion where, oxygen
preheating was done using recirculated flue gas using a plattenoder tube heat
exchanger. US Patent US 831684 B2, suggest a combustion system using
recirculated flue gas for pulverized coal transport to burner for the purpose of
coal drying and preheating of oxygen. Recirculated flue gas is also heated using
the flue gas before admitting to boiler.
US Patent US 6202574 B1 discloses an oxy-fuel combustion methodology, where,
compressing the end product portion of the flue gas to yield carbon in a liquid
phase for CO2 storage taught.
Indian Patent 8478/CHENP/2010 discusses about oxy-fuel combustion
methodology in which flue gas is utilized for preheating the economizer water
until flue gas temperature was little above acid dew point temperature and
thereby sending to main feed line.
WO 2013057661A1 describes a method in which flue gas after passing through
electrostatic precipitator and flue gas desulphurization was preheated using raw
flue gas.
SUMMARY OF THE INVENTION
Present invention describes a method of converting a carbonaceous fuel and air
fired power generating system to an oxy-fuel based carbonaceous fuel firing
system. The present invention provides a higher boiler efficiency than
conventional combustion system. However, to execute the new methodology, it

requires additional supporting systems such as Air Separating Unit (ASU), a
ducting system for recycling the flue gas, a Flue Gas Recirculation (FGR) fan for
injecting the flue gas to the firing system and carbon dioxide compression unit
for CO2 capture or utilization. According to the present invention, oxygen from
the ASU is preheated using existing air preheater to maximum possible
temperature, in which outgoing flue gas supplies heat to oxygen . The preheated
oxygen is mixed with CO2- rich recycled flue gas and then sent to furnace
through the burner. Here, the outgoing flue gas from air preheater passes
through a dust removal unit for fly ash removal. The dust free CO2-rich gas from
the dust removal unit is divided into two portions. The larger portion of CO2-rich
flue gas is utilized for flue gas recycle, which is then mixed with preheated
oxygen. The mixture of oxygen and recycled flue gas (oxidizer) is further divided
into two portions. The smaller portion of the oxidizer is used to transport solid
carbonaceous fuel to the firing system. The remaining portion of the oxidizer is
sent to the firing system for complete combustion of the carbonaceous fuel. The
smaller portion (flue gas remaining after segregation) of dust free C02-rich flue
gas after dust removal unit is sent to a multistage cooling system, where most of
the water vapour is removed from the flue gas. The flue gas now rich in CO2 is
sent for multistage CO2 compression unit, which is either sent for further
utilization or sent for permanent storage.
The amount of flue gas recirculation to the boiler should be adjusted to match
the radiative and convective heat flux of conventional combustion system in the
boiler. The present invention proposes that the oxygen concentration in the

mixture of flue gas and oxygen (oxidizer) should be greater than 21% by volume
and preferably in a range between 23% to 32% by volume. Therefore, the
amount of CO2-rich flue gas should be segregated accordingly from the exit of
the dust removal unit. Since, the quantity of heat extraction for preheating the
oxidizer is lower in oxy-fuel combustion methodology, the flue gas leaving the
boiler would be at higher temperature than the conventional combustion system.
Hence, it will increase the volume of flue gas to be handled in the dust collection
system.7 however, overall flue gas flow rate reduces in oxyfuel combustion
methodology due to higher specific heat of CO2 and H2O than N2. This will
compensate the rise in volumetric flow rate of flue gas due to temperature.
Eventually, the flue gas volumetric flow rate is more or less the same as a
conventional combustion system. Therefore, it does not require any further
modifications to convert an existing boiler set-up. The existing dust cleaning can
also be used as it is for oxy-fuel combustion flue gas. Similarly the existing
induced draft (ID) fan as used in conventional combustion, can also be used for
pumping the exhaust flue gas.
According to another aspect of the invention, the portion of flue gas after
segregation is cooled down partially before mixing with preheated oxygen.
to maintain same oxidizer temperature as that of conventional combustion
system. This is helpful in avoiding higher temperatures in the solid carbonaceous
fuel transportation line. This will prevent accidental firing in a fuel transportation
line.

According to another aspect of the invention, a part of the recycled flue gas,
without mixing with preheated oxygen, is directly sent for transporting the
pulverized solid carbonaceous fuel. This is useful in preventing the accidental fuel
firing in the fuel transportation line due to lack of enough oxygen in the solid
carbonaceous fuel and oxidizer mixture.
According to another aspect of the invention, a portion of the flue gas, which is
used for transportation of the solid carbonaceous fuel is cooled down to a very
low temperature up to 50°C, to remove water vapour. This is preheated with
available heat during flue gas cooling then sent for transporting solid
carbonaceous fuel to the firing system. This is advantageous in avoiding the
water formation during the contact of flue gas and solid carbonaceous fuel
hence, reduces the corrosion formation and coal line blockage in the solid
carbonaceous fuel transportation line.
According to another aspect of the invention, the portion of the flue gas which is
sent for CO2 compression or utilization is cooled down to low temperature using
low pressure economizer water with a pressure up to 30 bar in a multistage
cooling unit. This heated low pressure water is then sent to low pressure
economizer in the boiler, it will further enhance the boiler efficiency. Since water
has higher specific heat, it is also helpful in rapidly cooling the flue gas to lower
temperature. Where most of the water vapour gets condensed and removed, the
moisture free CO2-rich flue gas is then sent for multistage compression unit.

According to another aspect of the invention, a new variant of oxidizer is
possible. The flue gas after segregating at the end of dust cleaning system is
split into two portions. One portion is mixed with a small portion oxygen from
ASU in such a way that the oxygen concentration in the mixture should be lower
than 21% by volume. The remaining portion of oxygen and flue gas is mixed and
sent to the firing system for complete combustion.
According to another aspect of the invention, portion of the flue gas which is
unsegregated at the end of dust cleaning system is sent to Flue Gas
Desulphurization (FGD) to remove sulphur dioxide before sent for cooling and
CO2 compression.
The Experimental values of important parameters for a conventional (coal-air)
combustion system and an oxy-fuel combustion system are given below:
Coal - air case
Net calorific value of the coal = 3670 kcal/kg
Coal flow rate = 320 ton/hr
Total air flow rate to the boiler = 2030 ton/hr
Total flue gas flow rate to the chimney = 2250 ton/hr
Temperature of the flue gas leaving through chimney = 150 °C

Oxy-coal case:
For the same coal flow and properties, these are the following parameters for
oxy-fuel combustion methodology.
Oxygen flow rate (95% purity) = 430 ton/hr
Percentage of the flue gas recycle from total flue gas = 70.5
Recycle flow rate of the flue gas = 1550 ton/hr
Oxygen concentration in the oxidizer (O2 + RFG) = 26.2. % by volume
Temperature of the recycled flue gas = 280 °C
Pure oxygen (95%) preheated temperature = 290 °C
Flue gas send for compression and storage = 650 ton/hr
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure 1 illustrates a schematic lay-out of a retrofitted oxy-fuel combustion based
power plant according to the invention with flue gas recycle system, air
separation unit and CO2 compression unit.

Figure 2 illustrates a schematic lay-out of a retrofitted oxy-fuel combustion based
power plant according to the invention with an alternate flue gas recycle system,
air separation unit and CO2 compression unit.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT OF THE
INVENTION
The present invention describes a method to retrofit conventional steam based
power production boiler with oxy-fuel technology, which allows an easy CO2
sequestration from the exit flue gas.
Figures 1 and 2 show the detailed schematic lay-outs of the retrofitted oxy-fuel
combustion based power plant with different variants of flue gas recirculation
and heat utilization in the flue gas stream. In Figure 1, CO2 rich flue gas stream
(10) from a boiler (1) enters a gas/gas heat exchanger (2), where maximum
possible heat is extracted by pure oxygen stream (23). Here the pure oxygen
stream is produced by sending fresh air (22) to an air-separation unit (5). The
ASU (5) separates nitrogen from air and produces the pure oxygen stream (23).
Now, the CO2-rich flue gas stream (11) is sent to a dust removal unit (3), where
it separates the most of the dust particles from the flue gas stream and these
dust particles gets collected at the bottom (9). The dust free CO2-rich flue gas
(12) is passed through an induced draft (ID) fan (4) . The exit flue gas stream
(13) from the ID fan (4) is split into two portions, one portion (25) is sent for
CO2 compression and the other portion (14) is sent for recycling into the boiler
(1) inlet.

The recycle portion of the flue gas stream (14) is further pressurized in a FGR
fan (15). The exit flue gas (16) from the FGR fan (15) is mixed with a preheated
oxygen stream (2). The mixture of pure oxygen and flue gas stream (17) is
again split into two portions, the larger portion (18) is directly sent to the boiler
(1) for complete combustion of carbonaceous fuel (20) and the smaller portion
(19) is sent for conveying the carbonaceous fuel (20) to the furnace. The mixture
of oxidizer and solid carbonaceous fuel (21) from the raw coal storage bin (6) is
sent to the boiler (1) through burner port for combustion.
The non-recycled portion of CGvrich flue gas stream (25) is sent for cooling in
the heat exchanger (7) where, the CCVrich flue gas is cooled down to room
temperature hence, most of the water vapour present in the flue gas gets
condensed and removed at the bottom as water (26). The dust and moisture
free CCVrich flue gas stream (27) is sent for compression in a multistage
compressor (8). The compressed CO2 stream (28) is then sent either for
utilization or permanent storage or both depending upon demand and
requirement.
The heat liberated during the carbonaceous fuel combustion in the boiler (1) is
utilized to superheat water (29). The superheated steam (30) from the boiler is
then sent for steam turbine based power production.
The dotted line in Figure 1 indicates the new variant of heat utilization from the
dust free CO2-rich flue gas stream (25). The energy in the flue gas stream (25) is
recovered using low pressure water stream (33) in the multistage heat
exchanger (7). Here the fresh water (31) from condenser is pressurized up to 30

bar in a pump (32). The pressurized water (33) is then sent for heating in a
multistage heat exchanger (7). The preheated pressurized water (34) is then
sent to boiler (1) for further heating and to generate superheated steam for
power production. The cooled flue gas stream (27) which is now devoid of water
vapour sent to compression in multistage compressor (7).
Another variation is highlighted in Figure 1 with dotted lines, which relates to the
flue gas treatment. Based on the end user specifications or sulphur dioxide (SO2)
limits in compressed CO2, a FGD unit can also be incorporated in the process.
The flue gas stream (25) is sent to the FGD unit (35) to remove SO2 from the
flue gas stream. The treated flue gas stream (36), then sent for cooling in
multistage heat exchanger (7) and then to multistage compressor (8).
Figure 2 illustrates a new variant of flue gas recirculation in conducting the oxy-
fuel combustion technology in a boiler. The oxy-fuel combustion methodology in
Figure 2 is same as Figure 1 except that the CCVrich flue gas stream (16) after
pressurizing in the FGR fan (15) is again split into two portions, the larger
portion of flue gas stream (17) is mixed with preheated pure oxygen stream (24)
and is sent to boiler for complete combustion of carbonaceous fuel (20). The
smaller portion of flue gas stream (19) is utilized for transporting the
carbonaceous fuel (20) from the raw coal storage bin (6). The mixture of CO2-
rich flue gas and carbonaceous fuel stream (21) is sent to boiler (1) for
combustion.

The dotted lines in the Figure 2 represents the another variant of above
mentioned methodology where the smaller portion of flue gas stream (19) is sent
to a heat exchanger (31) to cool the flue gas close to acid dew point of the
gaseous mixture. The cooled flue gas stream (33) is sent to another heat
exchanger (32) where the flue gas (33) is further cooled down to low
temperature hence water vapour gets condensed and removed at the bottom
(35). The moisture free CXVrich flue gas (34) is heated with the low grade
energy available in heat exchangers (32) and the exit moisture free flue gas (36)
is sent to the other heat exchanger (31) to gain further heat. The moisture free
CO2 rich flue gas stream (37) is then utilized for transporting the carbonaceous
fuel (20) from the raw coal storage bin (6). The mixture of moisture free CO2-
rich flue gas and carbonaceous fuel stream (21) is sent to the boiler (1) for
combustion.

WE CLAIM :
1. A method of converting existing carbonaceous fuel and air fired boiler to
oxy-fuel fired boiler, the method comprising:
(a) generating a stream of pure oxygen;
(b) allowing passing of a carbonaceous fuel stream;
(c) segregating a stream of dust free flue gas at the end of a dust
removal unit;
(d) preheating the pure oxygen stream in a preheater unit;
(e) recycling the flue gas in a duct system;
(f) circulating the flue gas through a recirculation fan;
(g) a multistage cooling of the flue gas in a multistage heat
exchanger; and
(h) multistage compression of carbon dioxide rich flue gas in a
compressor unit.
2. The method as claimed in claim 1, wherein the purity of oxygen is varied
from 85% to 100%.
3. The method as claimed in claim 1, wherein carbonaceous fuel is a
combination of different carbonaceous fuels.
4. The method as claimed in claim 1, wherein the preheater unit is used to
preheat pure oxygen with minor modifications.

5. The method as claimed in claim 1, wherein an Electrostatic Precipitator
(ESP) or a bag house filter or a fabric filter is adapted for removal of
dust.
6. The method as claimed in claim 1, wherein existing boilers'
carbonaceous fuel handling system, firing system, steam generating
system, air preheating system, dust removal system, ID fan can be used
for conducting oxy-fuel combustion methodology with little/no
modifications.
7. The method as claimed in claim 1, wherein oxygen concentration in
the mixture of recycled flue gas and oxygen is in the range between
23% to 32% by volume.
8. The method as claimed in claim 1, wherein the segregated flue gas is
either directly mixed with the preheated pure oxygen stream or partially
cooled before mixing with preheated pure oxygen stream.
9. The method as claimed in claim 1, wherein the segregated flue gas is
split into two portions, wherein a first portion is used for transporting
pulverized carbonaceous fuel to the firing system and the second portion
is mixed with the preheated oxygen before being sent to the
carbonaceous fuel firing system.

10. The method as claimed in claim 9, wherein the flue gas stream is cooled
below 100°C to condense most of the water vapour and then used to
convey solid pulverized carbonaceous fuel.
11.The method as claimed in claim 9, wherein the flue gas stream is mixed
with a part of the preheated pure oxygen in such a way that oxygen
concentration is less than 21% by volume in the mixture of flue gas and
oxygen before being used for transporting solid carbonaceous fuel.
12. The method as claimed in claim 10, wherein the water vapor free flue
gas is preheated with heat available during flue gas cooling and then
sent for solid carbonaceous fuel transportation.
13. The method as claimed in claim 12, wherein the water vapor free flue
gas is partially heated and mixed with the preheated pure oxygen
stream, in such a way that oxygen concentration is lower than 21% by
volume in the mixture of the gas and oxygen..
14. The method as claimed in claim 1, wherein the remaining flue gas after
segregating the recycled portion is sent for cooling to remove water
vapour before sending for multistage compression.
15. The method as claimed in claim 14, wherein for cooling of flue gas, a
pressurized feed water (pressure of water up to 30 bar) to the boiler is
used to recover available energy.

16. The method as claimed in claim 15, wherein the heated pressurized feed
Water (pressure of water up to 30 bar) is sent to boiler for further
heating.
17. The method as claimed in claim 14, wherein the cooled flue gas, which
is now rich in carbon dioxide (CO2) is sent for high pressure compressor,
the CO2 rich compressed flue gas being either sent for permanent storage
or for further utilization.
18. The method as claimed in claim 14, wherein the flue gas before sending
for cooling is, first sent to a flue gas desulphurization unit to remove
sulphur dioxide from the flue gas.
19. The method as claimed in claim 14, wherein the flue gas after sulphur
dioxide removal is sent for multistage cooling and compression.

ABSTRACT

The invention relates to a method of converting existing carbonaceous fuel and
air fired boiler to oxy-fuel fired boiler, the method comprising generating a
stream of pure oxygen; allowing passing of a carbonaceous fuel stream;
segregating a stream of dust free flue gas at the end of a dust removal unit;
preheating the pure oxygen stream in a preheater unit; recycling the flue gas in
a duct system; circulating the flue gas through a recirculation fan; a multistage
cooling of the flue gas in a multistage heat exchanger; and multistage
compression of carbon dioxide rich flue gas in a compressor unit.