OXYGEN FIRED CIRCULATING
FLUIDIZED BED STEAM GENERATOR
BACKGROUND OF THE INVENTION
The present invention relates to a circulating fluidized bed steam generator
and a method for operating the circulating fluidized bed steam generator for
producing a carbon dioxide end product.
US Patent No. 5,175,995 to Pak et al describes a conventional power
generation plant operable to burn fuel with air in a combustor so as to thereby
provide combustion gas energy to drive a steam or gas turbine. In those versions
of such conventional power generation plants which combust natural gas,
petroleum fuel gas or coal gas as the fuel for the combustion process, this fuel
includes carbon components such as carbon (C), carbon monoxide (CO), and
other hydrocarbons (CmHn). Accordingly, the flue gas produced by a combustion
process which combusts the fuel in the presence of air is comprised of carbon
dioxide (CO2), nitrogen oxide (NOX), and sulfur oxide (SOX) as well as nitrogen
gas (N2).
The '995 Pak et al patent further notes that the release of gases such as
NOX, SO,, and CO2 into the atmosphere creates environmental pollution.
Conventional power generation plants have conventionally countered such
pollution by the deployment of removal equipment such as scrubbers to remove
the NOX and SOX pollutants. Moreover, removal equipment has been deployed to
remove the carbon dioxide (CO2) present in the flue gas including removal
equipment of the type which utilizes a sorbent to selectively absorb the carbon
dioxide (CO2) from the flue gas. However, this solvent approach, according to the
'995 Pak et al patent, disadvantageously requires additional heat energy to heat
the solvent and it is not practical to provide the relatively long contact time
between the solvent and the carbon dioxide (CO2) for the solvent to fully absorb
the carbon dioxide (CO2).

The '995 Pak et al patent discloses several versions of a closed combined
cycle type power generation plant which purports to ameliorate some of the
disadvantages of the carbon dioxide (CO2) absorbing solvent approach. In each
version of the power generation plant disclosed in this reference, fuel is supplied
to a combustor in the presence of oxygen instead of in the presence of air so as to
produce a combustion gas (flue gas) which mainly includes a water component
and carbon dioxide (CO2). This combustion gas is handled so as to separate the
water component and the carbon dioxide (CO2) with the separated carbon dioxide
(CO2) being recycled as the working fluid for driving a turbine of the combined
cycle power generation plant. Since the combustion of the fuel in presence of
oxygen instead of air substantially eliminates the creation of NO, and, further,
since the carbon dioxide (CO2) is retained within the closed cycle as a working
fluid, the approach disclosed by the '995 Pak et al patent advantageously avoids
the discharge of NOX as well as carbon dioxide (CO2).
US Patent No. 4.498.239 to Osgersby also discloses a power system which
combusts a hydrocarbonic fuel in the presence of oxygen in lieu of air so as to
obtain a working fluid comprised of carbon dioxide (CO2). While the '995 Pak et
al patent and the '289 Osgersby patent each disclose an arrangement for reducing
carbon dioxide (CO2) emissions via the combustion of fuel in the presence of
oxygen instead of air, the art could still benefit from an arrangement for new or
existing power generation system designs which offers the flexibility to both to
produce carbon dioxide (CO2) as a desirable end product and to use carbon
dioxide (CO2) as support to the combustion process. Also, the art could benefit
from an arrangement comprising a circulating fluidized bed steam generator
whose combustion temperature can be controlled with the aid of up to no more
than one-half the typical requirement for flue gas recirculation required by a
comparably performing pulverized coal steam generator.

SUMMARY OF THE INVENTION
It is an object of the present invention to provide an arrangement for new
or existing power generation system designs which offers the flexibility both to
produce carbon dioxide (CO2) as a desirable end product and as sugport to the
combustion process.
It is another object of the present invention to provide an arrangement for
producing liquid carbon dioxide (CO2) which improves the heat output of a fossil
fuel fired power generating system.
According to one aspect of the present invention, a method for operating a
circulating fluidized bed steam generator is provided for new or existing power
generation system designs which offers the flexibility to use carbon dioxide (CO2)
both as a desirable end product and as support to the combustion process. The
method includes the step of introducing a substantially pure oxygen feed stream
into the circulating fluidized bed steam generator and the step of combusting a
fuel in the presence of the substantially pure oxygen feed stream to produce a flue
gas having carbon dioxide and water vapor as its two largest constituent elements
by volume.
The method for operating a circulating fluidized bed steam generator in
accordance with the present invention includes the steps of introducing a
substantially pure oxygen feed stream into the circulating fluidized bed steam
generator, combusting a fuel in the presence of the substantially pure oxygen feed
stream to produce a flue gas having carbon dioxide and water vapor as its two
largest constituent elements by volume, passing the flue gas through an oxygen
feed stream pre-heater at which heat from the flue gas is transferred to the oxygen
feed stream, separating the flue gas into an end product portion and a recycling
portion, and directing the recycling portion of the flue gas to the circulating
fluidized bed steam generator to contribute to the combustion process therein.
Preferably, the method also includes cooling and compressing the end product
portion of the flue gas so as to yield carbon dioxide in a liquid phase.

According to another aspect of the present invention, there is provided a
circulating fluidized bed steam generator which includes means for introducing a
substantially pure oxygen feed stream into the circulating fluidized bed steam
generator, means for combusting a fuel in the presence of the substantially pure
oxygen feed stream to produce a flue gas having carbon dioxide and water vapor
as its two largest constituent elements by volume, means for passing the flue gas
through an oxygen feed stream pre-heater at which heat from the flue gas is
transferred to the oxygen feed stream, means for separating the flue gas into an
end product portion and a recycling portion, and means for directing the recycling
portion of the flue gas to the circulating fluidized bed steam generator to
contribute to the combustion process therein. Preferably, the circulating fluidized
bed steam generator also includes means for cooling and compressing the end
product portion of the flue gas so as to yield carbon dioxide in a liquid phase.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 is a schematic view of a circulating fluidized bed steam generator;
and
Figure 2 is a schematic view of a combined cycle power generating unit
comprising the circulating fluidized bed steam generator shown in Figure 1 for
producing a carbon dioxide end product.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring initially to Figure 2 of the drawings, there is depicted therein an
embodiment of the oxygen fired circulating fluidized bed steam generator (CFB)
of the present invention. The circulating fluidized bed steam generator (CFB),
generally designated by the reference numeral 10, uses oxygen in lieu of air for
combustion to thereby advantageously minimize the amount of recirculated flue
gas in a cost favorable manner. However, before providing a detailed description
of the entirety of the circulating fluidized bed steam generator 10 and, thereafter, a
detailed description of a combined cycle power generating unit comprising the

circulating fluidized bed steam generator 10, reference will first be had to Figure 1
of the drawings to provide a general description of a sub group of combustion and
hot solids-gas separator components of the circulating fluidized bed steam
generator 10.
It is to be understood that the configuration of the circulating fluidized bed
steam generator 10, including the presence or absence, the placement, and the
interconnection of its assorted elements, as illustrated and described herein, is to
be understood as merely exemplary of one configuration in which an oxygen fired
circulating fluidized bed system in accordance of the present invention may be
employed. For this reason, it is noted that the following discussion of the
circulating fluidized bed steam generator 10 discloses merely one possible
operational arrangement and it is contemplated that, as desired or as dictated by
circumstances, the configuration of the circulating fluidized bed steam generator
10, including the presence or absence, the placement, and the interconnection of
its assorted elements, may be changed while nonetheless representing an
embodiment of the circulating fluidized bed system_of the present invention.
As illustrated in Figure 1, the circulating fluidized bed steam
generator 10 includes a furnace volume, denoted therein by the reference numeral
12, the latter being defined by waterwall tubes, denoted therein by the reference
numeral 14; a first section of ductwork, denoted therein by the reference numeral
16; a combined hot solids-gas separator, denoted therein by the reference numeral
18; an intermediate section of backpass ductwork, denoted therein by the
reference numeral 20; and a backpass volume, denoted therein by the reference
numeral22, from which further ductwork, denoted therein by the reference
numeral 24, extends.
The furnace volume 12 is water cooled via water transported
through the waterwall tubes 14 whereas the combined hot solids-gas separator 18
and the backpass volume are steam cooled via tubes integrated into their wall
structures.

The lower segment of the combined hot solids-gas separator 18, which can
be, for example, a conventional cyclone, is connected in fluid flow relation with
the lower segment of the furnace volume 12 through a fluid flow system
consisting, in accordance with the illustration thereof in Figure 1 of an initial
collection path, denoted therein by the reference numeral 26; a direct return
measured feed device, denoted therein by the reference numeral 28; a direct return
path, denoted therein by the reference numeral 30; a fluidized bed heat exchanger
(FBHE) inlet, denoted therein by the reference numeral 32; an ash control valve,
denoted therein by the reference numeral 34; a fluidized bed heat exchanger
(FBHE), denoted therein by the reference numeral 36; and a fluidized bed heat
exchanger (FBHE) outlet, denoted therein by the reference numeral 38. For
purposes of the discussion that follows hereinafter, the ductwork 16, the combined
hot solids-gas separator 18 and the fluid flow system 26, 28, 30. 32, 34, 36, 38
will be referred to as a hot solids circulation path, denoted by the reference
numerals 40, 42, 44. Further, it is to be understood that the fluid flow system 26,
28, 30, 32, 34, 36, 38 is typical of the fluid flow system, which is cooperatively
associated with the combined hot solids-gas separator 18. It can be seen from a
reference to Figure I of the drawing that the furnace volume 12 is in
communication with a source, denoted therein by the reference numeral 46, of fuel
and sorbent through a supply line, denoted therein by the reference numeral 48, as
well as with a source, denoted therein by the reference numeral 50, of oxygen
through a supply line, denoted therein by the reference numeral 52.
With regard to Figure 1 of the drawing, it will be understood from
reference thereto that in the lower segment of the furnace volume 12 a mixture of
fuel and sorbent, denoted therein by the reference numeral 54, is mixed for
purposes of the combustion thereof with oxygen, denoted therein by the reference
numeral 56. Preferably, fluidizing media comprising the oxygen 56 is fed through
a floor grate on which the fluidized bed of the furnace volume 12 is disposed and
additional oxygen is fed at two levels above the floor grate. Moreover, it is
preferred to configure the feed and sorbent supply line 48 to include air-assisted

fuel and sorbent feed nozzles to thereby advantageously minimize waterwall
penetration opening size and to minimize fuel chute plugging potential. Ash can
be drained from the lower volume 12 of the circulating fluidized bed steam
generator 10 via a conventional ash cooler-58, shown in Figure 2.
In known fashion, from this combustion, hot combustion gases, denoted
therein by the reference numeral 40 are produced and hot solids, denoted therein
by the reference numeral 42, are entrained in the hot combustion gases 40. These
hot combustion gases 40 with the hot solids 42 entrained therewith rise within the
furnace volume 12 whereupon at the top of the furnace volume 12 the hot
combustion gases 40 with the hot solids 42 entrained therewith are made to flow
through the duct 16 to the combined hot solids-gas separator 18.
Within the combined hot solids-gas separator 18. the hot solids 42 that are
made to flow thereto, which are above a predetermined size, are separated from
the hot combustion gases 40 in which they are entrained. The separated hot solids
42 which contain unburned fuel, flyash and sorbent, as well as carbon dioxide
(CO2) and water vapor (H2O), flow through the combined hot solids-gas separator
18. From the combined hot solids-gas separator 18, the hot solids 42 are
discharged under the influence of gravity into the initial collection path 26, from
whence a portion of the hot solids 42 flow through the initial collection path 26 to
and through the direct return measured feed device 28. Thereafter, from the direct
return measured feed device 28, this portion of the hot solids 42 is reintroduced by
means of a corresponding direct return path 30 into the lower segment of the
furnace volume 12 whereupon this portion of the hot solids 42 are once again
subjected to the combustion process that takes place in the circulating fluidized
bed steam generator (CFB) 10. The remainder of the hot solids 42 which are
above a predetermined size, denoted as heat exchanger hot solids 44, are diverted
from the combined hot solids-gas separator 18 to the fluidized bed heat exchanger
(FBHE) 36 by way of the heat exchanger inlet 32 and thence to the lower segment
of the furnace volume 12 via a corresponding heat exchanger outlet 38. The hot
solids 42 diverted through the fluidized bed heat exchanger (FBHE) 36 are cooled

in a heat exchange process in which the hot solids transfer heat to a working fluid
which flows through the fluidized bed heat exchanger (FBHE) 36 in conventional
manner. The temperature in the circulating fluidized bed steam generator (CFB)
10 can thus be controlled by properly splitting the flow of hot recirculated solids
42 leaving the cyclone such that an uncooled stream of solids flows directly back
to the circulating fluidized bed steam generator (CFB) 10 or is therebefore cooled
by the fluidized bed heat exchanger (FBHE) 36 before flowing to the circulating
fluidized bed steam generator (CFB) 10.
Continuing, on the other hand, the hot combustion gases 40 leaving the
combined hot solids-gas separator 18, hereinafter referred to as flue gases, are
directed from the combined hot solids-gas separator 18 via the intermediate
backpass ductwork 20 to the backpass volume 22, where additional heat transfer
duty is performed therewith as will be described more fully hereinafter. From the
backpass volume 22, the flue gases 40 exit through the ductwork 24 to a sub group
of downstream flue gas treatment components which will be described in more
detail hereinafter with reference to Figure 2.
Figure 2 is a schematic view of an exemplary combined cycle power
generating unit 110 comprising the circulating fluidized bed steam generator 10
which is operable to both generate electrical power and produce a carbon dioxide
(CO2) end product as well as, optionally, a nitrogen (N2) product. Details of the
arrangement - hereinafter generally designated as the end product and recyclable
group EPRG - will now be provided commencing with a description of the details
concerning the combined cycle power generating unit 110. Attention is now
drawn to Figure 2 which illustrates the exemplary combined cycle power
generating unit 110 having a fuel fired combustion vessel in the form of the
circulating fluidized bed steam generator 10 and additionally including the sub
group of the downstream flue gas treatment components and a sub group of
oxygen supplying components, these latter two sub groups being hereinafter
referred to as the end product and recyclable group EPRG. One of the oxygen

supplying components treats a stream of air 112 to render oxygen therefrom of a
desired purity.
The combined cycle power generating unit 110 also includes a turbine 136
for generating electricity under the motive action of steam passed thereover.
Steam is conducted from the circulating fluidized bed steam generator 10 to the
turbine 136 via a plurality of ducts 138 and injected thereagainst to drive the
turbine.
With reference again to Figure 2, the end product and recyclable group
EPRG also includes an oxygen source 140 for supplying the particular variety of
combustion gas via appropriate means such as. for example, via oxygen
introducing elements 142. The oxygen introducing elements 142 comprise the
supply line 50, and the supply line which supplies the additional two upper levels
of oxygen, which respectively introduce oxygen into the lower volume of the
circulating fluidized bed steam generator 10 and into at least one location above
the mixture of fuel and sorbent 54. The oxygen (O2) supplied into the circulating
fluidized bed steam generator 10 reacts with the fuel being fed into the circulating
fluidized bed steam generator 10, such fuel preferably being a fossil fuel with a
high carbon content such as, for example, coal or petcoke, or biomass.
The oxygen (O2) supplied by the oxygen source 140 is preferably created
by an air separation process performed by an air separation unit which separates
oxygen (O2) from an ambient air feed stream and, in this regard, the oxygen
source 140 can be configured, for example, as a cryogenic plant having the
capability of producing oxygen (O2) of a purity of at least ninety-five percent
(95%). The air separation unit can be configured, if desired, to produce as well a
nitrogen (N2) product 141. The oxygen source 140 can alternatively be
configured as an apparatus comprising an oxygen transport membrane.
The oxygen (O2) supplied by the oxygen source 140 is pre heated upstream
of the oxygen introducing elements 142 by a pure oxygen pre heater 144 having a
cold side inlet communicated with an exit duct 146 of the oxygen source 140 and
a cold side outlet communicated with a duct 148 which, in turn, is connected via a

duct manifold arrangement with the oxygen introducing elements 142. The hot
side of the pure oxygen pre heater 144 is supplied with flue gas which has exited
the back pass volume 22 via the ductwork 24.
The flue gas which had been supplied from the back pass volume flows
through a duct 150 communicated with the hot side inlet of the pure oxygen pre
heater 144. The flue gas then gives up further heat to the oxygen (O2) flowing
through the pure oxygen pre heater 144 enroute to the circulating fluidized bed
steam generator 10.
The two largest constituent elements by volume of the flue gas exiting the
back pass 22 are carbon dioxide (CO2) and water vapor (H2O). This composition
of the flue gas results from the combustion of the coal within the circulating
fluidized bed steam generator 10 in the presence of the pure or nearly pure oxygen
supplied from the oxygen source 140 and in the presence of recycled solids which
are fed to the circulating fluidized bed steam generator 10 by the fluidized bed
heat exchanger (FBHE) 36.
The end product and recyclable group EPRG additionally includes, as seen
in Figure 1, a particulate removal system for removing relatively fine, particulate
matter in the form of an electrostatic precipitator 152 operable to remove, in
conventional manner, selected solids entrained with the flue gas. The electrostatic
precipitator 152 is communicated with the pure oxygen pre heater i44 via a duct
154 for receiving the flue gas following its passage through the pure oxygen pre
heater 144. The flue gas exiting the electrostatic precipitator 152 next flows via a
duct 156 to a gas cooler 160 whereat some of the water vapor (H2O) is condensed
out before the flue gas is further flowed downstream to an induced draft fan 162.
The gas cooler 160 cools the flue gas to the lowest temperature possible before
recycling to minimize fluidizing air blower power requirements. The gas cooler
160 contacts the flue gas with relatively colder water in countercurrent fashion
and this contact causes a relatively significant proportion of the water vapor in the
flue gas to condense into water and the water is then separated from the flue gas.

The flue gas exiting the induced draft fan 162 in a stream 164, which is
mainly comprised of carbon dioxide (CO2), is split or segregated such that the
majority of the flue gas is guided to a location 166 at which the flue gas can be
further processed, used, of sequestered. For example, the end product and
recyclable group EPRG may include a liquid recovery assembly 168 which is
operable to liquefy a portion of the carbon dioxide (CO2) of the flue gas so as to
render a liquid carbon dioxide product suitable for a commercial operation such
as, for example, enhanced oil recovery (EOR). Additionally, the nitrogen (N2)
product 141 produced by the oxygen source 140 (if it is so configured to produce
this product) can be used for enhanced oil recovery (EOR) as well.
A relatively small portion of the flue gas which is diverted to the location
166 is ultimately recycled into the circulating fluidized bed steam generator 10 in
a recycle stream 170. Typically, this portion of the flue gas is only a small
fraction of the total flue gas flowed through the gas cooler 160 and the
requirement for this flue gas portion is selected as a function of the amount
required for fluidization purposes in the circulating fluidized bed steam generator
10. Such fluidization is performed at least in part by a fluidizing air blower 172
which directs this flue gas portion to the fluidized bed heat exchanger (FBHE) 36.
The end product and recyclable group EPRG thus provides a system which
can be operated in accordance with the method of the present invention to produce
a liquid carbon dioxide (CO2) end product and a recyclable flue gas for supporting
the combustion process. Additionally, the recirculation of solids is employed in
accordance with the method of the present invention to control the combustion
temperatures in the circulating fluidized bed steam generator. Moreover, there is a
relative reduction in the creation of nitrogen as compared to a conventional
circulating fluidized bed steam generator without oxygen firing. Due to the use of
oxygen instead of air and the minimization of recirculated flue gas, relatively
more compact designs of the circulating fluidized bed steam generator, the gas-hot
solids separator, and the back pass heat exchanger can be realized in accordance
with the present invention.

While an embodiment and vanations of the present invention have been
shown, it will be appreciated that modifications thereof, some of which have been
alluded to hereinabove, may still be readily made thereto by those skilled in the
art. It is, therefore, intended that the appended claims shall cover the'
modifications alluded to herein as well as all the other modifications which fall
within the true spirit and scope of the present invention.

WE CLAIM
1. A method for operating a circulating fluidized bed steam generator (10)
comprising the steps of:
introducing a substantially pure oxygen feed stream into the circulating
fluidized bed steam generator (10);
combusting a fuel in the presence of the substantially pure oxygen feed
stream to produce a flue gas having carbon dioxide and water vapor as
its two largest constituent elements by volume;
passing the flue gas through an oxygen feed stream pre-heater (144) at
which heat from the flue gas is transferred to the oxygen feed stream;
separating the flue gas into an end product portion and a recycling
portion;
directing the recycling portion of the flue gas to the circulating flukiized
bed steam generator (10) to contribute to the combustion process
therein
characterized by comprising the step of cooling and compressing the end
product portion of the flue gas so as to yield carbon dioxide in a liquid
phase.

2. The mtthod as claimed in claim 1, wherein the step of directing a
recycling portion of the flue gas to the circulating fluidlztd bad steam
ganarator (10) comprises diverting at least part of the recycling portion
of the flue gas through a solids heat exchanger to effect a transfer of
heat from the recycling portion of the flue gas to the solids being flowed
through the solids heat exchanger.
3. A circulating fluidized bed staam generator ( 10) comprising'
means for Introducing a substantially pure oxygen feed stream into the
circulating fluidized bed steam generator (10);
means for combusting a fuel in the presence of the substantially pure
oxygen feed stream to produce a flue gas having carbon dioxide and
water vapor as its two largest constituent elements by volume;
means for passing the flue gas through an oxygen feed stream;
means for separating the flue gas into an end product portion and a
recycling portion;

means for directing the recycling portion of the flue gas to the circulating
fluidized bed steam generator (10) to contribute to the combustion process
therein, characterlied by comprising means for cooling and compressing
the end product portion of the flue gas so as to yield carbon dioxide in
in a liquid phase.

A circulating fluidized bed steam generator 10 and a method for
operating the circulating fluidized bed steam generator are provided
which offer the flexibility to use carbon dioxide (CO2) both as a
desirable end product and as support to the combustion process. The
method includes the step of introducing a substantially pure oxygen
feed stream into the circulating fluidized bed steam generator 10 and
the step of combusting a fuel in the presence of the substantially
pure oxygen feed steam to produce a flue gas having carbon dioxide
and water vapor as its two largest constituent elements by volume.
The method also includes the step of passing the flue gas through an
oxygen feed stream preheater 144 at which heat from the flue gas is
transferred to the oxygen feed stream. Furthermore, the method
includes the step of separating the flue gas into an end product
portion and a recycling portion. The method additionally includes
cooling and compressing the end product portion of the flue gas so
as to yield carbon dioxide in a liquid phase and directing the recycling
portion of the flue gas to the circulating fluidized bed steam
generator 10 to contribute to the combustion process therein.