APPARATUS AND METHOD OF CONTROLLING THE THERMAL PERFORMANCE OF AN OXYGEN-FIRED BOILER

Abstract

A method of controlling the operation of an oxy-fired boiler includes combusting a fuel that comprises oil heavy residues in a boiler, the oil heavy residues including hydrocarbon molecules having a number average molecular weight from approximately 200 to approximately 3000 grams per mole, discharging flue gas from the boiler, recycling a portion of the flue gas to the boiler, combining a first oxidant stream with the recycled flue gas to form a combined stream, splitting the combined stream into a plurality of independent split streams, introducing each independent split stream at a different elevation of the boiler, and controlling independently a parameter of each of the independent split streams to adjust the heat release at each respective elevation of the boiler to vary the heat release profile of the boiler by adding a second oxidant stream to each respective independent split stream to form respective independent oxygen enriched split streams.

Description

CROSS-REFERENCE

This Application is a Continuation-in-Part of U.S. patent application Ser. No. 13/298,147, filed on Nov. 16, 2011, which claims the benefit of U.S. Provisional Application Ser. No. 61/414,175, filed on Nov. 16, 2010, both of which are hereby incorporated by reference herein in their entireties.

BACKGROUND

Technical Field

This disclosure relates generally to oxy-fired boilers, and more specifically to an oxy-fired boiler that burns heavy oil residues as fuel.

Discussion of Art

Oxy-combustion has been developed for carbon dioxide capture and sequestration in fossil fuel fired power plants. The concept of oxy-combustion (also sometimes referred to as ‘oxyfuel’ and ‘oxy-firing’) is to replace combustion air with a mixture of oxygen and recycled flue gas, thereby creating a high carbon dioxide content flue gas stream that can be more simply processed for sequestration. A simplified exemplary schematic of the oxy-combustion process for pulverized coal (pc) power plants is shown in prior art depicted in FIG. 1.

FIG. 1 depicts an oxy-combustion system 100, comprising an air separation unit 102, a boiler 104, a pollution control system 106 and a gas processing unit 108. The air separation unit 102 is located upstream of the boiler 104, which is located upstream of the pollution control system 106 and the gas processing unit 108. The pollution control system 106 is located upstream of the gas processing unit 108. Gas recycle is shown taken after the pollution control system, but could be taken from any location between the boiler and the gas processing unit.

The boiler 104 may be a tangentially fired boiler (also known as a T-fired) or a wall fired boiler. T-firing is different from wall firing in that it utilizes burner assemblies with fuel admission compartments located at the corners of the boiler furnace, which generate a rotating fireball that fills most of the furnace cross section. Wall firing (not shown), on the other hand, utilizes burner assemblies that are perpendicular to a side (of the shell) of the boiler.

FIG. 2 depicts a tangentially fired boiler 104. Tangentially fired boilers have a rectangular cross-section and have burner assemblies 105 positioned at the corners. Fuel and transport air are introduced into the boiler 104 via the burner assemblies 105 and are directed tangentially to an imaginary circle located at the center of the furnace and with a diameter greater than zero. This generates a rotating fireball that fills most of the furnace cross section. The fuel and air mixing is limited until the streams join together in the furnace volume and generate a rotation. This has often been described, as “the entire boiler is the burner.” Global boiler aerodynamics and mixing is much more important to the combustion process and the resulting boiler performance during T-firing as compared with wall-firing. During wall-firing, fuel and air/oxygen mixing occurs in or near the burners and less mixing occurs in the boiler.

With reference now once again to FIG. 1, in one method of operating the oxy-combustion system 100, oxygen is first separated from nitrogen in the air separation unit 102. The nitrogen is discharged separately from the air separation unit. The air separation unit 102 extracts oxygen from the atmosphere.

The oxygen is then discharged from the air separation unit 102 to combine with recycled flue gas, the combination of which is fed to the boiler 104. The boiler 104 uses the oxygen present in the flue gas stream to combust a fuel (e.g., coal, oil, or the like) to generate heat and flue gases. As a result of combusting the fuel with oxygen instead of with air, the flue gas produced has a high carbon dioxide content. The other constituents of the flue gas are water vapor and small amounts of oxygen, nitrogen, and pollutants such as sulfur oxides, nitrogen oxides, and carbon monoxide. Removing the water and other components produces a very pure carbon dioxide stream suitable for sequestration or other use.

The heat is used to generate steam, which may be used to drive a generator (not shown) to produce electricity, while the flue gases are discharged to the pollution control system 106 where particulate matter and other pollutants (e.g., NOx, SOx, and the like) are removed. A portion of the purified flue gases is recycled to the boiler 104 as shown in FIG. 1. The remaining flue gases (that substantially comprise carbon dioxide) are discharged to the gas processing unit 108 from where it is sequestered.

As will be readily appreciated, recycling large amounts of flue gases to the boiler 104 require large. On the other hand, burning the fuel with pure oxygen generally produces flame temperatures much too high for practical boiler materials, so a portion of the high-carbon dioxide flue gas is used to dilute the oxygen and moderate the boiler temperature. The amount of oxygen added to the recycled flue gas is based on the amount of fuel combusted in the boiler. The fuel uses a certain amount of oxygen in addition to some amount of excess oxygen to ensure complete combustion.

While most of the aforementioned discussion has been directed to the oxy-fired boilers that use coal for combustion, it is desirable to use other fuels, such as, for example, liquid fuels in oxy-fired boilers.

One such liquid fuel is oil heavy residue. Oil heavy residues comprise primarily high molecular weight hydrocarbon products of crude oil that may be unsuitable for use in other applications or that cannot easily be converted into lower molecular weight products that can be used in other applications. Oil refineries convert crude oil into a range of useable products (e.g., gasoline, diesel and fuel oil components) that are used commercially. The first step in the manufacture of petroleum products is the separation of crude oil into the main fractions by atmospheric distillation. When crude oil is heated, the lightest and most volatile hydrocarbons boil off as vapors first and the heaviest (i.e., those having higher molecular weights) and least volatile last. The vapors are then cooled and condensed back into liquids, which are then supplied for commercial use.

The residue from atmospheric distillation is sometimes referred to as long residue and to recover more distillate product, further distillation is carried out at a reduced pressure and high temperature. Vacuum distillation is one such process and is used to further recover useful products from the long residue. The percentage of residue varies depending on the composition of crude processed. For a typical “light” North African crude the residue is 28%, whilst for a “heavy” Venezuelan crude it is as high as 85%. The proportion of products produced does not always match the product demand and is primarily determined by the particular composition of crude oil.

Further refining such as thermal cracking at temperatures of 450 to 750° C. and pressures from atmospheric to 70 bar are used to convert the long residue into useful commercial product. The temperature and pressure depends on the type of feedstock and the product requirement. At these elevated temperatures, the large hydrocarbon molecules become unstable and spontaneously break into smaller molecules. Several different thermal cracking processes may be performed on the residues to convert them to useful commercial products. However, not all of the high molecular weight hydrocarbon molecules can be converted to lower weight molecules that can be marketed commercially. The high molecular weight hydrocarbon molecules that are finally left behind after all of the useful product is extracted for commercial use is called oil heavy residue. It is desirable to find uses for the oil heavy residue that cannot be used for conventional commercial products such as gasoline and fuel oil.

It is therefore desirable to use the oil heavy residue in oxy-fired boilers to reduce waste and to reduce the environmental impact by employing these liquids in and efficient combustion processes that has a very low environmental signature as compared with other comparative combustion processes.

BRIEF DESCRIPTION

In an embodiment, a method of controlling the operation of an oxy-fired boiler is provided. The method includes combusting a fuel that comprises oil heavy residues in a boiler, the oil heavy residues including hydrocarbon molecules having a number average molecular weight from approximately 200 to approximately 3000 grams per mole, discharging flue gas from the boiler, recycling a portion of the flue gas to the boiler, combining a first oxidant stream with the recycled flue gas to form a combined stream, splitting the combined stream into a plurality of independent split streams, introducing each independent split stream at a different elevation of the boiler, and controlling independently a parameter of each of the independent split streams to adjust the heat release at each respective elevation of the boiler to vary the heat release profile of the boiler by adding a second oxidant stream to each respective independent split stream to form respective independent oxygen enriched split streams.

In another embodiment, a method is provided. The method includes the steps of combusting a fuel that comprises oil heavy residues in a boiler, where the oil heavy residues that comprise hydrocarbon molecules having a number average molecular weight from 200 to 3000 grams per mole, discharging flue gas from the boiler, recycling a portion of the flue gas to the boiler, combining a first oxidant stream with the recycled flue gases to form a first combined stream, splitting the first combined stream into a plurality of independent split streams, combining a second oxidant stream to each respective independent split stream provided to the boiler to form respective independent oxygen enriched split streams, introducing each independent oxygen enriched split stream to a different elevation of the boiler, and controlling independently the amount of the second oxidant stream added to each respective independent split stream to adjust the heat release at each respective elevation of the boiler to vary the heat release profile of the boiler. The first combined stream, the independent split streams, and the independent oxygen enriched split streams do not carry the fuel for the boiler.

In yet another embodiment, a system is provided. The system includes an air separation unit, a boiler configured to combust oil heavy residues, the oil heavy residues comprising hydrocarbon molecules having a number average molecular weight from 200 to 3000 grams per mole, a pollution control system, a gas processing unit and a control system. The air separation unit is upstream of the boiler, the pollution control system and the gas processing unit. The boiler is upstream of the pollution control system and the gas processing unit. The control system is configured to control the addition of a first oxidant stream to the recycled flue gas to form a combined stream and to control the addition of a second oxidant stream to a plurality of independent split streams formed from the combined stream to vary the heat release profile of the boiler. Each of the independent split streams to which the second oxidant stream is added is introduced to a different elevation of the boiler.

DRAWINGS

The present invention will be better understood from reading the following description of non-limiting embodiments, with reference to the attached drawings, wherein below:

FIG. 1 represents the prior art and depicts a combustion system where flue gases are recycled to the boiler;

FIG. 2 depicts a prior art tangentially fired boiler;

FIG. 3 is a depiction of the various points at which a combined stream that comprises a first oxidant stream (that comprises substantially oxygen) and a second stream (that comprises substantially recycled flue gases) can be introduced into the boiler;

FIG. 4 is another depiction of an exemplary embodiment of introducing oxygen into the flue gas stream into the boiler;

FIG. 5 represents a depiction of one embodiment of the introduction of the combined stream into a tangentially fired boiler;

FIG. 6 depicts one embodiment of nozzle orientation that is used for the combustion of oil heavy residues in concentric firing boilers;

FIG. 7 depicts a nozzle for injecting highly concentrated oxygen streams in a concentric firing system;

FIG. 8 is a graph that shows improved carbon burnout/carbon heat loss with oxygen enrichment in the fuel compartment during 15 MW testing; and

FIG. 9 is a graph that illustrations the impact of oxygen enrichment on heat flux to the furnace wall near the burner/windbox during 15 MW testing.

DETAILED DESCRIPTION

Reference will be made below in detail to exemplary embodiments of the invention, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference characters used throughout the drawings refer to the same or like parts. While embodiments of the invention are suitable for use with a tangentially fired boiler, embodiments of the invention may also be utilized in connection with any oxygen fired boiler, including an oxygen wall fired boiler.

Embodiments of the invention relate to an oxyfuel combustion system that uses oil heavy residue (OHR) such as vacuum bottoms from petroleum refineries or high asphaltene materials, for boiler applications to enhance thermal performance and reduce emissions. The method disclosed herein includes applying selective oxygen injection into oxidant streams, into the furnace windbox and into over fire air (OFA) nozzles to control combustion rate and furnace heat flux distribution. Features include local oxygen enrichment to promote combustion and to improve the gaseous environment near the waterwall tubes to reduce corrosion. Features also include options for injection through special oxygen sub-compartments through the burner/windbox into the furnace allowing more of the windbox and associated equipment to be fabricated from lower cost material such as carbon steel.

As used herein, “oil heavy residue” refers to primarily high molecular weight hydrocarbon products of crude oil that may be unsuitable for use in other applications or that cannot easily be converted into lower molecular weight products that can be used in other applications. The oil heavy residues have number average molecular weights of 200 to 3,000 grams per mole and comprise primarily hydrocarbon molecules. Substituted and unsubstituted hydrocarbon molecules may be used. In an embodiment, the oil heavy residues comprise heavy oils, oil sands, bitumen, biodegraded oils, and the like, some of which may contain asphaltene. Asphaltenes are molecular substances that are found in crude oil, along with resins, aromatic hydrocarbons, and saturates (i.e., saturated hydrocarbons such as alkanes). Asphaltenes consist primarily of carbon, hydrogen, nitrogen, oxygen, and sulfur, as well as trace amounts of vanadium and nickel. The carbon to hydrogen (C:H) ratio is approximately 1:1.2, depending on the asphaltene source. Asphaltenes are defined operationally as the n-heptane (C₇H₁₆)-insoluble, toluene (C₆H₅CH₃)-soluble component of a carbonaceous material such as crude oil, bitumen, or coal. Asphaltenes generally have a distribution of molecular weight in the range of 400 to 1500 grams per mole.

In an embodiment, the method includes varying the amount, proportion and/or distribution of oxygen, the amount, proportion and/or distribution of recycled flue gases, or both the amount, proportion and/or distribution of both oxygen and recycled flue gases in a combined stream that is fed to various inputs to the boiler, an input stream provided to the boiler and/or various zones of the boiler. For example, the volume of oxygen sufficient for the desired amount of combustion of the fuel provided to the boiler in accordance with desired stoichiometric parameters may be portioned or distributed to different zones or locations of the boiler to provide a desired heat release profile in the boiler. Further, the recycled flue gas and/or volume of oxygen may be proportioned and/or distributed to different areas within a zone of the boiler to provide a desired heat release profile in that zone. Furthermore, the proportion and/or distribution of addition of a volume or proportion of oxygen and/or recycled flue gas to an input stream to the boiler may be controlled to provide a desired heat release profile.

In one embodiment similar to that shown in FIG. 3, a system and method includes supplying a first combined stream of recycled flue gas and a first oxidant stream to different sections or zones of the boiler. The first combined stream may be supplied to a hopper zone, the windbox zone and/or one or more overfired oxidant compartments at different volumes, which are each controlled by a respective fluid flow control device. In this embodiment, the ratio of oxygen to the recycled flue gas is constant in any of the zones of the boiler to which it is introduced, however, the distribution of the first combined stream is controlled by providing varying portions of the first combined stream to different zones of the boiler and/or different locations within a particular zone to provide a desired heat release profile.

In another embodiment similar to that shown in FIG. 4, a system and method comprises combining the first combined stream with a second oxidant stream to form a second combined stream that may be supplied to the boiler at the hopper zone, the windbox zone and/or one or more overfired oxidant compartments at different volumes amounts, wherein the volumetric flow of the second oxidant is controlled by a fluid flow control device. This method of enriching the first combined stream is conducted just prior to the introduction of the second combined stream into the boiler. In this system and method, the amount of oxygen to the hopper zone, the windbox zone, the hopper zone and/or the overfired oxidant compartments is varied relative to the amount of the recycled flue gas. This system and method can be advantageously used to vary the heat release pattern in the boiler.

This system and method of controlling the distribution of oxygen and/or recycled flue gas to the boiler is advantageous in that it permits localized oxygen enrichment of the atmosphere in the boiler and hence increasing the localized heat release and modifying the temperature profiles in desired areas of the boiler.

In yet another embodiment similar to that in FIGS. 3 and 4, the amount of flue gas in the combined stream may be varied instead of or in addition to varying the amount of the oxygen. In yet another embodiment, the invention details modulating or changing the proportion or distribution of the recycled flue gas admitted to the boiler at varying elevations relative to the furnace outlet plane. This method of controlling the flow rates of flue gases is advantageous in that it allows for maintaining a constant steam temperature control as fuel properties or furnace conditions vary. This provides a means of steam temperature control as loads vary. Another method of steam temperature control can be achieved by modulating the amount of oxygen to the varying elevations.

An advantage of the present invention the amount of oxygen and flue gas provided to a fluid stream in an oxygen fired boiler may be independently controlled to provide great flexibility to optimize the operation of the boiler and provide or modify the heat release profile of the boiler. One skilled in the art will appreciate that an increase of oxygen with an input fluid stream to the boiler will result in an increase heat flux at the location of the input fluid stream.

FIG. 3 is a depiction of a boiler 200, such as a T-fired boiler, having a control system 290 that controls the proportion or distribution of a combined stream 320 to various locations or zones of the boiler. The combined stream 320 comprises a first oxidant stream 310 (that comprises 0-100 weight % of oxygen, wherein in one embodiment stream 310 is substantially oxygen) and a second stream 350 (that comprises substantially recycled flue gases). The volumetric flow of the first oxidant stream 310 and second stream 350 are controlled by respective fluid flow control devices 311, such as baffles, fans, dampers, valves, and eductors. These flow control devices may be controlled in an open loop or closed loop control system, which will be described in greater detail hereinafter.

The oxidant is injected into three main zones (all of which are detailed below): 1) a windbox/main burner zone, where it is injected at the lowest stoichiometric ratio of flue gas to oxygen, (2) a furnace located above the windbox (also termed an over-fire zone), where the stoichiometric ratio of flue gas to oxygen is more than 1; and 3) a furnace hopper located below the windbox.

The bulk of the oxygen for combustion is injected and mixed into the main gas recirculation flow taken from after a pollution control system (not shown) to form a premix oxidant 320 which is heated in a regenerative gas-gas heater 390 and sent to the windbox and over fire air system. The total quantity of flue gas recirculation is established based on the size and heat transfer surfacing of the boiler.

With reference again to the FIG. 3, the boiler 200 includes a hopper zone 210 located below the main burner zone 208 from which ash can be removed, a main burner zone 208 (hereinafter windbox 208) where an oxidant and an oxidant-fuel mixture (or alternatively a gas-fuel mixture) is introduced into the boiler 200, a burnout zone 216 where any oxygen or fuel that is not combusted in the main burner zone gets combusted, a superheater zone 212 where steam can be superheated, and an economizer zone 214 where water can be preheated prior to entering the superheater zone 212. The burnout zone 216 can utilize a lower overfired oxidant compartment 206 and an upper overfired oxidant compartment 204. The boiler 200 also includes a horizontal boiler outlet plane 304 and a vertical boiler outlet plane 302. The boiler 200 also includes waterwalls 202 in which the water is transformed to steam.

As noted above, a first oxidant stream 310 and a second stream 350 are combined to form the combined stream 320 that is then fed to the boiler. The combined stream 320 can comprise about 15 to about 40 volume percent oxygen, with the remainder being recycled flue gases. As can be seen in FIG. 3, the combined stream 320 can be fed to the boiler 200 into the hopper zone 210, into the windbox 208, into the lower overfired oxidant compartment 206 and/or into an upper overfired oxidant compartment 204. In other words, the combined stream 320 can be split up and distributed into several split streams (320A, 320B, 320C and/or 320D) and fed into different parts of the boiler to vary the heat release profile in the boiler and to improve its thermal performance, whereby the volumetric flow rate of one or more of the split streams 320A, 320B, 320C, 320D is controlled by a respective fluid flow control device 312. For example a higher percentage of the combined stream 320 may be provided to the windbox 208 to increase the heat release profile in this zone, or vice versa. This method of enriching the second stream 350 with oxygen and splitting the combined stream 320 into different streams 320A, 320B, 320C, 320D permits varying the amount of flue gas and oxygen into different parts of the boiler to improve its thermal performance or provide a desired heat release profile.

For purposes of identification, the combined stream 320 that is fed into the boiler 200 at the hopper zone 210 is identified as 320A and can comprise up to about 25 weight percent of the total weight of the combined stream 320. In one embodiment, the combined stream 320A stream can comprise about 0 to about 10 weight percent of the total weight of the combined stream 320. In another embodiment, the combined stream 320 that is fed into the boiler 200 at the windbox zone 208 is identified as 320B and can comprise about 50 weight percent to about 100 weight percent of the total weight of the combined stream 320. In one embodiment, the combined stream 320B stream can comprise about 50 to about 80 weight percent of the total weight of the combined stream 320. In yet another embodiment, the combined stream 320 that is fed into the boiler 200 at the lower overfired oxidant compartment 206 is identified as 320C and can comprise up to about 50 weight percent of the total weight of the combined stream 320. In one embodiment, the combined stream 320C stream can comprise about 10 to about 30 weight percent of the total weight of the combined stream 320.

In yet another embodiment, the combined stream 320 that is fed into the boiler 200 at the upper overfired oxidant compartment 204 is identified as 320D and can comprise up to about 50 weight percent of the total weight of the combined stream 320. In one embodiment, the combined stream 320D stream can comprise about 10 to about 30 weight percent of the total weight of the combined stream 320.

FIG. 4 depicts another embodiment of a boiler 200, such as a T-fired boiler, having a control system 291 that controls the proportion or distribution of a combined stream 360 and the oxygen ratio of each split stream 360A, 360B, 360C, 360D to various locations or zones of the boiler, using a combined stream 360 and a second oxidant stream 370 to enrich or deplete the flue gas of oxygen of each respective input stream supplied to the boiler 200 to define or vary the heat release profile in the boiler and to improve its thermal performance or provide a desired heat release profile.

The control logic for a boiler that burns oil heavy residues requires controlling the specific oxygen concentrations to predetermined values for each of the three main furnace zones—the oxidant compartments 204 and 206, the windbox zone 208 and the hopper zone 210. This includes controlling oxygen flow rates and oxygen concentrations in each split stream 360A, 360B, 360C, and 360D as well any further subdivisions of these streams (not shown here). The flow rate and oxygen concentration of each of the streams is adjusted to provide a final value based on the oxygen concentration in the flue gas leaving the boiler in order to maintain the optimize amount of oxygen efficient combustion.

The distribution of this oxygen into the furnace is specifically optimized in dependence upon the specific composition and properties of the oil heavy residue fuel and the design of the boiler. Control of the oxygen added to specific oxidant or flue gas recirculation streams provides a means to control combustion rate and heat release allowing control of furnace heat flux profiles and steam temperature control. Moreover, it provides a means to optimize and improve combustion performance including flame stability/turndown and carbon burnout. It also provides a means to improve combustion performance allowing more aggressive staging for NOx control and lower NOx emissions.

In an embodiment, streams having different oxygen concentrations are fed to the hopper zone 210, the windbox zone 208, the upper overfired oxidant zone 204 and the lower overfired oxidant zone 206. One skilled in the art will appreciate that the control oxygen concentration or ratio of each zone may be controlled in any configuration or combination of locations or zones of the boiler. The recycled flue gases 350 may be first pre-mixed with an oxidant stream 310 to form a first combined stream 360. The first combined stream 360 is then discharged towards different locations or zones of the boiler in different amounts or volumes. However, each combined stream 360 is enriched with oxygen from a respective second oxidant stream 370 to provide each respective input (split) stream 360A, 360B, 360C, 360D with the desired concentration of oxygen as well as the desired overall volumetric flow for each input stream. The ratio of oxygen in the different split streams can therefore be the same or different from one another.

As shown in FIG. 4, the control system 291 controls the concentration of oxygen and volumetric flow of the combined stream 360 by controlling the fluid flow of stream 350 and oxidant stream 310 using the respective fluid flow control devices 311. The control system 291 further controls the concentration of oxygen and volumetric flow rate for each respective input stream 360A, 360B, 360C, 360D by controlling the respective fluid flow control devices 312 to control the flow of the combined stream 360 and controlling respective fluid flow control devices 313 to control the flow of respective fluid flow control devices 313. The fluid flow control device 312 may be disposed upstream or downstream of the point that the second oxidant stream 370 is added. However, when the fluid flow control device 312 is disposed upstream of the point that the second oxidant stream 370 is added, the control system 291 provides greater flexibility and concentration range to locally control both the concentration of oxygen and the overall volume of input stream 360A, 360B, 360C, 360D to the boiler. In summary as shown in FIG. 4, the fluid flow control devices 311, 312, 313 of the control system 291 can control the oxygen concentration of each input stream 360A, 360B, 360C, 360D the distribution of oxygen to each input stream and thus zone of the boiler, and the desired volumetric gas flow of each input stream.

With reference once again to FIG. 4, a first oxidant stream 310 of the total added oxygen is mixed with a second stream 350 that comprises recycled flue gases to form a first combined stream 360. In an exemplary embodiment, the first oxidant stream 310 comprises about 50 to about 95 percent of the total added oxygen, specifically about 80 to about 90 percent. The remaining percentage of oxygen necessary for the desired amount of combustion in the boiler 200 is provided in the second oxidant stream 370. Note that the recycled flue gas and the transport gas may include a small percentage of oxygen that may need to be considered in the control of the input streams 360A, 360B, 360C, 360D.

As can be seen in FIG. 4, the second combined stream 360A which comprises the first combined stream 360 and the second oxidant stream 370 comprising up to 20% of the total added is fed to the hopper zone 210. In an exemplary embodiment, the second combined stream 360A can comprise about 0 to about 18%, and more specifically about 2 to about 15% of the total added oxygen.

In another embodiment, a second oxidant stream 370 that comprises oxygen in an amount of up to 100% of the total added oxygen is combined with the first combined stream 360 and fed to the windbox 208. In an exemplary embodiment, the second oxidant stream comprises oxygen in an amount of about 50 to about 80% of the total added oxygen is combined with the first combined stream 360 and fed to the windbox 208.

In yet another embodiment, a second oxidant stream 370 that comprises oxygen in an amount of up to 50 wt % is combined with the first combined stream 360 and fed to the lower overfired oxidant compartment 206. In an exemplary embodiment, the second oxidant stream comprises oxygen in an amount of about 10 to about 30% of the total added oxygen is combined with the first combined stream 360 and fed to the lower overfired oxidant compartment 206 and/or to the upper overfired oxidant compartment 204.

The second oxidant stream 370 is generally mixed with the first combined stream 360 to form the combined stream 360A, 360B, 360C or 360D as close as possible to the boiler 200. A finer level of control over oxygen distributions can be achieved by mixing in oxygen closer to the boiler, for example adding additional oxygen at the positions depicted in FIG. 4 to locally enrich the oxygen content in one area of the windbox 208. This mode of enrichment of the first combined stream 360 can be used in the tangentially fired boilers as well as in wall fired boilers.

While the control systems 290, 291 of FIGS. 3 and 4 control the distribution and concentration of oxygen to particular zones of the boiler 200, the present invention contemplates that each zone having a plurality of separate input streams may also be controlled by the control systems. FIG. 5 depicts one exemplary apparatus and method of introducing the combined stream 320B (from FIG. 3) or 360B (from FIG. 4) into the windbox 208 of the boiler 200. FIG. 5 illustrates the details of the input compartments or input streams of a windbox 208 of a tangentially fired boiler, and the apparatus and method of controlling the oxygen concentration and volumetric flow of respective input streams provided by the windbox 208. Varying oxygen concentrations are introduced in different compartments of the windbox 208.

FIG. 5 depicts a plurality of assemblies, e.g. primary nozzles 402, 404, 406, in the windbox 208 of a tangentially fired boiler 200. FIG. 5 contains an expanded view of the windbox 208 of the boiler 200 to illustrate a configuration of the nozzles 2a through 2k of nozzle assemblies 402, 404, 406. In one embodiment, the windbox 208 can comprise about 2 to about 10 such assemblies of nozzles. It is to be appreciated that a number of different configurations between the nozzles that introduce the combined stream 360B, and those that deliver heavy oil residues to the windbox and those that deliver auxiliary air to the windbox may be used.

Oil heavy residue and transport gas, along with a mixture of recycled flue gas and oxygen (e.g., the combined stream 360B or 320B) can be introduced into the respective nozzles. As can be seen in the FIG. 5, nozzles that supply the fuel to the windbox 208 may be alternated with nozzles that supply a mixture of recycled flue gas and oxygen to the windbox. Further nozzles that supply auxiliary air may also be alternated with nozzles that supply recycled flue gas and oxygen to the windbox.

The nozzles can be arranged into assemblies. For example, a first nozzle assembly 402 can contain a first nozzle that introduce the combined stream 360B, a second nozzle that introduces the oil heavy residue and a third optional nozzle that introduces auxiliary air. A second nozzle assembly 404, a third nozzle assembly 406, and so on may be arranged in a similar fashion to the first nozzle assembly 402 in order to introduce the combined stream 360B, the oil heavy residue and the auxiliary air into the windbox 208. It is to be noted that the second and third nozzle assemblies may alternatively have different configurations from the first nozzle assembly 402, if desired.

In an embodiment, it is desirable to introduce the combined stream 360B (with localized oxygen enrichment) into the nozzles 2a, 2c, 2e, 2g, 2i and 2k respectively, while nozzles 2b, 2f, and 2j deliver oil heavy residues to the windbox 208. In an embodiment, auxiliary air is supplied to the windbox 208 via nozzles 2d and 2h. From FIG. 5 it may therefore be seen that the first nozzle assembly 402 contains the nozzles 2a, 2b, 2c and 2d, while the second nozzle assembly 404 contains nozzles 2e, 2f, 2g and 2h, and third nozzle assembly 406 contains nozzles 2i, 2j, 2k, and so on.

The ratio of oxygen to the recycled flue gas in the combined stream 360B that is fed to the respective assemblies 402, 404, and so on can be varied similar to the configuration shown and described in FIGS. 3 and 4. Specifically, control systems 290, 291 of FIGS. 3 and 4 may have the same configuration of fluid flow control devices to control the concentration, proportion and/or distribution of each respective split input stream that feeds into each nozzle 2a, 2c, 2e, 2g, 2i and 2k. In other words, the locations of nozzles (i.e., windbox zones) of the windbox 208 may be controlled in the same or similar manner as each zone of the boiler 200, whereby the input stream 360B would be functionally the same as the combined stream 360 in FIGS. 3 and 4. While this functionality has been shown for the windbox 208, one will appreciate that this level of control is contemplated by the invention for other zones of the boiler 200.

For example, the first nozzle 2a can receive a first ratio of oxygen to recycled flue gases, while the second nozzle 2c can receive a second ratio of oxygen to the recycled flue gases, and so on. In one embodiment, the first ratio can be the same as the second ratio. In another embodiment, the mass ratio of the combined stream 360B to the oil heavy residue fed to the first assembly 402 can be the same or different from the mass ratio fed to the second assembly 404. By changing the ratio of oxygen to the recycled flue gas, the heat release profile at different portions of the windbox 208 can be varied.

In an embodiment, the system used for the combustion of oil heavy residues may use concentric firing system (CFS) compartments which increase the oxidizing environment at the waterwall, thereby reducing corrosion potential. FIG. 6 depicts one such embodiment. In this embodiment, the combined stream 360B of the FIG. 4 (or 320B in the FIG. 3) is directed through a concentric firing nozzle system that is angled towards the walls (i.e., the waterwalls) of the furnace rather than towards the center of the furnace. The effect of this angular adjustment of the nozzles is enhanced by supplying enriched oxygen concentrations as part of the stream 360B through the nozzles. The enriched oxygen concentrations in the streams through the concentric firing system (CFS) compartments increase the oxidizing environment at the waterwall thereby reducing corrosion potential.

FIG. 6 illustrates CFS nozzles 506 that are adjusted to direct the stream 360B away from the center fireball circle 504 more toward the waterwalls 508 of the boiler 502. Enriching the oxygen in this concentric firing system flow increases the oxygen concentration near the waterwalls 508. This is particularly valuable for high sulfur, high nitrogen content oil heavy residues, where the nozzle angles could be staged more aggressively for NOx control while controlling corrosive behavior as well.

Enrichment of oxygen concentrations in the stream 360B requires the selection of a appropriate materials for use in nozzle components that contact the stream 360B (see FIG. 4). Appropriate materials such as stainless steel (SS 304, SS 316, and the like) are desirable for use in the windbox compartments, and other components exposed to heated (>300° F.) oxygen streams that have oxygen concentrations of greater than 23.5 wt %, based on the total weight of the stream. In certain embodiments, material savings can be achieved by restricting the oxygen flow in a separate conduit through the windbox compartment to the existing nozzles into the furnace.

FIG. 7 depicts a nozzle 600 for transporting highly concentrated oxygen streams through the concentric firing system of FIG. 6. As shown therein, the nozzle 600 includes an outer wall 602 that houses a conduit 604 for transporting the stream 360B that may contain amounts of oxygen in amounts of greater than 23 wt % (based on the total weight of the stream 360B) into the windbox 208 (see FIGS. 3 and 4) of the boiler. In an embodiment, only the conduit 604 that is exposed to nearly pure oxygen would need to be of higher grade, oxygen compatible material, and the other windbox compartments could remain constructed using conventional air-fired design and materials. In an embodiment, a concentric conduit 606 may be used for transporting the oil heavy residues.

Referring once again to FIG. 4, in one embodiment, the combined stream 360 and the second oxidant stream 370 may be introduced into the boiler 200 at the upper overfired oxidant compartment 204 or at the lower overfired oxidant compartment 206. The enrichment with oxygen can thus take place in the upper overfired oxidant compartment 204 relative to the lower overfired oxidant compartment 206, the windbox 208 and/or the hopper zone 210. In another embodiment, the enrichment with oxygen can take place in the lower overfired oxidant compartment 206 relative to the upper overfired oxidant compartment 204, the windbox 208 and/or the hopper zone 210. Referring to FIG. 4, one embodiment where the combined stream 360 and the secondary oxidant stream 370 is introduced into the upper or lower overfired oxidant compartment 204 or 206 respectively. The upper overfired oxidant compartment 204 is closest to the horizontal boiler outlet plane 304, while the lower overfired compartment 206 is the compartment farthest from the horizontal boiler outlet plane 304.

When the combined stream 360 is introduced into the lower overfired oxidant compartment 206, the second oxidant stream 370 is introduced into the upper overfired oxidant compartment 204 and vice versa. By introducing the combined stream 360 into the lower overfired oxidant compartment 206, the oxidant stream in the lower overfired oxidant compartment 206 is enriched in oxygen relative to the upper overfired oxidant compartment 204, the windbox 208 and the inlet header zone 210.

Sufficient oxygen is used in the overfired oxidant compartments so that the combustion process may continue from the lower boiler while allowing for the lower boiler to operate at a ratio of oxygen to fuel lower than the stoichiometric ratio than the combustion process requires. The purpose of enriching the flue gas stream to the overfired oxidant compartments is to control the amount of nitrogen oxides (NOx) formed as well as to control the temperatures in the lower furnace.

Referring to FIG. 4, another embodiment related to varying oxygen concentrations in the upper and lower overfire oxidant compartments 204 and 206 is illustrated. The oxygen concentration of the upper overfired oxidant compartment 204 can be depleted relative to the bulk of the second oxidant stream 370 by the introduction of a supplemental flue gas recirculation stream 380 to the upper overfired oxidant compartment 204. Furthermore, depletion of the upper overfired oxidant compartment 204 relative to the bulk of the secondary oxidant stream 370 may be accomplished by introducing the combined stream 360 into the lower overfired oxidant compartment 206 and/or the windbox 208. In one embodiment, the second oxidant stream 370 can be introduced into the windbox 208, while the supplemental flue gas recirculation stream 380 is fed to the upper overfired oxidant compartment 204.

Upper overfire oxidant compartments depleted in oxygen relative to the global oxygen concentration (i.e., 15 to 40 wt %) will allow for higher combustion temperatures and result in higher heat transfer rates in the lower portions of the boiler where there is a lower working fluid temperature, while decreasing the combustion temperature and resultant heat transfer rates higher in the boiler.

Due to the energy required to increase the temperature of the upper overfire oxidant, the temperature of the combustion gases will decrease (most of the combustion will have been completed). At a decreased temperature of the combustion gases, the resultant flux to the boiler walls in the portion of the boiler closest to the outlet plane will decrease. The resulting alteration in the heat transfer profile will be beneficial for waterwall materials, in particular for supercritical steam generators. The primary benefit is to reduce the heat transfer in the boiler close to the boiler outlet plane where the working fluid temperatures are highest.

The use of the additional oxygen in the combustion of oil heavy residues has a number of advantages. Adding oxygen to the oxidant stream located below the lowest burner assembly alters the heat absorption profile in the boiler. The ability to alter and control the heat absorption profile can increase utilization of heat transfer surfaces located in the lower boiler. This allows for more total heat absorption in the radiant section of the boiler. This could also reduce peak temperatures and heat transfer rates which generally occur above the windbox, and thereby reduce material requirements and the potential for ash slagging problems.

Altering the heat release profile in the boiler can decrease peak boiler material temperatures at a constant thermal heat input and the flue gas recirculation rate. The advantage is that flue gas recirculation rates can be lowered without peak heat fluxes that causes slagging problems and/or waterwall tube overheating. Another beneficial result of altering of the heat release profile in the boiler is to allow for a more efficient utilization of the heat transfer surface. The benefits for a retrofit boiler is an increase in thermal heat input and thus working fluid power, while for a new boiler it results in a decrease in boiler size.

A further beneficial result is an improvement in emission characteristics, including carbon monoxide emissions, excess oxygen required, unburned carbon, and mineral matter properties. Another result is a beneficial impact on ash fouling properties in the convective section of the boiler, by controlling the boiler outlet temperature. Yet another advantageous result is a beneficial impact on ash slagging properties in the lower section of the boiler. Another benefit is that ductwork used in the first combined streams 360 to the boiler do not need to tolerate increased oxygen concentrations. The benefit being that ductwork can be constructed from a wider variety of materials thereby decreasing cost. Only the shorter ductwork containing the second combined streams 360A, and the like, need tolerate higher oxygen concentrations after mixing with the second oxidant stream 370. Another benefit for retrofit applications is utilizing existing plant ductwork.

The control systems 290, 291 of the present invention may be an open loop system, whereby fluid flow control devices are adjusted or set at predetermined settings or set by an operator, or may be a closed loop system. As a closed loop system, the fluid flow control devices may be adjusted or set in response to an operation and/or conditional parameter of the boiler and/or boiler island. For example, the fluid flow control devices may control the fluid flow in response to thermal parameters of the boiler or boiler island such as steam temperature, boiler temperature, or other thermal zones of the boiler or boiler island. Similarly the fluid flow control devices may control fluid flow in response operational parameters such as system load or changes to the load of the boiler or boiler island. The present invention contemplates that a processor or DCS may provide a respective control signal to a respective fluid flow control device in response to a sensed input signal, such as an operational or system condition parameter.

FIG. 8 illustrates an example of the carbon heat loss that can be achieved by varying the oxygen content in the stream 360B. The tests were conducted in a 15 MW pilot plant. The tests were conducted using an oxygen stoichiometry of 1 and 0.85 respectively in the fuel compartment (the windbox 208—see FIG. 4). A test was also conducted using air-fired asphalt (the oil heavy residue). As shown therein, improved carbon burnout/carbon heat loss with oxygen enrichment in the fuel compartment during 15 MW testing was achieved. Significant improvement in carbon heat loss is achieved by increasing the concentration of oxygen in the oxidant flow through the fuel compartment from 25% to 30%. Control of the oxygen concentration and total flow of oxygen into the fuel compartment is a therefore a useful aspect in optimizing thermal performance when oil heavy residues are used as fuels.

The enrichment of oxygen in the oxidant to the fuel compartment also impacts heat release and heat absorption in the furnace waterwalls. FIG. 9 is a graph that illustrates the impact of oxygen enrichment on heat flux to the furnace wall near the burner/windbox during 15 MW testing. The system controls the oxygen concentrations and distribution of oxidant along the height of the windbox allowing for adjustment of the heat flux profile in the furnace walls. This is done in concert with the oxygen concentration and oxygen flows to the OFA locations to optimize overall furnace waterwall heat flux while maintaining desired combustion efficiency and emission levels. Further adjustment of this oxygen distribution can provide an active control of superheat reheat steam temperature by shifting furnace waterwall heat absorption and convective section absorption.

In an embodiment, a method of controlling the operation of an oxy-fired boiler is provided. The method includes combusting a fuel that comprises oil heavy residues in a boiler, the oil heavy residues including hydrocarbon molecules having a number average molecular weight from approximately 200 to approximately 3000 grams per mole, discharging flue gas from the boiler, recycling a portion of the flue gas to the boiler, combining a first oxidant stream with the recycled flue gas to form a combined stream, splitting the combined stream into a plurality of independent split streams, introducing each independent split stream at a different elevation of the boiler, and controlling independently a parameter of each of the independent split streams to adjust the heat release at each respective elevation of the boiler to vary the heat release profile of the boiler by adding a second oxidant stream to each respective independent split stream to form respective independent oxygen enriched split streams. In an embodiment, the oil heavy residues comprise asphaltene. In an embodiment, the boiler is a tangentially fired boiler. In an embodiment, the step of controlling independently the parameter of each independent split stream further includes changing a heat absorption in the boiler to a desired heat absorption pattern. In an embodiment, at least one of the split streams is introduced into the boiler at a hopper zone located below a windbox, at the windbox and/or in an overfire compartment located above the windbox. In an embodiment, at least a portion of the combined stream is introduced into the boiler in a lower portion of the windbox. In an embodiment, the at least one split stream that is introduced into the boiler at the windbox is about 50 to about 100 weight percent of the combined stream. In an embodiment, the at least one split stream is introduced into the boiler in a lower portion of an overfire compartment. In an embodiment, the at least one split stream is introduced into the boiler in an upper portion of an overfire compartment.

In another embodiment, a method is provided. The method includes the steps of combusting a fuel that comprises oil heavy residues in a boiler, where the oil heavy residues that comprise hydrocarbon molecules having a number average molecular weight from 200 to 3000 grams per mole, discharging flue gas from the boiler, recycling a portion of the flue gas to the boiler, combining a first oxidant stream with the recycled flue gases to form a first combined stream, splitting the first combined stream into a plurality of independent split streams, combining a second oxidant stream to each respective independent split stream provided to the boiler to form respective independent oxygen enriched split streams, introducing each independent oxygen enriched split stream to a different elevation of the boiler, and controlling independently the amount of the second oxidant stream added to each respective independent split stream to adjust the heat release at each respective elevation of the boiler to vary the heat release profile of the boiler. The first combined stream, the independent split streams, and the independent oxygen enriched split streams do not carry the fuel for the boiler. In an embodiment, the boiler is a tangentially fired boiler. In an embodiment, adding the second oxidant stream to form the respective oxygen enriched split streams is conducted at a position proximate to a point of entry into the boiler. In an embodiment, the respective split streams are sequentially introduced into the boiler. In an embodiment, at least one respective oxygen enriched split stream is introduced into the boiler at a hopper zone located below a windbox. In an embodiment, the oxygen enriched split stream introduced into the boiler at the windbox comprises about 50 to about 100 wt % oxygen, based on the total weight of the stream. In an embodiment, each oxygen enriched split stream is introduced into the boiler via an annular space disposed around an inner port, where the inner port introduces the fuel and transport air into the boiler. In an embodiment, the boiler is a wall fired boiler. In an embodiment, the step of controlling independently the parameter of each respective oxygen enriched split stream introduced to the boiler changes the heat pattern of the boiler. In an embodiment, at least one respective oxygen enriched split stream is introduced into the boiler at an overfire compartment at the hopper zone, wherein the oxygen enriched split stream introduced into the overfire compartment at the hopper zone comprises up to 50 wt % oxygen based on the total weight of the oxygen enriched split stream.

In yet another embodiment, a system is provided. The system includes an air separation unit, a boiler configured to combust oil heavy residues, the oil heavy residues comprising hydrocarbon molecules having a number average molecular weight from 200 to 3000 grams per mole, a pollution control system, a gas processing unit and a control system. The air separation unit is upstream of the boiler, the pollution control system and the gas processing unit. The boiler is upstream of the pollution control system and the gas processing unit. Flue gas is recycled from the gas processing unit to the boiler via the air separation unit. The control system is configured to control the addition of a first oxidant stream to the recycled flue gas to form a combined stream and to control the addition of a second oxidant stream to a plurality of independent split streams formed from the combined stream to vary the heat release profile of the boiler. Each of the independent split streams to which the second oxidant stream is added is introduced to a different elevation of the boiler.

As used herein, an element or step recited in the singular and proceeded with the word “a” or “an” should be understood as not excluding plural of said elements or steps, unless such exclusion is explicitly stated. Furthermore, references to “one embodiment” of the present invention are not intended to be interpreted as excluding the existence of additional embodiments that also incorporate the recited features. Moreover, unless explicitly stated to the contrary, embodiments “comprising,” “including,” or “having” an element or a plurality of elements having a particular property may include additional such elements not having that property

This written description uses examples to disclose several embodiments of the invention, including the best mode, and also to enable one of ordinary skill in the art to practice the embodiments of invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention is defined by the claims, and may include other examples that occur to one of ordinary skill in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal languages of the claims.

Claims

1. A method of controlling the operation of an oxy-fired boiler, the method comprising:

combusting a fuel that comprises oil heavy residues in a boiler, the oil heavy residues including hydrocarbon molecules having a number average molecular weight from approximately 200 to approximately 3000 grams per mole;

discharging flue gas from the boiler;

recycling a portion of the flue gas to the boiler;

combining a first oxidant stream with the recycled flue gas to form a combined stream;

splitting the combined stream into a plurality of independent split streams;

introducing each independent split stream at a different elevation of the boiler; and

controlling independently a parameter of each of the independent split streams to adjust the heat release at each respective elevation of the boiler to vary the heat release profile of the boiler by adding a second oxidant stream to each respective independent split stream to form respective independent oxygen enriched split streams.

2. The method according to claim 1, wherein:

the oil heavy residues comprise asphaltene.

3. The method according to claim 1, wherein:

the boiler is a tangentially fired boiler.

4. The method according to claim 1, wherein:

controlling independently the parameter of each independent split stream further includes changing a heat absorption in the boiler to a desired heat absorption pattern.

5. The method according to claim 1, wherein:

at least one of the split streams is introduced into the boiler at a hopper zone located below a windbox, at the windbox and/or in an overfire compartment located above the windbox.

6. The method according to claim 1, wherein:

at least a portion of the combined stream is introduced into the boiler in a lower portion of the windbox.

7. The method according to claim 6, wherein:

the at least one split stream that is introduced into the boiler at the windbox is about 50 to about 100 weight percent of the combined stream.

8. The method according to claim 1, wherein:

the at least one split stream is introduced into the boiler in a lower portion of an overfire compartment.

9. The method according to claim 1, wherein:

the at least one split stream is introduced into the boiler in an upper portion of an overfire compartment.

10. A method comprising:

combusting a fuel that comprises oil heavy residues in a boiler, where the oil heavy residues that comprise hydrocarbon molecules having a number average molecular weight from 200 to 3000 grams per mole;

discharging flue gas from the boiler;

recycling a portion of the flue gas to the boiler;

combining a first oxidant stream with the recycled flue gases to form a first combined stream;

splitting the first combined stream into a plurality of independent split streams;

combining a second oxidant stream to each respective independent split stream provided to the boiler to form respective independent oxygen enriched split streams;

introducing each independent oxygen enriched split stream to a different elevation of the boiler; and

controlling independently the amount of the second oxidant stream added to each respective independent split stream to adjust the heat release at each respective elevation of the boiler to vary the heat release profile of the boiler;

wherein the first combined stream, the independent split streams, and the independent oxygen enriched split streams do not carry the fuel for the boiler.

11. The method according to claim 10, wherein:

the boiler is a tangentially fired boiler.

12. The method according to claim 10, wherein:

adding the second oxidant stream to form the respective oxygen enriched split streams is conducted at a position proximate to a point of entry into the boiler.

13. The method according to claim 10, wherein:

the respective split streams are sequentially introduced into the boiler.

14. The method according to claim 10, wherein:

at least one respective oxygen enriched split stream is introduced into the boiler at a hopper zone located below a windbox.

15. The method according to claim 14, wherein:

the oxygen enriched split stream introduced into the boiler at the windbox comprises about 50 to about 100 wt % oxygen, based on the total weight of the stream.

16. The method according to claim 14, wherein:

each oxygen enriched split stream is introduced into the boiler via an annular space disposed around an inner port, where the inner port introduces the fuel and transport air into the boiler.

17. The method according to claim 10, wherein:

the boiler is a wall fired boiler.

18. The method according to claim 10, wherein:

controlling independently the parameter of each respective oxygen enriched split stream introduced to the boiler changes the heat pattern of the boiler.

19. The method according to claim 14, wherein:

at least one respective oxygen enriched split stream is introduced into the boiler at an overfire compartment at the hopper zone; and

wherein the oxygen enriched split stream introduced into the overfire compartment at the hopper zone comprises up to 50 wt % oxygen based on the total weight of the oxygen enriched split stream.

20. A system comprising:

an air separation unit;

a boiler configured to combust oil heavy residues, the oil heavy residues comprising hydrocarbon molecules having a number average molecular weight from 200 to 3000 grams per mole;

a pollution control system;

a gas processing unit, wherein the air separation unit is upstream of the boiler, the pollution control system and the gas processing unit, wherein the boiler is upstream of the pollution control system and the gas processing unit, and wherein flue gas is recycled from the gas processing unit to the boiler via the air separation unit; and

a control system configured to control the addition of a first oxidant stream to the recycled flue gas to form a combined stream and to control the addition of a second oxidant stream to a plurality of independent split streams formed from the combined stream to vary the heat release profile of the boiler;

wherein each of the independent split streams to which the second oxidant stream is added is introduced to a different elevation of the boiler.